Optical connection structure and method for manufacturing optical connection structure

Abstract

In an optical connection structure (1), a first metallic holder (18) holds a multicore fiber (12) and a first lens (15). A second metallic holder (24) is welded to the first metallic holder (18). The second metallic holder holds a second lens (22). A third metallic holder (48) holds an optical fiber bundle (41). A metallic sleeve (30) is welded individually to the second metallic holder and the third metallic holder. The metallic sleeve joins the second metallic holder and the third metallic holder. The shape of the arrangement of a plurality of second fiber cores (71) at a second end face (42a) and the shape of the arrangement of a plurality of first fiber cores (61) at a first end face (12a) are similar to each other. The focal lengths (f1, f2) of the first and second lenses, as well as the pitches (P1, P2) and the mode field diameters (MFD1, MFD2) of the first and second fiber cores satisfy a prescribed relationship.

Description

Optical connection structure and method for manufacturing the optical connection structure

[0001] The present disclosure relates to an optical connection structure and a method for manufacturing an optical connection structure. This application claims priority to Japanese Application No. 2023-211170 filed on December 14, 2023, and incorporates by reference all of the contents of said Japanese application.

[0002] An optical connection structure is known that optically couples a multicore fiber with an optical fiber bundle having a plurality of single-core fibers. The optical fiber bundle includes a plurality of single fibers. Hereinafter, the multicore fiber is also referred to as an MCF. The single-core fiber is also referred to as an SCF. For example, in Patent Document 1, the MCF and the optical fiber bundle are optically coupled via a lens. The MCF includes a plurality of fiber cores, and each SCF also includes a fiber core. For example, the MCF and the SCF each have an end face from which the fiber cores are exposed. The lens is disposed between the end face of the MCF and the end face of the SCF.

[0003] International Publication No. 2022 / 004220

[0004] An optical connection structure according to one aspect of the present disclosure includes a multicore fiber, an optical fiber bundle, a first lens, a second lens, a first metal holder, a second metal holder, a third metal holder, and a metal sleeve. The multicore fiber includes a plurality of first fiber cores. The multicore fiber has a first end face from which the plurality of first fiber cores are exposed. The optical fiber bundle includes a plurality of single-core fibers arranged in parallel in a direction perpendicular to the longitudinal direction. Each of the plurality of single-core fibers includes a second fiber core and has a second end face from which the second fiber core is exposed. The first lens and the second lens are arranged in order from the first end face to the second end face between the first end face and the second end face. The first metal holder holds the multicore fiber and the first lens so that light emitted from the multicore fiber is collimated. The second metal holder is welded to the first metal holder. The second metal holder holds the second lens. The third metal holder holds the optical fiber bundle. The metal sleeve is welded to the second metal holder and the third metal holder, respectively. The metal sleeve connects the second metal holder and the third metal holder. The shape of the arrangement of the plurality of second fiber cores at the second end face and the shape of the arrangement of the plurality of first fiber cores at the first end face are similar to each other. The focal length of the first lens is "f1". The focal length of the second lens is "f2". The pitch of at least one set of first fiber cores at the first end face is "P1". The mode field diameter of light of a specific wavelength at the first end face for at least one set of first fiber cores is "MFD1". The pitch of at least one set of second fiber cores at the second end face is "P2". The mode field diameter of light of a specific wavelength at the second end face for at least one set of second fiber cores is "MFD2". In this case, the following relationships are satisfied: (P1 / P2)×0.9 ≦ f1 / f2 ≦ (P1 / P2)×1.1 (P1 / P2)×0.9 ≦ MFD1 / MFD2 ≦ (P1 / P2)×1.1

[0005] A manufacturing method of an optical connection structure according to another aspect of the present disclosure includes laser welding a first metal holder and a second metal holder, laser welding the second metal holder welded to the first metal holder to a metal sleeve, and laser welding the metal sleeve welded to the second metal holder to a third metal holder holding an optical fiber bundle. The first metal holder holds the multicore fiber and a first lens facing the first end face so that light emitted from the multicore fiber is collimated. The multicore fiber includes a plurality of first fiber cores and has a first end face from which the plurality of first fiber cores are exposed. The second metal holder holds a second lens facing the first lens. In the optical fiber bundle, a plurality of single-core fibers are arranged in parallel in a direction perpendicular to the longitudinal direction. Each of the plurality of single-core fibers includes a second fiber core and has a second end face from which the second fiber core is exposed. The shape of the arrangement of the plurality of second fiber cores at the second end face and the shape of the arrangement of the plurality of first fiber cores at the first end face are similar to each other. The focal length of the first lens is "f1". The focal length of the second lens is "f2". The pitch of at least one set of first fiber cores at the first end face is "P1", and the mode field diameter of light of a specific wavelength at the first end face for the at least one set of first fiber cores is "MFD1". The pitch of at least one set of second fiber cores at the second end face is "P2", and the mode field diameter of light of the specific wavelength at the second end face for the at least one set of second fiber cores is "MFD2". In this case, the following relationships are satisfied: (P1 / P2) x 0.9 ≦ f1 / f2 ≦ (P1 / P2) x 1.1 (P1 / P2) x 0.9 ≦ MFD1 / MFD2 ≦ (P1 / P2) x 1.1

[0006] FIG. 1 is a perspective view showing an optical connection structure in an embodiment. FIG. 2 is a cross-sectional view of the optical connection structure. FIG. 3 is a cross-sectional view of a multicore fiber in a direction perpendicular to the optical axis direction. FIG. 4 is a cross-sectional view of an optical fiber bundle in a direction perpendicular to the optical axis direction. FIG. 5 is a partial conceptual view of the optical connection structure. FIG. 6 is a conceptual view showing the mode field diameter of a second fiber core. FIG. 7 is a diagram for explaining the procedure of laser spot welding. FIG. 8 is a graph showing the relationship between X-axis misalignment and loss increase for each mode field diameter. FIG. 9 is a graph showing the relationship between Z-axis misalignment and loss increase for each mode field diameter. FIG. 10 is a diagram for explaining the incidence of light from a lens into multiple single-core fibers.

