Research On Distributed Optical Fiber Vibration Sensing Technology Based On Φ-OTDR

Distributed fiber optic sensor collects and analyzes scattered light at continuous locations along the fiber to achieve distributed measurement of physical parameter changes,such as ambient temperature and strain changes.Phase sensitive optical time domain reflectometer(Φ-OTDR)can detect the external disturbance along the sensing fiber by the phase change of the coherent Rayleigh backscattering signal(RBS)generated in the fiber,which has high sensitivity and can recover the waveform information of disturbed signal.However,the reliability and accuracy of Φ-OTDR are seriously affected by the weak intensity of RBS,especially the existence of interference fading.A novel scheme combining intensity-coding technology and an interference-fading-suppressed method called Spectrum Extraction and Rotated-Vector-Sum(SERVS)is proposed.In this scheme,acousto-optic modulator(AOM)is used to generate intensity-coded pulses to increase the incident power,and SERVS method is applied in digital signal processing to overcome the interference fading problem.Firstly,based on the theoretical model of intensity-golay-coding ΦOTDR,a simulation platform is established with MATLAB language.When the vibration signal is simulated to applied to the sensing fiber,the location of disturbance position and the recovery of waveform information can be realized.The interference-fading-suppressed algorithm is also applied in the simulation,verifying the feasibility of the proposed scheme.On the basis of simulation,an experimental scheme of intensity-golaycoding Φ-OTDR with AOM is further proposed.Compared with other schemes of phase-coding pulses,this scheme avoids the I/Q modulator and other components which are complicated to operate and expensive.The experimental results show that SERVS method can not only maintain the signal to noise ratio(SNR)gain of pulse coding,but also eliminates phase demodulation errors caused by interference fading along the sensing fiber effectively,thus avoiding the false alarm phenomenon of current pulsecoding Φ-OTDR system.When using 8-bit Golay coded pulses,SERVS method can reduce the proportion of interference fading points from 2.98%to 0.33%and decrease the averaged standard deviation(stedv)of differential phase from 0.6556 rad to 0.2901 rad by 55.8%in the sensing range of 2 km.In addition,the minimum level of averaged power spectral density(PSD)of the differential phase at whole locations along the fiber can be reduced by 6 dB/Hz.The above experimental results demonstrate the ability of SERVS method in pulse-coding Φ-OTDR to suppress the interference fading.Compared with the conventional single-pulse ΦOTDR system,the SNR gain of demodulated disturbance signal is 8.6 dB,the strain sensitivity is 2.38 pε/√Hz.