fiber optic cable

Abstract

To provide an optical fiber cable that is excellent in handleability without lowering manufacturability greatly.SOLUTION: In a cross section vertical to a longitudinal direction of an optical fiber cable 1, a first tension member 9a is provided in a facing portion in a center of a core 5. Also, a plurality of second tension members 9b are disposed in a circumferential direction between the first tension members 9a. The second tension member 9b is lower in tension rigidity (axis rigidity=axis direction force / an amount of displacement in an axial direction) than the first tension member 9a. A sheath 13 is provided so as to cover the core 5 (a plurality of coated optical fibers 3), the first tension member 9a, the second tension member 9b and the like. The shape of the sheath 13 is almost circle. The sheath 13 is, for example, polyolefin-based resin. Herein, the sheath 13 in at least an outer peripheral part of the first tension member 9a is composed of foamed resin 13a.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical fiber cable comprising a plurality of optical fiber core wires. [Background technology]

[0002] As an optical fiber cable, a core made up of a large number of coated optical fibers, a tension member arranged on the outer periphery of the core, and an outer jacket are used (for example, Patent Document 1).

[0003] In such optical fiber cables, a pair of tension members are positioned opposite each other with the core at the center, making them easy to bend in the direction of the bending center on the line connecting the tension members, but difficult to bend in other directions. In other words, optical fiber cables have a directional tendency to bend. In particular, they are most difficult to bend in the direction of the bending center on the center line perpendicular to the line connecting the tension members.

[0004] As such, optical fiber cables such as those in Patent Document 1 are very prone to bending in directions with the straight line connecting the tension members as the bending center, so when the optical fiber cable is pressure-fed into a pipe, for example, there is a risk that the optical fiber cable will become bent or wavy and become clogged inside the pipe.

[0005] In response to this, an optical fiber cable has been proposed in which tension members are arranged at positions facing each other across the core, and similar tension members are also arranged perpendicular to the line connecting these tension members (Patent Document 2).In other words, an optical fiber cable has been proposed in which tension members are arranged in mutually orthogonal directions, sandwiching the core. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2019-109400 A [Patent Document 2] Japanese Patent Publication No. 2020-204752 Summary of the Invention [Problem to be solved by the invention]

[0007] The optical fiber cable of Patent Document 2 has tension members arranged in the above-mentioned bending direction, so it has high overall rigidity and is difficult to bend in either direction. As a result, it cannot be wound onto a small-diameter drum, and winding it onto a drum larger than necessary requires excessive packaging. Furthermore, forcibly winding it onto a small-diameter drum may cause the tension members to buckle.

[0008] In response to this, the inventors discovered a method for suppressing such buckling and further improving bending flexibility by reducing the adhesive force between the tension member and the outer sheath. By reducing the adhesive force between the tension member and the outer sheath in this way, for example, when bending in a direction that is difficult to bend, the tension member and the outer sheath can shift longitudinally, thereby easing the difficulty in bending and suppressing buckling of the tension member.

[0009] However, when the amount of resin extruded for the jacket is large, such as in the case of an optical fiber cable with a large outer diameter, the resin temperature during extrusion tends to rise, and the adhesive strength between the tension member and the jacket tends to increase. On the other hand, if you try to lower the resin temperature and thereby reduce the adhesive strength between the jacket and the tension member, you need to lower the extrusion temperature setting of the extruder or slow down the manufacturing line speed, but this hardens the resin, making molding difficult and reducing manufacturing efficiency.

[0010] The present invention has been made in view of the above problems, and has as its object to provide an optical fiber cable that is easy to handle without significantly reducing manufacturability. [Means for solving the problem]

[0011] In order to achieve the above-mentioned object, the present invention provides an optical fiber cable comprising a core consisting of a plurality of optical fiber cores, first tension members arranged in positions facing each other around the core in a cross section perpendicular to the longitudinal direction of the optical fiber cable, a plurality of second tension members arranged circumferentially between the first tension members and having a lower tensile rigidity than the first tension members, and an outer sheath arranged to cover the core, the first tension member, and the second tension member, wherein at least the outer sheath on the outer periphery of the first tension member is made of a foamed resin.

