Multi-core optical fiber

Abstract

A multi-core optical fiber (1) is provided with a plurality of cores (10) and a cladding (20), the outer peripheral surface of the cladding (20) has a first outer peripheral surface (21) that is a portion in the circumferential direction and a second outer peripheral surface (22) that is another portion in the circumferential direction, and the first outer peripheral surface (21) bulges outward with a larger radius of curvature than the second outer peripheral surface (22).

Description

Technical Field

[0001] The present invention relates to multi-core optical fibers. Background Art

[0002] In recent years, with the widespread adoption of optical fiber communication systems, the amount of information transmitted by optical fibers has dramatically increased. Against this backdrop, multi-core optical fibers, consisting of multiple cores surrounded by a single cladding, have become increasingly popular. Multi-core optical fibers can transmit multiple signals using light propagating through each of the multiple cores, thereby increasing the transmission capacity of each fiber.

[0003] When using a multi-core optical fiber for long-distance optical signal transmission, it is sometimes connected to another multi-core optical fiber. In this case, from the perspective of reducing light loss at the connection part of the multi-core optical fiber, it is desirable to improve the arrangement accuracy of each core of the multi-core optical fiber.

[0004] Patent Document 1 below describes a multi-core optical fiber having a non-circular cladding shape to achieve good optical coupling. Patent Document 1 describes a multi-core optical fiber having a so-called D-shaped cladding shape in which a portion of the outer peripheral surface is formed flat.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2010-286548 Summary of the Invention

[0008] (1) Technical issues to be resolved

[0009] When aligning a multi-core optical fiber with a D-shaped cladding along the rotational direction, alignment is sometimes performed by illuminating the cladding with light from the side and observing the position of the core, etc. However, when aligning the fiber from the side using a multi-core optical fiber with a D-shaped cladding, there are rotational positions where spherical aberration is large and side-view alignment is difficult. Examples of such rotational positions include positions where light is incident from a planar portion of the outer circumference of the cladding. Therefore, alignment is sometimes difficult.

[0010] Therefore, an object of the present invention is to provide a multi-core optical fiber that can be easily aligned.

[0011] (2) Technical solution

[0012] One method of the present invention for solving the above-mentioned technical problem is a multi-core optical fiber, characterized in that it has multiple fiber cores and a cladding surrounding the fiber cores, the outer peripheral surface of the cladding has a first outer peripheral surface as a part in the circumferential direction and a second outer peripheral surface as another part in the circumferential direction, and at least a part of the first outer peripheral surface bulges outward from the cladding with a larger curvature radius than the second outer peripheral surface.

[0013] When performing side-view centering on such a multi-core optical fiber, even when light is incident from the first outer peripheral surface, spherical aberration can be suppressed compared to when light is incident from a planar portion of the outer peripheral surface of the D-type cladding. Therefore, the multi-core optical fiber of the present invention can be easily aligned.

[0014] In addition, the multi-core optical fiber of the second embodiment of the present invention is a multi-core optical fiber of the first embodiment, characterized in that a plurality of cores are arranged on the outer peripheral side of the cladding, and at least one of the parts of the outer peripheral surface facing the plurality of cores is the first outer peripheral surface.

[0015] The portion of the outer peripheral surface opposite the core is the portion of the outer peripheral surface closest to the core. Therefore, the distance between the core positioned on the outer peripheral side and the first outer peripheral surface opposite the core is smaller than the distance between the core and the second outer peripheral surface when the portion of the outer peripheral surface opposite the core positioned on the outer peripheral side is the second outer peripheral surface. Consequently, unwanted high-order modes of light propagating in the core opposite the first outer peripheral surface can be attenuated by the influence of the first outer peripheral surface. Unwanted high-order modes of light are, for example, light not used for communication.

[0016] Furthermore, a multi-core optical fiber according to a third aspect of the present invention is a multi-core optical fiber according to the second aspect, characterized in that the aforementioned portions are each the first outer peripheral surface.

