ANALYSIS OF DISPERSION COMPENSATION ... OF DISPERSION COMPENSATION TECHNIQUES AND FIBER NON-LINEAR EFFECTS IN DWDM OPTICAL NETWORKS A PROJECT REPORT Submitted by SATHYA V Register

ANALYSIS OF DISPERSION COMPENSATION

TECHNIQUES AND FIBER NON-LINEAR

EFFECTS IN DWDM

OPTICAL NETWORKS

A PROJECT REPORT

Submitted by

SATHYA V

Register No: 14MCO021

in partial fulfillment for the requirement of award of the degree

of

MASTER OF ENGINEERING

in

COMMUNICATION SYSTEMS

Department of Electronics and Communication Engineering

KUMARAGURU COLLEGE OF TECHNOLOGY

(An autonomous institution affiliated to Anna University, Chennai)

COIMBATORE-641049

ANNA UNIVERSITY: CHENNAI 600 025

APRIL 2016

ii

BONAFIDE CERTIFICATE

Certified that this project report titled “ANALYSIS OF DISPERSION

COMPENSATION TECHNIQUES AND FIBER NON-LINEAR EFFECTS IN

DWDM OPTICAL NETWORKS” is the bonafide work of SATHYA.V [Reg. No.

14MCO021] who carried out the research under my supervision. Certified further that, to

the best of my knowledge the work reported herein does not form part of any other project

or dissertation on the basis of which a degree or award was conferred on an earlier

occasion on this or any other candidate.

HHHH

The Candidate with Register No. 14MCO021 was examined by us in the project viva –

voice examination held on ............................

INTERNAL EXAMINER EXTERNAL EXAMINER

SIGNATURE

Ms. R.HEMALATHA

PROJECT SUPERVISOR

Department of ECE

Kumaraguru College of Technology

Coimbatore-641 049

SIGNATURE

Dr. A.VASUKI

HEAD OF THE DEPARTMENT

Department of ECE

Kumaraguru College of Technology

Coimbatore-641 049

iii

ACKNOWLEDGEMENT

First, I would like to express my praise and gratitude to the Lord, who has

showered his grace and blessings enabling me to complete this project in an excellent

manner.

I express my sincere thanks to the management of Kumaraguru College of

Technology and Joint Correspondent Shri Shankar Vanavarayar for his kind

support and for providing necessary facilities to carry out the work.

I would like to express my sincere thanks to our beloved Principal

Dr.R.S.Kumar Ph.D., Kumaraguru College of Technology, who encouraged me with

his valuable thoughts.

I would like to thank Dr.A.Vasuki Ph.D., Head of the Department, Electronics

and Communication Engineering, for her kind support and for providing necessary

facilities to carry out the project work.

In particular, I wish to thank with everlasting gratitude to the Project

Coordinator Dr.M.Alagumeenaakshi Ph.D., Assistant Professor- III, Department of

Electronics and Communication Engineering, throughout the course of this project

work.

I am greatly privileged to express my heartfelt thanks to my project guide

Ms.R.Hemalatha M.E.,(Ph.D), Associate Professor, Department of Electronics and

Communication Engineering, for her expert counselling and guidance to make this

project to a great deal of success and I wish to convey my deep sense of gratitude to

all teaching and non-teaching staff of ECE Department for their help and cooperation.

Finally, I thank my parents and my family members for giving me the moral

support and abundant blessings in all of my activities and my dear friends who helped

me to endure my difficult times with their unfailing support and warm wishes.

iv

ABSTRACT

Dense Wavelength Division Multiplexing (DWDM) is an extension of optical

networking. DWDM devices combine the output of more than eight optical

transmitters for transmission across a single optical fiber. Problems like dispersion,

cross talk and other non-linear effects occur in optical networks. Dispersion is the

spreading out of a signal as it travels down the fiber. Chromatic dispersion and

polarization mode dispersion (PMD) affects the DWDM. Chromatic Dispersion (CD)

is a major factor in the transmission of data over a long haul application. Many

techniques can be used to overcome the losses caused by CD.

The proposed work focuses on the dispersion and its compensation techniques

and methods to alleviate/suppress fiber Non linear effects on DWDM networks. In

order to compensate for the dispersion, various compensation techniques like Fiber

Bragg Grating (FBG), Dispersion Compensation Fiber (DCF), Electronic Dispersion

Compensation (EDC), and Optical filter are employed. The DWDM architecture is

implemented using OptSim. The simulation results show the performance of the

DWDM system in terms of Bit Error Rate (BER), Q-factor and Eye diagram.

Optical Nonlinearities give rise to many ubiquitous effects in optical

transmission system. Power level carried by fiber increases which generates nonlinear

effect such as Self Phase Modulation (SPM), Cross Phase Modulation (XPM) and

Four Wave Mixing (FWM). The effect of Cross phase modulation in DWDM system

is reduced inducing the negative optical dispersion effect. The effect of Self Phase

Modulation is eliminated through parametric runs from -10ps/nm/km to

10ps/nm/km.The FWM leads to crosstalk in DWDM system whose channel spacing is

narrow. By introducing appropriate positive/negative effects and by providing unequal

spacing between channels it is possible to suppress FWM crosstalk.

TABLE OF CONTENTS

CHAPTER

NO TITLE

PAGE

NO

ABSTRACT iv

LIST OF TABLES vii

LIST OF FIGURES viii

LIST OF ABBREVIATIONS xi

1 INTRODUCTION 1

1.1 Overview of optical fiber 1

1.2WDM 1

1.3 DWDM 3

1.4 DWDM systems 3

1.5 Introduction to Optsim tool 5

1.5.1 Features 6

2 LITERATURE SURVEY 8

3 PROPOSED SYSTEM 11

3.1Dispersion 11

3.2 Chromatic Dispersion 11

3.2.1 Waveguide Dispersion 11

3.2.2 Material dispersion 12

3.3 Polarization mode dispersion 12

3.4 Compensation Techniques 13

3.4.1 Fiber Bragg Grating 13

3.4.2 Dispersion Compensation Fiber 14

3.4.2.1 Pre-Compensation technique 15

3.4.2.2 Post-Compensation technique 15

3.4.2.3Symmetric-Compensation technique 15

3.4.3 Electronic Dispersion compensation 16

3.4.4 Optical Filter 17

3.5 Non Linear Effects 18

3.5.1 Self phase modulation 18

3.5.2 Cross phase modulation 20

3.5.3 Four wave mixing 22

4 RESULTS AND DISCUSSION 24

4.1 DWDM-(without Compensation) 24

4.2 DWDM with Dispersion Compensation 29

4.2.1 Fiber Bragg Grating 29

4.2.2 Dispersion Compensation Fiber 33

4.2.2.1 Pre-Compensation 33

4.2.2.2 Post-Compensation 36

4.2.2.3 Symmetric Compensation 38

4.2.3 Electronic Dispersion Compensation 40

4.2.4 Optical filter 42

4.3 Non Linear Effects 49

4.3.1 Cross Phase Modulation 49

4.3.2 Self Phase Modulation 52

4.3.3 Four Wave Mixing 56

5 CONCLUSION 62

6 REFERENCES 64

LIST OF PUBLICATIONS 67

vii

LIST OF TABLES

S.No. CAPTION PAGE NO.

4.1

Analysis of various Dispersion Compensation techniques

using BER as the performance indicator

48

4.2 Analysis of different dispersion compensation techniques

using Q parameter as a performance metric

49

viii

LIST OF FIGURES

FIGURE

NO. CAPTION PAGE NO.

