BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an optical cable with an expandable fiber capacity. More particularly, the present invention relates to an expandable optical cable having conduits capable of receiving additional optical fibers as needs arise after the cable is installed.  
           [0002]    Typically, optical fiber cables are installed underground and, in many situations, in ducts of an optical-cable passageway. When installed in ducts, the cables may be pulled through the ducts or blown through them. In a pulled installation, a leading end of a cable is attached to a line and pulled through the duct. In an air-blown installation, a gas is flowed into an end of the duct along with the cable and in a direction in which the cable is installed. In this manner, frictional forces produced between the flowing gas and the cable help to move the cable through the duct. The length of cable that can be blown through a duct can be increased by applying an additional pulling force or by a parachute-like device attached to the leading end of the cable. Unlike pulled cables, however, air-blown cables do not have to withstand high tensile loads, and, consequently, their structure can be less substantial. Air-blown cables do require a degree of stiffness, though, to facilitate their movement through ducts.  
           [0003]    When installing optical fiber cables, it is often desirable to provide for future growth in demand along a particular communication route. An optical fiber cable installed today may not provide sufficient capacity to meet tomorrow&#39;s needs. Currently, future demand is typically accommodated by including additional ducts in a cable assembly along the particular route that are unused initially. The additional ducts meet future demand by providing a pathway through which an additional cable can be installed at some point in the future. This method of installing a separate duct and cable along a route, however, has a few disadvantages. For example, two different products (a duct and a cable) are required which often come from two different manufacturers. Additionally, installation time is increased as the two separate components necessitate additional handling.  
           [0004]    A second option for addressing future demand is to install a cable with greater capacity than that required at the time of installation. In particular, a cable containing more optical fibers than is immediately necessary can be installed. But this is an expensive alternative as the capital investment for predicted future growth must be made at the time of initial installation. Deferring capital investment until the time of expansion is generally more desirable.  
           [0005]    A third option is disclosed in U.S. Pat. No. 6,101,304. The &#39;304 patent describes a so-called air blown fiber tube assembly that combines several ducts into one unit and surrounds it with an outer jacket. In particular, the assembly has a central innerduct surrounded by a plurality of strength elements and air blown fiber tubes. The assembly is installed underground using conventional boring or direct burial methods. After installation, a conventional cable (optical fiber, power, etc.) may be pulled through the central innerduct of the tube assembly. As well, fiber assemblies may be air blown into the air blown fiber tubes.  
           [0006]    PCT application GB01/GB03343, having a priority date of Aug. 7, 2000, discloses an expandable optical fiber cable having a tubular central strength member. In this PCT application, the cable has optical fibers in buffer tubes that are stranded around the central strength member. Additional optical fibers may be blown into the tubular central strength member. PCT application GB01/GB03343 and the present application were, at the time the present invention was made, owned by the same entity or subject to an obligation of assignment to the same entity.  
           [0007]    Applicants have found that the proposals in the known art do not provide sufficient capacity for expansion while minimizing installation costs. For example, the tube assembly of the &#39;304 patent requires that installation take place in two steps before any operation can occur. First, the tube assembly must be installed underground, then the cables must be installed by pulling and blowing into the assembly conduits. Applicants have also noted that this tube assembly is impractical for upgrading a communication route that already has ducts installed underground.  
         SUMMARY OF THE INVENTION  
         [0008]    Applicants have found that an optical fiber cable having within it the capability of expanding capacity provides a desirable alternative to prior arrangements. In particular, Applicants have discovered that a cable populated with optical fibers can help meet current capacity demands and provide for future growth along a given route by including at least one conduit outside the central strength member for receiving additional blown fibers.  
           [0009]    In one aspect, a cable consistent with the present invention includes a central strength member, a buffer tube, a first conduit, and an outer jacket. The buffer tube is stranded around the central strength member and loosely houses at least one optical fiber. A first conduit is configured to receive at least one first blown optical fiber and is positioned external to the central strength member. The central strength member may include a bore configured to receive at least one central blown optical fiber. The outer jacket is positioned at the periphery of the cable.  
