Single-layer silicon nitride-based coupler with sub-dB coupling loss to a standard SMF-28 by an engineering...

With the explosion of AI, the requirements for system performance of silicon photonic chips are becoming increasingly higher. High-reliability, low-loss edge couplers are very important for the application of silicon photonic chips. It can support longer-distance transmission and reduce the requirements for the optical source's output power. This paper conducts an in-depth study on a high-reliability, low-loss edge coupler. We numerically studied the factors that affect the coupling loss between standard SMF-28 (Ultra single-mode optical fibers) and Si₃N₄ (Silicon nitride) based edge coupler, and found that the refractive index of the upper cladding oxide is one of the most important factors. By engineering this refractive index, we experimentally demonstrated that ∼ 0.5 dB coupling loss for TE and ∼ 0.7 dB for TM polarization can be achieved across the full O-band (1250-1350 nm). To our knowledge, this result represents one of the lowest coupling losses reported for a solid edge coupler directly interfacing with SMF-28 fiber across the full O-band.