optical fiber base material

Abstract

To provide an optical fiber preform that enables efficient manufacturing of optical fibers.SOLUTION: An optical fiber preform 1P comprises: a clad rod 20R that is provided with holes 25 extending along the longitudinal direction, and includes a cladding glass body 20 P which is at least a part of clad 20 in optical fiber 1; and core rods 10R, each of which is held by the hole 25 and includes a large diameter rod section 11R containing a core glass body 10P that serves as a core 10 in the optical fiber 1 and a small diameter rod section 12R that is connected to one end of the large diameter rod section 11R and is thinner than the large diameter rod section 11R, wherein one end of the clad rod 20R encloses at least a portion of the small diameter rod section 12R.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a preform for an optical fiber.

Background Art

[0002] As a method for manufacturing a preform for an optical fiber for manufacturing an optical fiber, for example, a hole opening method is known, and the following Patent Document 1 discloses the method. In the hole opening method, a through hole is provided in a clad rod that becomes a clad using a drill or the like, and a core rod that becomes a core is inserted into the through hole, thereby manufacturing a preform for an optical fiber. This preform for an optical fiber is subjected to a fusing process in which both ends are respectively fused and sealed portions that are welded to the core rod at both ends of the clad rod to close the ends of the through holes are formed, and then used for drawing the optical fiber.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Since such a fusing process for a preform for an optical fiber takes time, there is a desire to shorten the time. Further, when drawing the preform for an optical fiber from one end, at one end, the clad rod and the core rod are melted to form a tapered molten portion, and one end side of the hole of the clad rod is blocked by the molten portion. As the drawing progresses, the preform for the optical fiber becomes shorter from one end side, and the space between the clad rod and the core rod, which is the space inside the hole, becomes smaller. When the space becomes smaller, the pressure of the gas in the space rises, and bubbles are likely to be included in the manufactured optical fiber. Therefore, there is also a desire to suppress the inclusion of bubbles in the optical fiber and improve the productivity of the optical fiber. Therefore, there is a desire to manufacture an optical fiber efficiently.

[0005] Therefore, the present invention aims to provide a preform for optical fibers that can efficiently manufacture optical fibers. [Means for solving the problem]

[0006] To achieve the above objective, the optical fiber base material of the present invention comprises a clad rod having a hole extending along the longitudinal direction and including a clad glass body that forms at least a part of the cladding in the optical fiber; a large-diameter rod portion including a predetermined glass body that forms a predetermined part different from the cladding in the optical fiber; and a glass rod that is connected to one end of the large-diameter rod portion and includes a small-diameter rod portion that is thinner than the large-diameter rod portion, and is held in the hole, wherein one end of the clad rod surrounds at least a part of the small-diameter rod portion.

[0007] At one end of this optical fiber base material, one end of the clad rod surrounds at least a portion of the small-diameter rod section, and the small-diameter rod section is thinner than the large-diameter rod section. Therefore, the heat capacity per unit length at one end of this optical fiber base material is smaller than when the thickness of the small-diameter rod section is the same as the thickness of the large-diameter rod section. As a result, this optical fiber base material can shorten the time required for cutting one end of the optical fiber base material compared to this case. Alternatively, when drawing this optical fiber base material from the end where the large-diameter rod section is located, a molten area is formed at that end, and as the drawing progresses, this molten area approaches the end where the small-diameter rod section is located. Therefore, with this optical fiber base material, by drawing from the end where the large-diameter rod section is located, the space between the clad rod and the glass rod can be increased when the drawing has progressed by the same amount, compared to when the glass rod is composed only of a large-diameter rod section of the same length. Therefore, this optical fiber base material can reduce the increase in gas pressure within the space compared to the above case, suppress the inclusion of air bubbles in the optical fiber, and improve the productivity of optical fibers.

[0008] The clad rod includes a first section along the longitudinal direction including the one end, and a second section connected to the first section. The sum of the cross-sectional areas of the clad rod and the glass rod in the cross-section of the first section may be smaller than the sum of the cross-sectional areas of the clad rod and the glass rod in the cross-section of the second section. In this optical fiber base material, the heat capacity per unit length in the first section of the optical fiber base material is smaller than the heat capacity per unit length in the second section of the optical fiber base material. Therefore, with this optical fiber base material, the heat capacity per unit length at one end of the optical fiber base material can be reduced compared to the case where the sum of the cross-sectional areas in the first section is equal to or greater than the sum of the cross-sectional areas in the second section. Therefore, with this optical fiber base material, the heat capacity per unit length at one end of the optical fiber base material can be reduced compared to the above case, and the time required for the cutting process of one end of the optical fiber base material can be shortened.

[0009] In this case, the diameter of the hole in the first section may be larger than the diameter of the hole in the second section. The diameter of the hole in the clad rod can be changed, for example, by changing the diameter of the drill that forms the hole. Therefore, with this configuration, it is possible to shorten the time required for cutting one end of the optical fiber base material while suppressing the difficulty in manufacturing the clad rod.

[0010] The clad rod may have a first sealing portion at one end that is welded to the side of the small-diameter rod portion opposite to the side of the large-diameter rod portion to close one end of the hole, and a second sealing portion at the other end that is welded to the side of the large-diameter rod portion opposite to the side of the small-diameter rod portion to close the other end of the hole.

