Control of eigenmode localization and coupling anisotropy by multiscan femtosecond laser writing
The multiscan method is widely used in femtosecond laser writing of waveguide systems to increase the refractive index contrast and to control the eigenmode shape. Here, we investigate multiscan writing with a constant effective writing speed and show that the displacement between adjacent scans provides an additional degree of freedom for controlling eigenmode confinement. By optimizing the multiscan parameters, the effective mode area can be reduced, enabling an up to eightfold increase in fabrication speed compared to conventional single-scan writing. We further demonstrate that careful tuning of the multiscan geometry substantially reduces the anisotropy of inter-waveguide coupling. These findings are applied to the fabrication of three-dimensional waveguide arrays operating at wavelengths of 808 nm and 1450 nm, where reduced coupling anisotropy is achieved. The proposed approach may be useful for three-dimensional waveguide arrays employed in topological photonics experiments, where low coupling anisotropy is desired.