Cutting tool and optical fiber preform manufacturing method

Abstract

This cutting tool (40) comprises a shank part (41), and a cutting part (42) provided to one end of the shank part (41). The cutting part (42) has a first region (51) provided to one end of the cutting tool (40), and a second region (52) positioned closer to the center of the cutting tool (40) than is the first region (51). Abrasive grains are anchored to the first region (51) and the second region (52). The average grain diameter of the abrasive grains in the second region (52) is less than the average grain diameter of the abrasive grains in the first region (51).

Description

[Technical field]

[0001] The present disclosure relates to a cutting tool and a method for manufacturing an optical fiber preform. This application claims priority to Japanese Application No. 2019-062449, filed on March 28, 2019, and incorporates by reference all of the contents of said Japanese application. [Background technology]

[0002] An optical fiber preform having a core extending in the longitudinal direction may be manufactured by the rod-in-collapse method. In the rod-in-collapse method, a jacket material is manufactured by forming a hole extending in the longitudinal direction in, for example, a cylindrical glass body. Next, a core rod is inserted into the hole, and the jacket material is heated from the outside to integrate the core rod and the jacket material, thereby manufacturing an optical fiber preform.

[0003] For example, Patent Document 1 discloses a technique for manufacturing an optical fiber preform having one core extending in the longitudinal direction (hereinafter referred to as a single-core optical fiber preform), and Patent Document 2 discloses a technique for manufacturing an optical fiber preform having multiple cores (hereinafter referred to as a multi-core optical fiber preform). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 63-2826 [Patent Document 2] Japanese Patent Publication No. 61-201633 Summary of the Invention

[0005] A cutting tool according to one aspect of the present disclosure includes a shank portion to which a rotational force about an axis extending in a longitudinal direction is applied, and a cutting portion located in front of the shank portion and inserted into a glass body to form a hole in the glass body, the cutting portion having a first region including a tip of the cutting portion and having a constant outer diameter in the longitudinal direction, and a second region located in a region closer to the shank portion than the first region; The outer diameter of the second region is larger than the outer diameter of the first region, Abrasive grains are secured to the first region and the second region, and the average grain size of the abrasive grains in the second region is smaller than the average grain size of the abrasive grains in the first region.

[0006] A method for manufacturing an optical fiber preform according to one aspect of the present disclosure is a method for manufacturing an optical fiber preform having a core extending in a longitudinal direction, the method comprising the steps of: forming a hole in a glass body from one end to the other end in an axial direction thereof using a cutting tool according to the present disclosure to produce a jacket material; The process of Inserting a core rod into the hole The process of The jacket material is heated to integrate the jacket material with the core rod. The process includes . [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a cross-sectional view showing an example of a multi-core optical fiber. [Figure 2A] FIG. 2A is a front view showing a glass body used in a method for producing an optical fiber preform according to one embodiment of the present disclosure. [Figure 2B] FIG. 2B is a cross-sectional view showing a glass body used in the method for producing an optical fiber preform according to one embodiment of the present disclosure. [Figure 3A] FIG. 3A is a side view illustrating an example of a cutting tool according to one embodiment of the present disclosure. [Figure 3B] FIG. 3B is a front view illustrating an example of a cutting tool according to one embodiment of the present disclosure. [Figure 3C] FIG. 3C is an enlarged perspective view of the cutting portion of the cutting tool of FIGS. 3A and 3B, to which abrasive grains are attached. [Figure 4]FIG. 4 is a cross-sectional view including the central axis of a glass body in a process for producing a jacket material according to an embodiment of the present disclosure. [Diagram 5] FIG. 5 is a cross-sectional view including a central axis of a jacket material according to an embodiment of the present disclosure. [Figure 6] FIG. 6 is a cross-sectional view including the central axis of the jacket material into which a core rod has been inserted, in a step of inserting the core rod according to an embodiment of the present disclosure. [Figure 7] FIG. 7 is a cross-sectional view including the central axis of a glass body in a process of producing a jacket material according to a modified embodiment of the present disclosure. [Figure 8] FIG. 8 is a cross-sectional view including the central axis of a jacket material according to a modified embodiment of the present disclosure. [Figure 9] FIG. 9 is a cross-sectional view including the central axis of the jacket material into which a core rod has been inserted, in a step of inserting the core rod according to a modified example of the embodiment of the present disclosure. [Figure 10] FIG. 10 is a conceptual diagram illustrating the integration process according to an embodiment of the present disclosure. [Figure 11] FIG. 11 is a cross-sectional view showing an example of a jacket material. [Figure 12A] FIG. 12A is a front view showing another example of a cutting tool. [Figure 12B] FIG. 12B is a front view showing still another example of the cutting tool. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] [Problem that this disclosure aims to solve] In a jacket pipe for manufacturing a single-core optical fiber preform, a hole is provided at the center of the jacket pipe. When the jacket pipe is heated from the outside, the jacket pipe deforms while maintaining a symmetric state with respect to the central axis of the jacket pipe. In other words, the inner circumference of the hole shrinks uniformly toward the central axis of the hole, and the inner wall of the hole comes into contact with the core rod. At the same time as the inner circumference of the hole shrinks, the inner wall of the hole becomes smooth. However, if the inner wall of the hole comes into contact with the core rod before the roughness of the inner wall of the hole becomes sufficiently small, air bubbles may remain at the boundary between the inner wall of the hole and the core rod. When an optical fiber preform containing air bubbles is drawn, the outer diameter fluctuation of the optical fiber increases, and the mechanical strength of the optical fiber decreases.

