Reseach On Mode Transmission And Sensing Characteristics Of Few Mode Fiber

Compared with traditional electric sensor,optical fiber sensor has unique advantages,such as strong electromagnetic interference resistance,high integration and small size.It has attracted more and more attention from scholars at home and abroad in recent years.Compared with single mode fiber(SMF)and multimode fiber,few mode fiber(FMF)has the advantages of limited number of modes,low nonlinearity and large area of mode field,widely used in optical communication,optical sensing,fiber device production and other fields.The sensor based on FMF has gradually become the research focus of new sensor.Based on the conclusion of FMF sensor technology and status,this dissertation analyzes the characteristics of FMF transmission mode and proposes mode interferometer and mode coupling sensor.Then,the theoretical analysis and experimental research on sensing characteristics are carried out.The main contents of the paper are as follows:Firstly,the theoretical model of FMF is established by using scalar method.The characteristic equation of FMF is deduced through the continuous condition of the electromagnetic field in the optical field,and the transmission mode in FMF is analyzed in detail by numerical solution of the characteristic equation.This work will provide a theoretical basis for the optimization design of the structure parameters of the mode interferometric sensor.Secondly,the mode coupling principle of FMF grating is studied.Combined with the coupled mode theory,the coupling equation of FMF Bragg grating and superimposed long period FMF grating is derived,and the reason for multi resonant peaks is explained.The sensing principle of temperature,curvature and refractive index of the FMF grating is analyzed theoretically,which provides a theoretical basis for the preparation of the mode coupling sensor.Thirdly,a mode interferometer sensor is designed,which is composed of a section of FMF sandwiched between single-mode fibers with small lateral core-offset in the input end.The transmission spectrum of the sensor is simulated,and its sensing principle is expounded.Based on the optical fiber fusion technology,the core-offset structure sensor is prepared.By Fourier transform of the transmission spectrum,two modes of participation in interference are analyzed.The sensitivity of the temperature and refractive index of the sensor is obtained by the experiment,and the experimental results are analyzed and discussed in detail.At last,the influence factors of the transmission spectra of FMF bragg grating and superimposed long period FMF grating are simulation study.Three mode coupling FMF sensors,including FMF Bragg grating,up-taper cascade Bragg grating and superimposed long period grating,are prepared,and the sensing characteristics of temperature,curvature and refractive index are further studied.Based on the characteristics of multi resonant peaks of the FMF grating,the double-parameters measurement is realized through the demodulation of the coefficient matrix,and the interference of temperature in refractive index sensing is avoided through the demodulation of wavelength difference.The paper does theoretically analyzes and experiment work on the FMF sensor characteristics of the refractive index,temperature,curvature,and the work lays the foundation for the further research and application of the FMF sensor.