Subwavelength waveguide grating for mode conversion and light coupling in integrated optics

TLDR

The authors propose a subwavelength grating coupler that converts fiber modes to sub‑micrometer waveguide modes with periods below the first‑order Bragg period. The coupler operates by gradually altering the waveguide effective index through the SWG effect, suppressing diffraction and minimizing reflection, as demonstrated by 2‑D FDTD simulations on SOI waveguides with 0.3 µm cores. FDTD results show a coupling loss of only 0.9 dB for a 50 µm device, with misalignment tolerances of 0.07 dB for ±1 µm transverse and 0.24 dB for ±2° angular shifts, and a 10 µm‑long coupler is also feasible, underscoring the design’s low loss and fabrication advantages.

Abstract

We propose a new method for mode conversion and coupling between an optical fiber and a sub-micrometer waveguide using a subwavelength grating (SWG) with a period less than the 1st order Bragg period. The coupler principle is based on gradual modification of the waveguide mode effective index by the SWG effect that at the same time frustrates diffraction and minimizes reflection loss. We demonstrate the proposed principle by two-dimensional Finite Difference Time Domain (FDTD) calculations of various SWG structures designed for the silicon-on-insulator (SOI) platform with a Si core thickness of 0.3 microm. We found a coupling loss as small as 0.9 dB for a 50 microm-long SWG device and low excess loss due to fiber misalignment, namely 0.07 dB for a transverse misalignment of +/-1 microm, and 0.24 dB for an angular misalignment of +/-2 degrees. Scaling of the SWG coupler length down to 10 microm is also reported on an example of a 2D slab waveguide coupling structure including aspect ratio dependent etching and micro-loading effects. Finally, advantages of the proposed coupling principle for fabricating 3D coupling structures are discussed.