OPTICAL FIBER CABLE

An optical fiber cable includes optical fibers, a wrapping tube that wraps around the optical fibers and contacts outermost ones of the optical fibers, and a sheath that covers the wrapping tube and has recesses on an inner circumferential surface of the sheath. The recesses are recessed toward a radially outer side of the optical fiber cable such that a space exists between the wrapping tube and the sheath in each of the recesses.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent Application No. 2021-83719, filed May 18, 2021. The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND

Technical Field

The present invention relates to a slotless-type optical fiber cable.

Description of the Related Art

The slotless-type optical fiber cable including the twisted optical fibers, the wrapping tape covering the optical fibers, and the jacket covering the wrapping tape is known (refer to, for example, Patent Document 1).

PATENT DOCUMENT

In the above-described slotless-type optical fiber cable, it is possible to suppress deterioration of the transmission characteristics of the optical fiber due to shrinkage of the jacket at low temperatures by reducing the packing density of the optical fiber. Then, it is possible to lower the packing density by increasing the outer diameter of the optical fiber cable itself. However, when installing the optical fiber cable in the existing duct, the outer diameter of the optical fiber cable is limited.

Further, when manufacturing the optical fiber cable, the optical fiber is fed in a state where a certain tension is applied to the optical fiber. Therefore, if the pull-out force of the optical fiber in the optical fiber cable (the force required to start the relative movement of the optical fiber with respect to the optical fiber cable when the optical fiber is pulled) is weak, a defective product may be manufactured. Further, if the pull-out force of the optical fiber is weak, the optical fiber may stick out from the end of the optical fiber cable during or after installing the optical fiber cable. Therefore, it is necessary to secure a pull-out force equal to or greater than a predetermined value. However, when the packing density of the optical fiber in the above-described slotless-type optical fiber cable decreases, the pull-out force of the optical fiber may also decrease.

SUMMARY

One or more embodiments provide an optical fiber cable capable of reducing the packing density while maintaining the outer diameter and the pull-out force.

An optical fiber cable according to one or more embodiments is an optical fiber cable comprising: optical fibers; a wrapping tube that wraps around the optical fibers and contacts outermost optical fibers included in the optical fibers; and a sheath covering the wrapping tube, wherein the sheath has recesses formed on an inner circumferential surface of the sheath and recessed toward a radially outer side of the optical fiber cable, and each of the recesses forms a space between the wrapping tube and the sheath.

In the above embodiments, each of the recesses may include a bottom having an arc shape.

In the above embodiments, each of the recesses may include first and second sidewalls connected to the bottom, and an angle between the first sidewall and the second sidewall may be equal to or greater than 90 degrees.

In the above embodiments, each of the recesses may include first and second sidewalls connected to the bottom, and each of ends of the first and second sidewalls on a radially inner side of the optical fiber cable may have an arc shape.

In the above embodiments, the optical fiber cable may further comprise a tensile strength member embedded in the sheath, and one of the recesses and the tensile strength member may overlap with each other toward a radial direction of the optical fiber cable.

In the above embodiments, the sheath may have protrusions formed on an outer circumferential surface of the sheath and protruded toward the radially outer side of the optical fiber cable, and one of the recesses and one of the protrusions may overlap with each other toward a radial direction of the optical fiber cable.

In the above embodiments, the optical fiber cable may be a slotless-type optical fiber cable having no rod with slots.

In the above embodiments, a wrapping tape may be longitudinally wound around the optical fibers to form the wrapping tube, and a lap portion in which ends of the wrapping tape are laid on each other may not overlap with the recesses toward a radial direction of the optical fiber cable.

In the above embodiments, the sheath may include main surfaces interposed between the recesses adjacent to each other along a circumferential direction of the optical fiber cable, and a following formula (1) may be satisfied.

In the above formula (1), CL0 is the length of a virtual inscribed circle inscribed in the main surfaces, and CL1 is the total length of the main surfaces.

According to one or more embodiments, because the recesses that are recessed toward the radially outer side of the optical fiber cable is formed on the inner circumferential surface of the sheath and each of the recesses forms a space between the wrapping tube and the sheath, it is possible to reduce the packing density while maintaining the outer diameter of the optical fiber cable and the pull-out force or the optical fiber.

