CHAPTER 1 Introduction - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/32897/10/10_chapter 1.p… · The backbone of modern telecommunication industry consists of wavelength

Introduction 1

Study on Interchannel Crosstalk in WDM Optical Fiber Communication Link

CHAPTER 1

Introduction

1.1. WAVELENGTH DIVISON MULTIPLEXED SYSTEM

The backbone of modern telecommunication industry consists of wavelength

division multiplexed (WDM) system. A WDM system transmits information by

multiplexing number of independent information carrying wavelengths on a single fiber

and demultiplexing at the receiver. WDM system thus facilitates tremendous increase in

transport capacity of optical transmission system with equivalent decrease in the cost of

transmitted bit. With continuous demand for increase in bandwidth, extensive research

is undergoing in the field of optical communication (Keiser, 2010; Agrawal, 2002). At

the transmitter end, optical carrier generated by laser modulates data sequence. The

separated wavelengths are multiplexed using multiplexer such as arrayed waveguide

which typically has 100 GHz wavelength separation (Keiser, 2010). The capacity of

WDM system is further increased by multiplexing wavelengths with an optical

interleaver (INT). This results in 80 channel WDM system with 50 GHz wavelength

separation. Thus transmission system with aggregate capacity of 1Tb/s is being

deployed with per channel bit rate of 10 Gbps (Keiser, 2010). Spectral efficiency,

which is the transmitted capacity per unit bandwidth, expresses the bit rate and channel

spacing of a WDM transmission system. Next generation transmission system is

expected to have spectral efficiency of 0.8 b/s/Hz as compared to currently deployed

transmission system which has a spectral efficiency of 0.2 b/s/Hz (Keiser, 2010). At the

receiver end, a demultiplexer separates the multiplexed signal into appropriate detection

for processing by the receiver.

WDM is basically frequency division multiplexing at optical frequencies.

International Telecommunication Union (ITU) has specified WDM standards in terms

of frequency spacing between channels. Frequency was used for stating the channel

separation and not wavelength because when a laser is locked to a particular operating

mode, it’s the frequency of the laser that is fixed. The ITU-T specifications for

Introduction 2


multimode and single mode telecommunication fiber are as follows: G.651 (revised Feb

1998), G.652 (revised June 2005), G. 653 (version 5), G.654 (revised June 2004), G.

655 (revised March 2006) and G.656 (June 2004) (Keiser, 2010). These

recommendations have been made by ITU-T for deployments of single mode fibers,

optical interface, undersea applications, long haul links application etc (Keiser, 2010). It

also specifies wavelength separation for coarse WDM, dense WDM and WDM

operation in S-, C- and L- bands for metro area network and wide area network. In

WDM system due to high power confinement, nonlinear effects come into play.

Moreover due to inherent nature of silica fiber, optical signal undergo attenuation on

transmission through the fiber.

1.2. NONLINEAR EFFECTS

In spite of low nonlinear index of silica, two factors enhance the optical

nonlinearities: core size and length of fiber.

Fiber nonlinearities are not evenly spread over the fiber but rather dominate the first part

of it due to fiber attenuation. The effective length over which the nonlinearity dominates

is defined as the length over which a signal would propagate through the fiber if it had

constant amplitude over the length and zero amplitude beyond (Toulouse, 2005).

where is the attenuation co-efficient and L is the length of fiber.

The effective area is the area the core would have, if the optical intensity was uniformly

distributed within it and zero outside (Toulouse, 2005).

For conventional single mode fiber Aeff is approximately equal to 80µm2, and for

dispersion compensating fiber is 20 µm2.

The physics of nonlinearities can be understood from the material (silica or

glass) response to propagation of large optical field in terms of polarization P.

where E is the optical electrical field, is the nth

order susceptibility. In glass the

second order susceptibility is zero due to optical isotropy. The real and imaginary part

of susceptibility gives rise to various types of nonlinearities. The real part of

Introduction 3


susceptibility is associated with the refractive index and gives rise to Self Phase

Modulation (SPM), Cross Phase Modulation (XPM) and Four Wave Mixing (FWM).

