Optical fiber and optical fiber cable

Abstract

A cladding 20 of an optical fiber 1 includes four individual claddings 21 and a common cladding 22. Each of the four individual claddings 21 includes a low-refractive-index layer 23. The four individual claddings 21 include a plurality of types of structures that, in a cross-section orthogonal to a central axis AX, differ in terms of at least one from among the inner diameter of the low-refractive-index layer 23, the outer diameter of the low-refractive-index layer 23, the difference between the outer diameter of the low-refractive-index layer 23 and the inner diameter of the low-refractive-index layer 23, and the difference between the inner diameter of the individual cladding 21 and the inner diameter of the low-refractive-index layer 23. In a cross-section orthogonal to the central axis AX, the arrangement pattern of the plurality of types of structures of the individual claddings 21 is not rotationally symmetric about an axis GC which passes through the geometric center of the positions of four cores 10.

Description

Optical fibers and optical fiber cables

[0001] This application claims priority to Japanese Patent Application No. 2024-128698, filed August 5, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] There is known an optical fiber having a plurality of cores each extending along a central axis and a cladding covering the plurality of cores (for example, Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2). The cladding has a refractive index lower than that of the plurality of cores.

[0003] Japanese Patent Application Laid-Open No. 2022-66064

[0004] “2.15 Pb / s Transmission Using a 22 Core Homogeneous Single-ModeMulti-Core Fiber and Wideband Optical Comb” Ecoc 2015,BJ Puttnam RS “228-spatial-channel Bi-directional Data Communication System Enabledby 39-core 3-mode Fiber” IEEE Xplore

[0005] An optical fiber according to an embodiment of the present disclosure includes four cores and a cladding having a refractive index lower than that of the four cores. The four cores each extend along a central axis. The cladding includes four individual claddings and a common cladding. Each of the four individual claddings covers a different one of the four cores. The common cladding covers the four individual claddings and forms a circular outer periphery in a cross section perpendicular to the central axis. Each of the four individual claddings includes a low refractive index layer. The low refractive index layer covers a corresponding one of the four cores and has a refractive index lower than that of the common cladding. The four individual claddings include a plurality of types of structures that differ in at least one of the inner diameter of the low refractive index layer, the outer diameter of the low refractive index layer, the difference between the outer diameter of the low refractive index layer and the inner diameter of the low refractive index layer, and the difference between the inner diameter of the individual cladding and the inner diameter of the low refractive index layer in a cross section perpendicular to the central axis. The arrangement pattern of the multiple types of structures of the individual cladding is rotationally asymmetric about an axis passing through the geometric center of the positions of the four cores in a cross section perpendicular to the central axis.

[0006] FIG. 1 is a diagram showing the structure of an optical fiber cable. FIG. 2 is a diagram showing the cross-sectional structure of an optical fiber. FIG. 3 is a diagram showing an example of a refractive index profile around the core of an optical fiber. FIG. 4 is a diagram showing the arrangement of cores of an optical fiber in a modified example of this embodiment. FIG. 5 is a diagram showing the cross-sectional structure of an optical fiber in a modified example of this embodiment. FIG. 6 is a diagram showing an example of a refractive index profile around the core of an optical fiber in a modified example of this embodiment. FIG. 7 is a diagram for explaining the cross-sectional structure of an optical fiber in a modified example of this embodiment. FIG. 8 is a diagram for explaining the cross-sectional structure of an optical fiber in a modified example of this embodiment.

[0007] In a multi-core optical fiber including multiple cores, there are cases where it is required to identify each core. Patent Document 1 describes a configuration in which four cores are formed into a trapezoidal shape for core identification. An optical fiber with four cores is suitable for communication in terms of the number of spatial channels. In this case, compared to a case in which four cores are arranged in a square lattice pattern, there are sides where the inter-core distance is shorter or longer, which raises concerns about increased crosstalk between cores or increased leakage loss toward the outer periphery of the optical fiber (toward the coating).

[0008] Patent Document 1 also describes a configuration in which a marker is provided in addition to the core. In this case, the number of manufacturing steps for manufacturing the configuration in which the marker is provided increases, which may result in a decrease in production throughput.

