Design and Simulation of CSRZ Modulated 40 Gbps

DWDM System in Presence of Kerr Non Linearity

Lucky Sharan1 & V K Chaubey2
EEE, BITS-Pilani
Pilani, India
luckysharan@bits-pilani.ac.in1, vkc@bits-pilani.ac.in2

Abstract— Research in optical communications is being These non-linear effects of the optical fiber play a
constantly driven by requirements for higher data rates and detrimental role for a high speed data transmission through the
better spectral efficiency. On one hand, higher bit rates per fiber. The problem becomes severe in case of multichannel
channel involve the deployment of high-order modulation transmission due to four-wave mixing (FWM), a parametric
formats, which require increased signal-to-noise ratio and, hence, interaction among optical waves. In such multi-channel system,
higher power per channel. On the other hand, higher spectral the beating between two or more channels causes generation of
efficiency also demands closely spaced channels to optimize the one or more new frequencies at the expense of power depletion
operational bandwidth of optical amplifiers. The above of the original channels. Since these mixing products can fall
requirements clearly point to a scenario of increased nonlinearity
directly on the propagating channels, proper FWM suppression
in the form of intra- and inter-channel crosstalk. Therefore, the
mitigation or compensation of fiber impairments caused by Kerr
is necessary to avoid significant interference between signal
nonlinearity becomes crucial. In this paper the performance of channels and FWM frequency components.
CSRZ modulation scheme is analyzed for WDM systems in the Chromatic dispersion, which broadens the pulses, can
presence of Kerr Non linearity to estimate the effect of input be reduced by using dispersion-shifted fibers at the 1550-
optical power for various transmission distances under different nm wavelength range, but low chromatic dispersion
dispersion compensation schemes and the performance is studied enhances some nonlinear effects of fiber, especially four-wave
with the help of Q value and eye diagrams. This paper reports
mixing (FWM) [2-3]. FWM is of particular concern on account
the superiority of CSRZ format over the conventional NRZ /RZ
of its relatively low threshold power and rises very quickly as
formats. The investigation is based on comprehensive simulative
analysis.
the number of channels increased. The efficiency of FWM
crosstalk generation can be reduced by increasing the
Keywords-Four Wave Mixing, CSRZ, Wavelength Division frequency separation between the various channels [4],
Multiplexing( WDM), Q value , Dispersion Compensation however increased channel separation would preclude dense
WDM systems. To suppress FWM-induced crosstalk in WDM
systems using dispersion-shifted fiber (DSF), the unequal
I. INTRODUCTION channel-spacing scheme was proposed which works quite well
Long-haul optical communication systems with very high but makes the transmitter very costly and complex. Moreover,
capacity are made possible by the extremely wide bandwidth of the use of non-uniform channel spacing is not always practical
optical fibers, which is best exploited by wavelength division because many optical components, such as optical filters and
multiplexing (WDM) [1].The performance of long-distance waveguide-grating routers require equal channel spacing. A
optical communication systems is limited due to the interactive practical solution is to use periodic dispersion management
and accumulative effects of chromatic dispersion and nonlinear technique which is discussed in section III.
phenomena. Nonlinear effects have become much more In modern optical communication systems, modulation
important since the development of the optical fiber amplifier format has a great impact on system performance. By
which can boost the power in a number of channels at different manipulating the spectrum of the optical signal, the available
wavelengths simultaneously rather than having a separate bandwidth can be used more efficiently. Till now NRZ (non-
repeater for each channel. This allows many more channels to return-to-zero) modulation format is widely used in the 10 Gb/s
be multiplexed into a single fiber than was economically viable and lower rate optical communication system owing to its
with optoelectronic repeaters. Although the individual power in lower bandwidth demand, simple transmitter architecture and
each channel maybe below the required threshold value to lower cost. However, for 40 Gb/s and higher data rate system,
produce nonlinearities, but the accumulative effects of all the capacity is restricted due to poor optical signal-to-noise
channels can initiate non-linear complexities significantly [2]. ratio (OSNR) tolerance and nonlinear effects of NRZ, making
Thus, in a multi-channel, closely spaced WDM system many it impractical for 40 Gb/s long-distance transmission systems.
kinds of nonlinear effects appear, including:
As an improvement over NRZ, RZ (Return-to-Zero)
• Cross-Phase Modulation (XPM) modulation format has been proposed and implemented with
• Four-Wave Mixing (FWM) superior performance [4, 5]. Other proposed formats are
• Stimulated Brillouin Scattering (SBS) carrier-suppressed return-to-zero (CSRZ), optical duo binary
• Stimulated Raman Scattering (SRS) (ODB), differential phase shift keying (DPSK), and

