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Study On Long Distance Brillouin Distributed Optical Fiber Sensor

Posted on:2006-07-20Degree:DoctorType:Dissertation
Country:ChinaCandidate:D R LiuFull Text:PDF
GTID:1118360182990583Subject:Electronic Science and Technology
Abstract/Summary:PDF Full Text Request
A lot of interest has been focused on Brillouin distributed optical fiber sensor (B-DOFS) for the recent several years because of its many advantages and potential applications. In this thesis, based on a review of the distributed optic fiber sensors, the characterization of Brillouin scattering in the 25km single-mode fiber pumped by the pulses with the different width is investigated by theoretical analysis and numerical simulation;The relationships of modulation and demodulation between Brillouin spectrum and stress and temperature of the sensing fiber are founded;The measurement principles, equipment configurations, and experimental tests of the distributed stress and temperature along the 25km sensing fiber have been studied. In addition, the effect of the stress gradient on the accuracy of Brillouin spectrum, and the polarization-induced fading elimination technique for improving the performance of B-DOFS have been studied in detail. The main contributions of this dissertation can be briefly summarized as follows:(1) The Brillouin spectrum and its threshold power of the 25km sensing fiber pumped with the different width pulse are analyzed by the effective numerical simulation model and method. It provides the theoretical basis for the parameter optimization of equipment design.(2) In order to reduce the effect of polarization mismatch in the coherent detection system for B-DOFS, based on the feature of Brillouin scattering light and the principles of the polarization spreading and the polarization diversity, the time domain polarization diversity (TDPD) technique is introduced for the first time. The theoretical analysis shows that more than half of signal power is heterodyned with the local light no matter how their polarizations vary, and the received signals are insensitive to the random variation of the polarization of Brillouin backscattering light. The experimental comparison between the system with and without TDPD shows that the fluctuation of the received signal light power with TDPD is 4.2 times less than that without TDPD.(3) Based on the modulation relationship between Brillouin scattering spectrum and stress and temperature, It is presented that simultaneous temperature and stress along the sensing fiber are demodulated from Brillouin spectrum by the weighted least square method, and our special method todetermine the weighted number.(4) The experimental tests of the distributed stress, stress gradient and temperature along 25km fiber by B-DOFS are investigated. The experimental results demonstrate that the stress and temperature vary linearly with Brillouin frequency shift. Its spatial resolution and resolution of stress and temperature separately are 10m,100u.e,and 5°C. In addition, The analytic relationship between Brillouin backscattering spectrum and stress gradient is set up to investigate the influence of the stress gradient within the resolution on the measurement accuracy of Brillouin frequency shift. The numerical results show that the increasing stress gradient results in the linear increasing of the Brillouin frequency shift, the nonlinear decreasing of the peak and the flatting of Brillouin spectrum. With the spatial resolution of 10m and the stress gradient of lOOue/m, the measurement error of Brillouin frequency shift is 2.04 times larger than that without the stress gradient. The Brillouin spectra and stress along the test fiber are experimentally measured at different stress gradients with the sensing fiber fixed on a cantilever beam. Experimental results are in agreement with the theoretical prediction.
Keywords/Search Tags:Brillouin scattering, Distributed measurement, optical fiber sensor, polarization, stress, temperature
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