Optical fiber cable

Abstract

This optical fiber cable (1) is provided with: an optical fiber bundle (BD) that includes an optical fiber (FB) having a plurality of cores (CR) through which light propagates; and a cladding (CT) that covers the periphery of the optical fiber bundle (BD). In any combination of adjacent first and second cores selected from among the plurality of cores (CR) included in the same optical fiber (FB), the effective bend radius of the optical fiber (FB) is larger than the peak radius, wherein the peak radius is a value obtained by dividing the product of the interval between the first core and the second core and the effective refractive index of the first core by the effective refractive index difference between the first and second cores.

Description

fiber optic cable

[0001] The present disclosure relates to fiber optic cables.

[0002] Non-Patent Document 1 discloses that inter-core crosstalk, which is leakage of light waves between cores, changes depending on the bending state of a multi-core optical fiber, and that the inter-core crosstalk reaches a maximum value at a specific bending radius.

[0003] Non-Patent Document 2 discloses a small-diameter, high-density terrestrial cable equipped with a four-core optical fiber, and discloses that when comparing the state of the optical fiber in a bare wire state, a state after cable formation, and a state after installation, the inter-core crosstalk changes by up to about 8 dB.

[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.

[0005] In the optical fibers described in Non-Patent Documents 1 and 2, the inter-core crosstalk causes degradation of the signal quality of signals passing through the cores. In addition, the inter-core crosstalk increases as the transmission distance increases, which is a problem that it becomes a limiting factor when optical fiber cables are made longer in distance and capacity.

[0006] The present disclosure has been made in view of the above-mentioned problems, and has an object to provide an optical fiber cable that can suppress inter-core crosstalk and achieve long distances and high capacity.

[0007] In order to solve the above-mentioned problems, an optical fiber cable according to the present disclosure includes an optical fiber bundle including an optical fiber having a plurality of cores through which light propagates, and an outer jacket surrounding the optical fiber bundle, wherein, for any combination of adjacent first and second cores selected from a plurality of cores included in the same optical fiber, the peak radius is defined as the product of the spacing between the first and second cores and the effective refractive index of the first core divided by the effective refractive index difference between the first and second cores, and the effective bend radius of the optical fiber is greater than the peak radius.

[0008] According to the present disclosure, it is possible to provide an optical fiber cable that can suppress inter-core crosstalk and achieve long distances and high capacity.

[0009] FIG. 1 is a cross-sectional view of an optical fiber cable according to an embodiment of the present disclosure; FIG. 2 is a schematic diagram of an optical fiber bundle formed by twisting optical fibers; FIG. 3 is a cross-sectional view showing an example of an arrangement of cores in an optical fiber; FIG. 4 is a diagram showing an example of a relationship between the bending radius of an optical fiber and inter-core crosstalk; FIG. 5 is a diagram showing an example of a relationship between a deviation in core radius and a peak radius; FIG. 6 is a diagram showing an example of a relationship between a deviation in relative refractive index difference of cores and a peak radius; FIG. 7 is a cross-sectional view showing an example of setting of core radius.

[0010] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and redundant description will be omitted.

[0011] [1. Configuration of Optical Fiber Cable] Fig. 1 is a cross-sectional view of an optical fiber cable according to an embodiment of the present disclosure. Fig. 2 is a schematic diagram of an optical fiber bundle formed by twisting optical fibers together. Fig. 3 is a cross-sectional view showing an example of the arrangement of cores in optical fibers.

[0012] Fig. 1 shows a cross-sectional view of the optical fiber cable 1 taken along a plane perpendicular to the longitudinal direction of the optical fiber cable 1. Fig. 2 shows a plan view of the optical fiber bundle BD as viewed from a direction perpendicular to the longitudinal direction (left-right direction in the drawing). Fig. 3 shows a cross-sectional view of the optical fiber FB taken along a plane perpendicular to the longitudinal direction of the optical fiber FB.

