1A. Each protrusion 7 may be provided continuously along the longitudinal direction, or may be provided intermittently. The two protrusions 7 are provided so as to face each other across the center of the optical fiber cable 1A in the circumferential direction of the sheath 4 in the radial cross section of the optical fiber cable 1A. In this example, the protrusions 7 are provided on the straight line L connecting the tear strings 6 and the center of the optical fiber cable 1A. The protrusions 7 are formed on the outer peripheral portion of the sheath 4 so as to protrude in the radial direction of the optical fiber cable 1A. The protrusion 7 has a curved surface 7a in the protruding direction. The protrusions 7 are integrally formed with the sheath 4 by extrusion. The positions of the tear strings 6 can be easily recognized by the presence of the protrusions 7. Further, since the friction between the sheath 4 and the duct is reduced, the pneumatic distance can be extended.

As described above, in the present embodiment, since at least four sets 50 of tensile strength members spaced apart from one another are embedded in the sheath 4, the non-uniformity in rigidity of the optical fiber cable 1A due to the positions where the sets 50 of tensile strength members are embedded is improved. Therefore, the optical fiber cable 1A is less likely to buckle during pneumatic feeding. Further, since the optical fiber cable 1A of the present embodiment includes the central tensile strength member 5A as well as the sets 50 of tensile strength members embedded in the sheath 4, the optical fiber cable 1A is less likely to stretch when pulled. Therefore, wiring of the optical fiber cable 1A by pneumatic feeding and pulling is facilitated.

Since the central tensile strength member 5A or the sets 50 of tensile strength members of the present embodiment includes aramid fiber reinforced plastic, the optical fiber cable 1A has a lightweight and highly flexible tensile strength member, thereby facilitating wiring by pneumatic feeding and pulling.

Since the central tensile strength member 5A of the present embodiment includes aramid fiber reinforced plastic, the optical fiber cable 1A includes a lightweight and highly flexible tensile strength member. In addition, if the central tensile strength member 5A is too thick for the diameter of the optical fiber cable 1A, it is more likely that the plurality of optical fiber ribbons 2 in the cable are mounted without gaps as lateral pressure is applied on each other. As a result, the cable transmission losses can be increased. However, since the diameter of the central tensile strength member 5A of the present embodiment is 3 mm or less, it is possible to suppress stretching of the cable during pulling while suppressing the cable transmission loss.

Since the sheath of the present embodiment includes 1.5% by mass or more of release agent, wiring of the optical fiber cable 1A by pneumatic feeding is facilitated.

Since the sheath 4 of the present embodiment includes the flame retardant inorganic materials with the oxygen index of 50 or higher, the flame retardancy of the optical fiber cable 1A is further improved.

Since the optical fiber cable 1A of the present embodiment can use the optical fiber ribbon with fiber adhesive parts and non-adhesive parts, it is excellent in cable housing and can be easily separated into single fibers.

In the plurality of optical fiber ribbons 2 of the present embodiment, each of the optical fibers 11A to 11L can also use multi-core fibers, so that the transmission capacity of the optical fiber cable can be increased. Each optical fiber of the optical fiber ribbon 2 may have two cores or eight cores. For example, when the optical fiber cable 1A houses 36 optical fiber ribbons 2, each optical fiber of each optical fiber ribbon 2 has two cores, and when the cable outer diameter of the optical fiber cable 1A is 10 mm, mounting at high density of 11 cores/mm²or more per cross-sectional area of the cable can be achieved.

Since the optical fiber cable 1A of the present embodiment is provided with the tear strings 6 arranged along the plurality of optical fiber ribbons 2, an operator can tear the sheath 4 with the tear strings 6 and easily take out the individual optical fiber ribbons 2. Furthermore, since the four sets 50 of tensile strength members of the optical fiber cable 1A are arranged symmetrically with respect to the straight line L connecting the tear strings 6 and the center of the optical fiber cable 1A, the non-uniformity of the rigidity of the cable is further improved, and the wiring work during pneumatic feeding is further improved.

According to the optical fiber cable 1A of the present embodiment, the plurality of protrusions 7 protruding in the radial direction of the optical fiber cable 1A are provided on the outer peripheral portion of the sheath 4. Therefore, when the optical fiber cable 1A is pneumatically fed through the duct, the protrusions 7 come into contact with the inner wall of the duct, and accordingly, the contact area between the sheath 4 and the duct can be reduced. As a result, the friction between the sheath 4 and the duct can be reduced, and the pneumatic distance can be extended.

In FIG. 1, one set 50 of tensile strength members includes four tensile strength members 5B, but the number of tensile strength members 5B is not limited. One set 50 of tensile strength members may include one, two, or five or more tensile strength members 5B.

Although FIG. 2 shows the optical fiber ribbon with adhesive parts and non-adhesive parts of 12 optical fibers, the number of optical fibers is not limited thereto. In addition, the plurality of optical fibers may be formed by twisting a plurality of single-core optical fibers rather than the ribbons.

