Optical connection structure

Abstract

The mode field diameter of each of a plurality of second fiber cores (73) on a third end surface (14b) is smaller than the mode field diameter of a first fiber core (71) corresponding to a second fiber core (73), among the mode field diameters of a plurality of the first fiber cores (71) on a first end surface (12a). The core pitch of a plurality of third fiber cores (81) on a fourth end surface (22a) is greater than the core pitch of the plurality of second fiber cores (73) on the third end surface (14b). A first lens (42) and a second lens (52) are arranged in order from the third end surface (14b) to the fourth end surface (22a), between the third end surface (14b) and the fourth end surface (22a). The shape of the arrangement of the plurality of second fiber cores (73) on the third end surface (14b) and the shape of the arrangement of the plurality of third fiber cores (81) on the fourth end surface (22a) are similar to each other.

Description

Optical connection structure

[0001] The present disclosure relates to an optical connection structure. This application claims priority to Japanese Patent Application No. 2023-097921, filed on June 14, 2023, and incorporates by reference all the contents of the Japanese application.

[0002] Patent Document 1 discloses an optical connection structure. The optical connection structure includes a multi-core fiber (hereinafter also referred to as "MCF") and a fiber bundle including a plurality of single-core fibers (hereinafter also referred to as "SCF"). The MCF includes a plurality of fiber cores.

[0003] International Publication No. 2023 / 008341

[0004] An optical connection structure according to one aspect of the present disclosure includes a first multicore fiber, a second multicore fiber, a fiber bundle, a first lens, and a second lens. The first multicore fiber includes a plurality of first fiber cores. The first multicore fiber has a first end face from which the plurality of first fiber cores are exposed. The second multicore fiber includes a plurality of second fiber cores. The second multicore fiber has a second end face from which the plurality of second fiber cores are exposed and a third end face from which the plurality of second fiber cores are exposed. The third end face is located opposite the second end face. Each of the plurality of second fiber cores is optically coupled to a corresponding first fiber core among the plurality of first fiber cores. The fiber bundle includes a plurality of single-core fibers, each including a third fiber core. The fiber bundle has a fourth end face from which the third fiber core is exposed. The exposed surface of each of the plurality of second fiber cores at the second end face and the exposed surface of each of the plurality of first fiber cores at the first end face are optically coupled to each other. The exposed surfaces of the plurality of third fiber cores at the fourth end face are optically coupled to the exposed surfaces of the plurality of second fiber cores at the third end face. The mode field diameter (MFD) of each of the plurality of second fiber cores at the third end face is smaller than the mode field diameter of a first fiber core corresponding to the second fiber core among the mode field diameters of the plurality of first fiber cores at the first end face. The core pitch of the plurality of third fiber cores at the fourth end face is larger than the core pitch of the plurality of second fiber cores at the third end face. The first lens and the second lens are arranged in order from the third end face to the fourth end face between the third end face and the fourth end face. When viewed in the extending direction of the plurality of second fiber cores, the shape of the arrangement of the plurality of second fiber cores at the third end face and the shape of the arrangement of the plurality of third fiber cores at the fourth end face are similar to each other.

[0005] Fig. 1 is a cross-sectional view along the optical axis direction of an optical connection structure according to an embodiment. Fig. 2 is a partially enlarged view of the optical connection structure. Fig. 3 is a cross-sectional view of a first multicore fiber in a direction perpendicular to the optical axis direction. Fig. 4 is a cross-sectional view of a second multicore fiber in a direction perpendicular to the optical axis direction. Fig. 5 is a cross-sectional view of a fiber bundle in a direction perpendicular to the optical axis direction. Fig. 6 is a cross-sectional view along the optical axis direction of an optical connection structure according to a modified example of this embodiment. Fig. 7 is a partially enlarged view of an optical connection structure according to a modified example of this embodiment. Fig. 8 is a partially enlarged view of an optical connection structure according to a modified example of this embodiment. Fig. 9 is a cross-sectional view along the optical axis direction of an optical connection structure according to a modified example of this embodiment.

[0006] [Problem to be Solved by the Present Disclosure] The core pitch of a fiber bundle may be larger than the core pitch of an MCF. Here, the core pitch refers to the spacing between fiber cores. For example, if the MCF is a coupled multicore fiber in which the modes of propagating light between fiber cores are randomly coupled, the optical power per fiber core is reduced. However, the core pitch of the MCF is very small, and it is difficult to fabricate a fiber bundle that matches that pitch.

[0007] In the optical connection structure disclosed in Patent Document 1, a fiber bundle and an MCF having different core pitches are optically coupled via a tapered component. The tapered component converts the core pitch between the fiber bundle and the MCF. However, it is difficult to manufacture a tapered component that ensures the accuracy of the core pitch conversion between the fiber bundle and the MCF, and optical loss also occurs in the tapered component.

[0008] Effect of the Present Disclosure According to the present disclosure, it is possible to provide an optical connection structure that is easier to manufacture and can suppress optical loss in optical fibers.

[0009] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0010] (1) An optical connection structure according to an embodiment of the present disclosure includes a first multicore fiber, a second multicore fiber, a fiber bundle, a first lens, and a second lens. The first multicore fiber includes a plurality of first fiber cores. The first multicore fiber has a first end face from which the plurality of first fiber cores are exposed. The second multicore fiber includes a plurality of second fiber cores. The second multicore fiber has a second end face from which the plurality of second fiber cores are exposed and a third end face from which the plurality of second fiber cores are exposed. The third end face is located opposite the second end face. Each of the plurality of second fiber cores is optically coupled to a corresponding first fiber core among the plurality of first fiber cores. The fiber bundle includes a plurality of single-core fibers, each including a third fiber core. The fiber bundle has a fourth end face from which the third fiber core is exposed. The exposed surface of each of the plurality of second fiber cores at the second end face and the exposed surface of each of the plurality of first fiber cores at the first end face are optically coupled to each other. The exposed surfaces of the plurality of third fiber cores at the fourth end face are optically coupled to the exposed surfaces of the plurality of second fiber cores at the third end face. The mode field diameter of each of the plurality of second fiber cores at the third end face is smaller than the mode field diameter of a first fiber core corresponding to the second fiber core among the mode field diameters of the plurality of first fiber cores at the first end face. The core pitch of the plurality of third fiber cores at the fourth end face is larger than the core pitch of the plurality of second fiber cores at the third end face. The first lens and the second lens are arranged in order from the third end face to the fourth end face between the third end face and surface 34. When viewed in the extension direction of the plurality of second fiber cores, the shape of the arrangement of the plurality of second fiber cores at the third end face and the shape of the arrangement of the plurality of third fiber cores at the fourth end face are similar to each other.

