New research from the Illinois Grainger College of Engineering and collaborators at Clemson University demonstrates a new method of co-doping erbium into glass, an approach that will improve the efficiency of erbium-doped fiber amplifiers and may scale to high power lasing applications. Their findings appear in Optica and Optics Letters.
Silica glass is the most frequently used material in fiber-optic technology, but its use is complicated by the clustering effect common among erbium ions. This clustering limits the performance of erbium-doped fiber amplifiers, which boost optical signal strength in fiber-optic communications systems. Notably, clustering can lead to significant reductions in quantum efficiency, or the efficiency with which signal photons are produced by the amplifier. Scientists have traditionally addressed this problem by co-doping erbium into fibers with aluminum oxide. But researchers wondered: Is there a better way?
“The set of fibers we were working on originally started in ytterbium,” said Jennifer Campbell, an Illinois physics PhD student and lead author of the paper. “After our partners at Clemson University produced ytterbium versions of the fibers for a different project and noted their unique capabilities, we wondered if similar things would happen in erbium. That’s when we discovered it had an improved efficiency, and it snowballed from there.”
Most erbium-doped fibers are used to develop amplifiers or lasers. Many of these fibers are used in telecommunications optical fiber because erbium doped into glass is perfect for operation in the C-band near 1550 nm, a portion of the electromagnetic spectrum where transmission loss in silica is minimized. Erbium’s role is in transmitting signal from end to end by amplifying the signal at each step, ensuring nothing is lost in transit.
The current challenge facing scientists is how to increase the efficiency of these erbium-doped fibers. Shorter fibers are important for cost efficiency and for the decreased likelihood of nonlinear contributions, or detrimental effects that occur as a signal travels further down the fiber. The Illinois researchers — backed by Peter Dragic, associate professor of electrical & computer engineering — wondered if barium could be used to further force apart erbium ions, improving its efficiency when doped into silica glass.
“When we started looking at this fiber, we didn’t have any expectations as to what it would do,” Campbell said. “We just hoped it would do something productive. So we put it through the wringer. We characterized the entire fiber as much as possible and when that was done, we decided to hook it up and test its capabilities as an amplifier.”
Their process determined the length of fiber needed and how much conversion of pump energy to output energy was produced, indicating an increase in efficiency. Erbium-barium fibers showed comparable or better characteristics compared to commercial erbium-alumina fibers and required a shorter length to boot.
The novelty of the team’s research comes from a combination of chemical and physical approaches. By combining aluminum salts, barium salts, and erbium salts and the use of a barium-erbium fluoride nanoparticle structure, barium was able to hold the erbium ions apart long enough for the glass to cool and settle, decreasing the amount of erbium ions finding each other in the glass and sticking together.
“It’s kind of like bracing the gates before the nanoparticles are transformed and hoping the erbium ions don’t have enough time to run back together,” Campbell said.
Going forward, the researchers will be focused on a two-pronged approach. The first step was adapting this new fiber as if it were a telecom fiber, demonstrating that a fiber developed with a higher concentration of erbium as well as increased efficiency can result in a shorter amplifier with a broader operation range.
Now, the group will attempt to increase the amount of erbium while altering the physical shape of the fiber in hopes of supporting high power lasing. The most powerful element used in lasing is ytterbium, which lases at a wavelength not safe for human eyes. In comparison, erbium can lase at an eye-safer wavelength, but has yet to approach kW-level lasing like ytterbium before it.
“If we can do this, we’d have the combination of an eye-safer wavelength with even more power,” Campbell said. “Now we can produce lasers that won’t be as harmful.”