opti system project

FOUR WAVE MIXING NONLINEARITY EFFECT IN WAVELENGTH

DIVISION MULTIPLEXING RADIO OVER FIBER SYSTEM

HAFIZ ABD EL LATIF AHMED HABIB

UNIVERSITI TEKNOLOGI MALAYSIA

FOUR WAVE MIXING NONLINEARITY EFFECT INWAVELENGTH DIVISION MULTIPLEXING RADIOOVER FIBER SYSTEM


2006/2007-III

DR. RAZALI BIN NGAH

√

Elmitdaad, 3rd Extension,Khartoum, Sudan.

JULY 2007JULY 2007

i

“I hereby declare that I have read this project report and in

my opinion this project report is sufficient in terms of scope and

quality for the award of the degree of Master of Engineering

(Electrical- Electronics and Telecommunications)”

Signature : ………………………………………...

Name of Supervisor : DR. RAZALI BIN NGAH

Date : ………………………………………...

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FOUR WAVE MIXING NONLINEARITY EFFECT IN WAVELENGTH DIISION

MULTIPLEXING EADIO OVER FIBER SYSTEM


A project report submitted in partial fulfillment of the requirements

for the award of the degree of Master of Engineering

(Electrical-Electronics and Telecommunications)

Faculty of Electrical Engineering

Universiti Technologi Malaysia

JULY 2007

iii

I declare that this project report entitled “Four Wave Mixing Nonlinearity Effect in

Wavelength Division Multiplexing Radio over Fiber System” is the result of my own

research except as cited in references. The project has not been accepted for any

degree and is not concurrently submitted in candidature of any other degree.

Signature : ……………………………………

Name : Hafiz Abd El Latif Ahmed

Date :

iv

DEDICATION

To my beloved late mother, may her soul rest in Paradise.

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ACKNOWLEDGEMENTS

Praise and thanks to Allah (SWT) who gave me the strength and courage to

complete this project.

I would like to express sincere thanks to my supervisor Dr. Razali bin Ngah

for his invaluable guidance throughout the course of this project. His guidance, ideas,

encouragement, affable nature, kindness and support were greatly helpful. Even with

his busy schedule, he spent considerable amount of time helping me through the

different phases of this project.

Special acknowledgement to Dr. Abdelwahab Mohammed Ali, for his

valuable suggestions, and revision of this project. Special thanks to Eng. Mr.

Abdelrazig Saeed Mohamed for his assistance and support and to Eng. Mr. Reza

Abdolee for the many interesting discussions I have had with him.

I wish to thank my parents, for their daily prayers, giving me the motivation

and strength, and encouraging me to accomplish and achieve my goals.

A special acknowledgment must be given to my brothers and sisters for their

motivation help and support during my academic period at UTM. I am indebted to

them and words will never express the gratitude I owe to them.

Last but not least, sincere thanks and gratitude to my lovely wife Najat and

my sons Mohammed and Awab who inspired me by their, courage, support and

patience throughout the period of my study.

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ABSTRACT

The integration of wireless and optical networks is a potential solution for the

increasing capacity and mobility as well as decreasing costs in the access networks.

Optical networks are fast, robust and error free, however, there are nonlinearity

obstacles preventing them from being perfect media. The performance of wavelength

division multiplexing (WDM) in radio over fiber (RoF) systems is found to be

strongly influenced by nonlinearity characteristics in side the fiber. The effect of four

wave mixing (FWM) as one of the influential factors in the WDM for RoF has been

studied here using Optisystem and Matlab. From the results obtained, it is found that

the FWM effects have become significant at high optical power levels and have

become even more significant when the capacity of the optical transmission line is

increased, which has been done by either increasing the channel bit rate, and

decreasing the channel spacing, or by the combination of both process. It is found

that when the channel spacing is 0.1 nm, 0.2 nm and 0.5 nm the FWM power is

respectively, becomes about -59 dBm, -61 dBm and -79 dBm. This result confirms

that the fiber nonlinearities play decisive role in the WDM for RoF system. The

simulation results obtained here are in reasonable agreement as compared with other

numerical simulation results obtained, elsewhere, using different simulation tools.

vii

ABSTRAK

Integrasi talian tanpa wayar dan rangkaian optik menjadi potensi kepada

penyelesaian untuk peningkatan kapasiti dan mobiliti dan seterusnya mengurangkan

kos capaian rangkaian. Rangkaian optik adalah pantas, berkesan, dan tidak

mempunyai masalah. Namun begitu halangan ‘nonlinearity’ menghalangnya menjadi

media yang sempurna. Prestasi jarak gelombang pembahagi pemultipleksan (WDM)

dalam radio melalui fiber (RoF) sistem amatlah dipengaruhi oleh ciri-ciri

‘nonlinearity’ didalam fiber. Kesan ‘four wave mixing’ (FWM) yang menjadi salah

satu faktor berpengaruh dalam WDM untuk RoF telah dikaji menggunakan

Optisystem dan Matlab. Keputusan yang diperolehi mendapati bahawa kesan FWM

menjadi penting pada optik kuasa aras tinggi dan sangat penting apabila kapasiti

talian penghantaraan optik bertambah, sama ada dengan meningkatkan kadar bit

saluran, mengurangkan penjarakan saluran, ataupun kedua-duanya sekali. Ianya

didapati bahawa apabila penjarakan saluran adalah 0.1 nm, 0.2 nm, dan 0.5 nm kuasa

FWM masing-masing adalah lebih kurang –59 dBm, -61 dBm, dan –79 dBm.

Keputusan ini mengesahkan bahawa ‘fiber nonlinearities’ memainkan peranan utama

dalam WDM untuk sistem RoF. Keputusan simulasi berangka yang diperolehi juga

bersamaan dengan keputusan model analisis yang diperolehi melalui Matlab.

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION iii

DEDICATION iv

ACKNOWLEGEMENT v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENTS viii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF ABBREVIATION xv

LIST OF SYMBOLS xvii

LIST OF APPENDICES xix

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Problem Background 2

1.3 Problem Statement 3

1.4 Objective of the Project 4

1.5 Scope of the Project 5

1.6 Organization of the Thesis 5

2 RADIO OVER FIBER TECHNOLOGY 7

2.1 Introduction 7

2.2 What is Radio over Fiber? 8

2.3 Benefits of RoF Technology 9

ix

2.3.1 Low Attenuation Loss 9

2.3.2 Large Bandwidth 10

2.3.3 Immunity to Radio Frequency Interference 11

2.3.4 Easy Installation and Maintenance 11

2.3.5 Reduced Power Consumption 12

2.3.6 Multi-operator and Multi-service Operation 12

2.3.7 Dynamic Resource Allocation 13

2.4 The Application of Radio over Fiber Technology 13

2.4.1 Cellular Networks 13

2.4.2 Satellite Communications 14

2.5 RoF Multiplexing Techniques 15

2.5.1 Sub-Carrier Multiplexing in RoF System 15

2.5.2 Wavelength Division Multiplexing in RoF System 17

3 NON-LINEAR EFFECTS 19

3.1 Introduction 19

3.2 Types of Fibers 20

3.3 Fiber Losses 20

3.4 Fiber Nonlinearities 21

3.4.1 Self Phase Modulation 22

3.4.2 Cross Phase Modulation 23

3.4.3 Four Wave Mixing 25

3.4.3 Stimulated Brillouin Scattering 29

3.4.5 Stimulated Raman Scattering 30

4 METHODOLOGY 32

4.1 Introduction 32

4.2 Simulation using Optisystem Software 32

4.3 The Simulation Model 33

4.4 Simulation of the Four Wave Mixing effect 35

4.5 Simulation of FWM for Higher Number of Channels 40

4.6 Effect of Different Power Level of the Signal Sources 41

4.7 Effect of Increase Dispersion of the Fiber Optic 41

4.8 Effect of Increase Effective Area f the Fiber Optic 42

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4.9 Modelling the Effect of FWM 42

5 RESULTS AND DISCUSSIONS 47

5.1 Introduction 47

5.2 Simulation of the Four Wave Mixing Effect 47

5.3 Simulation Results without the External Modulated Signal 48

5.3.1 Effect of Channel Spacing 48

5.3.2 Effect of Different Power Level of the Signals Sources 51

5.3.3 Effect of Increase Dispersion of the Fiber Optic 53

5.4 Simulation Results with the External Modulated Signal 54

5.4.1 Effect of Channel Spacing 55

5.4.2 Effect of Different Power Level of the Signals Sources 58

5.4.3 Effect of Increase Dispersion of the Fiber Optic 61

5.4.4 Effect of Increase Effective Area of the Fiber Optic 61

5.5 Simulation of FWM for Higher Number of Channels 62

5.5.1 Simulation Results for Four Signal Source without External

Modulated Signal 63

5.5.2 Simulation Results for Four Signal Source without External

Modulated Signal 65

5.6 Discussion 67

5.7 Analytical Modelling 68

5.8 FWM reduction 70

5.8.1 Effect of Unequal Channel Spacing 70

5.8.1 Effect of Increase Effective Area of the Fiber Optic 72

6 CONCLUSIONS AND RECOMMENDATIONS 73

6.1 Conclusion 73

6.2 Recommendations for Future Work 74

REFERENCE 75

Appendix 77

xi

LIST OF TABLES

TABLE NO. TITLE PAGE

3.1 Comparison between SBS and SRS 31

4.1 Global parameters 37

4.2 CW Laser sources parameters 38

4.3 DM 2x1 multiplexer parameters 38

4.4 Main tab and dispersion tab parameters for optical fiber 38

4.5 Nonlinear tab parameters for optical fiber 39

4.6 Numerical tab and PMD tab parameters for optical fiber 39

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LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 The Radio over Fiber System Concept 8

2.2 Operating regions of optical fiber 10

2.3 These robust RAPs are connected to the central base

station via the ROF 14

2.4 SubCarrier Multiplexing of Mixed Digital and Analogue

Signals 16

2.5 WDM system using multiple wavelength channels and

optical amplifiers 17

3.1 Frequency chirping effect 23

3.2 Four wave mixing products 26

3.3 The arising new frequency components due to FWM 27

3.4 FWM products versus channel count 28

3.5 FWM mixing efficiency in single-mode fibers 29

4.1 Direct Modulation 33

4.2 External modulation 33

4.3 Simulation model with external modulated signal 34

4.4 Simulation model without external modulated signal 35

4.5 Simulation model with three channels 40

4.6 flowchart illustrate the modeling steps 40

4.7 The phase matching condition of two different

wavelengths 45

5.1 Optical spectrum at the input of the fiber when channel

spacing is set to 0.1 nm 48

5.2 Optical spectrum at the onput of the fiber when channel


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5.6 Optical spectrum at the onput of the fiber when channel


