Optical fiber base material

Abstract

This optical fiber preform (1P) is provided with: a clad rod (20R) which has, provided therein, a hole (25) extending along the length direction and open at both ends thereof, and which includes a clad glass body (20P) that forms at least a part of cladding (20) in an optical fiber (1); and a core rod (10R) which includes a core glass body (10P) that forms a core (10) in the optical fiber (1) and which is held in the hole (25). The diameter of the hole (25) gradually increases from one end side toward the other end side.

Description

[Technical Field]

[0001] The present invention relates to an optical fiber preform. [Background technology]

[0002] One known method for manufacturing an optical fiber preform for producing an optical fiber is the drilling method, and this method is disclosed in Patent Document 1 listed below. In the drilling method, a through hole is formed in a cladding rod, which becomes the cladding, using a drill or the like, and a core rod, which becomes the core, is inserted into the through hole to manufacture an optical fiber preform. This optical fiber preform is then subjected to a fusion cutting process in which both ends are fused to form sealing portions, which are fused to the core rod at both ends of the cladding rod and seal the ends of the through hole, before being used to draw an optical fiber.

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-031427 Summary of the Invention

[0004] In the fusion cutting process of such an optical fiber preform, for example, the end portion is fused while the clad rod is rotated around its central axis with the central axis of the clad rod in a generally horizontal state. Furthermore, a glass rod or the like may be welded to the end face of the clad rod before the fusion cutting process, and this welding may also be performed while the optical fiber preform is rotated around its central axis. When the optical fiber preform is rotated in this manner, the core rod held in the through hole may move longitudinally relative to the clad rod. In an optical fiber preform in which the through hole of the clad rod is not inclined in the longitudinal direction of the optical fiber preform and the core rod is present at an angle within the through hole of the clad rod, such movement of the core rod may cause one end of the core rod to protrude from the through hole of the clad rod. In this case, the fusion position on the side of the optical fiber preform opposite the side from which the core rod protrudes shifts toward the side from which the core rod protrudes as the core rod moves, resulting in a shortened length of the optical fiber preform after the fusion cutting process. In order to prevent the core rod from protruding from the through hole before the fusion cutting process, for example, it is possible to block the openings at both ends of the through hole, but this increases the number of processing steps and reduces the processing efficiency. Therefore, there is a demand for an optical fiber preform that suppresses the reduction in processing efficiency and enables the efficient production of optical fibers.

[0005] Furthermore, when the optical fiber preform after fusion cutting is drawn from one end, the cladding rod and the core rod fuse together at the one end, forming a tapered molten portion, which closes one end of the hole in the cladding rod. As drawing progresses, the optical fiber preform shortens from the one end, and the space between the cladding rod and the core rod, which is the space within the hole, becomes smaller. As this space becomes smaller, the pressure of the gas in this space increases, making it easier for bubbles to be contained in the optical fiber produced. For this reason, there is a demand for an optical fiber preform that can suppress the inclusion of bubbles in the optical fiber and enable efficient production of optical fiber.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical fiber preform that allows efficient production of optical fibers.

[0007] In order to achieve the above-mentioned object, a first aspect of the present invention is an optical fiber base material comprising: a clad rod having a hole extending along the longitudinal direction and open at both ends, the clad rod including a clad glass body that will become at least a part of the clad in the optical fiber; and a glass rod held in the hole, the glass rod including a predetermined glass body that will become a predetermined part different from the clad in the optical fiber, the diameter of the hole increasing from one end to the other end.

[0008] The optical fiber preform of the first embodiment is drawn after being subjected to fusion cutting. In the optical fiber preform of the first embodiment, the diameter of the hole increases from one end to the other end. Therefore, when the optical fiber preform is rotated around the central axis of the clad rod while the central axis of the clad rod is approximately horizontal, the glass rod may tilt downward from one end to the other end due to its own weight. Therefore, during such rotation, a force may be applied to the glass rod from one end to the other end. Therefore, with the optical fiber preform of the first embodiment, the direction in which the glass rod protrudes from the hole in the clad rod can be identified as the other end. Therefore, even if the openings at both ends of the hole are not blocked before fusion cutting, when at least a portion of the opening on the other end side of the hole is blocked with a blocking member or the like, the blocking member can block the glass rod. As a result, it is possible to prevent the glass rod from protruding from the hole in the clad rod, and compared to the case where the openings at both ends of the hole are blocked before the fusion cutting process, it is possible to prevent a decrease in processing efficiency, and it is possible to realize an optical fiber preform that can efficiently produce optical fibers.