DESCRIPTION OF THE EMBODIMENTS

FIG.1is a cross-sectional view showing the optical fiber cable1in one or more embodiments.FIG.2is an enlarged cross-sectional view showing the inner recess in one or more embodiments and is an enlarged view of II portion ofFIG.1.FIG.1andFIG.2are cross-sectional views of the optical fiber cable1cut along the direction substantially perpendicular to the longitudinal direction (axial direction) of the optical fiber cable1.

As shownFIG.1, the optical fiber cable1of one or more embodiments includes the optical fiber assembly10including the optical fibers11, the wrapping tube20wrapping around the optical fiber assembly10, the sheath30covering the wrapping tube20, and the tensile strength members60embedded in the sheath30. The optical fiber cable1is a so-called slotless-type optical fiber cable having no rod with slots. Therefore, the wrapping tube20wrapping around the optical fiber assembly10directly contacts the outermost optical fibers11of the optical fiber assembly10.

The optical fiber cable1of one or more embodiments is an optical fiber cable that is installed in an already installed duct or flow path. Therefore, the outer diameter of the sheath30of the optical fiber cable1is limited due to restrictions such as the inner diameter of the existing duct. The use of the optical fiber cable1is not particularly limited to the above.

The optical fiber assembly10is formed by twisting the optical fiber units together. Each of the optical fiber units is formed by bundling the optical fiber ribbons. As an example of the optical fiber ribbon, a so-called intermittently fixed optical fiber ribbon in which the optical fibers11arranged in parallel are intermittently connected by an adhesive portion can be exemplified.

In one or more embodiments, the optical fiber units constituting the optical fiber assembly10are twisted together in the SZ twisting manner. The SZ twisting manner is a method of twisting linear bodies while reversing the twisting direction at predetermined intervals. The method of twisting the optical fiber units is not particularly limited to this. For example, the optical fiber units constituting the optical fiber assembly10may be twisted in the unidirectional twisting manner. The unidirectional twisting manner is a twisting method having only one direction as a twisting direction and is a twisting method in which the linear bodies are spirally twisted together.

The configuration of the optical fiber unit is not particularly limited to the above configuration. For example, the optical fiber unit may be configured by simply bundling the optical fibers (optical fiber strands)11without using an optical fiber ribbon. Alternatively, the optical fiber unit may be configured by twisting the optical fibers11together. Alternatively, the optical fiber unit may be configured by winding a linear body around the optical fibers11to bundle the optical fibers11. Also, the configuration of the optical fiber assembly10is not particularly limited to the above. For example, the optical fiber assembly10may be configured by simply twisting the optical fibers11together without using an optical fiber unit.

The optical fiber assembly10is covered with the wrapping tube20. In one or more embodiments, the wrapping tape21is longitudinally wound around the outer periphery of the optical fiber assembly10to form the wrapping tube20. Specifically, the wrapping tape21is wound around the outer periphery of the optical fiber assembly10in a state in which the longitudinal direction of the wrapping tape21corresponds to the axial direction of the optical fiber cable1and the width direction of the wrapping tape21corresponds to the circumferential direction of the optical fiber cable1. The winding method of the wrapping tape21is not particularly limited to the above and may be, for example, horizontal winding (spiral winding).

Here, when the wrapping tape21is wound around the optical fiber assembly10, both ends of the wrapping tape21may not be laid on each other (that is, the lap portion22may not be formed), or both ends of the wrapping tape21may be laid to each other to from the lap portion22. The effect of the inner recesses22can be enhanced by not forming the lap portion22in the wrapping tube20. When the lap portion22is formed in the wrapping tube20, the effect of the inner recesses42can be enhanced as the width of the lap portion22is narrower. The thickness of the lap portion22may be set to be equal to or less than the thickness of the non-lap portion by thinning both ends of the wrapping tape21that become the lap portion22.

Further, when the wrapping tape21is longitudinally wound, as shown inFIG.1, the lap portion22may not be overlapped with the inner recess42of the inner circumferential surface40by overlapping the lap portion22with the main surface41of the inner circumferential surface40of the sheath30toward the radial direction of the optical fiber cable1. As a result, since the following of the shape of the wrapping tube20to the inner recess42at the time of shrinkage of the sheath30is not hindered, it is possible to obtain an effect close to the case where the wrapping tube20does not have the lap portion22.

The wrapping tape21is constituted by a nonwoven fabric or a film. Although not particularly limited, as specific examples of the nonwoven fabric constituting the wrapping tape21, nonwoven fabrics made of fibers such as polyester, polyethylene (PE), and polypropylene (PP) can be exemplified. On the other hand, although not particularly limited, as specific examples of the film constituting the wrapping tape21, a film made of a resin such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and nylon can be exemplified.

