Bill Klipstein

The standard way for spacecraft to communicate with teams on the ground has been to use radio waves. NASA, however, will test the use of lasers to increase data communication rates by as much as 100 times. In a Technology Leaders Q&A, JPL physicist Bill Klipstein explains his role as project manager of NASA's Deep Space Optical Communications (DSOC) mission.

Photonics & Imaging Technology: How will lasers be used to improve data transmission from space?

Bill Klipstein: On the ground, optical communications, fiber optic links, and lasers have revolutionized our ability to move data around, and they're doing that in space, near-Earth, as well. There are a few different benefits. One of them is related to bandwidth. We actually are running out of spectrum in RF, limiting how much data we can squeeze in between different RF channels and even get allocated to NASA's needs. Optical frequencies are ten thousand times higher than radio frequencies, so we can fit in far more data in a channel, and the channels are not all used up. We're working on extending that capability to deep space.

P&IT: What must be overcome to achieve deep-space communication?

Klipstein: In deep space, even with the sharpness of the laser beam, you still have very few photons that you receive at Earth. We need to have very high peak-to-average power. We use pulsed lasers, and we use a technique called pulse-position modulation (PPM) to encode the data. Because we're photon-starved, we take the average power of the laser and put all the energy in a narrow pulse to ensure detection of even a few photons. Instead of sending many photons for every digital “one,” we represent a sequence of ones and zeros with a single laser pulse, with the sequence identified based on the time a pulse arrives. The timing of the pulse tells you, essentially, the bits you're sending.

P&IT: How are you able to direct the laser?

Klipstein: With an RF system, you just point in the general direction of the Earth and you're bathing the whole Earth in radio waves. With lasers, it's not that simple. The beam does spread out, but much less, so we have to point to a particular spot on the Earth, in a much better way than spacecraft usually point.

We send a beacon laser up from a transmit station on the Earth, and the terminal will find that beacon and point back towards the beacon. When you get light from the beacon, it's telling you where the Earth was when it sent the light. By knowing how fast the spacecraft is moving relative to the Earth, we “point ahead” the downlink beam relative to the beacon so it hits the downlink station where it will be when the light gets there.

Several upcoming NASA missions will use lasers to increase data transmission from space. (Image Credit: NASA's Goddard Space Flight Center/Amber Jacobson, producer)

P&IT: What are the big applications that are possible with these kinds of increased data rates?

Klipstein: Optical communications enjoys broad support within NASA. The human spaceflight team is very interested in being able to support video links and high data rates, just to allow a lot of information flow back and forth. With our robotic missions, we're starved on the information we can send back to Earth.

On the science side, there are instruments that return an increasingly large amount of data. Radars around Earth, for example, produce huge amounts of data; as we send radars to distant planets, they'll generate a lot of data. Photographs take a lot of information. When we talk deep-space optical communications, we're still talking about relatively low data rates compared to what you might get at your house, but an order-of-magnitude or more better than what you can get with a comparable RF system.

P&IT: What is the Deep Space Optical Communications (DSOC) mission?

Klipstein: I'm the project manager of the Deep Space Optical Communications mission. DSOC is going to fly on the Psyche mission, part of NASA's Discovery program. DSOC is a technology demonstration. We're funded for a one-to two-year demonstration of optical communications from deep-space distances.

Psyche is going to one of the Trojan asteroids, outside of the Mars orbit. We'll do our demonstration during the Psyche cruise. If we are still operational when we get to [the asteroid], then we will have the ability to send down data from the spacecraft, and from the science instruments. [DSOC is scheduled to launch in 2023].

P&IT: With lasers, what are your biggest challenges when it comes to data transmission from space?

Klipstein: In the near-Earth environment, it's very easy to have a photon-rich environment, even out to the Moon. But when you go out to deep space, you get very few photons go up and down, so we have to have a photon counting camera. We're using a detector and a readout circuit, pioneered at [MIT Lincoln Laboratory, based in Cambridge, MA], which is a big player in optical communications.

On the ground, we use a very interesting detector. Again, we're still trying to count single photons arriving, and get the arrival time very well. We have a superconducting nanowire array; the material is called tungsten silicide. It's superconducting at about 1 Kelvin. When a single photon hits the material and deposits enough energy, you get a very strong signal. So, every 40 nanoseconds, you can detect a single pulse. Atmospheric disturbances steer the beam enough so that a larger-area detector than normal is required.

P&IT: What is most exciting to you about this work and its possibilities?

Klipstein: We're used to the free flow of information; in the deep space environment, that's just not the reality. In a deep space environment, data rates like dialup are not uncommon. We're going to want to transfer a lot of information at comfortable rates for people to communicate and stay in touch. On the science side, there will be a science return of having a larger type of information coming down.

Eventually optical communications will be taken for granted, but not yet. It's rare that you have an opportunity to do the first of something, to do something challenging, and to be the first mark in the sand of what will eventually become commonplace.

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