Development of fiber optic BOTDA sensor for intrusion detection
Introduction
For the purpose of protection against intruders, IR fiber sensor, magnetic sensor buried under ground and leakage coaxial cable sensor are widely used [1]. IR fiber sensors are sensitive to the dust and the water molecules in the air and their detection lines must be constructed in straight lines. Magnetic sensors and leakage coaxial cable sensors cannot be used in a harsh environment suffering electromagnetic interference. Fiber optic sensors have no such disadvantages and were developed for intrusion detection of the surroundings, such as the outskirts of an airport and the buildings. One of the fiber optic sensors detects the loss of light associated with the cutting of optical fiber, which is inevitable for an intrusion attempt [2]. Another type of optical fiber sensor detects the change in the polarization state of light occurred when the multimode optical fiber gets bent by an intruder [3]. The multimode optical fiber sensor has a very short detection range of several meters. Fiber optic sensor utilizing speckle pattern caused by interference among propagating modes has a very high sensitivity, however, its detection range is still limited to several hundred meters [4], [5].
In 1976, Barnoski and Jensen [6] reported a method to measure the loss of light nondestructively by an analysis of Rayleigh back scattering in time domain. Dakin [7] suggested that optical time domain reflectometry (OTDR) utilizing Rayleigh back scattering can be applied to the intrusion detection. Since it measures the back-scattered light, this sensor cannot detect such intrusions that were located behind a certain intrusion whose disturbance is large enough to obscure all the later events. Sensor utilizing stimulated Brillouin scattering has overcome this problem. Stimulated Brillouin scattering fiber optic sensor employs a pumping pulse and a CW probe beam running along a single mode optical fiber in opposite direction and detects the stimulated Brillouin back scattering signal amplified by two light beam and acoustic wave mixing [8], [9]. In this method, the frequency of CW probe beam differs from the pump beam by the amount of Brillouin frequency of optical fiber to enable the amplification and high intensity Brillouin scattering signal can be obtained [10], [11]. The Brillouin optical time domain analysis (BOTDA) sensor system equipped with one electro-optic modulator has been studied for measuring distributed strain and temperature, however, its signal analysis duration is too long to use in intrusion detection [8].
In this study we developed a BOTDA sensor system, which is capable of detecting and locating intrusion attempts over several tens of kilometers long paths. Simulation of an intrusion effect was achieved by use of a strain-inducing setup installed on an optical table. We report experimental results that confirmed the distance resolution of 3 m for the fiber length of 4.81 km within 1.5 s detection time.
Section snippets
Operating principle
When the power of optical signal which propagates along the single-mode optical fiber is larger than the Brillouin threshold power, the backward stimulated Brillouin scattering (SBS) signal is generated. SBS can be described as a parametric interaction among the incident light, the Stokes light, and an acoustic wave. The Brillouin frequency shift νB of the backward scattering light of the propagating light in an optical fiber is given by [12]where va is the acoustic velocity, n the
Experimental setup
The experimental setup for the present fiber optic BOTDA sensor is shown in Fig. 3. The system consists of an optical source assembly, two modulators, and the detector part. Optical source is composed of a DFB diode laser of its maximum output of 30 mW and maximum bandwidth of 5 MHz, and an optical amplifier of its maximum output of 18 dBm. Pump pulse is generated at the electro-optic modulator 1 (EOM1, 2.5 Gb/s modulation), which is driven by a pulse generator (HP81110A). Pump pulses of two
Intrusion detection experiments and results
The experimental setup for inducing strain on the single mode optical fiber to simulate intrusion effects is shown in Fig. 5. The total length of the optical fiber was 4.81 km and a tension jig is placed to induce strain of 100 μm over about 10 m long section of the fiber. The measured pulse shape of two different pump pulses of 30 and 50 ns long is shown in Fig. 6. The peak power of pump pulse was about 720 mW when measured after the amplifier. The bias voltage of EOM2 was adjusted to convert all
Conclusion
In order to develop a fiber optic BOTDA sensor applicable for intrusion detection, an in-lab experiment has been performed. The sensor system consists of a laser diode, fiber amplifier, two electro-optic modulators, and a detector. Simulation of an intrusion effect was achieved by use of a strain-inducing setup installed on the table. The experimental results confirmed the distance resolution of 3 m for the fiber length of 4.81 km within 1.5 s detection time. Actual intrusion detection experiment
Acknowledgements
This work has been supported by Dual Use Technology Program of Ministry of Science and Technology. One of the authors has also been supported by the Brain Korea 21 Project in 2001.
Il-Bum Kwon received his PhD degree in aerospace engineering from Korea Advanced Institute of Science and Technology, Taejon, Korea, in 1997. He worked at Steel Research Division at Research Institute of Industrial Science and Technology (RIST/POSCO), Korea, during 1989–1992. He is now a senior researcher of Industrial Metrology Division at Korea Research Institute of Standards and Science (KRISS), Taejon, Korea. He is a member of SPIE. His current fields of interests are smart structures,
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Cited by (0)
Il-Bum Kwon received his PhD degree in aerospace engineering from Korea Advanced Institute of Science and Technology, Taejon, Korea, in 1997. He worked at Steel Research Division at Research Institute of Industrial Science and Technology (RIST/POSCO), Korea, during 1989–1992. He is now a senior researcher of Industrial Metrology Division at Korea Research Institute of Standards and Science (KRISS), Taejon, Korea. He is a member of SPIE. His current fields of interests are smart structures, fiber optic sensor, structural mechanics, and application of artificial intelligence.
Se-Jong Baik received the PhD degrees in physics from Chonnam National University, Gwangju, Korea in 2002. He is now a chip researcher of R&D Center SEGI Engineering Co. Ltd., Gwangju, Korea. He is a member of KPS, KOSA. His current fields of interest are fiber optic sensors and photonic devices.
Kiegon Im received his PhD degree in physics from University of New Mexico at Albuquerque. He is now a professor of Department of Physics and the director of Photonics Research Center, Chonnam National University, Gwangju, Korea. He is a member of KPS, KOSA, OSA, and IEEE/LEOS.
Jae-Wang Yu received the BS and MS degrees in electrical engineering from Jeonbuk National University, Jeonju, Korea, in 1996 and 1998, respectively. He is currently working towards the PhD degree in the area of optical fiber devices and sensors at the Department of Information and Communications in Gwangju Institute of Science and Technology. His current fields of interest are optical fiber devices and sensors.