[0007] [Problem to be solved by the present disclosure]

[0008] It is conceivable to manufacture an optical connection structure by optically coupling an MCF collimator, a lens, and an optical fiber bundle that are aligned in three positions. The MCF collimator is a unit in which an MCF and a lens are combined so that light emitted from the MCF is collimated. In this case, the influence of misalignment of components that occurs during the assembly of the MCF collimator can be compensated for by aligning the lens and the optical fiber bundle. For example, Patent Document 1 describes assembling the MCF collimator, the lens, and the optical fiber bundle that are aligned in three positions using an adhesive. However, when an adhesive is used for assembly, reliability is a concern in high-temperature or high-humidity environments.

[0009] It is also possible to assemble the MCF collimator, lens, and optical fiber bundle by laser spot welding. However, when the metal melts and solidifies due to laser irradiation, there is a risk of misalignment due to metal contraction. In particular, a triple-body core structure is sensitive to axial misalignment, and the impact of misalignment on optical coupling loss is significant.

[0010] An object of the present disclosure is to provide an optical connection structure that ensures environmental reliability and can suppress optical coupling loss, and a method for manufacturing the optical connection structure.

[0011] According to the present disclosure, it is possible to provide an optical connection structure and a method for manufacturing an optical connection structure that can ensure environmental reliability and suppress optical coupling loss. [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0012] [1] An optical connection structure according to an embodiment of the present disclosure includes a multicore fiber, an optical fiber bundle, a first lens, a second lens, a first metal holder, a second metal holder, a third metal holder, and a metal sleeve. The multicore fiber includes a plurality of first fiber cores. The multicore fiber has a first end face from which the plurality of first fiber cores are exposed. The optical fiber bundle includes a plurality of single-core fibers arranged in parallel in a direction perpendicular to the longitudinal direction. Each of the plurality of single-core fibers includes a second fiber core and has a second end face from which the second fiber core is exposed. The first lens and the second lens are arranged in order from the first end face to the second end face between the first end face and the second end face. The first metal holder holds the multicore fiber and the first lens so that light emitted from the multicore fiber is collimated. The second metal holder is welded to the first metal holder. The second metal holder holds the second lens. The third metal holder holds the optical fiber bundle. The metal sleeve is welded to the second metal holder and the third metal holder, respectively. The metal sleeve connects the second metal holder and the third metal holder. The shape of the arrangement of the plurality of second fiber cores at the second end face and the shape of the arrangement of the plurality of first fiber cores at the first end face are similar to each other. The focal length of the first lens is "f1". The focal length of the second lens is "f2". The pitch of at least one set of first fiber cores at the first end face is "P1". The mode field diameter of light of a specific wavelength at the first end face for at least one set of first fiber cores is "MFD1". The pitch of at least one set of second fiber cores at the second end face is "P2". The mode field diameter of light of a specific wavelength at the second end face for at least one set of second fiber cores is "MFD2". In this case, the following relationships are satisfied: (P1 / P2)×0.9 ≦ f1 / f2 ≦ (P1 / P2)×1.1 (P1 / P2)×0.9 ≦ MFD1 / MFD2 ≦ (P1 / P2)×1.1

[0013] In this optical connection structure, the first metal holder holds the multicore fiber and the first lens so that light emitted from the multicore fiber is collimated. The second metal holder is welded to the first metal holder. The second metal holder holds the second lens. The third metal holder holds the optical fiber bundle. The metal sleeve is welded to the second metal holder and the third metal holder, respectively. The metal sleeve connects the second metal holder and the third metal holder. With this structure, since the components are joined by welding, environmental reliability can be ensured compared to when adhesives are used. Furthermore, with this configuration, the optical connection structure is constructed by, for example, laser spot welding, and positional misalignment is less likely to occur. Therefore, environmental reliability can be ensured and optical connection loss can be suppressed.

[0014] [2] In the optical connection structure of [1] above, the MFD 2 may be 20 μm or more and 40 μm or less at a specific wavelength. In this case, misalignment and optical coupling loss can be further suppressed.

[0015] [3] In the optical connection structure of [1] or [2] above, the second end face may be inclined at an angle of 4 degrees or less with respect to a plane perpendicular to the optical axis direction of the second lens. In this case, defocusing in the optical axis direction of light incident from the second end face can be suppressed.

[0016] [4] In the optical connection structure of any one of [1] to [3] above, the second lens may protrude from the second metal holder toward the optical fiber bundle. In this case, the distance between the end face of the second lens and the plurality of single-core fibers is narrowed.

[0017] [5] In the optical connection structure of any one of [1] to [4] above, the second metal holder and the metal sleeve may have a cylindrical shape. The metal sleeve may include a first cylindrical portion and a second cylindrical portion. The first cylindrical portion has an inner circumferential surface welded to the second metal holder. The second cylindrical portion may extend from an edge of the first cylindrical portion toward the optical axis. The second cylindrical portion has an inner circumferential surface overlapping the second lens when viewed in a direction perpendicular to the optical axis direction, and a surface welded to the third metal holder.

[0018] [6] In the optical connection structure of any one of [1] to [5] above, the second lens may be a rod lens. The rod lens may have an outer peripheral surface held by the second metal holder, a first end face facing the first lens, and a second end face facing the optical fiber bundle. The first end face may be a curved surface. The second end face may be a flat surface. In this case, the second lens can be more stably held by the second metal holder.

[0019] [7] In the optical connection structure of any one of [1] to [6] above, the second lens may include a GRIN lens. The GRIN lens may have an outer peripheral surface that is held by the second metal holder. In this case, the second lens may be more stably held by the second metal holder.

