| In the information age, with the explosive growth of information, technology and the rapid development of information industry, human demand for information transmission bandwidth growing at an alarming rate, with the information transmission capacity significantly increasing, the existing frequency resources can not meet the requirements and the existing system of transmission rate and capacity are also to be increased dramatically, so in a high-speed large-capacity situation, the advantages of optical fiber communication technology become increasingly apparent. However, in optical communication systems, optical fiber dispersion limits the transmission capacity and transmission distance, fiber dispersion has become the realization of large capacity, ultra-long haul optical fiber communications "bottleneck". The orthogonal frequency division multiplexing which can effectively combat the chromatic dispersion and polarization mode dispersion, maximize the use of spectrum resources of the optical link and improve the performance of optical fiber communication, combining OFDM modulation technology and optical fiber communication can construct high-speed, long distance and low-cost optical transmission network. However, due to the Peak-to-Average Power Ratio of OFDM technology is relatively large, would make signal severe nonlinear distortion, optical fiber links and a variety of devices have raised the high linearity requirements, so the study of OFDM Optical Fiber Communication nonlinear effect and its compensation algorithm is essential.In order to compensate nonlinear effects of OFDM fiber communication better, this paper uses the Matlab software and Optisystem7.0 to built a combination of OFDM simulation model of optical fiber communications. Based on deeply study of nonlinear optical effects of OFDM communications, with its characteristics, issue some compensation algorithms according to the causes of nonlinear effects: Mach-Zehnder electro-optic modulator nonlinear pre-compensation algorithm and the nonlinear optical OFDM communication link electric post-compensation algorithm. Pre-compensation algorithm and the electric post-compensation algorithm reduce the error rate, improve the system performance, especially the electric post-compensation algorithm has important theoretical and practical significance. The main work is as follows:First, established the simulation model of OFDM optical fiber communication using Optisystem7.0 and Matlab. Simulated and analyzed the performance of the system ,compared the received and transmitted signal of OFDM in Optisystem7.0 with the graph in Matlab to prove the correctness of the interface program, and also proved that the combination of two kinds of softwares can be used approach to OFDM optical fiber simulation model of communication structures.Secondly, we analyzed the reason of optical OFDM nonlinear: nonlinear of electro-optic modulator and nonlinear of transmission links. Aspects of electro-optic modulator to Mach-Zehnder electro-optic modulator as an example of the input and output is derived nonlinear theory and the Mach-Zehnder electro-optic modulator quadrature bias point drift theory is verified by computer simulation, Mach-Zehnder electro-optic modulator modulation index value of the correctness of the theoretical derivation and analysis of quadrature bias point drift to the system mean square error. Transmission link introduces a nonlinear effect of optical OFDM link, fiber optic communication link OFDM derived the nonlinear Schrodinger equation, and the transport equation is improved, the simulation of the nonlinear effect on the system at last.Then, for the Mach-Zehnder electro-optic modulator input and output of the nonlinear characteristics, we issua a Taylor polynomial based pre-compensation algorithm, and simulation results obtained with different precision, this algorithm are the Mach-Zehnder electro-optic modulator modulation index of 0.65 and 0.9, the modulator input signal added to the original dynamic range of 2 times and 3 times, is to enhance the dynamic range of the input signal, can reduce the high PAPR which is the damage to OFDM optical fiber communication.Finally, for the nonlinear optical OFDM transmission link, we issue a nonlinear post-compensation algorithm. Describe the principle and derivation of the algorithm, and then proceed three post-compensation of nonlinear performance simulation varying different nonlinear coefficient, transmission links of different lengths, different transmission power of OFDM signals. After a post-compensation system, reducing the mean square error of two orders of magnitude, the length of transmission link increased from the original 300km to 500km, proved that it can be applied in optical OFDM communication system with a high nonlinear effect, and it has significant meaning in both research and application for solving the nonlinear effect in OFDM optical fiber communication system. |
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