Nonlinear Compensation Investigate Of The CO-OFDM System Based On Optical Phase Conjugation

As a core technology for the fourth generation of wireless communication, orthogonal frequency division multiplexing (OFDM) technology has been widely applied. It can efficiently improve the system capacity, anti-jamming capability and spectral efficiency. Coherent optical communication exhibits several advantages, such as long transmission distance, large system capacity and good signal quality. Combining the advantages of coherent optical communication technology and OFDM technology, coherent optical orthogonal frequency division multiplexing (CO-OFDM) technology, has become a key technology of ultra-long haul optical network transmission in recent years, as it is not sensitive to chromatic dispersion and polarization mode dispersion. However, there are also some limitations in the CO-OFDM system. Firstly, additional amplifier is required to resist the loss of the fiber, which can generate nonlinear phase noise due to the spontaneous emission noise. Secondly, the effect of dispersion cannot be ignored as the CO-OFDM is not sensitive to dispersion for limited condition. Thirdly, because of the dense sub-carriers, the nonlinear effects, especially the four wave mixing (FWM) process are very serious, and the accumulation of these nonlinear effects would degrade the performance significantly.The accumulation and suppression of the FWM noise in the CO-OFDM transmission system are investigated based on the nonlinear Schrodinger equation model. Firstly, the principles and some key technologies commonly used in the CO-OFDM system are summarized for subsequent chapters; Secondly, the influences of the fiber loss, chromatic dispersion, number of fiber segments, the length of each fiber segment and phase mismatch in CO-OFDM system on the FWM effect are theoretically analyzed; Thirdly, the CO-OFDM is analyzed with an introduced optical phase conjugation. Finally, numerical simulations have been performed, which agree well with the theoretical analysisSeveral interesting results are obtained. Firstly, in the CO-OFDM system, the FWM noise mainly depends on the fiber loss, chromatic dispersion, optical fiber number of segments, each fiber length and the phase mismatch parameters, the influences of which are correlative. For example, when the phase mismatch is different, the number of fiber segments bring different influences to FWM noise; Secondly, by introducing an OPC into the CO-OFDM system, the FWM noise can be suppressed effectively, with a small phase mismatch parameter value. In the simulation, when the phase mismatch parameter is zero, the FWM light intensity is the strongest under the condition that other parameters are determined. In addition, the FWM noise intensity accumulated in the system would be changed regularly with the increase of the absolute value of this parameter. Besides, when the phase mismatch parameters with different orders are used, the fiber segment number and the length of each fiber have a different influence on the FWM light intensity. Especially, if the phase mismatch parameters is less than or equal to1.7x10-8m-1, all the parameters except the phase mismatch aren’t of influence on the FWM light intensity. In virtue of the OPC technology, the FWM noise has been efficiently suppressed, especially when the absolute value of the phase mismatch parameter is less thanl.7x10-7m-1. Moreover, when the phase matching is fixed, by adopting the OPC technology, the influence of other parameters on the system performance can also be significantly improved.