kamlesh report
TRANSCRIPT
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CHEPTER:-1
INTRODUCTION
1. INTRODUCTION
The need of communication is an all time need of human beings. For communication some
channel is needed. Fiber is one channel among many other channels for communication. The
dispersion phenomenon is a problem for high bit rate and long haul optical communication
systems. An easy solution of this problem is optical solitonspulses that preserve their shape
over long distances. Soliton based optical communication systems can be used over distances
of several thousands of kilometers with huge information carrying capacity by using optical
amplifiers. Recently optical fiber drawing technology has become so much potential that less
than 102db / km loss is possible.
We know also that the loss depends on the intensity of the
input pulse. For lower intensity based input pulse, only the linear refractive index term
contributes the loss and dispersion. Optical losses due to many other reasons in the fiber cavity
play a major role in the wave propagation. For high intensity coherent pulse, both linear and
non-linear refractive index will influence the optical pulse in its propagation [1-4]. In optical
fiber an important non-linear effect caused by intensity dependent refractive index goes
according to the following equation
n= n1+n2I
Where 0 n is the linear refractive index of the core medium of the optical fiber and 2 n is the
non-linear correction term and I is the effective intensity. The intensity dependent refractive
index leads to the phenomenon known as self-phase modulation and it is the main reason for
spectral broadening, which signifies the generation of additional frequencies. Optical pulse
propagation through a linear dispersive medium undergoes temporal broadening as well as
chirping at wavelength higher than the zero dispersion wavelengths, the instantaneous
frequency decreases with increasing time.
A pulse propagating through a non-linear non-
dispersive medium undergoes no temporal broadening but undergoes only chirping. A solitary
pulse is a pulse that travels without dependence of other pulses and a soliton is the solitary
wave pulse whose shape and speed are not altered by a collision. However the term soliton
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indicates in general a peculiar solitary wave whose propagation is framed by non-linear
dispersive equation. Here as the non-linearity and dispersion effect balance each other to
maintain the temporal and spectral profile unaltered. Such a pulse would broaden neither in the
time domain (as in linear dispersion) nor in the frequency domain (as in self-phase modulation)
and is called a soliton.
Soliton has tremendous potentialities because it does not broaden
during its journey through the fiber so it can be used in super high bandwidth and very low loss
optical communication system. Thus optical soliton is very much useful in optical fiber based
telecommunication system over hundreds of thousands kilometers in reality. For a soliton
propagating around a 1550 nm wavelength, the peak power t P in the pulse (in mw) and pulse
duration are related by the equation,
Where D is the dispersion coefficient in ps/Km-nm and f is the FWHM (full width half
maxima) of the pulse in picoseconds. Thus for a 10 ps soliton operating in a dispersion
shifted fiber with D=1 ps/km-nm the required peak power will be approximately 15 mw. In this
communication we propose a new concept of interacting soliton pulses at a long distant point inthe fiber to organize logic operation there. This operation is basically controlled from several
kilometers of distance from the place of operation.
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CHEPTER:-2
SOLITON BASED OPTICAL COMMUNICATION
2.1 SELF-PHASE MODULATION
Self-phase modulation (SPM) is the frequency change caused by a phase shift induced by the
pulse itself. SPM arises because the refractive index of the fiber has an intensity dependent
component. When an optical pulse travels through the fiber, the higher intensity portions of an
optical pulse encounter a higher refractive index of fiber compared with the lower intensity
portions.
