Optical connection structure

Abstract

In this optical connection structure, the following formula is satisfied. Regarding at least one first fiber core (61) among a plurality of first fiber cores (61), the mode field diameter at a first MCF end surface (11a) is "D1". Regarding a second fiber core (71) corresponding to the at least one first fiber core (61) among a plurality of second fiber cores (71), the mode field diameter at a second MCF end surface (21a) is "D2". The core pitch of the plurality of first fiber cores (61) at the first MCF end surface (11a) is "P1". The core pitch of the plurality of second fiber cores (71) at the second MCF end surface (21a) is "P2". (P1 / P2)×0.8 ≤ D1 / D2 ≤ (P1 / P2)×1.2

Description

Optical connection structure

[0001] The present disclosure relates to an optical connection structure. This application claims priority to Japanese Patent Application No. 2023-099171, filed on June 16, 2023, and incorporates by reference all the contents of the above-mentioned Japanese application.

[0002] An optical connection structure for optically coupling a pair of multi-core fibers is known. Hereinafter, the multi-core fiber is also referred to as an MCF. For example, in Patent Document 1, two MCFs are optically coupled via a lens. The MCF includes multiple fiber cores. For example, the MCF has an end face from which the multiple fiber cores are exposed. The lens is disposed between the end faces of the two MCFs.

[0003] Japanese Patent Application Laid-Open No. 2022-39849

[0004] 2015 Institute of Electronics, Information and Communication Engineers General Conference Proceedings "Core pitch conversion connection technology for multi-core fibers using spatial coupling" Optoquest

[0005] An optical connection structure according to one aspect of the present disclosure includes a first multicore fiber, a second multicore fiber, a first lens, and a second lens. The first multicore fiber includes a plurality of first fiber cores. The first multicore fiber has a first end face from which the plurality of first fiber cores are exposed. The second multicore fiber includes a plurality of second fiber cores. The second multicore fiber has a second end face from which the plurality of second fiber cores are exposed. Each of the plurality of second fiber cores is optically connected to a corresponding first fiber core among the plurality of first fiber cores. 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 exposed surfaces of each of the plurality of second fiber cores at the second end face and the exposed surfaces of each of the plurality of first fiber cores at the first end face are optically coupled to each other. The core pitch of the plurality of first fiber cores at the first end face is greater than the core pitch of the plurality of second fiber cores at the second end face. The mode field diameter of each of the multiple first fiber cores at the first end face is larger than the mode field diameter of the multiple second fiber cores at the second end face that is optically coupled to the first fiber core. The shape of the arrangement of the multiple first fiber cores at the first end face and the shape of the arrangement of the multiple second fiber cores at the second end face are similar to each other. The following formula is satisfied. In the following formula, the mode field diameter at the first end face of at least one first fiber core among the multiple first fiber cores is "D1". The mode field diameter at the second end face of a second fiber core among the multiple second fiber cores that is optically coupled to the at least one first fiber core is "D2". The core pitch of the multiple first fiber cores at the first end face is "P1". The core pitch of the multiple second fiber cores at the second end face is "P2". (P1 / P2) × 0.8 ≦ D1 / D2 ≦ (P1 / P2) × 1.2

[0006] Fig. 1 is a schematic cross-sectional view along the optical axis direction of an optical connection structure according to an embodiment. Fig. 2 is a partial cross-sectional view of the optical connection structure. Fig. 3 is a cross-sectional view of a pair of multi-core fibers in a direction perpendicular to the optical axis direction. Fig. 4 is a schematic cross-sectional view along the optical axis direction of an optical connection structure according to a modified example of this embodiment. Fig. 5 is a schematic cross-sectional view along the optical axis direction of an optical connection structure according to a modified example of this embodiment.

[0007] [Problem to be Solved by the Present Disclosure] Regarding optical coupling of MCFs, it is conceivable to optically couple two MCFs having different core pitches. In this case, if multiple lenses are arranged according to the ratio of the core pitches of the two MCFs, the pitch of the light beams emitted from each fiber core can be adjusted by the multiple lenses, thereby achieving optical coupling of these MCFs. In this specification, the "core pitch" corresponds to the distance between the centers of the fiber cores in a cross section perpendicular to the optical axis direction D.

[0008] In this case, the inventors of the present application discovered that if the mode field diameter of one MCF does not match the diameter of the focused beam spot formed by the lens, optical coupling loss occurs. Hereinafter, the mode field diameter is also referred to as MFD. When the pitch of the light beams emitted from each fiber core is adjusted by multiple lenses, the focused beam spot diameter also changes at the multiple lenses. Therefore, even if the pitch of the light beams emitted from each fiber core is adjusted by lenses according to the ratio of the core pitches of the two MCFs, there is a risk that the MFD of the MCFs will not match the diameter of the focused beam spot formed by the lenses, resulting in optical coupling loss.

[0009] Effect of the Present Disclosure According to the present disclosure, it is possible to provide an optical connection structure that can suppress optical coupling loss when joining a pair of optical fibers having different core pitches.

