Silicon Nitride Spot Size Converter With Very Low-Loss Over the C-Band

Photonic Integrated Circuits (PIC) provide a solution to overcome the main limitations of electronics, such as the operating frequency and heat generation, pushing the so-called "More than Moore" concept to increase the capacity and the speed of data transmission. In large data centers and optical transmission systems, PICs are interconnected by using fiber-to-chip couplers, which are crucial to improve system performance. An ultra-low loss interconnection (< 1 dB) over a wide bandwidth (≈50 nm) is the gold standard. In this context, the silicon nitride (Si3N4) platform is a promising candidate, with propagation losses of the order of dB/m at 1550 nm. Here, we propose the design and the experimental results for a silicon nitride-based fiber-to-chip interconnect, acting as a high aspect ratio waveguide Spot-Size Converter (SSC). A coupling loss less than 0.20 dB over the entire C-band and within a footprint of 1, 800~\mu \text{m}^{2} has been experimentally demonstrated, suggesting the proposed interconnect as a promising solution for next-generation high-density PICs.