Multicore fiber and optical connector

Abstract

An objective of the present invention is to provide a multicore fiber and an optical connector in which phase can be controlled.　A multicore fiber (20) is provided with a plurality of cores (22) and a cladding (21) surrounding the plurality of cores (22), wherein a recessed section (21b) or protruding sections (21c, 21d) are formed on the outer periphery of the cladding (21), and the recessed section (21b) or the protruding sections (21c, 21d) are present in different locations and different phases in the longitudinal direction.

Description

Multicore fiber and optical connectors

[0001] The present invention relates to a multicore fiber and an optical connector. This application claims priority to Japanese Patent Application No. 2024-130786, filed on August 7, 2024, the contents of which are incorporated herein by reference.

[0002] Patent Document 1 discloses a structure for optically coupling a multi-core fiber with a plurality of single-core optical fibers. The multi-core fiber has a plurality of cores and a cladding that surrounds these cores.

[0003] Japanese Patent Application Publication No. 2013-125195

[0004] When the phase of a multi-core fiber changes, the position of the core changes. Here, the phase refers to the angle in the circumferential direction of the multi-core fiber. In order to optically couple the multi-core fiber to another optical fiber, it is necessary to appropriately control the phase of the multi-core fiber. Patent Document 1 does not disclose a method for controlling the phase of a multi-core fiber.

[0005] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a multicore fiber and an optical connector that are capable of controlling the phase.

[0006] A multicore fiber according to aspect 1 of the present invention comprises a plurality of cores and a cladding surrounding the plurality of cores, wherein recesses or protrusions are formed on the outer periphery of the cladding, and the recesses or protrusions are present at different positions and in different phases in the longitudinal direction.

[0007] A second aspect of the present invention is the multicore fiber according to the first aspect, wherein a plurality of recesses or a plurality of protrusions including the recess or protrusion are formed on the outer periphery of the cladding.

[0008] A third aspect of the present invention is the multi-core fiber according to the first aspect, wherein the recessed portion or the protruding portion extends spirally around the outer periphery of the cladding.

[0009] A fourth aspect of the present invention is a multicore fiber according to any one of the first to third aspects, wherein the recess is formed on the outer periphery of the cladding, and in a cross section perpendicular to the longitudinal direction, the distance from the center of the cladding to the recess is greater than the maximum distance from the center of the cladding to the center of each of the plurality of cores.

[0010] A fifth aspect of the present invention is a multicore fiber according to any one of the first to fourth aspects, wherein, in a cross section perpendicular to the longitudinal direction, the centers of the plurality of cores are arranged at equal intervals on a circle centered at the center of the cladding, and in the cross section, a first line connecting a predetermined point on the recess or protrusion and the center of the cladding, and a second line connecting the centers of two circumferentially adjacent cores among the plurality of cores are arranged to be approximately parallel to or approximately perpendicular to each other.

[0011] A sixth aspect of the present invention is a multicore fiber according to the fifth aspect, wherein the plurality of cores are four cores, and in the cross section, the centers of the four cores are located at the four corners of a square, respectively, and the sides of the square are planes that the concave or convex portion has, and extend in a direction that is approximately parallel to or approximately perpendicular to a plane that includes the predetermined point.

[0012] An optical connector according to aspect 7 of the present invention comprises a multi-core fiber according to any one of aspects 1 to 6, a connection end face, and a ferrule having a fiber hole that opens to the connection end face and into which the multi-core fiber is inserted, and an engaging convex portion or engaging concave portion is formed on the inner circumference of the fiber hole, the engaging convex portion or engaging concave portion being engageable with the concave portion or the convex portion.

[0013] According to the above aspects of the present invention, it is possible to provide a multicore fiber and an optical connector capable of controlling the phase.

