Shake it off: taking the twinkle out of a laser beam

We steered a laser beam through atmospheric turbulence to show how this technology could benefit fundamental science and spacecraft communications.

Light can carry a lot more information than radio waves can. The bandwidth (data rate) of a transmission is limited by the frequency of that transmission. This is the main difference between 4G and 5G mobile phone technology — 5G uses higher frequencies, and so can transmit a lot more data. Lasers are much higher in frequency than any radio wave, and so can transmit data thousands of times faster. This is why the internet uses lasers transmitted through fibre optic cables to haul vast amounts of data across continents. The higher frequency of lasers also means that scientific measurements made with lasers are much higher precision.

Trading radio transmitters for laser links promises to revolutionize scientific measurements and communications with spacecraft.

The most precise scientific measurements are currently provided by optical atomic clocks, which use ultra-stable frequencies of light to make ultra-precise timing measurements. With a network of these clocks in space and on the ground, we can make precise tests of fundamental physics, such as Einstein’s General Theory of Relativity, searches for dark matter, and measurements for geoscience and precision navigation.

Laser links from spacecraft to the ground would also overcome a massive bottleneck facing satellite operators. As technology improves, each satellite is generating more data, and as launch costs come down, we are launching more and more of them. Radio communications do not have enough bandwidth to get all of the valuable data down to the ground.

Laser links are the only way to compare modern optical atomic clocks at their full precision. However, when trying to link clocks on spacecraft to clocks on the ground, atmospheric turbulence distorts the laser beam, causing it to fade, and also introduces timing errors.

The fading of the laser beam also severely limits the data rate that can be achieved with laser communications from spacecraft.

To make full use of the atomic clocks, and enable high-bandwidth laser communications, we need to develop systems that can counteract the effects of atmospheric turbulence on the laser beam.

We combined optical phase stabilization technology, which suppresses the timing errors caused by atmospheric turbulence, with a small adaptive optics system which detects and counteracts the distortion and deflection of the laser beam.

We setup a 2.4 km laser link between two buildings, and measured the ability of these systems to stabilize our laser signal through the turbulent air. While 2.4 km is much less than the roughly 1,000 km we would need to achieve a link to a spacecraft, because the atmosphere rapidly gets less dense as you increase altitude, 2 km of air horizontally at ground level has the same amount of turbulence as a link up through the atmosphere to space.

We achieved world-record phase stability for our laser and showed, thanks to this horizontal link having very similar turbulence to a ground-to-space link, that the stability of our laser would be easily enough to compare atomic clocks between spacecraft and the ground. Our adaptive optics system greatly reduced the number of times the laser faded out because of turbulence, allowing us to establish a more robust and stable laser link, and which could support much higher data rates.

However, spacecraft move, meaning a robust and stable link to a satellite is much harder than a link between buildings. We are now improving our optics and control systems in order to track and lock onto a drone that we will use to fly simulated satellite passes. Because spacecraft move so quickly (7.6 km/s in low Earth orbit), a laser signal coming down from a spacecraft passes through different turbulent air to a signal going up from the ground. We will also need to investigate what effect this has on our system’s ability to suppress the timing noise caused by atmospheric turbulence.

The technologies we have developed pave the way to robust and stable laser links between spacecraft and the ground, and will enable ultra-precise scientific measurements and high-speed laser communications, which will revolutionize a wide range of fundamental and applied science. We are currently commissioning the Western Australian Optical Ground Station, the first laser communications ground station in the southern hemisphere, to test and develop these technologies, with the aim of establishing practical laser links to satellites in the next few years.

The published paper is available here.
You can read more about this work here.