The wrapping tape21has such rigidity that the wrapping tape21can follow the deformation of the sheath30and the shape change of the optical fiber assembly10and the cross-sectional shape of the space surrounded by the wrapping tube20can be deformed. The rigidity of the wrapping tape21can be set according to the material and thickness of the wrapping tape21. Further, the wrapping tape21may have a strength enough to protect the optical fiber11from the blade when the sheath30is cut.

In the case where the wrapping tape21is constituted by nonwoven fabric, the wrapping tube20may function as a water absorbing layer for stopping water into the optical fiber cable1by adding a water absorbing powder to the nonwoven fabric. At the time of water immersion, the water absorbing powder swells and seals the gap in the optical fiber cable1to stop the water inside the optical fiber cable1.

Although not particularly limited, as specific examples of the water absorbing powder, material having high absorbency such as a starch-based material, a cellulose-based material, a polyacrylic acid-based material, a polyvinyl alcohol-based material, and a polyoxyethylene-based material, and mixtures thereof can be exemplified. As a method of adding the water absorbing powder to the nonwoven fabric, the water absorbing powder may be attached (applied) to the surface of the nonwoven fabric or may be interposed between two nonwoven fabrics.

The sheath (jacket)30is a cylindrical member covering the outer periphery of the wrapping tube20. The optical fiber assembly10wrapped in the wrapping tube20is housed in the inner space of the sheath30. As examples of the material of which the sheath30is made, a resin material such as polyvinyl chloride (PVC), polyethylene (PE), nylon, ethylene fluoride, and polypropylene (PP) can be exemplified.

Multiple (sixteen (16) in one or more embodiments) tensile strength members60are embedded in the sheath30. Each of the tensile strength members60is a linear member for suppressing distortion and bending applied to the optical fiber11due to shrinkage of the sheath30. In one or more embodiments, the tensile strength members60are arranged along the circumferential direction of the optical fiber cable1and are disposed at substantially equal intervals.

The number of tensile strength members60included in the optical fiber cable1is not particularly limited to the above. In one or more embodiments, each of the tensile strength member60is constituted by a single rod. However not particularly limited to the above, each of the tensile strength members60may be constituted by a plurality of rods. The tensile strength members60may not be embedded in the sheath30.

In one or more embodiments, since the twisting method of the optical fiber assembly10is the SZ twisting manner as described above, the tensile strength members60also extend in the axial direction of the optical fiber cable1while reversing the rotational direction at a predetermined cycle following the twisting of the optical fiber assembly10. The tensile strength members60extend substantially parallel to each other. In the case where the twisting method of the optical fiber assembly10is the unidirectional twisting manner, the tensile strength members60spirally extend along the axial direction of the optical fiber cable1following the twisting of the optical fiber assembly10. Alternatively, the tensile strength members60may extend substantially parallel to the axial direction of the optical fiber cable1without following the twisting of the optical fiber assembly10.

As examples of the material of which the tensile strength member60is made, a non-metallic material and a metallic material can be exemplified. Although not particularly limited, as specific examples of the non-metallic material, fiber reinforced plastics (FRP) such as glass fiber reinforced plastic (GFRP), aramid fiber reinforced plastic (KFRP) reinforced with Kevlar (registered trademark), and polyethylene fiber reinforced plastic reinforced with polyethylene fiber can be exemplified. On the other hand, although not particularly limited, as specific examples of the metallic material, a metal wire such as a copper wire can be exemplified.

The sheath30in one or more embodiments includes multiple inner recesses42formed on the inner circumferential surface40of the sheath30, and multiple outer protrusions51formed on the outer circumferential surface50of the sheath30.

Each of the inner recesses42is a linear groove extending in the axial direction of the optical fiber cable1while reversing the rotation direction at a predetermined cycle following the twisting of the optical fiber assembly10. Each of the outer protrusions51is also a linear protrusion extending in the axial direction of the optical fiber cable1while reversing the rotation direction at a predetermined cycle following the twisting of the optical fiber assembly10. In the case where the twisting method of the optical fiber assembly10is the unidirectional twisting manner, the inner recesses42and the outer protrusions51spirally extend along the axial direction of the optical fiber cable1following the twisting of the optical fiber assembly10.

