Multicore fiber

Abstract

This multicore fiber (1) comprises multiple cores (10) and a cladding (20), wherein an outer circumferential surface of the cladding (20) has a first outer circumferential surface (21) forming one circumferential portion thereof and a second outer circumferential surface (22) forming another circumferential portion thereof, and the first outer circumferential surface (21) bulges outward with a radius of curvature larger than that of the second outer circumferential surface (22).

Description

Multicore Fiber

[0001] The present invention relates to a multicore fiber.

[0002] In recent years, with the spread of optical fiber communication systems, the amount of information transmitted by optical fibers has increased dramatically. Against this background, multicore fibers, in which the outer peripheries of multiple cores are surrounded by a single cladding, are being used. Multicore fibers can transmit multiple signals using light propagating through each of the multiple cores, thereby increasing the transmission capacity per optical fiber.

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

[0004] Patent Document 1 listed below describes a multi-core fiber in which the cladding has a non-circular outer shape in order to achieve good optical coupling. For example, Patent Document 1 describes a multi-core fiber having a cladding with a so-called D-shaped outer shape in which part of the outer circumferential surface is formed into a flat shape.

[0005] JP 2010-286548 A

[0006] When aligning a multi-core fiber whose cladding has a D-shaped outer shape in the rotational direction, the alignment may be performed by side-view alignment, in which light is irradiated from the side surface of the cladding to observe the positions of the cores, etc. However, when performing side-view alignment on a multi-core fiber whose cladding has a D-shaped outer shape, there are rotational positions where spherical aberration is large and side-view alignment is difficult. An example of such a rotational position is a position where light is incident from a flat portion of the outer peripheral surface of the cladding. For this reason, alignment may be difficult.

[0007] Therefore, an object of the present invention is to provide a multi-core fiber that can be easily aligned.

[0008] A first aspect of the present invention for solving the above problem is a multi-core fiber comprising a plurality of cores and a cladding surrounding the cores, wherein the outer surface of the cladding has a first outer surface that is a part in the circumferential direction and a second outer surface that is another part in the circumferential direction, and at least a part of the first outer surface bulges outward from the cladding with a larger radius of curvature than the second outer surface.

[0009] When performing side-view alignment on such a multi-core fiber, even when light is incident from the first outer peripheral surface side, spherical aberration can be suppressed compared to when light is incident from a flat portion of the outer peripheral surface of the D-shaped cladding. Therefore, the multi-core fiber of the present invention can be easily aligned.

[0010] Aspect 2 of the present invention is the multi-core fiber of Aspect 1, characterized in that a plurality of the cores are arranged on the outer circumferential side of the cladding, and at least one of the portions of the outer circumferential surface facing the plurality of cores is the first outer circumferential surface.

[0011] The portion of the outer circumferential surface facing the core is the portion of the outer circumferential surface closest to the core. Therefore, the distance between the core arranged on the outer circumferential side and the first outer circumferential surface facing the core is smaller than the distance between the core and the second outer circumferential surface when the portion of the outer circumferential surface facing the core arranged on the outer circumferential side is the second outer circumferential surface. Therefore, unnecessary higher-order mode light propagating through the core facing the first outer circumferential surface can be attenuated by the influence of the first outer circumferential surface. The unnecessary higher-order mode light is, for example, light not used for communication.

[0012] A third aspect of the present invention is the multi-core fiber according to the second aspect, wherein each of the portions is the first outer circumferential surface.

[0013] In this case, unnecessary light of higher order modes propagating through each core arranged on the outer periphery side can be attenuated by the influence of the first outer periphery surface.

[0014] A fourth aspect of the present invention is the multi-core fiber according to any one of the first to third aspects, wherein the outer circumferential surface of the cladding has a rotationally asymmetric shape.

[0015] In this case, it may be possible to roughly align the multi-core fiber so that the rotation direction of the cladding is in a specific direction depending on the outer shape of the cladding.

[0016] Aspect 5 of the present invention is the multicore fiber according to any one of Aspects 1 to 4, characterized in that the radius of curvature of the first outer peripheral surface is 1.5 times or more and 20 times or less the radius of curvature of the second outer peripheral surface.