The leading edge will experience a positive refractive index gradient (dn/dt) and
trailing edge a negative refractive index gradient (dn/4dt). This temporally varying index
change results in a temporally varying phase change. The optical phase changes with time inexactly the same way as the optical signal [1, 15]. Different parts of the pulse undergo different
phase shift because of intensity dependence of phase fluctuations. This results in frequency
chirping. The rising edge of the pulse finds frequency shift in upper side whereas the trailing
edge experiences shift in lower side as shown in Figure 1. Hence primary effect of SPM is to
broaden the spectrum of the pulse, keeping the temporal shape unaltered. For a fiber containing
high-transmitted power, the phase () introduced by a field
over a fiber lengthL is given by
(1)
where effective length
Linear and nonlinear refractive indices are nl and nnl respectively and is attenuation
coefficient. The first term on right hand side refers to linear portion of phase constant (l) and
second term provides nonlinear phase constant (nl). This variation in phase with time is
responsible for change in frequency spectrum. For a Gaussian pulse, the optical carrier
frequency (say) is modulated and the new instantaneous frequency becomes,
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(2)
The sign of the phase shift due to SPM is negative because of the minus sign in the expression
for phase i.e., (tkz). Using equations (1) and (2) _can be written as,
(3)
At leading edge of the pulse dI/dt > 0;
(4)
And at trailing edge dI/dt < 0;
(5)
Where,
(6)
Thus chirping phenomenon (frequency variation) is generated due to SPM, which leads to the
spectral broadening of the pulse without any change in temporal distribution. The SPM induced
chirp can be used to modify the pulse broadening effects of dispersion. SPM phenomenon also
can be used in pulse compression. In the wavelength region where chromatic dispersion is
positive, the red-shifted leading edge of the pulse travels slower and moves toward the center of
pulse. Similarly, the blue shifted trailing edge travels faster, and also moves toward the center
of the pulse. In this situation SPM causes the pulses to narrow. Another simple pulse
compression scheme is based on filtering self-phase modulation-broadened spectrum.
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Figure 2.1 Spectral broadening of a pulse due to SPM.
The performance of self-phase modulation-impaired system can be improved significantly by
adjusting the net residual (NRD) of the system. For SPM-impaired system the optimal NRD
can be obtained by minimizing the output distortion of signal pulse. The NRD of SPM-
impaired dispersion-managed systems can be optimized by a semi analytical expression
obtained with help of perturbation theory. This method is verified by numerical simulations for
many SPM-impaired systems.
2.2 GROUP VELOCITY DISPERSION
Any information-carrying signal, by necessity, contains components from a range of
wavelengths. The group velocity of a signal is function of wavelength, therefore each spectral
component can be assumed to travel independently and to undergo a group delay, which
ultimately results in pulse broadening. This GVD will eventually cause a pulse to overlap with
neighboring pulses. After a certain amount of overlap, adjacent pulses can not be identified at
the receiver and error will occur. In this way the dispersive characteristics determine theinformation carrying capacity of fibers. The group delay (g) per unit length in direction of
propagation is given by
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(7)
where,L is the distance traveled by the pulse,is the propagation constant along fiber axis,
wave propagation constant
and group velocity
The delay difference per unit wavelength can be approximated as dg/d if taken optical source
is not of too vide spectral width. For spectral width , total delay difference over a distance
L, can be written as
(8)
where is angular frequency. The factor
is the GVD parameter, which determines how much a light pulse broadens in time as it is
transmitted over the fiber.
2.3 EVOLUTION OF SOLITON
The nonlinear Schrodinger equation (NLSE) is an appropriate equation for describing the
propagation of light in optical fibers. Using normalization parameters such as: the normalized
time T0, the dispersion length LD and peak power of the pulse P0 the nonlinear Schrodinger
equation in terms of normalized coordinates can be written as,
(9)
where u(z, t) is pulse envelope function, z is propagation distance along the fiber, N is an
integer designating the order of soliton and is the coefficient of energy gain per unit length,
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with negative values it represents energy loss. Here s is 1 for negative2 (anomalous GVD
Bright Soliton) and +1 for positive2 (normal GVD-Dark Soliton) as shown in Figures 2 and 3
and
with nonlinear parameter and nonlinear lengthLNL.
Figure 2.2 Evolution of soliton in normal dispersion regime.
Figure 2.3 Evolution of soliton in anomalous dispersion regime.
