Optical waveguides – Optical transmission cable – Loose tube type
Reexamination Certificate
2002-01-29
2003-04-08
Font, Frank G. (Department: 2877)
Optical waveguides
Optical transmission cable
Loose tube type
C385S100000, C385S106000, C385S107000, C385S114000
Reexamination Certificate
active
06546176
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical fiber cables and, more particularly, to optical fiber cables comprising a central buffer tube containing one or more optical fibers, reinforced by longitudinal strength members to protect the optical fibers from forces, such as installation and thermally induced expansion and contraction of the buffer tubes.
BACKGROUND OF THE INVENTION
Optical fibers are relatively fragile and must be protected during the manufacture, handling and installation of cables including such fibers. A variety of protective measures are therefore provided in cables containing optical fibers. For example, to allow the cable to move or be flexed a certain degree by external forces or by thermal expansion and contraction without stressing or microbending the optical fibers, the optical fiber or fibers are typically enclosed in a plastic buffer tube having a bore of a cross-sectional area larger than the cross-sectional area of the fiber or fibers within it. This is referred to as a “loose” configuration. The material of the tube typically has a relatively high temperature coefficient of expansion and a relatively low tensile strength. Frequently, the length of the optical fibers is greater than the length of the tube, referred to as excess fiber length. An optical fiber cable may include one or several buffer tubes, each containing one or a plurality of optical fibers. The plurality of optical fibers may be in the form of individual fibers, an optical fiber ribbon or a stack of optical fiber ribbons, for example.
To further resist thermal expansion and contraction, as well as longitudinal loads which may be applied during manufacture, handling and installation of the optical fiber cable, strength members of metal wires or high strength non-metallic rods or fibers, such as glass rods or fibers may be embedded in the material of the buffer tube. See, for example, U.K. Patent Application GB 2,215,081, which provides two diametrically opposed wires or high tensile plastic filaments entirely within the walls of the buffer tubes. U.S. Pat. No. 4,898,451 also shows a plurality of flexible fibers of aramid, steel or glass, completely encircled by the plastic of the buffer tubes contained within an optical fiber cable.
Strength members have also been provided in grooves in the outer surface of the buffer tubes. See, for example, U.S. Pat. Nos. 4,804,245 and 4,610,505. In both patents, the strength members are held in the groves by an outer wrapping. In the '245 patent, the shape of the grooves and the physical properties of the strength members are such that there is little resistance to compression forces. Also, the optical fibers are not encircled by, or centrally disposed with respect to the buffer tube. The '505 patent relies on the wrapping to provide coupling between the strength members and the tube and the optical fiber is not loosely contained in a buffer tube. The requirement of such a wrapping complicates the structure of the cable and could increase its diameter.
In an alternative configuration disclosed in U.S. Pat. No. 5,509,097 (“the '097 patent”), assigned to the assignee of the present invention and incorporated by reference herein, an optical fiber cable is disclosed including a buffer tube loosely enclosing optical fibers. Two diametrically opposed, longitudinally extending strength members are tightly coupled to the buffer tube by an adhesive or a cord to resist tensile and compression forces applied to the buffer tube and prevent buckling of the strength members. The core of the cable can also include a layer of extruded plastic which presses the strength members toward the tube to increase the coupling between the strength members and the buffer tube and which opposes buckling and kinking of the strength members.
When the strength members are at the outside of the buffer tube as shown in the '097 patent, the structure is not circular in cross-section. If the structure shown in the '097 patent is to be covered by other layers, the outer surface may have “lumps” at the location of the strength member, and it may be desirable to have an outer surface corresponding substantially to the outer surface of the cylinder. Furthermore, if the strength members are entirely outside the outer surface of the buffer tube, special apparatus is required in applying a plastic layer over the combination of the buffer tube and the strength members, and the diameter if the jacket may be increased. If the diameter of the layer is increased, the diameter of the cable is increased which is undesirable. If the layer is omitted, it becomes difficult to apply the armoring, etc., because the strength members, being at the outer surface of the buffer tube, constitute protrusions and deviations from a cylindrical surface to which the armoring, etc., must conform. It can be desirable, in some cases, to omit the layer and to minimize the extent to which the strength members extend outwardly from the buffer tube without losing the benefits of the structure shown in the '097 patent and particularly, the tight coupling of the strength members to the buffer tube.
If the strength members are completely encircled by a substantial amount of the buffer tube material, it is difficult to main the strength members in the correct positions within the buffer tube, and it can be necessary to increase the thickness of the buffer tube wall because of the presence of the strength members therein which increases the amount of buffer tube material required and increases the diameter of the buffer tube and hence, the diameter of an armored cable incorporating such structure. Completely embedded strength members can also interfere with gaining midspan access to the optical fibers during splicing because the location of the strength member is not visible. Further, attempting to remove these strength members necessarily damages the structural integrity of the tube, increasing the risk of damage (i.e. kinking, crushing) to the optical fibers as the midspan operation continues. Examples of cables with completely embedded strength members have been described hereinbefore.
It is known to include water blocking compound within the buffer tubes for moisture protection. The water blocking compound, which permits the optical fibers to move within the buffer tube, may be a gel or grease-like, and non-hygroscopic and/or thixotropic. Any spaces between the buffer tube and the outer jacket can also be filled with a water blocking compound. Further moisture protection is provided by water blocking yarn and/or water blocking tape disposed between the buffer tube and the outer jacket. Such yarn or tape may be provided within the buffer tube and the outer jacket. Such yarn or tape may be provided within the buffer tube instead of the water blocking compound. See, for example, U.S. Pat. No. 5,071,221.
Additional layers of materials, such as armoring for crushing and rodent protection, can also be provided between the buffer tube and outer jacket. Longitudinal ripcords are typically provided to assist in opening the optical fiber cable jacket and armor, if provided.
In a telecommunications network, certain optical fibers within an optical fiber cable may be spliced to other optical fibers, such as optical fibers leading to particular terminal locations. The optical fibers may also need to be accessed to check for faults. To achieve midspan access to the particular optical fibers to be spliced or checked at a particular location along the cable, the jacket and the buffer tube containing the optical fibers and any intervening layers must be opened. Typically, the optical fiber cable is opened by accessing and pulling the ripcord to open the jacket and armor if present, and cutting through or removing other layers of material, such as water blocking tape, to expose the buffer tube of interest.
When strength members are embedded in the cable jacket, they are not tightly coupled to the buffer tube and the strength members in the region of the jacket to be opened must be removed befo
Anderson Aaron M.
Vaughn Roger
Wells Ben H.
Brooks L. P.
Font Frank G.
Mooney Michael P.
Norris & McLaughlin & Marcus
Pirelli Cable Corporation
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