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Studies on Placement of SemiconductorOptical Amplifiers in Wavelength Division

Multiplexed Star and Tree Topology Networks

by

Yatindra Nath Singh

submitted

in fulfilment of the requirement of degree ofDoctor of Philosophy (Ph.D.)

to

Electrical Engineering DepartmentIndian Institute of Technology, Delhi

Hauz Khas, New Delhi 110016India

September 1996

Certificate

This is to certify that the thesis entitled"Studies on Placement of Semiconductor Optical

Amplifiers in Wavelength Division Multiplexed Star and Tree Topology Networks"being

submitted by Mr. Yatindra Nath Singh to the Department of Electrical Engineering, Indian

Institute of Technology, Delhi is the record of the bonafide research work carried out by him.

He has worked under our supervision and guidance during the period December 1992 to

August 1996. He has fulfilled all the requirements for submission of the thesis which has

reached the requisite standard.

The results contained in this thesis have not been submitted either in part or in full to

any other university or institute for the award of any degree or diploma.

(Prof. Hari Mohan Gupta)

(Dr. Virander Kumar Jain)Thesis Supervisors

Department of Electrical EngineeringIndian Institute of Technology, Delhi

Hauz Khas, New DelhiIndia

i

Acknowledgement

I am highly indebted to my thesis supervisors Prof. Hari Mohan Gupta and

Dr. Virander Kumar Jain for giving me the opportunity to work under their supervision. They

had encouraged me in period of distress and anxiety and guided me to accomplish this

research work.

I must acknowledge the staff of optical communication laboratory, IIT Delhi

(Mr. A. P. Thukral, Ms. Neeru Asija and Mr. J. P. Naudiyal) for their support and help during

the research work.

My parents had kept me free from my duties at home so that I can devote more time

to my research work. I acknowledge them for their blessings and support.

My friends Mr. Brejesh Lall and Mr. Bharat Gupta must also be acknowledged for

their patience while listening to me about my research work and for their suggestions and

encouragement.

I sincerely thank all the persons who have helped and supported me directly or

indirectly in the course of this research work.

(Yatindra Nath Singh)

iii

Abstract

This thesis is mainly concerned with the use of semiconductor optical amplifiers

(SOAs) in wavelength division multiplexed (WDM) broadcast topologies viz. star and tree.

The aim of investigations is to determine the increase in the number of users when SOAs are

placed in the above topologies. Various placement options have been considered in the above

investigations.

Error correcting codes can also be used in optical networks to increase the number of

users by improving the available power budget. Therefore, an on-off keying (OOK)

communication system has been investigated with SOA as preamplifier and an error

correcting code. The coding gain in the single channel point-to-point link has been computed.

In this study, the baseband filter in the receiver is assumed to be optimum for uncoded

system. Hence, the coding gain in the above includes the effect of increased intersymbol

interference (ISI). It is compared with the gain obtained due to placement of SOA in the link.

The suitability of SOA over coding has been discussed based on the above comparison. It is

shown that SOA provides much higher gain than the use of error correcting code.

A detailed study on the use of SOAs in WDM star topology is also carried out. Two

schemes have been considered viz. SOAs as postamplifiers and preamplifiers. In the

postamplifier scheme, SOAs are placed after the transmitter and in the preamplifier scheme

before the receiver. In the postamplifier scheme, three cases have been investigated. First case

corresponds to unsaturated SOAs, second to gain saturated SOAs and in third case effect of

reflection noise is studied. Similarly, in the preamplifier case unsaturated SOAs, average gain

v

saturated SOAs with and without gain fluctuations have been considered. Further, reflection

noise is considered as in postamplifier scheme. It is observed that for typical values of system

parameters, SOAs as preamplifiers perform better than the postamplifiers.

The study on placement of SOAs has been extended to WDM tree topology passive

broadcast network i.e. WDM tree-net. The tree-net consists of star as main topology and

folded bus as auxiliary topology. Star portion consists of a star coupler. A tree-net with

b branches usesbxb star coupler. The SOAs can be placed in the star portion of the tree-net

to increase the supportable number of users. However, the number of SOAs can be smaller

than b to support the same number of users as in star. In this study unsaturated SOAs and

average gain saturated SOAs with and without gain fluctuations have been considered. On

comparison with star, it is observed that a tree-net can support more users than a star for a

given number of SOAs.

It is concluded that SOAs as preamplifiers perform better in star topology. When

SOAs are used in tree topology, number of users supported can be more than in star. Further,

the required number of SOAs would be lesser in tree than in star for a given number of users.

vi

Table of Contents

List of Figures ix

List of Tables xiii

List of symbols xv

Abbreviations xix

Chapter 1Introduction 1

1.1 Need for Optical Communications. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Fiber Optic Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Multiplexing Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4 WDM Broadcast Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.5 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Chapter 2Optical Amplifiers in Broadcast Networks 13

2.1 Optical Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2 Optical Amplifiers in Broadcast Networks . . . . . . . . . . . . . . . . . . . . . 20

2.2.1 Bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.2 Dual Bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.3 Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.2.4 Star . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Chapter 3Semiconductor Optical Amplifier and Coding in OOK System 37

3.1 Uncoded System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.2 Coded System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.3 Uncoded System with Optical Amplifier . . . . . . . . . . . . . . . . . . . . . . 453.4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Chapter 4Semiconductor Optical Amplifiers in WDM Star Topology 51

4.1 System Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524.2 Amplifier Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544.3 Evaluation of Minimum Required Transmitter Power . . . . . . . . . . . . 564.4 Star without Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 564.5 Star with Postamplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4.5.1 Unsaturated Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.5.2 Effect of Gain Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . 624.5.3 Effect of Reflection Noise. . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.6 Star with Preamplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.6.1 Unsaturated Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

vii

4.6.2 Effect of Gain Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.6.3 Effect of Reflection Noise. . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.7 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Chapter 5Semiconductor Optical Amplifiers In WDM Tree-net 91

5.1 System Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 925.2 Analytical Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965.3 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Chapter 6Conclusions 109

References 113

Appendix - I 123

Appendix -II 129

viii

List of Figures

Fig.2-1 psds of (i) ASE-signal beat noise, (ii) ASE-ASE beat noise and (iii)ASE-shot noise components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Fig.2-2 Forward pumped doped fiber amplifier. . . . . . . . . . . . . . . . . . . . . . 18

Fig.2-3 Optical amplifiers in bus topology.. . . . . . . . . . . . . . . . . . . . . . . . . 21

Fig.2-4 Dual bus network using fiber as point-to-point link.. . . . . . . . . . . . . 23

Fig.2-5a Schematic of non-regenerative photonic dual bus (NI is the nodeinterface).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Fig.2-5b Node interface for head of the bus (node 1).. . . . . . . . . . . . . . . . . . . 24

Fig.2-5c Node interface for node 2 to N.. . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Fig.2-6 Ring network with distributed EDFA.. . . . . . . . . . . . . . . . . . . . . . . 25

Fig.2-7 4x4 star coupler with single EDFA.. . . . . . . . . . . . . . . . . . . . . . . . . 27

Fig.2-8 8x8 coupler based on 4x4 star couplers.. . . . . . . . . . . . . . . . . . . . . . 27

Fig.2-9 Implementation of distributedNu x Nu (Nu=m2) reflective star couplerbased onm number ofm x m couplers. . . . . . . . . . . . . . . . . . . . . . . 28

Fig.2-10 16x16 distributed reflective star coupler.. . . . . . . . . . . . . . . . . . . . . 29

Fig.2-11 Implementation of distributedNu x Nu (Nu= 2 m2) coupler based on 2mnumber ofmxm couplers.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Fig.2-12 32x32 distributed reflective star coupler.. . . . . . . . . . . . . . . . . . . . . 31

Fig.2-13 FDM/OFDM distributed expandable amplified star couplerconfiguration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Fig.2-14 Star coupler with amplifiers and without bandpass and bandstopfilters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Fig.2-15 8x1 coupler based on seven 2x2 couplers.. . . . . . . . . . . . . . . . . . . . 34

Fig.2-16 Forward pumped EDFA.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Fig.2-17 mmc x mmc reflective star. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Fig.2-18 mc x mc reflective star coupler based onmcx1 tree coupler.. . . . . . . . . 36

Fig.3-1a The system block diagram for optical communication system withcoding scheme.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Fig.3-1b The system block diagram for optical communication system with SOAas preamplifier.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Fig.3-2 Worst-case waveform patterns for (a) bit 1 and (b) bit 0 before andafter the Gaussian filter.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

ix

Fig.3-3 Variations of BER vs received power level (dBm) for (i) uncodedsystem, (ii) coded system without decoding, (iii) coded system withdecoding and (iv) uncoded system with SOA as preamplifier.. . . . . . . 49

Fig.4-1 An 8x8 star coupler consisting of 3 dB 2x2 couplers.. . . . . . . . . . . . 53

Fig.4-2 Star network without optical amplifier.. . . . . . . . . . . . . . . . . . . . . . . 58

Fig.4-3 Star network with postamplifiers (A, B, ...F is echo path).. . . . . . . . . 60

Fig.4-4 Star network with preamplifiers (A, B, ...G is echo path).. . . . . . . . . 77

Fig.4-5 Variations of minimum required average transmitter powerPta withnumber of usersNu for ε = 0.0 in postamplifier scheme. . . . . . . . . . . 80

Fig.4-6 Variations of minimum required average transmitter powerPta withnumber of usersNu for ε = 0.05 in postamplifier scheme. . . . . . . . . . 81



Fig.4-9 Variations of minimum required average transmitter powerPta withnumber of usersNu for ε = 0.0 in preamplifier scheme. . . . . . . . . . . 84




Fig.4-13 Variations ofPta with ε for different Nu in postamplifier scheme. Solidlines correspond to gain saturated SOAs and dashed lines to gainsaturated SOAs with reflection noise.. . . . . . . . . . . . . . . . . . . . . . . . 88

Fig.4-14 Variations ofPta with ε for different Nu in preamplifier scheme. Solidlines correspond to gain saturated SOAs and dashed lines to gainsaturated SOAs with reflections.. . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Fig.5-1 Basic tree-net topology.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Fig.5-2a Structure of star coupler in tree-net without amplifiers.. . . . . . . . . . . 93

Fig.5-2b Structure of branches in a tree-net without amplifiers.. . . . . . . . . . . . 94

Fig.5-3a A 4x4 star coupler with one amplifier in tree-net.. . . . . . . . . . . . . . . 94

Fig.5-3b A 4x4 star coupler with two amplifiers in tree-net.. . . . . . . . . . . . . . 95

Fig.5-3c A 4x4 star coupler with four amplifiers in tree-net.. . . . . . . . . . . . . . 95

x

Fig.5-4a Variations ofNu with Na (unsaturated SOAs) forn=2. The values in thebrackets are corresponding requiredPta in dBm. . . . . . . . . . . . . . . . 102

Fig.5-4b Variations ofNu with Na (unsaturated SOAs) forn=3. The values in thebrackets are corresponding requiredPta in dBm. . . . . . . . . . . . . . . . 102

Fig.5-5a Variations ofNu with Na (average gain saturated SOAs) forn=2. Thevalues in the brackets are corresponding requiredPta in dBm. . . . . . . 103

Fig.5-5b Variations ofNu with Na (average gain saturated SOAs) forn=3. Thevalues in the brackets are corresponding requiredPta in dBm. . . . . . . 103

Fig.5-6a Variations ofNu with Na (average gain saturated SOAs with gainfluctuations) forn=2. The values in the brackets are correspondingrequiredPta in dBm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Fig.5-6b Variations ofNu with Na (average gain saturated SOAs with gainfluctuations) forn=3. The values in the brackets are correspondingrequiredPta in dBm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Fig.A-1 Reflected and transmitted signals in a Fabry-Perot amplifier.. . . . . . 124

xi

List of Tables

Table 4-1 Number of users supported in the postamplifier scheme forε=0.10. . . 86

Table 4-2 Number of users supported in the preamplifier scheme forε=0.10. . . . 90

Table 5.1 Number of users and requiredPta for the tree-net without SOAs. . . . 105

Table 5.2 Maximum number of users andPta for a given number of SOAs.Numbers in brackets are the corresponding values ofn. . . . . . . . . . . 107

Table 5.3 Comparison of star network and tree-net in terms of number of SOAsrequired for a given number of users.. . . . . . . . . . . . . . . . . . . . . . 108

xiii

List of symbols

a0(τ) Worst-case waveform for bit 0 in the presence of instersymbol interference(ISI)

a1(τ) Worst-case waveform for bit 1 in the presence of ISI

b Number of branches in the tree-net

bc Bit in the desired channel

br Bit whose echo produces the reflection noise

B Bit rate per user

Be Bandwidth of electrical filter in receiver

Bo Optical filter/demultiplexer bandwidth

Cf Velocity of light in fiber

Dth Decision threshold level

e Electronic charge

eecho Echo signal electric field

er Signal electric field at the receiver

erfc(x)

Ei Input electric field amplitude at Fabry-Perot cavity

Eo Output electric field amplitude at Fabry-Perot cavity

Er Reflected electric field amplitude from Fabry-Perot cavity

g Gain coefficient of SOA

go Unsaturated gain coefficient of SOA

G Amplifier gain

G0 Unsaturated amplifier gain

Gav Average gain of SOA when multiple channels are being amplified

GFP Gain of F-P amplifier

Gp Gain of SOA when signal corresponding to bitbr is amplified

Gp(br) Gain of SOA when signal corresponding to bitbr is amplified

h Planck’s constant

xv

h(τ) Impulse response of electrical filter in the receiver

H(f) Frequency response of electrical filter in the receiver

iecho-sig Echo and signal beat noise current

Isig Signal current at photodetector

k Number of data symbols in a coded block

kb Boltzmann’s constant

L Length of fiber from user to star coupler

Las1 Loss between SOA and splice 1

Lc Coherence length of laser linewidth

Lcp Amplifier coupling loss

Lcv Star coupler variability

Li Insertion loss of 3 dB 2x2 coupler

Lfi Insertion loss of filter or demultiplexer

Lg Length of gain medium

Lsp Fiber splice loss

Lss Distance between two consecutive users on a branch in the tree-net

Lti Total insertion loss of star coupler

Ltr Loss between transmitter and receiver

mc Size of central coupler

M Maximum number of users on a bus segment without SOA

n Number of users per branch in the tree-net

nsp Spontaneous emission factor

N Number of symbols in a block

Na Number of amplifiers

Nu Number of users (stations)

P Optical power

Pe Probability of error

Pes(ad) Probability of symbol error after decoding

Pes(bd) Probability of symbol error before decoding

Pin Input optical power to SOA

Pint Total input power to amplifier

xvi

Pout Output power of SOA

Pr Received optical signal power at the photodetector

Pr(0) Received optical signal power at the photodetector for bit 0

Pr(1) Received optical signal power at the photodetector for bit 1

Pra Average received optical power at the photodetector

Psat Saturation power level of SOA

Pt Transmitted optical power

Pta Average transmitted optical power

Pt(0) Transmitted optical power for bit 0

Pt(1) Transmitted optical power for bit 1

R Data rate

R1 Reflection coefficient of facet 1

R2 Reflection coefficient of facet 2

Ramp Reflectivity of SOA

Rf Facet reflectivity of SOA (Rf = R1 = R2)

RL Receiver load resistance

Ro Responsivity of photodetector

Rsp Reflection coefficient of splice

S0 Signal level for bit 0 at timet=0 for worst-case bit sequence in presence of ISI