978-1-4673-1989-8/12/$31.00 ©2012 IEEE

differential quadrature phase shift keying (DQPSK), etc. [6, 7, can be implemented either by electronically generating RZ
8, 9]. As a result, among other enabling technologies advanced waveforms, which are then modulated onto an optical carrier,
optical modulation formats have become key factor in the or by carving pulses out of an NRZ signal using an additional
design of modern WDM system. Compared to NRZ, the most modulator, called pulse carver.
significant advantage of new modulation formats is to improve
the N × 40Gb/s WDM transmission system for the most B. Carrier Suppressed Return-to-Zero (CS-RZ) Format
limiting factors (such as dispersion, nonlinear effects, back to
back OSNR, etc.), and at the same time significantly expanding CS-RZ format is a modification of RZ format and is
the transmission distance.
discussed in detail in this paper. A number of transmission
This paper presents a simulation analysis to reveal the experiments have employed this format as it is highly tolerant
features and application conditions to analyze 40 Gb/s 32 to the mixed effect of self phase modulation (SPM) and group
channel WDM CSRZ modulation based transmission systems velocity dispersion (GVD) ,and has a narrower pedestal shape
over standard-fiber lines. Till now, this format has been studied of the optical spectrum than the conventional RZ format
for WDM systems with 50 GHz channel separation [10], but [5,6,11]. Fig. 2 (a) shows the block of the 40 Gb/s CS-RZ
with up to 16 channels [11] or at a lower data rate [12]. Here, transmitter
we make an attempt to improve the bandwidth efficiency and
find the most suitable format for higher transmission distance
working at 40 Gb/s for 32 channel DWDM system with 50 PRBS 40 Gbps 20-GHz
GHz channel spacing. The CSRZ format at 40Gbps data rate is NRZ data Clock
analyzed using pre, post and symmetrical dispersion
compensation schemes on the basis of Q value and eye opening
for multiple transmission spans for varying signal input
powers. CW MZM Dual arm
This paper is organized as follows. Section II describes the Laser MZM
40 Gbps
design architecture of transmitters of the NRZ,RZ and CSRZ Optical CSRZ
format. Section III mentions the assumptions used in the II. SIMULATION SETUP signal
transmitter, optical link and receiver structures. In Section IV, Fig.2. (a) Block diagram of CSRZ. transmitter
comparative results have been reported using different
dispersion compensation methods and, finally Section V
concludes the findings. This transmitter exploits two concatenated MZMs. The first
MZM modulates the intensity of the light from a CW laser
II. MODULATION FORMATS source with a 40 Gb/s NRZ data. Then, this signal is re-
modulated by the second MZM that is driven by a clock at the
A. Non return-to-Zero (NRZ) / Return-to-Zero (RZ) Format half bit-rate, 20 GHz in this case. That introduces a pi phase
shift between any two adjacent bits and the spectrum gets
The NRZ format has the simplest configuration and is modified such that the central peak at the carrier frequency is
widely used in commercial products so far. A schematic block suppressed as shown in the bottom diagram in Fig. 2(b).
diagram of the 40 Gb/s NRZ transmitter is shown in Fig. 1. An
optical signal generated by the continuous-wave (CW) laser
source is ON–OFF keyed by the Mach–Zehnder intensity
modulator (MZM) driven by 40-GHz NRZ data signal.

