Air at the center of optical fibers to guide light

With requirements for very high-speed internet on the increase, several research teams are developing a new generation of optical fibers called “hollow-core fibers” that could help to improve certain properties of light propagation compared to conventional glass fibers. This technology, whose performances are progressing rapidly, could be used in low latency communications in 5G networks for example, or in certain optical instruments in medicine or in industry that require high optical power.

“Guiding light through air could make it possible to be freed of the limits of glass and thus open up a whole range of possibilities.”

The use of silica glass in optical fibers has had profound impacts in many areas, from telecommunications to imaging, through cutting, laser welding, or lighting. In the area of telecommunications, single-mode fibers are solid glass tubes that enable the transmission of data at very high speeds over long distances.

Over the last half a century, glass fibers have facilitated the technological feats necessary for connecting continents and now each and every one of us. Around 500 kilometers of fiber are produced every year. Subject to the limitations inherent to the properties of glass, light propagation can have technical limits. The diffusion of light in glass can have strong optical power impacts.

Guiding light through air – that is the principal of hollow-core fibers – could make it possible to be freed of this limit and thus open up a whole range of possibilities.

Hollow-core fibers vs. conventional fibers

An optical fiber is made up of a core in which light travels, of a tube that keeps the light within the core, and a protective layer. Whereas conventional fibers are generally made up of a silica core, hollow fibers are composed of a hollow core surrounded by several air channels (also known as microstructured fibers).

The light-guidance properties of this category of fibers comes from their structure rather than from the properties of the material used in the core. They are dependent notably on the number of tubes surrounding the core, on their diameter, and their spacing. By varying these values, it is possible to then vary many parameters. Several types of microstructure have thus been developed over time, as is described in this history drawn up by the Optical Society.

Fast progress

Although the propagation loss rates of hollow-core fibers were initially much higher than those of conventional fibers, several methods have been developed to reduce this gap. Over the past few years, research teams have revealed hollow-core fibers that have highly reduced loss, getting closer to the performances of glass fibers, and at wavelengths that are appropriate for several commercial applications.

In a study published in “Nature Communications”, researchers from the University of Southampton in the United Kingdom claim that it is possible to overcome the attenuation limit set by glass thanks to a new family of hollow fibers that have been under development for over ten years within the Optoelectronics Research Centre under the name “Nested Antiresonant Nodeless Fibers” (NANFs).

They have developed three hollow-core fibers with wavelengths between 600 and 1,100 nm (non-telecoms application) which they claim have losses comparable or lower than is achievable in solid glass fibers. For information they specify that “while the minimum absolute loss achieved at 1550 nm (telecoms application) is only 0.142 dB/km7, the lowest reported loss at the shorter technologically relevant wavelengths of 1060, 830, and 630 nm increase to 0.57, 1.6, and 4.5 dB km−1 respectively”. Records at this level can only be achieved with very high quality fibers.

Telecommunications: from 5G networks to high frequency trading

These advances allow us to imagine the possibility of developing fibers of this type for telecommunications in the coming years. An important part of the research and development being carried out on hollow-core fibers is effectively dedicated to their applications for telecommunications, whereby several data transmission experiments have been carried out over the past few years to reduce latency. In effect, the refractive index of air (around 1) being less than that of silica (around 1.5), light travels faster through a hollow-core fiber.

In June 2021, British telecoms operator BT announced that it had started trials with a 10-kilometer-long hollow-core fiber cable provided by Lumenisity, a Southampton University spin-off company. This cable is to be used for testing a variety of use cases, in particular in the area of 5G networks or that of ultra-secure communications, such as Quantum Key Distribution (QKD). According to BT, this technology could potentially reduce latency by up to 50%, which “would enable a variety of benefits, from high-frequency trading to lowering mobile network costs”.

In fact, hollow-core fiber is already being used to connect the Chicago and New York stock exchanges where “high-frequency traders” are constantly pushing back the limits (in particular thanks to algorithms) to reduce the time needed to complete transactions and gain an edge on rivals.

Improving endoscopic instruments in medicine

Today, optical fiber is used in medicine as much for diagnosis as for treatment. Among other things it has enabled the development of new endoscopic instruments that help to improve the quality of surgery whilst making it less invasive.

Researchers from the University of Bath (United Kingdom) are studying how hollow-core fiber could facilitate the improvement of spectroscopy techniques (the study of electromagnetic radiation emitted, absorbed, or diffused by matter), specifically Raman instruments used in endoscopy.

Raman spectroscopy is based on the analysis of the interaction of light with molecules in order to retrieve information on the composition or characteristics of a sample. More specifically, this technique makes it possible to determine the vibrational signature of a molecule that has been excited by a monochromatic light (guided by fiber optic cables), which provides information on its structure and its mode of interaction with surrounding molecules. In conjunction with other tools, in medical diagnosis this can help identify certain diseases; for example, doctors are hopeful that Raman spectroscopy will be used to better detect cancers.

Currently, the glass composing the core of the fiber used in Raman probes generates a background noise that can drown out the target signal. The research team therefore hopes that a hollow-core fiber will make it possible to minimize this noise.

Other tools could also benefit from these new advances in the design of optical fibers, particularly environmental sensors and the high-power lasers used in industry.

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