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It is obvious that SPM dominates forN > 1 while forN < 1 dispersion effects dominates. ForN
1 both SPM and GVD cooperate in such a way that the SPM-induced chirp is just right to
cancel the GVD-induced broadening of the pulse. The optical pulse would then propagate
undistorted in the form of a soliton. By integrating the NLSE, the solution for fundamental
soliton(N= 1) can be written
(10)
where sech (t) is hyperbolic secant function. Since the phase term exp (iz/2) has no influence on
the shape of the pulse, the soliton is independent ofz and hence is nondispersive in time
domain. It is this property of a fundamental soliton that makes it an ideal candidate for optical
communications. Optical solitons are very stable against perturbations; therefore they can be
created even when the pulse shape and peak power deviates from ideal conditions (values
corresponding toN= 1).
2.4 INFORMATION TRANSMISSION
A digital bit stream can be generated by two distinct modulation formats i.e., non-return-to-zero
(NRZ) and return-to-zero (RZ). The solution of NLS equation for soliton holds only when
individual pulses are well separated [5, 6]. This can be ensured by keeping soliton width a
small fraction of the bit slot. To achieve this, RZ format (Figure 4) has to be used instead of
NRZ format when solitons are used as information bits.
Figure 2.4 Soliton bit stream in an RZ format.
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The bit rateB and the width of the bit slot TB can be related as
where 2s0 = TB/T0 is the normalized separation between neighboring solitons.
2.5 SOLITON TRANSMITTERS
Soliton communication systems require an optical source capable of producing chirp free pico-
second pulses at a high repetition rate with a shape closest to the sech shape. The source
should operate in the wavelength region near 1.55 m. Early experiments on soliton
transmission used the technique of gain switching for generating optical pulses of 2030 ps
duration by biasing the laser below threshold and pumping it high above threshold periodically.
A problem with the gain switching technique is that each pulse becomes chirped because of the
refractive-index changes governed by the linewidth enhancement factor.
Mode-locked
semiconductor lasers can also be used for soliton communication and are often preferred
because the pulse train is emitted from such a laser is being nearly chirp-free. The grating also
offers a self-tuning mechanism that allows mode-locking of the laser over a wide range of
modulation frequencies. Such a source produces soliton like pulses of widths 1218 ps at a
repetition rate as large as 40 Gb/s . A compact, synchronously diode-pumped tunable fiber
Raman source of subpico second solitons can also be used which employs synchronous Raman
amplificat9ion in dispersion shifted fiber.
Wavelength tunability of 16201660nm is exhibited
through simple electronic variation of the gain-switching repetition frequency and solitons as
short as 400 fs are obtained. The use of femtosecond pulses enhances the capacity of the soliton
systems to a great extent. However, with the femtosecond optical pulses, it is necessary to
control their propagation characteristics. In the femtosecond pulse duration regime, the main
higher order nonlinear contribution comes from stimulated Raman scattering.
The Raman self-
frequency shift that results from the energy exchange between the propagating pulses and
optical vibrational modes of the glass precludes the stable propagation of subpicosecond
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solitons along the fiber, leading to rapid displacement of the pulse spectrum to the red (lower
frequency side) as it propagates. As a result, practical solitons in fibers often have durations of
1 ps or longer. A shorter pulse suffers self-frequency shifts of = L/T2 0 , where L and T0
are as defined as earlier. An adaptive feedback can be used to control the Raman frequency
shift of the output pulse, preserving its duration and intensity. Compact erbium doped fiber
lasers are promising sources for pulse generation because of their high stability, ease of use and
cost efficiency. One such laser was recently reported [22] which generates 30 ps pulses.
2.6 AMPLIFICATION OF SOLITONS
In long distance soliton propagation, the energy of soliton decreases because of fiber losses.
This would produce soliton broadening because a reduced peak power weakens the SPM effect
necessary to counteract the effect of GVD. Therefore, to overcome the effect of fiber losses
soliton must be amplified periodically using either lumped or distributed amplification scheme.
