Optical fibers lit in the shape of the world map. 3D image concept of global communication by optical fiber.
Fibre optics may be mainstream, but researchers are making vast improvements in the technology. Photo illustration: Thinkstock

Glass fibres with potential far beyond transmitting light

Fibre optics are at the heart of today’s communication systems, a number of medical devices and more. But when researchers put a silicon-germanium mix at the core of the fibre and treated it, they made something with potential far beyond transmitting light.

Glass fibres do everything from connecting us to the internet to enabling keyhole surgery by delivering light through an endoscope. But as versatile as today’s fibre optics are, scientists around the world have been working to expand their capabilities by adding semiconductor core materials to the glass fibres.

Now, a team of researchers has created glass fibres with single-crystal silicon-germanium cores. The process used to make these could assist in the development of high-speed semiconductor devices and expand the capabilities of endoscopes, says Ursula Gibson, a physics professor at the Norwegian University of Science and Technology and senior author of the paper.

“This paper lays the groundwork for future devices in several areas,” Gibson said, because the germanium in the silicon core allows researchers to locally alter its physical attributes.

Ursula Gibson, a professor of physics at the Norwegian University of Science and Technology, holds a tiny glass fibre in her hand. A new approach could enhance its performance. Photo: Nancy Bazilchuk, NTNU

Ursula Gibson, a professor of physics at the Norwegian University of Science and Technology, holds a tiny glass fibre in her hand. A new approach could enhance its performance. Photo: Nancy Bazilchuk, NTNU

The article, “Laser recrystallization and inscription of compositional microstructures in crystalline SiGe-core fibres,” was published in Nature Communications on October 24.

Melting and recrystallizing

To understand what the researchers did, you need to recognize that silicon and germanium have different melting points. When the two substances are combined in a glass fibre, flecks of germanium-rich material are scattered throughout the fibre in a disorderly way because the silicon has a higher melting point and solidifies, or “freezes” first. These germanium flecks limit the fibre’s ability to transmit light or information. “When they are first made, these fibres don’t look very good,” Gibson said

But rapidly heating the fibre by moving it through a laser beam allowed the researchers to melt the semiconductors in the core in a controlled fashion. Using the difference in the solidification behaviour, the researchers were able to control the local concentration of the germanium inside the fibre depending upon where they focused the laser beam and for how long.

Improving the ability of glass fibres to transmit light is just one of the benefits of the new approach. Photo: Thinkstock

Improving the ability of glass fibres to transmit light is just one of the benefits of the new approach. Photo: Thinkstock

“If we take a fibre and melt the core without moving it, we can accumulate small germanium-rich droplets into a melt zone, which is then the last thing to crystalize when we remove the laser slowly,” Gibson said. “We can make stripes, dots… you could use this to make a series of structures that would allow you to detect and manipulate light.”

When the researchers periodically interrupted the laser beam as it moved along their silicon-germanium fibre, they were also able to make a potentially useful structure. This created a series of germanium-rich stripes across the width of the 150-micrometer diameter core. That kind of pattern creates something called a Bragg grating, which could help expand the capability of long wavelength light-guiding devices. “That is of interest to the medical imaging industry,” Gibson said.

Rapid heating, cooling key

Another key aspect of the geometry and laser heating of the silicon-germanium fibre is that once the fibre is heated, it can also be cooled very quickly as the fibre is carried away from the laser on a moving stage.

Controlled rapid cooling allows the mixture to solidify into a single uniform crystal the length of the fibre — which makes it ideal for optical transmission.

Previously, people working with bulk silicon-germanium alloys have had problems creating a uniform crystal that is a perfect mix, because they have not had sufficient control of the temperature profile of the sample.

“When you perform overall heating and cooling, you get uneven composition through the structure, because the last part to freeze concentrates excess germanium,” Gibson said. “We have shown we can create single crystalline silicon-germanium at high production rates when we have a large temperature gradient and a controlled growth direction.”

Transistors that switch faster

Gibson says the laser heating process could also be used to simplify the incorporation of silicon-germanium alloys into transistor circuits.

“You could adapt the laser treatment to thin films of the alloy in integrated circuits,” she said.

Using a glass fibre with a siicon-germanium core that has been treated could led to faster silicon-based transistors. Photo: Thinkstock

Using a glass fibre with a siicon-germanium core that has been treated could led to faster silicon-based transistors. Photo: Thinkstock

Traditionally, Gibson said, electronics researchers have looked at other materials, such as gallium arsenide, in their quest to build ever-faster transistors. However, the mix of silicon and germanium, often called SiGe, allows electrons to move through the material more quickly than they move through pure silicon, and is compatible with standard integrated circuit processing.

“SiGe allows you to make transistors that switch faster” than today’s silicon-based transistors, she said, “and our results could impact their production.”

Gibson’s collaborators are at the KTH Royal Institute of Technology in Sweden, Newcastle University and the University of Southampton in the UK and Clemson University in the USA.

Reference: Laser recrystallization and inscription of compositional microstructures in crystalline SiGe-core fibres. David A. Coucheron, Michael Fokine, Nilesh Patil, Dag Werner Breiby, Ole Tore Buset, Noel Healy, Anna C. Peacock, Thomas Hawkins, Max Jones, John Ballato & Ursula J. Gibson. Nature Communications 7, 13265 (2016) doi:10.1038/ncomms13265.