Patent Publication Number: US-2023161125-A1

Title: Optical fiber cable

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority is claimed on U.S. patent application Ser. No. 17/260,792, which is a National Stage Entry of International Patent Application No. PCT/JP2019/034515, whose priority is claimed on Japanese Patent Application No. 2018-169597 filed in Japan on Sep. 11, 2018, Japanese Patent Application No. 2018-194103 filed in Japan on Oct. 15, 2018, and Japanese Patent Application No. 2018-211366 filed in Japan on Nov 9, 2018, the contents of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an optical fiber cable. 
     BACKGROUND 
     In the related art, an optical fiber cable in which fillings are disposed around the optical fiber unit has been used. 
     For example, in the optical fiber cable of Patent Document 1, a plurality of ribbons are stacked and a unit coating layer is provided around the ribbons to form an optical fiber unit. By providing fillings around the optical fiber unit, it is easy to make the shape of the cross section of the optical fiber cable circular. 
     Further, in the optical fiber cable of Patent Document 2, fillings are disposed so as to be sandwiched between the optical fiber units. Thus, the movement of the optical fiber unit in the optical fiber cable is suppressed. 
     PATENT LITERATURE 
     Patent Document 1 
     Japanese Unexamined Patent Application, First Publication No. 2001-51169 
     Patent Document 2 
     Japanese Patent No. 6255120 
     In this type of optical fiber cable, the optical fiber unit may be twisted in an SZ shape. Here, when the optical fiber units are twisted in an SZ shape, “untwisting” occurs in which the optical fiber unit moves in the direction in which the twisting is canceled. In the optical fiber cable in the related art, the suppression of untwisting may be insufficient. 
     One or more embodiments of the present invention provide an optical fiber cable in which untwisting is suppressed. 
     SUMMARY 
     An optical fiber cable according to one or more embodiments of the present invention includes: a plurality of optical fiber units each having a plurality of optical fibers; a wrapping tube that wraps around the plurality of optical fiber units; at least one filling disposed inside the wrapping tube; and a sheath that covers the wrapping tube, in which a plurality of outer units included in the plurality of optical fiber units that are located in an outermost layer are twisted in an SZ shape around a cable central axis, and the filling is sandwiched between one of the outer units and the wrapping tube in a cross-sectional view. 
     According to the above embodiments of the present invention, it is possible to generate a frictional force between the outer unit and the fillings and between the fillings and the wrapping tube by using a force of an outer unit to expand radially outward. This makes it possible to provide an optical fiber cable in which untwisting is suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view of an optical fiber cable according to a first embodiment. 
         FIG.  2    is a cross-sectional view of an optical fiber cable according to a modified example of the first embodiment. 
         FIG.  3    is a cross-sectional view of an optical fiber cable according to another modification of the first embodiment. 
         FIG.  4    is a cross-sectional view of an optical fiber cable according to a second embodiment. 
         FIG.  5    is a cross-sectional view of an optical fiber cable according to a third embodiment. 
         FIG.  6    is a schematic view showing the dimensions of each part of the optical fiber cable of  FIG.  4   . 
         FIG.  7    is a cross-sectional view of an optical fiber cable according to a modified example of the second embodiment. 
         FIG.  8    is a cross-sectional view of an optical fiber cable according to another modification of the second embodiment. 
         FIG.  9    is a cross-sectional view of an optical fiber cable according to a fourth embodiment. 
         FIG.  10    is a cross-sectional view of an optical fiber cable according to a fifth embodiment. 
         FIG.  11    is a schematic view showing the dimensions of each part of the optical fiber cable of  FIG.  9   . 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     Hereinafter, an optical fiber cable of the first embodiment will be described with reference to the drawings. 
     As illustrated in  FIG.  1   , an optical fiber cable  100  includes a core  20  having a plurality of optical fiber units  10 , a sheath  55  accommodating the core  20  inside, and a pair of tensile strength members  56  (tension members) and a pair of wire bodies  57 , which are embedded in the sheath  55 . The core  20  has a wrapping tube  54  that wraps around a plurality of optical fiber units  10 . 
     Direction Definition 
     In the present embodiment, the central axis of the optical fiber cable  100  is referred to as the cable central axis O. Further, the direction along the cable central axis O (longitudinal direction of the optical fiber unit  10 ) is simply referred to as the longitudinal direction. A cross section orthogonal to the cable central axis O (a cross section orthogonal to the longitudinal direction) is referred to as a cross section. In the cross-sectional view ( FIG.  1   ), the direction intersecting the cable central axis O is called the radial direction, and the direction rotating around the cable central axis O is called the circumferential direction. 
     When the optical fiber cable  100  is non-circular in the cross-sectional view, the cable central axis O is located at the center of the optical fiber cable  100 . 
     The sheath  55  is formed in a cylindrical shape centered on the cable central axis O. As the material of the sheath  55 , polyolefin (PO) resin such as polyethylene (PE), polypropylene (PP), ethylene ethyl acrylate copolymer (EEA), ethylene vinyl acetate copolymer (EVA), and ethylene propylene copolymer (EP), polyvinyl chloride (PVC), or the like can be used. 
     As the material of the wire body  57 , a columnar rod made of PP or nylon can be used. Further, the wire body  57  may be formed of yarns in which fibers such as PP or polyester are twisted, and the wire body  57  may have water absorbency. 
     The pair of wire bodies  57  is disposed so as to sandwich the core  20  in the radial direction. Each wire body  57  is in contact with the outer peripheral surface of the core  20  (the outer peripheral surface of the wrapping tube  54 ). The number of wire bodies  57  embedded in the sheath  55  may be 1 or 3 or more. 
     As the material of the tensile strength member  56 , for example, a metal wire (such as steel wire), a tension fiber (such as aramid fiber), FRP or the like can be used. 
     The pair of tensile strength members  56  is disposed so as to sandwich the core  20  in the radial direction. Further, the pair of tensile strength members  56  is disposed at intervals in the radial direction from the core  20 . The number of tensile strength members  56  embedded in the sheath  55  may be 1 or 3 or more. Further, the tensile strength member  56  may not be embedded in the sheath  55 . 
     A pair of protrusions  58  projecting radially outward is formed on the outer peripheral surface of the sheath  55 . The protrusion  58  extends along the longitudinal direction. 
     The protrusion  58  and the wire body  57  are disposed at the same position in the circumferential direction. The protrusion  58  serves as a mark when the sheath  55  is incised in order to take out the wire body  57 . Instead of the protrusion  58 , a mark indicating the position of the wire body  57  may be provided, for example, by making a part of the sheath  55  different in color from the other parts. 
     The core  20  includes a plurality of optical fiber units  10 , a plurality of fillings  3   a  to  3   c,  and a wrapping tube  54 . The wrapping tube  54  wraps the optical fiber unit  10  and fillings  3   a  to  3   c.  Each of the optical fiber units  10  has a plurality of optical fiber core wires or optical fiber strands (hereinafter, simply referred to as optical fibers  1 ), and a binding material  2  for binding the optical fibers  1 . The optical fiber unit  10  and fillings  3   a  to  3   c  extend along the longitudinal direction. 
     The optical fiber unit  10  of the present embodiment is a so-called intermittently-adhered optical fiber ribbon, and when a plurality of optical fibers  1  are pulled in a direction orthogonal to the longitudinal direction, the optical fibers  1  are adhered to each other so as to spread in a mesh form (spider web shape). Specifically, one optical fiber  1  is adhered to adjacent optical fibers  1  on both sides thereof at different positions in the longitudinal direction, and the adjacent optical fibers  1  are spaced apart from each other at a fixed interval in the longitudinal direction and are adhered to each other. 
     The mode of the optical fiber unit  10  is not limited to the intermittently-adhered optical fiber ribbon, and may be changed as appropriate. For example, the optical fiber unit  10  may be obtained by simply binding the plurality of optical fibers  1  with the binding material  2 . 
     As illustrated in  FIG.  1   , the optical fiber unit  10  is divided into two layers, that is, a radially inner layer and a radially outer layer. Hereinafter, the optical fiber unit  10  located in the outermost layer is referred to as an outer unit  10 A. The optical fiber unit  10  other than the outer unit  10 A is referred to as an inner unit  10 B. That is, the outer unit  10 A and the inner unit  10 B are included in the plurality of optical fiber units  10 . 
     In the example of  FIG.  1   , three inner units  10 B are twisted together in an SZ shape or a spiral shape around the cable central axis O. Further, the nine outer units  10 A are twisted in an SZ shape around the cable central axis O so as to surround the three inner units  10 B. The number of optical fiber units  10  can be changed as appropriate. 
     In the cross-sectional view, the inner unit  10 B located in the inner layer is formed in a fan shape, and the outer unit  10 A located in the outermost layer is formed in square. Not limited to the illustrated example, the optical fiber unit  10  having a circular, elliptical, or polygonal cross section may be used. Further, the cross-sectional shape of the optical fiber unit  10  may be deformed. Further, the core  20  may be composed of one layer (layer of the outer unit  10 A) without the inner unit  10 B. 
     The binding material  2  has a long string shape and is wound around the plurality of optical fibers  1 . The optical fiber  1  is partially exposed from the gap between the binding materials  2 . Therefore, when the sheath  55  is incised and the wrapping tube  54  is removed, it is possible to visually recognize the optical fiber  1  from the gap between the binding materials  2 . The binding material  2  is made of a thin and highly flexible material such as resin. Therefore, even in the state where the optical fibers  1  are bound with the binding material  2 , the optical fibers  1  are appropriately moved to a vacant space in the sheath  55  while deforming the binding material  2 . Therefore, the cross-sectional shape of the optical fiber unit  10  in an actual product may not be arranged as illustrated in  FIG.  1   . 
     The wrapping tube  54  is formed in a cylindrical shape centered on the cable central axis O. The inner peripheral surface of the wrapping tube  54  is in contact with the radially outer end of the outer unit  10 A. Further, the inner peripheral surface of the wrapping tube  54  is in contact with the fillings  3   b  and  3   c.  As the wrapping tube  54 , a non-woven fabric, a plastic tape member, or the like can be used. The wrapping tube  54  may be made of materials having water absorbency, such as a water-absorbent tape. 
     The fillings  3   a  to  3   c  are formed of a fibrous material such as polyester fiber, aramid fiber, and glass fiber. In addition, the fillings  3   a  to  3   c  may be yarns having water absorbency or the like. In this case, it is possible to enhance the waterproof performance inside the optical fiber cable  100 . 
     In the cross-sectional view, the fillings  3   a  are sandwiched between the outer unit  10 A and the inner unit  10 B. The filling  3   b  is sandwiched between the outer units  10 A adjacent to each other in the circumferential direction, and are in contact with the wrapping tube  54 . The filling  3   c  is sandwiched between one outer unit  10 A and the wrapping tube  54 . 
     The filling  3   a  is twisted together with the inner unit  10 B. The fillings  3   b,    3   c  are twisted together with the outer unit  10 A. 
     The fillings  3   b,    3   c  are in contact with the outer unit  10 A. The filling  3   a  is in contact with the outer unit  10 A and the inner unit  10 B. Here, the binding material  2  has a thin and long string shape, and is wound around a bundle of the optical fibers  1  in a spiral shape, for example. Therefore, a part of the optical fiber  1  which is not covered with the string-shaped binding material  2  is partially in contact with the fillings  3   a  to  3   c.    
     The optical fiber  1  usually has a structure in which an optical fiber bare fiber formed of glass is coated with a coating material such as a resin. Therefore, the surface of the optical fiber  1  is smooth, and the friction coefficient when the optical fibers  1  come into contact with each other is relatively small. On the other hand, fillings  3   a  to  3   c  are formed of a fibrous material. Therefore, the friction coefficient when the fillings  3   a  to  3   c  are in contact with the optical fibers  1  is larger than the friction coefficient when the optical fibers  1  are in contact with each other. 
     From the above, it is possible to increase the frictional resistance when the optical fiber units  10  move relative to each other, by disposing the fillings  3   a  to  3   c  so as to be sandwiched between the plurality of optical fiber units  10 . This makes it possible to suppress the movement of the optical fiber unit  10  in the optical fiber cable  100 . 
     Incidentally, the plurality of optical fiber units  10  are twisted together, with the cable central axis O as the center of twisting. When the optical fiber unit  10  tends to untwist, the bundle of the optical fiber unit  10  tends to expand radially outward. That is, the outer unit  10 A is pressed against the wrapping tube  54  by the force trying to untwist. Here, in the present embodiment, fillings  3   b  and  3   c  are sandwiched between the outer unit  10 A and the wrapping tube  54  in the cross-sectional view. 
     According to this configuration, when the bundle of the optical fiber unit  10  tends to expand radially outward, fillings  3   b  and  3   c  are compressed in the radial direction between the outer unit  10 A and the wrapping tube  54 . That is, the fillings  3   b  and  3   c  twisted together with the outer unit  10 A are pressed against the wrapping tube  54 . Since the fillings  3   b  and  3   c  are formed of a fibrous material, the friction coefficient between the optical fiber  1  and the fillings  3   b  and  3   c,  and the friction coefficient between the fillings  3   b  and  3   c  and the wrapping tube  54  are larger than the friction coefficient between the optical fiber  1  and the wrapping tube  54 . Therefore, the frictional force generated when the outer unit  10 A is pressed against the wrapping tube  54  with the fillings  3   b  and  3   c  sandwiched between them is larger than the frictional force generated when the outer unit  10 A is directly pressed against the wrapping tube  54 . 
     That is, in the present embodiment, when the outer unit  10 A tends to expand radially outward, the fillings  3   b  and  3   c  generate a large frictional force. Due to this frictional force, the outer unit  10 A is less likely to move with respect to the wrapping tube  54 , and it is possible to suppress the untwisting of the outer unit  10 A. 
     Further, in the present embodiment, the filling  3   c  is located on the straight line L passing through the center point X of the outer unit  10 A and the cable central axis O in the cross-sectional view. With this configuration, the force that the outer unit  10 A tends to expand radially outward can be more efficiently converted into a frictional force. Therefore, it is possible to more reliably suppress the untwisting of the outer unit  10 A. 
     Further, in the present embodiment, in the cross-sectional view, the filling  3   c  is surrounded by one outer unit  10 A and the wrapping tube  54 . Therefore, when the bundle of the optical fiber unit  10  tends to expand radially outward, the fillings  3   c  are more reliably sandwiched between the outer unit  10 A and the wrapping tube  54 . Further, the outer unit  10 A prevents the fillings  3   c  from moving radially inward, so that the state in which the fillings  3   c  are in contact with the wrapping tube  54  can be more reliably maintained. Therefore, it is possible to more reliably generate the frictional force due to the fillings  3   c,  and to suppress the untwisting. 
     The center point X in the present specification is the center of the outer unit  10 A in a cross-sectional view. Since the outer unit  10 A is twisted around the cable central axis O, the outer unit  10 A tends to expand radially outward due to untwisting. The direction in which the outer unit  10 A expands is a direction that starts from the cable central axis O and passes through the center point X (center of the outer unit  10 A). Therefore, by locating the fillings  3   c  on the straight line L passing through the cable central axis O and the center point X, the frictional force generated by the fillings  3   c  due to the force that the outer unit  10 A tends to expand becomes large, and it is possible to effectively suppress the untwisting. 
     EXAMPLES 
     Hereinafter, the above first embodiment will be described with reference to specific examples. The present invention is not limited to the following examples. 
     Example 1 
     As Example 1, an optical fiber cable having a cross-sectional structure as illustrated in  FIG.  1    is prepared. 
     The number of optical fibers  1  included in each optical fiber unit  10  is 144. The three inner units  10 B are twisted in an SZ shape, and the nine outer units  10 A are twisted in an SZ shape on the outer circumference thereof. That is, the total number of optical fiber units  10  is 12, and the total number of optical fibers  1  is 1728. Water-absorbent yarns are used as fillings  3   a,    3   b,  and  3   c.  Three fillings  3   a,  eight fillings  3   b,  and one filling  3   c  are disposed. 
     The set angle of the twisting device (oscillator) when twisting the optical fiber unit  10  is adjusted such that the twist angle (introduction angle) actually introduced is ±150°. The “set angle” is in a range of angles at which the oscillator is oscillated. For example, when the set angle is ±500°, the oscillator repeats the operation of oscillating 500° in the CW direction and then oscillating 500° in the CCW direction. 
     The manufactured optical fiber cable is cut at predetermined intervals in the longitudinal direction, and the position of the specific outer unit  10 A or the optical fiber  1  included in the outer unit  10 A in the circumferential direction is measured on each cut surface. The rotation angle of a specific outer unit  10 A or the optical fiber  1  included in the outer unit  10 A with respect to the cable central axis O is defined as the introduction angle. The larger the difference between the set angle and the introduction angle, the larger the outer unit  10 A is untwisted. 
     The twisted optical fiber unit  10  is wrapped with a wrapping tube  54  and further covered with a sheath  55  to prepare an optical fiber cable. 
     Example 2 
     As Example 2, an optical fiber cable is prepared in which the number of fillings  3   b  and  3   c  is changed from Example 1. Three fillings  3   a,  six fillings  3   b,  and three fillings  3   c  are disposed. Other conditions are the same as in Example 1. 
     Example 3 
     As Example 3, an optical fiber cable is prepared in which the numbers of the fillings  3   a,    3   b,  and  3   c  are changed from those in Example 1. No fillings  3   a  and  3   b  are disposed, and only six fillings  3   c  are disposed. Other conditions are the same as in Example 1. 
     Example 4 
     As Example 4, an optical fiber cable is prepared in which the numbers of the fillings  3   a,    3   b,  and  3   c  are changed from those in Example 1. The fillings  3   a  are not disposed, and six fillings  3   b  and three fillings  3   c  are disposed. Further, three fillings  3   d  as illustrated in  FIG.  2    are disposed. The fillings  3   d  are radially sandwiched between the inner unit  10 B and the outer unit  10 A. Each filling  3   d  is disposed between one outer unit  10 A and one inner unit  10 B. Other conditions are the same as in Example 1. 
     Example 5 
     As Example 5, an optical fiber cable is prepared in which the numbers of the fillings  3   a,    3   b,  and  3   c  are changed from those in Example 1. The fillings  3   b  are not disposed, and three fillings  3   a  and nine fillings  3   c  are disposed. Other conditions are the same as in Example 1. 
     Comparative Example 1 
     As Comparative Example 1, an optical fiber cable  100  provided with fillings  3   a  and  3   b  without fillings  3   c  is prepared. Three fillings  3   a  and nine fillings  3   b  are disposed. Other conditions are the same as in Example 1. 
     Table 1 shows the results of checking the introduction angle and sheath twisting of each of the optical fiber cables of Examples 1 to 5 and Comparative Example 1. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Number of fillings (pieces) 
                 Set 
                 Introduction 
                 Sheath 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 3a 
                 3b 
                 3c 
                 3d 
                 Total 
                 angle[°] 
                 angle[°] 
                 twisting 
                 Determination 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 3 
                 8 
                 1 
                 0 
                 12 
                 ±500 
                 ±150 
                 ±10°  
                 OK 
               
