Multi-core fiber, method for manufacturing multi-core fiber, multi-core fiber preform, and method for manufacturing multi-core fiber preform

Abstract

To provide a multicore fiber having high breaking strength, a method for manufacturing the multicore fiber, a multicore fiber base material, and a method for manufacturing the multicore fiber base material.SOLUTION: A multicore fiber 10 comprises a plurality of core portions 1 made from glass, and a cladding portion 2 which is made from glass and which surrounds the outer peripheries of the plurality of core portions. The cladding portion has a tensile stress of 20 MPa or less in a region on the outer peripheral side relative to the core portion closest to the outer periphery of the cladding portion among the core portions. The region may include at least one element of a halogen, alkali metal, and boron.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a multi-core fiber, a method for manufacturing a multi-core fiber, a multi-core fiber preform, and a method for manufacturing a multi-core fiber preform.

Background Art

[0002] As a method for manufacturing a multi-core fiber, which is an optical fiber having a plurality of core portions, a drilling method is known (Patent Document 1). In the drilling method, first, a plurality of holes are drilled in a cylindrical glass rod that becomes a part of the cladding portion using a drill to form a cladding preform. Next, a core preform containing a core portion is inserted into each of these plurality of holes to form a multi-core fiber preform. The multi-core fiber can be manufactured by drawing the multi-core fiber from the multi-core fiber preform.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As a practical problem of the multi-core fiber, there is a problem that the breaking strength may be lower compared to, for example, a single-core single-mode optical fiber (hereinafter sometimes referred to as a standard SMF) defined by ITU (International Telecommunication Union)-T G.652. In particular, a multi-core fiber manufactured using the drilling method may have a relatively low breaking strength.

[0005] The present invention has been made in view of the above, and an object thereof is to provide a multi-core fiber having a high breaking strength, a method for manufacturing the same, a multi-core fiber preform, and a method for manufacturing the same.

Means for Solving the Problems

[0006] In order to solve the above-described problems and achieve the object, one aspect of the present invention includes a plurality of core portions made of glass, and a clad portion made of glass that surrounds the outer periphery of the plurality of core portions, and the clad portion is a multi-core fiber having a tensile stress of 20 MPa or less in a region on the outer peripheral side of the core portion closest to the outer periphery of the clad portion among the core portions.

[0007] The region may contain at least one element among halogen, alkali metal, and boron.

[0008] On the central side of the region, it may have a region where the maximum value of the tensile stress is greater than the maximum value of the tensile stress in the region.

[0009] The plurality of core portions may be arranged concentrically around the central axis of the multi-core fiber.

[0010] One aspect of the present invention includes a plurality of core portions made of glass, and a clad portion made of glass that surrounds the outer periphery of the plurality of core portions, and the clad portion has a low softening point region on the outer peripheral side of the core portion closest to the outer periphery of the clad among the core portions, and the softening point of the low softening point region is lower than the softening point of the region on the central side of the low softening point region, and it is a multi-core fiber base material.

[0011] The low softening point region may contain at least one element among halogen, alkali metal, and boron.

[0012] The clad portion includes a first clad portion, a second clad portion, and a third clad portion. The first clad portion has a cylindrical shape that surrounds the outer periphery of the core portion, and together with the core portion, it constitutes a core base material. The second clad portion has a cylindrical shape that constitutes the low softening point region. The third clad portion may be columnar with grooves for accommodating the plurality of core base materials provided on the side surface along the longitudinal direction.

[0013] The third cladding portion may be configured to support each of the core base materials and restrict movement.

[0014] The groove of the third cladding portion may have a shape in which the bottom surface of the groove and a region approximately half of the outer peripheral surface of the core base material are inscribed.

[0015] One aspect of the present invention is a method for manufacturing a multi-core fiber, which heats and melts the multi-core fiber base material and draws the multi-core fiber.

[0016] One aspect of the present invention includes a step of housing each of a plurality of core base materials, each having a core portion and a first cladding portion surrounding the outer periphery of the core portion, in each groove of a columnar second cladding portion provided with a plurality of grooves along the longitudinal direction on the surface, and a step of inserting the second cladding portion together with the plurality of core base materials into a cylindrical third cladding portion, wherein the softening point of the third cladding portion is lower than the softening points of the core base material and the second cladding portion, and is a method for manufacturing a multi-core fiber base material.