[0017] By having such a relationship between the ratio of the radius of curvature of the first outer surface and the radius of curvature of the second outer surface, rough alignment based on the outer surface of the cladding can be performed more easily, and spherical aberration can be suppressed when performing side-view alignment.

[0018] As described above, according to the present invention, a multi-core fiber that can be easily aligned is provided.

[0019] Fig. 1 is a diagram showing a cross section perpendicular to the longitudinal direction of a multi-core fiber according to a first embodiment of the present invention. Fig. 2 is a diagram showing spherical aberration when light is incident on a second outer peripheral surface from a direction perpendicular to the longitudinal direction of the multi-core fiber of Fig. 1 and is emitted from a first outer peripheral surface. Fig. 3 is a diagram showing spherical aberration when light is incident on a first outer peripheral surface from a direction perpendicular to the longitudinal direction of the multi-core fiber of Fig. 1 and is emitted from a second outer peripheral surface. Fig. 4 is a diagram showing a cross section perpendicular to the longitudinal direction of a multi-core fiber according to a second embodiment of the present invention. Fig. 5 is a diagram showing a cross section perpendicular to the longitudinal direction of a multi-core fiber according to a third embodiment of the present invention.

[0020] Preferred embodiments of the multicore fiber according to the present invention will be described in detail below with reference to the drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved from the embodiments within the scope of the claims without departing from the spirit thereof. Note that, for ease of understanding, the scales of the drawings may differ from those described in the following description.

[0021] (First embodiment) Fig. 1 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber according to this embodiment. The multicore fiber 1 of this embodiment includes a plurality of cores 10, markers 15, cladding 20 that tightly surrounds the outer peripheral surfaces of each of the cores 10 and markers 15, an inner coating layer 31 that coats the outer peripheral surface of the cladding 20, and an outer coating layer 32 that coats the outer peripheral surface of the inner coating layer 31. In the example of Fig. 1, an example with four cores 10 is shown.

[0022] In the multicore fiber 1 of this embodiment, the cores 10 are arranged on a circumference centered on a reference position 20R, which is approximately the center of the cladding 20. In this embodiment, the cores 10 are arranged on the outermost side of the cladding 20. In the multicore fiber 1 of this embodiment, the distances between the cores 10 are equal to each other, and the cores 10 are arranged at positions that are approximately four-fold rotationally symmetric about the reference position 20R. The diameter of the cores 10 is, for example, 4 μm or more and 14 μm or less.

[0023] The markers 15 are arranged outside the circumference where the cores 10 are arranged. The refractive index of the markers 15 may be higher or lower than that of the cladding 20, provided that the refractive index is different from that of the cladding 20.

[0024] The refractive index of each core 10 is higher than the refractive index of the cladding 20, and the relative refractive index difference of each core 10 with respect to the cladding 20 is, for example, 0.2% or more and 2.0% or less. Such a core 10 is made of silica glass doped with a dopant such as germanium that increases the refractive index, and the cladding 20 is made of silica glass that does not have any dopant added. Alternatively, the core 10 may be made of silica glass that does not have any dopant added, and the cladding 20 may be made of silica glass doped with a dopant such as fluorine that decreases the refractive index. The marker 15 is made of silica glass that has a refractive index different from that of the cladding 20.

[0025] The cladding 20 has a non-circular outer shape and includes a first outer peripheral surface 21, which is a portion of the cladding 20 in the circumferential direction, and a second outer peripheral surface 22, which is another portion of the cladding 20 in the circumferential direction. In this embodiment, one portion of the cladding 20's outer peripheral surface facing each core 10 is designated the first outer peripheral surface 21, and the other portion of the cladding 20's outer peripheral surface is designated the second outer peripheral surface 22. The portion of the outer peripheral surface facing each 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 with a portion of a predetermined circumference 20C centered at a reference position 20R of the cladding 20. Note that, for ease of viewing the drawings, the circumference 20C indicated by a dashed line and the second outer peripheral surface 22 are depicted slightly offset from each other. 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 and is connected to the second outer peripheral surface 22. Therefore, the first outer peripheral surface 21 is located inside the circumference 20C.

[0026] In the embodiment, the length of a 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. Furthermore, in the present embodiment, when the first outer peripheral surface 21 is viewed along a direction perpendicular to the longitudinal direction of the multicore fiber 1, the core 10 facing the first outer peripheral surface 21 completely overlaps the first outer peripheral surface 21.

