31 Jul 2015
Lawrence Livermore team's oblong-patterned photonic crystal fibers look set to help pulsed applications first.
by Andy Extance
The inventors of an unconventional, ribbon-shaped optical fiber geometry say that it could be set to drive fiber-laser power output dramatically, while maintaining beam quality.
Based at the Lawrence Livermore National Laboratory (LLNL) in California, the team is now working to bring its ribbon-core fibers to pulsed-laser applications, and could help to make commercial high-power fiber-laser systems more scalable.
LLNL’s photonic crystal fiber (PCF) scientists have found that large-diameter fibers with thin, flat core patterns circumvent problems experienced with conventional cylindrical cores. The new designs could handle higher powers without being thermally damaged, and minimize the impact of non-linear stimulated Brillouin and Raman scattering effects that otherwise broaden emission wavelength ranges.
While the new technology feeds activities including the US National Ignition Facility (NIF), famed for its laser fusion studies, ribbon-core fibers specifically address defense-related research. In such work, single-mode emission is often needed to combine fiber lasers to reach the highest total powers possible.
The highest power fiber lasers available today with the kind of beam quality needed for combining stand at around 1.5 kW, underlined Mike Carter, a program manager for photon science at LLNL. When disregarding beam quality, LLNL’s modelling suggests the upper theoretical power limit from a single conventional fiber is around 40 kW.
Ribbon-core fibers, made by placing a line of doped silica rods in the center of a cylinder of undoped rods, can raise those limits. In such designs the approximately 8 x 108 µm central ribbon core has a higher refractive index than its surroundings, and thus preferentially guides light. Meanwhile, the surrounding 245 µm-diameter air-clad fiber extracts heat more efficiently.
One challenge is that this design is difficult to use with a conventional laser beam shape. Consequently, the LLNL team has developed a system to efficiently convert beams into a profile that propagates into the ribbon fiber and is converted back after it is amplified.
Making bigger better
“You can set up similar mode-guiding properties to a small-diameter single fiber, spread out over a larger geometry,” explained Carter. “The higher-order modes that would normally creep into a large-diameter fiber are directed to a loss region and thus don’t exit the fiber.”
LLNL’s analysis suggests that by using this approach it should be possible to produce individual single-mode fiber lasers with an output power of 10 kW, and push multi-mode limits to at least 100 kW.
Reducing scattering losses and thermal damage could make fiber lasers even more reliable and efficient than they already are, and the higher powers would mean fewer systems need to be combined.
That could make commercial systems more compact and easier to scale up. “Fiber lasers’ currently low-power building blocks result in complicated integration challenges, large-volume systems and large infrastructure,” Carter says. “The ribbon fiber can simplify that.”
This complements advantages already offered by similar commercial technology, notes Christian Poulsen, CTO at the Birkerød, Denmark, PCF specialist NKT Photonics.
“PCFs enable efficient pump schemes as the pump numerical aperture is greatly increased, allowing more pump light to be converted to signal light,” he said. “This enables much more compact module/system design and potentially a much lower $/W cost of the laser.”
Poulsen adds that the LLNL team's efforts in this area are a good example of the seemingly exponential growth of such technology happening at the moment.
For the majority of commercial applications, such as welding and cutting, the short working distances employed mean that beam quality is not particularly important. Electrical efficiency and compactness are important but not critically, Carter said. Consequently he doesn’t feel that there’s currently a strong commercial pull for PCF-based lasers, even though such systems may have advantages once perfected.
“Sustainment of beam quality allows for more precise laser operations or the same operations at longer working distances,” Carter asserted. “Also, if we can reduce the number of building block fiber lasers needed to reach high powers by making each ribbon fiber carry more power, then they are likely to be broadly adopted if the cost of the fibers is the same.
“The ribbon fibers are still at the research stage so we are not at the right performance levels yet to assess cost comparisons. But if it is successful, scalability at the same price would mean this technology could potentially completely replace current high-power fiber laser technologies.”
On the pulse
The LLNL researchers haven’t yet demonstrated high powers, having published ribbon-core fiber systems with outputs of just 40 W. However, this range already has practical applications. For example, in a NASA project where LLNL is collaborating with the San Jose, California fiber laser developer PolarOnyx.
Funded through a Small Business Innovation Research (SBIR) phase II project, they are working on 20 W, 1550 nm wavelength fiber lasers with very high repetition rates.
The particular challenge in this project is achieving 100 MHz repetition rates with a wall-plug efficiency higher than 20 per cent, for high-rate optical communications in deep space. “In NASA’s deep-space missions power efficiency is very important,” Carter stressed.
Conventional fiber limitations mean that current commercial systems work well for tasks in which emission energies remain steady over time, but not for applications that require energy bursts. To deliver quality fiber-laser pulses at 100 MHz, PolarOnyx is exploiting LLNL’s PCFs.
NKT is likewise benefiting from such capabilities, albeit at higher powers. “Within materials processing requiring high peak and pulse energies, PCFs are being incorporated into numerous commercial laser platforms,” Poulsen said.
Average annual growth rates for such systems are in the high double-digit range, he revealed. “Today, PCF is no longer an upcoming technology, but has actually matured into numerous products with real industrial applications, especially in the bio-imaging and materials processing segments.”
About the Author
Andy Extance is a freelance writer based in Exeter, UK.