Optical fiber base material production method

Abstract

The optic fiber base material production method includes melting and connecting glass pipes to both ends of a clad pipe, in which holes are provided, so as to surround the holes, heating the connections of the clad pipe and glass pipes at a heating temperature below the melting temperature of the connections, and slowly cooling the connections from the heating temperature after heating.

Description

[Technical field]

[0001] The present disclosure relates to a method for manufacturing an optical fiber preform. This application claims priority to Japanese Application No. 2022-102859 filed on June 27, 2022, and incorporates all of the contents of said Japanese application by reference. [Background technology]

[0002] A rod-in-collapse method is known as a method for manufacturing an optical fiber preform (for example, Patent Documents 1 and 2). In the rod-in-collapse method, a core rod is inserted into a cladding pipe having a hole, and the cladding pipe and the core rod are integrated by heating. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 61-201633 [Patent Document 2] JP 2011-168464 A Summary of the Invention

[0004] A method for manufacturing an optical fiber preform according to one embodiment of the present disclosure includes melt-connecting glass pipes to both ends of a cladding pipe having a hole therein so as to surround the hole, heating the connection between the cladding pipe and the glass pipe at a heating temperature lower than the melting temperature of the connection, and slowly cooling the connection from the heating temperature after heating. [Brief description of the drawings]

[0005] [Figure 1] FIG. 1 is a cross-sectional view of an optical fiber preform manufactured in an embodiment. [Diagram 2] FIG. 2 is a flowchart showing a method for manufacturing an optical fiber preform according to the embodiment. [Diagram 3] FIG. 3 is a cross-sectional view for explaining the connection step. [Figure 4] FIG. 4 is a cross-sectional view illustrating the heating step and the slow cooling step. [Diagram 5] FIG. 5 is a cross-sectional view for explaining the etching step. [Figure 6] FIG. 6 is a cross-sectional view for explaining the insertion step. [Figure 7] FIG. 7 is a cross-sectional view illustrating the heating step and the slow cooling step according to the modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] [Problem that this disclosure aims to solve] In the rod-in-collapse method, glass pipes are fused to both ends of a clad pipe, and process gas is supplied to and exhausted from the clad pipe through the glass pipes. In the conventional method, the connection between the clad pipe and the glass pipe may come off or break.

[0007] An object of the present disclosure is to provide a method for manufacturing an optical fiber preform capable of suppressing separation and cracking of the connection between the cladding pipe and the glass pipe.

[0008] [Effects of this disclosure] According to the present disclosure, it is possible to provide a method for manufacturing an optical fiber preform capable of suppressing separation and cracking of the connection between the cladding pipe and the glass pipe.

[0009] [Description of the embodiments of the present disclosure] First, the embodiments of the present disclosure will be described. (1) A manufacturing method of an optical fiber preform according to one embodiment of the present disclosure includes melt-splicing glass pipes to both ends of a cladding pipe having a hole so as to surround the hole, heating the joint between the cladding pipe and the glass pipe at a heating temperature lower than the melting temperature of the joint, and slowly cooling the joint from the heating temperature after heating.

[0010] In the above-mentioned method for manufacturing an optical fiber preform, the glass strain introduced into the joint by the fusion connection between the cladding pipe and the glass pipe is released by heating, and the microcracks remaining in the joint are repaired by heating. By slowly cooling the joint without quenching, the joint can be cooled to room temperature while preventing the generation of new glass strain. Therefore, the joint can be prevented from coming loose and cracking.

[0011] (2) In the above (1), the heating time may be 5 minutes or more, which makes it easier to release glass distortion and repair microcracks.

[0012] (3) In the above (1) or (2), the heating temperature may be 900° C. or higher. In this case, the glass distortion is easily released and microcracks are easily repaired.

[0013] (4) In any of the above (1) to (3), the slow cooling may be performed at a maximum cooling rate of 100° C. / min or less until the temperature of the joint reaches 400° C. In this case, glass distortion is easily released and microcracks are easily repaired.

[0014] (5) In any of the above (1) to (4), in the slow cooling, the difference between the surface temperature of the clad pipe at a position 10 mm axially away from the joint and the surface temperature of the glass pipe at a position 10 mm axially away from the joint may be 0° C. or more and 200° C. or less. In this case, the occurrence of glass distortion at the joint is suppressed.

