ALL OPTICAL SIGNAL REGENERATION TECHNIQUE DESIGN AND...

ALL OPTICAL SIGNAL REGENERATION TECHNIQUE DESIGN AND REAL

TIME IMPLEMENTATION FOR DIFFERENT MODULATION SCHEMES

USING ULTRASCALE FPGA

BHAGWAN DAS

UNIVERSITI TUN HUSSEIN ONN MALAYSIA

ALL OPTICAL SIGNAL REGENERATION TECHNIQUE DESIGN AND REAL

TIME IMPLEMENTATION FOR DIFFERENT MODULATION SCHEMES

USING ULTRASCALE FPGA

BHAGWAN DAS

A thesis submitted in

fulfillment of the requirement for the award of the

Doctor of Philosophy

Faculty of Electrical and Electronic Engineering

UNIVERSITI TUN HUSSEIN ONN MALAYSIA

86400 Parit Raja, Johor

MAY 2017

iii

I would like to dedicate this thesis to

Almighty “GOD”

(Who gave me strength, knowledge, patience and wisdom)

My “Parents”

(Their pure love, devotion, cares and prays helps me to achieve this target)

My “Wife, Mehak; daughter, Heer and son, Yash”

(Their love, care, commitment and sincerity motivate me to finish this valuable work)

iv

ACKNOWLEDGEMENT

I am grateful to Almighty GOD who is the most congenital, most sympathetic and

sustainer for the worlds for giving me the potency and the ability to do this research

work.

I would like to express my special appreciation and thanks to my academic

supervisor Professor Dr. Mohammad Faiz Liew bin Abdullah and Co-supervisor

Dr. Nor Shahida Mohd Shah for their continuous support, encouragements at every

stage of study with patience and unlimited guidance results, the completion of this

study within time. Their advice on both research as well as on my career have been

priceless.

I express my gratitude to my parent university Quaid -e- Awam University of

Engineering, Science and Technology Nawabshah Pakistan who sponsor me for

this research work, without the financial support it was impossible to complete this

study. I am thankful to Universiti Tun Hussein Onn Malaysia who has provided me

a platform where, I can undergo for my higher studies.

I would like to thank Prof. Dr. Bhawani Shaker Chowdhury, Dean of

Faculty of Electrical, Electronic and Computer Engineering, Mehran University

of Engineering and Technology, Jmashoro for his endless support and guidance.

I am also thankful to all my parents and dearest friends for their moral support and

motivation at the every step of this study.

v

ABSTRACT

The all-optical signal regeneration is a demanding research area for long haul optical

communication systems. Electronic signal regeneration is limited due to its real-time

infeasibility in terms of data rate and accumulated losses; therefore, all-optical signal

regeneration is utilized to overcome these issues. The existing all-optical signal

regeneration techniques have not been able to facilitate low power consumption,

demonstration of real-time low cost commercial based design systems and application

for the optical systems. In this research, a new all-optical signal regeneration technique

is developed using single- pump Phase Sensitive Amplification, designed Optical

Frequency Locked Model and noise mitigation model. The designed technique

consumes less power than existing signal regeneration techniques for 10Gb/s optical

degraded signal for different amplitude and phase modulation formats transmitted at

different transmission distances between 50 km to 250 km. The designed all-optical

signal regeneration technique is realized using numerically and verified using

Simulink model. A real-time demonstration and commercial design based application

is developed using Xilinx KCU105 UltraScale FPGA. The new all-optical signal

regeneration technique has achieved a very low Bit Error Rate (BER) of 10-13 at low

received power of -16 dBm averagely for different transmission distances between 50

km to 250 km via simulation and experiment. The new all-optical signal regeneration

technique consumes low power of -16dBm, compared to existing all-optical signal

regeneration techniques that consumes -9dBm. The new all-optical signal regeneration

technique consumes 45% less power; with low BER and low received power compared

to existing technique. The new all-optical signal regeneration system offers, real time

implementation, live monitoring and commercial based design for Differential Phase

Shift keying (DPSK) Non-Return-to-Zero (NRZ), DPSK-Return-to-Zero (RZ), Binary

PSK (BPSK), Differential BPSK, Quadrature PSK, Orthogonal Frequency Division

Multiplexing (OFDM), Quadrature Amplitude Modulation (QAM), Binary Frequency

Shift Keying (BFSK), 8-PSK, and On-Off Keying (OOK).

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ABSTRAK

Penjanaan semula isyarat semua optik merupakan kawasan penyelidikan yang mencabar bagi

jarak jauh sistem komunikasi optik. Penjanaan semula isyarat elektronik adalah terhad

disebabkan oleh kemudahan nyata untuk mengawal kadar penghantaran data dan kehilangan

data terkumpul, Oleh itu, penjanaan semula isyarat optik diperlukan untuk mengatasi isu-isu

ini. Teknik-teknik penjanaan semula isyarat semua optik yang sedia ada tidak mampu untuk

mengawal penggunaan kuasa yang rendah, demonstrasi komersial sistem reka bentuk

berasaskan masa nyata yang kos rendah dan aplikasi untuk sistem optik. Dalam kajian ini,

teknik penjanaan semula isyarat optik dibangunkan menggunakan “Single Phase sensitive

amplification”, direka oleh model optik frekuensi dan model bunyi pengurangan. Teknik yang

direkabentuk menggunakan kuasa yang kurang daripada teknik penjanaan semula isyarat yang

sedia ada untuk isyarat optik 10Gb/s untuk amplitud dan fasa modulasi format yang berbeza

dan dihantar pada jarak penghantaran yang berbeza iaitu antara 50 km hingga 250 km. Teknik

penjanaan semula isyarat semua optik yang direkabentuk dapat direalisasikan menggunakan

kaedah berangka dan disahkan menggunakan model Simulink. Satu demonstrasi masa sebenar

dan aplikasi komersial berdasarkan reka bentuk yang dibangunkan menggunakan Xilinx

KCU105 UltraScale FPGA. Teknik penjanaan semula isyarat semua optik yang baru ini telah

mencapai kadar yang sangat rendah iaitu Bit Error (BER) daripada 10-13 pada kuasa penerima

rendah iaitu -16 dBm, secara purata untuk jarak penghantaran yang berbeza antara 50 km

hingga 250 km melalui kaedah simulasi dan eksperimen. Teknik penjanaan semula isyarat

semua-optik yang baru ini menggunakan kuasa rendah -16dBm, berbanding dengan semua-

optik teknik penjanaan semula isyarat sedia ada yang menggunakan -9dBm.

Teknik penjanaan semula isyarat semua-optik yang baru ini menggunakan kurang 45% kuasa;

dengan BER yang rendah dan kuasa penerima yang rendah berbanding dengan teknik yang

sedia ada. Reka bentuk semua-optik sistem penjanaan semula isyarat menawarkan,

pelaksanaan masa sebenar, pemantauan secara langsung dan komersil berdasarkan rekabentuk

untuk Differential Phase Shift keying (DPSK) Non-Return-to-Zero (NRZ), DPSK Return to

Zero (RZ), Binary PSK (BPSK), Differential BPSK, Quadrature PSK, Orthogonal Frequency

Division Multiplexing (OFDM), Quadrature Amplitude Modulation (QAM), Binary

Frequency Shift Keying (BFSK), 8-PSK, and On-Off KEying (OOK).

vii

TABLE OF CONTENTS

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xiii

LIST OF FIGURES xv

LIST OF APPENDICES xxiv

LIST OF SYMBOLS xxv

LIST OF ABBREVIATIONS xxix

LIST OF AWARDS xxxii

LIST OF PUBLICATIONS xxxiii

CHAPTER 1 INTRODUCTION 1

1.1 Background 1

Why optical signal regeneration? 3

1.2 Motivation 3

1.3 Problem statement 5

1.4 The research question and objectives 6

1.5 Aim of the study 7

1.6 Scope of study 7

1.7 Limitation of the study 9

1.8 Thesis organization 10

CHAPTER 2 LITERATURE REVIEW 11

2.1 Introduction 11

2.2 Optical fiber communication system 11

2.3 Optical transmitter 12

viii

Differential Phase Shift Keying (DPSK) signal

format 13

Laser signal 14

Optical modulator 15

2.4 Optical fiber and optical signal propagation 18

Optical signal propagation 19

Optical fiber impairments 21

2.4.2.1 Attenuation 22

2.4.2.2 Dispersion 22

Non-Linear fiber effects 24

2.4.3.1 Stimulated Brillioun Scattering (SBS)

and Stimulated Raman Scattering (SRS)

