Optical fiber connection structure

Abstract

An optical fiber connection structure according to one embodiment is provided with: a multi-core fiber; a plurality of single-core fibers arranged in parallel in a direction orthogonal to a longitudinal direction; a first lens having a focal length of f1 (mm); and a second lens having a focal length of f2 (mm). The placement of the respective center positions of cores in a cross section orthogonal to the longitudinal direction of the multi-core fiber and the placement of the respective center positions of cores in cross sections orthogonal to the longitudinal direction of the plurality of single-core fibers are similar in shape. The pitch between the cores of the multi-core fiber is denoted by P1 (μm), the mode field diameter at a first end face of each of the cores is denoted by MFD1 (μm), the pitch between the cores of the plurality of single-core fibers is denoted by P2 (μm), the mode field diameter of light at a second end face of each of the cores is denoted by MFD2 (μm), and (P1 / P2)×0.9 ≤ f1 / f2 ≤ (P1 / P2)×1.1 and (P1 / P2)×0.9 ≤ MFD1 / MFD2 ≤ (P1 / P2)×1.1 are satisfied.

Description

[Technical field]

[0001] The present disclosure relates to optical fiber connection structures. This application claims priority to Japanese Application No. 2020-111739 filed on June 29, 2020, and incorporates by reference all of the contents of said Japanese application. [Background technology]

[0002] Patent Document 1 describes a multi-core fiber coupling device. The multi-core fiber coupling device includes a multi-core fiber (hereinafter also referred to as "MCF"), a first lens, an optical functional element, a number of prisms equal to the number of cores of the MCF, and a collimator including a number of single-core fibers (hereinafter also referred to as "SCF") equal to the number of cores of the MCF. The light emitted from each core of the MCF is converted into collimated light by the first lens and spatially separated. Each of the spatially separated lights enters each prism and is coupled to the SCF of each collimator with the pitch between the multiple optical paths being widened in each prism.

[0003] Non-Patent Document 1 describes a spatial coupling structure of MCF. This spatial coupling structure includes an MCF fixed to one end of a housing, multiple SMF collimators fixed to the other end of the housing, a lens provided inside the housing, and multiple parallelogram prisms. Light emitted from each core of the MCF is coupled to each SMF collimator via the lens and each of the multiple parallelogram prisms.

[0004] Non-Patent Document 2 describes a spatial optical system multicore fiber coupling device. This device includes a 7-core MCF, a single lens, a deflection prism, a lens array, and an SMF array. The lens array includes the same number of lens elements as the SMFs that make up the SMF array. In this device, light emitted from each core of the MCF is coupled to each SMF of the SMF array via the single lens, the deflection prism, and each lens element of the lens array.

[0005] Non-Patent Document 3 describes a core spacing conversion device. This device includes a first MCF, a second MCF, and two lenses interposed between the first and second MCFs. The shape of the core array of the first MCF and the shape of the core array of the second MCF are similar to each other. The core spacing (pitch) of the first MCF and the pitch of the second MCF are different from each other. In this device, optical coupling with the same pitch is realized by magnifying MCFs with different pitches with two lenses. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2013-182222 A [Non-patent literature]

[0007] [Non-Patent Document 1] IEICE Technical Report, vol. 112, no. 448, OFT2012-72, pp. 17-22, February 2013. "Multi-core fiber connection technology using spatial coupling," Optoquest, Tottori, et al. [Non-Patent Document 2] IEICE Technical Report, vol. 113, no. 448, OPE2013-211, pp. 35-40, February 2014. "Free-space optical system multi-core fiber coupling device and its scalability" Optoquest, Kobayashi et al. [Non-Patent Document 3] IEICE Technical Report, vol. 114, no. 452, OFT2014-78, pp. 113-116, February 2015. "Miniaturization of spatially coupled fan-in / fan-out devices and their applications," Optoquest, Tottori, et al. Summary of the Invention

[0008] An optical fiber connection structure according to one aspect of the present disclosure includes a multicore fiber, a plurality of single-core fibers arranged in parallel in a direction perpendicular to the longitudinal direction, a first lens facing a first end face that is a tip face of the multicore fiber and having a focal length of f1 (mm), and a second lens disposed between second end faces that are tip faces of the plurality of single-core fibers and the first lens and having a focal length of f2 (mm). The arrangement of the central positions of each core in a cross section perpendicular to the longitudinal direction of the multicore fiber and the arrangement of the central positions of each core in a cross section perpendicular to the longitudinal direction of the plurality of single-core fibers are arranged to have similar shapes. When the pitch of a set of cores in a multicore fiber is P1 (μm) and the mode field diameter of light of a specific wavelength at a first end face of each core is MFD1 (μm), and the pitch of a set of cores in a plurality of single-core fibers located at a position corresponding to the set of cores in the multicore fiber is P2 (μm) and the mode field diameter of light of a specific wavelength at a second end face of each core is MFD2 (μm), the following conditions are satisfied: (P1 / P2)×0.9≦f1 / f2≦(P1 / P2)×1.1 and (P1 / P2)×0.9≦MFD1 / MFD2≦(P1 / P2)×1.1. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing an optical fiber connection structure according to an embodiment. [Diagram 2] FIG. 2 is a diagram showing a plurality of SCFs in the optical fiber connection structure of FIG. [Diagram 3] FIG. 3 is a diagram showing an MCF of the optical fiber connection structure of FIG. [Figure 4] FIG. 4 illustrates a core extension of an exemplary SCF. [Diagram 5] FIG. 5 is a diagram illustrating an exemplary SCF GRIN lens. [Figure 6] FIG. 6 shows another optical fiber connected to an SCF. [Figure 7] FIG. 7 is a diagram showing an optical fiber connection structure according to a modified example. [Figure 8]FIG. 8 is a diagram showing an optical fiber connection structure according to another modified example. [Figure 9] FIG. 9 is a perspective view showing an example of an assembled optical fiber connection structure. [Figure 10] FIG. 10 is a cross-sectional view of the optical fiber connection structure of FIG. [Figure 11] FIG. 11 is a perspective view showing another example of an assembled optical fiber connection structure. [Figure 12] FIG. 12 is a cross-sectional view of the optical fiber connection structure of FIG. [Figure 13] FIG. 13 is a perspective view showing yet another example of an assembled optical fiber connection structure. [Figure 14] FIG. 14 is a cross-sectional view of the optical fiber connection structure of FIG. [Figure 15] FIG. 15 is a perspective view of an optical fiber connection structure according to a modified example. [Figure 16] FIG. 16 is a cross-sectional view of the optical fiber connection structure of FIG. [Figure 17] FIG. 17 is a cross-sectional view showing a ferrule and a lens according to a comparative example. [Figure 18] FIG. 18 is a cross-sectional view showing a ferrule and a lens of the optical fiber connection structure of FIG. [Figure 19] FIG. 19 is a diagram for explaining a method of assembling the optical fiber connection structure of FIG. [Figure 20] FIG. 20 is a diagram for explaining a method of assembling the optical fiber connection structure of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Incidentally, conventional multicore fiber coupling structures and spatial coupling structures of MCFs are provided with multiple prisms, and the SCF is further made a collimator with a lens. Therefore, the number of parts is large, which may lead to an increase in size. The spatial optical system multicore fiber coupling device described above requires special parts such as a deflection prism and a lens array. In the core conversion device described above, when the lens magnification is converted together with the core pitch, the mode field diameter (hereinafter sometimes referred to as "MFD") also changes, and therefore a mismatch in MFD occurs between a pair of optical fibers for light of the same wavelength. Therefore, it is expected that optical loss will increase.

