Abstract:
A fiber optic cable comprises at least one elongated optical fiber situated in a fiber nest having a plurality of filaments collectively surrounding the optical fiber. The cable further includes a structural member at least partially surrounding the optical fiber but spaced apart from the optical fiber in a radial direction such that at least some of the filaments of the fiber nest are positioned between the optical fiber and the structural member. The foregoing elements are encased in an outer jacket.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to fiber optic cable. More particularly, the present invention relates to an improved fiber optic cable that is well-suited for retrofit use in residential applications and the like. 
         [0002]    The ability of high-quality optical fiber to transmit large amounts of information without appreciable signal degradation is well known. As a result, optical fibers have found widespread use in many applications, such as voice and data transmission. Initially, optical fiber was often limited to such uses as trunk line communications or commercial settings requiring high rates of data throughput. More recently, however, the need for greater bandwidth in residential settings has brought optical fibers directly into homes and multiple dwelling units (MDUs). Such applications have generally come to be known by the acronym FTTH (“Fiber To The Home”). As one skilled in the art will appreciate, retrofitting an existing structure with optical fiber can present various challenges not present when optical fiber is installed during construction. 
         [0003]    Optical fiber is typically supplied and installed as fiber optic cable. The term “fiber optic cable” refers to the combination of the actual optical fiber plus the structure in which it is carried and protected during and after installation. Generally, a fiber optic cable includes the optical fiber, aramid fibers or other strength members, and an outer jacket. Two common types of fiber optic cable used in FTTH and similar applications are “simplex cable” and “flat type cable.” 
         [0004]    Both simplex cable and flat type cable have certain advantages and disadvantages. Simplex cable, with a diameter generally about 3.0 millimeters, will not fit through some tight spaces. Furthermore, simplex cable has good flexibility—which is advantageous in some situations but can lead to difficulties in other situations. For example, the flexibility of simplex cable allows easy installation inside walls. This flexibility, however, makes it difficult for the installer to push simplex cable through conduit. 
         [0005]    Flat type cable, which has two strength members of aramid fiber reinforced polymer (FRP) located on lateral sides of the optical fiber, exhibits better stiffness than simplex cable. As a result, it can be more easily pushed through conduit. With a width of only about 2.0 millimeters, flat type cable is also smaller than typical simplex cable. This allows it to be inserted into gaps and other openings through which a simplex cable might not fit. The two strength members also prevent excessive signal attenuation at low temperatures or due to bending. As disadvantages, flat type cable has a limited bend radius and does not easily bend in the side-to-side direction. As a result, great care must be taken when installing flat type cable into a wall. 
         [0006]    The present invention recognizes the foregoing considerations, and others, of the prior art. 
       SUMMARY OF THE INVENTION 
       [0007]    According to one aspect, the present invention provides a fiber optic cable comprising at least one elongated optical fiber. A fiber nest having a plurality of filaments collectively surrounding the optical fiber is also provided. The cable further includes a structural member at least partially surrounding the optical fiber but spaced apart from the optical fiber in a radial direction such that at least some of the filaments of the fiber nest are positioned between the optical fiber and the structural member. The foregoing elements are encased in an outer jacket. 
         [0008]    In exemplary embodiments, the structural member comprises a fiber reinforced polymer member. Fibers of the fiber reinforced polymer member and the fiber nest may be of the same fiber type, such as aramid fibers. The structural member may completely surround the optical fiber or partially surround the optical fiber in various embodiments. Where the structural member partially surrounds the optical fiber, it may have a C-shaped configuration. 
         [0009]    To facilitate bending in any direction, the outer jacket is preferably configured to have a substantially round outer periphery. Also, the outer jacket is preferably sized so that the cable will fit into small holes and other tight spaces. For example, the outer jacket may preferably have a diameter no greater than about 1.8 millimeters. The outer jacket is also preferably provided with at least one inwardly-directed notch configured to facilitate removal of the outer jacket. In this regard, a pair of inwardly-directed notches situated at opposing locations on the outer jacket may be provided. 