The imaginary part of susceptibility is associated with phase delay in material response

and gives rise to Stimulated Raman scattering (SRS) and stimulated Brillouin scattering

(SBS) (Agrawal, 2007).

The nonlinear processes occurring inside fiber during optical signal propagation

can be basically categorized into three types (Agrawal, 2007). The first type is known as

non resonant process and is defined as an intensity dependent variation in the refractive

index of fiber. Nonlinearities arising from nonlinear change in refractive index are also

known as Kerr effect. The refractive index is equal to n0 + n2I where n0 is the linear

refractive index, n2 is the nonlinear coefficient (in silica n2 = 2.6 x 10-16

cm2/W) and I is

the intensity of optical signal. Nonlinear co-efficient γ determines the corresponding

nonlinear effects and is given by γ =

. The nonlinear phase shift induced in the

propagating signal is ΦNL = γ P0 z. The impact of this nonlinear process can be very

dominant in WDM system and has been demonstrated in number of research

experiments (Wegener et. al., 2004; Wu and Way, 2004; Chraplyvy, 1990). The field of

co-propagating optical signals polarized along x axis at frequency ω1 and ω2 is given as

(Agrawal, 1995)

E =

(1.4)

XPM and SPM induced nonlinear phase shift of optical signal at frequency ω1 is given

as

ΦNL = n2k0L (|E1|2 +2 |E2|

2) (1.5)

where L is the length of fiber and k0 is the free space wave number and is equal to

2 /λ. SPM and XPM are the two consequences of Kerr nonlinear processes which will

be studied in this thesis. Other associated phenomena are FWM, Intrachannel XPM and

FWM. Second type of nonlinear process is the resonant process or non-elastic scattering

process. In such processes part of the energy is transferred from optical field to the

dielectric medium. SRS and SBS are examples of resonant process. Effect of SRS on

WDM system will be studied in this thesis.

Third type of nonlinear process is the nonlinear interaction between signal and

noise.

Introduction 4


In the following subsections, detailed description of three of these nonlinearities

i.e. SRS, XPM and SPM, their specific examples and references of their recent

applications in optical communication system are discussed.

1.2.1. SELF PHASE MODULATION

SPM refers to the self induced phase shift experienced by an optical signal as it

propagates in optical fibers. In Eq. (1.5), the first term gives the SPM induced phase

shift. The modulation of phase leads to frequency chirping and hence SPM is essentially

a pulse effect causing leading edge of pulse to be red shifted and trailing edge to be blue

shifted (Agrawal, 2007).

If the initial pulse is negatively chirped, SPM causes spectral compression of the

pulse. Picoseconds second pulses with peak power in kilowatts have been obtained by

using pulse compression technique (Limpert et. al., 2002; Washburn et. al., 2000;

Planas et. al., 1993). Similarly, by using two step processes temporal compression of

pulse has also been achieved. A spectrally broadened and linearly chirped pulse

generated in normal dispersion fiber using SPM is temporally compressed in a section

of fiber with anomalous dispersion. A number of research experiments (Shen et. al.,

1999; Schiess, 1995; Olsson, 2002) have demonstrated pulse compression using SPM.

In one such experiment pulse broadening and recompression ratio greater than 300 has

been achieved in 2.5 km transmission distance.

Soliton propagation is achieved by balance between chromatic dispersion and

SPM induced chirp, resulting in signal propagation undistorted by chromatic dispersion

or nonlinearity (Mollenauer et. al., 1980). As soliton propagation facilitates error free

transmission over trans-oceanic distance, it has been an extensive field of research

(Dudley et. al., 2001; Haus et. al., 1996; Mollenauer et. al., 1997; Mitschke and

Mollenauer, 1986).

1.2.2. CROSS PHASE MODULATION

The phenomenon of cross phase modulation is similar to self phase modulation

except that two optical signals are involved. XPM refers to the nonlinear phase shift

induced in an optical signal by co-propagating field at different wavelength. In Eq.

