Optical fiber bundle structure, method for manufacturing an optical fiber bundle structure, and optical fiber connection structure

Abstract

The present invention provides an optical fiber bundle structure that can improve optical coupling efficiency, a method for manufacturing the optical fiber bundle structure, and an optical fiber connection structure. [Solution] An optical fiber bundle structure according to one embodiment comprises a plurality of single-core fibers and a ferrule having a holding hole into which the plurality of single-core fibers are inserted. Each of the plurality of single-core fibers has an MFD (Mode Field Diameter) enlargement portion at its tip, which is a portion in which the mode field diameter is enlarged. The cladding of each single-core fiber in the MFD enlargement portion is thinner than the cladding of each single-core fiber in the portion other than the MFD enlargement portion. At least two of the plurality of MFD enlargement portions are in contact with each other in the holding hole.

Description

Technical Field

[0001] The present disclosure relates to an optical fiber bundle structure, a method for manufacturing an optical fiber bundle structure, and an optical fiber connection structure.

Background Art

[0002] Patent Document 1 describes an optical fiber bundle structure. The optical fiber bundle structure includes a multi-core fiber, a plurality of single-core fibers, a first lens having a focal length of f1 (mm), and a second lens having a focal length of f2 (mm). The arrangement of the center positions of the cores in a cross section orthogonal to the longitudinal direction of the multi-core fiber and the arrangement of the center positions of the cores in a cross section orthogonal to the longitudinal direction of the plurality of single-core fibers are similar shapes. When the pitch of the cores of the multi-core fiber is P1 (μm), the mode field diameter of the first end face of each core is MFD1 (μm), the pitch of the cores of the plurality of single-core fibers is P2 (μm), and the mode field diameter of the light at the second end face of each core is MFD2 (μm), (P1 / P2)×0.9≦f1 / f2≦(P1 / P2)×1.1 and (P1 / P2)×0.9≦MFD1 / MFD2≦(P1 / P2)×1.1 are satisfied.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Incidentally, in optical fiber bundle structures, multiple single-core fibers are inserted into the holding holes of a ferrule. To accommodate the insertion of multiple single-core fibers, the inner diameter of the holding holes is sometimes made larger. In this case, gaps may form between the multiple single-core fibers within the holding hole. When gaps form between multiple single-core fibers, the pitch of the single-core fibers becomes larger than the design value, which can lead to a decrease in optical coupling efficiency.

[0005] The purpose of this disclosure is to provide an optical fiber bundle structure that can improve optical coupling efficiency, a method for manufacturing an optical fiber bundle structure, and an optical fiber connection structure. [Means for solving the problem]

[0006] The optical fiber bundle structure according to this disclosure comprises a plurality of single-core fibers and a ferrule having a holding hole into which the plurality of single-core fibers are inserted. Each of the plurality of single-core fibers has an MFD (Mode Field Diameter) expansion portion at its tip, which is a portion in which the mode field diameter is enlarged. The cladding of each single-core fiber in the MFD expansion portion is thinner than the cladding of each single-core fiber in the portion other than the MFD expansion portion. At least two of the plurality of MFD expansion portions are in contact with each other in the holding hole. [Effects of the Invention]

[0007] According to this disclosure, the photocoupling efficiency can be increased. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 shows an optical fiber connection structure according to an embodiment. [Figure 2] Figure 2 is a cross-sectional view showing an optical fiber bundle structure according to an embodiment. [Figure 3] Figure 3 shows the steps of the manufacturing method for the optical fiber bundle structure according to the embodiment. [Figure 4]Figure 4 shows the steps of the manufacturing method for the optical fiber bundle structure according to the embodiment. [Figure 5] Figure 5 shows the steps of the manufacturing method for the optical fiber bundle structure according to the embodiment. [Figure 6] Figure 6 shows a magnified view of the MFD (Multi-Function Display) section. [Figure 7] Figure 7 shows the steps for manufacturing an optical fiber bundle structure. [Figure 8] Figure 8 shows a modified optical fiber bundle structure. [Figure 9] Figure 9 shows the steps for manufacturing a modified optical fiber bundle structure. [Figure 10] Figure 10 shows the steps of the manufacturing method for a modified optical fiber bundle structure. [Figure 11] Figure 11 shows the steps for manufacturing a modified optical fiber bundle structure. [Figure 12] Figure 12 is a cross-sectional view showing a modified optical fiber bundle structure. [Modes for carrying out the invention]

