A new type of hollow optical fibre is the first to be more transparent than the materials it is made from.

The crucial element in this apparent paradox is a novel mirror structure lining the fibre, which confines light in the hollow core. By keeping the light in air rather than a solid, it offers a way to transmit signals over longer distances, at higher speeds, and with less noise than conventional communications fibres.

A standard fibre consists of a solid glass core surrounded by a cladding made from glass with a lower refractive index. A phenomenon called total internal reflection keeps light directed along the higher-index core from leaking into the lower-index cladding. However, this approach does not work with conventional hollow fibres, because all solids have a higher refractive index than air.

To solve the problem, Yoel Fink and colleagues at the Massachusetts Institute of Technology used a novel mirror made from thin alternating layers of two materials with a large difference between their refractive indexes. This acts as a “photonic bandgap”, which rejects light at certain wavelengths and keeps it trapped in the hollow core.


Roll and stretch

Fink’s group deposited the alternating layers of arsenic triselenide and a plastic called polyether sulphone on a flat layer of the plastic, which they rolled into a tube. Then they heated the tube until it softened, and stretched it to make a thin fibre.

They have made fibres up to 50 metres long, with transmission peaking at wavelengths between 0.75 and 10.6 micrometres. Fibres with peak reflection at 10.6 micrometers could transmit more than 80 per cent of light through a one-metre length, much more than could be achieved through solid fibres of plastic or arsenic triselenide.

Transmission remains far short of conventional silica glass fibres for the wavelengths used in communication systems. These can carry 80 per cent of light up to six kilometres at 1.55 micrometres. Yet the hollow fibres may avoid some of the problems afflicting solid fibres, such as the stretching and distorting of light pulses that limits signal speed and distance.

With the telecommunications market currently “in the pits”, Fink hopes to take advantage of another feature of the hollow fibre – it can be designed to work at wavelengths for which no good fibres are yet available.

His first target is 10.6 micrometres, the wavelength of carbon-dioxide lasers used for surgery and laser machining. A flexible fibre would be much easier for a surgeon to manipulate than the articulated arm currently used to deliver the beam.

Journal reference: Nature (vol 420, p 650)