Metal mirrors with metal-dielectric HR-coating for ultrashort laser pulses applied in scanner applications

Metal mirrors are an attractive solution for scan mirrors working with ultra-short pulse lasers. Small mechanical inertia and a small mirror mass are required. Therefore, the mirrors have to be very stiff and a high quality optical surface has to be provided. This can be achieved with lightweight AlSi based mirrors with diamond-turned NiP polishable plating. Different coating options were evaluated in order to provide the necessary high reflectivity and a satisfactory laser damage threshold for ultrashort laser pulses in the few ps to fs regime at λ = 1030 nm. High-reflective metal layers enhanced by dielectric HfO 2 /SiO 2 stacks were found to be the most advantageous coating option due to their comparatively small thickness and measured damage thresholds above 1 J/cm 2 @8ps.