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Structured Light Advancement Clears Way to Image Through Noisy Channels

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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 distinct pattern could form an encoding alphabet for optical communication, or each 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 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.
When light passes through a noisy channel such as the atmosphere, it gets distorted, but there exist complex forms of light that come out distortion-free, so that the output pattern is the same as the input. Courtesy of Klug, Peters, and Forbes, doi: 10.1117/1.AP.5.1.016006.
When light passes through a noisy channel such as the atmosphere, it gets distorted. Researchers in South Africa targeted complex forms of light that come out of a noisy channel distortion-free so that the output pattern is the same as the input. The work points to future work in imaging and communicating through noisy channels, which is particularly relevant when the structured forms of light are fragile quantum states. Courtesy of Klug et al., doi: 10.1117/1.AP.5.1.016006.
Researchers in South Africa have shown how to find distortion-free forms of light that come out of a noisy channel identical to how they were put in. Using atmospheric turbulence as an example, they showed that these special forms of light, called eigenmodes, can be found for even very complex channels, emerging undistorted, while other forms of structured light would be unrecognizable.


In applications such as free-space optics, sensing, and energy delivery, it’s necessary to pass light through the atmosphere. Traditionally, a trial-and-error approach has been used to find the most robust forms of light in some particular noisy channel. To date, however, all forms of familiar structured light have shown to be distorted as the medium becomes progressively noisier.  

In experiments, the researchers addressed the example of the severe case of distortion due to atmospheric turbulence. They revealed previously unrecognized forms light, that is, not in any well-known structured light family, but nevertheless completely robust to the medium. They then confirmed their results experimentally and theoretically for weak and strong turbulence conditions.

Corresponding author Andrew Forbes, distinguished professor at the University of the Witwatersrand in Johannesburg, said, “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.

“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, which is particularly relevant when the structured forms of light are fragile quantum states.

The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.5.1.016006).

Published: February 2023
Glossary
structured light
The projection of a plane or grid pattern of light onto an object. It can be used for the determination of three-dimensional characteristics of the object from the observed deflections that result.
adaptive optics
Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effects of atmospheric distortions. The Earth's atmosphere can cause light passing through it to experience distortions, resulting in image blurring and degradation in various optical applications, such as astronomical observations, laser communications, and imaging systems. Adaptive optics systems actively adjust the optical elements in real-time to compensate for these distortions. Key...
distortion
A general term referring to the situation in which an image is not a true-to-scale reproduction of an object. The term also is used to connote the temporal alteration of the signal's waveform shape. There are many types of distortion. See also anamorphic distortion; curvilinear distortion; keystone distortion; panoramic distortion; perspective distortion; radial distortion; stereoscopic distortion; tangential distortion; wide-angle distortion.
noise
The unwanted and unpredictable fluctuations that distort a received signal and hence tend to obscure the desired message. Noise disturbances, which may be generated in the devices of a communications system or which may enter the system from the outside, limit the range of the system and place requirements on the signal power necessary to ensure good reception.
Research & TechnologyOpticsLasersstructured lightadaptive opticsdistortioncomplex medianoiseinputoutputUniversity of WitwatersrandAfricaadvanced photonics

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