The Brillouin sensing technology in multimode optical fibers has garnered significant attention due to its capability for simultaneous modal transmission of multiple parameters, such as temperature and strain, which confer it higher information capacity and transmission efficiency. Additionally, lithium niobate, with their excellent electro-optical properties, show potential application value in the sensing field and are expected to provide higher sensitivity and precision. However, owing to the maturity of manufacturing processes, current research on fiber optic sensing predominantly focuses on silicon-based materials, with relatively fewer studies dedicated to fibers using lithium niobate as the core material, thus underestimating its application potential. This paper investigates the theoretical aspects of Brillouin scattering effects in lithium niobate optical fibers. We simulate the intra-mode backward Brillouin scattering characteristics of the first five orders of LP modes in micrometer-scale lithium niobate fibers by means of finite-element simulation, in order to explore its intrinsic law.
First of all, the relationship between the Brillouin frequency shift and gain for the first five optical mode interactions is analyzed in detail. The results showed that in the case of intra mode BSBS, the peak of BFS would exhibit a significant redshift, ranging from 20.63 GHz to 18.747 GHz. The Brillouin gain coefficient would also decrease from 13.503 m ⁻¹·W ⁻¹to 4.0115 m ⁻¹·W ⁻¹with increasing mode order, in which mode LP ₀₁having the strongest gain intra modal interaction means the best sensing sensitivity. In addition, compared with ordinary silica fiber, the Brillouin gain of lithium niobate fiber is increased by about 5 orders of magnitude, which means that fibers with lithium niobate as the core can have higher sensing sensitivity. In addition, we found that although there are significant differences in the Brillouin frequency shift values of each order of optical modes under intra modal interactions, the sound velocity of their corresponding acoustic modes is always consistent under the same acoustic mode. In data processing, we noticed that this is because as the mode order changes, the corresponding effective refractive index also decreases to ensure that each acoustic mode of the material always has the same sound velocity. These findings provide the basis for lithium niobate fiber sensors with electro-optic properties.