Impact of Nonlinearities on Fiber Optic Communications

Volume 7 of the series Optical and Fiber Communications Reports pp 325-341


Cross-Phase Modulation-Induced Nonlinear Phase Noise for Quadriphase-Shift-Keying Signals

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For a quadriphase-shift keying (QPSK) or differential QPSK (DQPSK) signal in a wavelengthdivision- multiplexed (WDM) system, cross-phase modulation (XPM) induces nonlinear phase noise from adjacent WDM channels. The impact of XPM-induced nonlinear phase noise is investigated when the adjacent WDM channels are either constant-intensity phase-modulated or on-off keying (OOK) signals. XPM-induced nonlinear phase noise is Gaussian distributed. For both QPSK and DQPSK signals, the phase error standard deviation (STD) should be less than 4o to 6o for a raw bit-error-rate (BER) between 10-5 to 10-3 before forward error correction (FEC). For DQPSK signals with mean nonlinear phase shift up to 0.5 rad, the SNR penalty is less than 0.5 dB due to the XPM-induced nonlinear phase noise from adjacent OOK WDM channels without guard-band. For QPSK signals with feedforward carrier recovery, the smoothing filter must be designed to minimize the phase error. The optimal Wiener filter can lower the phase error of QPSK signals to within 4o to 6o even when the adjacent channels are OOK signals without guardband.