Comparison of Nonlinear Phase Noise and Intrachannel Four-Wave-Mixing   for RZ-DPSK Signals in Dispersive Transmission Systems
  Self-phase modulation induced nonlinear phase noise is reduced with the increase of fiber dispersion but intrachannel four-wave-mixing (IFWM) is increased with dispersion. Both degrading DPSK signals, the standard deviation of nonlinear phase noise induced differential phase is about three times that from IFWM even in highly dispersive transmission systems. 
Keang-Po Ho and Hsi-Cheng Wang