[0010] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0011] (1) An optical connection structure according to an embodiment of the present disclosure includes a first multicore fiber, a second multicore fiber, a first lens, and a second lens. The first multicore fiber includes a plurality of first fiber cores. The first multicore fiber has a first end face from which the plurality of first fiber cores are exposed. The second multicore fiber includes a plurality of second fiber cores. The second multicore fiber has a second end face from which the plurality of second fiber cores are exposed. Each of the plurality of second fiber cores is optically connected to a corresponding first fiber core among the plurality of first fiber cores. 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 exposed surfaces of each of the plurality of second fiber cores at the second end face and the exposed surfaces of each of the plurality of first fiber cores at the first end face are optically coupled to each other. The core pitch of the plurality of first fiber cores at the first end face is larger than the core pitch of the plurality of second fiber cores at the second end face. The mode field diameter of each of the multiple first fiber cores at the first end face is larger than the mode field diameter of the multiple second fiber cores at the second end face that corresponds to the first fiber core. The shape of the arrangement of the multiple first fiber cores at the first end face and the shape of the arrangement of the multiple second fiber cores at the second end face are similar to each other. The following formula is satisfied. In the following formula, the mode field diameter at the first end face of at least one first fiber core among the multiple first fiber cores is "D1". The mode field diameter at the second end face of a second fiber core among the multiple second fiber cores that is optically coupled to the at least one first fiber core is "D2". The core pitch of the multiple first fiber cores at the first end face is "P1". The core pitch of the multiple second fiber cores at the second end face is "P2". (P1 / P2) × 0.8 ≦ D1 / D2 ≦ (P1 / P2) × 1.2

[0012] In this optical connection structure, the mode field diameter D1 of the first fiber core, the mode field diameter D2 of the second fiber core, the core pitch P1 of the first fiber core, and the core pitch P2 of the second fiber core have the relationship expressed by the above formula. As a result of extensive research, the inventors of the present application have determined that optical coupling loss can be reduced if the MFD is adjusted so that the relationship expressed by the above formula is satisfied. As a result, optical coupling loss can be suppressed when splicing a pair of optical fibers having different core pitches.

[0013] (2) In the optical connection structure of (1), in each of the plurality of first fiber cores, the mode field diameter at the first end face may be larger than the mode field diameter at a position a predetermined distance away from the first end face. This configuration can be realized, for example, by subjecting an existing MCF to a process of enlarging the mode field diameter only in a portion corresponding to the first end face. Therefore, while the configuration reduces costs and labor, leakage of incident light from the second multicore fiber to the first multicore fiber can be suppressed.

[0014] (3) In the optical connection structure of either (1) or (2), each of the plurality of first fiber cores may include a portion whose mode field diameter decreases with increasing distance from the first end face. In this case, leakage of incident light into the first multicore fiber at the first end face can be further suppressed compared to a configuration in which the mode field diameter changes abruptly.

[0015] (4) In the optical connection structure of any one of (1) to (3) above, when the focal length of the first lens is "f1" and the focal length of the second lens is "f2", the following relational expression may be satisfied. In this case, the optical coupling loss of the optical fiber can be further suppressed: (P1 / P2) x 0.9 ≦ f1 / f2 ≦ (P1 / P2) x 1.1

[0016] (5) In the optical connection structure according to any one of (1) to (4), when the refractive index of the first lens and the refractive index of the second lens are the same, the angle at which the second end face is inclined relative to the optical axis of the second lens may be larger than the angle at which the first end face is inclined relative to the optical axis of the first lens. In this case, optical coupling loss of the optical fiber can be further reduced.

[0017] (6) In the optical connection structure of (5) above, when the angle at which the first end face is inclined with respect to the optical axis of the first lens is "θ1" and the angle at which the second end face is inclined with respect to the optical axis of the second lens is "θ2", the following relational expression may be satisfied. In this case, reflection at each end face can be reduced, and optical coupling loss of the optical fiber can be further suppressed: (P1 / P2) x 0.8 ≦ θ2 / θ1 ≦ (P1 / P2) x 1.2

[0018] (7) In the optical connection structure according to any one of (1) to (6), the first end face and the second end face may be arranged such that a first imaginary plane including the first end face and a second imaginary plane including the second end face form a V-shape. In this case, optical coupling loss of the optical fiber can be further reduced.

[0019] (8) In the optical connection structure according to any one of (1) to (7), the focal lengths of the first lens and the second lens may be 0.5 or more and 4.0 or less. In this case, the distance between the lenses is ensured, which can ensure ease of manufacturing.

[0020] [Details of the embodiments of the present disclosure] Specific examples of the embodiments of the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted as appropriate.

[0021] First, the optical connection structure according to this embodiment will be described in more detail with reference to Fig. 1 to Fig. 3. Fig. 1 is a schematic cross-sectional view of an optical connection structure according to one embodiment taken along the optical axis direction. Fig. 2 is a partial cross-sectional view of the optical connection structure. Fig. 3 is a cross-sectional view of a multi-core fiber.

