DISPERSION COMPENSATED LONG- HAUL OPTICAL ...

DISPERSION COMPENSATED LONG-

HAUL OPTICAL COMMUNICATION

SYSTEM EMPLOYING MODIFIED SLPCF

AND EDFA

MD. AZIZUL HAKIM Student ID: 15202035-P

DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING

DHAKA UNIVERSITY OF ENGINEERING & TECHNOLOGY, GAZIPUR

June, 2021

iii

Declaration

It is hereby declared that this thesis or any part of it has not been submitted

elsewhere for the award of any degree or diploma.

Signature of the candidate

------------------------------

Md. Azizul Hakim

(Student ID. 15202035-P)

iv

ACKNOWLEDGEMENT

At first, I would like to express my gratitude to almighty. Then, I would

like to express my sincere gratitude to my thesis supervisor, Prof. Dr. Md.

Shaheen Hasan Chowdhury and I am grateful to Prof. Dr. E J Zinat Mahol

Sathi, for her help, support and guidelines. Their continuous encouragement

with extreme enthusiasm toward research has motivated me during my entire

research life. I am eternally grateful for the things both academic and

nonacademic which I have learnt from my supervisors. I will always remember

the countless hours we spent together discussing the research work with ideas.

I am indebted to Prof. Dr. Ruma, Head, Department of Electrical and

Electronic Engineering, DUET, for her caring and guidelines. My sincere

gratitude and thanks to Prof. Dr. Masuma Akter and Prof. Dr. Mohammad

Jakir Hossain for their valuable feedback and comments throughout the

research period. It is my pleasure to thank Dr. Arifur Rahman for his

continuous inspiration and comments regarding this research. I wish to thank

the member of my thesis committee Prof. Dr. Pran Kanai Saha for his

invaluable feedback on my work. I am obliged to Prof. Dr. Md. Sharafat

Hossain for his motivation and suggestions.

Finally, thanks to all the faculty and staff of the EEE department for

their help and motivation throughout the research.

At last, I would like to thank my father and my mother for their

continuous support, love and positive attitude towards my research life.

v

ABSTRACT

In this thesis, photonic crystal fibre (PCF) is designed and assessed as dispersion

compensating fibre (DCF) to compensate dispersion. The PCF, termed as modified square

lattice photonic crystal fibre (SLPCF), is designed with square in outer shape having

circular and elliptical air holes, and is proposed as a new approach of dispersion

compensation. Here, various PCF structures are designed and modelled by varying the

shape and lattice type of air holes in the core and cladding by using COMSOL

Multiphysics. Spectral characteristics such as effective mode index, mode field

distribution, effective refractive index and light propagation through various PCFs of

square, circular and elliptical lattice structures are analyzed. The effective mode index

(EMI), responsible for improving light confinement within the fibre, was not very high in

earlier work resulting in a higher confinement loss. Therefore, the term modified SLPCF

remarks square core within the square lattice consisting of circular and elliptical air holes

and compared this one with other PCF structures to find suitable DCF with improved EMI.

The optical characteristics of these PCFs are evaluated and compared at 1.55 μm (or

193.414 THz).

Besides dispersion compensation, reduction of bit error rate (BER) is essential in

communication link since higher BER gives a slower data rate that degrades the quality of

data link. Radio over fibre (RoF) link at different data rates is used for information

transmission in single mode fibre where signal weakens during transmission and needs to

be amplified. In this research, a RoF link is designed with modified square lattice PCF and

erbium doped fibre amplifier (EDFA) and evaluated its performance by varying the pump

power of the continuous wave (CW) laser that is used to excite the EDFA. Here, EDFA is

used as an in-line optical amplifier to boost up the weak optical signal in the proposed RoF

link.

It is observed that the proposed RoF link shows remarkable lower dispersion as -

0.90E-14 for modified SLPCF where as it is found in previous -0.5E-14 (Circular Core), -

0.47E-14 (Elliptical Core), 30.76 (Square Core), and BER, sharper eye diagram and both

the improved Q-factor and eye height. Therefore, the proposed model for PCF and RoF

link can be used for designing cost-effective long-haul optical communication system.

vi

LIST OF ABBREVIATIONS

AM Amplitude Modulation

ARQ Automatic Repeat Request

AMB Access of Mobile Broadband

BER Bit Error Rate

CW Continuous Wave

CPCF Chalcogenide Photonic Crystal Fibre

CD Chromatic Dispersion

CN Cellular Networks

DCF Dispersion Compensating Fibre

EDF Erbium Doped Fibre

ECC Error Correcting Code

EDFA Erbium Doped Fibre Amplifier

EMI Effective Mode Index

FDTD Finite Difference Time Domain Method

FBG Fibre Bragg Grating

FWM Four Wave Mixing

FEC Forward Error Correction

FM Frequency Modulation

FEM Finite Element Method

HNDSF Highly Nonlinear Dispersion Shifted Fibre

HARQ Hybrid Schemes Combining ARQ and ECC

LASER Light Amplification by Stimulated Emission of Radiation

LEAF Large Effective Area Fibre

MRI Magnetic Resonance Imaging

MIMO Multiple-Input Multiple-Output

NA Numerical Aperture

NRZ Non Return to Zero

OA Optical Attenuator

OSA Optical Spectrum Analyzer

vii

OPM Optical Power Meter

OPG Optical Phase Conjugator

PCF Photonic Crystal Fibre

PRBS Pseudo-Random Bit Sequence

PWE Plane Wave Expansion Method

PCM Phase Conjugate Mirror.

PBG Photonic Bandgap

RA Raman Amplifier

RF Radio Frequency

RE Rare Earthed

RADAR Radio Detection and Ranging

RoF Radio over Fibre

SLPCF Square Lattice Photonic Crystal Fibre

SMF Single Mode Fibre

SONAR Sound Navigation and Ranging

SC Satellite Control

SC Square in Circular Core

SS Square in Square Core

SE Square Elliptical Core

TIR Total Internal Reflection

VDS Video Distribution Systems

VC Vehicle Communication

WLAN Wireless Local Area Network

WDM Wavelength Division Multiplexing

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LIST OF FIGURES

Figure 2.1 Basic block diagram of a simple radio over fibre (RoF) link. ........................... 7

Figure 2.2 Chromatic dispersion in an optical fibre communication system. .................... 9

Figure 2.3 Dispersion compensation by means of using DCF to have zero dispersion.... 10

Figure 2.4 Optical communication system using EDFA. ................................................. 11

Figure 2.5 Pump radiation with respect to three energy levels for EDFA. ....................... 11

Figure 3.1 Use of PCF in sensing application................................................................... 15

Figure 3.2 Photonic crystal fibre fabrication. ................................................................... 16

Figure 3.3 Microscope picture of birefringent PCF at 1.55 µm. ...................................... 17

Figure 3.4 Schematic of (a) solid core PCF, (b) hollow core PCF, and (c) SMF. ............ 18

Figure 3.5 Light gathering capacity of PCF. ..................................................................... 20

Figure 3.6 Yee cell. ........................................................................................................... 21

Figure 3.7 A fabricated hollow-core Hexagonal cladding PCF. ....................................... 22

Figure 3.8 Triangular lattice photonic crystal fibre. ......................................................... 23

Figure 3.9 A typical elliptical PCF with hexagonal core. ................................................. 25

Figure 3.10 Modeling outlines of a typical square in square PCF with hollow core. ....... 27

Figure 4.1 COMSOL modeling Flowchart. ...................................................................... 29

Figure 4.2 Space dimension selection. .............................................................................. 30

Figure 4.3 Physics Selection. ............................................................................................ 30

Figure 4.4 Study selection................................................................................................. 31

Figure 4.5 Creating a new model using Blank Model. ..................................................... 31

Figure 4.6 Select the geometry. ........................................................................................ 32

Figure 4.7 Materials selection. .......................................................................................... 33

Figure 4.8 Mesh Analysis. ................................................................................................ 34

Figure 4.9 Preform the study. ........................................................................................... 34

Figure 4.10 Effective mode index of Circular PCF. ......................................................... 37

Figure 4.11 Effective mode index of Elliptical PCF......................................................... 38

Figure 4.12 Effective mode index of SLPCF.................................................................... 39

Figure 4.13 Cross section of proposed SLPCF. ................................................................ 40

Figure 4.14 Circular shape PCF. ....................................................................................... 41

Figure 4.15 Light Propagation through SLPCF. ............................................................... 42

ix

Figure 4.16 Cross sectional view of light propagation through modified SLPCF............ 42

Figure 4.17 Light propagation through an elliptical core PCF. ........................................ 44

Figure 4.18 Light propagation inside square core PCF. ................................................... 45

Figure 4.19 Light propagation inside circular core PCF. .................................................. 46

Figure 4.20 Total Internal Reflection (TIR). .................................................................... 47

Figure 4.21 Total Internal Reflection inside SLPCF. ....................................................... 48

Figure 5.1 Model of a typical Elliptical core PCF with circular air hole only. ................ 49

Figure 5.2 Model of a typical circular core PCF with circular air hole only. ................... 52

Figure 5.3 Model of a typical square core PCF (SLPCF) with circular air hole only. ..... 54

Figure 5.4 Modified SLPCF. ............................................................................................ 56

Figure 5.5 Effective mode index for circular core PCF with circular air hole only. ........ 56

Figure 5.6 Effective mode index for elliptical core PCF with circular air hole only. ...... 57

Figure 5.7 EMI for square core PCF with both circular and elliptical air hole. ............... 58

Figure 5.8 Comparison of EMI for various PCFs. ............................................................ 58

Figure 5.9 BER characteristics. ........................................................................................ 60

Figure 5.10 Dispersion of various PCF structures. ........................................................... 61

Figure 5.11 Dispersion of various PCFs structures .......................................................... 62

Figure 5.12 Effective Area of various PCF structures ...................................................... 62

Figure 5.13 Summarized effective Area of various PCF structures ................................. 63

Figure 5.14 Nonlinearity, γ of various PCF structures. .................................................... 64

Figure 5.15 Summarized nonlinearity, γ of various PCF structures ................................. 64

Figure 5.16 Flow chart of the RoF link analysis. .............................................................. 66

Figure 5.17 Flowchart for RoF link analysis using OptiSystem. ...................................... 68

Figure 5.18 Proposed Model for improved RoF link. ....................................................... 69

Figure 5.19 Simulation model using conventional SMF. ................................................. 70

Figure 5.20 Optical spectra at 1 dBm ............................................................................... 70

Figure 5.21 Optical spectra at 1dBm ................................................................................ 71

Figure 5.22 Optical spectra at 10 dBm ............................................................................. 71

Figure 5.23 Optical spectra at 10dBm .............................................................................. 72

Figure 5.24 Optical spectra at 15 dBm ............................................................................. 72

Figure 5.25 Optical spectra at 15dBm .............................................................................. 73

x

Figure 5.26 Q-factor for conventional SMF. .................................................................... 74

Figure 5.27 Eye pattern for conventional SMF. ............................................................... 74

Figure 5.28 Q- factor and BER pattern for previously used SLPCF. ............................... 75

Figure 5.29 Q-factor for 1dBm CW laser input power ..................................................... 75

Figure 5.30 Eye pattern for 1dBm CW laser input power. ............................................... 76

Figure 5.31 Q-factor for 10dBm CW laser input power. .................................................. 76

Figure 5.32 Eye pattern for 10dBm CW laser input power. ............................................. 77

Figure 5.33 Q- factor for 15dBm CW laser input power. ................................................. 77

Figure 5.34 Eye pattern for 15dBm CW laser input power. ............................................. 78

xi

LIST OF TABLES

Table 3-1 Development of PCFs....................................................................................... 14

Table 3-2 Modeling outlines of a typical hexagonal PCF ................................................ 21

Table 3-3 Modeling outlines of a Triangular lattice PCF ................................................. 23

Table 3-4 Parameters of elliptical PCF hexagonal core with circular air hole only. ........ 24

Table 3-5 Parameters of a typical square core PCF with circular air hole only. .............. 26

Table 4-1 Electromagnetic mode analysis (multimode analysis) of various PCFs. ......... 39

Table 4-2 Materials properties required materials. ........................................................... 40

Table 4-3 Modeling outlines of a TIR-Total Internal Reflection ..................................... 47

Table 5-1 Parameters of a typical Elliptical core PCF with circular air hole only. .......... 50

Table 5-2 Modeling outlines of a typical circular core PCF. ............................................ 51

Table 5-3 Parameters of a typical square core PCF with circular air hole only. .............. 53

Table 5-4 Parameters of a square core PCF with circular and elliptical air hole.............. 55

Table 5-5 Summary of PCF parameters from simulation. ................................................ 65

Table 5-6 Component Properties used in modeling RoF link........................................... 68

Table 5-7 Summary of performance of RoF link from simulation results. ...................... 78

xii

TABLE OF CONTENTS

Acknowledgement ............................................................................................................. iv

Abstract ............................................................................................................................... v

List of Abbreviations ......................................................................................................... vi

List of Figures .................................................................................................................. viii

List of tables ....................................................................................................................... xi

CHAPTER 1 INTRODUCTION

1.1 Introduction ............................................................................................................... 1

1.2 Problem statement ..................................................................................................... 2

1.3 Aim of the thesis ....................................................................................................... 5

1.4 Chapter Orientation ................................................................................................... 6

