Method for manufacturing optical fiber preform

Abstract

This method for manufacturing an optical fiber preform includes: a step of chamfering a corner part 20c between a first end surface 20a of a dummy tube 20 and an outer peripheral surface 20p of the dummy tube 20; and a step of welding the first end surface 20a of the dummy tube 20 after the chamfering to an end surface 10a of a clad tube 10.

Description

Optical fiber preform manufacturing method

[0001] This disclosure relates to a method for manufacturing an optical fiber preform. This application claims priority to Japanese Application No. 2024-086233, filed May 28, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] Optical fiber preforms are sometimes manufactured using the rod-in-tube method. The rod-in-tube method involves, for example, preparing a glass material that will become the cladding of the optical fiber, drilling a hole where the core is to be located, inserting a core rod into the hole, and integrating the cladding tube and the core rod using a heat source such as an induction furnace, resistance furnace, or oxyhydrogen burner. To reduce manufacturing costs, the rod-in-tube method may involve welding a relatively inexpensive dummy tube to the end of the cladding tube, and then holding the dummy tube to move the cladding tube and integrate it with the core rod. In this case, a process of fusion-connecting (welding) the cladding tube and the dummy tube is required. In this process, the end faces of both tubes are heated and melted, and the ends are then directly pressed against each other to weld them.

[0003] Patent Documents 1 and 2 describe methods for welding a dummy tube to a clad tube.

[0004] JP 2000-128559 A JP 2023-146915 A

[0005] A method for manufacturing an optical fiber preform according to one aspect of the present disclosure includes a step of chamfering a corner between an end face of a dummy tube and an outer peripheral surface of the dummy tube, and a step of welding the chamfered end face of the dummy tube to an end face of a cladding tube.

[0006] Fig. 1 is a flowchart showing a method for manufacturing an optical fiber preform according to a first embodiment. Fig. 2 is a cross-sectional view for explaining a welding step according to the first embodiment. Fig. 3 is a diagram for explaining the area of ​​a first portion and the area of ​​a second portion. Fig. 4 is a cross-sectional view for explaining an integration step according to a comparative example. Fig. 5 is a cross-sectional view for explaining a welding step according to a second embodiment.

[0007] In the process of welding the clad tube and the dummy tube, the glasses may be pressed together, forming a protrusion on the outside of the clad tube. In this case, in the collapse process of integrating the clad tube and the core rod, the protrusion may collide with the airtight parts of the induction furnace, causing damage to the clad tube, the dummy tube, or both, and there is a risk of internal gas leaking.

[0008] The methods of Patent Documents 1 and 2 can reduce the risk of forming a convex portion on the inside of the cladding tube, but cannot reduce the risk of forming a convex portion on the outside of the cladding tube.

[0009] The present disclosure provides a method for manufacturing an optical fiber preform that is less likely to form a convex portion on the outside of the cladding tube during the process of welding the cladding tube and the dummy tube.

[0010] According to the present disclosure, it is possible to provide a method for manufacturing an optical fiber preform in which a convex portion is less likely to be formed on the outside of the cladding tube in the step of welding the cladding tube and the dummy tube.

[0011] First, an embodiment of the present disclosure will be described. (1) A method for manufacturing an optical fiber preform according to a first aspect of the present disclosure includes a step of chamfering a corner between an end face of a dummy tube and an outer peripheral surface of the dummy tube, and a step of welding the chamfered end face of the dummy tube to an end face of a cladding tube. In the method for manufacturing an optical fiber preform, the outer corner of the first end face of the dummy tube is chamfered, so that a space corresponding to the portion removed by chamfering is formed outside the chamfered surface of the dummy tube. Therefore, a protrusion is unlikely to be formed on the outside of the cladding tube during the welding step.