[0009] An object of the present disclosure is to provide an optical fiber and an optical fiber cable in which identification of the four cores is easily achieved.

[0010] According to the present disclosure, it is possible to provide an optical fiber and an optical fiber cable in which the identification of the four cores can be easily achieved.

[0011] An embodiment of the present disclosure will be described.

[0012] [1] An optical fiber according to an embodiment of the present disclosure includes four cores and a cladding having a refractive index lower than that of the four cores. The four cores each extend along a central axis. The cladding includes four individual claddings and a common cladding. Each of the four individual claddings covers a different one of the four cores. The common cladding covers the four individual claddings and forms a circular outer periphery in a cross section perpendicular to the central axis. Each of the four individual claddings includes a low-refractive index layer. The low-refractive index layer covers a corresponding one of the four cores and has a refractive index lower than that of the common cladding. The four individual claddings include a plurality of types of structures that differ in at least one of the inner diameter of the low-refractive index layer, the outer diameter of the low-refractive index layer, the difference between the outer diameter of the low-refractive index layer and the inner diameter of the low-refractive index layer, and the difference between the inner diameter of the individual cladding and the inner diameter of the low-refractive index layer in a cross section perpendicular to the central axis. The arrangement pattern of the multiple types of structures of the individual cladding is rotationally asymmetric about an axis passing through the geometric center of the positions of the four cores in a cross section perpendicular to the central axis.

[0013] In this optical fiber, the cladding includes four individual claddings and a common cladding, and the four individual claddings include multiple types of structures. The arrangement pattern of the multiple types of structures in the individual claddings is rotationally asymmetric with respect to an axis passing through the geometric centers of the positions of the four cores in a cross section perpendicular to the central axis. Each core can be identified by checking the arrangement pattern of the multiple types of structures in the individual claddings. Therefore, identification of the four cores can be easily achieved.

[0014] [2] In the optical fiber of [1] above, the axis passing through the geometric center of the four core positions may pass through the geometric center of the common cladding. The arrangement pattern of the multiple types of structures is symmetrical about a line that does not pass through the centers of the four cores in a cross section perpendicular to the central axis. In this case, when connecting this optical fiber to another optical fiber, the optical fiber can be connected appropriately without having to identify which of the two end faces of the optical fiber is connected. Therefore, core identification is easy.

[0015] [3] In the optical fiber according to [1] or [2], the four cores may be arranged in a square lattice pattern in a cross section perpendicular to the central axis, thereby reducing crosstalk or leakage loss to the coating.

[0016] [4] In the optical fiber according to [3] above, the positions of the four cores have four-fold rotational symmetry about an axis passing through the geometric center of the common cladding in a cross section perpendicular to the central axis. In this case, the stress that each core receives from the cladding is uniform. Therefore, the cores are easily distinguishable, and non-uniformity in the optical properties of the four cores is reduced.

[0017] [5] In the optical fiber of [3] above, the positions of the four cores may be rotationally asymmetric about an axis passing through the geometric center of the common cladding in a cross section perpendicular to the central axis. In this case, when this optical fiber is spliced ​​with another optical fiber, a misalignment in the direction of rotational alignment is detected and the cores are identified by detecting a splice loss.

[0018] [6] In the optical fiber according to any one of the above [1] to [5], the refractive index of at least one of the four cores may be different. In this case, since the refractive indexes of the cores are different, it is easier to identify the cores.

[0019] [7] In the optical fiber according to any one of [1] to [6] above, the refractive index of at least one low-refractive-index layer among the four individual claddings may be different. In this case, the number of types of individual claddings increases, making it easier to identify the core.

[0020] [8] In the optical fiber according to any one of [1] to [7] above, at least one of the four individual claddings may include a low refractive index layer and an optical cladding. In this case, the number of types of individual claddings increases, making it easier to identify the core.

