Abstract:
A fiber optic cable is provided having a at least one fiber element, a layer of aramid strength members, and a jacket disposed over said layer of aramid strength members. The layer of aramid strength members is wound at a lay length that is equal to or lesser than a predetermined bend radius.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present arrangement relates to fiber optic cables. More particularly, the present arrangement relates to a fiber optic cable with a modified construction for cordage or tactical applications. 
         [0003]    2. Description of the Related Art 
         [0004]    Cordage and tactical applications for fiber optic cables are typically required to meet very stringent testing requirements, such as being able to operate under extreme temperatures, have good tensile strength, higher resistance to abrasion and crushing, etc. However, at the same time tactical fiber optic cables also need to maintain a certain level of flexibility, so as to be wound and unwound from relatively small drums for fast and easy field deployment. 
         [0005]    For example, a typical tactical fiber optic cable may be constructed as a tight buffer optical fiber(s), surrounded by longitudinal or slightly stranded aramid fibers and enclosed within an outer jacket of polyurethane. The tight buffer optical fiber is generally a more protected fiber than normal UV optical fibers. The aramid fibers provide strength to the cable, such as tensile strength, and the polyurethane jacket provides a tough but flexible outer casing that can endure severe temperatures. See prior art  FIG. 1 . 
         [0006]    However, such a cable, although flexible, still has certain drawbacks associated with its ability to wind and unwind around tight cable drums, such as for application requiring highly portable fiber drums. Because of the properties of the polyurethane under partial pressure extrusion, including its melt-flow properties (and which has no measured shrinkage after the jacket is removed an exposed to 110° C. for 2 hrs), the jacket minutely encapsulates some of the layer of aramid fibers/strength members causing the outer portion of the strength layer to “weld” into the inside diameter of the polyurethane jacket as shown for example in prior art  FIG. 2 . In other words, the aramid and jacket combination, at least in part forms an aramid reinforced polymer at their interface. 
         [0007]    This welding of the strength layer to the inside of the jacket partially fuses the two layers, reducing flexibility, particularly when the cable is turned around a non-standard tight or reduced diameter drum, tent post, or mandrel during testing. In an ideal non-welded situation the surface of the jacket (particularly at the inside portion of the tightest bending) is able to stretch and the aramid fibers therein may re-position so that the jacket and strength fibers do not transfer the bending stresses down/up onto the fibers therein. However, as illustrated in prior art  FIG. 3 , when the aramid fibers are welded into the jacket as shown in  FIG. 2  above, and when the cable is bent around a drum or mandrel, the shorter path (inner surface of the jacket against the drum) must absorb all of the shortening since aramid-welded polymer cannot stretch. This results in the inner radius jacket having to collapse on itself in an accordion fashion and the inner uncoupled constituents of strength yarn filaments or aramid fibers, optical fibers, tight buffered optical fibers, subunits or fillers being forced to adapt or collapsed to a reduced longitudinal space or to be longitudinally “crushed” into a sine-wave shape. Additionally, the strength yarn filaments or fibers welded in along the top surface of the bend (away from the drum or mandrel) cannot reposition and are pulled down on the upper surface of the fiber in the middle of the cable. The distorted jacket and welded aramid fiber combination pushes in towards the strength filaments or fibers, optical fibers, tight buffered optical fibers, subunits or fillers in the center causing either unacceptable levels of attenuation of even outright failure of the cable. 
         [0008]    In an exemplary calculation using a tactical cable with an outer diameter (OD) of 0.310″ being wrapped around a 3″ mandrel the following equation shows the approximate crushing percentage (length differential caused by bending around the mandrel) that must be entirely absorbed on the inner diameter of the bent cable when the welded aramid jacket cannot reposition or stretch along the outer diameter. 
         [0000]      π(3+0.310)·π(3)/π(3)=0.310/3=0.1033=10.33%
 
         [0000]      [(π*diameter of outside bend)·(π*diameter of inside bend)/(π*mandrel diameter) ]
 
