PERFORMANCE OF MULTICHANNEL FIBER OPTIC PARAMETRIC

APMLIFIER

NOR ‘IZZATI BINTI MOHD NOOR

A thesis submitted in partial

fulfillment of the requirement for the award of the

Degree of Master of Electrical Engineering

Faculty of Electrical and Electronic Engineering

Universiti Tun Hussein Onn Malaysia

JANUARY, 2015

v

ABSTRACT

Optical networks have a significant role to play in the present and future global

telecommunication networking scenario due to the increasing demand for larger

transmission capacity. In fiber optic communication systems, Dense Wavelength

Division Multiplexing (DWDM) is very popular in which multiple optical signals at

various wavelengths are combined and transmitted through a single fiber. DWDM

technology provides a cost effective deployment strategy. One of the key components in

DWDM system is an optical amplifier. Fiber optical parametric amplifier (FOPA) can

be used for several signal processing application including optical amplification, phase

conjugate and wavelength conversion. FOPA operate based on a fiber nonlinearity

known as four wave mixing (FWM). Fiber optical parametric amplifiers are based on

the third-order susceptibility of the glasses making up the fiber core. It happens when at

least two waves with the different frequencies co-propagate in the fiber. In this

simulation is to show the ability of a single pump parametric amplifier in the eight

channels DWDM transmission system and performance of FOPA in order to ensure

higher level of amplification coped with less amplifier produced signal impairments.

The simulation were done by software OptiSystem 13, the fiber optical amplifier is

perform by simulation of 10 Gbit/s each channel. Furthermore NRZ encoding

technique, intensity OOK modulation format has been used in this simulation. The

frequencies of channel carrier was chooses in the region from 193.1 THz to 193.8 THz.

Eight modulated signal are transmitter over 220 km span long single mode fiber. The

single pump combination with four signal radio frequency, 180 MHz, 420 MHz, 1.087

GHz and 2.133 GHz are used to show higher level of amplification and mitigating the

impact of simulated Brillouin scattering. As a result, the maximum 22.134 dB gain and

lower noise figure 2.84 dB is achieved.

vi

ABSTRAK

Rangkaian optik mempunyai peranan penting dalam senario rangkaian telekomunikasi

global pada masa kini dan juga pada masa hadapan berikutan permintaan yang semakin

meningkat untuk kapasiti penghantaran yang lebih besar. Dalam sistem komunikasi

gentian optik, Dense Wavelength Division Multiplexing (DWDM) adalah sangat

popular di mana pelbagai isyarat optik dalam berbagai panjang gelombang digabungkan

dan dihantar melalui gentian tunggal. Teknologi DWDM menyediakan strategi

perlaksaan yang lebih effektif. Salah satu komponen utama dalam teknologi DWDM

ialah penguat optik. Gentian parametrik penguat optik (FOPA) boleh digunakan dalam

beberapa aplikasi isyarat pemprosesan termasuk penguatan optik, fasa konjugate dan

penukaran panjang gelombang. FOPA beroperasi berdasarkan gentian tidak linear yang

dikenali sebagai Four-wave Mixing, (FWM). Serat penguat parametrik optik adalah

berdasarkan kecenderungan ketiga-susunan kaca yang membentuk teras gentian. Ia

berlaku apabila sekurang-kurangnya dua gelombang dengan frekuensi yang berbeza

tersebar bersama di dalam gentian. Dalam proses simulasi ini menunjukkan keupayaan

penguat pam parametrik tunggal dalam sistem penghantaran DWDM lapan saluran dan

prestasi FOPA untuk memastikan tahap penguatan yang lebih tinggi diatasi dengan

kurangnya penguat yang menghasilkan kecacatan pada isyarat. Proses simulasi ini

dilakukan menggunakan perisisn OptiSystem 13, gentian penguat optik menghantar 10

Gbit/s setiap channel dianalisis. Tambahan pula, teknik NRZ dan format modulasi

intesiti OOK telah digunakan dalam simulasi ini. Spectrum 193.1 THz hingga 193.8

THz telah dipilih untuk digunakan dalam simulasi ini. Lapan isyarat yang dimodulasi

dihantar pada jarak 220 km melalui gentian tunggal. Kombinasi pump tunggal dengan

bantuan empat isyarat frekuensi radio iaitu 180 MHz, 420 MHz, 1.087 GHz dan 2.133

GHz telah menunjukkan peningkat penguatan dan mengurangkan kesan stimulated

vii

Brillouin scattering. Keputusan menunjukkan maksimum gain iaitu 22.134 dB dan

noise figure yang rendah dapat dicapai.

