Gordon and Mollenauer (G-M) proposed in 1990 that the interplay between the nonlinear Kerr effect and amplified spontaneous emission noise can generate an enhanced level of noise and degrade the performance of optical phase-modulated systems. We systematically investigate the G-M effect in 112 Gbit/s coherent dual polarization quaternary phase shift keying (DP-QPSK) systems through comprehensive simulation. The results show that in the presence or absence of inline dispersion compensation, the G-M effect will seriously damage the system performance. Some of our important conclusions are listed as follows: with the increase in the input power into each span, the G-M effect is enhanced simultaneously; the smaller the span length, the stronger the G-M effect caused by more inter-span interaction; for non-zero dispersion-shifted fibers, different coefficient values will cause a similar amount of G-M effect penalty; by comparing the system performance with different dispersion compensation ratios, one can conclude that different residual dispersions of the same magnitude, no matter whether they are under-compensated or over-compensated, can derogate the overall system performance to a similar degree. In brief, the G-M effect cannot be ignored. We also have a short discussion on how to reduce and compensate for the G-M effect in DP-QPSK systems.

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The mid-nonlinearity temporal inversion (MNTI) technology based on optical phase conjugate (OPC) is firstly proposed to compensate the chromatic dispersion and nonlinearity effect of the fiber link in coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. System performance is analyzed under different transmission distance and different dispersion coefficient of fiber link. Simulation results show that, compared with mid-span spectral inversion (MSSI) method, at least 2-dB Q-factor improvement is achieved for the CO-OFDM system.