Abstract:
A novel fiber optic tube and method of forming the fiber optic tube. The method includes placing at least one optical fiber onto a tape with lateral ends and forming the tape into a tube. The length of the at least one optical fiber is longer than the length of the tape. Thus, the fiber optic tube has at least one optical fiber placed on the inside wall of the tube.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention generally relates to fiber optic tubes and cables. Optical fibers in general have a low tensile strength and a very low tolerable elongation. Therefore, limiting the strain on the optical fibers increases the optical fiber&#39;s useful life. In addition, the transmission characteristics of an optical fiber are optimum when the optical fiber is stress free because the ability of optical fibers to propagate light is very sensitive to lateral stresses and deformations. This is generally discussed in U.S. Pat. No. 4,640,576 to Eastwood et al.  
           [0002]    Due to these concerns, optical fibers within a fiber optic tube are produced having an excess fiber length (“EFL”). This protects the optical fibers from the stresses that would be applied to it in the fiber optic tube.  
           [0003]    A conventional process of forming an optical fiber tube with optical fibers having an appropriate EFL includes stretching the tube in a longitudinal direction, inserting the fibers into the pre-stretched tube, coupling fibers to the stretched tube and relaxing the tube so that when the tube returns to its normal length, the optical fibers within the tube have a length in excess of the tube length. This process, however, limits the materials from which a fiber optic tube can be made because some materials, such as aluminum and other metals, may not completely return to their original length after stretching.  
           [0004]    For example, soft, low grade metals must be used in order for the metal to withstand the deformation required to form a tube that may then be stretched in the manner discussed above. This is because low grade metals usually have relatively high elongation at their breaking point. However, these metals also have a relatively low elongation when they reach their elastic limit. Therefore, when this type of metal is stretched, it does not return to its original length unless elongation is kept low enough to stay below its elastic limit.  
           [0005]    As is shown in FIG. 1, it is also known to form fiber optic bands  1  with EFL in which optical fibers  4  are embedded in a matrix material or in between two synthetic thermoplastic films  2 ,  3 . As is shown in FIG. 2, these bands  1 , with embedded optical fibers  4 , are wound around a carrying element  20  to form a telecommunications cable  19 . These types of bands are generally discussed in U.S. Pat. No. 3,937,559 to Ferrentino et al. These bands  1 , however, are only used as structures winded around a carrying element and the structure obtained does not feature excess fiber length with respect to the tape.  
         SUMMARY OF THE INVENTION  
         [0006]    This invention is a novel fiber optic tube and method of forming a fiber optic tube including fibers having an excess length with respect to the tube. The method includes placing at least one optical fiber onto a tape with lateral ends and forming the tape into a tube such that the at least one optical fiber is longer than the tape after it has been placed on the tape. Thus, the resulting fiber optic tube has at least one optical fiber placed on the inside wall of the tube.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiment of the invention which is schematically set forth in the drawings, in which:  
         [0008]    [0008]FIG. 1 shows a prior art optical fiber band including two thermoplastic material films with optical fibers imbedded between them;  
         [0009]    [0009]FIG. 2 shows a prior art telecommunications cable including the optical fiber band shown in FIG. 1;  
         [0010]    [0010]FIG. 3 shows a tape and a plurality of optical fibers attached to the tape according to an embodiment of the present invention;  
         [0011]    [0011]FIG. 4 shows an embodiment of the fiber optic tube after the lateral ends of the tape of FIG. 3 have been attached to each other;  
         [0012]    [0012]FIG. 5 shows another embodiment of the fiber optic tube in which the lateral ends of the tape of FIG. 3 overlap one another;  
         [0013]    [0013]FIG. 6 shows a cross section of a fiber optic cable having an optical fiber tube in the core of the fiber optic cable;  
         [0014]    [0014]FIG. 7 shows a cross section of a fiber optic cable in which more than one fiber optic tube is used in the core of the fiber optic cable; and  
         [0015]    [0015]FIG. 8 shows a cross section of an fiber optic cable having an inner thermoplastic jacket, strengthening elements, and an outer thermoplastic jacket.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    An embodiment of the invention will be explained in further detail by making reference to the accompanying drawings, which do not limit the scope of the invention in any way.  
