Using Air to Amplify Light

Illustration of a hollow-core optical fiber. (Source: M. Galal et al., EPFL)

“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.” But now, Luc Thévenaz, the head of the Fiber Optics Group in EPFL’s School of Engi­neering, has finally made it happen: his lab has deve­loped a tech­nology to amplify light inside the latest hollow-core optical fibers.

Today’s optical fibers usually have a solid glass core, so there’s no air inside. Light can travel along the fibers but loses half of its inten­sity after 15 kilometers. It keeps weakening until it can hardly be detected at 300 kilometers. So to keep the light moving, it has to be amplified at regular intervals. Thévenaz’s approach is based on new hollow-core optical fibers 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,” says the professor. 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.”

So what did the researchers do? “We just added pressure to the air in the fiber to give us some controlled resistance,” explains Fan Yang, postdoctoral student. “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 explains.

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

Reference: F. Yang et al.: Intense Brillouin amplification in gas using hollow-core waveguides, Nat. Phot., online 10 August 2020; DOI: 10.1038/s41566-020-0676-z

Link: Group for Fibre Optics, Ecole Polytechnique Fédérale de Lausanne EPFL, Lausanne, Switzerland

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