         [0009]    Using the same size OD cable 0.310 around a 2″ mandrel 
         [0000]      π(2+0.310)·π(2)/π(2)=0.310/2=0.155=15.5%
 
         [0010]    Likewise, in an exemplary calculation using a tactical cable with an outer diameter (OD) of 0.175″ being wrapped around a 2.5″ mandrel the following equation shows the approximate crushing percentage that must be entirely absorbed on the inner diameter of the bent cable when the welded aramid jacket cannot reposition or stretch along the outer diameter. 
         [0000]      π(2.5+0.175″)·π(2.5)/π(2.5)=0.175/2.5=0.07=7%
 
         [0011]    Using the same site OD cable 0.175 around a 2″ mandrel 
         [0000]      π(2+0.175)·π(2)/π(2)=0.175/2=0.0875=8.75%
 
         [0012]    Using the same size OD cable 0.175 around a 1″ mandrel 
         [0000]      π(1+0.175)·π(1)/π(1)=0.175/1=0.175=17.5%
 
         [0013]    As such, in a cable according to the prior art, with the aramid strength fibers welded into the jacket, and with the outer surface of the jacket on the bend unable to stretch, the two surfaces share the amount that must be absorbed by the bend, the inner surface of the bend must absorb roughly 50% of the approximated 10% -15% length differential (or 5% to 7.5%) as shown in  FIG. 3 . 
       OBJECTS AND SUMMARY 
       [0014]    The present invention overcomes these drawbacks by altering the design of the aramids and jacket so as to minimize this “welding in” effect or otherwise preventing it all together so that when tactical or cordage application tight buffer optical fiber cables are bent around relatively smaller drums or mandrels, the aramid fibers and jacket do not crush into the optical fiber therein. 
         [0015]    To this end, the present arrangement includes a fiber optic cable having a at least one fiber element, a layer of aramid&#39;s strength members, and a jacket disposed over said layer of aramid strength members. The layer of aramid strength members is wound at a lay length that is equal to or lesser than a predetermined bend radius. 
         [0016]    Alternatively, the present arrangement includes a fiber optic cable having at least one fiber element, a layer of aramid strength members, an aramid coating layer, and a jacket disposed over the layer of aramid strength members, where the aramid coiling layer is non-slip with respect to the jacket. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention can be best understood through the following description and accompanying drawings, wherein: 
           [0018]      FIG. 1  shows a prior art tactile or cordage application fiber optic cable; 
           [0019]      FIG. 2  shows a prior art tactile or cordage application fiber optic cable of  FIG. 1  with the aramid strength cables welded into the jacket; 
           [0020]      FIG. 3  shows the prior art cable of  FIGS. 1 and 2  bent around a mandrel and exhibiting the results of the welded aramid/jacket combination; 
           [0021]      FIG. 4  shows a cut away view of a tactical fiber optic cable according to one embodiment; 
           [0022]      FIG. 5  shows a cut away view of a tactical fiber optic cable according to an alternative embodiment; 
           [0023]      FIG. 6  shows a profile view of the tactical fiber optic cable of  FIG. 4  in a side cut-away view; 
           [0024]      FIG. 7  shows a profile view of the present aramid fibers in the strength portion of a tactical fiber optic cable in overlay over a prior art longer lay length; 
           [0025]      FIG. 8  shows a side view of the cable of  FIGS. 4 and 6  being pulled around a 3″ mandrel according to one embodiment; 
           [0026]      FIG. 9  shows a close up diagram the inter fiber stretching of the polymer of jacket occurring in  FIG. 8  according to one embodiment; 
           [0027]      FIG. 10  shows an alternative arrangement of the cable of  FIGS. 4 and 6  using a two layer aramid layer in accordance with one embodiment; 
