Performance of atmospheric coherent laser receiver and bandwidth for adaptive optics system

For atmospheric links, the free space optical communication is challenged by the effects of turbulence. Wave-front aberration and power scintillation of the signal light are the main factors which lead to the degradation of system performance. In this paper, the bit-error-rate performance of atmospheric coherent laser receiver with binary phase shift keying modulation is performed. Beside the turbulence induced wave front phase error and the power scintillation, we investigated the time-delay phase error due to the insufficient of servo bandwidth of adaptive optics system. Our simulation results show that Greenwood frequency can meet the requirement for the servo bandwidth of adaptive optics system when the detected photons per bit are more than 100. However, if there are 70 or less photons per bit detected, 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, the aperture averaging effects is analyzed and we found that the optimal size of receiving aperture may be around twice of the coherent length r₀ when there is no adaptive optics employed in system.