[0020] [8] In the optical connection structure according to any one of [1] to [7], the mode field diameter of each of the plurality of second fiber cores at the second end face may be larger than the mode field diameter at a position a predetermined distance away from the second end face, thereby reducing optical coupling loss at the second end face.

[0021] [9] A manufacturing method of an optical connection structure according to an embodiment of the present disclosure includes laser welding a first metal holder and a second metal holder, laser welding the second metal holder welded to the first metal holder to a metal sleeve, and laser welding the metal sleeve welded to the second metal holder to a third metal holder holding an optical fiber bundle. The first metal holder holds a multicore fiber and a first lens facing the first end face so that light emitted from the multicore fiber is collimated. The multicore fiber includes a plurality of first fiber cores and has a first end face from which the plurality of first fiber cores are exposed. The second metal holder holds a second lens facing the first lens. In the optical fiber bundle, a plurality of single-core fibers are arranged in parallel in a direction perpendicular to the longitudinal direction. Each of the plurality of single-core fibers includes a second fiber core and has a second end face from which the second fiber core is exposed. The shape of the arrangement of the plurality of second fiber cores at the second end face and the shape of the arrangement of the plurality of first fiber cores at the first end face are similar to each other. The focal length of the first lens is "f1". The focal length of the second lens is "f2". The pitch of at least one set of first fiber cores at the first end face is "P1", and the mode field diameter of light of a specific wavelength at the first end face for the at least one set of first fiber cores is "MFD1". The pitch of at least one set of second fiber cores at the second end face is "P2", and the mode field diameter of light of the specific wavelength at the second end face for the at least one set of second fiber cores is "MFD2". In this case, the following relationships are satisfied: (P1 / P2) x 0.9 ≦ f1 / f2 ≦ (P1 / P2) x 1.1 (P1 / P2) x 0.9 ≦ MFD1 / MFD2 ≦ (P1 / P2) x 1.1 [Details of the embodiment of the present disclosure]

[0022] Specific examples of embodiments of the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted as appropriate.

[0023] First, the optical connection structure according to this embodiment will be described in more detail with reference to Fig. 1 to Fig. 5. Fig. 1 is a perspective view showing an optical connection structure according to one embodiment. Fig. 2 is a cross-sectional view of the optical connection structure.

[0024] The optical connection structure 1 has a configuration for optically coupling, for example, an MCF and a plurality of optical fiber bundles, and includes an MCF collimator unit 10, an SCF lens unit 20, a metal sleeve 30, and an SCF bundle unit 40.

[0025] The MCF collimator unit 10 includes an MCF unit 11, a first lens 15, a sleeve 16, and a first metal holder 18. The MCF unit 11 includes an MCF 12 and a ferrule 13.

[0026] As shown in Fig. 3, the MCF 12 includes a plurality of first fiber cores 61 and a first fiber cladding 62. Fig. 3 is a cross-sectional view of the MCF in a direction perpendicular to the optical axis direction. The MCF 12 has an MCF end face 12a. A plurality of first fiber cores 61 are exposed at the MCF end face 12a. Each of the first fiber cores 61 extends in the optical axis direction D. In the example shown in this embodiment, the MCF 12 includes four first fiber cores 61. The four first fiber cores 61 are arranged in a square lattice pattern.

[0027] The ferrule 13 holds the MCF 12. The ferrule 13 has an accommodating hole 13a and an end face 13b. The ferrule 13 accommodates a portion of the MCF 12 inside the accommodating hole 13a. The MCF 12 is exposed from the accommodating hole 13a.

[0028] The first lens 15 has a pair of surfaces 15a, 15b facing each other in the optical axis direction D and an outer peripheral surface 15c connecting the pair of surfaces 15a, 15b. The surface 15b of the first lens 15 faces the MCF end face 12a. The first lens 15 has the same outer diameter as the ferrule 13. The end face 13b of the ferrule 13 and the surface 15b of the first lens 15 face each other. The surface 15a of the first lens 15 faces the metal sleeve 30. The first lens 15 is, for example, a rod lens, and the surface 15a is spherical and the surface 15b is flat. As a modification of this embodiment, the first lens 15 may be a GRIN lens, a biconvex lens, or the like. The first lens 15 may be fixed to the first metal holder 18 with an adhesive or by press-fitting.

[0029] The sleeve 16 holds the ferrule 13 and the first lens 15. The sleeve 16 holds the ferrule 13 and the first lens 15 so that the light emitted from the MCF 12 is collimated. The sleeve 16 has a cylindrical shape. The sleeve 16 has an accommodating hole 16a. The sleeve 16 accommodates at least a portion of the ferrule 13 inside the accommodating hole 16a. The sleeve 16 holds the ferrule 13 and an outer peripheral surface 15c of the first lens 15 in the accommodating hole 16a.

[0030] The first metal holder 18 holds the sleeve 16. In other words, the first metal holder 18 holds the MCF 12 and the first lens 15 so that the light emitted from the MCF 12 is collimated. The first metal holder 18 has a cylindrical shape. The first metal holder 18 has end faces 18a and 18b that face each other in the optical axis direction D, and an accommodating hole 18c. The accommodating hole 18c extends in the opposing direction of the end faces 18a and 18b. The accommodating hole 18c penetrates between the end faces 18a and 18b. The first metal holder 18 accommodates at least a portion of the sleeve 16 inside the accommodating hole 18c. The first metal holder 18 contacts the sleeve 16 at the accommodating hole 18c.

[0031] The SCF lens unit 20 includes a second lens 22 and a second metal holder 24. The second lens 22 has a pair of surfaces 22a, 22b facing each other in the optical axis direction D, and an outer peripheral surface 22c connecting the pair of surfaces 22a, 22b. The second lens 22 is rod-shaped, and the longitudinal direction of the second lens 22 is along the optical axis direction D. The surface 15a of the first lens 15 and the surface 22b of the second lens 22 face each other. The surface 22a of the second lens 22 faces the SCF bundle unit 40. The second lens 22 is, for example, a GRIN lens, and the surfaces 22a and 22b are flat surfaces.

