High-sensitivity Distributed Optical Fiber Pressure Sensing Technology Based On Brillouin And Rayleigh Scattering

Distributed optical fiber sensing technology has not only the advantages of general optical fiber sensors,but also continuous measurement capability of spatial position information.It is broadly exploited in large-scale building structure health monitoring,perimeter security,power system,aerospace,and mostly focus on temperature,strain and vibration measurement,and there are few reports on pressure measurement.Besides,the distributed Brillouin sensing system using single-mode fiber(SMF)as the sensing fiber has low pressure sensitivity,and the research on dynamic distributed,large-range pressure measurement is rarely reported.In this thesis,SMF is used as the sensing fiber,and the high-sensitivity,dynamic,and large-range distributed pressure sensing research is carried out by using coated fiber and different sensing technologies.In high-sensitivity Brillouin optical time-domain analysis(BOTDA)scheme,aiming at the problems of low pressure sensitivity and large measurement error of standard SMF,this thesis proposes double-coated SMF method to enhance pressure sensitivity of Brillouin frequency shift(BFS).The pressure-induced strain model of multi-coated fibers based on Lame formula is established to analyze its enhancement mechanism.Simulation results show that BFS pressure sensitivity can be enhanced by increasing outer coating radius or decreasing outer coating Young’s modulus and Poisson’s ratio.To eliminate the influence of temperature on pressure measurement,the temperature sensitivities of three double-coated SMFs are analyzed and measured for subsequent temperature compensation.The experimentally measured BFS pressure sensitivities of three double-coated fibers in the pressure range of 0 to30 MPa after temperature compensation are-1.65MHz/MPa,-2.66MHz/MPa and-3.61MHz/MPa,respectively.The maximum pressure sensitivity is about 5 times that of standard SMF,and the pressure measurement error is 0.09 MPa.In high-sensitivity dynamic BOTDA scheme,aiming at the problems of slow pressure measurement and low sensitivity of traditional BOTDA scheme,this thesis uses double-coated SMF as the sensing fiber,and proposes a high-sensitivity and fast BOTDA pressure sensing scheme based on optical frequency-agile technique.Two electro-optic modulators are connected in series on an optical path to achieve the generation of frequency-agile pulse,and injection locking method is used to achieve filtering and amplifying of frequency-agile pulse,while it can make peak power of frequency-agile pulse more flat.The static experimentally measured BFS pressure sensitivity is-3.46MHz/MPa within range of 0 to 30 MPa,and single measurement time only takes 1.92 ms after an average of 64 times.Subsequently,the dynamic pressure measurement capability of the proposed sensing scheme is tested,and the measurement of dynamic pressure change of 30 MPa is achieved with a measurement error of 0.03 MPa.In high-sensitivity dynamic phase-sensitive optical time-domain reflectometry(phase-OTDR)scheme,aiming at the problem that the lower BFS pressure sensitivity of BOTDA system,this thesis uses standard SMF as the sensing fiber and proposes a high-sensitivity dynamic pressure sensing scheme based on frequency-scanning phase-OTDR.Multiple frequency pulsed light generated by arbitrary waveform generator(AWG)is filtered with a high extinction ratio through the injection locking scheme,and the sensing signal is extracted by cross-correlation algorithm.In the pressure range of 0 to 1.8MPa,the pressure coefficient of frequency shift of Rayleigh scattering spectrum is 702.50MHz/MPa,which is about1000 times that of BOTDA method with standard SMF.Subsequently,the dynamic pressure measurement experiment from 2.0 to 0MPa is carried out,and distributed measurement of pressure sampling rate of 33.33 k Hz is achieved with a measurement error of 0.61 k Pa.In high-sensitivity and large-range dynamic measurement scheme,aiming at the trade-off between pressure sensitivity and measurement range,this thesis takes standard SMF as the sensing fiber and proposes a distributed pressure sensing method combining BOTDA and phase-OTDR techniques.BOTDA with absolute measurement capability provides pressure calibration for the system,while the phase-OTDR with relative measurement is used to achieve high-sensitivity pressure measurement.Multiple frequency fast switching pulses are generated by AWG,and it is filtered and amplified by injection locking method to perform fast measurement.The experimentally obtained BFS pressure coefficient is-0.74MHz/MPa with ranging from 0 to 20 MPa,and the pressure dependence of Rayleigh scattering spectrum shift is 705.50MHz/MPa with ranging from 0 to 2.0MPa.Through the combination of two sensing techniques,the measurement of dynamic pressure change of 0～20MPa is achieved,and the measurement range of phase-OTDR is improved by an order of magnitude from 0～2.0MPa to 0～20MPa,while achieving a measurement error of 0.30 k Pa,which means the measurement accuracy is more than2600 times higher than that of BOTDA technique.