Distributed Fiber Sensing Technique Employing Vector Brillouin Optical Time Domain Analyzer

Distributed fiber sensing technology based on Brillouin scattering provides a powerful tool for continuous monitoring of strain and temperature over the sensing fiber with high precision,long sensing distance and high spatial resolution.Therefore it has a wide range of applications in the national economic development.Recently,vector Brillouin optical time domain analyzer(VBOTDA)is proposed as a new research field in Brillouin sensors.In this scheme,the vector signal analysis technology is introduced into the Brillouin sensors,which can simultaneously measure the Brillouin gain and phase-shift.Due to the superiority of high signal to noise ratio(SNR),it exhibits outstanding technical advantages in long-haul fiber distributed sensing and fiber distributed dynamic strain measurement,which keeps in step with the development trend of the Brillouin sensors and attracts a lot of attention.In this dissertation,the Brillouin gain and phase-shift profiles of the sensing fiber are studied in detail,and the operation principle of the VBOTDA is analyzed.Then an experimental setup of the VBOTDA is established to measure the Brillouin gain and phaseshift profiles of the sensing fiber.Based on this setup,Brillouin fiber long-haul distributed sensing and distributed dynamic strain measurement are realized.The main results are summarized as follows:1.The basic concepts of stimulated Brillouin scattering(SBS)in optical fibers are described.The principle of heterodyne detection in VBOTDA and its mechanism of temperature and strain sensing are studied.Based on the nonlinear wave equation and the acoustic wave equation,the three wave coupling equations that describe SBS in optical fibers are deduced in detail under the condition of slow-varying amplitude.Starting from the three wave coupling equations,an integral model of Brillouin gain and phase-shift in optical fibers is derived in consideration of the practical application of fiber Brillouin distributed sensors.This model can be used to study the Brillouin gain and phase shift characteristics of a pump with an arbitrary pulse,and can provide a theoretical guidance for improving the performance of fiber Brillouin sensors by designing the pump pulse waveform.2.A signal processing algorithm is designed for VBOTDA.The influence of the signal analysis bandwidth on the spatial resolution of the sensing system is numerically analyzed,obtaining an empircal formula of the two factors in inverse proportion.This formula is very useful to design the signal processing algorithm for VBOTDA.The experiment system of VBOTDA is analyzed in detail,in which the study focuses on the generation and performance of single side band modulation.The distributed sensing experiment is carried out successfully in a 1.42-km long sensing fiber.The spatial resolution of the sensing system is about 1 m,and the precisions of the temperature and strain are respectively about 0.5?C and 10 ??.Finally,the distributed temperature and strain sensing experiments are carried out.The temperature and strain coefficients of the Brillouin frequency shift are estimated to be about 1.05?C/MHz and 0.047 MHz/??,respectively.3.With the advantage of high SNR,the heterodyne-based fiber Brillouin long-haul sensor is realized and studied,employing the IQ demodulation and the envelope detection technique based on the generalized wavelet transform,respectively.Theoretical analysis and experimental results show that the main advantage of heterodyne detection is filtering out the Rayleigh scattering noise introduced by the pump.Compared with the tranditional Brillouin optical time domain analyzer,the heterodyne detection can improve the SNR by about 7 dB,when the pump and probe powers are equal.Based on the heterodyne detection,the distributed sensing experiment is carried out on a 24.4-km sensing fiber with a spatial resolution of about 2 m and the temperature and strain precisions of about 0.7?C and 14 ??.And the distributed temperature sensing is achieved with the spatial resolution of about 3 m and the temperature precision of about 2.5?C on a 48.7-km sensing fiber.The Brillouin fiber sensor with the envelope detection technique based on the generalized wavelet transform realizes a comparable performance of that with the IQ demodulation.Besides,the parameters of generalized harmonic wavelet transform can be adjusted in real time in order to meet the needs of the spatial resolution and sensing precision.This is very useful in the application of Brillouin fiber long-haul distributed sensor.4.The basic principle of the slope-assisted Brillouin fiber dynamic strain measurement is described,and the optimal work point and the strain sensitivity are analyzed.An experimental setup is built up for distributed Brillouin dynamic strain measurement.Dynamic strains with frequencies of 40,60,80,100 Hz are successfully measured with an effective sensing rate of 1 kHz and a spatial resolution of 1 m over a 46-m sensing fiber.The experiment results show that the dynamic ranges of these two schemes are measured to be about 47 MHz(940 ??),with the 1-MHz precision.Both theoretical and experimental results show that the optimal work point of the Brillouin dynamic strain sensing using the Brillouin gain is ?B±?3??B/6,rather than the commonly known ?B ±??B/2,where?Bcorresponds to the Brillouin frequency shift and ??B is the Brillouin linewidth.These results,especially the determination of the optimum work point for Brillouin gain,will provide guidelines for practical dynamic strain measurements and further improvement of the performance of the Brillouin fiber sensor for dynamic strain measurement.