performance of multichannel fiber optic parametric
TRANSCRIPT
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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