[0015] (6) In any of the above (1) to (5), the heating and slow cooling may be performed using a heating furnace. In this case, a wide area can be heated more easily than with a flame, so the joint can be heated uniformly. Therefore, the glass distortion in the joint can be easily released.

[0016] (7) In any of the above (1) to (5), the heating and slow cooling may be performed using a flame. In this case, it is easier to visually check the connection than in a heating furnace.

[0017] (8) In any of the above (1) to (7), if the joint is rapidly cooled after annealing, heating and annealing may be repeated. In this case, the glass distortion caused by the annealing can be released.

[0018] (9) In any of the above (1) to (7), the method for producing the optical fiber preform further includes, after the annealing, flowing a gas into the hole to perform a gas-phase treatment on the inner surface of the hole or inserting a core rod into the hole, and heating and annealing may be performed again after the gas-phase treatment or insertion. In this case, the glass distortion caused by the quenching during the gas-phase treatment can be released.

[0019] (10) In any of the above (1) to (9), the optical fiber preform may be a multi-core optical fiber preform. In this case, the glass pipe needs to surround all of the multiple holes provided in the cladding pipe, so it is likely to have a larger diameter than the single-core optical fiber preform. A large-diameter glass pipe has a larger change in heat capacity and thermal expansion coefficient due to its shape than a small-diameter glass pipe. Therefore, glass distortion is likely to be introduced into the joint, and microcracks are likely to remain. Therefore, the effect of heating and slow cooling is large.

[0020] [Details of the embodiment of the present disclosure] Specific examples of the manufacturing method of the optical fiber preform of the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicated descriptions will be omitted.

[0021] Fig. 1 is a cross-sectional view of an optical fiber preform manufactured in an embodiment. This figure shows a cross section perpendicular to the central axis of the optical fiber preform 1. The optical fiber preform 1 is a multi-core optical fiber preform including a plurality of cores 2 and a common cladding 3. In this embodiment, the number of cores 2 is four.

[0022] The core portions 2 extend along the central axis of the optical fiber preform 1. The core portions 2 are arranged at positions rotationally symmetrical with respect to the central axis in a cross section perpendicular to the central axis. The cross-sectional shapes of the core portions 2 are the same circular shapes. The diameter of the core portions 2 may be, for example, 4 mm or more and 50 mm or less, 5 mm or more and 30 mm or less, or 6 mm or more and 12 mm or less. In optical fibers formed from a multi-core optical fiber preform The diameter of the core is 6 μm or more and 15 μm or less. The cladding portion 3 surrounds the multiple core portions 2. The diameter of the cladding portion 3 may be, for example, 40 mm or more and 180 mm or less, or 40 mm or more and 120 mm or less. In optical fibers formed from a multi-core optical fiber preform The diameter of the cladding is 124 μm or more and 126 μm or less.

[0023] The refractive index of the core 2 is higher than that of the cladding 3. The core 2 and the cladding 3 are made of a silica-based glass material. Each of the core 2 and the cladding 3 contains silica glass as a main component and a dopant for adjusting the refractive index.

[0024] 2 is a flowchart showing a method for manufacturing an optical fiber preform according to an embodiment of the present invention. As shown in FIG. 2, the method for manufacturing the optical fiber preform 1 includes a splicing step S1, a heating step S2, a slow cooling step S3, an etching step S4, a first baking step S5, an insertion step S6, a second baking step S7, an integration step S8, and a removal step S9.

[0025] 3 is a cross-sectional view for explaining the connecting step. In this figure, a cross section along the axial direction of the clad pipe 10 (hereinafter also simply referred to as the "axial direction") is shown. The connecting step S1 is a step of melt-connecting a first glass pipe 21 to a first end 11 of the clad pipe 10 and melt-connecting a second glass pipe 22 to a second end 12 of the clad pipe 10. The connecting step S1 forms a connection part 31 (see FIG. 4) between the first end 11 and the first glass pipe 21, and also forms a connection part 32 (see FIG. 4) between the second end 12 and the second glass pipe 22. The connecting step S1 is performed using the flame of a burner 30.

