Near-field diffraction analysis of color-separating gratings under high-power laser condition

Abstract

Color Separating Grating (CSG) is one of the important Diffractive Optical Elements (DOE) used in the final optical system of high power laser system. Its periodic step-phase structure can separate the third harmonic frequency from base- and second-harmonic waves in the far field. But the structure abbreviations of CSG, caused by the fabrication process, generate great modulations to the laser beam, which may lead to severe optical damages to CSG itself and the system. In this paper, a comprehensive error model is built, in which each structural parameter of CSG is expressed as a variable, and the structural parameter error induced by fabrication process is expressed as minute disturb of relative variable. With this error model, the near field diffraction pattern of CSG is calculated based on Fresnel diffractive theory. Through simulation and analysis, we obtain the amplitude modulation of different harmonic waves in near field, and also the relationship between the amplitude modulation and the fabrication error. The results show that beam modulations are mainly caused by the stair depth error and the slope error. This study provides a convenient way to estimate the possibility of optical damages induced by CSG.