Research Of The Cross-phase Modulation Effect In Coherent Optical OFDM System

XPM originates from the Kerr effect in optical fiber, where optical intensity variations of an optical wave in WDM system modulate the optical phases of other co-propagating waves through XPM. In the meantime, due to the GVD, phase modulation can also be converted into intensity modulation. The noise of signal phases and amplitude fluctuations induced by XPM effect will damage the performance of coherent optical communication system, and the impact is inversely proportional to the channel spacing and proportional to the signal power. The research of XPM crosstalk in the past is limited to single-carrier phase modulation system. Its generation mechanism and compensation method has been studied extensively. But, there are little research on multi-carriers such as CO-OFDM systems. Due to its high PAPR and strict requirement for orthogonality, CO-OFDM system will be very sensitive to XPM induced phase noise. Thus, XPM effect must not be negligible in CO-OFDM system. In this thesis, we will study the impact of XPM crosstalk on the performance of CO-OFDM systems.Firstly, we introduce the nonlinear impairment mechanism in CO-OFDM system, including SPM, XPM and FWM. A mathematical model of inter-channel XPM based on the Volterra series can be established by solving the nonlinear Schrodinger equation. The analysis of XPM effect based on Volterra series, can calculate the dispersion of the fiber and XPM effect at the same time. There is no need to establish the transformation between signal amplitude noise and phase noise in the presence of group velocity dispersion. Secondly, this XPM mathematical model is expanded to the dispersion management system, then a transmission filter based on Volterra series is established which can be used to calculate the effect of XPM. Then, this filter model is applied to the OFDM/OOK hybrid system, and it can be used to calculate the XPM crosstalk effect on the performance of OFDM system.Finally, considering XPM induced phase noise seriously affect the performance of CO-OFDM systems. Therefore a combination of OFDM and double sideband modulation can be used to mitigate the XPM induced phase noise in CO-OFDM systems. With the increase of DSB modulation frequency, XPM induced phase noise can be minimized gradually and even can be neglected. Then, the OFDM WDM system is used to verify the accuracy of this method.