Novel Fiber Bragg Grating Sensors

In the field of oil industry and oil-gas exploration, the research of novel fiber sensors and the establishment of a complete optical fiber sensor network system for real-time monitoring will contribute to a fully grasp of the most primary data such as temperature, pressure, vibration and other physical quantities. While, optical fiber sensors still remain some big challenges and difficulties such as the capabilities of all-fiber integrating, multi-parameter monitor simultaneously and separately, distributed-sensing-system constructing et al. Using FBG based sensors can solve this problem efficiently owing to their advantages such as small size, light weight, electromagnetic immunity, long-distance transmission capacity and corrosion resistance. Besides, they also exhibit some unique advantages such as wavelength multiplexing ability and multi-parameter sensitivity. As such, FBG sensors are regarded as one of the most prospectivity in such areas mentioned above, as well as many other in-field applications. The thesis presented here was focused on the research of novel FBG based sensors. The content included the grating coupling mechanism, the design of novel fiber grating based sensing structure and the measurement of different physical quantities.1. In the theoretical part, the mode coupling theories of optical fiber waveguides and FBG were introduced. FBG writing techniques and their characteristics were discussed, respectively;2. Three kinds of "specially-designed sensing structure"-based standard FBG bending and vibration sensor were proposed. ?1? Firstly, a compact FBG accelerometer based on a hybridization of two cantilever beams was proposed. Two FBGs were prestressed and fixed on the beam longitudinal symmetrically. When acceleration signals were exerted on the accelerometer, the Bragg wavelength of the two gratings shifted sharply in opposite directions without unwanted chirp. Experimental results show a high cross-axis response ?3.2%? from the main axiscontribute to the cantilever beam of the sensor. The stastic and dynamic sensitivities are 4.8nm/mm and 218.4 pm/G, respectively.?2? Secondly, an FBG based three-dimensional acceleration sensor was proposed. In the sensing structure, the mass was centrally located in three-dimensional space, each elastic metal bellows were fixed along each face of the mass, respectively, acting as a buffer medium connected gratings with the mass. In the experiment, three of FBGs located in different axis were coupled to the spectrum demodulation analyzer. The experimental results indicated that the proposed structure has orientation dependence to space vibration. ?3? A non-uniform section beam based fiber Bragg grating sensor was introduced. The sensing structure consisted of a semi-circle bending beam, with a free end acting as the inertial mass. Through the simulation of the structure, the beam showed the feather of linearity to strain distribution along the surface of non-uniform beam in the range of the half-hole. As such, with the increase of deformation on its surface, FBG spectrum was broadened obviously and the peak wavelengths drifted as well. Experimental results showed that when detecting spectral power and peak wavelength simultaneously, the bending and temperature sensitivities of the sensor were separately obtained with values of 1.9dBm/mm and 11.85pm/?, respectively. Furthermore, a series vibration tests were presented and the results exhibited the resonance frequency of 250Hz and the vibration response range of 0 to 3.5G, respectively.3. Based on the "specially-designed grating structure" principle, a novel and even simpler approach to fabricate off-axis FBGs by conventional UV photosensitivity, using an unmodified FBG writing set-up was discussed.The off-axis FBG was easily formed by displacing the fiber core away from the beam center (the position of the fiber relative to the beam center was easily visualized from the diffraction pattern of the UV light going through the fiber observed on a screen located behind the fiber. As such, the diffraction grating fringe induced by UV exposure irradiated on the optical fiber along the cross-section was asymmetric. Experimental result showed that the maximum bending sensitivities are found to vary from+1.17dB/m"1 at 0° to-1.25 dB/m^-1 at 180°, in the important curvature range from 0 to 1 m^-1 without cross sensitivity to temperature or surrounding media.4. The theoretical analysis of TFBG transmission spectrum by simulation was discuessed and two TFBG based all-fiber sensors were presented, respectively. ?1? By using two weakly titled TFBG, a novel 3D shape optical sensor with two gratings spliced together such that their tilt plane directions are oriented 90° from each other was proposed and demonstrated. The results demonstrated that bending directions from 0 to 180 degrees and curvature magnitudes between 0 and 3 m"1 can be extracted from each pair of resonance transmission values in a single measurement using unpolarized light. The measurement sensitivities were obtained from the two TFBGs range from-0.33dB/m^-1 to+0.21dB/m^-1, depending on orientation. For the third dimension in shape measurement, the axial strain was obtained from the wavelength of the Bragg peak similarly as for FBGs, with a sensitivity of 1.06 pm/?.The temperature was also measured, respectively. This sensor was compact with total length less than 3cm, obtaining the integrated the space shape signal in one sensing head. In a packaged device, a mirror could be coated on the end cleave to increase the signal coming back to the interrogation system. ?2? The principle of surface plasmon resonance ?SPR? and sensing characteristics were studied, and the basic method using TFBG to excite SPR was discussed, followed by simulations of four layer fiber waveguide. The results showed that the higher-order cladding mode which had plasma characteristics presented unique mode field distribution of energy. Connecting this feature with the orientation of grating tilt plane we developed a new, simple coating method to TFBG-SPR fiber optic sensor. The results provided that the depositions are suitably oriented along the tilt plane direction. Furthermore, it is shown that even a ?properly oriented? single-sided coating ?over only half of the fiber circumference? is sufficient to provide a theoretically perfect SPR response Finally, using a pair of adjacent TFBG resonances within the SPR response envelope, a power detection scheme is used to demonstrate an extremely high power sensitivity of 3.34×10⁴dB/RIU. The fabricated SPR sensor reduced the complexity of manufacturing to a large extent.