viii

CONTENTS

TITLE i

DECLAIRATION ii

DEDICATION iii

ACKNOWLEDGMENT iv

ABSTRACT v

ABSTRAK vi

CONTENTS viii

LIST OF FIGURES xi

LIST OF TABLE xiv

LIST OF ABBREVIATIONS xv

CHAPTER 1 INTRODUCTION

1.1 Background of study 1

1.2 Problem Statement 3

1.3 Objective 4

1.4 Scope of project 4

CHAPTER 2 LITERATURE REVIEW

2.1 Theoretical Background 5

2.2 Optical Communication System 5

2.3 Fiber Optic ParametricbAmplifier Theory 6

2.4 Fiber Nonlinearities 7

2.5 Nonlinear Refrective Index Effect 9

2.5.1 Four Wave Mixing 9

ix

2.5.2 Self Phase Modulation 11

2.5.3 Cross Phase Modulation 12

2.6 Inelastic Scattering Effect 13

2.6.1 Stimulated Raman Scattering 13

2.6.2 Stimulated Brillioun Scattering 14

2.5.2.1 SBS Mitigation Techniques 16

2.7 DWDM System Function 16

2.8 Highly Nonlinear Fiber (HNLF) 17

2.9 Modulation Format 19

2.10 Channel Spacing 20

2.11 Q-factor 22

2.12 Previous Work 22

2.12.1 Fiber Optical Parametric Amplifier

Performance in a 1-Tb/s DWDM

Communication System 22

2.12.2 BER Performance of a Lumped Single-pump

Fiber Optical Parametric Amplifier in a

10 Gbit/s 4-channel S-band DWDM System 22

2.13 Summary 23

CHAPTER 3 METHODOLOGY

3.1 Methodology 24

3.2 Stages 24

3.3 Design Methodology 25

3.3.1 Transmitter 26

3.3.2 Pseudo Random Bit Generator 27

3.3.3 Continuous Wave (CW) Laser 27

3.3.4 Non Return-to-Zero (NRZ) Pulse Generator 28

3.3.5 Mach Zehnder Modulation (MZM) 30

3.3.6 Photodetector PIN 30

3.3.7 Optical Bessel Filter 31

x

3.3.8 Optical Fiber 32

3.4 Fiber Optical Parametric Amplifiers 33

3.5 Parametric Amplification 34

3.6 Bit Error Rate 35

3.7 Eye Pattern 35

3.8 Simulation setup 36

CHAPTER 4 RESULT AND ANALYSIS

4.1 Introduction 41

4.2 Quality factor (Q-factor) and minimum bit error

rate (min BER) of without amplification and with

amplification 41

4.3 Spectrum of eight channels 47

4.4 Optimization pumping power 48

4.5 Gains in FOPA 50

4.6 Noise figure (dB) in FOPA 52

4.7 Data pump regenerated signal 53

4.8 Different input power signal 55

4.9 Different length of highly nonlinear fiber (HNLF) 56

CHAPTER 5 CONCLUSION

5.0 Conclusion 57

REFERENCES 59

xi

LIST OF FIGURES

2.1 Schematic basic of optical transmission 6

2.2 Amplifier and idler signal after HNLF 7

2.3 Linear and nonlinear interaction 8

2.4 Nonlinear effect in optical fiber 8

2.5 Four wave mixing in a waveguide to generate a new

colour of light 9

2.6 Nonlinear process, transfer energy of pump to signal

and idler waves 11

2.7 Effect of self-phase modulation on a transform

limited pulse 11

2.8 Shown is the principle of cross-phase-modulation

wavelength conversion. 13

2.9.1 Spontaneous Brilliouin scattering 15

2.9.2 Stimulated Brillioun scattering phenomenon 15

2.10 DWDM function schematic 17

2.11 Set-up of DWDM system with recommended points

for reference measurements, OFA: Optical fiber

amplifier, E/O: Transmitter, O/E: Receiver 18

2.12 Here a ‘0’ is represented by having the carrier ‘off’

(reducing its amplitude to zero), and a ‘1’ by having

the carrier ‘on’ (giving it a chosen amplitude) 19

2.12 Typical Optical Characteristics for DWDM Channels 20

2.13 Channel spacing 0.8 nm 21

2.14 0.4 nm Channel Spacing DWDM Fiber Bragg Grating 21

3.1 DWDM system 25

xii

3.2.1 Component of optical transmitter 26

3.2.2 Transmitter simulation in OptiSystem 13 software 26

3.3.1 Non Return-to-Zero signal 28

3.3.2 NRZ block diagram 28

3.4 Compare of digital bit stream coded by using (a) return

to zero (RZ) and (b) nonreturn-to-zero (NRZ) formats 29

3.5 Mach-Zehnder Modulator 30

3.6 Bessel filter 32

3.7 Bessel Optical Filter component in OptiSystem 13

Software 24

3.8 Single Mode Fiber 33

3.9 Single pump FOPA configuration 35

3.10 Different for eye height and amplitude comes from

histogram data of the one and zero level 36

3.11 Simulation setup of multi-wavelength FOPA 37

3.12 The spectrum of amplify DWDM signal: a) before

HNLF and b) after HNLF 39

3.13 Position of signal and idler (wavelength, nm) 40

3.14 The signal that shown by BER analyzer will measure

the value of Q Factor and min bit error rate (BER) 40

4.1a and b 42

4.1c and d 43

4.1e and f 43

4.1g and h 44

4.1i and j 44

4.1k and l 45

4.1m and n 45

4.1o and p 46

4.2 Power spectrum used as input for eight channels

10 Gb/s simulation 47

4.3 Pump power in dBm versus min BER 48

4.4 Gain (dB) at eight wavelengths (nm) 51

4.5 Noise figure versus wavelength (nm) 52

xiii

4.6.1 FWM principle using data pump 53

4.6.2 After regenerated 54

4.7 Gain at different input signal power 55

4.8 Q-Factor versus wavelength (nm) 56

xiv

LIST OF TABLES

1 Parameter of HNLF 23

2 HNLF parameters 38

3 Value of pump power and min BER 49

4 Value of wavelength and gain 51

5 Value of wavelength and noise figure 52

xv

LIST OF ABBREVIATIONS

EDFA - Erbium Doped Fiber Amplifier

WDM - Wavelength Division Multiplexer

DWDM - Dense wavelength division multiplexing

C-band - Convensional Band

L-band - Long Length Wideband

OSA - Optical Isolator Analyzer

ASE - Amplifier Spontaneous Emission

SNR - Signal Noise Ratio

