Optical Fiber
Construction
and
Theory
Optical Fiber Components
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The core, which carries the light, is the smallest and most
fragile part of the optical fiber. The optical fiber core is
usually made of glass, although some are made of plastic.
In the manufacturing process, the glass used in the core
has impurities such as germanium or
phosphorous added to raise the refractive index under
controlled conditions.
Optical fiber cores are manufactured
In different diameters for different
applications
Cladding
 Surrounding
and protecting the core, and
providing the lower refractive index to make
the optical.
 The
manufacturing process adds different
amounts of impurities to the core and the
cladding to maintain a difference in refractive
indices between them of about 1 percent.
Coating
 The
coating is the true protective layer of the
optical fiber.
 Generally made of plastic , the coating absorbs
the shocks, and moisture that could damage the
cladding.
 The coating is solely protective. It does not
contribute to the light-carrying ability of the optical
fiber in any way.
Tensile Strength
 One
characteristic of optical fiber that deserves
special attention is its tensile strength.
 Tensile
strength is important for several reasons.
It affects the way fiber must be handled during
installation.
 The outer layer of the cladding provides much of the
fiber’s tensile.
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 Once
the outer layer is scratched or cracked,
however, the tensile strength is gone at that
location.
 A scratch
or crack compromises the integrity of
the glass and allows the fiber to break more
easily under stress.
Modes
 One
of the most important characteristics used to
distinguish types of fiber is the number of potential
paths light can take through it.
 As shown, the light can follow modes ranging from a
straight line through the fiber (zero-order mode) to a
low number of reflections (low-order mode) to a high
number of reflections (high-order mode).
Optical Fiber
Characteristics

In this chapter, we describe the characteristics of optical
fiber that affect the way it is selected, handled, installed
Dispersion
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o
Dispersion is the spreading of light as it travels away from
its source.
The light spreads because different components of it
travel at slightly different velocities, depending on
the conditions in the medium through which it is traveling
and the wavelengths that make up the light.
There are different kinds of dispersion, however, and the
kind that is taking place depends on several factors in the
fiber and in the light itself
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• The greatest effect of dispersion is that as the light
spreads, it can degrade or destroy the distinct pulses
of the digital signals in the light by making them
overlap each other.
• Blurring and blending them to the point that they are
unusable. The effect grows more pronounced as the
distance the light travels increases.
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 To
prevent signal loss due to dispersion, it is
necessary to keep the pulses far enough apart to
ensure that they do not overlap.
 Restricting the bit rate places a limit on the fiber’s
bandwidth , or the amount of information it can carry.
 The types of dispersion that affect optical fiber are:
Modal dispersion
 Methods for overcoming modal dispersion include:
Lower bit rate 
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 Lowering the bit rate increases the gap
between bits in the signal.
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 Graded index fiber: Graded index fiber
gradually reduces the refractive index of the fiber
core from the center toward the cladding, allowing
the light that follows a more angled path to speed up
as it leaves the center and causing it to slow down
again as it reaches the center.
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 Single-mode
fiber
Single-mode fiber has a core that is narrow enough for only one
mode to propagate, eliminating the problems caused by multiple
modes.
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 Material
dispersion
 Waveguide dispersion
 Chromatic
dispersion
 Polarization-mode
dispersion
Material Dispersion
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o
Material dispersion is the result of
different wavelengths of light traveling
at different velocities in the fiber
When the different wavelengths travel
at different velocities, the slower
wavelengths begin to lag behind as the
light travels down the fiber core,
causing the light to spread
If the light must travel a great distance,
the lag in the slower wavelengths can
cause them to overlap the faster
wavelengths of the bits following them.
As with modal dispersion, these
overlaps can degrade and ultimately
destroy the signal.
Waveguide Dispersion
 Occurs in single-mode fiber as the light passes through not
only the core, but also part of the cladding.
 Because, by design, the core has a higher refractive index
than the cladding, the light will be traveling more slowly
through the core than through the cladding.