Experimental And Applied Research On Ultra-long Distance Phase-sensitive Optical Time-domain Reflectometer

Comparing with other sensors, the distributed optical fiber sensor (DOFS), whichuses optical fiber as the sensing and transmission medium, has advantages ofresistance to electromagnetic interference, large-scale and long distance measurement,corrosion resistance, and etc. Thus it is widely researched and reported. In this paper,it is studied a kind of DOFS-the phase-sensitive optical time-domain reflectometer(Φ-OTDR) system with characteristics of quick response, high sensitivity, simplestructure, and so on. The Φ-OTDR has the structure similar to the conventionalOTDR, however, the difference is the injecting light pulse is strong coherent, and thepower of interference light detected is induced by backward Rayleigh scattering oflight from all parts of the optical fiber using the photonic detector. Thus the Φ-OTDRsystem has the ability to detect weak disturbances, and can be applied in securitymonitoring of important fields, such as oil and gas pipelines, the border line, militarybases and so on.The research on the Φ-OTDR is mainly concentrated on how to improve thespatial resolution and extend the sensing distance. In this paper, an amplifiedΦ-OTDR system is studied based on the hybrid EDFA and distributed optical fiberRaman amplifier. The transmission loss of detecting light can be compensated withRaman pump by making use of bi-directional pump structure which can balance theoptical power distribution along the fiber route. A longer distance Φ-OTDR sensingsystem is realized by this way.The following work is carried out, which includes from sensing distanceextending, system software developing, hardware development and the applicationtests. The details are listed as follows:(1) The distributed optical fiber sensing technology and research is summarized,especially that using Φ-OTDR technology. The key factors influencing theperformances of Φ-OTDR sensing system are analyzed theoretically. At the same time,different amplifying technologies commonly used and their technical principles areintroduced.(2) A distributed optical fiber sensing system using Φ-OTDR based on EDFAamplification and acousto-optic modulation is achieved, with spatial resolution of 20m and sensing distance of20km.(3) The gain of the distributed optical fiber Raman amplification is simulatednumerically. Based on the first-order and second-order optical fiber Ramanamplification, the Φ-OTDR system with sensing distance of74km and84km, areachieved respectively, which is the longest Φ-OTDR sensing system reported so far.(4) The system software of Φ-OTDR is developed with structure of menu/viewbased on VC++, MFC, and the alarm log is designed and its data base is managed byMSSQL Sever.(5) A prototype of Φ-OTDR is realized for practical uses. Fileds experimentsbased on this prototype is studied for validating the intrusion detection effect indifferent installation cases for the fiber cable.