FOUR WAVE MIXING NONLINEARITY EFFECT IN WAVELENGTH

DIVISION MULTIPLEXING RADIO OVER FIBER SYSTEM

HAFIZ ABD EL LATIF AHMED HABIB

UNIVERSITI TEKNOLOGI MALAYSIA

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DEDICATION

To my beloved late mother, may her soul rest in Paradise.

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ACKNOWLEDGEMENTS

Praise and thanks to Allah (SWT) who gave me the strength and courage to

complete this project.

I would like to express sincere thanks to my supervisor Dr. Razali bin Ngah

for his invaluable guidance throughout the course of this project. His guidance, ideas,

encouragement, affable nature, kindness and support were greatly helpful. Even with

his busy schedule, he spent considerable amount of time helping me through the

different phases of this project.

Special acknowledgement to Dr. Abdelwahab Mohammed Ali, for his

valuable suggestions, and revision of this project. Special thanks to Eng. Mr.

Abdelrazig Saeed Mohamed for his assistance and support and to Eng. Mr. Reza

Abdolee for the many interesting discussions I have had with him.

I wish to thank my parents, for their daily prayers, giving me the motivation

and strength, and encouraging me to accomplish and achieve my goals.

A special acknowledgment must be given to my brothers and sisters for their

motivation help and support during my academic period at UTM. I am indebted to

them and words will never express the gratitude I owe to them.

Last but not least, sincere thanks and gratitude to my lovely wife Najat and

my sons Mohammed and Awab who inspired me by their, courage, support and

patience throughout the period of my study.

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ABSTRACT

The integration of wireless and optical networks is a potential solution for the

increasing capacity and mobility as well as decreasing costs in the access networks.

Optical networks are fast, robust and error free, however, there are nonlinearity

obstacles preventing them from being perfect media. The performance of wavelength

division multiplexing (WDM) in radio over fiber (RoF) systems is found to be

strongly influenced by nonlinearity characteristics in side the fiber. The effect of four

wave mixing (FWM) as one of the influential factors in the WDM for RoF has been

studied here using Optisystem and Matlab. From the results obtained, it is found that

the FWM effects have become significant at high optical power levels and have

become even more significant when the capacity of the optical transmission line is

increased, which has been done by either increasing the channel bit rate, and

decreasing the channel spacing, or by the combination of both process. It is found

that when the channel spacing is 0.1 nm, 0.2 nm and 0.5 nm the FWM power is

respectively, becomes about -59 dBm, -61 dBm and -79 dBm. This result confirms

that the fiber nonlinearities play decisive role in the WDM for RoF system. The

simulation results obtained here are in reasonable agreement as compared with other

numerical simulation results obtained, elsewhere, using different simulation tools.

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ABSTRAK

Integrasi talian tanpa wayar dan rangkaian optik menjadi potensi kepada

penyelesaian untuk peningkatan kapasiti dan mobiliti dan seterusnya mengurangkan

kos capaian rangkaian. Rangkaian optik adalah pantas, berkesan, dan tidak

mempunyai masalah. Namun begitu halangan ‘nonlinearity’ menghalangnya menjadi

media yang sempurna. Prestasi jarak gelombang pembahagi pemultipleksan (WDM)

dalam radio melalui fiber (RoF) sistem amatlah dipengaruhi oleh ciri-ciri

‘nonlinearity’ didalam fiber. Kesan ‘four wave mixing’ (FWM) yang menjadi salah

satu faktor berpengaruh dalam WDM untuk RoF telah dikaji menggunakan

Optisystem dan Matlab. Keputusan yang diperolehi mendapati bahawa kesan FWM

menjadi penting pada optik kuasa aras tinggi dan sangat penting apabila kapasiti

talian penghantaraan optik bertambah, sama ada dengan meningkatkan kadar bit

saluran, mengurangkan penjarakan saluran, ataupun kedua-duanya sekali. Ianya

didapati bahawa apabila penjarakan saluran adalah 0.1 nm, 0.2 nm, dan 0.5 nm kuasa

FWM masing-masing adalah lebih kurang –59 dBm, -61 dBm, dan –79 dBm.

Keputusan ini mengesahkan bahawa ‘fiber nonlinearities’ memainkan peranan utama

dalam WDM untuk sistem RoF. Keputusan simulasi berangka yang diperolehi juga

bersamaan dengan keputusan model analisis yang diperolehi melalui Matlab.