[0011] When drawing this optical fiber base material from the end on the second sealing side, a neck-down section is formed at that end, and this neck-down section moves closer to the first sealing side as drawing progresses. In this optical fiber base material, the small-diameter rod section, which is thinner than the large-diameter rod section, is located on the first sealing side. Therefore, with this optical fiber base material, by drawing from the end on the second sealing side, the space between the clad rod and the glass rod can be made larger when drawing has progressed by the same amount compared to the case where the glass rod is composed only of a large-diameter rod section of the same length. Therefore, with this optical fiber base material, the amount of pressure increase of the gas in this space can be reduced compared to the above case, the inclusion of air bubbles in the optical fiber can be suppressed, and the productivity of optical fibers can be improved.

[0012] In this case, the main body of the clad rod between the first sealing portion and the second sealing portion consists of a first section along the longitudinal direction from the end on the first sealing portion side toward the second sealing portion side, and a second section connected to the first section. In the cross-section of the first section, the area of ​​the space surrounded by the clad rod and communicating with the holding space for holding the glass rod in the second section may be larger than the area of ​​the holding space in the cross-section of the second section. With this optical fiber base material, the space between the clad rod and the glass rod can be made larger when the line drawing has progressed by the same amount, compared to the case where the lengths of the first and second sections are the same and the area of ​​the space in the cross-section of the first section is less than or equal to the area of ​​the holding space in the cross-section of the second section. Therefore, with this optical fiber base material, the amount of pressure increase of the gas in this space can be reduced compared to the above case, and the inclusion of air bubbles in the optical fiber can be suppressed. In this case, the diameter of the hole in the first section may be larger than the diameter of the hole in the second section.

[0013] Alternatively, if the clad rod has a first sealing portion and a second sealing portion, the clad rod may include a first section along the longitudinal direction including the one end and a second section connected to the first section, wherein the sum of the cross-sectional areas of the clad rod and the glass rod in the cross-section of the first section is smaller than the sum of the cross-sectional areas of the clad rod and the glass rod in the cross-section of the second section.

[0014] The optical fiber base material described above may comprise a plurality of glass rods, and the cladding rod may be provided with a plurality of holes in which the plurality of glass rods are individually held. By adopting such a configuration when the predetermined glass body described above is the core glass body, it is possible to manufacture a multicore fiber.

[0015] When a clad rod has multiple holes and includes a first section and a second section, the clad rod may be provided with a communication hole that extends along the longitudinal direction over at least a portion of the first section and is formed by multiple interconnected holes, so that when viewed along the longitudinal direction, all of the multiple holes in the second section are located within the communication hole. With this configuration, the heat capacity per unit length in the first section is reduced compared to when no communication hole is provided, and the time required for cutting one end of the optical fiber base material can be shortened. In this view, however, when viewed along the longitudinal direction, some of the multiple holes in the second section do not need to be located within the communication hole.

[0016] In a clad rod having multiple holes and comprising a first sealing portion, a second sealing portion, and a main body portion, the clad rod may be provided with a communication hole that extends along the longitudinal direction over at least a portion of the first section and is formed by multiple holes communicating with each other. With this optical fiber base material, the space between the clad rod and the glass rod can be made larger when the drawing has progressed by the same amount compared to when no communication hole is provided. Therefore, with this optical fiber base material, the amount of increase in gas pressure in this space can be reduced compared to the above case, and the inclusion of air bubbles in the optical fiber can be suppressed. Furthermore, with this optical fiber base material, compared to when no communication hole is provided, it is possible to suppress the pressure in the space between the clad rod and a specific glass rod when the drawing has progressed from being higher than the pressure in the space between the clad rod and other glass rods. Note that when viewed along the longitudinal direction, the entirety of the multiple holes in the second section may be located within the communication hole.

[0017] The large-diameter rod portion and the small-diameter rod portion may be made of the same material, and the maximum refractive index may be constant from the large-diameter rod portion to the small-diameter rod portion. Such a glass rod can be obtained, for example, by stretching the large-diameter rod portion to form the small-diameter rod portion from a part of the large-diameter rod portion. Compared to the case where the small-diameter rod portion is welded to one end of the large-diameter rod portion, such a glass rod can increase the strength of the connection between the large-diameter rod portion and the small-diameter rod portion, and can suppress the glass rod from breaking at the connection due to vibration or the like.

[0018] It is preferable that the large-diameter rod portion is longer than the small-diameter rod portion. With this configuration, it is possible to manufacture longer optical fibers compared to the case where the large-diameter rod portion is shorter than the small-diameter rod portion. [Effects of the Invention]

[0019] As described above, the present invention provides a preform for optical fibers that can efficiently manufacture optical fibers. [Brief explanation of the drawing]

[0020] [Figure 1] It is a cross-sectional view perpendicular to the longitudinal direction of an optical fiber manufactured from a preform for an optical fiber according to the first embodiment of the present invention. [Figure 2] It is a cross-sectional view along the longitudinal direction of a preform for an optical fiber for manufacturing the optical fiber shown in FIG. 1. [Figure 3] It is a cross-sectional view perpendicular to the longitudinal direction of the preform for an optical fiber shown in FIG. 2. [Figure 4] It is another cross-sectional view perpendicular to the longitudinal direction of the preform for an optical fiber shown in FIG. 2. [Figure 5] It is a view showing a state where a fusing process is applied to the preform for an optical fiber shown in FIG. 2. [Figure 6] It is a flowchart showing the steps of a method for manufacturing an optical fiber including a method for manufacturing a preform for an optical fiber according to the first embodiment. [Figure 7] It is a view showing the state of the dummy glass tube welding process. [Figure 8] It is a view showing the state of the etching process. [Figure 9] It is a view showing the state after the insertion process. [Figure 10] It is a view showing the state of the closing process. [Figure 11] It is a view showing the state of the first fusing process. [Figure 12] It is a view showing the state after the first fusing process. [Figure 13] It is a view showing the state of the second fusing process. [Figure 14] It is a view showing the state of the scribing process. [Figure 15] It is a view showing a preform for an optical fiber in the second embodiment in the same manner as FIG. 4. [Figure 16] It is a view showing the clad rod of the preform for an optical fiber shown in FIG. 15. [Figure 17] It is a view showing the state where a fusing process is applied to the preform for an optical fiber shown in FIG. 15 in the same manner as FIG. 5. [Figure 18] It is a view showing a preform for an optical fiber in a modification in the same manner as FIG. 15. [Figure 19]Figure 18 shows the cladding rod for optical fiber preforms, similar to Figure 16. [Figure 20] Figure 18 shows the optical fiber base material after it has been cut using a thermal cutting process, similar to Figure 17. [Modes for carrying out the invention]

