Guiding Intense Laser Pulses Through Thin Air

LLNL’s fiber draw tower, a state-of-the-art facility for fabricating specialized fiber optics, is being put to one of its sternest tests yet: producing complex multichannel fibers with ultrathin webbing and tiny filaments a few billionths of a meter thick.

The 8.2-meter-tall fiber draw tower was commissioned in February 2012 to provide the Laboratory with the capability of producing optical fiber structures for a wide variety of research projects. Since then, the NIF & Photon Science Fiber Group has been working on ways to overcome the inherent power limitations of conventional silica fibers, including developing ribbon-shaped photonic crystal fibers (PCFs) that increase the surface area for heat removal, thus extending the thermal limit for power scaling.

The latest project, funded by the Laboratory Directed Research and Development (LDRD) Program, is developing a new approach to hollow-core fiber (HCF) technology, in which the laser light is primarily guided in the air in the empty core instead of by solid fiber material.

In a hollow-core PCF, tiny and closely spaced air holes running the length of the fiber act as reflectors to confine the light in the empty core. Using a hollow-core fiber testbed, European scientists and engineers recently achieved the world’s highest data transmission rate of 57.6 terabytes per second—50 times faster than the previous record.

“In conventional fiber lasers, the light is guided by total internal reflection,” said LLNL researcher Mike Messerly. “In the hollow-core fibers it’s guided by a constructive interference phenomenon. The great advantage of these fibers is that the breakdown thresholds for air are much higher than for glass. The light intensity is not limited by damage in the glass, so it can transport more energetic pulses or higher-power laser beams.”

The idea for hollow-core lasers originated in the United Kingdom, at Bath and Southampton universities, a little more than a decade ago. Along with their use in data transmission and precision measurement systems, hollow-core fibers are being studied for potential application in optical sensing, especially chemical and biological sensing. Filling the hollow cores of the fibers with liquid or gas enables direct interaction between the propagating light and the material being analyzed, enhancing the sensitivity of the sensors.

“You get a long interaction length,” Messerly said, “because the fiber guides a highly focused spot, so the beam can be very intense and can interact intensely with a gas over a long length. The gas might be a sample from the environment, and you could be looking for traces of certain chemicals.” The fibers already have been used to detect low levels of CO₂, biomedically significant molecules such as glucose, and bacteria.

Another potential application is laser-based particle accelerators. Jay Dawson, who leads the Fiber Group, said traditional accelerators based on radio-frequency technology “would be scaled down to optical wavelengths by taking a hollow-core optical fiber and shooting light into it to make an accelerator.” Dawson said the Department of Energy and scientists at SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory have expressed interest in the application of lasers to accelerators.

Hollow-core fibers, however, can have substantially higher propagation losses than solid-core fibers. The LLNL project is working to mitigate those losses by developing “negative curvature” HCFs, in which the core boundary has a convex shape when seen from the center of the fiber.

“It’s now widely accepted that negative curvature is the key to low loss,” Messerly said. “However, it’s difficult to fabricate. We need to fabricate thin webs of glass on the order of a wavelength thick. The challenge is to control the thickness as you draw the glass out of the (draw tower) furnace.”

To shape the negative curvature structure, different pressures have to be applied to the core and the cladding during the fiber drawing process. “A difference of one-tenth of a pound per square inch can be the difference between too little and too much pressure,” Messerly said. “We need to control the pressures to half that, or .05 psi.”

To brush up on current practices, the Fiber Group’s Reggie Drachenberg attended a recent University of Bath fabrication workshop, which covered techniques for applying differential pressures, drawing large capillaries, and “snuggly” stacking filaments to avoid water contamination.

“The big fabrication challenge is controlling the pressure and making the fibers reproducible, and that’s where we are now,” Messerly said. “It’s a matter of mastering the process.”

Joining Messerly, Dawson, and Drachenberg on the project are LLNL researchers Paul Pax and Nick Schenkel. NIF&PS Summer Scholar Celine Ledesma, who has returned to school, also participated.

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