Multi-core fiber and optical device

Abstract

To achieve a multi-core fiber in which the inclination directions of end faces are appropriately set in consideration of connection.SOLUTION: In a multi-core fiber (MF), the inclination directions (v1, v2) of a first end face (σ1) and a second end face (σ2) are set such that, when the first end face (σ1) and the second end face (σ2) are brought into surface contact with each other to minimize an angle formed by the extension direction of cores (a1-an) in the first end face (σ1) and the extension direction of cores (a1-an) in the second end face (σ2), each of the cores (a1-an) in the first end face (σ1) at least partially overlaps any one of the cores (a1-an) in the second end face (σ2).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a multicore fiber or an optical device including a multicore fiber. [Background technology]

[0002] In the field of optical communications, multi-core fibers having multiple cores are widely used. Patent Document 1, for example, is an example of a document disclosing a multi-core fiber. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-152866 Summary of the Invention [Problem to be solved by the invention]

[0004] In optical fibers, in order to reduce reflection at the end face, it is common to tilt the end face so that it is not perpendicular to the extending direction of the cores. However, when tilting the end face of a multicore fiber, for example, the following problems may occur.

[0005] That is, when attempting to connect two multi-core fibers with inclined end faces so that the extending directions of the cores at the end faces of the respective multi-core fibers are aligned, there are cases in which the cores of one multi-core fiber cannot be overlapped with the cores of the other multi-core fiber. This is because the inclination directions of the end faces of the respective multi-core fibers are not appropriately set with connection in mind.

[0006] One aspect of the present invention has been made in consideration of the above problems, and an object of the present invention is to realize a multicore fiber in which the inclination direction of an end face is appropriately set in consideration of connection, or an optical device including such a multicore fiber. [Means for solving the problem]

[0007] A multicore fiber according to one aspect of the present invention has a clad, a plurality of cores formed in the clad, a marker formed in the clad, and a first end face and a second end face inclined so as not to be perpendicular to an extending direction of the plurality of cores, wherein when the first end face and the second end face are brought into surface contact so as to minimize an angle formed between the extending direction of the plurality of cores at the first end face and the extending direction of the plurality of cores at the second end face, each of the plurality of cores at the first end face is inclined toward the extending direction of the plurality of cores at the second end face. an inclination direction of the first end face and the second end face is set so as to at least partially overlap one of the plurality of cores arranged in the first end face and the second end face, the plurality of cores are arranged in line symmetry with respect to a virtual axis perpendicular to the inclination direction, the virtual axis does not intersect with any of the plurality of cores in the first end face and the second end face, and the center of the marker in each of the first end face and the second end face is included between the virtual axis and a straight line that passes through a center of the core that is farthest from the virtual axis among the plurality of cores and is parallel to the virtual axis.

[0008] A multicore fiber according to one aspect of the present invention has a cladding, a plurality of cores formed in the cladding, a marker formed in the cladding, and a first end face and a second end face inclined so as not to be perpendicular to an extending direction of the plurality of cores, wherein when the first end face and the second end face are brought into surface contact so as to minimize an angle formed between the extending direction of the plurality of cores at the first end face and the extending direction of the plurality of cores at the second end face, each of the plurality of cores at the first end face is inclined toward a front end of the second end face. The inclination direction of the first end face and the second end face is set so as to at least partially overlap with any of the multiple cores, the multiple cores are arranged in line symmetry with respect to a virtual axis perpendicular to the inclination direction at the first end face and the second end face, the virtual axis does not intersect with any of the multiple cores at the first end face and the second end face, and the core of the multiple cores closest to the marker and the core of the multiple cores second closest to the marker are arranged on either side of the virtual axis at each of the first end face and the second end face. Effect of the Invention

[0009] According to one aspect of the present invention, it is possible to realize a multi-core fiber in which the inclination direction of the end face is appropriately set in consideration of connection, or an optical device including such a multi-core fiber. [Brief description of the drawings]

[0010] [Figure 1] 1A is a diagram illustrating a configuration of a multi-core fiber according to a first embodiment of the present invention, in which (a) is a side view of the multi-core fiber, (b) is a front view of one end face of the multi-core fiber, (c) is a front view of the other end face of the multi-core fiber, and (d) is a perspective view of the multi-core fiber. [Diagram 2]Fig. 2 is a diagram showing modified examples of the multi-core fiber shown in Fig. 1. (a) is a front view of two end faces of a multi-core fiber according to a first modified example, (b) is a front view of two end faces of a multi-core fiber according to a second modified example, and (c) is a front view of two end faces of a multi-core fiber according to a third modified example. [Diagram 3] 1A is a diagram illustrating a configuration of a multi-core fiber according to a second embodiment of the present invention, in which (a) is a side view of the multi-core fiber, (b) is a front view of one end face of the multi-core fiber, (c) is a front view of the other end face of the multi-core fiber, and (d) is a perspective view of the multi-core fiber. [Figure 4] Fig. 4 is a diagram showing modified examples of the multi-core fiber shown in Fig. 3. (a) is a front view of two end faces of a multi-core fiber according to a first modified example, (b) is a front view of two end faces of a multi-core fiber according to a second modified example, and (c) is a front view of two end faces of a multi-core fiber according to a third modified example. [Diagram 5] 1A is a diagram illustrating a configuration of a multi-core fiber according to a third embodiment of the present invention, in which (a) is a side view of the multi-core fiber, (b) is a front view of one end face of the multi-core fiber, (c) is a front view of the other end face of the multi-core fiber, and (d) is a perspective view of the multi-core fiber. [Figure 6] Fig. 6 is a diagram showing modified examples of the multi-core fiber shown in Fig. 5. (a) is a front view of two end faces of a multi-core fiber according to a first modified example, (b) is a front view of two end faces of a multi-core fiber according to a second modified example, and (c) is a front view of two end faces of a multi-core fiber according to a third modified example. [Figure 7] Fig. 11 is a diagram illustrating a configuration of a multi-core fiber according to a fourth embodiment of the present invention, in which (a) is a side view of the multi-core fiber, (b) is a front view of one end face of the multi-core fiber, (c) is a front view of the other end face of the multi-core fiber, and (d) is a perspective view of the multi-core fiber. [Figure 8]8A and 8B are diagrams illustrating modified examples of the multi-core fiber illustrated in Fig. 7. (a) is a front view of two end faces of a multi-core fiber according to a first modified example, (b) is a front view of two end faces of a multi-core fiber according to a second modified example, and (c) is a front view of two end faces of a multi-core fiber according to a third modified example. [Figure 9] 10A and 10B are diagrams illustrating a configuration of an optical device according to a fifth embodiment of the present invention, in which (a) is a side view of the optical device, (b) is a front view of one end face of the optical device, and (c) is a front view of the other end face of the optical device. [Figure 10] 13A and 13B are diagrams illustrating a configuration of an optical device according to a sixth embodiment of the present invention, in which (a) is a side view of the optical device, (b) is a front view of one end face of the optical device, and (c) is a front view of the other end face of the optical device. [Figure 11] 11A and 11B are diagrams illustrating modified examples of the optical device shown in Fig. 10. (a) is a front view of one end face of an optical device according to a first modified example, (b) is a front view of one end face of an optical device according to a second modified example, and (c) is a front view of one end face of an optical device according to a third modified example. [Figure 12] 13A and 13B are diagrams illustrating a configuration of a multi-core fiber assembly MFS according to a seventh embodiment of the present invention, where (a) is a side view of the multi-core fiber assembly, and (b) is a perspective view of the multi-core fiber assembly. [Figure 13] 13A and 13B are diagrams illustrating a configuration of a multi-core fiber assembly MFS according to an eighth embodiment of the present invention, where (a) is a side view of the multi-core fiber assembly, and (b) is a perspective view of the multi-core fiber assembly. [Figure 14] FIG. 1A is a block diagram showing the configuration of an FI / FO device including a multi-core fiber according to an embodiment of the present invention, and FIG. 1B is a block diagram showing the configuration of an FI / FO device to which a multi-core fiber according to an embodiment of the present invention is connected. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [First embodiment] (Multicore fiber configuration) A configuration of a multi-core fiber MF according to a first embodiment of the present invention will be described with reference to Fig. 1. In Fig. 1, (a) is a side view of the multi-core fiber MF. Also, (b) is a front view of one end face (hereinafter referred to as "first end face") σ1 of the multi-core fiber MF seen from the line of sight E1. Also, (c) is a front view of the other end face (hereinafter referred to as "second end face") σ2 of the multi-core fiber MF seen from the line of sight E2. Also, (d) is a perspective view of the multi-core fiber MF in a state in which the first end face σ1 and the second end face σ2 are butted together.

[0012] The multi-core fiber MF includes n cores a1 to an and a clad b. Here, n is any natural number equal to or greater than 2 (FIG. 1 illustrates the case where n=4). The clad b is a cylindrical member. The clad b is made of, for example, silica glass. Each core ai (i is a natural number equal to or greater than 1 and equal to or less than n) is a cylindrical region provided inside the clad b, has a higher refractive index than the clad b, and extends in the same direction as the clad b. Each core ai is made of, for example, silica glass doped with an up-dopant such as germanium. Note that the clad b may have any cross-sectional shape as long as it is cylindrical. The cross-sectional shape of the clad b may be, for example, a polygonal shape such as a rectangle or a hexagon. Note that the multi-core fiber MF may further include a core other than the n cores a1 to an of interest in this embodiment. For example, the multi-core fiber MF may include a core provided at the center of the clad b as a core other than the n cores a1 to an of interest. The n cores a1 to an of interest are, for example, cores used for communication, and in this case, it is preferable that they are cores that meet the standards set by ITU-T. Cores other than the n cores a1 to an of interest may be cores used for communication, or may be cores not used for communication (dummy cores), and in the latter case, may be cores that do not meet the standards set by ITU-T.

[0013] The multi-core fiber MF further includes a marker c for identifying the core numbers 1 to n of the cores a1 to an. The marker c is a columnar region provided inside the cladding b, has a different refractive index from that of the cladding b, and extends in the same direction as the cladding b. The cross-sectional shape of the marker c is arbitrary, for example, a circular shape, a triangular shape, a rectangular shape, etc. The marker c is made of, for example, quartz glass doped with a down dopant such as fluorine or boron. In this case, the refractive index of the marker c is lower than that of the cladding b. The marker c is alternatively made of quartz glass doped with an up dopant such as germanium, aluminum, phosphorus, or chlorine. In this case, the refractive index of the marker c is higher than that of the cladding b. The marker c may be formed by, for example, a hole drilling method or a stack and draw method. The outer diameter of the marker c is usually smaller than that of the core ai. The marker c may be an air hole. In this case, the refractive index of the marker c is lower than the refractive index of the cladding b. Moreover, the multi-core fiber MF may further include markers other than the marker c of interest in this embodiment.

