German Researchers Guide Light Along Coreless Photonic Crystal Fibers

By Jof Enriquez,
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Scientists at the Max Planck Institute for the Science of Light (MPL) in Erlangen, Germany are the first to successfully guide light through a coreless photonic crystal fiber (PCF).

Conventional optical fibers feature a light-guiding core made of solid glass, with a higher refractive index than the glass of the enclosing outer cladding, which reflects the light back to the core, where it stays “trapped.” In comparison, instead of being solid throughout, photonic crystal fibers (PCFs) – first made in 1996 by MPL Director Philip Russell and his team – feature hollow channels arranged in a periodic pattern that run along its entire length. These air-filled channels already give the glass a refractive index different from the one it would have if completely solid. A cross section view reveals a PCF to resemble a sieve with holes measuring just one-thousandth of a millimeter across.

Now, MPL researchers have discovered that twisting the hollow channels inside and along the length of a PCF guides the light in the innermost central region of the fiber, demonstrating that a core with a different refractive index is not needed to trap light at the very center of the fiber. This phenomenon is similar to Einstein's general theory of relativity, which posits that light travels along the shortest path between two points in space-time (a geodesic), like the gravitationally curved path around a celestial mass, such as the Sun.

“By twisting the fibre, the ‘space’ in our photonic crystal fibre becomes twisted as well,” explains MPL researcher Dr. Gordon Wong. "This leads to helical geodesic lines along which light travels. This can be intuitively understood by taking into account the fact that light always takes the shortest route through a medium. The glass strands between the air-filled channels describe spirals, which define possible paths for the light rays. The path through the wide spirals at the edge of the fibre is longer than that through the more closely wound spirals in its centre, however, resulting in curved ray paths that at a certain radius are reflected by a photonic crystal effect back towards the fibre axis."

The more the PCF is twisted, the narrower is the space within which the light is confined in a "topological channel," the German scientists found. If purposefully made less twisted at certain intervals along the length of the PCF, light could escape to the outside. This opens up possibilities, such as collecting data over large areas as an environmental sensor.

MPL researchers in 2015 tested a tiny glass bead as a multi-purpose measuring probe and guided it through the central hollow channel of a PCF. The flying bead was able to measure temperature, vibrations, and electric fields with high spatial resolution, as well as radioactivity levels.

The MPL team also experimented on how the PCF can generate light from the ultraviolet to the mid-infrared region of the spectrum. The light produced with a very broad spectrum could facilitate many investigations in biomedical research, in physics and chemistry, according to the scientists.