Patent Publication Number: US-2023142376-A1

Title: Optical fiber cable

Description:
TECHNICAL FIELD 
     The present disclosure relates to an optical fiber cable. 
     The present application is based on and claims priority from Japanese Patent Application No. 2020-111836 filed on Jun. 29, 2020, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND ART 
     In the related art, as a trunk optical fiber cable or a micro duct cable wired to a thin pipe by air pressure feeding, a loose tube-type cable in which optical fiber units each obtained by bundling a plurality of optical fibers are respectively covered with a resin tube and are covered with a cable sheath is known. Further, a slotless cable in which a resin tube is omitted and optical fibers are mounted in a cable sheath at a high density is also known (for example, Patent Literatures 1, 2, and 3). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-T-2015-517679 
     Patent Literature 2: JP-A-2010-008923 
     Patent Literature 3: JP-A-2014-071441 
     SUMMARY OF INVENTION 
     In order to solve the above problem, an optical fiber cable according to the present disclosure includes: 
     a plurality of optical fibers; 
     a tensile strength member set that is disposed along the plurality of optical fibers; and 
     a sheath that covers the plurality of optical fibers from outside and encloses the tensile strength member set therein, 
     at least four tensile strength member sets are embedded in the sheath in a manner of being apart from each other, 
     the sheath contains a flame-retardant inorganic substance and a release agent, and 
     a distance from one of the tensile strength member sets to a surface layer of the sheath is 0.5 mm or more. 
     Further, an optical fiber cable according to the present disclosure includes: 
     a plurality of optical fibers; 
     a tensile strength member set that are disposed along the plurality of optical fibers; and 
     a sheath that covers the plurality of optical fibers from outside and encloses the tensile strength member set therein, 
     at least four tensile strength member sets are embedded in the sheath in a manner of being apart from each other, 
     the sheath includes an outer layer and an inner layer, 
     the inner layer contains a flame-retardant inorganic substance, and 
     the outer layer contains a release agent. 
    
    
     
