Optical fiber cable

Abstract

The outer circumference of a slot rod (5) has rib parts (9) and groove parts (7) that alternate in the circumferential direction such that the groove parts (7) are formed between adjacent rib parts (9). A plurality of optical fiber core wires (15) are accommodated in each of the groove parts (7) of the slot rod (3), and a wrap (11) is wound around the outer circumference of the slot rod (5). The outer circumference of the wrap (11) is covered with a jacket (13). The shortest distance (b) from the center of the slot rod (5) to the surface of the jacket (13) as covering the groove parts (7) is shorter than the distance a from the center of the slot rod (5) to the surface of the jacket (13) as covering the rib parts (9).

Description

fiber optic cable

[0001] The present invention relates to a slotted optical fiber cable.

[0002] Conventionally, optical fiber cables have been used that use a slotted rod, in which a plurality of grooves are formed on the outer circumferential surface, and a plurality of optical fiber cores are housed.

[0003] The grooves on the outer peripheral surface of the slotted rod trace a spiral locus at a certain pitch in one direction (S-type) or in alternating directions (SZ-type) relative to the longitudinal direction of the slotted rod, i.e., the position of the optical fiber core wire in the circumferential direction of the optical fiber cable changes relative to the longitudinal direction of the optical fiber cable.

[0004] By using such a slot rod, when the optical fiber cable is bent, each optical fiber can be positioned both on the inside and outside of the bending center of the cable at each longitudinal portion of the optical fiber cable, which makes the transmission loss of the optical fiber uniform and suppresses an increase in transmission loss.

[0005] When laying such an optical fiber cable in a duct, the wiring work becomes difficult due to friction between the optical fiber cable and the inner surface of the duct. For this reason, a method has been proposed in which the cross-sectional shape of the optical fiber cable is made polygonal (a cross-sectional shape consisting of multiple flat surfaces) rather than circular, thereby reducing the contact area with the inner surface of the duct and reducing friction (Patent Document 1).

[0006] International Publication No. 2018 / 230618

[0007] However, even with such polygonal optical fiber cables, deformation of the jacket can increase the contact area with the inner surface of the duct, resulting in increased frictional resistance. Also, even if the contact area is reduced and friction is reduced with the inner surface of a circular duct, when the cable is laid in a rectangular duct or trough, one side of the polygon comes into surface contact with the inner surface of the duct or trough, increasing frictional resistance.

[0008] The present invention has been made in view of the above problems, and has as its object to provide a slotted optical fiber cable that can reduce frictional resistance when laid in a duct or the like.

[0009] In order to achieve the above-mentioned object, the present invention provides an optical fiber cable comprising: a slotted rod having a plurality of rib portions on its outer periphery and a plurality of groove portions formed between the rib portions; a plurality of optical fiber cores housed in the groove portions; and an outer jacket covering the outer periphery of the slotted rod, wherein, in a cross section perpendicular to the longitudinal direction of the slotted rod, b / a, which is the ratio of the distance a from the center of the slotted rod to the surface of the outer jacket covering the rib portions to the shortest distance b from the center of the slotted rod to the surface of the outer jacket covering the groove portions, is less than 1.

[0010] It is desirable to satisfy the relationship 0.7≦(b / a)≦0.95.

[0011] It is desirable that the winding tension of the pressure winding be 4N or more and less than 30N.

[0012] The number of cores in the optical fiber core wire is preferably 100 or more and 640 or less.

[0013] In a cross section perpendicular to the longitudinal direction of the slot rod, it is desirable that i / h, which is the ratio of the distance h from the center of the slot rod to the inner surface of the pressure winding covering the rib portion to the shortest distance i from the center of the slot rod to the inner surface of the pressure winding covering the groove portion, be less than 1.

[0014] In a cross section perpendicular to the longitudinal direction of the slot rod, it is desirable that a gap be formed between the inner surface of the pressure winding covering the groove and the optical fiber accommodated in the groove.

