The absorption and luminescence spectra of 4f-4f transitions of Yb3+ ions in crystals of CaF2, SrF2, and BaF2 fluorides have been studied. The crystal splitting of the 2F7/2, 2F5/2 levels in Tm2+, Yb3+ by the cubic field decreases linearly with increasing lattice constant. Both lines intersect at the energy of 2F7/2 - 2F5/2 transitions in unperturbed Tm2+, Yb3+ ions. The positions of the f13 levels of Tm2+ or Yb3+ in fluoride crystals are well described by ab-initio relativistic calculations

Upconversion processes were studied in MeF2-ErF3 crystals (Me - Ca, Sr, Ba) in the range of ErF3 concentrations from 0.01 to 10 mol% when excited in infrared bands of the Er3 + ion by laser diodes of 808, 978, 1542 nm. Upon excitation of 1542 nm, the 4I11/2-4I15/2 band at 990 nm predominates in the luminescence spectrum of MeF2-Er; the fraction of visible bands is less than 0.1. The energy yield of the upconversion luminescence MeF2-10% ErF3 upon excitation of 1542 nm with a density of about 1 W/cm2 is in the range of 14-26%.