Optical fiber bundle structure, connection structure between optical fiber bundle structure and multi-core fiber, method for manufacturing optical fiber bundle structure

Abstract

To provide an optical fiber bundle structure having excellent manufacturability and capable of being fused and connected to a multi-core fiber with low loss.SOLUTION: An optical fiber bundle structure 3 is formed by collecting a plurality of optical fiber cores 7 and inserting them into a capillary 9. At a tip part of each optical fiber core 3, a glass fiber 13 is exposed from a coating resin. A rear end part of the capillary 9 is made into a large diameter part 12, and a resin coating part of the optical fiber core 7 is arranged. At a tip part of the capillary 9, a reduced diameter part 11 having a smaller diameter than the large diameter part 12 is formed. At the reduced diameter part 11 of the capillary 9, a part of the tip part of the optical fiber core 7 (a small diameter part 15 of the glass fiber 13) is fused to an inner surface of the capillary 9 at a fusion part 25a.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical fiber bundle structure and the like that can be connected to a multicore fiber in which cores are arranged at predetermined intervals. [Background technology]

[0002] The recent rapid increase in traffic in optical communications has led to a demand for increased transmission capacity. As a means of further expanding communication capacity, multi-core fibers, in which multiple cores are formed in a single optical fiber, have been proposed instead of single-core optical fibers.

[0003] When a multicore fiber is used as a transmission line, each core of the multicore fiber must be connected to a different optical fiber, optical element, etc. to send and receive transmission signals. In other words, fan-in and fan-out are required to connect the multicore fiber to multiple single-core fibers.

[0004] As a method for connecting such a multi-core fiber and a single-core fiber, a method has been proposed in which a plurality of single-core fibers are drawn and bundled together (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-1673 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when the optical fiber is stretched significantly as in Patent Document 1, the variation in core pitch and deformation of the core become large, which raises concerns about reliability.

[0007] On the other hand, to regulate the core pitch, there is a method of inserting the optical fiber core into a multi-hole capillary and adhesively fixing it, but this method is difficult to manufacture and has problems with the accuracy of the hole position. Furthermore, since fusion splicing with a multi-core fiber is desirable to strengthen high-power durability, it is also desirable to fix the optical fiber and the capillary by fusion.

[0008] The present invention has been made in view of the above problems, and has an object to provide an optical fiber bundle structure or the like that is easy to manufacture and that can be fusion-spliced ​​to a multi-core fiber with low loss. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the first invention is an optical fiber bundle structure in which a plurality of optical fiber core wires are assembled, wherein the plurality of optical fiber core wires are inserted into a single-hole capillary, a tapered portion is formed at the tip of the capillary, and the optical fiber core wires are fused to the inner surface of the capillary in the tapered portion.

[0010] It is desirable that at least some of the optical fibers inside the capillary are fused together.

[0011] The capillary may be substantially circular at the reduced diameter portion.

[0012] The capillary may have a flattened shape at the reduced diameter portion.

[0013] In the reduced diameter portion, it is desirable that the fusion length between the capillary and the optical fiber be 50 mm or less.

[0014] In the reduced diameter portion, the optical fiber may have a flattened shape, and the ratio of the difference between the diameter on the major axis side and the diameter on the minor axis side to the diameter on the major axis side may be 30% or less.

[0015] A small diameter portion having an outer diameter smaller than that of other portions may be formed in a predetermined length of the tip of the optical fiber core, and a part of the tip of the small diameter portion may be fused to the capillary at the reduced diameter portion.

[0016] A large diameter portion, a first reduced diameter portion having a diameter smaller than that of the large diameter portion, and a second reduced diameter portion having a diameter smaller than that of the first reduced diameter portion may be formed in this order from the rear end side to the tip end side of the capillary, and the optical fiber core may be fused to the inner surface of the capillary at the second reduced diameter portion.