CHAPTER 2 IPMAIRMENTS IN ROF LINK

2.1 Introduction ............................................................................................................... 7

2.2 Radio over fibre (RoF) link....................................................................................... 7

2.3 Dispersion in long-haul optical communication ....................................................... 8

2.4 Dispersion compensation .......................................................................................... 9

2.4.1 Dispersion compensating fibre (DCF) ............................................................. 9

2.5 Erbium doped fibre amplifier (EDFA) ................................................................... 10

2.6 Conclusion .............................................................................................................. 12

CHAPTER 3 STUDY OF PHOTONIC CRYSTAL FIBRE (PCF)

3.1 Introduction ............................................................................................................. 13

3.2 PCF basics ............................................................................................................... 13

3.3 Development of PCF............................................................................................... 13

3.4 Applications of PCF ................................................................................................ 14

3.5 Methods of PCF fabrication .................................................................................... 16

xiii

3.6 Types of PCFs ......................................................................................................... 17

3.6.1 PCFs with solid core ...................................................................................... 17

3.6.2 PCFs with highly birefringent ........................................................................ 17

3.6.3 PCFs with large mode area ............................................................................ 17

3.6.4 Operation Modes ............................................................................................ 18

3.7 Optical properties of PCF ....................................................................................... 18

3.7.1 Properties in terms of dispersion.................................................................... 19

3.7.2 Properties in terms of effective Area ............................................................. 19

3.7.3 Properties in terms of light gathering capacity .............................................. 19

3.7.4 Properties in terms of non-linearity coefficient, γ ......................................... 20

3.8 Methods of PCF modeling ...................................................................................... 20

3.9 PCF modeling ......................................................................................................... 21

3.10 Conclusion ............................................................................................................ 27

CHAPTER 4 DESIGN OF VARIOUS PCF STRUCTURES USING COMSOL

MULTIPHYSICS

4.1 Introduction ............................................................................................................. 28

4.2 Modeling Flowchart using COMSOL Multiphysics............................................... 28

4.3 New Model Creation ............................................................................................... 29

4.3.1 Using model wizard creation of a new model ............................................... 29

4.3.2 Using Blank Model creation of a new model ................................................ 31

4.4 Parameters, Variables, and scope............................................................................ 31

4.4.1 Global Definition ........................................................................................... 31

4.4.2 Geometry Unit ............................................................................................... 32

4.4.3 Selection of materials ..................................................................................... 32

4.5 Boundary Selection with other geometric conditions ............................................. 33

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4.5.1 Performing the mesh condition ...................................................................... 33

4.5.2 Performing the study condition ...................................................................... 34

4.6 Modeling specifications of various PCF Lattice Structures. .................................. 35

4.6.1 Label: Material properties .............................................................................. 35

4.6.2 Label: Electromagnetic Waves Analysis (Frequency Domain) ..................... 35

4.6.3 Label: Mesh ................................................................................................... 35

4.6.4 Label: Mode Analysis .................................................................................... 36

4.6.5 Label: Eigenvalue Solver ............................................................................... 36

4.6.6 Mode analysis solver...................................................................................... 36

4.7 Electromagnetic mode analysis of various PCF structures ..................................... 36

4.8 Characteristics of SLPCF ........................................................................................ 40

4.9 Electromagnetic mode analysis for various structure (single mode analysis) at 1550

nm or 193.4 THz ........................................................................................................... 43

4.10 Conclusion ............................................................................................................ 48

CHAPTER 5 ANALYSIS OF RESULTS OF MODIFIED SLPCF

5.1 Introduction ............................................................................................................. 49

5.2 Modeling and analysis of various SLPCF structures .............................................. 49

5.2.1 Square lattice PCF with elliptical core ........................................................... 49

5.2.2 Circular core PCF with circular air hole ........................................................ 50

5.2.3 Analysis of square core PCF .......................................................................... 52

5.2.4 Square core PCF with circular and elliptical air hole .................................... 54

5.3 Effective mode analysis for various photonic crystal fibre (PCF) .......................... 56

5.4 Numerical Methods and Equations ......................................................................... 58

5.5 Result of simulation of modification with respect to other PCFs ........................... 60

5.6 Performance improvement in RoF link using SLPCF and EDFA .......................... 65

xv

5.8 Flow chart for RoF link design ............................................................................... 66

5.9 Design and analysis of RoF link using OptiSystem................................................ 67

5.10 Performance evaluation of RoF link ..................................................................... 69

5.11 Conclusion ............................................................................................................ 80

CHAPTER 6 CONCLUSION AND DISCUSSION

6.1 Conclusion .............................................................................................................. 81

6.2 Scopes for future research ....................................................................................... 81

REFERENCES ................................................................................................................. 83

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

This chapter is an introduction to the overall thesis work on dispersion compensated

optical fibre link using special type of photonic crystal fibre (PCF) and erbium doped fibre

amplifier (EDFA). The motivation behind research on this topic is briefed by describing

the present state of research on this area from analysis on recent literatures followed by

specific aim of the thesis. An outline of thesis chapters is presented at the end of this

chapter. The dispersion and nonlinear effects are the vital impairment in long-haul optical

communication for RoF link. The dispersion (optical pulses get widened) reduces the data

transmission rate. As a result cost of RoF communication will be increased due to lower

data transmission rate to meet up the huge consumer demand. That is why dispersion

should be compensated for better RoF communication. There are various dispersion

compensation techniques are exist such as fibre Bragg grating (FBG), phase conjugation,

dispersion compensation fibre (DCF), photonic crystal fibre etc. are used to reduce

dispersion impairments in fibre link. PCF structures have many advantageous

characteristics over other dispersion compensation methods because dispersion; non-

linearity are often controlled individually by varying the air-hole diameter (d) and pitch

(Λ), and by changing the lattice structures of PCF. Also SMF can propagate the light in

small distance but PCFs can propagate the light in long-haul optical communication.

Furthermore SLPCF was found better dispersion compensation characteristics over other

PCF structures. But the modification of SLPCF is needed over other PCFs including

previously used SLPCF to improve the overall performance of long-haul optical

communication like RoF link communication. The modification of SLPCF design was

presented by varying the lattice structures, material types and combination of lattice in core

and cladding. Also another RoF communication link impairments BER, lower Q factor,

closer eye height etc. are studied in this research.

2

1.2 Problem statement

The evolution of communication system prospers with time and technological

improvement. Communication devices and medium play significant impact on overall

communication system. However, numerous impairments are present within the optical

communication network hampering the event towards an efficient communication system.

Various sorts of PCF structures have become attractive among scientific groups for their

optimistic optical properties. For example, by varying the dimensions and size or shape of

the cladding holes and/or the core, the dispersion, nonlinearity, transmission spectrum,

mode shape, air filling fraction and birefringence, etc. are tuned to design suitable PCFs.

These feature in PCFs show an enormous amount in mint conditional and ameliorate

applications within the fibre optic communication system [1].

PCF offers design simplicity over single mode fibres (SMFs). SMFs have stubborn

arrangement parameter such that the optical characteristics cannot be maintained

independently. But in PCFs, various optical impairments like dispersion; non-linearity are

often controlled individually by varying the air-hole diameter (d) and pitch (Λ), and by

changing the lattice structures. In symmetric PCF, the air-holes are arranged symmetric to

the middle core. In asymmetric PCF, one or more holes near the core is removed. By

suitable design it is possible to shift the zero dispersion wavelengths closer to

telecommunication wavelength. The obtained zero dispersion wavelengths found to be

much smaller than normal silica. The core diameter in symmetric structures is far reduced

to nanometer range while the encompassing air hole diameter is large than core. Supported

light guiding mechanism photonic crystal fibres has two types. (i) Hollow core fibre and

(ii) Index guiding fibre. The index guiding fibres are suitable for telecommunication

application [2]. The applications of hollow-core fibres are: narrow line width delivery,

power delivery, pulse shaping and compression, nonlinear optics, gas spectroscopy, fibre

optic gyroscopes, and sensors [3]. Consistent with the sunshine guiding mechanisms in

photonic crystal fibre are as follows (i) index-guiding mechanism, (ii) photonic band gap

guiding mechanism, (iii) Hybrid guiding mechanism (combination of index-guiding

mechanism and photonic band gap guiding mechanism) [4, 5].

For all kinds of PCFs, fibre properties like effective area [6], dispersion [7],

nonlinearity [8], birefringence [9], confinement loss [10], and propagation constant [11]

3

etc. are varied by changing hole’s size, arrangement of spacing and shape. Various optical

properties of PCFs have contributed to the development of these structures in numerous

applications within the optical communications [7], [10], nonlinear optics [8] and high-

power technology [12]. Effective index of refraction is a crucial term for PCF design which

has relation with the evaluation of various properties of optical communication [13]. This

index of refraction is obtained in complex form and can simply be varied with the

wavelength of PCF. Again, this variation of index of refraction also varies these important

optical properties e.g. effective area, waveguide dispersion, confinement loss, propagation

constant etc. [14]. Confinement loss depends on the imaginary value of effective index of

refraction and effective area, waveguide dispersion also as propagation constant depend

upon the important a part of effective index of refraction. So, it is possible to style

application oriented guiding properties, by means of tuning the diameter and pitch of the

air holes within the cladding [15]. The massive effective mode area PCFs are required in

optical transmission systems [16] which they're required to support broadband optical

transmission also on minimize the coupling losses through the fibres.

A design of a highly nonlinear dispersion-shifted fibre (HNDSF) with an efficient

area of 9.3 μm2 is obtainable [17], and this advanced HNDSF structure offers also low

attenuation, and bending losses. Besides, fibre dispersion and confinement loss also play

vital roles in broadband communications systems. This is often rigorously achieved by

ensuring ultra-flattened dispersion characteristics of fibres [18] also as low confinement

loss [7]. Hence, there is only variety of published papers handling PCFs having both better

effective area and dispersion-flattened characteristics at an equivalent time [19, 20]. So,

there's still scope to style PCF of various structures with low confinement loss also a slow

dispersion.

Photonic crystals structures are unnatural structures those have a regular variation

of the refractive index in one, two, or three dimensions. Like an electronic band gap

between conduction and valence bands, photonic crystals structures create a photonic band

gap. Therefore, the electromagnetic waves cannot propagate through the device within the

band gap with band gap frequencies [21, 22]. This property is very attracting, giving new

chances for guiding light which is never think about before [23].

4

Finite Element Method (FEM), Plane Wave Expansion Method (PWE) and Finite

Difference Time Domain Method (FDTD) can be employed for designing and manipulate

the PCFs [24, 26]. For the development of long-haul optical communication systems

including RoF link, minimization of these impairments is necessary. Among the lower

promising advancements towards cost-effective long-haul transmission are the use of

Fibber Bragg Gratings (FBGs) and Optical Phase Conjugator (OPC) showing some

identical improvement in the link performance due to lower value of optical impairments

like dispersion [27]. The special optical light guide PCF shows the refractive index of core

region (silica glass) is higher than that of cladding region along the fibre [28]. Single mode

fibres have raspy modeling where the size of the core is bounded where light propagation

occurs using TIR-total internal reflection method [29], as a result light can propagate a

small distance. But PCFs can propagate the light in long-haul optical communication with

the help of EDFA [30].

Furthermore square-lattice based PCF (SLPCF) is found better than triangular-

lattice PCF for certain characteristics [31, 32]. SLPCF consists of single-mode fibre and

cladding of photonic crystal lattice like square in shape. Square-lattice PCF shows wider

range of single mode operation with the same 𝑑/Λ value compared to the triangular one

[31]. The effective area of square-lattice PCF is higher than triangular one, making the

former better for high power management [32]. Square-lattice PCF can better compensate

the inline dispersion around the 1550 nm wavelength than the triangular-lattice PCF [32].

In recent times, a square-lattice PCF preform has been realized with a standard fabrication

process, stack and draw, in order to study the localization and control of high frequency

sound by introducing two solid defects in the periodic distribution of air-holes [33]. So,

square-lattice PCF can be experimentally realized like that of the usual triangular-lattice

PCF. Finally, one can choose SLPCF for better long-haul optical communication. But the

modification of SLPCF is needed over other PCFs including previously used SLPCF to

improve the overall performance of long-haul optical communication like RoF link

communication. The modification of SLPCF design was presented by varying the lattice

structures, material types and combination of lattice in core and cladding. For instance,

GeS2 was found to minimize dispersion in square lattice PCF (SLPCF) compared to that

using GeO2 [34]. The premature technique is based on regular SLPCF with all the air-hole

5

of same uniform diameter and the effective size of the air-holes are modified with a

selective infiltration of the air-holes with liquids [35]. A fabrication problem arises between

previously used doped core (GeO2 or GeS2) and background material (fused silica), which

is insufferable due to some basic rule of physics, since the molecules of doping material

diffuse from its original location to adjacent undoped areas [36]. Therefore, the furthermore

modification is needed which is described in this work later. Where modification was done

by considering square in square with circular and elliptical air hole in cladding structure.