[0017] In this case, light in an unnecessary high-order mode propagating through each core arranged on the outer peripheral side can be attenuated by the influence of the first outer peripheral surface.

[0018] Furthermore, a multi-core optical fiber according to a fourth aspect of the present invention is the multi-core optical fiber according to any one of the first to third aspects, characterized in that the outer peripheral surface of the cladding has a rotationally asymmetric shape.

[0019] In this case, the multi-core optical fiber can be roughly aligned so that the rotation direction of the cladding becomes a specific direction according to the outer shape of the cladding.

[0020] Furthermore, a multi-core optical fiber according to a fifth aspect of the present invention is the multi-core optical fiber according to any one of aspects 1 to 4, wherein the curvature radius of the first outer peripheral surface is 1.5 to 20 times the curvature radius of the second outer peripheral surface.

[0021] By setting the ratio of the curvature radius of the first outer peripheral surface to the curvature radius of the second outer peripheral surface in such a relationship, rough centering of the outer peripheral surface using the cladding can be performed more easily, and spherical aberration during side view centering can be suppressed.

[0022] (3) Beneficial effects

[0023] As described above, according to the present invention, a multi-core optical fiber that can be easily aligned is provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a diagram showing a cross section perpendicular to the longitudinal direction of the multi-core optical fiber according to the first embodiment of the present invention.

[0025] Figure 2 It means that the light is Figure 1 A diagram showing spherical aberration when light is incident on the second outer peripheral surface in a direction perpendicular to the longitudinal direction of the multi-core optical fiber and is emitted from the first outer peripheral surface.

[0026] Figure 3 It means that the light is Figure 1 A diagram showing spherical aberration when light is incident on the first peripheral surface in a direction perpendicular to the longitudinal direction of the multi-core optical fiber and is emitted from the second peripheral surface.

[0027] Figure 4 It is a diagram showing a cross section perpendicular to the longitudinal direction of a multi-core optical fiber according to a second embodiment of the present invention.

[0028] Figure 5 It is a diagram showing a cross section perpendicular to the longitudinal direction of a multi-core optical fiber according to a third embodiment of the present invention. DETAILED DESCRIPTION

[0029] Below, preferred embodiments of the multi-core optical fiber of the present invention are described in detail with reference to the accompanying drawings. The following exemplary embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. Within the scope of the claims, the present invention may be modified or improved from the exemplary embodiments without departing from its spirit. Furthermore, for ease of understanding, the scales of the various figures may differ from those described in the following description.

[0030] (First embodiment)

[0031] Figure 1 : This is a diagram showing a cross section perpendicular to the longitudinal direction of a multi-core optical fiber of this embodiment. The multi-core optical fiber 1 of this embodiment comprises: a plurality of cores 10, a marker 15, a cladding 20 surrounding the outer circumference of each core 10 and the marker 15 without a gap, an inner covering layer 31 covering the outer circumference of the cladding 20, and an outer covering layer 32 covering the outer circumference of the inner covering layer 31. Figure 1In the example shown, there are four cores 10.

[0032] In the multi-core optical fiber 1 of this embodiment, the cores 10 are arranged on a circumference centered at a reference position 20R, which is approximately the center of the cladding 20. In this embodiment, the cores 10 are arranged at the outermost periphery of the cladding 20. In the multi-core optical fiber 1 of this embodiment, the distances between the cores 10 are equal, and the cores 10 are arranged at positions that are approximately fourfold rotationally symmetrical about the reference position 20R. The diameter of the cores 10 is, for example, not less than 4 μm and not more than 14 μm.

[0033] The marks 15 are arranged outside the circumference of the arrangement of the cores 10. The refractive index of the marks 15 may be higher or lower than that of the cladding 20 as long as it is different from that of the cladding 20.

