Low-loss three-dimensional waveguide crossings using adiabatic interlayer coupling

Three-dimensional silica-based waveguide crossings using a triple waveguide layer structure are proposed and demonstrated. Interlayer coupling is achieved among the three waveguide layers by using adiabatic modefield converters with a tapered double/stacked-core structure. With this structure, we successfully suppress insertion loss of 30 waveguide crossings to 0.4 dB (TE and TM) for the pass-through ports, and 1.5 dB (TE) and 2.5 dB(TM) for the add ports, from 3 dB for the conventional two-dimensional waveguide crossings.