Research On Key Technologies Of Optical Fiber Vibration Sensing Based On Phase Demodulation

The distributed optical fiber vibration sensing system has the advantages of light weight,small size,high sensitivity and strong anti-interference ability,so it has been widely used in military,national defense,industrial and other fields.Because when the fiber is disturbed,the intensity and amplitude of the scattered light of Rayleigh will change accordingly,and this phase change is linearly related to the frequency,amplitude and other information of the disturbance force,and the phase-sensitivity optical time domain reflectometer(φ-OTDR)technology based on this principle has become a research hotspot for distributed optical fiber vibration sensing because of its strong sensitivity to the phase information of vibration signals.In this paper,the basic principles of φ-OTDR systems based on direct detection and coherent detection and the mathematical forms of their demodulation methods are derived,and the infinite scattering is introduced into the φ-OTDR system based on the traditional coherent detection structure to optimize the spatial resolution of the system,and finally its feasibility is experimentally verified.1.Based on the basic principle of the traditional φ-OTDR system,infinite scattering is introduced into the φ-OTDR system,the basic principle is introduced,and a mathematical model is established,several main performance parameters affecting the system performance are analyzed,and the conclusion that spatial resolution,frequency response range and dynamic range will be mutually restricted,and the four methods of differential positioning,simple average,differential sliding average and total variational alternating direction multiplier denoising are used to locate and denoise the vibration signal respectively.It is concluded that the noise reduction effect of sliding average method is the same as that of simple average method,but the high frequency response range can still be retained.The combination of differential positioning and sliding average method makes the signal-to-noise ratio of the system reach 5.95 d B,and finally the basic principle of demodulation of vibration signals based on orthogonal differential multiplication and Hilbert transform and its mathematical model under infinite scattering structure are introduced.2.The structure of the φ-OTDR experimental system based on infinite scattering was constructed,the key instruments of the experimental structure were introduced,and the vibration of different amplitudes(2.5 V-10 V)and frequency(200 Hz-3 k Hz)was located and demodulated under the system structure,and the positioning performance of the system would be limited under high frequency and weak amplitude vibration was obtained,and the multi-point detection ability of the system was tested by setting two vibration points,and the spatial resolution of the system was obtained to be 1.2 m.Finally,the Hilbert transform algorithm is used to restore the phase information of the vibration signal with a vibration amplitude of 5 V and a vibration frequency of 200 Hz,500 Hz and1 k Hz,respectively.