The Internet has created a boom in long-distance optical communications. As a result the demand for capacity in undersea optical-fibre communications is escalating. A simple way to increase the capacity is to send many separate optical wavelengths through the same fibre, a technique known as wavelength division multiplexing. Currently all optical fibres rely on total internal reflection to confine the light in the core of the glass fibre. However, as the light propagates through the glass, it is subject to optical nonlinearities that can shift the various frequencies in the signal and introduce errors. This imposes a limit on the capacity of the fibre.
The optical nonlinearities of glass are responsible for this problem. If light could be confined in air or in a vacuum, then the permissible power levels could be much higher. Four years ago Philip Russell and co-workers at Bath University in the UK proposed that the answer could lie in a concept called "photonic band-gap confinement". In photonic crystal structures, the band-structure ideas of solid-state physics are applied to electromagnetic waves. Light cannot propagate in a photonic band gap, in much the same way that electrons are forbidden within the electronic band gap of semiconductors. By exploiting photonic band-gap confinement, a well defined wavelength of light can be trapped and guided through a hollow fibre surrounded by a material with a photonic crystal structure.
Photonic band-gap structures are inspired by the 2-D and 3-D geometry of both natural crystals, and artificial crystals that can arise only in the human imagination. However, it is not straightforward to determine the structures needed for some applications.
In the November issue of Physics World magazine Eli Yablonovitch from the University of California at Los Angeles describes how Russell and co-workers at Bath, the UK Defence Evaluation and Research Agency in Malvern, and the optical fibre manufacturer Corning in New York (R Cregan et al. 1999 Science 285 1537) discovered the 2-D crystal structure that can confine light in the core of a hollow fibre.