Stimulated Brillouin Scattering Sensing Properties In Stepped-Shaped Single Mode Fiber

Distributed optical fiber sensors have received wide attention in recent years.It is widely used in civil engineering,environmental monitoring,aerospace and other fields,and is especially suitable for environments with limited working space.They have the advantages of high measurement accuracy,small error,long life,etc.The distributed optical fiber sensors based on stimulated Brillouin scattering can simultaneously measure two parameters of temperature and strain,but there are cross-sensitive problems in simultaneous sensing of temperature and strain,and it is difficult to improve measurement accuracy.In this paper,the stimulated Brillouin scattering of stepped-shaped single mode fiber is theoretically studied,and its temperature and strain simultaneous sensing characteristics are systematically analyzed.The main contents of this paper are arranged as follows.Firstly,for the stepped-shaped single mode fiber doped with germanium or fluorine,we systematically studied the effects of doping concentration of germanium and fluorine and core structure on the Brillouin gain spectrum based on the stimulated Brillouin scattering theory.Secondly,by optimizing the structure of the stepped-shaped refractive index distribution single mode fiber,the Brillouin frequency shift difference between the L01 acoustic mode and the L03 acoustic mode is 400MHz,the Brillouin gain between the L01 acoustic mode and the L03 acoustic mode is on the same order of magnitude.Thirdly,using the optimized stepped-shaped refractive index distribution single mode fiber for simultaneous sensing of temperature and strain.By analyzing the temperature coefficient and strain coefficient of different acoustic modes,the maximum strain error is 23.41με,the maximum temperature error is 0.74℃.At last,We systematically studied the relationship between longitudinal acoustic velocity and temperature change and strain change in doped fiber.The relationship between longitudinal acoustic velocity and strain and temperature was obtained by linear fitting,and the relationship was verified.This research work in this paper has certain guiding significance for the analysis of stimulated Brillouin scattering in single mode fiber,the sensing applications based on stimulated Brillouin scattering and the design and optimization of the fiber structure.