         [0017]    As is shown in FIG. 3, in order to form a fiber optic tube  14 , optical fibers  10  are first placed onto a side of a tape  12 . The tape is preferably flat but can include other configurations, such as a corrugated tape. Next, in order to form the tape  12  into a tube  14 , the lateral edges  16   a  of the lateral ends  16  of the tape  12  are attached to each other or the tape  12  is maintained in a tube shape by one or several binders. The resulting structure is a tube  14  with optical fibers  10  placed on its inside walls, as is shown in FIGS. 4 and 5.  
         [0018]    Optical fibers  10  are generally made of a glass or synthetic plastic material, but the choice of optical fiber material is not critical to the invention.  
         [0019]    In one embodiment, the optical fibers  10  are fed onto the tape  12  in a manner that ensures that the length of the optical fibers  10  is longer than the length of the tape  12 . By feeding and positioning optical fibers  10  so that they have an increased length compared to the length of the tape  12 , optical fibers having an EFL are produced.  
         [0020]    Preferably, the tension on the optical fiber  10  should be kept as low as possible when the fibers  10  are fed onto the surface of the tape  12 . This ensures that once the tension on the fibers  10  is released, there is no back tension created. A back tension might cause the fibers  10  to contract when they are attached to the tape  12 , which would reduce the fibers&#39; EFL.  
         [0021]    One way to produce a tape  12  with optical fibers  10  involves feeding the tape  12  and the optical fibers  10  simultaneously. In order to feed the optical fibers  10  so that the fibers  10  have an excess length with respect to the tape  12 , the fibers  10  are fed at a feeding speed that is greater than the feeding speed of the tape  12 . The point of contact between the fibers  10  and the tape  12  is moved from side to side with respect to the width of the tape  12  so that the extra feed length of the optical fibers  10 , due to their greater feeding speed, provides optical fibers  10  placed onto the tape  12  in an oscillating pattern.  
         [0022]    The manner in which the optical fibers  10  are fed onto the side surface of the tape  12  creates the specific elongated pattern of the fiber  10 . One manner of producing fibers with an EFL is to feed and place the fibers  10  onto the tape  12  in an undulating path, as is shown in FIG. 3. The amplitude and frequency of the undulating pattern are used to control the amount of EFL of the optical fibers. Other patterns could be used such as a triangular shaped pattern, or a trapezoidal pattern as long as the length of the optical fibers  10  is greater than that of the tape  12 .  
         [0023]    Alternatively, the deposition of the fibers  10  onto the tape  12  can be facilitated by the application of a “pre-deformation”, or a curvature radius in the same direction as that of the tube-forming process, to the tape  12 . The fibers  10  are then fed on the concave side of the tape  12 .  
         [0024]    The tape  12  can also include side portions  18  near the lateral ends  16  of the tape  12  on which optical fibers  10  are not placed. These empty side portions  18  provide space for attaching the lateral ends  16  to each other.  
         [0025]    The tape  12  can be made of either an electrically conductive material, such as steel, aluminum, or copper, or of a dielectric material, such as Mylar. Of course other materials may also be used while remaining within the spirit of the invention. Applications exist for fiber optic cables formed by either electrically conductive or dielectric tape.  
         [0026]    Electrically conductive materials provide several advantages for optical fiber systems. For example, optical ground wire systems often use a metallic housing because, when compared to dielectric housings, the metallic housings provide a better resistance to high temperatures, offer a better resistance to water and Hydrogen permeation, are mechanically stronger, and can contribute to some degree to the electrical properties of the fiber optic cable.  