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION iii

DEDICATION iv

ACKNOWLEGEMENT v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENTS viii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF ABBREVIATION xv

LIST OF SYMBOLS xvii

LIST OF APPENDICES xix

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Problem Background 2

1.3 Problem Statement 3

1.4 Objective of the Project 4

1.5 Scope of the Project 5

1.6 Organization of the Thesis 5

2 RADIO OVER FIBER TECHNOLOGY 7

2.1 Introduction 7

2.2 What is Radio over Fiber? 8

2.3 Benefits of RoF Technology 9

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2.3.1 Low Attenuation Loss 9

2.3.2 Large Bandwidth 10

2.3.3 Immunity to Radio Frequency Interference 11

2.3.4 Easy Installation and Maintenance 11

2.3.5 Reduced Power Consumption 12

2.3.6 Multi-operator and Multi-service Operation 12

2.3.7 Dynamic Resource Allocation 13

2.4 The Application of Radio over Fiber Technology 13

2.4.1 Cellular Networks 13

2.4.2 Satellite Communications 14

2.5 RoF Multiplexing Techniques 15

2.5.1 Sub-Carrier Multiplexing in RoF System 15

2.5.2 Wavelength Division Multiplexing in RoF System 17

3 NON-LINEAR EFFECTS 19

3.1 Introduction 19

3.2 Types of Fibers 20

3.3 Fiber Losses 20

3.4 Fiber Nonlinearities 21

3.4.1 Self Phase Modulation 22

3.4.2 Cross Phase Modulation 23

3.4.3 Four Wave Mixing 25

3.4.3 Stimulated Brillouin Scattering 29

3.4.5 Stimulated Raman Scattering 30

4 METHODOLOGY 32

4.1 Introduction 32

4.2 Simulation using Optisystem Software 32

4.3 The Simulation Model 33

4.4 Simulation of the Four Wave Mixing effect 35

4.5 Simulation of FWM for Higher Number of Channels 40

4.6 Effect of Different Power Level of the Signal Sources 41

4.7 Effect of Increase Dispersion of the Fiber Optic 41

4.8 Effect of Increase Effective Area f the Fiber Optic 42

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4.9 Modelling the Effect of FWM 42

5 RESULTS AND DISCUSSIONS 47

5.1 Introduction 47

5.2 Simulation of the Four Wave Mixing Effect 47

5.3 Simulation Results without the External Modulated Signal 48

5.3.1 Effect of Channel Spacing 48

5.3.2 Effect of Different Power Level of the Signals Sources 51

5.3.3 Effect of Increase Dispersion of the Fiber Optic 53

5.4 Simulation Results with the External Modulated Signal 54

5.4.1 Effect of Channel Spacing 55

5.4.2 Effect of Different Power Level of the Signals Sources 58

5.4.3 Effect of Increase Dispersion of the Fiber Optic 61

5.4.4 Effect of Increase Effective Area of the Fiber Optic 61

5.5 Simulation of FWM for Higher Number of Channels 62

5.5.1 Simulation Results for Four Signal Source without External

Modulated Signal 63

5.5.2 Simulation Results for Four Signal Source without External

Modulated Signal 65

5.6 Discussion 67

5.7 Analytical Modelling 68

5.8 FWM reduction 70

5.8.1 Effect of Unequal Channel Spacing 70

5.8.1 Effect of Increase Effective Area of the Fiber Optic 72

6 CONCLUSIONS AND RECOMMENDATIONS 73

6.1 Conclusion 73

6.2 Recommendations for Future Work 74

REFERENCE 75

Appendix 77

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LIST OF TABLES

TABLE NO. TITLE PAGE

3.1 Comparison between SBS and SRS 31

4.1 Global parameters 37

4.2 CW Laser sources parameters 38

4.3 DM 2x1 multiplexer parameters 38

4.4 Main tab and dispersion tab parameters for optical fiber 38

4.5 Nonlinear tab parameters for optical fiber 39

4.6 Numerical tab and PMD tab parameters for optical fiber 39

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LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 The Radio over Fiber System Concept 8

