Simulation Framework for Athermal Silicon Nitride Waveguide Design

We present a simulation framework to engineer athermal integrated waveguides in silicon nitride photonic platforms. Starting from realistic Si3N4/SiO2 stacks with optional overlay layers, we compute the temperature dependence of the guided modes by combining electromagnetic eigenmode solvers with temperature-dependent refractive index models. Thermo-optic finite-element simulations provide the local temperature distribution allowing us to find contributions to the effective index shift. By sweeping core geometry, cladding composition, and overlay thickness, we map the design space where the net thermo-optic coefficient of the fundamental mode approaches or crosses zero, and we quantify the spectral bandwidth over which this athermal condition is maintained. The methodology provides a practical, simulation-driven route to identify and optimize athermal Si3N4/SiO2 waveguide geometries for temperature-stable operation in precision photonic integrated circuits.