| The distributed optical fiber sensing (DOFS) is a kind of remote on-line monitoringtechnology that uses the optical fiber as means of transmission and perception mediumsimultaneously. Perimeter security system based on DOFS is a new generation of security system.Compared with the traditional mechanical and electrical security system, its advantages are highsensitivity, small size, light weight, resistance to electromagnetic interference, etc. DOFS can beapplied to a variety of harsh environmental conditions. But the common DOFS based on M-Z,Sagnac and other interferometric optical fiber sensor system usually can only detect one singleintrusion or disturbance at some point. To detect and locate the multi-intrusions ormulti-disturbance, the research of multi-functional optical fiber security system based on the phasesensitive optical time-domain reflectometer (Φ-OTDR) has been carried out in this paper. As anew security detection system based on Φ-OTDR, it can detect and locate multi-intrusions at thesame time. It has good concealment, positioning precision, and is suitable for long and large rangeof monitoring. There is no doubt that it has broad application prospects in the field of security forits outstanding features.This paper is engaged in the research on practical technique of the multi-functional opticalfiber security system based on Φ-OTDR, the main research tasks presented in this paper are asfollows:(1) The author expounds DOFS based on OTDR and that based on interferometer respectively, and then compares their differences. The advantages and prospects of the Φ-OTDRtechnique which combines the interferometer principle with OTDR are analyzed.(2) The light scattering in optical fiber is introduced, and the detection and location principleof the Φ-OTDR system are analyzed. Then the theoretical analysis and simulation of the Φ-OTDRsystem are carried out by using the discrete backward scattering field model of optical fiber andMonte Carlo method in combination.(3) The influences of the laser linewidth, accumulative average number, and pulse width onthe performance of the system have been investigated theoretically and experimentally. Thefollowing conclusions are made. The attenuation of noise decreases gradually with the increasingof accumulative average number. And the signal-to-noise ratio (SNR) increases firstly, and thendecreases as the pulse width increasing. Then a further study for the influence of the invasionsignal on the performance of the Φ-OTDR system has been made. And the periodic change rulesof the SNR and position precision with the invasion signal phase are presented, as well as theinfluence on the missing report which decided by the invasion signal frequency. Finally, thepreliminary solutions have been proposed and proved based on some defects existed in thesystems.(4) A multi-functional optical fiber security system based on Φ-OTDR which can be used formulti-intrusions detection has been built, and the detection experiments have been accomplished.A temperature-compensated optical fiber Bragg grating with a filtering accuracy of0.14pm/oC isused to filter the backscattering light. And several signal processing methods which suitable forΦ-OTDR system are applied. A location accuracy of2m and a SNR of15dB have been achievedunder a spatial resolution of8m on the15km optical fiber with four invasions detected at thesame time, so far, which is the most simultaneous detection points reported in the world. Finallythe author puts forward a practical multifunctional structural health and border security opticalfiber system, and has carried on a preliminary study on it.(5) A new kind of signal processing and demodulation method for the distributedmulti-functional optical fiber security monitoring system has been put forward. And the Φ-OTDRmonitoring software based on LabVIEW platform has been developed that can realize the newdemodulation scheme of data acquisition, analysis, storage and alarm. |
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