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Hollow-Core Fibers Outperform Silica Glass Counterparts

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JAKE SALTZMAN, NEWS EDITOR
[email protected]

SOUTHAMPTON, England, Dec. 2, 2020 — Researchers from the University of Southampton Optoelectronics Research Centre (ORC) have introduced three unique hollow-core fiber designs that each exhibit losses comparable to or lower than those that solid glass fibers are able to achieve around wavelengths of 660, 850, and 1060 nm. The fibers, in addition to reducing losses of power that standard glass fibers currently experience, withstood testing at high lasing intensities, such as those needed to melt rock and drill oil wells.

Silica glass optical fibers have traditionally served as transmission media for high-speed optical communications, powering the internet and cloud-based services. Due to light scattering inside the glass, however, a fraction of power is lost in the transmission process; the phenomenon is known as attenuation.

As light’s wavelength shortens, attenuation intensifies. Higher transmission loss through a fiber limits the performance capabilities of applications requiring shorter light wavelengths.

In guiding light through air-filled fibers, the Southampton-based researchers overcame attenuation, as well as the constraints it causes. Due to the fact that light propagates in media with high densities and refractive indices, the ability to confine it effectively — and guide it through a gaseous medium surrounded by solid, glass-like materials as happens in a hollow-core fiber — requires highly intentional techniques, said Francesco Poletti, a professor from ORC.

“First, we surround the hollow core with multiple thin glass membranes of a chosen thickness that act like mirrors for specific wavelengths, and help confine light in the hollow center,” Poletti told Photonics Media. “This is called antiresonance. Second, we make these membranes long enough so that light guided therein is prevented from coupling to the light guided in air — this is called inhibited coupling.”

Hollow-core fibers, developed at the University of Southampton, could reduce loss of power currently experienced in standard glass fibers. Courtesy of the University of Southampton.
Hollow-core fibers, developed at the University of Southampton, could reduce loss of power currently experienced in standard glass fibers. Courtesy of the University of Southampton.
A final step involved designing membranes to feature a physical shape like that of nested or nest tubes. The design ensured the elimination of all modes (there are hundreds) that the researchers guided through the air core of their fiber(s).

The one exception is the Gaussian-shaped fundamental mode.

“This is essential to ensure that light guided along the fiber propagates with a spatial distribution that is temporally stable and not affected by bends or external perturbations,” Poletti said.

The hollow-core fibers transmitted laser pulses with power levels so high that they would be unusable if transmitted by standard glass fibers.

“Two main advantages that every hollow-core fiber type offer are the significant suppression of nonlinear effects, and the considerable increase in damage threshold,” Poletti said. The nested antiresonant hollow-core fibers (NANFs) — hollow-core fibers that rely on thin glass membranes surrounding the core to hold light in their central voids — with which ORC tested, and those that scientists at the center have designed, additionally delivered a pure spatial single-mode propagation. This helped maintain and, in some cases, increase the brightness of the launched laser light, Poletti said, with low propagation loss (minimizing the number of photons lost in propagation).

They also preserved the degree of light polarization necessary to increase existing sensing technology and endoscope devices. “It is not inconceivable now to think of sending high brightness KW scale average power through kilometers of optical fiber with no damage, nonlinear broadening, or brightness deterioration,” Poletti said.

The researchers tested with a standard fiber optics test kit, relying on the fiber fabrication process to innovate their designs, using custom-developed fluid dynamics models of the fluid dynamic process. “The fluid dynamic process of ‘drawing’ a macroscopic hollow glass tube the size of a vacuum cleaner pipe into hollow optical fibers the size of a human hair is a complicated interplay of surface tension, viscous forces, and differential pressures inside a very hot furnace,” Poletti said.

The significance of application around the 660-, 850-, and 1060-nm wavelengths ties into a range of potential applications for the fibers, and their suitability to those that are quantum-based, semiconductor-based, and more. Wavelengths around 660 nm are of particular concern to the efficient transport of quantum states from a source, such as an ion trap, color vacancy center in a diamond, or even a semiconductor, to an application, such as a gravitometer or quantum computer. Applications at 850 nm involve short-reach data transmission from VCSELs, as well as the transfer of ultrashort pulses from TI:sapphire lasers.

Beyond 1000 nm, the primary interests involving use of the hollow-core fibers are high-average and peak-power laser delivery from solid-state or Yb fiber lasers, Polleti said.

As the researchers move toward device commercialization and volume production, the group’s most advanced discussions are taking place in the field of low-latency data transmission. They have spun out a company, Lumenisity Ltd., to commercialize hollow-core, fiber-based cables for high-bandwidth connections. The study of applications in areas such as biophotonics and quantum sensing and communications remains ongoing. The Southampton team is supported by funding from the European Research Council project “LightPipe.”

The current work stems from more than 10 years of ORC research aimed at developing and optimizing NANFs. Through the course of the team’s research, technological enhancements have enabled it to move from constructing fibers with attenuations of 5 dB, or approximately 30% of light transmission for every meter of fiber, to fibers that improve on those quantities by a factor of 10,000.

Francesco Poletti. Courtesy of the University of Southampton.
Francesco Poletti. Courtesy of the University of Southampton.
The fibers described in the recent work achieve an attenuation of just 5 dB every 10 km.

"The transmission capacity of optical fibers is so large that we never thought we'd reach the point where we would use it all up,” said Sir David Payne, director of ORC. “In the last five to 10 years, we’ve realized that we’re now close to doing just that, and the impact of COVID-19 has accelerated this further. This means that no longer can we tweak conventional fibers to mine more capacity but must resort to the sledgehammer approach of installing huge numbers of new fiber cables. This is possible but drives up costs.

"A faster, more reliable internet with larger bandwidth would help us sustain our current levels of online work and socializing and also enable us to take this further in areas like 3D video conferencing and virtual reality."

The research was published in Nature Communications (www.doi.org/10.1038/s41467-020-19910-7).

Photonics.com
Dec 2020
GLOSSARY
quantum
Smallest amount into which the energy of a wave can be divided. The quantum is proportional to the frequency of the wave. See photon.
Research & TechnologyeducationEuropefiber opticsquantumultrafast lasersBiophotonicsCommunicationsenergysemicondcutorUniversity of SouthamptonNature CommunicationsEnglandoptical fiberssilica glass fibersindustrialsupercomputing

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