Cryogenic Eyesafer Laser Optimization for Use Without Liquid Nitrogen

Report No. ARL-TR-6826
Authors: Larry D. Merkle and Nikolay Ter-Gabrielyan
Date/Pages: February 2014; 24 pages
Abstract: We report our efforts and findings on a High Energy Lasers Joint Technology Office (HEL-JTO)-funded project to identify and optimize Er-doped laser materials and lasing schemes for cryogenic operation without the need for liquid cryogens. This calls for optimal performance around 125–150 K—high enough for reasonably efficient operation of a Stirling cooler. We investigated Er-doped yttrium aluminum garnet (Er:YAG) and Er-doped sesquioxides. We observe that the most favorable pump line in Er:YAG, at 1532 nm, is extremely narrow at 77 K, but that by 120—150 K it is broad enough for pumping by line-narrowed diodes. That material's laser output under narrow-band diode pumping is most efficient around 120 K and its laser wavelength is 1618 nm. Among the sesquioxides, Er:Sc2O3 looks most promising. It has strong emission lines at shorter wavelengths, yet long enough that at cryogenic temperatures their absorption is weak. The 1535 nm pump line of Er:Sc2O3 is sufficiently wide for efficient pumping by line-narrowed diodes. We observe efficient laser operation at 1581 nm and 1558 nm. Surprisingly, the latter line outperforms the former as the temperature is increased above liquid nitrogen temperature. Spectroscopic studies have clarified the reason for this encouraging small-quantum defect behavior.
Distribution: Approved for public release
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Last Update / Reviewed: February 1, 2014