[0011] An object of the present disclosure is to provide an optical fiber connection structure capable of reducing optical loss.

[0012] According to the present disclosure, optical loss can be reduced.

[0013] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described. An optical fiber connection structure according to one embodiment includes a multicore fiber, a plurality of single-core fibers arranged in parallel in a direction perpendicular to the longitudinal direction, a first lens facing a first end face that is a tip face of the multicore fiber and having a focal length of f1 (mm), and a second lens disposed between a second end face that is a tip face of the plurality of single-core fibers and the first lens and having a focal length of f2 (mm). The arrangement of the central positions of each core in a cross section perpendicular to the longitudinal direction of the multicore fiber and the arrangement of the central positions of each core in a cross section perpendicular to the longitudinal direction of the plurality of single-core fibers are arranged to have similar shapes. When the pitch of a set of cores in a multicore fiber is P1 (μm) and the mode field diameter of light of a specific wavelength at a first end face of each core is MFD1 (μm), and the pitch of a set of cores in a plurality of single-core fibers located at a position corresponding to the set of cores in the multicore fiber is P2 (μm) and the mode field diameter of light of a specific wavelength at a second end face of each core is MFD2 (μm), the following conditions are satisfied: (P1 / P2)×0.9≦f1 / f2≦(P1 / P2)×1.1 and (P1 / P2)×0.9≦MFD1 / MFD2≦(P1 / P2)×1.1.

[0014] In this optical fiber connection structure, the pitch of each core of the multicore fiber is P1, and the mode field diameter of each core of the multicore fiber is MFD1 (μm). The pitch of each core of the multiple single-core fibers is P2, and the mode field diameter of light of the same specific wavelength at the second end face of each single-core fiber is MFD2 (μm). If the focal length of the first lens facing the multicore fiber is f1 (mm), and the focal length of the second lens facing the single-core fiber is f2 (mm), (f1 / f2) is within ±10% of (P1 / P2). And (MFD1 / MFD2) is within ±10% of (P1 / P2). Therefore, since the magnification conversion of the lens and the mode field diameter is performed together with the core pitch, it is possible to eliminate the mismatch of the mode field diameter between the multicore fiber and the single-core fiber. Therefore, it is possible to suppress the optical loss. Furthermore, since this optical fiber connection structure includes a multi-core fiber, a plurality of single-core fibers, a first lens, and a second lens, special parts such as the above-mentioned deflection prism or lens array are not required. Therefore, an increase in the number of parts can be suppressed and miniaturization can be achieved.

[0015] Each of the multiple single-core fibers may have a beam expanding portion at a second end face capable of expanding a mode field diameter of light propagating through the core of the single-core fiber. In this case, the second end face of each single-core fiber is an exit face and serves as an exit end. The mode field diameter of the light propagating through each single-core fiber is expanded at the second end face. Therefore, the value of MFD2 can be increased according to the expansion of the light from the second lens, thereby reducing optical loss.

[0016] The beam expansion section may be a core expansion section in which the core diameter of each single-core fiber is expanded. In this case, the value of MFD2 can be easily increased at the second end face of the single-core fiber.

[0017] The beam expanding section may be a GRIN lens or a GI (Graded Index) fiber connected to each single-core fiber. In this case, the end face of the GRIN lens or the GI fiber that is not connected to each single-core fiber is the exit surface of each single-core fiber, and serves as the exit end. When the outer diameter of the GRIN lens or the GI fiber is the same as the outer diameter of the single-core fiber, the GRIN lens or the GI fiber can be easily connected to the single-core fiber.

[0018] The beam expanding section may be an optical fiber having a core with a diameter larger than that of the single-core fiber. The core may expand from the single-core fiber toward the optical fiber in a region including the interface between the single-core fiber and the optical fiber. In this case, an end face of the optical fiber having a core with a diameter larger than that of the single-core fiber, which is not connected to the single-core fiber, is the exit surface and the exit end of each single-core fiber.

[0019] The normal to the second end face may be inclined with respect to the optical axis direction of the single-core fiber. The normal to the first end face may be inclined with respect to the optical axis direction. When the inclination angle of the normal to the first end face with respect to the optical axis direction is θ1 and the inclination angle of the normal to the second end face with respect to the optical axis direction is θ2, (f1 / f2)×0.8≦θ2 / θ1≦(f1 / f2)×1.2 may be satisfied. In this case, the optical axes of both the single-core fiber and the multi-core fiber can be made closer to being parallel.

[0020] The first lens and the second lens may be plano-convex lenses having curved surfaces facing each other, which makes it possible to use inexpensive plano-convex lenses and facilitates assembly.

[0021] The first lens and the second lens may be GRIN lenses. In this case, the use of a GRIN lens having a planar shape has the advantage of being easy to handle and easy to assemble.

[0022] The first lens and the second lens may be aspheric lenses, in which case the coupling efficiency between the multi-core fiber and the single-core fiber can be further increased, resulting in lower coupling loss.

[0023] Each of the multiple single-core fibers may be thinned. In this case, the magnification of the second lens can be kept low by using a single-core fiber that has a small diameter in advance or a single-core fiber whose diameter has been reduced by etching or the like.

[0024] The optical fiber connection structure may include a first collimator in which a multi-core fiber and a first lens are integrated, a second collimator in which a plurality of single-core fibers and a second lens are integrated, and a cylindrical member into which the first collimator and the second collimator are inserted so that the first lens and the second lens face each other. The first collimator and the second collimator may be fixed to the cylindrical member by an adhesive in a state where they are inserted into the cylindrical member.

[0025] The above-mentioned optical fiber connection structure may include a first collimator in which a multi-core fiber and a first lens are integrated, a second collimator in which a plurality of single-core fibers and a second lens are integrated, and a metal tube that fixes the first collimator and the second collimator by welding so that the first lens and the second lens face each other.

