Multicore Fiber

Abstract

To provide a multicore fiber which can reduce a stress generated in a core by bending.SOLUTION: A multicore fiber 1 includes a core 11 and a plurality of single core fibers (10A and 10B) having a clad 12 surrounding an outer peripheral surface of the core 11, and an inside coating layer 30 formed of a resin surrounding the outer peripheral surface of the clad 12 of the single core fibers (10A and 10B), wherein at least the one single core fiber 10B is spirally arranged so as to rotate around a central axis 30ca of the inside coating layer 30.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a multi-core fiber.

Background Art

[0002] Currently, the optical fibers used in generally popular optical fiber communication systems have a structure in which the outer periphery of a single core is surrounded by a cladding, and information is transmitted by the propagation of an optical signal through this core. And in recent years, with the popularization of optical fiber communication systems, the amount of information transmitted has increased dramatically.

[0003] In order to realize an increase in the transmission capacity of such optical fiber communication systems, as described in Patent Document 1 below, a multi-core fiber in which the outer peripheries of a plurality of cores are surrounded by a single cladding is used, and a plurality of signals are transmitted by the light propagating through each core. It is known.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Such a multi-core fiber has cores located outside the central axis of the cladding in a cross-section perpendicular to the longitudinal direction. Therefore, when the multi-core fiber is bent, compressive stress occurs in the core located inside the central axis at the bent portion, and tensile stress occurs in the core located outside the central axis. Since the refractive index of the core where compressive stress occurs tends to increase and the refractive index of the core where tensile stress occurs tends to decrease, for example, there is a risk of skew. Therefore, there is a demand to reduce the stress generated in the core. To meet this demand, for example, it is conceivable to form a multi-core fiber by surrounding the outer peripheral surfaces of a plurality of single-core fibers each having a core and a cladding surrounding the outer peripheral surface of the core with a coating layer containing resin to integrate the plurality of single-core fibers. In such a configuration, since a coating layer containing resin is interposed between the plurality of single-core fibers, the coating layer buffers when bent, and the stress generated in the core of the single-core fiber can be reduced. However, even in such a configuration, compressive stress occurs in the core of the single-core fiber located inside the central axis at the bent portion, and tensile stress occurs in the core of the single-core fiber located outside the central axis. Therefore, there is a demand to further reduce the stress generated in the core by bending.

[0006] Therefore, an object of the present invention is to provide a multi-core fiber capable of reducing the stress generated in the core by bending.

Means for Solving the Problem

[0007] To achieve the above object, the multi-core fiber of the present invention includes a plurality of single-core fibers each having a core and a cladding surrounding the outer peripheral surface of the core, and a coating layer containing resin surrounding the outer peripheral surface of the cladding of each of the single-core fibers, and at least one of the single-core fibers is arranged in a spiral shape so as to rotate around the central axis of the coating layer.

[0008] In this multi-core fiber, in the bent portion when the multi-core fiber is bent, the spiral single-core fiber has an inner portion located inside the central axis of the coating layer and an outer portion located outside, and the inner portion and the outer portion can be alternately present along the longitudinal direction. As described above, the spiral single-core fiber is surrounded by a coating layer containing resin. Therefore, the coating layer is deformed by the compressive stress generated in the inner portion and the tensile stress generated in the outer portion adjacent to the inner portion, and the compressive stress in the inner portion and the tensile stress in the outer portion can weaken each other. Therefore, compared with the case where the single-core fiber is linearly arranged along the central axis direction, the stress generated in the core of the single-core fiber due to bending can be reduced.

[0009] Also, the pitch at which the spiral single-core fiber rotates around the central axis may be set to be 0.1 turns / m or more on average.

[0010] At least one of the single-core fibers may be spaced apart from the other single-core fibers.

[0011] When the single-core fibers have a portion where they are in contact with each other, no coating layer is interposed between the single-core fibers at that portion. Therefore, according to this multi-core fiber, compared with the case where each single-core fiber has a portion in contact with another single-core fiber, the stress generated in at least one single-core fiber when bent can be reduced by the buffering of the coating layer, and the stress generated in the core of the at least one single-core fiber can be reduced.

