Studies on Semiconductor Optical Amplifier Placements in WDM Star and Tree Topology Networks

154 Pages·2000·2.08 MB·English

Studies on Placement of Semiconductor
Optical Amplifiers in Wavelength Division
Multiplexed Star and Tree Topology Networks
by
Yatindra Nath Singh
submitted
in fulfilment of the requirement of degree of
Doctor of Philosophy (Ph.D.)
to
Electrical Engineering Department
Indian Institute of Technology, Delhi
Hauz Khas, New Delhi 110016
India
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 Engineering
Indian Institute of Technology, Delhi
Hauz Khas, New Delhi
India
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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)
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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
beforethereceiver.In thepostamplifierscheme,threecaseshave been investigated.Firstcase
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
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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 uses bxb 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.
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Table of Contents
List of Figures ix
List of Tables xiii
List of symbols xv
Abbreviations xix
Chapter 1
Introduction 1
1.1 Need for Optical Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Fiber Optic Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Multiplexing Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 WDM Broadcast Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Chapter 2
Optical Amplifiers in Broadcast Networks 13
2.1 Optical Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Optical Amplifiers in Broadcast Networks . . . . . . . . . . . . . . . . . . . . . 20
2.2.1 Bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.2 Dual Bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.3 Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2.4 Star . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Chapter 3
Semiconductor Optical Amplifier and Coding in OOK System 37
3.1 Uncoded System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.2 Coded System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.3 Uncoded System with Optical Amplifier . . . . . . . . . . . . . . . . . . . . . . 45
3.4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Chapter 4
Semiconductor Optical Amplifiers in WDM Star Topology 51
4.1 System Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2 Amplifier Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.3 Evaluation of Minimum Required Transmitter Power . . . . . . . . . . . . 56
4.4 Star without Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.5 Star with Postamplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.5.1 Unsaturated Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.5.2 Effect of Gain Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.5.3 Effect of Reflection Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.6 Star with Preamplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.6.1 Unsaturated Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
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4.6.2 Effect of Gain Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.6.3 Effect of Reflection Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.7 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Chapter 5
Semiconductor Optical Amplifiers In WDM Tree-net 91
5.1 System Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.2 Analytical Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.3 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Chapter 6
Conclusions 109
References 113
Appendix - I 123
Appendix -II 129
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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 node
interface). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 distributed N x N (N =m2) reflective star coupler
u u u
based on m number of m x m couplers. . . . . . . . . . . . . . . . . . . . . . . 28
Fig.2-10 16x16 distributed reflective star coupler. . . . . . . . . . . . . . . . . . . . . . 29
Fig.2-11 Implementation of distributed N x N (N = 2 m2) coupler based on 2m
u u u
number of mxm couplers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Fig.2-12 32x32 distributed reflective star coupler. . . . . . . . . . . . . . . . . . . . . . 31
Fig.2-13 FDM/OFDM distributed expandable amplified star coupler
configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Fig.2-14 Star coupler with amplifiers and without bandpass and bandstop
filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Fig.2-15 8x1 coupler based on seven 2x2 couplers. . . . . . . . . . . . . . . . . . . . . 34
Fig.2-16 Forward pumped EDFA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Fig.2-17 mm x mm reflective star. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
c c
Fig.2-18 m x m reflective star coupler based on m x1 tree coupler. . . . . . . . . . 36
c c c
Fig.3-1a The system block diagram for optical communication system with
coding scheme. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Fig.3-1b ThesystemblockdiagramforopticalcommunicationsystemwithSOA
as preamplifier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Fig.3-2 Worst-case waveform patterns for (a) bit 1 and (b) bit 0 before and
after the Gaussian filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
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