A Theoretical Study on Rib-Type Photonic Wires Based on LiNbO₃ Thin Film on Insulator

A theoretical study on rib-type photonic wires (PW) based on lithium niobate (LiNbO₃)-on-insulator (LNOI) using finite element method is performed. Aiming at 1.55 μm wavelength, effective refractive indices of several lower-order transverse-electric/magnetic (TE/TM) modes in the central rib region and the fundamental mode in the slab waveguide in the side region are calculated as a function of geometric parameters of the PW. By comparing effective refractive index of the first-order mode in the central rib region with that of the fundamental mode in the slab waveguide, single-mode condition in the central rib region is determined in terms of geometric parameters of the PW. Electric field distributions of fundamental TE and TM modes are also simulated and discussed. In addition, dispersion relation of effective group refractive index of the fundamental mode in the central rib region is calculated and discussed in comparison with those of ridge-type LNOI PW, traditional Ti-diffused LiNbO₃ strip waveguide, and bulk LiNbO₃ single-crystal. For a fundamental TE mode in a rib-type LNOI PW with a LiNbO₃ film thickness 700 nm, a rib width 1 μm and an etching depth 100-200 nm, nearly zero group index dispersion in the wavelength range 1.31.7 μm can be realized.