[0013] 1, the optical fiber cable 1 includes an optical fiber bundle BD and a jacket CT. The jacket CT covers the optical fiber bundle BD. The optical fiber cable 1 may include only one optical fiber bundle BD, or may include multiple optical fiber bundles BD. Furthermore, in the optical fiber cable 1, multiple optical fiber bundles BD may be twisted together.

[0014] The optical fiber bundle BD includes an optical fiber FB. The optical fiber bundle BD may include only one optical fiber FB, or may include multiple optical fibers FB. The optical fiber bundle BD may include other wires (e.g., reinforcing wires) in addition to the optical fiber FB. The optical fiber bundle BD may be formed by twisting together optical fibers FB, or by twisting together the optical fiber FB and other wires.

[0015] As shown in Fig. 2, the optical fiber bundle BD may be formed by twisting together one or more optical fibers FB. Fig. 2 shows two optical fibers FB twisted together and wound around each other in a spiral shape.

[0016] 2 shows the radius r of the optical fiber bundle BD and the twisting interval P of the optical fibers FB in the optical fiber bundle BD. By twisting the optical fibers FB, the optical fibers FB have a bending radius.

[0017] The effective bending radius R of the optical fiber FB has the following relationship with the radius r of the optical fiber bundle BD and the twist interval P of the optical fiber FB: The twist interval P is the length measured in the longitudinal direction of the optical fiber bundle BD for one period of the structure of the helically twisted optical fiber FB.

[0018] Therefore, it can be seen that the effective bending radius R of the optical fiber FB can be increased by increasing the twist interval P of the optical fiber FB.

[0019] The optical fiber FB has a plurality of cores CR through which light propagates. In addition, the optical fiber FB has a cladding CD that surrounds the cores CR. While Fig. 3 shows an example in which the optical fiber FB has four cores CR, the present invention is not limited to this example.

[0020] The core CR may have a step-index type refractive index profile, a trench-assisted type refractive index profile, or a W-type refractive index profile. The refractive index profile of the core CR is not limited to the examples given here.

[0021] For example, a trench-assisted refractive index profile may be achieved by doping a fluorine-containing material with a low refractive index trench region surrounding a region with a high refractive index. A W-type refractive index profile may be achieved by providing a core with a refractive index similar to that of silica and doping a fluorine-containing material with a low refractive index trench region outside the core. The materials of the core CR, cladding CD, and other components of the optical fiber FB are not limited to the examples given here.

[0022] 2. Setting the bending radius of the optical fiber Fig. 4 is a diagram showing an example of the relationship between the bending radius of the optical fiber and the inter-core crosstalk. Fig. 4 shows the calculation results for an optical fiber FB in which the core radius of the core CR is 4.1 µm, the relative refractive index difference of the core CR with respect to the cladding CD is 0.39%, the interval between adjacent cores CR is 40 µm, and the correlation length is 0.02 m.

[0023] 4 also shows the relationship between the bending radius of the optical fiber and the inter-core crosstalk when the core radius deviation Δa (the difference in radius between adjacent cores CR) is set to different conditions. Specifically, the relationship is shown when Δa = 0.01 μm, 0.05 μm, 0.1 μm, and 0.2 μm.

[0024] The optical fiber FB is made of glass, and is subjected to external pressure when twisted. This causes stress to act on the glass that makes up the optical fiber FB, changing the density of the glass. This change results in a change in the refractive index profile.

[0025] Even in optical fibers FB manufactured using the same core preform, deviations in core radius and relative refractive index difference occur between cores due to manufacturing, resulting in an effective refractive index difference Δneff. In the following description, of two adjacent cores, the core with the smaller refractive index is referred to as core A, and the core with the larger refractive index is referred to as core B.

[0026] For example, when the optical fiber FB is straight or when the bending radius R is large, the effective refractive index of core B is larger than the effective refractive index of core A, and there is an effective refractive index difference Δneff. In FIG. 4, this corresponds to the situation where the bending radius R is larger than the peak radii PK1 to PK3.

