Optical Fibre Manufacture
Communications are increasingly based on electro-optic systems in which telephones, television and computers are linked by fibre optic cables which carry information by light.
Making glass optical fibres is a highly specialised aspect of glass manufacture. Optical fibres consist of two distinct glasses, core of highly refracting glass surrounded by a sheath of glass with lower refractive index between the two glasses, it is guided by total reflection at the core-sheath interface to the other end of the fibre. In theory, a wide range of glasses can be used as long as the difference in refractive index is appropriate but the higher the refractive index of the core relative to that of the sheath glass, the greater the carrying capacity of the fibre. A typical system available commercially comprises a germanium doped silica core and a borosilicate cladding.
The aim in manufacture is to produce a fibre of glass which is so pure and free form defects that light inserted at one end will emerge at the other end a distance of 1 kilometre or more away. There are many manufacturing processes being used to produce cored fibre; two of these will illustrate the principles. All the processes require ultra-pure starting materials.
Chemical vapour deposition - high silica glass fibres are prepared by chemical vapour deposition in which layers of SiO, are deposited to make a preform, either on the outside of a mould or on the inside of a fused silica tube. The layers are doped during the deposition to control the refractive index. The preform is then drawn to a rod and subsequently to a fibre of 100-125mm diameter. The surface is protected from damage by a plastic coating.
The double crucible method
The double crucible uses purified glasses in separate crucibles in a controlled atmosphere furnace. Fibre drawn from the tip consists of a uniform core drawn from the central crucible and a cladding drawn from the outer crucible.
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