In-line phase-sensitive amplification of QPSK signal using multiple quasi-phase matched LiNbO₃ waveguide.

Phase-sensitive amplifiers (PSA) using periodically poled (PPLN) LiNbO₃ waveguides are promising as low-noise optical amplifiers. However, it is difficult to realize in-line operation for multi-level phase modulated signals using a PPLN based PSA with the conventional configuration. In this paper, we report a PPLN based in-line PSA that can regenerate quadrature phase shift keying (QPSK) signals. Multi-stage frequency mixing in a multiple quasi-phase matched LiNbO₃waveguide allows carrier phase recovery from a QPSK signal. Non-degenerate parametric amplification enables the phase-sensitive amplification of a QPSK signal. Amplitude and phase regeneration is examined utilizing gain saturation and phase squeezing capability.