[0015] It is desirable that the gap be 0.2 mm or more and 4.5 mm or less.

[0016] It is desirable that the pressure winding be wound so as to cover the outer peripheral surface of the slotted rod without any gaps.

[0017] The optical fiber core wire is preferably an intermittent optical fiber ribbon in which a plurality of single-core optical fiber core wires are arranged in parallel and intermittently bonded in the longitudinal direction.

[0018] According to the present invention, in a cross section perpendicular to the longitudinal direction of the slotted rod, the shortest distance b from the center of the slotted rod to the surface of the outer covering covering the groove portion is smaller than the distance a from the center of the slotted rod to the surface of the outer covering covering the rib portion, so that surface contact between the outer covering at the outer surface of the groove portion and the inner surface of a duct, etc. is suppressed. In other words, the outer covering at the outer surface of the groove portion is slightly concave, so that the contact area between the inner surface of the duct, etc. and the outer covering at that portion is reduced, thereby suppressing frictional resistance. This makes it easier to perform wiring work into the duct, etc.

[0019] Such an effect can be obtained particularly when the relationship 0.7≦(b / a)≦0.95 is satisfied.

[0020] In particular, by setting the winding tension of the pressure winding to 4N or more and less than 30N, the optical fiber cable can be obtained efficiently.

[0021] The present invention is also suitable for an optical fiber cable having an optical fiber core count of 100 to 640 cores.

[0022] Furthermore, in a cross section perpendicular to the longitudinal direction of the slot rod, by making the shortest distance i from the center of the slot rod to the inner surface of the pressure winding that covers the groove portion smaller than the distance h from the center of the slot rod to the inner surface of the pressure winding that covers the rib portion, it is possible to make the above-mentioned b / a less than 1 when the pressure winding member is wound.

[0023] In addition, by forming a gap between the inner surface of the pressure winding covering the groove portion and the optical fiber core wire housed in the groove portion in a cross section perpendicular to the longitudinal direction of the slot rod, it is possible to prevent the optical fiber core wire from receiving lateral pressure from the outer sheath.

[0024] Furthermore, if this gap is 0.2 mm or more and 4.5 mm or less, the lateral pressure on the optical fiber can be suppressed, and a sufficient space factor of the optical fiber in the groove can be ensured.

[0025] Furthermore, if the pressure winding is wound so as to cover the outer peripheral surface of the slot rod without any gaps, post-processing such as branching work becomes easier.

[0026] Furthermore, if the optical fiber core is an intermittent optical fiber ribbon core in which multiple single-core optical fiber cores are arranged in parallel and intermittently bonded in the longitudinal direction, loss increases can be suppressed even when the cable is bent.

[0027] According to the present invention, it is possible to provide a slotted optical fiber cable that can reduce frictional resistance when laid in a duct or the like.

[0028] 1 is a cross-sectional view showing an optical fiber cable 1. FIG. 2 is a partially enlarged view of the optical fiber cable 1. FIG. 3 is a diagram showing a state in which the optical fiber cable 1 is installed in a duct 17. FIG. 4 is a diagram showing a state in which the optical fiber cable 1 is installed in a duct 17.

[0029] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view perpendicular to the longitudinal direction of an optical fiber cable 1. The optical fiber cable 1 is composed of a tension member 3, a slot rod 5, a pressure winding 11, an outer jacket 13, an optical fiber 15, etc.

[0030] A tension member 3, which is a tensile body, is provided in the center of the slot rod 5. The tension member 3 is made of, for example, a steel wire or FRP that has resistance to tension and compression.

[0031] Furthermore, a plurality of grooves 7 are provided on the outer periphery of the slotted rod 5. The spaces between adjacent grooves 7 form rib sections 9. That is, in a cross section perpendicular to the longitudinal direction of the slotted rod 5, a plurality of rib sections 9 and groove sections 7 are alternately arranged in the circumferential direction on the outer periphery of the slotted rod 5, and groove sections 7 are formed between adjacent rib sections 9.