The inner recesses42may extend substantially parallel to the axial direction of the optical fiber cable1without following the twisting of the optical fiber assembly10. However, by following the inner recesses42to the twisting of the optical fiber assembly10, the wrapping tube20and the optical fiber11can easily enter (i.e., accommodated within) the space45formed by the inner recess42at the time of the shrinkage of the sheath30.

The inner recesses42are formed on the inner circumferential surface40of the sheath30and are disposed at substantially equal intervals along the circumferential direction of the sheath30. Each of the inner recesses42is recessed toward the radially outer side of the optical fiber cable1. In one or more embodiments, the radially outer side of the optical fiber cable1is a direction from the center of the optical fiber cable1toward the outer side of the sheath30.

The main surface41of the inner circumferential surface40is interposed between the inner recesses42adjacent to each other along the circumferential direction of the optical fiber cable1. The main surface41has a gentle arc shape, and the multiple main surfaces41forms a circumference that is concentric with the optical fiber cable1by arranging the multiple main surfaces41in the circumferential direction. The virtual inscribed circle43inscribed in all the main surfaces41defines the outer periphery of the wrapping tube20wrapping around the optical fiber assembly10. Therefore, a space45is formed between the wrapping tube20and the sheath30by the inner recess42.

Here, since the space45of the inner recess42that can be effectively utilized at the time of the shrinkage of the sheath30is reduced by the thickness of the wrapping tape21, the cross-sectional area, the width, the depth, and the like of the inner recess42may be designed in consideration of the shrinkage amount of the sheath30and the thickness of the wrapping tube20. The width of each main surface41in the inner circumferential surface40of the sheath30, and the ratio and number of main surfaces41in the inner circumferential surface40are preferably set to such an extent that the wrapping tube20maintains the inscribed circle43.

For example, the ratio P of the main surface41with respect to the above inscribed circle43(the ratio P (P-CL1/CL0×100) of the total CL1 of the lengths of all the main surfaces41with respect to the total length CL0 of the inscribed circle43) can be 20% or more and 80% or less (20%≤P≤80%), preferably 40% or more and 60% or less (40%≤P≤60%).

InFIG.1, the inscribed circle43is illustrated as being separated from the main surface41of the inner circumferential surface40for convenience, but the inscribed circle43actually matches the main surface41. Further, inFIG.1, the inscribed circle43is illustrated as being separated from the wrapping tube20for convenience, but the inscribed circle43actually contacts the outer circumferential surface of the wrapping tube20.

As shown inFIG.2, each of the inner recesses42has a substantially triangular cross-sectional shape in which an apex toward the radially outer side of the optical fiber cable1has an arc shape. Specifically, the inner recess42has a bottom421and a pair of sidewalls422and423.

In one or more embodiments, the bottom421has an arc shape. The curvature R1 of the arc shape of the bottom421is preferably 0.1 mm or more (R≥120.1 mm). As a result, it is possible to suppress the occurrence of cracks in the bottom421of the inner recess42due to the stress concentration as compared with a case where the bottom of the inner recess has an angular apex. The curvature R1 of the arc shape of the bottom421is preferably less than or equal to 1.0 mm (R1≤1.0 mm), thereby the connecting portions between the bottom421and the sidewalls422and423do not form angular apexes.

The first and second sidewalls422and423are connected to both ends of the bottom421. The first and second side walls422and423are inclined with respect to the radial direction of the optical fiber cable1. Specifically, the first and second sidewalls422and423are inclined away from each other toward the radially inner side of the optical fiber cable1.

In one or more embodiments, the angle θ formed between the first sidewall422and the second sidewall423is preferably 90 degrees or more)(θ≥90°, thereby it is possible to further suppress the occurrence of cracks in the bottom421of the inner recess32due to stress concentration. Further, the angle θ formed between the first and second sidewalls422and423is preferably 150 degrees or less (θ≤150 degrees), thereby it is possible to secure the main surface41having a sufficient width for pressing the wrapping tube20by the inscribed circle43.

In one or more embodiments, each of the side portions422and423have a linear shape, and the cross-sectional shape of the inner recess42has a substantially triangular shape, but not particularly limited to the above. For example, each of the side portions422and423may have a curved shape, and the cross-sectional shape of the inner recess42may be a substantially convex curved shape toward the radially outer side of the optical fiber cable1. Although the cross-sectional shape of the inner recess42is not particularly limited, it may form a substantially sinusoidal shape together with the main surface41of the inner circumferential surface40of the sheath30. In the case, the angle θ is an angle between the tangent lines of the side portions422and423, and the tangent lines are tangent lines of the side portions422and423at a middle point in the depth direction from the main surface41.

