Design & Analysis of Secure Optical

Communication System with Advance

Modulation Formats

i

Design & Analysis of Secure Optical Communication

System with Advance Modulation Formats

ii

Certificate of Approval

iii

iv

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ACKNOWLEDGEMENT

I am thankful to Almighty Allah who gave me knowledge & wisdom to achieve this

milestone. Sincere gratitude and thanks to my supervisor Prof. Dr. Muhammad Khawar

Islam whose guidance & support remained with me throughout the process. I dedicate

this thesis to my parents/family for their patience & support throughout my life.

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SUMMARY

The growth in web traffic has driven the rise in demand for information measure and high

data rates. ‘Advance optical modulation formats’ are taken into account as a promising

feature to satisfy the enhanced demand for information measure in broadband and other

type of services. Lots of modulation formats have been proposed to increase the data

rates but security implementation in optical communication system explicit to these

modulation formats is still an open area to explore. In this thesis, the performance of

some advance modulation formats as applicable to chaotic/secure optical communication

is analyzed. The research work includes the generation of high bandwidth optical chaos

through semiconductor lasers by controlling different parameters to get the optimum

chaos. Initially, the basic chaotic communication system by synchronizing transmitted

and received chaos is developed and tested for ordinary modulation schemes like RZ and

NRZ to check validity of the scheme. Later, the long haul secure communication models

based on Duobinary modulation and QAM format are designed and their performance is

tested against various parameters. The Duobinary format is chosen due to its effective

bandwidth utilization and robust performance against the intersymbol interference

whereas QAM scheme is selected due to its better support against high data rates. In

comparison to chaos produced by fiber lasers which are inherited by low bandwidth,

semiconductor laser generated chaos is exploited for embedding the message. The

performance of modulation schemes is studied for long distance communication models

including non-linear and higher order effects. Various simulations experiments are

conducted on latest versions of Optisystem 14.0 and MATLAB. This work can be

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considered as a first effort related to security implementation explicit to the advance

modulation formats using optical chaos.

Keywords: Semiconductor Lasers, Advance Modulation Formats, Chaos

Synchronization,

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Dedicated to my parents & family…

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TABLE OF CONTENTS

TITLE PAGE……………………………………...……………………………………………………. i

APPROVAL PAGE ............................................................................................................ ii

PLAGIARISM UNDERTAKING………………………………………………………………….. iii

AUTHOR DECLARATION……………………………………………………………. iv

ACKNOWLEDGEMENT ................................................................................................... v

SUMMARY........................................................................................................................ vi

TABLE OF CONTENTS .................................................................................................. ix

LIST OF FIGURES ......................................................................................................... xii

LIST OF TABLES .......................................................................................................... xvi

Chapter 1 ............................................................................................................................ 1 Introduction ................................................................................................................................................ 1

1.1 Chaos Basics ..................................................................................................................................... 1

1.2 Basic Chaotic Communication System............................................................................................. 2

1.3 Optical Chaos Based Communication .............................................................................................. 5

1.4 Requirements of Chaotic Communication System ........................................................................... 7

1.5 Advantages & Disadvantages of Chaos Based Communication: ..................................................... 7

1.6 Importance of Modulation Formats .................................................................................................. 9

1.6.1 Bandwidth Expansion ...............................................................................................................11

1.6.2 Spectral Efficiency Enhancement .............................................................................................12

1.7 Security Implementation of Advance Modulation Formats .............................................................13

1.8 Thesis Outline ..................................................................................................................................14

1.9 References .......................................................................................................................................16

Chapter 2 .......................................................................................................................... 20 Literature Review ......................................................................................................................................20

2.1 Conclusion .......................................................................................................................................29

2.2 References .......................................................................................................................................30

Chapter 3 .......................................................................................................................... 34 Semiconductor Laser Chaos Generation ...................................................................................................34

3.1 Optical Feedback .............................................................................................................................34

3.2 Optical Coupling and Injection Mechanism ....................................................................................35

3.3 External Modulation ........................................................................................................................36

3.4 Insertion of Nonlinear Element .......................................................................................................37

3.5 Chaos Generation ............................................................................................................................37

3.5.1 Changing Bias Current (Route from Quasi-Periodic to Chaos) ................................................38

3.5.2 Changing Modulation Peak Current (High Amplitude Pulses) .................................................40

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3.5.3 Changing Frequency of Current Source (Random Amplitude Pulses) .....................................42

3.6 Conclusion .......................................................................................................................................44

3.7 References .......................................................................................................................................45

Chapter 4 .......................................................................................................................... 46 Performance Analysis of Intensity Modulation Formats ...........................................................................46

4.1 Introduction .....................................................................................................................................46

NRZ Modulation Format ............................................................................................................48

RZ Modulation Format ...............................................................................................................48

4.2 Mathematical Model ........................................................................................................................50

4.3 Simulation Model ............................................................................................................................52

4.4 Results and Discussion ....................................................................................................................53

4.5 Conclusion .......................................................................................................................................57

4.6 References .......................................................................................................................................58

Chapter 5 .......................................................................................................................... 61 Secure Duobinary Optical Transmission Model ........................................................................................61

5.1 Introduction .....................................................................................................................................61

5.1.1 Duobinary Modulation Format .................................................................................................62

5.2 Mathematical Model ........................................................................................................................63

5.3 Proposed Scheme .............................................................................................................................68

5.4 Results & Discussion .......................................................................................................................69

5.5 Conclusion .......................................................................................................................................76

5.6 References .......................................................................................................................................78

Chapter 6 .......................................................................................................................... 83 Secure Optical QAM Transmission Model ...............................................................................................83

6.1 Introduction .....................................................................................................................................83

6.2 Mathematical Model ........................................................................................................................84

6.2.1 Laser Rate Equations ................................................................................................................85

6.2.2 QAM Signal ..............................................................................................................................86

6.3 Proposed Scheme .............................................................................................................................88

6.4 Results and Discussion ....................................................................................................................90

6.5 Conclusion .......................................................................................................................................98

6.6 References .....................................................................................................................................100

Chapter 7 ........................................................................................................................ 105 Performance Evaluation of Chaotic QAM-128 Dual Polarization System for Long Haul Secure Optical

Communication .......................................................................................................................................105

7.1 Introduction ...................................................................................................................................105

7.2 Proposed Model .............................................................................................................................108

7.3 Mathematical Model: .....................................................................................................................112

7.4 Simulations and Results: ...............................................................................................................113

7.5 Conclusion: ....................................................................................................................................125

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7.6 References: ....................................................................................................................................126

Chapter 8 ........................................................................................................................ 129 Conclusion ...............................................................................................................................................129

8.1 Summary of Accomplishments..........................................................................................................129

8.2 Future Work.......................................................................................................................................132

Abbreviations .................................................................................................................. 133

List of Publications ........................................................................................................ 135

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LIST OF FIGURES

Fig. 1.1 Message encoding schemes CMS, ACM & CSK .............................................................. 3

Fig. 1.2 Increase in internet users .................................................................................................. 10

Fig. 1.3 2016 Projection of video services percentage of internet traffic [26] .............................. 11

Fig. 3.1 Optical Feedback .............................................................................................................. 35

Fig. 3.2 Optical Coupling and Injection ........................................................................................ 36

Fig. 3.3 External Modulation......................................................................................................... 36

Fig. 3.4 Insertion of Non-linear Element ....................................................................................... 37

Fig. 3.5 Semiconductor laser output at bias current = 3 mA ......................................................... 39

Fig. 3.6 Semiconductor laser output at bias current = 13 mA ....................................................... 39

Fig. 3.7 Semiconductor laser output at bias current = 33 mA ....................................................... 39

Fig. 3.8 Semiconductor laser output at modulation peak current = 5 mA ..................................... 41

Fig. 3.9 Semiconductor laser output at modulation peak current = 7 mA ..................................... 41

Fig. 3.10 Semiconductor laser output at modulation peak current = 10 mA ................................. 41

Fig. 3.11 Semiconductor laser output at 0.75 GHz frequency of current source .......................... 43

Fig. 3.12 Semiconductor laser output at 0.8 GHz frequency of current source ............................ 43

Fig. 3.13 Semiconductor laser output at 1.0 GHz frequency of current source ............................ 43

Fig. 4.1 Classification of different Intensity modulation formats ................................................. 49

Fig. 4.2 NRZ, RZ, DB, AMI and CSRZ optical signal generation ............................................... 50

Fig. 4.3 Optical spectrum of different modulation formats ........................................................... 51

Fig. 4.4 Proposed GPON implemented with different modulation schemes ................................. 52

Fig. 4.5 Performance analysis of different NRZ formats .............................................................. 54

Fig. 4.6 Performance analysis of different RZ formats ................................................................. 54

Fig. 4.7 Performance analysis at 10 Gbps ..................................................................................... 55

Fig. 4.8 Performance analysis at 40 Gbps ..................................................................................... 56

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Fig. 4.9 Performance analysis at 100 Gbps ................................................................................... 56

Fig. 4.10 BER analysis at 10 Gbps ................................................................................................ 56

Fig. 5.1 Basic CMS communication model. .................................................................................. 64

Fig. 5.2. Chaotic optical communication model using duobinary format. .................................... 69

Fig. 5.3 Duobinary message with data rate 10GB/s. ..................................................................... 70

Fig 5.4(a) Time domain plot of generated chaos. .......................................................................... 70

Fig 5.4(b) Time domain plot of generated chaos on larger scale. ................................................. 71

Fig. 5.5(a) Duobinary message generated by transmitter. ............................................................. 71

Fig. 5.5(b) Chaotic waveform hiding duobinary message. ............................................................ 71

Fig. 5.6(a) Optical spectrum of duobinary message. ..................................................................... 72

Fig. 5.6(b) Optical spectrum of chaos embedding duobinary message. ........................................ 72

Fig. 5.7(a) Transmitted vs. received chaos without synchronization. ........................................... 73

Fig. 5.7(b) Transmitted vs. received chaos after synchronization & delay matching. .................. 73

Fig. 5.8 Transmitted signal vs. received Signal. ............................................................................ 75

Fig. 5.9. Amplifier Response on chaotic waveform at different SMF lengths & corresponding

gains............................................................................................................................................... 75

Fig. 5.10 Eye-diagrams of duobinary signal at different SMF lengths. ........................................ 76

Fig. 5.11 Q-factor vs Length of fiber. ........................................................................................... 76

Fig. 6.1 Basic QAM Model ........................................................................................................... 87

Fig. 6.2 Chaotic optical communication model using QAM format ............................................. 89

Fig. 6.3 QAM module ................................................................................................................... 89

Fig. 6.4 QAM Message with Data Rate 10 GB/s .......................................................................... 90

Fig. 6.5 Time Domain Plot of Generated Chaos ........................................................................... 91

Fig. 6.6 Zoomed Plot of Chaos in Time domain ........................................................................... 91

Fig. 6.7 4-QAM Optical Spectrum ................................................................................................ 92

Fig. 6.8 Optical Spectrum of Chaos Embedding 4-QAM ............................................................. 92

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Fig. 6.9 Time Delay between Transmitted and Received Chaos ................................................... 93

Fig. 6.10 Time Delay between Transmitted and Received Chaos ................................................. 93

Fig. 6.11 Time Delay between Transmitted and Received Chaos ................................................. 94

Fig. 6.12 Optical spectrum of QAM signal unmasked from chaos ............................................... 94

Fig. 6.13 Optical spectrum of received QAM signal after filtering ............................................... 95

Fig. 6.14 Retrieved Message ......................................................................................................... 95

Fig. 6.15 Constellation diagram of 4-QAM signal with different SMF lengths ............................ 96

(a) 10 km (b) 50 km (c) 80 km .............................................................................................. 96

Fig. 6.16 BER vs. Laser Power (without adjusting gain of amplifier) .......................................... 96

Fig. 6.17 Constellation diagram of 16-QAM signal with different SMF lengths .......................... 97

(a) 0 km (b) 10 km (c) 50 km (d) 80 km .................................................................................. 97

Fig. 6.18 Constellation diagram of 64-QAM signal with different SMF lengths .......................... 98

(a) 0 km (b) 10 km (c) 50 km (d) 80 km .................................................................................. 98

Fig. 7.1 128-QAM Chaotic Communication Model (Dual polarization) .................................... 109

Fig. 7.2 Transmitter Design ......................................................................................................... 109

Fig. 7.3 Receiver Design ............................................................................................................. 110

Fig. 7.4 Optical spectrum of 128-QAM signal at Tx ................................................................... 115

Fig. 7.5 Time domain plot of 128-QAM signal at Tx ................................................................. 116

Fig. 7.6 Chaos produced by semiconductor laser ........................................................................ 116

Fig. 7.7 Chaotic waveform hiding message ................................................................................ 117

Fig. 7.8 Constellation diagram of message embedding chaos ..................................................... 117

Fig. 7.9 Signal after eliminating chaos at Rx side ....................................................................... 118

Fig. 7.10 Optical spectrum of received 128-QAM signal ........................................................... 118

Fig. 7.11 Transmitted message vs received message .................................................................. 119

Fig. 7.12 Constellation diagram of 128-QAM signal at Tx side

a) x-polarization state b) y-polarization state ........................................................................ 120

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Fig. 7.13 Constellation diagram of 128-QAM signal at 100 km (after controlling dispersion)

a) x-polarization state b) y-polarization state ........................................................... 121

Fig. 7.14 Constellation diagram of 128-QAM signal at 100 km (without controlling dispersion)

a) x-polarization state b) y-polarization state ........................................................... 121

Fig. 7.15 Constellation diagram of 128-QAM signal at 160 km (without controlling dispersion )

a) x-polarization state b) y-polarization state ........................................................... 122

Fig. 7.16 Constellation diagram of 128-QAM signal after 220 km (without controlling dispersion)

a) x-polarization state b) y-polarization state ........................................................... 122

Fig. 7.17 Bit errors at different fiber lengths ............................................................................... 123

Fig. 7.18 EVM values at different fiber lengths .......................................................................... 124

Fig. 7.19 BER values at different fiber lengths ........................................................................... 124

Fig. 7.20 Log of BER at different fiber lengths ........................................................................... 124

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LIST OF TABLES

Table 1.1 World Internet Usage Statistic [23] ................................................................................. 9

Table 3.1 Semiconductor Laser Parameters (with different bias current) ……………………...38

Table 3.2 Semiconductor Laser Parameters (with different modulation peak current) ................. 40

Table 3.3 Current Source Parameters (with different frequencies) ............................................... 42

Table 4.1 System Parameters ... …………………………………………………………………53

Table 5.1 Performance comparison of message encoding schemes .............................................. 63

Table 5.2 Physical parameters of chaotic laser.............................................................................. 66

Table 5.3 Operating parameters of chaotic laser ........................................................................... 67

Table 5.4 Operating parameters of Duobinary model ................................................................... 69

Table 6.1 Parameters of semiconductor laser ................................................................................ 86

Table 6.2 Other parameters of QAM model .................................................................................. 88

Table 7.1 System parameters ....................................................................................................... 114

Table 7.2 Chaotic laser parameters ............................................................................................. 114

Table 7.3 Current source parameters ........................................................................................... 115

Table 7.4 System performance indicators ................................................................................... 123

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Intentionally Left Blank

1

CHAPTER 1

Introduction

1.1 Chaos Basics

Chaos being a ubiquitous phenomenon, researchers have not only been exploring chaos

properties for scientific understanding but also have been successfully employing it for useful

purposes. The interesting applications of chaos include but are not limited to secure

communication [1], ultrafast random number generation [2], secure LIDAR [3], image

processing [4] and water marking and steganography [5-7].

Chaos is the 3rd most important and vital discovery of 21st century besides quantum

mechanics and theory of relativity. It was discovered by Lorenz while numerically simulating

weather model [8]. Chaos is a random looking phenomenon, characterized by sensitive

dependence on initial conditions, whose generation mechanism is not stochastic but is

deterministic.

According to Poincare-Bendixon [9], minimum three nonlinear differential equations are

necessary for three degrees of freedom or two differential equations and one periodic

perturbation to meet the three degrees of freedom criteria. Nonlinear systems exhibiting chaos

also exhibit periodicity, quasi-periodicity and stable behavior.

The chaos is generated at certain range of parameters and till now this must be determined

numerically or experimentally with no closed form expressions. Researchers have explored

nonlinear systems and plotted bifurcation diagrams which exactly tell when a system switches

from one region to other once a specific parameter is varied.

Chaos can be identified and measured in two ways i.e. qualitatively and quantitatively.

The main qualitative measure of chaos is phase plots direct observation approach [10] in which

2

we shall observe circulating trajectories with compressing and expanding nature. The

quantitative measure of chaos is Lyapunov Exponent [11] which measures the average

divergence of trajectories in phase space once these starts nearby.

In continuous systems, Chaos is only exhibited by nonlinear systems with cross-coupled

terms in differential equations. The cross-coupled terms are responsible for bouncing of energy

from one to other state.

Irregularities due to chaos should be different from the random fluctuations which follow

stochastic process as the chaos based communication system can be defined by the set of precise

equations termed as deterministic equations.

1.2 Basic Chaotic Communication System

It was discovered by Pecorra and Caroll [12] that two similar chaotic systems with same

set of parameters can be synchronized if seed is fed from the transmitter to receiver. It was an

important milestone discovery which established that chaos can be used for secure

communication. So, a receiver almost identical to transmitter is synchronized to transmitter by

feeding a seed from transmitter to receiver and then message is encoded and decoded in one of

three possible ways [13].

• CMS

• ACM

• CSK

CMS (Chaos Message masking): In this scheme, message is mixed with chaotic waveform

before sending it over the communication channel. The receiver must be synchronized with the

transmitter to recover the original message at the transmitting side. Message can be recovered

after subtraction. A little mismatch in synchronization between the transmitter side and receiver

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side can degrade the overall performance of optical communication system. In this scheme, a

high bandwidth chaotic carrier is required to transmit high bit rate. Also, the amplitude of

original message should be small enough as compared to chaotic carrier so that message could be

hidden by chaotic carrier easily.

CSK (Chaos Shift Keying): In this scheme, transmitter side laser’s bias current is modulated with

original message, but the receiving side laser is not modulated. As digital message is used for

modulation so two lasers can be synchronized when “0” is transmitted and reversely the lasers

are desynchronized. Message can be recovered by calculating the synchronization error.

ACM (Additive Chaos Modulation): In this scheme, the message is added within the

transmitter’s optoelectronic loop to change the dynamics of the transmitter. The message can be

made to enter in loop additively as in case of ACM or multiplicatively as in MCM

(Multiplicative Chaos Modulation). ACM scheme can be used to add more complexity in the

system in addition to the synchronization control before and after the message encoding.

The above-mentioned schemes have different effects on system parameters such as

synchronization, dynamics and communication performance. The schematic to implement these

schemes is shown in Fig 1.1.

Fig. 1.1 Message encoding schemes CMS, ACM & CSK

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A chaotic communication system must be able to meet following requirements from security

perspective.

• Confidentiality

• Integrity

• Availability

Availability:

This defines the availability of network components all the time. Components may include

hardware, software, complete system or subsystem. In case of service interruption, these

components should be recovered immediately and quickly. Ideally, these components should not

be considered as susceptible during DoS (Denial of Service) attacks.

Integrity:

This is related to the accuracy of the data transmission. Thus, it defines the ability to protect or

hide data, information or transmission from unwanted, unauthorized or accidental alteration.

Sometimes, this term is also used in context of system, application or network functioning.

Confidentiality:

This means information should not be disclosed to unauthorized parties. In today’s world

information may have so much value. Personal information, debit card or credit card information,

bank account information, visa or passport information, business or defense secrets are some

common type of important information which must be confidential. Protecting such important

information requires encryption.

Electronics based communication is more vulnerable to security attacks as compared to

optical security. Cellular phones, landline conversation can be easily tapped or intercepted by the

attackers. Although lots of security algorithms are proposed in order to ensure the security of

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data. However, advanced high-speed computing machines are also evolved with the passage of

time which can perform decryption of data in a very limited time.

Over the past few years many cryptographic solutions that were initially thought to be safe

have finally been cracked. One of notable example is the cracking of DES (Data Encryption

Standard) and security gaps observed in WEP (Wireless Equivalent Privacy) Protocol [3]. In this

situation optical cryptography is one the most suitable solution which is explored by the

scientists.

1.3 Optical Chaos Based Communication

Optical fibers have so many advantages as compared to other communication mediums,

these include their light weight, small size, electrical isolation, very low attenuation, huge

bandwidth, low cost, reliability and flexibility [14]. One major property of secure optical

communication is the encryption of data. The system can be made secure by implementing

optical cryptography.

Lasers and especially semiconductor lasers are a very good source of generating very high

speed optical chaos used for secure optical communication. Semiconductor lasers can be utilized

for chaos generation in Optical feedback mechanism [15], Optoelectronic feedback [16] and

Direct modulation of laser current [17]. Besides semiconductor lasers, Erbium doped fiber ring

lasers (EDFRLs) are also used to produce optical chaos. The three main schemes to generate

chaos are loss modulation [18], pump modulation [19] and loop nonlinearities [20].

Chaos produced by semiconductor laser is of great importance in daily life applications since

the chaos produced in this way is very high due to frequency of non-uniform/irregular

oscillations in GHz range, which is also much greater as compared to chaos frequency in

traditional electronic circuits. In other words, semiconductor laser chaos is at least two digits

6

faster than the chaos attained in the fastest electronic circuits. In modern communication system,

light is the carrier for fast data transmission and these fast chaotic oscillations meet well with the

higher data rates. Although semiconductor lasers which are mostly described by signal’s field

and carrier density, are considered as stable lasers during their operation. However, these lasers

can be destabilized easily by launching perturbation from outside like optical injection, optical

feedback mechanism and modulation of different laser parameters. In such lasers, irregularities

in oscillations can be induced significantly when the reflected light from external

mirrors/reflectors is fed into the cavity to couple with original field.

