Research On The Characteristics Of Composite Multimode Fiber Bragg Grating Based Sensor Used For Measuring Multi Parameters

Optical fiber sensing technology can detect external signals based on light and optical fiber.With the rapid development of dense wavelength division multiplexing,optical amplifiers and optical time division multiplexing technology,optical fiber sensor is widely used in all walks of life because of its small size,immunity to electromagnetic interference,easy to multiplex and other advantages.Optical fiber grating is one of the most commonly device used in fiber optic sensors.Sensors based on single mode fiber grating can measure temperature,strain,pressure,acceleration,vibration,and so on.Compared to the single mode fiber grating,multimode fiber grating has broad application prospects for its more modes,richer reflection peaks information and flexible parameters.Control the modes excited in the multimode fiber by launching light from single mode fiber(SMF),and made the reflection spectra of each resonance peaks relatively independent,have great practical significance for practical application of multimode fiber gratings.In this thesis,a detailed study of the characteristics of multimode fiber gratings and preliminary analysis for its sensing application was in research.Firstly,optical fiber model theory and coupled-mode theory were introduced in this thesis.The possible modes of a step index multimode fiber(SI MMF)and gradient index multimode fiber(GI MMF)was in analysis and the corresponding effective refractive index was solved based on optical fiber model theory.In the case of equal energy for each mode,the reflection and transmission spectra of the SI MMF grating and GI MMF grating are solved based on the coupled-mode theory with side exposure.Compared to the SI MMF grating,the reflection spectra of each resonance peaks are relatively independent in GI MMF grating because of the larger spacing of the resonance wavelength between each modes.Secondly,the mode excitation characteristics of SI MMF launched by single mode fiber(SMF)were studied in this thesis.There were only LP01 ~ LP06 modes in SI MMF when the SMF and SI MMF were splicing stranded.Based on this,the coupling coefficient between each modes with side exposure were in analysis,and the corresponding coupled-mode equations were given to solve the reflection and transmission spectra of the MMF gratings.A FBG-in-SMS was proposed,and the reflection and transmission spectra and the sensing characteristics of SI FBG-in-SMS and GI FBG-in-SMS with standard splicing wereresearched in detail.Thirdly,the sensing characteristics of FBG-in-SMS with offset splicing were researched in detail.Compared to the FBG-in-SMS with standard splicing,different modes excited in the case of offset splicing,which induced the reflection and transmission spectra of FBG-in-SMS changed.The temperature sensing characteristics and strain sensing characteristics were analyzed,which verify the feasibility for multi-parameter sensing based on FBG-in-SMS with offset splicing.Finally,the characteristics of Fizeau cavity sensor based on SMF and Fizeau cavity sensor based on MMF were analyzed in detail.Because of the interference signal of the Fizeau cavity sensor are sensitive to the length of Fizeau cavity and insensitive to temperature,and the reflection spectrum of fiber Bragg grating is sensitive to temperature.Based on this,fiber sensors to measure temperature and strain or stress simultaneous are designed and tested.