Fabrication technologies for vertically coupled microring resonator with multilevel crossing busline and ultracompact-ring radius

To eliminate the scattering loss at the crossing points of cross-grid busline waveguides, the multilevel crossing structure of the busline waveguides was introduced into a vertically coupled microring resonator (VCMRR) filter. To achieve this structure, two fabrication technologies were newly developed; one is a method to planarize perfectly the top surface of each buried waveguide, and the other is a method to fabricate microring waveguides with very smooth sidewalls. Using the latter method, an ultracompact VCMRR with a ring radius of 5 μm was realized and a free spectral range of 37 nm was successfully demonstrated. Next, using the former method, single-ring and quadruple series-coupled ring resonators with multilevel crossing busline waveguides were fabricated. A clear filter response was obtained for the single-microring resonator, and a boxlike filter response was obtained for the quadruple series-coupled microring resonator with multilevel crossing busline waveguides.