Multi-level modulation formats offer a higher spectral efficiency than on-off keying at the same data rate. Most popular is quadrature phase-shift keying (QPSK). However, it is susceptible to amplitude noise and also to phase noise. Two schemes for QPSK phase regeneration have been previously proposed. The first one [1] is based on phase diversity (PD) and uses two phase-sensitive amplifiers in a Mach-Zehnder configuration. The operation of the second scheme [2] is based on four-wave mixing (FWM) of the signal with its higher-order idler (HOI) and two pumps. We have studied the regeneration ability of the two schemes for QPSK phase regeneration with the emphasis on their sensitivity to amplitude noise in the incoming signal. This can be especially important in high-bit-rate transmission systems with strong high-frequency components of the relative intensity noise due to intra-and inter-channel nonlinear effects.
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