Page 51 - Optical Switching And Networking Handbook

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36 Chapter 2

Table 2-2 Impairment Discussion
Problems and
Impairments Attenuation Like an electrical signal moving on copper, the light pulse
Found in Fiber will attenuate on the fiber. The signal gets weaker because
Systems a certain portion of the light is absorbed by the glass.
The actual frequency determines the amount and speed
of absorption. Attenuation is stated in the form of
decibels (dB).
Dispersion When the pulse is sent down the fiber, it spreads out during
the transmission. The short pulse becomes longer and joins
with the pulse behind it. This makes it difficult (or impossi-
ble) for the receiving equipment to separate the pulses.
There are different forms of dispersion, including
Material—A range of frequencies is produced by the LED
and laser. The materials used to create the fiber cable use
different refractive indices; therefore, each wavelength
moves at a different speed inside the fiber cable. This means
that some wavelengths arrive before others and a signal
pulse disperses over a broader range. This is also called
smearing.
Waveguide—The center core creates the waveguide (it
guides the wave inside the core). The shape and the refrac-
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tive index inside the core can create the dispersion or
spreading of the pulse.
Modal—Multimode fiber creates many different modes
(paths) for the light to travel down the fiber. The length of
the path can be different depending on the mode taken,
meaning the light beams may take different paths of differ-
ing lengths. Portions of the light may arrive out of sequence.
This means that the spreading over time causes the fiber
receiver to have to deal with this. The longer the route, the
bigger is the problem.
Noise Modal noise is usually associated with multimode fiber. The
mismatch of the connectors and the modes in a cable can
cause loss of some of the modes. This causes signal loss,
which is defined as noise.
Polarization Optical fibers in normal systems are cylindrical and sym-
metric. Light traveling on the fiber can change in its polar-
ity (positive to negative). At the higher speeds, this may
pose problems.

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