5.7 Optical spectrum at the output of the fiber when input

power is set to 20 dBm 52


power is set to 20 dBm 52


power is set to -10 dBm 53

5.10 Output optical spectrum when the dispersion of fiber

optic is set to 16.75 ps/nm/km 54

5.11 Optical spectrum at the input of the fiber when the

channel spacing is set at 0.1 nm 55

5.12 Optical spectrum at the output of the fiber when the











power is set at 20 dBm 59



xiv


power is set at -10 dBm 60




effective area of the fiber optic is set at 76.5 μm2 62

5.22 Four optical spectrum at the input of the fiber when the


5.23 Four output optical spectrum channels when the channel

spacing is set at 0.1 nm 64



5.25 Four input optical spectrum channels when the channel






5.28 Power per channel vs. FWM power 69

5.29 Channel spacing versus FWM power 69


channel spacing is unequal 71

5.31 Optical spectrum at the output of the fiber with unequal

channel spacing 71


effective area of the fiber optic is set at 76.5 μm2 72

xv

LIST OF ABBREVIATIONS

RoF - Radio over Fiber

SPM - Self Phase Modulation

XPM - Cross Phase Modulation

FWM - Four Wave Mixing

SRS - Stimulated Raman Scattering

SBS - Stimulated Brillouin Scattering

WDM - Wavelength Division Multiplexing

DWDM - Dense Wavelength Division Multiplexing

SMF - Single Mode Fiber

nm - nanometer

E/O - Electrical-To-Optical Converter

O/E - Optical - To Electrical- Converter

RF - Radio Frequency

IF - Intermediate Frequency

CW - Continuous Wave

RAU - Radio Antenna Unit

THz - Teri hertz

OTDM - Optical Time Division Multiplexing

SCM - Sub-Carrier Multiplexing

EMI - ElectroMagnetic Interference

IM-DD - Intensity Modulation and Direct Detection

OFM - Optical Frequency Multiplication

GSM - Global System for Mobile communication

MVDS - Multipoint Video Distribution Service

MBS - Mobile Broadband System

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GHz - Gigahertz

RHD - Remote Heterodyne Detection

TDM - Time Division Multiplexing

OADM - Optical Add-Drop Multiplexer

LED - Light Emitting Diode

GVD - Group velocity dispersion

ITU - International Telecommunication Union

MUX - Multiplexer

NDSF - Non Dispersion Shifted Fiber

PMD - Polarization Mode Dispersion

NRZ - Non-Return to zero

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LIST OF SYMBOLS

A - Pulse amplitude

Aeff - Effective area of optical fiber

c - Speed of light

co - Speed of light in vacuum

D - Dispersion parameter

dijk - Degeneracy factor

E - Electric field, vector

E - Electric field, scalar

f - Frequency

I - Intensity

L - Length

Leff - Effective length

n - Refractive index

no - Wavelength dependent refractive index

n2 - Nonlinear refractive index

n2/Aeff - Nonlinear coefficient

P - Total polarization, vector

P - Power

pi - Input power

r - Radius

t - Time

z - Distance

α - Attenuation constant [1/m]

αdB - Attenuation constant [db/km]

βi - Propagation constant of the mode i

γ - Nonlinear parameter

xviii

εo - Vacuum permittivity

λ - Wavelength

λo - Center wavelength

τ - Normalized time constant

ωi - Angular frequency

χ(j) - jth order susceptibility

xix

LIST OF APPENDICES

APPENDIX TITLE PAGE

A A Matlab Program for FWM power and channel

spacing 78

1

CHAPTER 1

INTRODUCTION

1.1 Introduct ion

In the past, dating back to the beginning of the human civilization,

communication was done through signals, voice or primitive forms of writing and

gradually developed to use signaling lamps, flags, and other semaphore tools.

As time passed by, the need for communication through distances, to pass

information from one place to another, became necessary and the invention of

telegraphy brought the world into the electrical-communication. The major

revolution that affected the world however was the invention of the telephone in

1876. This event has drastically transformed the development of communication

technology. Today’s long distance communication has the ability to transmit and

receive a large amount of information in a short period of time.

Since the development of the first-generation of optical fiber communication

systems in the early 80’s [4], the optical fiber communication technology has

developed fast to achieve larger transmission capacity and longer transmission

distance, to satisfy the increased demand of computer network. Since the demand on

the increasing system and network capacity is expected, more bandwidth is needed

because of the high data rates application, such as video conference and real-time

image transmission, and also to achieve affordable communication for everyone, at

2

anytime and place [1]. The communication capabilities allow not only human to

human communication and contact, but also human to machine and machine to

machine interaction. The communication will allow our visual, audio, and touch

sense, to be contacted as a virtual 3-D presence [3].

To keep up with the capacity increasing requirement, new devices and

technologies with high bandwidth are greatly needed by using both electronic and

optical technologies together to produce a new term Radio over Fiber (RoF). The

progress made so far has been impressive, where information rate at 1 terabits/s can

be handled by a single fiber [5].

RoF is a technology used to distribute RF signals over analog optical links. In

such RoF systems, broadband microwave data signals are modulated onto an optical

carrier at a central location, and then transported to remote sites using optical fiber.

The base-stations then transmit the RF signals over small areas using microwave

antennas and. Such a technology is expected to play an important role in present and

future wireless networks since it provides an end user with a truly broadband access

to the network while guaranteeing the increasing requirement for mobility. In

addition, since it enables the generation of millimeter-wave signals with excellent

properties, and makes effective use of the broad bandwidth and low transmission loss

characteristics of optical fibers, it is a very attractive, cost-effective and flexible

system configuration.

1.2 Problem Background

Normally light waves or photons transmitted through RoF have little

interaction with each other, and are not changed by their passage through the fiber

(except for absorption and scattering). However, there are exceptions arising from

the interactions between light waves and the material transmitting them, which can

affect optical signals in RoF. These processes generally are called nonlinear effects

because their strength typically depends on the square (or some higher power) of

intensity rather than simply on the amount of light present. This means that nonlinear

3

such as self phase modulation (SPM), cross phase modulation (XPM), four wave

mixing (FWM), stimulated raman scattering (SRS), and stimulated brillouin

scattering effects (SBS) are weak at low powers, but can become much stronger

when light reaches high intensities [7]. This can occur either when the power is

increased, or when it is concentrated in a small area-such as the core of an optical

fiber. Nonlinear optical devices have become common in RoF applications, such as

to convert the output of lasers to shorter wavelengths by doubling the frequency. The

nonlinearities in RoF are small, but they accumulate as light passes through many

kilometers of fiber. Nonlinear effects are comparatively small in optical fibers

transmitting a single optical channel. They become much larger when wavelength-

division multiplexing (WDM) packs many channels into a single fiber [9].

WDM puts many closely spaced wavelengths into the same fiber where they

can interact with one another. It also multiplies the total power in the fiber. A single-

channel system may carry powers of 3 milliwatts near the transmitter. DWDM

multiplies the total power by the number of channels, so a 40-channel system carries

120 mW. That's a total of 2 mW per square micrometer-or 200,000 watts per square

centimeter [11]. Several nonlinear effects are potentially important in RoF, although

some have produce more troublesome than others. Some occur in systems carrying

only a single optical channel, but others can occur only in multichannel systems.

1.3 Problem Statement

The rapid development of the wireless communication networks has

increased the need of the optical signal processing. The link lengths have grown to

thousands of kilometers without need to convert optical signals back and forth to

electric form, and the transmission speeds of terabits per second are feasible today

[5]. This ever-growing demand for the high speed communication has forced to use

higher bit rates as well as transmission powers.

Nonlinear effects on communication have become significant at high optical

power levels and have become even more important since the development of

4

erbium-doped fiber amplifier (EFDA) and DWDM systems. By increasing the capacity

of the optical transmission line, which can be done by increasing channel bit rate,

decreasing channel spacing or the combination of both, the fiber nonlinearities come

to play even more decisive role.

The origin of the nonlinearities is the refractive index of the optical fiber,

which is varies with the intensity of the optical signal. This intensity-dependent

component of the refractive index includes several nonlinear effects, such as SPM,

XPM, FWM, SRS, and SBS, and becomes significant when high powers are used.

Although the individual power in each channel may be below the level needed to

produce nonlinearities, the total power summed over all channels can quickly

become significant. The combination of high total optical power and large number of

channels at closely spaced wavelengths is a source for many kinds of nonlinear

interactions.

Form the above-mentioned reasons, this study is aimed to gain insight into

nonlinear effect caused specifically by FWM in the WDM for RoF system and

measure the coefficient behind these nonlinear effects. Nonlinear coefficient of the

RoF may become an important parameter, when new optical long-haul transmission

lines and networks are being deployed.

1.4 Object ive of the Projec t

The main objective of this project is to evaluate the FWM in WDM for RoF

technology, in order to calculate the impairments associated with long-distance high-

bit rate optical fiber communication systems. In order to achieve the objective,

optisystem and matlab programming software will be used respectively in the

numerical simulation and the analytical modelling will be verified through

comparison with optisystem simulation.

5

1.5 Scope of the Project

To study the efficiency of the FWM in WDM for RoF optical network, two

approaches were followed in this project. The first approach is the numerical

simulation using Optisystem software which almost replicates a real system. The

second aproach is the analytical modeling, which is simple and faster to analyze its

performance. MATLAB programming is used to implement the analytical model. To

verify the analytical system, a comparison is made with the Optisystem software.

Since Routing and wavelength assignment algorithm (RWA) needs to set up the path

immediately to reduce network delays, the analytical model developed in this project

can be used to calculate the impairments fast enough so that the routing decisions can

be made efficiently, to achieve optimal systems.

