Unitube optical fiber cable

Abstract: The optical fiber cable has a plurality of centrally located optical fibers. Carrying the optical fibers is a gel-filled plastic buffer tube. A plurality of radial strength yarn threads extend along the outside of the buffer tube in a contra-helical pattern. In the armored cable embodiment, a strength and protective member in the form of a corrugated steel armor layer may be provided for additional protection against possible rodent damage. The armor is usually coated with a thin layer of plastic material in the form of a water blockable flooding compound. A ripcord is provided to assist in removal of the armor. A final polyethylene outer jacket is provided for additional cable protection. Embedded within the outer jacket is a pair of diametrically opposed, longitudinally extending strength members.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to optical fiber cables and, more particularly, to optical fiber cables having a monotube design. Description of the Prior Art Optical fiber cables have been used for the past several years to transmit information at high rates and long distances. The transmission medium is composed of hair-thin optical fibers which are protected from external forces by precisely designed and manufactured cable structures. Cable structure families which are currently being used are: A. Loose Tube: Structure in which several gel filled buffer tubes containing optical fibers are stranded around a central strength member. B. Slotted Core: Structure in which optical fibers are precisely placed in gel filled channels or slots. The channels are symmetrical and form a helical path along the longitudinal axis of the cable. A strength member is located in center of the cable structure. C. Monotube: Structure in which all of the optical fibers are in a single, centrally located, gel filled buffer tube. All of the cable structures listed above also incorporate additional protection which may consist of radially applied strength members, corrugated armor, and plastic sheaths. Two monotube cable structures are currently being manufactured by AT&T (LXE design) and Siecor (Maxitube design). Cable cross-sectional views of the AT&T LXE design generally indicated by the numeral 10 and Siecor Maxitube design generally indicated by the numeral 30 can be found in Figures 1 and 2, respectively. The AT&T LXE cable 10 consists of a single large plastic, gel filled buffer tube 11 which can contain up to 96 optical fibers 12. The buffer tube 11 is surrounded by water-swellable tape 14 and corrugated armor 16. A ripcord 18 is placed under the corrugated armor 16 to aid in sheath removal. A strip of water-swellable tape 15 is helically wrapped around the corrugated armor. Two steel strength members 20 are located 180 degrees apart outside the corrugated armor 16. The armor 16 and strength members 20 are encapsulated by a high-density polyethylene jacket 22 which bonds to the armor 16 and completes the structure. The cable 10 has several weaknesses in design, processibility and installation. First, the two steel strength members 20 with outer diameters of 0.063" produce an extremely stiff cable. Secondly, the steel strength members have a tendency to "piston" during installation and termination. The term "pistoning" describes the longitudinal movement of the steel strength members relative to the other cable components. Thirdly, the sheathing process is more complex due to the steel strength members 20 impregnated in the jacket 22. Fourthly, water penetration susceptibility exists between the steel strength members 20 and armor 16, and in areas void of water-swellable tape 15. Finally, the non-metallic AT&T LXE cable design is significantly more expensive to produce than the Siecor Maxitube design. The Siecor Maxitube cable 30 consists of a single, large, gel filled, dual-layer buffer tube 32 which can contain up to 12 optical fibers 34. The buffer tube 32 is surrounded by radial strength yarns 36 which are impregnated with filling compound. Flooded, corrugated armor 38 is applied over the radial strength yarns 36. A ripcord 40 is placed over the corrugated armor 38 to aid in the removal of the outer jacket 42. A medium-density polyethylene jacket 42 is applied over the armor to complete the structure. The cable 30 also has a number of drawbacks. First, the buffer tube 32 contains up to 0.6% fiber overlength, which is difficult to process in buffering. Secondly, the cable 30 has the capability of containing only 12 fibers. Thirdly, the buffer tube filling compound has a drip susceptibility at 65°C due to low viscosity. Fourthly, cable flexibility causes reduced sheave spacing during an aerial installation. Fifthly, the two-layer buffer tube 32 is difficult to process. Finally, taunt mid-span entry of the cable 30 is difficult. EP-A-0 284 900 describes an optical fiber cable comprising a single hollow buffer tube enclosing optical fiber units and a plastic outer jacket. Between the buffer tube and the outer jacket two layers of rod-like strength members are arranged. The strength members of the two layers are helically wrapped in the opposite helical direction to one another. The rod-like strength members are made of