Optical connection structure and method for manufacturing optical connection structure

Abstract

An optical connection structure according to one embodiment comprises: four single-core fibers; a multicore fiber having two cores; a first ferrule having a first optical fiber holding hole into which the four single-core fibers are inserted; and a second ferrule having a second optical fiber holding hole into which the multicore fiber is inserted. In a cross-section orthogonal to the direction in which the four single-core fibers extend, the four single-core fibers are arranged in a square shape. Two single-core fibers positioned on one side of the square shape among the four single-core fibers arranged in the square are optically coupled to the two cores of the multicore fiber.

Description

Optical connection structure and method for manufacturing the optical connection structure

[0001] This disclosure relates to an optical connection structure and a method for manufacturing the same. This application claims priority to Japanese Application No. 2024-105383, filed on June 28, 2024, and incorporates by reference all the contents of said Japanese application.

[0002] Patent Document 1 describes an optical fiber bundle structure. This optical fiber bundle structure includes a plurality of optical fiber cores and a capillary that holds the plurality of optical fiber cores. The optical fiber cores include signal optical fibers and dummy optical fibers. Inside the capillary, a plurality of signal optical fiber groups in which a plurality of optical fibers including the signal optical fibers are aligned, and a dummy fiber group made of dummy optical fibers are stacked in multiple stages.

[0003] Patent Document 2 describes a connector system that converts a multicore fiber into a plurality of single-core fibers. The plurality of single-core fibers is housed in a ferrule. The plurality of single-core fibers is closely packed inside the ferrule. The plurality of single-core fibers includes a central single-core fiber and six single-core fibers that are arranged in a regular hexagon shape so as to surround the central single-core fiber.

[0004] Patent Document 3 describes a fiber splicing component. This fiber splicing component includes a thin fiber and a grooved substrate having a groove into which the thin fiber is filled. A V-shaped groove is formed in the grooved substrate.

[0005] International Publication No. 2016 / 021589 US Patent Application Publication No. 2019 / 0285810 JP 2014-10403 A

[0006] The optical connection structure according to the present disclosure includes four single-core fibers, a multi-core fiber having two cores, a first ferrule having a first optical fiber holding hole into which the four single-core fibers are inserted, and a second ferrule having a second optical fiber holding hole into which the multi-core fiber is inserted. In a cross section perpendicular to the direction in which the four single-core fibers extend, the four single-core fibers are arranged in a square shape. Of the four single-core fibers arranged in a square shape, two single-core fibers located on one side of the square are optically coupled to two cores of the multi-core fiber.

[0007] FIG. 1 is a side view showing an example of an optical connection structure according to an embodiment. FIG. 2 is a cross-sectional view showing a first optical connector according to an embodiment. FIG. 3 is a side view showing a single-core fiber according to an embodiment. FIG. 4 is a cross-sectional view along the extension direction of a first ferrule having a transition portion, and a cross-sectional view along the extension direction of a first ferrule without a transition portion. FIG. 5 is a diagram schematically showing tip faces of four single-core fibers and an end face of a first ferrule. FIG. 6 is a diagram schematically showing a tip face of a multi-core fiber and an end face of a second ferrule. FIG. 7 is a diagram showing tip faces of a plurality of single-core fibers. FIG. 8 is a diagram showing a tip face of a multi-core fiber. FIG. 9 is a diagram in which the tip face of the first optical connector is projected onto the tip face of the second optical connector. FIG. 10 is a front view showing the tip face of a second optical connector of an optical connection structure according to a first modified example. FIG. 11 is a cross-sectional view showing an optical connection structure according to a second modified example. FIG. 12 is a front view showing a second optical connector of an optical connection structure according to a third modified example.

[0008] In the optical connection structure, each of the multiple single-core fibers is optically coupled to each of the multiple cores of the multi-core fiber. However, there are cases where the multiple single-core fibers are misaligned, and in such cases, optical coupling is not easily achieved, which may result in optical loss.

[0009] An object of the present disclosure is to provide an optical connection structure that allows easy optical coupling and reduces optical loss, and a method for manufacturing the optical connection structure.

[0010] According to the present disclosure, optical coupling can be easily achieved and optical loss can be reduced.

[0011] The contents of the embodiments of the present disclosure will be described.

[0012] (1) An optical connection structure according to one embodiment includes four single-core fibers, a multi-core fiber having two cores, a first ferrule having a first optical fiber holding hole into which the four single-core fibers are inserted, and a second ferrule having a second optical fiber holding hole into which the multi-core fiber is inserted. In a cross section perpendicular to the direction in which the four single-core fibers extend, the four single-core fibers are arranged in a square shape. Of the four single-core fibers arranged in a square shape, two single-core fibers located on one side of the square are optically coupled to two cores of the multi-core fiber.

[0013] In this optical connection structure, four single-core fibers are inserted into the first optical fiber holding hole of the first ferrule, and a multi-core fiber having two cores is inserted into the second optical fiber holding hole of the second ferrule. Of the four single-core fibers arranged in a square shape in a cross section perpendicular to the direction in which the single-core fibers extend, two single-core fibers located on one side of the square are optically coupled to two cores of the multi-core fiber. By arranging the four single-core fibers in a square shape, the four single-core fibers can be packed closest in density in the first optical fiber holding hole. Therefore, two of the four single-core fibers can be easily optically coupled to two cores of the multi-core fiber, and optical loss can be reduced.

[0014] (2) In the above (1), the outer diameter of the single-core fiber may be the same as the distance between two cores of the multi-core fiber. In this case, the distance between two cores of two adjacent single-core fibers is equal to the distance between two cores of the multi-core fiber.

[0015] (3) In the above (1) or (2), the first ferrule may have a cylindrical shape, and the second ferrule may have a cylindrical shape.

[0016] (4) In any of the above (1) to (3), the single-core fiber may have a first diameter portion, a second diameter portion having a diameter larger than that of the first diameter portion, and a tapered diameter portion connecting the second diameter portion to the first diameter portion, where the diameter of the single-core fiber decreases toward the first diameter portion. In this case, since the second diameter portion has a diameter larger than that of the first diameter portion, the single-core fiber can be easily manipulated by gripping the second diameter portion. As a result, the single-core fiber can be easily inserted into the first optical fiber holding hole.