[0017] According to the first invention, a tapered portion is formed at the tip of the capillary, and the optical fiber core is fused to the inner surface of the capillary at the tapered portion, so that the optical fiber core can be securely fixed inside the tapered portion.

[0018] Furthermore, if at least some of the optical fibers inside the capillary are fused together, the optical fibers can be positioned more reliably.

[0019] Furthermore, if the capillary has a substantially circular shape at the reduced diameter portion, the precision of the core pitch of the coated optical fiber within the capillary is high.

[0020] If the capillary has a flattened shape such as a substantially elliptical shape at the reduced diameter portion, the gap between the capillary and the optical fiber core can be reduced.

[0021] Furthermore, by setting the fusion length between the capillary and the optical fiber at the reduced diameter portion to 50 mm or less, crosstalk can be reduced.

[0022] In addition, when the optical fiber core is flattened in the tapered portion, the loss at the connection portion can be reduced by setting the ratio of the difference between the diameter on the long axis side and the diameter on the short axis side to the diameter on the long axis side to 30% or less.

[0023] Furthermore, by forming a small diameter portion whose outer diameter is smaller than the outer diameter of other portions at a predetermined length of the tip of the optical fiber core, it is possible to use an optical fiber core of a general diameter and approach the core pitch of the multi-core fiber at the connection portion.

[0024] Furthermore, by forming a reduced diameter portion at the tip of the capillary in advance and then further drawing the tip after inserting the optical fiber core, the amount of drawing can be reduced. In this way, an optical fiber bundle structure can be obtained in which, from the rear end side to the tip end side of the capillary, a large diameter portion, a first reduced diameter portion whose diameter is smaller than that of the large diameter portion, and a second reduced diameter portion whose diameter is smaller than that of the first reduced diameter portion are formed in this order.

[0025] The second invention is a connection structure in which the optical fiber bundle structure according to the first invention and a multi-core fiber are fusion-connected, characterized in that a core of the optical fiber coated wire is optically connected to each core of the multi-core fiber.

[0026] It is desirable that the deviation in core pitch between the multicore fiber and the optical fiber be 1.5 μm or less.

[0027] According to the second invention, since the optical fiber bundle structure and the multi-core fiber are fusion-spliced, an optical connection structure with high high-output resistance can be obtained.

[0028] Furthermore, if the core pitch difference between the multicore fiber and the optical fiber is 1.5 μm or less, an optical connection structure with small connection loss can be obtained.

[0029] The third invention is a method for manufacturing an optical fiber bundle structure, comprising: step a) of inserting a plurality of optical fiber core wires into a single-hole capillary; step b) of heating and stretching the tip of the capillary to form a tapered portion; and step c) of cutting or polishing the tip of the tapered portion to expose the end face of the optical fiber core wire, wherein in step b) the tapered portion and the optical fiber core wire are fused together.

[0030] In the step b, the tip of the capillary may be heated by three-phase discharge generated from three electrodes.

[0031] Before step a, step d may be included in which the tip of the capillary is reduced in diameter to form a first reduced diameter section, and in step a, the tip of the optical fiber core is inserted into the first reduced diameter section, and in step b, the tip of the first reduced diameter section is extended to form a second reduced diameter section, and the second reduced diameter section and the optical fiber core are fused together.

[0032] According to the third invention, the tip of the capillary is heated and stretched to form a narrowed portion, and the narrowed portion is fused to the optical fiber core, thereby bringing the optical fiber cores close to each other and fixing the capillary and the optical fiber cores together without stretching the optical fiber cores significantly.

[0033] Furthermore, by heating the tip of the capillary by three-phase discharge generated from three electrodes, the capillary can be heated almost uniformly and elongated.

