Method for producing multi-core optical fiber preform

Abstract

This method for producing a multi-core optical fiber preform includes a step in which a first end that is an end of a cladding tube and a second end that is an end of a dummy tube are melted by heating and a step in which the first end and the second end which are melted are connected to each other. The melting step is performed while an inert gas is introduced from an introduction part attached to a third end, which is the end of the cladding tube on the reverse side from the first end, to the inside of a plurality of first holes provided to the cladding tube. The introduction part has a flow regulation plate provided with a plurality of second holes.

Description

Method for manufacturing a multi-core optical fiber preform

[0001] This disclosure relates to a method for manufacturing a multi-core optical fiber preform. This application claims priority to Japanese Patent Application No. 2024-097905, filed on June 18, 2024, and incorporates the entire contents of said Japanese application by reference.

[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, a resistance furnace, or an oxyhydrogen burner. In this rod-in-tube method, a relatively inexpensive dummy tube is connected to the cladding tube to reduce manufacturing costs, and the cladding tube is moved by holding the dummy tube. Alternatively, the cladding tube and the core rod are integrated. In these cases, a process is required in which the ends of the cladding tube and the dummy tube are melted by heating and connected to each other.

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

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

[0005] The manufacturing method of the MCF base material according to the present disclosure includes a step of melting a first end, which is an end of a clad tube, and a second end, which is an end of a dummy tube, by heating, and a step of connecting the melted first end and second end to each other, wherein the melting step is carried out while introducing an inert gas into a plurality of first holes provided in the clad tube from an introduction part attached to a third end, which is the end of the clad tube opposite the first end, and the introduction part has a straightening plate provided with a plurality of second holes.

[0006] Fig. 1 is a flowchart showing a method for manufacturing an MCF base material according to an embodiment. Fig. 2 is a cross-sectional view illustrating a welding process. Fig. 3 is a plan view of a straightening plate and a clad tube viewed along the axial direction. Fig. 4 is a diagram showing the flow of inert gas when an introduction section according to a comparative example is used. Fig. 5 is a diagram showing the flow of inert gas when an introduction section according to an embodiment is used.

[0007] When the ends of the cladding tube and the dummy tube are heated using an oxyhydrogen burner, some of the hydrogen and oxygen gases flow into the cladding tube and the dummy tube, which can easily generate water inside the tube. If water remains, this can cause pipe failure or a small explosion. Therefore, to prevent the hydrogen and oxygen gases from flowing into the cladding tube and the dummy tube, heating is sometimes performed while an inert gas such as nitrogen is flowed into the inside of the cladding tube and the dummy tube from the end opposite the end being heated.

[0008] The cladding tube for a multi-core optical fiber (MCF) preform has multiple holes, so it is necessary to flow an inert gas through each of the multiple holes. When there are many holes, it is laborious to install pipes for flowing the inert gas for the same number of holes. Since the position and number of holes vary depending on the optical design of the MCF, it is also laborious to create dedicated parts to branch the pipes each time.

[0009] The present disclosure provides a method for manufacturing an MCF preform that allows easy flow of an inert gas into each of the multiple holes in the cladding tube.

[0010] According to the present disclosure, it is possible to provide a method for manufacturing an MCF preform that allows an inert gas to easily flow into each of a plurality of holes in a cladding tube.

[0011] First, embodiments of the present disclosure will be described. (1) A method for manufacturing an MCF preform according to one aspect of the present disclosure includes the steps of: melting a first end portion of a cladding tube and a second end portion of a dummy tube by heating; and connecting the melted first and second end portions to each other. The melting step is performed while introducing an inert gas into a plurality of first holes provided in the cladding tube from an introduction portion attached to a third end portion of the cladding tube opposite the first end portion. The introduction portion has a flow straightening plate having a plurality of second holes. In the above-described method for manufacturing an MCF preform, the inert gas is introduced into the plurality of first holes in the cladding tube from the introduction portion having the flow straightening plate. The inert gas hits the flow straightening plate and diffuses, allowing the inert gas to easily flow into each of the plurality of first holes in the cladding tube.