[0021] [9] An optical fiber cable according to an embodiment of the present disclosure includes a plurality of optical fibers, including the optical fiber according to any one of [1] to [8] above. In this case, it is easy to identify the cores even in an optical fiber cable including a plurality of optical fibers. [Details of the embodiment of the present disclosure]

[0022] Specific examples of embodiments of the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted as appropriate.

[0023] The optical fiber and optical fiber cable of this embodiment will be described in more detail with reference to Figures 1 to 3. Figure 1 is a diagram showing the structure of the optical fiber cable. The optical fiber cable CB corresponds to a multi-core optical fiber cable (MCF cable). The optical fiber cable CB includes a jacket 3 including an MCF storage space extending along the longitudinal direction of the optical fiber cable CB, and a plurality of multi-core fibers (MCFs) 5. Two tension members 4A, 4B extending along the MCF storage space are embedded in the jacket 3. Each of the plurality of MCFs 5 includes an optical fiber 1 whose outer circumferential surface is covered with a resin coating. The optical fiber 1 is, for example, a glass fiber.

[0024] 2 is a diagram showing the cross-sectional structure of the optical fiber 1. The central axis AX is the central axis of the optical fiber 1. The optical fiber 1 has a circular shape in a cross section perpendicular to the central axis AX. The optical fiber 1 is a multi-core fiber. The optical fiber 1 has four cores 10 and a cladding 20.

[0025] The four cores 10 each extend along a central axis AX. Each core 10 has a circular shape in a cross section perpendicular to the central axis AX. The central axis CX is the central axis of each core 10. An axis GC passing through the geometric center of the positions of the four cores 10 corresponds to the central axis AX. The four cores 10 are arranged in a square lattice pattern in a cross section perpendicular to the central axis AX. That is, the central axes CX of adjacent cores 10 among the four cores 10 are located equidistant from each other in a cross section perpendicular to the central axis AX. In this specification, the positions of the four cores 10 refer to the positions of the central axes CX of each core 10.

[0026] The cladding 20 covers the four cores 10 and extends along the central axis AX of the optical fiber 1. The cladding 20 is in contact with each of the four cores 10. The refractive index of the cladding 20 is lower than that of the cores 10. The refractive indexes of the cores 10 and the cladding 20 can be measured, for example, using an IFA-100 Multiwavelength Optical Fiber Analyzer, a measuring instrument manufactured by Interfiber Analysis. The cladding 20 includes four individual claddings 21 and a common cladding 22. Each of the four individual claddings 21 covers a different one of the four cores 10. The four individual claddings 21 surround the different cores 10 around the central axis CX of the cores 10. Each of the individual claddings 21 contacts a corresponding one of the four cores 10 .

[0027] Each of the four individual claddings 21 includes a low-refractive index layer 23. The low-refractive index layer 23 is a concentric ring shape around the central axis CX of the corresponding core 10 among the four cores 10. The low-refractive index layer 23 covers the corresponding core 10 among the four cores 10 and has a refractive index lower than that of the common cladding 22. The refractive indexes of the low-refractive index layer 23 and the common cladding 22 can be measured, for example, using an IFA-100 Multiwavelength Optical Fiber Analyzer, a measuring instrument manufactured by Interfiber Analysis. The low-refractive index layer 23 surrounds the core 10 around the central axis CX of the core 10. In the example shown in this embodiment, the low-refractive index layer 23 contacts the corresponding core 10 among the four cores 10. The low-refractive index layer 23 corresponds to a depressed region.

[0028] The common clad 22 covers the four individual clads 21 and forms a circular outer periphery in a cross section perpendicular to the central axis AX. The common clad 22 fixes the four individual clads 21 so that the four individual clads 21 are spaced apart from each other. The common clad 22 is in contact with each of the four individual clads 21.

[0029] In the example shown in this embodiment, an axis GC passing through the geometric center of the positions of the four cores 10 passes through the geometric center of the common cladding 22. The geometric center of the common cladding 22 corresponds to the central axis AX. The positions of the four cores 10 have four-fold rotational symmetry about the central axis AX passing through the geometric center of the common cladding 22 in a cross section perpendicular to the central axis AX.

