Comparison Optical Amplifiers Optical Communication

Canadian Journal on Electrical and Electronics Engineering Vol. 2, No. 11, November 2011

505

A review paper on comparison of optical amplifiers in optical

communication systems

Bhawna Utreja, Hardeep Singh, Thapar University

Abstract - In this paper, several

technologies of optical amplifiers have

been discussed with their applications

that are suitable for the low-cost,

moderate performance application

space. These amplifiers must be small in

size and easy to control to allow their

use in many places in the network. The

different technologies, such as EDFA,

Raman amplifiers and SOA, have

different properties making them

suitable for a variety of applications.

Key-words : Optical communication

system, semiconductor optical amplifier,

EDFA, Raman amplifier.

1. Introduction:

In order to transmit signals over long

distances (>100 km) it is necessary to

compensate the attenuation losses within

the fiber. Initially this was accomplished

with an optoelectronic module consisting

of an optical receiver, regeneration and

equalization system, and an optical

transmitter to send the data. Although

functional this arrangement is limited by

the optical to electrical and electrical to

optical conversions. Several types of

optical amplifiers have since been

demonstrated to replace the OE– electronic

regeneration systems [1]. These systems

eliminate the need for E-O and O-E

conversions. This is one of the main

reasons for the success of today’s optical

communications systems shown in Figure

1.

Electronic AMP (regeneration, equalization) Fiber Fiber

Optical signal In Optical Signal Out

Figure 1: Optical Communication System

Optical Amplifiers: The general form of an

optical amplifier is shown in Figure 2:

Pump Power

Fiber Fiber

Optical Signal In Optical Signal Out

Figure 2: Optical Amplifier

Optical amplifiers can be divided into two

classes: optical fiber amplifiers (OFA) and

semiconductor optical amplifiers (SOAs).

The former has tended to dominate

conventional system applications such as

in-line amplification used to compensate

for fiber losses. However, due to advances

in optical semiconductor fabrication

techniques and device design, the SOA is

showing great promise for use in evolving

optical communication networks. It can be

utilised as a general gain element but also

has many functional applications including

an optical switching and wavelength

conversion. These functions, where there

is no conversion of optical signals into the

electrical domain, are required in

transparent optical networks. The optical

OE Rx EO Tx

Optical AMP Medium


506

fiber amplifiers are EDFA and Raman

amplifiers.

2. Semiconductor Optical Amplifiers:

Semiconductor optical amplifiers (SOAs)

are essentially laser diodes, without end

mirrors, which have fiber attached to both

ends. They amplify any optical signal that

comes from either fiber and transmit an

amplified version of the signal out of the

second fiber. SOAs are typically

constructed in a small package, and they

work for 1310 nm and 1550 nm systems.

In addition, they transmit bidirectionally,

making the reduced size of the device an

advantage over regenerators of EDFAs [2].

However, the drawbacks of SOAs include

high-coupling loss, polarization

dependence, and a higher noise figure.

Figure 3 illustrates the basics of a

Semiconductor optical amplifier.

Figure 3: Semiconductor Optical

Amplifier

The gain of an SOA is influenced by the

input signal power and internal noise

generated by the amplification process. As

the output signal power increases the gain

decreases as shown in Figure 4. This gain

saturation can cause significant signal

distortion [3]. It can also limit the gain

achievable when SOAs are used as

multichannel amplifiers in wavelength

division (WDM) multiplexed systems.

3 dB

Gain (dB)

Po, sat

Output Signal Power

Figure 4: Typical SOA gain versus output

signal power.

SOAs are normally used to amplify

modulated light signals. If the signal

power is high then gain saturation will

occur. This would not be a serious

problem if the amplifier gain dynamics

were a slow process. However in SOAs the

gain dynamics are determined by the

carrier recombination lifetime (few

hundred picoseconds). This means that the

amplifier gain will react relatively quickly

to changes in the input signal power. This

dynamic gain can cause signal distortion,

which becomes more severe as the

modulated signal bandwidth increases.

These effects are even more important in

multichannel systems where the dynamic

gain leads to interchannel crosstalk [4].

This is in contrast to optical fiber

amplifiers, which have recombination

lifetimes of the order of milliseconds

leading to negligible signal distortion.

