Use air to amplify light

In a promising breakthrough for the future of communications, EPFL researchers have developed a technology that can amplify light in the latest optical head-core fibers.

“The idea had been on my mind for about 15 years, but I never had the time or the means to do it,” said Luc Thévenaz, head of the Fiber Optics Group at EPFL’s School of Engineering. Now his laboratory has developed a technology to amplify light in the latest optical head-core fibers.

Square the circle

Today’s optical fibers usually have a solid glass core with no air inside. Light can travel along the fibers, but loses half of its intensity after 15 kilometers. It remains attenuated until it can barely be detected at 300 kilometers. So to keep the light moving, it needs to be amplified at regular intervals.

Thévenaz’s approach is based on new optical fibers with hollow cores filled with air or gas. “The air means there is less attenuation, so the light can travel over a longer distance. That is 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 is less resistance, but at the same time it’s harder to act on. Fortunately, our discovery has squared that circle.”

From infrared to ultraviolet

So what did the researchers do? “We just added pressure to the air in the glass fiber to give us some controlled resistance,” explains Van Yang, a postdoctoral fellow. It works in the same way as optical tweezers – the air molecules are compressed and form into regularly spaced clusters. This creates a sound wave that increases in amplitude and effectively diffuses the light from a powerful source to the attenuated 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. Their findings have just been published in Nature Photonics.

A very accurate thermometer

Going forward, the technology could serve other purposes besides light amplification. Hollow-core as well as compressed gas optical fibers could be used, for example, to make extremely accurate thermometers. “We will be able to measure temperature distribution at any point along the glass fiber. So when a fire starts along a tunnel, we will know exactly where it started based on the elevated temperature at a particular point,” says Flavien Gyger, Ph.D. . student. The technology could also be used to create a temporary optical memory by stopping the light in the glass fiber for a microsecond – which is ten times longer than is currently possible.