[0011] This optical connection structure includes a second multicore fiber optically coupled to the first multicore fiber. The mode field diameter of each of the plurality of second fiber cores at the third end face is smaller than the mode field diameter of a first fiber core corresponding to the second fiber core among the plurality of first fiber cores at the first end face. Furthermore, a first lens and a second lens are arranged between the second multicore fiber and the fiber bundle, and the second multicore fiber and the fiber bundle are optically coupled. In this case, optical coupling is performed via the first lens and the second lens, which improves ease of manufacture.

[0012] (2) In any of the optical connection structures described in (1) above, the focal length of the first lens is "f1". The focal length of the second lens is "f2". The core pitch of the plurality of second fiber cores at the third end face is "P1". The core pitch of the plurality of third fiber cores at the fourth end face is "P2". The mode field diameter of at least one second fiber core among the plurality of second fiber cores at the third end face is "D1". The mode field diameter of a third fiber core corresponding to at least one second fiber core among the plurality of third fiber cores at the fourth end face is "D2". In this case, the relationships formed by the following formulas may be satisfied. (P2 / P1) x 0.9 ≦ f2 / f1 ≦ (P2 / P1) x 1.1 (P2 / P1) x 0.8 ≦ D2 / D1 ≦ (P2 / P1) x 1.2

[0013] In this case, leakage of incident light into the fiber bundle can be further suppressed, and optical loss in the optical fiber can be further suppressed.

[0014] (3) In the optical connection structure of (1) or (2), the mode field diameter of each of the plurality of third fiber cores at the fourth end face may be larger than the mode field diameter of a second fiber core corresponding to the third fiber core among the plurality of second fiber cores at the third end face. In this case, even if the mode field diameter is converted along with the core pitch between the fiber bundle and the second multicore fiber by the first lens and the second lens, leakage of incident light into the fiber bundle can be suppressed. Therefore, optical loss in the optical fiber can be suppressed.

[0015] (4) In the optical connection structure according to any one of (1) to (3), in each of the plurality of third fiber cores, the mode field diameter at the fourth end face may be larger than the mode field diameter at a position a predetermined distance away from the fourth end face. In this case, leakage of incident light from the second multicore fiber to the fiber bundle at the fourth end face can be further suppressed while using an existing fiber bundle.

[0016] (5) In the optical connection structure of (4), each of the plurality of third fiber cores may include a portion whose mode field diameter decreases with increasing distance from the fourth end face. In this case, leakage of incident light into the fiber bundle at the fourth end face can be further suppressed compared to a configuration in which the mode field diameter changes abruptly.

[0017] (6) In the optical connection structure according to any one of (1) to (5), in each of the plurality of second fiber cores, the mode field diameter at the second end face may be larger than the mode field diameter at the third end face. In this case, the mode field diameter of the first multicore fiber is converted in the second multicore fiber.

[0018] (7) In the optical connection structure of (6), each of the plurality of second fiber cores may include a portion whose mode field diameter decreases with increasing distance from the second end face. In this case, leakage of incident light into the second multicore fiber at the second end face can be further suppressed compared to a configuration in which the mode field diameter changes abruptly.

[0019] (8) The optical connection structure of any one of (1) to (7) above may further include a ferrule. The ferrule may have an accommodating hole that accommodates at least a portion of the first multicore fiber and the second multicore fiber therein. The first end face of the first multicore fiber and the second end face of the second multicore fiber may abut against each other. The ferrule may accommodate the first end face and the second end face in the accommodating hole. In this case, the optical connection structure can be made smaller.

[0020] (9) The optical connection structure according to any one of (1) to (8) above may further include a ferrule having a receiving hole for receiving at least a part of the plurality of single-core fibers therein. In this case, the optical connection structure can be made compact, and the arrangement of the plurality of single-core fibers can be stabilized.

[0021] (10) In the optical connection structure of any one of (1) to (9) above, when the refractive index of the first lens and the refractive index of the second lens are the same, the angle at which the third end face is inclined with respect to the optical axis of the first lens may be larger than the angle at which the fourth end face is inclined with respect to the optical axis of the second lens. In this case, the optical axes of the lenses and the optical axes of the optical fibers can be kept nearly parallel to each other, improving the ease of assembly of the optical connection structure.

[0022] (11) In the optical connection structure of (10) above, the angle at which the third end face is inclined with respect to the optical axis of the first lens is "θ1", and the angle at which the fourth end face is inclined with respect to the optical axis of the second lens is "θ2". In this case, the relationship formed by the following formula may be satisfied: (P2 / P1) x 0.8 ≦ θ1 / θ2 ≦ (P2 / P1) x 1.2

[0023] In this case, reflection at each end face can be reduced, and the coupling efficiency between optical fibers can be improved.

[0024] (12) In the optical connection structure according to any one of (1) to (11), the fourth end face and the third end face may be arranged such that a V-shape is formed by a first imaginary plane including the third end face and a second imaginary plane including the fourth end face. In this case, the coupling efficiency between the optical fibers may be improved.

[0025] (13) In the optical connection structure according to any one of (1) to (12), the focal lengths of the first lens and the second lens may be 0.5 or more and 4.0 or less. In this case, the distance between the lenses is ensured, which can ensure ease of manufacturing.

[0026] (14) In the optical connection structure according to any one of (1) to (13) above, the plurality of first fiber cores may include coupled cores that are optically coupled to one another. In this case, even though the configuration uses a coupled multicore fiber, optical loss between the multicore fiber and the fiber bundle is easily suppressed. If a coupled multicore fiber is used, the optical power in each fiber core can be reduced, thereby suppressing degradation of the communication signal caused by optical power density.

[0027] [Details of the embodiments of the present disclosure] Specific examples of the embodiments of the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted as appropriate.

[0028] Fig. 1 is a cross-sectional view showing an optical connection structure according to an embodiment. Fig. 2 is a partially enlarged view of the optical connection structure shown in Fig. 1. Figs. 3 and 4 are cross-sectional views of a multi-core fiber. Fig. 5 is a cross-sectional view of a fiber bundle.

[0029] The optical connection structure 1 constitutes, for example, a fan-in / fan-out (FIFO) device of a lens-coupled multicore fiber, and includes a first optical fiber unit 10, a second optical fiber unit 20, a metal tube 30, a first lens unit 40, and a second lens unit 50.

[0030] The first optical fiber unit 10 includes an MCF 12, an MCF 14, a ferrule 16, and a sleeve 18. The second optical fiber unit 20 includes a fiber bundle 22 and a sleeve 28. For example, the MCF 12 corresponds to the first multi-core fiber, and the MCF 14 corresponds to the second multi-core fiber.