[0030] The configuration of the four individual claddings 21 will be described in detail with reference to Fig. 3. Fig. 3 is a diagram showing an example of a refractive index profile around the core of an optical fiber. The four individual claddings 21 include, in a cross section perpendicular to the central axis AX, a plurality of types of structures that differ in at least one of the inner diameter of the low refractive index layer 23, the outer diameter of the low refractive index layer 23, the difference between the outer diameter of the low refractive index layer 23 and the inner diameter of the low refractive index layer 23, and the difference between the inner diameter of the individual cladding 21 and the inner diameter of the low refractive index layer 23. The arrangement pattern of the plurality of types of structures of the individual cladding 21 does not have rotational symmetry about an axis GC that passes through the geometric center of the positions of the four cores 10 in a cross section perpendicular to the central axis AX.

[0031] In the example shown in this embodiment, the four individual claddings 21 include four types of individual claddings 21a, 21b, 21c, and 21d. FIG. 3 shows refractive index profiles of four cores 10, four types of individual claddings 21a, 21b, 21c, and 21d, Type 1 to Type 4, surrounding each of the four cores 10, and the common cladding 22. In each of the refractive index profiles of Type 1 to Type 4 in FIG. 3, the vertical axis represents the refractive index k, and the horizontal axis represents the distance r from the central axis CX of the core 10. The refractive index of the core 10 is "k1," the refractive index of the low-refractive-index layer 23 is "k2," and the refractive index of the common cladding 22 is "kc." In different types of individual claddings 21, "k1" and "k2" do not have to have the same value. The error in "k2" between the different types of individual claddings 21 may be within a range of −0.01% to +0.01% of the respective "k2" values.

[0032] In the example shown in this embodiment, the distance ra corresponds to the outer diameter of the core 10 and the inner diameter of the individual cladding 21. The distance rb corresponds to the outer diameter of the individual cladding 21. The distance ra corresponds to the inner diameter of the low refractive index layer 23, and the distance rb corresponds to the outer diameter of the low refractive index layer 23.

[0033] In the example shown in this embodiment, the four individual claddings 21 include a plurality of types of structures that are different in at least one of the distance r and the difference between the distance r and the distance ra in a cross section perpendicular to the central axis AX. In this case, the four cores 10 have approximately the same mode field diameter and chromatic dispersion characteristics.

[0034] 3, the difference between the distance rb and the distance ra is different for each of the individual claddings 21a, 21b, 21c, and 21d. In the example shown in this embodiment, the difference between the distance rb and the distance ra corresponds to the thickness of the low-refractive-index layer 23. The thickness of the low-refractive-index layer 23 is the length of the low-refractive-index layer 23 in the radial direction from the central axis CX.

[0035] An optical fiber 1A according to a modification of this embodiment will be described with reference to Fig. 4. Fig. 4 is a diagram showing the arrangement of cores of an optical fiber according to a modification of this embodiment. This modification is generally similar to or the same as the embodiment described above. This modification differs from the embodiment described above in terms of the arrangement of the four cores. Below, the differences between the embodiment and the modification described above will be mainly described.

[0036] The central axis AX is the central axis of the optical fiber 1A. The optical fiber 1A has a circular shape in a cross section perpendicular to the central axis AX. The optical fiber 1A is a multi-core fiber.

[0037] In the optical fiber 1A, the axis GC passing through the geometric center of the positions of the four cores 10 is offset from the central axis AX. In Fig. 4, the positions of each of the four cores 10 are indicated by the central axis CX of the core 10. The axis GC passing through the geometric center of the positions of the four cores 10 does not pass through the geometric center of the common cladding 22. In a cross section perpendicular to the central axis AX, the positions of the four cores 10 do not have rotational symmetry about the central axis AX passing through the geometric center of the common cladding 22. That is, in a cross section perpendicular to the central axis AX, the positions are rotationally asymmetric about the central axis AX passing through the geometric center of the common cladding 22.

[0038] In this modification, the four cores 10 are also arranged in a square lattice pattern in a cross section perpendicular to the central axis AX. That is, the central axes CX of adjacent cores 10 of the four cores 10 are equidistant in a cross section perpendicular to the central axis AX.

