comparative analysis of self phase modulation spm and cross phase modulation cpm

ISSN: 2277 – 9043

International Journal of Advanced Research in Computer Science and Electronics Engineering

Volume 1, Issue 3, May 2012

97 All Rights Reserved © 2012 IJARCSEE

Abstract---- Use of optical fiber communication is widely

use due to its better bit rate and bandwidth and high

carrier frequency with low power consumption. So, in

this paper, we have analyzed non linear modulation

techniques that are self phase modulation (SPM) and

cross phase modulation (CPM) in an optical fiber

system and discussed how these cause dispersion in

input signal. These effects are simulated using

OPTISYSTEM tool at a bit rate of 10Gbps and

analyzed by eye pattern method with respect to bit

error rate and Q factor. Simulation results of self phase

modulation and cross phase modulation obtained in

OPTISYSTEM tool which is made by OPTIWAVE

INC. are compared with each other. Formula for bit

error rate (BER) is implemented in MATLAB and its

value is obtained by taking the value of Q factor from

the design implemented in OPTISYSTEM and further

variations in the value of BER are studied for both

types of non linear effects and see that which type of

modulation is better for long transmission in single

mode optical fiber.

Keywords--- Self phase modulation, Cross phase

modulation, Fiber nonlinearities, Optisystem tool.

I .INTRODUCTION

The developments of low loss optical fiber, optical

transmitter, optical detector and optical amplifier

with compact size and high efficiency have

dominated the field of telecommunication. When

optical signal is transmitted at distances typically

longer than 100 km, they suffer from attenuation,

temporal broadening and even interact with each

other through non linear effects in the optical fiber.

Transmitter capacity and performance of the system

are greatly affected by the non linear effects. The

main requirement of the optical system is to increase

the spacing between optical repeaters in the link,

which in turn requires higher optical power to

achieve the desired signal to noise ratio (SNR). With

the increase in optical power, bit rate, and number of

Wavelength channels, the total optical power

propagating through the optical fiber increases and

hence, results in non linear effects. These non linear

effects include self phase modulation (SPM), cross

phase modulation (XPM), four wave mixing (FWM),

stimulated brillounin, stimulated raman scattering

(SRS). Although, these effects have several

disadvantages but there are certain advantages also,

such as, formation of dispersionless pulses (solitons)

with the help of SPM; realization of low noise optical

amplifier using SRS; in signal processing using

XPM; or in the realization of wavelength converter

using FWM.

Fig.1 types of non linearity effects

This paper deals with the analysis of reducing non

linear dispersion, induced distortion in single mode,

Comparative analysis of self phase modulation

(SPM) and cross phase modulation (CPM)

Ruby Verma, Pankaj Garg

B.Tech ECE, Lovely Professional University, Punjab

ISSN: 2277 – 9043




non linear fiber and erbium doped fiber amplifier

(EDFA). Also, analysis of various fiber non linear

designs are done and compared with each other over

long haul distance of 100 km.

A. Self phase modulation

Non linear phase modulation of beam, caused by its

own intensity by the kerr effect. Due to kerr effect

high optical intensity in medium causes a non linear

phase delay which has same temporal shape as

optical intensity. This can be described as a non

linear change in refractive index [1]. phase

modulation of an optical signal by itself is known as

SPM. SPM generally occurs in single wavelength

system. it occurs through interaction of rapidly

varying and time dependent laser pulse with non

linear intensity dependent change in refractive index

of an optical material. At high bit rate SPM tends to

cancel dispersion , but it increases with signal power

level.

Phase shift by field over fiber length is given by:

2 nL

[5]

Where, n= refractive index of the medium; L= length

of the fiber; = Wavelength of the optical pulse

B Cross phase modulation

CPM is the change in optical phase of light beam

caused by interaction with other beam in non linear

medium, especially kerr medium. CPM results from

different carrier frequencies of independent channels,

including the phase shift of one another. The induced

phase shift is due to the walkover effect, where two

pulses at different bit rate or with different group

velocities walk across each other. CPM converts

power fluctuation in particular wavelength channel to

phase fluctuation in other co-propagating channel.

Expression for phase shift caused by non linear effect

is given as:

2N

i

nl nl eff i n

n i

k L P P

[5]

N= N-channel transmission system, n= 1, 2, 3…...N,

effL = Effective length of link nlk = Propagation

constant

The designs of CPM and SPM are stimulated using

Optisystem tool. Coding of non linear schrodinger

equation is done in Matlab and analysis of Eye

diagram, bit error rate (BER), and Q factor is done.

II. SIMULATION MODEL

Simulation model of SPM and CPM is implemented

in optisystem. It consists of a transmitter block, fiber

channel and a receiver block.

A. Scenario 1: Model of SPM

Conceptual design of SPM consists of an optical

transmitter, channel and receiver.

Fig.2 Conceptual model of SPM

1) Transmitter block:

It consists of a pseudo random generator, NRZ

modulator, continuous wave laser, Mach-Zehnder

amplitude modulator and EDFA amplifier. Each

component block has its own parameters apart from

the parameters of the design called as global

parameters, which are helpful if we want to use the

same parameter for two or more components in the

model. Wavelength, frequency, power of the signal is

initialized and phase parameter of signal is set to

random in CW laser block. We placed electrical and

optical oscilloscope to observe waveforms. The

transmission rate used is 10 Gbps, power of light

ISSN: 2277 – 9043




wave is 3.98mW, fiber length is 100 km, wavelength

is 1550nm and frequency is 193.1THz.

