Wavelength Conversion of 25-Gbit/s PAM-4 Signals Using a Quantum-Dot SOA

We demonstrate all-optical wavelength conversion of a 4-level pulse-amplitude modulation (PAM-4) signal by cross gain modulation (XGM) in a quantum-dot semiconductor optical amplifier (QD-SOA). In this scheme, the center passband of a conventional optical filter located at the QD-SOA output is slightly shifted to the shorter wavelength side [blue-shifted filtering (BSF)], which enables us to accelerate the gain recovery time of the QD-SOA and reduce the bandwidth limitation in the process of the wavelength conversion. By using the combination of XGM and BSF, we successfully achieve 25-Gbit/s PAM-4 wavelength conversion, with good conversion performance in terms of error-vector magnitude. We also investigate the conversion performance in a wide wavelength-hopping range and the high tolerance to the shift in input signal wavelength for use in data center applications.