[0012] (2) In the above (1), the dummy tube may be chamfered so that the area of ​​the first portion removed by chamfering in a cross section including the central axis of the dummy tube is 20% or more of the sum of the area of ​​the first portion and the area of ​​the second portion overlapping the chamfered surface of the dummy tube as viewed in a direction perpendicular to the central axis. In this case, it is even more difficult to form a convex portion on the outside of the cladding tube.

[0013] (3) In the above (1), the dummy tube may be chamfered so that the thickness of the end face of the dummy tube after chamfering is 60% or less of the thickness of the end face of the dummy tube before chamfering, which further reduces the likelihood of formation of a protrusion on the outside of the cladding tube.

[0014] (4) A method for manufacturing an optical fiber preform according to a second aspect of the present disclosure includes the steps of chamfering corners between the end face of a dummy tube and the outer peripheral surface of the dummy tube and corners between the end face of a cladding tube and the outer peripheral surface of the cladding tube, and welding the chamfered end face of the dummy tube to the chamfered end face of the cladding tube. In this method for manufacturing an optical fiber preform, the outer corners of the dummy tube and the outer corners of the cladding tube are chamfered, so that spaces corresponding to the portions removed by chamfering are formed outside the chamfered surfaces of the dummy tube and the cladding tube. Therefore, protrusions are less likely to be formed on the outside of the cladding tube during the welding process.

[0015] (5) In the above (4), the dummy tube may be chamfered so that the area of ​​the first portion removed by chamfering in a cross section including the central axis of the dummy tube is 10% or more of the sum of the area of ​​the first portion and the area of ​​the second portion overlapping the chamfered surface of the dummy tube as viewed in a direction perpendicular to the central axis. In this case, it is even more difficult to form a convex portion on the outside of the cladding tube.

[0016] (6) In the above (4), the dummy tube may be chamfered so that the thickness of the end face of the dummy tube after chamfering is 80% or less of the thickness of the end face of the dummy tube before chamfering, which makes it even more difficult for a protrusion to be formed on the outside of the cladding tube.

[0017] (7) In any of (1) to (6) above, the dummy tube may be chamfered so that, in a cross section including the central axis of the dummy tube, the area of ​​the first portion removed by chamfering is 20% or more of the sum of the area of ​​the first portion and the area of ​​the second portion overlapping the chamfered surface of the dummy tube when viewed along a direction perpendicular to the central axis. If too much of the first portion is removed by chamfering, the end of the dummy tube is likely to deform when the dummy tube is heated and melted, which could result in clogging of the holes in the cladding tube. By setting the area of ​​the first portion to less than 40% of the sum, the holes in the cladding tube are less likely to be clogging.

[0018] (8) In any of (1) to (6) above, the dummy tube may be chamfered so that the thickness of the dummy tube at the end face after chamfering is more than 20% of the thickness of the dummy tube at the end face before chamfering. If too much of the first portion is removed by chamfering, the end of the dummy tube is likely to deform when the dummy tube is heated and melted, which could result in blocking of the holes in the cladding tube. By having the thickness of the dummy tube at the first end face after chamfering be more than 20% of the thickness of the dummy tube at the first end face before chamfering, the holes in the cladding tube are less likely to be blocked.

[0019] [Details of the embodiments 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. The present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted.

[0020] (First embodiment) Fig. 1 is a flowchart showing a method for manufacturing an optical fiber preform according to a first embodiment. Fig. 2 is a cross-sectional view for explaining a welding step according to the first embodiment. As shown in Fig. 1, the method for manufacturing an optical fiber preform according to the first embodiment includes a chamfering step S1, a holding step S2, a welding step S3, a slow cooling step S4, an insertion step S5, and an integration step S6. The optical fiber preform is manufactured by carrying out these steps S1 to S6 in this order.