[0009] When a multi-core optical fiber preform is manufactured by the rod-in-collapse method, the positional accuracy of the cores is high if the clearance between the inner diameter of the hole and the outer diameter of the core rod is small. However, if the clearance is small, the inner wall of the hole is likely to come into contact with the core rod before the roughness of the inner wall of the hole becomes sufficiently small, so that air bubbles are likely to remain.

[0010] Some multi-core optical fiber preforms have holes provided at locations other than the center of the jacket pipe. When the jacket pipe is heated from the outside, the vicinity of the outer periphery of the jacket pipe is heated more strongly than the center of the jacket pipe. For this reason, it is difficult to maintain the roughness of the inner walls of the holes provided at locations other than the center of the jacket pipe uniform over the entire circumference of the inner walls in one cross section of the jacket pipe. In particular, in the case of holes provided near the outer periphery of the jacket pipe, the inner walls of the holes may come into contact with the core rod before the roughness of the inner walls of the holes near the center of the jacket pipe becomes sufficiently small, so that air bubbles are likely to remain.

[0011] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described. (1) A cutting tool comprising a shank portion to which a rotational force about an axis extending in a longitudinal direction is applied, and a cutting portion located in front of the shank portion and inserted into a glass body to form a hole in the glass body, the cutting portion having a first region including a tip of the cutting portion and having a constant outer diameter in the longitudinal direction, and a second region located in a region closer to the shank portion than the first region; The outer diameter of the second region is larger than the outer diameter of the first region, Abrasive grains are fixed to the first region and the second region, and the average grain size of the abrasive grains in the second region is smaller than the average grain size of the abrasive grains in the first region. Since the grain size of the abrasive grains in the second region is smaller than the grain size of the abrasive grains in the first region, the present disclosure can ensure the productivity of the hole drilling in the first region while reducing the roughness of the inner wall of the hole in the second region. Therefore, the present disclosure can obtain an optical fiber preform in which air bubbles are less likely to remain at the boundary between the inner wall of the hole and the core rod without impairing the productivity of the hole drilling. In addition, since the outer diameter of the second region is larger than the outer diameter of the first region, the second region can reliably process the inner wall of the hole after the first region processes the hole, and therefore the present disclosure can reliably obtain a hole with a smooth inner wall.

[0012] (2) In one embodiment of the cutting tool of the present disclosure, the abrasive grains are diamond grains, which makes it easy to form holes with smooth inner walls in the glass body.

[0013] (3) In one embodiment of the cutting tool of the present disclosure, the average grain size of the abrasive grains in the first region is 100 μm or more, and the average grain size of the abrasive grains in the second region is less than 100 μm. Since the average grain size of the abrasive grains in the first region is 100 μm or more, the present disclosure can maintain a high processing speed for drilling holes. Furthermore, since the average grain size of the abrasive grains in the second region is less than 100 μm, the present disclosure can obtain a smooth inner wall even at the above processing speed.

[0015] ( 4In one embodiment of the cutting tool of the present disclosure, the difference between the outer diameter of the second region and the outer diameter of the first region is in the range of 10 μm to 300 μm. Since the difference between the outer diameter of the second region and the outer diameter of the first region is 10 μm or more, even if the abrasive grains in the second region wear out, the second region can continue to process the inner wall of the hole. Furthermore, since the difference between the outer diameter of the second region and the outer diameter of the first region is 300 μm or less, the load on the second region during processing does not increase, and wear of the abrasive grains in the second region is reduced. ( 5 ) In one embodiment of the cutting tool of the present disclosure, the second region has a constant outer diameter in the longitudinal direction. ( 6 ) In one embodiment of the cutting tool of the present disclosure, it is in the shape of a hollow round bar.

