Investigation Of Microwave Modulation Based Quasi-distributed Fiber Sensing And The Interrogation Techniques

Optical fiber sensing technology has attracted a great deal of attention in academia and industry in the past decades, due to its significant advantages such as compactness, high-sensitivity and electromagnetic interference immunity. There are about hundreds of environmental parameters that could be detected using fiber sensing technology, which has made them became highly important in a wide range of industrial applications. Using fiber sensing technology for distributed sensing is particularly promising for many practical applications requiring a large scale real-time monitoring, such as structural health monitoring. However, the high-cost of the implementations, relatively low measuring speed and low SNR are disadvantages that still limit its widespread applications in these applications.In this dissertation, a type of distributed fiber sensing networks using microwave modulation are proposed and studied. Also, several novel interrogation schems for FBG sensors are designed and investigated. The main achievements of this dissertation are as the following:(1) A remote strain monitoring system using microwave-domain interference is proposed. In this novel system, the fiber plays not only the transmission link, but also the dispersion media. It exhibits important advantages of low-cost, high reliability and simplicity.(2) A novel FBG interrogation scheme using crossed optical Gaussian filters is proposed. The wavelength shifts of the sensors are transferred to intensity changes, leading to an ultra-high speed of the detection and thus a potential ability for real-time monitoring. Another feature for this proposal is its significant flexibility, that is the sensitivity and the dynamic range can be adjusted by simply changing the offset between the Gaussian filters. Besides, different from the conventional linier edge filtering methods, the proposed interrogation uses filters with broad Gaussian spectra, which gives it a potential of a much bigger dynamic range compared to conventional approaches.(3) An ultra-short FBG(USFBG) based distributed sensing network is proposed and demonstrated, where the crossed Gaussian filtering concept is firstly exploited as a powerful interrogation for distributed sensors. The remarkable features of self-referencing ability, tunable sensitivity & dynamic range, and high SNR are experimental validated. In addition, a comprehensive numeral studies of the sensing networks parameters, showing that such an USFBG based distributed sensing network possesses significant advantages compared to conventional ones in terms of power budget, crosstalk, capacity, etc. It could be expected that this new type of sensing networks could provide significant implications to their future applications for large-scale monitoring.