[0014] 6A is a perspective view of an optical connector equipped with a multicore fiber. FIG. 6B is a perspective view of a multicore fiber of a first embodiment. FIG. 6C is a cross-sectional view corresponding to line IIIa-IIIa in FIG. 1. FIG. 6D is a cross-sectional view corresponding to line IIIb-IIIb in FIG. 1. FIG. 6E is a cross-sectional view corresponding to line IIIc-IIIc in FIG. 1. FIG. 6F is a cross-sectional view of a multicore fiber of a first embodiment. FIG. 6G is a cross-sectional view of a multicore fiber of a second embodiment. FIG. 6H is a cross-sectional view of the optical connector of the second embodiment, in the vicinity of the tip end of the fiber hole. FIG. 6I is a cross-sectional view of the optical connector of the second embodiment, closer to the base end than FIG. 6A. FIG. 6I is a perspective view of a multicore fiber of a third embodiment. FIG. 6H is a cross-sectional view of a multicore fiber of a modified example of the second embodiment. FIG. 6I is a cross-sectional view of a multicore fiber of a modified example of the first embodiment. FIG. 6I is a cross-sectional view of a multicore fiber of a modified example of the second embodiment.

[0015] First Embodiment A multicore fiber and an optical connector according to this embodiment will be described below with reference to the drawings. As shown in Fig. 1, the optical connector 1 includes a ferrule 10, a multicore fiber 20, an adhesive 30, and two positioning pins 40. Note that the optical connector 1 does not necessarily have to include the positioning pins 40.

[0016] The ferrule 10 has a connection end face 10a, a rear end face 10b, a fiber hole 11, an injection hole 12, and two positioning holes 13. The connection end face 10a is a surface that is abutted against another connector or the like when the optical connector 1 is connected to another connector or the like. The fiber hole 11 and the two positioning holes 13 open at the connection end face 10a. An introduction hole (not shown) that communicates with the fiber hole 11 opens at the rear end face 10b. The multi-core fiber 20 is introduced into the ferrule 10 through the introduction hole. A positioning pin 40 is inserted into each of the two positioning holes 13.

[0017] (Direction Definition) In this specification, the direction in which the fiber holes 11 extend is referred to as the longitudinal direction Z. The longitudinal direction Z is also the direction in which the multicore fiber 20 extends (i.e., the longitudinal direction of the multicore fiber 20). A direction perpendicular to the longitudinal direction Z is referred to as the first direction X. The first direction X is also the direction in which the two positioning holes 13 are arranged side by side. A direction perpendicular to both the longitudinal direction Z and the first direction X is referred to as the second direction Y. The direction from the rear end face 10b of the ferrule 10 toward the connection end face 10a along the longitudinal direction Z is referred to as the +Z direction, the front, or the tip side. The direction opposite to the +Z direction is referred to as the -Z direction, the rear, or the base end side. The direction perpendicular to the central axis O of the fiber hole 11 when viewed from the longitudinal direction Z is referred to as the radial direction. The direction along the radial direction approaching the central axis O is referred to as the radially inner direction, and the direction away from the central axis O is referred to as the radially outer direction. The direction going around the central axis O as viewed from the longitudinal direction Z is referred to as the circumferential direction. A cross section perpendicular to the longitudinal direction Z is referred to as a transverse cross section. In other words, a transverse cross section is a cross section extending along the first direction X and the second direction Y.

[0018] In the connection end face 10a, the fiber hole 11 is arranged so as to be sandwiched between two positioning holes 13. The ferrule 10 may have a plurality of fiber holes 11. The injection hole 12 opens to one end face of the ferrule 10 facing the second direction Y. The injection hole 12 communicates with the internal space of the ferrule 10 and the fiber holes 11. When the optical connector 1 is assembled, an adhesive 30 is injected into the ferrule 10 through the injection hole 12. The injected adhesive 30 also enters the inside of the fiber holes 11. The adhesive 30 can suppress rotation of the multicore fiber 20 relative to the ferrule 10 and a change in phase. However, because the phase of the multicore fiber 20 can be controlled by a method described below, the adhesive 30 may be unnecessary.

[0019] As shown in Fig. 2, the multicore fiber 20 has a cladding 21, a plurality of cores 22, a marker 23, and a coating 24. The core 22 is also called a communication core. The core 22 is used to propagate an optical signal for communication. The refractive index of the core 22 is higher than the refractive index of the cladding 21. The core 22 is formed of, for example, silica glass doped with a dopant (such as germanium) that increases the refractive index. The cladding 21 is formed of, for example, pure silica glass.

[0020] In this embodiment, the core 22 operates in a single mode in the optical communication wavelength band. For example, the core 22 has a cutoff wavelength of 1.26 μm and a mode field diameter of 8.6 μm at a wavelength of 1.31 μm.