[0014] The core numbers 1 to n of the cores a1 to an can be identified based on the distance from the marker c. For example, when the cores a1 to an are arranged on a circumference, the core numbers 1 to n of the cores a1 to an can be identified as follows. First, the core number of the core a1 closest to the marker c is set to "1". Next, the core number of the core a2 second closest to the marker c is set to "2". Then, when the circumference is traced so as to pass through the cores a1 and a2 in this order, the core number of the third core a3 is set to "3", the core number of the fourth core a4 is set to "4", ..., the core number of the nth core an is set to "n".

[0015] In the present embodiment, the configuration is adopted in which the core numbers 1 to n are specified by referring to the marker c, but the structure to be referred to for specifying the core numbers 1 to n is not limited to the marker c. For example, a configuration may be adopted in which the core numbers 1 to n are specified by referring to a mark (e.g., a symbol or a letter) formed on the surface of the cladding b. In addition, when the multi-core fiber MF is covered with a coating, the core numbers 1 to n may be specified by referring to a mark formed on the surface of the coating. In addition, when a connector is attached to the end of the multi-core fiber MF, a configuration may be adopted in which the core numbers 1 to n are specified by referring to a key (e.g., a protrusion) formed on the connector. In addition, for example, when a flat portion is provided on the side surface of the cladding b so that the cross-sectional shape of the cladding b has an anisotropic shape (e.g., a D-shape), the core numbers 1 to n may be specified by referring to the flat portion. In addition, when a notch is provided on the side surface of the cladding b, the core numbers 1 to n may be specified by referring to the notch.

[0016] In the multi-core fiber MF, the first end face σ1 is inclined so as not to be perpendicular to the extending direction of the cores a1 to an. Hereinafter, when a plane perpendicular to the extending direction of the cores a1 to an is regarded as a horizontal plane, a direction in which the downward gradient of the first end face σ1 is maximum is described as the inclination direction v1 of the first end face σ1, and the maximum value of the downward gradient of the first end face σ1 is described as the inclination angle θ1 of the first end face σ1. The inclination angle θ1 of the first end face σ1 is preferably 2° or more and 88° or less, more preferably 4° or more and 12° or less, more preferably 7° or more and 9° or less, and even more preferably 7.8° or more and 8.2° or less. The inclination angle θ1 of the first end face σ1 is, for example, 8° or 6°. Note that the gradient in the non-inclined direction is preferably 0° or more and 2° or less, and more preferably 0° or more and 1° or less.

[0017] In the multi-core fiber MF, the second end face σ2 is inclined so as not to be perpendicular to the extending direction of the cores a1 to an. Hereinafter, when the plane perpendicular to the extending direction of the cores a1 to an is regarded as a horizontal plane, the direction in which the downward gradient of the second end face σ2 is maximum is described as the inclination direction v2 of the second end face σ2, and the maximum value of the downward gradient of the second end face σ2 is described as the inclination angle θ2 of the second end face σ2. The inclination angle θ2 of the second end face σ2 is preferably 2° or more and 88° or less, more preferably 4° or more and 12° or less, more preferably 7° or more and 9° or less, and more preferably 7.8° or more and 8.2° or less. The inclination angle θ2 of the second end face σ2 is, for example, 8° or 6°. The gradient in the non-inclined direction is preferably 0° or more and 2° or less, and more preferably 0° or more and 1° or less.

[0018] The inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so as to satisfy the following condition 1.

[0019] Condition 1: When the first end face σ1 and the second end face σ2 are brought into surface contact so as to minimize the angle between the extension direction of the cores a1-an at the first end face σ1 and the extension direction of the cores a1-an at the second end face σ2, each of the cores a1-an at the first end face σ1 at least partially overlaps with one of the cores a1-an at the second end face σ2.

[0020] In addition, situations in which two cores at least partially overlap include situations in which only a portion of one core overlaps with only a portion of the other core, situations in which only a portion of one core overlaps with the entirety of the other core, and situations in which one core entirely overlaps with the entirety of the other core (i.e., situations in which the two cores overlap each other just enough).

[0021] This enables self-alignment when connecting two multi-core fibers MF. That is, when the first end face σ1 of one multi-core fiber MF and the second end face σ2 of the other multi-core fiber MF are connected so that the two multi-core fibers MF are arranged as straight as possible, it becomes possible to optically couple the cores a1 to an of these two multi-core fibers MF. Here, connecting the first end face σ1 and the second end face σ2 "so that the two multi-core fibers MF are arranged as straight as possible" refers to connecting the first end face σ1 and the second end face σ2 "so that the angle formed between the extending direction of the cores a1 to an at the first end face σ1 of one multi-core fiber MF and the extending direction of the cores a1 to an of the MF at the second end face σ2 of the other multi-core fiber is minimized."

[0022] From this viewpoint, the inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 are preferably 45° or more, more preferably 60° or more, and even more preferably 70° or more. The larger the inclination angles θ1 and θ2 are, the more extreme the bending that occurs at the connection point of the two multi-core fibers MF when the cores a1 to an of the two multi-core fibers MF are not optically coupled, and as a result, self-alignment when connecting the two multi-core fibers MF becomes easier. In addition, the larger the inclination angles θ1 and θ2 are, the more the surface areas of the end faces σ1 and σ2 can be increased, and as a result, the surface contact of the end faces σ1 and σ2 can be made more stable, and the connection of the end faces σ1 and σ2 can be made stronger.

[0023] The inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 may or may not be the same. However, it is preferable that the inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 are equal (substantially equal), and it is more preferable that they are equal (completely equal). Here, the inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 being equal (substantially equal) means, for example, that the difference |θ1-θ2| is 2° or less, or 0.4° or less.

[0024] Also, in each of the first end face σ1 and the second end face σ2, n×(n-1) / 2 straight lines passing through the centers of two cores ai, aj selected from the cores a1 to an are considered. The gradient of each of the first end face σ1 and the second end face σ2 with respect to the direction parallel to these straight lines is preferably 5° or less, and more preferably 2° or less.

[0025] When the inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 are the same, the minimum angle between the extension direction of the cores a1-an at the first end face σ1 and the extension direction of the cores a1-an at the second end face σ2 is 0°. In other words, minimizing the angle between the extension direction of the cores a1-an at the first end face σ1 and the extension direction of the cores a1-an at the second end face σ2 is equivalent to matching the extension direction of the cores a1-an at the first end face σ1 and the extension direction of the cores a1-an at the second end face σ2. Therefore, the above condition 1 is equivalent to the following condition 1'.

[0026] Condition 1': When the first end face σ1 and the second end face σ2 are brought into surface contact so that the extending direction of the cores a1-an at the first end face σ1 coincides with the extending direction of the cores a1-an at the second end face σ2, each of the cores a1-an at the first end face σ1 at least partially overlaps with one of the cores a1-an at the second end face σ2.

[0027] As a result, when two multi-core fibers MF are connected such that the first end face σ1 of one multi-core fiber MF and the second end face σ2 of the other multi-core fiber MF are arranged on a straight line, it becomes possible to optically couple the cores a1 to an of the two multi-core fibers MF. Here, connecting the first end face σ1 and the second end face σ2 "so that the two multi-core fibers MF are arranged on a straight line" refers to connecting the first end face σ1 and the second end face σ2 "so that the extending direction of the cores a1 to an of one multi-core fiber MF and the extending direction of the cores a1 to an of the other multi-core fiber MF coincide with each other." Note that even if the inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 do not strictly coincide with each other, as long as the inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 are equivalent (substantially equivalent), an equivalent effect can be obtained.

[0028] In the multi-core fiber MF according to this embodiment, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so that their projections onto a plane perpendicular to the optical axis L0 are in opposite directions to each other. Therefore, the multi-core fiber MF according to this embodiment satisfies the following condition 2 in addition to the above condition 1.

[0029] Condition 2: When the first end face σ1 and the second end face σ2 are brought into surface contact so as to minimize the angle between the extension direction of the cores a1-an at the first end face σ1 and the extension direction of the cores a1-an at the second end face σ2, the core numbers of the cores that at least partially overlap will match.

[0030] 1, the pairs of cores that at least partially overlap are (1) a pair of core a1 at the first end face σ1 and a pair of core a1 at the second end face σ2, (2) a pair of core a2 at the first end face σ1 and a pair of core a2 at the second end face σ2, (3) a pair of core a3 at the first end face σ1 and a pair of core a3 at the second end face σ2, and (4) a pair of core a4 at the first end face σ1 and a pair of core a4 at the second end face σ2. For any of these four pairs, the core numbers of the two cores that make up the pair are the same.

[0031] As a result, when two multi-core fibers MF are connected such that the first end face σ1 of one multi-core fiber MF and the second end face σ2 of the other multi-core fiber MF are arranged as closely as possible in a straight line, it becomes possible to optically couple cores having the same core number.

[0032] Moreover, in the multi-core fiber MF according to the present embodiment, at the first end face σ1, the cores a1 to an are arranged in line symmetry (substantially line symmetry) with respect to a virtual axis L1 perpendicular to the tilt direction v1. Here, the fact that the cores a1 to an are arranged in line symmetry with respect to the virtual axis L1 means that when the first end face σ1 is inverted with respect to the virtual axis L1, each of the cores a1 to an at least partially overlaps with one of the cores a1 to an. As is clear from the description "at the first end face σ1" at the beginning of this paragraph, the virtual axis L1 is a straight line within the first end face σ1. In this embodiment, the virtual axis L1 passes through the center of the first end face σ1 (the center of the cladding). However, the virtual axis L1 only needs to be perpendicular to the tilt direction v1, and does not need to pass through the center of the first end face σ1.

[0033] As a result, when two multi-core fibers MF are connected such that the first end face σ1 of one multi-core fiber MF and the first end face σ1 of the other multi-core fiber MF are arranged as closely as possible on a straight line, it becomes possible to optically couple the cores a1 to an of these two multi-core fibers MF.

[0034] It is more preferable that the cores a1-an are arranged in line symmetry (complete line symmetry) with respect to a virtual axis L1 perpendicular to the tilt direction v1 on the first end surface σ1. Here, the fact that the cores a1-an are arranged in line symmetry with respect to the virtual axis L1 means that when the first end surface σ1 is inverted with respect to the virtual axis L1, each of the cores a1-an overlaps with one of the cores a1-an without any excess or deficiency. Also, on the first end surface σ1, the cores among the cores a1-an that are paired in line symmetry with respect to the virtual axis L1 are the same distance from the virtual axis L1.