[0027] When the optical fiber FB is bent to a moderate bending radius R, the refractive index changes, and a situation occurs in which the effective refractive indices of core A and core B become equal. In this situation, the inter-core crosstalk is maximized. In Figure 4, this corresponds to the situation in which the bending radius R is one of the peak radii PK1 to PK3.

[0028] When the optical fiber FB is further bent and the bending radius R becomes smaller, a situation arises in which the effective refractive index of core B becomes smaller than the effective refractive index of core A. In this situation, the inter-core crosstalk becomes smaller. In FIG. 4, this corresponds to a situation in which the bending radius R is smaller than the peak radii PK1 to PK3. In other words, this corresponds to a situation in which the bending radius R is within the range of region RG.

[0029] Here, the peak radius can be evaluated by a value obtained by dividing the product of the spacing between adjacent cores CR and the effective refractive index of one of the cores CR by the effective refractive index difference Δneff between the adjacent cores CR. By making the bending radius R of the optical fiber FB larger than the peak radius evaluated as above, it is possible to suppress inter-core crosstalk.

[0030] Since there may be two or more cores CR in the optical fiber FB, by making the bending radius R of the optical fiber FB larger than the peak radius in any combination of adjacent first and second cores selected from the multiple cores CR included in the same optical fiber FB, it is possible to suppress inter-core crosstalk in all cores CR included in the optical fiber FB.

[0031] 5 is a diagram showing an example of the relationship between the deviation of the core radius and the peak radius. As shown in FIG. 5, the peak radius R ｐｋ changes depending on the core radius deviation Δa, where the core radius deviation Δa is the difference between the radii of adjacent cores CR.

[0032] The effective bending radius R (unit: mm) and the radius deviation Δa (unit: μm) of two adjacent cores CR are expressed as the peak radius R ｐｋ can be evaluated by the following formula:

[0033] Therefore, inter-core crosstalk can be suppressed by satisfying the following relationship between the effective bending radius R (unit: mm) and the radius deviation Δa (unit: μm) of two adjacent cores CR included in the same optical fiber FB:

[0034] 6 is a diagram showing an example of the relationship between the deviation of the relative refractive index difference of the core and the peak radius. As shown in FIG. 6, the peak radius R ｐｋ changes depending on the deviation Δd of the relative refractive index difference of the core, where the deviation Δd of the relative refractive index difference of the core is the difference between the relative refractive index differences of the adjacent cores CR.

[0035] The effective bending radius R (unit: mm) and the deviation Δd (unit: μm) of the relative refractive index difference between two adjacent cores CR are expressed as the peak radius R ｐｋ can be evaluated by the following formula:

[0036] Therefore, inter-core crosstalk can be suppressed by having the following relationship between the effective bending radius R (unit: mm) and the deviation Δd (unit: %) of the relative refractive index difference between two adjacent cores CR included in the same optical fiber FB:

[0037] 7 is a cross-sectional view showing an example of core radius setting. Optical fiber FB has cores CR1 to CR4. Cores CR2 and CR4 are adjacent to core CR1. Cores CR2 and CR4 are adjacent to core CR3. The relative refractive index difference of cores CR1 to CR4 with respect to cladding CD is assumed to be 0.39%.

[0038] In the manufacturing of optical fibers, even when the same core preform is used, deviations of about 0.001% to 0.02% can occur. Therefore, in order to reduce deviations in inter-core crosstalk, it is desirable to set the radii of the cores CR1 to CR4 in a pattern such as that shown in the first and second examples described below.

[0039] [5-1. Comparative Example] First, a comparative example will be described in which the deviation in inter-core crosstalk cannot be reduced, resulting in a problem. Here, it is assumed that the radii of cores CR1 to CR4 are 4.1 μm, 4.25 μm, 4.3 μm, and 4.0 μm, respectively.