[0012] (2) In the above (1), when viewed along the axial direction of the cladding tube, the area ratio A2 / A1 of the area A2 of the portion overlapping with the plurality of second holes to the entire area A1 of each of the plurality of first holes may be 0.5 or more. In this case, the flow rate of the inert gas flowing through the plurality of first holes of the cladding tube is less likely to be uneven.

[0013] (3) In the above (1) or (2), the area ratio A2 / A1 may be 0.95 or less. In this case, the flow rate of the inert gas flowing through the plurality of first holes of the cladding tube is less likely to be uneven.

[0014] (4) In any of the above (1) to (3), the material of the rectifying plate may be silicone resin, fluororesin, or stainless steel.

[0015] (5) In any of the above (1) to (4), the shape of each of the plurality of second holes may be circular. In this case, it is possible to easily process the second holes using a tool such as a drill.

[0016] [Details of the embodiments of the present disclosure] Specific examples of the manufacturing method of the MCF 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.

[0017] 1 is a flowchart showing a method for manufacturing an MCF base material according to an embodiment. As shown in the figure, the method for manufacturing an MCF base material according to an embodiment includes a first melting step S1, a first connecting step S2, a first annealing step S3, a second melting step S4, a second connecting step S5, a second annealing step S6, an inserting step S7, and an integrating step S8. The MCF base material is manufactured by performing these steps S1 to S8 in this order.

[0018] FIG. 2 is a cross-sectional view illustrating the melting step. As shown in the figure, the first melting step S1 involves heating and melting the end 10a (first end) of the cladding tube 10 and the end 20a (second end) of the first dummy tube 20. The cladding tube 10 is a glass tube that serves as the cladding portion of the MCF preform. The cladding tube 10 is made of silica-based glass. The cladding tube 10 has an end 10a and an end 10b (third end) opposite the end 10a. The cladding tube 10 is provided with multiple holes 10h (first holes) for inserting core rods. The multiple holes 10h extend along the axial direction D1 of the cladding tube 10 and have a circular cross-section. The multiple holes 10h open to an end face 10c included in the end 10a of the cladding tube and an end face 10d included in the end 10b. The diameters of the multiple holes 10h may all be the same or may be different.

[0019] The first dummy tube 20 is a relatively inexpensive glass tube welded to the cladding tube 10. The first dummy tube 20 is made of silica-based glass. The first dummy tube 20 is a cylindrical glass tube with one hole 20h. The hole 20h extends along the axial direction of the first dummy tube 20. The cross section of the hole 20h is circular. The inner diameter of the first 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 multiple holes 10h of the cladding tube 10. The outer diameter of the first dummy tube 20 is, for example, equal to or smaller than the outer diameter of the cladding tube 10. The first dummy tube 20 has an end 20a and an end 20b opposite to the end 20a. The end 20a is the portion welded to the cladding tube 10. The end 20b is the portion gripped by a gripping portion.

[0020] To heat the end 10 a of the cladding tube 10 and the end 20 a of the first dummy tube 20, for example, an oxyhydrogen burner is used as a heat source 30. The first melting step S1 is performed while introducing an inert gas IG into a plurality of holes 10 h formed in the cladding tube 10. The inert gas IG is, for example, nitrogen gas. An inlet 40 for introducing the inert gas IG is attached to the end 10 b of the cladding tube 10.

[0021] The introduction part 40 has a cap-shaped case 41 that covers the end 10b of the cladding tube 10, a port 42, and a rectifying plate 43 provided in the internal space IS of the case 41. The case 41 has a cylindrical shape with a bottom. The inner diameter of the case 41 is equal to the outer diameter of the cladding tube 10. The end 10b of the cladding tube 10 is fitted inside the opening of the case 41, thereby attaching the introduction part 40 to the end 10b. The case 41 is made of a material such as silicone resin, a fluororesin such as Teflon (registered trademark), or stainless steel, for example.