[0012] The outer covering on the outer periphery of the second tension member may be made of a non-foaming resin.

[0013] The entire outer cover may be made of foamed resin.

[0015] According to the present invention, by arranging first tension members facing each other around the core and arranging second tension members, which have lower tensile rigidity than the first tension members, between the first tension members in the circumferential direction, it is possible to reduce the ease of bending in the bending direction (hereinafter sometimes simply referred to as the "easy-to-bend direction") with the center line of the facing direction of the first tension members as the bending center. This increases the overall rigidity and makes it possible to suppress unintended bending of the optical fiber cable.

[0016] Furthermore, compared to the bending direction centered on the center line perpendicular to the opposing direction of the first tension members (hereinafter sometimes simply referred to as the "difficult to bend direction"), bending in the easy to bend direction is relatively easy, so the optical fiber cable can be easily bent when winding it around a drum, etc. Furthermore, because the ease of bending in the easy to bend direction is reduced, excessive bending of the optical fiber cable in this direction can be prevented.

[0017] That is, the present invention increases the rigidity in the easy-to-bend direction to make it harder to bend compared to the optical fiber cable of Patent Document 1, and intentionally provides a directionality that is relatively easier to bend compared to the hard-to-bend direction compared to the optical fiber cable of Patent Document 2. In this way, the present invention can achieve an appropriate balance of ease of bending in directions that are perpendicular to each other, thereby achieving both the advantages of the conventional optical fiber cables of Patent Documents 1 and 2.

[0018] Furthermore, by using a foamed resin for the outer sheath that covers the outer periphery of the first tension member, the adhesion force between the first tension member and the sheath can be efficiently reduced even when the resin temperature during extrusion is raised. This results in excellent manufacturability. Furthermore, because the adhesion force between the first tension member and the sheath is low, the first tension member and the sheath can partially shift when the optical fiber cable is bent, thereby suppressing buckling of the first tension member. This prevents damage to the optical fiber cable when it is being pressure-fed or wound up.

[0019] In this case, only the outer periphery of the first tension member may be made of foamed resin, with the other parts made of non-foamed resin. This allows the outer sheath of the first tension member, except for the outer periphery, to be made of ordinary non-foamed resin, which not only looks good but also ensures the strength and wear resistance of the sheath.

[0020] Furthermore, the entire outer jacket can be made of foamed resin, rather than just the outer periphery of the first tension member. This allows the jacket to be made of a single resin material, which improves manufacturability and reduces the weight of the optical fiber cable. [Effects of the Invention]

[0022] According to the present invention, an optical fiber cable with excellent handleability can be provided without significantly reducing manufacturability. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a cross-sectional view of an optical fiber cable 1. FIG. [Figure 2] FIG. 2 is a cross-sectional view of the optical fiber cable 1a. [Figure 3] FIG. 2 is a cross-sectional view of the optical fiber cable 1b. [Figure 4] FIG. 1C is a cross-sectional view of an optical fiber cable 1c. DETAILED DESCRIPTION OF THE INVENTION

[0024] (First embodiment) The first embodiment will be described below with reference to the drawings. Fig. 1 is a cross-sectional view of an optical fiber cable 1. The optical fiber cable 1 is a slotless cable that does not use slots, and is composed of a core 5, a first tension member 9a, a second tension member 9b, an outer jacket 13, etc.

[0025] The core 5 is made up of a plurality of optical fiber cores 3. More specifically, a plurality of optical fiber cores 3 are twisted together to form an optical fiber unit, and a plurality of optical fiber units are further twisted together to form the core 5. The optical fiber cores 3 may be, for example, intermittently bonded optical fiber ribbons that are bonded intermittently in the longitudinal direction.

