Optical fiber cable and manufacturing method therefor

Abstract

An optical fiber cable (10) is provided with one or a plurality of optical fiber bundles (11) including a plurality of optical fibers (12) that are coupled multicore optical fibers. At least one of the plurality of optical fibers (12) and the optical fiber bundle (11) is twisted to form a helix, and at least one of the radius and the pitch of the helix changes along the axial direction of the helix.

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 bundles including a plurality of optical fibers that are coupled multi-core optical fibers, wherein at least one of the plurality of optical fibers and the optical fiber bundle is twisted to form a spiral, and at least one of the radius and spacing of the spiral varies along the axial direction of the spiral.

[0009] A method for manufacturing an optical fiber cable according to one aspect of the present disclosure includes twisting at least one of a plurality of optical fibers that are coupled multi-core optical fibers and an optical fiber bundle including the plurality of optical fibers so that a spiral is formed and at least one of the radius and spacing of the spiral varies along the axial direction of the spiral.

[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. 4 is a side view of an example optical fiber bundle. FIG. 5 is a diagram illustrating the effective bending radius R of an optical fiber. FIG. 6 is a graph showing the relationship between helix spacing and helix radius for several effective bending radii R. FIG. 7 is a graph showing analysis results of power coupling efficiency versus effective bending radius R. FIG. 8 is a graph showing changes 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-2D are cross-sectional views of several example optical fibers 12. FIG. 3 is a cross-sectional view of an example optical fiber cable 10. FIG. 4 is a side view of an example optical fiber bundle 11. As shown in FIG. 1, the optical fiber cable 10 includes one or more optical fiber bundles 11. The optical fiber bundle 11 includes a plurality of optical fibers 12. At least one of the optical fibers 12 and the optical fiber bundle 11 is twisted to form a helix 13 (see FIG. 5).

[0014] In the example shown in Fig. 1, each of the two optical fiber bundles 11 forms a spiral, and the four optical fibers 12 constituting each optical fiber bundle 11 are twisted together to form a spiral. That is, the optical fiber 12 shown in Fig. 1 has a double spiral structure.

[0015] 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).

[0016] 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.

[0017] 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.

[0018] 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.

[0019] The number of optical fiber bundles 11 is arbitrary. For example, as shown in Fig. 3, an optical fiber cable 10 includes ten optical fiber bundles 11 and an outer jacket 17 that houses and protects the optical fiber bundles 11. Each optical fiber bundle 11 is disposed in a space 18 within the outer jacket 17.

[0020] When manufacturing the optical fiber bundle 11 according to this embodiment, at least one of the optical fibers 12 and the optical fiber bundle 11 is twisted to form a spiral 13, and at least one of the radius r and spacing P of the spiral 13 changes along the axial direction (extension direction) of the spiral 13. Hereinafter, for convenience of explanation, the radius r of the spiral 13 will be referred to as the spiral radius r, and the spacing P of the spiral 13 will be referred to as the spiral spacing P.

[0021] For example, a plurality of optical fibers 12 aligned approximately parallel to one another are prepared. The optical fibers 12 are gripped at two locations spaced apart in the Z direction, and twisted together until a desired helical shape is obtained. Then, another two locations are gripped again, and the optical fibers 12 are twisted together until another desired helical shape is obtained. By performing or repeating these steps, an optical fiber bundle 11 can be formed in which the optical fibers 12 are wound around each other in a helical shape. In this case, at least one of the radius and spacing of the helix formed by each optical fiber 12 varies along the axial direction of the helix.

[0022] In another example, a plurality of optical fiber bundles 11 including a plurality of optical fibers 12 aligned approximately in parallel are prepared. The plurality of optical fiber bundles 11 are gripped at two locations spaced apart in the Z direction, and the plurality of optical fiber bundles 11 are twisted until a desired helical shape is obtained. Then, another two locations are gripped again, and the plurality of optical fiber bundles 11 are twisted until another desired helical shape is obtained. By performing these steps or repeating these steps, it is possible to form optical fiber bundles 11 in which optical fibers 12 are wound around each other in a helical shape. Furthermore, by performing the steps shown in these examples above in a combined manner, it is possible to twist the optical fiber bundles 11 in a helical shape (or twist the optical fiber bundles 11 together) while twisting the optical fibers 12 together in a helical shape.