1.1 Wavelength-division multiplexing (WDM) 2

3.1 Chromatic Dispersion 11

3.2 Different fiber brag gratings structures 13

3.3 Block diagram of pre-compensation 15

3.4 Block diagram of post compensation 15

3.5 Block diagram of Symmetric compensation 16

3.6 Block diagram of Electronic Dispersion compensation 16

3.7 Self Phase Modulation 18

3.8 Formation of Fourth Spurious Component 22

4.1 Design of Dense Wavelength Division Multiplexing (without

compensation) 24

4.2 Dispersion Map for without Compensation 27

4.3 Eye diagram of receiver 8 28

4.4 BER plot for without Compensation 28

4.5 Q-factor plot for without Compensation 29

4.6 Design of DWDM using fiber bragg grating 29


4.8 BER plot for FBG without EDFA 31

4.9 Q-factor plot for FBG without EDFA 31


4.11 BER plot for FBG with EDFA 32

4.12 Q- plot for FBG with EDFA 33

4.13 Design of DWDM using DCF (pre-compensation) 34

ix

LIST OF FIGURES

FIGURE



4.15 BER plot for pre compensation 35

4.16 Q-factor plot for pre compensation 35

4.17 Design of DWDM Using DCF (Post Compensation) 36


4.19 BER plot for post compensation 37

4.20 Q- factor plot for post compensation 38

4.21 Design of DWDM Using DCF (Symmetric Compensation) 38


4.23 BER plot for Symmetric compensation 39

4.24 Q-factor plot for Symmetric compensation 40

4.25 Design of DWDM Using Electronic Dispersion Compensation 41


4.27 BER plot for Electronic dispersion compensation 42

4.28 Q-factor plot for Electronic dispersion compensation 42

4.29 Design of DWDM Using optical filter 43



4.32 BER plot for lorentzian filter 45

4.33 Q-factor plot for lorentzian filter 45


4.35 BER plot for fabry- perotfilter 46

4.36 Q-factor plot for fabry- perot filter 47

x

LIST OF FIGURES

FIGURE


4.37 Simulation Setup of Cross Phase Modulation (XPM) 50



4.40 Dispersion value vs BER &Q-factor 52

4.41 Simulation Setup of Self Phase Modulation (SPM) 53

4.42 Eye diagram for before Self Phase Modulation 54

4.43 Eye diagram for after Self Phase Modulation of distance 50km

and dispersion value = -5ps/nm/km 54

4.44 Eye diagram for after Self Phase Modulation of distance

100km and dispersion value = -5ps/nm/km 55

4.45 Eye diagram for after Self Phase Modulation of distance

140km and dispersion value = -5ps/nm/km 55

4.46 Length of fiber vs BER &Q-factor 56

4.47 Simulation Setup of Four Wave Mixing (FWM) 57

4.48 Input spectrum 57

4.49 Output spectrum Dispersion value=1ps/nm/km 58



4.52 Output spectrum Dispersion value =4ps/nm/km 59



4.55 Equal channel spacing 61

4.56 Unequal channel spacing 61

xi

LIST OF ABBREVIATIONS

WDM Wavelength Division Multiplexing

DWDM Dense Wavelength Division Multiplexing

TDM Time Division Multiplexing

CW LASER Continuous Wave LASER

MUX Multiplexer

DEMUX Demultiplexer

FBG Fiber Bragg Grating

DCF Dispersion Compensation Fiber

EDC Electronic Compensation Fiber

EDFA Erbium Doped Fiber Amplifier

PBRS Pseudo Random Bit Sequence

BER Bit Error Rate

PON Passive Optical Network

XPM Cross Phase Modulation

SPM Self Phase Modulation

FWM Four Wave Mixing

1

CHAPTER 1

INTRODUCTION

1.1 OVERVIEW OF OPTICAL FIBER

Fiber-optic communication is a method of transmitting information from one

place to another by sending pulses of light through an optical fiber. The light forms an

electromagnetic carrier wave that is modulated to carry information. Because of its

advantages over electrical transmission, optical fibers have largely replaced copper

wire communications in core networks in the developed world. Optical fiber is used

by many telecommunications companies to transmit telephone signals, internet

communications and cable television signals. Researchers at Bell Labs have reached

internet speeds of over 100 data bits per second using fiber-optic communication. The

process of communicating using fiber-optics involves the following basic steps:

Creating the optical signal involving the use of a transmitter, relaying the signal along

the fiber, ensuring that the signal does not become too distorted or weak, receiving the

optical signal, and converting it into an electrical signal.

The important features are,

Immunity to electromagnetic interference, including nuclear electromagnetic

pulses (although fiber can be damaged by alpha and beta radiation).

High electrical resistance, making it safe to use near high-voltage equipment or

between areas with different earth potentials.

Lighter weight – important, for example, in aircraft.

Much smaller cable size – important where pathway is limited, such as

networking an existing building, where smaller channels can be drilled and

space can be saved in existing cable ducts and trays.

1.2 WDM

In fiber-optic communications, wavelength-division multiplexing (WDM) is a

technology which multiplexes a number of optical carrier signals onto a single optical

2

fiber by using different wavelengths (i.e., colors) of laser light as shown in Fig 1.1.

This technique enables bidirectional communications over one strand of fiber, as well

as multiplication of capacity.

Fig 1.1 Wavelength-division multiplexing (WDM)

A WDM system uses a multiplexer at the transmitter to join several signals

together and a demultiplexer at the receiver to split them apart. With the right type of

fiber it is possible to have a device that does both simultaneously and can function as

an optical add-drop multiplexer. The optical filtering devices used have

conventionally been etalons (stable solid-state single-frequency Fabry–Pérot

interferometers in the form of thin-film-coated optical glass).

Most WDM systems operate on single-mode fiber optical cables, which have a

core diameter of 9 µm. Certain forms of WDM can also use multi-mode fiber cables

(also known as premises cables) which have core diameters of 50 or 62.5 µm.

WDM systems are divided into different wavelength patterns,

conventional/coarse wavelength division multiplexing (CWDM) and Dense

wavelength division multiplexing (DWDM). Conventional WDM systems provide up

to 8 channels in the 3rd transmission window (C-Band) of silica fibers around 1,550

nm. Dense wavelength division multiplexing (DWDM) uses the same transmission

window but with denser channel spacing. Channel plans vary, but a typical system

would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing.

Some technologies are capable of 12.5 GHz spacing (sometimes called ultra dense

3

WDM). New amplification options (Raman amplification) enable the extension of the

usable wavelengths to the L-band, more or less doubling these numbers.

1.3 DWDM

Dense Wavelength Division Multiplexing (DWDM) is a technology that allows

multiple information streams to be transmitted simultaneously over a single fiber. This

provides a cost effective method to increase the capacity of the existing networks

without the need to add additional fiber [3].

1.4 DWDM systems

The basic DWDM system contains several main components:

1. DWDM terminal multiplexer: The terminal multiplexer contains a wavelength-

converting transponder for each data signal, an optical multiplexer and if necessary an

optical amplifier (EDFA). Each wavelength-converting transponder receives an

optical data signal from the client-layer, such as Synchronous optical networking

[SONET /SDH] or another type of data signal, converts this signal into the electrical

domain and re-transmits the signal at a specific wavelength using a 1,550 nm band

laser. These data signals are then combined together into a multi-wavelength optical

signal using an optical multiplexer, for transmission over a single fiber (e.g., SMF-28

fiber). The terminal multiplexer may or may not also include a local transmit EDFA

for power amplification of the multi-wavelength optical signal. In the mid-1990s

DWDM systems contained 4 or 8 wavelength-converting transponders; by 2000 or so,

commercial systems capable of carrying 128 signals were available.

2. Intermediate line repeater: It is placed approximately every 80–100 km to

compensate for the loss of optical power as the signal travels along the fiber. The

'multi-wavelength optical signal' is amplified by an EDFA, which usually consists of

several amplifier stages.

3. Intermediate optical terminal or optical add-drop multiplexer: This is a remote

amplification site that amplifies the multi-wavelength signal that may have traversed

up to 140 km or more before reaching the remote site. Optical diagnostics and

telemetry are often extracted or inserted at such a site, to allow for localization of any

4

fiber breaks or signal impairments. In more sophisticated systems (which are no

longer point-to-point), several signals out of the multi-wavelength optical signal may

be removed and dropped locally.

4. DWDM terminal demultiplexer: At the remote site, the terminal de-multiplexer

consisting of an optical de-multiplexer and one or more wavelength-converting

transponders separates the multi-wavelength optical signal back into individual data

signals and outputs them on separate fibers for client-layer systems (such as

SONET/SDH). Originally, this de-multiplexing was performed entirely passively,

except for some telemetry, as most SONET systems can receive 1,550 nm signals.

However, in order to allow for transmission to remote client-layer systems (and to

allow for digital domain signal integrity determination) such de-multiplexed signals

are usually sent to O/E/O output transponders prior to being relayed to their client-

layer systems. Often, the functionality of output transponder has been integrated into

that of input transponder, so that most commercial systems have transponders that

support bi-directional interfaces on both their 1,550 nm (i.e., internal) side, and

external (i.e., client-facing) side. Transponders in some systems supporting 40 GHz

nominal operation may also perform forward error correction (FEC) via digital

wrapper technology, as described in the ITU-T G.709 standard.

5. Optical Supervisory Channel (OSC): This is data channel which uses an

additional wavelength usually outside the EDFA amplification band (at 1,510 nm,

1,620 nm, 1,310 nm or another proprietary wavelength). The OSC carries information

about the multi-wavelength optical signal as well as remote conditions at the optical

terminal or EDFA site. It is also normally used for remote software upgrades and user

(i.e., network operator) Network Management information. It is the multi-wavelength

analogue to SONET's DCC (or supervisory channel). ITU standards suggest that the

OSC should utilize an OC-3 signal structure, though some vendors have opted to use

100 megabit Ethernet or another signal format. Unlike the 1550 nm multi-wavelength

signal containing client data, the OSC is always terminated at intermediate amplifier

sites, where it receives local information before re-transmission.

