Scientists go mad for optical sensors

How to tell if your building is exploding

Physicists at Heriot-Watt university have developed a robust new type of sensor that could help engineers design buildings capable of withstanding huge explosions. Professor Julian Jones, who led the research, presented the findings at the Physics 2005 conference in Warwick today.

The specially engineered optical fibres can monitor blast-waves from high explosives, structural safety in tunnels, bridges and buildings, bending in critical aircraft components, and deterioration in weapons stockpiles.

Electronic sensors generally work on the principle that temperature, pressure or stress affects the electrical behaviour of the sensor. They are not always safe to use in explosive environments, are not well suited for some medical applications, and are prone to interference when used in strong electromagnetic fields.

The new breed of sensors developed at Heriot-Watt are not so limited by their environment, because they use light rather than electricity to take their measurements.

The underlying principle is fairly straightforward: the fibres work in pairs: one to take the measurement, the other to act as a reference beam. Light travels down the length of the fibres and is reflected back to its origin. The interference pattern produced by the merging of two beams provides information about the nature of the environment the sensor is in.

Professor Jones suggests that optical fibres can be custom designed for specific sensing tasks, and has developed three specially engineered kinds of fibre that illustrate his ideas.

The first class of fibre has multiple cores, with gratings etched along the length of the fibre to produce a mirror tuned to a particular colour of light. When white light is sent along the fibre and its component colours will be reflected from different gratings, providing information about how the fibre is bending.

The second type of fibre is made from plastic, rather than glass, making it suitable for use as an optical strain gauge, measuring the stresses in plastic and composite structures, for example. It is only recently that plastic fibres have become slim enough to replace glass fibres for this kind of application.

Finally, the researchers have developed new, highly responsive pressure sensors. These are made from standard communication optical fibres with a tiny hole drilled, by laser, into the tip, and then capped with a lightweight membrane.

Professor Jones says he thinks they could be the fastest-reacting pressure sensors in the world. "They're so robust that we'll be using them to measure blast waves," he went on. "In the current climate of increased terror threat, there’s a huge demand for technology which could help to design bomb-proof buildings." ®

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