Ultra-low stress and high laser damage threshold optical coatings for large-aperture component

Abstract

In high-power laser systems, various optical components must possess high damage thresholds and excellent surface figure quality. However, some large-aperture ultra-thin optical elements are prone to significant changes in their surface figure due to the residual stress in high-reflective coatings. Therefore, it is imperative to mitigate the residual stress in thin films while keep high damage threshold. This paper theoretically analyzes the impact of the stress state in the films on its damage threshold and introduces a multilayer coating design strategy aimed at decoupling the stress characteristics from the anti-damage performance of the HR films. Then, for the large-aperture ultra-thin mirror, the stress optimal targets for thin film stress reduction were identified through numerical simulation. Finally, an ultralow-stress, high-damage-threshold HR films technology were successfully developed, its equivalent residual stress is only about 6.36 MPa. By employing this technology, exceptional deformation control was achieved on ultra-thin large-aperture mirrors (diameter-thickness ratio: 78), which surface shape only changed by less than 1 μm (PV) after coating.