| The exploration of oil-gas field and production log are of great significance in resource exploration and oil recovery. Underground multi-parameter detection is a contribution to an overall understanding of geological underground features, discovery and evaluation of hydrocarbon reservoir, and access to data on well drilling. Fiber sensor will show a wide application prospect because of its own advantages (features:small size, anti-electromagnetic interference, high sensitivity, and easy to multiplex). In this dissertation, interference-based fiber sensors and fiber Bragg grating (FBG)-based fiber sensors were proposed and used to measure multi-parameters. Meanwhile, structure properties and experimental measurements of these fiber devices were also studied in depth. The main research contents of this dissertation include as follows:1. Two kinds of fiber temperature sensors:(1) An interferometer was proposed based on Fabry-Perot (FP) interference by coating a PVA film on the end face of a single mode fiber (SMF). Because of the large thermal expansion coefficient of PVA, this interferometer presented a high sensitivity to temperature change, besides, which could be further improved up to 319 pm/℃ by changing the length of the cavity. However, PVA would appear an alcoholysis reaction at high temperature (>100℃), which was bad for sensing property. (2) A polarization-maintaining photonic crystal fiber (PM-PCF) that could resist high temperature up to 1100℃ was employed as the sensing fiber to realize high temperature measurement. The device was formed by splicing a 6 cm-long PM-PCF with the SMF, which was based on the Sagnac interference. In the experiment, a well-defined interference spectrum was obtained. Moreover, this interferometer has been used to measure high temperature measurement up to 1050℃. The high temperature hysteresis of the sensor was eliminated by the repeated annealings, as a result, the temperature measurement repeatability was improved. The experimental results showed the temperature sensitivity of the sensor was 12 pm/℃ which was about 30 times higher than that of the conventional PM-PCF-based temperature sensor.2. Several kinds of refractometer were proposed, among them, two quasi MZ interferometers were used to realize the liquid level measurement. These devices mainly include the follows. (1) A multi-mode interferometer comprised of a 10 mm-long multimode fiber (MMF) sandwitched between two SMFs, i.e. SMS configuration. And then a FBG was cascaded with the interferometer. The cascade structure was capable to measure refractive index (RI) and temperature simultaneously. To further improve RI sensitivity of SMS structure, the middle MMF was etched by hydrofluoric acid. After several minutes etch, the RI sensitivity of this interferometer could be improved up to 800 nm/RIU. (2) A Mach-Zehnder (MZ) interferometer was formed by a SMF sandwiched between two 2 mm-long thin-core fibers (TCFs), i.e. TCF-SMF-TCF configuration. The transmission cladding modes within the cladding of the center SMF were sensitive to surround RI. Therefore, the interferometer could be demonstrated for the RI measurement with a high sensitivity of 158.88 nm/RIU. The above refractometers were wavelength-referenced sensors which might be affected by the ambient temperature. This temperature cross-talk was just compensated by cascading other fiber device. (3) A novel refractometer was proposed, which consisted of a short section of TCF followed with a MMF tip inscribed with a FBG. In which, the core/cladding diameter of step-index MMF are 50μm/125μm. The reflection spectrum of this device presented clearly separated multi-wavelengths (with respect to multiple cladding modes). The reflection intensities of the selected cladding modes have been employed to measure RI with high sensitivity of 116.27 dB/RIU. Meanwhile, ambient temperature only changed the reflection wavelengths and not affected the reflection intensities. (4) An in-fiber quasi-Michelson interferometer (IFQMI) working on reflection has been proposed. The configuration was composed of a 2 mm-long MMF followed by a 50 mm-long SMF terminated by a thick silver film as a reflection mirror. The cladding modes propagating in the SMF cladding were sensitive to RI. And this device has been used to realize the three liquid level measurements with the RI values of 1.3341,1.3672 and 1.4018). The corresponding sensitivities were 49.8 pm/mm,68.7 pm/mm and 88.7 pm/mm, respectively. Meanwhile, the liquid level sensitivity could be employed for the measurement of liquid RI. Furthermore,2 mm-long TCF was used to replace the MMF to obtain another IFQMI, which also could function as a sensor for liqiuid level measurement. The experiment result presented that the liquid level sensitivity of this interferometer was 68.3 nm/mm with respect to the RI sensitivity of 1200.61 (nm/mm)/RIU.3. Several kinds of fiber strain sensors. (1) A 6 mm-long TCF was spliced to a standard FBG, i.e. TCF-FBG configuration, where the gap between TCF and grating was small. The TCF acted as a bridge for the coupling and recoupling of core-to-cladding modes. The intensity of the reflected first-order odd mode was sensitive to fiber bending, and the reflected wavelength was sensitive to surroundings temperature. This reflection probe has been demonstrated experimentally to measure displacement and temperature simultaneously by the information recovery of reflection intensity and wavelength of the first-order odd mode. But the length of the TCF had a great influence on the reflective spectrum so that the structure was not easy to be reproduced. (2) A FBG has been written into a TCF (where the core diameter is 3 pm) using a 248 nm UV laser, and the structure was spliced to a SMF. The core-mismatching was as a bridge for the coupling and recoupling of core-to-cladding modes. The reflection spectrum of this FBG included two wavelength bands corresponding to cladding modes and core mode, and presented clearly separated reflection wavelength. The recouling intensities of cladding modes have been employed for the temperature-independent displacement measurement with a sensitivity of 6.34 dB/mm. (3) An in-fiber Sagnac interference-based device has been formed by splicing a short piece of PM-PCF with a lead-in SMF. The interferometer was sensitive to fiber bending since the increasing fiber bending angle might change the birefringence of PM-PCF, resulting in the interference wavelength shift. The fiber bending range of the device was from -80° to 80°. Because of orientation-dependence property of the PM-PCF, the sensor can be used to recognize the orientation of fiber bending angle. However, the inclinometer presented a low sensitivity, especially presented a temperature cross-talk to bending angles as wavelength-referenced sensor, which needed to be compensated by another fiber device. (4) A hybrid interference structure, i.e. multimode interferometer casacaded with a FP interferometer, was formed a by a simple fiber splice between a 5 mm-long TCF (4.4 μm core diameter and 80 μm cladding diameter) and a hollow-core fiber (HCF,30 μm core diameter and 150 μm cladding diameter). As the TCF appeared bending, which cause the loss change of cladding modes, the interference intensity changed. In the experiment, the interferometer has shown a high sensitivity ranging from -4° to 4°. (5) A FBG has been written into a bow-tie PMF using a 248 nm UV laser. The reflective spectrum has shown two separated wavelengths (corresponding to two orthogonal polarizations of the fundamental mode). Based on the coupling between the polarization states, the device could be employed to realize temperature-independent vector pressure measurement by a 10 mm-long upstream PMF as the sensing head. In the experiment, the device presented a maximum pressure linear sensitivity of 23.16 dB/N. Those fiber sensors mentioned above mainly focused on the static strain measurement. (6) Femtosecond laser side-illumination technique was utilized to realize grating inscription over TCF cladding FBGs, (where the core and cladding diameters of TCF are the 4.4 μm and 120 μm. The "cladding FBG" reflection was sensitive to fiber bending. Meanwhile, the grating was only distributed on one side of the fiber core, which makes it as an orientation-recognized fiber-optic accelerometer.
|
PDF Full Text Request