Online digital compensation of phase and amplitude distortions in cascaded FOPA transmission links

Today mitigating stimulated Brillouin scattering (SBS) remains a significant challenge for the eventual penetration of fibre-optical parametric amplifiers (FOPAs) into future communication systems. The pump phase modulation that is often used for SBS mitigation causes temporal variation of the parametric gain, which is primarily a source of phase distortion for coherently detected quadrature-amplitude modulation (QAM) signals. Here, we introduce a new and fully online digital signal processing (DSP) scheme that enables compensation of the intrinsic dithering-induced phase distortion and its conversion into amplitude distortion when interacting with the dispersive fibre channel in transmission systems with cascaded FOPAs operating with multi-tone pump dithering to support sufficient SBS-limited gain. Through extensive numerical simulations, we identify the required dimensioning for performance improvement and evaluate the scheme's robustness against non-ideal conditions. We demonstrate that enhancing a conventional DSP chain with the proposed algorithm achieves significant performance benefits in 28-Gbaud 16-QAM transmission. We believe that our scheme may become a key component of future FOPA-amplified links, where pump dithering will be necessary to achieve high amplification gains.