[0027] As described above, the cladding 20 of this embodiment has the first outer peripheral surface 21 formed at only one location, and therefore the outer peripheral surface of the cladding 20 has a rotationally asymmetric shape.

[0028] The radius of curvature of the first outer peripheral surface 21 is preferably 1.5 to 20 times the radius of curvature of the second outer peripheral surface 22. In this way, when the radius of curvature of the first outer peripheral surface is 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 even more planar. Therefore, rough alignment based on the outer peripheral surface of such cladding 20 can be performed even more easily. Furthermore, when the radius of curvature of the first outer peripheral surface is 20 times or less the radius of curvature of the second outer peripheral surface, spherical aberration can be suppressed during side-view alignment.

[0029] The inner coating layer 31 and the outer coating layer 32 are each made of a resin such as an ultraviolet curable resin, and the inner coating layer 31 and the outer coating layer 32 are made of different resins.

[0030] To manufacture the multi-core fiber 1 having the cladding 20 of such a shape, for example, a part of the outer circumferential surface of a cladding rod that becomes the cladding 20 in the preform of the multi-core fiber 1 is cut. By drawing the preform having such a cladding rod into the multi-core fiber 1, the cut part of the outer circumferential surface of the cladding rod becomes the first outer circumferential surface 21, and the other part becomes the second outer circumferential surface 22.

[0031] 2 is a diagram showing spherical aberration when light is incident on the second outer peripheral surface 22 from a direction perpendicular to the longitudinal direction of the multi-core fiber 1 and exits from the first outer peripheral surface 21. When light is transmitted through the multi-core fiber 1 in this manner, the inner coating layer 31 and the outer coating layer 32 are peeled off. In the case of FIG. 2 , since the first outer peripheral surface 21 is located on the light emission side, the multi-core fiber 1 can be understood as a biconvex lens similar to a flat lens, and the light emitted from the first outer peripheral surface 21 has spherical aberration of magnitude a1. A flat lens is a lens in which the light incidence surface is formed convex and the light emission surface is formed flat, and a biconvex lens is a lens in which both the light incidence surface and the light emission surface are formed convex. Furthermore, FIG. 2 also shows the magnitude a0 of spherical aberration when similar light is transmitted through a multi-core fiber having a circular cladding. A multi-core fiber having a circular cladding can be understood as a biconvex lens. In this way, by transmitting light through the multi-core fiber 1 of the present embodiment as described above, it is possible to reduce the spherical aberration of light incident on a circular outer peripheral surface having a radius equal to the radius of curvature of the second outer peripheral surface, which is smaller than the spherical aberration of light when light is incident on the circular outer peripheral surface from a direction perpendicular to the longitudinal direction of the circular cladding and emitted from the circular outer peripheral surface, compared to when light is transmitted through the multi-core fiber having a circular cladding having a radius equal to the radius of curvature of the second outer peripheral surface. Such a small spherical aberration of light allows for more accurate alignment of the multi-core fiber 1 in the rotational direction.

[0032] 2 also shows the magnitude aD1 of spherical aberration when light is similarly transmitted through a multicore fiber having a so-called D-shaped cladding in which the portion corresponding to the first outer peripheral surface 21 of the cladding is flat. This plane is indicated by a dotted line in Fig. 2. In this case, the multicore fiber can be understood as a flat lens, and the magnitude aD1 of spherical aberration is smaller than the magnitude a1 of spherical aberration when light is transmitted through the multicore fiber 1 of this embodiment as described above.

[0033] 3 is a diagram showing spherical aberration when light is incident on the first outer peripheral surface 21 from a direction perpendicular to the longitudinal direction of the multicore fiber 1 and is emitted from the second outer peripheral surface 22. As shown in FIG. 3 , in this case, the multicore fiber 1 can be understood as a biconvex lens similar to a plano-convex lens, and the light emitted from the second outer peripheral surface 22 has spherical aberration of magnitude a2. Note that a plano-convex lens is a lens in which the surface on which light enters is formed flat and the surface from which light exits is formed convex. The magnitude a2 of the spherical aberration is larger than the magnitude a1 of the spherical aberration described above. FIG. 3 also shows the magnitude a0 of the spherical aberration when similar light is transmitted through a multicore fiber having a circular cladding. In this case, the magnitude a2 of the spherical aberration tends to be larger than the magnitude a0 of the spherical aberration.

