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The University of Southampton

ORC researchers create ‘Speed of light’ fibre optic

Published: 25 April 2013
Professor David Richardson

By exploiting a characteristic of hollow-core optical fibres, researchers at the University of Southampton claim to have brought data transmission rates within touching distance of the speed of light.

In recent years, fibre optic cables have transformed the delivery of internet services due to their ability to transmit data over greater distances and at faster speeds than traditional copper wire.

In a recent research paper in the Journal 'Nature Photonics' , researchers at the University of Southampton reported that they have developed a hollow, air-filled fibre that transmits light far quicker in the absence of material that previously slowed it down.

A conventional fibre is made from two types of glass. At its centre lies a thin silica glass core that carries the light and is surrounded by a thicker layer of glass cladding, which is coated in polymer and then cabled for protection.

Because the cladding has a lower refractive index than the core, light is continually reflected back and forth in one direction down the core. This guidance mechanism, referred to as total internal reflection, slows the light down and means it propagates at roughly 70 per cent of its full potential speed in a vacuum.

A hollow-core fibre replaces the core and cladding with a single surround made of a fine mesh of struts made of silica glass, which confines the light in a hollow air-carrying region in the centre of the fibre. This allows light to propagate 31 per cent faster than in a conventional fibre, the researchers showed, and shortens the time it takes to travel from one end to another, known as latency.

“One way to increase the speed of the light in the fibre is to ensure that it is propagating in air rather than glass,” explains Professor David Richardson , Deputy Director at the University's Optoelectronics Research Centre (ORC). “We’ve developed a fibre where the light is confined by another guidance mechanism that results from light reflection at the multiple air:glass interfaces within the fibre cladding. This more complex mechanism, referred to as bandgap guidance, allows the light to be guided in an air rather than glass-core.”

The University is not the first to have produced hollow-core fibres. However, Professor Richardson says its researchers improved the characteristics of the fibre allowing it to carry light over a range of wavelengths using multiple spatial patterns of light vibration known as modes.

“The structure of conventional fibres typically only supports one fibre mode. Hollow structures generally support multiple modes, and in our research we managed to demonstrate that by controlling the injection of the data signals into the fibre, we could excite just a single mode, thereby readily allowing for high-fidelity data transmission.”

The researchers successfully demonstrated the first high-capacity, low-latency data transmission experiment performed using a hollow-core fibre. In this, they found that light propagated 31 per cent quicker than in a solid core fibre, increasing from 70 per cent of its full speed in a vacuum to 99.7 per cent. To put this in context, this means that data propagating in this fibre would arrived 1.54 microseconds/per km earlier that it would in an equivalent length of conventional solid fibre. Not only did the light almost travel at its fastest possible speed, but it did so with a very low loss of 3.5 dB per kilometre.

Exploiting more modes in a single fibre strand, each a separate information channel, yielded even better results. Professor Richardson adds:

“We have now launched independent data signals onto several modes of the fibre. By using three modes we were able to increase the capacity threefold,” he says. “We felt and feared that the hollow core structures wouldn't support more modes without degrading signal quality, but by exploiting a few tricks borrowed from wireless communications this proved not to be the case.”

Researchers at the University, who were supported with funding both from the European Union MODEGAP project and the UK Government's Photonics HyperHighway project to carry out the work, will now continue to push loss limits down further.

“We are optimistic that those loss limits can be reduced to values comparable with conventional fibre,” Professor Richardson adds. “If theory is believable, then we should be able to get them even lower than that, which would be significant.”

Eventually, the technology could see commercial adoption in a range of sectors. For traders, the low latency afforded by the hollow-core fibre would prove valuable in the race to get information first.

Next-generation number crunching super computers in data centres, too, could benefit from the combination of low latency and faster data transmission rates, which Richardson says reached up to 73.7 Terabits per second during tests carried out with Nokia Siemens in a German test lab earlier this year.

“For the technology to be adopted commercially, many issues still need resolving, such as how you reliably splice the fibres together and terminate the ends of the fibres to stop moisture getting into the holes and producing such fibres in high volume,” explains Professor Richardson. “At this stage, it's still very early in the research phase and we've just proven that the cables can work over kilometre scale distances, not hundreds of kilometres.”

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