BER - Bit Error Rate

CW - Continuous Wave

DCF - Dispersion compensating fiber

DeMux - Demultiplexer

DSF - Dispersion shifted fiber

FOPA - Fiber optical parametric amplifier

FWM - Four Wave Mixing

HNLF - Highly Nonlinear Fiber

MZM - Mach Zehnder Modulator

NF - Noise Figure

NRZ - Nonreturn-to-Zero

O/E/O - Optical electrical optical

OOK - On-off keying

OF - Optical Fiber

PMD - Polarization mode dispersion

xvi

PRBS - Pseudo random binary sequence

RZ - Return-to-zero

SBS - Stimulated Brillouin scattering

SMF - Single mode Fiber

SPM - Self-phase modulation

SRS - Stimulated Raman scattering

SSMF - Standard Single Mode Fiber

XPM - Cross-phase Modulation

ZDW - Zero-dispersion wavelength

1

CHAPTER 1

Introduction

1.1 Background of study

Optical fiber communication systems have attracted remarkable attention in the past

two decades, as they are up to now the most promosing technology in response to the

exponentially growing demand for higher transmission capacity. This is due to the

technique feature of optical fibers, which is their relatively low attenuation over a wide

frequency range in third telecommunication window around 1550 nm. This

telecommunication window offers a bandwidth of a few tens of THz. However, most of

the installed fiber optic communication systems are only using a fraction of this huge

available bandwidth.

The technique of multiplex several signals at different wavelengths and transmit

them in a single fiber, which is called wavelength division multiplexing (WDM) can

used to fully utilize the available bandwidth of optical fibers. [1]. By increasing the

number of wavelengths, the transmission capacity will be enhanced using this

technology. WDM became possible only with the emergence of the erbium-doped fiber

amplifier (EDFA) [2], which provided amplification of multiple wavelengths

simultaneously due to its relatively large bandwidth and low crosstalk [2]. The EDFA

uses fiber doped with erbium as a gain medium to amplify an optical signal.

Amplification is a result of stimulated emission of photons from Erbium ions in the

2

fiber. The dopant ions are pumped (usually at 980 nm or 1480 nm) to a higher energy

state from where they can release their energy by stimulated emission of a photon which

has the same wavelength as the input signal. This release energy will bring the excited

ions back to the lower energy level and meanwhile will amplify the input optical signal.

Another, the fiber Raman amplifier (FRA) also one of optical amplifiers based

on fiber nonlinearities. FRA can utilizes a nonlinear phenomenon in fibers known as the

stimulated Raman scattering (SRS) for amplification [3,4]. In SRS the incident pump

photon gives up its energy to create another photon with a lower energy at a lower

frequency, while the remaining energy is absorbed by the medium in the form of

molecular vibration. When the pump and signal beams are injected into the fiber,

energy is transferred from the pump to signal through SRS, which gives rise to Raman

gain. The Raman gain can be distributed in the transmission fiber which usually reduces

the noise.

Fiber optical parametric amplifier (FOPA) operated based on another fiber

nonlinearity known as four wave mixing (FWM). These amplifiers used as another

possible solution on fiber nonlinearities. Lately these amplifiers are investigated by

many researches. FWM arises from the third order nonlinearity in a fiber and occurs

when at least two waves with different frequencies co-propagate in the fiber. A

refractive index modulation occurs at the difference frequency of the input waves,

which creates an additional frequency component. FOPA can operate at any arbitrary

wavelength. In the meantime, the FOPA gain is approximately twice as high as that of

the FRA of the same parameter [5]. The higher gain makes FOPA more suitable for

lumped amplification and all-optical signal processing applications. An important, the

generation of an idler in the parametric process can be used for wavelength conversion

[6]. FOPA are uni-directional amplifiers which makes them less sensitive to saturation

effects arising from the generated internal amplified spontaneous emission (ASE) [7].

Furthermore, FOPA are used for amplification of WDM communication signals

is that nonlinearity of the amplifying medium may lead to detrimental inter-channel

crosstalk [8], which arises from spurious FWM and cross-gain modulation (XGM) due

to pump depletion [9]. FOPA have characteristic which make them have the potential to

be used for a variety of applications [5]. In additional to being an amplifier, due to the

3

generation of a new wave, FOPA can simultaneously be used as a wavelength converter

[10,11] which is required for wavelength routing in optical works. The intrinsic

spectrum inversion of idlers in FOPA can be used for dispersion compensation in long

haul transmission system. Due to the requirement of higher network capacity, the data

bit rate and channel number of WDM system have been rapidly increasing.

