Monday, November 05, 2018

Next-Gen Satellites May Use Lasers to Part the Clouds

Scientists of the future huddle around a computer, waiting for an HD live stream of the incoming asteroid. As the probe that will provide the crucial communication slowly moves into view of the asteroid, they know that every second counts. In a surprising move, they tune their receivers not to radio frequencies, like we do today, but to a much higher frequency—somewhere in the near-infrared. But they think nothing of it—infrared and visible light allows for a much better transmission of data, and all of the leading satellite producers have switched over by now. But at the last minute, a cloud rolls in above the station, scattering the message from the relay satellite in all directions and cutting off the receiver.

That’s where the work of Dr. Jean-Pierre Wolf comes in.

He’s a professor at the Université de Genève in Switzerland and his research could be the missing link that makes this kind of high-speed communication with satellites possible. He’s been working for years on a specialized laser that stations can aim at the sky to dispel any water droplets in the air—effectively boring a hole through the cloud and permitting the satellite’s signal to pass through.

A visualization of the laser that bores through clouds.
Image Credit: UNIGE, Xavier Ravinet
But let’s take a step back. Today, all communication with satellites is done using radio frequency electromagnetic waves, largely because they're stable over large distances and aren’t prone to scattering in Earth’s atmosphere. However, they sprawl out as they travel down to Earth and tend to cover a large area by the time they reach land, requiring that nearby satellites use frequency bands that don’t overlap—otherwise any listener on the ground would receive an unintelligible mixture of information coming from a variety of sources. Because of this, a patchwork of national and international organizations relegate so-called “frequency bands” (a narrow range of frequencies) to various agencies for a fee. However, in our increasingly connected world, unused frequency bands are harder and harder to come by—which means the price has skyrocketed.

Because of these constraints, a number of researchers are turning to a different mode of communication: near-infrared and optical frequencies beamed down from space using powerful lasers. While laser beams are still composed of electromagnetic waves, the waves oscillate thousands of times faster than radio, carrying up to 10,000 times the amount of data. The pinpoint accuracy of a laser beam also allows for more direct communication without the risk of a third party intercepting messages simply by tuning in to a fortuitous frequency. Instead, the laser ensures what Wolf calls “end-to-end connection”: one end comes from the satellite and the other end lands on a single receiver, with a strongly reduced possibility of leakage. In addition, at higher frequencies the quantum nature of the photons present in the electromagnetic wave becomes more apparent, making extra security features like quantum cryptography possible. Oh, and lasers use much less power than radio sources.

In short, there are plenty of reasons to move away from radio communications.

The biggest obstacle to the switch, surprisingly, doesn’t lie in current laser or satellite technology—in fact, NASA used lasers to beam a video down from the space station back in 2014. Instead, the problem is a more fundamental one: bad weather.

Unlike radio waves, the higher frequency lasers are easily scattered by the water droplets that make up clouds or fog, rendering any satellites that use them mute the moment a storm hits. One popular solution to this problem has been to simply build more receiving stations and hope that at least one will see clear skies at any given time, but this is every bit as costly and inconvenient as it sounds. Fortunately, in light of Wolf’s recent publication, it may be unnecessary.

Believe it or not, Wolf isn’t the first to tackle the project of whisking clouds away with lasers. In the 1970s and 1980s the Soviet Union and U.S. military experimented with using high-intensity carbon dioxide lasers to locally boil away the water contained in clouds to improve troop visibility on the battlefield. Although they did succeed in clearing a hole roughly 100 m long, the electricity draw was eventually determined to be prohibitive.

An image from the '80s depicting a fifteen meter long haze caused by a carbon dioxide laser.
Image Credit: Zuev A A, Zemlyanov Yu, Kopytin D and Kuzikovskii A V 1985 High Power Laser Radiation in Atmospheric Aerosols (Dordrecht: VE Reidel Publ.)
Instead, Wolf decided to take a different approach that would be more energy efficient using pulsed lasers. This kind of laser has a rapidly oscillating power, rather than a constant one; as it turns out, when the peak power—the power delivered when the pulse is at its strongest—is sufficiently high, a special property emerges that isn’t seen in other lasers.

First, the power is not uniformly concentrated across the tiny width of the laser beam; instead it is highest at the center of the beam and tapers off towards the edges. The variation in electromagnetic radiation actually affects the index of refraction (a measure of how much light is bent) of the surrounding air, effectively turning the air itself into a lens—the technical term for this phenomenon is a Kerr lens. This “lens” bends the beams in towards itself, narrowing its breadth and refocusing it until it is strong enough to rip electrons from the surrounding atoms in a process called ionization.

At this point the beam diverges again, and the process repeats itself faster and faster until eventually the laser beam holds a stable width, at which point it is referred to as a “filament”. The lone electrons that were stripped off of their atoms don’t remain that way for long though; they quickly recombine with the positively-charged ions, releasing energy in the form of a shock wave and a crackling noise. This is where the magic happens: the shock waves pushes water droplets out of the way, opening a channel roughly a centimeter in diameter. But that’s enough to allow the passage of a laser transmitting all the information it contains. (Eventually, we will need to get to approximately 10 cm to receive information from a satellite.)

An illustration of the filamentation process. To the upper left, the diagram shows how the index of refraction (n) varies with distance, creating a "lens" and causing the beam to converge. To the upper right, the ionization changes the index of refraction once more, this time causing the beam to diverge. As the process repeats itself over and over, the laser beam self-stabilizes and creates a laser filament (lower diagram).
Image Credit: Jean-Pierre Wolf.

Since the laser is merely displacing the water droplets rather than boiling them off like the military tried to do, the power draw comes out to a much more reasonable 100 watts. Even so, this laser is still incredibly powerful. When asked about safety concerns, Wolf nods. Since the cleaning beams are sent up to the satellite and not down to the earth, there isn’t a big risk associated with them—after all, the energy (and the danger) of a laser is concentrated inside the beam. He does acknowledge that the only (manmade) objects that might pass through the beam—aircraft—will need to be diverted. “But there are already a number of no-fly zones,” he points out. We would just need to be strategic about where we build receiving stations; the middle of a flight corridor might be out of the question. But even if a plane did fly through the beam, it wouldn’t be at risk unless it were flying sideways or upside down. “But if that is the case, you have other problems,” he chuckles.

Wolf envisions equipping receiving facilities with one of these self-stabilizing lasers, possibly by coupling them to the focusing lasers that are already required to get a satellite’s attention. Then, any time intense humidity threatens the quality of communication the stations will be ready to blast it out of the way.

So far, Wolf has demonstrated his laser’s effectiveness in a laboratory setting using an artificial cloud only 50 cm deep, but with 10,000 the water concentration of a typical cloud. He says he is currently in close contact with large aerospace consortia and hopes to test the technology out on a real cloud soon. If that goes well—as he believes it will—he hopes to see a global rollout by 2025.
If that’s the case, it might not actually be long before those scientists of the future become the scientists of the present...let’s just hope there’s no asteroid.

—Eleanor Hook

1 comment:

  1. The thought enters my mind if it is possible to determine if similar transmissions/beams are currently coming and going from Earth. That would certainly be revealing.