[0034] 3 also shows the magnitude aD2 of spherical aberration when light is similarly transmitted through a multicore fiber having a so-called D-shaped cladding in which the portion corresponding to the first outer peripheral surface 21 of the cladding is flat. This plane is indicated by a dotted line in Fig. 3. In this case, the multicore fiber can be understood as a plano-convex lens, and the magnitude aD2 of spherical aberration is larger than the magnitude a2 of spherical aberration when light is transmitted through the multicore fiber 1 of this embodiment as described above.

[0035] From the descriptions of FIGS. 2 and 3 , when light is transmitted through the multi-core fiber 1 of this embodiment in a direction perpendicular to the longitudinal direction and the multi-core fiber 1 is rotated, the amount of change in spherical aberration can be suppressed more than in a multi-core fiber having a D-shaped cladding.

[0036] As long as the radius of curvature of the first outer peripheral surface 21 is larger than the radius of curvature of the second outer peripheral surface 22, the first outer peripheral surface 21 may have a constant radius of curvature that overlaps with a portion of the circumference of an imaginary circle (not shown), or the radius of curvature may not be constant. For example, if the radius of curvature of the first outer peripheral surface 21 is larger 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.

[0037] As described above, in the multi-core fiber 1 of this embodiment, the outer peripheral surface of the cladding 20 has a first outer peripheral surface 21 that is a part in the circumferential direction and a second outer peripheral surface 22 that is another part in the circumferential direction, and 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, when performing side-view alignment, even when light is incident from the first outer peripheral surface 21 side, spherical aberration can be suppressed compared to when light is incident from a planar portion of the outer peripheral surface of the D-shaped cladding, and when the multi-core fiber 1 is rotated, the amount of change in spherical aberration can be suppressed more than in a multi-core fiber having a D-shaped cladding. Therefore, the multi-core fiber 1 of this embodiment can be easily aligned. Furthermore, because the shape of the outer peripheral surface of the cladding 20 is non-circular, rough alignment can be performed based on the outer shape of the cladding 20 before side-view alignment.

[0038] Furthermore, if the outer surface of the cladding 20 has a rotationally asymmetric shape as in the multi-core fiber 1 of this embodiment, the alignment position can be determined to be one when performing the above-mentioned rough alignment, making alignment easier.

[0039] Moreover, in the multicore fiber 1 of this embodiment, the first outer peripheral surface 21 is a portion of the outer peripheral surface of the cladding 20 that faces the cores 10 arranged on the outer peripheral side. Therefore, the distance between the first outer peripheral surface 21 and the cores 10 that face the first outer peripheral surface 21 is smaller than the distance between the second outer peripheral surface 22 and the cores 10 that face the second outer peripheral surface 22. Here, the closer the distance between the outer peripheral surface of the cores 10 and the outer peripheral surface of the cladding 20, the more likely it is that higher-order modes propagating through the cores 10 will propagate toward the cladding 20. Therefore, higher-order mode light that is unnecessary for communications, etc., that propagates through the cores 10 that face the first outer peripheral surface 21 can be attenuated by the influence of the first outer peripheral surface 21.

[0040] Second Embodiment Next, a second embodiment of the present invention will be described in detail with reference to Fig. 4. Note that components that are the same as or equivalent to those in the first embodiment will be given the same reference numerals and will not be described again unless otherwise specified.

[0041] As shown in Figure 4, the multi-core fiber 1 of this embodiment differs from the multi-core fiber 1 of the first embodiment in that each portion of the outer surface of the cladding 20 facing the core 10 is designated as a first outer surface 21, and the other portion of the outer surface of the cladding 20 is designated as a second outer surface 22.

[0042] In the multicore fiber 1 of this embodiment, each of the portions of the outer surface of the cladding 20 that face the cores 10 is made into the first outer surface 21, and therefore, compared to a multicore fiber having a circular cladding that overlaps with the second outer surface 22, unnecessary higher-order mode light propagating through each core 10 can be attenuated by the influence of the first outer surface 21.