(1.5), the second term gives the XPM induced phase shift. It is evident from Eq. (1.5)

Introduction 5


that XPM contribution is double that of SPM contribution in nonlinear phase shift. If the

strength of one of the two signals is much higher than the other, in XPM, stronger signal

acts as a pump beam for the weaker signal. Another requirement for XPM to occur is

that the two signals must overlap in space and time. Combined effect of XPM and

dispersion induces multi-peak temporal structure in optical pulses (Agrawal, 2007).

XPM has been used for number of optical applications such as wavelength

conversion (Olsson and Blumenthal, 2001), demultiplexing (Perlin and Winful, 2002; Li

et. al. 2003), switching (Perlin and Winful, 2001) and other optical control applications.

XPM is used for wavelength conversion as it is a very fast process, can scale to high bit

rates, and convert multiple signal conversion simultaneously. XPM along with SPM has

been used for simultaneous demultiplexing and regeneration of optical signals (Yu and

Jeppesen, 2001). In the interaction of femtosecond pulse train with continuous beam,

XPM causes generation of a comb of frequencies centred at an arbitrary wavelength

(Jones et. al., 2001).

XPM causes problems in WDM communication system because it causes

crosstalk in terms of nonlinear phase shift between nearby signals and will thus affect

the phase and amplitude of optical signals (Hui et. al., 1999; Collings and Boivin,

2000).

1.2.3. STIMULATED RAMAN SCATTERING

Raman scattering is defined as the non-elastic scattering of light due to which

part of photon energy is transferred to the silica molecules of the optical fiber. This type

of scattering occurs in one in millionth photon-molecule collisions (Toulouse, 2005).

Due to Raman scattering, a lower frequency photon is generated and the frequency shift

of the generated photon with respect to colliding photon depend on the propagating

material. In WDM system, when co-propagating channel are present within the Raman

bandwidth, Raman scattering causes stimulated emission and is known as stimulated

Raman scattering (SRS). SRS has been used for number of applications like wavelength

conversion, amplification, optical modulation and switching (Dahan et. al., 2003;

Barmenkov et. al., 2003; Ahmed and Kishi, 2003).

SRS gain is used in cavity configuration for laser applications. In one particular

experiment, initial pump beam generated at 1050 nm in an ytterbium doped fiber, as a

result of SRS gain, output light at 1210 nm was generated (Karpov et. al., 1999). SRS

Introduction 6


provides gain at a wavelength shift depending on the material under observation. For

fused silica this shift is 100 nm or 13.2 THz from pump. This property is used for

wavelength conversion and has been demonstrated in experiment conducted in highly

nonlinear fiber (HNLF) (Dahan et. al., 2003). In the experiment, pump signal modulated

by data stream up shifted the CW probe by 13 THz. The situation was reversed in all

optical switching or modulation in which the signal acted as control beam. The

switching or modulation of pump was controlled by the presence or absence of control

beam signal (Barmenkov et. al., 2003; Ahmed and Kishi, 2003).

SRS can combine with SPM or XPM giving rise to intrapulse Raman scattering

which in anomalous dispersive medium gives rise to Raman soliton (Dianov, 1985) and

self frequency shifts (Mitschke and Mollenauer, 1986; Gordon 1986). This phenomenon

has been employed for generation of tunable subpicosecond soliton pulses (Reeves and

Taylor, 2001).

The process of amplification of signal due to SRS in fiber was first demonstrated

in early 1970s by Stolen and Ippen (Stolen and Ippen, 1973). SRS is the fundamental

working principle of Raman amplifiers which will be discussed later in section 1.3.1. In

WDM transmission SRS causes power transfer from lower wavelength to higher

wavelength resulting in unwanted crosstalk.

1.3. OPTICAL AMPLIFIERS

When signal propagates in optical fiber it suffers intrinsic power loss similar to

propagation in any medium except vacuum. The signal power attenuation can be

expressed by the following equation.