Lumped amplification is useful provided the spacing between amplifiers LA is less than
dispersion length LD (LA _ LD). For systems having bit rates greater than 10 Gb/s, the
conditionLA _ LD can not be satisfied. In such situation distributed amplification scheme is a
better alternative. This scheme
Figure 2.5 (a) Lumped and (b) Distributed amplification of solitons.
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is inherently superior to lumped amplification because it compensates losses locally at every
point along fiber link. Raman fiber amplifiers can be used for distributed gain when fiber
carrying the signal is pumped at wavelength about 1480 nm. Another way is to dope lightly the
transmission fiber with erbium ions and pump periodically to provide distributed gain. Solitons
can be propagated in such active fiber over long distances.
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CHEPTER:-3
INTERACTION OF SOLITON PULSES IN LONG DISTANCE
SWITCHING
3.1 INTERACTION BETWEEN TWO SOLITON PULSES
One of the main features in soliton communication system is the interaction between adjacent
pulses. To overcome this problem, several effective methods have been proposed. One main
factor in limiting the full utilization of bandwidth offered by soliton guided signal system is the
soliton-soliton interaction. The interaction occurs due to overlapping of frequency components
of either pulse. Depending on the phase difference between the soliton pulses non-linear
interaction may be either destructive or constructive. These interactions alter the phases and
positions of soliton pulses. While their carrier frequencies and amplitudes remains unaffected.To understand the interactions of soliton pulses, following cases should be studied must where
the interaction between the soliton pulses having
a. Equal amplitude with equal phase
b. Equal amplitude with unequal phase
c. Unequal amplitude with equal phase
d. Unequal frequency with equal phase
e. Equal frequency with unequal phase
3.2 EFFECT OF INTERACTION BETWEEN TWO SOLITON PULSES WITH
DIFFERENT FREQUENCIES BUT IN SAME PHASE
In a linear dielectric medium the electric polarization is assumed to be linear function of the
electric field, that is,
(1)
where (1) is the linear dielectric susceptibility,
E is the electric field and
P is the polarization.
At very high optical field intensity, if the guiding media behave as a non-linear one, then
polarization is expressed as,
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(2)
For non-crystalline isotropic media, such as silica-based glass optical fiber, (2) = 0 and lowest
order non-linearity arises due to the term (3) . Now considering two plane optical waves
propagating in z direction, the electric field variation of the waves form becomes
Therefore, using equation (3a) and (3b) in equation (2) we can get
(4)
Neglecting second harmonic and third harmonic generation of frequencies due to phase
mismatching in optical fiber, equation (4) becomes,
(5)
Also we know that
(6)
Where I1and I2 are intensities of the individual waves having the frequencies 1 and 2
respectively. Using equation (6) in equation (5) we get
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(7a)
=P1+P2 (7b)
where
(7c)
And (7d)
We know general relationship between polarization and refractive index as
(8)
Now comparing equation (7a) with equation (8) we get,
(9)
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(10)
Therefore refractive index
(11)
(12)
Where linear refractive index,
(13a)
and non-linear correction term
(13b)
Here it can be found that refractive index increases after interaction between two such soliton
pulses.
3.3 EFFECT OF INTERACTION AMONG THREE SOLITON PULSES WITH
DIFFERENT FREQUENCIES BUT IN SAME PHASE
Now we consider three plane optical waves propagating in z-direction and their electric field
variation of the form of,
(14a)
(14b)
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(14c)
Therefore polarization of equation (4) becomes
(15)
due to phase mismatching other than the frequencies 1 2 3 , and are to be neglected and
the integrated polarization at the place where the interactions happening, the equation (15) can
be written as,
(16)
Also we can write equation (16) as,
(17)
WhereI1 ,I2 and I3 are the intensities of the individual waves having the frequencies 1 ,2
and3 respectively. Now using equation (17) in equation (16) we get
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(18a)
(18b)
(18c)
(18d)
(18e)
We can rewrite equation (18) as the form of
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(19)
Now comparing equation (19) with equation (18a) one may get,
(20)
or
(21)
or
(22)
Here non-linear correction term becomes
(23)
Here also the refractive index of the media increases with increasing intensity (that is with the
number of increasing interacting soliton pulses having same phase).
