The Lunar Laser Communications Demonstration (LLCD) mission made history in October 2013 when it succeeded in transferring data at 622 Megabits per second, a rate six times that of comparable radio frequency systems, like going from dial up to a high-speed Internet connection. But this technological achievement in laser communications was at risk had it not been for the “push” researchers experienced when an important component, a photodiode detector, failed to perform as necessary during testing.
In the world of emerging technologies, a “push” is any activity attempting to expand on advancements to current challenges or limitations. Within NASA’s Space Technology Mission Directorate (STMD), projects like Deep Space Optical Communications (DSOC) seek to do just that. When LLCD was faced with the detector failure, a potential replacement was identified—one with a challenge: it was still under development with DSOC.
The LLCD experiment, now well known for its achievement, launched onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) from NASA’s Wallops Flight Facility in Virginia on September 6, 2013. A series of LLCD experiments began in late September with the first successful downlink from LADEE on September 28, just before LADEE reached lunar orbit. LLCD mission operations began in mid-October, and by October 21 six links were successfully completed.
Getting to that successful point, however, was not a straightforward path and required numerous collaborative efforts among individuals and organizations across NASA and industry.
Early in the mission life cycle, it became evident that there was a high probability of limited or no communications link opportunities for the LADEE launch due to clouds or inclement weather during the monsoon season at the optical ground station at White Sands Center in New Mexico. NASA’s Space Communications and Navigation (SCaN) Office stepped in by funding a back-up ground station at the NASA/Jet Propulsion Laboratory (JPL) Optical Communications Telescope Laboratory. The JPL back-up ground station project is referred to as LLOT, or the Lunar Lasercom OCTL Terminal. The JPL ground station has a telescope specifically designed for space optical communications experiments. The back-up station project required a demonstration only at the lowest downlink rate of 39 Mb/s. During early testing of that capability, the baselined commercial intensified photodiode detector failed to adequately detect data at 39 Mb/s.
The need to overcome this limitation was clear; fortunately the answer was already in the works.
Back in the summer of 2011, under SCaN funding, Bill Farr and Jeff Stern of JPL had begun WSi detector development in collaboration with the National Institute of Standards and Technology, building on what Farr described as NIST’s “ground-breaking achievements.”
“This naturally flowed into STMD’s Game Changing Development DSOC project starting in the fall of 2011,” said Farr. “Our DSOC project goal has been to make large arrays of WSi detectors to go behind 5- to 12-m diameter telescopes. We are presently fabricating 64-pixel arrays. At an interim step we fabricated the 8- and 12-pixel devices, which were suitable for use behind a 1-m telescope, such as at the JPL ground station.”
Farr and Stern fabricated and began testing their first WSi devices at the start of March 2012.
“In collaboration with NIST, by the end of April 2012 we had a record setting 93-percent system detection efficiency with single-pixel devices, and under the DARPA-funded InPho program performed a record setting 13-bits per photon demonstration using pulse-position-modulation (the preferred deep-space optical communications modulation format) with one of these devices,” Farr said of the testing results.
In September 2012, after the critical nature of issues with the commercial photodiode detector was deemed insurmountable, the challenge was firmly set. The LLOT project found that to succeed, it would be necessary to switch to the WSi detector and moving forward was review-board approved.
With that approval, the push was now truly on.
Farr’s own words best describe the dynamic collaborative efforts:
“I knew a local vendor, Photon Spot, Inc., (Monrovia, Ca.) starting a business in superconducting nanowire detectors. The LLOT project worked with Photon Spot to quickly assemble and lease a cryostat that would achieve the required 1-K operating temperature for the WSi detectors.
“The cryostat was delivered to JPL in April 2013. Matt Shaw and Kevin Birnbaum at JPL then led the effort under the LLOT project to get the detector array installed into this cryostat and then interfaced to the data acquisition system, which was originally selected to operate with the photodiode detector. Kevin came up with a novel interface using only off-the-shelf electronic modules in order to meet the tight project schedule and budget.”
By June, the LLOT project demonstrated error-free communications and successfully completed compatibility testing of the WSi-based LLOT receiver with the Lunar Lasercomm Space Terminal engineering unit.
“An amazing 2-month integration effort by Matt and Kevin and the rest of the LLOT team,” said Farr.
John Rush, director for the Technology and Standards Division of NASA’s Space Communications Office, visited the JPL ground station for a final check before the LLCD experiment started. Discussions included the list of challenges the team faced in getting ready on time. “The biggest challenge was the detectors where everyone agreed that the original detectors would not have worked. But the tungsten silicide detectors that STMD invested in saved the day,” Rush said.
“The new detectors now hold the world record for efficiency at 93 percent and for a mind-boggling 13 bits per photon,” Rush added. “This is an excellent example of how working together we can achieve things that we can’t achieve by ourselves.”
Denise M. Stefula
NASA’s Langley Research Center