effects 24

2.4.3.2 Self-Phase Modulation (SPM) and

Cross Phase Modulation (XPM) 25

2.4.3.3 Four Wave Mixing (FWM) 25

2.4.3.4 Nonlinear phase matching 27

Highly Nonlinear Fiber (HNLF) 27

Four Wave Mixing (FWM) in Highly Nonlinear

Fiber (HNLF) 28

Parametric amplification 29

2.4.6.1 Fiber Optic Parametric Amplifiers

(FOPA) 29

Phase Sensitive Amplification (PSA) 32

2.4.7.1 Types of PSA 33

2.5 Optical filtering 34

Directional coupler 36

Optical filter types 36

2.6 Optical receiver and performance evaluation of optical

communication system 38

Bit Error Rate and Q-factor 39

Optical Signal-to- Noise Ratio (OSNR) 40

Eye diagram 41

2.7 Field Programming Gate Array (FPGA) 42

ix

The Xilinx KCU105 UltraScale FPGA

Evaluation board 44

2.8 All-optical signal regeneration 45

Existing all-optical signal regeneration systems

and their related issues 46

2.9 Research gap 49

2.10 Summary 52

CHAPTER 3 NEW ALL-OPTICAL SIGNAL REGENERATION

TECHNIQUE 54

3.1 Introduction 54

3.2 System model and algorithm development of the all-

optical signal regeneration technique 54

3.3 Numerical design of new all-optical signal regeneration

technique 57

Phase Sensitive Amplification (PSA) model 57

3.3.1.1 Optical filtering 61

Optical frequency locked signal model 63

Noise mitigation model 65

3.4 Simulation setup for all-optical signal regeneration

technique 69

Simulink model for PSA model 69

Simulink model for optical frequency locked

signal model 72

Noise mitigation using Autoregressive optical

filter 75

3.5 Summary 77

CHAPTER 4 IMPLEMENTATION OF NEW ALL-OPTICAL

SIGNAL REGENERATION TECHNIQUE 78

4.1 Introduction 78

4.2 Implementation of the developed all-optical signal

regeneration for DPSK transceiver systems 78

4.3 Implementation of all-optical signal regeneration

Differential Phase Shift keying Non-Return-to-Zero

(DPSK-NRZ) transceiver system using Simulink model 79

x

10Gb/s optical Differential Phase Shift Keying

Non-Return-to-Zero (DPSK-NRZ) transmitter 81

Propagation of 10Gb/s optical DPSK-NRZ

signal over noisy Single Mode Fiber (SMF) link 85

Implementation of the developed all-optical

signal regeneration over degraded 10Gb/s optical

DPSK-NRZ 88

Receiver model 101

Performance evaluation for the developed all-

optical signal regeneration technique for 10Gb/s

optical DPSK-NRZ transceiver system 104

4.3.5.1 BER analysis 105

4.3.5.2 Q-factor and Eye diagram analysis 107

4.3.5.3 Optical Signal to Noise ratio (OSNR)

analysis 109

Performance analysis of the developed all-optical

signal regeneration technique for 10Gb/s

degraded DPSK-NRZ signal propagated at

different transmission distances 110

4.4 Implementation of the developed all-optical signal

regeneration for Differential Phase Shift keying- Return-

to-Zero (DPSK-RZ) transceiver system using Simulink

model 114

4.5 Simulink design and performance analysis of the

developed all-optical signal regeneration technique for

different modulation formats 133

4.6 Summary 138

CHAPTER 5 HARDWARE EXECUTION AND SYSTEM

VALIDATION OF THE NEW ALL-OPTICAL SIGNAL

REGENERATION SYSTEM 141

5.1 Introduction 141

5.2 System model for experimental setup 141

5.3 Design process of hardware implementation 143

xi

5.4 HDL design entry of the developed all-optical signal

regeneration for DPSK- NRZ transceiver system 145

HDL code generation for Simulink models of

DPSK-NRZ all-optical signal regeneration

system 146

HDL code generation for Simulink models of

DPSK-RZ all-optical signal regeneration system 147

RTL schematic diagram of the developed all-

optical signal regeneration for DPSK- NRZ

transceiver system 147

RTL schematic diagram of DPSK- RZ

transmitter 148

Export the RTL design of the developed all-

optical signal regeneration technique for DPSK-

NRZ and DPSK-RZ system to Xilinx KCU105

UltraScale FPGA board 148

5.5 Design verification process 149

5.6 Real time hardware setup view 150

5.7 Real-time implementation and performance analysis of

the developed all-optical signal regeneration technique

for DPSK-NRZ and RZ system implemented on Xilinx

KCU105 UltraScale FPGA board 151

Experiment 1: Real time implementation of the

developed all-optical signal regeneration

technique for DPSK-NRZ system 151

Performance analysis 152



technique for DPSK-RZ system 164



technique for other modulation formats 172

5.8 System validation 174

5.9 Discussion 178

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5.10 Summary 180

CHAPTER 6 CONCLUSION AND FUTURE WORK 182

6.1 Conclusion 182

6.2 Research contributions 184

6.3 Future work 185

REFERENCES 186

APPENDIX A1 198

APPENDIX A2 201

APPENDIX B1 204

APPENDIX B2 210

APPENDIX C1 212

APPENDIX C2 227

VITA 229

xiii

LIST OF TABLES

2.1 Literature overview and comparison with the designed system 50

3.1 Feasible parameters and values used for developing the PSA

Simulink model 71

3.2 Feasible parameters and values used for optical frequency locked

signal model 75

3.3 Feasible parameters and values used for noise mitigation model 76

4.1 Feasible parameters and values used for developing the designed

10Gb/s optical DPSK-NRZ transmitter model 83

4.2 Feasible parameters and values of developed all-optical signal

regeneration technique for 10Gb/s degraded optical DPSK- NRZ

signal 89

4.3 Feasible parameters and values used in receiver model 102

4. 4 Analysis in terms of power penalty at BER 10-9 106

4.5 BER versus received power analysis 129

4.6 BER versus OSNR analysis 131

4.7 BER versus OSNR analysis of selected modulation formats

implemented for all-optical signal regeneration technique 136

4.8 Summary of performance of the developed all-optical signal

regeneration technique 139

5.1 Experimental BER analysis before implementing the developed

all-optical signal regeneration technique over degraded links of

DPSK-NRZ system 157

5.2 Experimental BER analysis after implementing the developed all-

optical signal regeneration technique over degraded links of

DPSK-NRZ system 161

xiv

5.3 Experimental BER analysis before implementing the developed

all-optical signal regeneration technique over degraded links of

DPSK-RZ system 165



DPSK-RZ system 171



different formats 173

5.6 Summary of comparing the results of simulation and experiment

of performance of the developed all-optical signal regeneration

technique 179

xv

LIST OF FIGURES

1.1 Exponential growth in internet traffic across the globe [1] 1

1.2 Optical signal regeneration requirement [2] 3

1.3 Capacity distance product versus modulation formats usage [13] 4

1.4 Scope of the study using K-Chart 8

2.1 A typical optical fiber long haul communication system [13] 12

2.2 An optical transmitter block diagram [13] 12

2.3 Classification of phase and intensity digital formats [17] 13

2.4 DPSK signal format: (a) signal generation, (b) signal constellation

diagram [22] 14

2.5 Laser model using laser rate equation [24] 15

2.6 Optical phase modulator [28] 16

2.7 Optical intensity modulator [28] 16

2.8 MZIM phasor and electric to optical characteristics: (a) P-V

characteristics curve, and (b) phasor diagram [28] 17

2.9 Optical DPSK transmitter: (a) Optical DPSK-NRZ, and (b)

Optical DPSK-RZ 18

2.10 Different transmission impairments effects in optical fiber [37] 21

2.11 Dispersion in SMF due to material dispersion (DM) and

waveguide dispersion (DW) [38] 23

2.12 FWM components generated when three inputs are present [44] 26

2.13 FOPA schemes: (a) single pump, and (b) dual pump [46] 29

2.14 Difference between conventional amplification and PSA: (a)

conventional amplification, and (b) Phase Sensitive Amplification

[53] 33

2.15 Implementation of PSA using different schemes [57] 34

2.16 Direction coupler parameters [61] 36

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2.17 MA optical filter schematic:(a) MZI and (b) waveguide model

[61] 37

2.18 AR optical filter schematic:(a) MZI model and (b) waveguide

model [61] 37

2.19 PDF of received binary signal [65] 39

2.20 Typical eye diagram [67] 41

2.21 Field Programming Gate Array (FPGA) architecture [71] 42

2.22 FPGA advantages: (a) applications [72], and (b) software support

[73] 43

2.23 Latest FPGA performance and productivity offered by Xilinx

[75] 43

2.24 Xilinx UltraScale KCU105 Evaluation board block diagram [76] 44

2.25 Xilinx UltraScale KCU105 evaluation board and key features

[77] 44

2.26 Illustration of existing all-optical signal regeneration techniques 47

2.27 Recent technologies for optical nonlinear signal processing [110] 48

2.28 Experimental setup of high-speed optical link using FPGA [143] 49

3.1 System model for the developed optical signal regeneration

technique 55

3.2 Flow chart of algorithm design of the developed all-optical signal


3.3 Block diagram of PSA model 57

3.4 Generation of phase harmonics via FWM in frequency domain 57

3.5 Schematic design of single stage Moving Average optical filter 61

3.6 Block diagram of optical frequency locked signal model 64

3.7 Block diagram of noise mitigation model 66

3.8 Schematic diagram of Autoregressive (AR) optical filter 68

3.9 Simulation setup for the all-optical signal regeneration technique

using MATLAB Simulink 70

3.10 Simulink model of MZIM modulator 72

3.11 Simulink model of internal design of MZIM modulator 73

3.12 Characteristics curves of MZIM 74

3.13 Detector circuit model 74

xvii

4.1 System model of implementing the developed all-optical signal

regeneration for DPSK transceiver systems 79

4.2 Simulink model of the developed all-optical signal regeneration

technique for DPSK-NRZ transceiver system 80

4.3 Simulink model for the designed 10Gb/s optical DPSK-NRZ

transmitter 82

4.4 Response of the designed optical DPSK-NRZ transmitter: (a)

10Gb/s PRBS sequence, (b) 10Gb/s PRBS DPSK sequence, (c)