[0021] The chamfering step S1 is a step of chamfering a pair of dummy tubes 20 shown in FIG. 2 . In FIG. 2 , only the first dummy tube 20 of the pair of dummy tubes 20 is shown in the axial direction D1, sandwiching the cladding tube 10, and the second dummy tube 20 is not shown. The dummy tube 20 is a relatively inexpensive glass tube welded to the cladding tube 10. The cladding tube 10 is a glass tube that serves as the cladding portion of the optical fiber preform. The cladding tube 10 is made of silica-based glass. The cladding tube 10 has a pair of end faces 10a facing the axial direction D1 along the central axis 10x of the cladding tube 10. The cladding tube 10 has an outer peripheral surface 10p. The cladding tube 10 has a pair of corners 10c spaced apart from each other in the axial direction D1 between the pair of end faces 10a and the outer peripheral surface 10p. The cross section shown in FIG. 2 includes the central axis 10x.

[0022] The cladding tube 10 is provided with a hole 10h for inserting a core rod. The hole 10h extends along the central axis 10x and has a circular cross section. In this embodiment, the cladding tube 10 is a cladding tube for a multi-core optical fiber preform and is provided with a plurality of holes 10h. The number of holes 10h is, for example, four. In FIG. 2, the number of holes 10h is simplified to two. Note that the cladding tube 10 may also be a cladding tube for a single-core optical fiber preform. In this case, the number of holes 10h is one.

[0023] The dummy tube 20 is made of silica-based glass. The dummy tube 20 is a cylindrical glass tube provided with one hole 20h. The hole 20h extends along the central axis 20x of the dummy tube 20 and has a circular cross section. The inner diameter of the dummy tube 20, i.e., the diameter of the hole 20h, is large enough to allow core rods to be inserted into all of the holes 10h of the cladding tube 10. When the cladding tube 10 is a cladding tube for a multi-core optical fiber preform, the holes 10h are provided over a wider area of ​​the end face 10a than in a cladding tube for a single-core optical fiber preform, making it easier to increase the size of the dummy tube 20. The outer diameter of the dummy tube 20 is, for example, equal to or smaller than the outer diameter of the cladding tube 10.

[0024] The dummy tube 20 has a first end face 20a and a second end face 20b facing the axial direction D2 along the central axis 20x. The dummy tube 20 has an outer circumferential surface 20p. The first end face 20a is the end face that is welded to the clad tube 10. The end of the dummy tube 20 in the region close to the second end face 20b is the portion that is gripped by the gripping part 30. Before chamfering, the dummy tube 20 has a corner 20c between the first end face 20a and the outer circumferential surface 20p.

[0025] In the chamfering process S1, the corners 20c of the dummy tube 20 are chamfered. The chamfering forms a chamfered surface 20d between the first end face 20a and the outer peripheral surface 20p. The chamfered surface 20d is an inclined surface inclined with respect to the axial direction D2. The angle θ of the inclination of the chamfered surface 20d with respect to the axial direction D2 is, for example, 20 degrees or more and 70 degrees or less. The chamfered surface 20d has a tapered shape that approaches the central axis of the dummy tube 20 in a direction perpendicular to the axial direction D2 as it approaches the first end face 20a in the axial direction D2.

[0026] The length La of the chamfered surface 20d in the axial direction D2 is, for example, 1 mm or more and 10 mm or less. The length Lb of the chamfered surface 20d in the direction perpendicular to the axial direction D2, i.e., the length of the chamfered surface 20d in the radial direction of the dummy tube 20, is, for example, 1 mm or more and 10 mm or less. The chamfering is performed uniformly around the entire circumference of the corner 20c. Therefore, the angle θ, length La, and length Lb are uniform around the entire circumference of the chamfered surface 20d.

[0027] The dummy tube 20 is chamfered, for example, so that the thickness t of the dummy tube 20 at the first end face 20a after chamfering is 60% or less of the thickness T of the dummy tube 20 at the first end face 20a before chamfering. That is, t / T≦0.6. That is, the dummy tube 20 is chamfered so that the thickness of the dummy tube 20 at the first end face 20a is reduced by 40% or more due to the chamfering. That is, (T−t) / T≧0.4, T−t=Lb. The dummy tube 20 is chamfered, for example, so that the thickness t is more than 20% of the thickness T. That is, t / T>0.2. That is, the dummy tube 20 is chamfered so that the thickness reduction of the dummy tube 20 at the first end face 20a due to the chamfering is less than 80%. That is, (T−t) / T<0.8.