[0026] The optical fiber connection structure may include a first optical fiber unit having a multi-core fiber and a second optical fiber unit having a plurality of single-core fibers. The optical fiber connection structure may further include a first lens unit including a first lens and fixed to the first optical fiber unit by welding, a second lens unit including a second lens and fixed to the second optical fiber unit by welding, and a metal tube that fixes the first lens unit and the second lens unit by welding so that the first lens unit and the second lens unit face each other.

[0027] [Details of the embodiment of the present disclosure] Specific examples of the optical fiber connection structure of the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to the examples described below, but is intended to include all modifications within the scope of the claims and equivalents thereto. In the description of the drawings, the same or corresponding elements are given the same reference numerals, and duplicated descriptions are omitted as appropriate. The drawings may be partially simplified or exaggerated to facilitate understanding, and the dimensional ratios and angles are not limited to those in the drawings.

[0028] 1 is a diagram showing an optical fiber connection structure 1 according to an embodiment. The optical fiber connection structure 1 constitutes, for example, a fan-in fan-out device (Fan-In / Fan-Out: FIFO) of a lens-coupled multicore fiber. In the following description, the multicore fiber is referred to as an MCF and the single-core fiber is referred to as an SCF. The optical fiber connection structure 1 includes an MCF 10, a plurality of SCFs 20, a first lens 30 interposed between the MCF 10 and the plurality of SCFs 20, and a second lens 40 interposed between the plurality of SCFs 20 and the first lens 30.

[0029] The MCF 10 has a plurality of (seven, for example) cores 11 and clads 12, and each SCF 20 has a core 21 and a clad 22. For the sake of simplicity, in FIG. 1 and in FIGS. 7 and 8 described later, the cores of the optical fibers are represented by lines, and the boundaries between the cores and the clads are omitted. In contrast, in FIGS. 2 to 6 described later, the cores are depicted as having widths. The optical fiber connection structure 1 is a fan-in / fan-out device that splits light L passing through the MCF 10 into a plurality of SCFs 20, or couples light L passing through each of the plurality of SCFs 20 to one MCF 10. The light L has a wavelength in the 1.55 μm band, for example.

[0030] The optical fiber connection structure 1 may be used, for example, in an optical amplifier that splits the light L passing through each core 11 of the MCF 10 toward each of a plurality of SCFs 20 and amplifies each of the split lights L. The optical fiber connection structure 1 may be used in an optical transmitter that transmits the light L from each of the plurality of SCFs 20, or in an optical receiver that receives the light L from each of the plurality of SCFs 20.

[0031] In the optical fiber connection structure 1, the MCF 10, the first lens 30, the second lens 40, and the SCF 20 are arranged in this order so as to be aligned along the optical axis direction D in which the optical axis of the SCF 20 extends. The MCF 10 and the SCF 20 are optically coupled (spatially coupled) via a space.

[0032] 2 is a cross-sectional view of the SCF 20 taken along a plane perpendicular to the optical axis direction D. As shown in FIGS. 1 and 2, for example, a plurality of SCFs 20 are bundled in a ferrule 23. As an example, seven SCFs 20 are packed in the ferrule 23. In the cross section of the plurality of SCFs 20, six SCFs 20 are arranged around one SCF 20 at the center. In the cross section of the plurality of SCFs 20, the seven SCFs 20 are arranged in a triangular lattice pattern.

[0033] Fig. 3 is a cross-sectional view of the MCF 10 cut along a plane perpendicular to the optical axis direction D. As shown in Fig. 1 and Fig. 3, the MCF 10 is held by, for example, a ferrule 13. In the cross section of the MCF 10 cut along a plane perpendicular to the optical axis direction D, six cores 11 are arranged around one core 11 at the center. In the cross section of the MCF 10, seven cores 11 are arranged in a triangular lattice pattern.

[0034] In a cross section cut by a plane perpendicular to the optical axis direction D, the arrangement shape of the multiple cores 21 of the SCF 20 and the arrangement shape of the multiple cores 11 of the MCF 10 are similar to each other. For example, if the pitch of the cores 11 of the MCF 10 is P1 (μm) and the pitch of the cores 21 of the SCF 20 is P2 (μm), P2 is larger than P1. For example, P2 is equal to the diameter of the SCF 20 (cladding 22).

[0035] As an example, P1 is 42 (μm) and P2 is 126 (μm). If the MFD of light of a specific wavelength at the output end of core 11 of MCF 10 is MFD1 (μm) and the MFD of light of the same specific wavelength at the output end of core 21 of SCF 20 is MFD2 (μm), MFD2 is larger than MFD1. As an example, MFD1 is 10 (μm) and MFD2 is 30 (μm). P1, P2, MFD1 and MFD2 satisfy the following formula (1). (P1 / P2)×0.9≦MFD1 / MFD2≦(P1 / P2)×1.1 (1)

[0036] The MCF 10 has a first end face 14 facing the first lens 30. For example, the first end face 14 is flat and inclined at an angle θ1 with respect to a plane perpendicular to the optical axis direction D. A normal line H1 to the first end face 14 is inclined at an angle θ1 with respect to the optical axis direction D. The SCF 20 has a second end face 24 facing the second lens 40. The second end face 24 is flat like the first end face 14, and a normal line H2 to the second end face 24 is inclined at an angle θ2 with respect to the optical axis direction D.

[0037] The first lens 30 is disposed at a position facing the MCF 10 along the optical axis direction D. The first lens 30 collects the multiple light beams L emitted from each of the multiple cores 11 of the MCF 10 on the side opposite to the MCF 10. The second lens 40 is disposed at a position facing the SCF 20 along the optical axis direction D. The first lens 30 and the second lens 40 are both, for example, biconvex lenses. When the first lens 30 and the second lens 40 are aspheric lenses, the loss of the light beam L can be reduced, which contributes to reducing optical loss.

[0038] If the focal length of the first lens 30 is f1 (mm) and the focal length of the second lens 40 is f2 (mm), for example, f2 is larger than f1. The second lens 40 may be larger than the first lens 30. The above-mentioned P1 and P2, and f1 and f2 satisfy the relationship of the following formula (2). (P1 / P2)×0.9 ≦ f1 / f2 ≦ (P1 / P2)×1.1 ···(2) Furthermore, the above-mentioned θ1 and θ2, and f1 and f2 may satisfy the following formula (3). (f1 / f2)×0.8 ≦ θ2 / θ1 ≦ (f1 / f2)×1.2 (3)

[0039] As mentioned above, when P1 is 42 (μm), P2 is 126 (μm), MFD1 is 10 (μm), and MFD2 is 30 (μm), (P1 / P2) = (MFD1 / MFD2) = 1 / 3, so formula (1) is satisfied. In this case, the magnification of SCF20 with respect to MCF10 is three times. Also, the diameter of a typical SCF20 (cladding 22) is 125 (μm), so formula (1) is satisfied in this case as well.