Although the oscillations of different semiconductor lasers are different even if made up of

same wafer however these oscillations can be adjusted to the order of GHz frequency by

manipulating injection current in the cavity. So, a light source is required in injection locking of

different type of lasers in addition to the feature of accurate frequency detuning. The injection

locking phenomenon has been investigated thoroughly in semiconductor lasers for stable laser

output and amplification. In such applications, when laser is locked to some source it means that

laser copies almost same spectrum as that of injected light. At the other hand instabilities,

unsteady injection of light and detuned frequencies mixing occurs outside the stable region. As

far as chaos is concerned, this external injection offers an additional degree of freedom inside the

semiconductor lasers which can produce chaotic oscillations in laser’s output [21].

Optical signal produced by semiconductor laser exhibits following four main parameters to

carry useful information.

• Phase

• Amplitude

• Wavelength

7

• Polarization

Information can be made secure by working on above parameters or combination of

parameters at the same time.

1.4 Requirements of Chaotic Communication System

To implement chaotic communication system, two fundamental requirements cannot be ignored.

These include

• Synchronization

• High Bandwidth

Transmitting and receiving side of optical communication system must be synchronized

with each other. A little mismatch in the parameters of transmitting and receiving side can result

in huge errors. To implement the synchronization between chaos at transmitting and receiving

side, same hardware with same parameter settings are needed at both side of communication

system. This symmetry is key for an identical synchronous solution of the coupled laser systems.

In addition to this, the identical synchronous system must also be stable.

Another important requirement is of high bandwidth in the implementation of chaos based

optical communication system. The chaos produced by chaotic source must be atleast 10 times

higher than the bandwidth of the original message to be sent securely. Thus, the chaotic schemes

are well suitable for their implementation on optical fiber specially using SMF because of its

larger capacity as a transmission medium.

1.5 Advantages & Disadvantages of Chaos Based Communication:

Although chaos has different applications in daily life however few advantages of chaos-

based communication can be listed as [22],

8

• Classical cryptography is restricted to limited integer numbers while chaos-based

communication uses the concept of broad continuous numbers. By using this property of

chaotic communication, more encryption functions can be applied at the same time.

• Traditional cryptographic solutions require digitization of data before their implication.

Chaos concept can be applied without performing digitization of data.

• Chaos based communication can be implemented by using analog components such as

lasers which is not possible in traditional cryptosystems.

• Usually, traditional cryptographic system requires two components i.e digital component

for encryption of data and analog circuit for performing modulation. These two tasks can

be implemented by using single circuit in chaos-based communication system.

• Pseudo-random waveforms which are non-periodic in nature can be utilized to hide

message.

Some other applications of chaos which are not directly concerned with the communication

but where non-periodic pseudo-random waveforms generated by chaos can be used in random

number generation that can be further used in many applications such as gaming, statistics,

gambling etc.

Besides some important applications in communication, few disadvantages of chaos-based

communication are [22],

• Chaos based communication systems require high bandwidth. The bandwidth of chaotic

carrier must be too high as compared to message amplitude. Typical bandwidth

requirement of chaos is 10 times than the size of the message. Although, it requires extra

bandwidth for security feature implementation but due to huge capacity of optical fiber

channel this disadvantage can be overcome.

9

• Probability of inducing error during transmission is much greater than traditional

cryptographic schemes. Many error encoding schemes can be in traditional cryptographic

system which is not easily possible in chaos-based communication.

• Circuit complexity specially in achieving synchronization, is another problem which is

directly related to the overall cost of the system.

• Chaos based communication requires high power transmitters.

1.6 Importance of Modulation Formats

Internet usage & multimedia fever is increasing day by day not only in developed countries

but also in under-developed/third world countries, resulting in enormous requirement of

bandwidth. Greater than 50% of the total population of the world is using internet facility & its

demand is further increasing day by day. Table 1.1 shows the increase in internet users with

respect to population over the few years whereas year wise increase in internet users can be seen

in Fig. 1.2 [23].

Table 1.1 World Internet Usage Statistic [23]

WORLD INTERNET USAGE & POPULATION STATISTICS

(2017 UPDATE)

Regions

Population

(2017 Approx.) Population % Internet Users

Penetration

(Population %)

Growth %

(2000-2017 Approx.)

Africa 4,148,177,672 55.20% 1,856,212,654 44.70% 1523.90%

Asia 822,710,362 10.90% 630,708,269 76.70% 500.10%

Europe 647,604,645 8.60% 384,766,521 59.40% 2029.40%

Middle East 1,246,504,865 16.60% 335,453,374 26.90% 7330.70%

North America 363,224,006 4.80% 320,067,193 88.10% 196.10%

Latin America 250,327,574 3.30% 141,489,765 56.50% 4207.40%

Australia 40,479,847 0.60% 27,540,655 68.00% 261.40%

World Total 7,519,028,971 100.00% 3,696,238,431 49.20% 923.90%

10

Fig. 1.2 Increase in internet users

The development in web activity, which incorporates information, voice, and video

administrations, has driven the expanded request in transmission capacity and high information

rates. In this scenario, optical fiber is the only choice of medium for the researchers for high data

rates and long distances at the same time. Although WiMAX, Wifi & 3G are becoming popular

due to their flexibility and scalability [24] but the importance of optical fiber as a backbone

medium cannot be ignored. As indicated by Cisco organization, the web activity from 2009 to

2016 will be four folded [25]. This expansion in web movement is driven by the blast in online

recordings. Figure 1.3 shows as a 2016 projection of the video benefits as a rate of all web

movement. Furthermore, an extensive variety of online applications are being developed and

there is an expansion in separate learning. The greater part of this will expand the transmission

capacity requests later on.

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Fig. 1.3 2016 Projection of video services percentage of internet traffic [26]

Overall innovative work endeavors are being directed to meet the high limit request in

transport network, for the most part for 100G Ethernet and beyond. The two principle issues that

should be recognized to expand the information rate to 100Gb/s per wavelength are

• Bandwidth Expansion

• Spectral Efficiency Enhancement

1.6.1 Bandwidth Expansion

One direct way to deal with increment in the capacity of the whole system is to expand the

transmission data rates per wavelength either optically or electronically. In optical

correspondence, two strategies are generally utilized to expand the limit of the transmission data

rates. The principal strategy is to expand the bandwidth of the system by including a large

number of optical carriers i.e. Wavelength Division Multiplexing (WDM). The WDM uses the

separation of 1 nm between the carrier wavelengths inside single fiber and has attracted a huge

deal of interest in the area of optical communication in the past few years [27]. The technology

12

can broaden the transmission transfer speed by including numerous transceivers for the current

optical fiber links without the need to introduce other fiber links. WDM is viewed as a standout

amongst the most cost proficient ways to deal with increment the optical fiber interface

throughput. The second system is to expand the electronic transfer speed per wavelength

depending on the CMOS innovation. It is important to note that existing digital to analog

converters (DAC)/(ADC) can just help a 6 GHz bandwidth. It is a test to acknowledge 100 Gb/s

transmission in a financially savvy way. Yet, as of late DAC/ADC accomplished more than 30

GS/s with more than a 20 GHz analog bandwidth. This can help to transmit high data rate

transmission.

1.6.2 Spectral Efficiency Enhancement

In optical communications, the spectral potency, that is the data capacity per unit

bandwidth, is that the most significant figure of advantage. Currently, optical networks use

intensity modulation and direct detection for transmission, and additionally use binary

modulation to minimize the transceiver complexness. However, with binary modulation, the

spectral potency won't exceed one bits/s/Hz. Recently, many advanced modulation formats in

signal amplitude, phase, and polarization are investigated to extend the capability of the system

[28]. The other techniques that can be combined with advance modulation formats and have

currently huge focus of researchers are

• Coherent Detection

• Dual Polarization

• Signal Processing

• DWDM communication

• Impairments Control

13

1.7 Security Implementation of Advance Modulation Formats

Higher order modulation formats can play a vital role in making optical communication

networks more efficient along with carrying higher data rates, however security concerns

explicitly need to be studied in these formats for reliable communication along with efficiency.

By using Advance Modulation Formats a waveform can be made more complex in the

presence of some chaotic source before sending it on physical medium. So, our idea is to exploit

the complexity of waveforms of advance modulation formats in the presence of some chaotic

source i.e. semiconductor lasers. The advantage of using semiconductor lasers is to get increased

bandwidth of chaos as discussed in Section 1.3.

Introducing chaos in advance modulation formats has some pros and cons which can be listed as,

Pros

• High data rate along with security

• Good spectral efficiency

• Effective use of optical fiber

Cons

• Complex transmitter & receiver design

• More effected by transmission impairments

• High power required

• Difficult to match initial waveform

• Filtration/Signal processing techniques required.

14

1.8 Thesis Outline

This thesis is composed of seven chapters. This outline gives brief description of all chapters

which construct this thesis.

Chapter 1 gives a short overview of security implementation in optical communication system

implemented with advance modulation formats. This chapter begins with basics of chaos and

includes introduction of optical chaotic communication system. This chapter not only covers

fundamental requirements of chaotic communication system but also includes advantages and

disadvantages of chaos based optical communication. In the end importance of advance

modulation formats is highlighted which can be made secured by high bandwidth chaos

produced by semiconductor lasers to take the advantage of high data rate and security at the same

time.

Chapter 2 provides the related work which is done in the field of chaos and advance modulation

formats. This section begins with the work done when chaos was first utilized in the field of

communication for security purposes. It contains the references related to chaos generation and

chaos synchronization techniques. Some work is included related to chaos enhancement. Finally,

some papers are included which deals with the advance modulation formats, their comparisons

and the performance of optical communication system using these modulation formats.

Chapter 3 deals with the chaos generation through semiconductor lasers. Different parameters

are considered and simulated to see the impact of chaos produced through semiconductor lasers.

Factors affecting degree and enhancement of chaos are also studied and simulated in this chapter.

Chapter 4 shows detailed performance comparison of different modulation formats. Intensity

based modulation formats are selected for this section and end to end optical communication

system is implemented by using these formats. Performance is measured in terms of Bit error

15

rate and Q-factor. Different distances have been tested against these factors. Among all the

formats, duobinary model is selected for the next stage to implement security due to its multi-

level.

Chapter 5 is related to our work which is done to hide multilevel message format i.e duo-binary

by using optical chaos produced through semiconductor laser. A long distance secure optical

communication model based on duo-binary modulation format is proposed. Performance of RZ

& NRZ-duobinary format is measured in chaotic environment. Impact of linear and nonlinear

impairments on secure signal propagation is investigated.

Chapter 6 describes the implementation of security measures while transmitting QAM signal.

Laser rate equations are used for mathematical modeling. Whereas, masking technique is used

for signal security. In this chapter propagation of secure QAM signal is studied for long haul

communication. Propagation model is implemented for 4-QAM which is extended to 16-QAM &

64-QAM model.

Chapter 7 shows the performance analysis of chaotic QAM-128 dual polarization model which

is designed for long distance communication. Transmitter and receiver design are presented in

this chapter for proposed scheme. Chaos is generated by using semiconductor laser and

performance of model is investigated for different lengths of fiber by using constellation

diagrams.

All the simulations are made in licensed version of Optisystem 14.0 and signal analysis is done

in MATLAB.

16

1.9 References

[1] Wang, Anbang, Longsheng Wang, and Yuncai Wang. "Applications of chaotic laser in

optical communications." Optical Communications and Networks (ICOCN), 2016 15th

International Conference on. IEEE, 2016.

[2] Dal Bosco, Andreas Karsaklian, et al. "Random number generation from intermittent

optical chaos." IEEE Journal of Selected Topics in Quantum Electronics 23.6 (2017): 1-

8.

[3] Lin, Fan-Yi, and Jia-Ming Liu. "Chaotic lidar." IEEE journal of selected topics in

quantum electronics 10.5 (2004): 991-997.

[4] Aubert, Gilles, and Pierre Kornprobst. Mathematical problems in image processing:

partial differential equations and the calculus of variations. Vol. 147. Springer Science &

Business Media, 2006.

[5] Guyeux, Christophe, and Jacques M. Bahi. "A new chaos-based watermarking

algorithm." Security and Cryptography (SECRYPT), Proceedings of the 2010

International Conference on. IEEE, 2010.

[6] Tong, Xiaojun, et al. "A novel chaos-based fragile watermarking for image tampering

detection and self-recovery." Signal Processing: Image Communication 28.3 (2013):

301-308.

[7] Wang, Bo, and Jiuchao Feng. "A chaos-based steganography algorithm for H. 264

standard video sequences." Communications, Circuits and Systems, 2008. ICCCAS

2008. International Conference on. IEEE, 2008.

[8] Uchida, Atsushi. "Basics of Chaos and Laser." Optical Communication with Chaotic

Lasers: Applications of Nonlinear Dynamics and Synchronization: 19-57.

17

[9] Ivancevic, Vladimir G., and Tijana T. Ivancevic. Complex nonlinearity: chaos, phase

transitions, topology change and path integrals. Springer Science & Business Media,

2008.

[10] Hilborn, Robert C. Chaos and nonlinear dynamics: an introduction for scientists and

engineers. Oxford University Press on Demand, 2000.

[11] Gilstrap, Donald L. "Quantitative research methods in chaos and complexity: from

probability to post hoc regression analyses." Complicity 10.1/2 (2013): 57.

[12] Carroll, Thomas L., and Louis M. Pecora. "Synchronizing chaotic circuits." IEEE

Transactions on circuits and systems 38.4 (1991): 453-456.

[13] Kanakidis, Dimitris, Apostolos Argyris, and Dimitris Syvridis. "Performance

characterization of high-bit-rate optical chaotic communication systems in a back-to-

back configuration." Journal of lightwave technology 21.3 (2003): 750-758.

[14] Domingues, Maria de Ftima F., and Ayman Radwan. "Optical Fiber Sensors for loT and

Smart Devices." (2017)

[15] Wang, Anbang, Yuncai Wang, and Hucheng He. "Enhancing the bandwidth of the

optical chaotic signal generated by a semiconductor laser with optical feedback." IEEE

Photonics Technology Letters 20.19 (2008): 1633-1635.

[16] Xia, Guang-Qiong, Sze-Chun Chan, and Jia-Ming Liu. "Multistability in a

semiconductor laser with optoelectronic feedback." Optics Express 15.2 (2007): 572-

576.

[17] Ohtsubo, Junji. Semiconductor lasers: stability, instability and chaos. Vol. 111. Springer,

2012.

18

[18] Liu, Yue, et al. "Frequency locking, quasiperiodicity, and chaos in dual-frequency loss-

modulated erbium-doped fiber lasers." Chinese Optics Letters 7.8 (2009): 699-702.

[19] Pisarchik, A. N., and Yu O. Barmenkov. "Locking of self-oscillation frequency by pump

modulation in an erbium-doped fiber laser." Optics communications 254.1-3 (2005):

128-137.

[20] Lewis, Clifford Tureman, et al. "Synchronization of chaotic oscillations in doped fiber

ring lasers." Physical Review E 63.1 (2000): 016215.

[21] Vanwiggeren, Gregory D., and Rajarshi Roy. "Communication with chaotic lasers."

Science 279.5354 (1998): 1198-1200.

[22] Tenny, R., Tsimring, L. S., Abarbanel, H. D. I., and Larson, L. E. Security of chaos-

based communication and encryption. In Digital Communications Using Chaos and

Nonlinear Dynamics (Institute for Nonlinear Science). Springer, 2006, pp. 191–229.

[23] http://internetworldstats.com./stats.htm

[24] Ji, Wei, Dejun Feng, Qingjie Huang, and Jun Chang. "The design of linearly polarized

dual-wavelength fiber laser which used in WDM-RoF-PON." Optik-International

Journal for Light and Electron Optics 124, no. 21 (2013): 5146-5148.

[25] https://www.cisco.com

[26] http://www.internetphenomena.com/

[27] Chu, Pak Lim, Boris A. Malomed, and Gang-Ding Peng. "Soliton WDM system using

channel-isolating notch filters." Photonics East'99. International Society for Optics and

Photonics, 1999.

19

[28] Winzer, Peter J., and Ren-Jean Essiambre. "Advanced modulation formats for high-

capacity optical transport networks." Journal of Lightwave Technology 24.12 (2006):

4711-4728.

20

CHAPTER 2

Literature Review

Pecorra and Caroll [1] first laid the foundation of secure communication using optical chaos

by demonstrating the idea of synchronization between transmitter and receiver. They linked

chaotic systems by using common signal source and proved that system will synchronize if the

signs of Lyapunov exponent are all negative for the subsystems. Their discovery opened new and

interesting ways in the field of chaos communication by exploiting the attractive features of

chaotic signals.

Kevin and Oppenheim [2] implemented chaos based analog circuit using Lorenz system.

They showed that Lorenz system synchronization is result of very stable errors dynamics

between sending source and receiver. They took identical coefficients of transmitter and receiver

lasers to achieve maximum synchronization. This was the first time to show potential use of

Lorenz electrical circuits in the field of secure communication. Their work was not only the

implementation of Lorenz equation but also used for the implementation of message hiding using

Chua’s circuit.

In [3], Zafar et al. showed comparative analysis of chaos produced by Erbium doped fiber

ring lasers and semiconductor lasers. The results revealed that chaos produced by semiconductor

laser using optoelectronic feedback exhibit higher degree of optical chaos and has greater

bandwidth as compared to chaos generated through EDRFL schemes. They used the Lyapunov

exponents to measure the degree of chaos and found that greater the Lyapunov exponent values

towards positive side indicates more unpredictability i.e. higher degree of chaos. They further

investigated that performance of pulsed chaos is much better than non-pulsed chaos in terms of

unpredictability.

21

In [4] Zafar et.al worked on the security of physical layer to see the impact of dispersion,

fiber non-linearities and noise of EDFA on chaos synchronization. They used two separate

configurations and implemented in lumped and distributed fashion. The results are evaluated by

using noise figure and sync diagram. This was the first time that they introduced sync diagram in

their work for this purpose. According to their results linear impairments can be compensated

however it is difficult to compensate non-linearities which should be avoided. They further

demonstrated that chaos after propagation worked better in distributed amplification

configuration as compared to lumped amplification.

J. Mork et al. [5] presented a detailed experimental & theoretical work in the investigation

of nonlinear behavior of semiconductor lasers with the optical feedback mechanism. According

to their work, more chaotic level can be achieved simply by increasing the optical feedback. At

start the behavior of laser found as quasi-periodic however, with the increase in optical feedback

mechanism laser is entered into chaotic regime which is then interfered by frequency locking.

They further demonstrated that laser dynamics show typical properties of chaotic transmitter

within the regime of coherence collapse.

Wang et al. [6] worked on enhancement of chaos generated by semiconductor laser. They

used the optical feedback property of DFB (Distributed feedback lasers) for chaos generation and

experimentally proved that bandwidth of chaotic carrier can be increased by continuous wave

optical injection inside the cavity. Experimental results showed that by using optical injection

phenomenon bandwidth can be increased upto three times of the original chaotic signal. They

found that relaxation frequency enhancement and high frequency oscillations are the physical

factors behind the enhancement of the chaotic lasers by stable and instable injection locking

respectively.

22

Authors work in [7] is based on the idea of synchronization of chaos with the suppression

of time-delay signature. Due to the greater advantages of optical chaos comparable with

electrical they preferred to hide their message in optical domain by using chaos masking scheme.

Their research is carried out on security of long distance optical communication in which effect

of feedback time of VCSEL laser is evaluated for the overall security of system. Three cascaded

VCSELs are used for time delay signature suppression. Four 10 Gbit/sec messages are

transmitted simultaneously and BER values are found to be smaller than 10-9. Q-factor is

maintained having values greater than 6. Experiment is successfully done at SMF length of 20

km. After using DCF, fiber length is further increased to 132 km.

In [8], authors performed work to enhance the bandwidth of chaotic optical carrier by

introducing a concept in which sinusoidal phase modulated feedback is used with the

semiconductor laser in closed loop configuration. They claimed overall enhancement of security

of system while improving bit error rate. Two external cavity lasers are deployed in such a way

that Master semiconductor laser (MSL) serves at transmitting side while Slave semiconductor

laser (SSL) serves at receiving side in closed loop configuration. Differing from conventional

closed loop configuration they added phase modulator in both feedback loops of master and

slave lasers which is provided with sinusoidal signal. They used chaos modulation add message

with chaotic carrier.

In [9], authors presented their work useful for secure communication, chaotic radar and

generation of random numbers. They proposed PM to IM (Phase modulation to intensity

modulation) conversion based on analog/digital feedback loop. Analog part takes input from

shift registers in the form of digital sequences and coverts them into random analog signal like

noise, which is further used to determine new data bits. The complexity and effective bandwidth

23

of chaos can be adjusted by setting appropriate parameters of the system. They concluded that

PM to IM conversion as fairly efficient process for converting random digital sequences into

noise like random analog signal, thus saving overall digital computational cost.

In [10], authors used semiconductor laser with feedback mechanism to make chaos

generator. By using master-slave configuration they generated chaotic oscillations in the range of

GHz. Synchronization was achieved by locking through optical injection phenomenon. A 1.5

GHz message is encoded by masking technique which is recovered at the receiving side by

applying subtraction rule. They also demonstrated degree of signal reconstruction using narrow

band-pass filter at receiving side centered at frequency of the transmitted signal.

To test the real-world performance of chaotic communication system, Argyris et al. [11]

performed experiment on already deployed optical fiber network infrastructure. A single mode

optical fiber metropolitan area network belonging to Athens was chosen for this purpose. Two

schemes were tested at fiber length of 120 km using all optical and electro optical feedback. In

the first case laser’s output is fed to nonlinear intensity Mach-Zehnder modulator. In the other

case coherent optical feedback phenomenon is provided by using external mirror. In both cases a

high chaotic output is obtained. In case of all optical, message is modulated with the amplitude

of the chaotic carrier. In case of electro-optical, encoding of message is done with the chaotic

carrier inside the non-linear environment of feedback loop.

