SOME STUDIES ON SEMICONDUCTOR OPTICAL ...shodhganga.inflibnet.ac.in/bitstream/10603/50262/1/thesis...iii The University of Burdwan Department of Physics Golapbag, Burdwan, West Bengal,

SOME STUDIES ON SEMICONDUCTOR OPTICAL

AMPLIFIER AND KERR TYPE OPTICAL SWITCHING

Thesis submitted to

The University of Burdwan, Burdwan

for the award of the degree

of

Doctor of Philosophy

by

Soma Dutta

under the guidance of

Prof. Sourangshu Mukhopadhyay

DEPARTMENT OF PHYSICS

THE UNIVERSITY OF BURDWAN, BURDWAN

DECEMBER 2012 © 2012, Soma Dutta. All rights reserved.

ii

Dedicated to

My Family

iii

The University of Burdwan Department of Physics

Golapbag, Burdwan, West Bengal, India-713104 Phone: 91-342-2657800 Fax: 91-342-2657800

……………………………………………………………………………………..........

CERTIFICATE FROM THE SUPERVISOR

This is to certify that the thesis entitled “SOME STUDIES ON SEMICONDUCTOR

OPTICAL AMPLIFIER AND KERR TYPE OPTICAL SWITCHING”, submitted

by Soma Dutta to The University of Burdwan, Burdwan, is a record of bona fide

research work under my supervision. The contributions comprised in the thesis are done

by her and it was not submitted to any other University or Institute for any degree or

diploma. It is my pleasure to mention that Soma Dutta did her Ph. D. work with full

sincerity and dedication. I wish every success in her life.

Date: Place: Burdwan

(Sourangshu Mukhopadhyay) Professor,

Dept. of Physics, The University of Burdwan,

Burdwan.

iv

Acknowledgements

I wish to express my deep sense of gratitude to my supervisor Prof. Sourangshu

Mukhopadhyay for his invaluable guidance and inspiration throughout this course of

research work. His advice, encouragement, patience and ample experience were

invaluable and vital to the completion of this work.

My wholehearted thanks to my laboratory colleague,, for their

cooperation, adjustment and encouragement during this long period of research. I am

also thankful to west Bengal state fund, for providing financial support during the course

of this study.

Above all, I would like to express my gratitude to my parents Mr. Mrinal Dutta

and Mrs. Sujata Dutta, my younger sister Sayani Dutta, my husband Supravat Karak and

all other family members and friends for their constant encouragement and love.

The University of Burdwan, Burdwan December, 2012 Soma Dutta

v

Preface

Optics has a strong and very potential role in information and data processing

because of its inherent parallelism. It has several advantages over electronics in super-fast

computation and data processing. Last few decades, several all optical data processors

were proposed based on Boolean logic. Those optical systems and optical logic devices

based on optical switches are found very much useful than electronic ones in connection

to speed and many other aspects. Different types of all-optical methods have been

proposed for implementation of the optical logic and arithmetic processors and devices.

These types of optical systems require some optical switches like non-linear material

based switches, electro-optic material based switches, optical bistable materials, optical

filters, optical converters and beam splitters, Semiconductor Optical Amplifier (SOA)

based optical switches etc.

There are several types of encoding principle to implement the all-optical logic

and arithmetic devices. These are the intensity encoding principle, polarization encoding

principle, phase encoding and frequency encoding principle. Above all these encoding

principles, frequency encoding principle is the most faithful and reliable one.

Frequency is the basic characteristics of light. In optical computing and data

processing therefore the most important point is the very high speed of processing. The

interesting point is that this problem in long distance communication can be solved by

using frequency encoding of light. One can encode and decode two different states of

information by two different frequencies. These frequencies are remaining unaltered

during the reflection, refraction, absorption etc when data is to be transmitted. Using this

vi

frequency encoding principle various types of all-optical logic operation are reported

earlier. These operations are based on different types of optical switches like SOA based

optical switches, optical modulator based switches, non-linear based switches etc. To use

these types of optical switches different types of all-optical logic processors and devices

are developed by scientists through all over the world.

Observing the tremendous importance of optics in computation and

communication I felt too much interested to contribute some research works in this area

of optical computing and parallel processing. The current thesis encloses my research

contributions. The work contained in this thesis is original and has been done by me

under the guidance of my supervisor. The work has not been submitted to any other

Institute for any degree or diploma.

(Soma Dutta) The University of Burdwan

Burdwan-713104 West Bengal

India

vii

CONTENTS

Title Page Some studies on Semiconductor Optical Amplifier and KERR type optical switching

i

Certificate iii Acknowledgements iv Preface v Contents vii Chapter 1 A challenge of optical information processing with

frequency encoded principle: An Introduction

1

1.1 Advantages of optics over electronics in information processing, computing and data handling

1

1.2 Optical logic, Arithmetic and algebraic operations with optical switching technology

3

1.3 Various types of encoding for all optical logic operations

4

1.4 Frequency encoding/ Wavelength encoding and its advantages

7

Optical switches for conducting frequency encoded logic system

9

1.5.1 Fundamentals characteristics of SOA

9

1.5.2 Using SOA as an Ultra fast Nonlinear Medium

11

1.5.3 Frequency conversion utilizing Nonlinear Polarization Rotation (NPR) of SOA probe beam

13

1.5.4 SOA based add/drop multiplexer

15

1.5.5 SOA based wavelength converter

17

1.5

1.5.6 SOA acting as a polarization-switch (PSW)

18

1.6 Objective of the thesis 20

1.7 Conclusion 21

References 23

Chapter 2 A method of implementing frequency encoded all-optical latch with semiconductor optical amplifier

33

viii

2.1 Introduction 34

2.2 Optical implementation of a NOT based latch 35

2.3 Optical implementation of two-bit memory cell 38

2.4 Conclusion 41

References 42

Chapter 3 Some new approaches of conducting all-optical frequency/wavelength encoded logic operations, programmable logic units and RS flip-flops with semiconductor optical amplifiers

44

3.1 Introduction 46

3.2 Scheme of realization of frequency encoded optical OR logic operation

47

3.2.1 Principle of operation of optical OR gate

48

3.3 Scheme of realization of frequency encoded Optical AND gate

51

3.3.1 Principle operation of optical AND gate

53

3.4 Scheme of realization of frequency encoded Optical NAND gate

55

3.4.1 Principal operation of optical NAND gate

56

3.5 Scheme of realization of frequency encoded Optical R-S flip-flop

59

3.5.1 Principle operation of optical R-S flip-flop

60

3.6 Frequency encoded programmable logic unit scheme 63

3.6.1 Optical OR logic gate controlled by an optical signal

63

3.6.1.1 Scheme of realization of optical OR logic gate controlled by the light signal

64

3.6.2 Optical AND logic gate controlled by the light signal

67

3.6.2.1 Scheme of realization of optical AND logic operation controlled by the light signal

68

3.6.3 Optical NAND logic gate controlled by the light signal

71

ix

3.6.3.1 Scheme of realization of optical NAND logic operation controlled by the light signal

71

Optical NOT logic gate controlled by the light signal

75 3.6.4

3.6.4.1 Scheme of realization of optical NOT logic operation controlled by the light signal

75

Optical XOR logic gate controlled by the light signal

77 3.6.5

3.6.5.1 Scheme of realization of optical XOR logic operation controlled by the light signal

77

3.6.6 Scheme of realization of frequency encoded programmable logic unit

82

3.7 Important requirements for the switching of SOA

84

3.8 Conclusion 85

References 86

Chapter 4 A new approach of implementing all-optical frequency/wavelength encoded clocked S-R flip-flop

88

4.1 Introduction 89

4.2 Optical implementation of clocked S-R flip-flop 90

4.3 Conclusion 94

References 96

Chapter 5 A new method for transmission of frequency encoded

parallel data

98

5.1 Introduction 99

5.2 A new method of frequency encoded parallel data transmission through optical waveguide

100

5.3 Essential requirements for implementation of the practical transmission of data

103

5.4 Conclusion 106

References 107

Chapter 6 A new approach of developing Universal all-optical multiplexer with frequency encoded mechanism

109

x

6.1 Introduction 110

6.2 Method of developing a frequency encoded universal multiplexer with SOA

111

6.3 Method of developing double triggering universal multiplexer

113

6.4 Practical realization 116

6.5 Conclusion 117

References 119

Chapter 7 Use of all-optical Kerr Cell for super fast conversion of a binary number having a fractional part to its decimal counterpart and vice-versa

121


7.2 Optical tree architecture 122

7.3 Non-linear material as an optical switch 124

7.4 Optical conversion method of a binary number having a fractional part to its equivalent decimal number

126

7.5 Optical conversion method of a decimal number having a fractional part to its binary equivalent

130

7.6 Alternating approach of non-linear switch 133

7.7 Conclusion 135

References 136

General conclusion and future scope of the work 138 Chapter 8


8.2 Conclusion of the thesis 138

8.3 Future scope of the work in this area 141

1

CHAPTER-1

A challenge of optical information processing with frequency encoding

principle: An Introduction

1.1 Advantages of optics over electronics in information processing, computing and

data handling:

Optics has already been established as a potential and promising candidate in

information and data processing. Many all-optical logical algebraic and arithmetic

processors have been proposed since the middle of the decade of seventy’s [1.1-1.9]. Several

schemes of all optical logic gates, optical digital memory units, optical algebraic

processors, image processors are proposed by the scientists around the globe since the

decade of seventy. The choice of optical signal in replacement of conventional electronic

signal in a data processor is mainly because of the inherent parallelism in optics, which

leads a suparfast up gradation of computing technology. Different types of all-optical

methods have been proposed for implementation of the optical logic and arithmetic

processors and devices. These types of optical systems are required some optical switches

like non-linear material based switches, electro-optic material based switches, optical bi-

stable materials, optical filters, optical converters and beam splitters, Semiconductor

Optical Amplifier (SOA) based optical switches etc. Again to support the increasing

demand and rapid growth information, optics has been proved as proper alternative for

very high speed communication with high bit rate and low bit error rate. In optical

communication network the response time at the nodes is a very important issue for

setting high speed communication. For managing the tremendously increasing day to day

2

data traffic, it is very necessary to enhance the transmission link capacity as well as the

speed of the switching networks at the nodes. The realization of a network node with

throughput at the order of 100 GB/s is not far away. SOA grating combination has been

successfully used for 160 GHz wavelength conversion. Again using a cross-correlation

system and non-linear polarization rotation 200 GBPS wavelength conversion at temporal

resolution at 1.5 PS is also reported. Some logic gates are also implemented based on the

four-wave mixing character of SOA with the mechanism of polarization shift keying and

with tri-state operation logic [1.10-1.14].

Communication and data processing is totally turned over to electronic

communication and data processing when electron was found in nineteenth century.

Actually twentieth century can be regarded as a century of electronics. But at the end of

the last century scientists and technologists found some big problems of electronic

systems which are relating to the speed of operation and many others also. These

problems are written serially below.

I. Electronic systems can not raise its speed greater than few GHz limit.

II. Being a charged particle it has coulomb interaction with other charged particle or

even among them. Because of these character an electron when behaves as an

information carrier, it faces Von-Neumann bottle neck problems, problems related

to cross-talk, interaction with external electric field, magnetic field and

electromagnetic field and many others.

III. Electronic massage can not be sent through a high magnetic field without

interaction.

3

IV. It has also traffic jam problem in information broad ways with increase of data in

the information banks.

The problems can be solved with optics. In optical system photon is used as an optical

signal carrier. It has many advantages over electronics. The advantages are-

I. Photon is a basically charge less particle having no rest mass and obeys Bose-

Einstein statistics and some times follows the classical passion distribution. For

these reason problems like cross-talk, interaction with other charged particle or

even among themselves can be removed.

II. It was proved that photonic system can be extended an operation far more than

THz limit.

III. It has high degree of inherent parallelism.

IV. In Electronic system I am forced to follow the only Boolean algebra. In photonic

system one can code binary, ternary, quaternary, decimal, hexadecimal or even

multi-valued data with optical signal. Different types of encoding system are there

in photonic system.

Except the above advantages there can be found many other points by which one can

infer that optics is very much suitable as an information carrier over electronics.

1.2 Optical logic, Arithmetic and algebraic operations with optical switching

technology:

There has been significant research attempt in the general area of information

processing by optical techniques over the last twenty years [1.15-1.21]. Recently optical

computation is the new addition part of this work. To perform numerical computations on

one-dimensional or multidimensional data that are generally not images optical

4

computing is defined as the use of optical systems. In optical computation one can

design, simulate, or analyze optical systems with optics and also can form the images.

Optical computing systems have many prospective advantages, and also many

disadvantages. Optical processors are fundamentally two-dimensional and parallel

systems and can have high space-bandwidth and time-bandwidth products. Without any

interaction optical signals can propagate through each other in separate channels, and can

transmit in parallel channels without interference and crosstalk. Due to inherent

parallelism of optics, in computation and arithmetic data processing it has made

successful contributions. Several types of operation schemes for arithmetic data

processing have already been reported by scientists over last few decades [1.22-1.32]. Even

though in wide and local area networks optical technologies are playing increasingly

important roles. Several types of all-optical logic and algebraic operations are reported by

many scientists. In recent years, some low-level all-optical logic gates mostly based on

the nonlinearity of semiconductor optical amplifiers (SOA) are reported by several

scientists [1.33-1.37].

1.3 Various types of encoding for all optical logic operations:

The most important thing to implement some successful optical logic and arithmetic

processors is how to encode your optical system. Several types of encoding systems are

used to develop optical logic systems. These encoding systems are depends on intensity

of light, polarization of light, phase change of light, frequency of light etc. Using these

characters of light some popular optical encoding systems are made as for example

Intensity encoding, Polarization encoding, Phase encoding and Frequency encoding

principles etc. In intensity encoding principle different intensities of light is encoded as

5

logic state 1 and 0. If I intensity of light represents the logic state ‘1’ then 2I intensity of

light represents the logic state ‘0’. In this intensity encoding method some non-linear

materials are used as a switch. For some isotropic material the well established Kerr non-

linearity equation is

Innn o 2+= (1.1)

Where n is the refractive index (r.i) of the concerned non-linear material, n2 is the non

linear correction term, n0 is a constant linear refractive index term and I is the intensity of

light passing through the material. As for example of some non-linear material working

as a optical switches are Pure silica glass (SiO2), carbon di-sulfide (CS2), gallium

arsenide (GaAs), etc. This material shows this type of non-linearity and also follows this

Kerr non-linearity equation. As for example for carbon di-sulfide the constant linear

refractive index term n0 = 1.62 and n2 = 0.22 × 10−19m2/W, whereas for pure silica the

values of constant linear refractive index term is n0 = 1.46 and n2 = 3.2 × 10−20m2/W. The

refractive index (n) is changed when the intensity of incident light is changed and then

the path of output light is also changed according to their changing r.i. Here any one can

measure the amount of lateral change of channel due to refraction of light from NLM.

This lateral change is measured by the term Δθ. Again it is seen that for use of input

signal pulse LASER have a significant value of Δθ but continuous LASER does not have

any significant value of Δθ. As for example for pure SiO2 a laser pulse of duration 10−7 s

and of power 100 mW incident light is made double then an angular separation of the

refracted beam (from the NLM) becomes 0.0130◦ whereas the input incident angle fixed

at 45◦ [1.38-1.44]. In polarization encoding principle polarization of light is encoded as logic

states 1 and 0. If un-polarized light represents the logic state ‘0’ then polarized light

6

represents the logic state ‘1’. To implement the Boolean logic operations the logic states

are indicated by the intensity level of the light signal where nonlinear material is working

as a switching element. For that reason it is required to maintain a constant intensity level

of the light signal to represent any logic state. However the intensity of the signal is

changed due to long distance communication of optical signal. Then the logic processor

whose working principle is based on the intensity level of the signal fails to work

properly in the detecting side. This problem may be avoided using the principle of

polarization based encoding/decoding technique. Using of optical polarization based

encoding systems for implementing logic family was already proposed by Lohmann et-al

[1.45-1.54]. In phase encoding principle two different phase of light is encoded as logic

states 1 and 0. In optical phase encoding process, a co-sinusoidal light signal having,

Acosωt represents the logic state 1 and with the help of a phase modulator introducing a

phase π in a co-sinusoidal signal Acosωt represents the logic state 0. In the decoding

process if the phase of the output wave is zero with respect to a reference signal

√3Acosωt then the output bit will be 1 and it will be 0 if the phase of the output wave is π

with respect to the reference signal √3Acosωt [1.55-1.58]. To implement Boolean logic one

can mention a frequency encoded technique which is more advantageous in contrast to

above proposals. It is very well known that frequency of light remains unchanged during

transmission of light signal. If the frequency n1 represents the logic state ‘1’ state and the

frequency n2 represents the logic state‘0’ then n1 and n2 will remain unaffected through

out the transmission of data. Similarly, if ν1 frequency of light represents the logic state

‘0’ then ν2 frequency of light represents the logic state ‘1’. Several types of all-optical

7

logic operation and logic devices is already developed successfully using these all

encoding techniques [1.59-1.60].

1.4 Frequency encoding/ Wavelength encoding and its advantages:

Frequency is the fundamental character of light. So it remains unchanged in

reflection, refraction, absorption etc. In optical computation photon is found to be a very

suitable information carrier than electron not only in the connection of super fast speed

but in many other aspects of information processing also. Thus these photonic systems

can successfully replace the electronic systems. Again it is also seen that in case of

optical data processing the conventional methodologies can not be followed always as it

is done in electronics. Scientists and technologists are deeply involved in research to

overcome the speed related difficulties to realize all-optical logic, arithmetic and

algebraic operations with Boolean mechanism. There are found several popular reports

on the development of optical logical systems where the logic gates are the basic building

blocks. To implement of optical Boolean logic systems using the coding norms, as the

presence of optical signal is equal to 1 and absence of optical signal is 0, face several

problems. For these many alternative approaches are reported to encode 1 and 0 with

optical signal. For example, several areas like modulation, spatial input/output encoding;

symbolic substitution etc. which deal with different coding principles. Again it is also

seen that many proposals are reported where the same coding mechanism (i.e. presence

of optical signal is regarded as 1 and absence as 0) is followed to implement the optical

logic gates in Boolean mechanism. In most of the cases, the presence of optical signal at

the input or output of a logical system is encoded as ‘1’ bit and the absence of the signal

is regarded as ‘0’ logic state, i.e. by intensity variation mechanism. As the intensity of

8

light decreases with the increase of optical path through a medium, so the intensity of

light may drop down below the reference level of the concerned logic state ‘1’ and may

enter into the reference level of logic state ‘0’. So this is not problem free encoding

system. Similarly polarization encoding principle has some disadvantages. In this

polarization encoding method two orthogonal polarized states of light are represented by

1 and 0 logic state respectively. During transmission and propagation the state of

polarization may change at reflecting points and refracting points, so this reason may

entree several problems in implementation of the optical logic gates. In wide range of

data processing, Boolean system has some own limitations. That’s why to implement

Boolean logic gates lots of different proposals may be mentioned for the different coding

norms. All the encoding norms and techniques mentioned above have some advantages

but all of them widen the loss dependent problem. In compare to those proposals one can

mention an advantageous proposal to implement Boolean logic which is called the

frequency encoded technique. The frequency encoding principle can be used as a more

reliable candidate than other encoding principles for the development of super-fast

optical processors, as the frequency of a signal remains unchanged even after different

optical transformations. To overcome all the above problems one can use the frequency

of light for encoding a bit. If the presence of a specific frequency of light is treated as ‘1’

state and then other specific one represents ‘0’ state i.e. if ‘1’ state is encoded by

frequency ‘ν2’ then that of the ‘0’ state is done by another frequency by ‘ν1’ where ν1 and

ν2 remain unaltered throughout the transmission of data. Frequency encoding techniques

is free from such transmission problems. This encoding technique has some more

9

advantages like very low BER effect, many data can be transmitted parallel through one

optical channel and it is not attenuated in long distance communication [1.61-1.64].

1.5 Optical switches for conducting frequency encoded logic system:

To develop the all-optical frequency encoded logic and arithmetic processors

scientists are used some optical switches like SOA based wavelength converters;

add/drop multiplexers, some optical filters, some optical prisms etc. The semiconductor

optical amplifier (SOA) is one of the most frequently used devices for ultra fast signal

processing. This is because the device technologies are already well established and it is

commercially available. Ultra fast signal processing depends on how to take out the

beneficial features of SOA [1.65-1.67]. In this chapter I briefly review about the basic

properties of SOA, their features and applications as an ultra fast nonlinear switch.

1.5.1 Fundamentals characteristics of SOA:

The full form of SOA is semiconductor optical amplifier. The basic structure of

an SOA is very simple and it has some similarity with semiconductor laser diodes. But

SOA is different from semiconductor laser diodes for an anti-reflection (AR) coating on

both facets and it is also sometimes used as the pointed structure of the waveguide close

to the facets. These are to minimize the reflection at both surfaces. The active layer is

surrounded as a gain medium and it can either be bulk, quantum well, or quantum dots.

Using the following equation optical gain of the SOA with bulk or quantum well active

layer can be achieved [1.68]

( ) ( ) ( ) ( )( )[ ]∫∞=

=

−=k

k

dkkkDcn

wg0

1Im χω

10

( )( )( )

( ) ( )( )dkkfkfk

kDcn vc

CVcv

cvCVk

k

−+−

= ∫∞=

=2222

2

00 γωω

μγεω

hh

h (1.2)

Here c is the velocity of light, ω is the light wave angular frequency, ε0 is the vacuum

dielectric constant, n is the refractive index and D(k) is the density of state expressed in

terms of wave number k, ωcv(k) is the k-dependent transition frequency between

conduction band (C) and valence band (V), γCV is the dephasing time and fC, fV are

Fermi–Dirac distribution functions. How to comprehend large output saturation power,

small noise and polarization insensitive gain is the main important thing in the

development of SOA. The output power saturation takes place for the intense optical

power due to reduced population inversion. It consumes the population inversion carriers.

If I use the SOA at an output power above the saturation power, I cannot get good eye

opening for random pattern modulation, known as the pattern effect. In general the

equation of saturation power is given by [1.68]

τωωρ

ds g

dC 1Γ

= h (1.3)

Where d is the thickness of active layer, C is the fiber-chip coupling efficiency, w is the

width of active layer, gd is differential gain, Γ is the optical confinement factor, τ is

carrier lifetime and Γωd corresponds to the mode cross-section.

For a good semiconductor optical amplifier the most essential things are output saturation

power, polarization intensive gain and low noise. The large saturation power depends on

the design of the active layer, optical confinement factors and carrier lifetime.

Polarization intensive gain also depends on the active layers. A waveguide with a thin

active layer gives a small loss to the TE mode when compared with that for the TM mode

11

for polarization dependence. The rectangular cross section of the waveguide and the

active layer give polarization independent waveguide loss. Because of degenerate heavy-

hole and light-hole bands the polarization dependence is small in the bulk active layer. In

the quantum well active layer polarization-dependent optical gain occurs for the removal

of degeneracy. For minimizing polarization dependence there are several types of designs

of active layer. They consist of geometry and strain of active layer and the waveguide

structure. Several types of SOA have been developed. The bulk active layer SOA may be

the most suitable available device for large saturation power and polarization insensitive

response.

1.5.2 SOA as an ultra fast non-linear optical switch:

Semiconductor optical amplifier is acting as an ultra fast non-linear medium. The

electron-hole pair density dependence of the nonlinear refractive index of GaAs is

investigated by Lee et al. Absorption reduces and becomes negative for high excitation

levels when increase the photo-excitation, correspondingly, the refractive index changes.

This analysis was carried out based on the Kramers–Kronig relation which is also

applicable for SOA. The refractive index changes with the optical gain correspondingly

because of the current injection. Slow response associated with carrier recombination is

one of the most inconvenient features of the SOA for their ultra fast signal processing. So

far two methods are reported to overcome this problem. One of them is the direct use of

the ultrafast response by selecting only the fast response using a wavelength filter[1.69-1.75]

and the other one is the use of a symmetric Mach–Zehnder configuration with SOA at

each arm to cancel out the slow response component [1.76,-1.78].

12

Four types of non-linearity’s are found in SOA, which are cross gain modulation

(XGM), cross phase modulation (XPM), self phase modulation (SPM), and four wave

mixing (FWM). Cross gain modulation (XGM) is a result of gain saturation in SOA. It

occurs when lights of two different wavelengths, a pump and a probe, are injected into

the semiconductor optical amplifier when operated under gain saturation conditions; the

available optical gain is distributed between the two wavelengths depending on their

relative photon densities. The changes in the power level of the pump wavelength have

an inverse effect on the gain available to the probe wavelength and results in data

transfer. When the pump (λ2 = 1550.68nm) is not present, the gain to the probe

wavelength (λ1 = 1552.24nm) is high so that the output power of the probe is very high.

When the pump (λ2) and probe (λ1) are injected into the SOA at the same time, the pump

power is so high that it saturates the gain of the SOA. The available gain to the probe (λ1)

will be reduced. So the output power of the probe is much lower. When the power of the

pump is modulated with data, the gain of the probe is also modulated. Thus the output

power of the probe is modulated. This results in transfer of data from pump to probe.

Thus wavelength conversion is achieved.

A cross-phase modulation (XPM) accompanies the cross gain modulation when

two optical signals are simultaneously present in the SOA. An interferometer

configuration can be used to convert the phase modulation to an intensity modulation.

XPM in a semiconductor optical amplifier (SOA) used in an interferometer configuration

has been used for all-optical wavelength conversion, optical demultiplexing and for

optical clock recovery. The scheme has high conversion efficiency and high signal to

13

noise ratio. Generally a Mach-Zehnder or Michelsen interferometer configuration

integrated on a single chip is used to convert phase modulation to intensity modulation.

Semiconductor amplifiers are known for optical nonlinear effects, such as the four-wave

mixing (FWM). FWM in SOA’s has been used as a technique for performing wavelength

conversion due to its good conversion efficiency and high speed response for wavelength

division multiplexing (WDM) networks. Four-wave mixing (FWM) is a process by which

optical signals at different (but closely spaced) wavelengths mix to produce new signals

at other wavelengths, but with lower power. In the FWM process, light at two

frequencies, ω0 and ω1, are injected into the amplifier. These injected signals are

generally referred to as pump and probe beams. The pump and probe beams can be

obtained from two single wavelength distributed feedback (DFB) lasers. The pump signal

is of higher power than the probe signal. Consider the case when both the pump and the

probe signals are CW. Propagation through the SOA results in the generation of two

additional FWM signals with frequencies 2ω0 − ω1 and 2ω1 − ω0. The intensity of light at

these wavelengths is measured using a spectrometer. The FWM signal at frequency

2ω0−ω1 has higher power if the pump signal strength (at frequency ω0) is higher than that

of the probe signal. If I0 and I1 are the intensities of the signals at frequencies ω0 and ω1,

the intensities of the signals at frequencies 2ω0−ω1 and 2ω1−ω0 are proportional to I02 I1

and I0I12 respectively.

1.5.3 Frequency conversion utilizing Nonlinear Polarization Rotation (NPR) of

probe beam in SOA.

Non-linear polarization rotation of the probe beam is the basic property of SOA.

Optically induced nonlinear refractive index in a bulk SOA by highly intense pump

14

beams are responsible for this Non-linear polarization rotation of the probe beam. The

intense pump beam can modify the optical properties of the SOA during the interaction of

the intense pump beam with probe beam in nonlinear SOA which in turn modify the

intensity of probe beam as well as its SOP. When a linearly polarized light is coupled in a

SOA then its SOP will change after leaving the SOA. To measure the non-linear rotation

in terms of intensity difference a polarization beam splitters (PBS) is present at the output

end. The whole scheme is shown in figure-1.1.

Now for the first case with suitable current the SOA has to be biased and also ‘X’

and ‘Y’ are the input pump beams are adjusted into the proper power level. ‘Z’ is the

linearly polarized weak intensity probe beam of frequency ν0. This weak probe beam is

coupled with the pump beams in SOA. The polarizer is adjusted in such a way that the

pass axis of the polarization beam splitter (PBS) is crossed with respect to SOP of the

linearly polarized probe beam (Z) when there is no input beams i.e. the absence of both

the input pump beams ‘X’ and ‘Y’. In this situation no light is obtained at the output end

(O). Again no light is obtained at the output end (O) when only one input pump beam is

present at the input (X/Y). So ν0 frequency is obtained at the final output end (O) only

when two pump beams are present at the input end (X, Y). It is to be noted that if only

one pump beam has such intensity which is equal to sum of both input pimp beam

intensity then one pump beam of such intensity can change the state of polarization of the

probe beam of SOA. So, therefore using that pump beam of such intensity as a control

beam one can get the transmitted probe beam to input end to output end.

15

Figure-1.1: Frequency conversion of probe beam by NPR method of SOA.

1.5.4 SOA based add/drop multiplexer

To achieve a successful routing of wavelengths, the ability of SOA to add and

drop a specific wavelength channels in a wavelength-division multiplexed (WDM)

network is also a great important function. This is an Add/Drop multiplexing unit of

SOA. The function of an Add/Drop multiplexer (ADM) is to select one particular

wavelength of light without interfering with the adjacent wavelengths. Several types of

add/drop multiplexers are reported for developing several optical devices, some of them

use grating filters and circulators and others use different light wave technology. The

filters can be tuned by changing the biasing input current into the SOA. The tunable filter

has the transfer function of the spectral width 0.9 nm around the selected wavelength.

The selected specific wavelength is reflected by the filter and amplified by the multiple

SOA XY

O

INPUT PUMP BEAMS LINEARLY

POLARIZED BEAM

OPTICAL FILTER PBS

16

quantum wells (MQW) and the circulator is used to drop the selected wavelength in a

required direction. Others wavelength which comes parallel at the input along with the

specific one will pass through the SOA made filter. So one can separate a particular

wavelength of light using this add/drop multiplexer from a band of wavelengths. The

system is shown in figure-1.2 schematically. As for example if one wants to get particular

ν1 frequency light from a stream of data of frequencies ν1, ν2, ν3,….νn then SOA has to be

adjusted or tuned at the proper biasing current of ν1 frequency then all the frequencies

passes through the ADM only ν1 frequency is reflected form the SOA and is collected by

the circulator [1.65, 1.79].

Figure-1.2: SOA based ADD/DROP multiplexer

ADD/DROP Multiplexer

Input signal λ1 (ν1), λ2 (ν2), λ3 (ν3)…λn(νn)

Reflected signal λ1 (ν1)

Circulator (C)

Biasing terminal for ν1 frequency

λ2(ν2), λ3 (ν3)… λn(νn)

MQW amplifier Grating filter

Output signal λ1 (ν1) collected by the circulator

17

1.5.5 SOA based wavelength converter

In modern communication those switching devices are be very much effective

where one light signal is switched by another light signal. Wavelength conversion is

based on the XGM character of SOA is a result of gain saturation phenomenon. A weak

CW probe beam (at a specific wavelength) and a strong pump beam (at another specific

wavelength) of light are injected jointly into the SOA. At a suitable biasing current in the

amplifier the probe beam will be treated as a strong beam output from the SOA because

of XGM character. Thus one can refer this incidence as wavelength conversion. There

are two types of basic schemes used in XGM based wavelength conversion, one is co-

propagating and another one is counter propagating schemes. In the first case the pump

and probe beams are injected from the same side of the SOA and in the second scheme

pump and probe beams are injected in mutually opposite directions into the SOA. Co-

propagating scheme has better noise performance. A weak CW (continuous wave) probe

light of wavelength λ2 and a strong pump beam of wavelength λ1 are injected into the

input terminals of the SOA having an anti-reflecting surface at its input side for λ2 and a

highly reflecting surface for λ1 at the output terminal. In this situation the strong pump

beam transfers its total power to the weak probe beam and thus the weak probe beam

being a stronger one and comes to the output terminal. The scheme is shown in figure-

1.3. If there lays no pump beams at the input side, no conversion is allowed [1.65, 1.66].

18

Figure-1.3: SOA based wavelength converter

1.5.6. SOA acting as a polarization-switch (PSW)

Semiconductor optical amplifier can work as a polarization switch. To design the

non-linear polarization switching (PSW) one can use the properties of polarization SOA–

gain saturation [1.80-1.83]. To implement this switching system two laser sources of

different frequencies, one strained bulk SOA, a power meter, three polarization

controllers, an attenuator and one polarization beam splitter (PBS) are needed. The whole

scheme of the polarization switching is shown in Fig-1.4. The polarization controllers

PC1, PC2, PC3 are controlled the probe beam (this is a CW laser of ν1 frequency), pump

beam (highly intense beam of frequency ν2) and output beam respectively. Now to

reduced the power of input probe beam (-15 dBm) it is applied to one input terminal of

SOA via an attenuator. The polarization direction of the input probe beam be

Semiconductor optical

amplifier

SOA

Strong pump beam

CW weak probe beam

λ1 (ν1)

λ2 (ν2)

λ1 (ν1)

Output strong light beam of frequency ν1

19

approximately 450 to the orientation of SOA layer because the orientation of linearly

polarized probe beam is adjusted by PC1 in that a way. The polarization beam splitter

(PBS) is combined the output beam of SOA. The output beam from SOA is divided into

two parts by the PBS, one part is the horizontal (H) and another part is the vertical

polarization component (V). The vertical component of SOA output is received at port-1

and horizontal component at port-2 respectively. The two modes transverse electric field

(TE) and transverse magnetic field (TM) components which are decomposed by the

optical field of linearly polarized light due to absence of pump beam propagate through

SOA independently. This propagation should be amplified by the biasing current in SOA.

When the maximum gain of TE and TM modes are almost equal because of the biasing

current value (162 mA) then under this condition PC3 orients the state of polarization of

output beam of SOA and the beam at the output port-1 becomes zero i.e. vertical

component (V) of the output beam of SOA is absent and as a consequence maximum

power is delivered at port-2.

SOA have the property of polarization dependent gain saturation. So in the case

the polarization dependent gain saturation character give rise to different refractive index

change for TE and TM when highly intense pump beam is present.

So one can say probe beam will appear at port-2 (ON-state) when the pump beam is

absent. The probe beam will be suppressed in port- 2(OFF-state) when the pump beam of

specific intensity is present and it is Obvious that the state of port-1 will be

complementary with respect to port-2 i.e. power will develop at port-1 when the pump

beam is present [1.65, 1.66].

20

Figure-1.4: SOA acting as a polarization switch 1.6 Objective of my thesis:

From the above discussion, it is clear from the study of optical switches and encoding

systems that there exists considerable scope for investigation to improve the performance

of the systems used in optical computation and communication. This dissertation contains

seven chapters with a focus on some of the above issues.

Chapter 1 gives a detail introduction on the need of optical frequency encoding and

different types of optical switches. This chapter also discusses on some back ground

review works in this area.

Chapter 2 reports on the implementation of all-optical frequency encoded NOT latch

using semiconductor optical amplifier based switches.

Chapter 3 mainly deals with frequency encoded logic gates, R-S flip-flop and

programmable logic unit with semiconductor optical amplifier based switches.

SOA ATTENUATOR

PUMPING CURRENT

PORT 2

PROBE BEAM ν1

PUMP BEAM ν2

PORT 1

PBSPC3PC1

PC2

H

ν

21

Chapter 4 presents a frequency encoded all-optical clocked S-R flip-flop based on

semiconductor optical amplifier switches.

