Optical fiber cable and manufacturing method therefor

Abstract

An optical fiber cable (10) is provided with one or a plurality of optical fiber units (11). The optical fiber unit (11) includes one or a plurality of optical fibers (12), which are coupled multi-core optical fibers, and a linear member (13). The optical fiber (12) and the linear member (13) are spirally wound around each other. A winding state of the linear member (13) changes along a longitudinal direction of the optical fiber cable (10).

Description

Optical fiber cable and its manufacturing method

[0001] The present disclosure relates to optical fiber cables and methods of manufacturing the same.

[0002] As one of the transmission technologies for next-generation optical transmission systems that will support large-capacity communications, space division multiplexing (SDM) technology using coupled multi-core fiber (C-MCF) is being investigated. C-MCF is one of various types of multi-core optical fibers and is an optical fiber that tolerates crosstalk (mode coupling) between cores. C-MCF allows for a shorter spacing between adjacent cores, allowing for high-density packaging of multiple cores without increasing the outer diameter of the cladding.

[0003] In optical transmission systems using C-MCF, it is essential to introduce MIMO DSP (Multiple-Input Multiple-Output Digital Signal Processing) technology to compensate for crosstalk at the receiving side. The circuit scale of the MIMO DSP depends on the degree of spatial mode dispersion (SMD) between cores.

[0004] Non-Patent Document 1 shows that the SMD of C-MCF can be reduced by bending and twisting the optical fiber. Non-Patent Document 2 shows that the SMD can be reduced by intentionally bending and twisting the optical fiber using the tension of the bundle tape wound around a two-core C-MCF.

[0005] T. Sakamoto, T. Mori, M. Wada, T. Yamamoto, F. Yamamoto and K. Nakajima, "Fiber Twisting- and Bending-Induced Adiabatic / Nonadiabatic Super-Mode Transition in Coupled Multicore Fiber," in Journal of Lightwave Technology, vol. 34, no. 4, pp. 1228-1237, 15 Feb.15, 2016, doi: 10.1109 / JLT.2015.2502260.Y. Yamada et al., "Design of High-Density Cable Parameters for Controlling Spatial-Mode Dispersion of Randomly Coupled Multi-Core Fibers," in Journal of Lightwave Technology, vol. 39, no. 4, pp. 1179-1185, 15 Feb.15, 2021, doi: 10.1109 / JLT.2020.3045761.

[0006] To realize a larger capacity optical transmission system using C-MCF, it is necessary to expand the operating wavelength range of C-MCF, while suppressing the increase in SMD, which leads to an increase in the circuit scale of MIMO DSP.

[0007] An object of the present disclosure is to provide an optical fiber cable including a coupled multi-core optical fiber that can expand the operating wavelength range, and a method for manufacturing the same.

[0008] An optical fiber cable according to one aspect of the present disclosure comprises one or more optical fiber units, each of which includes one or more optical fibers that are coupled multi-core optical fibers and a linear member, the optical fibers and the linear member being spirally wound around each other, and the winding state of the linear member varying along the longitudinal direction of the optical fiber cable.

[0009] A method for manufacturing an optical fiber cable according to one aspect of the present disclosure includes winding a linear member around one or more optical fibers that are coupled multi-core optical fibers, applying tension to the linear member so that the optical fibers and the linear member are spirally wound around each other, and adjusting the tension so that the winding state of the linear member changes along the longitudinal direction of the optical fiber cable.

[0010] According to the present disclosure, it is possible to provide an optical fiber cable including a coupled multi-core optical fiber capable of expanding the operating wavelength range, and a method for manufacturing the same.

[0011] FIG. 1 is a side view of an optical fiber cable according to an embodiment of the present disclosure. FIG. 2A is a cross-sectional view of an example optical fiber. FIG. 2B is a cross-sectional view of an example optical fiber. FIG. 2C is a cross-sectional view of an example optical fiber. FIG. 2D is a cross-sectional view of an example optical fiber. FIG. 3 is a cross-sectional view of an example optical fiber cable. FIG. 4A is a diagram illustrating a manufacturing process of an optical fiber unit. FIG. 4B is a diagram illustrating a manufacturing process of an optical fiber unit. FIG. 5A is an enlarged view of the optical fiber cable shown in FIG. 1. FIG. 5B is a diagram illustrating the effective bending radius R of an optical fiber. FIG. 6 is a graph showing the relationship between the twist spacing and twist radius of an optical fiber (fiber bundle) for several effective bending radii R. FIG. 7 is a graph showing an analysis result of power coupling efficiency versus effective bending radius R. FIG. 8 is a graph showing the change in effective bending radius R versus twist rate γ. FIG. 9 is a cross-sectional view of an example optical fiber cable along the longitudinal direction.