[0009] In addition, in order to achieve the above-mentioned object, a second aspect of the present invention is an optical fiber base material comprising: a clad rod having a hole extending along the longitudinal direction and including a clad glass body that will become at least a part of the clad in an optical fiber; and a glass rod held in the hole, the glass rod including a predetermined glass body that will become a predetermined part different from the clad in the optical fiber, wherein the diameter of the hole increases from one end side to the other end side, and the clad rod has a first sealing portion fused to the glass rod at an end on the other end side to seal the end of the hole on the other end side, and a second sealing portion fused to the glass rod at an end on the one end side to seal the end of the hole on the one end side.

[0010] When the optical fiber preform of Example 2 is drawn from the end on the second sealing portion side, a tapered molten zone is formed at the end, and the molten zone approaches the first sealing portion side as drawing proceeds. In the optical fiber preform of Example 2, the hole becomes larger from the second sealing portion side toward the first sealing portion side. Therefore, according to the optical fiber preform of Example 2, by drawing from the end on the second sealing portion side, the space between the cladding rod and the glass rod can be increased as drawing proceeds without increasing the minimum diameter of the hole, compared to when the diameter of the hole is constant in the longitudinal direction. Therefore, according to the optical fiber preform of Example 2, compared to the above case, the increase in gas pressure in the space as drawing proceeds can be reduced while suppressing positional deviation of the predetermined portion, and the inclusion of air bubbles in the optical fiber can be suppressed. Therefore, the optical fiber preform of Example 2 can realize an optical fiber preform that allows for efficient production of optical fiber.

[0011] A third aspect of the present invention is the optical fiber preform of the first or second aspect, characterized in that the diameter of the hole increases stepwise from the one end to the other end. A fourth aspect of the present invention is the optical fiber preform of the first or second aspect, characterized in that the diameter of the hole increases gradually from the one end to the other end.

[0012] A fifth aspect of the present invention is the optical fiber preform according to any one of the first to fourth aspects, characterized in that it comprises a plurality of the glass rods, and the cladding rod is provided with a plurality of the holes for individually holding the plurality of glass rods. By adopting such a configuration when the predetermined glass body is a core glass body, it is possible to manufacture a multi-core fiber.

[0013] As described above, the present invention provides an optical fiber preform that allows efficient production of optical fibers. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view perpendicular to the longitudinal direction of an optical fiber manufactured using an optical fiber preform according to an embodiment of the present invention. [Figure 2] 2 is a cross-sectional view taken along the longitudinal direction of an optical fiber preform for producing the optical fiber shown in FIG. 1. FIG. [Figure 3] 3 is a cross-sectional view perpendicular to the longitudinal direction of the optical fiber preform shown in FIG. 2. [Figure 4] 3 is a diagram showing a state in which the optical fiber preform shown in FIG. 2 has been subjected to fusion cutting. FIG. [Figure 5] 1 is a flowchart showing steps of a method for manufacturing an optical fiber, including a method for manufacturing an optical fiber preform according to an embodiment of the present invention. [Figure 6] 10A and 10B are diagrams showing the dummy glass tube welding process. [Figure 7] FIG. 10 is a diagram showing an etching process. [Figure 8] FIG. 10 is a diagram showing the state after the insertion step. [Figure 9] FIG. 10 is a diagram showing a closing process. [Figure 10] FIG. 10 is a diagram showing a first fusing step. [Figure 11] FIG. 10 is a diagram showing the state after the first fusing step. [Figure 12] FIG. 10 is a diagram showing a second fusing step. [Figure 13] FIG. [Figure 14] 10 is a view similar to FIG. 2 showing an optical fiber preform according to a modified example. [Figure 15] 15 is a view similar to FIG. 4 showing the state of the optical fiber preform shown in FIG. 14 after fusion cutting. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments for carrying out the optical fiber preform according to the present invention will be described with reference to the accompanying drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified or improved from the following embodiments without departing from the spirit of the present invention. Furthermore, in this specification, the dimensions of each component may be exaggerated to facilitate understanding.

[0016] FIG. 1 is a cross-sectional view perpendicular to the longitudinal direction of an optical fiber manufactured using an optical fiber preform according to an embodiment of the present invention. As shown in FIG. 1, the optical fiber 1 according to this embodiment is a multi-core fiber, and is mainly composed of a plurality of cores 10, cladding 20 surrounding the outer peripheral surfaces of each of the cores 10, and a coating layer 30 covering the outer peripheral surface of the cladding 20. In this embodiment, the number of cores 10 is four, and the cores 10 are arranged at approximately equal intervals on a circumference centered on the central axis of the optical fiber 1. Furthermore, although the outer shapes of the cores 10 and the cladding 20 in the cross section are circular, these outer shapes may also be non-circular, such as elliptical. Furthermore, the number of cores 10 is not particularly limited; for example, the optical fiber 1 may be a single-core fiber having one core 10.