[0012] All of the plurality of single-core fibers may be spirally arranged so as to rotate around the central axis.

[0013] With such a configuration, the stress generated by bending can be reduced for the cores of all the single-core fibers.

[0014] Alternatively, at least one of the single-core fibers may be arranged along the central axis.

[0015] The total length of the single-core fiber arranged along the central axis is shorter than the total length of the helical single-core fiber. Therefore, for example, an optical signal propagating through the core of a specific single-core fiber can be used as a header signal to demodulate an optical signal propagating through the core of the helical single-core fiber.

[0016] In this case, one of the single-core fibers may be located on the central axis.

[0017] With such a configuration, compared with the case where the single-core fiber along the central axis is not located on the central axis, the stress generated in the core of the single-core fiber along the central axis when bent can be reduced, and the loss associated with the stress of the light propagating through the core of the single-core fiber can be reduced. Also, this single-core fiber tends to have less stress generated in the core when bent compared to the helical single-core fiber. Therefore, the loss associated with the stress of the light propagating through the core of this single-core fiber can be made smaller than the loss associated with the stress of the light propagating through the core of the helical single-core fiber.

[0018] The direction in which the helical single-core fiber rotates around the central axis may periodically reverse along the longitudinal direction, and the outer diameter of the helical single-core fiber may alternately repeat increasing and decreasing in a period synchronized with the period in which the rotation of the helical single-core fiber reverses.

[0019] The direction in which the helical single-core fiber rotates around the central axis may periodically reverse along the longitudinal direction, and the distance between the helical single-core fiber and the central axis may alternately repeat increasing and decreasing in a period synchronized with the period of the reversal.

Advantages of the Invention

[0020] As described above, according to the present invention, there is provided a multi-core fiber capable of reducing the stress generated in the core by bending.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0022] Hereinafter, embodiments for implementing the multi-core fiber according to the present invention will be illustrated together with the accompanying drawings. The embodiments illustrated below are for facilitating the understanding of the present invention and are not for limiting the interpretation of the present invention. The present invention can be changed and improved from the following embodiments without departing from the gist thereof. In the drawings referred to below, the dimensions of each member may be changed for easy understanding.

[0023] FIG. 1 is a diagram schematically showing a cross-sectional view perpendicular to the longitudinal direction of a multi-core fiber according to an embodiment of the present invention. As shown in FIG. 1, in the present embodiment, the multi-core fiber 1 mainly includes a plurality of single-core fibers (10A, 10B), an inner coating layer 30 as a coating layer, and an outer coating layer 40. Each of the plurality of single-core fibers (10A, 10B) has a core 11 and a cladding 12 surrounding the outer peripheral surface of the core 11. The outer shape of the core 11 and the outer shape of the cladding 12 of the single-core fibers (10A, 10B) in a cross-section perpendicular to the longitudinal direction are circular, and the core 11 is arranged at the center of the cladding 12. In the present embodiment, the outer diameters of the plurality of single-core fibers (10A, 10B) are substantially the same, for example, 80 μm. Note that the outer diameters of the single-core fibers 10A and 10B are not particularly limited, and may be, for example, 60 μm, 100 μm, or 125 μm. Also, the outer diameters of at least two single-core fibers (10A, 10B) may be different from each other. Further, the outer shape of the core 11 in a cross-section perpendicular to the longitudinal direction is not limited to a circular shape, and the outer shape of the cladding 12 in a cross-section perpendicular to the longitudinal direction is not limited to a circular shape. Also, the single-core fiber (10A, 10B) only needs to have a core 11 and a cladding 12, and for example, the cladding 12 may have a multilayer structure.

[0024] The refractive index of the core 11 is made higher than the refractive index of the cladding 12. In the present embodiment, the core 11 is made of silica glass without any additives, and the cladding 12 is made of silica glass to which a dopant such as fluorine (F) that lowers the refractive index is added. Note that the core 11 may be made of silica glass to which a dopant such as germanium (Ge) that increases the refractive index is added, and the cladding 12 may be made of silica glass without any additives. Also, the core 11 may be made of silica glass to which a dopant that increases the refractive index is added, and the cladding 12 may be made of silica glass to which a dopant that lowers the refractive index is added. Also, the dopants that increase the refractive index and the dopants that lower the refractive index are not particularly limited.

