Method for producing optical fiber preform

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

[0001] This disclosure relates to a method for manufacturing an optical fiber preform. This application claims priority to Japanese Application No. 2024-049091, filed on March 26, 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, grasping the dummy tube, and moving the cladding tube to integrate the cladding tube and the core rod. In this case, a process of fusion-splicing (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 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.

[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 illustrating a welding process. Fig. 3 is a cross-sectional view illustrating a welding process according to a comparative example. Fig. 4 is a cross-sectional view illustrating a welding process according to a second embodiment.

[0007] [Problem to be Solved by the Present Disclosure] In order to reduce the cost of optical fibers, 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 surface of the dummy tube increases. In addition, the weight of the dummy tube itself increases, which increases 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 such problems occur, the manufacturability of the optical fiber preform decreases.

[0008] The present disclosure provides a method for manufacturing an optical fiber preform that can improve manufacturability.

[0009] Effect 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.

[0010] [Description of 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 unit to a cladding tube, the dummy tube having a first end welded to the cladding tube and a second end held by the holding unit, and the thickness of the dummy tube excluding the first end varies along the length of the dummy tube. In the method for manufacturing the optical fiber preform, for example, a dummy tube that is thickened only in areas requiring strength and is lightweight can be used. This improves the manufacturability of the optical fiber preform.

[0011] (2) In the above (1), the thickness of the dummy tube may be greatest at the second end, which prevents the second end from cracking due to stress from the gripping portion.

[0012] (3) In the above (1) or (2), the dummy tube may have a plurality of glass tubes connected to each other, at least two of which may have different thicknesses, which makes it easy to form a configuration in which the thickness of the dummy tube varies along the length.

[0013] (4) In the above (3), 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 cladding tube. Furthermore, the second end is the thickest, preventing the second end from cracking due to stress from the gripping portion.

[0014] (5) In the above (3), the 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, which also improves the manufacturability of the optical fiber preform.

[0015] (6) In any of the above (1) to (5), the first end portion may have an end face to be welded to the cladding tube, and R1 / R2 may be 1.28 or less, where R1 [mm] is the outer diameter of the end face and R2 [mm] is the inner diameter of the end face. In this case, the manufacturability of the optical fiber preform can be reliably improved.

[0016] (7) In any of the above (1) to (6), 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.

[0017] [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.

[0018] As mentioned above, in the rod-in-tube method, a dummy tube may be used to reduce manufacturing costs and increase yield. The dummy tube must meet the following requirements: 1: The dummy tube must have a certain length 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 rod to be inserted into all of the holes in the cladding tube. 3: The dummy tube must have a thickness that will not crack due to stress from the holding part.

[0019] 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, which may increase the risk of insufficient heating. Furthermore, the weight of the dummy tube will increase, which will increase the stress on the welded joint and may cause cracks on the weld surface under conventional welding conditions. Furthermore, the weight of the dummy tube will increase, which will increase the stress on the dummy tube at the gripping portion due to the principle of leverage, which will increase the risk of cracking the dummy tube.

[0020] The inventors came up with the idea of ​​making the dummy tube lighter while satisfying requirement 3 by making the shape of the dummy tube have a thickness that varies along the length rather than a uniform thickness along the length.

[0021] (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. 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.

[0022] The preparation step S1 is a step of preparing the cladding tube 10 and a pair of dummy tubes 20A shown in FIG. 2 . In FIG. 2 , only the first dummy tube 20A of the pair of dummy tubes 20A is shown, and the second dummy tube 20A is not shown. The cladding tube 10 is a glass tube that serves as the cladding portion of an optical fiber preform. The cladding tube 10 is made of silica-based glass. The cladding tube 10 is provided with 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 is provided with multiple holes 10h. The number of holes 10h is, for example, four. (FIG. 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.

[0023] 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 cladding tube 10. The second end 20b is the end gripped by the gripping part 30. The first end 20a has a first end face 20c welded to the end face 10a of the cladding tube 10. The second end face 20b has a second end face 20d opposite the first end face 20c in the longitudinal direction DL.