[0035] As a result, when two multi-core fibers MF are connected such that the first end face σ1 of one multi-core fiber MF and the first end face σ1 of the other multi-core fiber MF are arranged as closely as possible on a straight line, it is possible to further improve the coupling efficiency of the cores a1 to an of these two multi-core fibers MF.

[0036] Moreover, in the multi-core fiber MF according to this embodiment, at the first end face σ1, the above-mentioned virtual axis L1 does not intersect with any of the cores a1 to an.

[0037] For this reason, when two multi-core fibers MF are connected such that a first end face σ1 of one multi-core fiber MF and a first end face σ1 of the other multi-core fiber MF are arranged on a straight line as much as possible, cores with different core numbers are connected to each other. In the example shown in Fig. 1, a core a1 of one multi-core fiber MF and a core a2 of the other multi-core fiber MF are connected, and a core a3 of one multi-core fiber MF and a core a4 of the other multi-core fiber MF are connected.

[0038] Moreover, in the multi-core fiber MF according to the present embodiment, at the second end face σ2, the cores a1 to an are arranged line-symmetrically (substantially line-symmetrically) with respect to a virtual axis L2 perpendicular to the tilt direction v2. Here, the fact that the cores a1 to an are arranged line-symmetrically with respect to the virtual axis L2 means that, when the second end face σ2 is inverted with respect to the virtual axis L2, each of the cores a1 to an at least partially overlaps with one of the cores a1 to an. As is clear from the description "at the second end face σ2" at the beginning of this paragraph, the virtual axis L2 is a straight line within the second end face σ2. In this embodiment, the virtual axis L2 passes through the center of the second end face σ2 (the center of the cladding). However, the virtual axis L2 only needs to be perpendicular to the tilt direction v2, and does not need to pass through the center of the second end face σ2.

[0039] As a result, when two multi-core fibers MF are connected such that the second end face σ2 of one multi-core fiber MF and the second end face σ2 of the other multi-core fiber MF are arranged as closely as possible on a straight line, it becomes possible to optically couple the cores a1 to an of these two multi-core fibers MF.

[0040] It is more preferable that the cores a1-an are arranged in line symmetry (complete line symmetry) with respect to the virtual axis L2 perpendicular to the tilt direction v2 on the second end surface σ2. Here, "arranged in line symmetry with respect to the virtual axis L2" means that when the second end surface σ2 is inverted with respect to the virtual axis L2, each of the cores a1-an overlaps with one of the cores a1-an without any overlap. Also, on the second end surface σ2, the cores a1-an that are paired in line symmetry with respect to the virtual axis L1 are equally distant from the virtual axis L2.

[0041] As a result, when two multi-core fibers MF are connected such that the second end face σ2 of one multi-core fiber MF and the second end face σ2 of the other multi-core fiber MF are arranged as closely as possible on a straight line, it is possible to further improve the coupling efficiency of the cores a1 to an of these two multi-core fibers MF.

[0042] Moreover, in the multi-core fiber MF according to this embodiment, at the second end face σ2, the above-mentioned virtual axis L2 does not intersect with any of the cores a1 to an.

[0043] For this reason, when two multi-core fibers MF are connected such that the second end face σ2 of one multi-core fiber MF and the second end face σ2 of the other multi-core fiber MF are arranged as closely as possible on a straight line, cores with different core numbers are connected to each other. In the example shown in Fig. 1, a core a1 of one multi-core fiber MF is connected to a core a2 of the other multi-core fiber MF, and a core a3 of one multi-core fiber MF is connected to a core a4 of the other multi-core fiber MF.

[0044] In the multi-core fiber MF shown in Fig. 1, the core a1 closest to the marker c and the core a2 second closest to the marker c are arranged to sandwich the virtual axis L1 at the first end face σ1. Therefore, the marker c is arranged near the virtual axis L1. This makes it easy to observe the marker c together with the cores a1 to an when observing the first end face σ1 of the multi-core fiber MF from the front using a microscope or the like. This is because, when the focus of the objective lens is set on the virtual axis L1 in order to uniformly suppress the defocus of the cores a1 to an, the defocus of the marker c can be suppressed to be small. The same can be said about the multi-core fibers MF shown in each of Figs. 3 and 5 described later.

[0045] Similarly, in the multi-core fiber MF shown in Fig. 1, at the second end face σ2, the core a1 closest to the marker c and the core a2 second closest to the marker c are arranged to sandwich the virtual axis L2. Therefore, the marker c is arranged in the vicinity of the virtual axis L2. This makes it easy to observe the marker c together with the cores a1 to an when observing the second end face σ2 of the multi-core fiber MF from the front using a microscope or the like. This is because, when the focal point of the objective lens is set on the virtual axis L2 in order to uniformly suppress the defocus of the cores a1 to an, the defocus of the marker c can be suppressed to be small.

[0046] (Modification of multi-core fiber) A first modified example of the multi-core fiber MF according to the first embodiment will be described with reference to Fig. 2. In Fig. 2, (a) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the first modified example. (b) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the second modified example. (c) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the third modified example.

[0047] As shown in Fig. 2(a), in the multi-core fiber MF according to the first modification, the virtual axis L1 does not intersect with any of the cores a1 to an at the first end face σ1, similarly to the multi-core fiber MF shown in Fig. 1. However, while the core a1 and the core a2 are adjacent to each other across the virtual axis L1 at the first end face σ1 in the multi-core fiber MF shown in Fig. 1, the core a1 and the core a4 are adjacent to each other across the virtual axis L1 at the first end face σ1 in the multi-core fiber MF according to the first modification. The same can be said about the second end face σ2.

[0048] For this reason, for two multi-core fibers MF according to the first modified example, when the first end face σ1 of one multi-core fiber MF and the first end face σ1 of the other multi-core fiber MF are connected so that these two multi-core fibers MF are arranged as close to a straight line as possible, for all of the cores a1 to an, the cores with different core numbers are connected to each other. In the example shown in (a) of Fig. 2, the core a1 of one multi-core fiber MF is connected to the core a4 of the other multi-core fiber MF, and the core a2 of one multi-core fiber MF is connected to the core a3 of the other multi-core fiber MF. The same can be said about the second end face σ2.

[0049] As shown in Fig. 2(b), in the multi-core fiber MF according to the second modified example, unlike the multi-core fiber MF shown in Fig. 1, the virtual axis L1 intersects with the core a2 and the core a4 at the first end face σ1. The same can be said about the second end face σ2.

[0050] For this reason, for two multi-core fibers MF according to the second modified example, when the first end face σ1 of one multi-core fiber MF and the first end face σ1 of the other multi-core fiber MF are connected so that these two multi-core fibers MF are arranged on a straight line as much as possible, cores with different core numbers are connected to each other for some of the cores a1 to an. In the example shown in (b) of Fig. 2, the core a1 of one multi-core fiber MF is connected to the core a2 of the other multi-core fiber MF. Note that the core a2 of one multi-core fiber MF is connected to the core a2 of the other multi-core fiber MF, and the core a4 of one multi-core fiber MF is connected to the core a4 of the other multi-core fiber MF. The same can be said about the second end face σ2.

[0051] In the multi-core fiber MF shown in (b) of Fig. 2, the core a2 second closest to the marker c is arranged on the virtual axis L1 at the first end face σ1. Therefore, the marker c is arranged in the vicinity of the virtual axis L1. This makes it easy to observe the marker c together with the cores a1 to an when observing the first end face σ1 of the multi-core fiber MF from the front using a microscope or the like. This is because, when the focus of the objective lens is set on the virtual axis L1 in order to uniformly suppress the defocus of the cores a1 to an, the defocus of the marker c can be suppressed to be small. The same can be said about the multi-core fibers MF shown in each of (b) of Fig. 4 and (b) of Fig. 6 described later.

[0052] As shown in Fig. 2(c), in the multi-core fiber MF according to the third modified example, unlike the multi-core fiber MF shown in Fig. 1, the virtual axis L1 intersects with the core a1 and the core a3 at the first end face σ1. The same can be said about the second end face σ2.

[0053] For this reason, for two multi-core fibers MF according to the second modified example, when the first end face σ1 of one multi-core fiber MF and the first end face σ1 of the other multi-core fiber MF are connected so that these two multi-core fibers MF are arranged on a straight line as much as possible, for some of the cores a1 to an, cores with different core numbers are connected to each other. In the example shown in (c) of Fig. 2, a core a2 of one multi-core fiber MF and a core a4 of the other multi-core fiber MF are connected. Note that the core a1 of one multi-core fiber MF is connected to the core a1 of the other multi-core fiber MF, and the core a3 of one multi-core fiber MF is connected to the core a3 of the other multi-core fiber MF. The same can be said about the second end face σ2.

[0054] In the multi-core fiber MF shown in (c) of Fig. 2, the core a1 closest to the marker c at the first end face σ1 is arranged on the virtual axis L1. Therefore, the marker c is arranged in the vicinity of the virtual axis L1. This makes it easy to observe the marker c together with the cores a1 to an when observing the first end face σ1 of the multi-core fiber MF from the front using a microscope or the like. This is because, when the focus of the objective lens is set on the virtual axis L1 in order to uniformly suppress the defocus of the cores a1 to an, the defocus of the marker c can be suppressed to be small. The same can be said about the multi-core fibers MF shown in each of (c) of Fig. 4 and (c) of Fig. 6 described later.

[0055] Similarly, in the multi-core fiber MF shown in (c) of Fig. 2, at the second end face σ2, the core a1 closest to the marker c is arranged on the virtual axis L2. Therefore, the marker c is arranged in the vicinity of the virtual axis L2. This makes it easy to observe the marker c together with the cores a1 to an when observing the second end face σ2 of the multi-core fiber MF from the front using a microscope or the like. This is because, when the focus of the objective lens is set on the virtual axis L2 in order to uniformly suppress the defocus of the cores a1 to an, the defocus of the marker c can be suppressed to be small.