Chapter 5 reports on the transmission on the frequency encoded parallel data.

Chapter 6 deals with the frequency encoded universal all-optical multiplexer system.

Chapter 7 reports on all-optical Kerr Cell for super fast conversion of a binary number

having a fractional part to its decimal counterpart and vice-versa.

Optical systems are very important for computation and communication and it

will be very much essential and effective for the future communication. So the other

objective of the thesis is to get a review work on the use of different types of optical

switches for implementation of different types of optical logic systems. Optical systems

with frequency encoding principle are very new concept and have many advantages over

others. So, I make a review work on this topic. The objective is to know about the

different types of optical switches which are very essential to develop optical logic and

arithmetic systems. Again I also take a look of those optical devices which are already

been established successfully. In this thesis I want to get an overview of all-optical

frequency encoded logic operations.

1.7 Conclusion;

By reviewing in this work in the area of frequency encoded optical logic and

arithmetic operations I received the knowledge about the various kinds of optical

operations and different optical systems and also got the idea of using light in high speed

communication systems. I also reported the advantages and disadvantages of different

types of optical encoding and decoding processes, especially the advantages of frequency

encoding principle and different types of frequency encoded optical switches, devices and

22

methods are discussed here. The prime beauty of using frequency of light for encoding

the logic states to implement the optical system is that the frequency is the fundamental

character of light, so it is unchanged during reflection, refraction, absorption and

transmission etc. this frequency encoding technique is more reliable than other encoding

techniques to implement the optical logic and arithmetic operations.

23

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32

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33

CHAPTER-2

A method of implementing frequency encoded all-optical latch with

semiconductor optical amplifier

ABSTRACT

This chapter includes a method of implementing frequency encoded all-optical NOT

latch unit. Optical logic gates based on the principle of frequency conversion of some

non-linear materials play the key role for the implementation of a frequency encoded data

processing system. Memory is one of the most important units to implement the logic

operations. Using semiconductor optical amplifier based switches and frequency

encoding principle make the latch unit to be more faithful and reliable one.

Work reported in this chapter was published in:

S. Dutta and S. Mukhopadhyay, “An all optical approach of frequency encoded NOT

based Latch using semiconductor optical amplifier” J Opt 39 (1), 39–45, 2010.

34

2.1 Introduction:

Increasing demand for a faster and reliable data processor has given birth of the

concept of all optical super-fast computers. In last few decades, there are several

proposed optical and photonic devices, which can be run with operation speed far above

the conventional GHz limit. Many of those devices have been dedicated for performing

logic, arithmetic and algebraic operations to achieve the goal of all-optical computer and

data processor [2.1-2.6]. Among all other components, memory is an essential one for any

data processor. Different types of volatile and non-volatile memories have already been

developed successfully for the present electronic data processing systems. Similarly

optical memories are also of great importance for the development of optical computing

technologies. Many scientists are involved in deep research for the realization of digital

volatile and non-volatile optical memories. Though some successes have been achieved

in the last few decades to develop optical memories, flip-flop, bi-stable multivibrators

and latches, still the ultimate goal of developing an optical computer has not been

fulfilled yet.

In this chapter, a methodology of developing optical NOT based latch using a

specific non-linear behavior of semiconductor optical amplifiers (SOA) [2.7-2.10]

accommodating with frequency encoding technique are reported. This all-optical latch

system is developed by the use of one special type of semiconductor optical amplifier

based switch that is wavelength converter. Several types of memory units are reported

earlier [2.11, 2.12]. For using SOA based switch and frequency encoding principle a reliable,

super-fast and not attenuated operation is achieved.

35

2.2 Optical implementation of a NOT based latch:

Memory is the basic requirement to construct any electronic or optical processor.

To develop a complete unit of digital optical memory, the first step is to develop a latch

or a 1-bit memory unit as it can store a single bit. The proposed system described here is

based on frequency encoding principle (fig.2.1). Here two different frequencies are used

for encoding 1 and 0 i.e. if ν1 frequency represents the state 0 then ν2 represents the state

‘1’. A is the input terminal and Q and Q are the two output terminals. To implement the

optical NOT based latch logic some beam splitters (BS), and mirrors (M) and some SOA

based wavelength converters (WC) are used at different position of the system. Here two

frequency selecting filters are used, where one is ν1 optical pass filter and other is ν2

optical pass filter. When ν1 frequency is applied at A, the light beam enters only to WC1,

but not in WC4. Thus the ν1 frequency of light beam which behaves as a week probe

beam now falls on to the WC1 from the input terminal A. Another proper constant beam

strong pump light (CW) of ν2 frequency is injected to the WC1. For which the ν1

frequency of light can be obtained which is again injected as a strong pump beam to the

input terminal of WC2. A constant weak probe beam of frequency ν2 (CW) is applied at

the input of WC2, for which an output of ν2 frequency is obtained at the terminal Q for

the application of ν1 at A. Here CP’s (cross polarizer’s) are used in the output of every

WCs to block the unwanted light beam i.e. for this CP at the time of absence of pump

beam or probe beam no light beam will come out from the WCs. This output light beam

is feedback as a pump beam to the WC3 and a constant weak light of ν1 frequency is also

applied on the other input of the WC3 to get the output of at ν1 frequency which is divided

into two parts by a beam splitter (BS). One part is feedback to the input of the WC1 and

36

the other part is sent to the output terminalQ . Thus ν1 is obtained at A, ν2 at Q and ν1

atQ .

Now ν2 frequency of a week probe beam of light is given at input A then it falls

directly to the WC4, but it will not at all enter at WC1 and a constantly supplied strong

pump beam of ν1 frequency is applied on the other input channel of the WC4. In this

situation the ν2 frequency of light will come out from the output of WC4 which is again

treated as a strong pump beam to fall on the WC5. A constant week probe beam of

frequency ν1 is kept in the input channel of WC5 which helps to get the intense ν1

frequency of light at the output of WC5 (X). This is divided again into two more parts;

one is feedback to the input of the WC6 as a strong pump beam where the other part

comes at the output X, which is ultimately connected with Q by BS and M. Again a

constant week probe beam of ν2 frequency given to the 2nd input of WC6 which ensures

the output of ν2 frequency at the output Y which is ultimately connected withQ . A

portion of the output at Y is feedback to the input of WC4. This is the overall connection

of the whole latch unit. Now to describe the operation it can be said that when ν1

frequency of light i.e. logic state (0) is applied at the input terminal A, the upper portion

of the system described in figure-2.1, activated but lower half does not because of

presence of the ν1 optical pass filter which only allows the ν1 frequency of light in the

upper half of the unit and the ν2 pass filter only passes the ν2 frequency of light in the

lower half of the system described in figure-2.1. So when ν1 frequency of light i.e. logic

state (0) is given to input terminal A the ν2 frequency of light is obtained at output Q and

ν1 frequency of light at Q i.e. Q =1 and Q =0 when A=0. Similarly when ν2 frequency of

light i.e. logic state 1 is applied at the input terminal A, the lower portion of the system

37

takes the major role instead of upper portion and the ν1 frequency of light is obtained at

Q and ν2 at Q i.e. Q =0 and Q =1 when A=1.

Now when ν1 frequency of light is applied into the input terminal A, then light

beam passes through the optical ν1 pass filter and is applied into the WC1 as a weak probe

beam. Here a strong pump beam is already present at the input terminal of WC1, so the ν1

frequency of light is achieved at the output of WC1 which is applied as a strong pump

beam to the WC2. Presence of ν2 frequency as a weak probe beam at the input of WC2 the

conversion is obtained and the ν2 frequency is achieved at the output terminal of WC2.

This light beam is divided into two parts, one part is applied as a strong pump beam into

the WC3 and another part comes at the outputQ . Where as in presence of ν1 frequency at

the input of WC3 the conversion is obtained and the ν1 frequency is achieved at the output

of WC3. Here the light beam is divided into two parts, one is feedback to the WC1 and

another part comes to theQ . So Q =1 and Q =0 when A=0. The most important and

interesting point here is that if the input ν1 and ν2 frequency of light are withdrawn, the

system will continue to show the last attended values of Q and Q at the final output

because of the feedback mechanisms. At this situation the feedback light will continue to

excite the respective WCs. So, this system can behave as a frequency encoded optical one

bit memory.

38

Figure-2.1: Frequency encoded optical one-bit memory cell based on latch logic. (WC,

M, CP, BS are represents the wavelength converter, mirror, cross polarizer and beam

splitter or combiner respectively).

2.3 Optical implementation of two-bit memory cell:

Now slightly extending the one-bit memory cell circuit a two-bit memory cell

(fig.-2.2) can be developed. Here I have attached two similar optical circuits of one-bit

memory unit. Here A and B are the input terminals and X1 and Y1 are the output

terminals. The block diagram of this scheme is shown in figure-2.2. Here A* and B* are

the two unit memory cells (latches), Q and Q are the output terminals of A* block and

X1 and Y1 are the output terminals of B* block. X1 is then combined with Q and Y1 is

combined withQ . So, an overall two-bit memory cell is developed. Now two inputs are

WC1

WC2

WC3

WC4

WC5

WC6

Probe beam Pump beam

ν1 or ν2 A

ν1 pass filter

ν2 pass filter

Q

Q

BS

BS

M

M

M

M

M

M

M

M

BS

BS M

M

M

M

M BS

M

M

MM

M

ν1

ν1

ν1

ν1

ν1

ν1

ν1

ν1

ν1

ν1

ν2

ν2 ν2

ν2

ν2

ν2

ν2

ν2ν2

ν2

ν2

X

Y

CP

CP

CP

CP

CP

CP

39

A and B and two outputs are Q and Q , if A=1(ν2) and B=0(ν1) then Q =0(ν1) and

Q =1(ν2) whereas if A=0(ν1) and B=1(ν2) then Q =1(ν2) and Q =0(ν1). The states of the

last output will be attended if there is no light signal present in the input terminal i.e. for

the withdrawal of optical signal, the system to continue to show its last attended values at

the output. The truth table of this memory unit is given in table 2.1.

Table-2.1 Truth table of optical two bit memory cell

INPUTS OUTPUTS

A B Q Q

ν 2(1) ν 1(0) ν 1(0) ν 2(1)

ν 1(0) ν 2(1) ν 2(1) ν 1(0)

No light No light Last state attended Last state attended

40

A

B

BLOCK-A*

BLOCK-B*

Q

X1

Y1

Input terminals

Output terminals

Q

Figure-2.2: Block diagram of frequency encoded optical two-bit memory cell based on

NOT latch logic. Here each block comprise the memory or latch as given fig. 2.1.

41

2.4 Conclusion:

Frequency encoded technique based optical NOT latch or memory cell has been

proposed here. The whole system is all-optical one and is expected to support a high

speed operation (far above GHz limit) which is a potential advantage of this mechanism.

The coded information (0 or 1) of an output signal remains unaltered in reflection,

refraction, absorption etc. due to this encoding principle. Therefore this technique will be

very much useful in reliable optical communication. To achieve the faithful amplification

the pump beam of WC should lie between 4dB to 10 dB. The proposed system does not

only offer a high speed operation but also offers noise free conversion to provide a high

signal to noise (S/N) ratio. Using this latch logic and the frequency encoded technique

one can implement many other operations like digital types of flip-flops, multiplexer,

demultiplexer etc. with some modifications of the scheme.

42

References:

2.1 Kuladeep Roy Chowdhury, S. Mukhopadhyay, “A new method of binary addition

scheme with massive use of non-linear material based system,” Chinese Optics Letters,

1(4), April 20, 2003, 241-242.

2.2 Partha Ghosh, Partha Pratima Das, S. Mukhopadhyay, “New proposal for optical flip-

flop using residue arithmetic”, ITCOM-2001, (4534B-22), SPIE Proceedings

(Optoelectronic and Wireless Data Management, Processing, Storage and Retrieval),

4534, 8 November, 2001, 148-154.

2.3. Archan Kumar Das, S. Mukhopadhyay, “General approach of spatial input encoding

for multiplexing and De-multiplexing”, Optical Engineering (USA), 43(1), 1 January,

2004, 126-131.

2.4 Kuladeep Roy Chowdhury, S. Mukhopadhyay, “Binary optical arithmetic operation

scheme with tree architecture by proper accommodation of optical nonlinear materials,”

Optical Engineering, 43(1), ,1 January, 2004, 132-136.

2.5 Nirmalya Pahari, Debendra Nath Das, S. Mukhopadhyay, “All-optical method for the

addition of binary data by non-linear materials’, Applied Optics, 43(33), 2004, 6147-

6150.

2.6 Nirmalya Pahari, S. Mukhopadhyay, “An all-optical R-S flip-flop by optical non-

linear material”, Journal of Optics, 34(3), 2005, 108-114.

2.7 Sisir Kumar Garai, Debajyoti Samanta, S. Mukhopadhyay, “All-optical

implementation of inversion logic operation by second harmonic generation and wave

mixing character of some nonlinear material,” Optics and Optoelectronic Technology

(China), 6(4), 2008, 39-42.

43

2.8 L. Q. Guo and M. J. Connelly, “A poincare approach to investigate nonlinear

polarization rotation in semiconductor optical amplifiers and its applications to all-optical

wavelength conversion”, Proc. of SPIE Vol.6783, 2007, 678325(1-5).

2.9 H. J. Dorren, Daan Lenstra, Yong Liu, Martin, T. Hill, Giok-Djan Khoe, “Nonlinear

polarization rotation in semiconductor optical amplifiers: theory and application to all-

optical flip-flop memories “, IEEE Journal of Quantum Electronics, 39(1), Jan-2003,

141-148.

2.10 M. A. Karim and A. S. Awwal, John Wiley and Sons, “Optical Computing An

Introduction” INC.,1992

2.11 S.K. Garai, S. Mukhopadhyay, “A method of optical implementation of frequency

encoded different logic operations using second harmonic and difference frequency

generation techniques in non-linear material”, Opt. Int. J. Light Electron. Opt. (2008),

doi: 10.1016/ j.ijleo.2008.10.011, 121(8), 2010, 7.

2.12 Bikash Chakraborty, S. Mukhopadhyay, “Alternative approach of conducting

phase-modulated all optical logic gates,” Optical Engineering, 48(3), 2009, 035201.

44

CHAPTER-3

Some new approaches of conducting all-optical frequency/wavelength

encoded logic operations, programmable logic units and RS flip-flops

with semiconductor optical amplifiers

ABSTRACT

In this chapter includes the frequency encoded logic operations, programmable logic

units and also RS flip-flop. In conduction of parallel logic, arithmetic and algebraic

operations, optics has already proved its successful role. Since last few decades a number

of established methods on optical data processing were proposed and to implement such

processors different data encoding/decoding techniques have also been reported. Memory

is the most important criteria of any fundamental computation and also communication.

Here in this chapter I want to explore the memory unit and logic operation as well as the

programmable logic operations also. Using this programmable logic system any types of

logic operation have been obtained just changing the controlling signal within a one

system.


1. S. Dutta, S. Mukhopadhyay, Alternating approach of implementing frequency encoded

all optical logic gates and flip-flop using semiconductor optical amplifier, Optik-Int. J.

Light Electron Opt.(2010), 122 (2011) 1088–1094.

45

2. S.Dutta and S.Mukhopadhyay, “Application of Semiconductor Optical Amplifier for

development of ultra fast programmable unit” Frontiers in Materials Science-2010,

December-06, National Institute of Technology Durgapur, INDIA, Paper was presented

(poster) by S. Dutta on 6th December and awarded for the best paper.

3. S.Dutta and S.Mukhopadhyay, “A new approach of all-optical frequency encoded

programmable unit” en2c, 21st January-2011.

46

3.1 Introduction:

All optical signal and data processing is especially attractive for high speed and

high capacity computation to avoid the speed related problems in optoelectronic

processing systems. Because of the inherent character of parallelism of light can show

more strong and potential applications in information processing, computing, data

handling and image processing. In optical computation photon is found to be a very

suitable information carrier than electron not only in the connection of super fast speed

but in many other aspects of information processing also. Thus these photonic systems

can successfully replace the electronic systems. Again it is also seen that in case of

optical data processing the conventional methodologies can not be followed always as it

is done in electronics. There are found several popular reports on the development of

optical logical systems where the logic gates are the basic building blocks.

In previous chapter I have focused about the NOT based memory latch unit. In

this chapter I have propose frequency encoded optical logic gates, programmable logic

unit and RS flip-flop. Several types of optical mechanism are there for implementing

several types of optical logic gates and optical flip-flops [3.1-3.8]. Those systems are like

non-linear material, polarization based, phase encoded based etc. But those systems have

some problems due to long distance, interaction etc. In this chapter I have used frequency

encoding principle to overcome all problems and make the operations with greater

accuracy. Different types of frequency encoded logic systems are reported earlier using

different kinds of semiconductor optical amplifier based switches [3.9-3.12]. In this chapter I

have proposed alternative and new approach of frequency encoded logic operations, RS

flip-flop and programmable logic operations in a very simple way using simple kind of

47

SOA based switches [3.13]. Some logic gates with controlled light signals are combined to

assemble a programmable logic unit. By controlling the light signals of a programmable

logic unit one can operate the corresponding logic operation and this is the beauty of this

system.

3.2 Scheme of realization of frequency encoded optical OR logic operation:

Optical logic gates are the basic building block to implement any optical logical

functions or operations. OR gate is one of the most important and a basic unit of integral

logical system. To develop the system some ADD/DROP multiplexers, wavelength

converters, mirrors (M) and beam splitters (BS) are used which is shown in fig-3.1. Here

the input beam A, and B may have either the frequency of ν1 or ν2 (wavelength λ1 or λ2

respectively), where ν1 frequency of light is encoded for ‘0’ state and ν2 frequency of

light encoded for ‘1’ state. Now a light beam of frequency ν1 or ν2 from point A falls on

the 1st ADM which is tuned at the biasing frequency ν2 then only ν2 frequency of light is

reflected by the ADM and is captured by the optical circulator C1 and on the other hand

the ν1 frequency of light passes through the ADM and falls on the BS and one part of

light falls on the 1st wavelength converter (WC) as a pump beam and another part of light

falls on the 2nd wavelength converter (WC) as a pump beam, so for the case of the 1st WC

if the probe beam is present then the WC works and a converted strong probe beam is

obtained at the output which is the input of the 3rd ADM and in absence of the probe

beam no output is obtained. Now in the 2nd WC there is a constant weak probe beam so if

there is found a strong pump beam then a converted strong beam is obtained at the output

which is injected again to the 3rd WC as a pump beam. The captured tuned frequency of

light from the first circulator C1 of 1st input terminal A is injected to the 3rd ADM with

48

the help of some mirrors and beam couplers. Here for the 2nd input terminal the similar

type of input light (ν1 or ν2 frequency) is injected to the 2nd ADM which is also tuned with

the biasing frequency (ν2). So only this frequency of light is reflected and captured by the

circulator C2 and given to the 1st WC as a weak probe beam. Another frequency of light

passes through the 2nd ADM and falls in the 3rd WC as a weak probe beam. If the pump

beam and probe beam are present then only a strong probe beam is obtained at the output

otherwise absence of any probe or pump beam no output can be generated. The output

injected to the 3rd ADM. So the final output result of frequency encoded OR logic

operation is obtained from the terminal Y. The scheme is shown in figure-3.1.

3.2.1 Principle of operation of optical OR gate:

Now when the ν1 frequency of light is applied to the first input terminal A the

light passes through the 1st ADM and it is divided into two parts by the BS, where one

part is treated as a strong pump beam to the 1st RSOA and other part is delivered to the

2nd RSOA also as a strong pump beam. One constant weak probe beam of ν2 frequency is

delivered to the 2nd RSOA, so that at the output it gives a strong light beam of ν2

frequency which is again applied as a strong pump beam to the 3rd RSOA. Here a light of

ν1 frequency is applied to the second input terminal B which passes through the 2nd ADM

and delivered as weak probe beam to the 3rd RSOA. This light beam which is emerged an

output beam of ν1 frequency light and is applied to the 3rd ADM. Here to select the proper

frequency of light one optical filter is kept. Again the 3rd ADM is tuned at its biasing

frequency ν1 so it reflects the output beam from 3rd RSOA and it is collected by the

circulator and combined with the output terminal Y with the help of mirrors. Thus ν1

frequency of light is obtained an output result. In this case (A=B= ν1) there is no probe

49

beam in the 1st RSOA, so 1st RSOA can not work and the output beam is obtained from

the 1st RSOA. In the second case ν1 frequency of light is given at the input terminal A and

ν2 frequency of light is to the second input terminal B. Here the 1st ADM passes the light

and it is divided into two parts by the BS where one part is given to the 1st RSOA as a

strong pump beam and other part is given also as a strong pump beam to the 2nd RSOA

and for which the output result of ν2 frequency of light is obtained from the 2nd RSOA,

which serves as a strong pump beam to the 3rd RSOA but as there is no probe beam in the

3rd RSOA because B=ν2, so no ν1 frequency of light comes to the 3rd RSOA and it does

not convert any signal so no output result is obtained from the 3rd RSOA. As the second

input terminal B receives ν2 frequency of light so the 2nd ADM (tuned at its biasing

frequency ν2) reflects the light beam and it is collected by the circulator C2 as shown in

the figure-3.1. This light is applied as a weak probe beam to the 1st RSOA. So the 1st

RSOA works and the output result of ν2 frequency of light is obtained, which is delivered

to the 3rd ADM and I get the ν2 frequency of light at the output terminal Y. In the third

case when A=ν2 and B=ν1, 1st ADM reflects the light beam and it is collected by the

circulator C1 and applied to the 3rd ADM. Now the beam passes through the 3rd ADM and

the result of ν2 frequency of light is obtained at the output Y. The rest of circuit can not

take part in this conversion. Now when both the two input takes ν2 frequency of light the

1st ADM reflects the light beam and it is collected by the circulator C1 and delivers it to

the 3rd ADM, and passed through this ADM and the converted light beam of frequency ν2

is obtained at the output terminal Y. Similarly in the 2nd ADM the light is reflected by the

ADM and collected by the circulator C2 and is delivered as a probe beam to the 1st

RSOA, but as there is no pump beam in the 1st RSOA it can not work and no converted

50

light beam is obtained from it. So that Y= ν2 when A = ν1 and B=ν2; when A=ν2 and B=

ν1; Y=ν2; when A=ν2 and B=ν1; Y=ν2 and finally when A=B=ν2; Y= ν2. This verifies the

truth table of OR gate which is shown in table-3.1, if ν2 is encoded as 1 and ν1 by 0.

Table-3.1: Truth table of optical logic OR gate

A B Y

ν1 (0) ν1 (0) ν1 (0)

ν2 (1) ν1 (0) ν2 (1)

ν1 (0) ν2 (1) ν2 (1)

ν2 (1) ν2 (1) ν2 (1)

51

Figure-3.1: Frequency encoded optical OR gate

3.3 Scheme of realization of frequency encoded Optical AND gate:

To implement optical AND gate two channels are taken A and B and they may

have either ν1 (corresponding wavelength λ1) frequency of light or ν2 (corresponding

wavelength λ2) frequency of light. This whole system is shown in figure-3.2. Now a

beam of light of frequency ν1 or ν2 falls on the 1st ADM which is tuned in the biasing

ADM1

RSOA3 ADM2

RSOA2

RSOA1

ADM3

C1 C3

C2

1

PROBE BEAM

PUMP BEAM

OPTICAL PASS FILTER

MIRROR

C CIRCULATOR

1, 2 –ν2 and 3-ν1 optical pass filter

A

B

Y

A, B INPUT TERMINALS

Y OUTPUT TERMINAL

3

ν2

ν2

ν1

2

52

frequency ν2 so only ν2 frequency of light is reflected back by ADM and captured by the

circulator C1 and the ν1 frequency passing through the ADM is injected to the 1st

wavelength converter (WC) as a weak probe beam. A constant ν2 frequency of strong

pump beam of light is given to the 1st WC. If both the pump and probe beam is present in

the RSOA (WC) then the converted strong probe beam i.e. ν1 frequency of light is

achieved at the output. Now the reflected ν2 frequency of light from the 1st ADM is

captured by the circulator C1 and is reflected by the mirror and is injected as a weak

probe beam to the 2nd RSOA. Here from the channel B the input beam of light of

frequency ν1 or ν2 falls on the 2nd ADM which is also tuned at its biasing frequency ν2. So

it reflects the ν2 frequency of light and passes the ν1 frequency of light through it. This

light beam merges with the probe beam of light of 1st RSOA with the help of mirror. The

reflected ν2 frequency of light from the 2nd ADM is captured by the circulator C2 and is

given to the 3rd RSOA as a strong pump beam and ν1 frequency of weak probe constant

beam light is given to the 3rd RSOA. So when both pump or probe beam is present a

converted strong probe beam is obtained at its output. This output beam is injected to the

2nd RSOA as a strong pump beam with the help of mirrors. So if both pump or probe

beam is present in the 2nd RSOA one gets the output beam as converted weak probe beam

into a strong one which is added with the output beam of light of 1st RSOA using some

properly oriented mirrors and these two light beams again are injected to the 3rd ADM

which is tuned at its biasing frequency ν2. So it passes the ν1 frequency of light and

reflects the ν2 frequency of light and it is collected by the circulator C3. This reflected

beam of light is ultimately added with the output beam from 3rd ADM by the use of

properly oriented mirrors. The whole system is shown in figure-3.2. Here optical filters

53

are used to select the proper frequency of light beam. Thus when A=ν1 and B=ν1; Y=ν1,

when A=ν1 and B=ν2; Y= ν1, for A=ν2 and B=ν1; Y=ν1 and finally when A=B=ν2; Y=ν2.

This satisfies the truth table of AND gate.

3.3.1 Principle of operation of optical AND gate:

The AND logic system is shown in figure-3.2. Now in the case of AND logic

when two input beams are of ν1 frequency then 1st ADM passes the light beam and 2nd

ADM also passes the light beam and then they are combined together by mirror and beam

splitter. The combined beams are serving as a weak probe beam to the 1st RSOA. A

constant strong pump beam is present in the 1st RSOA so it gives a converted output light

beam which is delivered to the 3rd ADM and it passes through it. Thus a result of ν1

frequency of light is obtained at the output end Y. The conversion of 2nd and 3rd RSOA

can not take part due to the absence of the either pump or probe beam. Again when A=ν1

and B=ν2, 1st ADM passes the light beam and the light is delivered as a probe beam to the

1st RSOA and similarly due to the above operation the converted output light beam of ν1

frequency is obtained at the output end Y. The rest of the circuit does not take part in the

conversion process due to absence of either pump beam or probe beam. Now when A=ν2

and B=ν1 then 1st ADM reflects the light beam and the light is collected by the circulator

C1 and delivered as a weak probe beam to the 2nd RSOA. As the pump beam is absent

here so conversion can not take part in 2nd RSOA. Again as B= ν1, so the 2nd ADM passes

this light beam and is combined with the output terminal of the 1st ADM and serves as a

probe beam to the 1st RSOA. Thus a similar conversion is occurred in this time and as the

output light beam of frequency ν1 is obtained at Y. Finally when the ν2 frequency of light

is applied to both input terminal A and B, 1st ADM reflects the light beam and is

54

delivered as a probe beam to the 2nd RSOA. So, 1st RSOA can not take part any role in

this conversion. Now as B=ν2 so the 2nd ADM reflects the light beam and the light is

collected by the circulator, which is delivered again as a strong pump beam to the 3rd

RSOA where probe beam of ν2 frequency is already present. So the conversion takes

place and the output light beam of ν1 frequency is obtained and it is delivered as a pump

beam to the 2nd RSOA, for which the output light beam of ν2 frequency from 2nd RSOA

comes and it is applied to the 3rd ADM. This light is reflected back by the ADM and

collected by the circulator C3. The light from the circulator is connected to the output

terminal Y. So then the ν2 frequency of light is obtained at the output Y. Thus when

A=ν1 and B=ν1; Y=ν1, when A=ν1 and B=ν2; Y= ν1, when A=ν2 and B=ν1; Y=ν1 and

finally when A=B=ν2; Y=ν2. This satisfies the truth table of AND gate which is shown in

Table-3.2, if ν1 is encoded as 0 and ν2 as 1.

Table-3.2: Truth table of optical logic AND gate

A B Y

ν1 (0) ν1 (0) ν1 (0)

ν1 (0) ν2 (1) ν1 (0)

ν2 (1) ν1 (0) ν1 (0)

ν2 (1) ν2 (1) ν2 (1)

55

Figure-3.2: Frequency encoded optical AND gate

3.4 Scheme of realization of frequency encoded Optical NAND gate:

NAND gate is the most important logic gate in the logics family as it is universal

gate. Again here two input channels are taken A and B as sources of input light beams of

frequency ν1 or ν2. The whole set up is much closed to the AND logic set up except three

ADM1

RSOA3

ADM2

RSOA2

RSOA1

ADM3

C1 C3

C2

1

2

PROBE BEAM

PUMP BEAM

OPTICAL PASS FILTER

MIRROR

C CIRCULATOR

1, 3 -ν1 and 2-ν2 optical pass filter

A

B

Y


Y OUTPUT TERMINAL

3

ν2

ν2

ν2

56

main changes seen in this system. Here the output light from 1st ADM is injected as a

pump beam instead of the probe beam to the 1st RSOA and the reflected beam of light

from 1st ADM is injected as a pump beam instead of a probe beam to the 2nd RSOA.

Finally reflected beam from the 2nd ADM is applied as a pump beam to the 3rd RSOA

(figure-3.3). Thus the truth table of the NAND logic is developed and it is shown in table-

3.3. So when A=B=ν1 then Y=ν2, when A= ν1 and B= ν2 then Y= ν2, for A= ν2 and B= ν1;

Y= ν2 and finally for A=B= ν2; Y= ν1. This supports the truth table of universal gate

NAND gate.

3.4.1 Principle of operation of optical NAND gate:

The diagram of frequency encoded NAND logic is shown in figure-3.3. Now in

the case of optical NAND logic gate at first ν1 frequency light is applied in both of the

inputs A and B. So ν1 frequency of light falls on the 1st ADM from A, and as it is tuned at

its biasing frequency ν2, so ν1 frequency light passes through the ADM and is given as a

strong pump beam to the 1st RSOA, one weak probe beam of ν2 frequency is also given to

the 1st RSOA for which a converted strong light beam of ν2 frequency will be generated.

This is again delivered to the 3rd ADM which is tuned also at ν2 frequency, so the light

beam reflected from the 3rd ADM is captured by the circulator C3 and connected with the

output Y by the use of mirrors and the ν2 frequency of light is obtained at the output Y.

Now ν1 frequency light is also given to the 2nd ADM from B and the ADM is tuned at

same frequency ν2, so the ADM passes the light beam which is combined with the output

beam of the 1st ADM by the mirrors etc. Now when A=ν1 and B=ν2 then similar operation

goes on in the 1st ADM, but in this case of second input terminal B, the light beam falls

on the 2nd ADM and gets reflected from it and being captured by the circulator C2 it falls

57

on the 3rd RSOA as a pump beam. So the converted ν1 frequency of light is obtained in

presence of ν1 probe beam at the output and it is again applied to the 2nd RSOA as a probe

beam. Due to absence of pump beam the conversion does not occur and hence the ν2

frequency of light is obtained at the output Y for the operation of the 1st section of the

system. When A=ν2 and B=ν1 then 1st ADM reflects the light beam and it is then captured

by the circulator C1. This is given to the 2nd RSOA as a strong pump beam. No other

conversion takes place now because of the absence of the probe beam. Here ν1 frequency

of light falls on the 2nd ADM and passing through it, this delivers a strong pump beam to

the 1st RSOA and the conversion is occurred (due to presence of ν2 frequency probe beam

of light) and one receives the ν2 frequency light at the output and it is then delivered to

the 3rd ADM. Thus the final ν2 frequency light is found at the output terminal Y. Finally

when A=B=ν2; 1st ADM reflects the light beam and is delivered to the 2nd RSOA as a

strong pump beam. Again the light beam also falls on the 2nd ADM and gets reflected. It

is captured then by the circulator C3 and is given to the 3rd RSOA as a strong pump beam

and one gets the converted output beam of light of ν1 frequency in the presence of a weak

probe beam of ν1 frequency. This converted beam of ν1 frequency of light is delivered as

a probe beam again to the 2nd RSOA and one thus gets the converted output beam of ν1

frequency of light which is delivered to the 3rd ADM. This 3rd ADM passes it and a ν1

frequency of light is obtained at the output end Y. Thus surely the logical NAND output

is obtained from Y from the system described in fig-3.3. Hence one finds Y=ν2 when

A=B=ν1, again when A= ν1 and B= ν2 then Y= ν2, for A= ν2 and B= ν1, Y= ν2 and finally

when A=B= ν2 then Y= ν1. This supports the truth table of a universal frequency encoded

gate NAND gate which is shown in table-3.3. Here ν1 is encoded as 0 and ν2 as 1.

58

Figure-3.3: Frequency encoded optical NAND gate

ADM1

RSOA3

ADM2

RSOA2

RSOA1

ADM3

C1 C3

C2

1

2

PROBE BEAM

PUMP BEAM

OPTICAL PASS FILTER

MIRROR

C CIRCULATOR

1-ν2 and 2, 3 –ν1 optical pass filter

A

B

Y


Y OUTPUT TERMINAL

3

ν2

ν2

ν2

59

Table-3.3: Truth table of optical logic NAND gate

3.5. Scheme of realization of frequency encoded Optical R-S flip-flop:

Memory is the fundamental criteria for developing an all of optical processor. To

realize this system with optics I have considered two input light channels R and S, each

of them may take either ν1 (0) or ν2 (1) frequency of light. The outputs light channels are

Q and Q respectively. The outputs are feedback to the input i.e. it is connected with input

R and Q is connected with the other input S by the help of some mirrors and beam

splitters. The whole system is shown in figure-3.4. Now a beam of light of frequency ν1

or ν2 falls on the 1st ADM through point R. As the ADD/DROP multiplexer is tuned with

its biasing frequency ν2 so it reflects this frequency of light and passes the ν1 frequency of

light. This light is then introduced to the 1st RSOA as a strong pump beam. There already

presents a constant weak probe beam of ν2 frequency. So if both pump and probe beam is

present the converted strong probe beam of light with the respective frequency is

obtained which again falls on the 3rd ADM. The absences of any pump or probe beam in

the RSOA makes the conversion stop. Here one optical filter is used to select the proper

output light beam. The 3rd ADM is tuned with its biasing frequency ν1 i.e. it reflects only

ν1 frequency of light and passes all other frequencies. The reflected beam is captured by

A B Y

ν1 (0) ν1 (0) ν2 (1)

ν1 (0) ν2 (1) ν2 (1)

ν2 (1) ν1 (0) ν2 (1)

ν2 (1) ν2 (1) ν1 (0)

60

the circulator C3 and added with the output Q by the mirrors. Now the reflected light

beam from the 1st ADM is separated by the circulator C1 and is introduced to the 2nd

RSOA as a strong pump light beam. A weak probe beam is also given to the 2nd RSOA

and the converted output light beam passes through the respective optical filter and

merges with the output beam of 1st RSOA. Ultimately the output from the 1st RSOA is

given to the 3rd ADM. Same process is happened with the second input S and finally two

outputs Q andQ are obtained as shown in figure-3.4. Now when one applies the light

beam in the two input channels i.e. when R=ν1 and S=ν2 then Q= ν2 and Q = ν1

respectively and when R= ν2 and S=ν1 then Q=ν1 and Q = ν2 but when light is withdrawn

from the inputs the last state is obtained in the outputs Q andQ . So this follows the truth

table of optical RS flip-flop which is shown in table-3.4.