[0021] The markers 23 are also called marker cores. The markers 23 are used to recognize the phase of the multicore fiber 20. It is desirable that the refractive index of the markers 23 is smaller than the refractive index of the cores 22. The type or amount of dopant added may be different between the cores 22 and the markers 23. The markers 23 are formed at positions that break the symmetry of the multiple cores 22. This makes it easier to understand the phase of the multicore fiber 20. Although the number of cores 22 is four in FIG. 2 and other figures, the number of cores 22 may be changed.

[0022] A recess 21b is formed in the outer peripheral surface 21a of the cladding 21. The recess 21b is a portion recessed radially inward relative to the outer peripheral surface 21a. Typically, the cladding of an optical fiber has a cylindrical shape with a substantially uniform radius. The recess 21b is formed by partially removing the surface of this cylinder. More specifically, the recess 21b may be formed by partially removing the cylindrical cladding 21 by an etching process. Alternatively, the recess 21b may be formed by cutting or the like.

[0023] As a specific example of the etching step, for example, the multi-core fiber 20 may be immersed in an etching solution. When the clad 21 is made of quartz glass, hydrofluoric acid or buffered hydrofluoric acid (BHF) may be used as the etching solution. To form the recesses 21b, the outer peripheral surface 21a of the clad 21 excluding the recesses 21b may be masked, and the multi-core fiber 20 may be immersed in the etching solution.

[0024] The coating 24 is provided on the outer periphery of the clad 21. The material of the coating 24 is a resin having a higher refractive index than the clad 21. That is, the refractive index of the coating 24 is higher than the refractive index of the clad 21. This makes it easier for the coating 24 to absorb light leaking from the cores 22, thereby reducing crosstalk in the multicore fiber 20. Furthermore, the coating 24 is not provided at the front end of the multicore fiber 20, and the clad 21 is exposed. The recess 21b is provided in a portion of the clad 21 that is exposed from the coating 24.

[0025] When the coating 24 is provided on the outer periphery of the clad 21, a coating removal step may be performed before the etching step. In the coating removal step, a portion of the coating is removed. The method for removing the portion of the coating is not particularly limited, and for example, laser processing or cutting processing may be used. After the coating removal step, in the etching step, the multi-core fiber 20 together with the remaining coating is immersed in an etching solution to etch the clad 21. At this time, the etching solution also penetrates into the portion where the coating remains, and the clad 21 is etched. However, etching progresses faster in the portion where the coating has been removed than in the portion where the coating remains. Therefore, the outer shape of the clad 21 is etched to correspond to the shape after the coating has been removed in the coating removal step.

[0026] In this embodiment, each recess 21b is formed spirally along the longitudinal direction Z. Therefore, the phase (circumferential position) of the recess 21b changes in the longitudinal direction Z. In other words, the recess 21b exists at different positions in the longitudinal direction Z. FIG. 3A is a cross-sectional view taken along the arrows IIIa-IIIa in FIG. 1. Similarly, FIG. 3B is a cross-sectional view taken along the arrows IIIb-IIIb in FIG. 1, and FIG. 3C is a cross-sectional view taken along the arrows IIIc-IIIc in FIG. 1. The line IIIa-IIIa is located near the tip (end on the +Z side) of the ferrule 10. The line IIIb-IIIb is closer to the base end (the -Z side) than the line IIIa-IIIa, and the line IIIc-IIIc is further closer to the base end. When the recess 21b is formed by etching, the coating may be removed spirally in the coating removal step described above.

[0027] As shown in FIG. 3A , a convex portion 11a (engaging convex portion) is formed at the tip end of the fiber hole 11. The convex portion 11a protrudes radially inward from the inner circumferential surface of the fiber hole 11. As shown in FIGS. 3B and 3C , no convex portion 11a is formed at any position other than the tip end of the fiber hole 11. The convex portion 11a engages with a concave portion 21b of the multi-core fiber 20. More specifically, when the multi-core fiber 20 is inserted into the fiber hole 11 from the −Z side and protrudes from the splicing end face 10a toward the +Z side, the phases of the concave portion 21b and the convex portion 11a match. Furthermore, when the multi-core fiber 20 is attempted to protrude toward the +Z side, a rotational force is applied to the multi-core fiber 20 with the convex portion 11a as the base point. The rotational force is generated because the concave portion 21b is formed in a spiral shape.

