Improvement of transmission performance of phase-shift-keying signals by the use of ultrafast optical limiters (amplitude regenerators) based on four-wave mixing (FWM) in fibers is theoretically and numerically studied. Theoretical analysis focuses on the nonlinear phase noise (the Gordon-Mollenauer effect) and its reduction by the limiters. It is shown that the cubic growth of the phase variance as the distance extends can be suppressed by the limiters that are periodically inserted in the system, although some additional phase noise is introduced by them. Numerical simulation is performed for nonreturn-to-zero (NRZ) differential phase-shift-keying (DPSK) transmission in a quasi-linear highly dispersed-pulse system with and without limiters. The results show that the maximum transmission distance is extended by the limiters. A clear indication of nonlinear phase-noise reduction at the high-signal-power regime, however, is not seen. This is attributed to the existence of intrachannel-FWM-induced phase fluctuation, which cannot be effectively suppressed by the amplitude limiter.
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