Following the discovery by Kao and Hockman(1-3) that ultra-low-loss optical
fibres could be made from pure silica through the elimination of impuritie
s, the ability to guide signals effectively at optical wavelengths has been
assured. But there remains an important region of the spectrum-from 30 to
3,000 GHz (the millimetre-submillimetre band)-where low-loss waveguides are
unknown. The main problem here in finding low-loss solids is no longer one
of eliminating impurities, but is due to the presence of intrinsic vibrati
on absorption bands(4-6). And the use of highly conducting materials is als
o precluded owing to high skin-depth losses(7,8) in this part of the spectr
um. Here we show that a combination of material and waveguide geometry can
circumvent these difficulties. We adopt a ribbon-like structure with an asp
ect ratio of 10:1, fabricated from ceramic alumina (Coors' 998 Alumina), an
d the resulting waveguide has an attenuation factor of less than 10 dB km(-
1) in the millimetre-submillimetre band. This attenuation is more than 100
times smaller than that of a typical ceramic (or other dielectric) circular
rod waveguide and is sufficient for immediate application.