[0026] As shown in Fig. 1, a pressure wrap 7 is provided around the core 5 (plurality of coated optical fibers 3). The pressure wrap 7 is a tape-like member, a nonwoven fabric, or the like, and is arranged to cover the entire outer periphery of the core 5 by, for example, vertical splicing. That is, the pressure wrap 7 is vertically spliced around the outer periphery of the core 5 so that the longitudinal direction of the pressure wrap 7 substantially coincides with the axial direction of the optical fiber cable 1 and the width direction of the pressure wrap 7 is the circumferential direction of the optical fiber cable 1. Note that the pressure wrap 7 is not necessarily required, and the pressure wrap 7 may also be referred to as the core 5.

[0027] In a cross section perpendicular to the longitudinal direction of the optical fiber cable 1, first tension members 9a are provided in positions facing each other with the core 5 at the center. In addition, multiple second tension members 9b are arranged circumferentially between the first tension members 9a. The second tension members 9b have lower tensile rigidity (axial rigidity = axial force / axial deformation amount) than the first tension members 9a. For example, the diameter (cross-sectional area) of each first tension member 9a is larger than the diameter (cross-sectional area) of each second tension member 9b.

[0028] The first tension member 9a and the second tension member 9b bear the tension of the optical fiber cable 1. The first tension member 9a mainly bears the tension of the optical fiber cable 1, and the second tension member 9b secondarily bears the tension of the optical fiber cable 1. The material of the first tension member 9a and the second tension member 9b is not particularly limited, but for example, fiber reinforced plastic (FRP) made from aramid fiber, glass fiber, etc. can be used. The first tension member 9a and the second tension member 9b may also be made of different materials.

[0029] In the illustrated example, three second tension members 9b are arranged between each pair of first tension members 9a in the circumferential direction. That is, a total of six second tension members 9b are arranged between each pair of first tension members 9a in the circumferential direction. Note that it is desirable for the total cross-sectional area of the first tension members 9a to be larger than the total cross-sectional area of the second tension members 9b, and furthermore, the cross-sectional area of each first tension member 9a may be larger than the total cross-sectional area of the second tension members 9b.

[0030] The second tension members 9b and the first tension members 9a are arranged at approximately equal intervals in the circumferential direction. That is, the first tension members 9a are arranged at intervals of 180°, and the second tension members 9b and adjacent first tension members and second tension members 9b are arranged at intervals of approximately 45°. Note that there is no particular limitation on the number of second tension members 9b.

[0031] Furthermore, tear cords 11 are provided at circumferential positions different from the first tension member 9a and the second tension member 9b, but at positions facing each other across the core 5. Furthermore, an outer jacket 13 is provided on the outer periphery of the core 5. The first tension member 9a, the second tension member 9b, and the tear cord 11 are embedded in the outer jacket 13. In other words, the outer jacket 13 is provided so as to cover the core 5 (plurality of optical fiber cores 3), the first tension member 9a, the second tension member 9b, etc. The outer shape of the outer jacket 13 is approximately circular. The outer jacket 13 is made of, for example, a polyolefin-based resin.

[0032] Here, at least the outer sheath 13 on the outer periphery of the first tension member 9a is made of foamed resin 13a. In the illustrated example, the foamed resin 13a is exposed on the outer surface of the sheath 13. Furthermore, the outer sheath 13 in areas other than the first tension member 9a is made of non-foamed resin. In other words, the outer sheath 13 on the outer periphery of the second tension member 9b is made of non-foamed resin. In this way, by arranging the foamed resin 13a only in necessary areas, the amount of foaming agent used can be reduced and the reduction in the mechanical strength of the sheath 13 can be minimized.

[0033] Such partially foamed resin 13a can be easily formed by two-layer extrusion. For example, by adding a foaming agent to the resin extruded into the area corresponding to foamed resin 13a, the resin can be foamed by the heat generated during extrusion. In this case, even if the linear speed and extrusion temperature are increased, the adhesive force with the first tension member 9a can be reduced, which improves manufacturability.

[0034] The foaming ratio of the foamed resin is preferably between 14% and 50%. If the foaming ratio is too low, the effect of reducing the adhesive force with the first tension member 9a is small. If the foaming ratio is too high, the mechanical properties of the outer covering are reduced, and the durability as a protective layer is reduced.