1.6 Organizat ion of the Projec t

Chapter 1 provides the introduction to this project where brief background of

the study problem and to the statement of the problem. Followed by the objective,

and the scope of the study. Chapter 2 reviews the literature, which includes

introduction to the RoF, the benefits, and applications of the Radio over Fiber

Technology in both satellite and mobile radio communications. In addition various

types of RoF Multiplexing Techniques, such as Sub carrier multiplexing and

wavelength division multiplexing, have also bee covered. Chapter 3 provides

information about the fiber characteristics, and the non linear effects such as SPM,

FWM, SBS, SRS, and XPM.

Chapter 4 describes the methodological processes by showing detailed diagram of

the methods implemented as well as highlighting briefly the steps those have been

followed to achieve the objective of this project. Chapter 5 presents the results

derived from the methods explained where some analyses and simulations were done

based on the FWM effects. Finally the conclusions of the study, as well as some

suggestions for future work were summed up in Chapter 6.

6

CHAPTER 2

RADIO-OVER-FIBER TECHNOLOGY

2.1 Introduction

The integration of wireless and optical networks is a potential solution for

increasing capacity and mobility as well as decreasing costs in the access network,

by RoF. The concept of RoF means to transport information over optical fiber by

modulating the light with the radio signal. This modulation can be done directly with

the radio signal or at an intermediate frequency. RoF technique has the potentiality to

the backbone of the wireless access network. Such architecture can give several

advantages, such as reduced complexity at the antenna site, radio carriers can be

allocated dynamically to the different antenna sites, and Transparency and scalability

[10].

RoF technology is now ubiquitous in the telecommunications infrastructure.

Fiber optics and WDM technology have increased significantly the transmission

capacity of today's transport networks, and they are playing important roles in

supporting the rapidly increasing data traffic.

7

2.2 What is Radio over Fiber?

RoF technology entails the use of optical fiber links to distribute RF signals

from a central location (headend) to Remote Antenna Units (RAUs). In narrowband

communication systems and Wireless Local Area Network (WLANs), most of signal

processing (including coding, multiplexing, RF generation, modulation, etc) are

made in central stations (CS-s) rather than in the base station (BS-s) [1]. The signal

between CS and BS is transmitted in the optical band via a RoF network. This

architecture makes design of BS-s quite simple. In the simplest case, the BS consists

mainly from optical-to-electrical (O/E) and electrical-to-optical (E/O) converters, an

antenna and some microwave circuitry (two amplifiers and a diplexer).

The centralization of Radio Frequency (RF) signal processing functions

enables equipment sharing, dynamic allocation of resources, and simplifies system

operation and maintenance. These advantages could be translated into major system

installation and operational savings, especially in wide-coverage broadband wireless

communication systems, where a high density is necessary. Figure 2.1 shows that the

concept of RoF system.

Figure 2.1 The Radio over Fiber System Concept [5]

8

2.3 Benefits of RoF Technology

The RoF technology holds many advantages compared to the electronic

signal distribution. Some of these advantages will be given in the following sections.

2.3.1 Low Attenuation Loss

Electrical distribution of high-frequency microwave signals through either

free space or transmission lines always causes problems besides its high cost. In free

space, losses due to absorption and reflection increase with frequency, where as in

transmission lines, the rise of impedance with frequency leads to very high losses.

Therefore, distributing high frequency radio signals electrically over long distances

requires expensive regenerating equipment, as for mm-waves, their distribution via

the use of transmission lines is not feasible even for short distances.

The alternative solution for this problem is to distribute baseband signals or

signals at low intermediate frequencies (IF) from the switching centre (headend) to

the BS [1]. The baseband or IF signals are up-converted to the required microwave,

or mm-wave frequency at each base station and amplified before being radiated. This

system configuration is the same as the one used in the distribution of narrowband

mobile communication systems. Since optical fiber offers very low loss, RoF

technology can be used to achieve both low-loss distribution of mm-waves, as well

as simplification of RAUs at the same time.

Single Mode Fibers (SMFs) made from glass (silica), have attenuation losses

below 0.2 dB/km and 0.5 dB/km in the 1550 and 1300 nm windows, respectively as

shown in Figure 2.2 [6].

9

Figure 2.2. Operating regions of optical fiber [2]

2.3.2 Large Bandwidth

Optical fibers offer enormous bandwidth. There are three main transmission

windows, which offer low attenuation in the wavelength region of 850, 1310, and

1550 nm respectively [6] as shown in Figure 2.2.

For a single SMF optical fiber, the combined bandwidth of the three windows

is in the excess of 50 THz. commercial systems utilize only a fraction of this capacity

(1.6 THz) [5].

The high optical bandwidth enables high speed signal processing that may be

more difficult or impossible to do in electronic systems. Furthermore, signal

processing in the optical domain makes it possible to use cheaper low bandwidth

optical components such as laser diodes and modulators; in addition, it is still

capable to handle high bandwidth signals.

The utilization of enormous bandwidth, which is primary source of receiver

and transmission data, offered by optical fibers is however, severely hampered by the

limitation of bandwidth in electronic systems. This problem is referred to as the

“electronic bottleneck” [3]. The solution of the electronic bottleneck lies in effective

10

multiplexing Optical Time Division Multiplexing (OTDM) and DWDM techniques.

In analogue optical systems, including RoF technology, the Sub-Carrier Multiplexing

(SCM) is used to increase optical fiber bandwidth utilization. In SCM, several

microwave subcarriers, which are modulated with digital or analogue data, are

combined and used to modulate the optical signal, to be carried on a single fiber.

This makes RoF systems cost-effective.

2.3.3 Immunity to Radio Frequency Interference

Immunity to ElectroMagnetic Interference (EMI) is a very attractive property

of RoF technology, especially for microwave transmission. This is so because signals

are transmitted in the form of light through the fiber. Due to this immunity, fiber

cables are preferred even for short connections at mm-waves. EMI immunity is the

immunity to eavesdropping, which is an important characteristic of optical fiber

communications as it provides privacy and security.

2.3.4 Easy Installation and Maintenance

In RoF systems, complex and expensive equipments are kept at the headend,

thereby making the Remote Antenna Unit (RAUs) simpler. For instance, most RoF

techniques eliminate the need for a local oscillator and related equipments at the

RAU. In such cases a photodetector, an RF amplifier and an antenna make up the

RAU. Modulation and switching equipment is kept in the headend and is shared by

several RAUs. This arrangement leads to smaller and lighter RAUs by effectively

reducing system installation and maintenance costs. Easy installation and low

maintenance costs of RAUs are very important requirements for mm-wave systems,

because of the large number of the required RAUs. In applications where RAUs are

not easily accessible, the reduction in maintenance requirements leads to a major

operational cost savings [10]. The usage of smaller number of RAUs also leads to a

reduced environmental impact.

11

2.3.5 Reduced Power Consumption

Reduced power consumption is a consequence of having simple RAUs with

reduced equipments. Most of the complex equipments are kept at the centralized

headend. In some applications, the RAUs are operated in passive mode. For instance,

some 5 GHz Fiber-Radio systems employing pico-cells can have the RAUs operate

in a passive mode [10]. Reduced power consumption at the RAU is significant

considering that the RAUs are sometimes placed in remote locations and have not

been fed by the power grid.

2.3.6 Multi-Operator and Multi-Service Operation

RoF offers system operational flexibility. Depending on the microwave

generation technique, the RoF distribution system can be made signal-format

transparent. The Intensity Modulation and Direct Detection (IM-DD) technique can

be made to operate as a linear system and, therefore, as a transparent system. This

can be achieved by using low dispersion fiber (SMF) in combination with pre-

modulated RF subcarriers (SCM). In that case, the same RoF network can be used to

distribute multi-operator and multi-service traffic resulting in huge economic savings

[11]. The principle of Optical Frequency Multiplication (OFM) can also be used to

achieve multi-service operation in combination with either WDM or SCM, because

its tolerance to chromatic dispersion.

2.3.7 Dynamic Resource Allocation

Since the switching, modulation, and other RF functions are performed at a

centralized headend, it is possible to allocate the capacity dynamically. In a RoF

distribution system for Global System for Mobile communications (GSM) traffic,

more capacity can be allocated to a certain area during the peak times and then re-

allocated to other areas when off-peak. This can be achieved by allocating optical

12

wavelengths, through WDM [1]. Allocating the capacity dynamically as the need for

it arises, obviates the requirement for allocating permanent capacity, which would be

a waste of resources in the cases where traffic loads vary frequently by large

margins. Furthermore, having the centralized headend facilitates the consolidation of

other signal processing functions such as mobility functions and macro diversity

transmission [1].

2.4 The Applications of Radio-over-Fiber Technology

Some of the applications of RoF technology include satellite

communications, mobile radio communications, broadband access radio, Multipoint

Video Distribution Services (MVDS), Mobile Broadband System (MBS), vehicle

communications and control, and wireless LANs over optical networks. Two of the

main application areas of RoF technology are briefly discussed below.

2.4.1 Cellular Networks

The field of mobile networks is an important application area of RoF

technology. The ever-rising number of mobile subscribers coupled with the

increasing demand for broadband services have kept sustained pressure on mobile

networks to offer increased capacity. Therefore, mobile traffic (GSM) can be relayed

cost effectively between the SCs and the BSs by exploiting the benefits of SMF

technology. Other RoF functionalities such as dynamic capacity allocation offer

significant operational benefits to cellular networks.

13

Figure 2.3 These robust RAPs are connected to the central base station via the

RoF links [10]

2.4.2 Satellite Communications

Satellite communication was one of the first practical applications of RoF

technology. One of the applications involves the remoting of antennas to suitable

locations at satellite earth stations. In this case, small optical fiber links of less than

1km and operating at frequencies between 1 GHz and 15 GHz are used [10]. By

doing so, high frequency equipment can be centralized.

The second application involves the remoting of earth stations themselves.

With the use of RoF technology, the antenna needs not to be within the control area

(e.g. Switching Centre). They can be sited many kilometers away for the purpose of

improved satellite visibility or reduction of interference from other terrestrial

systems. The Switching equipment may also be appropriately sited, taking in to

consideration the environmental or accessibility reasons or reasons relating to the

cost of premises, without requiring to be in the vicinity of the earth station antennas.

CBS

14

2.5 RoF Multiplexing Techniques

RoF multiplexing techniques is the process of multiplexing wavelength of

different frequency onto a single fiber. This operation cerates many virtual fibers,

each capable of carrying different signal.

