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First, a cylindrical preform is made by depositing layers of specially formulated silicon dioxide on the inside surface of a hollow substrate rod. The layers are deposited by applying a gaseous stream of pure oxygen to the substrate rod. Various chemical vapors, such as silicon tetrachloride (SiCl 4 ), germanium tetrachloride (GeCl 4 ), and phosphorous oxychloride (POCl 3 ), are added to the stream of oxygen. As the oxygen contacts the hot surface of the rod-a flame underneath the rod keeps the walls of the rod very hot-silicon dioxide of high purity is formed. The result is a glassy soot, several layers thick, deposited inside the rod. This soot will become the core. The properties of these layers of soot can be altered depending on the types of chemical vapors used.
Fig.Illustration of MCVD (inside) process
After sufficient layers are built up, the tube is collapsed into a solid glass rod referred to as a preform. It is now a scale model of the desired fiber, but much shorter and thicker. The preform is then taken to the drawing tower, where it is pulled into a length of fiber up to 10 kilometers long.
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One of many variations of vapour deposition technique for fabricating optical fiber. Here an inert rod is layered with core and cladding glass deposits built up on the outside. Once enough layers are in place, the rod is removed and the layers consolidated into a solid preform which can be drawn into fiber. Silicon chloride, SiCl4 and germanium chloride, GeCl4 are oxidised to form silica and germania particles for the deposition.
Vapour Phase Axial Deposition (VPAD)
In this diagram, we see how the preform is made. A seed rod is slowly rotated and pulled upward. As the seed rod is pulled, two burners deposit fine glass soot. The lower burner in this diagram is depositing the core glass material, and above it is a burner depositing the cladding glass. The rate at which the seed rod is pulled is carefully controlled by servo mechanisms. After deposition the glass soot rod is dehydrated and sintered into a solid preform in a furnace.
Drawing the fiber
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