Research On Distributed Dynamic Strain Sensing Technology Based On Stimulated Brillouin Scattering

Distributed optical fiber sensors(DOFS)have become a research hotspot in the last decades due to the fiber's properties of small in size,low in weight,immune to electromagnetic interference and its ability of distributed sensing over tens to hundreds kilometers.Among them,Brillouin optical time domain analysis(BOTDA),which enables high spatial resolution distributed strain/temperature measurements over long distances,could play an important role in the health monitoring of oil pipeline and large-scale buildings.Generally,BOTDA reconstructs the Brillouin gain spectrum(BGS)of fiber by sweeping the frequency of probe to realize distributed strain/temperature measurement.However,the frequency sweeping process is time-consuming and larger number of averaging is required to improve the signal-to-noise ratio(SNR),which makes the sensing speed of BOTDA is fairly slow,one completed measurement takes from seconds to a few minutes.To overcome the problems mentioned above,various techniques are proposed in this paper to further improve the frequency response range,sensing length,spatial resolution and SNR of BOTDA system.The main contents are as follow:(1)The research progresses of DOFS based on stimulated Brillouin scattering(SBS)are reviewed,the research status of dynamic BOTDA is emphatically introduced and the technical challenges of dynamic BOTDA are discussed.The formation process and physical model of SBS are presented,the mechanism of temperature/strain sensing of SBS is studied.Then the principles of fast BOTDA(F-BOTDA)and slope-assisted BOTDA(SA-BOTDA)are expounded,as well as their improvement methods.(2)A high spatial resolution F-BOTDA based on pump pulse frequency modulation and differential pulse-width pair(DPP)is proposed.Firstly,the principle of pump pulse frequency modulation is introduced,the noise in double sideband pump pulse frequency modulation is analyzed,which provides guidance for the optimization of the modulation parameters.Subsequently,the F-BOTDA system is built.Dynamic sensing with a maximum sampling rate of 2.6 kHz is realized over 230 m polarization-maintaining fiber,the spatial resolution and dynamic range are 50 cm and 210 MHz,respectively.(3)A long range F-BOTDA based on cyclic coding and polarization diversity technique is proposed.Starting from the linear coding theory,the decoding theory of Simplex code and its coding gain are described.The encoding and decoding process of cyclic Simplex code is deeply studied,which provides a theoretical basis for the application of cyclic Simplex code in F-BOTDA.The polarization fading phenomenon of BOTDA in single-mode fiber is analyzed.Combined with the pump pulse frequency modulation in chapter three,polarization diversity technology based on double sideband modulation is proposed to eliminate the polarization fading.Then the F-BOTDA system based on cyclic coding and polarization diversity technology is built.By employing 71-bit cyclic Simplex code and polarization diversity technology,the sensing range of F-BOTDA is extended to 2 km for the first time and the trace averaging is avioded.The spatial resolution and sampling rate of the system is 1.5 m and 440 Hz,respectively.(4)A vector multi-slope assisted BOTDA based on Brillouin phase gain ratio(BPGR)is proposed.The working principle of vector BOTDA is introduced,as well as in-phase/quadrature(IQ)demodulation algorithm which is commonly used for signal demodulation in vector BOTDA.The effects of pump pulse width and SNR on the linear range of BPGR are analyzed by using theoretical simulation.On this basis,a vector multi-slope assisted BOTDA system based on BPGR is built.By simultaneously detecting the BPGR of each frequency component of the probe light,the Brillouin frequency shift of the optical fiber can be demodulated,thus the dynamic range of the SA-BOTDA is improved without any expense of sensing speed.Dynamic sensing with sampling rate of 1.5 kHz and spatial resolution of 2.5 m is achieved over 1.9 km single-mode fiber by adopting a three-tone probe light with 60 MHz interval and the dynamic range is extended to 180 MHz.In this paper,pump pulse frequency modulation is used to reduce the cost of F-BOTDA,cyclic coding and polarization diversity technology is used to extended the sensing distance of F-BOTDA and vector multi-slope assisted technology is used to improve the dynamic range of SA-BOTDA.Finally,the great improvement of the performance of dynamic BOTDA is achieved.