Page 60 - Optical Communications Essentials

Optical Fibers

50 Chapter Four

Figure 4.3. Electric field patterns of the three lowest-order guided modes as
seen in a cross-sectional view of an optical fiber.

fields are concentrated tightly near the axis of the fiber with little penetration
into the cladding. On the other hand, for higher-order modes the fields are dis-
tributed more toward the edges of the core and penetrate farther into the
cladding region. The importance of the characteristic that the power of a mode
extends partially into the cladding will be seen in later chapters which discuss
applications such as coupling of power from one fiber to another.

4.3. Variations of Fiber Types
Variations in the material composition of the core and the cladding give rise to the
two basic fiber types shown in Fig. 4.4a. In the first case, the refractive index of
the core is uniform throughout and undergoes an abrupt change (or step) at the
cladding boundary. This is called a step-index fiber. In the second case, the core
refractive index varies as a function of the radial distance from the center of the
fiber. This defines a graded-index fiber. Section 4.6 describes the advantages of
graded-index fibers over step-index fibers for high-speed data transfer when
using multimode fibers. More complex structures of the cladding index profile
allow fiber designers to tailor the signal dispersion characteristics of the fiber (see
Sec. 4.6). Figure 4.4b shows two of many different possible configurations.
Table 4.1 lists typical core, cladding, and buffer coating sizes of optical fibers
for use in telecommunications, in a metropolitan-area network (MAN), or in a
local-area network (LAN). The outer diameter of the buffer coating can be
either 250 or 500µm. Single-mode fibers are used for long-distance communi-
cation and for transmissions at very high data rates. The larger-core multimode
fibers typically are used for local-area network applications in a campus envi-
ronment, particularly for gigabit or 10-Gbit rate Ethernet links, which are
known popularly as GigE and 10GigE, respectively. Here the word campus
refers to any group of buildings that are within reasonable walking distance of
one another (see Sec. 2.5).

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