Patent Publication Number: US-2023152548-A1

Title: Optical cable and optical cable manufacturing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of International Patent Application No. PCT/JP2021/030854 filed Aug. 23, 2021, which claims the benefit of priority to Japanese Patent Application No. 2020-147296 filed Sep. 2, 2020. The full contents of the International Patent Application are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an optical cable and an optical cable manufacturing method. 
     BACKGROUND 
     There is a known technique for forming an optical cable by housing, inside a sheath, a plurality of optical fiber units each being an optical fiber assembly formed by a plurality of optical fibers brought together. Japanese Patent Application Publication No. 2020-76915 describes placing a filling inside a press-wrapping tape wrapping up a plurality of optical fiber units in order to reduce occurrence of “untwisting” in which, when the plurality of optical fiber units are twisted in an S-Z configuration together, optical fibers move in untwisting directions. 
     If the space inside the optical cable deforms when the cable is bent, the postures of the members in the space inside the optical cable cannot be easily maintained. 
     SUMMARY 
     One or more embodiments of the present disclosure easily maintain the postures of the members in the space inside the optical cable even if the space inside the optical cable deforms when the cable is bent. 
     According to one or more embodiments of the present disclosure, an optical cable comprises: a plurality of optical fiber units each having a fiber group formed by a plurality of optical fibers, wherein the plurality of optical fiber units are twisted, at least one optical fiber unit of the plurality of optical fiber units has a filling, and the filling is wrapped around an outer circumference of the fiber group. 
     Other features of the present disclosure will become apparent in the following description and the drawings. 
     One or more embodiments of the present disclosure reduce untwisting of the optical fiber units with less fillings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  are diagrams illustrating an optical cable  1 . 
         FIG.  2 A  is a diagram illustrating an optical fiber unit  11 .  FIG.  2 B  is a diagram illustrating another optical fiber unit  11 . 
         FIG.  3    is a diagram illustrating the placement of a filling  17 A for a given optical fiber unit  11 A. 
         FIG.  4    is a table showing evaluation results of easiness of dismantling the optical cable  1 . 
         FIG.  5    is a diagram illustrating a manufacturing system  40  for the optical cable  1 . 
         FIGS.  6 A and  6 B  are diagrams illustrating a comber board  44 . 
         FIGS.  7 A to  7 C  are diagrams illustrating how the comber board  44  rotates. 
         FIG.  8    is a diagram illustrating a manufacturing system  40  of a modification. 
         FIG.  9 A  is a diagram illustrating how a compression rate R is measured.  FIG.  9 B  is a diagram illustrating how the cross-sectional shape changes between before and after giving lateral pressure. 
         FIG.  10    is measurement results of the compression rates of fillings, a bundling member, and Kevlar. 
         FIG.  11    is a graph showing the relation between an applied load P and the compression rate R. 
         FIG.  12    is a diagram illustrating the cross section of a filling  17  in the optical cable  1 . 
     
    
    
     DETAILED DESCRIPTION 
     At least the following matters will be become apparent from the following description and the drawings. 
     An optical cable will become apparent, comprising: a plurality of optical fiber units each having a fiber group formed by a plurality of optical fibers, wherein the plurality of optical fiber units are twisted in an S-Z configuration, at least one optical fiber unit of the plurality of optical fiber units has a filling, and the filling is wrapped around an outer circumference of the fiber group. According to such an optical cable, untwisting of the optical fiber units can be reduced with less fillings. 
     Incidentally, in order to reduce “untwisting,” voids may be decreased inside an optical cable by placing many fillings around the optical fiber units. However, more fillings inside an optical cable means stronger lateral pressure acting on the optical fibers, which may lead to more microbending losses. Thus, conflicting goals of decreasing fillings and reducing untwisting of optical fiber units may be met. In respect to the above, according to the above optical cable, untwisting of the optical fiber units can be reduced with less fillings. 
     The optical fiber units may each include a bundling member bundling the plurality of optical fibers together. This makes it possible for the optical fibers to be bundled together and not to come apart. 
     The filling may be wrapped around an outer side of the bundling member. This helps the filling come into contact with the neighboring optical fiber units, and thus, untwisting of the optical fiber units can be further reduced with less fillings. 
     It may further comprise the optical fiber units having the filling and the optical fiber units not having the filling. This helps decrease the fillings. 
     An inner-layer unit may be formed by the optical fiber unit, an outer-layer unit may be formed by circumferentially placing a plurality of the optical fiber units outside the inner-layer unit, and the optical fiber unit forming the inner-layer unit may have the filling. In this situation, the outer-layer unit may include the optical fiber units having the filling and the optical fiber units not having the filling. This makes it possible for untwisting of the optical fiber units to be further reduced with less fillings. 
     The optical fiber unit not having the filling may be placed between two of the optical fiber units having the filling in the outer-layer unit. This makes it possible for untwisting of the optical fiber units to be further reduced with less fillings. 
     P1/P2 may be 0.1 or greater, where P1 is a pitch of wrapping of the filling and P2 is a pitch of twisting the plurality of optical fiber units. This improves easiness for dismantling the optical cable. 
     The filling may be wrapped around the outer circumference of the fiber group in an S-Z configuration without being joined to other members. Not having to have a function to bring the plurality of optical fibers together, the filling can be wrapped around the outer circumference of the fiber group in an S-Z configuration without being joined to other members. 
     A compression rate R may increase as a load P increases when the load P is 1 N or greater, where P(N) is a load applied to a winding member wound around an outer circumference of the filling, and R is a compression rate of the filling when the load P is applied to the winding member to give lateral pressure to the filling. This makes it possible for untwisting of the optical fiber units to be reduced. 
     When the load P is in a range from 1.5 N to 2.0 N, the compression rate R may increase as the load P increases. This makes it possible for untwisting of the optical fiber units to be reduced. 