[0021] Hereinafter, embodiments for carrying out the optical fiber preform according to the present invention are illustrated with accompanying drawings. The embodiments illustrated below are for the purpose of facilitating understanding of the present invention and are not intended to limit the interpretation of the present invention. The present invention can be modified and improved from the following embodiments without departing from its spirit. In addition, in this specification, the dimensions of each component may be exaggerated in order to facilitate understanding.

[0022] (First Embodiment) Figure 1 is a cross-sectional view perpendicular to the longitudinal direction of an optical fiber manufactured using a preform for optical fibers according to the first embodiment of the present invention. As shown in Figure 1, the optical fiber 1 of this embodiment is a multicore fiber and mainly comprises a plurality of cores 10, a cladding 20 surrounding the outer surface of each core 10, and a coating layer 30 covering the outer surface of the cladding 20. In this embodiment, there are four cores 10, and each core 10 is arranged at approximately equal intervals on a circumference centered on the central axis of the optical fiber 1. In addition, the outer shapes of the cores 10 and the cladding 20 in this cross-section are circular, but their outer shapes may 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.

[0023] The refractive index of the core 10 is higher than that of the cladding 20. In this embodiment, the core 10 is made of silica glass to which a dopant that increases the refractive index, such as germanium, is added, and the cladding 20 is made of silica glass without any additives. Alternatively, the core 10 may be made of silica glass without any additives, and the cladding 20 may be made of silica glass to which a dopant that decreases the refractive index, such as fluorine (F), is added, and the dopant that changes the refractive index is not particularly limited.

[0024] The coating layer 30 is made of a resin such as a thermosetting resin or an ultraviolet curing resin.

[0025] Figure 2 is a cross-sectional view along the longitudinal direction of the optical fiber base material used to manufacture the optical fiber 1 shown in Figure 1. As shown in Figure 2, the optical fiber base material 1P is a rod assembly consisting of a plurality of glass rods, and in this embodiment mainly comprises core rods 10R and cladding rods 20R including cladding glass body 20P which is at least a part of the cladding 20 of the optical fiber 1. The number of core rods 10R is four, the same as the number of cores 10 in the optical fiber 1.

[0026] Each core rod 10R has a similar configuration to the others, and includes a cylindrical large-diameter rod section 11R and a cylindrical small-diameter rod section 12R that is connected to one end of the large-diameter rod section 11R and is thinner than the large-diameter rod section 11R. The large-diameter rod section 11R is longer than the small-diameter rod section 12R. In addition, the diameters of the large-diameter rod section 11R and the small-diameter rod section 12R are approximately constant in the longitudinal direction.

[0027] Figure 3 is a cross-sectional view perpendicular to the longitudinal direction of the optical fiber base material 1P shown in Figure 2, and is a cross-sectional view of the portion where the large-diameter rod portion 11R and the second section of the clad rod 20R, which will be described later, are located. As shown in Figure 3, the large-diameter rod portion 11R includes a rod-shaped core glass body 10P that becomes the core 10 as a predetermined portion different from the clad 20 in the optical fiber 1. In this embodiment, the large-diameter rod portion 11R is a glass rod in which the outer circumferential surface of the core glass body 10P is covered with a coating layer 11RL made of the same glass material as the clad glass body 20P.

[0028] Figure 4 is another cross-sectional view perpendicular to the longitudinal direction of the optical fiber base material 1P shown in Figure 2, and is a cross-sectional view of the area where the small-diameter rod portion 12R and the first section of the clad rod 20R, which will be described later, are located. As shown in Figure 4, the small-diameter rod portion 12R is a glass rod in which a rod-shaped main body portion 12RB, made of the same glass material as the core glass body 10P, is covered with a coating layer 12RL made of the same glass material as the clad glass body 20P. One end of the main body portion 12RB is connected to one end of the core glass body 10P of the large-diameter rod portion 11R, and one end of the coating layer 12RL is connected to the coating layer 11RL of the large-diameter rod portion 11R. For this reason, the large-diameter rod portion 11R and the small-diameter rod portion 12R are made of the same material. Furthermore, the core glass body 10P and the main body portion 12RB are formed integrally, and the coating layer 11RL and the coating layer 12RL are formed integrally.

[0029] As shown in Figure 2, the connection portion 13R between the large-diameter rod portion 11R and the small-diameter rod portion 12R decreases in diameter from the large-diameter rod portion 11R side to the small-diameter rod portion 12R side. Furthermore, the maximum refractive index is constant from the large-diameter rod portion 11R to the small-diameter rod portion 12R. Such a core rod 10R can be obtained, for example, by extending the large-diameter rod portion 11R to form the small-diameter rod portion 12R from a part of the large-diameter rod portion 11R. Note that the large-diameter rod portion 11R and the small-diameter rod portion 12R do not necessarily have coating layers 11RL and 12RL. Also, the large-diameter rod portion 11R is longer than the small-diameter rod portion 12R.

