To send and receive data, orbiting spacecraft depend on radio-frequency communications. It’s been this way since the 1957 launch of Sputnik 1, which made use of two radio frequencies: 20.005 and 40.01 MHz. The United States’ first satellite, Explorer 1, had beacons for the frequencies 108.00 and 108.03 MHz. Space communication nowadays borrows a huge swath of the radio spectrum for all sorts of competing purposes, from weather satellite imagery to GPS to television broadcasts.
Radio communications have some serious limitations. The radio spectrum is only one small interference-prone slice of the larger electromagnetic spectrum, and it can only offer relatively slow data rates. Unfortunately, we also seem to be stuck with it as the radio band happens to be one of only two frequency ranges capable of transmitting across Earth’s atmosphere. Ultraviolet and higher frequencies are mostly absorbed, as are the infrared frequencies. We’re then left with two electromagnetic “windows” representing frequency ranges that can pass between Earth and space relatively unimpeded. Radio is one, while the other is the even narrower band of visible light.
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NASA engineers have taken a major step toward making communications possible via that second window in the form of the first-ever integrated-photonics modem. The “potentially revolutionary” technology, which is highlighted in the current Goddard Space Flight Center newsletter, offers data rates of 10 to 100 times that of radio communications and could have applications well beyond space communication, including Earth-bound telecommunications, medical imaging, advanced manufacturing methods, and national defense.
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The technology, known properly as the Integrated LCRD LEO (Low-Earth Orbit) User Modem and Amplifier (ILLUMA), will be put to the test beginning in 2020 as part of NASA’s multi-year Laser Communications Relay Demonstration (LCRD) mission, where it will be installed aboard the ISS. Unlike NASA’s prior laser-communications effort, which was sent into orbit with the Lunar Atmosphere and Dust Environment Explorer in 2013, the LCRD is a full system featuring not just the ISS payload but two dedicated ground stations. Together, the link should be capable of data rates of up to a gigabit per-second.
The key technology behind the LCRD is integrated photonics. The basic idea is of building circuits just as you would for a normal electronic device—using lithographic techniques—but trading electrons for photons, e.g. particles of light. This is how the modem is implemented and why it’s able to make sense of laser-based communication.
“We’ve pushed this for a long time,” explains Mike Krainak, who is leading the modem’s development at Goddard, in the newsletter. “The technology will simplify optical-system design. It will reduce the size and power consumption of optical devices, and improve reliability, all while enabling new functions from a lower-cost system. It is clear that our strategy to leverage integrated-photonic circuitry will lead to a revolution in Earth and planetary-space communications as well as in science instruments.”
Down here on Earth, photonics remains an important field of research for plenty of non-space applications. For one thing, photons are thought to be a necessary or at least ideal ingredient in quantum computing as they’re highly mobile, relatively easy to manipulate at the single qubit level, and are naturally resistant to interference and noise, which is crucial for preserving fragile quantum states. Photons also offer some general hope for making computers smaller and more energy efficient; compared to an electron, vast torrents of which are used to represent information in contemporary computers, it’s hard to even say that a photon has “size” at all.
One terrestrial application suggested by the NASA engineers is in data centers. Some large part of the internet’s backbone of fiber-optics is dedicated to managing the interface between optical information and the stuff that actually gets stashed in servers. LCRD promises that same hardware on a small chip.
“Google, Facebook, they’re all starting to look at this technology,” Krainak says. “As integrated photonics progresses to be more cost effective than fiber optics, it will be used. Everything is headed this way.”
