Research On Distributed Fiber-Optic White Light Interferometric Deformation Measuring Technique

With the development of our country, government invest large amount of money in building new infrastructures and renovations of old structure. As structures have become more and more complex and provided versatile functions, there has been an ever-demanding need to monitor them. The identification and monitoring of buildings, structures, and bridges etc. are meaningful to evaluate the lifetime and quality of these structures, especially for the large-scale or pivotal ones. The engineering values and importance have been realized in recent years and have become research topics.The real-time measuring of displacement, strain, crack, and sedimentation for structures is the precondition for these research activities and engineering work. Fiber optic white light interferometry (FOWLI) developed in the early of 1980s, which can be embedded directly into structures without increasing the overall size and is resistive to electromagnetism, is capable of measuring strain, temperature, and other key parameters. In this thesis the development and current research of FOWLI are reviewed. The main issues for practical implementation of FOWLI have been investigated in detail.The contents of the thesis are organized as follows: Firstly, the principle of FOWLI is discussed. Starting from the partial coherent light interference the white light interference theory is derived, which is then applied to measuring principle of strain and temperature. However, the FOWLI is sensitive to both the strain and temperature, which is detrimental to accurate strain measurement. The cross coupling of strain and temperature are discussed and practical temperature compensation schemes are also proposed.Secondly, in the strain measurement, it's inevitable to attach fiber to the structure no matter the fiber is pasted to the surface or embedded in the structure. The elastic mechanic between the fiber and the matrix with buffer materials between them is modeled and is confirmed by both the theoretical simulations of strain transfer coefficient of fiber sensors with different length and experiments. In addition, the apparent strain induced by temperature is discussed based on the modeling.Thirdly, for the practical implementation of FOWLI, the compatibility between fiber sensors and structures should be considered. The double-cladding and tri-cladding fiber sensors are designed and tested. The structures, packages, andtechniques related to the fiber sensors are discussed.Fourthly, in order to real-time monitor and measure the strain for the structures, a prototype version of optical fiber strain measurement instrument (OFSMI) is developed. OFSMI is composed of a host computer for data processing and visualization and measuring part which can interrogate the fiber sensors. OFSMIhas a resolution of ±2μm and accuracy of ±10μm. It can measure strainsignals no higher than 5Hz and can be multiplexed with 5 fiber sensors. Fifthly, at the end of the thesis the structure of distributed dual-loop optical fiber sensors and its related optical path matching, output signal and noise characteristics, and fiber sensor multiplexing capability are discussed. The dual-loop sensor structure is proven to be useful in increasing multiplexed sensor numbers and preventing from sensor damages. It has been shown that the optimized dual-loop structure with bi-directional interrogation is a promising method for practical measurement.Lastly, this paper has researched soil deformation measurement based on the techniques of fiber optical sensor and white-light interference. The calibration experiment of fiber optic soil sensor is carried out. Model of soil dam and land slide are designed and soil deformation has been researched with effect of outside loading.