Depth localization of subsurface defects by optical dark-field confocal microscopy

Subsurface defects (SSD) in optical components pose a significant challenge for enhancing the power density of high-energy laser devices. This study investigated the issue of systematic deviation between the measured and actual depths of SSD when employing optical dark-field confocal microscopy for three-dimensional measurements, which is attributed to refractive index disparities between the sample and the observation environment. This paper introduces geometric and diffraction optical models for correcting errors in the SSD depth, along with a calculation method for determining the correction coefficient. By comparing the experimental data and model simulations, a linear relationship between the measured and actual depths was identified with linearity errors below 2.5% and a minimum of 0.67%. The correction coefficients derived from the optical diffraction model are in good agreement with those obtained experimentally. These findings offer valuable insights for calculating SSD depth correction coefficients across various scenarios and requirements to ensure precise measurements.