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Implementation of an adaptive Shack-Hartmann sensor using a phase-modulated liquid crystal spatial light modulator

In this paper we demonstrate a closed-loop adaptive-optics system that uses a twisted nematic liquid-crystal television (LCTV) as an adaptive Shack-Hartmann wave front sensor (SHWS). This system writes a dynamic lenslet array onto the LCTV so that each subaperture generates a focal spot at the focal length of the lenslet array. The focal spots of the lenslet array are detected by a video CCD camera. The focal spots move around if turbulence exists in the system, therefore the locations of the distorted focal spots are computed using a centroid algorithm and used to correct the local tilted wave fronts. Using the centroid shift data from all of the subapertures of the lenslet array, the incident wave front can be calculated and the phase can be reconstructed. In this experiment we assume that the slope of the disturbed wave front in a subaperture is the spatially averaged wave front tilt, and a correcting LCTV uses a simplifying linear approximation to generate a compensating tilt at each subaperture.