Liquid-core fibers: Data flow in glycerol
Data and signals can be transmitted quickly and reliably with optical fibers - as long as the fiber does not break. Strong bending or tensile stress can quickly destroy it. An Empa team has now developed a fiber with a liquid glycerol core that is much more robust and can transmit data just as reliably. And such fibres can even be used to build micro-hydraulic components and light sensors.
"When it comes to optically conductive polymer fibers, we've tried all kinds of things," says Rudolf Hufenus. "But even with the best solid fiber cores, we can never achieve the kind of elasticity that we can with our liquid-filled fiber." The special combination of optical and mechanical properties could now open up new market niches for Empa's two-component fiber.
To assess what is at stake, a brief panoramic view of the scene helps: fiber optic cables are ideal for data transmission over long distances. The technology is tried and tested and is used on a large scale. But glass fibers can only be bent to a limited extent and are very sensitive to tensile stress. If the glass core of the fiber tears, data transmission is over.
Use liquid core for light transmission?
Plastic fibers are typically used for shorter transmission distances: for individual buildings, company premises or in vehicles. The core of these fibers is often made of PMMA - also known as Plexiglas - or the plastic polycarbonate. These transparent materials are more flexible than glass, but almost as sensitive to tensile forces. "As soon as a microcrack forms in the fiber core, light is scattered by it and lost," explains Hufenus. "So data transmission initially deteriorates, and later the fiber core can even tear completely at this weakened point."
This is where Empa's expertise comes into play: for seven years now, the laboratories of the "Advanced Fibers" research department in St. Gallen have had a machine capable of producing kilometer-long fibers filled with liquid. This know-how makes Empa a world leader. "Two-component fibers with a solid core have been around for over 50 years," says Hufenus. "But to fabricate a continuous liquid core is considerably more complex. Everything has to fit together exactly for it to succeed."
Couldn't this liquid core also be used to transmit light? the Empa researcher wondered. He was following in the footsteps of a good Swiss tradition: it was the Geneva physicist Jean-Daniel Colladon who first conducted light along the inside of a jet of water in 1842 - and thus discovered one of the physical foundations of today's fibre optic technology.
For light conduction in hollow fibers with a liquid core, however, everything must now fit together again. The difference in the refractive index between the liquid and the transparent cladding material is crucial: the refractive index of the liquid must be significantly greater than that of the cladding material. Only then will the light be cleanly reflected at the interface and remain trapped inside the liquid core.
At the same time, all the ingredients must be temperature-stable. "The two components of the fiber must run together through our spinneret under high pressure and at 200 to 300 degrees Celsius," says the Empa researcher. "So we need a liquid with a suitable refractive index for the functionality and with the lowest possible vapor pressure for the production of the fiber." The team decided on a liquid core made of glycerol and a shell made of a fluoropolymer.
Up to 10 percent reversible elongation
The experiment was a success: the fiber produced can withstand up to ten percent elongation and then returns to its original length - no other solid-core optical fiber can do that!
But the fibre is not only extremely stretchable, it can also measure how far it has been stretched. Hufenus and his team added a small amount of fluorescent dye to the glycerol and investigated the optical properties of this luminescent fibre during the stretching process. The result: When the fiber is stretched, the path of the light is lengthened, but the number of dye molecules in the fiber remains constant. This leads to a small change in the color of the emitted light, which can be measured using suitable electronics. In this way, the liquid-filled fiber can indicate a change in length or a tensile load that is occurring.
"We expect that our liquid-filled fibers can be used not only for signal transmission and sensor technology, but also for force transmission in micromotor and microhydraulics," says Hufenus. The exact composition of the fiber sheath and filling can then be specifically adapted to the requirements of the respective application.
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