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Glass fibre drawing

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Copper is no longer the first choice for modern information transmission. Glass fibre cables (optical fibres) have assumed this task over longer distances, as they are vastly superior to the previous metal solution. Only 125 µm thick, a single glass fibre cable is theoretically sufficient to transmit 100,000 million telephone conversations. In contrast to copper, no skin effects occur (forcing of the current with increasing frequency from the middle of the cable to the surface by eddy currents), so that considerably higher frequencies can also be utilised and high transmission rates achieved. They also do not require suppression measures against electromagnetic radiation and can thus be combined with high voltage cables to form inexpensive solutions. High-purity quartz glass forms the core of glass fibre cables, being coated with glass of lower refraction (doped quartz glass). Total reflection thus occurs in the area between the core and the jacket, so information to be transmitted is conveyed in the core in the form of (IR) light. The outer jacket forms a polymer coating that contributes to mechanical stability.

Glass fibre cables are manufactured in a drop tower at approx. 2000 °C through simultaneous drawing (and collapsing) of the core and jacket from the respective preform (tubes). Graphite and CFC resistance heating elements are suitable for heating these furnaces, as these can be produced in ultra-pure forms. However, even the smallest of impurities in glass fibres considerably increase the level of evaporation. They can even resist temperatures up to 2,800 °C, remaining absolutely free of distortion. NTC behaviour enables these materials to realise a high heating efficiency in a vacuum with minimum power.

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