High Laser Damage Threshold, Broadband Anti-Reflection Treatment Based on Surface Relief Microstructures

Multiple strategic military missions depend on the advancement of high power lasers that operate within the IR spectral region. For maximum effectiveness, laser systems under development for the CIRCM, ABL, and next generation UAV programs, require increased power and broad wavelength agility. Promising tunable mid-IR solid state laser sources depend on conventional thin-film coating technology to achieve critical optical functions such as anti-reflection (AR), high reflection (HR), and spectral or polarization filtering. Thin-film coatings are easily damaged within high power laser systems, and the threshold for coating damage decreases as the demand for higher performance or wider bandwidth increases. As a primary example of this performance/reliability tradeoff, developers scaling the power output and tuning range of metal-ion doped semiconductor lasers opt for more complex, less stable laser cavity configurations with reduced efficiency in order to avoid the use of thin-film AR and HR coatings directly on the laser gain material facets. Dramatic increases in the power output and tuning range of these laser sources could be attained by development of a more robust AR treatment applied to the gain material facets alone. An innovative AR treatment based on surface relief microstructures has been shown to have great potential for increasing the reliability and power handling capacity of optical components. AR microstructures (ARMs) etched directly in the surface of relevant IR transmitting materials have consistently exhibited damage thresholds 2 times higher than untreated surfaces, a value that equates to a 4-5 time increase over any equivalent performance broad-band thin-film AR coated surface. This Phase I project proposes to demonstrate robust, wide bandwidth, high performance ARMs textures built in the end facets of chromium ion (Cr2+) doped zinc selenide (ZnSe) and zinc sulfide (ZnS) laser gain material. Multiple ARMs design variants will be fabricated in ZnSe, ZnS, and Cr2+:ZnSe coupons and subjected to standardized pulsed and continuous wave laser damage testing. Additional ARMs treated coupons of the most promising designs will be delivered to the Government for further damage testing. In collaboration with IR laser manufacturer IPG Photonics, the new robust AR treatment will be integrated into critical AFRL systems during Phase II and Phase III commercialization projects. BENEFIT: It is anticipated that a dramatic increase in laser power handling capacity combined with enhanced operational lifetime will be achieved through the integration of anti-reflecting microstructures in solid state laser systems. In particular, the broad-band performance of ARMs will benefit tunable metal-ion doped ZnSe and ZnS lasers that find a wide range of applications throughout the mid-IR spectral region. Air Force applications requiring more reliable, higher power, wavelength agile mid-IR laser sources include laser communications, countermeasures, target designators, weapons, rangefinders, remote chemical sensors, and infrared scene projectors. Commercial applications include environmental chemical monitoring, industrial welding and cutting systems, and food processing. Significant advances in medical devices for surgery, noninvasive treatments, breath diagnostics, and imaging await higher power laser sources, as do astronomical instruments for spectroscopy, planetary exploration, earth and solar observations, and optical telecommunications. Although the proposed effort will immediately benefit metal ion doped ZnSe and ZnS laser materials, the robust nature of ARMs can be extended to any current or emerging laser technology such as quantum cascade lasers, zinc germanium phosphide lasers, and semiconductor microchip lasers, as well as silica and chalcogenide glass optical fiber laser delivery systems.