3.5.1 Principle of operation of optical R-S flip-flop:

To implement the circuit diagram of an optical RS flip-flop the light beam is

supplied first to both the input channel. This delivered light is of frequency ν1 to the 1st

ADM through the input end R. As the 1st ADM is tuned at its biasing frequency ν2, so ν1

frequency light beam passes through it and falls on the 1st RSOA where a strong beam of

ν2 frequency is already present. So one gets the converted ν2 frequency of light beam at

the output of 1st RSOA and it is again introduced to the 3rd ADM (tuning frequency ν1)

and light passing through the ADM comes to the output end Q. This output beam of ν2

frequency of light splits into two parts by the BS, one part is sent to the output end Q and

another part is feedback to the second input terminal S. Again if a ν2 frequency light is

applied to the S terminal, a combined beam of ν2 frequency of light is delivered to the 2nd

ADM (tuning frequency ν2). This ADM reflects by it and sends to the 4th RSOA by the

61

help of circulator C2 and mirrors. Due to presence of a weak probe beam of ν1 frequency

of light in 4th RSOA, the conversion is occurred and the converted output light beam of

frequency ν1 is obtained which is introduced to the 4th ADM (tuning frequency ν1) and is

reflected by it. This light is captured by the circulator C4 and is combined with the output

and splitted by the BS. One part goes to the other output end Q where another part is

feedback to the input terminal R and the process continues. Now when the R=ν2 and S=ν1

then reverse process is developed in this circuit and one gets Q= ν1 and Q = ν2 in the

output ends. Now when no light passes through both the input channel i.e. for the absence

of light in the input terminals, last state is attended i.e. If the last state is Q= ν1 and Q = ν2

for (R=ν2 and S=ν1) then the same result is obtained in the output terminals. As because

the output terminals are feedback to the reverse input terminals, so always there is found

a light present in the input terminals (even if no external input is applied) and that’s why

the conversion process continues. So certainly when R=ν1 and S=ν2 then Q= ν2 and Q =

ν1 respectively and when R= ν2 and S=ν1, Q=ν1 and Q = ν2, but when there is no input

light beam is applied at all i.e. when the inputs are withdrawn the last state is attended in

the output terminals Q andQ . So the truth table of an optical RS flip-flop is followed, this

is shown in table-3.4.

62

Figure-3.4: Frequency encoded optical RS flip-flop

ADM1

RSOA2

RSOA4

RSOA3

ADM2

RSOA1

ADM 3

ADM 4

1

2

3

4

C1 C3

C2 C4

R

S

Q

Q

BS

BS

C CIRCULATOR

OPTICAL PASS FILTER

1, 3 –ν2 and 2, 4-ν1 optical pass

PROBE BEAM

PUMP BEAM

MIRROR

R, S INPUT TERMINALS

Q, Q OUTPUT TERMINALS

ν2

ν2

ν1

ν1

BEAM SPLITTER

63

Table-3.4: Truth table of optical frequency encoded RS flip-flop

3.6. Frequency encoded programmable logic unit scheme:

In this programmable logic unit five logic gates, half adder and half subtractor are

present. Here seven control signals are present to activate their corresponding logic gates

(AND, OR, NAND, XOR, NOT), half adder and half subtractor unit. Here ν3 frequency

of light represent the control signal of OR gate, ν4 frequency of light represent the control

signal of AND gate, ν5 frequency of light represent the control signal of NAND gate, ν6

frequency of light represent the control signal of NOT gate and ν7 frequency represents

the control signal of XOR gate. Two input channels are present for application of the

input light signal and two output channels are present for obtaining the output data. The

whole scheme in block diagram is shown in figure-3.10. In this chapter first I have

explained the five controlled logic gates and then I integrate these logic units and make a

programmable logic unit.

3.6.1. Optical OR logic gate controlled by an optical signal:

Now in this chapter first I explain the OR gate logic controlled by the light signal.

To implement this logic gate some beam splitters, mirrors, optical channels and SOA

R S Q Q

ν1 (0) ν2 (1) ν2 (1) ν1 (0)

ν2 (1) ν1 (0) ν1 (0) ν2 (1)

No light No light Last state

attended

Last state

attended

64

based switches like add/drop multiplexer and wavelength converters have been used. The

whole scheme is shown in figure-3.5. Actually the whole circuit diagram of optical logic

gate is similar to the simple uncontrolled OR logic gates but the only difference in this

circuit is in this scheme one controlling signal CS1 is present. Only then OR logic gate

can be operate when CS1 is present. Absence of this CS1 OR logic operation can not be

operating whereas input signals A and B are present. This CS1 is the controlling channel

through which one selected frequency is introduced. This selected frequency is

controlling the whole OR gate operation. In this operation I choose ν3 frequency of a light

as a controlling signal.

3.6.1.1 Scheme of realization of optical OR logic gate controlled by the light signal:

Now if A=ν1, B=ν1 and CS1=ν3 then ADM1 passes the ν1 frequency and it is

divided into two parts. One part is applied as a strong pump beam to WC1 and another

part is applied also as a strong pump beam to WC3. Whereas incase of WC1, ν3 frequency

which is comes from controlled channel CS1 is applied as a weak probe beam to WC1. So

ν3 frequency is obtained at the output of WC1. This ν3 frequency light beam is applied as

a strong pump beam to the WC2. But absence of probe beam WC2 can not be conducted

and no output is obtained from WC2. Now ν1 frequency comes through the input terminal

B and passes to the ADM2 and it is applied as a weak probe beam to the WC4. Again one

part of CS1 signal is applied to the WC1and another part is applied as a weak probe beam

to the WC3. Here already ν1 frequency of light is present as a strong pump beam so

conversion is occurred and ν3 frequency of light is obtained at the output of WC3. This ν3

frequency of light is applied as a strong pump beam to WC4. So conversion is obtained

and ν1 frequency of light is obtained at the output of WC4 and it is applied to the ADM3.

65

It is tuned at a ν1 frequency so this light signal reflected by the ADM3 and collected by

the circulator C3 and the final output ν1 frequency of light is obtained at terminal Y. So,

when A=ν1, B=ν1 and CS1=ν3 then Y=ν1.

Now when A=ν2, B=ν1 and CS1=ν3 then ADM1 reflected the ν2 frequency and it is

collected by the circulator. This light beam is applied to the WC4 as a weak probe beam.

Here CS1 (ν3) light signal is applied as a strong pump beam and conversion is obtained

and ν2 frequency of light is obtained at the output of WC4 which is directly applied to the

ADM3 and the light beam passes through the ADM3. Again ν1 frequency from input

terminal B passes through the ADM2 and it is applied as a weak probe beam to the WC4

whereas pump beam is absent in WC3 so conversion does not obtain. So absence of pump

beam in WC4 conversion does not obtain. So ν2 frequency of light is obtained at the final

output terminal Y. So, when A=ν2, B=ν1 and CS1=ν3 then Y=ν2.

Now when A=ν1, B=ν2 and CS1=ν3 then the ADM1 passes the ν1 frequency and it

is applied as a strong pump beam to the WC1. So ν3 frequency of light from the CS1 is

applied as a weak probe beam to the WC1. Probe and pump beam are both present so

conversion is obtained from WC1 and ν3 frequency of light is obtained at the output of

WC1 which is again applied to the WC2 as a strong pump beam. Here ν2 frequency of

light (from the input channel B) is applied as a weak probe beam to the WC2 and

conversion is occurred. So ν2 frequency of light is obtained at the output of WC2 which is

applied to the ADM3 and it passes the light. Final output ν2 frequency of light is obtained

at the output terminal Y. So, when A=ν1, B=ν2 and CS1=ν3 then Y=ν2.

When A= ν2, B=ν2 and CS1=ν3 then the ADM1 reflects the ν2 frequency and it is

collected by the C1 and is applied to the WC4 as a weak probe beam. Here ν3 frequency

66

from CS1 is applied as a strong pump beam to the WC4. So, ν2 frequency is obtained at

the output of WC4 which is applied to the ADM3. This light beam passes through the

ADM3 and the final output ν2 frequency is obtained at the output terminal Y. Others WCs

does not take part in this operation due to the absence of any one light beam either probe

or pump beam. So, when A= ν2, B=ν2 and CS1=ν3 then Y=ν2. These all conditions are

satisfied the truth table of OR logic operation which is shown in table-3.5. Here if

controlling signal CS1 (ν3) is not present then whole logic operation will be stopped.

Table-3.5: Truth table of frequency encoded optical controlled OR gate

INPUT CHANNELS OUTPUT CHANNEL

CS1 A B Y

ν3 ν1(0) ν1(0) ν1(0)

ν3 ν1(0) ν2(1) ν2(1)

ν3 ν2(1) ν1(0) ν2(1)

ν3 ν2(1) ν2(1) ν2(1)

No light signal ν1(0) ν1(0) No Result




67

Figure-3.5: Frequency encoded optical OR logic operation controlled by external light signal

3.6.2. Optical AND logic gate controlled by the light signal:

Now I have explained of operation of optical AND logic controlled by an optical

signal. Whole operation is similar to optical AND logic operation but only difference is

the controlling signal CS2. This is an optical channel in which one particular frequency ν4

ADM1

WC4 ADM2

WC3

WC1

ADM3

C1 C3

C2

PROBE BEAM

PUMP BEAM

MIRRORS AND BEAM SPLITTERS

C CIRCULATOR

A

B

Y


Y OUTPUT TERMINAL

ν2

ν2

ν1WC2

CS1 (ν3)

CONTROL LIGHT FREQUENCY ν5

WC4

68

is chosen for controlling light signal. This light signal controlled the whole operation.

The scheme is shown in figure-3.6.

3.6.2.1. Scheme of realization of optical AND logic operation controlled by the light

signal:

In the first case, when A=ν1, B=ν1 and CS2=ν4 then ν1 frequency from input A

passes through the ADM1 and it is applied as a weak probe beam to input of WC1. Here

ν4 frequency from controlled channel CS2 is divided in to two parts by the beam splitters

and one part is applied as a strong pump beam to the WC2 and another part is applied as a

weak probe beam to the input of WC1. So, presence of both pump and probe beam in

WC1 the ν1 frequency is obtained at the output of WC1. This light beam passes through

the ADM3 and ν1 frequency is obtained at the final output terminal Y. Due to the absence

of either pump or probe beams into others wavelength converters they does not take any

par in this operation. So when A=ν1, B=ν1 and CS2=ν4 then Y=ν1.

When A=ν2, B=ν1 and Ct2=ν4 then ν2 frequency from input A is reflected by the

ADM1 and it is collected by the circulator C1 and is applied to the WC3 as a weak probe

beam. Again one part of ν4 frequency from CS2 is applied as a strong pump beam to the

input of WC1. Now ν1 frequency from input B passes through the ADM2 and falls on the

input of WC1 as a weak probe beam. So presence of both pump and probe beam ν1

frequency is obtained at the output of WC1. This light beam passes through the ADM3

and the ν1 frequency light is obtained at the final output terminal Y as shown in the

figure-3.6. Due to the absence of either pump or probe beams into others wavelength

converters they does not take any part in this operation. So when A=ν2, B=ν1 and CS2=ν4

then Y=ν1.

69

In the 3rd case, when A=ν1, B=ν2 and CS2=ν4 then ν1 frequency passes through the

ADM1 and is applied as a weak probe beam to input of the WC1. Again one part of ν4

frequency from CS2 is introduced to the input of WC1 as a weak probe beam. Presence of

both pump and probe beam ν1 frequency is obtained at the output of WC1 which is passes

through the ADM3 and finally ν1 frequency of light is obtained at the final output

terminal Y. Due to the absence of either pump or probe beams into others wavelength

converters they does not take any par in this operation. So when A=ν1, B=ν2 and CS2=ν4

then Y=ν1.

When A=ν2, B=ν2 and CS2=ν4 then ν2 frequency from input A is reflected from

the ADM1 and collected by the circulator C1. This light beam is applied as a weak probe

beam to the WC3. One part of ν4 frequency from CS2 is applied as a strong pump beam to

WC2. Here ν1 frequency of probe beam is already present so ν1 frequency is obtained at

the output of WC2 and it is applied as a weak probe to the WC4. Again ν2 frequency from

input B is reflected from ADM2 and is collected by the circulator C2. This light beam is

applied as a strong pump beam to the input of WC4 so presence of both pump and probe

beam in WC4 the ν1 frequency is obtained at the output of WC4. This light beam is

applied as a strong pump beam to the WC3. In the presence of both pump and probe beam

the ν2 frequency of light is obtained at the output of WC3 which is reflected by the ADM3

and collected by the circulator C3. So, ν2 frequency is obtained at the final output terminal

Y. When A=ν2, B=ν2 and CS2=ν4 then Y=ν2. The truth table of AND gate is shown in

table-3.6.

70

Figure-3.6: Frequency encoded optical AND logic operation controlled by external light signal

ADM1

WC4

ADM2

WC3

WC2

ADM3

C1 C3

C2 PROBE BEAM

PUMP BEAM

MIRROR AND BEAM SPLITTERS

C CIRCULATOR

A

B

Y


Y OUTPUT TERMINAL

ν2

ν2

ν2

WC1

CS2 (ν4)

CONTROL LIGHT FREQUENCY

ν5

71

Table-3.6: Truth table of frequency encoded optical controlled AND gate

3.6.3. Optical NAND logic gate controlled by the light signal:

To implement this logic operation some optical wavelength converters, mirrors,

beam splitters and add/drop multiplexer are needed. This operation is similar to normal

NAND operation but only difference is controlled light signal CS3 (ν5). This particular

light frequency ν4 is controlled the whole NAND logic operation. The whole scheme is

shown in figure-3.7.

3.6.3.1. Scheme of realization of optical NAND logic operation controlled by the

light signal:

In this circuit arrangements A and B are the inputs and y is the output terminal.

Here controlling light signal is CS3(ν5). Now when A=ν1, B= ν1 and CS3=ν5 then ν1 light

beam from A passes through the ADM1 and is applied as a strong pump beam to the


CS2 A B Y

ν4 ν1(0) ν1(0) ν1(0)

ν4 ν1(0) ν2(1) ν1(0)

ν4 ν2(1) ν1(0) ν1(0)

ν4 ν2(1) ν2(1) ν2(1)





72

WC1. Again ν5 light beam from CS3 is divided into two parts one part is applied as a

strong pump beam to the WC3 and another part is also applied as a strong pump beam to

the WC4. In presence of ν2 frequency light in WC3 conversion is occurred and ν2

frequency is obtained at the output of the WC3 which is applied as a weak probe beam to

the WC1. Here in WC1 conversion is obtained in presence of both pump and probe beam

and ν2 frequency is obtained at the output of WC1. This light beam is applied to the

ADM3 and it is reflected from the ADM. Then it is collected by the circulator C3 and ν2

frequency is obtained at the final output terminal Y. Other WCs does not conducted in

this operation due to absence of either pump or probe beam. So when A=ν1, B= ν1 and

CS3=ν5 then Y=ν2.

When A=ν2, B=ν1 and Ct3=ν5 then ν2 frequency from A is reflected by the ADM1

and it is collected by the C1 and is applied as a strong pump beam to the WC2 as is

describing in the figure-3.7. But absence of any probe beam WC2 does not work. Again

ν1 frequency from B passes through the ADM2 and is applied as a strong pump beam to

the WC1. Here ν2 frequency from the output of WC3 is present so conversion is happened

and ν2 frequency light is obtained at the output of WC1 which is reflected by the ADM3

and collected by the C3. Now ν2 frequency of light is obtained at the final output terminal

Y. Other WCs does not conducted in this operation due to absence of either pump or

probe beam. So when A=ν2, B= ν1 and CS3=ν5 then Y=ν2.

Again when A=ν1, B=ν2 and CS3=ν5 then ν2 frequency from B is reflected by the

ADM2 and is collected by the C2. This light beam is applied as a strong pump beam to the

WC5. Again ν5 frequency from CS3 is divided into two parts. Those are applied as strong

pump beam to the WC3 and WC4 respectively. In WC3 ν2 frequency is present as a weak

73

probe beam and in WC4 ν1 frequency is present as a weak probe beam. So conversion is

obtained in both WCs and ν2 frequency is achieved at the output of WC3 and ν1 frequency

at the output of WC4. This ν1 frequency is applied as a weak probe beam to the WC5. In

presence of both pump and probe beam ν1 frequency is obtained at the output of WC5

which is applied as a weak probe beam to the WC2. But absence of pump beam

conversion does not occurred. Again ν2 frequency from WC3 is applied as a weak probe

beam to the WC1 and ν1 frequency from A is applied as a strong pump beam to the WC1.

So conversion is obtained and ν2 frequency of light is obtained at the output of WC1

which is introduced to the ADM3 and reflected by it. This reflected beam is collected by

the C3 and ν2 frequency of light is obtained at the final output terminal Y. So when A=ν1,

B= ν2 and CS3=ν5 then Y=ν2.

Now when A=ν2, B=ν2 and CS3=ν5 then ν2 frequency from A is reflected by the

ADM1 and is collected by the C1. This light beam is applied as a strong pump beam to the

WC2. Again ν2 frequency from B is reflected by the ADM2 and is collected by the C2.

This light beam is applied as a strong pump beam to the WC5. Here in WC5 ν1 frequency

(from WC5) is applied as a weak probe beam. So conversion is obtained and ν1 frequency

is obtained at the output of the WC5 which is applied as a weak probe beam to the WC2.

In presence of both pump and probe beam in WC2 ν1 frequency is obtained at the output

of WC2 and this light beam passes through the ADM3. So ν1 frequency of light is

obtained at the final output Y. So, when A=ν2, B=ν2 and CS3=ν5 then Y= ν1. This whole

logic system can be operating only when this controlling signal is present. If I withdraw

the signal then this NAND logic system can not operate. The truth table of NAND

operation is shown in table-3.7.

74

Figure-3.7: Frequency encoded optical NAND logic operation controlled by external light signal

ADM1

WC5

ADM2

WC2

WC1

ADM3

C1 C3

C2 PROBE BEAM

PUMP BEAM

MIRROR

C CIRCULATOR

A

B

Y


Y OUTPUT TERMINAL

ν2

ν2

ν2

WC4

CS3 (ν5)

ν7 CONTROL LIGHT FREQUENCY

WC3

75

Table-3.7: Truth table of optical frequency encoded controlled NAND gate

3.6.4. Optical NOT logic gate controlled by the light signal:

To implement the NOT gate I have used some ADM’s, WCs, mirrors and beam

splitters. Here ν6 is the control light frequency. Again A is the input terminal and Y is the

output terminal. Here CS4 is the controlling light signal terminal. This scheme is shown

in figure-3.8.

3.6.4.1. Scheme of realization of optical NOT logic operation controlled by the light

signal:

Now when A=ν1 and CS4=ν6 then ν1 frequency of light is applied through the

input terminal A into the ADM1 which is tuned at ν2 frequency then ADM1 passes this

input ν1 frequency of light and introduce as a strong pump beam to the WC1. Here ν6

frequency is applied to the WC2 as a strong pump beam and ν2 frequency of week probe


CS3 A B Y

ν5 ν1(0) ν1(0) ν2(1)

ν5 ν1(0) ν2(1) ν2(1)

ν5 ν2(1) ν1(0) ν2(1)

ν5 ν2(1) ν2(1) ν1(0)





76

beam is already present in the input of WC2 then the ν2 frequency of light is obtained at

the output of WC2 and it is applied as a week probe beam to the WC1. The WC1 is

activated and the ν2 frequency of light is obtained at the output of WC1 and after passing

through the ADM2 the ν2 frequency of light is obtained at the output terminal Y. But if ν6

frequency of light is absence then the total conversion process will be stopped. Now

when A=ν2 then ADM1 reflects it and it is applied to WC4 and again ν6 frequency of light

is present it serves as a strong pump beam to WC3 where ν1 frequency of week probe of

light is present so conversion is obtained and the ν1 frequency of light is obtained at the

output of WC3 which is applied as a week probe beam to the WC4 and the ν1 frequency of

light is obtained at the output of WC4. This light beam is applied to the ADM2 which is

tuned at ν1 frequency so it reflects the light beam and the ν1 frequency of light beam is

obtained at the output terminal Y by using C1 and mirrors. But absence of ν6 frequency of

light the whole process will be stopped. So when A=ν1 then Y=ν2 and A=ν2 then Y=ν1,

only when ν6 frequency of light is present. It satisfies the truth table of NOT gate which is

shown in table-3.8 and ν6 frequency of light serve as a control light signal.

Table-3.8: Truth table of frequency encoded optical controlled NOT gate

INPUT CHANNELS OUTPUT CHANNELS

CS4 A Y

ν6 ν1(0) ν2(1)

ν6 ν2(1) ν1(0)

No light signal ν1(0) No Result

No light signal ν2(1) No Result

77

Figure-3.8: Frequency encoded optical NOT logic operation controlled by external light

signal

3.6.5. Optical XOR logic gate controlled by the light signal:

Similarly to implement the optical XOR logic operation some ADMs, WCs,

mirrors, beam splitters and optical channels are used. Here ν7 frequency is used as a

controlling light signal. This scheme is shown in figure-3.9.

3.6.5.1. Scheme of realization of optical XOR logic operation controlled by the light

signal:

In this operation system A and B are the input terminals. Y is the output terminal

and CS5 is the controlling light signal terminal. Now when I have introduced ν1 frequency

ADM1 ADM2 WC1

WC4

WC2

WC3

C C

A Y

CS4 (ν6) CONTROL LIGHT

FREQUENCY ν6

PROBE BEAM

PUMP BEAM

C CIRCULATOR

MIRRORS AND BEAM SPLITTERS

A INPUT TERMINAL

Y OUTPUT TERMINAL

ν2 ν1

78

at the A terminal, ν1 frequency at the B terminal and ν7 frequency at the CS5 terminal

then ν1 frequency from A passes through the ADM1 and divided into two parts by the use

of beam splitters. One part is applied as a strong pump beam to the WC3 and another part

is applied as a weak probe beam to the WC1. But due to absence of any strong pump

beam and also probe beam in WC1 and WC3 respectively they are not working. Here ν1

frequency from B passes through the ADM2 and is divided into two parts. One part is

applied as a strong pump beam to the WC8 and another one is applied also as a strong

pump beam to the WC7. Due to absence of weak probe beam in WC7 it does not work.

Again ν7 frequency from CS5 is divided into two parts both parts are applied as a strong

pump beam to WC5 and WC6 respectively as described in the figure-3.9. In presence of ν2

frequency weak probe beam in WC5 conversion process is obtained and ν2 frequency is

obtained at the output of WC5. This ν2 frequency light beam is divided into two parts one

part is applied as a weak probe beam to the WC2 and another one is applied as weak

probe beam to the WC8. In presence of ν1 frequency weak probe beam in WC6 conversion

process is obtained and ν1 frequency is obtained at the output of WC6. So in presence of

both pump and probe beam in WC8 ν2 frequency is obtained at the output of WC8 which

is applied as weak probe beam to the WC3. Both pump and probe beam are present in

WC3. Then ν2 frequency is obtained at the output of WC3 which is applied as a strong

pump beam to the WC4. Output ν1 frequency beam from WC6 is divided into two parts

both part are applied as a weak probe beam to WC4 and WC9 respectively. Both pump

and probe beam are present in the WC4. So, ν1 frequency is obtained at the output of

WC4. This light beam passes through the ADM3. So, ν1 frequency is obtained at the final

output terminal Y. It can be said that when A=ν1, B=ν1 and CS5= ν7 then Y=ν1. This is

79

satisfying the truth table of XOR logic operation. But in absence of CS5 (ν7), this whole

operating system does not work.

Now when ν1 frequency is applied in terminal A, ν2 frequency is applied in B and

ν7 frequency is applied in terminal CS5 then ν1frequency passes through the ADM1 and is

divided into two parts. One part is applied as a weak probe beam to the WC1 and another

part is applied as a strong pump beam to the WC3. Again ν2 frequency from B is reflected

from the ADM2 and is collected by the circulator C2. This light beam is divided into two

parts one part is applied as a strong pump beam to the WC1 and another one is applied

also as a strong pump beam to the WC9. So, presence of both pump and probe in WC1

conversion is obtained and ν1 frequency is obtained at the output of WC1. This ν1 is

applied as a strong pump beam to the WC2. Here ν2 frequency from the output of WC5 is

applied as a weak probe beam. So, ν2 frequency is obtained at the output of WC2. This

light beam is reflected by the ADM3 and is collected by the C3 and ν2 frequency is

obtained at the final output Y. So It can be said when A=ν1, B=ν2 and CS5= ν7 then Y=ν2.

This is satisfying the truth table of XOR logic operation. But in absence of CS5 (ν7), this

whole operating system does not work.

Again ν2 frequency is applied in terminal A, ν1 frequency is applied in terminal B

and ν7 frequency is applied in terminal CS5 then ν2 frequency from A is reflected by the

ADM1 and it is collected by the C1. This light beam is divided into two parts. One is

applied as a weak probe beam to the WC3 and another is also applied as a weak probe

beam to the WC7. Again ν1 frequency from B passes through the ADM2 and is divided

into two parts as stated before. Both are applied as a strong pump beam to the WC7 and

WC8. Here ν2 frequency present as a weak probe beam in WC7 so conversion is obtained

80

and ν2 frequency is obtained at the output of WC7. This ν2 frequency is applied to the

ADM3 and is reflected by the ADM3. This reflected beam is collected by the C3 and ν2

frequency is obtained at the final output terminal Y. So, when A=ν2, B=ν1 and CS5= ν7

then Y=ν2. This is satisfying the truth table of XOR logic operation. But in absence of

CS5 (ν7), this whole operating system does not work.

In the last consideration when ν2 frequency is applied into both terminal A and B

and ν7 frequency at the terminal CS5. Then ν2 frequency from A is reflected by the ADM1

and is collected by the C1. As stated before this light beam is divided into two parts, both

parts are applied as a weak probe beam to the WC3 and WC7 respectively. Again ν2

frequency from B is reflected by the ADM2 and is collected by the C2. This light beam is

divided into two parts and both are applied as a strong pump beam to the WC1 and WC9

respectively. In presence of both pump and probe beam in WC9 ν1 frequency is obtained

at the output of WC9 which is applied as a strong pump beam to WC3. Here ν2 frequency

as a weak probe beam is present so conversion is obtained and ν2 frequency is obtained at

the output of WC3. This frequency is again applied as a strong pump beam to the WC4.

Due to presence of ν1 frequency probe beam in WC4 conversion is obtained and ν1

frequency is obtained at the output of WC4 which is applied to the ADM3. This light

beam passes through the ADM3 and ν1 frequency is obtained at the final output terminal

Y. So, when A=ν2, B=ν2 and CS5= ν7 then Y=ν1. This is satisfying the truth table of XOR

logic operation which is shown in table-3.9. But in absence of CS5 (ν7), this whole

operating system does not work.

81

Figure-3.9: Frequency encoded optical XOR logic operation controlled by external light signal

ADM1

ADM2

ADM3

WC3 W

C4 WC5

WC6

WC7

WC8

WC9

WC2

WC1

C

C

C

A

B

Y

C CIRCULATOR

PROBE BEAM

PUMP BEAM

MIRRORS and BEAM SPLITTERS


Y OUTPUT TERMINAL

ν7 CONTROLE LIGHT FREQUENCY

CS5 (ν7)

82

Table-3.9: Truth table of frequency encoded optical controlled XOR gate

3.6.6. Scheme of realization of frequency encoded programmable logic unit:

To implement this programmable logic unit some optical channels, mirrors and

beam splitters have been used. The whole block diagram is shown in figure-3.10. Here in

this figure two common input channel. This channels are A and B. Channel A is divided

into five parts and is applied to the respective inputs of logic gates and B also is divided

into five parts and is applied to the respective inputs of the logic gates. In this system five

controlling terminals are CS1, CS2, CS3, CS4 and CS5. The outputs of five logic gates are

combined in one final output channel Y. So when I need to operate any one gate I have to

apply respective frequency at the inputs A and B and the corresponding controlling signal

then the output result is obtained. As for example, if the NAND logic operation has been

operated then they have to give the corresponding frequency to the controlling terminal


CS5 A B Y

ν7 ν1(0) ν1(0) ν1(0)

ν7 ν1(0) ν2(1) ν2(1)

ν7 ν2(1) ν1(0) ν2(1)

ν7 ν2(1) ν2(1) ν1(0)





83

CS3 (ν5 frequency) and just maintain the inputs according the NAND logic gate. If their

inputs frequencies are present but controlling signal is not present then this system does

not work. So, just controlling these control signal terminals one can get any types of logic

operation in the output terminal. The whole truth table is shown in table-3.10.

Table-3.10: Truth table of optical programmable logic system

INPUTS

A B CS1 CS2 CS3 CS4 CS5

OUTPUT

Present Present Present × × × × OR

Present Present × Present × × × AND

Present Present × × Present × × NAND

Present Present × × × Present × NOT

Present Present × × × × Present XOR

84

Figure-3.10: Block diagram of frequency encoded optical programmable unit

3.7 Important requirements for the switching of SOA:

To obtain the faithful operation the power of the input control light beam which,

serves as a pump beams for the SOA should lie between 2 and 4 dB. The performance of

the data transfer depends on the pomp beam energy. Energy of each probe beam is to be

maintained between -4 and -2 dB. The wavelengths of the selected inputs are 1555 and

1550 nm corresponding to frequency ν1 and ν2 respectively. Frequencies of CW input

signal should lie in C band (1536-1570 nm). This wavelength range is favorable for

optical communication.

OR AND NAND NOT XOR

CS1 CS2 CS3 CS4 CS5

A

B Y

INPUT TERMINALS

OUTPUT TERMINAL

CONTROL LIGHT SIGNAL TERMINALS

85

3.8 Conclusion:

To conclude, an all optical approach for the successful realization of high speed

(far above GHz range) optical logic gates, memories and programmable logic operations

have been proposed here. The potential advantage of these optical gates and flip-flop over

many other established optical gates was the use of frequency encoding technique, for

which the coded information (0, 1) in a signal remains unchanged in refraction,

reflection, absorption etc. for a long distance transmission of data. The proposed system

could offer also a noise free conversion to provide a high signal to noise (S/N) ratio.

Using such frequency encoded technology. In this chapter not only about the logic

operations have been discussed but also the programmable logic operations have been

discussed. Using this programmable unit one can get any types of logic operation just

only controlling the external controlling light signal. It is very helpful for any

computation and communication systems.

86

References:

3.1 N. Pahari, S. Mukhopadhyay, An all-optical R–S flip-flop by optical non-linear

material, J. Opt., 34 (3), 2005, 108–114.

3.2 J.M. Jeong, M.E. Marhic, All-optical logic gates based on cross-phase modulation in

a non-linear fiber interferometer, Opt. Commun., 85 (5–6), 1991, 430–436.

3.3 B.K. Jenkins, A.A. Sawchuk, T.C. Strand, R. Forchheimer, B.H. Soffer, Sequential

optical logic implementation, Appl. Opt. 23 (19), 1984, 3455–3464.

3.4 T.A. Ibrahim, R. Grover, L.-C. Kuo, S. Kanakaraju, L.C. Calkoun, P.-T. Ho, All

optical AND/NAND logic gates using semiconductor microresonators, IEE Photonics

Technol. Lett., 15 (10), 2003, 1422–1424.

3.5 P. Ghosh, P.P. Das, S. Mukhopadhyay, New proposal for optical flip-flop using

residue arithmetic, in: ITCOM-2001, (4534B-22), SPIE Proceedings (Optoelectronic and

Wireless Data Management, Processing, Storage and Retrieval), 4534, 8 November,

2001, 148–154.

3.6 A.K. Das, S. Mukhopadhyay, General approach of spatial input encoding for

multiplexing and De-multiplexing, Opt. Eng. (U.S.A.), 43, 2004, 126–131.

3.7 Y. Ichioka, J. Tanida, Optical parallel logic gates using a shadow-casting system for

optical digital computing, Proc. IEE, 72 (7), 1984, 787–801.

3.8 K. E. Zoiros, M. K. Das, D. K. Gayen, H. K. Maity, T. Chattopadhyay, J. N. Roy,

“All-optical pseudorandom binary sequence generator with TOAD-based D flip-flops”,

Optics Communications, 284(19), 2011, 4297-4306.

87

3.9 S.K. Garai, S. Mukhopadhyay, Method of implementing frequency encoded

multiplexer and demultiplexer systems using nonlinear semiconductor optical amplifiers,

Opt. Laser Technol. 41 (8), 2009, 972–976.

3.10 S.K. Garai, S. Mukhopadhyay, Method of implementation of all-optical frequency

encoded logic operations exploiting the propagation characters of light through

semiconductor optical amplifiers, J. Opt. (2009), doi:10.1007/s12596-009-0009-6.

3.11 S.K. Garai, A. Pal, S. Mukhopadhyay, All-optical frequency encoded inversion

operation with tristate logic using reflecting semiconductor optical amplifiers, Optik

(2009), doi:10.1016/j.ijleo.2009.02.011.

3.12 S.K. Garai, S. Mukhopadhyay, Amethod of optical implementation of frequency

encoded different logic operations using second harmonic and difference frequency

generation techniques in non-linear material, Optik Int. J. Light Electron. Opt. (2008),

doi:10.1016/j.ijleo.2008.10.011.

3.13 H.J. Dorren, D. Lenstra, Y. Liu, M.T. Hill, G.-D. Khoe, Nonlinear polarization

rotation in semiconductor optical amplifiers: theory and application to all-optical flip-flop

memories, IEEE J. Quantum Electron., 39, 2003, 141–148.

88

CHAPTER-4

A new approach of implementing all-optical frequency/wavelength

encoded clocked S-R flip-flop

ABSTRACT

In all optical networking and computing system, the role of all-optical flip-flops is very

much essential. For signal synchronization with a reference clock and for storage of

digital bits the flip-flop has no alternative. In this chapter I have proposed a method of

developing an all optical frequency encoded clocked RS flip-flop using the non-linear

character of semiconductor optical amplifiers. Frequency is the basic character of light

and several encoding/decoding problems in computations and communications can be

solved using the frequency encoding principle of optical data. The proposed system is all-

optical and therefore it can extend a super fast speed of operation (far above THz limit).


S. Dutta, S. Mukhopadhyay, “A new alternative approach of all optical frequency

encoded clocked S–R flip-flop exploiting the non-linear character of semiconductor

optical amplifiers”, Optik - Int. J. Light Electron Opt., 123 (2012) 2082– 2084.

89

4.1. Introduction:

In the previous chapter the logic operations, RS flip-flop and programmable logic

unit have been discussed. In this chapter I am interested how to make frequency encoded

better memory unit and how to overcome the problems related with RS flip-flop. That’s

why here I have proposed clocked SR flip-flop with SOA based optical switches. All-

optical signal processing has drawn much attention of scientific communities because of

its inherent parallelism and potential applications in high speed optical networks, optical

computing systems etc. Again all-optical signal processing is especially useful to

overcome the high bit rate problems in future communication systems. In recent years a

lot of efforts have been seen in this area where optics is tactfully used for the processing

of digital data.

