Learning About Options in Fiber - Cables Plus USA

THE OPTICAL FIBER
BASIC FIBER CONSTRUCTION
SECTION 2—FIBER-OPTIC BASICS
propagation is governed by the indices of the core
and cladding (and by Snell’s law.) Refer to section
3, Glossary of Terms, for a definition of Snell’s Law.
Optical fiber consists of a thin strand (or core) of
optically pure glass surrounded by another layer
of less pure glass (the cladding). The inner core
is the light-carrying part. The surrounding
cladding provides the difference in refractive
index that allows total internal reflection of light
through the core. The index of the cladding is less
than 1 percent lower than that of the core.
Most fibers have an additional coating around the
cladding. This coating, usually one or more layers of
polymer, protects the core and cladding from
shocks that might affect their optical or physical
properties. The coating has no optical properties
affecting the propagation of light within the fiber.
Thus the buffer coating serves as a shock absorber.
Figure 2-1 shows the idea of light traveling through
a fiber. Light injected into the fiber and striking the
core-to-cladding interface at greater than the critical
angle reflects back into the core. Since the
angles of incidence and reflection are equal, the
reflected light will again be reflected. The light will
continue zig zagging down the length of the fiber.
Light, however, striking the interface at less than
the critical angle passes into the cladding where it
is lost over distance. The cladding is usually inefficient
as a light carrier, and light in the cladding
becomes attenuated fairly rapidly.
Notice also in Figure 2-1 that the light is refracted
as it passes from air into the fiber. Thereafter, its
The specific characteristics of light propagation
through a fiber depends on many factors including:
The size of the fiber; the composition of the fiber;
and the light injected into this fiber. An understanding
of the interplay between these properties
will clarify many aspects of fiber optics.
Fiber is basically classified into three groups:
• Glass (silica) which includes single-mode step
index fibers, multimode graded index, and
multimode step index.
• Plastic clad silica (PCS).
• Plastic.
Most optical fibers for telecommunications are
made 99 percent of silica glass, the material from
which quartz and sand are formed. Figure 2-1 on
the previous page shows a fiber, which consists of
an inner core (about 8 to 100 micrometers, or
0.0003 to 0.004 inches, in diameter), a cladding
(125 to 140 micrometers outer diameter) and a
buffer jacket for protection.
The clad is made of glass of a slightly different
formula. This causes light entering the core at one
end of the fiber to be trapped inside, a phenomenon
called internal reflection. The light hits the
boundary between the core and the cladding
bouncing off the cladding much like a billiard ball
and at the same angle as it travels down the fiber.
Figure 2-1—Internal Reflection in an Optical Fiber
Jacket
Jacket
Cladding
Core
Core
Cladding
Light at less than critical
angle is absorbed in jacket
Angle of
Incidence
Cladding
Jacket
Angle of
Reflection
Light is propagated by
total internal reflection
2-1