In [12], Lin et al. numerically presented the non-linear behavior of semiconductor laser in

repeated optical injection. By using the train of iterative pulses, semiconductor laser found to be

have very complex behavior and it followed the period doubling rout to the chaos. They also

observed frequency locked states after varying the iterative frequency of the optical pulses.

24

Different dynamical states including oscillations & pulsation are investigated by changing the

intensity of optical pulses.

Atsushi Uchida et al. [13] generated power spectrum exceeding frequency range above 20

GHz by taking two semiconductor lasers which were commercially available, and they coupled

them in unidirectional master slave configuration. Master laser has used optical feedback

externally which induced optical chaos in the output of laser. Some part of chaotic laser light is

then fed into slave laser. Their investigation showed that high frequency signal generation

depends upon two factors i.e. ‘Flattening of spectrum’ due to insertion of chaotic pulses and

‘High frequency oscillations’ because of beating phenomenon between master and slave laser

pulses.

In [14], Takeuchi worked on optical feedback scheme based on polarization rotation from

transverse electric (TE) to transverse magnetic (TM) mode. The dynamics of this scheme found

to be very different from simple ordinary optical feedback mechanism. To excite transverse

magnetic mode’s chaotic oscillations in semiconductor laser, greater optical feedback is required

but there exists no lower level threshold in this polarization rotated feedback scheme and the

oscillations begins at the time of bias current injection. The chaotic waveform of transverse

magnetic mode found same as that of transverse electric mode however transverse magnetic

mode oscillates in anti-phase manner compared to transverse electric mode. They observed zero-

time difference between the two modes. Hence, dynamics of this system found to be changed

from the simple optical injection and locking mechanism.

In [15], authors studied nonlinear behavior of semiconductor laser using delayed negative

optoelectronic feedback mechanism. A comparison is made in the behavior of both negative

optoelectronic feedback and positive optoelectronic feedback. Both the systems followed the

25

route from quasi periodic to chaos, where regular pulses, quasi-periodic pulses and chaotic

behavior is observed. However, frequency locked phenomenon is observed only in negative

optoelectronic feedback.

In [16], Uchida et al. used chaotic pulses to produce random bit sequences at the rate of 1.7

GHz. High bandwidth chaotic semiconductor lasers are used for this purpose. Their system

generated random signals of high amplitude from very little noise, produced through nonlinear

amplification in addition to mixing mechanism. This was the first time that semiconductor lasers

were used for the generation of high bit rate sequence generation. Before this semiconductor

lasers were used for message hiding purposes only. In their experiment they used two

semiconductor lasers having chaos-based intensity oscillations. The output of each laser is

detected on photodiode, amplified and converted to binary output by using 1-bit A/D converter

driven by a fast clock.

In [17], Mirasso et al. successfully demonstrated the implementation of CSK (Chaos shift

keying) by using single receiver. They used rate equation model for two unidirectional coupled

single mode external cavity lasers working in chaotic mode. By slightly varying the injection

current, message is encrypted with the output of transmitter. Under appropriate conditions the

receiver synchronizes with transmitter for message extraction after filtering process. Their work

was different from the traditional implementation of CSK schemes which uses two receivers.

In [18], authors described the synchronization of frequency oscillations in two external

cavities of semiconductor lasers on the scale of nano-second time interval. The relaxation

oscillation frequencies of both lasers were kept same. The length of each cavity is set to 29 cm

and the two lasers were connected in master-slave configuration. Master laser alongwith the

mirror acted as chaotic transmitter whereas slave laser alongwith another mirror served as

26

chaotic receiver. The threshold currents for master laser and slave laser were set to 27 mA and

27.1 mA respectively, in free running condition, whereas bias current is controlled by using

stabilized current sources. Synchronization achieved from this method was somewhat different

from the traditional amplification or injecting phenomenon as lasers output can be made

synchronized just by little current, even without current injection the two lasers started

oscillations in individualistic way and coherence of the lasers is finished in chaotic oscillations.

In [19], S. Tang et al. used the concept of chaos modulation for message security. By using

chaos modulation scheme the complexity of chaotic system found to be increased. However, no

effect observed on overall synchronization quality due to message encoding. In their work two

chaotic communication systems were used in parallel. One was optical injection system used for

period doubling rout to chaos. Second system was based on optoelectronic feedback used for

chaos generation through the rout of quasi periodic. Performance of the system was tested for

different amplitudes of messages in terms of synchronization quality. Impact of channel noise

was investigated and synchronization problem due to channel and system noise found to be the

main reason behind the increase error rate.

In [20], Shuo.T & Jia-Ming measured the performance of message encoding schemes.

CMS, ACM and CSK are investigated for this purpose. Chaotic dynamics of the system are not

affected by message encoding in case of CMS scheme. In ACM & CSK a little amount of

message signal when injected to chaotic system, it increased the overall complexity of the

chaotic. Synchronization quality of ACM found to be the good one among the three. Easy

message recovery performed when CMS is used for message security.

In [21], Li Tao et al. presented use of advance modulation formats for short haul optical

communication networks. Unlike long haul communication where external modulators are used

27

alongwith coherent detection, they used directly modulated lasers with direct detection of signal.

They targeted Orthogonal frequency division multiplexing (OFDM) and Pulse amplitude

modulation (PAM) in their work. PAM4 and PAM8 found to be the optimal modulation formats

for their implementation in short haul network due to the simplicity of implementation. These

schemes were used for carrying 100Gbit/sec for total fiber length of 10 km. For higher data rates

and longer distances they suggested OFDM scheme.

In [22, 23], Peter J. Winzer and Rene Jean conducted extensive research on the use of

advance modulation formats in WDM systems. They worked on generation and detection of

gigabit intensity and phase modulated formats. Factors affecting performance of optical system

implemented with advance modulation formats, such as polarization mode dispersion (PMD),

multipath interference, chromatic dispersion, WDM cross talk and optical amplifier noise are

also investigated in their work. In addition, they also explored the effect of narrow band optical

filtering and Kerr nonlinearity in fiber optics.

In paper [24], authors reviewed the use of high spectral efficient optical modulation

formats in coherent digital systems. They focused on Quadrature amplitude modulation (QAM)

in WDM communication system to increase per channel as well as aggregated WDM capacity.

They concluded 16-QAM as a promising spot representing good tradeoff between various

performance effecting parameters and high speed/high capacity optical networking. To improve

the performance of optical communication system they used DSP techniques in their work. They

also highlighted the important tradeoff parameters pertaining to performance and design of

coherent optical receivers and the use of particular advance modulation format.

In paper [25], Xuelin Yang et al. presented novel security scheme to make the OFDM

signal secure. Security is implemented by introducing electro optic chaos on physical layer in

28

optical domain. Work is carried out explicitly for one advance modulation format i.e OFDM

only. Transmission quality as well as security level is evaluated for both intensity modulation

and direct modulation of OFDM signals. In addition, performance is evaluated for different

masking ratios in terms of SNR. Results are plotted in both time domain and frequency domain

after masking OFDM signal with chaotic carrier.

In [26], performance evaluation of RZ, NRZ and Duobinary modulation formats is carried

out in DWDM optical communication system. Quality factor, jitter and BER is calculated at

different fiber lengths. The RZ modulation format found to be optimal at a data rate of 10Gbps in

dispersion managed environment when Jitter and BER are considered. NRZ found feasible at

shorter distances and higher data rates. Duobinary modulation format scheme found to be most

dispersion tolerant among the three and recommended for higher data rates and greater distances.

In [27], Anu et al. performed simulative research on modulation formats i.e CSRZ,

Duobinary-RZ & Modified Duobinary-RZ (MDRZ) to check performance of optical

communication system for longer distances using DWDM setup. Results are evaluated by using

Pre, Post & Symmetrical dispersion compensation schemes. Channel spacing is set to 25Ghz

between DWDM channels. MDRZ found to be most suitable among the three for longer

distances. Further, maximum transmission distances obtained by using MDRZ using symmetrical

compensation scheme.

To meet the on-going demand of huge data rates for global communication, amplifiers and

lasers can play an important role if operate at inaccessible wavelengths in the range of 1200 nm

to 1700nm. In [28], Zareanborji, Jianzhong Zhang et. al worked on Bi/Er co-doped amplifier to

increase the broadband spectrum for the working of fiber based amplifiers and lasers. They

demonstrated the working of these components in the bands of O, C, S, E and L covering range

29

from 1100nm to 1600nm. As different dopants materials i.e Er, Tm, Pr and Bi are still being used

to enhance the bandwidth range of fiber-based amplifiers and lasers, their findings are useful to

meet the requirement of high data rates of current world.

In [29], authors targeted the security implementation of OFDM signal on physical layer of

the transmission medium. They used concept of Discrete-Fourier Transform spread of OFDM

signal in passive optical networks. The signal is made secured by using the digital chaotic signal

generated via 4-D hyper-digital method. The solution is implemented for 20 km of fiber length.

Authors improved the receiver sensitivity and got the advantages of less computational

complexity and no processing of redundant side bands. Signal is received at the photodiode side

by direct detection method. Total 13.3 Gb/s of data rate is encrypted via this method.

Mengfan chang et.al in [30] presented idea of hybrid chaotic scheme for the encryption of

OFDM signal in passive optical networks. Both optical and electrical fields are used for the

encryption of upstream and downstream data respectively. Due to hybrid approach it

circumvents the problems of both digital and analog chaos. Their results showed that time delay

signature is covered by digital chaos whereas periodicity in digital chaos is concealed by analog

chaos. Their approach can be considered as novel one for securing PON at physical layer.

2.1 Conclusion

This chapter is compiled based on references related to chaos generation, enhancement and

advance modulation formats which are the core parts of this thesis. Different techniques for the

generation of chaos and their applications for secure communication are studied which helped in

implementation of our proposed schemes discussed later in this thesis. In addition, performance

of some advance modulation formats is also studied which is addressed in this thesis to acquire

high data rates and security simultaneously.

30

2.2 References

[1] L. M. Pecorra and T. L. Carroll, “Synchronization in Chaotic Systems”, Phys. Rev.

Lett. ,Vol. 64 No. 8, pp. 821-824, February 1990.

[2] Cuomo, Kevin M., and Alan V. Oppenheim. "Circuit implementation of synchronized chaos

with applications to communications." Physical review letters 71.1 (1993): 65.

[3] Ali, S. Zafar, M. K. Islam, and M. Zafrullah. "Comparative analysis of chaotic properties of

optical chaos generators." Optik-International Journal for Light and Electron Optics 123.11

(2012): 950-955.

[4] Ali, Syed Zafar, Muhammad Khawar Islam, and Muhammad Zafrullah. "Effect of

transmission fiber and amplifier noise on optical chaos synchronization." Optical review 19.5

(2012): 320-327.

[5] Mork, Jesper, Bjarne Tromborg, and Jannik Mark. "Chaos in semiconductor lasers with

optical feedback: Theory and experiment." IEEE Journal of Quantum Electronics 28.1

(1992): 93-108.

[6] Wang, Anbang, Yuncai Wang, and Hucheng He. "Enhancing the bandwidth of the optical

chaotic signal generated by a semiconductor laser with optical feedback." IEEE Photonics

Technology Letters 20.19 (2008): 1633-1635.

[7] Liu, Yu-Zhu, et al. "Exploiting optical chaos with time-delay signature suppression for long-

distance secure communication." IEEE Photonics Journal 9.1 (2017): 1-12.

[8] Jiang, Ning, et al. "Secure chaos communication with semiconductor lasers subject to

sinusoidal phase-modulated optical feedback." Lasers and Electro-Optics Pacific Rim

(CLEO-PR), 2017 Conference on. IEEE, 2017.

31

[9] Luo, Chenkun, et al. "Broadband optical chaos generation by constructing a simple hybrid

feedback loop." Microwave Photonics (MWP), 2017 International Topical Meeting on.

IEEE, 2017.

[10] Kusumoto, Kenji, and Junji Ohtsubo. "1.5-GHz message transmission based on

synchronization of chaos in semiconductor lasers." Optics letters 27.12 (2002): 989-991.

[11] Argyris, Apostolos, et al. "Chaos-based communications at high bit rates using commercial

fibre-optic links." Nature 438.7066 (2005): 343-346.

[12] Lin, Fan-Yi, et al. "Nonlinear dynamics of semiconductor lasers under repetitive optical

pulse injection." IEEE Journal of Selected Topics in Quantum Electronics 15.3 (2009): 604-

611.

[13] Uchida, Atsushi, et al. "High-frequency broad-band signal generation using a semiconductor

laser with a chaotic optical injection." IEEE journal of quantum electronics 39.11 (2003):

1462-1467.

[14] Takeuchi, Yasutoshi, Rui Shogenji, and Junji Ohtsubo. "Chaotic dynamics in semiconductor

lasers subjected to polarization-rotated optical feedback." Applied Physics Letters 93.18

(2008): 181105.

[15] Lin, Fan-Yi, and Jia-Ming Liu. "Nonlinear dynamics of a semiconductor laser with delayed

negative optoelectronic feedback." IEEE journal of quantum electronics 39.4 (2003): 562-

568.

[16] Uchida, Atsushi, et al. "Fast physical random bit generation with chaotic semiconductor

lasers." Nature Photonics 2.12 (2008): 728-732.

32

[17] Mirasso, Claudio R., Josep Mulet, and Cristina Masoller. "Chaos shift-keying encryption in

chaotic external-cavity semiconductor lasers using a single-receiver scheme." IEEE

Photonics Technology Letters 14.4 (2002): 456-458.

[18] Fujino, Hitoshi, and Junji Ohtsubo. "Experimental synchronization of chaotic oscillations in

external-cavity semiconductor lasers." Optics letters 25.9 (2000): 625-627.

[19] Tang, S., et al. "Message encoding and decoding through chaos modulation in chaotic optical

communications." IEEE Transactions on Circuits and Systems I: Fundamental Theory and

Applications 49.2 (2002): 163-169.

[20] Tang, Shuo, and Jia-Ming Liu. "Effects of message encoding and decoding on synchronized

chaotic optical communications." IEEE journal of quantum electronics 39.11 (2003): 1468-

1475.

[21] Tao, Li, et al. "Advanced modulation formats for short reach optical communication

systems." IEEE network 27.6 (2013): 6-13.

[22] Winzer, Peter J., and R-J. Essiambre. "Advanced optical modulation formats." Proceedings

of the IEEE 94.5 (2006): 952-985.

[23] Winzer, Peter J., and Ren-Jean Essiambre. "Advanced modulation formats for high-capacity

optical transport networks." Journal of Lightwave Technology 24.12 (2006): 4711-4728.

[24] Winzer, Peter J. "High-spectral-efficiency optical modulation formats." Journal of Lightwave

Technology 30.24 (2012): 3824-3835.

[25] Yang, Xuelin, et al. "Secure optical OFDM signal transmission using electro-optic chaos."

Optical Communications and Networks (ICOCN), 2015 14th International Conference on.

IEEE, 2015.

33

[26] Phogat, Anju, Tarun Gulati, and Deepak Malik. "Q-Factor and Jitter performance in WDM

systems using RZ, NRZ and Duo binary modulation formats at different distances."

Communications International Journal of Latest Trends in Engineering and Technology 4

(2013): 1-5.

[27] Sheetal, Anu, Ajay K. Sharma, and R. S. Kaler. "Simulation of high capacity 40Gb/s long

haul DWDM system using different modulation formats and dispersion compensation

schemes in the presence of Kerr's effect." Optik-International Journal for Light and Electron

Optics 121.8 (2010): 739-749.

[28] Zareanborji, Amirhassan, et al. "Time-resolved emission characteristics of Bi/Er codoped

fiber for ultra-broadband applications." Workshop on Specialty Optical Fibers and their

Applications. Optical Society of America, 2013.

[29] Shen, Zanwei, et al. "Secure transmission of optical DFT-S-OFDM data encrypted by digital

chaos." IEEE Photonics Journal 8.3 (2016): 1-9.

[30] Cheng, Mengfan, et al. "Security-enhanced OFDM-PON using hybrid chaotic system." IEEE

Photon. Technol. Lett. 27.3 (2015): 326-329.

34

CHAPTER 3

Semiconductor Laser Chaos Generation

In this chapter chaos generation is discussed by using semiconductor lasers. For the

generation of chaos through semiconductor laser, dynamics of two important physical parameters

i.e. population inversion and electric field must be controlled. Time for the exchange of energy

between these two parameters is determined by oscillation frequency which is dependent on

following three parameters

1. Photon lifetime

2. Population Inversion time

3. Pumping rate

Lasers which depend only on electric field and population inversion, produce stable outputs.

In order to generate chaos atleast one extra degree of freedom is needed [1]. Different techniques

for generating chaos by using additional variables can be classified into following major

categories [2].

1. Optical feedback

2. Optical coupling & injection mechanism

3. External modulation

4. Insertion of nonlinear elements/devices.

3.1 Optical Feedback

Chaotic instability in the output of laser can be achieved by providing self-feedback signal in

the laser cavity. Semiconductor lasers dynamics using time delayed feedback in optical state is

one of the typical examples of feedback induced chaos inside the laser cavity [3]. In order to

35

produce optical feedback, an external mirror can be kept right in front of the cavity of laser, the

light of laser is reflected by this external mirror and then re-injected inside the cavity. Fig 3.1

shows the optical feedback mechanism using external mirror. The disturbance in the equilibrium

of photon-carrier interaction inside the laser cavity is done by self-feedback optical signal

resulting in the instability of laser output.

LD

Mirror

Length of external cavity

Laser Light

Fig. 3.1 Optical Feedback

3.2 Optical Coupling and Injection Mechanism

Chaotic instability in the output of laser can also be achieved by performing mutual or

unidirectional coupling from one laser cavity to another laser cavity [4]. Optical coupling

and injection phenomenon can be observed in Fig. 3.2. The frequencies of optical carrier are

detuned between the two lasers i.e. injected laser and the injection laser as per the order of

relaxation oscillation frequency so that interaction among the relaxation oscillation frequency

and optical carrier frequency detuning can take place resulting in the chaotic fluctuations. As

this interaction is nonlinear so the control of detuning of optical carrier frequency and

coupling strength is very important for generation of chaos by optical coupling and injection

mechanism.

36

LD LD

LD LDLaser Light

Cavity

Isolator

Fig. 3.2 Optical Coupling and Injection

3.3 External Modulation

Another way to produce the instability in the intensity of laser is external modulation. The

block diagram to generate chaos through external modulation is shown in Fig. 3.3. In this method

modulation is applied externally to the pumping source of the laser cavity [5, 6]. The frequency

of external modulation is set according to the laser’s relaxation oscillation frequency. Again, the

interaction among the relaxation oscillation frequency of laser and frequency of external

modulation is nonlinear which results in chaos generation. This mechanism can also be used for

the laser cavity loss which is called as “loss modulation”. As the strength of external modulation

increases the chaotic dynamics can be observed following the route from quasi-periodicity to

chaos.

LD Laser Light

Modulation

Fig. 3.3 External Modulation

37

3.4 Insertion of Nonlinear Element

The chaos generation by inserting non-linear element in laser system can be seen in Fig 3.4.

By inserting non-linear element chaotic dynamics can be achieved. As an example, intensity

fluctuations of chaotic nature can be seen in a system of laser (solid state) with the addition of

nonlinear element which is crystal of Second harmonic generation (SHG) [7]. The interaction

among the modes of SHG wavelength and laser’s fundamental wavelength takes place in a

nonlinear way to produce the chaotic dynamics. In case of gaseous state laser system saturable

absorber can be used to observe the complex chaotic dynamics. In this case more degree of

freedom increases the nonlinear interaction of photon-atom in the laser which results in intensity

fluctuations of chaotic nature.

Laser Medium

Non-linear Device

Laser Light

LD

Fig. 3.4 Insertion of Non-linear Element

Above mentioned methods can also be used in combination to generate the chaos through

semiconductor lasers.

3.5 Chaos Generation

In our work, we have utilized the concept of external modulation given in Section 3.3, in

which pumping power is supplied to the laser cavity through the external current source.

Operating wavelength of laser is kept to 1550 nm. It can be set to any other wavelength but for

38

convenience it is set to 1550 nm, chaos produced at same wavelength will be used later in

chapter 5 & 6. Semiconductor laser is used to generate the chaos in which threshold current is set

to 33.46 mA approximately while threshold power is kept as 0.015 mW. At 10 dBm, ‘Bias

current’ & ‘Modulation Peak Current’ of semiconductor laser and ‘Frequency’ of current source

are varied to see the impact of their values on chaotic pulses.

3.5.1 Changing Bias Current (Route from Quasi-Periodic to Chaos)

Parameters of semiconductor lasers to generate chaos are given in Table 3.1. At start when

the bias current is low i.e. 3 mA, quasi periodic behavior of chaotic pulses can be observed

where “Bunching” of chaotic pulses can be seen. The span of each bunch is approximately equal

to 1 ns. After 1 ns, new bunch of chaotic pulses starts. With the rise in bias current the frequency

of Bunches increase, which overlapped with each other giving rise to chaotic route. From bias

current 3 mA to 33 mA, route from quasi-periodic to chaos can be observed in Fig. 3.5, Fig. 3.6

and Fig. 3.7.