Several all-optical digital devices have been proposed which are supposed to run

with the operation speed far above the GHz range [4.1-4.5]. Many of those devices are

dedicated for performing logic gates, flip-flops, optical buffers, and arithmetic operations

to achieve the goal of all-optical computer/data processor. In particular, optical flip-flop

can attract a special interest as because it can serve as an optical memory. Optical

memory has also of great impact for the development of optical packet switches in

networks. Now several types of optical flip-flops are proposed with different types of

encoding mechanisms in last few years which are polarization encoding, phase encoding,

intensity encoding and also frequency encoding [4.6-4.8].

In this chapter a novel alternative approach of frequency encoded clocked SR flip-

flop based on SOA based switches has been proposed. Several types of all optical

memory units have been reported earlier. Among these memory units some are

90

polarization encoded, some are phase encoded, some are intensity encoded and some are

frequency encoded [4.9-4.11]. There also some frequency encoded memory units which have

already been reported by different scientists. MZI-SOA switches, add-drop multiplexers

and PBSs were used there to implement the optical memory units and as well as the tri-

state logic gates. Also all-optical frequency encoded one bit memory unit and two- bit

memory unit have been proposed earlier. These are based on SOA made switches,

wavelength converters and add/drop multiplexers [4.12-4.15]. The frequency encoded

memory unit is not run by clock signal there. In digital optical communication and

computation this frequency encoded flip-flop will take an important role. This type of

encoding is chosen because of very high signal to noise (S/N) ratio and very low bit error

problem.

4.2. Optical implementation of clocked S-R flip-flop:

The frequency encoded optical RS flip-flop and the functions of SOA based

switches are elaborately discussed in the previous chapters. Now to introduce a clock

based memory operation I have propose a new concept of frequency encoded optical

clocked RS flip-flop. To implement this clocked system some SOA based wavelength

converters (WC), add/drop multiplexers (ADM), optical filters, mirrors (M) and beam

splitters (BS) have been used. Now the proposed clock based RS flip-flop is given by a

schematic diagram shown in fig-4.1 and the truth table is shown in table-4.1. Here one

light source (serving as a clock signal) used as the clock (CLK) terminal. One optical

filter which only passes the ν2 frequency of light is placed in front of the clock (CLK). So

this clock channel will only send the ν2 frequency of light, when the CLK is 1. S and R

are the two input terminals. Now if ν2(1) frequency of light is applied to the CLK and S=

91

ν1(0), and R= ν2(1), then ν1 frequency of light from the terminal S falls on the ADM1

which is tuned at frequency ν2 and therefore the light passes through the ADM1 and it is

applied to the WC1 as a pump beam. Again ν2 frequency of light from CLK passes

through the optical filter and is splitted into three parts by the two beam splitters. One

part is applied to the WC1 as a probe beam where as the second part is applied to the WC4

also as a probe beam. Third part is applied to the WC3 as the pump beam. Now for WC1

both the pump and probe beam is present then the ν2 frequency of light is obtained at its

output which is directly applied to the input R1 terminal to the unlocked RS flip-flop and

the ν1 frequency of light is obtained at the output terminalQ (according to the principle

of R-S flip-flop). This RS flip-flop operation is already discussed in our previous chapter.

The figure of RS flip-flop is shown in figure-3.4 and truth table of this is shown in table-

3.4. For the WC3 a constant probe beam (of frequency ν1) is applied in its input then both

pump and probe beam is present the ν1 frequency of light is obtained from the output of

the WC3 which is divided again into two parts by the use of a BS. One part is applied as a

probe beam to the WC2, but due to absence of pump beam the conversion can not be

obtained. Another part of the output from WC3 is applied to the WC5 as a probe beam.

Again as ν2 frequency of light is applied at the input terminal R and it falls on the ADM2

(which is tuned also at ν2 frequency), so it blocks the passage of ν2 through it and reflects

the light which is received by the optical circulator. It then serves as a pump beam to the

WC5. So in presence of both pump beam and probe beam the conversion is obtained at

WC5 and the ν1 frequency of light is obtained at the output of WC5 which goes to the S1

terminal of RS flip-flop. Thus the ν2 frequency of light is received at the output

terminalQ according to the principle of R-S flip-flop (table-3.4 of previous chapter).

92

Here absence of pump beam does not support any conversion WC4. Thus when clock is

ν2 (1) and S=ν1(0) and R= ν2(1) the output of the whole flip-flop Q = ν1(0) and Q = ν2(1).

When the clock is at the frequency ν2 (1) and S= ν2 (1), and R= ν1(0) then WC2, WC3 and

WC4 are active and take part in conversion process and input of the RS flip-flip is

achieved of block A as, R1= ν1(0) and S1= ν2(1), which produces Q = ν2(1) and Q = ν1(0)

respectively at the final outputs. Now when clock= ν2(1) but the light beams are

withdrawn from the input terminals S and R then the clock system will not affect the

WCs and they stop the conversion, but due to the feedback mechanism in the R-S flip-

flop, the outputs Q and Q will attend with its last achieved values. Similarly when CLK

is set at o, i.e. when ν1 frequency is applied at the clock terminal the filter will not pass

any signal to the WCs. So instead of the presence of R-S inputs the WCs will not support

the conversion process. Thus R1 and S1 will get no signal from R and S for this reason Q

and Q will continue with its last attended values. The truth table of optical clocked SR

flip-flop is shown in table-4.1. Thus it can seen that when the clock is applied to the flip-

flop the outputs (Q , Q ) give the result according to the truth table, but when no clock is

applied or ν1(0) frequency is applied at the clock terminal the Q , Q outputs of the flip-

flop holds its last attended values.

93

Figure-4.1: Optical clocked SR flip-flop, with SOA based switches

ADM1

WC5

WC4

WC3

WC2

WC1

F

C

C

ADM2

QR1

S1 Q R

S

CLK

PUMP BEAM

PROBE BEAM

F OPTICAL ν2 PASS FILTER

BLOCK-A

BLOCK-A:OPTICAL RS FLIP-FLOP

C CIRCULATOR

94

Table-4.1: Truth table of optical clocked S-R flip-flop

4.3 Conclusion:

The method of optical implementation of clocked SR flip-flop with all optical

switching systems is illustrated here. The speed of operation can go far above the THz

limit as the SOA based switches operate at this speed. A high speed operation of the

proposed scheme is not the only a prospective advantage of the system, but the frequency

encoding mechanism also offers a great support. The most important application of the

system can be seen in digital communication, where the frequency encoded new data or

an (old data depending on the applied clock) have been send in the communication

channel. Even if the sender requires making a data to be continued for communication or

a new data is to be introduced in the channel, he can easily do it by the use of the above

system. The output signal from the clocked SR flip-flop can be sent to distant receiver

Clock S R Q Q

ν2 (1)

ν1 (0)

ν2 (1)

ν1 (0)

ν2 (1)

ν2 (1)

ν2 (1)

ν1 (0)

ν2 (1)

ν1 (0)

ν2 (1)


attended

Last state

attended

ν1 (0)


attended

Last state

attended

95

as it remains unaltered in reflection, refraction, absorption etc due to the nature of coding

of bits (0 or 1) with frequency variation of light. Therefore this technique will be very

much useful for conducting a reliable and faithful optical memory both in communication

and computation. To achieve a good amplification the pump beam of WCs should lie

between 4dB to 10 dB. The proposed system does not only offer a high speed operation

but it also offers an operation which provides a high signal to noise (S/N) ratio. The

accommodation of frequency encoding process is the main reason for obtaining the high

S/N ratio. For this reason bit error rate also goes down in comparison to conventional

intensity based encoding processes. This optical clocked SR flip-flop and frequency

encoding technique can be used for many other optical devices where clocked SR flip-

flop is an essential unit.

96

References:

4.1 M.T. Fatehi, K.C. Wasmundt, S.A. Collins, Optical flip-flops and sequential logic

circuits using a liquid crystal light valve, App. Optics 23, 1984, 2163–2171.

4.2 M.T. Hill, H. de. Waardt, G.D. Khoe, H.J.S. Dorren, All-optical flip-flop based on

coupled laser diodes, IEEE J. Quantum Elect. 37 (3), 2001, 405–413.

4.3 W. Wu, S. Campbell, S. Zhou, P. Yeh, Polarisation encoded optical logic operations

in photorefractive media, Opt. Lett. 174, 1993, 2–1744.

4.4 S. Dutta, S. Mukhopadhyay, All optical frequency encoding method for converting a

decimal number to its equivalent binary number using tree architecture, Optik, 122, 2011,

125–127.

4.5 D. Samanta, S. Mukhopadhyay, A method of maintaining the intensity level of a

polarization encoded light signal, J. Phys. Sci. (Vidyasagar Univ.) 11, 2007, 87–91.

4.6 B. Chakraborty, S. Mukhopadhyay, Alternative approach of conducting phase

modulated all optical logic gates, Opt. Eng. 48 (3), 2009, 035201.

4.7 S. Dutta, S. Mukhopadhyay, An all optical approach of frequency encoded NOT

based Latch using semiconductor optical amplifier, J. Opt. 39 (1), 2010, 35–41.

4.8 S. Dutta, S. Mukhopadhyay, Alternating approach of implementing frequency

encoded all-optical logic gates and flip-flop using semiconductor optical amplifier, Optik,

122, 2011, 1088–1094.

4.9 D. Samanta, S. Mukhopadhyay, Implementation of an optical S-R flip-flop with

polarization encoded light signal, Optoelectron. Lett. 5, January 1, 2009, 57–60.

4.10 N. Mitra, S. Mukhopadhyay, A new scheme of an all-optical J-K flip-flop using

nonlinear material, J. Opt. 37 (3), 2008, 85–92.

97

4.11 S.K. Garai, S. Mukhopadhyay, A novel method of developing all-optical frequency

encoded memory unit exploiting nonlinear switching character of semiconductor optical

amplifier, Opt. Laser Technol. 42 (5), 2010, 1122–1127.

4.12 A. Mecozzi, Small-signal theory of wavelength converters based on cross-gain

modulation in semiconductor optical amplifiers, IEEE Photon. Technol. Lett. 8, 1996

1471–1473.

4.13 Z. Li, G. Li, Gates based on four-wave mixing in a semiconductor optical amplifier,

IEEE Photon. Technol. Lett. 18, 2006, 1341–1343.

4.14 A. Kumpera, P. Honzathko, R. Slavik, Novel 160-GHz wavelength converter based

on a SOA and a long period grating, Opt. Commun. 282, 2009, 1775–1779.

4.15 G. Raybon, U. Koren, B.I. Miller, M. Chien, M.G. Young, R.J. Capik, K. Dreyer,

R.M. Derosier, A wavelength-tunable semiconductor amplifier/filter for add/drop

multiplexing in WDM networks, IEEE Photon. Technol. Lett. 9, 1997, 40–42.

98

CHAPTER-5

A new method for transmission of frequency encoded parallel optical

data

ABSTRACT

Semiconductor optical amplifier (SOA) is a well known non-linear device which can

exhibit Tera Hertz switching speed of operation. SOA based switching, therefore, has

wide application in fiber optic communication. In this chapter a new concept of

frequency encoded parallel data transmission with SOA for optical communication has

been proposed. To achieve the transmission SOA is suggested for the generation of

frequency encoded/decoded parallel data. It converts initially an intensity encoded optical

data to frequency encoded one; whereas at the receiving end it again returns the intensity

encoded data from frequency encoded one.


S. Dutta, S. Mukhopadhyay, “A new approach of parallel data transmission through

optical waveguide with SOA based frequency encoding/decoding technique”, Optik - Int.

J. Light Electron Opt., 123 (2012) 212– 216.

99

5.1 Introduction:

In the previous chapters the several types of logic operations and clocked and un-

clocked memory units have been discussed. In this chapter I extend the area of work and

interested to implement the frequency encoded parallel data transmission for the

communication system. To support the increasing demand and rapid growth information,

optics has been proved as proper alternative for very high speed communication with

high bit rate and low bit error rate [5.1-5.4]. In optical communication network the response

time at the nodes is a very important issue for setting high speed communication. For

managing the tremendously increasing day to day data traffic, it is very necessary to

enhance the transmission link capacity as well as the speed of the switching networks at

the nodes. The realization of a network node with throughput at the order of 100 Gb/s is

not far away. SOA grating combination has been successfully used for 160 GHz

wavelength conversion. Here a phase encoded signal is converted to amplitude modulated

signal. Again using a cross-correlation system and non-linear polarization rotation 200

GBPS wavelength conversion at temporal resolution at 1.5 PS is also reported. Some

logic gates are also implemented based on the four-wave mixing character of SOA with

the mechanism of polarization shift keying [5.5] and with tri-state operation logic [5.6].

Habib Fathallah et al proposed the concept of a high bandwidth optical communication

with fast optical frequency-hop code division multiple access (FFH-CDMA) system [5.7].

The encoding and decoding are done by all-fiber device.

Here different frequencies of light are encoded as different logic state. Here I

focus a principle for the successful realization of an efficient optical data transmission

based on the frequency encoding principle, which can be used as a more reliable one than

100

other conventional encoding principle towards the achievement of super-fast optical

communication and data processing [5.8-5.14].

5.2. A new method of frequency encoded parallel data transmission through optical

waveguide:

This communication system requires some specific frequencies (eight bit data

string) which represent the bit ‘1’ of each position of the data respectively. For example

λ1 represent the least significant bit (LSB) if it is ‘1’ whereas λ8 represents the most

significant bit if it is ‘1’ state. In this way all the other bits are represented by other

frequencies. The absence of light signal represents the logic state ‘0’. To implement the

whole system which is shown in fig-5.1, eight SOA based wavelength converters, eight

SOA based Add/Drop multiplexers, some beam couplers and mirrors are used. The

conversion method of the SOA based wavelength converter is discussed earlier. Now

using eight wavelength converters position-wise (e.g. for the first data bit WC1 is used)

the new mechanism of data transmission can be implemented. The pump beam of

frequency ν0 (corresponding wavelength λ0) is used in the other input terminals of all the

position-wise arranged WCs. The constant probe beams of light of wavelengths λ1, λ2, λ3,

λ4, λ5, λ6, λ7, λ8 are also applied to the respective WCs position-wise. The outputs of the

WCs are coupled by the beam couplers. This couplers output can be introduced to the

input of the optical fiber. If an eight bit data represented as 01101001 is to be sent

through the fiber through this proposed encoder, the light intensity of wavelength λ0 is

applied as pump beams to WC1, WC4, WC6 and WC7 which produces the signal λ1λ4λ6

and λ7 in the coupled output and finally this coupled beam comprised of λ1, λ4, λ6 and λ7

are introduced to the fiber. Following the same process any eight bit data can be encoded

101

as frequency encoded data and this frequency encoded data can be transmitted through

the fiber. In the receiving end eight Add/Drop multiplexers (ADM) are required for

decoding of frequency encoded data to intensity encoded one. Add/Drop multiplexers

(ADM) are arranged position-wise and each multiplexer is tuned for one particular

frequency by the application of proper bias current. ADM1 tuned at its biasing frequency

ν1, ADM2 at ν2, ADM3 at ν3 and all the other ADMs at ν4 to ν8 respectively. The output

obtained from the receiving end of the optical fiber is applied to the input of ADM8. Each

ADM has one circulator to collect the selected frequency of light. When the light beam

having frequency ν1 to ν8 applied to an ADM the respective frequency of light will be

reflected from the respective ADM for which it is tuned. The other frequencies will easily

pass through the ADM. Thus the reflected signals (having different frequencies) are

collected by the optical circulators. The output from the circulators gives the same

wavelengths in decoded data, which were sent originally at the input side. This data then

is passed parallely and bitwise through 8 wavelength converters. The probe beams of the

wavelength converters are at λ0. These wavelength converters returns the original

intensity encoded data with exactly position wise. In this output data all the ‘1’s are

represented by presence of light at the wavelength λ0 and ‘0’s by the absence of light.

The whole process can be illustrated by an example. Let an eight bit data 11100011is

used for encoding and decoding. For this light intensity of wavelength λ0 is applied as

pump beam to WC1, WC2, WC6, WC7 and WC8 only. Constant probe beams are also

applied to all the WCs. But the conversion takes place only in WC1, WC2, WC6, WC7,

WC8 whereas WC3, WC4, WC5 are not working for conversion. Thus the output beam of

wavelength λ1 is obtained from WC1, λ2 from WC2, λ6 from WC6, λ7 from WC7 and λ8

102

from WC8 and these light beams are coupled by the beam couplers. At the receiving end

the coupled beam is introduced to the ADM8. This ADM8 reflects the light having λ8

wavelength and passes λ1, λ2, λ6 and λ7 to the ADM7 which reflects the light of frequency

λ7 wavelength and sends λ1, λ2 and λ6 to the ADM6 which again reflects the light of λ6

wavelength. Similarly ADM2 reflects the light having λ2 wavelength and ADM1 reflects

the light having λ1 wavelength. ADM3, ADM4, ADM5 give no reflected light. These

output lights of different frequencies from the ADMs are applied to the eight wavelength

converters (WC9, WC10, WC11, WC12,…..WC16) as a pump beams and constant λ0 probe

beams are also applied to the WCs. So the output is obtained as λ0λ0λ0000λ0λ0 i.e. the

transmitted data 1110011 is received at the output. Thus any eight bit data can be sent

parallel with the system as described in fig.5.1. The system described in fig.5.1 enables of

transmitting eight bit data. For a sixteen bit data transmission 16 ADMs are required for

making the mechanism active. Any data of high number of bits can be sent in parallel

following this mechanism.

For another example a binary data string as (10011010) can be taken to sent

parallel through an optical fiber. In fig-5.2 the step by step result is described. In fig

5.2(a) the parallel data string is shown, whereas in fig 5.2(b) the bit wise intensity

encoded parallel data string is depicted .In fig 5.2(c) the bit wise wavelength encoded

data is shown, which is obtained from the series of wavelength converters as given in fig

5.1. This wavelength encoded data is sent through the optical fiber for parallel

communication. The data obtained at the outlet of the fiber is again sent through the

ADMs and wavelength converters, which are used for decoding the data. The wave

length converters at the receiving end reconvert the frequency encoded data to bitwise

103

intensity encoded data, which is shown in fig 5.2(d). The received data string is shown in

fig 5.2(e).

5.3 Essential requirements for implementation of the practical transmission of data:

The essential requirements for setting a good response from a SOA based optical

switch is pump beam for wavelength conversions should lie between 2 dB to 4 dB. An

optical filter can be used just after the SOA converters which only select the desired

probe beam frequency of light at the output. The 3 dB bandwidth of the filter should be in

the order of 1 nm. The performance of the data transfer depends upon the used pump

beam energy. The intensity level maintained at the each probe beam should be between -

4 to -2 dB. Again it is very important to mention that the wavelength of the both pump

and probe beam should lie in C band (1536-1570 nm). Based on all the above aspects

some wavelengths in C band are proposed for the consideration of pump beam and eight

different probe beams for encoding the bits of an eight bit byte. This is given in table-5.1.

Table-5.1: Some proposed wavelengths in C band for encoding the eight different ‘1’ bits of a byte and also for that of the probe beams

Pump beam λ0

Probe Beams (nm)

λ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8 1550 nm

1555 1557 1559 1561 1563 1565 1567 1569

104

Figure-5.1: Frequency encoded data transmission method based on SOA switching.

WC8 WC7

Input terminals for pump beam (λ0)

ADM1

CC C C

Output string of bits (λ0 represents 1 and no light represents 0)

WC2 WC1

ADM2 ADM7 ADM8

λ1 λ2 λ7 λ8

λ1 λ2 λ7 λ8

Represents Beam coupler

C Represents Circulator

Represents Optical fiber

WC9 WC10 WC15 WC16

λ0 λ0 λ0 λ0

………

………

……...

105

Figure-5.2 Graphical outputs of the frequency encoded data at different stages of the encoding/decoding system (a) Encoded intensity encoded eight bit data string, (b) Intensity distribution for the position-wise bits at different frequencies at initial stage, (c) Intensity distribution of the bit-wise coded light signals at the input of the optical fiber, (d) Intensity distribution of the position wise bits at the final output of the decoding system, (e) Decoded intensity encoded eight bit data string.

0.0

0.2

0.4

0.6

0.8

1.0

(a) 0000 111 1

8765432

λ8λ

6λ3

λ2

λ7

λ5

λ4

λ1

1

(b)

Inte

nsity

Bit position

0.0

0.2

0.4

0.6

0.8

1.0

λ8

λ6

λ3

λ2 λ

7λ5λ

4λ1

(c)

Frequency encoded light signal at the input of the optical fiber

Inte

nsity

0.0

0.2

0.4

0.6

0.8

1.0

(e) 01011001

87654321

λ0

λ0λ

0λ0

Position-wise output from the wavelength converters

λ0

λ0

λ0

λ0

(d)

Inte

nsity

106

5.4. Conclusion:

The above method exhibits a method for parallel transmission of data bits in case

of optical communication through fiber. The SOA takes the role of conversion of an

intensity encoded data to a frequency encoded one by the exploitation of its wavelength

conversion character. SOA can achieve the THz speed of operation for this conversion

process. This proposed method can, therefore, ensures a very high speed optical

communication over many other conventional communication techniques. If a data

accommodates eight bits or sixteen bits, then all the bits can be sent in parallel through

the optical fiber. It is important to mention here the wavelength all the wavelengths (λ1 to

λ8) should be selected at the c-band for setting the best conversion efficiency as well as

for low loss communication. The arrangement of SOA based wavelength converters and

ADMs in fig 1 are given in such a way that the form of the originally intensity encoded

data at the input of the fiber is maintained at the receiving end of the communication

system. At the final output end all the data bits are represented either by 1’s if an intensity

of light with wavelength λ0 presents otherwise by 0’s for no light.

107

References:

5.1 A. Kumpera, P. Honzathko, R. Slavik, Novel 160-GHz wavelength converter based

on a SOA and a long period grating, Opt. Commun. 282, 2009, 1775–1779.

5.2 J. Sakaguchi, T. Nishida, Y. Ueno, 200-Gb/s wavelength conversion using a delayed-

interference all-optical semiconductor gate assisted by nonlinear polarization rotation,

Opt. Commun. 282, 2009, 1728–1733.

5.3 C. Zhang, K. Qiu, B. Xu, Y. Ling, A novel all-optical label processing based on

multiple optical orthogonal coded sequences for optical packet switching networks, Opt.

Commun. 281, 2008, 2433–2442.

5.4 A. Argyris, D. Syvridis, L. Larger, V. Annovazzi-Lodi, P. Colet, I. Fischer, J. Garcia-

Ojalvo, C.R. Mirasso, L. Pesquera, K. Alan Shore, Chaos-based communications at high

bit rates using commercial fibre-optic links, Nature 438, 2005, 343–346.

5.5 Z. Li, G. Li, Gates based on four-wave mixing in a semiconductor optical amplifier,

IEEE Photon. Technol. Lett. 18, 2006, 1341–1343.

5.6 S.K. Garai, A scheme of developing frequency encoded tristate-optical logic

operations using Semiconductor Optical Amplifier, J. Mod. Opt. (Taylor and Francis) 57

(6), 2010, 419–428.

5.7 H. Fathallah, A. Rusch, S. Larochelle, Passive optical fast frequency-hop CDMA

communication system, J. Lightwave Technol. 17, 1999, 397–405.

5.8 S.K. Garai, S. Mukhopadhyay, Method of implementing frequency encoded

multiplexer and demultiplexer systems using nonlinear semiconductor optical amplifiers,

Opt. Laser Technol. 41, 2009, 972–976.

108

5.9 T. Durhuus, B. Mikkelsen, C. Joergensen, S. Lykke Danielsen, K.E. Stubkjaer, All-

optical wavelength conversion by semiconductor optical amplifiers, J. Lightwave

Technol. 14, 1996, 942–954.

5.10 D.A.O. Davies, Small-signal analysis of wavelength conversion in semiconductor

laser amplifiers via gain saturation, IEEE Photon. Technol. Lett. 7, 1995, 617–619.

5.11 E. Iannone, R. Sabella, L. De Stefano, F. Valeri, All-optical wavelength conversion

in optical multicarrier networks, IEEE Trans. Commun. 44, 1996, 716–724.

5.12 K. Obermann, S. Kindt, D. Breuer, K. Petermann, C. Schmidt, S. Diez, H.G. Weber,

Noise characteristics of semiconductor-optical amplifiers used for wavelength conversion

via cross-gain and cross-phase modulation, IEEE Photon. Technol. Lett. 9, 1997, 312–

314.

5.13 M.F.C. Stephens, D. Nesset, K.A. Williams, A.E. Kelly, R.V. Penty, I.H. White,

M.J. Fice, Wavelength conversion at 40 Gbit/s via cross-gain modulation in distributed

feedback laser integrated with semiconductor optical amplifier, Electron. Lett. 35, 1999

1762–1764.

5.14 D.D. Marcenac, A.E. Kelly, D. Nesset, D.A.O. Davies, Bandwidth enhancement of

wavelength conversion via cross-gain modulation by semiconductor optical amplifier

cascade, Electron. Lett. 31, 1995, 1442–1443.

109

CHAPTER-6

A new approach of developing Universal all-optical multiplexer with

frequency encoding mechanism

ABSTRACT

Multiplexing and demultiplexing are essential in any networking system. Different

encoding and decoding schemes have different advantages in optical multiplexing and

demultiplexing for all optical systems. Frequency encoding technique is established as a

faithful and reliable one among different encoding processes. Intensity encoded,

frequency encoded and many other multiplexers were proposed earlier. In all those

multiplexers the numbers of multiplexed input channels depend on the number of

triggering channels. In this chapter a novel concept of frequency encoded universal

multiplexer which can deal any number of input channels with the use of a single

triggering channel has been described.

110

6.1. Introduction:

In any information processing system as well as in communication networking

system, multiplexer and demultiplexer are key elements. Different types of optical

multiplexer and demultiplexer with various encoding mechanisms are reported by several

scientists [6.1-6.6]. Some frequency encoded multiplexers are reported with tri-state system

[6.7]. In this system some specials types SOA based optical switches BSOA has been used.

In previous studies the frequency encoded logic units, memory units and parallel

transmission of data have been studied. In this chapter a novel approach of implementing

frequency encoded universal triggered multiplexer using the semiconductor optical

amplifier based optical switches has been proposed. To implement this system frequency

encoded method but with Boolean logic system and SOA based wavelength converters

and some optical pass filters have been used [6.8-6.12]. Multiplexer means ‘many into one’,

as it follows the combinational logic to connect one output channel with one of many

input channels. This is selected by the signals in the triggering channels. In multiplexer, it

has one or many signals in triggering channels and these triggering signals select the

proper input channel to be connected with the output. A multiplexer having n numbers

of triggering channels can accommodate 2n input channels. In general Boolean triggering

is used in conventional multiplexer and demultiplexrer systems. The novelty of this

system is that any number of input channels can be accessed by a single triggering

channel. So using this special multiplexer any one can access enormous number of input

channels, if the triggering channels are increased. The frequency encoding mechanism

and SOA made optical switches are exploited full to extend their advantages for

implementation of such universal multiplexer.

111

6.2. Method of developing a frequency encoded universal multiplexer with SOA:

To implement the universal optical multiplexing system I have used some optical

pass filters (which passes selected one frequency of light and blocks others), some beam

splitters (BS), beam couplers (BC), mirrors (M), optical channels and SOA based

switches like wavelength converters (WC). The whole scheme is shown in figure-6.1.

According to figure-6.1 T is the single triggering channel. Here any one of n numbers of

frequencies ν0, ν1, ν2, ν3, ν4,……………..νn is used to select one from n inputs. There are

n pass filters (PF) where each can select a specific frequency of light. In fig-6.1, block-1

is a ν0 pass filter, block-2 is ν1 pass filter, and block-3 is ν2 pass filter and so on. Here I1,

I2,……….In are the input channels and O is the output channel. Now only when

triggering signal 0 i.e. ν0 frequency is applied in the optical triggering channel and it is

passed through the block-1 due to presence of optical pass filters which only passes the ν0

frequency of light, then only the ν0 frequency of light is applied into the WC1 as a strong

pump beam. Here the I1 input channel which consumes a weak probe beam i.e. when the

system is triggered by ν0 frequency of light, the pump beam is present in only WC1 and

for this I1 is connected to the output. Therefore the frequency of light applied at I1 passes

to the output cannel O. So the input signal of I1 is transmitted directly to the output being

controlled by the triggering signal ν0. Similarly when ν1 frequency of light is present in

the triggering channel T then only I2 input signal is transmitted to the output as ν2 being

filtered by the block-2 comes to WC2 as a pump beam and sends I2 (i.e. the frequency of

I2) to the output port O. In such a way a specific frequency (λi, i=1…….n) applied to the

triggering channel T activates the specific WCi and helps Ii to be transmitted to the

output. These n numbers of frequencies in the triggering channel select n inputs

112

respectively. It is important to mention that when a specific WC works others remain

nonfunctional because of absence of pump beams to those WCs.

Figure-6.1: Architecture of all-optical single triggered universal multiplexer (M, BS, WC represents the mirror, beam splitters and wavelength converter).

1 2 n-1 n

WC1

WC2

WCn-1

WCn

. . . . . . . . . . . . . .

.

.

. . . . . . . . . . . . . . . . . . . . . .

T

I1

I2

In

In-1

O

(ν0 to νn )

M

M

M

M’s

M’s

M’s

M’s

BC

BC

BC

BC

M M

M

M

M

BS

113

6.3. Method of developing double triggering universal multiplexer:

To implement a double triggered universal multiplexer some other sets of optical

beam splitters, beam couplers, mirrors, wavelength converters and optical pass filters

have been used. Here signals have been used in two triggering channels, where each

triggering channel can accommodate n numbers of possible triggering signals as shown in

figure-6.2. Here T1 and T2 are the triggering channels and blocks 1,2…………n, and

1′ , 2′…….. n′ are the optical pass filters respectively in two channels. Block 1,2…..

represents the ν1, ν2, ν3…. frequencies optical pass filters respectively and block-

1′ , 2′….. also represents the ν1, ν2, ν3……….frequencies optical pass filters respectively.

WC11, WC21……..WCij, WCnn and W1, W2,…..Wi,….. Wn, and 1′W ,

2′W ….. jW ′ ,…. nW ′ are the wavelength converters. I11, I21……Iij, Inn are the input

channels and O is the output channel of the whole multiplexing system which collects all

the output of the WCs. The whole scheme is shown in figure-6.2. Now when T1and T2

both is ‘ν1’ i.e. ν1 frequency is applied into the triggering channels T1 and T2 then only

block-1 and block-1′ pass this frequency and they are connected to the wavelength

converters W1 and 1′W respectively as a pump beam. The probe beams of each W1 and

1′W is ν0. So the output of W1 and 1′W are also ν0. The combined output of W1 and

1′W is applied into the WC11 as a strong pump beam. Here signal of I11 input channel

applied to the WC11 (which is a weak probe beam of WC11) and is transmitted to the

output channel O. Applied strong pump beam of the wavelength converters which are

coming from T1 and T2 must be in same frequency and have to satisfy the requirement of

pump power of the WC’s. If one frequency is different from other and it is applied to the

WC as pump beam, then it does not work and no transmitted signal is obtained to the

114

output terminal. Again if T1= ν1 and T2=ν2 then block-1 passes the ν1. In W1, ν1 frequency

is applied as a strong pump beam and a constant weak probe beam of ν0 frequency is

already present there. So conversion is obtained from ν1 to ν0 and one gets the ν0

frequency of light at the output terminal of W1. The ν2 frequency is passed through the

block- 2′ and it is also converted to the ν0 frequency by the use of 2′W . These both ν0

frequencies from W1 and 2′W are applied as a strong pump beam jointly to the WC21. I21

the input channel of WC21 takes the weak input probe beam as data. In presence of both

pump and probe beams data of I21 is transmitted to the output channel O in this

multiplexer. Each of T1 and T2 triggering channels can accept n inputs. Thus overall the

two channels jointly can handle n2 number of input channels for multiplexing. The whole

operation can be made clear by an example. Let T1 takes νi and T2 takes νj inputs then i th

filter allows only to pass the νi th frequency from T1 and similarly j′ th filter allows the νj

th frequency from T2 through it. Then νi and νj after passing through respective Wi and

jW ′are converted to ν0 frequency. The joint power of both the ν0 is adjusted in such a

way that they meet the requirement of pump powers for conversion of wavelength when

they fall on WCij th wavelength converter. Under this situation WCij th converter allows

Iij th input to pass through it and it comes then to the final output terminal of the

multiplexer.

The advantage of the proposed universal multiplexer is described by the table-6.1.

In this table the numbers of input channels multiplexed by the concerned number of

triggering channels are shown. From this table it can be concluded that if there are m

number of triggering channels in a multiplexer then it can access nm number of input

channels, if n-array encoding is adopted. By the use of frequency encoding technique as

115

proposed in this scheme, the advantage of handling maximum no. of input channels with

a fixed number of triggering channels can be achieved.

Figure-6.2: Architecture of all-optical double triggered universal multiplexer.

1 2 n

W1

1′ 2′ n′

W2 Wn 1′W nW ′

WC11 WC12 WC1n

WCij

WCnn

WCn1

.

.

.

.

.

.

.

.

.

………...

.

.

.

.

.

…………………

T1 T2

………... ……

…… ………...

O Output of Multiplexer

ν0ν0 ν0 ν0 ν0

I11

I1n

Iij

In1

Inn

.

.

.

.

116

Table-6.1: No. of input channels accessed by m number triggering channels for different encoding

6.4. Practical realization:

To set the desired operation pump beam for wavelength conversions of SOA

based optical switch should lie between 2 dB to 4 dB which is the essential requirement

for realization of an experimental set up. The 3 dB bandwidth of an optical pass filter,

which is used for selecting the particular frequency of light, should be in the order of 1

nm to provide a larger number of inputs channels. The pump beam energy actually

controls the performance of the data transfer so the selection of pump beam power is very

important. The each probe beam should lie between -4 to -2 dB for maintaining a

Encoding

schemes →

Binary

encoding

Trinary

encoding

……………… N-ary encoding

Number of

input channels

multiplexed by

m no. of

triggering

channels →

2m 3m …………….. nm

117

standard intensity level of light at the output of WCs. The important matter is to mention

here is that the wavelengths of the both pump and probe beams should be in C band

(1536-1570 nm). Thus if 20 input frequencies are taken for T1 and T2 triggering channel

both, the double triggering universal multiplexer then can accommodate 400 input

channels.

6.5. Conclusion:

The method of optical implementation of a universal multiplexer with all optical

switching systems has been discussed in this chapter. The speed of operation of each

SOA based wavelength converting switches is far above THz limit. So a high operational

speed (above THz) is expected from the whole scheme. It is not only the one advantage

of this proposed system but the frequency encoding mechanism also offers a great

advantage. The output signal from the multiplexer can be sent to distant receiver as

frequency remains generally unaltered in reflection, refraction, absorption etc for the

coding of information (0 or 1) with different frequency of light. Therefore this optical

technique will be very much useful for transmitting a reliable and faithful data for long

distance transmission. To achieve a good amplification the pump beam of WCs should lie

between 4dB to 10 dB. Also the scheme offers a high signal to noise (S/N) ratio. The

main reason for obtaining the high S/N ratio is due to the use of the frequency encoding

process in this proposed system. Bit error rate also goes to a very small value in

comparison to conventional intensity based or polarization based encoding processes.

