Optical fiber cable

Abstract

An optical fiber cable (10) is provided with: at least one optical fiber bundle (12) that includes a plurality of multi-core optical fibers (11); an outer sheath (13) that accommodates said at least one optical fiber bundle (12); and a pressure fluctuation suppression part (15) that is provided to the outer sheath (13) and suppresses fluctuation in pressure being applied to the multi-core optical fibers (11) due to expansion and contraction of the outer sheath (13) associated with temperature changes.

Description

Optical fiber cable 【0001】 The present disclosure relates to an optical fiber cable. 【0002】 In order to meet the increasing demand for large capacity and multi-core in submarine and terrestrial networks, multi-core optical fibers (MCFs) that improve space utilization efficiency in limited equipment space have been studied. Crosstalk between cores in a multi-core optical fiber is communication noise and degrades the signal quality. Crosstalk between cores increases as the transmission distance increases, which is a limiting factor for the long-distance and large-capacity communication networks using multi-core optical fibers. 【0003】 It is known that crosstalk between cores varies depending on the bending state of the multi-core optical fiber and reaches a maximum value at a specific bending radius (see Non-Patent Document 1). According to tests using conventional terrestrial multi-core optical fiber cables, crosstalk between cores increases after the cable is laid (see Non-Patent Document 2). Crosstalk between cores also varies with temperature changes (see Non-Patent Documents 3 and 4). 【0004】M. Koshiba et al., “Analytical expression of average power-coupling coefficients for estimating intercore crosstalk in multicore fibers,” IEEE Photon. J., vol. 4, no. 5, pp. 1987-1995, Oct. 2012. T. Mori et al., “Crosstalk variation in a standard cladding diameter multicore fiber during the cabling and installation processes,” in Proc. of SPIE Vol. 12429, 124290B Feb.15, 2023, doi: 10.1117 / 12.2649473.T. Mori et al., “Applicability of Standard Cladding Diameter Multi-Core Fiber Cables for Terrestrial Field,” in Journal of Lightwave Technology, vol. 42, no. 3, pp. 1044-1055, 1 Feb.1, 2024, doi: 10.1109 / JLT.2023.3333429.Y. Sasaki, R. Fukumoto, K. Takenaga, S. Shimizu and K. Aikawa, “Optical-Fiber Cable Employing 200-μm-Coated Four-Core Multicore Fibers,” in Journal of Lightwave Technology, vol. 40, no. 5, pp. 1560-1566, 1 March1, 2022, doi: 10.1109 / JLT.2022.3144505. 【0005】 Intercore crosstalk within multicore optical fibers is one of the phenomena that affects the characteristics of a transmission line. For example, the transmission distance of a transmission line is limited by the maximum value of intercore crosstalk. 【0006】 This disclosure aims to provide an optical fiber cable that can suppress the amount of change in inter-core crosstalk after installation. 【0007】 An optical fiber cable according to one aspect of the present disclosure comprises at least one optical fiber bundle including a plurality of multicore optical fibers, an outer sheath housing at least one of the optical fiber bundles, and a pressure fluctuation suppression unit provided on the outer sheath to suppress pressure fluctuations on the multicore optical fibers due to expansion and contraction of the outer sheath caused by temperature changes. 【0008】 According to this disclosure, it is possible to provide an optical fiber cable that can suppress the amount of change in inter-core crosstalk after installation. 【0009】 Figure 1 is a cross-sectional view of an optical fiber cable according to the first embodiment of this disclosure. Figure 2 is a cross-sectional view of an optical fiber cable according to the second embodiment of this disclosure. Figure 3 is a cross-sectional view of an optical fiber cable according to the third embodiment of this disclosure. Figure 4 is a cross-sectional view of an optical fiber cable according to the fourth embodiment of this disclosure. Figure 5 is a cross-sectional view of an optical fiber cable according to the fifth embodiment of this disclosure. Figure 6 is a cross-sectional view of an optical fiber cable according to the sixth embodiment of this disclosure. Figure 7 is a diagram for explaining the effective bending radius of the MCF. Figure 8 is a graph showing the relationship between the twist spacing and twist radius of the MCF (optical fiber bundle) for several effective bending radii. Figure 9 is a graph of numerical analysis results showing the amount of intercore crosstalk with respect to the effective bending radius of the MCF for several core deviations. Figure 10A is a graph showing an example of the change in effective bending radius with respect to the inner diameter of the outer sheath (maximum value of the twist radius). Figure 10B is a graph showing an example of the change in effective bending radius with respect to the inner diameter of the outer sheath (maximum value of the twist radius). Figure 10C is a graph showing an example of the change in effective bending radius with respect to the inner diameter of the outer sheath (maximum value of the twist radius). 【0010】The optical fiber cable 10 according to several embodiments of this disclosure will be described below. In each figure, common parts are denoted by the same reference numerals, and redundant explanations will be omitted. In the following description, "longitudinal direction" means the direction in which the optical fiber cable 10 extends. The circumferential direction and radial direction are both defined with respect to the central axis of the optical fiber cable 10. 