[0017] (5) In any of the above (1) to (4), the two cores of the multicore fiber may include a first core and a second core. When viewed along the extending direction, the distance from the center of the second optical fiber holding hole to the center of the second ferrule may be half the distance from the first core to the second core. The center of the second optical fiber holding hole is shifted with respect to the center of the second ferrule. Accordingly, the positions of the first core and the second core are shifted with respect to the center of the second ferrule. This makes it easier to align the positions of the cores of the two single-core fibers with the positions of the first core and the second core, respectively.

[0018] (6) In any of the above (1) to (5), the multicore fiber may have markers for identifying the two cores. In this case, the two cores of the multicore fiber can be easily identified.

[0019] (7) In any of the above (1) to (6), the two cores of the multicore fiber may include a first core and a second core. The center of the second optical fiber holding hole when viewed along the extending direction may be on a line segment connecting the center of the first core and the center of the second core, and may be offset from the midpoint of the line segment. The center of the second ferrule when viewed along the extending direction may coincide with the midpoint. In this case, even if the two cores of the multicore fiber are offset in a first direction with respect to the center of the multicore fiber, the position of the second optical fiber holding hole is offset in a direction opposite to the first direction. Therefore, two single-core fibers can be easily optically coupled to the two cores of the multicore fiber.

[0020] (8) In any of (1) to (7) above, the difference between the outer diameter of the first ferrule and the outer diameter of the second ferrule may be equal to or greater than half the distance between the two cores of the multicore fiber. For example, when the first ferrule and the second ferrule are inserted into a cylindrical part, the position of the first ferrule or the second ferrule with the smaller outer diameter can be adjusted inside the cylindrical part. As a result, the cores of the two single-core fibers can be easily optically coupled to the two cores of the multicore fiber.

[0021] (9) In any of the above (1) to (8), the second ferrule may have a viewing portion that is visible along the extending direction. When viewed along the extending direction, the center of the second optical fiber holding hole may be located between the viewing portion and the center of the second ferrule. In this case, by viewing the viewing portion, it is possible to evaluate the direction in which the second optical fiber holding hole is misaligned with respect to the center of the second ferrule. As a result, it is possible to easily evaluate the positions of the two cores of the multi-core fiber inserted in the second optical fiber holding hole.

[0022] (10) A manufacturing method of an optical connection structure according to the present disclosure is the manufacturing method of the optical connection structure of (7), comprising the steps of inserting four single-core fibers into a first optical fiber holding hole of a first ferrule, fixing the four single-core fibers in the first optical fiber holding hole, inserting a multi-core fiber into a second optical fiber holding hole of a second ferrule, fixing the multi-core fiber in the second optical fiber holding hole, bringing tip faces of the four single-core fibers into opposition to a tip face of the multi-core fiber, and rotating the four single-core fibers arranged in a square shape with respect to the multi-core fiber to optically couple two cores of two single-core fibers located on one side of the square to the first core and the second core of the multi-core fiber, respectively.

[0023] In this manufacturing method, even if the two cores of the multi-core fiber are misaligned in a first direction with respect to the center of the multi-core fiber, the position of the second optical fiber holding hole is misaligned in a direction opposite to the first direction. Therefore, by rotating the second ferrule, the positions of the first core and the second core can be rotationally moved. As a result, the cores of two of the four single-core fibers can be easily optically coupled to the two cores of the multi-core fiber.

[0024] [Details of the Embodiments of the Present Disclosure] Specific examples of optical connection structures and manufacturing methods of optical connection structures according to the embodiments will be described below with reference to the drawings. The present disclosure is not limited to these examples, but is intended to include all modifications set forth in the claims and within the scope equivalent to the claims. In the description of the drawings, identical or corresponding elements are denoted by the same reference numerals, and redundant description will be omitted as appropriate. For ease of understanding, some parts of the drawings may be simplified or exaggerated, and dimensional ratios and the like are not limited to those shown in the drawings.

[0025] FIG. 1 is a side view showing an example of an optical connection structure 1. As shown in FIG. 1, the optical connection structure 1 includes a first optical connector 10, a second optical connector 20, and a coupling unit 30. The first optical connector 10, the second optical connector 20, and the coupling unit 30 extend in a direction X. The direction X is the extension direction of a single-core fiber 12 (described later). The first optical connector 10 has a central axis CX extending in the direction X. The first optical connector 10 has an optical fiber bundle 11 including a plurality of (e.g., four) single-core fibers 12. The second optical connector 20 has an MCF (Multi-Core Fiber) unit 21 having a multi-core fiber 22. When the first optical connector 10 and the second optical connector 20 are connected to each other, each core of the plurality of single-core fibers 12 and each of the plurality of cores of the multi-core fiber 22 are optically coupled to each other. Here, optical coupling means that the coupling efficiency at a wavelength of 1550 nm is 50% or more and the loss is 3 dB or less.

[0026] For example, the configuration of the second optical connector 20 is the same as the configuration of the first optical connector 10, except that it has a multi-core fiber 22 instead of the multiple single-core fibers 12. In the following, descriptions of the configuration of the second optical connector 20 that overlap with the configuration of the first optical connector 10 will be omitted as appropriate. The second optical connector 20 has a central axis that overlaps with the central axis CX of the first optical connector 10.

[0027] 1 , the optical fiber bundle 11 is configured to optically couple a plurality of single-core fibers 12 to a multi-core fiber 22. The optical fiber bundle 11 includes a plurality of single-core fibers 12, a first ferrule 14, and a first flange 16.

[0028] The first flange 16 has, for example, a cylindrical shape. The first flange 16 is connected to the first ferrule 14. The first flange 16 holds, for example, a part of the first ferrule 14 and accommodates the multiple single-core fibers 12 therein.