Increasing the number of triggering channels one can access more number of input

channels for multiplexing if the span of frequency is restricted. The operation exploits the

inherent parallelism of optics as far as practicable. It is important to mention that each

118

WC in fig-6.2 are only operative for transmitting its probe beam to its output if and only

if the joint power of both ν0 frequency coming after conversion from T1 and T2 satisfy the

requirement of pump power for conversion operation by the WC. For this when only one

WC is dedicated to take part in conversion mechanism, other WCs in the matrix of WCs

are non operative. This multiplexer can be extended to develop a three or higher

triggering based channel universal multiplexer. If it becomes an m channel multiplexer it

then easily accommodates nm number of input channels for multiplexing. This is the

prime advantage of the proposed scheme as a whole.

119

References:

6.1. S.K. Garai, S. Mukhopadhyay, “Method of implementing frequency encoded

multiplexer and demultiplexer systems using nonlinear semiconductor optical

amplifiers”, Optics and Laser Technology, 41, 2009, 972-976.

6.2. Jitendra Nath Roy, Anup Kumar Maiti, S. Mukhopadhyay, “Designing of an all-

optical time division multiplexing scheme with the help of non-linear material based tree-

net architecture”, Chinese Optics Letters (China), 4(8), 2006,483-486.

6.3. Jitendra Nath Roy, Anup Maiti, S. Mukhopadhyay, “Exploitation of nonlinear

material based tree-net architecture in all optical demultiplexing scheme,” Journal of

Optics, 36(1), 2007, 1-7.

6.4. C. Dragone, “An N*N optical multiplexer using a planar arrangement of two star

couplers”, Photonics Technology Letters, 3 (9), 06 August 2002, 812 – 815.

6.5. Unnikrishnan Gopinathan, Thomas J. Naughton, and John T. Sheridan, “Polarization

encoding and multiplexing of two-dimensional signals: application to image encryption”,

Applied Optics, 45(22), 2006, 5693-5700.

6.6. Jose M. Castro, David F. Geraghty, Seppo Honkanen, Christoph M. Greiner, Dmitri

Iazikov, and Thomas W. Mossberg, “Optical add-drop multiplexers based on the

antisymmetric waveguide Bragg grating”, Applied Optics, 45(6), 2006, 1236-1243.

6.7. Ashish Pal and Sourangshu Mukhopadhyay, ‘An alternative approach of developing

a frequency encoded optical tri-state multiplexer with Broad area semiconductor optical

amplifier (BSOA)’, Optics and Laser Technology, 44, 2012, 281-284.

120

6.8. H.J. Lee, M. Sohn, K. Kim and H.G. Kirn, “Wavelength dependent performance of a

wavelength converter based on cross-gain modulation and birefringence of a

semiconductor optical amplifier”, IEEE Photon. Technol. Lett., 11, 1999, 185-187.

6.9. L. Deming, N.J. Hong and L. Chao, “Wavelength conversion based on cross-gain

modulation of ASE spectrum of SOA”, IEEE Photon. Technol. Lett, 12, 2000, 1222-

1224.

6.10. H. Yu, D. Mahgerefteh, P.S. Cho and J. Goldhar, “Improved transmission of

chirped signals from semiconductor optical devices by pulse reshaping using a fiber

Bragg grating filter”, J. Lightwave Technol., 17, 1999, , 898-903.

6.11. D.D. Marcenac, A.E. Kelly, D. Nesset and D.A.O. Davies, “Bandwidth

enhancement of wavelength conversion via cross-gain modulation by semiconductor

optical amplifier Cascade”, Electron. Lett., 31, 1995, 1442-1443.

6.12. X. Zheng, F. Liu and A. Kloch, “Experimental investigation of the cascadability of

a cross-modulation wavelength converter”, IEEE Photon. Technol. Lett., 12, 2000, 272-

274.

121

CHAPTER-7

Use of all-optical Kerr Nonlinearity for super-fast conversion of a

binary number having a fractional part to its decimal counterpart and

vice-versa

ABSTRACT

The role of optical tree architecture is an important approach for conversion of an optical

data from binary to decimal and vice-versa. In this chapter I have proposed a new concept

of converting of a binary number having some fraction (fractional value) to its equivalent

decimal counterpart and its vice-versa. To perform this operation optical tree and some

nonlinear material based switches are used properly.


S. Dutta and S. Mukhopadhyay, “All optical frequency encoding method for converting a

decimal number to its equivalent binary number using tree architecture”, Optik - Int. J.

Light Electron Opt., 122 (2011) 125–127.

122

7.1. Introduction:

In the previous chapters the frequency encoded all optical systems has been

discussed. Those systems are implemented by encode the state of working using

frequency of light. Several types of encoding systems are popular for implementing of

optical systems. Optical tree architecture is one of them. In connection to the new

developments of several all-optical data processing techniques, the role of optical tree

architecture can be mentioned specially [7.1-7.5]. This tree has already been used to convert

a position-wise encoded optical decimal data to its binary counterpart and from binary

data to its decimal counterpart. Not only are these conversions but also there several other

conversions which are possible to be conducted by this tree architecture for the need of

all-optical data processing and computing. Here in this chapter I have proposed a

modification of optical tree architecture by which a binary data having a fractional part

can be converted to its respective equivalent decimal value and from decimal number

having a fractional part to its equivalent binary form. Already optical tree has been used

for conversion of different forms of data, but this type of conversion of binary number

having fractional part to its decimal counterpart and its vice-versa is a completely new

concept. The beauty of this optical tree architecture is no use of any kind of switches. In

tree architecture only optical channels, beam splitters and mirrors have been used. That’s

why power conservation is so high and the system will be so fast.

7.2 Optical tree architecture:

The schematic diagram of an optical tree-architecture is shown in figure-7.1. Here

a light beam (preferably a laser) emitted from a point source (A) passes through the some

optical switches to break into two different light beams into BC and BD. Each of these

123

two beams breaks into two more parts individually as BC to CE and CF and BD to DG

and DH. Proceeding in this way ultimately eight spots (from I to P) are obtained from a

single beam AB. By arranging another splitting arrangement in the output plane one can

get 16 spots. To use this circuit some optical channels (A0, A1, A2) which control the

passing of the light from one main channel to a sub channel have been used. The control

channels also carries light beam. A2 control channel is connected to the switch B, A1

channel is connected to C and D switches, where as A0 channel is connected to E, F, G, H

switches. The function of the switches can be illustrated by an example. If light in the

control channel A2 is present then it activates the switch B to pass the light from AB to

BC (upper) sub channel otherwise the light of AB channel will go through the (lower)

sub channel BD. In the same way the other switches function. In this connection here this

optical system has been used for the conversion of a binary number having a fractional

part to its equivalent decimal number. In the present analysis all the switches are all-

optical in nature. For the conversion of decimal to binary data different arrangements of

mirrors, beam splitters and optical channels are used [7.1].

124

Figure-7.1 Optical tree architecture for binary to decimal conversion

7.3. Non-linear material as an optical switch:

In this system only non-linear type of material has been used as a switch. Now the

function of some isotropic non-linear material (NLM) has been discussed as an all-optical

switch. This Kerr non-linearity equation for some isotropic material is well established

Innn o 2+= (1.4)

where n is the refractive index (r.i) of the concerned non-linear material, n2 is the non-

linear correction term, n0 is the constant linear refractive index term and, I is the intensity

of light passing through the material. As for example of some non-linear materials is

pure silica glass (SiO2), gallium arsenide (GaAs), carbon di-sulfide (CS2), etc. The path

A

O

P

N

M

L

K

J I

A2 A1 A0

B

C

D

E

F

G

H

125

of output light is depended on the intensity of the input light. If the intensity of incident

light is increased or decreased the refractive index is also changed according this light

intensity for that reason the path of output light is also changed. It’s a very simple

switching system. According to this equation when a beam AB of some fixed intensity I

falls in the point O (O is a point in the boundary of linear material (LM) and non-linear

material (NLM) it passes through the DE channel as shown in fig.7.2. Now when one

beam of intensity I in AB channel and other beam of intensity I in CB channel fall jointly

on “O” , then according to the equation the non-linear refractive index of the material

becomes n=n0 + n2I, and for this reason light beam ultimately will pass through the FG

channel of fig.7.2. This can be compared as an optical switch. Many all-optical logic

devices have been proposed based on this principle [7.6-7.16].

Figure7.2: Non-linear material as an optical switch.

NLM

LM

A

B

C

M

O

F

D

G

E

126

7.4. Optical conversion method of a binary number having a fractional part to its

equivalent decimal number:

The optical method of conversion of binary to decimal number is an established

one. The next important function is the implementation of a system for transformation of

fractional binary number to its decimal counterpart. Here modified tree architecture can

be used for such conversion. The significant difference of this scheme from that of the

described earlier is the method of placement and position of NLM based switches, beam

splitters (BS) and mirrors (M). Placing these components in a properly different way the

modified system can be developed. The whole scheme is shown in fig.-7.3(a). Here the

control channels (B4B3B2B1B0) carry the bits of fractional binary number. The whole

things of the system can be made clear by an example. Let the fractional binary is

0.10110. So B4=B2=B1=1 and B3=B0=0 and for these type of input signal light beam from

constant light source (CLS) first passes through the upper channel from NLM1 then it

comes to the lower channel from NLM2 and again passing through the upper channel

from NLM4 it ultimately exits through the channel number 7. This indicates that the

conversion of 0.10110 gives the position-wise encoded decimal number 0.7. In this way

any binary number having fractional part can be converted to the respective decimal

number by the use of the system described in a figure-7.3(a). Now the whole scheme can

be integrated together to convert a binary number having both fractional and non-

fractional parts to its equivalent decimal value. Here the block A consist the system of

non-fractional part. A3A2A1A0 are the input channels i.e. the channels for placing the

binary non-fractional inputs. On the other hand block-B carries the same system as

described in figure-7.3(a) i.e. fractional part, where B4B3B2B1B0 are the input channels

127

for placing the binary fractional inputs. By the joint action of the two blocks (A and B)

one can convert any binary number (having both fractional and non-fractional parts) to its

equivalent decimal value which is encoded position-wise. The whole block diagram is

shown in figure-7.3(b) For example if 011.10011 are applied to this integrated system 3.6

is obtained at the output. All the converted fractional decimal numbers from its

equivalent binary numbers are shown in table-7.1.

Table-7.1: Converted numbers of fractional decimal to its equivalent binary code

FRACTIONAL DECIMAL

NUMBER

CORRESPONDING EQUIVALENT

FRACTIONAL BINARY NUMBER

0.1 0.00011

0.2 0.00110

0.3 0.01001

0.4 0.01100

0.5 0.10000

0.6 0.10011

0.7 0.10110

0.8 0.11001

0.9 0.11100

128

Figure-7.3(a): All optical system for converting fractional binary number to its decimal counterpart

CLS

NLM1

NLM2

NLM3

NLM4

NLM5

NLM6

NLM7

NLM8

NLM9

0

1

2

3

4

5

6

7

8

9

B0B1B2B3B4

REPRESENTS MIRROR

REPRESENTS BEAM SPLITTERS

OUTPUTS

129

Figure-7.3(b): All optical integrated system for converting a binary number having both fractional part and non-fractional part to its equivalent decimal number

CONVERSION METHODE FOR BINARY

TO DECIMAL

BLOCK-A

CONVERSION METHODE FOR BINARY

(FRACTIONAL) TO DECIMAL

BLOCK-B

0 1 2 3 4 5 6 7 8 9 . 0 1 2 3 4 5 6 7 8 9

.

OUTPUTS

A0

A1

A2

A3

B0

B1

B2

B3

130

7.5. Optical conversion method of a decimal number having a fractional part to its

binary equivalent:

Now I also propose a concept of converting a decimal number having a fractional

part to its binary equivalent in an all optical process. Here the decimal number has two

parts as a whole. First part is the non fractional where as second part is the fractional.

Second part extends the conversion of fractional part. The proposed system for

conversion of the fractional digit to its binary equivalent is shown in figure-7.4(a). Here

beam combiners have been used for coupling two light beams into a single one and also

for breaking a single beam into two parts. These are represented by BS in the figure. The

mirrors (M) are used for the reflection of light beam. Using these BS’s the converted

result is obtained at the output channel B4B3B2B1B0 of the system shown in fig. 7.4(a).

By the use of the modified tree structure B4B3B2B1B0 ultimately show the converted

equivalent binary number of the fractional decimal contribution which lies from 0 to 0.9.

To develop this system the channels marked by 0,1,2,3,…..,9 are used for applying the

fractional part of the decimal number. If 0.8 is to be converted the light is to be applied at

the channel marked by 8, and binary converted output is obtained in the channels marked

by B4B3B2B1B0.As for example-when the light goes through the channel 7, no light

comes channels B0 and B3 because of the absence of BS in the respective channel. So

B0=B3=0, but B4,B2,B1 channels get the necessary light reflected from the respective

BS’s. Hence B4=B2=B1=1. Ultimately B4B3B2B1B0=10110 is obtained for the application

of light in the input channel marked by 7. So it is the equivalent binary number of

decimal value 0.7. Similarly any decimal number from 0 to 0.9 can be converted to its

binary value. The non-fractional part and fractional part of a system can be combined

131

together to develop a single integrated system from which a decimal number having both

the non fractional part and the fractional part to its binary equivalent value can be

converted. The scheme is shown in figure-7.4(b). As for example- if it is required to

convert of the decimal number a.b to its equivalent binary number (where ‘a’ is the non

fractional decimal digit and ‘b’ is the fractional decimal digit), the light beams are to be

placed in the ‘a’ th channel of the block A and to ‘b’ th channel of the block B in the

system. Here block A and block B comprise the system of non-fractional part and the

system of fractional part respectively. The A3A2A1A0.B4B3B2B1B0 comes as the result of

the converted binary number from its decimal counterpart. If the decimal number is 3.7

then surely 0011.10110 has been received at the output stage i.e. A1=A0=B4=B2=B1=1

and A3=A2=B3=B0=0.

132

Figure-7.4(a) All-optical Conversion system from fractional decimal number to its binary counter part

1

2 3

4

5

6

7

8

9

0

M

M

M

M

M

M

M

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

B4 B3 B2 B1 B0

OUTPUTS

133

Figure-7.4(b): The block diagram of complete system for conversion of decimal number having both fractional part and non-fractional part to its binary equivalent

7.6. Alternating approach of non-linear switch:

In this chapter non-linear material has been used as an optical switch for

conducting the intensity encoded data based operation. In this approach the optical

switching operation is realized by the use of non-linear character of semiconductor

optical amplifier (SOA). Here the channel shifting operation in the tree architecture is

CONVERSION SYSTEM FOR

NONFRACTIONAL PART (0 TO 9)

BLOCK-A

CONVERSION SYSTEM FOR

FRACTIONAL PART (0 TO 9)

BLOCK-B

.

A3 A2 A1 A0 . B3 B2 B1 B4 B0

0123456789

789

6789

012345

OUTPUTS

134

conducted by SOA. As usual SOA based wavelength converter converts the wavelength

of the light which is described before in the introduction section. One prism is placed in

front of it. So λ1 wavelengths of light first passing through the SOA switch goes through

the prism and exist from a specific (channel 1). Now if a (λ2 wavelength) of weak probe

beam is applied to the input side of the SOA switch in addition to the existing pump

beam then a converted light beam of wavelength λ2 is obtained from the output of the

SOA switch and after passing through the prism it comes out through another channel

(channel 2). Thus two light beams come in two different channels of the output in the

system described in fig-7.5. When a light of wavelength λ1 is applied at the input side of

the SOA, it ultimately exits from channel-1 and when light of λ1 and λ2 wavelengths both

falls in the input side of SOA, light is achieved from another channel marked as channel-

2. This type of switch can be used in replacement of the non-linear material switch. It is

already seen that for the conversion of a decimal number (having some fractional part) to

the equivalent binary number and its vice-versa the systems proposed in fig-7.4(a) and

fig-7.3(a) will be useful. The conventional non-linear switches used in these two systems

may be replaced by the SOA based switches as described above to get a faithful

operation. The output power of the digits of the converted data becomes stronger here

sufficiently.

135

Figure-7.5: SOA based intensity encoded switch

7.7. Conclusion:

In this chapter the method of conversion of a binary number having a fractional

part to its equivalent decimal number and its vice-versa with all optical switching system

has been discussed. The speed of operation is real time. The scheme may be extended

both vertically as well as horizontally for the conversion of a higher valued binary

number to its equivalent decimal number and from decimal to binary. The inherent

parallelism of optics is exploited in the scheme as far as practicable to obtain superfast

operation speed. To get more accuracy in the conversion of the fractional binary number

one may use more number of control channels in an extended system. As a whole the

total system is all optical one and hence the advantages of using optics are achieved. A

suitable non-linear material like SOA and a suitable diode laser should be used, for

getting low optical power consumption and for reliable operational result.

λ1

λ2

INPUTS

SOA based wavelength converter

Optical prism

λ2

λ1

Biasing current

OUTPUTS

136

References:

7.1 An optical conversion system: From binary to decimal and decimal to binary, Optics

Communications (The Netherlands) 76(5–6), 1990, 309–312.

7.2 S. Mukhopadhyay, J. N. Roy, and S. K. Bera, ‘‘Design of a minimized LED array for

maximum parallel logic operations in optical shadow casting technique,’’ Opt. Commun.

99, 1993, 31–37.

7.3 Parallel Distributed Processing: Explorations in the Microstructure of Cognition, D.

E. Rumelhart and J. L. McClelland, Eds., Vols. 1 and 2, MIT Press, Boston 1986.

7.4 M. M. Mirsalehi, ‘‘History of optical parallel processing,’’ R. A. Mayer, Ed.,

Encyclopedia of Laser and Optical Tech., Academic Press Inc., New York 1991.

7.5 N. Peyghambarian and H. M. Gibbs, ‘‘Optical bistability for optical signal processing

and computing,’’ Opt. Eng. 24(1), 1985, 68–72.

7.6 R. Tripathi, G. S. Pati, and K. Singh, ‘‘Non-linear processing and fractional-order

filtering in a joint fractional Fourier transform correlator: performance evaluation in

multiobject recognition,’’ Appl. Opt. 40(17), 2001, 12844–2858.

7.7 S. D. Smith, I. Janossy, H. A. Mackenzie, J. G. H. Mathew, J. J. E. Reid, M. R.

Taghizadeh, F. A. P. Tooley, and A. C.Walker, ‘‘Nonlinear optical circuit elements, logic

gates for optical computers: the first digital optical circuits,’’ Opt. Eng. 24(4), 1985, 569–

573.

7.8 N. Pahari, D. Das and S. Mukhopadhyay, All-optical method for the addition of

binary data by non-linear materials, Applied Optics, 43(33), 2004, 6147–6150.

137

7.9 J. N. Roy, A. K. Maiti, D. Samanta and S. Mukhopadhyay, Tree-net architecture for

integrated all-optical arithmetic operations and data comparison scheme with optical

nonlinear material, Optical Switching and Networking, 4, 2007, 231–237.

7.10 N. Mitra and S. Mukhopadhyay, A new scheme of an all optical J-K Flip-flop using

non-linear material, Journal of Optics., 37(3), 2008, 85–92.

7.11 H. L. Minh, Z. Ghassemlooy and W. P. Ng, All-optical.ip–.opbased on asymmetric

Mach–Zehnder switch with a feedback loop and multiple forward set/reset signals, Opt.

Eng. 46, 2007, 040501-1–040501-3.

7.12. Mitra and S. Mukhopadhyay, A method of developing all-optical mono-stable

multivibrator system exploiting the Kerr non-linearity of medium, Optik —

International Journal for Light and Electron Optics, 122, 2011, 92–94.

7.13. Srivastava, S. Medhekar, “Switching of one beam by another in a Kerr type

nonlinear Mach–Zehnder interferometer”, Optics and Laser Technology, vol. 43, no.

1, 2011, pp. 29-35.

7.14. Srivastava, S. Medhekar, “Switching behavior of a nonlinear Mach–Zehnder

interferometer: Saturating nonlinearity”, 43(7), 2011, 1208-1211.

7.15. Medhekar, Rajkamal, S. Konar, “Successive uptapering and stationary self-trapped

propagation of a laser beam in a saturating nonlinear medium” Laser and Particle

Beams, 13(4), 1995.

7.16. Medhekar, S. Konar, Rajkamal, “Self tapering and uptapering of a self guided laser

beam in an absorbing/gain medium with nonlinearity”, Pramana-journal of Physics,

44(3), 1995, 249-256.

138

CHAPTER-8

General conclusion and future scope of the work

ABSTRACT

Here in this chapter I have included an overall conclusion of the whole thesis, where the

advantages and limitations of the proposed schemes are discussed. Accordingly I have

discussed also the remaining scope of work in this field where some contribution may be

done.

8.1 Introduction:

The need of optical computation and communication has stimulated new scientific

research for flexible, efficient and super fast optical techniques and optical switches.

Optical systems based on semiconductors optical amplifiers switches have shown the

potential of obtaining the cheap and easy techniques to implement the super fast optical

computation and communication systems. In this dissertation, I have undertaken a

systematic study to improve the all-optical logic and arithmetic operations using a

problem free encoding method. The major conclusions from the dissertation are

summarized below:

8.2 Conclusion of the thesis

Chapter-1 contains the brief past history of optical computation and

communication systems as well as the current status in this field. In these chapter

advantages of optics over electronics has been described. The most important

property of the frequency encoding principle is that, frequency is the primary

character of the wave and during its propagation throughout the communicating

media it can protect its uniqueness irrespective of the absorption, reflection,

139

transmission. Over all other conventional encoding techniques this is the most

prospective advantage of the frequency encoding technique and these advantages

are described in this chapter.

Chapter 2 reports on the implementation of all-optical frequency encoded NOT

latch using semiconductor optical amplifier based switches. Here the advantages

of frequency encoded NOT latch has been described. Using frequency encoding

principle this NOT latch scheme will be more reliable and faster comparatively

other techniques. Using this latch logic and the frequency encoded technique one

can implement many other operations like digital types of flip-flops, multiplexer,

demultiplexer etc. with some modification of the scheme.

Chapter 3 mainly deals with frequency encoded logic gates, R-S flip-flop and

programmable logic unit with semiconductor optical amplifier based switches.

Logic gates are the building blocks of any memory unit. The potential advantage

of these optical gates and flip-flop over many other established optical gates was

the use of frequency encoding technique, for which the coded information (0, 1)

in a signal remains unchanged in refraction, reflection, absorption, etc. for a long

distance transmission of data. The proposed system can offer also a noise free

conversion to provide a high signal to noise (S/N) ratio.

Chapter 4 presents a frequency encoded all-optical clocked S-R flip-flop based

on semiconductor optical amplifier switches. The most important application of

the system can be seen in digital communication, where one can send the

frequency encoded new data or an (old data depending on the applied clock) in

the communication channel. Even if the sender requires making a data to be

140

continued for communication or a new data is to be introduced in the channel, he

can easily do it by the use of the above system. The output signal from the

clocked S–R flip-flop can be sent to distant receiver as it remains unaltered in

reflection, refraction, absorption, etc. due to the nature of coding of bits (0 or 1)

with frequency variation of light. Therefore this technique will be very much

useful for conducting a reliable and faithful optical memory both in

communication and computation. This optical clocked S–R flip-flop and

frequency encoding technique can be used many other optical devices where

clocked S–R flip-flop is an essential unit.

Chapter 5 reports on the transmission on the frequency encoded parallel data. The

SOA takes the role of conversion of an intensity encoded data to a frequency

encoded one by the exploitation of its wavelength conversion character. SOA can

achieve the THz speed of operation for this conversion process. This proposed

method can, therefore ensure a very high speed optical communication over many

other conventional communication techniques. If a data accommodates eight bits

or sixteen bits, then all the bits can be sent in parallel through the optical fiber.

Chapter 6 deals with the frequency encoded universal all-optical multiplexer

system. Here to implement this universal some wavelength converters and some

ADD/DROP multiplexers has been used. Using frequency encoding technique

will be very much useful for transmitting a reliable and faithful data for long

distance transmission. Also the scheme offers a high signal to noise (S/N) ratio.

The main reason for obtaining the high S/N ratio is due to the use of the frequency

encoding process in this proposed system. Bit error rate also goes to a very small

141

value in comparison to conventional intensity based or polarization based

encoding processes. Increasing the number of triggering channels one can access

more number of input channels for multiplexing if the span of frequency is

restricted. The operation exploits the inherent parallelism of optics as far as

practicable.

Chapter 7 reports on all-optical Kerr Cell for super fast conversion of a binary

number having a fractional part to its decimal counterpart and vice-versa. The

speed of operation is real time. The scheme may be extended both vertically as

well as horizontally for the conversion of a higher valued binary number to its

equivalent decimal number and also from decimal to binary. The inherent

parallelism of optics is exploited in the scheme as far as practicable to obtain

super fast operation speed. Increasing the controlling channel the can be extended.

Again this conversion system with non-linear material can be replaced with the

SOA based switches. This is the advantageous point of view of this whole

scheme.

8.3 Future scope of the work in this area

Although the present dissertation reports a detailed study on semiconductor optical

amplifier and kerr type optical switching for developing some systems in the area of

optical computation and communication to improve efficiency and speed of the proposed

systems, but several aspects (as mentioned below) could not be taken up , which are

worth for further investigations.

• There are possibilities to implement all these proposed works physically and

experimentally.

142

• All other building blocks which are not been proposed yet can be implemented in

future works.

• Implementing the optical frequency encoded register and counter based on

semiconductor optical amplifier based switches.

• Implementation of some all-optical frequency encoded multiplier, tristate optical

processors, optical softwares, optical nano processors etc.

143

List of published papers Section A: Journal papers

1. S. Dutta and S. Mukhopadhyay, “All optical frequency encoding method for

converting a decimal number to its equivalent binary number using tree

architecture”, Optik 122 (2011) 125–127.

2. S. Dutta and S. Mukhopadhyay, “An all optical approach of frequency encoded

NOT based Latch using semiconductor optical amplifier”, J Opt 39 (1) (2010) 39–

45.

3. S. Dutta, S. Mukhopadhyay, Alternating approach of implementing frequency

encoded all optical logic gates and flip-flop using semiconductor optical

amplifier, Optik 122 (2011) 1088–1094.

4. S. Dutta and S.Mukhopadhyay, “All-optical approach for conversion of a binary

number having a fractional part to its decimal equivalent and vice-versa”, Optics

and Photonics Letters, 3(1) (2010) 51–59.

5. S. Dutta, S. Mukhopadhyay, A new approach of parallel data transmission

through optical waveguide with SOA based frequency encoding/decoding

technique, Optik 123 (2012) 212– 216.

6. S. Dutta, S. Mukhopadhyay, “A new alternative approach of all optical frequency

encoded clocked S–R flip-flop exploiting the non-linear character of

semiconductor optical amplifiers”, Optik 123 (2012) 2082– 2084.

144

Section B: Conference papers

1. S.Dutta and S.Mukhopadhyay, ‘Optical conversion system: from any fractional

decimal number to its equivalent binary form”, 16th West Bengal State Science &

Technology Congress, The University of Burdwan, Bardhaman, India (28th

February -1st March, 2009). Paper was presented (oral) by S. Dutta on 28th

February.

2. Soma Dutta, S. Mukhopadhyay, “All optical frequency encoding method for

converting a decimal number to its equivalent binary number using tree

architecture”, International Conference on Optics & Photonics (ICOP-2009),

XXXIV Symposium of the Optical Society of India, Central Scientific

Instruments Organisation (CSIO), Sector -30 C, Chandigarh, India, October 30 –

November 1, 2009. Paper was presented (poster) by S. Dutta on 31st December.

3. Soma Dutta, S. Mukhopadhyay, “Method of developing an all optical frequency

encoded clocked R-S flip-flop exploiting the nonlinear character of

semiconductor optical amplifiers”, International Conference on Radiation Physics

and its Applications (ICRPA 2010), organized by Department of Physics, The

University of Burdwan; held at Science Centre, Golapbag, Bardhaman, 16th and

17th January, 2010. Paper was presented (poster) by S. Dutta on 17th January.


conversion from a binary data to the decimal one using SOA based switches”,

International Conference on Computing and Systems – 2010 (ICCS – 10),

organized by the Department of Computer Science, The University of Burdwan,

145

held at Science Centre, Golapbag, Bardhaman, 19th and 20th November, 2010.

Paper was presented (oral) by S. Dutta on 20th November.

5. S.Dutta and S.Mukhopadhyay, “Application of semiconductor optical amplifier

for development of ultra fast programmable unit”, Frontiers in Materials Science-

2010, December- 06, National Institute of Technology Durgapur, INDIA, Paper

was presented (poster) by S. Dutta on 6th December and awarded for the best

paper.


programmable unit”, en2c, 21st January-2011.

7. S.Dutta and S.Mukhopadhyay, “All-optical frequency encoded programmable

adder and subtractor unit”, National Workshop on Quantum Perspective of

advanced Material (QPAM-11), Organized by the dept. of physics and

technophysics, Vidyasagar University, 23rd to 25th March, 2011. Paper was

presented (poster) by S.Dutta on 24th March.


memory unit based on semiconductor optical amplifier”, International conference

on laser, materials science & communication (ICLMSC-2011), Paper was

presented (poster) by S. Dutta on 9th December, 2011

Author's personal copy

Optik 122 (2011) 125–127

Contents lists available at ScienceDirect

Optik

journa l homepage: www.e lsev ier .de / i j leo

All optical frequency encoding method for converting a decimal number to itsequivalent binary number using tree architecture

Soma Dutta ∗, Sourangshu MukhopadhyayDepartment of Physics, The University of Burdwan, Burdwan 713104, West Bengal, India

a r t i c l e i n f o

Article history:Received 21 May 2009Accepted 5 November 2009

Keywords:Optical computingOptical frequency encoding principleOptical tree architectureSemiconductor optical amplifier (SOA)Optical nonlinearity

a b s t r a c t

In any kind of computing and data processing system the use of binary numbers are found very muchsuitable and reliable. On the other hand several natural representations have been realized using dec-imal numbers. So conversion of a decimal number to its binary equivalent and vise-versa are of greatimportance in the field of computation technology. There lie already a number of established methodsregarding such conversion processes. Again optical tree architecture is one of the most promising sys-tems for realizing the optical conversion of any decimal number to its equivalent binary. Here in thiscommunication the authors propose a new method for optical conversion of a decimal number to itsbinary equivalent using tree architecture based system and frequency encoding principle. In frequencyencoding system, frequency of light is used for encoding of decimal digits or binary bits instead of inten-sity variation. For example 0 and 1 bits of binary number are coded by two different frequencies of lightsignal, instead of representing the presence of light as 1 and absence by 0. The proposed conversionprocess has multifaceted advantages in communication, as well as in data processing. To implement theabove conversion some characteristic features of semiconductor optical amplifier (SOA) have been usedmassively. The wavelength conversion property, cross gain modulation and some nonlinear propertiesof SOA are exploited to get the frequency encoded response. The proposed system carries all the basicadvantages of optical processing as well as those of frequency encoding also.

© 2010 Elsevier GmbH. All rights reserved.

1. Introduction

Optics has a strong and very potential role in information anddata processing because of its strong inherent parallelism. It hasseveral advantages over electronics in superfast computation anddata processing. Last few decades, several all optical data pro-cessors were proposed based on the Boolean logic. Those opticalsystems and optical logic devices based on optical switches arefound very much useful than electronic ones in connection to speedand many other aspects. There lie several types of optical switches.Semiconductor optical amplifier (SOA) is one of them which canbe used as a potential high speed optical switch. Again it is wellestablished that like electronic computation, in optical computa-tion also the conversion from decimal number to binary numberor vise-versa is very much important as data represented in binaryare found most suitable for computation. Here in this communica-tion the authors propose a new concept to implement all opticalconversion from decimal to binary with optical tree architectureusing the frequency encoding principle [1,2]. The advantages of

∗ Corresponding author.E-mail addresses: [email protected] (S. Dutta), [email protected]

(S. Mukhopadhyay).

frequency encoding are that as the frequency is a fundamental char-acter of any signal, so it remains unchanged in reflection, refraction,absorption, etc. during transformation of signal. The high reflectingand frequency diverting properties of optical add and drop multi-plexer (ADM) and high wavelength conversion property of reflectedsemiconductor optical amplifier (RSOA) because of its four wavesmixing character have been exploited here to implement the aboveconversion [3,4].

2. Frequency encoding principle

Frequency is the basic characteristics of light. In optical comput-ing and data processing therefore most important point is the veryhigh speed of processing. In most of the cases the presence of opticalsignal at input/output end are encoded as ‘1’ and absence of opti-cal signal as ‘0’. But for long distance in communication intensity ofoptical signal may significantly change, it may dropdown below therespective reference level. The interesting point is that this problemcan be solved by using frequency of light which is the fundamentalcharacteristics of light. One can encode and decode two differentstates of information by two different frequencies [5,6]. Here if ‘1’state is represented by a frequency �2 then ‘0’ state is represent byanother frequency �1. These frequencies are remaining unaltered

0030-4026/$ – see front matter © 2010 Elsevier GmbH. All rights reserved.doi:10.1016/j.ijleo.2009.11.018


126 S. Dutta, S. Mukhopadhyay / Optik 122 (2011) 125–127

Fig. 1. Add/drop multiplexer (ADM) by RSOA (here �3 is selected for reflection back).

during the reflection, refraction, absorption, etc. when data is to betransmitted. Here in this proposal we encode and decode the stateof information ‘0’ by a beam of wavelength �1 (which is correspond-ing to �1) and the state of information ‘1’ by another wavelength�2 (which is corresponding to frequency �2).

3. Switching operation of semiconductor optical amplifier(SOA)

Semiconductor optical amplifier (SOA) is an optoelectronicdevice that under suitable operating conditions (i.e. at proper biascurrent) can amplify an input optical signal. Semiconductor opti-cal amplifier SOAs can be classified into two main types, the FabryPerot (FP-SOA) where reflections from the end facets are significantwhereas in the traveling wave SOA (TW-SOA) type reflections arenegligible. Anti-reflecting coating can be used to create SOAs forfacet reflection. In optical transport action network these SOAs canbe successfully used. Many of the functional applications of SOAare based on its nonlinearity. SOAs can be used as different types ofoptical switches which are based on four wave mixing, wavelengthconversion, Add/drop multiplexing (ADM), etc. Here in this com-munication the authors use the add/drop multiplexing character ofSOA for selecting the proper frequency from a band of frequencies,not disturbing others. The wavelength conversion property of SOAcan be used for the conversion of the selected frequency or wave-length to another desired one. There are specifically of some typesof ADM. Here in this paper the WDM add/drop multiplexer is par-ticularly used. A mixer of frequencies of pulse or continuous signal(�0 to �9) falls on the ADM which is tuned at any particular fre-quency by the application of the proper bias current. The ADM thenpasses all the frequencies not allowing that particular frequency topass through it. One can receive that particular frequency of lightat output by the use of an optical circulator. This is shown in Fig. 1.The reflecting semiconductor optical amplifier (RSOA) on the otherhand is a wavelength converter [7,8]. In this switch a particular fre-quency of week light signal is given as a probe beam and anothersignal of another frequency is given as a pump beam to the inputchannels of RSOA. If only those two specific light signals are presentin the input channels then one can get the power of the pump beamis delivered to the week probe beam at output, i.e. the probe beamis exchanged by a power factor. The scheme is shown in Fig. 2. Usingthe ADM a specific frequency of light is selected and using RSOA theselected frequency is amplified [9–12].

