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What are the differences among the types of
dispersions?
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Modal dispersion
Material dispersion
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Waveguide dispersion
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Chromatic dispersion=

Material dispersion + Waveguide dispersion
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Polarization-Mode Dispersion
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Modal Dispersion
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The input light pulse is made up of a group of
modes occurs because each mode travels a
different distance.

Modal dispersion is the dominant source of
dispersion in multimode fibers.

Modal dispersion does not exist in single mode
fibers.

Each mode enters the fiber at a different angle
and thus travels at different paths in the fiber
Solutions ( see lecture 4)
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Material dispersion because
an optical pulse emitted by a light source
has a certain spectral width.
Spreading of a light pulse is
dependent on the wavelengths'
interaction with the refractive
index of the fiber core.
Different wavelengths travel at
different speeds in the fiber
material.
So
each
frequency
component
travels at a slightly different
speed.
Its effects in a fiber are
measured in picoseconds per
nanometer of spectral width per
kilometer (ps/nm/km)
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http://upload.wikimedia.org/wikipedia/com
mons/f/f5/Light_dispersion_conceptual_wav
es.gif
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In single mode fiber, the wavelength of the light
is not that much bigger than the core of the
fiber as a result the light traveling down the
fiber actually travels in an area that exceeds
the diameter of the core
With longer wavelengths traveling in a larger
mode field diameter. Thus part of the light is
traveling in the geometric core of the fiber and
part is traveling in the cladding.
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Since the core is made of a higher index of refraction
glass than the cladding, the light in the cladding
travels faster than the light in the core. Longer
wavelengths have larger mode field diameters so
they suffer more material dispersion.

Why?

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

In
multimode
fibers,
waveguide
dispersion and material dispersion
are basically separate properties.
Multimode waveguide dispersion is
generally
small
compared
to
material dispersion.
Waveguide dispersion is usually
neglected.
Why?
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Occurs in single-mode fiber,
and results from the
combination of effects from:
1- Material dispersion and
2- Waveguide dispersion
When chromatic dispersion
occurs, the effects of material
dispersion,
compound
the
effects
of
waveguide
dispersion
One way to reduce chromatic dispersion is by taking advantage of the
fact that the relationship between wavelength, refractive index, and
velocity is not linear

Somewhere in the 1300 nm to 1550
nm range there is a crossover where,
depending on the specific composition
of the fiber, the refractive index is the
same. for the wavelengths within the
narrow spectral width of the light being
transmitted
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This zero-dispersion point normally occurs at 1300 nm in a standard
single-mode fiber.
Other characteristics of the fiber attenuate the signal at this
wavelength, making it unusable for long-distance runs

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

There are two ways to reduce chromatic
dispersion in fiber
Dispersion-Shifted Fiber
shifts the zero-dispersion point to 1550 nm
the signal can travel a greater distance
through the fiber without significant attenuation
Reduced Spectral Width
Because material dispersion is caused by an
overabundance of wavelengths in the optical
signal. So, the simplest solution is to reduce
the number of wavelengths by reducing the
spectral width.
 The light consists of wave variations in electric and
magnetic fields.
 Many applications require that the electric and
magnetic fields have a particular direction
 If the direction of one of these fields is known, the other direction must
specified since the two are always perpendicular.
 The orientation of the two waves along the path of propagation
determines the light’s polarization mode , or polarity
 Different polarities of light may travel through the
fiber in a signal, occupying different parts of the fiber as they pass through
it
 Because no fiber is perfect, there will be obstacles in one part of the
fiber that are not present in another.
 So, the light having one polarity may pass an area without
interference, while another polarity may pass through a defective
region, slowing it down
.

Polarization-mode dispersion is a result of

1-irregularities,
2- damage, or
3-environmental conditions such as temperature
The exact amount of PMD changes with external
conditions, the physical condition of the fiber, and the
polarization state of the light passing through it at any given
moment.
PMD is measured in terms of the total difference in the
travel time between the two polarization states, referred to
as the differential group delay (DGD) and measured in
picoseconds
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

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While different types of dispersion have
different causes and are measured at different
rates all of them have one effect in common:
they place a limit on the bandwidth of optical
fibers.