Abstract:
A fiber optic buffer tube containing fiber optic ribbons centrally located within the buffer tube and a gel compound surrounding the fiber optic ribbons. Disposed within the gel compound, between the walls of the buffer tube and the fiber optic ribbons are yarns. The yarns provide greater surface area which helps to hold the gel compound in place when the gel compound is heated. Also, the yarns provide a cushion which helps prevent the fiber optic ribbons from coming into contact with the walls of the buffer tube, thereby preventing signal attenuation problems.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to gel compounds within conduits or buffer tubes and more specifically to the reduction of dripping of the gel compounds at higher temperatures. 
     2. Description of Related Art 
     Fiber optic cables have been used by the telecommunications industry for a number of years to transmit information at very high rates over long distances. Fiber optic cables come in a variety of configurations, including: cables with a centrally located single buffer tube containing one or more optical fibers; cables with a plurality of buffer tubes stranded in a helical or alternating helical arrangement about a central strength member; and cables with slotted cores in which a plurality of optical fibers reside. 
     The buffer tubes within the ribbon cable generally contain one or more fiber optic ribbons centrally located within the buffer tube and a gel compound surrounding the optical fiber ribbons. An example of this can be seen in FIGS. 1-4. As shown in these figures, the fiber optic ribbons  3  are centrally located within buffer tube  1 . As can be further seen from FIGS. 1-4, a gel compound  2  surrounds the fiber optic ribbons  3 . The gel compound  2  serves a number of purposes. One purpose is to provide a cushioning media between the buffer tube  1  and the fiber optic ribbons  3  to thereby prevent the fiber optic ribbons  3  from contacting the sides of the buffer tube  1 . The cushioning media dissipates radial crushing force and in addition, the gel compound  2  provides delayed motion response to the fibers under scanning bending loads. Such loads occur during the installation, when cables are pulled around the corners of the ducts or over the sheaves. The same applies to the earlier stages of manufacture when buffer tube  1  is bent over the sheaves and radially compressed by caterpillars. The artificial increase in the inertia of the ribbons  3  is provided by the viscous gel media and results in time delay for fibers to accommodate the load and to move slower than in a non-gel media toward the tube walls  1 . When the fiber optic ribbons  3  contact the sides of the buffer tube  1 , signal attenuation problems occur due to micro-bending and high stress. The gel compound  2  also serves to prevent exterior items from coming into contact with the fiber optic ribbons  3  if the buffer tube  1  is penetrated. For example, the gel compound  2  protects the fiber optic ribbons  3  from water that might penetrate the buffer tube  1 . 
     Several problems occur in these conventional buffer tubes, especially ones in which the buffer tube  1  diameter is large (for example, greater than 0.310 inches). First, when the temperature of the gel compound  2  increases, the viscosity and yield stress of the gel compound  2  decreases. If the yield stress of the gel compound  2  decreases below a critical value, the gel compound  2  may begin to flow. For example, if the buffer tube  1  is physically positioned in a vertical manner, as shown in FIG. 5, and the buffer tube  1  is heated, the gel compound  2  within the buffer tube  1  may begin to flow towards the bottom of the buffer tube  1 , leaving a cavity  4 . 
     In more detail, as the temperature of the buffer tube  1  increases, the buffer tube  1  expands, thereby increasing the diameter and length of the buffer tube  1 , according to the difference between the coefficient of thermal expansion (“CTE”) of the buffer tube material  1  and gel compound  2 . As for the gel compound  2 , as noted above, as its temperature increases, the viscosity and yield stress of the gel compound  2  decreases. As shown in FIG. 5, gravity provides a downward force to the gel compound  2  while frictional forces (F 1  and F 2 ) with the tube wall are transmitted through the material by the yield stress of the gel compound  2 . Friction between the gel compound  2  and the buffer tube  1  is labeled F 1  while the fiction between the gel compound  2  and the fiber optic ribbons  3  is labeled F 2 . Consequently, as the temperature of the gel compound  2  increases, the yield stress of the gel compound  2  decreases and the ability of the gel to transmit friction forces F 1  and F 1  through the gel compound  2  decreases. Since the downward force of gravity remains constant during an increase in temperature of the gel compound  2 , when the temperature of the gel compound  2  increases, the downward force of gravity on the material becomes greater than the upward force that can be transmitted through the material through the yield stress of the gel compound  2 . As a result, the gel compound  2  may flow downward. 
     Additionally, gel compound  2  may be “forced” out of the buffer tube  1  when heated due to the difference between the CTE of the buffer tube  1  and the CTE of the gel compound  2 . As stated earlier, when heated, both the buffer tube  1  and the gel compound  2  expand according to their respective CTE. If the CTE of the buffer tube  1  is less than the CTE of the gel compound  2 , then the gel compound  2  expands more than the buffer tube  1 . Since the gel compound  2  is expansionally limited in the radial direction by the buffer tube  1 , if the gel compound  2  expands more than the buffer tube  1  when heated, the additional expansion of the gel compound  2  is directed in the axial direction. As a result, gel compound  2  is “forced” out of the ends of the buffer tube  1 . 