[0034] Furthermore, by forming a reduced diameter portion at the tip of the capillary in advance, and then further drawing the tip portion after inserting the optical fiber core, the amount of drawing can be reduced. [Effects of the Invention]

[0035] According to the present invention, it is possible to provide an optical fiber bundle structure or the like that is easy to manufacture and that can be fusion-spliced ​​to a multi-core fiber with low loss. [Brief explanation of the drawings]

[0036] [Figure 1] 1A is a cross-sectional view showing an optical fiber connection structure 1, FIG. 1B is a cross-sectional view taken along line AA in FIG. 1A, and FIG. 1C is a cross-sectional view taken along line BB in FIG. [Figure 2] 10(a) to 10(c) are cross-sectional views of other forms of the optical fiber bundle structure 3. [Figure 3] 1A is a diagram showing a state in which an optical fiber core 7 is inserted into a capillary 9, and FIG. 1B is a cross-sectional view of FIG. 1A taken along the line EE. [Figure 4] 10(a) and 10(b) are diagrams showing a process of drawing a capillary 9. FIG. [Figure 5] 10(a) to 10(c) are diagrams showing another method for manufacturing an optical fiber bundle structure. [Figure 6] 10(a) and 10(b) are cross-sectional views of other forms of the optical fiber bundle structure 3. FIG. [Figure 7] 10 is a cross-sectional view of another embodiment of the optical fiber bundle structure 3. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0037] The optical fiber connection structure will be described below. Fig. 1(a) is a cross-sectional view showing the optical fiber connection structure 1, Fig. 1(b) is a cross-sectional view taken along line AA in Fig. 1(a), and Fig. 1(c) is a cross-sectional view taken along line BB in Fig. 1(a). The optical fiber connection structure 1 is a connection structure in which an optical fiber bundle structure 3 and a multi-core fiber 5 are fusion-spliced.

[0038] The optical fiber bundle structure 3 is constructed by assembling a plurality of optical fiber cores 7 and inserting them into a single-hole capillary 9. In the illustrated example, two optical fiber cores 7 are inserted into the single-hole capillary 9. The optical fiber cores 7 are single-core optical fibers having a core 19 and a cladding covering the core 19. At a predetermined length at the tip of each optical fiber core 7, a glass fiber 13 is exposed from the coating resin.

[0039] In the illustrated example, a small diameter portion 15 having an outer diameter smaller than the outer diameter of the remaining portion of the glass fiber 13 is formed in a predetermined length at the tip of the glass fiber 13. The small diameter portion 15 is formed by, for example, etching. If the optical fiber core 7 is a small-diameter fiber, the glass fiber 13 may have the same outer diameter without the small diameter portion 15.

[0040] The capillary 9 has a large diameter portion 12 at its rear end, and a part of the resin coating of the optical fiber 7 is disposed inside the large diameter portion 12. The capillary 9 has a reduced diameter portion 11 at its front end, the reduced diameter portion 11 having a diameter smaller than that of the large diameter portion 12. The reduced diameter portion 11 is formed by drawing. A method for manufacturing the optical fiber bundle structure 3 will be described later.

[0041] 1(b), the multi-core fiber 5 is configured by integrating a plurality of cores 21 with a cladding. In the illustrated example, two cores 21 are arranged at a predetermined interval. Note that the multi-core fiber 5 is fixed to, for example, a capillary 17, but the capillary 17 is not necessarily required.

[0042] 1(c), in the reduced diameter portion 11 of the capillary 9, a part of the tip of the optical fiber 7 (small diameter portion 15 of the glass fiber 13) is fused to the inner surface of the capillary 9 at a fusion portion 25a. In the reduced diameter portion 11, the fusion length in the axial direction between the inner surface of the capillary 9 and the optical fiber 7 (small diameter portion 15 of the glass fiber 13) is preferably 50 mm or less from the tip of the optical fiber bundle structure 3 (the connection end face with the multi-core fiber 5). By setting the fusion length between the capillary 9 and the optical fiber 7 in the reduced diameter portion 11 to 50 mm or less, crosstalk can be reduced, for example, to −60 dB or less.

