In situ thermal trimming of waveguides in a standard active silicon photonics platform

We present suspended heater structures fabricated in a standard C- and O-band silicon (Si) photonics platform that can achieve sufficiently high local temperatures to induce effective refractive index trimming of Si and silicon nitride (SiN) waveguides with 30 - 90 mW of applied electrical power. Following thermal trimming at moderate powers (≤40 mW), maximum changes in the averaged waveguide effective refractive index of -5.18 × 10^-3 and -7.9 × 10^-4 are demonstrated in SiN and Si waveguides, respectively, at a wavelength of 1550 nm. At higher powers, SiN waveguides exhibit positive averaged effective index changes up to ≈0.02, demonstrating bi-directional index trimming. As an example application, we demonstrate bias point trimming of a carrier injection Mach-Zehnder switch. Through investigations of the origin of the thermal trimming effect, we hypothesize that changes in the silica (SiO₂) waveguide cladding may be a primary underlying mechanism at temperatures ≳300°C, with significant trimming of SiN waveguide cores occurring at larger temperatures ≳510°C. As the trimming experiments represent a form of accelerated thermal aging, we estimate the aging behavior of the suspended heaters by fitting and extrapolating the measured datasets to 100 - 200°C operating temperatures over five years.