RoF multiplexing uses wavelengths to transmit data parallel by bit or serial

by character, which increases the capacity of the fiber by assigning incoming optical

signals to specific frequency (wavelengths) within designated frequency band and

then multiplexing the resulting signal out on to one fiber.

2.5.1 Sub-Carrier Multiplexing in RoF Systems

Subcarrier Multiplexing (SCM) is a simple and cost effective approach for

exploiting optical fiber bandwidth in analogue optical communication systems in

general and in RoF systems in particular. In SCM, the RF signal is used to modulate

an optical carrier at the transmitter’s side. This results in an optical spectrum

consisting of the original optical carrier f0, plus two side-tones located at f0 ± fSC, ,

where fSC is the subcarrier frequency. If the subcarrier itself is modulated with data

(analogue or digital), then sidebands centered at f0 ± fSC are produced as illustrated in

Figure 2.4.

Figure 2.4 SubCarrier multiplexing of mixed digital and analogue signals [11]

2.4 GHz

15

In order to multiplex multiple channels of mixed digital and analogue signals

to one optical carrier, the multiple sub-carriers are first combined and then used to

modulate the optical carrier as shown in Figure 2.3. At the receiver’s side the sub-

carriers are recovered through direct detection and then radiated. Different

modulation schemes may be used on separate sub-carriers. One sub-carrier may carry

digital data, while the other may be modulated with an analogue signal such as video

or telephone traffic. therefore, SCM is found to support the multiplexing of various

kinds of mixed mode broadband data. Modulation of the optical carrier may be

achieved by either directly modulating the laser, or by using external modulators.

SCM may be used in both IM-DD and Remote Heterodyne Detection (RHD)

RoF techniques. SCM in combination with IM-DD has been used in RoF systems fed

by multimode fiber. However, these systems have been used mainly for transmitting

WLAN signals at frequencies below 6 GHz [11].

2.5.2 Wavelength Division Multiplexing in RoF Systems

WDM are passive devices that combine light signals with different

wavelengths, coming from different fibers, onto a single fiber. They include dense

wavelength division multiplexers (DWDM), devices that use optical (analog)

multiplexing techniques to increase the carrying capacity of fiber networks beyond

levels that can be accomplished via time division multiplexing (TDM)

The use of WDM for the distribution of RoF signals as illustrated in figure,

has gained importance recently. WDM enables the efficient exploitation of the fiber

network’s bandwidth. These systems can achieve capacities over 1 Tb/s over a single

fiber. At the same time, bit rates on a single channel have risen to 10 Gb/s and

systems operating at 40 Gb/s channel rates are becoming commercially available.

The channel spacing in WDM can be decreased to 50 GHz or even to 25 GHz and

thus, it is possible to use hundreds of channels. However, if the channel spacing is

dropped to 50 GHz instead of 100 GHz, it will become much harder to upgrade the

systems to operate at 40 Gb/s due to the nonlinear effects.

16

Figure 2.5 WDM system using multiple wavelength channels and optical

amplifiers [10]

However, the transmission of RF signals is seen as inefficient in terms of

spectrum utilization, since the modulation bandwidth is always a small fraction of the

carrier signal frequency. Therefore, methods to improve the spectrum efficiency have

been proposed. RoF on WDM systems have been reported. Carriers modulated with

mm-waves are dropped from and added to a fiber ring using Optical Add-Drop

Multiplexers (OADM). The OADM are placed at base stations and tuned to select

the desired optical carriers to drop [10] [11].

17

CHAPTER 3

NON-LINEAR EFFECTS

3.1 Introduction

The fundamental component that makes the optical communication possible

is the optical fiber. The phenomenon which guides the light along the optical fiber is

the total internal reflection. It is an optical phenomenon which occurs when the

incident light is completely reflected. In case of materials with different refractive

indices, light will be reflected and refracted at the boundary surface. This will occur

only from higher refractive index to a lower refractive index such as light passing

from glass to air. This phenomenon forms the basis of optical communication

through fibers.

An optical fiber is a dielectric waveguide, it is cylindrical, and guides the light

parallel to the axis. The cylindrical structure is dielectric with a radius “a” and

refractive index of “n1” is the called the core of the fiber and the layer that

encompasses this structure is called the cladding. The Cladding has a refractive index

“n2” which is lesser than “n1”. This helps in providing mechanical strength and

reducing scattering losses. It also prevents the core from surface contamination.

cladding doesn’t take part in light propagation.

18

3.2 Types of Fibers

Fibers can be classified according to the core’s material composition. If the

refractive index of the core is uniform and changes abruptly at the cladding boundary,

then it is called as Step-index fiber. If the refractive index changes at each radial distance,

then it is called as Graded-index fiber. These fibers can be divided into single mode and

multi mode fibers. The single mode fibers operate in only one mode of propagation.

Multimode fibers can support hundreds of modes.

Both laser diodes and light emitting diodes (LED) can be used as light wave

sources in fiber-optical communication systems. When compared to Laser diodes, LEDs

are less expensive, less complex and have a longer lifetime; however, their optical

powers are typically small and spectral linewidths are much wider than that of laser

diodes. In multimode fibers different modes travel with different speed, which is

commonly referred to as intermodal dispersion, giving room to pulse spreading. In single

mode fibers, different signal frequency components travel in different speed within the

fundamental mode and this result in chromatic dispersion. Since the effect of chromatic

dispersion is proportional the spectral linewidth of the source, laser diodes are often used

in high-speed optical systems because of their narrow spectral linewidth.

3.3 Fiber Losses

For efficient recovery of the received signal, the signal to noise ratio at the

receiver must be considerably high. Fiber losses will affect the received power eventually

reducing the signal power at the receiver. Hence optical fibers suffer heavy loss and

degradation over long distances. To overcome these losses, optical amplifiers were

invented, which significantly boosted the power in the spans in between the source and

receiver. However, optical amplifiers introduce amplified spontaneous emission (ASE)

noises which are proportional to the amount of optical amplifications they provide; low

19

loss in optical fibers is still a critical requirement in long distance optical systems to

efficiently recover the signal at the receiver.

Attenuation Coefficient is a fiber-loss parameter, which expressed in the units of

dB/Km. For short wavelengths; the loss may exceed 5 dB/Km and makes it unsuitable for

long distance transmission [2]. These losses are mainly due to material absorption and

Rayleigh scattering. Material absorption is the phenomenon exhibited by silica fibers.

The intrinsic absorption is caused by the fused silica and the extrinsic absorption is

caused by the impurities in silica. The other contributing factor is the Rayleigh scattering

which is caused by the density fluctuations in the fiber. These fluctuations change the

refractive index on a smaller scale. Light scattering in such medium is called Rayleigh

scattering [7].

In multi-mode fibers, intermodal dispersion is the dominant contributor of signal

waveform distortion. Although intermodal dispersion is eliminated in single mode fibers,

different frequency component of optical signal carried by the fundamental mode still

travel in slightly different speed giving rise to a wavelength-dependent group delay. As

the group delay depends on wavelength, different amount of time is taken for the

different spectral components to reach a certain distance. Due to this effect, the optical

signal with a certain spectral width spreads with time when it travels through the fiber.

This pulse spreading is important and needs to be determined.

3.4 Fiber Nonlinearities

Even though optical networks are fast, robust, and error free, still nonlinearity

obstacles exist, which prevent it from being a perfect medium.

The nonlinear effects of the fibers play a detrimental role in the light propagation.

Nonlinear Kerr effect is the dependence of the refractive index of the fiber on the power

20

that propagating through it. This effect is responsible for self phase modulation (SPM),

cross phase modulation (XPM) and four wave mixing (FWM). The other two important

effects are stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS).

3.4.1 Self Phase Modulation

In fibers, the refractive index always has some dependence on the optical intensity

which is the optical power per effective area. This relation can be given as [6]:

0 2 0 2 ................................................................ (3......... ). 1eff

Pn n n I n nA

where no is the ordinary refractive index , n2 is the nonlinear refractive index co-efficient,

Aeff is the effective core area, and P is the power of the optical signal.

This nonlinearity is known as Kerr nonlinearity. This produces Kerr effect in

which the propagating signal is phase modulated by the carrier. This leads to a

phenomenon called Self-phase modulation that converts power fluctuations into phase

fluctuations in the same wave [8].

In a material where the refractive index depends on a varying signal intensity

propagating along the fiber, it will produce time varying refractive index. Higher

refractive index at the peak of the pulse is produced, when compared to the edges of the

pulse. These results a time varying phase change dθ/dt. Due to this change, the frequency

of the optical signal undergoes a frequency shift from its initial value. This effect is

known as frequency chirping, in which different parts of the pulse undergo different

phase change as shown in Figure 3.1 [8]. The rising edge experiences a shift towards the

higher frequency and the trailing edge experiences a shift towards the lower frequency.

Since this effect depends heavily on the signal intensity, SPM has more effect on high

intensity signal pulses.

21

Figure 3.1 Frequency chirping effect

In case of fibers having the group velocity dispersion (GVD) effects, the pulse

normally broadens which leads to difficulty in the receiver side to decode the signal.

When the chromatic dispersion is negative, the edges of frequencies which experienced

higher shifts tend to move away from the centre of the pulse. The edges of the

frequencies which experienced lower shifts tend to move away from the centre in the

opposite direction. Thus the GVD affected pulse will be broadened at the end of the fiber,

and the chirping worsens due to this effect. Therefore the SPM can worsen the

performance of the optical system in the case of long haul transmission.

3.4.2 Cross Phase Modulation

As with Equation 3.2, the refractive index of the fiber depends on the time

varying signal intensity, which results time varying refractive index. This phenomenon

22

leads to an effect called XPM. XPM has more pronounced effect in the case of WDM

systems in which more optical channels are transmitted simultaneously. In the case of

XPM, the phase shift depends on the power of other channel. The total phase shift can be

represented as [6].

..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .2 ..... ..... ..... .... (. 3.2 )NLj eff j m

m j

L P P

where is the non-linear phase shift for the jth channel, M=½(N3-N2) varies from 1 to

5 W-1

/Km, Leff is the effective length of the fiber, and Pj and Pm are the power for the

channels i and j respectively.

