Compact silicon microring resonators with ultra-low propagation loss in the C band

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 mum, we demonstrate as low as 3-4 dB/cm propagation losses in compact silicon microring resonators with a small bending radius of 5 mum, 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 mum, 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 mum band. These loss numbers are believed to be among the lowest ones ever achieved in silicon microring resonators with similar sizes.