Research On Distributed Fiber Brillouin Sensing Technology With Large Dynamic Strain Range

Distributed fiber sensing technology based on the Brillouin scattering can achieve distributed sensing of temperature and strain due to the Brillouin frequency shift(BFS)depends on the temperature and strain,which has great application potential in the field of structural health monitoring of large-scale infrastructure.However,the sampling process of the traditional Brillouin optical time-domain reflectometer(BOTDR)and Brillouin optical time-domain analyzer(BOTDA)includes the frequency sweeping for the Brillouin gain spectrum(BGS),which is very time-consuming,induced that the traditional BOTDA/R system can only measure the static parameter.For achieving the dynamic strain measurement,the BOTDR/A system based on slope-assisted method(SA-BOTDA,or SA-BOTDR)has been proposed,which utilizes the approximate linear region of the BGS to make the received signal power change linearly with the change of the BFS,which can realize high-speed dynamic strain sensing.Nevertheless,the strain measurement range of SA method depends on the bandwidth of the BGS,which is about 40 MHz in the single-mode fiber(SMF)and the corresponding strain range is less than 1000 με.To solve this problem,this paper has extended the strain measurement range of SA method from the shape of the BGS and measurement method,and has successfully expanded the strain measurement range of the SA method by more than three times.The main works of this paper are summarized as follows:(1)According to the mathematical model of the BGS in multi-mode fiber,the BGS in step-index(SI-)and graded-index(GI-)multi-mode fiber(MMF)is numerically simulated.The frequency characteristics of the optical and acoustic modes and the coupling characteristics of optic-acoustic-optic in two kinds of fibers are analyzed.The mode excitation in the fibers is studied by numerical simulation of offset splicing.It is found that the optical and acoustic modes in the GI-MMF form the mode group,and the effective refractive index(ERI)or frequencies between the modes within the group are almost equal,while the difference between the groups is great which leads to the obvious change in the shape of the superposed BGS.Therefore,the shape of the BGS can be regulated by offset splicing.(2)Based on the simulation results that the shape of BGS in GI-MMF can be adjusted by offset splicing between SMF and GI-MMF.The 1 km GI-MMF is used as the sensing fiber,and the BGS is broadened by offset splicing between the SMF and the GI-MMF,resulting in the slope range expand.The SA-BOTDR system is set up for measuring large dynamic strain.The maximum strain of 3000 με is measured which is three times than traditional SA-BOTDR,and the measured frequency errors are less than 1.5 Hz.The advantages of this system are simple and relatively low cost.(3)A novel SA measurement strategy based on the frequency equalizer(FEQ)is proposed for expanding the dynamic strain measurement range of the SA-BOTDR system.The complete BGS information in the bandwidth of the FEQ is taken as the detection signal,and the frequency gain is modulated linearly,so that the strain measuring range is not dependent on the slope range of the BGS,but on the modulation range of the FEQ.Based on this strategy,the dynamic strain with the maximum strain of 3108 με and the frequency error of less than 1 Hz is measured using SA-BOTDR system based on 1.94 km SMF,and the spatial resolution is 5 m.This measurement strategy make the strain measurement range of the SA method get rid of the limit from the bandwidth of the BGS,and the system is simple and relatively low cost.