On the right-hand-side of Equation 3.2, the first term represents effect of SPM

and the second term represents that of XPM. The factor of 2 in this equation implies that

XPM is twice as effective as SPM for the same amount of power [7]. The phase shift

which is directly created by XPM at the end of the fiber depends on the bit patterns and

powers of the neighboring channels. The effect of XPM also depends on the wavelength

separation between the signal channel and the neighboring channel. If the channels are

separated widely, then the XPM effects are relatively weak because the two bit streams

walk-off from each other quickly. In case of the DWDM systems, the channel

wavelength separation is very narrow, which leads to strong XPM effect. Since XPM

results in an inter channel crosstalk, its effect, to some extent, also depends on the bit

pattern of the two channels.

To analyze the effect of XPM and SPM, the nonlinear Schrödinger equation can

be used which represented as [2]:

222

2 ...............................................................(3.3)2 2

iA AA i A A

z t

23

By increasing the effective area, nonlinearities can be reduced. Aeff is about 80

µm2 for standard fibers and 50 µm2 for dispersion shifted fibers [6].

3.4.3 Four Wave Mixing

FWM is a phenomenon that occurs in the case of DWDM systems in which the

wavelength channel spacing are very close to each other. This effect is generated by the

third order distortion that creates third order harmonics. As shown in Figure 3.2, these

cross products interfere with the original wavelength and cause the mixing. In fact, these

spurious signals fall right on the original wavelength which results in difficulty in

filtering them out. In case of 3 channel system, there will be 9 cross products, where 3 of

them will be on the original wavelength. This is caused by the channel spacing and fiber

dispersion. If the channel spacing is too close, then FWM occurs. If the dispersion is

lesser, then FWM is higher since dispersion is inversely proportional to mixing

efficiency. As can be seen in Figure 3.2, the cross product lies right on the original signal

which poses problem when filtering.

In general, for N wavelengths input channel there will be M cross mixing

products and are given by [22]

2

.........................................................................................1 (3.4)2

.N

M N

24

Figure 3.2 Four wave mixing products

If the WDM system is considered as a sum of N monochromatic plane waves, it is

possible to solve the arising channels angular frequencies. Considering a simple three-

wavelength (1, 2, and 3) system that is experiencing FWM distortion, nine cross

products are generated near 1, 2 , and 3 see Figure 3.3 that involve two or more of the

original wavelengths. There are additional products generated, however they fall well

away from the original input wavelengths.

25

Figure 3.3 (a) two input signals ω1 and ω2 (b) three input signals ω1, ω2 and ω3

and the arising new frequency components due to FWM

Assuming that the input wavelengths are 1 = 1551.72 nm, 2 = 1552.52 nm, and

3 = 1553.32 nm. The interfering wavelengths generated around the original three

wavelength system are:

1 + 2-3 = 1550.92 nm

1-2 +3 = 1552.52 nm

2+31 = 1554.12 nm

1-2+3 =1552.52 nm

21-3 = 1550.12 nm

23- 1 = 1554.92 nm

2+ 3-1 = 1554.12 nm

22- 1 = 1553.32 nm

23 -2 = 1554.12 nm

It can be seen that three of the interfering products fall right on top of the original three

signals and the remaining six products fall outside of the original three signals. These six

wavelengths can be optically filtered out. The three interfering products that fall on top of

the original signals are mixed together; and cannot be removed by any means. Figure 3.2

shows the results graphically. The three tall solid bars are the three original signals. The

shorter cross-hatched bars represent the nine interfering products. The number of the

interfering products increases as ½ • (N3-N2) where N is the number of signals.

26

Figure 3.4 show that the number of the interfering products rapidly becomes a very large.

Since there is no way to eliminate the products that falling on top of the original signals,

the priority is to prevent them from forming in the first place.

Figure 3.4 FWM products versus channel count [22]

Therefore two factors strongly influence the magnitude of the FWM products, referred to

as the FWM efficiency. The first factor is the channel spacing; where the mixing

efficiency increases dramatically as the channel spacing becomes closer. Fiber dispersion

is the second factor, and the mixing efficiency is inversely proportional to the fiber

dispersion, being strongest at the zero-dispersion point. In all cases, the FWM mixing

efficiency is expressed in dB, and more negative values are better since they indicate a

lower mixing efficiency.

Figure 3.5 shows the magnitude of FWM mixing efficiency versus fiber

dispersion and channel spacing. If a system design uses NDSF with dispersion of 17

ps/nm/km and the minimum recommended International Telecommunication Union

(ITU) DWDM spacing of 0.8 nm, then the mixing efficiency is about -48 dB and will

have little impact. On the other hand, if a system design uses DSF with a dispersion of 1

ps/nm/km and a non-standard spacing of 0.4 nm, then the mixing efficiency becomes -12

dB and will have a severe impact on the system performance, perhaps, making the

recovery of the transmitted signal impossible. The magnitude of the mixing efficiency

27

will vary widely as these parameters vary. The data presented is intended to illustrate the

principles only.

Figure 3.5 FWM mixing efficiency in single-mode fibers [22]

FWM is independent of the used bit rate; however, it is critically dependent on channel

spacing and chromatic dispersion. Therefore, the effects of FWM must be considered

even at moderate-bit-rate systems, if the channel spacing is small or the chromatic

dispersion of the fiber is low. Thus, it is possible to minimize the effects of FWM by

increasing the channel spacing and the chromatic dispersion of the fiber.

3.4.4 Stimulated Brillouin Scattering (SBS)

SBS falls under the category of inelastic scattering in which the frequency of the

scattered light is shifted downward. This results in the loss of the transmitted power along

the fiber. At low power levels, this effect will become negligible. SBS sets a threshold on

the transmitted power, above which considerable amount of power is reflected. This back

reflection will make the light to reverse direction and travel towards the source. This

usually happens at the connector interfaces where there is a change in the refractive

28

index. As the power level increases, more light is backscattered since the level would

have crossed the SBS threshold. The parameters which decide the threshold are the

wavelength and the line width of the transmitter.

Lower line width experiences lesser SBS and the increase in the spectral width of

the source will reduce SBS. In the case of bit streams with shorter pulse width, no SBS

will occur. The value of the threshold depends on the RZ and NRZ waveforms, which are

used to modulate the source. It is typically 5 mW and can be increased to 10 mW by

increasing the bandwidth of the carrier greater than 200 MHz by phase modulation [8].

3.4.5 Stimulated Raman Scattering (SRS)

SRS occurs when the pump power increases beyond the threshold, however SRS

can happen in either direction, forward and backward. The molecular oscillations set in at

the beat frequency and the amplitude of the scattering increases with the oscillations. The

equations that govern the feedback process are [8]:

..............................................................................(3.9)pR p s p p

dIg I I I

dz

............................................................................ (.. 3.10)sR p s s s

dIg I I I

dz

where gR is the SRS gain. Ip and Is are intensities of Pump and stokes field.

In case of the threshold power, the Pth is given by [8],2 ........................................................................16 ( ) / . (3.11)........th RP w g

where πw2 is the effective area of the fiber core and w is the spot size.

Even though there are some detrimental effects posed by these two effects,

SBS and SRS can also be used in a positive way. Since both deal with transferring

energy to the signal from a pump, they can be used to amplify the optical signal.

Raman gain is also used in compensating losses in the fiber transmission. Table 3.1

shows comparison of property behavior under the influence of SBS and SRS.

Table 3.1 Comparison between SBS and SRS

Property SBS SRS

Direction of

scatter

Only in backward

direction

In both forward and backward

direction

Frequency shift About 10 GHz About 13 THz

Spectrum width Narrow width Broad spectrum width

29

CHAPTER 4

METHODOLOGY

4.1 Introduction

This chapter highlights the techniques and methods employed to study the

nonlinear effects of FWM in WDM for RoF as well as to analyze the modelling

results obtained. Details of the methods will be given in the proceeding sections.

4.2 Simulation using Optisystem Software

OptiSystem software is a numerical simulation enables users to plan, test and

simulate almost every type of optical link in the physical layer across the broad

spectrum of optical networks. Algorithms are included for dispersion map design, bit

error rate calculation, system penalty estimations, and link budget calculations.

Each layout can have certain component parameters assigned to be in sweep

mode. The number of sweep iterations to be performed on the selected parameters

could be defined. The value of the parameter changes through each sweep iterations;

which produces a series of different calculation results, based on the parameter

values. These processing parameters effect on the results are channel pacing, input

power, effective area and dispersion of the fiber

30

4.3 The Simulation Model

There are two technologies for modulation, direct or without external

modulation as shown in Figure 4.1 which the RF signal directly varies the bias of a

semiconductor laser diode

Figure 4.1 Direct modulation

The other technology is the external modulators are typically either integrated

Mach-Zehnder interferometers or electroabsorption modulators as shown in Figure

4.2 which the constant wave (CW) laser (always on bright), and the light is

modulated by an external lithium-niobate electro-optic modulator. External

modulation is currently preferred over any other form of modulation because it has

best performance, in spite of high cost.

Figure 4.2 External modulation

31

Using Optisystem software, two types of simulation models have been

developed to study FWM effects. The two models are with external modulated signal

and without external modulated signal as shown in the Figure 4.3 and 4.4,

respectively.

The frequency of the phase modulator drive signal was kept at 2.4 GHz. The

phase modulator has been used to sweep the optical frequency, it was necessary to

first integrate the drive signal [11].

.

Figure 4.3 Simulation model with external modulated signal

32

Figure 4.4 Simulation model without external modulated signal

The simulation models were modified according to the related parameters or

components for different types of simulation process as given below

i. Effect of channel spacing.

ii. Effect of different Power Level of the signals Sources

iii. Effect of increase dispersion of the Fiber Optic

iv. Effect of Increase Effective Area of the Fiber optic

4.4 Simulation of the Four Wave Mixing effect

Each component in both simulation models, shown in Figures 4.3 and 4.4,

has its own role, to play in the process.

The Pseudo Random Bit Sequence Generator is a device or algorithm, which

outputs a sequence of statistically independent and unbiased binary digits.

33

NRZ Pulse Generator (non-return-to-zero) refers to a form of digital data

transmission in which the binary low and high states, represented by numerals 0 and

1, are transmitted by specific and constant DC (direct-current) voltages.

In positive-logic NRZ, the low state is represented by the more negative or less

positive voltage, and the high state is represented by the less negative or more

positive voltage. In negative-logic NRZ, the low state is represented by the more

positive or less negative voltage, and the high state is represented by the less positive

or more negative voltage.

