Protecting the fibers

You might think that optical fibers are quite fragile-they're made of glass, after all. But optical fibers are a lot stronger than fine glassware, and, once they're enclosed in protective cables, they can be even tougher than copper communication cables.
To achieve this strength, all optical fibers are designed in three concentric layers. At the center is the core, the central piece of glass that carries the light. Surrounding the core is the cladding, a second layer of glass, which keeps the light from escaping the core. And around both of these lies the buffer, an outer layer of plastic, which provides the real protection and strength. There are two basic types of buffers in use today: tight buffers and loose tubes.
Tight buffers consist of a thin layer of plastic extruded over the individual fiber, much like the insulation placed on copper wires (except a lot smaller). Tight-buffer configurations are generally used with indoor cables, though they can also be found on some outdoor cables.
Loose tubes, meanwhile, are used in most outdoor cables. In a loose-tube setup, one or more fibers (usually up to 12) are installed inside of a plastic tube that resembles a drinking straw, and the assembly is then bound into the cable. Frequently these tubes are filled with a water-block gel (usually and descriptively called "icky-pick" on the job site), which prevents water from entering the cable.
The loose-tube assembly provides particularly good protection for the fibers, but such cables are messy to terminate and, because the fibers have a tendency to fall out of the vertical cables, should never be used as risers.
For added strength during pulling, most fiber cables also contain a strength member. Except for very short, easy runs (like 20 feet, between two outlets in the same room), every time you pull a fiber cable, you must attach the pull string of the fish tape to the strength member. If you do not, you stand a good chance of ruining the cable.
With larger cables, strength members may be made of a steel rod, steel wires, or fiberglass/epoxy rods. Most strength members, however, consist of a layer of Kevlar yarn in the cable. (Warning: Use special scissors to cut the Kevlar. This is the stuff they make bullet-proof vests out of, and cutting it with your lineman's pliers will ruin them quickly.)



Source:www.elec-toolbox.com/communications/classifications/fiber/fiber.htm