To artists and romantics, the twinkling of stars is visual poetry; a dance of distant light as it twists and bends through a turbulent ocean of air above our heads.
Not everybody is so enamoured with our atmosphere’s distortions. To many scientists and engineers, a great deal of research and ground-to-satellite communication would be a whole lot easier if the air simply wasn’t there.
Losing our planet’s protective bubble of gases isn’t exactly a popular option. But Australian and French researchers have teamed up to design the next best thing – a system that guides light through the tempestuous currents of rippling air with the flick of a mirror.
The result is a laser link capable of holding its own through the atmosphere with unprecedented stability.
While astronomers have a few tricks up their sleeve to correct for the atmosphere’s distortions on incoming light, it’s been a challenge to emit a coherent beam of photons from the ground to a distant receiver so they keep together and on point.
Keeping transmissions on target and coherent – with their phases remaining neatly in line – through hundreds of kilometres of shifting air would allow us to link highly precise measurement tools and communications systems.
Satellites could probe for ores or evaluate water tables with improved precision. High-speed data transfer could require less power, and contain more information.
Lead author Ben Dix-Matthews, an electrical engineer with the International Centre for Radio Astronomy Research in Australia, explained the technology to ScienceAlert.
“The active terminal essentially uses a small four-pixel camera, which measures the sideways movement of the received beam,” says Dix-Matthews.
“This position measurement is then used to actively control a steerable mirror that keeps the received beam centred and removes the sideways movement caused by the atmosphere.”
In effect, the system can be used to compensate for the warping effects of the moving air in three dimensions – not just up and down, or left and right, but along the beam’s trajectory, keeping the link centred and its phases in order.
So far it’s only been tested across a relatively short distance of 265 metres (about 870 feet). About 715 metres (just under half a mile) of optical fibre cable was run underground between the transmitter and receiver to carry a beam for comparison.
The results were so stable they could be used to connect the kinds of optical atomic clocks used to test fundamental physics, such as Einstein’s theories of relativity.
With the proof of concept demonstrated, there’s no reason to think a similar technique won’t one day be aiming for the sky, and beyond. Though there are a few hurdles that need to be overcome first.
“During this experiment we had to do the initial alignment by hand, using a visible guide laser that was in line with the stabilised infrared beam,” Dix-Matthews told ScienceAlert.
“When making links between optical atomic clocks, it would be good to have a way of doing this coarse alignment more easily.”
Fortunately Dix-Matthews’ French collaborators are working on a device that will speed up the initial coarse alignment process, promising a second generation of laser link technology that won’t require such an involved set-up.
The team also found temperature variations in the equipment affected the phase’s stability, limiting the duration of the signal to around 100 seconds. This hurdle will also be the focus of future improvements.
We might not need to wait long. The researchers are already making headway on upgrades for their system.
“We have started using a high-power laser amplifier that should help us deal with the larger power losses expected over longer distances, such as to space,” says Dix-Matthews.
“We have also completely rebuilt our active terminal to make it more sensitive to low received powers and make it more effective at cancelling out the movement of the received beam.”
With orbiting technology rapidly becoming a major focus for many data providers, potentially filling our skies with satellites, innovations that make linking communications systems across our atmosphere will only become more sought after.
As useful as our atmosphere is for, well, keeping us all alive, there are certainly some downsides to being buried under a restless blanket of warm gas.
This research was published in Nature Communications.