[0028] The portion of the end of the dummy tube 20 that is removed by chamfering is referred to as a first portion 21, and the portion that remains after chamfering is referred to as a second portion 22. The second portion 22 includes the entire chamfered surface 20d but does not include the outer circumferential surface 20p. The second portion 22 can also be said to be the portion that overlaps with the chamfered surface 20d when viewed in a direction perpendicular to the central axis 20x.

[0029] The dummy tube 20 is chamfered, for example, so that the area A1 of the first portion 21 is 20% or more of the sum A1 + A2 of the area A1 and the area A2 of the second portion 22 in a cross section including the central axis 20x shown in FIG. 2 . That is, A1 / (A1 + A2)≧0.2. The dummy tube 20 is chamfered, for example, so that the area A1 is less than 40% of the sum A1 + A2 in a cross section including the central axis 20x. That is, A1 / (A1 + A2)<0.4.

[0030] 3 is a diagram for explaining the area of ​​the first portion and the area of ​​the second portion. In FIG. 3, the first portion 21 is hatched, and the second portion 22 is not hatched. In a cross section including the central axis 20x, the first portion 21 and the second portion 22 are each divided into two and disposed on either side of the central axis 20x. Therefore, the area A1 is calculated as A1 = La × Lb. The area A2 is calculated as 2 × La × T - La × Lb.

[0031] The dummy tube 20 is chamfered, for example, so that the volume V1 of the first portion 21 is 20% or more of the sum V1 + V2 of the volume V1 and the volume V2 of the second portion 22. That is, V1 / (V1 + V2) ≥ 0.2. The dummy tube 20 is chamfered, for example, so that the volume V1 is less than 40% of the sum V1 + V2. That is, V1 / (V1 + V2) < 0.4.

[0032] If the end portion of the dummy tube 20 consisting only of the first portion 21 and the second portion 22 is defined as the chamfering target portion, the area ratio of the chamfering target portion to the first portion 21 to be removed in a cross section including the central axis 20x is 20% or more and less than 40%. It can be said that the dummy tube 20 is chamfered so that, for example, 20% or more and less than 40% of the volume of the chamfering target portion is removed. In other words, the volume ratio of the chamfering target portion to the first portion 21 to be removed is 20% or more and less than 40%.

[0033] The holding step S2 is a step of holding the clad tube 10 and the pair of dummy tubes 20. The clad tube 10 is held, for example, by a lathe. The dummy tube 20 is held by a holding unit 30. The holding unit 30 is, for example, a chuck provided on the lathe. The clad tube 10 and the pair of dummy tubes 20 are held so that the end faces 10a and the first end faces 20a face each other. That is, the first end face 10a of the clad tube 10 faces the first end face 20a of the first dummy tube 20, and the second end face 10a of the clad tube 10 faces the first end face 20a of the second dummy tube 20. At this time, the clad tube 10 and the pair of dummy tubes 20 are held so that the central axes 10x and 20x coincide with each other and the axial directions D1 and D2 are parallel to each other.

[0034] 2, the welding step S3 is a step of welding the chamfered first end face 20a of the dummy tube 20 held by the holding unit 30 to the end face 10a of the clad tube 10. In the welding step S3, the end face 10a of the clad tube 10 and the first end face 20a of the dummy tube 20 are simultaneously heated and melted by a heat source 31, and then the melted end faces are welded together. The pair of dummy tubes 20 may be welded to the pair of end faces 10a of the clad tube 10 simultaneously or sequentially.

