Comparison of dispersion compensation with Fiber Braggs Grating at Transmitter and Receiver end of a single channel optical communication system

Er. Abhishek Sharma, Er. Sukhbir Singh, Er. Rajeev Thakur, Er. Bhubneshwar Sharma M. Tech student (ECE), Eternal University, H.P (India) sharma.abhishek174@gmail.com Assistant Professor (ECE), Eternal University, H.P (India) sukh_ece@yahoo.co.in Lecturer (ECE),Eternal University, H.P (India) thakur.rajeev.1984@gmail.com Assistant Professor (ECE), Eternal University, H.P (India) bhubnesh86@gmail.com


1.INTRODUCTION
Optical fiber is used in fiber-optic communication to transmit data in form of light pulses with higher rates over long distances as compare to other form of communication. Dispersion is the main factor which affects the performance of fiber in optical fiber communication. Dispersion compensating fiber (DCF) is a standard solution for dispersion compensation in long hall transmission system. But DCF are bulky [1] and by using these fibers nonlinear affects [2], cost of optical system [2], and insertion losses increases [4].
Placement of components in optical communication system plays a very important role in dispersion reduction. Thus performance of system is checked by changing the position of FBG along with EDFA (using FBG after EDFA) at transmitter end (before fiber) and receiver end (after the fiber).
In the third section of this paper the performance (Minimum BER) of two systems (FBG before EDFA (at transmitter end before the fiber) and FBG before EDFA (at receiver end after the fiber) in the optical transmission system) is investigated by plotting the graph between input power and BER. In fourth section performance is investigated by plotting the graph between FBG length and BER for two systems.

SYSTEM DESCRIPTION
The diagrams of optical transmission system are shown in Fig. 1and    The function of Pseudo-random bit sequence generator (generating 10 Gbps) is to scramble data signal in terms of bit rates [10]. NRZ-pulse Generator produces the electrical data signal for modulation process [11]. Continuous Wave (CW) laser is operating at frequency 193.1 THz is applied to the system and it is modulated externally with non-return-zero (NRZ) pseudorandom binary sequence in a Mach-Zehnder modulator (Mach-Zehnder has extinction ratio of 30 dB). Signal flows through single mode optical fiber. FBG is used to compensate the chromatic dispersion of optical fiber (working principle-the longer wavelengths are transmitted through the last part of grating and short wavelengths are J u n e 2 5 , 2 0 1 3 reflected by the first part of grating, due to this results longer wavelength have to travel a longer distance, so they are delayed and allowing the shorter wavelength to catch up, and dispersion gets reduced).The function of erbium-doped fiber amplifier (EDFA) is to compensate the losses in optical transmission system. Function of photodector is detection of light (photon) at the receiver. It directly converts light into current. Optical power meter is used is used for checking the received signal power level. Eye Diagram Analyzer is used for checking the minimum bit error rate.

COMPARISON ON THE BASIS OF INPUT POWER VERSES BER
This section describes comparison between transmitted power with BER at different values of distances (10, 20, 30, 40, 50 Km). For 10km distance, power is varied from -10dBm to 10dBm and FBG length is also varied from 2mm to 14mm. Then minimum BER is observed and is plotted against transmitted power. The same process is repeated for other distances (20, 30, 40, 50 km).
The results shows that system achieves a low BER for a particular power, then afterward it increases. Graph between input power and BER for dispersion compensation with FBG after EDFA at transmitter end is shown in Fig. 3.

Fig 3: Graph between input power and BER for dispersion compensation with FBG after EDFA at transmitter end.
For 10km minimum BER of 2.52095e-313 is observed at 2dBm input power. For 20 km minimum BER of 3.02516e-320 is observed at 0dBm input power. For 30km minimum BER of 1.00295e-321 is observed at 10dBm input power. For 40km minimum BER of 2.09258e-233 is observed at 2dBm. For 50km minimum BER of 9.57499e-201 is observed at 8dBm.
Graph between input power and BER fordispersion compensation with FBG after EDFA at receiver end is shown in Fig. 4. J u n e 2 5 , 2 0 1 3