glass roving or yarn impregnated with a resinous material or - for a better flexibility of the strength members - with a polyurethane matrix material. A water-resistant communications cable with an impregnated tape being interposed between the cable core and the outer jacket and wrapped to form a longitudinal overlapped seam is described in EP-A-0 314 991. The tape comprises a substrate tape impregnated with a superabsorbent material which swells upon contact with water and inhibits the further movement of the water. The cable may comprise two longitudinally linearly extending strength members which are diametrically opposed or two superimposed layers of strength members, the strength members of the outer layer being wrapped in an opposite direction to the strength members of the inner layer. EP-A-0 286 349 deals with an optical fiber cable comprising two strength members which extend linearly longitudinally along the cable. The single hollow buffer tube is filled with a water-blocking material and is enclosed by a water-absorbent tape. Optical fiber cables comprising strength members which extend longitudinally along the cable are also known from FR-A-2 497 964 and AT&T Technology Vol. 4, No. 1, pages 62 and 63, the latter showing water-swellable tape enclosing the cable core. SUMMARY OF THE INVENTION The present invention is designed to overcome the limitations that are attendant upon the use of the prior art cables described above, and toward this end, it contemplates the provision of a novel optical fiber cable which minimizes the drawbacks of the prior art cables. The cable of the present invention is provided with a buffer tube which contains between 0.15% and 0.35% fiber over length in the buffer tube whereby a high fiber count (comparable to AT&T LXE) in the buffer tube is possible. High viscosity filling compounds are used which will not drip at temperatures up to 80°C. The single layer polyester (PBT) buffer tube combines high strength, good flexibility, and improved processibility. It is anticipated that the present invention would be applicable for use in many different types of installations including all types of outdoor installation. It is an object of the present invention to provide a cable design which reduces the overall diameter and weight thereof as compared to many of the prior art cables to facilitate installation in ducts. It is another object of the present invention to provide an optical fiber cable with radial strength yarns around the buffer tube to inhibit excessive tube stretch during the sheathing process. A further object is to provide such a cable which may be readily and economically fabricated and will enjoy a long life in operation. The invention (see claim 1) will be more fully understood when reference is made to the following detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of the prior art AT&T LXE optical fiber cable design; Figure 2 is a cross-sectional view of the prior art Siecor Maxitube optical fiber cable design; Figure 3 is a perspective cutaway section of an optical fiber cable made in accordance with the present invention. Figure 4 is a cross-sectional view taken along 4-4 line of Figure 3; and Figure 5 is a cross-sectional view similar to Figure 4 but of a second embodiment of the optical fiber cable of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Turning first to Figures 3 and 4, therein is illustrated the armored cable design of the present invention generally indicated by the numeral 50 and having a single, large, gel-filled buffer tube 52 made of polyester (PBT). The gel is a thixotropic, water blocking gel. The gel-filled buffer tube 52 contains a plurality of optical fibers 54. Radial strength yarns 56 made of either aramid or fiberglass materials are contra-helically stranded around the buffer tube 52 and impregnated with filling compound such as a petroleum based hot melt filling compound manufactured by Witco Corporation, New York, New York or Amoco Chemical Company, Chicago. Corrugated steel armor 58 is applied over the radial strength yarns 56. The corrugated armor 58 is flooded with a water blocking flooding compound 59 such as a hot melt adhesive or a petroleum based flooding compound. A high strength ripcord 60 is applied under the armor 59 to aid in sheath removal. Two steel strength members 62, 0.055" in diameter, are located 180 degrees apart in circumferential direction on the outside of the corrugated armor 58. A medium-density polyethylene (MDPE) outer jacket 64 encapsulates the steel strength members 62 and corrugated armor 58 to complete the structure. Ideally, the strength members 62 are at least 0.5 mm from the outer surface of the jacket 64. The water blocking flooding compound 59 is disposed between the corrugated armor 58 and the outer jacket 64. The non-armored cable design of the present invention is shown in Figure 5 and generally indicated by the numeral 70. It includes a single, large, gel-filled buffer tube 72 made of polyester (PBT). The gel is a thixotropic, water-blocking gel. The gel filled buffer tube 72 contains a plurality of optical fibers 74. Radial strength yarns 76 made of either aramid or fiberglass materials are contra-helically stranded around the