2.2 Operating regions of optical fiber 10

2.3 These robust RAPs are connected to the central base

station via the ROF 14

2.4 SubCarrier Multiplexing of Mixed Digital and Analogue

Signals 16

2.5 WDM system using multiple wavelength channels and

optical amplifiers 17

3.1 Frequency chirping effect 23

3.2 Four wave mixing products 26

3.3 The arising new frequency components due to FWM 27

3.4 FWM products versus channel count 28

3.5 FWM mixing efficiency in single-mode fibers 29

4.1 Direct Modulation 33

4.2 External modulation 33

4.3 Simulation model with external modulated signal 34

4.4 Simulation model without external modulated signal 35

4.5 Simulation model with three channels 40

4.6 flowchart illustrate the modeling steps 40

4.7 The phase matching condition of two different

wavelengths 45

5.1 Optical spectrum at the input of the fiber when channel

spacing is set to 0.1 nm 48

5.2 Optical spectrum at the onput of the fiber when channel

spacing is set to 0.1 nm 49

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5.3 Optical spectrum at the input of the fiber when channel

spacing is set to 0.2 nm 49

5.4 Optical spectrum at the input of the fiber when channel

spacing is set to 0.2 nm 50

5.5 Optical spectrum at the input of the fiber when channel

spacing is set to 0.5 nm 50

5.6 Optical spectrum at the onput of the fiber when channel

spacing is set to 0.5 nm 51

5.7 Optical spectrum at the output of the fiber when input

power is set to 20 dBm 52

5.8 Optical spectrum at the output of the fiber when input

power is set to 20 dBm 52

5.9 Optical spectrum at the output of the fiber when input

power is set to -10 dBm 53

5.10 Output optical spectrum when the dispersion of fiber

optic is set to 16.75 ps/nm/km 54

5.11 Optical spectrum at the input of the fiber when the

channel spacing is set at 0.1 nm 55

5.12 Optical spectrum at the output of the fiber when the

channel spacing is set at 0.1 nm 56

5.13 Optical spectrum at the input of the fiber when the

channel spacing is set at 0.2 nm 56

5.14 Optical spectrum at the output of the fiber when the

channel spacing is set at 0.2 nm 57

5.15 Optical spectrum at the input of the fiber when the

channel spacing is set at 0.5 nm 57

5.16 Optical spectrum at the output of the fiber when the

channel spacing is set at 0.5 nm 58

5.17 Optical spectrum at the output of the fiber when input

power is set at 20 dBm 59

5.18 Optical spectrum at the output of the fiber when input

power is set at 10 dBm 59

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5.19 Optical spectrum at the output of the fiber when input