[0017] The refractive index of the core 10 is higher than the refractive index of the cladding 20. In this embodiment, the core 10 is made of silica glass doped with a dopant such as germanium that increases the refractive index, and the cladding 20 is made of silica glass with no dopants. Note that the core 10 may be made of silica glass with no dopants and the cladding 20 may be made of silica glass doped with a dopant such as fluorine (F) that decreases the refractive index, and there are no particular restrictions on the dopant that changes the refractive index.

[0018] The coating layer 30 is made of a resin such as a thermosetting resin or an ultraviolet curing resin.

[0019] Fig. 2 is a cross-sectional view along the longitudinal direction of an optical fiber preform for manufacturing the optical fiber 1 shown in Fig. 1. As shown in Fig. 2, the optical fiber preform 1P is a rod assembly made up of a plurality of glass rods, and in this embodiment, it mainly comprises a core rod 10R and a cladding rod 20R including a cladding glass body 20P that will become at least a part of the cladding 20 of the optical fiber 1. The number of core rods 10R is four, which is the same as the number of cores 10 of the optical fiber 1.

[0020] Fig. 3 is a cross-sectional view perpendicular to the longitudinal direction of the optical fiber preform 1P shown in Fig. 2. As shown in Fig. 3, each core rod 10R has the same configuration and includes a rod-shaped core glass body 10P that becomes the core 10 as a predetermined part different from the cladding 20 in the optical fiber 1. The core rod 10R of this embodiment is a glass rod in which the outer peripheral surface of the core glass body 10P is covered with a coating layer 10RL made of the same glass body as the cladding glass body 20P, and the diameter of the core rod 10R is approximately constant in the longitudinal direction.

[0021] As shown in Figures 2 and 3, the clad rod 20R of this embodiment is made of a clad glass body 20P. The cross-sectional outer shape of the clad rod 20R is circular, and the outer diameter is approximately constant in the longitudinal direction. The length of the clad rod 20R is approximately the same as the length of the core rod 10R. The clad rod 20R has four holes 25 extending along the longitudinal direction, and these holes 25 correspond one-to-one to the four core rods 10R. Therefore, when there is one core rod 10R, the clad rod 20R has only one hole 25. These holes 25 are through-holes whose both ends open to both end faces of the clad rod 20R.

[0022] A core rod 10R is held in each hole 25. In this embodiment, one end of the clad rod 20R surrounds one end of the core rod 10R, and the other end of the clad rod 20R surrounds the other end of the core rod 10R. Note that, hereinafter, the one end may be referred to as the one end, and the other end as the other end. The positions of the holes 25 relative to the central axis of the clad rod 20R are generally similar to the positions of the cores 10 relative to the central axis of the optical fiber 1. The diameter of each hole 25 increases from one end to the other end, and in this embodiment, the diameter increases in stages. The number of steps in which the diameter changes is not limited, and FIG. 2 shows an example in which the diameter increases in two stages.

[0023] Such an optical fiber preform 1P is used after being subjected to a fusion cutting process in which both ends are fused to form sealing portions that close the ends of the holes 25 at both ends of the clad rod 20R.

[0024] Next, the optical fiber preform in the state in which this fusion cutting process has been performed will be described.

[0025] Fig. 4 is a diagram showing a state in which the optical fiber preform 1P shown in Fig. 2 has been subjected to fusion cutting, and is a cross-sectional view along the longitudinal direction of the optical fiber preform in the fusion cutting state. In the following, in order to easily distinguish between the optical fiber preform 1Pa after fusion cutting and the optical fiber preform 1P before fusion cutting, the optical fiber preform 1P before fusion cutting will be referred to as a rod assembly.

[0026] As shown in FIG. 4, the optical fiber preform 1Pa includes four core rods 10Ra and a cladding rod 20Ra, similar to the rod assembly 1P.

[0027] The core rod 10Ra is a part of the core rod 10R of the rod assembly 1P, and differs mainly from the core rod 10R of the rod assembly 1P in that its length is shortened.

[0028] The clad rod 20Ra differs from the clad rod 20R of the rod assembly 1P mainly in that it has a shorter length and includes a first sealing portion 23, a second sealing portion 24, and a main body portion 20RaB. The clad rod 20Ra has four holes 25a extending along its longitudinal direction, which are part of the holes 25 of the clad rod 20R of the rod assembly 1P. A core rod 10Ra is held in each hole 25a. The diameter of each hole 25a increases from one end to the other end, and in this embodiment, the diameter increases in two steps.

[0029] The clad rod 20Ra has a first sealing portion 23 at the other end where the diameter of the hole 25a is larger, and a second sealing portion 24 at the other end where the diameter of the hole 25a is smaller, and the portion between the first sealing portion 23 and the second sealing portion 24 is the main body portion 20RaB. The cross-sectional outer shape of the main body portion 20RaB is circular, and the outer diameter is approximately constant in the longitudinal direction.