           [0010]    Preferably, the cable also includes an inner jacket positioned inside the outer jacket. The inner jacket may surround at least the central strength member and the buffer tube. The first conduit may be positioned inside or outside the inner jacket. As well, the first conduit and the inner jacket may have substantially identical inner diameters. The cable may include a second conduit configured to receive at least one second blown optical fiber and positioned outside the inner jacket.  
           [0011]    In another aspect, a cable consistent with the present invention includes a central strength member and a conductor of electrical energy stranded around the central strength member. The central strength member may be either hollow or solid. A first conduit is configured to receive at least one first blown optical fiber and is positioned external to the central strength member. Finally, an outer jacket is positioned at the periphery of the cable.  
           [0012]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.  
         [0014]    [0014]FIG. 1 is a perspective view of a cable assembly with a central strength member, multiple cables, and multiple ducts consistent with the principles of the present invention;  
         [0015]    [0015]FIG. 2 is a cross-sectional view of a further embodiment of a cable assembly with multiple ducts and cables and an additional inner jacket consistent with the principles of the present invention;  
         [0016]    [0016]FIG. 3 is a cross-sectional view of a further embodiment of a cable assembly with multiple ducts, multiple cables, and a single outer jacket consistent with the principles of the present invention; and  
         [0017]    [0017]FIG. 4 is a cross-sectional view of a further embodiment of a cable assembly with multiple ducts, multiple cables, an additional inner jacket and a lashing element consistent with the principles of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Reference will now be made to various embodiments according to this invention, examples of which are shown in the accompanying drawings and will be obvious from the description of the invention. In the drawings, the same reference numbers represent the same or similar elements in the different drawings whenever possible.  
         [0019]    Consistent with the general principles of the present invention, a cable allowing for future expansion includes a central strength member, a buffer tube stranded around the central strength member, a first conduit configured to receive at least one optical fiber, and an outer jacket.  
         [0020]    As herein embodied and illustrated in FIG. 1, a cable  100  includes strength member  104 , first conduit  116 , buffer tube  124 , and outer jacket  132 . In this embodiment, strength member  104  forms the center of cable  100 . Buffer tube  124  and first conduit  116  are stranded around strength member  104 . As shown, multiple buffer tubes  124  (and conduits  116 ) may be disposed around strength member  104 . The buffer tubes loosely house at least one optical fiber  128 , which with other fibers may arranged, for example, individually, in bundles, or in ribbons. While first conduit  116  and buffer tube  124  are depicted as being the same size in FIG. 1, they may be of different shapes or sizes. Preferably, however, the size of first conduit  116  is substantially similar to that of buffer tube  124  to ease manufacture and to make the diameter of cable  100  uniform. Buffer tubes  124 , preferably, are formed from a polyester material, and most preferably from a polybutylene terapthalate (PBT) resin or polypropylene, and alternatively from medium density or high density polyethylene.  
         [0021]    An outer jacket  132  forms the outer barrier of cable  100  at its periphery, surrounding the plurality of outer strength members  144 . Outer jacket  132  is preferably made of a medium density polyethylene but can be made of any suitable material. For example, outer jacket  132  may be made of a reinforced plastic. In a flame retardant application, outer jacket  132  can, for example, be made of a PVC material or a low smoke halogen material. Outer ripcord  148  may be provided under outer jacket  132  or integral to outer jacket  132  to assist in opening cable  100  to access internal elements.  
         [0022]    As shown in FIG. 1, additional protective elements may be included in cable  100 . For example, a first layer of water blocking material  136  may be applied between strength member  104  and the stranded arrangement of first conduit  116  and buffer tube  124 . Material  136  may be a water blocking tape or water swellable material. In another aspect, water swellable yarn may be used instead of tape. Additionally, water swellable material can be placed inside buffer tube  124  surrounding optical fibers  128 . A second layer of water blocking material  140  of the same or similar material as  136  may surround the stranded arrangement of first conduit  116  and buffer tube  124 .  