[0035] The adhesive strength between the tension members and the resin tends to decrease as the foaming ratio of the resin increases. That is, the adhesive strength between the foamed resin 13a and the tension members is weaker than that between the non-foamed resin. Therefore, in this embodiment, the adhesive strength between the first tension member 9a and the outer sheath 13 is weaker than the adhesive strength between the second tension member 9b and the outer sheath 13.

[0036] By doing so, when the optical fiber cable 1 is bent in a direction that makes it difficult to bend, the restraining force that the first tension member 9a receives from the jacket 13 can be reduced, allowing misalignment to occur between the tension member and the jacket. This prevents buckling of the first tension member 9a. Meanwhile, because the second tension member 9b and the jacket 13 are in close contact, loss fluctuations due to expansion and contraction of the jacket 13 in response to changes in environmental temperature can be suppressed, stabilizing characteristics.

[0037] Here, the optical fiber cable 1 is easiest to bend in the direction (up and down in the figure) with the center line (center line A in the figure) parallel to the opposing direction of the first tension member 9a as the bending center, and is difficult to bend in other directions (for example, center line B perpendicular to center line A).

[0038] In this case, compared to a conventional optical fiber cable without the second tension member 9b (for example, the optical fiber cable of Patent Document 1), the optical fiber cable 1 has a reduced bending strength in the easy bending direction (i.e., it becomes harder to bend) due to the second tension member 9b. In other words, the bending rigidity in the easy bending direction can be increased.

[0039] On the other hand, compared to an optical fiber cable in which the first tension members 9a are arranged to face each other in directions perpendicular to each other and are difficult to bend in either direction (for example, the optical fiber cable of Patent Document 2), the optical fiber cable 1 has directions in which it is difficult to bend and directions in which it is easy to bend. In other words, the direction in which the center line A is the bending center is relatively easier to bend than other directions, and is the easy bending direction.

[0040] As described above, in this embodiment, the outer sheath 13 around the outer periphery of the first tension member 9a is made of foamed resin 13a, which reduces the adhesive force between the first tension member 9a and the sheath 13 (foamed resin 13a). This reduces the constraint of the first tension member 9a by the sheath 13, and prevents buckling of the first tension member 9a.

[0041] Furthermore, the second tension member 9b can increase the bending rigidity in the conventional direction where the cable is easy to bend, thereby preventing bending of the optical fiber cable during pumping, and the resulting clogging, etc. Furthermore, it can prevent the optical fiber cable from bending beyond the allowable bending radius during installation, etc.

[0042] Furthermore, when winding the optical fiber cable around a drum or the like, the optical fiber cable can be bent in the direction in which it is easiest to bend and then wound around the drum or the like, thereby preventing the size of the drum or the like from increasing. In this way, by using different tension members in combination, it is possible to increase the bending rigidity in the easy-to-bend direction while leaving both the easy-to-bend direction and the hard-to-bend direction, thereby achieving an appropriate balance of ease of bending depending on the direction, and thereby obtaining an optical fiber cable that is easy to handle and easy to install.

[0043] Furthermore, since the second tension members 9b are arranged at approximately equal intervals in the circumferential direction, the direction of the ease of bending due to the second tension members 9b can be changed smoothly.

[0044] The spacing (arrangement angle) between the second tension members 9b and the spacing (arrangement angle) between the first tension member 9a and the second tension member 9b do not have to be the same. For example, the second tension members 9b may be arranged unevenly in a direction perpendicular to the opposing direction of the first tension members 9a. For example, in the case where three second tension members 9b are arranged between each pair of first tension members 9a, the angle between the second tension members 9b may be approximately 30°, and the angle between the first tension member 9a and the second tension members at both ends may be approximately 60°.

[0045] As mentioned above, the effect of improving the bending rigidity in the bending direction around the center line A increases with increasing distance from the center line A. Therefore, by disposing the second tension members 9b unevenly in a direction perpendicular to the opposing direction of the first tension members 9a, the bending rigidity can be efficiently improved in the direction in which the optical fiber cable 1b is most easily bent.

