Buried waveguides in Nd:YLF crystals obtained by femtosecond laser writing under double line approach

In this paper, we present buried waveguides fabricated by fs laser writing in Nd3+ doped YLF crystal under double line approach (Miura et al. in Appl. Phys. Lett. 71: 3329-3331, 1997). The waveguides were made by focusing two consecutive optical breakdown tracks (OBT) separated by about 20 mu m. To make the optimal OBT, we focused the fs-laser pulses 200 mu m below surface at intensities above the OB threshold for the material and controlled the writing speed. The guiding structures were fabricated by using a Chirped Pulse Amplification (CPA) femtosecond (fs) laser system. We chose the optimal writing parameters in order to obtain suitable waveguides, using around 3 mu J energy and writing speed from 15 to 50 mu m/s. After optically exploring the waveguides by end-fire coupling, the guiding structures showed good optical performance. Guiding index profiles were retrieved from modal analysis by using Beam-Prop (RSoft) commercial software. This spatial distribution of the index increment, taking into account a lower refractive barrier on the OBT region plus the compressed region between the tracks, was obtained correctly fitting profiles modes. Finally, optical spectroscopy measurements were also performed in the waveguides. The results showed that the luminescence properties of Nd3+ ions are preserved in the waveguides compared with the values obtained for bulk.