Introduction to Fiber Optics:
The optical fiber in a fiber optic cable acts as a waveguide, which transmits a light signal from one end to the other. The light source can either be a light-emitting diode (LED) or a laser. This form of conduction offers many advantages over standard copper interconnections – signals can be transmitted at a much higher frequency, with minimal attenuation or signal loss over long distances. They also provide greater shielding from electromagnetic noise such as motors or other nearby cables, and they cost much less to maintain.
While a fiber optic cable itself is cheaper than an equivalent length of copper cable, fiber optic cable connectors and the associated components needed to install them make the overall cabling system more expensive. Fiber optic cables are often used in high-speed data communication systems.
The optical fiber is very sensitive to external forces, such as tensile stresses or bending, and cannot operate when they are excessive. Optical fibers are packaged in cable structures for handling and protection. Sometimes a cable has to provide several layers of protection to isolate a fiber from any external forces.
Types of Fiber Optics:
Fiber optic cables are available in a wide variety of physical constructions. Two standard types of cable constructions are used: loose tube and tight buffer (shown in below figures). There are also two basic fiber structure configurations that are commonly used for these two construction types: 900um tight buffered fibers and 250um coated fibers, where the coating is a thin layer of soft plastic. The only difference between these two is that the 900um tight buffered fibers are basically 250um coated fibers covered with a thick layer of hard plastic, which adds protection.
The 250um coated fiber (bare fiber) consists of a central core which carries the light signal, a cladding material with a 125um diameter, and a soft plastic coating with a 250um (sometimes 400um) diameter. The core is either 9um for standard single mode fibers, or 50um or 62.5um for multimode fibers. The 900um tight buffered fiber just adds a 900um hard plastic layer over the 250um soft plastic coating. 900um tight-buffered cables are often used as pigtails, patch cords and jumpers to terminate loose-tube cables.
What about the construction of these variants?
Loose Tube Fiber Optics Construction
In loose tube construction, the fiber (usually the 250um coated fiber) is contained in a color-coded, flexible plastic tube that has an inner diameter considerably larger than the fiber itself. Up to 12 of these fibers can be put inside a tube, and is usually filled with a gel material that prevents water penetration. These tubes are stranded around a dielectric or steel central member to provide more flexibility. Aramid yarn is used as the primary tensile strength member. An outer polyethylene jacket is wrapped over the assembly. Polyethylene is the most common for outdoor cables due to high moisture resistance, abrasion resistance, and stability over a wide temperature range.
The loose tube provides the protection needed for the inner fiber (or fibers) from the exterior mechanical forces acting on a cable by isolating them. Several loose tube cables are often combined with strength members to form a multi-fiber assembly, and this provides additional protection from stress, and minimizes elongation and contraction. By cutting several fibers slightly longer than the loose tube length and braiding them inside the loose tube, the shrinkage due to temperature variation can be controlled while insulating the fibers from the stresses of installation and environmental loading. This would be a big advantage in outdoor applications, which is why most outdoor fiber optic systems have a loose tube configuration. Loose-tube cables are normally used for outside-plant installation in aerial, duct and direct-buried applications.
The loose tube configuration is ideal for modular designs, as each tube can hold up to 12 fibers. With multiple tubes in a cable, this could add up to more than 200 fibers. With a modular design, certain groups of fibers can be routed to intermediate points without interfering with other protected buffer tubes being routed to other locations. The color-coded scheme in loose tube cable assemblies allows for easy identification and administration of fibers in the system.
Tight Buffered Fiber Optics Cable
In tight buffered cable construction, several 900um tight buffered fibers are stranded around a central strength member to form a cable assembly. Aramid Yarn (Kevlar) covers the fibers for added tensile strength and physical protection. An outer jacket (usually PVC) is wrapped over this assembly. PVC is most common for indoor cables due to high flexibility and easy extrusion characteristics.
There are no loose tube coverings, and the protection is provided instead by the 900um hard plastic layer. This type of construction can withstand much greater crush and impact forces without fiber breakage than the loose tube variant. Tight buffered cables are also more flexible, although sharp bends should be avoided to reduce optical losses due to micro-bending.
The tight buffer design does not isolate the internal fibers from the stresses of temperature variation as well as the loose tube design. Therefore, tight buffered cables are almost always used for indoor applications. Tight buffered cables are typically found connecting outside plant cables to terminal equipment, and in general building applications where various devices are linked together.
Another form of tight buffer construction is the breakout cable. In a breakout cable, a single tightly buffered fiber is wrapped with aramid yarn and a jacket. Several of these single-fiber cables are then covered by a common sheath to form the breakout cable. This design provides a rugged cable structure which protects the individual fibers when handling or routing the cable. This method also simplifies connector attachment and installation.
Each of these two basic constructions has its own inherent advantages and disadvantages. The loose tube type offers a higher level of isolation from external forces, and when subjected to continuous mechanical stress, it has more stable transmission characteristics. For any given fiber, the effects of micro-bending on cable attenuation are lower. But the main reason the loose tube type is used outdoors is that it exhibits greater stability over temperature variations. The tight buffer construction is generally lower in cost, and it is smaller and a lighter in weight than a similar loose tube configuration. This cable design offers better flexibility, and is basically a rugged, crush resistant cable.
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