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CHEPTER:-4
ANALYSIS OF OPTICAL NETWORK WITH SOLITON
PROPAGATION
4.1 ERBIUM DOPED FIBER AMPLIFIER (EDFA)
In real time situations, as the pulse propagates through the fiber, the loss of energy due to the
fiber loss (absorption of energy by the fiber) has to be taken into account. This loss in energy
can be restored by the amplification of pulses using erbium-doped fiber amplifier (EDFA) and
it requires periodic pumping along the fiber length. EDFAs are the active components in optical
communication field due to its promising advantages such as high polarization insensitive gain,
low noise, wider bandwidth and high saturation output power.
EDFA helps to boost the optical
signal used in fiber optics. Fiber is doped with rare earth erbium so that it can absorb light at
one frequency and emit at another frequency. Standard fiber have low loss in 1.55m
wavelength region and dispersion of 17 ps/km/nm in 1.55m region. EDFAs operate in 1.55m
communication window and they do not retime or reshape the optical signal. These amplifiers
are widebandwidth amplifiers and data rate transparent and hence regenerators are replaced by
EDFAs. But the other design problems due to the dispersion of the optical fiber in the 1.55m
must be considered. Erbium amplifiers offer attractive prospects for the implementation ofultralong lossless transmission lines for wider applications. It can act as a pre-amplifier,
repeater and power amplifier.
The gain bandwidth of EDFA extends from 1525 to 1565nm and
hence it covers considerable part of the low loss window. Amplifier spacing plays an important
role in long-haul soliton communication systems. Although fiber losses are compensated by
amplifiers but the amplification process is accompanied by the spontaneous emission noise.
However, by using an appropriate filter, the noise that is outside the bandwidth of the optical
signal can be removed.
4.2DISPERSION COMPENSATING FIBER (DCF)In 1991, the idea of using
dispersion compensation for soliton transmission was first reported by Ellis et al. There are
several techniques to resolve the issue of dispersion. Some of them are the usage of dispersion
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compensating fiber and fiber bragg grating. Dispersion compensation using DCF is simple and
effective but requires large additional fiber lengths and greater optical gain leading to
additional amplifier noise. DCF has high negative dispersion in the 1.55 m window. The
average dispersion of each of the amplifier link can be reduced by placing a length of DCF
prior to the active fiber. Length of the DCF must be short because the loss introduced by them
is higher than the standard fiber. The loss is around 0.5 dB/km at 1.55m and hence the length
of the DCF is shorter. Around 1km of DCF is required for 10 to 12km of standard fiber reduce
the dispersion. The length of DCF LDCF required is calculated using the relation
,
(1)
where
DSMF is the dispersion of SMF
LSMF is the length of SMF segment and
DDCF is the dispersion of DCF segment
The average dispersion across the transmission link is given by,
4.3 SYSTEM DESIGN
The system model used for soliton propagation with dispersion compensation using DCF is
shown in Fig.
Transmitter section includes PRBS generator and a soliton/Non-Return-to-Zero (NRZ)-
rectangular transmitter. Soliton transmitter consists of a mode locked laser source (MLL) to
generate optical signal, electrical signal generator and a modulator. Fundamental soliton (N=1)
is considered for simulation.
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Figure 4.1 Simulation system model of soliton communication system with dispersion
Compensation
Fundamental soliton (N=1) is considered for simulation. The laser source peak power is set to
11.56mW and the pulse type is chosen to be sech. The relative intensity noise (RIN) is set to
-150dB/Hz. To generate rectangular pulse, NRZ-rectangular transmitter is used, which consists
of a continuous wave laser source with peak power of 1mW, electrical signal generator and
modulator. Wavelength of the laser source is 1550nm which is the low loss optical window.
PRBS generator generates binary sequence of pattern length 128bits. The simulation is
performed for four different data rate: 10 to 40Gbps in steps of 10Gbps.