[0009] [Description of Embodiments of the Present Invention] First, embodiments of the optical fiber bundle structure, the method for manufacturing the optical fiber bundle structure, and the optical fiber connection structure according to this disclosure will be listed and described. (1) The optical fiber bundle structure according to this embodiment comprises a plurality of single-core fibers and a ferrule having a holding hole into which the plurality of single-core fibers are inserted. Each of the plurality of single-core fibers has an MFD (Mode Field Diameter) enlargement portion at its tip, which is a portion in which the mode field diameter is enlarged. The cladding of each single-core fiber in the MFD enlargement portion is thinner than the cladding of each single-core fiber in the portion other than the MFD enlargement portion. At least two of the plurality of MFD enlargement portions are in contact with each other in the holding hole.

[0010] In this optical fiber bundle structure, each of the multiple single-core fibers inserted into the ferrule's holding hole has an MFD (Modular Function Diameter) expansion portion, which is a portion with an enlarged mode field diameter. The MFD expansion portion is formed at the tip of the single-core fiber. The cladding in the portion where the MFD expansion portion is formed is thinner than the cladding in the portion where the MFD expansion portion is not formed, and the multiple MFD expansion portions are in contact with each other in the holding hole. At least two of the multiple MFD expansion portions are in contact with each other in the holding hole, which reduces the gap between the multiple single-core fibers in the holding hole. Therefore, it is possible to prevent the pitch of the single-core fibers from becoming larger than the design value, thereby improving the optical coupling efficiency.

[0011] (2) In (1) above, the difference between the cladding diameter of each single core fiber in the part other than the MFD enlargement section and the cladding diameter of each single core fiber in the MFD enlargement section may be 5 μm or less. In this case, it is possible to prevent the cladding in the MFD enlargement section from becoming too thin.

[0012] (3) In (1) or (2) above, each of the multiple MFD magnification sections may be in contact with at least one of the multiple MFD magnification sections.

[0013] (4) The method for manufacturing the optical fiber bundle structure according to this embodiment comprises the steps of: heating a single core fiber to form an MFD enlargement portion on the single core fiber, which is a portion in which the mode field diameter is enlarged and the cladding is narrowed; cutting the MFD enlargement portion; inserting the MFD enlargement portions of a plurality of single core fibers obtained by cutting the MFD enlargement portion into a holding hole of a ferrule, and arranging the plurality of single core fibers in the holding hole such that the direction in which the plurality of MFD enlargement portions bend is in a direction that approaches each other.

[0014] In this manufacturing method, an MFD enlargement portion is formed by heating a single-core fiber, and a plurality of single-core fibers having the MFD enlargement portion are obtained by cutting the MFD enlargement portion. The MDF enlargement portion formed by heating and cutting is bent, and the plurality of MFD enlargement portions are inserted into the holding holes of the ferrule. In the holding holes, the plurality of single-core fibers are arranged such that the directions in which the plurality of MFD enlargement portions are bent approach each other. Since the directions in which the plurality of MFD enlargement portions are bent approach each other, the plurality of MFD enlargement portions contact each other in the holding holes. Therefore, the gap between the plurality of single-core fibers in the holding holes can be reduced, and the optical coupling efficiency can be increased.

[0015] (5) In the above (4), the manufacturing method may include a step of reducing the diameter of the MFD enlargement portion after the step of cutting the MFD enlargement portion and before the step of arranging the plurality of single-core fibers. In this case, by reducing the diameter of the MFD enlargement portion, the optical coupling efficiency can be further increased.

[0016] (6) In the above (5), in the step of reducing the diameter, the MFD enlargement portion may be reduced in diameter by chemically etching the MFD enlargement portion. In this case, the reduction in diameter of the MDF enlargement portion can be easily performed.

[0017] (7) The optical fiber connection structure according to the present embodiment includes a multi-core fiber, the optical fiber bundle structure described above, a first lens facing the first end surface which is the end surface of the multi-core fiber, and a second lens disposed between the second end surface which is the end surfaces of the plurality of single-core fibers of the optical fiber bundle structure and the first lens. From this optical fiber connection structure, the same effects as those of the optical fiber bundle structure described above can be obtained.

