Multi-parameter control in fs-laser written waveguides: enhancing loss and coupling characteristics via exposure tuning

Recently, femtosecond laser direct-written (FLDW) waveguides have garnered significant attention due to their true 3D structuring, rapid fabrication, and flexibility in material selection. However, controlling the evanescent field ratio (EFR) of waveguide and its loss characteristics remains a significant challenge. In this work, we propose a loss estimation method based on the evanescent field intensity distribution, enabling rapid evaluation of coupling and propagation losses by analyzing the evanescent field size and leaking mode excitation. We demonstrate continuous tuning of the waveguide mode-field size from 5.04 μm to 18.42 μm, and EFR range from 25% to 96%. This method offers potential improvements in FLDW waveguide performance and could benefit applications in nonlinear optics, biosensing, photonic integrated circuits, quantum computing, and micro-optical imaging.