[0040] As described above, one of the means for making the MFD2 larger than the MFD1 is to use a TEC (Thermally Expanded Core) fiber 20A as the SCF 20, as shown in Fig. 4. The TEC fiber 20A has a core expansion section 25 in which the core diameter of the core 21 is expanded at the second end face 24. For example, the core expansion section 25 includes a tapered section 25a in which the core 21 is expanded in a tapered shape. In this case, since the core 21 is expanded at the second end face 24, the mode field diameter of the light propagating through the SCF 20 expands in the core expansion section 25 in a direction perpendicular to the optical axis direction D, and the MFD2 is easily achieved to be 30 (μm). The core expansion section 25 functions as the beam expansion section 15.

[0041] Here, the direction of light heading from inside the SCF 20 toward second end face 24 is considered as the reference, and the beam diameter near second end face 24 is "expanded" compared to the beam diameter inside the SCF 20, and is referred to as "beam expansion." However, if the direction of light heading from second end face 24 toward the inside of the SCF 20 is considered as the reference, the beam diameter inside the SCF 20 is "reduced" compared to the beam diameter near second end face 24. In other words, this portion becomes a "beam reduction section."

[0042] In the above, an example was described in which P1 is 42 (μm), P2 is 126 (μm), MFD1 is 10 (μm), and MFD2 is 30 (μm). However, the values ​​of P1, P2, MFD1, and MFD2 are not limited to the above. For example, when P1, which is the pitch of the core 11 of the MCF 10, is 35 (μm) and is enlarged three times as described above, P2, which is the pitch of the core 21 of the SCF 20, can be 105 (μm).

[0043] The SCF 20 may be, for example, a thin optical fiber having a diameter (P2) of 80 (μm). When the SCF 20 is an optical fiber having a thin diameter, it is possible to keep the lens magnification from the first lens 30 to the second lens 40 low in order to satisfy the above-mentioned formula (2).

[0044] In the present disclosure, "thinned" SCFs include both the use of SCFs with small diameters and the use of SCFs with a small diameter by processing. An example of using SCFs with a small diameter is the use of SCFs with a diameter of less than 125 μm. An example of the use of SCFs with a small diameter by processing is the use of SCFs with a small diameter by etching using hydrofluoric acid. "Thinned" SCFs further include both SCFs with a small diameter over their entire length and SCFs with a small diameter only in part. In addition, the diameter of the SCFs in the "thinned" portion may be smaller than the diameter of the MCF 10.

[0045] The beam expansion section 15 of the SCF 20 is not limited to the core expansion section 25 of the TEC fiber 20A. As shown in Fig. 5, the beam expansion section 15 may be a GRIN lens 26 attached to the second end face 24 of the SCF 20. The GRIN lens 26 is, for example, fusion-spliced ​​to the second end face 24 of the SCF 20. In this case, the end face of the GRIN lens 26 on the side not connected to the SCF 20 (the end face of the GRIN lens 26 opposite the second end face 24, the right end face in Fig. 5) serves as the output face and output end. When the diameter of the GRIN lens 26 matches the diameter of the SCF 20, the fusion-splicing of the GRIN lens 26 to the SCF 20 can be easily performed.

[0046] Instead of the GRIN lens 26, a GI fiber (graded index fiber) may be provided. As shown in FIG. 6, the beam expansion section 15 of the SCF 20 may be an optical fiber 27 having a core 27a with a diameter larger than that of the core 21 of the SCF 20. The optical fiber 27 is, for example, fusion-spliced ​​to the SCF 20, and the end face of the optical fiber 27 on the side not connected to the SCF 20 (the end face of the optical fiber 27 opposite to the second end face 24, the right end face in FIG. 6) is the emission face and the emission end. The vicinity of the second end face 24, which is the interface between the SCF 20 and the optical fiber 27, may be subjected to TEC (Thermally Expanded Core) treatment. In this case, the vicinity of the second end face 24 of the SCF 20 that has been TEC-treated and the optical fiber 27 are the beam expansion section 15.

[0047] 7 and 8 show optical fiber connection structures 1A and 1B according to the modified examples. The optical fiber connection structure 1A includes a first lens 30A and a second lens 40A, which are cylindrical plano-convex lenses, instead of the first lens 30 and the second lens 40, which are biconvex lenses. For example, one end of each of the first lens 30A and the second lens 40A is a convex lens. Furthermore, the first lens 30A and the second lens 40A have a cylindrical portion continuing to the convex lens portion and a flat portion located at the other end. The first lens 30A is connected to the first end surface 14 of the MCF 10 at its flat portion, and the second lens 40A is connected to the second end surface 24 of the SCF 20 at its flat portion. When the outer diameters of the cylindrical portions of the first lens 30A and the second lens 40A are the same as the outer diameters of the ferrules 13 and 23, the first lens 30A and the MCF 10, and the second lens 40A and the SCF 20, respectively, can be easily connected.

[0048] The optical fiber connection structure 1B includes a first lens 30B and a second lens 40B, which are GRIN lenses, instead of the first lens 30 and the second lens 40. For example, an anti-reflection (AR) coating may be applied between the first lens 30B and the first end face 14 of the MCF 10, and a small gap may be formed between them. Also, an adhesive (a gel-like adhesive, for example) may be interposed between the first lens 30B and the first end face 14. The same applies to the gap between the second lens 40B and the second end face 24 of the SCF 20.

[0049] As described above, when using the first lens 30A which is a cylindrical plano-convex lens and the first lens 30B which is a GRIN lens, it is possible to easily align the first lenses 30A and 30B with respect to the MCF 10 by matching the outer diameter of the first lens 30A or the outer diameter of the first lens 30B with the outer diameter of the ferrule 13 which holds the MCF 10. For example, it is possible to align the axis with the MCF 10 by inserting the ferrule 13 and the first lens 30A into a glass tube. The same effect as above can be obtained from the second lens 40A and the second lens 40B.

[0050] Next, an example of an assembled optical fiber connection structure will be described. As shown in Fig. 9 and Fig. 10, an optical fiber connection structure 50 includes a first collimator 51 in which an MCF 10, a ferrule 13, a first lens 30, and a sleeve 53 are integrated, a second collimator 52 in which a plurality of SCFs 20, a ferrule 23, a second lens 40, and a sleeve 54 are integrated, and a cylindrical member 55 that connects the first collimator 51 and the second collimator 52 to each other. The cylindrical member 55 is, for example, a glass tube.