[0043] In the illustrated example, the optical fiber cores 7 (tip portions of the glass fibers 13) are fused to each other at the fusion portion 25b inside the reduced diameter portion 11 of the capillary 9. Note that the optical fiber cores 7 (tip portions of the glass fibers 13) do not necessarily have to be fused to each other.

[0044] As described above, in the optical fiber connection structure 1, the cores 19 of the optical fiber 7 are optically connected to the cores 21 of the multicore fiber 5. Here, the pitch of the cores 19 of the pair of optical fiber cores 7 (glass fibers 13) at the connection part is approximately the same as the pitch of the cores 21 of the multicore fiber 5, and the deviation in core pitch between the multicore fiber 5 and the optical fiber 7 is, for example, 1.5 μm or less. In this way, loss at the connection part can be suppressed.

[0045] As described above, the reduced diameter portion 11 of the capillary 9 is formed by drawing the capillary 9. At this time, the glass fiber 13 (small diameter portion 15) inside is also slightly processed by fusion with the capillary 9. Therefore, the outer diameter of the small diameter portion 15 at the fused portion with the capillary 9 is slightly smaller than the outer diameter of the small diameter portion 15 other than the fused portion.

[0046] For example, when the pitch of the cores 21 of the multicore fiber 5 is 50 μm, the pitch of the cores 19 of the glass fiber 13 (small diameter portion 15) in the optical connection portion also needs to be 50 μm, but the outer diameter of the small diameter portion 15 other than the fused portion can be, for example, about 55 μm to 60 μm. That is, the diameter of the small diameter portion 15 is reduced by, for example, about 9 to 17% at the fused portion with the capillary 9 (connection portion with the multicore fiber 5). In this way, if the diameter of the small diameter portion 15 in the fused portion is 80% or more (diameter reduction rate is 20% or less) of the diameter of the small diameter portion 15 in the portion other than the fused portion, the amount of processing of the glass fiber 13 is small, so that the change in the optical properties of the glass fiber 13 is small, and the glass fiber 13 can be connected to the multicore fiber 5 with lower loss.

[0047] In the example shown in Fig. 1(c), the cross-sectional shape of the tapered portion 11 of the capillary 9 in the optical fiber bundle structure 3 is approximately circular, but is not limited to this. For example, as shown in Fig. 2(a), the cross-sectional shape of the tapered portion 11 may be flattened depending on the arrangement of the glass fibers 13 therein.

[0048] In this way, by making the capillary 9 (reduced diameter portion 11) flat so as to fit the arrangement of the glass fibers 13 inside, it is possible to reduce the gap (space) between the capillary 9 and the glass fibers 13. Therefore, it is possible to more reliably suppress positional deviation of the glass fibers 13 during fusion splicing, and to fusion splice with the multicore fiber 5. Note that, in the reduced diameter portion 11 (tip portion), the cross-sectional area (void ratio) of the space other than the glass fibers 13 relative to the cross-sectional area inside the capillary 9 is preferably less than 50%.

[0049] 2(a), the glass fiber 13 inside the diameter-reducing portion 11 has a substantially circular cross section, but as shown in FIG. 2(b), the cross section of the glass fiber 13 may be flattened in the diameter-reducing portion 11. That is, like the capillary 9, the internal glass fiber 13 may also be flattened.

[0050] In this case, it is desirable that the ratio of the difference between the diameter on the major axis side (C in the figure) and the diameter on the minor axis side (D in the figure) to the diameter on the major axis side is 30% or less. In other words, it is desirable that (CD) / C≦0.3. By doing so, it is possible to reduce the loss at the splice, for example, to 0.5 dB or less. Note that the outer edges of the glass fiber 13 are not necessarily clear due to the fusion splices, but it is possible to measure the diameters of each by dividing the boundaries of each fusion splice with a straight line.

