Using vibronic transitions in rare-earth doped crystals as a case-study, we present detailed analysis for the optimum operation of radiation-balanced lasers. In particular, conditions for achieving highest output power and highest optical-to-optical efficiency in Yb:YAG and Yb:YLF thin disc RBLs are given. Finally, we extend our analysis to two-tone RBL systems involving Ho-Tm doped crystals.
Detailed characterization of Tm- and Ho-doped crystals is performed to identify optimum operation parameters for reaching cryogenic temperatures. The energy-gap scaling law states that these materials can deliver double the cooling efficiency compared with the Yb-doped systems. Here, we show our recent measurements of external quantum efficiency 𝜂ext and background absorption 𝛼b in Ho- and Tm-doped YLF and BYF crystals. Together with temperature-dependent spectroscopy, these data are then used to determine the minimum achievable temperature and the optimum cooling wavelengths for each crystal. Finally, the potential of these crystals for implementing mid-IR radiation balanced lasers is discussed.
Laser cooling in Tm:YLF and Tm:BYF crystals has recently been reported. We investigate high power laser cooling of Tm doped crystals under high vacuum using multiple-pass Herriott cell configuration. We also model potential mid-IR Radiation Balanced Lasers (RBLs) in available Tm:YLF and Tm:BYF crystals. Our experiments and modelling shows that our 1% wt. Tm:BYF sample is a promising 2 µm RBL candidate, since it has high gain and high external quantum efficiency as well as good room temperature cooling efficiency. We will attempt to demonstrate the first mid-IR RBL experimentally in Tm:BYF crystal as well.
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.
We investigate high power laser cooling in Tm:YLF crystals both in ambient pressure and high vacuum. For this purpose, we have constructed a high power CW OPO broadly tunable from 1755 nm to 2000 nm. By using this tunable source, laser cooling for (3 mm3) 1% doped Tm:YLF crystal was observed from 1801 nm to 2000 nm. Cooling efficiency of the sample, external quantum efficiency (EQE), background absorption and optimum laser cooling wavelength are extracted from laser induced temperature modulation spectrum (LITMoS) test on the cooling sample. To improve cooling performance, we have designed multiple pass non-resonant cavities to maximize the absorption of the laser light inside the sample. We setup multiple pass cavities in a high vacuum chamber to reduce convective heat load and enhance laser cooling results.
We report the first observation of laser cooling in 1% doped Tm:YLF by 0.5 K and in 0.8% doped Ho:YLF crystals by
0.1 K starting from room temperature in air. To achieve this, we designed and constructed a high power, broadly tunable
(1735 nm-2086 nm) continuous wave singly-resonant optical parametric oscillator. (OPO). The cooling experiments were
performed at ambient pressure, and temperature changes were measured using a thermal camera.