An All-Digital Optical Phase-Locked Loop Suitable for Satellite Downlinks

Abstract

The optical signal propagation used in satellite uplinks and downlinks is influenced by absorption, scattering, and changes in the atmospheric refractive index or turbulence, causing optical signal attenuation. A free space optics (FSO) communications system using coherent communication can improve the link sensitivity and reach higher distances. This article proposes a new architecture for the phase detector in an all-digital optical phase-locked loop (OPLL) for coherent optical detection. Firstly, the performance of the proposed phase detector is evaluated under Gaussian noise, where the best operation point is found for the OPLL working with two sample rates: 625 MSa/s and 10 GSa/s. The system analyses also take a non-negligible delay into account. Then, it will be evaluated and compared with an OPLL using an analog phase detector in the presence of atmospheric turbulence. Finally, in three different atmospheric turbulence conditions, the effect of wind speed on communication quality is investigated through the obtained bit error rate (BER) from the recovered data for a bit rate of 20 Gbps. The results show that the proposed digital phase detector can track a signal under longer feedback loop delays and fading signals.