The conversion of infrared radiation of a Tm 3+ :YAP laser at a wavelength of 1.93 µm into visible light in [(ZrF 4 ) 0.4 -(BiF 3 ) 0.6 ] 100−x -(HоF 3 ) x (x = 1.5 or 5 wt%) ceramic samples was investigated. The emission bands at wavelengths of 650 nm, 900 nm and 1100 nm corresponding to transitions from 5 F 5 , 5 I 5 and 5 I 6 levels to the ground state 5 I 8 were observed in upconversion luminescence spectra. From the experimental data, an estimate of the crossrelaxation coefficient from 5 S 2 was obtained. It was K cr ≈ 1 × 10 −16 cm 3 s −1 . The threshold power density of Tm 3+ :YAP laser, at which a luminescence was observed on the ceramic samples, was about 0.85 W cm −2 .
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