Waveguide grating coupler with subwavelength microstructures

TLDR

The authors propose a silicon waveguide‑fiber grating coupler that employs a subwavelength microstructure to provide continuously variable grating strength while being fabricated with a single etch step. By tailoring the subwavelength microstructure along the grating, the coupler achieves high field overlap with the optical fiber mode and suppresses backreflections, and is optimized for quasi‑TM mode in a silicon photonic‑wire waveguide for evanescent‑field sensing. The optimized coupler attains up to 94 % field overlap with a standard SMF‑28 fiber, reduces backreflection to ~0.1 %, and predicts a 50 % total photonic‑wire‑to‑fiber coupling efficiency.

Abstract

We propose a silicon waveguide-fiber grating coupler that uses a subwavelength microstructure to achieve a continuously variable grating strength yet can be fabricated using only a single etch step. By adjusting the subwavelength microstructure at every point along the grating, the grating coupler can be optimized to give high field overlap with the optical fiber mode and also minimize backreflections along the incident waveguide path. Our design example is optimized for quasi-TM mode in a silicon photonic-wire waveguide, as required for waveguide evanescent-field-sensing applications. A field overlap of up to 94% with a standard single-mode optical fiber (SMF-28) is achieved by coupler apodization. Backreflection from the grating is reduced to ~0.1%, and the total predicted photonic wire to fiber coupling efficiency is 50%.