               
                 Example 2 
                 3 
                 6 
                 3 
                 0 
                 12 
                 ±400 
                 ±150 
                 ±5° 
                 OK 
               
               
                 Example 3 
                 0 
                 0 
                 6 
                 0 
                 6 
                 ±400 
                 ±150 
                 ±5° 
                 OK 
               
               
                 Example 4 
                 0 
                 6 
                 3 
                 3 
                 12 
                 ±400 
                 ±150 
                 ±5° 
                 OK 
               
               
                 Example 5 
                 3 
                 0 
                 9 
                 0 
                 12 
                 ±300 
                 ±150 
                 ±4° 
                 OK 
               
               
                 Comparative 
                 3 
                 9 
                 0 
                 0 
                 12 
                 ±700 
                 ±150 
                 ±45°  
                 NG 
               
               
                 Example 1 
               
               
                   
               
            
           
         
       
     
     “Sheath twisting” in Table 1 indicates the degree of sheath twisting in the prepared optical fiber cable. More specifically, it shows how much the position of the protrusion  58  in the circumferential direction changes along the longitudinal direction. For example, when the sheath twisting is ±10°, the position of the protrusion  58  in the circumferential direction changes within a range of ±10° around the cable central axis O. When the degree of sheath twisting is large, the optical fiber cable meanders, leading to a decrease in installing workability and a decrease in the length of an optical cable that can be wound around the drum. 
     In the “Determination” field, the result is good (OK) when the sheath twisting is ±10° or less, and the result is insufficient (NG) when the sheath twisting exceeds ±10°. The sheath twisting increases as the set angle increases. This is because the larger the set angle, the stronger the twisted optical fiber unit  10  tends to untwist, and the sheath  55  is twisted around the cable central axis O. 
     As shown in Table 1, in Examples 1 to 5, the sheath twisting is ±10° or less, and good results are obtained. On the other hand, in Comparative Example 1 in which the fillings  3   c  are not disposed, the sheath twisting is ±45°, and the result is insufficient. 
     It is considered that the reason why good results are obtained in Examples 1 to 5 is that the set angle for setting the introduction angle to ±150° is ±500° or less, and the set angle is relatively small. The reason why the set angle is able to be reduced in such a manner is that the untwisting of the outer unit  10 A can be reduced by the filling  3   c.  That is, when the optical fiber unit  10  including the outer unit  10 A tends to untwist and expand radially outward, the filling  3   c  is sandwiched between the outer unit  10 A and the wrapping tube  54  to generate a frictional force. 
     On the other hand, in Comparative Example 1, since the fillings  3   c  are not provided, the frictional force generated between the outer unit  10 A and the wrapping tube  54  when the optical fiber unit  10  tends to untwist is relatively small. Therefore, untwisting is likely to occur, and the set angle for setting the introduction angle to ±150° is ±700°, and the set angle is relatively large. Then, it is considered that the larger the set angle, the stronger the force with which the outer unit  10 A twists the sheath  55 , so that the angle of the sheath twisting becomes larger. 
     From the above results, it is checked that it is possible to reduce the untwisting of the outer unit  10 A by providing at least one filling  3   c  on the straight line L passing through the cable central axis O and the outer unit  10 A. Further, it is found that as a result of reducing the untwisting of the outer unit  10 A, it is possible to reduce the set angle, and to suppress the twisting generated in the sheath  55 . 
     Further, comparing Example 2 and Example 5, the total number of fillings  3   b  and  3   c  in contact with the wrapping tube  54  is the same, but the set angle for setting the introduction angle to ±150° is smaller in Example 5. Further, the twisting generated in the sheath  55  is also smaller in Example 5. That is, untwisting is more effectively suppressed in Example 5 than in Example 2. This is because that the fillings  3   c  are located on a straight line passing through the cable central axis O and the center point X of the outer unit  10 A, so that the fillings  3   c  are less likely to move radially inward, and it is possible to more reliably maintain the state in which the fillings  3   c  are in contact with the wrapping tube  54 . Thus, it is possible to effectively convert the force that the outer unit  10 A tends to expand radially outward into a frictional force. 
     Further, in Example 3, good result is obtained even when the total number of fillings is smaller, as compared with the other Examples 1, 2, 4, and 5. Then, in Example 3, only the fillings  3   c  are disposed. From this result, it is checked that the effect of suppressing untwisting by the fillings  3   c  is larger than that of other fillings. 
     It should be noted that the technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. 
     For example, in the example of  FIG.  1   , the core  20  includes a two-layer optical fiber unit  10 . However, the number of layers of the optical fiber unit included in the core  20  may be 1 or 3 or more. 
     Further, when the core  20  includes a plurality of layers of optical fiber units, no fillings may be disposed between the optical fiber units (inner unit  10 B in the example of  FIG.  1   ) included in the layers other than the outermost layer. 
     Further, in the above-described embodiments, the filling  3   c  is sandwiched between one outer unit  10 A and the wrapping tube  54 . However, as illustrated in  FIG.  3   , the fillings  3   c  may be sandwiched between the plurality of outer units  10 A and the wrapping tube  54 . Even in this case, due to the force that the outer unit  10 A tends to expand radially outward, it is possible to generate a frictional force between the outer unit  10 A and the fillings  3   c  and between the fillings  3   c  and the wrapping tube  54 . Further, since the fillings  3   c  are located on the straight line L passing through the cable central axis O and the center point X of the outer unit  10 A, the force that the outer unit  10 A tends to expand radially outward is able to be converted into frictional force more efficiently. Therefore, it is possible to more reliably suppress the untwisting of the outer unit  10 A. 
     Second Embodiment 
     Hereinafter, an optical fiber cable of a second embodiment will be described with reference to the drawings. The same members as in the first embodiment are denoted by the same reference numerals, and a description thereof is omitted. 
     As illustrated in  FIG.  4   , an optical fiber cable  100 A includes a core  20  having a plurality of optical fiber units  10 , a sheath  55  accommodating the core  20  inside, and a pair of tensile strength members  56  (tension members) and a pair of wire bodies  57 , which are embedded in the sheath  55 . The core  20  has a wrapping tube  54  that wraps around a plurality of optical fiber units  10 . 
     Direction Definition 
     In the present embodiment, the central axis of the optical fiber cable  100 A is referred to as the cable central axis O. Further, the longitudinal direction of the optical fiber cable  100 A (longitudinal direction of the optical fiber unit  10 ) is simply referred to as the longitudinal direction. A cross section orthogonal to the longitudinal direction (a cross section orthogonal to the cable central axis O) is called a cross section. In the cross-sectional view ( FIG.  4   ), the direction intersecting the cable central axis O is called the radial direction, and the direction rotating around the cable central axis O is called the circumferential direction. 
     When the optical fiber cable  100 A is non-circular in the cross-sectional view, the cable central axis O is located at the center of the optical fiber cable  100 A. 
     The sheath  55  is formed in a cylindrical shape centered on the cable central axis O. As the material of the sheath  55 , polyolefin (PO) resin such as polyethylene (PE), polypropylene (PP), ethylene ethyl acrylate copolymer (EEA), ethylene vinyl acetate copolymer (EVA), and ethylene propylene copolymer (EP), polyvinyl chloride (PVC), or the like can be used. 
     As the material of the wire body  57 , a cylindrical rod made of PP, nylon, or the like can be used. Further, the wire body  57  may be formed of yarns in which fibers such as PP or polyester are twisted, and the wire body  57  may have water absorbency. 
     The pair of wire bodies  57  is disposed so as to sandwich the core  20  in the radial direction. Each wire body  57  is in contact with the outer peripheral surface of the core  20  (the outer peripheral surface of the wrapping tube  54 ). The number of wire bodies  57  embedded in the sheath  55  may be 1 or 3 or more. 
     As the material of the tensile strength member  56 , for example, a metal wire (such as steel wire), a tension fiber (such as aramid fiber), FRP or the like can be used. 
     The pair of tensile strength members  56  is disposed so as to sandwich the core  20  in the radial direction. Further, the pair of tensile strength members  56  is disposed at intervals in the radial direction from the core  20 . The number of tensile strength members  56  embedded in the sheath  55  may be 1 or 3 or more. Further, the tensile strength member  56  may not be embedded in the sheath  55 . 
     A pair of protrusions  58  projecting radially outward is formed on the outer peripheral surface of the sheath  55 . The protrusion  58  extends along the longitudinal direction. 
     The protrusion  58  and the wire body  57  are disposed at the same position in the circumferential direction. The protrusion  58  serves as a mark when the sheath  55  is incised in order to take out the wire body  57 . Instead of the protrusion  58 , a mark indicating the position of the wire body  57  may be provided, for example, by making a part of the sheath  55  different in color from the other parts. 
     The core  20  includes a plurality of optical fiber units  10 , a plurality of fillings  13   a  to  13   d,  and a wrapping tube  54 . The wrapping tube  54  wraps the optical fiber unit  10  and fillings  13   a  to  13   d.  Each of the optical fiber units  10  has a plurality of optical fiber core wires or optical fiber strands (hereinafter, simply referred to as optical fibers  1 ), and a binding material  2  for binding the optical fibers  1 . The optical fiber unit  10  and fillings  13   a  to  13   d  extend along the longitudinal direction. 
     The optical fiber unit  10  of the present embodiment is a so-called intermittently-adhered optical fiber ribbon, and when a plurality of optical fibers  1  are pulled in a direction orthogonal to the longitudinal direction, the optical fibers  1  are adhered to each other so as to spread in a mesh form (spider web shape). Specifically, one optical fiber  1  is adhered to adjacent optical fibers  1  on both sides thereof at different positions in the longitudinal direction, and the adjacent optical fibers  1  are spaced apart from each other at a fixed interval in the longitudinal direction and are adhered to each other. The mode of the optical fiber unit  10  is not limited to the intermittently-adhered optical fiber ribbon, and may be changed as appropriate. For example, the optical fiber unit  10  may be obtained by simply binding the plurality of optical fibers  1  with the binding material  2 . 
     As illustrated in  FIG.  4   , the optical fiber units  10  are disposed so as to be divided into two layers, that is, a radially inner layer and a radially outer layer. In the present specification, the optical fiber unit  10  located in the outermost layer is referred to as an outer unit  10 A. Further, the optical fiber unit  10  located radially inside the outer unit  10 A is referred to as an inner unit  10 B. 
     In the example of  FIG.  4   , three inner units  10 B are twisted together in an SZ shape or a spiral shape around the cable central axis O. Further, the nine outer units  10 A are twisted in an SZ shape around the cable central axis O so as to surround the three inner units  10 B. The number of optical fiber units  10  can be changed as appropriate. 
     In the cross-sectional view, the inner unit  10 B located in the inner layer is formed in a fan shape, and the outer unit  10 A located in the outermost layer is formed in square. In addition, the present invention is not limited to the illustrated example, the optical fiber unit  10  having a circular, elliptical, or polygonal cross section may be used. Further, the cross-sectional shape of the optical fiber unit  10  may be deformed. Further, the core  20  may be composed of one layer (layer of the outer unit  10 A) without the inner unit  10 B. 
     The binding material  2  has a long string shape and is wound around the plurality of optical fibers  1 . The optical fiber  1  is partially exposed from the gap between the binding materials  2 . Therefore, when the sheath  55  is incised and the wrapping tube  54  is removed, it is possible to visually recognize the optical fiber  1  from the gap between the binding materials  2 . The binding material  2  is made of a thin and highly flexible material such as resin. Therefore, even in the state where the optical fibers  1  are bound with the binding material  2 , the optical fibers  1  are appropriately moved to a vacant space in the sheath  55  while deforming the binding material  2 . Therefore, the cross-sectional shape of the optical fiber unit  10  in an actual product may not be arranged as illustrated in  FIG.  4   . 
     The wrapping tube  54  is formed in a cylindrical shape centered on the cable central axis O. The inner peripheral surface of the wrapping tube  54  is in contact with the radially outer end of the outer unit  10 A. Further, the inner peripheral surface of the wrapping tube  54  is in contact with the fillings  13   a.  As the wrapping tube  54 , a non-woven fabric, a plastic tape member, or the like can be used. The wrapping tube  54  may be made of materials having water absorbency, such as a water-absorbent tape. 
     The fillings  13   a  to  13   d  are formed of a fibrous material such as polyester fiber, aramid fiber, and glass fiber. The fillings  13   a  to  13   d  may be yarns having water absorbency or the like. In this case, it is possible to enhance the waterproof performance inside the optical fiber cable  100 A. 
     In the cross-sectional view, the fillings  13   a  are sandwiched between the outer units  10 A adjacent to each other in the circumferential direction and are in contact with the inner peripheral surface of the wrapping tube  54 . The filling  13   a  is disposed between two outer units  10 A and the wrapping tube  54 . The fillings  13   b  are sandwiched between the outer units  10 A adjacent to each other in the circumferential direction, but are not in contact with the wrapping tube  54 . The fillings  13   a  and  13   b  are twisted together with the outer unit  10 A in an SZ shape around the cable central axis O. 
     The fillings  13   c  are sandwiched between the inner units  10 B adjacent to each other in the circumferential direction. The fillings  13   c  are located radially inside the fillings  13   a  and  13   b,  and are not in contact with the inner peripheral surface of the wrapping tube  54 . The fillings  13   c  are twisted together with the inner unit  10 B in an SZ shape or a spiral shape around the cable central axis O. The fillings  13   c  may not be disposed. 
     The filling  13   d  is located at the center of the optical fiber cable  100 A. In the example of  FIG.  4   , one filling  13   d  is disposed coaxially with the cable central axis O. However, as illustrated in  FIG.  7   , a plurality of fillings  13   d  may be disposed at the center of the optical fiber cable  100 A. Further, the fillings  13   d  may not be located coaxially with the cable central axis O. The fillings  13   d  may be twisted together with the inner unit  10 B in an SZ shape or a spiral shape around the cable central axis O. Alternatively, the fillings  13   d  may not be twisted together with the inner unit  10 B. Further, the fillings  13   d  may not be disposed. 
     The fillings  13   a  and  13   b  are in contact with the outer unit  10 A. The fillings  13   c  and  13   d  are in contact with the inner unit  10 B. Here, the binding material  2  has a thin and long string shape, and is wound around a bundle of the optical fibers  1  in a spiral shape, for example. Therefore, a part of the optical fiber  1  which is not covered with the string-shaped binding material  2  is partially in contact with the fillings  13   a  to  13   d.    
     The optical fiber  1  usually has a structure in which an optical fiber bare fiber formed of glass is coated with a coating material such as a resin. Therefore, the surface of the optical fiber  1  is smooth, and the friction coefficient when the optical fibers  1  come into contact with each other is relatively small. On the other hand, the fillings  13   a  to  13   d  are formed of a fibrous material. Therefore, the friction coefficient when the fillings  13   a  to  13   d  are in contact with the optical fibers  1  is larger than the friction coefficient when the optical fibers  1  are in contact with each other. 
     From the above, it is possible to increase the frictional resistance when the optical fiber units  10  move relative to each other, by disposing the fillings  13   a  to  13   d  so as to be sandwiched between the plurality of optical fiber units  10 . This makes it possible to suppress the movement of the optical fiber unit  10  in the optical fiber cable  100 A. 
     Third Embodiment 
     Hereinafter, an optical fiber cable of a third embodiment will be described with reference to the drawings. The same members as in the first embodiment are denoted by the same reference numerals, and a description thereof is omitted. 
       FIG.  5    indicates an optical fiber cable  100 B according to the third embodiment. The third embodiment has the same basic configuration as the second embodiment, but the optical fiber cable  100 B is different from the optical fiber cable  100 A of  FIG.  4    in having fillings  3   c.    
     In the optical fiber cable  100 B, the core  20  includes a plurality of optical fiber units  10 , a plurality of fillings  13   a  to  13   c,    3   c,  and a wrapping tube  54 . The wrapping tube  54  wraps the optical fiber unit  10  and the fillings  13   a  to  13   c,    3   c.    
     The filling  3   c  is sandwiched between one outer unit  10 A and the wrapping tube  54 . The fillings  3   c  are twisted together with the outer unit  10 A in an SZ shape. The fillings  3   c  are in contact with the wrapping tube  54  and the outer unit  10 A. 
     Further, the part of the optical fiber  1  which is not covered with the string-shaped binding material  2  is partially in contact with the filling  3   c.    
     Further, in the cross-sectional view, the fillings  3   c  may be located on the straight line L passing through the center point X of the outer unit  10 A and the cable central axis O. 
     Incidentally, in the second and third embodiments, the outer unit  10 A is twisted in an SZ shape. As a result, when the optical fiber cables  100 A and  100 B are bent, it is possible to improve the workability of the mid-span branching, while suppressing the action of tension or strain on the optical fiber  1  included in the outer unit  10 A. 
     On the other hand, when the outer unit  10 A is twisted in an SZ shape, it may be insufficient to suppress the untwisting of the outer unit  10 A. When a compressive force acts on the optical fiber cables  100 A and  100 B, it is also required to suppress the lateral pressure acting on the outer unit  10 A. 
     Therefore, in the second and third embodiments, the amounts of fillings  13   a  and  13   b  disposed between the outer units  10 A and the fillings  3   c  disposed between one outer unit  10 A and the wrapping tube  54  are optimized. Hereinafter, the second and third embodiments will be described with reference to specific examples. The present invention is not limited to the following examples. 
     Positions of Fillings 
     First, the result of checking the effect of disposing the fillings  13   a  and  13   b  between the outer units  10 A will be described. Here, eight optical fiber cables (Examples 6 to 9 and Comparative Examples 2 to 5) shown in Table 2 are prepared. In Examples 6 to 9 and Comparative Examples 2 to 5, water-absorbent yarns are used as fillings  13   a  to  13   d . 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Number of fillings (pieces) 
                 Set 
                 Introduction 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 13a 
                 13b 
                 13c 
                 13d 
                 angle[°] 
                 angle[°] 
                 Determination 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 6 
                 8 
                 0 
                 0 
                 0 
                 ±400 
                 ±150 
                 OK 
               
