CaF2:Er thin films grown by molecular beam epitaxy on CaF2 substrates have been characterized as optical waveguides. The characterization has been carried out by measuring the synchronous angles in attenuated total reflection experiments. It has been found that the incorporation of Er3+ ions to the CaF2 crystal produces an increase of the refractive index of the material giving rise to the formation of a steplike planar optical waveguide. The refractive index increase shows a linear dependence with Er3+ concentration up to 35 mol %, at which point saturation is observed. The obtained results compared well with previously reported data on the lattice parameter of CaF2:Er3+ bulk crystals and fluorescence quenching from Er3+ ions in these layers. The results of this work show that the preparation of CaF2:Er3+ layers on CaF2 substrates by molecular beam epitaxy is an ideal method to produce monomode active optical waveguides useful for optoelectronic devices.