Fiber-Optic Techniques - LAN Wiring - page 249

TABLE 11.1
Optical Fiber Types Approved for LAN Wiring
Refractive Core Cladding Optical
Modal type type diameter, µm diameter, µm windows, nm
Single mode N/A* 4-8 125 1310, 1550
Multimode Graded index 50 125 850, 1300
Multimode Graded index 62.5 125 850, 1300
*Single-mode optical fiber is technically a step-index refractive type, although the term is primarily used to distin-
guish multimode construction.
Single-mode fiber has a very narrow core, so that only a narrow beam of light can
propagate down the fiber, essentially in a single mode of light. Multimode fiber has
a much larger core diameter, and multiple modes of light propagate either by reflec-
tion from the core walls or by a combination of reflection and refraction in the core.
Table 11.1 lists optical fiber types approved for LAN wiring.
The single-mode fiber used in LAN technology has a core diameter of only 4
to 8 µm and is surrounded by a cladding that is generally 125 µm in diameter.
Multimode fiber used for LANs has a relatively large core diameter that is either 50
µm or 62.5 µm in diameter, again surrounded by a cladding of 125 µm in diameter.
These dimensions hold for most LAN and campus fiber currently in use, although a
variety of other types are available for other applications.
Multimode fiber is further subdivided into two primary types, step-index and
graded-index fiber. These terms refer to the gradation of the refractive index, as
measured radially from the center of the core. As illustrated in Fig. 11.2, the rays of
light bounce off the “walls” formed by the radical step in refractive index between
the core and the cladding of the step-index multimode fiber. In contrast, in gradu-
ated-index multimode fiber, the rays primarily bend because of the gradual change
in refractive index from the center of the core to the edge of the cladding.
Fiber-Optic Cable Configurations
Fiber-optic cables are available in a wide variety of physical constructions. Fiber
cable can be anything from simple two-fiber zip cord used for jumpers to 144-fiber
cable (often layered in rows of 12) for intercity transmission. Cable for outdoor use
(called outside plant ) may have integral steel cables or even jacket armor to resist
rodents. However, most of the fiber used in LAN wiring is ordinary indoor-rated
multifiber cable of one or two configurations. This discussion supplements the
information in Chapter 5, “Wire and Cable Technology for LANs.”