         [0010]    According to another aspect, the present invention provides a fiber optic cable comprising at least one elongated optical fiber. A structural member formed of fiber reinforced polymer and at least partially surrounding the optical fiber is also provided. The structural member is spaced apart from the optical fiber in a radial direction such that the optical fiber can move within the structural member. In addition, a fiber nest formed of a plurality of filaments collectively surrounding the optical fiber may be provided. At least some of the filaments in such embodiments are positioned between the optical fiber and the structural member. The cable further includes an outer jacket having a substantially round outer periphery. 
         [0011]    In exemplary embodiments, the structural member may comprise a plurality of aramid fibers interconnected by a reinforcing resin. For example, the reinforcing resin may be selected from a group consisting of epoxy, thermal cure silicone resin and UV-cure urethane resin. 
         [0012]    A further aspect of the present invention provides a method of making a fiber optic cable. According to one step of the method, an elongated optical fiber is provided. The optical fiber is situated in a fiber nest having a plurality of individual filaments. According to another step, a reinforcing resin is infused into an outer part of the fiber nest. The reinforcing resin is then processed to become hardened. As a result, a combination in which a structural member of fiber reinforced polymer is formed from the outer part of the fiber nest is produced. In addition, an outer jacket may be formed to encase the combination. 
         [0013]    Another aspect of the present invention provides a fiber optic cable comprising at least one elongated optical fiber. A fiber nest having a plurality of aramid filaments collectively surrounding the optical fiber is also provided. The fiber optic cable according to this aspect of the present invention also comprises a structural member formed of a plurality of aramid fibers interconnected by a reinforcing resin. The structural member at least partially surrounds the optical fiber but is spaced apart from the optical fiber in a radial direction such that at least some of the filaments of the fiber nest are positioned between the optical fiber and the structural member. The fiber optic cable further includes an outer jacket having a substantially round outer periphery of a diameter of no greater than about 1.8 millimeters. 
         [0014]    Other objects, features and aspects of the present invention are provided by various combinations and subcombinations of the disclosed elements, as well as methods of practicing same, which are discussed in greater detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  is a transverse cross-sectional view of a fiber optic simplex cable in accordance with the prior art; 
           [0017]      FIG. 2  is a perspective view of the prior art cable of  FIG. 1  with layers cut away; 
           [0018]      FIG. 3  is a transverse cross-sectional view of a flat type fiber optic cable in accordance with the prior art; 
           [0019]      FIG. 4  is a perspective view of the prior art cable of  FIG. 3  with jacket halves separated; 
           [0020]      FIG. 5  is a transverse cross-sectional view of a fiber optic cable in accordance with an embodiment of the present invention; 
           [0021]      FIG. 6  is a perspective view of the cable of  FIG. 5  with jacket halves separated; 
           [0022]      FIG. 7  illustrates a wiring conduit of an existing building structure through which a fiber optic cable of the present invention is being pushed; 
           [0023]      FIG. 8  is an enlarged fragmentary view showing a portion of a fiber optic cable constructed in accordance with an embodiment of the present invention; 
           [0024]      FIG. 9  is a diagrammatic representation of an exemplary process for making the cable of  FIG. 8 ; 
           [0025]      FIG. 10  is a transverse cross-sectional view of a fiber optic cable in accordance with a further embodiment of the present invention; and 
           [0026]      FIG. 11  is a perspective view of the cable of  FIG. 10  with the outer jacket partially cut away to better show certain internal details. 
       
    
    
       [0027]    Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions. 
         [0029]    Before turning to preferred embodiments of the present invention, certain additional aspects of the prior art will be described in greater detail. In this regard,  FIGS. 1 and 2  illustrate a simplex cable  10  in accordance with the prior art. As shown, cable  10  includes an optical fiber unit  12  extending along its central axis. Optical fiber unit  12  comprises a glass fiber  14  for the transmission of optical signals. A protective sheath  16  is located around the glass fiber  14 , as shown. Typically, sheath  16  will be formed of a fluoropolymer, such as PVC. As used herein, the term “optical fiber” is intended to be synonymous with the optical fiber unit including the glass fiber and sheath. 