         [0023]    Outer strength members  144  are optionally provided to reinforce and stiffen cable assembly  100 . A layer of outer strength members  144 , disposed beneath outer jacket  132 , may comprise aramid or fiberglass yarns, though any convenient reinforcement may be employed.  
         [0024]    Central strength member  104  in FIG. 1 may be solid (not shown), but preferably is hollow. If hollow, it contains a longitudinal bore  108  of any desirable shape. In this manner, an interior surface of strength member  104  defines a continuous bore  108  through which optical fibers can be passed, preferably by blowing air. Strength member  104  can be made of any suitable material. For example, strength member  104  may be formed from fiber-reinforced plastic. Preferably, strength member  104  is made of polyethylene. The polyethylene may be embedded with dielectric strength rods or, for example, e-glass. The reinforcement may also be accomplished by adding fiberglass rods, fiberglass yarn, or other suitable fillers to the plastic base material that provide the necessary anti-buckling and strength characteristics for a particular installation. In a flame retardant application, strength member  104  can, for example, be made of a PVC material or a low smoke halogen material.  
         [0025]    First conduit  116  contains a cavity  120  for receiving optical fibers. In this manner, an interior surface of first conduit  116  forms a passageway  120  for optical fibers. First conduit  116  is preferably suited for accepting optical fibers through a blown optical fiber installation process. As mentioned, cable  100  may contain a plurality of conduits  116  just as it may contain a plurality of buffer tubes  124 . First conduit  116  can be made from a fiber reinforced plastic but may be manufactured from any suitable material including the same material chosen for buffer tubes  124 . First conduit  116  is preferably made from a water-resistant material, such as polyethylene. In a flame retardant application, first conduit  116  can, for example, be made of a PVC material or a low smoke halogen material.  
         [0026]    In accordance with the present invention, optical fibers  128  within buffer tubes  124  provide an initial communication capacity for cable  100 . Optical fibers  128  are installed in buffer tubes  124  upon manufacture following standard assembly techniques. Typically, bore  108  and/or first conduit  116  are empty after the manufacture of cable  100 . These cavities enable cable  100  to have an expandable capacity for optical fibers. If cable  100  is installed in a communication route that later requires more fiber paths, additional fibers can be blown into bore  108  and/or first conduit  116  to accommodate that requirement. Fibers can be blown into bore  108  and/or first conduit  116  in any manner known in the art, including with the assistance of a pulling force or a pushing force like a parachute-type device.  
         [0027]    One skilled in the art will understand that the diameter of cable  100  may be constrained by the application in which cable  100  is used. For example, if cable  100  is to be installed in an existing duct with a certain diameter, then the components comprising cable  100  need to be sized accordingly. In this manner, the diameter of strength member  104  and the number of buffer tubes  124  and conduits  116  that can be arranged within outer jacket  132  may be adjusted. Alternatively, if the installation does not have specific diameter constraints, first bore  108  of strength member  104  may be of a large diameter so as to accommodate a greater number of optical fibers. In a further configuration not illustrated but readily apparent to one of skill in the art from the present disclosure, a plurality of conduits  116  and buffer tubes  124  may be positioned around strength member  104  in multiple layers.  
         [0028]    It will be recognized that the capacity of first conduit  116  to receive an optical fiber is dependent upon the lay length of the conduit and its inner diameter. In general, the distance a fiber can be blown is defined by the frictional characteristics of the conduit and of the cable itself, the volume of air blown through the conduit, the relative diameters of the cable and conduit, and the amount of bending required in the installation. In general, the smaller the inner diameter of first conduit  116 , the more difficult it would be to blow an optical fiber through it.  
