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DISPERSION
There are two different types of dispersion in optical fibers.
The types are intramodal and intermodal dispersion.
Intramodal, or chromatic,
dispersion occurs in all types of fibers. Intermodal, or modal, dispersion occurs only in
multimode fibers. Each type of dispersion mechanism leads to pulse spreading. As a pulse
spreads, energy is overlapped. This condition is shown in figure 2-24. The spreading of
the optical pulse as it travels along the fiber limits the information capacity of the
fiber.
Figure 2-24. - Pulse overlap.
Intramodal Dispersion
Intramodal, or chromatic, dispersion depends primarily on fiber materials. There are
two types of intramodal dispersion. The first type is material dispersion. The second type
is waveguide dispersion.
Intramodal dispersion occurs because different colors of light travel through
different materials and different waveguide structures at different speeds.
Material dispersion occurs because the 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. Different wavelengths of a
light pulse that enter a fiber at one time exit the fiber at different times. Material
dispersion is a function of the source spectral width. The spectral width specifies the
range of wavelengths that can propagate in the fiber. Material dispersion is less at
longer wavelengths.
Waveguide dispersion occurs because the mode propagation constant (β) is a
function of the size of the fiber's core relative to the wavelength of operation.
Waveguide dispersion also occurs because light propagates differently in the core than in
the cladding.
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.
However, in single mode fibers, material and waveguide dispersion are interrelated.
The total dispersion present in single mode fibers may be minimized by trading material
and waveguide properties depending on the wavelength of operation.
Q.46 Name the two types of intramodal, or chromatic, dispersion.
Q.47 Which dispersion mechanism (material or waveguide) is a function of the size of the
fiber's core relative to the wavelength of operation?
Intermodal Dispersion
Intermodal or modal dispersion causes the input light pulse to spread. The input light
pulse is made up of a group of modes. As the modes propagate along the fiber, light energy
distributed among the modes is delayed by different amounts. The pulse spreads because
each mode propagates along the fiber at different speeds. Since modes travel in different
directions, some modes travel longer distances. Modal dispersion occurs because
each mode travels a different distance over the same time span, as shown in figure 2-25.
The modes of a light pulse that enter the fiber at one time exit the fiber a different
times. This condition causes the light pulse to spread. As the length of the fiber
increases, modal dispersion increases.
Figure 2-25. - Distance traveled by each mode over the same time span.
Modal dispersion is the dominant source of dispersion in multimode fibers. Modal
dispersion does not exist in single mode fibers. Single mode fibers propagate only the
fundamental mode. Therefore, single mode fibers exhibit the lowest amount of total
dispersion. Single mode fibers also exhibit the highest possible bandwidth.
Q.48 Modes of a light pulse that enter the fiber at one time exit the fiber at
different times. This condition causes the light pulse to spread. What is this condition
called?
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