[0028] According to the present embodiment, it is possible to control the phase of the multi-core fiber 20 relative to the ferrule 10 by using the recess 21b and the protrusion 11a. Moreover, it is also possible to rotate the multi-core fiber 20 and adjust the phase by pressing the multi-core fiber 20 toward the +Z side relative to the ferrule 10. After adjusting the phase of the multi-core fiber 20, the adhesive 30 may be filled into the ferrule 10. In this case, it is possible to prevent the adjusted phase of the multi-core fiber 20 from changing subsequently.

[0029] In cross section, the cladding 21 has a circular shape centered on a center O1, except for the portion where the recess 21b is provided. The center O1 of the cladding 21 coincides with the central axis O of the fiber hole 11. The multiple cores 22 are arranged such that the centers O2 of the multiple cores 22 are located on an imaginary circle C centered on the center O1 of the cladding 21. The centers O2 of the multiple cores 22 are arranged on the imaginary circle C at equal intervals in the circumferential direction.

[0030] In the cross section of the ferrule 10 near the tip shown in FIG. 3A , a first line L1 connecting a predetermined point P of the recess 21b and the center O1 of the cladding 21, and second lines L2 and L3 connecting the centers O2 of two circumferentially adjacent cores 22 among the multiple cores 22, are arranged to be substantially parallel or substantially perpendicular to each other. Specifically, in the cross section, the first line L1 and the second line L2 are arranged to be substantially parallel to each other, and the first line L1 and the second line L3 are arranged to be substantially perpendicular to each other. In this embodiment, the recess 21b has a plane 21b1 in the cross section. The predetermined point P is the midpoint of the plane 21b1 of the recess 21b in the cross section. In this specification, "substantially parallel" means that the angle between the two lines is within 0 degrees ±1 degree, and "substantially perpendicular" means that the angle between the two lines is within 90 degrees ±1 degree. More specifically, in this embodiment, the number of cores 22 is four. The centers O2 of the four cores 22 are arranged at equal intervals on an imaginary circle C, i.e., located at the four corners of an imaginary square S. In a cross section near the tip of the ferrule 10, the recesses 21b are provided so that the sides of the imaginary square S extend in a direction substantially parallel to or substantially perpendicular to the plane 21b1. With this configuration, the arrangement of the cores 22 on the connection end face 10a can be made constant, and the recesses 21b can be used to more accurately control the phase relative to the ferrule 10.

[0031] Dimensions of the multicore fiber 20 will be described with reference to FIG. 4 . In the cross section, the distance D1 from the center O1 of the cladding 21 to the recess 21b is preferably greater than the maximum distance D2 among the distances from the center O1 of the cladding 21 to the center O2 of each of the multiple cores 22. In this embodiment, the distances from the center O1 of the cladding 21 to the center O2 of each of the multiple cores 22 are equal. In addition, in the cross section, the distance D3 from the center O2 of the core 22 closest to the recess 21b to the recess 21b is preferably three times or more the half of the mode field diameter of the core 22 in the communication wavelength band (i.e., the radius of the mode field). In this embodiment, the phase (circumferential position) of the recess 21b changes in the longitudinal direction Z, and therefore the distance D3 also changes in the longitudinal direction Z (see FIGS. 3B and 3C ). At any position in the longitudinal direction Z, the distance D3 is preferably three times or more the half of the mode field diameter of the core 22 in the communication wavelength band. By setting the dimensions of the multi-core fiber 20 as described above, the transmission loss of the cores 22 can be reduced.

[0032] Second Embodiment Next, a second embodiment of the present invention will be described, which has the same basic configuration as the first embodiment. Therefore, the same components are denoted by the same reference numerals, and their description will be omitted, and only the differences will be described.

[0033] 5, in the multi-core fiber 20 of this embodiment, a plurality of convex portions 21c, 21d are formed on the outer peripheral surface 21a of the cladding 21. The first convex portion 21c is located at the tip of the multi-core fiber 20. The second convex portion 21d is located closer to the base end (-Z side) than the first convex portion 21c. The convex portions 21c, 21d are located at different positions in the circumferential direction.