buffer tube and impregnated with filling compound such as a petroleum based hot melt filling compound manufactured by Witco Corporation, New York, New York or Amoco Chemical Company, Chicago. Two metallic or dielectric strength members 78, 0.055" in diameter, are located 180 degrees apart in circumferential direction on the outside of the radial strength yarns 76. A high strength ripcord 80 is applied over the radial strength yarns 76 to aid in sheath removal. A medium-density polyethylene (MDPE) outer jacket 82 encapsulates the strength members 78 and radial strength yarns 76 to complete the structure. Ideally, the strength members 78 are at least 0.5 mm from the outer surface of the jacket 82. Both the armored and non-armored design exhibit improved characteristics over the prior art. The buffer tube contains between 0.15% and 0.35% fiber overlength, which facilitates processing of the cable on a consistent basis. With low fiber overlength in the buffer tube, a high optical fiber count (comparable to AT&T LXE) in the buffer tube is possible while high viscosity filling compounds can be used which will not drip at temperatures to 80°C. The single layer polyester (PBT) buffer tube combines high strength with good flexibility. The cable of the present invention has a smaller diameter and is lighter in weight than the AT&T LXE design, which improves the flexibility and the ease of installation in ducts. The two steel strength members produce a cable which is stiffer than the Siecor Maxitube design, which facilitates aerial installations. The radial strength yarns stranded around the buffer tube inhibit excessive tube stretch during the sheathing process. The combination of steel strength members and radial strength yarns help to better distribute installation loads throughout the cable and improve outer jacket "turtlenecking". The amount of radial strength yarns can be increased in the cable design of the present invention to produce a non-metallic cable which is less expensive than the AT&T LXE non-metallic design. The core filling and flooding compounds used in the cable design of the present invention to give better, less expensive water penetration resistance over water-swellable tape. The medium-density polyethylene outer jacket used in the present design is easier to process, more flexible, and less expensive than high-density polyethylene used in the prior art AT&T LXE cable. Furthermore, the present cable is designed for mid-span entry under taut conditions since the strength members do not have to be cut to access the optical fibers. Thus, it can be seen from the foregoing specification and attached drawings that the optical fiber cable of the present invention provides an effective cable design which emphasizes the positive features and minimizes the negative features of the prior art. The cable is ruggedly built and will function adequately for a very long period of time. The preferred embodiments admirably achieve the objects of the present invention; however, it should be appreciated that departures can be made by those skilled in the art without departing from the scope of the invention which is limited only by the claims.

Claims: An optical fiber cable (50; 70) comprising a core with at least one optical fiber (54; 74), a single hollow buffer tube (52; 72) enclosing said core and an outer plastic jacket (64; 82), whereby stranded strength elements (56; 76) are arranged between the outer jacket (64; 82) and the buffer tube (52; 72), said cable being characterized in that said strength elements (56; 76) are radial strength yarns contra-helically stranded around the buffer tube (52; 72) and impregnated with a water blocking filling compound, in such a way that they provide access to the optical fibers (54; 74) without being cut, two strength members (62; 78) are longitudinally embedded in said jacket (64; 82) near its inner surface and approximately 180 degrees apart in circumferential direction, said buffer tube (52; 72) is a single layer polybutylenterephthalate (PBT) buffer tube and is filled with a high-viscosity filling compound and said optical fiber (54; 74) has a longitudinal length between 0,15 % and 0,35 % longer than said buffer tube longitudinal length. A cable as described in claim 1, wherein a metallic, shield (58) is disposed between said strength yarns (56) and said jacket (64). A cable as described in claim 2, wherein said metallic shield (58) is corrugated. A cable as described in claim 3, wherein a water blocking flooding compound (59) is disposed between said metallic shield (58) and said jacket (64). A cable as described in claim 1, wherein the distance between said steel strength members (62; 78) and the outer surface of said jacket (64; 82) is at least 0.5 mm. A cable as described in claim 1, further including a ripcord (60; 80) longitudinally disposed over said strength yarns (56; 76). A cable as described in claim 6 wherein said ripcord (60; 80) is positioned approximately 90 degrees apart from said steel strength members (62; 78) in circumferential direction.

Inventors: NILSSON RICHARD CORY, NILSSON, RICHARD CORY
Applicants: ALSTHOM CGE ALCATEL, ALCATEL
Application year: 1990
Main classification: G02B6
IPC codes: G02B6