|Fiber optic cable|
|Monday, 04 December 2006|
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with relatively low signal loss as compared to standard wire or cable networks. The use of optical fibers in today's technology has developed into many widespread areas, such as: medicine, aviation, communications, etc. An optical fiber is an elongated glass or plastic filament having a core region surrounded by one or more layers of cladding, with the core having a higher index of refraction than the cladding so that light introduced at one end of the fiber will be internally reflected for transmission longitudinally within the core channel to the other end of the fiber. The glass optical fibers of fiber optic cables have very small diameters, which are susceptible to external influences such as mechanical stress and environmental conditions. The optic fibers cannot be subjected to damage or stresses from bending, stretching, moisture or chemical intrusion, excessive rotational or longitudinal stresses on the cable or any other circumstance that could affect the optical transmission characteristic of the individual optic fibers. Damage to the optic fibers may result from excessive rotational and radial stresses involved in linking the cable to the intended operational equipment. The index of refraction of the core is higher than the index of refraction of the cladding to promote internal reflection of light propagating down the core. Thus optical fibers require cabling to protect the relatively fragile silica-based optical fibers and to preserve the optical performance thereof. Most applications of optical fibers require the individual fibers to be placed into groupings, such as in fiber optic cables. In general, a fiber optic cable consists of a number of separate optical fibers that are stranded together, and may include a central metallic strength member to provide support to the set of optical fibers. Surrounding the fibers is a metallic sheath, used to provide outer mechanical support to the cable, as well as to provide an electrical signal path that is used to send certain operational tones along the length of the cable. The number of optical fibers included in the cable, and the materials and thicknesses thereof used to form the protective layers, are selected based on the type of application or installation of the cable. The optical fibers are carried in fiber optic cables which range from one to as many as 216 optical fibers. Normally, the fiber optic cable contains groups of 12 optical fibers in buffer tubes, either loose or in ribbon form. If a cable is to be joined to another cable or hardware in the field by connectors, it is common to attach the connectors to the cable at the factory before the cable is shipped to the installation site. This process is called "connectorization." Typically, optical fibers are spliced within an enclosure or housing. The splice closure protects the optical fibers, such as from moisture or other forms of environmental degradation. The splice closure also isolates or otherwise protects the optical fibers within the splice closure from strain or torque imparted to a portion of the fiber optic cable outside of the splice closure in order to maintain proper alignment and spacing between the spliced optical fibers and to prevent undesirable signal attenuation.
A fiber optic cable is typically formed from a plurality of optical fibers, each of which has its own protective jacket. The fibers are bundled together within a larger protective jacket. The fibers are linked to optical transmitters and optical receivers. The transmitters typically include electronic circuits that drive a light source such as a laser diode. The laser diode emits a modulated light beam that travels through the fiber optic cable to a photodetector. The optical fibers are typically encased in protective buffer tubes which are formed of a flexible plastic material and may be color coded for ease of installation. A fiber optic cable has a central metallic strength member to provide outer mechanical support to the cable, as well as to provide an electric signal path that is used to send certain operational tones along the length of the cable. Plastic coatings may then be used to cover the metallic sheath, particularly when the cable is to be buried in the ground. Fiber optic cables can generally be classified into two categories, namely, trunk and distribution cables that are designed to span relatively long distances, and drop cables that span much shorter distances and that typically terminate at a home or business. Trunk and distribution cables are generally relatively large and rigid. Trunk and distribution cables can include one or more strength members resisting sharp twists or turns of the cable. Fiber optic drop cables are typically relatively flexible. This flexibility facilitates twisting and turning of the drop cable during installation. Fiber optic drop cables generally include fewer optical fibers and extend across shorter distances than fiber optic trunk and distribution cables, fiber optic drop cables are comparatively smaller and less expensive. Optical fiber communication cables can be typically grouped into three main categories distinguished by the location of the optical fibers within the cable. All three types of optical fiber cables contain an outer protective covering or polymeric jacket. Loose-tube cables typically include a number of relatively small tubes that are positioned around a central strength member, and each tube encloses a number of optical fibers. The fiber-containing tubes are longitudinally stranded around the central member, which is to say that the tubes are rotated around the central member along the length of the cable. In loose tube fiber optic cables, the optical fibers lie in one or more buffer tubes that are placed about an elongated central strength member. Each of the buffer tubes usually includes a water-blocking material, such as a gel, that prevents moisture intrusion. The second category of fiber optic cables is monotube cable. In monotube cables, the optical fibers are contained within a central buffer or core tube, which contains a water-blocking agent. Slotted core cables are another category of fiber optic cables. In slotted core cables, the optical fibers reside in channels or grooves that have been formed on a surface of a rod-shaped polymeric core. The grooves typically follow a helical path along the surface of the core, thereby reducing compressive and tensile forces on the optical fibers whenever the cable is twisted, stretched, bent or compressed.
