Researchers at the University of the Witwatersrand (Wits) and the University of Bordeaux in France have demonstrated a new way of sending information through the atmosphere without having to correct for the distortions that normally disrupt optical communication.
In a world-first experiment conducted across the Wits West Campus in Johannesburg, the researchers showed that information encoded in a special property of light known as topology remains intact even when the laser beam carrying it is strongly distorted by atmospheric turbulence.
The findings, published in Science Advances , could help pave the way for more reliable long-distance optical communication, including links to satellites and spacecraft and, potentially, improved connectivity in remote and underserved areas.
Information that survives distortion
Modern communication systems use different properties of light, including colour, intensity and polarisation, to carry information. Fibre-optic cables protect these signals from many outside disturbances, but sending light through open air is more difficult.
Changes in air temperature and pressure create atmospheric turbulence, which can distort a laser beam as it travels. This can damage the information it carries.
The Wits-led research takes a different approach by encoding information in topology — a mathematical idea describing properties that remain unchanged even when an object is stretched or deformed.
“To explain its benefits, we can liken it to how a coffee mug can be reshaped into the form of a doughnut,” says Prof Andrew Forbes, Head of the Structured Light Lab in the Wits School of Physics.
“Despite their very different shapes, both have a single hole. You can stretch or distort them without changing that fundamental property. In the same way, the light beam can become badly distorted while its topological information remains unchanged.”
The researchers used an optical structure known as a skyrmion to encode this information into laser beams.
They then transmitted the beams between two buildings on the Wits campus over a distance of hundreds of metres, exposing them to naturally occurring atmospheric turbulence.
Although the shape of the light arriving at the receiver was significantly different from the transmitted beam, the information contained in its topology remained intact.
“Small laboratory experiments have previously suggested that topology could provide a robust way of carrying information,” says lead author Cade Peters. “This is the first time we have demonstrated that robustness across a real-world optical link and under naturally occurring atmospheric conditions.”
No need to correct the beam
Conventional free-space optical communication systems often need to measure atmospheric distortion and then compensate for it using specialised hardware and complex calculations.
The new approach does not require the distortion to be measured or corrected before the information can be recovered.
This could make future systems simpler and reduce the technical and computing demands involved in transmitting information through difficult environments.
“The ability to encode and transmit information using light has transformed the way the world communicates,” says Prof Mitchel Cox of the Optical Communications Laboratory in the Wits School of Electrical and Information Engineering.
“As our demand for data continues to grow, we need new ways to increase the capacity, reliability and performance of communication systems. This work shows that topology is a largely untapped resource that could contribute to the next generation of optical communications.”
The researchers also explored applications for both conventional and quantum communication, where maintaining the integrity of information is particularly important.
For South Africa, Peters says the research could eventually have wider social benefits.
“South Africa still faces major challenges in providing reliable access to information and communication technologies, particularly in remote and underserved communities,” he says.
“By developing new approaches to long-range optical communication, we hope this work can contribute to the broader effort to bridge the digital divide.”
Science Advances