Adaptive wavefront control based on scintillation-resistant phase aberration sensing for free-space optical communication applications

Atmospheric adaptive optics (AO) system architectures utilizing a scintillation resistant wavefront sensor based on iterative phase retrieval techniques for closed-loop mitigation of atmospheric turbulence-induced wavefront aberrations in bidirectional free space optical (FSO) communication links are introduced and analyzed through wave-optics based numerical simulations. Laser power coupled into single-mode fibers utilized for both laser beam transmission and received light measurements was considered as a FSO link performance metric (power-in-the-fiber, PIF). An AO system based on a scintillation-resistant advanced phase contrast (SAPCO) wavefront sensor [1] and piston type deformable mirror (DM) composed of nDM x nDM square array of actuators (nDM ranging from 8 to 32) was considered in the optical train of one FSO communication terminal. The corresponding FSO terminal at the other end of the bidirectional link was represented by a passive optical transceiver system. Numerical simulations were conducted for horizontal propagation distances ranging from 0.5 km to 10 km, various turbulence strengths, and for FSO terminals with aperture diameters 30 cm and 15 cm. Analysis of AO control efficiency was based on Ntrial =100 computer simulated AO control trials composed of sequential wavefront aberration sensing and control update steps. Simulations were performed using identical initial conditions and different (statistically independent) turbulence realizations represented by equally spaced 2D random thin phase screens corresponding to the Kolmogorov turbulence power spectrum. Performance metric values computed in these AO trials were further averaged and the obtained dependencies characterizing atmospheric-averaged PIF metric evolution during AO control were utilized for AO control system parameter optimization and efficiency assessment. Numerical simulation results show that the adaptive beam control technique proposed here can provide a significant increase in received laser power coupling into the single mode fibers at both FSO communication terminals in the presence of strong intensity scintillations (Rytov number up to 10).