P(z) = P(0).exp(-αz) (1.6)

Where P(z) is the power at distance z, α is the attenuation co-efficient and z is the

transmission distance. The minimum fiber loss occurs between 1.5 µm and 1.6 µm at

around 1.55 µm. The band of wavelength from 1260 nm to 1625 nm has been defined

for optical communication by the International Telecommunication Union

Standardization (ITU_T). Lower bound of the optical band is bounded by fiber cut-off

wavelength and upper bound is bounded by bending and SiO2 absorption losses. The

band is further subdivided into six sub bands namely Ordinary (O), Extended (E), Short

(S), Conventional (C), Long (L) and Ultra-long (U). In long haul transmission system,

Introduction 7


periodic amplification of signal is required to compensate for fiber loss. Two commonly

used amplification techniques are Raman amplifier and Erbium doped fiber amplifier.

1.3.1. RAMAN AMPLIFIER

Raman amplifier works on the principle of SRS where signal is amplified by

pump at 13.2 THz frequency shift (~ 100 nm) where the peak of Raman gain spectrum

for silica occurs. The most commonly used Raman amplifier is distributed Raman

amplifier (DRA) in which the transmission fiber itself acts as gain medium (Agrawal,

2002). A Raman amplifier can be forward pumped, backward pumped or bi-directional

pumped depending on the direction of flow of pump and signal. In forward pumped

DRA, signal and pump co-propagate in the fiber. In backward pumped DRA, signal and

pump counter propagate in the fiber. In bi-directional pumped DRA both forward and

backward pumps propagate with the signal. The 3 dB bandwidth of Raman amplifier is

approximately 55 nm making it an apt choice for WDM transmission system.

Multi Raman pump at different wavelength can be used to flatten and broaden

the Raman gain spectrum. This technique is also known as “WDM pumping scheme”

and has been used in a number of research experiments (Namiki and Emori, 2001;

Namiki et. al., 2006; Krummrich et. al., 2001 and Nielsen et. al., 2000). It further gives

flexibility in terms of choice of pump and signal wavelength. Another advantage of

Raman amplifier is that both amplification and dispersion compensation can be

combined in the same fiber length (Islam, 2002). In the present thesis, WDM pumping

scheme has been considered to realize broad and flat gain spectrum.

1.3.2. ERBIUM DOPED FIBER AMPLIFIER

Erbium Doped Fiber (EDF) is fabricated by doping a SMF with Erbium ions.

EDFAs are configured by pumping the fiber with external pump source. EDFA

revolutionized the field of optical communication after its invention in 1987 by the

group at University of Southampton (Mears et. al., 1987). They have become a very

important tool for improving system performance of long haul networks, local

telephone network, cable television network and local area networks (Keiser, 2010). It

offers a unique combination of features that have the potential to revolutionize optical

communications (Delavaux et. al., 1995). It has a number of advantages such as:

transparent to bit rates and transmission format, gain insensitive to polarization,

Introduction 8


simultaneous amplification of a large number of channels, gain stability over 100oC

temperature range (Keiser, 2010). Several research experiment has demonstrated high

transmission capacity exceeding 10 Tb/s using EDFA (Walker et. al., 1991; Shrivastava,

2005).

These attractive features have led to many important applications. Typical length

of EDFA in which optical amplification takes place is about 10 m in C- Band. The

signals and pumps are combined using passive wavelength coupler to minimize

insertion loss. Similarly isolators are used to prevent back reflections. The wavelength

of pump is either 980 nm or 1480 nm. The three pumping configurations that can be

used in EDFAs, the forward pumping, the backward pumping and the bidirectional

pumping are as shown in Fig. 1.1, 1.2 and 1.3 respectively (Keiser, 2010).

Fig. 1.1: Forward Pumped EDFA

Fig. 1.2: Backward Pumped EDFA

Fig. 1.3: Bidirectional Pumped EDFA

1.4. MOTIVATION

10-Gbps WDM optical systems have been installed on a large scale over the last

decade. But the optical industry has become more cost oriented after the telecom burst

in 2001 than being performance oriented that was prevalent during the boom period.

The first generation 40 Gbps transmission system could not compete on a cost basis

Introduction 9


with the 10 Gbps system that were already deployed. This has hindered the commercial

deployment of 40 Gbps system and limited their use to research experiments. However,

the use of advanced optical modulation formats and electronic equalization can enable

cost effective deployment of 40 Gbps optical transmission systems. This has stimulated

development efforts by the research labs into developing these technologies for future.