The performance of Fiber Optical Parametric Amplification (FOPA) today has

been significantly improved by the advances in both highly nonlinear fiber (HNLF),

which acts as the gain medium, as well as improved pump sources offering high

coherence and high power. FOPA are an innovative type of amplifiers with many

promising amplification [1]. In FOPA there is several factor effected for their

performances, which is polarization dependence of the gain, stimulated Brillion

scattering, transferred from the pump to the amplified optical signal and related

intensity noise (RIN) [2]. Fiber optic parametric amplifiers (FOPA), based on four-

wave mixing (FWM) occurring inside optical fibers, are attracting considerable

attention because they can provide signal for the entire wavelength band used in optical

communication system and generation of phase conjugated idler harmonics that can be

used for wavelength conversion [3]. There has several key of components in FOPA, it is

high power, single wavelength pump laser and a specifically tailored highly nonlinear

and low dispersion optical fiber [4]. The maximum gain can be achieved by using

single-pumped in FOPA. Furthermore, for low-penalty amplification improvement of

DWDM signal by means of FOPA is achieved by reducing signal-signal FWM crosstalk

using short highly nonlinear fiber (HNLF) in conjunction with a high power pump.

1.2 Problem statement

DWDM system required rapidly demand in internet traffic due to emerging

multimedia application. In order to fulfill such required system, FOPA can be useful for

communication system because it is offering prospect for amplification over large

bandwidth outside the EDFA band. However the nonlinearity of the amplifying medium

and good phase matching are necessary for parametric amplification which is occur

deleterious nonlinear crosstalk in DWDM system. Since of these issues, the crosstalk

4

can be reduced by using highly uniform fibers, low signal power, polarization

multiplexing and one pump FOPA. By using single pump of FOPA can provide higher

gain and also can reduces crosstalk.

1.3 Objectives

Most of this project is focused on the fiber optical parametric amplifier (FOPA)

in a DWDM transmission. An objective of this project is

i) To design and simulate the setup of FOPA.

ii) To exhibit the performance with higher gain and lower noise figure in

FOPA.

iii) To evaluate the length of HNLF, pump power and input power signal by

single pump of FOPA.

1.4 Scope of project

i) Using non return zero encoding technique with phase modulation format

ii) Optimization the pump power to get highest gain and better bit error rate

BER to get better quality in data transmission.

iii) Channel spacing is set at 100 GHz to separates eight signals channel

multiplex.

5

CHAPTER 2

Literature Review

2.1 Theoretical Background

This part will describe about theoretical background of fiber optical parametric

amplifier performance by reviewing the previous project related to this research.

2.2 Optical communication system

In optical communication system, the data is transported into optical fiber by light

signals that travel through suitable channel that used. This provides transmission of data

over long distances with higher speed and bandwidth as compared to the other media

used for telecommunications. A electrical signal is modulated onto a carrier light such

as the light from continuous wave laser using opto-electronic modulator (Mach Zehnder

modulator). The light wave carrying signal then travels in the optical fiber and then

received at the other end by a light wave receiver such as photo detector which then

converts the light back to the electrical signal [12]. In order to utilize the capacity of an

optical fiber to the maximum extent, several electrical channels are multiplexed onto a

signal fiber channel using either wavelength division multiplexing (WDM) or optical

time division multiplexing (OTDM).

6

Figure 2.1: Schematic basic of optical transmission

Over long spans of optical fiber channel, the optical signal attenuates due to

dispersion, bending losses, fiber imperfection and polarization mode. Amplifier is used

to regenerate the signal after a certain distance [13]. Figure 2.1 depicts the schematic of

the basic optical communication system.

2.3 Fiber Optic Parametric Amplifier Theory

Optical Parametric Amplifier relay on the third order nonlinearity of the fiber material

and it can in principle be operated at an arbitrary center wavelength, corresponding to

the zero-dispersion wavelength of the fiber. Their bandwidth depends on pump power,

fiber nonlinearity and fiber dispersion. The presence of a frequency-shifted idler

indicates that such devices can also be used as broadband wavelength converters,

possibly exhibiting high conversion efficiency. The effect of dispersion fluctuation on

noise properties of FOPA's with use of highly non-linear dispersion shift fiber

(HNLDSF), it was found that zero dispersion wavelengths (ZDWL) variation became

very significant with high gain of more than 25 dB [14]. When the input signal power

equals a power called the saturation power, the parametric gain will amplify the signal

and idler waves equally. The idler wave is produced in all parametric amplifiers (PA)

with a frequency less than the pump by an amount equals exactly the frequency shift

between the signal and that of the pump as shown in Figure 2.2

Laser

Transmitter Receiver

Electric

signal

input

Electric

signal

output

Optical data signal

Optical

fiber

7

Figure 2.2: Amplifier and idler signal after HNLF

2.4 Fiber Nonlinearities

The response of any dielectric (silica) to light becomes nonlinear for intense

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

origin of nonlinear response is related to a harmonic motion of bound electrons under

the influence of an applied field. When an electric field is applied to a dielectric

material, which contains negative (electrons) and positive (nuclei) charges, it drives

these charges and polarizes atoms in the dielectric material. For intense electric field

(high power) applied, a harmonic motion of the electrons is observable. As a result the

total polarization P induced by electric dipoles is not linear in the electric field E, but

satisfies the more general relation in a Taylor expansion as [15].

