Extremely Cold And Highly Efficient: Scientists Present New Light Fiber

Researchers have developed a new type of optical fiber by freezing a glass capillary filled with liquid. It guides light and sound waves simultaneously and enables highly efficient coupling between them. The high coupling strength lowers the energy consumption of photonic neuromorphic computing schemes and quantum signal processing applications by several orders of magnitude.

When volcanoes erupt, one can observe the liquid streams of lava cool down and solidify into rock formations at the bottom of the volcanoes. The same physical process – a liquid changing into the solid phase when cooling – can be observed when lakes start freezing over cold winters. These phase changes always come with changes in the physical properties of the material, for example the density or the refractive index that govern how sound and light move through it. This fundamental physical process is also used during the melting of glass preforms to loosen their structure while pulling optical fiber. These fibers then guide light through their cores, allowing the transmission of information via light over long distances very quickly, which is why they are used widely for telecommunication applications. For more specialized applications, for example fiber lasers, fiber endoscopes or fiber sensors, other types of optical fibers have been developed. Hollow core fibers, for instance, can be filled with different gases or liquids and measure temperature distributions, or act as microscopic chemistry labs.

In a collaboration, researchers at the Max Planck Institute of the Science of Light (MPL) in Erlangen, the Leibniz University Hannover (LUH) and the Leibniz Institute for Photonic Technologies (IPHT) in Jena have developed a new type of optical fiber by freezing such liquid core optical fibers (LiCOF) in nitrogen at -196 °C, leading to a phase change in the fiber core from liquid to solid. “The key point is, that the frozen section of the LiCOF retains its ability to guide light. Not only that, but both the liquid and the frozen section of the fiber also guide hypersonic sound waves,” says Simon Seiderer, one of the three lead authors of the article and a researcher in the “Quantum Optoacoustics” research group of Prof. Dr. Birgit Stiller, who leads the project.

The researchers utilize the extremely efficient coupling between light and sound in their new fiber, an effect known as Brillouin-Mandelstam scattering. The effect is already well known in traditional optical fibers, however, with the phase transition to the frozen LiCOF, the researchers create an extreme, highly confined and dense environment. Here, the optoacoustic coupling becomes more than 1000 times stronger than in standard optical fibers. By harnessing this efficient coupling, the researchers demonstrated optoacoustic memory. This fundamental building block for photonic neuromorphic computing in fibers works by utilizing the drastic differences in velocities between light and sound waves. Information is transferred from the fast light wave to the much slower sound waves, and later converted back into light. The efficient optoacoustic coupling in the frozen LiCOF opens up new avenues for drastically reducing the energy consumption of photonic computing architectures.

The project was possible thanks to the established cooperation with Prof. Markus Schmidt and Prof. Mario Chemnitz from the IPHT Jena, who pioneered the research with liquid core optical fibers. With this additional step of freezing the LiCOFs core, higher nonlinearities became possible. “By freezing the liquid core, we have created an entirely new physical platform that provides extreme nonlinearities while being easy to handle,” says Stiller. “While demonstrating a highly efficient optoacoustic memory is a fantastic first step, this level of light-sound coupling not only opens up exciting new possibilities for neuromorphic computing, but also for quantum information processing, microwave photonics and high-precision sensing.”