[0026] The clad pipe 10 is a glass pipe that becomes the clad portion 3 and has a shape corresponding to the clad portion 3 . Clad Pipe 10 The cross section perpendicular to the axial direction has a circular outer shape, and the outer diameter may be, for example, 40 mm or more and 180 mm or less, or 40 mm or more and 120 mm or less. The clad pipe 10 is provided with one or more holes 13 (core holes). The holes 13 are through holes extending in the axial direction. The clad pipe 10 is produced, for example, by drilling the holes 13 in a cylindrical glass member. The holes 13 have a circular cross section. In this embodiment, the number of holes 13 is four.

[0027] The first glass pipe 21 and the second glass pipe 22 are clad pipes. 10 They are work pipes for supporting the clad pipe 10 and are provided to introduce process gas into the hole 13 of the clad pipe 10 in a subsequent process. For example, the first glass pipe 21 is connected to a process gas supply system, and the second glass pipe 22 is connected to a process gas exhaust system.

[0028] The first glass pipe 21 and the second glass pipe 22 have, for example, the same shape. The first glass pipe 21 and the second glass pipe 22 are made of, for example, the same glass material. The first glass pipe 21 and the second glass pipe 22 have a cylindrical shape. In this embodiment, the outer diameter of the first glass pipe 21 and the second glass pipe 22 is approximately the same as the outer diameter of the clad pipe 10, but may be smaller than the outer diameter of the clad pipe 10.

[0029] The first glass pipe 21 surrounds the plurality of holes 13 at the first end 11, and is fusion-connected to the first end 11 so as to be coaxial with the clad pipe 10. That is, when viewed in the axial direction, the plurality of holes 13 are entirely disposed inside the first glass pipe 21. The second glass pipe 22 surrounds the plurality of holes 13 at the second end 12, and is fusion-connected to the second end 12 so as to be coaxial with the clad pipe 10. That is, when viewed in the axial direction, the plurality of holes 13 are entirely disposed inside the second glass pipe 22.

[0030] If there is a difference between the heat capacity of the clad pipe 10 and the heat capacity of the first glass pipe 21, or if there is a difference between the thermal expansion coefficient of the clad pipe 10 and the thermal expansion coefficient of the first glass pipe 21, glass distortion may occur in the connection part 31 during the heating and cooling process. If there is a difference between the heat capacity of the clad pipe 10 and the heat capacity of the second glass pipe 22, or if there is a difference between the thermal expansion coefficient of the clad pipe 10 and the thermal expansion coefficient of the second glass pipe 22, glass distortion may occur in the connection part 32 during the heating and cooling process. There is also a risk that microcracks introduced into the processed surfaces during pre-processing of the clad pipe 10, the first glass pipe 21, and the second glass pipe 22 may remain in the connection parts 31 and 32. As a result of the investigation, it was found that such glass distortion and microcracks may cause the connection parts 31 and 32 to come off and break in the post-process in which the connection parts 31 and 32 are heated and cooled.

[0031] 4 is a cross-sectional view for explaining the heating step and the slow cooling step. In this figure, a cross section along the axial direction of the clad pipe 10 is shown. The heating step S2 is a step of heating the connection part 31 at a heating temperature lower than the melting temperature of the connection part 31, and heating the connection part 32 at a heating temperature lower than the melting temperature of the connection part 32. The glass distortion generated in the connection parts 31 and 32 in the connection step S1 is released by heating, and the microcracks remaining in the connection parts 31 and 32 are repaired by heating. The heating step S2 is performed by using the flame of the burner 30 to directly heat the connection parts 31 and 32 with the flame of the burner 30.

[0032] Here, the melting temperature is the temperature at which the viscosity of the glass increases by 10 6.6 It is the lowest temperature at which the viscosity is less than Pa s. When the melting temperature of the clad pipe 10 and the melting temperature of the first glass pipe 21 are different from each other, the melting temperature of the joint 31 is the lower of the melting temperatures of the clad pipe 10 and the first glass pipe 21. When the melting temperature of the clad pipe 10 and the melting temperature of the second glass pipe 22 are different from each other, the melting temperature of the joint 32 is the lower of the melting temperatures of the clad pipe 10 and the second glass pipe 22. The viscosity of glass can be measured by, for example, the penetration method or the parallel plate method.

[0033] The closer the heating temperature in the heating step S2 is to the melting temperature of the connection parts 31, 32, the easier it is to alleviate glass distortion and microcracks. Therefore, the heating temperature is appropriately set according to the melting temperature of the connection parts 31, 32. The heating temperature is, for example, 900° C. or higher. The temperature of the connection parts 31, 32 is measured as the temperature of the very surface of the connection parts 31, 32, for example, by a pyrometer.