[0022] The optical connection structure 1 has a configuration for optically coupling, for example, a pair of multi-core fibers, and includes a first optical fiber unit 10, a second optical fiber unit 20, a first lens unit 30, a second lens unit 40, and a cylindrical member 50.

[0023] The first optical fiber unit 10 includes a first MCF 11, a ferrule 15, and a sleeve 17. The second optical fiber unit 20 includes a second MCF 21 and a ferrule 25. For example, the first MCF 11 corresponds to the first multi-core fiber, and the second MCF 21 corresponds to the second multi-core fiber.

[0024] The ferrule 15 holds the first MCF 11. The ferrule 15 has an accommodating hole 15a. The ferrule 15 accommodates a portion of the first MCF 11 inside the accommodating hole 15a. The sleeve 17 holds the ferrule 15. The sleeve 17 accommodates at least a portion of the ferrule 15 inside the accommodating hole 17a. The sleeve 17 has a flange portion 17b at one end in the axial direction. The flange portion 17b corresponds to the portion of the first optical fiber unit 10 that is fixed to the first lens unit 30.

[0025] The ferrule 25 holds the second MCF 21. The ferrule 25 has an accommodating hole 25a. The ferrule 25 accommodates a portion of the second MCF 21 inside the accommodating hole 25a.

[0026] For example, the first lens unit 30 includes a first lens 32 and a cylindrical first lens holding member 34 that surrounds and holds the first lens 32. The second lens unit 40 includes a second lens 42 and a cylindrical second lens holding member 44 that surrounds and holds the second lens 42.

[0027] The cylindrical member 50 has an accommodation hole 50a. The cylindrical member 50 accommodates the second lens holder 44 and at least a portion of the ferrule 25 inside the accommodation hole 50a. For example, the cylindrical member 50 and the ferrule 25 are fixed with an adhesive. If the cylindrical member 50 and the ferrule 25 are both made of metal, they can also be fixed by welding. The ferrule 25 may be fixed by press-fitting into the cylindrical member 50. The first lens holder 34, the second lens holder 44, and the cylindrical member 50 each have an opening. The first lens holder 34, the second lens holder 44, and the cylindrical member 50 each form two openings arranged in a straight line. The adhesive is, for example, a thermosetting adhesive or an ultraviolet (UV) curing adhesive.

[0028] For example, the first optical fiber unit 10 is fixed to the end of the cylindrical member 50 via the first lens holding member 34. For example, the first optical fiber unit 10 is fixed to the first lens holding member 34 by welding. For example, the flange portion 17b is welded to the end of the first lens holding member 34 so that the opening of the sleeve 17 and the opening of the first lens holding member 34 are in communication with each other.

[0029] The first lens holding member 34 holds the first lens 32 on the inner surface 30a. The first lens holding member 34 is connected to the end of the cylindrical member 50 so that the opening of the first lens holding member 34 communicates with the opening of the cylindrical member 50. For example, the first lens holding member 34 is fixed to the cylindrical member 50 by welding. The second lens holding member 44 holds the second lens 42 on the inner surface 40a. The second lens holding member 44 is connected to the end of the ferrule 25 and is housed inside the cylindrical member 50. For example, the second lens holding member 44 is fixed to the ferrule 25 by welding. For example, welding is performed by irradiating a YAG laser. The above-mentioned fixing may also be performed by, for example, an adhesive.

[0030] The first lens unit 30 and the ferrule 15 are aligned with respect to the integrated second optical fiber unit 20 and cylindrical member 50. At this time, the first lens unit 30 is aligned in each of the X and Y directions. The ferrule 15 is aligned in each of the X, Y, Z, and θz directions. Once the alignment is complete, the cylindrical member 50 and the first lens unit 30 are fixed by welding. After that, the ferrule 15 is again aligned in each of the directions perpendicular to the optical axis direction D and around the optical axis direction, and then the ferrule 15 and the sleeve 17 are fixed by welding or adhesive. Next, the ferrule 15 fixed to the sleeve 17 is aligned in each of the X, Y, and θz directions, and then fixed to the first lens unit 30 by welding. The welding is performed, for example, by irradiating a YAG laser. The adhesive is, for example, a thermosetting adhesive or a UV-curing adhesive.

[0031] In the optical connection structure 1, the light L passing through the first MCF 11 is incident on the second MCF 21, or the light L passing through the second MCF 21 is incident on the first MCF 11. The light L is, for example, light having a wavelength in the 1.55 (μm) band.

[0032] In the optical connection structure 1, the first MCF 11, the first lens 32, the second lens 42, and the second MCF 21 are arranged in this order along the optical axis direction D. The optical axis direction D is the extension direction of the cylindrical member 50. The first MCF 11 and the second MCF 21 are optically coupled (spatially coupled) via space, the first lens 32, and the second lens 42.

[0033] The first MCF 11 includes a plurality of first fiber cores 61 and a first fiber cladding 62. The first MCF 11 has a first MCF end face 11a. The plurality of first fiber cores 61 are exposed at the first MCF end face 11a. The first MCF end face 11a faces the first lens 32.