[0025] The inner coating layer 30 fills the space between the respective single-core fibers (10A, 10B) and surrounds the outer peripheral surface of the cladding 12 of each single-core fiber (10A, 10B). Further, the outer coating layer 40 covers the outer peripheral surface of the inner coating layer 30. In the present embodiment, the outer shapes of the inner coating layer 30 and the outer coating layer 40 in a cross section perpendicular to the longitudinal direction are circular, and the outer diameter of the outer coating layer 40 is, for example, 250 μm. Note that the outer diameter of the outer coating layer 40 is not particularly limited and may be larger or smaller than 250 μm. Also, the outer shapes of the inner coating layer 30 and the outer coating layer 40 in a cross section perpendicular to the longitudinal direction may be elliptical, polygonal, or the like. Further, the outer coating layer 40 may have a multilayer structure, and the multi-core fiber 1 may not include the outer coating layer 40.

[0026] The inner coating layer 30 is made of a material containing resin, and examples of the resin include ultraviolet curable resin and thermosetting resin. The material constituting the inner coating layer 30 may contain, in addition to the resin, for example, an inorganic substance as a filler. The Young's modulus of the inner coating layer 30 is smaller than the Young's modulus of the cladding 12 of the single-core fiber (10A, 10B), and is, for example, 0.1 MPa or more and 10 MPa or less.

[0027] The outer coating layer 40 is made of a material containing a resin different from the resin constituting the inner coating layer 30, and examples of the resin include ultraviolet curable resin and thermosetting resin. The material constituting the outer coating layer 40 may contain, in addition to the resin, for example, an inorganic substance as a filler. The Young's modulus of the outer coating layer 40 is smaller than the Young's modulus of the cladding 12 of the single-core fiber (10A, 10B), and the Young's modulus is, for example, larger than the Young's modulus of the inner coating layer 30 described above. For example, the Young's modulus of the outer coating layer 40 is 100 MPa or more and 2000 MPa.

[0028] In this embodiment, the number of single-core fibers is seven. One single-core fiber 10A is arranged at the center of the inner coating layer 30, and the other six single-core fibers 10B are arranged at equal intervals along the outer periphery of the inner coating layer 30. The central single-core fiber 10A and each of the single-core fibers 10B on the outer peripheral side are arranged in a triangular lattice. Therefore, the center-to-center distance between adjacent single-core fibers 10A and 10B is equal to each other, and the plurality of single-core fibers (10A, 10B) are arranged at positions that are rotationally symmetric with respect to the central axis 30ca of the inner coating layer 30.

[0029] Next, the arrangement of the plurality of single-core fibers (10A, 10B) in the longitudinal direction of the multi-core fiber 1 will be described.

[0030] FIG. 2 is a diagram schematically showing the states of the single-core fibers 10A and 10B in the longitudinal direction of the multi-core fiber 1 in FIG. 1. In FIG. 2, for ease of understanding, the inner coating layer 30 is shown by a dashed line, the outer coating layer 40 is omitted, and the scales in the longitudinal direction and the radial direction of the multi-core fiber 1 are described with changes from the actual multi-core fiber.

[0031] As shown in FIG. 2, the central single-core fiber 10A is arranged along the central axis 30ca of the inner coating layer 30 and is located on the central axis 30ca. However, the single-core fiber 10A may be separated from the central axis 30ca without being located on the central axis 30ca. The single-core fibers 10B on the outer peripheral side are spirally arranged around the central axis 30ca of the inner coating layer 30 so as to rotate in the same direction as each other. In FIG. 2, the single-core fibers 10B on the outer peripheral side are arranged to rotate counterclockwise along the direction of arrow A. In such a multi-core fiber 1, the single-core fibers 10B are solidified in a spiral state, and a permanent twist is imparted to the core 11 of the single-core fibers 10B. Note that the imparting of this permanent twist means that a permanent twist is imparted in the state where the inner coating layer 30 is present.