[0023] Next, the effective bending radius R of the optical fiber 12 will be described. Fig. 5 is a diagram for explaining the effective bending radius R of the optical fiber 12. For convenience of explanation, a central axis 19 of the helix 13 is defined, and the axial direction, which is the extension direction of the helix 13, is referred to as the Z direction.

[0024] As described above, in this embodiment, at least one of the optical fibers 12 and the optical fiber bundle 11 is twisted to form the spiral 13. This spiral 13 can be explained by the simple model shown in Fig. 5. That is, as shown in Fig. 5, the spiral radius r is the radius of the circle formed by the spiral 13 when viewed from the axial direction of the spiral 13 (i.e., the Z direction). On the other hand, the spacing P of the spiral 13 is the distance traveled in the Z direction when the spiral 13 makes one rotation when viewed from the axial direction of the spiral 13.

[0025] When the optical fibers 12 or the optical fiber bundle 11 are deformed from a linear shape to a spiral shape, the optical fibers 12 are 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): Here, r and P are the helix radius r and the helix spacing P, respectively. When the optical fiber bundle 11 as a whole forms the helix 13, the effective bending radius R may be the average value of the effective bending radii R of the multiple optical fibers 12.

[0026] When the optical fiber 12 is deformed into the shape of the helix 13, 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 2πrad / m, this means that the optical fiber 12 (optical fiber bundle 11) is twisted once per meter, and if the twist rate γ is 20πrad / m, this means that the optical fiber 12 (optical fiber bundle 11) is twisted 10 times per meter.

[0027] 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 at least one of the helix radius r and helix spacing P formed between the optical fibers 12 or by the optical fiber bundle 11 itself.

[0028] In this embodiment, the helix radius r and helix spacing P described above vary along the axial direction of the helix 13. For example, in the optical fiber cable 10 illustrated in FIG. 1 , the helix spacing P of the optical fiber bundle 11 differs between sections S1 and S2. Alternatively, as illustrated in FIG. 4 , the helix spacing P of the optical fibers 12 in the optical fiber bundle 11 differs between sections S3 and S4. By varying the shape of the helix for each section, the optical characteristics of the optical fiber 12 in each section are changed, thereby varying the wavelength range (i.e., center wavelength) at which a power coupling efficiency equal to or greater than a desired value is obtained for each section. Note that the shape of the helix may vary continuously or stepwise. In either case, various values ​​of the effective bending radius R (varying within a certain range) can be obtained.

[0029] 6 is a graph showing the relationship between the helix spacing P and the helix radius r for several effective bending radii R. The lower horizontal axis of the graph represents the helix spacing P, and the vertical axis represents the helix radius r. The upper horizontal axis also represents the twist rate γ of the helix 13. By selecting a combination of the helix spacing P and the helix radius r based on the graph of FIG. 6, the effective bending radius R of the optical fiber 12 can be determined. Specific examples of this will be described later.

[0030] 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.

[0031] 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 20πrad / m (helix 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 2πrad / m (helix 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.

[0032] In this way, when the twist rate γ of the optical fiber 12 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.

[0033] If the SMD has wavelength dependency, processing such as MIMO DSP on the receiving side requires a circuit scale that matches the maximum 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 spiral shape along the longitudinal direction of the optical fiber cable 10, i.e., by setting multiple sections with different effective bending radii R, the wavelength range in which the desired power coupling efficiency can be obtained can be expanded, thereby reducing the wavelength dependency of the SMD in that wavelength range. 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.

[0034] The twist rate γ may be provided by twisting the optical fibers 12 together, and the distribution of the effective bending radius R may be provided by twisting the fiber bundle. Or vice versa. That is, the optical fibers 12 may be twisted together. In other words, the optical fibers 12 may be given two types of twists: one that gives the optical fibers their own twist, and another that gives them a bending distribution. In these cases, the fiber bundle will have a double helix structure.

