tenary, can be built into the cable sheath to give a figure-of-eight at a core cladding boundary where the refractive index of the section and clamped to each supporting pole. Cable joint housings cladding is less than that in the core. A multimode fibre supports are usually mounted part way down the pole. Two techniques are many hundreds of modes which can be conceptually considered as used for underground installation, by duct and by direct burial. light rays. Each ray is totally reflected at the core/cladding boundary Ducts have the advantage that additional cables can be added and and progresses along the fibre by multiple reflections back into the obsolete cables removed relatively easily. Ducts are the normal core. Axially injected rays experience fewer reflections than rays solution in urban areas and are preferred by some operating com- injected at an angle to the axis. panies (e.g. British Telecom) for virtually all applications. For many Before considering the properties of optical fibres, it is useful to years lead was the preferred cable sheath as it is totally impervious have some appreciation of how fibres are made. and waterproof joints can be made. Plastic, particularly polye- Fibre manufacture requires a high purity glass which is generally thylene offers lower cost, greater toughness and lighter weight, but achieved by fabrication from compounds in a gaseous form. A its widescale adoption had to wait until sheath jointing by either typical process is by chemical vapour deposition (CVD) the prin- welding or epoxy adhesives were developed. The polyethylene ciples of which are as follows: silicon tetrachloride, oxygen and sheath is applied over and bonded to an aluminium/polyethylene germanium chloride gases are fed into a silica tube which is rotating laminate moisture barrier. This aluminium foil also provides the in a lathe. A hot burner traverses along the tube causing a chemical equivalent electrical screen to a lead sheath. reaction at that point and glass to be deposited on the inside wall o