[0034] The refractive index of each core 10 is higher than that of the cladding 20. The relative refractive index difference (specific refractive index difference) of each core 10 relative to the cladding 20 is, for example, 0.2% to 2.0%. Such cores 10 are composed, for example, of silica glass doped with a refractive index-raising dopant such as germanium, while the cladding 20 is composed, for example, of undoped silica glass. Alternatively, the cores 10 may be composed of undoped silica glass, while the cladding 20 may be composed of silica glass doped with a refractive index-lowering dopant such as fluorine. Marking 15 is composed of silica glass having a different refractive index from that of the cladding 20.

[0035] The cladding 20 has a non-circular shape and includes a first outer peripheral surface 21, which is one portion of the circumference, and a second outer peripheral surface 22, which is the other portion of the circumference. In this embodiment, one portion of the outer peripheral surface of the cladding 20 that faces each core 10 is referred to as the first outer peripheral surface 21, and the other portion of the outer peripheral surface of the cladding 20 is referred to as the second outer peripheral surface 22. The portion of the outer peripheral surface facing the core 10 is the portion of the outer peripheral surface closest to the core 10. Therefore, the distance between the core 10 facing the first outer peripheral surface 21 and the first outer peripheral surface 21 is smaller than the distance between the core 10 and the second outer peripheral surface 22. The second outer peripheral surface 22 overlaps a portion of a predetermined circle 20C centered at a reference position 20R of the cladding 20. For ease of reading in the drawings, the circle 20C and the second outer peripheral surface 22, shown with dashed lines, are depicted with a slight offset. The first outer peripheral surface 21 bulges outward from the cladding 20 with a larger curvature radius than the second outer peripheral surface 22 and is connected to the second outer peripheral surface 22. Therefore, the first outer peripheral surface 21 is located inside the circumference 20C.

[0036] In an embodiment, the length of the straight line connecting both ends of the first outer peripheral surface 21 is equal to or greater than the diameter of the core 10 facing the first outer peripheral surface 21. Further, in the present embodiment, when the first outer peripheral surface 21 is observed in a direction perpendicular to the length direction of the multi-core optical fiber 1, the core 10 facing the first outer peripheral surface 21 completely overlaps with the first outer peripheral surface 21.

[0037] Thus, since the cladding 20 of the present embodiment has the first outer peripheral surface 21 formed only at one part, the outer peripheral surface of the cladding 20 has a non-rotationally symmetric shape.

[0038] The radius of curvature of the first outer peripheral surface 21 is preferably 1.5 times or more and 20 times or less the radius of curvature of the second outer peripheral surface 22. Thus, by making the radius of curvature of the first outer peripheral surface 1.5 times or more the radius of curvature of the second outer peripheral surface, the outer peripheral surface of the cladding can be made closer to a planar shape. Therefore, it is possible to more easily perform coarse alignment based on the outer peripheral surface of such a cladding 20. Further, by making the radius of curvature of the first outer peripheral surface 20 times or less the radius of curvature of the second outer peripheral surface, spherical aberration during side viewing alignment can be suppressed.

[0039] The inner covering layer 31 and the outer covering layer 32 are each made of a resin such as an ultraviolet curable resin, and the inner covering layer 31 and the outer covering layer 32 are made of different resins.

[0040] To manufacture the multi-core optical fiber 1 having such a shaped cladding 20, for example, a part of the outer peripheral surface of the cladding rod that becomes the cladding 20 in the base material of the multi-core optical fiber 1 is cut. The multi-core optical fiber 1 is formed by drawing (in Japanese: 線引き) the base material having such a cladding rod, and the cut part of the outer peripheral surface of the cladding rod becomes the first outer peripheral surface 21, and the other part becomes the second outer peripheral surface 22.

