Two-Beam Ф-OTDR Distributed Fiber Sensing Technology And Application

With the rapid development of optical communication technology, optical fibersensing technology has been in-depth studied in recent years and has been widelyused in such fields as oil and gas, aerospace, civil infrastructure, transportation andothers. With its unique technical advantages, distributed fiber sensing technology caneffectively replace the point type sensors, which needs a large amount in the regionalmonitoring applications. The distributed sensor can capture vibration signals causedby structural damage or external intrusion along optical fiber, precisely point out thelocation, and thus has the ability of structural health monitoring, pipeline safetypre-warning and perimeter security. It is important for the safety of industryproduction and has a wide application prospect.Aiming at the shortage of traditional Ф-OTDR system that the detectionsensitivity is influenced by the laser linewidth, this paper proposes a two-beamФ-OTDR distributed optical fiber vibration sensor which combines the interferometrytechnology and OTDR technology. The main contents are as follows:(1) A two-beam Ф-OTDR distributed optical fiber sensing technology isproposed. Similar to the traditional Ф-OTDR system, the system analyzes thevibration location by the change of Rayleigh backscattering light. The difference liesin that the interference of two-beam system comes from two light paths, not thescattering center of optical pulse. As the initial phase of optical is small, therequirement of laser source becomes lower. The detection sensitivity can be increasedwhen utilize the non-ultra narrow linewidth laser source.(2) The location method used in traditional OTDR is to average thebackscattering trace and then subtract each other to get the vibration position. Basedon the traditional method, this paper proposes an interval moving averaging methodand a new differential method. The lab experiment result indicates that the method canfigure out vibration precisely. The standard deviation of localization is less than5mand the spatial resolution can reach10m under the pulse width of100ns.(3) Against the shortage of the averaging-differential method in long distanceattenuation, a new signal processing method is proposed which analyzes the signal intime domain. Firstly, the time domain signal of one sensor point is collected and thefrequency characteristic is analyzed using fast Fourier transform (FFT) and short-timeFourier transform (STFT). Then wavelet filtering is used to get a standard deviation curve and a time-space figure. The results show the standard deviation curve canaccurately locate the disturbance events in a complex environment and maskinterference incident such as car-passing. Besides, the time-space figure achievesmonitoring from both space-domain and time-domain.(4) Since the system uses dual-beam interference principle, the polarization stateinduced degradation needs to be considered. Firstly, the equivalent model of thesystem is then established and analyzed to find the reason of phase shift andpolarization fading. Based on the simulation analysis, an active compensation methodis proposed which uses a polarization controller to to find a suitable polarization stateof the source light so that the interference signal remains large vibration amplitude.Experimental results show that after applying a polarization controller, the signal tonoise ratio has been improved significantly, which verifies the validity of this method.(5) Based on the theoretical analysis, the pipeline safety pre-warning systemusing two-beam Ф-OTDR sensing technology is proposed and put into fieldexperiments. Experimental results show that the monitoring distance can reach20kmand the disturbance events can be effectively detected and positioned from theStandard deviation curve. In addition, from the space-time diagram we can clearlydistinguish the target disturbance events and the interference events.