Table 3.1 Semiconductor Laser Parameters (with different bias current)

Parameter Name Value Unit

Wavelength 1550 nm

Power 10 dBm

Bias current 3, 13, 33 mA

Power (at bias current) 0 dBm

Threshold power 0.0154 mW

Threshold current 33.46 mA

Modulation peak current 10 mA

39

Fig. 3. 5 Semiconductor laser output at bias current = 3 mA

Fig. 3. 6 Semiconductor laser output at bias current = 13 mA

Fig. 3. 7 Semiconductor laser output at bias current = 33 mA

40

3.5.2 Changing Modulation Peak Current (High Amplitude Pulses)

Table 3.2 shows the parameters to generate chaos through semiconductor lasers with

variations in Modulation peak current. Keeping the bias current same, Modulation peak current

is changed to from 5 mA to 10 mA. At low Modulation peak current, amplitude of generated

pulses is extremely low which not only deviate these pulses from chaotic behavior but also of no

use for any suitable fiber length. With the increase in value of Modulation peak current, clear

variation in amplitude of pulses can be observed in Fig. 3.8, Fig. 3.9 and Fig. 3.10 in which

many low amplitude pulses are followed by high amplitude pulses.

Table 3. 2 Semiconductor Laser Parameters (with different modulation peak current)

Parameter Name Value Unit

Wavelength 1550 nm

Power 10 dBm

Bias current 33 mA

Threshold power 0.0154 mW

Power (at bias current) 0 dBm

Modulation peak current 5,7,10 mA

Threshold current 33.46 mA

41

Fig. 3. 8 Semiconductor laser output at modulation peak current = 5 mA

Fig. 3. 9 Semiconductor laser output at modulation peak current = 7 mA

Fig. 3. 10 Semiconductor laser output at modulation peak current = 10 mA

42

3.5.3 Changing Frequency of Current Source (Random Amplitude Pulses)

Suitable pumping power or current is required to achieve the chaotic nature of pulses. At low

frequency of current source when the strength of electric field is not very high, pulses of same

amplitude are observed. Increasing the frequency results in the variation of amplitude of pulses

due to increase in field strength. Table 3.3 shows the parameters of current source applied for

external modulation by setting the different frequencies. Fig. 3.11, Fig. 3.12 and Fig. 3.13 are the

outputs of semiconductor lasers in response to change of frequency of current source.

Table 3.3 Current Source Parameters (with different frequencies)

Parameter Name Value Unit

Frequency 0.75, 0.8, 1.0 GHz

Amplitude 1 a.u.

Phase 90 deg

Bias 0 a.u.

In the Fig 3.13, ‘Gain Quenching’ which is one of the properties of chaotic pulses can be

observed in which few pulses of high amplitude are immediately followed by pulses of very low

amplitude. The pulses produced in this way are Gaussian in nature and can be checked by

applying Gaussian Fit.

43

Fig. 3.11 Semiconductor laser output at 0.75 GHz frequency of current source

Fig. 3.12 Semiconductor laser output at 0.8 GHz frequency of current source

Fig. 3.13 Semiconductor laser output at 1.0 GHz frequency of current source

44

3.6 Conclusion

In this chapter the different techniques to generate optical chaos through semiconductor

lasers are studied. The chaos produced through semiconductor laser is simulated by using direct

modulation in which a current source is directly connected to the semiconductor laser and effect

on chaos is observed by varying different parameters of laser and current source. The laser and

current source parameters are adjusted and optimized to produce useful chaos to be utilized as

carrier to hide high data rate message for long distance communication.

45

3.7 References

[1] Strogatz, Steven, et al. "Nonlinear dynamics and chaos: With applications to physics,

biology, chemistry, and engineering." Computers in Physics 8.5 (1994): 532-532.

[2] Ohtsubo, Junji. Semiconductor lasers: stability, instability and chaos. Vol. 111. Springer,

2012.

[3] Bogris, A., et al. "Enhancement of the encryption efficiency of chaotic communications

based on all-optical feedback chaos generation by means of subcarrier modulation." Lasers and

Electro-Optics, 2007 and the International Quantum Electronics Conference. CLEOE-IQEC

2007. European Conference on. IEEE, 2007.

[4] Uchida, Atsushi, et al. "High-frequency broad-band signal generation using a semiconductor

laser with a chaotic optical injection." IEEE journal of quantum electronics 39.11 (2003): 1462-

1467.

[5] Gastaud, Nicolas, et al. "Electro-optical chaos for multi-10 Gbit/s optical transmissions."

Electronics letters 40.14 (2004): 898-899.

[6] Sciamanna, Marc, and K. Alan Shore. "Physics and applications of laser diode chaos."

Nature Photonics 9.3 (2015): 151.

[7] Mao, Hanying, et al. "Chaotic characteristics analysis of simulation signal of second

harmonic generation effect." (2016).

46

CHAPTER 4

Performance Analysis of Intensity Modulation Formats

4.1 Introduction

In this chapter, performance analysis of some efficient modulation formats like Chirped-RZ,

Chirped-NRZ, Duobinary, AMI and RZ-50 %, is performed for G-PON (Gigabit passive optical

networks). The performance of these modulation formats which are binary in nature is

questioned in terms of bit rates versus fiber link length. The quality of the signal at receiving side

is judged by analyzing the parameters like bit error rate and signal’s quality factor (Q-factor).

In current scenario optical networks are gaining immense attention of the researchers due to

their high bandwidth, high data rates and increased reliability. With the increase in requirement

of huge bandwidth each consumer would demand data rate of Gbps for triple play services i.e.

data transfer, video and audio. The services like high definition TV and video on demand require

high bandwidth with high speed internet access (HSI). Optical fibers to subscribers (FTTx)

provide the most optimum solution to cope with all these demands. Most of the FTTx models are

designed on the fashion of passive optical networks (PON). GPON is the most emerging

architecture in next generation networks (NGN). For long haul optical transmission many optical

modulation formats have been proposed [1, 2]. Appropriate type of digital modulation is the

significant part of optical communication system. Higher spectral efficiency can be achieved by

selecting most suitable modulation format in PON. In this paper comparison of various

modulation formats is investigated for different data rates and transmission distances. PON is a

cost-effective approach consuming less energy per bit. It is point-to-multipoint topology having

splitter based optical distribution network (ODN) [3, 4]. The PON optical line terminal (OLT)

located at the central office (CO) is connected with numerous optical network units (ONUs)

47

placed at far end through one or more passive optical splitters [5, 6].

The Next Generation Networks necessitates the exploration of the optimum schemes for

increased capacity and spectral efficiency (b/s/Hz) using different modulation formats. To

improve the system performance, telecom networks and communication systems require the

exploitation/implementation of advanced modulation formats to minimize the effect of sources

responsible for degradation. To choose the suitable modulation format depends on different

factors like channel spacing, per channel data rates, channel impairments and fiber types etc [7,

8]. Direct modulation also called as intensity modulation is simplest method for performing

optical modulation. In this type laser drive current is used to modulate the binary data which

switches on and off states of laser light (OOK). Fig. 4.1 shows the classification of different

intensity modulation formats [9, 10].

In this work different intensity modulation formats are investigated for different lengths of

fiber at different data rates. These modulation formats include

• Duobinary

• Alternate mark inversion (AMI)

• Chirped-NRZ

• Chirped-RZ

• Carrier suppressed-RZ (CSRZ)

• Vestigial side band NRZ (VSB-NRZ)

• RZ-33%

• RZ-50%

• RZ-67%

48

NRZ Modulation Format

NRZ dominates the other intensity modulation formats due to the following reasons.

1. As far as the design of transmitter and receiver is concerned it requires much lower

electrical bandwidth than the RZ format.

2. In comparison of phase shift keying (PSK), NRZ format is less sensitive to the phase

noise of laser.

3. NRZ format has the least broad spectrum as compared to other formats [8].

Some disadvantages of using NRZ modulation scheme are

1. NRZ scheme is not efficient in DWDM communication.

2. NRZ has bad tolerance against XPM (cross phase modulation) and chromatic dispersion

in case of WDM system [10].

RZ Modulation Format

The spectrum of RZ modulation format is broader than NRZ. Linear effects is optical fiber

communication particularly dispersion causes the broadening of RZ pulse more rapidly. But in

some situations, this effect proves to be the beneficial because when the pulse gets broaden its

peak amplitude decreases. This makes the performance of RZ format much better and robust

against the non-linear impairments. Non-linear effects are directly proportional to the intensity of

the light [11, 12].

Dual port configuration of Mach-Zehnder modulator (MZM) can be used for

implementation of both NRZ & RZ modulation formats. The function of MZM is to transform

the incoming electrical signal (binary coded) into OOK signal which is the optical form of signal

after modulation with laser light. Now by changing the drive conditions of this modulator

different intensity modulation formats can be implemented. For example, by manipulating the

49

bias voltage of modulator different RZ formats can be achieved with duty cycle of 67%, 50%

and 33% [13].

Duobinary modulation can be classified as the subtype of correlative coding as indicated by

the Fig. 4.1. In this type intersymbol interference (ISI) is introduced intentionally in the

transmitted signal so that to achieve required rate as per Nyquist. Decoding of signal is

performed at the receiving side in order to eliminate the ISI effect [14, 15]. The tolerance of

duobinary format against non-linear effect is same as that of RZ and NRZ but the advantage of

using this format is to limit the chromatic dispersion as compared to other type of modulation

formats. This scheme can be implemented in the system where non-linear effects are weaker.

Also, for the data rates greater than 40 Gbps duobinary modulation is preferred over OOK [16].

Intensity based Modulation

Binary MultilevelPseudo-

Multilevel

Chirped Free Chirped

M-ASKNRZ-VSB RZ-VSB

Chirped-NRZ Chirped-RZ

CSRZ DB

With Memory

Correlative Coding

AMI

Fig. 4.1 Classification of different Intensity modulation formats

50

Laser diode MZ Modulator

Electrical SignalEncoded Data

MZ/Phase Modulator

RZ/NRZ Optical Signal

RZ/NRZ Data

DB/AMI/CSRZ Modulated Data

Fig. 4.2 NRZ, RZ, DB, AMI and CSRZ optical signal generation

CSRZ is type of modulation format in which for every bit interval phase of optical carrier is

made to change. Thus, in CSRZ scheme half bits have negative phase whereas the other half

have positive. Due to this phase change the higher power components does not exist in the

spectrum of signal. For higher data rates particularly above 40 Gbps, CSRZ has excellent

tolerance against the nonlinearities. In order to generate CSRZ modulated optical signal two

Mach-Zander modulators are required as shown in Fig 4.2. The first modulator generates the RZ

optical signal which is non-chirped whereas other modulator is used to provide optical phase

shift of 180 degree in the adjacent bits. Optical spectrum of different modulation formats can be

seen in Fig. 4.3.

4.2 Mathematical Model

Mathematically, the baseband envelope of transmitted optical duobinary signal can be

represented as [17, 18].

Ed(τ) = ∑ Om1g(τ − 𝑚1tb)m1 (4.1)

In (4.1), Om1 shows transmitted data in binary form which can be -1 or +1, having even

occurrence probability. The expression g(τ) can be represented as,

𝑔(𝜏) =1

2[ℎ1(𝜏) + ℎ1(𝜏 − 𝑡𝑏)] (4.2)

51

In (4.2), ℎ1(τ) is the pulse shape of transmitted signal. From (4.1) and (4.2) it can be shown

that Power spectral density (PSD) of duobinary signal Ed(τ) is represented by

Pd(f) = |𝐻1(𝑓)|2cos2(πftb) (4.3)

Here in (4.3), 𝐻1(𝑓) is Fourier transform of pulse ℎ1(τ) which is time limited.

One of the major differences between AMI and duobinary modulation schemes is of

encoding process. AMI modulation format can be implemented without inverter which makes it

less costly. AMI modulation is less effected by nonlinearities and dispersion. The equation of

baseband envelop for the AMI transmitted signal is same as that of (4.1). But equation of g(τ) for

AMI signal can be written as

𝑔(𝜏) =1

2[ℎ1(𝜏) − ℎ1(𝜏 − 𝑡𝑏)] (4.4)

Fig. 4.3 Optical spectrum of different modulation formats

52

Using (4.1) and (4.4), the expression for power spectral density of AMI signal can be written as

𝑃𝑑(𝑓) = |𝐻1(𝑓)|2𝑠𝑖𝑛2(𝜋𝑓𝑡𝑏) (4.5)

4.3 Simulation Model

Basic working of GPON is given in Fig. 4.4. The GPON model contains ‘N’ nodes of optical

network units linked with optical line terminal through optical fiber. Depending upon the

consumer density ‘N’ may be 16, 32, 64 or 128. Performance of different modulation scheme is

investigated from OLT to remote node (ONU). At OLT side, laser light is modulated with binary

data through MZ modulator by using one of the above discussed modulation schemes i.e. NRZ,

RZ, AMI or DB. Modulated signal is then propagated along the single mode optical fiber.

Dispersion losses are compensated by using dispersion compensation fiber. EDFA is used

alongwith the optical fiber to deal with the attenuation losses [19, 20]. Covering broad range of

applications in telecommunications, EDFA is one of the feasible choices to work at C-Band [21,

22]. At the remote node or ONU side, photodetector is used to get the incoming optical signal

and which converts signal into electrical pulses for further processing. Eye-diagram analyzer is

used to analyze the quality of the signal via quality factor and bit error rate. The parameters used

for different components are listed in Table 4.1.

Fig. 4.4 Proposed GPON implemented with different modulation schemes

53

Table 4. 1 System Parameters

Parameters Value

Laser power 0-10 dBm

Modulator extinction ratio 20 dB

Channel wavelength 1550 nm

DCF attenuation 0.6 dB/km

Amplifier gain 0-15 dB

Noise figure 4-6 dB

DCF dispersion -80 ps/nm/km

Responsivity of receiver 1 A/W

4.4 Results and Discussion

Based on the proposed system prototype, different modulation formats including RZ, NRZ,

AMI and DB are compared, and their behavior is tested for different data rates and lengths of

optical fiber. The comparison of different NRZ schemes such as VSB-NRZ, Chirped-NRZ and

non-Chirped NRZ can be seen in Fig. 4.5. The analysis is carried out at data rate of 10 Gbps. By

varying the fiber lengths from 40 km to 140 km, quality factors are observed against these

modulation schemes in the range from 0 to 140. The objective of this analysis is to select best

NRZ format for its further comparison with other proposed modulation schemes for GPON

network. Fig. 4.5 clearly shows that chirped-NRZ scheme outperforms the other NRZ formats.

In Fig. 4.6, performance comparison of different RZ modulation formats i.e. Chirped-RZ,

CSRZ-67%, RZ-50%, RZ-33%, Alternate Chirp-RZ and VSB-CSRZ is presented for data rate of

10 Gbps. At different lengths of optical fiber, behaviour of different RZ schemes can be seen in

this graph. Ranging from 40 km to 140 km, RZ-50% performance is better than other RZ

schemes. Performance of different RZ formats are comparable except Alternate Chirp-RZ whose

performance found much lower throughout the length of optical fiber.

54

Fig. 4.5 Performance analysis of different NRZ formats

Fig. 4.6 Performance analysis of different RZ formats

In Fig. 4.7 performance of different modulation formats is shown by varying fiber length at

data rate of 10 Gbps. Chirped-NRZ and RZ-50 % modulation formats which shows excellent

performance in Fig. 4.5 and Fig. 4.6 are further tested and compared with DB and AMI schemes.

55

Fig. 4.7 Performance analysis at 10 Gbps

Our aim is to select the best modulation format for reliable and secure communication for

GPON. It is clear from the graph that chirped-NRZ shows good tolerance against linear and

nonlinear impairments which results in better quality factor and large eye-opening. This means

by using Chirped-NRZ between OLT and ONU, good quality signal can be sent with low bit

error rate. Further, the proposed modulation formats are analyzed and their performance is

investigated at data rates of 40 Gbps and 100 Gbps as shown in Fig. 4.8 and Fig. 4.9

respectively. Again, the chirped-NRZ gives acceptable quality factor at these data rates. At 100

Gbps, due to residual dispersion and nonlinearities the overall quality of signal degrades due to

high bit error rates values. In Fig. 4.10, the effect of BER on different modulation formats is

shown. BER shows the ratio between the effected bits and transmitted bits. According to [18],

for telecommunication systems it should be in range of 10−9 to 10−12. It is clear from the graph

that for the case of DB, AMI and RZ-50 % modulation schemes the value of BER increases

gradually with the increase in fiber length in GPON. Upto 90 km Chirped-NRZ and RZ-50 %

modulation formats have lower BER but at the same length DB and AMI have high values of

BER. Among all the modulation formats chirped NRZ shows the best BER values while AMI

has the worst BER results.

56

Fig. 4.8 Performance analysis at 40 Gbps

Fig. 4.9 Performance analysis at 100 Gbps

Fig. 4.10 BER analysis at 10 Gbps

57

4.5 Conclusion

The comparison & analysis of different modulation formats for next generation passive

optical networks like non-chirped NRZ, VSB-NRZ and chirped-NRZ on the basis of their Q-

factor values & BER shows that chirped NRZ outperforms all the other NRZ modulation

formats. The detailed comparison of different RZ modulation formats like RZ-33 %, RZ-50 %,

RZ-67 %, Chirped-RZ and VSB-CSRZ shows that RZ-50 % is best among all the RZ modulation

formats. Further RZ-50 %, Chirped-NRZ, DB and AMI analyzed at 10 Gbps of data rate by

varying fiber lengths, shows that Chirped-NRZ performs best in terms of low BER and higher

quality factor.

58

4.6 References

[1] Winzer, Peter J. "High-spectral-efficiency optical modulation formats." Journal of

Lightwave Technology 30.24 (2012): 3824-3835.

[2] Astar, W., et al. "Conversion of 10 Gb/s NRZ-OOK to RZ-OOK utilizing XPM in a Si

nanowire." Optics express 17.15 (2009): 12987-12999.

[3] Wong, Elaine. "Next-generation broadband access networks and

technologies."Lightwave Technology, Journal of 30.4 (2012): 597-608.

[4] Baliga, Jayant, et al. "Energy consumption in optical IP networks." Journal of Lightwave

Technology 27.13 (2009): 2391-2403.

[5] Rajniti, Anita Suman, and Anu Sheetal. "Comparison of RZ and NRZ Data Formats for

2.5 Gb/s Bidirectional WDM/TDM-PON Using Narrowband AWG."International

Journal of VLSI and Signal Processing Applications 1.2 (2011): 95-101.

[6] Xinsheng, Wang. "Insights into Next Generation PON Evolution." ZTE

Technologies14.4 (2012).

[7] Hui, Ron, et al. "Advanced optical modulation formats and their comparison in fiber-

optic systems." Technical Reports University of 11-7 CFY 2004-7 R-15666-01 (2004).

[8] Hui, Zhan-Qiang, Bo Zhang, and Jian-Guo Zhang. "All-optical NRZ-to-RZ format

conversion at 10 Gbit/s with 1-to-4 wavelength multicasting exploiting cross-phase

modulation & four-wave-mixing in single dispersion-flattened highly nonlinear photonic

crystal fiber." Journal of Modern Optics (2015): 1-11.

[9] Koch, Thomas L. "Laser sources for amplified and WDM light wave systems." Optical

fiber telecommunications IIIB (1997): 115-162.

59

[10] Ackerman, D. A., et al. "Telecommunication lasers." Optical fiber telecommunications,

IV A (2002).

[11] Hasegawa, Akira. "Soliton-based optical communications: An overview."Selected

Topics in Quantum Electronics, IEEE Journal of 6.6 (2000): 1161-1172.

[12] Nakazawa, Masataka. "Solitons for breaking barriers to terabit/second WDM and OTDM

transmission in the next millennium." Selected Topics in Quantum Electronics, IEEE

Journal of 6.6 (2000): 1332-1343.

[13] Zhu, B., et al. "High spectral density long-haul 40-Gb/s transmission using CSRZ-DPSK

format." Journal of Lightwave technology 22.1 (2004): 208.

[14] Proakis, John G. "Digital communications. 1995." McGraw-Hill, New York.

[15] Couch, I. I., and W. Leon. Digital and analog communication systems.Prentice Hall

PTR, 1990.

[16] Grobe, Klaus, and Michael Eiselt. Wavelength Division Multiplexing: A Practical

Engineering Guide. John Wiley & Sons, 2013.

[17] Shtaif, Mark, and Alan H. Gnauck. "The relation between optical duobinary modulation

and spectral efficiency in WDM systems." Photonics Technology Letters, IEEE 11.6

(1999): 712-714.

[18] Bobrovs, Vjačeslavs, J. Porins, and G. Ivanovs. "Influence of nonlinear optical effects on

the NRZ and RZ modulation signals in WDM systems."Elektronika irElektrotechnika

76.4 (2015): 55-58.

[19] Neto, B., et al. "Enhanced optical gain clamping for upstream packet based traffic on

hybrid WDM/TDM-PON using fiber Bragg grating." Optics Communications 284.5

(2011): 1354-1356.

60

[20] Neto, B., et al. "Assessment and mitigation of EDFA gain transients in hybrid

WDM/TDM PON in the presence of packet-based traffic." IET Optoelectron 4 (2010):

219-226.

[21] Chu, Yushi, et al. "Ce3+/Yb3+/Er3+ triply doped bismuth borosilicate glass: a potential

fiber material for broadband near-infrared fiber amplifiers." Scientific reports 6 (2016).

[22] Chang, Jun, Qing-Pu Wang, and Gang-Ding Peng. "Optical amplification in Yb 3+-

codoped thulium doped silica fiber." Optical Materials 28.8 (2006): 1088-1094.

61

CHAPTER 5

Secure Duobinary Optical Transmission Model

5.1 Introduction

A numerical study is presented in this chapter based on semiconductor laser chaos

generation which is being used to hide multi-level data signal i.e. duo-binary message to take the

advantage of secure environment & higher data rates at the same time. Duobinary message is

generated by using the combination of duobinary precoder, duobinary generator & RZ/NRZ

pulse generator. Laser rate equations are used to model chaos generation through semiconductor

laser whereas message is made secured by hiding it through chaos masking scheme. Propagation

of chaos hiding the multi-format message is studied for long distance communication model.