[0012] Hereinafter, an optical fiber cable 10 according to an embodiment of the present disclosure will be described. Note that common parts in the various figures are given the same reference numerals, and duplicated explanations will be omitted.

[0013] Fig. 1 is a side view of an optical fiber cable 10. Figs. 2A to 2D are cross-sectional views of several examples of optical fibers 12. Fig. 3 is a cross-sectional view of one example of an optical fiber cable 10. As shown in Fig. 1, the optical fiber cable 10 includes one or more optical fiber units 11. The optical fiber unit 11 includes one or more optical fibers 12 and a linear member 13. Furthermore, the one or more optical fibers 12 and the linear member 13 in each optical fiber unit 11 are spirally wound around each other.

[0014] As shown in Fig. 2A, the optical fiber 12 is a multi-core optical fiber having a cladding 14 and a plurality of cores 15 provided in the cladding 14. The optical fiber 12 illustrated in Fig. 2A has two cores 15, 15. The two cores 15, 15 are separated by a core spacing Λ, which is the spacing between their centers. The outer peripheral surface 14a of the cladding 14 may be covered with at least one layer of coating (not shown).

[0015] The optical fiber 12 has, for example, a step-index type refractive index profile. In this case, each core 15 has a refractive index n １ and the cladding 14 has a refractive index n ２ The refractive index n １ is the refractive index n ２ The refractive index profile may be a graded index profile, a trench-assisted refractive index profile, or any other known refractive index profile.

[0016] The optical fiber 12 according to this embodiment is a coupled multi-core optical fiber. Therefore, the core spacing Λ is set to a value that allows mode coupling. For example, the core spacing Λ is set to a value in the range of 10 μm to 30 μm, which allows random mode coupling to be obtained. In this case, the accumulation of SMD (spatial mode dispersion) can be reduced to the ½ power of the fiber length, and an excessive increase in the size of the optical circuit that executes MIMO DSP can be suppressed.

[0017] The number and positions of the cores 15 are not limited to those shown in Fig. 2A. For example, the number of cores may be 4, 8, or 12, as shown in Figs. 2B to 2D. As shown in Figs. 2A to 2D, the multiple cores 15 are arranged at positions with a periodicity such as a line, a ring, a square lattice, or a triangular lattice, depending on the number of cores n. The multiple cores 15 may also be positioned so as to have a predetermined rotational symmetry around the central axis of the optical fiber 12. Note that when the number of cores 15 exceeds two, the distance between the two most adjacent cores 15 among the multiple cores 15 is the core distance Λ.

[0018] The linear member 13 is a flexible member that extends in a strip or thread shape. For example, the linear member 13 is formed of a synthetic resin film or fiber, and has a thickness or diameter similar to that of the optical fiber 12. An example of a linear member 13 that extends in a strip shape is a so-called bundle tape. The linear member 13 wraps around the optical fiber 12, holds the optical fiber 12, and deforms the optical fiber 12 into a spiral shape. When there are multiple optical fibers 12, the optical fibers 12 are bundled by the linear member 13 into a fiber bundle 16.

[0019] The linear member 13 has a winding radius r ｔ or winding interval P ｔ That is, the width of the linear member 13 may vary along the longitudinal direction of the linear member 13. As will be described later, the linear member 13 may have a winding interval (pitch) P ｔ The winding interval P ｔ The winding interval P can be set by changing the width of the linear member 13 in accordance with the ｔ This can widen the range of winding, or make it easier to obtain a desired winding state.

[0020] The number of optical fiber units 11 is arbitrary. For example, as shown in Fig. 3, an optical fiber cable 10 includes ten optical fiber units 11 and an outer jacket 17 that houses and protects the optical fiber units 11. Each optical fiber unit 11 is disposed in a space 18 within the outer jacket 17, with one or more optical fibers 12 and linear members 13 spirally wound around each other.

