High-efficiency off-chip grating couplers for silicon nitride photonics

Silicon nitride (Si3N4) is a leading platform in integrated photonics, providing unique passive functionalities, while maintaining compatibility with complementary metal-oxide-semiconductor processes and scalable and low-cost high-volume production. However, the moderate index contrast of the Si3N4 platform makes it difficult to implement efficient vertical surface grating couplers. In this work, we present efficient and robust fiber-chip grating couplers on native and hybrid Si3N4 platforms. Minimum coupling losses between -5 dB and -3 dB were measured for single-etch fabricated devices near 1.55 m wavelength. Moreover, by leveraging the amorphous-silicon overlay on top of the Si3N4 platform, we develop hybrid single-etch grating couplers, with a coupling loss approaching 1 dB. The demonstrated grating couplers are promising for SiN integrated photonics, enabling a rapid and cost-effective chip interfacing with standard optical fibers. This opens up novel opportunities for rising applications, including optical communications, nonlinear optics, or quantum information sciences.