A new laser technology could improve the quality of deep-space communication, making it easier for humans to push the boundaries of the final frontier.
Much of today's space communication relies on radio signals. But these diffract and broaden as they travel, as does light or any other electromagnetic wave. A radio beam fired from the moon to Earth “would typically diverge to the size of a continent,” says Peter Andrekson, a photonics researcher at Chalmers University of Technology in Sweden and co-author of a new study in Light: Science and Applications. In contrast, he notes, “a laser beam would diverge to a two-kilometer radius or so.”
Catching enough of a spacefaring radio signal from somewhere like Mars requires a really big dish. NASA's widest receivers stretch 70 meters across, says Bryan Robinson, an optical communications engineer at the MIT Lincoln Laboratory, who was not involved in the study: “It's like a football field that's on a gimbal pointing to Mars.”
Laser communication could work with receivers about 20 centimeters across—the size of a personal pizza—and condensed laser beams can carry much more information than radio. But laser signals are transmitted at a lower power level, and processing them once they are received requires a daunting level of amplification.
The researchers' new receiver manipulates interactions between photons to magnify an incoming signal without reducing its quality, a technique called phase-sensitive amplification (PSA). This approach is “very interesting,” Robinson says, because today's amplifiers add distorting “noise.” The experimental PSA system was sensitive enough to receive an unprecedented 10.5 gigabits of information per second, noise-free, through a lab setup that mimics the vacuum of deep space and adds diffraction to simulate distance. The next challenge will be to overcome distortion caused by Earth's atmosphere.
In 2013 the Lincoln Laboratory and NASA successfully tested another type of laser transmission between a spacecraft and Earth. That method used a photon-counting receiver, which tallies individual light particles as they strike a detector. It is extremely efficient for transmitting data, which can be numerically encoded—but the counter works only at –454 degrees Fahrenheit. PSA receivers operate at room temperature.
Despite the challenges, refining optical communications systems such as these would be “a pretty big deal,” says planetary scientist Tanya Harrison, who was not involved with either project. Harrison is mapping Mars by satellite and has been frustrated with the limitations of radio transmissions. Radio data currently travel from Mars to Earth with all the speed and fidelity of an early-1990s modem. A satellite orbiting the Red Planet, Harrison says, “can take an order of magnitude more data than it's able to actually send back. Basically we could be doing a lot more science if we had optical communications.”