【0011】 The optical fiber cable 10 according to this embodiment comprises an optical fiber bundle 12 including a plurality of multicore optical fibers 11 and a pressure fluctuation suppression unit 15 provided on the outer sheath 13 (see Figure 1). The optical fiber cable 10 is laid, for example, in an optical transmission network. The environment in which the optical fiber cable 10 is laid may be on land or on the seabed (lakebed). 【0012】 The pressure fluctuation suppression unit 15 suppresses pressure fluctuations on the optical fiber bundle 12 caused by the expansion and contraction of the outer sheath 13 due to temperature changes. In other words, the pressure fluctuation suppression unit 15 prevents excessive bending changes of the multicore optical fiber 11 due to the expansion or contraction of the outer sheath 13 due to temperature changes, and suppresses pressure fluctuations transmitted from the outer sheath 13 to the multicore optical fiber 11. As a result, excessive bending changes of the optical fiber cable 10 due to temperature changes are suppressed, and the increase in the amount of change in intercore crosstalk is suppressed. 【0013】 (First Embodiment) Figure 1 is a cross-sectional view of an optical fiber cable 10 according to the first embodiment. As shown in Figure 1, the optical fiber cable 10 according to the first embodiment comprises at least one optical fiber bundle 12, an outer sheath 13 housing at least one optical fiber bundle 12, and a pressure fluctuation suppression part 15 provided on the outer sheath 13. 【0014】 The optical fiber bundle 12 includes a plurality of multicore optical fibers 11. The multicore optical fibers 11 may be uncoupled multicore optical fibers that do not assume mode coupling between cores, or coupled multicore optical fibers that allow mode coupling between cores. Furthermore, the core radius, core spacing, number of cores, and core arrangement of the multicore optical fibers 11 are arbitrary, as long as the desired mode coupling characteristics are obtained. For the sake of convenience in this explanation, the multicore optical fibers 11 will be referred to as MCF 11 below. 【0015】Multiple MCFs 11 in each optical fiber bundle 12 are wrapped and bundled with binding tape 17. As shown in Figure 1, if multiple optical fiber bundles 12 are housed in an outer sheath 13, some of the multiple optical fiber bundles 12 may be further wrapped and bundled with binding tape 17. 【0016】 The outer sheath 13 houses the optical fiber bundle 12 radially inward. The outer sheath 13 has at least an outer layer 14. The outer layer 14 is circumferentially extended and surrounds the entire outer circumference of the optical fiber bundle 12. The outer layer 14 is made of a durable and waterproof material. Such materials are, for example, synthetic resins such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PUR), or high-density polyethylene (HDPE). 【0017】 The pressure fluctuation suppression section 15 according to the first embodiment is provided radially inward of the outer layer 14 and is slidable relative to the outer layer 14. The pressure fluctuation suppression section 15 is formed in a layered manner that extends in the circumferential direction and has, for example, an annular cross-section surrounding the optical fiber bundle 12. The pressure fluctuation suppression section 15 is, for example, an elastomer (elastic body) and has a larger expansion ratio and a higher coefficient of thermal expansion than the outer layer 14. That is, the pressure fluctuation suppression section 15 is made of a material that is more flexible than the outer layer 14. Such materials are, for example, synthetic resins such as silicone rubber and polyurethane. The thickness of the pressure fluctuation suppression section 15 along the radial direction is such that it elastically receives the expanding or contracting outer layer 14 and the stress transmitted to the MCF 11 (optical fiber bundle 12) due to its radial displacement is less likely to be transmitted. 【0018】The outer sheath 13 may include an inner layer 16 provided radially inside the pressure fluctuation suppression section 15. That is, the outer sheath 13 may have a three-layer structure consisting of an outer layer 14, a pressure fluctuation suppression section 15 as an intermediate layer, and an inner layer 16. In this case, the inner layer 16 is provided so as to be slidable relative to the pressure fluctuation suppression section 15 and protects the MCF 11 (optical fiber bundle 12) from deformation of the outer sheath 13. The inner layer 16 is formed of a material having a low coefficient of friction and has a smaller expansion / contraction ratio than the outer layer 14 and the pressure fluctuation suppression section 15. Such a material is, for example, a fluororesin such as polytetrafluoroethylene (PTFE). This reduces friction between the inner layer 16 and the MCF 11. The inner layer 16 may also be provided in the optical fiber cable 10 according to the second to sixth embodiments described later. In any case, the inner layer 16 is provided on the radially innermost part of the outer sheath 13. 