4. Optical tree architecture

For optical conversion from a decimal to binary number and itsvise-versa are very important. Several types of conversion methodsare well known in computation process. Optical tree architectureis one of them. It is also an established optical technique. No

Fig. 2. Transfer of power from pump beam to a weak probe beam by wavelengthconverting mode of RSOA (here HR and AR represent the high reflection and anti-reflection coating).

switches are required here for converting a decimal digit to itsbinary counter-part. Some beam splitters (BS) and mirrors (M) areused in proper positions of the tree to execute this conversion. Someoptical channels are used here to get an output. Light beams aresplited by the BS’s and reflected by the M’s and go through theoptical channels. In Fig. 3 the tree architecture is shown. Here anydecimal digit (positionally represented) is converted to its respec-tive binary counter-part. For example if the decimal value (which issupposed to be converted) is 5, then one have to apply light to theinput channel of the tree marked by 5. At that time one can receivethe output 0101, i.e. light will come at X2 and X0 but no light willcome at X3 and X1. The conversion scheme is shown in Fig. 3.

5. Conversion method

Here �0 to �9 the ten frequencies are taken for representing 10decimal digits. A light beam having frequency (�0 to �9) fall on theADM1 (the 1st add drop multiplexer) where this ADM1 is tunedfor reflecting the light of frequency �1 by the application of properbias current to it. So, only �1 frequency of light will not pass throughthe 1st SOA whether all other will easily pass through it. The �1 fre-quency will come out from the ADM1 by the optical circulator (C).Similarly nine other ADMs are used in series which are tuned for dif-ferent other frequencies (�2 to �9), respectively. So, those selectedfrequencies of light will come from 10 different channels of the 10SOA blocks in the series of ADMs. Now using some optical beamsplitters BS’s and mirrors M’s one can convert a decimal numberto an appropriate binary number follow the principle of conver-sion by tree architecture. For this conversion one can use now thefrequency encoding principle. Here the light frequency is used tocode signal bit. If it is to convert the decimal number 5 one shoulduse the �5 light frequency at initial input. As for example – 101 isthe equivalent binary number of the decimal number 5. So first the�5 light frequency is selected and applied as initial input for con-version. It passes through the ADM1, ADM2, ADM3, and ADM4 butcannot pass through ADM5 as it is tuned for the frequency �5. So�5 cannot pass through the ADM5 and the �5 frequency is receivedfrom the respective circulator. Then passing through the properly

Fig. 3. Optical tree architecture for conversion from decimal to its binary counter-part.


S. Dutta, S. Mukhopadhyay / Optik 122 (2011) 125–127 127

Fig. 4. An integrated scheme of converting a frequency encoded decimal numberto its binary counter-part.

oriented BSs and Ms (as shown in Fig. 4) one can get the binaryoutput from the respective channels. These outputs are A3A2A1A0.In the particular case one can get �5 signal at A2 and A0 and no lightat A3 and A1 which indicates 0101. Now these outputs A3A2A1A0treated as inputs of the wavelength converter switches (the RSOAs).Here these outputs are used pump beams and another constant �1frequency light beam is given as probe beam to all the wavelengthconverters. For different input frequencies �0 to �9 (representingdifferent decimal inputs) the A3A2A1A0 are converted in such away that the bits (A3 or A2 or A1 or A0) takes that particular fre-quency when it is 1 and it takes no light when it is 0. For examplewhen the decimal input 7 is applied for conversion, it indicates �7frequency is applied. Therefore A3A2A1A0 is 0111 which is binaryequivalent of 7. So here A3 represent 0 by consuming no light in thechannel whereas A2A1A0 takes the light of frequency �7 to repre-sent 1. Now for conversion of decimal 6, �6 is applied as decimalinput. The binary equivalent of 6 is 0110. To represent it the A2 andA1 takes light of �6 frequency to represent 1 and no light to repre-sent 0. Therefore for different conversion 1 bit are represented bydifferent frequencies. To get rid of this problem, the outputs of A3,A2, A1 and A0 are passed through four different RSOA’s for conver-sion of light frequencies to a specific frequency �1 to represent 1.So when 9 is to be converted, the bits of the converted binary num-ber are represented by frequency �1, by the use of RSOA. Thus thefinal outputs are B3B2B1B0, which comes after the operations fromRSOAs. Therefore one can get �1 frequency of light in B3 and B0 andno light B2 and B1. In the same way for application of all the decimalinputs the 1 bits of the converted binary number is represented the�1 frequency of light and 0 bits by no signal (Fig. 4).

6. Conclusion

The whole operation system is all optical one and extends a veryhigh speed operation (far above GHz limit). This method can be

extended also for converting a decimal number of higher values(greater than 9) by enlarging the tree architecture. The potentialadvantage of the method over any other electronic and optical oneis the frequency depending input and output encoding. For that rea-son the coded information (1 or 0) in a signal remain unchangedin reflection, refraction, transmission, absorption, etc. For the sameconverters based on the intensity based encoding/decoding of dec-imal numbers, the most disadvantageous point is the fluctuationof intensity of light during transition which may alter the refer-ence level of bit value. Again due to the fluctuation of the lightintensity the direction of the output may be changed in case ofimplementation of the conversion by Kerr type nonlinear mate-rials switch based optical switch with intensity based encoding,decoding mechanism. So in the case of nonlinear material basedoptical switches it always requires constant intensity light source.Whereas in frequency based encoding/decoding system this disad-vantage is removed. Here the coded frequency remains unalteredin all the above conditions. Therefore, this proposed system doesnot only give a high speed operation, but also offers a trouble freeand noise free conversion.

References

[1] S.K. Garai, S. Mukhopadhyay, A method of optical implementation of frequencyencoded different logic operations using second harmonic and difference fre-quency generation techniques in non-linear material, Opt. Int. J. Light Electron.Opt. (2008), doi:10.1016/j.ijleo.2008.10.011.

[2] S. Mukhopadhyay, An optical conversion systems from binary to decimal anddecimal to binary, Opt. Commun. (The Netherland) 76 (May (5–6)) (1990)309–312.

[3] S.K. Garai, D. Samanta, S. Mukhopadhyay, All-optical implementation of inver-sion logic operation by second harmonic generation and wave mixing characterof some nonlinear material, Opt. Optoelectr. Technol. China 6 (August (4))(2008) 43–46.

[4] J.P.R. Lacey, M.A. Summerfield, S.J. Madden, Tunability of polarization-intensivewavelength converters based on four wave mixing in semiconductor opticalamplifiers, J. Lightwave Technol. 16 (12) (1998) 2419–2427.

[5] M.J. Connelly, Semiconductor Optical Amplifiers, Kluwer Academic publishers,2002.

[6] N.K. Dutta, Q. Wang, Semiconductor Optical Amplifier, World Scientific Pub-lishing, Singapore, 2006.

[7] L.Q. Guo, M.J. Connelly, A novel approach to all-optical wavelength con-version by utilizing a reflective semiconductor optical amplifier in copropagation scheme, Opt. Commun. 281 (September (17)) (2008) 4470–4473.

[8] Q. Guo, M.J. Connelly, All-optical and gate using induced nonlinear polariza-tion rotation in a bulk in a bulk semiconductor optical amplifier, in: TechnicalDigest: Optical Amplifiers and Their Applications, The Optik Society of America,Washington, DC, 2005 (Press no. SuB9).

[9] L.Q. Guo, M.J. Connelly, A poincare approach to investigate nonlinearpolarization rotation in semiconductor optical amplifiers and its applica-tions to all-optical wavelength conversion, Proc. SPIE 6783, 678325 (1–5)(2007).

[10] H.J. Dorren, D. Lenstra, Y. Liu, M.T. Hill, G.-D. Khoe, Nonlinear polarization rota-tion in semiconductor optical amplifiers: theory and application to all-opticalflip-flop memories, IEEE J. Quantum Electron. 39 (January (1)) (2003) 141–148.

[11] H. Soto, D. Erasme, G. Guekos, Cross-polarization modulation semicon-ductor optical amplifiers, IEEE Photon. Technol. Lett. 11 (1999) 970–972.

[12] L.Y. Lin, J.M. Wiesenfeld, J.S. Perino, A.H. Gnauck, Polarization-intensive wave-length amplifier, IEEE Photon. Technol. Lett. 10 (7) (1998) 955–957.

39J Opt 39(1) : 39–45

13

J Opt 39 (1) : 39–45

RESEARCH ARTICLE

An all optical approach of frequency encoded NOT based Latch using

semiconductor optical amplifier

Soma Dutta · Sourangshu Mukhopadhyay

Received: 01 September 2009 / Accepted: 28 January 2010

© Optical Society of India 2009

Soma Dutta(�) · Sourangshu Mukhopadhyay

Department of Physics, The University of Burdwan, Golapbag, Burdwan, pin-713104,

West Bengal, India

E-mail: [email protected]

Abstract In case of super-fast optical computation and communication, frequency encoding techniques are found to

be very promising and reliable one. Optical logic gates based on the principle of frequency conversion of some non-

linear materials play the key role for the implementation of a frequency encoded data processing system. Again

semiconductor optical amplifier has already been established successfully for frequency conversion. In frequency

encoding system, different frequencies of light signal are used for representation of binary bits 0 or 1 instead of

intensity variation. For example 0 and 1 bits of Boolean logic can be coded by two different frequencies of light signa�

��and�

��respectively. In this communication, we propose the method of developing an optical memory or a NOT

based latch. Several types of phase encoded, polarization encoded and intensity encoded optical memories have been

reported earlier, including latch also, whereas this proposal have been planned to develop an all optical latch logic

using frequency encoded principle and it offers a reliable and faithful processing rather than other established

techniques.

Key words Optical computation · Non-linear materials · Semiconductor optical amplifier.

Introduction

Increasing demand for a faster and reliable data processor has given birth of the concept of all optical super-fast

computers. In last few decades, there are several proposed optical and photonic devices, which can be run with

operation speed far above the conventional GHz limit. Many of those devices have been dedicated for performing

40 J Opt 39(1) : 39–45

13

logic, arithmetic and algebraic operations to achieve the goal of all-optical computer and data processor [1-6]. Among

all other components, memory is an essential one for any data processor. Different types of volatile and non-volatile

memories have already been developed successfully for the present electronic data processing systems. Similarly

optical memories are also of great importance for the development of optical computing technologies. Many scientists

are involved in deep research for the realization of digital volatile and non-volatile optical memories. Though some

successes have been achieved in the last few decades to develop optical memories, flip-flop, bi-stable multivibrators

and latches, still the ultimate goal of developing an optical computer has not been fulfilled yet. In this communication

we have proposed a methodology of developing optical NOT based latch using a specific non-linear behavior of

semiconductor optical amplifiers (SOA) accommodating with frequency encoding technique [7–11]. In most of the

encoding cases, presence of optical signal at the input or output of a logical device is encoded as ‘1’ logic state and

absence of optical signal as ‘0’ logic state. As the intensity of light decreases with the increase of optical path of a light

signal, so the intensity of light may drop down below the reference level of logic state ‘1’ and may enter into the

reference level of logic state ‘0’. To overcome this problem one can use the frequency of light signal to encode a logic

bit. Here the presence of a specific frequency of light is treated as ‘1’ state and then other specific one represents ‘0’

state i.e. if ‘1’ state is encoded by a frequency ‘�’ then ‘0’ state is done by another frequency ‘

�’. So

� and

� will

remain unaltered throughout the transmission of data as frequency is a fundamental characteristic of a light signal and

it remains unchanged under reflection, refraction, and absorption etc. Thus the frequency encoded technology can be

more promising and reliable one for the successful realization of super-fast computation and also for communication

with optics over many other conventional techniques [12–13]. There lies also many other encoding techniques like

polarization encoding, phase encoding etc. [14–15]. But all these encoding principles have the same problem of

changing the encoded state during transmission. Frequency encoding technique is free from such problems.

Semiconductor optical amplifier based optical switching

Semiconductor optical amplifier (SOA) is generally based on specially GaAs material. It is used for developing

several optoelectronic devices which under suitable operating condition can amplify an input signal. SOA can be

classified into two main types; one is the Fabry Perot SOA (FP-SOA), where reflections from the end surfaces are

important the signal undergoes many passes through the amplifier. On the other side in Traveling Wave (TW-SOA)

the reflection is negligible a signal undergoes only a single passes through the amplifier. This type of SOA can be used

in optical transparent networking.

The non-linearity of SOA can also be used successfully in many functional applications, which are caused by

carrier density induced by the amplifier’s input signal. There are four types of non-linearities in SOA, which are cross

gain modulation (XGM), cross phase modulation (XPM), self phase modulation (SPM), and four wave mixing

(FWM).

Here in this communication the authors exploit the cross gain modulation character of SOA. Changing carrier

density of the amplifier will affect all of the input signals entered into the SOA. Carrier lifetime depends upon the

temporal response of the carrier density. A weak CW probe light of wavelength 1 and a strong pump beam of

wavelength 2, with a small-signal harmonic modulation at angular frequency , are injected to the input terminals of

the SOA. The strong pump beam transfer its total power to the weak probe beam and then the weak probe beam

becomes strong and comes out to the output terminal the SOA acts as a wavelength converter [16]. It transfers

41J Opt 39(1) : 39–45

13

information from one wavelength to another signal at a different wavelength. The scheme is shown in Fig.1. There are

two basic schemes used in XGM based wavelength converters; where one is the co-propagating and the other is

counter-propagating schemes. In this communication we use the co-propagating type XGM wavelength converter.

This type of converters having anti-reflecting coating at the front surface which provides no reflection for 1 but

supports transmission for 1 and 2 and a highly-reflecting surface at the output which provides a very good

reflection for 2 and good transmission for 1 wavelength. If 1 does not exist at one input probe terminal, this

conversion is not allowed. Now for the conversion process the roles of the above coatings are very much important.

This coating basically ensures the obtaining of 1 signal at the output. Thus this SOA behaves as perfect optical

switch. The wavelengths of the pump and probe inputs are generally selected as 1555 nm ( 1) and 1550 nm ( 2)

corresponding to frequency � and

� respectively when GaAs is used as the concerned SOA.

Fig.1 A schematic diagram of semiconductor optical amplifier (SOA) used for wavelength conversion. Anti-

reflection and high-reflection coatings are for 1 and 2 wavelengths respectively.

Optical implementation of a NOT based latch:

Memory is the basic requirement to construct any electronic or optical processor. To develop a complete unit of

digital optical memory, the first step is to develop a latch or a 1-bit memory unit as it can store a single bit. The

proposed system described here is based on frequency encoding principle (Fig.2). Here two different frequencies are

used for encoding 1 and 0 i.e. if � frequency represents the state 0 then

� represents the state ‘1’.A is the input

terminal and Q and Q– are the two output terminals. To implement the optical NOT based latch logic some beam

splitters (BS), and mirrors (M) and some SOA based wavelength converters (WC) are used at different position of the

system. Here two frequency selecting filters are used, where one is � optical pass filter and other is

� optical pass

filter. When � frequency is applied at A, the light beam enters only to WC1, but not in WC4. Thus the

� frequency

42 J Opt 39(1) : 39–45

13

of light beam which behaves as a week probe beam now falls on to the WC1 from the input terminal A. Another proper

constant beam strong pump light (CW) of � frequency is injected to the WC1. For which one can obtain the

�

frequency of light which is again injected as a strong pump beam to the input terminal of WC2. A constant weak probe

beam of frequency � (CW) is applied at the input of WC2, for which an output of

� frequency is obtained at the

terminal Q for the application of � at A. Here CP’s (cross polarizer’s) are used in the output of every WCs to block

the unwanted light beam i.e. for this CP at the time of absence of pump beam or probe beam no light beam will come

out from the WCs. This output light beam is feedback as a pump beam to the WC3 and a constant weak light of �

frequency is also applied on the other input of the WC3 to get the output of at � frequency which is divided

into two parts by a beam splitter (BS). One part is feedback to the input of the WC1 and the other part is sent to the

output terminal Q–. Thus for � at A one can get

� at Q and

��at Q–.

Now � frequency of a week probe beam of light is given at input A then it falls directly to the WC

�, but it will

not at all enter at WC� and a constantly supplied strong pump beam of

� frequency is applied on the other input

channel of the WC�. In this situation the

� frequency of light will come out from the output of WC

� which is again

treated as a strong pump beam to fall on the WC�. A constant week probe beam of frequency

� is kept in the input

Fig.2 Frequency encoded optical one-bit memory cell based on latch logic. (WC, M, CP, BS are represents the

wavelength converter, mirror, cross polarizer and beam splitter or combiner respectively).

43J Opt 39(1) : 39–45

13

channel of WC� which helps to get the intense

� frequency of light at the output of WC

� (X). This is divided again

into two more parts; one is feedback to the input of the WC� as a strong pump beam where the other part comes at

the output X, which is ultimately connected with Q by BS and M. Again a constant week probe beam of � frequency

given to the 2nd input of WC� which ensures the output of

� frequency at the output Y which is ultimately

connected with Q–. A portion of the output at Y is feedback to the input of WC�. This is the overall connection of the

whole latch unit. Now to describe the operation it can be said that when � frequency of light i.e. logic state (0) is

applied at the input terminal A, the upper portion of the system described in figure-2, activated but lower half does

not because of presence of the � optical pass filter which only allows the

� frequency of light in the upper half of

the unit and the � pass filter only passes the

� frequency of light in the lower half of the system described in Fig.2.

So when � frequency of light i.e. logic state (0) is given to input terminal A one can get the

� frequency of light at

output Q and � frequency of light at Q– i.e. Q=1 and Q– = 0 when A=0. Similarly when

� frequency of light i.e. logic

state 1 is applied at the input terminal A, the lower portion of the system takes the major role instead of upper

portion and one can ensure the � frequency of light at Q and

� at Q–

i.e. Q=0 and Q– =1 when A=1. The most

important and interesting point here is that if the input � and

� frequency of light are withdrawn, the system will

continue to show the last attended values of Q and Q– at the final output because of the feedback mechanisms. At this

situation the feedback light will continue to excite the respective WCs. So, this system can behave as a frequency

encoded optical one bit memory.

Optical implementation of two-bit memory cell

Now slightly extending the one-bit memory cell circuit one can develop a two-bit memory cell (Fig.3). Here we

attached two similar optical circuits of one-bit memory unit. Here A and B are the input terminals and X1 and Y1 are

the output terminals. The block diagram of this scheme is shown in Fig.3. Here A* and B* are the two unit memory

cells (latches), Q and Q– are the output terminals of A* block and X1 and Y1 are the output terminals of B* block. X1

is then combined with Q– and Y1 is combined with Q. So, an overall two-bit memory cell is developed. Now two

inputs A and B and two outputs Q and Q–, if A=1(�) and B=0(

�) then one can obtain Q=0(

�) and Q–

=1(�) whereas

if A=0(�) and B=1(

�) then Q=1(

�) and Q–

=0(�). The states of the last output will be attended if there is no light

signal present in the input terminal i.e. for the withdrawal of optical signal, the system to continue to show its last

attended values at the output. The truth table of this memory unit is given in table 1.

Table 1 Truth table of optical two bit memory cell.

Input Output

A B Q Q–

�(1)

�(0)

�(0)

�(1)

�(0)

�(1)

�(1)

�(0)

0 0 Last Last

state state

attended attended

44 J Opt 39(1) : 39–45

13

Conclusion

To conclude, we have proposed here a frequency encoded technique based optical NOT latch or memory cell. The

whole system is all-optical one and is expected to support a high speed operation (far above GHz limit) which is a

potential advantage of this mechanism. The coded information (0 or 1) of an output signal remains unaltered in

reflection, refraction, absorption etc. due to this encoding principle. Therefore this technique will be very much useful

in reliable optical communication. To achieve the faithful amplification the pump beam of WC should lie between 4dB

to 10 dB. The proposed system does not only offer a high speed operation but also offers noise free conversion to

provide a high signal to noise (S/N) ratio. Using this latch logic and the frequency encoded technique one can

implement many other operations like digital types of flip-flops, multiplexer, demultiplexer etc. with some modifications

of the scheme.

References

1. K. R. Chowdhury and S. Mukhopadhyay, “A new method of binary addition scheme with massive use of non-

linear material based system,” Chinese Opt Letts, 1(4), 241–242 (2003)

Fig.3 Block diagram of frequency encoded optical two-bit memory cell based on NOT latch logic. (A and B receive

the frequency encoded inputs, Q and Q– give the frequency encoded outputs). Here each block comprise the memory

or latch as given Fig.2.

45J Opt 39(1) : 39–45

13

2. P. Ghosh, P. P. Das and S. Mukhopadhyay, “New proposal for optical flip-flop using residue arithmetic”,

ITCOM-2001, (4534B-22), SPIE Proceedings (Optoelectronic and Wireless Data Management, Processing,

Storage and Retrieval), 4534, 148-154 (2001)

3. A. K. Das and S. Mukhopadhyay, “General approach of spatial input encoding for multiplexing and De-

multiplexing”, Opt Engineering (USA), 43(1), 126–131 (2004)

4. K. Roy Chowdhury and S. Mukhopadhyay, “Binary optical arithmetic operation scheme with tree architecture

by proper accommodation of optical nonlinear materials,” Opt Engineering, 43(1), 132–136 (2004)

5. N. Pahari, D. N. Das and S. Mukhopadhyay, “All-optical method for the addition of binary data by non-linear

materials”, Appl Opt, 43(33), 6147–6150 (2004)

6. N. Pahari and S. Mukhopadhyay, “An all-optical R-S flip-flop by optical nonlinear material”, J of Opt, 34(3),

108–114 (2005)

7. S. K. Garai, D. Samanta and S. Mukhopadhyay, “All-optical implementation of inversion logic operation by

second harmonic generation and wave mixing character of some nonlinear material,” Optics and Optoelectronic

Technology (China), 6(4), 39–42(2008)

8. L. Q. Guo and M. J. Connelly, “A novel approach to all-optical wavelength conversion by utilizing a reflective

semiconductor optical amplifier in co propagation scheme”, Opt. Communication, 281(17), 4470–4473 (2008)

9. L. Q. Guo and M. J. Connelly, “A poincare approach to investigate nonlinear polarization rotation in semiconductor

optical amplifiers and its applications to alloptical wavelength conversion”, Proc. of SPIE 6783, 678325 (1–5)

(2007)

10. H. J. Dorren, D. Lenstra, Y. Liu, Martin, T. Hill, G. D. Khoe, “Nonlinear polarization rotation in semiconductor

optical amplifiers: theory and application to all-optical flip-flop memories “, IEEE Journal of Quantum Electronics,

39(1), 141–148 (2003)

11. M. A. Karim and A. S. Awwal, John Wiley and Sons, “Optical Computing An Introduction” INC. (1992)

12. S. Dutta and S. Mukhopadhyay, “All optical frequency encoding method for converting a decimal number to its

equivalent binary number using tree architecture”, Presented and Published in the proceedings of International

Conference Of Optics and Photonics (ICOP-2009) in chandigar, 31st OCT. to 2nd NOV (2009)

13. S. K. Garai, S. Mukhopadhyay, “A method of optical implementation of frequency encoded different logic

operations using second harmonic and difference frequency generation techniques in non-linear material”, Opt Int

J Light Electron Opt, doi: 10.1016/ j.ijleo.2008.10.011 (2008)

14. D. Samanta and S. Mukhopadhyay, “A method of maintaining the intensity level of a polarization encoded light

signal,” Journal of Physical Sciences (Vidyasagar University), 11, 87–91 (2007)

15. B. Chakraborty and S. Mukhopadhyay, “Alternative approach of conducting phase-modulated all optical logic

gates,” Opt Engineering, 48(3), 035201(2009)

16. M. J. Connelly, “Semiconductor Optical Amplifiers”, Kluwer Academic publishers (2002)


Optik 122 (2011) 1088–1094


Optik

journa l homepage: www.e lsev ier .de / i j leo

Alternating approach of implementing frequency encoded all-opticallogic gates and flip-flop using semiconductor optical amplifier

Soma Dutta ∗, Sourangshu MukhopadhyayDepartment of Physics, Burdwan University, Golapbag, Burdwan 713104, West Bengal, India

a r t i c l e i n f o

Article history:Received 18 December 2009Accepted 16 June 2010

Keywords:Optical computationNon-linear opticsSemiconductor optical amplifierFiber optics

a b s t r a c t

In conduction of parallel logic, arithmetic and algebraic operations, optics has already proved its success-ful role. Since last few decades a number of established methods on optical data processing were proposedand to implement such processors different data encoding/decoding techniques have also been reported.Currently frequency encoding technique is found be a promising as well as a faithful mechanism for theconversion of all-optical processing as the frequency of light remains unaltered after refection, refraction,absorption, etc. during the transmission of light. There are already proposed some frequency encodedoptical logic gates. In this communication the authors propose a new and different concept of frequencyencoded optical logic gates and optical flip-flop using the non-linear function of semiconductor opticalamplifier.


1. Introduction

All optical signal and data processing is especially attractivefor high speed and high capacity computation to avoid the speedrelated problems in optoelectronic processing systems. Because ofthe inherent character of parallelism of light can show more strongand potential applications in information processing, computing,data handling and image processing. In optical computation photonis found to be a very suitable information carrier than electron notonly in the connection of super fast speed but in many other aspectsof information processing also. Thus these photonic systems cansuccessfully replace the electronic systems. Again it is also seen thatin case of optical data processing the conventional methodologiescannot be followed always as it is done in electronics. Scientistsand technologists are deeply involved in research to overcome thespeed related difficulties to realize all-optical logic, arithmetic andalgebraic operations with Boolean mechanism. There are found sev-eral popular reports on the development of optical logical systemswhere the logic gates are the basic building blocks. In this presentcommunication, we focused on the successful realization of someefficient optical logic gates based on the frequency encoding prin-ciple, which can be used as a more reliable candidate than otherencoding principles for the development of super-fast optical pro-cessors, as the frequency of a signal remains unchanged even afterdifferent optical transformations. In most of the cases, the presence

∗ Corresponding author. Tel.: +91 9232656039.E-mail addresses: [email protected] (S. Dutta),

[email protected] (S. Mukhopadhyay).

of optical signal at the input or output of a logical system is encodedas ‘1’ bit and the absence of the signal is regarded as ‘0’ logic state.As the intensity of light decreases with the increase of optical paththrough a medium, so the intensity of light may drop down belowthe reference level of the concerned logic state ‘1’ and may enterinto the reference level of logic state ‘0’. To overcome this problemone can use the frequency of light for encoding a bit. If the presenceof a specific frequency of light is treated as ‘1’ state and then otherspecific one represents ‘0’ state i.e. if ‘1’ state is encoded by fre-quency ‘�2’ then that of the ‘0’ state is done by another frequencyby ‘�1’ where �1 and �2 remain unaltered throughout the trans-mission of data. There lies also many other encoding techniqueslike polarization encoding, phase encoding, etc. All these encodingprinciples have the same problem of changing the encoded stateduring transmission. Frequency encoding techniques is free fromsuch transmission problems. There are reported some frequencyencoded optical logic systems [1–3]. Here in this communicationwe propose an alternative and new approach for the implementa-tion of some all-optical logic gates and flip-flops using frequencyencoding principle [4–8]. To implement these logic gates and flip-flops different methods are adopted, which show the switchingadvantages of implementation.

Like logic gates different types of memories are also the essentialpart of any computing system. Already different types of volatileand non-volatile memories have been successfully developed forelectronic data processing systems. On the other hand optical mem-ories are also of great importance for the development of opticalcomputing technology. There are found many reports where theschemes for developing all-optical memories, and flip-flops arereported [9–14]. Here in this communication the authors also pro-

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pose a method of developing all optical frequency encoded RSflip-flop based on some all optical frequency encoded logic gateswith the potential uses of the switching character of semiconductoroptical amplifier.

2. Frequency encoding principle

We have already discussed above that the frequency of lightis the fundamental character of light and it is unaltered after thereflection, refraction, absorption, etc. In optical computing and dataprocessing the most important point is the very high speed ofprocessing. In most of the cases the presence of optical signal atinput/output end is encoded as ‘1’ and absence of optical signal as‘0’. The interesting point is that the problem of variation of inten-sity of light in data processing due to increasing distance can besolved by using the frequency encoding principle of light. One canencode and decode two different states of information by two dif-ferent frequencies. Here if ‘1’ state is represented by a frequency�2 then ‘0’ state is represented by another frequency �1. These fre-quencies remain generally unaltered during transmission. Here inthis communication we encode and decode the state of informa-tion ‘0’ by a beam of wavelength �1 (which is corresponding to �1)and the state of information ‘1’ by another wavelength �2 (whichis corresponding to another frequency �2) for the implementationof optical logic gates and based on this logic gates we proposed amethod of implementing all optical frequency encoded RS flip-flop.

3. Semiconductor optical amplifier based switches (SOA)

Semiconductor optical amplifier is generally based on GaAsmaterial. It is used for developing several optoelectronic devices,which under suitable operating conditions can amplify an inputoptical signal [15–19]. SOA can be classified into two main types,one is the Fabry Perot SOA (FP-SOA), where reflection from the endsurface is important i.e. the signal undergoes many passes throughthe amplifier, on the other side in case of Traveling Wave SOA (TW-SOA) the reflection is negligible i.e. a signal undergoes only a singlepasses through the amplifier. These behaviors of SOA can be usedin optical transparent networking and digital optical processing.

The non-linearity of SOA can also be used in many functionalapplications which are caused by carrier density induced by theamplifiers signal. There are found four types of non-linearity’s inSOA, which are cross gain modulation (XGM), cross phase modu-lation (XPM), self phase modulation (SPM), and four wave mixing

Fig. 1. Reflecting semiconductor optical amplifier (RSOA).

(FWM). Here in this communication the authors exploit the crossgain modulation character of SOA. A weak CW probe light of wave-length �1 and a strong pump beam of wavelength �2 are injectedto the input terminals of the SOA having an anti-reflecting surfaceat the input for �1 and a highly reflecting surface for �2 at the out-put terminal. The strong pump beam transfer its total power to theweak probe beam and then the weak probe beam becomes strongerand comes out to the output terminal i.e. the SOA acts as a properwavelength converter. The scheme is shown in Fig. 1. This type ofSOA switch is called RSOA. The switching action of RSOA can be usedto develop optical logic gates and flip-flop. Again optical ADD/DROPmultiplexer is a frequency selecting network. It is tuned in a par-ticular biasing frequency and it reflects that particular frequency oflight through it and passes all other frequencies of light. The wholescheme is shown in Fig. 2.

4. Scheme of realization of frequency encoded optical ORlogic operation

Optical logic gates are the basic building block to implement anyoptical logical functions or operations. OR gate is one of the mostimportant and a basic unit of integral logical system. To developthe system some ADD/DROP multiplexers, wavelength converters,mirrors (M) and beam splitters (BS) are used which is shown inFig. 3. Here the input beam A, and B may have either the frequencyof �1 or �2 (wavelength �1 or �2 respectively), where �1 frequencyof light is encoded for ‘0’ state and �2 frequency of light encoded for‘1’ state. Now a light beam of frequency �1 or �2 from point A falls onthe 1st ADM which is tuned at the biasing frequency �2 then only�2 frequency of light is reflected by the ADM and is captured by theoptical circulator C1 and on the other hand the �1 frequency of lightpasses through the ADM and falls on the BS and one part of light fallson the 1st wavelength converter (WC) as a pump beam and another

Fig. 2. ADD/DROP multiplexer.



Fig. 3. Frequency encoded optical OR gate.

part of light falls on the 2nd wavelength converter (WC) as a pumpbeam, so for the case of the 1st WC if the probe beam is present thenthe WC works and one can get a converted strong probe beam atthe output which is the input of the 3rd ADM and in absence of theprobe beam one cannot get any output. Now in the 2nd WC thereis a constant weak probe beam so if there is found a strong pumpbeam one can get then a converted strong beam at the output whichis injected again to the 3rd WC as a pump beam. The captured tunedfrequency of light from the first circulator C1 of 1st input terminal Ais injected to the 3rd ADM with the help of some mirrors and beamcouplers. Here for the 2nd input terminal the similar type of inputlight (�1 or �2 frequency) is injected to the 2nd ADM which is alsotuned with the biasing frequency (�2). So only this frequency oflight is reflected and captured by the circulator C2 and given to the1st WC as a weak probe beam. Another frequency of light passesthrough the 2nd ADM and falls in the 3rd WC as a weak probebeam. If the pump beam and probe beam are present then onlyone can get a strong probe beam at the output otherwise absenceof any probe or pump beam one cannot generate any output. Theoutput injected to the 3rd ADM. So one can get the final outputresult of frequency encoded OR logic operation from the terminalY. The scheme is shown in Fig. 3.

5. Principle of operation of optical OR gate

Now when the �1 frequency of light is applied to the first inputterminal A the light passes through the 1st ADM and it is dividedinto two parts by the BS, where one part is treated as a strong pumpbeam to the 1st RSOA and other part is delivered to the 2nd RSOAalso as a strong pump beam. One constant weak probe beam of�2 frequency is delivered to the 2nd RSOA, so that at the output itgives a strong light beam of �2 frequency which is again applied as astrong pump beam to the 3rd RSOA. Here one can apply a light of �1frequency to the second input terminal B which passes through the2nd ADM and delivered as weak probe beam to the 3rd RSOA. Thislight beam which is emerged an output beam of �1 frequency lightand is applied to the 3rd ADM. Here to select the proper frequency

of light one optical filter is kept. Again the 3rd ADM is tuned atits biasing frequency �1 so it reflects the output beam from 3rdRSOA and it is collected by the circulator and combined with theoutput terminal Y with the help of mirrors. Thus one can get anoutput result of �1 frequency of light. In this case (A = B = �1) thereis no probe beam in the 1st RSOA, so 1st RSOA cannot work andone cannot get any output from the 1st RSOA. In the second case �1frequency of light is given at the input terminal A and �2 frequencyof light is to the second input terminal B. Here the 1st ADM passesthe light and it is divided into two parts by the BS where one part isgiven to the 1st RSOA as a strong pump beam and other part is givenalso as a strong pump beam to the 2nd RSOA and for which one canget the output result of �2 frequency of light from the 2nd RSOA,which serves as a strong pump beam to the 3rd RSOA but as there isno probe beam in the 3rd RSOA because B = �2, so no �1 frequency oflight comes to the 3rd RSOA and it does not convert any signal so onecannot get any output result from the 3rd RSOA. As the second inputterminal B receives �2 frequency of light so the 2nd ADM (tuned atits biasing frequency �2) reflects the light beam and it is collectedby the circulator C2 as shown in Fig. 3. This light is applied as a weakprobe beam to the 1st RSOA. So the 1st RSOA works and one canget the output result of �2 frequency of light, which is deliveredto the 3rd ADM and we get the �2 frequency of light at the outputterminal Y. In the third case when A = �2 and B = �1, 1st ADM reflectsthe light beam and it is collected by the circulator C1 and applied tothe 3rd ADM. Now the beam passes through the 3rd ADM and onecan get the result of �2 frequency of light at the output Y. The restof circuit cannot take part in this conversion. Now when both thetwo input takes �2 frequency of light the 1st ADM reflects the lightbeam and it is collected by the circulator C1 and delivers it to the3rd ADM, and passed through this ADM and we get the convertedlight beam of frequency �2 at the output terminal Y. Similarly inthe 2nd ADM the light is reflected by the ADM and collected bythe circulator C2 and is delivered as a probe beam to the 1st RSOA,but as there is no pump beam in the 1st RSOA it cannot work andwe cannot obtained any converted light beam from it. So that onecan get Y = �2 when A = �1 and B = �2; when A = �2 and B = �1; Y = �2;



Table 1Truth table of optical logic OR gate.

