Low-loss 3-dimensional waveguide crossing with a parabolic taper interlayer coupler based on a SiN-SiN-Si three-layer platform.

We demonstrated a SiN-SiN-Si three-layer silicon waveguide crossing with low-loss crossings and interlayer couplers. The underpass and overpass crossings exhibited ultralow loss (<0.82/1.16 mdB) and cross talk (<-56/-48 dB) in the wavelength range of 1260-1340 nm. To reduce the loss and length of the interlayer coupler, a parabolic interlayer coupling structure was adopted. The measured interlayer coupling loss was less than 0.11 dB from 1260 to 1340 nm, which is, to the best of our knowledge, the lowest loss reported for an interlayer coupler based on a SiN-SiN-Si three-layer platform. The total interlayer coupler length was only 120 µm.