Light speed. Optical fibre tests the laws of physics

By exploiting a characteristic of hollow-core optical fibres, our researchers have raised data transmission rates to within touching distance of the speed of light.

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 copper wire. In a recent research paper in the journal Nature Photonics, University researchers 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: a fine mesh of silica glass struts, which confines the light in a hollow air-carrying region in the centre of the fibre. This allows light to propagate significantly faster than in a conventional fibre 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 of 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 researchers demonstrated 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. This means that data propagating in this fibre arrived 1.54 microseconds/km earlier that it would in an equivalent length of conventional solid fibre. Not only did the light travel at virtually its ultimate 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 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.”

The technology could see commercial adoption in a range of sectors. Next-generation supercomputers 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 earlier in 2013.

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Professor David Richardson

Professor David Richardson

Deputy Director at the University's Optoelectronics Research Centre (ORC).