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  • Fiber Optic Intrusion Sensor Protects Perimeters
Jan 2005
COLLEGE STATION, Texas, Jan. 28 -- Two Texas A&M University engineers are developing a fiber optic intrusion-detection sensor for protecting long perimeters.

The sensor is commercial cable containing a fiber made of fused silica, a type of glass that's specially doped to guide light. The cable is buried eight to 20 inches underground along the monitored perimeter. Because the same type of cable is used in the telecommunications industry, it is readily available and cheap -- about 15 cents a foot.

"Since the 9/11 terrorist attacks, the demand for perimeter security systems has increased tremendously," said Henry Taylor, a professor of electrical engineering, who is leading the effort with PhD student Juan Carlos Juarez. "We're projecting that this new type of distributed sensor will be able to monitor perimeters at about one-tenth the cost of other methods. We envision application along national borders, and for fixed installations such as airports, nuclear plants and military bases." The Texas A&M university system holds a patent on the technology.

Digging the trenches in Yuma. (Photo: Texas A&M University)
Taylor and Juarez have established a permanent test bed at the Texas A&M Riverside Campus and have twice transported their system to the US Marine Corps Air Station in Yuma, Ariz., at the invitation of the Defense Advanced Research Projects Agency (DARPA), to conduct further tests in a desert environment.

Using their novel system, Taylor and Juarez have observed the effect of the pressure of someone walking directly over the cable and up to seven feet on either side. They have also been able to detect cars driving hundreds of feet away from the cable and even helicopters flying overhead, because seismic waves produced by cars and helicopters are picked up by the sensor.

In their scheme, a light pulse from a specially designed, ultrastable laser is sent into the sensing fiber. As the pulse advances along the fiber, a phenomenon known as Rayleigh backscattering causes some of the light to reverse its direction. An optical receiver collects this backscattered light and converts it to an electrical signal, which is then processed with a personal computer. The system determines not only the presence of an intruder, but his or her location along the cable as well.

In the Yuma tests, Taylor and Juarez demonstrated that their system works up to ranges of 19 kilometers, and they believe that this can be extended to 40 kilometers or more. Taylor said it's even feasible to cover the entire US-Mexico border or the US-Canada border from one location, provided that optical amplifiers are used to boost the light signals.

With the sensor performing better than had ever been anticipated, the researchers are now working on user-friendly software to project the location of disturbances on a map of the cable path and identify them as people, vehicles or animals.

"As far as we know, the kinds of results we're getting with the system have never been seen before by anyone," said Juarez. "It's exciting to think that this sensor could soon be of benefit to society and our national interests."

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