     Once the gel compound  2  flows out of the buffer tube  1 , it does not provide adequate protection to the fiber optic ribbons  3 . The fiber optic ribbons  3  tend to contact the buffer tube walls  1 , which in turn may cause attenuation problems. Additionally, gel compound  2  flowing out of the buffer tubes  1  will flow into splice enclosures which make later access to the closures problematic. Therefore, it is an object of the present invention to improve the compound flow performance of gel-filled fiber optic cables. 
     Another problem with the conventional buffer tubes is the stability or integrity of the ribbon stack shape under the bending loads. In particular, when the stack twist laylength is long and the tube or cable is bent about a small-radius object, the ribbon stack may collapse. The ribbons  3  may slide sideways and “fall” sideways causing ribbon damage and fiber attenuation. Thus, it is desirable to provide means for holding the stack with initial, typically rectangular shape of its cross section. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, optical fibers are provided in a conduit along with yarns and a gel compound. In one of the preferred embodiments, the conduit is a buffer tube. 
     More specifically, the present invention solves the above-described problems and limitations by placing yarns with whiskers within the gel-filled conduits or buffer tubes. The yarns provide many beneficial effects. First, the yarns themselves provide additional surface area for the gel compound to contact which in turn provides additional friction forces that help to keep the gel compound from “running” downward. Second, the yarns with radially oriented whiskers provide additional cushioning effect to slow down the stack motion toward the buffer tube wall and to “smooth” contact pressure on the fibers when the stack is in the close proximity of the tube wall. Third, the whiskered yarns, helically wrapped around the ribbon stack provides for enhanced stack stability i.e. minimizes lateral ribbon sliding and falling off the stack, thus minimizing ribbon damage and fiber attenuation. The stack stability is especially important in the cases when the cables or buffer tubes are bent around small-radius sheaves or corners during cable manufacture and installation. Fourth, the yarns help to compensate for the expansion of the buffer tube walls caused by the increase in temperature by making the expansion and GTE of the gel-yarn system closer to that of the buffer tube material. 
     In a preferred embodiment of the present invention, yarns having whiskers are disposed between the fiber optic ribbon stacks and the walls of the buffer tube. The yarns vary in size and surround the fiber optic ribbon stacks. 
     Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above aspects of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings, in which: 
     FIG. 1 is a plan view of a conventional buffer tube; 
     FIG. 2 is a side view of a conventional buffer tube with a transparent front of the buffer tube; 
     FIG. 3 is a side view of a conventional buffer tube; 
     FIG. 4 is a side view of a conventional buffer tube taken along the IV—IV line of FIG. 1; 
     FIG. 5 is side view of a conventional buffer tube taken along the IV—IV line of FIG. 1 showing the forces acting on the gel compound when the temperature of the buffer tube is increased; 
     FIG. 6 is a perspective view of a buffer tube according to a preferred embodiment of the present invention; 
     FIG. 7 is a side view of the buffer tube of FIG. 6 showing the forces acting on the gel compound when yarns are embedded in the gel compound; 
     FIG. 8 is a plan view of the buffer tube of FIG.  6 . 
     FIG. 9 is a perspective view of a buffer tube of the present invention having a yarn helically disposed around optical fibers; and 
     FIG. 10 is a plan view of a buffer tube of the present invention having optical fibers circumferentially arranged within the buffer tube. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. The present invention is not restricted to the following embodiments, and many variations are possible within the spirit and scope of the present invention. The embodiments of the present invention are provided in order to more completely explain the present invention to one skilled in the art. 
     Referring to FIG. 6, the present invention solves many of the problems created when a buffer tube  1  containing a single ribbon or single stack of ribbons  3  centrally located and surrounded by gel compound  2 , is heated. The buffer tube  1  can be made of any type material and can be any shape. Generally, the buffer tube  1  is cylindrical in shape. The fiber optic ribbons  3  can be assembled in stacks (as shown) or can be individual if necessary. The gel compound  2  is also not limited in any manner. 
     The present invention, as shown in FIG. 6, embeds yarns  5  in the gel compound  2 , between the walls of the buffer tube  1  and the fiber optic ribbon stack  3 . The yarns  5  can be disposed in a number of ways. For example, the yarns  5  can run axially parallel to the fiber optic ribbon  3  or even be wrapped around the fiber optic ribbon  3  in a helical manner. The yarns  5  do not have to be evenly dispersed within the buffer tube  1 . For example, the yarns  5  can be places closer to the buffer tube walls  1  than to the fiber optic ribbons  3 . Although FIG. 6 shows only one yarn  5 , any number of yarns can be used. As shown in FIG. 8, the size and shape of the yarns can vary as well as the type. For example, although the figures show the yarns  5  with whiskers, the present invention can be practiced using yarns  5  with and without whiskers. 