Laser signal of 1550.8 nm, and (d) 10Gb/s optical DPSK-NRZ

modulated signal 84

4.5 Simulink model for the designed Single Mode Fiber link together

with noise and dispersion model 85

4.6 Single Mode Fiber response 87

4.7 Response of degraded 10Gb/s optical signal received at end of

noisy fiber link of 150 km transmission distance 87

4.8 Frequency spectrum of degraded 10Gb/s optical DPSK-NRZ

signal 88

4.9 PSA performance analysis using different pump power and pump

wavelengths at different HNLF fiber lengths 91

4.10 PSA performance analysis using different pump power at 1550 nm

at different HNLF fiber lengths 92

4.11 PSA performance analysis using output signal power and noise

figure at 100mW pump power for HNLF of 0.5 km 93

4.12 Response of regenerated signal received at the end of HNLF fiber

94

4.13 Response of regenerated PSA signal and different spectral

components 94

4.14 Phase response improvement using PSA model 95

4.15 Response of Moving Average Optical Filter to filter out the

regenerated in-phase gain signal 96

4.16 MA optical filter coefficients response to filter out the regenerate

in-phase gain signal 97

4.17 Response of optical frequency locked model, before and after

locking 98

xviii

4.18 Phase error detection for locked in-phase gain regenerated signal 98

4.19 Phase error minimization of in-phase regenerated signal using

CLT algorithm 99

4.20 Response of AR optical filter to filter out the noises from phase

error minimized in-phase gain regenerated signal 99

4.21 Final response of the signal regenerate using the developed all-


4.22 Simulink setup for receiver and performance evaluation

model 101

4.23 Response of detection process in electrical domain of 10Gb/s

optical signal 102

4.24 Low pass filter parameters initialization using MATLAB filter

design tool 103

4.25 Response of received 10Gb/s regenerated signal using the

developed all-optical signal regeneration technique in electrical

domain 104


4.27 BER versus received power analysis for different receiver

responsivity 106

4.28 Q-factor versus received power analysis for receiver responsivity

= 1 107

4.29 Q-factor versus received power analysis for different receiver

responsivity 108

4.30 Eye diagram analysis for the received signal: (a) before, and (b)

after implementation of the developed all-optical signal



4.32 BER versus received power analysis for different transmission

distances: (a) 50 km, (b) 100 km, (c) 200 km, and (d) 250 km 111

4.33 BER versus OSNR analysis for different transmission distances 113

4.34 BER versus Eb/No analysis for different transmission distances 114

4.35 Simulink model of the developed all-optical signal regeneration

technique for DPSK-RZ transceiver system 115

xix

4.36 Simulink model of the designed 10Gb/s optical DPSK-RZ

transmitter 116

4.37 Response of the designed optical DPSK-RZ transmitter (a) 10Gb/s

PRBS sequence, (b) 10Gb/s PRBS DPSK sequence, (c) Laser

signal of 1550.8 nm, and (d) 10Gb/s optical DPSK-RZ modulated

signal 117

4.38 Response of: (a) 10Gb/s optical DPSK-RZ signal after

propagation at 150 km noisy fiber link, and (b) its frequency

spectrum of signal 118

4.39 PSA performance analysis using different pump power and pump

wavelengths at different HNLF fiber lengths 119

4.40 PSA performance analysis using different pump power of 1550

nm at different HNLF fiber lengths 120

4. 41 PSA performance analysis using output signal power and noise

figure at 100mW pump power for HNLF of 0.5 km 120

4.42 Response PSA model that generates different spectral components

121

4.43 Phase improvement in the signal using PSA model 122

4.44 Response of Moving Average Optical Filter to filter out the

regenerated in-phase gain signal 122

4.45 MA optical filter coefficients response to filter out the regenerate

in-phase gain signal 123

4.46 Response of optical frequency locked model, before and after

frequency locking 123

4.47 Phase error detection for locked in-phase gain regenerated signal 124

4.48 Phase error minimization of in-phase regenerated signal 124

4.49 Response of AR optical filter to filter out the noises from phase

error minimized in-phase gain regenerated signal 125

4.50 Final response of the regenerated signal using the developed all-


4.51 Response of detection of 10Gb/s electrical modulated signal 126

4.52 Response of the received 10Gb/s regenerated signal using the

developed all-optical signal regeneration technique in electrical

form 126

xx


4.54 Q-factor versus received power analysis for receiver responsivity

= 1 128

4.55 Eye diagram analysis for the received signal: (a) before and (b)

after implementation of the developed all-optical signal



4.57 BER versus received power analysis for different transmission

distances: (a) 50 km, (b) 100 km, (c) 200 km, and (d) 250 km 130

4.58 BER versus OSNR analysis for different transmission distances 131


4.60 BER versus Eb/No comparison for the developed all-optical signal

regeneration implemented on DPSK-RZ and DPSK-NRZ

transceiver systems 133

4.61 Simulink models of the developed all-optical signal regeneration

technique for different modulation formats: (a) QPSK, (b) BPSK,

(c) DBPSK, (d) OOK/ASK, (e) 8-PSK, (f) BFSK, (g) QAM, and

(h) OFDM 134


4.63 BER versus OSNR analysis of the developed all-optical signal

regeneration implemented on different modulation schemes at

different transmission distances 137

5.1 System model for experimental demonstration 142

5.2 Procedure for converting the Simulink code in VHDL and

exporting in to Xilinx KCU105 UltraScale FPGA 143

5.3 Design process of hardware implementation of the developed all-

optical signal regeneration systems using Xilinx KCU105

UltraScale FPGA board 144

5.4 HDL code generation steps 146

5.5 Design steps to generate the RTL schematic 147

5.6 Sequence of experiment execution 148

5.7 Demonstration of hardware setup 150

5.8 Experimental steps 152

xxi

5.9 GUI data recorded for DPSK-NRZ system before implementing

the developed all-optical signal regeneration technique 153

5.10 BER and IO diagram using serial IO analyzer for 50 km fiber link

before implementing the developed all-optical signal regeneration










technique for DPSK- NRZ system 156



technique for DPSK NRZ system 156

5.15 GUI data recorded for DPSK-NRZ system after implementing the

developed all-optical signal regeneration technique 158


after implementing the developed all-optical signal regeneration














xxii

5.21 Timing analysis for the developed DPSK-NRZ all-optical signal

regeneration system on Xilinx UltraScale FPGA 162

5.22 Timing histogram for the developed DPSK-NRZ all-optical signal

regeneration system on Xilinx UltraScale FPGA 163

5.23 FPGA Power analysis for the developed DPSK-NRZ all-optical

signal regeneration system on Xilinx UltraScale FPGA 164

5.24 GUI data recorded for DPSK-RZ of the developed all-optical

signal regeneration technique: (a) before, and (b) after 165



technique for DPSK- RZ system 166






















xxiii







5.35 BER versus received power validation of simulation and hardware

setup of the developed all-optical signal regeneration DPSK-NRZ

system: (a) for 50 km, (b) 100 km, (c) 150 km, (d) 200 km, and (e)

250 km 175

5.36 BER versus received power validation of simulation and hardware

setup of the developed all-optical signal regeneration DPSK- RZ

system: (a) for 50 km, (b) 100 km, (c) 150 km, (d) 200 km, and (e)