[0031] The ferrule 16 has an accommodating hole 16a. The ferrule 16 accommodates at least a portion of the MCF 12 in the accommodating hole 16a. In the example shown in this embodiment, the ferrule 16 accommodates a portion of the MCF 12 in the accommodating hole 16a. The ferrule 16 accommodates at least a portion of the MCF 14 in the accommodating hole 16a. In the example shown in this embodiment, the ferrule 16 accommodates the entire MCF 14 in the accommodating hole 16a. The sleeve 18 has a flange portion 18b at one end in the axial direction. The flange portion 18b corresponds to the portion of the first optical fiber unit 10 that is fixed to the first lens unit 40. The fiber bundle 22 includes a ferrule 26 and a plurality of SCFs 29. The ferrule 26 has an accommodating hole 26a. The ferrule 26 accommodates at least a portion of the plurality of SCFs 29 in the accommodating hole 26a. The sleeve 28 has a flange portion 28b at one axial end thereof. The flange portion 28b corresponds to the portion of the second optical fiber unit 20 that is fixed to the second lens unit 50.

[0032] For example, the first lens unit 40 includes a first lens 42 and a cylindrical lens holding member 44 that surrounds and holds the first lens 42. The lens holding member 44 holds the first lens 42 at its inner surface 40a. The second lens unit 50 includes a second lens 52 and a cylindrical lens holding member 54 that surrounds and holds the second lens 52. The lens holding member 54 holds the second lens 52 at its inner surface 50a. The lens holding member 44, the lens holding member 54, and the metal tube 30 each have an opening. The lens holding member 44, the lens holding member 54, and the metal tube 30 each form two openings that are arranged on a straight line.

[0033] The lens holding member 44 and the lens holding member 54 are each connected to an end of the metal tube 30 so that their openings communicate with the openings of the metal tube 30. The lens holding member 44 and the lens holding member 54 are each fixed to one of a pair of ends of the metal tube 30 that are located opposite each other. For example, the lens holding member 44 and the lens holding member 54 are each fixed to the metal tube 30 by welding.

[0034] For example, the first optical fiber unit 10 is fixed to the end of the metal tube 30 via the lens holding member 44. The second optical fiber unit 20 is fixed to the end of the metal tube 30 via the lens holding member 54. For example, the first optical fiber unit 10 is fixed to the lens holding member 44 by welding. The second optical fiber unit 20 is fixed to the lens holding member 54 by welding. For example, the flange portion 18b is welded to the end of the lens holding member 44 so that the opening of the sleeve 18 and the opening of the lens holding member 44 are in communication with each other. The flange portion 28b is welded to the end of the lens holding member 54 so that the opening of the sleeve 28 and the opening of the lens holding member 54 are in communication with each other.

[0035] The second lens unit 50 and the ferrule 26 are aligned with respect to the integrated first optical fiber unit 10 and metal tube 30, respectively. At this time, the second lens unit 50 is aligned in a direction perpendicular to the optical axis direction D. The ferrule 26 is aligned in both a direction perpendicular to the optical axis direction D and a direction around the optical axis direction. Once the alignment is complete, the metal tube 30 and the second lens unit 50 are fixed by welding. Thereafter, the ferrule 26 is again aligned in both a direction perpendicular to the optical axis direction D and a direction around the optical axis direction, and then the ferrule 26 and the sleeve 28 are fixed by welding. Next, the ferrule 26 fixed to the sleeve 28 is aligned in both a direction perpendicular to the optical axis direction D and a direction around the optical axis direction, and then fixed to the second lens unit 50 by welding. The welding is performed, for example, by irradiating a YAG laser.

[0036] The optical connection structure 1 is a fan-in / fan-out device in which light L passing through an MCF 12 is split into multiple SCFs 29 via an MCF 14, or light L passing through each of multiple SCFs 29 is coupled to one MCF 12 via one MCF 14. The light L is, for example, light having a wavelength in the 1.55 (μm) band.

[0037] In the optical connection structure 1, the MCF 12, the MCF 14, the first lens 42, the second lens 52, and the fiber bundle 22 are arranged in this order along the optical axis direction D. The optical axis direction D is the extension direction of the MCF 14. The MCF 12 and each SCF 29 are optically coupled (spatially coupled) via space, the first lens 42, the second lens 52, and the MCF 14.

[0038] The MCF 12 includes a plurality of fiber cores 71 and a cladding 72. The MCF 12 has an end face 12a. The plurality of fiber cores 71 are exposed at the end face 12a. The end face 12a faces the first lens 42. For example, the end face 12a is flat and parallel to a plane perpendicular to the optical axis direction D. A normal to the end face 12a is perpendicular to the optical axis direction D. For example, the plurality of fiber cores 71 include coupled cores that are optically coupled to each other. In other words, the MCF 12 is, for example, a coupled multicore fiber.

[0039] The MCF 14 includes a plurality of fiber cores 73 and a cladding 74. The MCF 14 has an end face 14a and an end face 14b. A plurality of fiber cores 73 are exposed at each of the end faces 14a and 14b. The plurality of fiber cores 73 extend in the optical axis direction D. Each of the plurality of fiber cores 73 is optically coupled to a corresponding one of the plurality of fiber cores 71. For example, the relative refractive index difference between the plurality of fiber cores 73 and the cladding 74 is larger than the relative refractive index difference between the plurality of fiber cores 71 and the cladding 72. In other words, the MCF 14 is an MCF in which the fiber core 71 has a high Δ. For example, the relative refractive index difference in the MCF 14 is 3% or more.

[0040] The end face 14a faces the end face 12a. In the example shown in this embodiment, the end face 14a abuts against the end face 12a. For example, the end face 12a and the end face 14a are fusion-spliced ​​to each other. For example, the end face 14a is flat and parallel to a plane perpendicular to the optical axis direction D. The end face 14a is perpendicular to the optical axis direction D. The end face 14b is located on the opposite side of the end face 14a. The end face 14b faces the first lens 42. For example, the end face 14b is flat and inclined with respect to the optical axis direction D and the plane perpendicular to the optical axis direction D. The end face 14b is inclined at an angle θ1 with respect to the plane perpendicular to the optical axis direction D.

[0041] The fiber bundle 22 includes a plurality of SCFs 29. Each of the plurality of SCFs 29 includes a fiber core 81 and a cladding 82. The diameter of the cladding 82 is, for example, 125 μm. The diameter of the cladding 82 may be a value other than this, for example, 80 μm. For example, the fiber core 71 corresponds to the first fiber core, the fiber core 73 corresponds to the second fiber core, and the fiber core 81 corresponds to the third fiber core.