[0056] Note that the multi-core fibers MF shown in Fig. 1, Fig. 2(b), and Fig. 2(c) have a common feature that "at the first end face σ1, the center of the marker c is included in a region between the virtual axis L1 and a straight line passing through the center of the core farthest from the virtual axis L1 among the cores a1 to an and parallel to the virtual axis L1". Therefore, compared with the multi-core fiber MF shown in Fig. 2(a), when observing the first end face σ1 of the multi-core fiber MF from the front using a microscope or the like, it becomes easier to observe the marker c together with the cores a1 to an. The same can be said about the second end face σ2 of the multi-core fiber MF shown in Fig. 1, Fig. 2(b), and Fig. 2(c). Furthermore, the same can be said about the first end face σ1 of the multi-core fiber MF shown in each of Figs. 3, Fig. 4(b), Fig. 4(c), Fig. 5, Fig. 6(b), Fig. 6(c), Fig. 7, Fig. 8(b), and Fig. 8(c), which will be described later. The same can be said about the second end faces σ2 of the multi-core fibers MF shown in each of Figures 3, 4(b), 4(c), 6(a), 6(b), 6(c), 8(a), 8(b), and 8(c) described below.

[0057] Moreover, the multi-core fiber MF shown in FIG. 1 has a feature that "at the first end face σ1, the center of the marker c is included in a region between the virtual axis L1 and a straight line passing through the center of the core closest to the virtual axis L1 among the cores a1 to an and parallel to the virtual axis L1". Therefore, since the center of the marker c is disposed closer to the virtual axis L1, it becomes easier to observe the marker c together with the cores a1 to an when observing the first end face σ1 of the multi-core fiber MF from the front using a microscope or the like, compared with the multi-core fibers MF shown in FIG. 2(a), FIG. 2(b), and FIG. 2(c). The same can be said about the second end face σ2 of the multi-core fiber MF shown in FIG. 1. Furthermore, the same can be said about the first end face σ1 of the multi-core fiber MF shown in each of FIG. 3, FIG. 5, and FIG. 7 described later. The same can be said about the second end face σ2 of the multi-core fiber MF shown in each of FIG. 3, FIG. 6(a), and FIG. 8(a) described later.

[0058] Second Embodiment (Multicore fiber configuration) A configuration of a multi-core fiber MF according to a second embodiment of the present invention will be described with reference to Fig. 3. In Fig. 3, (a) is a side view of the multi-core fiber MF. Also, (b) is a front view of one end face (hereinafter referred to as "first end face") σ1 of the multi-core fiber MF seen from the line of sight E1. Also, (c) is a front view of the other end face (hereinafter referred to as "second end face") σ2 of the multi-core fiber MF seen from the line of sight E2. Also, (d) is a perspective view of the multi-core fiber MF in a state in which the first end face σ1 and the second end face σ2 are butted together.

[0059] In the multi-core fiber MF according to the first embodiment, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so that their projections onto a plane orthogonal to the optical axis L0 are in opposite directions. In contrast, in the multi-core fiber MF according to the second embodiment, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so that their projections onto a plane orthogonal to the optical axis L0 are in the same direction. As a result, the multi-core fiber MF according to the second embodiment satisfies the following condition 3 in addition to the above condition 1.

[0060] Condition 3: When the first end face σ1 and the second end face σ2 are brought into surface contact so as to minimize the angle between the extension direction of the cores a1-an at the first end face σ1 and the extension direction of the cores a1-an at the second end face σ2, the cores that at least partially overlap have different core numbers.

[0061] 3, the pairs of cores that at least partially overlap are (1) a pair of core a1 at the first end face σ1 and a core a3 at the second end face σ2, (2) a pair of core a2 at the first end face σ1 and a core a4 at the second end face σ2, (3) a pair of core a3 at the first end face σ1 and a core a1 at the second end face σ2, and (4) a pair of core a4 at the first end face σ1 and a core a2 at the second end face σ2. For any of these four pairs, the core numbers of the two cores that make up the pair are different.

[0062] As a result, when two multi-core fibers MF are connected such that the first end face σ1 of one multi-core fiber MF and the second end face σ2 of the other multi-core fiber MF are arranged as closely as possible in a straight line, it becomes possible to optically couple cores having different core numbers.

[0063] (Modification of multi-core fiber) A first modified example of the multi-core fiber MF according to the second embodiment will be described with reference to Fig. 4. In Fig. 4, (a) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the first modified example. (b) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the second modified example. (c) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the third modified example.

[0064] Just as it is possible to configure the three modified examples shown in Fig. 2 for the multi-core fiber MF shown in Fig. 1, it is possible to configure the three modified examples shown in Fig. 4 for the multi-core fiber MF shown in Fig. 3. The effects obtained by the multi-core fiber MF according to each modified example shown in Fig. 4 are similar to the effects obtained by the multi-core fiber MF according to each modified example shown in Fig. 2.

[0065] Third embodiment (Multicore fiber configuration) A configuration of a multi-core fiber MF according to a third embodiment of the present invention will be described with reference to Fig. 5. In Fig. 5, (a) is a side view of the multi-core fiber MF. Also, (b) is a front view of one end face (hereinafter referred to as "first end face") σ1 of the multi-core fiber MF seen from the line of sight E1. Also, (c) is a front view of the other end face (hereinafter referred to as "second end face") σ2 of the multi-core fiber MF seen from the line of sight E2. Also, (d) is a perspective view of the multi-core fiber MF in a state in which the first end face σ1 and the second end face σ2 are butted together.

[0066] In the multi-core fiber MF according to the first embodiment, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so that their projections onto a plane orthogonal to the optical axis L0 are in opposite directions. In contrast, in the multi-core fiber MF according to the third embodiment, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so that their projections onto a plane orthogonal to the optical axis L0 are orthogonal. As a result, the multi-core fiber MF according to the third embodiment satisfies the following condition 3, in addition to the above condition 1.

[0067] Condition 3: When the first end face σ1 and the second end face σ2 are brought into surface contact so as to minimize the angle between the extension direction of the cores a1-an at the first end face σ1 and the extension direction of the cores a1-an at the second end face σ2, the cores that at least partially overlap have different core numbers.

[0068] 5, the pairs of cores that at least partially overlap are (1) a pair of core a1 at the first end face σ1 and a core a2 at the second end face σ2, (2) a pair of core a2 at the first end face σ1 and a core a3 at the second end face σ2, (3) a pair of core a3 at the first end face σ1 and a core a4 at the second end face σ2, and (4) a pair of core a4 at the first end face σ1 and a core a1 at the second end face σ2. For any of these four pairs, the core numbers of the two cores that make up the pair are different.

[0069] As a result, when two multi-core fibers MF are connected such that the first end face σ1 of one multi-core fiber MF and the second end face σ2 of the other multi-core fiber MF are arranged as closely as possible in a straight line, it becomes possible to optically couple cores having different core numbers.

[0070] (Modification of multi-core fiber) Three modified examples of the multi-core fiber MF according to the third embodiment will be described with reference to Fig. 6. In Fig. 6, (a) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the first modified example. (b) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the second modified example. (c) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the third modified example.

[0071] Just as it is possible to configure the three modified examples shown in Fig. 2 for the multi-core fiber MF shown in Fig. 1, it is possible to configure the three modified examples shown in Fig. 6 for the multi-core fiber MF shown in Fig. 5. The effects obtained by the multi-core fiber MF according to each modified example shown in Fig. 6 are similar to the effects obtained by the multi-core fiber MF according to each modified example shown in Fig. 2.

[0072] [Fourth embodiment] (Multicore fiber configuration) A configuration of a multi-core fiber MF according to a fourth embodiment of the present invention will be described with reference to Fig. 7. In Fig. 7, (a) is a side view of the multi-core fiber MF. Also, (b) is a front view of one end face (hereinafter referred to as "first end face") σ1 of the multi-core fiber MF seen from the line of sight E1. Also, (c) is a front view of the other end face (hereinafter referred to as "second end face") σ2 of the multi-core fiber MF seen from the line of sight E2. Also, (d) is a perspective view of the multi-core fiber MF in a state in which the first end face σ1 and the second end face σ2 are butted together.

[0073] In the multi-core fiber MF according to the first embodiment, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so that their projections onto a plane orthogonal to the optical axis L0 are in opposite directions. In contrast, in the multi-core fiber MF according to the fourth embodiment, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so that their projections onto a plane orthogonal to the optical axis L0 are orthogonal. As a result, the multi-core fiber MF according to the fourth embodiment satisfies the following condition 3, in addition to the above condition 1.

[0074] Condition 3: When the first end face σ1 and the second end face σ2 are brought into surface contact so as to minimize the angle between the extension direction of the cores a1-an at the first end face σ1 and the extension direction of the cores a1-an at the second end face σ2, the cores that at least partially overlap have different core numbers.

[0075] 7, the pairs of cores that at least partially overlap are (1) a pair of core a1 at the first end face σ1 and a core a4 at the second end face σ2, (2) a pair of core a2 at the first end face σ1 and a core a1 at the second end face σ2, (3) a pair of core a3 at the first end face σ1 and a core a2 at the second end face σ2, and (4) a pair of core a4 at the first end face σ1 and a core a3 at the second end face σ2. For any of these four pairs, the core numbers of the two cores that make up the pair are different.

[0076] As a result, when two multi-core fibers MF are connected such that the first end face σ1 of one multi-core fiber MF and the second end face σ2 of the other multi-core fiber MF are arranged as closely as possible in a straight line, it becomes possible to optically couple cores having different core numbers.

[0077] (Modification of multi-core fiber) Three modified examples of the multi-core fiber MF according to the fourth embodiment will be described with reference to Fig. 8. In Fig. 8, (a) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the first modified example. (b) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the second modified example. (c) is a front view of the first end face σ1 and the second end face σ2 of the multi-core fiber MF according to the third modified example.

[0078] Just as it is possible to configure the three modified examples shown in Fig. 2 for the multi-core fiber MF shown in Fig. 1, it is possible to configure the three modified examples shown in Fig. 8 for the multi-core fiber MF shown in Fig. 7. The effects obtained by the multi-core fiber MF according to the modified examples shown in Fig. 8 are similar to the effects obtained by the multi-core fiber MF according to the modified examples shown in Fig. 2.