[0024] The thickness (wall thickness) of the dummy tube 20A excluding the first end 20a varies along the longitudinal direction DL. That is, the thickness of the dummy tube 20A only needs to vary along the longitudinal direction DL at least in the portion excluding the first end 20a. The thickness of the dummy tube 20A may be uniform along the longitudinal direction DL within the first end 20a, or may vary along the longitudinal direction DL. The first end 20a may be chamfered, for example. In this case, the thickness of the dummy tube 20A gradually decreases along the longitudinal direction DL within the first end 20a as it approaches the first end surface 20c.

[0025] The thickness of the dummy tube 20A increases stepwise from the first end 20a to the second end 20b, for example, and is thickest at the second end 20b and thinnest at the first end 20a.

[0026] 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 a cladding tube for a single-core optical fiber preform, so the dummy tube 20A is likely to be large. The outer diameter of the dummy tube 20A is, for example, equal to or smaller than the outer diameter of the cladding tube 10.

[0027] The dummy tube 20A includes a plurality of glass tubes 21, 22 connected to each other. In this embodiment, the dummy tube 20A includes two glass tubes 21, 22. The glass tubes 21, 22 are connected to each other by, for example, welding. The glass tube 21 includes a first end 20a. The glass tube 22 includes a second end 20b. The glass tubes 21, 22 have different thicknesses. The glass tubes 21, 22 are connected such that the thickness of the dummy tube 20A gradually increases from the first end 20a toward the second end 20b.

[0028] The thickness of the glass tube 22 is thicker than the thickness of the glass tube 21. The glass tube 21 has a uniform thickness along the length direction DL, at least in a portion excluding the first end 20a. The thickness of the glass tube 21 is, for example, the thickness of the portion excluding the first end 20a. The glass tube 22 has a uniform thickness along the length direction DL. The outer diameters of the glass tubes 21 and 22 are, for example, the same as each other. The inner diameter of the glass tube 21 is, for example, larger than the inner diameter of the glass tube 22. Therefore, a step corresponding to the connection portion of the glass tubes 21 and 22 exists on the inner surface of the hole 20h.

[0029] 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.

[0030] The holding step S2 is a step of holding the clad tube 10 and the pair of dummy tubes 20A. The clad tube 10 is held, for example, by a lathe. The dummy tube 20A is held at its second end 20b 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 20A are held so that the end faces 10a and the first end faces 20c face each other. That is, 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.

[0031] 2, the welding step S3 is a step of welding the dummy tube 20A held by the holding part 30 to the clad tube 10. In the welding step 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 by a heat source, and then the melted end faces are 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 sequentially.

[0032] The slow cooling step S4 is a step of maintaining the welded portion between the clad tube 10 and the dummy tube 20A at a temperature equal to or higher than the slow cooling point for a predetermined time after the welding step S3, and then lowering the temperature. The slow cooling step S4 can relieve stress remaining after welding.

[0033] 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 20A and be inserted into the multiple holes 10h from the openings of the end face 10a.

[0034] The integration step S6 is a step of, for example, melting the cladding tube 10 and the core rod by heating using a rod-in-tube method to integrate them together, thereby producing an optical fiber preform.

[0035] 3 is a cross-sectional view illustrating a welding process according to a 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 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. That is, the thickness of the dummy tube 120 is uniform along the length direction of the dummy tube 120. The dummy tube 120 has a first end 120a welded to the cladding tube 10 and a second end 120b gripped by the gripping part 30.

[0036] If the thickness of the dummy tube 120 is determined to satisfy the above-mentioned requirement 3, the thickness of the dummy tube 120 becomes thick throughout the dummy tube 120, which increases the weight of the dummy tube 120. 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.

[0037] 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. This allows the weight of the dummy tube 20A to be reduced while satisfying the above-mentioned requirement 3 with the second end 20b. The reduced weight of the dummy tube 20A reduces the risk of cracks occurring at the welded surface between the dummy tube 20A and the cladding tube 10 and at the gripping portion 30. Furthermore, because the thickness of the glass tube 21 having the first end 20a is thinner than that 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.

[0038] Second Embodiment FIG. 4 is a cross-sectional view illustrating a welding process according to a second embodiment. The manufacturing method of an optical fiber preform according to the second embodiment differs from the manufacturing method of an 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.

[0039] 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 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 such 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, or the thickness of the glass tube 25 may be thicker than the thickness of the glass tube 23, for example.

