Research On Distributed Optical Fiber Vibration Sensor Based On Rayleigh Backscattering Light

In the field of inshore defence system, the large-scale fiber optic hydrophone arrayand related long-distance transmission fiber cable are the important parts of land-basedunderwater acoustic defence system. But the underwater fiber optic hydrophone array isoften destroyed by submarine current, halobios, the activities of human beings and so on.The long-distance transmission fiber cable is often impacted by construction on the landor digging. When the underwater acoustic defence system is destroyed, it usually costsmuch to repair it. Then developing a kind of sensing system of high resulotion, longsensing distance and low cost becomes a matter of vital importance.Distributed optical fiber sensor has being researched as a kind of vital securityalarm system based on its various virtue, such as low cost and long-distance detection.However, Traditional distributed optical fiber sensors can’t get the waveform of thedisturbance signal. In this thesis, we study a kind of novel distributed optical fibersensor, which is based on optical time domain reflection of coherent Rayleighbackscattering light, which provides a method for long-distance detection of the eventsalong the fiber cable. And the distributed fiber sensor demonstrated in this thesis canattain the waveform of vibration signal effectively. Besides, the system use theremaining communication fiber cable as the sensing fiber, which cuts the cost greatly.Combined with the research on optical fiber hydrophone in our group, especiallythe opcial heterodyne and time division multiplexing technology in large-scale fiberoptic hydrophone array, we investigate the key technology of distributed optical fibersensor based on the interference of Rayleigh backscattering. We develop the study inthe following fields, including the theory of Rayleigh scattering in the fiber, the theoryof OTDR for location, the theory of phase modulation by outer vibration and the designand experiment research of the sensor system. The thesis includes the followingcontents in detail:(1) The situation of the current research and application is analyzed. The feasibilityand value in the future of the novel distributed fiber sensor we demonstrate inthe thesis is illuminated based on the sum-up of the virtue and defects oftraditional distributed fiber sensors;(2) The theory of Rayleigh scattering and OTDR is illuminated, and the theoryabout phase modulation by outer vibration is demonstrated;(3) The structure of the fiber sensor system is designed. The theory for sensing andlocation is analyzed theoretically. The dynamic range and the influence ofoptical noise on heterodyne demodulation is analyzed in detail. We find thatthe upper limit of dynamic range of heterodyne demodulation based onarctangent algorithm is higher than PGC algorithm; moreover, arctangent algorithm can eliminate the effect of intensity perturbation efficiently;(4) The system is constructed and the sensing experiment is taken on. Before thedemodulation and locating experiment, we analyze the maximum pulseintensity influenced by the stimulated Brillouin scattering. Experiment basedon sensing fibers of different length is developed. The vibration signal of500Hz and800Hz of different amplitudes is detected at the location of225mfrom the start point of the sensing fiber in an experiment when the fiber lengthis1.8km; and the waveform and frequency of the vibration signal isdemodulated effectively; and the spatial resolution is25m.