[0042] The fiber bundle 22 has an end face 22a. A plurality of fiber cores 81 are exposed at the end face 22a. The end face 22a faces the second lens 52. Like the end face 22a, the end face 22a is flat and inclined with respect to the optical axis direction D and a plane perpendicular to the optical axis direction D. The end face 22a is inclined at an angle θ2 with respect to the plane perpendicular to the optical axis direction D. The end face 22a and the end face 14b are arranged such that a V-shape is formed by a first imaginary plane 77 including the end face 14b and a second imaginary plane 78 including the end face 22a. For example, the end face 12a corresponds to the first end face, the end face 14a corresponds to the second end face, the end face 14b corresponds to the third end face, and the end face 22a corresponds to the fourth end face. In this specification, "arranged so as to form a V-shape" means that the first normal vector of the first imaginary plane 77 including the end face 14b and the second normal vector of the second imaginary plane 78 including the end face 22a are on the same plane, and that within this same plane, the optical axis direction component of the first normal vector and the optical axis direction component of the second normal vector are components that move toward each other, and the directional component perpendicular to the optical axis direction of the first normal vector and the directional component perpendicular to the optical axis direction of the second normal vector are components that move in the same direction.

[0043] The optical connection structure 1 may, for example, split the light L passing through each fiber core 71 of the MCF 12 toward each fiber core 81 of the plurality of SCFs 29, and transmit each of the split light L to an optical amplifier that amplifies the split light L. Furthermore, the optical connection structure 1 may be used in an optical transmitter that transmits the light L to each of the fiber cores 81 of the plurality of SCFs 29, or in an optical receiver that receives the light L from each of the plurality of SCFs 29.

[0044] 3 is a cross-sectional view of the MCF 12 cut along a plane perpendicular to the optical axis direction D. As shown in FIG. 3, the MCF 12 is held by, for example, a ferrule 16. Four fiber cores 71 are arranged in the cross section of the MCF 12 cut along a plane perpendicular to the optical axis direction D. The four fiber cores 71 are arranged in a square lattice pattern.

[0045] 4 is a cross-sectional view of the MCF 14 cut along a plane perpendicular to the optical axis direction D. As shown in FIG. 4, the MCF 14 is held by, for example, a ferrule 16. Four fiber cores 73 are arranged in the cross section of the MCF 14 cut along a plane perpendicular to the optical axis direction D. The four fiber cores 73 are arranged in a square lattice pattern. The fiber cores 71 are thicker than the fiber cores 73 in a tapered portion 75, which will be described later.

[0046] 5 is a cross-sectional view of the fiber bundle 22 cut along a plane perpendicular to the optical axis direction D. In the fiber bundle 22, for example, a plurality of SCFs 29 are bundled in a ferrule 26. As an example, four SCFs 29 are packed in the ferrule 26. The four SCFs 29 are arranged in a square lattice pattern.

[0047] The exposed surfaces of the plurality of fiber cores 73 at end face 14a and the exposed surfaces of the plurality of fiber cores 71 at end face 12a are optically coupled to each other. The exposed surfaces of the plurality of fiber cores 81 at end face 22a and the exposed surfaces of the plurality of fiber cores 73 at end face 14b are optically coupled to each other.

[0048] When viewed from the optical axis direction D, the shape of the arrangement of the plurality of fiber cores 73 on the end face 14b and the shape of the arrangement of the plurality of fiber cores 81 on the end face 22a are similar to each other. In other words, the shape of the arrangement of the plurality of fiber cores 73 when the end face 14b is orthogonally projected in the optical axis direction D and the shape of the arrangement of the plurality of fiber cores 81 when the end face 22a is orthogonally projected in the optical axis direction D are similar to each other. In a cross section cut by a plane perpendicular to the optical axis direction D, the shape of the arrangement of the plurality of fiber cores 73 of the MCF 14 and the shape of the arrangement of the plurality of fiber cores 81 of the fiber bundle 22 are similar to each other. In a cross section cut by a plane perpendicular to the optical axis direction D, the shape of the arrangement of the plurality of fiber cores 71 of the MCF 12 and the shape of the arrangement of the plurality of fiber cores 81 of the fiber bundle 22 are similar to each other.

[0049] The core pitch of the multiple fiber cores 81 at the end face 22a is larger than the core pitch of the multiple fiber cores 73 at the end face 14b. The core pitch of the multiple fiber cores 81 at the end face 22a is larger than the core pitch of the multiple fiber cores 71 at the end face 14b. The core pitch of the multiple fiber cores 73 at the end face 14a is the same as the core pitch of the multiple fiber cores 71 at the end face 12a. In this specification, the term "core pitch" corresponds to the distance between the centers of the fiber cores in a cross section perpendicular to the optical axis direction D. In this specification, "same" includes deviations within the range of manufacturing error.

[0050] In each of the multiple fiber cores 73, the MFD at the end face 14a is larger than the MFD at the end face 14b. MFD is a mode field diameter. In each of the multiple fiber cores 73, the MFD at the end face 14a is larger than the MFD at a position a predetermined distance away from the end face 14a in the optical axis direction D. Each of the multiple fiber cores 73 includes a portion in which the MFD decreases with increasing distance from the end face 14a in the optical axis direction. In other words, the multiple fiber cores 73 include a tapered portion 75 in which the MFD is formed in a tapered shape. The end face 12a and the end face 14a are abutted against each other.

[0051] In each of the plurality of fiber cores 81, the MFD at the end face 22a is larger than the MFD at a position a predetermined distance away from the end face 22a in the optical axis direction D. Each of the plurality of fiber cores 81 includes a portion where the MFD decreases with increasing distance from the end face 22a in the optical axis direction D. In other words, the plurality of fiber cores 81 includes a tapered portion 85 where the MFD is formed in a tapered shape.

[0052] The MCF 14 and the SCF 29 may be, for example, a thermally expanded core (TEC) fiber, and may be formed with a shape that has a partially different MFD by TEC processing. For example, the above-mentioned tapered portion may be formed by TEC processing.

[0053] The MFD of each of the plurality of fiber cores 73 at the end face 14b is smaller than the MFD of the fiber core 71 corresponding to the fiber core 73 among the MFDs of the plurality of fiber cores 71 at the end face 12a. The MFD of each of the plurality of fiber cores 73 at the end face 14a is the same as the MFD of the fiber core 71 corresponding to the fiber core 73 among the MFDs of the plurality of fiber cores 71 at the end face 12a. The MFD of each of the plurality of fiber cores 81 at the end face 22a is larger than the MFD of the fiber core 73 corresponding to the fiber core 81 among the MFDs of the plurality of fiber cores 73 at the end face 14b. The ferrule 16 accommodates the end face 12a and the end face 14a in the accommodation hole 16a. The ferrule 26 has an accommodation hole 26a.

