A 10 Million Mile Message: Behind NASA’s Deep Space Optical Comms Laser
The optical communication system has successfully received "first light" after sending data by laser to and from far beyond the Moon.
On November 14, 2023, NASA received the first light from the new deep space optical communications (DSOC) experiment piggybacked on the Psyche asteroid probe. The Psyche mission, launched October 13, 2023, on a Falcon Heavy rocket, is headed to its namesake in the asteroid belt via a gravity assist from Mars. The DSOC is pioneering laser-based optical communication system for use in future deep space missions.
Artist’s concept of Psyche at its mission target. Image used courtesy of NASA via Wikimedia Commons
The DSOC system consists of a ground-based laser, a ground-based optical receiver, a laser, and an optical receiver on the spacecraft.
First, the Transition From Radio to Laser
Since Sputnik beamed a signal to Earth on October 4, 1957, radio has been the primary communication method between this planet and spacecraft. Back on Earth, much of our communications, especially high-speed data, has switched to optics over the last few decades. The highest-speed systems use solid-state lasers connected to optical fiber with modulated data. Fiber on earth delivers data on the order of 100 Gbs, while our robots on Mars eke out 0.5 to 4 Mbit/s, depending on the orbital locations of the two planets.
Comparison of radio frequency and laser communications. Graphic used courtesy of NASA
With DSOC, NASA is testing out what it hopes will be the future of high-speed deep space communications. Faster data does not mean that the data travels faster than 186,000 miles per second (300,000 km/s). Rather, it means that more data bandwidth can be encoded in near-infrared light than in MHz radio signals. Near-infrared is the optimal portion of the spectrum for the “first mile/last mile” leg of the trip because it is less susceptible to interference from the lower Earth's atmosphere. While data travel will still take the same time for the carrier, laser transmits 10 to 100 times more data over the same time period than radio transmission.
A High-Flying Transceiver
On the probe, the DSOC is a 25-kg package with a power budget of 75 W. It is equipped with an 8.6-in (22-cm) aperture telescope to both transmit and receive laser signals. Data is sent to Psyche from a 5-kw laser on a ground station at JPL’s Table Mountain, California, location. For this mission, the ground station sends low-rate data that the probe will use to fine-tune its alignment. DSOC uses a photon-counting camera designed around Geiger-mode avalanche photodiodes to pick up weak signals. The focal plane array detector simultaneously points, acquires, tracks, and communicates. The return signal from the probe includes high-rate data.
NASA's Psyche spacecraft with DSOC's integrated flight laser transceiver. Image used courtesy of NASA
A conventional light sensor produces electric current proportional to the amount of light that hits it. A Geiger-mode avalanche photodiode, on the other hand, will produce a signal when any of its cells go into breakdown mode. In optimal conditions, a single photon on one cell in an array can lead to the breakdown condition, thus creating a signal that can be easily amplified and decoded into digital data.
The downlink on Earth requires an even more sensitive receiver to clearly read the high-data-rate signal through the planet’s atmosphere. From 200 million or more miles away, the light signal is fairly weak. That’s where the superconducting nanowire single-photon detectors (SNSPD) come into play. SNSPDs are the highest-performing detectors in the ultraviolet to mid-infrared wavelengths. The SNSPD is used with the 200 inch (5.1 meter) Caltech Palomar Observatory in San Diego, California, 100 miles from the Table Mountain laser transmitter.
64-pixel superconducting nanowire single-photon detector array. Image used courtesy of NASA JPL
The SNSPD operates cooled to -458°F, or 1ºK. In that state, it is sensitive enough to not only detect single photons but also detect them fast enough to collect high-speed data. The superconducting detector is fast enough to determine photon arrival time within 100 picoseconds. This capability allows the detector to receive signals from the Psyche probe at its destination, 240 million miles (390 million kilometers) away. Despite its extreme sensitivity, the SNSPD can also detect large streams of photons without being oversaturated, allowing the sensor to be used for the entire mission.
More Laser Comms to Come
The next step in the experiment is to refine the algorithms to improve spacecraft and Earthbound alignment. The DSOC will be tested throughout the mission, including at the Mars gravity assist stage in 2026. Upon the probe’s arrival at Psyche in 2029, it will be approximately 20 light-minutes distant. It was only 50 light seconds away on the November 14 test, so it still has a lot of capability to prove, but so far, everything is going to plan.
Ultimately, NASA and its partners plan to use laser communications systems to deliver hundreds of Gb/s to terabytes/s in low Earth orbit (LEO) and multiple Gb/s throughout much of the solar system.