[0032] In FIG. 2, the spiral single-core fiber 10B rotates around the central axis 30ca of the inner coating layer 30 at the same pitch, and the number of rotations of the single-core fiber 10B per unit length of the multi-core fiber 1 is substantially constant. However, the spiral single-core fiber 10B may have a section where the pitch of rotation around the central axis 30ca changes. For example, in a certain predetermined section, the pitch of rotation of the spiral single-core fiber 10B may be 1 rotation / m, and in another section, it may be 0.5 rotation / m, and in still another section, it may rotate at another pitch. Also, the pitch of the spiral single-core fiber 10B may constantly change.

[0033] Also, although not particularly shown, the direction in which the spiral single-core fiber 10B rotates around the central axis 30ca is not constant and may periodically reverse or non-periodically reverse along the longitudinal direction. That is, the spiral single-core fiber 10B may be arranged to rotate clockwise in a predetermined section and counterclockwise in a section adjacent to the predetermined section. In this specification, the periodic reversal of the rotation direction means that a specific section composed of a clockwise rotation section where the spiral single-core fiber 10B rotates clockwise and a counterclockwise rotation section where the spiral single-core fiber 10B rotates counterclockwise adjacent to the clockwise rotation section is continuously repeated. For example, in a 1m section of the multi-core fiber 1, it rotates once clockwise, in the subsequent 0.5m section, it rotates once counterclockwise, in the subsequent 1m section, it rotates once clockwise, and in the subsequent 0.5m section, it rotates once counterclockwise, which is included in the periodic reversal.

[0034] Note that the single-core fiber 10B preferably rotates around the central axis 30ca of the inner coating layer 30 at a pitch of 0.1 rotation / m or more on average, and more preferably at a pitch of 1 rotation / m or more on average. Also, in each section of right or left rotation, it is preferable that the fiber rotates 0.5 rotations or more. When the single-core fiber 10B has a right-rotation section and a left-rotation section, this pitch is calculated by adding together the number of rotations in the clockwise direction and the number of rotations in the counterclockwise direction, both of which are taken as positive. For example, when the helical single-core fiber 10B rotates once clockwise around the central axis 30ca in a 0.5 m section of the multi-core fiber 1 and then rotates once counterclockwise in the subsequent 0.5 m section, the number of rotations per meter is 1 + 1 = 2 rotations, and the pitch of the rotation of the single-core fiber 10B in this case is 2 rotations / m.

[0035] Also, although not particularly shown, the outer diameter of the single-core fiber (10A, 10B) may vary in the longitudinal direction of the multi-core fiber 1. Further, when the direction in which the helical single-core fiber 10B rotates around the central axis 30ca periodically reverses along the longitudinal direction, this outer diameter may alternately repeat increases and decreases in a cycle synchronized with the cycle in which the rotation reverses. For example, this outer diameter decreases from the reversal site where the rotation direction reverses toward a predetermined site within the section adjacent to the reversal site where the rotation direction is reversed. Also, this outer diameter increases from a specific site on the side opposite the above-mentioned reversal site from the predetermined site within this section toward the next reversal site where the rotation direction reverses. Then, along the longitudinal direction of the multi-core fiber 1, such increases and decreases in the outer diameter of the helical single-core fiber 10B may alternately repeat. Note that the specific site and the predetermined site may be the same.

[0036] Further, although not particularly illustrated, the distance between the helical single-core fiber 10B and the central axis 30ca may vary in the longitudinal direction of the multi-core fiber 1. Also, when the direction in which the helical single-core fiber 10B rotates around the central axis 30ca periodically reverses along the longitudinal direction, this distance, like the outer diameter of the helical single-core fiber 10B, may alternately repeat increases and decreases in a cycle synchronized with the cycle in which the rotation reverses. For example, this distance decreases from the inversion site where the rotating direction reverses toward a predetermined site within a section adjacent to the inversion site and where the rotating direction is reversed. Also, this distance increases from a specific site on the opposite side of the inversion site from the predetermined site within this section toward the next inversion site where the rotating direction reverses. Then, along the longitudinal direction of the multi-core fiber 1, such increases and decreases in the distance between the helical single-core fiber 10B and the central axis 30ca may alternately repeat. Note that the specific site and the predetermined site may be the same. Also, the distance between the helical single-core fiber 10B and the central axis 30ca is the distance between the center of the single-core fiber 10B and the central axis 30ca in a cross-section perpendicular to the longitudinal direction of the multi-core fiber 1.

