| With the development of inscription technology, fiber-optic grating becomes one of the most typical and potential passive components, which has been extensively applied in optical communication and optical sensing system. To meet the increasing demands for multi-functionalities, multi-functional fiber-optic gratings with novel structure, new characteristics have become an increasingly important trend for new photonic devices. The eigen-structures, modes coupling, transmission and some other characteristics have been thoroughly analyzed domestically for the first time. This dissertation focused on the coupling mode and side-radiation theory, inscription technology in BFBGs, and then the factors affecting on the intrinsic structure parameters are profoundly discussed, including the effective index of core and cladding, tilted angle, grating period, etc. The spectral characteristics, mode couplings and potential applications are illustrated as well.The contents of the dissertation include the followings in detail:1. Based on the mode coupling theory the dissertation investigates mode coupling characteristics of BFBG, demonstrating mode coupling equations and optical parameters under different coupling conditions. At the same time, mode coupling characteristics are investigated using volume current method (VCM); the spectra are also simulated through the above theories which guide us to research on mode coupling, directional side-radiation and the other characteristics.2. By combining the inscription theory with our experience, the fabrication processes of BFBG are developed successfully, which provides a good experimental platform for the entire study. The transmission spectra of BFBG under different conditions are analyzed in detail, providing a theoretical guidance for the fabrication of high quality BFBGs.3. Single-parameter and multi-parameter sensing characteristics of BFBG based on eigen-structure are studied respectively. The dissertation discusses elsto-optic, thermo-optic and thermal expansion effect of fibers on the structure parameter of BFBG, and its temperature, strain, bending sensing characteristics are analyzed though theoretical and experimental approaches. Special structure and thermal feature of BFBG are combined to achieve temperature/strain, temperature/bend and the other multi-parameter sensing technology using only one BFBG for the first time in domestic research. Therefore, not only does BFBG achieve highly precise measurement, and but also it plays an important role in the area of multi-parameter measurement. Consequently, it is a new style of multi-function fiber grating.4. It is first reported that the transmission characteristics of BFBG for a range of curvatures with different axial rotational orientations. BFBG shows strong spectral dependence not only on the curvature but also on the direction along which bending is applied, which provides the basis to construct other vector sensors.5. Two schemes are proposed in order to solve temperature-cross problem and enhance sensitivity respectively. According to the same temperature sensing characteristic between core mode and cladding modes, we can achieve precise measurement without the influence of temperature by subtracting the resonant wavelength of any cladding mode from that of core mode. By reducing the cladding diameter, high sensitivity can be achieved because of more susceptibility of cladding modes to surrounding material refractive index.6. A novel BFBG liquid-level sensor based on refractive index sensitivity and the relation between the length and coupling strength is proposed and demonstrated. The results show that liquid level has an influence on the coupling of cladding modes, and a linear relationship is kept pretty well between variation of transmission power and that of liquid level. This sensor is able to detect the liquid level in real time.7. A real-time air relative humidity sensor based on BFBG is proposed for the first time. By combining the advantages of BFBG and PVA as a smart material, the proposed sensor achieves high sensitivity under different ambient humidity levels ranging from 20%RH to 98%RH. The transmission power of BFBG presents different linear change within two humidity range, and sensitivity in the high humidity range is much higher. This scheme extends the applications of BFBG to biochemical and the other cross-discipline areas.8. A novel edge filtering method with dynamic temperature compensation based on BFBG is proposed in a fiber sensor system. Based on the superior linear edge feature of cladding modes envelope of BFBG and the similar temperature characteristics to FBG, the strain sensor demodulation system with dynamic temperature compensation can be achieved. This demodulation system has lots of advantages such as good wavelength linearity, design flexibility, different sensitivity, and different wavelength dynamic demodulation range. Therefore, it can be applied for different sensing signals. The proposed scheme provides a basis for the optimization of all-fiber wavelength demodulation technology. In addition, the principle of side-radiation wavelength demodulation is analyzed. It is proved that different wavelengths correspond to different location, resulting in the simultaneous multi- wavelength demodulation. This system is constructed by all-optical devices, which ensures its various advantages such as simple structure, good stability and compatibility.
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