[0041] Figure 2 is a diagram showing spherical aberration when light is incident on the second outer peripheral surface 22 from a direction perpendicular to the length direction of the multi-core optical fiber 1 and the light is emitted from the first outer peripheral surface 21. Further, when light passes through the multi-core optical fiber 1 in this way, the inner covering layer 31 and the outer covering layer 32 are peeled off. In Figure 2 In this case, since the first outer peripheral surface 21 is located on the light emission side, the multi-core optical fiber 1 can be understood as a biconvex lens close to a plano-lens, and the light emitted from the first outer peripheral surface 21 has spherical aberration of size a1. Further, a plano-lens means a lens in which the surface on which light is incident is formed convex and the surface on which light is emitted is formed planar, and a biconvex lens means a lens in which both the surface on which light is incident and the surface on which light is emitted are formed convex. Further, in Figure 2The spherical aberration size a0 when the same light is passed through a multi-core optical fiber with a circular cladding is shown in FIG. The multi-core optical fiber with a circular cladding can be understood as a double convex lens. In this way, by passing the light through the multi-core optical fiber 1 of the present embodiment as described above, the spherical aberration can be reduced compared to the case where the light is passed through a multi-core optical fiber with a circular cladding with a radius of curvature of the second outer peripheral surface, wherein the spherical aberration of the light is smaller than that when the light is incident from a direction perpendicular to the longitudinal direction of the circular cladding onto the circular outer peripheral surface with a radius of curvature of the second outer peripheral surface and the light is emitted from the circular outer peripheral surface. By making the spherical aberration of the light smaller in this way, the centering of the rotation direction of the multi-core optical fiber 1 can be performed with higher precision.

[0042] In addition, Figure 2 , the magnitude of spherical aberration aD1 when light is transmitted in the same manner in a multi-core optical fiber having a so-called D-type cladding in which the portion corresponding to the first outer peripheral surface 21 of the cladding is flat. Figure 2 In this case, the multi-core optical fiber can be understood as a plane lens, and the magnitude of spherical aberration aD1 is smaller than the magnitude of spherical aberration a1 when light is transmitted through the multi-core optical fiber 1 of this embodiment as described above.

[0043] Figure 3 : is a diagram showing spherical aberration when light is incident on the first peripheral surface 21 from a direction perpendicular to the longitudinal direction of the multi-core optical fiber 1 and light is emitted from the second peripheral surface 22. Figure 3 As shown, in this case, the multi-core optical fiber 1 can be understood as a biconvex lens close to a plano-convex lens, and the light emitted from the second peripheral surface 22 has a spherical aberration of size a2. In addition, a plano-convex lens refers to a lens in which the surface where light enters is formed into a plane and the surface where light exits is formed into a convex shape. The size of the spherical aberration a2 is larger than the size of the spherical aberration a1 mentioned above. In addition, in Figure 3 ] shows the magnitude a0 of spherical aberration when the same light is transmitted through a multi-core optical fiber having a circular cladding. In this case, the magnitude a2 of spherical aberration tends to be larger than the magnitude a0 of spherical aberration.

[0044] In addition, Figure 3 , the magnitude of spherical aberration aD2 when light is transmitted in the same manner in a multi-core optical fiber having a so-called D-type cladding in which the portion corresponding to the first outer peripheral surface 21 of the cladding is flat. Figure 3 In this case, the multi-core optical fiber can be understood as a plano-convex lens, and the magnitude of spherical aberration aD2 is larger than the magnitude of spherical aberration a2 when light is transmitted through the multi-core optical fiber 1 of this embodiment as described above.

[0045] according to Figure 2 、 Figure 3 As described above, when light is transmitted through the multi-core optical fiber 1 of this embodiment in a direction perpendicular to the longitudinal direction and the multi-core optical fiber 1 is rotated, the change in spherical aberration can be suppressed compared to a multi-core optical fiber having a D-type cladding.

[0046] Furthermore, as long as the radius of curvature of the first outer peripheral surface 21 is greater than the radius of curvature of the second outer peripheral surface 22, the first outer peripheral surface 21 may have a fixed radius of curvature that overlaps a portion of the circumference of an imaginary circle (not shown), or it may not have a fixed radius of curvature. For example, if the radius of curvature of the first outer peripheral surface 21 is greater than the radius of curvature of the second outer peripheral surface 22, the shape of the first outer peripheral surface 21 may be a portion of an ellipse or a portion of a perfect circle.

