Silicon-on-Insulator All-Pass Microring Resonators Using a Polarization Rotating Coupling Section

We propose a novel microring resonator concept that uses a polarization rotating asymmetrical directional coupler. The quasi-transverse electric polarized mode in the bus waveguide is used to excite the quasi-transverse magnetic (TM) polarized mode in the microring. This enables the realization of microrings with a high quality factor due to the lower scattering loss of the quasi-TM polarized mode. We demonstrate the operation principle with two all-pass microrings with radius 30 μm, working in the critically coupled regime with a quality factor of 31 000 and in the under-coupled regime with a quality factor of 125 000.