Theoretical Study On The Liquid Refractive Index Sensing Based On Brillouin Scattering In Micro-nano Optical Waveguide

With the development of optical fiber sensing technology,people try to apply it to the measurement of more parameters.Liquid refractive index,an important optical quantities for environmental and biochemical properties,has draw people's attention.Generally,when single-mode fiber is used as the sensing fiber based on Brillouin method,only physical parameters such as temperature and strain can be directly measured,it's still difficult to measure the refractive index of the liquid.However,the evanescent field of the micro-nano optical waveguide can be coupled with the external environment,which provides the possibility of refractive index sensing based on Brillouin method.The change of the external liquid refractive index leads to the change of the Brillouin scattering characteristics of the micro-nano optical waveguide,which provides a feasible solution for measuring the liquid refractive index using the Brillouin scattering effect.In this paper,the theoretical research of liquid refractive index sensing based on micro-nano optical waveguide Brillouin scattering is developed.First,the mathematical models of the acoustic and optical wave fields in the micronano optical waveguide are analyzed,the complete Brillouin scattering model induced by the photo-elastic effect and the boundary movement effect are comprehensively considered.What's more,the influencing factors of the model in the process of refractive index sensing are analyzed.Then,for different structure and size of the microfiber,the gain coefficient,the dynamic range,sensitivity of the Brillouin refractive index sensing are analyzed,and the theoretical upper limit sensitivity of Si O2 microfiber for liquid refractive index sensing is pointed out.Then we analyze the chalcogenide microfiber,which has a high Brillouin gain coefficient.And it was compared with the Si O2 microfiber,so we can find how the material parameters effect the sensitivity and gain coefficient.A structure of microfiber which can expand the measurement dynamic range is proposed.Finally,two common microstructure of optical waveguide are analyzed,of which the liquid refractive index sensing characteristics are discussed respectively.It is found that the slot-guided waveguide structure are more sensitive,and has better dynamic range and gain coefficient after modification.With the analysis of the substrate structure,we find that the miniature support structure can be a good solution for the integration of photonic chip on the substrate when it is used for Brillouin refractive index sensing.