Because optical fibers are not ductile they must be protected from external forces such as tensile forces. Additionally, optical fibers require protection from macro-bending and/or micro-bending to inhibit undesired optical degradation. In order to meet these requirements, fiber optic cables designed for indoor, outdoor, or indoor/outdoor applications typically have a cable core surrounded by a sheath system that generally includes a cable jacket. The strength element of a fiber optic cable is intended to carry tensile loads applied to the fiber optic cable inhibiting, for example, tensile stress and/or strain from being applied to the optical fibers within the cable. Different types of strength members may be used in fiber optic cables, for example, metal wires, glass-reinforced plastics, and/or aramid fibers. Self-supporting fiber optic cables typically include a strength member in the form of a messenger section supporting the fiber optic cable, and a carrier section that includes optical fibers, or optical fibers and electrical conductors. One type of self-supporting fiber optic cable is a "figure 8 " configuration that includes a pair of cable sections connected by a web, wherein the messenger section forms one of the sections and the carrier section forms the other cable section. Fiber optic cables are generally buried under ground and typically constructed in a tubular fashion with numerous fiber optic conductors surrounded by a conductive ground shield which is in turn surrounded by a protective jacket of tough flexible plastic or rubber. Many fiber optic cables also include steel cords running the length of the cable, positioned between the conductive shield and the protective jacket, which protect the fragile inner conductors and reinforce the cable. Fiber optic cable generally must have different properties and meet different requirements depending upon the intended application of the cable. Fiber optic cable that is intended for outdoor use generally includes water blocking elements to prevent the infiltration of water and, in instances in which water does penetrate the cable, to mitigate the effects of the water. Fiber optic cables that are intended for indoor use do not generally have to be designed to be water blocking, but are commonly required to be flame retardant. Moreover, some fiber optic cables are intended for indoor/outdoor use and, as such, must have both water blocking and flame retardant properties. Fiber optic cables designed for indoor, outdoor, or indoor/outdoor applications may include one or more ripcords and may include one or more binders. Fiber optic cables usually interface with repeaters at periodic intervals so that the optical signals carried by the fibers can be restored to their desired levels after having suffered attenuation over long propagation distances. The optical transmission path may include optical filters to shape and flatten composite signal response. Often the individual fibers carry multiple independent channels through the use of, for example, dense wavelength division multiplexing (DWDM) technology. A fiber optic cable having a layer of tape may be required to pass a cable twist test and a water-blocking test. The cable twist test may be used to determine a cable's ability to resist jacket zippering.
The process of terminating the fiber optic cable in a connector is commonly referred to as "connectorization." Fibre optic connection systems are used to easily connect a fibre optic cable to another cable or to an optical or electro-optical device. Fibre optic connection systems typically have two pieces: a male connector and a female connector. The female connector, in turn, is formed to receive and hold the male connector. Fiber optic connectors of a wide variety of designs have been employed to terminate optical fiber cables and to facilitate connection of the cables to other cables or other optical fiber transition devices. A typical fiber optic connector includes a ferrule which mounts and centers an optical fiber or fibers within the connector. The ferrule may be fabricated of such material as ceramics. In the telecommunications industry, fiber-optic cables are often routed through rooms and buildings using ceiling-mounted or overhead cable routing systems. Such cable routing systems may be used to provide overhead cable routing between and among the various communication equipment in a data processing center, central office, and telecommunications room. A typical component of such overhead routing systems is a segmented aluminum trough that may be manufactured in short segments that may be pieced together to form all or part of the overhead cable routing system. No matter which type of fiber optic cable is used, the cables must inevitably be spliced. Splicing is required during initial installation between cable runs and at points where the cable branches. After installation splices are often required to repair breaks in the cable. A single fiber optic cable can contain hundreds of individual fibers which must be separately spliced, either by fusion or by mechanical connectors. Signals transmitted through optical fibers are subject to various distorting and attenuating non-linear effects that limit their practical transmission distance. Optical fibers are employed in fiber optic systems that consist of an optical terminal and a plurality of amplifiers/repeaters connected by optical fibers. The amplifier/repeaters are typically situated at regular intervals along a transmission path, and serve to boost the strength of the signal pulses, thereby overcoming the effects of attenuation. The total length of the transmission path is limited by phase shifts in the pulsed signals resulting from the optical nonlinearity of the optical fibers. Fiber optic cables can be installed in conduits that are disposed within the ground or aerially by being suspended between utility poles. Self-supporting fiber optic cables typically include a messenger section including optical fibers in a tube, and a carrier section including a support member a steel wire.