An exciting prospect in modern optics is to exploit patterns of light, how the light looks in its many degrees of freedom, often referred to as structured light. Each pattern could form an encoding alphabet for optical communi­cation or might be used in manu­facturing to enhance performance and produc­tivity. Unfortunately, patterns of light get distorted when they pass through noisy channels, for instance, stressed optical fiber, aberrated optics, turbid living tissue, and perhaps a very severe example, atmospheric turbu­lence in air. 

In all these examples, the distorted pattern can deteriorate to the point that the output pattern looks nothing like the input, negating the benefit. Now researchers from the University of the Witwaters­rand in South Africa have shown how it is possible to find distortion-free forms of light that come out of a noisy channel exactly the same as they were put in. Using atmo­spheric turbulence as an example, they showed that these special forms of light, the eigenmodes, can be found for even very complex channels, emerging undistorted, while other forms of structured light would be unrecog­nisable.

“Passing light through the atmo­sphere is crucial in many appli­cations, such as free-space optics, sensing and energy delivery, but finding how best to do this has proved challenging,” says Andrew Forbes, head of the Structured Light Laboratory at Wits University. Tradi­tionally a trial-and-error approach has been used to find the most robust forms of light to some particular noisy channel, but to date all forms of familiar structured light have shown to be distorted as the medium become progres­sively more noisy. The reason is that we see the distortion.

To establish whether it is possible to create light that doesn’t see the distortion, passing through as if it wasn’t there the researchers treated the noisy channel as a mathe­matical operator and asked a simple question: “what forms of light would be invariant to this operator?”. In other words, what forms of light appear as the natural mode of the channel that it is in, so that it don’t see the distortion. This can also be called the true eigenmodes of the channel.  

The example tackled was the severe case of distortions due to atmo­spheric turbulence. The answer to the problem revealed unrecog­nizable forms of light – light that is not in any well-known structured light family, but nevertheless completely robust to the medium. This fact was confirmed experimentally and theo­retically for weak and strong turbu­lence conditions. “What is exciting about the work is that it opens up a new approach to studying complex light in complex systems, for instance, in transporting classical and quantum light through optical fiber, underwater channels, living tissue and other highly aber­rated systems,” says Forbes. 

Because of the nature of eigenmodes, it doesn’t matter how long this medium is, nor how strong the pertur­bation, so that it should work well even in regimes where tradi­tional corrective procedures, such as adaptive optics, fail. “Main­taining the integrity of structured light in complex media will pave the way to future work in imaging and communi­cating through noisy channels, parti­cularly relevant when the structured forms of light are fragile quantum states.” (Source: U. Wits)

Reference: A. Klug et al.: Robust structured light in atmospheric turbulence, Adv. Phot. 5, 016006 (2023); DOI: 10.1117/1.AP.5.1.016006

Link: Structured Light Laboratory, University of the Witwatersrand, South Africa