[0034] The second MCF 21 includes a plurality of second fiber cores 71 and a second fiber clad 72. The second MCF 21 has a second MCF end face 21a. The plurality of second fiber cores 71 are exposed at the second MCF end face 21a. The plurality of second fiber cores 71 extend in the optical axis direction D. Each of the plurality of second fiber cores 71 is optically coupled to a corresponding one of the plurality of first fiber cores 61. The first fiber core 61 corresponds to the first fiber core, and the second fiber core 71 corresponds to the second fiber core. The optical connection structure 1 may be used in an optical transmitter that transmits light L to each of the first fiber cores 61 of the first MCF 11, or an optical receiver that receives light L from each of the first fiber cores 61 of the first MCF 11.

[0035] The second MCF end face 21a and the first MCF end face 11a face each other in the optical axis direction D. The first MCF end face 11a faces the first lens 32. The second MCF end face 21a faces the second lens 42. The first MCF end face 11a and the second MCF end face 21a are inclined with respect to the optical axis direction D. The first MCF end face 11a and the second MCF end face 21a are flat and are inclined with respect to the optical axis direction D and with respect to a plane perpendicular to the optical axis direction D.

[0036] The first MCF end face 11a is inclined at an angle θ1 with respect to a plane perpendicular to the optical axis direction D. The second MCF end face 21a is inclined at an angle θ2 with respect to a plane perpendicular to the optical axis direction D. The first MCF end face 11a and the second MCF end face 21a are arranged so that a V-shape is formed by a first imaginary plane 77 including the first MCF end face 11a and a second imaginary plane 78 including the second MCF end face 21a. For example, the first MCF end face 11a corresponds to the first end face, and the second MCF end face 21a corresponds to the second end face. In this specification, "arranged so as to form a V-shape" means that the first normal vector of the first imaginary plane 77 including the first MCF end face 11a and the second normal vector of the second imaginary plane 78 including the second MCF end face 21a 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.

[0037] FIG. 3 is a cross-sectional view of the first MCF 11 and the second MCF 21 taken along a plane perpendicular to the optical axis direction D. As shown in FIG. 3 , four first fiber cores 61 are arranged in a rectangular lattice pattern in the cross section of the first MCF 11 taken along a plane perpendicular to the optical axis direction D. Similarly, four second fiber cores 71 are arranged in a rectangular lattice pattern in the cross section of the second MCF 21 taken along a plane perpendicular to the optical axis direction D. The four second fiber cores 71 are optically coupled to the exposed surfaces of the plurality of second fiber cores 71 at the second MCF end face 21 a and the exposed surfaces of the plurality of first fiber cores 61 at the first MCF end face 11 a. For example, the second fiber core 71 has the same MFD as the first fiber core 61 in a portion other than the tapered portion 65 (described later). Here, "the same MFD" means that the difference in MFD is less than ±10%. In addition to the above, in this specification, the term "same" includes deviations within the range of manufacturing tolerances.

[0038] The shape of the arrangement of the multiple first fiber cores 61 on the first MCF end face 11a and the shape of the arrangement of the multiple second fiber cores 71 on the second MCF end face 21a are similar to each other when viewed from the optical axis direction D. In other words, the shape of the arrangement of the multiple second fiber cores 71 when the first MCF end face 11a is orthogonally projected in the optical axis direction D and the shape of the arrangement of the multiple second fiber cores 71 when the second MCF end face 21a is orthogonally projected in the optical axis direction D are similar to each other.

[0039] The core pitch P1 of the multiple first fiber cores 61 is larger than the core pitch P2 of the multiple second fiber cores 71 .

[0040] In each of the multiple first fiber cores 61, the MFD at the first MCF end face 11a is larger than the MFD at a position a predetermined distance away from the first MCF end face 11a. Each of the multiple first fiber cores 61 includes a portion in which the MFD gradually decreases continuously or stepwise with increasing distance from the first MCF end face 11a in the optical axis direction D. In other words, the multiple first fiber cores 61 include a tapered portion 65 in which the MFD changes in a tapered manner in the optical axis direction D. The MFD of the second fiber core 71 is, for example, the same as the MFD of the first fiber core 61 in a portion other than the tapered portion 65. In this case, the MFD of each of the multiple first fiber cores 61 at the first MCF end face 11a is larger than the MFD of the second fiber core 71 optically coupled to the first fiber core 61, among the MFDs of the multiple second fiber cores 71 at the second MCF end face 21a.

[0041] The first MCF 11 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.

[0042] The first lens 32 is interposed between the first MCF 11 and the second MCF 21. The second lens 42 is interposed between the second MCF 21 and the first lens 32. The first lens 32 and the second lens 42 are arranged between the first MCF end face 11 a and the second MCF end face 21 a in the following order from the first MCF end face 11 a toward the second MCF end face 21 a: first MCF end face 11 a, first lens 32, second lens 42, second MCF end face 21 a.

