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A new wavefront sensing technique for satellite-ground laser communication

Adaptive optics plays an important role to compensate the atmosphere turbulence therefore concentrate the laser energy for satellite-ground laser communication. However, as the satellite especially LEO (Low-Earth orbit) moves, the communication laser from downlink (satellite to ground) and uplink (ground to satellite) will experience a different turbulence path, called the point ahead angle (PAA). PAA can be much larger than the atmosphere isoplanatic angle for strong turbulence or fast moving satellites, causing the AO system not working. For now there is no simple and effective way to solve this problem. In this paper, a new wavefront sensing technique called Projected Pupil Plane Pattern (PPPP) is used, where the Rayleigh backscattered light of the uplink laser is used to sense the uplink path of the turbulence. Specifically, PPPP uses at least two scattered images from two different heights to reconstruct the integrated turbulence phase due to the TIE (transport-of-intensity). As PPPP uses the uplink laser itself, the PAA problem is solved automatically. We demonstrate that PPPP method can be effectively used as a simple wavefront sensor in the adaptive optics system for satellite-ground laser communication by numerical simulation for 1m class ground telescope and AO system. Several important PPPP coefficients such as the propagation heights, number of Zernike Modes for reconstruction are studied, and their optimum choices are given.