[0021] The optical fiber unit 11 according to this embodiment can be manufactured, for example, through the following steps. Figures 4A and 4B are diagrams for explaining the manufacturing steps of the optical fiber unit 11. Hereinafter, the extension direction of the optical fiber unit 11 is defined as the Z direction, and the radial direction is defined based on the central axis 19 of the optical fiber unit 11.

[0022] For convenience of explanation, the following description will be given taking as an example a case where the optical fiber unit 11 has a plurality of optical fibers 12. Therefore, the plurality of optical fibers 12 are bundled by a linear member 13 to form a fiber bundle 16.

[0023] First, as shown in Fig. 4A, a fiber bundle 16 is prepared. A predetermined tension is applied to the fiber bundle 16 along the Z direction, and the fiber bundle 16 is maintained in a state of being linearly stretched in the Z direction. However, this tension is set to a value that allows the fiber bundle 16 to be deformed into a spiral shape by the pressure of the linear member 13. Therefore, the tension applied to the fiber bundle 16 is sufficiently smaller than the tension applied to the linear member 13. Next, the fiber bundle 16 is wound spirally around the linearly stretched linear member 13.

[0024] Next, as shown in FIG. 4B , tension is applied to the linear member 13 so that the fiber bundle 16 and the linear member 13 are wound around each other in a spiral shape. For example, two locations of the linear member 13 spaced apart in the Z direction are grasped, and the linear member 13 is pulled with a predetermined tension in opposite directions along the Z direction. The linear member 13 extends along the Z direction while widening the spacing of its own spiral. At this time, the linear member 13 shifts radially inward (i.e., so that the radius of the spiral decreases). Meanwhile, the portion of the fiber bundle 16 in contact with the linear member 13 is subjected to pressure associated with this shift and shifts radially outward. As a result, the fiber bundle 16 is deformed into a spiral shape with a twist spacing equal to the winding spacing of the linear member 13. The fiber bundle 16 and the linear member 13 then spirally wind around each other to form the optical fiber unit 11.

[0025] The tension applied to the linear member 13 is adjusted so that the winding state of the linear member 13 changes along the longitudinal direction of the optical fiber cable 10. The winding state may be, for example, a change in the winding radius r ｔ and winding interval Pｔ (See FIG. 5A ). ｔ and the winding radius r ｔ can be controlled by the tension applied to the linear member 13. Similarly, the twist interval P ｆ and twist radius r ｆ (see FIG. 5A) can also be controlled by the tension applied to the linear member 13. ｔ and the twist interval P of the fiber bundle 16 ｆ On the other hand, the twist radius r of the fiber bundle 16 is ｆ varies depending on the tension applied to the linear member 13 and the bending rigidity of the fiber bundle 16. For example, when the tension of the linear member 13 is increased, the radius of the spiral of the fiber bundle 16 becomes smaller. On the other hand, when the interval between the spirals of the linear member 13 (i.e., the winding interval P ｔ ) and the spacing of the helices of the fiber bundle 16 (i.e., the twist spacing P ｆ ) are equal to each other regardless of the tension of the linear member 13.

[0026] The radius and spacing of each spiral of the linear member 13 and the fiber bundle 16 are obtained in the section between the two gripped locations. Then, two other locations of the linear member 13 other than the section where the spirals were formed are gripped again, and the linear member 13 is pulled in the Z direction with a different predetermined tension. This forms a linear member 13 and a fiber bundle 16 having a different radius or spacing, or both, from the spirals already formed. The manufactured optical fiber unit 11 is inserted into an outer jacket 17 (see FIG. 3), thereby forming an optical fiber cable.

[0027] Next, a description will be given of the effective bending radius R of the optical fiber 12. Fig. 5A is an enlarged view of the optical fiber cable 10 shown in Fig. 1. Fig. 5B is a diagram for explaining the effective bending radius R of the optical fiber 12.