       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 partially developed view showing an optical fiber ribbon accommodated in the optical fiber cable in a longitudinal direction thereof. 
         FIG.  3    is a cross-sectional view of an optical fiber cable according to a first modification of the first embodiment. 
         FIG.  4    is a cross-sectional view of an optical fiber cable according to a second modification of the first embodiment. 
         FIG.  5    is a cross-sectional view of an optical fiber cable according to a third modification of the first embodiment. 
         FIG.  6    is a cross-sectional view of an optical fiber cable according to a fourth modification of the first embodiment. 
         FIG.  7    is a schematic diagram showing an experiment for measuring dynamic friction coefficients of the optical fiber cables of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Technical Problem 
     The loose tube-type cable includes a tension member at a center thereof. Therefore, wiring based on air pressure feeding is easy without anisotropy in a bending direction. However, in general, a film of the resin tube covering the optical fiber unit is formed to be thick. Therefore, it is difficult to reduce an outer diameter of the loose tube-type cable and to mount the optical fibers at a high density. 
     On the other hand, since the slotless cable does not include a resin tube, the optical fibers may be mounted at a high density. However, in the slotless cable, since a plurality of tension members are embedded in the sheath, rigidity of the cable is not uniform depending on positions where the tension members are embedded. As a result, anisotropy in the bending direction is generated, and buckling may occur at the time of air pressure feeding. 
     In recent years, there has been an increasing demand for a flame-retardant and low-smoking micro duct cable due to strengthening of regulations on buildings. In order to satisfy the requirement of flame retardance, a flame-retardant resin is used for the optical fiber cable. However, since the flame-retardant resin is generally formed of a soft material having a high friction coefficient, it is difficult to satisfy a low friction property required for the air pressure feeding. 
     The present disclosure provides a flame-retardant optical fiber cable that enables easy wiring based on air pressure feeding. 
     DESCRIPTION OF EMBODIMENT OF PRESENT DISCLOSURE 
     First, an embodiment of the present disclosure will be listed and described. 
     (1) An optical fiber cable according to an aspect of the present disclosure includes: 
     a plurality of optical fibers; 
     a tensile strength member set that is disposed along the plurality of optical fibers; and 
     a sheath that covers the plurality of optical fibers from outside and encloses the tensile strength member set therein, 
     at least four tensile strength member sets are embedded in the sheath in a manner of being apart from each other, 
     the sheath contains a flame-retardant inorganic substance and a release agent, and 
     a distance from one of the tensile strength member sets to a surface layer of the sheath is 0.5 mm or more. 
     According to the optical fiber cable of the present disclosure, since the sheath contains the flame-retardant inorganic substance, a flame-retardant optical fiber cable may be provided. Since the sheath contains the release agent, wiring based on air pressure feeding is facilitated. Further, since at least four tensile strength member sets are embedded in the sheath in a manner of being apart from each other, unevenness in rigidity of the cable due to positions where the tensile strength member sets are embedded is improved, and the cable is less likely to buckle during air pressure feeding. 
     In general, the tensile strength member is made of a combustible material. When the tensile strength member is embedded in the vicinity of the surface layer of the sheath, flame retardance of the optical fiber cable decreases. However, in the present disclosure, since the distance from one tensile strength member set to the surface layer of the sheath is 0.5 mm or more, the flame retardance of the optical fiber cable is improved. 
     (2) The tensile strength member set may be one tensile strength member or two tensile strength members that are paired. 
     According to the present disclosure, since the tensile strength member set is one tensile strength member or two tensile strength members that are paired, rigidity of the optical fiber cable is maintained. In the case where the tensile strength member set is two tensile strength members that are paired, the rigidity of the optical fiber cable is further increased. 
     (3) The sheath may contain magnesium hydroxide or aluminum hydroxide as the flame-retardant inorganic substance. 
     According to the present disclosure, since the sheath contains magnesium hydroxide or aluminum hydroxide as the flame-retardant inorganic substance, the flame retardance of the optical fiber cable is improved. 
     (4) The tensile strength member sets may be arranged at equal intervals. 
     According to the present disclosure, since four or more tensile strength member sets are disposed at equal intervals, unevenness in rigidity of the cable due to positions where the tensile strength member sets are disposed is improved, and the cable is less likely to buckle during air pressure feeding. 
     (5) The optical fiber cable may further include at least one fibrous filler arranged along the plurality of optical fibers. 
     The tensile strength member sets may be arranged in line symmetry with respect to a straight line connecting the fibrous filler and a center of the optical fiber cable in a cable cross-sectional view. 
