Research On Distributed Vibration Sensing System Fused With Weak Grating Array

Distributed fiber optic vibration sensing technology is one of the research hotspots in the field of fiber optic sensing.It can acquire vibration waveform information along the fiber link and has advantages such as resistance to harsh environments,electromagnetic interference,and corrosion.It has a wide range of applications in earthquake wave detection,infrastructure structural health monitoring,perimeter intrusion monitoring,and other areas.The distributed fiber optic vibration sensing system is mainly based on a phase-sensitive optical time domain reflectometer(φ-OTDR)constructed using the backward Rayleigh scattering(RBS)in the fiber as the signal light.However,the weak RBS of single-mode fibers fundamentally affects the signal-to-noise ratio of the distributed fiber optic sensing system based on Rayleigh scattering,limiting the sensing distance and sensitivity of the system.Therefore,in this paper,a weak fiber Bragg grating(WFBG)array is used as the sensing unit to replace the RBS light in the fiber with WFBG reflected light to improve the strength and signal-to-noise ratio of the received optical signal.Combined with the external coherent detection φ-OTDR technology,a self-coherent vibration sensing model based on WFBG and double pulses is constructed.A distributed vibration sensing system incorporating weak grating arrays is designed,achieving high sensitivity linear demodulation of vibration signals over long distances.The main content is as follows:The mechanism of weak reflection grating sensing was investigated,and the spectral characteristics of weak reflection gratings were analyzed using the grating mode coupling theory.A fiber grating axial strain sensing model was derived.The sensing principle of the phase-sensitive optical time-domain reflectometry(φ-OTDR)technology was studied,and a φ-OTDR sensing system based on heterodyne coherent detection was established.Furthermore,a vibration sensing model based on the double-pulse interference between weak fiber Bragg grating(WFBG)arrays was proposed,elucidating the principle that the fixed carrier frequency light pulses induced by the interference of double-pulse light between the WFBG arrays can sense fiber vibrations.A dual-pulse φ-OTDR system incorporating weak fiber Bragg gratings(FBGs)was constructed and optimized.The effects of the FBG array spacing and pulse width on the system signal were studied experimentally.A continuous and uniform improvement in signal quality was achieved by adopting a pulse width consistent with the FBG spacing.The central wavelength drift of the weak reflection FBG was calibrated with temperature,and a 1.5 nm wide-spectrum weak FBG was selected to effectively reduce the influence of external environmental temperature changes on the FBG reflection spectrum.A mathematical model based on the discrete Fourier transform was proposed to extract the phase of a single-frequency pulse signal.A digital simulation was conducted for phase demodulation of low signal-to-noise ratio signals,achieving high-precision phase demodulation.A vibration-sensitive sensing unit was designed,and a physical model of the sensing unit was established.The relationship between the bending radius of the optical fiber and the fiber bending loss was theoretically derived.Combining elasticity mechanics with the fiber-optic elasticity effect,a calculation formula for the fiber-optic sound pressure sensitivity of a thin-walled cylinder was derived,and the effects of the thin-walled cylinder material and structural parameters on the fiber-optic sound pressure sensitivity were analyzed by simulation.A distributed vibration sensing system was built,and the performance of the sensing system was experimentally verified.The system was found to have high phase demodulation linearity,good frequency response,and multi-point detection capability.The sensing system was deployed for field testing,and the experimental results showed that the distributed sensing system could accurately locate vibrations along the optical fiber,and the sensing system still had good stability after long-term testing.In terms of long-distance sensing,the system can realize vibration sensing at the end of a 51 km optical fiber,and the fitting degree of the 200 Hz sinusoidal signal demodulation results is as high as 0.974.The amplification effect of the acoustic vibration sensitivity unit was experimentally verified,and the measurement results showed that the signal-to-noise ratio of the sound signal demodulated using the acoustic vibration sensitivity unit was 55.21 d B,which was 21.53 d B higher than that of direct fiber detection.