               
                 Example 7 
                 5 
                 0 
                 3 
                 0 
                 ±500 
                 ±150 
                 OK 
               
               
                 Example 8 
                 1 
                 0 
                 3 
                 4 
                 ±600 
                 ±150 
                 OK 
               
               
                 Example 9 
                 1 
                 4 
                 3 
                 0 
                 ±500 
                 ±150 
                 OK 
               
               
                 Comparative 
                 0 
                 0 
                 3 
                 5 
                 ±600 
                 ±75  
                 NG 
               
               
                 Example 2 
               
               
                 Comparative 
                 0 
                 0 
                 6 
                 2 
                 ±600 
                 ±75  
                 NG 
               
               
                 Example 3 
               
               
                 Comparative 
                 0 
                 0 
                 3 
                 0 
                 ±600 
                 ±70  
                 NG 
               
               
                 Example 4 
               
               
                 Comparative 
                 0 
                 4 
                 3 
                 1 
                 ±600 
                 ±90  
                 NG 
               
               
                 Example 5 
               
               
                   
               
            
           
         
       
     
     Example 6 
     In the optical fiber cable of Example 6, the number of optical fibers  1  included in one optical fiber unit  10  is 144. The three inner units  10 B are twisted in an SZ shape, and the nine outer units  10 A are twisted in an SZ shape on the outer circumference thereof. That is, the total number of optical fiber units  10  is 12, and the total number of optical fibers  1  is 1728. Eight fillings  13   a  are provided, but fillings  13   b  to  13   d  are not provided. One filling  13   a  is disposed each between the outer units  10 A. 
     The optical fiber unit  10  is twisted with the set angle of the twisting device (oscillator) set to ±400°. The “set angle” is in a range of angles at which the oscillator is oscillated. For example, when the set angle is ±400°, the oscillator repeats the operation of oscillating 400° in the CW direction and then oscillating 400° in the CCW direction. In this way, the twisted optical fiber unit  10  is wrapped with a wrapping tube  54  and further covered with a sheath  55  to prepare an optical fiber cable. 
     Example 7 
     As Example 7, an optical fiber cable is prepared in which the number of fillings  13   a  to  13   d  is changed from Example 6. Five fillings  13   a  are provided and three fillings  13   c  are provided. The set angle is ±500°. Other conditions are the same as in Example 6. 
     Example 8 
     As Example 8, an optical fiber cable is prepared in which the number of fillings  13   a  to  13   d  is changed from Example 6. As illustrated in  FIG.  7   , one filling  13   a,  three fillings  13   c,  and four fillings  13   d  are provided. Among the four fillings  13   d,  one filling is disposed coaxially with the cable central axis O, and the remaining three fillings are disposed along the circumference of the one filling. The set angle is ±600°. Other conditions are the same as in Example 6. 
     Example 9 
     As Example 9, an optical fiber cable is prepared in which the number of fillings  13   a  to  13   d  is changed from Example 6. As illustrated in  FIG.  8   , one filling  13   a,  four fillings  13   b,  and three fillings  13   c  are provided. No fillings  13   d  are provided. The set angle is ±500°. Other conditions are the same as in Example 6. 
     Comparative Example 2 
     As Comparative Example 2, an optical fiber cable  100 A provided with three fillings  13   c  and five fillings  13   d  without fillings  13   a  and  13   b  is prepared. The set angle is ±600°. Other conditions are the same as in Example 6. 
     Comparative Example 
     As Comparative Example 3, an optical fiber cable  100 A is prepared in which the numbers of fillings  13   c  and  13   d  are changed from Comparative Example 2. Other conditions are the same as in Comparative Example 2. 
     Comparative Example 4 
     As Comparative Example 4, an optical fiber cable  100 A is prepared in which the numbers of fillings  13   c  and  13   d  are changed from Comparative Example 2. Three fillings  13   c  are provided and no fillings  13   d  are provided. Other conditions are the same as in Comparative Example 2. 
     Comparative Example 5 
     As Comparative Example 5, an optical fiber cable  100 A is prepared in which the number of fillings  13   b  to  13   d  are changed from Comparative Example 2. Four fillings  13   b,  three fillings  13   c,  and one filling  13   d  are provided. Other conditions are the same as in Comparative Example 2. 
     Table 2 shows the results of checking the SZ twist angle (introduction angle) actually introduced into the outer unit  10 A, for the optical fiber cables of Examples 6 to 9 and Comparative Examples 2 to 5. The manufactured optical fiber cable is cut at predetermined intervals in the longitudinal direction, and the position of a specific optical fiber or optical fiber unit in the circumferential direction is measured on each cut surface. The rotation angle of a specific optical fiber or optical fiber unit with respect to the cable central axis O is defined as the introduction angle. The larger the difference between the set angle and the introduction angle, the larger the outer unit  10 A is untwisted. 
     In the “Determination” field of Table 2, the result is good (OK) when the introduction angle is ±135° or more, and the result is insufficient (NG) when the introduction angle is less than ±135°. The reason why the determination criterion is that the introduction angle is ±135° or more is as follows. For example, in a case where the outer unit  10 A is not twisted, when the optical fiber cable is bent, the outer unit  10 A is compressed inside the bend of the optical fiber cable and stretched outside the bend of the optical fiber cable. On the other hand, when the outer unit  10 A is twisted in an SZ shape at an introduction angle of ±135° or more, one outer unit  10 A is reliably disposed across both the compressed part and the stretched part. Since the introduction angle of ±135° or more is satisfied, it is possible to cancel out the tension and compression acting on the outer unit  10 A, and to suppress the tension or strain acting on the optical fiber  1 . 
     As shown in Table 2, it is possible to make the introduction angles of Examples 6 to 9 larger than the introduction angles of Comparative Examples 2 to 5. Further, in Examples 6 to 9, the introduction angle is ±135° or more, and good results are obtained. This is because the fillings  13   a  are in contact with the wrapping tube  54 , and the frictional force between the fillings  13   a  and the wrapping tube  54  is able to suppress the outer unit  10 A from untwisting. 
     From the comparison between Examples 6 to 9 and Comparative Examples 2 to 5, it is checked that the fillings  13   a  in contact with the wrapping tube  54  are able to suppress the untwisting of the outer unit  10 A located in the outermost layer. 
     Further, from the comparison between Example 9 and Comparative Example 5, it is checked that it is possible to obtain a large untwisting suppressing effect by providing at least one filling  13   a.    
     Further, from the comparison between Example 8 and Example 9, the filling  13   b  sandwiched between the outer units  10 A has a more effective effect of suppressing untwisting than the filling  13   d  sandwiched between the inner units  10 B. 
     Further, from Comparative Examples 2 to 5, it is checked that the untwisting suppressing effect is less affected by the change in the number and arrangement of the fillings  13   b  to  13   d.    
     Further, in the optical fiber cable  100 B of the third embodiment, fillings  3   c  are disposed in addition to fillings  13   a  and  13   b.  The filling  3   c  is sandwiched between one outer unit  10 A and the wrapping tube  54 . Therefore, the fillings  3   c  are less likely to move radially inward, and it is possible to more reliably maintain the state in which the fillings  3   c  are in contact with the wrapping tube  54 . Thus, it is possible to effectively convert the force that the outer unit  10 A tends to expand radially outward into a frictional force, and to obtain a more reliable untwisting suppressing effect. 
     Next, the result of examining the optimum density when fillings  13   a,    13   b,  and  3   c  are provided will be described. 
     Here, the parameter of “Outer layer filling density D” is used. The outer layer filling density D is the density of fillings sandwiched between the outer units  10 A among the plurality of optical fiber units  10  included in the core. 
     Here, the outer layer filling density D will be described in more detail with reference to  FIG.  6   . The virtual circle C 1  illustrated in  FIG.  6    is an arc connecting the radially inner ends of the plurality of outer units  10 A located in the outermost layer. The virtual circle C 2  is an arc connecting the radially outer ends of the plurality of outer units  10 A located in the outermost layer. The virtual circle C 2  substantially overlaps the inner peripheral surface of the wrapping tube  54 . 
     Dimension r 1  is the radius of the virtual circle C 1  and dimension r 2  is the radius of the virtual circle C 2 . In other words, the dimension r 1  is the distance between the radially inner end of the outer unit  10 A located in the outermost layer and the cable central axis O. The dimension r 2  is the distance between the radially outer end of the outer unit  10 A located in the outermost layer (the inner circumferential surface of the wrapping tube  54 ) and the cable central axis O. 
     Regarding the plurality of outer units  10 A located in the outermost layer, the positions of the radially inner ends may be non-uniform (the virtual circle C 1  in  FIG.  6    is non-circular). In that case, the average value of the distance between the radially inner end of each outer unit  10 A and the cable central axis O is defined as the dimension r 1 . The same applies when the virtual circle C 2  is non-circular. That is, the average value of the distance between the radially outer end of each outer unit  10 A and the cable central axis O is defined as the dimension r 2 . 
     Here, the twisted states are different in the outermost layer (layer of the outer unit  10 A) and the inner layer (layer of the inner unit  10 B). Further, the fillings  13   a,    13   b , and  3   c  located in the outermost layer and the fillings  13   c  and  13   d  located in the inner layer have different roles. More specifically, the fillings  13   a  and  3   c  are in contact with the wrapping tube  54  to suppress untwisting. Further, although the fillings  13   b  do not come into contact with the wrapping tube  54 , the fillings  13   b  are sandwiched between the outer units  10 A and have the effect of suppressing the relative movement of the outer units  10 A. On the other hand, since the fillings  13   c  and  13   d  are not in contact with the wrapping tube  54  and are not sandwiched between the outer units  10 A, the effect of suppressing the untwisting of the outer unit  10 A is small. Therefore, for the fillings  13   a ,  13   b,  and  3   c  disposed in the outermost layer, the density in the outermost layer is set to an appropriate value. 
     Therefore, the cross-sectional area A of the outermost layer is defined by the following Equation (1). In other words, the cross-sectional area A is the area of the region surrounded by the virtual circle C 1  and the virtual circle C 2 . 
       A=π×r 2   2 −π×r 1   2    (1)
 
