Study On Fiber Optic Sensors Based On Waist-Enlarged

The fiber optic sensors based on waist-enlarged have been mainlystudied and several applications have been proposed in this thesis. The fiber opticsensors based on waist-enlarged not only have the advantages of resistance toelectromagnetic interference, corrosion resistance and small size, but also thestructure is simple, low cost and easy to processing. Firstly, in this paper, theprinciple of the proposed sensors based on waist-enlarged is presented in detail. Thena new optical fiber sensor has been designed through combining the waist-enlargedstructure and single-mode-multimode-single-mode (SMS) fiber structure. Finally, theproposed optical fiber sensors based on waist-enlarged for measuring strain, relativehumidity and refractive index is proposed and experimentally demonstrated and thefiber optic sensors based on waist-enlarged could have a wide application in varioussensing fields.The main work of this thesis is as follows:1. The principle of Mach-Zehnder interferometer is analyzed, which providesimportant theoretical foundation for the following studies of the fiber sensors. Thentwo main demodulation methods: wavelength and intensity demodulation of theMach-Zehnder interferometer are introduced.2. The beam propagation method was employed for modeling the claddingmode stimulated by fiber waist-enlarged structure. First, The influence of differentdiameter size and length of fiber waist-enlarged structure is studied in detail. Thenthe strain and humidity sensor are introduced. The strain sensitivity of0.00215dBm/ε with a measurement range of0~3870ε, and the relative humiditysensitivity of0.0983nm/%RH with a relative humidity range of40%~90%RH areachieved.3. SMS theory and Mach-Zehnder interferometer theory provide theoreticalbasis for designing the optical fiber sensors based on SMS structure withwaist-enlarged tapers. Compared with the SMS structure, the sensors based on SMSstructure with waist-enlarged taper highlights the advantages of waist-enlarged tapers.The multi-mode fiber in the SMS structure based on waist-enlarged tapers is not needto etch with hydrofluoric acid in the measurement of refractive index and humidity,and there are more interference pattern. 4. The fiber refractive index and relative humidity sensor based on the SMSstructure with waist-enlarged tapers are proposed. It is fabricated by splicing twocascaded waist-enlarged fiber tapers to form a novel optical fiber sensor. When theexternal refractive index changes, the interference spectrum drift, the refractive indexor relative humidity can be obtained using the wavelength demodulation method.The refractive index sensitivity of152.237nm/RIU with a measurement range of1.3725~1.462, and the relative humidity sensitivity of0.223nm/%RH with a relativehumidity range of35%~85%RH are achieved, compared with SMS structure, it hashigher sensitivity.