5

Applications:

Long-haul optical networks either in point-to-point or ring topology.

Expanding the capacity of an existing optical network.

Capacity leasing for network wholesalers.

Advantages:

Transparency—Because DWDM is a physical layer architecture, it can

transparently support both TDM and data formats such as ATM, Gigabit

Ethernet, ESCON, and Fiber Channel with open interfaces over a common

physical layer.

Scalability—DWDM can leverage the abundance of dark fiber in many

metropolitan area and enterprise networks to quickly meet demand for capacity

on point-to-point links and on spans of existing SONET/SDH rings.

Dynamic provisioning—Fast, simple, and dynamic provisioning of network

connections give providers the ability to provide highbandwidth services in

days rather than months.

Disadvantages:

Dispersion

Cross-phase modulation(XPM)

Crosstalk etc.

1.5 INTRODUCTION TO OPTSIM TOOL

OptSim, Rsoft’s award-winning software tool for the design and simulation of

optical communication systems at the signal propagation level empowers the users

with models and simulation techniques that are specifically designed for PM-QPSK

and other advanced modulation formats including OFDM, D(QPSK) and duo binary.

It is basically an advanced optical communication system designed for professional

engineers. It can be used to design optical communication systems and simulate them

6

to determine their performance given various component parameters. With user

friendly simulation techniques and easy-to-use graphical user interface, OptSim

provides unmatched flexibility and usability.

1.5.1 FEATURES

1. Performance analysis (e.g. Q value, BER, Power spectra and OSNR, eye diagram).

2. Wide and complete choice of measurement (e.g. jitter, eye opening/closure,

electrical/optical spectra, chirp, optical instantaneous phase/frequency and power).

3. Link optimization: power budget, dispersion map, tailoring of pulse shape and chip,

transmitter pre-emphasis, amplifier positioning.

4. Transmission impairment analysis and assessment of countermeasures (e.g. All-

order PMD, SPM, XPM, FWM, Stimulated Raman Scattering effect).

5. Edge design and validation system sensitivity evaluation.

6. Extensive library of predefined manufacturer components makes it easy to model

commercially available devices.

OptSim works on the theory of ―blocks‖. An optical communication system is

represented as an interconnection of various blocks. Each block in this set represents a

component or subsystem in the communication system. Each block model is presented

graphically as an icon, has own set of parameters which can be modified by user. As

physical signals are passed between components in a real world communication

system, ―signal‖ data is passed between component models in the OptSim simulation.

Each block is simulated independently using the parameters specified by the user for

that block and the signal information passed into it from other blocks. This is known

as a block-oriented simulation methodology. These blocks are graphically represented

as icons in OptSim. Internally, they are represented as data structures and

sophisticated numerical algorithms.

The twin simulation engines support two complementary simulation approaches.

7

1. Block mode simulation engine: signal data is represented as one block of data and

is passed between block to block. Nonlinear fiber is simulated using the Split Step

Fourier technique in this mode.

2. Sample mode simulation engine: signal data is represented as single sample that is

passed between block to block.

Results Analysis and Post Processing:

Stage 1: General Model (Modelling preliminaries)

Stage 2: Select optimum parameters (Performance Evaluation)

Stage 3: See results after simulation (Optsim Validation).

8

CHAPTER 2

LITERATURE SURVEY

2.1 INTRODUCTION

This chapter presents the literature surveyed in the area of DWDM.The merits

and demerits of different methods are discussed in terms of complexity, performance

and speed of computation. Dispersion is a big factor which degrades the performance

of optical communication networks. Due to this several fiber based and devices based

compensation techniques had been developed to limit their effect. The fiber and

device based compensation techniques are Dispersion Compensation Fiber, Fiber

Bragg Grating and Electronic Dispersion Compensation.

[1] An overview of fiber dispersion and nonlinearity compensation techniques in

optical orthogonal frequency division multiplexing systems

T. Ilavarasan et al,says the performance of analogue RoF suffers from noise

and linearity issues and digital RoF is degraded by fiber dispersion and nonlinearity

due to high rate of transmission .Several techniques were discussed to overcome fiber

dispersion and nonlinear effects in WDM systems. No specific compensation

technique is suited for all kind of optical systems. Therefore, the fiber dispersion and

nonlinearity compensation technique should be carefully chosen according to various

system requirements and the applications.

[2] Performance analysis of hybrid TDM/DWDM optical communication system

in the presence of FWM and ASE noise.

R.Hemalatha et al,explained the hybrid Dense Wavelength Division

Multiplexing (DWDM) and Time Division Multiplexing (TDM) Passive Optical

Network (PON).This architecture increases total number of sensors that can be

supported in a single fiber, at the same time it behaves well when compared with the

individual architectures of TDM and DWDM. The system performance degrades due

to Four-Wave Mixing (FWM) and Amplified Spontaneous Emission (ASE) noise.

The ASE noise is filtered out using a Mach-Zehnder Interferometer at receiver end

and the performance has been analyzed with eye patterns.

9

[3] Various dispersion compensation techniques for optical system

N.K. Kahlon et al,says the dispersion compensation is the most important

feature required in optical fiber communication system because, absence of it leads to

pulse spreading that cause the output pulses to overlap. Various dispersion

compensation techniques are discussed.

[4] Performance analysis of dispersion compensation using fiber bragg grating

(FBG) in optical communication

Kaushal Kumar et al, analyzed the dispersion compensation using Fiber

Bragg Grating at different fiber lengths. By varying input power (dBm), fiber cable

length (km), FBG Length (mm) and attenuation coefficient (dB/km) at cable section,

four different parameters had been investigated which are output power (dBm), noise

figure (dB), gain (dB) and Q-Factor(dB) at receiver. When input power (dBm) and

output power (dBm) are increased, gain (dB) and Q-Factor (dB) decrease. When FBG

Length (mm) is increased the output power (dBm), noise figure (dB), gain (dB), and

Q-Factor(dB) became nonlinear due to EDFA and also the gain has been

compressed.[6]

[5]An improved methodology for dispersion compensation and synchronization

in optical fiber communication networks

Ajeet Singh Verma et al,says in long haul application, dispersion is the main

parameter which needs to be compensated in order to provide high level of reliability

of service (ROS). Fiber Braggs Grating (FBG) is one of the most widely used element

to compensate it, however its performance slows down with the increase in distance.

Dispersion compensation method offers improved value of performance parameters

such as Q-facter, Min BER and threshold value compared to FBG compensation

technique. Eye diagram shows better value of threshold and eye height. DCF method

offers reduced dispersion and improved synchronization in long haul applications [4].

[6] An overview of DWDM technology &network

Reena Antil et al, says about functions and applications of DWDM system

components. The operation of each component is discussed individually.DWDM

10

terminology like Attenuation, dispersion, and optical signal to noise ratio (OSNR) are

measures of optical signal quality and are the key factors involved in DWDM system

design and operation [9].

[7 ] Architecture to integrate multiple pons with long reach DWDM backhaul

D.P.Shea et al, says about feasibility of an architecture that consolidates a

number of deployed Passive Optical Network (PON) infrastructures into a long-reach,

high-split ratio system which further increases equipment sharing between users. The

demonstrated system allows the use of uncooled lasers with possible wavelength drift

across a CWDM band (20 nm) with optical amplification and narrow optical filtering

with no performance degradation. Complete study of potential implementation was

performed with experimental results showing that a target performance of

BER(10−10) could be achieved over 120km of standard fiber with transmitter

wavelengths from 1542 to 1558 nm and DWDM back haul wavelengths from 1520 to

1535 nm.This gives the potential to support up to 2560 users.

[8] Highly scalable amplified hybrid TDM/DWDM array architecture for

interferometric fiber-optic sensor systems

Yi Liao et al, experimented a Hybrid Dense Wavelength Division

Multiplexing (DWDM) and Time Division Multiplexing (TDM) array architecture for

large scale interferometric fiber-optic sensor array systems. This architecture employs

a distributed Erbium Doped Fiber Amplifer (EDFA) scheme to decrease the

distribution loss among multiplexed wavelengths and employs TDM at each

wavelength to increase the total number of sensors that can be supported. The first

experimental demonstration of this system is reported including results which show

the potential for multiplexing and interrogating up to 4096 sensors using a single

telemetry fiber pair with good system performance. The number of interrogation

sensors could be further increased by increasing the number of wavelength channels.

