The new temperature-monitoring devices should be able to take the heat. Credit: © Punchstock

A trio of newly developed sensors could help make buildings and transport infrastructure safer from fire or structural failure, say their creators. The fibre-optic sensors can survive situations of extreme heat and pressure, where ordinary electronic ones might fail.

Most modern sensors use electronic circuits to collect and convey information. Temperature sensors, for example, monitor the voltage at a junction of two different metals. As the conditions get warmer, the electrical properties of these materials change and alter the electric flow.

Systems such as these can alert people to irregularities, but they lose function when exposed to strong electromagnetic fields and extreme heat. A team of researchers led by Julian Jones of Heriot-Watt University in Edinburgh, UK, is developing a radically different class of sensors that still work under these conditions.

Seeing the light

Jones and his colleagues turned to optical fibres, which have transformed telecommunications since they became available in the late 1970s. Made of glass and typically as thin as a human hair, they carry information as pulses of light along the fibre because the acute angle at which the light hits the fibre's walls ensures that it does not escape.

Fibre-optic sensors are already used to keep tabs on wing strain in airplanes and to detect movement in bridges. The new trio of sensors offer even more possibilities, such as investigating the strain to which structures are subjected in explosions.

Jones's team worked on a temperature sensor that works by assessing how far the light travels in the optical fibres at different temperatures. Light is emitted from a laser diode similar to those in compact disc players. The beam is then split, and half travels down a normal optical fibre while the other half goes through a fibre specially engineered to lengthen with increasing temperature.

The two parcels of light eventually recombine in a single channel. By measuring how the two sets of light waves match up, researchers can measure the difference in the distance they travelled and thus any temperature changes.

Shaping up

Another of the team's fibre-optic sensors, developed in collaboration with NASA, tackles the problem of measuring change in shape. Whereas typical telecommunications fibres contain only one core pipe - which carries the light inside a protective tube - the new sensors have four. With this inner bundle of tubes, the cross-section of the beefed-up fibre resembles a four-leaved clover.

If the fibre is pulled to one side, the cores inside become distorted to different degrees. The sensor picks up on any such strain by contrasting the length that light must travel in each of the cores. Jones says that this technology could detect signs of earth movement inside railway tunnels - a task that currently requires an unfeasibly large array of electronic gauges.

The third type of sensor offers a new way to monitor pressure changes. To make the sensors, the engineers used a powerful laser to drill a hole in one end of the optical fibre and capped it with a lightweight membrane. This creates an airspace that acts as a pressure sensor, bending in and out like a drum. Jones presents the sensors this week at a meeting of the Institute of Physics in Warwick, UK.