Page 65 - Optical Communications Essentials

Optical Fibers

Optical Fibers 55

Since often fibers need to be bent into very tight loops within component
packages, special fibers that are immune to bending losses have been developed
for such applications. This class of specialty fibers is described in Sec. 4.7.

4.6. Fiber Information Capacity

The information-carrying capacity of the fiber is limited by various distortion
mechanisms in the fiber, such as signal dispersion factors and nonlinear effects.
The three main dispersion categories are modal, chromatic, and polarization
mode dispersions. These distortion mechanisms cause optical signal pulses to
broaden as they travel along a fiber. As Fig. 4.7 shows, if optical pulses travel
sufficiently far in a fiber, they will eventually overlap with neighboring pulses,
thereby creating errors in the output since they become indistinguishable to the
receiver. Nonlinear effects occur when there are high power densities (optical
power per cross-sectional area) in a fiber. Their impact on signal fidelity
includes shifting of power between wavelength channels, appearances of spuri-
ous signals at other wavelengths, and decreases in signal strength. Chapter 15
discusses these nonlinear-induced degradations in greater detail.
Modal dispersion arises from the different path lengths associated with vari-
ous modes (as represented by light rays at different angles). It appears only in
multimode fibers, since in a single-mode fiber there is only one mode. By look-
ing at Fig. 4.8 it can be deduced that rays bouncing off the core-cladding inter-
face follow a longer path compared to the fundamental ray that travels straight
down the fiber axis. For example, since ray 2 makes a steeper angle than ray 1,

Figure 4.7. Broadening and attenuation of two adjacent pulses as they travel along
a fiber.

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