High-efficiency edge couplers enabled by vertically tapering on lithium-niobate photonic chips
Di Jia, Qiang Luo, Chen Yang, Rui Ma, Xuanyi Yu, Feng Gao, Qi‐Fan Yang, Fang Bo, Guoquan Zhang, Jingjun Xu
Abstract
In the past decade, photonic integrated circuits (PICs) based on thin-film lithium niobate (TFLN) have made substantial progress in various fields, including optical communication, nonlinear photonics, and quantum optics. A critical component is an efficient edge coupler facilitating the connection between PICs and light sources or detectors. Here, we propose an innovative edge coupler design with a wedge-shaped TFLN waveguide and a silicon oxynitride cladding. Experimental results show a low coupling loss between the TFLN PIC and a 3-μm mode field diameter (MFD) lensed fiber, measuring at 1.52 dB/facet, with theoretical potential for improvement to 0.43 dB/facet. Additionally, the coupling loss between the edge coupler and a UHNA7 fiber with an MFD of 3.2 μm is reduced to 0.92 dB/facet. This design exhibits robust fabrication and alignment tolerances. Notably, the minimum linewidth of the TFLN waveguide of the coupler (600 nm) can be readily achieved using commercially available i-line stepper lithography. This work benefits the development of TFLN integrated devices, such as on-chip electro-optic modulators, frequency combs, and lasers.