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Optical Fiber Cables

Optical Fiber Cables 69

where long lengths of cable need to be unrolled and retrieved rapidly, LAN links
where fiber cables are placed in cable trays above suspended ceilings, and inter-
building links where cables often need to be pulled through crowded cable ducts.

5.2. Cable Materials and Structures

In most cases there are some common fundamental materials that are used in
the cabling process. To see what they are, let us examine the generic cable con-
figuration shown in Fig. 5.1. Here there is a central strength member, which can
be strong nylon yarns, steel wires, or fiberglass rods. A commonly used yarn is
Kevlar, which is a soft but tough yellow synthetic nylon material belonging to a
generic yarn family known as aramids. Note that aramid yarns also are used for
making bulletproof vests, trampolines, and tennis rackets. Individual fibers or
modules consisting of fiber groupings are wound loosely around the central
member. Optionally, a cable wrapping tape and another strength member then
encapsulate these fiber groupings.
Surrounding all this is a tough polymer jacket that provides crush resistance
and handles any tensile stresses applied to the cable so that the fibers inside are
not damaged. The jacket also protects the fibers inside against abrasion, mois-
ture, oil, solvents, and other contaminants. The jacket type defines the applica-
tion characteristics; for example, heavy-duty cables for direct-burial and aerial
use have thicker, tougher jackets than light-duty cables for indoor use.
An important factor for using a cable in a building is the flammability rating.
The National Electrical Code (NEC) in the United States establishes flame rat-
ings for cables, while on a global scale the Underwriters Laboratories (UL) has
developed cable test procedures. For example, the NEC requires that all cables
which run through plenums (the air-handling space between walls, under
floors, and above drop ceilings) must either be placed in fireproof conduits or be
constructed of low-smoke and fire-retardant materials. Table 5.1 lists some pop-
ular jacket materials and their properties.
One of two basic structures is used to house individual fibers for any type of
fiber optic cable design. The structures are the tight-buffered fiber cable design and
the loose-tube cable configuration. Cables with tight-buffered fibers nominally

Figure 5.1. Cross section of a generic optical fiber cable.

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