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Green lantern shoots straight

Scientists have set a world record for the most stable transmission of a laser signal through the atmosphere, effectively eliminating atmospheric turbulence.

While twinkling stars and shimmering mirages over hot roads are fascinating sights, many scientists would find research a whole lot easier if these phenomena didn’t exist. The atmosphere is composed of pockets of air of varying temperatures, which distorts the path of any optical signal that passes through.

But these twists and turns of light make it difficult for astronomers attempting to accurately study the cosmos, or engineers communicating with satellites.

Now, in a study published this month in the journal Nature Communications, a joint Australian-French research team has developed the technology to send laser signals from one point to another with record-breaking stability.

“We can correct for atmospheric turbulence in 3D – that is, left-right, up-down and, critically, along the line of flight,” says lead author Benjamin Dix-Matthews, from the International Centre for Radio Astronomy Research (ICRAR) at the University of Western Australia (UWA).

“It’s as if the moving atmosphere has been removed and doesn’t exist.”

Benjamin dix-mathhews

In the early months of 2020 – before international travel became a distant fantasy – the Australian team from flew from Perth to France to test this technology at the French National Centre for Space Studies (CNES) in Toulouse.

They took with them Australian-developed equipment, including phase stabilisation technology created for the Square Kilometre Array (SKA) and self-guiding optical terminals. Dix-Matthews describes each terminal as “a fancy telescope with a few extra abilities” – including using tip-tilt mirrors to correct the wandering laser light.

They set these terminals up on the rooftops of two buildings 265 metres apart and sent laser signals between them. Crucially, the buildings were also linked by a 715 metre underground optical fibre cable, which was used to measure the performance of the system in an independent way.

Dix-Matthews explains that each terminal is equipped with “a position sensitive diode – basically a small camera – that…can see the spot of the light being moved around as the atmosphere moves”.

One of the self-guiding optical terminals on its telescope mount at the CNES campus in Toulouse. Credit: ICRAR/UWA

These terminals then cancel out these movements, maintaining the quality of the original laser signal. The tests proved that the system is, to date, the most precise and stable method of transmitting laser signals.

While the system has many similarities to adaptive optics – which use sophisticated deformable mirrors to remove atmospheric distortions from astronomical observations – it focuses more precisely on removing lateral movement and has different applications. It’s designed for transmitting optical signals rather than imaging.

Co-author Sascha Schediwy, an ICRAR-UWA researcher, explains that the technology can be used to make ultra-precise measurements across many fields. 

“If you have one of these optical terminals on the ground and another on a satellite in space, then you can start to explore fundamental physics,” he says. “Everything from testing Einstein’s theory of general relativity more precisely than ever before, to discovering if fundamental physical constants change over time.”

It could also be used in earth sciences, improving satellite studies of the water table, or to look for underground ore deposits.Stable laser signals will also be a boon to high-speed optical communications, which uses light to carry information.

Currently, radio waves are the dominant form of communication with satellites, but optical communications could securely transmit data between satellites and the Earth with much higher precision.

“Our technology could help us increase the data rate from satellites to ground by orders of magnitude,” Schediwy says. “The next generation of big data-gathering satellites would be able to get critical information to the ground faster.”