The continues wave (CW) Generator is a generator of continuous-wave

millimeter-wave optical signals. The spectral linewidth of the generated millimeter-

wave signals is 2 kHz. The power of the measured cw millimeter-wave signals is

almost in proportion to the power multiplication of the two input optical signals.

The Mach-Zehnder Modulator, is a modulator, which has two inputs, one for

the laser diode and the other for the data from the channels.

The WDM Multiplexer is a method of transmitting data from different

sources over the same fiber optic link at the same time whereby each data channel is

carried on its own unique wavelength.

The Optical Fiber is a component, used in the simulation is a single mode fiber

(SMF-28), where the dispersive and nonlinear effects are taken into account by a

direct numerical integration of the modified nonlinear Scrödinger (NLS) equation.

Besides the above components there are three types of components, which used

for visualizing purposes:

i. Optical Power Meter Visualizer

ii. Optical Spectrum Analysis

iii. WDM analyzer

Below are the tables for parameters setting. Table 4.1 shows the set of the

global parameters; and Table 4.2 shows the parameters, set for the CW laser sources.

The parameters set in the WDM MUX are shown in Table 4.3. There are many tabs

34

for the optical fiber parameter settings, where Table 4.4 gives the setting for the main

and the dispersion tabs, Table 4.5 gives the setting for the nonlinear tab, and Table

4.6 gives the setting for the numerical and PMD tabs in optical fiber respectively.

Table 4.1 Global parameters

Table 4.2 CW Laser sources parameters

35

Table 4.3 WDM 2x1 multiplexer parameters

Table 4.4 Main tab and dispersion tab parameters are set for optical fiber

Table 4.4: Main tab and Dispersion tab Parameters for Optical Fibers

36

Tables 4.5 Nonlinear tab parameters for optical fiber

Table 4.6 Numerical tab and PMD tab parameters for optical fiber

37

4.5 Simulation of FWM for higher number of channels

Sources in the simulation model were increased to three or four channels.

Figures 4.5 and 4.6 show the sources increased in the new simulation model based on

direct modulation [22].

Figure 4.5 Simulation model with three channels

Figure 4.6 Simulation model with four channels

38

4.6 Effect of Different Power Level of the Signals Sources

The main requirement from a wireless communication system is that the

transmitted electro magnetic (EM) wave must reach the receiver with ample power to

allow the receiver to distinguish the wave from the background noise.

Another common property used to describe signal strength is the S/N ratio.

The S/N ratio does not describe the absolute power in the signal, but instead

describes the power of the signal in comparison to the power of the background

noise. The higher the S/N ratio, the better or more powerful the signal. Since the S/N

ratio accounts for the level of background noise, it is a very valuable and widely used

indicator of signal strength.

In the simulation process, the power at the simulation model sources was

varied from 20 dBm to -10 dBm with step of -10 dBm to in order try different

simulations.

4.7 Effect of Increase dispersion of the Fiber Optic

Wavelength dispersion, is a signal dispersion, which occurrs primarily in

single-mode fiber. A significant amount of the light launched into the fiber is leaked

into the cladding. This leaked amount is wavelength dependent and also influences

the speed of propagation. High volume communication lines have carefully timed

spacings between individual signals. Fortunately, wavelength dispersion can be

minimized by careful designation of fiber refractive index.

The dispersion parameter of the fiber optic in the simulation model was

varied from 1 ps/nm/km to 16.75 ps/nm/km. This has been done in order to compare

the results with different dispersion parameters and the power level of sources set at

0 dBm.

39

4.8 Effect of Increase Effective Area of the Fiber optic

The effective area (Aeff) of the single-mode fiber is an important

measurement parameter. It is the area of the cross section of the beam arrived into

the fiber. The effective area evaluation requires the measurement of the field

distribution in the fundamental mode

The effective area parameter of the fiber optic in the simulation model has

been changed from 64 μm2 to 76.5 μm2 , in order to compare the results with different

effective area parameters as the power level of sources set at 0 dBm.

.

4.9 Modelling the Effect of FWM

Matlab program is used to develop the analytical model of the effect of FWM

in WDM for RoF. The modelling is meant to study the nonlinear effects due to the

FWM in WDM for RoF when the light passing through the medium. Figure 4.6

shows the steps that will be followed in the modeling process.

The total polarization P is nonlinear with respect to the electric field E,

however, it can be written as:

(1) (2) (3)0 ....................... . . . . . . ........ ............... .........(4.1)P

whereε0 is the vacuum permittivity and χ(j) (j = 1,2,…) is jth order susceptibility.

When light propagates in a transparent medium, its electric field causes some

amount of polarization in the medium. While at low light intensities the polarization

is linear with the electric field, nonlinear contributions become important at high

optical intensities, so the polarization equation consists linear terms as well as

nonlinear terms. The first order susceptibility χ(1) represents the linear term, and

nonlinearities can have strong effects in fibers at the third order susceptibility χ(3). So,

only the nonlinear effects in the optical fibers, which originate from the third-order

susceptibility χ(3), will be considered and the other terms will be neglected. The

40

programming will start from the third-order susceptibility χ(3). Thus the electric field of

the signal can be written as [6]:

1

................................................................., cos .(4.2)....N

i i ii

r t E t z

Where β is the propagation constant, and ω is angular frequencies

Substituting Equation 4.2 into Equation 4.1, and if only the term of the third

order susceptibility is taken into account, the nonlinear dielectric polarization

( ,NLP r t ) can be written as [6]:

3

1 1 1

, cos cos cosn n n

NL o i i i j j j k k ki j k

P r t E t z E t z E t z

3

2

1 1

..............................3

2 .( 1)cos ..........4

n no

i i j i i ii j

E E E E t termz

3

3

1

.......................................................cos ....3 34

.. ( ). .. . 2n

oi i i

i

E t z term

3

2

1 1

...................3

cos 2 3 ( 3)2 .4

... .n n

oi j i i i j

i j

E E t z t z term

3

2

1 1

3cos 2 3 .......................

4.( 4)2

n no

i j i i i ji j

E E tet z t z rm

3

1

........................................ ...

34

cos c 5. )s (o

n n no

i j ki j i k j

i j k i j k

E E E

t z term

................ .........cos cos ............... .( 6.. )i j k i j k tert z m

................ .........cos cos ............... ( 7... )i j k i j k tert z m

............. .........cos cos ............ (...... 8)i j k i j k z rmt te

(4.3)

The nonlinear susceptibility of the optical fiber generates new waves at the

angular frequencies ωr ± ωs ± ωt (r, s, t = 1, 2,…). Term 1, in the above equation

represents the effects of SPM and XPM.

41

Terms 2, 4 and 5 can be neglected, due to lack of phase matching. The remaining terms can

satisfy the phase matching condition. The power transferred due to the FWM to new

frequencies after light has propagated distance L in the fiber can be estimated from equation

4.4 [6]:

232................................................................................. (4......... )

8.. 4ijk ijk

ijk i j keff eff

dP PP P L

A n c

where neff is the effective index, Aeff is the effective area, Pi, Pj and Pk are the input

powers at ωi, ωj and ωk. The factor dijk depends on the number of channels affecting

the FWM

The efficiency of FWM and noise performance are analyzed, taking into

account the effects of difference channel spacing. Equation 4.5 is presented to

evaluate the efficiency of the FWM [23].

2

22( )eff

nA D

(4.5)

Equation 4.6 is used to investigate the relationship between the efficiency and

the power of the FWM [23].

2 2 2exp( )

9ijk ijk i j k effP d p p p L L

(4.6)

where Leff is effective length, which can be calculated by using Equation 4.7.

1 l

effeL

(4.7)

where ωis the Angular frequency, d is the degeneracy factor, (3) is the third order

susceptibility, Aeff is the effective Area, n2 is the nonlinear reflective index, c is the

speed of light, D is the dispersion, λ is the channel space, αis the fiber loss

coefficient and L is total fiber length.

42

The third order susceptibility χ(3), which includes self-phase modulation

(SPM) and cross-phase modulation (XPM) as well as four-wave mixing (FWM).

Therefore, the SPM and XPM will be considered as zero, thus, their effects on FWM

modeling are neglected. Term1 representing XPM and SPM will be considered as of

zero effect and will be neglected too.

The four-wave mixing, require the phase matching to be efficient. Essentially

this is mean to ensure a proper phase relationship between the interacting waves.

FWM will be a peak at the phase matching spectrum. Equation 4.8 satisfies the

condition of phase matching:

β= β(ω1)+β(ω2)- β(ω3)- β(ω4) (4.8)

Where βj is the propagation constant. If β= 0 the phase matching condition is

satisfied, otherwise mismatching occurs.

The model in this study will use only two wavelengths, therefore the phase matching

condition will be β= β(ω2) - 2 β(ω1) =0 in order to satisfy the phase matching

requirement as shown in Figure 4.7.

Figure 4.7 The phase matching condition of two different wavelengths [8]

Term2, term4, and term5 in the polarization Equation 4.3 are considered as

mismatching terms. After neglecting the terms representing the effects of SPM, XPM

that lack phase matching, the remaining terms in the nonlinear equation, which

satisfy the phase matching condition, will be used later to model the FWM.

β1

β2

β1 β

43

CHAPTER 5

RESULTS AND DISCUSSIONS

5.1 Introduction

This chapter presents and discusses the results obtained from the simulation

model by using Optisystem as numerical simulation and Matlab as analytical

simulation. The numerical simulation is simulated accordingly as mentioned in the

previous chapter, with and without external modulated laser.

5.2 Simulation of the Four Wave Mixing Effect

In the FWM simulation model layout, two types of visualiser tools have been

used. The optical spectrum analyzer and the WDM analyzer were fixed after MUX

and at the end of the fiber optic. The results obtained after the multiplexer are same

as the input power level shown before the nonlinear effect. The nonlinear effect

occurs only during the propagation of signals through the fiber. The optical spectrum

analyzer has been used to show the waveform whereby the WDM analyzer has been

used to display signal power (dBm), noise power (dBm) and OSNR (dB).

44

5.3 Simulation Results without the External Modulated Signal

In this simulation two CW lasers were used as signals sources, the

frequencies were set at 1550 and 1550.1 nm, where as the power was set at 0 dBm.

