Design and Optimization of a High-Efficiency 3D Multi-Tip Edge Coupler Based Lithium Niobate on Insulator Platform

Abstract

Fiber-chip edge couplers can minimize mode mismatch in integrated lithium niobate (LiNbO3) photonics via facilitating broad optical bandwidth coupling between optical fibers and waveguide circuits. We designed a high-efficiency multi-tip edge coupler utilizing the lithium niobate on insulator (LNOI) platform for achieving superior fiber-to-chip coupling. The device comprises a bilayer LN inversely tapered waveguide, three 3D inversely tapered waveguides, and a silicon oxynitride (SiON) cladding waveguide (CLDWG). Finite difference method (FDM) and eigenmode expansion (EME) simulations were utilized to simulate and optimize the edge coupler structure specifically within the 1550 nm band. This coupler demonstrates a low fiber-chip coupling loss of 0.0682/0.0958 dB/facet for TE/TM mode at 1550 nm when interfaced with a commercially cleaved single-mode fiber (SMF) with a mode field diameter (MFD) of approximately 8.2 μm. Moreover, the 1 dB bandwidth of the coupler is 270 nm for the TE mode and 288 nm for the TM mode. Notably, the coupler exhibits a relatively large tolerance for optical misalignment owing to its large mode spot size of up to 4 μm. Given its ultra-low loss, high-efficiency ultra-broadband capabilities, and substantial tolerance features, this proposed device provides a paradigm for fiber-to-chip edge coupling within lithium niobate photonics.