Recent advances in cryogenic optical refrigeration have demonstrated cooling of a payload to below 125 K, enabled by several improvements. The parasitic load from pump and fluorescence scattering on the payload are minimized using a textured-MgF2 thermal link. Amplified Spontaneous Emission and lasing are suppressed by spectrally selective coatings on the multi-pass pump circulator. Power scaling of the cooling lift at cryogenic temperatures is yet to be observed, prompting us to investigate the negative contribution of pump saturation on a material’s cooling efficiency and, thus, Minimal Achievable Temperature. A systematic study of the power-dependent performance of the cryocooler is reported.
A record cooling to <125 K of an all-solid-state optical cryocooler by the anti-Stokes fluorescence cooling of a 10% Yb:YLF crystal. The record cooling achievable by the employment of a novel textured-MgF2 thermal link which improves the thermal transport and fluorescence escape. Spectrally selective coatings on the multi-pass pump circulator mirrors tuned to be highly reflective for the pump wavelengths yet transmit longer wavelengths in the Stokes regime. The loss introduced prevents sufficient gain from building up leading to amplified spontaneous emission and lasing. The roles of other potential nonlinearities, such as pump saturation are investigated.
Optical refrigeration of Yb:YLF is used to cool an arbitrary payload. An astigmatic Herriott cell enhances the total pump laser absorption by keeping the average pump intensity below the saturation while minimizing the leakage from the cavity. A spectrally-selective reflection coating mitigates the effects of amplified spontaneous emission and parasitic lasing, which limit the power scaling for temperatures <140 K. Direct cooling of the entire clamshell and shielding stray fluorescence prevents adverse heating of the crystal from its surroundings. Finally, an improved Differential Luminescence Thermometry (DLT) technique is used to measure the crystal temperature with higher accuracy and precision.
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