Therefore, compared with various PCF structure like circular, elliptical and conventional

SLPCF. For dispersion compensation in RoF link improved technique is necessary. SLPCF

is used to provide smooth hole diameter and hole pitch and to achieve lower nonlinearity,

low insertion loss, and its relative dispersion slope copes well with the formal SMF

assuring lower confinement loss which were not achievable from DCF [37, 39]. In addition

to dispersion compensation, reduction of BER is essential in communication link; since

higher BER gives a slower data rate degrades the quality of data link. Moreover, the

quality-factor (Q-factor) shows the system tolerance in dB. The Q-factor and BER have a

single valued function a larger Q-factor provides smaller BER and better RoF link

performance. Usually, three techniques are considered to reduce BER: automatic repeat

request (ARQ), forward error correction (FEC) or Error correcting code (ECC) and Hybrid

schemes combining ARQ and ECC (HARQ). Although SLPCF is preferred over SMF or

other PCFs in RoF link due to compensate dispersion and moreover, the eye diagram in

RoF with modified SLPCF is found sharper than that of conventional SMF or other PCFs,

the bit error rate is increased, Q-factor and eye height are not improved in SMF or other

PCFs [40].

1.3 Aim of the thesis

The thesis aims to understand the performance of the dispersion compensated long-

haul optical communication system employing modified SLPCF and EDFA. In this work,

performance of PCFs with various lattice structures namely square, circular, elliptical etc.

are analyzed and compared. Then, a modified SLPCF is designed for improved dispersion

characteristics to use in optical communication systems. A RoF link incorporating EDFA)

and CW laser of definite power is proposed as a new approach of forward error correction

6

(FEC). In particular, the RoF model operates with CW laser driven at various input powers.

The RoF link is designed to produce reduced dispersion and BER, sharper eye diagram and

both the improved Q-factor and eye height. Thus, the performance of the RoF link using

modified SLPCF incorporating EDFA and CW laser is aimed for cost-effective RoF link

[40, 41].

It focuses following objectives in the thesis:

• To analyze the optical performance of PCFs designed with various lattice

structures.

• To modify the SLPCF for obtaining improved dispersion property.

• To design and to evaluate the performance of RoF link in terms of dispersion, BER,

Q-factor, eye diagram using SLPCF and EDFA.

1.4 Chapter Orientation

The overall thesis is presented in the following way:

Chapter 1 gives the introduction of the research work by outlining the motivation and aim

of the thesis.

Chapter 2 presents the literature review on RoF link and discusses the impairments of long-

haul optical communication systems with an indication to solve them.

Chapter 3 describes the fundamentals of PCF design and characterization methods for

dispersion compensation.

Chapter 4 presents the research work on design, analysis and performance comparison

among various PCF lattice structures using COMSOL Multiphysics.

Chapter 5 presents the design and characterization of modified SLPCF and evaluates its

performance over other PCF structures for dispersion compensation. Describes the design

and analysis on RoF link using SLPCF and EDFA for dispersion compensated long-haul

optical communication system using OptiSystem software.

Chapter 6 presents the conclusion of the thesis and briefs possible scopes for further

research in future.

7

CHAPTER 2

IPMAIRMENTS IN ROF LINK

2.1 Introduction

In this chapter, fundamentals on RoF link are discussed. The impairments of RoF

link such as dispersion and non-linearity effects are described and methods to compensate

those impairments are presented.

2.2 Radio over fibre (RoF) link

Microwave signals are transmitted as radio waves through optical fibre link in RoF

(Radio-over-fibre) [42] as shown in Figure 2.1 [44]. Where the RW signal is converted into

light signal, then transmitted through optical fibre. In RoF link, signals are usually acquired

via FM [43]. When a conventional optical communication system that follows AM is

interconnected with a RoF system that follows FM, AM-to-FM up conversion is required.

AM-to-FM up conversion is obtained by using optically injected semiconductor laser

namely CW laser. Then, the FM signal is transmitted through RoF link generally using

single mode fibre (SMF). However, chromatic dispersion in fibre link is the critical

impairment in RoF transmission system due to pulse broadening.

Figure 2.1 Basic block diagram of a simple RoF link.

Various dispersion compensation techniques such as fibre Bragg grating (FBG),

phase conjugation and dispersion compensation fibre (DCF) etc. are used to mitigate

dispersion impairments in fibre link. Although DCF is more effective among these, it has

8

some negative aspect for compensating dispersion, for instance, high insertion loss and

bulk size. Hence, for dispersion compensation in RoF improved technique is necessary.

The two main threats in the modeling of higher rate long-haul optical fibre communication

system are as follows [45]: (i) radio frequency (RF) signals distribution [46]; and the

solution is to employ RoF link system and (ii) the production of the RF signals in higher

order [47]; and it depends on the production of RF signals within the optical signal

processing [48]. The important merits of a RoF communication system are namely,

(a) Bandwidth is high.

(b) Low attenuation loss.

(c) Using cells of small size.

(d) Use the multiple-input multiple-output (MIMO) method.

(e) Low power loss.

(f) Wireless coverage is wide where backhaul wireless is not coverable.

(g) More economic.

Another benefit of the RoF system over the wireless backhaul is the no difficulty with

which it can be install, uninstall, and insertion anywhere one can desire [49]. Main

applications RoF link are:

• SC (Satellite Control)

• VC (Vehicle Communication)

• WLAN (Wireless LAN)

• AMB (Access of Mobile Broadband)

• VDS (Video Distribution Systems)

• CN (Cellular Networks)

2.3 Dispersion in long-haul optical communication

The dispersion and nonlinear effects are the vital impairment in long-haul optical

communication. In long-haul optical communication system including the RoF link,

signals in the form of optical pulses are transmitted through single mode fibre (SMF).

While transmission, optical pulses get widened limiting the data transmission rate. This

unwanted phenomenon is known as dispersion or more specifically, chromatic dispersion

(CD) as shown in Figure 2.2 [50].

9

Figure 2.2 Chromatic dispersion in an optical fibre communication system.

The reason of increasing the spectral width, after propagating the signals through optical

fibre is the wave guide and material property of the link. That is Dispersion occurs since

the speed of light depends on the wavelength and propagation mode. While travelling long

distances, slight differences in speed accumulate resulting in BER. Thus, CD decreases the

system performance and makes the system more costly.

2.4 Dispersion compensation

Accompanied by modal dispersion and polarization mode dispersion, dispersion

consists of two main components - material dispersion and waveguide dispersion. Material

dispersion, which is positive for silica glass at telecom wavelengths, is related to material

properties, fibre length and the width of spectrum of an optical source. Waveguide

dispersion depends on the geometry of fibre. It is possible to obtain overall negative CD of

the fibre through high negative waveguide dispersion [51].

2.4.1 Dispersion compensating fibre (DCF)

Dispersion compensating fibre (DCF) is widely used as the solution for

compensation of dispersion in long-haul optical transmission links such as RoF link.

However, DCF has some limitations like bulky, nonlinearity and very high insertion loss.

In DCFs, material and waveguide dispersion is designed to combine them in such a way

that results zero CD at the required wavelength. Figure 2.3 shows zero CD produced at

1.50 µm by designing material dispersion and waveguide dispersion in a DCF [52].

10

Figure 2.3 Dispersion compensation by means of using DCF to have zero dispersion

at 1.50 µm wavelength.

In previous Fiber Br agg Gratings (FBGs) and Optical Phase Conjugator (OPC) are

considered as the dispersion compensating method [53].

2.5 Erbium doped fibre amplifier (EDFA)

In addition to dispersion effects causing pulse spreading, optical communication

systems are affected by absorption mechanisms within the optical link causing signal

weakening [54]. In long-haul optical communication system, signals travel through fibres

for very large distances and becomes weaker and need to strengthen that is to amplify the

signal. Electrical repeaters were used to overcome the above problem earlier [55]. The

general types of amplifiers used in optical system are: semiconductor optical amplifier [56,

57], Raman amplifier (RA) [58, 59] and rare-earth doped fibre amplifier (DFA) [60]. The

commonly used rare earthed doped fibre amplifier is erbium-doped fibre amplifier (EDFA)

due to the broad window and low attenuation properties [61]. EDFAs are pumped by a

laser. Types of pumping methods are as follows: (a) forward, (b) backward, (c) bi-

directional [62].

11

The Figure 2.4 below shows a typical configuration of EDFA based long-haul optical

communication network operating at 1550 nm [63]. First of all, the signal is passed via an

optical isolator through an optical coupler that can fend off reflection. The mechanism

which is allowed the pumping signal input to the link with lower loss is called optical

coupler. Being a rare-earthed (RE) material namely Erbium (Er) when doped in silica glass

makes amplifier that is EDFA.

IsolatorCoupler Isolator

Pumping Laser

(980/1480 nm)

EDFA

Signal 1550 nm

Output

(Amplified)

Figure 2.4 Optical communication system using EDFA.

The Figure 2.5 shows energy levels of erbium doped fibre [63]. It may be noted

that the energy levels form three groups of energy levels marked with their spectroscopic

notations. Here, the configuration energy states are divided into three types of energy levels

as the ground states (g), the metastable (m) and the upper (u).

Figure 2.5 Pump radiation with respect to three energy levels for EDFA.

When the radiation that is beam of light of desired frequency is not present, the ions

are located at ground states (g). If a radiation is appeared on the model, the ions will then

12

excited from lower energy levels to the higher energy levels. This is known as pump

radiation.

In a third transmission window EDFA can be used to amplify signal in two bands

system. First band is called C-band or the conventional band having wavelength range 1530

nm to 1565 nm and the second band is called as the L band or the long band having

wavelength range from 1565 nm to 1625 nm.

2.6 Conclusion

Basic understanding on RoF link are described in this chapter. The impairments of

RoF link and methods to compensate them namely the dispersion are presented. Microwave

signals is introduced. The important merits and applications of RoF communication system

are described. The use of frequency modulation (FM) is introduced in RoF link. Amplitude

modulation (AM)-to-FM up conversion is described in the interconnected RoF system by

using optically injected semiconductor laser namely CW laser. The general types of

amplifiers used in optical system are: semiconductor optical amplifier, Raman amplifier

(RA) and rare-earth doped fibre amplifier (DFA) are shown. Then, fundamentals of the

commonly used rare earthed doped fibre amplifier (DFA) is erbium-doped fibre amplifier

(EDFA) are discussed.

13

CHAPTER 3

STUDY OF PHOTONIC CRYSTAL FIBRE (PCF)

3.1 Introduction

In this chapter the evolution and structural features of various types of photonic

crystal fibres (PCF) along with their applications in optical system are described. Design

issues, essential parameters and considerations behind PCF design are illustrated.

3.2 PCF basics

PCF is one kind of optical fibre based on the characteristics of photonic crystals

lattice. At University of Bath, UK in 1996 PCF was first surveyed. PCF has confinement

properties that signal can be confined into the core of PCF but which is not possible in

SMF. Photonic crystal fibres have vast applications in optical fibre communications

like lasers fibre, nonlinear optics, high-power applications, sensing applications etc. PCF

has many structures like photonic-bandgap (PBG) fibre, Hollow Core, hole-assisted fibre

and Bragg fibre. PCFs are micro structured arrangement of high refractive-index and low

refractive index materials based on the properties of photonic crystals. PCFs can also be

classified according to its microstructure.

3.3 Development of PCF

According to the Table 3-1 it is shown that PCF introduction was held in 1992

which is applicable for high power applications, multi-wavelength fibre laser development

etc. Then in 1997 PCF endlessly single mode absence of higher order modes was

developed. Which is applicable in sensing, mode filtering applications, interferometers etc.

In 2000 PCF with more birefringence and PCF with super continuum source were

developed for high data rate, mirror, RADAR, SONAR, Echography, wavelength division

multiplexing etc. applications. In 2001-2005 PCF with double cladding, PCF with ultra-

flattened dispersion, Chalcogenide Photonic Crystal Fibres (CPCF), Kagome Lattice PCF

were introduced for high power, spectrophotometry in Cancer, diagnostics pulse

transmission. In 2006 Photonic Crystal Fibre in Hybrid lattice was introduced for

polarization, sensing etc. In 2014 PCF based Nano displacement sensor was introduced for

https://en.wikipedia.org/wiki/Optical_fiber

https://en.wikipedia.org/wiki/Photonic_crystal

https://en.wikipedia.org/wiki/Fiber-optic_communication

https://en.wikipedia.org/wiki/Fiber_laser

14

nano-displacement sensor applications. In 2015 Combination of Fibre Laser with Photonic

Crystal Fibres (PCF) was introduced for high power applications. Therefore, various hybrid

structure like SLPCF, Hexagonal PCF, triangular PCF, circular PCF, elliptical PCF,

butterfly PCF etc. were developed for various applications like sensing, optoelectronics,

digital displays etc.

Table 3-1 Development of PCFs

Year Development Applications

1992 PCF introduction [64] • high power applications

• multi-wavelength fibre laser

development

1997 PCF - endlessly single mode [65]

absence of higher order modes

• Sensor development

• mode filtering applications

• interferometers

2000 PCF with more birefringence [66] • high data rates

• mirror (fibre loop type)

2000 PCF with Super continuum

source [67]

Radar, sonar and echography,

wavelength-division multiplexing etc.

2001 -2005 • PCF with double cladding

[68]

• PCF with ultra-flattened

dispersion [69]

• Chalcogenide Photonic

Crystal Fibres (CPCF) [70]

• Kagome Lattice PCF

introduction [71]

• High power.

• Spectrophotometry in Cancer

Diagnostics

• pulse transmission

2006-2015 • Photonic Crystal Fibre in

Hybrid lattice [72]

• PCF based Nano

displacement sensor [73]

• Combination of Fibre Laser

with Photonic Crystal Fibres

(PCF) [74]

• Polarization, sensing etc.

• Nano-displacement sensor

• High Power Applications.

