App Notes

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 Kevlar. 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.

 Fiber Optic construction

 Cross Section of fiber

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