         [0029]    The following describes further detail about one example of the embodiment of FIG. 1 consistent with the present invention. In this case, strength member  104  is polyethylene and has an outer diameter of 6.2 mm and an inner diameter of 4.0 mm. First bore  108 , formed by an interior surface of strength member  104 , has a diameter of 4.0 mm. In this configuration, first bore  108  can accommodate an optical fiber unit with  12  fibers (not shown) after the cable has been installed. To bolster the strength of member  104 , eighteen glass reinforcement rods  112  are disposed longitudinally along and preferably within polyethylene member  104 . Each of these reinforcement members  112  has a diameter of 0.7 mm. A first water swellable tape layer  136  surrounds strength member  104 .  
         [0030]    Stranded tubes surround central strength member  104 . In particular, five buffer tubes  124 , each with a diameter of 6.2 mm, are S-Z stranded around strength member  104 . A conduit  116  with an inner diameter of 4.0 mm and an outer diameter of 6.2 mm is stranded with the buffer tubes  124  around strength member  104 . The buffer tubes  124  and the conduit  116  are made of materials with similar stiffness characteristics. Conduit  116  can accommodate an optical fiber unit with  12  fibers (not shown) after the cable has been installed. The lay length of the stranded conduit would preferably range from about 40 mm to about 1100 mm.  
         [0031]    Outside the stranded tubes, additional water blocking and strength materials protect the optical fibers of the cable. Water swellable yarns are contra-helically wound around buffer tubes  124  and conduit  116  to form second water blocking layer  140 . A layer of aramid strength yarns, forming outer strength members  144 , are longitudinally disposed over second water blocking layer  140 . Outer MDPE jacket  132  surrounds the preceding structures to provide a cable assembly  100  with an overall diameter of 21.35 mm. This is but one example of an embodiment of the present invention and is not intended to limit the scope of the claims.  
         [0032]    In a variation to the embodiment illustrated in FIG. 1, cable  100  may comprise a conductor of electrical energy in place of one or more of buffer tubes  124 . In particular, one or more metallic conductors (not shown), typically copper or aluminum, may be positioned where buffer tubes  124  are shown in FIG. 1. In this way, cable  100  may accommodate both optical communications and electric power as a hybrid cable.  
         [0033]    In this hybrid alternative to FIG. 1, central strength member  104  may be hollow as shown in FIG. 1, or it may be a solid rod. The choice of whether to use a hollow or a solid strength member depends on factors such as the need for additional capacity reserve for future expansion and the need for cable stiffness.  
         [0034]    [0034]FIG. 2 is a cross-sectional view of a further embodiment consistent with the present invention. As herein embodied and illustrated in FIG. 2, a cable assembly  200  includes strength member  104 , first conduit  116 , buffer tubes  124 , an inner jacket  220 , second and third conduits  208  and  216 , and an outer jacket  132 . Several other basic elements of the cable in this embodiment are shown in FIG. 1 but, for convenience, are not repeated in FIG. 2.  
         [0035]    In the embodiment of FIG. 2, strength member  104  and buffer tubes  124  are arranged in the same configuration as in FIG. 1. Cable  200 , however, further comprises an inner jacket  220  surrounding at least central strength member  104  and buffer tubes  124 . First conduit  116  is positioned external to the central strength member  104  and outside inner jacket  220 . In this embodiment, as depicted in FIG. 2, first conduit  116  has substantially the same diameter as inner jacket  220 , although it may be of any size. First conduit  116  has a cavity  120  for accepting blown or pulled optical fibers.  
