Investigation On Theories And Applications Of High Sensitive Fiber Optical Sensing Based On The Evanescent Field Of Micro/Nanofiber

Micro/nanofibers (MNF) featured with small size, low optical loss, tight opticalconfinement, high fraction of evanescent fields, large waveguide dispersion, and highmechanical strength and flexibility, are suitable for variety of functional, miniaturization, andhigh performance photonic devices. The optical property of high fractional evanescent fields,allowing strong evanescent wave coupling between MNF and ambient environment, makesthe MNFs applicable to compact and high sensitive optical sensors. In this thesis, we focuson investigating the MNF or micro-structures based high sensitive sensors for applications ofrefractive index and temperature monitoring in composites cure process.Firstly, we set up the mathematic model of MNF, and perform the simulations by usingMatlab to investigate the evanescent field enhancement mechanism. The fabrication andmeasurement platform of microfiber is established. The flame-heated taper drawing ofoptical fibers is performed to successfully fabricate microfiber with uniform diameter andlow transmission loss.The optical sensors based on the evanescent field of single microfiber are proposed,including the intensity modulation refractometer, the phase modulation refractive index andtemperature sensor based on Mach-Zehnder interferometer with a single microfiber as thesensing arm, and miniature refractive index sensor based on cutoff wavelength of singlemicro/nanofiber. We experimentally realize high sensitive temperature and refractive indexmeasurement.Compact sensors based on micro/nanofiber Bragg grating (MNFBG) are investigated.The characteristics of refractive index sensing and temperature sensing are numericallysimulated using the software of OptiGrating. By employing the refractive index matchingliquid with high thermo-optic coefficient as sensing carrier, a novel temperature sensor basedon MNF with very high sensitivity is proposed and discussed.Finally, we present two miniature Fabry-Perot interferometers (MFPIs), which areformed by FBG–MNF-FBG structure or two cascaded MNF Sagnac loops. The optical andsensing properties of the two MFPI structures are analyzed, and the laboratory fabricationtechniques of them are performed. By using the MNF based MFPI as the reflection filterelement of the fiber laser, dual wavelength laser as well as beat frequency interrogation for refractive index sensing is experimentally realized. The refractive index sensitivity of856MHz/RIU can be achieved.