     α may be 0.17 or greater, where α(N −1 ) is a ratio of an amount of increase in the compression rate R to an amount of increase in the load P when the load P is in a range from 1.5 N to 2.0 N. This makes it possible for untwisting of the optical fiber units to be reduced. 
     The filling may be configured to deform such that the compression rate R is 0.57 or greater, where P(N) is a load applied to a winding member wound around an outer circumference of the filling, and R is a compression rate of the filling when the load P is applied to the winding member to give lateral pressure to the filling. This makes it possible for untwisting of the optical fiber units to be reduced. 
     An optical cable manufacturing method will become apparent, comprising: inserting fiber groups into a plurality of respective insertion holes in a comber board; inserting a filling into at least one of the insertion holes in the comber board; and oscillating the comber board to wrap the filling around an outer circumference of the fiber group. According to such an optical cable manufacturing method, an optical cable can be manufactured in which untwisting of the optical fiber units is reduced with less fillings. 
     First Embodiments 
     &lt;Configuration of an Optical Cable  1 &gt; 
       FIGS.  1 A and  1 B  are diagrams illustrating an optical cable  1 . For the sake of illustration, an optical fiber unit  11  having a distorted cross-sectional shape as shown in  FIG.  1 B  may be depicted hereinbelow as having a circular cross section as shown in  FIG.  1 A . Similarly, a filling  17  having a distorted cross-sectional shape as shown in  FIG.  1 B  may be depicted hereinbelow as having a clean cross section such as a circular or oval shape. 
     The optical cable  1  is a cable that houses optical fibers. The optical cable  1  of one or more embodiments is what is called a slot-less optical cable, which is an optical cable that does not have a slotted rod having slots (grooves for housing optical fibers) formed therein. Alternatively, the optical cable  1  may be a slotted optical cable having a slotted rod. The optical cable  1  has a core  10  and a sheath  20 . 
     The core  10  is a member housed in the sheath  20 . The core  10  has a plurality of optical fiber units  11  ( 11 A to  11 J) and a press-wrapping tape  18 . Although the core  10  of one or more embodiments has ten optical fiber units  11  as shown in  FIG.  1 A  (or  FIG.  1 B ), the number of optical fiber units  11  is not limited to ten. Also, the core  10  of one or more embodiments is formed by the plurality of optical fiber units  11  twisted in an S-Z configuration together. The press-wrapping tape  18  is a member wrapping up the plurality of optical fiber units  11 . 
     In one or more embodiments, the plurality of optical fiber units  11  forming the core  10  form an inner-layer unit  12  and an outer-layer unit  13 . The inner-layer unit  12  is the optical fiber units  11  placed in a center portion of the core  10 . The outer-layer unit  13  is the optical fiber units  11  placed outside the inner-layer unit  12 . In one or more embodiments, the inner-layer unit  12  is formed by three optical fiber units  11 , and the outer-layer unit  13  is formed by seven optical fiber units  11 . However, the number of optical fiber units  11  forming the inner-layer unit  12  or the outer-layer unit  13  is not limited to the above. In the following description, the reference numerals for the optical fiber units  11  forming the inner-layer unit  12  may be followed by indices A to C, and the reference numerals for the optical fiber units  11  forming the outer-layer unit  13  may be followed by indices D to J. Also, in the following description, as to members associated with the optical fiber units  11  (e.g., insertion holes  441  in  FIGS.  7 A to  7 C ), their reference numerals may be followed by the same indices corresponding to the optical fiber units  11 . 
     The sheath  20  is a member covering the plurality of optical fiber units  11  (and the press-wrapping tape  18 ). The outer shape of the sheath  20  is substantially circular in section here, but the outer shape of the sheath  20  is not limited to a circular shape. A tension member  21  is embedded in the sheath  20 . Members other than the tension member  21  (e.g., a rip cord  22 ) may be embedded in the sheath  20  as well. 
       FIG.  2 A  is a diagram illustrating the optical fiber unit  11 . 
     The optical fiber unit  11  is a structure formed by a plurality of optical fibers  15  brought together. The optical fiber unit  11  shown in  FIG.  2 A  has a fiber group  14  and a bundling member  16 . The fiber group  14  is a group of the plurality of optical fibers  15 . In one or more embodiments, the fiber group  14  is formed by bringing together a plurality of intermittently-coupled optical fiber ribbons. However, the fiber group  14  does not have to be formed by a plurality of intermittently-coupled optical fiber ribbons, and for example, may be formed by a single intermittently-coupled optical fiber ribbon or a plurality of single optical fibers. The bundling member  16  is a member that bundles the plurality of optical fibers  15  forming the fiber group  14 . The bundling member  16  is wrapped around the outer circumference of the fiber group  14 . The plurality of optical fibers  15  forming the fiber group  14  are thus brought together so as not to come apart. In one or more embodiments, the optical fiber unit  11  has a pair of bundling members  16 , and the bundling members  16  are wrapped around the outer circumference of the fiber group  14  in an S-Z configuration in such a manner that their wrapping directions are reversed at points where they are joined. However, the bundling members  16  are not limited to being wrapped in an S-Z configuration and may be wrapped around the outer circumference of the fiber group  14  in one direction helically. Also, the number of bundling members  16  is not limited to two. Also, in a case where the optical fiber unit  11  is formed by a single intermittently-coupled optical fiber ribbon, the optical fiber unit  11  does not need to include the bundling members  16  because the group of the optical fibers  15  would not come apart. 
       FIG.  2 B  is a diagram illustrating another optical fiber unit  11 . 
     Some optical fiber units  11  ( 11 A to  11 C,  11 G,  11 J; see  FIG.  1 A ) of one or more embodiments further include the filling  17 . 