Advantages of the Invention

[0017] According to the present invention, a multi-core fiber having a high breaking strength can be realized.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

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Figure 9

BEST MODE FOR CARRYING OUT THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments described below. In each drawing, the same or corresponding components are appropriately denoted by the same reference numerals. Also, in this specification, terms not particularly defined shall follow the definitions and measurement methods in ITU-T G.650.1 and G.650.2.

[0020] (Embodiment 1) [Structure of Multi-Core Fiber] FIG. 1 is a schematic cross-sectional view of the multi-core fiber according to Embodiment 1, which is a cross-sectional view in a plane perpendicular to the longitudinal direction. The multi-core fiber 10 includes four core portions 1 as a plurality of core portions and a clad portion 2 surrounding the outer periphery of each core portion 1. Each of the core portion 1 and the clad portion 2 is made of silica-based glass.

[0021] The four core portions 1 are arranged on a circle C centered on the central axis X1 of the multi-core fiber 10. The clad portion 2 has a refractive index lower than the maximum refractive index of each core portion 1.

[0022] The cladding portion 2 has four first regions 2a, second regions 2b, and third regions 2c. Each first region 2a is a region having an annular cross-section that surrounds the outer periphery of each core portion 1. The second region 2b is a region having an annular cross-section that is located on the outer peripheral side of all the core portions 1. The outer edge of the second region 2b coincides with the outer edge of the cladding portion 2. The first region 2a is inscribed in the second region 2b. The third region 2c is a region other than the first region 2a and the second region 2b.

[0023] Each core portion 1 is made of silica glass containing a dopant that increases the refractive index, such as germanium. Each of the first region 2a and the third region 2c of the cladding portion 2 is made of, for example, pure silica glass. Pure silica glass is an extremely high-purity silica glass that substantially does not contain a dopant that changes the refractive index and has a refractive index of about 1.444 at a wavelength of 1550 nm.

[0024] The second region 2b contains at least one element among, for example, a halogen, an alkali metal, and boron. The halogen is, for example, fluorine, chlorine, or bromine. The alkali metal is, for example, lithium, sodium, or potassium.

[0025] Here, the cladding portion 2 has a tensile stress of 20 MPa or less in a region on the outer peripheral side of the core portion 1 closest to the outer periphery of the cladding portion 2. In the multi-core fiber 10, since the four core portions 1 are at the same distance from the outer periphery of the cladding portion 2, all the core portions 1 correspond to the core portion 1 closest to the outer periphery, and the second region 2b, a part of the first region 2a, and a part of the third region 2c correspond to the region where the tensile stress is 20 MPa or less.

[0026] As a result of intensive studies, the inventors have found that when the cladding portion 2 has such a region with a stress of 20 MPa or less, the breaking strength of the multi-core fiber 10 is relatively high.

[0027] For example, FIG. 2 is a schematic cross-sectional view of a multi-core fiber according to a comparative form. This multi-core fiber 10A has the same configuration as the multi-core fiber 10 according to Embodiment 1, except that in the cladding portion 2A, the third region 2d extends to the outer edge of the cladding portion 2A without the presence of the second region 2b.

[0028] FIG. 3 is a diagram showing the stress distribution in the radial direction of the multi-core fibers 10 and 10A. Curve L1 shows the stress distribution of the multi-core fiber 10, and curve L2 shows the stress distribution of the multi-core fiber 10A. In FIG. 3, when the stress is tensile stress, it is shown as a positive value, and when the stress is compressive stress, it is shown as a negative value.

[0029] As shown in FIG. 3, in the cladding portion 2A of the multi-core fiber 10A, there exists a region where the tensile stress exceeds 20 MPa on the outer peripheral side of the core portion 1. On the other hand, in the cladding portion 2 of the multi-core fiber 10, there is no region where the tensile stress exceeds 20 MPa on the outer peripheral side of the core portion 1, and it is 20 MPa or less.

[0030] In the cladding portion 2A of the multi-core fiber 10A, since the tensile stress exceeds 20 MPa on the outer peripheral side of the core portion 1, cracks are likely to occur from the outer edge of the cladding portion 2A, and the breaking strength becomes low. On the other hand, in the cladding portion 2 of the multi-core fiber 10, since the tensile stress is 20 MPa or less on the outer peripheral side of the core portion 1, cracks from the outer edge of the cladding portion 2 are less likely to occur, and the breaking strength becomes high.

[0031] Note that in the multi-core fiber 10, as compared with the multi-core fiber 10A, a relatively large compressive stress is generated at the position of the core portion 1, and it is also different in that the tensile stress is higher at the position of the center of the cladding portion 2 than at the position of the center of the cladding portion 2A of the multi-core fiber 10A. The region at the center of the cladding portion 2 is an example of a region where the maximum value of the tensile stress is larger than the maximum value of the tensile stress in the region on the outer peripheral side of the core portion 1 and on the center side of the core portion 1.

