Evaluating middle-high frequency phase distortion of optics by fractal method

Based on the fractal theory, this paper analyzed statistical distribution features of middle-high frequency phase distortion of the optics for Shenguang-Ⅲ laser facility. The middle-high frequency phase distribution is found to be consistent with the model of fractional Brownian random field. The fractal dimension was calculated by structure function. The fractal dimension of large-aperture neodymium-doped phosphate amplifier glass is distributed between 2.34 and 2.43. A method of phase simulation by random midpoint-displacement was presented. In this method, the fractal dimension can be viewed as a parameter for estimating phase distortion, which helps to evaluate the high laser optics.