         [0030]    Optical fiber unit  12  is located at the center of a yarn  18  formed of a plurality of loose aramid fibers, as shown. The aramid fibers provide strength to the overall cable  10 . Yarn  18  and optical fiber unit  12  are encased by an outer jacket  20 . The outer jacket is made of a material such as plenum-rated PVC, riser-rated PVC or LSZH. 
         [0031]    As noted above, simplex cable provides good flexibility which is advantageous in some situations. The flexibility of simplex cable is often a disadvantage, however, because it cannot be effectively pushed through long conduits. In addition, the diameter of some simplex cable (typically 3.0 millimeters) prevents it from being used in some installations where space is very limited. 
         [0032]    Turning now to  FIGS. 3 and 4 , a typical flat type cable  30  in accordance with the prior art is illustrated. As can be seen, cable  30  has an optical fiber unit  32  extending along its central axis. Optical fiber unit  32  includes a glass fiber  34  and a sheath  36  similar to that described above in connection with simplex cable  10 . Unlike simplex cable  10 , however, flat type cable  30  includes a pair of strength members  38  and  40  running alongside optical fiber unit  32 . Strength members  38  and  40  are formed of fiber reinforced polymer (FRP) made by infusing aramid yarn with a hardening polymer, such as epoxy. Strength members  38  and  40  thus add rigidity to cable  30 . 
         [0033]    Cable  30  is encased in an outer jacket  42  which may be typically formed of FR-PE (flame resistant polyethylene) material. FR-PE is typically used as an outer jacket of this type of drop cable in Japan and in other Asian regions in order that the cable meets regional standards (e.g. Japanese Industrial Standard C3005 in Japan). As can be seen, outer jacket  42  in this embodiment is configured having a first half portion  44  and a second half portion  46  defined by inwardly-directed notches  48  and  50 . As illustrated in  FIG. 4 , notches  48  and  50  allow jacket  42  to be opened in zipper-like fashion in order to access optical fiber unit  32  for termination. 
         [0034]    As discussed above, flat type fiber optic cable has several desirable qualities. In particular, the presence of strength members  38  and  40  allows it to be pushed through existing conduit. In addition, the relatively small 2.0 millimeter width of typical flat type cable is smaller than the diameter of typical simplex cable. As a result, flat type cable can often be fit into tighter spaces than is the case with simplex cable. However, one significant drawback of flat type cable in some installations is its bending capability. In particular, flat type cable  30  is generally limited to bending in the top-to-bottom direction (indicated by arrow  52 ). The shape and location of strength members  38  and  40  make side-to-side bending very difficult. As a result, there are some situations where the use of flat type cable is not feasible. 
         [0035]    Preferred embodiments of a fiber optic cable constructed in accordance with the present invention will now be described. As will become apparent from the ensuing discussion, fiber optic cables of the present invention provide numerous advantages in comparison with various cables of the prior art. In this regard, fiber optic cables of the present invention will typically exhibit good stiffness to facilitate pushing of the cable into conduits. In addition, the cables will effectively bend in most any direction, thus facilitating installation in both walls and conduits. Preferred embodiments also have smaller diameter than many conventional cables, thus allowing installation in small holes and other tight spaces. Moreover, the optical fiber unit in the interior of the cable is well protected such that there is minimal signal attenuation due to bending or low temperatures. 
         [0036]    Referring now to  FIGS. 5 and 6 , a fiber optic cable  60  constructed in accordance with a first embodiment of the present invention is illustrated. As shown, fiber optic cable  60  includes an optical fiber unit  62  extending along its central axis. Optical fiber unit  62  includes a glass fiber  64  encased in a sheath  66 . 
         [0037]    Optical fiber unit  62  is located at the approximate center of a “fiber nest”  68  formed by a plurality of loose aramid fibers (filaments). As used herein, the term “loose” indicates that the filaments are not interconnected with one another using a reinforcing polymer. Instead, the individual filaments may be tightly packed, but are capable of independent movement. 