[0051] In the first collimator 51, the first lens 30 is exposed from one side of the sleeve 53, and the ferrule 13 holding the MCF 10 is exposed from the other side of the sleeve 53. The MCF 10 extends from the ferrule 13 to the side opposite the first lens 30 and the cylindrical member 55. In the second collimator 52, similar to the first collimator 51, the second lens 40 is exposed from one side of the sleeve 54, and the ferrule 23 holding a bundle of multiple SCFs 20 is exposed from the other side of the sleeve 54. The multiple SCFs 20 extend from the ferrule 23 to the side opposite the second lens 40 and the cylindrical member 55.

[0052] Each of the first collimator 51 and the second collimator 52 configured as above is fixed to a cylindrical member 55 such that the first lens 30 and the second lens 40 face each other along the optical axis direction D. For example, each of the first collimator 51 and the second collimator 52 is fixed to the cylindrical member 55 by an adhesive.

[0053] Regarding a method of assembling the optical fiber connection structure 50, first, a first collimator 51, a second collimator 52, and a cylindrical member 55 are prepared. Then, the first collimator 51 is inserted from one side of the cylindrical member 55 so that the first lens 30 enters inside the cylindrical member 55, and the second collimator 52 is inserted from the other side of the cylindrical member 55 so that the second lens 40 enters inside the cylindrical member 55. Then, one of the first collimator 51 and the second collimator 52 is fixed to the cylindrical member 55 with an adhesive, and the other of the first collimator 51 and the second collimator 52 is aligned and then fixed to the cylindrical member 55 with an adhesive.

[0054] The above-mentioned alignment is performed in six directions, namely, 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 the Y direction are directions perpendicular to the Z direction. The θx direction, the θy direction, and the θz direction respectively indicate the direction around the X axis, the direction around the Y axis, and the direction around the Z axis. The above-mentioned adhesive is, for example, a UV-curing adhesive.

[0055] Fig. 11 is a perspective view showing an optical fiber connection structure 60 according to a modified example, and Fig. 12 is a cross-sectional view showing the optical fiber connection structure 60. As shown in Fig. 11 and Fig. 12, the optical fiber connection structure 60 includes a first collimator 61, a second collimator 62, and a metal tube 65. The first collimator 61 differs from the first collimator 51 in that it includes a sleeve 63 having a different shape from the sleeve 53, and the second collimator 62 differs from the second collimator 52 in that it includes a sleeve 64 having a different shape from the sleeve 54.

[0056] The sleeve 63 has an insertion portion 63a that enters the metal tube 65, and the outer diameter of the insertion portion 63a is reduced and the inner surface protrudes radially inward of the sleeve 63. The sleeve 64 also has an insertion portion 64a similar to the insertion portion 63a. The metal tube 65 has a cylindrical main body portion 65a and annular inserted portions 65b connected to one and the other axial ends of the main body portion 65a.

[0057] Regarding the method of assembling the optical fiber connection structure 60, first, the first collimator 61, the second collimator 62, and the metal tube 65 are prepared. The main body 65a and the inserted portion 65b are separate before assembly. One of the first collimator 61 and the second collimator 62, one inserted portion 65b, and the main body 65a are fixed to each other by welding. Then, the other of the first collimator 61 and the second collimator 62, and the other inserted portion 65b are aligned in the X direction, the Y direction, the Z direction, the θx direction, the θy direction, and the θz direction, and then fixed to the main body 65a by welding. The welding is performed, for example, by irradiating a YAG laser.

[0058] Fig. 13 is a perspective view showing an optical fiber connection structure 70 according to a further modified example, and Fig. 14 is a cross-sectional view showing the optical fiber connection structure 70. As shown in Fig. 13 and Fig. 14, the optical fiber connection structure 70 includes a first optical fiber (MCF) unit 71, a second optical fiber (SCF) unit 72, a metal tube 75, a first lens unit 76 including a first lens 30, and a second lens unit 77 including a second lens 40.

[0059] 11 and 12 in that the first optical fiber unit 71 does not have the first lens 30, and is provided with a sleeve 73 having a different shape from the sleeve 53. The second optical fiber unit 72 also differs in that it does not have the second lens 40, and is provided with a sleeve 74 having a different shape from the sleeve 54.

[0060] The sleeve 73 has a flange portion 73a at one end in the axial direction. The sleeve 74 also has a flange portion 74a similar to the flange portion 73a. The flange portion 73a and the flange portion 74a correspond to portions fixed to the first lens unit 76 and the second lens unit 77, respectively. For example, the first lens unit 76 includes a first lens 30 and a cylindrical lens holding member 76a that surrounds and holds the first lens 30. The second lens unit 77 includes a second lens 40 and a cylindrical lens holding member 77a that surrounds and holds the second lens 40.

[0061] A method of assembling the optical fiber connection structure 70 will be described. First, a first optical fiber unit 71, a second optical fiber unit 72, a metal tube 75, a first lens unit 76, and a second lens unit 77 are prepared. Here, the first optical fiber unit 71 refers to an assembly of an MCF 10, a ferrule 13 that holds the MCF 10, and a sleeve 73 that surrounds the ferrule 13. The second optical fiber unit 72 refers to an assembly of a plurality of SCFs 20, a ferrule 23 that holds the SCFs 20, and a sleeve 74 that surrounds the ferrule 23. Note that each of the ferrules 13 and 23 can move in the optical axis direction inside each of the sleeves 73 and 74. This allows each of the optical fibers (MCF 10, multiple SCFs 20) to change the distance from the lens (first lens, second lens) that faces it. Then, one of the first optical fiber unit 71 and the second optical fiber unit 72, and one of the first lens unit 76 and the second lens unit 77 facing the one are fixed to one opening of the metal tube 75 (one end in the axial direction of the metal tube 75) by welding. Here, adjacent first optical fiber unit 71, first lens unit 76, and metal tube 75 are fixed to each other by separate welding.

[0062] The second lens unit 77 and the ferrule 23 are aligned with respect to the integrated first optical fiber unit 71, the first lens unit 76, and the metal tube 75. The second lens unit 77 is aligned in the X direction and the Y direction. The ferrule 23 is aligned in the X direction, the Y direction, the Z direction, and the θz direction. After the alignment is completed, the metal tube 75 and the second lens unit 77 are fixed by welding. Thereafter, the ferrule 23 is aligned again in the X direction, the Y direction, the Z direction, and the θz direction, and then the ferrule 23 is fixed to the sleeve 74 by welding. Next, the ferrule 23 fixed to the sleeve 74 is aligned in the X direction, the Y direction, and the θz direction, and then the second lens unit 77 is fixed by welding. The welding is performed, for example, by irradiating a YAG laser.