[0030] The first sealing portion 23 is formed in a tapered shape with an outer diameter decreasing from one end to the other end, surrounds the other end of each core rod 10Ra, and is welded to the other end to block the other end of each hole 25a, and the core rod 10Ra is connected to the first sealing portion 23. The portion of the core rod 10Ra surrounded by the first sealing portion 23 is formed in a tapered shape with an outer diameter decreasing from one end to the other end. One end of a support rod 40 made of silica glass is welded to the tip of this first sealing portion 23, and the central axis of the support rod 40 and the central axis of the clad rod 20Ra are approximately aligned.

[0031] The second sealing portion 24 is formed in a tapered shape with an outer diameter decreasing from the other end side toward the one end side, surrounds one end of each core rod 10Ra, and is welded to that end to block one end of each hole 25a, and the core rod 10Ra is connected to the second sealing portion 24. The portion of the core rod 10Ra surrounded by the second sealing portion 24 is formed in a tapered shape with an outer diameter decreasing from the other end side toward the one end side. In this way, both ends of each hole 25a are blocked by the first sealing portion 23 and the second sealing portion 24, and the inside of each hole 25a is a closed space. The pressure in the space between the clad rod 20Ra and the core rod 10Ra, which is the space inside these holes 25a, is lower than atmospheric pressure, and for example, -5 Pa to 10 -8 It is about Pa.

[0032] Next, a method for manufacturing the optical fiber preform 1Pa and a method for manufacturing the optical fiber 1 will be described.

[0033] 5 is a flowchart showing steps of a method for manufacturing an optical fiber 1, including a method for manufacturing an optical fiber preform 1Pa according to this embodiment. As shown in FIG. 5, the method for manufacturing an optical fiber preform 1Pa according to this embodiment includes a preparation step P1, a closing step P2, a first fusing step P3, and a second fusing step P4. The method for manufacturing the optical fiber 1 also includes a drawing step P5 for drawing the manufactured optical fiber preform 1Pa.

[0034] <Preparation process P1> This step is a step of preparing a rod assembly 1P shown in Fig. 2. This step in this embodiment includes a glass member preparation step P11, a dummy glass tube welding step P12, an etching step P13, and an insertion step P14.

[0035] <Glass component preparation process P11> This step is a step of preparing a plurality of glass members. In this embodiment, the plurality of glass members prepared are the four core rods 10R and the clad rod 20R included in the rod assembly 1P. These members may be washed in advance using pure water, ethanol, hydrofluoric acid, or the like.

[0036] <Dummy glass tube welding process P12> This step involves welding a dummy glass tube to each of the two end faces of the clad rod 20R prepared in preparation step P1. The dummy glass tubes in this embodiment are cylindrical tubes made of silica glass and have an outer diameter roughly equal to that of the clad rod 20R. In the following description, the dummy glass tube welded to the other end face of the clad rod 20R, where the hole 25 has a larger diameter, is referred to as the first glass tube, and the dummy glass tube welded to the one end face, where the hole 25 has a smaller diameter, is referred to as the second glass tube.

[0037] 6 shows the process of this step. The clad rod 20R is rotated around its central axis by a lathe (not shown) while keeping the central axis of the clad rod 20R approximately horizontal, while the other end of the clad rod 20R is heated by an oxyhydrogen burner. Next, a first glass tube 41, one end of which faces the other end of the clad rod 20R at a predetermined distance, is rotated around its central axis by a lathe (not shown). The rotations of the clad rod 20R and the first glass tube 41 are synchronized, and in this state, the other end of the clad rod 20R and the end of the first glass tube 41 on the clad rod 20R side are heated by an oxyhydrogen burner 50. Next, one end face of the first glass tube 41 is butted against the other end face of the clad rod 20R, and the first glass tube 41 is fused to the other end face of the clad rod 20R so that the clad rod 20R and the first glass tube 41 are roughly coaxial. Then, in the same manner as the first glass tube 41, a second glass tube is fused to the one end face of the clad rod 20R. With the first glass tube 41 and the second glass tube fused, the other end of each hole 25 in the clad rod 20R opens into the internal space of the first glass tube 41, and one end of each hole 25 opens into the internal space of the second glass tube.

[0038] <Etching process P13> This process involves etching the inner circumferential surfaces of the clad rod 20R that define the holes 25. Figure 7 illustrates this process. In this embodiment, the clad rod 20R, to which the glass tubes 41 and 42 are welded, is rotated around its central axis by a lathe (not shown) while the central axis is kept substantially horizontal. An etching gas, such as sulfur hexafluoride (SF6) gas, is then passed through the second glass tube 42 and into the holes 25 of the clad rod 20R. During this process, an oxyhydrogen burner 50 is traversed along the longitudinal direction of the clad rod 20R to heat the clad rod 20R. In this manner, the inner circumferential surfaces that define the holes 25 are etched. The etching method is not particularly limited; for example, etching may be performed using an etching solution such as hydrofluoric acid (HF).