250 km 177

xxiv

LIST OF APPENDICES

APPENDIX A1 Derivation of PSA model 198

APPENDIX A2 Derivation for optical locked signal model 201

APPENDIX B1 Numerical design of 10Gb/s Differential Phase

Shift Keying Non-Return-to-Zero transceiver

system

204

APPENDIX B2 Complex MATLAB function for generating the

PSK signal

210

APPENDIX C1 The Design step for generating the HDL Code 212

APPENDIX C2 VHDL code DPSK-NRZ and DPSK-RZ

regeneration transceiver System

227

xxv

LIST OF SYMBOLS

𝑎𝑜 Gain coefficient

𝐴(𝑧, 𝑡) Slowly varying envelope of electric field

𝐴𝑝 Amplitudes of pump

𝐴𝑠 Amplitudes of signal

𝐴𝑖 Amplitudes of idler

𝐴𝑒𝑓𝑓 Effective area of core of optical fiber

𝐴𝑠(𝑧) Filtered out the regenerated in-phase gain signal

𝐴∗ Normalized values for the noise transfer function

𝐴11, 𝐴22 , 𝐴12 and 𝐴21 Filter coefficients

𝐴𝑉𝐶𝑂 VCO gain used for tuning the laser signal

𝛽 Fraction of spontaneous emission

𝛽(𝜔) propagation constant

∆𝛽 Power propagation mismatch factor

𝛽2 Second order propagation constant

𝛽3 Third order propagation constant

𝛽𝑁𝐿 Nonlinear phase mismatch factor

𝛽𝐿 Linear phase mismatch factor

𝛽𝑠 Mode propagation constants for signal

𝛽𝑖 Mode propagation constants for idler signal

𝛽𝑝 Mode propagation constants for pump signal

respectively

c Speed of light

𝐶𝑅1 and 𝐶𝑅2 Coupling coefficients for the optical filters arms 1 and

arms 2 respectively

𝐶𝑅 Ratio of 𝐶𝑅1 and 𝐶𝑅2

𝐷 Dispersion parameter

xxvi

𝐸(𝑟) Envelope of the field

𝐸𝑜 Output optical filed

𝐸𝑖 Input electrical field

𝜖 Gain compression

휀𝑜 Vacuum permittivity

𝐺 Parametric Gain

𝑔 Parametric gain coefficient

𝐺𝑃𝑆𝐴 Gain of Phase Sensitive Amplifier

ℎ𝑓 Photon energy

ℎ Plank’s constant

𝐻𝑠(𝑧) Filter transfer function

𝐼𝑃 Photocurrent

𝑘 Phase mismatch factor

∆𝑘 Change in phase mismatch factor

𝐿 Length of optical fiber

𝐿𝑒𝑓𝑓 Effective fiber length

∆L Difference in waveguide lengths

𝐿1 and 𝐿2 Lengths of waveguides that are connecting the couplers

𝐿𝑜(𝑧) Laser signal

𝑁𝐹 Noise figure of the receiver,

𝑁𝑜 Carrier density

𝑛2 Nonlinear index co-efficient

𝑃𝑝 Pump power

𝑃𝑠 Signal power

𝑃𝑖 Idler power

P Polarization density for electric field

𝑃𝑀 Modulator power

𝑃𝑁𝐿 Nonlinear polarization

𝑃𝑖 Input power launched in fiber,

𝑃𝑜 Output power at end of optical fiber

𝑃𝑆0 and 𝑃𝑆1 Probability of symbol 𝑆0 and 𝑆1

𝑃𝑒 Total probability function

𝑃𝐿 Linear polarization

xxvii

𝑞 Electric charge

𝑄 Q-factor

𝑅 Responsivity of the receiver

𝑆𝑅 Slope of Raman gain

𝑆0, 𝑆1 Symbol 𝑆0 and 𝑆1

𝑆𝑛𝑜𝑖𝑠𝑒(𝑓) Single side power spectral density

𝑆∗ Normalized values for the power spectral density of

noise

𝑇𝑑𝑒𝑙𝑎𝑦 Unit delay

𝜇𝑜 Vacuum permeability

𝜇1 and 𝜎1 Mean and standard deviation for symbol 𝑆1

𝜇0 and 𝜎0 Mean and standard deviation for symbol 𝑆0

𝜇 Total mean

𝑉𝑏𝑖𝑎𝑠 Bias voltage

𝑉𝜋 Drive voltage

𝑉(𝑡) Time varying input voltage

𝑉𝑎 Active volume of lasing

𝑣 Optical frequency

𝑣𝑔 Group velocity

𝜔𝑖𝑑𝑙𝑒𝑟 Idler signal angular frequency

𝜔𝑝𝑢𝑚𝑝 Pump signal angular frequency

𝜔𝑠𝑖𝑔𝑛𝑎𝑙 Signal angular frequency

𝜔𝑐 Center angular frequency

𝑋(1) First order susceptibility

𝑋(3) Third order susceptibility

𝑋1 and 𝑋2 Input arms of optical filter

𝑌1 and 𝑌2 Output arms of optical filter

𝑌𝑜𝑢𝑡 Output signal for phase locked signal mechanism

z Direction of propagation

α Fiber loss co-efficient

γ Nonlinear coefficient

ɼ Optical confinement factor

xxviii

r Polarization dependent

𝜏𝑝 Photon life time

𝜏𝑛 Electrons life time

𝜆 Wavelength

𝜆𝑜 Center wavelength,

∅𝑁𝐿 Nonlinear phase shift

∅𝑠 Phase of the signal

∅𝑖 Phase of the idler signal

∅𝑝 Phase of pump signal

∅𝑒 Detected phase error

∅𝑅 Phase of filtered regenerated signal

∅𝑟𝑒𝑙 Relative phase difference

ɸ𝑑𝑒𝑎𝑙𝑦 Filter delay

ɸ𝑀𝑍𝐼 Phase of Modulator used for filter design

휃 Relative phase difference due to change in power

𝛿 Normalized angular frequency

𝜗 Laser linewidth

𝜎2𝐸 Phase error variance

xxix

LIST OF ABBREVIATIONS

2R regeneration

3R regeneration

8-PSK

ACO

AR

ARMA

ASE

BBG

BER

BPSK

CLT

C-NRZ

CPM

C-RZ

CW

DBPSK

DFB

DPSK

DQPSK

EAM

EDFA

EOM

ERV

FIR

FOPA

FOPO

FPGA

Reshaping and Reamplification

Reshaping, Reamplification and Retiming

8- bit Phase Shift Keying Signal

Ant Colony Optimization

Autoregressive

Autoregressive Moving Average

Amplitude Spontaneous Nosie

Bernoulli Binary Generator

Bit Error Rate

Binary Phase Shift Keying

Center-Limit Theorem

Chirp Non Return- to- Zero

Cross Phase Modulation

Chirp Return- to- Zero

Continuous Wave

Differential Binary Phase Shift Keying

Distributed-Feedback

Differential Phase Shift Keying

Differential Quadrature Phase Shift Keying

Electro Absorption Modulator

Erbium Doped Fiber Amplifier

Electro Optic Modulator

Error Vector

Finite Impulse Response

Fiber Optical Parametric Amplifiers

Fiber Optic Parametric Oscillator

Field Programming Gate Array

xxx

FSK

FSR

FWM

GA

GAWBS

Gbps

GUI

HDL

HNLF

ICT

IF

IIR

IO block

ISI

ITU

LiNbO3

LTI

LUTs

MA

M-ASK

MMF

MZI

MZIM

NALM

NF

NLSE

NRZ

O-E-O

OF

OFDM

OIM

OOK

OPM

Frequency Shift Keying

Free Spectral Range

Four Wave Mixing

Genetic Algorithm

Guided Acoustic-Wave Brillouin Scattering

Giga bit per second

Graphical User Interface

Hardware Description Language

Highly Nonlinear Fiber

Information and Communication Technologies

Intermediate Frequency

Infinite Impulses response

Input/output block

Inter Symbol Interference

International Telecommunication Union

Lithium Niobate

Linear-time Invariant

Look-Up Tables

Moving Average

Multilevel- Amplitude Shift Keying

Multi-Mode Fiber

Mach-Zehnder Interferometer

Mach-Zehnder Interferometer Modulator

Nonlinear Amplifying Loop Mirror

Noise Figure

Non Linear Schrodinger Equation

Non- return –to - Zero

Optical- Electrical-Optical

Off-set Filtering

Orthogonal Frequency Division Multiplexing

Optical Intensity Modulator

On-Off Keying

Optical Phase Modulator

xxxi

OSNR

PCF

PDF

PI

PPLN

PRBS

PS

PSA

PSK

PS-QPSK

QAM

QPSK

RF

RZ

SBS

SFP

SMF

SNR

SOA

SoC

SPM

SRS

SSFM

Optical Signal –to- Noise Ratio

Photonic Crystal Fiber

Probability Density Function

Phase Insensitive

Periodically poled lithium niobate

Pseudo Random Binary Sequence

Phase Sensitive

Phase Sensitive Amplifiers

Phase Shift Keying

Phase- Sensitive- Quadrature Phase Shift Keying

Quadrature Amplitude Modulation

Quadrature Phase Shift Keying

Radio Frequency

Return- to – Zero

Stimulated Brillioun Scattering

Small Form-factor Pluggable

Single Mode fiber

Signal –to- Noise Ratio

Semiconductor Optical Amplifier

System on Chip

Self Phase Modulation

Stimulated Raman Scattering

Split Step Fourier Method

xxxii

LIST OF AWARDS

1. Best Man Inventor Award in ITEX 2016 (27th International Invention &

Innovation Exhibition) from 12-14 May 2016 at Kuala Lumpur Convention

Centre, Malaysia for “Development of Optical Signal Regeneration System

Using Xilinx UltraScale FPGA”.

2. Gold Medal in ITEX 2016 (27th International Invention & Innovation

Exhibition) from 12-14 May 2016 at Kuala Lumpur Convention Centre,

Malaysia for “Development of Optical Signal Regeneration System Using

Xilinx UltraScale FPGA”.

3. Gold Medal in Research and Innovation Festival 2015 [R&I 2015] for

“Development of Optical Signal Regeneration System Using Xilinx UltraScale

FPGA KCU105 Evaluation Board”.

4. Patent filled with application No. (PI. 2016701652) by PINTAS IP Group.

5. Young Scientist Award in international Conference on Green Computing and

Engineering Technologies (ICGCET 2015) from 25-56 July, 2015 in Dubai.

6. 1st Position in Poster competition, Hari Transformi Minda 2015, FKEE,

Universiti Tun Hussein Onn Malaysia.

xxxiii

LIST OF PUBLICATIONS

Book Chapter

1. B. Das, and M.F.L Abdullah. “Low Power Design of High Speed

Communication System Using IO Standard Technique over 28 nm Chip” in

Design and Modeling of Low Power VLSI Systems, M. Sharma, R.Gautam,

M.A. Khan, Eds. USA: IGI publisher, June 2016.

Lecture Notes (ISI indexed published by Springer)

2. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “All optical Signal restoration

for 10G DPSK System” in Advanced Computer and Communication

Engineering Technology, 1st Ed. Vol. 362. H.A. Sulaiman, M.A. Othman,

M.F.I. Othman, Y.A. Rahim, N.C. Pee Eds. Switzerland: Springer

International Publishing, 2016, pp. 545-556. I.F 0.4

3. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “Current Mode Logic based

Semiconductor Laser Driver Design for Optical Communication System,”. In.

Journal of Sci, and Technol., 9(10), pp. 1-6, 2016. I.F 1.3

4. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “Development of All-Optical

Signal Regeneration Method for 100Gb/s Differential Phase Shift Keying

Degraded Signal” Lecture Notes in Electrical Engineering, 1st Ed. H.A.

Sulaiman, M.A. Othman, M.F.I. Othman, Y.A. Rahim, N.C. Pee Eds.

Switzerland: Springer International Publishing, August 2016. I.F 0.4

5. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “Development of New All-

Optical Signal Regeneration,”. In. Wireless Personnel Communication,

Springer International Publishing, Jan 2017. I.F 0.98

6. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “Energy Efficient Design of

100Gb/s Optical DPSK Transmitter Design Using UltraScale FPGA,”. In.

Journal of Sci, and Technol., September, 2016. I.F 1.3

xxxiv

Journal Articles (ISI and Scopus indexed)

7. B. Das, M.F.L Abdullah, M.S. Nor Shahida, Q. Bakhsh, and B. Pandey. “Power

Optimization of Semiconductor Laser Driver Using Voltage Scaling

Technique Voltage Scaling Technique,” ARPN Journal of Engineering and

Applied Sciences, Vol. 10(18), pp. 8379-8387, 2015.

8. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “All Optical Regeneration for

Optical Communication Network Using 3R Regeneration and Phase sensitive

amplifier” International Journal of Control and Automation, Vol. 8 (8), pp.

87-94, 2015.


Hyper Transport Protocol based Laser Driver using Low-Voltage Differential

Signaling” International Journal of Control and Automation, Vol. 8(9), pp.

131-138, 2015.


Semiconductor Laser Driver Using Voltage Scaling Technique” ARPN Journal

of Engineering and Applied Sciences.

11. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “A New All-Optical Signal

Regeneration Technique for 10 GB/S DPSK Transmission System,”

International Journal of Electrical and Computer Engineering, Vol. 6(2),

2016.

