THE trouble with most solid optical fibres is that they soak up some of the
light energy as it zips along, so less power gets to the other end. But now a
team from the University of Bath has developed optical fibres with a big hole in
the middle which transmit huge amounts of energy compared with conventional
fibres, just as developer Philip Russell said he hoped to do a few weeks ago in New Scientist
(12 June, p 38).
Conventional optical fibres rely on total internal reflection to guide the
light through the core, which has a higher refractive index than the surrounding
cladding. Gases inevitably have much lower refractive indexes than solids. This
means the core of these fibres cannot be hollow, and the light must pass through
But even when the glass is extremely clear, it can only transmit a limited
amount of power. Light concentrated into a tiny core interacts with the glass
and itself, generating noise, reducing power at the desired wavelength, and at
high power levels there’s catastrophic damage to the glass, usually at the ends
of the fibre.
Russell’s group uses a different mechanism, called a “photonic band gap”,
which they create by alternating layers of two materials with different
refractive indices. To make their fibres, they align many thin glass tubes, melt
them together, then stretch the resulting cylinder into a thin fibre with many
long, thin holes along its length. The band-gap effect happens when light is
scattered at the boundaries of air and glass. The effect depends on the
wavelength and angle of the light and the thickness and refractive indices of
the layers. At certain angles, the scattering prevents light of certain
wavelengths from penetrating the multilayer material so it goes straight down
the centre of the fibre.
Early photonic band-gap fibres made by the Bath team were built like a a
honeycomb. While the fibre guided light, it was confined to the glass region
along the fibre axis—so it had power limits similar to a conventional thin
The real breakthrough happened when they removed a single tube from the
centre to guide the light through a central hole. “The key breakthrough for us
was to realise that the air hole had to be quite large for it to work,” Russell
says. Removing the central seven tubes left a central hole that was 14.8
micrometres across when the glass was stretched into a fibre—large enough
to act as a light waveguide in its own right (see Diagram).
The band gap guides certain wavelength of light along the hollow core. “In
many respects, it behaves like a hollow metal waveguide,” says Russell. It’s an
optical wavelength analogue of the metal tubes used in microwave antennas. When
white light illuminates one end of the structure, the central hole glows with
red light, while the smaller 3-micrometre holes glow white as the light leaks
through them (Science, vol 285, p 1537).
The fibres could dramatically boost the power of industrial laser cutters.
And one day, they might guide atoms over long distances, forming the basis of
fibre-optic devices that move matter.