[0039] <Insertion process P14> This step involves inserting the core rod 10R prepared in the glass member preparation step P11 into the hole 25 of the clad rod 20R. FIG. 8 shows the state after this step. In this embodiment, first, a portion of the first glass tube 41 is fused using an oxyhydrogen burner 50 to shorten the first glass tube 41. Next, each core rod 10R is inserted into the hole 25 corresponding to that core rod 10R. In this embodiment, the core rod 10R is inserted into the hole 25 so that the other end of the core rod 10R is surrounded by the other end of the clad rod 20R. In this way, the rod assembly 1P shown in FIG. 2 is obtained. Note that in this rod assembly 1P, the first glass tube 41 and the second glass tube 42 are fused to the clad rod 20R. Furthermore, since the length of the clad rod 20R and the length of the core rod 10R are approximately the same, one end of the core rod 10R is surrounded by one end of the clad rod 20R.

[0040] <Closure process P2> This step involves attaching a blocking member to the end face on the other end of the clad rod 20R so as to block at least a portion of the opening on the other end of each hole 25. FIG. 9 shows the process of this step. In this embodiment, the blocking member is a cylindrical dummy rod 43 made of silica glass, and the diameter of the dummy rod 43 is smaller than the inner diameter of the first glass tube 41. As shown in FIG. 9, the dummy rod 43 is inserted into the internal space of the first glass tube 41 so that one end face of the dummy rod 43 contacts the end face on the other end of the clad rod 20R. The central axes of the clad rod 20R and the dummy rod 43 are generally aligned, and in this embodiment, the entire opening on the other end of each hole 25 is covered by the dummy rod 43. Next, the rod assembly 1P and the dummy rod 43 are synchronously rotated around their central axes by a lathe (not shown) with the central axes kept generally horizontal, while the other end of the rod assembly 1P, the end of the dummy rod 43 on the rod assembly 1P side, and the first glass tube 41 are heated by an oxyhydrogen burner 50. Then, the first glass tube 41 is welded to the dummy rod 43, and the dummy rod 43 is welded to the other end of the rod assembly 1P. In this manner, the dummy rod 43 is attached to the end face on the other end side of the clad rod 20R. As a result, in this embodiment, the entire opening on the other end side of each hole 25 of the clad rod 20R is blocked by the dummy rod 43 and the first glass tube 41.

[0041] <First fusing process P3> In this process, the other end of the rod assembly 1P is fused to form a first sealing portion 23 at the other end of the clad rod 20R, sealing the other end of the hole 25. FIG. 10 illustrates this process, and FIG. 11 illustrates the state after this process. As shown in FIG. 10, a vacuum pump (not shown) connected to a second glass tube 42 is used to evacuate the holes 25 of the clad rods 20R of the rod assembly 1P, while a lathe (not shown) is used to rotate the rod assembly 1P around its central axis, with the central axis of the clad rod 20R positioned approximately horizontal. In this state, the other end of the rod assembly 1P is heated by an oxyhydrogen burner 50 to evaporate the other end of the clad rod 20R from the outer peripheral surface side, thereby forming a necked portion 26 with a reduced outer diameter at the other end of the clad rod 20R. Then, the second glass tube 42 and the dummy rod 43 are moved relatively apart in the longitudinal direction, and the other end of the rod assembly 1P is fused starting from the constricted portion 26 to form a first sealing portion 23, as shown in Fig. 11. In this embodiment, the other end of the clad rod 20R surrounds the other end of the core rod 10R, so that the core rod 10R is positioned at the other end of the rod assembly 1P that is fused, and the formed first sealing portion 23 is welded to the core rod 10R. Then, one end of the support rod 40 is welded to the tip of this first sealing portion 23.

[0042] <Second fusing process P4> In this step, one end of the rod assembly 1P is fused to form a second sealing portion 24 that blocks one end of the hole 25 at one end of the clad rod 20R. FIG. 12 illustrates this step. As shown in FIG. 12, similar to the first fusion-cutting step P3, the rod assembly 1P is rotated by a lathe (not shown) while a vacuum is drawn inside each hole 25 using a vacuum pump (not shown). In this state, one end of the rod assembly 1P is heated by an oxyhydrogen burner 50 to form a necked portion 27 with a reduced outer diameter at one end of the clad rod 20R. The second glass tube 42 and the support rod 40 are then moved relatively apart in the longitudinal direction to fused one end of the rod assembly 1P starting from the necked portion 27, thereby forming the second sealing portion 24. In this embodiment, because one end of the clad rod 20R surrounds one end of the core rod 10R, the formed second sealing portion 24 is fused to the core rod 10R.