[0043] The first lens 32 is disposed at a position facing the first MCF 11 along the optical axis direction D. The second lens 42 is disposed at a position facing the second MCF 21 along the optical axis direction D. The second lens 42 and the first lens 32, for example, collimate the multiple light beams L emitted from each of the multiple second fiber cores 71 of the second MCF 21 with the second lens 42, and then focus the collimated light beams on the first MCF end face 11a with the first lens 32 to couple them to each first fiber core 61. Conversely, the first lens 32 collimates the multiple light beams L emitted from each of the multiple first fiber cores 61 of the first MCF 11, and then focus the collimated light beams on the second MCF end face 21a with the second lens 42 to couple them to each second fiber core 71. The first lens 32 and the second lens 42 are, for example, biconvex aspherical lenses. When the first lens 32 and the second lens 42 are aspherical lenses, the coupling loss of the light L to the first fiber core 61 or the coupling loss to the second fiber core 71 can be reduced, which contributes to reducing optical loss.

[0044] The focal length of the first lens 32 is "f1", and the focal length of the second lens 42 is "f2". When the core pitch of the multiple first fiber cores 61 at the first MCF end face 11a is "P1", the core pitch of the multiple second fiber cores 71 at the second MCF end face 21a is "P2", the MFD of at least one first fiber core 61 among the multiple first fiber cores 61 at the first MCF end face 11a is "D1", and the MFD of a second fiber core 71 optically coupled to the at least one first fiber core 61 among the multiple second fiber cores 71 at the second MCF end face 21a is "D2", the relationships shown in the following formulas (1) and (2) are satisfied. (P1 / P2) x 0.9 ≦ f1 / f2 ≦ (P1 / P2) x 1.1 ... (1) (P1 / P2) x 0.8 ≦ D1 / D2 ≦ (P1 / P2) x 1.2 ... (2)

[0045] As an example, the lens system between the first MCF end face 11a and the second MCF end face 21a has a magnification of 1.29. In this case, for example, "P1" is 45 (μm) and "P2" is 35 (μm). The cladding diameter of each of the first MCF 11 and the second MCF 21 is, for example, 125 (μm). For example, "D1" in the TEC processing section is 12.5 (μm) for light with a wavelength of 1.55 (μm), and "D2" is 9.7 (μm) for light with a wavelength of 1.55 (μm).

[0046] 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 37.5 μm or more. For example, when the cladding diameter is 125 μm, the core pitch is preferably 50 μm or less. In this case, by reducing the core pitch, the distance between the fiber core and the outer periphery of the cladding is secured, and light leakage to the outside is suppressed.

[0047] For example, "f1" is 2.7 (mm) and "f2" is 2.1 (mm). For example, the focal length "f1" of the first lens 32 and the focal length "f2" of the second lens 42 are preferably 0.5 (mm) or more and 4.0 (mm) or less. If the focal length is 0.5 (mm) or more, the distance between the lenses and the distance between the fiber and the lens are also ensured, making manufacturing easier.

[0048] In the example shown in this embodiment, the MFD at the first MCF end face 11a of each of the multiple first fiber cores 61 corresponds to "D1", and the MFD at the second MCF end face 21a of each of the multiple second fiber cores 71 corresponds to "D2", and the relationship shown in the above equations (1) and (2) is satisfied.

[0049] When the refractive index of the first lens 32 and the refractive index of the second lens 42 are the same, the angle at which the first MCF end face 11a is inclined with respect to the optical axis of the first lens 32 is larger than the angle at which the second MCF end face 21a is inclined with respect to the optical axis of the second lens 42. When the angle at which the first MCF end face 11a is inclined with respect to the optical axis of the first lens 32 is "θ1" and the angle at which the second MCF end face 21a is inclined with respect to the optical axis of the second lens 42 is "θ2", the relationship shown in the following formula (3) is satisfied: (P1 / P2)×0.8≦θ2 / θ1≦(P1 / P2)×1.2 (3)

[0050] For example, if the core pitches of the first MCF 11 at the first MCF end face 11a and the second MCF 21 at the second MCF end face 21a are 45 μm and 35 μm, respectively, "θ1" is 6.2 degrees and "θ2" is 8.0 degrees. Since the MFD of the first MCF 11 at the first MCF end face 11a is larger than the MDF of the second MCF 21 at the second MCF end face 21a, the numerical aperture (NA) of the fiber is smaller. Therefore, even if the end face angle is small, deterioration of return loss is suppressed.

[0051] For example, when the relationship shown in the following formula (4) is satisfied, the ferrules 16 and 25 are unlikely to deviate from the horizontal during alignment: θ2·f2=θ1·f1 (4)

[0052] Next, an optical connection structure 1A according to a modified example of this embodiment will be described with reference to Fig. 4. Fig. 4 is a schematic cross-sectional view of the optical connection structure according to this modified example along the optical axis direction. This modified example is generally similar to or the same as the example shown in the above-described embodiment. This modified example differs from the example shown in the above-described embodiment in that the first optical fiber unit 10A and the second optical fiber unit 20A each include a lens, and that the first optical fiber unit 10A and the second optical fiber unit 20A are held by the same cylindrical member. Below, the differences between this modified example and the example shown in the above-described embodiment will be mainly described.