[0032] [Manufacturing Method] An example of the manufacturing method of the multi-core fiber 10 will be described with reference to FIGS. 4 to 6. First, as shown in FIG. 4, a cylindrical glass rod 110 is prepared. The glass rod 110 is a portion that will become the third region 2c of the cladding portion 2 of the multi-core fiber 10 and is made of the same glass material as the third region 2c. Then, four grooves 121 are formed along the longitudinal direction on the side surface of the glass rod 110 by cutting or the like to produce a columnar third cladding portion 120. Each groove 121 has a substantially equal width from the side surface of the third cladding portion 120 to a certain depth, and a bottom surface with a round bottom on the center side thereof.

[0033] However, the third cladding portion 120 is not limited to being prepared by processing a cylindrical glass rod 110 and may be prepared by molding.

[0034] Subsequently, as shown in FIG. 5, a step of inserting the third cladding portion 120 into the hollow portion of the cylindrical second cladding portion 130 is performed. The second cladding portion 130 is a portion that will become the second region 2b of the cladding portion 2 of the multi-core fiber 10 and is made of the same glass material as the second region 2b.

[0035] Subsequently, as shown in FIG. 6, a step of accommodating the core base material 140 in each groove 121 of the third cladding portion 120 is performed. The core base material 140 is composed of a core portion 141 and a cylindrical first cladding portion 142 that surrounds the outer periphery of the core portion. The core portion 141 is a portion that will become the core portion 1 of the multi-core fiber 10 and is made of the same glass material as the core portion 1. The first cladding portion 142 is a portion that will become the first region 2a of the cladding portion 2 of the multi-core fiber 10 and is made of the same glass material as the first region 2a. Thereby, the multi-core fiber base material 100 is manufactured.

[0036] The four core parts 1 are arranged to be equidistant from the central axis X2 which is the central axis of the third cladding part 120 and also the central axis of the second cladding part 130. Each groove 121 of the third cladding part 120 is shaped to position the core base material 140 such that the four core parts 1 are equidistant from the central axis X2.

[0037] By heating and melting this multi-core fiber base material 100 and drawing the multi-core fiber, the multi-core fiber 10 can be manufactured. Note that before drawing, the multi-core fiber base material 100 may be heated to integrate the core base material 140, the third cladding part 120, and the second cladding part 130.

[0038] The multi-core fiber base material 100 will be described more specifically with reference to FIG. 6. The multi-core fiber base material 100 includes a plurality of four core parts 141 and a cladding part 150 surrounding the outer periphery of the four core parts 141. The cladding part 150 includes a first cladding part 142, a second cladding part 130, and a third cladding part 120.

[0039] The second cladding part 130 constitutes a low softening point region located on the outer peripheral side of the core part 141 in the cladding part 150. The softening point of this low softening point region is lower than the softening point of the region on the central side of the low softening point region. Specifically, the softening point of the second cladding part 130 is lower than the softening points of the core part 141, the first cladding part 142, and the third cladding part 120, which are regions on the central side (central axis X2 side) of the second cladding part 130. Such a relationship of softening points can be realized as follows. That is, the first cladding part 142 and the third cladding part 120 are made of pure silica glass, and the addition amount of at least one of halogen, alkali metal, and boron in the silica glass-based second cladding part 130 is adjusted so that the softening point is lower than that of the core part 141.

[0040] Since the softening point of the second cladding part 130 is lower than the softening points of the core part 141, the first cladding part 142, and the third cladding part 120, when the multi-core fiber base material 100 is heated and melted and drawn into a wire, the fluidity of the third cladding part 120 becomes lower than that inside it. As a result, it is considered that a region with a tensile stress of 20 MPa or less is formed on the outer peripheral side of the drawn multi-core fiber 10.

[0041] Here, when manufacturing a multi-core fiber base material using the drilling method, a clearance for insertion is required between the holes of the core base material and the cladding base material. Therefore, the inner diameter of the hole is formed larger than the outer diameter of the core base material. As a result of having such a clearance, when the core base material moves toward the center within the hole during heating and melting wire drawing, the relative positional relationship of the four core parts may deviate from the design.