2) Fiber channel:

It is shown as iterative loop component. The iterative

loop component consists of an optical fiber, fiber

compensating techniques and a pre-amplifier. Output

of fiber is sent to fiber bragg grating which is used to

compensate the distortion of signal by inducing

dispersion after each stage. Dispersion coefficients

used are 0ps/nm, -500ps/nm, -1000ps/nm, -

1500ps/nm, -2000ps/nm.

3) Receiver block:

It consists of EDFA, photodiode, low pass Bessel

filter whose cut off frequency is 0.7 * bit rate, BER

analyzer and an electrical oscilloscope.

B. Result analysis

Input signal is shown in figure 3 which is visualized

as almost a sinusoidal waveform. the output of CW

laser is sent to Mech-Zehnder modulator which is an

electro-optical modulator, used to modulate the light

wave with respect to transmitted electrical signal and

generate an optical signal at output of modulator. The

optical signal before and after the booster block with

factor 10 is shown in fig. 5 and fig. 6 respectively.

Fig. 7 shows the output of pin diode , Fig. 8 shows

the output of low pass Bessel filter and finally the

results are analysed using bit error rate and eye

diagram in Fig. 9.

Fig.3 Electrical signal of NRZ modulator

Fig.4 Signal of CW laser block

Fig.5 Output of Mach-Zehnder modulator

Fig.6 Output signal after EDFA

ISSN: 2277 – 9043




Fig.7 Output of PIN diode

Fig.8 Output of Low Pass Bessel filter

Fig.9 BER waveform and EYE diagram

C. Scenario2: Cross Phase Modulation

In CPM all the parameters (pseudo random generator,

NRZ modulator, continuous wave laser, Mach-

Zehnder amplitude modulator and EDFA amplifier)

are same except the usage of two transmitter WDM

system and after modulating signal through Mach-

Zehnder modulator, signal is sent to the ideal MUX

which combine the two different signals of two

different frequencies of 193.1 THz and 192.4 THz. In

CPM we use band pass filter whose bandwidth is

1.5*bit rate. Rest is same like the SPM.

Figure10. Conceptual model of CPM

D. Result analysis

The output of Mach-Zehnder modulator is shown in

Fig. 11, output signal after EDFA block and output of

pin diode are shown in Fig. 12 and Fig. 13

respectively. Then the signal passes through band

pass bessel filter and output obtained is shown in

Fig. 14 and then results are analysed using eye

diagram in Fig. 15.

Fig.11 Output of Mach-Zehnder modulator

ISSN: 2277 – 9043




Fig.12 Output signal after EDFA

Fig.13 Output of PIN diode

Fig.14 Output of Band Pass Bessel filter

Fig.15 BER waveform and EYE diagram

E. Comparison between SPM and CPM

We have compared two nonlinear effects and

concluded which nonlinear effect is more

advantageous and why. Comparisons are based on Q

factor. The nonlinear dispersion due to SPM is

considered advantageous only when one WDM

channel is used. . One has to compromise with the

transmission rate and number of channels in the

design. More channels leads to reduction in the

transmission rate for an optimized design. The Q

factor for SPM design model is 5.849. Q factor is

related to SNR and is inversely proportional to BER.

To calculate BER value from Q factor generated

from design, we wrote a software algorithm in

Matlab to calculate BER with any value of Q factor.

High Q factor shows that the signal is less immune to

noise and received signal is similar to input signal

with less noise. In today’s world, challenge is to have

more than one channel and higher transmission rate

to receive the signal without distortion when

transmitted through fiber link. To achieve above, we

will be considering CPM dispersion effects. As

discussed earlier, CPM designs have two WDM

channels in the design. The eye pattern for CPM has

Q factor of 5.0109. We have the same power for both

channels; can see the improvement in the Q factor.

Some of the parameters observed using electrical

scope and BER analyzer in Optisystem tool is given

in Table.

ISSN: 2277 – 9043




Parameters SPM CPM

Bit Rate 10Gbps 10Gbps

Channel 1 2

Q factor(linear) 5.849 5.0109

Q factor(in db) 7.6708 6.9992

EYE Opening 0.00101139 0.00488261

BER 2.41907*10-9 2.70798*10-7

Table I. Comparison of SPM and CPM

Parameters SPM CPM

Q factor(in db) 7.6708 6.9992

BER(in

Optisystem)

2.41907*10-9 2.70798*10-7

BER(in Matlab) 8.6870*10-15 1.3124*10-12

Table II. Comparison of BER in MATLAB

III. Conclusion

This project dealt with analysis of self phase and

cross phase nonlinear effects in optical system. Non-

linear effects have disadvantages in the form of

limiting the transmission rate but have an advantage

of improving performance of transmitted signal in the

system. Q factor is known as digital SNR and it is

defined as ratio of signal current to noise current.

Optical communication system bit error rate less than

10-12 is to be achieved which corresponds for

obtaining Q > 7. If BER <10-9 then Q>6. By

theoretical implementation of SPM in Matlab bit

error rate obtained is 8.6870*10-15, but by practical

analysis of SPM in optisystem BER obtained is

2.41907 * 10-9. In CPM in Matlab bit error rate

obtained is 1.3124*10-12, but in optisystem BER

obtained is 2.70798 * 10-7. So from above result we

can conclude that SPM is better than CPM for single

mode fiber channel.

REFERENCES

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[3] Govind P Agarwal, “Fiber Optic communication

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152

[4] Optiwave,“Optisystem user guide and application

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[6] Govind P Agarwal, “Nonlinear fiber optics.” Springer-

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comparative analysis of self phase modulation spm and cross phase modulation cpm

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