     Since the optical fiber cable of the present disclosure includes the fibrous filler, the optical fibers may be easily taken out from the optical fiber cable. Further, since four or more tensile strength member sets are arranged in line symmetry with respect to the straight line connecting the fibrous filler and the center of the optical fiber cable, the unevenness in rigidity of the cable is improved, and the cable is less likely to buckle during air pressure feeding. 
     (6) The optical fiber cable may further include an upper wound tape that covers the plurality of optical fibers from outside. 
     The upper wound tape may contain the flame-retardant inorganic substance. 
     The upper wound tape is disposed between the sheath and the plurality of optical fibers. 
     According to the present disclosure, since the upper wound tape containing the flame-retardant inorganic substance is disposed between the sheath and the plurality of optical fibers, the flame retardance of the optical fiber cable is further improved. 
     (7) An optical fiber cable according to another aspect of the present disclosure includes: 
     a plurality of optical fibers; 
     a tensile strength member set that are disposed along the plurality of optical fibers; and 
     a sheath that covers the plurality of optical fibers from outside and encloses the tensile strength member set therein, 
     at least four tensile strength member sets are embedded in the sheath in a manner of being apart from each other, 
     the sheath includes an outer layer and an inner layer, 
     the inner layer contains a flame-retardant inorganic substance, and 
     the outer layer contains a release agent. 
     According to the present disclosure, since the inner layer of the sheath contains the flame-retardant inorganic substance, flame retardance of the optical fiber cable may be improved. Further, since the outer layer of the sheath contains the release agent, wiring based on air pressure feeding is facilitated. 
     (8) A distance from one of the tensile strength member sets to a surface layer of the outer layer may be 0.5 mm or more. 
     According to the present disclosure, since the distance from one of the tensile strength member sets to the surface layer of the outer layer is 0.5 mm or more, the flame retardance of the optical fiber cable is improved. 
     (9) The optical fiber cable includes: 
     the plurality of optical fibers in a form of an intermittent-connection-type optical fiber ribbon in which, in a state where the plurality of optical fibers are arranged adjacently in a direction orthogonal to a longitudinal direction of the plurality of optical fibers, a connected portion in a state where adjacent optical fibers are connected and a non-connected portion in a state where adjacent optical fibers are not connected are intermittently provided in the longitudinal direction in a part or all of the plurality of optical fibers. 
     Since the optical fiber cable of the present disclosure may also use the optical fiber ribbon of intermittent connection type, the optical fiber cable has an excellent cable accommodation property and enables easy single-core separation. 
     Advantageous Effects of Invention 
     According to the present disclosure, a flame-retardant optical fiber cable that enables easy wiring based on air pressure feeding may be provided. 
     DETAILS OF EMBODIMENTS OF PRESENT DISCLOSURE 
     A specific example of an optical fiber cable according to an embodiment of the present disclosure will be described with reference to the drawings. 
     The present disclosure is not limited to these examples but defined by the scope of the claims, and is intended to include all modifications within the scope and meaning equivalent to the scope of the claims. 
     First Embodiment 
     An optical fiber cable  1 A according to a first embodiment will be described with reference to  FIGS.  1  and  2   . 
       FIG.  1    is a cross-sectional view perpendicular to a longitudinal direction of the optical fiber cable  1 A. As shown in  FIG.  1   , the optical fiber cable  1 A includes a plurality of optical fibers in a form of a plurality of optical fiber ribbons  2 , a water absorbing tape  3  covering a periphery of the optical fiber ribbons  2 , a sheath  4  covering the periphery of the water absorbing tape  3 , tensile strength members  5  (tensile strength member sets  50 ) embedded in the sheath  4 , at least one tearing string  6  (fibrous filler), and a plurality of protrusions  7 . A cable outer diameter of the optical fiber cable  1 A is, for example, 14 mm. 
     The water absorbing tape  3  is wound around the entire periphery of the plurality of optical fiber ribbons  2 , for example, longitudinally or spirally. The water absorbing tape  3  is, for example, a tape obtained by performing water absorption processing by adhering water absorbing powders to a base cloth made of polyester or the like. A thickness of the water absorbing tape  3  is, for example, 0.3 mm. In the present embodiment, the optical fiber cable  1 A includes the water absorbing tape  3 , but the optical fiber cable  1 A may not include the water absorbing tape  3 . 
     The sheath  4  is provided to cover the plurality of optical fiber ribbons  2  from outside and to enclose the tensile strength members  5  (tensile strength member sets  50 ) therein. A thickness of the sheath  4  is, for example, 1.5 mm. The sheath  4  is mainly made of, for example, a vinyl resin such as polyvinyl chloride (PVC) or a polyolefin resin such as polyethylene (PE). Further, the sheath  4  contains a flame-retardant inorganic substance and a release agent. Examples of the flame-retardant inorganic substance include magnesium hydroxide and aluminum hydroxide. Examples of the release agent include silicon-based release agents such as silicon or siloxane. 
