Broadband-Source-Based Composite Fiber-Optic Sensors

Smart structure has become a focus research area and has magnificent theoretical and practical meaning because more and more old and new buildings and structures need to be monitored in order to avoid the huge loss caused by collapse.Fiber-optic sensors as part of smart structure are fitful to be applied to sensing strain and temperature and other parameter of structures. They can be embedded in structures directly because of the small size and flexibility shape. Fiber-optic sensors are also resistive to electromagnetism, so they can be used in some special application where electronic sensor can not. While, one kind of fiber-optic sensors has limited sensing abilities, it needs to combine different types of fiber-optic sensors in order to satisfy complex sensing requirements. The thesis focuses on developing composite fiber-optic sensing structures by combining the fiber-optic sensors that can be interrogated by broadband source.First of all, two type of broadband-interrogation-based fiber-optic sensors, fiber Bragg grating (FBG) and broadband-source-based fiber-optic interferometry (BSFOI), are reviewed and discussed, including their basic theories, couple mode theory and low coherence theory, and their demodulation schemes.Secondly, two methods of multiplexing FBG and long-gauge BSFOI are proposed. One is to combining FBG and long-gauge BSFOI to construct a novel sensor unit. The other one is to multiplexing FBG and long-gauge BSFOI parallel. Theoretic analysis of this two types topologies is presented, and the experimental results are also given.Thirdly, A composite-cavity-based fiber-optic sensor (CCFOS) structure that can also be interrogated by broadband source is proposed to be worked as a strain sensor and a corresponding double-peak-based FFT demodulation scheme is given. There are two application schemes for CCFO. One of them is to use a CCFO as a quasi-point sensor as Fizeau sensors. Compared with traditional Fizeau sensors, CCFO gains the higher multiplexing ability because the limitation of cavity length in Fizeau sensors is eliminated. At the same time, one attractive characteristic of Fizeau sensors as strain sensors, the minimum cross sensitivity to temperature, is remained. The feasibility of the other scheme, as a strain sensor that can realise long-and-short gauge strain monitoring simultaneous, is discussed. Combining FBG and CCFO is also analysed at last.At the end of thesis, some designed experiments are given to show the feasibility of our designed composite fiber-optic sensor. And a demodulation system is developed to realise monitoring Como Railway Bridge and some concrete samples based on FBG. The experimental results coming from the applications are given to show the reliableness of system. The resolution and accuracy of system are 1μεand 5με, respectively.