Synchronization between transmitter & receiver is achieved to obtain the acceptable Eye-

diagrams & Quality factor (Q-factor). Q-factor is function of optical signal to noise ratio (OSNR)

that gives a qualitative performance of receiver. It provides the minimum signal to noise ratio

(SNR) to obtain specific bit error rate (BER) of signal. A comparison is made with & without

deployment of CMS scheme on RZ & NRZ duobinary optical system to observe the penalty in

terms of Q-factor. In addition, response of amplifier on chaotic signal due to its nonlinearities is

investigated by varying the gain of amplifier.

Now-a-days, telecom networks are going through various technological changes to

support huge data traffic. Newly developed technologies & applications such as internet services,

interactive games, telemedicine, large-scale computing, IPTV etc. along with existing video and

voice services are making traffic enormous day by day. In this situation, the concept of multi-

data formats has gained the attention of researchers to efficiently utilize the bandwidth of

channel [1].

62

5.1.1 Duobinary Modulation Format

Duobinary, type of multi-level format, is a proficient optical modulation scheme which

is the area of interest due to its increased spectral efficiency. It is being used to increase the

channel capacity by improving the bandwidth utilization and increased system functionality [2].

Its simplicity of implementation, tolerance to high chromatic dispersion [3] & increased spectral

efficiency [4] are some of the attractive features which can be used in long haul communication

[5] Although the concept of using duo-binary modulation scheme in long haul optical

communication is already deployed [6-8] but the security issues explicit to these formats are still

need to be studied in detail. In this chapter, we analyzed & addressed the security features for the

first time in multi-level format i.e. Duobinary.

Due to some important features of chaotic waveform such as noise-like time domain

signal and broad spectrum, chaos communication can be used for secure communication [9-16].

An attacker, who can get the modulated chaotic signal, cannot read the message information

without having the original chaotic signal produced by the chaotic lasers. Many important

discoveries are made in last few years in the field of optical chaos generation [17]. Potential

sources which are being used to generate chaos includes Semiconductor lasers [18-24],

Semiconductor ring lasers [25], Erbium doped fiber ring lasers [26-27], Vertical cavity surface

emitting Lasers [28-29], Random feedback fiber distributed lasers [30], Optoelectronic

oscillators [31-32] etc. Of these chaotic sources semiconductor lasers are most commonly used in

the field of secure optical communication when high bandwidth chaos is intended. As multi-level

formats are suitable for high data rates, so our proposed study gives an idea to ensure the security

of multi-level formats by using semiconductor chaotic lasers. Major contributions related to this

chapter can be listed as follow:

63

1.This is the first time that we combined the advantages of chaos masking & duobinary schemes

(multi-level format) to take the advantage of security & high data rate at the same time.

2.Performance comparison of RZ-duobinary & NRZ-duobinary format is done when both are

used in chaotic environment.

3.Amplifier response on chaotic signal due to its nonlinearities is investigated by varying the

gain of amplifier according to different lengths of optical fiber.

4.Also, a complete step by step approach can be seen in this chapter starting from duo-binary

signal generation, security feature addition, effects of channel & amplifier on chaotic signal &

finally the retrieval of original message from chaos.

This chapter is arranged in following sections. Section-I comprises introduction of

chapter. Section-II covers mathematical model, operating parameters & their values. Section-

III shows proposed setup for simulations. Results & discussions are included in Section-IV.

Finally, the work is concluded in Section-V.

5.2 Mathematical Model

Three schemes which are used to make signal secure are chaos masking scheme (CMS),

chaos shift keying (CSK) & chaos modulation (CM). The performance comparison of these three

schemes can be observed in Table 5.1.

Table 5. 1 Performance comparison of message encoding schemes

Ref Properties CMS CSK CM

[33] Easy Message Recovery ✓ [34] Simplicity ✓

[35] Noise Immunity

✓

[34] Low Cost ✓ [36] Exact Message Recovery

✓

[37] Synchronization

Required ✓ ✓ ✓

[37] Lowest Q-Factor

✓

64

As our goal is to design low cost efficient secure system for long haul communication so

CMS is chosen among the three. The basic chaotic communication model using CMS is shown

in Fig. 5.1. In this scheme, the message m(t) is simply added to chaotic waveform c(t) generated

by semiconductor laser & then transmitted over channel. The transmitted chaotic waveform is

similar to noise signal n(t) which hides message in it. At the receiving side, the original message

m(t) is recovered by subtracting chaos produced by the locally available chaotic laser.

Mathematically this model can be expressed as,

𝑛(𝑡) = 𝑚(𝑡) + 𝑐(𝑡) (5.1)

𝑟(𝑡) = 𝑛(𝑡) − 𝑐(𝑡) (5.2)

𝑟(𝑡) = [𝑚(𝑡) + 𝑐(𝑡)] − 𝑐(𝑡) (5.3)

𝑟(𝑡) = 𝑚(𝑡) (5.4)

Where, n(t) is transmitted signal over the channel and r(t) is received signal.

The essence of secure optical communication by using chaos particularly in CMS lies in the fact

that two spatially deployed chaotic lasers must be synchronized with each other [38-39].

Synchronization of chaotic lasers is the irregular optical pulses evolution of the transmitter side

Fig. 5.1 Basic CMS communication model.

65

laser that is well reproduced by the receiver side laser in similar fashion. In our study, this task is

achieved by closely matching the parameters of these two chaotic lasers. Also, the operating

conditions of both the lasers are also kept same for perfect synchronization.

Chaos produced through this model is of pulsating nature which exhibits more chaotic

behavior as compared to non-pulsating chaos. The chaos degree is measured by calculating

Lyapunov exponents, which shows larger values towards positive side for the pulsating chaos

[40]. As the larger values of Lyapunov exponents (towards +ive side) show greater instability in

system so on this basis we can suggest that the generated chaos is highly unpredictable. Optical

chaos generated by semiconductor laser can be represented by following laser rate equations

[41]:

𝑑𝑛

𝑑𝑡=

𝐽

𝑒𝑑− 𝐺(𝑛)𝑆 −

𝑛

𝜏𝑛 (5.5)

𝑑𝑆

𝑑𝑡= 𝐺(𝑛)𝑆 −

𝑆

𝜏𝑝ℎ+ 𝛽𝑠𝑝

𝑛

𝜏𝑟 (5.6)

In eq (5.5), ‘𝑛’ is the concentration of carrier, ‘𝐽’ is the injection current density (in active layer it

is electric current flowing per unit area), ‘ 𝑒 ’ represents the elementary charge, ‘ 𝑑 ’ is the

thickness of active layer. ‘ 𝐺(𝑛) ’ defines the mode amplifying rate because of stimulated

emission & ‘𝜏𝑛’ is the lifetime of carrier.

Whereas in eq (5.6), ‘𝑆’ is photon density, ‘𝜏𝑝ℎ’ represents photon lifetime, ‘𝛽𝑠𝑝’ defines

coupling factor due to spontaneous emission & ‘𝜏𝑟’ is radiative recombination lifetime because

of spontaneous emission. Eq (5.5) can be written as:

𝑑𝑛

𝑑𝑡=

𝑑(𝑁/𝑉𝑎)

𝑑𝑡=

1

𝑉𝑎

𝑑𝑁

𝑑𝑡 (5.7)

Where 𝑛 = 𝑁/𝑉𝑎 and ‘𝑁’ is the carrier’s number in active layer. ‘𝑉𝑎’ is volume of active layer.

By solving eq (5.7)

66

=1

𝑉𝑎[

𝐼

ℯ− 𝑔(𝑛)𝛤𝑎𝑁𝑝ℎ −

𝑁

𝜏𝑛] (5.8)

In eq (5.8), ‘I’ is the injected current that is running through the active layer. ‘𝑁𝑝ℎ’ are the

number of photons. ‘𝑔(𝑛)’ represents the amplifying rate because of stimulated emission in the

active layer.

=I/𝑉𝑎

ℯ− 𝑔(𝑛)

𝑉𝑎

𝑉𝑚

𝑁𝑝ℎ

𝑉𝑎−

N/𝑉𝑎

𝜏𝑛 (5.9)

Where, 𝛤𝑎 = 𝑉𝑎/𝑉𝑚

By solving eq (5.9)

= 𝐽

ℯ𝑑− 𝑔(𝑛)𝑠 −

𝑛

𝜏𝑛 (5.10)

Now, modifying eq (5.6)

𝑑𝑠

𝑑𝑡=

𝑑(𝑁𝑝ℎ/𝑉𝑚)

𝑑𝑡=

1

𝑉𝑚

𝑑𝑁𝑝ℎ

𝑑𝑡 (5.11)

=1

𝑉𝑚[ 𝑔(𝑛)𝛤𝑎𝑁𝑝ℎ −

𝑁𝑝ℎ

𝜏𝑝ℎ+ 𝛽𝑠𝑝

𝑁

𝜏𝑟] (5.12)

= 𝑔(𝑛)𝛤𝑎𝑁𝑝ℎ

𝑉𝑚−

𝑁𝑝ℎ/𝑉𝑚

𝜏𝑝ℎ+ 𝛽𝑠𝑝

𝑁/𝑉𝑚

𝜏𝑟 (5.13)

= 𝑔(𝑛)𝑠𝛤𝑎 −𝑠

𝜏𝑝ℎ+ 𝛤𝑎𝛽𝑠𝑝

𝑛

𝜏𝑟 (5.14)

Eq (5.10) & Eq (5.14) are the required solved rate equations for producing chaos through

semiconductor lasers. Parameters & their values which are used in this setup to control chaos are

listed in Table 5.2 and Table 5.3 respectively:

Table 5.2 Physical parameters of chaotic laser

Symbol Physical Parameters Value Unit

N Carrier density (at transparency) 1.0x1018 cm-3

Mode confinement factor 0.40 -

Fraction of spontaneous emission 8.0x10-7 -

67

(coupled into the lasing mode)

Photon lifetime 3.0x10-12 s

Volume of Active layer 1.5x10-10 cm3

Λ Linewidth enhancement factor 5.0 -

Electron lifetime 1.0x10-9 s

Table 5.3 Operating parameters of chaotic laser

Parameter Values

Wavelength 1550 nm

Power 20 dBm

Power at Bias Current 0 dBm

Threshold Power 0.146 mW

Bias Current 30 mA

Modulation Peak Current 35 mA

Threshold Current 33.46 mA

Duobinary modulation scheme is a combination of two shift keying techniques, amplitude

shift keying (ASK) and phase shift keying (PSK) [2]. The duobinary scheme can transmit ‘R’

bits/sec of signal data by using bandwidth less than ‘R/2’ Hz. As the Nyquist theorem suggests

that minimum bandwidth needed to transmit ‘R’ bits/sec is atleast ‘R/2’ Hz with no inter-symbol

interference (ISI). This shows that duobinary pulses will have the ISI but in such a way that it

will be induced in controlled manner to recover the original signal.

In duobinary modulation scheme the drive signal for modulator can be constructed by

combining 1-bit delayed data to current data bit which enhance it upto three levels i.e. 0, +1, and

-1. A similar result can be obtained by applying a filter (low pass) to the signal having binary

values. Duobinary modulation is obtained by 100 percent over-driving a MZM with duobinary

encoded signal. So, in this way level ‘+1’ and ‘-1’ allow 100% transmission with different

optical phases while level ‘0’ allows 0% transmission. This 3-level duobinary signal can be

68

demodulated by using an optical direct detection receiver into a binary signal again. The main

advantage of this approach is that the duobinary modulated optical signals have narrower

bandwidth in contrast to NRZ modulated binary signals. Due to this, the net effect of optical

fiber dispersion is decreased and thus will be feasible in long haul or ultra-dense wavelength

division multiplexing (WDM) systems applications.

Transmitted signal can be represented by the following equation [2].

x(t) = ∑ dk q(t – kt),∞k=−∞ 𝑑𝑘 = 0,1 (5.15)

Where, 𝑑𝑘is data bits, q(t) is transmitted pulse, and (T =1/R) is bit period

q(kT) = {1 𝑖𝑓 𝐾 = 0,10 𝑜𝑡ℎ𝑒𝑟𝑤𝑖𝑠𝑒

Transmitted pulse will be overlapping in time domain due to ISI, narrowing the pulse spectrum.

Now, the duobinary pulse affected by ISI can be written as,

x(t) = ∑ 𝐶𝑘 q(t − kt)∞

𝑘=−∞ (5.16)

5.3 Proposed Scheme

The proposed scheme for chaos masking of duobinary signal is shown in Fig.2.

Duobinary generator is used to convert data into duobinary pulses coming from data source.

Precoder ensures the data integrity by making it error free. Semiconductor chaotic laser is

driven into chaotic mode to generate chaos. A semiconductor laser at the receiver end seeded

by the transmitted chaos is driven under the same parameters to generate identical chaos for

synchronization of transmitter and receiver. The duobinary signal is recovered from chaos

through subtraction rule. The link consisting of SMF-28 is varied from 110 to 170 km to

investigate the system performance. An erbium doped fiber amplifier (EDFA) with

controllable gain and DCF of appropriate length is used for loss management and dispersion

69

compensation of the broadened pulses. Direct detection optical receiver is used at the end to

receive the signal. Table 5.4 shows the operating parameters of our proposed scheme.

Data

Source

Duobinary

GeneratorMZM

CW Laser Chaotic

Laser (R)

Optical

Reciever

Low Pass

Filter

Duobinary

Precoder

Chaotic

Laser (T)

NRZ/RZ Pulse

Generator

Electrical Gain

Fig. 5.2. Chaotic optical communication model using duobinary format.

Table 5.4 Operating parameters of Duobinary model

Parameters Value

Data Rate 10 Gb/s

Optical Fiber 110 km, 150 km, 170 km

DCF Value -83.75 ps/nm/km

DCF Lengths 22 km, 30 km, 34 km

CW Laser Wavelength 1550 nm

CW Laser Power 20 dBm

Optical Amplifier Gain 22-34 dB

5.4 Results & Discussion

Simulations and analysis are made by using Optisystem 14.0 & MATLAB respectively.

Initially, propagation of NRZ-duobinary format is analyzed due to its increased efficiency over

RZ. The input NRZ-duobinary coded message at the rate of 10 Gb/sec is shown in Fig. 5.3. This

message is 3-level and fed to the modulator to convert it to the 2-level optical signal. A

continuous wave (CW) laser is used for this purpose whose power is set to 20 dBm and which

operates at 1550 nm.

70

Fig. 5.3 Duobinary message with data rate 10 GB/s.

At the next level this optical signal is mixed with chaotic waveform generated by chaotic

laser. The chaotic waveform and their zoomed plots before mixing with duobinary message can

be seen in Fig. 5.4(a) and Fig. 5.4(b) respectively. The duobinary message shown in Fig. 5.5(a) is

embedded in chaos through CMS. The power of chaotic laser is also set to 20 dBm and it also

operates at 1550 nm to hide the signal completely. The resultant noise-like waveform produced

after masking duobinary message with chaotic waveform can be seen in Fig 5.5(b).

Fig 5.4(a) Time domain plot of generated chaos.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

x 10-8

0

0.1

0.2

0.3

0.4

0.5

0.6

Time (Sec)

Powe

r (W

)

71

Fig. 5.4(b) Time domain plot of generated chaos on larger scale.

Fig. 5.5(a) Duobinary message generated by transmitter.

Fig. 5.5(b) Chaotic waveform hiding duobinary message.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

x 10-9

0

0.1

0.2

0.3

0.4

0.5

0.6

Time (Sec)

Powe

r (W

)

0 1 2 3 4 5 6 7

x 10-8

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Time (Sec)

Ampli

tude (

a.u.)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

x 10-9

0

0.1

0.2

0.3

0.4

0.5

Time (Sec)

Powe

r (W

)

72

The effect of introducing security feature in duobinary message using chaos can also be

seen by analyzing the optical spectrums of original duobinary message and chaos embedding

duobinary message. Fig. 5.6(a) shows the optical spectrum of duobinary message at the

transmitter side before mixing it with chaotic waveform. Fig. 5.6(b) shows the total change in

spectrum due to applied chaos which is not discernible for the intruders.

Fig. 5.6(a) Optical spectrum of duobinary message.

Fig. 5.6(b) Optical spectrum of chaos embedding duobinary message.

1.5475 1.548 1.5485 1.549 1.5495 1.55 1.5505 1.551 1.5515 1.552 1.5525

x 10-6

-80

-70

-60

-50

-40

-30

-20

-10

0

10

Wavelength(m)

Pow

er(d

Bm

)

1.5475 1.548 1.5485 1.549 1.5495 1.55 1.5505 1.551 1.5515 1.552 1.5525

x 10-6

-70

-60

-50

-40

-30

-20

-10

0

10

Wavelength(m)

Pow

er(d

Bm

)

73

The transmitted chaos after propagation through the channel gets deteriorated as it is

evident from the scatter plot between transmitted and received chaos shown in Fig. 5.7(a). After

adjusting the amplifier gain, insertion of appropriate length of DCF and handling the delay

corresponding to link parameters, the improved scatter plot between transmitted and received

chaos is shown in Fig. 5.7(b).

Fig. 5.7(a) Transmitted vs. received chaos without synchronization.

Fig. 5.7(b) Transmitted vs. received chaos after synchronization & delay matching.

74

The final message retrieved by the receiver after subtracting chaos can be seen in Fig. 5.8.

Waveform shown in Fig. 5.8 can be compared with the waveform of original message also

shown in Fig. 5.5(a).

Response of amplifier can be seen by taking the scatter-plots between the transmitted chaos

through amplifier and its output. The response is plotted for the three different lengths of SMF-

28 which are used in our setup i.e. 110 km, 150 km and 170 km. The power of laser is set to 20

dBm for all the three lengths whereas the gain of amplifier is adjusted according to the length of

fiber by taking the standard value of loss as 0.2 dB/km. Fig. 5.9 shows that the increase in gain

of amplifier also increases the effect of nonlinearities in amplifier which makes the signal

distorted. The reason behind the distortion is that as the amplifier amplifies the optical signal, the

amplitude of signal increases which undergoes the Kerr effect resulting in the increase of

nonlinearities of the amplifier. The usual waveguides made-up of silica have very low Kerr-

linearities [42] but when these waveguides are exposed to high power of laser, the Kerr effect

increases. The effect of amplifier nonlinearities has been studied in our previous work [43]. The

eye-diagrams for different lengths of fiber are depicted in Fig. 5.10. The eye-opening decreases

due to link and amplifier impairments incurred with the increase in fiber length. The eye-diagram

up to fiber length of 110 km shows suitable results at the receiver side as compared to the 150

km and 170 km lengths of fiber.

The Q-factor as a function of fiber length is shown in Fig. 5.11. This figure clearly shows that

as the length increases from 110 km, there is drastic decrease in Q-factor. This is because the

linear impairments are easily controlled but the non-linear impairments such as amplifier and

fiber nonlinearities cannot be fully avoided which results in drastic drop of Q-factor with the

increase in fiber length. A comparison is also shown in this figure between the NRZ/RZ

75

duobinary systems with & without the deployment of CMS. Results show that CMS resulted in

penalty of Q-factor. Also, NRZ-duobinary showed better performance than RZ-duobinary when

used in combination with CMS.

Fig. 5.8 Transmitted signal vs. received Signal.

a) 22dB b) 30dB c) 34dB

Fig 5.9. Amplifier Response on chaotic waveform at different SMF lengths & corresponding

gains.

76

a) 110 km

b) 150 km

c) 170 km

Fig. 5.10 Eye-diagrams of duobinary signal at different SMF lengths.

Fig. 5.11 Q-factor vs Length of fiber.

5.5 Conclusion

In this chapter, we demonstrated the security implementation of multilevel data format by

explicitly targeting the duobinary modulation format due to its increased efficiency as compared

to simple RZ & NRZ. By implementing our proposed scheme, we have not only taken the benefit

of increased throughput of communication system but also made it secured at the same time.

Thus, our work gives detailed view of implementation of secure duobinary optical

communication system, starting from duobinary message generation till restoration of original

-0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.40

1

2

x 10-4

Time(bit period)

Am

plit

ude(a

.u.)

-0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.40

1

2

3

4

5

6

7

8

9x 10

-5

Time(bit period)

Am

plit

ude(a

.u.)

-0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.40.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10

-5

Time(bit period)

Am

plit

ude(a

.u.)

77

message at the receiver side after its transmission over the optical fiber in chaotic waveform

which ensures its security & integrity. Duobinary message is added to the chaotic waveform

generated by the chaotic laser through message masking scheme due to the simplicity of this

scheme. The chaotically masked duobinary signal is transmitted over different SMF lengths

i.e.110 km, 150 km and 170 km. An analysis is made for RZ/NRZ-duobinary signal deviation

with the link parameters before and after applying CMS. Synchronization is successfully

achieved between transmitter & receiver by matching the physical & operating parameters of

chaotic lasers at transmitting & receiving side. Dispersion compensation fiber is used to nullify

the dispersion effects before synchronizing and subtracting the receiver chaos to recover the

duobinary signal. Optical amplifier is used to increase the optical fiber length & effect of

amplifier’s nonlinearities on chaotic waveform propagation to limit the fiber length, is observed

by increasing the gain of amplifier w.r.t fiber length. The future work will include higher order

modulation schemes i.e. QAM & OFDM with other chaos message encoding schemes to ensure

transmission of high data rates in secure environment.

78

5.6 References

[1] Kaur, Rupinder, and Sanjeev Dewra. "Duobinary modulation format for optical system-a

review." Int. J. Adv. Res. Electr. Electron. Instrum. Eng. Vol.3, pp. 11039, 2014.

[2] Vujicic, Z., et al. "Considerations on performance, cost and power consumption of candidate

100G EPON architectures." Transparent Optical Networks (ICTON), 2016 18th International

Conference on. IEEE, 2016.

[3] Šalík, Pavol, Filip Čertík, and Rastislav Róka. "Duobinary Modulation Format in Optical

Communication Systems." Advances in Signal Processing, vol. 3, no. 1, pp. 1-7, 2015.