【0019】 As described above, the pressure fluctuation suppression section 15 is located radially inward from the outer layer 14 and has a greater expansion / contraction ratio than the outer layer 14. Therefore, when the outer sheath 13 expands or contracts due to temperature changes in the optical fiber cable 10, the pressure fluctuation suppression section 15 elastically receives the radially expanding and contracting outer layer 14. The expansion or contraction of the pressure fluctuation suppression section 15 is relatively small compared to that of the outer layer 14. Therefore, the deformation of the pressure fluctuation suppression section 15 remains small compared to the deformation of the outer layer 14, mitigating the change in shape of the space radially inward of the pressure fluctuation suppression section 15. In other words, pressure fluctuations to the MCF 11 due to the displacement of the outer layer 14 are suppressed. Thus, it is possible to prevent excessive bending changes of the multicore optical fiber 11 due to the expansion or contraction of the outer sheath 13 accompanying temperature changes, and to suppress pressure fluctuations transmitted from the outer sheath 13 to the multicore optical fiber 11. 【0020】 The pressure fluctuation suppression section 15 is slidably mounted relative to the outer layer 14. Therefore, the effect of deformation of the pressure fluctuation suppression section 15 due to displacement (movement) of the outer layer 14 along the circumferential direction can be mitigated. Furthermore, the elasticity of the pressure fluctuation suppression section 15 can contribute to the absorption (mitigation) of pressure fluctuations caused by displacement (movement) of the outer layer 14 along the radial direction. 【0021】(Second Embodiment) A second embodiment will now be described. Figure 2 is a cross-sectional view of the optical fiber cable 10 according to the second embodiment. Components similar to those in the first embodiment are denoted by the same reference numerals, and their descriptions are omitted. 【0022】 As shown in Figure 2, the pressure fluctuation suppression unit 15 according to the second embodiment is composed of a double layer 18. The double layer 18 is provided radially inward of the outer layer 14 and extends circumferentially. The double layer 18 is located, for example, in the radially innermost part of the outer sheath 13. The optical fiber cable 10 according to the second embodiment may also have a plurality of tensile strength lines 21 provided inside the outer layer 14. The tensile strength lines 21 are located symmetrically with respect to the center of the optical fiber cable 10. The other configurations are the same as those of the first embodiment. 【0023】 The double layer 18 has a lower coefficient of thermal expansion than the outer layer 14 and includes a first layer 19 and a second layer 20 provided radially inward of the first layer 19. Both the first layer 19 and the second layer 20 are thermally shrinkable, but are made of different materials. Therefore, the expansion and contraction properties (e.g., shrinkage temperatures) of the first layer 19 and the second layer 20 may be different. The materials of the first layer 19 and the second layer 20 are, for example, elastomers such as silicone rubber, fluoropolymers, or plastics (FRP) containing reinforcing fibers such as aramid fibers or glass fibers. The material of each layer is selected considering both rapid response to temperature changes and long-term stability of properties against temperature changes. 【0024】 As described above, the double layer 18 has a smaller coefficient of thermal expansion than the outer layer 14. Therefore, when the temperature of the optical fiber cable 10 changes, the expansion and contraction of the double layer 18 are smaller than those of the outer layer 14. Thus, similar to the first embodiment, the effects of expansion and contraction by the outer layer 14 are mitigated, and the change in shape of the space formed radially inside the double layer 18 is mitigated. As a result, the same effects as in the first embodiment are obtained. 【0025】Furthermore, when the temperature of the optical fiber cable 10 reaches the contraction temperature of either the first layer 19 or the second layer 20, the outer layer 14 expands, while the first layer 19 or the second layer 20 that has reached the contraction temperature contracts. This contraction also helps to prevent excessive bending changes in the multicore optical fiber 11 and suppress fluctuations in pressure transmitted from the outer sheath 13 to the multicore optical fiber 11. 【0026】 The pressure fluctuation suppression section 15 according to the second embodiment may be composed of a single layer, or of three or more layers formed of different materials. That is, the pressure fluctuation suppression section 15 according to the second embodiment may be composed of at least one layer that is thermally shrinkable and stretches in the circumferential direction. In either case, when the pressure fluctuation suppression section 15 rises to its shrinkage temperature, it suppresses further expansion of the outer layer 14 and achieves the above-described effect. 【0027】 (Third Embodiment) A third embodiment will now be described. Figure 3 is a cross-sectional view of the optical fiber cable 10 according to the third embodiment. Components similar to those in the first embodiment are denoted by the same reference numerals, and their descriptions are omitted. 