[0051] 2(c), not only the optical fiber 7 (glass fiber 13) that is optically connected to the multi-core fiber 5 but also a dummy fiber 23 may be inserted into the capillary 9. In this case, a part of the dummy fiber 23 is also fused to the capillary 9. Also in this case, the dummy fiber 23 and the glass fiber 13 may or may not be fused.

[0052] By using such a dummy fiber 23, it is possible to reduce the gap (space) between the capillary 9 and the glass fiber 13. Therefore, it is possible to more reliably suppress the positional deviation of the glass fiber 13 during fusion splicing, and to fusion splice it to the multi-core fiber 5.

[0053] Next, a method for manufacturing the optical fiber bundle structure 3 will be described. First, a predetermined length of the coating at the tip of the optical fiber core 7 is stripped off to expose the glass fiber 13. If necessary, a small diameter portion 15 is formed at the tip of the glass fiber 13 by chemical etching or the like. For example, for a glass fiber 13 with a cladding diameter of 125 μm, the small diameter portion 15 has a diameter of 55 μm to 60 μm.

[0054] 3(a), the small diameter portions 15 of the multiple optical fiber cores 7 are inserted into a single-hole capillary 9. In this state, the coating portions of the optical fiber cores 7 may be adhesively fixed to the capillary 9 at the rear end of the capillary 9. Thereafter, the tip end of the capillary 9 is heated and stretched to form a reduced diameter portion.

[0055] 3(b) is a schematic diagram showing the heating process in the cross section taken along line EE in FIG. 3(a). As shown in the figure, the heating of the vicinity of the tip of the capillary 9 is preferably performed by three-phase discharge discharged from three electrodes 27. In this way, a substantially uniform heating zone is formed over a relatively wide range by the discharge between the electrodes 27, and the capillary 9 can be heated substantially uniformly.

[0056] 4(a) is a conceptual diagram showing a process of drawing forward the tip of the capillary 9 softened by heating. The drawing process reduces the diameter of the tip of the capillary 9, forming a reduced diameter portion 11. Furthermore, the reduction in diameter brings the inner surface of the capillary 9 into contact with the outer surface of the glass fiber 13, forming a fused portion 25a.

[0057] 4(b), drawing is performed until the pitch of the cores 19 of the glass fibers 13 (small diameter portions 15) reaches a predetermined pitch (until the capillary 9 and the glass fibers 13 come into contact with each other). At this time, as described above, the small diameter portions 15 may be slightly drawn together with the capillary 9. Furthermore, fused portions 25b may be formed in parts of the contact portions between the glass fibers 13.

[0058] Thereafter, the end face of the optical fiber core 7 (glass fiber 13) is exposed by cutting or polishing the tip of the reduced diameter portion 11. In this way, the optical fiber bundle structure 3 is formed.

[0059] Note that instead of the above-described manufacturing method, the following method may be used. First, as shown in Fig. 5(a), before inserting the optical fiber 7, the tip of the capillary 9 is reduced in diameter to form a reduced diameter portion 11a, which is a first reduced diameter portion. That is, one end of the capillary 9 becomes a large diameter portion 12, and the other end becomes a reduced diameter portion 11a. Note that the capillary 9 has a tapered shape so that the diameter gradually decreases from the large diameter portion 12 to the reduced diameter portion 11a.

[0060] 5(b), the optical fiber 7 is inserted into the capillary 9 from the large diameter portion 12 side, and the glass fiber 13 (small diameter portion 15) at the tip of the optical fiber 7 is placed in the reduced diameter portion 11a. Here, the inner diameter of the reduced diameter portion 11a is such that the small diameter portions 15 of the multiple glass fibers 13 can be inserted therethrough. That is, in this state, some gaps are formed between the multiple glass fibers 13 inside the reduced diameter portion 11a or between the glass fibers 13 and the capillary 9.