[0029] The first optical connector 10 will be described in detail with reference to Figures 2, 3, and 4. Figure 2 is a cross-sectional view showing the first optical connector 10. Figure 3 is a side view showing the single-core fiber 12. Figure 4 is a cross-sectional view of a first ferrule 14 having a transition portion 14d along the direction X, and a cross-sectional view of a first ferrule 14A having no transition portion along the direction X. The single-core fiber 12 has a first diameter portion 12a, a second diameter portion 12b having a diameter larger than that of the first diameter portion 12a, a reduced diameter portion 12c connecting the second diameter portion 12b to the first diameter portion 12a, and a coating portion 12d having a diameter larger than that of the second diameter portion 12b.

[0030] In the tapered diameter portion 12c, the diameter of the single-core fiber 12 decreases from the second diameter portion 12b to the first diameter portion 12a. The diameter of the first diameter portion 12a is, for example, 35 μm. The diameter of the second diameter portion 12b is, for example, 125 μm. The diameter of the coating portion 12d is, for example, 250 μm. The multiple single-core fibers 12 are fixed inside the first ferrule 14 by an adhesive 40, and are also fixed inside the first flange 16 by the adhesive 40. The first flange 16 contains, for example, metal as a constituent material. The first flange 16 may also contain resin as a constituent material, and the material of the first flange 16 is not particularly limited.

[0031] The first ferrule 14 holds the tip portion of the single-core fiber 12. The first ferrule 14 is tubular (for example, cylindrical). The first ferrule 14 fixes the tip portion of the single-core fiber 12 to the first optical fiber holding hole 14a with adhesive 40 so that the tip surface 12e of the single-core fiber 12 is exposed at an end surface 14h of the first ferrule 14. The first ferrule 14 includes, for example, ceramic such as zirconia, glass, or metal as a constituent material. The length of the first ferrule 14 in the direction X is, for example, 6.5 mm. The length of the first ferrule 14 in the direction X may be 6 mm or more and 8 mm or less.

[0032] The first ferrule 14 has a first optical fiber holding hole 14a into which a plurality of single-core fibers 12 are inserted. In a cross section perpendicular to the direction X, the first optical fiber holding hole 14a is circular. For example, the inner diameter of the first optical fiber holding hole 14a varies depending on the position in the direction X in the first optical fiber holding hole 14a. For example, the first ferrule 14 has a portion where the inner diameter of the first optical fiber holding hole 14a varies.

[0033] The first optical fiber holding hole 14a includes a small diameter portion 14b, a large diameter portion 14c having an inner diameter larger than the inner diameter of the small diameter portion 14b, and a transition portion 14d connecting the large diameter portion 14c to the small diameter portion 14b. At least a portion of the first diameter portion 12a is disposed in the small diameter portion 14b. As an example, the inner diameter of the small diameter portion 14b is 84.5 μm. The inner diameter of the small diameter portion 14b may be 85.5 μm or less. At least a portion of the second diameter portion 12b is disposed in the large diameter portion 14c. As an example, the inner diameter of the large diameter portion 14c is 302 μm. The inner diameter of the large diameter portion 14c may be 310 μm or less. At least a portion of the reduced diameter portion 12c is disposed in the transition portion 14d.

[0034] The first optical connector 10 may have a first ferrule 14A instead of the first ferrule 14. The first ferrule 14A does not have the transition portion 14d. The first ferrule 14A has a first optical fiber holding hole 14e into which multiple single-core fibers 12 are inserted. The first optical fiber holding hole 14e has a first diameter portion 14f and a diameter-changing portion 14g extending from the first diameter portion 14f in the direction opposite to the X direction. The first diameter portion 14f extends from the X-direction end of the diameter-changing portion 14g to the X-direction end of the first ferrule 14A. The first diameter portion 14f has a constant inner diameter. The diameter-changing portion 14g extends from the first diameter portion 14f to the X-direction end of the first ferrule 14A in the direction opposite to the X direction. The diameter-changing portion 14g has an inner diameter that decreases as it approaches the first diameter portion 14f.

[0035] 1 , the MCF unit 21 has a multi-core fiber 22, a second ferrule 24, and a second flange 26. For example, the shape and material of the second flange 26 are the same as those of the first flange 16 described above. The second flange 26 is connected to the second ferrule 24. The second flange 26 holds, for example, a portion of the second ferrule 24 and houses the multi-core fiber 22 therein. The multi-core fiber 22 is fixed inside the second ferrule 24 by an adhesive, and is also fixed inside the second flange 26 by an adhesive.

[0036] The coupling portion 30 is, for example, a split sleeve. The coupling portion 30 accommodates at least a portion of the first ferrule 14 and at least a portion of the second ferrule 24 and fixes the first ferrule 14 and the second ferrule 24. The coupling portion 30 is a component that holds the first ferrule 14 and the second ferrule 24 from the outside so that the central axes of the first ferrule 14 and the second ferrule 24 coincide with each other. Inside the coupling portion 30, the multiple single-core fibers 12 of the optical fiber bundle 11 and the multiple cores of the multi-core fiber 22 are optically coupled to each other.

[0037] Although not shown, the optical connection structure 1 may have a spring attached to the first flange 16 and a spring attached to the second flange 26. These springs urge the first flange 16 and the second flange 26 toward each other. For example, the first ferrule 14 and the second ferrule 24 are pressed against each other inside the coupling portion 30. The single-core fiber 12 and the multi-core fiber 22 may be pressed against each other by the elastic force of the above-mentioned springs.

[0038] 5 is a diagram schematically showing the tip faces 12e of the four single-core fibers 12 and the end face 14h of the first ferrule 14. In a cross section of the four single-core fibers 12 perpendicular to the direction X, the first optical fiber holding hole 14a has a circular shape. As an example, the outer diameter of the first ferrule 14 is 1.25 mm. The outer diameter of the first ferrule 14 is equal to the outer diameter of the second ferrule 24. This makes it possible to insert the first ferrule 14 and the second ferrule 24 into a common coupling portion 30.

[0039] The direction X in which the four single-core fibers 12 extend coincides with the direction in which the multi-core fiber 22 extends. The single-core fiber 12 has a core 12 f and a cladding 12 g that covers the core 12 f. The single-core fiber 12 is an optical fiber that is optically coupled to the multi-core fiber 22. The core 12 f is exposed at a tip surface 12 e of the single-core fiber 12. The outer diameter of the cladding 12 g may be constant over the entire length of the single-core fiber 12. The tip surface 12 e of the single-core fiber 12 includes the tip of the core 12 f and the tip of the cladding 12 g. For example, the tip surface 12 e of the single-core fiber 12 extends along a plane that is perpendicular to the direction X.