[0043] By forming the second sealing portion 24 in this manner, the clad rods 20R of the rod assembly 1P become the clad rods 20Ra of the optical fiber preform 1Pa, the holes 25 of the clad rods 20R of the rod assembly 1P become the holes 25a of the clad rods 20Ra of the optical fiber preform 1Pa, and the core rods 10R of the rod assembly 1P become the core rods 10Ra of the optical fiber preform 1P, thereby obtaining the optical fiber preform 1P shown in Figure 4.

[0044] <Drawing process P5> This step involves drawing the optical fiber preform 1Pa to obtain the optical fiber 1. FIG. 13 illustrates the process. As shown in FIG. 13, in this step, the end of the optical fiber preform 1Pa opposite the first sealing portion 23 is heated in a spinning furnace 60 to form a tapered fused portion ND at the end where the cladding rod 20Ra and the core rod 10Ra are fused and integrated. Glass is then drawn from the tip of the fused portion ND. This fused portion ND closes the end of the hole 25a of the cladding rod 20Ra opposite the first sealing portion 23. The glass drawn from the fused portion ND immediately solidifies, and the core glass body 10P becomes the core 10, and the cladding glass body 20P becomes the cladding 20, resulting in a bare optical fiber 1N composed of the core 10 and the cladding 20. A coating layer 30 is provided on the outer circumferential surface of the bare optical fiber 1N using a coating device 70, thereby obtaining the optical fiber 1 shown in FIG. 1.

[0045] As described above, the rod assembly 1P, which is an optical fiber preform according to this embodiment, includes a clad rod 20R and a core rod 10R serving as a glass rod. The clad rod 20R has a hole 25 extending longitudinally and open at both ends, and the core rod 10R is held in the hole 25. The diameter of the hole 25 in the clad rod 20R increases from one end to the other end. Therefore, when the rod assembly 1P is rotated around the central axis of the clad rod 20R with the central axis of the clad rod 20R approximately horizontal, the core rod 10R may tilt downward from one end to the other end due to its own weight. Therefore, during such rotation, a force from one end to the other end may be applied to the core rod 10R. Therefore, according to the rod assembly 1P according to this embodiment, the direction in which the core rod 10R protrudes from the hole 25 in the clad rod 20R can be identified as the other end. For this reason, even if the openings at both ends of the hole 25 are not blocked before the fusion cutting process, as in the above-described closing step P2 and first fusing step P3, when the opening at the other end of the hole 25 is blocked, the blocking member can block the core rod 10R. As described above, it is possible to prevent the core rod 10R from protruding from the hole 25, and compared to the case where the openings at both ends are blocked before the fusion cutting process, it is possible to prevent a decrease in processing efficiency and realize a rod assembly 1P that can efficiently produce the optical fiber 1. Note that in the above-described closing step P2 and first fusing step P3, the entire opening at the other end of the hole 25 is blocked, but from the perspective of preventing the core rod 10R from protruding from the hole 25, it is sufficient to block at least a part of the opening at the other end of the hole 25.

[0046] The inclination angle of the core rod 10R with respect to the central axis of the clad rod 20R when the central axis of the clad rod 20R is horizontal is preferably, for example, 0° or more and 0.1° or less. In this embodiment, the diameter of the hole 25 increases stepwise from one end to the other, and the lengths 25L1, 25L2, and 25L3 of the portions of the hole 25 where the diameter is constant are preferably, for example, 200 mm or more and 2000 mm or less. While FIG. 2 shows three portions of the hole 25 where the diameter is constant, this number is not limited to three in this embodiment. In this case, when the hole 25 increases stepwise, the increase in the diameter of the hole 25 at the portion where the diameter increases is preferably, for example, 0.01 mm or more and 5 mm or less.

[0047] The optical fiber preform 1Pa of this embodiment includes a clad rod 20Ra and a core rod 10Ra serving as a glass rod. The clad rod 20Ra has a hole 25a extending along the longitudinal direction, and the core rod 10Ra is held in the hole 25a. The diameter of the hole 25a increases from one end to the other end. The clad rod 20Ra has a first sealing portion 23 at its other end that closes the other end of the hole 25a, and a second sealing portion 24 at its one end that is fused to the core rod 10Ra and closes the one end of the hole 25a. When the optical fiber preform 1Pa of this embodiment is drawn from the end on the second sealing portion 24 side, a fusion zone ND is formed at that end, as shown in FIG. 13 . The fusion zone ND closes the other end of the hole 25a and moves closer to the first sealing portion 23 as drawing progresses. In the optical fiber preform 1Pa of this embodiment, the hole 25a becomes larger from the second sealing portion 24 side toward the first sealing portion 23 side. Therefore, according to the optical fiber preform 1Pa of this embodiment, by drawing from the end on the second sealing portion 24 side, the space between the cladding rod 20Ra and the core rod 10R can be increased as the drawing progresses without increasing the minimum diameter of the hole 25a, compared to when the diameter of the hole 25a is constant in the longitudinal direction. Therefore, according to the optical fiber preform 1Pa of this embodiment, the increase in gas pressure in the space as the drawing progresses can be reduced while suppressing positional deviation of the core 10 as the predetermined portion, compared to the above case, and the inclusion of air bubbles in the optical fiber 1 can be suppressed. Therefore, the optical fiber preform 1Pa of this embodiment can efficiently produce the optical fiber 1.

