Glass optical fiber is drawn as one continuous thread from a single cylinder of purified glass called a preform . The glass preform is manufactured by a process of chemical vapor deposition (CVD), whereby purified, gaseous glass vapor (silica) is accreted onto the surface of a uniform glass cylinder in thin layers under conditions of controlled heat and pressure. As the glass accumulates, impurities may be mixed into the glass vapor to modify the index of refraction of the finished product.
The two most common CVD processes for making a preform are outside vapor deposition (OVD) and inside vapor deposition  ,  ,  . The outside vapor deposition process deposits doped silica on the outside of a rotating glass mandrel, with growth taking place radially. The inside vapor deposition process works backwards , depositing doped silica onto the inner surface of a pure silica tube until it is almost filled. The resulting inside deposition preform has a hollow core , which is squeezed shut when the fiber is drawn.
Either process results in a large cylinder of pure glass with a carefully controlled radial variation in the index of refraction (Figure 11.1). A typical preform has a diameter in the range of 1 cm to 6 cm and a length of 1 m to 2 m  . The inside portion of the cylinder, where light will eventually flow, is called the core. The outer portion of the cylinder, which acts as a mirror to keep the light centered in the core, is called the cladding. Only a small percentage of the light power carried in a glass fiber travels in the cladding.
Figure 11.1. The ideal refractive-index profile for a graded-index multimode preform is almost perfectly parabolic .
What happens next is, to me, the truly amazing part. The preform is heated in a drawing apparatus, and a finished fiber is pulled from the bottom (Figure 11.2). As the preform sinks into the drawing funnel, the profile of the index of refraction is squeezed down to microscopic dimensions but retains its shape . A single fiber pulled from a preform may be as long as 100 km  . The finished fiber is given a protective polymer coating, reeled, and tested .
Figure 11.2. A finished fiber is pulled from the bottom of a heated preform.
POINT TO REMEMBER