Research On Optical Fiber Refractive Index Sensors And Magnetic Field Sensors Based On Mode-interference

Optical fiber sensors based on mode interference take full advantage of the mode characteristics of optical fiber.Because of featuring the implementation simplicity,flexible design,and all-fiber structure etc.,they have become one of research hotspots in the optical fiber sensing field in recent years.In this thesis,aiming to realize the sensing to refrective index(RI)and magnetic field,we will focus our work on two types of optical fiber sensors,namely the singlemode-no-core-singlemode(SNS)structure based on multimode interference and the bent single-mode fiber(SMF)structure based on core-whispering-gallery modes interference.Firstly,we researched the applications of the two types of sensors on the RI sensing.Then,using the magnetic-field-dependent complex RI of magnetic fluid,we proposed and studied two novel magnetic field optical fiber sensors.The main work in this thesis can be summarized as:1.The theoretical research on the SNS and bent SMF structures were conducted.Deriving from the mode theory of optical fiber and employing the mode expansion method,we built the theoretical model of light traveling in SNS structure and simulated the output spectra under different conditions.The phenomenon of the spectral broadening or compressing behavior caused by the change of surrounding RI was observed for the first time,leading to the conclusion that the RI sensitivity is proportional to the wavelength.Then,from the perspectives of geometrical optics and mode theory,we elaborated the phenomenon of core-whispering-gallery modes interference and the principle of RI sensing in the bent SMF structure,providing theoretical support for the further research.2.We realized the self-compensation to the remperature for the SNS-based refractometer for the first time,and the simultaneous measurement of RI and temperature based on bent SMF structure and FBG.Based on the conclusion that the RI sensitivity is proportional to the wavelength,we proposed and demonstrated the method of self-compensation to temperature by simultaneously monitoring two characterictic wavelengths.Besides,by combing the droplet-like bent SMF structure and FBG together,we proposed and comparatively studied the cascaded and embedded structures for the simultaneous sensing to RI and temperature.3.By coating MF to the surfaces of SNS and bent SMF structures,two kinds of optical fiber magnetic field sensors based on magnetic fluid(MF)and mode interference were proposed and studied in theory and experiments.Through introducing the absorption term of MF to evanescent field into the original theoretical model of SNS strcuture,we studied the working principles and characteristics of sensors by simulations.In experiments,the two kinds of sensors were demonstrated and studied from the following three perspectives: the dynamic response of the sensors,the influence of bent diameter to the sensors based on MF and bent SMF,and the realization of simultaneous measurement of magnetic field and temperature.4.Employing the fiber loop ring-down(FLRD)technology,we realized the demodulation for the sensors based on MF and mode interference.FLRD technology can eliminate the influence of the fluctuation of light source and enhance the dection sensitivity.Aiming at the higher sensitivity,we detailedly analyzed the selections of key parameters in the fiber loop,the influence and selecting criterion of input wavelength.Finally,the sensing system was studied in experiments.