| Phase Optical Time Domain Reflectometer(?-OTDR)has the advantages of continuous distributed monitoring,fast response speed and high detection sensitivity.It has important potential application value in the field of health monitoring of large infrastructure.Two adjacent front and rear sections of optical fiber are usually selected as the reference regions,and the quantitative measurement of the external vibration is realized by extracting the phase difference of the back Rayleigh scattering light(RBS)of the two reference regions with time.Because of the uneven spatial distribution of refractive index in the fiber,the influence of coherent fading effect can't be ignored by ?-OTDR with narrow linewidth laser light source,and the coherent fading effect causes the randomly intensity of RBS fluctuate.If the region with low RBS intensity is selected,the results of phase discrimination are prone to the sharp deterioration of signal-to-noise ratio(SNR).Researchers have proved that the accuracy of phase discrimination results is related to the SNR of RBS amplitude,but the numerical relationship between them is not clear,so there is no method to use the essential principle of fading to suppress noise,and there is no basis for judging the accuracy of phase discrimination results in practical engineering applications.Phase demodulation result of ?-OTDR usually used for subsequent signal processing,such as pattern recognition and feature extraction.However,after noise suppression,no proper solution has been given to eliminate the interference.This paper presents a method to achieve elimination of fading induced false alarms for?-OTDR system.The numerical relationship between accuracy of phase demodulation result and SNR of RBS amplitudes has been revealed,which confirmed that the amplitudes must be high enough if a good demodulation result for the phase difference change is desired.Based on the relationship,a suppression mask is created to identify the inaccurate phase demodulation result of low amplitude regions,so as to eliminate false alarms induced by fading.Performances of the proposed method have been experimentally demonstrated in both laboratory environment and external testing field or monitoring power line breaking events With a commercial ?-OTDR system equipment,False Alarms Rate(FAR)can be reduced to around 0.15%,an order of magnitude from 4.81%.By setting a proper accuracy threshold of phase demodulation result,both low FAR and Missing Alarms Rate(MAR)can be achieved at the same time.The improved ?-OTDR equipment can provide reliable early warning function for external force damage,guaranteeing the safe operation of power systemIn order not to affect the subsequent signal processing,this paper introduces patching methods from the field of image patching,which are based on one-dimensional linear interpolation,least squares and discrete cosine transform(DCT),and the Patch Match algorithm.The patching effect of these three methods is evaluated from the spectrum and the region variance.Results show that the Patch Match algorithm method can effectively restore the information of the suppressed phase demodulation result,achieving the effect of transparent suppression.The model established in this paper has certain guiding significance for the selection of RBS amplitude threshold in engineering application.The method based on amplitude dynamic evaluation can effectively suppress fading noise.Besides,the transparent repair method can connect the subsequent signal processing.These works proposed in this paper are expected to promote the further engineering application of ?-OTDR sensing system... |
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