The input binary
sequence is converted into NRZ format electrical signal using electrical signal generator.
Modulator of Mach-Zehnder type is used which modulates the light to represent the binary data
that is received from the source. Signal analyzer which represents the signal waveform of the
incoming signal, is connected to the modulator and it displays the signal that is transmitted
through the fiber.
Soliton pulses are generated in 1.55m region and it is transmitted through
fibers arranged in a recirculating loop which will repeat for 200 times. Each loop consist of
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four optical amplifiers (EDFA) which overcomes the fiber losses and two segments of single
mode fiber (SMF) and two segments of DCF to compensate the accumulated dispersion in
SMF. The total length of the fiber link is 12,000km.The length of the fiber where dispersion is
to be compensated is of length 25Km and the length of the DCF is 5Km. The dispersion slope
of SMF is 0.07 ps/km/nm2 and the dispersion parameter is 16.5 ps/km/nm. Loss of the SMF
segment is 0.2 dB/km. The gain of the EDFA placed after the fiber to compensate the fiber loss
is 6 dB. The dispersion slope of DCF is -0.07 ps/km/nm2 and the dispersion parameter is - 82.5
ps/km/nm2. Thus the total accumulated dispersion in the loop is zero. The dispersion
wavelength is set to 1.55 m. The loss of the DCF segment is 0.22 dB/km and the gain of the
EDFA placed after this segment is 0.66 dB. The saturation power of EDFA is 100dBm.
The
constant nonlinear refractive value of the fiber is chosen as 2.6x10-20 m2/W. The core
diameter is 8.2 m. The effective core area of SMF is 80m2 and for DCF is 20m2. The
nonlinearities such as SPM, crossphase modulation (XPM) and stimulated raman scattering
(SRS) are included for simulation.
Property map is connected to the fibers and
EDFAs in the loop and it generates dispersion, optical power and pulse width (FWHM) map
from span to span along the transmission link. At the output of the loop, optical normalizer is
connected, which normalizes the optical signal power by attenuating the input optical signal to
the specified average output power level. It attenuates all input optical signal to the same
average output power irrespective of their different average input powers and hence the signal
with the largest average input power will have the specified average output power. In our
simulation, the average output power of the normalizer with uniform attenuation is considered
in the range of -25 dBm to -20 dBm. The optical signal from the normalizer is given to the
receiver which is followed by BER tester and eye diagram analyzer. Receiver section is
composed of a photodetector, a preamplifier and a postamplifier/filter. An avalanche
photodiode (APD) is used to convert the optical signal into an electrical current. Its quantum
efficiency is 0.8. The preamplifier model converts the photocurrent into voltage. Postamplifier
consist of set of baseband filters to shape out the waveform. Low pass Bessel filter of order 4 is
used to reduce the inter symbol interference (ISI).
The performance of the system is evaluated using
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BER, Q-factor, OSNR and Eye-diagrams. BER tester computes BER, quality factor of the input
electrical signal and electrical eye properties such as the height, width, area and closure. PRBS
generating the binary data is connected to the BER tester and the binary value of every
transmitted bit is compared against the value of the same bit at the receiver. It counts the events
in which the comparison fails. Q-factor specifies the signal-noise ratio of the received data eye
and it is directly related to BER. Q-factor is given by
(12)
where 1,0 specifies the mean value of the marks/spaces rail in the eye diagram and 1,0
specifies the standard deviation. BER indicates the faithfulness of the link in transmitting the
data from the source to the receiver and it is obtained from Q-factor. BER can be obtained
using the relation
(13)
For the BER of 10-9, Q must be greater than 6. The above equation (13) specifies the upper
limit for BER. Large value of the Q-factor indicates the pulse is free from noise. OSNR is
computed by the optical monitor connected at the end of the re-circulating loop. It specifies the
quality of the signal. OSNR, ratio of the signal power to the noise power, is computed using the
relation,
(14)
where Pin is the input power, is the loss (attenuation) of the fiber, NF is the amplifier noise
figure and N is the number of amplifiers in the transmission link. SNR in optical system is
degraded by various factors such as optical noise, fiber chromatic dispersion, polarization mode
dispersion and fiber nonlinearities. The Eye diagram analyzer displays the eye diagram which
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represents the visual depiction of a waveform. It allows quick verification of the signal
performance and specifies the data handling ability of the transmission system. An open eye
pattern indicates minimum signal distortion. Distortion due to ISI and noise results in closure of
the eye pattern.