[0040] FIG. 6 is a diagram schematically illustrating a tip surface 22e of the multicore fiber 22 and an end surface 24h of the second ferrule 24. As illustrated in FIG. 6, the multicore fiber 22 has a plurality of (e.g., two) cores 22f and a cladding 22g covering the plurality of cores 22f. The tip surface 22e includes the tips of the plurality of cores 22f and the tip of the cladding 22g. An optical signal propagates through each of the plurality of cores 22f. In a cross section of the multicore fiber 22 perpendicular to the direction X, the two cores 22f are arranged in point symmetry with respect to the center of the multicore fiber 22. As an example, the outer diameter of the cladding 22g is 125 μm. The diameter of the cladding 22g (cladding diameter) may be 200 μm or less, or may be 80 μm or less. The cladding diameter may be 50 μm or more.

[0041] A portion of the configuration of the second ferrule 24 is the same as a portion of the configuration of the first ferrule 14 described above. Below, descriptions of the second ferrule 24 that overlap with the configuration of the first ferrule 14 will be omitted as appropriate. The second ferrule 24 holds the tip portion of the multicore fiber 22. The second ferrule 24 is tubular (for example, cylindrical). For example, the outer diameter of the second ferrule 24 is 1.25 mm. The second ferrule 24 has a second optical fiber holding hole 24a into which the multicore fiber 22 is inserted. In a cross section perpendicular to direction X, the second optical fiber holding hole 24a is circular. For example, the inner diameter of the second optical fiber holding hole 24a is 126 μm.

[0042] The second optical fiber holding hole 24a accommodates, for example, a tip portion of the multi-core fiber 22. The second optical fiber holding hole 24a is defined by the inner surface of a cylindrical second ferrule 24. The second ferrule 24 fixes the tip portion of the multi-core fiber 22 to the second optical fiber holding hole 24a with an adhesive so that a tip surface 22e of the multi-core fiber 22 is exposed at an end surface 24h of the second ferrule 24.

[0043] The inner diameter of the second optical fiber holding hole 24a is the same as or larger than the outer diameter of the multicore fiber 22. For example, the tip portion of the multicore fiber 22 is inserted into the second optical fiber holding hole 24a to be fitted into the second optical fiber holding hole 24a. For example, the length of the second ferrule 24 in the direction X of the multicore fiber 22 is 6.5 mm. For example, the length of the second ferrule 24 in the direction X is 6 mm or more and 8 mm or less. The second ferrule 24 includes, for example, ceramic such as zirconia, glass, or metal as a constituent material.

[0044] Fig. 7 is a diagram showing the tip surfaces 12e of the plurality of single-core fibers 12. Fig. 8 is a diagram showing the tip surface 22e of the multi-core fiber 22. The optical connection structure 1 has four single-core fibers 12. The four single-core fibers 12 are arranged two-dimensionally in a cross section 50 orthogonal to the direction X. Here, "arranged two-dimensionally" means that the centers of the four single-core fibers 12 are arranged at positions that become vertices of a two-dimensional figure in the cross section 50. The four single-core fibers 12 are arranged in a square shape of two rows and two columns. The cores 12f of the plurality of single-core fibers 12 are optically coupled to the cores 22f of the multi-core fiber 22 by being rotated and adjusted about the central axis CX.

[0045] In a cross section 50 perpendicular to the direction X, the four single-core fibers 12 are arranged in a square shape. "Arranged in a square shape" means that the centers of the four single-core fibers 12 are arranged at positions that become the vertices of a square in the cross section 50. In the first optical fiber holding hole 14a of the first ferrule 14, the four single-core fibers 12 are arranged in a close-packed manner. Each single-core fiber 12 is in contact with the inner surface of the first optical fiber holding hole 14a and with the other single-core fibers 12.

[0046] The optical connection structure 1 has an adhesive 18 that fixes the single-core fiber 12 to the first ferrule 14. The adhesive 18 is filled inside the first optical fiber holding hole 14a and between the multiple single-core fibers 12. For example, the adhesive 18 includes a resin as a constituent material. The adhesive 18 is, for example, a thermosetting resin, a room-temperature curing resin, an ultraviolet curing resin, or a combined ultraviolet and thermosetting resin. Examples of the thermosetting resin include epoxy-based resins. Examples of the ultraviolet curing resin include epoxy-based resins and acrylic-based resins. For example, the Shore D hardness of the adhesive 18 is 60 or higher.

[0047] In the cross section 50, of the four single-core fibers 12 arranged in a square, the distance D1 between two cores 12f in two single-core fibers 12 lined up on one side of the square is equal to twice the radius r of the single-core fiber 12. The distance D1 is equal to the outer diameter of the single-core fiber 12. The distance D1 indicates the distance between the core centers of the two cores 12f. As an example, the distance D1 is 35 μm.

[0048] Hereinafter, the multiple single-core fibers 12 may be described as a single-core fiber 12A, a single-core fiber 12B, a single-core fiber 12C, and a single-core fiber 12D. When viewed along the direction X, the single-core fibers 12A, 12B, 12C, and 12D are arranged clockwise.

[0049] The optical connection structure 1 includes a multicore fiber 22 having two cores 22 f. For example, in a cross section perpendicular to the direction X, a line segment connecting the two cores 22 f extends perpendicular to the direction X and parallel to an imaginary line L1 passing through the center C1 of the second ferrule 24. The imaginary line L1 extends in a direction Y perpendicular to the direction X. The two cores 22 f include a first core 22 f1 and a second core 22 f2. The multicore fiber 22 includes a center C2 of the tip surface 22 e. When viewed along the direction X, the center C2 coincides with the central axis of the second optical fiber holding hole 22 a. When viewed along the direction X, the center C2 of the multicore fiber 22 is located on a line segment 60 connecting the first core 22 f1 and the second core 22 f2. When viewed along the direction X, the center C2 overlaps with the midpoint of the line segment 60. The first core 22f1, the center C2, and the second core 22f2 are arranged in a straight line in this order. The distance between the first core 22f1 and the center C2 is equal to the distance between the second core 22f2 and the center C2.