S1 Signal level for bit 1 at timet=0 for worst-case bit sequence in presence of ISI

SASE-shot(f) psd of shot noise due to ASE

SN(f) psd of additive Gaussian noise

Sshot psd of shot noise

Ssp Single sided ASE noise psd due to amplifier

Sspi psd of noise at the receiver due toith amplifier in postamplifier scheme in starnetwork

Sth psd of thermal noise

t Number of correctable errors in a coded block

T Absolute temperature in Kelvin

Tamp Transmittance of amplifier cavity

Tb Bit period

Tmax Maximum value ofTamp

xvii

W1 Wavelength band 1 in WDM/FDMA system

α Attenuation coefficient of fiber in dB/km

β Propagation constant of electric field in a given media

ε Extinction ratio

∆ν Laser linewidth

∆νL Longitudinal mode spacing of FP cavity

η Quantum efficiency of photodetector

λ Operating wavelength

λc Wavelength for clock broadcast in photonic dual bus

λd Wavelength for data broadcast in photonic dual bus

λp Pump wavelength

λs Signal wavelength

ν Optical frequency

νm Resonant frequency ofmth peak in FP cavity transmission spectrum

σ2(1) Noise variance for bit 1

σ2(0) Noise variance for bit 0

σ2(bc) Noise variance for bitbc

σ2ASE-ASE ASE-ASE beat noise variance

σ2ASE-shot Shot noise variance due to ASE noise

σ2ASE-sig ASE-signal beat noise variance

σ2n Noise variance

xviii

Abbreviations

ASE Amplifier Spontaneous Emission

BER Bit Error Rate

BPF Band Pass Filter

BSF Band Stop Filter

CDM Code Division Multiplexing

CDMA Code Division Multiple Access

dc Direct Current

DFA Doped Fiber Amplifier

DQDB Distributed Queue Dual Bus

EDFA Erbium Doped Fiber Amplifier

FCS Fiber Channel Standard

FDDI Fiber Distributed Data Interface

FDM Frequency Division Multiplexing

FPA Fabry-Perot Amplifier

FT-FR Fixed Transmitter-Fixed Receiver

FT-TR Fixed Transmitter-Tunable Receiver

HIPPI HIgh Performance Parallel Interface

ISI Inter Symbol Interference

LAN Local Area Network

MAC Media Access Control

MAN Metropolitan Area Network

OA Optical Amplifier

O/E Optical-to-Electronic

OOK On-Off Keying

psd Power Spectral Density

RATO Random Access Time Out

SAN Subscriber Access Network

SDH Synchronous Digital Hierarchy

SOA Semiconductor Optical Amplifier

xix

SONET Synchronous Optical NETwork

TDM Time Division Multiplexing

TT-FR Tunable Transmitter-Fixed Receiver

TT-TR Tunable Transmitter-Tunable Receiver

TWA Travelling Wave Amplifier

WAN Wide Area Network

WDM Wavelength Division Multiplexing

xx

Chapter 1

Introduction

1.1 Need for Optical Communications

Technical breakthroughs are affecting the human society for ages. It is well known that

discovery of fire, agriculture and wheel has made the human beings superior over all the other

species on the earth. In the last two centuries, industrial revolution started by the invention

of steam engine by James Watt has changed the face of humanity. Nowadays, we are

witnessing information revolution which is affecting the humanity.

Information revolution implies that the information can be presented, analyzed,

transported in an efficient manner. Computers are facilitating the presentation and analysis

of information, while the transportation of information is achieved using communication

networks. With the passage of time, communication networks are becoming independent of

type of information. Now, the information moving over the networks can be voice, video,

computer data or text. The networking for supporting such information is termed as

multimedia networking. Multimedia networks are designed to carry a large amount of

information bits per second. The high bit rates are especially needed by the upcoming high

bandwidth real-time video services [58] (e.g. video conferencing and video on demand). At

present, optical fiber is the only transmission medium offering such large bandwidth

[27, 28, 39]. Optical fiber also provides the low loss communication links as compared to

1

radio or electrical cables. In addition to large bandwidth and low loss, there are many other

advantages of using optical fiber. Optical fibers are immune to electromagnetic interference;

comparatively lighter and cheaper than copper cables required to carry the same amount of

information. As the transmission of information is through dielectric media, transmitting and

receiving ends are electrically isolated. Optical links are more reliable and can support future

application due to inherently large available capacity.

Optical fiber links are best suited for fixed user locations. These links require

specialized tools and skills for installation and maintenance. Further, the technology of optical

devices and components, especially optical sources and detectors is still evolving to match

the system requirements.

Optical fiber is finding increasing use in the communication networks due to above

advantages [42]. The telecommunication service providers have laid down the optical fiber

links on the land and in the oceans for trunk traffic. Optical fibers are finding increasing use

in subscriber loop too [20, 43, 52, 65, 85, 95]. It is also used in computer networking [6, 66].

1.2 Fiber Optic Networks

Optical fiber has found use in switched communication networks by replacing the

other media viz. microwave, electrical cable. Nowadays, almost all the trunk lines of existing

networks are using optical fiber. This large scale use of optical fiber has led to the

development of synchronous digital hierarchy (SDH)/ synchronous optical network (SONET)

standards for switched networks [35, 73, 82]. In future, the link between the user and the

2

switch i.e. subscriber loop will also be based on fiber. Consequently, new services (e.g.

videophony, video on demand) can be made available by telecommunication service providers.

The fiber has also replaced the media in the broadcast and select networks which are

commonly used as local area networks (LANs). Consequently, ethernet, token bus and token

ring have been implemented on optical fiber. As the fiber is used to replace conventional

media, these networks should be designed to harness the full fiber capabilities. The broadcast

networks can be classified as active or passive. When the signal is optoelectronically

regenerated or amplified, the network is active broadcast network otherwise it is passive

broadcast network. Ethernet and token bus are the examples of passive broadcast network,

while token ring is of active broadcast network. In passive broadcast networks, the transmitted

optical power is split for distribution among the receivers. Therefore, increase in number of

users implies that each receiver will receive reduced amount of optical power for a fixed

transmitted power level. The reduction in received optical power level will worsen the

receiver performance. To keep the receiver performance acceptable, the number of users have

to be limited. Such a limitation on number of users does not exist in active broadcast

networks, but the active broadcast networks suffer with other problems e.g. low reliability,

higher cost etc..

New networks e.g. fiber distributed data interface (FDDI) [25, 26, 74] and distributed

queue dual bus (DQDB) have also been deployed with optical fiber as media. These networks

operate at higher bit rates and cover larger distances. FDDI uses ring topology. It contains

two counter propagating rings for high reliability. In case of failure of a node or a link, the

fault is bypassed using the other ring. DQDB uses dual bus or dual ring as topology. Each

3

of the two bus or ring is unidirectional and carries data in opposite directions. In the above

networks, optical fiber is used as point-to-point link connecting two consecutive nodes.

However, it is possible to use fiber as passive broadcast media by using passive taps in

DQDB network. These networks operate at bit rates of the order of 100 Mb/s due to optical

media. The network standards with still higher bit rates (of the order of Gb/s) have been

developed e.g. fiber channel standard (FCS) and high performance parallel interface (HIPPI).

HIPPI uses multiple parallel links, each of which is operating at bit rates of the order of 100

Mb/s. The parallel links can use electrical cable for shorter distances (few meters) or optical

fiber for longer distances (few kms). Further increase in capacity of broadcast networks for

efficient utilization of optical fiber has been hindered by electronic bottleneck. The transmitter

and receiver become extremely costly with the increase in bit rates above Gb/s. At high bit

rates, optical multiplexing and demultiplexing techniques are used to attain large capacity

networks. Large capacity switched optical networks are also possible [34, 93]. In these

networks, many switches are interconnected. The signal remains optical from transmitting to

receiving end. Therefore, switching is optical in nature and optical multiplexing and switching

techniques will be used for increasing the capacity [2].

1.3 Multiplexing Schemes

The bandwidth requirement is expected to increase with time. Therefore, research in

optical fiber networks is directed towards attaining high capacity optical networks. As

mentioned above, high capacities in these networks can be achieved by using various

multiplexing techniques. There are three possible approaches: (i) wavelength division

4

multiplexing (WDM), (ii) time division multiplexing (TDM) and (iii) code division

multiplexing (CDM). Combinations of the above can also be used.

A WDM system is similar to frequency division multiplexed (FDM) radio broadcasting

system. The optical spectrum is sliced in multiple channels centred at different wavelengths

[8, 9, 53, 77, 78, 79]. Ideally, the information in various channels do not interfere with each

other. However, interchannel interference may occur when the information passes through

optical amplifiers (OAs) or any other nonlinear device in the link. Implementation of WDM

systems require couplers [69], wavelength multiplexers and demultiplexers, tunable or fixed

wavelength sources and detectors capable of operating over the entire wavelength range. Also

the various parameters characterizing the OAs, couplers, fibers etc. are desired to be uniform

in the operating wavelength range. There are many issues which must be tackled in a WDM

system. Two of these are: (i) wavelength stability of sources and (ii) channel spacing. If the

transmitter source wavelength is not stable, it may drift towards the neighbouring wavelength

channels. Consequently, channels spacing will reduce resulting in crosstalk. Various

wavelength stabilizing schemes have been proposed to avoid drift [70]. Most of these schemes

compare the transmitting wavelength with a reference and adjust the transmitting source

wavelength continuously. Channel spacing is an important issue [81]. Smaller spacing implies

more channels in a given spectral range. But the demultiplexers resolution decides the

spacing. For smaller channel spacing, narrow bandwidth filters are required. When coherent

receivers are used, channel spacing can be very small (2 to 4 times the signal bandwidth) as

compared to when noncoherent receivers are used [10, 11]. The WDM systems with a smaller

channel spacing are termed as optical FDM systems [33].

5

In TDM systems, optical pulses from various sources carrying the information are time

multiplexed [4, 41]. In these systems, the pulse width reduces with the increase in number

of users when bit rate per user is fixed. It must be≤ 1/(Nu·B) whereNu is number of users

andB the bit rate per user. Such a system requires the optical sources which generate very

narrow width optical pulses. Generally, mode locked lasers can be used for this purpose. The

output of these are then externally modulated depending upon the information to be

transmitted. If internal modulation is used on commonly used laser sources, pulse compression

has to be applied to generate narrow pulses. Such a compression is generally achieved using

nonlinear processing. In optical TDM, the pulses from various sources require very tight delay

adjustment so that pulses can be multiplexed together. Further, exact timing information is

required at all the receivers for demultiplexing [3] which can be obtained by centralized clock

or synchronized distributed clocks. Therefore, implementation of optical TDM system requires

tight clock jitter control and clock synchronisation at very high frequencies (≥ 1 THz).

In CDM systems, each user is assigned a unique waveform code to represent the

information bit [1, 21, 51]. The codes are selected in such a manner that code sequence of

each user has very small correlation with code sequences of all the other users. Thus codes

are equivalent to independent channels. At the receiving end, optical correlators are used for

identifying the information destined for it. As the number of waveform codes are equal to

number of users, increase in number of users implies more number of pulses in waveform

code for a bit. Consequently, for a given bit rate the pulse width will decrease with the

increase in number of users. In a code division multiple access (CDMA) broadcast network

having more than 100 users, bit rate of 1 Gb/s per user implies pulses of the order of

10 picoseconds. These short pulses can be generated by a mode locked laser. With these

6

narrow pulses and proper optical delay lines, suitable waveform code can be generated [40].

At the receiver end, delay lines are used to decode the required waveform. In a broadcast

situation either encoders or decoders or both can be tunable. Each waveform code is used as

an independent channel. In encoders and decoders, optical fiber delay lines which require tight

fiber length tolerances are used. To make the delay lines, tunable switching can be used. Fine

tuning of delay can be achieved using Piezoelectric tuning of delay lines.

It may be mentioned that for implementing a WDM system, fixed/tunable sources,

wavelength multiplexers and demultiplexers and fixed/tunable receivers are required. These

devices are available at the current state of the art technology [5, 15, 16, 29, 56].

In comparison, implementation of encoders/decoders and time multiplexers/demultiplexers at

Tb/s is very difficult. Therefore, WDM implementation is comparatively easier than that of

TDM and CDM [13]. It is expected that WDM switched as well as broadcast networks will

dominate the future for quite some time. As WDM systems can provide large bandwidth,

complexity of switched network can be avoided by using broadcast network topologies.

1.4 WDM Broadcast Networks

Many issues must be investigated for implementing a WDM broadcast network. Some

of the important issues are: (i) topology, (ii) tunability characteristics of transmitter,

(iii) tunability characteristics of receiver, (iv) size of the network, (v) number of channels and

(vi) media access control protocols. Some issues are inter related also.

7

Fiber layout to connect the different users constitute the physical topology of the

networks [66]. A physical broadcast topology must have some desirable features. A few of

these are given below.

(i) In a WDM system, there must be large number of users in the topology to utilize the

available bandwidth. Ideally, each receiver must receive minimum required power for

a given bit error rate (BER) to maximize the number of users supported. A topology

must also provide each receiver with equal amount of power so that receivers with

smaller dynamic range can be used. Sometimes, equal distribution of power can also

lead to more number of users (e.g. star topology supports more users than the bus).

(ii) Fiber used in the network topology must be minimum. This is an important cost

consideration.

(iii) Transmitter to receiver loss within the topology must be minimum. This would mean

more power is available to receivers. Therefore, number of users in the network can

be increased.

(iv) Topology must be extendible. It means that number of users can be increased easily.

When the above is not met, the topology is designed for more number of users than

actually present. The unused ends are latter assigned to the new users. It is also

desired that media access control (MAC) protocol is independent of number of users

in the network.

8

(v) WDM optical network are generally very high speed networks [12]. In such networks,

transmission time is much less than the propagation time. Therefore, the average

propagation time in the network must be minimized while selecting a topology. This

is extremely important for better performance.

(vi) Minimization of number of couplers, multiplexers and demultiplexers, transmitters and

receivers and optical amplifiers in the network topologies is also required. This is an

important cost reduction measure in a large network.

In the broadcast network topologies, star, bus, tree or ring can be used. Among these,

star is the best option as it can support maximum number of users for a given power budget.

The reason for the above is uniform distribution of transmitted power by a star coupler.

In WDM networks, efficient methods (i.e. MAC protocols) for accessing the

bandwidth are desired [31, 36, 44, 45, 60, 64, 71, 103]. In these protocols, contention for the

wavelength channels to be used and time when the packet is to be transmitted are resolved.

There are several possibilities depending on the tunability of transmitters and receivers. One

can have fixed transmitter - fixed receiver (FT-FR), fixed transmitter - tunable receiver

(FT-TR), tunable transmitter - fixed receiver (TT-FR) and tunable transmitter - tunable

receiver (TT-TR). The protocols for the cases when tunable components are used are required

to take into consideration the tuning range and tuning time of the tunable components. When

the tunable range of components is small, it may not be possible to implement the MAC

protocol on all the channels simultaneously. Therefore, the protocol needs to be modified

depending upon the number of channels covered by the tunable range.

9

High capacity WDM networks [97] should be able to support large number of users.