[0022] The port 42 is formed integrally with the case 41. The material of the port 42 is, for example, the same as the material of the case 41, silicone resin, fluororesin such as Teflon (registered trademark), or stainless steel. The port 42 has a cylindrical shape with a smaller diameter than the case 41. The port 42 protrudes from the bottom of the case 41 to the outside of the case 41. The inside of the port 42 is in communication with the internal space IS of the case 41. A pipe (not shown) connected to a supply unit for inert gas IG is inserted into the port 42.

[0023] 3 is a plan view of the current plate and the clad tube as viewed along the axial direction. This view shows the end face 10c of the clad tube 10, with the clad tube 10 positioned in front of the current plate 43. The current plate 43 is, for example, disk-shaped. The current plate 43 is, for example, flat and of uniform thickness. The current plate 43 is attached to the case 41, for example, by fixing the outer edge of the current plate 43 to the inner peripheral surface of the case 41. The current plate 43 faces the end face 10c of the clad tube 10 in the axial direction D1. The current plate 43 is spaced apart from the end face 10c of the clad tube 10 in the axial direction D1. The current plate 43 is made of, for example, silicone resin, fluororesin such as Teflon (registered trademark), or stainless steel.

[0024] The straightening plate 43 has a plurality of holes 43h (second holes). The holes 43h are, for example, circular. When viewed along the axial direction D1, for each of the plurality of holes 10h, the area ratio A2 / A1 of the area A2 of the portion overlapping with the plurality of holes 43h to the entire area A1 is 0.5 or more. The area ratio A2 / A1 may be 0.6 or more. The area ratio A2 / A1 is 0.95 or less. The area ratio A2 / A1 may be 0.9 or less. When the holes 43h are circular, they can be easily machined using a tool such as a drill.

[0025] 4 is a diagram showing the flow of inert gas when an inlet section according to a comparative example is used. As shown in the figure, the inlet section 140 according to the comparative example differs from the inlet section 40 in that it does not have a flow straightening plate. In the inlet section 140, the inert gas IG flows easily into the holes 10h closer to the port 42, but does not easily flow into the holes 10h farther from the port 42. Therefore, hydrogen gas and oxygen gas generated by the heat source 30 flow into the holes 10h farther from the port 42 from the end 10a (not shown), which tends to generate water.

[0026] 5 is a diagram showing the flow of inert gas when using the inlet portion according to the embodiment. As shown in the figure, the inlet portion 40 according to the embodiment has a rectifying plate 43, so that the inert gas IG diffuses against the rectifying plate 43. This facilitates the inert gas IG to flow evenly into the multiple holes 10h of the cladding tube 10. As the inert gas IG flows into each hole 10h in this manner, the hydrogen gas and oxygen gas generated in the heat source 30 are less likely to flow into each hole 10h. As a result, water is less likely to be generated in any of the holes 10h.

[0027] 2, the first melting step S1 is performed while introducing an inert gas IG into the first dummy tube 20, i.e., into the hole 20h. The inert gas IG is, for example, nitrogen gas. A pipe connected to a supply of the inert gas IG is inserted into the end 20b of the first dummy tube 20. This makes it difficult for water to be generated inside the first dummy tube 20.

[0028] The first connecting step S2 is a step of connecting the melted end 10a of the cladding tube 10 and the melted end 20a of the first dummy tube 20. The first connecting step S2 is performed immediately after the first melting step S1.

[0029] The first annealing step S3 is a step performed after the first connecting step S2, in which the connection (weld) between the clad tube 10 and the first dummy tube 20 is maintained at a temperature equal to or higher than the annealing point for a predetermined time, and then cooled. The first annealing step S3 can relieve stress remaining at the time of connection (welding).