[0034] 6A and 6B are cross-sectional views of the optical connector 1 of this embodiment cut at different positions in the longitudinal direction Z. FIG. 6A is a cross-sectional view of the vicinity of the tip of the fiber hole 11, and FIG. 6B is a cross-sectional view of the base end side (-Z side) of FIG. 6A. As shown in FIG. 6A, a first recess 11b (engagement recess) is formed in the ferrule 10 of this embodiment. The recess 11b is recessed radially outward from the inner circumferential surface of the fiber hole 11. A first protrusion 21c of the multi-core fiber 20 is located inside the recess 11b. The recess 11b engages with the first protrusion 21c.

[0035] 6B , a second recess 11c (engagement recess) is formed in the ferrule 10. The second recess 11c is disposed at a different position and in a different phase from the first recess 11b in the longitudinal direction Z. A second protrusion 21d of the multi-core fiber 20 is located inside the second recess 11c. The gap between the recess 11c and the protrusion 21d is larger than the gap between the recess 11b and the protrusion 21c.

[0036] According to this embodiment, the two recesses 11b, 11c and the two protrusions 21c, 21d can control the phase of the multi-core fiber 20 with respect to the ferrule 10. Furthermore, the protrusion 21d and the recess 11c with a large gap can roughly adjust the phase of the multi-core fiber 20 at a position far from the splicing end face 10a, and the protrusion 21c and the recess 11b can highly accurately control the phase of the multi-core fiber 20 at a position close to the splicing end face 10a.

[0037] The protrusions 21c and 21d of this embodiment can be formed by etching, cutting, or the like, similarly to the first embodiment. When etching is used, in the coating removal step, the coating may be left on the portions corresponding to the protrusions 21c and 21d, and the coating on the remaining portions may be removed.

[0038] In this embodiment, the cladding 21 has a circular cross section centered at the center O1, except for the portions where the protrusions 21c and 21d are provided. The cores 22 are arranged such that the centers O2 of the cores 22 are located on an imaginary circle C centered at the center O1 of the cladding 21. The centers O2 of the cores 22 are arranged at equal intervals in the circumferential direction on the imaginary circle C.

[0039] In addition, in the cross section near the tip of the ferrule 10 shown in FIG. 6A , a first line L1 connecting a predetermined point P of the protrusion 21c and the center O1 of the cladding 21, and second lines L2 and L3 connecting the centers O2 of two circumferentially adjacent cores 22 among the multiple cores 22, are arranged to be approximately parallel or approximately perpendicular to each other. In this embodiment, in the cross section, the protrusion 21c has a plane 21c1. The predetermined point P is the midpoint of the plane 21c1 of the protrusion 21c in the cross section. More specifically, in the cross section near the tip of the ferrule 10, the protrusion 21c is provided so that the sides of the imaginary square S extend in a direction approximately parallel to or approximately perpendicular to the plane 21c1. With this configuration, the arrangement of the cores 22 at the connection end face 10a can be made constant, and the protrusion 21c can be used to more accurately control the phase relative to the ferrule 10.

[0040] Third Embodiment Next, a third embodiment of the present invention will be described, which has the same basic configuration as the first embodiment. Therefore, the same components are denoted by the same reference numerals, and the description thereof will be omitted, and only the differences will be described.

[0041] As shown in Fig. 7 , the multi-core fiber 20 of this embodiment has a plurality of recesses 21b. The recesses 21b are arranged at different positions and at different phases in the longitudinal direction Z. Although not shown, a plurality of protrusions 11a (see Fig. 3A ) are formed in the fiber hole 11 of the ferrule 10 at positions corresponding to the recesses 21b. By engaging the plurality of recesses 21b and the plurality of protrusions 11a, the phase of the multi-core fiber 20 with respect to the ferrule 10 can be controlled. The gap between the recess 21b located on the most distal end side (+Z side) and the protrusion 11a may be smaller than the gap between the recess 21b located on the more proximal end side (-Z side) and the protrusion 11a.

[0042] As described above, the multi-core fiber 20 according to the present disclosure includes a plurality of cores 22 and a clad 21 surrounding the plurality of cores 22, and the concave portions 21b or the convex portions 21c, 21d are formed on the outer periphery of the clad 21. In the first embodiment, the concave portion 21b is spiral-shaped and exists at different positions and in different phases in the longitudinal direction Z. In the second embodiment, the convex portions 21c, 21d exist at different positions and in different phases in the longitudinal direction Z. In the third embodiment, the concave portions 21b exist at different positions and in different phases in the longitudinal direction Z. Furthermore, the optical connector 1 according to the present disclosure includes the multi-core fiber 20, and a ferrule 10 having a connection end face 10a and a fiber hole 11 that opens to the connection end face 10a and into which the multi-core fiber 20 is inserted. The inner periphery of the fiber hole 11 is formed with convex portions 11a (engaging convex portions) or concave portions 11b (engaging concave portions) that can engage with the concave portions 21b or the convex portions 21c. According to such a multi-core fiber 20 and optical connector 1, it is possible to control the phase relative to the ferrule 10 by using the recess 21b or the protrusions 21c and 21d.