(2.0)

Where εₒ is the vacuum permittivity and χ(j)

is jth order susceptibility. In general, χ(j)

is a

tensor of rank j+1. The linear susceptibility χ

(1) represent dominant contribution to P.

the second order susceptibility χ(2)

is responsible for such nonlinear effects as second-

harmonic generation and sum frequency generation. However, it is nonzero only for

media that lack of inversion symmetry at the molecular level. As SiO2 is a symmetric

molecule, χ(2)

vanish for silica glasses. As a result, optical fiber do not normally exhibit

second order nonlinear effects. Nonetheless, the electrical-quadrupole and magnetic-

dipole moments can generate weak second order nonlinear effects.

P = Ɛ0χ(1)

. E + Ɛ0χ(2)

: EE + Ɛ0χ(3)

⁞EEE + … ,

8

Figure 2.3 : Linear and nonlinear interaction

Figure 2.4: Nonlinear effect in optical fiber

Linear

Nonlinear

Input

Output

Nonlinear Effects in Optical Fibers

Nonlinear Refractive

Index Effects

Inelastic Scattering

Effects

SPM CPM FW

M

SRS SBS

9

2.5 Nonlinear Refractive Index Effect

2.5.1 Four Wave Mixing

Figure 2.5: Four wave mixing in a waveguide to generate a new colour of light

Four-wave mixing (FWM) also called four photon mixing is one of the major in

limiting factor in WDM optical fiber communication system that use the low dispersion

fiber or narrow channel spacing. FWM is very harmful for a multichannel fibers optics

transmission system such as wavelength division multiplexing (WDM). Normally,

multiple optical channels passing through the same fiber interact with each other very

weakly. However, these weak of interaction in glass become significant over long fiber-

transmission distance. The most important is FWM in which three wavelength interact

to generate fourth [16].

The term of four-wave mixing is usually reserved for the interaction of four

spatially or spectrally distinct fields. FWM reduces to the previously discussed

processes when two or more of the frequencies are degenerate. FWM may be used to

probe either one photon resonances or two photon resonances in a material by

measuring the resonant enhancement as one or more of the frequencies are tuned. By

ω idler

ω signal

ω pump

ω pump

ω idler

ω signal

ω pump

ω pump

10

tuning the frequencies to multiple resonances in the material, excited state cross

sections, lifetimes, and linewidth may be measured. [17]

The process of FWM is when two intense pump waves at frequencies ω1 and ω2

co-propagate inside a silica fiber, electrons, which only have a tiny mass, can be driven

almost instantaneously at any frequency stemming from the mixing of these waves.

Even though the potential provided by silica molecules confines electrons top their

original atom, electrons respond to the applied electromagnetic field by emitting

secondary waves not only at the original frequencies ω1 and ω2 (linear response), but

also at two new frequency components denoted as ω3 and ω4 (third order nonlinear

response). Physically, two photons at the original frequencies are scattered elastically

into two new photons at frequencies ω3 and ω4. In the absence of absorption, the total

energy and momentum is conserved during FWM.

In practice, the efficiency of the FWM process is enhanced by seeding it.

Seeding is accomplished by lunching a signal wave at the frequency ω3. The probability

of creating photons at the frequency ω4 depends on how many photons at ω3 already

exist inside the fiber. As a result, the FWM process is stimulated and new photons at ω3

and ω4 are created with an exponential growth rate provided the phase matching

condition is nearly satisfied. The fourth wave at the frequency ω4 is commonly referred

as the idler wave following the terminology used in the microwave literature [18].

The process of four-wave mixing is when photon at frequencies ω1 and ω2 are

annihilated and create two photons at frequencies ω3 and ω4 such that

(2.1)

The FWM process can be described by Eq.(2.1) relies on a phase matching

condition given by

(2.2)

Condition (2.2) is easily satisfied in optical fibers for the partially degenerate

case when ω1 = ω2. The FWM effect is used in many important fiber applications,

∆𝜅 = 𝜅3 + 𝜅4 − 𝜅1 − 𝜅2 = 0

ω3 + ω4 = ω1 + ω2

11

including wavelength conversion, parametric amplification and optical regeneration,

among others.

Figure 2.6: Nonlinear process, transfer energy of pump to signal and idler

waves.

2.5.2 Self-Phase Modulation (SPM)

Self-phase modulation (SPM) refers to the phenomenon in which the laser beam

propagating in medium interacts with the medium and imposes a phase modulation on

itself.

Figure 2.7: Effect of self-phase modulation on a transform limited pulse

The refractive index of a material is not only dependent on the wavelength of

the light traveling through it but also dependent on the intensity of the incident light.

SPM

Transform-limited pulse Spectrally broadened

pulse with negative chirp

12

For a very intense optical pulse, the fiber refractive index changes with the relation

[Agrawal 2001]:

(2.3)

Where n0 is the refractive index of the material, n2 is the Kerr nonlinear

refractive index and I is the intensity of the pulse. An optical pulse will experience

phase retardation, which is proportional to the intensity-induced changes in fiber

refractive index [Agrawal, 2002], so that the phase delay (negative phase) is maximum

at the time of peak intensity and decreases in magnitude towards both ends of the pulse.