power is set at -10 dBm 60

5.20 Optical spectrum at the output of the fiber when input

power is set at 0 dBm 61

5.21 Optical spectrum at the output of the fiber when the

effective area of the fiber optic is set at 76.5 μm2 62

5.22 Four optical spectrum at the input of the fiber when the

channel spacing is set at 0.1 nm 63

5.23 Four output optical spectrum channels when the channel

spacing is set at 0.1 nm 64

5.24 Four output optical spectrum channels when the channel

spacing is set at 0.5 nm 64

5.25 Four input optical spectrum channels when the channel

spacing is set at 0.1 nm 65

5.26 Four output optical spectrum channels when the channel

spacing is set at 0.1 nm 66

5.27 Four output optical spectrum channels when the channel

spacing is set at 0.5 nm 66

5.28 Power per channel vs. FWM power 69

5.29 Channel spacing versus FWM power 69

5.30 Optical spectrum at the input of the fiber when the

channel spacing is unequal 71

5.31 Optical spectrum at the output of the fiber with unequal

channel spacing 71

5.32 Optical spectrum at the output of the fiber when the

effective area of the fiber optic is set at 76.5 μm2 72

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LIST OF ABBREVIATIONS

RoF - Radio over Fiber

SPM - Self Phase Modulation

XPM - Cross Phase Modulation

FWM - Four Wave Mixing

SRS - Stimulated Raman Scattering

SBS - Stimulated Brillouin Scattering

WDM - Wavelength Division Multiplexing

DWDM - Dense Wavelength Division Multiplexing

SMF - Single Mode Fiber

nm - nanometer

E/O - Electrical-To-Optical Converter

O/E - Optical - To Electrical- Converter

RF - Radio Frequency

IF - Intermediate Frequency

CW - Continuous Wave

RAU - Radio Antenna Unit

THz - Teri hertz

OTDM - Optical Time Division Multiplexing

SCM - Sub-Carrier Multiplexing

EMI - ElectroMagnetic Interference

IM-DD - Intensity Modulation and Direct Detection

OFM - Optical Frequency Multiplication

GSM - Global System for Mobile communication

MVDS - Multipoint Video Distribution Service

MBS - Mobile Broadband System

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GHz - Gigahertz

RHD - Remote Heterodyne Detection

TDM - Time Division Multiplexing

OADM - Optical Add-Drop Multiplexer

LED - Light Emitting Diode

GVD - Group velocity dispersion

ITU - International Telecommunication Union

MUX - Multiplexer

NDSF - Non Dispersion Shifted Fiber

PMD - Polarization Mode Dispersion

NRZ - Non-Return to zero

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LIST OF SYMBOLS

A - Pulse amplitude

Aeff - Effective area of optical fiber

c - Speed of light

co - Speed of light in vacuum

D - Dispersion parameter

dijk - Degeneracy factor

E - Electric field, vector

E - Electric field, scalar

f - Frequency

I - Intensity

L - Length

Leff - Effective length

n - Refractive index

no - Wavelength dependent refractive index

n2 - Nonlinear refractive index

n2/Aeff - Nonlinear coefficient

P - Total polarization, vector

P - Power

pi - Input power

r - Radius

t - Time

z - Distance

α - Attenuation constant [1/m]

αdB - Attenuation constant [db/km]

βi - Propagation constant of the mode i

γ - Nonlinear parameter

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εo - Vacuum permittivity

λ - Wavelength

λo - Center wavelength

τ - Normalized time constant

ωi - Angular frequency

χ(j) - jth order susceptibility

xix

LIST OF APPENDICES

APPENDIX TITLE PAGE

A A Matlab Program for FWM power and channel

spacing 78

1

CHAPTER 1

INTRODUCTION

1.1 Introduct ion

In the past, dating back to the beginning of the human civilization,

communication was done through signals, voice or primitive forms of writing and

gradually developed to use signaling lamps, flags, and other semaphore tools.

As time passed by, the need for communication through distances, to pass

information from one place to another, became necessary and the invention of

telegraphy brought the world into the electrical-communication. The major

revolution that affected the world however was the invention of the telephone in

1876. This event has drastically transformed the development of communication

technology. Today’s long distance communication has the ability to transmit and

receive a large amount of information in a short period of time.

Since the development of the first-generation of optical fiber communication

systems in the early 80’s [4], the optical fiber communication technology has

developed fast to achieve larger transmission capacity and longer transmission

distance, to satisfy the increased demand of computer network. Since the demand on

the increasing system and network capacity is expected, more bandwidth is needed

because of the high data rates application, such as video conference and real-time

image transmission, and also to achieve affordable communication for everyone, at

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anytime and place [1]. The communication capabilities allow not only human to

human communication and contact, but also human to machine and machine to

machine interaction. The communication will allow our visual, audio, and touch

sense, to be contacted as a virtual 3-D presence [3].

To keep up with the capacity increasing requirement, new devices and

technologies with high bandwidth are greatly needed by using both electronic and

optical technologies together to produce a new term Radio over Fiber (RoF). The

progress made so far has been impressive, where information rate at 1 terabits/s can

be handled by a single fiber [5].

RoF is a technology used to distribute RF signals over analog optical links. In

such RoF systems, broadband microwave data signals are modulated onto an optical

carrier at a central location, and then transported to remote sites using optical fiber.

The base-stations then transmit the RF signals over small areas using microwave

antennas and. Such a technology is expected to play an important role in present and

future wireless networks since it provides an end user with a truly broadband access

to the network while guaranteeing the increasing requirement for mobility. In

addition, since it enables the generation of millimeter-wave signals with excellent

properties, and makes effective use of the broad bandwidth and low transmission loss

characteristics of optical fibers, it is a very attractive, cost-effective and flexible

system configuration.

1.2 Problem Background

Normally light waves or photons transmitted through RoF have little

interaction with each other, and are not changed by their passage through the fiber

(except for absorption and scattering). However, there are exceptions arising from

the interactions between light waves and the material transmitting them, which can

affect optical signals in RoF. These processes generally are called nonlinear effects

because their strength typically depends on the square (or some higher power) of

intensity rather than simply on the amount of light present. This means that nonlinear

3

such as self phase modulation (SPM), cross phase modulation (XPM), four wave

mixing (FWM), stimulated raman scattering (SRS), and stimulated brillouin

scattering effects (SBS) are weak at low powers, but can become much stronger

when light reaches high intensities [7]. This can occur either when the power is

increased, or when it is concentrated in a small area-such as the core of an optical

fiber. Nonlinear optical devices have become common in RoF applications, such as

to convert the output of lasers to shorter wavelengths by doubling the frequency. The

nonlinearities in RoF are small, but they accumulate as light passes through many

kilometers of fiber. Nonlinear effects are comparatively small in optical fibers

transmitting a single optical channel. They become much larger when wavelength-

division multiplexing (WDM) packs many channels into a single fiber [9].

