Grating-coupled input/output interfaces for silicon nitride integrated photonics

Integrated photonics emerges as a foundational technology for a broad range of applications. This includes high-speed optical communications, ultrafast computing, and quantum information processing, among others. A persistent challenge across these domains is the efficient coupling of light between standard single-mode optical fibers and sub-micrometer planar photonic waveguides. Despite significant progress in on-chip integration, fiber-chip optical coupling remains a key bottleneck, constraining the power budget and limiting overall system performance. Direct fiber-chip interfacing is lossy, primarily due to mismatches in geometry, material properties, and mode field diameters, along with stringent spatial alignment requirements. In this work, we report on recent advancements in the development of compact and efficient surface grating couplers implemented on the silicon nitride (Si3N4) photonic platform. We introduce and compare several CMOS-compatible design strategies, including both native and hybrid surface grating architectures, tailored for improved mode matching and radiation efficiency at short-wave-infrared wavebands. Single-etch Si3N4 devices demonstrated coupling losses between -6 dB and -3 dB, while hybrid grating designs show great promise to reduce fiber-chip losses close to -1 dB level. This marks a significant step towards practical and low-loss interconnections for next-generation photonic integrated circuits.