On-chip optical fiber-to-nanophotonic waveguide adiabatic coupler

We report a simple design of an on-chip optical fiber-to-nanophotonic waveguide adiabatic coupler. An inverse-tapered waveguide can be fabricated on an etched oxide layer to achieve index-crossing with a tapered optical fiber without severe light leakage into the substrate. Our design involves simple fabrication steps without substrate releasing and does not rely on cladding the tapered fiber in a higher-index material. The simulated transmission efficiency could reach beyond 95% (−0.2 dB) with robust alignment tolerance for quasi-transverse magnetic (quasi-TM) polarization. The coupler is expected to show reasonably wide bandwidth and high coupling efficiency for both of the fundamental quasi-TM and quasi-transverse electric (quasi-TE) polarizations. Our new design can extend the scalability and stability of adiabatic couplers in photonic integrated circuits.