Quantitative Measurement Of Dynamic Nano-strain Based On ?-OTDR

In recent years,with the improvement of people's security awareness in production and living,the demands of structure health monitoring of bridge,tunnel,highway and other civil structures have increased rapidly.In this case,strain is an extremely important parameter of these physical quantities.However,for presently available technologies,their sensitivity of strain measurement is normally on the order of ??,which makes it hard to satisfy the demands of special circumstances.Therefore,developing a dynamic and quantitative nano-strain measurement technology has a practical research value and broad application prospect.In this thesis,the quantitative measurement technology of dynamic nano-strain is realized by phase-sensitive optical time-domain reflectometer(?-OTDR),which has advantages of high precision,long distance,distributed sensing and has the ability to perceive extremely faint external stress or temperature changes.The concrete research contexts are as follows:Firstly,starting from the theory of optical fiber scattering,the basic principle of conventional OTDR and ?-OTDR are analyzed in detail.By establishing the theoretical model,the strain sensing mechanism of ?-OTDR is deduced from phase and intensity of scattered light particularly.Secondly,the ?-OTDR scattering process is visually presented through numerical simulations with the mathematical model.At the same time,the quantitative relationship of strain with phase and intensity of the scattered light is simulated.According to the simulation results,the linear relationship between strain and phase differences at fixed interval is proved.Besides,the linear relationship between strain and standard deviation of scattered light intensity is also demonstrated by numerical simulations.Finally,a sensing system is proposed for quantitative measurement of largerange dynamic nano-strain based on ?-OTDR,where the coherent detection and IQ demodulation are employed to demodulate both the phase and the amplitude of the Rayleigh scattering light in real-time.The quantitative relationship between strain and standard deviation of scattered light intensity is demonstrated experimentally,the strain sensitivity at each position of the fiber can be mitigated by increasing the length of sample data.This method can achieve a strain resolution of 6 n?.In addition,by measuring phase differences between two adjacent sections,the quantitative nanostrain with a large measurement range is demonstrated,while it is also a novel method to measure the strain-parameter of refractive index.For the Panda polarizationmaintaining fiber under test in the experiment,the strain parameter of refractive index is measured to be-0.375 ?-1,the strain sensitivity parameter of system is measured to be 8.71 mrad/(n?·m),while the strain resolution is 1 n? or 2 n? corresponding to 5 m or 2.5 m spatial resolution,respectively.Based on the above research,the dynamic measurement performance of the sensing system is demonstrated.The experimental measurement shows a triangular wave with a 24-Hz vibrating frequency and a 50-n? strain amplitude as well as a 188-Hz resonant signal of the tensile section,which can prove that this sensing system has the ability to measure dynamic nano-strain quantificationally.