Ultracompact vertically coupled microring resonator with buried vacuum cladding structure

We developed a novel fabrication process for the stacked dense integration of a microring resonator (MRR) buried in a SiO₂ cladding with vacuum cladding in the lateral direction. Owing to the vertical coupling and the lateral vacuum cladding, the radiation loss can be greatly reduced without affecting the coupling strength between the busline waveguide and the MRR. The reduction of radiation loss by ten orders of magnitude compared with the SiO₂ buried cladding was theoretically and experimentally demonstrated. Using this technique, an ultracompact vertically coupled MRR with the ring radius of 5 μm was realized, and the free spectral range of 29 nm was successfully obtained.