Inter-mode backward stimulated Brillouin scattering in lithium niobate core fiber

Abstract

Brillouin sensing in multimode optical fibre enables the simultaneous modal transmission of multiple modulus, such as temperature and strain, and helps to promote the development of simultaneous multi-property sensing techniques. Among these, intermodal backward Brillouin scattering induced by higher order modes (HOMs) is of great importance for understanding the phonon-photon interaction in multimode fibre. In addition, lithium niobate (LiNbO₃) materials have potential applications in sensing due to their excellent electro-optical properties, which are expected to provide higher sensitivity and accuracy. In this paper, the inter-mode BSBS induced by higher order optical modes in micron scale LiNbO₃ optical fibre is investigated. The inter-mode excited Brillouin scattering characteristics of the first 10 LP modes in the fibre are simulated and analysed by finite element simulation. During backward Brillouin scattering, several longitudinal acoustic modes (LAMs) are excited. However, only a few specific low-order LAMs are involved in the interaction between different optical mode pairs, resulting in Brillouin amplification. In this paper, we elucidate the differences in the gain characteristics of the Brillouin gain spectra between different modes and reveal the acoustic mode law that contributes most to the BGS in the inter-mode BSBS. The results show that both the effective refractive index (2.1847–2.2072) and the Brillouin scattering frequency shift (20.6–20.9 GHz) decrease with the increase of mode order in this structure. And the Brillouin gain reaches 0.4244 m⁻¹·W⁻¹, which is a significant improvement compared with the conventional SiO₂ fiber, predicting the improvement of sensitivity in sensing.