[0079] [Additional Notes] In each of the above-described embodiments, a multi-core fiber MF that does not include a splice point, i.e., a multi-core fiber in which the marker c is continuous from the first end face σ1 to the second end face σ2, is considered, but the present invention is not limited thereto. That is, a multi-core fiber that includes at least one splice point, i.e., a multi-core fiber in which the marker c is not continuous from the first end face σ1 to the second end face σ2, is also included in the scope of the present invention. For example, two multi-core fibers MF are prepared, and the first end face σ1 of one multi-core fiber MF is connected to the first end face σ1 of the other multi-core fiber MF. In this way, a multi-core fiber in which both end faces are the second end face σ2 can be obtained. At this time, the first end face σ1 of each multi-core fiber MF may or may not be inclined. Alternatively, two multi-core fibers MF are prepared, and the second end face σ2 of one multi-core fiber MF is connected to the second end face σ2 of the other multi-core fiber MF. In this way, a multi-core fiber in which both end faces are the first end face σ1 can be obtained. In this case, the second end face σ2 of each multicore fiber MF may or may not be inclined. A multicore fiber obtained in this manner is also included in the scope of the present invention so long as it satisfies the above-mentioned condition 1. In such a multicore fiber, when the first end face σ1 and the second end face σ2 are brought into surface contact so as to minimize the angle formed between the extending direction of the cores a1 to an at the first end face σ1 and the extending direction of the cores a1 to an at the second end face σ2, there may be a form in which, among the combinations of cores that at least partially overlap, some combinations have the same core numbers and the remaining combinations have different core numbers.

[0080] Fifth embodiment (Optical Device Configuration) The configuration of an optical device OD1 according to a fifth embodiment of the present invention will be described with reference to Fig. 9. In Fig. 9, (a) is a side view of the optical device OD1, (b) is a front view of one end face of the optical device OD1 seen from the line of sight E1, and (c) is a front view of the other end face of the optical device OD1 seen from the line of sight E2.

[0081] The optical device OD1 includes a multi-core fiber MF, and single-core connectors C1, C2 provided on both ends of the multi-core fiber MF.

[0082] The multi-core fiber MF may be any of the multi-core fibers MF shown in Fig. 1 to Fig. 8. In Fig. 9, the multi-core fiber MF shown in Fig. 1 is illustrated as an example of the multi-core fiber MF.

[0083] The first single-core connector C1 is provided at one end of the multi-core fiber MF. An end face of the first single-core connector C1 is inclined so as to be flush with the first end face σ1 of the multi-core fiber MF. In addition, a key K1 is provided on the side face of the first single-core connector C1 that is located beyond the inclination direction v1 of the first end face σ1, among the four side faces of the first single-core connector C1. The key K1 is, for example, a rectangular parallelepiped protrusion protruding from the side face of the first single-core connector C1.

[0084] The second single-core connector C2 is provided at the other end of the multi-core fiber MF. The end face of the second single-core connector C2 is inclined so as to be flush with the second end face σ2 of the multi-core fiber MF. In addition, of the four side faces of the second single-core connector C2, a key K2 is provided on the side face at the end of the inclination direction v2 of the second end face σ2. The key K2 is, for example, a rectangular parallelepiped protrusion protruding from the side face of the second single-core connector C2.

[0085] As a result, when two optical devices OD1 are connected such that the first single-core connector C1 of one optical device OD1 and the second single-core connector C2 of the other optical device OD1 are in the opposite positions to the key K1 and the key K2, it becomes possible to optically couple the cores a1 to an of the multi-core fibers MF included in these two optical devices OD1.

[0086] Here, the configuration in which the single-core connectors are provided at both ends of the multi-core fiber MF has been described, but the present invention is not limited thereto. That is, the scope of the present invention also includes a configuration in which a single-core connector is provided at one end of the multi-core fiber MF. That is, the scope of the present invention also includes a configuration in which either the first single-core connector C1 or the second single-core connector C2 is omitted from the optical device OD1 shown in FIG. 9. In this case, the end face of the multi-core fiber MF on the side where the single-core connector is not provided may or may not be inclined. Also, the multi-core fiber MF may be a multi-core fiber 103 constituting an FI / FO (Fan-In / Fan-Out) device FD1, as shown in (a) of FIG. 14. Here, the FI / FO device FD1 includes an optical path changing unit 101 having an optical path changing function, a multi-core fiber 103 provided on one side of the optical path changing unit, and a plurality of single-core fibers 102a to 102d provided on the other side of the optical path changing unit. The number of the single-core fibers 102a to 102d corresponds to the number of cores of the multi-core fiber 103, and is, for example, the same as the number of cores of the multi-core fiber. Furthermore, an end face of the multi-core fiber MF on which the single-core connector is not provided may be connected to the multi-core fiber 103 (sometimes called a pigtail fiber) constituting the FI / FO device FD2, as shown in (b) of Fig. 14. In either case, it is possible to suppress end face reflection caused by the end face of the multi-core fiber MF and the end face of the optical path changing unit 101, and it is possible to facilitate management of port numbers in the optical path changing unit.

[0087] Sixth embodiment (Optical Device Configuration) The configuration of an optical device OD2 according to a sixth embodiment of the present invention will be described with reference to Fig. 10. In Fig. 10, (a) is a side view of the optical device OD2, (b) is a front view of one end face of the optical device OD2 seen from the line of sight E1, and (c) is a front view of the other end face of the optical device OD2 seen from the line of sight E2.

[0088] The optical device OD2 includes a multi-core fiber bundle MFB consisting of a plurality of multi-core fibers MF, and multi-core connectors C3 and C4 provided on both ends of the multi-core fiber bundle MFB.

[0089] In Fig. 10, the multi-core fiber MF shown in Fig. 1 is shown as the multi-core fiber MF constituting the multi-core fiber MFB. However, the multi-core fiber MF constituting the multi-core fiber bundle MFB may be any of the multi-core fibers MF shown in Fig. 1 to Fig. 8. For example, it may be the multi-core fiber MF shown in (b) of Fig. 2, or the multi-core fiber MF shown in (c) of Fig. 2.

[0090] The first multi-core connector C3 is provided at one end of the multi-core fiber bundle MFB. Each multi-core fiber MF constituting the multi-core fiber bundle MFB is fixed to the first multi-core connector C3 so that the inclination direction (in the illustrated example, the first end face σ1) of the end face on the first multi-core connector C3 side (in the illustrated example, the first end face σ1) is aligned in a specific direction and these end faces are flush with each other. In addition, among the four side faces of the first multi-core connector C3, a key K3 is provided on the side face that is ahead of the inclination direction (in the illustrated example, the inclination direction v1) of the end face (in the illustrated example, the first end face σ1) on the first multi-core connector C3 side of each multi-core fiber MF constituting the multi-core fiber bundle MFB. The key K3 is, for example, a rectangular parallelepiped protrusion protruding from the side face of the first multi-core connector C3.

[0091] The second multi-core connector C4 is provided at the other end of the multi-core fiber bundle MFB. Each multi-core fiber MF constituting the multi-core fiber bundle MFB is fixed to the second multi-core connector C4 so that the inclination direction (in the illustrated example, the second end face σ2) of the end face on the second multi-core connector C4 side (in the illustrated example, the second end face σ2) is aligned in a specific direction and these end faces are flush with each other. In addition, among the four side faces of the second multi-core connector C4, a key K4 is provided on the side face that is ahead of the inclination direction (in the illustrated example, the inclination direction v2) of the end face (in the illustrated example, the second end face σ2) of each multi-core fiber MF constituting the multi-core fiber bundle MFB on the second multi-core connector C4 side. The key K4 is, for example, a rectangular parallelepiped protrusion protruding from the side face of the second multi-core connector C4.

[0092] As a result, when two optical devices OD2 are connected such that the first multi-core connector C3 of one optical device OD2 and the second multi-core connector C4 of the other optical device OD2 are in the opposite positions to the key K3 and the key K4, it becomes possible to optically couple the cores a1 to an of each multi-core fiber MF included in these two optical devices OD2.

[0093] Instead of the first multi-core connector C3, a single-core connector may be provided at one end of each multi-core fiber MF, and these single-core connectors may be integrated. Similarly, instead of the second multi-core connector C4, a single-core connector may be provided at the other end of each multi-core fiber MF, and these single-core connectors may be integrated.

[0094] Further, although the configuration in which the multi-core connectors are provided at both ends of the multi-core fiber bundle MFB has been described here, the present invention is not limited thereto. That is, a configuration in which a multi-core connector is provided at one end of the multi-core fiber bundle MFB is also included in the scope of the present invention. That is, a configuration in which either the first multi-core connector C3 or the second multi-core connector C4 is omitted from the optical device OD2 shown in FIG. 10 is also included in the scope of the present invention. In this case, the end face of each multi-core fiber MF on the side where the multi-core connector is not provided may or may not be inclined. Moreover, each multi-core fiber MF may be a multi-core fiber 103 constituting an FI / FO (Fan-In / Fan-Out) device FD1 as shown in (a) of FIG. 14. Moreover, the end face of each multi-core fiber MF on the side where the multi-core connector is not provided may be connected to a multi-core fiber 103 (sometimes called a pigtail fiber) constituting an FI / FO (Fan-In / Fan-Out) device FD2 as shown in (b) of FIG. 14.

[0095] In the first multi-core connector C3, the arrangement direction of the multi-core fibers MF constituting the multi-core fiber bundle MFB is arbitrary. However, the arrangement direction of the multi-core fiber bundle MFB is preferably a direction perpendicular to the inclination direction of the end face of the first multi-core connector C3 (i.e., the inclination direction v1 of the end face σ1 of the multi-core fiber MF) as shown in (b) of Fig. 10. In particular, when the multi-core fiber bundles MFB are arranged in a row, it is more preferable that the arrangement direction is perpendicular to the inclination direction of the end face of the first multi-core connector C3. Alternatively, it is preferable that the arrangement direction is parallel to the inclination direction of the first multi-core connector C3. This is because, when connecting two first multi-core connectors C3 or connecting the first multi-core connector C3 to the second multi-core connector C4, the connectors are mated so that the inclination directions of the end faces match, thereby enabling the rotational alignment of the multi-core fibers MF. The same can be said about the second multi-core connector C4.

[0096] (Modification of Optical Device) Modified examples of the optical device OD2 will be described with reference to Fig. 11. In Fig. 11, (a) is a front view of the optical device OD2 according to a first modified example. In Fig. 11, (b) is a front view of the optical device OD2 according to a second modified example. In Fig. 11, (c) is a front view of the optical device OD2 according to a third modified example.

[0097] In the optical device OD2 according to the first modified example, as shown in (a) of FIG. 11, the multi-core fiber bundle MFB includes at least two multi-core fibers MF in which cores having the same core numbers at least partially overlap each other when the end face on the first multi-core connector C3 side is translated and inverted. (a) of FIG. 11 illustrates that the leftmost multi-core fiber MF and the second multi-core fiber MF from the left satisfy this relationship. This allows the same core numbers to be swapped in the at least two multi-core fibers MF when the first multi-core connectors C3 are connected to each other. In the example illustrated in (a) of FIG. 11, in the leftmost multi-core fiber MF and the second multi-core fiber MF from the left, the core a1 and the core a2 are connected (core number 1 and core number 2 are swapped), and the core a3 and the core a4 are connected (core number 3 and core number 4 are swapped). This also makes it easier to manage the core numbers of the multi-core fibers when two multi-core fibers MF are connected to each other when building a network.

