Kamlesh Report

7/30/2019 Kamlesh Report

1/33

1

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


2/33

2

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.


3/33

3

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,


4/33

4

(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.


5/33

5

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


6/33

6

(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,


7/33

7

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.


8/33

8

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.


9/33

9

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


10/33

10

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.


11/33

11

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.


12/33

12

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,


13/33

13

(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


14/33

14

(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)


15/33

15

(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)


16/33

16

(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


17/33

17

(18a)

(18b)

(18c)

(18d)

(18e)

We can rewrite equation (18) as the form of


18/33

18

(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).


19/33

19

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


20/33

20

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.


21/33

21

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


22/33

22

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


23/33

23

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


24/33

24

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


25/33

25

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.


26/33

26

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


27/33

27

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.


28/33

28

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.


(a)-25 dBm (b) -20 dBm.


29/33

29

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.


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


30/33

30

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.


31/33

31

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.


32/33

32

REFERENCES

1. Stolen, R. H. and C. Lin, Self-phase modulation in silica optical fibers, Physical Review

A, Vol. 17, No. 4, 14481453, 1978.

2. Agrawal, G. P., Fiber Optic Communication Systems, 2nd edition, Wiley, New York, 1997.3. Hasegawa, A. and Y. Kodma, Soliton in Optical Communication, Clarendon Press, Oxford,

1995.

4. Haus, H. and W. S. Wong, Soliton in optical communications, Rev. Mod. Phys., Vol. 68,

432444, 1996.

5. Haus, H. A., Optical fiber solitons: Their properties & uses, Proc. IEEE, Vol. 81, 970983,

1993.

6. Chraplyvy, A. R. and R. W. Tkach, Terabit/second transmission experiments, IEEE J.

Quantum Electron., Vol. 34, 21032108, 1998.

7. Tauser, F., F. Adler, and A. Leitenstorfer, Widely tunable sub - 20 fs pulses from a compact

erbium-doped fiber source, Opt. Lett., Vol. 29, 516, 2004.

8. Yang, Y., C. Lou, H. Zhou, J. Wang, and Y. Gao, Simple pulse compression scheme based

on filtering self-phase modulationbroadened spectrum and its application in an optical time-

division multiplexing systems, Appl. Opt., Vol. 45, 75247528, 2006.

9. Biswas, A. and S. Konar, Soliton-solitons interaction with kerr law non-linearity, Journal

of Electromagnetic Waves and Applications, Vol. 19, No. 11, 14431453, 2005.

10. Sawetanshumala, S. J. and S. Konar, Propagation of a mixture of modes of a laser beam in

a medium with saturable nonlinearity, Journal of Electromagnetic Waves and Applications,

Vol. 20, No. 1, 6577, 2006.

11. Mandal, B. and A. R. Chowdhary, Spatial soliton scattering in a quasi phase matched

quaderatic media in presence of cubic nonlinearity, Journal of Electromagnetic Waves and

Applications, Vol. 21, No. 1, 123135, 2007.

12. Xiao, X. S., S. M. Gao, Y. Tian, and C. X. Yang, Optimization of the net residual

dispersion for self- phase modulation-impaired systems by perturbation theory, J. lightwave.

Tech., Vol. 25, No. 3, 929937, 2007.


33/33

13. Xiao, X. S., S. M. Gao, Y. Tian, and C. X. Yang, Analytical optimization of net residual

dispersion in SPM-limited dispersionmanaged systems, J. Lightwave. Tech., Vol. 24, No. 5,

20382044, 2006.

14. Yang, Y., C. Lou, H. Zhou, J. Wang, and Y. Gao, Simple pulse compression scheme based

on filtering self-phase modulationbroadened spectrum and its application in an optical time-

division multiplexing systems, Appl. Opt., Vol. 45, 75247528, 2006.

15. Singh, S. P. and N. Singh, Nonlinear effects in optical fibers: Origin, management and

applications, Progress In Electromagnetics Research, PIER 73, 249275, 2007.

16. Optical Communications Essentials, The McGraw- Hill companies, 2004.

17. K. Porsezian, A.Hasegawa, V. N. Serkin, T. L. Belyaeva and R. Ganapathy, Dispersion

and nonlinear management for femtosecond optical soliton, ScienceDirect, Physics letters A

361, pp504-508, 2007.

18. V. C. Kuriakose, K. Porsezian, Elements of optical soliton: an overview, Resonance, pp

643-666, July 2010.

19. Linn. F. Mollenauer and James. P. Gordon, Solitons in optical fibers: fundamentals and

applications, Elsevier, 2006.

20. Rajiv Ramaswami, Kumar N. Sivarajan Optical Networks: A Practical Perspective,

Morgan Kauffman, 2000.

21. Finlay M. Knox , Wladek Forysiak, and Nicholas J. Dorm, 10 -Gbt/s soliton

communication systems over standard fiber at 1.55 m and the use of dispersion

compensation, Journal of Lightwave Technology, Vol 13, No. 10, pp. 1955-1962, October

1995.

Documents

Kamlesh Report