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PROPERTIES OF OPTICAL FIBER TRANSMISSION
The principles behind the transfer of light along an optical fiber were discussed
earlier in this chapter. You learned that propagation of light depended on the nature of
light and the structure of the optical fiber. However, our discussion did not describe how
optical fibers affect system performance.
In this case, system performance deals with signal loss and bandwidth.
Signal loss and system bandwidth describe the amount of data transmitted over a
specified length of fiber. Many optical fiber properties increase signal loss and reduce
system bandwidth. The most important properties that affect system performance are fiber
attenuation and dispersion.
Attenuation reduces the amount of optical power transmitted by the fiber. Attenuation
controls the distance an optical signal (pulse) can travel as shown in figure 2-19. Once
the power of an optical pulse is reduced to a point where the receiver is unable to detect
the pulse, an error occurs. Attenuation is mainly a result of light absorption, scattering,
and bending losses. Dispersion spreads the optical pulse as it travels
along the fiber. This spreading of the signal pulse reduces the system bandwidth or the
information-carrying capacity of the fiber. Dispersion limits how fast information is
transferred as shown in figure 2-19. An error occurs when the receiver is unable to
distinguish between input pulses caused by the spreading of each pulse. The effects of
attenuation and dispersion increase as the pulse travels the length of the fiber as shown
in figure 2-20.
Figure 2-19. - Fiber transmission properties.
Figure 2-20. - Pulse spreading and power loss along an optical fiber.
In addition to fiber attenuation and dispersion, other optical fiber properties affect
system performance. Fiber properties, such as modal noise, pulse broadening, and
polarization, can reduce system performance.
Modal noise, pulse broadening, and polarization are too complex to discuss as
introductory level material. However, you should be aware that attenuation and dispersion
are not the only fiber properties that affect performance.
Q.37 Attenuation is mainly a result of what three properties?
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