– The propagation loss in compact silicon microring resonators is optimized with varied ring widths as well as bending radii. At the telecom band of 1.53–1.57 μm, we demonstrate as low as 3-4 dB/cm propagation losses in compact silicon microring resonators with a small bending radius of 5 μm, corresponding to a high intrinsic quality factor of 200,000-300,000. The loss is reduced to 2-3 dB/cm for a larger bending radius of 10 μm, and the intrinsic quality factor increases up to an ultrahigh value of 420,000. Slot-waveguide microring resonators with around 80% optical power confinement in the slot are also demonstrated with propagation losses as low as 1.3±0.2 dB/mm at 1.55 μm band. These loss numbers are believed to be among the lowest ones ever achieved in silicon microring resonators with similar sizes.
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