Plastic optical fibers (POF) consist of a plastic core of anywhere from 50 microns on up, surrounded by a plastic cladding of a different index of refraction. Generally speaking, these are the lowest-quality optical fibers and are seldom sufficient to transmit light over long distances. Plastic optical cables are used for very short-distance data transmissions or for transmission of visible light in decorations or other specialty lighting purposes not related to data transmission. Recently, POF has been promoted as a horizontal cable in LAN applications for residential systems. However, the difficulty in manufacturing a graded-index POF, combined with a low bandwidth-for-dollar value, has kept POF from being accepted as a horizontal medium in commercial applications.
Buffer
The buffer, the second-most distinguishing characteristic of the cable, is the component that provides additional protection for the optical fibers inside the cable. The buffer does just what its name implies: it buffers, or cushions, the optical fiber from the stresses and forces of the outside world. Optical fiber buffers are categorized as either tight or loose tube.
With tight buffers, a protective layer (usually a 900 micron PVC or Nylon covering) is applied directly over the coating of each optical fiber in the cable. Tight buffers make the entire cable more durable, easier to handle, and easier to terminate. Figure 1 shows tight buffering in a single-fiber (simplex) construction. Tight-buffered cables are most often used indoors because expansion and contraction caused by outdoor temperature swings can exert great force on a cable. Tight-buffered designs tend to transmit the force to the fiber strand, which can damage the strand or inhibit its transmission ability, so thermal expansion and contraction from temperature extremes is to be avoided. However, there are some specially designed tight-buffered designs for either exclusive outdoor use or a combination of indoor/outdoor installation.
A loose-tube buffer, on the other hand, is essentially a tough plastic pipe about 0.125″ in diameter. One or several coated fibers can be placed inside the tube, depending on the cable design. The tube can then be filled with a protective substance, usually a water-blocking gel, to provide cushioning, strength, and protection from the elements if the cabling is used outdoors. More commonly, water-blocking powders and tapes are used to waterproof the cable. A loose-tube design is very effective at absorbing forces exerted on the cable so that the fiber strands are isolated from the damaging stress. For this reason, loose-tube designs are almost always seen in outdoor installations.
Multiple tubes can be placed in a cable to accommodate a large fiber count for high-density communication areas such as large cities. They can also be used as trunk lines for long-distance telecommunications.
Figure 2 shows a loose-buffered fiber-optic cable. Notice that the cable shown uses water-blocking materials.
Strength Members
Fiber-optic cables require additional support to prevent breakage of the delicate optical fibers within the cable while pulling them into place. That's where the strength memberscome in. The strength member of a fiber-optic cable is the part that provides additional tensile (pull) strength. Strength elements can also provide compression resistance. Compression is encountered when the temperature drops below room temperature.
The most common strength member in tight-buffered cables is aramid yarn, the same material found in bulletproof vests. Thousands of strands of this material are placed in a layer, called a serving, around all the tight-buffered fibers in the cable. When pulling on the cable, tensile force is transferred to the aramid yarn and not to the fibers.
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Aramid yarn is extremely durable, so cables that use it require a special cutting tool, called aramid scissors. Aramid yarn cannot be cut with ordinary cutting tools.
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Loose-tube fiber-optic cables sometimes have a strand of either fiberglass or steel wire as a strength member. These strands can be placed around the perimeter of a bundle of optical fibers within a single cable, or the strength member can be located in the center of the cable with the individual optical fibers clustered around it. As with aramid yarn in tight-buffered cable, tensile force is borne by the strength member(s), not the buffer tubes or fiber strands. Unlike aramid yarns, glass or steel strength members also have the ability to prevent compression-induced microbending caused by temperatures as low as −40°C.
Shield Materials
In fiber-optic cables designed for outdoor use, or for indoor environments with the potential for mechanical damage, metallic shields are often applied over the inner components but under the jacket. The shield is often referred to as armor. A common armoring material is 0.006″ steel with a special coating that adheres to the cable jacket. This shield should not be confused with shielding to protect against EMI. However, when present, the shield must be properly grounded at both ends of the cable in order to avoid an electrical-shock hazard should it inadvertently come into contact with a voltage source such as a lightning strike or a power cable.
Cable Jacket
The cable jacket of a fiber-optic cable is the outer coating of the cable that protects all the inner components from the environment. It is usually made of a durable plastic material and comes in various colors. As with copper cables, fiber-optic cables designed for indoor applications must meet fire-resistance requirements of the NEC.
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