[0034] The longer the heating time in the heating step S2, the easier it is to alleviate the glass distortion and microcracks. The heating time can be, for example, 5 minutes or more. When the joints 31, 32 have a large diameter, it takes time to alleviate the glass distortion and microcracks. For example, when the outer diameter of the joints 31, 32 is 80 mm or more, the heating time can be 15 minutes or more. The longer the heating time, the lower the manufacturing efficiency of the optical fiber preform 1, so the heating time can be, for example, 60 minutes or less.

[0035] The slow cooling step S3 is carried out after the heating step S2 and consecutively to the heating step S2, and is a step of slowly cooling the connection parts 31, 32 from the heating temperature in the heating step S2. If the connection parts 31, 32 are rapidly cooled by natural cooling, glass distortion is likely to occur in the connection parts 31, 32. The slow cooling step S3 suppresses the occurrence of glass distortion in the connection parts 31, 32. In the slow cooling step S3, Each The time required for the temperature to reach 400° C. is 5 minutes or more. The longer this time is, the easier it is to alleviate glass distortion and microcracks, but the lower the manufacturing efficiency of the optical fiber preform 1. This time can be, for example, 60 minutes or less.

[0036] In the slow cooling step S3, the cooling rate until the temperature of the joints 31, 32 reaches 400°C is 100°C / min or less. The lower the cooling rate, the easier it is to alleviate glass distortion and microcracks, but the manufacturing efficiency of the optical fiber preform 1 decreases. The cooling rate can be, for example, 15°C / min or more. The slow cooling step S3 is performed using the flame of the burner 30 while directly heating the joints 31, 32 with the flame of the burner 30.

[0037] In the slow cooling step S3, the joint 3 1 or Difference in surface temperature at ±10 mm positions in the axial direction , and the difference in surface temperature at positions ±10 mm in the axial direction from the connection portion 32 teeth, each,It is 0°C or more and 200°C or less. That is, the difference between the surface temperature of the clad pipe 10 at a position 10 mm axially away from the joint 31 and the surface temperature of the first glass pipe 21 at a position 10 mm axially away from the joint 31 is 0°C or more and 200°C or less. In addition, the difference between the surface temperature of the clad pipe 10 at a position 10 mm axially away from the joint 32 and the surface temperature of the second glass pipe 22 at a position 10 mm axially away from the joint 32 is 0°C or more and 200°C or less.

[0038] 5 is a cross-sectional view for explaining the etching step. In this figure, a cross section along the axial direction of the clad pipe 10 is shown. The etching step S4 is a step in which a gas (process gas) is caused to flow through the hole 13 of the clad pipe 10 to etch the inner surface of the hole 13 (gas phase treatment). The etching step is performed, for example, while rotating the clad pipe 10 around the axial direction and heating the outer circumferential surface of the clad pipe 10 with an external heat source 33 such as a burner. The etching step removes impurities from the inner surface of the hole 13 and smoothes the inner surface of the hole 13.

[0039] The gas introduced into the hole 13 is, for example, an etching gas such as SF6. The gas is supplied to the hole 13 of the clad pipe 10 through the first glass pipe 21. After flowing through the hole 13, the gas is discharged to the outside through the second glass pipe 22. The etching process is carried out when the first glass pipe 21 and the second glass pipe 22 are G The lathe is rotated by a gripping portion (not shown) of the lathe. The gripping portion grips the first glass pipe 21 and the second glass pipe 22, not the clad pipe 10 which is heated by an external heat source, so that the thermal influence on the gripping portion can be suppressed.

[0040] The first baking process S5 is a process in which a gas (process gas) is flowed through the hole 13 of the clad pipe 10 to perform a baking process (gas phase process) on the inner surface of the hole 13. As in the etching process, the first baking process is performed, for example, while rotating the clad pipe 10 around its axial direction and heating the outer circumferential surface of the clad pipe 10 by an external heat source. By the first baking process, foreign matter on the inner surface of the hole 13 is removed and the inner surface of the hole 13 is smoothed.