【0028】 The outer sheath 13 according to the third embodiment is formed of the same material as the outer layer 14. The pressure fluctuation suppression section 15 according to the third embodiment is made of the same material as the double layer 18 according to the second embodiment and has thermal shrinkability. However, the pressure fluctuation suppression section 15 according to this embodiment is located at circumferential intervals inside the outer sheath 13. The pressure fluctuation suppression section 15 is located symmetrically with respect to the center of the optical fiber cable 10. The pressure fluctuation suppression section 15 may also serve as the tensile strength line of the optical fiber cable 10. 【0029】 In the third embodiment, the same effect as in the second embodiment can be obtained. That is, when the temperature of the optical fiber cable 10 changes, the expansion and contraction of the pressure fluctuation suppression section 15 according to the third embodiment are smaller than those of the outer sheath 13. Therefore, as in the first embodiment, the effect of expansion and contraction by the outer sheath 13 is mitigated, and the change in shape of the space formed radially inside the outer sheath 13 is mitigated. Therefore, the same effect as in the first embodiment can be obtained. 【0030】Furthermore, when the temperature of the optical fiber cable 10 reaches the contraction temperature of the pressure fluctuation suppression section 15, the outer sheath 13 expands, while the pressure fluctuation suppression section 15, having reached its contraction temperature, contracts. This contraction also helps prevent excessive bending changes in the multicore optical fiber 11 and contributes to suppressing pressure fluctuations transmitted from the outer sheath 13 to the multicore optical fiber 11. 【0031】 (Fourth Embodiment) A fourth embodiment will now be described. Figure 4 is a cross-sectional view of the optical fiber cable 10 according to the fourth embodiment. Components similar to those in the first embodiment are denoted by the same reference numerals, and their descriptions are omitted. 【0032】 As shown in Figure 4, the pressure fluctuation suppression section 15 according to the fourth embodiment is embedded in the outer sheath 13. The pressure fluctuation suppression section 15 is located, for example, in the innermost radial part of the outer sheath 13. The pressure fluctuation suppression section 15 extends in the circumferential direction. For example, the pressure fluctuation suppression section 15 has an annular cross-section surrounding the optical fiber bundle 12. 【0033】 The pressure fluctuation suppression section 15 according to the fourth embodiment is made of a shape memory alloy such as nickel-titanium alloy (Nitinol). The shape memory alloy is formed into a shape such that the pressure on the MCF 11 falls within a desired range of core crosstalk fluctuations by undergoing plastic deformation at a temperature lower than the shape recovery temperature. 【0034】 In the fourth embodiment, the same effect as in the first embodiment can be obtained. That is, when the temperature of the optical fiber cable 10 changes, the expansion and contraction of the pressure fluctuation suppression section 15 according to the fourth embodiment are smaller than those of the outer sheath 13. Therefore, as in the first embodiment, the effect of expansion and contraction by the outer sheath 13 is mitigated, and the change in shape of the space formed radially inside the outer sheath 13 is mitigated. Therefore, the same effect as in the first embodiment can be obtained. 【0035】 Furthermore, when the temperature of the optical fiber cable 10 rises to a temperature above the shape recovery temperature of the shape memory alloy, the shape of the pressure fluctuation suppression section 15 returns to its original (i.e., pre-deformation) shape. Therefore, further expansion of the outer sheath 13 is suppressed, and the shape change of the space formed radially inward of the outer sheath 13 is mitigated. Thus, the same effects as in the first embodiment can be obtained. 【0036】 (Fifth Embodiment) A fifth embodiment will now be described. Figure 5 is a cross-sectional view of the optical fiber cable 10 according to the fifth embodiment. Components similar to those in the first embodiment are denoted by the same reference numerals, and their descriptions are omitted. 【0037】 In the fifth embodiment, the pressure fluctuation suppression section 15 is formed as a hollow layer (cavity) inside the outer sheath 13. The hollow layer extends in both the circumferential and longitudinal directions. Spacers (not shown) are provided in the hollow layer at intervals in the circumferential direction, and the spacers connect the radially outer portion of the outer sheath 13 to the radially inner portion. The hollow layer may be treated with a moisture-proof and waterproof coating. In this case, the durability of the optical fiber cable 10 against moisture can be improved. 【0038】 The function of the pressure fluctuation suppression section 15, which is formed as a hollow layer, is similar to the function of the pressure fluctuation suppression section 15, which is formed as an elastic body according to the first embodiment. That is, when the outer sheath 13 expands or contracts due to temperature changes of the optical fiber cable 10, a part of the expanded or contracted outer sheath 13 is retained in the hollow layer. This mitigates the change in shape of the space radially inside the hollow layer, and suppresses pressure fluctuations to the MCF 11 due to the displacement of the outer sheath 13. 【0039】 (Sixth Embodiment) The sixth embodiment will now be described. Figure 6 is a cross-sectional view of the optical fiber cable 10 according to the sixth embodiment. Components similar to those in the first embodiment are denoted by the same reference numerals and their descriptions are omitted. 