A B Y

�1 (0) �1 (0) �1 (0)�2 (1) �1 (0) �2 (1)�1 (0) �2 (1) �2 (1)�2 (1) �2 (1) �2 (1)

when A = �2 and B = �1; Y = �2 and finally when A = B = �2; Y = �2. Thisverifies the truth table of OR gate, if �2 is encoded as 1 and �1 by 0(Table 1).

6. Scheme of realization of frequency encoded optical ANDgate

To implement optical AND gate we take two channels A and Band they may have either �1 (corresponding wavelength �1) fre-quency of light or �2 (corresponding wavelength �2) frequency oflight. Now a beam of light of frequency �1 or �2 falls on the 1st ADMwhich is tuned in the biasing frequency �2 so only �2 frequency oflight is reflected back by ADM and captured by the circulator C1and the �1 frequency passing through the ADM is injected to the1st wavelength converter (WC) as a weak probe beam. A constant�2 frequency of strong pump beam of light is given to the 1st WC.If both the pump and probe beam is present in the RSOA (WC) thenone can get the converted strong probe beam i.e. �1 frequency oflight at the output. Now the reflected �2 frequency of light fromthe 1st ADM is captured by the circulator C1 and is reflected by themirror and is injected as a weak probe beam to the 2nd RSOA. Herefrom the channel B the input beam of light of frequency �1 or �2falls on the 2nd ADM which is also tuned at its biasing frequency �2.So it reflects the �2 frequency of light and passes the �1 frequencyof light through it. This light beam merges with the probe beam oflight of 1st RSOA with the help of mirror. The reflected �2 frequencyof light from the 2nd ADM is captured by the circulator C2 and isgiven to the 3rd RSOA as a strong pump beam and �1 frequency ofweak probe constant beam light is given to the 3rd RSOA. So whenboth pump or probe beam is present one can get a converted strongprobe beam at its output. This output beam is injected to the 2ndRSOA as a strong pump beam with the help of mirrors. So if bothpump or probe beam is present in the 2nd RSOA one gets the out-put beam as converted weak probe beam into a strong one which isadded with the output beam of light of 1st RSOA using some prop-erly oriented mirrors and these two light beams again are injectedto the 3rd ADM which is tuned at its biasing frequency �2. So itpasses the �1 frequency of light and reflects the �2 frequency oflight and it is collected by the circulator C3. This reflected beam oflight is ultimately added with the output beam from 3rd ADM bythe use of properly oriented mirrors. The whole system is shownin Fig. 4. Here optical filters are used to select the proper frequencyof light beam. Thus when A = �1 and B = �1; Y = �1, when A = �1 andB = �2; Y = �1, for A = �2 and B = �1; Y = �1 and finally when A = B = �2;Y = �2. This satisfies the truth table of AND gate.

7. Principle of operation of optical AND gate

The AND logic system is shown in Fig. 4. Now in the case ofAND logic when two input beams are of �1 frequency then 1st ADMpasses the light beam and 2nd ADM also passes the light beamand then they are combined together by mirror and beam splitter.The combined beams are serving as a weak probe beam to the 1stRSOA. A constant strong pump beam is present in the 1st RSOA soit gives a converted output light beam which is delivered to the3rd ADM and it passes through it. Thus a result of �1 frequency oflight is obtained at the output end Y. The conversion of 2nd and 3rd

Table 2Truth table of optical logic AND gate.

A B Y

�1 (0) �1 (0) �1 (0)�1 (0) �2 (1) �1 (0)�2 (1) �1 (0) �1 (0)�2 (1) �2 (1) �2 (1)

RSOA cannot take part due to the absence of the either pump orprobe beam. Again when A = �1 and B = �2, 1st ADM passes the lightbeam and the light is delivered as a probe beam to the 1st RSOAand similarly due to the above operation one can get the convertedoutput light beam of �1 frequency at the output end Y. The restof the circuit does not take part in the conversion process due toabsence of either pump beam or probe beam. Now when A = �2 andB = �1 then 1st ADM reflects the light beam and the light is collectedby the circulator C1 and delivered as a weak probe beam to the 2ndRSOA. As the pump beam is absent here so conversion cannot takepart in 2nd RSOA. Again as B = �1, so the 2nd ADM passes this lightbeam and is combined with the output terminal of the 1st ADM andserves as a probe beam to the 1st RSOA. Thus a similar conversionis occurred in this time and as the output light beam of frequency�1 is obtained at Y. Finally when the �2 frequency of light is appliedto both input terminal A and B, 1st ADM reflects the light beamand is delivered as a probe beam to the 2nd RSOA. So, 1st RSOAcannot take part any role in this conversion. Now as B = �2 so the2nd ADM reflects the light beam and the light is collected by thecirculator, which is delivered again as a strong pump beam to the3rd RSOA where probe beam of �2 frequency is already present. Sothe conversion takes place and one can get the output light beam of�1 frequency and it is delivered as a pump beam to the 2nd RSOA,for which the output light beam of �2 frequency from 2nd RSOAcomes and it is applied to the 3rd ADM. This light is reflected backby the ADM and collected by the circulator C3. The light from thecirculator is connected to the output terminal Y. So then one canget the �2 frequency of light at the output Y. Thus when A = �1 andB = �1; Y = �1, when A = �1 and B = �2; Y = �1, when A = �2 and B = �1;Y = �1 and finally when A = B = �2; Y = �2. This satisfies the truth tableof AND gate which is shown in Table 2, if �1 is encoded as 0 and �2as 1.

8. Scheme of realization of frequency encoded opticalNAND gate

NAND gate is the most important logic gate in the logics familyas it is universal gate. Again here we take two input channels A andB as sources of input light beams of frequency �1 or �2. The wholeset up is much closed to the AND logic set up except three mainchanges seen in this system. Here the output light from 1st ADMis injected as a pump beam instead of the probe beam to the 1stRSOA and the reflected beam of light from 1st ADM is injected asa pump beam instead of a probe beam to the 2nd RSOA. Finallyreflected beam from the 2nd ADM is applied as a pump beam tothe 3rd RSOA (Fig. 5). Thus the truth table of the NAND logic isdeveloped and it is shown in Table 3. So when A = B = �1 then Y = �2,when A = �1 and B = �2 then Y = �2, for A = �2 and B = �1; Y = �2 andfinally for A = B = �2; Y = �1. This supports the truth table of universalgate NAND gate.

9. Principle of operation of optical NAND gate

The diagram of frequency encoded NAND logic is shown in Fig. 5.Now in the case of optical NAND logic gate at first �1 frequency lightis applied in both of the inputs A and B. So �1 frequency of light fallson the 1st ADM from A, and as it is tuned at its biasing frequency



Fig. 4. Frequency encoded optical AND gate.

�2, so �1 frequency light passes through the ADM and is given asa strong pump beam to the 1st RSOA, one weak probe beam of�2 frequency is also given to the 1st RSOA for which a convertedstrong light beam of �2 frequency will be generated. This is againdelivered to the 3rd ADM which is tuned also at �2 frequency, so thelight beam reflected from the 3rd ADM is captured by the circulatorC3 and connected with the output Y by the use of mirrors and onecan get the �2 frequency of light at the output Y. Now �1 frequencylight is also given to the 2nd ADM from B and the ADM is tunedat same frequency �2, so the ADM passes the light beam which iscombined with the output beam of the 1st ADM by the mirrors, etc.Now when A = �1 and B = �2 then similar operation goes on in the1st ADM, but in this case of second input terminal B, the light beamfalls on the 2nd ADM and gets reflected from it and being capturedby the circulator C2 it falls on the 3rd RSOA as a pump beam. Soone can get the converted �1 frequency of light in presence of �1probe beam at the output and it is again applied to the 2nd RSOA as aprobe beam. Due to absence of pump beam the conversion does notoccur and hence the �2 frequency of light is obtained at the outputY for the operation of the 1st section of the system. When A = �2 andB = �1 then 1st ADM reflects the light beam and it is then capturedby the circulator C1. This is given to the 2nd RSOA as a strong pumpbeam. No other conversion takes place now because of the absenceof the probe beam. Here �1 frequency of light falls on the 2nd ADMand passing through it, this delivers a strong pump beam to the1st RSOA and the conversion is occurred (due to presence of �2frequency probe beam of light) and one receives the �2 frequencylight at the output and it is then delivered to the 3rd ADM. Thus thefinal �2 frequency light is found at the output terminal Y. Finally

Table 3Truth table of optical logic NAND gate.

A B Y

�1 (0) �1 (0) �2 (1)�1 (0) �2 (1) �2 (1)�2 (1) �1 (0) �2 (1)�2 (1) �2 (1) �1 (0)

when A = B = �2; 1st ADM reflects the light beam and is delivered tothe 2nd RSOA as a strong pump beam. Again the light beam alsofalls on the 2nd ADM and gets reflected. It is captured then by thecirculator C3 and is given to the 3rd RSOA as a strong pump beamand one gets the converted output beam of light of �1 frequency inthe presence of a weak probe beam of �1 frequency. This convertedbeam of �1 frequency of light is delivered as a probe beam again tothe 2nd RSOA and one thus gets the converted output beam of �1frequency of light which is delivered to the 3rd ADM. This 3rd ADMpasses it and a �1 frequency of light is obtained at the output endY. Thus one can ensure the logical NAND output from Y from thesystem described in Fig. 5. Hence one finds Y = �2 when A = B = �1,again when A = �1 and B = �2 then Y = �2, for A = �2 and B = �1, Y = �2and finally when A = B = �2 then Y = �1. This supports the truth tableof a universal frequency encoded gate NAND gate which is shownin Table 3. Here �1 is encoded as 0 and �2 as 1.

10. Scheme of realization of frequency encoded optical R–Sflip-flop

Memory is the fundamental criteria for developing an all of opti-cal processor. To realize this system with optics we consider twoinput light channels R and S, each of them may take either �1 (0)or �2 (1) frequency of light. One can get the output in the channelsQ and Q̄ , respectively. The outputs are feedback to the input i.e. Q̄is connected with input R and Q is connected with the other inputS by the help of some mirrors and beam splitters. Now a beam oflight of frequency �1 or �2 falls on the 1st ADM through point R.As the ADD/DROP multiplexer is tuned with its biasing frequency�2 so it reflects this frequency of light and passes the �1 frequencyof light. This light is then introduced to the 1st RSOA as a strongpump beam. There already presents a constant weak probe beamof �2 frequency. So if both pump and probe beam is present onecan get the converted strong probe beam of light with the respec-tive frequency which again falls on the 3rd ADM. The absences ofany pump or probe beam in the RSOA makes the conversion stop.Here one can use one optical filter to select the proper output lightbeam. The 3rd ADM is tuned with its biasing frequency �1 i.e. it



Fig. 5. Frequency encoded optical NAND gate.

Table 4Truth table of optical frequency encoded RS flip-flop.

R S Q Q̄

�1 (0) �2 (1) �2 (1) �1 (0)�2 (1) �1 (0) �1 (0) �2 (1)0 0 Last state attended Last state attended

reflects only �1 frequency of light and passes all other frequencies.The reflected beam is captured by the circulator C3 and added withthe output Q by the mirrors. Now the reflected light beam fromthe 1st ADM is separated by the circulator C1 and is introduced tothe 2nd RSOA as a strong pump light beam. A weak probe beam isalso given to the 2nd RSOA and the converted output light beampasses through the respective optical filter and merges with theoutput beam of 1st RSOA. Ultimately the output from the 1st RSOAis given to the 3rd ADM. Same process is happened with the sec-ond input S and finally we get the two outputs Q and Q̄ . The wholescheme is shown in Fig. 6. Now when one applies the light beamin the two input channels i.e. when R = �1 and S = �2 then Q = �2and Q̄ = �1 respectively and when R = �2 and S = �1 then Q = �1 andQ̄ = �2 but when light is withdrawn from the inputs one can getthe last state attended in the outputs Q and Q̄ . So this follows thetruth table of optical RS flip-flop which is shown in Table 4 and thefigure of whole scheme of optical RS flip-flop is shown in Fig. 6.

11. Principle operation of optical R–S flip-flop

To implement the circuit diagram of an optical RS flip-flop thelight beam is supplied first to both the input channel. This deliveredlight is of frequency �1 to the 1st ADM through the input end R. Asthe 1st ADM is tuned at its biasing frequency �2, so �1 frequencylight beam passes through it and falls on the 1st RSOA where astrong beam of �2 frequency is already present. So one gets theconverted �2 frequency of light beam at the output of 1st RSOAand it is again introduced to the 3rd ADM (tuning frequency �1)and light passing through the ADM comes to the output end Q. Thisoutput beam of �2 frequency of light splits into two parts by the BS,one part is sent to the output end Q and another part is feedback to

the second input terminal S. Again if a �2 frequency light is appliedto the S terminal, a combined beam of �2 frequency of light is deliv-ered to the 2nd ADM (tuning frequency �2). This ADM reflects by itand sends to the 4th RSOA by the help of circulator C2 and mirrors.Due to presence of a weak probe beam of �1 frequency of light in 4thRSOA, the conversion is occurred and one can get the converted out-put light beam of frequency �1 which is introduced to the 4th ADM(tuning frequency �1) and is reflected by it. This light is capturedby the circulator C4 and is combined with the output Q̄ and splittedby the BS. One part goes to the other output end Q̄ where anotherpart is feedback to the input terminal R and the process continues.Now when the R = �2 and S = �1 then reverse process is developedin this circuit and one gets Q = �1 and Q̄ = �2 in the output ends.Now when no light passes through both the input channel i.e. forthe absence of light in the input terminals, last state is attendedi.e. If the last state is Q = �1 and Q̄ = �2 for (R = �2 and S = �1) thenone gets the same result in the output terminals. As because theoutput terminals are feedback to the reverse input terminals, soalways there is found a light present in the input terminals (evenif no external input is applied) and that’s why the conversion pro-cess continues. So certainly when R = �1 and S = �2 then Q = �2 andQ̄ = �1 respectively and when R = �2 and S = �1, Q = �1 and Q̄ = �2,but when there is no input light beam is applied at all i.e. whenthe inputs are withdrawn one can ensure the last state attended inthe output terminals Q and Q̄ . So the truth table of an optical RSflip-flop is followed.

12. Important requirements for the switching of SOA

To obtain the faithful operation the power of the input controllight beam which, serves as a pump beams for the SOA should liebetween 2 and 4 dB. The performance of the data transfer dependson the pomp beam energy. Energy of each probe beam is to bemaintained between −4 and −2 dB. The wavelengths of the selectedinputs are 1555 and 1550 nm corresponding to frequency �1 and �2,respectively. Frequencies of CW input signal should lie in C band(1536–1570 nm). This wavelength range is favorable for opticalcommunication.



Fig. 6. Frequency encoded optical RS flip-flop.

13. Conclusion

To conclude, we have proposed an all optical approach for thesuccessful realization of high speed (far above GHz range) opticallogic gates and memories. The potential advantage of these opticalgates and flip-flop over many other established optical gates wasthe use of frequency encoding technique, for which the coded infor-mation (0, 1) in a signal remains unchanged in refraction, reflection,absorption, etc. for a long distance transmission of data. The pro-posed system could offer also a noise free conversion to provide ahigh signal to noise (S/N) ratio. Using such frequency encoded tech-nology based optical logic gates and flip-flops one can implementmany other digital operations like multiplexer, demultiplexer, mul-tivibrators, etc.

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[6] J.M. Jeong, M.E. Marhic, All-optical logic gates based on cross-phase modula-tion in a non-linear fiber interferometer, Opt. Commun. 85 (5–6) (1991) 430–436.

[7] B.K. Jenkins, A.A. Sawchuk, T.C. Strand, R. Forchheimer, B.H. Soffer, Sequentialoptical logic implementation, Appl. Opt. 23 (19) (1984) 3455–3464.

[8] T.A. Ibrahim, R. Grover, L.-C. Kuo, S. Kanakaraju, L.C. Calkoun, P.-T. Ho, All-optical AND/NAND logic gates using semiconductor microresonators, IEEPhotonics Technol. Lett. 15 (10) (2003) 1422–1424.

[9] P. Ghosh, P.P. Das, S. Mukhopadhyay, New proposal for optical flip-flop usingresidue arithmetic, in: ITCOM-2001, (4534B-22), SPIE Proceedings (Optoelec-tronic and Wireless Data Management, Processing, Storage and Retrieval), vol.4534, 8 November, 2001, pp. 148–154.

[10] A.K. Das, S. Mukhopadhyay, General approach of spatial input encoding formultiplexing and De-multiplexing, Opt. Eng. (U.S.A.) 43 (2004) 126–131.

[11] K.R. Chowdhury, S. Mukhopadhyay, Binary optical arithmetic operation schemewith tree architecture by proper accommodation of optical nonlinear materials,Opt. Eng. 43 (2004) 132–136.

[12] N. Pahari, D.N. Das, S. Mukhopadhyay, All-optical method for the additionof binary data by non-linear materials, Appl. Opt. 43 (33) (2004) 6147–6150.

[13] N. Pahari, S. Mukhopadhyay, An all-optical R–S flip-flop by optical non-linearmaterial, J. Opt. 34 (3) (2005) 108–114.

[14] K.R. Chowdhury, S. Mukhopadhyay, A new method of binary addition schemewith massive use of non-linear material based system, Chin. Opt. Lett. 1 (April(4)) (2003) 241–242.

[15] M.J. Connelly, Semiconductor Optical Amplifiers, Kluwer Academic publishers,2002.

[16] L.Q. Guo, M.J. Connelly, A novel approach to all-optical wavelength conver-sion by utilizing a reflective semiconductor optical amplifier in co propagationscheme, Opt. Commun. 281 (2008) 4470–4473.

[17] L.Q. Guo, M.J. Connelly, A poincare approach to investigate nonlinear polar-ization rotation in semiconductor optical amplifiers and its applications toall-optical wavelength conversion, Proc. SPIE 6783 (1–5) (2007) 678325.

[18] H.J. Dorren, D. Lenstra, Y. Liu, M.T. Hill, G.-D. Khoe, Nonlinear polarization rota-tion in semiconductor optical amplifiers: theory and application to all-opticalflip-flop memories, IEEE J. Quantum Electron. 39 (2003) 141–148.

[19] M.A. Karim, A.S. Awwal, Optical Computing An Introduction, John Wiley andSons, Inc., 1992.

December 24, 2010 11:12 WSPC/241-OPL S1793528810000104

Optics and Photonics LettersVol. 3, No. 1 (2010) 51–59c© World Scientific Publishing CompanyDOI: 10.1142/S1793528810000104

ALL-OPTICAL APPROACH FOR CONVERSIONOF A BINARY NUMBER HAVING A FRACTIONAL

PART TO ITS DECIMAL EQUIVALENTAND VICE-VERSA

SOMA DUTTA∗ and SOURANGSHU MUKHOPADHYAY†

Department of Physics, The University of BurdwanBurdwan, pin-713104, West Bengal, India

∗[email protected]†[email protected]

Received 25 October 2010

Optics is found as a very potential candidate in information processing and computing. Several alloptical methods have been proposed for implementation of all optical logic and arithmetic devicesduring the last few decades. In this regard, the role of optical tree architecture can be mentioned asan important approach for conversion of an optical data from binary to decimal and vice-versa. Inthis communication, the authors propose a new concept of conversion of a binary number havingsome fraction (fractional value) to its equivalent decimal counterpart and its vice-versa. To performthis operation, optical tree and some nonlinear material based switches are used properly.

Keywords: Non-linear optics; optical computation; optical tree-architecture.

1. Introduction

Optics has already been established as a potential and promising candidate in informationand data processing. Many all-optical logical, algebraic and arithmetic processors have beenproposed since the middle of the decade of seventy.1–5 Several schemes of all optical logicgates, optical digital memory units, optical algebraic processors, image processors etc wereproposed by scientists and technologists.6–11 The choice of optical signal in replacement ofconventional electronic signal in data processor is mainly because of the inherent parallelismin optics, which can lead to a super fast up-gradation of computing technology. The majordevelopments conducted in this area are also discussed in the 1st and 2nd InternationalConferences on Photonic Computing.12,13

In connection to the new developments of several all-optical data processing techniques,the role of optical tree architecture can be mentioned specially. This tree has already beenused to convert a position-wise encoded optical decimal data to its binary counterpart andfrom binary data to its decimal counterpart.14 Not only these conversions but there are alsoseveral other conversions which are possibly conducted by the use of this tree architecture for

51

http://dx.doi.org/10.1142/S1793528810000104


52 S. Dutta & S. Mukhopadhyay

the purpose of all-optical data processing and computing. Here in this communication, theauthors propose a modification of optical tree architecture by which a binary data having afractional part can be converted to its respective equivalent decimal value and from decimalnumber having a fractional part to its equivalent binary form. Already optical tree has beenused for conversion of different forms of data, but the optical conversion of binary numberhaving fractional part to its decimal counterpart and its vice-versa is a completely newconcept.

2. Optical Tree-Architecture

The schematic diagram of an optical tree-architecture is shown in Fig. 1. Here a light beam(preferably a laser) is emitted from a point source (A). Then the light beam passes throughsome optical channels. Here each channel is divided into two different light paths e.g. ABinto BC and BD by the use of some beam splitters. Each of BC and BD is divided intotwo more parts individually as BC to CE and CF and BD to DG and DH. Proceedingin this way ultimately eight spots (from I to P) are obtained from a single beam AB. Byarranging another set of splitting arrangement in the output plane one can get 16 spots.To use this circuit as a data processor, one can use some optical channels (A0, A1, A2)which can control the way of passing of the light beam from one main channel to a subchannel. These control channels also carries light beam. A2 control channel is connected tothe switching point B and A1 channel is connected to C and D switching points. A0 channelis connected to E, F, G, H switches. The function of the switches can be illustrated byan example. If light in the control channel A2 is present then it activates the switch B topass the light from AB to BC (upper) sub channel otherwise the light of AB channel willgo through the (lower) sub channel BD. The other switches operate in the same way. Onecan use this optical system for the conversion of a binary number having a fractional part

A

O

P

N

M

L

K

J

I

A2 A1 A0

B

C

D

E

F

G

H

Fig. 1. Optical tree architecture for binary to decimal conversion.


Conversion of BN Having a Fractional Part 53

to its equivalent decimal number. In the present analysis, all the switches are all-opticalin nature. Now for the conversion of a fractional decimal number to its binary equivalentsystem, a different arrangement of mirrors, beam splitters are needed.

3. Non-Linear Material as an Optical Switch

The Kerr type of non-linear materials like CS2, pure silica glass etc can show its isotropicnon-linear behavior of the refractive index of the material for passing the light through it.Due to this behavior, the refractive index of the material (n) maintains the equation asn = n0 + n2I, where n0 is a constant refractive index term, n2 is a non-linear correctionterm and I is the intensity of the light passing through it. According to this equation, whena beam AB of some fixed intensity I falls in the point O (O is a point in the interface ofa linear material (LM) and non-linear material (NLM)), it then passes through the DEchannel as shown in Fig. 2. Now when one beam of intensity I in AB channel and otherone beam of intensity I in CMB channel fall jointly on “O” , then according to the aboveequation, the non-linear refractive index of the material becomes n = n0 +2n2I, and for thisreason light beam ultimately will pass through the FG channel of Fig. 2. Thus the behaviorof the NLM can be compared as an optical switch. Already many all-optical logic deviceshave been proposed based on this principle.15–18

NLM

LM

A

B

C

M

O

F

D

G

E

Fig. 2. Non-linear material as an optical switch.



4. Optical Conversion Method of a Binary Number Having a FractionalPart to Its Equivalent Decimal Number

The optical method of conversion of binary to decimal number is an established one.14 Thenext important function is the implementation of a system for transformation of a fractionalbinary number to its decimal counterpart. Here modified tree architecture can be used forsuch conversion. The significant difference of this scheme from that described earlier is themethod of placement and the position of NLM based switches, beam splitters (BS) and mir-rors (M). Placing these components in a properly different way, one can develop the modifiedsystem. The whole scheme is shown in Fig. 3. Here the control channels (B4B3B2B1B0) carrythe bits of fractional binary number. The whole things of the system can be made clear byan example. Let the fractional binary be 0.10110. So B4 = B2 = B1 = 1 and B3 = B0 = 0and for these type of control/input signals, the beam originally generated from a constantlight source (CLS) first passes through the upper channel from NLM1 then it comes tothe lower channel from NLM2 and again passing through the upper channel from NLM4

it ultimately exits through the channel number 7. This indicates that the conversion of0.10110 gives the position-wise encoded decimal number 0.7. In this way, any binary num-ber having a fractional part can be converted to the respective decimal number by the useof the system described in a Fig. 3. Now the whole scheme can be integrated together toconvert a binary number having both fractional and non-fractional parts to its equivalentdecimal value. This integrated scheme is shown in Fig. 4. Here the block-A consists of the

0

1

2

3

4

56

7

8

9

BS

BS

BS

BS

BS

BS

BS

BS

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

B4 B3 B2 B1 B0

NLM1

NLM2 NLM4

NLM3

NLM6

NLM7

NLM8

NLM9

NLM5

CLS

Fig. 3. All optical system for converting fractional binary number to its decimal counterpart.



A3

A2

A

A0

B4

B3

B2

B1

B0

Conversion method from binaryto decimal

BLOCK - A

Conversion method from binary( fractional ) to decimal

BLOCK - B

.

Output

.

1

0 1 2 3 4 5 6 7 89 0 1 2 3 4 5 6 7 8 9

Fig. 4. All optical integrated system for converting a binary number having both fractional part and non-fractional part to its equivalent decimal number.

system of non-fractional part.14 A3A2A1A0 are the input channels i.e. the channels for plac-ing the binary non-fractional inputs. On the other hand block-B carries the same system asdescribed in Fig. 3 i.e. fractional part, where B4B3B2B1B0 are the input channels for placingthe binary fractional inputs. By the joint action of the two blocks (A and B) one can con-vert any binary number (having both fractional and non-fractional parts) to its equivalentdecimal value which is encoded position-wise. For example if 011.10011 are applied to thisintegrated system, one can obtain 3.6 at the output. All the converted fractional decimalnumbers from its equivalent binary numbers are shown in Table 1.

5. Optical Conversion Method of a Decimal Number Havinga Fractional Part to Its Binary Equivalent

Now we also propose a concept of converting a decimal number having a fractional part toits binary equivalent in an all optical process. Here the decimal number has two parts as awhole. First part is non fractional14 whereas second part is the fractional one. The secondpart extends the conversion of fractional part. The proposed system for conversion of thefractional digit to its binary equivalent is shown in Fig. 5. Here we use beam combiners forcoupling two light beams into a single one and also for breaking a single beam into two parts.These are represented by BS in the figure. The mirrors (M) are used for the reflection of lightbeam. Using these BSs, one can get the converted result at the output channel B4B3B2B1B0

of the system shown in Fig. 5. By the use of the modified tree structure, ultimately onecan show the converted equivalent binary number of the fractional decimal contribution,which lies from 0 to 0.9 in the channels B4B3B2B1B0. To develop this system, the channelsmarked by 0, 1, 2, 3, . . . , 9 are used for applying the fractional part of the decimal number.If 0.8 is to be converted, then the light should be applied at the channel marked by 8,



1

23

4

5

6

7

8

9

0

M

M

M

M

M

M

M

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

B4 B3 B2 B1 B0

Fig. 5. All-optical conversion system from fractional decimal number to its binary counter part.

and the converted binary output is obtained in the channels marked by B4B3B2B1B0. Asan example, when the light goes through the channel 7, no light comes into the channelsB0 and B3 because of the absence of BS in the respective channel. So B0 = B3 = 0,but B4, B2, B1 channels get the necessary light reflected from the respective BS. HenceB4 = B2 = B1 = 1. Ultimately one can obtain B4B3B2B1B0 = 10110 for the application oflight in the input channel marked by 7. So it is the equivalent binary number of decimalvalue 0.7. Similarly, any decimal number from 0 to 0.9 can be converted to its binary value.The non-fractional part and fractional part of a system can be combined together to developa single integrated system from which one can convert a decimal number having both thenon fractional part and the fractional part to its binary equivalent value. The scheme isshown in Fig. 6. As an example, if it is required to convert the decimal number a.b toits equivalent binary number (where ‘a’ is the non fractional decimal digit and ‘b’ is thefractional decimal digit), the light beams are to be placed in the ‘a’th channel of the block Aand to ‘b’th channel of the block B in the system described in Fig. 6. Here block A and blockB comprise the system of non-fractional part and the system of fractional part respectively.The A3A2A1A0.B4B3B2B1B0 comes as the result of the converted binary number from itsdecimal counterpart. If the decimal number is 3.7, one can surely receive 0011.10110 at theoutput stage i.e. A1 = A0 = B4 = B2 = B1 = 1 and A3 = A2 = B3 = B0 = 0.

6. Alternating Approach of Non-Linear Switch

In this communication, we have used Kerr type of non-linear material as an optical switchfor conducting the intensity encoded data based optical conversion. In addition to thismethod, we can propose the use of an alternative optical switch, which can be realized by



A3 A2 A1 A0 . B4 B3 B2 B1 B0

Conversion system for non fractional part (0 to 9)BLOCK-A

9

0

.

Conversion system for fractional part (0 to 0.9) BLOCK-B

0

9

Output channels

Fig. 6. The block diagram of complete system for conversion of decimal number having both fractional partand non-fractional part to its binary equivalent.

SOA basedwavelengthconverter

(λ2)

(λ1)

(λ2) Channel -2

(λ1) Channel - 1

Biasing current

Output

Input

Optical Prism

Fig. 7. SOA based intensity encoded switch.

the use of non-linear character of semiconductor optical amplifier (SOA). Here the channelshifting operation in the tree architecture is conducted by SOA. In this process, one strongpump beam of λ1 wavelength is applied into the input terminal of a properly biased SOAbased wavelength converter (Fig. 7). One prism is placed in front of it. So λ1 wavelengths oflight first passing through the SOA switch goes through the prism and exits from a specific(channel 1). Now if a weak probe beam (λ2 wavelength) is applied to the input side of theSOA switch in addition to the existing pump beam, then one can get a converted light beamof wavelength λ2 from the output of the SOA switch and after passing through the prism, itcomes out through another channel (channel 2). Thus two light beams come in two differentchannels of the output in the system described in Fig. 7. When a light of wavelength λ1

is applied at the input side of the SOA, it ultimately exits from channel-1 and when lightof λ1 and λ2 wavelengths both falls in the input side of SOA, one gets light from another



channel marked as channel-2. This type of switch can be used as replacement of the switchshown in Fig. 2. It is already seen that for the conversion of a decimal number (havingsome fractional part) to the equivalent binary number the system proposed in Fig. 3 andFig. 4 will be useful with this alternative switch. The conventional non-linear switches usedin these two systems may be replaced by the SOA based switches as described above to geta faithful operation. The output power of the digits of the converted data becomes strongerhere sufficiently.

In the case of decimal (with fractional number) to binary conversion, no optical switchesare required.

7. Conclusion

To conclude, the method of conversion of a binary number having a fractional part to itsequivalent decimal number and its vice-versa with all optical switching system have beendiscussed here. The speed of operation is real time. The scheme may be extended bothvertically as well as horizontally for the conversion of a higher valued binary number toits equivalent decimal number and also from decimal to binary. The inherent parallelism ofoptics is exploited in the scheme as far as practicable to obtain superfast operation speed.To get more accuracy in the conversion of the fractional binary number, one may use agreater number of control channels in an extended system. As a whole, the total systemis an all optical one and hence the advantages of using optics are achieved. We should usea suitable non-linear material like SOA and a suitable diode laser, for getting low opticalpower consumption, superfast speed of operation and for reliable operational result.

References

1. M. A. Karim and A. S. Awwal, Optical Computing An Introduction (John Wiley and Sons,Canada, 1992).

2. M. A. Karim, A. A. S. Awwal and A. K. Cherri, Polarisation-Encoded Optical Shadow-CastingLogic Unit:Design, Appl. Opt. 26 (1987) 2720–2725.

3. T. Houbavlis and K. Zoiros, 10-GHZ all-optical recirculating shift register with semiconductoroptical amplifier (SOA)-assisted sagnac switch and SOA feedback, Opt. Engg. 42(9) (2003)2483–2484.

4. K. R. Chowdhury, A. Sinha and S. Mukhopadhyay, An all optical comparison scheme betweentwo multi-bit data with optical nonlinear material, Chinese Optics Letters 6(9) (2008) 693.

5. K. R. Chowdhury, P. P. Das and S. Mukhopadhyay, All-optical time-domain multiplexing-demultiplexing scheme with non-linear material, Optical Engineering 44(3) (2005) 035201.

6. M. T. Hill, H. de Waardt, G. D. Khoe and H. J. S. Dorren, Fast optical flip-flop by use ofMach-Zehnder interferometers, Microwave Optical Technology Letters 31 (2001) 411–415.

7. M. T. Hill, H. de Waardt, G. D. Khoe and H. J. S. Dorren, All-optical flip-flop based on coupledlaser diodes, Journal of Quantum Electronics 37 (2001) 405–413.

8. T. Houbavlis and K. Zoiros, 10-GHZ all-optical recirculating shift register with semiconductoroptical amplifier (SOA)-assisted sagnac switch and SOA feedback, Opt. Engg. 42(9) (2003)2483–2484.

9. S. Dutta and S. Mukhopadhyay, An all optical approach of frequency encoded NOT based Latchusing semiconductor optical amplifier, Journal of optics 39(1) (2010) 35–41.



10. S. Dutta and S. Mukhopadhyay, All optical frequency encoding method for converting a decimalnumber to its equivalent binary number using tree architecture, Optik-Int. J. Light Electron Opt.(2010), 10.1016/j.ijleo.2009.11.018.

11. S. Dutta and S. Mukhopadhyay, Alternating approach of implementing frequency encoded all-optical logic gates and flip-flop using semiconductor optical amplifier, Optik-Int. J. Light ElectronOpt. (2010), doi:10.1016/j.ijleo.2010.06.046. Article in press.

12. “Optical super computing”, First International Workshop, OSC 2008 Vienna Austria, August26, 2008 proceedings, Published by Springer Berlin/Heidelberg, Volume 5172/2008.