[0063] Next, the effects obtained from the optical fiber connection structure according to the embodiment will be described. In the optical fiber connection structure 1, the pitch of each core 11 of the MCF 10 is P1 and the mode field diameter of each core 11 of the MCF 10 is MFD1 (μm), and the pitch of each core 21 of the SCF 20 is P2 and the mode field diameter of each core 21 of the SCF 20 is MFD2 (μm). If the focal length of the first lens 30 facing the MCF 10 is f1 (mm) and the focal length of the second lens 40 facing the SCF 20 is f2 (mm), (f1 / f2) is within ±10% of (P1 / P2). And (MFD1 / MFD2) is within ±10% of (P1 / P2).

[0064] Therefore, since the lens and the magnification conversion of the mode field diameter are performed together with the core pitch, it is possible to eliminate the mismatch of the mode field diameter between the MCF 10 and the SCF 20. Therefore, the optical loss can be suppressed. Furthermore, since the optical fiber connection structure 1 includes the MCF 10, the multiple SCFs 20, the first lens 30, and the second lens 40, it is possible to eliminate the need for special parts such as a deflection prism or a lens array. Therefore, it is possible to suppress an increase in the number of parts and realize a compact size.

[0065] Each of the multiple SCFs 20 may have a beam expansion portion 15 on the second end face 24 facing the second lens 40. In this case, the mode field diameter of the light propagating through the core 21 of each SCF 20 is expanded on the exit surface of each SCF 20, so that the value of MFD2 can be increased according to the expanded light L from the second lens 40, thereby reducing optical loss.

[0066] Each of the multiple SCFs 20 may have an expanded core portion 25 in which the core diameter is expanded at the second end face 24 facing the second lens 40. In this case, since the core 21 is expanded at the second end face 24 of each SCF 20, the value of MFD2 can be increased according to the expanded light L from the second lens 40, thereby reducing optical loss.

[0067] The beam expansion section may be a GRIN lens 26 or a GI fiber connected to each SCF 20. In this case, the end face of the GRIN lens 26 or the GI fiber on the side not connected to each SCF 20 is the exit surface of each SCF, and serves as the exit end. If the outer diameter of the GRIN lens 26 or the GI fiber matches the outer diameter of the SCF 20, the GRIN lens 26 or the GI fiber can be easily connected to the SCF 20.

[0068] The beam expansion section may be an optical fiber 27 having a core 27a with a diameter larger than that of the core 21 of the SCF 20, and the core may expand from the SCF 20 toward the optical fiber 27 in a region including the interface between the SCF 20 and the optical fiber 27. In this case, the end face of the optical fiber 27 having a core 27a with a diameter larger than that of the core 21 of the SCF 20, on the side not connected to the SCF 20, is the exit surface and exit end of each SCF.

[0069] A normal H2 to the exit surface of the SCF 20 may be inclined with respect to the optical axis direction D of the SCF 20, and a normal H1 to the first end face 14 of the MCF 10 facing the first lens 30 may be inclined with respect to the optical axis direction D. When the inclination angle of the normal H1 to the first end face 14 of the MCF 10 with respect to the optical axis direction D is θ1 and the inclination angle of the normal H2 to the exit surface of the SCF 20 with respect to the optical axis direction D is θ2, then, (f1 / f2)×0.8 ≦ θ2 / θ1 ≦ (f1 / f2)×1.2 (3) In this case, the optical axes of the SCF 20 and the MCF 10 can be made closer to being parallel.

[0070] The first lens 30A and the second lens 40A may be plano-convex lenses having curved surfaces facing each other. In this case, inexpensive plano-convex lenses can be used, and assembly can be easily performed.

[0071] The first lens and the second lens may be a first lens 30B and a second lens 40B that are GRIN lenses. In this case, by using a GRIN lens having a planar shape, there is an advantage that it is easy to handle and that assembly can be easily performed.

[0072] The first lens and the second lens may be aspheric lenses, which allows for lower loss coupling.

[0073] Each of the plurality of SCFs 20 may have a small diameter. In this case, the magnification ratio of the second lens 40 can be kept low by using SCFs 20 that already have a small diameter or SCFs 20 whose diameter has been reduced by etching or the like.

[0074] Like the optical fiber connection structure 50 described above, the optical fiber connection structure may include a first collimator 51 in which the MCF 10 and the first lens 30 are integrated, a second collimator 52 in which a plurality of SCFs 20 and the second lens 40 are integrated, and a cylindrical member 55 into which the first collimator 51 and the second collimator 52 are inserted so that the first lens 30 and the second lens 40 face each other. The first collimator 51 and the second collimator 52 may be fixed to the cylindrical member 55 with an adhesive while being inserted into the cylindrical member 55.

[0075] Like the optical fiber connection structure 60 described above, the optical fiber connection structure may include a first collimator 61 in which the MCF 10 and the first lens 30 are integrated, a second collimator 62 in which a plurality of SCFs 20 and a second lens 40 are integrated, and a metal tube 65 that fixes the first collimator 61 and the second collimator 62 by welding so that the first lens 30 and the second lens 40 face each other.

[0076] Like the optical fiber connection structure 70 described above, the optical fiber connection structure may include a first optical fiber unit 71 having an MCF 10, a second optical fiber unit 72 having a plurality of SCFs 20, a first lens unit 76 including a first lens 30 and fixed to the first optical fiber unit 71 by welding, a second lens unit 77 including a second lens 40 and fixed to the second optical fiber unit 72 by welding, and a metal tube 75 that fixes the first lens unit 76 and the second lens unit 77 by welding so that the first lens unit 76 and the second lens unit 77 face each other.

[0077] Fig. 15 is a perspective view showing an optical fiber connection structure 80 according to yet another modified example. Fig. 16 is a cross-sectional view showing the optical fiber connection structure 80. As shown in Figs. 15 and 16, the optical fiber connection structure 80 includes a first collimator 81, a second collimator 82, and a tubular member 85. The tubular member 85 is made of, for example, glass. The tubular member 85 fixes the first collimator 81 and the second collimator 82 such that the first lens 30A and the second lens 40A face each other.

[0078] The first collimator 81 has an MCF 10, a ferrule 13 (first ferrule) that holds the MCF 10, a first lens 30A that faces the ferrule 13 via a gap S1, and a first sleeve 83 that internally holds the first lens 30A and the ferrule 13. The second collimator 82 has a plurality of SCFs 20, a ferrule 23 (second ferrule) that holds the plurality of SCFs 20, a second lens 40A that faces the ferrule 23 via a gap S2, and a second sleeve 84 that internally holds the second lens 40A and the ferrule 23.