[0043] Moreover, in the second and third embodiments, a plurality of recesses 21b or a plurality of protrusions 21c, 21d are formed on the outer periphery of the cladding 21. In this case, by varying the size of the gap between the multicore fiber 20 and the ferrule 10, it is possible to roughly adjust the phase of the multicore fiber 20 at a position far from the splicing end face 10a, and to highly accurately control the phase of the multicore fiber 20 at a position close to the splicing end face 10a.

[0044] Furthermore, the recess 21b according to the first embodiment extends in a spiral shape on the outer periphery of the cladding 21. Therefore, by engaging the protrusion 11a of the ferrule 10 with the recess 21b, it is possible to rotate the multi-core fiber 20 and adjust the phase. Note that instead of the multiple protrusions 21c and 21d as shown in Fig. 5, a single protrusion 21c may be formed in a spiral shape. In this case, too, it is possible to rotate the multi-core fiber 20 by engaging the spiral protrusion 21c (not shown) with the recess 11b of the ferrule 10 (see Fig. 6A ).

[0045] In the first embodiment, in the cross section, the distance D1 from the center O1 of the cladding 21 to the recess 21 b is greater than the maximum distance D2 among the distances from the center O1 of the cladding 21 to the center O2 of each of the multiple cores 22. This configuration can reduce the transmission loss of the cores 22.

[0046] In addition, in the cross section, the centers O2 of the multiple cores 22 are arranged at equal intervals on an imaginary circle C centered on the center O1 of the cladding 21, and a first line L1 connecting a predetermined point P of the recessed portion 21 b or the protruding portion 21 c to the center O1 of the cladding 21 and a second line L2 connecting the centers O2 of two circumferentially adjacent cores 22 among the multiple cores 22 are arranged so as to be substantially parallel to or substantially perpendicular to each other. Specifically, the multiple cores 22 are four cores 22, and in the cross section, the centers O2 of the four cores 22 are located at four corners of an imaginary square S, respectively. The sides of the imaginary square S may be planes 21 b1 and 21 c1 of the recessed portion 21 b or the protruding portion 21 c, and may extend in a direction substantially parallel to or substantially perpendicular to the planes 21 b1 and 21 c1 including the predetermined point P. With this configuration, the phase with respect to the ferrule 10 can be controlled with higher precision using the recessed portion 21 b or the protruding portion 21 c.

[0047] Furthermore, an example of a manufacturing method of the multi-core fiber 20 may include an etching step of immersing the multi-core fiber 20 in an etching solution. In the etching step, by removing a part of the outer periphery of the cladding 21, it is possible to form the recesses 21 b or the protrusions 21 c, 21 d at different positions and in different phases in the longitudinal direction Z.

[0048] In the manufacturing method of the multicore fiber 20, a coating removal step may be performed before the etching step to remove a part of the coating provided on the outer periphery of the cladding 21. In this case, in the etching step, the recess 21b or the protrusions 21c and 21d can be formed due to the difference in etching rate between the part where the coating has been removed and the part where the coating remains.

[0049] The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0050] For example, in the manufacturing method of the multi-core fiber 20 in the above embodiment, so-called wet etching using an etching solution has been described. However, dry etching may also be employed. Dry etching is a method of etching the cladding 21 using a gas. Even when dry etching is employed in the etching step, a shape corresponding to the removed coating can be formed on the outer periphery of the cladding 21 by performing a coating removal step in advance.

[0051] Although the number of cores 22 in the above embodiment is four, the present invention is not limited to this. For example, as shown in FIG. 8 , the number of cores 22 may be two. In this case, too, the centers O2 of the two cores 22 are arranged on an imaginary circle C at equal intervals in the circumferential direction. In addition, in a cross section near the tip of the ferrule 10, a first line L1 connecting a predetermined point P (the midpoint of the plane 21c1) of the protruding portion 21c and the center O1 of the cladding 21 and a second line L2 connecting the centers O2 of the two cores 22 are arranged substantially parallel to each other. The second line L2 is arranged to pass through the center O1 of the cladding 21. The first line L1 and the second line L2 may be arranged substantially perpendicular to each other.