[0050] When viewed along the direction X, the center C1 of the outer shape of the second ferrule 24 does not overlap with the center C2 of the multicore fiber 22 (the tip end surface 22e). When viewed along the direction X, both the centers C1 and C2 are included inside the tip end surface 22e of the multicore fiber 22. The centers C1 and C2 are aligned along an imaginary straight line L2. The imaginary straight line L2 extends in a direction Z that is perpendicular to both the direction X and the direction Y. The imaginary straight line L2 passes through the center C2 of the tip end surface 22e. The position of the center C1 in the direction Y is shifted from the position of the first core 22f1 in the direction Y and the position of the second core 22f2 in the direction Y. The positions of the first core 22f1 in the direction Z and the second core 22f2 in the direction Z are shifted from the position of the center C1 in the direction Z. The distance dy from the center C1 to the center C2 is half the distance D2, which is the center-to-center distance between the two cores 22f.

[0051] The distance D2 is equal to the distance D1. The distance D2 is equal to twice the radius r. The distance D2 between the two cores 22f of the multicore fiber 22 is the same as the outer diameter of the single-core fiber 12. The distance dy is equal to the radius r. When viewed along the direction X, the distance from the center C2 of the second optical fiber holding hole 24a to the center C1 of the outer shape of the second ferrule 24 is half the distance from the first core 22f1 to the second core 22f2. "Equal distances" and "same distances" are not limited to cases where the distances are completely equal, but also include cases where the distances are different to the extent that the effect is not changed.

[0052] The multicore fiber 22 has markers 22h for identifying the first core 22f1 and the second core 22f2. By providing the markers 22h, it is possible to identify which of the two cores is the first core 22f1 and which is the second core 22f2. The distance from the first core 22f1 to the marker 22h is different from the distance from the second core 22f2 to the marker 22h. Since the marker 22h is close to either the first core 22f1 or the second core 22f2, it is possible to easily identify the first core 22f1 and the second core 22f2.

[0053] 9 is a diagram in which the tip surface of the first optical connector 10 is projected onto the tip surface of the second optical connector 20. Of the four single-core fibers 12 arranged in a square shape, cores 12f of two single-core fibers 12 located on one side of the square are optically coupled to two cores 22f of the multi-core fiber 22. In this embodiment, the core 12f of the single-core fiber 12A is optically coupled to the second core 22f2 of the multi-core fiber 22, and the core 12f of the single-core fiber 12B is optically coupled to the first core 22f1 of the multi-core fiber 22.

[0054] When the first ferrule 14 and the second ferrule 24 are inserted into the coupling portion 30 with the tip surface 12e and the tip surface 22e facing each other, two of the four single-core fibers 12 are optically coupled to the core 22f of the multi-core fiber 22. The other single-core fibers 12 of the four single-core fibers 12 are not optically coupled to the core 22f. In this embodiment, the single-core fibers 12C and 12D are not optically coupled to the core 22f. The two single-core fibers 12 that are not optically coupled to the core 22f are also located on one side of the square.

[0055] As described above, in the optical connection structure 1, four single-core fibers 12 are inserted into the first optical fiber holding hole 14a of the first ferrule 14, and a multi-core fiber 22 having two cores 22f is inserted into the second optical fiber holding hole 24a of the second ferrule 24. Of the four single-core fibers 12 arranged in a square shape in a cross section perpendicular to the direction X in which the single-core fibers 12 extend, two single-core fibers 12 located on one side of the square are optically coupled to two cores 22f of the multi-core fiber 22. By arranging the four single-core fibers 12 in a square shape, the four single-core fibers 12 can be packed closest in density in the first optical fiber holding hole 14a. Two of the four single-core fibers 12 can be easily optically coupled to the two cores 22f of the multi-core fiber 22, thereby reducing optical loss.

[0056] For example, if single-core fibers are arranged in a square shape and the distance between the two cores of two single-core fibers located on the diagonal of the square is equal to the distance between the two cores 22f of the multi-core fiber 22, it may be necessary to make the outer diameter of the single-core fiber smaller than that of the single-core fiber 12 of this embodiment. If the outer diameter of the single-core fiber is small, there are concerns that the strength of the single-core fiber will decrease, bending loss will increase, and the amount of light leaking from the single-core fiber when light passes through it will increase. If the distance between the cores 12f of two single-core fibers 12 arranged on one side of a square is equal to the distance between the two cores 22f of the multi-core fiber 22, as in this embodiment, the outer diameter of the single-core fiber 12 can be kept large. As a result, it is possible to reduce the decrease in strength of the single-core fiber 12, the increase in bending loss, and the increase in light leaking from the single-core fiber 12 when light passes through the single-core fiber 12.

[0057] The outer diameter of the single-core fiber 12 is the same as the distance between two cores 22f of the multi-core fiber 22. The distance between two cores 12f of two adjacent single-core fibers 12 is equal to the distance between two cores 22f of the multi-core fiber 22. By inserting four single-core fibers 12 into the first optical fiber holding hole 14a, the distance between two cores 12f can easily be made equal to the distance between two cores 22f.

[0058] The first ferrule 14 has a cylindrical shape, and the second ferrule 24 has a cylindrical shape. With the first ferrule 14 and the second ferrule 24 housed in the coupling portion 30, the second ferrule 24 can be rotated relative to the first ferrule 14. This makes it easier to adjust the positions of the cores 12f of the multiple single-core fibers 12 with respect to the positions of the cores 22f of the multi-core fiber 22.

[0059] The single-core fiber 12 has a first diameter portion 12a, a second diameter portion 12b having a diameter larger than that of the first diameter portion 12a, and a tapered diameter portion 12c connecting the second diameter portion 12b to the first diameter portion 12a and in which the diameter of the single-core fiber 12 decreases toward the first diameter portion 12a. Because the second diameter portion 12b has a diameter larger than that of the first diameter portion 12a, the single-core fiber 12 can be easily manipulated by gripping the second diameter portion 12b. As a result, the single-core fiber 12 can be easily inserted into the first optical fiber holding hole 14a.

