All-Optical Format Conversion of 2-Dimensional MQAM to MPSK Based on Nonlinear Mach-Zehnder Interferometer With Wavelength Preservation

Abstract

An all-optical format conversion scheme of 2-dimensional multiple-quadrature amplitude modulation (MQAM) to multiple-phase shift keying (MPSK) signals is proposed and numerically simulated by using the nonlinear Mach-Zehnder interferometer (MZI) configuration with wavelength preservation. The input octal-quadrature amplitude modulation (8QAM) signal is generated through the phase modulator (PM) and amplitude modulator (AM) and injected into the nonlinear MZI configuration. The designed nonlinear MZI configuration includes two 3 dB optical couplers (OCs), a piece of high nonlinear fiber (HNLF) and a variable optical attenuator (VOA) in the upper interfering arm and a piece of standard single mode fiber (SSMF) or tunable optical delay line (TODL) and a variable phase shifter (VPS) in the lower interfering arm. The converted octal-phase shift keying (8PSK) signal can be obtained from the nonlinear MZI configuration through vector superposition between the input optical signal and itself with some nonlinear phase shift. The needed asymmetric nonlinear phase shift is mainly induced by the big nonlinear index difference between HNLF and SSMF. The error vector magnitude (EVM) and bit error rate (BER) performance of the scheme are calculated before and after format conversion with the different input and receiver optical-signal-noise-ratios (OSNRs). The scheme can be used in the intermediate node to connect the different networks which adopt 8QAM and 8PSK signals, respectively.