4.4 RESULTS AND DISCUSSION
Simulations were carried out for pulse width ranging from 5 ps to 50 ps for the distance of
12,000 km with zero cumulative dispersion and bit pattern length of 128 bits which is preceded
by two bits and followed by three bits. The impact on bit stream over a long- haul soliton
transmission link of 12,000 km without including the noise figure of the amplifier has been
analyzed. As the data rate increases, the soliton width is reduced and results in soliton
interaction which reduces the transmission distance. Thus the performance of the system is
considerably reduced.
Figure 4.1 Signal transmitted through the fiber
The optical soliton signal at the input end of the fiber is shown in Fig. 4.1 Dispersion map for a
single span with dispersion compensation is shown in Fig.4.1 The dispersion parameter of SMF
segment is 16.5 ps/km/nm and it increase along with the distance. The accumulated
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positive dispersion in SMF is 412.5 ps/nm and it is compensated by DCF segment resulting in
zero cumulative dispersion at the end of the span.
The pulse width (FWHM) map obtained
with dispersion compensation is shown in Fig. 4.1 Due to the positive dispersion, the pulse
broadening takes place and the pulse width ranges from 25 ps to 57.4 ps for 10 Gbps data rate.
But the negative dispersion of the DCF segment compresses the pulse width considerably to 25
ps. Using the BER tester, it is found that the BE achieved by the soliton pulse for the receiver
sensitivity of -25dBm ranges from 5.45x10-16 for 10 Gbps rate to 1.34x10-01 for 40 Gbps data
rate. For -20dBm normalize average power output, BER ranges from . BER increases and the
Q-factor degrade at higher data rates due to pulse broadening.
Figure 4.2 Dispersion map (single span) and pulse width (FWHM) map of soliton
communication system with dispersion compensation
Fig. 4.2 shows the BER obtained for soliton pulse with normalizer average power output of -
20dBm. Fig. 4.2 shows the OSNR obtained for soliton pulse. For 10Gbps data rate, error rate
increases from 5.45x10-16 to 5.52x10-129 and the Q-factor increases from 18 dB to 27 dB.Almost 50% performance improvement can be achieved with the increase I receiver sensitivity.
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Figure 4.3 (a) Error rate for normalizer average power output of -20 dBm
(b) OSNR obtained for soliton pulse for different data rates over a distance of
12,000km with dispersion compensation.
In long-haul transmission system, Forward error correction (FEC) coding techniques help
optical links to achieve higher performance and also to increase the transmission distance and
the repeater spacing. Higher the FEC redundancy, larger the FEC coding gain. Fig.4.3 shows
the Q-factor obtained for soliton pulse without FEC and with FEC with the normalizer average
power output of -25dBm. By employing FEC, the error rate is found to be reduced from
5.45x10-16 to 6.04x10-11
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Figure 4.4 Q-factor (dB) obtained for soliton pulse for different data rates over a
distance of 12,000km with dispersion compensation.(a)without FEC (b)with FEC
Fig. 4.4 specifies the eye diagram obtained for soliton pulse with average power output of the
normalizer as -25 dBm and -20 dBm for 10 Gbps data rate. Completely open eye indicates
error-free communication with minimal distortion. The eye opening (eye height) of soliton
pulse in Fig. 4.4 is 11.3 V and the eye width is 36 ps. The eye opening in Fig. 4.4(b) is 42.78
V and the eye width is 45.22 ps. The height of the eye opening indicates the immunity to
noise.