[0018] [Details of Embodiments of the Present Disclosure] Specific examples of the optical fiber bundle structure, the method for manufacturing the optical fiber bundle structure, and the optical fiber connection structure of this disclosure will be described below with reference to the drawings. The present invention is not limited to the examples described below, but is intended to be limited to those shown in the claims and to include all modifications within the scope equivalent to the claims. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant descriptions are omitted as appropriate. The drawings are simplified or exaggerated in part for ease of understanding, and the dimensional ratios are not limited to those shown in the drawings.

[0019] Figure 1 shows an optical fiber connection structure 1 according to an embodiment. The optical fiber connection structure 1 is, for example, a fan-in / fan-out device (FIFO) for a lens-coupled multicore fiber. Hereinafter, multicore fibers will be described as MCF and single-core fibers as SCF. The optical fiber connection structure 1 comprises an MCF 10, an optical fiber bundle structure 20, a first lens 30 interposed between the MCF 10 and the optical fiber bundle structure 20, and a second lens 40 interposed between the optical fiber bundle structure 20 and the first lens 30.

[0020] The MCF10 has multiple (e.g., two) cores 11 and cladding 12. The MCF10 is held by a ferrule 13. The optical fiber bundle structure 20 has, for example, two SCF21. Each SCF21 has a core 22 and cladding 23. The optical fiber connection structure 1 is a fan-in / fan-out device that separates the light L passing through the MCF10 into multiple SCF21, or combines the light L passing through each of the multiple SCF21 into a single MCF10. The light L is, for example, light having a wavelength in the 1.55 μm band.

[0021] The optical fiber connection structure 1 may be used, for example, as an optical amplifier that splits the light L passing through each core 11 of the MCF 10 toward each of the multiple SCF 21 and amplifies each of the split light L. The optical fiber connection structure 1 may be used as an optical transmitter that transmits light L from each of the multiple SCF 21, or as an optical receiver that receives light L from each of the multiple SCF 21.

[0022] In the optical fiber connection structure 1, the MCF10, the first lens 30, the second lens 40, and the optical fiber bundle structure 20 are arranged in this order along the optical axis direction D1, which is the direction in which the optical axis of the SCF21 extends. The MCF10 and SCF21 are optically coupled (spatially coupled) through space.

[0023] Figure 2 is a cross-sectional view of the optical fiber bundle structure 20, obtained by cutting the optical fiber bundle structure 20 with a plane perpendicular to the optical axis direction D1. As shown in Figures 1 and 2, multiple SCFs 21 are bundled together in a ferrule 24. The ferrule 24 has retaining holes 24b into which multiple SCFs 21 are inserted. As an example, two SCFs 21 are packed into the ferrule 24.

[0024] In a cross-section cut by a plane perpendicular to the optical axis direction D1, the arrangement shape of the multiple cores 22 of SCF21 and the arrangement shape of the multiple cores 11 of MCF10 are similar. If the pitch (center-to-center distance) of the cores 11 of MCF10 is P1 (μm) and the pitch of the cores 22 of SCF21 is P2 (μm), then P2 is larger than P1. For example, P2 is equal to the diameter of SCF21 (cladding 23).

[0025] If MFD1 (μm) is the mode field diameter of light L of a specific wavelength at the exit end of core 11 of MCF10, and MFD2 (μm) is the mode field diameter of light L of the same specific wavelength at the exit end of core 22 of SCF21, then MFD2 is larger than MFD1. Hereafter, "mode field diameter" may be referred to as MFD. P1, P2, MFD1, and MFD2 satisfy the following equation (1). (P1 / P2)×0.9≦MFD1 / MFD2≦(P1 / P2)×1.1 (1)

[0026] MCF10 has a first end face 14 facing the first lens 30. SCF21 has a second end face 25 facing the second lens 40. For example, both the first lens 30 and the second lens 40 are rod lenses. The first lens 30 has, for example, a flat surface 31 facing the MCF10 and a curved surface 32 opposite to the flat surface 31. The second lens 40 has, for example, a flat surface 41 facing multiple SCF21s and a curved surface 42 opposite to the flat surface 41. If the focal length of the first lens 30 is f1 (mm) and the focal length of the second lens 40 is f2 (mm), then, for example, f2 is greater than f1. The aforementioned P1 and P2 and f1 and f2 satisfy the following equation (2). (P1 / P2)×0.9 ≦ f1 / f2 ≦(P1 / P2)×1.1 ···(2)

[0027] For example, SCF21 is a TEC (Thermally Expanded Core) fiber. SCF21 has an MFD expansion section 26 at its tip, which is a portion where the MFD is expanded. For example, the MFD expansion section 26 includes a core expansion section 26b, which is a portion where the core 22 is expanded in a tapered shape. Because the core 22 is expanded at the tip of SCF21, the MFD of the light L that has propagated through SCF21 spreads in the core expansion section 26b in a direction perpendicular to the optical axis direction D1.