[0061] From this state, as shown in Figure 5(c), the tip of the reduced diameter portion 11a is heated and a drawing process is performed to form a second reduced diameter portion, namely a reduced diameter portion 11b. At this time, the reduced diameter portion 11b and the optical fiber core 7 (glass fiber 13) are fused together. Thereafter, the tip of the reduced diameter portion 11 is cut or polished to expose the end face of the optical fiber core 7 (glass fiber 13). In this way, the optical fiber bundle structure 3 is formed.

[0062] The optical fiber bundle structure 3 obtained by this method has a large diameter portion 12, a reduced diameter portion 11a having a diameter smaller than that of the large diameter portion 12, and a reduced diameter portion 11b having a diameter smaller than that of the reduced diameter portion 11a, formed in this order from the rear end side to the front end side of the capillary 9. The optical fiber core 7 is fused to the inner surface of the capillary 9 at the reduced diameter portion 11b.

[0063] As described above, according to this embodiment, the capillary 9 is drawn to form a tapered portion, and the capillary 9 is fused to the glass fiber 13 therein at the tapered portion, so that the glass fiber 13 can be reliably fixed to the capillary 9 and displacement can be suppressed. Furthermore, since the drawing (melting) of the glass fiber 13 itself is hardly performed or only slightly performed, the core pitch can be easily controlled with high precision.

[0064] For example, the outer diameter of the glass fiber 13 at the fused portion is 80% or more of the outer diameter of the small-diameter portion 15 other than the fused portion (other than the reduced diameter portion 11), and the change in diameter of the glass fiber 13 due to the drawing process is small, so the effects of core pitch fluctuations and core deformation, etc. are small, and the change in the optical properties of the glass fiber 13 is small, allowing it to be connected to the multi-core fiber 5 with lower loss.

[0065] Furthermore, by forming the reduced diameter portion 11a in advance in the capillary 9, the amount of drawing processing can be made small when the glass fiber 13 is inserted therethrough. This allows processing to be performed with higher precision.

[0066] In the above-described embodiment, the bundle structure is made up of two optical fiber cores 7, but the number of optical fiber cores 7 is not particularly limited and is set according to the number of cores and core arrangement of the multi-core fiber 5 to be connected.

[0067] For example, as shown in Fig. 6(a), a bundle structure of four optical fiber cores 7 (glass fibers 13) may be used. In the illustrated example, the four glass fibers 13 are arranged so that the cross section is square. Even in this case, a fused portion 25a is formed at the contact point between the inner surface of the capillary 9 and the glass fibers 13. Furthermore, a fused portion 25b may be formed at the contact point between the glass fibers 13, or the glass fibers 13 may not be fused to each other.

[0068] 6(b), a bundle structure of seven optical fiber cores 7 (glass fibers 13) may also be used. In the illustrated example, the seven glass fibers 13 are arranged in a so-called close-packed arrangement in cross section. Even in this case, a fused portion 25a is formed at the contact point between the inner surface of the capillary 9 and the glass fibers 13. In this case, for example, a fused portion 25b is formed at the contact point between the six glass fibers 13 on the outer periphery, and the central glass fiber 13 may not be fused to the surrounding glass fibers 13. In this way, since there is no need to fused all the way to the inside, the drawing process can be performed without excessive heating.

[0069] 7, a bundle structure of 19 optical fiber cores 7 (glass fibers 13) may also be used. In the illustrated example, the 19 glass fibers 13 are arranged in a so-called close-packed arrangement in cross section. Even in this case, a fused portion 25a is formed at the contact portion between the inner surface of the capillary 9 and the glass fibers 13. Also in this case, for example, a fused portion 25b is formed at the contact portion between the 12 glass fibers 13 on the outermost side, and the seven glass fibers 13 in the center and second layer may not be fused to the other glass fibers 13. Even in this case, since it is not necessary to fused all the way to the inside, the drawing process can be performed without excessive heating.