[0037] Next, a method for manufacturing the multi-core fiber 1 according to the present embodiment will be described.

[0038] FIG. 3 is a flowchart showing the steps of the method for manufacturing the multi-core fiber 1 according to the present embodiment. As shown in FIG. 3, the method for manufacturing the multi-core fiber 1 of the present embodiment mainly includes a bundling step SP1 and a drawing step SP2.

[0039] (Bundling step SP1) In this project, first, a plurality of columnar base materials 10P for single-core fibers, which will become the single-core fibers (10A, 10B) in the multi-core fiber 1 shown in FIG. 1, are prepared. In this embodiment, the base material 10P for single-core fiber includes a columnar core glass body 11R that will become the core 11 and a cladding glass body 12R that will become the cladding 12, and the cladding glass body 12R surrounds the outer peripheral surface of the core glass body 11R. As shown in FIG. 1, in this embodiment, since the number of single-core fibers (10A, 10B) is seven, seven base materials 10P for single-core fibers are prepared. Also, each base material 10P for single-core fiber has the same size and the same configuration as each other. The core glass body 11R is composed of the same material as the core 11, and the cladding glass body 12R is composed of the same material as the cladding 12.

[0040] Next, these base materials 10P for single-core fibers are arranged at the position where they are bundled. In this embodiment, the base materials 10P for single-core fibers are arranged in the densest arrangement so that the center-to-center distance of the core glass bodies 11R is the same. Then, the base materials 10P for single-core fibers arranged at the bundling position are bundled, for example, by a bundling band 51. The bundling band 51 may be made of resin or metal. Thus, as shown in FIG. 4, the base materials 10P for single-core fibers are in a bundled state.

[0041] Next, after fixing a dummy glass rod 52 to one end of each of the bundled base materials 10P for single-core fibers, the bundling band 51 is removed. In FIG. 4, the dummy glass rod 52 is shown by a broken line. It is preferable to fix the dummy glass rod 52 by welding. By fixing by welding, it is possible to effectively suppress the adhesion of impurities to the base material 10P for single-core fiber. By fixing the dummy glass rod 52 to one end of each of the base materials 10P for single-core fibers in this way, these base materials 10P for single-core fibers are maintained in a bundled state and become the base material for multi-core fiber.

[0042] In this step, after bundling the base materials 10P for single-core fibers using the binding bands 51, the dummy glass rods 52 are welded to the base materials 10P for single-core fibers respectively. However, it is not necessarily required to follow such a procedure. For example, fix the dummy glass rod 52 at an appropriate position at one end of one base material 10P for single-core fiber. Next, arrange another base material 10P for single-core fiber adjacent to the base material 10P for single-core fiber already fixed to the dummy glass rod 52, and fix one end of the arranged other base material 10P for single-core fiber to the dummy glass rod 52. Repeat this to fix all the base materials 10P for single-core fibers to the dummy glass rod 52 at appropriate positions. Also, the base materials 10P for single-core fibers may be bundled so that gaps are formed between adjacent base materials 10P for single-core fibers.

[0043] (Wire-drawing process SP2) This process is a wire-drawing process in which the lower end of the base material for multi-core fiber in which a plurality of base materials 10P for single-core fibers are bundled is heated, and the single-core fiber strands are drawn while rotating from the respective base materials 10P for single-core fibers.

