Patent Document

BACKGROUND  
       [0001]     Optical fiber cables are typically composed of a variety of longitudinal elements which are terminated and constrained longitudinally with respect to each other. These elements may include the optical fiber itself, tubular sheathing materials, linear strength members, and outer layers for sealing the other elements from environmental damage from rain or other moisture. Each of these elements may have different thermal coefficients of expansion. At temperatures near the ambient temperature present when the cable is assembled and terminated, the differences in thermal expansion of the various elements is not significant enough to cause any attenuation or insertion loss to optical signals being transmitted by the cable.  
         [0002]     However, as these cables are exposed to temperatures more extreme with respect to the ambient temperature at the time of assembly and termination, the differing thermal expansion coefficients may become more significant. Optical fiber cables may be exposed to operating temperatures up to one hundred degrees Fahrenheit removed from the ambient temperature of assembly and termination. At these temperatures, the differing degrees of elongation or contraction among the elements of the cable may damage the fiber or may cause unacceptable amounts of attenuation or insertion loss of signals being transmitted over the cable. Improvements to known optical fiber cables to address temperature-induced stresses are desirable.  
       SUMMARY  
       [0003]     The present invention relates to a fiber optic cable assembly with an optical fiber extending through first and second overlapping cable jackets. The first cable jacket is slidably received within a longitudinal opening of the second cable jacket. A first end of the optical fiber is fixed with respect to the first cable jacket and a second optical fiber is fixed with respect to the second cable jacket. The two cable jackets overlap and are slidably movable with respect to each other. The present invention further relates to a method forming a cable assembly.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     The accompanying drawings, which are incorporated in and constitute a part of the description, illustrate several aspects of the invention and together with the detailed description, serve to explain the principles of the invention. A brief description of the drawings is as follows:  
         [0005]      FIG. 1  is a cross-sectional view of a prior art optical fiber cable segment.  
         [0006]      FIG. 2  is a cross-sectional view of the prior art optical fiber cable segment of  FIG. 1  at a reduced ambient temperature where the ends of the fiber and the cable jacket are not constrained with respect to each other.  
         [0007]      FIG. 3  is a cross-sectional view of the prior art optical fiber cable segment of  FIG. 1  at a reduced temperature where the ends of the fiber and the cable jacket are constrained with respect to each other.  
         [0008]      FIG. 4  is a cross-sectional view of a fiber optic cable assembly according to the present invention.  
         [0009]      FIG. 5  is a side view of a fiber optic cable to be used in construction of a fiber optic cable assembly according to the present invention.  
         [0010]      FIG. 6  is the fiber optic cable of  FIG. 5 , with the first cable jacket cut and a segment of the jacket removed from about the optical fiber of the cable.  
         [0011]      FIG. 7  is the fiber optic cable of  FIG. 6 , with a second cable jacket positioned about the exposed optical fiber.  
         [0012]      FIG. 8  is the fiber optic cable of  FIG. 6 , with the second cable jacket overlapping the first cable jacket to form a cable assembly in accordance with the present invention.  
         [0013]      FIG. 9  is an alternative embodiment of the cable assembly of  FIG. 8 . 
     