     The tensile strength members  5  are disposed in the longitudinal direction of the optical fiber cable  1 A along the plurality of optical fiber ribbons  2 , and are provided to be embedded in the sheath  4 . A diameter of the tensile strength member  5  is, for example, 0.5 mm. The tensile strength member  5  is made of, for example, fiber-reinforced plastic (FRP) such as aramid FRP, glass FRP, or carbon FRP. The tensile strength member  5  may be made of a liquid crystal polymer. The tensile strength member  5  is preferably non-inductive. The fiber-reinforced plastic (FRP) is generally a combustible material. From the viewpoint of improving flame retardance of the entire optical fiber cable  1 A, it is preferable that the tensile strength members  5  are disposed inside the sheath  4  at a position close to a center of the optical fiber cable  1 A, but not in the vicinity of a surface layer of the sheath  4 . 
     The tensile strength member  5  is formed to have a radial cross section of a circular shape. In the optical fiber cable  1 A, at least four (four in the present example) tensile strength members  5  are provided. As to be described later, in the present disclosure, the plurality of tensile strength members  5  may be provided such that every two tensile strength members are paired. In the following description, one tensile strength member  5  or two tensile strength members  5  that are paired are collectively referred to as the tensile strength member set  50 . 
     The tensile strength member  50  may be a set including three tensile strength members  5  or four or more tensile strength members  5 . 
     In the present example, four tensile strength member sets  50  are embedded in the sheath  4  in a manner of being apart from each other. In the optical fiber cable  1 A, four tensile strength members  5  (four tensile strength member sets  50 ) are arranged at equal intervals. Specifically, in the radial cross section of the optical fiber cable  1 A, the four tensile strength members  5  (tensile strength member sets  50 ) are provided at positions facing each other across a center of the optical fiber cable  1 A. The four tensile strength members  5  (the tensile strength member sets  50 ) in the radial cross section of the optical fiber cable  1 A are arranged such that two straight lines each connecting the facing tensile strength members  5  (the tensile strength member sets  50 ) are orthogonal to each other. 
     In the present example, a distance d from a position on an outer periphery of one tensile strength member  5  (tensile strength member set  50 ) to the surface layer of the sheath  4  is 0.5 mm or more. Here, the position on the outer periphery of the tensile strength member  5  is a position at which a straight line L 1  connecting the center of the optical fiber cable  1 A and a center of the tensile strength member  5  intersects with the outer periphery of the tensile strength member  5  on a cable outer peripheral side in the cable cross-sectional view. In the present example, the distance d is 0.5 mm or more for each of all the tensile strength members  5  (tensile strength member sets  50 ). 
     The tearing string  6  is provided to tear the sheath  4 , and is disposed inside the sheath  4  along the plurality of optical fiber ribbons  2  in the longitudinal direction of the optical fiber cable  1 A. In the present example, two tearing strings  6  are provided. The two tearing strings  6  are provided to face each other such that each is at a substantially intermediate position between adjacent tensile strength members  5  (tensile strength member sets  50 ). In addition, the four tensile strength members  5  (tensile strength member sets  50 ) are arranged in line symmetry with respect to a straight line L 2  connecting the tearing strings  6  and the center of the optical fiber cable  1 A in the cable cross-sectional view. An operator may tear the sheath  4  in the longitudinal direction and take out the optical fiber ribbons  2  by pulling out the tearing strings  6 . The tearing string  6  is fibrous, and is made of, for example, a plastic material (for example, polyester) resistant to tension. 
     A plurality of (two in the present example) protrusions  7  are provided. The two protrusions  7  are provided along the longitudinal direction of the optical fiber cable  1 A. Each of the protrusions  7  may be provided continuously or intermittently along the longitudinal direction. In addition, the two protrusions  7  are provided to face each other across the center of the optical fiber cable  1 A in a peripheral direction of an outer peripheral portion of the sheath  4  in the radial cross section of the optical fiber cable  1 A. In the present example, the protrusions  7  are provided on the straight line L 2  connecting the tearing strings  6  and the center of the optical fiber cable  1 A. The protrusion  7  is formed on the outer peripheral portion of the sheath  4  in a state of protruding in the radial direction of the optical fiber cable  1 A. A surface  7   a  of the protrusion  7  in a protruding direction is formed as a curved surface. The protrusions  7  are formed integrally with the sheath  4  by extrusion molding. 
       FIG.  2    is a partially developed view showing the optical fiber ribbons  2  accommodated in the optical fiber cable  1 A in the longitudinal direction thereof. As shown in  FIG.  2   , the optical fiber ribbon  2  is an intermittent-connection-type optical fiber ribbon in which, in a state where a plurality of optical fibers  11 A to  11 L are arranged adjacently in a direction orthogonal to the longitudinal direction, connected portions  12  at each of which adjacent optical fibers are connected and non-connected portions  13  at each of which adjacent optical fibers are not connected are intermittently provided in the longitudinal direction in a part or all of the plurality of optical fibers  11 A to  11 L. An outer diameter of each of the optical fibers  11 A to  11 L is, for example, 200 μm, and may be 250 μm or 180 μm. 