[0079] The ferrule 13 and the first sleeve 83 are made of glass, for example. The first lens 30A is, for example, a bulb-tipped rod lens. The outer diameter of the first lens 30A is approximately the same as the outer diameter of the ferrule 13. The inner diameter of the first sleeve 83 is slightly larger than the outer diameter of the first lens 30A and the outer diameter of the ferrule 13. A gap of, for example, 0 μm or more and 20 μm or less is formed between the outer peripheral surface of the first lens 30A and the outer peripheral surface of the ferrule 13 and the inner peripheral surface of the first sleeve 83. The end face 30c of the first lens 30A facing the tubular member 85 is recessed further into the first sleeve 83 than the end face 83b of the first sleeve 83 facing the tubular member 85. This makes it possible to avoid the first lens 30A from interfering with the tubular member 85 when the first collimator 81 is aligned with the tubular member 85 in the XY direction.

[0080] For example, the ferrule 23 and the second sleeve 84 are made of glass, and the second lens 40A is a spherical rod lens. A part of the second lens 40A is inserted inside the tubular member 85. The outer diameter of the second lens 40A is slightly smaller than the inner diameter of the tubular member 85, smaller than the outer diameter of the ferrule 23, and smaller than the inner diameter of the second sleeve 84. The outer diameter of the ferrule 23 is larger than the inner diameter of the tubular member 85 and slightly smaller than the inner diameter of the second sleeve 84.

[0081] The length of the gap S1 formed between the MCF 10 (ferrule 13) and the first lens 30A is adjusted so that the light propagating between the first lens 30A and the second lens 40A becomes collimated light. Fig. 18 is a diagram showing the MCF 10, ferrule 13, and first lens 30A according to this modification, and Fig. 17 is a diagram showing the MCF 110, ferrule 113, and first lens 130A according to a comparative example.

[0082] Incidentally, the refractive index of the first lens 130A may be different from that of the MCF 110, and may be higher than that of the MCF 110, for example. In this case, as shown in Fig. 17, when an end face 113b (tip face of the MCF 110) of the ferrule 113 facing the first lens 130A and a surface 130b of the first lens 130A facing the ferrule 113 are parallel to each other, the light L emitted from the MCF 110 may be inclined from the optical axis direction D (longitudinal direction) of the MCF 110 at the first lens 130A. In contrast, as shown in Fig. 18, when an end face 13b (tip face of the MCF 10) of the ferrule 13 facing the first lens 30A and a surface 30b of the first lens 30A facing the ferrule 13 are non-parallel to each other, the light L emitted from the MCF 10 is emitted parallel to the optical axis direction D at the first lens 30A. As a result, the light L is emitted from the first lens 30A toward the second lens 40A in parallel to the optical axis direction D, so that optical coupling loss can be suppressed. When the refractive index of the first lens 30A is higher than that of the MCF 10, the inclination angle of the end face 13b of the ferrule 13 with respect to the plane perpendicular to the optical axis direction D is larger than the inclination angle of the face 30b of the first lens 30A with respect to the plane perpendicular to the optical axis direction D. Note that the end face of the ferrule 23 facing the second lens 40A and the face of the second lens 40A facing the ferrule 23 may be non-parallel to each other. In this case, the same effects as those of the first lens 30A and the ferrule 13 described above can be obtained.

[0083] Next, a method of assembling the optical fiber connection structure 80 will be described. As shown in FIG. 19, the second lens 40A is fixed to the tubular member 85. Specifically, the spherical portion of the second lens 40A is inserted into the inside of the tubular member 85, and the second lens 40A inserted into the tubular member 85 is fixed to the tubular member 85 with an adhesive. The first lens 30A and the ferrule 13 are inserted into the first sleeve 83 and fixed to the first sleeve 83 with an adhesive to prepare the first collimator 81. Then, the first collimator 81 is aligned with respect to the second lens 40A fixed to the tubular member 85. For example, the first collimator 81 is aligned in the X direction and the Y direction, and after the alignment, the end of the tubular member 85 and the end of the first sleeve 83 are fixed with an adhesive to fix the first collimator 81 to the tubular member 85.

[0084] As shown in Fig. 20, the second collimator 82 is aligned in each of the X direction, Y direction, Z direction, and θz direction. When aligning the second collimator 82 in each of the X direction and Y direction, the ferrule 23 and the second sleeve 84 move together. When aligning the second collimator 82 in each of the Z direction and θz direction, for example, the ferrule 23 slides inside the second sleeve 84. After the alignment of the second collimator 82, the ferrule 23 is fixed to the second sleeve 84 by adhesive, and the end of the second sleeve 84 and the end of the tubular member 85 are fixed by adhesive. Through the above steps, the optical fiber connection structure 80 is completed.

[0085] 16, in the optical fiber connection structure 80, the end face 30c of the first lens 30A facing the tubular member 85 is located further inside the first sleeve 83 than the end face 83b of the first sleeve 83 facing the tubular member 85. That is, the first lens 30A is located inside the first sleeve 83. Therefore, when aligning the first collimator 81 with the tubular member 85, it is possible to prevent the first lens 30A from coming into contact with the tubular member 85.

[0086] As shown in FIG. 18, the inclination angle of the end face 13b of the ferrule 13 with respect to the plane perpendicular to the optical axis direction D (longitudinal direction) is different from the inclination angle of the face 30b of the first lens 30A (face 30b facing the ferrule 13) with respect to the plane perpendicular to the optical axis direction D, and the light L is emitted from the first lens 30A to the second lens 40A parallel to the optical axis direction D. Therefore, the coupling loss of the light L can be suppressed. The inclination angle of the end face of the ferrule 23 with respect to the plane perpendicular to the optical axis direction D is different from the inclination angle of the face of the second lens 40A with respect to the plane perpendicular to the optical axis direction D, and the light may be emitted from the second lens 40A to the first lens 30A parallel to the optical axis direction D. In this case, the same effect as above can be obtained. In addition, the outer diameter of the ferrule 23 is larger than the outer diameter of the second lens 40A. In this case, the second sleeve 84 and the ferrule 23 can be moved in the X direction and the Y direction with respect to the second lens 40A fixed to the tubular member 85.

[0087] Although the optical fiber connection structure 80 has been described above, it is possible to further modify the configuration of the optical fiber connection structure 80. For example, in the optical fiber connection structure 80, the ferrule 13 inserted into the first sleeve 83 of the first collimator 81 holds the MCF 10, and the ferrule 23 inserted into the second sleeve 84 of the second collimator 82 holds a plurality of SCFs 20. However, the ferrule 13 of the first collimator 81 may hold a plurality of SCFs, and the ferrule 23 of the second collimator 82 may hold an MCF. In other words, the type of optical fiber held by the ferrule is not particularly limited.