[4] Wei, Jinlong, et al. "40 Gb/s Lane Rate NG-PON using Electrical/Optical Duobinary, PAM-4

and Low Complex Equalizations." Optical Fiber Communication Conference. Optical Society

of America, pp.Tu3C--5 2016.

[5] Yu, C. X., et al. "10.7 Gbit/s transmission over> 200 km of standard single mode fibre using

forward error correction and duobinary modulation." Electronics Letters, vol. 39, no.1, pp. 76-

78, 2003.

[6] C. Sun, S. Bae, H. Kim, and Y. Chung, “Transmission of 28-Gb/s duobinary signals over 45-

km SSMF using 1550-nm directly modulated laser,” in Proc. OECC, paper ThB1-4, pp.1-3,

2016.

[7] Z. Al-Qazwini and H. Kim, “Directly modulated laser driven by low bandwidth duobinary

signals,” IEEE Photon. Technol. Lett., vol. 22, no. 17, pp. 1306–1308, Sep. 1, 2010

[8] Sun, Chuanbowen, S. H. Bae, and Hoon Kim. "Transmission of 28-Gb/s Duobinary and

PAM-4 Signals Using DML for Optical Access Network." IEEE Photonics Technology

Letters. Vol. 29, no.1, pp. 130-133, 2017.

79

[9] J. Ohtsubo, Semiconductor Lasers: Stability, Instability and Chaos, Third Edition, Springer-

Verlag, Berlin Heidelberg, 2013.

[10] F. C. M. Lau and C. K. Tse, Chaos-based Digital Communications Systems. Heidelberg:

Springer Verlag, 2003.

[11] A. Uchida, S. Yoshimori, M. Shinozuka, T. Ogawa, and F. Kannari, "Chaotic on-off keying

for secure communications," Optics Letters, vol. 26, no.12, pp. 866-868, 2001.

[12] Annovazzi-Lodi, Valerio, et al. "Message encryption by phase modulation of a chaotic optical

carrier." IEEE Photonics Technology Letters, Vol. 19, no.2, pp. 76-78, 2007.

[13] C. R. Mirasso, P. Colet, and P. García-Fernández, “Synchronization of chaotic semiconductor

lasers: application to encoded communications,” IEEE Photonics Technology Letters, vol. 8,

no. 2, pp. 299-301, 1996.

[14] A. Argyris, E. Grivas, M. Hamacher, A. Bogris, and D. Syvridis, "Chaos-on-chip secures data

transmission in optical fibre links," Optics Express, vol. 18, pp. 5188-5198, 2010.

[15] H. Yanhua, et al., "GHz bandwidth message transmission using chaotic vertical-cavity

surface-emitting lasers," Journal of Lightw. Technol., Vol. 27, no.22, pp. 5099-5015. 2009.

[16] A. Argyris, D. Syvridis, L. Larger, V. Annovazzi-Lodi, P. Colet, I. Fischer, J. Garcia-Ojalvo,

C. R. Mirasso, L. Pesquera, and K. A. Shore, “Chaos-based communications at high bit rates

using commercial fiber optic links,”Nature, vol. 437, no. 17, pp. 343–346, Nov. 2005

[17] Xiang, Shui Ying, et al. "Effect of gain nonlinearity on time delay signature of chaos in

external-cavity semiconductor lasers." IEEE Journal of Quantum Electronics, Vol. 52, no. 4,

pp. 1-7, 2016.

[18] M. Sciamanna and K. A. Shore, “Physics and applications of laser diode chaos,”Nature

Photon., vol. 9, no. 3, pp. 151–162, Feb. 2015.

80

[19] M. C. Soriano, J. García-Ojalvo., C. R. Mirasso, and I. Fischer, “Complex photonics:

Dynamics and applications of delay-coupled semiconductors lasers,”Rev. Mod. Phys., vol. 85,

no. 1, pp. 421–470, Mar. 2013.

[20] V. Annovazzi-Lodi, G. Aromataris, and M. Benedetti, “Multi-user private transmission with

chaotic lasers,”IEEE J. Quantum Electron.,vol. 48, no. 8, pp. 1095–1101, Aug. 2012.

[21] N. Jianget al., “Chaos synchronization and communication in mutually coupled

semiconductor lasers driven by a third laser,”J.Lightw Technol., vol. 28, no. 13, pp. 1978–

1986, Jul. 1, 2010

[22] I. Reidler, Y. Aviad, M. Rosenbluh, and I. Kanter, “Ultrahigh-speed random number

generation based on a chaotic semiconductor laser,” Phys.Rev.Lett., vol. 103, no. 2, pp.

024102-1–024102-4, Jul. 2009

[23] A. Uchida et al., “Fast physical random bit generation with chaotic semiconductor lasers,”

Nature Photon., vol. 2, no. 12, pp. 728–732, Dec. 2008.

[24] A. Argyris et al., “Chaos-based communications at high bit rates using commercial fibre-optic

links,”Nature, vol. 438, pp. 343–346, Sep. 2005.

[25] R. M. Nguimdo, G. Verschaffelt, J. Danckaert, and G. Van der Sande, “Loss of time-delay

signature in chaotic semiconductor ring lasers,”Opt. Lett., vol. 37, no. 13, pp. 2541–2543, Jul.

2012.

[26] Ali, Syed Zafar, Muhammad Khawar Islam, and Muhammad Zafrullah. "Effect of parametric

variation on generation and enhancement of chaos in erbium-doped fiber-ring lasers." Optical

Engineering, Vol. 49, no. 10, pp. 105002-105002, 2010.

81

[27] Ali, S. Zafar, M. K. Islam, and M. Zafrullah. "Generation of higher degree chaos by

controlling harmonics of the modulating signal in EDFRL." Optik-International Journal for

Light and Electron Optics, Vol. 122, no. 21, pp. 1903-1909, 2011.

[28] S. Xianget al., “Conceal time-delay signature of chaotic vertical-cavity surface-emitting lasers

by variable-polarization optical feedback,” Opt. Commun., vol. 284, no. 24, pp. 5758–5765,

Dec. 2011.

[29] S. Priyadarshi, Y. Hong, I. Pierce,and K. A. Shore, “Experimental investigations of time-delay

signature concealment in chaotic external cavity VCSELs subject to variable optical

polarization angle of feedback,”IEEE J. Sel. Topics Quantum Electron., vol. 19, no. 4,

Jul/Aug, Art. no. 1700707, 2013.

[30] W. Zhang et al., “Random distributed feedback fiber laser based on combination of er-doped

fiber and single-mode fiber,”IEEE J. Sel. Topics Quantum Electron., vol. 21, no. 1, Jan/Feb,

Art. no. 0900406, 2015.

[31] R. Lavrov, M. Peil, M. Jacquot, L. Larger, V. Udaltsov, and J. Dudley, “Electro-optic delay

oscillator with nonlocal nonlinearity: Optical phase dynamics, chaos, and synchronization,”

Phys.Rev.E, vol. 80, no. 2, p. 026207, Aug. 2009.

[32] K. E. Callan, L. Illing, Z. Gao, D. J. Gauthier, and E. Schöll, “Broadband chaos generated by

an optoelectronic oscillator,”Phys.Rev.Lett., vol. 104, no. 11, p. 11390, Mar. 2010.

[33] Li, Yanli, Yuncai Wang, and Anbang Wang. "Message filtering characteristics of

semiconductor laser as receiver in optical chaos communication." Optics Communications,

Vol. 281, no. 9, pp. 2656-2662, 2008.

[34] Cuomo, Kevin M., and Alan V. Oppenheim. "Circuit implementation of synchronized chaos

with applications to communications." Physical review letters, Vol 71, no. 1, pp. 65, 1993.

82

[35] Uchida, Atsushi. Optical communication with chaotic lasers: applications of nonlinear

dynamics and synchronization. John Wiley & Sons, 2012.

[36] Li, Shujun, et al. "Analog chaos-based secure communications and cryptanalysis: A brief

survey." arXiv preprint arXiv:0710.5455, 2007.

[37] Kanakidis, Dimitris, Apostolos Argyris, and Dimitris Syvridis. "Performance characterization

of high-bit-rate optical chaotic communication systems in a back-to-back

configuration." Journal of lightwave technology, Vol. 21, no. 3, pp. 750-758, 2003.

[38] K. Kusumoto and J. Ohtsubo, “1.5-GHz message transmission based on synchronization of

chaos in semiconductor lasers,”Opt. Lett, Vol. 27, no. 12, pp. 989–991, 2002.

[39] Donati, Silvano, and Claudio R. Mirasso. "Introduction to the feature section on optical chaos

and applications to cryptography." IEEE Journal of Quantum Electronics, Vol. 38, no. 9, pp.

1138-1140, 2002.

[40] Ali, S. Zafar, M. K. Islam, and M. Zafrullah. "Comparative analysis of chaotic properties of

optical chaos generators." Optik-International Journal for Light and Electron Optics, Vol. 123,

no. 11, pp. 950-955, 2012.

[41] Numai, Takahiro. "Fundamentals of semiconductor lasers." Fundamentals of Semiconductor

Lasers. Springer Japan, pp. 89-186, 2015.

[42] Ankiewicz, A., and G. D. Peng. "Generalized Gaussian analysis of fibres with non-Kerr-law

nonlinearities." Optical and quantum electronics 25.2 (1993): 147-155.

[43] Ali Syed Zafar, Muhammad Khawar Islam, and Muhammad Zafrullah. "Effect of

transmission fiber and amplifier noise on optical chaos synchronization."Optical review,

Vol. 19, no. 5, pp. 320-327, 2012

83

CHAPTER 6

Secure Optical QAM Transmission Model

6.1 Introduction

This chapter presents the joint use of advance modulation scheme i.e. m-QAM and optical

chaos to combine the best of the two worlds i.e. higher data rates and security. A semiconductor

laser diode is driven into chaotic region using direct modulation scheme and 4-QAM signal is

added by Chaos message masking (CMS). The chaotically masked data stream is transmitted

over an optical communication link to investigate the propagation issues and synchronization of

chaos at the receiver. The transmitted chaos is synchronized at the receiver to unmask the QAM

stream and binary data by using subtraction rule and conventional QAM demodulator

respectively. The deterioration of constellation diagrams and bit error rate depends upon

transmitter/receiver synchronization and link parameters. The use of 4-QAM and chaos is

extendable to m-QAM. In our work it is extended to 16-QAM and 64-QAM schemes.

The demand of high data rates and bandwidth hungry applications in the field of optical

communication has led to the interest in implementation of advance modulation formats in

optical networks. Next Generation Networks (NGN), High Definition (HD) videos, video-on-

demand and wireless high-speed file transfer are some bandwidth hungry applications that are

made possible by using advance modulation formats [1–5]. Although using advance modulation

formats is the key to obtain highest possible data rates [1, 2] however the security issues still

need to be addressed in these formats [6]. Optical chaos being the important discovery of this

century finds its application in secure communication [7–14], chaotic lidar [15], ultra-fast

random number generators (RNG) [16–20], optical time domain reflectometer (OTDR) [21] and

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photonic ultra-wideband signal generator [22]. Optical chaos for secure optical communication

has been explored with basic modulation formats successfully [23]. However, higher data rates

and security can be achieved simultaneously by combining the strength of QAM modulation

formats and chaotic optical communication [2, 14].

The optical chaos can be generated by using semiconductor lasers and fiber lasers [7, 9]. The

issues related to generation and control of chaos, effect of fiber transmission, effect of amplifier

noise on optical chaos synchronization and DWDM applications are discussed in our previously

reported work [24]. The optical QAM message can be made secure by using Additive Chaos

Modulation (ACM), Chaos Shift Keying (CSK) or Chaos Message Masking (CMS) [25,26].

However, chaos masking is selected in this work due to its simplicity of implementation [27]. In

chaos masking technique the message is mixed with the chaotic waveform and is transmitted

over the communication link [11, 28]. Chaos control being an important aspect of secure

communication [29] is successfully done to achieve the desired result. The message is recovered

by synchronizing the transmitter and receiver and using subtraction [30, 31]. The objective of

this work is to demonstrate secure 4-QAM signal transmission over an optical channel by using

Chaos Message Masking scheme and chaos generated through Semiconductor lasers.

6.2 Mathematical Model

The chaotic communication system requires generation of identical chaos both at

transmitter and receiver. Optical chaotic waveforms reported so far in existing literature are of

two types: (i) the non-pulsed chaos is continuous oscillation generated by semiconductor laser

diodes or nonlinearity based EDFRL arrangements [32], (ii) the Pulsed chaos is generated by

directly modulated semiconductor lasers or modulated EDFRLs including loss and pump

modulated EDFRLs [33]. The pulsed chaos is based on the buildup of population inversion for

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some time and then sudden release of photons energy in form of chaotic light intensity pulse.

Since all the stored energy in population inversion is not released in a single pulse, the remaining

energy which is the starting point for the build-up of next pulse changes in a random fashion and

hence each chaotic pulse is different in amplitude. This behavior becomes more random if firing

time of chaotic pulses is not fixed. Pulsed chaos can be taken as optical chaos whose amplitude

goes to zero after each consecutive chaotic amplitude pulse, the pulse being Gaussian in shape

[33] and population inversion building up during zero amplitude time for generation of the next

pulse. Pulsed chaos itself is of two types i.e. chaotic in amplitude only with constant time

interval and the one chaotic in amplitude and time interval both. The latter is more unpredictable

and hence more useful for secure communication.

6.2.1 Laser Rate Equations

The security level in chaos-based communication system depends on chaos degree which

can be measured by Lyapunov Exponents. The pulsed chaos shows larger values Lyapunov

Exponents towards positive side, which in other terms offer more security in optical

communication system [32,34]. Thus, by using pulsed chaos, security of optical communication

system is increased as compared to non-pulsed chaos. The chaos generated by direct modulation

through semiconductor lasers offer higher bandwidth which is essential to implement high bit

rate modulation formats [7]. Keeping these factors in view, we have utilized optical chaos of

pulsating nature which is generated through direct modulation of semiconductor laser and

modelled by the laser rate equations given below [24]:

dN(t)

dt=

I(t)

q(V)− Go(N(t) − Nt)

1

1 + εS(t)S(t) (6.1)

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dS(t)

dt= Γg0(N(t) − Nt)

1

(1 + εS(t))S(t) −

S(t)

τp+

ΓβN(t)

τn (6.2)

dφ

dt=

1

2α ⌊Γg0(N(t) − Nt) −

1

τp⌋ (6.3)

where,

g0 = vga0 (6.4)

I(t) = IDC + Iin. IPK (6.5)

The chaos produced by the semiconductor lasers can be controlled by the following

parameters given in Table 6.1

Table 6.1 Parameters of semiconductor laser

Symbol Title Value Unit

a0 Active Layer coefficient 1.50x10-10 cm3

ε Gain Compression Factor 1.0x10-17 -

vg Group Velocity 8.50x109 cm/sec

β Fraction of spontaneous emission

(coupled into the lasing mode) 8.0x10-7

-

Nt Carrier density (at transparency) 1.0x1018 cm-3

V Volume of active layer 1.5x10-10 cm3

Γ Mode confinement factor 0.40 -

τn Electron lifetime 1.0x10-9 sec

τp Photon lifetime 3.0x10-12 sec

Λ Line width enhancement factor 5.0 -

6.2.2 QAM Signal

The basic QAM model includes two carrier signals having phase shift of 90 degree between

each other as shown in Fig. 6.1. Two data streams known as ‘I’ (In-phase) and ‘Q’ (Quadrature

amplitude) are used to modulate carrier signals. Two resultant signals are then added and

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processed in RF signal domain for conversion to the required level of frequency. QAM

demodulator performs the inverse function of the QAM modulator.

Fig. 6.1 Basic QAM Model

A transmitting QAM signal can be mathematically modeled as [35]:

𝑆(𝑡) = 𝑅𝑒[{𝐼(𝑡) + 𝑖𝑄(𝑡)}𝑒𝑖2𝜋𝑓0𝑡] = 𝐼(𝑡)cos (2𝜋𝑓0𝑡) − 𝑄(𝑡)sin (2𝜋𝑓0𝑡) (6.6)

Where, I(t) and Q(t) represents the modulating signal and ‘𝑓𝑜’ is the frequency of carrier. At

the receiving end the above modulating signals can be demodulated by using QAM demodulator.

Due to orthogonal property of these two signals it is possible to receive the signals separately. In

the ideal scenario, I(t) can be demodulated by multiplying received signal with cosine waveform:

𝑟(𝑡) = 𝑠(𝑡)co s(2𝜋𝑓𝑜𝑡) = 𝐼(𝑡)co s(2𝜋𝑓0𝑡) co s(2𝜋𝑓0𝑡) − 𝑄(𝑡)si n(2𝜋𝑓0𝑡) 𝑐𝑜𝑠(2𝜋𝑓0𝑡) (6.7)

𝑟(𝑡) =1

2𝐼(𝑡)[1 + cos(4𝜋𝑓𝑜𝑡)] −

1

2 𝑄(𝑡)[sin(4𝜋𝑓𝑜𝑡)] (6.8)

𝑟(𝑡) =1

2 𝐼(𝑡) +

1

2𝐼(𝑡)[cos(4𝜋𝑓𝑜𝑡)] − 𝑄(𝑡)[sin(4𝜋𝑓𝑜𝑡)] (6.9)

The optical power from the laser can be written as:

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𝑃𝑎𝑣𝑔 =1

𝑁 ∑ Pn𝑁𝑛=1

(6.10)

Pn−1 − Pn|𝑛=𝑁 = 2ƞ𝑜

A/(N − 1) (6.11)

Where, ‘η𝑜’ is the internal efficiency of laser. ’N’ shows modulation level (N=2, 3, 4... for 4-

QAM, 8-QAM, 16-QAM... respectively). ’A’ represents the number of levels in the signal. All

the levels are assumed equally likely probable. The QAM and link parameters are given in Table

6.2.

Table 6. 2 Other parameters of QAM model

Parameters Value

Data Rate 10 Gb/s

Optical Fiber Length 10 km, 50 km, 80 km

DCF Value -83.75 ps/nm/km

DCF Lengths 2 km, 10 km, 16 km

CW Laser Wavelength 1550 nm

CW Laser Power 10 dBm

QAM Transmitter 2 b/sym

Optical Amplifier Gain 16 dB

6.3 Proposed Scheme

The proposed scheme for chaos masking of m-QAM signal is shown in Fig. 6.2. QAM

modulation technique characterized by M-symbols can be described as m-QAM, e.g. 4-QAM,

16-QAM, 64-QAM, 256- QAM.

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Fig. 6.2 Chaotic optical communication model using QAM format

The 4-QAM transmitting module consisting of two MZMs is shown in Fig. 6.3. Higher

order QAM transmitter can be implemented by exploiting series combination of MZMs.

Fig. 6.3 QAM module

The optical signal generated by the optical laser, is divided (by an optical fiber coupler)

into two paths/arms and then again coupled into single fiber [36] and onto the optical signal

analyzer. The signal approaching the analyzer is modulated as per optical path difference

between the two fiber paths/arms. Semiconductor chaotic laser is driven into chaotic mode to

generate chaos. A semiconductor laser at the receiver end seeded by the transmitted chaos is

driven under the same parameters to generate identical chaos for synchronization of

transmitter and receiver. The 4-QAM signal is recovered from chaos through subtraction

rule. The link consisting of SMF-28 is varied from 10 to 80 km to investigate the system

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performance. An EDFA with controllable gain and DCF of appropriate length is used for loss

management and dispersion compensation of the broadened pulses. Coherent m-QAM

receiver consists of multiple photodiodes for better signal detection. Low pass filter, decision

threshold unit and sequence decoder are used in the end for message retrieval.

6.4 Results and Discussion

The 4-QAM input message at the rate of 10 Gb/sec is shown in Fig. 6.4. The generated chaos

and their zoomed plots are shown in Fig. 6.5 and Fig. 6.6 respectively. The message is embedded

in chaos through Chaos Message Masking scheme.

Fig. 6.4 QAM Message with Data Rate 10 GB/s

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Fig. 6.5 Time Domain Plot of Generated Chaos

Fig. 6.6 Zoomed Plot of Chaos in Time domain

The optical spectrum of 4-QAM signal before embedding with chaos is shown in Fig. 6.7.

After masking QAM signal with chaotic pulses, this optical spectrum is totally changed and

become unpredictable as depicted in Fig. 6.8.

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Fig. 6.7 4-QAM Optical Spectrum

Fig. 6.8 Optical Spectrum of Chaos Embedding 4-QAM

The received chaos is lagging in time as compared to the transmitted chaos, time domain plot

of transmitted and received chaos is shown in Fig. 6.9. A time delay corresponding to link length

is adjusted to acquire the synchronization.

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Fig. 6.9 Time Delay between Transmitted and Received Chaos

The transmitted chaos after propagation through the channel gets deteriorated as it is evident

from the scatter plot between transmitted and received chaos shown in Fig. 6.10

Fig. 6.10 Time Delay between Transmitted and Received Chaos

After adjusting the amplifier gain, insertion of appropriate DCF length and introducing the

delay corresponding to link parameters, the scatter plot between transmitted and received chaos

is shown in Fig. 6.11.

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Fig. 6.11 Time Delay between Transmitted and Received Chaos

The demodulation accuracy relies on the synchronization of transmitter and receiver. The

chaos is affected by fiber loss, dispersion and nonlinearities. The parameters of EDFA and

Dispersion compensators are optimized to counter the impairments caused by the channel. The

optical spectrum of received QAM signal unmasked from chaos is shown in Fig. 6.12. The signal

is then filtered and passed through signal processing unit to retrieve the original QAM spectrum

as shown in Fig. 6.13.

The retrieved message in time domain is shown in Fig. 6.14. This message is same which was

sent through transmitter before adding it to chaotic laser.