[0030] The clad rod 20R in this embodiment is made of a clad glass body 20P. The cross-sectional shape of the clad rod 20R is circular, and its 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 is provided with four holes 25 extending along its longitudinal direction, and these holes 25 correspond one-to-one with the four core rods 10R. Therefore, when there is one core rod 10R, the number of holes 25 provided in the clad rod 20R is one. These holes 25 are through holes that open at both end faces of the clad rod 20R.

[0031] A core rod 10R is held in each hole 25, and one end of the clad rod 20R surrounds at least a portion of the small-diameter rod portion 12R. The position of the hole 25 relative to the central axis of the clad rod 20R is approximately similar to the position of the core 10 relative to the central axis of the optical fiber 1.

[0032] The clad rod 20R includes a first section 21 along its longitudinal direction, which includes one end, and a second section 22 connected to the first section 21. The diameter of each hole 25 in the first section 21 is greater than the diameter of the corresponding hole 25 in the second section 22. Therefore, the cross-sectional area of ​​the clad rod 20R in the cross-section of the first section 21 is smaller than the cross-sectional area of ​​the clad rod 20R in the cross-section of the second section 22. In this embodiment, the first section 21 is shorter than the second section 22, and the first section 21 surrounds a part of the small-diameter rod portion 12R, while the second section 22 surrounds the other part of the small-diameter rod portion 12R and the large-diameter rod portion 11R. As described above, the length of the clad rod 20R is approximately the same as the length of the core rod 10R, so the other end of the clad rod 20R surrounds the large-diameter rod portion 11R. Furthermore, the sum of the cross-sectional areas of the clad rod 20R and the core rod 10R in the cross-section of the first section 21 is smaller than the sum of the cross-sectional areas of the clad rod 20R and the core rod 10R in the cross-section of the second section.

[0033] Such optical fiber base material 1P is used after being subjected to a heat-cutting process, in which both ends are cut to form sealing portions that close the ends of the holes 25 at each end of the clad rod 20R. Next, the optical fiber base material in the state after this heat-cutting process will be described.

[0034] Figure 5 shows the optical fiber base material 1P shown in Figure 2 after thermal cutting has been performed. It is a cross-sectional view along the longitudinal direction of the optical fiber base material after thermal cutting. In the following, in order to make it easier to distinguish between the optical fiber base material 1Pa after thermal cutting and the optical fiber base material 1P before thermal cutting, the optical fiber base material 1P before thermal cutting will be referred to as a rod assembly.

[0035] As shown in Figure 5, the optical fiber base material 1Pa, like the rod assembly 1P, comprises four core rods 10Ra and cladding rods 20Ra.

[0036] The core rod 10Ra differs from the core rod 10R of the rod assembly 1P mainly in that its length is shorter. The core rod 10Ra has a large-diameter rod section 11Ra and a small-diameter rod section 12Ra. The large-diameter rod section 11Ra is part of the large-diameter rod section 11R of the core rod 10R of the rod assembly 1P, and the small-diameter rod section 12Ra is part of the small-diameter rod section 12R of the core rod 10R of the rod assembly 1P. The large-diameter rod section 11Ra is longer than the small-diameter rod section 12Ra, and the connection between the large-diameter rod section 11Ra and the small-diameter rod section 12Ra is the connection section 13R of the core rod 10R of the rod assembly 1P.

[0037] The clad rod 20Ra differs from the clad rod 20R of the rod assembly 1P mainly in that it is shorter in length and has a first sealing portion 23, a second sealing portion 24, and a main body portion 20RaB. The clad rod 20Ra is provided with four holes 25a extending along its longitudinal direction, and these holes 25a are part of the holes 25 of the clad rod 20R of the rod assembly 1P. The core rod 10Ra is held in each hole 25a such that a small-diameter rod portion 12Ra is located at one end and a large-diameter rod portion 11Ra is located at the other end.

[0038] The clad rod 20Ra has a first sealing portion 23 at one end on the side where the small-diameter rod portion 12Ra is located, and a second sealing portion 24 at the other end on the side where the large-diameter rod portion 11Ra is located, with the portion between the first sealing portion 23 and the second sealing portion 24 being the main body portion 20RaB. The first sealing portion 23 is formed in a tapered shape with its outer diameter decreasing from the other end to the one end, and surrounds the side of the small-diameter rod portion 12Ra opposite to the large-diameter rod portion 11Ra, welding it to the small-diameter rod portion 12Ra and sealing one end of each hole 25a, so that the small-diameter rod portion 12Ra is connected to the first sealing portion 23. For this reason, it can be understood that one end of the clad rod 20Ra surrounds at least a part of the small-diameter rod portion 12Ra. Furthermore, the portion of the small-diameter rod portion 12Ra surrounded by the first sealing portion 23 is formed in a tapered shape with its outer diameter decreasing from the other end to the one end. One end of a support rod 40 made of silica glass is welded to the tip of the first sealing portion 23, and the central axis of the support rod 40 and the central axis of the clad rod 20Ra are roughly aligned.

