Impact of pump phase modulation on QAM signals in polarization-insensitive fiber optical parametric amplifiers

We examine mitigation of stimulated Brillouin scattering (SBS) in fiber optical parametric amplifiers (FOPAs) by pump phase modulation in the context of QAM signals and the state-of-art polarization-insensitive FOPA architecture. We characterize the impact of the pump phase modulation on QAM signals in the cases of single-polarization FOPAs and polarization-insensitive FOPAs, and study pathways to minimization of this impact. We find that the impact of pump phase modulation on signals is more complicated in polarization-insensitive FOPAs than in single-polarization FOPAs, and it can be mitigated by using the features of the polarization-insensitive FOPA architecture. We use the required optical signal to noise ratio (rOSNR) to evaluate the impact of pump phase modulation on QAM signals and find that the induced signal phase modulation makes significantly more contribution than the induced signal amplitude modulation. Our numerical and experimental study has revealed a distribution of the rOSNR penalty across the FOPA gain spectrum which is distinct from previously reported for on/off signals. We explore and optimize the pump phase modulation schemes to reduce the signal rOSNR penalty whilst providing more SBS mitigation at the same time. We demonstrate that a receiver digital signal processing plays a significant role in contribution to rOSNR penalty.