Optimized Dispersion Using Dispersion
Compensation Bank In WDM Optical System
GAITRI RAI
M.TECH SCHOLAR
Galgotias University
Mr.S.Sugumaran
Assistant Professor III
SEECE Galgotias university
ABSTRACT
In this paper demonstrates the possibility for dispersion compensation in a 10 Gbps
WDM with the help of fiber Bragg Grating created with the Fiber Grating component.
Chromatic dispersion also called group velocity dispersion (GVD), is a very serious
issue for communication engineers to design a wavelength division multiplexing (WDM)
system. The pulse gets broader with increasing length of fiber due to chromatic
dispersion and this pulse broadening effect stimulates the adjacent bit periods to
overlap which causes inter symbol interference (ISI). The chromatic dispersion also has
inverse relation with Four wave mixing (FWM) and Four wave mixing is minimum
when chromatic dispersion is at highest level. The pulse broadening is a function of
length of the fiber which has been shown in this work through simulation results for 10
Gbps fiber optics system at various lengths (50-200 km) using standard single mode
fiber (SMF) in Optisystem software. In this work, DCB (dispersion compensation bank)
has been used to reduce the effect of chromatic dispersion and it has been shown
that the dispersion effect get mitigates more effectively by using DCB.
Index Term- Dispersion, chromatic dispersion,GVD, SMF,GOF,DCB,IFBG,FBG,BER;
I. INTRODUCTION
Wavelength division multiplexing (WDM) is a technology which multiplexes a number of
optical carrier signals onto a single optical fiber by using different wavelengths (i.e. colours)
of laser light. This technique enables bidirectional communications over one strand of fiber,
as well as multiplication of capacity [1]. The term wavelength-division multiplexing is
commonly applied to an optical carrier (which is typically described by its wavelength),
whereas frequency-division multiplexing typically applies to a radio carrier (which is more
often described by frequency). Since wavelength and frequency are tied together through a
simple directly inverse relationship, the two terms actually describe the same concept. From
transmission point of view our main goal is to increase the length of fiber. But with
increasing length of fiber many losses occur during the transmission. These losses and
dispersion badly affect the performance of optical fiber. When the transmission length of
optical fiber increases these losses and dispersion also increase. The Erbium doped fiber
amplifiers (EDFA) are very help full for the amplification of optical signals at various
lengths of SMF. Since the EDFA operates on wave band of 1550 nm and in this band the
average dispersion for SMF is almost 15-20ps/ nm/km [4]. The dispersion is a dominant
factor which restricts the length of optical fiber for long distance transmission [5]. The
dispersions in high speed optical communication systems can be controlled by DCF or FBG.
II. DESCRIPTION OF THE OPTICAL DESIGN
In this optical design, 4-channel transmits 10Gb/s, is simulated with variety of extinction
ratio and fiber lengths. The design consists of 4 transmitters and 4 receivers. In the
transmitter side, return to zero (RZ) pulse generator is used in order to generate pseudorandom bit sequence (PRBS). Then, Mach-Zehnder is used to modulate the electric signal
from NRZ with continuous wave (CW) laser. An optical fiber with different distances are
used to link between the transmitter and the receiver by using an EDFA and DCB for long
distances. In the receiver side, Avalanche photodetectors (APDs) are used to convert the
received optical signals to electrical signals are filtered through a low pass Bessel filter. Bit
Error Rate (BER) analyzers are used to realize the quality of the output signal for each
channel. A schematic design of four channels WDM is presented. In this scheme various
dispersion compensation components and optical filters are used to compensate the effect of
chromatic dispersion. The implementation is shown in figure.
Figure:Block diagram
DCB (dispersion compensation bank) consist of either three or two dispersion
compensation components.
 The ideal fiber Bragg grating (IFBG)
 Fiber Bragg grating (FBG)
 Gaussian optical filter (GOF)
Figure2:DCB System
DCB is just like fiber segment having attributes of two dispersion compensation components
in a single package.DCB is a combination of FBG or IFBG and GOF available in single
package .The Grating is just like a dielectric mirror which reflects back particular
wavelengths and passes all other wavelengths. The grating is used in small segment of fiber
and it is obtained by making gradual variations in the core of fiber. In this system, we have
used the ideal dispersion compensation grating and FBG, operating at frequency of 193.1
THz with Effective index of 1.45 . In this scheme, we also used GOF (Gaussian optical
filter) to make the response of Ideal FBG more flat and smooth, because GOF has property to
minimize the rise and fall time and minimizing group delay which is very helpful to reduce
the effect of chromatic dispersion. GOF which has been used here is operating at frequency
of 193.1 THz and bandwidth of 10 GHz. The EDFA is most important component in fiber
optics communication systems and it can efficiently amplify the wavelength in region of
1.5µm.
III. SIMULATION DESIGN
The simulation is performed by the use of Optisystem software. Optisystem13 gives the
ability to the designer to change the design and the parameters to get better results. As shown
in figure 2 the transmitter side consists of 4 transmitters at 193.1, 193.2, 193.3, and 193.4
THz with almost 100GHz spacing between the channels. Then, the media is WDM MUX,
optical fiber with different lengths, EDFA, DCB and WDM DEMUX. In the receiver side,
photodetector APDs, low pass Bessel filters and BER analyzers. The total accumulated
dispersion of the SMF is 16x80 = 1280 ps/nm. We swept the total dispersion of FBG from 30 to -3000 ps/nm. The bit rate is set to 10 Gbps. WDM network consist an optical span
which has a loop control. This loop control can be used to run the transmission of signal one
or more time in optical fiber. In this simulation, we want to investigate the dispersion-limited
performance of the system. The designed system is shown in Figure 3.