At the current level of technology, the broadcast optical networks support very few users

(≤ 64 users). The number of users can be increased by (i) increasing the power budget,

(ii) reducing the loss in the network, (iii) using the opto-electronic regenerators and (iv) using

optical amplifiers (OAs). The available power budget can be improved by using transmitters

with higher power sources and receivers with better sensitivity. Higher transmitter power and

better receiver sensitivity need improvement in device technology. The error control coding

can be used to provide coding gain and hence the improvement in the receiver sensitivity

which in turn increases the available power budget [38]. Coding techniques require the

encoder and decoder blocks in the transmitter and receiver respectively. The reduction of loss

in the network requires the improvement in splicing techniques and optical components. The

optoelectronic regenerators involve optical-to-electronic (O/E) conversion and vice-versa. This

reduces the reliability of the network as the regenerator is an active device. Further, the

upgradation of multichannel WDM network will require separate regenerator for each channel.

In contrast to the above, an optical amplifier can amplify multiple WDM channels.

1.5 Thesis Outline

This thesis is mainly concerned with the use of semiconductor optical amplifiers

(SOAs) in WDM broadcast topologies viz. star and tree. The aim of investigations is to

determine the increase in the number of users in the above topologies when SOAs are placed.

Further, various SOA placement options are studied. In chapter 2, the characteristics and

suitability of SOAs in networks has been reviewed. The relevant SOA model and parameters

have been presented.

10

In chapter 3, the improvement in an on-off keying (OOK) communication system due

to coding and semiconductor optical preamplifier has been investigated. The coding gain in

the single channel point-to-point link has been computed. In this study, the baseband filter is

assumed to be optimum [37] for uncoded system. Hence, the coding gain in the above

includes the effect of increased intersymbol interference (ISI). It is compared with the gain

obtained due to placement of SOA in the link. The suitability of SOA over coding has been

discussed based on the above comparison [91, 92, 96, 99, 102].

Chapter 4 contains a detailed study on the use of SOAs in WDM star topology. Two

placement schemes, postamplifier and preamplifier, have been considered. In the postamplifier

scheme, SOAs are placed after the transmitter and in the preamplifier scheme before the

receiver. In the postamplifier scheme, three cases have been investigated. First case

corresponds to unsaturated SOAs, second to gain saturated SOAs and in third case reflection

noise is incorporated. Similarly, in the preamplifier scheme unsaturated SOAs and average

gain saturated SOAs with and without gain fluctuations have been considered. Further, the

effect of reflection noise is also investigated like in postamplifier scheme [98, 100].

In chapter 5, the study on placement of SOAs is extended to WDM tree topology

passive broadcast network i.e. WDM tree-net. The tree-net consists of star as main topology

and folded bus as auxiliary topology. Star portion consists of a star coupler. The deployable

number of SOAs has been investigated in the tree-net. This number varies from 1 tob, where

b is number of branches in the tree-net. In this study, unsaturated SOAs and average gain

saturated SOAs with and without gain fluctuations have been considered [101].

11

In chapter 6, conclusions of the thesis are presented. The limitations and shortcomings

of approach in the thesis are discussed. Further possibilities of research and extensions have

been suggested.

12

Chapter 2

Optical Amplifiers in BroadcastNetworks

Optical amplifiers (OAs) can increase the number of users supported by a broadcast

network. Many investigations have been carried out to increase the network size using OAs.

The work reported in this chapter has resulted in the following publication

1. Y. N. Singh, V. K. Jain and H. M. Gupta, "WDM Data Network," Presented inIXth NationalConvention of Electronics and Telecommunication Engineers, University of Roorkee, Roorkee,India, March 30-31, 1994.

Some of the schemes are based on semiconductor optical amplifiers (SOAs) and others on

doped fiber amplifiers (DFAs). This chapter consists of review of OAs in broadcast

networks. Section 2.1 describes operational principles of OAs. Various types of OAs are

compared in this section. Specifically, SOAs are modelled for their subsequent analysis in

networks. The effectiveness of OAs in various schemes, as reported, has been discussed in

section 2.2.

2.1 Optical Amplifiers

Optical amplifier is a device which amplify the input optical signal. This device

works on the principle of stimulated emission [7]. There are two types of OAs which are

used in communication system; (i) semiconductor optical amplifiers (SOAs) and (ii) doped

13

fiber amplifiers (DFAs). The SOAs are basically semiconductor lasers which operate below

lasing threshold [7]. These devices require population inversion so that stimulated emission

and hence amplification can take place. The population inversion is achieved by means of

electrical energy. A SOA has two facets and reflectivities of these facets decide whether

the device will operate as a SOA or a semiconductor laser. When the facet reflectivities are

zero, input signal passes through the device only once. Such an amplifier is called travelling

wave amplifier (TWA). When reflectivities are non-zero but quite small, the signal passes

through the cavity several times. The signal within the cavity reduces with the successive

passes and dies out ultimately. This type of SOA is called Fabry-Perot amplifier (FPA).

In SOAs, gain of amplifier reduces as the input signal power increases. This

phenomenon is called gain saturation. The gain coefficientg is given by [30]

(2.1)

where g0 is the unsaturated gain coefficient,Psat the saturation power level,P the optical

power to the amplifier. The rate of increase of optical power with distance is given by

(2.2)

14

For a TWA, the output power is obtained by using Eqn.2.1 in Eqn.2.2 and integrating

Eqn.2.2 from 0 toLg with initial condition P(0)=Pin, whereLg is the length of gain medium

and Pin the input optical power to SOA. The output power is obtained as

(2.3)

whereG0 (= exp (g0L)) is unsaturated amplifier gain. The amplifier gainG (=Pout/Pin) from

the above equation is given by

(2.4)

A semiconductor laser amplifier alongwith the amplified signal, also produces

amplified spontaneous emission (ASE) noise. It is modelled as a white Gaussian noise with

single sided power spectral density (psd) [30]

(2.5)

where nsp is the spontaneous emission factor,h the Planck’s constant andν the optical

frequency. Typically, the unsaturated gain,G0, of SOAs is in the range 8 - 29 dB and the

Psat in the range 3.1 - 15.6 dBm. The noise figure for SOAs ranges from 6 to 8.5 dB which

corresponds tonsp of 2 - 3.5 [19, 47, 62, 87]. When the amplified signal and accompanying

ASE noise are made to incident at the photodetector, the noise beats with itself and also

with the signal. Thus ASE-ASE and ASE-signal beat noise components are produced. The

15

psds of the above noise components and ASE shot noise which arises due to dc component

of ASE-ASE beat noise are shown in Fig.2-1 [63]. In this figure,Bo is the bandwidth of

optical filter used for limiting the ASE noise of OA,Ro the responsivity of photodetector

and Pr the received optical power level at the photodetector.

Fig.2-1 psds of (i) ASE-signal beat noise, (ii) ASE-ASE beat noise and (iii) ASE-shotnoise components

In a FPA, the gain is given by [30]

(2.6)

In the above equation,R1 and R2 are the facet reflectivities,G(ν) the gain spectrum when

the reflectivities are zero,∆νL the longitudinal mode spacing andνm the resonant

16

frequencies for the cavity. When the FPA can be approximated as TWA

[30].

In DFAs, optical fiber core is doped by rare earth elements e.g. erbium [17, 18, 55],

praseodymium [48, 88] etc.. These amplifiers like SOAs also use stimulated emission for

amplifying the optical signals. In contrast to SOAs, population inversion in these amplifiers

is achieved by optical pumping. The basic scheme for optical pumping is as shown in

Fig.2-2. A coupler is used for combining the information signal and the pump signal. When

these signals travel in a doped fiber, power at the pump wavelengthλp is absorbed and it

creates population inversion. The signal atλs, the signal wavelength, amplifies as it passes

through the doped fiber core having population inversion. The pump power is generated by

semiconductor laser diodes at suitable wavelengths (e.g. 980 nm and 1480 nm for EDFAs).

In Fig.2-2, both pump and information signals propagate in the same direction. Such type

of pumping scheme is called forward pumping. It is possible to introduce the pump power

at the second coupler so that pump and information signal travel in the opposite direction.

This pumping scheme is referred as backward pumping. Both types of pumping can also

be used simultaneously and referred as bidirectional pumping. As the pump power reduces

along the propagation direction, the population inversion also reduces. The signal

amplification depends upon the population inversion profile along the length of doped fiber.

For higher amplification, more population inversion is desired over the larger length. For

a given population inversion profile, there is an optimum length of doped fiber which

maximises the overall gain [30].

17

The relaxation time of excited ions/atoms is an important parameter in determining

Fig.2-2 Forward pumped doped fiber amplifier

the effect of gain variations on the incoming information signal. In DFAs, it is of the order

of few milliseconds. Therefore, signal for both bit 1 and 0 experiences same amplifier gain

when very high bit rate signals are amplified. In contrast to this, the relaxation time in

SOAs is of the order of few nanoseconds. This means that at high bit rates (≈109 b/s), gain

for bit 1 and 0 would be different. Due to this reason, fluctuations in gain for bit 1 and 0

are more in SOAs than in DFAs. This results in comparatively less cross-saturation effect

in DFAs. The cross-saturation effect means the reduction of gain in desired channel due to

the presence of other channels when wavelength division multiplexed (WDM) signals are

amplified by OAs.

The DFAs can be used as in-line component in fiber optic links because these can

be easily spliced with the fiber. Consequently, there is smaller coupling loss. The signal

polarisation does not affect the gain of these amplifiers because of their circularly

symmetric cross-section. In SOAs, there is a large coupling loss (≈3.5 dB/facet typically)

and gain varies with input signal polarisation. The gain sensitivity to signal polarisation in

SOAs can be reduced by using various techniques [30, 89]. SOAs have two distinct

18

advantages over doped fiber amplifiers. These are: (i) SOAs can be easily integrated in

integrated optic transmitter/ receiver and (ii) SOAs are available over a wide wavelength

range (0.8 µm - 1.55 µm).

The most significant application of optical amplifiers is for amplifying the optical

signals in communication links. This increases the regenerator spacing in power budget

limited optical fiber links. In fiber optic broadcast networks, the transmitter power is

distributed among all the users. The number of users are restricted by the transmitter power

level. Use of OAs will increase the available power budget and hence the number of users

supported. This aspect of SOAs has been considered in detail in the following chapters of

the thesis.

In a point-to-point link, OA can be used as (i) postamplifier, (ii) preamplifier and

(iii) in-line amplifier. Postamplifier implies that an OA is used just after the transmitter. It

is also possible that OA is integrated with the source to form a high power optical source.

In this application, OA with higher saturation power is desired to reduce the effect of gain

saturation. DFAs can be used for this application because of their high saturation power

level. However, such a high power optical source would be very bulky. When an optical

amplifier is used as preamplifier, it is placed just before the receiver. This can also be

integrated with the receiver to form a high sensitivity receiver module. The OAs in this

application must produce low noise. Higher value of saturation power level is not required

as in postamplifier. In-line amplifiers are used in the optical fiber link itself [23, 24, 49,

80]. DFAs are better suited for this application because these can be easily spliced in the

link. Remote pumping can also be used to avoid electrical supply to the amplifier.

19

2.2 Optical Amplifiers in Broadcast Networks

As mentioned above, OAs can be used in passive broadcast networks for increasing

the number of users supported. The number of OAs in a network should be minimum for

a given number of supportable users. Conversely, number of supportable users must be

maximized for a given number of OAs. The optimal utilization of OAs can be achieved by

(i) proper placement of OAs in a given network topology and (ii) modifying the existing

topologies to utilize the OAs effectively. In view of the above, many studies have been

made to investigate the effectiveness of OAs in various topologies e.g. bus, ring, star and

some multilevel topologies. In the following, above studies and their results are reviewed.

2.2.1 Bus

Bus topology is very popular in copper based networks. It requires less copper cable

as compared to other topologies. Further, standard media access protocols e.g. IEEE 802.3

can be used. When the bus is implemented on optical fiber, there is a limitation on the

number of users supported by it. This is because of non-uniform distribution of power

among the users, requiring large dynamic range for the receivers. With limited dynamic

range, number of users are limited on bus network.

Bus topology supports very few users (typically < 20) [84]. The optical amplifier

can be used to keep the distribution of received power level more uniform. This results in

increase in supportable number of users. Wagner [84] has considered the use of SOAs in

a single channel bus network with uniformly distributed users. The SOAs are assumed to

20

be placed such that there areM users in between as shown in Fig.2-3. The maximum value

of M depends upon the dynamic range of received and transmitted power levels. It is equal

to number of users which can be supported by the bus without any SOA.

Fig.2-3 Optical amplifiers in bus topology.

There are two factors which limit the increase of number of users when SOAs are

used: (i) ASE noise and (ii) gain saturation. Initially, only the effect of ASE noise is

studied assuming the SOAs to be unsaturated. When a signal passes through many SOAs,

the ASE noise accumulates and degrades the system performance. It limits the number of

SOAs and hence number of supportable users. When the gain saturation is considered,

accumulated ASE noise saturates the SOAs alongwith the signal. This further reduces the

number of supportable users.

Use of heterodyne detection alongwith SOAs has also been studied [84]. In the

study, it is considered that in each section havingM users and a amplifier, end-to-end

signal gain compensates the loss. Therefore, whatever signal power enters in a section also

emerges out of the section. To study the degrading effect of gain saturation, gain saturation

due to last amplifier is considered and for all other amplifiers the gain equal to the loss is

21

assumed. With this assumption, the study gives an upper bound on number of users. The

analysis is limited to single channel bus. It can be modified for a WDM network.

2.2.2 Dual Bus

Dual bus topology is used for implementing distributed queue dual bus (DQDB)

metropolitan area networks (MANs). It consists of two buses which carry data in opposite

direction. The data is transferred in time slots which are generated by the Head of the Bus

(HoB). The two buses use optical fiber for connecting the two consecutive nodes on the

bus. The fiber is basically used as point-to-point link (Fig.2-4). This network has a

disadvantage of being unreliable as it does not use passive bus over the whole network. It

can be made reliable by connecting the nodes to the passive bus using 2x2 couplers. This

limits the number of nodes which can be supported by the network. Study on the use of

OAs in such a passive dual bus for increasing the number of nodes has been carried out

[50]. The above scheme is shown in Fig.2-5. Two wavelengthsλc and λd are used in this

scheme (Fig.2.5b and 2.5c). The wavelengthλc is used to broadcast the clock signal and

λd to transmit/receive the data. As the network is to operate at 10 Gb/s, broadcast of clock

signal on λc is essential. This is because at such a high bit rate, clock recovery and

synchronisation is very difficult. Use of clock atλc and data atλd, requires phase delay

compensation to compensate for smaller phase delays due to environmental variations (e.g.

temperature). The type of OAs although not specified have been modelled for noise and

gain saturation as in SOAs. It has been shown that a photonic dual bus using OAs (with

unsaturated gain of 12 dB and saturation power level of 4 dBm) with 100 nodes spanning

22

hundreds of kilometres is possible. Therefore, such a network is suitable as wide area

network (WAN) or MAN.

Fig.2-4 Dual bus network using fiber as point-to-point link.

Fig.2-5a Schematic of non-regenerative photonic dual bus (NI is the node interface).

2.2.3 Ring

Optical fibers have been used for connecting consecutive nodes in rings. Fiber

distributed data interface (FDDI) is such a ring network designed with fiber as the

23

transmission medium [25, 26]. Ring has also been proposed for self healing synchronous

Fig.2-5b Node interface for head of the bus (node 1).

