Optical fiber ribbon cable
RELATED APPLICATION 
     This application is a continuation-in-part of application, Ser. No. 07/837,465, filed on Feb. 14, 1992 now U.S. Pat. No. 5,189,721. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of optical waveguide fiber cables, particularly to cables embodying a multiplicity of optical fibers configured as a flat ribbon cable that is specifically designed to mate with mass termination systems. 
     BACKGROUND OF THE INVENTION 
     Optical glass fibers for communication are made from glass such as quartz glass or doped silica glass and are extremely small in diameter and hence weak in strength. Under normal conditions of use, such fibers may be subjected to considerable bending strain and other tensile forces during the cable manufacturing process and while being pulled through ducts and otherwise bent, twisted, or manipulated during reeling or during installation. In the transmission of light signals through optical fiber cables, the signals are readily attenuated in response to relatively small distortions in the cable, such as those caused by the above stresses, sharp bends or roughness in the surface of the fiber to produce light signal distortions or attenuation at an unacceptable level. 
     To confine the optical signals inside the signal-transmitting fiber core, a glass or silica fiber core is coated with a glass cladding or an amorphous fluoropolymer coating always of a lower refractive index from that of the optical fiber. The coating on the glass cladding may optionally be a silicone, acrylic, polyimide, or other release agent and a polymer coating, which is usually a hard or soft polymer coating which is coated on the fiber from a melt or a solution of the polymer, or extruded onto the fiber. 
     These considerations also apply to flat ribbon cables embodying a multiplicity of optical fibers arranged parallel to each other. Problems associated with ribbon cables occur in aligning the optical fibers parallel to each other at known and fixed distances apart. It is also crucial for a ribbon cable that is designed to mate with a mass termination system to have exact spacing with respect to a reference point. Crush resistance, flex life, and temperature and flame resistance of the cable can also be problems to be overcome. 
     Coating and buffering of optical fibers for various purposes are described in U.S. Pat. Nos. 5,002,359, 4,113,350, 4,105,284, 4,380,367, 4,072,400, 3,930,103, 4,463,329, 4,099,837, and 4,642,265 and in a paper in the Bell System Technical Journal, in Volume 54, No. 2, pages 245-262, February 1975, by D. Gloge. 
     SUMMARY OF THE INVENTION 
     The cable of the invention comprises an optical fiber ribbon cable having the layers inside to outside: 
     (a) a multiplicity of polymer coated optical fibers arranged parallel to each other at controlled fixed distances apart; 
     (b) surrounding the optical fibers as a unit on at least the top and bottom sides of the unit, a first layer of porous expanded polytetrafluoroethylene, each side of which is coated by an adhesive; 
     (c) surrounding the first layer a second layer of polyester tape, which is coated on at least its inner side with a polyester adhesive layer; 
     (d) surrounding the polyester tape layer a braided sheath of aramid fibers; 
     (e) surrounding said braided sheath of aramid fibers a layer of tape-wrapped binder tape; and 
     (f) the braided sheath surrounded by an extruded jacket of flame retardent thermopolymer. 
     In an alternative cable of the invention, the braided sheath layer (d) and the tape-wrapped binder tape layer (e) are interchanged. This interchange of layers allows some movement of the layers along the cable to allow for easier termination of the cable to a connector and allows the aramid fiber braid to provide better strain relief if an accidental pull on the cable occurs. 
     A second alternative form of the invention substitutes a woven aramid fiber cloth for the braided sheath of aramid fibers to provide a high break strength along the length of the cable under tension of an order of magnitude of about four hundred fifty pounds per square inch of pull. 
     A preferred form of the cable of the invention embodies acrylic-polymer coated or clad glass or silica optical fibers, a polyester adhesive-coated porous expanded polytetrafluoroethylene (ePTFE) first tape layer, coated on at least one side with the adhesive, a polyester adhesive-coated polyester second tape layer, coated on both sides with the adhesive, and a flame retardant, preferably polyvinyl chloride (PVC) jacket. The braided sheath strength member is of a strong aramid fiber, such as Kevlar® (manufactured by E. I. duPont de Nemours and Company). 
