Re-analysis of single-mode conditions for thin-film lithium niobate rib waveguides

Single-mode waveguide devices are necessary in photonic integrated circuits. Previous studies on the single-mode conditions (SMCs) of the thin film lithium niobate (TFLN) either only focus on wire waveguide, or lack comprehensive discussions on the influence of the rib waveguide with different cross-section area at a certain height. In this paper, a systematical study on the SMCs of rib waveguide based on TFLN is carried out by using finite difference method (FDM). By comparing the effective refractive index between the first-order mode in the central rib region and the fundamental mode in the slab waveguide, and the fraction of the mode power flux between the TFLN rib region and the whole waveguide region, the SMCs in the central rib region are analyzed in terms of geometric parameters of the TFLN rib waveguide. The simulation results suggest that no single equation could meet the SMCs of different waveguide structures simultaneously in a small TFLN with the height no more than 900 nm. Ultimately, new approximate expressions as functions of rib-waveguide width and etching depth are proposed to precisely describe the single-mode condition at these dimensions.