Experimental demonstration of waveguide-coupled round-cornered octagonal microresonators in silicon nitride.

We report laterally waveguide-coupled octagonal microresonators with designed round resonator corners in silicon nitride. We demonstrate nearly single-mode add-drop filter characteristics, when 50-microm-size round-cornered octagonal microresonators are used, with an optimum coupling efficiency exceeding 96%, a finesse of approximately 30, and a Q of approximately 6400. Our experiments also reveal two unforeseen phenomena in certain microresonator shapes: a pronounced drop in coupling efficiency and resonance notches in the drop spectrum. These device characteristics hint at the recently postulated multiple orbit interference in waveguide-coupled polygonal microresonators.