Fig.2-5c Node interface for node 2 to N.

optical network (SONET) architectures. In these self healing rings, reliability is increased

by creating alternate paths and providing isolation of faulty links. FDDI uses two counter

rotating rings. It employs a loopback mechanism for high reliability in case of node failures.

Passive optical rings can provide high reliability because failure of a node does not break

the ring. But these have power budget problem limiting the number of nodes which can be

24

used on it. This problem can be overcome to a great extent by using optical amplifiers. A

passive ring having distributed erbium doped fiber amplifier has been proposed [22]. The

scheme is shown in Fig.2-6. In the analysis of this ring structure,g/α is considered to be

a constant along the ring. Here,g is gain coefficient andα the attenuation coefficient of

the fiber in the ring. It was shown that wheng is less thanα, ASE noise psd within the

ring attains a constant value which is quite low even whenα-g << 1. Thus distributed

optical amplification can be used in the ring to increase the supportable number of users.

It has been shown that a 300 km ring at 2.5 Gb/s cannot support a single user without

amplifier, but with the distributed amplification over 450 users can be supported. In the

study, quite a few assumptions have been made. Pump power is assumed to cause such a

population inversion profile thatg/α is constant. Some other factors which restrict the

number of users are ignored e.g. (i) dynamic range of receivers, (ii) back reflection at

various points and (iii) remnants of recirculating optical signals.

Fig.2-6 Ring network with distributed EDFA.

25

2.2.4 Star

Ideally, star topology distributes optical power equally at output ports. Therefore, it

can support maximum number of users without OAs as compared to any other topology.

At the present level of technology, number of users supported by star is small (typically

≤ 64). Many modified topologies which use star couplers and OAs have been proposed

[59, 94, 104]. These can support higher number of users as compared to a passive star

topology. The star topology can use either transmissive or reflective stars or both. In

reflective star coupler, input and output ports are same. The user transmits the signal in a

given port and receives the signal from all other users from the same port [67]. The

propagation direction of transmitted and received signals are opposite in the port. Two

schemes of distributed reflective star couplers which use the erbium doped fiber amplifiers

(EDFAs) have been proposed [94]. In both the schemes, 4x4 couplers having an EDFA as

shown in Fig.2-7 is used as basic element. An 8x8 star coupler can be made using 4x4

couplers and 2x2 couplers as shown in Fig.2-8. In the same manner, star coupler of larger

size can be made using smaller size couplers. In the first scheme (Fig.2-9),m number of

mxm star couplers are used. Implementation of eachmxm coupler requirem/4 EDFAs and

6m/4 + (m/2) log2(m/4) number of 2x2 couplers [94]. It can be seen from Fig.2-9 that

reflective star made ofm number ofmxm star couplers can supportNu=m2 number of users.

One output port of each coupler is terminated with a mirror and other (m-1) ports are

connected to remaining (m-1) number ofmxm couplers. A 16x16 distributed reflective star

coupler based on this scheme is shown in Fig.2-10

26

In the second scheme (Fig.2-11), 2m number ofmxm couplers are used to implement

Fig.2-7 4x4 star coupler with single EDFA.

Fig.2-8 8x8 coupler based on 4x4 star couplers.

a distributed reflective star coupler which can supportNu=2m2 number of users [94]. The

mxm couplers are being grouped to formm pairs. In each pair, one output of eachmxm

coupler is connected using 2x2 reflective star coupler. Each pair ofmxm couplers is

connected to every other pair by a 2x2 coupler. A 32x32 reflective star coupler based on

this scheme is shown in Fig.2-12. In these star couplers, less fiber is required as compared

27

Fig.2-9 Implementation of distributedNu x Nu (Nu=m2) reflective star coupler based onm number ofm x m couplers.

28

to a centralized star coupler. The EDFAs are shared by all the users. In the above

Fig.2-10 16x16 distributed reflective star coupler.

proposition, no mention has been made regarding the utility of this star in a WDM system.

The use of above star in a WDM system will require wide bandpass filters in EDFAs.

Further, the required gain of EDFAs for a given number of users is not given, which is

needed in design of such systems. Also the injection of pump in EDFAs will require

wavelength selective coupler.

Another star coupler configuration using EDFAs for WDM/FDMA network

(Fig.2-13) has also been proposed [59]. WDM/FDMA network means that the signal

spectrum consists of many frequency band at different wavelengths. Each band contains

many channels which are wavelength multiplexed with very narrow spacings. In this

29

scheme, there is one central coupler of dimensionmcxmc. All other couplers are auxiliary

Fig.2-11 Implementation of distributedNu x Nu (Nu= 2 m2) coupler based on 2m numberof mxm couplers.

couplers. There aremc wavelength bands and a unique wavelength band is allocated to the

users on auxiliary coupler. Each wavelength band has multiple channels to be used for

transmission by the users on the corresponding auxiliary coupler. When a user on auxiliary

coupler AC1 transmits the signal on a wavelength channel in the assigned bandW1, it is

broadcast to all the users on the same coupler. A part of the signal is transmitted to central

coupler. At the input, only the wavelength bandW1 is allowed to pass through by means

of an optical bandpass filter (BPF). This filter allows only the signals from users on AC1

30

to be passed, which are then amplified and broadcast to the other auxiliary star couplers.

Fig.2-12 32x32 distributed reflective star coupler.

The output fiber connected to AC1 from central star contains a bandstop filter (BSF) for

wavelength bandW1. Ideally, the use of BSF prevents the recirculation of the optical signal

in wavelength bandW1. One of the output ports ofmcxmc central coupler can be used as

the input for the pump power for all the EDFAs. This scheme has the advantage that

existing networks on auxiliary stars are easily interconnected. The network size is easily

expandable by using auxiliary stars until all the ports of the central star are filled. Sharing

of pump source, amplification of a part of spectrum by EDFAs are the main features of the

scheme. The disadvantage is that recirculating signals can produce interference in practical

BSFs. The interference can be minimized by suitably matching the BPFs and BSFs. The

31

mxm star coupler must be wideband as the pump signal also traverse through the central

coupler.

Fig.2-13 FDM/OFDM distributed expandable amplified star coupler configuration.

A few other configurations using star couplers with OAs have been reported [59,

104]. The star coupler proposed in [59] uses BPFs and BSFs for each wavelength band.

This coupler is modified by Yung-Kuanget.al [104] and the modified version is shown in

Fig.2-14. This is ammcxmmc centralized star coupler. The 1xmc and mcx1 couplers can be

made using (mc-1) number of 2x2 couplers as shown in Fig.2-15. Themcxmc coupler is a

transmissive star coupler made using (mc/2)log2(mc) number of 2x2 couplers. Each OA is

a forward pumped EDFA as shown in Fig.2-16. This scheme supports more number of

users thanm(mc-1)xm(mc-1) coupler [59]. The number of required 2x2 couplers is less than

32

that of star coupler [59] form ≥ 32. This coupler does not require dedicated BPF and BSF

as in [59].

Fig.2-14 Star coupler with amplifiers and without bandpass and bandstop filters.

As each user must be connected to the centralized star coupler, there is a large fiber

requirement. The loss between transmitter and receiver is more than the loss in the coupler

with BPFs and BSFs. The fiber requirement can be reduced by making the above star

coupler distributed. This can be achieved by putting the 1xm and mx1 couplers with the

group of m users. It implies that fiber is required only to connect the 1xm and mx1

couplers tomcxmc star. This saves the required fiber as well as the required conduit length

for fiber layout. Further, the number of 1xm couplers can be reduced by using reflective

mcxmc star coupler. The resultingmmcxmmc coupler is shown in Fig.2-17. The above

modification also reduces the required fiber to half as the transmitted signal and signal to

33

be received share the same fiber in opposite direction. These signals can be separated by

using diplexers at the user ends. Themcxmc reflective star coupler can be made by using

a mcx1 coupler whose single port is terminated by a mirror (Fig.2-18). There are also two

other possible ways to form amxm reflected star coupler. These are shown in Figs.2-9 and

2-11.

Fig.2-15 8x1 coupler based on seven 2x2 couplers.

In all the above studies, degrading effects due to OAs have not been considered.

Therefore, these give an upper bound on the supportable number of users. In some of the

studies, new schemes have been proposed which use the OAs more efficiently. But their

practical implementation require some special devices e.g. mirrors for terminating the ports

and 2x2 reflective star couplers.

34

An accurate analysis is needed to compute the supportable number of users which

Fig.2-16 Forward pumped EDFA.

Fig.2-17 mmc x mmc reflective star.

is expected to be lower than the upper bound. Therefore, the previous performance bounds

cannot be used to compare the various schemes. Only a qualitative comparison can be

made. All recent studies are mostly based on EDFA due to its popularity. It is remarked

that the doped fiber can only be used in distributed star coupler or in bus and ring

35

networks. In centralized star couplers, SOAs can be easily integrated with integrated optic

Fig.2-18 mc x mc reflective star coupler based onmcx1 tree coupler.

implementation. Further, SOAs are also better suited as post or preamplifier since these can

be integrated with either the transmitter or the receiver chips. Therefore, SOAs have been

considered in the studies reported in the following chapters.

36

Chapter 3

Semiconductor Optical Amplifier andCoding in OOK System

Fiber optic broadcast networks can support only a limited number of users because of

constraint on the transmitter power. It is desirable to have large number of users to utilize the

The work reported in this chapter has resulted in the following publications.

1. V. K. Jain, Y. N. Singh and H. M. Gupta, "Power Penalty due to Optical Amplifier InducedCrosstalk in Non-Coherent OOK Transmission Systems,"Journal of Optical Communications,Vol.16, No.5, Oct.1995, pp.194-196.

2. Y. N. Singh, V. K. Jain and H. M. Gupta, "Effect of Reed-Solomon Code on Laser LinewidthRequirements of BPSK Homodyne Optical Communication Systems,"Journal of OpticalCommunications,Vol.16, No.6, Dec.1995, pp.207-210.

3. Y. N. Singh, V. K. Jain and H. M. Gupta, "Reed Solomon Code and Semiconductor OpticalPreamplifier in OOK Communication System: A Comparative Study,"Journal of OpticalCommunications(accepted for publication).

4. Y. N. Singh, V. K. Jain and H. M. Gupta, "Effect of Error Correcting Codes on Laser LinewidthRequirements in Optical Binary Phase Shift Keying Communication Systems," Presented inIXth

National Convention of Electronics and Telecommunication Engineers,University of Roorkee,Roorkee, India, March 30-31, 1994.

5. V. K. Jain, Y. N. Singh and H. M. Gupta, "Effect of Optical Amplifier Induced Crosstalk in TwoChannel Non-Coherent OOK Transmission System," Presented inIXth National Convention ofElectronics and Telecommunication Engineers, University of Roorkee, Roorkee, India,March 30-31, 1994.

capacity of network. The number of users can be increased by increasing the power budget,

reducing the loss in the network, using electronic repeater and optical amplifiers. The power

budget can be improved by using coding techniques. The use of coding and semiconductor

optical amplifier (SOA) to increase the network size have been considered in this chapter. For

37

their comparative study, a point-to-point on-off keying (OOK) communication link has been

considered. The receiver structure considered is shown in Fig.3-1. It consists of a PIN

photodetector followed by a Gaussian filter with the frequency response

(3.1)

Here, f is the frequency andBe the electrical bandwidth of the filter. This filter is followed

by sampling, threshold and decision circuits.

Fig.3-1a The system block diagram for optical communication system with codingscheme.

Fig.3-1b The system block diagram for optical communication system with SOA aspreamplifier.

38

The electrical signal at the receiver output is corrupted by additive shot and thermal

noise of photodetector. Both types of noise are considered to be white Gaussian with power

spectral densities (psds) [30]

(3.2a)

and

(3.2b)

respectively. In the above equations,Ro is the responsivity of the photodetector,Pr the

received optical power level which becomesPr(1) for bit 1 andPr(0) for bit 0. These are

given by

(3.3a)

and

(3.3b)

whereε is the extinction ratio andPra (= [Pr(1) + Pr(0)]/2) the average received optical power

level. In the above equations,e is the electron charge,kb the Boltzmann’s constant,T the

temperature of receiver in Kelvin andRL the receiver load resistance.

39

The receiver performance in terms of required recieved optical power level has been

computed for (i) uncoded system, (ii) coded system with decoding and (iii) uncoded system

with optical preamplifier. The Gaussian filter in the receiving system is optimized for uncoded

transmission. The coding in the system increases the bit rate and hence intersymbol

interference (ISI). In order to evaluate the degradation due to ISI, performance of coded

system without decoding has been evaluated. Any coding gain should be measured with

respect to the above performance. Section 3.1 presents the analysis of uncoded system. In

section 3.2, coded system without decoding and with decoding has been considered. Uncoded

system with SOA has been analyzed in section 3.3. A comparative study of above systems

based on a practical example is given in section 3.4. Section 3.5 contains the observations and

conclusions of this study.

3.1 Uncoded System

The Gaussian filter in the receiver reduces noise by removing noise outside the

frequency band of interest. But it also causes ISI. Increase inBe results in decrease in ISI and

increase in noise after the filter and vice-versa. Hence, there exists an optimal filter

bandwidth.

Worst-case degradation due to ISI can be modelled by considering worst-case

waveforms for bit 1 and 0. These waveforms decide the eye opening. For bit 1 and 0, the

worst-case waveforms correspond to bit sequences ...0001000... and ...1110111... respectively.

The waveforms before and after the filter are shown in Fig.3-2. When the sampling is done

40

in the middle of bit, signal levelsS1 andS0 corresponding to worst-case sequences for bit 1

and 0 are given by

(3.4a)

and

(3.4b)

where

(3.5a)

and

(3.5b)

In the above equations,h(τ) is the impulse response of the Gaussian filter,Tb the bit duration.

The a1(τ) and a0(τ) are the worst-case waveforms for bit 1 and 0 respectively. The eye

opening will beS1-S0.

41

For an additive Gaussian noise with psdSN(f), variance after the Gaussian filter is

Fig.3-2 Worst-case waveform patterns for (a) bit 1 and (b) bit 0 before and after theGaussian filter.

given by

(3.6a)

If the noise is white i.e.SN(f)=SN. Using Eqn.3.1 in the above equation,σn2 is given by

42

(3.6b)

In an uncoded system,SN corresponding to bit 1 and 0 is given by sum ofSshot andSth. Noise

variancesσ2(1) andσ2(0) corresponding toS1 andS0 are determined from Eqn.3.6b. The bit

error rate (BER) which equalises the error rates forS1 andS0 is given by

(3.7)

The above analysis is quite general and the system performance can be evaluated for any bit

rateR (= 1/ Tb). The filter can be optimized to minimize the effect of ISI at a given bit rate.

When bit rate is increased (as in coding), the above optimization is not valid and the system

performance degrades due to increased ISI. The degradation in performance is evaluated using

the above general model (Eqns.3.4 to 3.7).

3.2 Coded System

Coding is introduced in the system by encoding the input signal to the transmitter and

decoding the receiver output signal (Fig.3-1a). As mentioned earlier, the bit rate increases in

the system due to coding. The increase depends on the amount of redundancy introduced by

coding scheme. When the data rate isR b/s and (N,k,t) block code is used, the coded bit rate

becomesR(N/k) b/s. Here,N is the number of symbols in the block,k the data symbols in the

block andt the number of symbols which can be corrected by the error correcting code. The

43

increased bit rate leads to more ISI as the receiver is same as in the uncoded system. It results

in increase in BER. Some errors are corrected in the decoding process leading to decrease in

BER. If the decrease in BER is more than the increase due to ISI, there will be a coding gain.

