High Efficiency and Compact Spot Size Converter for LNOI Integrated Device

In this paper, we present a new design of spot size converter (SSC) for coupling between UHNA (Ultra-High Numerical Aperture) fiber and lithium niobate on insulator (LNOI) photonic wire waveguide. We demonstrate a novel structure in a cross-like arrangement composed by a central LN waveguide and four LN waveguides outside around. The cross-like structure collects the fiber spot power with high power overlap efficiency and transmits with low loss, which converts the large-scale spot to a small size one. The SSC is designed using Bidirectional Eigenmode Expansion solver (EME solver) of MODE Solutions. It is very compact with a total length as short as 35 μm. The simulation result shows that the power overlap efficiency at the fiber-to-chip facet of proposed device can reach 93.8% (92.7%) and coupling loss is 1.85 dB (1.20 dB) for the TE (TM) mode at 1550 nm.