Femtosecond Laser Direct-written Type I Waveguides in LiTaO<sub>3 </sub>crystal

This work employs Type I single-line and multi-line optical waveguides in LiTaO3 crystals by femtosecond laser direct writing. Investigating the critical factors influencing the success of high-performance waveguide fabrication. By systematically varying pulse energy, scanning speed, and focal depth, the effects of femtosecond laser direct writing modifications in LiTaO3 crystals were investigated, achieving a low insertion loss of 1.39 dB at a writing speed of 4 mm/s and a pulse energy of 0.42 μJ. Through multi- scan technique, three sets of multi-line optical waveguides with varying track spacings under parallel and perpendicular polarization orientations were fabricated. By tailoring the waveguide cross-sections into square or circularly symmetric profiles via scanning parameter optimization, precise control over modal distribution was achieved, enabling efficient guided-mode propagation at 1550 nm with minimized scattering losses. This work provides a straightforward and versatile approach for fabricating complex optical waveguide devices, enabling their applications in on-chip quantum information processing with enhanced integration flexibility.