[0032] The grooves 7 form a spiral locus that is unidirectional (S-shaped) or alternates in direction (SZ-shaped) at a certain pitch relative to the longitudinal direction of the slotted rod 5. The number and depth of the grooves 7 are not limited to the example shown in the drawing, but for example, 4 to 12 grooves 7 (rib portions 9) are provided in the circumferential direction.

[0033] A plurality of optical fiber core wires 15 are housed inside each groove 7 of the slot rod 5. The form of the optical fiber core wire 15 is not particularly limited, but in the illustrated example, the optical fiber core wire 15 is an optical fiber ribbon in which a plurality of single-core optical fibers are arranged in parallel and integrated. Because of its tape-like shape, the optical fiber ribbon is resistant to external forces, easy to handle, and highly suitable for post-processing. The optical fiber ribbon may be, for example, an intermittent optical fiber ribbon in which a plurality of single-core optical fiber core wires are arranged in parallel and intermittently bonded in the longitudinal direction. In this case, it is preferable that the bonding portions bonding the single-core optical fiber core wires adjacent in the width direction are not aligned in a single row in the width direction, but that the bonding portions bonding the single-core optical fiber core wires adjacent in the width direction are shifted in the longitudinal direction. Alternatively, a plurality of optical fiber ribbon core wires may be bundled to form an optical fiber unit, and the plurality of optical fiber units may be twisted together and housed in the groove 7. The total number of fibers in the optical fiber core wire 15 is not particularly limited, but is preferably 100 to 640.

[0034] Although the shapes of the grooves 7 and the ribs 9 are not particularly limited, it is desirable that the grooves 7 and the ribs 9 are both formed so that their widths are narrower toward the center of the optical fiber cable 1 and gradually increase toward the periphery, as shown in the figure. By making the width of the bottom of the grooves 7 close to the ribbon width of the optical fiber 15 in this way, the optical fiber 15 can be stably installed in the grooves 7. Furthermore, by making the width of the ribs 9 increase toward the periphery, the optical fiber 15 can be inserted more easily into the grooves 7.

[0035] With a plurality of optical fiber cores 15 housed in each groove 7, a pressure wrap 11 is wound around the outer periphery of the slot rod 5. The pressure wrap 11 is formed by longitudinally or spirally wrapping a tape-like material, such as a nonwoven fabric, around the outer periphery of the slot rod 5. Furthermore, a tear cord is arranged on the outer periphery of the pressure wrap 11 as necessary, and the outer periphery of the pressure wrap 11 is covered with an outer jacket 13.

[0036] For example, when the pressure wrap 11 is wound longitudinally around the outer periphery of the slotted rod 5, the pressure wrap 11 is wound around the slotted rod 5 so that the widthwise ends of the pressure wrap 11 overlap. At this time, the overlapping portion of the pressure wrap 11 continues in a substantially linear manner in the longitudinal direction. Also, when the pressure wrap 11 is wound spirally around the outer periphery of the slotted rod 5, the pressure wrap 11 is wound around the slotted rod 5 so that the widthwise ends of the pressure wrap 11 overlap. At this time, the overlapping portion of the pressure wrap 11 continues in a spiral manner in the longitudinal direction. In other words, the pressure wrap 11 is wound so as to cover the outer periphery of the slotted rod 5 without any gaps. By doing so, the outer jacket 13 and the slotted rod 5 do not come into contact with each other, improving the workability of branching operations, etc.

[0037] The thickness of the outer jacket 13 is, for example, about 1.5 mm to 2.2 mm, and is substantially uniform around the entire circumference. That is, in cross section, the inner and outer surfaces of the outer jacket 13 are substantially parallel. Furthermore, tracer marks that allow the positions of the ribs 9 and grooves 7 to be identified may be provided on the surface of the outer jacket 13 corresponding to the ribs 9 or the outer surface of the outer jacket 13 corresponding to the grooves 7.