WDM puts many closely spaced wavelengths into the same fiber where they

can interact with one another. It also multiplies the total power in the fiber. A single-

channel system may carry powers of 3 milliwatts near the transmitter. DWDM

multiplies the total power by the number of channels, so a 40-channel system carries

120 mW. That's a total of 2 mW per square micrometer-or 200,000 watts per square

centimeter [11]. Several nonlinear effects are potentially important in RoF, although

some have produce more troublesome than others. Some occur in systems carrying

only a single optical channel, but others can occur only in multichannel systems.

1.3 Problem Statement

The rapid development of the wireless communication networks has

increased the need of the optical signal processing. The link lengths have grown to

thousands of kilometers without need to convert optical signals back and forth to

electric form, and the transmission speeds of terabits per second are feasible today

[5]. This ever-growing demand for the high speed communication has forced to use

higher bit rates as well as transmission powers.

Nonlinear effects on communication have become significant at high optical

power levels and have become even more important since the development of

4

erbium-doped fiber amplifier (EFDA) and DWDM systems. By increasing the capacity

of the optical transmission line, which can be done by increasing channel bit rate,

decreasing channel spacing or the combination of both, the fiber nonlinearities come

to play even more decisive role.

The origin of the nonlinearities is the refractive index of the optical fiber,

which is varies with the intensity of the optical signal. This intensity-dependent

component of the refractive index includes several nonlinear effects, such as SPM,

XPM, FWM, SRS, and SBS, and becomes significant when high powers are used.

Although the individual power in each channel may be below the level needed to

produce nonlinearities, the total power summed over all channels can quickly

become significant. The combination of high total optical power and large number of

channels at closely spaced wavelengths is a source for many kinds of nonlinear

interactions.

Form the above-mentioned reasons, this study is aimed to gain insight into

nonlinear effect caused specifically by FWM in the WDM for RoF system and

measure the coefficient behind these nonlinear effects. Nonlinear coefficient of the

RoF may become an important parameter, when new optical long-haul transmission

lines and networks are being deployed.

1.4 Object ive of the Projec t

The main objective of this project is to evaluate the FWM in WDM for RoF

technology, in order to calculate the impairments associated with long-distance high-

bit rate optical fiber communication systems. In order to achieve the objective,

optisystem and matlab programming software will be used respectively in the

numerical simulation and the analytical modelling will be verified through

comparison with optisystem simulation.

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1.5 Scope of the Project

To study the efficiency of the FWM in WDM for RoF optical network, two

approaches were followed in this project. The first approach is the numerical

simulation using Optisystem software which almost replicates a real system. The

second aproach is the analytical modeling, which is simple and faster to analyze its

performance. MATLAB programming is used to implement the analytical model. To

verify the analytical system, a comparison is made with the Optisystem software.

Since Routing and wavelength assignment algorithm (RWA) needs to set up the path

immediately to reduce network delays, the analytical model developed in this project

can be used to calculate the impairments fast enough so that the routing decisions can

be made efficiently, to achieve optimal systems.

1.6 Organizat ion of the Projec t

Chapter 1 provides the introduction to this project where brief background of

the study problem and to the statement of the problem. Followed by the objective,

and the scope of the study. Chapter 2 reviews the literature, which includes

introduction to the RoF, the benefits, and applications of the Radio over Fiber

Technology in both satellite and mobile radio communications. In addition various

types of RoF Multiplexing Techniques, such as Sub carrier multiplexing and

wavelength division multiplexing, have also bee covered. Chapter 3 provides

information about the fiber characteristics, and the non linear effects such as SPM,

FWM, SBS, SRS, and XPM.

Chapter 4 describes the methodological processes by showing detailed diagram of

the methods implemented as well as highlighting briefly the steps those have been

followed to achieve the objective of this project. Chapter 5 presents the results

derived from the methods explained where some analyses and simulations were done

based on the FWM effects. Finally the conclusions of the study, as well as some

suggestions for future work were summed up in Chapter 6.