Closed-Loop Control for Adaptive Optics Wavefront Compensation in Highly Scintillated Conditions

Conventional adaptive optics methods for controlling the phase control element based on least-squares reconstructions of the measured residual phase error exhibit poor performance as scintillation becomes strong. This paper compares the performance of various closed-loop control methods for different phase sensor types (self-referencing interferometer, shearing, and Shack-Hartmann interferometers), and for both conventional and segmented piston-type deformable mirrors (DMs). Significant performance improvements are demonstrated using a weighted least-squares reconstructor that adaptively optimizes the weights at each frame based on the intensities associated with each phase difference measurement and their sums around closed loops. Although the reconstructors considered do not explicitly place branch-cuts in the reconstructed residual phase, branch-cut like features can appear in both the single frame reconstructions and the closed-loop actuator commands. It is also found that at higher Rytov numbers, segmented piston-type DMs outperform conventional deformable mirrors. It is believed that conventional DMs suffer a fitting error associated with branch cuts in the actuator commands that the piston-type DMs are immune to. Performance trends corresponding to self-referencing interferometers provides a useful benchmark since, unlike Shack-Hartmann and shearing interferometers, the phase measurements are not corrupted by scintillation effects.