[0030] The second melting step S4 is a step of heating and melting the end 10b of the cladding tube 10 and the end 20a of the second dummy tube 20. The second dummy tube 20 has, for example, the same shape as the first dummy tube 20, and is therefore not shown. The second melting step S4 is performed after the introduction section 40 is removed from the cladding tube 10 to expose the end 10b. To heat the end 10b and the end 20a, for example, an oxyhydrogen burner is used as the heat source 30, as in the first melting step S1.

[0031] The second melting step S4 is performed while introducing an inert gas IG into the multiple holes 10h provided in the cladding tube 10 and into the second dummy tube 20. The inert gas IG is, for example, nitrogen gas. Pipes connected to a supply of the inert gas IG are inserted into the end 20b of the first dummy tube 20 and the end 20b of the second dummy tube 20. This makes it difficult for water to be generated inside the cladding tube 10, the first dummy tube 20, and the second dummy tube 20, even during the second melting step S4.

[0032] The second connecting step S5 is a step of connecting the melted end 10b of the cladding tube 10 and the melted end 20a of the second dummy tube 20. The second connecting step S5 is performed immediately after the second melting step S4.

[0033] The second slow cooling step S6 is a step performed after the second connecting step S5, in which the connection (weld) between the clad tube 10 and the second dummy tube 20 is maintained at a temperature equal to or higher than the slow cooling point for a predetermined time, and then cooled. The second slow cooling step S6 can relieve stress remaining at the time of connection (welding).

[0034] The insertion step S7 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 first dummy tube 20 and be inserted into the multiple holes 10h from the openings in the end face 10c.

[0035] The integration step S8 is a step in which the cladding tube 10 and the core rod are heated and melted by, for example, a rod-in-tube method to integrate them. An induction furnace, a resistance furnace, or an oxyhydrogen burner is used as a heat source. The integration step S8 is also called a collapse step. The integration step S8 produces an MCF preform.

[0036] As described above, in the manufacturing method of the MCF preform according to the embodiment, an inert gas is introduced into the multiple holes 10h in the cladding tube 10 from the inlet 40 having the rectifying plate 43. The inert gas diffuses when it hits the rectifying plate 43. This allows the inert gas to easily flow into each of the multiple holes 10h in the cladding tube 10. There is no need to install pipes for the inert gas flow equal to the number of holes 10h, or to create dedicated parts for branching the pipes according to the position and number of holes 10h. Since water accumulation in the holes 10h is reduced, the effort of removing water during the manufacturing of the MCF preform is eliminated. This improves productivity.

[0037] Residual water may affect the quality of the MCF preform. In the manufacturing method of the MCF preform according to the embodiment, the generation of water is reduced, thereby improving the quality of the MCF preform. Accordingly, the quality of the MCF drawn from the MCF preform can also be improved.

[0038] 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.

[0039] For example, the shape of the holes 43h of the rectifying plate 43 is not limited to a circular shape and may be, for example, a square or triangular shape. The rectifying plate 43 is not limited to a flat plate shape with a uniform thickness and may be curved or have a non-uniform thickness. Triangular and square holes 43h may be able to reduce turbulence more than circular holes 43h for a specific gas type or flow rate.

[0040] 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.

[0041] (Test Examples 1 to 12) In each of Test Examples 1 to 12, a glass tube having an outer diameter of 80 mm, an inner diameter of 70 mm, a thickness of 10 mm, and a length of 1000 mm was prepared as a first dummy tube. A cladding tube was prepared by drilling five holes with a diameter of 20 mm in a glass material having an outer diameter of 80 mm and a length of 1000 mm.

