Abstract:
A hybrid optical fiber cable for bearing trunk traffic as well as local traffic has at least one tubular sheath member containing a fiber ribbon stack and surrounded by a sheath member and one or more smaller tubular sheath members within the surrounding sheath member in a configuration amenable to breaking out one or more fibers for local traffic without disturbing the fiber ribbon stack or stacks.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains to the field of high fiber density optical fiber cables.  
         BACKGROUND OF THE INVENTION  
         [0002]    The optical communications field is growing at a rapid rate, requiring continuing advances in the development of the numerous components required in optical communications as well as continuing improvements in existing components. It is anticipated that in the near future, optical fibers will replace metallic conductors substantially completely in communication signal transmission. One genus of optical fiber components is the transmission means used to transmit signals to their ultimate destination. Usually, for long distance or trunk usage, cables are required which have a large fiber capacity, while for local use, only a few fibers encased in a suitable means, i.e., stranded cable or small fiber ribbons, are required.  
           [0003]    It is common in the prior art to form ribbon cables for use in loop or trunk arrangements. An optical fiber ribbon is usually formed as a planar array of closely adjacent parallel fibers embedded in a suitable supporting medium, to form a ribbon having, for example, twelve parallel fibers. A cable is formed by stacking a number of ribbons together, such as twelve ribbons, to create a structure that contains, for example, one hundred and forty-four fibers. The ribbon stack is, generally, enclosed in a loose sheath or tube, which most often is filled with a suitable water blocking filling compound. Additional protective sheath and strength members may surround the tubular sheath containing the stack, the core tube. In U.S. Pat. No. 4,744,631 of Eichenbaum, et al, the disclosure of which is incorporated herein by reference, there is shown such a cable wherein the filling compound is a thixotropic material having a low yield stress. A high density cable can have one or more stacks of, for example, twelve ribbons per stack, with each ribbon having twelve fibers. Thus, a cable stack has, for example, one hundred and forty-four (144) fibers. In U.S. Pat. No. 5,857,051 of Travieso, et al., the disclosure of which is incorporated herein by reference, there are shown cables with several ribbon stacks, each enclosed in its own sheath, and the several sheaths enclosed in an outer sheath or jacket.  
           [0004]    In the wiring of premiums, such as, for example, an office building, such a high density cable of stacks can be used, with individual ribbons and individual fibers being broken out for use at individual stations within the building. However, the breaking out operation in such a stack containing cables is tedious and subject to error. Where such cables are used to supply individual residences, for example, the break out operation can entail the breaking out of only one or two individual fibers from the entire collection of stacks, which, again, is tedious and subject to error. Obviously, cables containing stacked ribbons are better suited for long hand or trunk transmission, where, at each destination, large numbers of fibers are to be broken out.  
           [0005]    In local use situations, where there are numerous stations, e.g., customer premises, it is common practice to use loose tube cabling, where a plurality of fibers (such as twelve) are loosely contained within a tubular sheath which, usually, is filled with a suitable water-proofing compound, and accurate break-out of individual fibers is relatively easy. Loose tube cabling is not generally amenable to long distance or trunk usage, such as a stacked ribbon cable, but, on the other hand, is to be preferred where local usage requires a large number of break-outs.  
           [0006]    In urban areas, high cable density is a desideratum that is difficult to attain where trunk operation and local operation are both required.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is a hybrid cable for combining trunk or long haul transmission with local area transmission which utilizes both a stacked ribbon configuration and a piggy-back loose tube configuration, wherein both types of transmission are optimally realized, and, for a given cable diameter, the fiber density is greater than heretofore realized in the prior art.  
           [0008]    In a first preferred embodiment of the invention, the cable has a central member having a plurality of stacked fiber ribbon such as, for example, twelve ribbons bearing one hundred forty-four fibers, encased in a sheath which, preferably, has a waterproofing material, such as a gel therein. Immediately surrounding the sheath are a plurality, such as twelve, loose tube cables, each containing twelve fibers in a loose tube configuration. The assembly is surrounded by a jacket which may comprise a protective sheath of, for example, high density polyethylene having strength members embedded therein, a polyester tape, and a second protective sheath having strength members embedded therein. The strength members may be steel wire, fiber glass roving, aramid fiber or yarn, or other suitable strength providing material. It is to be understood that the jacket may comprise other layers, or only one layer, than those shown herein. The cable of the embodiment has a central tubular member of a first diameter and a plurality of surrounding tubes of a second lesser diameter, and contains two hundred and eighty-eight (288) fibers. The cable core central tube diameter contains a twelve fiber ribbon stack and therefore normally contains one hundred and forty-four fibers. The twelve smaller diameter outer tubes contain a total of one hundred and forty-four (144) fibers, there being twelve loose fibers in each tube, and thus a fiber density increase of forty-eight fibers is achieved in approximately the same diametric dimension as a prior art two stack cable, and one-hundred and forty-four of the fibers contained in the smaller diameter tubes are readily accessible for breaking out.  