[0098] The above-mentioned inversion may be an inversion about an axis parallel to the tilt direction of each multi-core fiber MF, or an inversion about an axis perpendicular to the tilt direction of each multi-core fiber MF. In either case, the above-mentioned effect is achieved. In addition, the above-mentioned parallel movement may be a parallel movement in a direction parallel to the tilt direction of the end face of the first multi-core connector C3, or a parallel movement in a direction perpendicular to the tilt direction of the end face of the first multi-core connector C3. In the former case, the above-mentioned at least two multi-core fibers MF are arranged parallel to the tilt direction of the end face of the first multi-core connector C3. On the other hand, in the latter case, the above-mentioned at least two multi-core fibers MF are arranged perpendicular to the tilt direction of the end face of the first multi-core connector C3. In either case, the above-mentioned effect is achieved. In addition, particularly in the latter case, the distances from a virtual axis perpendicular to the tilt direction of the end face of the first multi-core connector C3 to the centers of the markers c of the above-mentioned at least two multi-core fibers MF are equal. As a result, when observing the end face of the multi-core connector C3 from the front using a microscope or the like, it becomes easy to simultaneously observe the markers c of at least two of the multi-core fibers MF described above by adjusting the focus onto this virtual axis.

[0099] It is preferable that all the multi-core fibers MF constituting the multi-core fiber bundle MFB satisfy the above-mentioned translation-reversal relationship. This makes it possible to cause the same core number replacement in all the multi-core fibers MF constituting the multi-core fiber bundle MFB. In particular, when the above-mentioned parallel movement is a parallel movement in a direction perpendicular to the tilt direction of the end face of the first multi-core connector C3, it becomes easy to simultaneously observe the markers c of all the multi-core fibers MF constituting the multi-core fiber bundle MFB.

[0100] In the optical device OD2 according to the second modification, as shown in (b) of FIG. 11, the multi-core fiber bundle MFB includes at least two multi-core fibers MF in which cores having the same core numbers at least partially overlap each other when the end face on the first multi-core connector C3 side is translated and rotated. (b) of FIG. 11 illustrates that the leftmost multi-core fiber MF and the second multi-core fiber MF from the left satisfy this relationship. This allows different core numbers to be swapped in the at least two multi-core fibers MF when the first multi-core connectors C3 are connected to each other. In the example illustrated in (b) of FIG. 11, in the leftmost multi-core fiber MF, the core a1 and the core a2 are connected (core number 1 and core number 2 are swapped), and the core a3 and the core a4 are connected (core number 3 and core number 4 are swapped). On the other hand, in the second multicore fiber MF from the left, the core a1 and the core a4 are connected (core number 1 and core number 4 are swapped), and the core a2 and the core a3 are connected (core number 2 and core number 3 are swapped). This can improve the degree of freedom in wiring when constructing a network using the optical device OD2.

[0101] Note that, as the above-mentioned rotation, when the arrangement of the cores a1 to an of the multi-core fiber MF has n-fold symmetry, a rotation of m×360° / n (m is a natural number not less than 1 and not more than n−1) is possible. For example, when the arrangement of the cores a1 to a4 of the multi-core fiber MF has 4-fold symmetry, a rotation of 90°, a rotation of 180°, and a rotation of 270° are possible.

[0102] In the optical device OD2 according to the third modified example, as shown in (c) of Fig. 11, the multi-core fiber bundle MFB is composed of a first multi-core fiber group MFB1 and a second multi-core fiber group MFB2. The first multi-core fiber group MFB1 is a collection of a plurality of (four in the illustrated example) multi-core fibers MF whose end faces on the first multi-core connector C3 side are arranged along the axis L on one side of the axis L. The second multi-core fiber group MFB2 is a collection of a plurality of (four in the illustrated example) multi-core fibers MF whose end faces on the first multi-core connector C3 side are arranged along the axis L on the other side of the axis L. Here, the axis L is an axis perpendicular to the inclination direction of the end faces of the multi-core fibers MF on the first multi-core connector C3 side.

[0103] Each of the multi-core fibers MF constituting the first multi-core fiber group MFB1 and each of the multi-core fibers MF constituting the second multi-core fiber group MFB2 are rotationally aligned such that the distance from the marker c to the axis L is shorter than the distance from the cores a1 to an to the axis L. For this reason, in each of the multi-core fibers MF, the marker c is disposed in the vicinity of the axis L. When observing the end face on the first multi-core connector C3 side from the front using a microscope or the like, the focus of the objective lens is set on the axis L. At this time, since the marker c is disposed in the vicinity of the axis L, observation of the marker c becomes easy.

[0104] In the first multi-core connector C3, the arrangement of the multiple multi-core fibers MF constituting the multi-core fiber bundle MFB may be arbitrary. However, when the multi-core fibers MF are arranged in a matrix with the direction parallel to the tilt direction of the end face of the first multi-core connector C3 as the row direction and the direction perpendicular to the tilt direction of the end face of the first multi-core connector C3 as the column direction, it is preferable that the number of rows is smaller than the number of columns. This is because, when observing the end face of the first multi-core connector C3 from the front using a microscope or the like, it becomes easier to observe the markers c of each multi-core fiber MF.

[0105] Seventh embodiment A multi-core fiber assembly MFS according to a seventh embodiment of the present invention will be described with reference to Fig. 12. In Fig. 12, (a) is a side view of the multi-core fiber assembly MFS, and (b) is a perspective view of the multi-core fiber assembly MFS.

[0106] The multicore fiber assembly MFS includes a first multicore fiber MF1 and a second multicore fiber MF2. The first multicore fiber MF1 and the second multicore fiber MF2 may or may not be connected to each other. When the first multicore fiber MF1 and the second multicore fiber MF2 are connected to each other, a first end face Σ1 (to be described later) and a second end face Σ2 (to be described later) are connected (for example, by a connector connection or fusion splicing).

[0107] The first multi-core fiber MF1 is configured similarly to the multi-core fiber MF shown in Fig. 1. However, in the first multi-core fiber MF1, it is essential that the first end face σ1 is inclined, but it is not essential that the second end face σ2 is inclined. Hereinafter, the end face of the first multi-core fiber MF1 that is essential to be inclined, i.e., the end face corresponding to the first end face σ1 of the multi-core fiber MF shown in Fig. 1, will be referred to as the first end face Σ1.

[0108] The second multi-core fiber MF2 is configured similarly to the multi-core fiber MF shown in Fig. 1. However, in the second multi-core fiber MF2, it is essential that the second end face σ2 is inclined, but it is not essential that the first end face σ1 is inclined. Hereinafter, the end face of the second multi-core fiber MF2 that is essential to be inclined, i.e., the end face corresponding to the second end face σ2 of the multi-core fiber MF shown in Fig. 1, will be referred to as the second end face Σ2.

[0109] The inclination direction v1 of the first end face Σ1 and the inclination direction v2 of the second end face Σ2 are determined so as to satisfy the following condition 1.

[0110] Condition 1: When the first end face Σ1 and the second end face Σ2 are brought into surface contact so as to minimize the angle between the extension direction of the cores a1-an at the first end face Σ1 and the extension direction of the cores a1-an at the second end face Σ2, each of the cores a1-an at the first end face Σ1 at least partially overlaps with one of the cores a1-an at the second end face Σ2.

[0111] As a result, when the first end face Σ1 of the first multi-core fiber MF1 and the second end face Σ2 of the second multi-core fiber MF2 are connected so that the first multi-core fiber MF1 and the second multi-core fiber MF2 are arranged as close to a straight line as possible, it becomes possible to optically couple the cores a1-an of the first multi-core fiber MF1 and the cores a1-an of the second multi-core fiber MF2. Here, connecting the first end face Σ1 and the second end face Σ2 "so that the first multi-core fiber MF1 and the second multi-core fiber MF2 are arranged as close to a straight line as possible" refers to connecting the first end face Σ1 and the second end face Σ2 "so that the angle formed between the extending direction of the cores a1-an of the first multi-core fiber MF1 and the extending direction of the cores a1-an of the second multi-core fiber MF2 is minimized."

[0112] The inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 may or may not be the same. However, it is preferable that the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 are equal (substantially equal), and it is more preferable that they are equal (completely equal). Here, the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 being equal (substantially equal) means, for example, that the difference |θ1-θ2| is 2° or less, or 0.4° or less.

[0113] When the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 are the same, the minimum angle between the extension direction of the cores a1-an at the first end face Σ1 and the extension direction of the cores a1-an at the second end face Σ2 is 0°. In other words, minimizing the angle between the extension direction of the cores a1-an at the first end face Σ1 and the extension direction of the cores a1-an at the second end face Σ2 is equivalent to matching the extension direction of the cores a1-an at the first end face Σ1 and the extension direction of the cores a1-an at the second end face Σ2. Therefore, the above condition 1 is equivalent to the following condition 1'.

[0114] Condition 1': When the first end face Σ1 and the second end face Σ2 are brought into surface contact so that the extension direction of the cores a1-an at the first end face Σ1 coincides with the extension direction of the cores a1-an at the second end face Σ2, each of the cores a1-an at the first end face Σ1 at least partially overlaps with one of the cores a1-an at the second end face Σ2.

[0115] As a result, when the first end face Σ1 of the first multi-core fiber MF1 and the second end face Σ2 of the second multi-core fiber MF2 are connected so that the first multi-core fiber MF1 and the second multi-core fiber MF2 are arranged on a straight line, it becomes possible to optically couple the cores a1-an of the first multi-core fiber MF1 and the cores a1-an of the second multi-core fiber MF2. Here, connecting the first end face Σ1 and the second end face Σ2 "so that the first multi-core fiber MF1 and the second multi-core fiber MF2 are arranged on a straight line" refers to connecting the first end face Σ1 and the second end face Σ2 "so that the extending direction of the cores a1-an of the first multi-core fiber MF1 and the extending direction of the cores a1-an of the second multi-core fiber MF2 coincide with each other". Even if the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 do not strictly match, as long as the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 are equivalent (substantially match), a similar effect can be obtained.

[0116] In the multi-core fiber assembly MFS of this embodiment, the inclination direction v1 of the first end face Σ1 of the first multi-core fiber MF1 and the inclination direction v2 of the second end face Σ2 of the second multi-core fiber MF2 are determined so as to satisfy the following condition 2, in addition to the above condition 1.

