Research On The Measurement Method Of Optical Fiber Brillouin Scattering Characteristics And Its Application

The distributed sensing technology based on Brillouin scattering in optical fiber has the ability to extract large range information of the measurement field which is particularly suitable for large power engineering structure,and especially for the online temperature and strain monitoring of power cables and electrical equipment,etc.It may be applied extensively in the near future.On the basis of a thorough review of Brillouin scattering characteristics about the optical fiber,the acousto-optic coupling model of optical fiber were built with the finite element method,and the corresponding Brillouin gain spectrums were numerically calculated.Focus on the characteristics of the multimode optical fiber Brillouin scattering,and the parameters extraction methods for the Brillouin scattering spectrum with single peak and multi peak.Finally,the online monitoring system of submarine cable and the fault location method were realized based on distributed optical fiber Brillouin scattering sensing technology.The main novelties are presented as follows:(1)The mathematical modeling method of optical fiber Brillouin scattering spectrum with the finite element was proposed,and the acousto-optic coupling model of single-mode fiber and multimode fiber were built,and the corresponding Brillouin gain spectrums were numerically calculated.The system of Brillouin scattering spectrum measurement was designed and built.The Brillouin scattering spectrum of single-mode fiber,50/125 ?m and 62.5/125 ?m graded index multimode fiber were measured.The experiment results show that the Brillouin frequency shift of the multimode optical fiber is lower than that of the single-mode fiber,but the linewidth of Brillouin scattering spectrum of the former is obviously larger than that of the latter.The experimental results validate the built simulation model.(2)The method for measuring the stimulated Brillouin scattering threshold of multimode fiber was proposed.The system of stimulated Brillouin scattering threshold measurement was designed and constructed.The stimulated Brillouin scattering threshold of single-mode fiber and multimode fiber was studied theoretically and experimentally.The experimental results reveal that the threshold of multimode fiber is much larger than that of single-mode fiber under the same fiber length.Compared with step index fiber,the gradient refractive index optical fiber is prone to generating stimulated Brillouin scattering.(3)The method for simultaneous temperature and strain measurement based on Brillouin scattering in multimode fiber was proposed.The system about temperature and strain distributed sensing system of multimode optical fiber was designed.The distributed measurement of multimode optical fiber is realized by using commercial Brillouin optical time domain reflectometer.The obtained temperature and strain coefficient of Brillouin frequency shift are 1.026+0.04 MHz/°C and 0.05288+0.001 MHz/??.The strain coefficient of Brillouin frequency shift in multimode fiber is larger than that of single-mode fiber.Therefore,the former is more suitable for strain sensing measurement.(4)To further improve the real-time performance of distributed Brillouin sensing system,the linear optimization model for parameters extraction of Brillouin scattering spectrum was proposed,and compared with the nonlinear model,the scope of application of the two models was investigated.For Brillouin scattering spectrum with single peak and multi peak,the fast and high precision extraction methods for parameters extraction were proposed.(5)The on-line monitoring and fault location method based on distributed optical fiber Brillouin scattering sensing technology was proposed.The on-line monitoring system of submarine cable was constructed.The condition of submarine cable was monitored and evaluation by the system.By combining submarine cable route information,the topography,geological structure and construction details,a method for accurately positioning submarine cable fault was proposed and the accuracy is 5 m.This study lays the foundation for fully exploring the sensing ability of communication fiber in the world,and provides valuable theoretical and technical reference for the development and construction of the full coverage optical fiber sensing network.