Page 78 - Optical Communications Essentials

Optical Fiber Cables

68 Chapter Five

5.1.1. Fiber strength
Glass is quite a strong material and typically withstands large pulling forces.
However, it does have its strength limits, as any other material. Thus, one
important mechanical property is the maximum allowable axial load on the
cable, that is, how hard one can pull on the cable before something snaps. In
copper-based cables the wires themselves generally are the principal load-bearing
members of the cable, and elongations of more than 20 percent are possible. On
the other hand, typical high-quality optical fibers break after stretching around
0.5 to 1.0 percent. Since damage occurs very quickly at axial stress levels above
40 percent of the permissible stretching point and very slowly below 20 percent,
fiber elongations during cable installation and afterward when it is in operation
should be limited to 0.1 to 0.2 percent.
To prevent excessive stretching, the cabling process usually includes the
incorporation of strength members into the cable design. This is especially
important in the design of aerial cables that can experience severe stresses due
to factors such as wind forces or ice loading. Of course if a tree falls on an aer-
ial transmission line during a storm or if someone with a backhoe or a bulldozer
accidentally encounters a buried cable, then the chances of cable survivability
are extremely small! Common strength members are strong nylon yarns, steel
wires, and fiberglass rods. Some examples of strength members are described in
Sec. 5.2 in the discussions on cable materials and structures.

5.1.2. Dielectric nature of fibers
Although a variety of cable designs use steel strength members, in certain appli-
cations it is advantageous to have a completely nonmetallic cable. Such cables
have a low weight, are immune to ground-loop problems, are resistant to elec-
tromagnetic coupling arising from adjacent electronic equipment or nearby
lightning strikes, and can be run through explosive environments where elec-
tric sparks would not be very welcome.
In some cases signal amplification equipment located within the optical link
requires electric power. In that case copper wires may be integrated into the
optical cable, since the fibers cannot carry electric power to run the equipment.

5.1.3. Small size and low weight
Cable designs must take into account the small size of optical fibers both from
the perspective of handling ease and from the desire to have strong, low-weight
cables. Their size and weight characteristics make optical cables smaller and eas-
ier to handle than their copper counterparts, which tend to be heavier and
bulkier. The cabling process itself also color-codes each fiber by means of differ-
ent jacket colors and arranges the small fibers systematically within the cable
(see Sec. 5.5). This allows them to be seen and identified easily when one is
attaching connectors or splicing fibers. In addition, low weight and small size are
advantageous for applications such as tactical military communication links

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.