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Optical fiber is mostly made from silicon dioxide(SiO 2 ) but some little amount of other materials such as fluorozirconate, fluoroaluminate, and chalcogenide glasses as well as crystalline materials like sapphire, are used for longer-wavelength infrared or other specialized applications.Chemical compounds such as germanium tetrachloride (GeCl4 ) and phosphorus oxychloride (POC1 POCl3 ) can be used to produce core fibers and outer shells, or claddings, with function-specific optical properties.
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Silica, which be drawn into fibers at reasonably high temperatures, has a fairly broad glass transformation range. One other advantage is that fusion splicing and cleaving of silica fibers is relatively effective. Silica fiber also has high mechanical strength against both pulling and even bending, provided that the fiber is not too thick and that the surfaces have been well prepared during processing. Even simple cleaving (breaking) of the ends of the fiber can provide nicely flat surfaces with acceptable optical quality. Silica is also relatively chemically inert. In particular, it is not hygroscopic (does not absorb water) also it can be doped with various materials. Silica fiber also exhibits a high threshold for optical damage. But, pure silca is not best suitable for optical fiber, because it exhibits a low solubility for rare earth ions. This can lead to quenching effects due to clustering of dopant ions. These properties makes silca most widely use material for optical fibers.
Process of Manufacutring
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There are two main steps in the process of converting raw materials into optical fiber ready to be shipped.
- Manufacturing of the pure glass preform
- Drawing of the preform
The first step in manufacturing glass optical fibers is to make a solid glass rod, known as a preform. Ultra-pure chemicals -- primarily silicon tetrachloride (SiCl4) and germanium tetrachloride (GeCl4) -- are converted into glass during preform manufacturing. These chemicals are used in varying proportions to fabricate the core regions for the different types of preforms. The basic chemical reaction of manufacturing optical glass is:
- SiCl4 (gas) + O2 > SiO2 (solid) + 2Cl2 (in the presence of heat)
- GeCl4 (gas) + O2 > GeO2 (solid) + 2Cl2 (in the presence of heat)
The core composition of all standard communication fibers consists primarily of silica, with varying amounts of germania added to increase the fiber's refractive index to the desired level. Single-mode fibers typically have only small amounts of germania and have a uniform composition within the core. Multimode fibers typically have a much higher refractive index, and therefore much higher germania content. Also, the core composition and the refractive index of graded-index multimode fibers changes across the core of the fiber to give the refractive index a parabolic shape.
Today Three methods are used to manufacture optical fiberperform.
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Modified Chemical Vapor Deposition(
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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 (POC1 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.
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