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Optical Fibre: principles.
An optical fibre is a flexible light guide consisting of two different materials, one for the core, one for the optical cladding, surrounding the first one.
This production is more or less complex, according to the type of fibres that you want to obtain.
The core production
The most currently produced optical fibres are made from a synthetic fused silica glass.
This extremely pure synthetic glass is obtained by thermal oxidation of silicon tetrachloride (SiCl4), oxidation taking place at very high temperature (2100°C approx.), following the chemical formula: SiCl4 + O2 > SiO2 + 2Cl2.
This production consists of multi deposition of glass layers on a starting silica block so as to obtain a cylinder of glass of several centimetres (10 to 15) on a given length. The obtained block, of extreme purity (99,9999 % of SiO2) will be the basic core material of the different types of speciality fibres used in many applications, including medical.
Due to the different thermal methods used for the material synthesis (H2/O2 flame, dry plasma torch, wet plasma torch), different types of silica glasses will be obtained. This will be due to the residual amount of water contained in the glass (in ppm - parts per million), amount (from 0,1 to 1600 ppm) which will define the transmission characteristics of the glass.
A "dry" silica will be optimised for near-IR and Visible, a "wet" silica will be optimised for UV and Visible. This silica glass, in an ingot form, cannot be directly drawn into fibres. It will be either drawn into rods (preforms) for the production of PCS or HCS fibres or kept as such for further transformation, so as to produce AS fibres (All Silica Fibres).
