Research On Key Methods Of Optical Fiber Distributed Acoustic Sensing Based On Weak FBG And Dual-Pulse

As one of the frontiers in the development of optical fiber sensing measurement technology,the optical fiber distributed acoustic sensing technology can realize the waveform recovery of the sound / vibration signal along the optical fiber link,and has the advantages of resistance to harsh environments,electromagnetic interference,and corrosion.Optical fiber distributed acoustic sensing system is mainly based on phasesensitive optical time domain reflectometry technology.Achieving high-bandwidth and high-sensitivity linear demodulation of signal phase over long distance is critical for high-precision extraction of acoustic strain information of optical fiber.In order to solve this problem,this thesis uses the weak reflection fiber Bragg grating(WFBG)and the double pulse interference method to carry out research of fiber distributed acoustic sensing,and analyzes three aspects of sensing distance,sensing bandwidth and sensing enhancement.The major research work of this thesis as follows:(1)The principle of using WFBG and dual-pulses to generate fixed-frequency interference pulses to sense the strain on the sensing fiber is expounded.By using the reflected light pulse provided by WFBG to replace the back Rayleigh scattering light in the sensing fiber to improve the intensity and signal-to-noise ratio(SNR)of the received signal,a mathematical model for extracting the phase of a single frequency pulse signal based on discrete Fourier transform(DFT)is proposed.Digital simulations are performed for phase demodulation of low SNR signals,and the high-precision phase solution of the low signal-to-noise ratio signal is realized.(2)In the aspect of ultra-long-distance distributed optical fiber sensing,a model in which a single chirped pulse signal generates the same dual-chirped-pulse through a time-delayed fiber is established.Generate interference pulses with a fixed carrier frequency by the interaction of dual-chirped-pulse and WFBG arrays to sense strain in sensing fibers,and use the DFT-based phase extraction method to extract the phase change of the interference pulse to correspond to the strain on the sensing fiber.The proposed system can realize acoustic sensing at the end of the 101.64 km sensing fiber,the spatial resolution is 10 m,where the SNR of the signal returned from the end of the sensing fiber is as low as 3.234 d B,and the fitting degree of the demodulated sinusoidal signal can reach 0.9681.(3)In the aspect of sensing bandwidth enhancement of distributed acoustic sensing,based on the frequency division multiplexing of different frequency dual-pulses,a distributed acoustic sensing frequency division multiplexing method based on WFBG and different frequency dual-pulses is proposed.The frequency separation design of pulse pairs in different frequency ranges is carried out,and the sensing bandwidth of distributed acoustic sensing is increased by 4 times by multiplexing 4 pairs of different frequency dual-pulses in different frequency ranges.The resulting phase stitching verifies the accuracy of the proposed method for demodulating signals at various frequencies.Experimental results show that the proposed method can achieve frequency measurements a 2 k Hz bandwidth on a 70 km sensing fiber.(4)In terms of acoustic sensitization sensing,a fiber optic sensing acoustic sensitization method based on cylindrical ring pillars and polyester resin film sensitizing cylinders is proposed,which solves the problem that the actual sound signal causes the sensing fiber to have small strain and cannot realize actual sound measurement.Using the DFT-based phase extraction method,the calculated sensitivity of the optical fiber sensing sensitizing cylinder is 10.2 rad/pa,and the minimum measured sound pressure is 0.0025 pa.The optical fiber sensing intensifier tube is used to perceive the actual vocal signal recorded by the Bluetooth audio playback,and the accurate recovery of the vocal signal is realized.The correlation coefficient between the recovered vocal signal and the recorded signal reaches 0.6956.The ability of acoustic sensitization and accurate measurement of real acoustic signal are verified.