Phase compensation considerations on coherent, free-space laser communications system

Free-space optical communications have distinct advantages over conventional RF and microwave systems by virtue of their high carrier frequencies that permit high modulation bandwidth, enhanced security, freedom from interference, and low power. However, the turbulent atmosphere causes phase variations along the path that are manifested in intensity variations (scintillation) and beam profile distortion and boresight wander. These variations manifest as a multiplicative noise source that reduces the ability of the receiver to determine the information contained in the modulation. For many years, the emphasis throughout this area has been on elucidating those implications of the atmospheric propagation problem that bear on the design and performance of optical communication systems. In this work, it is our intention toelucidate how the addition of adaptive optics to the transmitter or receiver can reduce the effects of atmospheric propagation and, in so doing, to quantify the improvement on the performance of optical communications systems regarding coherent detection.