【0040】 As shown in Figure 6, the pressure fluctuation suppression section 15 according to the sixth embodiment is configured as the inner circumferential surface 13a of the outer sheath 13, which includes a plurality of peaks 23. The plurality of peaks 23 are arranged in the circumferential direction, and each extends in the longitudinal direction of the optical fiber cable 10. The peaks 23 have a predetermined width in the circumferential direction and protrude radially inward. The predetermined width is, for example, 1 to 3 mm, and it is desirable that it is about 1 / 10 or less of the length (circumferential length) of the inner circumferential surface 13a of the outer sheath 13. Note that the cross-sectional shape of the peaks 23 perpendicular to the longitudinal direction is not limited to the triangle shown in Figure 6. That is, the cross-sectional shape of the peaks 23 may be a semicircle, a rectangle, or the like. 【0041】 Figure 6 shows the state before the outer cover 13 contracts. At this time, the apexes of the respective peak portions 23 are substantially located on the circle A indicated by the dotted line. On the other hand, when the outer cover 13 contracts, the apexes of the respective peak portions 23 move radially inward beyond the circle A. On the other hand, the positions of the connection points between the peak portions 23, which are located radially outward with respect to the circle A, hardly change before and after contraction. 【0042】 Compared with the case where the inner peripheral surface of the outer cover 13 is flat, the apexes of the peak portions 23 are more likely to remain within the circle A even after contraction. Therefore, the pressure fluctuation before and after contraction from the outer cover 13 to the MCF11 becomes smaller. In other words, even if the shape of the wavy structure formed by the peak portions 23 changes due to a temperature change, the pressure can be evenly distributed to each MCF11. As a result, the change in the shape of the space radially inside the inner peripheral surface 13a is alleviated, and the pressure fluctuation to the MCF11 due to the displacement of the outer cover 13 is suppressed. 【0043】 (Seventh Embodiment) The seventh embodiment will be described. FIG. 7 is a diagram for explaining the effective bending radius R of the MCF11. The helix of the MCF11 that occurs after cable laying can be explained by a simple model shown in FIG. 7. That is, as shown in FIG. 7, the radius r of the helix is the radius of the circle formed by the helix when viewed from the extending direction of the helix. On the other hand, the pitch P of the helix is the longitudinal distance advanced when the helix makes one rotation when viewed from the extending direction of the helix. Hereinafter, for convenience of explanation, the radius r of the helix is referred to as the "twist radius r", and the pitch P of the helix is referred to as the "twist pitch P". 【0044】 When the MCF11 is deformed from a linear shape to a helical shape, the MCF11 is bent with a radius of curvature corresponding to the deformation. This radius of curvature is referred to as the effective bending radius R. The effective bending radius R of the MCF11 is expressed by the following formula (1). Here, r and P are the twist radius and the twist pitch of the MCF11, respectively. When a plurality of MCF11s are assumed, the effective bending radius R may be the average value of the effective bending radii R of the plurality of MCF11s. This average value is substantially equal to the effective bending radius of the optical fiber bundle 12. 【0045】When MCF11 transforms from a linear shape to a helical shape, MCF11 is twisted about its central axis. The twist rate γ is a ratio indicating how much it is twisted per unit length and is represented by the following formula (2). For example, when the twist rate γ is 2π rad / m, it means that MCF11 (optical fiber bundle 12) is twisted one turn per meter. Also, when the twist rate γ is 20π rad / m, it means that MCF11 (optical fiber bundle 12) is twisted ten turns per meter. 【0046】 Figure 8 is a graph showing the relationship between the twist interval P and the twist radius r of MCF11 (optical fiber bundle 12) for each of several effective bending radii R. The lower horizontal axis of the graph is the helix interval P, and the vertical axis is the helix radius r. Also, the twist rate γ is shown on the upper horizontal axis. In a general optical fiber cable, the twist interval P, the twist radius r, and the effective bending radius R of the optical fiber often take values within the region B indicated by the dotted line. For example, in the case of an onshore optical fiber cable, the effective bending radius R is usually about 1000 mm. In the case of an undersea optical fiber cable, since the linearity of the fiber is enhanced, the effective bending radius R is about 10000 mm. 【0047】 Figure 9 is a graph of the numerical analysis results showing the amount of core-to-core crosstalk with respect to the effective bending radius R of MCF11 for each of several core deviations Δa. In this analysis, it is assumed that the core radius a is 4.1 μm, the relative refractive index difference Δ is 0.39%, the core interval Λ is 40 μm, and the correlation length lc is 0.02 m. 