[0035] The slow cooling step S4 is a step that follows the welding step S3, in which the welded portion between the clad tube 10 and the dummy tube 20 is maintained at a temperature equal to or higher than the slow cooling point for a predetermined time, and then the temperature is lowered. The slow cooling step S4 can relieve stress remaining during welding.

[0036] The insertion step S5 is a step of inserting core rods into the holes 10h of the cladding tube 10. In this embodiment, multiple core rods are inserted into the multiple holes 10h one by one. For example, the glass rods may pass through the holes 20h of the dummy tube 20 and be inserted into the multiple holes 10h from the openings of the end face 10a.

[0037] The integration step S6 is a step in which the cladding tube 10 and the core rod are heated and melted to be integrated with each other by, for example, a rod-in-tube method. An induction furnace, a resistance furnace, an oxyhydrogen burner, or the like is used as a heat source. The integration step S6 is also called a collapse step. An optical fiber preform is manufactured by the integration step S6.

[0038] 4 is a cross-sectional view illustrating the integration process according to the comparative example. The manufacturing method of the optical fiber preform according to the comparative example differs from the manufacturing method of the optical fiber preform according to the first embodiment in that it does not include the chamfering process S1. As a result, in the manufacturing method of the optical fiber preform according to the comparative example, a protrusion 40 that protrudes outside the cladding tube 10 can be easily formed in the welding process S2. The protrusion 40 protrudes outside the cladding tube 10 when viewed along the axial direction D1.

[0039] In the integration step S6, the induction furnace 50 is moved along the axial direction D1 to integrate the cladding tube 10 and the core rod by heating. A pair of airtight components 51, the inner diameter of which can be adjusted according to the outer diameter of the cladding tube 10, are provided at both ends of the induction furnace 50 in the axial direction D1. The airtight components 51 prevent atmospheric air from entering the induction furnace 50 from the outside and also prevent inert gas introduced into the induction furnace 50 from leaking to the outside. This effect of the airtight components 51 is enhanced as the inner diameter of the airtight components 51 approaches the outer diameter of the cladding tube 10, but this increases the likelihood of the protrusions 40 coming into contact with the airtight components 51. Contact between the protrusions 40 and the airtight components 51 may damage the cladding tube 10 and the dummy tube 20. As a result, there is a risk of gas leaking from the inside of the cladding tube 10. Furthermore, this reduces the productivity of the optical fiber preform.

[0040] In contrast, in the manufacturing method of the optical fiber preform according to the first embodiment, the outer corner 20c of the dummy tube 20 is chamfered in the chamfering step S1. As a result, a space corresponding to the first portion 21 removed by the chamfering is formed outside the chamfered surface 20d of the dummy tube 20. In the welding step S3, molten glass can be accommodated in this space, making it difficult for a protrusion 40 to form on the outside of the cladding tube 10. In other words, no protrusion 40 is formed, or even if a protrusion 40 is formed, the size of the protrusion 40 can be made smaller than the size of the protrusion 40 formed in the comparative example. Therefore, in the integration step S6, damage to the cladding tube 10, the dummy tube 20, or both due to the protrusion 40, which would cause internal gas leakage, can be prevented. Furthermore, the productivity of the optical fiber preform is improved.

[0041] The dummy tube 20 is chamfered, for example, so that the thickness t is 60% or less of the thickness T. This makes it even more difficult for the convex portion 40 to be formed. The dummy tube 20 is chamfered, for example, so that the area A1 is 20% or more of the sum A1 + A2. This makes it even more difficult for the convex portion 40 to be formed. The dummy tube 20 is chamfered, for example, so that the volume V1 is 20% or more of the sum V1 + V2. This makes it even more difficult for the convex portion 40 to be formed.

