This week's stories from New Scientist available online. 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 glass . 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. 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.