     In the optical fiber ribbon  2  of the present example, 12 optical fibers  11 A to  11 L are arranged adjacently. The connected portion  12  and the non-connected portion  13  may be intermittently provided between a part of the optical fibers (intermittently provided between every two optical fibers) as shown in  FIG.  2   , or between all of the optical fibers (intermittently provided every one optical fiber).  FIG.  2    shows an example in which the connected portion  12  and the non-connected portion  13  are intermittently provided between every two fibers, and the non-connected portion  13  is not provided between the optical fibers  11 A and  11 B,  11 C and  11 D,  11 E and  11 F,  11 G and  11 H,  11 I and  11 J, and  11 K and  11 L. 
     The connected portion  12  in the optical fiber ribbon  2  is formed by applying a connecting resin  14  made of, for example, an ultraviolet curable resin, or a thermosetting resin between the optical fibers. By applying the connecting resin  14  between predetermined optical fibers, the connected portions  12  and the non-connected portions  13  are intermittently provided, and the optical fibers  11 A to  11 L are integrated adjacently. The connecting resin  14  may be applied to only one side of the adjacent surfaces formed by the optical fibers  11 A to  11 L arranged adjacently, or may be applied to both sides. In addition, the optical fiber ribbon  2  may be manufactured such that, for example, a tape resin is applied to one surface or both surfaces of all of the optical fibers  11 A to  11 L arranged adjacently, all of the optical fibers  11 A to  11 L are connected, and then a part of the optical fibers are cleaved by a rotary blade or the like to form the non-connected portions  13 . 
     As described above, in the present embodiment, since the sheath  4  contains the flame-retardant inorganic substance, the optical fiber cable  1 A excellent in flame retardance may be provided. In addition, since the sheath  4  also contains the release agent, the optical fiber cable  1 A may be smoothly fed even in a duct during air pressure feeding. Accordingly, the wiring is facilitated. 
     In the present embodiment, since four tensile strength member sets  50  (four tensile strength members  5 ) are embedded in the sheath  4  in a manner of being apart from each other, unevenness in rigidity of the cable due to the positions where the tensile strength member sets  50  are embedded is improved. Therefore, the optical fiber cable  1 A which is less likely to be bent during air pressure feeding may be provided. 
     In general, the tensile strength member is a combustible material. When the tensile strength member is embedded in the vicinity of the surface layer of the sheath, flame retardance of the optical fiber cable decreases. However, in the present disclosure, since the distance from one tensile strength member set  50  to the surface layer of the sheath  4  is 0.5 mm or more, the flame retardance of the optical fiber cable  1 A is improved. 
     In the present embodiment, since the tensile strength member set  50  is one tensile strength member  5  or two tensile strength members  5  that are paired, the rigidity of the optical fiber cable  1 A is maintained. In a case where the tensile strength member set  50  includes the two tensile strength members  5  that are paired, the rigidity of the optical fiber cable  1 A is further increased. 
     In the present embodiment, the sheath  4  contains the flame-retardant inorganic substance. Specifically, the sheath  4  contains magnesium hydroxide or aluminum hydroxide. Therefore, the flame retardance of the optical fiber cable  1 A may be improved. 
     In the present embodiment, since the four tensile strength member sets  50  are arranged at equal intervals from each other, unevenness in rigidity of the cable due to the positions where the tensile strength member sets  50  are disposed is further improved. Therefore, the optical fiber cable  1 A is less likely to be bent during air pressure feeding, and the wiring work may be improved. 
     Since the optical fiber cable  1 A of the present embodiment includes the tearing strings  6  arranged along the plurality of optical fiber ribbons  2 , the operator may tear the sheath  4  with the tearing strings  6  and easily take out the individual optical fiber ribbons  2 . Further, since the four tensile strength member sets  50  of the optical fiber cable  1 A are arranged in line symmetry with respect to the straight line L 2  connecting the tearing strings  6  and the center of the optical fiber cable  1 A, the unevenness in rigidity of the cable is further improved, and the wiring work during air pressure feeding is further improved. 
     According to the optical fiber cable  1 A of the present embodiment, a plurality of protrusions  7  protruding in the radial direction of the optical fiber cable  1 A are provided on the outer peripheral portion of the sheath  4 . Therefore, when the optical fiber cable  1 A is air pressure-fed in the duct, the protrusions  7  come into contact with an inner wall of the duct, so that a contact area between the sheath  4  and the duct may be reduced. Accordingly, friction between the sheath  4  and the duct is reduced, and a pressure feeding distance may be extended. 
     In  FIG.  2   , the intermittent-connection-type optical fiber ribbon including 12 optical fibers is shown, but the number of optical fibers is not limited thereto. In addition, the plurality of optical fibers may be in a form in which a plurality of single-core optical fibers are twisted, instead of an optical fiber ribbon. 
     First Modification of First Embodiment 
     An optical fiber cable  1 B according to a first modification of the first embodiment will be described with reference to  FIG.  3   . The same components as those of the optical fiber cable  1 A according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted. 
       FIG.  3    is a cross-sectional view perpendicular to a longitudinal direction of the optical fiber cable  1 B. The tensile strength member set  50  of the optical fiber cable  1 A according to the first embodiment includes one tensile strength member  5 , whereas the tensile strength member set  50  of the optical fiber cable  1 B according to the first modification includes two tensile strength members  5 . 