[0047] As described above, in the multi-core optical fiber 1 of this embodiment, the outer peripheral surface of the cladding 20 includes a first outer peripheral surface 21, which is one portion of the circumferential direction, and a second outer peripheral surface 22, which is another portion of the circumferential direction. The first outer peripheral surface 21 bulges outward from the cladding 20 with a larger radius of curvature than the second outer peripheral surface 22. Therefore, during side-view centering, even when light is incident from the first outer peripheral surface 21, spherical aberration is suppressed compared to when light is incident from a planar portion of the outer peripheral surface of a D-shaped cladding. Furthermore, when the multi-core optical fiber 1 is rotated, the change in spherical aberration is suppressed compared to a multi-core optical fiber having a D-shaped cladding. Therefore, the multi-core optical fiber 1 of this embodiment facilitates centering. Furthermore, since the outer peripheral surface of the cladding 20 is non-circular in shape, rough centering can be performed based on the outer shape of the cladding 20 before side-view centering.

[0048] Furthermore, if the outer peripheral surface of the cladding 20 has an asymmetric rotational shape as in the multi-core optical fiber 1 of the present embodiment, the centering position can be determined to be one during the rough centering, thereby making centering easier.

[0049] Furthermore, in the multi-core optical fiber 1 of this embodiment, the first outer peripheral surface 21 is the portion of the outer peripheral surface of the cladding 20 that faces the core 10 disposed on the outer peripheral side. Therefore, the distance between the core 10 facing the first outer peripheral surface 21 and the first outer peripheral surface 21 is smaller than the distance between the core 10 facing the second outer peripheral surface 22 and the second outer peripheral surface 22. The closer the distance between the outer peripheral surface of the core 10 and the outer peripheral surface of the cladding 20, the more likely high-order modes propagating in the core 10 are to propagate toward the cladding 20. Therefore, the first outer peripheral surface 21 can attenuate light in high-order modes that are not necessary for communication, etc., that propagate in the core 10 facing the first outer peripheral surface 21.

[0050] (Second embodiment)

[0051] Next, refer to Figure 4The second embodiment of the present invention will be described in detail. Components identical or equivalent to those of the first embodiment are denoted by the same reference numerals unless otherwise specified, and duplicate descriptions will be omitted.

[0052] like Figure 4 As shown, the multi-core optical fiber 1 of this embodiment differs from the multi-core optical fiber 1 of the first embodiment in that each portion of the outer peripheral surface of the cladding 20 opposite to the core 10 is a first outer peripheral surface 21, and the other portions of the outer peripheral surface of the cladding 20 are a second outer peripheral surface 22.

[0053] In the multi-core optical fiber 1 of this embodiment, each portion of the outer peripheral surface of the cladding 20 opposite to the core 10 is the first outer peripheral surface 21. Therefore, compared with the multi-core optical fiber having a circular cladding overlapping with the second outer peripheral surface 22, the unnecessary high-order mode light propagating in each core 10 can be attenuated due to the influence of the first outer peripheral surface 21.

[0054] (Third embodiment)

[0055] Next, refer to Figure 5 The third embodiment of the present invention will be described in detail. Components identical or equivalent to those in the first embodiment are denoted by the same reference numerals unless otherwise specified, and duplicate descriptions will be omitted.

[0056] like Figure 5 As shown, the multi-core optical fiber 1 of this embodiment is different from the multi-core optical fiber 1 of the first embodiment in that a plurality of cores 10 are arranged in a straight line.

