The light is guided within a dielectric waveguide. This consists of a core of denser glass surrounded by a cladding glass of lower refractive index.
The hair-thin fibres are drawn from a thick glass preform heated at one end.
The preforms can be made in various ways. The earlier methods introduced serious losses and are now largely abandoned.
The first fibre preforms were made by the “Rod in Tube ” process, in which a rod of denser glass was inserted into a tube of less dense glass. These fibres suffered from high scattering loss due to the imperfect interface between rod and tube.
Some unsuccessful attempts were made to draw fibres from liquid rather than a solid preform rod. The “Double Crucible” method offered the possibility of a continuous process, but the fibres made by this method were lossy due to impurities absorbed from the crucible material.
Crude large core fibres were made by simply drawing down a silica rod, and coating it with a plastic of lower refractive index. This was an easy way of making large core (Multimode) fibres but they were unsuitable for long distance communication.
The succesful methods of manufacture used some form of chemical vapour deposition. The glass was created from oxidation of ultra-pure chemicals, such as Silicon TetraChloride and Germanium TetraChloride.
The heated preform end is then drawn down into a thin fibre.