Scientists Apply Einstein's Theory to Guide Light Through Photonic Crystal Fibers

Coreless optical fiber: If a photonic crystal fiber is twisted, it does not require a core with a different refractive index to trap light at its centre. CREDIT: Science 2016/MPI for the Science of Light.

An innovative mechanism to guide light through photonic crystal fiber (PCF) has been developed by scientists from the Max Planck Institute for the Science of Light in Erlangen.

PCF is a glass fiber with hair-like thickness and comprising a systematic array of hollow channels arranged along its length. As outlined by the general theory of relativity, the spiraling array of hollow channels, when helically twisted, influence the light rays analogous to bending of the rays when the rays move through the gravitationally curved space surrounding a star.

With respect to light, optical fibers act as pipes. Similar to the inside of a pipe being surrounded by a wall, optical fibers generally include a light-guiding core made of glass with higher refractive index than the glass with which the enclosing outer cladding formed.

This difference in the refractive index is the reason for the reflection of light at the cladding interface and subsequent trapping of the light in the core similar to water in a pipe. For the first time, a research group led by Philip Russell, Director at the Max Planck Institute for the Science of Light, has been triumphant in guiding light through a PCF that has no core.

Photonic crystals give butterflies their color and can also guide light

In a conventional photonic crystal, there is a piece of glass with holes that are positioned throughout its volume in a regular periodic pattern. The difference in the refractive indices of glass and air results in a periodic structure of refractive index of the material, and hence the name crystals - atoms in such materials form an ordered, three-dimensional lattice similar to that seen in crystalline salt or silicon, for instance.

In a conventional crystal, the accurate 3D structural design governs the electron behavior, resulting in, for instance, conductors, electrical insulators, and semiconductors.

Likewise, the periodic 3D microstructure largely decides the optical characteristics of a photonic crystal, where the 3D microstructure is the reason behind the shimmering colors of specific butterfly wings, for instance. The ability to regulate the optical characteristics of materials is very helpful in a broad array of applications.

For instance, the photonic crystal fibers created by Philip Russell and his colleagues at the Max Planck Institute can be applied to filter particular wavelengths from the visible spectrum or even to generate very white light.

Similar to all optical fibers used for telecommunication purposes, all the typical photonic crystal fibers include a core and a cladding with differing refractive indices or optical characteristics. The air-filled channels arranged in a PCF provide the glass with a refractive index different from that of a completely solid glass.

The holes define the space in a photonic crystal fiber

“We are the first to succeed in guiding light through a coreless fibre,” stated Gordon Wong, a scientist at the Max Planck Institute for the Science of Light in Erlangen. Philip Russell and his team have developed a photonic crystal fiber with the entire cross-section closely packed with innumerable air-filled channels extending along its entire length. Diameter of each channel is nearly one-thousandth of a millimeter.

In contrast to the core of a traditional PCF, which is solid glass, the cross-sectional view of the new optical fiber is identical to a sieve. The holes are arranged so that they have regular separations and a regular hexagon of adjoining holes surrounds each hole.

“This structure defines the space in the fibre,” explained Ramin Beravat, lead author of the study. The holes can be regarded similar to distance markers, and the inner structure of the fiber includes a type of artificial spatial structure formed by the regular lattice of holes.

“We have now fabricated the fibre in a twisted form,” added Beravat. The twisting renders the hollow channels to be wound around the fiber’s length in helical lines. Then, the research team transmitted laser light into the fiber.

For a regular, coreless cross-section, the light is originally anticipated to be evenly distributed itself between the holes in the sieve based on their pattern, that is, they are distributed nearly similar at the edge as well as at the center. However, the researchers achieved astonishing findings: the light was found to be concentrated in the central region at which the core of a traditional optical fiber is positioned.

In a twisted PCF, the light follows the shortest path in the interior of the fiber

“The effect is analogous to the curvature of space in Einstein’s general theory of relativity,” explained Wong. This shows that a heavy mass like the Sun is sure to distort the space around it similar to distortion of a lead sphere placed into a rubber sheet.

To state it more clearly, there is a space-time distortion, that is, distortion of the combination of the three spatial dimensions with the fourth dimension, i.e. time. This curvature is followed by light, and the shortest path between the two points no longer remains a straight line, but transforms into a curve.

This phenomenon is reason why stars which are actually expected to be hidden behind the Sun during a solar eclipse really become visible. Such shortest connecting paths are termed as “geodesics” by the physicists.

“By twisting the fibre, the ‘space’ in our photonic crystal fibre becomes twisted as well,” stated Wong, which results in the formation of helical geodesic lines along which light travels. This phenomenon can be instinctively perceived by considering that light always travels in the shortest route through a medium.

The glass strands positioned between the air-filled channels are in the form of spirals defining probable paths for the light rays. However, the path along the wider spirals located at the edge of the fiber is longer when compared to the path through the more tightly wound spirals at the center, which leads to curved ray paths that are reflected by a photonic crystal effect, at a specific radius, back toward the axis of the fiber.

A twisted PCF as a large-scale environmental sensor

The extent to which the fiber is twisted is directly proportional to the narrowness of the space within which the light is concentrated. When comparing this phenomenon with Einstein’s theory, it denotes to a stronger gravitational force and consequently a greater deflection of light. The researchers report that they have developed a “topological channel” for the passage of light. (Topology is related to the characteristics of space that are conserved under continuous distortion.)

The researchers reiterate that the study is just a basic research, and that they are among very few research teams across the globe conducting research in this area. However, the researchers can put their findings to good use in various applications. For instance, a twisted fiber that is twisted less at specific intervals can permit a portion of light to escape to outer side.

Then, the light will have the potential to interact with the surroundings at such defined locations. “This could be used for sensors which measure the absorption of a medium, for instance.” Data can be collected by a network of these fibers, functioning as an environmental sensor, over larger areas.

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