Automatic mode-matching of a Fabry-Pérot cavity with a single photodiode and spatial light modulation

Conventional cavity alignment sensing uses multi-element photodiodes to sense the matching of a laser beam to the mode of a cavity. We present a new approach that uses a single-element photodiode combined with deformable mirrors to modulate orthogonal codes across distinct spatial regions of the laser wavefront. By decoding the photodiode signal it is possible to map the mode mismatch and produce error signals for feedback to the mirrors to optimize the coupling of the laser to the cavity. We characterise this sensing and control system experimentally and demonstrate closed-loop enhancement of cavity alignment and mode matching.