     The filling  17  is a member that fills a gap in the space inside the optical cable  1 . Placing the fillings  17  inside the optical cable  1  can increase the packaging density of the optical fibers  15 . Note that the packaging density of the optical fibers  15  is the ratio of the cross-sectional areas of the plurality of optical fibers  15  to an area obtained by subtracting the cross-sectional areas of members other than the optical fibers  15  (such as the press-wrapping tape  18 , the bundling members  16 , and the fillings  17 ) from the entire cross-sectional area of the space inside the optical cable  1 . Specifically, the packaging density of the optical fibers  15  is expressed as ρ=Sf/(S0−S1), where S0 is the entire cross-sectional area of the space inside the optical cable  1 , S1 is the sum of the cross-sectional areas of the members inside the optical cable  1  other than the optical fibers  15  (such as the press-wrapping tape  18 , the bundling members  16 , and the fillings  17 ), Sf is the sum of the cross-sectional areas of the optical fibers  15  inside the optical cable  1 , and ρ is the packaging density of the optical fibers  15 . 
     A low packaging density of the optical fibers  15  means many voids in the space inside the optical cable  1 , and therefore there is a concern that the plurality of optical fiber units  11  twisted in an S-Z configuration may move in untwisting directions. In other words, in a case where the packaging density of the optical fibers  15  is low, “untwisting” of the optical fiber units  11  may occur. Meanwhile, placing too many fillings  17  inside the optical cable  1  to reduce the “untwisting” increases lateral pressure acting on the optical fibers  15  and may increase microbending losses of the optical fibers  15 . In particular, low-loss optical fibers employed to increase the length of the transmission section of the optical cable  1  (e.g., optical fibers having low-loss characteristics conforming to ITU-T G.654.E) have microbending characteristics inferior to those of optical fibers conforming to ITU-T G.657.A1. Thus, placing many fillings  17  inside the optical cable  1  employing such optical fibers makes it likely to increase the microbending losses. For this reason, as will be described next, one or more embodiments reduce the “untwisting” of the optical fiber unit  11  with less fillings  17 . 
     The filling  17  of one or more embodiments is an elongated member and is wrapped around the outer circumference of the fiber group  14  in the longitudinal direction in a helical or S-Z configuration. The filling  17  of one or more embodiments is a cord-shaped member, but is not limited to a cord shape and may be, for example, a ribbon shape. Although the filling  17  of one or more embodiments is formed by a polypropylene cord, the material of the filling  17  is not limited to polypropylene and may be a different material. For example, the filling  17  may be a water-absorbent member such as a water-absorbent yarn. When the filling  17  is water-absorbent, water running inside the optical cable  1  can be reduced. In FIG.  2 B, the filling  17  is wrapped around the outer circumference of the fiber group  14  in one direction helically. Alternatively, the filling  17  may be wrapped around the outer circumference of the fiber group  14  in an S-Z configuration by being reversed in its wrapping direction midway. 
     The filling  17  may be a member which has a higher cushioning property than the bundling member  16 . Thus, the filling  17  is a member whose cross-sectional shape largely changes when receiving lateral pressure (by contrast, the bundling member  16  is a member with a small amount of deformation and its cross-sectional area deforms very little when receiving lateral pressure). Also, the filling  17  has properties such that its cross-sectional shape deforms even with a small lateral pressure and is easily restored to the original form when the lateral pressure is removed (a high restoration rate). Even if the space inside the optical cable  1  deforms when the cable is bent, the filling  17  having such a cushioning property can follow the deformation of the internal space and keep filling a gap inside the optical cable  1 . Thus, the filling  17  can maintain the postures of the members in the space inside the optical cable  1  (e.g., the optical fiber units  11 ) and reduce the “untwisting” of the optical fiber units  11 . 
     Although  FIG.  2 B  depicts that the optical fiber units  11  (and the fiber groups  14 ) extend linearly in the longitudinal direction, the longitudinal directions of the optical fiber units  11  extend in an S-Z configuration along the longitudinal direction of the optical cable  1  because the plurality of optical fiber units  11  are twisted together inside the optical cable  1  in one or more embodiments. In other words, the optical fiber units  11  (and the fiber groups  14 ) are placed inside the optical cable  1  in an S-Z configuration in the longitudinal direction, and the filling  17  of one or more embodiments is wrapped in a helical or S-Z configuration along the longitudinal direction of such an S-Z configuration fiber group  14 . 
       FIG.  3    is a diagram illustrating the placement of the filling  17 A for a given optical fiber unit  11 A. Only the filling  17 A for the given optical fiber unit  11 A is shown here, and the fillings  17  for the other optical fiber units  11  are not shown.  FIG.  3    shows cross sections of the optical cable  1  at different locations in the longitudinal direction of the optical cable  1 . Note that  FIG.  3    is depicted with the circumferential position of the cross section of the optical cable  1  being changed so that the plurality of optical fiber units  11  inside the optical cable  1  may be at the same positions (because the plurality of optical fiber units  11  are twisted in an S-Z configuration inside the optical cable  1  in one or more embodiments, the positions of the optical fiber units  11  on a cross section of the optical cable  1  are different depending on the longitudinal position of the optical cable  1 ). 
