Abstract:
A ruggedized cable has an inner and an outer jacket. The cable also includes two layers of aramid strength elements for tensile strength. The cable can be pulled through various environments due to the jacketing and strength elements. The outer jacket and strength elements can be stripped away at a transition point, and secured at an entry point of a housing of an FDT, ONT, etc. The remaining inner cable element is then routed through the hardware housing and terminated.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application PCT/US2009/062468, filed Oct. 29, 2009, which claims the benefit of U.S. Provisional App. No. 61/110,311, filed Oct. 31, 2008, the entire contents of which are incorporated by reference as if presented herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application relates to rugged drop cables. 
       BACKGROUND 
       [0003]    Bend-insensitive optical fibers provide greater flexibility and ease of installation than standard single- and multi-mode fiber cables incorporating conventional optical fibers. While the conveyed optical signal itself suffers little or no attenuation over tight bends, the optical fibers are subject to tensile, bending, and crush stresses, etc. 
         [0004]    Rugged drop cable designs have been utilized to protect bend-insensitive fibers and optical fibers in general. One conventional rugged drop cable has a tightly buffered optical fiber surrounded by aramid fibers (for tensile strength), and a heavy, fire retardant polymer jacket. Such cables are installed using standard techniques and may be subjected to tensile loads of 50 pounds (220 Newtons) or more. Such high tensile loads can cause the aramid fibers to break free from the cable jacket, which may cause the jacket to elongate elastically. During elongation of the cable jacket, the strength elements and the optical fiber may remain at their original lengths. A length difference ΔL therefore exists between the strength elements/fiber and the elongated jacket. When the tensile load is removed and the cable jacket returns to its original length, ΔL does not change, and the strength elements and optical fiber are compressively deformed, causing a phenomenon known as “wavy fiber.” Wavy fiber can be generally described as severe sinusoidal or serpentine bending of one or more optical fibers within the cable jacket. 
         [0005]    Phenomena such as wavy fiber can be mitigated by further ruggedization of the drop cable, such as by the use of thicker cable jackets and heavier strength elements. Each additional protective element, however, increases cost, adds bulk to the cable, and may increase the difficulty in processing the fiber, e.g. connectorizing. 
       SUMMARY 
       [0006]    According to one embodiment of the present invention, a cable comprises an inner cable having an inner jacket of a first diameter, and an outer jacket surrounding the inner cable and having a second diameter. One or more layers of strength elements may also be included adjacent to the jackets. 
         [0007]    According to one aspect of the embodiment, the dual jackets and strength element layers provide ruggedness for installation, so that the cable can be pulled through relatively problematic environments. The outer jacket can be removed to expose the inner cable, which can be pulled through smaller, confined spaces, such as within a connection enclosure. The inner cable can also be sized so that it can be connectorized using existing parts and procedures within the connection enclosure. The cable therefore provides the installer with the installation advantages of heavier cables outside of a connection enclosure, and the routing and connectorization advantages of smaller cables inside the enclosure. 
         [0008]    According to another aspect, transition elements can be placed at the point of transition from the larger diameter cable to the inner cable. The transition elements can be used for strain relief and for securing the cable to a fixed assembly such as a connection enclosure. 
         [0009]    According to yet another aspect of the embodiment, cable is sufficiently rugged so as to mitigate or eliminate the effects of wavy fiber. 
         [0010]    It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a cross section a cable according to a first embodiment. 
           [0012]      FIG. 2  illustrates an end of the cable shown in  FIG. 1 . 
           [0013]      FIGS. 3-5  illustrate stripping the end of the cable shown in  FIG. 1  and the application of transition elements. 
           [0014]      FIG. 6  illustrates an alternative transition element. 
           [0015]      FIG. 7  illustrates another alternative transition element. 
           [0016]      FIG. 8  illustrates yet another alternative transition element. 
           [0017]      FIG. 9  shows an alternative method of securing the end of the cable. 
           [0018]      FIGS. 10 and 11  illustrate the effects of tensile loading on the cable. 
           [0019]      FIG. 12  illustrates a cable assembly including the cable of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Reference will now be made in detail to the present embodiments, examples of which are illustrated in the accompanying drawings. When practical, the same or similar reference numerals are used throughout the drawings to refer to the same or like parts. 