[0016] ( 7 ) An optical fiber preform manufacturing method according to one aspect of the present disclosure is a method for manufacturing an optical fiber preform having a core extending in the longitudinal direction, the method including the steps of: forming a hole from one end of the axial direction of the glass body to the other end using the cutting tool of the present disclosure to prepare a jacket material; inserting a core rod into the hole; and heating the jacket material to integrate the jacket material and the core rod. Since the average grain size of the abrasive grains in the second region is smaller than the average grain size of the abrasive grains in the first region, the present disclosure can reduce the roughness of the inner wall of the hole in the second region while ensuring the productivity of hole drilling in the first region. Thus, the present disclosure can obtain an optical fiber preform in which air bubbles are less likely to remain at the boundary between the inner wall of the hole and the core rod without impairing the productivity of hole drilling. The present disclosure also allows the optical fiber preform thus manufactured to be drawn to produce an optical fiber with less outer diameter variation and no decrease in mechanical strength.

[0017] [Effects of this disclosure] An object of the present disclosure is to provide an optical fiber preform manufacturing method and cutting tool that are less likely to leave air bubbles at the boundary between the inner wall of a hole and a core rod, without impairing productivity in drilling holes. [Details of the embodiment of the present disclosure]

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the optical fiber preform manufacturing method and cutting tool according to the present disclosure will be described with reference to the accompanying drawings.

[0019] 1 is a cross-sectional view showing an example of a multi-core optical fiber 1. The multi-core optical fiber 1 has, for example, seven cores 11 in a cladding 10. The cores 11 extend in the longitudinal direction of the multi-core optical fiber 1. The cores 11 include a central core arranged on the central axis of the optical fiber and outer cores arranged on the vertices of a hexagon around the central axis of the optical fiber. Each core 11 includes a region having a refractive index higher than the refractive index of the cladding 10, and is configured to propagate light.

[0020] One method for manufacturing an optical fiber preform is the rod-in-collapse method, which includes the steps of forming a hole in, for example, a cylindrical glass body from one end to the other end in the axial direction to form a jacket material, inserting a core rod into the hole in the jacket material, and heating the jacket material to integrate the jacket material and the core rod.

[0021] Fig. 2A is a front view of a glass body 20 used in the optical fiber preform manufacturing method according to one embodiment of the present disclosure, seen from one end 21. Fig. 2B is a cross-sectional view taken along line XX in Fig. 2A. The glass body 20 is, for example, silica glass doped with fluorine or pure silica glass, and has a cylindrical shape. When the optical fiber preform is manufactured by the rod-in-collapse method to obtain the multi-core optical fiber 1, seven holes are formed in the glass body 20 from one end 21 toward the other end 22 in the axial direction using a drill-shaped tool.

[0022] 3A to 3C are diagrams illustrating a cutting tool 40 used in a method for producing an optical fiber preform according to an embodiment of the present disclosure. FIG. 3A is a side view showing an example of the cutting tool 40. FIG. 3B is a front view showing an example of the cutting tool 40. FIG. 3C is an enlarged perspective view of a cutting part of the cutting tool 40 to which abrasive grains are fixed. The cutting tool 40 includes a shank part 41 and a cutting part 42. The shank part 41 is, for example, a hollow round bar made of metal, and is configured so that a rotational force about an axis extending in the longitudinal direction is applied to the shank part 41. The cutting part 42 is located in front of the shank part 41 (one end of the cutting tool 40, on the right side in FIG. 3A) and is configured to rotate together with the shank part 41.

[0023] The cutting part 42 is, for example, a hollow round bar, and a discharge passage 50a is provided at the center of the cross section of the cutting part 42, which is concentric with the shank part 41. The outer peripheral surface of the cutting part 42 has a first region 51 provided at one end of the cutting tool 40, and a second region 52 located closer to the center of the cutting tool 40 than the first region 51. Specifically, the second region 52 is located rearward (leftward in FIG. 3A) than the first region 51. The front end of the second region 52 is connected to the rear end of the first region 51, for example. The length L1 of the first region 51 and the length L2 of the second region 52 are both, for example, 5 mm. Abrasive grains (for example, diamond grains) are fixed to the first region 51 (including the tip surface 50) and the second region 52, for example, by a multilayered electroplating structure.