[0048] In the optical fiber preform 1Pa of this embodiment, the first sealing portion 23 is welded to the core rod 10Ra. If the first sealing portion 23 is not welded to the core rod 10Ra and drawing is performed from the end portion on the second sealing portion 24 side, the distance between the end of the core rod 10Ra on the first sealing portion 23 side and the first sealing portion 23 may vary during drawing. In this case, the amount of core glass body 10P drawn per unit time changes, and the diameter of the core 10 of the optical fiber 1 varies in the longitudinal direction. However, according to the optical fiber preform 1Pa of this embodiment, the longitudinal variation in the diameter of the core 10 can be suppressed compared to the above case.

[0049] In the optical fiber preform 1Pa of this embodiment, the pressure inside the hole 25a is lower than atmospheric pressure. Therefore, with the optical fiber preform 1Pa of this embodiment, the pressure in the space between the cladding rod 20Ra and the core rod 10Ra can be lowered as the drawing progresses, compared to when the pressure inside the hole 25a is equal to or higher than atmospheric pressure, and the inclusion of air bubbles in the optical fiber 1 can be suppressed. Note that the pressure inside the hole 25a may be equal to or higher than atmospheric pressure.

[0050] The rod assembly 1P and the optical fiber preform 1Pa of this embodiment include a plurality of core rods 10R, 10Ra, and the cladding rods 20R, 20Ra are provided with a plurality of holes 25, 25a that individually hold the plurality of core rods 10R, 10Ra. Therefore, as described above, the rod assembly 1P and the optical fiber preform 1Pa of this embodiment can be used to manufacture the optical fiber 1, which is a multi-core fiber.

[0051] The manufacturing method of the optical fiber preform 1Pa of this embodiment also includes a preparation step P1 and a first fusing step P3. In the preparation step P1, a rod assembly 1P is prepared. In the first fusing step P3, the rod assembly 1P is rotated around the central axis of the clad rod 20R with at least a portion of the opening on the other end of the hole 25 blocked and the central axis of the clad rod 20R held substantially horizontal, and the other end of the rod assembly 1P is heated and fusing. In the manufacturing method of this embodiment, the diameter of the hole 25 in the clad rod 20R increases from one end to the other end. Therefore, even if the opening on one end of the hole 25 is not blocked in the first fusing step P3, the core rod 10R can be prevented from protruding from the hole 25. Therefore, the manufacturing method of this embodiment can prevent a decrease in processing efficiency compared to when both openings of the hole 25 are blocked in the first fusing step P3.

[0052] Although the present invention has been described above using the above embodiment as an example, the present invention is not limited to this.

[0053] For example, in the above embodiment, the clad rods 20R and 20Ra were described as examples in which the diameters of the holes 25 and 25a gradually increased from one end to the other end. However, the diameters of the holes 25 and 25a may also increase from one end to the other end. For example, as shown in FIGS. 14 and 15 , the diameters of the holes 25 and 25a may gradually increase from one end to the other end. In this case, the inner circumferential surfaces defining the holes 25 and 25a are inclined toward the side opposite the central axes of the holes 25 and 25a from one end to the other end. Note that FIG. 14 is a diagram similar to FIG. 2 illustrating an optical fiber preform 1P according to a modified example. The optical fiber preform 1P illustrated in FIG. 14 is a rod assembly 1P before fusion cutting. Similarly to FIG. 4, FIG. 15 is a diagram similar to FIG. 4 illustrating the optical fiber preform 1P illustrated in FIG. 14 after fusion cutting. According to the rod assembly 1P of this modification, as in the above embodiment, when the rod assembly 1P is rotated around the central axis of the clad rod 20R with the central axis of the clad rod 20R being approximately horizontal, the opening on the other end side of the opening of the hole 25 is blocked, thereby making it possible to prevent the core rod 10R from protruding from the hole 25. Furthermore, according to the optical fiber preform 1Pa of this modification, as in the above embodiment, it is possible to prevent air bubbles from being included in the optical fiber 1.

[0054] In the above embodiment, the clad rods 20R, 20Ra are each made of a clad glass body 20P. However, the clad rods 20R, 20Ra only need to include the clad glass body 20P, and may further include a predetermined glass body that serves as a predetermined portion different from the clad 20 in the optical fiber 1. Examples of such predetermined portions include the core 10, a marker, and a stress-applying portion that applies stress to the core 10. The clad rod 20R may also be provided with a hole extending along the longitudinal direction in which no glass rod is housed.

