All-Optical Format Conversion for Multichannel QPSK Signals

All-optical format conversion is a key function in providing flexible management and interface for optical networks with different formats. In this paper, we propose and demonstrate multichannel mutual format conversions between the nonreturn-to-zero quadrature phase-shift keying (NRZ-QPSK) and the return-to-zero QPSK (RZ-QPSK) signals using a single semiconductor optical amplifier and/or a delay interferometer (DI). The conversion from the NRZ-QPSK to RZ-QPSK is based on the cross phase modulation (XPM) effect, which is introduced by a synchronous optical clock signal with high input power, and subsequent detuned filtering. The amplitude of the NRZ-QPSK signal is converted to the RZ-QPSK signal with identical phase shift onto every input bit by the XPM effect. The conversion from the RZ-QPSK to NRZ-QPSK is based on the constructive interference in a single DI with a half-bit delay. The principle for the mutual conversions is analyzed theoretically, and simulations validate the feasibility of the proposed schemes. Parallel 4 channels format conversions between the NRZ-QPSK and the RZ-QPSK signals are experimentally achieved well with small Q penalty less than 2 dB.