[0042] On the other hand, each groove 121 of the third cladding part 120 has a shape in which the bottom surface 121a of each groove 121 and a region approximately half of the outer peripheral surface of each core base material 140 are inscribed. That is, the bottom surface 121a of each groove 121 has a round bottom shape along the shape of the outer peripheral surface of each core base material 140. Thereby, each core base material 140 is supported by the third cladding part 120 and the movement toward the radial center and in the circumferential direction is restricted. Therefore, even if a force that moves the core base material 140 toward the center acts within the groove 121 during heating and melting wire drawing, the change in the relative position is restricted. As a result, the relative positional relationship of the four core parts 141 is maintained, so that the relative positional relationship of the four core parts 1 in the multi-core fiber 10 can be maintained with high precision with respect to the design.

[0043] Also, in the stacking method, which is one of the methods for manufacturing a multi-core fiber base material other than the drilling method, the core base materials are positioned under the restraint of each other and from glass rods, so the core base materials are prone to displacement and the accuracy of the positional relationship of the core portions tends to be relatively low. Furthermore, in the stacking method, since the core portions need to be arranged at lattice points, there are also restrictions on the design of the positions of the core portions. Therefore, the manufacturing method of this example has advantages in terms of the degree of freedom in arranging the core portions and the positional accuracy compared to the drilling method and the stacking method.

[0044] Also, in the multi-core fiber base material 100, each groove 121 of the third cladding portion 120 can be formed by cutting the side surface of the glass rod 110 or the like, so there is no restriction on the length of the cladding base material due to the length of the drill as in the drilling method. As a result, the multi-core fiber base material 100 is likely to be enlarged, which is advantageous in terms of cost reduction of the multi-core fiber 10.

[0045] The multi-core fiber 10 according to Embodiment 1 configured as described above has a high breaking strength and can achieve, for example, a breaking strength equivalent to that of a standard SMF. Also, the multi-core fiber 10 has high positional accuracy and degree of freedom in arranging the core portions 1. Also, the multi-core fiber base material 100 configured as described above can be used for manufacturing the multi-core fiber 10 and is likely to be enlarged.

[0046] Specifically, according to the manufacturing method of this example, compared with the drilling method, the multi-core fiber base material is likely to be enlarged in both length and outer diameter. For example, in the drilling method, about 1000 mm is the upper limit of the length that can be drilled, but in the manufacturing method of this example, since the third cladding portion 120 can be processed by external cutting, there are fewer restrictions on the length.

[0047] In addition, in the drilling method, when the outer diameter of the multi-core fiber base material becomes thick and the drilling diameter becomes thick, the load on the drilling tool (drill, etc.) increases, and the difficulty of processing increases. On the other hand, according to the manufacturing method of this example, since the third cladding portion 120 can be processed by external cutting, even if the portion to be externally cut becomes large, the difficulty level of the processing process is not likely to increase. Furthermore, for the second cladding portion 130, large-sized members have already been put into practical use both in terms of the outer diameter and the length, and there are few restrictions on increasing the size.

[0048] As an example of the present invention, a multi-core fiber having the configuration of the multi-core fiber 10 according to Embodiment 1 was manufactured using the above manufacturing method. The transmission losses of the four core portions of the manufactured multi-core fiber of the example were all 0.21 dB / km or less, and the crosstalk between the cores was also -27 dB or less per 100 km.

[0049] In addition, as a comparative example, a multi-core fiber having the configuration of the multi-core fiber 10A according to the comparative form was manufactured. The stress distributions of the multi-core fiber of the example and the multi-core fiber of the comparative example were measured with a stress distribution measuring instrument (manufactured by INTERFIBER ANALYSIS, model: IFA-100). Then, in the multi-core fiber of the comparative example, there was a region where the tensile stress was 30 MPa or more on the outer peripheral side of the core portion, but in the multi-core fiber of the example, there was no region where the tensile stress exceeded 20 MPa on the outer peripheral side of the core portion, and it was 20 MPa or less.

[0050] In addition, regarding the positional accuracy of the core portion with respect to the design value, there was a maximum deviation of 0.3 μm in the multi-core fiber of the comparative example, but the maximum deviation in the multi-core fiber of the example was 0.2 μm.

[0051] In addition, when a proof test was performed in which a stress of 1% elongation was applied to the multi-core fiber, the average survival length of the multi-core fiber of the example was improved by 30% or more compared to the case of the multi-core fiber of the comparative example.