2.1. Applications:


507

The principal applications of SOAs in

optical communication systems can be

classified into three areas: (a)

Postamplifier or booster amplifier to

increase transmitter laser power, (b) in-line

amplifier to compensate for fiber and other

transmission losses in medium and long-

haul links and (c) preamplifier to improve

receiver sensitivity. These amplifiers have

been shown in Figure 5. The incorporation

of optical amplifiers into optical

communication links can improve system

performance and reduce costs. Booster amplifier Preamplifier

Transmitter In-line amplifier Receiver

Fiber

Figure 5: Optical amplifiers used for signal

amplification in optical fiber transmission.

(a) Booster or postamplifier :

The function of a booster amplifier is to

increase a high power input signal prior to

transmission. In long-haul links the use of

a booster amplifier can increase the link

power budget and reduce the number of in-

line amplifiers or regenerators required.

Booster amplifiers are also useful in

distribution networks shown in Figure 6,

where there are large splitting losses or a

large number of taps. Booster amplifiers

are also needed when it is required to

simultaneously amplify a number of input

signals at different wavelengths, as is the

case in WDM transmission.

Modulated Laser

Booster amplifier

Optical Receivers

Figure 6 : Booster amplifier application in

optical distribution networks

(b) In-Line amplifier: In-line optical

amplifiers can be used to compensate for

fiber loss across lengths of fiber cable,

such that optical regeneration of the signal

is unnecessary. Under regimes of linear

gain, where the bit rate is low enough such

that saturation is negligible, SOAs can be

used for in-line amplification. The

advantages of using SOAs for in-line are:

transparency to modulation format, bi-

directionality, WDM capability, low power

consumption and compactness.

(c) Preamplifier: The purpose of a

preamplifier is to increase the power level

of the optical signal prior to the detection

and demodulation by receiver. By using a

preamplifier, the sensitivity of the receiver

can be greatly increased. Similar to the use

of booster amplifiers, pre-amplification

can reduce the number of in-line amplifiers

needed over a distance of fiber. Receiver

units typically used in optical fiber

systems consists of an optical preamplifier,

a narrowband optical filter and a

photodiode used for detection. The signal

from the photodiode is then connected to

circuitry used for demodulation.

(d) Wavelength Conversion:

SOAs exhibit non-linear properties due to

carrier density changes induced by

differences in power of the input signal.

While these non-linear properties create

problems for the use of SOAs as simple


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linear gain elements, they can be exploited

to perform functions that are typically

carried out by electronic signal processing

circuits. All-optical wavelength converters

will play an important role in broadband

optical networks. They will be used

primarily to avoid wavelength blocking in

cross-connects in WDM systems [5]. In

packet switching networks, wavelength

converters can be used to change the

wavelength of certain signals so as to

avoid packet contention and reduce the

need for optical buffering. There are three

primary ways of exploiting the non-linear

properties of SOAs for wavelength

conversion: cross gain modulation (XGM),

cross phase modulation (XPM) and four-

wave mixing (FWM). Wavelength

conversion can induce by injecting a

strong signal with a harmonic modulation

at a certain angular frequency along with a

weaker data signal into an SOA. Due to

cross gain modulation (XGM), the

stronger signal will force the weaker signal

to its modulation [6]. The result is scheme

by which the wavelength of a signal can be

converted to that of another input signal

with a single wavelength as shown in

Figure 7.

Figure 7: Simple wavelength converter

using SOA XGM

Other applications of SOA are optical

gates and multiplexers. Future high speed

WDM and time division multiplexed

(TDM) optical networks will require high

speed all optical gates that can be either

optically or electronically controlled.

These optical gates can be implemented

using SOAs, where turning on or off the

current to the SOA can control the

functionality of the gate. Due to their

compact size and fitness for integration,

SOAs can be used to form gate arrays

3. EDFA :

A typical setup of a simple erbium-doped

fiber amplifier (EDFA) is shown in

Figure 8. Its core is the erbium-doped

optical fiber, which is typically a single-

mode fiber. In the shown case, the active

fiber is “pumped” with light from two

laser diodes (bidirectional pumping),

although unidirectional pumping in the

forward or backward direction (co-

directional and counter-directional

pumping) is also very common.

