Fiber Optic Sensors Research Based On Modal Interferometer

In recent decades, the fiber optical sensors have been developed for a fewdistinguished features such as small size, electromagnetic immunity, and resistance tocorrosive, light weight, and high sensitivity. Therefore, the sensors based on modalinterferometer have been widely provided. In this paper, a series of novel modalinterferometers based on waist-enlarge have been proposed. And their practicalapplications in sensing field were verified by experiment.The main work of this thesis is as follows:1. The modal interferometer based on waist-enlarge splicing with all SMF hasbeen proposed. A section of single mode fiber (SMF) is spliced with waist-enlargebetween two SMFs to construct a modal interferometer. The sensor can measure thephysical quantity by the wavelength shift. Moreover, the proposed sensor has theadvantages of easy fabrication, low cost, and hard robustness.2. A novel sensor based on Polarization-maintaining fiber (PMF) modalinterferometer is presented. A section of PMF is spliced with waist-enlarge betweentwo SMFs. Owing to the advantage of polarization-maintaining stabilitycharacteristics, the sensor can be achieved high-precision measurement of physicalquantities. The results show that the PMF-based sensor has a higher sensitivity thanthe all SMF-based one. And different lengths of sensors have been experimented.3. The modal interferometer based on waist-enlarge splicing with long periodfiber grating (LPG) has been proposed. High-frequency CO2laser in the SMF iswritten long period fiber grating, and the LPG is spliced with the SMF. The sensorachieved high-precision measurement of physical quantities by the wavelengthchange.4. A new structure of long-period grating (LPG) sensor is introduced forsimultaneous measurement of the refractive index (RI) and temperature. This type ofgrating device consists of a LPG and two waist-enlarge tapers. A section of LPG isspliced with waist-enlarge between two SMFs to construct a modalinterferometer.The sensor can measure the double parameter at the same time, so itcan eliminate the influence of temperature on the other physical quantities in the daily practical production and engineering structure. The proposed sensor is able tomeasure refractive index and temperature simultaneously, so it expands the practicalapplication.