[0038] 2 is a partially enlarged view of FIG. 1. The outer peripheral surface of the rib portion 9 of the slotted rod 5 is approximately flat. Therefore, the outer peripheral portion of the outer jacket 13 at a portion corresponding to the rib portion 9 of the slotted rod 5 is approximately flat. That is, in the cross section, a substantially straight line is formed between portions e and d, which are positions on the outer peripheral surface of the outer jacket 13 corresponding to the boundary between the rib portion 9 and the groove portion 7. Note that the outer peripheral surface of the rib portion 9 does not necessarily have to be flat as a whole; for example, both ends (near the boundary with the groove portion 7) may be tapered so that the diameter decreases toward the groove portion 7.

[0039] The outer periphery of the jacket 13 at a portion corresponding to the groove 7 of the slot rod 5 has a gently concave shape. That is, the portion between portions f and g, which is the position on the outer periphery of the jacket 13 corresponding to the boundary between the rib portion 9 and the groove 7, has an arc shape toward the center of the optical fiber cable 1. At this time, the pressure winding 11 is also pressed into the groove 7 and has an arc shape toward the center of the optical fiber cable 1.

[0040] Here, in a cross section perpendicular to the longitudinal direction of the slotted rod 5, the distance from the center of the slotted rod 5 to the surface of the outer jacket 13 covering the rib portion 9 is defined as a. For example, the distance between the center of the slotted rod 5 and the midpoint between portions e and d in the figure is defined as a. Also, the shortest distance from the center of the slotted rod 5 to the surface of the outer jacket 13 covering the groove portion 7 is defined as b. For example, the distance between the center of the slotted rod 5 and the midpoint on the arc between portions f and g in the figure is defined as b.

[0041] In this case, the shortest distance b from the center of the slotted rod 5 to the surface of the outer jacket 13 covering the groove portion 7 is smaller than the distance a from the center of the slotted rod 5 to the surface of the outer jacket 13 covering the rib portion 9. In other words, the ratio b / a of the distance a to the distance b is less than 1.

[0042] Furthermore, the shortest distance i from the center of the slot rod 5 to the inner surface of the pressure winding 11 that covers the groove portion 7 is smaller than the distance h from the center of the slot rod 5 to the inner surface of the pressure winding 11 that covers the rib portion 9. In other words, the ratio i / h of the distance h to the distance i is also less than 1.

[0043] When the optical fiber cable 1 is substantially polygonal, the portions f and g in the figure become substantially straight lines. In this case, the distance c from the center of the slotted rod 5 to the surface of the jacket 13 covering the groove portion 7 is substantially equal to the distance a from the center of the slotted rod 5 to the surface of the jacket 13 covering the rib portion 9. In contrast, by making the outer surface of the jacket 13 corresponding to the groove portion 7 concave, it is possible to make the ratio b / a of the distance a to the distance b and the ratio i / h of the distance h to the distance i less than 1.

[0044] The distance j to the outer surface of the optical fiber 15 in the groove 7 is smaller than the shortest distance i from the center of the slot rod 5 to the inner surface of the pressure wrap 11 covering the groove 7. That is, a gap is formed between the pressure wrap 11 and the optical fiber 15, and the inner surface of the pressure wrap 11 does not contact the optical fiber 15 in the groove 7. This gap is preferably 0.2 mm or more and 4.5 mm or less. By providing such a gap, the lateral pressure from the jacket 13 to the optical fiber 15 can be suppressed. A gap of more than 4.5 mm is undesirable because it reduces the space factor.

[0045] In this way, by making the outer surface of the part corresponding to the groove portion 7 concave, the outer surface of the outer covering 13 outside the groove portion 7 is prevented from coming into surface contact with the inner surface of a duct or the like, thereby reducing frictional resistance.