[0044] 5 is a diagram showing the state of the drawing step SP2. First, a multicore fiber preform 1P consisting of a plurality of single-core fiber preforms 10P bundled in the bundling step SP1 is placed in a spinning furnace 110. In this embodiment, the multicore fiber preform 1P is rotated such that the central axis 10ca of the single-core fiber preform 10P located at the center becomes the rotation axis, and the heating section 111 of the spinning furnace 110 is heated to heat the lower end portion of each single-core fiber preform 10P. The lower end portion of each single-core fiber preform 10P is in a molten state, and glass is drawn from each single-core fiber preform 10P. The drawn molten glass solidifies immediately upon leaving the spinning furnace 110, and the core glass body 11R becomes the core 11 and the clad glass body 12R becomes the clad 12, thereby forming a single-core fiber wire composed of the core 11 and the clad 12. Here, as described above, the multi-core fiber preform 1P made up of the bundled single-core fiber preforms 10P rotates around the central axis 10ca of the single-core fiber preform 10P located at the center. By rotating the multi-core fiber preform 1P in this manner, a rotational force about the central axis 10ca is transmitted to the single-core fiber wires drawn from each of the single-core fiber preforms 10P. Therefore, the single-core fiber wires drawn from the six single-core fiber preforms 10P located on the outer side are formed in a spiral shape so as to rotate around the central axis 10ca, and the single-core fiber wire drawn from the single-core fiber preform 10P located at the center is formed along the central axis 10ca. Thereafter, these single-core fiber wires pass through the cooling device 120 and are cooled to an appropriate temperature. For example, they are cooled to 40°C to 50°C.

[0045] These single-core fiber wires coming out of the cooling device 120 pass through a first coating device 131 containing a material containing a first thermosetting resin that will become the inner coating layer 30, and are surrounded by the material containing the first thermosetting resin, and the spaces between each single-core fiber wire are filled with the material containing the first thermosetting resin. These single-core fiber wires that have passed through the first coating device 131 pass through a first heating furnace 132 and are heated in the first heating furnace 132. This heating causes crosslinking of the material that forms the first thermosetting resin, hardening the first thermosetting resin, and forming the inner coating layer 30.

[0046] When the inner coating layer 30 is formed from a material containing an ultraviolet curable resin, each single-core fiber is passed through a first coating device 131 containing a material containing a first ultraviolet curable resin. Then, each single-core fiber is surrounded by the material containing the first ultraviolet curable resin, and the space between each single-core fiber is filled with the material containing the first ultraviolet curable resin. After that, the material containing the first ultraviolet curable resin is irradiated with ultraviolet light to cure the first ultraviolet curable resin.

[0047] Next, the bundle of single-core fiber wires on which the inner coating layer 30 has been formed passes through a second coating device 133 containing a material containing a second thermosetting resin that will become the outer coating layer 40, and the outer surface of the inner coating layer 30 is coated with the material containing the second thermosetting resin. The bundle of single-core fiber wires that has passed through the second coating device 133 passes through a second heating furnace 134 and is heated in the second heating furnace 134. This heating causes crosslinking of the material that forms the second thermosetting resin, hardening the second thermosetting resin and forming the outer coating layer 40.

[0048] In addition, when the outer coating layer 40 is formed from a material containing an ultraviolet curable resin, the inner coating layer 30 is coated with the second ultraviolet curable resin using a second coating device 133 containing a material containing the second ultraviolet curable resin, and then ultraviolet light is irradiated onto the material containing the second ultraviolet curable resin to harden the second ultraviolet curable resin.

[0049] In this way, the multi-core fiber 1 shown in FIG. 1 is formed.

[0050] Thereafter, the direction of the multi-core fiber 1 is changed by the turning pulley 141 and it is wound up by the reel 142.

[0051] Note that by reversing the rotation direction of the base material 1P for the multi-core fiber, the direction of rotation around the central axis 30ca of the spiral single-core fiber 10B is reversed. Also, by changing the rotation speed of the base material 1P for the multi-core fiber, the pitch at which the spiral single-core fiber 10B rotates around the central axis 30ca changes.

[0052] As described above, the multi-core fiber 1 of the present embodiment includes a plurality of single-core fibers (10A, 10B) having cores 11 and a cladding 12 surrounding the outer peripheral surfaces of the cores 11, and an inner coating layer 30 containing a resin as a coating layer. The inner coating layer 30 surrounds the outer peripheral surfaces of the claddings 12 of the respective single-core fibers (10A, 10B). Six single-core fibers 10B are arranged in a spiral so as to rotate around the central axis 30ca of the inner coating layer 30.