[0028] 5A, the linear member 13 and the optical fiber 12 wound around each other both have a spiral shape. For convenience of explanation, the radius and spacing of the spiral of the linear member 13 will be referred to as the winding radius r ｔ and winding interval P ｔ Similarly, the radius and spacing of the helix of the optical fiber 12 are defined as the twist radius rｆ and twist interval P ｆ It is defined as:

[0029] The spirals of the optical fiber 12 and the linear member 13 can be explained by the simple model shown in Fig. 5B. That is, as shown in Fig. 5B, the radius r of a spiral is the radius of a circle formed by the spiral when viewed from the extending direction of the spiral. On the other hand, the spacing P of the spiral is the distance traveled in the Z direction when the spiral makes one rotation when viewed from the extending direction of the spiral.

[0030] When the optical fiber 12 is deformed from a straight shape to a spiral shape, the optical fiber 12 is bent with a radius of curvature corresponding to the deformation. This radius of curvature is called the effective bending radius and is denoted by R. The effective bending radius R of the optical fiber 12 is expressed by the following equation (1): where r and P are the radius of the helix of the optical fiber 12 (i.e., the twist radius r ｆ ) and spacing (twist spacing). When a plurality of optical fibers 12 are provided in the optical fiber unit 11, the effective bending radius R may be the average value of the effective bending radii R of the plurality of optical fibers 12. This average value is substantially equal to the effective bending radius of the fiber bundle 16.

[0031] When the optical fiber 12 is deformed from a straight shape to a spiral shape, the optical fiber 12 is twisted around its central axis. The twist ratio γ is a ratio indicating how much the optical fiber 12 is twisted per unit length, and is expressed by the following equation (2): For example, if the twist rate γ is 2π rad / m, this means that the optical fiber 12 (fiber bundle 16) is twisted once per meter, and if the twist rate γ is 20π rad / m, this means that the optical fiber 12 (fiber bundle 16) is twisted 10 times per meter.

[0032] When the core diameter and core spacing Λ are constant, the power coupling efficiency between cores with respect to wavelength varies depending on the optical characteristics of the optical fiber 12, such as the refractive index profile and SMD. These optical characteristics vary depending on the effective bending radius R of the optical fiber 12. Furthermore, the effective bending radius R varies depending on the twisting radius r of the optical fiber 12. ｆ , twist interval P ｆand its torsion.

[0033] The twist radius r of the optical fiber 12 is ｆ and twist interval P ｆ varies depending on the winding state of the linear member 13 that determines the helical shape of the optical fiber 12. In this embodiment, this winding state varies along the longitudinal direction of the optical fiber cable 10. In other words, by changing the winding state for each section, the optical characteristics of the optical fiber 12 in each section are changed, and thereby the wavelength range (in other words, the center wavelength) at which a power coupling efficiency equal to or greater than a desired value is obtained is changed for each section. Note that the winding state along the longitudinal direction may change continuously or in steps.

[0034] As described above, the winding state of the linear member 13 is, for example, the winding radius r ｔ and winding interval P ｔ For example, as shown in FIG. 1, the optical fiber cable 10 has sections S1 and S2 with different winding states. In this case, for example, the winding radius r ｔ and winding interval P ｔ In the example shown in FIG. 1, at least one of the winding intervals P ｔ is set to a value longer in the section S1 than in the section S2. ｔ and winding interval P ｔ can be determined, the twist radius r ｆ and twist interval P ｆ can be determined, and the effective bending radius R of the optical fiber 12 can be calculated. ｔ and / or winding interval P ｔ may vary continuously or stepwise, and may be constant or vary along the length. In either case, various values ​​of the effective bending radius R (varying within a certain range) are obtained.

[0035] FIG. 6 shows the twist interval P ｆ and twist radius r ｆ1 is a graph showing the relationship between the twist interval P and the effective bending radius R. ｆ , the vertical axis is the twist radius r ｆ The twist ratio γ is also plotted on the upper horizontal axis. By selecting a combination of the twist interval P and twist radius r based on the graph in Figure 6, the effective bending radius R of the optical fiber 12 (fiber bundle 16) can be determined. Specific examples of this will be described later.

[0036] Figure 7 is a graph showing the analysis results of the power coupling efficiency versus the effective bending radius R. The horizontal axis of the graph represents the effective bending radius R, and the vertical axis represents the power coupling efficiency between cores. This analysis assumes a two-core optical fiber. The power coupling efficiency was calculated for three wavelengths (1530 nm, 1565 nm, and 1625 nm) at two different twist rates γ (2π rad / m and 20π rad / m). Specifically, the C-band, which is the communication wavelength band from 1530 nm to 1565 nm, and the L-band, which is the communication wavelength band from 1565 nm to 1625 nm, are assumed. The C-band is the conventional communication wavelength band. The L-band is assumed as an additional communication wavelength band, anticipating a shortage of communication capacity and an increase in transmission capacity.

