?-OTDR Sensing Technology Application In Railway Safety Monitoring

The distributed optical fiber sensing system based on phase sensitive optical time domain reflectometer is one of the most widely used optical fiber vibration measurement schemes at present.Nowadays,this technology has great application value and potential in fields of perimeter security,petroleum exploration,seismic wave detection,structural health,transportation pipeline monitoring and so on.For this technology,it has great significance of demodulating the light phase of the Rayleigh scattered light.Compared with the direct light intensity change caused by external disturbance,the light phase has a linear relationship with the change of external environment on the whole.Among them,digital orthogonal demodulation algorithm based on heterodyne coherent detection(D-OTDR is a commonly used method to obtain the light phase.But due to the nature of the beat signal itself,if the original signal is collected for digital demodulation processing,a high-speed acquisition card higher than the beat frequency rate 2 times or more to obtain the beat signal is necessary.Based on the undersampling theorem with band-pass signal,the possibility of using the sampling rate lower than Nyquist frequency to obtain the beat signal is discussed in this paper,and the strict constraint relation between the sampling rate and the bandwidth of the beat signal is given.Experimental results show that even with lower sampling rate,we can also complete and get beat frequency signal phase information and vibration positioning information without distortion.It means we can reduce the sampling rate requirements of heterodyne coherent ?-OTDR system and the computational burden,which makes the performance of the real-time online monitoring of the system to further improvement.The main contents of this thesis include:introduces the current research status of distributed fiber optic sensing system at home and abroad,the advantages and current application status of phase-sensitive optical time domain reflectometer are illustrated,and the research progress of phase demodulation technology is introduced;Based on the basic principle of Rayleigh scattering,the phase modulation and demodulation principles of ?-OTDR technology are deduced,and the two structures of ?-OTDR system and their corresponding demodulation algorithms are analyzed in detail;After discretization of the ?-OTDR system,the modularized simulation is carried out.The simulation results show that undersampled phase demodulation of the beat signal can be achieved well;The subsequent demodulation experiments verify the feasibility of using low speed sampling rate for heterodyne coherence detection ?-OTDR and the accuracy of phase demodulation and differential positioning results.Meanwhile,the demodulation algorithm of intensity signal reconstruction in time domain based on direct detection type ?-OTDR system is also verified;Finally,an integrated system based on heterodyne coherence detection ?-OTDR is built and applied to the actual detection of a certain section of domestic railway.The experimental results show that the system is accurate for the detection and positioning of the train running state.