Satellite Downlink Coherent Laser Communications

Abstract

Free-space coherent laser downlinks can potentially provide high data rates in space-to-Earth communications, and are a key enabler for future optical information systems in space. There is growing interest in the high sensitivity and spectral efficiency of coherent optical receivers for downlink systems, despite a higher cost and complexity than direct-detection receivers. In practice, however, when the link passes through the atmosphere, clear-air turbulence induces serious phase distortions and fading that impair coherent receivers. The impact of turbulence can be mitigated conveniently by using channel matched array receivers, which comprise multiple subapertures. Alternatively, this impact can be mitigated using a single-aperture receiver with adaptive optics to correct wave front distortion. Here we provide a comprehensive, unified analysis of these two fundamental techniques in atmospheric downlink coherent systems. Our results demonstrate how array receivers and adaptive optics are expected to perform in tracking and correcting atmospherically distorted signals.