         [0021]      FIG. 1  is a cross section of a cable  10  according to a first embodiment.  FIG. 2  illustrates an end of the cable  10 . Referring to  FIGS. 1 and 2 , the cable  10  has at least one optical fiber  12 , a tight buffer layer  14  surrounding the optical fiber  12 , an inner jacket  16  surrounding the tight buffer layer  14 , and an outer jacket  18  surrounding the inner jacket  16 . An inner strength element layer  20  can be disposed between the tight buffer layer  14  and the inner jacket  16 , and an outer strength element layer  22  can be disposed between the inner jacket  16  and the outer jacket  18 . The dual jackets and dual strength element layers provide the cable  10  with the necessary ruggedness for ease of installation, and the versatility to be routed in confined spaces, while also being capable of termination using existing parts and methods. 
         [0022]    The optical fiber  12  can be, for example, a bend-insensitive optical fiber, such as fibers sold under the name ClearCurve™, available from Corning Incorporated. Other bend-insensitive fibers and conventional optical fibers can also be used. The inner jacket  16  and the outer jacket  18  can be polymeric and can include materials, such as, for example, flame-retardant polymers conforming to NEC® OFNR and CSA OFN FT-4 for riser rated cables. The terms “polymer” and “polymeric” as used in this specification allow for the presence of additives, such as are commonly used in flame-rated jacket materials. 
         [0023]    Any suitable jacket material may be used for the inner jacket  16  and the outer jacket  18 , such as, for example, polyurethanes (PU), polyvinylchloride (PVC), polyethylenes (PE), polyproplyenes (PP), UV-curable materials, and other polymer materials. The inner strength element layer  20  and the outer strength element layer  22  provide tensile strength to the cable  10 . The strength element layers  20 ,  22  can be comprised of high tensile strength fibers aligned generally along the length of the cable  10 . The fibers can be aramid or para-aramid synthetic fibers such as, for example, KEVLAR™, though other suitable materials may include fiberglass, polyester, high tensile polypropylene, and the like. The use of a layer of discrete tensile fibers provides tensile strength to the cable while also providing high flexibility for the cable  10 . The outer strength element layer  22  can be arranged so that some of the tensile fibers in the layer  22  contact the exterior of the inner jacket  16  and so that some of the tensile fibers contact the interior of the outer jacket  18 . 
         [0024]    Optical fibers used in the present embodiments may be coated with the tubular tight buffer layer  14 , which can be polymeric. In the illustrated example, a single optical fiber  12  is a bend-insensitive ClearCurve™ fiber capable of bending to a 5 mm radius without appreciable attenuation, and the tight buffer layer  14  is a 900 μm thick layer of flame-retardant PVC material. Other buffer layer thicknesses, such as 500 μm are also possible. The inner strength element layer  20  can be arranged so that some of the tensile fibers in the layer  20  contact the tight buffer layer  14  and so that some of the fibers contact the interior of the inner jacket  16 . 
         [0025]    As shown in  FIG. 1 , the cable  10 , and accordingly the outer jacket  18 , have an average or optimal diameter D 2 . The inner jacket  16 , inner strength element layer  20 , and the buffered optical fiber  12  form an inner cable  24  having an average or optimal outside diameter D 1  that is smaller than D 2 . The diameters D 1  and D 2  should be considered average diameters for the illustrated cross-section because some ovality and other defects may occur in manufacturing that cause the jackets to be non-circular to some degree. The diameter D 1  may be, for example, 80% or less of the diameter D 2 . In the illustrated embodiment, the diameter D 1  is 70% or less of the diameter D 2 . 
         [0026]    To manufacture the cable  10 , the tight-buffered optical fiber  12  is paid out through a non-stranded layer of tensile aramid yarn fibers that form the inner strength element layer  20 . Flame-retardant polymer material is then pressure-extruded over the inner strength element layer  20  to form the inner jacket  16 . The inner jacket  16 , inner strength element layer  20 , and the buffered optical fiber  12  form the inner cable  24 . In the exemplary embodiment, the nominal diameter D 1  is 2.9 mm. The land length of the extrusion die used to extrude the polymer material for the inner jacket  16  is controlled to achieve a reasonable bonding force of the aramid yarns of the inner strength element layer  20  to the inner jacket  16 . Bonding force can be determined by measuring the force required to remove a 10 inch section of the inner jacket  16  from the inner strength element layer  20 . According to one embodiment, the force can be in the range of 5 to 15 lbs (22-66 Newtons). The 2.9 mm diameter inner cable  24  is paid out through a helically stranded layer of tensile fibers to form the outer strength element layer  22 . In the illustrated embodiment, the fibers are aramid yarns of KEVLAR™. The inner drop cable  24  with helically stranded outer strength element layer  22  may then be passed through an applicator where a layer of mineral particulates (such as talc) is applied. The polymer outer jacket  18  is then extruded over the outer strength element layer  22 . In the exemplary embodiment, the outer jacket  18  has an outer diameter D 2  of 4.8 mm. The amount of mineral particulates applied and the amount of pressure in the extrusion die can be controlled to provide a moderate stripping force of the outer jacket  18  from the outer strength element layer  22 . The stripping force can be in the range of 5 to 40 lbs (22-178 Newtons) for the removal of a 10 inch section of outer jacket  18 . 