[0070] Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the technical scope of the present invention is not limited to the above-described embodiments. It is clear that those skilled in the art can conceive of various modifications and alterations within the scope of the technical ideas described in the claims, and it is understood that these modifications and alterations also fall within the technical scope of the present invention. [Explanation of symbols]

[0071] 1...Optical fiber connection structure 3...Optical fiber bundle structure 5. Multicore fiber 7....Optical fiber core 9, 17...Capillary 11, 11a, 11b……Reduced diameter part 12...Large diameter section 13...Glass fiber 15……Small diameter part 19, 21... Core 23...Dummy fiber 25a, 25b……fused part 27... Electrode

Claims

1. An optical fiber bundle structure in which a plurality of optical fiber core wires are assembled, A plurality of the optical fiber core wires are inserted into a single-hole capillary, a reduced diameter portion is formed at the tip of the capillary; An optical fiber bundle structure characterized in that the optical fiber core is fused to the inner surface of the capillary in the reduced diameter portion.

2. 2. The optical fiber bundle structure according to claim 1, wherein at least some of the optical fibers inside the capillary are fused together.

3. 2. The optical fiber bundle structure according to claim 1, wherein the capillary has a substantially circular shape at the reduced diameter portion.

4. 2. The optical fiber bundle structure according to claim 1, wherein the capillary has a flat shape at the reduced diameter portion.

5. 2. The optical fiber bundle structure according to claim 1, wherein the fused length between the capillary and the optical fiber core in the reduced diameter portion is 50 mm or less.

6. 2. The optical fiber bundle structure according to claim 1, wherein the optical fiber core is flattened in the tapered portion, and the ratio of the difference between the diameter on the long axis side and the diameter on the short axis side to the diameter on the long axis side is 30% or less.

7. 2. The optical fiber bundle structure according to claim 1, wherein a small diameter portion having an outer diameter smaller than the outer diameter of other portions is formed at a predetermined length of the tip of the optical fiber core, and a part of the tip of the small diameter portion is fused to the capillary at the reduced diameter portion.

8. a large diameter portion, a first reduced diameter portion having a diameter smaller than that of the large diameter portion, and a second reduced diameter portion having a diameter smaller than that of the first reduced diameter portion are formed in this order from the rear end side to the front end side of the capillary; 2. The optical fiber bundle structure according to claim 1, wherein the optical fiber core is fused to the inner surface of the capillary at the second reduced diameter portion.

9. A connection structure in which the optical fiber bundle structure according to any one of claims 1 to 8 and a multi-core fiber are fusion-spliced, A connection structure between an optical fiber bundle structure and a multi-core fiber, characterized in that a core of the optical fiber coated wire is optically connected to each core of the multi-core fiber.

10. 10. The connection structure between an optical fiber bundle structure and a multi-core fiber according to claim 9, wherein a deviation in core pitch between the multi-core fiber and the optical fiber core is 1.5 μm or less.

11. 1. A method for manufacturing an optical fiber bundle structure, comprising: a step a of inserting a plurality of optical fiber cores into a single-hole capillary; a step b of heating and extending the tip of the capillary to form a reduced diameter portion; a step c of cutting or polishing the tip of the reduced diameter portion to expose the end face of the optical fiber; Equipped with A method for manufacturing an optical fiber bundle structure, wherein in the step (b), the reduced diameter portion and the optical fiber core are fused together.

12. 12. The method for manufacturing an optical fiber bundle structure according to claim 11, wherein in the step (b), the tip of the capillary is heated by three-phase discharge generated from three electrodes.

13. a step d of reducing the diameter of the tip of the capillary to form a first reduced diameter portion before the step a; In the step (a), the tip of the optical fiber is inserted into the first reduced diameter portion; 12. The method for manufacturing an optical fiber bundle structure according to claim 11, wherein in step b, a tip of the first reduced diameter portion is extended to form a second reduced diameter portion, and the second reduced diameter portion and the optical fiber core are fused together.

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