[0052] The arrangement of the core 22 is not limited to the above example. For example, the center O2 of the core 22 may be arranged at a position other than on the imaginary circle C.

[0053] The shapes of the recess 21b and the protrusions 21c, 21d are not limited to those of the above embodiment. For example, as shown in FIG. 9 , the recess 21b may be V-shaped in cross section. In this case, the predetermined point P is located at the apex of the V-shaped recess 21b. In a cross section near the tip of the ferrule 10, a first straight line L1 connecting the predetermined point P of the recess 21b to the center O1 of the cladding 21 and second straight lines L2, L3 connecting the centers O2 of two circumferentially adjacent cores 22 among the multiple cores 22 are arranged to be substantially parallel or substantially perpendicular to each other. Furthermore, as shown in FIG. 10 , the protrusions 21c, 21d may be arc-shaped in cross section. In this case, the predetermined point P is located at the apex of the arc-shaped protrusion 21c. In a cross section near the tip of the ferrule 10, a first straight line L1 connecting a predetermined point P of the protrusion 21c and the center O1 of the cladding 21 and second straight lines L2, L3 connecting the centers O2 of two circumferentially adjacent cores 22 among the multiple cores 22 are arranged to be substantially parallel or substantially perpendicular to each other. Furthermore, in the cross section, the recess 21b may be arc-shaped. In the cross section, the protrusions 21c, 21d may be V-shaped.

[0054] In addition, it is possible to replace the components in the above-described embodiments with well-known components as appropriate, and the above-described embodiments and variations may be combined as appropriate, without departing from the spirit of the present invention.

[0055] DESCRIPTION OF SYMBOLS 1... Optical connector 10... Ferrule 10a... Connection end face 11... Fiber hole 11a... Convex portion (engaging convex portion) 11b... Concave portion (engaging concave portion) 20... Multi-core fiber 21... Cladding 21b... Concave portion 21b1... Concave surface 21c... Convex portion 21c1... Convex surface 22... Core O1... Center of cladding O2... Center of core Z... Longitudinal direction

Claims

1. A multicore fiber comprising: a plurality of cores; and a cladding surrounding the plurality of cores, wherein recesses or protrusions are formed on the outer periphery of the cladding, and the recesses or protrusions are present at different positions and in different phases in the longitudinal direction.

2. A multi-core fiber according to claim 1, wherein a plurality of recesses or a plurality of protrusions, including the recess or protrusion, are formed on the outer periphery of the cladding.

3. The multi-core fiber according to claim 1, wherein the recessed portion or the protruding portion extends spirally around the outer periphery of the cladding.

4. A multicore fiber according to any one of claims 1 to 3, wherein the recess is formed on the outer periphery of the cladding, and in a cross section perpendicular to the longitudinal direction, the distance from the center of the cladding to the recess is greater than the maximum distance from the center of the cladding to the center of each of the plurality of cores.

5. A multicore fiber according to any one of claims 1 to 4, wherein, in a cross section perpendicular to the longitudinal direction, the centers of the multiple cores are arranged at equal intervals on a circle whose center is the center of the cladding, and in the cross section, a first line connecting a predetermined point on the recess or protrusion and the center of the cladding, and a second line connecting the centers of two circumferentially adjacent cores among the multiple cores, are arranged so as to be approximately parallel to or approximately perpendicular to each other.

6. The multicore fiber according to claim 5, wherein the plurality of cores is four cores, the centers of the four cores are located at the four corners of a square in the cross section, and the sides of the square are planes that the recessed or protruding portion has, and extend in a direction that is approximately parallel to or approximately perpendicular to a plane that includes the predetermined point.

7. An optical connector comprising: a multi-core fiber according to any one of claims 1 to 6; and a ferrule having a connection end face and a fiber hole that opens into said connection end face and into which said multi-core fiber is inserted, wherein an engaging convex portion or engaging concave portion that can engage with said concave portion or said convex portion is formed on the inner periphery of said fiber hole.

Images