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.
We cooled a 10% Yb:YLF crystal to temperatures of approximately 130 K using a fiber-coupled laser diode module emitting at 1020 nm. Previous approaches to optical refrigeration typically relied on high-power fiber lasers and intricate multi-pass schemes to reach cryogenic temperatures. In the current experiments a combination of dielectric mirrors deposited onto the sample and total internal reflection confined the laser light inside the crystal. The divergent beams of laser diodes produce relatively low average intensity inside the crystal, mitigating nonlinearities like absorption saturation and lasing. We have designed a full diode-pumped cryo-cooler with a coldfinger to cool arbitrary loads.
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.
The effects of amplified spontaneous emission (ASE) as well as absorption saturation in cryogenic optical refrigeration in rare-earth doped materials are analyzed theoretically. It is seen that ASE may pose a limitation on power scaling under strong feedback conditions (i.e. in high finesse pump circulator cavities). Absorption saturation may have similar effect depending on the value of the background absorption.
High purity is a fundamental requirement to enable laser-cooling-grade materials. The vertical Bridgman method is well suited for crystal growth on the few-grams scale, which is compatible with purification techniques that aim to exceed the typical 99.999% to 99.9999% purity of commercial precursor materials. Here, we present advances in the Bridgman crystal growth of cooling-grade LLF:Yb single crystals in a radio-frequency heated furnace. Optical spectroscopy, cooling efficiency, and power cooling characterization are reported. COMSOL simulations were used to investigate the thermal gradient inside the crucible as the crystal growth proceeds.
In radiation balanced lasers, anti-Stokes fluorescence is used to minimize the heat generated by the quantum defect and other non-radiative processes. Thermo-optics distortions can be minimized, enabling the scaling to high power. Here, experimental results of radiation balanced operation in various disk gain materials (i.e. YLF:Yb and LLF:Yb ) are presented. Different multipass pumping schemes are investigated for pump beam area scaling towards high power CW operation. Laser cavity design and thermal management issues are also discussed.
The role of pump saturation in high-power optical refrigeration is investigated. Employing both Z-scan and intensity-dependent PL techniques, we measure the pump saturation intensity in Yb:YLF versus the temperature. We find that the absorption efficiency, and consequently the cooling efficiency can be limited at cryogenic temperatures under power scaling (when substantial heat lift is desired) unless multi-pass pumping schemes are tailored to control the average pump intensity inside the crystal.
Optical refrigeration of a Yb:YLF crystal can be used to cool an arbitrary payload, utilizing a novel MgF2 thermal link in a completely vibration free optical cryocooler. We discuss the latest advances in the design and implementation of an astigmatic Herriott cell to optimize pump laser absorption in the cooling crystal, and are studying the adverse effects of absorption saturation. We investigate novel spectrally selective coatings of the clamshell to reduce the parasitic heat load on the crystal. By overcoming these challenges, we work towards cooling a silicon reference cavity to a temperature of 124 K.
We present the experimental progress in the investigation of radiation-balanced lasers in Yb:YAG and Yb:YLF thin discs. Due to low absorption of the pump beam, we explored pump absorption schemes including VECSEL intracavity pumping as well as multipass approach. The results are compared with theory and predictions are made for prospects of these lasers for high power (multi kW) operation.
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