Comparison of different in situ optical temperature probing techniques for cryogenic Yb:YLF

: We present, what is to our knowledge, the first detailed set of experiments comparing different in situ optical temperature estimation methods for Yb:YLF (Yb:LiYF 4 ) crystals used in cryogenic laser applications. The proposed temperature estimation methods are based on the temperature dependence of emission spectra of Yb:YLF in E//c axis, and looks at either the variation of the spectral intensity ratio of different wavelengths, or to the full-width half-maximum (FWHM) of the emission lines, or to the overall absolute integrated spectral change with respect to a reference temperature (also known as Differential Luminescence Thermometry: DLT). We have shown that by using the DLT method we can estimate the temperature of Yb:YLF crystals in the 78-300 K range with an accuracy better than ± 1 K. The other methods work well in the 78-150 K range, and provide a fast temperature estimation with ± 2 K accuracy. The benefit of the proposed technique has been demonstrated via evaluation of thermal contact quality of different Yb:YLF crystals, where we have seen that, a temperature estimation accuracy of ± 5 K is feasible even for samples under nonhomogeneous thermal load. We hope the findings presented in this work to be useful to laser engineers and scientists working with cryogenic Yb:YLF systems. of the OSA Open Access Publishing Agreement