Journal Articles (indexed by IET Inspec)

12. B. Das, M.F.L Abdullah, M.S. Nor Shahida, Q. Bakhsh, and B. Pandey.

“Pseudo Noise Generator Based Optical Transmitter at High Speed

Transceiver Logic IO Standard,” Journal of Automation and Control, Vol.

4(1), pp. 28-32, 2016.

13. B. Das, M.F.L Abdullah, M.S. Nor Shahida, Q. Bakhsh, and B. Pandey.

“Temperature Regulations of Pseudo Noise Generator Based Optical

Transmitter using Airflow and Heat Sink Profile at High Speed Transceiver

Logic IO Standard,” International Journal of Materials, Mechanics and

Manufacturing, Vol. 5(1), pp. 64-67, 2017.

xxxv

Conference presented

14. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “DSP Techniques For

Reducing Chromatic Dispersion In Optical Communication Systems,” Int.

Conf. on Computer, Comm., and Control Technology, 2014, pp. 305-309.

15. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “Energy-Efficient Pseudo

Noise Generator Based Optical Transmitter for Ethernet (IEEE 802.3az),” Int.

Conf. on Computer, Comm., and Control Technology, 2014, pp. 142-146.

16. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “Frequency domain

Technique For Reducing Chromatic Dispersion,” 7th Electrical Power,

Electronics, Comm. Control and Informatics International Seminar, 2015, pp.

56-61.

17. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “Development All Optical

Regeneration for Optical Communication Network,” International Conference

on Engineering & Emerging Technologies, 2015, pp. 1-6.

18. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “Power Optimization of

Pseudo Noise Based Optical Transmitter Using LVCMOS IO Standard,” 2nd

International Conference Power Generation Systems and Renewable Energy

Technologies, 2015, pp. 1-7.

19. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “I/O Standard based Low-

Energy Pseudo Noise Generator for optical Communication,” International

Multi Topic Conference, 2015, pp. 200-210.

20. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “New All-Optical Signal

Regeneration Technique,” KICS-Korea and Southeast Asia ICT International

Workshop, Siem Reap, Cambodia, December 21-22, 2015.

21. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “Development and Testing of

A New All-Optical Signal Regeneration Technique,” IEEE 6th International

Conference on Photonics, Kuching, Sarawak, 14-16 April 2016.

22. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “High Performance Design of

100Gb/s DPSK Optical Transmitter,” IEEE INDIAcom, Delhi, India, 16-18

March 2016.

23. B. Das, M.F.L Abdullah, and M.S. Nor Shahida. “High Performance and High

Range Design of 100Gb/s Optical Differential Phase Shift keying

xxxvi

Transmitter,” International Conference on Recent Trends in Computer Science

and Electronic, 02-03 January 2016, Parkroyal KualaLampur, Malaysia.

CHAPTER 1

INTRODUCTION

1.1 Background

Information and Communication Technologies (ICT) are continuously escalating,

as means of global communication using telecommunication network infrastructure.

Recently, the broadband internet access and the smartphones’ bandwidth-hungry

multimedia applications have become ubiquitous. Consequently, bandwidth demand

continues to grow as demonstrated in Figure 1.1.

Figure 1.1: Exponential growth in internet traffic across the globe [1]

2

The increasing demand of ICT has increased the power consumption that about 25%

of total global power is utilized by ICT networks. Therefore, search for a solution to

the “green communication systems” lead to the alternative approaches to design and

develop the energy efficient communication systems, techniques, and algorithms that

consume less power for high-speed communications [1].

In telecommunication networks, fiber optic communication systems are

lightwave systems that employ optical fibers for the exchange of information at a long

distance. The optical fiber communication systems have revolutionized the

communication technology and have subsequently became the backbone of

telecommunications infrastructures. The fiber optic communication systems have

numerous advantages over existing electrical data transmission systems such as:

immunity to noise, lightweight, and high bandwidth. The recent advancement in

optical communication systems has improved reliability and data rate for transmission

systems.

The optical fiber communication systems offer ultra-high data rates due to the

high optical frequencies, which make it possible to utilize broad optical bandwidths

using telecommunication windows. These telecommunication windows are classified

as: Original (O) band (1260-1360 nm), Extended (E) band (1360-1460 nm), Short

wavelengths (S) band (1460-1530 nm), Conventional (C) band (1530-1565 nm), Long

(L) band (1565-1625 nm), and Ultra long (U) band (1625-1675 nm) [1].

The performance of high-speed optical systems is limited due to the collective

effect of amplifier noise accumulation, chromatic and polarization-mode dispersion,

fiber nonlinearity, inter-channel crosstalk, multipath interference, long distance,

transmission of high power and frequency signal, and other impairments. These

transmission impairments mainly produce amplitude and phase noise. The phase noise

is further divided into linear and nonlinear phase noise. The linear phase noise and

amplitude noise are easy to mitigate by controlling the dispersion. However, nonlinear

phase noise is produced by the conversion of the Amplitude Spontaneous Emission

(ASE) noise into phase noise through Kerr nonlinearities and is difficult to mitigate.

These degradation mechanisms, present in optical fiber communication systems, are

necessary to clean up the data signals, in order to provide noiseless communication.

The mitigation of transmission impairments and noise from transmitted signal is

termed as signal regeneration.

3

Why optical signal regeneration?

The optical communication systems are the interconnection of optical fiber between

different access points in long haul communication as described in Figure 1.2 [2].

Figure 1.2: Optical signal regeneration requirement [2]

In Section 1.1, it is explained that optical communication system performance

is limited due to several factors and malfunctions, misconfigurations and traffic

affecting signal impairments, which leads to a high demand for fault recognition and

correction. The use of optical signal regeneration become very important to supervise

the network and fault management. The optical signal regeneration can be defined as

the process of restoring the optical signal quality for the long haul communication and

it can be performed in either electronic domain or in all-optical domain [2].

1.2 Motivation

The first commercial signal regenerator utilized a process referred to as an Optical-

Electrical-Optical (O-E-O) converter that converts an optical signal into analog

electrical signal and re-digitize the analog signal for noise removal and re-modulate

4

this digital stream into optical signal. An efficient signal regenerator should be able to

perform the noise mitigation, signal amplification and regeneration at appropriate

wavelength. However, these O-E-O regenerators have the limited capabilities such as:

ability to be used in different transmission for real time link speed of few Gbps data

rate. The real time and commercial based design of these electronics signal regenerator

for 10Gb/s is not reported yet, due to complex modulation formats and single

bit/symbol input shown in Figure 1.3. On contrary, the signal regeneration in optical

domain processes at least 12 times as many symbols for higher order modulation

formats [3].

Figure 1.3: Capacity distance product versus modulation formats usage [13]

Since, two decades, research literature on all-optical signal processing has

stated that all-optical techniques are more power efficient, and consequently greener

in power consumption than O-E-O signal regenerators. The development of all-optical

signal regeneration has been initiated in 1978 [4]; and until now, numerous all-optical

signal regeneration techniques have been reported and still the number is going on.

The all-optical signal regeneration can be achieved using nonlinear signal processing

[4]; that offers parametric amplification via Self Phase Modulation (SPM), Cross

Phase Modulation (CPM), and Four Wave Mixing (FWM) using different types of

fiber such as: Highly Nonlinear Fibers (HNLF), microstructure fiber, and non-silica

fibers. The all-optical signal regeneration has been achieved using FWM via HNLF

using nonlinear amplifying loop mirror (NALM), optical logic gates, Fiber Optical

Parametric Amplifiers (FOPA) and, Phase Sensitive Amplifiers (PSA) [5], [6].

5

Several problems are also reported with existing all-optical signal regeneration system

such as; requirements of data rate of input signal, narrow linewidth, and high power

pumps that are phase locked to the signals (and idlers) [7]-[11]. Furthermore, being

amplified, high Bit Error Rate (BER), low Optical Signal–to-Noise Ratio (OSNR) and

other real time monitoring issues has limited the research efforts in this field for a long

time [12]-[13].

Recent issues are explored by Das (2015) [14]; such as power consumption and

cost of the experimental demonstration for all-optical signal regeneration systems. The

alternate solution for lowering the power consumption and cost of optical

communication system’s experimental demonstration is to implement the high-speed

optical links using Field Programming Gate Array (FPGA) [15], [16]. An experimental

study was carried out of utilizing the FPGA for optical fiber link in [16]; and this type

of system design for optical communication system can reduce the cost and power

consumption of the future optical communication systems. Therefore, the philosophy

of this research study is to introduce a new all-optical signal regeneration technique

that can overcome the problems for existing all-optical signal regeneration system.

1.3 Problem statement

The development in the all-optical signal regeneration system demand more power

efficiency, accuracy in noise mitigation, and regeneration for the high data rate

(10Gb/s) degraded optical signals in real time at low cost. In progress towards, various

limitation were reported such as: limitations of data rate of input signal for signal

regeneration up to 5 Gbps in real time, high power consumption of all-optical signal

regeneration techniques due to usage of high power pumps, and due to complex design.

The designing of signal regeneration is also a challenge, because of usage of most

expensive highly equipped optical and photonics laboratories and costly commercial

software such as: Optisystem, VPIphotonicsTM.

In addition, the all-optical signal regeneration for BPSK, FSK, OFDM, QAM,

DBPSK are unexplored area. In the performance analysis of the existing all-optical

signal regeneration system are only capable to provide the lowest BER of 10-12 and not

lower than -9 dBm power, this BER and power consumption is high when multiuser

are using the network at the same time. The demand of today’s all-optical regeneration

6

systems in terms of power consumption, and BER is less than this range. Additionally,

the real time commercial package for energy efficient all-optical signal regeneration is

the need of future all-optical regenerative system.

1.4 The research question and objectives

The overall quest of the research work can be formulated as follows: an improved all-

optical signal regeneration technique and its real-time implementation for commercial

based design is achieved that consumes less power for signal regeneration of degraded

signals. The following four research questions are formulated that defines the

investigation of research work:

1) What is the purpose of designing the new all-optical signal regeneration

technique for noisy high-speed degraded optical signals?