[0039] An optical fiber 1B according to a modification of this embodiment will be described with reference to Figures 5 and 6. Figure 5 is a diagram showing the cross-sectional structure of an optical fiber according to a modification of this embodiment. Figure 6 is a diagram showing an example of a refractive index profile around the core of an optical fiber according to a modification of this embodiment. This modification is generally similar to or the same as the above-described embodiment. This modification differs from the above-described embodiment in terms of the configuration of the cladding. Below, differences between the above-described embodiment and the modification will be mainly described.

[0040] The central axis AX is the central axis of the optical fiber 1B. The optical fiber 1B has a circular shape in a cross section perpendicular to the central axis AX. The optical fiber 1B is a multi-core fiber. The optical fiber 1B has four cores 10 and a cladding 20A.

[0041] The cladding 20A covers the four cores 10 and extends along the central axis AX of the optical fiber 1B. The cladding 20A is in contact with each of the four cores 10. The refractive index of the cladding 20A is lower than the refractive index of the cores 10. The cladding 20A includes four individual claddings 25 and a common cladding 22. Each of the four individual claddings 25 covers a different core 10 among the four cores 10. The four individual claddings 25 surround the different cores 10 around the central axis CX of the cores 10. Each of the individual claddings 25 is in contact with a corresponding core 10 among the four cores 10.

[0042] Each of the four individual claddings 25 includes a low refractive index layer 26 and an optical cladding 27. The low refractive index layer 26 and the optical cladding 27 are concentric rings about the central axis CX of the corresponding core 10 of the four cores 10. The low refractive index layer 26 covers the corresponding core 10 of the four cores 10 and has a refractive index lower than the refractive index of the common cladding 22. The low refractive index layer 26 surrounds the optical cladding 27 around the central axis CX of the core 10. The low refractive index layer 26 is in contact with the optical cladding 27 that covers the corresponding core 10 of the four cores 10. The low refractive index layer 26 corresponds to a trench.

[0043] The optical cladding 27 is located between the core 10 and the low refractive index layer 26. The optical cladding 27 covers the corresponding core 10 of the four cores 10 and has a refractive index higher than the refractive index of the common cladding 22. The optical cladding 27 surrounds the core 10 around the central axis CX of the core 10. The optical cladding 27 is in contact with the corresponding core 10 of the four cores 10. The refractive indexes of the low refractive index layer 26, the common cladding 22, and the optical cladding 27 can be measured, for example, by an IFA-100 Multiwavelength Optical Fiber Analyzer, a measuring instrument manufactured by Interfiber Analysis.

[0044] The common clad 22 covers the four individual clads 25 and forms a circular outer periphery in a cross section perpendicular to the central axis AX. The common clad 22 fixes the four individual clads 25 so that the four individual clads 25 are spaced apart from each other. The common clad 22 is in contact with each of the four individual clads 25.

[0045] In this modification, an axis GC passing through the geometric center of the positions of the four cores 10 passes through the geometric center of the common cladding 22. The geometric center of the common cladding 22 corresponds to the central axis AX. The positions of the four cores 10 have four-fold rotational symmetry about the central axis AX passing through the geometric center of the common cladding 22 in a cross section perpendicular to the central axis AX.

[0046] 6 , the four individual claddings 25 include, in a cross section perpendicular to the central axis AX, a plurality of types of structures that differ in at least one of the inner diameter of the low refractive index layer 26, the outer diameter of the low refractive index layer 26, the difference between the outer diameter of the low refractive index layer 26 and the inner diameter of the low refractive index layer 26, and the difference between the inner diameter of the individual cladding 25 and the inner diameter of the low refractive index layer 26. The arrangement pattern of the plurality of types of structures of the individual cladding 25 does not have rotational symmetry about an axis GC that passes through the geometric center of the positions of the four cores 10 in a cross section perpendicular to the central axis AX. That is, the arrangement pattern of the plurality of types of structures of the individual cladding 25 is rotationally asymmetric about an axis GC that passes through the geometric center of the positions of the four cores 10 in a cross section perpendicular to the central axis AX.