Fig. 6.12 Optical spectrum of QAM signal unmasked from chaos

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Fig. 6.13 Optical spectrum of received QAM signal after filtering

Fig. 6.14 Retrieved Message

The constellation diagrams for different link parameters are depicted in Fig. 6.15. The

circumference of constellation dots/points increases because of transmission link impairments

incurred with the extension in optical fiber length. The constellation diagrams are showing much

lesser dispersive effects up to fiber length of 50 km as compared to the length exceeding 50 km.

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Fig. 6.15 Constellation diagram of 4-QAM signal with different SMF lengths

(a) 10 km (b) 50 km (c) 80 km

The BER as a function of laser power and different link parameters is shown in Fig. 6.16. In

order to maintain the BER at constant value, laser power is increased from 10 dBm to 20 dBm

for 10 km to 80 km respectively.

Fig. 6.16 BER vs. Laser Power (without adjusting gain of amplifier)

In the next stage, simulations are made for 16-QAM and 64-QAM secure optical

communication model following the same steps as in case of 4-QAM. Again, the chaos produced

by semiconductor laser is used to hide the higher order QAM message. Dispersion and

97

attenuation are controlled to send it to the maximum fiber length. The data rate is increased to

double for these simulations. For this, bandwidth of chaos is also increased to cater higher data

rate. Constellation diagrams obtained for both the models can be seen in Fig 6.17 and Fig 6.18 at

different lengths of fiber. When using 64-QAM model digital signal processing unit is further

added as an additional component at the receiving side and is optimized to significantly decrease

the errors in transmission as compared to 16-QAM model.

(a) (b)

(c) (d)

Fig. 6.17 Constellation diagram of 16-QAM signal with different SMF lengths

(a) 0 km (b) 10 km (c) 50 km (d) 80 km

98

(a) (b)

(c) (d)

Fig. 6.18 Constellation diagram of 64-QAM signal with different SMF lengths

(a) 0 km (b) 10 km (c) 50 km (d) 80 km

6.5 Conclusion

Chaos based secure optical communication has remained limited in bandwidth in the past

without experimenting with the advanced modulation formats. The numerical experiments for

utilizing the advantage of 4-QAM to increase data throughput in a secure chaotic system using

direct modulated semiconductor laser is demonstrated. Chaotic masking scheme is preferred to

add the 4-QAM message to the chaotic laser as it is the simplest scheme of the three, Additive

chaos modulation (ACM) and Chaos shift keying (CSK) being the other two schemes.

Synchronization between transmitter and receiver is achieved by controlling the channel

99

parameters and dispersion compensation. The QAM signal deviation varies with the link

parameters. The work is then extended to 16-QAM and 64-QAM for message security. The data

rate transmission is increased by enhancing the bandwidth of the carrier produced by chaotic

laser.

100

6.6 References

[1] E. Lach and W. Idler, “Modulation formats for 100g and beyond,”Optical Fiber Technology,

vol. 17, no. 5, pp. 377–386, 2011.

[2] M. L. Abbade, M. Cvijetic, C. A. Messani, C. J. Alves, and S. Tenenbaum, “Double all-

optical encryption of m-qam signals based on spectrally sliced encoding keys,” in 2015 17th

International Conference on Transparent Optical Networks (ICTON). IEEE, 2015, pp.1–4.

[3] N. Jiang, C. Zhang, and K. Qiu, “Secure passive optical network based on chaos

synchronization,” Optics letters, vol. 37, no. 21, pp.4501–4503, 2012.

[4] A. Ragheb and H. Fathallah, “Candidate modulation schemes for next generation-passive

optical networks (ng-pons),” in High Capacity Optical Networks and Emerging/Enabling

Technologies. IEEE,2012, pp. 226–231.

[5] M. Nölle, F. Frey, R. Elschner, C. Schmidt-Langhorst, A. Napoli, and C. Schubert,

“Performance comparison of different 8qam constellations for the use in flexible optical

networks,” in Optical Fiber Communication Conference. Optical Society of America, 2014,

pp.W3B–2.

[6] X. Yang, Q. Chen, Z. Shen, X. Hu, H. He, and W. Hu, “Secure optical ofdm signal

transmission using electro-optic chaos,” in Optical Communications and Networks (ICOCN),

2015 14th International Conference on. IEEE, 2015, pp. 1–3.

[7] G. D. Vanwiggeren and R. Roy, “Communication with chaotic lasers,” Science, vol. 279, no.

5354, pp. 1198–1200, 1998.

[8] S. Sivaprakasam and K. Shore, “Message encoding and decoding using chaotic external-

cavity diode lasers,” IEEE journal of quantum electronics, vol. 36, no. 1, pp. 35–39, 2000.

101

[9] S. Tang and J. Liu, “Message encoding–decoding at 2.5 gbits/s through synchronization of

chaotic pulsing semiconductor lasers,”Optics letters, vol. 26, no. 23, pp. 1843–1845, 2001.

[10] R. Vicente, C. R. Mirasso, and I. Fischer, “Simultaneous bidirectional message transmission

in a chaos-based communication scheme,” Optics letters, vol. 32, no. 4, pp. 403–405, 2007.

[11] J. Ohtsubo, “Chaos synchronization and chaotic signal masking in semiconductor lasers

with optical feedback,” IEEE Journal of Quantum Electronics, vol. 38, no. 9, pp. 1141–1154,

2002.

[12] C. R. Mirasso, J. Mulet, and C. Masoller, “Chaos shift-keying encryption in chaotic

external-cavity semiconductor lasers using a singlereceiver scheme,” IEEE Photonics

Technology Letters, vol. 14, no. 4,pp. 456–458, 2002

[13] N. Jiang, W. Pan, L. Yan, B. Luo, X. Zou, S. Xiang, L. Yang, and D. Zheng, “Multiaccess

optical chaos communication using mutually coupled semiconductor lasers subjected to identical

external injections,” IEEE Photonics Technology Letters, vol. 22, no. 10, pp. 676–678, 2010.

[14] A. Argyris, D. Syvridis, L. Larger, V. Annovazzi-Lodi, P. Colet,I. Fischer, J. Garcia-Ojalvo,

C. R. Mirasso, L. Pesquera, and K. A.Shore, “Chaos-based communications at high bit rates

using commercial fibre-optic links,” Nature, vol. 438, no. 7066, pp. 343–346,2005.

[15] F.-Y. Lin and J.-M. Liu, “Chaotic lidar,” IEEE journal of selected topics in quantum

electronics, vol. 10, no. 5, pp. 991–997, 2004.

[16] A. Uchida, K. Amano, M. Inoue, K. Hirano, S. Naito, H. Someya, I. Oowada, T. Kurashige,

M. Shiki, S. Yoshimori et al., “Fast physical random bit generation with chaotic semiconductor

lasers,” Nature Photonics, vol. 2, no. 12, pp. 728–732, 2008.

102

[17] I. Reidler, Y. Aviad, M. Rosenbluh, and I. Kanter, “Ultrahigh-speed random number

generation based on a chaotic semiconductor laser,” Physical review letters, vol. 103, no. 2, p.

024102, 2009.

[18] I. Kanter, Y. Aviad, I. Reidler, E. Cohen, and M. Rosenbluh, “An optical ultrafast random

bit generator,” Nature Photonics, vol. 4, no. 1, pp. 58–61, 2010.

[19] T. Harayama, S. Sunada, K. Yoshimura, P. Davis, K. Tsuzuki, and A. Uchida, “Fast

nondeterministic random-bit generation using onchip chaos lasers,” Physical Review A, vol. 83,

no. 3, p. 031803, 2011.

[20] A. Argyris, S. Deligiannidis, E. Pikasis, A. Bogris, and D. Syvridis, “Implementation of 140

gb/s true random bit generator based on a chaotic photonic integrated circuit,” Optics express,

vol. 18, no. 18, pp. 18 763–18 768, 2010.

[21] Y. Wang, B. Wang, and A. Wang, “Chaotic correlation optical time domain reflectometer

utilizing laser diode,” IEEE Photon. Technol. Lett, vol. 20, no. 19, pp. 1636–1638, 2008.

[22] M.-J. Zhang, T.-G. Liu, A.-B. Wang, J.-Y. Zheng, L.-N. Meng, Z.-X. Zhang, and Y.-C.

Wang, “Photonic ultrawideband signal generator using an optically injected chaotic

semiconductor laser,” Optics letters, vol. 36, no. 6, pp. 1008–1010, 2011.

[23] R. Reddy, T. Srinivas, and A. Nadarajan, “Comparative study between multi-level

modulations in chaotic optical communications,” in 2006 IFIP International Conference on

Wireless and Optical Communications Networks. IEEE, 2006, pp. 5–pp.

[24] S. Z. Ali, M. Islam, and M. Zafrullah, “Effect of transmission fiber on dense wavelength

division multiplexed (dwdm) chaos synchronization,” Optik-International Journal for Light and

Electron Optics, vol.124, no. 12, pp. 1108–1112, 2013.

103

[25] S. Tang and J.-M. Liu, “Effects of message encoding and decoding on synchronized chaotic

optical communications,” IEEE journal of quantum electronics, vol. 39, no. 11, pp. 1468–1475,

2003.

[26] C. Chian-Song, T.-S. Chiang, and L. Peter, “Robust chaotic message masking

communication over noisy channels: The modified chaos approach,” IEICE Transactions on

Fundamentals of Electronics, Communications and Computer Sciences, vol. 89, no. 4, pp. 1092–

1099, 2006.

[27] A. Uchida, Optical communication with chaotic lasers: applications of nonlinear dynamics

and synchronization. John Wiley & Sons, 2012.

[28] E. Okamoto, “A chaos mimo transmission scheme for channel coding and physical-layer

security,” IEICE transactions on communications, vol. 95, no. 4, pp. 1384–1392, 2012.

[29] A. Masaki, “Concept of chaos-based hierarchical network control and its application to

transmission rate control,” IEICE Transactions on Communications, vol. 98, no. 1, pp. 135–144,

2015.

[30] T. Miyano, K. Nishimura, and Y. Yoshida, “Chaos-based communications using open-plus-

closed-loop control,” IEICE transactions on fundamentals of electronics, communications and

computer sciences, vol. 94, no. 1, pp. 282–289, 2011

[31] J. Kawata, Y. Nishio, H. Dedieu, and A. Ushida, “Performance comparison of

communication systems using chaos synchronization,” IEICE TRANSACTIONS on

Fundamentals of Electronics, Communications and Computer Sciences, vol. 82, no. 7, pp. 1322–

1328,1999.

104

[32] S. Z. Ali, M. Islam, and M. Zafrullah, “Comparative analysis of chaotic properties of optical

chaos generators,” Optik-International Journal for Light and Electron Optics, vol. 123, no. 11,

pp. 950–955,2012.

[33] S. Z. Ali, M. K. Islam, and M. Zafrullah, “Effect of parametric variation on generation and

enhancement of chaos in erbium-doped fiber-ring lasers,” Optical Engineering, vol. 49, no. 10,

pp. 105 002–105 002, 2010.

[34] R. Vicente, J. Daudén, P. Colet, and R. Toral, “Analysis and characterization of the

hyperchaos generated by a semiconductor laser subject to a delayed feedback loop.” Institute of

Electrical and Electronics Engineers, 2005

[35] R. Sambaraju, “16-qam photonic vector modulator for gigabit wireless links,” 2013.

[36] Domingues, Maria de Ftima F., and Ayman Radwan. "Optical Fiber Sensors for loT and

Smart Devices." (2017).

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CHAPTER 7

Performance Evaluation of Chaotic QAM-128 Dual Polarization

System for Long Haul Secure Optical Communication

7.1 Introduction

To send high data rate contents/applications over the optical fiber channel, higher order

modulation formats are one of the important techniques along with coherent detection and good

error coding methods. Among the higher order modulation formats QAM modulation scheme is

one of the known and most commonly used modulation formats now-a-days for optical networks.

Due to more number of symbols per second higher order modulation formats are much more

bandwidth efficient than lower order formats. In this chapter performance of 128-QAM

modulation format along with dual polarization is investigated for long haul communication in

chaotic environment. We have done our investigations at data rate of 112 Gb/s and at different

fiber lengths. Simulation analysis helped us to study propagation of secure signal both in time

and frequency domain at the same time. The effects of different transmission impairments such

as attenuation and dispersion are also analyzed and combated in our proposed model. In addition,

designs of 128-QAM transmitter and receiver are also presented in this work.

The demand of higher data rates in optical communication continues to expand at an

exponential rate. In the history of optical fiber communication, new advancements and

technologies capable of handling high bandwidth have always been remained an area of interest

[1]. In an optical communication system, one of the important tasks is to reduce power

consumption so that maximum transmission efficiency could be achieved for which QAM is an

optimum choice [2]. Significant improvements in network capacity and high data rates have been

observed due to some latest research i.e. due to the coherent detection of digital signal,

106

implementation of more sophisticated multidimensional and multilevel advance modulation

formats has become possible, thus opening a new way to further enhance the fiber capacity [3].

As multicarrier modulation format is an efficient way to achieve broadband capabilities and huge

bandwidth. By implementing higher order modulation format such as 128-QAM we can transmit

more bits per symbol i.e. greater points on constellation diagram. QAM is also one of the most

used higher order modulation scheme in other communication systems. For example, QAM is

also being used in radio communication system to increase the efficiency.

QAM modulates data by varying amplitude and phase of a carrier signal/wave (usually a

sinusoidal) in response to a data signal. QAM efficiency is much better than PSK/QPSK

modulation schemes. Few points which make QAM efficient for optical communication are

• Interference due to continuous carrier close to modulation sidebands can be eliminated or

minimized by using QAM.

• Due to greater number of sub-channels, QAM can carry more number of data bits per

symbol.

• QAM is the receiver embodiment and recovery is relatively easy.

• The data transfer rate of a transmission link can be made greater by moving towards

higher order of QAM.

• It is more adaptive towards channel change.

• Digital modulation such as M-ary QAM modulation is very much advantageous in noise

immunity and robustness to channel impairments [4].

• QAM modulation scheme is preferable because it provides larger bandwidth and it

includes two-dimensional multi-level coding which has advantage of larger noise margin

as compared to single dimensional multi-level coding [5].

107

Due to these advantages QAM modulation scheme is commonly used in Digital video

broadcast over cable (DVBC) technology, Wireless LAN systems, Broadband services and in

DOCSIS modem routers. QAM also finds its application in WiMAX communication. For

broadcast communication, 256-QAM and 64-QAM are being used in cable modem applications

and for digital cable television. Implementation of 64-QAM and 16-QAM are very common in

United Kingdom in terrestrial digital TVs employing Digital video broadcasting (DVB) systems.

As per the standards set by SCTE and ANSI (07-2000), 256-QAM and 64-QAM are compulsory

modulation formats for digital television in United States. Other than these applications, some

QAM variants are also part of LTE, GSM and wide-band CDMA technologies.

Another advantage is that QAM coding and decoding circuits are commercially available.

Quadrature amplitude modulation has some major applications digital radio communication

system and data communication networks. It is an attractive method of increasing the spectral

efficiency in optical communications [6]. QAM is the best candidate for the spectrally efficient

modulation format, because we can decrease the power penalty due to the increase in the level of

modulation [7]. A variety of QAM forms are already available for many common applications

and some of the other common forms include 32-QAM, 64-QAM, 128-QAM, and 256-QAM.

128-QAM is a variation of the basic quadrature amplitude modulation (QAM) scheme. In

128-QAM, 128 signal combinations are possible, with each symbol having seven bits (2^7 =

128). The overall outcome of the complex 128-QAM modulation scheme is that the transmission

rate becomes seven times greater than the signaling rate.

As with many digital modulation techniques, constellation diagram is useful tool for

representation [8]. For every possible symbol, it shows the graphical representation of complex

envelop. QAM order can be visualized by number of dots on the constellation, i.e. the number of

108

possible states that may exist. On the constellation diagram the distance between dots represents

the difference among the modulated waveform and related random noise defined by the receiver

ability to differentiate between symbols. QAM can also be considered as combination of PSK

and ASK. Its constellation diagrams are generally composed of multiple rings [9]. QAM

modulation provides a better transmission performance because QAM constellation points are

much more converged than PSK constellation points with larger distances between constellation

points [10]. In this work, we have analyzed the performance of 128-QAM chaotic model

propagating through optical fiber using dual polarization technique. The signal strength and

recovery is made possible by eliminating chaos at receiving side and by using some important

elements in the model such as optical amplifier and DSP module.

7.2 Proposed Model

Fig 7.1 shows the block diagram of 128-DP-QAM chaotic model. This model has QAM

optical transmitter which is connected to preamplifier. Preamplifier boosts the transmitting signal

so to reduce the effect of attenuation in the channel to provide a readable signal at the receiver

end. Amplifier is then connected to single mode fiber which is the signal propagating medium.

Standard single-mode fiber has dispersion value of 16.75 ps/(nm.km), optical fiber PMD is taken

as 0.2 ps/km1/2 and attenuation loss is set to 0.2 dB/km [11]. The non-linear Kerr effect in optical

fiber restricts the capacity of optical fiber links, resulting a drop in highest achievable data rate

when we increase the transmitter power too much [12]. So, another amplifier is placed just after

DCF to limit the transmitter power. After amplifier, filter is applied to cancel out unwanted

signals in the form of noise. At the receiving side, filter is connected to optical receiver, from

where the signal is fed into universal DSP module. DSP module is responsible for performing

signal processing on signal before sending it to the decision polarization circuit. After decision

109

polarization circuit, QAM demodulation is applied on the signal so to recover the original signal.

Semiconductor chaotic lasers are used both at transmitting and receiving side to hide and recover

the signal respectively. Parallel to serial converter can be used in the end to get original bits in

serial form.

Transmitter

Amplifier

ReceiverFilter

Amplifier

QAM

QAM

Parallel to

Serial

Converter

BER Test

Set

Universal

DSP

Decision

Polarization

Type = Dual

SMF DCF

Chaotic

Laser (T)

Chaotic

Laser (T)

Fig. 7. 1 128-QAM Chaotic Communication Model (Dual polarization)

Fig. 7. 2 Transmitter Design

110

Fig. 7.2 shows the design of 128-QAM optical transmitter. It consists of following parts

• CW laser

• LiNB Mach-Zehnder modulator

• M-ary pulse generator

• Polarization splitter

• Polarization combiner

Fig 7.3 shows the design of 128-QAM receiver. Major components of receiver are

• Photodetector

• Polarization Splitter

• Amplifier

• Phase shifter

• Electrical subtractor

CW

Laser

Polarization Split ter

Optical Null

Phase Shif t

Phase Shif t

Photodetector PIN

Electrical Subtractor

Electrical Amplifier

Optical Null

Optical Null

Optical Null

Optical Null

Optical Null

Electrical Subtractor

Electrical Subtractor

Electrical Subtractor

Electrical Amplifier

Electrical Amplifier

Electrical Amplifier

Photodetector PIN

Photodetector PIN

Photodetector PIN

Fig.7.3 Receiver Design

111

The transmitter applies the discretionary pulse shaping and up-conversion on the QAM

symbols converted from the bits. Among number of digital modulation formats, QAM format

helps to be visualized through constellation diagrams. Complex digital modulation schemes can

be best seen through a scattered constellation diagram. Scatter diagram permits visualizing the

real (in-phase) and imaginary (quadrature) components of a complex signal individually.

In the design of optical communication system, pulse shaping is another fundamental aspect.

The filter used for pulse shaping should be bandwidth efficient and must be duration limited on

time scale. Pulses wider in time domain will overlap other pulses resulting in ISI. This filtering

or pulse shaping can be done by using Gaussian filter, Root-raised cosine (RRC) filter or Bessel

filter. Further, the eye diagram allows understanding the time domain properties of signal in

addition to its susceptibility against timing errors of symbols.

In transmitter design, serial to parallel converter is an important component that is linked

with QAM sequence generator. It converts serial data into parallel form so that it can be given to

M-ary pulse generator. This pulse generator is responsible to convert parallel data into QAM

electrical pulses. Electrical gain is then applied on these pulses before entering in the LiNb

Mach-Zehnder modulator. The function of modulator is to combine/modulate the QAM electrical

pulses with the optical signal generated by CW laser. The complexity of QAM receiver is much

greater than the complexity of transmitter. It performs reverse function of the transmitter. Its

function is to match the detected pulses with the original pulses as closely as possible.

There are so many factors which can come in contact to deteriorate the original signal at the

receiver side. Noise addition by the electronic circuitry of receiver and the multipath effects of

reflection, diffraction and scattering plays an important role in deteriorating the signal at receiver

end. Therefore, the simulated and actual results can never be the same unless these factors are

112

accounted for [13]. In the proposed communication model simulations are carried out at 112

Gbits/s. Transmission above 100 Gbits/s has a few challenges in the form of attenuation and

noises created by nonlinearities. Appropriate amplifier gain and laser power is required to meet

the challenges.

While performing simulations, the sequence length is taken as 65536 bits in the global

parameters. We are using 4 samples per bit, so the total number of samples is 262144. Total

number of guard bits is set as 10. For optical filtering, Gaussian filter is used with frequency

centered at 1550 nm. Its value is set according to the different values of signal at different fiber

lengths. DSP block which is used for signal processing is operated in dual polarization mode.

DSP module is configured according to 128-QAM signal. Constellation type is set as square for

the QAM signal.

Decision circuit is another important component in this scheme. The purpose of decision

circuit is to measure a probable value of signal and make an output signal decision depending on

the value of input signal and predetermined criteria.

