WO2025204837A1 - Method for producing optical fiber preform - Google Patents

Abstract

This method for producing an optical fiber preform comprises a step for welding a dummy tube gripped by a gripping part to a cladding tube. The dummy tube has a first end part to be welded to the cladding tube and a second end part gripped by the gripping part. The thickness of the dummy tube excluding the first end part varies along the length direction of the dummy tube.

Description

Optical fiber preform manufacturing method

This disclosure relates to a method for manufacturing an optical fiber preform. This application claims priority to Japanese Application No. 2024-049091, filed March 26, 2024, and incorporates by reference all of the contents of that Japanese application.

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 optical fiber cladding, drilling a hole where you want the core to be, inserting a core rod into the hole, and integrating the cladding tube and 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 sometimes involves welding a relatively inexpensive dummy tube to the end of the cladding tube, then holding the dummy tube and moving the cladding tube to integrate it with the core rod. In this case, a process is required to fusion-connect (weld) the cladding tube and dummy tube. In this process, the end faces of both tubes are heated and melted, and the ends are then pressed directly against each other to weld them.

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

Japanese Patent Application Laid-Open No. 2000-128559 Japanese Patent Application Laid-Open No. 2023-146915

A method for manufacturing an optical fiber preform according to one aspect of the present disclosure includes a step of welding a dummy tube held by a holding portion to a cladding tube, the dummy tube having a first end welded to the cladding tube and a second end held by the holding portion, and the thickness of the dummy tube excluding the first end varies along the length of the dummy tube.

FIG. 1 is a flowchart showing a method for manufacturing an optical fiber preform according to the first embodiment. FIG. 2 is a cross-sectional view for explaining the welding process. FIG. 3 is a cross-sectional view for explaining a welding process according to a comparative example. FIG. 4 is a cross-sectional view for explaining a welding process according to the second embodiment.

[Problem to be solved by the present disclosure]
To reduce the cost of optical fiber, optical fiber preforms are becoming larger. Accordingly, cladding tubes and dummy tubes are also becoming larger. As the dummy tube becomes larger, the risk of insufficient heating of the end face of the dummy tube increases. In addition, the weight of the dummy tube itself increases, increasing the risk of the weld between the cladding tube and the dummy tube coming loose, and the dummy tube cracking at the grip due to the principle of leverage. When these problems occur, the manufacturability of optical fiber preforms decreases.

This disclosure provides a method for manufacturing an optical fiber preform that can improve manufacturability.

[Effects of the present disclosure]
According to the present disclosure, it is possible to provide a method for manufacturing an optical fiber preform that can improve productivity.

Description of the embodiments of the present disclosure
First, embodiments of the present disclosure will be listed and described.
(1) A method for manufacturing an optical fiber preform according to one aspect of the present disclosure includes a step of welding a dummy tube held by a holding portion to a cladding tube, the dummy tube having a first end welded to the cladding tube and a second end held by the holding portion, and the thickness of the dummy tube excluding the first end varies along the length of the dummy tube.
In the above-described manufacturing method of the optical fiber preform, for example, a dummy tube can be used that is thickened only in the portion where strength is required, thereby reducing the weight, thereby improving the manufacturability of the optical fiber preform.

(2) In (1) above, the thickness of the dummy tube may be greatest at the second end. In this case, the second end is prevented from cracking due to stress from the gripping portion.

(3) In (1) or (2) above, the dummy tube may have a plurality of glass tubes connected to each other, and at least two of the plurality of glass tubes may have different thicknesses. In this case, it is easy to form a configuration in which the thickness of the dummy tube varies along the length.

(4) In (3) above, the multiple glass tubes may be connected so that the thickness of the dummy tube gradually increases from the first end to the second end. In this case, the first end is the thinnest, allowing for a strong weld between the dummy glass tube and the clad tube. Furthermore, the second end is the thickest, preventing the second end from cracking due to stress from the gripping portion.

(5) In (3) above, the multiple glass tubes may be connected so that the thickness of the dummy tube excluding the first end is thinnest between the first end and the second end. Even in this case, the manufacturability of the optical fiber preform can be improved.

(6) In any of (1) to (5) above, the first end portion may have an end face that is welded to the cladding tube, and where the outer diameter of the end face is R1 [mm] and the inner diameter of the end face is R2 [mm], R1/R2 may be 1.28 or less. In this case, the manufacturability of the optical fiber preform can be reliably improved.