     The process for manufacture of the cable of the invention also forms a part of the invention in that it provides a cable having a precise, stable spacing of the optical fibers in the cable which are buffered and decoupled from the mechanical and temperature environment surrounding the cable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cutaway perspective view of a cable of the invention to display all the layers thereof. 
     FIG. 2 is a schematic representation of the process for making a cable of the invention. 
     FIG. 3 is a perspective view of an accumulator, useful to align a multiplicity of clad optical fibers side by side in the process of manufacture. 
     FIG. 4 is a cross-sectional view of a guide for aligning a multiplicity of optical fibers from the accumulator in a fixed spacing along with guide wires before entering the nip of the rollers used to apply insulation under pressure around the clad optical fibers. 
     FIG. 5 is a partial sectional view of the grooved rollers used to form the insulation layers around the clad optical fibers of the cable. 
     FIG. 6 is a cutaway perspective view of a cable of the invention to display the layers of an alternative version of the cable. 
     FIG. 7 is a cutaway perspective view of a cable of the invention to display the layers of another alternative version of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The cable and the process of the invention are now described with reference to the drawings to more clearly delineate the materials and structure of the cable of the invention and the process for its manufacture. 
     FIG. 1 is a perspective view of the cable of the invention with various layers cutaway to display the materials and structure of the cable. The clad optical fibers are aligned parallel to each other at known fixed distances apart, its being generally desirable that the separating distance between the fibers approach zero as nearly as possible. The glass or silica optical fibers 1 are clad with preferably a polymer cladding 2, an acrylic polymer being preferred, although other polymers known to be effective for cladding an optical fiber may be used. 
     Fibers 1 with cladding 2 are surrounded by a layer of porous expanded polytetrafluoroethylene (ePTFE) tape 4 which has on both sides a coating of adhesive 3 and 5, which is preferably a polyester-based adhesive. The adhesive is coated on the ePTFE film from a solvent, such as methylene chloride, or is adhered to the ePTFE as a film. Other adhesives may be used, such as polyurethane, fluorinated ethylene-propylene copolymers (FEP), perfluoroalkyl ethers of polytetrafluoroethylene (PTFE) polymers (PFA), and silicone-urethane polymers. 
     Applied over the adhesive-coated ePTFE tape is a strong polyester tape 7 which is adhesive 6 coated on at least the inner side adjacent the ePTFE tape. Polyester tape 7 is surrounded by a braided sheath 8 of strong aramid fibers, such as Kevlar (E. I. duPont de Nemours &amp; Company). Other materials useful for the braided fibers include RASTEX® PTFE fiber (W. L. Gore &amp; Associates, Inc.), Vectran® (Hoechst Celanese), Technora® (Tejin America Inc.), and Spectrafiber® (Allied Chemical), for example. A binder ribbon 9 is wrapped around the braided sheath 8. Ribbon 9 is formed from ePTFE or a highly compressed and densified almost transparent PTFE tape (HSCR). 
     The cable is completed by extrusion of a flame-retardant jacket 10 around it, such as PVC. 
     In FIG. 2 is diagrammed the process of making the cable of the invention. Spools 20 of guide wires and spools 21 of clad optical fibers have been set up on &#34;trees&#34; for feeding the wires and optical fibers into an accumulator 22, which contains banks of parallel rollers set in vertical and horizontal rows to direct the wires and fibers into a flat parallel row in similar configuration to their arrangement in the ribbon cable. Great care must be exercised to provide spools of optical fibers which have even back tension and which will pass evenly through the accumulator 22 smoothly to form a flat bundle 31 of fibers and guide wires in which all components are of even tension passing into guide 26. 