40 Gbps
PRBS NRZ data

Fig.2 (b) Spectrum of CSRZ signal

CW MZM 40 Gbps
Laser Opitcal NRZ
III. SIMULATION SETUP
signal The complexity of optical communication systems
employing WDM requires a comprehensive computer aided
Fig.1 Block diagram of NRZ.transmitter
modeling platform in order to optimize design, experimental
costs and evaluate the performance of the implemented
In RZ format for the logical 1 bit, the power level returns to 0 networks. In this work, we have used the OptiSytem 10
after half of the period, whereas for the 0 bit, the power level simulator that gives us an environment identical to the physical
is 0 continuously. Binary 0 is represented by the absence of an realization of a fiber-optic transmission system. Matlab 7.0 has
optical pulse during the entire bit duration. RZ transmitters been used for graphical analysis.
 The proposed 32 channel DWDM system consists of a adjacent channels. Extinction ratio of Mach-Zehnder intensity
transmitter, fiber and an optical receiver as shown in Fig.3 with modulators (MZM) is set at 30 dB. To each output port of each
the central frequency of the first channel= 193.1 THz. The CW laser a data modulator shown in Fig 2 has been connected.
design of transmission link has been done using periodic Optical signals from 32 data modulators are fed to the 32
dispersion management technique wherein fibers with normal input ports of an optical multiplexer. To ensure separation
and anomalous GVD are combined to form a dispersion map between the channels in the frequency domain (linear cross-
such that the GVD is high locally all along the link length, but talk suppression), before multiplexing, each channel is
keeping an overall low average value [13]. The parameters of optically filtered with narrow transmission optical filter [14].
Dispersion Compensating Fiber(DCF) and Single Mode Here, a second order Gaussian filter with a bandwidth equal to
Fiber(SMF) are chosen such that the first-order dispersion is 50 GHz has been considered. The channel spacing and
compensated exactly (D = 0) i.e. DSMF LSMF = DDCF LDCF where operating wavelengths are as defined by ITU-T standards.
D means the first-order dispersion parameter [ps/nm/km] of the
corresponding fiber and L stands for the total SMF or DCF B. Fiber section
length per span. The simulation parameters used are given in The combined optical signal is fed into the SMF. The
Table 1. model in OptiSystem takes into account the unidirectional
Table 1: Simulation parameters signal flow, stimulated and spontaneous Raman scattering,
Kerr-nonlinearity and dispersion. The fiber parameters have
Bit rate 40Gbps been specified in the Table 2.The gain of the erbium doped
Sequence length 64 fiber amplifier (EDFA) placed after each fiber is to compensate
Samples/bit 256 the losses of the preceding fiber. The noise figure of the
DWDM channel spacing 50 GHz
amplifiers is constant and set to 6 dB. Scalar model of both the
fiber has been used to avoid polarization mode dispersion
Central frequency of the 193.1 THz
1st channel
(PMD). The signal is then launched over N spans of standard
Capacity 32-channel 40-Gbps single mode fiber (SSMF) of 30 km each. The proposed WDM
system has been simulated for pre, post and symmetrical
Distance 30 Km X N Spans
dispersion compensation scheme. In pre-compensation scheme,
Input Power -10 dBm as shown in Fig. 3(a), to compensate for the dispersion and the
nonlinearities, DCF fiber of 5 km is used prior to the SSMF
fiber of 25 km length. Also, two in-line-EDFA with gain 2.5
dB and 5.5 dB, respectively, have been used in the link. The
post-compensation scheme has been shown in Fig. 3(b) where
DCF fiber of 5 km is used after the SSMF fiber of 25 km length
to combat the accumulated dispersion. In symmetrical-
compensation scheme, as shown in Fig. 3(c), DCF fiber of 5
km is used in the middle of the SSMF fiber of 25 km length.
Here, three in-line-EDFA’s with gain 2.75 dB, 2.5 dB and 2.75
dB have been used.
Table 2. Fiber parameters
Fiber Attenuation Dispersion Dispersion Effecti Non linear
(dB/Km). (ps/km nm) Slope ve core refractive
(ps/km Area index
nm 2) (µm 2) (n2)

