Chemical and Laser Damage Resistant, High Performance AR Microstructures For DPALs

Compact, efficient, mega-watt power level lasers are being developed by the MDA for the mission of destroying targets over a range of many hundreds of kilometers. A candidate laser technology with the promise of meeting this mission goal is based on alkali gas that is optically pumped by multiple, electrically driven diode lasers. One current problem limiting the potential operational lifetime of these Diode Pumped Alkali Lasers, or DPALs is that the windows of the cell containing the alkali vapor, become fouled with deposits or fogged by damage from chemical reactions with the gas, reactions that increase as the laser optical power is scaled up. In addition, DPALs cannot operate efficiently in the presence of laser cavity losses such as reflections from the gas cell windows, and therefore some form of anti-reflection (AR) treatment must be applied. Conventional thin-film material coatings designed to suppress reflections are typically less resistant to chemical attack from alkali compounds than the cell window material, and thin-film AR coatings suffer optical damage at laser power levels well below the mission requirements. A roadblock to major increases in the power output and reliability of DPALs can be removed by development of a more robust AR treatment for the windows of the gas cell. The innovative solution proposed is to eliminate thin-film coatings completely, creating the critical AR function by fabricating microstructures directly in sapphire windows that have been shown to be naturally resistant to alkali chemical attack. AR microstructures (ARMs) etched in sapphire windows have exhibited higher transmission and pulsed laser damage thresholds 2-5 times higher than thin-film AR coatings. This Phase I project proposes to demonstrate high transparency ARMs textures built in sapphire and other chemically resistant window materials suitable for DPAL vapor cells. Multiple ARMs design variants will be fabricated in sapphire coupons and subjected to standardized pulsed laser damage testing and alkali vapor exposure testing using the flowing DPAL system at Kirtland AFB. Additional ARMs treated vapor cell windows will be delivered to the Government for further evaluation. As part of a Phase I Option, a design for a microstructure-based, all sapphire output coupler will be investigated and prototyped as a means for eliminating other DPAL cavity thin-film coatings. Potential DPAL manufacturers such as General Atomics, Northrup Grumman, and Boeing will be engaged to enable the new robust AR treatment to be integrated into MDA test platforms during Phase II and Phase III commercialization projects.