7.3 Mathematical Model:

As QAM has both phase and amplitude modulation. Mathematically, M-ary QAM signal can

be defined as [14];

𝑠(𝑖)𝑡 = √2𝐸𝑚𝑖𝑛

𝑇𝑠𝑎𝑖 cos(2𝜋𝑓𝑐𝑡) + √

2𝐸𝑚𝑖𝑛

𝑇𝑠𝑏𝑖 cos(2𝜋𝑓𝑐𝑡) , 0 ≤ 𝑡 ≤ 𝑇, (7.1)

whereas, 𝑖 = 1,2, … … 𝑀

′𝐸𝑚𝑖𝑛′ represents signal energy with lowest amplitude and 𝑎𝑖 , 𝑏𝑖 shows the set of independent

integers as per position of specific signal point.

′𝑓𝑐′ is carrier’s frequency.

113

′ 𝑇𝑠′ represents period of symbol.

At value M > 4, least Euclidean distance of signal modulated with M-QAM is greater than

M-PSK, M-ASK and many other modulation schemes. At M = 2 or 4, Euclidean distance of M-

QAM is same as that of Euclidean distance of M-PSK signals.

Mathematically, any M-QAM signal can be written by using following equation:

𝑠𝑚𝑛(𝑡) = 𝐴𝑚 cos(2𝜋𝑓𝑐 + 𝜃𝑛) 𝑚 = 1,2, … , 𝑀1 & 𝑛 = 1,2, … , 𝑀2 (7.2)

Simultaneous transmission of log2M1M2 bits/symbol is resulted due to the combined

modulation of phase and amplitude.

In an M-QAM system,

(i) Points will become more and more closer to each other as we increase the value of M. In this

case the distance between vector points is reduced which is more susceptible to errors due to

cross talk.

(ii) Initially, the original data is in digital form and represented by binary values. To generate the

M-ary QAM signal, binary signals must be first converted into M-signals and then quadrature

modulation can be applied.

7.4 Simulations and Results:

In this section, we describe the parameters used in our simulations and corresponding

results observed during transmission. Analysis is made by taking the signal plots in time domain

and frequency domain both. Signal deterioration is observed by using the constellation diagrams.

Proposed system is tested at three different fiber lengths. Different system impairments show

their effect on the transmitted signal which are mitigated by applying different techniques. A

comparison is made before sending and after receiving signal from the channel to demonstrate

114

the minimization of the unwanted effects on the signal by the adopted techniques. Finally, the

system is optimized at 100 km of fiber length.

System parameters used at different fiber lengths are given in Table 7.1 whereas Table 7.2

and Table 7.3 shows the parameters and values of chaotic laser and current source respectively.

Table 7. 1 System parameters

Parameters Values

SMF Lengths 100 km,160 km,220 km

DCF Lengths 20 km, 32 km, 44 km

DCF Values -83.75 ps/nm/km

CW Laser (Power) 10 dBm

CW Laser (Wavelength) 1550 nm

Amplifier Gain 20 dB, 32 dB, 44 dB

Gaussian Optical Filter

(Bandwidth) 10 – 35 GHz

Gaussian Optical Filter

(Centered Frequency) 1550 nm

Modulation Type 128-QAM

Polarization Type Dual

Table 7. 2 Chaotic laser parameters

Parameter Values

Power 10 dBm

Power at Bias Current 0 dbm

Bias Current 38 mA

Threshold Power 0.154 mW

Modulation Peak Current 28 mA

Threshold Current 33.46 mA

115

Table 7. 3 Current source parameters

Parameter Values

Frequency 5 GHz

Amplitude 1 a.u

Phase 90 degrees

Bias 0 a.u

Optical spectrum of 112 Gbit/s signal at transmitter side can be seen in Fig. 7.4. The

spectrum is centered at 1550 nm frequency with peak power of -45 dBm. Time domain plot of

same signal before mixing it with chaotic signal can be observed in Fig. 7.5. Square pulses show

the null effect of channel impairments as signal is at the start of fiber length. When the signal

passes through the channel it deteriorates due to linear & nonlinear effects. Post and preamplifier

used in our proposed schemes are used to recover the lost strength of signal.

Fig. 7.4 Optical spectrum of 128-QAM signal at Tx

116

Fig. 7.5 Time domain plot of 128-QAM signal at Tx

Fig. 7.6 Chaos produced by semiconductor laser

117

Fig. 7.7 Chaotic waveform hiding message

Chaos produced by semiconductor laser is given in Fig.7.6. The amplitude of this chaos is

high enough to hide the message given in Fig 7.5. After mixing of original message with chaos,

resultant pulses in time domain can be seen in Fig 7.7. Fig 7.7 shows total mess and has

apparently no information in it. Same is evident by the constellation diagram of signal obtained

after mixing original signal with chaos as given in Fig 7.8.

Fig. 7.8 Constellation diagram of message embedding chaos

118

Fig. 7.9 Signal after eliminating chaos at Rx side

Secure 128-QAM message in the form of chaos is propagated throughout the length of the

fiber. However, at the receiving side, another chaotic semiconductor laser chaos operating at

same parameters and timing (as that of transmitting side) is used to subtract chaos from original

message. Fig. 7.9 and Fig 7.10 are the recovered signal in time domain and frequency domain

respectively.

Recovered message can also been seen in comparison with transmitted message in Fig 7.11.

Fig. 7.10 Optical spectrum of received 128-QAM signal

119

Fig. 7.11 Transmitted message vs received message

Constellation diagram of 128-QAM dual polarization signal can be seen in Fig. 7.12. The

diagram has two parts. The first constellation of diagram presents the x-polarization state of the

signal whereas second constellation diagram is the y-polarization state of the signal. As the

signal is just produced by the transmitter and neither traversed through fiber nor mixed with

chaos, so all the 128-points on the constellation diagram are extremely converged showing the

null effect of transmission impairments and chaos.

120

a) b)

Fig. 7.12 Constellation diagram of 128-QAM signal at Tx side

a) x-polarization state b) y-polarization state

Constellation diagram of 128-QAM signal after passing 100 km of fiber length is shown in

Fig. 7.13. At 100 km the signal received by the receiver is not very much distorted when

compared to original transmitted signal. This is because distortion caused mostly due to linear

impairments, is controlled by setting appropriate gain of amplifier & taking suitable DCF length.

Single mode fiber has 0.2 dB/km loss and to neutralize this loss, optical amplifier having 20 dB

gain is used. Similarly, as dispersion value of single mode fiber is 16.75 ps/nm/km [15] so 20 km

of DCF is used by taking value of dispersion as -83.75 ps/nm/km. These measures resulted in

better and clear constellation diagram at receiver side. Without taking appropriate measures the

constellation diagram at same fiber length can be seen in Fig. 7.14

121

Fig. 7.13 Constellation diagram of 128-QAM signal at 100 km (after controlling dispersion)

a) x-polarization state b) y-polarization state

a) b)

Fig. 7.14 Constellation diagram of 128-QAM signal at 100 km (without controlling dispersion)

a) x-polarization state b) y-polarization state

At 160 km, fiber losses act significantly, and nonlinear fiber impairments come into

contact which results in a poor-quality signal at the receiver side. Although the linear

impairments are controlled by increasing the gain of amplifier from 20 dB to 32 dB and

increasing the DCF length from 20 km to 32 km but as the amplifier gain is increased it resulted

in an additional non-linear effect in the amplifier i.e. Kerr effect. Fig. 7.15 is the constellation

diagram at 160 km of fiber length.

122

a) b)

Fig. 7.15 Constellation diagram of 128-QAM signal at 160 km (without controlling dispersion )

a) x-polarization state b) y-polarization state

On further extending the distance upto 220 km, constellation diagram clearly shows that the

signal becomes so dispersive that it is not possible to reconstruct the original signal at the

receiving side. Due to further increase in Kerr effect at 44 dB the behavior of amplifier became

more nonlinear resulting in total noise like signal. Results can be seen in Fig 7.16.

a) b)

Fig. 7.16 Constellation diagram of 128-QAM signal after 220 km (without controlling dispersion)

a) x-polarization state b) y-polarization state

123

The performance of chaotic dual polarization 128-QAM communication system is also

measured by calculating BER, Bit errors, Symbol errors and Error magnitude value as given in

Table 7.4. Indicators clearly shows that as the fiber length increase from 160 km to 220 km, non-

linear impairments badly effect the dual polarization 128-QAM chaotic model causing lot of

errors in both the polarization states. It is also concluded from the results that one polarization

state which is x-polarization state in our case is affected more than the other polarization state i.e.

y-polarization state. Plots of Bit Errors, EVM values, BER & log of BER can be seen in Fig.

7.17, Fig 7.18, Fig 7.19 & Fig 7.20 respectively.

Table 7. 4 System performance indicators

Fiber Lengths

Bit Errors

BER log of BER

Bit Errors (X Polarization)

Bit Errors (Y Polarization)

No of Symbol Errors

No of Symbol

Errors (X)

No of Symbol

Errors (Y)

EVM %

100 km 0 0 -1000 0 0 0 0 0 2.39

160 km 55 0.000839 -3.075 50 5 12 11 1 4.43

220 km 11101 0.1694 -0.77 9158 1943 3229 2745 484 7.4

0 55

11101

0

2000

4000

6000

8000

10000

12000

100 Km 160 Km 220 Km

Bit Errors

Fig. 7.17 Bit errors at different fiber

lengths

124

Fig. 7.18 EVM values at different fiber lengths

Fig. 7.19 BER values at different fiber lengths

Fig. 7.20 Log of BER at different fiber lengths

2.39

4.43

7.4

0

1

2

3

4

5

6

7

8

100 Km 160 Km 220 Km

EVM %

0 0.000839

0.1694

0

0.05

0.1

0.15

0.2

100 Km 160 Km 220 Km

BER

-1200

-1000

-800

-600

-400

-200

0

100 Km 160 Km 220 Km

log of BER

125

7.5 Conclusion:

A long haul secure optical communication model is proposed by using dual polarization

128-QAM format. Simulations are made at data rate of 112 Gb/sec. QAM transmitter and

receiver design is presented for generation and reception of 128-QAM high data rate signal.

Chaos produced by semiconductor lasers is used to hide the message. Performance of secure

128-QAM signal is tested at three different fiber lengths i.e. 100 km, 160 km and 220 km by

calculating BER, EVM & Symbol error rate. 128-QAM model is optimized by controlling the

linear impairments and setting the appropriate gain of amplifier for 100 km of fiber length.

126

7.6 References:

[1] Sano, Akihide, et al. "102.3-Tb/s (224× 548-Gb/s) C-and extended L-band all-Raman

transmission over 240 km using PDM-64QAM single carrier FDM with digital pilot

tone." Optical Fiber Communication Conference and Exposition (OFC/NFOEC), 2012

and the National Fiber Optic Engineers Conference. IEEE, 2012.

[2] Lee, Jaeyoon, Dongweon Yoon, and Kyongkuk Cho. "Error Performance Analysis of

M-ary θ-QAM." IEEE Transactions on Vehicular Technology 61.3 (2012): 1423-1427.

[3] Zhou, Xiang, et al. "Transmission of 32-Tb/s capacity over 580 km using RZ-shaped

PDM-8QAM modulation format and cascaded multimodulus blind equalization

algorithm." Journal of Lightwave Technology 28.4 (2010): 456-465.

[4] Haque, Md, MdRashed, and M. HasnatKabir. "A comprehensive study and

performance comparison of M-ary modulation schemes for an efficient wireless mobile

communication system." arXiv preprint arXiv: 1203.1778 (2012).

[5] Youssef, Tamer, and Eman Abdelfattah. "Performance evaluation of different QAM

techniques using Matlab/Simulink." Systems, Applications and Technology Conference

(LISAT), 2013 IEEE Long Island. IEEE, 2013.

[6] Khallaf, Haitham S., et al. "Performance Analysis of a Hybrid QAM-MPPM

Technique Over Turbulence-Free and Gamma–Gamma Free-Space Optical

Channels." Journal of Optical Communications and Networking 9.2 (2017): 161-171.

Design & Analysis of Secure Optical Communication System with Advance Modulation Formats

127

[7] Mori, Yojiro, et al. "Unrepeated 200-km transmission of 40-Gbit/s 16-QAM signals

using digital coherent receiver." Optics Express 17.3 (2009): 1435-1441.

[8] Shashidharan, Sreenesh, and JincyJohny. "Design and simulation of a 16 QAM radio

over fiber link." 2015 International Conference on Electrical, Electronics, Signals,

Communication and Optimization (EESCO). 2015.

[9] Winzer, P. J., et al. "Spectrally efficient long-haul optical networking using 112-Gb/s

polarization-multiplexed 16-QAM." Journal of lightwave technology 28.4 (2010): 547-

556.

[10] Wang, Jingjing, et al. "Capacity of 60 GHz wireless communications based on

QAM." Journal of Applied Mathematics 2014 (2014).

[11] Jiang, Wen, et al. "Blind and Simultaneous Polarization and Phase Recovery for

Time Domain Hybrid QAM Signals Based on Extended Kalman Filtering." Asia

Communications and Photonics Conference. Optical Society of America, 2015.

.

[12] Essiambre, René-Jean, et al. "Capacity limits of optical fiber networks." Journal of

Lightwave Technology 28.4 (2010): 662-701.

[13] Riche, Larry. "The performance of high-order quadrature amplitude modulation

schemes for broadband wireless communication systems." (2012).

[14] Surekha, T. P., et al. "Modeling and Performance Analysis of QAM

System." International Conference on Network Security and Applications. Springer

Berlin Heidelberg, 2011.

Design & Analysis of Secure Optical Communication System with Advance Modulation Formats

128

[15] Vujicic, Z., N. B. Pavlovic, and A. Teixeira. "Optical filtering optimization for NRZ

coding format in RSOA-based DWDM PON." Transparent Optical Networks (ICTON),

2012 14th International Conference on. IEEE, 2012.

Design & Analysis of Secure Optical Communication System with Advance Modulation Formats

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CHAPTER 8

Conclusion

This work is based on performance analysis and security implementation of optical

communication system based on advance modulation formats. Thesis begins with the

study of advance modulation formats and security implementation through chaos

generation by semiconductor lasers. Different parameters of semiconductor lasers and

pumping/current source are tested to analyze the effects on chaos generation. A detailed

comparison is performed on the performance of intensity-based modulation formats and

their variants i.e. compressed signal formats. A secure optical communication system is

then implemented using Duobinary modulation format. Optical chaos generated by the

semiconductor laser is used to hide Duobinary message. In the next phase, QAM

modulation format is selected to implement security through optical chaos which includes

both phase and amplitude in contrast to Duobinary scheme which involves only

amplitude of signal. A complete end to end communication model is proposed for 4-

QAM and then results are obtained for 16-QAM and 64-QAM model using the same

approach. Finally, the work is extended to dual polarization 128-QAM model so to

ensure security and achieve more data rates at the same time.

This chapter includes the accomplishments and future directions in the field of chaos and

advance modulation formats.

8.1 Summary of Accomplishments

1. Simulations are made to generate high bandwidth chaos through semiconductor

laser. Different laser and current source parameters are tested for this purpose.

Design & Analysis of Secure Optical Communication System with Advance Modulation Formats

130

Parameters such as modulation peak current, bias current and frequency of current

source are optimized to find route from quasi-periodic to chaos. Chaotic pulses of

acceptable amplitude are produced so that to use them for long distance

communication and to hide message modulated with advance formats.

2. Detailed comparison is performed between different intensity modulation formats.

In this comparison, non-chirped-NRZ, VSB-NRZ and chirped-NRZ are selected

initially for comparison based on their Q-factor values & BER. Simulations are

made by implementing these schemes on passive optical networks. Results proved

that chirped-NRZ outperforms all the other NRZ modulation formats.

3. Further, a detailed comparison of different RZ modulation formats like RZ-33 %,

RZ-50 %, RZ-67 %, Chirped-RZ and VSB-CSRZ is performed, results show that

RZ-50 % is best among all the proposed RZ modulation formats. In the next step

RZ-50 %, Chirped NRZ, DB and AMI are analyzed at 10 Gbps of data rate by

varying fiber lengths, results show that chirped-NRZ performs best in terms of

low BER and higher quality factor. Earlier works didn’t study the performance

comparison of these compressed modulation formats in detail for passive optical

networks.

4. At next stage duobinary modulation format is selected due to its multilevel and

increased spectral efficiency for long haul communication model. Security is

implemented by producing chaos through semiconductor lasers. Performance of

RZ & NRZ-duobinary format is measured in chaotic environment for the first

time. Impact of linear and nonlinear impairments on secure signal propagation is

also investigated in this model. Results shows penalty in terms of Q-factor while

Design & Analysis of Secure Optical Communication System with Advance Modulation Formats

131

implementing security in duo-binary communication model. NRZ-duobinary

format shows better performance in chaotic environment as compared to RZ-

duobinary format.

5. At next level, long haul communication model is tested with QAM modulation

performance and chaos masking technique is applied to analyze the results for

long haul secure communication. Results proved that laser power need to be

increased (upto 20 dBm) alongwith distance to keep the value of BER constant.

The secure model is designed for 4-QAM which is then extended to higher order

QAM i.e. 16-QAM and 64-QAM. Results shown that till 80 Km of fiber length,

signal can be read without deploying specialized DSP units.

6. Long distance secure communication model based on dual polarization 128-QAM

modulation scheme is also proposed. Simulations are made to carry 112 Gb/s of

data rate. Propagation of signal is analyzed for different lengths of fiber.

Optimization of this model is done by controlling the linear impairments and

amplifier nonlinearities. DSP unit is used to increase the data rates and fiber

length upto 220 km in chaotic environment.

7. Transmitter and receiver designs are presented for dual polarization 128-QAM

optical communication model to generate and receive high data rate message

respectively.

8. Amplifier gain is optimized for all the proposed chaotic optical communication

model to avoid the Kerr effect.

9. As advance modulation formats are much prone to noises, appropriate filtering

which is key to eliminate errors, is done to achieve the desired results.

Design & Analysis of Secure Optical Communication System with Advance Modulation Formats

132

10. A step by step approach can be seen in all the proposed scheme to study and

analyze the effect of chaos on optical communication system implemented with

advance modulation formats.

8.2 Future Work

1. Future work will include security implementation of higher order QAM formats

i.e. QAM-256 and QAM-512.

2. Still no literature is reported on study of propagation of OFDM signal mixed with

optical chaos.

3. New methods of chaos generation need to be discovered to support high data rate

and complex waveform of advance modulation formats.

4. The photodetector and signal processing required to recover the original

waveform of advance modulation format once subtracted from chaos at receiver

side needs further improvements.

5. FSO systems implemented with advance modulation formats still need to be

explored in terms of security either implemented via optical chaos or in some

other ways.

Design & Analysis of Secure Optical Communication System with Advance Modulation Formats

133

ABBREVIATIONS

ACM: Additive Chaos Modulation

CSK: Chaos Shift Keying

CMS: Chaos Message Masking

QAM: Quadrature Amplitude Modulation

DWDM: Dense Wavelength Division Multiplexing

SOA: Semiconductor Operational Amplifier

SMF: Single Mode Fiber

MMF: Multimode Fiber

EDFA: Erbium Doped Fiber Amplifier

DCF: Dispersion Compensation Fiber

MZM: MachZander Modulator

LiNb : Lithium Niobate

CW: Continuous Wave

LPF: Low Pass Filter

Q-factor: Quality Factor

BER: Bit Error Rate

OSNR: Optical Signal to Noise Ratio

RZ: Return to Zero

NRZ: Non Return to Zero

DFB: Distributed Feedback Laser

VCSEL: Vertical Cavity Surface Emitting Laser

PAM: Pulse Amplitude Modulation

Design & Analysis of Secure Optical Communication System with Advance Modulation Formats

134

OFDM: Orthogonal Frequency Division Multiplexing

SNR: Signal to Noise Ratio

DoS: Denial of Service

DM: Duo-binary Modulation

PSK: Phase Shift Keying

IM: Intensity Modulation

DSP: Digital Signal Processing

DP: Dual Polarization

DD: Direct Detection

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LIST OF PUBLICATIONS

• Farhan Qamar, Muhammad Khawar Islam, Syed Zafar Ali Shah, Romana Farhan,

and Mudassar Ali. "Secure Duobinary Signal Transmission in Optical

Communication Networks for High Performance & Reliability." IEEE Access 5

(2017): 17795-17802. (Published)

• Farhan Qamar, Muhammad Khawar Islam, Romana Farhan, Mudassar Ali, Syed

Zafar Ali, and Asim Shahzad. "DWDM system analysis by varying different

erbium doped fiber parameters." In Engineering and Emerging Technologies

(ICEET), 2018 International Conference on, pp. 1-4. IEEE, 2018. (Published)

• Ambreen Niaz, Farhan Qamar, Mudassar Ali, Romana Farhan, and Muhammad

Khawar Islam. "Performance analysis of chaotic FSO communication system

under different weather conditions. "Transactions on Emerging

Telecommunications Technologies: e3486. (Published)

• Rashid, M. F., Farhan Qamar, F. Rashid, and S. Ahmad. "Implementation of

OFDM-RoF at 60 GHz with DCF for Long Haul Communication." The Nucleus

53, no. 3 (2016): 195-199. (Published)

• Performance Analysis and Comparison of QPSK and DP-QPSK Based Optical

Fiber Communication Systems. Ambreen Niaz, Farhan Qamar, Khawar Islam,

Asim Shahzad, Romana Shahzadi, Mudassar Ali. ITEE journal, Vol 7, No. 9,

2018. (Published)

• Secure Optical QAM Transmission Using Chaos Message Masking. Farhan

Qamar, Muhammad Khawar Islam. (Submitted)

Design & Analysis of Secure Optical Communication System with Advance Modulation Formats

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• Performance Analysis of Modulation Formats for Next Generation Passive

Optical Networks, Farhan Qamar, Umair, Muhammad Khawar Islam.

(Submitted)

• Optimization of Gain Flattening Filter to Achieve Flat Gain of EDFA for DWDM

Chaotic Communication. Farhan Qamar, Saleha Maqsood, Muhammad Khawar

Islam. (Submitted)