Suppression of Gain Variation in a PSA-Based Phase Regenerator Using an Additional Harmonic

Phase regeneration of an M-ary phase shift keyed signal through coherent addition with its (M-1)th phase harmonic, despite successfully regenerating phase, exhibits a strongly phase-dependent gain. By applying to this scheme, a small alteration that makes use of both the (M-1)th and conjugated (M+1)th phase harmonics, this undesirable gain variation may be reduced. We experimentally characterize both systems for the case of four-level phase quantization in terms of their complex transfer profiles and demonstrate a 6.5dB reduction in gain variation using the altered scheme. To perform measurements of the phase transfer profile of each scheme, we synthesize the systems from an optical comb, proposing and implementing a novel method to measure the relative phase between neighboring spectral lines.