Theoretical And Experimental Investigation On The High Sensitive Interferometric Fiber-optic Sensors

As one of the most important investigation area, the interferometric fiber-optic sensor has attracted considerable interest. Compared with other fiber-optic sensors, it has extremely high sensitivity, ultra-large dynamic measuring range and broadband frequency response range. Recently, interferometric fiber-optic sensor has been widely used in structure health and environment monitoring, biological and human health monitoring. Accompanied by the development of the nanotechnology and integration technique, interferometric fiber-optic sensor has been moving in the direction of more sensitive, simpler structure and tinier dimension.Aiming at the development needs of the interferometric fiber-optic sensor, this thesis has mainly discussed and investigated several kinds of novel high sensitive interferometric fiber-optic sensing structure based on the key technologies of fiber laser, micro/nano-fiber and all-fiber interferometric structure. The main research results include:According to the coupled-mode theory of the fiber, we have investigated the basic theory of the fiber Bragg grating and fiber laser. Based on the successful fabrication of the FBG written onto the active fiber and the theoretical analysis of the linear cavity erbium doped fiber laser, an ultra-short cavity fiber distributed Bragg grating (DBR) laser with the cavity length of13mm has been fabricated and tested.On the basis of the analysis and successful fabrication of the DBR fiber laser, the transverse force response of the laser has been stimulated and experimentally demonstrated by utilizing the principle of the beat frequency demodulation and the sensitive characteristic to the transverse force of the fiber birefringence. Meanwhile, by cascading two fiber laser cavities along a single fiber, the possibility of the quasi-distributed fiber sensing based on the DBR fiber laser has also been certified. Furthermore, according to the transverse force response of the laser cavity, the high sensitive measuring system for the liquid level, human health parameters has been proposed and experimentally demonstrated with the help of specific fiber packaging structures. In addition, since the fiber birefringence is also sensitive to the fiber twist, the fiber twist response has been achieved.The waveguide field equation of the micro/nano-fiber, the characteristic of the evanescent field and the coupling effect between guided modes especially the high-order modes and the environment have been carefully investigated. Then, the micro/nano-fiber fabrication platform has been constructed and the fiber drawing technique has been introduced. Moreover, a novel refractive index sensor with Mach-Zehnder interferometric structure and micro/nano-fiber as the sensing arm has been proposed. The device can not only achieve linear measurement, but also can improve the measuring sensitivity efficiently. The sensitivity can achieve as high as7159μm/RIU at the microfiber diameter of2.0μm.The all-fiber in line interferometric sensing structure as well as its mode theory and interferometric mechanism have been thoroughly studied. According to the theoretical analysis, a "multimode-singlemode-multimode" fiber mode interferometric structure has been designed and fabricated. The temperature, axial strain and ambient refractive index response of this structure have been tested. Meanwhile, by tapering down the central single mode fiber of the structure, a novel fiber bending sensing structure in terms of the mode coupling enhancing characteristic has been proposed. The bending measurement could be achieved simply by measuring the fringe contrast of the interferometric spectrum. The experimental results show that the device could not only achieve a really high bending sensitivity, but also overcome the influence of the fluctuation of temperature and light source power.