2) Which modulation schemes will be supported by the designed new all-optical

signal regeneration technique?

3) How the system designed is achieved for the new all-optical signal

regeneration technique?

4) How the performance of the new all-optical signal regeneration technique is

compared to existing all-optical signal regeneration techniques?

The research objectives pursued in order to answer the research questions are:

1) To design an improved all-optical signal regeneration technique for noise

mitigation and regeneration of the degraded optical signal for 10Gb/s high-

speed degraded optical signals.

2) To utilize the designed all-optical signal regeneration for DPSK-NRZ, DPSK

RZ transceiver systems and for BPSK, DBPSK, QPSK, OFDM, QAM, BFSK,

8-PSK, and OOK modulation formats.

3) To test the real time implementation and commercial based design of the

designed all-optical signal regeneration technique for DPSK-NRZ transceiver

system, DPSK-RZ transceiver system and for advanced digital modulation

formats using Xilinx KCU105 UltraScale FPGA.

7

4) To validate the performance of the designed all-optical signal regeneration

systems by comparing the simulation and hardware results of power

consumption and BER with existing all-optical signal regeneration systems.

1.5 Aim of the study

The aim of this study is to develop a new all-optical signal regeneration technique,

which is able to provide noise mitigation, amplification and regeneration for different

modulation formats such as: Differential Phase Shift Keying (DPSK), Quadrature

Phase Shift Keying (QPSK), Differential Quadrature Phase Shift Keying (DQPSK),

Phase Shift Keying (PSK), On-Off Keying (OOK), Binary Phase Shift Keying

(BPSK), Binary Frequency Shift Keying (BFSK), Orthogonal Frequency Division

Multiplexing (OFDM), Quadrature Amplitude Modulation (QAM), Differential

Binary Phase Shift Keying (DBPSK) for high data rate degraded signal. The designed

system consumes less power for the designed all-optical signal regeneration technique;

will provide the low BER with low received power. It is also projected to enable

modern functionalities of all-optical signal regeneration system by exploiting the

Xilinx KCU105 UltraScale FPGA, this type of FPGA provides the low power optical

system design, debugging, troubleshooting, testing and monitoring at low cost solution

for real time commercial based design package for all-optical signal regeneration

system.

1.6 Scope of study

The Scope of this study is represented using the K-Chart described in Figure 1.4 that

defines the study model. This chart illustrates the relationship of the main optical signal

regeneration technique and the research work focusing in this area. The highlighted

text boxes in pink color indicate the direction of this work carried out in order to

achieve the research objectives.

In this research, the numerical design has been developed to analyze the need

of each parameter required for developing the new all-optical signal regeneration

technique. The numerical design of new all-optical signal regeneration has been

verified using MATLAB Simulink model. The Simulink model of developed all-

8

optical signal regeneration technique is implemented for different modulation formats

such as; DPSK-NRZ, DPSK RZ, BPSK, PSK, DBPSK, QPSK, OFDM, BFSK, QAM,

and OOK to test the developed all-optical signal regeneration technique.

Figure 1.4: Scope of the study using K-Chart

Numerical design

Degenerated PSA Optical filtering Optical frequency locked

model

Noise mitigation

model

Simulation design Implementation

DPSK- NRZ System DPSK- RZ System Other formats

Real time experiment using Xilinx KCU105 UltraScale FPGA

FOPA PI- FOPA PS- FOPA SOA PSA

New All-optical signal regeneration technique

Optical signal regeneration

Electronic signal regeneration All-optical signal regeneration

Optical

Tx.

Noisy Optical

fiber Optical

Rx.

Performance Analysis

NAOSR

BER OSNR Q -

factor

Eye diagram

QPSK FSK DBPSK QAM OFDM BPSK

System Validation

Experiment

9

The performance of proposed signal regeneration system is analyzed using BER,

OSNR, Q-factor, and eye diagram before and after implementation of all-optical signal

regeneration for degraded optical signal transmitted at long distance noisy fiber link.

The designed Simulink model of all-optical signal regeneration for DPSK- Non

Return-to-Zero (NRZ) and DPSK- Return-to-Zero (RZ) is converted to Hardware

Description Language (HDL) code to determine the real-time demonstration

performance using Xilinx KCU105 UltraScale FPGA. The real-time demonstration of

the designed all-optical signal regeneration system is analyzed using BER and IO

analyzer. The system validation is performed by comparing the simulation and

hardware results.

1.7 Limitation of the study

This study has the following limitations.

Pre-designed modulation formats are employed for testing of new all-optical

signal regeneration technique.

The developed all-optical signal regeneration need to be reconfigured using

programming in order to perform the signal regeneration for each modulation

format.

The developed all-optical signal regeneration is not tested for multilevel format

of signals.

The parameters of pump frequency, HNLF, optical filter and an optically

locked frequency model need to be specified for data format of the signal to

perform the all-optical signal regeneration.

The entire simulation for the developed system is carried out in MATLAB

Simulink, using a highly fast computer such as; Core i7 processor to run the

simulation.

The technique is specially designed for 10Gb/s optical degraded signals due to

widely utilized data for high speed communication in real time and commercial

based design.

The real-time implementation of new all-optical signal regeneration is carried

out for DPSK NRZ and DPSK- RZ data formats.

10

The real-time implementation of the system need the conversion of Simulink

model to HDL codes for each model

The real-time implementation of designed all-optical signal regeneration

demonstrates the live monitoring of only data rate, eye diagram and link speed

using Xilinx KCU105 UltraScale FPGA.

1.8 Thesis organization

The thesis is organized in six chapters. Chapter 1 includes a brief introduction about

optical communication and optical signal regeneration systems followed by

motivation, problem statement, objective, scope, and limitation of the study.

The brief introduction and discussion concerned about all-optical signal

regeneration technologies and the research gap was highlighted in Chapter 2. The

development of new all-optical signal regeneration technique is discussed in Chapter

3 that narrates the accomplishment of objective 1. The implementation of new all-

optical signal regeneration technique for different modulation systems is discussed in

Chapter 4 to realize objective 2. In chapter 5, the real-time hardware implementation

as an experimental stage is carried out to achieve objective 3. In the same chapter 5

the system validation is performed to validate the simulation and experimental findings

of the designed system. Furthermore, the results are validated with existing all-optical

signal regeneration systems to realize objective 4. Lastly, the conclusions with future

suggestions are discussed in Chapter 6.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter presents the review of fundamental knowledge of optical communication,

optical fiber, all-optical signal regeneration, optical transmitter, optical receiver, and

optical system performance analysis. The review addresses the relative merits of

existing commercially viable design of all optical signal regeneration system for future

generation optical networks. The concurrence of various techniques and systems in the

shape of several earlier studies has been discussed in detail. The state of the art of all-

optical signal regeneration techniques has been presented herewith, which highlights

the approaches, methods, and techniques that were utilized during their pursuit of

development.

2.2 Optical fiber communication system

The optical fiber communication systems are used for long distance information

transmission from one point to another. A typical long haul communication system

can be divided in three subsystems: optical transmitter, optical fiber as a channel, and

optical receiver as shown in Figure 2.1 [13].

12

Figure 2.1: A typical optical fiber long haul communication system [13]

The first subsystem is the optical transmitter that generates the lights signal

with particular modulation format. The second subsystem is the optical fiber in which

light signals are guided inside with a minimum of attenuation. The last subsystem is

the receiver that detects the optical signal and converts in electrical signal. These

subsystems are discussed in the subsequent sections.

2.3 Optical transmitter

An optical transmitter consists of laser source (narrow line width), optical modulator,

data sequence, and signal generator. The generic block diagram for optical transmitter

is defined in Figure 2.2.

Figure 2.2: An optical transmitter block diagram [13]

The data signal is binary sequence of n-bits, where n is the number of bits. The

signal generator converts binary signal in a specific format, such as: phase shift

(represents data in terms of phase as binary “1” represents 180° phase change and

binary “0” represents 0° change) frequency shift, amplitude shift. This conversion can

13

be termed as digital modulation [13]. There are different digital modulation formats

exist for the scope of optical communication system as demonstrated in Figure 2.3

[17].

Figure 2.3: Classification of phase and intensity digital formats [17]

These modulation formats are mainly, categorized in intensity and phase

formats that are further categorized in different types [18]-[21]. In the next section, the

DPSK signal generation, laser signal used as carrier, optical modulator, and finally,

optical NRZ-DPSK modulator are discussed in detail respectively.

Differential Phase Shift Keying (DPSK) signal format

Differential Phase Shift Keying (DPSK) is a discrete phase modulation type

that indicates the state of phase of light carrier that can be switched using phasor. For

example (0 to π) is binary PSK, (0, π/2, - π/2, π) is Quadrature PSK and etc. In DPSK

format, the information is encoded in the phase of light carrier [22]. The DPSK signal

generation using differential encoding can be described using Figure 2.4, which states

that binary "0" is encoded if existing input bit and penetrable encoded bit are of

opposite logic, whereas a binary "1" is encoded if the logic are alike. The DPSK signal

generation using differential encoding is similar to an XOR logic operation, which

operates as differential encoder. The phase encoded in DPSK signals are defined in

such a way that binary "1" designates π or 180ᵒ phase change between sequential data

in the optical data bits, whereas "0" designates no phase or 0ᵒ change between

14

sequential data bits. In the next section, the laser signal, which is used as carrier is

discussed.

(a) (b)

Figure 2.4: DPSK signal format: (a) signal generation, (b) signal constellation

diagram [22]

Laser signal

Lasers in the optical communication systems are used as source to transmit signal

inside optical fiber. The semiconductor lasers are widely used for optical

communication. The semiconductor lasers have wide range of spectrum, which is

suitable for parameters of the C-band (1530-1565 nm) transmission system. The

semiconductors laser are further classified by their structure due to narrow spectral

width and minimum rise time to achieve the high bit rate transmission [23].