     In one or more embodiments, the filling  17  is wrapped around the outer circumference of the fiber group  14  in the longitudinal direction in a helical or S-Z configuration (see  FIG.  2 B ). Thus, as shown in  FIG.  3   , the filling  17 A for the given optical fiber unit  11 A can be adjacent not only to a particular optical fiber unit  11  but also to the neighboring other optical fiber units  11 . For example, the filling  17 A for the given optical fiber unit  11 A shown in  FIG.  3    is, in a certain cross section, adjacent to the optical fiber unit  11 D of the outer-layer unit  13  and is, in another cross section, adjacent to the optical fiber unit  11 B of the inner-layer unit  12 . In this way, the filling  17 A for the given optical fiber unit  11 A shown in  FIG.  3    is adjacent to a plurality of different optical fiber units  11 . Similarly, the fillings  17  for the other optical fiber units  11  are also adjacent to a plurality of different optical fiber units  11  by being wrapped around the outer circumferences of the fiber groups  14  in the longitudinal direction in a helical or S-Z configuration. The filling  17  for a given optical fiber unit  11  thus being adjacent to a plurality of neighboring different optical fiber units  11  means that the filling  17  is adjacent to many optical fiber units  11 . The filling  17  helps the optical fiber units  11  that are adjacent to the filling  17  maintain their postures (postures twisted in an S-Z configuration) and therefore can inhibit “untwisting” of the optical fiber units  11  by being adjacent to many optical fiber units  11 . In other words, one or more embodiments can reduce the “untwisting” of the optical fiber units  11  with less fillings  17  by wrapping the filling  17  around the outer circumference of the fiber group  14  in the longitudinal direction in a helical or S-Z configuration. 
     Also, in one or more embodiments, as shown in  FIG.  2 B , the filling  17  is wrapped around the outer side of the bundling member  16 . This makes the filling  17  come easily into contact with the neighboring optical fiber units  11  compared to a case where the filling  17  is placed to the inner side of the bundling member  16 , and for this reason, thus one or more embodiments can further reduce the “untwisting” of the optical fiber units  11 . 
     As shown in  FIG.  1 A , in one or more embodiments, not all the optical fiber units  11  have the filling  17 , and there are both optical fiber units  11  having the filling  17  ( 11 A to  11 C,  11 G,  11 J; see  FIG.  2 B ) and optical fiber units  11  not having the filling  17  ( 11 D to  11 F,  11 H,  11 I; see  FIG.  2 A ). Thus, compared to a case where all the optical fiber units  11  have the filling  17 , the fillings  17  inside the optical cable  1  can be decreased. Note that the “untwisting” of the optical fiber units  11  not having the filling  17  is reduced because the optical fiber units  11  not having the filling  17  are adjacent to the fillings  17  for the adjacent optical fiber units  11 , and for this reason, it is permissible that some of the optical fiber units  11  do not have the filling  17 . However, all the optical fiber units  11  may have the filling  17 . 
     Also, in one or more embodiments, as shown in  FIG.  1 A , the three optical fiber units  11  ( 11 A to  11 C) forming the inner-layer unit  12  each have the filling  17 . The optical fiber units  11  forming the inner-layer unit  12  have more adjacent optical fiber units  11  therearound than the optical fiber units  11  ( 11 D to  11 J) forming the outer-layer unit  13 . Thus, when the optical fiber units  11  forming the inner-layer unit  12  have the fillings  17  like in one or more embodiments, the fillings  17  are adjacent to many optical fiber units  11 . This helps the optical fiber units  11  maintain their postures twisted in an S-Z configuration and thus helps reduce the “untwisting” of the optical fiber units  11 . However, the optical fiber units  11  forming the inner-layer unit  12  do not have to have the filling  17 . 
     Further, in one or more embodiments, as shown in  FIG.  1 A , the outer-layer unit  13  includes both optical fiber units  11  having the filling  17  and optical fiber units  11  not having the filling  17 . This can decrease the fillings  17  inside the optical cable  1 . Note that because the optical fiber units  11  of the inner-layer unit  12  have the filling  17  in one or more embodiments, even if some of the optical fiber units  11  of the outer-layer unit  13  do not have the filling  17 , those optical fiber units  11  of the outer-layer unit  13  are at least adjacent to the fillings  17  for the optical fiber units  11  of the inner-layer unit  12 , and therefore the “untwisting” of the optical fiber units  11  of the outer-layer unit  13  can be reduced. 
     Note that, in a case where the outer-layer unit  13  has both optical fiber units  11  having the filling  17  and optical fiber units  11  not having the filling  17 , each optical fiber unit  11  not having the filling  17  may be placed circumferentially between two optical fiber units  11  having the filling  17 . This helps reduce “untwisting” of the optical fiber units  11  even with less fillings  17  inside the optical cable  1 , compared to a case where two optical fiber units  11  having the filling  17  are adjacent circumferentially in the outer-layer unit  13 . 
     Incidentally, in a case where the filling  17  is wrapped around the fiber group  14 , if the filling  17  is wrapped at a small pitch, the work for removing the filling  17  becomes cumbersome, which may make branching work for the optical cable  1  time-consuming. In this respect, a plurality of types of optical cables  1  were created each with a different P1/P2, where P1 is the pitch of wrapping the filling  17  and P2 is the pitch of twisting the plurality of optical fiber units  11 , and dismantlement easiness was evaluated for each of the optical cables  1 . Note that the wrapping pitch P 1  of the filling  17  is the longitudinal length of the fiber group  14  over which the filling  17  helically wrapped around the outer circumference of the fiber group  14  makes a full circle circumferentially around the outer circumference of the fiber group  14 . Also, the twisting pitch P 2  is the longitudinal length of the optical cable  1  between reversion of the twisting direction of the optical fiber units  11  twisted in an S-Z configuration and next reversion of the twisting direction of the optical fiber units  11  in the same direction. The optical cables  1  created had the structure shown in  FIG.  1 A  (or  FIG.  1 B ) and were each formed by twisting in an S-Z configuration ten optical fiber units  11  each formed by five intermittently-coupled four-fiber optical fiber ribbons. Also, five of the ten optical fiber units  11  had the filling  17  wrapped therearound helically. The dismantlement easiness for the optical cable  1  in which the filling  17  was not wrapped around the fiber group  14  but laid longitudinally therealong (P1 is infinity) was used as a reference, and the dismantlement easiness was evaluated for each of the optical cables  1  as “excellent” when the dismantlement easiness was almost the same, “good” when the dismantlement easiness was unproblematic, and “passable” when the dismantlement was possible but time-consuming. 
