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Platinum Metals Rev., 1973, 17, (3), 88

Production of Optical Glass Fibres

A Special Fibre Optic Grade of Platinum

  • By A. E. Heywood
  • Johnson Matthey Metals Limited, London
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The use of flexible transparent materials having a high refractive index in the form of thin rods or fibres as a means of transmitting thin beams of light is well established. Perhaps the most widely used material for the production of these fibres ‘ light guides’ is polymethyl methacrylate, which is used in applications such as surgical endoscopes, automobile warning light function indicators, decorative lighting systems and also, in the form of regularly bunched fibres, as image transmission devices. This relatively new technology has been given the name of Fibre Optics.

With the advent of the laser, interest in the use of fibre optics as a method of transmitting information has increased considerably.

In the telecommunications field the use of laser beams to carry telephonic messages has considerable attractions in that more than one message may be carried on each channel. By using thin fibre light guides thousands of messages can be carried in one small diameter cable with considerable cost savings over conventional copper cables. The material used for the fibres must be flexible, must give the highest possible internal reflectance and must have a high transmissivity. Glass can meet these requirements and is widely used.

This bushing for the production of optical glass fibres, some of which are illustrated, is fabricated from the new high purity Fibre Optic grade of platinum

This bushing for the production of optical glass fibres, some of which are illustrated, is fabricated from the new high purity Fibre Optic grade of platinum

To prevent unacceptable attenuation of the laser signals, the glass fibre must be of extremely high purity. The starting materials from which the glass is made must also therefore be of the highest purity, particularly with respect to the presence of transition metals which can cause the maximum attenuation at minimum levels with the particular wavelengths used. With starting materials of very high purity it is essential to ensure that this is not degraded on contact with the walls of the apparatus used in the manufacture of optical fibres.

Until recently the purest grades of platinum available for the fabrication of apparatus, while being extremely pure by commercial standards have contained very minor quantities of other elements, sufficient to cause some contamination of these high purity glasses.

To meet this new need a high purity grade of platinum is now available that is suitable for the fabrication of apparatus to be used for the production of these high purity glasses. The total maximum permitted metallic impurity content of this Fibre Optic grade of platinum is 10 p.p.m., with combined iron and copper content limited to a maximum of 5 p.p.m., and zinc, lead, tin and nickel to less than 1 p.p.m. each.

Optical quality glass fibres have been satisfactorily produced by melting and drawing the glass in apparatus fabricated from this high purity platinum and, because of the workability of platinum, there are almost no limits to the designs of apparatus and plant that can be built from this new high purity grade.

It is essential that the manufacturing processes used in the fabrication of such apparatus complement the purity of the platinum and, accordingly, these processes are designed to ensure that the minimum of contact with base metal occurs. As a final safeguard, all apparatus produced from this high purity platinum is, on completion, given a hot chemical etch to remove any traces of base metal that might have been picked up during manufacture despite all precautions.

Apparatus produced from this high purity grade of platinum ensures the successful, consistent and economic production of ultra high purity glasses and enables development of this technology to proceed, overcoming what might otherwise have been a major obstacle to progress.

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