The single mode Distributed-Feedback (DFB) semiconductor laser is widely

in use, because in a DFB laser, the optical energy is distributed throughout the cavity

length. An internal corrugated grating leads a periodic perturbation of refractive index

produces the Bragg diffraction to couple the waves in backward and forward

directions. The operating characteristics DFB semiconductor laser is described using

laser rate equation. In laser rate equation, the number of photons (N) and the number

of electrons change (S) with time inside active region are illustrated in Figure 2.5.

15

Figure 2.5: Laser model using laser rate equation [24]

The laser rate equations are defined in equation (2.1)–(2.3) [25];

𝜕𝑁(𝑡)

𝜕𝑡=

𝐼(𝑡)

𝑞𝑉𝑎−𝑁(𝑡)

𝜏𝑛− 𝑣𝑔𝑎𝑜

𝑁(𝑡)−𝑁𝑜

1+𝜖𝑆(𝑡)𝑆(𝑡) (2.1)

𝜕𝑆(𝑡)

𝜕𝑡= (ɼ𝑣𝑔𝑎𝑜

𝑁(𝑡)−𝑁𝑜

1+𝜖𝑆(𝑡)−

1

𝜏𝑝) 𝑆(𝑡) +

𝛽ɼ𝑁(𝑡)

𝜏𝑛 (2.2)

𝜕∅(𝑡)

𝜕𝑡=

𝛼

2(ɼ𝑣𝑔𝑎𝑜𝑁(𝑡) − 𝑁𝑜 −

1

𝜏𝑝) (2.3)

where ɼ is an optical confinement factor, 𝑣𝑔 is the group velocity, 𝑎𝑜 is the gain

coefficient, 𝑁𝑜 is the carrier density, 𝜖 is the gain compression, 𝜏𝑝 is the photon life

time, 𝛽 is the fraction of spontaneous emission, 𝜏𝑛 is the electrons life time, 𝑞 is the

electric charge, 𝑉𝑎 is the active volume of lasing, 𝛼 is the optical linewidth factor and

∅ is the optical phase. The DPSK signal and laser signal discussed above are fed into

the optical modulator to produce an optical signal.

Optical modulator

The optical modulation can be achieved by means of external modulation and direct

modulation. The direct modulation has limited application because this retain

unwanted chirps, fluctuations in intensity that produce the relative intensity noise and

yields the broaden signal spectrum that results in dispersion penalties. The external

modulators are preferred over direct modulation due to broaden signal issue. The

external modulator are further categorized in Electro Absorption Modulator (EAM)

and Electro Optic Modulator (EOM). EOM is widely used because it has linear

16

response, high extinction ratio and most importantly, it can control the amplitude,

frequency and phase of the optical carrier [26]. The EOM are developed using lithium

niobate (LiNbO3) materials [27] as it has low attenuation. The EOM has mainly two

modulator; one is the Optical Phase Modulator (OPM) and second is the Optical

Intensity Modulator (OIM). The OPM is developed using single electrode as shown in

Figure 2.6 [28];

Figure 2.6: Optical phase modulator [28]

The phase vibration ∅(𝑡) in optical modulator is induced according to Radio

Frequency (RF) signal, drive voltage 𝑉𝜋, bias voltage 𝑉𝑏𝑖𝑎𝑠 and time varying input

voltage 𝑉(𝑡) as expressed in equation (2.4) [28];

∅(𝑡) = 𝜋𝑉(𝑡)+𝑉𝑏𝑖𝑎𝑠

𝑉𝜋 (2.4)

The complex-envelope optical field can be defined as 𝐸𝑜(𝑡) = 𝐸𝑖(𝑡)𝑒∅𝑗(𝑡),

where 𝐸𝑜 is the output optical field and 𝐸𝑖 is the input electrical field. The OIM is

composed of two OPM in the parallel form to develop a Mach-Zehnder Interferometer

Modulator (MZIM) as presented in Figure 2.7 [28].

Figure 2.7: Optical intensity modulator [28]

The optical field is distributed in two arms of MZIM, where, each arm

represents the OPM for modulating the phase of optical carrier. The OPM uses the

17

constructive and destructive interference, where both fields are coupled at output

terminal to enable the ON-OFF modulation of carrier intensity. The mathematical

derivation for single stage MZIM is derived with assumption that on first arm 1 there

is no drive voltage and drive voltage is applied to arm 2. The output optical field for

single stage MZIM can be defined as in equation (2.5) [28];

𝐸𝑜(𝑡) =𝐸𝑖(𝑡)

2(1 + 𝑒

𝑗𝜋𝑉(𝑡)+𝑉𝑏𝑖𝑎𝑠

𝑉𝜋 ) = 𝐸𝑖(𝑡) cos (𝜋

2

𝑉(𝑡)+𝑉𝑏𝑖𝑎𝑠

𝑉𝜋) 𝑒

−𝑗𝜋

2

𝑉(𝑡)+𝑉𝑏𝑎𝑖𝑠𝑉𝜋 (2.5)

The output field of MZIM is characterized using the phasor and transfer characteristics

of MZIM, this is because MZIM has two waveguides splits into two arms and then

combine into single output waveguide. The electrodes are biased with two voltages as;

𝑉𝑏𝑖𝑎𝑠1 and 𝑉𝑏𝑖𝑎𝑠2 and according to that phase extracted are defined as ∅1 = 𝜋𝑉𝑏𝑖𝑎𝑠1

𝑉𝜋=

−∅2. The output optical field can be obtained as in equation (2.6) [28];

𝐸𝑜(𝑡) =1

2𝐸𝑖𝑅𝑀𝑆𝑒

𝑗𝜔𝑐𝑡(𝑒𝑗∅1(𝑡) + 𝑒𝑗∅1(𝑡)) (2.6)

where 𝜔𝑐 is the carrier angular frequency. The phases are swinging with respect to

magnitude and sign of voltage applied to electrodes either constructively or

destructively. The response of MZIM electrical to optical signal is described in Figure

2.8 [28];

(a) (b)

Figure 2.8: MZIM phasor and electric to optical characteristics: (a) P-V

characteristics curve, and (b) phasor diagram [28]

The MZIM power can be defined as 𝑃𝑀 = 𝛼𝑃𝑖 cos2 𝜋𝑉(𝑡)

𝑉𝜋, where 𝑃𝑀 is the

output modulator power, 𝛼 is the insertion loss. The MZIM modulator can further be

18

divided in two categorized based signal coding using Non Return-to-Zero (NRZ) and

Return -to-Zero (RZ) formats. Figure 2.9 illustrates the optical modulator designed for

NRZ and RZ formats [29], [30];

(a) (b)

Figure 2.9: Optical DPSK transmitter: (a) Optical DPSK-NRZ, and (b) Optical

DPSK-RZ

The optical DPSK-NRZ is modulating the phase of optical carrier using MZIM

that is defined as data modulator. In optical DPSK-RZ, the phase is modulated; firstly,

with intensity modulator and then by a synchronized pulse train with the same data

rate as of the data modulator [29], [30]. The RZ optical signal is more tolerant to

nonlinearity than NRZ optical signal. In the next section, the optical fiber

characteristics, types and its signal propagation is discussed.

2.4 Optical fiber and optical signal propagation

Optical fibers transmit the light using different layers; the core and the cladding,

composed of fine threads of glass (or other glass material). The optical fiber is

categorized in mainly two types, Single Mode fiber (SMF) and Multi-Mode Fiber

(MMF). The SMF has smaller core area than MMF that allows only one mode of light

at a time through the core [31]. SMF are preferred because they provide better signal

quality transmission over longer distances due to high modal dispersion. The

International Telecommunication Union (ITU) is a global standardization body for

telecommunication systems and vendors, and defines different types of fibers [31].

The SMF has various categories that include nondispersion-shifted (G.652),

19

dispersion shifted (G.653), 1550-nm loss minimized (G.654), and nonzero-dispersion

fiber (G.655) [31].

Optical signal propagation

The optical wave’s propagation in a single mode fiber is directed by Maxwell’s

equations i.e. wave equation (2.7) [33];

∆2𝐸 −1

𝑐2−𝜕2𝐸

𝜕2𝑡= −𝜇𝑜

𝜕2𝑃(𝐸)

𝜕2𝑡 (2.7)

where E is an electric field vector, 𝜇𝑜 is the vacuum permeability, c is the speed of

light, and P is the polarization density for electric field. When weak optical power is

achieved the induced polarization of the linear relation with electric field vector can

be defined in equation (2.8) [33];

𝑃𝐿(𝑟, 𝑡) = 휀𝑜 ∫ 𝑋(1)(𝑡 − 𝜏) ∙ 𝐸(𝑟, 𝜏)∞

−∞𝑑𝜏 (2.8)

where 휀𝑜 is the vacuum permittivity, 𝑋(1) is the first order susceptibility. The

polarization itself is composed of two parts one is linear and second is nonlinear, which

can be defined using equation (2.9) [34];

𝑃(𝑟, 𝑡) = 𝑃𝐿(𝑟, 𝑡) + 𝑃𝑁𝐿(𝑟, 𝑡) (2.9)

This nonlinear part of the polarization exists in silica fiber and usually comes

from the third order susceptibility can be defined using equation (2.10) [34];

𝑃𝑁𝐿(𝑟, 𝑡) = 휀𝑜∭ 𝑋(3)∞

−∞(𝑡 − 𝜏1)(𝜏 − 𝑡2)(𝜏 − 𝑡3) ∙ 𝐸(𝑟, 𝜏1)𝑑𝜏1 ∙

𝐸(𝑟, 𝜏2)𝑑𝜏2 ∙ 𝐸(𝑟, 𝜏3)𝑑𝜏3 (2.10)

The third order susceptibility 𝑋(3) is a 4th tensor, and could have more than 80

different terms. However, a single mode fiber (isotropic media), the third order

susceptibility, the number of independent terms are reduced to one [35]. The

propagation equation in nonlinear dispersive fibers can be obtained by solving wave

equation (2.7) for equation (2.8)-(2.10) using few assumption [35]. These assumptions

are:

𝑃𝑁𝐿 is a small perturbation of 𝑃𝐿 with maintain field polarization.

