High-alignment-tolerance optical coupling based on high-Δ light-induced self-written waveguides

Abstract

High-density multichannel optical components, such as silicon photonics and multicore fibers, have attracted significant attention for high-density photonic systems. These systems require low-loss coupling between optical components and single-mode fibers, for which light-induced self-written waveguide (LISW) connections represent a promising approach owing to their high alignment tolerance. Previous studies have shown that the alignment tolerance of LISW connections can be increased by extending the gap between the waveguides, thereby forming longer LISWs. However, the formation of long LISWs can increase propagation loss and require more stringent fabrication conditions. In this study, we investigated an alternative design approach for improving the alignment tolerance of LISW connections at a relatively small gap. Analytical modeling and numerical calculations were performed to examine the factors governing the alignment tolerance, and the results indicated that an LISW with a high relative refractive-index difference (Δ) can effectively improve the alignment tolerance. Based on these findings, we experimentally demonstrated a 1-dB alignment tolerance of approximately ±7.4 μm using a high-Δ LISW connection at a relatively small gap of 250 μm. These results indicate that high-Δ LISW connections can improve the alignment tolerance without substantially increasing the gap, making them promising for coupling to small-core devices used in high-density optical components.