The opening424of the inner recess42is defined by the end422aon the radially inner side of optical fiber cable1in the first side wall422and the end423aon the radially inner side of optical fiber cable1in the second side wall423.

Each of the ends422aand423aof the first and second side walls422and423also has an arc shape. The curvature R2 of the arc shape of each of the ends422aand423ais preferably equal to or greater than 0.1 mm (R≥220.1 mm) and preferably equal to or less than 5.0 mm (R2≤5.0 mm). By setting the curvature R2 of the arc shape of each of the first and second sidewalls422and423to be within the above-described limits, it is possible to prevent the stresses from being concentrated on the optical fibers11due to the abutment of the ends422aand423aof the first and second sidewalls422and423at the time of the shrinkage of the sheath30.

On the other hand, as shown inFIG.1, the outer protrusions51are formed on the outer circumferential surface50of the sheath30and are disposed at substantially equal intervals along the circumferential direction of the sheath30. Each of the outer protrusions51protrudes toward the radially outer side of the optical fiber cable1. Further, each of the outer recesses52is complementarily formed between the outer protrusions51adjacent to each other along the circumferential direction of the optical fiber cable1. Each of the outer recesses52is relatively recessed toward the radially inner side of the optical fiber cable1as compared with the outer protrusion51.

Each of the outer protrusions51has a tip that faces toward the radially outer side of the optical fiber cable1, and the tip has an arc shape. When installing the optical fiber cable1in the existing duct, it is possible to reduce the friction occurring with the inner wall surface or the like of the duct by the sheath30having such multiple outer protrusions51. In a case where the effect of reducing friction is not required, the outer protrusions51may not be formed in the sheath30.

The sheath30of one or more embodiments has the same number (sixteen (16) in one or more embodiments) of the inner recess42as the number of the tensile strength60. As shown inFIG.1, the inner recesses42are disposed so as to overlap with the tensile strength members60toward the radial direction of the optical fiber cable1, and the tensile strength members60are positioned on the radially outer side of the optical fiber cable1with respect to the inner recesses42. Accordingly, when the stress is concentrated in the inner recess42and the crack progresses, it is possible to stop the progress of the crack by the tensile strength member60positioned on the radially outer side with respect to the inner recess42. Although not particularly limited, it is preferable that the center of the inner recess42and the center of the tensile strength member60substantially match with each other toward the radial direction of the optical fiber cable1.

The sheath30has the same number (sixteen (16) in one or more embodiments) of the outer protrusion51as the number of the tensile strength member60. The outer protrusions51are disposed so as to overlap with the tensile strength members60toward the radial direction of the optical fiber cable1, and the outer protrusions51are positioned on the radially outer side of the optical fiber cable1with respect to the tensile strength members60. That is, in one or more embodiments, the inner recess42, the tensile strength member60, and the outer convex portion51overlap with each other toward the radial direction of the optical fiber cable1. It is possible to increase the thickness of the portion of the sheath30where the space is formed by the inner recess42by adopting such an arrangement. Although not particularly limited, it is preferable that the center of the outer protrusion51and the center of the tensile strength member60substantially match with each other toward the radial direction of the optical fiber cable1.

As described above, in one or more embodiments, the inner recesses42that are recessed toward the radially outer side of the optical fiber cable1are formed on the inner circumferential surface40of the sheath30, and the space45is formed between the wrapping tube20and the sheath30by the inner recesses42. Accordingly, in one or more embodiments, since the inner area of the sheath30can be increased while the outer diameter of the optical fiber cable1is maintained, it is possible to reduce the packing density of the optical fiber11.

Even if the sheath30shrinks at a low temperature, the wrapping tube20and the optical fiber11can enter the space45formed by the inner recess42. Therefore, it is possible to suppress the application of the stress to the optical fiber11due to the shrinkage of the sheath30, and it is possible to suppress the deterioration of the transmission characteristics of the optical fiber11at a low temperature.

Further, in one or more embodiments, the wrapping tube20wrapping around the optical fiber assembly10is pressed by the main surfaces (contact surfaces)41of the inner circumferential surface40of the sheath30, and the outer periphery of the wrapping tube20is the inscribed circle43having an inner diameter equivalent to the sheath having no inner recess42. Therefore, even if the inner area of the sheath30is increased for reducing the packing density of the optical fiber11, it is possible to maintain the pull-out force of the optical fiber.

