Page 64 - Optical Switching And Networking Handbook

03_200023_CH02/Batesx 1/17/01 8:17 AM Page 49

Basic Fiberoptics Technologies 49

Fiber Cable Conditions

Fibers themselves are very small, with an outside diameter of 125
microns due to the cladding. Although they are also very strong
under tension (greater tensile strength than steel), they break very
easily when subjected to lateral pressure or any kind of rough han-
dling. Therefore, to use fiber in “hostile” communications environ-
ments, the fiber needs to be enclosed in a cable. Depending on the
location and type of installation, fiber cables vary widely in their
characteristics. They are made to satisfy a specific need. The goal of
using a cable is to protect the fiber from things that can harm it. Sev-
eral risks pose problems for the installers and operators of fiber-
based networks, including the following:
Tensile stress Fiber itself is very strong under tension.
Stress causes a significant increase in light attenuation
and creates a number of other problems. One has to be
careful not to stress the cable too much. The fibers also can
stretch, causing a change in the reflective and refractive
indices and creating major problems.
Bends Tight bends in the fiber cause signal loss because
the light escapes through the cladding material. Crimping
the cable also causes signal loss because the microbends
create the wrong angle of incidence for the light to bounce
down the fiber. By placing the fibers in a cable, the bending
radius is better maintained because of the additional
materials inside the cable (strengthening members and
other cladding components).
Physical damage The type of protection used in cables
varies with the risks posed. Indoor conditions include the
risk of rodent damage. The cables are a food supply that
rodents cannot resist. They will chew through the outer
cladding (and then some), causing damage and loss of
reflective materials (the light will escape from inside the
fibers). Rodent damage is not limited to indoor installations.
Gophers, rabbits, termites, and fire ants all may eat through
cables.