Laser damage resistant anti-reflection microstructures for mid-infrared metal-ion doped ZnSe gain media

Abstract

Power scaling of mid-infrared laser systems based on chromium and iron doped zinc selenide (ZnSe) and zinc sulfide (ZnS) crystals is being advanced through the integration of surface relief anti-reflection microstructures (ARMs) etched directly in the facets of the laser gain media. In this study, a new ARMs texture fabrication process is demonstrated for polycrystalline ZnSe and ZnS material that results in a significant increase in pulsed laser damage resistance combined with an average reflection loss of less than 0.5% over the wavelength range of 1.9-3.0μm. The process was utilized to fabricate ARMs in chromium-doped zinc selenide (Cr₂₊:ZnSe) materials supplied by IPG Photonics and standardized pulsed laser induced damage threshold (LiDT) measurements at a wavelength of 2.09μm were made using the commercial testing services of Spica Technologies. It was found that the pulsed LiDT of ARMs etched in ZnSe and Cr²⁺:ZnSe can match or even exceed the level of a well-polished surface, a survivability that is many times higher than an equivalent performance broad-band thin-film AR coating. The results also indicate that the ARMs plasma etch process may find use as a post-polish damage mitigation technique similar to the chemical immersion used to double the damage resistance of fused silica optics. ARMs etched in Cr²⁺:ZnSe were also evaluated by IPG Photonics for survivability under continuous wave (CW) laser operation at a pump laser wavelength of 1.94μm. Catastrophic damage occurred between power levels of 400-500 kilowatt per square centimeter for both as polished and ARMs textured samples indicating no reduction in CW damage resistance attributable to surface effects.