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| An optical fibre is typically
only 125 µm in diameter, and has a core of a different
refractive index running exactly down its centre. However,
it starts life as a rod of solid glass about 25 mm in diameter
and about 1 metre in length. This is called a preform and
is an exactly scaled up version of the core and cladding
of the final fibre. |
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The preform is
usually made using a process called modified chemical
vapour deposition (MCVD).
A cylinder of pure silica is spun in a special lathe,
and heated from the outside. Meanwhile, a vapour
created by forcing oxygen through solutions of silica
and dopant is sprayed on the inside of the cylinder.
The heat causes the glass to deposit on the inside
of the cylinder, building the core of the preform
inwards. The level of dopant can be tightly controlled
to create the desired refractive index profile.
Finally the cylinder is collapsed to form the preform
rod
The fibre itself is drawn from the preform, which
is loaded at the top of a fibre drawing tower and
heated to about 2050°C. The fibre diameter is
monitored as it is drawn, and the speed of drawing
adjusted to keep the diameter constant. As the fibre
cools, it is also fed through coating cups to apply
the required buffer coatings, which are cured in
ultraviolet ovens, before the coated fibre is wound
onto spools.
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