         [0027]      FIG. 2  shows an end  28  of the cable  10  ready for processing steps such as connectorization and/or optical coupling to another fiber, either within the factory or in the field. According to the present invention, the dual jackets  16 ,  18  and dual strength element layers  20 ,  22  provide the cable  10  with the necessary ruggedness for installation in relatively harsh conditions. The inner cable  24  can be separated from the exterior remainder of the cable  10  by stripping away an end portion of the outer jacket  18  and layer  22 . The exposed portion of the inner cable  24  may then be routed through smaller, relatively confined spaces, where the additional ruggedness of the jacket  18  and layer  22  may not be required. Further, the inner cable  24  can be configured for connectorization using known parts and procedures, and is suitable for interface housings for multiple dwelling units (MDU). For example, the reduced diameter inner cable  24  can be installed in a housing, such as an Optisheath® MDU Terminal available from Corning Cable Systems, LLC. The ruggedness of the cable  10  also mitigates or eliminates the effects of wavy fiber in the cable  10 . The cable therefore provides the installer with the installation advantages of heavier cables and the routing, connectorization, and other processing advantages of smaller cables. An exemplary method of processing the cable  10  is described below with reference to  FIGS. 3-5 . 
         [0028]      FIG. 3  shows an end portion  30  of the outer jacket  18  stripped away intact. The end portion  30  can be stripped away using a ring cut, using a commercially available wire stripper. A length of 24 inches to 48 inches of the end of the outer jacket  18  may be removed. Mineral particulate applied on the outer strength element layer  22  and the controlled extrusion of the outer jacket  18  can be selected to obtain a desired coupling strength. The coupling strength, or, the force required to pull the outer jacket  18  off of the inner jacket  16  and the outer strength element layer  22  can be selected so that is it less than the force required to elastically deform the inner jacket  16 . Elastic deformation of the inner jacket  16  may have the adverse effect of decoupling the bond between the inner layer of strength elements  20  and the inner jacket  16 . 
         [0029]    Referring to  FIG. 4 , after removal of the end portion  30  of the outer jacket  18 , an end portion (not shown) of the inner jacket  16  is removed in the same manner as the end portion  30  to expose a sufficient length of tightly buffered layer  14  and fiber  12 . The tensile strength fibers of the inner and outer strength element layers  20 ,  22  can be trimmed and laid back along the respective inner and outer jackets,  16  and  18 , as shown in  FIG. 4 . 
         [0030]    Referring to  FIG. 5 , prior to connectorizing the cable  10 , one or more transition elements can be secured to the furcated end  36  of the cable. The transition elements, which include an outer transition element  40  and an inner transition element  60  in the illustrated embodiment, can be included for strain relief. Strain relief can be advantageous at locations such as at the point where the cable  10  enters an MDU and is abruptly of reduced diameter. The exemplary outer transition element  40  is a machined metallic piece with an interior threaded bore  44 . The threaded bore  44  can be threaded over the laid back fibers of the outer strength element layer  22  and the outer jacket  18  so that it is firmly secured to the jacket  18 . Similarly, the inner transition element  60  can be a crimp ring tightened around the laid back fibers of the inner strength element layer  20  and the inner jacket  16  so that it is secured to the jacket  16 . In the exemplary embodiment, the outer transition element  40  transitions the cable  10  from the 4.8 mm diameter outer jacket  18  to the 2.9 diameter inner jacket  16 . The inner transition element  60  can be part of the connectorization of the inner cable  24  of 2.9 mm diameter inner jacket  16  with a connector (shown in  FIG. 12 ). 