For a (N,k,t) block code, the symbol error rate after decodingPes(ad) is given by

[72, 99, 102]

(3.8a)

wherePes(bd) is the symbol error rate before decoding. It is related to bit error ratePe by

(3.8b)

In the above,n is the number of bits in a symbol. For example, in systematic Reed-Solomon

(R-S) (31,27,2) code, there are 31 five bit symbols, 27 data symbols and 4 parity symbols.

Therefore, in Eqn.3.8, values ofN, k, t andn are 31, 27, 2 and 5 respectively.

In the coded system, the bit rate becomes higher and the BER can be determined using

the same approach as in the uncoded system. This BER is used to determine the symbols

error rate before decodingPes(bd) from Eqn.3.8b. The R-S code will reducePes(bd). The reduced

symbol error rate after decodingPes(ad) can be determined using Eqn.3.8a. To compare the

44

performance of coded system with the uncoded system, BER for coded system is required to

be known. This can be determined from Eqn.3.8b by replacingPes(bd) with Pes(ad) and then

determiningPe using a suitable search technique. The BER also includes the errors in the

parity symbols which are discarded after decoding. Hence, the BER after discarding parity

symbols isPe(k/N). The system performance for R-S (31,27,2) code is evaluated using the

above approach and results are shown in Fig.3-3.

3.3 Uncoded System with Optical Amplifier

In this analysis, an SOA has been considered as preamplifier. It is modelled as

travelling wave amplifier (TWA). The SOA alongwith the amplified signal also produces ASE

noise which beats with signal and itself at the photodetector. The psd of various noise

components at the photodetector is shown in Fig.2-1.

There is a dc component due to ASE-ASE beat noise which gives rise to shot noise

with psd

(3.9)

The above is modelled as white Gaussian noise [63]. It adds to shot noise produced by the

signal. In Eqn.3.9,Ssp is the ASE noise psd andBo the bandwidth of optical filter. TheSsp is

given by

45

(3.10)

wherensp is the spontaneous emission factor,G the gain of OA,h the Planck’s constant and

ν the optical frequency.

The noise variance at the filter output due to ASE beat noise components is

determined by using Eqn.3.6a and noise psds are shown in Fig.2-1. These are given by

(i) ASE-ASE beat noise

(3.11a)

(ii) ASE-signal beat noise

(3.11b)

46

(iii) ASE shot noise

(3.11c)

In the above,Pin is the optical power at the input of optical preamplifier. The total noise

variance at filter output is given by

(3.12)

In order to compute the BER, received power levels for bit 1 and 0 are determined for

a given average received power level and extinction ratio from Eqn.3.3. Corresponding

amplifier gainG(1) andG(0) for bit 1 and 0 respectively are determined using these received

power levels and the amplifier gain saturation formula given in Eqn.2.4 [30]. The optical

power levels falling on photodetector for bit 1 and 0 arePin(1)G(1) and Pin(0)G(0)

respectively. Here,Pin(1) andPin(0) are input optical power levels to SOA for bit 1 and 0

respectively. The signal current after the Gaussian filter is determined for bit 1 and 0

following the same approach as in section 3.1. There is an optical bandpass filter of

bandwidthBo between SOA and photodetector (refer to Fig.3-1b). It is used to limit the ASE

noise. The noise variances for bit 1 and 0 are determined from Eqns.3.11 and 3.12 using the

ASE noise psd, optical signal power levels for bit 1 and 0 and optical filter bandwidthBo.

These are used to determine BER from Eqn.3.7.

47

3.4 Example

The results for a sample system are computed and analyzed in this section. Following

are the system parameters:

Bit rate (uncoded system),R 1 Gb/s

Optimum BW of Gaussian filter,Be 1.5 GHz

Operating wavelength,λ 1.55 µm

Quantum efficiency of photodetector,η 0.95

Coding scheme R-S (31,27,2)

Bit rate after coding 1.14 (=32/27) Gb/s

Unsaturated gain of SOA,G0 29 dB

Saturation power level,Psat 10 dBm

BW of optical filter, Bo 10 GHz

Extinction ratio,ε 0

The BER is determined as a function of average received signal power level for (i) uncoded

system, (ii) coded system without decoding, (iii) coded system with decoding and (iv)

uncoded system with optical preamplifier. Variations of log10(BER) with received signal

power levelPra (in dBm) are shown in Fig.3-3.

It is seen from the above figure that a coded system without decoding performs worse

than the uncoded system. This is expected as coding increases bit rate which in turn increases

the ISI and therefore the performance becomes worse. Once the received signal is decoded,

the performance improves because of error corrections. R-S(31,27,2) code for a BER of 10-9

48

provides coding gain of 1.7 dB. When optical preamplifier is used instead of coding,

Fig.3-3 Variations of BER vs received power level (dBm) for (i) uncoded system, (ii)coded system without decoding, (iii) coded system with decoding and (iv)uncoded system with SOA as preamplifier.

improvement in receiver sensitivity is about 21.2 dB. It implies that use of optical amplifier

improves the system performance much more than the use of coding.

3.5 Conclusions

It is concluded that in an OOK communication system, use of SOA is advantageous

as it provides more improvement ( > 20 dB) as compared to coding. Although the above

conclusion is based on R-S code, it will remain valid for all other codes too. The level of

49

improvement depends upon the received optical power level and coding scheme used. Use of

coding cannot provide coding gains in the vicinity of 20 dB.

Another advantage of SOA over coding is that it can amplify signals at high bit rates

(> 1 Gb/s). The practical implementation of encoder and decoder at such a high bit rate is

difficult. Further, a single SOA can handle multiple WDM channels. Therefore, in chapters 4

and 5 of the thesis only the use of SOAs in network has been considered.

50

Chapter 4

Semiconductor Optical Amplifiers inWDM Star Topology

In a star topology network, the transmitted signal from all the users are combined

The work reported in this chapter has appeared in the following publications.

1. Y. N. Singh, V. K. Jain and H. M. Gupta, "Semiconductor Optical Amplifiers in WDM StarNetworks,"IEE-Proceedings Optoelectronics,Vol.143, No.2, April 1996, pp.144-152.

2. Y. N. Singh, V. K. Jain and H. M. Gupta, "On Placement of Semiconductor Laser Amplifiers inWDM Star Networks,"Proc. National Conference on Optical Communications, J.K.Institute ofApplied Physics & Technology, University of Allahabad, Allahabad, India, Feb.22-24, 1995,pp.66-78.

in a star coupler and then distributed to all the receivers. The star coupler can be

centralized or distributed. Generally, the star coupler distribute the transmitted signal power

equally among the users. Consequently, star topology supports maximum number of users

as compared to other passive broadcast topologies viz. bus, ring and tree.

The semiconductor optical amplifiers (SOAs) can be placed in the star network to

amplify the optical signal. These will increase the supportable number of users. The SOAs

can be placed in a star either after the transmitters as postamplifiers or before the receivers

as preamplifiers. In case of WDM star networks, postamplifiers amplify single channel

signal only, while preamplifiers amplify signals in multiple channels. In the latter case,

there may be interchannel crosstalk.

51

In this chapter, the above SOA placement schemes are analyzed. Section 4.1 presents

the description of the star network. Section 4.2 discusses the reflection due to amplifier.

Noise variances due to ASE and signal beat noises are determined when the receiver uses

an ideal filter. In section 4.3, approach used in analyses is described. The analyses of star

without SOA, with postamplifier and preamplifier are presented in sections 4.4, 4.5 and 4.6

respectively. In section 4.7, an example with practical system parameters has been

considered. Finally, conclusions are given in section 4.8.

4.1 System Description

In a NuxNu star network, every user is provided with a transmitter and receiver pair.

In the simplest arrangement [32, 61], each user is assigned a dedicated transmit wavelength

in the 1.5 µm waveband. The modulation scheme considered is the binary intensity

modulation. When extinction ratioε is zero, it becomes an on-off keying system. The

transmitted signal power is equally distributed to all the receivers by the star coupler. Each

user receives the signals on all the wavelengths. One of the wavelengths is selected using

an optical filter and then detected using a photodetector. The above mentioned star coupler

is made of 3 dB 2x2 couplers. An 8x8 star coupler consisting of 2x2 couplers is shown in

Fig.4-1. In general, aNuxNu star coupler has log2Nu stages and requires (Nu/2)log2Nu number

of 3 dB 2x2 couplers. The total insertion lossLti of a NuxNu star coupler is given by

(4.1)

52

whereLi is the insertion loss of an individual 3 dB 2x2 coupler. The power split due to star

coupler may not be uniform due to practical limitations. This nonuniformity has been

considered in the analysis in terms of coupler variability,Lcv, which is the worst-case

reduction of power in the output port. Each user is connected to star coupler using a single-

mode dispersion flattened fiber. Consequently, the input and output ports ofNuxNu coupler

are spliced to optical fiber. Each of these splices introduces a loss ofLsp dB.

Fig.4-1 An 8x8 star coupler consisting of 3 dB 2x2 couplers.

At receivers, ideal optical filters are used. Therefore, there is no interchannel

crosstalk due to these filters. These filters introduce a insertion loss ofLfi dB. The filters

can be tunable or fixed wavelength type depending on the media access control (MAC)

protocol. When receiver arrays are used to receive more than one channel at a time,

demultiplexers can be employed instead of filters. Optical filters are followed by PIN

photodetectors in each of the receivers to detect the optical signal.

53

The use of SOAs is expected to increase the number of users or decrease the

required transmitter power in the star network. However, the presence of amplifier noise

and gain saturation effect is expected to partially reduce the above advantages. When SOAs

amplify multiple channels, total optical power at the SOA input varies randomly since light

is independently modulated in each channel. This results in interchannel crosstalk. The

signal power at the amplifier output in a channel varies according to the gain fluctuations

induced by modulation in other channels even when the input power in the channel is

constant. This is the crosstalk induced by gain saturation in the amplifier and is often

referred as cross-saturation. The cross-saturation effect increases with the increase in the

number of wavelength channels. Therefore, it is more severe when larger number of

wavelengths are used in the system. In the above system, maximum cross-saturation occurs

as Nu wavelengths are used. Hence this will provide upper bound on the degradation due

to cross-saturation for the WDM star network.

4.2 Amplifier Model

The gain of SOAs reduces with the increase in input power level. This reduction

arises due to gain saturation. The saturated gain of SOAs is given by Eqn.2.4. The

amplified spontaneous emission (ASE) noise is also present in the amplifier output

alongwith the amplified signal. The psd of ASE noise is given by Eqn.2.5. At the

photodetector, ASE noise beats with itself, signal and echo signal if present. The psds of

ASE-ASE and ASE-signal beat noise components are shown in Fig.2-1. The noise variances

of the various noise components, assuming an ideal electrical filter of bandwidthBe in the

receiver, are given by

54

(i) ASE-ASE beat noise [63, 90]

(4.2a)

(ii) ASE-signal beat noise [63, 90]

(4.2b)

When an echo signal is present, ASE-echo beat noise variance is given by Eqn.4.2b by

replacing Pr by the echo powerPecho. ASE-ASE beating also produces a dc component

resulting in shot noise. The variance of this shot noise is given by

(4.3)

In addition to signal amplification, the amplifiers also reflect signals. The reflection

coefficient of the amplifier is given by (Appendix-I).

(4.4)

In the above,R1 and R2 are the facet reflectivities andG the single pass gain of SOA.

55

4.3 Evaluation of Minimum Required Transmitter Power

Once the relationship between bit error rate (BER) and transmitter power,Pta, is

known for a given number of users,Nu, minimum requiredPta can be determined using this

relationship. Therefore, models to determine BER as a function ofPta andNu are formulated

in the following sections. These models are used to evaluate the performance of star

network without amplifier, with postamplifier and with preamplifier. In the analysis,

reflection noise due to filters and coupler have been neglected. Optical filters are assumed

to be ideal so that these do not introduce any crosstalk. Further, responsivity of

photodetectors is assumed to be independent of wavelength. The effect of finite extinction

ratio of transmitted pulse has been considered in the analysis.

4.4 Star without Amplifier

The WDM star topology is shown in Fig.4-2. Each transmitter is assigned an unique

operating wavelength and the coupler distributes signals of all wavelengths to all the

receivers. The power received at the photodetector for bitbc (bc is either 1 or 0) is

given by

(4.5)

where

56

(4.6a)

(4.6b)

In the above equations,Pta represents the average transmitter power andε the extinction

ratio. The lossLtr between transmitter and receiver excluding the split loss of star coupler

is given by

(4.7a)

or equivalently

(4.7b)

The above does not include the split loss of star coupler which has been included by the

factor 1/Nu in Eqn.4.5. In the above equation,α is the attenuation coefficient of fiber in

dB/km, L the length of fiber from user to star coupler in km,Lfi the filter insertion loss,

Lcv the loss due to nonuniformity in the power splitting by the star coupler andLsp the

splice loss in dB.

57

The signal current and noise variance for bitbc (bc = 0 or 1) at photodetector output

Fig.4-2 Star network without optical amplifier.

are given by

(4.8a)

and

(4.8b)

wherekb is the Boltzmann’s constant,T the temperature in Kelvin,R0 the responsivity and

RL the load resistance of the photodetector. The first term on right hand side of Eqn.4.8b

represents the shot noise and second term the thermal noise. The average probability of

error with the threshold level which equalises the BER for bit 1 and 0 is given by

58

(4.9)

It is obvious thatPe is determined fromIsig(1) and Isig(0), σ(1) andσ(0) which in turn are

determined fromPr(bc) and hence fromPt(bc). The Pt(bc) is determined fromPta. ThusPta

determinesPe. Alternatively, for a givenPe, Pta can be computed by a suitable search

technique.

4.5 Star with Postamplifier

In order to reduce the requiredPta, SOAs can be used in the star topology. In this

section, SOAs are placed immediately after the transmitters (postamplifiers). The

performance of star network with unsaturated postamplifiers has been analyzed in this

section. Subsequently, the effects of amplifier gain saturation and reflection have been

included in the analysis.

4.5.1 Unsaturated Amplifiers

As the gain of unsaturated postamplifier isG0, the received signal powerPr(bc) is

given by

59

Fig.4-3 Star network with postamplifiers (A, B, ...F is echo path).

(4.10)

whereLta is the loss between transmitter and amplifier andLar the loss between amplifier

and receiver. These are given by

(4.11a)

and

(4.11b)

In Eqn.4.11b,Lcp is the coupling loss of an amplifier facet. The psd of ASE noise at the

receiver will be

60

(4.12)

This psd is not reduced by the factor 1/Nu due to the following reason. LetSspi be noise psd

at the receiver due toi th amplifier. It is given by

(4.13)

The total noise psd at the receiver should be sum of all the psds due toNu amplifiers.

HenceSsp will be

(4.14)

This is same as given in Eqn.4.12.

The signal current and noise variance for bitbc using Eqns.4.2 and 4.3 are given by

(4.15a)

and

61

(4.15b)

The first term in Eqn.4.15b corresponds to shot noise due to signal and ASE, second term

to ASE-signal beat noise, third term to ASE-ASE beat noise and fourth term to thermal

noise. The signal currentsIsig(bc) and noise varianceσ2(bc) from Eqn.4.15 are used in

Eqn.4.9 to determine average probability of errorPe.

