Compact integrated design of low loss silicon nitride grating coupler based on apodized quasi-bound states in the continuum

Grating couplers are key components for realizing optical interconnection between photonic integrated circuits and optical fibers. The coupling efficiency mainly depends on the spatial overlap between fiber modes and grating near-fields, which is fundamentally determined by apodized grating structures. However, existing apodized methods have limitations in directional enhancement and are still suboptimal in achieving peak efficiency. In this study, we propose a compact, low-loss silicon nitride grating coupler, which features an integrated design of apodized gratings and quasi-bound states in the continuum (quasi-BIC) gratings. It incorporates bottom distributed Bragg reflectors (Bottom-DBR) and side distributed Bragg reflectors (Side-DBR) to enhance coupling efficiency, maintaining high directionality and ideal mode matching while having a compact device area. Finite-difference time-domain (FDTD) simulation results show that this novel design achieves a high coupling efficiency of -0.33 dB with a 3-dB bandwidth of 25 nm while the footprint scale of the whole device is less than 10 micrometers. Our device is designed for a wavelength of 934 nm and is intended for use in hybrid quantum photonic integrated circuits with single-photon light sources.