IJRIT International Journal of Research in Information Technology, Volume 2, Issue 3, March 2014, Pg: 316-321

International Journal of Research in Information Technology (IJRIT) www.ijrit.com

ISSN 2001-5569

Performance Enhancement of the Optical Link with Use of Soliton Pulse and Dispersion Compensating Fiber Nirav Purohit1, Paras Gosai2, Rohit Patel3 and Gaurang Patel4 1

PG Sudent, Wireless Communication System and Networks Department, Dr. S. & S. S. Ghandhy Government Engineering College, Surat, Gujarat Technological University, Ahmedabad, Gujarat, India [email protected] 2

Associate Professor, Wireless Communication System and Networks Department, Dr. S. & S. S. Ghandhy Government Engineering College, Surat, Gujarat Technological University, Ahmedabad, Gujarat, India [email protected] 3 Associate Professor, Electronics and Communication Department, U. V. Patel College of Engineering, Kherva, Mehsana, Ganpat University, Kherva, Mehsana, Gujarat, India [email protected] 4

Lecturer, Electronics & Communication Department, Government Polytechnic, Gandhinagar, Gujarat Technological University, Ahmedabad, Gujarat, India [email protected]

Abstract The Optical communication is the revolution in the data transmission because of the better bandwidth and high speed communication. The performance of the optical communication is limited due to the dispersion and other nonlinearities. There are several techniques used to minimize the effect of dispersion and to improve the performance of the link. Soliton is one of the pulse shaping techniques in which the shape of the pulse intact as it is as the signal travels through the optical fiber, means shape is self-changing such that it encounters the effect of dispersion. DCF technique is also effective technique to minimize the dispersion. Dispersion Compensating Fiber (DCF) has a negative dispersion coefficient. So, it minimizes the dispersion effect. In this paper, we have simulated one 10 Gb/s long haul optical link and used soliton pulse as well as DCF scheme to improve the performance. In this paper, the performance of the soliton pulse with DCF scheme is compared with the normal (NRZ) pulse in 10 Gb/s long haul optical link. From the simulation results, it has been shown that use of soliton pulse with DCF scheme provides higher value of Q-factor and lower value of BER than normal (NRZ) pulse without DCF scheme. Keywords: Dispersion, NRZ pulse, Soliton pulse, DCF, Q-factor, BER.

1. Introduction Now a day, Optical communication is mainly used for the data transmission because of the better bandwidth and high speed communication. But the dispersion is the major problem in the optical communication system. Due to this dispersion effect, the shape of the pulse becomes broaden. So that the Q-factor of the system decreases and BER (bit error rate) increases. That degrades the performance of the optical communication system. Nirav Purohit,

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There are several techniques are used to minimize the effect of dispersion in optical communication system and to achieve the better performance of the system. One of them is to use the soliton pulse in the optical link instead of the normal (NRZ) pulse and another one is to use the dispersion compensating fiber (DCF). Optical Solitons are the special type of pulse which can propagate over long distance without changing in its shape and velocity. Soliton pulse is obtained by the cancellation of linear (dispersion) effect and nonlinear (Kerr) effect in an optical fiber. This pulse has a property of self-changing its shape while transmission to encounter the effect of dispersion. So the dispersion effect can be minimized in the optical communication system. The dispersion compensating fiber (DCF) has a negative dispersion coefficient. So, it minimizes the dispersion effect. In this paper, first we have used the normal (NRZ) pulse in the10Gb/s long haul optical link and measured the Q-factor and BER for various distances. Then NRZ pulse is used with the DCF scheme. Second time, we have generated the soliton pulse and then it is transmitted through the same 10Gb/s long haul optical link for the same various distances and measured the Q-factor and BER. Then Soliton pulse is used with the DCF scheme. From the simulation results, we have made the comparison between normal (NRZ) pulse and soliton pulse with and without DCF scheme in terms of Q-factor and BER. For a good Transmission scheme, Q factor should be greater than 6 and BER should well below 10 . -9