[0054] The first lens 42 is interposed between the MCF 12 and the plurality of SCFs 29. The second lens 52 is interposed between the plurality of SCFs 29 and the first lens 42. The first lens 42 and the second lens 52 are arranged between the end face 22a and the end face 14b in the order of the end face 14b, the first lens 42, the second lens 52, and the end face 22a.

[0055] The first lens 42 is disposed in a position facing the MCF 14 along the optical axis direction D. The second lens 52 is disposed in a position facing the fiber bundle 22 along the optical axis direction D. The first lens 42 and the second lens 52 focus the multiple light beams L emitted from each of the multiple fiber cores 73 of the MCF 14 on the opposite side of the MCF 14 from the first lens 42 and the second lens 52. The second lens 52 and the first lens 42 focus the light beams L on each fiber core 73 on the end face 22 a, for example. The first lens 42 and the second lens 52 are, for example, biconvex aspherical lenses. When the first lens 42 and the second lens 52 are aspherical lenses, the coupling loss of the light beams L can be reduced.

[0056] The core pitch of the fiber cores 71 of the MCF 12 is "P0", the core pitch of the fiber cores 73 of the MCF 14 is "P1", and the core pitch of the fiber cores 81 of the fiber bundle 22 is "P2".

[0057] The focal length of the first lens 42 is "f1", and the focal length of the second lens 52 is "f2". When the core pitch of the plurality of fiber cores 73 at the end face 14b is "P1", the core pitch of the plurality of fiber cores 81 at the end face 22a is "P2", the MFD at the end face 14b for at least one fiber core 73 among the plurality of fiber cores 73 is "D1", and the MFD at the end face 22a for at least one fiber core 81 among the plurality of fiber cores 81 corresponding to the fiber core 73 is "D2", the relationships shown in the following formulas (1) and (2) are satisfied: (P2 / P1)×0.9≦f2 / f1≦(P2 / P1)×1.1 (1) (P2 / P1)×0.8≦D2 / D1≦(P2 / P1)×1.2 (2)

[0058] As an example, the lens system between the end face 14b of the MCF 14 and the end face 22a of the fiber bundle 22 has a magnification of 6.25 times. In this case, for example, "P0" is 20 (μm), "P1" is 20 (μm), and "P2" is 125 (μm). The cladding diameter of each of the fiber bundle 22, the MCF 12, and the MCF 14 is, for example, 125 (μm). For example, if the MFD at the end face 12a is "D0," "D0" is 10 (μm) for light with a wavelength of 1.55 (μm), and the MFD at the end face 14a is also the same. For example, "D1" is 5 (μm) for light with a wavelength of 1.55 (μm), and "D2" is 31 (μm) for light with a wavelength of 1.55 (μm).

[0059] The core pitch is preferably 30 μm or more. In this case, crosstalk between fiber cores is suppressed. The shortest distance between the center of the fiber core and the outer periphery of the cladding is preferably 37.5 μm or more. For example, when the cladding diameter is 125 μm, the core pitch is preferably 50 μm or less. In this case, by reducing the core pitch, the distance between the fiber core and the outer periphery of the cladding is secured, and light leakage to the outside is suppressed.

[0060] For example, "f1" is 2.1 (mm) and "f2" is 2.7 (mm). For example, the focal length "f1" of the first lens 42 and the focal length "f2" of the second lens 52 are 0.5 (mm) or more and 4.0 (mm) or less. If the focal length is 0.5 (mm) or more, the distance between the lenses and the distance between the fiber and the lens are ensured, making manufacturing easier. If the focal length is 4.0 (mm) or less, sensitivity to losses due to tilt angle misalignment of the fiber and lens is suppressed, and the device can be made more compact.

[0061] In the example shown in this embodiment, the MFD at the end face 14b of each of the multiple fiber cores 73 corresponds to "D1", and the MFD at the end face 22a of each of the multiple fiber cores 81 corresponds to "D2", and the relationship shown in the above equations (1) and (2) is satisfied.

[0062] When the refractive index of the first lens 42 and the refractive index of the second lens 52 are the same, the angle at which the end face 14b is inclined with respect to the optical axis of the first lens 42 is larger than the angle at which the end face 22a is inclined with respect to the optical axis of the second lens 52. When the angle at which the end face 14b is inclined with respect to the optical axis of the first lens 42 is "θ １ ", and the angle at which the end face 22a is inclined with respect to the optical axis of the second lens 52 is "θ ２ ", the relationship shown in the following formula (3) is satisfied: (P2 / P1)×0.8≦θ １ / θ ２ ≦ (P2 / P1)×1.2 ... (3)

[0063] For example, when the core pitches of the fiber bundle 22 and the MCF 14 at the end face 14b are 45 (μm) and 35 (μm), respectively, １ " is 8°, and "θ ２ The angle .alpha." is 6.2 degrees. Since the MFD of the fiber bundle 22 is larger than the MDF of the MCF 14 at the end face 14b, the numerical aperture (NA) of the fiber is ensured. Therefore, even if the angle of the end face is small, the deterioration of the return loss is suppressed.

[0064] For example, when the relationship shown in the following formula (4) is satisfied, the central axes of the ferrules 16 and 26 are unlikely to deviate from the horizontal during alignment. ２ f2 = θ １ f1 ... (4)

[0065] Next, an optical connection structure 1A according to a modified example of this embodiment will be described with reference to Figures 6 and 7. Figure 6 is a cross-sectional view showing the optical connection structure according to this modified example. Figure 7 is a partially enlarged view of the optical connection structure. This modified example is generally similar to or the same as the example shown in the above-described embodiment. This modified example differs from the example shown in the above-described embodiment in that the first optical fiber unit 10A and the second optical fiber unit 20A each include a lens, and that the first optical fiber unit 10A and the second optical fiber unit 20A are held by the same cylindrical tube. Below, the differences between this modified example and the example shown in the above-described embodiment will be mainly described.

[0066] The optical connection structure 1A includes a first optical fiber unit 10A, a second optical fiber unit 20A, and a tubular member 30A. The first optical fiber unit 10A includes an MCF 12, an MCF 14, a ferrule 16, a sleeve 18A, and a first lens 42A. The second optical fiber unit 20A includes a fiber bundle 22, a ferrule 26, a sleeve 28A, and a second lens 52A. The tubular member 30A connects the first optical fiber unit 10A and the second optical fiber unit 20A to each other. The tubular member 30A is, for example, a glass tube.