[0046] In particular, in a cross section perpendicular to the axial direction of the optical fiber cable 1, the width of the jacket 13 corresponding to the opening of the groove 7 (between f-g in FIG. 2) is wider than the width of the jacket 13 corresponding to the rib 9 (between d-e in FIG. 2). Therefore, when laid in a duct or the like, the jacket 13 corresponding to the groove 7 is in stable contact with the inner surface of the duct. Therefore, if the outer surface of the jacket 13 corresponding to the groove 7 is linear, a wider area will come into contact with the inner surface of the duct. In contrast, in this embodiment, even if the jacket 13 corresponding to the groove 7 is disposed opposite the inner surface of the duct, the contact area is small, and frictional resistance can be reduced.

[0047] Furthermore, as a secondary effect, contact with a duct or the like can suppress lateral pressure on the optical fiber 15 inside the groove 7. Fig. 3A is a diagram showing the state when the optical fiber cable 1 is placed in a duct 17 with a flat inner surface. In the state shown in Fig. 3A, the outer surface of the jacket 13 in the portion corresponding to the rib portion 9 contacts the inner surface of the duct 17. In this case, for example, the reaction force X of the optical fiber cable 1's own weight is transmitted to the rib portion 9 of the slot rod 5 via the jacket 13 (and the pressure winding 11). In this case, since the rib portion 9 receives the force, transmission of the force to the optical fiber 15 housed in the groove 7 is suppressed.

[0048] 3B , which is a more stable posture, the outer surface of the outer covering 13 at the portion corresponding to the groove 7 faces the inner surface of the duct 17. However, as described above, the outer surface of the outer covering 13 at the portion corresponding to the groove 7 has a concave shape, and therefore, the outer covering 13 comes into contact with the inner surface of the duct 17 only near the boundary between the groove 7 and the rib 9.

[0049] In this case, for example, the reaction force Y of the optical fiber cable 1's own weight is transmitted to the end of the rib portion 9 of the slot rod 5 via the jacket 13 (and the pressure winding 11). That is, the transmission of force to the groove portion 7 is suppressed. Therefore, the transmission of force to the optical fiber 15 housed in the groove portion 7 is suppressed. In particular, the inner surface of the pressure winding 11 in the groove portion 7 does not come into contact with the optical fiber 15, so the transmission of force to the optical fiber 15 is more reliably suppressed.

[0050] In this way, by making the outer surface of the outer jacket 13 corresponding to all groove portions 7 concave, the force acting on the optical fiber core 15 housed in the groove portion 7 can be suppressed regardless of the posture of the optical fiber cable 1, and an increase in transmission loss due to lateral pressure on the optical fiber core 15 can also be suppressed.

[0051] It is desirable that the ratio b / a mentioned above satisfy the condition 0.7≦(b / a)≦0.95. If the ratio b / a exceeds 0.95, the friction reduction effect will be reduced due to deformation of the jacket 13, etc. Furthermore, if the ratio b / a is less than 0.7, the pressure winding 11 will be excessively pressed into the groove 7, reducing the storage volume of the groove 7.

[0052] Next, we will explain the method for manufacturing the optical fiber cable 1. The optical fiber cable 1 can be manufactured in a manner similar to that of conventional optical fiber cables. For example, first, a plurality of optical fiber cores 15 are twisted together, and one or more bundle members are wound around them for identification purposes to manufacture an optical fiber unit.

[0053] Thereafter, a plurality of optical fiber units are accommodated in each groove 7 of the slot rod 5, and then a pressure wrap 11 is wound around the outer periphery of the slot rod 5, and an outer jacket 13 is extrusion coated to manufacture the optical fiber cable 1.