    
     DETAILED DESCRIPTION  
       [0014]     Reference will now be made in detail to the exemplary aspects of the present invention that are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
         [0015]     Optical fiber cables may be installed within telecommunications networks and exposed to the extremes of outside air temperatures. These optical fiber cables are made of a variety of materials, including but not limited to the optical fiber itself, jacketing and cladding, and strength members. Each of these constituent materials may have a different thermal coefficient of expansion, meaning that the materials will expand or contract at different rates due to temperature changes. The prior art optical fiber cables in FIGS.  1  to  3  show the effect of reduced temperature on an optical fiber cable  10  including an outer jacket  12  and an optical fiber  14 . Fiber  14  is slidably held within a hollow opening  16  defined by jacket  12 . Jacket  12  includes a first end  18  and an opposing second end  20  and fiber  14  includes corresponding first and second ends  22  and  24 .  
         [0016]     In  FIG. 1 , cable  10  is exposed to a first temperature such that the ends of fiber  14  and jacket  12  are aligned with each other. If fiber  14  and jacket  12  were originally the same length at the time of their assembly, this indicates that the first temperature is approximately equal to the ambient temperature at which cable  10  was assembled. Cable  10  may be an optical fiber drop cable where fiber  14  is freely slidable within opening  16  of jacket  14 . The first ends  18  and  22  and the second ends  20  and  24  are not fixed or constrained with respect to each in cable  10 .  
         [0017]     In FIG,  2 , cable  10  has now been exposed to a second temperature below the first temperature. Fiber  14  has a thermal coefficient of expansion which is relatively smaller than a thermal coefficient of expansion of jacket  12 . At the second temperature, jacket  12  has contracted much more than fiber  14 . Ends  22  and  24  of fiber  14  extend beyond ends  18  and  20 , respectively, of jacket  12 . Ends  22  and  24  of fiber  14  are unconstrained at ends  18  and  20 , respectively, and are free to move beyond ends  18  and  20 , as shown. Ends  22  and  24  extend beyond ends  18  and  20  to define an excess length  15  of fiber  14 .  
         [0018]     Alternatively, one of the first or second ends of fiber  14  and jacket  12  might be constrained with respect to each other provided the opposite ends are unconstrained and fiber  14  is freely movable within opening  16  of jacket  12 .  
         [0019]     In  FIG. 3 , cable  10  is now terminated at each of the first and second ends with an optical fiber connector  26 . Such optical fiber connectors are well known in the art. To terminate cable  10  at connector  26 , jacket  12  and fiber  14  are constrained with respect to each other. While optical fiber connector  26  may provide some degree of movement in compression of fiber  14 , connector  26  does not permit fiber  14  to extend beyond connector  26 . As shown in  FIG. 3 , cable  10  is exposed to the second, lower temperature and jacket  12  has contracted to the same extent shown in  FIG. 2 . In  FIG. 3 , however, ends  22  and  24  of fiber  14  are now constrained at ends  18  and  20  of jacket  12  by connectors  26 . Thus, the contraction of jacket  12  compresses fiber  14  into the same length as jacket  12 . Known materials suitable for making optical fiber  12  are essentially incompressible. Excess length  15  of fiber  14  is forced to fit within a shorter length of jacket  12  and is forced into a series of microbends  28  within opening  16 . These microbends  28  may cause excess signal loss within cable  10 . While cable  10  is shown as a single fiber cable and connectors  26  are described as fiber connectors, it is anticipated that a cable including multiple optical fibers could be substituted for cable  10  and a cable breakout at the end of such a multifiber cable could be substituted for connector  26  within the present invention.  
         [0020]     Referring now to  FIG. 4 , a cross-sectional view of a cable assembly  110  includes a first tubular cable jacket  112 , a second tubular cable jacket  114  and optical fiber  14  extending between connectors  26 . Optical fiber  14  extends within an inner longitudinal opening  116  of first jacket  112  and an inner longitudinal opening  118  of second jacket  114 . A first end  120  of optical fiber  14  is terminated in a ferrule  121  of one of the connectors  26  and a second end  122  of optical fiber  14  is terminated in ferrule  121  of the other connector  26 . First cable jacket  112  includes a first end  124  that is terminated at the connector  26  holding first end  120  of optical fiber  14  and an opposite second end  126 . Second cable jacket  114  includes a first end  128  and a second end  130  terminated at the connector  26  holding second end  122  of optical fiber  14 .  
         [0021]     As shown in  FIG. 4 , first end  120  of optical fiber  14  and first cable jacket  112  are fixed with respect to each other by their common termination at one of the connectors  26 . Second end  122  of optical fiber  14  and second cable jacket  114  are fixed with respect to each other by their common termination at the other connector  26 . Second end  126  of first cable jacket  112  extends within opening  118  of second cable jacket  114  through first end  128  of second cable jacket  114 . Opening  118  is sized to fit about first cable jacket  112  adjacent second end  126  and to permit first cable jacket  112  to freely slide within second cable jacket  114 . If cable assembly  110  is exposed to different environmental conditions, differential shrinkage or expansion of either or both jackets  112  and  114  may occur, resulting in length changes of the jackets with respect to optical fiber  14 . Any differential in length caused by such shrinkage or expansion is accommodated by sliding of cables jackets  112  and  114  with respect to each other, so that microbending of fiber  14  is avoided.  
         [0022]      FIG. 5  shows an optical fiber cable  140  with first cable jacket  112  and optical fiber  14  with a second end  122  adjacent a second end  126  of jacket  112 .  FIG. 6  shows first cable jacket  112  cut to form a new second end  142  and the portion of jacket  112  between second end  122  and new second end  142  removed to expose a segment  150  of fiber  14  between new second end  142  and second end  122 . In  FIG. 7 , second end  122  of fiber  14  has been extended through a first end  146  of a second tubular cable jacket  144  and first end  146  has been positioned adjacent new second end  142 . Second cable jacket  144  is sized to fit about first cable jacket  112  so that first cable jacket  112  is slidably received within second cable jacket  144 . This is shown in  FIG. 8 , where new second end  142  and a portion of first cable jacket  112  of cable  140 , along with fiber  14 , are shown in dashed lines within a central opening  152  of second cable segment  144 . This forms a cable assembly  154 .  
         [0023]     Once cable jackets  112  and  144  are so positioned, second end  122  of fiber  14  can be terminated and fixed in position with respect to second end  130  of cable jacket  144 , such as by a connector  26  as shown in the FIGS., above. Fiber  14  may also include first end  120  which is fixed with regard to first cable jacket  112 , such as by terminating with a connector  26 .  
         [0024]      FIG. 9  shows an alternative embodiment cable assembly  156  with a boot  148  positioned about second jacket  144  adjacent first end  146  (not visible within boot  148 ). Boot  148  extends also about a portion of first cable jacket  112 . Boot  148  may be made of a heat shrink material and placed about one of the cable jackets at a point offset from either of the ends to be overlapped. Once the cable jackets have been overlapped as shown in  FIG. 8 , boot  148  can be moved to the position shown in  FIG. 9 , and heated to shrink boot  148  tightly about both jackets. Boot  148  may provide a temporary fixed connection between the cable jackets to facilitate termination of second end  122  of fiber  14  and second end  130  of second cable jacket  144 . Once this termination is completed, second cable jacket  144  can be cut at a position  158  adjacent boot  148  about first cable jacket  112 . Once the cut is made at position  158 , cable jackets  112  and  144  are free to move with respect to each other, as described above, to compensate for differential shrinkage or expansion.

Technology Category: 3