Around the tube is a layer of Kevlar which provides tensile strength, and on the outside is an overall sheath made of high density plastic. One or more fibers with 250 micron jackets are installed in the tubes, and the fibers are a little longer than the cable itself, so that strain on the cable which might stretch the cable does not stretch the fibers. In most cases, all the spaces inside the tubes and other places in the cable are filled with a gel which serves to block water and support the fibers in the tubes.
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 standed 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 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 multifiber 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 installtion 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.
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 principle disadvantage of the loose tube cable relates to connectorization. Because of the small fiber jacket size and the use of multiple fibers in a tube, connectors “can not” be installed directly on the fibers. (I have seen it done, but it is not recommended because the very small diameter plastic jacket gives virtually no protection to the fiber, and provides little surface area for the connector to grip.) As a result, the extra step of splicing pre-connectorized pigtails to the cable is commonly used. This makes connectorizing the loose tube cable more expensive than the tight buffer cables. Note that this does not apply to splices joining two cables together, though removing the gel from the fibers and tubes does slow the splicing and connectorization processes. Note also that the majority of loose tube cables are NOT fire rated for indoor use at all, so when a loose tube cable enters a building, it either has to be run in conduit or spliced to a fire rated cable.
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