[0040] According to the comparative example, the radius deviation between cores CR1 and CR2 is 0.15 μm. The radius deviation between cores CR2 and CR3 is 0.05 μm. The radius deviation between cores CR3 and CR4 is 0.3 μm. The radius deviation between cores CR4 and CR1 is 0.1 μm.

[0041] Regarding the inter-core crosstalk in the cores CR1 to CR4, it is necessary to consider the sum of the inter-core crosstalk from the cores adjacent to the core of interest.

[0042] - For core CR1, the sum of inter-core crosstalk (0.15 μm, 0.1 μm) from core CR2 and core CR4 will be considered. - For core CR2, the sum of inter-core crosstalk (0.15 μm, 0.05 μm) from core CR1 and core CR3 will be considered. - For core CR3, the sum of inter-core crosstalk (0.05 μm, 0.3 μm) from core CR2 and core CR4 will be considered. - For core CR4, the sum of inter-core crosstalk (0.1 μm, 0.3 μm) from core CR1 and core CR3 will be considered.

[0043] According to the core radius pattern of the comparative example, the deviation of the core radii of the cores CR1 to CR4 differs, which causes a problem that the deviation of the inter-core crosstalk cannot be reduced. As a result, the magnitude of the inter-core crosstalk differs between the cores, which hinders the optical fiber cable from being longer in distance and having a larger capacity.

[0044] [5-2. First Example of Reducing Deviation in Inter-Core Crosstalk] Next, a first example of reducing deviation in inter-core crosstalk will be described. Here, it is assumed that the radii of cores CR1 to CR4 are 4.1 μm, 4.3 μm, 4.1 μm, and 4.3 μm, respectively.

[0045] In this way, by making the radii of the cores CR1 and CR3, and the radii of the cores CR2 and CR4 the same, it is possible to reduce the difference in deviation between the cores.

[0046] According to the first example, the radius deviation between cores CR1 and CR2 is 0.2 μm. The radius deviation between cores CR2 and CR3 is 0.2 μm. The radius deviation between cores CR3 and CR4 is 0.2 μm. The radius deviation between cores CR4 and CR1 is 0.2 μm.

[0047] Therefore, for each of the cores CR1 to CR4, the total inter-core crosstalk from the cores adjacent to the core of interest is equal. Therefore, even if the magnitude of inter-core crosstalk varies at a specific bending radius, the deviation of inter-core crosstalk can be reduced. As a result, optical fiber cables with longer distances and higher capacities can be realized.

[0048] [5-3. Second Example of Reducing Deviation in Inter-Core Crosstalk] Next, a second example of reducing deviation in inter-core crosstalk will be described. Here, it is assumed that the radii of the cores CR1 to CR4 are 4.1 μm, 4.11 μm, 4.32 μm, and 4.31 μm, respectively.

[0049] According to the second example, the radius deviation between cores CR1 and CR2 is 0.01 μm. The radius deviation between cores CR2 and CR3 is 0.21 μm. The radius deviation between cores CR3 and CR4 is 0.01 μm. The radius deviation between cores CR4 and CR1 is 0.21 μm.

[0050] - For core CR1, the sum of inter-core crosstalk (0.01 μm, 0.21 μm) from core CR2 and core CR4 will be considered. - For core CR2, the sum of inter-core crosstalk (0.01 μm, 0.21 μm) from core CR1 and core CR3 will be considered. - For core CR3, the sum of inter-core crosstalk (0.21 μm, 0.01 μm) from core CR2 and core CR4 will be considered. - For core CR4, the sum of inter-core crosstalk (0.21 μm, 0.01 μm) from core CR1 and core CR3 will be considered.

[0051] Therefore, for each of the cores CR1 to CR4, the total inter-core crosstalk from the cores adjacent to the core of interest is equal. Therefore, even if the magnitude of inter-core crosstalk varies at a specific bending radius, the deviation of inter-core crosstalk can be reduced. As a result, optical fiber cables with longer distances and higher capacities can be realized.