           [0028]      FIG. 11  shows an alternative arrangement of the cable of  FIGS. 4 and 6  using a barrier layer of talc or powder; 
           [0029]      FIG. 12  shows an alternative arrangement of the cable of  FIGS. 4 and 6  using a two layer strength arrangement with both an aramid layer and a flexible layer in accordance with one embodiment; and 
           [0030]      FIG. 13  shows an alternative arrangement of the cable of  FIGS. 4 and 6  using a barrier layer of polymer between the jacket and the aramid layer in accordance with one embodiment. 
           [0031]      FIG. 14  shows an alternative arrangement of the cable of  FIGS. 4 and 6  using copper wires in addition to the fiber elements in accordance with one embodiment. 
           [0032]      FIG. 15  shows an alternative arrangement of a high fiber count cable in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    In one arrangement, as shown in  FIG. 4  a tactical or cordage application fiber optic cable  10  includes four tight buffer optical fibers  12  a plurality of helically wound aramid yarns  14  around optical fiber  12  and a jacket  16  over aramid yarns  14 . 
         [0034]    For the purposes of illustration, the present arrangement will be described with four tight buffer optical fibers  12 . However, the invention is not limited in his respect The features of the invention may be implemented with other signal carrying fibers such as a single tight buffer fiber, grouped (jacketed) fiber subunits, UV coated optical fibers within a loose tube arrangement and the like. For example,  FIG. 5  shows an alternative arrangement with cable  12  having multiple fiber sub-units (fibers within independent breakout jackets independent of cable jacket  10 ). 
         [0035]    For the purposes of illustration, the present arrangement will be described as advantageously employing a polyurethane jacket  16 , such as a TPU (Thermoplastic Polyurethane) Estane 58202 TPU polyether-type TPU. However, the invention is not limited in this respect. The features of the invention may be implemented with other polymers used for jacket  16  including any polymers that, as per their melt characteristics would exhibit a strong welding to the underlying aramid fibers. 
         [0036]    Turning to aramid yarns  14 , these are the primary strength member of cable  10  that provide it with its tensile strength and other physical strength characteristics. In the present arrangement, aramid yarns  14  are described as 15-24 yarns of 1500 denier aramid fibers. However, the invention is not limited in this respect. Depending on the size of the aramids, more or less yarns or differing sizes could be used within the context of this invention. Moreover, although aramid yarns  14  are typically used in tactical or cordage fiber optic cable applications, the invention also contemplates the use of other strength elements, either along with aramids  14  or in place of them. 
         [0037]    It is noted that the aramid fibers that make up aramid yarns  14  are typically available in different forms including with a coating at 0.3, 0.7 or 1.3% (by weight) or even generous water swellable power coverage (at up to 10% by weight or greater). In one arrangement. the present cable  10  exhibits a high degree of welding between aramids  14  and jacket  16  by using the lesser coated 0.3% finish. The low coating or water swellable aramids fibers of aramid yarns  14  results in a high level of jacket  16  adhesion. It is understood that using aramid fibers with heavier de-adhesive coating (such as with silicon oil or paraffin oil) within aramid yarns  14  may have a reduced weld with jacket  16 , but the features of the present invention may be equally applied using such fibers for aramid yarns  14 . 
