Three-Dimensional Cross-Coupled Silicon Nitride Racetrack Resonator-Based Tunable Optical Filter

A tunable three-dimensional silicon nitride racetrack resonator was experimentally demonstrated, with a microring resonator on a bottom layer and a feedback cross-coupled waveguide on a top layer. At a 1559.3-nm wavelength, an extinction ratio of more than 14 dB and free spectral range of 2.7 nm can be realized with a quality factor of about $1.88\times 10^{4}$ . The filter performance, such as the resonance wavelength and extinction ratio, can be thermo-optically tuned based on the electrical control of a heater above the feedback waveguide. The presented device has a potential to be applied as a tunable modulator/switch as well as a highly sensitive sensor.