       FIG.  4    is a table showing the evaluation results of the dismantlement easiness for the optical cables  1 . As shown in  FIG.  4   , P1/P2 may be 0.1 or greater (P1/P2≥0.1). Further, P1/P2 may be 0.5 or greater (P1/P2≥0.5). 
     Note that the filling  17  may be wrapped around the outer circumference of the fiber group  14  in an S-Z configuration instead of being wrapped around the outer circumference of the fiber group  14  helically. When the filling  17  is wrapped around the outer circumference of the fiber group  14  in an S-Z configuration, the work for removing the filling  17  would be easier than in a case where the filling  17  is wrapped helically. Further, the filling  17  may be wrapped around the outer circumference of the fiber group  14  in an S-Z configuration without being joined to other members (e.g., another filling  17  in a case where the optical fiber unit  11  has two or more fillings  17 ). This facilitates the work for removing the filling  17 . Note that it is possible to form the optical fiber unit  11  by wrapping the filling  17  around the outer circumference of the fiber group  14  in an S-Z configuration without the filling  17  being joined to other members because the filling  17  does not need to have a function to bring together the plurality of optical fibers  15  unlike the bundling member  16 . Also, because the filling  17  does not need to have a function to bring together the plurality of optical fibers  15  unlike the bundling member  16 , the number of the fillings  17  wrapped around the outer circumference of the fiber group  14  of the optical fiber unit  11  may be one, and that single filling  17  may be wrapped around the outer circumference of the fiber group  14  in an S-Z configuration. The bundling member  16 , when wrapped around in an S-Z configuration, is joined to another corresponding bundling member  16  in order to bundle the plurality of optical fibers  15 , whereas the number of fillings  17  can be one even when the filling  17  is wrapped around in an S-Z configuration. This helps decrease the number of fillings  17  included in the optical cable  1 . 
     As thus described above, the optical cable  1  of one or more embodiments includes a plurality of optical fiber units  11  each having the fiber group  14  formed by a plurality of optical fibers  15 , and the plurality of optical fiber units  11  are twisted together in an S-Z configuration with the filling  17  being wrapped around the outer circumference of at least one fiber group  14 . According to the optical cable  1  having such a configuration, as shown in  FIG.  3   , the filling  17  can be adjacent to a plurality of neighboring different optical fiber units  11 . Thus, the filling  17  helps the optical fiber units  11  adjacent to the filling  17  maintain their postures twisted in an S-Z configuration. As a result, the “untwisting” of the optical fiber units  11  can be reduced with less fillings  17 . Although five optical fiber units  11  include the filling  17  in the embodiments described above, the “untwisting” of the optical fiber units  11  can be reduced with less fillings  17  as long as at least one of the plurality of optical fiber units  11  includes the filling  17  and the filling  17  is wrapped around the outer circumference of the fiber group  14 . 
     &lt;Manufacturing Method&gt; 
       FIG.  5    is a diagram illustrating a manufacturing system  40  for the optical cable  1 . The manufacturing system  40  has fiber supply sections  41 , bundling apparatuses  42 , filling supply sections  43 , a comber board  44 , an extrusion molding section  45 , and a take-up section  47 . 
     The fiber supply sections  41  are apparatuses (supply sources) configured to supply the optical fibers  15 . In one or more embodiments, the fiber supply sections  41  are each an apparatus (a supply source) configured to supply an intermittently-coupled optical fiber ribbon and is capable of supplying a plurality of optical fibers  15 . Specifically, the fiber supply section  41  is formed of a drum (or a bobbin) around which an intermittently-coupled optical fiber ribbon is wound in advance. Note that the fiber supply section  41  may be formed of an apparatus that manufactures an intermittently-coupled optical fiber ribbon. In one or more embodiments, optical fiber ribbons supplied from the fiber supply sections  41  are supplied as the fiber group  14  to the bundling apparatus  42 . 
     The bundling apparatuses  42  are each an apparatus configured to wrap the bundling member  16  around the outer circumference of the fiber group  14 . In one or more embodiments, the bundling apparatus  42  wraps two bundling members  16  in an S-Z configuration in opposite directions from each other while joining the two bundling members  16  at locations where the wrapping directions are reversed. Alternatively, the bundling apparatus  42  may wrap the bundling member  16  around the outer circumference of the fiber group  14  in one direction helically. As a result of the bundling apparatus  42  wrapping the bundling members  16  around the fiber group  14 , the optical fiber unit  11  shown in  FIG.  2 A  is formed. Note that the bundling apparatus  42  may be omitted in a case where the optical fiber unit  11  is formed without the bundling members  16 . 
     The filling supply sections  43  are each an apparatus (a supply source) configured to supply the filling  17 . For example, the filling supply section  43  is formed of a drum (or a bobbin) around which the filling  17  is wound in advance. 
       FIGS.  6 A and  6 B  are diagrams illustrating the comber board  44 .  FIG.  6 A  is a diagram illustrating the comber board  44 .  FIG.  6 B  is a diagram illustrating a state where the optical fiber units  11  (the fiber groups  14  and the bundling members  16 ) and the fillings  17  are inserted through the insertion holes  441  in the comber board  44 . 
     The comber board  44  is a plate-shaped member having the plurality of insertion holes  441 . The insertion holes  441  are through-holes penetrating through the comber board  44  and are holes for inserting the fiber groups  14  and the fillings  17 . In one or more embodiments, the insertion holes  441  are formed in a circular shape. Toward the respective insertion holes  441  in the comber board  44 , the optical fiber units  11  (the fiber groups  14 ) are supplied from the bundling apparatuses  42 , while the fillings  17  are supplied from the filling supply sections  43 . As shown in  FIG.  5   , the direction in which the optical fiber units  11  are supplied to the comber board  44  and the direction in which the fillings  17  are supplied to the comber board  44  are different. Specifically, the direction in which the optical fiber units  11  are supplied to the comber board  44  is almost perpendicular to the comber board  44 , whereas the direction in which the fillings  17  are supplied to the comber board  44  is oblique to the direction perpendicular to the comber board  44 . 