[0060] The two cores 22f of the multicore fiber 22 include a first core 22f1 and a second core 22f2. When viewed along the direction X, the distance from the center C2 of the second optical fiber holding hole 24a to the center C1 of the second ferrule 24 is half the distance from the first core 22f1 to the second core 22f2. The center C2 of the second optical fiber holding hole 24a is shifted from the center C1 of the second ferrule 24. Accordingly, the positions of the first core 22f1 and the second core 22f2 are shifted in the direction Z from the center C1 of the second ferrule 24. This makes it easier to align the positions of the cores 12f of the two single-core fibers 12 with the positions of the first core 22f1 and the second core 22f2, respectively.

[0061] The multi-core fiber 22 has markers 22h for identifying the two cores. When the markers 22h are provided, the two cores of the multi-core fiber can be easily identified.

[0062] Various modified examples of the optical connection structure according to the present disclosure will be described. Some configurations of the optical connection structures according to the modified examples are the same as some configurations of the optical connection structure 1 described above. Therefore, in the following description, the description of the configurations that overlap with the configuration of the optical connection structure 1 will be omitted as appropriate by assigning the same reference numerals.

[0063] An optical connection structure 1A according to a first modified example will be described. Fig. 10 is a front view showing the tip end surface of a second optical connector 20A according to the modified example. The optical connection structure 1A differs from the optical connection structure 1 described above in that, instead of the second optical connector 20, the second optical connector 20A includes a multi-core fiber 22A whose cores are offset. In the multi-core fiber 22A, the arrangement of the two cores is asymmetric with respect to the center C2 of the second optical fiber holding hole 24a to make it easier to distinguish the two cores. The second optical connector 20A includes a multi-core fiber 22A whose cores are offset and a second ferrule 24A that holds the multi-core fiber 22A.

[0064] The multicore fiber 22A has a first core 22f3 and a second core 22f4. The position of the first core 22f3 is closer to the center C2 of the second optical fiber holding hole 24a by a predetermined distance compared to the position of the second core 22f4. The midpoint M of the line segment connecting the first core 22f3 and the second core 22f4 is shifted in the direction Y from the center C2. When viewed along the direction X, the center C2 is located on the line segment connecting the first core 22f3 and the second core 22f4. In the following description, the center-to-center distance between the first core 22f3 and the second core 22f4 is referred to as distance D2, and the distance from the center of the first core 22f3 to the midpoint M is referred to as distance d2. The distance d2 is the same as the distance from the center of the second core 22f4 to the midpoint M. The distance D2 is twice the distance d2.

[0065] The midpoint M is offset from the center C2 by a distance dx along the direction Y. The distance from the center of the first core 22f3 to the center C2 is the distance d2 minus the distance dx. The distance from the center of the second core 22f4 to the center C2 is the distance d2 plus the distance dx.

[0066] The center C2 of the second optical fiber holding hole 24a is offset in the opposite direction to the direction Y from the center C1 of the outer shape of the second ferrule 24A. When viewed along the direction X, the center C2 is not aligned with the center C1 along the direction Z. The center C2 is offset from the center C1 by a distance dx in the opposite direction to the direction Y from the center C1. The midpoint M is aligned with the center C1 along the direction Z. When viewed along the direction X, the position of the center C1 of the second ferrule 24A in the direction Y coincides with the position of the midpoint M in the direction Y. The midpoint M and the center C1 are aligned along an imaginary straight line L3. The imaginary straight line L3 extends in the direction Z. The imaginary straight line L3 passes through the center C1.

[0067] The midpoint M is a distance dx away from the center C2 in the direction Y. The center C2 is a distance dx away from the center C1 in the opposite direction to the direction Y. The positions of the first core 22f3 and the second core 22f4 are asymmetric with respect to an imaginary line L2 that passes through the center C2 and extends in the direction Z. The second optical fiber holding hole 24a is shifted in the opposite direction to the direction Y with respect to an imaginary line L3 that passes through the center C1. As a result, the position of the center C1 in the direction Y coincides with the position of the midpoint M in the direction Y. Therefore, the distance from the center C1 to the center of the first core 22f3 is equal to the distance from the center C1 to the center of the second core 22f4. The distance from the center C2 to the center C1 is the square root of the sum of the square of the distance dx and the square of the distance dy.

[0068] The distance d2 is half of the distance D2, and the distance dy is half of the distance D2. The distance d2 is equal to the distance dy. The distance dy is equal to the radius r (see FIG. 7 ). The distances d2, dy, and r are equal to one another. When the first ferrule 14 is opposed to the second ferrule 24A so that the central axis of the first ferrule 14 and the central axis of the second ferrule 24A overlap, the cores 12f of two of the four single-core fibers 12 face the first core 22f3 and the second core 22f4, respectively.

[0069] A manufacturing method of the optical connection structure according to this embodiment will be described. An example of a manufacturing method of the optical connection structure 1A will be described below. Four single-core fibers 12 are inserted into the first optical fiber holding hole 14a of the first ferrule 14 (first insertion step). As a specific example, the single-core fibers 12 are passed through the through-hole of the first flange 16, and the single-core fibers 12 extending from the first flange 16 are inserted into the first optical fiber holding hole 14a. The single-core fibers 12 are inserted into the first optical fiber holding hole 14a so that the first diameter portion 12a of the single-core fibers 12 is located in the small diameter portion 14b of the first ferrule 14, the second diameter portion 12b is located in the large diameter portion 14c, and the reduced diameter portion 12c is located in the transition portion 14d. The single-core fibers 12 are inserted into the first optical fiber holding hole 14a so that the position of the tip surface 12e of the single-core fibers 12 is aligned with the end surface 14h of the first ferrule 14. Before, during, or after inserting the four single-core fibers 12 into the first ferrule 14, adhesive is injected into the first optical fiber holding hole 14a or applied to the four single-core fibers 12. The four single-core fibers 12 are fixed inside the first optical fiber holding hole 14a with the adhesive. Then, the tip surfaces 12e of the single-core fibers 12 and the end surface 14h of the first ferrule 14 are polished.