[0032] In a variation of this embodiment, second lens 22 may be a rod lens, in which case surface 22b is curved and surface 22a is flat. In another variation of this embodiment, second lens 22 may be a biconvex lens.

[0033] The second metal holder 24 holds the second lens 22. The second lens 22 may be fixed to the second metal holder 24 with an adhesive or by press-fitting. The second metal holder 24 has a cylindrical shape. The second metal holder 24 has end faces 24a and 24b facing each other in the optical axis direction D, an outer circumferential surface 24c, and an accommodating hole 24d. The outer circumferential surface 24c extends in the optical axis direction D and connects the end faces 24a and 24b. The accommodating hole 24d extends in the opposing direction of the end faces 24a and 24b. The accommodating hole 24d penetrates between the end faces 24a and 24b. The second metal holder 24 accommodates at least a portion of the second lens 22 inside the accommodating hole 24d. The second metal holder 24 holds the outer circumferential surface 22c of the second lens 22 in the accommodating hole 24d. The second lens 22 protrudes along the optical axis direction D from the end surface 24 a of the second metal holder 24 toward the SCF bundle unit 40 .

[0034] The metal sleeve 30 has a cylindrical shape and includes a first cylindrical portion 31 and a second cylindrical portion 32. The first cylindrical portion 31 has an inner peripheral surface 31a that contacts the outer peripheral surface 24c of the second metal holder 24.

[0035] The second cylindrical portion 32 extends from the edge of the first cylindrical portion 31 toward the optical axis of the second lens 22. In other words, the second cylindrical portion 32 protrudes from the edge of the first cylindrical portion 31 toward the optical axis of the second lens 22. The second cylindrical portion 32 has principal surfaces 32a, 32b and an inner peripheral surface 32c that face each other. The principal surfaces 32a, 32b are, for example, flat surfaces. The first cylindrical portion 31 is bonded to the principal surface 32b of the second cylindrical portion 32. The inner peripheral surface 32c overlaps with the second lens 22 when viewed in a direction perpendicular to the optical axis direction D. The inner peripheral surface 32c is spaced apart from the second lens 22.

[0036] The SCF bundle unit 40 includes an SCF bundle 41, a sleeve 45, and a third metal holder 48. The SCF bundle 41 corresponds to an optical fiber bundle. The SCF bundle 41 includes a plurality of SCFs 42 and a ferrule 43. In the SCF bundle 41, the plurality of SCFs 42 are arranged in parallel in a direction perpendicular to the optical axis direction D. The optical axis direction D corresponds to the longitudinal direction of the plurality of SCFs 42.

[0037] As shown in Fig. 4, each of the multiple SCFs 42 includes a second fiber core 71 and a second fiber clad 72. Fig. 4 is a cross-sectional view of the SCF bundle in a direction perpendicular to the optical axis direction. Each SCF 42 has an SCF end face 42a. The second fiber core 71 is exposed at the SCF end face 42a. The SCF end face 42a faces the surface 22a of the second lens 22. Each second fiber core 71 extends in the optical axis direction D. The distance between the SCF end face 42a and the surface 22a of the second lens 22 is, for example, 100 to 300 μm.

[0038] In the example shown in this embodiment, the SCF bundle 41 includes four second fiber cores 71. The four second fiber cores 71 are arranged in a square lattice pattern. As a modification of this embodiment, when the SCF bundle 41 includes seven second fiber cores 71, the MCF 12 may include seven first fiber cores 61.

[0039] The ferrule 43 holds the plurality of SCFs 42. The ferrule 43 has an accommodating hole 43a and an end face 43b. The ferrule 43 accommodates a portion of each SCF 42 in the accommodating hole 43a. The plurality of SCFs 42 are exposed from the accommodating hole 43a.

[0040] The sleeve 45 holds the ferrule 43. The sleeve 45 has a cylindrical shape. The sleeve 45 has an accommodating hole 45a. The sleeve 45 accommodates a portion of the ferrule 43 and a portion of the plurality of SCFs 42 extending from the ferrule 43 inside the accommodating hole 45a. The sleeve 45 contacts the ferrule 43 at the accommodating hole 45a.

[0041] The third metal holder 48 holds the ferrule 43. The ferrule 43 is fixed to the third metal holder 48 by, for example, adhesive or press-fitting. The third metal holder 48 has a cylindrical shape. The third metal holder 48 has end faces 48a and 48b that face each other in the optical axis direction D, and an accommodating hole 48c. The accommodating hole 48c extends in the opposing direction of the end faces 48a and 48b. The accommodating hole 48c penetrates between the end faces 48a and 48b. The third metal holder 48 accommodates at least a portion of the ferrule 43 in the accommodating hole 48c. The end face 43b of the ferrule 43 may be flush with the end face 48a of the third metal holder 48, or may be disposed within the accommodating hole 48c.

[0042] The MCF collimator unit 10 and the SCF lens unit 20 are bonded to each other. More specifically, the end face 18a of the first metal holder 18 and the end face 24b of the second metal holder 24 are fixed to each other by welding. The SCF lens unit 20 and the metal sleeve 30 are bonded to each other. More specifically, the outer peripheral surface 24c of the second metal holder 24 and the inner peripheral surface 31a of the metal sleeve 30 are fixed to each other by welding. The metal sleeve 30 and the SCF bundle unit 40 are bonded to each other. More specifically, the main surface 32a of the metal sleeve 30 and the end face 48a of the third metal holder 48 are fixed to each other by welding. The metal sleeve 30 connects the second metal holder 24 and the third metal holder 48. For example, laser spot welding is used for these weldings.

