Sensitive optical free-space receiver architecture for coherent combining of deep-space communication signals through atmospheric turbulence

High data rate optical deep space communication links for future space missions need large capture area receivers that can efficiently couple light into a single-mode fiber. Coherent detection is attractive as it offers both high spectral efficiency and sensitivity. Here, we numerically investigate two such large area receivers in the context of weak signal reception; the multi-aperture array and the multi-mode fiber-coupled receiver, together with optical coherent combining. We find that the number of speckles captured by the aperture should match the number of modes supported by the receiver-fiber for high efficiency and sensitivity. Using an optically preamplified dither-optical phase locked loop for tip-tilt, phase, and amplitude compensation, we predict that efficient reception of signals can be maintained down to -80 dBm of received power per mode for realistic atmospheric channels.