'Twisted light' idea makes for terabit rates in fibre

Artwork of twisted light concept

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A novel way of boosting data rates in optical communication using "twisted light" has been shown to work in optical fibres.

The light is effectively corkscrew-shaped, and more data can be encoded in differently twisted beams.

The concept had been shown off over "free space" but it remained unclear if it would work in fibres.

Now a team reporting in Science has demonstrated data rates of 1.6 terabits per second over 1km of optical fibre.

This is still short of the "over-the-air" rate of 2.5 Tb/s demonstrated by members of the same team in 2012. But it is a powerful proof of principle for adapting the technique to use with fibres in, for example, data centres.

The idea behind twisted light is based on the fact that photons, the most basic units of light, carry two kinds of momentum - a kind of energy in their movement.

"Spin angular momentum" is better known as polarisation. Photons "wiggle" along a particular direction, and different polarisations can be separated out by, for example, polarising sunglasses or 3D glasses.

But they also carry what is called orbital angular momentum. This is best explained in analogy to the Earth-Sun system: our planet spinning around its axis manifests spin angular momentum, while the orbital angular momentum is seen in our revolution around the Sun.

"Twisted light" approaches use this orbital angular momentum, essentially encoding more data in varying degrees of twist.

The technique has drawn controversy when applied to radio-frequency waves, but its application at frequencies used in telecommunications has been going from strength to strength.

The problem is that these twisted beams get scrambled in standard fibres and lose their capacity to carry data. What was needed is a new design - that of report co-author Siddharth Ramachandran of Boston University, US.

Images of the phase of "twisted" light The "twist" of the waves could lead to vastly greater amounts of data - but only by using novel fibres

In 2011, Dr Ramachandran collaborated with fibre company OFS Fitel to produce a kind of fibres-within-fibres design, adding different chemicals to each concentric ring that changed the speed of light in each concentric fibre.

These novel fibres effectively provide different paths for different beam twists.

To put the fibres to work, Dr Ramachandran joined forces with Alan Willner, of the University of Southern California, who led the team behind the 2012 "over-the-air" demonstration.

The team demonstrated rates of 400Gb/s using a single colour of light with four levels of twist, and 1.6Tb using 10 colours, each with two levels of twist.

"It was a nice collaboration between a fibre expert and a systems communications group, to demonstrate that not only is orbital angular momentum able to propagate, but that the data contained within it would be of high quality," said Prof Willner.

Just how widespread the technique could become, however, remains to be seen - given that it has to be done on novel fibres very different from the billion kilometres of fibre already underground and under the sea globally.

"There may be certain areas where there are more or less closed systems where you need more bandwidth," Prof Willner told BBC News.

"If you have a Google data centre, say, where you need terabits between servers, you envision that might be where newer types of fibres might find a place."

The team is currently working to increase the number of colours and levels of twist that they can reliably produce and detect, increasing further the promise of increased data rates.

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