[0043] In the optical connection structure 1, the MCF 12 and the plurality of SCFs 42 are optically coupled (spatially coupled) via space, a first lens 15, and a second lens 22. In the optical connection structure 1, light L passing through the MCF 12 is incident on the plurality of SCFs 42, or light L passing through the plurality of SCFs 42 is incident on the MCF 12. The light L is, for example, light having a wavelength in the 1.55 μm band. The optical connection structure 1 may be used in an optical transmitter that transmits the light L to each of the first fiber cores 61 of the MCF 12, or in an optical receiver that receives the light L from each of the first fiber cores 61 of the MCF 12.

[0044] Each SCF end face 42a and each MCF end face 12a face each other in the optical axis direction D. The MCF end face 12a and each SCF end face 42a are inclined with respect to the optical axis direction D. The MCF end face 12a and each SCF end face 42a are flat and are inclined with respect to the optical axis direction D, and are also inclined with respect to a plane perpendicular to the optical axis direction D. In this specification, "inclined" does not include being perpendicular.

[0045] As shown in Fig. 5, the MCF end face 12a is inclined at an angle θ1 with respect to a plane perpendicular to the optical axis direction D. Fig. 5 is a diagram for explaining the incidence of light from a lens into multiple single-core fibers. In Fig. 5, the first lens 15 and the second lens 22 are schematically shown as convex lenses. Each SCF end face 42a is inclined at an angle θ2 with respect to a plane perpendicular to the optical axis direction D.

[0046] The MCF end face 12a and each SCF end face 42a are arranged so that a V-shape is formed by a first imaginary plane 77 including the MCF end face 12a and a second imaginary plane 78 including each SCF end face 42a. For example, the MCF end face 12a corresponds to the first end face, and each SCF end face 42a corresponds to the second end face.

[0047] In this specification, "arranged so as to form a V-shape" means that the first normal vector of the first imaginary plane 77 including the MCF end face 12a and the second normal vector of the second imaginary plane 78 including each SCF end face 42a are on the same plane, and that within this same plane, the optical axis direction component of the first normal vector and the optical axis direction component of the second normal vector are components that move toward each other, and the directional component perpendicular to the optical axis direction of the first normal vector and the directional component perpendicular to the optical axis direction of the second normal vector are components that move in the same direction.

[0048] The exposed surfaces of the second fiber cores 71 at each SCF end face 42a and the exposed surfaces of the first fiber cores 61 at the MCF end face 12a are optically coupled to each other. For example, the second fiber cores 71 have the same MFD as the first fiber cores 61 in a portion other than the tapered portion 65 described below. Here, "the same MFD" means that the difference in MFD is less than ±10%. In addition to the above, in this specification, "the same" includes deviations within the range of manufacturing error.

[0049] The shape of the arrangement of the multiple first fiber cores 61 on the MCF end face 12a and the shape of the arrangement of the multiple second fiber cores 71 on each SCF end face 42a are similar to each other when viewed along the optical axis direction D. In other words, the shape of the arrangement of the multiple first fiber cores 61 when the MCF end face 12a is orthogonally projected in the optical axis direction D and the shape of the arrangement of the multiple second fiber cores 71 when each SCF end face 42a is orthogonally projected in the optical axis direction D are similar to each other.

[0050] The core pitch P1 of the multiple first fiber cores 61 is smaller than the core pitch P2 of the multiple second fiber cores 71. As shown in FIG. 6 , in each of the multiple second fiber cores 71, the mode field diameter at the SCF end face 42a is larger than the mode field diameter at a position a predetermined distance away from the SCF end face 42a. Hereinafter, the mode field diameter will also be referred to as "MFD." FIG. 6 is a conceptual diagram showing the mode field diameter of the second fiber core. Each of the multiple second fiber cores 71 includes a portion in which the MFD gradually decreases continuously or stepwise with increasing distance from the SCF end face 42a in the optical axis direction D. In other words, the multiple second fiber cores 71 include a tapered portion 65 in which the MFD changes taperedly in the optical axis direction D.

[0051] The SCF 42 may be, for example, a thermally expanded core (TEC) fiber, and may be formed with a shape that has a partially different MFD by TEC processing. For example, the tapered portion 65 described above may be formed by TEC processing.

[0052] The first lens 15 is interposed between the MCF 12 and the SCF bundle 41. The second lens 22 is interposed between the SCF bundle 41 and the first lens 15. The first lens 15 and the second lens 22 are arranged between the MCF end face 12 a and each SCF end face 42 a in the following order from the MCF end face 12 a toward each SCF end face 42 a: MCF end face 12 a, first lens 15, second lens 22, and each SCF end face 42 a.

[0053] The first lens 15 is disposed at a position facing the MCF 12 along the optical axis direction D. The second lens 22 is disposed at a position facing the SCF bundle 41 along the optical axis direction D. The second lens 22 and the first lens 15 collimate the multiple light beams L emitted from each of the multiple second fiber cores 71 of the SCF bundle 41, and then focus the collimated light beams on the MCF end face 12a by the first lens 15, and couple the collimated light beams to the corresponding first fiber cores 61 among the multiple first fiber cores 61. Conversely, the first lens 15 collimates the multiple light beams L emitted from each of the multiple first fiber cores 61 of the MCF 12, and then focus the collimated light beams on the SCF end face 42a by the second lens 22, and couple the collimated light beams to the corresponding second fiber cores 71 among the multiple second fiber cores 71.