     Further, the outer layer filling density D is defined by the following Equation (2). 
       D=S÷A   (2)
 
     In Equation (2), S is the sum of the cross-sectional areas of the fillings  13   a,    13   b , and  3   c  disposed in the region between the virtual circles C 1  and C 2 . In other words, S is the sum of cross-sectional areas of parts of the fillings  13   a  to  13   d,  and  3   c  that are disposed in a region of which the distance from the cable central axis O is in a range of r 1  to r 2 . 
     The Equation (2) can also be expressed as the following Equation (2)′. 
       D=S÷(π×r 2   2 −π×r 1   2 )   (2)′
 
     Table 2 shows the results of preparing a plurality of optical fiber cables by changing the outer layer filling density D. The conditions other than the amounts of fillings  13   a  are the same as the conditions in Example 6 above. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Set  
                 Introduction  
                 Transmission  
                 Overall 
               
               
                   
                 D 
                 angle 
                 angle 
                 loss 
                 determination 
               
               
                   
                   
               
             
            
               
                   
                 0.00 
                 ±600° 
                  ±75° 
                 OK 
                 NG 
               
               
                   
                 0.05 
                 ±600° 
                 ±135° 
                 OK 
                 OK 
               
               
                   
                 0.10 
                 ±600° 
                 ±150° 
                 OK 
                 OK 
               
               
                   
                 0.15 
                 ±600° 
                 ±150° 
                 OK 
                 OK 
               
               
                   
                 0.20 
                 ±600° 
                 ±150° 
                 OK 
                 OK 
               
               
                   
                 0.25 
                 ±600° 
                 ±160° 
                 NG 
                 NG 
               
               
                   
                   
               
            
           
         
       
