Low-loss double-layer silicon nitride grating coupler for blue wavelength band

To address the challenge of fiber coupling in the blue light band, a low-loss double-layer silicon nitride grating coupler operating at a blue light wavelength of 473 nm is presented. The device employs a double-layer grating design, in which a SiO2/SiN distributed Bragg reflector (DBR) is integrated on the side connected to the silicon nitride waveguide to effectively suppress backward radiation loss. Meanwhile, a shallow-etched top grating layer and a bottom main grating layer collectively form an interferometric cavity. High directivity coupling is achieved through precise control of the lateral offset and vertical spacing between the grating layers. The simulation results demonstrate a coupling efficiency of -1.57 dB and a 3-dB bandwidth of 13.2 nm at the wavelength of 473 nm. The structure exhibits a large fabrication tolerance, with a minimal additional loss of only 0.9 dB for a grating layer misalignment of up to ±30 nm. The proposed device provides a highly integrated on-chip coupling solution for applications such as blue-light quantum sources, high-resolution biosensing, and neural probes.