[0024] The average grain size of the abrasive grains is evaluated by the grain size specified in JIS_B_4130. The average grain size of the diamond grains in the first region 51 is 100 μm or more (grain size #140 or less in JIS_B_4130), preferably 150 μm or more (grain size #100 or less in JIS_B_4130). The average grain size of the diamond grains in the second region 52 is smaller than the average grain size of the diamond grains in the first region 51. Specifically, the average grain size of the diamond grains in the second region 52 is less than 100 μm, preferably 50 μm or less (grain size #270 or more in JIS_B_4130). The average grain size is generally determined by a method of sorting grains using, for example, multiple types of sieves. An average grain size of 105 μm corresponds to grain size #140, an average grain size of 149 μm corresponds to #100, and an average grain size of 53 μm corresponds to #270, respectively.

[0025] In this way, since the average grain size of the diamond grains in the first region 51 is 100 μm or more, this embodiment can maintain a high processing speed for drilling holes. If the average grain size of the diamond grains in the first region 51 is 150 μm or more, this embodiment can further increase the processing speed for drilling holes. Furthermore, since the average grain size of the diamond grains in the second region 52 is less than 100 μm, this embodiment can obtain a smooth inner wall of the hole even at the above processing speed. If the average grain size of the diamond grains in the second region 52 is 50 μm or less, this embodiment can further smooth the inner wall of the hole.

[0026] The abrasive grains fixed to the cutting portion 42 are adjusted in protruding amount by dressing to form a cutting edge. The diamond may be synthetic diamond or natural diamond. Diamond is suitable for processing glass, but CBN (Cubic Boron Nitride) may be used for the abrasive grains of the present disclosure.

[0027] In the illustrated example, the first region 51 and the second region 52 are connected to each other. However, a region to which the abrasive grains are not adhered may be provided between the first region 51 and the second region 52, and the first region 51 and the second region 52 may be disposed apart from each other. This embodiment is not limited to the two regions, the first region 51 and the second region 52, and three or more regions to which the abrasive grains are adhered may be provided. In this case, the average grain size of the abrasive grains in the region located furthest rearward is the smallest.

[0028] In the cutting portion 42 that forms a hole in the glass body 20, the outer diameter of the second region 52 and the outer diameter of the first region 51 may be the same size. However, as shown in Fig. 3C, the outer diameter D2 of the second region 52 is preferably larger than the outer diameter D1 of the first region 51. This is because, after the first region 51 forms a hole in the glass body 20, the second region can reliably process the inner wall of the hole. As a result, in this embodiment, a hole with a smooth inner wall can be obtained.

[0029] Specifically, the difference (D2-D1) between the outer diameter D2 of the second region 52 and the outer diameter D1 of the first region 51 is in the range of 10 μm to 300 μm. Since the difference between the outer diameter D2 of the second region 52 and the outer diameter D1 of the first region 51 is 10 μm or more, even if the diamond grains of the second region 52 are worn down by multiple uses of the cutting tool 40, the second region 52 can continue to process the inner wall of the hole. In addition, since the difference between the outer diameter D2 of the second region 52 and the outer diameter D1 of the first region 51 is 300 μm or less, the load on the second region 52 during processing is not large, and wear of the diamond grains of the second region 52 is reduced.

[0030] 1, cutting tool 40 is rotated and inserted into glass body 20 from one end 21 to the other end 22 of glass body 20, with cutting part 42 disposed at the front as the leading edge. The glass material cut by cutting part 42 is sent rearward through discharge path 50a and discharged, for example.

[0031] 4 to 6 are cross-sectional views including the central axis of the glass body 20 and the jacket material 27 in the manufacturing method of the optical fiber preform. A total of seven ring-shaped holes 28 are formed in the glass body 20 by a hollow round bar-shaped cutting tool 40. FIG. 4 shows an intermediate step in which three of the seven ring-shaped holes 28 are formed on the cross section. An uncut rod 24 remains in the center of each ring-shaped hole 28. When the ring-shaped hole 28 reaches the other end 22, the rod 24 falls off, and the ring-shaped hole 28 becomes a through hole 29 (FIG. 5). The through hole 29 corresponds to the hole of the present disclosure. The inner surface of the through hole 29 is cleaned using a fluorine-based gas or the like.

