Fiberoptic sensors monitor structural changes

April 15, 2005
Scientists have developed a new breed of sensors that utilize fiberoptics to survive incredible levels of pressure and heat, helping researchers determine how to make buildings that could survive massive explosions.

WARWICK, UK - Scientists have developed a new breed of sensors that utilize fiberoptics to survive incredible levels of pressure and heat, helping researchers determine how to make buildings that could survive massive explosions. Professor Julian Jones, professor of Engineering Optics and Head of the School of Engineering and Physical Sciences at Heriot-Watt University (Edinburgh, Scotland), discussed the new sensing devices at the Institute of Physics conference Physics 2005 in Warwick, UK, in mid-April.

The three new types of sensor use specially-engineered optical fibers that respond to changes in their environment. They can monitor blast-waves from high explosives, structural safety in tunnels, bridges and buildings, bending in critical aircraft components, and deterioration in weapons stockpiles.

Most modern sensors are electronic and work on the principle that temperature, pressure or stress affects the electrical behaviour of the sensor. But electronic sensors can be impractical, unreliable and even dangerous when used in the wrong conditions. In recent years, fiberoptic alternatives have attracted serious interest and are beginning to monitor data that could never have been measured electronically. In recent years, fiberoptic sensors have been used to measure strain in airplane wings and detect movements in large civil engineering projects such as bridges and dams, via interferometry.

But at the Physics 2005 conference, Jones described some very different kinds of fibers, custom designed for specific sensing tasks and promising a whole new range of high accuracy sensors. The first has multiple cores, which he says makes them ideal for measuring how something changes over short distances, by comparing the difference between adjacent cores. One simple application is to measure how a structure bends, where one side of the fiber stretches more than the other. Previously, such a measurement would have required hundreds or even thousands of electrical sensors. The new sensors are already being developed in collaboration with NASA to monitor flexible aerodynamic wings and, closer to home, for safety monitoring of tunnels by measuring changes in their shape.

The second class of fibers Jones described are made of plastic. Glass fibers have their limits, and optical strain gauges could be used in many more situations if only the fibers were more resilient. Modern plastic and composite structures are excellent for saving weight, but need to be monitored for excessive stresses. For this application, plastic fibers are ideal, but only now are they being made sufficiently slender for interferometry.

Third, in situations where communications fibers are still best, scientists can make them even more versatile by constructing tiny structures at their tips. In one such example, they use a powerful laser to drill a hole just thousandths of a millimeter wide in the end of the fibre and then cap it with a lightweight membrane.

“These microsensors may be the fastest-reacting pressure sensors in the world,” Jones said. “And they’re so robust that we’ll be using them to measure blast waves. In the current climate of increased terror threat, there’s a huge demand for technology which could help to design bomb-proof buildings.”

Jones added that the spread of this technology from the laboratory into everyday use has barely begun, and he believes that, in the near future, fiberoptic sensors will begin to be used for applications in power generation, for air and sea guidance systems, and in food safety and medicine.

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