Unambiguous phase retrieval as a cophasing sensor for phased array telescopes

Cophasing a multiple-aperture optical telescope (MAOT) or optical interferometer requires the knowledge of the tips/tilts and of the differential pistons on its subapertures. In this paper we demonstrate in the case of a point source object that a single focal-plane image is sufficient for MAOT cophasing. Adopting a least-square approach allows us to derive an analytic estimator of the subaperture aberrations, provided that these are small enough (typically for closed-loop operation) and that the pupil is diluted noncentrosymmetric. We then provide the validation of this estimator by simulations as well as a performance comparison with a more conventional iterative algorithm of phase retrieval. Finally, we present the experimental validation of both estimators on a laboratory test bench; our results, especially subnanometric repeatability, demonstrate that focal-plane sensors are appropriate for the cophasing of phased array telescopes.