[0032] Next, a total of seven core rods 26 are inserted into the through holes 29, respectively. Fig. 6 shows three core rods 26 in cross section. In this case, for example, the core rod 26 located at the center of the multi-core optical fiber 1 is arranged concentrically with the through hole 29 arranged on the central axis of the jacket material 27. Also, the multiple core rods 26 located at the outer core of the multi-core optical fiber 1 are arranged closer to the central axis of the jacket material 27 in the corresponding through holes 29.

[0033] It should be noted that the holes of the present disclosure do not have to be completely through. In this case, a total of seven ring-shaped bottomed holes 23 are formed in the glass body 20. FIG. 7 shows three ring-shaped bottomed holes 23 in cross section. The ring-shaped bottomed holes 23 correspond to the holes of the present disclosure. The ring-shaped bottomed holes 23 extend along the longitudinal direction and reach a position from the other end 22 where a predetermined thickness remains. Within the glass body 20, an uncut rod 24 surrounded by the ring-shaped bottomed holes 23 remains.

[0034] Next, when the glass body 20 is heated from the outside, the bottom portion of the rod 24 softens and melts, and by cutting the bottom portion of the rod 24, a circular bottomed hole 25 is formed in the glass body 20 (FIG. 8). Next, for example, a kneading tool is used or a carbon dioxide laser is irradiated to remove residue from the bottom portion of the circular bottomed hole 25. Thereafter, the inside of the circular bottomed hole 25 is cleaned with a fluorine-based gas or the like to form a jacket material 27.

[0035] Next, a total of seven core rods 26 are inserted into the respective circular bottomed holes 25. Fig. 9 shows three core rods 26 in cross section. In this case, for example, the core rod 26 located at the center of the multi-core optical fiber 1 is arranged concentrically with the circular bottomed hole 25 arranged on the central axis of the jacket material 27. Also, the multiple core rods 26 located at the outer core of the multi-core optical fiber 1 are arranged closer to the central axis of the jacket material 27 in the corresponding circular bottomed holes 25.

[0036] The core rod 26 is a glass rod having a higher refractive index than the jacket material 27, and is produced by a vapor phase glass synthesis method such as a VAD (Vapor Phase Axial Deposition) method. When the jacket material 27 is silica glass doped with fluorine, the core rod 26 is a core rod including a central core including pure silica glass (which may contain chlorine) and an optical cladding including silica glass doped with fluorine that surrounds the central core. On the other hand, when the jacket material 27 is pure silica glass, the core rod 26 is a core rod including a central core including silica glass doped with GeO2 and an optical cladding including pure silica glass not doped with GeO2 that surrounds the central core.

[0037] 10 is a conceptual diagram showing an integration process according to an embodiment of the present disclosure. Next, the jacket material 27 is heated to integrate the core rod 26 and the jacket material 27. In detail, the jacket material 27 into which the core rod 26 is inserted rotates, for example, around the central axis of the jacket material 27, and the heat source moves in the axial direction of the jacket material 27 (moving from right to left in FIG. 10). When the jacket material 27 is heated, the inner diameter of the through hole 29 or the circular bottomed hole 25 shrinks due to surface tension, and the jacket material 27 is welded to the core rod 26.

[0038] A-A' in Fig. 10 shows a position before the heat source passes. The core rods 26 and the jacket material 27 are not yet integrated. B-B' in Fig. 10 shows a position during the passage of the heat source. The core rods 26 located at the outer cores of the multi-core optical fiber 1 are already integrated with the jacket material 27. However, the core rods 26 located at the center of the multi-core optical fiber 1 are not yet integrated with the jacket material 27. C-C' in Fig. 10 shows a position after the heat source passes. All the core rods 26 and the jacket materials 27 are integrated. That is, at the position C-C' in Fig. 10, the cross-sectional structure of the multi-core optical fiber preform 3 is as shown in Fig. 11, and the cladding portion 30 and the core portion 31 are integrated.

[0039] In this way, in the rod-in-collapse method, since the outer periphery of the jacket material 27 is closer to the heating source, the outer periphery of the jacket material 27 is heated earlier than the center of the jacket material 27, and deformation progresses. For this reason, as described at the position B-B' in Fig. 10, the through hole 29 or the circular bottomed hole 25 provided near the outer periphery of the jacket material 27 shrinks before the through hole 29 or the circular bottomed hole 25 provided at the center of the jacket material 27 shrinks. Generally, as the inner periphery of the hole shrinks, the inner wall of the hole becomes smooth, but in the case of the through hole 29 or the circular bottomed hole 25 provided near the outer periphery of the jacket material 27, the through hole 29 or the circular bottomed hole 25 may come into contact with the core rod 26 before the roughness of the inner wall of the through hole 29 or the circular bottomed hole 25 near the center of the jacket material 27 becomes sufficiently small. In addition, if the clearance between the inner diameter of the through hole 29 or the circular bottomed hole 25 and the outer diameter of the core rod 26 is small, the positional accuracy of the core portion 31 described in Figure 11 will be high, but it will be more likely to come into contact with the core rod 26 before the roughness of the inner wall of the through hole 29 or the circular bottomed hole 25 becomes sufficiently small.

