Light traveling through 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 Mexico and Scotland, has made a new discovery on the behavior of light in complex media, media that tends to distort light considerably. They demonstrated that “distortion” is a matter of perspective, describing a simple rule that applies to all light and a wide range of media, including underwater, fiber optics, transmission in the atmosphere and even through living biological samples.
Their new quantum approach to the problem resolves an ongoing debate over whether or not certain forms of light are robust, correcting some misconceptions in the community. Above all, the work emphasizes that all light has a property that remains unchanged, an idea that holds the key to unraveling the rest of the perceived distortion. To validate the finding, the team showed robust transport through otherwise highly distorting systems, using the result for error-free communication over noisy channels.
Nature Photonics today published the research of the Wits team led by Professor Andrew Forbes of the Wits University School of Physics online. In their article titled: Reveal the invariance of vector structured light in complex media, the team explains the simple rules that govern the complex propagation of light in complex media. First, they find that all these 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 light beam was treated as a special case, this is no longer the case — the new general theory covers everything. Second, they show that despite the distortion, there is a property of light – its “vectority” – which remains unchanged, invariant to the medium. This is still true and had not been noticed before. It holds the key in harnessing the light even in less than ideal conditions.
If you pass light through an imperfect medium, like the atmosphere, it gets distorted. For example, the shimmering mirage effect near hot roads or the twinkling of stars are two examples of light that gets distorted due to the turbulence of the atmosphere. Light can also sometimes be deliberately distorted, like mirrors at a fairground making you look taller, thinner or rounder. In this case, we all understand that distortion is just a matter of perspective – a quick glance at ourselves without the mirror reveals reality – but is this also true in other distorting systems? Is there a way to look at the light so the distortion goes away? The team led by Wits shows that yes, some properties are never distorted, while others can be unraveled by a change in perspective.
The question is how to understand what happens to light, how it is distorted, and how to find the new perspective? To answer these questions, the team used the most general form of light possible, vector light. Light has an electric whose direction can vary across the field, sometimes pointing up, down, left, right, etc. The “vectority” of a light is how the direction of a light’s electric field is mixed. In other words, it is a measure of the similarity of the directions of the electric fields of a light at different places: if it is everywhere the same (homogeneous), the value is 0, and if it is everywhere different (inhomogeneous ), the value is 1. This vector homogeneity never changes, even if the model of the electric field itself changes. The reason is rooted 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’ve discovered is that vectority is the only attribute of light that doesn’t change as it passes through complex media,” says Professor Andrew Forbes, from the Wits School of Physics. “It means we have something special that can be harnessed when using light for communications or sensing.”
“It’s a particular aspect of the pattern of light – what the polarization pattern looks like,” says Forbes. “‘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 distortions across media in a way that costs nothing. In other words, there is no loss.
“We show that even if the light is very distorted, the distortion is just a matter of perspective. Light can be seen in such a way that it regains its original ‘undistorted’ properties. It is remarkable that complex light in complex media can be universally understood based on very simple rules.”
For example, by simply changing the way a measurement is made, any communication over a highly distorted medium can be rendered “distortion-free”. The team showed this to be true experimentally across a range of systems, from turbulence to liquid or fiber optics.