[0037] According to the analysis results in Figure 7, the effective bending radius R at which the power coupling efficiency per location is -10 dB is as follows: When the twist rate γ is 20π rad / m (twist spacing P = 100 mm), the effective bending radius R in the C-band wavelength band is in the range of 1500 to 3000 mm. The effective bending radius R in the C-band to L-band wavelength bands is in the range of 900 to 3000 mm. On the other hand, when the twist rate γ is 2π rad / m (twist spacing P = 1000 mm), the effective bending radius R in the C-band wavelength band is in the range of 100 to 200 mm. The effective bending radius R in the C-band to L-band wavelength bands is in the range of 70 to 200 mm.

[0038] In this way, when the twist rate γ of the fiber changes, the distribution of the effective bending radius R required to achieve that twist rate γ also changes. By providing a range for the effective bending radius R of the optical fiber 12 as described above, it is possible to obtain a desired power coupling efficiency over a wide wavelength range, and as a result, a low SMD can be obtained over a wide wavelength range.

[0039] If the SMD has wavelength dependency, processing such as MIMO DSP on the receiving side requires a circuit scale that matches the largest SMD in the operating wavelength range. In other words, as SDM increases, the circuit scale increases, leading to increased manufacturing and operating costs. On the other hand, according to this embodiment, by varying the winding state along the longitudinal direction of the optical fiber cable 10, i.e., by setting multiple sections with different effective bending radii R, the wavelength band in which the desired power coupling efficiency can be obtained can be expanded, thereby reducing the wavelength dependency of the SMD in that wavelength band. Therefore, even if the operating wavelength range is expanded, an excessive increase in the circuit scale required for processing such as MIMO DSP can be suppressed. This also prevents excessive increases in manufacturing and operating costs.

[0040] In addition, when the optical fiber unit 11 includes multiple optical fibers 12, the twist rate γ may be determined by twisting the optical fibers together, and the distribution of the effective bending radius R may be determined by twisting the fiber bundle. Or vice versa. That is, the multiple optical fibers 12 may be twisted together. In other words, the optical fibers may be twisted in two ways: one to give the optical fibers their own twist, and another to give them a bending distribution. In these cases, the fiber bundle has a double helix structure. By twisting the optical fibers together, the twist rate γ and the effective bending radius R can be changed from the values ​​determined by the winding state of the linear member 13 and adjusted to desired values.

[0041] From the analysis results of FIGS. 6 and 7, the specific twist radius r ｆ and twist interval P ｆ For example, when the twist rate γ is 20π rad / m, the effective bending radius R is selected to be 2000 to 3000 mm. In this case, the operating wavelength band includes the C band. The dotted ellipse in Figure 6 indicates the region A where the effective bending radius R is 2000 to 3000 mm. In other words, the twisting radius r in region A ｆ and twist interval P ｆ By selecting the combination of these, an effective bending radius R of 2000 to 3000 mm can be obtained.

[0042] For example, the twist radius r ｆ When is 1mm, 2mm, 3mm, 4mm, the twist interval P ｆ are set in the ranges of 280 to 350 mm, 400 to 490 mm, 490 to 600 mm, and 560 to 690 mm, respectively. ｆ When the twist radius r is 500mm, 1000mm, and 1500mm ｆ are set in the ranges of 2 to 3 mm, 6 to 8 mm, and 19 to 29 mm, respectively. In this example, the twist radius r applicable to the C band is ｆ and twist interval P ｆ For the L band and other wavelength bands, the twist radius r according to the wavelength band can be calculated using the above calculation method. ｆ and twist interval P ｆ The combination of the following can be calculated.

