Postcompensation of Nonlinear Distortions of 64-QAM Signals in a Semiconductor-Based Wavelength Converter

We experimentally investigate postcompensation of nonlinear distortions induced by a wavelength converter (WC) based on four-wave mixing in a semiconductor optical amplifier. The technique exploits a low-complexity digital filter-based backpropagation (DFBP) method. We perform postcompensation of nonlinear distortions following single stage wavelength conversion of 5 Gbd 64-quadrature amplitude modulation (QAM). We examine the DFBP performance in the presence of a degraded optical signal-to-noise ratio at the WC input, and explore the WC optimal operating conditions. Also, we experimentally demonstrate for the first time in the literature the dual stage wavelength conversion of QAM signals, in particular, 5 Gbd 64-QAM, and show that bit error rate below hard-decision forward error correction threshold is only possible with postcompensation of nonlinear distortions. These results are of importance for the development of wavelength-routed networks requiring successive wavelength conversion stages to enhance routing capabilities.