[0042] Second Embodiment A method for manufacturing an optical fiber preform according to a second embodiment will be described, focusing on differences from the method for manufacturing an optical fiber preform according to the first embodiment. FIG. 5 is a cross-sectional view illustrating a welding process according to the second embodiment. In the second embodiment, the chamfering process S1 is a process for chamfering corners 20c of a pair of dummy tubes 20 and a pair of corners 10c of a cladding tube 10 shown in FIG. 5. In the chamfering process S1, either the chamfering of the corners 20c or the chamfering of the corners 10c may be performed first, or both may be performed simultaneously. FIG. 5 shows only the first dummy tube 20, and the second dummy tube 20 is not shown.

[0043] The chamfering of the corners 10c forms chamfered surfaces 10d between the pair of end faces 10a and the outer circumferential surface 10p. The chamfered surfaces 10d are inclined with respect to the axial direction D1. The chamfered surfaces 10d have a tapered shape that approaches the central axis of the cladding tube 10 in a direction perpendicular to the axial direction D1 as they approach the end faces 10a in the axial direction D1.

[0044] The chamfering of the corners 20c forms chamfered surfaces 20d similar to those in the first embodiment. In the second embodiment, the dummy tube 20 is chamfered, for example, so that the thickness t is 80% or less of the thickness T. That is, t / T≦0.8. That is, the dummy tube 20 is chamfered so that the thickness of the dummy tube 20 at the first end surface 20a is reduced by 20% or more. That is, (T−t) / T≧0.2. The dummy tube 20 is chamfered, for example, so that the thickness t is more than 20% of the thickness T. That is, t / T>0.2. That is, the dummy tube 20 is chamfered so that the thickness reduction of the dummy tube 20 at the first end surface 20a is less than 80%. That is, (T−t) / T<0.8.

[0045] The dummy tube 20 is chamfered, for example, so that the area A1 in a cross section including the central axis 20x is 10% or more of the sum A1 + A2. That is, A1 / (A1 + A2) ≧ 0.1. The dummy tube 20 is chamfered, for example, so that the area A1 in a cross section including the central axis 20x is less than 40% of the sum A1 + A2. That is, A1 / (A1 + A2) < 0.4. The dummy tube 20 is chamfered, for example, so that the volume V1 is 10% or more of the sum V1 + V2. That is, V1 / (V1 + V2) ≧ 0.1. The dummy tube 20 is chamfered, for example, so that the volume V1 is less than 40% of the sum V1 + V2. That is, V1 / (V1 + V2) < 0.4.

[0046] If the end portion of the dummy tube 20 consisting only of the first portion 21 and the second portion 22 is defined as the chamfering target portion, then the dummy tube 20 can be said to be chamfered so that, for example, 10% or more and less than 40% of the area of ​​the chamfering target portion is removed in a cross section including the central axis 20x. In other words, in a cross section including the central axis 20x, the area ratio of the removed first portion 21 to the chamfering target portion is 10% or more and less than 40%. In other words, the volume ratio of the removed first portion 21 to the chamfering target portion is 10% or more and less than 40%.

[0047] In the welding step S3, as shown in FIG. 5, the chamfered first end surface 20a of the dummy tube 20 held by the holding portion 30 is welded to the chamfered end surface 10a of the clad tube 10.

[0048] In the method for manufacturing an optical fiber preform according to the second embodiment, the outer corners 20c of the dummy tube 20 are also chamfered in the chamfering step S1, making it difficult for convex portions 40 to form on the outside of the cladding tube 10. In the second embodiment, the outer corners 10c of the cladding tube 10 are also chamfered, so that a space corresponding to the portion removed by chamfering is also formed outside the chamfered surface 10d of the cladding tube 10. This provides a larger space for accommodating molten glass than in the first embodiment. As a result, it is more difficult for convex portions 40 to form.

[0049] 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 of the present disclosure.

[0050] The present disclosure will be described in more detail below by showing the results of evaluation tests using examples and comparative examples according to the present disclosure, but the present disclosure is not limited to these examples.