4.5.2 Effect of Gain Saturation

In general, gain of SOAs is not constant and it is input powerPin dependent. The

gain reduces with the increase inPin. This will degrade the receiver performance which has

been investigated below. Let the amplifier gain corresponding to bit 1 and 0 beG(1) and

G(0) respectively. Therefore, the received signal power level for bitbc will be

(4.16)

The received ASE noise psd is different for bit 1 and 0. When the desired channel

has bit 1, one half of the remaining (Nu-1) channels are expected to have bit 0 and the

other half bit 1. The ASE noise from each amplifier gets distributed equally among the

62

outputs. Therefore, ASE noise psd from each amplifier will get reduced by a factor of 1/Nu

at the receiver input. The total ASE noise psd at the receiver for bitbc is

(4.17)

The first term in the above equation corresponds to the desired channel, second term to

channels having bit 0 and the third term to channels having bit 1.

The signal current and noise variance for bitbc are given by

(4.18a)

and

(4.18b)

The first term in Eqn.4.18b correspond to shot noise due to signal and ASE, second term

to ASE-signal beat noise, third term to ASE-ASE beat noise and last term to thermal noise.

The averagePe in this case can be determined using Eqn.4.18 alongwith Eqn.4.9.

63

4.5.3 Effect of Reflection Noise

Splices in the network produce back reflections resulting in echoes at the receivers.

A signal which has undergone 2i reflections will generatei-pass echo. Generally, the

reflection coefficient of splices is below -20 dB, therefore the echoes with two and more

passes can be neglected. When the delays suffered by echoes are greater than the coherence

time of optical signal, coherence between signal and echoes need not be considered. In the

network under consideration, distance between the reflection points is much greater than

coherence length. Hence, incoherent addition of signal and echo power has been considered.

In the network shown in Fig.4-3, reflection can occur at amplifier or at splice 1 or

at splice 2. Consequently, there would be three possible echo signals. Out of these, the echo

signals due to SOA and splice 1 will be strongest. This is so because loss between splice 1

and splice 2, and SOA and splice 2 (insertion loss of star coupler + splice loss + fiber loss)

is much higher than loss between SOA and splice 1. Hence, echo signal arising due to

splice 2 can be neglected.

Worst-case occurs when the signal bit and echo signal bit arrive synchronously at

the receiver. This gives upper bound on degradation due to echo. The probability of error

in this worst-case is obtained as follows.

Let the echo signal of bitbr interfere with the signal corresponding to bitbc. The

echo signal power at various points in the echo path A, B, ..., F (Fig.4-3) are given by

64

(4.19a)

(4.19b)

(4.19c)(4.19c)

(4.19d)

(4.19e)

(4.19f)

The received echo power,Pecho, is same as echo power at point F (Eqn.4.19f). In the above

equation,Las1 is the fiber loss between amplifier and splice 1,Rsp the splice reflection

coefficient. The parameterGp(br) represents the gain of amplifier when signal corresponding

to bit br passed through the amplifier. It is obtained by replacingPin with Pin(br, brr) in

Eqn.2.4. ThePin(br, brr) is given by

65

(4.20)

where Rf is the facet reflectivity of amplifier (Rf=R1=R2). In the above equation, it is

presumed that echo of bitbrr interfered with the signal corresponding to bitbr. The

parameterG′p is the gain when signal corresponding to bitbrr was amplified. In the worst-

case situation, it is assumed that no echo was present when signal corresponding tobrr was

amplified. Further,brr is assumed to be zero for maximum echo signal. In the above

equation, signal corresponding tobr and echo ofbrr are added together and used asPin in

Eqn.2.4. As such, the two optical signal powers cannot be added as these are entering the

amplifier from opposite ends. But the above addition corresponds to the worst-case scenario.

The amplifier gain G for the signal corresponding to bitbc under the same

assumption is given by

(4.21a)

where

(4.21b)

66

It is seen from Eqn.4.4 and the above equation thatRamp is function of G which in turn

depends uponbc and br.

The received signal power at the photodetector input will be

(4.22a)

The ASE noise psd at the photodetector is given by

(4.22b)

In the above equation, first term corresponds to bitbc in the desired channel, second term

to bit br in the desired channel, third term to signals from the remainingNu-1 amplifiers

and last term to echo signals from the remainingNu-1 amplifiers. Further, bitsbci, bri and

brri are bits in thei th channel corresponding to the bitsbc, br and brr respectively in the

desired channel. In the worst-case situation, bitsbrr and brri can be taken as 0. Thebci and

67

bri can be either 1 or 0 with equal probability. WhenPr, Pecho andSsp are known, the signal

current and noise variance are given by

(4.23a)

and

(4.23b)

In the Eqn.4.23b, additional fourth term represents the signal-echo beat noise.

It is evident from Eqn.4.23a that there are two signal levels corresponding to bit 1

as well as bit 0 depending on bitbr. The threshold is obtained using the highest level

corresponding to bit 0, the lowest level corresponding to bit 1 and the corresponding noise

variances. The threshold which equalizes the BERs for these levels is given by

(4.24)

With this threshold, the probabilities of error for bit 1 and 0 are given by

68

(4.25a)

and

(4.25b)

For equiprobable bit 1 and 0, average probability of error is given by

(4.25c)

4.6 Star with Preamplifier

In this scheme, all the wavelengths are amplified by each preamplifier and one of

these is selected by the filter. As in the postamplifier scheme, performance of star network

with ideal preamplifiers has been analyzed first. This analysis is extended to include the

effect of amplifier gain saturation and reflection noise.

69

4.6.1 Unsaturated Amplifier

The received power for bitbc is given by

(4.26)

where

(4.27)

is the loss between transmitter and amplifier without the split loss of coupler. TheLar

represents the loss between amplifier and receiver which consists of only the insertion loss

of filter, Lfi, and amplifier coupling loss,Lcp.

The ASE noise psd,Ssp, at the photodetector is determined from Eqn.4.12. It will

be same for bit 1 and bit 0. SubstitutingPr(bc) and Ssp in Eqn.4.15, the signal currents

(Isig(1) and Isig(0)) and noise variances (σ2(1) and σ2(0)) for bit 1 and 0 are obtained and

averagePe is computed using Eqn.4.9.

4.6.2 Effect of Gain Saturation

The cross-saturation leads to decrease in the average gain because of increase in the

number of channels and gain fluctuations due to randomness in the number of channels

70

having bit 1. This degrades the performance [46, 75, 76]. In this section, degradation due

to average gain reduction and gain fluctuations has been analyzed.

(i) Average Gain Reduction

The input power to the amplifier corresponding to bitbc is

(4.28)

Let N1 channels out of totalNu channels are having bit 1. Therefore, the total input power

to the amplifier is N1Pin(1) + (Nu-N1)Pin(0). The corresponding saturated gain of the

amplifier is given by

(4.29)

In order to determine the average gain, probability distribution of number of channels

having bit 1 is considered to be binomial. The probability ofN1 channels having bit 1 will

be

(4.30)

Therefore, the average gain is given by

71

(4.31)

The signal power received for bitbc will be

(4.32)

As Gav is independent of signal bit, the ASE noise psd for both bit 1 and 0 will be same.

It is given by

(4.33)

Eqns.4.32 and 4.33 are used to determine signal currents (Isig(1) and Isig(0)) and noise

variances (σ2(1) and σ2(0)) in conjunction with Eqn.4.15. ThenPe is computed using

Eqn.4.9.

(ii) Gain Fluctuations

Let N1 channels out ofNu-1 interfering channels are having bit 1. The corresponding

total amplifier input power for the bit bc in the desired channels is

Pin(bc) + N1Pin(1) + (Nu-1-N1)Pin(0). Therefore, the saturated gain of the amplifier will be

72

(4.34)

The probability thatN1 channels out ofNu-1 channels are having bit 1 is given by

(4.35)

For bit bc, the signal power and ASE noise psd at the photodetector is given by

(4.36a)

and

(4.36b)

The above equation is used to determine the signal current and noise variance corresponding

to bit bc. These are given by

(4.37a)

and

73

(4.37b)

Both Isig(1) andIsig(0) will have Nu levels depending uponN1. The threshold corresponding

to highest level ofIsig(0) and lowest level ofIsig(1) will be

(4.38)

The probability of error under the condition thatN1 interfering channels are having bit 1

is given by

(4.39a)

The averagePe will be

(4.39b)

74

4.6.3 Effect of Reflection Noise

In order to analyze the effect of reflection noise, the echo path A, B, ..., G as shown

in Fig.4-4 has been considered. Reflection between SOA and splice 2 has only been

considered because the loss between splice 1 and splice 2 and splice 1 and SOA is much

higher. The amplifier is saturated by the signal and echo signal power in the desired

channel and also the signal and echo signal power in the interfering channels. Let the echo

of bit br affect the signal corresponding to bitbc. Further, let theM1 interfering channels,

with probability P′M1, were having bit 1 when the signal corresponding tobr was reflected

by SOA to form the echo signal. The average echo signal power at the amplifier input in

the desired channel is given by

(4.40)

The average echo signal power in the remaining interfering channels will be

(4.41)

The P′M1 is obtained from Eqn.4.35 by replacingN1 by M1. The Ls2a represents the loss

between splice 2 and amplifier (fiber and amplifier coupling loss). TheRamp depends on the

gain G(br,M1) which is given by

75

(4.42a)

where

(4.42b)

Let N1 channels out ofNu-1 interfering channels have bit 1 when signal corresponding to

bit bc is amplified. The gain of amplifier for this signal is

(4.43a)

where

(4.43b)

In Eqn.4.43b, first term is the signal power in the desired channel, second term the echo

signal power in the desired channel, third term (in the square bracket) the signal power in

the interfering channels and last term the echo signal power in the interfering channels.

The signal power, echo signal power and ASE noise psd at the receiver are given

by

76

Fig.4-4 Star network with preamplifiers (A, B, ...G is echo path).

(4.44a)

(4.44b)

and

(4.44c)

The signal current and noise variance are determined using the above equations, which are

given by

(4.45a)

and

77

(4.45b)

As before there are many signal levels for both bit 1 and 0 and the threshold is computed

which equalizes the probability of error for the highest level for bit 0 and the lowest level

for bit 0. The probability of errorPe(br,N1) for a givenbr and N1 will be

(4.46)

The averagePe will be

(4.47)

whereP′N1 is given by Eqn.4.35.

78

4.7 Example

In this example, a star network withNu users (Nu=2, 4, 8, 16, ...) has been

considered. Data rate for each channel has been assumed to be 1 Gb/s. Typical values of

different parameters in the network are as follows [19, 47, 62, 87].

Quantum efficiency of photodetector,η 0.95

Length of fiber from user to star coupler,L 1 km

Attenuation coefficient of fiber,α 0.2 dB/km

Unsaturated amplifier gain,G0 29 dB

Saturation power level of amplifier,Psat 10 dBm

Optical filter bandwidth,Bo 10 GHz

Insertion loss of each 2x2 coupler,Li 0.5 dB

Insertion loss of splice,Lsp 0.5 dB

Output power variability ofNuxNu coupler,Lcv 0.5 dB

Amplifier facet reflectivities,Rf 10-5 (-50 dB)

Amplifier coupling loss,Lcp 3 dB

Receiver temperature,T 3000 K

Load resistance,RL 100 Ohms

Spontaneous emission factor,nsp 3.0

Electrical bandwidth of receiver,Be 1 GHz


Filter insertion loss,Lfi 0.5 dB

79

For this network, numerical computations have been made for various values ofNu

and a specified BER of 10-9. The required transmitter power is determined (i) without SOA,

(ii) with optical postamplifier and (iii) optical preamplifier. The above is computed for

different values of extinction ratio i.e.ε =0.0, 0.05, 0.10 and 0.15. Numerical computations

are also made to include the effect of gain saturation and reflection noise. These results are

shown in Figs.4-5 to 4-14. In the following, results and inferences based on the above

figures are discussed.

Fig.4-5 Variations of minimum required average transmitter powerPta with number ofusersNu for ε = 0.0 in postamplifier scheme

i. Use of an ideal amplifier reduces the required minimum transmitter power level in

both postamplifier and preamplifier schemes. The reduction is more in the postamplifier

80

than in the preamplifier because ASE noise undergoes higher attenuation in the former


scheme.

ii. Gain saturation in the amplifier results in an increase in the required transmitter

power Pta. The difference in power level is referred to as power penalty. In the

postamplifier scheme, the penalty becomes more severe with increasingNu. It can be

explained as follows. AsNu increases, split loss of the star coupler also increases.

Therefore, more transmitter power is required to compensate for this loss. It results in

decrease in gain of the amplifier owing to an increase in gain saturation. The penalty

becomes so severe afterNu=512 that it overcomes the amplifier gain resulting in

degradation of system performance. The reason for gain saturation penalty exceeding

81

unsaturated amplifier gain is that for largeNu, gain for bit 1 is almost unity due to


saturation effect and hence ASE noise psd is negligible. However for bit 0, gain is much

more than unity and this results in substantial ASE noise psd. Therefore, on an average the

noise level is more than the noise level without amplifier. It degrades the system

performance for largeNu. The same trend is observed for all values of extinction ratio,ε.

The gain saturation effect increases with the increase inε.

In the preamplifier scheme, the effect of average gain saturation is small for low

values ofNu. It increases with an increase inNu. It is observed that degradation due to gain

saturation effect is much less in this scheme than in the postamplifier scheme because of

much lower average power at the amplifier input.

82

The effect of gain fluctuations in the preamplifier first increases withNu and after


attaining a maximum value decreases with the increase inNu (Figs.4-9 to 4-12). Since the

effect of gain fluctuations depends on the variance of probability density function of gain

which increases withNu, for low value of Nu. However, for largeNu average gain shifts

towards the lower extreme of the range (1 toG0) and the levels are cluttered together. This

results in a decrease in variance withNu at largeNu. For some intermediate value ofNu,

the variance will be maximum. This explains the variations of effect of gain fluctuations

with Nu.

iii. The effect of reflection noise reduces with the increase inNu for both post and

preamplifier schemes. AsNu increases, split loss of the star coupler also increases which

83

results in increased required transmitter power. In the postamplifier scheme, it will give rise

Fig.4-9 Variations of minimum required average transmitter powerPta with number ofusersNu for ε = 0.0 in preamplifier scheme

to gain saturation and hence a reduction in amplifier gain, while in the preamplifier scheme,

the total power at the amplifier input increases with the increase in number of channels.

Therefore, amplifier gain reduces with an increase inNu in the preamplifier scheme also.

A reduction in amplifier gain means a decrease in amplifier reflectance which reduces the

degrading effect of reflection. It is also observed that preamplifier scheme is more severely

affected by reflection noise than the postamplifier scheme. This can be attributed to more

gain saturation in the postamplifier.

84

iv. Figs.4-13 and 4-14 show the variations of average required transmitter power,Pta,


with ε. It is observed thatPta increases with an increase inε as expected.

Figs.4-7 and 4-11 are used to present the data in tabular form in Tables 4-1 and 4-2.

The following observations are made from these tables.

v. With the use of unsaturated SOAs in the network, number of users is increased for

both the post and preamplifier schemes. Gain saturation and reflection noise reduce the

above advantages. For example, in the postamplifier scheme,Nu increases to 4096 from 64

for transmitter power level of 0 dBm. When gain saturation and reflection noise are

85

considered,Nu is 256. In general, the number of users will increase as the reflection


Table 4-1 Number of users supported in the postamplifier scheme forε=0.10.

