 Fiber Optic Cable
Belden's Fiber Optic cable line answers the diverse, and often complex, needs of today's advanced networks. Not only do these cables future-proof your network, they also fully organize your network while protecting it from the environment.  Basic Elements
The three basic elements of a fiber optic cable are the core, the cladding and the coating. Core This is the light transmission area of the fiber, either glass or plastic. The larger the core, the more light that will be transmitted into the fiber. Cladding The function of the cladding is to provide a lower refractive index at the core interface in order to cause reflection within the core so that light waves are transmitted through the fiber. Coating Coatings are usually multi-layers of plastic applied to preserve fiber strength, absorb shock and provide extra fiber protection. These buffer coatings are available from 250 microns to 900 microns. Fiber Size The size of the optical fiber is commonly referred to by the outer diameter of its core, cladding and coating. Example: 50/125/250 indicates a fiber with a core of 50 microns, cladding of 125 microns, and a coating of 250 microns. The coating is always removed when joining or connecting fibers. A micron (µm) is equal to one-millionth of a meter. 25 microns are equal to 1/1000 of an inch. (A sheet of paper is approximately 25 microns thick). Fiber Types Fiber can be identified by the type of paths that the light rays, or modes, travel within the fiber core. There are two basic types of fiber: multimode and single-mode. Multimode fiber cores may be either step index or graded index. Step index multimode fiber derives its name from the sharp step like difference in the refractive index of the core and cladding. In the more common graded index multimode fiber the light rays are also guided down the fiber in multiple pathways. But unlike step index fiber, a graded index core contains many layers of glass, each with a lower index of refraction as you go outward from the axis. The effect of this grading is that the light rays are speeded up in the outer layers, to match those rays going the shorter pathway directly down the axis. 
Graded index fibers are commercially available with core diameters of 50, 62.5 and 100 microns. The single mode fiber allows only a single light ray or mode to be transmitted down the core. This virtually eliminates any distortion due to the light pulses overlapping. The core of the single mode fiber is extremely small, approximately five to ten microns. The single mode has a higher capacity and capability than either of the two multimode types. For example, undersea telecommunications cables can convey 60,000 voice channels on a pair of single mode fibers. Design Considerations Considerations of tensile strength, ruggedness, durability, flexibility, size, resistance to the environment, flammability, temperature range and appearance are important in constructing optical fiber cable. First Level of Fiber Protection The optical fiber is a very small waveguide. In an environment free from stress or external forces, this waveguide will transmit the light launched into it with minimal loss, or attenuation. 
In the loose buffer construction, the fiber is contained in a plastic tube that has an inner diameter considerably larger than the fiber itself. The interior of the plastic tube is usually filled with a gel material. The loose tube isolates the fiber from the exterior mechanical forces acting on a cable. For multifiber cables, a number of these tubes, each containing single or multiple fibers, are combined with strength members to keep the fibers free of stress, and to minimize elongation and contraction. By varying the amount of fibers inside the tube during the cabling process, the degree of shrinkage due to temperature variation can be controlled, and therefore the degree of attenuation over a temperature range is minimized. The other fiber protection technique, tight buffer, uses a direct extrusion of plastic over the basic fiber coating. Tight buffer constructions are able to withstand much greater crush and impact forces without fiber breakage. The tight buffer design, however, results in lower isolation for the fiber from the stresses of temperature variation. While relatively more flexible than loose buffer, if the tight buffer is deployed with sharp bends or twists, optical losses are likely to exceed nominal specifications due to microbending. A refined form of tight buffer construction is breakout cable. In breakout cable, a tightly buffered fiber is surrounded by aramid yarn and a jacket, typically PVC. These single-fiber subunit elements are then covered by a common sheath to form the breakout cable. "This cable within a cable" offers the advantage of direct, simplified connector attachment and installation. Each construction has inherent advantages. The loose buffer tube offers lower cable attenuation from microbending in any given fiber, plus a high level of isolation from external forces. Under continuous mechanical stress, the loose tube permits more stable transmission characteristics. he tight buffer construction permits smaller, lighter weight designs for similar fiber configuration, and generally yields a more flexible, crush resistant cable. Currently, due to the expense of multiplexing equipment, separate, dedicated fibers are typically utilized for each application. So, for example, if two buildings were to be networked with a FDDI backbone, four fibers would be required in the cable connecting the buildings - two to transmit, two to receive. Further, it is recommended that at least two times the number of fibers needed are actually placed in the backbone to accommodate expansion requirements. For purposes of example, assume 3 applications to each floor: 1. FDDI Data (4 Fibers) 2. Interactive Video (2 Fibers) 3. CCTV Security (1 Fiber) These applications would indicate a need for 7 fibers to each wiring closet. It is recommended that 2 times the number of fibers required are actually run to each wiring closet to allow future network expansion. | 