3.4 Applications of PCF

PCFs are being considered for use in many applications, including tunable sensors,

high power communication, and medical applications. PCFs are widely used in light

guidance for λ that strongly absorbs infra-red range, gas lasers (hollow core), fibre lasers

(doped core). In fibre nanoparticle transportation ion the hollow core that is tweezers etc.

15

Besides the PCF has vast applications [75, 76]:

i) Sensing Application: The use of PCF in sensing applications are shown in Figure 3.1 as

follows:

Figure 3.1 Use of PCF in sensing application.

ii) Applications in medical sector:

a) Imaging (MRI, X-Ray, CT scan etc.).

b) Electrochemical sensor, biosensors, nano chemical sensors etc.

iii) Applications in communication sector.

iv) Applications in LASER sector.

v) Interconnection in optical networks.

vi) Applications in multi-structured fibre like square in square, square in circular, square

in elliptical.

Sensors

Physical Bio-chemical

Vibration

Electric

Refractive Index

Pressure

Torsion

Displacement

Curvature

Temperature

Gas

Molecular

pH Sensor

16

3.5 Methods of PCF fabrication

PCF is an important topic in the modern time. But it’s modeling and accurate design

is very necessary to consider before fabrication. To obtain one’s goal one have to accurate

design the PCF. To model the accurate PCF, there are some important steps to follow, [75,

76].

1. The neatly arrangement and draw procedure.

2. Projection and substantial technology.

3. For fabricating asymmetrical shaped photonic crystal fibre Sol-gel method is applied.

The master technic for fabrication of PCFs is multiple thinning whether the type of

glass and the type of structure are less important than that. The projection process is applied

for fabrication of PCF of soft glass. Initially, creation of individual capillaries is needed.

Then formation of the preform, drawing of intermediate preform by extra glass rods, finally

drawing of the final fibre. The entire process is shown in Figure 3.2 [77] as follows.

Figure 3.2 Photonic crystal fibre fabrication.

The Figure 3.2 (a) shows creation of individual capillaries, (b) shows formation of

the preform, (c) shows drawing of intermediate preform, (d) shows drawing of the final

fibre. Finally, extra layers of polymer are applied to make a coating of the fibre for

protection of fibre. The main problems in fabrication process are (i) appearance of

deformed air holes, (ii) need of extra holes, and (iii) disorder of the lattice symmetry [78].

17

Various fibres made from silicate [78], chalcogenide [79], and tellurite glass [80] have been

mentioned.

3.6 Types of PCFs

Various types of PCFs are described in this section.

3.6.1 PCFs with solid core

PCF of index guiding types with a cladding of air hole and solid core shows various

advantageous features to the application of fibre optics. Inside the microstructure, PCFs

cladding has the lower effective refractive index and solid core having higher refractive

index as a result of low birefringence, lower dispersion, low attenuation loss etc.

3.6.2 PCFs with highly birefringent

If the two orthogonal polarized mode SMF propagate at different rates, then this

phenomenon is called birefringent fibres. They are used to control the polarization states.

In this case the propagated modes are symmetric. The cross-section of a birefringent PCF

is shown in Figure 3.3.

3.6.3 PCFs with large mode area

To make a PCF having large effective area it is necessary to change the geometrical

behavior of the fibre structure. Thus, one can fabricate the large mode area PCFs as shown

in the Figure below.

Figure 3.3 Microscope picture of birefringent PCF at 1.55 µm.

The Figure 3.3 describes the effective area of the PCFs: the cross-section (left) and the core

region of a highly birefringent PCF (right).

18

Figure 3.4 Schematic of (a) solid core PCF, (b) hollow core PCF, and (c) SMF.

The Figure 3.4 shows the micro structured PCFs core modeling where L is the fibre

length, d is diameter of core and h is the pitch of air hole. To avoid complexity the Figure

3.4 [81] has represented hereby.

3.6.4 Operation Modes

According to the confinement mechanism the PCFs could be classified as two

types. A solid core PCF having a core of high refractive index than the cladding refractive

index can be maintained as a SMF. Therefore, applications in polarization controlling

fibres, nonlinear optics etc. On the other hand, PBG that is photonic bandgap (PBG) fibre,

can be made with reverse structure of index guiding structure. That in PBG core having

lower refractive index than the refractive index for cladding. The advantages of a hollow

core is finding out the presence of some substance like finding out the greenhouse gas,

fibre connection at house etc.

3.7 Optical properties of PCF

In the early 1860’s the nature of mode fields was developed by Maxwell. It shows

that, the fundamental relationship between electricity and magnetism. The electromagnetic

fields governed by the Maxwell’s equations is adopted to the mode observed in a

waveguide is a physical property. The studies of wave propagation in the fibres is added in

our study for the background material which is described by Maxwell’s equations. Finite

difference frequency domain method helps to numerical analysis. Waveguide properties

such as the mode analysis, light propagation, and dispersion mechanisms are also studied

in this work.

19

3.7.1 Properties in terms of dispersion

Dispersion can be defined as the broadening of optical pulses. Dispersion is related

with effective refractive index as shown in equation 1. It is known that if the value of

effective mode index is increased then the value of effective refractive index increased and

vice versa. Thus, the effective refractive index can vary the dispersion property of PCF.

There are various types of dispersion as follows:

1. Group-Velocity Dispersion

2. Material Dispersion

3. Waveguide Dispersion

4. Chromatic Dispersion

3.7.2 Properties in terms of effective Area

For a photonic crystal fibre, the effective area is defined as the area where the light

is enclosed in a fibre. It can measure the spreading field within the cladding. Effective area

is expressed as,

= dxdyEdxdyEAeff

4

2

22

1 /)(

Where, E1 is the electric field amplitude in the medium. If the value of effective

mode index (EMI) is increased according to Maxwell’s equation, then the electric field

amplitude in the medium will be increased. As a result of increasing the value of effective

area of PCF. In photonic crystal fibre, the effective area, Aeff combines the region of core

and the region of a small portion of areas in the cladding. Directly it is stated if EMI is

increased then Aeff will be increased according to the above equation.

3.7.3 Properties in terms of light gathering capacity

Light gathering capacity or the numerical aperture (NA) remarks about the capacity

of gathering light of the fibre. It is known as properties of the range of angles over which

the system can absorb or release the light. Figure 3.5 [82] shows the light gathering capacity

of PCFs. In this Figure Light gathering capacity=𝑆𝑖𝑛𝜃 where the divergence angle is 𝜃 and

the refractive index of air is 1. It is a dimensionless parameter.

20

Figure 3.5 Light gathering capacity of PCF.

For short wavelengths the modal field is tightly confined in silica glass region i.e.

core region that’s why, light gathering capacity is less in this case, however, at higher

wavelengths it cuts into the air hole of the cladding region and hence increase light

gathering capacity. So, NA is also related to effective area.

3.7.4 Properties in terms of non-linearity coefficient, γ

The third order susceptibility (χ(3)) can produce the effect nonlinearity. Wavelength

is related with nonlinear coefficient and it is expressed as,

)/()2( 2 effAn =

Where, n2 = 1.45 refractive index of silica glass. Nonlinearity is related with

effective refractive index, refractive index of background material and wavelength.

Everyone can find out the value of nonlinearity using the required parameter as described.

If Aeff is increased then, nonlinearity (γ) will be decreased according to the above equation.

3.8 Methods of PCF modeling

Generally, the method which is used in modeling of SMF or DCF that method

cannot applied for modeling PCF. Because there need to adapt the electromagnetic fields

mode and micro structures. The finite difference time domain (FDTD) is vastly used in

modeling purpose of PCF [83]. The propagation of light wave can be found from Maxwell's

equations. Time and Space analysis is needed in this case. The assessment of the various

mode field is analyzed on a Yee cell is shown in Figure 3.6 [84] below. The boundary

conditions are necessary in modeling, such as impedance boundary condition, scattering

boundary condition, transition boundary condition etc.

21

Figure 3.6 Yee cell.

The Yee cell describes all components of electrical and magnetic field in a cube.

Every component of the electromagnetic fields defined only in one place in the unit Yee

cell. The full vector plane wave expansion (PWE) method very effective method to model

PCFs [85, 86]. Here the signal can be confined by making the defect in the lattice [87].

Within the optical pulse sources the yield diffraction gives the consumption of low

electrical power [88]. Due to low electrical power consumption the value of nonlinearity

become lower [89].

3.9 PCF modeling

Table 3-2 Modeling outlines of a typical hexagonal PCF

Modeling outlines of a typical hollow core hexagonal cladding PCF

Size of PCF (µm) Geometry Materials

Radius • Circle-1 for

air hole in

cladding.

• Circle-2 for

core.

• Circle-3

and 4 for

outer

circle.

• Hexagonal

cladding

and core.

Air Silica Glass

• circular air

hole

(cladding)=

1.5×10-4m

• circular air

hole (core)=

0.167×10-4m

• Outer most

circle=

15×10-4m

• Outer circle=

14.325×10-4m

Relative permeability = 1

Relative permittivity = 1

Electric Conductivity: 0 S/m

Ratio of specific heats = 1.4

Refractive index (real part)

= 1

Refractive index (imaginary

part) = 0


Relative permittivity = 3.75

Electrical conductivity = 1e-

14[S/m]

Density=2203 kgm-3

Coefficient of thermal

expansion: 0.55e-6[1/K]

Thermal Conductivity =

1.38[W/(m*K)]

Heat capacity at constant

pressure= 703[J/(kg*K)]

Refractive index (real part) = 1.5


part) = 0

22

The background material is typically silica glass (undoped) having high refractive

index while the low refractive index region is usually provided by air holes. The Table 3-

2 shows the Modeling outlines of a typical hollow core hexagonal cladding PCF. The

Figure 3.7 shows structure of a typical hollow core hexagonal core PCF with circular air

hole only. The design has done with the help of COMSOL Multiphysics 5.3a. Where the

desired modeling input parameter has shown in Table 3-2. Here, the desired parameters

have selected from global definitions. The geometry from component block is square in

circular solid core with circular air hole in cladding. The air hole (cladding) radius, r is 150

µm, outer most radius is 0.0015 µm, outer circle radius is 0.0014 µm. Circular core having

radius 0.0000167 µm. Types of air hole in cladding is circular only. Then select the

materials from component block as air and silica glass. The properties of air and silica glass

is shown in Table 3-2. The electromagnetic waves, frequency domain conditions from

component block would be selected as desired. Here the boundary has mentioned if

required. Then the mesh is selected as user defined as finer. Then perform the mode

analysis from study block. Select the mode analysis frequency is 193.4 THz or 1.55 µm.

Figure 3.7 A fabricated hollow-core Hexagonal cladding PCF.

The Table 3-3 shows the Modeling outlines of a Modeling outlines of a Triangular

lattice PCF as follows.

Air hole

radius, r is

150 µm

23

Table 3-3 Modeling outlines of a Triangular lattice PCF

Modeling outlines of a Triangular lattice PCF


Radius • Circle-1 for

air hole in

cladding.

Air Silica Glass

• circular air

hole=

1.5×10-4m






= 1


part) = 0



Electrical conductivity = 1e-

14[S/m]

Density=2203 kgm-3




1.38[W/(m*K)]





part) = 0

The Figure 3.8 shows structure of a typical Triangular lattice PCF design of size air

hole is 1.5×10-4m with circular air hole only.

Figure 3.8 Triangular lattice photonic crystal fibre.

The design has done with the help of COMSOL Multiphysics 5.3a. Where the

desired modeling input parameter has shown in Table 3-3. Firstly the desired parameters

have selected from global definitions. The geometry from component block is triangular

24

with circular air hole in cladding. The air hole (cladding) radius, r is 150 µm, Types of air

hole in cladding is circular only. The number of air hole in cladding are 15. Then select the





analysis from study block. Select the mode analysis frequency is 193.4 THz or 1.55 µm.

Table 3-4 Parameters of elliptical PCF hexagonal core with circular air hole only.

Modeling outlines of a typical elliptical shape hexagonal core PCF with circular air hole only

Size of PCF

(µm)

Geometry Materials No. of

cladding

layer

Types of

air hole

in

cladding

No. of

air holes

in

claddin

g

• Air hole

(cladding)

Radius=

150 µm

• Outer shape

square size

300×300 µm

• Elliptical

core:

a-semi

axis=1050 µm

b-semi axis=850

µm

• Circle for

air hole in

cladding.

• Elliptical

solid core

Air Silica Glass 3 Circular

only

155

Relative

permeability = 1

Relative

permittivity = 1

Electric

Conductivity: 0

S/m

Ratio of specific

heats = 1.4

Refractive index

(real part) = 1

Refractive index

(imaginary part)

= 0

Relative

permeability = 1

Relative

permittivity = 3.75

Electrical

conductivity = 1e-

14[S/m]

Density=2203

kgm-3

Coefficient of

thermal expansion:

0.55e-6[1/K]

Thermal

Conductivity =

1.38[W/(m*K)]

Heat capacity at

constant pressure=

703[J/(kg*K)]

Refractive index

(real part) = 1.5

Refractive index

(imaginary part) =

0

25

The Table 3-4 shows the modeling outlines of a typical elliptical shape hexagonal

core PCF with circular air hole only as follows. The Figure 3.9 shows structure of a typical

elliptical shape with hexagonal core PCF with circular air hole only. The design has done

with the help of COMSOL Multiphysics 5.3a. Where the desired modeling input parameter

has shown in Table 3-4. Firstly, the desired parameters have selected from global

definitions. The geometry from component block is elliptical in hexagonal core with

circular air hole in cladding. The air hole (cladding) radius is 150 µm, elliptical shape

having a-semi axis=1050 µm, b-semi axis=850 µm. Types of air hole in cladding is circular

only. Then select the materials from component block as air and silica glass. The properties

of air and silica glass is shown in Table 3-4. The electromagnetic waves, frequency domain

conditions from component block would be selected as desired. Here the boundary has

mentioned if required. Then the mesh is selected as user defined as finer. Then perform the

mode analysis from study block. Select the mode analysis frequency is 193.4 THz or 1.55

µm.

