Compensation of propagation distortions using coherent optical adaptive techniques /COAT/

The paper examines the design concepts and hardware of COAT systems intended to determine the propagation phase distortions automatically in real time and then apply a time-dependent predistortion to the transmitted beam wavefront so that the net result after propagation through the atmosphere is a diffraction-limited wavefront, even for time-varying distortions. Three types of transmitting COAT systems that can perform the phase determination and correction task are considered: phase-conjugate, image compensation, and multidither outgoing-wave. The fundamental difference between these systems is the manner of determining the wavefront propagation phase errors. The COAT hardware is discussed in terms of optical receiver, servo electronics, and optical phase shifting element. Relevant COAT experiments and computer simulations are reviewed. COAT systems prove suitable for removing 80 to 95% of linear propagation distortions (turbulence, fixed optical system errors) as well as nonlinear distortions (thermal blooming) to a significant degree.