         [0038]    Another characteristic of the fibers of aramid yarns  14  is the twist rate of the fibers within yarn  14 . A minimal twist of the fibers that form aramid yarns  14  render those fibers practically parallel to one another, and thus free and somewhat individual. This results in the plastic of jacket  16 , during extrusion, flowing around the circumference of the fibers of aramid yarns  14 , particularly adjacent to the plastic flow as jacket  16  is being applied. 
         [0039]    It is noted that the low twist of individual fibers forming aramid yarns  11  refers to the twist rate of the individual aramid fibers within the aramid strength members/yarn  14  and not to the rate of helical application of the yarns  14  themselves around the other components within cable  10  (ie. fibers  12 ). Strength members/yarns  14  are actually twisted at a high rate/short lay length within cable  10  as discussed in more detail below. 
         [0040]    Turning to the arrangement of aramid yarns  14  within cable  10  in a first arrangement as shown in  FIG. 6 , for an exemplary cable  10  having an OD of 0.310, aramid strength yarns  14  are helically applied in a lay length of approximately 2.5″-3″ as opposed to a more typical lay length of 5″-8″.  FIG. 7  shows a comparison of a typical prior art aramid lay length versus the present 2.5″-3.0″ tighter lay lengths. As will he discussed below in more detail the actual lay length used for aramid yarns  14  are changed based on the desired flexibility/bending requirements. The present example of 2.5″-3.0″ lay length for aramid yarns  14  is for a desired bending of 3″ mandrel/drum for a 0.310″ OD cable  10 . 
         [0041]    This arrangement of the tighter lay length for aramid yarns  14  provides a significant advantage over prior art tactical cables, particularly with its ability to withstand tighter bend radiuses without incurring the problems described above in the background sections. 
         [0042]    For example, normally a cable according to the prior art designs would be expected to meet a potential bend radius (or bend radius testing) around a mandrel or drum that is roughly 20 times its OD. In other words using the 0.310″ OD tactical cable as described in the present example, in the prior art design, the cable may be expected to meet a bend radius testing around qua approximately 6″ diameter testing mandrel. However, with ever increasing demands for tighter bending, a cable with an OD 010.310″ may be required to meet a 10× its OD (or 3″) bending test radius which is half the diameter that was previously expected. 
         [0043]    In cable  10  of the present arrangement, by shortening the lay length of aramid yarns  14  to no greater than 10 times the OD of cable  10  (ie. 3″ lay length for a 0.310″ OD), cable  10  can pass the necessary bend radius testing with an acceptable attenuation on bending around a mandrel of 10 times the OD of cable  10 . In other words, for a g yen OD of cable the lay length of the aramid yarns  14  should be set no greater than the minimum desired bend radius. 
         [0044]    Thus, according to the present arrangement as shown in  FIGS. 6 and 7 , by wrapping the aramid with either the same lay length or a lesser lay length (such as 2.5″-3.0″ in the present example) than the target diameter of the mandrel in the bend testing (which in most specifications is set by the OD of the cable), the aramid attached to the jacket would act more like a spring and not contract the jacket along its inner surface (as opposed to prior art  FIG. 3 ). 
         [0045]    This arrangement results in a broader angle for the fibers relative to the longitudinal direction of fiber and jacket. For example using the examples above in  FIG. 7 , with a cable having 0.310″ OD the angle of aramid yarns  14  relative to the longitudinal direction of cable  10  is denoted by the following triangle and equations 