[0070] The multicore fiber 22A is inserted into the second optical fiber holding hole 24a of the second ferrule 24A (second insertion step). The second insertion step may be performed before the first insertion step. As a specific example, the multicore fiber 22A is passed through a through-hole of the second flange 26, and the multicore fiber 22A extending from the second flange 26 is inserted into the second optical fiber holding hole 24a. The multicore fiber 22A is inserted into the second optical fiber holding hole 24a so that the position of the tip face 22e of the multicore fiber 22A is the same as the position of the end face 24h of the second ferrule 24A. At this time, the positions of the first core 22f3 and the second core 22f4 in the rotation direction around the axis of the multicore fiber 22A may be adjusted according to the position of the center C1 of the second ferrule 24 and the position of the center C2 of the second optical fiber holding hole 24a. Before, during, or after inserting the multi-core fiber 22A into the second optical fiber holding hole 24a, an adhesive is injected into the second optical fiber holding hole 24a or applied to the multi-core fiber 22A. The multi-core fiber 22A is fixed in the second optical fiber holding hole 24a. Subsequently, the tip face 22e of the multi-core fiber 22A and the end face 24h of the second ferrule 24A are polished.

[0071] The tip surface 22 e of the multicore fiber 22 A is opposed to the tip surfaces 12 e of the four single-core fibers 12. As a specific example, the first ferrule 14 and the second ferrule 24 are inserted into the connecting portion 30, thereby making the tip surface 12 e face the tip surface 22 e.

[0072] The four single-core fibers 12 arranged in a square are rotated around the central axis CX, and two cores 12f of two single-core fibers 12 located on one side of the square are optically coupled to the first core 22f3 and the second core 22f4 of the multi-core fiber 22A, respectively (alignment process). At this time, the first ferrule 14 inserted inside the coupling portion 30 is rotated around the central axis CX relative to the second ferrule 24A. The first ferrule 14 is rotated 90 degrees at a time around the central axis CX relative to the second ferrule 24A, and the cores 12f of two of the four single-core fibers 12 are made to face the first core 22f3 and the second core 22f4. By repeating this 90-degree rotation, a combination of two single-core fibers 12 with the smallest optical loss is found. In order to prevent the tip surface 12 e of the single-core fiber 12 and the tip surface 22 e of the multi-core fiber 22 from being damaged when the first ferrule 14 is rotated, oil for protecting the tip surfaces 12 e and 22 e may be applied to the tip surfaces 12 e and 22 e. Instead of applying oil to the tip surfaces 12 e and 22 e, a refractive index matching agent may be applied to protect the tip surfaces.

[0073] After alignment, a force is applied in a direction in which the first flange 16 and the second flange 26 approach each other, for example, using a leaf spring or clip (not shown) that sandwiches the first flange 16 and the second flange 26. This fixes the first ferrule 14 and the second ferrule 24A inside the coupling portion 30.

[0074] An adhesive is filled between the inside of the coupling portion 30 and the outer peripheral surface of the first ferrule 14, and between the inside of the coupling portion 30 and the outer peripheral surface of the second ferrule 24A. When the adhesive hardens, the first ferrule 14 and the second ferrule 24A are fixed inside the coupling portion 30. Through the above steps, the series of steps in the method for manufacturing an optical connection structure is completed.

[0075] When viewed along the direction X, the center C2 of the second optical fiber holding hole 24a is on the line segment connecting the center of the first core 22f3 and the center of the second core 22f4, and is displaced from the midpoint M of the line segment. When viewed along the direction X, the position of the center C1 of the second ferrule 24A in the direction Y coincides with the position of the midpoint M in the direction Y. Even if the two cores of the multi-core fiber 22A are displaced in the Y direction with respect to the center of the multi-core fiber 22A, the position of the center C2 of the second optical fiber holding hole 24a with respect to the center C1 of the second ferrule 24A is displaced in the direction opposite to the Y direction. Therefore, two single-core fibers 12 can be easily optically coupled to the two cores of the multi-core fiber 22A.

[0076] The two cores of the multi-core fiber 22A are offset in the Y direction with respect to the center of the multi-core fiber 22A, and the position of the center C2 of the second optical fiber holding hole 24a is offset in the direction opposite to the Y direction with respect to the center C1 of the second ferrule 24A. By rotating the second ferrule 24A, the positions of the first core 22f3 and the second core 22f4 can be rotationally moved. As a result, the cores 12f of the two single-core fibers 12 can be easily optically coupled to the two cores of the multi-core fiber 22.

[0077] The multi-core fiber 22A shown in FIG. 10 does not have markers for identifying the two cores, but may have markers.

[0078] An optical connection structure 1B according to a second modified example will be described. FIG. 11 is a cross-sectional view showing the optical connection structure 1B. The optical connection structure 1B differs from the optical connection structure 1 described above in that it includes a first ferrule 14B having an outer diameter smaller than that of the first ferrule 14. The inner diameter of the coupling portion 30 is approximately equal to the outer diameter of the second ferrule 24. This fixes the second ferrule 24 inside the coupling portion 30. In contrast, the outer diameter of the first ferrule 14B is smaller than the inner diameter of the coupling portion 30. The first ferrule 14B is movable in a direction intersecting with direction X. The difference between the outer diameter Φ1 of the first ferrule 14B and the outer diameter Φ2 of the second ferrule 24 is equal to or greater than half the distance between the two cores 22f of the multicore fiber 22.

[0079] When the first ferrule 14B and the second ferrule 24 are inserted into the coupling portion 30, the position of the first ferrule 14B in the direction Y or the direction Z can be adjusted inside the coupling portion 30. As a result, the cores 12 f of the two single-core fibers 12 can be easily optically coupled to the two cores 22 f of the multi-core fiber 22.

