Buried channel waveguides in Yb-doped KY(WO₄)₂ crystals fabricated by femtosecond laser irradiation

We report on the fabrication of optical channel waveguides inside $KY{(WO_4)}_2$ crystals, both undoped and Yb-doped, for applications in compact solid-state lasers and three-dimensional photonic devices. Nonlinear absorption of femtosecond laser pulses has been employed in order to induce refractive-index changes in these crystals. The irradiation damage results in a decrease of the material density compared to the surrounding bulk. In this work, two types of buried channel waveguides have been fabricated in the $KY{(WO_4)}_2$ crystals. First ones (type I) were formed in the vicinity of the irradiated regions due to the presence of an induced compressive lattice strain, which causes an increase of the local refractive index. Light can be better confined in the second (type II) of channel waveguides that are created between pairs of damaged regions. For the best confined channels (type II), the propagation loss value measured at 1 mm amounted to 2–2.5 dB/cm.