Research On BOTDA Multi-parameter Sensing Method Based On Polarization State Analysis

Brillouin optical time domain analysis(BOTDA)system has the advantages of distributed,long distance,multi-parameter sensing,and is widely used for monitoring temperature and strain information along the sensing fiber.On the basis of temperature measurement,the monitoring of vibration signals along the optical fiber based on stimulated Brillouin scattering has become one of the research hotspots.Researchers have proposed many schemes,such as slope-assisted BOTDA,frequency-agility BOTDA and BOTDA based on chirped pump / probe to monitor the vibration signal.But these methods to achieve vibration signal measurement are faced with the limitation of sensing distance.Therefore,this paper explores the method of adding a polarization detection module to the BOTDA system to realize the measurement of the vibration signal along the long-distance single-mode fiber,and then realizes the multi-parameters measurement of temperature,static strain and vibration measured by BOTDA.In order to monitor the vibration signal along the fiber,this paper analyzes the polarization characteristics of the single-mode fiber in the BOTDA system,and the polarization vector model considering the birefringence effect and stimulated Brillouin scattering effect is established.The relationship between the Stokes light and the vibration signal along the fiber is obtained through simulation analysis.By analyzing the response of the Brillouin gain spectrum distributed along the fiber distance to the vibration signal,it is found that the Brillouin gain,frequency shift and full width at half maximum change with the polarization state caused by vibration,and the Brillouin gain spectrum parameters are different when different vibration signals are applied.In this paper,a BOTDA sensing experimental system combined with polarization state detection is built,and the sensing experimental analysis of the vibration signal along the optical fiber is carried out.By analyzing the relationship between the Stokes light and the vibration signal,it is verified that the added polarization detection module can identify the occurrence of vibration and the waveform,frequency and period of the vibration signal,and realize the monitoring of the vibration signal along the optical fiber.Based on the BOTDA sensing system constructed in this paper,the temperature sensing experiment along the fiber is carried out,the Brillouin gain spectrum distributed along the fiber length is obtained,the Brillouin frequency shift parameter is extracted,and the temperature information is further demodulated.Aiming at the problem that noise affects the accuracy of Brillouin frequency shift extraction in this measurement,the Brillouin gain spectrum is reconstructed by compressive sensing algorithm.The compressive sensing algorithm can reconstruct the Brillouin gain spectrum using the measurement data at large frequency sweep intervals,which improves the measurement accuracy through noise reduction and ensures real-time performance.The analysis results show that the system signal-to-noise ratio is improved form 3.966 d B to 10.34 d B.Finally,Lorentzian curve fitting was performed on the noise reduction data,and the temperature coefficient was obtained as 1.1MHz/℃.