[0117] Condition 2: When the first end face Σ1 and the second end face Σ2 are brought into surface contact so as to minimize the angle between the extension direction of the cores a1-an at the first end face Σ1 and the extension direction of the cores a1-an at the second end face Σ2, the core numbers of the cores that at least partially overlap will match.

[0118] 12, the pairs of cores that at least partially overlap are (1) a pair of core a1 at the first end face Σ1 and a core a1 at the second end face Σ2, (2) a pair of core a2 at the first end face Σ1 and a core a2 at the second end face Σ2, (3) a pair of core a3 at the first end face Σ1 and a core a3 at the second end face Σ2, and (4) a pair of core a4 at the first end face Σ1 and a core a4 at the second end face Σ2. For any of these four pairs, the core numbers of the two cores that make up the pair match.

[0119] As a result, when the first end face Σ1 of the first multi-core fiber MF1 and the second end face Σ2 of the second multi-core fiber MF2 are connected so that the first multi-core fiber MF1 and the second multi-core fiber MF2 are arranged as closely as possible in a straight line, it becomes possible to optically couple cores having the same core number.

[0120] Note that a first single-core connector C1 may be provided at an end portion on the first end face Σ1 side of the first multi-core fiber MF1 as shown in (c) of Fig. 12. Similarly, a second single-core connector C2 may be provided at an end portion on the second end face Σ2 side of the second multi-core fiber MF2 as shown in (c) of Fig. 12. The configurations of the first single-core connector C1 and the second single-core connector C2 have been described with reference to Fig. 9, and therefore will not be repeated here.

[0121] In this embodiment, a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the midpoint between the core a4 and the core a1 is used as the first multi-core fiber MF1, but the first multi-core fiber MF1 is not limited to this. (1) A multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the midpoint between the core a1 and the core a2, (2) A multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the midpoint between the core a2 and the core a3, or (3) A multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the midpoint between the core a3 and the core a4 can also be used as the first multi-core fiber MF1. In addition, (1) a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the core a1, (2) a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the core a2, (3) a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the core a3, and (4) a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the core a4 can also be used as the first multi-core fiber MF1. The same can be said for the second multi-core fiber MF2.

[0122] Eighth embodiment A multi-core fiber assembly MFS according to a seventh embodiment of the present invention will be described with reference to Fig. 13. In Fig. 13, (a) is a side view of the multi-core fiber assembly MFS, and (b) is a perspective view of the multi-core fiber assembly MFS.

[0123] The multicore fiber assembly MFS includes a first multicore fiber MF1 and a second multicore fiber MF2. The first multicore fiber MF1 and the second multicore fiber MF2 may or may not be connected to each other. When the first multicore fiber MF1 and the second multicore fiber MF2 are connected to each other, a first end face Σ1 described later and a second end face Σ2 described later are connected (for example, by fusion splicing).

[0124] The first multi-core fiber MF1 is configured similarly to the multi-core fiber MF shown in Fig. 1. However, in the first multi-core fiber MF1, it is essential that the first end face σ1 is inclined, but it is not essential that the second end face σ2 is inclined. The end face of the first multi-core fiber MF1 that is essential to be inclined, i.e., the end face corresponding to the first end face σ1 of the multi-core fiber MF shown in Fig. 1, is referred to as the first end face Σ1.

[0125] The second multi-core fiber MF2 is configured similarly to the multi-core fiber MF shown in Fig. 1. However, in the second multi-core fiber MF2, it is essential that the first end face σ1 is inclined, but it is not essential that the second end face σ2 is inclined. The end face of the second multi-core fiber MF2 that is essential to be inclined, i.e., the end face corresponding to the first end face σ1 of the multi-core fiber MF shown in Fig. 1, is called the second end face Σ2.

[0126] In the multi-core fiber assembly MFS according to the sixth embodiment, the inclination direction v1 of the first end face Σ1 and the inclination direction v2 of the second end face Σ2 are determined so as to satisfy the above condition 2 in addition to the above condition 1. On the other hand, in the multi-core fiber assembly MFS according to the seventh embodiment, the inclination direction v1 of the first end face Σ1 and the inclination direction v2 of the second end face Σ2 are determined so as to satisfy the following condition 3 in addition to the above condition 1.

[0127] Condition 3: When the first end face Σ1 and the second end face Σ2 are brought into surface contact so as to minimize the angle between the extension direction of the cores a1-an at the first end face Σ1 and the extension direction of the cores a1-an at the second end face Σ2, the core numbers of the cores that at least partially overlap are different.

[0128] 13, the pairs of cores that at least partially overlap are (1) a pair of core a1 at the first end face Σ1 and a core a2 at the second end face Σ2, (2) a pair of core a2 at the first end face Σ1 and a core a1 at the second end face Σ2, (3) a pair of core a3 at the first end face Σ1 and a core a4 at the second end face Σ2, and (4) a pair of core a4 at the first end face Σ1 and a core a3 at the second end face Σ2. For any of these four pairs, the core numbers of the two cores that make up the pair are different.

[0129] As a result, when the first end face Σ1 of the first multi-core fiber MF1 and the second end face Σ2 of the second multi-core fiber MF2 are connected so that the first multi-core fiber MF1 and the second multi-core fiber MF2 are arranged as closely as possible in a straight line, it becomes possible to optically couple cores having different core numbers.

[0130] Note that a first single-core connector C1 may be provided at an end portion on the first end face Σ1 side of the first multi-core fiber MF1 as shown in (c) of Fig. 13. Similarly, a second single-core connector C2 may be provided at an end portion on the second end face Σ2 side of the second multi-core fiber MF2 as shown in (c) of Fig. 13. The configurations of the first single-core connector C1 and the second single-core connector C2 have been described with reference to Fig. 9, and therefore will not be repeated here.

[0131] In this embodiment, a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the midpoint between the core a4 and the core a1 is used as the first multi-core fiber MF1, but the first multi-core fiber MF1 is not limited to this. (1) A multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the midpoint between the core a1 and the core a2, (2) A multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the midpoint between the core a2 and the core a3, or (3) A multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the midpoint between the core a3 and the core a4 can also be used as the first multi-core fiber MF1. In addition, (1) a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the core a1, (2) a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the core a2, (3) a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the core a3, and (4) a multi-core fiber in which the inclination direction v1 is from the center of the first end face Σ1 toward the core a4 can also be used as the first multi-core fiber MF1. The same can be said for the second multi-core fiber MF2.

[0132] 〔summary〕 A multicore fiber according to a first aspect of the present invention is a multicore fiber having a cladding, a plurality of cores formed in the cladding, and first and second end faces inclined so as not to be perpendicular to an extending direction of the plurality of cores, wherein the inclination directions of the first end face and the second end face are set such that, when the first end face and the second end face are brought into surface contact so as to minimize an angle formed between the extending direction of the plurality of cores at the first end face and the extending direction of the plurality of cores at the second end face, each of the plurality of cores at the first end face at least partially overlaps with any of the plurality of cores at the second end face.

[0133] A multicore fiber according to a second aspect of the present invention employs, in addition to the configuration of the first aspect, a configuration in which the inclination angle of the first end facet and the inclination angle of the second end facet are equal to each other.

[0134] A multicore fiber according to a third aspect of the present invention has, in addition to the configuration of the first or second aspect, a configuration in which core numbers of the plurality of cores can be identified at the first end face and the second end face, and when the first end face and the second end face are brought into surface contact so as to minimize the angle, the core numbers of at least partially overlapping cores match.

[0135] A multicore fiber according to a fourth aspect of the present invention employs, in addition to the configuration of the first or second aspect, a configuration in which core numbers of the plurality of cores can be identified at the first end face and the second end face, and the core numbers of at least partially overlapping cores are different when the first end face and the second end face are brought into surface contact so as to minimize the angle.

[0136] A multicore fiber according to a fifth aspect of the present invention has, in addition to the configuration of any one of the first to fourth aspects, a configuration in which the multiple cores at the first end face and the second end face are arranged line-symmetrically with respect to a virtual axis that is perpendicular to the tilt direction and passes through the center of the cladding.

[0137] A multicore fiber according to a sixth aspect of the present invention employs, in addition to the configuration of the fifth aspect, a configuration in which the virtual axis does not intersect with any of the multiple cores at the first end face and the second end face.

[0138] A multicore fiber according to a seventh aspect of the present invention has, in addition to the configuration of the fifth or sixth aspect, a configuration in which the multicore fiber further has a marker formed in the cladding, and at each of the first end face and the second end face, the center of the marker is included between the virtual axis and a straight line that passes through the center of the core that is farthest from the virtual axis among the multiple cores and is parallel to the virtual axis.

[0139] An optical device according to an eighth aspect of the present invention has a configuration including a multicore fiber and a single-core connector provided at one end of the multicore fiber, and the multicore fiber is the multicore fiber according to any one of the first to seventh aspects.

[0140] An optical device according to a ninth aspect of the present invention includes a multicore fiber bundle comprising a plurality of multicore fibers bundled together, and a multicore connector or an integrated single-core connector group provided at one end of the multicore fiber bundle, each of the plurality of multicore fibers being a multicore fiber according to any of aspects 1 to 7, and being fixed to the multicore connector or the single-core connector group such that the inclination directions of the connector-side end faces, which are the end faces of the first end face and the second end face on the side where the multicore connector or the single-core connector group is provided, are aligned in a specific direction.

[0141] An optical device according to a tenth aspect of the present invention employs, in addition to the configuration of the optical device according to the ninth aspect, a configuration in which each of the plurality of multi-core fibers is capable of identifying the core numbers of the plurality of cores at the connector side end face, and the plurality of multi-core fibers include multi-core fibers in which cores having the same core numbers at least partially overlap each other when the connector side end face is translated and inverted.

[0142] An optical device according to an eleventh aspect of the present invention has the same configuration as the optical device according to the tenth aspect, and further has a configuration in which the inversion is an inversion about a virtual axis parallel to the tilt direction or a virtual axis perpendicular to the tilt direction.

[0143] An optical device according to a twelfth aspect of the present invention employs, in addition to the configuration of the optical device according to the ninth aspect, a configuration in which each of the plurality of multi-core fibers is capable of identifying the core numbers of the plurality of cores at the connector side end face, and the plurality of multi-core fibers include multi-core fibers in which cores having the same core numbers at least partially overlap when the connector side end face is translated and rotated.