[0039] The second sealing portion 24 is formed in a tapered shape, with its outer diameter decreasing from one end to the other. It surrounds the large-diameter rod portion 11Ra on the side opposite to the small-diameter rod portion 12Ra and fuses to the large-diameter rod portion 11Ra, sealing the other end of each hole 25a, and the large-diameter rod portion 11Ra is connected to the second sealing portion 24. The portion of the large-diameter rod portion 11Ra surrounded by the second sealing portion 24 is formed in a tapered shape, with its outer diameter decreasing from one end to the other. In this way, both ends of each hole 25a are sealed 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, for example, 10 -5 Pa to 10 -8 It's around Pa.

[0040] The outer diameter of the main body 20RaB is generally constant in the longitudinal direction, and the main body 20RaB consists of a first section 21a along the longitudinal direction from the end on the first sealing part 23 side toward the second sealing part 24 side, and a second section 22a connected to the first section 21a. The first section 21a is part of the first section 21 of the clad rod 20R in the rod assembly 1P. The second section 22a is part of the second section 22 of the clad rod 20R in the rod assembly 1P. Therefore, the cross-sectional view of the optical fiber base material 1Pa in the area where the first section 21a is located is the same as in Figure 4, and the cross-sectional view of the optical fiber base material 1Pa in the area where the second section 22a is located is the same as in Figure 3.

[0041] In the cross-section of the first section 21a, the space surrounded by the clad rod 20Ra is the space within the hole 25a in the first section 21a, and the cross-sectional area of ​​this space in the cross-section of the first section 21a is the area of ​​the hole 25a on that surface. Also, the holding space for holding the core rod 10Ra in the second section 22a is the space within the hole 25a in the second section 22a, and is in communication with the space within the hole 25a in the first section 21a. In this embodiment, the diameter of each hole 25a in the first section 21a is larger than the diameter of the corresponding hole 25a in the second section 22a. Therefore, in the cross-section of the first section 21a, the area of ​​the space surrounded by the clad rod 20Ra and in communication with the holding space for holding the core rod 10Ra in the second section 22a is understood to be larger than the area of ​​this holding space in the cross-section of the second section 22a. In this embodiment, the first section 21a is shorter than the second section 22a, the first section 21a surrounds a part of the small-diameter rod portion 12Ra, and the second section 22a surrounds the other part of the small-diameter rod portion 12Ra and a part of the large-diameter rod portion 11Ra.

[0042] Furthermore, the clad rod 20Ra can be understood to include a predetermined section 21aa, which is a longitudinal section including one end and consists of a first sealing portion 23 and a first section 21a, and a specific section 22aa, which is a section connected to the predetermined section and consists of a second section 22a. The predetermined section 21aa consists of a part of the first section 21 of the rod assembly 1P, and the specific section 22aa consists of a part of the second section 22 of the rod assembly 1P. In this embodiment, the sum of the cross-sectional areas of the clad rod 20Ra and the core rod 10Ra in the cross-section of the predetermined section 21aa is smaller than the sum of the cross-sectional areas of the clad rod 20Ra and the core rod 10Ra in the cross-section of the specific section 22aa.

[0043] Next, the manufacturing method of the optical fiber base material 1Pa and the manufacturing method of the optical fiber 1 will be described.

[0044] Figure 6 is a flowchart showing the 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 Figure 6, the method for manufacturing an optical fiber preform 1Pa according to this embodiment comprises a preparation step P1, a closure step P2, a first cutting step P3, and a second cutting step P4. The method for manufacturing an optical fiber 1 also includes a drawing step P5 for drawing the manufactured optical fiber preform 1Pa.

[0045] <Preparation process P1> This process involves preparing the rod assembly 1P shown in Figure 2. This process in this embodiment includes a glass member preparation process P11, a dummy glass tube welding process P12, an etching process P13, and an insertion process P14.

[0046] <Glass component preparation process P11> This step involves preparing multiple glass components. In this embodiment, the multiple glass components to be prepared are the four core rods 10R and the clad rod 20R provided in the rod assembly 1P. These components may be washed in advance using pure water, ethanol, hydrofluoric acid, etc.

[0047] <Dummy glass tube welding process P12> This step involves welding dummy glass tubes to each of the 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 with an outer diameter approximately the same as the outer diameter of the clad rod 20R. In the following description, the dummy glass tube welded to one end face of the clad rod 20R will be referred to as the first glass tube, and the dummy glass tube welded to the other end face will be referred to as the second glass tube.

[0048] Figure 7 shows the process. The clad rod 20R is rotated around its central axis using a lathe (not shown) so that its central axis is approximately horizontal, while one end of the clad rod 20R is heated with an oxyhydrogen burner. Next, the first glass tube 41, which is positioned so that one end face faces the end face of the clad rod 20R at a predetermined distance apart, is rotated around its central axis using a lathe (not shown). The rotation of the clad rod 20R and the first glass tube 41 are synchronized, and in this state, one end of the clad rod 20R and the end of the first glass tube 41 on the clad rod 20R side are heated with an oxyhydrogen burner 50. Next, one end face of the first glass tube 41 is butted against the end face of the clad rod 20R so that the clad rod 20R and the first glass tube 41 are approximately coaxial, and the first glass tube 41 is welded to the end face of the clad rod 20R. Furthermore, the second glass tube is welded to the other end face of the clad rod 20R in the same manner as the first glass tube 41. With the first glass tube 41 and the second glass tube welded together, one end of each hole 25 of the clad rod 20R opens into the internal space of the first glass tube 41, and the other end of each hole 25 opens into the internal space of the second glass tube.