Figure 8 : Schematic setup of a simple

erbium-doped fiber amplifier.

The setup shown also contains two “pig-

tailed” (fiber-coupled) optical isolators.

The isolator at the input prevents light

originating from amplified spontaneous

emission from disturbing any previous

stages, whereas that at the output

suppresses lasing (or possibly even

destruction) if output light is reflected

back to the amplifier. Without isolators,

fiber amplifiers can be sensitive to back-

reflections. Apart from optical isolators,

various other components can be contained

in a commercial fiber amplifier [7]. For

example, there can be fiber couplers and

photodetectors for monitoring optical


509

power levels, pump laser diodes with

control electronics and gain-flattening

filters.

3.1. Energy Levels:

Pumping is primarily done optically with

the primary pump wavelengths at 1480 nm

and 980 nm. As indicated atoms pumped

to the 4I (11/2) 980 nm band decays to the

primary emission transition band. Pumping

with 1480 nm light is directly to the upper

transition levels of the emission band.

Semiconductor lasers have been developed

for both pump wavelengths. 10-20 mW of

absorbed pump power at these

wavelengths can produce 30-40 dB of

amplifier gain. Pump Efficiencies of 11

dB/mW achieved at 980 nm. Pumping can

also be performed at 820 and 670 nm with

GaAlAs laser diodes. Pump efficiencies

are lower but these lasers can be made

with high output power. 4I (11/2),

4I (13/2) and 4I (15/2) indicates states as

shown in Figure 9.

Figure 9: Energy Level Diagram of Er3+

The shape of the erbium gain spectrum

depends on the absorption and emission

cross sections, which depend on the host

glass. Also, the spectral shape of the gain

and not only its magnitude is substantially

influenced by the average degree of

excitation of the erbium ions, because

these have a quasi-three-level transition.

Figure 10 shows data for a common type

of glass, which is some variant of silica

with additional dopants e.g. to avoid

clustering of erbium ions. Other glass

compositions can lead to substantially

different gain spectra.

Figure 10: Gain and absorption (negative

gain) of erbium (Er3+

) ions in a phosphate

glass for excitation levels from 0 to 100%

in steps of 20%.

Strong three-level behavior (with

transparency reached only for > 50%

excitation) occurs at 1535 nm. In that

spectral region, the unpumped fiber

exhibits substantial losses, but the high

emission cross section allows for a high

gain for strong excitation. At longer

wavelengths (e.g. 1580 nm), a lower

excitation level is required for obtaining

gain, but the maximum gain is smaller.

The maximum gain typically occurs in the

wavelength region around 1530–1560 nm,

with the 1530-nm peak being most

pronounced for high excitation levels,

whereas low excitation levels lead to gain

maxima at longer wavelengths. The local

excitation level depends on the emission


510

and absorption cross sections and on the

pump and signal intensity (apart from that

of ASE light). The average excitation level

over the whole fiber length, as is relevant

for the net gain spectrum, depends on the

pump and signal powers, but also on the

fiber length and the erbium concentration

[8]. Such parameters (together with the

choice of glass composition) are used to

optimize EDFAs for a particular

wavelength region, such as the telecom C

or L band. A good flatness of the gain in a

wide wavelength region (→ gain

equalization), as required e.g. for

wavelength division multiplexing can be

obtained by using optimized glass hosts

(e.g. telluride or fluoride fibers, or some

combination of amplifier sections with

different glasses) or by combination with

appropriate optical filters, such as long-

period fiber Bragg gratings.

4. Raman Amplifier:

Raman optical amplifiers differ in

principle from EDFAs or conventional

lasers in that they utilize stimulated Raman

scattering (SRS) to create optical gain [9].

Initially, SRS was considered too

detrimental to high channel count DWDM

systems. Figure 11 shows the typical

transmit spectrum of a six channel DWDM

system in the 1550 nm window. Notice

that all six wavelengths have

approximately the same amplitude.