[0046] (Second embodiment) Next, a second embodiment will be described. Fig. 2 is a cross-sectional view showing an optical fiber cable 1a according to the second embodiment. In the following description, components that have the same functions as those in the first embodiment are given the same reference numerals as those in Fig. 1, and redundant description will be omitted.

[0047] The optical fiber cable 1a has a configuration similar to that of the optical fiber cable 1, but the shape of the first tension members 9a is different. In the optical fiber cable 1 described above, two first tension members 9a were arranged one on each side of the core 5, but in the optical fiber cable 1a, a total of four first tension members 9a are arranged two on each side of the core 5. In this case, all of the first tension members 9a are collectively covered with foam resin 13a at each position. In other words, all of the first tension members 9a are in close contact with the foam resin 13a.

[0048] In each location, the first tension members 9a are arranged with gaps in the circumferential direction. In this way, by arranging multiple first tension members 9a at circumferentially spaced positions facing each other around the core 5, the first tension members 9a can be arranged at positions shifted from the center line A that is parallel to the facing direction of the first tension members 9a.

[0049] Even in this case, the cross-sectional area of each first tension member 9a is larger than the cross-sectional area of each second tension member 9b. Furthermore, the cross-sectional area of each first tension member 9a may be larger than the total cross-sectional area of the second tension members 9b. Also, even in this case, the tensile stiffness of each first tension member 9a is larger than the tensile stiffness of each second tension member 9b. Also, the tensile stiffness of each first tension member 9a may be larger than the sum of the tensile stiffnesses of the second tension members 9b.

[0050] Normally, when a member is bent, the outside of the bend undergoes tensile deformation and the inside of the bend undergoes compressive deformation, but the bending centerline serves as the neutral axis of tension and compression. Therefore, the amount of deformation near the centerline is small, and the effect of the structure on the centerline on bending rigidity is small. Therefore, as mentioned above, the bending direction around the centerline A, which is the opposing direction of the first tension member 9a, is the direction in which the optical fiber cable 1a is easiest to bend. In other words, by arranging the first tension member 9a on the centerline A, as in the optical fiber cable 1 described above, it is possible to minimize the improvement in bending rigidity due to the first tension member 9a when bending in this direction, making it easier to bend in that direction.

[0051] On the other hand, in the optical fiber cable 1a, the first tension member 9a is positioned at a position offset from the center line A, and therefore the first tension member 9a has a greater effect on bending rigidity than in the optical fiber cable 1. In other words, by positioning the first tension member 9a at a position offset from the center line A, bending rigidity in the bending direction around the center line A is improved, further complementing the effect of positioning the second tension member 9b.

[0052] When multiple first tension members 9a are arranged in each location, the central position in the circumferential direction is taken as the representative position of the first tension members 9a. For example, when three second tension members 9b are arranged between each of the first tension members 9a, the angle between the first tension members 9a (representative positions of the first tension members 9a) and the second tension members 9b is 45°.

[0053] According to the second embodiment, it is possible to obtain the same effects as in the first embodiment. Furthermore, by arranging multiple first tension members 9a at positions facing each other with the core 5 at the center and arranging the first tension members 9a at positions shifted from the center line A, it is possible to improve the bending rigidity in the direction in which it is easy to bend. Note that in this embodiment, even when combined with the effect of improving bending rigidity provided by the second tension members 9b, the bending rigidity in the bending direction about the center line A as the bending center is lower than the bending rigidity in the bending direction about the center line B, which is perpendicular to the center line A, and therefore this direction remains the direction in which it is easy to bend.

[0054] If there is an even number of first tension members 9a arranged in each opposing position, the line connecting their centers in the circumferential direction should be taken as the opposing center line A. If there is an odd number of first tension members 9a arranged in each opposing position, the line connecting the middle first tension members 9a should be taken as the opposing center line A. However, if three or more first tension members 9a are arranged in each location, they should be arranged at equal intervals.

[0055] In this way, when there is an odd number of first tension members 9a arranged at each opposing position, the central first tension member 9a is arranged on the center line A, but at least some of the other first tension members 9a are each arranged at a position shifted from the center line A in the opposing direction of the first tension members 9a, thereby achieving a similar effect.

