Pump-Free and Reconfigurable All-Optical Modulation Format Conversion for MQAM Signals by Parallel Nonlinear Mach-Zehnder Interferometers

A pump-free and reconfigurable all-optical modulation format conversion (MFC) scheme for multiple quadrature amplitude modulation (MQAM) signals by the designed parallel nonlinear Mach-Zehnder interferometers (MZIs) is proposed and theory simulated for the first time. In this scheme, the input 30 Gbit/s 8-ary quadrature amplitude modulation (8QAM) signal is divided into two branches by the first 3-dB optical coupler (OC) to get the upper and lower branches 8QAM signals. For the input upper branch 8QAM signal, when it is injected into the designed nonlinear MZI configuration, the first converted quadrature phase shift keying (QPSK1) signal can be obtained. Compared to the input upper branch 8QAM signal, the input lower branch 8QAM signal is firstly sent into an extra piece of highly nonlinear fiber (HNLF) to obtain the pre-perturbed 8QAM signal. When the pre-perturbed 8QAM signal is also injected into the designed nonlinear MZI configuration, the second converted QPSK signal (QPSK2) can be extracted. After adjusting the power ratio (PR) and the relative phase shift (RPS) between QPSK1 and QPSK2, several output optical signals including the aggregated 8QAM (agg-8QAM) signal, the 8-ary phase shift keying (8PSK) signal, the square 8QAM (squ-8QAM) signal, the 8-ary amplitude and phase shift keying (8APSK) signal and the atypical on-off keying (OOK) signal can be obtained from the bar port of the last 3-dB OC. The proposed MFC scheme can also be extended to the other advanced modulation formats including the 40 Gbit/s star 16-ary quadrature amplitude modulation (star-16QAM) signal. The error vector magnitude (EVM) and bit error rate (BER) performance of the input and output optical signals before and after MFC are calculated to analysis the scheme performance. The proposed MFC scheme doesn't need any extra pump which is phase-locked to the input signal. It can also be applied at the intermediate network nodes (INNs) to dynamically select modulation formats suitable for applied between different types of optical networks.