Abstract:
Cables have reduced freespace, reduced tube diameters, and reduced strength member diameters. The cables are designed to pass robustness testing such as GR-20 while using smaller amounts of raw materials to produce.

Description:
PRIORITY APPLICATION 
       [0001]    This application is a continuation of International Application No. PCT/US10/43222 filed Jul. 26, 2010 which claims the benefit of priority to U.S. Provisional Application No. 61/230,452, filed Jul. 31, 2009, both applications being incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to optical cables having reduced material costs while maintaining desired performance characteristics. 
       BACKGROUND 
       [0003]    Fiber optic cables utilize optical fibers to transmit signals such as voice, video and/or data information. Where fiber optic cables are subjected to forces, the optical fibers may be stressed and attenuation of the transmitted light may result. Industry standards address generic mechanical and optical performance requirements for fiber optic cables to ensure attenuation losses are within acceptable ranges. One such standard is the Generic Requirements for Optical Fiber and Optical Fiber standard GR-20. One way to ensure compliance with GR-20 and other standards is to increase the bulk of the cable, such as by increasing cable diameter, jacket thickness, etc. These measures, however, increase the cost of the cable. It is therefore important for fiber optic cables to be constructed in a robust manner so as to satisfy industry standards while maintaining costs within competitive ranges. 
       SUMMARY 
       [0004]    According to a first embodiment, a cable comprises a cable jacket, a buffer tube defining a cable interior, a plurality of optical fibers in the interior, and strength members embedded in the cable jacket. The optical fibers can be arranged, for example, as a ribbon stack. The cable jacket can be extruded onto the exterior of the buffer tube, and both the cable jacket and the buffer tube can be constructed wholly or partly from polymer materials. 
         [0005]    According to one aspect of the first embodiment, the ribbon stack freespace can be lower than that of conventional cables. Cables according to the present embodiments with reduced ribbon stack freespace can show minimal attenuation response and lower material costs. 
         [0006]    According to another aspect of the first embodiment, the strength member height on either side of the cable can be relatively close to the buffer tube inside diameter to facilitate access to the cable interior. The strength member height can be, for example, within 1 mm of the buffer tube inside diameter. 
         [0007]    According to yet another aspect of the first embodiment, jacket size and strength member size can be smaller than comparable conventional cables in order to reduce material costs. 
         [0008]    According to yet another aspect of the first embodiment, the ratio of the product of elastic modulus E and total cross-sectional area A (EA) for the fibers in the ribbon stack to the product of elastic modulus E and total cross-sectional area A of the strength members is higher than in conventional designs at various fiber counts. 
         [0009]    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 FIGURES 
         [0010]    The present embodiments are explained in more detail below with reference to figures which show the exemplary embodiments. 
           [0011]      FIG. 1  is a cross section of a cable according to a first embodiment. 
           [0012]      FIG. 2  is a plot of the ratio of jacket area to strength member area for the cable of  FIG. 1 . 
           [0013]      FIG. 3  is a plot of the ration of fiber EA to strength member EA for the cable of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  is a cross section of a micromodule cable  10  according to a first embodiment and having an outer diameter  14 . The optical cable  10  comprises a jacket  20  having a wall thickness  24  and an outside diameter  28  (also corresponding to the cable  10  diameter). The jacket  20  surrounds and contacts the outer surface of a buffer tube  30  having an inner diameter  34  and an outer diameter  38 . The jacket  20  can be formed from, for example, a polymer material such as polyethylene. The buffer tube  30  defines an interior  40  of the cable  10 . The cable interior  40  accommodates a plurality of optical waveguides  50 . In the illustrated embodiment, the optical waveguides  50  are arranged as a stack of fiber optic ribbons with a ribbon stack diagonal dimension  54 . Pairs of strength members  60  are arranged on opposite sides of the cable  10  cross section. The strength members  60  are wholly or substantially embedded in the cable jacket  20 , and may be adjacent to and/or abut the buffer tube  30 . In the illustrated embodiment, the strength members  60  are circular in cross-section with diameter  64  and with a strength member height  68  for each pair. The jacket  20 , the buffer tube  30 , the ribbon stack  50 , and the strength members  60  can all extend longitudinally along the entire or substantially all of the length of the cable  10 . 
