Single-Etch Silicon Nitride Grating Couplers for Multiband Applications in Quantum and Fiber Communications

Silicon nitride (Si3N4) is an attractive alternative to the silicon-on-insulator platform due to its broad spectral transparency window, low waveguide losses, and negligible two-photon absorption. However, the moderate refractive index contrast between the Si3N4 waveguide core and the cladding presents challenges, including limiting the efficiency and performance of surface grating fiber-chip coupling devices. Addressing this issue is crucial to fully leveraging the advantages offered by the silicon nitride platform. While various strategies have been developed to enhance the performance of surface grating couplers, they often come with increasingly complex fabrication requirements. In this work, we present a set of high-efficiency silicon nitride grating couplers using standard single-etch fabrication processes. The devices are designed for three spectral regions: 950 nm, 1310 nm, and 1550 nm. Uniform grating couplers demonstrated experimental coupling efficiencies between 5.9 dB and 3.1 dB. Record-breaking performance was achieved using subwavelength metamaterial apodization and beam focalization, resulting in fiber-chip coupling losses as low as 2.5 dB, all achieved through a straight-forward single-etch fabrication process.