     Guide 26 (FIG. 4) is machined in two halves to close tolerances to position the optical fibers 40 and guide wires on the desired center to center spacing as they enter the nip of grooved rollers 27 which will apply pressure on the layers of ePTFE tape 4 and polyester tape 7 which feed into the nip on each side of the carefully aligned optical fibers and guide wires. The flat slot 48 precisely matches the width of the row of optical fibers and the apertures 49 contain the guide wires on each side of the optical fibers. Two guide wires are used on each side of the optical fibers for stability and cable flatness during the manufacture process. Sources of hot air 25, or other heated gas, supply hot air to the area just in front of rollers 27 to heat and soften the adhesive layers 3, 5, and 6 on the ePTFE and polyester tapes before they contact the surface of the optical fibers and guide wires in the nip of the rollers 27 under pressure. Rollers 27 have grooves carefully machined into their impinging surfaces to match the spacing of the optical fibers and guide wires to aid in proper formation of the insulation layers around the optical fibers and guide wires under the pressure of the rollers. In use, guide 26 has the two halves 45 and 46 closed together to form the slot 48 to proper dimensions (thickness) for guiding the optical fibers into the nip of rollers 27. ePTFE tapes (adhesive 3 and 5 on each side) and polyester tapes (adhesive 6 on the inner side adjacent the ePTFE tapes) unreel from spools 23 and 24 to pass into the nip of rollers 27 on the outside of both sides of the optical fibers and guide wires where they are sealed in place around them. Alternatively, polyester tapes 7 can be used without adhesive layer 6. 
     The formed cable 32 passes from rollers 27 under cold air (gas) streams from supplies 29 into slitter 25, where the edges of the cable and the guide wires are carefully trimmed from the cable to close tolerances to leave cable 33 containing the optical fibers. 
     Cable 33 then passes into a flat cable braider 34 of a type well known in the art of cable braiding wherein a braided sheath 8 of strong polymer fibers, such as Kevlar aramid fibers or fibers of equivalent strength properties, is applied. The now sheathed cable 30 then passes into a tape wrapper where a layer of binder tape 9 is wrapped around the sheath at about 50% overlap to separate the braided sheath from an outer jacket. The cable 36, now carrying an outer layer of binder tape 9, passes into an extruder, wherein an outer protective flame retardant jacket is applied surrounding the cable. The binder tape 9 is preferably ePTFE, but may be HSCR or other full density PTFE or other polymer tape. The extruded jacket is preferably flame resistant PVC, but may be other thermoplastic polymers known to be useful for jacketing, such as polyurethane, rubber, polyethylene, polypropylene or silicone, for example. The now jacketed cable 38 is taken up on storage reel 39. 
     FIG. 3 depicts in perspective view an accumulator 22 which receives on rows of parallel vertical 41 and horizontal 43 rollers optical fibers 40 and guide wires and aligns the fibers and wires into a flat configuration for easy passage into the guide 26 of FIG. 4 without appreciable torsion or back tension, which can distort both the shape and properties of the final cable after its manufacture. 
     FIG. 5 gives a partial view of the nip area between rollers 27 where the relationship and spacing between the grooves 54 to house clad optical fibers and grooves 52 to house guide wires is shown for a typical set of cable forming pressure rollers. Use of two sets of guide wires has been found to aid in achieving good stability and flatness properties in the ribbon cable. 
     FIG. 6 is a partial cutaway perspective view of an alternative cable of the invention wherein binder ribbon 9 is spiralled over polyester layer 7 and braided sheath 8 of strong aramid fibers is braided over the binder ribbon layer. This may be advantageous when the layers of the cable are pushed back from a cable end being terminated to an electrical connector to allow easier termination of the electrical conductors. Following completion of termination, the cable layers are pushed back into place on the terminated conductor ends. 
     It is also preferable if the angle of braiding of the aramid fibers is low, such as only about 5 to about 10% off parallel to the length of the cable. This low angle of braiding tends to moderate the tendency of a braid under strong linear pull to crush the flat cable into a round cable when the braid closes under tensile pull. 
     FIG. 7 is similar to FIG. 6 and FIG. 1 and describes an alternative version of the cable of the invention wherein an aramid fiber woven cloth 11 is used in place of a braid of aramid fibers. This version provides a cable having a tensile break strength of the order of about 450 psi. 
     The above process provides a well insulated and decoupled fiber optic ribbon cable which has edges of very stable dimensions in which the optical fibers are positioned precisely with reference to the edge such that when the datum edge of the cable abuts the edge of a connector, the optical fibers therein fall precisely into the grooves of the connector for easy and accurate termination of the cable to the connector.