(7) In any of (1) to (6) above, where the weight of the dummy tube is Wt [g] and the length of the dummy tube is Ln [mm], Wt/Ln [g/mm] may be 7.3 or less. In this case, the manufacturability of the optical fiber preform can be reliably improved.

[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. 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.

As mentioned above, in the rod-in-tube method, a dummy tube is sometimes used to reduce manufacturing costs and increase yields. The dummy tube must satisfy the following requirements:
1: The dummy tube must have a certain length or more so that the member (mechanism) holding the dummy tube is not damaged by heat from the heat source that integrates the cladding tube and the core rod.
2: The dummy tube must have an inner diameter that allows the core rods to be inserted into all of the holes in the cladding tube.
3: The dummy tube must be thick enough so that it will not crack due to the stress of the gripping part.

Requirement 1 determines the length of the dummy tube. Requirement 2 determines the inner diameter of the dummy tube. Requirement 3 determines the thickness of the dummy tube. However, if the dummy tube is too thick, it will take a long time to heat the end surface of the dummy tube when welding the clad tube and the dummy tube, potentially increasing the risk of insufficient heating. Furthermore, as the weight of the dummy tube increases, the stress on the weld increases, which could result in cracks on the weld surface if the same welding conditions as before are used. Furthermore, as the weight of the dummy tube increases, the stress on the dummy tube at the gripping part increases due to the principle of leverage, increasing the risk of the dummy tube cracking.

The inventors came up with the idea of making the dummy tube lighter while still satisfying requirement 3 by giving the dummy tube a shape that varies in thickness along its length, rather than a shape that has a uniform thickness along its length.

(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 illustrating a welding step. As shown in Fig. 1, the method for manufacturing an optical fiber preform includes a preparation 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.

Preparation step S1 is a step of preparing the cladding tube 10 and a pair of dummy tubes 20A shown in Figure 2. Figure 2 shows only the first dummy tube 20A of the pair of dummy tubes 20A, and omits the illustration of the second dummy tube 20A. 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 hole 10h for inserting a core rod. The hole 10h has a circular cross section. The cladding tube 10 has a pair of end faces 10a in the longitudinal direction of the cladding tube 10. In this embodiment, the cladding tube 10 is a cladding tube for a multi-core optical fiber preform and has multiple holes 10h. The number of holes 10h is, for example, four. (Figure 2 is simplified to two holes.) 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.

The dummy tube 20A is a cylindrical glass tube with one hole 20h. The dummy tube 20A is made of silica-based glass. The dummy tube 20A has a first end 20a and a second end 20b in the longitudinal direction DL of the dummy tube 20A. The first end 20a is the end welded to the clad tube 10. The second end 20b is the end gripped by the gripping portion 30. The first end 20a has a first end face 20c welded to the end face 10a of the clad tube 10. The second end face 20b has a second end face 20d opposite the first end face 20c in the longitudinal direction DL.

The thickness (wall thickness) of the dummy tube 20A excluding the first end 20a varies along the length direction DL. In other words, the thickness of the dummy tube 20A only needs to vary along the length direction DL at least in the portion excluding the first end 20a. The thickness of the dummy tube 20A may be uniform along the length direction DL within the first end 20a, or may vary along the length direction DL. The first end 20a may be chamfered, for example. In this case, the thickness of the dummy tube 20A gradually becomes thinner along the length direction DL within the first end 20a as it approaches the first end face 20c.

The thickness of the dummy tube 20A increases in stages, for example, from the first end 20a to the second end 20b. The thickness of the dummy tube 20A is, for example, thickest at the second end 20b and thinnest at the first end 20a.

The inner diameter of the dummy tube 20A 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 cladding tubes for single-core optical fiber preforms, making it easier for the dummy tube 20A to be large. The outer diameter of the dummy tube 20A is, for example, the same as or smaller than the outer diameter of the cladding tube 10.

The dummy tube 20A has a plurality of glass tubes 21, 22 connected to each other. In this embodiment, the dummy tube 20A has two glass tubes 21, 22. The plurality of glass tubes 21, 22 are connected to each other, for example, by welding. The glass tube 21 includes a first end 20a. The glass tube 22 includes a second end 20b. The plurality of glass tubes 21, 22 have different thicknesses. The plurality of glass tubes 21, 22 are connected so that, for example, the thickness of the dummy tube 20A gradually increases from the first end 20a toward the second end 20b.