[0053] FIG. 6 is a diagram showing the state of the single-core fibers (10A, 10B) in the state where the multi-core fiber 1 of FIG. 1 is bent, in the same manner as FIG. 2. As shown in FIG. 6, in the multi-core fiber 1 of the present embodiment, at the bending portion where the multi-core fiber 1 is bent, the helical single-core fiber 10B has an inner portion 10Ba located inside the central axis 30ca of the inner coating layer 30 and an outer portion 10Bb located outside, and the inner portion 10Ba and the outer portion 10Bb can be alternately present along the longitudinal direction. As described above, the helical single-core fiber 10B is surrounded by the inner coating layer 30 made of resin. Therefore, the inner coating layer 30 is deformed by the compressive stress generated in the inner portion 10Ba and the tensile stress generated in the outer portion 10Bb adjacent to the inner portion 10Ba. Then, the outer portion 10Bb moves toward the central axis ca side of the inner coating layer 30 so that the compressive stress in the inner portion 10Ba and the tensile stress in the outer portion 10Bb adjacent to the inner portion 10Ba can weaken each other. Therefore, according to the multi-core fiber 1 of the present embodiment, the stress generated in the core 11 of the single-core fiber 10B due to bending can be reduced as compared with the case where the single-core fiber 10B is linearly arranged in the direction along the central axis 30ca.

[0054] When single-core fibers are in contact with each other, there is no cladding layer between the single-core fibers at the contact site. On the other hand, in this embodiment, all of the plurality of single-core fibers (10A, 10B) are separated from each other. Therefore, according to the multi-core fiber 1 of this embodiment, compared with the case where each single-core fiber has a site in contact with another single-core fiber, the stress generated in the single-core fibers (10A, 10B) when bent can be reduced by the buffering of the inner cladding layer 30, and the stress generated in the core 11 of the single-core fibers (10A, 10B) can be reduced. Note that at least one single fiber may have a site in contact with at least one of the other single-core fibers. However, it is preferable that at least one single-core fiber is separated from the other single-core fibers. By adopting such a configuration, the stress generated in the core when bent can be reduced compared with the case where each single-core fiber has a site in contact with another single-core fiber.

[0055] In this embodiment, the single-core fiber 10A is arranged along the central axis 30ca. Therefore, the overall length of the single-core fiber 10A is shorter than the overall length of the other single-core fiber 10B. Thus, for example, an optical signal propagating through the core 11 of the single-core fiber 10A can be used as a header signal to demodulate an optical signal propagating through the core 11 of the other single-core fiber 10B.

[0056] In the present embodiment, the single-core fiber 10A is located on the central axis 30ca. Therefore, compared to when the single-core fiber 10A is not located on the central axis ca, the stress generated in the core 11 when the single-core fiber 10A is bent can be reduced, and the loss associated with the stress of light propagating through the core 11 of the single-core fiber 10A can be reduced. Furthermore, compared to the helical single-core fiber 10B, the stress generated in the core 11 of the single-core fiber 10A tends to be smaller when the single-core fiber 10A is bent. Therefore, the loss associated with the stress of light propagating through the core 11 of the single-core fiber 10A can be reduced compared to the loss associated with the stress of light propagating through the core 11 of the helical single-core fiber 10B. From these viewpoints, it is preferable that the core 11 of the single-core fiber 10A is located on the central axis 30ca.

[0057] Although the present invention has been described above using the above-mentioned embodiments as examples, the present invention is not limited to these.

[0058] For example, in the above embodiment, the drawing step SP2 has been described as an example in which the multicore fiber preform 1P is rotated around the central axis 10ca of the single-core fiber preform 10P located at the center, and single-core fiber bare wires are drawn from each single-core fiber preform 10P while rotating. However, in the drawing step SP2, the single-core fiber bare wires may be drawn from each single-core fiber preform 10P while rotating. For example, the multi-core fiber 1 may be rotated around the central axis 10ca of the multi-core fiber preform 10P by a spun roller disposed below the second heating furnace 134. With this configuration, the rotation of the multi-core fiber 1 by the spun roller propagates to the drawn molten glass located in the spinning furnace 110. Therefore, even with this configuration, the single-core fiber bare wires can be drawn from each single-core fiber preform 10P while rotating, and the outer circumferential single-core fiber 10B can be formed in a spiral shape so as to rotate around the central axis 30ca of the inner coating layer 30.