           [0009]    The embodiment of the invention achieves the aim of both long haul and short haul in a single fiber cable that is approximately equal in outside diameter to prior art cables, and which is of a higher fiber density than prior art cables of the same size.  
           [0010]    In a second embodiment of the invention, the structure is the same as in the first embodiment except that each of the smaller tubes contains a fiber ribbon stack of twelve fibers, for example, or a single twelve fiber ribbon.  
           [0011]    In a third embodiment of the invention, three ribbon fiber stacks are contained in three sheaths of a first diameter within an outer jacket. As will be seen hereinafter, in such an arrangement, where connection of the centers of the three sheaths forms a triangle, having at least three interstices into which can be fitted three fiber units of lesser diameter, each having approximately thirty-six fibers. Normally, to realize this many fibers in a small tubular member would preferably require the use of fiber ribbons stacked together, where the ribbons each contain, for example, six fibers. It is also possible to have the fibers loose in the tubes, as in the first embodiment. In this embodiment, the cable core has five hundred and forty (540) fibers in the same diameter cable as a prior art cable having four hundred and thirty-two (432) fibers, and gives a twenty-five percent (25%) increase in fiber density.  
           [0012]    In still another embodiment of the invention, the cable has four fiber ribbon stack units containing five hundred and seventy-six (576) fibers and five twenty-four fiber tubes of lesser diameter than the stack units for a total of six hundred and ninety-six (696) fibers contained within a cable jacket having the same diameter as a prior art five hundred and seventy-six (576) fibers cables, a twenty-one percent (21%) increase in fiber density in the same diameter cable as a prior art five hundred and seventy-six fiber cable. In both of these last two embodiments, the smaller tubes may have a loose tube fiber configuration or a ribbon stack configuration. In either case, break-out is much simpler than with the one hundred forty-four fiber stacks in the larger diameter units.  
           [0013]    Each of the foregoing embodiments is a high density hybrid cable having one or more large tubes containing stacked fiber ribbons, which serve as a trunk cable, and a plurality of small tubes of loose tube or smaller ribbon stack configuration which function as local area cables.  
           [0014]    The features and principles of the present invention will be more readily apparent from the following detailed description read in conjunction with the drawings. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a cross-sectional elevation view of a cable embodying the principles of the invention;  
         [0016]    [0016]FIG. 2 is a cross-sectional elevation view of a variation for use in the cables of the invention;  
         [0017]    [0017]FIG. 3 is a cross-sectional elevation view of another cable embodying the principles of the invention; and  
         [0018]    [0018]FIG. 4 is a cross-sectional elevation view of still another cable embodying the principles of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0019]    In FIG. 1 there is shown a preferred embodiment of the invention which comprises a hybrid cable  11  having a central core member  12  in which a ribbon stack  13  of, for example, twelve ribbons  14 , each having twelve fibers  16 , is enclosed in an inner tube member  17 . The empty space within tube member  17  is preferably filled with a suitable waterproofing material or gel  18 , such as, for example, the low yield stress thixotropic material disclosed in the aforementioned Eichenbaum, et al. patent. Tube member  17 , which forms an inner jacket around the stack  13  and gel  18  may be made of any of a number of suitable materials, such as polyethylene. For simplicity, only one of the ribbons  14  is shown with the fibers  16 , it being understood that each of the twelve ribbons  14  contains its complement of twelve substantially parallel fibers  16  encased in a suitable matrix material.  
         [0020]    Surrounding tube member  17  are twelve lose tubes  19 , each of which contains twelve loosely grouped fibers  21  which are preferably buffered with a suitable buffering material. Only one of the tubes  19  is shown containing fibers, but it is to be understood that each of the tubes  19  contains its complement of twelve fibers  21 . Surrounding the circular array of tubes  19  is a protective sheath  22  of, for example, high density polyethylene which may, for example, have embedded therein strength members  23  of aramid fiber, glass roving, or wire. Protective sheath  22  is surrounded by a polyester tape  24  and a second protective sheath  26 , which may or may not have strength members embedded therein, surrounds the tape  24 . The entire assembly as thus far described is preferably enclosed in a jacket  27 .  
