Zero Birefringence Condition in Lithium Niobate on Insulator Rib Waveguides

Abstract

Birefringence, quantified as the maximum effective index difference of two orthogonally polarized modes, is one of the key characteristics of Lithium Niobate on Insulator (LNOI) waveguide devices. As LNOI optical properties strongly depend on the polarization of the propagating light beam, achieving the zero‐birefringence condition (ZBC) is highly desired. In this work, a rib LNOI waveguide is designed, and the influence of geometrical dimensions on the birefringence is characterized. From the result, the slab's height is found to be the dominant geometrical factor that significantly contributes to the birefringence, with an optimum slab's height found at 50 nm. For this optimized value, the influence of width and wavelength on birefringence are characterized. The analysis is extended to investigate the properties of group indices that can result in the temporal walk‐off phenomenon. This phenomenon can significantly impact the performance of single‐photon devices, as it can cause a polarization mismatch between the incident photon and the polarization properties of the medium, resulting in reduced efficiency of devices such as single‐photon detectors. In the analysis, the zero birefringence and walk‐off are achieved at narrow waveguide width, 𝑤 of 250 nm and wavelength, = 1500 nm, which is within the telecommunication region band.