Fig 4.5 Eye diagrams of soliton pulse for 10Gbps data rate with normalizer power output of
(a)-25 dBm (b) -20 dBm.
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Figure 4.6 Eye diagrams for 20 Gbps data rate with normalizer power output of
(a)-25 dBm (b) -20 dBm.
Fig. 4.6 specifies the eye diagram obtained for soliton pulse with average power output of the
normalizer as -25 dBm and -20 dBm for 10 Gbps data rate. Completely open eye indicates
error-free communication with minimal distortion. The eye opening (eye height) of soliton
pulse in Fig.4.6(a) is 11.3 V and the eye width is 36 ps. The eye opening in Fig.4.6(b) is 42.78
V and the eye width is 45.22 ps. The height of the eye opening indicates the immunity to
noise.
Figure 4.7 Eye diagrams for 30 Gbps data rate with normalizer power output of
(a)-25 dBm (b) -20 dBm.
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Fig.4.8 shows the eye diagram of soliton pulse for 30 Gbps data rate. The height of the eye
opening for soliton pulse is 4.13 V and eye width is 0.93 ps. For-20 dBm, eye opening is
25.37 V and eye height is 14.37 ps.
Figure 4.8 Eye diagrams for 40 Gbps data rate with normalizer power output of
In Fig.4.9, eye diagram of soliton for 40 Gbps data rate is shown. The eye is completely closed
for -25 dBm average power output of the normalizer. But for average power output of -20 dBm,
the height of the eye opening is 12.54 V and eye width is 2.46 ps.
Figure 4.9 Eye diagrams of rectangular pulse (a) 10 Gbps (b) 20 Gbps.
Fig.4.10 shows the eye diagram of rectangular pulse for 10 and 20 Gbps data rate with
normalizer average power output of -25 dBm. The eye opening in Fig. 4.10 (a) is found to be
3.79 V and the eye width is 71.1 ps. In Fig. 10 (b),the eye height is 1.48 V and eye width is
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1.92 ps. The above measures indicate that the eye opening of soliton pulse is 2.9 times larger
than that of the rectangular pulse for 10Gbps data rate. As the data rate is increased, the height
of the eye opening is reduced. The above eye pattern also confirms the ability of the soliton
pulse to propagate for data rate upto 30 Gbps over a distance of 12,000km with dispersion
compensation scheme and improved receiver sensitivity. For higher data rates, the propagation
distance is limited due to pulse interaction. Further analysis can be made for higher data rates
and for long distance.
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CHEPTER:-5
CONCLUSION
The performance of the optical communication system with soliton pulse is compared with the
NRZrectangular pulse. The performance is analyzed using BER, Q-factor and eye diagrams
over 12,000km distance for data rates from 10 to 40 Gbps. To obtain improved performance,
the optimum placement of EDFA plays a vital role. The use of dispersion management along
with periodic amplification using EDFA, helps the soliton pulse to propagate for longer
distance, provided the system performance is not limited by other factors.
It is established in the above discussion that when number of
soliton pulses having same phases interact with each other the interaction enhances the
refractive index at the point where they meet. This enhancement depends on the intensities ofthe participating pulses individually. This character can be exploited very nicely in long
distance remote switching through optical fiber.
The change of refractive index in optical fiber
media by interaction between two or among more numbers of soliton pulses can lead leakage of
light from the fiber at the interaction point. Many other similar incidences can be organized by
this interaction and these can be directly utilized in long distance switching. Optical logic
operation can be conducted from a long distance if the interaction can be organized properly.
We can see also from the above discussion that the change of refractive index will not depend
at all of the frequencies of the carrier waves of the interacting soliton pulses.
Soliton based optical
fiber communication systems, using EDFAs, are more suitable for long haul communication
because of their very high information carrying capacity and repeater less transmission. These
systems are still to be developed for field applications. When transmission demand will
increase and device technology will improve, they will be certainly employed in field. By using
soliton based optical switches multi GBPS data rate can be achieved for optical computation
also.
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