         [0038]    As explained in more detail below, the fiber nest may be formed of a multifilament yarn into which the optical fiber is inserted. In this regard, the filaments of the yarn may be formed of any suitable synthetic or inorganic material. For example, the filaments in presently preferred embodiments may be aramid. The aramid yarn may have an overall size of about 4800 denier, with an 8×600 denier construction. However, it is contemplated that 400 denier, 600 denier, 1000 denier and 1420 denier yarns may also be used in some embodiments. For example, embodiments are contemplated in which twelve 400 denier yarns, or combinations of yarns having different deniers, are used. One skilled in the art will appreciate, however, that the size of the yarn and filaments, as well as the number of filaments making up the yarn, can be varied depending on tensile strength requirements. Embodiments are also contemplated in which the filaments may be glass fibers. 
         [0039]    Radially outside of fiber nest  68  is a structural member  70 . In this case, structural member  70  is formed in the configuration of a tube surrounding fiber nest  68  and optical fiber unit  62 . For example, structural member  70  may have an outer diameter of no more than about 0.8 millimeters in many preferred embodiments. As a result of this configuration, optical fiber unit  62  is capable of some movement within structural member  70 . 
         [0040]    Preferably, structural member  70  may be formed of fiber reinforced polymer (FRP). For example, aramid filaments may be infused with a reinforcing polymer and then hardened to yield structural member  70 . Any suitable polymer may be utilized for this purpose, including epoxy, thermal cure silicone resin, and UV-cure urethane resin. The presence of structural member  70  provides sufficient rigidity so that cable  60  can be easily pushed through wiring conduit. Structural member  70  also prevents shrinking under low temperature and otherwise protects optical fiber unit  62 . 
         [0041]    Structural member  70  and the components internal to it may be encased in a suitable outer jacket  72 . Preferably, outer jacket  72  may be formed of plenum-rated PVC, riser-rated PVC or LSZH material. The specific choice of material will often depend on the needs of the purchaser. As shown, outer jacket  72  comprises a first half portion  74  and a second half portion  76  defined by a pair of oppositely-directed notches  78  and  80 . In the illustrated embodiment, jacket  72  has a generally circular outer periphery with a diameter of preferably no more than about 1.8 millimeters. The configuration of cable  60  allows it to bend without difficulty in any direction, as indicated by crossing arrows  82 . 
         [0042]    Referring now to  FIG. 6 , it can be seen that notches  78  and  80  allow half portions  74  and  76  to be easily separated by the installer. As a result, the end of cable  60  can be opened in zipper-like fashion to reveal structural member  70  and optical fiber unit  62 . In this case, structural member  70  can be opened in any suitable manner. Because optical fiber unit  62  is carried inside of fiber nest  68 , it is easy to locate and remove after structural member  70  is opened. 
         [0043]    Installation of fiber optic cable  60  through wiring conduit  90  of a building is shown in  FIG. 7 . As is typical, wiring conduit  90  may have a plurality of existing wires or cables  92   a - d  located therein. Cables  92   a - d  may include telephone cables, coax cables, network cables or the like. In many cases, some of the existing cables may no longer be in use. Nevertheless, their presence in wiring conduit  90  limits the amount of available space into which fiber optic cable  60  can be inserted. Because of its characteristics, however, fiber optic cable  60  can be pushed through conduit  90  until it exits, as indicated at arrow  94 . In other words, fiber optic cable  60  will thus effectively traverse the entire length of the conduit. 
         [0044]    In some embodiments, it is contemplated that the structural member may comprise multiple layer-like portions. As shown in  FIG. 8 , for example, structural member  70 ′ is configured having an inner layer portion  96  and an outer layer portion  98 . Inner layer portion  96  may comprise a mixture of dry aramid filaments and resin-penetrated aramid filaments. As a result, portion  96  will be a bit “harder” than the loose filaments in fiber nest  68 . On the other hand, the interstices of the aramid filaments of outer layer portion  98  are fully penetrated by the epoxy. This will provide a harder outer shell to the assembly of components inside of the outer jacket. One skilled in the art will appreciate that the boundary between layer portions  96  and  98  may or may not be gradual depending on the manner in which structural member  70 ′ is made. 
