Study On Fiber Sensing Based On Coded DBA-BOTDA And Hybrid BOCDA/BOTDA

In this paper,the principle and research progress of distributed sensors based on stimulated Brillouin scattering are presented,including Brillouin optical time domain analysis(BOTDA)and Brillouin optical correlation domain analysis(BOCDA).Considering the impact of gain fluctuation on the sensing performance of BOTDA system based on distributed Brillouin amplification(DBA),a long-distance BOTDA system based on cyclic coding is proposed.Long-distance phase spectrum measurement of DBA-BOTDA is realized.A hybrid BOTDA/BOCDA sensing method based on noise modulation is proposed.The detailed works are summarized as follows:(1)The influence of Brillouin pump depletion and nonlinear amplification on sensing performance of coded DBA-BOTDA is studied.The effects of non-cyclic and cyclic coding on the Brillouin frequency shift(BFS)is compared under the same conditions.The results show that: For acyclic coding,the measured BFS exhibits overshoot and undershoot at positions where the BFS changes are large,such as hot-point.This phenomenon is related to the distortion of Brillouin gain spectrum(BGS)caused by pump depletion.The BFS error increases with the increase of peak power or coding length.Pulse gain fluctuation caused by pump depletion can be significantly reduced by cyclic coding.In addition,a simple logarithmic linearization method is proposed to compensate the BFS error caused by nonlinear amplification in DBA-BOTDA.Using 509 bits cyclic coding to overcome the sensing effect of pump depletion,combined with a logarithmic linearization compensation for non-linear amplification,sensing distance of ~123.6 km and the spatial resolution of ~5 m are achieved.The BFS standard deviation is ~1.6 MHz.(2)The phase spectrum(BPS)measurement technology of long-distance DBABOTDA is studied.The sensing distance of ~99 km and spatial resolution of ~5 m are realized.By averaging the BGS and BPS,the standard deviation of BFS is reduced from ~3 MHz measured separately to ~2 MHz.Because BPS is insensitive to the probe light power and the linear response with BFS variation by dividing BPS and BGS,this work is meaningful for improving the stability of BOTDA system and realizing the long-distance dynamic measurement.(3)A high spatial resolution hybrid BOCDA/BOTDA technology based on noise phase modulation(PM)is proposed and experimentally demonstrated.This system can effectively break through trade off of sensing distance and spatial resolution in traditional BOCDA.The results show that: Sufficient phonon excitation can be obtained by optimizing the amplitude of phase modulation and the width of pump pulse,which is helpful for increasing the resolution points of correlation peaks in a single scan.The sensing distance of ~198 m and spatial resolution of ~23.8 cm are demonstrated.The number of the resolved correlation peaks is 20 for single scanning.