[0047] In this modification, the four individual claddings 25 include four types of individual claddings 25a, 25b, 25c, and 25d. FIG. 6 shows the refractive index profiles of four cores 10, four types of individual claddings 25a, 25b, 25c, and 25d, Types 5 to 8, surrounding each of the four cores 10, and the common cladding 22. In each of the refractive index profiles of Types 5 to 8 in FIG. 6, the vertical axis represents the refractive index k, and the horizontal axis represents the distance r from the central axis CX of the core 10. In this modification, the refractive index of the core 10 is "k1'", the refractive index of the optical cladding 27 is "k2'", the refractive index of the low refractive index layer 23 is "k3'", and the refractive index of the common cladding 22 is "kc". In the different types of individual claddings 25, "k1'", "k2'", and "k3'" do not have to have the same values. The error of "k3'" between different types of individual claddings 25 may be within the range of -0.01% to +0.01% of the value of "k3'".

[0048] In this modified example, the distance ra corresponds to the outer diameter of the core 10 and the inner diameter of the individual cladding 25. The distance rc corresponds to the outer diameter of the individual cladding 25. The distance ra corresponds to the outer diameter of the core 10 and the inner diameter of the optical cladding 27. The distance rb corresponds to the outer diameter of the optical cladding 27 and the inner diameter of the low refractive index layer 26. The distance rc corresponds to the outer diameter of the low refractive index layer 26.

[0049] In this modification, the four individual claddings 25 include a plurality of types of structures that are different in at least one of the distance r, the distance rc, the difference between the distance rc and the distance rb, and the difference between the distance rb and the distance ra in a cross section perpendicular to the central axis AX. In this case, the four cores 10 have approximately the same mode field diameter and chromatic dispersion characteristics.

[0050] For example, in the example shown in Figure 6, the difference between the distance rc and the distance rb is different for the individual claddings 25a, 25b, 25c, and 25d. The difference between the distance rb and the distance ra is different for the individual claddings 25a, 25b, and 25c, and the difference between the distance rb and the distance ra is different for the individual claddings 25d. In this modification, the difference between the distance rb and the distance ra corresponds to the thickness of the optical cladding 27. The thickness of the optical cladding 27 is the length of the optical cladding 27 in the radial direction from the central axis CX. In this modification, the difference between the distance rc and the distance rb corresponds to the thickness of the low refractive index layer 26. The thickness of the low refractive index layer 26 is the length of the low refractive index layer 26 in the radial direction from the central axis CX.

[0051] Optical fibers 1C and 1D according to modifications of this embodiment will be described with reference to Figures 7 and 8. Figures 7 and 8 are diagrams illustrating the cross-sectional structures of optical fibers according to modifications of this embodiment. These modifications are generally similar to or the same as the above-described embodiment. These modifications differ from the above-described embodiment in terms of the configuration of the individual cladding. The following mainly describes the differences between the above-described embodiment and each modification.

[0052] The optical fibers 1C and 1D differ from the optical fiber 1 only in the arrangement pattern of the types of individual cladding 21. In this modification, the arrangement pattern of the types of individual cladding 21 does not have rotational symmetry about the axis GC passing through the geometric center of the positions of the four cores 10 in a cross section perpendicular to the central axis AX. In other words, the arrangement pattern of the types of individual cladding 21 is rotationally asymmetric about the axis GC passing through the geometric center of the positions of the four cores 10 in a cross section perpendicular to the central axis AX.

[0053] The optical fiber 1C includes only two types of individual claddings 21 as the four individual claddings 21. For example, as shown in Fig. 7 , the optical fiber 1C includes a pair of individual claddings 21a and a pair of individual claddings 21b. In the optical fiber 1C, the arrangement pattern of the types of individual claddings 21 is line-symmetric with respect to the axis of symmetry SX, which is a straight line that does not pass through the centers of the four cores 10, in a cross section perpendicular to the central axis AX. In Fig. 7 , two individual claddings 21a of the same type are arranged line-symmetric with respect to the axis of symmetry SX, and two individual claddings 21b of the same type are arranged line-symmetric with respect to the axis of symmetry SX.