         [0031]    With the transition elements attached, the fiber  12  can now be optically connected to a connector, such as connecting to a pigtailed connector, or, for directly connectorizing to a standard connector such as an SC connector, or to a field installable connector such as UniCam™ SC, ST and LC available from Corning Cable Systems, LLC. Connectorization can take place, for example, in a connection enclosure. 
         [0032]      FIGS. 6-9  illustrate alternative configurations and arrangements for the outer transition element.  FIG. 6  illustrates an outer transition element  90  in the form of plastic clip-on piece. Referring also to  FIG. 4 , opposing sections  92 ,  94  of the transition element  90  can be crimped over the tensile fibers of the outer strength element layer  22  and over the outer jacket  18 . The opposing halves  92 ,  94  fold about a living hinge  96  and attach to one another through engagement of projections  102  with apertures  104 .  FIG. 7  illustrates an outer transition element  110  in the form of a metallic U-clip. Referring also to  FIG. 4 , the transition element  110  can be crimped over the tensile fibers of the outer strength element layer  22  and over the outer jacket  18 .  FIG. 8  illustrates an outer transition element  130  in the form of a cable tie. Referring also to  FIG. 4 , the transition element  130  can be tightened around the tensile fibers of the outer strength element layer  22  and over the outer jacket  18 . 
         [0033]    As an alternative to securing the tensile fibers of the outer strength element layer  22  to the outer jacket  18 , the fibers can be secured to an exterior element.  FIG. 9 , for example, illustrates the tensile fibers of the outer strength element  22  secured under a screw terminal  150 . 
       Example 
       [0034]    A cable  10  as illustrated in  FIGS. 1-2  is installed to optically connect a fiber connection enclosure such as a fiber distribution terminal (FDT) to an apartment unit in an MDU. The cable  10  is pulled between the FDT and an apartment unit using conventional means. The cable  10  is prepared for connectorization as shown in  FIGS. 3-5 . The FDT can be of conventional design, having a housing enclosing hardware, and an entry point through which cables are connected to the FDT. At the entry point, the outer transition element is secured to a holding device (e.g., a crimp holder) at the entry point to the FDT. Any of the transition elements disclosed in this specification are suitable for securing the cable at the entry point. The smaller diameter inner cable  24  is then routed through the interior of the FDT. The inner cable  24  is terminated using an OptiSnap™ connector using crimp bands. 
         [0035]      FIGS. 10-11  illustrate the resistance of the cable  10  to the wavy fiber phenomenon.  FIG. 10  illustrates the cable  10  ready for installation, such as for pulling through ductwork, etc., when installing the cable  10  in a structure. A holding device such as a pressure clamp or a Kellum&#39;s grip can be used to securely couple to the outer jacket  18 , which enables the installer to exert the necessary force to pull the cable  10  through narrow openings or ducts. The presence of dual jackets  16 ,  18  and strength element layers  20 ,  22  provides high tensile strength and durability to the cable  10  during such installations. Initially, the cut end face of the cable  10  is substantially flush in the plane defined by axes x and y. Referring to  FIG. 11 , as the cable  10  is subjected to tensile pulling forces FP during installation, the elastic outer jacket  18  generally does not transfer excessive tensile loads to the inner jacket  16 . The outer jacket  18  may therefore elongate and extend past the inner jacket  16  and the optical fiber  12  by a distance ΔL. According to one aspect, when the outer jacket  18  retracts to its original length, little or none of the compressive load is transmitted to the inner jacket  16 , thereby inhibiting or precluding the creation of wavy fiber in the cable  10 . In other words, the outer jacket  18  bears most or all of the positive and negative tensile loads of installation such that fiber(s) in the inner cable  24  is not subject to the wavy fiber phenomenon. The limited space within the cable jacket also inhibits buckling of the fiber  12 . 
         [0036]      FIG. 12  illustrates the cable  10  of  FIG. 1  as part of a cable assembly  200 . The cable assembly  200  has an SC connector assembly  210  connected at the end of the cable  10 . 
         [0037]    Many modifications and other embodiments of the present invention, within the scope of the claims will be apparent to those skilled in the art. For instance, the concepts of the present invention can be used with any suitable composite cable designs and/or optical stub fitting assemblies. Thus, it is intended that this invention covers these modifications and embodiments as well those also apparent to those skilled in the art.