[0052] [Effects of the embodiment] As described above in detail, the optical fiber cable according to the present disclosure comprises an optical fiber bundle including an optical fiber having a plurality of cores through which light propagates, and an outer jacket surrounding the optical fiber bundle. In any combination of adjacent first and second cores selected from a plurality of cores included in the same optical fiber, the peak radius is defined as the product of the spacing between the first and second cores and the effective refractive index of the first core divided by the effective refractive index difference between the first and second cores, and the effective bend radius of the optical fiber is greater than the peak radius.

[0053] This makes it possible to provide an optical fiber cable that suppresses inter-core crosstalk and enables longer distances and higher capacities. Furthermore, the direction of change in inter-core crosstalk that occurs in each optical fiber when the optical fiber cable is bent is consistent, making it possible to unify the characteristics of the optical fibers that make up the optical fiber cable. As a result, limitations on transmission distances can be alleviated. Furthermore, the efficiency of operations using optical fiber cables can be improved.

[0054] With respect to the effective bending radius R, the radius r of the optical fiber bundle, and the twist interval P of the optical fibers in the optical fiber bundle, As a result, by increasing the twist interval P of the optical fiber, the effective bending radius R of the optical fiber can be increased. In addition, the range of the twist interval P that can be set can be determined.

[0055] Regarding the effective bending radius R (unit: mm) and the deviation Δa (unit: μm) between the radii of two adjacent cores included in the same optical fiber, This makes it possible to determine the range of the effective bending radius R of the optical fiber required to suppress inter-core crosstalk based on the radius deviation Δa of two adjacent cores CR. As a result, it is possible to more reliably suppress inter-core crosstalk.

[0056] Regarding the effective bending radius R (unit: mm) and the deviation Δd (unit: %) of the relative refractive index difference between two adjacent cores included in the same optical fiber, This makes it possible to determine the range of the effective bending radius R of the optical fiber required to suppress inter-core crosstalk based on the deviation Δd of the relative refractive index differences of two adjacent cores CR. As a result, it is possible to more reliably suppress inter-core crosstalk.

[0057] The optical fiber bundle may include a plurality of optical fibers. Also, the optical fiber cable may include a plurality of optical fiber bundles. As a result, it is possible to suppress inter-core crosstalk in various optical fiber cables used in actual operation.

[0058] Although the contents of the present disclosure have been described above based on the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which will be apparent to those skilled in the art. The descriptions and drawings that form part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0059] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description.

[0060] 1 Optical fiber cable BD Optical fiber bundle CD Cladding CR Core CT Outer jacket FB Optical fiber PK1 to PK3 Peak radius RG Area

Claims

1. An optical fiber cable comprising an optical fiber bundle including an optical fiber having a plurality of cores through which light propagates, and an outer jacket surrounding the optical fiber bundle, wherein, for any combination of adjacent first and second cores selected from the plurality of cores included in the same optical fiber, the effective bending radius of the optical fiber is greater than the peak radius, which is the value obtained by dividing the product of the distance between the first core and the second core and the effective refractive index of the first core by the effective refractive index difference between the first core and the second core.

2. Regarding the effective bending radius R, the radius r of the optical fiber bundle, and the twist interval P of the optical fibers in the optical fiber bundle, 2. The optical fiber cable of claim 1, wherein:

3. Regarding the effective bending radius R (unit: mm), and the deviation Δa (unit: μm) between the radii of two adjacent cores included in the same optical fiber, 3. The optical fiber cable according to claim 2, wherein the relationship is:

4. Regarding the effective bending radius R (unit: mm), and the deviation Δd (unit: %) of the relative refractive index difference between two adjacent cores included in the same optical fiber, 3. The optical fiber cable according to claim 1, wherein the relationship is:

5. An optical fiber cable according to any one of claims 1 to 4, wherein the optical fiber bundle comprises a plurality of the optical fibers.

6. An optical fiber cable according to any one of claims 1 to 5, comprising a plurality of said optical fiber bundles.

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