[0067] The first lens 42A and the second lens 52A are, for example, C lenses. The first lens 42A and the second lens 52A have, for example, the same refractive index and focal length as the first lens 42 and the second lens 52 described above, respectively. For example, the first lens 42A is a rod lens that has a spherical surface on the second lens 52A side, a flat surface on the MCF 14 side, and the same outer diameter as the ferrule 16. For example, the second lens 52A is a rod lens that has a spherical surface on the first lens 42A side, a flat surface on the fiber bundle 22 side, and the same outer diameter as the ferrule 26.

[0068] As a further modification of this embodiment, as shown in FIG. 8 , a first lens 42B and a second lens 52B may be used instead of the first lens 42A and the second lens 52A. For example, the first lens 42B and the second lens 52B are GRIN lenses. The first lens 42B and the second lens 52B have the same focal lengths as the first lens 42 and the second lens 52 described above, respectively. For example, the first lens 42A has the same outer diameter as the ferrule 16. For example, the second lens 52A has the same outer diameter as the ferrule 26.

[0069] In the first optical fiber unit 10A, a first lens 42A is exposed from one side of a sleeve 18A, and a ferrule 16 holding an MCF 12 is exposed from the other side of the sleeve 18A. The MCF 12 extends from the ferrule 16 to the side opposite the first lens 42A and the tubular member 30A. In the second optical fiber unit 20A, a second lens 52A is exposed from one side of a sleeve 28A, and a ferrule 26 holding an MCF 14 is exposed from the other side of the sleeve 28A. The SCF 29 extends from the ferrule 26 to the side opposite the second lens 52A and the tubular member 30A.

[0070] The MCF 14 extends from the ferrule 26 to the side opposite the first lens 42A and the cylindrical member 30A. The first lens 42A is fixed to the end face 14b of the MCF 14. The second lens 52A is fixed to the end face 22a of the fiber bundle 22. The sleeve 18A and the sleeve 28A are fixed to the cylindrical member 30A with an adhesive. The adhesive is, for example, a UV-curable adhesive.

[0071] The first optical fiber unit 10A is inserted into the cylindrical member 30A from one side and is held at one end of the cylindrical member 30A. The second optical fiber unit 20A is inserted into the cylindrical member 30A from the other side and is held at the other end of the cylindrical member 30A. The first optical fiber unit 10A is fixed to the cylindrical member 30A with an adhesive after the second optical fiber unit 20A has been aligned. The adhesive is, for example, a UV-curable adhesive.

[0072] The above-mentioned alignment is performed in six directions: X direction, Y direction, Z direction, θx direction, θy direction, and θz direction. The Z direction is the optical axis direction D, and the X direction and Y direction are directions perpendicular to the Z direction. The θx direction, θy direction, and θz direction indicate the direction around the X axis, the direction around the Y axis, and the direction around the Z axis, respectively.

[0073] Next, an optical connection structure 1C according to a modified example of this embodiment will be described with reference to Fig. 9. Fig. 9 is a cross-sectional view showing an optical connection structure according to this modified example. This modified example is generally similar to or the same as the example shown in the above-described embodiment and the modified example shown in Fig. 6. This modified example differs from the example shown in the above-described embodiment and the modified example in that it is provided with a sleeve having a shape different from the sleeves 18, 28 of the optical connection structures 1, 1A. Below, differences from the example shown in the above-described embodiment and the modified example shown in Fig. 6 will be mainly described.

[0074] The optical connection structure 1C includes a first optical fiber unit 10C, a second optical fiber unit 20C, and a metal tube 30C. The first optical fiber unit 10C includes an MCF 12, an MCF 14, a ferrule 16, a sleeve 18C, and a first lens 42A. The second optical fiber unit 20C includes a fiber bundle 22, a ferrule 26, a sleeve 28C, and a second lens 52A.

[0075] The sleeve 18C has an insertion portion 91 that fits into the metal tube 30C, where the outer diameter is reduced and the inner surface protrudes radially inward of the sleeve 18C. The sleeve 28C also has an insertion portion 92 similar to the insertion portion 91. The metal tube 30C has a cylindrical main body 95 and a pair of annular inserted portions 96a, 96b connected to the main body 95. The main body 95 and the pair of inserted portions 96a, 96b each have two openings that are arranged in a straight line.

[0076] The pair of inserted portions 96a, 96b are each connected to an end of the metal pipe 30C such that the openings of the respective inserted portions 96a, 96b communicate with the openings of the metal pipe 30C. The pair of inserted portions 96a, 96b are each fixed to one of a pair of ends of the metal pipe 30C that are located opposite each other. For example, the pair of inserted portions 96a, 96b are each fixed to the metal pipe 30C by welding.

[0077] For example, the first optical fiber unit 10C is fixed to the end of the metal tube 30C via the inserted portion 96a. The second optical fiber unit 20C is fixed to the end of the metal tube 30C via the inserted portion 96b. One of the first optical fiber unit 10C and the second optical fiber unit 20C is fixed to the main body 95 by welding after the inserted portions 96a, 96b are aligned in the X direction, Y direction, Z direction, θx direction, θy direction, and θz direction. The welding is performed by irradiating a YAG laser, for example.

[0078] Next, the effects obtained from the optical connection structure according to the embodiment will be described. This optical connection structure 1 includes an MCF 14 that is optically coupled to the MCF 12. The MFD of each of the multiple fiber cores 73 at the end face 14a is smaller than the MFD of the fiber core 71 corresponding to the fiber core 73 among the multiple fiber cores 71 at the end face 12a. Furthermore, a first lens 42 and a second lens 52 are arranged between the MCF 14 and the fiber bundle 22, and the MCF 14 and the fiber bundle 22 are optically coupled. In this case, optical coupling is performed via the first lens 42 and the second lens 52, which improves ease of manufacturing. Furthermore, because the MFD of the MCF 12 is changed by the MCF 14, optical loss in the optical fiber can be reduced. Similar effects are achieved with the optical connection structures 1A and 1C.

