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A concept of a Hybrid Wavefront-based Stochastic Parallel Gradient Decent (WSPGD) Adaptive Optics
(AO) system for correcting the combined effects of Beacon Anisoplanatism and Thermal Blooming is
introduced. This system integrates a conventional phase conjugate (PC) AO system with a WSPGD AO
system. It uses on-axis wavefront measurements of a laser return from an extended beacon to generate
initial deformable mirror (DM) commands. Since high frequency phase components are removed from the
wavefront of a laser return by a low-pass filter effect of an extended beacon, the system also uses off-axis
wavefront measurements to provide feedback for a multi-dithering beam control algorithm in order to
generate additional DM commands that account for those missing high frequency phase components.
Performance of the Hybrid WSPGD AO system was evaluated in simulation using a wave optics code.
Numerical analysis was performed for two tactical scenarios that included ranges of L = 2 km and L = 20
km, ratio of aperture diameter to Fried parameter, D/r0, of up to 15, ratio of beam spot size at the target to
isoplanatic angle, θB/θ0, of up to 40, and general distortion number characterizing the strength of Thermal
Blooming, Nd = 50, 75, and 100. A line-of-sight in the corrected beam was stabilized using a target-plane
tracker. The simulation results reveal that the Hybrid WSPGD AO system can efficiently correct the effects
of Beacon Anisoplanatism and Thermal Blooming, providing improved compensation of Thermal
Blooming in the presence of strong turbulence. Simulation results also indicate that the Hybrid WSPGD
AO system outperforms a conventional PC AO system, increasing the Strehl ratio by up to 300% in less
than 50 iterations. A follow-on laboratory demonstration performed under a separate program confirmed
our theoretical predictions.
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