Waveguide-based measurement of the silicon nitride refractive index and thermo-optic coefficient from the visible to the near-infrared

We describe a method to experimentally determine the refractive index and thermo-optic coefficient of waveguides fabricated from low-pressure chemical vapor deposition silicon nitride over a broad wavelength range (500–1600 nm) using Mach–Zehnder interferometers. Silicon nitride refractive index Sellmeier coefficients are extracted with a weighted root mean squared deviation that is typically better than 0.04% from the visible to the near-infrared. The thermo-optic coefficient of silicon nitride is measured over a similar wavelength range and shows a significant increase at visible wavelengths compared to the value in the near-infrared. Accurate knowledge of the waveguide core and cladding refractive indices and thermo-optic coefficients in foundry-based photonic integrated circuits allows for successful first-time design of passive and active components. This method can be used with any transparent waveguide material and can be automated for process monitoring and wafer-scale verification.