Pta

(dBm)

Number of users

Without SOAs UnsaturatedSOAs

GainsaturatedSOAs

Gain saturatedSOAs withreflection (Rsp=-20dB)

0 64 4096 256 256

-10 8 512 128 64

-20 0 32 32 0

-30 0 4 4 0

86

coefficient of splice decreases for a fixed transmitter power level.


vi. In the preamplifier scheme, greater number of users are supported except whenPta

is very low andRsp is very high e.g.Pta = -10 dBm andRsp = -20 dB. It is observed that

the supportable number of users reduces with the decrease inPta and the increase inRsp.

However, networks are usually not designed for very low transmitter powers and reflection

coefficients due to splices is much lower than -20 dB.

vii. For a typical Pta of 0 dBm, the preamplifier scheme performs better than the

postamplifier scheme in terms of an increase in the number of users. When the transmitter

power level is low (less than -10 dBm) and reflection coefficient is high, the postamplifier

87

scheme is better. However, networks are not implemented for such low transmitter power

Fig.4-13 Variations ofPta with ε for different Nu in postamplifier scheme. Solid linescorrespond to gain saturated SOAs and dashed lines to gain saturated SOAswith reflection noise.

levels and high reflection coefficients.

4.8 Conclusions

In this chapter, placement of optical amplifiers in a WDM star network has been

investigated. The number of users supported by the star topology for post and preamplifier

schemes has been determined. The effect of gain saturation and reflection noise in both

these schemes has been evaluated in terms of number of users supported or the required

transmitter power level for a given BER. The analyses also consider the effect of extinction

88

ratio. When there is no gain saturation and reflection noise in the network, postamplifier

Fig.4-14 Variations ofPta with ε for different Nu in preamplifier scheme. Solid linescorrespond to gain saturated SOAs and dashed lines to gain saturated SOAswith reflections.

scheme performs better. In presence of gain saturation, postamplifier scheme performs better

for low transmitter power levels. However, such power levels are not used in practice.

When number of users and consequently transmitter power increases, performance in

postamplifier scheme degrades, while in the preamplifier scheme it improves. The power

penalty due to reflection noise reduces in both the schemes with the increase in the number

of users.

Overall, the preamplifier is a better scheme than the postamplifier in WDM star

topology networks. The formalism developed in this chapter has been used to evaluate the

89

performance of tree-net topology in chapter 5. A comparative study of star network and

Table 4-2 Number of users supported in the preamplifier scheme forε=0.10.

Pta

(dBm)

Number of users

WithoutSOAs

UnsaturatedSOAs

AveragegainsaturatedSOAs

AveragegainsaturatedSOAs withgainfluctuation

Average gainsaturated SOAswith gainfluctuationsand reflection(Rsp=-20 dB)

0 64 1024 1024 512 512

-10 8 128 128 128 0

-20 0 16 16 16 0

-30 0 2 2 2 0

tree-net has been made in the same chapter.

90

Chapter 5

Semiconductor Optical Amplifiers InWDM Tree-net

In past few years, tree-net topology has received the attention of network designers.

The work reported in this chapter has resulted in the following communications

1 Y. N. Singh, H. M. Gupta and V. K. Jain, "Semiconductor Optical Amplifiers in WDM Tree-net,"IEEE/OSA Journal of Lightwave Technology(under review).

It provides large geographical area coverage with less amount of fiber and easy

expandability. The protocols can be designed for these networks to achieve low latency and

bounded delay [57]. Such networks can be configured in broadcast mode using passive

components or active regenerators. However, use of passive components is preferred for the

reasons of reliability and lower cost. The capacity of these networks can be tremendously

increased by making use of wavelength division multiplexing (WDM). The tree-net is a two

level topology (Fig.5.1) consisting of star as main topology and folded bus as auxiliary

topology. In a purely passive tree-net, the number of users supported is limited by split and

distribution losses in star couplers. The number of users supported can be increased by

using semiconductor optical amplifiers (SOAs).

Like in a star network, several SOA placement schemes are also possible in a tree-

net. As shown in the previous chapter, preamplifier scheme (SOAs as frontend of receivers)

is a better arrangement in the star topology [100]. With this in view, placement of SOAs

91

after the root (Fig.5-1) has been considered in the tree-net. In this chapter, tree-net has been

analyzed with and without SOAs. The results have been computed in terms of increase in

the number of users or reduction in the required transmitter power level with the placement

of SOAs. Subsequently, a comparative study of tree-net and star network has been made.

Fig.5-1 Basic tree-net topology.

5.1 System Description

In a tree-net withb (= 2i, i is a natural number) branches andn number of users

per branch, total number of supported users areNu (= bn). A tree-net topology withb=4

and n=4 is shown in Fig.5-1 [57]. In this topology, the users termed ’1’ are first order

users, ’2’ are second order users and so on. Therefore, order of users varies from 1 ton.

92

The power loss in the above network can be reduced by realising the star portion as

multistage star coupler (Fig.5-2) [66].

Fig.5-2a Structure of star coupler in tree-net without amplifiers.

In the tree-net topology,Na (=2k, k=0, 1, 2, ...) number of SOAs can be used. There

are three cases for incorporating SOAs in tree-net viz. (i)Na is one (20), (ii) Na ia less than

b and (iii) Na is equal tob. These configurations are shown in Fig.5-3a, 5-3b and 5-3c

respectively for 4x4 star portion of a tree-net. In these configurations, 3 dB couplers are

used to make the star coupler of dimensionNaxNa (Na > 1). The amplifiers are attached at

the output of this star coupler. Further, the dimension of the above is increased by using

the 3 dB 2x2 couplers as combiners at inputs (A, B, ...) and splitters at outputs (A’, B’,

...). The 3 dB 2x2 couplers are also used to connect the users to the folded bus.

93

Fig.5-2b Structure of branches in a tree-net without amplifiers.

Fig.5-3a A 4x4 star coupler with one amplifier in tree-net.

In the following analysis, number of wavelength channels in the tree-net are assumed

to be b. In a simple media access control (MAC) protocol, users on each branch transmit

on an unique wavelength channel. The contention for transmission among the users on same

branch can be resolved using variety of single channel protocols such as random access

time out (RATO) [14], ALOHA etc.. Each user can receive simultaneously on all the

94

wavelengths using an array of receivers. In contrast to WDM star [100], more than one user

Fig.5-3b A 4x4 star coupler with two amplifiers in tree-net.

Fig.5-3c A 4x4 star coupler with four amplifiers in tree-net.

can share a wavelength channel in tree-net. This reduces the required size of receiver arrays

at all the receivers. The available bandwidth in a wavelength channel is shared among the

users in a branch. Like in star network, the acknowledgement of packets is not required in

the tree-net also as the transmitting users listen to their own packet. If the transmitted

packet is received back correctly, it is expected that the packet has also been successfully

transmitted to its destination. It is also assumed that each transmitter in the tree-net adjusts

its transmitter power such that the power received from its own transmitter is same as that

95

received from highest order user (4th order user in Fig.5-2b). Thus a farthest (highest order)

user from the root on the bus transmits maximum power. This implies that the optical

power level at the amplifier input in each channel is same.

5.2 Analytical Model

In order to analyze the effectiveness of SOAs in tree-net, following four cases are

considered i.e. tree-net (i) without SOAs, (ii) with unsaturated SOAs, (iii) with SOAs

having average gain saturation and (iv) with SOAs having average gain saturation and gain

fluctuations. The optical amplifier model used in the analysis is same as used in previous

chapters.

For the worst-case analysis, the highest order transmitter has been considered. It is

remarked that the choice of receiver for worst-case analysis is not obvious. This is because

signal as well as the ASE noise is minimum at the highest order receiver. For the lowest

order receiver, both the signal as well as the ASE noise are at the maximum. Therefore,

BER for different receivers is computed and the receiver giving maximum BER or worst-

case receiver is identified. The computed results show that highest order receiver always

gives the worst performance. The BER for all the cases mentioned above are computed

following the same approach as in chapter 4 [100]. In the tree-net without SOAs, receiver

of highest order has been considered for the worst-case analysis as the received signal is

minimum for it. The lossLtr between transmitter and receiver depends upon number of

users, number of branches, attenuation in fiber, splice and insertion losses. It is given by

96

whereXtr is the above loss in dB and is given by

(5.1a)

(5.1b)

Here,Lss is the fiber loss between two consecutive users on a bus,Li the insertion loss of

a 3 dB 2x2 coupler,α the attenuation coefficient of fiber in dB/km,L the length of fiber

between order one user and star portion of the network in km,Lsp the splice loss andLfi

the insertion loss of wavelength demultiplexer. The received optical power is determined

using the given transmitter power level and lossLtr. The noise variances at the receiver

(sum of shot noise and thermal noise) are determined for received optical power levels for

bit 1 and 0. The BER which equalizes the probability of error for bit 1 and 0 is computed

using the signal currents and noise variances.

When SOAs without gain saturation are used in the tree-net, the optical power level

in a channel at the amplifier input is determined using given transmitter power level and

loss between transmitter and amplifierLta. In addition to parameters which affectLtr, the

Lta also depends on number of SOAs. It is given by

97

(5.2a)

whereXta is the above loss in dB and is given by

(5.2b)

The received signal and ASE noise at the receiver of orderj (1 ≤ j ≤ n) are determined

using the unsaturated gainG0 of the amplifier and loss,Lar, between amplifier and the

receiver of orderj. The lossLar is given by

(5.3a)

where

98

(5.3b)

At the receiver of orderj, noise variance due to ASE-signal beat noise, ASE-ASE beat

noise, shot noise (due to signal and dc component of ASE-ASE beat noise) and thermal

noise are determined for bit 1 and 0. The BER which equalises the probability of error for

bit 1 and 0 is determined using corresponding signal levels and noise variances.

In presence of average gain saturation, signal power at the input of SOA is

determined as mentioned above. It is presumed that bits in all the channels at the input of

SOA are synchronised which produce maximum degradation [75, 76]. For a given number

of channels having bit 1, total input power to SOA is determined and used for finding the

saturated gain. The average gain is determined by averaging this gain over the binomial

distribution of number of channels having bit 1. The received optical signal and ASE noise

at the input of the receiver are determined using this average gain. These are used to obtain

the signal levels and noise variances at the receiver output which are used to determine the

BER.

99

In the last case i.e. tree-net with SOAs having average gain saturation and gain

fluctuations, the BER is determined for a given bit pattern in the interfering channels. It

is then averaged using the binomial distribution of interfering channels having bit 1 to

determine overall BER. For a given channel, the power level at the input of SOA is

determined as in case of tree-net with unsaturated SOAs. The saturated gain of SOA for

bit 1 and 0 in desired channel is determined for a given number of interfering channels

having bit 1. Then received optical signal power and ASE noise psd are determined at the

input of receiver of orderj. These are used to determine signal levels and noise variances

at the receiver output for bit 1 and 0 in the desired channel. As there are (b-1) interfering

channels,b signal levels for both bit 1 and 0 will exist depending upon number of

interfering channels having bit 1. The highest signal level for bit 0 is always less than the

lowest level for bit 1 [75]. Using these two levels, threshold is determined which equalises

the probability of error for these levels. The probability of error for each signal level is

determined using the corresponding noise variance and the computed threshold. This

probability of error is averaged presuming equiprobable bit 1 and 0 in the desired channel

and the binomial distribution of number of interfering channels.

5.3 Example

The effectiveness of optical amplifiers on a sample tree-net is studied using the

above model. The tree-net has the following parameters:

Maximum allowed transmitter power,Pta 0 dBm

Desired BER 10-9

100

Insertion loss in 2x2 coupler,Li 0.5 dB

Unsaturated gain of SOAs,G0 29 dB

Saturation power level in SOAs,Psat 10 dBm

Distance between the root and first order user on a branch,L 1 km

Distance between consecutive nodes on a branch,Lss 100 m

Attenuation coefficient of fiber,α 0.2 dB/km

Load resistance of receiver,RL 100 Ω


Optical BW of a passband in wavelength demultiplexer,Bo 10 GHz

Electrical bandwidth of receiver,Be 1 GHz

Temperature of receiver,T 300 oK

Spontaneous emission factor,nsp 3.0

Insertion loss of splice,Lsp 0.5 dB

Insertion loss of wavelength demultiplexer,Lfi 0.5 dB

Extinction ratio,ε 0.10

Numerical results for the sample tree-net are computed using the above parameter

values. Table 5.1 shows the number of users supported,Nu, and minimum required

transmitter power level,Pta, for various values ofn without SOAs. Variations ofNu with

Na for three cases viz. tree-net with (i) unsaturated SOAs, (ii) average gain saturated SOAs

and (iii) average gain saturated SOAs with gain fluctuations are shown in Figs.5-4, 5-5 and

5-6 respectively. The numbers shown in brackets are the minimum required transmitter

power levels (dBm) for the corresponding values ofNu and Na. For example, in Fig.5-4a

whenn=2 andNa=32, Pta is -5.4 dBm and correspondingNu is 512. It is remarked that plot

101

for n=1 (not shown here) is similar to the ones forn=2. It is observed that for a given

Fig.5-4a Variations ofNu with Na (unsaturated SOAs) forn=2. The values in thebrackets are corresponding requiredPta in dBm.

Fig.5-4b Variations ofNu with Na (unsaturated SOAs) forn=3. The values in thebrackets are corresponding requiredPta in dBm.

value ofn, increase inNa may result in increasedNu or reducedPT or both. Increase inNa

102

does not necessarily increaseNu. Since in tree-net,Nu varies in discrete steps, the next

Fig.5-5a Variations ofNu with Na (average gain saturated SOAs) forn=2. The valuesin the brackets are corresponding requiredPta in dBm.

Fig.5-5b Variations ofNu with Na (average gain saturated SOAs) forn=3. The valuesin the brackets are corresponding requiredPta in dBm.

103

higher value may not be supported if the requiredPta is above 0 dBm. For example,Nu is

Fig.5-6a Variations ofNu with Na (average gain saturated SOAs with gain fluctuations)for n=2. The values in the brackets are corresponding requiredPta in dBm.

Fig.5-6b Variations ofNu with Na (average gain saturated SOAs with gain fluctuations)for n=3. The values in the brackets are corresponding requiredPta in dBm.

104

192 for n=3 andNa=4 (Fig.5-4b). When theNa is increased to 8,Nu is still 192 because

for the next admissible numberNu=384, requiredPta is more than 0 dBm. But the next

higher value ofNa i.e. 16 results in increase ofNu to 384. It is noticed that increase inNa

from 4 to 8 results in reduction inPta from -2.6 dBm to -5.7 dBm and no change inNu.

Therefore,Na=8 is not useful as the availablePta is 0 dBm and there is no increase inNu.

Table 5.1 Number of users and requiredPta for the tree-net without SOAs.