Figure 11 : DWDM Transmit Spectrum

with Six Wavelengths

By applying SRS the wavelengths, it is

obvious that the noise background has

increased, making the amplitudes of the

six wavelengths different. The lower

wavelengths have a smaller amplitude than

the upper wavelengths. The SRS

effectively robbed energy from the lower

wavelength and fed that energy to the

upper wavelength as shown in Figure 12.

Figure 12 : Received Spectrum After SRS

is on a Long Fiber

A Raman optical amplifier is little more

that a high-power pump laser, and a WDM

or directional coupler. The optical

amplification occurs in the transmission

fiber itself, distributed along the

transmission path. Optical signals are

amplified up to 10 dB in the network

optical fiber. The Raman optical amplifiers

have a wide gain bandwidth (up to 10 nm).

They can use any installed transmission

optical fiber [10]-[11]. Consequently, they

reduce the effective span loss to improve

noise performance by boosting the optical

signal in transit. They can be combined

with EDFAs to expand optical gain

flattened bandwidth. Figure 13 shows the

topology of a typical Raman optical

amplifier. The pump laser and circulator

comprise the two key elements of the


511

Raman optical amplifier. The pump laser,

in this case, has a wavelength of 1535 nm.

The circulator provides a convenient

means of injecting light backwards in to

the transmission path with minimal optical

loss.

Figure 13 : Typical Raman amplfier

Figure 14 illustrates the optical spectrum

of a forward-pumped Raman optical

amplifier. The pump laser is injected at the

transmit end rather than the receive end.

The pump laser has a wavelength of 1535

nm; the amplitude is much larger than the

data signals. Figure 14 shows example of

Raman Amplifier – Transmitted Spectrum

and Figure 15 shows example of Raman

Amplifier – Received Spectrum.

Figure 14 : Transmitted Spectrum

Figure 15: Received Spectrum

As before, applying SRS makes the

amplitude of the six data signals much

stronger. The energy from the 1535 nm

pump laser is redistributed to the six data

signals.

5. Comparison of SOA, EDFA and

Raman amplifiers:

Property SOA EDFA Raman

amplifier

Amplific

-ation

band

Depends

on pump

power

Depend

on

dopant

(Er, Y,

Th)

Depends

on pump

power

Gain

BW

60 nm ~ 90nm 20-25 nm

per pump

Flat

Gain

- - 15-20 nm

Noise

Figure

(dB)

8 5 5

Noise ASE ASE Raman

scatter,

double

Raleigh

Pump

wavelen-

gth

Electrica

-l pump

980/

1480

nm for

Erbium

By 100

nm

shorter

than

amplfied

signal

range

Pump

power

< 400

mW

~ 10-

300

mW

< 300

mW

Saturatio

-n power

Depends

on the

bias

current

Depend

on

dopant

and

gain

~ power

of pump

Directio-

n

Unidirec

-tional

Unidire

-ctional

Bidirecti-

onal

Simplici

-ty

Simpler More

comple-

x

Simpler

(no

special


512

fiber

needed)

Cost Low Mediu-

m

High

6. Conclusion:

SOAs offer certain advantages over more

commonly used optical fiber amplifier

such as low power consumption,

compactness and non-linear gain

properties. A particular attraction of

EDFAs is their large gain bandwidth,

which is typically tens of nanometers and

thus actually more than enough to amplify

data channels with the highest data rates

without introducing any effects of gain

narrowing [12]. A single EDFA may be

used for simultaneously amplifying many

data channels at different wavelengths

within the gain region; this technique is

called wavelength division multiplexing.

Before such fiber amplifiers were

available, there was no practical method

for amplifying all channels. The only

competitors to erbium-doped fiber

amplifiers in the 1.5-µm region are Raman

amplifiers, which profit from the

development of higher power pump lasers.

7. References:

[1]. Govind P. Agarwal, “Fiber Optic

Communication Systems”, John Wiley &

sons, Inc. Publication, 2003.

[2]. L. Guo and M.J. Connelly. “Signal

induced birefringence and dichroism in a

tensile-strained bulk semiconductor optical

amplifier and its application to wavelength

conversion”. IEEE Journal of Lightwave

Tech. Vol. 23, pp. 4037-4045, December

2005.

[3]. R.J. Manning et al, “Cancellation of

Nonlinear Patterning in Semiconductor

Amplifier Based Switches”, OTuC1,

Optical Amplifiers and their Applications,

Whistler, Canada, 2006.

