Performance Of Improved All-Optical OFDM System Based On AWG

The combination of optical orthogonal frequency-division multiplexing (OFDM) and fiber communication is one of the research hot topics for the past few years due to its prominent advantages. The advantages of OFDM are high spectrum efficiency, large tolerance to chromatic dispersion (CD) and polarization-mode dispersion, etc. However, if the OFDM technique is merely realized by using electrical circuit, the speed of system would be limited due to the processes of DAC/ADC, photoelectric/electric-photo conversion, and inverse/forward discrete Fourier transform (IDFT/DFT). To overcome this problem. All-optical OFDM technology, as a promising solution, has been proposed and demonstrated, where IDFT/DFT is achieved in optical domain. Furthermore, the all-optical OFDM system will be simpler and more practical benefited from the combination of arrayed waveguide gratings (AWGs) in DWDM, and IDFT/DFT.First, this paper illustrates the principle of all-optical OFDM based on OFDM theory, and compares it with that of AWG. Then, the FDFT/DFT is realized by AWG, since AWG has remarkable advantages in large-scale subcarrier systems. Next, the performance of the AWG based all-optical OFDM system is numerically simulated and analyzed, where the relation of Bit Error Rate (BER) and Signal Noise Ratio (SNR) with respect to dispersion is considered. In the simulations, we have changed the number of AWG subcarriers, so as to observe how subcarriers affect the system dispersion tolerance. From the analysis of the influence of impulse width which is broadened by dispersion, we have demonstrated that it will exacerbate all-optical OFDM signals. Compared with tradition OFDM system, the system can improve the dispersion tolerance with the introduction of cyclic postfix. By analyzing the structure and parameter of AWG, we have found that changing the number of arrayed waveguides of AWG will realize the CP in AWG. As a result of the above analysis, we have proposed an improved AWG based all-optical OFDM system by adding CP. Moreover, we have simulated the transfer performance of this system and analyzed the BER and SNR performance with respect to dispersion. Following that, we have focused on the CP length influence which will change the system performance. The proper CP length can also be determinate by analyzing the BER and SNR performance against dispersion. Finally, the application of all-optical OFDM to WDM system has been numerically investigated, where two cascaded AWGs were employed:one is for IDFT/DFT and the other for multiplexer/demultiplexer.The simulation results show, firstly, the number of subcarriers has significant effect on system dispersion tolerance. For the 200GHz all-optical OFDM system, as the number of subcarriers is changed from 16 to 32, the dispersion tolerance is increased by 85ps/nm. On the other hand, the AWG can reasonably increase the number of subcarriers because of its simple structure. Secondly, the system dispersion tolerance will be increased remarkably by adding CP. When the number of subcarrier is 16, the system dispersion tolerance will be improved by 220ps/nm where the CP length is 4. As the increment of CP length, the system dispersion tolerance will be improved as well. However, due to the influence of adjacent optical pulse, the CP length will not always improve the dispersion tolerance when it is increased to a critical value. Thirdly. the performance of all-optical OFDM applied in WDM system and that of the single channel are nearly comparable, so we can take cascaded AWGs to get access to WDM.