【0048】 Even for MCFs manufactured using the same core base material, deviations in the core radius and the relative refractive index difference occur between the cores during manufacturing, resulting in an effective refractive index difference. For example, in a two-core MCF having core 1 and core 2, consider the case where, in a state where the MCF extends in a straight line, the effective refractive index of core 2 is greater than the effective refractive index of core 1 due to the core deviation. 【0049】When bending is applied to this MCF, the effective refractive index difference decreases due to the change in refractive index, and at a certain curvature, the effective refractive indexes of core 1 and core 2 become equal. At this time, the amount of inter-core crosstalk reaches a maximum value. When bending is further applied to the fiber, the effective refractive index difference is generated and increases again, and the amount of inter-core crosstalk decreases. Thus, the amount of inter-core crosstalk in MCF 11 changes according to the effective bending radius R, and further changes according to the core deviation Δa. Each graph in Figure 9 shows the maximum value that reflects the above phenomenon. 【0050】 Once the permissible range for changes in the amount of inter-core crosstalk is determined, the maximum permissible twist radius r can be calculated from the twist spacing P and the effective bending radius R. The maximum value of the twist radius r corresponds to the outer diameter of the entire bundle of one or more optical fibers 12, or the inner diameter of the sheath 13. That is, the range of the inner diameter of the sheath 13 that is permissible within the permissible range for changes in the amount of inter-core crosstalk can be calculated from the twist spacing P and the effective bending radius R. 【0051】 For example, consider the case where the core deviation Δa of the MCF11 of a land-based optical fiber cable is 0.05 μm. As mentioned above, the effective bending radius R of a land-based optical fiber cable is usually around 1000 mm, and generally has a value in the range of approximately 700 to 2000 mm (see Figure 9). 【0052】 Here, assuming an acceptable range of change in the amount of inter-core crosstalk of about 2 to 3 dB, for example, the maximum value R of the amount of inter-core crosstalk... ｐｋ A region C smaller than and a maximum value R ｐｋ We can focus on region D, which is larger than region C. The effective bending radius R range in region C is approximately 700 to 900 mm. On the other hand, the effective bending radius R range in region D is approximately 1500 to 2000 mm. Within these regions, the upper limit of the effective bending radius R changes to a value of approximately 1.3 times the lower limit (i.e., about +30%), and the amount of intercore crosstalk changes by 2 to 3 dB. 【0053】Figures 10A to 10C are graphs showing the change in the effective bending radius R with respect to the inner diameter (maximum value of the twist radius r) of the outer sheath 13 for several twist intervals P. The assumed twist intervals P in Figures 10A, 10B, and 10C are 500 mm, 750 mm, and 1000 mm, respectively. Figures 10A and 10B show the change in the effective bending radius R in region C, and Figure 10C shows the change in the effective bending radius R in region D. These graphs can be derived based on the analysis results shown in Figure 8. 【0054】 From the graph examples in Figures 10A to 10C, the allowable range (allowable change rate) of the inner diameter (maximum value of the twist radius r) of the outer sheath 13 can be calculated when the allowable range of change in the amount of intercore crosstalk is kept to about 3 dB. For example, Figure 10A shows the change in the effective bending radius R in region C when the twist spacing P in region C is 500 mm. According to this graph, it can be seen that by keeping the change in the inner diameter of the outer sheath 13 within a range of about 7 to 9 mm, the allowable range of change in the amount of intercore crosstalk can be kept to about 3 dB. In other words, when the twist spacing P in region C is 500 mm, it can be seen that in order to keep the allowable range of change in the amount of intercore crosstalk to about 3 dB, the upper limit of the allowable change rate should be set to 30% for an inner diameter of 7 mm in the outer sheath 13. 【0055】 Similarly, the graph in Figure 10B shows that in region C, when the twist spacing P is 750 mm, in order to keep the allowable range of change in the amount of core crosstalk to about 3 dB, the upper limit of the allowable change rate should be set to 30% of the inner diameter of the outer sheath 13 (the inner diameter at this time is approximately 21 mm) relative to the inner diameter of 16 mm. Furthermore, the graph in Figure 10C shows that in region D, when the twist spacing P is 1000 mm, in order to keep the allowable range of change in the amount of core crosstalk to about 3 dB, the upper limit of the allowable change rate should be set to 30% of the inner diameter of the outer sheath 13 (the inner diameter at this time is approximately 17 mm) relative to the inner diameter of 13 mm. 【0056】 This disclosure naturally includes various embodiments and other features not described herein. Therefore, the technical scope of this disclosure is determined solely by the inventive features relating to the claims that are reasonable given the above description. 【0057】 10 Optical fiber cable 11 Multicore optical fiber 12 Optical fiber bundle 13 Outer sheath 13a Inner surface 14 Outer layer 15 Pressure fluctuation suppression section 16 Inner layer 17 Binding tape 18 Double layer 19 First layer 20 Second layer 21 Tensile strength line 23 Peak section