     At least eight (eight in the present example) tensile strength members  5  of the optical fiber cable  1 B according to the first modification are provided. The eight tensile strength members  5  in the present example are provided such that every two are paired. The two tensile strength members  5  that are paired are provided, for example, in a state of being close to each other or in a state of being at least partially in contact with each other. 
     In the present example, four tensile strength member sets  50  are embedded in the sheath  4  in a manner of being apart from each other. The four tensile strength member sets  50  are provided into two pairs such that two tensile strength member sets  50  at positions facing each other across a center of the optical fiber cable  1 B in a radial cross section of the optical fiber cable  1 B are paired. The positions of the four tensile strength member sets  50  in the radial cross section of the optical fiber cable  1 B are such positions that two straight lines respectively connecting the two tensile strength member sets  50  that are paired are orthogonal to each other. 
     In the present example, a distance d from a position on an outer periphery of one tensile strength member set  50  to a surface layer of the sheath  4  is 0.5 mm or more. Here, the position on the outer periphery of the tensile strength member set  50  is a position at which a straight line (a horizontal common normal line of the two tensile strength members  5  in  FIG.  3   ) L 1  connecting the center of the optical fiber cable  1 B and a substantially intermediate position between centers of the two tensile strength members  5  in the tensile strength member set  50  intersects with a straight line (a vertical common normal line of the tensile strength members  5  in  FIG.  3   ) which is perpendicular to the straight line and is in contact with the outer periphery on a cable outer peripheral side of each tensile strength member  5  in a cable cross-sectional view. 
     In the optical fiber cable  1 B according to the first modification, the same effects as those of the optical fiber cable  1 A according to the first embodiment may also be obtained. In particular, since the distance d from the position on the outer periphery of one tensile strength member set  50  to the surface layer of the sheath  4  is 0.5 mm or more, the flame retardance of the optical fiber cable  1 B is improved. 
     Second Modification of First Embodiment 
     An optical fiber cable  1 C according to a second modification of the first embodiment will be described with reference to  FIG.  4   . The same components as those of the optical fiber cable  1 A according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted. 
       FIG.  4    is a cross-sectional view perpendicular to a longitudinal direction of the optical fiber cable  1 C. As shown in  FIG.  4   , in addition to the configuration of the optical fiber cable  1 B according to the first modification, the optical fiber cable  1 C is provided with one more pair of tensile strength member sets  50  as compared with the optical fiber cable  1 B. 
     In the optical fiber cable  1 C according to the second modification, the same effects as those of the optical fiber cable  1 A according to the first embodiment may also be obtained. In particular, in the present example, the three tensile strength member sets  50  (six tensile strength members  5 ) are arranged in line symmetry with respect to the straight line L 2  connecting the tearing strings  6  and a center of the optical fiber cable  1 C in a cable cross-sectional view. Three tensile strength member sets  50  on one side of the line-symmetric straight line L 2  are arranged such that a distance between one tensile strength member set  50 A and an adjacent tensile strength member set  50 B is the same as a distance between the tensile strength member set  50 A and the other adjacent tensile strength member set  50 C. Similarly, three tensile strength member sets  50  on the other side of the line-symmetric straight line L 2  are arranged such that a distance between one tensile strength member set  50 D and an adjacent tensile strength member set  50 E is the same as a distance between the tensile strength member set  50 D and the other adjacent tensile strength member set  50 F. As a result, unevenness in rigidity of the cable due to the positions where the tensile strength member sets  50  are disposed is further improved, and the optical fiber cable  1 C is less likely to be bent during air pressure feeding, and wiring workability is improved. 
     In the present example, the distance d from a position on the outer periphery of one tensile strength member set  50  to a surface layer of the sheath  4  is also 0.5 mm or more. 
     Third Modification of First Embodiment 
     An optical fiber cable  1 D according to a third modification of the first embodiment will be described with reference to  FIG.  5   . The same components as those of the optical fiber cable  1 A according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted. 
       FIG.  5    is a cross-sectional view perpendicular to a longitudinal direction of the optical fiber cable  1 D. As shown in  FIG.  5   , in addition to the configuration of the optical fiber cable  1 B according to the first modification, the optical fiber cable  1 D further includes an upper wound tape  8  disposed between the sheath  4  and the plurality of optical fiber ribbons  2  and covering the plurality of optical fiber ribbons  2  from outside. In the present example, the upper wound tape  8  is disposed to cover a periphery of the water absorbing tape  3 . 
     The upper wound tape  8  contains, for example, polyimide, mica, magnesium hydroxide, or aluminum hydroxide as a flame-retardant inorganic substance. A thickness of the upper wound tape  8  is, for example, 0.3 mm. 