[0054] The focal length of the first lens 15 is "f1," and the focal length of the second lens 22 is "f2." When the core pitch of at least one set of first fiber cores 61 at the MCF end face 12a is "P1," the MFD of light of a specific wavelength at the MCF end face 12a for the at least one set of first fiber cores 61 is "MFD1," the core pitch of at least one set of second fiber cores 71 at the SCF end face 42a is "P2," and the MFD of each of the at least one set of second fiber cores 71 at the SCF end face 42a is "MFD2," the relationships shown in the following formulas (1) and (2) are satisfied. The at least one set of first fiber cores 61 and the at least one set of second fiber cores 71 are optically coupled to each other. (P1 / P2) x 0.9 ≦ f1 / f2 ≦ (P1 / P2) x 1.1 ... (1) (P1 / P2) x 0.9 ≦ MFD1 / MFD2 ≦ (P1 / P2) x 1.1 ... (2)

[0055] For example, the specific wavelength is 1.55 μm, and "MFD2" is 20 μm or more and 40 μm or less. As an example, the lens system between the MCF end face 12a and each SCF end face 42a has a magnification of 3.1 times. In this case, for example, "P1" is 40 μm, and "P2" is 125 μm. The cladding diameter of the MCF 12 and each SCF 42 is 125 μm. For example, "MFD1" in the TEC processing unit is 10 μm for light with a wavelength of 1.55 μm, and "MFD2" is 31 μm for light with a wavelength of 1.55 μm. In the example shown in this embodiment, the angle at which each SCF end face 42a is inclined with respect to a plane perpendicular to the optical axis of the second lens 22 is 4 degrees or less.

[0056] The pitch between nearest cores is preferably 30 μm or more. In this case, crosstalk between each of the multiple fiber cores is suppressed. The shortest distance between the center of the fiber core and the outer periphery of the cladding is preferably 30 μm or more. For example, when the cladding diameter is 125 μm, the core pitch is preferably 45 μm or less. In this case, by reducing the core pitch, the distance between the fiber core and the outer periphery of the cladding is secured, and light leakage to the outside is suppressed.

[0057] For example, "f1" is 1.0 [mm] and "f2" is 3.1 [mm]. For example, when a biconvex lens is used, the focal length "f1" of the first lens 15 and the focal length "f2" of the second lens 22 are preferably 0.5 [mm] or more and 5.0 [mm] or less. If the focal length "f2" is 0.5 [mm] or more, the distance between the lenses and the distance between the fiber and the lens are also ensured, making manufacturing easier. If the focal length "f2" is 5 [mm] or less, the overall length of the optical connection structure can be shortened.

[0058] Next, an example of a manufacturing method of the optical connection structure 1 will be described in detail with reference to Fig. 7 . In manufacturing the optical connection structure 1, first, the MCF collimator unit 10, the SCF lens unit 20, and the SCF bundle unit 40 are assembled. In the assembled MCF collimator unit 10, the first metal holder 18 holds the MCF 12 and the first lens 15 facing the MCF end face 12a so that the light emitted from the MCF 12 is collimated. In the assembled SCF lens unit 20, the second metal holder 24 holds the second lens 22. In the assembled SCF bundle unit 40, the third metal holder 48 holds the ferrule 43. The ferrule 43 holds a plurality of SCFs 42.

[0059] Subsequently, three-body collimation is performed by the MCF collimator unit 10, the SCF lens unit 20, and the SCF bundle unit 40.

[0060] Next, at position SP1 corresponding to the three body centers, end face 18a of first metal holder 18 and end face 24b of second metal holder 24 are welded and fixed to each other. At this time, end face 18a of first metal holder 18 and end face 24b of second metal holder 24 are welded to each other by laser spot welding. This fixes the relative positions of MCF unit 11 and first lens 15. At this time, surface 15a of first lens 15 and surface 22b of second lens 22 face each other.

[0061] Next, the metal sleeve 30 is arranged so that the outer peripheral surface 24c of the second metal holder 24 contacts the inner peripheral surface 31a of the metal sleeve 30, and alignment is performed by the MCF collimator unit 10 and the SCF lens unit 20, which are welded to each other, and the SCF bundle unit 40. In other words, alignment of the SCF bundle 41 is performed with respect to the MCF unit 11 and the first lens 15, whose relative positions are fixed to each other.

[0062] Next, at position SP2 according to the alignment of the MCF collimator unit 10 and the SCF lens unit 20, which have been welded together, with the SCF bundle unit 40, the outer peripheral surface 24c of the second metal holder 24 and the inner peripheral surface 31a of the metal sleeve 30 are welded and fixed to each other. At this time, the outer peripheral surface 24c of the second metal holder 24 and the inner peripheral surface 31a of the metal sleeve 30 are welded by laser spot welding. As a result, a part of the second lens 22 and the inner peripheral surface 32c of the metal sleeve 30 are arranged to overlap each other when viewed in a direction perpendicular to the optical axis direction D.

[0063] Subsequently, alignment is performed by the MCF collimator unit 10, the SCF lens unit 20, and the metal sleeve 30, which are welded together, and the SCF bundle unit 40. In other words, alignment of the SCF bundle 41 is performed again with respect to the MCF unit 11 and the first lens 15, whose relative positions are fixed.

[0064] Next, at position SP3 corresponding to the alignment of the MCF collimator unit 10, SCF lens unit 20, and metal sleeve 30, which have been welded together, with the SCF bundle unit 40, the main surface 32a of the metal sleeve 30 and the end surface 48a of the third metal holder 48 are welded and fixed to each other. At this time, the main surface 32a of the metal sleeve 30 and the end surface 48a of the third metal holder 48 are welded by laser spot welding. In this way, the optical connection structure 1 is assembled. For example, a YAG laser is used for the series of laser spot welding.

[0065] Next, the effects obtained from the optical connection structure according to the embodiment will be described. In this optical connection structure 1, the first metal holder 18 holds the MCF 12 and the first lens 15 so that the light emitted from the MCF 12 is collimated. The second metal holder 24 is welded to the first metal holder 18. The second metal holder 24 holds the second lens 22. The third metal holder 48 holds the SCF bundle 41. The metal sleeve 30 is welded to the second metal holder 24 and the third metal holder 48, respectively. The metal sleeve 30 connects the second metal holder 24 and the third metal holder 48. With this structure, since each component is joined by welding, environmental reliability can be ensured compared to when adhesives are used. Furthermore, with this configuration, the optical connection structure 1 is constructed by, for example, laser spot welding, but misalignment is less likely to occur. This ensures environmental reliability and reduces optical connection loss.