[0053] The optical connection structure 1A includes a first optical fiber unit 10A, a second optical fiber unit 20A, and a tubular member 50A. The first optical fiber unit 10A includes a first MCF 11, a ferrule 15A, and a first lens 32A. The second optical fiber unit 20A includes a second MCF 21, a ferrule 25A, and a second lens 42A. The tubular member 50A connects the first optical fiber unit 10A and the second optical fiber unit 20A to each other. The tubular member 50A is, for example, a glass tube.

[0054] The first lens 32A and the second lens 42A are, for example, C lenses. The first lens 32A and the second lens 42A have, for example, the same focal lengths as the first lens 32 and the second lens 42 described above, respectively. For example, the first lens 32A is a rod lens having a spherical surface on the second lens 42A side, a flat surface on the first MCF 11 side, and the same outer diameter as the ferrule 15A. For example, the second lens 42A is a rod lens having a spherical surface on the first lens 32A side, a flat surface on the second MCF 21 side, and the same outer diameter as the ferrule 25A. As a further modification of this embodiment, the first lens 32A and the second lens 42A may be GRIN lenses.

[0055] In the first optical fiber unit 10A, a first lens 32A is exposed from one side of a ferrule 15A, and a first MCF 11 is exposed from the other side of the ferrule 15A. The first MCF 11 extends from the ferrule 15A to the side opposite the first lens 32A and the cylindrical member 50A. In the second optical fiber unit 20A, a second lens 42A is exposed from one side of a ferrule 25A, and a second MCF 21 is exposed from the other side of the ferrule 25A. The second MCF 21 extends from the ferrule 25A to the side opposite the second lens 42A and the cylindrical member 50A.

[0056] The first lens 32A is fixed to the receiving hole 15a of the ferrule 15A. The second lens 42A is fixed to the receiving hole 25a of the ferrule 25. For example, the ferrules 15A and 25A are fixed to the cylindrical member 50A with an adhesive. The adhesive may be, for example, a thermosetting adhesive or a UV-curing adhesive.

[0057] The first optical fiber unit 10A is inserted into the cylindrical member 50A from one side and is held at one end of the cylindrical member 50A. The second optical fiber unit 20A is inserted into the cylindrical member 50A from the other side and is held at the other end of the cylindrical member 50A. The first optical fiber unit 10A is fixed to the cylindrical member 50A with an adhesive after the second optical fiber unit 20A has been aligned.

[0058] The above-mentioned alignment is performed in six directions: X direction, Y direction, Z direction, θx direction, θy direction, and θz direction. The Z direction is the optical axis direction D, and the X direction and Y direction are directions perpendicular to the Z direction. The θx direction, θy direction, and θz direction indicate the direction around the X axis, the direction around the Y axis, and the direction around the Z axis, respectively.

[0059] Next, an optical connection structure 1B according to a modified example of this embodiment will be described with reference to Fig. 5. Fig. 5 is a schematic cross-sectional view of the optical connection structure according to this modified example along the optical axis direction. This modified example is generally similar to or the same as the example shown in the above-described embodiment and the modified example shown in Fig. 4. This modified example differs from the example shown in the above-described embodiment and the modified example in that a member corresponding to the cylindrical member 50 or the cylindrical member 50A is not provided. Below, differences from the example shown in the above-described embodiment will be mainly described.

[0060] The optical connection structure 1B includes a first optical fiber unit 10, a second optical fiber unit 20B, and a first lens unit 30B. The second optical fiber unit 20B includes a second MCF 21, a ferrule 25B, and a second lens 42B. The first lens unit 30B includes a first lens 32B and a first lens holding member 34B.

[0061] The first lens 32B and the second lens 42B are, for example, C lenses. The first lens 32B and the second lens 42B have, for example, the same focal lengths as the first lens 32 and the second lens 42 described above, respectively. For example, the first lens 32B is a rod lens having a spherical surface on the second lens 42B side and a flat surface on the first MCF 11 side. For example, the second lens 42B is a rod lens having a spherical surface on the first lens 32B side and a flat surface on the second MCF 21 side. The second lens 42B has the same outer diameter as the ferrule 25B. As a further modification of this embodiment, the first lens 32B and the second lens 42B may be GRIN lenses.

[0062] The flange portion 17b corresponds to a portion of the first optical fiber unit 10 that is fixed to the first lens unit 30B. For example, the flange portion 17b is welded to the end of the first lens holding member 34B so that the opening of the sleeve 17 and the opening of the first lens holding member 34B communicate with each other.

[0063] The first lens holding member 34B holds the first lens 32B on the inner surface 30a. The first lens holding member 34B is connected to the end of the ferrule 25B so that the opening of the first lens holding member 34B communicates with the opening of the ferrule 25B. For example, the end of the first lens holding member 34B is fixed to the end of the ferrule 25B with an adhesive. The adhesive may be, for example, a thermosetting adhesive or a UV-curing adhesive.