A temporally varying phase delay implies that the instantaneous optical

frequency differ across the pulse from its center frequency, the self-frequency change

will be equivalent to the time derivative of the phase delay. Since the phase delay is

directly proportional to the pulse intensity, the self-frequency change is also

proportional to the derivative of the pulse intensity with respect to time. The variation

of the frequency is negative at the leading edge and becomes positive near the trailing

edge of the pulse. The variation of the pulse frequency can be treated as a frequency

chirp, which is due to the modulation of the pulses own phase as a result of Kerr

nonlinearity, known as SPM, which leads to spectral broadening. A more commonly

used measure of the nonlinearity of a fiber is its effective nonlinearity, γ = 2πn2 / (λAeff),

where λ is the optical wavelength and Aeff is the effective mode area of the fiber

[Agrawal, 2001]. For example, standard SMF-28 fiber has an Aeff of ~60 µm2 at 1550

nm and γ is around the range of 1.5 – 2.3 W-1

km-1

.

2.5.3 Cross Phase Modulation (XPM)

When at least two optical fields with different frequencies propagate inside a fiber

concurrently, they will not only experience an SPM induced Phase shift but will also

induce a phase shift on each other. This effect is known as cross phase modulation

(XPM). The reason behind XPM is that each optical wave will change the fiber

n(I) = n0 + n2I

13

refractive index seen by each optical wave not only depends on its own intensity but

also on the intensity of other co-propagating beams.

Figure 2.8 shows the principle, a continuous-wave probe experiences cross-

phase modulation from co-propagating signal pump pulses, which spectrally broadens

the probe. A bandpass filter subsequently selects a particular wavelength of the

broadened probe, and the information that was present on the signal is now present on

the new wavelength, λ2 + Δ in Figure 2.8.

Figure 2.8: Shown is the principle of cross-phase-modulation wavelength

conversion [http://www.photonics.com/Article.aspx?AID=30707]

2.6 Inelastic Scattering Effect

There are two nonlinear scattering phenomena in fiber and both are related to

vibrational excitation modes of silica. These phenomenon are known as stimulated

Raman scattering (SRS) and stimulated Brillouin scattering (SBS). The basic difference

is that, the optical phonons participate in SRS while SBS acoustic phonons. The

threshold power level for SBS is lower than the one for SRS, in the case of SBS the

stoke wave usually propagates in backward direction only while SRS can occurs in both

direction.

2.6.1 Stimulated Raman scattering (SRS)

Stimulated Raman scattering is a process in which the incident photon is scattered by a

molecule to a lower-frequency photon and is accompanied by a transition of the

14

molecule between two vibrational states. The SRS can strongly affect the performance

of multichannel transmission system by transferring the energy between channels. At

the same time, the SRS forms a basis of broadband Raman amplifiers and tunable

Raman lasers that are of foremost importance for energy restoration in optical

transmission systems and have numerous other applications as well. [19]

2.6.2 Stimulated Brillioun Scattering (SBS)

Stimulated Brillouin Scattering (SBS) is a strongest nonlinear effect in fiber optics and

is the most significant obstacle to the development of many practical FOPA system.

SBS is a nonlinear process that can occur in optical fibers at large intensity. The large

intensity produces compression (due to electric field also known as pump field) in core

of fiber through the process known as electrostriction. [20]

For an oscillating electrical field at the pump frequency ωP, the electrostriction

process generates a microscopic acoustic wave (involved phonons are coherent) at some

frequency ωB. The Brillouin scattering may be spontaneous or stimulated (figure 2.9.1

and figure 2.9.2). In spontaneous Brillouin scattering, there is annihilation of a pump

photon, which results in creation of Stokes photon and an acoustic phonon

simultaneously. The conversation laws for energy and momentum must be followed in

such scattering processes. For energy conservation, the Stokes shift ωB must be equal to

(ωP – ωS), where ωP and ωS are frequencies of pump and stokes waves. The momentum

conservation requires ƙA = (ƙP - kS), where ƙA, ƙP and ƙS are momentum vectors of

acoustic, pump and Stokes wave respectively. The phase matching conditions [21]

(2.4)

ωP – ωS = ωS ; ƙP - kS = ƙA

15

Figure 2.9.1: Spontaneous Brilliouin scattering

Figure 2.9.2: Stimulated Brillioun scattering phenomenon

When scattered wave is produced spontaneously, it is interferes with the pump

beam. This interference generates spatial modulation in intensity, which result in

amplification of acoustic wave by the electrostriction effect. The amplified acoustic

wave in turn raises the spatial modulation of intensity and hence the amplitude of

scattered wave. Again there is increment in amplitude of acoustic wave. This positive

feedback dynamics is responsible for the stimulated Brillouin scattering, which

ultimately, can transfer all power from the pump to the scattered wave. [22]

16

2.6.2.1 SBS Mitigation Techniques

In FOPA system under the condition of extremely intense optical pumps, the SBS

process has been seen to reflect the majority of the incident pump power. With

increasing demand for power delivered into fiber, a number of SBS suppression

techniques have arisen. These techniques seek to increase the SBS threshold by

manipulating either the Brillouin bandwidth and thus the peak gain or the incident laser

bandwidth in order to minimize the spectral overlap between the two, to limit the gain

experienced by the back reflection [23].