Modification Of First Embodiment

An optical fiber cable 1B according to a modification of the first embodiment will be described with reference to FIG. 3. It is to be noted that the same reference numerals are assigned to the same configurations as those of the optical fiber cable 1A according to the first embodiment, and the description thereof will be omitted.

FIG. 3 is a cross-sectional view of the optical fiber cable 1B viewed from a direction perpendicular to the longitudinal direction. As shown in FIG. 3, in the optical fiber cable 1B, the sheath 4 includes an inner layer 41 and an outer layer 42, in addition to the configuration of the optical fiber cable 1A according to the first embodiment. The cable outer diameter of the optical fiber cable 1B is 12 mm, for example.

The inner layer 41 of the sheath 4 is arranged outside the plurality of optical fiber ribbons and provided to enclose a plurality of sets 50 of tensile strength members. The plurality of sets 50 of tensile strength members and the tear strings 6 are embedded in the inner layer 41 of the sheath 4. For example, the inner layer 41 of the sheath 4 includes magnesium hydroxide or aluminum hydroxide as a flame retardant inorganic material. The thickness of the inner layer 41 is 1 mm, for example.

The outer layer 42 of the sheath 4 is arranged outside the inner layer 41. The protrusion 7 is provided on the outer peripheral portion of the outer layer 42. The outer layer 42 of the sheath 4 includes a silicone-based release agent such as silicone or siloxane as a release agent. The thickness of the outer layer 42 is 0.5 mm, for example.

As described above, in this modification, since the inner layer 41 includes the flame retardant inorganic material, it is possible to realize the optical fiber cable 1B having excellent flame retardancy. In addition, since the outer layer 42 includes the release agent, the optical fiber cable 1B can be fed smoothly even in the duct during pneumatic feeding. This facilitates wiring.

While certain embodiment and modification of the present disclosure have been described above, the present disclosure is not limited to the above embodiment or modification, and other modifications can be adopted for part of the configuration as necessary. For example, as shown in FIG. 4, the optical fiber cable 1A or the optical fiber cable 1B may be terminated. A cable 1X with a connector shown in FIG. 4 includes a multi-core connector 90 attached to the plurality of optical fiber ribbons 2 at one end of the optical fiber cable 1A or the optical fiber cable 1B. As shown in FIG. 4, the optical fibers 11 are exposed at an end of the multi-core connector 90. The number of optical fibers 11 shown in FIG. 4 is an example, and is not particularly limited as long as there are at least 12 of these. The optical fiber 1X with a connector may have 432 optical fibers, for example. By providing the multi-core connector 90 in advance at one end of the optical fiber cable 1A or the optical fiber cable 1B, workability can be facilitated when optically connecting the optical fiber cable 1A or the optical fiber cable 1B.

(Evaluation Test)

The allowable tension, pneumatic distance, cable transmission loss, and mounting density of the optical fiber cable 1A according to the first embodiment were evaluated. In the evaluation test, the central tensile strength member 5A and the plurality of tensile strength members 5B (set 50 of tensile strength members) are aramid fiber reinforced plastics with a tensile modulus of elasticity of 6300 kg/mm². As a method for evaluating the allowable tension, when tension was applied to the optical fiber cable 1A, the tension when the cable length was stretched by 0.3% was measured as the allowable tension.

As a method for evaluating the pneumatic distance, a microduct pumping test defined by the International Electrotechnical Commission (IEC) was used. A general-purpose microduct was used in the pneumatic distance test. The inner diameter of the duct was 20 mm. The total pneumatic distance in the duct was set to 1000 m or more, and the duct was arranged so as to be turned back every 100 m. The radius of curvature of the duct was 40 times the outer diameter of the duct. The pressure in the duct was 1.3 MPa to 1.5 MPa.

As a method for evaluating the cable loss characteristics, it was evaluated whether or not the transmission loss was less than 0.3 dB/km when light with a wavelength of 1.55 μm was incident on the fiber cable 1A.

In addition, as a method for evaluating the mounting density, the housing rate when 36 optical fiber ribbons 2 were housed in the optical fiber cable 1A was evaluated as a percentage. The cable outer diameter of the optical fiber cable 1A was 11 mm, and the diameter (core diameter) of the space housing the plurality of optical fiber ribbons 2 and the central tensile strength member 5A was 9 mm. In the optical fiber ribbon 2 in which 12 optical fibers having a diameter of 250 μm were arranged in parallel, the tape width of the optical fiber ribbon 2 was 3.1 mm and the tape thickness was 0.3 mm. The mounting density was calculated by assuming that the area of each optical fiber ribbon 2 was 0.786 mm²including the tolerance. Table 1 shows the evaluation results.

TABLE 1

Tensile strength
Central tensile

member 5B
strength member 5A
Allowable
Pneumatic
Cable
Mounting

Sample

Diameter
Number
Diameter
Number
tension
distance
transmission
density

No.
Cable structure
(mm)
(numbers)
(mm)
(numbers)
(N)
(m)
loss
(%)

1
Slotless cable
0.7
2
None
None
109
600
0.2 dB/km
45