BOTDA Sensing Employing A Brillouin Fiber Laser Probe Source For Frequency Shift

Besides the advantages of general optical fiber sensing technology,distributed fiber sensing technology also can realize distributed sensing,and it has become one of the important sensing technologies for large infrastructure structure health monitoring.Brillouin Optical time-domain Analysis(BOTDA)is one of the widely used distributed optical fiber sensing technologies.In order to realize the measurement of temperature and strain,the traditional BOTDA sensing system often need to use expensive high-frequency microwave signal source and high extinction ratio of electro-optic modulator(EOM)to realize frequency shift of the pump or detecting light frequency.In this way,the frequency difference between the detection light and the pump light can be controlled and the frequency sweep measurement of Brillouin gain spectrum(BGS)can be achieved.However,this kind of system has high cost,so the issue how to reduce the cost of BOTDA system has become an important research direction.In order to reduce the cost of the system,a ring cavity Brillouin Erbium-doped fiber laser(RC-BEFL)is proposed to replace the expensive high-frequency microwave signal source and electro-optic modulator(EOM)schemefor frequency shift,and a BOTDA sensor system employing a RC-BEFL for frequency shift is demonstrated in this thesis.The performance of BEFL and the sensing performance of BOTDA system based on BEFL are experimentally studied in detail.The main research contents of this thesis are as follows:(1)The research background and significance,the development status of Brillouin optical fiber laser and BOTDA sensor technology are summarized,and the related theories of Brillouin scattering in optical fiber and RC-BEFL,as well as the sensing mechanism and performance indexes of BOTDA are introduced in detail.(2)A Ring Cavity Brillouin Erbium-doped fiber laser(RC-BEFL)based on pumped pre-amplification was proposed and experimentally demonstrated.The line width,threshold,stability and tunability of the RC-BEFL are investigated experimentally and the results show that the line width of the laser can reach450 kHz.The wavelength stability of the laser is good,with the maximum wavelength drift of 0.006 nm in 160 mins,but the maximum peak power fluctuation is 0.8dbm in the measurement time.The laser can achieve a frequency tuning accuracy of 0.998MHz/? by using temperature control,which can meet the frequency shift needs of BOTDA system.(3)A BOTDA sensor system based on RC-BEFL frequency shift is proposed and experimentally studied.In this system,a RC-BEFL is proposed to replace the expensive high-frequency microwave signal source and electro-optic modulator(EOM)scheme for frequency shift,which can effectively reduce the cost of the BOTDA system.The temperature and strain sensing characteristics of the RC-BEFL–based BOTDA have been studied in detail.The spatial resolution of 9m is realized for the sensing fiber of about 5km.The temperature and strain measurement error can reach 0.64?and 38.30?? in our experiments.The performance of RC-BEFL-based BOTDA can be further improved by optimizing the sensing system.