Theoretical And Experimental Research On The Key Technology Of BOTDA System

In this thesis the research condition of Brillouin distributed optical fiber sensing technology is summarized, Brillouin scattering and its characteristic parameters in an optical fiber are analyzed, and the corresponding relations between Brillouin frequency shift and the temperature/strain in a fiber are studied. For the cross sensitivity characteristics of Brillouin shift to temperature and strain in BOTDA system, three methods to realize simultaneous measurement of temperature and strain have been summarized. According to the mechanism of SBS, the steady coupled wave equations and transient coupled wave equations have been analyzed.The basic theory of polarized light is introduced. The polarization control system of EOM’s input light in BOTDA is designed based on theoretical analysis of the internal structure of electro-optic modulator(EOM) and its relationship between input and output as well as the polarization characteristics of EOM.The relationship between input and output state of polarization of fiber squzzed polarization controller has been theoretically deduced, and the mathematic model of the adaptive polarization control system has been established. The PID industrial control algorithm has been theoretically analyzed and was proposed for the first time to apply to the polarization control system,and PID’s parameters have been selected properly, and the PID control algorithm has been programmed, and the polarization control system software platform based on the LabVIEW has been built. The design and test of polarized beam splitter detector’s pre filter amplifier circuit have been completed, and the hardware platform based on date acquisition and control system has been built. Theoretical analysis and experimental demonstration has been conducted to verify the feasibility of the proposed method.The experimental results show that the control system designed in this method not only can real-time monitor the output optical power of EOM, but also can keep the output power maximum. Compared with the performance of EOM connected directly to a narrow band laser with a polarization maintaining fiber, the output average power of EOM under the polarization control based on PID is increased by3.08 d B and the standard deviation of PD output voltage is reduced from 0.1214 to1.2375×10-4. The correction time for the state of polarization is in the order of ms when the system is working in steady control state, and the requirement of BOTDA’spolarization control system is satisfied.The principle of pulse pre-pump(PPP) BOTDA distributed sensing system is introduced, and the transient coupled wave equations of the SBS process in the system have been theoretically resolved with differential gain method, and the sensing system has been designed.