The Mechanism And Experimental Study Of High Sensitivity And Multi-Parameters Sensing Based On Micro/Nanofiber With Modes Coulpling

Micro/nano-fiber (MNF) is a kind of optical fiber waveguide with a subwavelength diameter. Owing to the large evanescent field, MNF is very sensitive to external environment change. Compared with common optical fiber sensors, MNF based sensors have properties of high sensitivities, small size, and easy coupling with other optical waveguideswhich have been widely researched and reported in the sensing areas of refractive index (RI), temperature, stress, acoustic wave, electric current, magnetic field, biochemistry, etc.This work supported by the National Natural Science Foundation of China which aims at developing fiber sensor with ultra high sensitivity and multi-parameters for material curing process monitoring. In this thesis, we mainly study two kinds of MNFs i.e. D-shaped microfiber and multimode microfiber (MMMF) in theory and experiment, including the theory of modes coulping, the mechanism of sensitivity enhancement, the experiment of RI and temperature sensing. The main research content includes:Firstly, we systematically introduce the fabrication process of D-shaped microfiber. With the assistance of commercial optical software COMSOL Multiphysics, the optical field distribution, birefringence, evanescent field proportion, and RI and temperature sensitivities of D-shaped microfiber are analyzed. Based on the strong evanescent filed of D-shaped microfiber, super-high sensitive RI and temperature sensing can be realized by utilizing Sagnac polarization interferometer.Secondly, based on the eigen equation of 2-Dimentional optical waveguide, we simulate the effective RI, optical field distribution, evanescent field proportion, and RI sensitivity of MMMF. We also prove the RI sensitivity enhancement mechanism based on multimode interference and strong evanescent field in MMMF. The turning point of RI sensitivity based on MMMF is firstly been found. On the turning point, RI sensitivity could reach to ±?, which has important significance on the super-high sensitive RI sensing. The experimental results exhibit high consistency with the theoretical predictions and the maximum RI sensitivity of 10777.8 nm/RIU is achieved with the diameter of 4.6 ?m.Thirdly, according to the requirements of multi-parameters sensing, we propose a MMMF based dual-arms MZI structure. By inserting the MMMF into the dual-arms MZI setup, the multimode interference in MMMF and the Mach-Zehnder interferometer (MZI) between the two arms are combined, and consequently simultaneous measurement of RI and temperature can be realized in the single MZI setup. Moreover, we use Gauss function fitting to accurately extract the resonant wavelength of the multimode interference in MMMF. By tracking wavelength shifts of the envelope dip and dense interference spectrum along with RI and temperature change, RI sensitivities of 2576.584 nm/RIU and 1001.864 nm/RIU, as well as temperature sensitivities of -0.193 nm/? and 0.239 nm/? are experimentally demonstrated. The sensitivities are the highest in simultaneous measurement of RI and temperature to the best of our knowledge.Forthly, to further reduce the size of the sensor, we propose an in-line MZI structure based on MMMF. By using the compound strcutrue of multimode fiber-single mode fiber-MMMF, the interference between the cladding mode and the core mode in single mode fiber (SMF) as well as the multimode interference in MMMF can be combined, resulting in the simultaneous measurement of RI and temperature in the single fiber. The sensing structure owns the advantage of compact and simple structure as the hybrid fibers are in a single line. Besides, the two MZIs are both in-line and no crosstalk exists between them, so the influence of vibration and air turbulence can be avoided. In dual parameters measurement, we obtain RI sensitivities of -23.672 nm/RIU and 3820.230 nm/RIU, as well as temperature sensitivities of 81.2 pm/? and -465.7 pm/?, respectuvely.Finally, in order to meet the demand of the sensor network access, we further design a microfiber interferometer assisted by Fresnel reflection. By cutting the fiber tail to form an endface, the multimode interference in MMMF and intensity modulation of Fresnel reflection can be easily combined. To demodulate the intensity of the reflection spectrum and the free spectrum range (FSR), we adopt sinusoidal function to fit the reflection spectrum. Then RI sensitivities of -72.247 dB/RIU and 68.122 nm/RIU, as well as temperature sensitivities of 0.0283 dB/? and -17 pm/? are achieved in the experimental demonstration. On account of the simple atrcuture of single reflection line, this sensor can be used for microsized probe in biochemical detection.