Number of users perbranch,n

Tree-net without SOAs

Nu=bn Pta (dBm)

1

2

3

64

32

12

-2.2

-2.2

-1.8

When average gain saturated SOAs are considered, a decrease inNu is observed

compared to above. For example, whenn=2, Na=16 supports 512 users for unsaturated

SOAs and 256 users for average gain saturated SOAs. This behaviour is true for all values

of n. Further, the gain fluctuations in SOAs either increasePta or reducesNu. For example,

when n=2, Na=16 still supports same number of users as in average gain saturation case,

but Pta has increased to -2.6 dBm from -3.8 dBm showing a power penalty of 1.2 dB. An

example of reduction inNu is n=3, Na=8 whenNu reduces from 192 to 96 (see Figs.5-5b

and 5-6b).

105

In tree-net topology,n=3 provides the values ofNu which are not admissible in star

topology. This is advantageous in certain cases as seen in Table 5.2. The table shows the

values ofNu corresponding to eachNa for all the three cases. For each case, minimum

requiredPta has also been computed and shown. It is observed that maximum number of

users are supported byn=2 in most of the cases. The worst-case losses inn=1 and 2 are

almost same, but the number of wavelengths used inn=2 are halved. This results in reduced

effect of average gain saturation and fluctuations. In some cases,n=3 supports more number

of users thann=2 (e.g.Na=32 supports a maximum of 384 users forn=3 in average gain

saturation case). This is due to the peculiarity of number of users admissible in tree-net for

n=3.

A comparison of star network with tree-net is given in Table 5.3. It is observed

from this table that both the networks can support 64 users without SOAs. Both star

network and tree-net with SOAs can support 128 users. However, the former needs 128

SOAs and the latter 4 SOAs. Similarly, star network can support 256 and 512 users with

256 and 512 SOAs respectively, while the tree-net requires 16 and 64 SOAs to support the

same number of users respectively. It is also observed that 1024 users are supported by the

tree-net only. The above is possible as the amplifiers are integrated within the star coupler

and the amplifier coupling loss is negligible. Therefore, the tree-net requires lesser number

of SOAs for a given number of supportable users. Both star network and tree-net cannot

support more than 1024 users irrespective of number of SOAs used.

106

Table 5.2 Maximum number of users andPta for a given number of SOAs. Numbers inbrackets are the corresponding values ofn.

Number ofamplifiers,Na

Unsaturated SOAs Average gain saturatedSOAs

Average gain saturatedSOAs with gain fluctu-ations

Nu (n) Pta (dBm) Nu (n) Pta (dBm) Nu (n) Pta (dBm)

1

2

4

8

16

32

64

128

256

512

1024

2048

128 (2)

128 (2)

256 (2)

256 (2)

512 (2)

512 (2)

1024 (2)

1024 (2)

2048 (2)

2048 (2)

2048 (2)

2048 (1)

-3.0

-6.1

-2.6

-5.7

-2.2

-5.4

-1.8

-3.8

-0.3

-1.5

-1.8

-1.8

64 (2)

64 (2)

128 (2)

192 (3)

256 (2)

384 (3)

512 (2)

512 (2)

1024 (2)

1024 (2)

2048 (2)

2048 (1)

-3.4

-10.0

-3.8

-0.3

-3.8

-0.3

-3.4

-6.9

-2.2

-4.6

-0.3

-1.1

64 (2)

64 (2)

128 (2)

128 (2)

256 (2)

256 (2)

512 (2)

512 (2)

1024 (2)

1024 (2)

1024 (2)

-

-0.7

-8.9

-1.8

-8.9

-2.6

-8.5

-3.0

-6.5

-1.8

-3.4

-3.0

-

5.4 Conclusions

It has been observed that the gain saturation in SOAs affects the performance of

tree-net. It reduces number of users supported as compared to tree-net with unsaturated

SOAs. The gain fluctuations in SOAs further deteriorate the performance. Since the star

topology supports maximum number of users without SOAs, star network and tree-net are

107

compared. It is shown that the tree-net require lesser number of SOAs as compared to star

Table 5.3 Comparison of star network and tree-net in terms of number of SOAs requiredfor a given number of users.

Number of users,Nu Number of SOAs requiredStar Tree

64

128

256

512

1024

0

128

256

512

-

0

4

16

64

256

for a fixed number of supportable users. This is because of topological advantage of tree-

net in terms of wider choices of users per SOA. It is remarked that in comparison to star

network, tree-net offers distinct advantages in terms of more geographical coverage, less

usage of fiber and transmitter sharing between more than one user [83, 86].

108

Chapter 6

Conclusions

In the research work reported in this thesis, usage of semiconductor optical amplifiers

(SOAs) in wavelength division multiplexed (WDM) star network and tree-net has been

studied. The SOAs are used to alleviate the limitation on power budget and hence number of

users supported. Error correcting codes can also be used for this purpose. The use of SOA

and an error correcting code has been investigated in a point-to-point OOK link. It has been

observed that the improvement in power budget due to SOA exceeds the improvement due

to coding by approximately 20 dB. Therefore, the use of SOA is a much better option than

the use of an error correcting code. In view of this, only the SOAs are considered for the

placement in WDM star and tree networks.

Two SOA placement schemes (i.e. postamplifier and preamplifier) in WDM star

network have been investigated. The number of SOAs in these schemes are always equal to

number of users. Each user is assigned an unique wavelength for transmission. The study

shows that when unsaturated SOAs are used, the postamplifier scheme is better than the

preamplifier scheme. The reason for the above is that the amplified spontaneous emission

(ASE) noise is attenuated more in the postamplifier scheme than in the preamplifier scheme.

Consequently, the effect of ASE noise is less in the postamplifier scheme. When gain

saturated SOAs are considered, the supportable number of users is reduced in both the

postamplifier and the preamplifier schemes. The degradation increases with an increase in the

number of users in both the schemes. This is quite expected. However, the degrading effect

of gain saturation is relatively more in the postamplifier scheme. The SOAs in the

109

preamplifier scheme are affected by both average gain saturation and gain fluctuations. The

average gain saturation is dependent on number of WDM channels and hence number of

active users. The gain fluctuations depend on bit patterns in active channels. The analysis in

chapter 4 shows that the degrading effect of average gain saturation increases with an increase

in the number of users. However, the degradation due to gain fluctuations first increases to

a maximum value and then decreases. When reflection noise due to splices is also considered,

the performance of both the schemes degrades. The above degradation is more severe in the

preamplifier scheme than in the postamplifier scheme. The effect of nonzero extinction ratio

has also been studied. As expected, it has been observed that the degradation increases with

an increase in extinction ratio. In general, the preamplifier scheme performs better than the

postamplifier scheme and hence supports more number of users.

The tree-net topology is a more general topology of which star is a special case. The

tree-net can support 2in (i=1,2,...,n=1,2,...) users while star supports only 2i users. In the tree-

net, there exists the possibility of sharing transmitter sources using passive access node (PAN)

interface [86]. In this thesis, number of wavelengths in the tree-net is chosen to be equal to

number of branches. The SOAs are placed in the star coupler part of the tree-net. In contrast

to the star network, number of SOAs in the tree-net can be less than the number of branches.

The number of users supported by tree-net with unsaturated SOAs and saturated SOAs is

determined. As expected, the supported number of users reduces when SOAs are gain

saturated. When tree-net is compared with the star, it is observed that the tree-net can support

more number of users than star for a given maximum transmitter power level and number of

amplifiers. This advantage accrues due to the topology of the tree-net which provides wider

possible assignment of number of users for a given number of SOAs. In brief, it is concluded

110

that use of SOAs is a better choice to increase the supportable number of users in both star

network and tree-net. Further, the tree-net utilizes the SOAs more effectively as compared to

the star network.

In the work presented in this thesis, following assumptions and simplifications have

been made. The noise of the transmitting sources has not been considered. The dispersion in

fiber is neglected as distances involved are small for an appreciable dispersion. Modal noise

due to fiber [68] has been assumed to be absent. The couplers used are considered to be

wavelength independent. Further, the demultiplexers and filters are taken to be ideal.

Therefore, no crosstalk occurs due to these components. The responsivity of photodetectors

has been assumed to be wavelength independent over the range of operation.

In the foregoing analyses, SOAs are considered as travelling wave amplifiers (TWAs)

and the gain profile is assumed to be uniform in the operating wavelength range. Further, the

crosstalk in the SOAs are considered to arise only due to the gain saturation.

In this thesis, use of SOAs in only two of the many topologies i.e. WDM star and

WDM tree-net has been studied. Therefore, OA placement options can also be explored in

other broadcast topologies including multilevel topologies. It is remarked that multilevel

topologies may be advantageous in terms of number of OAs required for a given number of

users.

As mentioned in chapter 1, switched optical networks will be attractive in the future.

Therefore, the placement of OAs in these networks requires investigations. These switched

111

networks include wavelength routed networks in which wavelengths are directed to those parts

of the network where these are needed the most. The proper routing of wavelengths allows

wavelengths reuse in different parts of network [54].

In this thesis, SOAs have been considered because these can be easily integrated in

star coupler, transmitter or receiver. However, doped fiber amplifiers (DFAs) are finding

applications in wide area networks as in-line amplifiers. Therefore, analysis presented in this

thesis can be extended to the networks using DFAs.

Another important area where optical amplifier would be very useful is subscriber

access networks (SANs). In these, both WDM techniques and OAs can be used for connecting

large number of users to a central office. The topologies used in SANs have different

requirement as compared to LANs and WANs. The OA placement studies can be extended

to SANs considering these requirements.

In the above, use of OAs as signal amplifier for the purpose of increasing the size of

network has been suggested. The OAs can also function as switches and wavelength

convertors. Therefore, studies of optical networks which include these additional

functionalities of OAs are suggested.

It is concluded from the above that OAs will form an important part of the future high

capacity optical networks.

112

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121

Appendix - I

Derivation of Ramp(G)

Let the reflectivities of two facets of SOA beR1 andR2 as shown in Fig.A-1. TheEi

is the amplitude of the input electric field andEo1, Eo2, ...Eop are the output electric field

amplitudes after 0, 2, ...2(p-1) reflections. Here,p is an integer. LetEr1, Er2, ...Erp be the

reflected electric field amplitudes after 1, 3, ...(2p-1) reflections (see Fig.A-1). The amplitudes

of the output electric fields are given by

(A-1a)

(A-1b)

and

(A-1c)

whereβ is the propagation constant in the gain media,G the single pass gain in cavity and

Lg the length of gain media. The total output electric field is given by

(A-2)

123

In the above equation,p has been taken as infinity. Normally, it must be an integer part of

Fig.A-1 Reflected and transmitted signals in a Fabry-Perot amplifier.

Lc/(2xLg). TheLc is the coherence length of laser light and is related to laser linewidth as [7]

(A-3)

whereCf is the speed of light in the fiber and∆ν the laser linewidth. The coherence length

of laser output for a linewidth of 1 MHz andCf=2x108 m/s in glass fiber will be 63.64 m [7].

Therefore,m would be 31,820 assuming the OA cavity length to be 1 mm. Further, the

amplitude of the output field decreases at each reflection. It means higher order reflections

will have negligibly small contribution in the output electric field. Therefore, the

approximation of extendingm to infinity is valid. Equation A-2 can be written as

124

(A-4)

When the above series converges toG R1R2 <1,

(A-5)

Substitution ofEo1 from Eqn.A-1a gives

(A-6)

The transmittanceTamp is given by

(A-7)

It is seen from the above equation thatTamp will be maximum when 2βLg = 2πi wherei is an

integer. The maximum value ofTamp is referred asTmax and is given by

125

(A-8)

Like the transmitted electric fields, reflected electric fields are given by

(A-9a)

(A-9b)

or

(A-9c)

and

(A-9d)

Following the same approach as used in the evaluation of the total output electric field, the

total reflected electric field is given by

126

(A-10)

This equation is used to determine the reflectanceRamp of the amplifier. As mentioned earlier,

Tamp will be maximum when 2βL=2πi. Under the same condition, reflectance of the amplifier

Ramp is given by

(A-11)

127

Appendix -II

Statistics of Signal and Echo Beat Noise

In presence of reflection, the information signal and echo signal beat at the

photodetector to produce the signal-echo beat noise. The electric fields due to echo and the

information signals are given by

(B-1a)

and

(B-1b)

At the photodetector,eecho ander beat and produce the beat noise current

(B-2a)

where

(B-2b)

AssumingΘ to be uniformly distributed random variable between -π andπ, its pdf is given

by

129

(B-3)

Therefore, the mean and variance of beat noise current,iecho-sigare given by

(B-4a)

(B-4b)

130

Biography of Author

The author was born in Delhi in 1969. He obtained B.Tech. in Electrical Engineering

with honours from Regional Engineering College, Hamirpur, Himachal Pradesh in July 1991

and M.Tech. in Optoelectronics & Optical Communications from Indian Institute of

Technology, Delhi in December 1992. Since then he is pursuing his Ph.D in Electrical

Engineering Department, Indian Institute of Technology, Delhi. He is an associate member

of Institution of Electronics and Telecommunication Engineers (IETE), India. His academic

interests include Optical Computing, Optical Networks, Photonic Switching and Optical

Communications. He is also interested in MAC Protocols and Philosophy of Science.

131

List of Publications Resulted from the Thesis Research

A. Journals

1. V. K. Jain, Y. N. Singh and H. M. Gupta, "Power Penalty due to Optical AmplifierInduced Crosstalk in Non-Coherent OOK Transmission Systems,"Journal of OpticalCommunications, Vol.16, No.5, Oct.1995, pp.194-196.

2. Y. N. Singh, V. K. Jain and H. M. Gupta,"Effect of Reed-Solomon Code on LaserLinewidth Requirements of BPSK Homodyne Optical Communication Systems,"Journal of Optical Communications,Vol.16, No.6, Dec.1995, pp.207-210.

3. Y. N. Singh, V. K. Jain and H. M. Gupta, "Semiconductor Optical Amplifiers inWDM Star Networks,"IEE-Proceedings Optoelectronics,Vol.143, No.2, April 1996,pp.144-152.

4. Y. N. Singh, V. K. Jain and H. M. Gupta, "Reed-Solomon Code and SemiconductorOptical Preamplifier in OOK Communication System: A Comparative Study,"Journalof Optical Communications(accepted for publication).

5. Y. N. Singh, H. M. Gupta and V. K. Jain, "Semiconductor Optical Amplifiers inWDM Tree-net,"IEEE/OSA Journal of Lightwave Technology(under review).

B. Conferences

6. Y. N. Singh, V. K. Jain and H. M. Gupta, "Effect of Error Correcting Codes on LaserLinewidth Requirements in Optical Binary Phase Shift Keying CommunicationSystems," Presented inIXth National Convention of Electronics and TelecommunicationEngineers,University of Roorkee, Roorkee, India, March 30-31, 1994.

7. V. K. Jain, Y. N. Singh and H. M. Gupta, "Effect of Optical Amplifier InducedCrosstalk in Two Channel Non-Coherent OOK Transmission System," Presented inIXth National Convention of Electronics and Telecommunication Engineers, Universityof Roorkee, Roorkee, India, March 30-31, 1994.

8. Y. N. Singh, V. K. Jain and H. M. Gupta, "WDM Data Network," Presented inIXth

National Convention of Electronics and Telecommunication Engineers, University ofRoorkee, Roorkee, India, March 30-31, 1994.

9. Y. N. Singh, V. K. Jain and H. M. Gupta, "On Placement of Semiconductor LaserAmplifiers in WDM Star Networks," Proc. National Conference on OpticalCommunications, J.K.Institute of Applied Physics & Technology, University ofAllahabad, Allahabad, India, Feb.22-24, 1995, pp.66-78.

133

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