[0040] 3A to 3C, however, the grain size of the diamond grains in the second region 52 of the cutting tool 40 is smaller than that in the first region 51, so that the first region 51 ensures the productivity of drilling holes, and the second region 52 reduces the roughness of the inner wall of the through hole 29 or the circular bottomed hole 25. Therefore, the present embodiment can obtain a multi-core optical fiber preform 3 in which air bubbles are less likely to remain in the boundary between the inner wall of the through hole 29 or the circular bottomed hole 25 and the core rod 26 without impairing the productivity of drilling holes. Then, in the present embodiment, by drawing the multi-core optical fiber preform 3 manufactured in this way, a multi-core optical fiber 1 with less outer diameter variation and no decrease in mechanical strength can be manufactured.

[0041] In the above embodiment, an example of a hollow round bar-shaped cutting tool 40 has been described. However, the present disclosure is not limited to this example. For example, a solid round bar-shaped cutting tool 40 as shown in Figs. 12A and 12B may be used. The cutting tool 40 shown in Fig. 12A has, for example, five discharge passages 50a on the outer circumferential surface of the cutting part 42. The tip surface 50 of the cutting part 42 is, for example, conical.

[0042] In the cutting tool 40 shown in Fig. 12B, the cutting part 42 has a smaller diameter than the through hole 29 or the bottomed circular hole 25. The cutting part 42 and the cut hole are not concentric, and the cutting part 42 rotates around a position eccentric to the center of the hole. In this case, since the cutting part 42 has a smaller diameter than the hole, it is not necessary to provide a discharge path for the glass material. The tip surface 50 of the cutting part 42 may be, for example, conical or cross-shaped.

[0043] In the above embodiment, the method for manufacturing the multi-core optical fiber preform 3 has been described, but the present disclosure is also applicable to the case of manufacturing a single-core optical fiber preform.

[0044] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is defined by the claims, not by the above meaning, and is intended to include all modifications within the scope and meaning equivalent to the claims. [Explanation of symbols]

[0045] 1...multi-core optical fiber, 3...multi-core optical fiber base material, 10...cladding, 11...core, 20...glass body, 21...one end, 22...other end, 23...ring-shaped bottomed hole, 24...uncut rod, 25...circular bottomed hole, 26...core rod, 27...jacket material, 28...ring-shaped hole, 29...through hole, 30...cladding portion, 31...core portion, 40...cutting tool, 41...shank portion, 42...cutting portion, 50...tip surface, 50a...discharge path, 51...first region, 52...second region.

Claims

1. A shank portion to which a rotational force is applied around an axis extending in a longitudinal direction; a cutting portion located in front of the shank portion and inserted into a glass body to form a hole in the glass body, The cutting portion has a first region including a tip of the cutting portion and having a constant outer diameter in the longitudinal direction, and a second region located in a region closer to the shank portion than the first region, The outer diameter of the second region is larger than the outer diameter of the first region, Abrasive grains are attached to the first region and the second region, A cutting tool, wherein an average grain size of the abrasive grains in the second region is smaller than an average grain size of the abrasive grains in the first region.

2. The cutting tool of claim 1 , wherein the abrasive grains are diamond grains.

3. 3. The cutting tool according to claim 1, wherein the abrasive grains in the first region have an average grain size of 100 μm or more, and the abrasive grains in the second region have an average grain size of less than 100 μm.

4. The cutting tool according to claim 1 , wherein a difference between an outer diameter of the second region and an outer diameter of the first region is in a range of 10 μm to 300 μm.

5. The cutting tool according to claim 1 , wherein the second region has a constant outer diameter in the longitudinal direction.

6. The cutting tool according to claim 1 , which is in the form of a hollow round bar.

7. 1. A method for manufacturing an optical fiber preform having a longitudinally extending core, comprising the steps of: A step of forming a hole from one end to the other end in an axial direction of a glass body using the cutting tool according to any one of claims 1 to 6 to prepare a jacket material; inserting a core rod into the hole; and heating the jacket material to integrate the jacket material with the core rod.

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