Fibre-optic cables, such as those used in internet infrastructure, could be used for eavesdropping. A device that can pick up tiny changes in signals sent through the cables can detect words spoken from over a kilometre away.
Optical fibres use beams of light to transport data across the world, underground and in the oceans. Researchers have previously found that these cables can also be used as sensors 鈥� for example, to detect earthquakes and track whales. Now, at Tsinghua University in China and his colleagues have built a device that uses optical fibres to listen in on people conversing close to them.
The device works by detecting changes in light that occur when someone speaks near an optical fibre. To test it, the researchers spoke the phrase 鈥渋t鈥檚 nine-fifteen鈥� near a fibre-optic cable that was transmitting data. About 3 metres of the fibre was exposed to the sound. The remaining 1.1 kilometres of cable was wound on a spool in another room where the device was connected.
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The team then listened to the sound waves picked up from the device. Though the words weren鈥檛 perfectly clear, Wang says they could be made easier to understand with existing computer speech enhancement methods.
Fibre-optic cables make great sensors because as light travels through the cable small changes in its properties accumulate and get amplified enough to be detectable, says at Delaware State University.
鈥淥ptical fibres are very sensitive to vibration so any fibre placed in a building is actually a sort of microphone that can tap any kind of conversation,鈥� says at the Swiss Federal Institute of Technology in Lausanne. Both he and Marcano Olaizola independently researched the idea 20 years ago, but most people are unaware of the possibility, he says.
Wang and his team had originally hit upon the idea themselves while trying to build a different fibre-optic sensor but found the sound of their voices kept showing up in the readings.
Though the new device is complicated, Wang says that a person with a background in engineering and enough resolve could use their method to spy on someone without having to install gadgets inside their home as long as there were some exposed fibre-optics lying around.
This could be prevented by adjusting the existing metal or glass coating of fibres to make them less sensitive to sound vibrations, he says. For instance, when he and his team wrapped steel wire around the fibres used in their lab, the device could no longer pick up any meaningful sound waves.
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