The thickness of glass tube 22 is thicker than the thickness of glass tube 21. Glass tube 21 has a uniform thickness along the length direction DL, at least in the portion excluding first end 20a. The thickness of glass tube 21 is, for example, the thickness of the portion excluding first end 20a. Glass tube 22 has a uniform thickness along the length direction DL. The outer diameters of glass tubes 21 and 22 are, for example, the same. The inner diameter of glass tube 21 is, for example, larger than the inner diameter of glass tube 22. Therefore, a step corresponding to the connection portion of glass tubes 21 and 22 exists on the inner surface of hole 20h.

If the outer diameter of the first end face 20c is R1 [mm] and the inner diameter of the first end face 20c is R2 [mm], the outer diameter/inner diameter ratio R1/R2 is, for example, 1.28 or less. R1/R2 is, for example, 1.19 or more. If the weight of the dummy tube 20A is Wt [g] and the length of the dummy tube 20A is Ln [mm], the weight/length ratio Wt/Ln [g/mm] is, for example, 7.3 or less. Wt/Ln [g/mm] is, for example, 6.7 or more.

The gripping process S2 is a process of gripping the clad tube 10 and the pair of dummy tubes 20A. The clad tube 10 is gripped, for example, by a lathe. The dummy tube 20A is gripped at the second end 20b by a gripping unit 30. The gripping unit 30 is, for example, a chuck provided on the lathe. The clad tube 10 and the pair of dummy tubes 20A are gripped so that the end face 10a and the first end face 20c face each other. In other words, the first end face 10a of the pair of end faces 10a of the clad tube 10 faces the first end face 20c of the first dummy tube 20A, and the second end face 10a of the clad tube 10 faces the first end face 20c of the second dummy tube 20A.

As shown in FIG. 2, welding process S3 is a process of welding the dummy tube 20A held by the holding portion 30 to the clad tube 10. In welding process S3, the end face 10a of the clad tube 10 and the first end face 20c of the dummy tube 20A are simultaneously heated and melted with a heat source, and the melted end faces are then welded together. The pair of dummy tubes 20A may be welded to the pair of end faces 10a of the clad tube 10 simultaneously, or they may be welded sequentially.

The slow cooling process S4 is a process that follows the welding process S3, in which the welded joint between the clad tube 10 and the dummy tube 20A is held at a temperature above the slow cooling point for a predetermined period of time, and then the temperature is lowered. The slow cooling process S4 relieves any residual stress that may have occurred during welding.

The insertion process S5 is a process 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. The glass rod may, for example, pass through the holes 20h of the dummy tube 20A and be inserted into the multiple holes 10h from the opening of the end face 10a.

The integration process S6 is a process in which the cladding tube 10 and the core rod are heated and melted to integrate them using, for example, the rod-in-tube method. This produces an optical fiber preform.

Figure 3 is a cross-sectional view illustrating the welding process according to the comparative example. The manufacturing method for the optical fiber preform according to the comparative example differs from the manufacturing method for the optical fiber preform according to the first embodiment in that a dummy tube 120 is used instead of the dummy tube 20A. The dummy tube 120 is composed of a single glass tube having a uniform shape along the length direction DL. In other words, the thickness of the dummy tube 120 is uniform along the length of the dummy tube 120. The dummy tube 120 has a first end 120a that is welded to the cladding tube 10 and a second end 120b that is gripped by the gripping portion 30.

If the thickness of the dummy tube 120 is determined to satisfy requirement 3 above, the thickness of the dummy tube 120 will be large throughout the entire dummy tube 120, and the weight of the dummy tube 120 will be heavy. This increases the risk of cracks occurring at the welded surface between the dummy tube 120 and the clad tube 10 and at the gripping portion 30.

In contrast, in the manufacturing method of the optical fiber preform according to the first embodiment, the thickness of the dummy tube 20A, excluding the first end 20a, varies along the longitudinal direction DL. The thickness of the dummy tube 20A is thickest at the second end 20b. As a result, the weight of the dummy tube 20A can be reduced while satisfying the above-mentioned requirement 3 using the second end 20b. Because the weight of the dummy tube 20A is reduced, the risk of cracks occurring at the welded surface between the dummy tube 20A and the cladding tube 10 and cracks occurring in the dummy tube 20A at the gripping portion 30 is reduced. Furthermore, because the thickness of the glass tube 21 having the first end 20a is thinner than the thickness of the glass tube 22, heating of the first end 20a is easier and the risk of insufficient heating is reduced. This improves the manufacturability of the optical fiber preform.