         [0015]    According to one aspect of the present embodiment, the ribbon stack freespace can be lower than that of conventional cables. Referring to  FIG. 1 , “ribbon stack freespace” is generally defined as the difference between the buffer tube  30  inside diameter  34  and the ribbon stack major dimension—in this case diagonal  54 . In conventional cables, the ribbon stack freespace has historically been above 2.0 mm, with some designs having a freespace as high as 3.38 mm. According to one aspect of the present embodiments, the ribbon stack freespace can be less than 1.5 mm, and more particularly less than 1.0 mm. In one embodiment, the cable  10  is a 48 fiber, 4.1 mm tube inner diameter  34  cable with a ribbon stack freespace of 0.71 mm. Cables according to the present embodiment with reduced freespace can show minimal attenuation response, especially when incorporating bend-improved fibers in the ribbon stack  50 . The ribbon stack  50  is free to move radially with respect to a center line  70  of the cable  10 , so the spacing between the buffer tube  30  in general will not be constant with respect to any of the corners of the ribbon stack  50 . 
         [0016]    According to another aspect of the present embodiment, the strength member height  68  can be relatively close to the buffer tube outside diameter  38  in order facilitate access to the cable interior  40 . For example, in one embodiment, the strength member height  68  is 3.2 mm, with each strength member  60  having a diameter of 1.60 mm, and the buffer tube inner diameter  34  is 4.1 mm. The difference between strength member height  68  and buffer tube inner diameter  34  can be relatively small—in the range of 1.3 mm or less, or more particularly in the range of 1.0 mm or less. Using the strength members  60  as a blade guide, the cable jacket  10  and buffer tube  30  may be shaved away from the cable  10  without damaging the ribbons in the stack  50 . Six strength members  60  of 1.25 mm diameter, for example, with three members on each side of the buffer tube  30 , would further decrease the difference between the buffer tube outer diameter  38  and the strength member height  68 . Also, if this feature is desired in the field, strength members  60  can be spaced or separated (in the vertical direction in  FIG. 1 ) in order to increase the overall strength member height  68 . In the illustrated embodiment, the strength members  60  are dielectric rigid/semi-rigid strength members, and can be glass-reinforced plastic (GRP) rods with circular cross-sections, although other materials (e.g. steel) and/or cross-sections can be used. Referring to  FIG. 1 , the “strength member height” is defined as the spacing between the outermost edges (shown as the uppermost and lowest edges in  FIG. 1 ) of the outermost strength members on one side of the cable. In the illustrated embodiment, the strength members  60  abut one another so the strength member height  68  is the sum of the diameters of the strength members  60  on each side of the cable  10 . It is generally preferable that the strength members  60  abut the buffer tube  30  to prevent jacket material from coming between the strength members  60  and the buffer tube  30 . 
         [0017]    According to another aspect of the present embodiment, by reducing the strength member diameter  64 , the jacket thickness  24  can also be reduced. For example, a 0.55 mm reduction in strength member diameter  64  was achieved for the cable  10  when compared with a conventional design. This corresponds to the same jacket thickness  24  reduction in the thick portions (or, portions not overlying the strength members  60 ) of the jacket  20 . Similar conventional cable arrangements require at least a 2.80 mm jacket wall to meet minimum jacket thickness requirements. Minimum jacket thickness is the thickness of the jacket required over the strength members  60 , indicated generally by the arrow  75  in  FIG. 1 . The cable  10  in the illustrated embodiment has a jacket  20  of about 2.30 mm thickness. The relatively thin jacket  20  significantly reduces material costs for the cable  10 . In an alternative embodiment, a cable having six 1.25 mm diameter strength members  60 —three strength members on each side—reduces the jacket thickness even further to 2.00 mm. Cables according to the present embodiments can be constructed to maintain a substantially round outer diameter while retaining the required minimum jacket thickness. The jacket thickness  24  can be, for example, in the range of 2.00 mm to 2.80 mm, or more particularly in the range of 2.30 mm to 2.80 mm. 
         [0018]    Another way to characterize the relationship between the jacket  20  and the strength members  60  is to compare the cross-sectional area of the jacket  20  with that of the strength members  60 . Jacket to strength member area ratio data are tabulated in  FIG. 2  for cables at room temperature. When using strength members  60  of round cross-section, the thickness of the jacket  20  is determined by the diameter of the strength members  60  plus the minimum jacket thickness  75  required over the strength members  60 . In the illustrated embodiment, the strength member diameter  64  is 1.60 mm, with two strength members  60  on each side of the jacket  20 . The minimum jacket thickness  75  is in the range of 0.7-1.0 mm. Reducing the size of the strength members  60  allows a reduction in jacket size, which reduces the costs of material for the cable. In this specification, the term “strength member area” refers to the sum of the cross-sectional areas of all of the strength members in the jacket, and the term “jacket area” refers to the total cross-sectional area for the jacket material. Referring to  FIG. 2 , the conventional design (the lowest data points on the plot, indicated by diamond data points), has lower jacket area to strength member area ratios for various fiber counts. Data describing the cable  10  illustrated in  FIG. 1  correspond to the intermediate values on the plot, and are indicated by square data points. For a cable  10  as shown in  FIG. 1 , with two 1.60 mm diameter strength members on each side of the cable, the ratio for 12-48 fiber count cables lies in the range of 6-8. For 48-72 fiber cables, the ratio lies in the range of 7-9. For 72-96 fiber cables, the ratio lies in the range of 7.5-9.5. For 96-144 fiber cables, the ratio lies in the range of 8-10. 
