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.
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.
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.