Compensation of laser beam projection through strong turbulence with a multi-mirror adaptive optics system

A technique for laser beam projection system using multi-mirror Adaptive Optics in strong turbulence is investigated. The goal of our research is to obtain the highest possible power density in a small region on the target. However, in strong turbulence, when the laser beam is propagated over longer, and possibly horizontal, paths, the turbulence-induced phase aberrations in one region are converted to amplitude fluctuations, or scintillation, due to wave propagation mechanics. In addition to the scintillation, beacon anisoplanatism degrades the laser beam projection system performance.{09}The beacon created by illuminating the target of interest with a laser beam transmitted from the platform is a randomly shaped, extended source instead of an ideal point-like source. In most cases of practical interest, the beacon subtends an angle that is many times larger than the isoplanatic angle. To compensate for scintillation as well as phase distortion, and to mitigate the effects of beacon anisoplanatism we introduced two approaches. The first method uses a two deformable mirror near field phase retrieval method to determine the control commands for the two deformable mirrors. Simulation results show that placing the second deformable mirror in the near field of the first mirror can provide a substantial improvement for encircled energy compared to one deformable mirror conventional adaptive optics with a Hartmann wave from sensor and phase only correction. On average, the 50% encircled energy radius decreased by a factor of three to four for the two-DM near field PR system compared to a conventional adaptive optics (AO) system using only one DM when the Rytov parameter s2c equals 1.4418. The second approach is an image mask method with two deformable mirrors. The control commands of the two deformable mirrors are based on optimization of image sharpness metric with the Quasi-Newton method. Our results indicate that we can deliver more energy in the same area by using the image mask method with two deformable mirrors compared to the image mask method with one deformable mirror, and conventional adaptive optics with Hartmann wave front sensor and phase only correction.