20

Apply weakly guiding approximation for small index difference between core

and cladding.

Apply quasi-monochromatic assumption in which the center frequency of

wave is greater than spectral width of wave also equivalently known as; slowly

varying envelope approximation in the time domain.

The propagation constant 𝛽(𝜔) few terms are approximated using Taylor

series expansion along carrier frequency of 𝜔𝑐 defined as in equation (2.11) [35];

𝛽(𝜔) = 𝛽𝑐 + 𝛽1(𝜔 − 𝜔𝑐) +1

2𝛽2(𝜔 − 𝜔𝑐)

2 +1

6𝛽3(𝜔 − 𝜔𝑐)

3 + ⋯⋯ (2.11)

Therefore, using Taylor series 𝛽𝑛 = (𝑑𝑛𝛽

𝑑𝜔𝑛) at 𝜔 = 𝜔𝑐, the cubic and higher-

order terms in (2.11) are negligible due to quasi-monochromatic assumption. The

second term for propagation constant i.e. 𝛽2 is defined as dispersion (ps2/km) effect in

optical communication that is discussed in Section 2.4.2.2. The dispersion region is

categorized in two regions; normal dispersion 𝛽2 > 0 and dispersion deviation 𝛽2 <

0. In the normal dispersion, high frequency components of signal travels slower than

low frequency components and opposite in dispersion deviation. The fiber dispersion

can be defined using dispersion parameter 𝐷 can be expressed numerically as 𝐷 =

𝑑

𝑑𝜆(1

𝑣𝑔) i.e. (ps/nm.km). The relationship between propagation constant and dispersion

parameter can be attained as in equation (2.12) [35];

𝛽2 = −𝜆2

2𝜋𝑐𝐷 (2.12)

where 𝜆 is the wavelength and 𝑣𝑔 is the group velocity. If the input electric field is

propagated in z- direction and it is polarized in x-direction then equation (2.7) will

become equation (2.13) [36];

𝜕

𝜕𝑧𝐴(𝑧, 𝑡) = −

𝛼

2𝐴(𝑧, 𝑡) + 𝑗

𝛽2

2

𝜕2

𝜕2𝑡𝐴(𝑧, 𝑡) +

𝛽3

6

𝜕3

𝜕3𝑡𝐴(𝑧, 𝑡) − 𝑗𝛾|𝐴(𝑧, 𝑡)|2𝐴(𝑧, 𝑡) +

𝑗𝛾𝑆𝑅𝜕

𝜕𝑡|𝐴(𝑧, 𝑡)|2𝐴(𝑧, 𝑡) −

𝛾

𝜔𝑐

𝜕

𝜕𝑡|𝐴(𝑧, 𝑡)|2𝐴(𝑧, 𝑡) … .. (2.13)

where 𝐴(𝑧, 𝑡) is the slowly varying envelope of electric field, 𝑧 is the direction of

propagation, t is = (τ-z)/ 𝑣𝑔, 𝛼 is the fiber loss co-efficient (1/km), 𝛽2 is the second

21

order propagation constant (ps2/km), 𝛽3 is the third order propagation constant

(ps3/km), γ is the nonlinear coefficient (2π𝑛2/𝜆𝑜𝐴𝑒𝑓𝑓), 𝑛2 is the nonlinear index co-

efficient, 𝜆𝑜 is the center wavelength, 𝜔𝑐 is center angular frequency and 𝑆𝑅 is the

slope of Raman gain. The first term in equation (2.13) defines the linear attenuation,

second term represents the second order dispersion, third term designates the third

order dispersion, fourth term denotes the Kerr effect, fifth term indicates the effect of

Stimulated Raman Scattering (SRS) and sixth term specifies the self-steepening effect.

The generalized Non Linear Schrodinger Equation (NLSE) equation (2.13) can

further be simplified by limiting the pulse width greater than 1 ps due to neglecting the

small terms SRS and self-steepening effect compared optical Kerr effect; then equation

(2.13) can be expressed as equation (2.14) [33];

𝜕

𝜕𝑧𝐴(𝑧, 𝑡) = −

𝛼

2𝐴(𝑧, 𝑡) + 𝑗

𝛽22

𝜕2

𝜕2𝑡𝐴(𝑧, 𝑡) +

𝛽36

𝜕3

𝜕3𝑡𝐴(𝑧, 𝑡)

− 𝑗𝛾|𝐴(𝑧, 𝑡)|2𝐴(𝑧, 𝑡) (2.14)

The solution of NLSE equation (2.14) is required to explore and understand

the various impairments occurring during signal transmission. The NLSE equation is

solved numerically using Split Step Fourier Method (SSFM) [36].

Optical fiber impairments

The signal transmission in optical fiber interacts with various impairments, thus

degrade the quality of the system. There are two main categories of transmission

impairments; linear and nonlinear effects as shown in Figure 2.10 [37];

Figure 2.10: Different transmission impairments effects in optical fiber [37]

22

The linear effects are produced due to variation in transmission characteristics, such

as: input power, distance, signal propagation, and due to different materials used in

optical fiber. These are categorized as attenuation, dispersion, and in polarization

dispersion. The nonlinear effects are classified as elastic and non-elastic. The elastic

effects are produced mainly, due to variation in refractive index. The high power

transmission in optical fiber produces fiber nonlinearity and non-elastic due to

radiation effect that causes the transmission. In the next subsections, these

transmission impairments are discussed briefly.

2.4.2.1 Attenuation

The loss in optical power due to signal propagation inside optical fiber is termed as

attenuation. It can be expressed as in equation (2.15) [37];

𝑃𝑜 = 𝑃𝑖𝑒−𝛼𝐿 (2.15)

where, 𝑃𝑖 is the input power launched in fiber, 𝑃𝑜 is the output light power received at

fiber end, 𝛼 is attenuation constant and 𝐿 is the length of optical fiber. The C-band has

minimum 𝛼 of 0.19 dB/km for bandwidth in THz [37]. The attenuation in optical fiber

is due to material absorption, scattering and geometric effects. The material absorption

is the dissipation amount of power as heat in optical fiber due to intrinsic absorption

(silica molecules) and extrinsic absorption (impurities such as: OH and metal ions).

The scattering is loss in optical power in terms of radiation. There are two main types

of scattering; Rayleigh and Mie. The Rayleigh scattering is produced due to small

dissimilarities in the density of glass during manufacturing. These dissimilarities are

microscopic in nature and smaller than the wavelengths used and consequently light

scatters in all directions. The irregularities in core cladding and its refractive index

produce the strains and bubbles that contribute to Mie scattering. The macro and micro

bending effects are considered to be geometric effects in optical fiber.

2.4.2.2 Dispersion

Dispersion is termed as chromatic dispersion. It is produced due to non-

monochromatic light source, thus different spectral components within the pulse will

23

travel with different velocities in result signal distortion is attained. In the optical fiber

transmission, the optical signal is the sequence of pulse that represents binary

information and due to dispersion, the pulse broadening effect is produced. This pulse

broadening effect degrades the system performance by Inter Symbol Interference (ISI)

with neighbor pulse and loss of energy within the bit slot. The reduction in pulse

energy decreases the Signal to Noise Ratio (SNR) at output. For quality performance,

the SNR should be constant and average received power should be enough for

detection process. The chromatic dispersion is composed of material and waveguide

dispersion as illustrated in Figure 2.11 [38].

Figure 2.11: Dispersion in SMF due to material dispersion (𝐷𝑀) and waveguide

dispersion (𝐷𝑊) [38]

The material dispersion produces the change in refractive index of optical fiber

that is function of wavelength and variation in refractive index lead to group delay in

each spectral component. The waveguide dispersion is the function core radius and

difference between refractive indices in fiber core and cladding. The numerical

representation of dispersion is illustrated in equation (2.12). The Polarization mode

dispersion is produced due to nonlinear polarization of electric fields [38].

24

Non-Linear fiber effects

The nonlinear effects in optical fiber are very crucial in high-speed optical

communication systems. The nonlinear effects are divided in two categories, elastic

and non-elastic. The elastic nonlinear effect are aroused due to interaction of optical

signal with phonons vibration in the silica medium. These effects are termed as

Stimulated Raman Scattering (SRS) and Stimulated Brillioun Scattering (SBS). The

elastic nonlinear effects are encountered in optical fiber due to dependency of

refractive index on the intensity of light signal. The most important effects are self-

phase modulation (SPM), Cross Phase Modulation (CPM), and Four Wave Mixing

(FWM). These nonlinear effects are named as optical Kerr nonlinearities of optical

Kerr effects.

2.4.3.1 Stimulated Brillioun Scattering (SBS) and Stimulated Raman Scattering

(SRS) effects

The SBS effects are produced, when optical signal interacts with matter via acoustic

waves and leads to optical signal power to a backward propagating stock wave at

satisfied threshold [38], [39]. The output power remains constant after SBS threshold

even though, when high power input power is given to fiber. The SBS threshold is

dependent on the effect area of fiber core 𝐴𝑒𝑓𝑓, effective fiber length 𝐿𝑒𝑓𝑓, the gain

coefficient of SBS and other pump parameters [40], [41] for modulated signal i.e. NRZ

or RZ format. The SBS is controlled by reducing SBS gain, increasing the laser

linewidth and according to Sakamoto [42], the launched power in optical fiber also

provides the control for SBS effect.

SRS arises in optical fiber when the optical pump signal is scattered by silica

molecules and this scattering occur isotropically. SRS scattered light at longer

wavelength than incident wave. If the signal exist, SRS light will amplify signal and

pump wavelength signal will decrease the power. SRS can be encountered in both

forward and in backward directions.

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