Second Embodiment
FIG. 4 is a cross-sectional view illustrating a welding process according to the second embodiment. The manufacturing method of the optical fiber preform according to the second embodiment differs from the manufacturing method of the optical fiber preform according to the first embodiment in that a dummy tube 20B is used instead of the dummy tube 20A. The following describes the dummy tube 20B, focusing on the differences from the dummy tube 20A. As shown in FIG. 4 , the dummy tube 20B has a plurality of glass tubes 23, 24, and 25 connected to each other. In this embodiment, the dummy tube 20B has three glass tubes 23, 24, and 25. The plurality of glass tubes 23, 24, and 25 are connected to each other by, for example, welding. The glass tube 23 includes a first end 20a. The glass tube 25 includes a second end 20b. The glass tube 24 is disposed between the glass tube 23 and the glass tube 25.

The thickness of the glass tubes 23, 25 is thicker than the thickness of the glass tube 24. That is, at least two of the multiple glass tubes 23, 24, 25 have different thicknesses. Therefore, the thickness of the dummy tube 20B excluding the first end 20a also varies along the longitudinal direction DL. The multiple glass tubes 23, 24, 25 are connected so that the thickness of the dummy tube 20B excluding the first end 20a is thinnest between the first end 20a and the second end 20b. The thickness of the dummy tube 20B is thickest at the second end 20b. The glass tubes 23, 25 may have the same thickness, for example, or the thickness of the glass tube 25 may be thicker than the thickness of the glass tube 23.

The outer diameters of glass tubes 23, 24, and 25 are, for example, the same. The inner diameter of glass tubes 23 and 25 is, for example, smaller than the inner diameter of glass tube 24. Therefore, on the inner surface of hole 20h, there is a step corresponding to the connection between glass tubes 23 and 24, and a step corresponding to the connection between glass tubes 24 and 25.

As explained above, in the method for manufacturing an optical fiber preform according to the second embodiment, the thickness of the dummy tube 20B, excluding the first end 20a, varies along the longitudinal direction DL. The thickness of the dummy tube 20B is thickest at the second end 20b. Therefore, the weight of the dummy tube 20B can be reduced while satisfying the above-mentioned requirement 3 at the second end 20b. Because the weight of the dummy tube 20B is reduced, the risk of cracks occurring at the welded surface between the dummy tube 20B and the cladding tube 10 and cracks occurring in the dummy tube 20B at the gripping portion 30 is reduced. This reduces the manufacturability of the optical fiber preform.

Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and variations, and various modifications are possible without departing from the spirit of the present disclosure. The above-described embodiments and variations may be combined as appropriate.

Below, the results of evaluation tests using examples and comparative examples related to this disclosure will be presented, and this disclosure will be explained in more detail. The present invention is not limited to these examples.

(Test Examples 1 to 5)
In Test Example 1, a dummy tube having an outer diameter of 100 mm, an inner diameter of 70 mm, a thickness of 15 mm, and a length of 1500 mm was prepared as a dummy tube corresponding to the dummy tube 120 according to the comparative example described above. In addition, a cladding tube having an outer diameter of 100 mm, a length of 1000 mm, and four holes with a diameter of 20 mm as openings was prepared.

Next, the clad tube and dummy tube were clamped in a lathe, and the end face of the clad tube and the first end face of the dummy tube were heated simultaneously for 20 minutes using an oxyhydrogen burner. Heating was carried out with the specified heating power and positional relationship maintained. The end faces, which had been melted by heating, were then welded together. After welding, the weld was held at a temperature above the annealing point for at least 20 minutes before being cooled.

A static load tensile strength test was conducted on the resulting welded joint between the clad pipe and the dummy pipe using a horizontal lathe. In this test, a load was applied to the end of the clad pipe to which the dummy pipe was not welded, while the second end of the dummy pipe was held by the lathe. A load smaller than the test limit of 2,000 kg caused the weld to break.

Test Examples 2 to 5 were conducted in the same manner as Test Example 1, except that the dummy tubes had the sizes shown in Table 1. Table 1 shows the specifications of the dummy tubes for Test Examples 1 to 5, as well as the types of failure as evaluation results. "Wt/Ln (g/mm)" is the weight/length ratio of the dummy tube. "R1/R2" is the outer diameter/inner diameter ratio at the first end face of the dummy tube. Failure type "A" indicates failure of the weld, i.e., the clad tube came off the dummy tube. Failure type "B" indicates failure of the gripping portion, i.e., the dummy tube cracked at the gripping portion.

As shown in Table 1, in Test Example 2, the dummy tube was thick, so the dummy tube itself did not break. However, because the dummy tube was thick, it is believed that the weld broke due to insufficient heating of the first end of the dummy tube or the weight of the dummy tube. In Test Examples 3 to 5, the dummy tube was thin and lacked strength, so it is believed that the dummy tube broke at the gripping portion.