     The comber board  44  is oscillated about its center rotation axis with the fiber groups  14  and the fillings  17  being inserted through the insertion holes  441 . By the oscillating of the comber board  44 , the plurality of optical fiber units  11  are twisted together in an S-Z configuration. The comber board  44  of one or more embodiments also has a function to place the fillings  17  around the outer circumference of the fiber groups  14  in an S-Z configuration by oscillating. 
       FIGS.  7 A to  7 C  are diagrams illustrating how the comber board  44  rotates. Here, for the sake of illustration,  FIGS.  7 A to  7 C  show a state where only one insertion hole  441 A has the fiber group  14  and the filling  17 A inserted therethrough. 
     As shown in  FIGS.  7 A to  7 C , by the oscillating of the comber board  44 , the circumferential position of the fiber group  14  changes. As a result, the fiber group  14  (the optical fiber unit  11 A) is placed in an S-Z configuration in the longitudinal direction inside the optical cable  1 . 
     As already described, in one or more embodiments, the direction in which the filling  17  is supplied to the comber board  44  is oblique to the direction perpendicular to the comber board  44 . This helps the filling  17  to be placed, inside the insertion hole  441 , toward the side where the filling supply section  43  is. For example, the filling supply section  43  (see  FIG.  5   ) for the filling  17  shown in  FIGS.  7 A to  7 C  is placed upward of the insertion hole  441  in the comber board  44  as seen in  FIG.  5   , which consequently helps the filling  17 A shown in  FIGS.  7 A to  7 C  to be placed, inside the insertion hole  441 A, toward the upper side (the upper edge) of the insertion hole  441 . In one or more embodiments, by oscillating of the comber board  44  with the filling  17  thus being placed toward a particular direction inside the insertion hole  441 , the filling  17  can be wrapped around the outer circumference of the fiber group  14  in the longitudinal direction in an S-Z configuration. 
     Note that in one or more embodiments, the insertion holes  441  are formed in a circular shape. Thus, when the comber board  44  is oscillated as shown in  FIGS.  7 A to  7 C , the fiber group  14  and the filling  17  can easily slide against the rim of the insertion hole  441  circumferentially (the direction along the rim of the insertion hole  441 ), which helps the filling  17  to be placed toward a particular direction (the upper side here) inside the insertion hole  441 . However, the shape of the insertion holes  441  does not have to be a circular shape and may be other shapes as long as the filling  17  can be placed toward a particular direction inside the insertion hole  441 . 
     Also, in one or more embodiments, by oscillating of the comber board  44  with the fillings  17  placed inside the insertion holes  441  in positions toward a particular direction, each filling  17  can be adjacent not only to a particular optical fiber unit  11 , but also to neighboring other optical fiber units  11 . For example, the filling  17 A shown in  FIG.  7 A  can, in the state shown in  FIG.  7 B , come adjacent to the optical fiber unit  11 D (not shown in  FIG.  7 B ; see  FIG.  3   ) inserted through the insertion hole  441 D and can, in the state shown in  FIG.  7 C , come adjacent to the optical fiber unit  11 B (not shown in  FIG.  7 C ; see  FIG.  3   ) inserted through the insertion hole  441 B. Similarly, the fillings  17  inserted through other insertion holes  441  can come adjacent to a plurality of different optical fiber units  11  when the comber board  44  is oscillated with the fillings  17  placed toward a particular direction inside the insertion holes  441 . 
     As shown in  FIG.  5   , the plurality of optical fiber units  11  having passed through the comber board  44  are supplied to the extrusion molding section  45  in the state of being twisted in an S-Z configuration. The extrusion molding section  45  is supplied not only with the plurality of optical fiber units  11 , but also with other members such as the press-wrapping tape  18 , the tension member  21 , and the rip cord  22 . 
     The extrusion molding section  45  is an apparatus that forms the sheath  20 . In the extrusion molding section  45 , the press-wrapping tape  18  is wrapped around the plurality of optical fiber units  11 , and a resin to be the sheath  20  is extruded, thereby manufacturing the optical cable  1  of one or more embodiments shown in  FIG.  1 A  (or  FIG.  1 B ). The optical cable  1  manufactured by the extrusion molding section  45  is cooled by a cooling apparatus  46  and is then taken up by the take-up section  47  (e.g., a drum). 
     As thus described, the method for manufacturing the optical cable  1  of one or more embodiments performs inserting the fiber groups  14  into the respective plurality of insertion holes  441  in the comber board  44 , inserting the filling  17  into at least one of the insertion holes  441  in the comber board  44 , and oscillating the comber board  44  to wrap the filling  17  around the outer circumference of at least one of the fiber groups  14 . Such a manufacturing method can manufacture the optical cable  1  while reducing the “untwisting” of the optical fiber units  11  with less fillings  17 . 
       FIG.  8    is a diagram illustrating a manufacturing system  40  of a modification. The manufacturing system  40  of the modification has the fiber supply sections  41 , the bundling apparatuses  42 , filling wrapping sections  43 ′, the comber board  44 , the extrusion molding section  45 , and the take-up section  47 . Compared to the manufacturing system  40  shown in  FIG.  5   , the manufacturing system  40  of the modification has the filling wrapping sections  43 ′ in place of the above-described filling supply sections  43  (see  FIG.  5   ). The filling wrapping sections  43 ′ are each an apparatus that wraps the filling  17  around the outer circumference of the fiber group  14 . Here, the filling wrapping section  43 ′ is an apparatus that wraps the filling  17  around the outer circumference of the fiber group  14  helically. The filling  17  may be wrapped around the outer circumference of the fiber group  14  in an S-Z configuration as long as the filling  17  does not come off. 
