Keeping very bright light behind bars

Light behind bars Colour bands and patterns achieved from white light transmitted through hollow-core photonic crystal fibre

Dr Fetah Benabid, Mr Francois Couny, Mr Phil Light, Dr Georges Humbert, Professor Jonathan Knight and Professor Philip Russell.
University of Bath.

A new type of optical fibre could revolutionise the use of light to carry data. Hollow-core photonic crystal fibres (HC-PCFs) being developed at the University of Bath use an entirely new way of containing light within optical fibres. The methodology could eventually see small-scale optical circuitry replacing current electronic circuits.

Fibres that carry light are familiar to many people, due to the popular 1970s fibre optic lamps and their use more recently in artificial Christmas trees. But such optical fibres have far more important applications as a highly efficient means of carrying data.

Optical fibres contain and guide light in a solid glass core by what is known as total internal reflection (TIR). Until 1991, it was thought that TIR was the only way to guide light over long distances. However, in that year, Philip Russell of the University of Bath, predicted that light could be guided in a hollow core, just like water travelling down a hose. 'Light behaves in a very weird way', says Fetah Benabid of the Photonics and Photonic Materials Group at Bath. 'Being able to contain it in a hollow core in effect guiding light in air was thought to be almost impossible'.

The key to guiding light in this way is the repetitive bar structure of the surrounding material which in effect creates a light trap that causes light to be reflected back into the core. 'With our system, a very bright laser beam is trapped behind bars just tens of nanometres thick, in a tubular cage only 10 millionths of a metre wide but kilometres in length', says Fetah. This breakthrough technique leads to dramatic reductions in transmission loss. In addition, because light can be made to interact with gases inside HC-PCFs, the technique has other applications. For instance, it may be used to generate laser-gas interactions, to produce differently coloured lasers,