Study of anti-Stokes luminescence of ZBLAN : Нo$^{3+}$ ceramics excited at 1908 nm

Visualization of IR radiation of a Tm:YLF laser with a wavelength of 1908 nm in 53ZrF$_{4}$–20BaF$_{2}$–3LaF$_{3}$–1HoF$_{3}$–3AlF$_{3}$–20NaF (mol%) ceramic samples is studied. The luminescence spectra of Но$^{3+}$-doped ZBLAN ceramics exhibit bands in the regions of 540, 650, and 900 nm, which correspond to the ${}^{5}S_{2}\to{}^{5}I_{8}$, $^{5}F_{5}\to{}^{5}I_{8}$, and ${}^{5}I_{5}\to{}^{5}I_{8}$ transitions, the red band (650 nm) being most intense. The population of the upper levels of these transitions can be explained using the cascade excitation mechanism. A visualization model is developed based on balance equations for populations of the upper energy states of Но$^{3+}$ ions. The population distributions are numerically estimated as functions of the excitation intensity. The obtained time dependences of the populations of the $^{5}S_{2}$ and $^{5}F_{5}$ states correlate with the experimental time dependences of the luminescence intensity at the $^{5}S_{2}\to{}^{5}I_{8}$ и $^{5}F_{5}\to{}^{5}I_{8}$ transitions upon pulsed excitation. The threshold power density of a Tm:YLF laser at which luminescence of the ceramic samples was observed was $\sim$2 W/cm$^2$.