     According to the third modification, since the upper wound tape  8  containing the flame-retardant inorganic substance is disposed between the sheath  4  and the plurality of optical fiber ribbons  2 , the optical fiber cable  1 D high in flame retardance may be provided. 
     Fourth Modification of First Embodiment 
     An optical fiber cable  1 E according to a fourth modification of the first embodiment will be described with reference to  FIG.  6   . The same components as those of the optical fiber cable  1 A according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted. 
       FIG.  6    is a cross-sectional view perpendicular to a longitudinal direction of the optical fiber cable  1 E. As shown in  FIG.  6   , in addition to the configuration of the optical fiber cable  1 B according to the first modification, the optical fiber cable  1 E further includes the sheath  4  including an inner layer  41  and an outer layer  42 . 
     The inner layer  41  of the sheath  4  is provided to cover the plurality of optical fiber ribbons  2  from outside and to enclose the plurality of tensile strength member sets  50 . The plurality of tensile strength member sets  50  and the tearing strings  6  are embedded in the inner layer  41  of the sheath  4 . The inner layer  41  of the sheath  4  contains, for example, magnesium hydroxide or aluminum hydroxide as a flame-retardant inorganic substance. A thickness of the inner layer  41  is, for example, 1.0 mm. 
     The outer layer  42  of the sheath  4  is provided to cover the inner layer  41  from outside. The protrusions  7  are provided on an outer peripheral portion of the outer layer  42 . The outer layer  42  of the sheath  4  contains, as a release agent, a silicon-based release agent such as silicon or siloxane. A thickness of the outer layer  42  is, for example, 0.5 mm. 
     In the present example, the distance d from a position on an outer periphery of one tensile strength member set  50  to a surface layer of the outer layer  42  is 0.5 mm or more. Here, the position on the outer periphery of the tensile strength member set  50  is the same as the position on the outer periphery described in the first modification. 
     As described above, in the present modification, since the inner layer  41  contains the flame-retardant inorganic substance, the optical fiber cable  1 E excellent in flame retardance may be provided. In addition, since the outer layer  42  contains the release agent, the optical fiber cable  1 E may be smoothly fed even in a duct during air pressure feeding. Accordingly, the wiring is facilitated. Further, since the distance d from the position on the outer periphery of one tensile strength member set  50  to the surface layer of the outer layer  42  is 0.5 mm or more, the flame retardance of the optical fiber cable  1 A is further improved. 
     (Evaluation Experiment) 
     Dynamic friction coefficients of the optical fiber cables  1 A,  1 B,  1 C,  1 D, and  1 E according to the first embodiment and the modifications were evaluated.  FIG.  7    is a schematic diagram of an experiment for measuring the dynamic friction coefficient of each optical fiber cable. As shown in  FIG.  7   , first, a plurality of optical fiber cables are attached adjacently on a lower plate  91  to form one optical fiber cable set  1 X. The plurality of optical fiber cables are the same type as the optical fiber cables to be measured, and are the optical fiber cables  1 A,  1 B,  1 C,  1 D, and  1 E. Further, similarly to the optical fiber cable set  1 X, another optical fiber cable set  1 Y in which a plurality of optical fiber cables of other types are arranged adjacently and bundled is attached in parallel to an upper plate  92 . In this state, one optical fiber cable ( 1 A,  1 B,  1 C,  1 D,  1 E) to be measured is sandwiched between the lower plate  91  and the upper plate  92  (between the optical fiber cable set  1 X and the optical fiber cable set  1 Y), and a load of 2 kg (19.6 N) is applied vertically and uniformly to the upper plate  92 . In a state where the load was applied, the one optical fiber cable to be measured was pulled at a tensile speed of 500 m/min, and tension at the start of movement was measured. A sample length of each optical fiber cable is 300 mm. As a result of the measurement experiment, it was confirmed that each of the optical fiber cables  1 A,  1 B,  1 C,  1 D, and  1 E had a dynamic friction coefficient of 0.25. 
     In addition, pressure feeding distances of the optical fiber cables  1 A,  1 B,  1 C,  1 D, and  1 E according to the first embodiment and the modifications were evaluated. As a method of evaluating the pressure feeding distance, a micro duct pressure feeding test defined by International Electrotechnical Commission (IEC) was used. In the pressure feeding test, a general-purpose micro duct was used. An inner diameter of the duct is 20 mm. A total pressure feeding distance in the duct is set to 1000 m or more, and the duct is folded back every 100 m. A radius of curvature of the duct is 40 times an outer diameter of the duct. A pressure in the duct is 1.3 MPa to 1.5 MPa. As a result of the measurement experiment, it was confirmed that the pressure feeding distance of each of the optical fiber cables  1 B,  1 C,  1 D, and  1 E was 1000 m or more, but the pressure feeding distance of the optical fiber cable  1 A was slightly less than 1000 m. 
     Further, flame retardance of each of the optical fiber cables  1 A,  1 B,  1 C,  1 D, and  1 E according to the first embodiment and the modifications was evaluated. As a method for evaluating the flame retardance, a test according to the standard defined by European Construction Products Regulation (CPR) was used. Evaluation results are shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Sample No. 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                   
               
