The primary material in optical fibers is silica glass 
. Impurities called dopants are purposely added to
modify the physical properties of the fiber, primarily the index of
refraction. Recall the index of refraction of the core has a slightly higher
index than the cladding. The nominal value for pure silica is n=1.458. In
what follows, let 
, 
denote the refractive indices for the core
and cladding, respectively, with 
defined as:
In the case of step-index fibers, is constant in the core. For
graded-index (GI) fibers, the refractive index is maximum at the center; in
this case, denotes this peak value. Single mode (SM) fibers have a
core radius of 
, and multi-mode (MM) step-index or graded-index
fibers have core diameter of about 
. The cladding diameter is
typically up to ten times the core diameter.
Current technology permits optical fibers with loss as low as 0.2dB/km in
the 
range (with the nominal center wavelength usually 
). This loss is near the the
oretical lower limit for silica glass,
so current research is focusing on developing optical systems (sources and
detectors as well as fibers) that operate in the mid-infrared ( 
)
and far-infrared ( 
) ranges. For example, losses as low as 0.01dB/km have been reported at 
in fluoride glass. Here,
however, we shall discuss conventional modern silica fibers operating in the
range.
The index of refraction is typically lowered by adding germanium 
or phosphorus 
in glass form, that is 
and 
, respectively, and is typically raised by adding fluorine 
in elemental form
or boron 
in glass
form as 
. Glass is a noncrystalline material that is usually
modeled as a very viscous liquid. Refer to Figure 1.
As a liquid is cooled, its volume decreases at a relatively rapid rate. When
the melting temperature 
is reached, a sudden
drop in volume occurs as the liquid crystallizes. The atoms or molecules get
locked into precise, repetitive symmetrical arrangements; removal of heat
does not decrease the temperature but instead increases the extent of
crystallization. After the liquid is solidified, further drops in
temperature cause significantly smaller changes in volume. If
crystallization does not occur at 
, then removal of heat causes further
contraction of the constituents atoms or molecules, resulting in a supercooled liquid. At the glass transition temperature 
, a sudden change in the sl
ope of the volume versus temperature
curve, called the coefficient of thermal expansion, occurs. Further
reduction in temperature affects the volume very slightly. The liquid flows
very slowly, and behaves much more like a solid than a liquid. There is no
long range crystalline order, but there is short range order. The thermal
agitation of the constituents atoms or molecules is sufficiently low that
attractive forces bind atoms or molecules across several atomic diameters,
thus accounting for the high viscosity. The process of forming a glass is
called vitrification.
The impurities in the silica are chemicals in solution, and so the
chemical formulas as denoted 
, or 
, and so forth. This notation means that or F,
respectively, are interspersed with 
, and substitute for uniformly through the m
aterial, much like dopants in a
semiconductor or other crystalline solid either substitute for substrate
atoms or occupy interstitial sites between substrate atoms, but do not
fundamentally change the crystalline structure. Table 1 summarizes the maximum change in index of refraction, 
, where 
is the index of refraction for pure , which can be obtained by adding various dopants.
Table 1: Change in Index of Refraction for Various Dopants
The most common methods for manufacturing optical fibers are classified as
vapor deposition (VD). The raw materials are liquids such as 
, 
, 
, 
or 
which are heated to gas
form (vapors). The hot vapors combine with oxygen gas 
and
solidify into the glass compounds. In most cases, vitrification does not
occur directly from the vapor form. Instead, the silica and other glasses
are deposited on a surface as a fine powder or particulate form, called
soot. The soot is heated and cooled to vitrify to a glass called the
preform rod. The preform rod is very thick (perhaps by several
orders of magnitude) compared to the final fiber. The process of converting
the preform rod to the final optical fiber is called drawing. The
preform rod is suspended and one end is heated to cause a thin glass stream
to flow under gravity. This stream is cooled and becomes the final fiber.
The fiber is spooled by wrapping it around a drum for storage.