Figure 3.9 A typical elliptical PCF with hexagonal core.

The Table 3-5 shows the modeling outlines of a typical square core PCF with circular air

hole only as follows.

26

Table 3-5 Parameters of a typical square core PCF with circular air hole only.

Modeling outlines of a typical square core PCF with circular air hole only

Size of PCF

(µm)


core

layer

Types of

air hole in

core

No. of air

holes in

core

Air hole

(cladding)

Radius=

150 µm

Outer shape

square size

300×300 µm

square solid

core:

size 100×100

µm

Circle for air

hole in

cladding.

Solid square

for core


only

155

Relative

permeability =

1

Relative

permittivity = 1

Electric

Conductivity: 0

S/m

Ratio of

specific heats =

1.4

Refractive

index (real part)

= 1

Refractive

index

(imaginary part)

= 0

Relative permeability

= 1

Relative permittivity

= 3.75

Electrical

conductivity = 1e-

14[S/m]

Density=2203 kgm-3


expansion: 0.55e-

6[1/K]

Thermal Conductivity

= 1.38[W/(m*K)]

Heat capacity at

constant pressure=

703[J/(kg*K)]

Refractive index (real

part) = 1.5

Refractive index

(imaginary part) = 0

The Figure 3.10 shows structure of a typical square core PCF with circular air hole

only. The design has done with the help of COMSOL Multiphysics 5.3a. Where the desired

modeling input parameter has shown in Table 3-5. Firstly the desired parameters have

selected from global definitions. The geometry from component block is square in square

core with circular air hole in core. The air hole (core) radius is 150 µm, Outer shape square

size is 300×300 µm, square core having size of 100×100 µm. Consider the number of core

layer 3. Types of air hole in core is circular only. Number of air hole in core are 25. Then

select the materials from component block as air and silica glass. The properties of air and

silica glass is shown in Table 3-5. The electromagnetic waves, frequency domain



27


µm.

Figure 3.10 Modeling outlines of a typical square in square PCF with hollow core.

A low refractive index material is needed for cladding around the PCF core.

3.10 Conclusion

Photonic crystal fibres (PCFs) integrate the characteristics of photonic crystals

microstructure and conventional optical fibres. Development of PCFs is still very new age

topic and one may explore new research, more accurate and efficient methods for modeling

and designing. In this chapter, typical properties of PCFs are discussed. The evolution and

applications of PCFs are summarized. Design method and consideration for various PCF

structures are illustrated.

28

CHAPTER 4

DESIGN OF VARIOUS PCF STRUCTURES USING COMSOL

MULTIPHYSICS

4.1 Introduction

In this chapter, the design and modeling of various PCF lattice structures and

analyze their performance in compensating dispersion are presented when they are used in

optical communication system. The PCFs are designed and analyzed using COMSOL

Multiphysics software of version 5.3a.

4.2 Modeling Flowchart using COMSOL Multiphysics

In this work COMSOL Multiphysics is used for modeling purpose. Firstly select

the model wizards from the following space dimensions: 3D, 2D Axisymmetric, 2D, 1D

Axisymmetric, 1D, and 0D. Then select the desired physics. Then added the required study

from study block. Then insert the parameters from global definition unit. Add the condition

of components and hence the geometry of the structure. Then add the materials as needed.

Then perform the mesh generation. Select user defined physics control condition. Select

finer as physics quality. Then built the mesh. Then select the boundary condition from

Electromagnetic waves, frequency domain. If a boundary is unselected, then its color is

typically gray. But if selected then color will be blue. First select Electromagnetic waves,

frequency domain, then right click on it and select the boundary condition. Then select the

boundary as needed from boundary list like impedance boundary condition, scattering

boundary condition, transition boundary condition etc. Then perform study from study unit.

Select the boundary mode analysis for three dimensional physics or mode analysis for two

dimensional physics. Here the frequency range must be selected. Select the desired mode

number. Then compute the solution. Then observe the post processing, visualization and

results. The Figure 4.1 shows the modeling and working procedure of photonic crystal fibre

in a flowchart as follows:

29

Figure 4.1 COMSOL modeling Flowchart.

4.3 New Model Creation

One may select a model from Model Wizard or start directly from a Blank Model.

4.3.1 Using model wizard creation of a new model

From Model Wizard the space dimension, physics, and study process are selected

within some following steps:

Select the model wizard

Added the physics interfaces

Added the required study

Insert the parameters in detailed

Add the condition of components and hence the geometry of the structure

Add the materials

Mesh generation

Compute the solution

Post processing

Visualization and Results

Unsatisfactory

30

Step-1. Select one from the following space dimensions: 3D, 2D Axisymmetric, 2D, 1D

Axisymmetric, 1D, 0D see Figure 4.2.

Figure 4.2 Space dimension selection.

Step-2: Select one or more physics from organized Physics block see the Figure 4.3.

Figure 4.3 Physics Selection.

Step-2: Select the Study from study block. Therefore, click on done. The desktop is now

shown with a blank model.

31

Figure 4.4 Study selection.

4.3.2 Using Blank Model creation of a new model

One can create a blank model without following previous steps. Just open

COMSOL Multiphysics 5.3a. Then only select the Blank Model.

Figure 4.5 Creating a new model using Blank Model.

4.4 Parameters, Variables, and scope.

4.4.1 Global Definition

The parameters on Global Definition block are defined by user. Where the user can

put the name, expression and value of the parameters.

32

4.4.2 Geometry Unit

Set the cursor on geometry and right click on mouse and select the necessary

geometrical structure. Select the type of object sizes and shapes are also be defined easily

in this block.

Figure 4.6 Select the geometry.

Location of user file path:

The file path for windows operating system is:

C:\Program Files\COMSOL\COMSOL5.3a\Multiphysics\applications.

Its location by default in Windows operating system is:

C:\ProgramFiles\COMSOL\COMSOL5.3\Multiphysics\applications\COMSOL_Multiph

ysics\ Structural Mechanics\wrench.mph\bin.

4.4.3 Selection of materials

Select the materials from materials library which are appeared in component block

or node. That is shown in Figure 4.7

33

Figure 4.7 Materials selection.

Select the add materials block. The materials can be added window in two steps:

Step-1: From the ribbon, select the Home tab and then click add material.

Step-2: Right-click on Comp. 1 > Materials in the model builder and select add material

4.5 Boundary Selection with other geometric conditions

From the electromagnetic waves, frequency domain boundary can be selected. If a

boundary is unselected, then its color is typically gray. But if selected then color will be

blue. First select electromagnetic waves, frequency domain, then right click on it and select

the boundary condition. That is shown in Figure 4.7.

4.5.1 Performing the mesh condition

Mesh is selected to mention the element size and sequence type. That is shown in

Figure 4.8.

34

Figure 4.8 Mesh Analysis.

4.5.2 Performing the study condition

This is the final stage. Here mode frequency, mode number etc are selected to

analyze the desired structures. That is shown in Figure 4.9.

Figure 4.9 Preform the study.

35

4.6 Modeling specifications of various PCF Lattice Structures.

Considering pressure: 1atm

Consider the thickness of boundary: 0.01m

Considering Temperature: 293.15[K]

Considering boundary condition: Transition Boundary Condition

Considering Materials: Material-1: Air, Material-2: Silica Glass

4.6.1 Label: Material properties

4.6.2 Label: Electromagnetic Waves Analysis (Frequency Domain)

Effective index or effective mode index (EMI) is the ratio of the waveguide

propagation constant to the free space propagation constant. There can be more than

one effective index depending on the modes that are excited in the waveguide.

Required wave equation for effective mode analysis:

0)/()( 0

2

0

1=−−

−EjKE rr

zj −−=

zik zezyxEzyxE −= ),,(),,(

4.6.3 Label: Mesh

Physics controlled Mesh.

Element size: Finer

Air Silica Glass





Refractive index (real part) = 1

Refractive index (imaginary part) = 0



Electrical conductivity = 1e-14[S/m]

Density=2203 kgm-3

Coefficient of thermal expansion: 0.55e-6[1/K]

Thermal Conductivity = 1.38[W/(m*K)]

Heat capacity at constant pressure= 703[J/(kg*K)]



36

4.6.4 Label: Mode Analysis

Mode Analysis Frequency: 193 GHz

Mode search method: Manual

Desired Number Modes: 5

4.6.5 Label: Eigenvalue Solver

Relative solver: 1E-6

Eigenvalue Transformation: Effective mode index

Desired number Eigenvalues: 6

4.6.6 Mode analysis solver

PARDISO Solver which having thread-safe, high-performance, robust, memory

efficient and easy to use software for solving large sparse symmetric and asymmetric linear

systems of equations with electromagnetic propagation on shared-memory and distributed-

memory multiprocessors.

4.7 Electromagnetic mode analysis of various PCF structures

The Figure 4.10 shows structure of a typical circular core PCF with circular air hole

only. The design has done with the help of COMSOL Multiphysics 5.3a. Firstly the desired

parameters have selected from global definitions. The geometry from component block is

square in circular solid core with circular air hole in cladding. The air hole (cladding)

radius, r is 150 µm, circular core having radius r/1.5 µm. Then select the materials from

component block as air and silica glass. The electromagnetic waves, frequency domain



mode analysis from study block and number of mode is 5. Select the mode analysis

frequency is 193.4 THz or 1.55 µm.

37

Figure 4.10 Effective mode index of Circular PCF.

The Figure 4.11 shows structure of a typical Elliptical core PCF with circular air

hole only. The design has done with the help of COMSOL Multiphysics 5.3a. Firstly the

desired parameters have selected from global definitions. The geometry from component

block is square in elliptical solid core with circular air hole in cladding. The air hole

(cladding) radius is 150 µm, elliptical core having a-semi axis=105 µm b-semi axis=85

µm. Then select the materials from component block as air and silica glass. The

electromagnetic waves, frequency domain conditions from component block would be

selected as desired. Here the boundary has mentioned if required. Then the mesh is selected

as user defined as finer. Then perform the mode analysis from study block and number of

mode is 5. Select the mode analysis frequency is 193.4 THz or 1.55 µm.

38

Figure 4.11 Effective mode index of Elliptical PCF.


only. The design has done with the help of COMSOL Multiphysics 5.3a. Firstly the desired


square in square solid core with circular air hole in cladding. The air hole (cladding) radius

is 150 µm, square core having size of 100×100 µm. Then select the materials from

component block as air and silica glass. The electromagnetic waves, frequency domain



mode analysis from study block and number of mode is 5. Select the mode analysis


39

Figure 4.12 Effective mode index of SLPCF.

The Table 4-1 shows the electromagnetic mode analysis or light propagation of

various PCF lattice structures without central mode.

Table 4-1 Electromagnetic mode analysis (multimode analysis) of various PCFs.

PCF Lattice Effective Mode Index

Circular 0.7894

Elliptical 0.84301

SLPCF 0.89832

PCFs [91, 92] shows a robust variety of possible geometries in shape, size, and

micro structured cladding. Obtaining very high negative values of dispersion is the target

in future [93, 105]. In this work to obtain high negative dispersion value with regular square

lattice have studied rigorously. Square-lattice-based PCF is better over other PCF [98, 105].

Modified SLPCF provide smooth hole diameter and hole pitch and to achieve lower

nonlinearity, low insertion loss, and lower dispersion [106, 107]. Modified Square-lattice

PCF shows wider range of single mode operation with the same value compared to the

40

others. The effective area of modified square-lattice PCF is higher than others. The Figure

4.13 shows the cross section of a typical PCF.

Figure 4.13 Cross section of proposed SLPCF.

Now a days, a SLPCF preform has a standard fabrication process, arranged and

design [108]. In another characteristic, the study of negative refraction has been studied

with modified SLPCF [109].

4.8 Characteristics of SLPCF

Multimode analysis (Multimode mode, 1550 nm – 193.4 THz).

Table 4-2 Materials properties required materials.

Air Silica Glass





Refractive index (real part) = 1





Density=2203 kgm-3

Coefficient of thermal expansion: 0.55e-6[1/K]

Thermal Conductivity = 1.38[W/(m*K)]

Heat capacity at constant pressure= 703[J/(kg*K)]



41

Figure 4.14 Circular shape PCF.

The Figure 4.14 light propagation of a typical circular core PCF with circular air

hole only. The design has done with the help of COMSOL Multiphysics 5.3a. Where the

desired modeling input parameter has shown in Table 4-2. Firstly the desired parameters

have selected from global definitions. The geometry from component block is square in

circular solid core with circular air hole in cladding. The air hole (cladding) radius, r is 150

µm, circular core having radius r/1.5 µm. Then select the materials from component block

as air and silica glass. The properties of air and silica glass is shown in Table 4-2. The





42

Figure 4.15 Light Propagation through SLPCF.

