Waveguide loss model and geometry optimization for linear and nonlinear applications

We present an experimentally validated numerical model to evaluate propagation losses in straight rib- and ridge-type waveguides. The model accounts for scattering contributions arising from all waveguide surfaces. It is subsequently employed to determine the optimal geometry of Lithium Niobate On Insulator (LNOI) waveguides for low propagation losses under different sidewall angle conditions. In each case, the associated fabrication constraints and loss scaling laws allow the optimization problem to be reduced to a two-dimensional study using as parameters a morphology scanning factor and the ratio between in-plane and sidewall roughness. The analysis reveals that the optimal geometry consistently tends toward shallow-etched waveguides with reduced losses, independently of the sidewall angle.