Phase-sensitive amplification in loop configuration for QPSK signal regeneration

In this paper, phase-sensitive amplification (PSA) in a loop configuration for phase regeneration of quadrature phase-shift keying (QPSK) signals is proposed. Unlike the conventional PSA schemes in which the signal output phase is locked to one of two values (e.g., 0 or π), the proposed scheme outputs signal light with one of four phase values different by π/2, owing to the loop configuration with two pump lights. All-optical QPSK signal regeneration can be achieved by using this PSA scheme.