The Figure 4.15 shows light propagation of a typical circular core PCF with circular

air hole only. The design has done with the help of COMSOL Multiphysics 5.3a. Where

the desired modeling input parameter has shown in Table 4-2.

Figure 4.16 Cross sectional view of light propagation through modified SLPCF.

Firstly, the desired parameters have selected from global definitions. The geometry

from component block is square in square solid core with circular air hole in cladding. The

43

air hole (cladding) radius is 150 µm, square core having size of 100×100 µm. Then select

the materials from component block as air and silica glass. The properties of air and silica

glass is shown in Table 4-2. The electromagnetic waves, frequency domain conditions from



analysis from study block and number of modes is 5. Select the mode analysis frequency

is 193.4 THz or 1.55 µm.

4.9 Electromagnetic mode analysis for various structure (single mode analysis) at

1550 nm or 193.4 THz

The Figure 4.17 is shown in below shows structure of a typical Elliptical core PCF

with circular air hole only. The design has done with the help of COMSOL Multiphysics

5.3a. Where the desired modeling input parameter has shown in Table 4-2. Firstly, the


block is square in elliptical solid core with circular air hole in cladding. The air hole

(cladding) radius is 150 µm, elliptical core having a-semi axis=105 µm b-semi axis=85

µm. Then select the materials from component block as air and silica glass. The properties

of air and silica glass is shown in Table 4-2. The electromagnetic waves, frequency domain



mode analysis from study block and number of modes is 1. Select the mode analysis


44

Figure 4.17 Light propagation through an elliptical core PCF.

The Figure 4.18 is shown in below shows the structure of a typical square core PCF

with circular air hole only. The design has done with the help of COMSOL Multiphysics

5.3a. Where the desired modeling input parameter has shown in Table 4-2. Firstly the


block is square in square solid core with circular air hole in cladding. The air hole

(cladding) radius is 150 µm, square core having size of 100×100 µm. Then select the





analysis from study block and number of mode is 1. Select the mode analysis frequency is

193.4 THz or 1.55 µm.

45

Figure 4.18 Light propagation inside square core PCF.



modeling input parameter has shown in Table 4-2. Firstly the desired parameters have

selected from global definitions. The geometry from component block is square in circular

solid core with circular air hole in cladding. The air hole (cladding) radius, r is 150 µm,

circular core having radius r/1.5 µm. Then select the materials from component block as

air and silica glass. The properties of air and silica glass is shown in Table 4-2. The





46

Figure 4.19 Light propagation inside circular core PCF.

By varying core diameter it is possible to shift zero dispersion wavelengths.

Dispersion can be calculated directly from effective mode index. In same modeling

condition SLPCF shows more effective mode index, which increase neff , a result of lower

dispersion, according to the following equation.

22 /][)/()( dnRdcD effe−=

Total Internal Reflection: When light pulse travels from one dense medium to

another dense it changes speed and is refracted. If the light rays are travelling for a less

dense material to a dense medium they are refracted towards the normal and if they are

travelling from a dense to less dense medium they are refracted away from the normal.

For total internal reflection to occur the light must travel from a dense medium to a less

dense medium (e.g. glass to air or water to air). As the angle of incidence increases so does

the angle of refraction. When the angle of incidence reaches a value known as the critical

angle the refracted rays travel along the surface of the medium or in other words are

refracted to an angle of 90°. The critical angle for the angle of incidence in glass is 42°.

The Figure 4.20 shows the Total Internal Reflection (TIR) within an optical fibre.

47

Figure 4.20 Total Internal Reflection (TIR).

When the angle of incidence of the light ray is greater than the critical angle then

no refraction takes place. Instead, all the light is reflected back into the denser material in

this case the glass. This is called total internal reflection.

Table 4-3 Modeling outlines of a TIR-Total Internal Reflection

Modeling outlines of a TIR-Total Internal Reflection [121, 122]


Rectangle

width= 5 µm

height= 10 µm

Polygon

P=0.1*height

x

vector=0,width,width,0

y vector=p,0.5*height,

height, height

Rectangle

Polygon

Air Silica Glass



Electric Conductivity: 0

S/m

Ratio of specific heats =

1.4

Refractive index (real

part) = 1

Refractive index





Density=2203 kgm-3

Coefficient of thermal expansion:

0.55e-6[1/K]


1.38[W/(m*K)]

Heat capacity at constant pressure=

703[J/(kg*K)]



Air

Glass

Incident

Ray

Reflected

Ray

48

Figure 4.21 Total Internal Reflection inside SLPCF.

The Figure 4.21 shows the Total Internal Reflection in the SLPCF. The geometry

is selected as a reactangle and a polygon. The height and width of rectangle 5 µm and

10 µm respectively. The Polygon parameter, p is 0.1*height. The x vector parameter is

0, width, width, 0 and y vector parameter is p, 0.5*height, height, height. Here the

incident angle is greater than the critical angle. Where the desired modeling input

parameter has shown in Table 4-3.

4.10 Conclusion

Various types of PCF structures and determined various optical properties of the

modeled PCFs using COMSOL Multiphysics are modeled. Especially, the TIR of the

PCFs are evaluated and compared for assessing the capability of PCF to use in long-

haul optical communication system.

49

CHAPTER 5

ANALYSIS OF RESULTS OF MODIFIED SLPCF

5.1 Introduction

In this chapter, the proposed square lattice PCF or the modified SLPCF is modeled

with an aim to mitigate dispersion in long-haul optical communication system. The optical

properties of the proposed SLPCF are evaluated to confirm its dispersion minimization

ability over other PCF structures.

5.2 Modeling and analysis of various SLPCF structures

The Figure 5.1 shows structure of a typical Elliptical core PCF with circular air hole


modeling input parameter has shown in Table 5-1.

5.2.1 Square lattice PCF with elliptical core

Figure 5.1 Model of a typical Elliptical core PCF with circular air hole only.

The optical parameters used for designing the elliptical core PCF is given in Table

5-1 below.

50

Table 5-1 Parameters of a typical Elliptical core PCF with circular air hole only.

Modeling outlines of a typical Elliptical core PCF with circular air hole only

Size of PCF

(µm)


cladding

layer

Types of

air hole

in

cladding

No. of air

holes in

cladding

Air hole

(cladding)

Radius=

150 µm

Outer shape

square size

300×300 µm

Elliptical core:

a-semi axis=105

µm

b-semi axis=85

µm

Circle for air

hole in

cladding.

Elliptical solid

core


only

155

Relative

permeability = 1

Relative

permittivity = 1

Electric

Conductivity: 0

S/m

Ratio of specific

heats = 1.4

Refractive index

(real part) = 1

Refractive index

(imaginary part)

= 0

Relative

permeability = 1

Relative

permittivity = 3.75

Electrical

conductivity = 1e-

14[S/m]

Density=2203

kgm-3

Coefficient of

thermal

expansion: 0.55e-

6[1/K]

Thermal

Conductivity =

1.38[W/(m*K)]

Heat capacity at

constant pressure=

703[J/(kg*K)]

Refractive index

(real part) = 1.5

Refractive index

(imaginary part) =

0

5.2.2 Circular core PCF with circular air hole


only.

51

Table 5-2 Modeling outlines of a typical circular core PCF.

Modeling outlines of a typical circular core PCF

Size of PCF

(µm)


cladding

layer

Types of

air hole in

cladding

No. of

air

holes

in

claddi

ng

Air hole

(cladding)

Radius=

150 µm

Outer shape

square size

300×300 µm

circular solid

core:

Radius = r/1.5

µm

Circle for air

hole in

cladding.

Solid Circle for

core


only

155

Relative

permeability = 1

Relative

permittivity = 1

Electric

Conductivity: 0

S/m

Ratio of specific

heats = 1.4

Refractive index

(real part) = 1

Refractive index

(imaginary part) =

0

Relative

permeability = 1

Relative

permittivity = 3.75

Electrical

conductivity = 1e-

14[S/m]

Density=2203

kgm-3

Coefficient of

thermal expansion:

0.55e-6[1/K]

Thermal

Conductivity =

1.38[W/(m*K)]

Heat capacity at

constant pressure=

703[J/(kg*K)]

Refractive index

(real part) = 1.5

Refractive index

(imaginary part) =

0

The design has done with the help of COMSOL Multiphysics 5.3a. Where the

desired modeling input parameter has shown in Table 5-2.

52

Figure 5.2 Model of a typical circular core PCF with circular air hole only.

The electromagnetic waves, frequency domain conditions from component block

would be selected as desired. Here the boundary has mentioned if required. Then the mesh

is selected as user defined as finer. Then perform the mode analysis from study block.

Select the mode analysis frequency is 193.4 THz or 1.55 µm.

5.2.3 Analysis of square core PCF

The desired modeling input parameter has shown in Table 5-3. In the design,, the


block is square in square solid core with circular air hole in cladding. The air hole

(cladding) radius, r is 150 µm, Outer shape square size is 300×300 µm, square core having

size of 100×100 µm. Consider the number of cladding layer 3. Types of air hole in cladding

is circular only. Number of air hole in cladding are 155. Then select the materials from

component block as air and silica glass. The properties of air and silica glass is shown in

Table 5-3. The electromagnetic waves, frequency domain conditions from component

block would be selected as desired. Here the boundary has mentioned if required. Then the

mesh is selected as user defined as finer. Then perform the mode analysis from study block.

Select the mode analysis frequency is 193.4 THz or 1.55 µm.

53

Table 5-3 Parameters of a typical square core PCF with circular air hole only.

Modeling outlines of a typical square core PCF with circular air hole only

Size of PCF

(µm)


cladding

layer

Types of

air hole in

cladding

No. of air

holes in

cladding

Air hole

(cladding)

Radius=

150 µm

Outer shape

square size

300×300 µm

square solid

core:

size 100×100

µm

Circle for air

hole in

cladding.

Solid square

for core


only

155

Relative

permeability =

1

Relative

permittivity = 1

Electric

Conductivity: 0

S/m

Ratio of

specific heats =

1.4

Refractive

index (real part)

= 1

Refractive

index

(imaginary part)

= 0

Relative

permeability = 1

Relative

permittivity = 3.75

Electrical

conductivity = 1e-

14[S/m]

Density=2203

kgm-3

Coefficient of

thermal

expansion: 0.55e-

6[1/K]

Thermal

Conductivity =

1.38[W/(m*K)]

Heat capacity at

constant pressure=

703[J/(kg*K)]

Refractive index

(real part) = 1.5

Refractive index

(imaginary part) =

0


only. The design has done with the help of COMSOL Multiphysics 5.3a.

54

Figure 5.3 Model of a typical square core PCF (SLPCF) with circular air hole only.

5.2.4 Square core PCF with circular and elliptical air hole

The Figure 5.4 shows structure of a typical square core PCF with circular and

elliptical air hole. The design has done with the help of COMSOL Multiphysics 5.3a.

Where the desired modeling input parameter has shown in Table 5-4. Firstly, the desired


square in square solid core with circular air hole in cladding. The circular air hole

(cladding) radius is 150 µm, elliptical air hole (cladding) having a-semi axis=25 µm b-semi

axis=10 µm Outer shape square size is 300×300 µm, square core having size of 100×100

µm. Consider the number of cladding layer 3. Types of air hole in cladding is circular and

elliptical. Where the circular cladding has 2 layer and elliptical has 1 layer. Number of air

hole in cladding are 155. Where circular air holes are 130 and elliptical air hole is 25. Then

select the materials from component block as air and silica glass. The properties of air and

silica glass is shown in Table 5-4. The electromagnetic waves, frequency domain




µm.

55

Table 5-4 Parameters of a square core PCF with circular and elliptical air hole.

Modeling outlines of a typical square core PCF with circular and elliptical air hole

Size of PCF (µm) Geometry Materials No. of

cladding

layer

Types of

air hole

in

cladding

No.

of air

holes

in

claddi

ng

Air hole (cladding)

Radius=

150 µm

Elliptical cladding

a-semi axis=25 µm

b-semi axis=10 µm

Outer shape

square size 300×300

µm

square solid core:

size 100×100 µm

Circle for

air hole in

cladding

Elliptical

cladding.

Solid

square for

core


(2 layer)

and

elliptical

(1 layer).

Circul

ar=13

0

Ellipti

cal=2

5

Total

=155

Relative

permeability = 1

Relative

permittivity = 1

Electric

Conductivity: 0

S/m

Ratio of specific

heats = 1.4

Refractive index

(real part) = 1

Refractive index

(imaginary part)

= 0


Relative permittivity =

3.75

Electrical conductivity =

1e-14[S/m]

Density=2203 kgm-3




1.38[W/(m*K)]




= 1.5

Refractive index


56

Figure 5.4 Modified SLPCF.

The Figure 5.4 shows the model of a typical square core PCF (Modified SLPCF)

with both circular elliptical air hole to analysis the compensation of dispersion property of

long-haul optical communication.

5.3 Effective mode analysis for various photonic crystal fibre (PCF)

In this work the proposed modeling shows the effective mode index for various

PCF for various required criteria described earlier in this chapter.

Figure 5.5 Effective mode index for circular core PCF with circular air hole only.

57

The Figure 5.5 shows the effective mode index for circular core PCF with circular

air hole only. Where the outer structure having the size of 300×300 µm. For circular core

PCF with circular air hole the effective mode index is shown by simulation is 0.98648.

