All-optical amplitude regeneration of non-return-to-zero differential-phase-shift-keying signal

We propose and experimentally demonstrate amplitude regeneration for non-return-to-zero differential-phase-shift-keying (NRZ-DPSK) signal at 40 Gb/s with two cascaded semiconductor optical amplifiers (SOAs) and a subsequent delay interferometer (DI). The amplitude regeneration can reduce the accumulation of nonlinear phase noise (NPN), which is one of the most severe impairments resulting from the intensity fluctuations of the signal in the long haul transmission. The regeneration course includes three stages, which are the NRZ-DPSK to return-to-zero DPSK (RZ-DPSK) format conversion achieved by the cross phase modulation (XPM) effect in a SOA, the amplitude regeneration of RZ-DPSK realized by using the gain saturation effect in a second SOA and the format conversion from RZ-DPSK back to NRZ-DPSK based on a constructive interferometer with a DI. The regenerative performance of the system is characterized in terms of bit-error rate against received power. A power penalty improvement up to 1.2 dB is obtained in our experiment.