[0054] Under normal manufacturing conditions, the outer surface of the jacket 13 corresponding to the groove 7 does not have a concave shape but is generally flat (the dotted line portion in FIG. 2 ). On the other hand, one method for forming the outer surface of the jacket 13 corresponding to the groove 7 into a concave shape is to adjust the winding conditions of the pressure wrap 11, for example.

[0055] The pressure wrap 11 is usually wound with a certain amount of tension so as not to slacken. By preventing slack from occurring in the pressure wrap 11 in this way, it is possible to ensure the storage volume of the groove 7 and also to shorten the winding length of the pressure wrap 11.

[0056] On the other hand, in this embodiment, the winding tension of the pressure winding 11 is made weaker than that under conventional manufacturing conditions. In other words, the pressure winding 11 is wound around the outer periphery of the slot rod 5 so that slack is intentionally formed in the pressure winding 11. In this state, by pushing the outer jacket 13 onto the outer periphery of the pressure winding 11, the outer jacket 13 is pushed onto the outer periphery of the pressure winding 11 that has loosened moderately inside the groove 7, and the outer surface can be made into a concave shape.

[0057] The winding tension of the pressure winding 11 is preferably 4 N or more and less than 30 N. If the winding tension of the pressure winding 11 is 30 N or more, it becomes difficult to form slack in the pressure winding 11. If the winding tension of the pressure winding 11 is less than 4 N, the slack becomes too large, reducing the capacity for accommodating optical fiber.

[0058] In addition to adjusting the tension of the pressure winding 11 as described above, in order to obtain a more optimal slack shape, it is also possible to adjust the diameter of the nozzle for extruding the outer jacket 13, the basis weight, the drawdown, etc. By adjusting the extrusion conditions of the outer jacket 13 so as to obtain a slack shape, the outer surface of the outer jacket 13 can be efficiently formed into a concave shape.

[0059] As described above, according to this embodiment, the outer sheath 13 on the outer surface side of the groove portion 7 has a concave shape, so when laid inside a duct or the like, the contact area with the optical fiber cable 1 can be reduced, thereby reducing frictional resistance.

[0060] In particular, by satisfying (b / a)≦0.95, the above-mentioned effects can be more reliably obtained. Furthermore, by satisfying 0.7≦(b / a), the pressure wrap 11 does not enter the groove 7 excessively, and a decrease in the storage volume that can be stored in the groove 7 can be suppressed.

[0061] As a secondary effect, the outer surface of the outer jacket 13 corresponding to the groove portion 7 does not come into surface contact with a duct or the like, and the lateral pressure on the optical fiber core 15 inside the groove portion 7 can be suppressed, thereby suppressing an increase in optical loss.

[0062] Furthermore, by making a portion of the jacket 13 fit into the groove 7 of the slot rod 5, the pull-out force of the core (the slot rod 5 and the pressure winding 11) from the jacket 13 can be increased. For example, if the outer surface of the jacket 13 corresponding to the groove 7 is not concave but is made substantially flat (dotted line portion in FIG. 2), the inner surface of the jacket 13 in that portion will also be substantially flat. In this case, the core may easily move relative to the jacket 13, which may result in a lower pull-out force. In contrast, by forming the inner surface of the jacket 13 in an arc shape rather than a straight line, it becomes possible to provide resistance to core movement and improve the pull-out force.

[0063] Various optical fiber cables were fabricated and their pull-out force and optical properties were evaluated. The friction resistance evaluation test was performed in accordance with the Telcodia standard (GR-356-Core section 4.2.5, R4-7). The results are shown in Table 1.

[0064]

[0065] The "Number of ribs" in the table refers to the number of ribs (i.e., the number of grooves) in the slot rod. The "Pressure winding tension" refers to the tension of the pressure winding during the manufacture of the optical fiber cable. The pressure winding was performed by longitudinally wrapping the fiber.

