Optical refrigeration of rare-earth-doped solids has reached the boiling point of argon, 87 K, and is expected to cool to that of nitrogen, 77 K, in the near future. This technology is poised to pave the way to compact, reliable, and vibrationfree all-solid-state optical cryocoolers. By attaching the Yb:YLF cooling crystal to a cold finger via a double 90° kink thermal link, we have cooled a silicon temperature sensor to below 151 K. An advanced design of the thermal link and the clamshell surrounding the cooled assembly successfully controlled the flow of heat and radiation to allow cooling of a payload to cryogenic temperatures. Key elements of the design were a low-absorption thermal link material, an optimized thermal link geometry, and a spectrally-selective coating of the clamshell.
Radiation-balanced lasers (RBL) combine solid-state optical refrigeration and lasing in one material to enable a net zero thermal load that allows for favorable scaling to high laser powers. A high-performance RBL material, therefore, has to first qualify as a high-performance laser-cooling material. This necessitates exquisite material purity in order to achieve the required near-unity external quantum efficiency and low background absorption. Solvent extraction, ion exchange, and electrochemical treatment of aqueous solutions or melts are some of the techniques available for the purification of starting materials used in the growth of RBL crystals. Scaling these methods to the 100s of gram scale needed for traditional Czochralski crystal growth while maintaining parts-per-billion level impurity concentrations however has proven challenging in several past efforts. In contrast, we have previously shown solvent extraction and electrochemical treatment to be effective on the several gram scale. This creates a need for exploring alternative methods for growing optical-cooling-grade fluoride crystals on the small scale. We will present results on growing Yb-doped YLiF4 (YLF) and LuLiF4 (LLF) single crystals using the vertical Bridgman method. The external quantum efficiency and background absorption of these samples will be reported and discussed in the context of RBL.
Optical refrigeration of rare-earth doped crystals has exceptional qualities that can be used for building a compact and vibration-free all-solid-state optical cooler. Estimating the lowest achievable temperature and cooling power of such a device requires accurate measurements of external quantum efficiency, mean fluorescence wavelength, and parasitic absorption. Here we discuss temperature dependent measurements of these parameters for a high quality Yb:YLF sample by performing a LITMoS test (Laser Induced Temperature Modulation Spectrum) combined with contact-free differential luminescence thermometry. These measurements are challenging at low temperatures, but by integrating these two methods, we can perform LITMoS test at any temperature.
Employing large surface-area-to-volume ratio gain, thin-disk lasers have shown great potential in power scaling. But thermal management for these devices is still challenging. One possible approach is to balance the heat load generated by the lasing process with cooling power from the anti-Stokes cooling process, forming radiation balanced lasers (RBLs). Compared to bulk RBLs, thin-disk RBLs can be better thermally balanced with reduced thermal gradients, promising higher output power and better beam quality. In this paper, we analyze and investigate radiation balanced disk lasers with Yb:YAG and Yb:YLF crystals in different pumping configurations.
Laser cooling of solids has advanced immensely in recent years and temperatures well below 100 K have been demonstrated in Yb:YLF crystals. We will discuss our progress towards developing a functional all-solid-state cryocooler based on this principle. We present data and analysis concerning laser coupling efficiency, thermal link between the cooling crystal and the cold-finger, shielding the load from the fluorescence, and overall thermal load management. Considerations for building a cooler prototype for specific applications will also be discussed.
Optical refrigeration of solids requires crystals with exceptional qualities. Crystals with external quantum efficiencies (EQE) larger than 99% and background absorptions of 4×10-4cm-1 have been cooled to cryogenic temperatures using non resonant cavities. Estimating the cooling efficiency requires accurate measurements of the above mentioned quantities. Here we discuss measurements of EQE and background absorption for two high quality Yb:YLF samples. For any given sample, to reach minimum achievable temperatures heat generated by fluorescence must be removed from the surrounding clamshell and more importantly, absorption of the laser light must be maximized. Since the absorption coefficient drops at lower temperatures the only option is to confine laser light in a cavity until almost 100% of the light is absorbed. This can be achieved by placing the crystal between a cylindrical and spherical mirror to form an astigmatic Herriott cell. In this geometry light enters through a hole in the middle of the spherical mirror and if the entrance angle is correct, it can make as many round trips as required to absorb all the light. At 120 K 60 passes and 150 passes at 100K ensures more than 95% absorption of the laser light. 5 and 10% Yb:YLF crystals placed in such a cell cool to sub 90K temperatures. Non-contact temperature measurements are more challenging for such a geometry. Reabsorption of fluorescence for each pass must be taken into account for accurate temperature measurements by differential luminescence thermometry (DLT). Alternatively, we used part of the spectrum that is not affected by reabsorption.
Cooling rare-earth-doped crystals to the lowest temperature possible requires enhanced resonant absorption and high-purity crystals. Since resonant absorption decreases as the crystal is cooled, the only path forward is to increase the number of roundtrips that the laser makes inside the crystal. To achieve even lower temperatures than previously reported, we have employed an astigmatic Herriott cell to improve laser absorption at low temperatures. Preliminary results indicate improvement over previous designs. This cavity potentially enables us to use unpolarized high-power fiber lasers, and to achieve much higher cooling power for practical applications.