Impact of manufacturing defects in wavelength-separation coatings on the optical field distribution at incipient laser damage

Wavelength separation coating can effectively serve as harmonic separation elements to isolate fundamental frequency (1ω) and third-harmonic (3ω) laser beams. However, compared to the bare substrate, the impurity defects introduced during the coating process significantly reduce the laser-induced damage threshold (LIDT) of the optical thin film. To investigate the influence of defect size, location and incident laser wavelength on the initial damage process in optical thin films, an early-stage light field modulation model for optical damage for SiO2/HfO2 multilayer dielectric films was established based on laser propagation theory, followed by numerical simulations. The results demonstrate that defect size, location and incident laser wavelength significantly affect the location of light-intensified hot spot during the initial damage stage. The LIEF of the 1ω laser peaks at a nodule defect diameter of 325nm. For a 10-nanometer fabrication defect at the 32nd layer, the LIEF for the 3ω laser reaches its maximum. This work holds significant theoretical importance and engineering application value for optimizing the manufacturing process of optical thin films to enhance performance.