Research On The Key Technologies Of Distributed Optical Fiber Sensing System Based On Stimulated Brillouin Scattering

For decades,distributed strain and temperature sensing technology based on Brillouin fiber scattering has attracted widespread attention.So far,a variety of methods have been adopted to obtain information about the Brillouin gain spectrum(BGS)distribution,from which the magnitude and location of the strain or temperature change can be derived.The performance of Brillouin Optical Time Domain Analysis(BOTDA)even reaches sub-centimeter spatial resolution.However,the practical BOTDA distributed strain and temperature sensing technology still has the disadvantages of long measurement time,simultaneous measurement of temperature and strain interference with each other,and cumbersome and complicated preparations for field testing.The paper compares the performance of various Brillouin sensors.The power threshold and main performance parameters of the BOTDA system in the Brillouin scattering process are analyzed.According to the stimulated Brillouin scattering theory,the Brillouin scattering spectrum and the three-wave coupling model are simulated.Designed for the BOTDA system,analyzed in detail the parameter selection and model selection of key experimental equipment such as IQ modulator,SOA pulse modulator,pulsed optical fiber amplifier,microwave signal source,and completed the preliminary parameter design of the system.Based on the above analysis,a BOTDA distributed sensing system with a test fiber length of 22.5km was built.For the noise caused by the continuous leakage of the pump pulse light,the polarization state of the optical signal and the non-local effect of the system,the reasons for the noise and the impact on the system are theoretically analyzed,and the improvement of the pulse is proposed.Optical extinction ratio,the polarization controller improves the polarization state and improves the suppression ratio of the single-side band signal of the probe light to reduce the noise experimental program.The pump pulse light with an extinction ratio of 40dB and the carrier rejection ratio of 30dB are obtained,and the harmonic rejection ratio is 24dB.The frequency shift probe light has a high rejection ratio.For the contradiction between the sampling rate of the data collector,the detection time and the signal-to-noise ratio,the spectrum and time-domain analysis of the time-domain signal collected by the photo detector is carried out,and the use of MATLAB for superimposed average denoising is proposed for the characteristics of the time-domain signal noise.The experimental results can well remove time-domain noise.The data processing flow of the BOTDA system is sorted out.The paper studies the relationship between Brillouin frequency shift and temperature/strain.The temperature measurement of optical fiber Brillouin intrinsic gain spectrum of BOTDA system is realized,and the linear relationship between the sensing distance of 22.5km and the Brillouin frequency shift and the obtained temperature change is verified.The location method of the temperature change area is verified by simulation,and the simulation results are consistent with the theoretical analysis,and a good simulation effect has been achieved.