High-speed internet access, high capacity data networking, multimedia broadcast

systems are some applications of broadband communication systems in modern

information society (Keiser, 2010). These communication systems have wide variety of

bandwidth demands which are met by different cost-effective communication

technologies. The performance of the various available technologies can be compared

using various methods. One such method is to compare the maximum data rates

supported by them for a given regeneration-free transmission distance. Optical

communication systems can support Tb/s capacities over long distances making them an

ideal technology for high capacity wireline networks. The transmission capacity of

long- haul optical networks has evolved tremendously over the past decades by adding

multiple wavelength channels through wavelength division multiplexing (WDM). With

the growing bandwidth demand, there is a tremendous interest in increasing the

transport capacity and transmission distance of WDM system with simultaneous

reduction in cost per transported information bit. Sharing of optical components among

WDM channels is a common technique for cost reduction; optical fibers and optical

amplifiers are well-known examples of shared optical components (Keiser, 2010;

Agrawal, 2002). Spectral efficiency of WDM system increases by sharing of

components as WDM channels are closely spaced in the available limited wavelength

range. Increasing data rate per channel is another technique of lowering cost per

information bit The advent of low loss optical components, EDFA, distributed Raman

amplifier (DRA), forward error correction, advanced modulation formats and other

cutting edge features has contributed in the tremendous growth of communication

capacity using WDM and DWDM.

Expanding network functionality into the optical domain is another aim of fiber-

optic communication research. Optical networks with high spectral efficiency are being

designed to have higher per fiber transport capabilities as well as low cost per

transmitted bits. Optical networks are rapidly evolving from point to point links to

Introduction 10


meshed optical networks. Implementation of meshed optical network requires routing of

the traffic through multiple optical add-drop multiplexer (OADM) nodes along the

transmission link. As the currently deployed long-haul transmission systems have 50

GHz spacing between the WDM channels, the width of the optical spectrum is not a

significant concern for 10 Gbps systems and WDM channels can be switched

transparently through an optical network. With the increase in symbol rate of the optical

signal, the spectral width of the signal scales accordingly. The spectral width of the

optical signal can become a matter of concern when 10 Gbps transmission systems are

upgraded to a 40 Gbps bit rate per WDM channel. Advanced modulation format is a key

technology that is tolerant to the narrowband optical filtering that occurs when multiple

OADMs are cascaded along a transmission link (Gnauck and Winzer, 2005). Thus,

advanced modulation format is a key technology that can enable more robust optical

transmission. Characteristics of the optical signal are completely defined by its

modulation format. Non-return-to-zero on-off keying has been the modulation format of

choice for optical communication as it combines a cost-effective transmitter and

receiver. But growing interest in robust optical transmission system has facilitated

deployment of different modulation format. This is more important for 40 Gbps and

100 Gbps bit rate optical transmission system with 50 GHz channel spacing where

NRZ-OOK modulation has reduced transmission tolerance.

Optical fibers have a unique characteristic of low threshold for nonlinear effects.

Impact of fiber nonlinearities in WDM system employing optical amplifiers for

advanced modulation formats have been studied using statistical methods (Ho 2000;

Yamamoto and Norimatsu, 2003), Volterra series method (Peddanarappagari and

Brandt-Pearce, 1998; Xu and Brandt-Pearce, 2002), digital backward propagation

technique (Ip et. al., 2008; Asif et. al., 2011), regular perturbation method (Secondini et.

al., 2009; Vannucci et. al., 2002). Statistical methods (Ho, 2000; Ho and Wang, 2006)

in which standard deviation of crosstalk are calculated for system performance

evaluation has been chosen as the method for the present work. The usage of the method

is widely known for studying the impact of inter channel nonlinearities such as XPM

(Ho, 2004), SPM ( Ho, 2004), SRS (Ho, 2000), FWM (Wu and Way, 2004) and intra

channel nonlinearities such as IFWM (Ho, 2005) for different modulation formats such

as OOK, DPSK, DQPSK etc.