 
         [0046]    As noted above,  FIG. 7  shows the present shorter lay length of aramid yarns  14  disposed over top of exemplary aramid yarns in prior art designs having longer lay lengths. Using this triangular representation of aramid yarns  14  laid across the components of cable  10 , the angle θ represents that angle relative to the longitudinal direction of the cable (and its component jacket  16  and fibers  12 ). This angle θ is larger in the present arrangement than prior art as it directly corresponds to the shorter lay length. 
         [0047]    For example, using an example of a 2.5″ lay length for aramids  14  in a 0.310″ OD cable  10   
         [0000]      tan θ=(opposite side/adjacent side) or tan θ=0.310″π(0.9738″)/2.5″
 
         [0000]      θ=about 21.202° angle relative to the longitudinal axis of the cable
 
         [0048]    on the other hand—using a 5″ lay length as in the prior art results in 
         [0000]      tan θ−opp/adj or tan θ=0.310″π(0.9738″)/5.0″
 
         [0000]      θ=about 11.021° angle relative to the longitudinal axis of the cable
 
         [0049]    Because aramid yarns  14  in the present arrangement are at a greater angle relative to the longitudinal direction of the jacket/optical fibers/cable, when cable  10  is pulled around a mandrel, even with the welding effect between jacket  16  and aramid yarns  14 , the polymer of jacket  16  prevents the bunching up, shown in prior art  FIG. 3 . The shorter lay of aramid yarns  14  renders the aramid  11  direction more perpendicular to the direction of stretch in jacket  16 , allowing the stretch between the filaments/fibers of aramid yarns  14 . This arrangement avoids the resultant attenuation seen in the prior art. See for example,  FIG. 8  showing they present arrangement cable  10  being pulled around a 3″ mandrel and  FIG. 9  showing a diagram of the inter-fiber stretching of the polymer of jacket  16 . 
         [0050]    Such an arrangement, even with the welding effect, produces cables  10  that meet the acceptable attenuation standards while still maintaining high pull off strength (eg. 90 lbs pull off). For example, an exemplary proposed military specification (mil spec MIL-PRF-84045/8B) requires that the long term minimum bend diameter be substantially 10 tines the cable OD. The present arrangement, with various arrangements of internal fiber optic components, is able to meet this standard in contrast to prior art cables with longer lay lengths for their aramid yarns which typically can only allow for a minimum bend radius of 20 times the OD of the cable using the same polyurethane jacket and aramid yarns. 
         [0051]    Owing to the affect produced above a general rule is developed that in order to ensure that cable  10  meets the required bend radius test, it is desirable to have the lay length of aramid yarns  14  to be within the range of 75%-100% (but not greater) than the diameter of the test radius. It is noted that typically the desired bend radius capability is set based on a multiple (eg. 10×) the OD of the cable. 
         [0052]    For example 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Desired radius 
                 lay length of aramids 
               
               
                   
                   
               
             
             
               
                   
                 4″ 
                 3″-4″ 
               
               
                   
                 3″ 
                 2.25″-3″   
               
               
                   
                 2″ 
                 1.5″-2″   
               
               
                   
                   
               
             
          
         
       
     
         [0053]    Although m theory even shorter lay lengths for aramid yarns  14  may be used, they are less desirable because of the resultant low line speeds they would cause. 
         [0054]    Optical fiber cable  10  for cordage or tactical configurations made according to the above description do not overly attenuate when wrapped 10-100 times around the target mandrel. The present arrangement, while being flexible, likewise continues to meet other typical tactical and cordage ruggedness standards such as being able to withstand 2000 repetitions are a multiple pass sheave machine after −60° C. temperatures (ie. remains flexible for tight bending, while still being of rugged design.) 
         [0055]    For example employing the design outlined above, using standard optical fibers for fiber  12 , the present cable  10  meets the following attenuation standards 
         [0056]    0.175″ cable OD having 2.5″ aramid  14  lay—
       10 turns around a 2.5″ mandrel resulted in only 0.2 dB attenuation@1550 nm       
 
         [0058]    0.301″ cable OD having 2.8″ aramid  14  lay—
       30 turns around a 3″ mandrel resulted in only 0.12 dB attenuation@1550 nm       
 
         [0060]    0.301″ cable OD having 2.8″ aramid  14  lay
       50 turns around a 2″ mandrel resulted in only 0.91 dB attenuation@1550 nm       
 