[4]. R.Giller et al, “Analysis of the

dimensional dependence of semiconductor

optical amplifier recovery speeds”, Optics

Express, Vol. 15, No. 4, 2007.

[5]. Tuan Nguyen Van and Hong Do Viet,

“Enhancing Optical to Signal Noise Ratio

in Terrestria Cascaded EDFAs Fiber optic

Communication Links Using Hybrid Fiber

Amplifier”, IEEE 2009.

[6]. Shinichi Aozasa, Hiroji Masuda,

Makoto Shimizu and Makoto Yamada,

“Novel Gain Spectrum Control Method

Employing Gain Clamping and Pump

Power Adjustment in Thulium- Doped

Fiber Amplifier” Journal of Lightwave

Tech. Vol. 26, No.10, May 2008.

[7]. Lijie Qiao and Paul J. Vella, “ASE

Analysis and Correction for EDFA

Automatic control,” Journal of Lightwave

Tech. Vol. 25, No.3, May 2007.

[8]. Tadashi Sakamoto, Shin- ichi Aozasa,

Makoto Yamada and Makoto Shimizu,

“Hybrid Fiber Amplifier Consisting Of

Cascaded TDFA and EDFA for WDM

Signals”, Journal of Lightwave Tech. Vol.

24, No.6 , June 2006.

[9]. Gerd Keiser, “Optical Fiber

Communications”, 4th Edition, Tata

McGraw-Hill Education Pvt. Ltd., New

Delhi, Inc. 2009, ISBN-13: 978-0-07-

064810-4.

[10]. A. A. Rieznik and H. L. Fragnito,

“Analytical solution for the dynamic

behavior of erbium-doped fiber amplifiers

with constant population inversion along

the fiber,” J. Opt. Soc. Amer. B, Opt.

Phys., Vol. 21, No. 10, pp. 1732–1739,

October 2004.


513

[11]. Z.G. Lu, J.R. Liu, S. Raymond, P.J.

Poole, P.J. Barrios, D. Poitras, F.G. Sun,

G. Pakulski, P.J. Bock, and T.J. Hall,

“Highly efficient non-degenerate four-

wave mixing in InAs/InGaAsP quantum

dots,” Electronics Letters, Vol. 48, No. 19,

pp. 1112-1114 , 2006.

[12]. C. Jiang et al., “Improved gain

performance of high concentration Er3+

–

Yb3+

-codoped phosphate fiber amplifier”,

IEEE J. Quantum Electron. Vol. 41, No. 5,

pp. 704, 2005.

Biography

Bhawna Utreja was born in Patiala,

Punjab, India, on 20 April 1984. She

obtained her Bachelor’s degree in

Electronics and Communication

Engineering from Chitkara University,

Punjab, India, in 2007 and her Master’s

Degree in Electronics and Communication

Engineering from Thapar University,

Patiala, Punjab, India, in 2010. She joined

as a Lecturer in the Department of

Electronics and communication

Engineering at Rayat and Bahra Institute

of Engineering and Technology, Mohali,

India.

Hardeep Singh was born in Jalandhar,

Punjab, India, on 15 February 1974. He

obtained his Bachelor’s Degree in

Electronics Engineering from Baba Banda

Singh Engineering College Fatehgarh

Sahib (Punjabi University, Patiala),

Punjab, India, in 1997 and his Master’s

Degree in Electronics Engineering from

Panjab University, Chandigarh, India, in

1999. He joined as a Lecturer in the

Department of Electrical and Electronics

Engineering at Thapar University, Patiala

(Formerly known as Thaper Institute of

Engineering and Technology, Patiala) in

July 1999. He obtained his Ph.D. degree

from Thapar University, Patiala, in 2007

during lecturer ship. In 2009 he became

Assistant Professor in the Department of

Electronics and Communication

Engineering. Presently he is working as

Assistant Professor in the same

department. His present interests are Fiber

Dispersion and Nonlinearities. He has over

30 research papers in Conferences and

Journals. His web site is located at

http://www.thapar.edu.

Documents

Comparison Optical Amplifiers Optical Communication