Here, optical fiber cables according to Example, Comparative Example 1, and Comparative Example 2 were produced. Example 1 is an optical fiber cable having the configuration shown inFIG.1and has eight hundred and sixty-four (864) optical fibers, and the cross-sectional area (inner area) of the inner space of the sheath is 89.0 mm2as shown in Table 1 below. On the other hand, Comparative Example 1 has the same configuration as that of Example 1 except that (1) the sheath does not have inner recesses. The inner area of the sheath of Comparative Example 1 is 87.3 mm2. Comparative Example 2 has the same configuration as that of Example 1 except that (1) the sheath does not have inner recesses and (2) the inner area of the sheath is substantially the same as the inner area of the sheath including the inner recesses in Example. The inner area of the sheath of Comparative Example 1 is 88.9 mm2.

TABLE 1ComparativeComparativeExampleExample 1Example 2Number of optical fibers864864864[number]Inner area of sheath [mm2]89.087.388.9Thickness of wrapping tube0.20.20.2[mm]Presence or absence of innerPresenceAbsenceAbsencerecessEquivalent linear expansion2.592.59—coefficient of cable jacket[×10{circumflex over ( )}−5/° C.]Inner area of sheath if ignoring87.2(87.3)(88.9)the inner recess [mm2]Pull-out force [N/10 m]5555—Pull-out of optical fiber∘∘xTemperature loss characteristics∘x—

Then, the pull-out force of the optical fiber was evaluated for the above Example, Comparative Example 1, and Comparative Example 2, and the transmission loss at the time of temperature change was evaluated for Example and Comparative Example 1.

In evaluating the pull-out force, an optical fiber cable having a length 10 m and having portions where the optical fiber protrudes from both ends was prepared, one end of the optical fiber was pulled by a load measuring device, and the load at which the other end of the optical fiber started to move was measured as the pull-out force. In Table 1 above, in the column of “Pull-out of optical fiber”, “o” means that pull-out force is equal to or greater than a predetermined value and no pull-out of the optical fiber occurs, and “×” means that the optical fiber was pulled out before the pull-out force reached the predetermined value.

As shown in Table 1 above, in both of Example and Comparative Example 1, the pull-out force was sufficient, and the optical fiber was not pulled out. On the other hand, in Comparative Example 2, since there was no inner recess and the packing density was low, the pull-out force was insufficient, and the optical fiber was pulled out immediately after starting to pull the optical fiber.

In the evaluation of the transmission loss at the time of temperature change, in accordance with the provision of “Temperature cycling” in the “Telcordia Technologies Generic Requirements GR-20-CORE Issue 4, July 2013”, the optical fiber cables of Example and Comparative Example 1 were subjected to two cycles of temperature change in the range of −40° C. to +70° C. to measure the maximum loss variation at the measurement wavelength of 1.55 μm. In Table 1 above, in the column of “temperature loss characteristics”, “∘” means that the transmission characteristics of the optical fiber cable at the time of temperature change is good, “×” means that the transmission characteristics of the optical fiber cable at the time of temperature change is insufficient.

In Example, the maximum loss variation in the transmission loss evaluation is equal to or less than 0.15 dB/km. On the other hand, in Comparative Example 1, since the inner area of the sheath was small, the maximum loss variation in the transmission loss evaluation was over 0.15 dB/km.

As described above, by forming the plurality of inner recesses42on the inner circumferential surface40of the sheath30, it is possible to reduce the packing density of the optical fiber11while maintaining the outer diameter of the optical fiber cable1and the pull-out force of the optical fiber11.

Further, in one or more embodiments, since the plurality of inner recesses42are formed on the inner circumferential surface40of the sheath30, the volume of the sheath30is reduced by an amount corresponding to the inner recesses42as compared with a sheath having no inner recesses42. Therefore, the amount of shrinkage of the sheath30itself at low temperature is also reduced.

Further, in the optical fiber cable of the type in which a plurality of tensile strength member is disposed in the sheath along the circumferential direction of the sheath, the exposing process of the optical fiber is performed by bending and cutting the sheath and the tensile strength member after cutting into the sheath. However, the portion of the sheath inside the tensile strength member hardly cut, and the workability of the exposing process is low. On the other hand, in one or more embodiments, since the portion of the sheath30inside the tensile strength member60is thinned by the inner recess42, the sheath30can be easily cut out, and the workability of the exposing process can be improved.

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