The Research On Distributed Stress Sensing Based On Polarization Mode Coupling

Distributed fiber sensing is a technique which aims to get the information of space distribution varying with time and space by sensing fiber. Combining with polarization mode coupling, the distributed stress sensing system based on optical frequency domain reflectometer(OFDR) has a higher spatial resolution of stress measurement. In this thesis, we firstly conducted a deep research of the relationship between the strength of polarization maintaining fiber coupling and stress; Then according to OFDR system structure, we made an analysis of the system performance parameters and phase noise model; Furthermore, we made thorough investigation of the non-linearity effects of tunable laser source in the system and finally we built an experimental platform for OFDR distributed stress sensing system which is later validated by experimental data.The main contents are exhibited as follows:1. According to elastic light effects and polarization mode coupling theory, this paper presents a theoretic deduce to polarization maintaining fiber coupling strength variation with respect to the magnitudes and angles of stress, and works out the analytical solutions to the polarization coupling strength and stress. Besides, the corresponding analysis and simulation experimental verification is also done. The result proved that, the general rule of the strength of polarization coupling varying with the stress is as follows: When stress magnitude F is constant, polarization maintaining fiber coupling strength h corresponds to the stress angle θ with a period of pi/2 sinusoidal curve; when the stress angle θ is constant, and the stress is small, the polarization maintaining fiber coupling strength h is approximately in a linear relationship with the stress F.2. We designed an OFDR system which adopted Mach-Zehnder interferometer optical structure, researched the phase noise model and analyzed the impacts of tunable laser source non-linear effects. Next, we discussed three kinds of compensation algorithms about non-linearity effect of light: non-uniform Fourier transform, Deskew filter and cubic spline interpolation, which will be later validated, the results implied that all of three compensation methods can compensate for nonlinear effects tunable light sources. The method of cubic spline interpolation is proved to be more effective because of simple principle and higher interpolation precision.3. We built and tested the distributed stress sensing experimental system based on OFDR. Through the non-linear compensation to the test data, the order of 4 centimeter spatial resolution has been realized, Based on which we carried out single-point and multi-point tests respectively on the situation for different stress strength and different stress angles disturbance. The results show that the system has a effect to monitor the location information of stress point and the changes of the stress. The position error of measurement system is ? 0.6 m, the size of stress measurement error is ? 9.7%.Through the several aspects of study above, we achieve the distribute stress sensor system based on OFDR, which combines with polarization-maintaining optical fiber polarization mode coupling. This system can effectively perform distributed stress tests with high spatial resolution, which will be of great significance in the industry of distributed optical fiber sensing technology.