Here the core size is 100×100 µm.

Figure 5.6 Effective mode index for elliptical core PCF with circular air hole only.

The Figure 5.6 shows the effective mode index for elliptical core PCF with circular

air hole only. Where the outer structure having the size of 300×300 µm. For elliptical core

PCF with circular air hole the effective mode index is shown by simulation is 0.92777.

Here the core size is a-semi axis is 25 µm and b-semi axis is 10 µm.

The Figure 5.7 shows the effective mode index (EMI) for square core PCF with

circular and elliptical air hole. Where the outer structure having the size of 300×300 µm.

For square core PCF with circular and elliptical air hole the effective mode index is shown

by simulation is 1.0003. Here the core size is 100×100 µm.

58

Figure 5.7 EMI for square core PCF with both circular and elliptical air hole.

The summarized values are shown in Figure 5.8 as follows.

Figure 5.8 Comparison of EMI for various PCFs.

5.4 Numerical Methods and Equations

The dispersion can be calculated or defined as the sum of wave-guide dispersions

and material dispersions (D = Dmaterial + Dwaveguide) [110]. The wave-guide dispersion in

PCFs can be easily controlled by modifying the air hole dimensions, numbers and center-

1 2 3

Series1 0.98648 0.92777 1.0003

0.98648

0.92777

1.0003

Effe

ctiv

e m

od

e in

de

x

1. Circular 2. Elliptical 3. Modified SLPCF

Effective mode analysis for various PCFs

59

to-center pitch [110]. In such configuration, the output waveform is distorted because of

noise, non-linear effects and dispersion. Dispersion D can be obtained using the following

relation [111]:

22 /][)/()( dnRdcD effe−= ……………………………………………....Equation (1)

Where, the effective refractive index (neff) is the measure of optical intensity

distribution.

For a photonic crystal fibre, the effective area is defined as the area where the light

is confined in a fibre. It can measure the spreading field within the cladding. Effective area

is expressed as,

= dxdyEdxdyEAeff

4

2

22

1 /)( ....................………………………………….Equation (2)

Where, E1 is the electric field amplitude in the medium. In photonic crystal fibre,

the effective area, Aeff combines the region of core and the region of a small portion of areas

in the cladding [112].

Wavelength is related with nonlinear coefficient as [90],

)/()2( 2 effAn = ....……………………………………………. …….Equation (3)

Where, n2 = 1.45 refractive index of silica glass. Because third order susceptibility

(χ(3)) the effect nonlinearity creates. If Aeff is increased then, nonlinearity (γ) will be

decreased according to equation (3).

If EMI is increased then Aeff will be increased according to equation (2). It is known

that, if Effective Mode Index (EMI) is increased as a result the measure of optical intensity

distribution i.e. neff is also be increased, therefore according to equation (1) dispersion D

(λ) will be will be decreased.

The Quality-factor (Q-factor) represents optical signal to noise ratio of a RoF link

when binary signals are transmitted. The Q-factor of a RoF link can be determined using

the following expression [113]:

)()( 0101 +−=Q ..………………… …………………….……………Equation (4)

Here, μ0 and σ0 refer to the mean and standard deviation, respectively, for the

Gaussian pulse, S0. Similarly, μ1 and σ1 refer to the mean and standard deviation,

respectively, for the Gaussian pulse, S1.

60

When two signals S0 and S1 with probability density functions (pdf) Pso and Ps1,

respectively, are transferred through a RoF link, the overlapping region between the pdf

represents the BER for S0 and S1, as shown in Figure 5.9 below.

Figure 5.9 BER characteristics.

The Figure 5.9 [114] shows the overlapping region between the probability density

function Pso and Ps1 of two symbols S0 and S1 represents the BER. In optical

communication, BER is generally defined as the ratio of total number of bits received in

error to the total number of bits transmitted through the transmission link and is given in

equation 5 [113]:

BER = Total bit received error Total bit transmitted………...……………Equation (5)

The Q factor and BER are related having a single valued function. Relation between

Q-factor and BER is given as follows [115]:

)2exp()2/1( 12 −−= QQBER ..………………………………...……...….....Equation (6)

If the relative permeability μr is increased for optoelectronic applications, then Q-

factor will be increased according to equation (5).

If Q-factor increased, then BER will be decreased according to equation (6).

5.5 Result of simulation of modification with respect to other PCFs

It is known that if the effective mode index (EMI) is increased then the value of

effective refractive index is increased and vice-versa. The simulation result shows that the

61

effective mode index (EMI) of modified SLPCF is noticeably higher than that of other

PCFs like circular photonic crystal fibre, elliptical photonic crystal fibre.

The Figure 5.10 shows that the dispersion properties for various PCF structures.

According to known method the effective refractive index is increased with the increased

of effective mode index (EMI). The dispersion for SLPCF is 30.76 [2], circular photonic

crystal fibre is -0.5E-14, elliptical photonic crystal fibre is -0.47E-14 and modified SLPCF

is -0.90E-14.

Figure 5.10 Dispersion of various PCF structures.

Due to the modification of core lattice it is found that the effective mode index

(EMI) of modified SLPCF is higher than that of other PCFs like circular photonic crystal

fibre, elliptical photonic crystal fibre. Therefore, according to the following equation the

value of dispersion is negatively increased that is dispersion is decreased.

22 /][)/()( dnRdcD effe−=

The summarized dispersion of various PCFs structures are shown in Figure 5.11.

PCF-Square

PCF-Elliptical

PCF-Circular

Modified SLPCF

62

Figure 5.11 Dispersion of various PCFs structures

The Figure 5.12 shows that the effective area properties for various PCF

structures.

Figure 5.12 Effective Area of various PCF structures

When Effective Mode Index (EMI) is increased then Aeff is also increased according

to the following equation.

= dxdyEdxdyEAeff

4

2

22

1 /)(

-5

0

5

10

15

20

25

30

35

1 2 3 4

Series1 30.76 -5.00E-15 -4.70E-15 -9.00E-15

30.76

-5.00E-15-4.70E-15

-9.00E-15

Dis

per

sio

n (

ps/

nm

/km

)

1. SLPCF 2. Circular 3. Elliptical 4. Modified SLPCF

Dispersion of various PCFs structures

63

The calculated value of the effective area from the designed structures for SLPCF

is 40000 µm2, circular photonic crystal fibre is 31416 µm2, elliptical photonic crystal fibre

is 280338 µm2 and modified SLPCF is 40785.4 µm2. Therefore, according to equation 2 it

is analyzed that the effective area of proposed modified SLPCF is higher than that of other

PCFs like circular photonic crystal fibre, elliptical photonic crystal fibre or ordinary

SLPCF. The summarized effective area of various PCF structures is shown in Figure 5.13

as follows.

Figure 5.13 Summarized effective Area of various PCF structures

The Figure 5.14 shows the nonlinearity properties for various PCF structures. It is

known that if effective area, Aeff is increased then, nonlinearity (γ) will be decreased

according to the following equation.

)/()2( 2 effAn =

Considering the value of refractive index of silica glass, n2 is 1.45. Wavelength is

1.55 µm. Then according to known method, the designed various structures show the

different values of nonlinearity. The nonlinearity of SLPCF is 1.47×1014 W

-1km-1, circular

photonic crystal fibre is 1.87×1014 W

-1km-1, elliptical photonic crystal fibre is 2.1×1014

W-

1km-1 and modified SLPCF is 1.40×1014

W-1km

-1.

1 2 3 4

Series1 40000 31416 28038 40785.4

40000

3141628038

40785.4

Effe

ctiv

e A

rea

of

PC

Fs (

µm

2)


Effective Area of various PCF structures

64

Figure 5.14 Nonlinearity, γ of various PCF structures.

From the modification it is found that the nonlinearity of modified SLPCF is much

lower than that of other PCFs like circular photonic crystal fibre, elliptical photonic crystal

fibre or ordinary SLPCF. The summarized nonlinearity, γ of various PCFs structures are

shown in Figure 5.15 as follows.

Figure 5.15 Summarized nonlinearity, γ of various PCF structures

1 2 3 4

Series1 1.47E+14 1.87E+14 2.10E+14 1.40E+14

1.47E+14

1.87E+142.10E+14

1.40E+14

No

nlin

ear

ity,

γ [

W-1

km-1

]


Nonlinearity of various PCF structures

PCF-Square

PCF-

Elliptical

PCF-Circular

Modified SLPCF

65

In a simple long-haul optical communication configuration is shown the output

waveform is distorted because of noise, non-linear effects and dispersion. A comparison

among various characteristics parameters obtained from simulation for the designed PCF

structures are summarized in the following table.

Table 5-5 Summary of PCF parameters from simulation.

Parameters D [ps2/km] Aeff [μm2] γ [W-1km

-1] Modified SLPCF -0.90E-14 40785.4 1.40E14

Circular Core -0.5E-14 31416 1.87E14

Elliptical Core -0.47E-14 28038 2.1E14

Square Core 30.76 [2] 40000 1.47E14

Ref. [32] - 1.9 -

Ref. [116] 1×10-17 - -

Ref. [117] 0.65 ± 0.05 - -

Ref. [118] 0.77, 0.99 - -

Ref. [119] -47500 - -

Ref. [120] - 16.2 -

It is observed that the modified SLPCF produces lower non-linearity, higher

effective area and higher negative dispersion. Whereas the circular core, elliptical core and

square core shows higher non-linearity, lower higher effective area and lower negative

dispersion. Therefore, the dispersion is significantly lower in the modified SLPCF than

those obtained earlier for asymmetrical structure, three rings of air-holes of elliptical

concentration and circular PCF. The effective area is high in the modified SLPCF than

those obtained earlier effective area of other PCFs.

5.6 Performance improvement in RoF link using SLPCF and EDFA

Now, a RoF link using SLPCF and EDFA is designed using OptiSystem software.

The optical performance of the designed RoF link is evaluated by varying the input power

about 1dBm, 10 dBm and 15 dBm of the CW laser used for driving the EDFA in a long-

haul optical link. By finding the BER, Q-factor, eye diagram and the optimum conditions

for minimized dispersion is evaluated from the simulation results. This work proposes a

modified SLPCF based RoF link incorporating EDFA and CW laser of definite power as a

new approach of FEC (or ECC) to improve the performance of RoF link [40].

66

5.8 Flow chart for RoF link design

A modified SLPCF Radio-over-fibre (RoF) link incorporating EDFA and CW laser

of definite power is proposed as a new approach of forward error correction (FEC). In

particular, the RoF model operates with CW laser driven at 1 dBm, 10 dBm and 15 dBm

input powers. The model is simulated considering 1010-bit sequence and the results are

compared with those using conventional SMF and previously described SLPCF

configuration.

Figure 5.16 Flow chart of the RoF link analysis.

It is observed that the proposed RoF link produces reduced dispersion and BER,

sharper eye diagram and both the improved Q-factor and eye height. Thus, the performance

of the RoF link using modified SLPCF incorporating EDFA and CW laser definite power

Mode analysis of square,

circular and elliptical

PCFs

Design of RoF link using

modified SLPCF and EDFA

(OptiSystem)

Assessment and

comparison with others

work

Optimized RoF link

Satisfactory

Modification of

Design

Unsatisfactory

Design and analysis of

square lattice PCF or

modified SLPCF

Analysis of Q-Factor,

BER and Eye

Diagram of RoF link

67

is noticeably improved, therefore, the model can be useful for cost-effective RoF link

design.

5.9 Design and analysis of RoF link using OptiSystem

The proposed model comprises user defined bit sequence generator, NRZ Pulse

generator, Mach-Zehnder as modulation technique, CW Laser, EDFA, a low pass Bessel

filter, BER analyzer, photodetector, SLPCF, optical spectrum analyzer (OSA), optical

power meter and a 50 km fibre link. A 50 km fibre link is discussed here; however, the

proposed model can be applicable for long-haul optical communication. The CW laser was

driven at optical input powers of 1dBm, 10 dBm and 15 dBm. The EDFA is used as a fibre

amplifier and its amplification window concurs with the third transmission window of

SLPCF. A complementary filter, namely, the Gaussian optical filter is used for arbitrary

pulse shape and filtering.

The modified SLPCF RoF link is simulated with Non-Return to Zero-bit sequence

1010 and initialized at 10 Gbps data sending rate and the simulated outputs are analyzed

for performance evaluation. From the BER analyzer, the Quality factor, BER pattern,

minimum BER, eye height is observed and from the OSA, the wave shapes of input power,

output power and noise are viewed. In addition, optical power meter gives the magnitude

of input and output powers, noise power, total power, signal power, sampled signal power

and parameterized signal power. The BER is varied based on the following components:

EDFA parameters, input power to the CW laser and the modulation technique. The Figure

5.18 shows flowchart for proposed RoF link using OptiSystem.

68

Figure 5.17 Flowchart for RoF link analysis using OptiSystem.

Table 5-6 Component Properties used in modeling RoF link.