11

CHAPTER 3

PROPOSED SYSTEM

3.1 DISPERSION

If a light signal is transmitted over a long haul optical fiber,its power is

dispersed with respect to time which widens shape of the pulse in the signal with time.

This is called as ―Dispersion‖(pulse broadening) of the signal. Fig 3.1 is the visual

representation of widening of shape of the pulse when transmitted through fiber.

Fig 3.1 Chromatic dispersion

Signal dispersion is seen due to multiple modes in the fiber, fiber material and

nonlinearities in fiber.Two general types of dispersion affect DWDM systems. One of

these effects, chromatic dispersion, is linear while the other, polarization mode

dispersion (PMD), is nonlinear [8].

3.2.CHROMATIC DISPERSION

Chromatic Dispersion (CD) is a phenomenon in optical fiber which is created

because of dependence of group index on wavelength which causes a temporal

broadening in optical pulses as they propagate through fiber. It can be divided into

two major components in standard single mode fibers [1].

3.2.1 Waveguide Dispersion

The Waveguide Dispersion is caused by physical structure of optical fiber core

and cladding. As a result of which different wavelengths propagate at different

velocities in optical fiber.

12

3.2.2 Material dispersion

Material dispersion is the dominant part of CD and is caused by change of

optical fiber refractive index ―n‖ with wavelength ―λ‖. After a certain propagation

distance, the broadening of pulses causes a significant number of errors at receiver.

3.3 POLARIZATION MODE DISPERSION

In an ideal optical fiber there is no distinguished optical axis, the material of

the core and of the cladding are isotropic, which means that a phenomenon of

birefringerence does not exist. In real optical fibers the tensions, change of thickness,

and the accidental changes of shape & core diameter cause an accidental formation of

distinguished optical axes and local birefringerence. As a consequence, two

orthogonal components travelling in a fiber as ordinary and extraordinary ray move in

the fiber with different velocities.

The different velocities of the two orthogonal components generate the phase

difference changing in time of propagation along fiber and change of polarization.

Besides the change of polarization with time of propagation, the different velocities of

ordinary ray (polarization vector is perpendicular to the plane of the optical axis) and

extraordinary ray (polarization in the same plane as the optical axis) cause that the

rays reach the end of a fiber in different time.

The changes of polarization are not essential, as long as a continuous light in a

fiber is propagated (Continuous Wave, CW) because the majority of detectors are not

sensitive to polarization state changes. However, in many applications the

maintenance of a constant polarization is essential, e.g. in optical interferometer,

optical lasers, sensors, optoelectrical modulators, in coherent transmission as well as

in the coupling of integrated optical circuits.

13

3.4 COMPENSATION TECHNIQUES

3.4.1 Fiber Bragg Grating

A Fiber Bragg Grating (FBG) is a type of distributed Bragg reflector

constructed in a short segment of optical fiber that reflects particular wavelengths of

light and transmits all others. This is achieved by creating a periodic variation in the

refractive index of the fiber core, which generates a wavelength-specific dielectric

mirror. A Fiber Bragg Grating can therefore be used as an inline optical filter to block

certain wavelengths, or as a wavelength-specific reflector.

3.4.1.1 Grating Structure

The structure of the FBG can vary via the refractive index, or the grating

period. The grating period can be uniform or graded, and either localised or

distributed in a superstructure. The refractive index has two primary characteristics,

the refractive index profile, and the offset[11]. Typically, the refractive index profile

can be uniform or apodized, and the refractive index offset is positive or zero. Fig 3.2

shows different kinds of FBG Strucures

Fig 3.2 Different fiber bragg gratings structures

14

There are six common structures for FBGs

1. Uniform positive-only index change

2. Gaussian apodized

3. Raised-cosine apodized

4. Chirped

5. Discrete phase shift

6. Superstructure

Chirped Fiber Bragg Gratings

The refractive index profile of the grating may be modified to add other

features, such as a linear variation in the grating period, called a chirp. The reflected

wavelength, changes with the grating period broadening the reflected spectrum. A

grating possessing a chirp has the property of adding dispersion—namely,different

wavelengths reflected from the grating will be subject to different delays.

Tilted Fiber Bragg Gratings

The grading or variation of the refractive index is along the length of the fiber

(the optical axis), and is typically uniform across the width of the fiber. In a tilted

FBG (TFBG), the variation of the refractive index is at an angle to the optical axis.

3.4.2 Dispersion Compensation Fiber

Dispersion compensation essentially means cancelling the chromatic dispersion of

some optical elements. The goal is to avoid excessive temporal broadening of ultra

short pulses and/or the distortion of signals[6].Dispersion compensation is used

mainly in mode-locked lasers and in telecommunication systems, but also sometimes

in optical fibers transporting light to or from some fiber-optic sensor[12]-[16].

There are two types of compensation schemes pre and post compensation, where

the Dispersion Compensation Fiber (DCF) is placed before and after the SMF or

symmetrically across the SMF. A DCF should have low insertion loss, low

polarization mode dispersion and low optical nonlinearity effects and also it should

15

have large chromatic dispersion coefficient to minimize the size of a DCF [7]-[20]. By

placing a DCF with negative dispersion after an SMF with positive dispersion, the net

dispersion should be zero.

3.4.2.1 Pre-Compensation technique

Fig 3.3 Block diagram of pre-compensation

In pre-compensation technique, the DCF is placed before the SMF as shown in

Fig.3.3. This scheme modifies the characteristics of input pulses at transmitter, before

they are sent into the fiber link, to compensate the effect of fiber dispersion.

3.4.2.2 Post-Compensation technique

In post-compensation technique, the DCF is placed after the SMF[19]. To

compensate the effect of fiber dispersion, this scheme modifies the characteristics of

optical pulses at the receiver as shown in Fig 3.4.

Fig 3.4 Block diagram of post compensation

16

3.4.2.3 Symmetric Compensation technique

In Symmetric-compensation technique, the DCF is placed before & after the

SMF,to compensate the effect of fiber dispersion as shown in Fig 3.5

.

Fig 3.5 Block diagram of Symmetric compensation

3.4.3 Electronic Dispersion Compensation

Electronic compensation technique makes use of electronics in conjunction with

optics in order to compensate dispersion [15]. Electronic Dispersion Compensation

(EDC) has become an important part of an optical transponder design. At present,

most of the installed optical fiber in the current metropolitan environment consists of

single mode fiber with a CD value of about 17ps/nm/km at a wavelength of 1550 nm.

In the current cost driven metro market, Electronic Dispersion Compensation can

become a very important tool in enhancing the existing fiber links to higher bit rates.

Fig.3.6 shows that block diagram of EDC using a feed forward equalizer and decision

feedback equalizer.

Fig 3.6 Block diagram of Electronic Dispersion compensation

17

3.4.4 Optical Filter

The use of Optical Filter to compensate dispersion is the most effective way of

dispersion compensation using fabry-perot and lorentzian filter. Fabry-Perot filter

have been widely used in optical fiber communications and Insertion Loss (IL) is one

of its important characteristics. This type of filter transmits a narrow band of

wavelengths and rejects wavelengths outside of that band. The filter has the ability to

"select" a different peak wavelength, as the filter is tilted. Fabry-perot filter can be

used to select wavelength for each channel in wavelength division multiplexing

(WDM) system. The main advantages of this filter are low loss, high tuning speed,

wide tuning range, high finesse, and flexible structures and different practical

applications [18].

3.4.4.1Fabry-perot Filter

The Fabry–Pérot interferometer is a pair of partially reflective glass optical

flats spaced micrometers to centimeters apart, with the reflective surfaces facing each

other. The flats in an interferometer are often made in a wedge shape to prevent the

rear surfaces from producing interference fringes, the rear surfaces often also have

an anti-reflective coating.

In a typical system, illumination is provided by a diffuse source set at the focal

plane of a collimating lens. A focusing lens after the pair of flats would produce an

inverted image of the source if the flats were not present, all light emitted from a point

on the source is focused to a single point in the system's image plane. In the

accompanying illustration, only one ray emitted from point A on the source is traced.

As the ray passes through the paired flats, it is multiply reflected to produce multiple

transmitted rays which are collected by the focusing lens and brought to point A' on

the screen. The complete interference pattern takes the appearance of a set of

concentric rings. The sharpness of the rings depends on the reflectivity of the flats. If

the reflectivity is high, it result in a high Q factor, monochromatic light produces a set

of narrow bright rings against a dark background. A Fabry–Pérot interferometer with

high Q is said to have high finesse.

18

3.5 NON LINEAR EFFECTS

Nonlinear effects in optical fibers occur due to

(1) change in the refractive index of the medium with optical intensity and,

(2) inelastic scattering phenomenon.

