Apr 29, 2020 8:00 AM

Physicists Clear the Air With a Sweet Frickin' Laser Beam

Fast laser pulses produce a shock wave in air that pushes water vapor aside. That clears channels in clouds for transmitting optical data from satellites.

In the first two months of 2019, Malte Schröder spent several weeks on a suburban Maryland college campus directing a dramatic scene that was set entirely inside a small styrofoam box.

First, he set the stage by filling the box with thick fog, a thousand times denser than a cumulus cloud. Then—the lights. To illuminate the scene, he used a special laser built by physicist Howard Milchberg and his team at the University of Maryland. Schröder, a physics graduate student at the University of Geneva in Switzerland, had traveled all the way to Maryland just to make use of that laser. “The Milchberg group is excellent at making these very accurate laser pulse trains, where you can clearly determine the time delay between each pulse,” says Schröder. The laser is so precise that it can produce pulses of red laser light 8.36 trillionths of a second apart—exactly what Schröder needed.

Beaming this steady train of red pulses through a small window in the styrofoam, Schröder monitored the fog under the intense illumination. In the end, he got the scene he wanted: The laser pushed vapor droplets out of the way to form a thin, clear vessel of air across the container.

This demonstration, recently published in Optics Express, is a crucial step in researchers’ grander ambitions to clear pathways in clouds and fog using lasers: “weather control,” as Schröder calls it, with a touch of mad scientist hamminess. In particular, fog-clearing may enable the widespread use of an emerging technology known as free-space optical communications, which delivers data in laser light through air instead of optical fiber. In free-space optical communications, a satellite or other transmission station on high sends information encoded in a laser beam down through the atmosphere to a receiver.

One advantage of this type of transmission is that it offers data rates comparable to fiber optics without needing to actually build a fiber network. For example, in 2013, the technology enabled a NASA mission called the Lunar Laser Communication Demonstration (LLCD) to beam high-definition video 239,000 miles from the moon to various ground stations on Earth. For NASA’s Artemis II mission, which plans to send a crewed spacecraft around the moon and back in 2022, engineers will install a similar laser communication system on the craft.

Photograph: Malte Schröder/University of Geneva

But successful downlinks through Earth’s atmosphere depend on the weather. Like a mob of tiny shadow puppets, cloud and fog droplets will dim and scatter the laser’s signal. To beat the clouds during the LLCD mission, NASA engineers had to build three receiver sites on Earth, of which two were usually in the line of sight between the spacecraft and the moon. “If there was a cloud at one station, we would steer our beam to the other station,” says Bryan Robinson, an optical engineer at MIT Lincoln Laboratory who worked on the LLCD mission. Occasionally, both sites would be clouded out “and you got nothing,” he says.

Because building multiple ground stations can get expensive, the technology hasn’t gone mainstream yet. “It’s an important area of communications, but it tends to be in niche applications,” says optical engineer Dan Kilper of the University of Arizona, who was not involved in the work. Typically, engineers use the technology in places where laying fiber would be difficult or impossible—hence, space. In addition, the research company X, formerly known as Google X, which operates under its parent company Alphabet, has installed these transmitters and receivers on balloons and building roofs to provide internet service in areas with limited connectivity, such as the state of Andhra Pradesh in India, and in Puerto Rico after Hurricane Maria.

Fog-clearing capability would allow optical communications to perform consistently without so many ground stations. But its costs are still unclear, as researchers are still understanding the basics of how high-power pulsed lasers might help. “Nobody was attempting to use any of this stuff to clear fog particles prior to a couple years ago,” says Milchberg.

This most recent demonstration builds on several years of experiments involving lasers and fog. In the past, physicist Jean-Pierre Wolf, Schröder’s University of Geneva adviser, had led experiments with laser pulses so intense that they would change the character of the surrounding air. Each of these fleeting pulses, occurring a few hundred times per second, beamed brighter than 10 trillion suns. This deluge of photons collided with the air molecules, stripping those molecules of their electrons to produce a fluid of loose electrical charges, a new state of matter known as a plasma. No longer the familiar diffuse gas that we normally bring into our lungs, the air briefly became a different material. Then, as the air cooled back into a gas, it vibrated, “launching a sound wave,” says Milchberg. This sound wave pushed the fog out of the air to form a fog-free channel.

Courtesy of University of Maryland and University of Geneva

But plasma is difficult to control, which means it’s hard to mold the air channel into the desired shape. In Milchberg and Schröder’s new work, they found that they could clear fog in a potentially more controlled way. This time, they fired the pulses at a much faster rhythm, which sent the air molecules spinning and produced the channel-clearing sound wave without creating the plasma. It’s crucial to time the pulses precisely so they hit the air molecules at just the right moment to get them spinning fast enough, says Milchberg. He compares it to pushing someone on a swing; for the biggest effect, you want to always push at that one moment at the top of the arc.

Fog-clearing could play an important role in optical communications in the future. In particular, Kilper highlights its utility in a budding technology known as quantum cryptography, which promises better security than existing encryption methods. In these protocols, two parties exchange information encoded in individual photons of light, perhaps relayed by a satellite in orbit. Because of the quirks of quantum mechanics, if a hacker tried to intercept those photons, they would instantly alter the information, rendering it unusable. But because individual photons are extremely dim, fog is a big problem for quantum cryptography.

While his group only demonstrated fog-clearing within the confines of the styrofoam box, Milchberg says they have the technical ability to clear fog over much longer expanses—100-meter stretches, which they could string together into much longer channels. Perhaps the bigger question is who would want to invest in this technology. Milchberg speculates that fog-clearing may end up being useful for laser weaponry. For example, in 2014 the US Navy installed a 30-kilowatt laser on a ship in the Persian Gulf and used it to shoot at practice targets such as drones. Navy engineers could conceivably install fog-clearing capability in such a laser, and the military has a lot more money to spend on experimental technologies like this, compared to civilian projects.

Meanwhile, Schröder’s group, back in Geneva, is also investigating whether the fog-clearing sound wave might help guide the path of lightning. The idea is for similar laser pulses to change the conductivity of the air and carve out the desired path of least resistance, which could potentially direct lightning away from airports, rocket launch sites, or even forests. “There is a huge interest in protecting sensitive infrastructure from lightning strikes,” says Schröder. Regardless of the application, they’re getting the light to go where they want.

Update 4-29-2020 11:15 AM: This story was updated to correct when the laser communication system will be installed on NASA's Artemis II craft.