SLPCF EDFA

Reference wavelength: 1550 nm

Length: 50 km

Attenuation: 0.2 dB/km

Dispersion slope: 0.075 ps/nm2/k

Differential Group Delay: 0.2 ps/km

Effective area: 80 μm2

Lower calculation limit: 1200 nm

Upper calculation limit: 1700 nm

Core radius: 2.2 μm

Er doping radius: 2.2 μm

Er metastable lifetime: 10 ms

Numeric Aperture: 0.24

Er ion density: 1e+025 m-3

Loss at 1550 nm: 0.1 dB/m

Loss at 980 nm: 0.15 dB/m

Length: 5m

Forward pump power: 100 mW

Backward pump power: 0 mW

Forward pump wavelength: 980 nm

Backward pump wavelength: 980 nm

Relative error: 0.0001

Maximum number of iterations: 50

Longitudinal steps: 100

Noise centre frequency: 193.4 THz

Noise Threshold: -100 dB

Start

Create Symbol Group

Calculated mean, standard

deviation and threshold

level

Plot BER Analyzer

Calculate BER and Q Factor

Perform Eye Diagram

End

69

The model of the modified SLPCF based RoF link incorporating EDFA and CW

laser of definite power is presented in Figure 5.19.

Figure 5.18 Proposed Model for improved RoF link.

In Figure 5.19 BER analyzer have three input terminals. Where one input is

connected to bit sequence generator (provides transmitted bit sequence), one input is

connected to low pass bessel filter (provide the received bit sequence), another input is

connected to NRZ pulse generator (allows users to create a sequence of non-return to zero

pulses that are coded by a digital signal input which makes the process continuous). As it

is known that BER analyzer calculates the transmitted bit sequence to the received bit

sequence and calculates the number of errors. BER is generally defined as the ratio of total

number of bits received in error to the total number of bits transmitted through the

transmission link.

5.10 Performance evaluation of RoF link

The configuration of a conventional SMF based RoF link is presented in Figure 5.19

and the simulated outputs for BER, Q-factor and eye-diagram can be understood from

Figure 5.20-5.28.

70

Figure 5.19 Simulation model using conventional SMF.

Here, Pseudo-Random Bit Sequence (PRBS) Generator: Sends the bit sequence to

the NRZ Pulse Generator. The user defined bit sequence generator can be used to generate

a bit sequence that is user defined. The transmitter is using a direct laser modulation scheme

the Photodetector PIN receives the optical signal attenuated by the Optical Attenuator

(OA), the Low Pass Bessel Filter filters the electrical signal. Optical Spectrum Analyzer

(OSA): Displays the modulated optical signal in the frequency domain.

Figure 5.20 Optical spectra at 1 dBm

The Figure 5.20 shows the power in digital format of the frequency domain

modulated optical signal for 1 dBm. At 1 dBm Optical Power Meter (OPM) shows the high

power loss and low active power as 39.708E-3W.

71

Figure 5.21 Optical spectra at 1dBm

The Figure 5.21 shows the frequency domain modulated optical signal for 1 dBm.

At 1 dBm Optical Spectrum Analyzer (OSA) shows the lower negative spectra than that of

at 1dBm at negative power in the range from-1dBm to -100dBm.



modulated optical signal for 10 dBm. At 10 dBm Optical Power Meter (OPM) shows the

lower power loss than that of 1 dBm and medium level of active power as 48.009E-3W.

72



At 10 dBm Optical Spectrum Analyzer (OSA) shows the lower negative spectra than that

of at 1dBm at power in the range from less than 10 dBm to -98 dBm.

\



modulated optical signal for 15 dBm. At 15 dBm Optical Power Meter (OPM) shows the

lower power loss than that of 1 dBm and 10 dBm and higher level of active power as

50.208E-3W.

73



At 15 dBm Optical Spectrum Analyzer (OSA) shows the lower negative spectra than that

of at 1dBm and 10 dBm at power in the range from 10 dBm to -93 dBm.

BER Analyzer: determine the BER of the system based on the signal propagation.

Double-click in an Analyzer show the result. The Mach-Zehnder Modulator is used for

signal modulation.

74

Figure 5.26 Q-factor for conventional SMF.

The Figure 5.26 shows the Q-factor for conventional SMF as 6.59497.

Figure 5.27 Eye pattern for conventional SMF.

The Figure 5.27 shows the eye diagram for conventional SMF as more noisy and

dispersed.

75

The Q-factor and BER pattern of the RoF link with previously used SLPCF, without

using EDFA and undefined input power of CW laser are shown in the following Figures:

Figure 5.28 Q- factor and BER pattern for previously used SLPCF.

The Figure 5.28 shows Q- factor and BER pattern for previously used SLPCF

without using EDFA and with undefined input power of CW laser.

Figure 5.29 Q-factor for 1dBm CW laser input power

76

The Figure 5.29 shows the Q-factor for 1dBm CW laser input power. The BER

pattern, Q-factor and eye diagram of the proposed RoF link for varied input powers to the

CW laser (1 dBm, 10 dBm and 15 dBm) are given in Figure 5.30-5.35.

Figure 5.30 Eye pattern for 1dBm CW laser input power.

The Figure 5.30 shows the Eye pattern for 1dBm CW laser input power.

Figure 5.31 Q-factor for 10dBm CW laser input power.

The Figure 5.31 shows the Q-factor for 10dBm CW laser input power.

77


The Figure 5.32 shows the Eye pattern for 10dBm CW laser input power.

Figure 5.33 Q- factor for 15dBm CW laser input power.

The Figure 5.33 shows the Q-factor for 15dBm CW laser input power. It is found

from Figure 5.33 and Figure 5.34 that the Q-factor of 40.7822, BER of tentatively 0 and

78

eye height of 8.29269×10-2 are produced for the RoF link incorporating EDFA and CW

laser driven at 15 dBm input power.


The Figure 5.34 shows the Eye pattern for 15dBm CW laser input power. From the

simulation results, the Q-factor, BER pattern and eye height of the modified SLPCF

incorporating EDFA and CW driven at definite power are extracted and compared with

those observed for conventional SMF and previously used SLPCF-the results are

summarized in Table 5-7.

Table 5-7 Summary of performance of RoF link from simulation results.

Parameters Q-factor BER Eye Height

Conventional SMF 6.59497 2.12369×10-11 6.56285×10-5

Previously used SLPCF 6.7715 5.78501×10-12 3.99937×10-2

Proposed

Modified

SLPCF

For 1dBm 26.7827 2.18494×10-158 5.66037×10-2

For 10dBm 30.7237 1.07765×10-207 7.33338×10-2

For 15dBm 40.7822 0 (Approximately) 8.29269×10-2

Ref. [43] - 10-9 -

Ref. [44] - 2.13×10-2 -

Ref. [113] - 3.9942x10-8 -

Ref. [116] - 1×10-17 -

It is found from Table 5-7 that when conventional SMF is used, the RoF link shows

a Q-factor of 6.59497, BER of 2.12369×10-11 and eye height of 6.56285×10-5 (Figure 5.26-

79

5.27). Therefore, high BER causes the signal noisy in conventional SMF (Figure 5.27) and

the dispersion becomes high. Previously, coventional SMF was replaced by SLPCF to

reduce dispersion and BER. Therefore, the simulated RoF link using SLPCF (without

EDFA and definite input power CW laser) as shown in Figure 6.14, then the Q-factor of

6.7715, BER of 5.78501×10-12 and eye height of 3.99937×10-2 are found. When compared

with the conventional SMF, the Q-factor and eye height are not significantly improved

(although the eye diagram is found sharper) in the SLPCF than those obtained in the

conventional SMF. Therefore, SLPCF without definite input power to CW laser and without

EDFA does not accurately compensate dispersion and as a result, a small amount of noise

is associated with the BER pattern (Figure 5.35). To overcome this dilemma, a RoF link is

proposed using SLPCF incorporating EDFA and CW laser operated at 1 dBm, 10 dBm and

15 dBm input powers. The simulation results of the RoF link are summarized next.

For 1 dBm input power to CW laser:

It is observed that the Q-factor of 26.7827, BER of 2.18494×10-158 and eye height of

5.66037×10-2 are achieved in the RoF link incorporating EDFA and CW laser driven at 1

dBm input power (Figure 5.29 and Figure 5.30). Therefore, the Q-factor and eye height are

improved than those obtained in previously used SLPCF and conventional SMF.

Furthermore, the BER becomes lower than those of previously used SLPCF and

conventional SMF.

For 10 dBm input power to CW laser and:

When the CW laser is driven at 10 dBm, the proposed RoF link produces Q-factor

of 30.7237, BER of 1.07765×10-207 and eye height of 7.33338×10-2 (Figure 5.31 and Figure

5.32). Here, the Q-factor and eye height are increased and the BER is decreased compared

to those obtained for previously used SLPCF and conventional SMF, and also for 1dBm

input power to CW laser.

For 15 dBm input power to CW laser:

It is found from (Figure 5.33 and Figure 5.34). that the Q-factor of 40.7822, BER of

tentatively 0 and eye height of 8.29269×10-2 are produced for the RoF link incorporating

EDFA and CW laser driven at 15 dBm input power. In this case, the Q-factor is improved

(as high as 40.7822 for 15 dBm to CW), which is higher than those obtained by using

conventional SMF (6.59497) and SLPCF (6.7715). Moreover, the Q-factor and eye height

80

are significantly increased than those for 1 dBm or 10 dBm input power. In addition, the

BER is found zero with a sharper eye diagram.

Therefore, the BER is significantly reduced (tentatively zero) in the modified RoF

link incorporating EDFA and CW laser than those obtained earlier: approximately 10-9 in

[43], 2.13×10-2 in [44], 3.9942x10-8 for 8 levels PAM in [113] and 1×10-17 in [116].

Moreover, the eye diagram becomes sharper in this model compared with those presented

earlier [114]. Therefore, numerical results of the modified SLPCF have shown a

remarkable reduction in BER, enhancements in the Q-factor as well as compensation of

dispersion.

5.11 Conclusion

Various dispersion compensating PCFs lattice structures are investigated. The

design parameters are shown in different table as necessary. The total modeling has been

done using COMSOL Multiphysics 5.3a. The structures modeled here are square in

elliptical core with three layer circular air hole only, square in circular core with three layer

circular air hole only, square in square core with three layer circular air hole only and the

modification has done by inserting elliptical air hole as first layer within the cladding and

rest of the cladding layer was inserted as circular air hole. Then compute the structures

according to description from the chapter 4. RoF link with SLPCF incorporating EDFA

and CW laser of definite power has been proposed as a new approach of forward error

correction (FEC). The model has been simulated considering 1010-bit sequence using CW

laser driven at definite powers of 1 dBm, 10 dBm and 15 dBm. The simulation results show

that the BER and dispersion of the modified SLPCF RoF link are significantly reduced

compared to those obtained using conventional SMF and previously described SLPCF

configuration. In addition, the eye diagram is found sharper and both the Q-factor and eye

height are improved in the proposed link. Therefore, due to the improved RoF link

performance such as lower BER, reduced dispersion, enhanced Q-factor and improved eye

height, along with the advantageous attributes of SLPCF, the modified SLPCF with EDFA

and CW laser of definite power could be very beneficial for cost effective RoF optical

communication and related applications.

81

CHAPTER 6

CONCLUSION AND DISCUSSION

6.1 Conclusion

The effect of dispersion, nonlinearity, and others losses play an important role in

modeling the PCFs for the formation of an effective pulse compressor. The solution could

be a modified SLPCF, is designed and the performance of SLPCF on dispersion

compensation is evaluated over other PCFs of various lattice structures. The performance

of dispersion compensated long-haul optical communication system employing SLPCF

and EDFA is analyzed. RoF link is designed for improvement in dispersion, BER, Q-factor,

eye diagram using SLPCF incorporating EDFA.

A modified SLPCF RoF link incorporating EDFA and CW laser of definite power

has been proposed as a new approach of forward error correction (FEC). The model has

been simulated considering 1010-bit sequence using CW laser driven at definite powers of

1 dBm, 10 dBm and 15 dBm. The simulation results show that the BER and dispersion of

the modified SLPCF RoF link are significantly reduced compared to those obtained using

conventional SMF and previously described SLPCF configuration. In addition, the eye

diagram is found sharper and both the Q-factor and eye height are improved in the proposed

link. Therefore, due to the improved RoF link performance such as lower BER, reduced

dispersion, enhanced Q-factor and improved eye height, along with the advantageous

attributes of SLPCF, namely, negative dispersion and high birefringence, the modified

SLPCF with EDFA and CW laser of definite power could be very beneficial for cost

effective RoF optical communication and related applications like an effective pulse

compressor..

6.2 Scopes for future research

The proposed modified SLPCF model is notably unsophisticated than other

structures proposed so far for compensating the dispersion and at the same time higher

effective mode index characteristics have been obtained at 1.55 μm. Additionally our

proposed modified SLPCF model shows low confinement losses, which are novel

properties in lower dispersion design. The main conclusion of this systematic approach is

82

that with a modest number of design parameters including Square in square with circular

and elliptical air hole which could optimize the dispersion properties of the PCF. Our

architecture is suitable for applications as a dispersion controller, dispersion compensator,

pulse compressor etc. Hence, there is scope of improving other properties. Besides, the

fabrication of proposed fibre which is also a big challenge. Moreover, there are also some

sensitivity properties which have great impact. These effects could not find because of

constraints of time and resource. Those properties should also be taken under consideration

while designing a sensing application.

However, the modified model will have an impact on compensating the way to

design better RoF link communication. The attempt of modifying SLPCF as a dispersion

compensation module will have a contribution of introducing effective RoF link

communication with most favorable characteristics like higher Q-factor, lower BER,

sharper eye diagram etc. The proposed modified model can also be implemented in

optoelectronic devices like electronic displays in future.

83

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