         [0045]      FIG. 9  illustrates an exemplary process which may be used to manufacture a fiber optic cable such as that illustrated in  FIG. 8 . In this case, multifilament aramid yarn is fed from a first spool located at position  100 . As indicated at  102 , optical fiber is fed such that it will be located along the central axis of the yarn. The resulting combination passes through an infusing station  104  where a suitable resin  106  is applied. Variables such as viscosity, pressure and dwell time can be used to control the penetration of resin into the yarn. As a result, the outermost filaments will be completely infused whereas the innermost filaments will not be infused at all. Those filaments located in between will be partially infused. 
         [0046]    After passing from infusing station  104 , the yarn enters a curing zone  108  where the resin is set or otherwise cured. In this case, curing zone  108  includes a pair of ovens  110  and  112  which provide heat for curing. Where a resin other than thermoset is used, such as a UV-cured resin, one skilled in the art will appreciate that curing zone  108  may comprise other types of suitable equipment. 
         [0047]    After the resin is cured, the resulting combination may be taken up on another spool, as indicated at  114 . Thus, in this example, the outer jacket is applied later in a separate process. One skilled in the art, however, will appreciate that a continuous process may also be utilized in which the outer jacket is applied immediately after curing zone  108  (and thus before take up). 
         [0048]      FIGS. 10 and 11  show a fiber optic cable  120  constructed in accordance with an alternative embodiment of the present invention. As can be seen, fiber optic cable  120  includes an optical fiber unit  122  extending along its central axis. Optical fiber unit  122  includes a glass fiber  124  encased with a sheath  126 . Optical fiber unit  122  is located at the axial center of a fiber nest  128  comprising a plurality of loose filaments formed of a suitable fiber material, such as aramid. Optical fiber unit  122  and fiber nest  128  are preferably similar to their counterparts previously described in connection with fiber optic cable  60 . 
         [0049]    In this embodiment, a structural member  130  is provided having a C-shaped configuration. As can be seen, the part of fiber nest  128  located inside of structural member  130  will serve to support optical fiber unit  122  and maintain it in position. Like the embodiment described above, the loose filaments of fiber nest  128  will allow some adjusting movement of optical fiber unit  122  as cable  120  is bent. 
         [0050]    Preferably, structural member  130  may be formed of FRP such as aramid fibers reinforced with a suitable hardening resin. Examples of suitable resins are epoxies, thermal cure silicone resins, or UV-cure urethane resins. One skilled in the art will appreciate that it may be necessary to produce structural member  130  in this embodiment in a separate process, and then join it with loose aramid yarn and the optical fiber unit. 
         [0051]    Fiber optic cable  120  further includes an outer jacket  132  formed of any suitable material, such as plenum-rated PVC, riser-rated PVC or LSZH material. Again, the specific choice of material will often depend on the needs of the purchaser. As shown, outer jacket  132  comprises half portions  134  and  136  defined by oppositely-directed notches  138  and  140 . Notches  138  and  140  allow half portion  134  and half portion  136  to be separated easily by the installer such that outer jacket  132  is opened in zipper-like fashion. As can be seen, jacket  132  defines a substantially circular outer periphery in this embodiment. The diameter of cable  120  may be small, such as no greater than about 1.8 millimeters. 
         [0052]    Generally, fiber optic cable  120  will have the same desirable characteristics of bendability plus rigidity that are present in fiber optic cable  60 . In addition, the shape of structural member  130  may provide an additional advantage during use. Specifically, as can be seen in  FIG. 11 , the longitudinal opening in structural member  130  allows the installer to easily locate and remove optical fiber unit  122  without cutting structural member  130 . As a result, there is little chance that optical fiber unit  122  may be inadvertently cut by the installer. 
         [0053]    It can thus be seen that the present invention provides an improved fiber optic cable having various advantages in comparison with the prior art. While preferred embodiments of the invention have been shown and described, modifications and variations may be made thereto by those of ordinary skill in the art without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to be limitative of the invention as further described in the appended claims.