A sensational prospect in advanced optics is to manipulate “patterns of light,” i.e., how the light appears in its numerous degrees of freedom.
Image Credit: University of the Witwatersrand
Each pattern can develop an encoding alphabet for optical communication or might be employed in manufacturing to improve productivity and performance. Regrettably, patterns of light become twisted when they travel via noisy channels, for example, aberrated optics, turbid living tissue, or stressed optical fibers.
In the examples mentioned above, the twisted pattern can worsen to the point where the output pattern appears nothing like the input.
Now, scientists from the University of the Witwatersrand (Wits University) in South Africa have demonstrated how it is possible to discover distortion-free patterns of light that travel through a noisy channel the same as they were sent in.
Using atmospheric turbulence as an illustration, they revealed that these unique patterns of light, known as eigenmodes, can be identified in even highly complex channels undistorted, where other patterns of structured light would be distorted. Their study has been published in the journal Advanced Photonics, the flagship journal of SPIE, the global society for optics and photonics.
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.
Andrew Forbes, Professor and Head, Structured Light Laboratory, University of the Witwatersrand
Usually, a trial-and-error method is applied to discover the most robust patterns of light, but thus far all patterns of familiarly structured light appear to be distorted as the medium becomes increasingly noisier. The reason is that distortion can be “seen”.
To prove if it is feasible to form light that does not “see” the distortion traveling through, the scientists assumed the noisy channel to be a mathematical operator and asked a basic question: “what forms of light would be invariant to this operator?”.
Simply put, what patterns of light appear as the natural mode of the channel that it is in, such that it does not see the distortion? This can also be called the true eigenmodes of the channel.
The answer to the issue exposed unfamiliar patterns of light—simply put, light that does not belong to any well-established structured light family but is still totally robust to the medium. This fact was verified theoretically and experimentally for strong and weak turbulence settings.
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.
Andrew Forbes, Professor and Head, Structured Light Laboratory, University of the Witwatersrand
Owing to the nature of eigenmodes, it does not matter how long this medium is, nor how robust the perturbation is, it should function well even in systems where conventional corrective processes, such as adaptive optics, do not work.
“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,” Forbes added.
Journal Reference
Klug, A., et al. (2023) Robust structured light in atmospheric turbulence. Advanced Photonics. doi.org/10.48550/arXiv.2205.07641.