[0066] When the MFD of an SCF is relatively small, the impact of axial misalignment in a direction perpendicular to the optical axis direction D on optical loss is also relatively large. For example, if the MFD of an SCF is 10 μm for light with a wavelength of 1.55 μm, an axial misalignment of about 1 to 2 μm in a direction perpendicular to the optical axis direction D may result in significant optical loss. Even an axial misalignment of 5 to 10 μm in the optical axis direction may result in significant optical loss. In contrast, the larger the MFD, the more likely it is that optical loss caused by axial misalignment will be reduced.

[0067] FIG. 8 is a graph showing the relationship between the axial misalignment (X-axis misalignment) in the X-axis direction perpendicular to the optical axis direction D and the optical loss increase amount (loss increase ΔIL). FIG. 9 is a graph showing the relationship between the axial misalignment (Z-axis misalignment) in the Z-axis direction along the optical axis direction D and the optical loss increase amount (loss increase ΔIL). In FIGS. 8 and 9 , data D1 shows the calculation results when the MFD of the SCF is 10 μm. Data D2 shows the calculation results when the MFD is 16 μm. Data D3 shows the calculation results when the MFD of the SCF is 20 μm. Data D4 shows the calculation results when the MFD of the SCF is 24 μm. Data D5 shows the calculation results when the MFD of the SCF is 28 μm. Data D6 shows the calculation results when the MFD of the SCF is 32 μm.

[0068] In the calculation results shown in Figure 8, when the MFD of the SCF is 10 μm, the loss increase is 0.17 dB with an X-axis misalignment of 1 μm, and 0.7 dB with an X-axis misalignment of 2 μm. When the MFD is 20 μm, the optical loss increase is suppressed to 0.04 dB with an X-axis misalignment of 1 μm, and even with an X-axis misalignment of 2 μm, the optical loss increase is suppressed to 0.17 dB. When the MFD is 32 μm, the optical loss increase is suppressed to 0.07 dB with an X-axis misalignment of 2 μm.

[0069] 9, when the MFD of the SCF is 10 μm, a Z-axis misalignment of 10 μm increases the loss by 0.04 dB. When the MFD is 20 μm, a Z-axis misalignment of 10 μm reduces the increase in optical loss to almost zero.

[0070] On the other hand, if the MFD exceeds 40 μm, the tail of the beam will reach the outer periphery of the cladding in the SCF, which may result in optical loss.

[0071] In the example shown in the above-described embodiment, the MFD 2 is, for example, 20 μm or more and 40 μm or less at a specific wavelength, which can further suppress misalignment and optical coupling loss.

[0072] As shown in Fig. 10 , the greater the angle at which the SCF end face 42a is inclined with respect to a plane perpendicular to the optical axis direction D of the second lens 22, the greater the defocusing in the optical axis direction D of the light incident from the SCF end face 42a between different second fiber cores 71, which may result in a decrease in optical coupling efficiency. In Fig. 10 , the second lens 22 is schematically shown as a convex lens. In the example shown in the above-described embodiment, the angle is 4 degrees or less. In this case, the defocusing in the optical axis direction D of the light incident from the SCF end face 42a can be suppressed.

[0073] When the MFD2 is 10 μm, the return loss at the end face 43 b is relatively high when the angle at which the SCF end face 42 a is inclined with respect to the plane perpendicular to the optical axis direction D of the second lens 22 is 8 degrees. When the MFD2 is 20 μm or more, the return loss at the end face 43 b is relatively high even when the angle at which the SCF end face 42 a is inclined with respect to the plane perpendicular to the optical axis direction D of the second lens 22 is 4 degrees or less. When the MFD2 is 30 μm or more, the return loss at the end face 43 b is relatively high even when the angle at which the SCF end face 42 a is inclined with respect to the plane perpendicular to the optical axis direction D of the second lens 22 is 2.7 degrees or less. This 2.7 degrees is calculated by 8 degrees × 10 μm / 30 μm.

[0074] In the example shown in the above-described embodiment, the second lens 22 protrudes from the second metal holder 24 toward the SCF bundle 41. In this case, the distance between the surface 22 a of the second lens 22 and the SCF end faces 42 a of the multiple SCFs 42 is narrowed.

[0075] In the example shown in the above-described embodiment, the second metal holder 24 and the metal sleeve 30 have a cylindrical shape. The metal sleeve 30 includes a first cylindrical portion 31 and a second cylindrical portion 32. The first cylindrical portion 31 has an inner circumferential surface 31a welded to the second metal holder 24. The second cylindrical portion 32 may extend from the edge of the first cylindrical portion 31 toward the optical axis. The second cylindrical portion 32 has an inner circumferential surface 32c that overlaps with the second lens 22 when viewed in a direction perpendicular to the optical axis direction D, and a main surface 32a that is welded to the third metal holder 48.

[0076] In the example shown in the above-described embodiment, the second lens 22 is a rod lens. The rod lens has an outer peripheral surface 24c held by the second metal holder 24, a surface 22b facing the first lens 15, and a surface 22a facing the SCF bundle 41. The surface 22b is a curved surface. The surface 22a is a flat surface. In this case, the second lens 22 can be held more stably by the second metal holder 24.

[0077] In the example shown in the above-described embodiment, the second lens 22 includes a GRIN lens. The GRIN lens has an outer peripheral surface 24c that is held by the second metal holder 24. In this case, the second lens 22 can be held more stably by the second metal holder 24.

[0078] In the example shown in the above-described embodiment, the MFD of each of the multiple second fiber cores 71 at the SCF end face 42 a is larger than the MFD at a position a predetermined distance away from the SCF end face 42 a, which can reduce optical coupling loss at the SCF end face 42 a.