[0066] "Distance a" and "distance b" in the table refer to distance a and distance b shown in Figure 2. That is, distance a is the distance from the center of the slotted rod to the surface of the jacket corresponding to the center of the rib portion, and distance b is the distance from the center of the slotted rod to the surface of the jacket corresponding to the center of the groove portion. Note that each distance was measured by capturing a cross-sectional image perpendicular to the longitudinal direction of the optical fiber cable using a microscope and measuring the dimensions on the enlarged image.

[0067] In addition, for "frictional resistance" in the table, if the frictional resistance was smaller than that of a conventional optical fiber cable without a dent (Comparative Example 1), it was rated as "good," while otherwise it was rated as "bad," meaning it was not acceptable. In other words, Comparative Example 1 was rated as not acceptable, and those with a frictional resistance smaller than that were rated as acceptable.

[0068] In addition, for the "fiber filling rate" in the table, a reduction of less than 30% in the number of optical fiber cores that can be filled in one groove compared to Comparative Example 1, which has no recess, was rated "good," and a reduction of 30% or more was rated "average."

[0069] The results showed that the frictional resistance was "good" in all of Examples 1 to 8, in which the outer surface of the sheath was concave (b / a<1). That is, all of Examples 1 to 8, in which a concave portion was provided on the outer surface of the groove portion 7, were able to reduce the frictional resistance compared to the conventional sheath with a flat outer surface.

[0070] In Example 8, the amount of recess was too large (b / a was too small), and the number of fiber cores was reduced by 30% or more compared to Comparative Example 1, so it was classified as "average."

[0071] Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the technical scope of the present invention is not limited to the above-described embodiments. It is clear that those skilled in the art can conceive of various modifications and alterations within the scope of the technical ideas described in the claims, and it is understood that these modifications and alterations also fall within the technical scope of the present invention.

[0072] REFERENCE SIGNS LIST 1 optical fiber cable 3 tension member 5 slot rod 7 groove 9 rib 11 pressure winding 13 jacket 15 optical fiber core 17 duct

Claims

1. An optical fiber cable comprising: a slot rod having on its outer periphery a plurality of rib portions and a plurality of groove portions formed between the rib portions; a plurality of optical fiber core wires housed in the groove portions; a pressure winding wound around the slot rod; and an outer jacket covering the outer periphery of the pressure winding; wherein, in a cross section perpendicular to the longitudinal direction of the slot rod, the ratio b / a, which is the ratio of the distance a from the center of the slot rod to the surface of the jacket covering the rib portions to the shortest distance b from the center of the slot rod to the surface of the jacket covering the groove portions, is less than 1.

2. The optical fiber cable according to claim 1, characterized in that 0.7≦(b / a)≦0.95 is satisfied.

3. The optical fiber cable according to claim 1, characterized in that the winding tension of the pressure winding is 4N or more but less than 30N.

4. The optical fiber cable according to claim 1, wherein the number of cores in the optical fiber core wires is 100 or more and 640 or less.

5. An optical fiber cable as described in claim 1, characterized in that in a cross section perpendicular to the longitudinal direction of the slot rod, i / h, which is the ratio of the distance h from the center of the slot rod to the inner surface of the pressure winding covering the rib portion to the shortest distance i from the center of the slot rod to the inner surface of the pressure winding covering the groove portion, is less than 1.

6. An optical fiber cable as described in claim 1, characterized in that, in a cross section perpendicular to the longitudinal direction of the slot rod, a gap is formed between the inner surface of the pressure winding covering the groove portion and the optical fiber core housed in the groove portion.

7. The optical fiber cable according to claim 6, wherein the gap is between 0.2 mm and 4.5 mm.

8. The optical fiber cable according to claim 1, wherein the pressure winding is wound so as to cover the outer peripheral surface of the slotted rod without any gaps.

9. An optical fiber cable according to claim 1, wherein the optical fiber core is an intermittent optical fiber ribbon core in which a plurality of single-core optical fiber cores are arranged in parallel and intermittently bonded in the longitudinal direction.

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