Claims

1. An optical fiber cable comprising: at least one optical fiber bundle containing multiple multicore optical fibers; an outer sheath housing at least one of the optical fiber bundles; and a pressure fluctuation suppression unit provided on the outer sheath to suppress pressure fluctuations on the multicore optical fibers due to expansion and contraction of the outer sheath caused by temperature changes.

2. The optical fiber cable according to claim 1, wherein the pressure fluctuation suppression portion is formed in a layered manner extending in the circumferential direction, is provided slidably with respect to the outer layer on the radially inward side of the outer layer of the outer sheath, and has a greater expansion and contraction ratio than the outer layer.

3. The optical fiber cable according to claim 1, wherein the pressure fluctuation suppression section is provided radially inward of the outer layer of the outer sheath and is composed of at least one layer that is thermally shrinkable and stretches in the circumferential direction.

4. The optical fiber cable according to claim 1, wherein the pressure fluctuation suppression portion is thermally shrinkable and is provided at circumferential intervals inside the outer sheath.

5. The optical fiber cable according to claim 1, wherein the pressure fluctuation suppression section is made of a shape memory alloy that is embedded in the outer sheath and extends in the circumferential direction.

6. The optical fiber cable according to claim 1, wherein the pressure fluctuation suppression portion is formed as a hollow layer extending circumferentially inside the outer sheath.

7. The optical fiber cable according to claim 1, wherein the pressure fluctuation suppression portion is configured as the inner circumferential surface of the outer sheath including a plurality of peaks, the plurality of peaks are arranged in the circumferential direction and each extends in the longitudinal direction of the optical fiber cable.

8. The optical fiber cable according to any one of claims 1 to 7, wherein the outer sheath includes an inner layer provided radially inward of the pressure fluctuation suppression portion, the inner layer is slidably provided with respect to the pressure fluctuation suppression portion and is formed of a material having a low coefficient of friction.

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