[0051] (Test Examples 1 to 8) In Test Example 1, a dummy tube with an outer diameter of 90 mm, an inner diameter of 70 mm, a thickness of 10 mm, and a length of 1000 mm was prepared. A cladding tube with an outer diameter of 90 mm, a length of 1000 mm, and four holes with a diameter of 20 mm as openings was prepared. Both the dummy tube and the cladding tube were made of silica-based glass.

[0052] Next, the clad tube and the dummy tube were clamped in a horizontal lathe, and the end face of the clad tube and the first end face of the dummy tube were simultaneously heated for 20 minutes using an oxyhydrogen burner. The heating was performed while maintaining a predetermined heating power and positional relationship. The heated, melted end faces were then pressed against each other to weld. After welding, the weld was held at a temperature above the annealing point for 20 minutes or more, and then cooled. In Test Example 1, a protrusion was formed on the outside of the clad tube when viewed along the axial direction of the clad tube. The height of the protrusion, i.e., the maximum height to which the protrusion protruded outside the clad tube when viewed along the axial direction of the clad tube, was 8 mm.

[0053] In Test Examples 2 to 8, except for chamfering the corner between the first end face and the outer peripheral surface of the dummy tube, the dummy tube and the clad tube were welded in the same manner as in Test Example 1. The amount of push-in (axial distance) between the end faces during welding was kept constant in Test Examples 1 to 8. In the chamfering process in Test Examples 2 to 8, the angle θ of the inclination of the chamfered surface relative to the axial direction of the dummy tube was fixed at 40 degrees, and the thickness t of the dummy tube at the first end face after chamfering was increased by 1 mm starting from 1 mm.

[0054] Table 1 shows the angle θ (degrees), thickness T (mm), length La (mm), and length Lb (mm) for Test Examples 1 to 8. Table 2 shows the shaved area A1 (mm ２ ), area ratio A1 / (A1+A2), thickness t (mm), thickness ratio t / T, and height of the convex portion. Because the chamfering is uniform over the entire circumference of the dummy tube, the area ratio A1 / (A1+A2) is substantially equal to the volume ratio V1 / (V1+V2).

[0055]

[0056] Test Example 1 corresponds to the above-mentioned comparative example, and since the dummy tube was not chamfered, a convex portion with a height of 8.0 mm was formed after connecting the cladding tube and the dummy tube. Test Examples 2 to 8 correspond to the above-mentioned first embodiment, and since the dummy tube was chamfered, it was confirmed that convex portions were less likely to form than in Test Example 1. In Test Examples 2 to 4, convex portions were formed, but the height of the convex portions was able to be kept to 4.0 mm or less. In Test Examples 5 to 8, no convex portions were formed. If the height was 0.0 mm or less, i.e., a concave portion was formed, it was recorded as 0.0 mm. In other words, if the cross-sectional area of ​​the scraped portion, i.e., the chamfered portion, was 9.5 mm ２ When the area ratio A1 / (A1+A2) was greater than 20%, sufficient space was created, and no convex portions were formed even after connecting the cladding tube and the dummy tube. From the above, it was confirmed that by setting the area ratio A1 / (A1+A2) to 20% or more, it becomes even more difficult for convex portions to form.

[0057] (Test Examples 9 to 16) In Test Examples 9 to 16, the dummy tube and the clad tube were welded in the same manner as Test Examples 1 to 8, except that the corner between the end face and the outer circumferential surface of the clad tube was also chamfered. The clad tube was chamfered so that the angle of the chamfered surface relative to the axial direction of the clad tube was 25 degrees, and the diameter of the end face of the clad tube after chamfering was φ80 mm.

[0058] Table 3 shows the angle θ (degrees), thickness T (mm), length La (mm), and length Lb (mm) for Test Examples 9 to 16. Table 4 shows the shaved area A1 (mm ２ ), area ratio A1 / (A1+A2), thickness t (mm), thickness ratio t / T, and height of the convex portion.

