These fibers are preferred over traditional electronic wiring because they can carry large amounts of data at very high speeds for very long distances. Because these signals are made of photons as opposed to electrons, they are immune to electromagnetic interference which can wreak havoc on electrical transmissions. With their low energy usage, light weight and high data integrity, optical fiber is ideal for use in space.
To Make a Better Fiber
Kelly Simmons-Potter, PhD, who holds professorships in the UA Department of Electrical and Computer Engineering as well as in the College of Optical Sciences, has been at the leading edge of this field for about 20 years. Coming from her initial work at Sandia National Labs, she has been a pioneer in understanding ionizing radiation effects in optics that are destined for use in extreme environments and in developing radiation-hardening materials for these applications. While some centers work on radiation-hardened electronics and others – some here at the UA – focus on optics for space, Simmons-Potter’s group is one of only a few in the nation working specifically on radiation-hardened optics. Another is NASA itself.
In her latest project, Simmons-Potter and PhD student Brian Fox examined optical fibers doped with erbium (Er, number 68 on the periodic table) and ytterbium (Yb, number 70 on the periodic table) with the goal of making them better for lasers and amplifiers in space-based telecommunications.
“The problem is that once you put elements into a fiber that are heavier or higher up in the periodic table, you start to see a lot of radiation susceptibility,” Simmons-Potter explains. “What happens in a radiation field is that if they’re not radiation hard, when they’re exposed to ionizing radiation, they do what’s called photo-darkening. They basically stop transmitting light.”
In other words, if your satellite up in space is dependent upon fiber-optics to work, photo-darkening can turn it into so much dead space junk.
Space-based vs. Earth-based Testing
To test how optical fibers perform, researchers do Earth-based tests at special facilities that can produce radiation environments that are similar to space. The problem is that these tests only expose the fiber to one kind of radiation at a time, such as x-rays or gamma rays.
“When you’re in space,” on the other hand, says Potter, “it’s a combined environment with all those different sources of radiation. You can’t recreate that on Earth.”
Sending an Experiment Skyward
Working with her connections at Sandia Labs and the Naval Research lab, Simmons-Potter was able to secure a spot on the Materials International Space Station Experiment-7 (MISSE-7).
“We sent our fibers over to NRL (Naval Research Laboratory), where they were loaded into the PEC (passive experiment container),” says Potter. Looking like an open metallic suitcase, the PEC was sent up to the space station aboard the Space Shuttle Atlantis, STS-120, on November 16, 2009. It was mounted on a truss where it was left to slowly bathe in the intense radiation present above the Earth’s protective atmosphere.
It returned to Earth on the Space Shuttle Endeavour, STS-134, on May 16, 2011. In a wonderful, double-connection for Tucson and the UA, STS-134 happened to be Endeavor’s last mission with Captain Mark Kelly, husband of Arizona Rep. Gabrielle Giffords, at the command.
Findings for the Future
When Simmons-Potter and her team tested the fibers after their 18-month sojourn in orbit, they found that indeed, the lasing and amplification properties of the fibers had changed, and certain compositions of fibers we’re much more durable than others.
“We’ve identified fibers that exhibit better radiation-hardening,” she says. “We also found that we’ve been able to identify the wavelength regions where the worst damage occurs, so that will give us a better handle on how to improve radiation hardness and what types of materials to stay away from. “
The third lesson they learned was that there is simply no substitute for true space-based testing.
“A combined environment – one in which all the damaging radiation sources are present at once – does lead to differences in the damage mechanisms in the materials,” she says. “It was a very important outcome in terms of trying to determine what materials are suitable for satellite applications and the results will help other groups as they take the research forward in other applications.”
Not only did Fox and Simmons-Potter enjoy the pleasure of seeing their project return successful results, but they have received appropriate kudos from the professional community as well. In September 2012, the paper they co-wrote about the project’s findings received the Outstanding Paper of 2012 Award at the Hardened Electronics and Radiation Technologies Conference.
Well Worth the Wait
Fox, a US citizen who lived for many years in Germany, moved back to the US in 2000. He spent two years at Pima College and then transferred to the UA. While he could have graduated over a year ago, he was compelled to have his own sojourn at the UA coincide with the completion of the MISSE-7 experiment.
“The fibers were only supposed to be in space for about 6 months,” explains Simmons-Potter, “but because of political glitches, the launches (for the return mission) were delayed and delayed and delayed.”
On the bad side, Fox wasn’t graduating. On the good side, the longer the fibers spent in space the better data the team would get when they returned to Earth.
“So every time they said it was delayed, we thought, yay! Another 6 months!” he remembers. “And the local thrill of having Mark Kelly command that mission was amazing.” While Fox might have completed his own mission in the normal 4 to 5 year period, he didn’t want to walk away when there was so much to do and so much to learn, so he kept on working.
“He just didn’t want to leave before he could get it done,” says Simmons-Potter. “It’s totally understandable. It’s not too often that you get to put your stuff up in space.”