多层电介质衍射光栅高功率激光辐照特性研究

Research on Characteristic of Multilayer Dielectric Diffraction Grating under High Power Laser Irradiation

多层电介质反射式衍射光栅作为光谱合成系统的核心元件,在高功率激光辐照下的表面形变将导致其衍射特性改变,进而影响系统合成光束质量和衍射效率。搭建泰曼-格林干涉仪探测在高功率激光辐照下衍射光栅表面的干涉条纹,经图像处理及泽尼克多项式拟合实现光栅表面形貌的重建。采用高功率光纤激光辐照加热光栅,测量另一光束经过热畸变光栅后的光束质量和衍射效率演变。结果表明,在高功率激光辐照下光栅表面形变高度分布为高斯型,辐照功率密度3kW/cm^2时,干涉条纹对应区域内的光栅表面在垂直方向上隆起,最大高度为127nm。表面形变引起衍射光束质量因子M2退化,这种退化主要来自衍射光远场发散角的增大。光栅衍射效率波动幅度小于1.8%。

Multilayer dielectric diffraction grating is the most important optical element in spectral beam combining system. The surface distortion of grating under high power laser irradiation will change its diffraction characteristic, which will influence the beam quality and diffraction efficiency of the combining system. We establish an experimental setup of Twyman-Green interferometer to measure the interference fringes on the diffraction grating surface, under the high power laser irradiation. Based on the image processing and Zernike polynomial fitting method, the grating surface profile is reconstructed. With the high power laser heating the grating and another laser measuring the beam quality and diffraction efficiency degradation, the results show that the grating surface deformation distribution conforms to Gaussian model, when the irradiance power density is 3 kW/cm^2, the maximum distortion is evaluated to 127 nm, which leads to the diffraction beam quality factor M2 degradation. The degradation is mainly due to the increase of angle divergence after diffraction. The change of diffraction efficiency is less than 1.8%, which is basically unchanged.