13. “Optical super computing”, Second International Workshop, OSC 2009 Bertinoro, Italy,November 18–20, 2009 Proceedings.

14. S. Mukhopadhyay, An optical conversion system: From binary to decimal and decimal to binary,Optics Communications (The Netherlands) 76(5–6) (1990) 309–312.

15. F. T. S. Yu, Q. W. Song and X. J. Lu, Implementation of Boolean logic gates using a macrochan-nel spatial light modulator with liquid-crystal television, Optics Lett. 12 (1987) 962.

16. F. T. S. Yu, S. Jutamulia and D. A. Gregory, Optical parallel logic gates using inexpensiveliquid crystal television, Optics Lett. 12 (1987) 1050.

17. K. M. Jhonson, M. R. Surette and J. Samir, Optical interconnection network using polarization-based ferroelectric liquid crystal gates, Appl. Optics 27 (1988) 1727.

18. T. Sato, K. Shiraishi, K. Tsuchida and S. Kawakami, Laminated polarization splitter with alarge split angle, ApPhL. 61 (1992) 2633S.

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Optik 123 (2012) 212– 216


Optik

j o ur nal homepage: www.elsev ier .de / i j leo

new approach of parallel data transmission through optical waveguide withOA based frequency encoding/decoding technique

oma Dutta ∗, Sourangshu Mukhopadhyayepartment of Physics, The University of Burdwan, Golapbag, Burdwan 713104, West Bengal, India

r t i c l e i n f o

rticle history:eceived 5 October 2010ccepted 15 February 2011

a b s t r a c t

Semiconductor optical amplifier (SOA) is a well known non-linear device which can exhibit Tera Hertzswitching speed of operation. SOA based switching, therefore, has wide application in fiber optic commu-nication. Here in this communication the authors propose a new concept of frequency encoded parallel

eywords:avelength conversion

emiconductor optical amplifieron-linear optics

data transmission with SOA for optical communication. To achieve the transmission SOA is suggestedfor the generation of frequency encoded/decoded parallel data. It converts initially an intensity encodedoptical data to frequency encoded one; whereas at the receiving end it again returns the intensity encodeddata from frequency encoded one.


ptical communication

. Introduction

To support the increasing demand and rapid growth informa-ion, optics has been proved as proper alternative for very highpeed communication with high bit rate and low bit error rate1–5]. In optical communication network the response time at theodes is a very important issue for setting high speed communi-ation. For managing the tremendously increasing day to day dataraffic, it is very necessary to enhance the transmission link capac-ty as well as the speed of the switching networks at the nodes.he realization of a network node with throughput at the order of00 Gb/s is not far away. SOA grating combination has been suc-essfully used for 160 GHz wavelength conversion. Here a phasencoded signal is converted to amplitude modulated signal [1].gain using a cross-correlation system and non-linear polarizationotation 200 GBPS wavelength conversion at temporal resolution at.5 PS is also reported [2]. Some logic gates are also implementedased on the four-wave mixing character of SOA with the mech-nism of polarization shift keying [6] and with tri-state operationogic [7].

Fathallah et al. proposed the concept of a high bandwidth opticalommunication with fast optical frequency-hop code division mul-
iple access (FFH-CDMA) system [8]. The encoding and decoding areone by all-fiber device.
∗ Corresponding author. Tel.: +91 342 2657800; fax: +91 342 2657800.E-mail addresses: [email protected] (S. Dutta), [email protected]

S. Mukhopadhyay).

030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved.oi:10.1016/j.ijleo.2011.02.028

In this communication we propose a novel concept of frequencyencoded data transmission through optical waveguide. Being a fun-damental characteristic, the frequency of light remains unchangedin reflection, refraction, absorption, etc. In the frequency encodingprinciple different frequencies of light are encoded as different logicstates [9–12]. In the present communication, we focus a principlefor the successful realization of an efficient optical data transmis-sion based on the frequency encoding principle, which can be usedas a more reliable one than other conventional encoding principletowards the achievement of super-fast optical communication anddata processing.

2. SOA based wavelength conversion

In modern communication those switching devices are verymuch effective where one light signal is switched by another lightsignal. In this connection semiconductor optical amplifiers (SOAs)can be referred as a promising switch because of many salientadvantages. SOA is a compact, reliable, low-cost all-optical one.Generally SOAs are prepared by some special treatment of GaAs likesemiconductors. Two main types of SOAs are generally focused, oneis the Fabry Perot SOA (FP-SOA) type and another one is TravelingWave SOA (TW-SOA) type.

In many functional applications in basic optical network SOAscan be potentially used. These applications are exploited by the
imported carrier density induced by SOA’s input current. Four fun-damental types of non-linearities in SOA are found which are crossgain modulation (XGM), cross phase modulation (XPM), self phasemodulation (SPM), and four wave mixing (FWM).
dx.doi.org/10.1016/j.ijleo.2011.02.028

http://www.sciencedirect.com/science/journal/00304026

http://www.elsevier.de/ijleo

mailto:[email protected]


dx.doi.org/10.1016/j.ijleo.2011.02.028

S. Dutta, S. Mukhopadhyay / Optik 123 (2012) 212– 216 213

SOA WC

STRONG PUMP BEAM

CW WEAK PROBE BEAM

ν1(λ1)

ν2(λ2) ν1(λ1)

mi(sata[wcbsdpliaoibss

3

iwiSpwffigprssmtwpww

4t

tesf

ADD/DROP Multiplexer

Input signa l λ (ν ), λ (ν ), λ (ν )

Reflected sign al λ (ν )

Circul ator (C)

Biasing ter minal for νfrequenc y

Λ (ν ), λ(ν )

MQW ampli fier Grating filt er

Dropped frequency λ (ν )

Fig. 1. Semiconductor optical amplifier (SOA) based wavelength converter.

Here in this communication the authors exploit the cross gainodulation (XGM) character of SOA [13,14]. XGM character of SOA

s a result of gain saturation phenomenon. A weak CW probe beamat a specific wavelength) and a strong pump beam (at anotherpecific wavelength) of light are injected jointly into the SOA. At

suitable biasing current in the amplifier the probe beam will bereated as a strong beam output from the SOA because of XGM char-cter. Thus one can refer this incidence as wavelength conversion15–25]. There are two types of basic schemes used in XGM basedavelength conversion, one is co-propagating and another one is

ounter propagating schemes. In the first case the pump and probeeams are injected from the same side of the SOA and in the secondcheme pump and probe beams are injected in mutually oppositeirections into the SOA. Co-propagating scheme has better noiseerformance. A weak CW (continuous wave) probe light of wave-

ength �2 and a strong pump beam of wavelength �1 are injectednto the input terminals of the SOA having an anti-reflecting surfacet its input side for �2 and a highly reflecting surface for �1 at theutput terminal. In this situation the strong pump beam transfersts total power to the weak probe beam and thus the weak probeeam being a stronger one and comes to the output terminal. Thecheme is shown in Fig. 1. If there lays no pump beams at the inputide, no conversion is allowed.

. SOA based add/drop multiplexer

Again to achieve a successful routing of wavelengths, the abil-ty of SOA to add and drop a specific wavelength channels in a

avelength-division multiplexed (WDM) network is also a greatmportant function [26]. This is an add/drop multiplexing unit ofOA. The function of an add/drop multiplexer (ADM) is to select onearticular wavelength of light without interfering with the adjacentavelengths. Several types of add/drop multiplexers are reported

or developing several optical devices, some of them use gratinglters and circulators and others use different light wave technolo-ies. In the present case we use the circulators for demultiplexingurpose. The filters can be tuned by changing the biasing input cur-ent into the SOA. The tunable filter has the transfer function of thepectral width 0.9 nm around the selected wavelength. The selectedpecific wavelength is reflected by the filter and amplified by theultiple quantum wells (MQW) and the circulator is used to drop

he selected wavelength in a required direction. Other wavelengthshich come parallel at the input along with the specific one willass through the SOA made filter. So one can separate a particularavelength of light using this add/drop multiplexer from a band ofavelengths. The system is shown in Fig. 2 schematically.

. A new method of frequency encoded parallel dataransmission through optical waveguide

For realization of the method of frequency encoded parallel data
ransmission, first it is necessary to explain about the frequencyncoding principle. In this principle different frequencies of lightignal represent different logic states i.e. if one particular selectedrequency (�2) of light is coded as Boolean logic state ‘1’ and then
Fig. 2. SOA based add/drop multiplexer.

another specific frequency (�1) of light is required to encode thelogic state ‘0’.

This communication system requires some specific frequencies(eight bit data string) which represent the bit ‘1’ of each positionof the data respectively. For example �1 represent the least signif-icant bit (LSB) if it is ‘1’ whereas �8 represents the most significantbit if it is ‘1’ state. In this way all the other bits are represented byother frequencies. The absence of light signal represents the logicstate ‘0’. To implement the whole system (shown in Fig. 3) eightSOA based wavelength converters, eight SOA based add/drop mul-tiplexers, some beam couplers and mirrors are used. The conversionmethod of the SOA based wavelength converter is discussed earlier.Now using eight wavelength converters position-wise (e.g. for thefirst data bit WC1 is used) the new mechanism of data transmissioncan be implemented. The pump beam of frequency �0 (correspond-ing wavelength �0) is used in the other input terminals of all theposition-wise arranged WCs. The constant probe beams of light ofwavelengths �1, �2, �3, �4, �5, �6, �7, �8 are also applied to therespective WCs position-wise. The outputs of the WCs are coupledby the beam couplers. This couplers output can be introduced tothe input of the optical fiber. If an eight bit data represented as01101001 is to be sent through the fiber through this proposedencoder, the light intensity of wavelength �0 is applied as pumpbeams to WC1, WC4, WC6 and WC7 which produces the signal�1�4�6 and �7 in the coupled output and finally this coupled beamcomprising �1, �4, �6 and �7 is introduced to the fiber. Followingthe same process any eight bit data can be encoded as frequencyencoded data and this frequency encoded data can be transmittedthrough the fiber. In the receiving end eight add/drop multiplex-ers (ADM) are required for decoding of frequency encoded data tointensity encoded one. Add/drop multiplexers (ADM) are arrangedposition-wise and each multiplexer is tuned for one particular fre-quency by the application of proper bias current. ADM1 tuned atits biasing frequency �1, ADM2 at �2, ADM3 at �3 and all the otherADMs at �4–�8 respectively. The output obtained from the receiv-ing end of the optical fiber is applied to the input of ADM8. EachADM has one circulator to collect the selected frequency of light.When the light beam having frequency �1–�8 is applied to an ADMthe respective frequency of light will be reflected from the respec-tive ADM for which it is tuned. The other frequencies will easilypass through the ADM. Thus the reflected signals (having differ-ent frequencies) are collected by the optical circulators. The outputfrom the circulators gives the same wavelengths in decoded data,which were sent originally at the input side. This data then is passedparallely and bitwise through 8 wavelength converters. The probebeams of the wavelength converters are at �0. These wavelengthconverters return the original intensity encoded data with exactly
position-wise. In this output data all the 1s are represented by pres-ence of light at the wavelength �0 and 0s by the absence of light.The whole process can be illustrated by an example. Let an eightbit data 11100011 is used for encoding and decoding. For this light

214 S. Dutta, S. Mukhopadhyay / Optik 123 (2012) 212– 216

WC8 WC7

Input terminals for pump beam ( λ0)

ADM1

CC C C

Output te rminals (λ0)

WC2 WC 1

ADM2 ADM7 ADM8

λ1 λ2 λ7 λ8

λ1 λ2 λ7 λ8

Beam coup ler

C Circulator

Optical fiber

WC9 WC10 WC15 WC16

λ0 λ0 λ0 λ0

………

………

………

Fig. 3. Frequency encoded data transmission method based on SOA switching.

iWtWvWarAalt�AdlbSdbdsmp

tb

ntensity of wavelength �0 is applied as pump beam to WC1, WC2,C6, WC7 and WC8 only. Constant probe beams are also applied

o all the WCs. But the conversion takes place only in WC1, WC2,C6, WC7, WC8 whereas WC3, WC4, WC5 are not working for con-

ersion. One can thus get the output beam of wavelength �1 fromC1, �2 from WC2, �6 from WC6, �7 from WC7 and �8 from WC8

nd these light beams are coupled by the beam couplers. At theeceiving end the coupled beam is introduced to the ADM8. ThisDM8 reflects the light having �8 wavelength and passes �1, �2, �6nd �7 to the ADM7 which reflects the light of frequency �7 wave-ength and sends �1, �2 and �6 to the ADM6 which again reflectshe light of �6 wavelength. Similarly ADM2 reflects the light having2 wavelength and ADM1 reflects the light having �1 wavelength.DM3, ADM4, ADM5 give no reflected light. These output lights ofifferent frequencies from the ADMs are applied to the eight wave-

ength converters (WC9, WC10, WC11, WC12, . . ., WC16) as a pumpeams and constant �0 probe beams are also applied to the WCs.o one can get the output as �0�0�0000�0�0 i.e. the transmittedata 1110011 is received at the output. Thus any eight bit data cane sent parallel with the system as described in Fig. 3. The systemescribed in Fig. 3 enables the transmission of eight bit data. For aixteen bit data transmission 16 ADMs are required for making theechanism active. Any data of high number of bits can be sent in

arallel following this mechanism.For another example we take a binary data string as (10011010)

o be sent parallely through an optical fiber. In Fig. 4 the stepy step result is described. In Fig. 4(a) the parallel data string is

shown, whereas in Fig. 4(b) the bitwise intensity encoded par-allel data string is depicted. In Fig. 4(c) the bitwise wavelengthencoded data is shown, which is obtained from the series of wave-length converters as given in Fig. 3. This wavelength encoded datais sent through the optical fiber for parallel communication. Thedata obtained at the outlet of the fiber is again sent through theADMs and wavelength converters, which are used for decodingthe data. The wavelength converters at the receiving end recon-vert the frequency encoded data to bitwise intensity encoded data,which is shown in Fig. 4(d). The received data string is shownin Fig. 4(e).

5. Essential requirements for implementation of thepractical transmission of data

The essential requirement for setting a good response from aSOA based optical switch is that the pump beam for wavelengthconversions should lie between 2 dB and 4 dB. An optical filter canbe used just after the SOA converters which only select the desiredprobe beam frequency of light at the output. The 3 dB bandwidth ofthe filter should be in the order of 1 nm. The performance of the datatransfer depends upon the used pump beam energy. The intensitylevel maintained at the each probe beam should be between −4
and −2 dB. Again it is very important to mention that the wave-length of the both pump and probe beam should lie in C band(1536–1570 nm). Based on all the above aspects some wavelengthsin C band are proposed for the consideration of pump beam and

S. Dutta, S. Mukhopadhyay / Optik 123 (2012) 212– 216 215

Table 1Some proposed wavelengths in C band for encoding the eight different ‘1’ bits of a byte and also for that of the probe beams.

Pump beam Probe beams

�0 �1 �2 �3 �4 �5 �6 �7 �8

1550 nm 1555 nm 1557 nm 1559 nm 1561 n

1 0 0 1 1 0 1 0

0.0

0.2

0.4

0.6

0.8

1.0

8765432

λ8λ6λ3λ2 λ7λ5λ4λ1

1

Inte

nsity

Bit position

a

b

c

d

e

0.0

0.2

0.4

0.6

0.8

1.0

λ8λ6λ3λ2 λ7λ5λ4λ1

Frequency encoded light signal at the input of the optical fiber

Inte

nsity

0.0

0.2

0.4

0.6

0.8

1.0

87654321

λ0λ0λ0λ0

Position-wise output from the wavelength converters

λ0λ0λ0λ0

Inte

nsity

1 0 0 1 1 0 1 0

Fig. 4. Graphical outputs of the frequency encoded data at different stages of theencoding/decoding system. (a) Encoded intensity encoded eight bit data string. (b)Intensity distribution for the position-wise bits at different frequencies at initialstage. (c) Intensity distribution of the bit-wise coded light signals at the input of theoo

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ptical fiber. (d) Intensity distribution of the position-wise bits at the final outputf the decoding system. (e) Decoded intensity encoded eight bit data string.

ight different probe beams for encoding the bits of an eight bityte [9,11]. This is given in Table 1.

. Conclusion

The above method exhibits a method for parallel transmissionf data bits in case of optical communication through fiber. TheOA takes the role of conversion of an intensity encoded data to

frequency encoded one by the exploitation of its wavelengthonversion character. SOA can achieve the THz speed of operationor this conversion process. This proposed method can, therefore,nsure a very high speed optical communication over many other
onventional communication techniques. If a data accommodatesight bits or sixteen bits, then all the bits can be sent in parallelhrough the optical fiber. It is important to mention here that all theavelengths (�1–�8) should be selected at the C-band for setting
[

m 1563 nm 1565 nm 1567 nm 1569 nm

the best conversion efficiency as well as for low loss communi-cation. The arrangement of SOA based wavelength converters andADMs in Fig. 3 is given in such a way that the form of the origi-nal intensity encoded data at the input of the fiber is maintainedat the receiving end of the communication system. At the finaloutput end all the data bits are represented either by 1s if an inten-sity of light with wavelength �0 presents otherwise by 0s for nolight.

References

[1] A. Kumpera, P. Honzathko, R. Slavik, Novel 160-GHz wavelength converterbased on a SOA and a long period grating, Opt. Commun. 282 (2009)1775–1779.

[2] J. Sakaguchi, T. Nishida, Y. Ueno, 200-Gb/s wavelength conversion using adelayed-interference all-optical semiconductor gate assisted by nonlinearpolarization rotation, Opt. Commun. 282 (2009) 1728–1733.

[3] C. Zhang, K. Qiu, B. Xu, Y. Ling, A novel all-optical label processing based onmultiple optical orthogonal coded sequences for optical packet switching net-works, Opt. Commun. 281 (2008) 2433–2442.

[4] A. Argyris, D. Syvridis, L. Larger, V. Annovazzi-Lodi, P. Colet, I. Fischer, J. Garcia-Ojalvo, C.R. Mirasso, L. Pesquera, K. Alan Shore, Chaos-based communicationsat high bit rates using commercial fibre-optic links, Nature 438 (2005) 343–346.

[5] L.Q. Guo, M.J. Connelly, A novel approach to all-optical wavelength conver-sion by utilizing a reflective semiconductor optical amplifier in co propagationscheme, Opt. Commun. 281 (September (17)) (2008) 4470–4473.

[6] Z. Li, G. Li, Gates based on four-wave mixing in a semiconductor optical ampli-fier, IEEE Photon. Technol. Lett. 18 (June) (2006) 1341–1343.

[7] S.K. Garai, A scheme of developing frequency encoded tristate-optical logicoperations using Semiconductor Optical Amplifier, J. Mod. Opt. (Taylor andFrancis) 57 (6) (2010) 419–428.

[8] H. Fathallah, A. Rusch, S. Larochelle, Passive optical fast frequency-hop CDMAcommunication system, J. Lightwave Technol. 17 (1999) 397–405.

[9] S. Dutta, S. Mukhopadhyay, An all optical approach of frequency encoded NOTbased Latch using semiconductor optical amplifier, J. Opt. 39 (1) (2010) 35–41.

10] S. Dutta, S. Mukhopadhyay, All optical frequency encoding method for convert-ing a decimal number to its equivalent binary number using tree architecture,Optik 122 (2) (2011) 125–127.

11] S. Dutta, S. Mukhopadhyay, Alternating approach of implementing frequencyencoded all-optical logic gates and flip-flop using semiconductor optical ampli-fier, Optik, in press.

12] S.K. Garai, S. Mukhopadhyay, Method of implementing frequency encodedmultiplexer and demultiplexer systems using nonlinear semiconductor opticalamplifiers, Opt. Laser Technol. 41 (2009) 972–976.

13] M.J. Connelly, Semiconductor Optical Amplifiers, Kluwer Academic Publishers,2002.

14] N.K. Dutta, Q. Wang, Semiconductor Optical Amplifiers, World Scientific Pub-lishing Co. Pte. Ltd., 2006.

15] T. Durhuus, B. Mikkelsen, C. Joergensen, S. Lykke Danielsen, K.E. Stubkjaer,All-optical wavelength conversion by semiconductor optical amplifiers, J.Lightwave Technol. 14 (1996) 942–954.

16] A. Mecozzi, Small-signal theory of wavelength converters based on cross-gainmodulation in semiconductor optical amplifiers, IEEE Photon. Technol. Lett. 8(1996) 1471–1473.

17] D.A.O. Davies, Small-signal analysis of wavelength conversion in semiconduc-tor laser amplifiers via gain saturation, IEEE Photon. Technol. Lett. 7 (1995)617–619.

18] E. Iannone, R. Sabella, L. De Stefano, F. Valeri, All-optical wavelength conversionin optical multicarrier networks, IEEE Trans. Commun. 44 (1996) 716–724.

19] K. Obermann, S. Kindt, D. Breuer, K. Petermann, C. Schmidt, S. Diez, H.G. Weber,Noise characteristics of semiconductor-optical amplifiers used for wavelengthconversion via cross-gain and cross-phase modulation, IEEE Photon. Technol.Lett. 9 (1997) 312–314.

20] M.F.C. Stephens, D. Nesset, K.A. Williams, A.E. Kelly, R.V. Penty, I.H. White,M.J. Fice, Wavelength conversion at 40 Gbit/s via cross-gain modulation in
distributed feedback laser integrated with semiconductor optical amplifier,Electron. Lett. 35 (1999) 1762–1764.
21] D.D. Marcenac, A.E. Kelly, D. Nesset, D.A.O. Davies, Bandwidth enhancementof wavelength conversion via cross-gain modulation by semiconductor opticalamplifier cascade, Electron. Lett. 31 (1995) 1442–1443.

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22] X. Zheng, F. Liu, A. Kloch, Experimental investigation of the cascadability of across-modulation wavelength converter, IEEE Photon. Technol. Lett. 12 (2000)272–274.

23] H.J. Lee, M. Sohn, K. Kim, H.G. Kirn, Wavelength dependent performance of a
wavelength converter based on cross-gain modulation and birefringence of asemiconductor optical amplifier, IEEE Photon. Technol. Lett. 11 (1999) 185–187.
24] L. Deming, N.J. Hong, L. Chao, Wavelength conversion based on cross-gainmodulation of ASE spectrum of SOA, IEEE Photon. Technol. Lett. 12 (2000)1222–1224.

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25] H. Yu, D. Mahgerefteh, P.S. Cho, J. Goldhar, Improved transmission ofchirped signals from semiconductor optical devices by pulse reshap-ing using a fiber Bragg grating filter, J. Lightwave Technol. 17 (1999)

26] G. Raybon, U. Koren, B.I. Miller, M. Chien, M.G. Young, R.J. Capik, K.Dreyer, R.M. Derosier, A wavelength-tunable semiconductor amplifier/filter foradd/drop multiplexing in WDM networks, IEEE Photon. Technol. Lett. 9 (1997)40–42.

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Optik 123 (2012) 2082– 2084

Contents lists available at SciVerse ScienceDirect

Optik

j o ur nal homepage: www.elsev ier .de / i j leo

new alternative approach of all optical frequency encoded clocked S–R flip-flopxploiting the non-linear character of semiconductor optical amplifiers

oma Dutta ∗, Sourangshu Mukhopadhyayepartment of Physics, The University of Burdwan, Golapbag, Burdwan 713104, West Bengal, India

r t i c l e i n f o

rticle history:eceived 28 April 2011ccepted 2 October 2011

a b s t r a c t

In all optical networking and computing system, the role of all-optical flip-flops is very much essential. Forsignal synchronization with a reference clock and for storage of digital bits the flip-flop has no alternative.In this communication the authors propose a method of developing an all optical frequency encoded

eywords:ptical computationemiconductor optical amplifieravelength conversion

clocked R–S flip-flop using the non-linear character of semiconductor optical amplifiers. Frequency is thebasic character of light and several encoding/decoding problems in computations and communicationscan be solved using the frequency encoding principle of optical data. The proposed system is all-opticaland therefore it can extend a super fast speed of operation (far above THz limit).


on-linear optics

. Introduction

All-optical signal processing has drawn much attention of scien-ific communities because of its inherent parallelism and potentialpplications in high-speed optical networks, optical computingystems, etc. Again all-optical signal processing is especially usefulo overcome the high bit rate problems in future communicationystems. In recent years a lot of efforts have been seen in this areahere optics is tactfully used for the processing of digital data.

Several all-optical digital devices have been proposed whichre supposed to run with the operation speed far above the GHzange [1–5]. Many of those devices are dedicated for performingogic gates, flip-flops, optical buffers, and arithmetic operations tochieve the goal of all-optical computer/data processor. In partic-lar, optical flip-flop can attract a special interest as because itan serve as an optical memory. Optical memory has also of greatmpact for the development of optical packet switches in networks.ow several types of optical flip-flops are proposed with differ-nt types of encoding mechanisms in last few years which areolarization encoding, phase encoding, intensity encoding and alsorequency encoding [6–8].

In this communication the authors have proposed a novel alter-
ative approach of frequency encoded clocked S–R flip-flop basedn SOA based switches. Frequency is a fundamental characterf light and it remains unchanged in reflection, refraction, and
∗ Corresponding author.E-mail addresses: [email protected] (S. Dutta), [email protected]

S. Mukhopadhyay).

030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved.oi:10.1016/j.ijleo.2011.10.011

absorption whereas in other encoding techniques the encodedparameters change with propagation of the signal. Due to this rea-son operation with frequency encoding principle is more reliableand faithful than other encoding systems. In most of the cases, pres-ence of optical signal at the input or at the output of an opticallogical device is represented as ‘1’ logic state and absence of opticalsignal as logic state ‘0’. The intensity of light falls with propagationdue to attenuation in the media. Hence the intensity of a light signalmay drop down below the prefixed reference level of logic state ‘1’and may enter into the reference level of logic state ‘0’. Same thinghappens for other encoding mechanism. To overcome this problemone can use the frequency of light to encode a bit. Here one spe-cific frequency (�2) of light is encoded as logic state ‘1’ and anotherspecific frequency (�1) of light encoded as logic state ‘0’.

2. SOA based wavelength conversion and ADD/DROPmultiplexing process

Semiconductor optical amplifier is an optoelectronic devicewhich under suitable operating condition can amplify an input lightsignal passing through it. This function of SOA has been used forseveral all optical implications [9]. Here we use two types of opticalswitches based on SOA. One is wavelength conversion switch andother is add/drop multiplexing switch [10–15]. Wavelength con-verters are used for conversion of specific wavelength of light toanother wavelength. The co-propagating type of XGM wavelength
converter is used in our proposed system (WC). If a suitable �2wavelength of light is applied to a WC as a strong pump beam andanother suitable �1 wavelength of light is applied to the WC asa weak probe beam into its input terminals then one can get the
dx.doi.org/10.1016/j.ijleo.2011.10.011

http://www.sciencedirect.com/science/journal/00304026

http://www.elsevier.de/ijleo



dx.doi.org/10.1016/j.ijleo.2011.10.011

/ Optik 123 (2012) 2082– 2084 2083

cnslc

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3

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Fig. 1. Optical R–S flip-flop (un-clocked) SOA based switches (WC and ADM).

ADM

WC

WC

WC

WC

WC

F

C

C

ADM

QR

S QR

S

CLK BLOCK -A

S. Dutta, S. Mukhopadhyay

onverted strong light beam of wavelength �1 at its output chan-el. The pump beam transfers its total power to the probe beam,o at the output one can obtain the strong probe beam, i.e. wave-ength is converted. In absence of any one pump or probe beam theonversion process is stopped.

In this communication we use also another type of optical switchhich is add/drop multiplexer and it is used to select a particular

requency of light from a spam of frequencies. If an add/drop mul-iplexer is tuned at a particular frequency by selecting its biasingurrent then it blocks the passage of that particular frequency ofight and admits the passing of all other frequencies of light. The fre-uency which is blocked by the multiplexer can be made reflectedrom it and collected by a circulator from its input side [16].

. All-optical memory unit

Several types of all optical memory units have been reportedarlier. Among these memory units some are polarization encoded,ome are phase encoded, and some are intensity encoded [17,18].here also some frequency encoded memory units which havelready been reported by different scientists [8,9,19]. MZI-SOAwitches, add/drop multiplexers and PBSs were used there tomplement the optical memory units and as well as the tri-stateogic gates. The current authors have also proposed all-optical fre-uency encoded one bit memory unit and two-bit memory unitarlier [8,9]. These are based on SOA made switches, wavelengthonverters and add/drop multiplexers. The frequency encodedemory unit is not run by clock signal there. In continuation of

heir earlier work now the authors propose a clocked S–R flip-flopith frequency/wavelength encoding. In digital optical communi-

ation and computation this frequency encoded flip-flop will taken important role. This type of encoding is chosen because of veryigh signal to noise (S/N) ratio and very low bit error problem.

. Optical implementation of clocked S–R flip-flop

The frequency encoded optical R–S flip-flop is already proposedy us and the functions of SOA based switches are elaborately dis-ussed in our previous work [9]. This SOA based optical R–S flip-flops given in Fig. 1. Now to introduce a clock based memory operation

e propose a new concept of frequency encoded optical clocked–S flip-flop. To implement this clocked system we use some SOAased wavelength converters (WC), add/drop multiplexers (ADM),ptical filters, mirrors (M) and beam splitters (BS). The truth tablef optical R–S flip-flop is shown in Table 1. Now the proposed clockased R–S flip-flop is given by a schematic diagram shown in Fig. 2.ere one light source (serving as a clock signal) used as the clock

erminal. One optical filter which only passes the �2 frequency ofight is placed in front of the clock. So this clock channel will onlyend the �2 frequency of light, when the clock is 1. S and R arehe two input terminals. Now if �2(1) frequency of light is appliedo the clock and S = �1(0), and R = �2(1), then �1 frequency of lightrom the terminal S falls on the ADM1 which is tuned at frequency2 and therefore the light passes through the ADM1 and it is applied
o the WC1 as a pump beam. Again �2 frequency of light from CLKasses through the optical filter and is splitted into three parts byhe two beam splitters. One part is applied to the WC1 as a probeeam where as the second part is applied to the WC4 also as a probe
able 1ruth table of optical R–S flip-flop.

R S Q Q

�1(0) �2(1) �2(1) �1(0)�2(1) �1(0) �1(0) �2(1)0 0 Last state attended Last state attended

PUMP BEAM

PROBE BEAM

F OPTICA L ν2 PASS FILTE R

BLOCK-A:OPTICAL RS FLIP-F LOP

C CIRCULATOR

Fig. 2. Optical clocked S–R flip-flop, with SOA based switches (WC and ADM).

2084 S. Dutta, S. Mukhopadhyay / Opt

Table 2Truth table of optical clocked S–R flip-flop.

Clock S R Q Q

�2(1) �1(0) �2(1) �1(0) �2(1)

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�2(1) �2(1) �1(0) �2(1) �1(0)�2(1) 0 0 Last state attended Last state attended�1(0) 0 0 Last state attended Last state attended

eam. Third part is applied to the WC3 as the pump beam. Now forC1 both the pump and probe beam is present then one can get

he �2 frequency of light at its output which is directly applied tohe input R1 terminal to the unlocked R–S flip-flop and the �1 fre-uency of light is obtained at the output terminal Q (according tohe principle of R–S flip-flop). For the WC3 a constant probe beamof frequency �1) is applied in its input then both pump and probeeam is present and one can get the �1 frequency of light from theutput of the WC3 which is divided again into two parts by these of a BS. One part is applied as a probe beam to the WC2, butue to absence of pump beam the conversion cannot be obtained.nother part of the output from WC3 is applied to the WC5 as arobe beam. Again as �2 frequency of light is applied at the inputerminal R and it falls on the ADM2 (which is tuned also at �2 fre-uency), so it blocks the passage of �2 through it and reflects the

ight which is received by the optical circulator. It then serves as pump beam to the WC5. So in presence of both pump beam androbe beam the conversion is obtained at WC5 and one can get the1 frequency of light at the output of WC5 which goes to the S1erminal of R–S flip-flop. Thus the �2 frequency of light is receivedt the output terminal Q according to the principle of R–S flip-flopTable 1). Here absence of pump beam does not support any con-ersion WC4. Thus when clock is �2(1) and S = �1(0) and R = �2(1)he output of the whole flip-flop Q = �1(0) and Q = �2(1). When thelock is at the frequency �2 (1) and S = �2(1), and R = �1(0) then WC2,

C3 and WC4 are active and take part in conversion process andne can get input of the R–S flip-flip of block A as, R1 = �1(0) and1 = �2(1), which produces Q = �2(1) and Q = �1(0) respectively athe final outputs. Now when clock = �2(1) but the light beams areithdrawn from the input terminals S and R then the clock systemill not affect the WCs and they stop the conversion, but due to the

eedback mechanism in the R–S flip-flop, the outputs Q and Q willttend with its last achieved values. Similarly when CLK is set at, i.e. when �1 frequency is applied at the clock terminal the filterill not pass any signal to the WCs. So instead of the presence of

–S inputs the WCs will not support the conversion process. Thus1 and S1 will get no signal from R and S for this reason Q and Qill continue with its last attended values. The truth table of opti-

al clocked S–R flip-flop is shown in Table 2. Thus it can seen thathen the clock is applied to the flip-flop the outputs (Q, Q ) give the

esult according to the truth table, but when no clock is applied or1(0) frequency is applied at the clock terminal the Q, Q outputs ofhe flip-flop holds its last attended values.

. Conclusion

The method of optical implementation of clocked S–R flip-flopith all optical switching systems is illustrated here. The speed of

peration can go far above the THz limit as the SOA based switchesperate at this speed. A high-speed operation of the proposedcheme is not the only a prospective advantage of the system, buthe frequency encoding mechanism also offers a great support. The

[

[

ik 123 (2012) 2082– 2084

most important application of the system can be seen in digitalcommunication, where one can send the frequency encoded newdata or an (old data depending on the applied clock) in the commu-nication channel. Even if the sender requires making a data to becontinued for communication or a new data is to be introduced inthe channel, he can easily do it by the use of the above system. Theoutput signal from the clocked S–R flip-flop can be sent to distantreceiver as it remains unaltered in reflection, refraction, absorp-tion, etc. due to the nature of coding of bits (0 or 1) with frequencyvariation of light. Therefore this technique will be very much use-ful for conducting a reliable and faithful optical memory both incommunication and computation. To achieve a good amplificationthe pump beam of WCs should lie between 4 dB and 10 dB. Theproposed system does not only offer a high speed operation butit also offers an operation which provides a high signal to noise(S/N) ratio. The accommodation of frequency encoding process isthe main reason for obtaining the high S/N ratio. For this reason biterror rate also goes down in comparison to conventional intensitybased encoding processes. This optical clocked S–R flip-flop and fre-quency encoding technique can be used many other optical deviceswhere clocked S–R flip-flop is an essential unit.

References

[1] M.T. Fatehi, K.C. Wasmundt, S.A. Collins, Optical flip-flops and sequen-tial logic circuits using a liquid crystal light valve, App. Optics 23 (1984)2163–2171.

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SOME STUDIES ON SEMICONDUCTOR OPTICAL ...shodhganga.inflibnet.ac.in/bitstream/10603/50262/1/thesis...iii The University of Burdwan Department of Physics Golapbag, Burdwan, West Bengal,