[0043] Figure 8 is a graph showing the change in effective bending radius R versus twist rate γ. This analysis assumes a two-core optical fiber with a core spacing Λ of 18 μm. The hatched area B in the graph in Figure 8 indicates the range of effective bending radius R where the coupling efficiency is -10 dB in the C band (1530-1565 nm) when the twist rate γ is changed. Specifically, the effective bending radius R satisfies the following equation (3) using the twist rate γ. That is, the effective bending radius R is distributed within the range shown in formula (3). Note that the left side of formula (3) represents the approximate straight line at a wavelength of 1565 nm shown in Fig. 8, and the right side represents the approximate straight line at a wavelength of 1530 nm shown in Fig. 8. Furthermore, the range of the effective bending radius R shown in region B is not limited to the case of two cores, and the core spacing Λ is not limited to the case of 18 μm, and can be applied to core spacings in the range of 10 to 30 μm, for example.

[0044] Fig. 9 is a cross-sectional view of an example of an optical fiber cable 10 taken along the longitudinal direction. As shown in Fig. 9, the optical fiber unit 11 includes a large-diameter portion 11a in which the winding radius of the linear member 13 is locally increased. When a plurality of optical fiber units 11 are provided in the optical fiber cable 10, the large-diameter portions 11a of the plurality of optical fiber units 11 may be arranged so that they are shifted relative to one another along the longitudinal direction. That is, the large-diameter portions 11a of the plurality of optical fiber units 11 may be positioned alternately in the longitudinal direction.

[0045] The formation of the large diameter portions 11a gives the fiber bundle 16 a tapered shape. By arranging the multiple optical fiber units 11 so that their large diameter portions 11a are offset from one another along the longitudinal direction, the contact area between the optical fiber units 11 is increased. The increased contact area increases frictional force, which prevents the optical fiber units 11 from moving unintentionally within the optical fiber cable 10 and prevents unnecessary lateral pressure and bending after installation. Therefore, the desired optical characteristics can be maintained.

[0046] When a plurality of optical fiber units 11 are provided in the optical fiber cable 10, the winding states of the plurality of optical fiber units 11 may be different from one another. In this case, the operating wavelength band set for each optical fiber unit 11 is different, so that, for example, the number of wavelengths to be assigned to each user can be increased.

[0047] REFERENCE SIGNS LIST 10 Optical fiber cable 11 Optical fiber unit 11a Large diameter portion 12 Optical fiber 13 Linear member 14 Cladding 15 Core 16 Fiber bundle 17 Jacket 18 Space 19 Central axis P Spacing P ｆ Twist spacing P ｔ Winding interval r Radius R Effective bending radius r ｆ Twist radius r ｔ Winding radius S1 section S2 section

Claims

1. An optical fiber cable comprising one or more optical fiber units, each of which includes one or more optical fibers that are coupled multi-core optical fibers and a linear member, the optical fiber and the linear member being wound around each other in a spiral shape, and the winding state of the linear member varying along the longitudinal direction of the optical fiber cable.

2. The optical fiber cable according to claim 1, wherein the winding state is at least one of the winding radius and winding interval of the linear member around the optical fiber.

3. The optical fiber cable according to claim 2, wherein the twist radius and twist spacing of the optical fiber when wound around the linear member are r [mm] and P [mm], respectively, and the effective bending radius R of the optical fiber satisfies the following formula (1) using the twist rate γ [rad / m] of the optical fiber. However, the effective bending radius R is expressed by the formula (2), and the twist rate γ is expressed by the formula (3).

4. The optical fiber cable according to claim 1, wherein the core spacing of the coupled multi-core optical fiber is set to a value in the range of 10 μm to 30 μm.

5. The optical fiber cable according to claim 2, wherein the one or more optical fiber units are a plurality of optical fiber units, each of the optical fiber units includes a large diameter portion where the winding radius of the linear member is locally increased, and the large diameter portions of the plurality of optical fiber units are arranged so as to be offset from one another along the longitudinal direction.

6. The optical fiber cable according to claim 2, wherein the linear member is a ribbon having a width that varies depending on the set winding radius or winding interval.

7. The optical fiber cable according to any one of claims 1 to 6, wherein the optical fiber in the optical fiber unit is a plurality of optical fibers twisted together.

8. A method for manufacturing an optical fiber cable, comprising: winding a linear member around one or more optical fibers that are coupled multi-core optical fibers; applying tension to the linear member so that the optical fibers and the linear member are spirally wound around each other; and adjusting the tension so that the winding state of the linear member changes along the longitudinal direction of the optical fiber cable.

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