     In the modification as well, the comber board  44  is oscillated about its center rotation axis with the fiber groups  14  and the filling  17  inserted through the insertion holes  441 . By the oscillating of the comber board  44 , the plurality of optical fiber units  11  are twisted together in an S-Z configuration. In the modification, the plurality of optical fiber units  11  are twisted together in an S-Z configuration with the filling  17  being wrapped around the outer circumference of the fiber group  14  in a helical or S-Z configuration. For this reason, in the modification, it is easier to separately set the pitch P 1  for wrapping the filling  17  and the pitch P 2  for twisting the plurality of optical fiber units  11 . 
     &lt;Cushioning Property of the Filling  17 &gt; 
     As already described, the filling  17  is a member whose cross-sectional shape changes greatly when receiving lateral pressure. A compression rate is an example of an index indicating a change in cross-sectional shape upon receipt of lateral pressure. A compression rate R of a member can be expressed as follows, where D1 (mm) is the diameter of the member before receiving lateral pressure and D2 (mm) is the diameter of the member when receiving lateral pressure. 
         R =( D 1− D 2)/ D 2
 
     Also, a compression rate R of a member can be expressed as follows, where L1 (mm) is the length of the outer circumference of the member before receiving lateral pressure (an initial circumferential length) and L2 (mm) is the length of the outer circumference of the member when receiving lateral pressure (a circumferential length). 
         R =( L 1− L 2)/ L 2
 
       FIG.  9 A  is a diagram illustrating how the compression rate R is measured.  FIG.  9 B  is a diagram illustrating how a cross-sectional shape changes before and after reception of lateral pressure. 
     As shown in  FIG.  9 A , a tension is given to a member to be measured  19  (e.g., the filling  17 ) while securing one end of the member to be measured  19  and attaching a weight to the other end thereof. Here, a 200 g weight is given (a tension of approximately 2 N is given) so that the same tension as that exerted to the filling  17  inside the optical cable  1  may be exerted. 
     The left side of  FIG.  9 B  shows a state before a load is applied to a winding member  53 . The right side of  FIG.  9 B  shows a state where a load is being applied to the winding member  53 . 
     As shown in  FIGS.  9 A and  9 B , the cord-shaped winding member  53  is wound around the outer circumference of the member to be measured  19  (e.g., the filling  17 ). Also, as shown in  FIGS.  9 A and  9 B , one end of the member to be measured  19  is secured, and the other end thereof has a measurement apparatus  52  attached thereto. The measurement apparatus  52  measures a load P(N) applied to the winding member  53  and a displacement X (mm) of an end portion of the winding member  53  relative to a reference position X 0 . 
     As shown in  FIG.  9 A , at the reference position X 0  before the application of a load to the winding member  53 , the member to be measured  19  has the initial circumferential length L 1  and the diameter D 1 . As shown in  FIG.  9 B , once the load P (a tensile load) is applied to the winding member  53 , lateral pressure is evenly given to the outer circumference of the member to be measured  19 , and the cross-sectional shape of the member to be measured  19  is compressively deformed, so that the member to be measured  19  has the circumferential length L 2  and the diameter D 2  (the member to be measured  19  becomes denser). As shown in  FIG.  9 B , when a load is applied to the winding member  53 , an end portion of the winding member  53  is displaced. By measuring the displacement X of the end portion of the winding member  53  relative to the reference position X 0  using the measurement apparatus  52 , the circumferential length L 2  (or the diameter D 2 ) of the member to be measured  19  can be measured, and the compression rate R can thus be calculated. 
       FIG.  10    is measurement results of the compression rates of fillings, a bundling member, and Kevlar. Here, three types of fillings (fillings  1  to  3 ), a bundling member, and Kevlar were measured as members to be measured. Note that in optical cables formed by wrapping the bundling member or Kevlar, which is a measurement target, around the outer circumferences of the optical fiber units, untwisting of the plurality of twisted optical fiber units occurred, whereas in optical cables formed by wrapping the filling (fillings  1  to  3 ), which is a measurement target, around the outer circumferences of the optical fiber units, untwisting of the plurality of optical fiber units was reduced. The fillings  2 ,  3  are water-absorbent fillings, or more specifically, water-absorbent yarns. Also, here, the applied load P was varied in the range from 0.0 N to 2.5 N, and as shown in  FIG.  9 B , the circumferential length L 2  when the load P was applied to the winding member  53  was measured, and then the compression rate R was measured based on the initial circumferential length L 1  and the circumferential length L 2 . 
       FIG.  11    is a graph showing the relation between the applied load P and the compression rate R. The horizontal axis of the graph represents the tensile load P(N) applied to the winding member  53  in  FIGS.  9 A and  9 B . The vertical axis of the graph represents the compression rate R of the member to be measured  19 . 
     As shown in  FIG.  11   , with the bundling member and the Kevlar, when the applied load P was 1.0 N or greater, the compression rate R changed very little, and the amount of change of the compression rate R was within the margin of measurement error. By contrast, with the fillings (the fillings  1  to  3 ), when the applied load P was 1.0 N or greater, the compression rate R increased as the applied load P increased. In other words, the graph shows that the bundling member and the Kevlar are members such that their cross-sectional shapes change not easily upon reception of lateral pressure, whereas the fillings are members such that their cross-sectional shapes change greatly upon reception of lateral pressure compared to the bundling member and the Kevlar. A member whose cross-sectional shape changes greatly when receiving lateral pressure can continue filling a gap inside the optical cable  1  and therefore can maintain the postures of members in the space inside the optical cable  1  (e.g., the optical fiber units  11 ). Thus, the filling may be such that its compression rate R increases as the applied load P increases as shown in  FIG.  11   . Particularly, the filling may be such that its compression rate R increases as the applied load P increases when the applied load P is 1.0 N or greater. Note that it is believed that the compression rate R increasing as the applied load P increases when the applied load P is in a range exceeding 1.0 N is also effective in reducing microbending losses of the optical fibers. 