         [0036]    A second conduit  208  and a third conduit  216  may be disposed in the spaces bounded by the exterior of inner jacket  220 , the exterior of first conduit  116 , and the interior of outer jacket  132 . As in cable  100  in FIG. 1, outer jacket  132  is positioned at the periphery of cable  200 . In this configuration, second conduit  208  and third conduit  216  may be sized to fit in the space formed above and below the point at which inner jacket  220  and first conduit  116  come in contact with each other, as shown in FIG. 2. Second conduit  208  and third conduit  216  may have a second cavity  204  and a third cavity  212 , respectively. These second and third cavities  204  and  212  are each capable of receiving a blown optical fiber after the cable is installed. It may also be envisioned that, in addition to or alternative to conduits  116 ,  208 , and  216 , conduits for receiving blown fibers may be installed alongside buffer tubes  124 , such as in the arrangement of FIG. 1.  
         [0037]    The following describes further detail about one example of an embodiment of FIG. 2 consistent with the present invention. In this case, strength member  104  is polyethylene and has an outer diameter of 6.2 mm and an inner diameter of 4.0 mm. First bore  108 , formed by an interior surface of strength member  104 , has a diameter of 4.0 mm. In this configuration, first bore  108  can accommodate an optical fiber unit with  12  fibers (not shown) after cable installation.  
         [0038]    Outside the stranded tubes, water blocking and strength materials (shown in FIG. 1 but not FIG. 2) protect the optical fibers of the cable. Eighteen glass reinforcement rods  112  are disposed longitudinally along polyethylene strength member  104 . Each of these reinforcement members  112  has a diameter of 0.7 mm. A first water swellable tape layer  136  surrounds strength member  104 . Six buffer tubes  124 , each with a diameter of 6.2 mm, are S-Z stranded around strength member  104 . An inner jacket  220  with a diameter of approximately 21.35 mm surrounds buffer tubes  124 .  
         [0039]    First conduit  116  is positioned parallel and contiguous to inner jacket  220 . First conduit  116  is formed of polyethylene, has an outer diameter 21.35 mm, and an inner diameter of 12.7 mm. With this interior diameter, first conduit  116  is capable of receiving approximately 144 optical fibers even after the cable has been installed.  
         [0040]    Outer MDPE jacket  132  surrounds first conduit  116  and inner jacket  220  to form a cable assembly  200  with overall dimensions of 24 mm by 45 mm. Optionally, second and third conduits  208  and  216 , preferably made of polyethylene, are located in the spaces bounded by the interior of outer jacket  132 , the exterior of first conduit  116 , and the exterior of inner jacket  220 . The second and third conduits  208  and  216  can have outer diameters of about 3 mm to about 15 mm with inner diameters of about 2 mm to about 10 mm. The second and third conduits  208  and  216  can receive one to approximately 60 optical fibers depending on their interior diameters. This example is but one embodiment of the present invention and is not intended to limit the scope of the claims.  
         [0041]    The arrangement of components mentioned in conjunction with the embodiment of FIG. 1 is also applicable to the embodiment of FIG. 2. For example, any number of conduits and cables can be arranged in any convenient configuration within outer jacket  132 . Additionally, the water blocking layers or outer strength members employed in the embodiment of FIG. 1 can also be used in the present embodiment. Likewise, the discussion of the preferred materials for the components of FIG. 1 is equally applicable to the cable assembly  200  of FIG. 2.  
         [0042]    [0042]FIG. 3 depicts a cross-sectional view of a further embodiment of a cable consistent with the principles of the present invention. In the embodiment of FIG. 3, strength member  104  and buffer tubes  124  are arranged in the same configuration as in FIGS. 1 and 2. Any one or all of buffer tubes  124  may be empty so as to form a conduit for future blown installation of optical fibers. In this embodiment, however, a first conduit  116  is disposed parallel to strength member  104 . First conduit  116  has a cavity  120  for accepting optical fibers. Outer jacket  132  forms two parallel tubes joined at connection point  304 . The first of these parallel tubes houses buffer tubes  124  and strength member  104 . The second of these parallel tubes surrounds first conduit  116 . In this embodiment, outer jacket  132  could be formed from two separate cable jackets connected together with a thin web of jacketing material at connection point  304 . Alternatively, both tubes of outer jacket  132  may be formed together with connection point  304 .  
