Elimination of the fundamental mode hybridization on an x-cut lithium-niobate-on-insulator by using a densely packed bent waveguide array

Abstract

Lithium niobate (LiNbO3, LN) is a promising material for integrated photonics due to its natural advantages. The commercialization of thin-film LN technology has revitalized this platform, enabling low-loss waveguides, micro-rings, and compact electro-optical modulators. However, the anisotropic birefringent nature of X-cut LN leads to mode hybridization of TE and TM modes, which is detrimental to most polarization-sensitive integrated optical waveguide devices. A novel structure, to the best of our knowldege, utilizing a densely packed bent waveguide array is presented in this paper to eliminate mode hybridization. The refractive index is modulated in a manner that eliminates the avoided crossing of the refractive index curves of the TE and TM fundamental modes; thus, mode hybridization is prevented. The structures are readily accessible in the full range of commercially available LN film thicknesses from 400 to 720 nm and in any etching depth. The proposed structures give a polarization extinction ratio of −30dB across all bend radii, while simultaneously maintaining low excess loss of less than −1dB after reaching a 100 µm bend radius.