[0064] In the second optical fiber unit 20B, the second MCF 21 is exposed from the other side of the ferrule 25B. The second MCF 21 extends from the ferrule 25B on the side opposite to the first lens 32B. The second lens 42B is accommodated inside the accommodation hole 25a of the ferrule 25B. For example, the second lens 42B is fixed to the accommodation hole 25a of the ferrule 25B with an adhesive. The adhesive may be, for example, a thermosetting adhesive or a UV-curing adhesive.

[0065] The first lens unit 30B and the ferrule 15 are aligned with respect to the second optical fiber unit 20B. At this time, the first lens unit 30B is aligned in each of the X and Y directions. The ferrule 15 is aligned in each of the X, Y, Z, and θz directions. Once the alignment is complete, the end of the ferrule 25B and the first lens unit 30 are fixed with an adhesive. After that, the ferrule 25B is again aligned in the direction perpendicular to the optical axis direction D and in the direction around the optical axis direction, and then the ferrule 15 and the sleeve 17 are fixed with welding or an adhesive. Next, the ferrule 15 fixed to the sleeve 17 is aligned in each of the X, Y, and θz directions, and then fixed to the first lens unit 30B by welding. The welding is performed, for example, by irradiating a YAG laser. The adhesive is, for example, a thermosetting adhesive or a UV-curing adhesive.

[0066] Next, the effects obtained from the optical connection structure according to the embodiment will be described. In this optical connection structure 1, the MFD of the first fiber core 61, the MFD of the second fiber core 71, the core pitch P1 of the first fiber core 61, and the core pitch P2 of the second fiber core 71 have the relationship expressed by the following formula (2): (P1 / P2) × 0.8 ≦ D1 / D2 ≦ (P1 / P2) × 1.2 (2) As a result of extensive research, the inventors of the present application have discovered that optical coupling loss can be reduced by adjusting the MFD so that the relationship expressed by the above formula (2) is satisfied. As a result, optical coupling loss can be suppressed when a pair of optical fibers having different core pitches are joined. An MFD that is too small or too large can lead to optical coupling loss. For example, if the MFD is too large, there is a risk of light leaking to the outside due to bending of the fiber. Similar effects can be achieved with the optical connection structures 1A and 1B.

[0067] In the optical connection structure 1, in each of the multiple first fiber cores 61, the MFD at the first MCF end face 11a may be larger than the MFD at a position a predetermined distance away from the first MCF end face 11a. This configuration can be realized, for example, by performing a process to enlarge the MFD on only the portion of the existing first MCF 11 corresponding to the first MCF end face 11a. Therefore, while achieving a configuration that reduces costs and effort, leakage of incident light from the second MCF 21 to the first MCF 11 can be suppressed. For example, the process to enlarge the MFD is a TEC process. Similar effects can be achieved with the optical connection structures 1A and 1B.

[0068] In the optical connection structure 1, each of the plurality of first fiber cores 61 may include a portion in which the MFD gradually decreases continuously or in steps with increasing distance from the first MCF end face 11a. In this case, leakage of incident light into the first MCF 11 at the first MCF end face 11a can be further suppressed compared to a configuration in which the MFD changes suddenly. The same effect can be achieved in the optical connection structures 1A and 1B.

[0069] In the optical connection structure 1, when the focal length of the first lens 32 is "f1" and the focal length of the second lens is "f2", the following formula (1) may be satisfied. In this case, the optical coupling loss of the optical fiber can be further suppressed. The same effect is achieved with the optical connection structures 1A and 1B. (P1 / P2) x 0.9 ≦ f1 / f2 ≦ (P1 / P2) x 1.1 (1)

[0070] In the optical connection structure 1, when the refractive index of the first lens 32 and the refractive index of the second lens 42 are the same, the angle at which the second MCF end face 21a is inclined relative to the optical axis of the second lens 42 may be larger than the angle at which the first MCF end face 11a is inclined relative to the optical axis of the first lens 32. In this case, optical coupling loss at each of the first MCF end face 11a and the second MCF end face 21a can be further suppressed. When the refractive indexes are the same, the same material can be used. If the same material is used, the coefficient of expansion according to the ambient temperature will also be the same. Similar effects are achieved for the optical connection structures 1A and 1B.

[0071] In the optical connection structure 1, when the angle at which the first MCF end face 11a is inclined relative to the optical axis of the first lens 32 is "θ1" and the angle at which the second MCF end face 21a is inclined relative to the optical axis of the second lens 42 is "θ2", the following formula (3) may be satisfied. In this case, reflections at each of the first MCF end face 11a and the second MCF end face 21a are reduced, and optical coupling loss of the optical fiber can be further suppressed. The same effect is achieved with the optical connection structures 1A and 1B. (P1 / P2) × 0.8 ≦ θ2 / θ1 ≦ (P1 / P2) × 1.2 ... (3)

[0072] In the optical connection structure 1, the first MCF end face 11a and the second MCF end face 21a may be arranged so that a V-shape is formed by a first imaginary plane 77 including the first MCF end face 11a and a second imaginary plane 78 including the second MCF end face 21a. In this case, optical coupling loss at each of the first MCF end face 11a and the second MCF end face 21a can be further suppressed. The same effect is achieved in the optical connection structures 1A and 1B.