[0059] Moreover, in the above embodiment, the multi-core fiber 1 including seven single-core fibers (10A, 10B) has been described as an example. However, the multi-core fiber 1 may have a plurality of single-core fibers, at least one of which may be a spiral single-core fiber. For example, the multi-core fiber 1 may have a configuration as shown in FIG. 7. FIG. 7 is a diagram schematically showing a cross section perpendicular to the longitudinal direction of a multi-core fiber according to a modified example. Note that components that are the same as or equivalent to those in the above embodiment will be assigned the same reference symbols and redundant description will be omitted unless otherwise specified.

[0060] As shown in FIG. 7 , the multi-core fiber 1 of this modification is mainly different from the multi-core fiber 1 of the above-described embodiment in that the number of single-core fibers is four. In this modification, four single-core fibers 10B are arranged at equal intervals along the outer periphery of the inner coating layer 30. The positions of these single-core fibers 10B are rotationally symmetric with respect to the central axis 30ca of the inner coating layer 30. Although not particularly shown, these single-core fibers 10B are arranged in a spiral shape so as to rotate in the same direction around the central axis 30ca of the inner coating layer 30. Therefore, the multi-core fiber 1 of this modification does not have any single-core fibers arranged along a direction parallel to the central axis 30ca, and all the single-core fibers 10B are arranged in a spiral shape so as to rotate around the central axis 30ca. Therefore, in the multi-core fiber 1 of this modification, stress caused by bending on the cores 11 of all the single-core fibers 10B can be reduced.

[0061] Note that one single-core fiber may be arranged along a direction parallel to the central axis 30ca of the inner coating layer 30, and the other three single-core fibers may be arranged in a spiral manner so as to rotate in the same direction around the central axis 30ca. Further, when the number of single-core fibers is two or three, for example, the plurality of single-core fibers may be arranged in a spiral manner so as to rotate in the same direction around the central axis 30ca of the inner coating layer 30. Further, the multi-core fiber may include a plurality of single-core fibers arranged along the central axis 30ca together with at least one spiral single-core fiber.

Industrial Applicability

[0062] As described above, according to the present invention, there is provided a multi-core fiber capable of reducing the stress generated in the core by bending, and it is expected to be used in fields such as optical fiber communication.

Explanation of Signs

[0063] 1 ··· Multi-core fiber 10A, 10B ··· Single-core fiber 11 ··· Core 12 ··· Clad 30 ··· Inner coating layer (coating layer) 30ca ··· Central axis of the inner coating layer 40 ··· Outer coating layer

Claims

1. A plurality of single-core fibers each having a core and a cladding surrounding the outer peripheral surface of the core, A coating layer that surrounds the outer peripheral surface of the cladding of each of the single-core fibers and contains resin, Comprising, At least one of the single-core fibers is spirally arranged so as to rotate around the central axis of the coating layer, At least one of the single-core fibers is spaced apart from the other single-core fibers A multi-core fiber characterized by this.

2. The pitch at which the spiral single-core fiber rotates around the central axis is 0.1 turns / m or more on average The multi-core fiber according to claim 1, characterized by this.

3. All of the plurality of single-core fibers are spirally arranged so as to rotate around the central axis The multi-core fiber according to claim 1 or 2, characterized by this.

4. At least one of the single-core fibers is arranged along the central axis The multi-core fiber according to claim 1 or 2, characterized by this.

5. One of the single-core fibers is located on the central axis The multi-core fiber according to claim 4, characterized by this.

6. The direction in which the spiral single-core fiber rotates around the central axis periodically reverses along the longitudinal direction, The outer diameter of the spiral single-core fiber alternately repeats increasing and decreasing in a cycle synchronized with the cycle in which the rotation of the spiral single-core fiber reverses The multi-core fiber according to any one of claims 1 to 5, characterized by this.

7. The direction in which the spiral single-core fiber rotates around the central axis periodically reverses along the longitudinal direction, The distance between the spiral single-core fiber and the central axis alternately repeats increasing and decreasing in a cycle synchronized with the cycle in which the rotation of the spiral single-core fiber reverses The multi-core fiber according to any one of claims 1 to 6, characterized by this.

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