[0079] Although the embodiments of the present disclosure have been described in detail above, the present invention is not limited to the above embodiments and can be applied to various embodiments. In the above example, the number of first fiber cores 61 and the number of second fiber cores 71 are four. However, the number of fiber cores is not limited to this. The number of first fiber cores 61 and the number of second fiber cores 71 may be different from each other.

[0080] DESCRIPTION OF SYMBOLS 1...Optical connection structure 2...MFD 10...MCF collimator unit 11...MCF unit 12...MCF 12a...MCF end face 13, 43...Ferrule 13a, 16a, 18c, 24d, 43a, 45a, 48c...Accommodating hole 13b, 18a, 18b, 24a, 24b, 43b, 48a, 48b...End face 15...First lens 15a, 15b...Surface 15c...Outer surface 16, 45...Sleeve 18...First metal holder 20...SCF lens unit 22...Second lens 22a...Surface 22b...Surface 22c...Outer surface 24...Second metal holder 24c...Outer surface 30...Metal sleeve 31...First cylindrical portion 31a...Inner peripheral surface 32...Second cylindrical portion 32a, 32b...main surface 32b...main surface 32c...inner surface 40...SCF bundle unit 41...SCF bundle 42...SCF 42a...SCF end face 43b...end face 48...third metal holder 61...first fiber core 62...first fiber clad 65...tapered portion 71...second fiber core 72...second fiber clad 77...first imaginary plane 78...second imaginary plane D...optical axis direction D1...data D2...data D3...data D4...data D5...data D6...data L...light P1, P2...core pitch SP1, SP2, SP3...position θ1...angle θ2...angle.

Claims

1. An optical fiber bundle including: a multicore fiber including a plurality of first fiber cores and having a first end face from which the plurality of first fiber cores are exposed; and a plurality of single-core fibers, each including a second fiber core and having a second end face from which the second fiber cores are exposed, arranged in parallel in a direction perpendicular to the longitudinal direction; a first lens and a second lens arranged in order from the first end face to the second end face between the first end face and the second end face; a first metal holder holding the multicore fiber and the first lens so that light emitted from the multicore fiber is collimated; a second metal holder welded to the first metal holder and holding the second lens; a third metal holder holding the optical fiber bundle; and metal sleeves welded to the second metal holder and the third metal holder, respectively, connecting the second metal holder and the third metal holder; an optical connection structure in which, when a focal length of the first lens is "f1", a focal length of the second lens is "f2", a pitch of at least one set of the first fiber cores at the first end face is "P1", a mode field diameter of light of a specific wavelength at the first end face for the at least one set of first fiber cores is "MFD1", a pitch of at least one set of the second fiber cores at the second end face is "P2", and a mode field diameter of light of the specific wavelength at the second end face for the at least one set of second fiber cores is "MFD2", the following relationships are satisfied: (P1 / P2)×0.9≦f1 / f2≦(P1 / P2)×1.1 (P1 / P2)×0.9≦MFD1 / MFD2≦(P1 / P2)×1.

1.

2. The optical connection structure according to claim 1, wherein the MFD2 is 20 μm or more and 40 μm or less at the specific wavelength.

3. An optical connection structure according to claim 1 or 2, wherein the angle at which the second end face is inclined with respect to a plane perpendicular to the optical axis direction of the second lens is 4 degrees or less.

4. An optical connection structure according to any one of claims 1 to 3, wherein the second lens protrudes from the second metal holder toward the optical fiber bundle.

5. An optical connection structure as described in any one of claims 1 to 4, wherein the second metal holder and the metal sleeve are cylindrical in shape, and the metal sleeve includes a first cylindrical portion having an inner surface welded to the second metal holder, and a second cylindrical portion extending from an edge of the first cylindrical portion toward the optical axis and having an inner surface overlapping the second lens when viewed along a direction perpendicular to the optical axis direction and a surface welded to the third metal holder.

6. An optical connection structure as described in any one of claims 1 to 5, wherein the second lens is a rod lens having an outer peripheral surface held by the second metal holder, a first end face facing the first lens, and a second end face facing the optical fiber bundle, the first end face being a curved surface, and the second end face being a flat surface.

7. An optical connection structure according to any one of claims 1 to 6, wherein the second lens is a GRIN lens having an outer peripheral surface that is held by the second metal holder.

8. An optical connection structure described in any one of claims 1 to 7, wherein the mode field diameter of each of the plurality of second fiber cores at the second end face is larger than the mode field diameter at a position a predetermined distance away from the second end face.

9. A multicore fiber includes a plurality of first fiber cores and has a first end face from which the plurality of first fiber cores are exposed, and a first metal holder holds a first lens facing the first end face, and a second metal holder holds a second lens facing the first lens, so that light emitted from the multicore fiber is collimated; laser welding the second metal holder welded to the first metal holder and a metal sleeve; laser welding the metal sleeve welded to the second metal holder and a third metal holder holding an optical fiber bundle in which a plurality of single-core fibers, each of which includes a second fiber core and has a second end face from which the second fiber cores are exposed, are arranged in parallel in a direction perpendicular to the longitudinal direction; and the shape of the arrangement of the plurality of second fiber cores at the second end face and the shape of the arrangement of the plurality of first fiber cores at the first end face are similar to each other; a focal length of the first lens is "f1", a focal length of the second lens is "f2", a pitch of at least one set of the first fiber cores at the first end face is "P1" and a mode field diameter of light of a specific wavelength at the first end face for the at least one set of first fiber cores is "MFD1", a pitch of at least one set of the second fiber cores at the second end face is "P2" and a mode field diameter of light of the specific wavelength at the second end face for the at least one set of second fiber cores is "MFD2", the following relationships are satisfied: (P1 / P2) x 0.9 ≦ f1 / f2 ≦ (P1 / P2) x 1.1 (P1 / P2) x 0.9 ≦ MFD1 / MFD2 ≦ (P1 / P2) x 1.1.

Images