[0073] In the optical connection structure 1, the focal lengths of the first lens 32 and the second lens 42 may be 0.5 or more and 4.0 or less. In this case, the distance between the lenses is ensured, which makes it possible to ensure ease of manufacturing. The same effects are achieved in the optical connection structures 1A and 1B.

[0074] Although the embodiments of the present disclosure have been described in detail above, the present invention is not limited to the above embodiments and can be applied to various embodiments. For example, the configurations of the optical connection structures 1, 1A, and 1B may be combined. For example, the configuration of the second MCF 21 side and the configuration of the first MCF 11 side may have different configurations from those of the examples shown by the optical connection structures 1, 1A, and 1B.

[0075] For example, the lenses arranged between the second MCF 21 and the first MCF 11 may be a combination of two or more lenses selected from a biconvex aspherical lens, a C lens, and a GRIN lens. In this case, for example, the lenses located at both ends correspond to the first lens and the second lens.

[0076] 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 four. For example, the number of first fiber cores 61 and the number of second fiber cores 71 may be seven. The number of first fiber cores 61 and the number of second fiber cores 71 may be different from each other.

[0077] In the above examples, the portions described as being connected by welding may be connected by adhesive, and the portions described as being connected by adhesive may be connected by welding.

[0078] DESCRIPTION OF SYMBOLS 1, 1A, 1B...Optical connection structure 10, 10A...First optical fiber unit 11...First MCF 11a...First MCF end face 21...Second MCF 21a...Second MCF end face 15, 25, 15A, 25A, 25B...Ferrule 15a, 25a...Accommodating hole 17...Sleeve 17b...Flange portion 20, 20A, 20B...Second optical fiber unit 30, 30B...First lens unit 30a, 40a...Inner surface 32, 32A, 32B...First lens 34, 34B...First lens holding member 40...Second lens unit 42, 42A, 42B...Second lens 44...Second lens holding member 50, 50A...Cylindrical member 50a...Accommodating hole 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 L... Light θ1, θ2... Angles

Claims

1. A first multi-core fiber including a plurality of first fiber cores and having a first end face at which the plurality of first fiber cores are exposed; a second multi-core fiber including a plurality of second fiber cores, each of which is optically connected to a corresponding first fiber core among the plurality of first fiber cores, and having a second end face at which the plurality of second fiber cores are exposed; and 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, wherein the exposed surface of each of the plurality of second fiber cores at the second end face and the exposed surface of each of the plurality of first fiber cores at the first end face are optically coupled to each other, and a core pitch of the plurality of first fiber cores at the first end face is greater than a core pitch of the plurality of second fiber cores at the second end face, a mode field diameter of each of the plurality of first fiber cores at the first end face is larger than a mode field diameter of the second fiber core optically coupled to the first fiber core, among the mode field diameters of the plurality of second fiber cores at the second end face; a shape of the arrangement of the plurality of second fiber cores at the second end face and a shape of the arrangement of the plurality of first fiber cores at the first end face are similar to each other; a mode field diameter of at least one first fiber core among the plurality of first fiber cores at the first end face is "D1", a mode field diameter of a second fiber core optically coupled to the at least one first fiber core among the plurality of second fiber cores at the second end face is "D2", a core pitch of the plurality of first fiber cores at the first end face is "P1", and a core pitch of the plurality of second fiber cores at the second end face is "P2", then (P1 / P2)×0.8≦D1 / D2≦(P1 / P2)×1.2 The optical connection structure satisfies the following relationship:

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

3. An optical connection structure according to claim 1 or 2, wherein each of the plurality of first fiber cores includes a portion in which the mode field diameter decreases with increasing distance from the first end face.

4. An optical connection structure described in any one of claims 1 to 3, wherein when the focal length of the first lens is "f1" and the focal length of the second lens is "f2", the relationship (P1 / P2) x 0.9 ≦ f1 / f2 ≦ (P1 / P2) x 1.1 is satisfied.

5. An optical connection structure described in any one of claims 1 to 4, wherein when the refractive index of the first lens and the refractive index of the second lens are the same, the angle at which the second end face is inclined relative to the optical axis of the second lens is greater than the angle at which the first end face is inclined relative to the optical axis of the first lens.

6. The optical connection structure according to claim 5, wherein when the angle at which the first end face is inclined with respect to the optical axis of the first lens is "θ1" and the angle at which the second end face is inclined with respect to the optical axis of the second lens is "θ2", the relationship (P1 / P2) x 0.8 ≦ θ2 / θ1 ≦ (P1 / P2) x 1.2 is satisfied.

7. An optical connection structure described in any one of claims 1 to 6, wherein the first end face and the second end face are arranged so that a V-shape is formed by a first imaginary plane including the first end face and a second imaginary plane including the second end face.

8. An optical connection structure according to any one of claims 1 to 7, wherein the focal lengths of the first lens and the second lens are not less than 0.5 and not more than 4.0.

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