Currently, there are active SBS suppression methods currently under study and

further presented in this work which seek to instead broaden the linewidth of the

incident light to reduce the amount of power experiencing gain in the generally narrow

SBS bandwidth. This can be done using either directly by frequency modulation at the

source or by phase modulation without introducing significant intensity modulation on

the laser concern [24]. In either case the laser is dithered in either phase or frequency by

a controlling electrical waveform. Typical waveform are used for dithering include

sinusoids, combinations of multiple sinusoid and pseudorandom bit sequences.

Linewidth broadening using phase modulation has been shown to be an extremely

effective SBS suppression method with threshold increase using four combination tones

up 1 GHz. Currently the active methods of SBS suppression are mode widely used due

to the performance advantage, the lack of effects on the fiber dispersive properties as

well as ease of implementation.

2.7 DWDM System Function

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

technology which multiplexes multiple optical carrier signals on a single optical fiber

by using different wavelengths of laser light to carry different signals. Figure 2.10 [25]:

17

Figure 2.10: DWDM function schematic [25].

A DWDM system performs the generating the signal where the source, a solid-

state laser, must provide stable light within a specific, narrow bandwidth that carries

digital data modulated as an analog signal. The modern DWDM systems employ

multiplexers to combine the signals. There is some inherent loss associated with

multiplexing and de-multiplexing. This loss is dependent on the number of channels but

can be mitigated with optical amplifiers, which boost all the wavelengths at once

without electrical conversion.

Moreover, when transmitting the signals, the effects of crosstalk and optical

signal degradation or loss must be considered in fiber-optic transmission. Controlling

variables such as channel spacing, wavelength tolerance, and laser power levels can

minimize these effects [25]. The signal might need to be optically amplified over a

transmission link. Separating the received signals used when the receiving end, the

multiplexed signals must be separated out. Next, the de-multiplexed signal is received

by a photodetector. In addition to these functions, a DWDM system must also be

equipped with client-side interfaces to receive the input signal. The client-side interface

function can be performed by transponders [26]. Interfaces on the DWDM side connect

the optical fiber to DWDM systems. In classical WDM system the transmission

capacity mostly only double, in DWDM system it increase for a factor n=4, 8, 16, 32,

64 or 128 depending on the configuration.

18

Figure 2.11: Set-up of DWDM system with recommended points for reference

measurements, OFA: Optical fiber amplifier, E/O: Transmitter, O/E: Receiver [Basic

Note DWDM system, Profile Optische System GmbH Gauss Str11 D-8575

Karlsfeld/Germany, copyright@

2000,Profile GmbH]

In a DWDM system the light of laser diode with wavelengths recommended by

the ITU is launched into the inputs of a wavelength multiplexer (MUX). At the output

of the wavelength multiplexer all wavelengths are then combined and coupled into a

single mode fiber (Figure 2.11). At the end of transmission link the optical channels are

separated again by means of a wavelength de-multiplexer (DMUX) and thus get to the

different output. In long transmission links it is necessary that the DWDM signals are

optically amplified by an optical fiber amplifier (OFA).

2.8 Highly Nonlinear Fiber, HNLF

HNLF allows only one mode of light at a time through the core and because of that, this

fiber have been widely used as platforms of all-optical signal processing and designed

to enhanced nonlinear effect that cause the spectrum to broaden while light is guided.

Nonlinear process depend on the intensity of the field hence, it is possible to enhance

their efficiency if the field is localized in a very small area. Due to the effectiveness of

nonlinear processes in these fibers, they offer the possibility of designing compact

devices that rely on nonlinear effects. The high nonlinearities are mainly caused by very

small effective areas [27]. The small area has the effect of increasing the energy density

19

to a point where the refractive index changes depending on the intensity of the light in

fiber. Hence, contrary to semiconductor optical amplifier (SOAs), no resonant physical

effect is exploited and the speed of signal processing in these devices is extremely high.

2.9 Modulation Format

Figure 2.12: Here a ‘0’ is represented by having the carrier ‘off’ (reducing its amplitude

to zero), and a ‘1’ by having the carrier ‘on’ (giving it a chosen amplitude)

[https://www.st-andrews.ac.uk]

In the case of analog modulation, the three modulation choices are known as amplitude

modulation (AM), frequency modulation (FM) and phase modulation (PM). The same

modulation techniques can be applied in the digital case and are called amplitude-shift

keying (ASK), frequency-shift keying (FSK) and phase-shift keying (PSK), depending

on whether the amplitude, frequency or phase of the carrier wave is shifted between the

two levels of a binary digital signal. The simplest technique consists of simply changing

the signal power between two levels, one of which is set to zero, and often called on-off

keying (OOK) to reflect the on-off nature of the resulting optical signal. In its simplest

form, the presence of a carrier for a specific duration represents a binary one, while its

absence for the same duration represents a binary zero. Some more sophisticated

20

schemes vary these durations to convey additional information. Another, RF carrier

waves, OOK is also used in optical communication system. Until recently, OOK was

the format of choice for most digital lightwave system [28].