             
            
               
                 Sample structure 
                 Optical fiber 
                 Optical fiber 
                 Optical fiber 
                 Optical fiber 
                 Optical fiber 
                 Optical fiber 
               
               
                   
                 cable 1B 
                 cable 1A 
                 cable 1B 
                 cable 1C 
                 cable 1D 
                 cable 1E 
               
               
                 Sheath configuration 
                 Flame-retardant 
                 Flame-retardant 
                 Flame-retardant 
                 Flame-retardant 
                 Flame-retardant 
                 Inner layer: flame- 
               
               
                   
                 HDPE(30 wt % 
                 HDPE(30 wt % 
                 HDPE(30 wt % 
                 HDPE(30 wt % 
                 HDPE(30 wt % 
                 retardant HDPE(30 wt % of 
               
               
                   
                 of magnesium 
                 of magnesium 
                 of magnesium 
                 of magnesium 
                 of magnesium 
                 magnesium hydroxide) 
               
               
                   
                 hydroxide, and 
                 hydroxide, and 
                 hydroxide, and 
                 hydroxide, and 
                 hydroxide, and 
                 Outer layer: non-flame- 
               
               
                   
                 2 wt % of silicon) 
                 2 wt % of silicon) 
                 2 wt % of silicon) 
                 2 wt % of silicon) 
                 2 wt % of silicon) 
                 retardant HDPE(2 wt % of 
               
               
                   
                   
                   
                   
                   
                   
                 silicon) 
               
               
                 Sheath 
                 1.5 mm 
                 1.5 mm 
                 1.5 mm 
                 1.5 mm 
                 1.5 mm 
                 Inner layer: 1.5 mm 
               
               
                   
                   
                   
                   
                   
                   
                 Outer layer: 0.5 mm 
               
               
                 Distance (d) from 
                 0.3 mm 
                 0.5 mm 
                 0.5 mm 
                 0.5 mm 
                 0.5 mm 
                 0.5 mm (to surface layer 
               
               
                 position on outer 
                   
                   
                   
                   
                   
                 of inner layer) 
               
               
                 periphery of 
               
               
                 tensile strength 
               
               
                 member to surface 
               
               
                 layer of sheath 
               
               
                 Absence or presence 
                 No 
                 No 
                 No 
                 No 
                 Yes 
                 No 
               
               
                 of upper wound tape 
               
               
                 Evaluation result 
                 Eca 
                 Dca 
                 Dca 
                 Dca 
                 Cca 
                 Dca 
               
               
                   
               
            
           
         
       
     
     In Table 1, Sample No. 1 is a comparative example. A basic configuration of a cable of Sample No. 1 is the same as that of the optical fiber cable  1 B according to the first modification, but the distance d from one tensile strength member set  50  to the surface layer of the sheath  4  is 0.3 mm. Sample No. 2 is the optical fiber cable  1 A according to the first embodiment, the number of the tensile strength members  5  is four, and the distance d from a position on the outer periphery of one tensile strength member  5  (the tensile strength member set  50 ) to the surface layer of the sheath  4  is 0.5 mm. Sample No. 3 is the optical fiber cable  1 B according to the first modification, and the distance d from one tensile strength member set  50  to the surface layer of the sheath  4  is 0.5 mm. Sample No. 4 is the optical fiber cable  1 C according to the second modification, and the distance d from one tensile strength member set  50  to the surface layer of the sheath  4  is 0.5 mm. Sample No. 5 is the optical fiber cable  1 D according to the third modification, the upper wound tape  8  containing the flame-retardant inorganic substance with a thickness of 0.3 mm is provided, and the distance d from one tensile strength member set  50  to the surface layer of the sheath  4  is 0.5 mm. 
     Sample No. 6 is the optical fiber cable  1 E according to the fourth modification, the sheath  4  includes the inner layer  41  and the outer layer  42 , and the distance d from one tensile strength member set  50  to the surface layer of the inner layer  41  is 0.5 mm. The thickness of the inner layer was 1.5 mm, and the thickness of the outer layer was 0.5 mm. To the sheath of Sample No. 1 to Sample No. 5, 30 wt % of magnesium hydroxide was added as the flame-retardant inorganic substance, and 2 wt % of silicon was added as the release agent. In Sample No. 6, 30 wt % of magnesium hydroxide was added as the flame-retardant inorganic substance to the inner layer  41 , and 2 wt % of silicon was added as the release agent to the outer layer  42 . 
     The flame retardance is evaluated in a class of seven stages including Aca, B1ca, B2ca, Cca, Dca, Eca, and Fca, and Aca means highest flame retardance and Fca means lowest flame retardance. As a result of the evaluation experiment, the sample No. 1 was classified as class Eca, and the flame retardance was the lowest. Sample No. 2 to Sample No. 4, and Sample No. 6 were classified as class Dca, and good flame retardance was confirmed. Sample No. 5 was classified as class Cca, and highest flame retardance was confirmed. 
     Although the present disclosure is described in detail with reference to a specific embodiment, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present disclosure. The numbers, positions, shapes or the like of components described above are not limited to the above embodiment, and may be changed to suitable numbers, positions, shapes or the like during carrying out the present disclosure. 
     REFERENCE SIGNS LIST 
     
         
           1 A to  1 E: optical fiber cable 
           2 : optical fiber ribbon 
           3 : water absorbing tape 
           4 : sheath 
           5 : tensile strength member 
           6 : tearing string (fibrous filler) 
           7 : protrusion 
           7   a : surface 
           8 : upper wound tape 
           91 : lower plate 
           92 : upper plate 
           11 A to  11 L: optical fiber 
           12 : connected portion 
           13 : non-connected portion 
           14 : connecting resin 
           41 : inner layer 
           42 : outer layer 
           50 : tensile strength member set