Signaling across the cosmos: Optical communications are poised for takeoff during NASA's "Decade of Light"

01 November 2022
Theresa Hitchens
LCRD laser communication link between Earth and the International Space Station
The LCRD could provide a laser communication link between Earth and the International Space Station. Photo credit: NASA

Most people’s mental image of infrared lasers likely comes from popular heist movies, where the hero (or anti-hero) acrobatically slips, leaps, and tumbles through a grid of red or green beams making up a sophisticated alarm system to get to the coveted prize.

But those pencil-thin beams—in actuality, invisible to the naked eye—also can be used to ferry large volumes of digital information across vast distances. And that is why the National Aeronautics and Space Administration (NASA) has set its sights on developing optical communications networks to link future astronaut-scientists—working in orbit and on the surface of the Moon, Mars, and asteroids—to each other, and back to their colleagues on Earth.

That research and development initiative, known as the “Decade of Light,” is now hitting critical mass, with a batch of foundational experiments set for liftoff through 2024.

Badri Younes, NASA’s deputy associate administrator for space communications and navigation (SCaN) and SCaN program manager, explains that the 2020s are to optical space communications what the 1980s were to terrestrial fiber—a period of rapid development leading to ubiquitous worldwide usage. “We expect the thing to take off this decade and be widely available next decade,” he says.

SCaN is a division of NASA’s Space Operations Mission Directorate, and it is responsible for a handful of ongoing demonstration projects using lasers to send voice, video, and data communications between satellites, between a satellite and the International Space Station (ISS), and between spacecraft and the ground—eventually all the way out to the Moon, Mars, and beyond.

Right now, radio-frequency-based (RF) communications with NASA’s Perseverance rover on Mars can take anywhere from eight to 45 minutes between call and response, because the Red Planet is 50 million km away at its closest point and 249.1 million km at its farthest. And while Perseverance can send color pictures back to Earth, high-volume data relay is beyond its traditional RF communications network.

The use of laser beams to transmit data, rather than traditional radio waves, will enable more data to be downloaded at once, revolutionizing deep space communications at a time when NASA is poised to populate the Solar System not just with cute anthropomorphic rovers like Perseverance, but also with people under its ambitious Artemis Moon-missions program.

“Optical communications…will provide significant benefits for missions, including bandwidth increases of 10 to 100 times more than radio frequency systems,” NASA says. What’s more, optical communications allow smaller size, lower weight, and decreased power, which in turn means more room on a satellite for payloads and less drain on its batteries. Finally, less weight means cheaper launch costs, since heavier payloads require bigger, more expensive rockets to break out of Earth’s gravity well and achieve orbital velocity.

The near-term goal of SCaN’s demonstration program is to prove laser communications technologies can be scaled up for actual use at a reasonable cost. “We have a roadmap that we‘re trying to execute to help us get over the hump in learning what it means to operationalize this technology,” says Jason Mitchell, director of SCaN’s Advanced Communications and Navigation Technology Division.

And while there is a bit of a boom in the development of optical intersatellite links, which he calls the lower hanging fruit, NASA is concentrating on perfecting optical communications between the Earth and space, whether it be spacecraft or celestial bodies.

Photo credit: NASA

“The benefits are sort of quite obvious, right? You go from the microwave a few orders of magnitude down in wavelength, and all of a sudden now you concentrate power in a much more direct beam, you get much more information transferred, and it’s much more directional,” says Mitchell.

Another big benefit over using radio communications, NASA experts explain, is that use of the optical spectrum is not regulated either nationally or internationally; neither is it scarce nor crowded. Indeed, there already have been so-called spectrum wars between commercial users in the US and government users, such as the US Department of Defense (DoD), about access to certain parts of the RF spectrum, a situation predicted to get worse as firms rush to build space-based internet networks and 5G wireless communications linked to the ground. “You get the benefit that you don‘t have to fight over spectrum,” Mitchell says.

Finally, optical communications systems are much more difficult to hack or spoof than those using RF waves—a factor that has drawn the interest of the DoD, as well as commercial satellite communications firms.

The fact of the matter is that if the long-term vision of a thriving off-Earth economy—including a human population—is to become a reality, optical communications networks will be required. Long-distance, high-volume RF communications systems are simply too capacity constrained, and their required hardware too massive, to make economic sense.

And that vision isn’t one limited to NASA, or even the United States. China and the European Union also are funding human space exploration with eyes set on the same prize of expanding their economies through the exploitation of outer space and its resources, such as rare earth elements and heavy metals. They, too, are putting R&D efforts into optical comms to underpin those goals.

NASA’s current demonstration project, the Laser Communications Relay Demonstration (LCRD), launched via a SpaceX Falcon 9 rocket last year as a hosted payload aboard DoD’s Space Test Program Satellite 6 stationed in geosynchronous orbit (GEO) at about 35,000 km above the Earth. The mini-fridge-sized LCRD is the agency’s latest technology demonstration of a two-way laser relay system. It was certified to start work in May.