[0028] Multiple MFD enlarged sections 26 are in contact with each other in the holding holes 24b. The cladding 23 of each SCF 21 in the MFD enlarged section 26 is thinner than the cladding 23 of each SCF 21 in the parts other than the MFD enlarged section 26. The difference between the cladding diameter of each SCF 21 in the parts other than the MFD enlarged section 26 and the cladding diameter of each SCF 21 in the MFD enlarged section 26 is, for example, 0.5 μm or more and 5 μm or less.

[0029] A method for manufacturing the optical fiber bundle structure according to this embodiment will be described below. The method for manufacturing the optical fiber bundle structure 20 described above will be described below. In Figures 3 to 7 and Figures 9 to 11, hatching is omitted for ease of understanding. As shown in Figure 3, an SCF 21 is prepared, and a part of the coating 27 of the SCF 21 is removed to expose the cladding 23. For example, tension F is applied to the part of the SCF 21 where the cladding 23 is exposed. The magnitude of this tension F is, as an example, 30 g or less.

[0030] As described above, the portion of the SCF21 cladding 23 that is exposed is heated while tension F is applied. In Figure 3, the portion to be heated is indicated by a shaded area. As shown in Figures 3 and 4, when the portion of the SCF21 cladding 23 that is exposed is heated, the germanium dopant of the core 22 diffuses in that portion, causing the MFD to expand. This heating causes the heated portion to stretch and narrow. For example, if the outer diameter of the cladding 23 is 125 μm, the outer diameter of the heated portion of the cladding 23 will be 122 μm or more and 124 μm or less. In this way, by heating the SCF21, an MFD-expanded portion 26 is formed in the SCF21, which is a portion in which the MFD is expanded and the cladding 23 is narrowed (step of forming the MFD-expanded portion).

[0031] As shown in Figures 5 and 6, the MFD enlargement portion 26 is cut (step of cutting the MFD enlargement portion). The heated and cut MFD enlargement portion 26 is bent. For example, the heated and cut MFD enlargement portion 26 bends downward due to gravity. As shown in Figure 7, the MFD enlargement portions 26 of the multiple SCF 21 obtained by heating are inserted into the holding holes 24b of the ferrule 24. The multiple SCF 21 are arranged in the holding holes 24b such that the direction D2 in which the multiple MFD enlargement portions 26 bend is towards each other (placement step). The multiple MFD enlargement portions 26 are brought into contact with each other in the holding holes 24b. After the holding holes 24b in which the multiple SCF 21 are arranged are filled with adhesive and the multiple SCF 21 are fixed in the holding holes 24b, the series of steps of the manufacturing method of the optical fiber bundle structure 20 is completed.

[0032] The effects obtained from the optical fiber connection structure 1, optical fiber bundle structure 20, and manufacturing method of the optical fiber bundle structure according to this embodiment will be described. In the optical fiber connection structure 1 and the optical fiber bundle structure 20, each of the multiple SCF 21 inserted into the holding hole 24b of the ferrule 24 has an MFD enlargement portion 26, which is a portion in which the MFD is enlarged. The MFD enlargement portion 26 is formed at the tip of the SCF 21. The cladding 23 of the portion of each SCF 21 in which the MFD enlargement portion 26 is formed is thinner than the cladding 23 of the portion in which the MFD enlargement portion 26 is not formed, and the multiple MFD enlargement portions 26 are in contact with each other in the holding hole 24b. At least two of the multiple MFD enlargement portions 26 are in contact with each other in the holding hole 24b. By at least two of the multiple MFD enlargement portions 26 being in contact with each other in the holding hole 24b, the gap between the multiple SCF 21 in the holding hole 24b can be reduced. Therefore, it is possible to prevent the pitch of the SCF 21 from becoming larger than the design value, and thus the optical coupling efficiency can be increased. Each of the multiple MFD magnification sections 26 may be in contact with at least one of the multiple MFD magnification sections 26.