The power dependence of the refractive index is responsible for the Kerr-effect.

Depending upon the type of input signal, the Kerr-nonlinearity manifests itself in

three different effects such as Self-Phase Modulation (SPM), Cross-Phase Modulation

(XPM) and Four-Wave Mixing (FWM) [5].

3.5.1 Self phase modulation(SPM)

SPM arises because the refractive index of the fiber has an intensity-dependent

component. This nonlinear refractive index causes an induced phase shift, which is

proportional to the intensity of the pulse. Thus different parts of the pulse undergo a

different phase shift, which gives rise to chirping of the pulses. The pulse chirping in

turn enhances the pulse broadening effects of chromatic dispersion. This chirping

effect is proportional to the transmitted signal power so the SPM effects are more

pronounced in systems using high-transmitted powers. The SPM-induced chirp affects

the pulse broadening effects of chromatic dispersion as shown in Fig 3.7 and thus is

important to consider for high-bit-rate systems that already have significant chromatic

dispersion limitations[13].

Fig 3.7 Self Phase Modulation

19

Methods to reduce SPM

1. Recovery of pulse by proper filtering.

2. Reduction in effective length.

3. Increase in core area of the fiber

Recovery of pulse by proper filtering:

The main effect of SPM is the broadening of pulse at high power level. One

way to reduce this effect is to slice the output broadened spectrum by proper choice of

filters. In this method dispersion shifted fiber is used as transmission media. The

pulses at 10 GHz from a ring laser with variable pulse width (9 - 20) ps at a

wavelength of 1541 nanometer is amplified to average power of 16dBm and

transmitted to distance of 5 km using DSF. At the output of DSF, an optical band pass

filter is used to slice the SPM broadened spectrum. The centre frequency of the band

pass filter is 1541.5 nm and the output pulse characteristics are measured with a

40GHz photo detector. It is observed that output pulse width is almost constant at 14.5

ps for input pulse width between (9-16) ps. This shows that the output pulse width is

comparable to input pulse width and hence SPM effect is reduced.

Limitations:

1. One of the main limitations to this approach is with regard to input power. The

input power is constant at 16dBm. Since the SPM effect comes into place at higher

power level, variation of input power from low to high level must be taken to clearly

understand the SPM effect.

2. The input pulse width is varied from (9-16) ps and remain constant at 14.5 ps In

fact, the input pulse width should remain constant and widening of pulse due to the

SPM effect must be reduced.

3. Length of DSF fiber is 5 km which is practically very short.

Reduction in effective length:

The nonlinear interaction depends upon the transmission length. The longer the link

length, the more the interaction and the worse the effect of nonlinearity. By

20

decreasing effective length of the fiber this effect may be reduced but by doing so the

maximum transmission distance is also reduced.

Increase in effective core area:

Another approach to reduce this effect is to increase the effective core area of the

fiber. The variation in refractive index at high power level is given by

𝑛′ = 𝑛𝑗 + 𝑛2 𝑃 𝐴𝑒𝑓𝑓 (3.1)

where j = 1, 2, n2 is the nonlinear index co-efficient, P is the optical power and Aeff is

the effective core area. So increasing core effective area can decrease the variation in

refractive index. This method is used in present work to reduce the SPM effect. The

DCF fiber is used and signal is transmitted up to 150 km.

This method over comes the limitation of first method in the following manner.

1. The input power is varied from 10 dBm to 17.5 dBm to show clearly the effect of

the SPM.

2. Input pulse width is constant (59 ps).

3. Distance of transmitting fiber is increased from 5 km to 100 km.

3.5.2 Cross Phase Modulation(XPM)

The response of any dielectric to light becomes Non-linear for intense

electromagnetic fields, and optical fibers are no exception. On a fundamental level,

the origin of Non-linear response is related to a harmonic motion of a bound electron

under the influence of an applied field. Cross phase Modulation (XPM) had been

derived from the fact that the refractive index of the fiber in nonlinearity converts the

optical intensity fluctuations in co-propagating channel. In addition, since the

refractive index seen by particular wavelength is influenced by both the optical

intensity of that wave and by the optical power fluctuation of the neighbouring

wavelength, SPM is always present when XPM occurs.

For a system,non-linearity coefficient is given by

21

𝛾 = 𝑛2𝜔𝑗

𝑐𝐴𝑒𝑓𝑓

(3.2)

Where n2is refractive index, ω is frequency, c is speed of light; i.e., 3*108m/s and

Aeff is effective core area. Analogous to SPM, for two interacting wavelength the

XPM induced frequency shift is given by

∆𝜑 = 𝛾𝑗 𝑃𝑗 +2𝑃3−𝑗 𝑍 (3.3)

Where nonlinear phase change and P is power (incident optical power). The

factor2 in above equation (3.3) has its origin in the form of nonlinear susceptibility

and indicates that XPM is twice as effective as SPM for the same amount of power.

The first term in equation (3.3) represents the contribution of SPM and second term is

that of XPM. It can be observed that XPM is effective only when the interacting

signals superimpose in time. XPM hinders the system performance through the same

mechanism as SPM i.e. chirping frequency and chromatic dispersion. But XPM can

damage the system performance even more than SPM. XPM influences the system

severely when number of channels are large. Theoretically, for a 100-channels system,

XPM imposes a power limit of 0.1mW per channel.

XPM appears only when two interacting light beams or pulses overlap in space. Time

pulses with two different wavelength channels will not remain superimposed since

each has different Group Velocity Dispersion (GVD). This greatly reduces the impact

of XPM for direct detection of optical fiber transmission systems. XPM could be the

problem for high rate ultra-dense WDM systems (2.5Gbps-10Gbps system with

wavelength spacing of 25GHz or less).

To avoid XPM, a fiber should carry the pulses that do not travel together for

longer distance. Thus it requires a large group velocity change as the function of

frequency. Large dispersion will give large velocity difference which will give small

walk-off time and thus reduction in XPM will be obtained. It can be greatly mitigated

in WDM systems operating over standard non dispersion shifted single mode fiber.

One more advantage of this kind of fiber is its effective core area, which is typically

80 μm2. This large effective area is helpful in reducing nonlinear effects because γ is

22

inversely proportional to Aeff. Like SPM, the XPM also depends on interaction length

of fiber (from equation 3.2). The long interaction length is always helpful in building

up this effect up to a significant level.

3.5.3 Four Wave Mixing

3.5.3.1 Basics of FWM

The interaction of two or more light waves can lead to a second kind of χ (3)

nonlinearities. These involve an energy transfer between waves and not simply a

modulation of the index seen by one of them due to the other. This interaction is often

referred to as ―parametric,‖ and these nonlinearities lead to parametric processes. Four

wave mixing (FWM) is one of the most troubling issues[2]. Three signals combine to

form a fourth spurious or mixing component, hence the name four wave mixing and is

shown in Figure 3.8 in terms of frequency ω:

Fig 3.8 Formation of Fourth Spurious Component

Four-wave mixing transfers energy from a strong pump wave to two waves up

shifted and down shifted in frequency from the pump frequency 𝜔1. If only the pump

wave is incident at the fiber and the phase-matching condition is satisfied, the Stokes

and anti-Stokes waves at the frequencies 𝜔3 and 𝜔4 can be generated from noise. On

the other hand, if a weak signal at 𝜔3 is also launched into the fiber together with the

pump, the signal is amplified while a new wave at 𝜔4 is generated simultaneously.

The gain responsible for such amplification is called the parametric gain [14].

Effects of FWM:

Four Wave Mixing (FWM) is one of the most troubling issues. Three signals

combine to form a fourth spurious or mixing component, hence the name four wave

mixing. Spurious components cause following problems:

23

1. Interference between wanted signal.

2. It generates additional noise and degrades system performance.

3. Power is lost from wanted signals into unwanted spurious signals.

The total number of mixing components increases dramatically with the number of

channels. The total number of mixing components, M is calculated from the equation

𝑀 = 12 𝑁2 𝑁 − 1 (3.4)

Thus three channels create 9 additional signals and so on. As N increases, M also

increases rapidly, where N is number of channels.

Effect of Dispersion and Channel Spacing on FWM

As dispersion increases, effect of four wave mixing decreases. For dispersion of

16ps/nm, FWM effect reduces but chromatic dispersion increases. At zero dispersion

FWM effect is more hence fiber having dispersion 4ps/nm is used where FWM effect

is less and fiber is called Non-Zero dispersion shifted fiber. Due to equal spacing

some FWM components overlap in the DWDM channels. But in unequal spacing

there is no overlapping signals in the DWDM channels.

