How Are Optical Fibers Made

Now that we know how fiber-optic systems work and why they are useful -- how do they make them. Optical fibers are made of extremely pure optical glass. We think of a glass window as transparent, but the thicker the glass gets, the less transparent it becomes due to impurities in the glass. However, the glass in an optical fiber has far fewer impurities than window-pane glass. One company's description of the quality of glass is as follows: If you were on top of an ocean that is miles of solid core optical fiber glass, you could see the bottom clearly.

Making optical fibers requires the following steps:1.Making a preform glass cylinder2.Drawing the fibers from the preform3.Testing the fibersThe glass for the preform is made by a process called modified chemical vapor deposition (MCVD).

In MCVD, oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4) and/or other chemicals. The precise mixture governs the various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside of the tube. The extreme heat from the torch causes two things to happen:

Lathe used in preparing the preform blankThe silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2).The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass.The lathe turns continuously to make an even coating and consistent blank. The purity of the glass is maintained by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by precisely controlling the flow and composition of the mixture. The process of making the preform blank is highly automated and takes several hours. After the preform blank cools, it is tested for quality control.

November 1, 2011

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