[0033] The difference between the cladding diameter of each SCF21 in areas other than the MFD magnification section 26 and the cladding diameter of each SCF21 in the MFD magnification section 26 may be 5 μm or less. In this case, it is possible to prevent the cladding 23 in the MFD magnification section 26 from becoming too thin.

[0034] In the manufacturing method of the optical fiber bundle structure according to this embodiment, an MFD (Multi-Function Display) expansion portion 26 is formed by heating an SCF (Single Fiber Optic) 21, and a plurality of SCFs 21 having the MFD expansion portion 26 are obtained by cutting the MFD expansion portion 26. The MFD expansion portion 26 formed by heating and cutting is curved, and the plurality of MFD expansion portions 26 are inserted into the holding hole 24b of the ferrule 24. In the holding hole 24b, the plurality of SCFs 21 are arranged such that the direction D2 in which the plurality of MFD expansion portions 26 bend is toward each other. Because the direction D2 in which the plurality of MFD expansion portions 26 bend is toward each other, the plurality of MFD expansion portions 26 come into contact with each other in the holding hole 24b. Therefore, the gap between the plurality of SCFs 21 in the holding hole 24b can be reduced, and the optical coupling efficiency can be increased.

[0035] The following describes the modified optical fiber connection structure, optical fiber bundle structure, and method for manufacturing the optical fiber bundle structure. Parts of the modified optical fiber connection structure, optical fiber bundle structure, and method for manufacturing the optical fiber bundle structure are the same as those described above for optical fiber connection structure 1, optical fiber bundle structure 20, and method for manufacturing the optical fiber bundle structure. In the following, descriptions of components identical to those previously described will be omitted as appropriate, using the same reference numerals.

[0036] Figure 8 shows modified optical fiber connection structures 1A and 1B. As shown in Figures 8(1) and (2), the optical fiber connection structures 1A and 1B have a first lens 30A and a second lens 40A that are different from the first lens 30 and second lens 40 described above. The first lens 30A has a plane 31A facing the MCF10 and a plane 32A opposite to plane 31. The second lens 40A has a plane 41A facing the plurality of SCF21 and a plane 42A opposite to plane 41A. The optical fiber connection structure 1B has the first lens 30A and the second lens 40 described above. Thus, various lenses can be used as the first lens and the second lens. The first lens and the second lens may each be a rod-shaped spherical lens, a GRIN lens, or an aspherical lens.

[0037] A method for manufacturing an optical fiber bundle structure according to a modified example will be described. As shown in Figures 9 and 10, after performing the step of cutting the MFD enlargement portion 26, the MFD enlargement portion 26 is reduced in diameter by chemical etching (step of reducing the diameter of the MFD enlargement portion). For example, etching is performed by immersing the MFD enlargement portion 26 in chemical solution E. Chemical solution E is, for example, a buffered hydrofluoric acid solution or a hydrofluoric acid solution.

[0038] As shown in Figure 11, by performing the aforementioned chemical etching, a tapered portion 28 including the MFD enlarged portion 26 is formed at the tip of the SCF 21. For example, the outer diameter of the tapered portion 28 in the portion other than the MFD enlarged portion 26 is 120 μm, and the outer diameter of the tapered portion 28 in the MFD enlarged portion 26 is 115 μm or more and 120 μm or less. After the step of tapering the MFD enlarged portion 26, multiple SCF 21s are placed in the holding hole 24b so that the bending direction D2 of the multiple MFD enlarged portions 26 are in a direction that approaches each other, similar to the manufacturing method described above (placement step). After that, the series of steps is completed.

[0039] The method for manufacturing the modified optical fiber bundle structure described above includes a step of reducing the diameter of the MFD magnification section 26 after the step of cutting the MFD magnification section 26 and before the step of arranging the multiple SCFs 21. In this case, the optical coupling efficiency can be further increased by reducing the diameter of the MFD magnification section 26. More specifically, by reducing the diameter of the MFD magnification section 26, the magnification of the first lens 30 and the second lens 40 can be reduced, thereby reducing distortion and increasing the coupling efficiency of the optical fiber L. In the diameter reduction step, the MFD magnification section 26 may be reduced in diameter by chemical etching. In this case, the diameter reduction of the MFD magnification section 26 can be easily carried out.

