A Cloud-Piercing Laser

Lasers can bore a hole in the clouds to pave the way for laser-transmitted data from a satellite. (Source: X. Ravinet, Unige)

We live in an age of long-range infor­mation, transmitted either by under­ground optical fibre or by radio frequency from satellites. But the throughput today is so great that radio frequency is no longer enough in itself. Research is turning towards the use of lasers which, although techni­cally complex, have several advan­tages, especially when it comes to security. However, this new tech­nology – currently in the testing phase – faces a major problem: clouds. Due to their density, clouds stop the laser beams and scramble the transfer of infor­mation. Researchers at the Univer­sity of Geneva, Switzerland, have devised an ultra-hot laser that creates a temporary hole in the cloud, which lets the laser beam containing the infor­mation pass through.

Although satellite radio communi­cation is powerful, it can no longer keep up with the daily demand for the flow of infor­mation. Its long wavelengths limit the amount of infor­mation trans­mitted, while the frequency bands available are scarce and increa­singly expensive. Furthermore, the ease with which radio frequencies can be captured poses ever more acute security problems. Which is why research is turning to lasers. “It’s a new tech­nology that is full of promise,” says Jean-Pierre Wolf, professor in the Physics Section at UNIGE’s Faculty of Science. “The very short wave­lengths can carry 10,000 times more items of infor­mation than radio frequency, and there aren’t any limits to the number of channels. Lasers can also be used to target a single person, meaning it’s a highly secure form of communi­cation.”

But there is a problem: the laser beams cannot pene­trate clouds and fog. So, if the weather is bad, it is impos­sible to transmit information using lasers. To counter this diffi­culty, current research is building more and more ground stations capable of receiving the laser signals in various parts of the world. The idea is to choose the station targeted by the satel­lite according to the weather. Although this solution is already opera­tional, it is still dependent on weather condi­tions. It also creates certain problems regarding the settings on the satellite, which have to be processed upstream of the communi­cation, without any assurance that there will not be any cloud cover at the appointed time.

“We want to get around the problem by making a hole directly through the clouds so that the laser beam can pass through,” explains Wolf. His team has developed a laser that heats the air over 1,500 degrees Celsius and produces a shock wave to expel sideways the suspended water droplets that make up the cloud. This creates a hole a few centi­metres wide over the entire thickness of the cloud. It is the disco­very of these ultra-powerful lasers that has just been awarded the Nobel Prize for Physics 2018. “All you then need to do is keep the laser beam on the cloud and send the laser that contains the infor­mation at the same time,” says Guillaume Schimmel, a researcher in the team led by Wolf. “It then slips into the hole through the cloud and allows the data to be trans­ferred.”

This “laser cleaner” is currently being tested on arti­ficial clouds that are 50 centi­meters thick but that contain 10,000 times more water per square centi­meter than a natural cloud – and it works, even if the cloud is moving. “Our experi­ments mean we can test an opacity that is similar to natural clouds. Now it’s going to be about doing it on thicker clouds up to one kilo­meter thick,” continues Wolf. “It’s also about testing different types of clouds in terms of their density and alti­tude,” adds Schimmel. This new tech­nology represents an important step towards the commer­cial use of satellite laser communi­cation. “We’re talking about possible global implemen­tation by 2025, and our idea is to be ready and to allow countries that are overcast to have this tech­nology,” concludes Wolf. (Source: Unige)

Reference: G. Schimmel et al.: Free space laser telecommunication through fog, Optica 5, 1338 (2018); DOI: 10.1364/optica.5.001338

Link: Group of Applied Physics (J.-P. Wolf), University of Geneva, Geneva, Switzerland

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