Modeling inter-subcarrier four-wave mixing noises in QAM coherent OFDM system using phase-conjugated twin waves with diversity implement domains

For coherent orthogonal frequency-division multiplexed (CO-OFDM) system, four-wave-mixing (FWM) noise is a major limiting factor. A scheme of phase-conjugated twin waves (PCTW) which co-propagates a signal with its phase-conjugated copy along optical fiber link, can be used to reduce the impact of FWM noise. Including walk-off between subcarriers, semi-analytic models of FWM noise are derived here for quadrature amplitude modulation (QAM) CO-OFDM system with three PCTW schemes implemented in time, subcarrier and polarization domains, respectively. We show that many phase-conjugated twin FWM components can be obtained in such system and they can be added destructively at the receiver, so the total FWM noise can be reduced, but some FWM components have not their phase-conjugated twins for the subcarrier-domain PCTW scheme. Some phase-conjugated twin FWM components are generated from different FWM collision processes for the time-domain PCTW scheme, as will reduce the effect of the PCTW scheme The calculative results obtained by using our semi-analytical model show that the polarization-domain PCTW scheme excels the other two schemes in improving the nonlinear tolerance of such system and the three PCTW schemes are all better than a partial carrier filling (PCF) technique.