Light traveling in a distorting medium can appear undistorted

A team led by researchers from the University of the Witwatersrand in Johannesburg, South Africa, with collaborators from the University of Pretoria (South Africa), as well as from Mexico and Scotland, has made a new discovery about how light behaves. in complex media, media that tends to significantly distort the light. They showed that “distortion” is a matter of perspective, outlining a simple rule that applies to all light and a wide variety of media, including underwater, optical fibers, transmission in the atmosphere and even through living biological samples.

Their new quantum approach to the problem resolves an ongoing debate about whether some forms of light are robust or not, and corrects some misconceptions in the community. Importantly, the work outlines that all light has a property that remains unchanged, an insight that is key to unraveling the rest of the perceived distortion. To validate the finding, the team demonstrated robust transport through otherwise highly distorting systems, using the result for error-free communication over noisy channels.

Nature Photonics today published online the research of the Wits team led by Professor Andrew Forbes of Wits University’s School of Physics. In their paper titled: Revealing the invariance of vectorially structured light in complex media, the team explains the simple rules that apply to complex light propagation in complex media. First, they find that all such media can be treated in the same way and that the analysis does not depend on the type of light used. Previously, each choice of media and beam of light was treated as a special case, no longer – the new general theory covers it all. Second, they show that despite the distortion, there is one property of light – its “vectorness” – that remains unchanged, immutable for the media. This is always true and had not been noticed before. It is the key to harnessing light even under non-ideal conditions.

When you pass light through an imperfect medium, such as the atmosphere, it gets distorted. For example, the glittering mirage effect on hot roads or the twinkling of stars are both examples of light being distorted by the turbulence in the atmosphere. Light can also sometimes be intentionally distorted, like the mirrors on a fairground that make you appear taller, thinner, or rounder. In this case, we all understand that the distortion is just a matter of perspective – a quick look at ourselves without the mirror reveals reality – but does this also apply to other distortion systems? Is there a way to look at the light so that the distortion disappears? The Wits-led team shows that yes, some traits are never distorted, while others can be unraveled through a change of perspective.

The question is how to understand what happens to the light, how it is distorted and how to find the new perspective? To answer these questions, the team used the most common form of light, vectorial light. Light has an electrical signal whose direction can vary across the field, sometimes up, down, left, right, and so on. The “vectorness” of a light indicates how confused the direction of a light’s electric field is. In other words, it is a measure of how similar the directions of a light’s electric fields are in different places: if it is the same everywhere (homogeneous) the value is 0, and if it is different everywhere (inhomogeneous) the value is is 1. This vectorial homogeneity never changes, even if the pattern of the electric field itself changes. The reason is embedded in quantum entangled states, a topic that seems to have little in common with optical distortions. The new discovery was made possible by applying tools from the quantum world to the world of optical distortions.

“What we found is that vectority is the only characteristic of light that does not change when it passes through complex media,” said Professor Andrew Forbes of the Wits School of Physics. “This means we have something special that can be exploited when using light for communication or sensing.”

“This is a specific aspect of the pattern of light — what the polarization pattern looks like,” says Forbes. “The ‘polarization’ is just a fancy way of describing the direction of the electric field that makes up light. The pattern is also distorted, but its intrinsic nature (homogeneous or inhomogeneous) is not.

The team’s approach allows researchers to identify how to correct any distortions through the media in a way that doesn’t cost any light. In other words, there is no loss.

“We show that although the light is very distorted, the distortion is only a matter of perspective. One can view the light in such a way that it regains its original ‘undistorted’ properties. It is remarkable that complex light in complex media can be universally understood from very simple rules.”

For example, by simply changing how a measurement is performed, any communication over a highly distorted medium can be made “distortion-free”. The team showed that this was experimentally true through a range of systems, from turbulence to liquid or optical fibers.

Reference:

  1. Isaac Nape, Keshaan Singh, Asher Klug, Wagner Buono, Carmelo Rosales-Guzman, Amy McWilliam, Sonja Franke-Arnold, Ané Kritzinger, Patricia Forbes, Angela Dudley, Andrew Forbes. Revealing the immutability of vectorially structured light in complex media. Nature Photonics, 2022; DOI: 10.1038/s41566-022-01023-w
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