Introduction 11


1.5. OBJECTIVES

The main motivation of the present research work was to investigate by

analytical method the crosstalk performance of WDM optical fiber link employing

lumped and distributed amplifier stemming from nonlinear effects such as stimulated

Raman scattering, cross phase modulation and self phase modulation. Yamamoto and

Norimatsu (2003) have developed closed form formulae for crosstalk in dispersion

managed system employing lumped amplifier. Motivation to investigate the impact of

fiber nonlinearity in terms of crosstalk in WDM optical fiber communication link

employing distributed amplifiers has led to derivation of novel closed form formulae.

The formulae provide a good estimate of individual crosstalk standard deviation of each

nonlinear effect and hence play a good role in fast–evolving areas of advanced

communication system. In dynamic WDM system channels add/drop results in variation

in input power to the EDFA causing power transients in surviving channel due to cross

gain saturation. Theses power transients are of large magnitude and can cause error due

to nonlinear effects. To investigate the impact of nonlinearity on dynamic WDM system

employing EDFA was another motivation of the research work.

Next, issues related to analysis of crosstalk due to SRS, XPM and SPM for

different WDM system parameters like input power, wavelength separation and bit rate

of the system has been investigated. The transmission capacity of a wavelength division

multiplexing optical system can be increased by increasing the data rate per wavelength

or increasing number of wavelength channels in fixed optical bandwidth. The first

method has been illustrated by the increase in data rate per wavelength from 2.5 Gbps to

10 Gbps and then to 40 Gbps. In the second method, the number of wavelengths in a

fixed optical band is significantly increased by decreasing the spacing between adjacent

wavelengths. The effects of variation in the above mentioned factors were investigated

in WDM system.

Design of an efficient WDM optical communication system depends on the

judicious choice of modulation format and pulse shape. To observe the impact of

nonlinearity on modulation formats like On Off Keying (OOK) and Differential Phase

Shift Keying (DPSK) is another objective of our present research work. Expanding

network functionality into the optical domain is another aim of fiber-optic

communication research. Modulation formats such as intensity and phase modulation is

Introduction 12


a key technology that enables the design of optical networks with high spectral

efficiency, higher per fiber transport capabilities as well as low cost per transmitted bits.

In this research work, analytical expressions have been derived for nonlinearity induced

crosstalk for two modulation formats - one intensity modulation format i.e. OOK and

other phase modulation format i.e. DPSK.

1.6. OUTLINE OF THESIS

1. Chapter 1 is the introductory chapter. It highlights the motivation and objectives of

thesis. It also contains the literature review of WDM system, nonlinear effects and its

impact on optical communication system and optical amplifiers.

2. In Chapter 2, existing closed form formulae for crosstalk in dispersion managed

system is applied to a typical fiber configuration and crosstalk performance for

different parameters of WDM system and of dynamic WDM system employing

EDFA has been quantitatively studied.

3. In Chapter 3, closed form formulae have been derived for crosstalk in WDM system

employing DRA. In DRA, high power pump co- and counter propagates with the

WDM signals providing a continuous gain to compensate for the attenuation of

signals. Using statistical methods, closed form formulae have been derived to study

crosstalk in single segment of fiber link employing DRA. Since the gain is

continuously increasing along the transmission line, crosstalk standard deviation has

been calculated for variable gain along the entire length of single mode fiber. The

formulae are then used to study the crosstalk performance of different pumping

scheme of DRA and of different methods to increase the transmission capacity of

WDM system.

4. In Chapter 4, closed form formulae for crosstalk are derived for two modulation

formats: one is an intensity modulation format (On Off Keying) and the other is a

phase modulation format (Differential Phase Shift keying). The performance of two

modulation formats are studied and analyzed for different pulse shapes.

5. Chapter 5 is the summary of the entire research work discussed in the previous

chapters. Some significant contributions by our research work have been described in

this chapter. It is followed by references of the previously published literatures.

Documents

CHAPTER 1 Introduction - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/32897/10/10_chapter 1.p… · The backbone of modern telecommunication industry consists of wavelength