[0055] In the above embodiment, the rod assembly 1P including the core rod 10R as a glass rod and the optical fiber preform 1Pa including the core rod 10Ra have been described as examples. However, the glass rods included in the rod assembly 1P and the optical fiber preform 1Pa may be glass rods containing a predetermined glass body that will become a predetermined portion different from the cladding 20 of the optical fiber 1. For example, the predetermined glass body contained in the glass rod may be a glass body that will become a marker as the predetermined portion or a glass body that will become a stress-applying portion as the predetermined portion. Furthermore, the glass rod may be a glass rod in which a core glass body that will become the core 10 is covered with a glass body that will become a low-refractive-index layer that surrounds the core 10. Furthermore, when there are multiple glass rods, the predetermined glass body contained in at least one glass rod among the multiple glass rods may be different from the predetermined glass body contained in at least one other glass rod.

[0056] In the above embodiment, an oxyhydrogen burner 50 was used to heat the glass member. However, the heating device for heating the glass member is not particularly limited and may be, for example, an electric furnace. From the perspective of reducing processing waste, it is preferable to use an oxyhydrogen burner 50 to fuse the rod assembly 1P in the first fusing step P3 and the second fusing step P4. Generally, the heat spot of an oxyhydrogen burner is narrower than that of an electric furnace. Therefore, using an oxyhydrogen burner can reduce the size of the necked portions 26 and 27 formed in the first fusing step P3 and the second fusing step P4 compared to using an electric furnace, thereby shortening the lengths of the first and second sealing portions 23 and 24 and reducing processing waste. The heat source may be a light source with high glass absorption, such as a CO laser. When using a laser as the heat source, the heat spot can be distributed as desired, enabling optimal heating to be achieved for the size and material of the base material.

[0057] In the above embodiment, the rod assembly 1P is described as an example in which both end portions of the clad rod 20R surround one end portion of the core rod 10R. However, at least one of the end portions of the clad rod 20R does not have to surround the core rod 10R. However, from the viewpoint of reducing processing waste, it is preferable that one end portion of the clad rod 20R surrounds one end portion of the core rod 10R and the other end portion of the clad rod 20R surrounds the other end portion of the core rod 10R.

[0058] Furthermore, in the manufacturing method of the optical fiber preform 1Pa of the above embodiment, the preparation step P1 including the glass member preparation step P11, the dummy glass tube welding step P12, the etching step P13, and the insertion step P14 has been described as an example. However, the preparation step P1 is not particularly limited as long as it can prepare the rod assembly 1P. For example, when preparing a clad rod 20R whose inner circumferential surface defining the hole 25 is etched, the etching step P13 may be omitted. Also, the closing step P2 has been described as an example in which the dummy rod 43 as a closing member completely closes the opening at one end of the hole 25 in the clad rod 20R. However, in the closing step P2, it is sufficient to close at least a portion of the opening at one end of the hole 25, and the closing member is not limited.

[0059] According to the present invention, an optical fiber preform that enables efficient production of optical fibers is provided, and can be used in various fields related to optical fibers.

Claims

1. a clad rod having a hole extending along a longitudinal direction and open at both ends, the clad rod including a clad glass body that will become at least a part of the clad of the optical fiber; a glass rod that includes a predetermined glass body that will become a predetermined portion different from the cladding of the optical fiber and is held in the hole; Equipped with The diameter of the hole increases from one end to the other end, The gap between the inner peripheral surface of the clad rod and the outer peripheral surface of the glass rod increases from the one end side to the other end side.

1. An optical fiber base material comprising:

2. a clad rod having a longitudinally extending hole and including a clad glass body that will become at least a part of the clad of an optical fiber; a glass rod that includes a predetermined glass body that will become a predetermined portion different from the cladding of the optical fiber and is held in the hole; Equipped with The diameter of the hole increases from one end to the other end, a gap between an inner peripheral surface of the clad rod and an outer peripheral surface of the glass rod becomes larger from the one end side toward the other end side, The clad rod has a first sealing portion welded to the glass rod at the other end and closing the other end of the hole, and a second sealing portion welded to the glass rod at the one end and closing the one end of the hole.

1. An optical fiber base material comprising:

3. The diameter of the hole increases stepwise from the one end to the other end.

3. The optical fiber preform according to claim 1 or 2.

4. The diameter of the hole gradually increases from the one end to the other end.

3. The optical fiber preform according to claim 1 or 2.

5. A plurality of the glass rods are provided, The clad rod is provided with a plurality of holes in which the plurality of glass rods are individually held.

3. The optical fiber preform according to claim 1 or 2.

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