Bandwidth of adaptive optics system in atmospheric coherent laser communication

The bit-error-rate performance of free space optical communication systems with binary phase shift keying modulation and coherent homodyne detection is performed. Besides the turbulence-induced wave-front phase error and the amplitude fluctuation, the servo bandwidth of adaptive optics system is investigated. It is shown that Greenwood frequency is large enough for the servo bandwidth of adaptive optics system when the detected photons per bit are more than 100. However, if the photons per bit are less than 70, the Greenwood frequency is only sufficient for weak scintillation. We should increase the servo bandwidth to almost twice the value of Greenwood frequency at least in order to obtain an acceptable BER performance when the scintillation index is larger than 0.7. In addition, we also investigate the aperture averaging effects when the receiving aperture is larger than the coherent length.