[0054] The optical fiber 1D includes only three types of individual claddings 21 as the four individual claddings 21. For example, as shown in FIG. 8 , the optical fiber 1D includes one individual cladding 21a, one individual cladding 21b, and two individual claddings 21c. In the optical fiber 1D, the arrangement pattern of the types of individual claddings 21 is not line-symmetric with respect to a line that does not pass through the center of the core 10 in a cross section perpendicular to the central axis AX. For example, FIG. 8 shows the symmetry axis SX shown in FIG. 7. Two individual claddings 21c of the same type are arranged line-symmetric with respect to the symmetry axis SX, but the types of the individual claddings 21a and the individual claddings 21b are different. In the optical fiber 1D, the arrangement pattern of the types of individual claddings 21 is not line-symmetric with respect to any of the lines that do not pass through the centers of the four cores 10 in a cross section perpendicular to the central axis AX. In the optical fiber 1 and optical fiber 1B described above, the arrangement pattern of the types of individual cladding 21 is not symmetrical with respect to any of the straight lines that do not pass through the centers of each of the four cores 10 in a cross section perpendicular to the central axis AX.

[0055] The effects obtained from the optical fibers 1, 1A, 1B, 1C, and 1D and the optical fiber cable CB in the embodiment and the modified example will be described.

[0056] In the optical fiber 1, the cladding 20 includes four individual claddings 21 and a common cladding 22, and the four individual claddings 21 include multiple types of structures. The arrangement pattern of the multiple types of structures of the individual claddings 21 is rotationally asymmetric with respect to an axis GC passing through the geometric center of the positions of the four cores 10 in a cross section perpendicular to the central axis AX. By checking the arrangement pattern of the types of the individual claddings 21, each core 10 can be identified without providing any marker structure other than the core 10. Therefore, identification of the four cores 10 is easily achieved. The same or similar effects can be obtained in the optical fibers 1A, 1B, 1C, and 1D.

[0057] In the optical fiber 1C, an axis GC passing through the geometric center of the positions of the four cores 10 passes through the geometric center of the common cladding 22. The arrangement pattern of the multiple types of structures is symmetrical with respect to a straight line that does not pass through the centers of the cores 10 in a cross section perpendicular to the central axis AX. In this case, when connecting the optical fiber 1C to another optical fiber, the optical fibers can be connected appropriately without having to identify which of the end faces of the optical fiber 1C is connected. Therefore, it is easy to identify the cores.

[0058] When the four cores 10 are arranged in a trapezoidal shape, crosstalk or leakage loss to the coating increases. In the optical fiber 1, the four cores 10 are arranged in a square lattice pattern in a cross section perpendicular to the central axis AX. In this case, the optical fiber 1 reduces an increase in crosstalk or leakage loss to the coating. When the four cores 10 are arranged in a square lattice pattern, increasing the trench diameter of only one core 10 may make it impossible to identify the core 10. According to the optical fiber 1, the cores 10 can be easily identified. The same or similar effects can be obtained in the optical fibers 1A, 1B, 1C, and 1D.

[0059] In the optical fiber 1, the positions of the four cores 10 have four-fold rotational symmetry about the central axis AX that passes through the geometric center of the common cladding 22 in a cross section perpendicular to the central axis AX. In this case, the stress that each core 10 receives from the cladding 20 is uniform. Therefore, the cores 10 are easily distinguishable, and non-uniformity in the optical properties of the four cores 10 is reduced. The same or similar effects can be obtained in the optical fibers 1B, 1C, and 1D.

[0060] In the optical fiber 1A, the positions of the four cores 10 are rotationally asymmetric about the central axis AX that passes through the geometric center of the common cladding 22 in a cross section perpendicular to the central axis AX. In this case, when the optical fiber 1A is spliced ​​with another optical fiber, a misalignment in the direction of rotational alignment is detected and the cores are also identified by detecting the splice loss.

