h impurities within a tube prior and during its heating. The soot produced by the heating process results in impurities that define the index of refraction (refrac- tive index). As we will note later in this chapter, the refractive index dif- ference between the core and the outer area of a fiber, which is referred to as the cladding, results in light pulses remaining inside the fiber. The deposit of chemical vapor or gas is referred to as modified chemical vapor deposition (MCVD). A second fiber fabrication process results in vapor deposits placed on the outside of the tube. This process is referred to as outside vapor deposition (OVD). Traversing Burning Both MCVD and OVD reference the location of the vapor with respect to the glass tube. The actual fabrication process that uses the glass tube to create a spooled wire employs a technique referred to as traversing burning This technique consists in rotating a large glass tube over a heat source while passing a series of gases through the tube. The soot from the gases is deposited either inside (MCVD) or outside (OVD) the tube until a certain level is reached where the flow of gases is stopped. At that time the heat is increased and the sooted glass tube is stretched. The resulting solid rod of glass is then placed in a furnace where it is melted and allowed to trickle down through an opening to form a fiber-optic strand. As the fiber exits the furnace, it is coated and spooled by a winder. The coating of the fiber results in an epoxy or plastic film added to the glass to provide protection. This process, which is referred to as a fiber-drawing tower, is illustrated in Figure 3.1. Chapter Three Figure 3.1 A popular method of manufacturing fiber glass strands employs a fiber-drawing tower through which a glass compound is heated, stretched, Furnace and spooled. primary fiber coating process coating curing winder The process illustrated in Figure 3.1 represents a general schematic of a fiber-drawing tower. Not shown but of extreme importance are feedback control sy