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1 September 2015 Radiation effects on Yb:YLF crystals used in cryogenic optical refrigerators
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Optical cooling of solids is a promising and innovative method to provide cryogenic cooling to infrared sensors. Currently insulator crystals, specifically ytterbium-doped yttrium- lithium-fluoride (Yb:YLF), have shown the most promise for cooling to low temperatures. This method has demonstrated cooling below the National Institute of Standards and Technology (NIST) cryogenic temperature definition of less than 123 K. Optical refrigeration utilizes a phenomenon called anti-Stokes fluorescence to generate cooling power. Incident laser light is absorbed by the cooling crystal and photons are spontaneously emitted at a higher, and thus more energetic, frequency. The difference in frequency is proportional to the cooling power of the crystal. Anti-Stokes cooling is highly dependent on doping percentages and YLF crystal purity and structure. Space based infrared sensors and their coolers are operated in a radiation environment where protons, gamma, rays, heavy ions, and other radiation species are common and of varying severities depending on operational orbit. To ensure that radiative effects on cooling crystal performance are minimal, we irradiated two samples with 63 MeV protons to a total of ionized dose of 100 Krad (Si) and 1 Mrad (Si), and compared cooling crystal efficiency parameters before and after dosing.
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K. W. Martin, S. Melgaard, V. M. Cowan, J. Hubbs, T. Fraser, and M. Sheik-Bahae "Radiation effects on Yb:YLF crystals used in cryogenic optical refrigerators", Proc. SPIE 9616, Nanophotonics and Macrophotonics for Space Environments IX, 96160V (1 September 2015);

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