[0035] From the analysis results in Figures 6 and 7, it is possible to calculate the specific helix radius r and helix spacing P for a given wavelength band. For example, suppose that an effective bending radius R of 2000 to 3000 mm is selected when the twist rate γ is 20πrad / m. In this case, the operating wavelength band includes Band C. The dotted ellipse in Figure 6 indicates Region A where the effective bending radius R is 2000 to 3000 mm. In other words, by selecting a combination of the helix radius r and helix spacing P within Region A, it is possible to obtain an effective bending radius R of 2000 to 3000 mm.

[0036] For example, when the spiral radius r is 1 mm, 2 mm, 3 mm, or 4 mm, the spiral spacing P is set to the ranges of 280 to 350 mm, 400 to 490 mm, 490 to 600 mm, and 560 to 690 mm, respectively. Furthermore, when the spiral spacing P is 500 mm, 1000 mm, or 1500 mm, the spiral radius r is set to the ranges of 2 to 3 mm, 6 to 8 mm, and 19 to 29 mm, respectively. While this example describes the settings of the spiral radius r and spiral spacing P applicable to the C band, the above calculation method can also be used to calculate combinations of the spiral radius r and spiral spacing P appropriate for the L band or other wavelength bands.

[0037] 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 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. For the core spacing Λ, for example, a range of 10 to 30 μm can be applied.

[0038] 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 bundle 11 may include a large-diameter portion 11a in which the helical radius r of the optical fiber bundle 11 itself is locally increased. When the optical fiber cable 10 is provided with a plurality of optical fiber bundles 11, the large-diameter portions 11a of the optical fiber bundles 11 may be arranged so that they are offset from one another along the longitudinal direction. That is, the large-diameter portions 11a of the optical fiber bundles 11 may be positioned alternately in the longitudinal direction.

[0039] The formation of the large diameter portions 11a gives the optical fiber bundle 11 a tapered shape. By arranging the multiple optical fiber bundles 11 so that their large diameter portions 11a are offset from one another along the longitudinal direction, the contact area between the optical fiber bundles 11 is increased. The increased contact area increases frictional force, which prevents the optical fiber bundles 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.

[0040] When a plurality of optical fiber bundles 11 are provided in the optical fiber cable 10, the helix radius r, the helix spacing P, or both may be different for each optical fiber bundle 11. In this case, the operating wavelength band set for each optical fiber bundle 11 will be different, and therefore, for example, it is possible to increase the number of wavelengths to be assigned to each user.

[0041] REFERENCE SIGNS LIST 10 Optical fiber cable 11 Optical fiber bundle 11a Large diameter portion 12 Optical fiber 13 Spiral 14 Cladding 15 Core 17 Outer jacket 18 Space 19 Central axis P Spiral spacing r Spiral radius R Effective bending radius S1 Section S2 Section

Claims

1. An optical fiber cable comprising one or more optical fiber bundles including a plurality of optical fibers that are coupled multi-core optical fibers, wherein at least one of the plurality of optical fibers and the optical fiber bundle is twisted to form a helix, and at least one of the radius and spacing of the helix varies along the axial direction of the helix.

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

3. The optical fiber cable according to claim 1 or 2, wherein the core spacing of the optical fibers is set to a value in the range of 10 μm to 30 μm.

4. The optical fiber cable according to claim 1, wherein the one or more optical fiber bundles are a plurality of optical fiber bundles twisted to form the spiral, each of the optical fiber bundles includes a large diameter portion whose radius is locally increased, and the large diameter portions of the plurality of optical fiber bundles are arranged so as to be offset from one another along the longitudinal direction of the optical fiber cable.

5. A method for manufacturing an optical fiber cable, comprising twisting at least one of a plurality of optical fibers that are coupled multi-core optical fibers and an optical fiber bundle including the plurality of optical fibers so that a spiral is formed and at least one of the radius and spacing of the spiral varies along the axial direction of the spiral.

Images