[0040] The outer diameters of the glass tubes 23, 24, and 25 are, for example, the same as each other. The inner diameter of the glass tubes 23 and 25 is, for example, smaller than the inner diameter of the glass tube 24. Therefore, on the inner surface of the hole 20h, there are steps corresponding to the connection portion of the glass tubes 23 and 24 and the connection portion of the glass tubes 24 and 25.

[0041] As described 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. This allows the weight of the dummy tube 20B to be reduced while satisfying the above-mentioned requirement 3 at the second end 20b. The reduced weight of the dummy tube 20B reduces the risk of cracks occurring at the welded surface between the dummy tube 20B and the cladding tube 10 and at the gripping portion 30. This improves the manufacturability of the optical fiber preform.

[0042] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the spirit of the present disclosure. The above-described embodiments and modifications may be combined as appropriate.

[0043] 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.

[0044] (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.

[0045] Next, the clad tube and the 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 simultaneously heated for 20 minutes using an oxyhydrogen burner. The heating was performed while maintaining a predetermined heating power and positional relationship. The end faces that had been melted by heating were then welded together. After welding, the weld was held at a temperature above the annealing point for 20 minutes or more, and then cooled.

[0046] The resulting welded joint between the clad pipe and the dummy pipe was subjected to a static load tensile strength test 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. The weld fractured when subjected to a load less than the test limit of 2000 kg.

[0047] Test Examples 2 to 5 were carried out 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, and 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.

[0048]

[0049] 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 fractured 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, it is believed that the dummy tube was thin and had insufficient strength, so the dummy tube broke at the gripping portion.

[0050] (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.

[0051] Test Examples 7 and 8 were carried out 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 sizes of the glass tubes in Test Examples 6 to 8. Table 3 shows the specifications of the dummy tubes in Test Examples 6 to 8, and the types of fractures as evaluation results.

[0052]

[0053] 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 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.

[0054] (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. A glass tube having an outer diameter of 100 mm, an inner diameter of 85 mm, a thickness of 7.5 mm, and a length of 500 mm was used as the glass tube to be welded to the cladding tube. A glass tube having an outer diameter of 100 mm, an inner diameter of 90 mm, a thickness of 5 mm, and a length of 500 mm was used as the central glass tube. A glass tube having an outer diameter of 100 mm, an inner diameter of 80 mm, a thickness of 10 mm, and a length of 500 mm was used as the clamped glass tube. 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.

[0055]

[0056] As shown in Table 5, in Test Example 9, no breakage occurred in the welded portion or the gripped portion even when a load of 2000 kg, the test limit, was applied. This shows that even if a thin portion is included in a portion along the length, breakage will not occur as long as the glass tube being gripped has a certain thickness.

[0057] 10... Clutch tube 10a... End surface 10h... Holes 20A, 20B... Dammy tube 20a... First end 20b... Second end 20c... First end surface 20d... Second end surface 20h... Holes 21, 22, 23, 24, 25... Glass tube 30... Gripping portion 120... Dammy tube 120a... First end 120b... Second end DL... Lengthwise direction S1... Preparation process S2... Gripping process S3... Welding process S4... Cooling process S5... Insertion process S6... Integration process

Claims

1. A method for manufacturing an optical fiber preform, comprising the step of welding a dummy tube held by a holding part to a cladding tube, wherein the dummy tube has a first end welded to the cladding tube and a second end held by the holding part, and the thickness of the dummy tube excluding the first end varies along the length of the dummy tube.

2. The method for manufacturing an optical fiber preform according to claim 1, wherein the thickness of the dummy tube is greatest at the second end.

3. A method for manufacturing an optical fiber preform according to claim 1 or claim 2, wherein the dummy tube comprises a plurality of glass tubes connected to each other, and at least two of the plurality of glass tubes have different thicknesses.

4. The method for manufacturing an optical fiber preform according to claim 3, wherein 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.

5. The method for manufacturing an optical fiber preform according to claim 3, wherein 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.

6. A method for manufacturing an optical fiber preform according to any one of claims 1 to 5, wherein the first end has an end face to be 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 is 1.28 or less.

7. A method for manufacturing an optical fiber preform according to any one of claims 1 to 6, wherein, 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.

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