SMF 0.22 17 0.08 80 2.6

DCF 0.5 -85 -0.45 30 2.6

C. Receiver section
In the receiver the signal is de-multiplexed, detected by
PIN detector, passed through the filter and 3R regenerator.
Fig.3. Schematic of simulation setups: (a) pre-compensation scheme, (b) post- Optical de-multiplexer used has 32 output ports Bessel band
compensation scheme and (c) symmetrical-compensation scheme. pass filters with filter parameters: 3 dB cut off frequency = 65
A. Transmitter section GHz, order of the filter = 4, depth = 100 dB have been used to
separate out the channels at the respective wavelengths. The
The WDM transmitter consists of a PRBS generator , CW filter parameters have been optimized to give the best result.
lasers, data modulators, filters and the optical multiplexer The The optical signal from each port is then passed through PIN
PRBS generator generates pseudo random bit sequences at the photodiode whose reference frequency ranges from 193.1–
rate of 40 Gbps with 27 –1 bits.. The emission frequencies of 194.65 THz respectively, responsivity [A/W] =1 and dark
CW laser are equally spaced and are in the range of 193.1– current = 0.1 nA. An electrical low pass Bessel filter follows
194.65 THz with the frequency spacing of 50 GHz between the the PIN photodiode whose cut-off frequency is determined by
the modulation used and is optimized at 40GHz with order 3
[15]. Thereafter, 3R regenerator is used to regenerate an
electrical signal connected directly to the BER analyzer
which is used as a visualizer to generate graphs and results
such as eye diagram, BER, Q value, eye opening etc.

IV. RESULTS AND DISCUSSIONS

The performance of CSRZ modulation format has been
compared for pre, post and symmetrical dispersion
compensation schemes for 32 X 40 Gb/s DWDM system in
terms of received maximum Q value[dB] and eye opening
using Split Step Fourier (SSF) method. For system analysis,
the results of the first channel have been considered as it (a)
corresponds to the worst-case scenario [16]. In this simulation,
a 30 Km span is designed by appropriately choosing SMF and
DCF fibers to compensate GVD. Then transmission spans
have been cascaded in series to realize lengths in multiples of
30 Km. Fig. 4 (a)–(d) shows the graphical representation of Q
value as a function of signal input power for spans 1 to 4
for pre, post and symmetrical-compensation schemes.
For a high data rate WDM system it is desirable that the
input power should be as low as possible to limit non-linear
effects. Keeping this in mind, we have varied the input power
from-20 dBm to 15 dBm. Though generally we do not operate
at power level greater than 5 dBm, the designed system works (b)
well even at 10 dBm but at 15 dBm power Q values falls
below the minimum required 15.6 dB but only for the 4th
Span. Also, it can be seen that as the signal input power
increases Q value is initially maintained for all the dispersion
compensation schemes till -10 dBm and then it starts to
decrease. This can be understood from the fact that for low
powers the DWDM system has very less non-linear effects
coming into play. However, at higher powers, the pulses tend
to overlap each other due to more dominance of non-linear
effects like XPM and FWM caused by optical Kerr’s effect
and thus reduces the Q value. This observation is in close
agreement with the reported results [4, 5, 17]. (c)
Further, it is observed that the worst performance is
shown by pre-compensation scheme. However, post and
symmetric compensation schemes follow each other closely
with post giving better results even at 15 dBm of input power.
The best Q value obtained is 45.46 dB at input power of -20
dBm using post-compensation scheme at a distance of 30 Km
while Q falls to 19.35 dB at a distance of 120 km. Since the
performance of post-compensation scheme is the best hence
we have further analysed this scheme to predict the eye
diagrams at the receiver as shown in fig.5.The shape of the eye
as well as the eye opening decreases with the distance.
The CSRZ format although robust against group velocity (d)
dispersion (GVD) and self-phase modulation (SPM) effect due
Fig.4. Q value as a function of signal input power for different
to narrow spectral width, but still DWDM transmission causes compensation schemes :( a) Span =1, (b) Span =2, (a) Span =3,
more degradation owing to inter-channel XPM and FWM due (d) Span= 4
to spectral broadening .The wave frequencies interacting
through FWM lead to the generation of sum and difference Thus, it is inferred that CSRZ format shows a better
frequencies, which further interact among each other leading suitability for WDM systems over the conventional NRZ /RZ
to increased bandwidth. To accommodate the expanded pulse as the former offers a higher tolerance to FWM, noise, inter-
bandwidth, the used filter bandwidth has been chosen three channel cross-talk and a better receiver sensitivity for lower
times so that the higher order FWM products are aliased. power.
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