Optical method of measuring the temperature of fluoride crystals activated by Yb$^{3+}$ and Tm$^{3+}$ ions

The fluorescence intensities ratio (FIR) method is applied for measuring the temperature inside the luminescent LiY$_{0.8}$Yb$_{0.2}$F$_{4}$:Tm$^{3+}$ crystal. The obtained results are used for monitoring of the sample temperature in the experiment when the exciting radiation waist is shifted by the focusing lens inside the sample. It is shown that a considerable change in the luminescence intensity of the LiY$_{0.8}$Yb$_{0.2}$F$_{4}$:Tm$^{3+}$ crystal (0.2%) is not accompanied by a significant change in the temperature. This verifies the hypothesis about the appearance of amplified spontaneous luminescence (ASL) of Yb$^{3+}$ ions during intense laser pumping in these crystals.