Theoretical And Experimental Research On Methods For Performance Improvement Of BOTDR Sensing System

Power cable and equipment are the important link to ensure power transmission,information exchange and electromagnetic energy conversion.Obtaining the real-time running condition of power cable and equipment and ensuring the safe and stable operation are of great significance for safeguarding economic and social development.Contrapose the problems of long laying distance and vulnerable to environment factors,Brillouin optical time domain reflectometry(BOTDR)with the characteristics of singleended measurement and simple construction can be used to monitor the operation conditions of power cable and equipment in real time.The characteristics of spontaneous Brillouin scattering and stimulated Brillouin scattering(SBS)were analyzed in the dissertation,and a method to measure fiber SBS threshold by employing Brillouin scattering spectrum width was proposed.A method for temperature measurement based on BOTDR with time-domain and spectral-domain reshaping was proposed,and the optimal design of step pulse was made by analyzing Brillouin scattering spectrum and power simultaneously.The effect of laser frequency instability on Brillouin signals was studied and a BOTDR temperature measurement method using self-heterodyne detection of Rayleigh and Brillouin scattering was presented.The effective methods for temperature measurement error reduction were also proposed.All the above studies can provide theoretical and technical supports for temperature measurement of power cables with high accuracy.The main research achievements are presented as follows:1.SBS coupled wave equations were solved by establishing mathematical model for describing SBS effect in optical fiber and the relationship of Brillouin scattering spectrum width with fiber input power was theoretically analyzed,a new method for fiber SBS threshold measurement by employing Brillouin spectrum width was proposed.The noise components and signal to noise ratio(SNR)of Brillouin spectrum measurement system based on heterodyne detection were studied,and a method using the variation characteristics of system SNR to determine the optimal fiber input and local reference powers was presented.2.The method to solve the contradiction between spatial resolution and measurement accuracy in BOTDR system by employing step pulse modulation was proposed.SBS models under steady-state were solved by transient analysis in the step pulse modulation BOTDR,and the reshaping of Brillouin spectrum and the amplification of sensing signal power were achieved through SBS interaction between pre-pump pulse and sensing pulse.The optimal design of step pulse was made by considering system performance,and the sensing system performance was compared with those in different pulse modulation structures.A temperature measurement system based on BOTDR with step pulse modulation was designed and constructed,and the experiment results showed that the contradiction between spatial resolution and measurement accuracy in BOTDR system was solved effectively by employing step pulse modulation.3.To solve the problem that performance improvement was limited by fiber SBS threshold in traditional BOTDR system with narrow-band laser,a BOTDR sensing system based on external modulation of multi-longitudinal mode Fabry-Perot laser and frequency-shifted local optical heterodyne detection was proposed.The dependence of peak power and bandwidth of superposed Brillouin spectrum,the system SNR and measurement accuracy of Brillouin frequency shift(BFS)on longitudinal mode number,and the relationship of longitudinal mode number with optimal measurement accuracy for different pulse widths and longitudinal mode intervals were theoretically studied.And the corresponding fitting formulas were obtained by calculation,which provided a theoretical basis for the modeling analysis and optimal design of BOTDR system using multi-wavelength light source.Considering the transmitting characteristics of superposed Brillouin spectrum in optical fiber and the requirement of feature extraction in high accuracy,a method to extract features of superposed Brillouin scattering spectrum based on Pseudo-Voigt basis function and Levenberg-Marquard optimized algorithm was presented,which offered an effective method for extracting useful information correctly in multi-wavelength Brillouin sensing system.4.The effect of laser frequency instability on heterodyne detection Brillouin scattering signals was analyzed,and the modeling analysis of Brillouin signals with laser frequency instability was conducted.To eliminate detrimental effect of laser frequency instability,a method employing self-heterodyne detection of Rayleigh and Brillouin scattering was presented.The relationships of self-heterodyne detection BFS and signal power with temperature and strain were studied,and a temperature measurement method based on BOTDR with broad-band laser and self-heterodyne detection was proposed,decreasing the influence of coherent Rayleigh noise on system performance effectively.The temperature measurement methods based on self-heterodyne detection BOTDR with multi-wavelength probe light and Golay pulse codes were respectively presented,reducing the fluctuations of BFS and Brillouin power generated by coherent Rayleigh noise effectively,which provided a theoretical and experimental basis for simultaneous measurement of temperature and strain in BOTDR sensing system based on selfheterodyne detection of Rayleigh and Brillouin scattering.