 is manufactured by melting and reshaping silica, IVD deposits various combinations of carefully selected compounds on the inside surface of a silica tube. The tube becomes the cladding of the fiber; the vapor- deposited compounds become the core. The compounds are typically sili- con chloride (SiCl4) and oxygen (O2), which are reacted under heat to form a soft, sooty deposit of silicon dioxide (SiO2), as shown in Figure 7- 17. In some cases, impurities such as germanium are added at this time to cause various effects in the finished product. In practice, the SiCl4 and O2 are pumped into the fused silica tube as gases; the tube is heated in a high-temperature lathe, causing the sooty deposit to collect on the inside surface of the tube. The continued heating of the tube causes the soot to fuse into a glasslike substance. This process can be repeated as many times as required to create a graded refractive index, if required. Ultimately, once the deposits are complete, the entire assembly is heated fiercely, which causes the tube to collapse, creating what is known in the optical fiber industry as a pre- form. An example of a preform is shown in Figure 7-18. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Chapter 7 Figure 7-17 Creating a multilayer preform SiCI4 Figure 7-18 Preforms, ready to be drawn An alternative manufacturing process is called outside vapor deposi- tion (OVD). In the OVD process, the soot is deposited on the surface of a rotating ceramic cylinder in two layers. The first layer is the soot that will become the core; the second layer becomes the cladding. Ultimately, the rod and soot are sintered to create a preform. The ceramic is then removed, leaving behind the fused silica that will become the fiber. There are a number of other techniques for creating the preforms that are used to create fiber, but