Research On Strain Sensor Based On Polarization Maintaining Fiber

The technique of oil and gas logging has important significance in the resources exploitation and development. Fiber-optics strain sensors, due to themselves advantages, will present wide application prospect in well logging. Polarization maintaining fiber (PMF) shows the intrinsic high-birefringence due to its circular-asymmetric cross-section structure. The coupling between polarization modes within PMF is sensitive to anisotropic disturbance inside PMF and, thus, PMF has the potential as sensing device. In this dissertation, there will be three kinds of strain sensors presented based-on the PMF.Based on the interference of the polarized light, a dual-PMFs-based interferometer has been proposed and demonstrated experimentally to measure strain. In theory, the propagation characteristics of the interferometer are analyzed by Jones matrix. According to the theoretical equations, we simulated the output spectra of this interferometer and achieved four-types interference spectra:sinusoidal, flat-top and narrow-band superposition, and line-shaped spectra. Furthermore, one of PMFs was employed to perceive the press and the torsion. The experiment results showed that the interference wavelength linearly responds to the press and, meanwhile, the longer the sensing fiber presented smaller press sensitivity. In addition, The interference spectral wavelength presents a sinusoidal response to torsion over 0° to 360°.Based on the typical mechanism of Sagnac loop interference, a dual-PMFs-based interferometer has been proposed and demonstrated experimentally. A method of Jones matrix was employed to characterize the interference mechanism and sensing properties. In the experiment, one of the PMFs was used as the sensing head to measure the torsion and fiber bending. Over the range of 0°-360° of the torsion angle, because of the periodic rotation of the E-field of PMF, the intensity of the output spectrum periodic changed as a sinusoidal. At the bending angle range from 0° to 16°, the intensity presented the linearly response with the sensitivity of 0.07dB/°.Based on the theories of the coupling between the polarized modes and radiation loss of the PMF, a reflection fiber inclinometer was proposed using the polarization-maintaining fiber Bragg grating (PM-FBG). The reflect spectrum of the device includes two peaks, which were corresponding to two orthogonally polarized modes of PMF. The experimental results showed that, at the bending angle range of 0° to ±40°, the intensities of two LP modes present opposite change; at the angle range of ±40° to ±88°, the intensities of two LP modes sharply decreased simultaneously, and linearly responded to bending angles with the sensitivities of -0.6872dB/deg and-0.7497dB/deg over ±60° to ±88°. In addition, we repeated the above angle measurements some times, and the results indicated the proposed sensor has excellent repeatability. In addition, the environment temperature only induced the wavelength shift of the interferometer. Therefore, this proposed device was capable to realize the measurements of fiber bending and temperature simultaneously.