EPFL team develop way to amplify light in hollow-core optical fibres

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A hollow-core optical fibre. Credit: Malak Galal, Fan Yang, Flavien Gyger, Luc Thévenaz/EPFL

As described in a Nature Photonics article published on 10 August, EPFL researchers have developed a technology that can amplify light in the latest hollow-core optical fibres. 

Today's optical fibres have a solid glass core, so there’s no air inside. Light can travel along the fibres but loses half of its intensity after 15 kilometres. It keeps weakening until it can hardly be detected at 300 kilometres. So to keep the light moving, it has to be amplified at regular intervals. 

A team led by Luc Thévenaz, head of the Fibre Optics Group in EPFL’s School of Engineering, has developed a technology to amplify light inside the latest hollow-core optical fibres.

'The idea had been going around my head for about 15 years, but I never had the time or the resources to do anything about it,' he said. 

Thévenaz’s approach is based on new hollow-core optical fibres that are filled with either air or gas. 'The air means there’s less attenuation, so the light can travel over a longer distance. That’s a real advantage,' he said. But in a thin substance like air, the light is harder to amplify. 'That’s the crux of the problem: light travels faster when there’s less resistance, but at the same time it’s harder to act on. Luckily, our discovery has squared that circle.' 

The team added pressure to the air in the fibre to create a controlled resistance. Fan Yang, postdoctoral student, explained: 'It works in a similar way to optical tweezers – the air molecules are compressed and form into regularly spaced clusters. This creates a sound wave that increases in amplitude and effectively diffracts the light from a powerful source towards the weakened beam so that it is amplified up to 100,000 times.' Their technique therefore makes the light considerably more powerful. 'Our technology can be applied to any type of light, from infrared to ultraviolet, and to any gas,' he explained. 

An extremely accurate thermometer

Going forward, the technology could serve other purposes in addition to light amplification. Hollow-core or compressed-gas optical fibres could, for instance, be used to make extremely accurate thermometers. 'We’ll be able to measure temperature distribution at any point along the fibre. So, if a fire starts along a tunnel, we’ll know exactly where it began based on the increased temperature at a given point,' said Flavien Gyger, PhD student. The technology could also be used to create a temporary optical memory by stopping the light in the fibre for a microsecond – that’s 10 times longer than is currently possible.

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