[0049] <Etching process P13> This process involves etching the inner circumferential surface that defines each hole 25 in the clad rod 20R. Figure 8 shows the process. In this embodiment, the clad rod 20R, to which glass tubes 41 and 42 are welded by a lathe (not shown), is rotated around its central axis while its central axis is approximately horizontal, and an etching gas such as sulfur hexafluoride (SF6) gas is flowed from the second glass tube 42 into the holes 25 of the clad rod 20R. At this time, an oxyhydrogen burner 50 is traversed along the longitudinal direction of the clad rod 20R to heat it. In this way, the inner circumferential surface that defines the holes 25 is etched. The etching method is not particularly limited, and etching may be performed using an etching solution such as hydrofluoric acid (HF), for example.

[0050] <Insertion process P14> This step involves inserting the core rods 10R prepared in the glass component preparation step P11 into the holes 25 of the clad rods 20R. Figure 9 shows the state after this step. In this embodiment, first, a portion of the first glass tube 41 is cut using an oxyhydrogen burner 50 to shorten the first glass tube 41. Next, each of the four core rods 10R is inserted into the corresponding hole 25 such that at least a portion of the small-diameter rod portion 12R of each core rod 10R is surrounded by one end of the clad rod 20R. In this way, a rod assembly 1P, which is the base material for optical fibers as shown in Figure 2, is obtained. The rod assembly 1P is in a state where the first glass tube 41 and the second glass tube 42 are welded 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, the end of the large-diameter rod section 11R opposite to the small-diameter rod section 12R is surrounded by the other end of the clad rod 20R.

[0051] <Closure process P2> This step involves attaching a closure member to one end face of the clad rod 20R so as to block at least a portion of the opening at one end of the hole 25. Figure 10 shows the process of this step. In this embodiment, the closure 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 Figure 10, 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 is in contact with the end face of the clad rod 20R. The central axis of the clad rod 20R and the central axis of the dummy rod 43 are roughly coincide, and in this embodiment, in this state, the entire opening at one end of each hole 25 is covered by the dummy rod 43. Next, the rod assembly 1P and the dummy rod 43 are rotated synchronously around the central axis of a lathe (not shown) with the central axis being approximately horizontal, while one 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 with an oxyhydrogen burner 50. The first glass tube 41 is then welded to the dummy rod 43, and the dummy rod 43 is also welded to one end of the rod assembly 1P. In this way, the dummy rod is attached to the end face of one end of the clad rod 20R, and as a result, in this embodiment, the entire opening at one end of each hole 25 of the clad rod 20R is blocked by the dummy rod 43 and the first glass tube 41.

[0052] <First cutting process P3> This process involves cutting one end of the rod assembly 1P and forming a first sealing portion 23 that closes one end of the hole 25 on one end of the clad rod 20R. Figure 11 shows the process, and Figure 12 shows the state after this process. As shown in Figure 11, while evacuating the holes 25 in each of the clad rods 20R of the rod assembly 1P with a vacuum pump (not shown) connected to the second glass tube 42, the rod assembly 1P is rotated around the central axis of the clad rod 20R with a lathe (not shown) such that the central axis is approximately horizontal. In this state, one end of the rod assembly 1P is heated with an oxyhydrogen burner 50 to evaporate one end of the clad rod 20R from the outer surface side, forming a constricted portion 26 with a reduced outer diameter on one end of the clad rod 20R. Then, the second glass tube 42 and the dummy rod 43 are moved relative to each other so as to move apart in the longitudinal direction, and one end of the rod assembly 1P is melted off starting from the constricted portion 26 to form the first sealing portion 23 as shown in Figure 12. As described above, since one end of the clad rod 20R surrounds at least a part of the small diameter rod portion 12R, at least a part of the small diameter rod portion 12R is located at the end of the rod assembly 1P that is melted off. For this reason, the formed first sealing portion 23 is welded to the small diameter rod portion 12R. In this embodiment, one end of the rod assembly 1P is melted off so that a part of the first section 21 of the clad rod 20R remains on the clad rod 20R without becoming the first sealing portion 23, and one end of the support rod 40 is welded to the tip of the first sealing portion 23.

[0053] <Second fusing process P4> This process involves cutting the other end of the rod assembly 1P and forming a second sealing portion 24 on the other end of the clad rod 20R that closes the other end of the hole 25. Figure 13 shows the process. As shown in Figure 13, similar to the first cutting process P3, the rod assembly 1P is rotated by a lathe (not shown) while the inside of each hole 25 is evacuated by a vacuum pump (not shown). In this state, the other end of the rod assembly 1P is heated by an oxyhydrogen burner 50 to form a constricted portion 27 with a reduced outer diameter on the other end of the clad rod 20R. Then, the second glass tube 42 and the support rod 40 are moved relative to each other so that they move apart in the longitudinal direction, and the other end of the rod assembly 1P is cut starting from the constricted portion 27 to form the second sealing portion 24. As described above, since the other end of the clad rod 20R surrounds the end of the large-diameter rod portion 11R opposite to the small-diameter rod portion 12R side, the formed second sealing portion 24 is welded to the side of the large-diameter rod portion 11R opposite to the small-diameter rod portion 12R side.

[0054] With the formation of the second sealing portion 24 in this manner, the clad rod 20R of the rod assembly 1P becomes the clad rod 20Ra of the optical fiber base material 1Pa, the hole 25 of the clad rod 20R of the rod assembly 1P becomes the hole 25a of the clad rod 20Ra of the optical fiber base material 1Pa, and the core rod 10R of the rod assembly 1P becomes the core rod 10Ra of the optical fiber base material 1Pa, thereby obtaining the optical fiber base material 1Pa shown in Figure 5.