24

CHAPTER 4

RESULTS AND DISCUSSIONS

4.1 DWDM-(without Compensation)

The model for the design of DWDM-(without Compensation) in Optsim (Optical

Simulator) tool is shown in Fig 4.1.

Fig 4.1 Design of Dense Wavelength Division Multiplexing (without

compensation)

Figure 4.1 represents eight channel DWDM link with wide channel spacing. The eight

channels are specified at the wavelengths from 1.5500 µm to 1.5501 µm. The random

25

binary signals are given to PBRS generator which is converted into electrical signal

using electrical generator. After the signals are generated in the direct modulated laser

models, they are multiplexed into a single optical signal by the MUX model block.

This model is set for ideal multiplexing. At the output of the MUX, the optical signal

passes through a length of fiber (140km). After demultiplexing, the individual signals

are sent to eight separate receivers. The received signals are analyzed using BER

tester, Q-Factor tester, dispersion analyzer and eye analyzer blocks.

PRBS Pattern Generator:

This model generates a binary sequence of several different types. The incoming

random bits of binary inputs are given to electrical generator to change it to electrical

signals. Each channel may have its own model instance configured to provide a

different pattern than the other model instances. PRBS Pattern Generator uses a Bit

rate of 10e-9.

Electrical Signal Generator:

This model converts an input binary signal into an output electrical signal. The output

signal may be specified as either voltage or current. The user parameters are used to

configure the electrical signal output. Different modulation formats available are RZ,

NRZ and Manchester. Manchester type modulation is used.

CW Laser:

This model produces the optical signal output of one or more CW lasers. It is most

commonly used in conjunction with the external modulator model to encode a binary

signal. For CW Laser the Peak power is 1.0e-3 watts.

Modulator:

This models an electro-optic modulator. The electrical signal is sent to an external

optical modulator and it is directly detected. The nonlinearity introduced by the

external modulator gives rise to clipping effects that can be seen both on the received

eye diagram and received signal spectrum.

26

Nonlinear Fiber:

This model provides a detailed implementation of propagation of one or more optical

channel in a Single Mode Fiber (SMF).SMF takes single path through the core. It has

no intermodal dispersion. It takes into account attenuation, dispersion, polarization

mode dispersion (PMD) and nonlinearities. When the Single Channel mode of the

MUX is used prior to the fiber model, it also takes into account four wave mixing.

Fiber length is varied from 10km to 140km.

Optical Multiplexer (Nx1 MUX):

This model represents an optical WDM multiplexer. It accepts multiple optical signals

at its input ports and produces a single WDM optical signal at its output port which

includes all the input WDM optical signals.

Physical EDFA:

This block models the operation of an Erbium-Doped Fiber Amplifier (EDFA) via a

set of well-established physical equations. The model supports component

specification at different levels of complexity, as well as a variety of pump and signal

configurations. Forward-propagating optical signals are launched into the EDFA via

the first input node, while backward-propagating signal enter via the second input

node. The EDFA may also be used to simulate bidirectional signal propagation, in

which case input signals are expected at both input nodes, and an additional backward

output appears at the backward output node.

Optical Splitter (1xN):

This model represents an ideal optical splitter. It takes a single input signal, and

divides it equally among 16 output ports with 1/16 splitting loss, plus excess loss

determined by the transmission model parameter.

Optical Demultiplexer (1xN DEMUX):

This model represents an optical WDM demultiplexer. It accepts single optical signals

at its input ports and produces a multiple WDM optical signal at its output port given

to different users that includes all the input WDM optical signals.

27

Signal Spectrum Analyzer:

The Signal Spectrum block is used to display the spectrum of a signal at the node

connected to its input ports. The signal plot of the summer as well as CW laser output

can be viewed.

Eye Diagram Analyzer:

This Eye Diagram block is used to display the eye diagram of a signal at the node

connected to its input ports. By default, it displays the magnitude of optical signals

and the real value of electrical signals.

Bit Error Rate Tester:

This model computes the Bit Error Rate (BER) for the input electrical signal(s) as

well as a number of useful parameters such as the Q factor and electrical properties

such as the height and width.

Dispersion Map

Fig 4.2 Dispersion Map for without Compensation

The output of dispersion analyzer block is shown in Fig 4.2.The eight different colors

correspond to eight DWDM channels with the different wavelength of 1.5501µm

to1.5507µm. As the length of the fiber increases, dispersion also increases.

28

Simulation Results

The eye diagram for the DWDM system without any dispersion compensation is

shown in Fig 4.3. Eye closure shows that the signal has been distorted due to

dispersion.

Fig 4.3 Eye diagram of receiver 1

Figure 4.4 and 4.5 shows that BER & Q parameter values for the eight channel

DWDM system without any dispersion compensation. It is found that the Bit Error

Rate (BER) is100 & Q parameter is 0.

Fig 4.4 BER plot for DWDM system without dispersion Compensation

29

Fig 4.5 Q-factor plot for DWDM system without dispersion Compensation

4.2 VARIOUS COMPENSATION TECHNIQUES

4.2.1 Fiber Bragg Grating

The model for the design of DWDM using Fiber Bragg Grating in Optsim (Optical

Simulator) tool is shown in Fig 4.6.

Fig 4.6 Design of DWDM using Fiber Bragg Grating

30

The eight channel DWDM link is designed with wide channel spacing. The eight

channels are used at the wavelengths 1.5501µm to1.5507µm.The PRBS block is to

generate the binary sequences of different types. The optical signals are generated in

direct modulated laser block and they are multiplexed using MUX model block. The

multiplexed optical signal is transmitted along the fiber of length 40 km. It is then

passed through an Fiber Bragg Grating to compensate the dispersion and then

demultiplexed using DEMUX model block, which outputs the individual signals to

eight separate receivers. The received signals are analyzed using BER tester, Q-Factor

tester and eye analyzer blocks.

Simulation Results

The eye diagram for FBG technique without EDFA is shown in Fig 4.7 and is

observed that eye opening is not wide and also has more interference.


31

BER & Q parameter values for the eight channel DWDM system using FBG (without

EDFA) is shown in Figure 4.8 and 4.9. and it is observed that the Bit Error Rate

(BER) is 10-29.5

& Q parameter is 11.3.

Fig 4.8 BER plot for FBG without EDFA

Fig 4.9 Q-factor plot for FBG without EDFA

32

The eye diagram for FBG technique with EDFA is shown in Fig 4.10 and is observed

that eye opening is wider and interference is also lesser.



DWDM system using FBG (with EDFA). It is viewed that the Bit Error Rate (BER) is

0 & Q parameter is 1410.

Fig 4.11 BER plot for FBG with EDFA

33

Fig 4.12 Q-factor plot for FBG with EDFA

4.2.2 Dispersion Compensation Fiber

4.2.2.1 Pre-Compensation

Figure 4.13 represents eight channel DWDM link with wide channel spacing. The

eight channels are specified at the wavelengths from 1.5500 µm to 1.5501 µm.The

PRBS block is to generate the binary sequences of different types. The optical signals

are generated in direct modulated laser block and they are multiplexed using MUX

model block. The multiplexed optical signal is transmitted along the fiber of length

140 km,in which 20 km of DCF with negative chromatic dispersion coefficient of

-5ps/nm/km is placed at the link front end. At the receiving end, demultiplexing

operation is done using DEMUX model block, which outputs the individual signals to

eight separate receivers. The received signals are analyzed using BER tester, Q-Factor

tester and eye analyzer blocks.

34

Fig 4.13 Design of DWDM using DCF (pre-compensation)

Simulation Results

The eye diagram for pre compensation technique is shown in Fig 4.14 and is observed

that eye opening is wider.


35


DWDM system using DCF (Pre-Compensation). It is found that the Bit Error Rate

(BER) is 0 & Q parameter is 1410

Fig 4.15 BER plot for pre compensation.

Fig 4.16 Q-factor plot for pre compensation

36

4.2.2.2 Post-Compensation

Figure 4.17 represents eight channel DWDM link with wide channel spacing.

The eight channels are specified at the wavelengths from 1.5500 µm to 1.5501

µm. The PRBS block is to generate the binary sequences of different types. The

optical signals are generated in direct modulated laser block and they are

multiplexed using MUX model block. The multiplexed optical signal is

transmitted along the fiber of length 140 km. In which 20 km DCF with positive

chromatic dispersion coefficient (5ps/nm/km) is placed at the link far end. At the

receiving end demultiplexing operation is done using DEMUX model block,

which outputs the individual signals to eight separate receivers. The received

signals are analyzed using BER tester, Q-Factor tester and eye analyzer blocks.

Fig 4.17 Design of DWDM Using DCF (Post Compensation)

37

Simulation Results

It is observed from the eye diagram that the eye opening is wider for post

compensation technique as shown in Fig 4.18



DWDM system using DCF (Post-Compensation). It is observed that the Bit Error

Rate (BER) is 0 & Q parameter is 1410.

