Planar waveguide structures based on SrF2 : Ho, Er, Tm. Dependence of the refractive index on the dopant concentration

Refractive indices of SrF₂ crystals doped with holmium, erbium, and thulium ions are measured. These crystals are used as active laser media, in particular, as materials for planar waveguides. The presence of active ions in these crystals may create a difference between the refractive indices of the cladding and the doped core of a planar waveguide. At dopant concentration to 4%, the difference Δn between the refractive indices of the doped core and undoped reflective layer can reach 0.007.