Optical fiber

An optical fiber is a transparent thin fiber, usually made of glass or plastic, for transmitting light. Fiber optics is the branch of science and engineering concerned with such optical fibers.

Manufacture

One technique for the manufacture of optic fibers is the Modified Chemical Vapour Deposition technique, where a hollow glass tube approximately 40cm in length known as a "preform" is placed horizontally and rotated slowly on a lathe. Gases such as Silicon Tetrachloride (SiCl4) or Germanium Tetrachloride (GeCl4) are injected with oxygen in the end of the tube. The gases are then heated by means of an external hydrogen burner, bringing the temperature of the gas up to 1900 Kelvin, where the tetrachlorides react with oxygen to produce silica or germania oxide particles.

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These particles then agglomerate to form large particle chains, which subsequently deposit on the walls of the tube as soot. The deposition is due to the large difference in temperature between the gas core and the wall causing the gas to push the particles outwards (this is known as thermophoresis). The torch is then traversed up and down the length of the tube to deposit the material evenly. After the torch has reached the end of the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to form a solid layer. This process is repeated until a sufficient amount of material has been deposited, for each layer the composition can be varied by varying the gas composition. This composition change results in a change in the finished fiber's optical properties.

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This preform is then placed in a device known as a drawing tower, where the preform tip is heated and the optic fiber is pulled out as a string. By measuring the resultant fiber width, the tension on the fiber can be used to maintain the fiber thickness.

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