     
     “Transmission loss” in Table 3 shows the measurement results according to ICEA S-87-640-2016. More specifically, for the single-mode optical fiber, the result is good (OK) when the transmission loss at a wavelength of 1550 nm is less than 0.30 dB/km, and the result is insufficient (NG) when the transmission loss is 0.30 dB/km or more. 
     The “Overall determination” in Table 3 is considered to be good (OK) when the results of both the introduction angle and the transmission loss are good. The determination criterion for the introduction angle is that it is good when the introduction angle is ±135° or more, as described in Example 6. 
     As shown in Table 3, in a case of 0.05≤D≤0.20, the overall determination is good. On the other hand, in a case of D=0.00, the transmission loss is good, but the introduction angle is less than the reference value (±135°), so that the overall determination is insufficient. This is because the fillings  13   a  and  3   c  are not disposed and the untwisting cannot be suppressed. 
     Further, in a case of D=0.25, the introduction angle is good, but the transmission loss is equal to or more than the reference value (0.30 dB/km), so that the overall determination is insufficient. This is because the lateral pressure acting on the optical fiber  1  of the outer unit  10 A is rather increased by disposing the fillings  13   a  and  3   c  excessively. 
     From the above results, it is found that by setting the outer layer filling density D to 0.05 or more and 0.20 or less, it is possible to suppress the lateral pressure acting on the optical fiber  1  to be small while suppressing the untwisting of the outer unit  10 A. 
     Further, even when the fillings  3   c  are disposed as in the third embodiment, by setting the outer layer filling density D to 0.05 or more and 0.20 or less, it is possible to suppress the lateral pressure acting on the optical fiber  1  to be small while suppressing the untwisting of the optical fiber unit  10 A. 
     As described above, the optical fiber cable  100 B includes: a plurality of optical fiber units  10  each having a plurality of optical fibers; a wrapping tube  54  that wraps around the plurality of optical fiber units  10 ; at least one filling  3   c  disposed inside the wrapping tube  54 ; and a sheath  55  that covers the wrapping tube  54 , in which a plurality of outer units  10 A included in the plurality of optical fiber units  10  that are located in an outermost layer are twisted in an SZ shape around a cable central axis O, and the filling  3   c  is sandwiched between one of the outer units  10 A and the wrapping tube  54  in a cross-sectional view. 
     According to this configuration, when the bundle of the optical fiber unit  10  tends to expand radially outward, fillings  13   a  and  3   c  are compressed in the radial direction between the optical fiber unit  10 A and the wrapping tube  54 . That is, the fillings  13   a  and  3   c  twisted together with the optical fiber unit  10 A are pressed against the wrapping tube  54 . Since the fillings  13   a  and  3   c  are formed of a fibrous material, the friction coefficient between the optical fiber  1  and the fillings  13   a  and  3   c,  and the friction coefficient between the fillings  13   a  and  3   c  and the wrapping tube  54  are larger than the friction coefficient between the optical fiber  1  and the wrapping tube  54 . Therefore, the frictional force generated when the optical fiber unit  10 A is pressed against the wrapping tube  54  with the fillings  13   a  and  3   c  sandwiched between them is larger than the frictional force generated when the optical fiber unit  10 A is directly pressed against the wrapping tube  54 . 
     That is, when the optical fiber unit  10 A tends to expand radially outward, the fillings  13   a  and  3   c  generate a large frictional force. Due to this frictional force, the optical fiber unit  10 A is less likely to move with respect to the wrapping tube  54 , and it is possible to suppress the untwisting of the optical fiber unit  10 A. 
     Further, in the cross-sectional view, the filling  3   c  is surrounded by one optical fiber unit  10 A and the wrapping tube  54 . Therefore, when the bundle of the optical fiber unit  10  tends to expand radially outward, the fillings  3   c  are more reliably sandwiched between the optical fiber unit  10 A and the wrapping tube  54 . Further, the optical fiber unit  10 A prevents the fillings  3   c  from moving radially inward, so that it is possible to more reliably maintain the state in which the fillings  3   c  are in contact with the wrapping tube  54 . 
     Further, in the cross-sectional view, the filling  3   c  may be located on a straight line passing through the cable central axis O and the center point X of one optical fiber unit  10 A. 
     With this configuration, it is possible to more efficiently convert the force that the optical fiber unit  10 A tends to expand radially outward into a frictional force. Therefore, it is possible to more reliably suppress the untwisting of the optical fiber unit  10 A. 
     Further, when the distance between the radially inner end of the outer unit  10 A and the cable central axis O is r 1 , the distance between the radially outer end of the outer unit  10 A and the cable central axis O is r 2 , and S is the sum of cross-sectional areas of parts the fillings disposed in a region of which a distance from the cable central axis is in a range of r 1  to r 2 , the outer layer filling density D represented by D=S÷(π×r 2   2 −π×r 1   2 ) may be 0.05 or more and 0.20 or less. 
     Thus, it is possible to suppress the lateral pressure acting on the optical fiber  1  to a small value while suppressing the untwisting of the optical fiber unit  10 A. 
     It should be noted that the technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. 
     For example, in the examples of  FIGS.  4  and  5   , the core  20  includes a two-layer optical fiber unit  10 . However, the number of layers of the optical fiber unit included in the core  20  may be 1 or 3 or more. 
     Further, when the core  20  includes a plurality of layers of optical fiber units, no fillings may be disposed between the optical fiber units (inner unit  10 B in the examples of  FIGS.  4  and  5   ) included in the layers other than the outermost layer. 
     Further, in the optical fiber cable  100 B, a plurality of fillings  13   d  may be disposed at the center of the cable. The fillings  13   d  may not be located coaxially with the cable central axis O. The fillings  13   d  may not be disposed. 
     Fourth Embodiment 
     Hereinafter, an optical fiber cable of the present embodiment will be described with reference to the drawings. The same members as in the first embodiment are denoted by the same reference numerals, and a description thereof is omitted. 
     As illustrated in  FIG.  9   , an optical fiber cable  100 C includes a core  20  having a plurality of optical fiber units  10 A,  10 B, a sheath  55  accommodating the core  20  inside, and a pair of tensile strength members  56  (tension members) and a pair of wire bodies  57 , which are embedded in the sheath  55 . The core  20  has a wrapping tube  54  that wraps around a plurality of optical fiber units  10 A and  10 B. 
     Direction Definition 
     In the present embodiment, the central axis of the optical fiber cable  100 C is referred to as the cable central axis O. Further, the longitudinal direction of the optical fiber cable  100 C (longitudinal direction of the optical fiber units  10 A and  10 B) is simply referred to as the longitudinal direction. A cross section orthogonal to the longitudinal direction is called a cross section. In the cross-sectional view ( FIG.  9   ), the direction intersecting the cable central axis O is called the radial direction, and the direction rotating around the cable central axis O is called the circumferential direction. 
     When the optical fiber cable  100 C is non-circular in the cross-sectional view, the cable central axis O is located at the center of the optical fiber cable  100 C. 
     The sheath  55  is formed in a cylindrical shape centered on the cable central axis O. As the material of the sheath  55 , polyolefin (PO) resin such as polyethylene (PE), polypropylene (PP), ethylene ethyl acrylate copolymer (EEA), ethylene vinyl acetate copolymer (EVA), and ethylene propylene copolymer (EP), polyvinyl chloride (PVC), or the like can be used. 
     As the material of the wire body  57 , a cylindrical rod made of PP, nylon, or the like can be used. Further, the wire body  57  may be formed of yarns in which fibers such as PP or polyester are twisted, and the wire body  57  may have water absorbency. 
     The pair of wire bodies  57  is disposed so as to sandwich the core  20  in the radial direction. Each wire body  57  is in contact with the outer peripheral surface of the core  20  (the outer peripheral surface of the wrapping tube  54 ). The number of wire bodies  57  embedded in the sheath  55  may be 1 or 3 or more. 
     As the material of the tensile strength member  56 , for example, a metal wire (such as steel wire), a tension fiber (such as aramid fiber), FRP or the like can be used. The pair of tensile strength members  56  is disposed so as to sandwich the core  20  in the radial direction. Further, the pair of tensile strength members  56  is disposed at intervals in the radial direction from the core  20 . The number of tensile strength members  56  embedded in the sheath  55  may be 1 or 3 or more. Further, the tensile strength member  56  may not be embedded in the sheath  55 . 
     A pair of protrusions  58  projecting radially outward is formed on the outer peripheral surface of the sheath  55 . The protrusion  58  extends along the longitudinal direction. 
     The protrusion  58  and the wire body  57  are disposed at the same position in the circumferential direction. The protrusion  58  serves as a mark when the sheath  55  is incised in order to take out the wire body  57 . Instead of the protrusion  58 , a mark indicating the position of the wire body  57  may be provided, for example, by making a part of the sheath  55  different in color from the other parts. 
     The core  20  includes a plurality of optical fiber units  10 A and  10 B, a plurality of fillings  23   a  to  23   c,  and a wrapping tube  54 . The wrapping tube  54  wraps the optical fiber units  10 A and  10 B and fillings  23   a  to  23   c.  Each of the optical fiber units  10 A,  10 B has a plurality of optical fiber core wires or optical fiber strands (hereinafter, simply referred to as optical fiber  1 ), and a binding material  2  for binding the optical fibers  1 . The optical fiber units  10 A and  10 B and fillings  23   a  to  23   c  extend along the longitudinal direction. 
     The optical fiber units  10 A and  10 B of the present embodiment is a so-called intermittently-adhered optical fiber ribbon, and when a plurality of optical fibers  1  are pulled in a direction orthogonal to the longitudinal direction, the optical fibers  1  are adhered to each other so as to spread in a mesh form (spider web shape). Specifically, one optical fiber  1  is adhered to adjacent optical fibers  1  on both sides thereof at different positions in the longitudinal direction, and the adjacent optical fibers  1  are spaced apart from each other at a fixed interval in the longitudinal direction and are adhered to each other. 
     The modes of the optical fiber units  10 A and  10 B are not limited to the intermittently-adhered optical fiber ribbon, and may be changed as appropriate. For example, the optical fiber units  10 A,  10 B may be simply a bundle of a plurality of optical fibers  1  with a binding material  2 . 
     As illustrated in  FIG.  9   , the optical fiber units  10 A and  10 B are divided into two layers, that is, a radially inner layer and a radially outer layer. The optical fiber unit  10 A is located in the outermost layer. The optical fiber unit  10 B is located in a layer inside the outermost layer (hereinafter referred to as an inner layer). The optical fiber unit  10 B is located radially inside the optical fiber unit  10 A. The optical fiber unit  10 A located in the outermost layer is also referred to as an outer unit  10 A. Further, the optical fiber unit  10 B other than the optical fiber unit  10 A is also referred to as an inner unit  10 B. In the example of  FIG.  9   , three optical fiber units  10 B are twisted together in an SZ shape or a spiral shape. Further, nine optical fiber units  10 A are twisted in an SZ shape so as to surround the three optical fiber units  10 B. The numbers of optical fiber units  10 A and  10 B can be changed as appropriate. 
     In the cross-sectional view, the optical fiber unit  10 B located in the inner layer is formed in a fan shape, and the optical fiber unit  10 A located in the outermost layer is formed in square. In addition, the present invention is not limited to the illustrated example, the optical fiber units  10 A and  10 B having a circular, elliptical, or polygonal cross section may be used. Further, the core  20  may be composed of one layer (layer of the optical fiber unit  10 A) without the optical fiber unit  10 B. 