[0080] An optical connection structure 1C according to a third modified example will be described. FIG. 12 is a front view showing a second optical connector 20C. The optical connection structure 1C differs from the optical connection structure 1 described above in that it includes a second ferrule 24C having a viewing portion 24k instead of the second ferrule 24. The second optical connector 20C includes the second ferrule 24C on which the viewing portion 24k is formed. The viewing portion 24k is visible when viewed along the direction X. When viewed along the direction X, the tip surface of the second ferrule 24C has a circular shape with a portion missing. The viewing portion 24k is a missing circular portion. The viewing portion 24k is a flat surface. When viewed along the direction X, the center C2 of the second optical fiber holding hole 24a is located between the viewing portion 24k and the center C1 of the outer shape of the second ferrule 24C.

[0081] The second ferrule 24C has a visual recognition portion 24m, which is, for example, a marker. The visual recognition portion 24m is provided on the tip surface of the second ferrule 24C. When viewed along the X direction, the center C2 of the second optical fiber holding hole 24a is located between the visual recognition portion 24m and the center C1 of the second ferrule 24C. The second ferrule 24C does not necessarily have to have the visual recognition portion 24m.

[0082] As described above, the second ferrule 24C has a viewing portion 24k that is visible along the direction X. When viewed along the direction X, the center C2 of the second optical fiber holding hole 24a is located between the viewing portion 24k and the center C1 of the second ferrule 24C. By viewing the viewing portion 24k, it is possible to evaluate the direction in which the second optical fiber holding hole 24a is misaligned with respect to the center C1 of the second ferrule 24C. As a result, it is possible to easily evaluate the positions of the two cores 22f of the multicore fiber 22 inserted in the second optical fiber holding hole 24a.

[0083] The above describes embodiments and various modifications of the optical connection structure according to the present disclosure. The optical connection structure according to the present disclosure is not limited to the contents of the above-described embodiments or modifications, 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 connection structure according to the present disclosure can be changed as appropriate within the scope of the above gist.

[0084] For example, the shape of the second optical fiber holding hole 24a when viewed along the direction X does not have to be circular. For example, the shape may be elliptical or rectangular. The second optical connector 20 may have a V-groove substrate in which a V-groove for positioning the multi-core fiber 22 is formed. In the above example, the second ferrule 24C having the viewing portion 24k, which is a circular notched portion and has a flat surface, has been described. The viewing portion does not have to be a circular notched portion and may be a mark. In this way, the form of the viewing portion is not particularly limited.

[0085] DESCRIPTION OF SYMBOLS 1, 1A, 1B, 1C...Optical connection structure 10...First optical connector 11...Optical fiber bundle 12...Single-core fiber 12a...First diameter section 12b...Second diameter section 12c...Reduced diameter section 12d...Coating section 12e...Tip surface 12f...Core 12g...Cladding 14, 14A, 14B...First ferrule 14a, 14e...First optical fiber holding hole 14b...Narrow diameter section 14c...Wide diameter section 14d...Transition section 14f...First diameter section 14g...Diameter changing section 14h...End surface 16...First flange 18...Adhesive 20, 20A, 20C...Second optical connector 21...MCF unit 22, 22A...Multi-core fiber 22e...Tip surface 22f...Core 22f1, 22f3...First core 22f2, 22f4... Second core 22g... Cladding 22h... Marker 24, 24A, 24C... Second ferrule 24a... Second optical fiber holding hole 24h... End face 24k, 24m... Visual recognition portion 26... Second flange 30... Connection portion 40... Adhesive 50... Cross section 60... Line segment

Claims

1. An optical connection structure comprising: four single-core fibers; a multi-core fiber having two cores; a first ferrule having a first optical fiber holding hole into which the four single-core fibers are inserted; and a second ferrule having a second optical fiber holding hole into which the multi-core fiber is inserted, wherein the four single-core fibers are arranged in a square shape in a cross section perpendicular to the direction in which the four single-core fibers extend, and two of the four single-core fibers arranged in a square shape, which are located on one side of the square, are optically coupled to two of the cores of the multi-core fiber.

2. The optical connection structure according to claim 1, wherein the outer diameter of the single-core fiber is the same as the distance between the two cores of the multi-core fiber.

3. An optical connection structure according to claim 1 or claim 2, wherein the first ferrule is cylindrical, and the second ferrule is cylindrical.

4. An optical connection structure according to any one of claims 1 to 3, wherein the single-core fiber has a first diameter portion, a second diameter portion having a diameter larger than that of the first diameter portion, and a tapered diameter portion connecting the second diameter portion to the first diameter portion, the diameter of the single-core fiber decreasing as it approaches the first diameter portion.

5. An optical connection structure according to any one of claims 1 to 4, wherein the two cores of the multicore fiber include a first core and a second core, and when viewed along the extending direction, the distance from the center of the second optical fiber holding hole to the center of the second ferrule is half the distance from the first core to the second core.

6. An optical connection structure according to any one of claims 1 to 5, wherein the multi-core fiber has a marker for identifying the two cores.

7. An optical connection structure according to any one of claims 1 to 6, wherein the two cores of the multicore fiber include a first core and a second core, the center of the second optical fiber holding hole when viewed along the extending direction is on a line segment connecting the center of the first core and the center of the second core and is offset from the midpoint of the line segment, and the center of the second ferrule when viewed along the extending direction coincides with the midpoint.

8. An optical connection structure according to any one of claims 1 to 7, wherein the difference between the outer diameter of the first ferrule and the outer diameter of the second ferrule is equal to or greater than half the distance between the two cores of the multicore fiber.

9. An optical connection structure described in any one of claims 1 to 8, wherein the second ferrule has a visible portion that is visible along the extending direction, and when viewed along the extending direction, the center of the second optical fiber holding hole is located between the visible portion and the center of the second ferrule.

10. A method for manufacturing an optical connection structure according to claim 7, comprising the steps of: inserting four of the single-core fibers into the first optical fiber holding hole of the first ferrule; fixing the four single-core fibers in the first optical fiber holding hole; inserting the multi-core fiber into the second optical fiber holding hole of the second ferrule; fixing the multi-core fiber in the second optical fiber holding hole; bringing the tip faces of the four single-core fibers into opposition to the tip face of the multi-core fiber; and rotating the four single-core fibers arranged in a square shape with respect to the multi-core fiber, and optically coupling two cores of two of the single-core fibers located on one side of the square to the first core and the second core of the multi-core fiber, respectively.

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