[0055] <Drawing process P5> This process involves drawing a wire from a pre-fabricated optical fiber material 1Pa to obtain an optical fiber 1. Figure 14 shows the process. As shown in Figure 14, in this process, the end of the pre-fabricated optical fiber material 1Pa on the side where the large-diameter rod portion 11Ra is located is heated in a spinning furnace 60, and a tapered molten portion ND is formed at this end where the clad rod 20Ra and the core rod 10Ra are melted and integrated. Glass is then drawn from the tip of this molten portion ND. This molten portion ND seals the end of the hole 25a of the clad rod 20Ra that is opposite to the first sealing portion 23. The glass drawn from the molten portion ND solidifies immediately, with the core glass body 10P becoming the core 10 and the clad glass body 20P becoming the clad 20, resulting in a bare optical fiber wire 1N composed of the core 10 and the clad 20. A coating layer 30 is applied to the outer surface of this bare optical fiber wire 1N using a coating device 70 to obtain the optical fiber 1 shown in Figure 1.

[0056] As described above, the rod assembly 1P, which is the optical fiber base material of this embodiment, comprises a clad rod 20R and a core rod 10R as a glass rod. The clad rod 20R is provided with a hole 25 extending along its longitudinal direction. The core rod 10R includes a large-diameter rod portion 11R and a small-diameter rod portion 12R connected to one end of the large-diameter rod portion 11R and thinner than the large-diameter rod portion 11R, and is held in the hole 25. One end of the clad rod 20R surrounds at least a part of the small-diameter rod portion 12R. Since the small-diameter rod portion 12R is thinner than the large-diameter rod portion 11R, the heat capacity per unit length of one end of the rod assembly 1P is smaller than when the thickness of the small-diameter rod portion 12R is the same as the thickness of the large-diameter rod portion 11R. For this reason, the rod assembly 1P of this embodiment can shorten the time required for the cutting process of one end of the rod assembly 1P compared to this case, and the optical fiber base material 1Pa can be manufactured efficiently.

[0057] In the rod assembly 1P of this embodiment, the clad rod 20R includes a first section 21 along the longitudinal direction including one end, and a second section 22 connected to the first section 21. The sum of the cross-sectional areas of the clad rod 20R and the core rod 10R in the cross-section of the first section 21 is smaller than the sum of the cross-sectional areas of the clad rod 20R and the core rod 10R in the cross-section of the second section 22. Therefore, in the rod assembly 1P of this embodiment, the heat capacity per unit length in the first section 21 of the rod assembly 1P is smaller than the heat capacity per unit length in the second section 22 of the rod assembly 1P. Thus, according to the rod assembly 1P of this embodiment, the heat capacity per unit length at one end of the rod assembly 1P can be reduced compared to the case where the sum of the cross-sectional areas in the first section 21 is equal to or greater than the sum of the cross-sectional areas in the second section 22. Therefore, according to the rod assembly 1P of this embodiment, the heat capacity per unit length at one end of the rod assembly 1P can be reduced compared to the above case, and the time required for the cutting process at one end of the rod assembly 1P can be shortened. Note that the cross-sectional area of ​​the clad rod 20R in the first section 21 may be greater than or equal to the cross-sectional area in the second section 22.

[0058] In the rod assembly 1P of this embodiment, the diameter of the hole 25 in the first section 21 is larger than the diameter of the hole 25 in the second section 22. The diameter of the hole 25 in the clad rod 20R can be changed, for example, by changing the diameter of the drill used to form the hole. Therefore, the rod assembly 1P of this embodiment can reduce the time required for cutting one end of the rod assembly 1P while suppressing the difficulty in manufacturing the clad rod 20R. Note that the diameter of each hole 25 in the first section 21 may be less than or equal to the diameter of the hole 25 in the second section 22.

[0059] Furthermore, the optical fiber base material 1Pa of this embodiment comprises a clad rod 20Ra and a core rod 10Ra as a glass rod. The clad rod 20Ra is provided with a hole 25a extending along its longitudinal direction. The core rod 10Ra includes a large-diameter rod portion 11Ra and a small-diameter rod portion 12Ra connected to one end of the large-diameter rod portion 11Ra and thinner than the large-diameter rod portion 11Ra, and is held in the hole 25a. One end of the clad rod 20Ra surrounds a part of the small-diameter rod portion 12Ra. When drawing the optical fiber base material 1Pa of this embodiment from the end where the large-diameter rod portion 11Ra is located, a molten portion ND is formed at that end, as shown in Figure 14. The molten portion ND closes the other end of the hole 25a and moves closer to the end where the small-diameter rod portion 12Ra is located as the drawing progresses. Therefore, with the optical fiber base material 1Pa of this embodiment, by drawing the wire from the end on the side where the large-diameter rod portion 11Ra is located, the space between the clad rod 20Ra and the core rod 10Ra can be made larger when the wire drawing has progressed by the same amount, compared to the case in which the core rod 10Ra is composed only of the large-diameter rod portion 11Ra of the same length. Therefore, with the optical fiber base material 1Pa of this embodiment, the amount of increase in gas pressure in this space can be reduced compared to the above case, the inclusion of air bubbles in the optical fiber 1 can be suppressed, and the productivity of the optical fiber 1 can be improved.