Fig 4: Graph between input power and BER for dispersion compensation with FBG after EDFA at receiver end.
For 10km minimum BER of 8.454e-310 is observed at 2dBm input power. For 20 km minimum BER of 1.73339e-314 is observed at 0dBm input power. For 30km minimum BER of 2.31627e-282 is observed at 0dBm input power. For 40km minimum BER of 5.59002e-304 is observed at 4dBm. For 50km minimum BER of 1.28559e-111 is observed at 10dBm. Fig. 5 and Fig.6 on the basis of minimum BER for particular distance is shown in table 1. After comparing the results (BER values) for the two configurations (using FBG after EDFA at transmitter end and FBG after EDFA at receiver end), it can be concluded that the optical system using FBG after EDFA at the transmitter end has a better performance (less BER) except for 40 km. Thus the placement of FBG after EDFA (by using FBG after EDFA) in optical transmission system plays an important role to reduce the BER of optical system. J u n e 2 5 , 2 0 1 3

Comparison of eye diagram
Comparison of eye diagram for 30km using dispersion compensation with FBG after EDFA at transmitter end and using dispersion compensation with FBG after EDFA at receiver end is shown in figures below. Fig 5.1 (Eye diagram using dispersion compensation with FBG after EDFA at transmitter end system) and Fig 5.2 (Eye diagram using dispersion compensation with FBG after EDFA at receiver end system) for 10 km. From comparison it can be seen that signal on Eye diagram using dispersion compensation with FBG after EDFA at transmitter end system is much clear (with Eye Height 0.00660574) than eye diagram using dispersion compensation with FBG after EDFA at transmitter end system (with Eye Height 0.000755223).

3.2Comparison of Q-factor
Comparison of Q-factor of system using dispersion compensation with FBG after EDFA at the transmitter end and system using dispersion compensation with FBG after EDFA at the at receiver end is shown in Table 2 below. From the comparison of Q-factor it is observed that system using FBG after EDFA at the transmitter gives a better performance for all distances (10, 20, 30, 50km), except for 40km.

COMPARISON ON THE BASIS OF FBG LENGTH AND BER
In this section performance is described by the comparison between FBG length with BER at different values of distances (10, 20, 30, 40, 50 km). The performance is investigated by varying the Input power of optical system is from -10 to 10 dBm and BER is observed by using BER analyser. Then resulting minimum BER (observed (available) at different input power level) obtained from BER analyser is plotted against FBG length.
From the graph in Fig. 6 and Fig. 7 it can seen that system achieves a low BER for a particular FBG length, and then afterward it increases. FBG length plays an important role in decreasing the BER. The results show that system achieves a low BER for a particular FBG length, and then afterward it increases.
Graph between FBG length and BER for dispersion compensation with FBG after EDFA at transmitter end is shown in Fig.  6.

Fig 6: Graph between FBG length and BER for dispersion compensation with FBG after EDFA at transmitter end.
Graph between FBG length and BER for dispersion compensation with FBG after EDFA at receiver end is shown in Fig. 7.

Fig 7: Graph between FBG length and BER for dispersion compensation with FBG after EDFA at receiver end.
Comparison of graphs in Fig. 5, Fig. 6 on the basis of minimum BER for particular distance is shown in Table 3. From the table, it is clear that the optical system using FBG after EDFA at the transmitter end achieves less BER as compare to optical system using FBG after EDFA at the receiver end. Thus the placement of FBG after EDFA along with proper selection of FBG length in optical transmission system plays an important role to reduce the BER of optical system.
If we use FBG before EDFA then at the output with increase in input power BER increases (because of fiber loses and ASE noise produced by EDFA during the amplification of signal). And if FBG used after EDFA then FBG supresses the ASE noise (produced by EDFA during amplification of signal) and this results less BER at the reception of signal. Therefore it is better to use FBG after the EDFA to reduce EDFA in single channel optical communication system.

CONCLUSION
The positioning of the FBG along with EDFA at transmitter end (before fiber) end and receiver end (after fiber) plays an important role in improvement of the optical systems performance. When FBG is used at transmitter side after the EDFA for dispersion compensation, it gives better performance (minimum BER) as compare to when used for compensation at receiver side (after fiber). For 10km BER reduces from 8.454e-310 to 2.52095e-313 with input power 2dB, for 20km BER reduces from 1.73339e-314 to 3.02516e-320 with input power 0dB, for 30km BER reduces from 2.31627e-282 to 1.00295e-321 with input power 10 dB, for 50 km bit error reduces from 1.28559e-111 to 9.57499e-201. Only for 40km BER increases from 5.59002e-304 to 2.09258e-233. The optimum FBG lengths (to get minimum BER) are 4, 8, 10, 14, 14mm for 10, 20, 30, 40, 50km respectively.