     The binding material  2  has a long string shape and is wound around the plurality of optical fibers  1 . The optical fiber  1  is partially exposed from the gap between the binding materials  2 . Therefore, when the sheath  55  is incised and the wrapping tube  54  is removed, it is possible to visually recognize the optical fiber  1  from the gap between the binding materials  2 . The binding material  2  is made of a thin and highly flexible material such as resin. Therefore, even in the state where the optical fibers  1  are bound with the binding material  2 , the optical fibers  1  are appropriately moved to a vacant space in the sheath  55  while deforming the binding material  2 . Therefore, the cross-sectional shapes of the optical fiber units  10 A and  10 B in the actual product may not be arranged as illustrated in  FIG.  9   . 
     The wrapping tube  54  is formed in a cylindrical shape centered on the cable central axis O. The inner peripheral surface of the wrapping tube  54  is in contact with the radially outer end of the optical fiber unit  10 A. Further, the inner peripheral surface of the wrapping tube  54  is in contact with the filling  23   a.  As the wrapping tube  54 , a non-woven fabric, a plastic tape member, or the like can be used. The wrapping tube  54  may be made of materials having water absorbency, such as a water-absorbent tape. 
     The fillings  23   a  to  23   c  are formed of a fibrous material such as polyester fiber, aramid fiber, and glass fiber. The fillings  23   a  to  23   c  may be yarns having water absorbency or the like. In this case, it is possible to enhance the waterproof performance inside the optical fiber cable  100 C. 
     In the cross-sectional view, the fillings  23   a  are sandwiched between the optical fiber units  10 A adjacent to each other in the circumferential direction and are in contact with the inner peripheral surface of the wrapping tube  54 . The fillings  23   a  are disposed between the two optical fiber units  10 A and the wrapping tube  54 . 
     The fillings  23   b  are sandwiched between the optical fiber units  10 A adjacent to each other in the circumferential direction. 
     The fillings  23   b  are located radially inside the fillings  23   a  and are not in contact with the inner peripheral surface of the wrapping tube  54 . The fillings  23   a  and  23   b  are twisted together with the optical fiber unit  10 A in an SZ shape. The fillings  23   a  and the fillings  23   b  are disposed at the same position in the circumferential direction. However, the position of the filling  23   b  in the circumferential direction may be different from the position of the filling  23   a  in the circumferential direction. 
     The fillings  23   c  are sandwiched between the optical fiber units  10 B adjacent to each other in the circumferential direction. 
     The fillings  23   c  are located radially inside the fillings  23   a  and  23   b,  and are not in contact with the inner peripheral surface of the wrapping tube  54 . The fillings  23   c  are twisted together with the optical fiber unit  10 B in an SZ shape or a spiral shape. The fillings  23   c  may not be disposed. 
     The fillings  23   a  and  23   b  are in contact with the optical fiber unit  10 A. The fillings  23   c  are in contact with the optical fiber unit  10 B. Here, the binding material  2  has a thin and long string shape, and is wound around a bundle of the optical fibers  1  in a spiral shape, for example. Therefore, a part of the optical fiber  1  which is not covered with the string-shaped binding material  2  is partially in contact with the fillings  23   a  to  23   c.    
     The optical fiber  1  usually has a structure in which an optical fiber bare fiber formed of glass is coated with a coating material such as a resin. Therefore, the surface of the optical fiber  1  is smooth, and the friction coefficient when the optical fibers  1  come into contact with each other is relatively small. On the other hand, the fillings  23   a  to  23   c  are formed of a fibrous material. Therefore, the friction coefficient when the fillings  23   a  to  23   c  are in contact with the optical fibers  1  is larger than the friction coefficient when the optical fibers  1  are in contact with each other. 
     From the above, it is possible to increase the frictional resistance when the optical fiber units  10 A and  10 B move relative to each other, by disposing the fillings  23   a  to  23   c  so as to be sandwiched between the plurality of optical fiber units  10 A and  10 B. This makes it possible to suppress the movement of the optical fiber units  10 A and  10 B in the optical fiber cable  100 C. 
     Incidentally, in the present embodiment, the optical fiber unit  10 A is twisted in an SZ shape. As a result, when the optical fiber cable  100 C is bent, it is possible to improve the workability of the mid-span branching, while suppressing the action of tension or strain on the optical fiber  1  included in the optical fiber unit  10 A. 
     On the other hand, when the optical fiber unit  10 A is twisted in an SZ shape, it may be insufficient to suppress the untwisting of the optical fiber unit  10 A. When a compressive force acts on the optical fiber cable  100 C, it is also required to suppress the lateral pressure acting on the optical fiber unit  10 A. 
     Therefore, in the present embodiment, the fillings  23   a  (second filling) and fillings  23   b  (third filling) are twisted together with the optical fiber unit  10 A. The fillings  23   a  are in contact with the wrapping tube  54  while being sandwiched between the optical fiber units  10 A, and the fillings  23   b  are located between the optical fiber units  10 A radially inside the fillings  23   a.    
     According to this configuration, since the fillings  23   a  are in contact with the wrapping tube  54 , untwisting is less likely to occur as compared with the case where only the optical fiber unit  10 A is in contact with the wrapping tube  54 . This is because the frictional force acting between the fillings  23   a  and the wrapping tube  54  is larger than the frictional force acting between the optical fiber unit  10 A and the wrapping tube  54 . More specifically, since the fillings  23   a  are formed of a fibrous material, the friction coefficient between the fillings  23   a  and the wrapping tube  54  is high. 
     Further, in addition to the fillings  23   a,  fillings  23   b  are disposed between the optical fiber units  10 A. The presence of the fillings  23   b  prevents the fillings  23   a  from moving radially inward, and it is possible to more reliably maintain the state in which the fillings  23   a  are in contact with the wrapping tube  54 . Therefore, it is possible to more reliably achieve the effect of suppressing untwisting by the fillings  23   a.    
     Further, the fillings  23   a  and the fillings  23   b  are disposed at the same position in the circumferential direction. With this configuration, it is possible to more reliably suppress the movement of the fillings  23   a  radially inward. Further, fillings  23   a  and  23   b  are disposed between the optical fiber units  10 A in a well-balanced manner. Thus, when a compressive force acts on the optical fiber cable  100 C, it is possible to reduce the lateral pressure acting on the optical fiber  1  included in the optical fiber unit  10 A, by the fillings  23   a  and  23   b  acting as cushioning materials. 
     Further, the optical fiber unit  10 A has a binding material  2  wound around the optical fibers  1 , and the optical fibers  1  are partially exposed from the gap between the binding materials  2 . Therefore, in the mid-span branching work, it is possible to easily visually recognize the optical fiber  1  by incising the sheath  55  and removing the wrapping tube  54 , and the workability is enhanced. 
     Fifth Embodiment 
     Hereinafter, an optical fiber cable of a fifth embodiment will be described with reference to the drawings. The same members as in the first embodiment are denoted by the same reference numerals, and a description thereof is omitted. 
       FIG.  10    illustrates an optical fiber cable  100 D according to the fifth embodiment. The fifth embodiment has the same basic configuration as the fourth embodiment, but the optical fiber cable  100 D is different from the optical fiber cable  100 C of  FIG.  9    in having fillings  3   c.    
     In the optical fiber cable  100 D, the core  20  includes a plurality of optical fiber units  10 A and  10 B, a plurality of fillings  23   a  to  23   c,    3   c,  and a wrapping tube  54 . The wrapping tube  54  wraps the optical fiber units  10 A and  10 B and fillings  23   a  to  23   c  and  3   c.    
     The filling  3   c  is sandwiched between one optical fiber unit  10 A and the wrapping tube  54 . The fillings  3   c  are twisted together with the optical fiber unit  10 A in an SZ shape. 
     The fillings  3   c  are in contact with the wrapping tube  54  and the optical fiber unit  10 A. Further, the part of the optical fiber  1  which is not covered with the string-shaped binding material  2  is partially in contact with the filling  3   c.    
     Further, in the cross-sectional view, the fillings  3   c  may be located on the straight line L passing through the center point X of the optical fiber unit  10 A and the cable central axis O. 
     EXAMPLES 
     Hereinafter, the fourth and fifth embodiments will be described with reference to specific examples. The present invention is not limited to the following examples. 
     In the present example, the optimum arrangement and amount of fillings are examined. 
     Example 10 
     As Example 10, an optical fiber cable having a cross-sectional structure as illustrated in  FIG.  9    is prepared. 
     The number of optical fibers  1  included in each optical fiber unit  10 A and  10 B is 144. Three optical fiber units  10 B are twisted in an SZ shape, and nine optical fiber units  10 A are twisted in an SZ shape on the outer circumference thereof. That is, the total number of optical fiber units  10 A and  10 B is 12, and the total number of optical fibers  1  is 1728. Water-absorbent yarns are used as fillings  23   a,    23   b,  and  23   c.  One filling  23   a , eight fillings  23   b,  and three fillings  23   c  are disposed. 
     The optical fiber units  10 A and  10 B are twisted with the set angle of the twisting device (oscillator) set to ±600°. The “set angle” is in a range of angles at which the oscillator is oscillated. For example, when the set angle is ±600°, the oscillator repeats the operation of oscillating 600° in the CW direction and then oscillating 600° n the CCW direction. In this way, the twisted optical fiber units  10 A and  10 B are wrapped with a wrapping tube  54  and further covered with a sheath  55  to prepare an optical fiber cable. 
     Example 11 
     As Example 11, an optical fiber cable is prepared in which the numbers of fillings  23   a  and  23   b  are changed from Example 10. Three fillings  23   a,  six fillings  23   b , and three fillings  23   c  are disposed. Other conditions are the same as in Example 10. 
     Example 12 
     As Example 12, an optical fiber cable having a cross-sectional structure as illustrated in  FIG.  10    is prepared. The optical fiber cable of Example 12 has the number of fillings  23   a  and  23   b  changed from that of Example 10 and further includes fillings  3   c . One filling  23   a,  seven fillings  23   b,  three fillings  23   c,  and one filling  3   c  are disposed. Other conditions are the same as in Example 10. 
     Comparative Example 6 
     As Comparative Example 6, an optical fiber cable  100 C provided with fillings  23   b  and  23   c  without fillings  23   a  is prepared. Nine fillings  23   b  and three fillings  23   c  are disposed. Other conditions are the same as in Example 10. 
     Table 4 shows the results of checking the SZ twist angle (introduction angle) actually introduced into the optical fiber unit  10 A, for the optical fiber cables of Examples 10 to 12 and Comparative Example 6. The manufactured optical fiber cable is cut at predetermined intervals in the longitudinal direction, and the position of a specific optical fiber or optical fiber unit in the circumferential direction is measured on each cut surface. The rotation angle of a specific optical fiber or optical fiber unit with respect to the cable central axis O is defined as the introduction angle. The larger the difference between the set angle and the introduction angle, the larger the optical fiber unit  10 A is untwisted. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                 Number of fillings (pieces) 
                 Set 
                 Introduction 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 23a 
                 23b 
                 23c 
                 3c 
                 total 
                 angle[°] 
                 angle[°] 
                 Determination 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Example 10 
                 1 
                 8 
                 3 
                 0 
                 12 
                 ±600 
                 ±150 
                 OK 
               