[0059]

[0060] In Test Examples 9 to 16, the cladding tube was also chamfered, so convex portions were less likely to form than in Test Examples 1 to 8. In Test Example 9, the dummy tube was not chamfered, so it was confirmed that larger convex portions were formed than in Test Examples 10 to 16. In Test Example 10, convex portions were formed, but the height of the convex portions was 3.0 mm or less. In Test Examples 11 to 16, no convex portions were formed. If the height was 0.0 mm or less, i.e., a concave portion was formed, it was recorded as 0.0 mm. In other words, if the cross-sectional area of ​​the scraped portion, i.e., the chamfered portion, was 2.3 mm ２ When the area ratio A1 / (A1+A2) was greater than 10%, sufficient space was created, and no convex portions were formed even after connecting the cladding tube and the dummy tube. From the above, it was confirmed that by setting the area ratio A1 / (A1+A2) to 10% or more, it becomes even more difficult for convex portions to form.

[0061] 10... Clad pipe 10a... End surface 10c... Corner 10d... Chamfered surface 10h... Hole 10p... Outer peripheral surface 10x... Center axis 20... Dummy tube 20a... First end surface 20b... Second end surface 20c... Corner 20d... Chamfered surface 20h... Hole 20p... Outer peripheral surface 20x... Center axis 21...First part 22...Second part 30...Gripping part 31...Heat source 40...Convex part 50...Induction furnace 51...Airtight part S1...Chamfering process S2...Gripping process S3...Welding process S4...Learning process S5...Insertion process S6...Integration process θ...Angle

Claims

1. A method for manufacturing an optical fiber preform, comprising: a step of chamfering a corner between an end face of a dummy tube and the outer peripheral surface of the dummy tube; and a step of welding the chamfered end face of the dummy tube to the end face of a cladding tube.

2. A method for manufacturing an optical fiber preform according to claim 1, wherein the dummy tube is chamfered so that the area of ​​the first portion removed by chamfering in a cross section including the central axis of the dummy tube is 20% or more of the sum of the area of ​​the first portion and the area of ​​the second portion that overlaps with the chamfered surface of the dummy tube when viewed along a direction perpendicular to the central axis.

3. The method for manufacturing an optical fiber preform according to claim 1, wherein the dummy tube is chamfered so that the thickness of the dummy tube at the end face after chamfering is 60% or less of the thickness of the dummy tube at the end face before chamfering.

4. A method for manufacturing an optical fiber preform, comprising: a step of chamfering a corner between an end face of a dummy tube and the outer peripheral surface of the dummy tube, and a corner between the end face of a cladding tube and the outer peripheral surface of the cladding tube; and a step of welding the chamfered end face of the dummy tube to the chamfered end face of the cladding tube.

5. A method for manufacturing an optical fiber preform according to claim 4, wherein the dummy tube is chamfered so that the area of ​​the first portion removed by chamfering in a cross section including the central axis of the dummy tube is 10% or more of the sum of the area of ​​the first portion and the area of ​​the second portion that overlaps with the chamfered surface of the dummy tube when viewed along a direction perpendicular to the central axis.

6. The method for manufacturing an optical fiber preform according to claim 4, wherein the dummy tube is chamfered so that the thickness of the dummy tube at the end face after chamfering is 80% or less of the thickness of the dummy tube at the end face before chamfering.

7. A method for manufacturing an optical fiber preform according to any one of claims 1 to 6, wherein the dummy tube is chamfered so that, in a cross section including the central axis of the dummy tube, the area of ​​the first portion removed by chamfering is less than 40% of the sum of the area of ​​the first portion and the area of ​​the second portion that overlaps with the chamfered surface of the dummy tube when viewed along a direction perpendicular to the central axis.

8. A method for manufacturing an optical fiber preform according to any one of claims 1 to 6, wherein the dummy tube is chamfered so that the thickness of the dummy tube at the end face of the dummy tube after chamfering is more than 20% of the thickness of the dummy tube at the end face of the dummy tube before chamfering.

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