         [0021]    The cable  11  of FIG. 1 thus is a hybrid structure in the sense that there are both ribbons arranged in a stack  13  for trunk usage, and loose tube fiber tubes  19  for break-out accessibility. The entire cable  11  has approximately the same dimensions, i.e., diameter as prior art cables having one ribbon stack of twenty ribbons, with two hundred and eighty-eight fibers arranged in the stack, and thus the cable of the invention yields considerably greater fiber density. A prior art cable having 288 fibers would require, for example, two twelve ribbon stacks, contained in a much larger diameter structure. It is to be understood that the arrangement of protective tubes may contain more or less tubes, fewer such tubes yielding a smaller diameter cable.  
         [0022]    In FIG. 2 there is shown a variation of the loose tube arrangement of FIG. 1 wherein each of the tubes  19  has a stack  31  of three ribbons  32  of four fibers  33  each. With the smaller ribbons  32  and consequent fewer fibers  33 , break-out is relatively easily performed while the fibers  33  have a greater protection against possible damage, being segregated from each other.  
         [0023]    In FIG. 3 there is shown a hybrid cable  36  embodying the principles of the present invention in which there are three large tubes or sheaths  37 , each containing, for example, a twelve ribbon stack  38  containing one hundred and forty-four fibers. For simplicity the units  37  are shown enclosed only in a first outer layer  39  which may comprise several layers such as are shown in FIG. 1. In addition, only one of the sheaths  37  is shown with a ribbon stack  38  therein, but it is to be understood that each of the tubes or sheaths contains a ribbon stack  38 . As thus far described, the cable  36  is basically similar to prior art cables containing three ribbons tacks  38  for a total of four hundred and thirty-two fibers, and is of substantially the same outer diameter, including any additional protective layers as shown in FIG. 1 and which are common to prior art cables also.  
         [0024]    In accordance with the present invention, three tubular members  39  are interspersed within the cable  36  in the interstices  41  formed by the triangular arrangement of the larger tubes  37 . As shown in FIG. 3, each of tubes  39  has, in a loose tube configuration, thirty-six fibers  42  for a total of one hundred and eight fibers, and the cable  36 , although of substantially the same diameter as prior art cables having three one hundred forty-four fiber stacks, or a total of four hundred thirty-two fibers. Thus, for the same dimension cable  36 , which has five hundred and forty fibers, there is a fiber density improvement of approximately twenty-five percent (25%). In addition, one hundred and eight fibers are, in the loose tube configuration, amenable to simplified break-out, as discussed hereinbefore. Instead of a loose tube configuration, as shown in FIG. 3, it is also feasible to have each of the tubes  39  contain ribbon stacks of thirty-six fibers, in the manner shown in FIG. 2, which would still afford a simplified break-out.  
         [0025]    [0025]FIG. 4 depicts a cable  46  having four large tubes  47  each containing, for example, a twelve ribbon stack  48  of one hundred forty-four fibers (not shown) for a total fiber count of five hundred and seventy-six fibers. Such an arrangement can be found in the prior art. In accordance with the principles of the invention, there are interspersed within the interstices  49  within the cable, five tubes  51 , each containing in a loose tube configuration, twenty-four fibers  50  for a total of one hundred and twenty fibers, giving cable  46  a total of six hundred and ninety-six fibers, a twenty-one percent (21%) improvement in fiber density within the same dimension cable as one containing five hundred and seventy-six fibers in four fiber stacks. As with cable  36  of FIG. 3, instead of a loose tube configuration as shown in FIG. 4, the tubes  51  may contain small fiber stacks of twenty-four fibers  50  each, without materially increasing the difficulty of break-out.  
         [0026]    The principles of the invention have been demonstrated in the foregoing embodiments. Such features as increased fiber density without increase in the external dimensions of the cable, and having both easily broken out fibers for local use and fiber stacks for trunk use represent a material improvement over prior art cable. These principles can be extended to apply to even larger cables having different fiber counts within the tubular members without departure from the basic principles set forth.  
         [0027]    It is to be understood that the various features of the present invention might be incorporated into other types of cables, and that other modifications or adaptations might occur to workers in the art. All such variations and modifications are intended to be included herein as being within the scope of the present invention as set forth. Further, in the claims hereinafter, the corresponding structures, materials, acts, and equivalents of all means or step-plus-function elements are intended to include any structure, material, or acts for performing the functions in combination with other elements as specifically claimed.