         [0062]    It is noted that the above discussed crushing caused by the welded jacket  16  and aramid yarns  14  may result in a protrusion of the internal constituents of optical cables (eg. fibers, subunits, yarns and fillers . . . ), when for a typical test condition, a 1 meter sample is wrapped around a test mandrel. For example, for a given radius test mandrel, the components of a typical prior fiber cable may protrude ⅜″ to 1″ after wrapping. On the other hand, a cable  10  according to the present arrangement with a high rate of twist/short lay length for aramid yarns  14 , experiencing no crushing effect and thus a lesser amount of cable component protrusion results, typically in the range of 0 to ¼″ under the same 1 meter test conditions. This provides another indicator of the effective compression of the jacket relative to the prior art. 
         [0063]    In another embodiment, as shown in  FIG. 10 , it was found that the key aramid strands  14 , are the ones next to jacket  16 . These outer yarns  14  are the ones among the plurality of yarns  14  that stuck or are welded to jacket  16  during extrusion. Therefore, these outer aramid yarns  14  closer to jacket  16  are the ones that are wrapped over the tight hullers at the desired shorter lay lengths (set by the desired target radius). 
         [0064]    For example, as shown in  FIG. 10 , aramid yarns  14  is divided into inner aramid layer  14   a  and outer aramid layer  14   b.  Inner aramid layer  14   a  need only he helically wound at the normal 5″-8″ lay length. However, outer aramid layer  14   b  should he wrapped at the tighter lay length that is between 75% and 100% of the diameter of the target bend radius as described above. Because the welding of jacket  16  to aramids occurs primarily the outer lavers it is possible that outer aramid layer  14   b  (of tighter lay length) need only he made from 4-8 yarns from the total 15-25 yarns, with the remaining yarns being in inner aramid layer  14   a.    
         [0065]    In another embodiment shown in  FIG. 11 , instead of reducing the lay length of aramid strength fiber layer  14 , a harrier layer  20  is introduced between jacket  16  and aramid layer  14 . For example, layer  20  is formed during the cabling/extrusion process as a relatively large amount of release powder or release agent placed on top of aramid  14  to prevent adhesion or “welding” with jacket  16 . This eliminates the high nodulus yarn&#39;s adhesion to jacket  16  and thus avoids the bunching issue shown in prior art  FIGS. 2 and 3 . Layer  20  is formed as a powder of high volume completely covering aramid  14  using a talc water swellable powder or a of a mix of the two where the powder is applied with a 100-500 micron thickness over aramid  14  to prevent adhesion to jacket  16 . 
         [0066]    In another embodiment shown in  FIG. 12  instead of using a two layer  14   a  and  14   b  aramid section as shown in  FIG. 10 , the outer layer  14   b  is replaced with a different form to strength yarn  22 . For example, in one exemplary arrangement, strength layer  22  is made from strength fibers of polyester or other low modulus yarns. Such polyester or low modulus yarns  22  allow the highly adhesive polyurethane jacket  16  to bond with a stretchable yarn rather than the less flexible aramid yarns of layer  14   a.  Thus jacket  16  does not weld or Contact the aramid yarns of  14   a  eliminating the high modulus yarn adhesion to jacket  16  preventing the contraction occurrence as shown in prior art  FIG. 3 . This arrangement, with low modulus yarn layer  22 , uses the higher than normal lay lengths so as to use higher cable  10  production line speeds. 
         [0067]    In another embodiment shown in  FIG. 13  instead of reducing the lay length of aramid strength fiber layer  14 , a polymer barrier layer  24  is introduced between jacket  16  and aramid layer  14 . For example, layer  24  is formed during the jacket  16  extrusion process, possibly by double extrusion, using PVC (polyvinyl chloride), PE (polyethylene) or PP (polypropylene). This polymer harrier layer  24  is made from a non-adhesive polymer (meaning its properties do not result in welding to the aramid yarns during extrusion) allows aramids  14  to move or re-distribute and allows the outer polyurethane jacket  16  to flex during bending. 
         [0068]    In another arrangement, as shown in  FIGS. 14 and 15 , cable  10  includes additional features typical of tactile cables. For example, in addition to the components discussed above,  FIG. 14  shows a cable  10  with tight buffer fiber optic elements  12 , aramid fibers  14  and jacket  16 . However, cable  10  additionally has copper wires  30  for power transmission within the cable as well as a ripcord  32  for assisting in removing jacket  16 . Additionally a central/dielectric strength member  31 , such as a GRP (Glass reinforced polymer) or other such central member may he included. In any event, aramid fibers  14  are helically wound around these cable  10  components at a short lay length, at or below the target bend radius.  FIG. 15  shows another exemplary high fiber count cable  10  with (12) tight buffer fiber units  12  around a central strength member  34 . Again, aramid fibers  14  arc helically wound around these cable  10  components at a short lay length, at or below the target bend radius. 
         [0069]    While only certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes or equivalents will now occur to those skilled in the art. It is therefore, to be understood that this application is intended to cover all such modifications and changes that fall within the true spirit of the invention.