     As described earlier, a problem arises when the buffer tube  1  and gel compound  2  become heated. By adding the yarns  5 , the problem of the gel compound  2  “flowing” downward can be diminished. As shown in FIG. 7, adding yarns  5  to the buffer tube  1  results in two additional upward forces F 3 , F 4  that help prevent the gel compound  2  from running downward. 
     More specifically, the addition of the yarns  5  increases the amount of surface area with which the gel compound  2  may contact. The additional surface area results in two additional forces F 3 , F 4  that act upon the gel compound  2 . As a result, more upward forces can be transmitted to the gel compound  2  when the temperature is increased. 
     Additionally, by selecting yarn having a CTE which is less than the CTE of the gel compound  2 , the CTE of the gel-yarn system is lowered. In fact, it is possible to select yarns having a negative CTE (i.e. yarns that volumetrically contract when heated). In a preferred embodiment, yarns are selected in such a manner that the resulting CTE of the gel-yarn system matches or is substantially equivalent to the CTE of the buffer tube  1 . Consequently, when heated, both the buffer tube  1  and the gel-yarn system expand by the same amount. As a result, gel compound  2  is not “forced” out of the buffer tube  1  in the axial direction. 
     Also, since the yarns  5  occupy some of the volume inside the buffer tube  1 , less gel compound  2  may consequently be used. Using less gel compound  2  results in at least two beneficial effects. First, since gel compound  2  is expensive, using less means the cost of manufacturing the fiber optic cable is decreased. Second, since the force acting in the downward direction (i.e. gravity) is a function of the mass of the gel compound  2 , replacing some of the gel compound  2  with yarns having less mass than the gel compound decreases the downward force due to gravity. A decrease in the force of gravity means that less upward force (i.e. friction forces F 1 , F 2 , F 3 , and F 4 ) is needed to keep the gel compound  2  from running down the fibers. When the gel compound  2  is held in place, it prevents the fiber optic ribbons  3  from contacting the walls of the buffer tube  1  and also prevents other materials (i.e. water) that might penetrate the buffer tube  1  from contacting the fiber optic ribbons  3 . 
     As shown in FIG. 8, the yarns  5  of the present invention can be oriented in a number of ways. The number, size and type of the strands of yarn used can also vary. These include both synthetic and “natural” yarns such as cotton, wool and polyester. For example, Nortex International produces twisted yarns which range from as coarse as 500 ypp (1.0 Nm) to as fine as 2500 ypp (5.0 NM) and which are made from fibers of acrylic, cotton, rayon, and combinations, with specialty fibers such as silk, wool, and linen. For example, in one embodiment, there may be three strands of yarns disposed within the buffer tube  1  with each of the three strands being a different size. Further, two of the strands may be oriented axially parallel to the fiber optic ribbons  3  while the remaining strand may be disposed helically around the fiber optic ribbons  3 . The yarns  5  in a preferred embodiment have whiskers, however, the present invention may be practiced with yarns  5  that do not have whiskers. The yarns  5  with whiskers may also be used to “drag” the gel compound  2  into the buffer tube  1  which speeds manufacturing of the buffer tubes  1 . Finally, the yarns may be oriented axially parallel or disposed helically around the fiber optic ribbons  3 . 
     The buffer tubes of the present invention can be made in a number of ways. Typically, an assembled stack of fiber ribbons is pulled through a die. Gel compound is injected in the die (from the inside) and a hot thermoplastic material is extruded over the gel-stack system (from the outside) to form a buffer tube with gel and ribbons inside. The buffer tube is then moved through a water-cooling channel and wound on the reel. In a preferred embodiment, the yarns  5  are pulled with the assembled stack of fiber ribbons through the die. 
     The yarns most likely will be applied during the buffering/stranding process of the ribbon buffer tubes. The yarns are wound onto a delivery spool and then delivered along with the other optical units into the tube while the polymer is being extruded. That is, ribbons are “stranded” and then “buffered” usually in a single process step. These terms, (stranded and buffered) are commonly used for manufacturing optical units in the industry. The advantages to this are 1) being able to control yarn tensions (which are critical to maintain desired stack integrity) and 2) applying them in the “stacks” “final” formation just prior to entering the buffer tube. Also, it allows you to apply them in the desired helical formation of the “stack”. The “stack” can then orient itself in an ideal center position cushions will help to center the stack within the polymer tube, which can prevent attenuation degradation due to the potential for the optical units to engage the tube wall. 
     The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. It is sought, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.