Nanotextured Antireflective YAG for High Energy Laser Systems

The Air Force utilizes yttrium aluminum garnet (YAG) laser crystals for critical applications including target designation, range finding, guidance, and directed energy weaponry; but has identified that thin film antireflection coatings on the laser facets reduce the laser-induced damage threshold (LiDT) of the crystal and are a primary limiting factor in achieving desired power levels. The Air Force recognizes that these coatings can by be replaced with an antireflective nano-texture that is etched directly into the critical laser facet, providing a rugged, low absorption solution with superior optical performance and laser damage resistance.  In the Phase II effort, YAG crystals will be nano-textured and characterized; showing bandwidth, reflection level, and angular and polarization insensitivity properties that far exceed the capabilities of coatings. Absorption measurements on the etched surfaces will confirm the absence of the absorptive defects that are seen in thin film coatings and lead to premature laser damage. Standardized pulsed and CW laser damage testing will demonstrate that the damage threshold of nanotextured YAG surfaces greatly exceeds the thresholds of military specification antireflection coatings for YAG.  Beyond the performance and durability metrics of nano-textures, improved cleaning and integration processes are required to increase product yield and drive down cost in laser manufacturing. This program will work closely with our commercial Phase II partners to simplify and streamline the methods of integrating nano-textured surfaces into thin disk and slab YAG lasers. Comprehensive analysis of laser function and stability will verify that antireflective nano-textures provide improved beam quality and stability, and provide significantly improved power handling. This Phase II effort demonstrating a reproducible and cost-effective manufacturing process for integrating nano-textures with YAG, is a critical milestone on the path to providing the U.S. Department of Defense with an industry base for providing critical nanotextured infrared materials, and a path to advancing power scaling in high energy lasers.