Fig 4.19 BER plot for post compensation

38

Fig 4.20 Q-factor plot for post compensation

4.2.2.3 Symmetric Compensation

Figure 4.21 describes DWDM link with wide channel spacing. The eight channels are

specified at the wavelengths from 1.5500 µm to 1.5501 µm.The random binary

signals are given to PBRS generator which is converted into electrical signal using

electrical generator. The optical signals are generated in direct modulated laser block

and they are multiplexed using MUX model block. Typically a 10 km of DCF is

placed before and after the single mode fiber of length 120km is achieved using

DEMUX model block, which outputs the individual signals to eight separate

receivers. The received signals are analyzed using BER tester, Q-Factor tester and eye

analyzer blocks.

Fig 4.21 Design of DWDM Using DCF (Symmetric Compensation)

39

Simulation Results

The eye diagram for symmetric compensation technique is shown in Fig 4.22.It is

observed that the eye opening is wider.



DWDM system using DCF (Symmetric-Compensation). It is examined that the Bit

Error Rate (BER) is 0 & Q parameter is 1410.

Fig 4.23 BER plot for Symmetric compensation

40

Fig 4.24 Q-factor plot for Symmetric compensation

4.2.3 Electronic Dispersion Compensation

Figure 4.25 represents eight channel DWDM link with wide channel spacing. The

eight channels are specified at the wavelengths from 1.5500 µm to 1.5501 µm. The

random binary signals are given to PBRS generator which is converted into electrical

signal using electrical generator. The optical signals are generated in direct modulated

laser block and they are multiplexed using MUX model block. The multiplexed

optical signal is transmitted along the fiber of length 50 km. The photo detector

converts the received optical signal to electrical signal and then an electrical filter is

used to further smooth the received output. Based on the analog or digital signal

processing techniques, the parameters may be adjusted automatically using the

feedback techniques, thus, minimizing the bit error rate. At the receiving end

demultiplexing operation is done using DEMUX model block, which outputs the

individual signals to eight separate receivers. The received signals are analyzed using

BER tester, Q-Factor tester and eye analyzer blocks.

41

Fig 4.25 Design of DWDM Using Electronic Dispersion Compensation

Simulation Results

The eye diagram for Electronic dispersion compensation technique is shown in

Fig4.26.Eye opening is wider in EDC technique.


42


DWDM system using Electronic Dispersion Compensation. It is noted that the Bit

Error Rate (BER) is 10-0.5


Fig 4.27 BER plot for Electronic dispersion compensation

.

Fig 4.28 Q- factor plot for Electronic dispersion compensation

4.2.4 Optical filter

Figure 4.29 eight channel DWDM link with wide channel spacing. The eight

channels are specified at the wavelengths from 1.5500 µm to 1.5501 µm. The

PRBS block is to generate the binary sequences of different types. The optical

signals are generated in direct modulated laser block and they are multiplexed

43

using MUX model block. The multiplexed optical signal is transmitted along the

fiber of length 50 km. Fabry perot and Lorentzian filter are used to smoothen the

received signal. At the receiving end demultiplexing operation is done using

DEMUX model block, which outputs the individual signals to eight separate

receivers. The received signals are analyzed using BER tester, Q-Factor tester

and eye analyzer blocks.

Fig 4.29 Design of DWDM Using optical filter

Simulation Results

The eye diagram for optical filter (Before compensation) technique is shown in Fig

4.30.The eye diagram is severely distorted before compensation.

44


The eye diagram for optical filter using lorentzian technique is shown in Fig 4.31.It is

observed that eye opening is clear and less interference.


45


DWDM system using Optical filter (lorentzian filter). It is shown that the Bit Error

Rate (BER) is 10-100


10-100

Fig 4.32 BER plot for lorentzian filter

Fig 4.33 Q-factor plot for lorentzian filter

46

The eye diagram for optical filter using fabry- perot technique is shown in Fig 4.34.It

is observed both upper lid and lower lid are nicely suppressed and eye opening is

wider.

.



DWDM system using Optical filter(fabry- perot filter). It is found that the Bit Error

Rate (BER) is 10-100


Fig 4.35 BER plot for fabry- perot filter

47

Fig 4.36 Q-factor plot for fabry- perot filter

BER & Q-factor Formula

𝐵𝐸𝑅 =1

2𝑒𝑟𝑓𝑐

𝑄

2 ≈

𝑒𝑥𝑝 −𝑄2 2

𝑄 2𝜋 (4.1)

Where the parameter Q is obtained from above equation

𝑄 =𝐼1 − 𝐼0

𝜎1 + 𝜎0

(4.2)

where erfc stands for the complementary error function

𝑒𝑟𝑓𝑐 𝑥 = 2

𝜋 𝑒𝑥𝑝 −𝑦2

∞

𝑥

𝑑𝑦 (4.3)

𝐼0, 𝐼1=Average value of current for bit 0 &bit 1 respectively.

𝜎0, 𝜎1=Noise variance for bit 0 &bit 1.

48

Analysis of Dispersion Compensation

Table4.1 Analysis of various Dispersion Compensation techniques using BER as

the performance indicator

S. No.

Various

Compensation

Techniques

Length of Fiber (km)

10 50 70 120 140

1 without compensation 100=1 10

0=1 10

0=1 10

0=1 10

0=1

2 FBG (without EDFA) 10-100

10-3.9

- - -

3 FBG (with EDFA) 0 0 0 10-3.9

-

4 EDC 10-0.5

10-0.5

10-0.5

- -

5

DCF (Pre , post and

Symmetric

compensation)

0 0 0 0 0

6

Op

tica

l F

ilte

r 1. Fabry perot 0 10-100

10-8.2

- -

2. Lorentzian 0 10-100

10-8.2

- -

Table4.1 shows that without any dispersion compensation, DWDM networks exhibit

maximum bit error rate. On using FBG (with and without EDFA) for compensation,

the former has better BER for increased fiber length. The performance of both fabry-

perot and lorentzian are considerably less. EDC provides a bit error rate of 10-0.5

even

for a fiber length 70km and is poorer when the distance is above 70km. Of all the

Compensation techniques, DCF achieve maximum transmission distance up to 140km

with best bit error rate performance.

49

Table 4.2 Analysis of different dispersion compensation techniques using Q

parameter as a performance metric

S. No.

Various Compensation

Techniques

Length of Fiber (km)

10 50 70 120 140

1 No compensation 0 0 0 0 0

2 FBG (without EDFA) 35.5 3.65 - - -

3 FBG (with EDFA) 1410 1410 1410 10.9 -

4 EDC 0.54 0.61 0.64 - -

5

DCF (Pre,post and

Symmetric

compensation)

1410 1410 1410 1410 1410

6

Op

tica

l F

ilte

r 1. Fabry perot 1410 18.6 5.52 - -

2. Lorentzian 1410 18.7 5.52 - -

Table 4.2 shows that dispersion compensation technique with less Q parameter value

has less BER. EDC provides a Q-factor of 0.61 even for a fiber length 70km and is

poorer when the distance is above 70km. Of all the Compensation techniques, DCF

achieve maximum transmission distance up to 140km with best Q-factor performance.

.

4.3 NON LINEAR EFFECTS

4.3.1 Cross Phase Modulation

Figure 4.37 shows the simulation setup for the analysis of Cross phase Modulation in

optical link. The XPM has been analyzed for the different values of dispersion.

Transmitter section consists of data source, modulator driver, laser source and

50

modulator. Data source produces a bit of pseudo random sequence of bits at the rate of

10Gbps. The output of the data source was given to modulator driver which produces

NRZ format pulse train. The transmitted signal was formed by modulating the light

carrier by NRZ data source. The light carrier is generated by lorentzian laser source.

Transmitter output is boosted up by the fixed gain erbium doped fiber amplifier

(EDFA). The channel section consists of Single Mode Fiber (SMF) and Fiber Bragg

Grating (FBG) with different dispersion values. In the receiver section, PIN receiver

has been used having quantum efficiency of 70%. The dark current is set at 0.1nA.

Output of the receiver section was given to the measurement devices with the help of

electrical splitter. Measurement device like electrical scope have been used.

Fig 4.37 Simulation Setup of Cross Phase Modulation (XPM)

51

Simulation Results

The eye diagram shows in Fig 4.38 that the effect of cross phase modulation is

adverse and eye is severely distorted.


The eye diagram as in Fig 4.39 shows that the cross phase modulation is eliminated at

a distance of 150km (Dispersion value= -8ps/nm/km)


52

Dispersion Value vs Q-factor & BER

Figure 4.40 shows BER & Q-factor for various dispersion values and is observed that

BER is constant and Q-factor increased.