[0052] (Embodiment 2) [Structure of Multicore Fiber] FIG. 2 is a schematic cross-sectional view of a multicore fiber according to Embodiment 2, which is a cross-sectional view in a plane perpendicular to the longitudinal direction. The multicore fiber 10B includes four core portions 1 as a plurality of core portions and a clad portion 2B surrounding the outer periphery of each core portion 1. Each of the core portion 1 and the clad portion 2B is made of silica glass.

[0053] The four core portions 1 are arranged on a circle centered on the central axis X3 of the multicore fiber 10B. The clad portion 2B has a refractive index lower than the maximum refractive index of each core portion 1.

[0054] The clad portion 2 has a plurality of first regions 2a, second regions 2Bb, and third regions 2Bc. Each first region 2a is a region with an annular cross-section surrounding the outer periphery of each core portion 1. The second region 2Bb is a region with an annular cross-section located on the outer peripheral side of all the core portions 1. The outer edge of the second region 2Bb coincides with the outer edge of the clad portion 2B. The first region 2a is not inscribed in the second region 2b and is arranged with a predetermined interval therebetween. The third region 2Bc is a region other than the first region 2a and the second region 2Bb.

[0055] The constituent materials of each core portion 1, each first region 2a, second region 2Bb, and third region 2Bc may be the same as the constituent materials of the corresponding elements, i.e., the first region 2a, second region 2b, and third region 2c, of the multicore fiber 10 according to Embodiment 1.

[0056] Also in the multicore fiber 10B, the clad portion 2B has a tensile stress of 20 MPa or less in a region on the outer peripheral side of the core portion 1 closest to the outer periphery of the clad portion 2B.

[0057] FIG. 8 is a diagram showing the stress distribution in the radial direction of the multicore fibers 10A and 10B. The curve L3 shows the stress distribution of the multicore fiber 10B, and the curve L2 shows the stress distribution of the multicore fiber 10A in a comparative form. In FIG. 8, when the stress is a tensile stress, it is shown as a positive value, and when the stress is a compressive stress, it is shown as a negative value.

[0058] As shown in Fig. 8, in the cladding portion 2B of the multi-core fiber 10B, there is no region where the tensile stress exceeds 20 MPa on the outer peripheral side of the core portion 1, and it is 20 MPa or less.

[0059] In the cladding portion 2B of the multi-core fiber 10B, since the tensile stress is 20 MPa or less on the outer peripheral side of the core portion 1, cracks from the outer edge of the cladding portion 2B are less likely to occur, and the breaking strength is increased.

[0060] In the multi-core fiber 10B, compared with the multi-core fiber 10A, the point where the stress is substantially zero at the position of the core portion 1 is the same, but at the position of the center portion of the cladding portion 2B, it is different in that the tensile stress is higher than the position of the center portion of the cladding portion 2A of the multi-core fiber 10A. The region of the center portion of the cladding portion 2B is an example of a region where the maximum value of the tensile stress is larger than the maximum value of the tensile stress in the region on the center side and the outer peripheral side of the core portion 1 with respect to the core portion 1.

[0061] The multi-core fiber 10B configured as described above has a high breaking strength like the multi-core fiber 10, and can achieve a breaking strength equivalent to that of a standard SMF, for example.

[0062] Fig. 9 shows an example of a method for manufacturing a multi-core fiber base material 200 for manufacturing the multi-core fiber 10B. First, a columnar third cladding portion 220 having a hole 221 drilled in a columnar glass rod is produced.

[0063] However, the third cladding portion 220 is not limited to being prepared by processing a columnar glass rod, and may be prepared by molding.

[0064] Subsequently, the third cladding portion 220 is inserted into the hollow portion of the cylindrical second cladding portion 230. Subsequently, a core base material 140 is inserted into each hole 221 of the third cladding portion 220. Note that the order of these two insertion steps is arbitrary. Thereby, the multi-core fiber base material 200 is manufactured.

[0065] By heating and melting the multi-core fiber base material 200 and drawing the multi-core fiber, the multi-core fiber 10B can be manufactured. Note that, before drawing, the multi-core fiber base material 200 may be heated to integrate the core base material 140, the third cladding portion 220, and the second cladding portion 230.

[0066] In the multi-core fibers 10 and 10B of the above embodiment, although the number of core portions 1 is four in each case, the number of core portions is not particularly limited. Also, for example, although the core portions 1 are arranged on a circle centered on the central axis of the multi-core fibers 10 and 10B, the arrangement is not particularly limited. Further, for example, when the number of core portions is even larger, for example, 8 or more, the core portions may be arranged concentrically around the central axis of the multi-core fiber. Also in this case, in the region on the outer peripheral side of the core portion closest to the outer periphery of the cladding portion, the tensile stress is 20 MPa or less.

