The Cross-Phase Modulation In Dispersion Management And Wavelength Division Multiplexing Systems

There are two main methods to improve transmission capability of optical communication systems. One is to increase bit rate in single channel. With the increase of bit rate, the impact that dispersion impose on pulses by widening pulses width strengthens gradually. Usually dispersion management (DM) is adopted to weaken the impact by reducing the average dispersion of transmission line. The other is to arrange more channels in same bandwidth. With the reducing of channel spacing, cross-phase modulation (XPM) strengthens gradually and the systems performance is degraded.This work focused on DM and XPM, which included the length optimization of compensation fiber in single channel DM systems, the intensity modulation and phase modulation induced by XPM in single segment fiber and multi-periods DM wavelength division multiplexing (WDM) systems, the DM system optimization for restraining XPM, the modulation instability (MI) in frequency-mixing process under the effect of loss, self-phase modulation (SPM) and XPM in WDM systems.With the example of single channel post-compensation DM systems, the length of compensation fiber was optimized by simulation. It is found that the slowly-varying-envelope narrows firstly, broaden and then. With the enhancement of compensation degree, the distance corresponding to the narrowest pulse width reduces and the least pulse width increases. It is found that the widened pulse width could be decreased effectively by reducing compensation degree comparing with single period and the max compression. The optimized compensation length increases with the enhancement of maximum power.For intensity modulation-direct detection (IM-DD) systems, the XPM under the effect of group velocity dispersion (GVD) and SPM were investigated by using the frequency domain transfer function (FDTF) and time domain intensity modulation index to evaluate systems performance. First, the FDTFs corresponding to XPM were derived in single segment and multi-segments amplification systems. The methods to calculate the time domain wave shape and time domain intensity modulation index were given when arbitrary pump signalswere inputted. Then, the effects of modulation frequency, fiber dispersion and SPM of pump signals on XPM were analysed. At last, compensation schemes (pre-compensation or post-compensation) and the length choice for compensation fiber were discussed for restraining XPM in DM systems. It is found that the optimized compensation scheme is variable. The choice of compensation scheme is affected by the length of compensation fiber, transmission distance or the number of DM periods.For coherent optical systems, the XPM were investigated by using the FDTF and time domain phase modulation index to evaluate systems performance when the wave shape of propagated pulses was changed by GVD. First, the FDTFs in single segment and multi-segments amplification systems were derived when the wave shape of propagated pulses was changed by GVD. The methods to calculate time domain phase shift and time domain phase modulation index in probe channel were given. Then, the effect of modulation frequency, transmission distance and fiber dispersion on XPM were analysed. At last, the different dispersion compensation schemes (under-compensation, full-compensation, over-compensation, pre-compensation, post-compensation) were compared for restraining the phase shift induced by XPM. The results show that the post-compensation scheme is superior to pre-compensation scheme. The average dispersion should be away from zero average dispersion when we choose the length of compensation fiber.For the MI under the effect of XPM in WDM systems, focusing on the two carried frequencies and sideband frequencies generated in the frequency-mixing process, the calculation method of MI gain in frequency-mixing process under the effect of loss, GVD, SPM and XPM was given by couple mode method. The expressions of MI gain caused by SPM or XPM of carried frequencies were given when one phase matched perfectly and others mismatched seriously. The maxima of gain are independent of GVD when the MI is caused by SPM or XPM mainly. The maxima of gain at different sidebands are equal when MI is caused by XPM. Only when the frequency shift is large enough, there is the MI gain after considering the fiber loss. The frequency shift corresponding to the maximal gain caused by SPM decreases after considering the fiber loss. With the increase of transmission distance, the MI gain peak induced by SPM closes up tocarries frequencies.The theory models and the results in this paper are useful for analysis and design of long distance and high capacity IM-DD or coherent detection WDM systems.