Research On The Influence Of Time-frequency Modulation Harmonics In Phase Sensitive OTDR Measurement

Distributed acoustic sensing based on Rayleigh scattering has become the most valuable detection technology with its advantages of high sensitivity,long sensing distance and wide response band,and has been gradually applied in perimeter security,structural health detection,oil and gas resources exploration.In order to break through the bottleneck of the measurement parameters in distributed acoustic sensing system and improve the comprehensive performance of the system,the research focus of distributed acoustic sensor system has turned totime-frequency joint modulation.One of the most representative schemes is Chirped-Pulse Phase-Sensitive Optical Time Domain Reflectometry(CP ?-OTDR).At present,this scheme has the best sensitivity in distributed acoustic sensing with a simple structure,and has a broad application prospect.However,due to the frequency modulation defects of electrical or optical devices,there are many useless harmonics in the linearly chirped light pulse which is used for detection in this scheme,and the harmonics will cause errors in the measurement process.Therefore,the quantitative sensing model of CP ?-OTDR is perfected in this thesis,and the influence mechanism of modulation harmonics is clarified under the guidance of this model.Finally,the CP ?-OTDR scheme based on injection locking is proposed,which improves the measurement performance of CP ?-OTDR based on improving the purity of linear frequency-swept light.The main research contents of this thesis are as follows:(1)In view of the incompletion of the current CP ?-OTDR scattering signal quantitative model,based on the principle of optical fiber interference and time-frequency modulation light vector process,this thesis deduced and improved the CP ?-OTDR quantitative scattering model,providing a simulation tool for subsequent theoretical research in related fields.(2)In view of the interference mechanism of modulation harmonics in the process of vibration demodulation,the interference peak introduced by harmonics in the process of crosscorrelation demodulation is theoretically analyzed in detail,and the degradation of measurement performance caused by harmonics in the process of dynamic and static strain demodulation is clarified.Finally,it is pointed out that the influence of frequency-swept harmonics is difficult to be eliminated by post-processing filtering,and the influence of harmonics should be solved by optical filtering.(3)The CP ?-OTDR experiment was carried out,using an arbitrary waveform generator(AWG)as an electric frequency sweep source,and the single side band mode of the IQ modulator to generate linear frequency sweep light with high harmonic suppression ratio.The inference about harmonic influence in this thesis was verified by the experimental data.The CP ?-OTDR scheme based on injection locking is proposed.The strain sensitivity of 4.5p?/Hz^1/2 is achieved by using a voltage controlled oscillator(VCO)with an electric harmonic suppression ratio of only 15 d B and injection locking technique.