[0040] Figure 12 is a cross-sectional view showing a modified optical fiber bundle structure. In the modified optical fiber bundle structure, the number of SCFs 21 and the shape of the retaining holes in the ferrule 24 differ from the embodiments described above. As shown in Figure 12, the optical fiber bundle structure may have four SCFs 21. As shown in Figure 12(1), the shape of the cross section perpendicular to the optical axis direction D1 in the retaining hole 24c of the ferrule of the optical fiber bundle structure is rectangular. As shown in Figure 12(2), the shape of the cross section perpendicular to the optical axis direction D1 in the retaining hole 24d of the ferrule of the optical fiber bundle structure may be circular. Thus, in the optical fiber bundle structure, the number of SCFs and the shape of the cross section of the retaining holes can be changed as appropriate.

[0041] Embodiments and various modifications of the optical fiber bundle structure, method for manufacturing the optical fiber bundle structure, and optical fiber connection structure according to this disclosure have been described above. However, the optical fiber bundle structure, method for manufacturing the optical fiber bundle structure, and optical fiber connection structure according to this disclosure are not limited to the embodiments or modifications described above, and may be further modified within the scope of the gist described in the claims. The shape, size, material, number and arrangement of each part of the optical fiber bundle structure and optical fiber connection structure according to this disclosure, as well as the content and sequence of steps in the method for manufacturing the optical fiber bundle structure, can be appropriately changed within the scope of the gist described above. For example, in the embodiment described above, an example was described in which tension F is applied to the exposed portion of the cladding 23 of the SCF21 in the method for manufacturing the optical fiber bundle structure. However, it is not necessary to apply tension F to the exposed portion of the cladding 23 of the SCF21. [Explanation of Symbols]

[0042] 1,1A,1B…Optical fiber connection structure 10…MCF (Multicore Fiber) 11... Core 12... Clad 13... Ferrule 14...First end surface 20…Optical fiber bundle structure 21…SCF (Single Core Fiber) 22... Core 23... Clad 24... Ferrule 24b, 24c, 24d…retention hole 25…Second end surface 26…MFD enlarged section 26b... Core enlargement section 27... Covering 28...Narrowed diameter part 30,30A…First lens 31,31A…Plane 32...Curved surface 32A…Plane 40, 40A…Second lens 41,41A…Plane 42...Curved surface 42A…Plane E...Medicinal solution

Claims

1. Multiple single-core fibers, A ferrule having a retaining hole into which a plurality of the single core fibers are inserted, Equipped with, Each of the multiple single-core fibers has an MFD enlargement portion at its tip, which is a portion in which the mode field diameter is enlarged. The cladding of each single-core fiber in the MFD enlargement section is thinner than the cladding of each single-core fiber in the section other than the MFD enlargement section. At least two of the multiple MFD enlargements are in contact with each other in the holding hole. Optical fiber bundle structure.

2. The difference between the cladding diameter of each single-core fiber in the portion other than the MFD enlargement section and the cladding diameter of each single-core fiber in the MFD enlargement section is 5 μm or less. The optical fiber bundle structure according to claim 1.

3. The optical fiber bundle structure according to claim 1 or claim 2, wherein each of the plurality of MFD enlargements is in contact with at least one of the plurality of MFD enlargements.

4. A step of heating a single-core fiber to form an MFD enlargement portion in the single-core fiber, which is a portion in which the mode field diameter is enlarged and the cladding is narrowed, The process of cutting the enlarged portion of the MFD, The steps include inserting the MFD enlarged portions of the multiple single core fibers obtained by cutting the MFD enlarged portions into the holding holes of the ferrule, and arranging the multiple single core fibers in the holding holes such that the bending directions of the multiple MFD enlarged portions are toward each other, Equipped with, A method for manufacturing an optical fiber bundle structure.

5. The process includes a step of reducing the diameter of the MFD enlargement portion after the step of cutting the MFD enlargement portion and before the step of arranging the multiple single core fibers. A method for manufacturing an optical fiber bundle structure according to claim 4.

6. In the process of reducing the diameter, the MFD enlargement portion is reduced in diameter by chemical etching. A method for manufacturing an optical fiber bundle structure according to claim 5.

7. Multicore fiber and The optical fiber bundle structure according to claim 1 or claim 2, A first lens facing the first end face, which is the tip surface of the multicore fiber, A second lens is disposed between the first lens and the second end face, which is the tip face of one of the multiple single-core fibers of the optical fiber bundle structure, Equipped with, Optical fiber connection structure.

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