2.10 Channel Spacing

Channel spacing can be determines by the performance of DWDM systems. Standard of

ITU channel spacing that often and reliable used is about 50 GHz to 100 GHz. Spacing

makes the channel can be used with the limitations of fiber amplifier. It is depends on

the system components that used on design. Channel spacing is the minimum frequency

system that separates two signals multiplex. An inverse proportion of frequency versus

wavelength of operation calls for different wavelengths to be introduced at each signal

[29]. Optical amplifiers and receiver are able to distinguish signal by determines the

spacing of two wave adjacent.

Figure 2.12: Typical Optical Characteristics for DWDM Channels [29].

WDM wavelengths are positioned in a grid having exactly 100 GHz (about 0.8

nm) spacing in optical frequency, with a reference frequency fixed at 193.10 THz

(1552.52 nm). The main grid is placed inside the optical fiber amplifier bandwidth, but

can be extended to wider bandwidths. DWDM systems use 50 GHz or even 25 GHz

channel spacing for up to 160 channel operation [30].

21

Figure 2.13: Channel spacing 0.8 nm [30].

Many designs have an increasingly difficult time separating the optical channels

as the spacing becomes very close. However, such as fiber Bragg gratings actually

appear better suited for closer channel spacing. The need for close optic channel spacing

is a trade-off between the performance required of the optical amplifiers used in the

system and the number of channels to be transmitted per fiber [29].

Figure 2.14: 0.4 nm Channel Spacing DWDM Fiber Bragg Grating [29].

22

2.11 Q-factor

The quality factor or Q factor is a dimensionless parameter that describes how under-

damped an oscillator is or equivalently, characterizes a resonators bandwidth relative to

its center frequency. Higher Q indicates a lower rate of energy loss relative to the stored

energy of the stored energy of the resonator, the oscillations die out more slowly. Q

factor (signal quality factor) can be evaluated from the measured eye diagram, by

evaluating the mean and the variance of the signal trances in the upper and lower part of

the eye diagram. Q factor is directly related to the bit error rate (BER).

2.12 Previous Work

2.12.1 Fiber Optical Parametric Amplifier Performance in a 1-Tb/s DWDM

Communication System [31].

This paper shows that at this time OPAs can handle amplification of WDM traffic in

excess of 1 Tb/s with little degradation. It also indicates that with further improvements,

fiber OPAs could be a contender for wideband amplification in future optical

communication networks. This consisted of 26 DWDM channels modulated in WDM

spectrum at 37.7 Gbps return to zero differential phased shift keyed (RZ-DPSK) with a

total data rate in excess of 1Tbps. This modulation format is known to yield reduced

crosstalk levels compared to OOK [32,33]. Typical communication system signal levels

were used throughout.

2.12.2 BER Performance of a Lumped Single-pump Fiber Optical Parametric

Amplifier in a 10 Gbit/s 4-channel S-band DWDM System [34].

In this paper discussed about their investigation with correlation to each other by means

of computer simulation. The gain curve of FOPAs varies considerably with different

parameters of optical fibers, such as zero-dispersion wavelength, PMD and temperature.

23

Besides, gain curve also depends on the pump laser power and wavelength and type and

length of the optical fiber utilized to stimulated the process of parametric amplification.

Table 1: Parameter of HNLF

Nonlinear Coefficient, (W-km)^-1 11.5

Effective Area, μm^2 11.7

Attenuation at 1550nm dB/km 0.9

Zero dispersion wavelength, nm 1510

From the result, it is necessary to determine the acceptable boundaries for HNLF

length and the pump power for each type of the FOPA. Besides, the accurate filtering

must be done to avoid unequal amplification of channels in the system. Lastly, FOPA

performance is very sensitive toward characteristic of DWDM systems such as the

number of channels, the power of each channel at the amplifier input and the necessary

gain.

2.13 Summary

A description of chromatic dispersion, due to its importance in the study of

nonlinear effects in fibers, along with fiber loss is given. Hence, a brief introduction to

various nonlinear effects arising from Kerr effect and stimulated inelastic scattering is

provided. The Kerr effect results from the dependence of the refractive index on the

optical intensity, which brings on SPM, XPM and FWM. The stimulated inelastic

scattering arises from the interaction of the optical field with acoustic phonons in the

fiber (SBS) or with phonons (SRS).

24

CHAPTER 3

Methodology

3.1 Methodology

There are several different method that used to carry out research in the field of optical

system like simulation and experimental. This research is focus on simulation

experiment based on OptiSystem software in order to ensure higher level of

amplification coped with less amplifier produced the signal distortion in multichannel

of signal transmission of Dense Wavelength Division Multiplexing (DWDM) fiber

optic communication system. The major aspect during methodology stage is

simulation process. The main focus of this simulation is to find the best configuration

of the system that can operate at optimum performance to be implemented it on the

application system.

3.2 Stage

At the first stage, the procedure used is to review the scope and objectives required by

the project are made. In addition, researches came made more focused on searching

the necessary information through reading of books, theses, journal, internet and so

on. Software selection process was conducted to carry out simulations on blocks

59

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