[0079] In the optical connection structure 1, the focal length of the first lens 42 is "f1". The focal length of the second lens 52 is "f2". The core pitch of the multiple fiber cores 73 at the end face 14b is "P1". The core pitch of the multiple fiber cores 81 at the end face 22a is "P2". The MFD at the end face 14b for at least one fiber core 73 out of the multiple fiber cores 73 is "D1". The MFD at the end face 22a for the fiber core 81 corresponding to at least one fiber core 73 out of the multiple fiber cores 73 is "D2". In this case, the relationship consisting of equations (1) and (2) may be satisfied. (P2 / P1) × 0.9 ≦ f2 / f1 ≦ (P2 / P1) × 1.1 (1) (P2 / P1) × 0.8 ≦ D2 / D1 ≦ (P2 / P1) × 1.2 (2)

[0080] In this case, leakage of incident light into the fiber bundle 22 is further suppressed, and optical loss in the optical fiber can be further suppressed. If the MFD is too small or too large, optical loss will occur. For example, if the MFD is too large, there is a risk of light leaking to the outside depending on the bending of the fiber. The optical connection structures 1A and 1C also have the same effect.

[0081] In the optical connection structure 1, the MFD of each of the plurality of fiber cores 81 at the end face 22a is larger than the MFD of the fiber core 73 corresponding to the fiber core 81 among the plurality of fiber cores 73 at the end face 14b. In this case, even if the MFD is converted along with the core pitch between the fiber bundle 22 and the MCF 14 by the first lens 42 and the second lens 52, leakage of incident light into the fiber bundle 22 can be suppressed. Therefore, optical loss in the optical fiber can be suppressed. The same effect can be achieved with the optical connection structures 1A and 1C.

[0082] In the optical connection structure 1, the MFD at the end face 22a of each of the plurality of fiber cores 81 is larger than the MFD at a position a predetermined distance away from the end face 22a. In this case, leakage of incident light from the MCF 14 to the fiber bundle 22 at the end face 22a can be further suppressed while using the existing fiber bundle 22. The same effect can be achieved with the optical connection structures 1A and 1C.

[0083] In the optical connection structure 1, each of the plurality of fiber cores 81 includes a portion where the MFD decreases with increasing distance from the end face 22a. In this case, compared to a configuration where the MFD changes abruptly, leakage of incident light from the end face 22a into the fiber bundle 22 can be further suppressed. The same effect is achieved in the optical connection structures 1A and 1C.

[0084] In the optical connection structure 1, the MFD at the end face 14b of each of the plurality of fiber cores 73 is larger than the MFD at the end face 14a. In this case, the MFD of the MCF 12 is converted in the MCF 14. The optical connection structures 1A and 1C also have the same effect.

[0085] In the optical connection structure 1, each of the plurality of fiber cores 73 includes a portion where the MFD decreases with increasing distance from the end face 14a. In this case, compared to a configuration where the MFD changes suddenly, leakage of incident light from the end face 14a into the fiber bundle 22 can be further suppressed. The same effect is achieved in the optical connection structures 1A and 1C.

[0086] The optical connection structure 1 further includes a ferrule 16. The ferrule 16 has a receiving hole 16a that receives at least a portion of the MCF 12 and the MCF 14 therein. The end face 12a of the MCF 12 and the end face 14a of the MCF 14 may be abutted against each other. The ferrule 16 receives the end face 12a and the end face 14a in the receiving hole 16a. In this case, the optical connection structure 1 can be made smaller. The same effects are achieved with the optical connection structures 1A and 1C.

[0087] The optical connection structure 1 includes a ferrule 26 having a receiving hole 26a. The receiving hole 26a receives at least a portion of the plurality of SCFs 29 therein. In this case, the optical connection structure 1 can be made smaller and the arrangement of the plurality of SCFs 29 is stabilized. The same effects are achieved with the optical connection structures 1A and 1C.

[0088] In the optical connection structure 1, when the refractive index of the first lens 42 and the refractive index of the second lens 52 are the same, the angle at which the end face 14b is inclined relative to the optical axis of the first lens 42 is greater than the angle at which the end face 22a is inclined relative to the optical axis of the second lens 52. In this case, the optical axes of the first lens 42 and the second lens 52 and the optical axes of the optical fibers can be kept nearly parallel to each other, improving the ease of assembly of the optical connection structure 1. Optical coupling loss of the optical fibers can be further reduced. When the refractive indexes are the same, the same material can be used. If the same material is used, the expansion coefficient according to the ambient temperature will also be the same. Similar effects are achieved with the optical connection structures 1A and 1C.

[0089] In the optical connection structure 1, the angle at which the end face 14b is inclined with respect to the optical axis of the first lens 42 is “θ １ ", and the angle at which the end face 22a is inclined with respect to the optical axis of the second lens 52 is "θ ２ In this case, the relationship represented by the formula (3) may be satisfied: (P2 / P1)×0.8≦θ １ / θ ２ ≦ (P2 / P1)×1.2...(3)

[0090] In this case, reflection at the end faces 14b and 22a can be reduced, and the coupling efficiency between the optical fibers can be improved. The same effects can be achieved with the optical connection structures 1A and 1C.

[0091] In the optical connection structure 1, the end face 22a and the end face 14b are arranged so that a V-shape is formed by a first imaginary plane 77 including the end face 14b and a second imaginary plane 78 including the end face 22a. In this case, the coupling efficiency between the optical fibers can be improved. The same effect can be achieved with the optical connection structures 1A and 1C.

[0092] In the optical connection structure 1, the focal lengths of the first lens 42 and the second lens 52 are equal to or greater than 0.5 and equal to or less than 4.0. In this case, the distance between the lenses is ensured. This ensures ease of manufacturing. The same effects are achieved with the optical connection structures 1A and 1C.

[0093] In the optical connection structure 1, the multiple fiber cores 71 include coupled cores that are optically coupled to each other. In this case, even though a coupled multicore fiber is used, optical loss between the MCF and the fiber bundle is easily suppressed. If a coupled multicore fiber is used, the optical power in each fiber core can be reduced, thereby suppressing degradation of communication signals caused by optical power density. The same effects are achieved in the optical connection structures 1A and 1C.

[0094] In a coupled multicore fiber, the core pitch is very narrow. For example, the core pitch of a coupled multicore fiber is about 20 μm. To optically couple this coupled multicore fiber with the fiber bundle 22, a connection method in which the tip of the SCF of the fiber bundle 22 is thinned can be considered. In this connection method, for example, by bundling fibers thinned, the same core arrangement as that of the coupled multicore fiber is realized, and the fiber bundle and the coupled multicore fiber are physically in contact with each other. However, even in the case of thinning, a core pitch of 30 μm or more is considered realistic from the viewpoints of mechanical strength and bending loss. According to the optical connection structures 1, 1A, and 1C, a configuration in which optical loss is easily suppressed can be realized in this case as well.