[0144] An optical device according to a thirteenth aspect of the present invention employs, in addition to the configuration of any one of the tenth to twelfth aspects, a configuration in which each of the plurality of multicore fibers includes a marker formed in the clad, and the plurality of multicore fibers are configured to include a first multicore fiber group whose connector side end faces are aligned along a virtual axis perpendicular to the specific direction, the first multicore fiber group having the connector side end faces arranged on one side of the virtual axis, and a second multicore fiber group whose connector side end faces are aligned along the virtual axis, the second multicore fiber group having the connector side end faces arranged on the other side of the virtual axis, and the distance from the marker to the virtual axis at the connector side end face of each of the plurality of multicore fibers is shorter than the distance from each of the plurality of cores to the virtual axis.

[0145] An optical device according to a fourteenth aspect of the present invention further includes an FI / FO device having an optical path changing unit and a multicore fiber, and the multicore fiber is the multicore fiber according to any one of the eighth to thirteenth aspects, or the multicore fiber is connected to the multicore fiber according to any one of the eighth to thirteenth aspects.

[0146] A multi-core fiber assemblies according to a fifteenth aspect of the present invention is a multi-core fiber assemblies including at least one first multi-core fiber having a cladding, a plurality of cores formed in the cladding, and a first end face inclined so as not to be perpendicular to the extending direction of each core, and at least one second multi-core fiber having a cladding, a plurality of cores formed in the cladding, and a second end face inclined so as not to be perpendicular to the extending direction of each core, wherein the inclination directions of the first end face and the second end face are set such that, when the first end face and the second end face are brought into surface contact so as to minimize an angle formed between the extending direction of the plurality of cores at the first end face and the extending direction of the plurality of cores at the second end face, each of the plurality of cores at the first end face at least partially overlaps with any of the plurality of cores at the second end face. A multicore fiber according to a sixteenth aspect of the present invention is a multicore fiber having a cladding, a plurality of cores formed in the cladding, a marker formed in the cladding, and a first end face and a second end face inclined so as not to be perpendicular to an extending direction of the plurality of cores, wherein when the first end face and the second end face are brought into surface contact so as to minimize an angle formed between the extending direction of the plurality of cores at the first end face and the extending direction of the plurality of cores at the second end face, each of the plurality of cores at the first end face is inclined toward the front end of the second end face. the inclination direction of the first end face and the second end face is set so as to at least partially overlap with any of the multiple cores, the multiple cores are arranged in line symmetry with respect to a virtual axis perpendicular to the inclination direction on the first end face and the second end face, the virtual axis does not intersect with any of the multiple cores on the first end face and the second end face, and the center of the marker on each of the first end face and the second end face is included between the virtual axis and a straight line parallel to the virtual axis that passes through a center of the core that is farthest from the virtual axis among the multiple cores. A multicore fiber according to a seventeenth aspect of the present invention is a multicore fiber having a cladding, a plurality of cores formed in the cladding, a marker formed in the cladding, and a first end face and a second end face inclined so as not to be perpendicular to an extending direction of the plurality of cores, wherein when the first end face and the second end face are brought into surface contact with each other so as to minimize an angle formed between the extending direction of the plurality of cores at the first end face and the extending direction of the plurality of cores at the second end face, each of the plurality of cores at the first end face is inclined relative to the extending direction of the plurality of cores at the second end face. a direction of inclination of the first end face and the second end face is set so that the cores at least partially overlap one of a number of cores, the multiple cores are arranged in line symmetry with respect to a virtual axis perpendicular to the direction of inclination at the first end face and the second end face, the virtual axis does not intersect with any of the multiple cores at the first end face and the second end face, and at each of the first end face and the second end face, a core of the multiple cores that is closest to the marker and a core of the multiple cores that is second closest to the marker are arranged to sandwich the virtual axis. A multicore fiber according to an eighteenth aspect of the present invention is the multicore fiber according to the seventeenth or eighteenth aspect, wherein an outer diameter of the marker is smaller than an outer diameter of the multiple cores. A multi-core fiber according to aspect 19 of the present invention is the multi-core fiber according to aspect 17 or 18 above, wherein the inclination direction of the first end face and the second end face is parallel to one of the straight lines passing through two cores selected from the plurality of cores. An optical device according to aspect 20 of the present invention comprises a multicore fiber and a single-core connector provided at one end of the multicore fiber, the multicore fiber being the multicore fiber according to either aspect 17 or 18 above. An optical device according to aspect 21 of the present invention comprises a multicore fiber bundle comprising a plurality of multicore fibers bundled together, and a multicore connector or an integrated group of single-core connectors provided at one end of the multicore fiber bundle, each of the plurality of multicore fibers being the multicore fiber according to either aspect 17 or 18 above, and being fixed to the multicore connector or the group of single-core connectors such that the inclination directions of the connector-side end faces, which are the end faces of the first end face and the second end face on which the multicore connector or the group of single-core connectors is provided, are aligned in a specific direction.

[0147] [Additional Notes] The present invention is not limited to the above-mentioned embodiment, and various modifications are possible within the scope of the claims. The technical scope of the present invention also includes embodiments obtained by appropriately combining the technical means disclosed in the above-mentioned embodiment. For example, the multicore fiber may be twisted. Regardless of whether it is twisted or not, a multicore fiber that satisfies the conditions described in the claims is included in the technical scope of the present invention. In addition, the shape of the connector is arbitrary. For example, an optical device having a connector of a type in which a multicore fiber is inserted and fixed in a fiber hole, or an optical device having a connector of a type in which a multicore fiber is accommodated and fixed in a V-groove is also included in the technical category of the present invention. In addition, the end face of the multicore fiber may be a flat surface, a curved surface that can be approximated by a flat surface (for example, a convex spherical surface or a concave spherical surface), or a curved surface that can be approximated by data of a virtual end face obtained by subtracting data of a virtual end face when the end face is fitted from data of an end face in the presence of surface roughness.

[0148] Furthermore, in addition to the above definition, the above-mentioned virtual axis may also be defined as follows, for example: That is, at the end face of a multicore fiber or the end face of a multicore fiber provided in an optical device, when the vector component in the above-mentioned tilt direction is divided into an X-axis direction component and a Y-axis direction component, a virtual axis extending in a direction perpendicular to the direction of the component with a larger scalar quantity may be defined as the virtual axis.

[0149] In addition, when an optical device has a prescribed structure capable of prescribing a certain direction and a direction perpendicular to the certain direction, a virtual axis that can be determined based on the prescribed structure can be defined as a virtual axis. Here, the above-mentioned prescribed structure can be, for example, two guide holes, two guide pins, or one guide hole and one guide pin provided on an end face of the optical device. In this case, a virtual axis passing through the centers of two guide holes, the centers of two guide pins, or the center of one guide hole and the center of one guide pin, or a virtual axis perpendicular to the virtual axis can be defined as a virtual axis. In addition, the above-mentioned prescribed structure can be, for example, two multi-core fibers exposed from an end face of the optical device. In this case, a virtual axis passing through the centers of two multi-core fibers, or a virtual axis perpendicular to the virtual axis can be defined as a virtual axis. In addition, the above-mentioned prescribed structure can be, for example, a ferrule, flange, array, housing, etc. having an outer shape that is polygonal, concave, convex, or anisotropic (barrel shape, shape with a notch, etc.) as an optical device. In this case, a virtual axis extending in a specific direction that can be determined based on the above-mentioned outer shape or a virtual axis perpendicular to the virtual axis can be defined as the virtual axis. [Explanation of symbols]

[0150] MF multi-core fiber a1~an core b Clad c Marker σ1 1st end face σ2 2nd end face v1,v2 Tilt direction θ1,θ2 Tilt angle OD1,OD2 Optical Device C1, C2 single-core connector C3, C4 multi-core connector MFB Multi-core fiber bundle MFS Multicore Fiber Assemble

Claims

1. A multi-core fiber having a cladding, a plurality of cores formed in the cladding, a marker formed in the cladding, and a first end face and a second end face inclined so as not to be perpendicular to an extending direction of the plurality of cores, inclination directions of the first end face and the second end face are set such that, when the first end face and the second end face are brought into surface contact with each other so as to minimize an angle between an extension direction of the plurality of cores in the first end face and an extension direction of the plurality of cores in the second end face, each of the plurality of cores in the first end face at least partially overlaps with any of the plurality of cores in the second end face; the plurality of cores are arranged in line symmetry with respect to a virtual axis perpendicular to the tilt direction on the first end surface and the second end surface, The virtual axis does not intersect with any of the plurality of cores at the first end surface and the second end surface, In each of the first end surface and the second end surface, the center of the marker is aligned with the virtual axis, and a straight line passing through a center of the core farthest from the virtual axis among the plurality of cores and parallel to the virtual axis. Multicore fiber.

2. A multi-core fiber having a cladding, a plurality of cores formed in the cladding, a marker formed in the cladding, and a first end face and a second end face inclined so as not to be perpendicular to an extending direction of the plurality of cores, inclination directions of the first end face and the second end face are set such that, when the first end face and the second end face are brought into surface contact with each other so as to minimize an angle between an extension direction of the plurality of cores in the first end face and an extension direction of the plurality of cores in the second end face, each of the plurality of cores in the first end face at least partially overlaps with any of the plurality of cores in the second end face; the plurality of cores are arranged in line symmetry with respect to a virtual axis perpendicular to the tilt direction on the first end surface and the second end surface, The virtual axis does not intersect with any of the plurality of cores at the first end surface and the second end surface, In each of the first end surface and the second end surface, a core among the plurality of cores that is closest to the marker and a core among the plurality of cores that is second closest to the marker are disposed to sandwich the virtual axis. Multicore fiber.

3. The outer diameter of the marker is smaller than the outer diameter of the cores. The multicore fiber according to claim 1 or 2.

4. a tilt direction of the first end surface and the second end surface is parallel to any one of straight lines passing through two cores selected from the plurality of cores; The multicore fiber according to claim 1 or 2.

5. A multi-core fiber and a single-core connector provided at one end of the multi-core fiber; It is equipped with The multi-core fiber is the multi-core fiber according to claim 1 or 2. An optical device comprising:

6. The present invention comprises a multi-core fiber bundle in which a plurality of multi-core fibers are bundled together, and a multi-core connector or an integrated group of single-core connectors provided at one end of the multi-core fiber bundle, Each of the plurality of multi-core fibers is the multi-core fiber according to claim 1 or 2, and is fixed to the multi-core connector or the single-core connector group such that the inclination directions of the connector-side end faces, which are the end faces on the side where the multi-core connector or the single-core connector group is provided, of the first end face and the second end face are aligned in a specific direction. An optical device comprising:

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