Stress-induced optical waveguides written by an ultrafast laser in Nd, Y co-doped SrF crystals.

In this paper, we report on the ultrafast laser-induced birefringence, refractive index changes, and enhanced photoluminescence properties in the volume of neodymium (Nd), yttrium (Y) co-doped strontium fluoride (SrF) and Nd, Y co-doped calcium fluoride (CaF) crystals. The optical waveguides written with 500 kHz repetition rate provided lowest propagation loss of 1.63±0.21  dB cm for transverse magnetic (TM) polarization at 632.8 nm in Nd,Y:SrF crystal. The measured retardance can be interpreted by stress-induced birefringence related to the permanent volume expansion, photo induced by a non-spherical irradiated zone. The absorption, steady-state, and time-resolved photoluminescence properties are also carried out in and out of the laser irradiated zone, enabling the local changes of the Nd and Y network in Nd,Y:SrF, as well as well-preserved Nd fluorescence in the written optical waveguides.