Upconversion of infrared emission in alkaline-earth fluoride crystals doped with Er$^{3+}$

Upconversion processes were studied in MeF$_2$–ErF$_{3}$ crystals (Me – Ca, Sr, Ba) in the range of ErF$_3$ concentrations from 0.01 to 10 mol% when excited in infrared bands of the Er$^{3+}$ ion by laser diodes of 808, 978, 1542 nm. Upon excitation of 1542 nm, the $^4I_{11/2}-^4I_{15/2}$ band at 990 nm predominates in the luminescence spectrum of MeF$_2$–Er; the fraction of visible bands is less than 0.1. The energy yield of the upconversion luminescence MeF$_2$–10% ErF$_3$ upon excitation of 1542 nm with a density of about 1 W/cm$^2$ is in the range of 14–26%.