         [0043]    The arrangement of components mentioned in conjunction with the embodiment of FIG. 1 is also applicable to the embodiment of FIG. 3. For example, any number of conduits and cables can be arranged in any convenient configuration within outer jacket  132 . Additionally, the water blocking layers or outer strength members employed in the embodiment of FIG. 1 can also be used in the present embodiment. Likewise, the discussion of the preferred materials for the components of FIG. 1 is equally applicable to the cable assembly  300  of FIG. 3.  
         [0044]    [0044]FIG. 4 is a cross-sectional view of a further embodiment of a cable assembly with multiple ducts, multiple cables, and a single outer lashing element consistent with the principles of the present invention. As herein embodied and illustrated in FIG. 4, a cable  400  includes a strength member  104 , a first conduit  116 , a buffer tube  124 , an inner jacket  220 , second and third conduits  208 ,  216 , and an outer lashing element  404 .  
         [0045]    In the embodiment of FIG. 2, strength member  104  and buffer tubes  124  are arranged in the same configuration as in FIG. 1. In this embodiment, however, cable  400  further comprises an inner jacket  220  surrounding at least central strength member  104  and buffer tubes  124 . A first conduit  116  is disposed adjacent to inner jacket  220 . In this embodiment, first conduit  116  has substantially the same diameter as inner jacket  220 , but it may be of any size. First conduit  116  has a cavity  120  for accepting optical fibers, either by pulling or blowing. A second conduit  208  and a third conduit  216  may be disposed in the spaces bounded by the exterior of inner jacket  220 , the exterior of first conduit  116 , and the interior of lashing element  404 , as in cable  200  of FIG. 2. In this configuration, second conduit  208  and third conduit  216  may be sized to fit in the space formed above and below the point at which inner jacket  220  and first conduit  116  come in contact with each other. Second conduit  208  and third conduit  216  may have a second cavity  204  and a third cavity  212 , respectively. These second and third cavities  204  and  212  are each capable of receiving an optical fiber.  
         [0046]    An outer lashing element  404  surrounds all of the preceding elements of cable  400 . Lashing element  404 , like outer jacket  132  in FIG. 2, serves to contain first conduit  116 , inner jacket  220 , second conduit  208 , and third conduit  216 . Lashing element  404 , for example, could be a 1500 Denier polyester binder, a 1125 Denier polypropylene binder, or a 0.001 inch clear polyester tape.  
         [0047]    The arrangement of components mentioned in conjunction with the embodiment of FIG. 1 is also applicable to the embodiment of FIG. 4. For example, any number of conduits and cables can be arranged in any convenient configuration within lashing element  404 . Additionally, the water blocking layers and outer strength members employed in the embodiment of FIG. 1 can also be used in the present embodiment. Likewise, the discussion of the preferred materials for the components of FIG. 1 is equally applicable to the cable  400  of FIG. 4.  
         [0048]    Other components and structures may be employed with the cable assembly of this invention without departing from the spirit and scope of the invention. Such components and structures may include various water blocking layers, jackets, reinforcements, conduits, cables, and other elements as known by those skilled in the art. In addition, the strength member may be interchangeable with the conduit or cable. For example, the conduit or cable may provide the proper strength characteristics for the cable assembly.  
         [0049]    It should be understood that the foregoing relates only to the exemplary embodiments of the present invention. For example, variations in the configuration of the conduits and buffer tubes are not restricted by the particular examples illustrated and described herein. Numerous changes may be made thereto without departing from the scope of the invention as defined by the following claims.
An optical fiber cable allows for future expansion with blown optical fibers through conduits. The cable assembly includes a central strength member, a conduit, and a buffer tube of optical fibers, all surrounded by an outer jacket. The central strength member may be tubular. While the buffer tube of optical fibers accommodates current capacity requirements, the conduit and possibly the tubular strength member provide paths for blowing additional optical fibers into the cable to meet future capacity needs.