The linewidth has been set at 0, due to the interest in measuring only the total power

of the sideband frequencies, where the shape of the spectrum is not required. The

input signals have propagated through 25 km of nonlinear fiber.

5.3.1 Effect of Channel Spacing variation

Figure 5.1 shows the signal at the input channel when the channel spacing is

set at 0.1 nm.

Figure 5.1 Optical spectrum at the input of the fiber when channel spacing is set

at 0.1 nm

The result obtained from the simulation is depicted in Figure 5.2. From this

figure, the FWM effect is obviousl because the simulation without external

modulated laser is simpler compared to the simulation model with external

modulated laser. The interfering wavelengths generated around the original two

45

wavelength systems are 1549.9 nm and 1550.2 nm, thereby the power of the each

FWM sideband is approximately -59 dBm

Figure 5.2 Optical spectrum at the output of the fiber when channel spacing is set

at 0.1 nm


set at 0.2 nm.


at 0.2 nm

When the channel spacing is increased to 0.2 nm, the result obtained from the

simulation is depicted in Figure 5.4. The interfering wavelengths generated around

47

the original two wavelength system are 1549.8 nm and 1550.4 nm, thereby the power

of the each FWM sideband is approximately -61 dBm.

Figure 5.4 Optical spectrum at the output of the fiber when channel spacing is setat 0.2 nm

Similarly, Figures 5.5 shows the signal at the input channel when the channel

spacing is increased to 0.5 nm.


at 0.5 nm

Figure 5.6 shows the interfering wavelengths generated around the original

two wavelength system of 1549.5 nm and 1551 nm; thereby the power of each FWM

sideband is approximately -71 dBm.

48

Figure 5.6 Optical spectrum at the output of the fiber when channel spacing is setat 0.5 nm

Therefore, as the spacing between channels is increased the effect of the

FWM is decreased

5.3.2 Effect of Different Power Level of the Signals Sources

In the following process, the power level of the input sources was varied from

20 dBm to -10 dBm with step -10 dBm while other parameters such as the dispersion

and the effective area were kept unchanged.

The result obtained from the simulation when the input source power is set at

20 dBm is depicted in Figure 5.7.

49

Figure 5.7 Optical spectrum at the output of the fiber when input power is set at

20 dBm



Figures 5.8 Optical spectrum at the output of the fiber when input power is set at

10 dBm


-10 dBm is depicted in Figure 5.9.

50

Figures 5.9 Optical spectrum at the output of the fiber when input power is set at

-10 dBm

From the results, given it is clear that when the power level is increased to 20

dBm the effect of the FWM becomes very severe as shown in the Figure 5.7. As the

power level of the signal sources is decreased to -10 dBm the FWM becomes less

effective, as shown in the Figure 5.9, therefore, the FWM becomes significantly

effective at high optical power levels.

5.3.3 Effect of Increase Dispersion of the Fiber Optic

The dispersion parameter of fiber optic was changed from 1.0 ps/nm/km

to16.75 ps/nm/km, at input power of 0 dBm. The results were taken at the end of the

fiber optic.

Simulation results at dispersion of 16.75 ps/nm/km at input power of 0 dBm

is shown in Figures 5.10.

51

Figure 5.10 Optical spectrum at the output of the optical when the dispersion of

fiber optic is set at 16.75 ps/nm/km

The results obtained at the end of the fiber when the power level is set at 0

dBm and the dispersion is set at 16.75 ps/nm/km as shown in Figure 5.10, was

compared with the result obtained at the same power level and dispersion of 1

ps/nm/km as shown in Figure 5.4, these result show that the FWM products were

reduced when the dispersion parameter is increased. It is important to mention that

the dispersion parameter can not be set at too high value because it does bring

limitation in bandwidth in the WDM model.

5.4 Simulation Results with the External Modulated Signal

In this simulation two CW lasers were used as signals sources, the

frequencies were set at 1550 and 1550.1 nm, as shown in Figure 4.1, where as the

power was set at 0 dBm, due to the interest in measuring only the total power of the

sideband frequencies, where the shape of the spectrum is not required. The input

signals have propagated through 25 km of nonlinear fiber.

52

5.4.1 Effect of Channel Spacing variation


set at 0.1 nm.

Figure 5.11 Optical spectrum at the input of the fiber when the channel spacing is

set at 0.1 nm

The result obtained from the simulation is depicted in Figure 5.12. The FWM

effect is not quite obvious because the external modulation produce sideband.

53

Figure 5.12 Optical spectrum at the output of the fiber when the channel spacing

is set at 0.1 nm


set at 0.2 nm.


set at 0.2 nm

From Figures 5.14, the FWM effect is quite obvious when the channel

spacing is increased to 0.2. The power of the FWM sideband is approximately -72

dBm

54


is set at 0.2 nm


set at 0.5 nm.


set at 0.5 nm

Also in Figures 5.16, the FWM effect is quite obvious when the channel

spacing is increased to 0.5 nm. The power of the FWM sideband is approximately -

87 dBm

55


is set at 0.5 nm

Therefore, as the spacing between channels is increased the effect of the

FWM is decreased

5.4.2 Effect of Different Power Level of the Signals Sources

In the following process, the power level of the input sources was varied from

20 dBm to -10 dBm with step -10 dBm while other parameters such as the dispersion

and the effective area were kept unchanged.



56


20 dBm




10 dBm


-10 dBm is depicted in Figure 5.19.

57

Figure 5.19 Optical spectrum at the output of the fiber when input power is set at -

10 dBm

From the results, given it is clear that when the power level is increased to 20

dBm the effect of the FWM becomes very severe as shown in the Figure 5.17. As the

power level of the signal sources is decreased to -10 dBm the FWM becomes less

effective, as shown in the Figure 5.19, therefore, the FWM becomes significantly

effective at high optical power levels.

The new generated mixing products have high possibilities of falling directly

on the original signal, which produce crosstalk.

5.4.3 Effect of Increase Dispersion of the Fiber Optic

Simulation results with the use of the external modulated laser at dispersion

of 16.75 ps/nm/km at input power of 0 dBm is shown in Figures 5.20.

58


0 dBm

The results obtained at the end of the fiber when the power level is set at 0

dBm and the dispersion is set at 16.75 ps/nm/km as shown in Figures from 5.20.

were compared with the result obtained at the same power level and dispersion of 1

ps/nm/km as shown in Figure 5.12, these result show that the FWM products were

reduced when the dispersion parameter is increased. It is important to mention that

the dispersion parameter can not be set at too high value because it does bring

limitation in bandwidth in the WDM model.

5.4.4 Effect of Increase Effective Area of the Fiber Optic

Simulation results with the use of the external modulated laser at effective

area of 76.5 μm2 at input power of 0 dBm are shown in Figure 5.21.

59

Figure 5.21 Optical spectrum at the output of the fiber when the effective area of

the fiber optic is set at 76.5 μm2

Results obtained at the end of fiber where the power level is set at 0 dBm, and

the effective area is increased to 76.5μm2 is shown in Figure 5.21 is compared with

Figure 5.12 which the effective area is set at 64 μm2. It is found that the increasing of

the effective area can reduce the FWM effect.

5.5 Simulation of Four Wave Mixing for Higher Number of Channels

This section presents the simulation results as the number of channels is

increased to four in the simulation model, with or without the use of external

modulated laser.

60

5.5.1 Simulation Results for Four Signal Source without External Modulated

Signal

The simulation results for four channels, without use of external modulated

laser, Figure 5.22 shows input signal when number of channels is increased to four

and the channel spacing is set at 0.1 nm.

Figure 5.22 Four optical spectrum at the intput of the fiber when the channel

spacing is set at 0.1 nm

The result obtained from the simulation when the number of channel is

increased is depicted in Figure 5.23. The number of FWM also is increased

61

Figure 5.23 Four output optical spectrum channels when the channel spacing is set

at 0.1 nm


increased and the channel spacing is set at 0.5 nm is depicted in Figure 5.24. The

number of FWM also is increased

Figure 5.24 Four output optical spectrum channels when the channel spacing is

set at 0.5 nm

62

5.5.2 Simulation Results for Four Signal Source with External Modulated

Signal

The simulation results for four channels, when using External modulated

Laser, at different channel spacing..

Figure 5.25 shows input signal when number of channels is increased to four

and the channel spacing is set at 0.1 nm.

Figure 5.25 Four Input optical spectrum channels when the channel spacing is set

at 0.1 nm



number of FWM is also increased.

63


at 0.1 nm



number of FWM is also increased but with less effect.


at 0.5 nm

64

5.6 Discussions

Based on the results presented, The FWM effects increase as the number of

channels is increased. The number of spurious signals due to FWM increase

geometrically and given by

M= (N3-N2)/2 (5.1)

where N is the number of channels and M is the number of the newly generated

sidebands. The new generated mixing products have high possibilities fall directly on

the original signal, this could produce crosstalk.

Therefore, as the spacing between channels is reduced or remained equal the

effect of the crosstalk is found to become greater. When the spacing between the

channels is unequal, showed that the mixing products have low power level and

highly possible not to falls on the original signal, which makes them easy to be

filtered, and in turn improve the system performance.


the effective area is increased to 76.5μm2 are shown in Figures 5.10. It is found that

the OSNR obtained is better than before increasing the effective area as shown in

Figure 5.4.

As general, the increase of the effective area can reduce the FWM effect and

give higher OSNR value compared to the simulation result obtained with the same

power level.

The effective area refers to the equivalent area of the fiber in which the

optical power is transmitted. In the case of single mode fiber, this is roughly

proportional to the core area. Fiber with a large effective area offers reduced optical

power density, which raises the power threshold for the FWM penalties.

In addition, the effective area parameter and the dispersion parameter can be

used to calculate the FWM efficiency as follows:

65

η= n2 /(Aeff x D xλ2 ) (5.2)

where ηis the FWM efficiency, n2 is the nonlinear index coefficient, Aeff is the

effective area, D is the dispersion and λ is the spectral width.

5.7 Analytical Modelling

Matlab based program has been developed using Equations 4.4 to 4.6 in order

to design analytical model (Appendix A), which assists to predict the expected FWM

power in different channel spacing. The designed model can give the expectation

value of the FWM power in different input signal power level.