[0056] (Third embodiment) Next, a third embodiment will be described. Fig. 3 is a cross-sectional view showing an optical fiber cable 1b according to the third embodiment. The optical fiber cable 1b has substantially the same configuration as the optical fiber cable 1a, but the arrangement of the foam resin 13a is different.

[0057] In the optical fiber cable 1b, the foamed resin 13a is not exposed on the outer surface of the jacket 13. That is, the entire outer surface of the jacket 13 is made of non-foamed resin.

[0058] According to the third embodiment, it is possible to obtain the same effects as those of the first embodiment. Furthermore, since the foamed resin 13a is not exposed on the outer surface, it has a good appearance and can prevent damage to the foamed resin 13a.

[0059] (Fourth embodiment) Next, a fourth embodiment will be described. Fig. 4 is a cross-sectional view showing an optical fiber cable 1c according to the fourth embodiment. Optical fiber cable 1c has a configuration similar to that of optical fiber cable 1a, but the entire outer jacket 13 is made of foamed resin 13a. In other words, no non-foamed resin is disposed in outer jacket 13, and second tension member 9b is also covered with foamed resin 13a.

[0060] According to the fourth embodiment, it is possible to obtain the same effects as those of the first embodiment. As described above, the entire outer cover 13 may be made of foamed resin 13a as long as sufficient durability can be ensured. This makes manufacturing easier. [Example]

[0061] We evaluated various optical fiber cables with different jacket configurations. The optical fiber cables used for the evaluation had cross-sectional shapes generally shown in Figure 2 (foamed resin only near the first tension member) or Figure 4 (foamed resin covering the entire jacket). First, 12 optical fibers with a diameter of 200 μm were intermittently bonded together to create a 12-fiber optical fiber ribbon. Twelve of these optical fiber ribbons were twisted together to create a 144-fiber optical fiber unit.

[0062] Forty-eight 144-core optical fiber units were supplied and twisted together, then pressure windings were attached vertically, the fiber was rolled up in a forming jig, and nylon pressure thread was wound around it to create a core with 6,912 cores.

[0063] The core, tension members, and rip cord for ripping the jacket were arranged straight along the length of the cable without twisting, and the jacket was extrusion coated. After sheathing, the cable was cooled to approximately 20°C in a water bath to create an optical fiber cable. The jacket material was LLDPE. The jacket was extruded in two layers, and LLDPE containing a sodium bicarbonate-based blowing agent was used for the outer periphery of the first tension member, and it was expanded at various expansion ratios depending on the oil content of the blowing agent and the heat during extrusion.

[0064] The optical fiber cable had an outer diameter of 29.5 mm and a jacket thickness of 4 mm. The tension members used were four φ2.0 mm first tension members made of G-FRP (glass fiber reinforced plastic) and six φ0.7 mm second tension members made of K-FRP (aramid fiber reinforced plastic). Two φ2.0 mm G-FRP first tension members were positioned opposite each other, and two φ0.7 mm K-FRP second tension members were positioned at equal intervals between the first tension members.

[0065] The adhesion between the tension member and the jacket was measured for each of the resulting optical fiber cables. The measurement method was as follows: First, a 20 mm wide, 100 mm long piece of jacket was cut out axially from the first tension member, including the first tension member. Next, a sample was created by removing 45 mm of the jacket from the front and rear of the first tension member, leaving a 10 mm long piece of jacket in the longitudinal direction. The exposed first tension member was passed through a drawing die with a 2.8 mm diameter hole and pulled at a tensile speed of 500 mm / min using an Instron tensile tester. The maximum stress upon removal was taken as the adhesion between the tension member and the jacket. The adhesion between the first tension member and the jacket can be adjusted by adjusting the foaming agent content (expansion ratio) and other manufacturing conditions, such as the jacket extrusion temperature. The second tension member was also passed through a drawing die with a 1.0 mm diameter hole in the same manner as the first tension member, and the adhesion between the tension member and the jacket was measured.