         [0027]    The use of dielectric materials, one the other hand, are preferable in systems in which in the fiber optic cables do not have an electrical function. This is because dielectric materials are not sensitive to external electrical or electromagnetic influences such as lightning.  
         [0028]    If the tape  12  is made of metal, the lateral edges  16   a  of the lateral ends  16  of a tape  12  can be attached by welding, as is shown in FIG. 4.  
         [0029]    In addition, for either metal or non-metal tapes  12 , the lateral ends  16  can be joined by folding one lateral end  16  over the other as is shown in FIG. 5. In this configuration, the interstices at the overlap between two ends  16  may be at least partially sealed with a hot-melt, glue, or viscous material and the tape may be maintained in shape by wrapping the tube with a binder such as a thread or a sheath. The lateral ends may also be configured to facilitate mechanical coupling of the ends. However, the invention is not limited in this respect.  
         [0030]    The tape  12  should be wide enough so that the material can be rolled into a tube  14 . It is envisioned that a tape with a width of at least about 20 mm would work well, but the invention is not limited in this respect either.  
         [0031]    The optical fibers  10  can be placed onto the tape  12  by flooding the surface of the tape  12  with a relatively thin layer of a viscous compound and/or by covering or coating the fibers with a viscous compound, the degree of adherence being however limited such that the fibers can be easily separated from the tape. However, other conventional means for placing the fibers onto the tape, such as an adhesive, can be used.  
         [0032]    When the tube  14  is formed, a hollow space is formed inside the tube  14 . The hollow space can be filled with a filling compound  50  such as, for example, a silicon based, a poly-alpha-olefine based, or a poly-isobutylene based compound as commonly used in fiber optic cables. The filling compound helps to stabilize the optical fibers  10  in their elongated pattern and prevents moisture from penetrating into the cable in the event of sheath damage or a cable break. Dry water swellable powders may also be disposed in the hollow space, in addition to or instead of, the filling compound  50 .  
         [0033]    One or more of the formed fiber optic tubes  14  can be used in the core of a fiber optic cable  100 . FIG. 6 shows an optical fiber cable  100  having a single tube  14  in its core. FIG. 7 shows an optical fiber cable having multiple tubes  14  within its core. Although these figures show the optical fibers  10  in contact with the tape  12 , which is their initial location after processing, in some embodiments of this invention, the optical fibers  10  may move to other positions within the tube  14  since the filling compound  50  does not prevent the optical fibers  10  from moving.  
         [0034]    With respect to the uni-tube cable, as is shown in FIG. 6, unlike conventional cables, the cable  100  does not need a additional protective layer since the tape portion  12  of the tube  14  is used both as a holder for the optical fibers  10  and as a protective layer. For example, if the fiber optic tube  14  is made of metal, then the cable  100  does not need an additional metal armor layer. A non-metallic layer, however, can also serve as both a holding tube and protector.  
         [0035]    Either of the cables discussed above, could also include additional cable protectors such as dielectric or metallic strengthening elements  32  which are longitudinally or helically applied and/or protective thermoplastic jackets  34 . Some non-limiting examples of these structures are a cable  100  with a simple thermoplastic jacket  34 , a cable with strengthening elements  32  disposed between the fiber optic tube  14 , and an outer surface of a thermoplastic jacket  34   a , and, as is shown in FIG. 8, a cable with an inner thermoplastic jacket  34   b , strengthening elements  32  disposed between the fiber optic tube  14  and an outer surface of an outer thermoplastic jacket  34   a , and the outer thermoplastic jacket  34   a . However, the invention is not limited by these examples.  
         [0036]    It is of course understood that departures can be made from the preferred embodiment of the invention by those of ordinary skill in the art without departing from the spirit and scope of the invention that is limited only by the following claims. For example, although the figures show a plurality of optical fibers attached to the tape, an optical fiber tube with a single optical fiber could be formed.