LCRD’s goal, according to SCaN’s program managers, is to demonstrate the benefits of optical technologies over current RF systems for data relay satellites, which already are widely used by NASA. Data relay satellites enable fast-moving spacecraft to communicate back to Earth without needing a direct line of sight to a ground station. For example, the International Space Station—which because it is in low Earth orbit (LEO) passes over the horizon in less than 10 minutes—uses RF links to satellites in GEO that match the Earth’s rotation and thus are always in sight of their ground stations.

LCRD initially is sending test data to and from its ground stations at Table Mountain, California, and Haleakalā, Hawaii. Younes explains that a diversity of ground stations is required to make sure that bad weather and clouds don’t disrupt the infrared laser’s signal. “We looked at the weather situation, we looked at the cloud coverage, and we have selected locations around the globe where at least you have one channel open.”

The initial test data, detailing the spacecraft’s health for example, will be sent up to LCRD via RF signals, and the spacecraft will reply using optical signals, according to mission plans.

Once the LCRD’s first-user platform, the Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal (ILLUMA-T), is launched to the ISS next year, it will begin serving as a relay satellite beaming scientific data from the station to the ground via laser. James Schier, SCaN’s chief architect, says that LCRD will be able to transmit a whopping 1.24 Gb of data per second. “So, you’re in the billion bits-per-second range.”

The optical system uses an erbium-doped fiber amplifier (EDFA), which emits a laser pulse at 1,550 nm, Schier says. (This is the same technology being used by the Pentagon for its two key optical intersatellite link efforts.) One advantage of using that wavelength, he explains, is that it is also used by terrestrial fiber optic cables. “When you’re plugging in your fiber optic cable to your laptop, you’re transmitting at 1,550 nm and probably using EDFA family components.”

Another advantage is safety, Schier adds, particularly when the laser is being used as an uplink where the high-density beam might accidentally hit an airplane—which, he notes, “is something the FAA calls a crime.”

Another laser pulse wavelength that could be used, he explains, is 1,064 nm. “The human eye does not react to a 1,064 wavelength, and you will literally burn out your eyeball before you detect damage to your eye. But 1,550 nm hitting an airplane will stimulate a pilot’s optical nerves, causing them to look away before their eyes are damaged by the beam. 

Another SCaN experiment also has been launched this year: the TeraByte InfraRed Delivery (TBIRD) payload lofted in May to LEO on a tiny CubeSat, courtesy of a Falcon 9 rocket. It will showcase 200 Gb/second data downlinks, the highest optical rate ever achieved by NASA. TBIRD will demonstrate the use of lasers to broadcast terabytes of data down to the ground in a single pass. This high-bandwidth capability will enable transmission of large, highly detailed images from the Moon.

Not only is TBIRD smaller and faster than LCRD, it also is much cheaper, explains Schier. “The big difference between the two in technology is LCRD is based on what you would consider in-house development: the government and government contractors developed the LCRD technology design, built it, tested it, launched it. The TBIRD is actually based on a commercial box that we bought, effectively, off the shelf.”

Terabyte InfraRed Delivery (TBIRD) laser communication. The TBIRD system is about the size of a tissue box and is integrated into a CubeSat. Photo credit: NASA

Laser communications also will be going to the Moon as part of the Artemis program goals to establish a permanent human presence there and in cislunar space. In fact, the agency’s 2013-2014 experiment involved establishing the feasibility of lunar communications via laser links, but, as Mitchell explains, it was a “one-off” experiment.

The Orion Artemis II Optical Communications System (O2O) will fly on board NASA’s Orion spacecraft during the Artemis II crewed spaceflight mission, now slated for May 2024. O2O will allow transmission of high-resolution images and video, with a downlink rate of up to 260 Mbit/sec. Mitchell explains that once these experiments are completed and the results evaluated, NASA will need to make decisions about where more investment is needed to bring demonstrated capabilities into actual operations—with an eye on the fact that the objective of the Artemis program is “industrial utilization of the Moon.”

Indeed, according to Schier, SCaN’s end-goal is to create what its program office is calling a full-up Moon-Earth internet, dubbed LunaNet. “LunaNet is the lunar Internet, a set of cooperating networks providing interoperable communications and navigation services for users on and around the Moon.”

LunaNet will involve not just optical communications, but RF communications too, for example, from the lunar surface to relay satellites stationed in orbit around the Moon. NASA was expected to release a request for proposals for RF communications links related to Artemis and LunaNet in October.

But NASA’s ambitions don’t stop with the dream of interplanetary optical communications, says Younes. The next big leap for human expansion into the Solar System and beyond will be based on quantum science, including quantum communications.

Quantum communications rely on optical communications and involve transmitting information via the “spooky action at a distance” of entangled photons that was bemoaned by Albert Einstein, but resolved to some extent by CERN physicist John Bell 30-odd years later.

“You know we have designated the 2020s as the Decade of Light, but the 2030s will be the Decade of Quantum,” Younes says. “There are a lot of technologies that are being developed now to enable us to move seamlessly to the next decade.”

But that, as the old saying goes, is a whole ’nother story.

Theresa Hitchens is the senior space reporter at Breaking Defense—a return to her first career as a journalist after having spent the previous 20-odd years researching arms control issues, including a six-year stint as director of the United Nations Institute for Disarmament Research in Geneva, Switzerland.

For more information from SPIE on laser communication, see the SPIE Conference on Free-Space Laser Communications XXXV.

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