(Test Examples 6 to 8)
In Test Example 6, a dummy tube consisting of two connected glass tubes was prepared as a dummy tube corresponding to the dummy tube 20A according to the first embodiment. The glass tube to be welded to the clad tube had an outer diameter of 100 mm, an inner diameter of 84 mm, a thickness of 8 mm, and a length of 500 mm. The glass tube to be gripped by the gripping portion had an outer diameter of 100 mm, an inner diameter of 75 mm, a thickness of 12.5 mm, and a length of 1000 mm. Test Example 6 was carried out in the same manner as Test Example 1, except that a different dummy tube was used. In Test Example 6, no fracture occurred in the welded portion or the gripping portion even when a load of 2000 kg, the test limit, was applied.

Test Examples 7 and 8 were conducted in the same manner as Test Example 6, except that the glass tubes to be welded had the sizes shown in Table 2. Table 2 shows the glass tube sizes for Test Examples 6 to 8. Table 3 shows the specifications of the dummy tubes for Test Examples 6 to 8, as well as the types of fractures that occurred as evaluation results.

As shown in Table 2, no fracture occurred in Test Examples 7 and 8. It is believed that fracture will not occur if the outer diameter/inner diameter ratio (R1/R2) at the first end face of the dummy tube to be welded to the clad tube is at least 1.28 or less. It is also believed that fracture will not occur if the weight/length ratio (Wt/Ln) is 7.3 or less.

(Test Example 9)
In Test Example 9, a dummy tube consisting of three connected glass tubes was prepared as a dummy tube corresponding to the dummy tube 20B according to the second embodiment. The glass tube to be welded to the cladding tube had an outer diameter of 100 mm, an inner diameter of 85 mm, a thickness of 7.5 mm, and a length of 500 mm. The central glass tube had an outer diameter of 100 mm, an inner diameter of 90 mm, a thickness of 5 mm, and a length of 500 mm. The gripped glass tube had an outer diameter of 100 mm, an inner diameter of 80 mm, a thickness of 10 mm, and a length of 500 mm. Test Example 9 was conducted in the same manner as Test Example 1, except for the dummy tube used. Table 4 shows the size of the glass tube in Test Example 9. Table 5 shows the specifications of the dummy tube in Test Example 9 and the type of fracture as an evaluation result.

As shown in Table 5, in Test Example 9, no breakage occurred at the weld or grip, even when a load of 2000 kg, the test limit, was applied. This shows that even if there are thin sections in the length direction, as long as the glass tube being gripped has a certain thickness, no breakage will occur.

10... Clad pipe 10a... End face 10h... Holes 20A, 20B... Dummy tube 20a... First end 20b... Second end 20c... First end face 20d... Second end face 20h... Holes 21, 22, 23, 24, 25... Rath tube 30...Gripping part 120...Dummy tube 120a...First end 120b...Second end DL...Length direction S1...Preparation process S2...Gripping process S3...Welding process S4...Learning process S5...Insertion process S6...Integration process

Claims (7)

welding the dummy tube held by the holding part to the clad tube;
the dummy tube has a first end welded to the clad tube and a second end gripped by the gripping portion;
The thickness of the dummy tube excluding the first end varies along the length of the dummy tube.
A method for manufacturing an optical fiber preform. The thickness of the dummy tube is greatest at the second end.
The method for manufacturing an optical fiber preform according to claim 1 . The dummy tube has a plurality of glass tubes connected to each other,
At least two of the glass tubes have different thicknesses.
3. The method for manufacturing an optical fiber preform according to claim 1. The plurality of glass tubes are connected so that the thickness of the dummy tube increases stepwise from the first end to the second end.
The method for manufacturing an optical fiber preform according to claim 3 . The plurality of glass tubes are connected so that the thickness of the dummy tube excluding the first end is thinnest between the first end and the second end.
The method for manufacturing an optical fiber preform according to claim 3 . the first end has an end surface that is welded to the cladding tube;
When the outer diameter of the end surface is R1 [mm] and the inner diameter of the end surface is R2 [mm], R1/R2 is 1.28 or less.
The method for manufacturing the optical fiber preform according to any one of claims 1 to 5. When the weight of the dummy tube is Wt [g] and the length of the dummy tube is Ln [mm], Wt/Ln [g/mm] is 7.3 or less.
The method for manufacturing the optical fiber preform according to any one of claims 1 to 6.