[0041] The gas introduced into the hole 13 is, for example, a cleaning gas (i.e., air-baking gas) such as chlorine or oxygen. The gas is supplied to the hole 13 of the clad pipe 10 through the first glass pipe 21. After flowing through the hole 13, the gas is discharged to the outside through the second glass pipe 22. As in the etching process, the first air-baking process is performed in a state in which the first glass pipe 21 and the second glass pipe 22 are rotatably held by a holding portion (not shown) of a glass lathe.

[0042] 6 is a cross-sectional view for explaining the insertion step. In this figure, a cross section along the axial direction of the clad pipe 10 is shown. In the insertion step S6, the core rod 40 Each of Clad pipe 10 hole 13 One corresponding hole The core rod 40 is a glass rod that will become the core portion 2. The outer diameter of the core rod 40 is slightly smaller than the diameter of the hole 13. The core rod 40 passes through the second glass pipe 22 and is inserted into the second end 1 of the cladding pipe 10. 2 or The wire is inserted into hole 13.

[0043] The second baking process S7 is a process in which a gas (process gas) is flowed through the hole 13 of the clad pipe 10 to perform a baking process (gas phase process) on the outer surface of the core rod 40. As in the etching process, the second baking process is performed, for example, while rotating the clad pipe 10 around the axial direction and heating the outer circumferential surface of the clad pipe 10 by an external heat source. By the second baking process, foreign matter on the outer surface of the core rod 40 and the inner surface of the hole 13 is removed, and the outer surface of the core rod 40 and the inner surface of the hole 13 are smoothed.

[0044] As in the first baking process, the gas introduced into the hole 13 is a cleaning gas (i.e., baking gas) such as chlorine or oxygen. The gas is supplied to the hole 13 of the clad pipe 10 through the first glass pipe 21. After flowing through the hole 13, the gas is discharged to the outside through the second glass pipe 22. As in the etching process, the second baking process is performed in a state in which the first glass pipe 21 and the second glass pipe 22 are rotatably held by a holding portion (not shown) of a glass lathe.

[0045] The integration step S8 is a step of integrating the clad pipe 10 and the core rod 40 by heating. The optical fiber preform 1 is produced by the thermal integration process. As in the etching process, the thermal integration process is performed, for example, while rotating the clad pipe 10 around its axial direction and heating the outer circumferential surface of the clad pipe 10 by an external heat source. The thermal integration process is performed in a state in which the first glass pipe 21 and the second glass pipe 22 are rotatably held by a holding portion (not shown) of a glass lathe.

[0046] The removing step S9 is a step of removing the first glass pipe 21 and the second glass pipe 22 from the optical fiber preform 1. In this manner, the optical fiber preform 1 is manufactured.

[0047] As described above, in the manufacturing method of the optical fiber preform 1, the glass strain introduced in the joints 31, 32 by the splicing step S1 is released by the heating step S2, and the microcracks remaining in the joints 31, 32 are repaired by the heating step S2. By slowly cooling the joints 31, 32 over a sufficient period of time without rapid cooling, the joints 31, 32 can be cooled to room temperature while preventing the generation of new glass strain. Therefore, the joints 31, 32 can be prevented from coming off or cracking.

[0048] In the heating step S2, the heating time is 5 minutes or more, and the heating temperature is 900° C. or more. This makes it easy to release the glass distortion and repair the microcracks. In the slow cooling step S3, the time required for the temperature of the joints 31 and 32 to reach 400° C. is 5 minutes or more. 31,32The cooling rate until the temperature reaches 400°C is 100°C / min or less. This makes it easy to release glass strain and repair microcracks. In the slow cooling process S3, the difference in surface temperature at positions ±10 mm in the axial direction from the joints 31, 32 is 0°C or more and 200°C or less. This prevents glass strain from occurring in the joints 31, 32.

[0049] Since the heating step S2 and the slow cooling step S3 are performed using the flame of the burner 30, it is easier to visually check the connections 31, 32 than when a heating furnace 34 (see FIG. 7) is used.

[0050] The optical fiber preform 1 is a multi-core optical fiber preform. Since the first glass pipe 21 and the second glass pipe 22 need to surround all of the holes 13 provided in the cladding pipe 10, they tend to have a larger diameter than the case of a single-core optical fiber preform. Since it takes time to connect a large-diameter glass pipe to the cladding pipe 10, glass distortion is likely to be introduced in the connection parts 31, 32, and microcracks are likely to remain. Therefore, the effect of relaxing the glass distortion and microcracks by heating and slow cooling is large.