The analytical results have been compared to the results obtained from the

numerical simulation, as shown in Figures 5.28 and 5.29.

Figure 5.28 Power per channel vs. FWM power

5 10 15 20 25 30 35 40-125

-120

-115

-110

-105

-100

-95

-90

power per channel in mill watt

FWM

pow

er(d

Bm

)

66

Figure 5.29 Channel spacing versus FWM power

These results show that when power per channel is increased the spurious

power increase, too. The power of the FWM produced is found to be inversely

proportional to the square of the channel spacing, when all channels have the same

input power. Furthermore, the FWM effects increase exponentially as the level of the

optical power from the signal sources is increased, as shown in the Figure 5.28

Based on results presented, it is clear that when the channel spacing is smaller

the FWM effect becomes more significant due to the phase matching, as shown in

Figure 5.29.

5.8 Four Wave Mixing Reduction

One way to combat the FWM process is to use unequal channel spacing, so

that the mixing products do not coincide with signal frequency, and to use low input

power, or high effective area. Fiber dispersion management is a very effective way,

helpful not for FWM but also is the case of other nonlinear phenomena, that degrade

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6-80

-75

-70

-65

-60

-55

c hannel s pac ing

FW

Mp

owe

r(d

Bm

)

A naly t ic al s im ula t ion

num eric al s im ula t ion

67

transmission performance in the fiber, also FWM can be mitigated by increasing the

effective area of the fiber [19].

5.8.1 Effect of Unequal Channels

Figure 5.30 shows input signal when the channel spacing is unequal.


unequal

When the spacing between the channels is unequal, showed that the mixing

products have low power level and highly possible not to falls on the original signal,

which makes them easy to be filtered, and in turn improve the system performance.

As shown Figure 5.31.

68

Figure 5.31 Optical spectrum at the output of the fiber with unequal channel

spacing

5.8.2 Effect of Increase Effective Area of the Fiber Optic

The effective area parameter of fiber optic has been changed from 64 μm2 to

76.5 μm2 at the power level set at 0 dBm. The results were taken at the end of the

fiber optic.

Simulation results at effective area of 76.5 μm2 at input power of 0 dBm are

shown in Figures 5.32.

Figure 5.32 Optical spectrum at the output of the fiber when the effective area of

the fiber optic is set at 76.5 μm2

69


the effective area is increased to 76.5μm2 s shown in Figure 5.32. It is found that the

increasing of the effective area can reduce the FWM effect.

70

CHAPTER 6

CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusion

Future wireless systems it will be targeting towards providing broadband

access and personal area multimedia services to large number of subscribers. Radio

over fiber (RoF) network accompanied with wavelength division multiplexing

(WDM) can provide a simple topology, easier network management, and an

increased capacity by allocating different wavelengths to individual remote nodes.

The performance of WDM networks is strongly influenced by nonlinearity

characteristic inside the fiber. Therefore the nonlinearity effects of fiber optics pose

additional limitation in WDM systems.

It is well known that FWM in WDM for RoF signals are mostly generated by

non-degenerate FWM process regardless of the number of input signals. In this study

only two and four input signals were launched into the optical fiber. The FWM effect

has been investigated analytically and numerically simulated. Simple equations to

determine the spectral linewidth, the FWM power due to channel spacing and the

power of the FWM components due to the input power have been deduced.

The numerical simulation results obtained have shown the spectral

characteristics of the FWM in WDM for RoF where the effects of FWM are

pronounced with decreased channel spacing of wavelengths or at high signal power

levels.

71

The numerical simulation model results and the analytical model results were

compared. The numerical simulated results clearly demonstrate that the degradation

due to FWM can be minimized by ensuring that the phase matching does not occur.

This has been achieved by increasing the channel separation and supplying low

signal power level. The high effective area is also found to the decrease FWM effect.

It is noticed that the FWM also causes inter-channel cross talk for equally spaced

WDM channels. Thus, FWM can be mitigated using unequal channel spacing.

It could be concluded that results obtained from this study will provide useful

information for identifying the fundamental limit of the capacity of the WDM

systems.

6.2 Recommendations for Future Work

FWM in WDM for RoF effects are likely to become the main source of

performance degradation in contemporary and future fiber optical communications,

therefore future studies in attempt to overcome such problems, the following could

be recommended.

Investigation of FWM effect using more than eight sources is essential

because most technologies nowadays use DWDM in order to meet the huge capacity

demands

Crosstalk is the transfer of power from one channel to another, can occurs

due to nonlinear effect. FWM can produce crosstalk between wavelength channels.

This crosstalk is strongly dependent on channel separation and optical power.

Therefore it is important to estimate how large the cross talk is.

72

7. References:

[1] Hamed Al-Raweshidy, “Radio over Fiber Technologies for Mobile

Communications Networks” , Artech House, 2002.

[2] Guo, Y., Kao, C. K. and Chiang, K. S. “Nonlinear photonics: nonlinearities

in optics, optoelectronics and fiber communications.” Springer-Verlag, 2002.

Berlin, Germany.

[3] Govind P.Agrawal, “Fiber-Optic communication system” McGraw-Hill

December 2001.

[4] Robert, C. E. “Optical Networking A beginer’s guide” McGraw-

Hill/Osborne 2004

[5] Yannis, L. G. “New optical Microwave Up-Conversion Solution in Radio

over Fiber Network” Journal of lightwave technology, 24 (3) (2006) 1277-

1282

[6] Antti Lamminpää, “Measurement of nonlinearity of optical fiber.” Master

thesis, Helsinki University of Technology, 2003.

[7] D. Marcuse, “Effect of Fiber Nonlinearity on Long-Distance Transmission”,

Journal of lightwave technology, 9 (1) (1991) 121-128.

[8] Remigius Zengerle, “Modeling of Nonlinear Phenomena in Optical Multi-

channel Trasmission system” Master thesis, University of Kaiserslautern,

2005.

[9] A. V. Yulin, D. V., and P. St. J. Russell “Four-wave mixing of linear waves

and solitons in fibers with higher-order dispersion” Journal of lightwave

technology, 29 (3) (2004) 950-955.

[10] Ajung Kim, Young Hun Joo, and Yungsoo Kim, “60GHz Wireless

Communication Systems with Radio-over-Fiber Links for Indoor Wireless

LANs” Journal of lightwave technology, 20, (4), (2004) 517-521.

[11] Anthong No’oma, “Radio over Fiber Technology for Broadband wireless

communication systems” Master thesis, Eindhoven University of Technology,

2005.

[12] Takuo Tanemura and Kazuro Kikuchi, “Unified analysis of modulational

instability induced by cross-phase modulation in optical fibers” Journal of

lightwave technology, 20 (12) (2003) 2502-2513.

73

[13] Caiqin Wu and Xiupu Zhang, “Impact of Nonlinear Distortion in Radio Over

Fiber Systems With Single-Sideband and Tandem Single-Sideband

Subcarrier Modulations ” Journal of lightwave technology, 24 (5) (2006)

2076 – 2090.

[14] M. J. Ablowitz and G. Biondini, “Four-wave mixing in wavelength– division

multiplexed soliton systems: ideal fibers”, Journal of Optical. Soc. Am. 14 (7)

(1997) 1788 – 1794.

[15] Hiroshi F., Koji Y., Tetsufumi S, and Sei-ichi Itabashi “Four-wave mixing in

silicon wire Waveguides ” Optics Express 13 (12) (2005) 4629-4637.

[16] J. Zhou, R. Hui, and N. Caponio, “Spectral Linewidth and Frequency Chirp

of Four-Wave Mixing Components in Optical Fibers”, Photonics Technology

Lelters, V 6, (3) (1994) 434-436.

[17] Y. S. Jang and Y. C. Chung, “Four-Wave Mixing of Incoherent Light in a

Dispersion-Shifted Fiber Using a Spectrum-Sliced Fiber Amplifier Light

Source” Photonics Technology Letters, 10 (2) (1998) 218-220.

[18] S. Betti, M. Giaconi, and M. Nardini, “Effect of Four-Wave Mixing on WDM

Optical Systems: A Statistical Analysis”, Photonics Technology Letters, 15

(8) (2003) 1079-1081.

[19] Vrizlynn L. L. Thing, P. Shum, and M. K. Rao, “Bandwidth-Efficient WDM

Channel Allocation for Four-Wave Mixing-Effect Minimization”,

Transactions On Communications, 52 (12) (2004) 2184-2089.

[20] Ivan Kaminow, and Tingye Li “Fiber Optic Telecommunication ”,

Academic Press 2002.

[21] M. L. F. Abbade, and E. A. M. Fagotto, R. S, “Optical Amplitude

Multiplexing Through Four-Wave Mixing in Optical Fibers” Photonics

Technology Letters, 17 (1) (2005) 151-153.

[22] Ooi Sock, “Four Wave Mixing Nonlinearity Effect in Wavelength Division

Multiplexing System for Radio Over Fiber.” Bachelor thesis, University

Technology Malaysia, 2007.

[23] A. V. Ramprasad, and M. Meenakshi, “Four Wave Mixing on Dense

wavelength division Multiplexing Optical system” Academic Open Internet

Journal, 17 (1) (2006) 1-8.

74

APPENDIX A

MATLAB PROGRAM FOR FWM POWER

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% this program is used to compute power per channel versus FWM power

% and to compute channe spaing versus FWM channel

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% variables

%

% X = third order susceptibility

% lemda = wavelength in vacuum

% c = speed og light in vacuum

% Aeff= effecive area of the optical fiber

% n= nonlinear reflective inex

% alfa = fiber loss

% D= degeneracy factor

% eff = FWM efficiency

% Leff = effctive length

%

x=6*10^-15;

lemda=0.5*10^-6;

c=3*10^8;

Aeff=6.4*10^-11;

n=1.48;

75

alfa=.0461;

eff=.05;

Leff=22*10^3;

D=3;

k=(32*(pi)^3*x)./(n^2.*lemda*c)*(Leff/Aeff)

P=eff*(D.*k).^2*(1*10 -̂3)^3*exp(-alfa*75)

Pdb= 10*log10(P/10 -̂3)

plot(x,y)

hold on

y1 = [-59.5 -61.2 -65.5 -68 -72.5 -80];

plot(x,y1,'r')

opti system project

Documents

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optical networks

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support andpatience

fiber rof systems

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