A real-time distributed fiber strain sensor for long-distance perimeter intruder detection

Abstract

A real-time distributed fiber sensor is described, which is based on Brillouin optical time-domain reflectometry using our propriety fiber laser technology. The sensor offers capabilities of protecting security sectors to larger perimeters using commercial fiber.Perimeter intrusion detection sensor is one of the most important elements in the country's ongoing efforts in homeland and national security. Various conventional technologies that make use of seismic, acoustic, infrared, and magnetic sensors have been developed for perimeter intrusion detection application. Optical fiber sensor technology that uses optical fiber as the sensor element, as one of the most promising technologies, offers a number of advantages over those conventional technologies. For example, optical fiber-based sensors are lightweight, compact, easily multiplexable, safe in hazardous environments, capable of distributed sensing, and immune to EMI (electromagnetic interference) radiation, resist to chemical corrosion, require no electrical power at the sensing point, and in most cases have the potential to be produced at low cost. This paper describes our recent development on a real-time long-distance distributed fiber strain sensor that could be very useful for perimeter intrusion detection.The sensor system is based on the fact that the frequency shift of Brillouin backscattering light in an optical fiber linearly depend on strain on the fiber. A schematic diagram of our system is shown in Fig. 1. A NP Photonics' propriety fiber laser is used as a light source in the sensor system. Part of the fiber laser beam is amplitude modulated by a modulator to generate pulsed light before launching into long sensing fiber. Another part of the beam is split to pump a Brillouin fiber ring laser. Since the Brillouin laser has extremely narrow linewidth and intensity noise, it is used as an optical local oscillator for coherent detection. Spontaneous Brillouin backscattering of the pulsed light in the sensing fiber, which carries local information of strain along the fiber, can be detected by coherent heterodyne detection. The innovation of our system is that the use of the Brillouin fiber laser as a frequency-shifted local oscillator allows bringing the coherent beat signal, which contains information of the Brillouin frequency shift, from expensive microwave (11 GHz) to low-cost RF (0.1 GHz) region. Furthermore, low-speed RF heterodyne detection offers the opportunity of real-time data acquisition and analysis by using low-cost electronics commercially available, instead of expensive microwave coherence detection that lies out of the bandwidth of a conventional heterodyne receiving system. This makes it possible to develop a low-cost real-time distributed fiber strain sensor in a very long (up to 100 km) optical fiber.A simulated experiment for intrusion detection is performed by using a step-on stage setup. A high-speed digitizer (1GS/sec) is used to take the data for the beat signal from sensing fiber at the repetition rate of 1kHz. The data-anal...

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A novel FBG-based security fence enabling to detect extremely weak intrusion signals from nonequivalent sensor nodes

“…With the development of fiber sensing technology, optical fiber sensors have been successfully applied in perimeter security field [1][2][3], including the optical time domain reflectometry (OTDR) [4][5][6] and Sagnac, Michelson, and Mach-Zehnder (M-Z) interferometers [7][8][9][10], and white light interferometers [11]. Comparing to the conventional perimeter security system which uses ultrasonic, radar, microwave, infrared or photo-electric sensors to detect intruders [12], fiber sensors have advantages of high sensitivity, simple structure, compactness, long-distance transmission capability, electro-magnetic immunity (EMI), and corrosion resistance.…”