               
                 Example 11 
                 3 
                 6 
                 3 
                 0 
                 12 
                 ±600 
                 ±160 
                 OK 
               
               
                 Example 12 
                 1 
                 7 
                 3 
                 1 
                 12 
                 ±600 
                 ±155 
                 OK 
               
               
                 Comparative 
                 0 
                 9 
                 3 
                 0 
                 12 
                 ±600 
                 ±110 
                 NG 
               
               
                 Example 6 
               
               
                   
               
            
           
         
       
     
     In the “Determination” field of Table 4, the result is good (OK) when the introduction angle is ±135° or more, and the result is insufficient (NG) when the introduction angle is less than ±135°. The reason why the determination criterion is that the introduction angle is ±135° or more is as follows. For example, in a case where the optical fiber unit  10 A is not twisted, when the optical fiber cable is bent, the optical fiber unit  10 A is compressed inside the bend of the optical fiber cable and stretched outside the bend of the optical fiber cable. On the other hand, when the optical fiber unit  10 A is twisted in an SZ shape at an introduction angle of ±135° or more, one optical fiber unit  10 A is reliably disposed across both the compressed part and the stretched part. Since the introduction angle of ±135° or more is satisfied, it is possible to cancel out the tension and compression acting on the optical fiber unit  10 A, and to suppress the tension acting on the optical fiber  1 . 
     As shown in Table 4, it is possible to make the introduction angles of Examples 10 to 12 larger than the introduction angles of Comparative Example 6. Further, in Examples 10 to 12, the introduction angle is ±135° or more, and good results are obtained. 
     This is because the filling  23   a  is in contact with the wrapping tube  54 , and the frictional force between the filling  23   a  and the wrapping tube  54  is able to suppress the optical fiber unit  10 A from untwisting. 
     From the comparison between Examples 10 to 12 and Comparative Example 6, it is checked that the fillings  23   a  in contact with the wrapping tube  54  are able to suppress the untwisting of the optical fiber unit  10 A located in the outermost layer. Further, from the comparison between Examples 10 and 12 and Comparative Example 6, it is checked that it is possible to obtain an appropriate introduction angle by disposing at least one filling  23   a  in contact with the wrapping tube  54 . 
     Further, in Examples 10 to 12, the total number of fillings  23   a  and  3   c  in contact with the wrapping tube  54  is 1 in Example 10, 2 in Example 12, and 3 in Example 11, and the number increases in this order. Further, the introduction angle is ±150° in Example 10, ±155° in Example 12, and ±160° in Example 11, and the larger the total number of fillings  23   a  and  3   c  in contact with the wrapping tube  54 , the greater the introduction angle is. 
     As described above, it is checked that the greater the number of fillings  23   a  and  3   c  in contact with the wrapping tube  54 , the greater the effect of suppressing untwisting. This is because the frictional force between the fillings  23   a  and  3   c  and the wrapping tube  54  increases as the total number of fillings  23   a  and  3   c  in contact with the wrapping tube  54  increases. 
     Next, the result of examining the optimum density when fillings  23   a,    23   b,  and  3   c  are provided will be described. 
     Here, the parameter of “Outer layer filling density D” is used. The outer layer filling density D is the density of fillings sandwiched between the optical fiber units located in the outermost layer among the plurality of optical fiber units included in the core. 
     Here, the outer layer filling density D will be described in more detail with reference to  FIG.  11   . The virtual circle C 1  illustrated in  FIG.  11    is an arc connecting the radially inner ends of the plurality of optical fiber units  10 A located in the outermost layer. The virtual circle C 2  is an arc connecting the radially outer ends of the plurality of optical fiber units  10 A located in the outermost layer. The virtual circle C 2  substantially overlaps the inner peripheral surface of the wrapping tube  54 . 
     Dimension r 1  is the radius of the virtual circle C 1  and dimension r 2  is the radius of the virtual circle C 2 . In other words, the dimension r 1  is the distance between the radially inner end of the optical fiber unit  10 A located in the outermost layer and the cable central axis O. The dimension r 2  is the distance between the radially outer end of the optical fiber unit  10 A located in the outermost layer (the inner circumferential surface of the wrapping tube  54 ) and the cable central axis O. 
     Regarding the plurality of optical fiber units  10 A located in the outermost layer, the positions of the radially inner ends may be non-uniform (the virtual circle C 1  in  FIG.  11    is non-circular). In that case, the average value of the distance between the radially inner end of each optical fiber unit  10 A and the cable central axis O is defined as the dimension r 1 . The same applies when the virtual circle C 2  is non-circular. That is, the average value of the distance between the radially outer end of each optical fiber unit  10 A and the cable central axis O is defined as the dimension r 2 . 
     Here, the twisted states are different in the outermost layer (layer of the optical fiber unit  10 A) and the inner layer (layer of the optical fiber unit  10 B). Further, the fillings  23   a,    23   b,  and  3   c  located in the outermost layer and the fillings  23   c  located in the inner layer have different roles. More specifically, the fillings  23   a  and  3   c  are in contact with the wrapping tube  54  to suppress untwisting, and the fillings  23   b  suppress the fillings  23   a  from moving radially inward. Therefore, for the fillings  23   a,    23   b,  and  3   c  disposed in the outermost layer, the density in the outermost layer is set to an appropriate value. 
     Therefore, the cross-sectional area A of the outermost layer is defined by the following Equation (1). In other words, the cross-sectional area A is the area of the region surrounded by the virtual circle C 1  and the virtual circle C 2 . 
       A=π×r 2   2 −π×r 1   2    (1)
 