[0051] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit and scope of the present disclosure.

[0052] In the above embodiment, the heating step S2 and the annealing step S3 are performed only once, but the heating step S2 and the annealing step S3 may be performed again after the annealing step S3. In the manufacturing method of the optical fiber preform 1, the joints 31, 32 may be heated and rapidly cooled after the annealing step S3. In this case, glass distortion may occur in the joints 31, 32. Therefore, if rapid cooling of the joints 31, 32 occurs, the heating step S2 and the annealing step S3 may be performed again. Here, rapid cooling means that the joints 31, 32 are cooled at a cooling rate exceeding a maximum of 100°C / min after the heating of the joints 31, 32 is stopped. Such repetition of the heating step S2 and the annealing step S3 may be performed, for example, every time the joints 31, 32 are rapidly cooled after the annealing step S3.

[0053] Specifically, the heating step S2 and the annealing step S3 may be performed again after the etching step S4, the first baking step S5, the insertion step S6, or the second baking step S7. More specifically, the heating step S2 and the annealing step S3 may be performed again after the etching step S4 and before the first baking step S5, after the first baking step S5 and before the insertion step S6, after the insertion step S6 and before the second baking step S7, or after the second baking step S7 and before the integration step S8. This makes it possible to release the glass distortion caused by rapid cooling.

[0054] Fig. 7 is a cross-sectional view for explaining the heating step and the slow cooling step according to the modified example. As shown in Fig. 7, the heating step S2 and the slow cooling step S3 may be performed using furnace heat from a heating furnace 34. The heating furnace 34 is, for example, an electric furnace, and has the ability to heat with a heat source other than a flame. The heating furnace 34 is disposed in a position where the connecting parts 31, 32 can be heated. When the heating furnace 34 is used, it is easier to heat a wider range than the flame of the burner 30, so the connecting parts 31, 32 can be heated uniformly. Therefore, the glass distortion of the connecting parts 31, 32 is easily released.

[0055] In the above embodiment, the optical fiber preform 1 is a multi-core optical fiber preform, but it may be a single-core optical fiber preform.

[0056] The above embodiments and modifications may be combined as appropriate. [Explanation of symbols]

[0057] 1...Optical fiber base material 2…Core section 3…Clad section 10...Clad pipe 11...first end 12…Second end 13...hole 21…First glass pipe 22…Second glass pipe 30…Burner 31…Connection 32…Connection 33...External heat source 34...Heating furnace 40…Core Rod

Claims

1. a glass pipe is fused to both ends of the clad pipe having the hole so as to surround the hole; heating a joint between the clad pipe and the glass pipe at a heating temperature lower than a melting temperature of the joint; cooling the connection from the heating temperature after the heating; Including, A method for manufacturing an optical fiber preform.

2. The heating time is 5 minutes or more. The method for producing the optical fiber preform according to claim 1 .

3. The heating temperature is 900° C. or higher.

3. The method for producing an optical fiber preform according to claim 1 or 2.

4. In the slow cooling, the maximum cooling rate until the temperature of the connection portion reaches 400° C. is 100° C. / min or less.

3. The method for producing an optical fiber preform according to claim 1 or 2.

5. In the slow cooling, the difference between the surface temperature of the clad pipe at a position 10 mm away from the joint in the axial direction and the surface temperature of the glass pipe at a position 10 mm away from the joint in the axial direction is 0° C. or more and 200° C. or less.

3. The method for producing an optical fiber preform according to claim 1 or 2.

6. The heating and the slow cooling are performed using a heating furnace.

3. The method for producing an optical fiber preform according to claim 1 or 2.

7. The heating and the gradual cooling are performed using a flame.

3. The method for producing an optical fiber preform according to claim 1 or 2.

8. If rapid cooling of the connection occurs after the slow cooling, the heating and the slow cooling are performed again.

3. The method for producing an optical fiber preform according to claim 1 or 2.

9. Further comprising, after the annealing, flowing a gas through the hole to perform a gas-phase treatment on the inner surface of the hole, or inserting a core rod into the hole; After the vapor phase treatment or the insertion, the heating and the slow cooling are performed again.

3. The method for producing an optical fiber preform according to claim 1 or 2.

10. The optical fiber preform is a multi-core optical fiber preform.

3. The method for producing an optical fiber preform according to claim 1 or 2.