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A photonic crystal fibre
A photonic crystal fibre - at the centre is a solid core, surrounded by a honeycomb of holes
A photonic crystal fibre
A photonic crystal fibre. Click to enlarge diagram

Press Release - 07 July 2005

A new way of controlling light

Light has enormous potential and versatility if its power can be controlled: lasers as delicate as those used to correct faulty eyesight or as powerful as those used to cut metal are examples. Conventional fibre optic cables which carry enormous amounts of data in telecommunications are another example.

But there has been one fundamental limitation when using light: it tends to rapidly spreads out from its source and be absorbed by matter. That’s why if you turn a torch on in a dark room you will be able to see, but as soon as the torch is turn off the room goes dark instantly – the photons of light from the torch have been absorbed by the matter in the room – its walls and furniture - and are no longer bouncing off them and into your eye, which is how you see objects.

In the 1980s optical fibres were developed. These were very long thin pieces of silica glass with a round central core running through them surrounded by a denser area. Light travelling through the central area bounces off denser surrounding, so keeping it ‘imprisoned’ as it travelled the length of the fibre. These fibres are now used in telecommunications such as carrying telephone calls and are many times more efficient than copper wires at transmitting data. However the light signals lose energy as they travel and need to be boosted on their journey.

Until 1991 the only way to transmit data using light was by these solid core fibres. But then Professor Philip Russell thought of the idea of the photonic crystal fibre, eventually built in 1996.

What makes the photonic crystal fibre – which can be a kilometre in length and the width of a human hair - so special is that it can trap light so that it doesn’t become absorbed by matter or lose its power. It does this by the structure of the tiny round holes running the length of the photonic crystal fibre, arranged in a honeycomb shape.

The holes are as tiny as a few tens of nanometres – that’s less than one hundred thousandth of a millimetre. At the centre is a larger hole, sometimes filled with silica glass or air, and this array - a large hole surrounded by smaller holes - is the key to the photonic crystal fibre’s power.

Light begins to behave in unexpected ways when it interacts with objects this tiny, and one effect is that light will enter the large central hole and travel down it the length of the photonic crystal fibre, but cannot escape into the smaller holes that surround it.

This gives the fibre the very important property of trapping light within the larger central hole so that it passes through the length of the fibre while losing almost none of its power. This is unlike conventional optical fibres, where there will be considerable loss of light as it passes down the fibre.

The consequences of this are fundamental: the ability to use light rather than electrical circuits to carry information will be greatly enhanced – it will make optical fibres many times more powerful and brings the day when computer memories consist of optical devices rather than less efficient silicon chips much closer.

It will also have an impact in many areas of engineering and technology, including light sources, optical telecommunications, ultra-violet light and x-ray generation, gas spectrometry analysis, atomic and quantum physics and astronomical imaging. Small particles such as virus cells can be transported down the hole, powered by the momentum of the moving light itself. This will help biological research.

Any device where light is important or can be used, photonic crystal fibres can make more efficient, sensitive and powerful.

Notes

For more information on photonics, click the links on the left hand column of this page.


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