Integrated waveguide coupled Si₃N₄ resonators in the ultrahigh-Q regime

The vast majority of work on waveguide-coupled reso-nators focuses on decreasing losses in the waveguide and coupling region. Here we present fully integrated reso-nators based on an ultralow-loss Si3N4 waveguide plat-form. By tailoring the directional coupler excitation to the resonators, we are able to achieve lower loss single-mode coupling to multimode waveguide widths com-pared to straight bus waveguide directional couplers. This allows us to demonstrate record-high integrated waveguide coupled intrinsic quality factor (Qint) values of 81 million at a 9.65 mm bend radius, with a future direction to both stronger and lower loss waveguide– resonator coupling. This result opens up integration possibilities for narrow linewidth integrated diode lasers, low noise feedback systems, microwave photonic research, and the ultrastable timing reference