Optical Coherent Combining in Multi-Aperture Free-Space Receivers through Atmospheric Turbulence via Optically Computed Alignment Gradients
Multi-aperture reception converts spatial diversity into a coherent-combining gain, but only if the relative phase and polarization of the collected field replicas are aligned before they are summed. In a satellite-to-ground downlink the atmosphere perturbs amplitude, phase and polarization, so the alignment must track a fluctuating channel rather than a fixed offset. This paper proposes a new optical-domain combining scheme in which the alignment information is computed by light itself: the ascent directions that maximize the combined output power are formed by optical interference and read out directly as differential photocurrents of balanced detection, so no estimate of the channel, no deliberate perturbation, and no per-branch digital processing is needed to obtain them. Every branch is aligned against one shared reference - a passive tap of the combined field itself - so no independent laser or optical frequency lock is required, and the update directions of all branches are available simultaneously in a single averaging window without frequency, code, or time multiplexing. The data path needs a single dual-polarization coherent receiver regardless of the aperture count, and the readout is built from elementary parts only - a 3 dB directional coupler, static phase shifters, a Faraday rotator, and balanced photodiodes - so no 90-degree optical hybrid, mixer, or polarization-diverse hybrid is required. The phase and polarization loops are fixed-gain and division-free, and each branch carries one phase modulator and one polarization controller with no amplitude actuator. A closed-form combining-gain and loss budget is derived, the assumptions and limitations of the scheme are stated explicitly, and experimental validation remains open.