[0088] The above describes the embodiment and the modified example of the optical fiber connection structure according to the present disclosure. However, the present invention is not limited to the above embodiment or modified example. That is, it is easily recognized by those skilled in the art that various modifications and changes are possible within the scope of the gist of the present invention described in the claims. For example, the shape, size, material, number and arrangement of each part of the optical fiber connection structure can be appropriately changed within the scope of the gist of the above. For example, in the above embodiment, an example was described in which the normal H1 of the first end face 14 of the MCF 10 and the normal H2 of the second end face 24 of the SCF 20 are inclined with respect to the optical axis direction D. However, at least one of the normal to the first end face of the MCF and the normal to the second end face of the SCF does not have to be inclined with respect to the optical axis direction. In this way, the orientation of the tip face can also be appropriately changed. [Explanation of symbols]

[0089] 1, 1A, 1B, 50, 60, 70...Optical fiber connection structure 10...MCF (multi-core fiber) 11. Core 12…Clad 13...Ferrule (first ferrule) 14...First end surface 15…Beam expansion section 20...SCF (single-core fiber) 20A…TEC fiber 21…Core 22…Clad 23...Ferrule (second ferrule) 24…Second end surface 25…Core expansion section 25a…Tapered section 26...GRIN lens 27...Optical fiber 27a…Core 30, 30A, 30B…First lens 40, 40A, 40B…2nd lens 51,61…First collimator 52,62…Second collimator 53,54,63,64,73,74…Sleeve 55...Cylindrical member 63a, 64a...insertion part 64a…insertion part 65,75…metal tube 65a…Main body 65b...Inserted part 71...First optical fiber unit 72...Second optical fiber unit 73a, 74a...Flange section 76…1st lens unit 76a, 77a...Lens holding members 77…2nd lens unit 80...Optical fiber connection structure 81…First collimator 82…Second collimator 83…First sleeve 83b...end face 84…Second sleeve 85...Tubular member D…Optical axis direction H1,H2…normal line L…Light θ1, θ2…Angle

Claims

1. A multicore fiber; A plurality of single-core fibers arranged in parallel in a direction perpendicular to the longitudinal direction; a first lens facing a first end face that is a tip face of the multi-core fiber and having a focal length of f1 (mm); a second lens having a focal length of f2 (mm) and disposed between the first lens and second end faces that are tip faces of the plurality of single-core fibers; Equipped with an arrangement of central positions of each core in a cross section perpendicular to a longitudinal direction of the multicore fiber and an arrangement of central positions of each core in a cross section perpendicular to a longitudinal direction of the plurality of single-core fibers are arranged to have similar shapes; a pitch of a set of cores in the multicore fiber is P1 (μm) and a mode field diameter of light of a specific wavelength at the first end face of each core is MFD1 (μm); a pitch of a set of cores of the plurality of single-core fibers at a position corresponding to a set of cores of the multi-core fiber is P2 (μm), and a mode field diameter of light of the specific wavelength at the second end face of each core is MFD2 (μm); (P1 / P2)×0.9≦f1 / f2≦(P1 / P2)×1.1, and (P1 / P2)×0.9≦MFD1 / MFD2≦(P1 / P2)×1.1, Fulfilling The P2 is greater than the P1, The f2 is greater than the f1, each of the plurality of single-core fibers has a beam expanding portion at the second end surface capable of expanding a mode field diameter of light propagating within a core toward the second end surface; the beam expansion section is a core expansion section in which a core diameter is expanded in a tapered shape in each of the single-core fibers, the plurality of single-core fibers are bundled in a ferrule; a normal line of the first end face is inclined at an inclination angle θ1 with respect to an optical axis direction of the single-core fiber, a normal line of the second end face is inclined at an inclination angle θ2 with respect to the optical axis direction, The first lens and the second lens are cylindrical plano-convex lenses or GRIN lenses having curved surfaces facing each other. Optical fiber connection structure.

2. The inclination angle θ1 and the inclination angle θ2 are (f1 / f2)×0.8 ≦ θ2 / θ1 ≦ (f1 / f2)×1.2, Fulfilling 2. The optical fiber connection structure according to claim 1.

3. an outer diameter of each of the single-core fibers at the second end face is smaller than an outer diameter of each of the single-core fibers at an end face opposite to the second end face; 3. The optical fiber connection structure according to claim 1 or 2.

4. a first collimator in which the multi-core fiber and the first lens are integrated; a second collimator in which the plurality of single-core fibers and the second lens are integrated; a cylindrical member into which the first collimator and the second collimator are inserted so that the first lens and the second lens face each other; Equipped with the first collimator and the second collimator are fixed to the cylindrical member by an adhesive in a state where they are inserted into the cylindrical member; The optical fiber connection structure according to any one of claims 1 to 3.

5. a first collimator in which the multi-core fiber and the first lens are integrated; a second collimator in which the plurality of single-core fibers and the second lens are integrated; a metal tube that fixes the first collimator and the second collimator by welding so that the first lens and the second lens face each other; Equipped with The optical fiber connection structure according to any one of claims 1 to 3.

6. a first optical fiber unit having the multi-core fiber; a second optical fiber unit having the plurality of single-core fibers; a first lens unit including the first lens and fixed to the first optical fiber unit by welding; a second lens unit including the second lens and fixed to the second optical fiber unit by welding; a metal tube that fixes the first lens unit and the second lens unit by welding so that the first lens unit and the second lens unit face each other; Equipped with The optical fiber connection structure according to any one of claims 1 to 3.

7. a first collimator including the first lens, a first ferrule that holds the multi-core fiber, and a first sleeve that holds the first lens and the first ferrule therein; a second collimator including the second lens, a second ferrule that holds the plurality of single-core fibers, and a second sleeve that holds the second lens and the second ferrule therein; a tubular member that fixes the first collimator and the second collimator so that the first lens and the second lens face each other; Equipped with An end of the first sleeve and one end of the tubular member, and an end of the second sleeve and the other end of the tubular member are fixed to each other. The optical fiber connection structure according to any one of claims 1 to 3.

8. an end surface of the first lens facing the tubular member is located further inside the first sleeve than an end surface of the first sleeve facing the tubular member; The optical fiber connection structure according to claim 7.

9. an inclination angle of an end face of the first ferrule of the first collimator with respect to a plane perpendicular to the longitudinal direction is different from an inclination angle of a surface of the first lens facing the first ferrule with respect to the plane perpendicular to the longitudinal direction, Light is emitted from the first lens toward the second lens in a direction parallel to the longitudinal direction.

9. The optical fiber connection structure according to claim 7 or 8.

10. an inclination angle of an end face of the second ferrule of the second collimator with respect to a plane perpendicular to the longitudinal direction is different from an inclination angle of a surface of the second lens facing the second ferrule with respect to the plane perpendicular to the longitudinal direction, Light is emitted from the second lens toward the first lens in a direction parallel to the longitudinal direction.

9. The optical fiber connection structure according to claim 7 or 8.

11. The outer diameter of the second ferrule is larger than the outer diameter of the second lens. The optical fiber connection structure according to any one of claims 7 to 10.

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