Characteristics Of Stimulated Brillouin Scattering And Sensing In M-shaped Few Mode Fiber

In recent years,research on the characteristics and applications of stimulated Brillouin scattering?SBS?in optical fibers has attracted much attention.As a very important nonlinear effect in optical fibers,SBS has been widely used in fiber amplifiers,lasers,sensors and so on.Among them,the SBS of the optical fiber has an important application value for distributed temperature and strain sensing.At present,the study on characteristics of SBS in single-mode fiber has been more perfect.With the emergence of mode division multiplexing technology,the application of few-mode fiber?FMF?has received much attention.The feature of SBS in the FMF needs to be further studied,and the sensing performance of temperature and strain based on SBS in the FMF needs to be further improved.In this paper,a FMF with M-shaped refractive index distribution?M-FMF?is proposed.The characteristics of simultaneous sensing of temperature and strain based on SBS in the M-shaped FMF are studied.The main research contents are as follows:1.Firstly,the mathematical model of SBS in FMF is deduced.The solution of scalar wave equations of optics and acoustics are briefly analyzed.The SBS in FMF is a process of interaction between different optical modes and acoustic modes,so it is very important to obtain the numerical solutions of the field distribution of optical mode and acoustic mode for the study of characteristics of SBS.2.Secondly,a FMF with M-shaped refractive index distribution is proposed.The acousto-optic effective area,which is obtained by the coupling between the different optical mode pairs and the excited acoustic modes in the M-FMF,is calculated.So the Brillouin gain spectrum?BGS?by the interaction of both intra-mode and inter-mode SBS in the M-FMF are analyzed.The results indicate that in the M-FMF,the BGS ofLP₀₁-LP₀₁ mode pair andLP₀₁-LP₁₁ mode pair have two scattering peaks with larger gain values and smaller peak differences,and the frequency interval of the scattering peaks is larger.For the BGS of other mode pairs,the scattering peaks are overlapped,the gain peaks are smaller and the peak difference is larger.Therefore,the BGS of these two mode pairs can be used to achieve the simultaneous sensing of temperature and strain.3.Finally,the influence of the fiber parameters?radius and refractive index?of M-FMF on the BGS of theLP₀₁-LP₀₁ mode pair andLP₀₁-LP₁₁ mode pair is analyzed,and the corresponding optimized structure of optical fiber is obtained.With this structure,the performance of temperature and strain sensing based on the BGS ofLP₀₁-LP₀₁ mode pair andLP₀₁-LP₁₁ mode pair are studied.The results show that the scattering peaks of the BGS by the interactions ofLP₀₁-LP₀₁ mode pair orLP₀₁-LP₁₁mode pair change inversely with temperature and strain increasing,and the frequency shift of the Brillouin frequency shift?BFS?are not same.By linearly fitting the relationship between BFS and temperature or strain,the temperature and strain sensing coefficients of the BGS based onLP₀₁-LP₀₁ mode pair andLP₀₁-LP₁₁ mode pair are obtained.The errors of temperature and strain are 0.21??5.20??and 0.23??5.67??,respectively.Compared with the existing research,the temperature and strain sensing based on SBS in the M-FMF,which is proposed in this paper,has higher sensitivity and lower error.These results of the thesis are of great significance for the study of SBS and simultaneous temperature and strain sensing in the FMF.