[0061] Although the embodiments of the present disclosure have been described in detail above, the present invention is not limited to the above embodiments and can be applied to various embodiments. The configurations of the optical fibers 1, 1A, 1B, 1C, and 1D and the optical fiber cable CB described above may be combined with each other.

[0062] For example, in the configuration of the optical fiber 1A, the plurality of individual claddings 21 may have the configuration shown in the optical fiber 1C or the optical fiber 1D.

[0063] For example, as shown in optical fiber 1B, at least one individual clad may be of a refractive index structure type including a low refractive index layer 26 and an optical clad 27, or multiple individual clads may have the configuration shown in optical fiber 1C or optical fiber 1D.

[0064] For example, the types of individual claddings in optical fiber 1C and optical fiber 1D do not have to be the types described above. For example, optical fiber 1C may include two types of individual claddings 21 a and 21 c, two types of individual claddings 21 a and 21 d, two types of individual claddings 21 b and 21 c, or two types of individual claddings 21 c and 21 d.

[0065] The optical fibers 1, 1A, 1C, and 1D may include individual claddings having refractive index profiles other than the individual claddings 21 a, 21 b, 21 c, and 21 d. The optical fiber 1B may include individual claddings having refractive index profiles other than the individual claddings 25 a, 25 b, 25 c, and 25 d.

[0066] 1, 1A, 1B, 1C, 1D... Optical fiber 3... Jacket 4A, 4B... Tensile strength wire 5... MCF 10... Core 20, 20A... Cladding 21, 21a, 21b, 21c, 21d, 25, 25a, 25b, 25c, 25d... Individual cladding 22... Common cladding 23, 26... Low refractive index layer k, kc, k1, k2, k1', k2', k3'... Refractive index 27... Optical cladding r, ra, rb, rc... Distance AX, CX... Center axis CB... Optical fiber cable GC... Axis

Claims

1. An optical fiber comprising four cores each extending along a central axis, and a cladding covering each of the four cores and having a refractive index lower than that of the four cores, wherein the cladding includes four individual claddings each covering a different one of the four cores, and a common cladding covering the four individual claddings and forming a circular outer periphery in a cross section perpendicular to the central axis, wherein each of the four individual claddings includes a low refractive index layer covering a corresponding one of the four cores and having a refractive index lower than that of the common cladding, and wherein the four individual claddings include a plurality of types of structures that differ in at least one of the inner diameter of the low refractive index layer, the outer diameter of the low refractive index layer, the difference between the outer diameter of the low refractive index layer and the inner diameter of the low refractive index layer, and the difference between the inner diameter of the individual cladding and the inner diameter of the low refractive index layer in the cross section perpendicular to the central axis, and wherein the arrangement pattern of the plurality of types of structures of the individual cladding is rotationally asymmetric about an axis passing through the geometric centers of the positions of the four cores in the cross section perpendicular to the central axis.

2. The optical fiber according to claim 1, wherein an axis passing through the geometric center of the positions of said four cores passes through the geometric center of said common cladding, and the arrangement pattern of said plurality of types of structures is symmetrical about a straight line that does not pass through the centers of each of said four cores in a cross section perpendicular to said central axis.

3. The optical fiber according to claim 1 or 2, wherein the four cores are arranged in a square lattice pattern in a cross section perpendicular to the central axis.

4. The optical fiber according to claim 3, wherein the positions of the four cores have four-fold rotational symmetry about an axis passing through the geometric center of the common cladding in a cross section perpendicular to the central axis.

5. The optical fiber according to claim 3, wherein the positions of the four cores are rotationally asymmetric about an axis passing through the geometric center of the common cladding in a cross section perpendicular to the central axis.

6. An optical fiber according to any one of claims 1 to 5, wherein the refractive index of at least one of the four cores is different.

7. The optical fiber according to any one of claims 1 to 6, wherein the refractive index of the low refractive index layer of at least one of the four individual claddings is different.

8. The optical fiber according to any one of claims 1 to 7, wherein at least one of said four individual claddings includes said low refractive index layer and an optical cladding.

9. An optical fiber cable comprising a plurality of optical fibers including the optical fiber according to any one of claims 1 to 8.

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