[0043] Third Embodiment Next, a third embodiment of the present invention will be described in detail with reference to Fig. 5. Note that components that are the same as or equivalent to those in the first embodiment will be given the same reference numerals and will not be described again unless otherwise specified.

[0044] As shown in FIG. 5, the multi-core fiber 1 of this embodiment differs from the multi-core fiber 1 of the first embodiment in that the multiple cores 10 are arranged linearly.

[0045] In this embodiment, the cores 10 located at both ends are cores arranged on the outer circumferential side of the clad 20. Furthermore, in this embodiment, the portions of the outer circumferential surface of the clad 20 facing the cores 10 located at both ends are defined as first outer circumferential surfaces 21, and the other portions of the outer circumferential surface of the clad 20 are defined as second outer circumferential surfaces 22. The shape of the outer circumferential surface of the clad 20 may be the same as the shape of the outer circumferential surface of the clad 20 in the first embodiment. In this case, only one of the portions of the outer circumferential surface of the clad 20 facing the cores 10 located at both ends is defined as the first outer circumferential surface 21, and the other portions of the outer circumferential surface of the clad 20 are defined as the second outer circumferential surface 22.

[0046] Although the present invention has been described above using the above-mentioned embodiment as an example, the present invention is not limited thereto. For example, the first outer surface 21 may be provided on the outer surface of the cladding 20 at a location other than the portion facing the core 10.

[0047] Furthermore, the first outer peripheral surface 21 may be provided at some of a plurality of portions on the outer peripheral surface of the cladding 20 that face the cores 10. The multicore fiber 1 of the first embodiment is an example of this form. Furthermore, in the first embodiment, the first outer peripheral surface 21 may be provided at two or three of four portions on the outer peripheral surface of the cladding 20 that face the cores 10. That is, some of the portions on the outer peripheral surface of the cladding 20 that face the cores 10 and are arranged on the outer periphery side may be the first outer peripheral surface 21, and some other of these portions may be the second outer peripheral surface.

[0048] In the first and second embodiments, all of the cores 10 are positioned on a circumference centered on the reference position 20R, but other cores may be positioned inside the circumference. For example, a core may be positioned on the reference position 20R.

[0049] In addition, in the above embodiment, an example has been described in which 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. However, the length of the straight line connecting both ends of the first outer peripheral surface 21 may be smaller than the diameter of the core 10. However, when 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, as in the above embodiment, higher-order mode light unnecessary for communication can be more efficiently absorbed.

[0050] Furthermore, in the above embodiment, when the first outer peripheral surface 21 is viewed along a direction perpendicular to the longitudinal direction of the multicore fiber 1, the cores 10 facing the first outer peripheral surface 21 completely overlap with the first outer peripheral surface 21. However, when viewed in the same manner, it is also possible that a portion of the cores 10 facing the first outer peripheral surface 21 overlaps with the first outer peripheral surface 21, and another portion does not overlap with the first outer peripheral surface 21. However, when the cores 10 facing the first outer peripheral surface 21 completely overlap with the first outer peripheral surface 21, light in higher modes unnecessary for communication can be absorbed more efficiently.

[0051] As described above, according to the present invention, a multi-core fiber that can be easily aligned is provided, and can be used in the field of optical communications and other devices that use multi-core fibers.

Claims

1. a plurality of cores and a cladding surrounding the cores; the outer peripheral surface of the clad has a first outer peripheral surface that is a part in the circumferential direction and a second outer peripheral surface that is another part in the circumferential direction, The first outer peripheral surface bulges outward from the cladding with a larger radius of curvature than the second outer peripheral surface. A multicore fiber characterized by:

2. Two or more of the cores are arranged on the outer circumferential side of the cladding, At least one of the portions of the outer circumferential surface facing the core is the first outer circumferential surface. The multicore fiber according to claim 1 .

3. Each of the portions is the first outer circumferential surface. The multi-core fiber according to claim 2 .

4. The outer peripheral surface of the cladding has a non-rotationally symmetric shape. The multicore fiber according to any one of claims 1 to 3.

5. The radius of curvature of the first outer peripheral surface is 1.5 times or more and 20 times or less than the radius of curvature of the second outer peripheral surface. The multicore fiber according to any one of claims 1 to 3.