[0067] Also, for example, the core portion, the first region, and the third region may contain at least one element among halogen, alkali metal, and boron. In that case, the content of at least one element among halogen, alkali metal, and boron in the second region may be adjusted so that the tensile stress becomes 20 MPa or less in the region on the outer peripheral side of the core portion closest to the outer periphery of the cladding portion.

[0068] Also, the present invention is not limited by the above embodiment. Those configured by appropriately combining the above-described components are also included in the present invention. Further, additional effects and modifications can be easily derived by those skilled in the art. Therefore, a broader aspect of the present invention is not limited to the above embodiment, and various changes are possible.

Explanation of Reference Numerals

[0069] 1, 141: Core portion 2, 2A, 2B, 150: Cladding portion 2a: First region 2b, 2Bb: Second region 2c, 2Bc, 2d: Third region 10, 10A, 10B: Multicore fiber 100, 200: Multicore fiber base material 110: Glass rod 120, 220: Third cladding part 121: Groove 121a: Bottom surface 130, 230: Second cladding part 140: Core base material 142: First cladding part 221: Hole C: Circle X1, X2, X3: Central axis

Claims

1. A multi-core fiber comprising a plurality of core portions made of glass, a cladding portion made of glass surrounding the outer periphery of the plurality of core portions, wherein the cladding portion includes a plurality of first regions having an annular cross-section that surround the outer periphery of each core portion and are spaced apart from each other, a second region having an annular cross-section located on the outer peripheral side of all the core portions, and a third region that is a region other than the first region and the second region, includes the central axis of the multi-core fiber, and is located inside the second region, the plurality of core portions are all spaced apart from the central axis of the multi-core fiber, in the second region of the cladding portion, the tensile stress is 20 MPa or less, on the central side of the second region and on the central side of the core portion, there is a region where the maximum value of the tensile stress is greater than the maximum value of the tensile stress in the second region, the maximum value of the tensile stress is greater than 20 MPa, the transmission loss is 0.21 dB / km or less, and the inter-core crosstalk is -27 dB or less per 100 km multi-core fiber.

2. The second region contains at least one element among halogen, alkali metal, and boron The multi-core fiber according to claim 1.

3. The plurality of core portions are arranged concentrically around the central axis of the multi-core fiber The multi-core fiber according to claim 1 or 2.

4. A multi-core fiber preform for manufacturing the multi-core fiber according to any one of claims 1 to 3, comprising a plurality of core portions made of glass, a cladding portion made of glass surrounding the outer periphery of the plurality of core portions, wherein the cladding portion has a low softening point region on the outer peripheral side of the core portion closest to the outer periphery of the cladding, and the softening point of the low softening point region is lower than the softening point of the region on the central side of the low softening point region multi-core fiber preform.

5. The low softening point region contains at least one element among halogen, alkali metal, and boron The multi-core fiber preform according to claim 4.

6. The cladding portion includes a first cladding portion, a second cladding portion, and a third cladding portion, the first cladding portion is a portion that becomes the first region, has a cylindrical shape surrounding the outer periphery of the core portion, and constitutes a core preform together with the core portion, ​ ​ The second cladding portion is a portion that becomes the second region, and is cylindrical and constitutes the low softening point region. The third cladding portion is a portion that becomes the third region, and is columnar with grooves for accommodating the plurality of core base materials provided on the side surface along the longitudinal direction. The multi-core fiber base material according to claim 4 or 5.

7. The third cladding portion is configured to support each of the core base materials and restrict movement. The multi-core fiber base material according to claim 6.

8. The groove of the third cladding portion has a shape in which the bottom surface of the groove and substantially half of the outer peripheral surface region of the core base material are inscribed. The multi-core fiber base material according to claim 6 or 7.

9. Melting the multi-core fiber base material according to any one of claims 4 to 8 by heating and drawing the multi-core fiber. A method for manufacturing a multi-core fiber.

10. A method for manufacturing a multi-core fiber base material according to any one of claims 4 to 8, a step of accommodating each of a plurality of core base materials including a core portion and a first cladding portion surrounding the outer periphery of the core portion in each groove of a columnar second cladding portion provided with a plurality of grooves on the surface along the longitudinal direction; a step of inserting the second cladding portion together with the plurality of core base materials into a cylindrical third cladding portion; comprising, wherein the softening point of the third cladding portion is lower than the softening points of the core base material and the second cladding portion. A method for manufacturing a multi-core fiber base material.

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