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-um-size roundcornered octagonal microresonators are used, with an optimum coupling efficiency exceeding 96%, a finesse of --30, and a Q of --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.
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