[0057] In this embodiment, the cores 10 located at the two ends are arranged on the outer periphery of the cladding 20. Furthermore, in this embodiment, the portions of the outer periphery of the cladding 20 facing the cores 10 at the two ends are each designated as a first outer periphery 21, while the remaining portions of the outer periphery of the cladding 20 are designated as a second outer periphery 22. Furthermore, the shape of the outer periphery of the cladding 20 may be the same as that of the outer periphery of the cladding 20 in the first embodiment. In this case, only one portion of the outer periphery of the cladding 20 facing the cores 10 at the two ends is designated as the first outer periphery 21, and the remaining portions of the outer periphery of the cladding 20 are designated as the second outer periphery 22.

[0058] The present invention has been described above using the above embodiment as an example, but the present invention is not limited thereto. For example, the first outer peripheral surface 21 may be provided at a portion of the outer peripheral surface of the cladding 20 other than the portion facing the core 10 .

[0059] Alternatively, the first outer peripheral surface 21 may be provided at a portion of the plurality of locations on the outer peripheral surface of the cladding 20 that face the core 10. The multi-core optical fiber 1 of the first embodiment is an example of this approach. Furthermore, in the first embodiment, the first outer peripheral surface 21 may be provided at two or three of the four locations on the outer peripheral surface of the cladding 20 that face the core 10. In other words, a portion of the locations on the outer peripheral surface facing the core 10 disposed on the outer peripheral side of the cladding 20 may serve as the first outer peripheral surface 21, while another portion of these locations may serve as the second outer peripheral surface.

[0060] In the first and second embodiments, all the cores 10 are located on a circumference centered at the reference position 20R, but other cores may be arranged inside the circumference. For example, a core may be arranged at the reference position 20R.

[0061] In the above embodiment, an example is described in which the length of the straight line connecting the two ends of the first outer peripheral surface 21 is equal to or greater than the diameter of the core 10 facing the first outer peripheral surface 21. However, the length of the straight line connecting the two ends of the first outer peripheral surface 21 may be less than the diameter of the core 10. However, as in the above embodiment, when the length of the straight line connecting the two ends of the first outer peripheral surface 21 is equal to or greater than the diameter of the core 10, light in high-order modes unnecessary for communication can be efficiently absorbed.

[0062] Furthermore, in the above-described embodiment, when the first outer peripheral surface 21 is viewed in a direction perpendicular to the longitudinal direction of the multi-core optical fiber 1, the core 10 facing the first outer peripheral surface 21 completely overlaps with the first outer peripheral surface 21. However, under the same observation, a portion of the core 10 facing the first outer peripheral surface 21 may overlap with the first outer peripheral surface 21, while another portion may not overlap with the first outer peripheral surface 21. However, when the core 10 facing the first outer peripheral surface 21 completely overlaps with the first outer peripheral surface 21, light in higher-order modes unnecessary for communication can be efficiently absorbed.

[0063] As described above, according to the present invention, a multi-core optical fiber that can be easily aligned is provided, and the multi-core optical fiber can be used in the field of optical communications and other equipment using multi-core optical fibers.

Claims

1. A multi-core optical fiber, characterized in that: A plurality of fiber cores and a cladding surrounding the fiber cores are provided. The outer peripheral surface of the cladding has a first outer peripheral surface as a portion in the circumferential direction and a second outer peripheral surface as another portion in the circumferential direction. The first outer peripheral surface bulges outward from the cladding with a larger curvature radius than the second outer peripheral surface.

2. The multi-core optical fiber according to claim 1, wherein Among the plurality of cores, two or more of the cores are arranged on the outer periphery of the cladding, At least one of the portions of the outer peripheral surface facing the core is the first outer peripheral surface.

3. The multi-core optical fiber according to claim 2, wherein: The parts are respectively the first outer peripheral surfaces.

4. The multi-core optical fiber according to any one of claims 1 to 3, wherein The outer peripheral surface of the cladding has a non-rotationally symmetrical shape.

5. The multi-core optical fiber according to any one of claims 1 to 4, wherein The curvature radius of the first outer peripheral surface is not less than 1.5 times and not more than 20 times the curvature radius of the second outer peripheral surface.

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