Fig 4.40 Dispersion value vs BER &Q-factor

4.3.2 Self Phase Modulation

The figure 4.41 shows the simulation setup for the analysis of Self Phase Modulation

in optical link having single channel. The SPM is analysed for six values of dispersion

from -10ps/nm/km to 10ps/nm/km. The transmitter and receiver section are connected

by the dispersive fiber link. The transmitter section consists of data source, modulator

driver, laser source and modulator. Data source produces a pseudo-random sequence

of bits at a rate of 10Gbps. The output of data source is given to modulator driver

which produces a NRZ (Non return to zero) format pulse train. The transmitted signal

is formed by modulating the light carrier by the NRZ data source. Confinement factor

is 0.35, insertion loss is 3 dB and output insertion loss is 3 dB. The various parameters

are Raman fiber length of 10 km, operating temperature of 300 K, pump wavelength

of 1480 nm and pump power of 300 mW. The light carrier is generated by Lorentzian

-8 -7 -6 -5 -4 -3 -2 -1

0

5

10

15

20

25

Dispersion Value

Qua

lity

Fac

tor

BER

Quality Factor

53

laser source at the 1550 nm wavelength. The transmitter output is boosted up by the

fixed gain Erbium Doped Fiber Amplifier (fixed_output_power).

There are two types of optical amplifiers-Semiconductor Optical Amplifier

(SOA) and Erbium Doped Fiber Amplifier (EDFA). Due to its high gain

characteristics EDFA is used these days. The shape of the gain graph is flat having a

gain of 25 dB. The noise figure value is set at 4.5 dB. The transmission medium used

is a standard single mode fiber of 100kms length. The receiver used in the system is a

PIN receiver, which uses the PIN (p-intrinsic-n) diode as a detector with 70%

quantum efficiency is used. The dark current is set at 0.1 nA. The output of the

receiver is given to the measurement devices which are fed through the electrical

splitter, the electrical scope and the Q estimator. The optical spectrum of the signal is

observed from optical spectrum analyzer (input and output) by splitting the signal

from fiber link with the use of optical splitters. Q-factor & BER are analyzed using

simple single mode fiber without any dispersion effect and are compared with Q-

factor & BER obtained using dispersion effects for various distances.

Fig 4.41 Simulation Setup of Self Phase Modulation(SPM)

54

Simulation Results

The figure 4.42 to 4.45 below shows the eye opening for the different distances with

optical dispersion as -5ps/nm/km. It is observed that as the distance varies the Quality

factor becomes nonlinear due to the effect of Self Phase Modulation (SPM).

Eye Diagram

Fig4.42 Eye diagram for before Self Phase Modulation

Fig 4.43 Eye diagram for after Self Phase Modulation of distance

50km and dispersion value = -5ps/nm/km

55

Fig 4.44 Eye diagram for after Self Phase Modulation of distance 100km and

dispersion value = -5ps/nm/km

Fig 4.45 Eye diagram for after Self Phase Modulation of distance 140km and

dispersion value = -5ps/nm/km

56

In figure 4.46 shows BER & Q-factor for different length of fiber and is observed that

BER is 0 throughout where Q parameter varies non-linearly.

Fig 4.46 Length of fiber vs BER & Q-factor

4.3.3 FOUR WAVE MIXING

The simulation setup for the analysis of Four Wave Mixing in DWDM optical link is

shown in Fig 4.47. The FWM has been analyzed for the different value of dispersion.

Transmitter section consists of data source, modulator driver, laser source and

modulator. Data source produces a bit of pseudo random sequence of bits at the rate of

10Gbps. The output of the data source is given to modulator driver which produces

NRZ format pulse train. The transmitted signal is formed by modulating the light

carrier by NRZ data source. The light carrier is generated by lorentzian laser source.

Transmitter output is boosted up by the fixed gain Erbium Doped Fiber Amplifier

(EDFA). The channel section consists of Single Mode Fiber (SMF) and Fiber Brags

Grating (FBG) with different dispersion values. In the receiver section, PIN receiver

has been used having quantum efficiency of 70%. The dark current is set at

0.1nA.Wavelength of sources are set to 1551 nm and dispersion is varied from 0 to 4

ps/nm/km and optical spectrum is observed. Each channel is set to central wavelength

1550nm firstly for equal channel spacing of 0.2nm and then unequal spacing of 0.1,

50 100 150

0

10

20

30

40

50

Length of the fiber

Qua

lity

Fac

tor

BER

Quality Factor

57

0.2, 0.1, 0.2 nm respectively. Optical spectrum and eye diagram are observed for

equal and unequal spacing keeping dispersion constant.

Fig 4.47 Simulation Setup of Four Wave Mixing (FWM)

Simulation Results

The observed input optical spectrum shown in Figure 4.48.

Fig 4.48 Input spectrum

58

Figure 4.49 to 4.54 show the output spectrum with varying dispersion values from

1ps/nm/km to 6ps/nm/km. It is observed that as the dispersion increases, the effects of

FWM is minimized.

Figure 4.49 Output spectrum dispersion value=1ps/nm/km


59


Figure 4.52 Output spectrum dispersion value =4ps/nm/km

60

Figure 4.53 Output spectrum dispersion value =5ps/nm/km

Figure4.54 Output spectrum dispersion value =6ps/nm/km

61

Eye Diagram

Figure 4.55 shows the eye diagram of the system with equal channel spacing and is

observed that eye is fully closed with no opening.

Figure4.55 Equal channel spacing

Figure 4.56 shows the eye diagram of the system with unequal channel spacing and is

observed that both upper lid and lower lid are nicely suppressed and eye opening is

wider

Figure4.56 Unequal channel spacing

62

CHAPTER 5

CONCLUSION

There are many techniques that can be utilized to compensate dispersion in an

optical fiber communication link. When the transmission distance increases up to

140km in DWDM networks, dispersion increases and the Bit Error Rate is high

and therefore Q-factor is poor. To overcome this problem, various dispersion

compensation techniques are dealt. Fiber Bragg Grating is a very compact device

with low insertion loss and compensates dispersion by compressing the pulse

which passes through it. This achieved wider eye opening at a transmission

distance upto 50km.Another technique, Electronic equalizer used in Electronic

Dispersion Compensation (EDC) makes use of feed forward equalizer to

compensate dispersion upto 70km of transmission distance.

Optical filters such as fabry-perot and lorentzian filter are considered for

dispersion compensation as they are capable of providing both fixed and tuneable

compensation of dispersion for DWDM systems. This technique achieved the

transmission distance upto 70km. Dispersion compensating fiber are be the best

technique for dispersion compensation for long haul applications. DCF achieved

maximum transmission distance upto 140km and bit error rate is very low

(BER=0) and provides better Q-factor (1410). On reaching transmission distance

greater than 200km, placing a DCF (Pre or post compensation techniques) for

every 100km provides better compensation.

Non linear effect-cross phase modulation is analyzed for various fiber length

and the fiber dispersion value was varied from -1 to -8 ps/nm/km through

parametric runs. It is observed that BER is constant and Q-factor has increased.

Further it is observed that XPM is reduced with increase in dispersion.

The effect of self phase modulation is reduced by varying the dispersion value

from 10 to -10 ps/nm/km through parametric runs. These effects are seen in the eye

63

diagrams for the different values of dispersion. It is observed that BER is constant

and Quality Factor is varying nonlinearly.

FWM generates additional noise and degrades system performance. By

varying the dispersion from 0 to 4 ps/nm/km we observed effect of dispersion on

FWM. Also effect of equal and unequal spacing on FWM is observed. For the

DWDM system with fiber having dispersion value of 4 ps/nm/km, if unequal

spacing is used among channels, FWM effect is suppressed.

64

CHAPTER 6

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67

LIST OF PUBLICATIONS

Presented a paper titled ―Analysis of Dispersion Compensation Techniques and

Fiber Non linear Effects in DWDM optical networks‖ in IEEE Sponsored 3rd

International Conference on Innovations in Information Embedded and

Communication Systems on 17th

and18th

March 2016 at Karpagam College of

Engineering, Coimbatore.

The paper is accepted to be published in a Scopus Indexed (Anna University

Annexure-II) Journal, Pakistan Journal of Biotechnology (S.no: 15755, Print

ISSN: 18121837, University of Sindh).

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ANALYSIS OF DISPERSION COMPENSATION ... OF DISPERSION COMPENSATION TECHNIQUES AND FIBER NON-LINEAR EFFECTS IN DWDM OPTICAL NETWORKS A PROJECT REPORT Submitted by SATHYA V Register