     Further, the outer layer filling density D is defined by the following Equation (2). 
       D=S÷A   (2)
 
     In Equation (2), S is the sum of the cross-sectional areas of the fillings  23   a,    23   b , and  3   c  disposed in the region between the virtual circles C 1  and C 2 . 
     The Equation (2) can also be expressed as the following Equation (2)′. 
       D=S÷(π×r 2   2 −π×r 1   2 )   (2)′
 
     Table 5 shows the results of preparing a plurality of optical fiber cables by changing the outer layer filling density D. The conditions other than the amounts of fillings  23   a  and  23   b  are the same as the conditions in Example 10. Further, the fillings  23   a  and  23   b  are disposed such that the amounts are equal to each other. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                 Set  
                 Introduction  
                 Transmission  
                 Overall 
               
               
                   
                 D 
                 angle 
                 angle 
                 loss 
                 determination 
               
               
                   
                   
               
             
            
               
                   
                 0.00 
                 ±600° 
                  ±75° 
                 OK 
                 NG 
               
               
                   
                 0.05 
                 ±600° 
                 ±135° 
                 OK 
                 OK 
               
               
                   
                 0.10 
                 ±600° 
                 ±150° 
                 OK 
                 OK 
               
               
                   
                 0.15 
                 ±600° 
                 ±150° 
                 OK 
                 OK 
               
               
                   
                 0.20 
                 ±600° 
                 ±150° 
                 OK 
                 OK 
               
               
                   
                 0.25 
                 ±600° 
                 ±160° 
                 NG 
                 NG 
               
               
                   
                   
               
            
           
         
       
     
     “Transmission loss” in Table 5 shows the measurement results according to ICEA S-87-640-2016. More specifically, for the single-mode optical fiber, the result is good (OK) when the transmission loss at a wavelength of 1550 nm is less than 0.30 dB/km, and the result is insufficient (NG) when the transmission loss is 0.30 dB/km or more. 
     The “Overall determination” in Table 5 is considered to be good (OK) when the results of both the introduction angle and the transmission loss are good. The determination criterion for the introduction angle is set such that the result is good when the introduction angle is ±135° or more, as described in Example 10. 
     As shown in Table 5, when 0.05≤D≤0.20, the overall determination is good. On the other hand, in a case of D=0.00, the transmission loss is good, but the introduction angle is less than the reference value (±135°), so that the overall determination is insufficient. This is because the fillings  23   a  and  23   b  are not disposed and the untwisting cannot be suppressed. 
     Further, in a case of D=0.25, the introduction angle is good, but the transmission loss is equal to or more than the reference value (0.30 dB/km), so that the overall determination is insufficient. This is because the lateral pressure acting on the optical fiber  1  of the optical fiber unit  10 A is rather increased by disposing the fillings  23   a  and  23   b  excessively. 
     From the above results, it is found that by setting the outer layer filling density D to 0.05 or more and 0.20 or less, it is possible to suppress the lateral pressure acting on the optical fiber  1  to be small while suppressing the untwisting of the optical fiber unit  10 A. 
     Further, even when the fillings  3   c  are disposed as in Example 12, by setting the outer layer filling density D to 0.05 or more and 0.20 or less, it is possible to suppress the lateral pressure acting on the optical fiber  1  to be small while suppressing the untwisting of the optical fiber unit  10 A. 
     As described above, the optical fiber cable  100 D includes: a plurality of optical fiber units  10 A,  10 B each having a plurality of optical fibers; a wrapping tube  54  that wraps around the plurality of optical fiber units  10 A,  10 B; at least one filling  3   c  disposed inside the wrapping tube  54 ; and a sheath  55  that covers the wrapping tube  54 , in which a plurality of outer units  10 A included in the plurality of optical fiber units  10 A,  10 B that are located in an outermost layer are twisted in an SZ shape around a cable central axis O, and the filling  3   c  is sandwiched between one of the outer units  10 A and the wrapping tube  54  in a cross-sectional view. 
     According to this configuration, when the bundle of the optical fiber unit  10  tends to expand radially outward, fillings  23   a  and  3   c  are compressed in the radial direction between the optical fiber unit  10 A and the wrapping tube  54 . That is, the fillings  23   a  and  3   c  twisted together with the optical fiber unit  10 A are pressed against the wrapping tube  54 . Since the fillings  23   a  and  3   c  are formed of a fibrous material, the friction coefficient between the optical fiber  1  and the fillings  23   a  and  3   c,  and the friction coefficient between the fillings  23   a  and  3   c  and the wrapping tube  54  are larger than the friction coefficient between the optical fiber  1  and the wrapping tube  54 . Therefore, the frictional force generated when the optical fiber unit  10 A is pressed against the wrapping tube  54  with the fillings  23   a  and  3   c  sandwiched between them is larger than the frictional force generated when the optical fiber unit  10 A is directly pressed against the wrapping tube  54 . 
     That is, when the optical fiber unit  10 A tends to expand radially outward, the fillings  23   a  and  3   c  generate a large frictional force. Due to this frictional force, the optical fiber unit  10 A is less likely to move with respect to the wrapping tube  54 , and it is possible to suppress the untwisting of the optical fiber unit  10 A. 
     Further, in the cross-sectional view, the filling  3   c  is surrounded by one optical fiber unit  10 A and the wrapping tube  54 . Therefore, when the bundle of the optical fiber unit  10  tends to expand radially outward, the fillings  3   c  are more reliably sandwiched between the optical fiber unit  10 A and the wrapping tube  54 . Further, the optical fiber unit  10 A prevents the fillings  3   c  from moving radially inward, so that it is possible to more reliably maintain the state in which the fillings  3   c  are in contact with the wrapping tube  54 . 
     Further, in the cross-sectional view, the filling  3   c  may be located on a straight line passing through the cable central axis O and the center point X of one optical fiber unit  10 A. 
     With this configuration, it is possible to more efficiently convert the force that the optical fiber unit  10 A tends to expand radially outward c into a frictional force. Therefore, it is possible to more reliably suppress the untwisting of the optical fiber unit  10 A. 
     Further, at least one second filling  23   a  and at least one third filling  23   b  located between the adjacent optical fiber units  10 A may be further provided, and the second filling  23   a  may be in contact with the wrapping tube  54  and the third filling  23   b  may be located inside the second filling  23   a  in the radial direction. 
     The presence of the fillings  23   b  prevents the fillings  23   a  from moving radially inward, and it is possible to more reliably maintain the state in which the fillings  23   a  are in contact with the wrapping tube  54 . Therefore, it is possible to more reliably achieve the effect of suppressing untwisting by the fillings  23   a.    
     Further, the fillings  23   a  and the fillings  23   b  may be disposed at the same position in the circumferential direction. With this configuration, it is possible to more reliably suppress the movement of the fillings  23   a  radially inward. Further, fillings  23   a  and  23   b  are disposed between the optical fiber units  10 A in a well-balanced manner. Thus, when a compressive force acts on the optical fiber cable  100 D, it is possible to reduce the lateral pressure acting on the optical fiber  1  included in the optical fiber unit  10 A, by the fillings  23   a  and  23   b  acting as cushioning materials. 
     Further, when the distance between the radially inner end of the optical fiber unit  10 A and the cable central axis O is r 1 , the distance between the radially outer end of the optical fiber unit  10 A and the cable central axis O is r 2 , and S is the sum of cross-sectional areas of parts of the fillings  23   a  to  23   c,  and  3   c  disposed in a region of which a distance from the cable central axis O is in a range of r 1  to r 2 , the outer layer filling density D represented by D=S÷(π×r 2   2 −π×r 1   2 ) may be 0.05 or more and 0.20 or less. 
     Thus, it is possible to suppress the lateral pressure acting on the optical fiber  1  to a small value while suppressing the untwisting of the optical fiber unit  10 A. 
     The fillings  23   a  to  23   c  and  3   c  may be formed of a fibrous material. As a result, it is possible to increase the frictional force when the fillings  23   a  to  23   c  and  3   c  come into contact with the optical fiber  1  and the wrapping tube  54 . 
     The optical fiber units  10 A,  10 B may have binding materials  2  wound around a plurality of optical fibers  1 , and the optical fiber  1  may be partially exposed from a gap between the binding materials  2 . Thus, it is possible to bring into contact the optical fibers  1  exposed from the gap of the binding material  2  with the fillings  23   a  to  23   c  and  3   c.    
     It should be noted that the technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. 
     For example, in the examples of  FIGS.  9  and  10   , the core  20  includes two layers of optical fiber units  10 A and  10 B. However, the number of layers of the optical fiber unit included in the core  20  may be 1 or 3 or more. 
     Further, when the core  20  includes a plurality of layers of optical fiber units, no fillings may be disposed between the optical fiber units (optical fiber units  10 B in the examples of  FIGS.  9  and  10   ) included in the layers other than the outermost layer. 
     In addition, without departing from the spirit of the present invention, it is possible to appropriately replace the constituent elements in the above-described embodiments with well-known constituent elements, and the above-described embodiments and modification examples may be appropriately combined. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. 
     REFERENCE SIGNS LIST 
     
         
           1  Optical fiber 
           2  Binding material 
           3   a  to  3   c,    13   a  to  13   d,    23   a  to  23   c  Filling 
           10  Optical fiber unit 
           10 A Outer unit 
           20  Core 
           54  Wrapping tube 
           55  Sheath 
           100 ,  100 A,  100 B,  100 C,  100 D Optical fiber cable 
         X Center point of outer unit 
         L Straight line 
         O Cable central axis