[0042] In Test Example 1, an inlet without a straightening vane was attached to the end of the clad tube, and the first melting step was performed while introducing inert gas into the clad tube through the inlet. After the first melting step, the total amount of water accumulated in the multiple holes in the clad tube was measured. In Test Example 2, the first melting step was performed in the same manner as Test Example 1, except that an inlet with a straightening vane was attached to the end of the clad tube, and the total amount of water was measured. In Test Example 2, the area ratio A2 / A1 of the straightening vane in the inlet was set to 0.10. In Test Examples 3 to 12, the area ratio A2 / A1 of the straightening vane in the inlet was changed from 0.20 to 0.98, and the first melting step was performed in the same manner as Test Example 2, and the total amount of water was measured. In Test Examples 1 to 12, a straightening vane with a thickness of 5 mm was used. Experiments were also performed with straightening vanes with thicknesses of 10 mm and 20 mm, and the total amount of water was reduced by approximately 10% to 20% compared to the case of a straightening vane with a thickness of 5 mm. It is believed that the area ratio A2 / A1 governs the total amount of water.

[0043] Table 1 shows the area ratio A2 / A1 of the straightening plate and the total amount (ml) of water accumulated in the holes of the cladding tube for Test Examples 1 to 12.

[0044] As shown in Table 1, the use of a straightening plate can reduce water generation. When more than 50 ml of water accumulates in the cladding tube, if the MCF preform is manufactured through subsequent processes without removing the water from the cladding tube, abnormalities presumably caused by the residual water may occur. For example, abnormalities may occur at the interface between the core and cladding of the MCF obtained after drawing, resulting in increased transmission loss. Furthermore, for example, the outer diameter of the glass may become unstable during drawing.

[0045] In each of Test Examples 2 to 10, the amount of water in the multiple holes was similar, with a difference of ±10%. This suggests that the inert gas was evenly distributed among the multiple holes. In Test Examples 11 and 12, differences in the amount of water among the multiple holes began to appear, and water began to accumulate in some holes. This is thought to be because the diffusion effect of the flow straightening vane becomes smaller when the total area of ​​the holes is too large. This suggests that, as in the case without a flow straightening vane, a large amount of inert gas flows into the holes in the cladding tube closest to the introduction port.

[0046] DESCRIPTION OF SYMBOLS 10...Clad tube 10a...End (first end) 10b...End (third end) 10c...End surface 10h...Hole (first hole) 20...First dummy tube, second dummy tube 20a...End (second end) 20b...End 20h...Hole 30...Heat source 40...Inlet portion 41...Case 42...Port 43...Flow plate 43h...Hole (second hole) 140...Inlet portion D1...Axial direction IG...Inert gas IS...Internal space S1...First melting step S2...First connecting step S3...First gradual cooling step S4...Second melting step S5...Second connecting step S6...Second gradual cooling step S7...Insertion step S8...Integration step

Claims

1. A method for manufacturing a multi-core optical fiber preform, comprising: a step of melting a first end portion, which is an end portion of a cladding tube, and a second end portion, which is an end portion of a dummy tube, by heating; and a step of connecting the melted first end portion and the second end portion to each other, wherein the melting step is performed while introducing an inert gas into a plurality of first holes provided in the cladding tube from an introduction part attached to a third end portion, which is the end portion of the cladding tube opposite to the first end portion, and the introduction part has a straightening plate provided with a plurality of second holes.

2. The method for manufacturing a multi-core optical fiber preform according to claim 1, wherein, when viewed along the axial direction of said cladding tube, for each of said plurality of first holes, an area ratio A2 / A1 of an area A2 of a portion overlapping with said plurality of second holes to an entire area A1 is 0.5 or more.

3. The method for manufacturing a multi-core optical fiber preform according to claim 1 or 2, wherein the area ratio A2 / A1 is 0.95 or less.

4. The method for manufacturing a multi-core optical fiber preform according to any one of claims 1 to 3, wherein the material of the rectifying plate is silicone resin, fluororesin, or stainless steel.

5. The method for manufacturing a multi-core optical fiber preform according to any one of claims 1 to 4, wherein each of the plurality of second holes has a circular shape.

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