Performance evaluation of large-aperture highly reflective mirrors for high-power laser applications with reflectance mapping

Abstract

The performance evaluation of large-aperture highly reflective (HR) mirrors, which are widely used in high-power laser systems, is essential for the long-term operation of these laser systems. In this study, the development of the HR mirror's reflectance distribution, which is measured via cavity ring-down (CRD) technique, to be a general quality control tool for the large-aperture HR mirrors is attempted. Various parameters and data analysis methods are defined and developed to evaluate the performance of the large-aperture HR mirrors. Specifically, we introduced the maximum-probability reflectance (R_max-p), which describes the reflectance distribution excluding the influence of defects, and the maximum-residual-probability reflectance (R_max-r-p), which characterizes the reflectance distribution affected by defects. Additionally, we discussed the full-width at half-maximum (FWHM) of the residual reflectance distribution, which quantifies the variation in reflectance due to defects, as well as the reflectance difference (ΔR) defined as the difference between R_max-p and R_max-r-p. These parameters collectively provide a comprehensive assessment of the HR mirrors' performance. The developed parameters and data analysis methods were applied to the performance evaluation of 68 HR mirrors with diameters ranging from 200 mm to 400 mm, with 0- or 45-degree angle of incidence (AOI) and fused silica or sapphire substrates. Statistical analysis revealed that HR mirrors with a 0-degree AOI have higher reflectance than that with a 45-degree AOI, while HR mirrors deposited on fused silica substrates showed slightly better reflection performance than that deposited on sapphire substrates. The statistical results demonstrated that the proposed parameters and data analysis methods are helpful to evaluate the performance of large-aperture HR mirrors.