         [0019]    Jacket area can be further reduced by using only two strength members, of 2.05 mm diameter, one on each side of the jacket  20 . In  FIG. 2 , data describing this cable correspond to the highest values on the plot, and are indicated by round data points. For this embodiment, the jacket to strength member ratio for 12-48 fiber count cables lies in the range of 10-12. For 48-72 fiber cables, the ratio lies in the range of 11-13. For 72-96 fiber cables, the ratio lies in the range of 12-14. For 96-144 fiber cables, the ratio lies in the range of 12-15. 
         [0020]    According to another aspect of the present embodiment, the ratio of the product of elastic modulus E and total area A (EA) for the fibers in the ribbon stack  50  and the strength members  60  is higher than in conventional designs. In this specification, the term “fiber area” refers to the sum of the cross-sectional areas of all of the optical fibers in the cable, including the fiber coatings, and, for ribbonized fibers, includes the total cross-sectional area of the fibers plus coatings in the fiber ribbons. The term “ribbon stack fiber area” could also be used to describe the total cross-sectional area of the optical fibers plus coatings in the fiber ribbons.  FIG. 3  is a plot of fiber area multiplied by fiber elastic modulus (or, “fiber EA”) divided by the strength member EA. The fiber elastic modulus E is typically calculated to include the fiber and coating(s) applied thereto. In  FIG. 3 , data showing the ratio of fiber EA to strength member EA for the cable  10  illustrated in  FIG. 1  is indicated by diamond data points, while data for a conventional cable is indicated by round data points. According to one embodiment, the ratio of fiber EA (or “ribbon stack EA” for ribbonized fibers) to strength member EA is at least 0.0015× fiber count. In the illustrated embodiment, the ratio is about 0.0021× fiber count. Common matrix material used to cover multiple fibers in a fiber optic ribbon has a relatively low elastic modulus and is not used to calculate ribbon stack fiber area or ribbon stack EA. 
         [0021]    The interior  40  of the cable  10  can be filled with a filling compound such as, for example, a waterblocking material such as thixotropic gel or grease. Gel-free designs with or without foam tapes can also be used. 
         [0022]    It is understood in this specification that values for jacket thickness  24 , cable diameter  28 , buffer tube inside diameter  34  and outside diameter  38 , ribbon stack diagonal  54 , strength member diameter  64 , strength member height  68 , etc. may vary to some degree according to manufacturing tolerances. The values in this specification may therefore be considered to be averages for a typical cross-section of the cable. The cross-sections in the cable may not necessarily be perfect geometric shapes; for example, the illustrated circular cross-sections may have some degree of ovality in the manufactured cable. Diameter values may therefore be considered to the average diameter of a cross-section at any point along the length of the cable. 
         [0023]    The cable  10  can be constructed of materials similar to Single-Tube Ribbon (SST-Ribbon™) Cables available from Corning Cable Systems, Inc. of Hickory N.C. The cable  10  can include one or more ripcords (not illustrated). An armored version of the cable  10  can include metallic or dielectric armor coatings. 
         [0024]    The present cable embodiments may utilize tensile yarns as tension relief elements that provide tensile strength to the cables. A preferred material for the tensile yarns is aramid (e.g., KEVLAR®), but other tensile strength materials could be used. For example, high molecular weight polyethylenes such as SPECTRA® fiber and DYNEEMA® fiber, Teijin Twaron® aramids, fiberglass, etc. may also be used. The yarns may be stranded to improve cable performance 
         [0025]    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 fiber optic cable design and/or method of manufacture. For instance, the embodiments shown can include other suitable cable components such as an armor layer, coupling elements, different cross-sectional shapes, or the like. Thus, it is intended that this invention covers these modifications and embodiments as well those also apparent to those skilled in the art.