What kinds of fiber optic cables are available and how rugged are they?
Like conventional copper wire, fiber optic cable is available in almost as
many physical variations. There are single and multiple conductor cables,
aerial, direct burial, plenum and riser versions and even ultra-rugged military
type tactical cables that will withstand severe mechanical abuse. Regardless of
the final application, all fiber optic cable contains one or more hair-thin
optical fiber, usually made of glass, which, as we have mentioned is what the
light actually travels through. Since these fibers are made of glass the cable
also contains various strength members to protect the delicate fiber from the
environmental stresses that the cable will be subjected to. These stresses
include pulling forces often encountered during installation, extremes in
temperature, water, corrosive atmospheres, the resistance to normal handling and
the effect of the earth and rodents in underground installations to name a few.
In fact, the stresses are no different that the ones copper cable encounters,
but unlike copper, glass is more fragile therefore the internal construction of
fiber optic cables is necessarily different.
The two most common methods in use today to protect the fiber are to either
enclose it in a loose fitting "buffer" tube or to coat the fiber with a tight
fitting rugged coating. In the loose tube method the fiber is enclosed in a
plastic tube that is larger in inner diameter than the outer diameter of the
fiber itself. This tube is also sometimes filled with a silicone gel to prevent
the buildup of moisture as well. Since the fiber is basically free to "float"
within the buffer tube, mechanical forces acting on the outside of the cable do
not usually reach the fiber. Cable containing loose buffer-tube fiber is
generally very tolerant of axial forces of the type encountered when pulling
through conduits or where constant mechanical stress is present such as cables
employed for aerial use. In the tight buffer construction, a thick coating of a
plastic-type material is applied directly to the outside of the fiber itself.
This results in a smaller overall diameter of the entire cable and one that is
more resistant to crushing or overall impact- type forces. However, because the
fiber is not free to "float", tensile strength is not as great. Tight buffer
cable is normally lighter in weight and more flexible than loose-tube cable and
is usually employed for less severe applications such as to use within a
building or to interconnect individual pieces of equipment.
The buffered fiber is then usually wrapped with a tough synthetic yarn such
as KevlarÔ . This material takes the brunt of the
force caused by stretching when the cable is pulled. Sometimes an additional
fiberglass or similar "stiff" member is also included to prevent the cable from
being forced into too tight a bend, or for additional support when the cable is
strung between buildings or on telephone-type poles. Finally, everything is
enclosed in an extruded outer jacket of polyethylene, rubber or PVC, depending
on the final application. Figure 2 is a diagram of the basic construction of
both loose-tube and tight-buffered fiber optic cable while figure 3 is a drawing
of the cross section details of a single and a two conductor fiber optic cable
as well as a more complex multi-fiber cable.
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