2. Types of Dispersion Dispersion is the major effect for the performance degradation of the optical communication system. In optical fiber, different modes are travels with the different velocity because of the material and waveguide property of the fiber. So, all the modes reach at the different times at the end of the fiber. That results into the pulse broadening i.e. dispersion. Different types of dispersion are classified as below. Dispersion

Modal Dispersion

Group Velocity Dispersion

Material Dispersion

Polarization mode Dispersion

Waveguide Dispersion Fig. 1 Types of dispersion

3. Dispersion Compensation Techniques There are several methods to compensate the dispersion effect. The various dispersion compensation techniques are as follows: • Feed Forward Equalizer (FFE) • Feed Forward-Decision Feed Back Equalizer (FFE-DFE) • Non Linear Feed Forward- Decision Feedback Equalizer (NL-FFE-DFE) • Maximum Likelihood Sequence Estimator (MLSE) • Pulse shaping techniques (e.g. Soliton pulse) • Dispersion compensation fiber (DCF) Nirav Purohit,

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• • •

fiber Brag grating (FBG) Optical phase conjugation (OPC) Polarization Mode Dispersion (PMD) compensation

In DCF scheme, there are three schemes are available: (i) Pre-compensation scheme, (ii) post-compensation scheme and (ii) mix-compensation scheme. Mix - compensation scheme provides the better results than pre and post compensation schemes. In this paper, we have used the hybrid approach of the two techniques to reduce the dispersion effect. One is to use the Soliton pulse and another is DCF technique. In this paper, Soliton pulse is used with combination of mix-compensation DCF scheme to minimize the dispersion effect.

4. Simulation Set-up Fig. 2 represents the block diagram of simulation setup. Fig. 2(a) shows the simulation setup for the generation and transmission of the NRZ pulses through the long haul optical link. Here, the bit rate of the link is 10 Gb/s. Data generator generates the user defined bit sequence which given to the NRZ pulse generator. Power and frequency of the CW laser is 0 dBm and 193.1 THz respectively. These NRZ pulse generator and CW laser output is launched into the Mach-Zehnder Modulator. Modulated optical signal is transmitted through the optical fiber. The Single mode optical fiber with attenuation of 0.2 dB/km and Dispersion of 1.67 ps/nm/km has been used here. Here the various lengths of the fiber is used to determine the performance of the system. Output signal from the fiber is given to the EDFA to amplify the attenuated pulses with gain 10 dB. Then PIN photo detector is used to detect the optical signal and convert it into the electrical signal. This electrical signal is passed through the filter. The filtered electrical signal is connected to the Eye Diagram Analyzer which is used as a visualizer to generate graphs and results such as eye diagram, BER, Q-factor value, eye opening etc. From the eye diagram, the performance of the optical link is determined. If eye opening is larger, the performance of the optical link is better and vice-versa. Fig. 2(b) shows the simulation setup for the generation and transmission of the NRZ pulses through the long haul optical link with use of dispersion compensating fiber (DCF) technique to reduce the dispersion. Here, Mix-Compensation DCF scheme is used. In Mix-Compensation DCF scheme, the DCF Module is included before and after the single mode optical fiber. Fig. 2(c) shows the simulation setup for the generation and transmission of the Soliton pulses through the long haul optical link. Here, the user defined bit sequence of the data generator is given to the Optical Sech Pulse Generator. Hyperbolic Secant pulses of power 5.8 mW are generated by the Optical Sech Pulse Generator at User Defined Bit Rates in the wavelength 1300 nm. These Soliton pulses are transmitted through the single mode optical fiber. After that, the simulation setup from optical fiber to the Eye diagram Analyzer is same as in the case of NRZ pulses. Fig. 2(d) shows the simulation setup for the generation and transmission of the Soliton pulses through the long haul optical link with use of dispersion compensating fiber (DCF) technique to reduce the dispersion. Here, Mix-Compensation DCF scheme is used. In MixCompensation DCF scheme, the DCF Module is included before and after the single mode optical fiber. The simulation parameters are listed in Table 1.