[0095] Although the embodiments of the present disclosure have been described in detail above, the present invention is not limited to the above embodiments and can be applied to various embodiments. For example, the configurations of the optical connection structures 1, 1A, and 1C may be combined. For example, the configuration on the fiber bundle side and the configuration on the MCF side may have different configurations from those of the examples shown in the optical connection structures 1, 1A, and 1C.

[0096] For example, the lenses arranged between the fiber bundle 22 and the MCF 14 may be a combination of two or more lenses selected from a biconvex aspherical lens, a C lens, and a GRIN lens. In this case, for example, the lenses located at both ends correspond to the first lens and the second lens.

[0097] In the above example, the number of fiber cores 71, 73, and 81 is four each. However, the number of fiber cores is not limited to four. For example, the number of fiber cores 71, 73, and 81 may be seven each. The number of fiber cores 71, 73, and 81 may be different from each other.

[0098] In the above examples, the portions described as being connected by welding may be connected by adhesive, and the portions described as being connected by adhesive may be connected by welding.

[0099] DESCRIPTION OF SYMBOLS 1, 1A, 1C...Optical connection structure 10, 10A, 10C...First optical fiber unit 12, 14...MCF 12a, 14a, 14b, 22a...End face 16, 26...Ferrule 16a, 26a...Accommodating hole 18, 18C...Sleeve 18b...Flange portion 20, 20A, 20C...Second optical fiber unit 22...Fiber bundle 28, 28C...Sleeve 28b...Flange portion 29...SCF 30, 30C...Metal tube 30A...Cylindrical member 40...First lens unit 42, 42A, 42B...First lens 44, 54...Lens holding member 50...Second lens unit 52, 52A, 52B...Second lens 71, 73, 81...Fiber core 72, 74, 82...Cladding 75, 85...Tapered portion 77...First virtual plane 78...Second virtual plane 91, 92...Insertion part 95...Main body part 96a, 96b...Inserted part D...Optical axis direction L...Light 29...SCF θ １ , θ ２ …angle

Claims

1. A first multicore fiber having a plurality of first fiber cores and a first end face on which the plurality of first fiber cores are exposed, A second multicore fiber, each containing a plurality of second fiber cores that are optically coupled to a corresponding first fiber core among the plurality of first fiber cores, and having a second end face on which the plurality of second fiber cores are exposed, and a third end face on which the second fiber cores are exposed and located on the opposite side of the second end face, A fiber bundle comprising a plurality of single-core fibers, each containing a third fiber core, and having a fourth end face in which the third fiber core is exposed, The device comprises a first lens and a second lens arranged sequentially from the third end face toward the fourth end face between the third end face and the fourth end face, The exposed surfaces of each of the plurality of second fiber cores at the second end face and the exposed surfaces of each of the plurality of first fiber cores at the first end face are optically coupled to each other. The exposed surfaces of each of the plurality of third fiber cores on the fourth end face and the exposed surfaces of each of the plurality of second fiber cores on the third end face are optically coupled to each other. The mode field diameter of each of the plurality of second fiber cores at the third end face is smaller than the mode field diameter of the first fiber core corresponding to the second fiber core among the plurality of first fiber cores at the first end face. The core pitch of the plurality of third fiber cores at the fourth end face is greater than the core pitch of the plurality of second fiber cores at the third end face. An optical connection structure in which, when viewed from the extending direction of the plurality of second fiber cores, the shape of the arrangement of the plurality of second fiber cores at the third end face and the shape of the arrangement of the plurality of third fiber cores at the fourth end face are similar in shape to one another.

2. If the focal length of the first lens is "f1", the focal length of the second lens is "f2", the core pitch of the plurality of second fiber cores at the third end face is "P1", the core pitch of the plurality of third fiber cores at the fourth end face is "P2", the mode field diameter at the third end face is "D1" for at least one of the plurality of second fiber cores, and the mode field diameter at the fourth end face is "D2" for the third fiber core corresponding to at least one of the plurality of third fiber cores, (P2 / P1)×0.9 ≦ f2 / f1 ≦ (P2 / P1)×1.1 (P2 / P1)×0.8 ≦ D2 / D1 ≦ (P2 / P1)×1.2 The optical connection structure according to claim 1, wherein the following relationship is satisfied.

3. The optical connection structure according to claim 1 or 2, wherein the mode field diameter of each of the plurality of third fiber cores at the fourth end face is larger than the mode field diameter of the second fiber core corresponding to the third fiber core among the plurality of second fiber cores at the third end face.

4. The optical connection structure according to claim 1 or 2, wherein in each of the plurality of third fiber cores, the mode field diameter at the fourth end face is greater than the mode field diameter at a position a predetermined distance away from the fourth end face.

5. The optical connection structure according to claim 4, wherein each of the plurality of third fiber cores includes a portion in which the mode field diameter decreases as it moves away from the fourth end face.

6. The optical connection structure according to claim 1 or 2, wherein in each of the plurality of second fiber cores, the mode field diameter at the second end face is greater than the mode field diameter at the third end face.

7. The optical connection structure according to claim 6, wherein each of the plurality of second fiber cores includes a portion in which the mode field diameter decreases as it moves away from the second end face.

8. The ferrule further comprises a storage hole that accommodates at least a portion of the first multicore fiber and the second multicore fiber inside, The first end face of the first multicore fiber and the second end face of the second multicore fiber are in contact with each other. The optical connection structure according to claim 1 or claim 2, wherein the ferrule accommodates the first end face and the second end face in the receiving hole.

9. The optical connection structure according to claim 1 or 2, further comprising a ferrule having a housing hole for housing at least a portion of the plurality of single-core fibers.

10. The optical connection structure according to claim 1 or claim 2, wherein, when the refractive index of the first lens and the refractive index of the second lens are the same, the angle at which the third end face is inclined with respect to the optical axis of the first lens is greater than the angle at which the fourth end face is inclined with respect to the optical axis of the second lens.

11. The angle at which the third end face is inclined with respect to the optical axis of the first lens is "θ １ The fourth end face is inclined at an angle of "θ" with respect to the optical axis of the second lens. ２ If that is the case, (P2 / P1)×0.8 ≦ θ １ / θ ２ ≦ (2 / / 1)×1.2 The optical connection structure according to claim 10, wherein the following relationship is satisfied.

12. The optical connection structure according to claim 1 or claim 2, wherein the fourth end face and the third end face are arranged such that a V-shape is formed by a first virtual plane including the fourth end face and a second virtual plane including the third end face.

13. The optical connection structure according to claim 1 or claim 2, wherein the focal lengths of the first lens and the second lens are 0.5 or more and 4.0 or less.

14. The optical connection structure according to claim 1 or claim 2, wherein the plurality of first fiber cores include coupled cores that optically couple with each other.