     As shown in  FIGS.  10  and  11   , with the bundling member and the Kevlar, the compression rate R did not change when the applied load P was in a range from 1.5 N to 2.0 N. This means that once the applied load P reached 1.5 N to 2.0 N, the cross-sectional shapes of the bundling member and the Kevlar did not change even if lateral pressure changed, and therefore means that the cross-sectional shapes of the bundling member and the Kevlar did not change once the applied load P exceeded 1.5 N. By contrast, with the filling (the fillings  1  to  3 ), when the applied load P was in a range from 1.5 N to 2.0 N, the compression rate R increased as the applied load P increased. This means that, unlike the bundling member and the Kevlar, the cross-sectional shape of the filling (the fillings  1  to  3 ) was able to change even after the applied load P exceeded 1.5 N. In this way, when the filling (the fillings  1  to  3 ) is compared with the bundling member and the Kevlar, when the applied load P was in a range from 1.5 N to 2.0 N, there was a great difference in change in the compression rate R, and based on such a difference in change in the compression rate R, it is believed that untwisting of optical fiber units was reduced in the optical cable formed by wrapping the filling (the fillings  1  to  3 ) around the outer circumferences of the optical fiber units. Thus, the fillings may have a property such that the compression rate R increases as the applied load P increases at least when the applied load P is in a range from 1.5 N to 2.0 N. 
     The right side of  FIG.  10    shows the rate of change α of the compression rate when the applied load P is in a range from 1.5 N to 2.0 N (the ratio of the amount of increase in the compression rate R to the amount of increase in the applied load P). A member having a large rate of change α of the compression rate has a property such that its compression rate R increases easily as the applied load P increases. The rate of change α of the compression rate of each member shown in  FIG.  10    corresponds to the slope of the graph in  FIG.  11    when the applied load P is in a range from 1.5 N to 2.0 N and corresponds to the slope of a line connecting two measurement results in  FIG.  11    when the applied load P is in a range from 1.5 N to 2.0 N. The rates of change α of the compression rate of the fillings are 0.17 to 0.26 (unit: N −1 ) (by contrast, the rate of change α of the compression rate of the bundling member or Kevlar is almost zero). In this way, the filling  17  wrapped around the outer circumference of the fiber group  14  described above may be a member such that the rate of change α of the compression rate (the ratio of the amount of increase in the compression rate R to the amount of increase in the applied load P) when the applied load P is in a range from 1.5 N to 2.0 N is 0.17 or greater. 
     As shown in  FIGS.  10  and  11   , the maximum value of the compression rate R of the bundling member was 0.40, and the maximum value of the compression rate R of the Kevlar was 0.33. By contrast, the maximum values of the compression rate R of the fillings  1  to  3  were 0.91, 0.57, and 0.66, respectively, and the compression rate R was large compared to those of the bundling member and the Kevlar. This is because with the bundling member and the Kevlar, the compression rate R changed very little when the applied load P was 1.0 N or greater, whereas with the filling (the fillings  1  to  3 ), the compression rate R changed even when the applied load P was 1.0 N or greater and its cross section greatly changed. As thus demonstrated from this point, while the bundling member and the Kevlar are members such that their cross-sectional shapes change not easily upon reception of lateral pressure, the filling is, compared to the bundling member and the Kevlar, a member whose cross-sectional shape greatly changes when receiving lateral pressure. As described above, a member whose cross-sectional shape greatly changes when receiving lateral pressure can continue filling a gap inside the optical cable  1  and therefore can maintain the postures of the members in the space inside the optical cable  1  (e.g., the optical fiber units  11 ). For this reason, the filling may be configured to deform such that the compression rate R is 0.57 or greater. 
     It was confirmed that when the load applied to the winding member  53  was canceled (when the lateral pressure applied to the member to be measured  19  was removed) after the measurement of the compression rate, the cross-sectional shape of the filling (the fillings  1  to  3 ) greatly changed and was restored almost to the pre-measurement cross-sectional shape. In this way, the filling  17  wrapped around the outer circumference of the fiber group  14  described above may have a property such that its cross-sectional shape is easily restored after removal of lateral pressure (a high restoration rate). 
       FIG.  12    is a diagram illustrating a cross section of the filling  17  in the optical cable  1 .  FIG.  12    is a diagram enlarging and illustrating an area around a certain filling  17  in the optical cable  1  shown in  FIG.  1 B . The cross section of the filling  17  in  FIG.  12    is hatched according to density. Here, a high-density part of the filling  17  is hatched densely, and a low-density part thereof is hatched less densely. 
     As shown in  FIG.  12   , a cross section of the filling  17  has regions having different densities from each other. The filling  17  has a relatively low density in a part filling a relatively large gap inside the optical cable  1  (the less densely hatched region) and has a relatively high density in a part filling a relatively small gap inside the optical cable  1  (the densely hatched region). Thus, the high-density area is receiving a larger lateral pressure from the surroundings than the low-density area, and as a result, is believed to be greatly compressively deformed. A given cross section of the filling  17  may have regions having different densities from each other in this way. In other words, in a given cross section of the filling  17 , the density of the filling  17  may be uneven. To put it differently, the filling  17  may be configured to deform so that there may be regions having different densities from each other in a given cross section (the filling  17  may be configured to deform so that the density may be different depending on a region). By using such a filling  17 , the filling  17  can continue filing a gap inside the optical cable  1  by following the deformation of the internal space, and can maintain the postures of the members in the space inside the optical cable  1  (e.g., the optical fiber units  11 ). 
     Other Embodiments 
     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 disclosure. Accordingly, the scope of the disclosure should be limited only by the attached claims.