(a)

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(c)

(d)

Fig. 2 Block diagram of simulation setup: (a) NRZ pulse transmission through the optical link without DCF scheme (b) NRZ pulse transmission through the optical link with DCF scheme (c) Soliton pulse transmission through the optical link without DCF scheme (d) Soliton pulse transmission through the optical link with DCF scheme Table 1 Simulation Parameters Bit Rate Power of CW Laser Frequency of CW Laser Modulator Dispersion Coefficient of SMF Attenuation Factor of SMF DCF scheme Dispersion Coefficient of DCF Attenuation Factor of DCF Amplifier and Gain Wavelength (λ) of Optical Sech Pulse Generator Power of Optical Sech Pulse Generator

Values 10 Gbps 0 dBm 193.1 THz Mach-Zehnder Modulator 1.67 ps/nm/km 0.2 dB/km Mix-Compensation scheme -80 ps/nm/km 0.6 dB/km EDFA and 10 dB 1300 nm 5.8 mW

5. Simulation Results and Discussion In this paper, we have used two different techniques to minimize the dispersion effect. (i) pulse shaping technique (Soliton pulse) (ii) DCF technique. Initially, simulation is done for the NRZ pulse transmission with and without use of DCF technique and Q-factor and BER are measured for various distances. From the simulation results, it is clear that when we apply DCF technique, the Q-factor increases and BER decreases. It means the performance of the system improves. Then, the simulation is done for the Soliton pulse transmission with and without use of DCF technique and Q-factor and BER are measured for the same distances as in the case of NRZ pulse. Initially, Soliton pulses are transmitted without use of DCF technique. At that time, the performance of the system is better than that of the NRZ pulse. It means Soliton pulse provides the better performance than NRZ pulse by minimizing the effect of dispersion by maintaining its shape. Then, DCF technique is included with the Soliton pulse transmission. In this case, the Q-factor increases and BER decreases. It means the performance of the system improves. So, we are getting the best performance when Soliton pulse and DCF technique is used together. It means, the hybrid approach of Soliton pulse and DCF technique provides the very good performance of the system. So, It can be used for long distance communication.The below figure shows the eye diagrams for different cases for 50 km distance.

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Fig. 3 Eye diagram of NRZ pulse transmission for 50 km without use of DCF technique

Fig. 4 Eye diagram of NRZ pulse transmission for 50 km with use of DCF technique

Fig. 5 Eye diagram of Soliton pulse transmission for 50 km without use of DCF technique

Fig. 6 Eye diagram of Soliton pulse transmission for 50 km with use of DCF technique

The values of Q-factor and BER, corresponding to different distances for NRZ pulse transmission and Soliton pulse transmission with and without use of DCF technique, are shown in Table 2.

NRZ pulse (without DCF)

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Table 2 NRZ pulse Soliton pulse (with DCF) (without DCF)

Soliton pulse (with DCF)

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Distance (km) Q-factor

50

100

50

100

50

100

50

100

10.53

7.52

18.42

9.76

179.47

164.87

744.86

729.75

BER

2.49e-026

2.31e-014

3.84e-076

7.76e-023

0

0

0

0

6. Conclusion In this paper, the generation and transmission of the Soliton pulse and NRZ pulse through the 10 Gb/s long haul optical link with and without use of DCF technique for the various distances has been studied. From the simulation results, it is clear that the soliton pulse provides the higher Q-factor and lower BER as compared to normal (NRZ) pulse. It means the performance of the optical link will be better in case of Soliton pulse. If we use the DCF technique in the optical link, then further improvement of the system can be achieved in terms of Q-factor and BER for NRZ pulse as well as Soliton pulse. So, among all these cases, when Soliton pulse and DCF technique is used together, the best performance of the system can be achieved.

Acknowledgments I would like to thank Mr. Paras S. Gosai, the head of the Wireless Communication System and Networks Department, Government Engineering College, Surat for providing me invaluable guidance and continuous support for this paper. I would also like to thank Mr. Rohit B. Patel, the head of the Eletronics and Communication Department, U.V. Patel College of Engineering, Kherva, Mehsana for providing me invaluable guidance and support.

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