[0066] To evaluate the ease of installation, a bending test was conducted. The bending test was conducted by preparing semicircular mandrels with radii of 600 mm, 300 mm, and 200 mm, and wrapping the cable 180 degrees around the mandrel, with the bending direction centered on a line that forms a 90-degree angle with the facing direction of the first tension member (the bending direction is difficult, with center line B in Figure 2 as the bending center). The presence or absence of buckling of the first tension member during this test was checked, and a rating of ○ was given for cases where no buckling was observed under any condition, a △ was given for cases where buckling occurred only under the 200 mm radius condition, and an × was given for cases where buckling was observed under all other conditions.

[0067] To evaluate the temperature characteristics, the increase in transmission loss during a heat cycle test from -30 to 70°C was evaluated. 1000m of cable was wound around a drum with a body diameter of 1400mm, and temperature fluctuations were repeated in a thermostatic chamber for 3 cycles, with a temperature holding time of 6 hours and a temperature change time of 6 hours. The maximum increase in transmission loss at a wavelength of 1550nm compared to the initial value was measured. Results of 0.15dB / km or less were rated as good.

[0068] Additionally, to evaluate the mechanical properties, damage to the jacket when subjected to lateral pressure was checked. A load of 2200 N was applied to a 100 mm long jacket, and those that showed no damage to the jacket were marked as ◯. The increase in loss at a wavelength of 1550 nm was also evaluated, and the maximum increase in transmission loss at a wavelength of 1550 nm relative to the initial value was measured. Results of less than 0.10 dB / km were marked as ◯, and those exceeding 0.10 dB / km were marked as △. The results are shown in Tables 1 and 2.

[0069] [Table 1]

[0070] [Table 2]

[0071] The results show that, for Examples 1 to 8, normal manufacturing conditions (extrusion resin temperature of 185°C or higher, linear speed of 10 m / min or higher) were used, but the adhesion force between the first tension member and the outer jacket was 70 N / 10 mm or less. As a result, the temperature characteristics were all rated as ○, and the installation ease was also rated as △ or better. In particular, Examples 2 to 8, in which the adhesion force between the first tension member and the outer jacket was 50 N / 10 mm or less, were also rated as ○ in the installation ease evaluation.

[0072] In Example 5, where the foaming rate of the foamed resin was over 60%, the adhesion force between the first tension member and the outer sheath was 10 N / 10 mm or less, and the loss increase under lateral pressure was evaluated as fair. Similarly, in Example 8, where the adhesion force between the first tension member and the outer sheath for the entire outer sheath and the foamed resin was 20 N / 10 mm or less and the adhesion force between the second tension member and the outer sheath was 160 N / 10 mm or less, the loss increase under lateral pressure was also evaluated as fair. The other examples, Examples 1 to 4 and Examples 6 and 7, also received an excellent rating for mechanical properties.

[0073] On the other hand, since Comparative Examples 1 and 2 do not use foamed resin, although there are some differences depending on the manufacturing conditions, the adhesion force between the first tension member and the outer sheath in both cases exceeds 70 N / 10 mm, so buckling was observed in the installation and the results were rated as ×.

[0074] 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. [Explanation of symbols]

[0075] 1, 1a, 1b, 1c....Optical fiber cable 3....Optical fiber core 5...Core 7. Press down 9a………First tension member 9b...Second tension member 11...Tear cord 13……Outer cover 13a: Foam resin

Claims

1. a core consisting of a plurality of optical fiber cores; a first tension member provided at a position facing the core in a cross section perpendicular to the longitudinal direction of the optical fiber cable; a plurality of second tension members arranged between the first tension members in the circumferential direction and having a lower tensile rigidity than the first tension members; an outer covering provided to cover the core, the first tension member, and the second tension member; Equipped with 10. An optical fiber cable, wherein the outer sheath at least on the outer periphery of the first tension member is made of foamed resin.

2. 2. The optical fiber cable according to claim 1, wherein the outer jacket on the outer periphery of the second tension member is made of a non-foaming resin.

3. 2. The optical fiber cable according to claim 1, wherein the entire outer jacket is made of foamed resin.

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