Figure 3: WDM system topology with DCB at 8o km SMF
IV RESULTS AND DISCUSSION
Eye diagram or also known as eye pattern in telecommunication is an old technique used in
order to evaluate the received signal. Totally opened eye pattern represents the lowest level of
distortion. BER analyzers create eye diagrams by making a pseudorandom arrangement of 1s
and 0s in a symmetric rate but in an arbitrary manner. In Optisystem, eye diagram, which can
be found in the BER analyzer show various traces of modulated signal in order to create an
eye diagram. Because of the shape of the pattern that looks as if it is an eye, it is called an eye
diagram. We can analyze different parameters like Q factor, Min BER, and BER pattern. In
this simulation Q factor is better as shown
Figure 4: graph for Quality factor
Figure 5: Graph for Min BER
Figure 6: Graph for Threshold
Figure 8:Optical spectrum after WDM
Figure 10: Graph for BER pattern
.
Figure 7: Graph for Height
Figure 9:. Optical spectrum at receiving end
Figure 11: Eye diagram
COMPARISION OF WDM SYSTEM WITH OR WITHOUT DCB AT
80KM SMF
In this system, we have used the ideal dispersion fibre bragg grating and operating at
frequency of 193.1 THz with effective index of 1.45 and length of 2mm. the signal is original
signal with an 80 Km long optical fiber cable is used as transmission medium
Figure 12:EYE Pattern without DCB
Figure 13:EYE Pattern with DCB
In this simulation we are observing that the Q factor without DCB is 22.3677 which is
increased to 23.3415.Min BER without DCB is 2.4454e-094 which is improved 5.12435e121.EYE height without DCB is .21008 and after DCB it is .211907.Threshold without DCB
is .0139233 and after implementing DCB we get .0137181 which is reduced in a very less
amount.
BER ANALYSIS AT VARIOUS LENGHTS
Now we move towards the analysis of dispersion at various lengths of single mode fiber and
we have used the ideal dispersion fiber bragg grating and operating at frequency of 193.1
THz with effective index of 1.45 and length of 2mm and GOF as a DCB(dispersion
compensation bank) . We have used BER analyzer to analyze the effect various lengths on
BER, maximum Quality factor etc at 20dB power level. The signal is transmitted on SMF at
various lengths.
1. At fiber length of 50 km.
Figure 14: EYE Pattern of WDM system using DCB at 50km SMF
In this simulation we are observing that the Q factor is23.6626,Min BER 2.49579e-124,EYE
height .840911,Threshold .0344497.Q factors is highest than DCB AT 80,100 ,200km SMF
and without DCB at 80km SMF.OSNR is also improved. Noise is also reduced. EYE height
maximum, so dispersion is less.
2. At fiber length of 100 km
Figure 15: EYE Pattern of WDM system using DCB at 100km SMF
In this simulation we are observing that the Q factor is20.6256, Min BER 4.90473e-095, EYE
height .0827458, Threshold .00581346.Q factors is decreased than DCB AT 50,80 kms
SMF and without DCB at 80km SMF.OSNR is also reduced . EYE height is less that all other
method except 200km SMF, so dispersion is less than 200km WDM system with DCB.
3. At fiber length of 200 km
Figure 16: EYE Pattern of WDM system using DCB at 200km SMF
In this simulation we are observing that the Q factor is 7.87062, Min BER 9.40547e-016,EYE
height .048016,Threshold .00114661.Q factors is decreased than DCB AT 50,80,100 kms
SMF and without DCB at 80km SMF.OSNR is also reduced. EYE height is also reduced.
Threshold is also minimum in this case. It means that overall quality of system decreases by
increasing the length of the fibre, so dispersion is also increased in this system.
CONCLUSION
This paper briefly presents the analysis of chromatic dispersion at various lengths (50 km,
100 km & 200 km) of SMF for 10 Gbps fiber optics communication system. The analysis is
done on the basis of simulation results obtained from Optisystem software. Then a dispersion
compensation technique using DCB (dispersion compensation bank) is briefly discussed in
this paper and it has been shown that the dispersion effect get mitigates more effectively by
using DCB by using BER analyzer eye diagram to analyze the BER and maximum Q. factor
etc; also it has been shown that BER decreases by increasing lengths at the same input power.
we can conclude that chromatic dispersion effect can be mitigated using DCB by inducing the
attributes of IFBG or FBG and GOF in an appropriate proportion in the same segment of
single mode fiber. So instead of using IFBG or FBG and GOF separately, we can use DCB
which is more efficient to mitigate the effect of chromatic dispersion in SMF having 10 Gbps
data rate DCB is also cost effective because it has the attributes of two dispersion
compensation components in a single package. The idea of DCB is presented for first time in
this work and DCB will be very helpful to compensate the effect of chromatic dispersion in
future.
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