[0060] In the optical fiber base material 1Pa of this embodiment, the clad rod 20Ra has a first sealing portion 23 that is welded at one end to the side opposite to the large-diameter rod portion 11Ra of the small-diameter rod portion 12Ra, and closes one end of the hole 25a. If the first sealing portion 23 is not welded to the core rod 10Ra, and a line is drawn from the end opposite to the first sealing portion 23, the distance between the end of the core rod 10Ra on the side of the first sealing portion 23 and the first sealing portion 23 may fluctuate during the line 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 fluctuates in the longitudinal direction. However, with the optical fiber base material 1Pa of this embodiment, the fluctuation of the diameter of the core 10 in the longitudinal direction can be suppressed compared to the above case.

[0061] In the optical fiber base material 1Pa of this embodiment, the clad rod 20Ra has a second sealing portion 24 that is welded at one end to the side of the large-diameter rod portion 11Ra opposite to the small-diameter rod portion 12Ra side, thereby closing the other end of the hole 25a. Furthermore, the main body portion 20RaB between the first sealing portion 23 and the second sealing portion 24 of the clad rod 20Ra consists of a first section 21a along the longitudinal direction from the end on the first sealing portion 23 side toward the second sealing portion 24 side, and a second section 22a connected to the first section 21a. In the cross-section of the first section 21a, the area of ​​the space surrounded by the clad rod 20Ra and communicating with the holding space for holding the core rod 10Ra in the second section 22a is larger than the area of ​​this accommodating space in the cross-section of the second section 22a. Therefore, with the optical fiber base material 1Pa of this embodiment, the space between the clad rod 20Ra and the core rod 10Ra can be made larger when the line drawing has progressed by the same amount, compared to the case where the lengths of the first section 21a and the second section 22a are the same and the area of ​​the space in the cross-section of the first section 21a is less than or equal to the area of ​​the holding space in the cross-section of the second section 22a. Therefore, with the optical fiber base material 1Pa of this embodiment, the amount of pressure increase of the gas in this space can be reduced compared to the above case, and the inclusion of air bubbles in the optical fiber 1 can be suppressed. Note that the area of ​​the space in the cross-section of the first section 21a may be less than or equal to the area of ​​the holding space in the cross-section of the second section 22a. For example, in the first cutting step P3, by cutting one end of the rod assembly 1P so that the first section 21 of the clad rod 20R does not remain in the clad rod 20R, the diameter of the hole 25a can be made constant in the longitudinal direction, and the area of ​​the space in the cross section of the first section 21a and the area of ​​the holding space in the cross section of the second section 22a can be made the same.

[0062] In the optical fiber base material 1Pa of this embodiment, the pressure inside the hole 25a is lower than atmospheric pressure. Therefore, with the optical fiber base material 1Pa of this embodiment, the pressure in the space between the cladding rod 20Ra and the core rod 10Ra when the drawing process has progressed can be lowered compared to the case where the pressure inside the hole 25a is equal to or greater 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 greater than atmospheric pressure.

[0063] The rod assembly 1P and optical fiber base material 1Pa of this embodiment are provided with 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 optical fiber base material 1Pa of this embodiment make it possible to manufacture an optical fiber 1 which is a multicore fiber.

[0064] In the rod assembly 1P and optical fiber base material 1Pa of this embodiment, the large-diameter rod sections 11R, 11Ra and the small-diameter rod sections 12R, 12Ra are made of the same material, and the maximum refractive index is constant from the large-diameter rod sections 11R, 11Ra to the small-diameter rod sections 12R, 12Ra. As mentioned above, such core rods 10R, 10Ra can increase the strength of the connection section 13R compared to the case where the small-diameter rod sections 12R, 12Ra are welded to one end of the large-diameter rod sections 11R, 11Ra, and can suppress the breakage of the core rods 10R, 10Ra at the connection section 13R due to vibration, etc.

[0065] The material constituting the small-diameter rod portions 12R and 12Ra is not particularly limited. For example, the small-diameter rod portions 12R and 12Ra may be made of silica glass and welded to the large-diameter rod portions 11R and 11Ra. The core glass body 10P that forms the core 10 may be made of silica glass to which a dopant that increases the refractive index, such as germanium, has been added. Such a core glass body 10P is more expensive than silica glass. Therefore, by using the above configuration in this embodiment, the core rods 10R and 10Ra can be made cheaper compared to the case where the large-diameter rod portions 11R and 11Ra and the small-diameter rod portions 12R and 12Ra each contain the core glass body 10P.

[0066] In the rod assembly 1P and optical fiber base material 1Pa of this embodiment, the large-diameter rod portions 11R and 11Ra are longer than the small-diameter rod portions 12R and 12Ra. Therefore, compared to the case where the large-diameter rod portions 11R and 11Ra are shorter than the small-diameter rod portions 12R and 12Ra, it is possible to manufacture longer optical fibers 1. However, the large-diameter rod portions 11R and 11Ra may be shorter than the small-diameter rod portions 12R and 12Ra.

[0067] In the rod assembly 1P and optical fiber base material 1Pa of this embodiment, the first sections 21 and 21a surround only the small-diameter rod sections 12R and 12Ra. Therefore, compared to the case where the first sections 21 and 21a surround the small-diameter rod sections 12R and 12Ra and a part of the large-diameter rod sections 11R and 11Ra, it is possible to manufacture longer optical fibers 1. Also, from this viewpoint, it is preferable that the first sections 21 and 21a are shorter than the second sections 22 and 22a, as in the rod assembly 1P and optical fiber base material 1Pa of this embodiment. Note that one end of the clad rod 20R included in the first section 21 of the rod assembly 1P should surround at least a part of the small-diameter rod section 12R. For example, the first sections 21 and 21a may surround the small-diameter rod sections 12R and 12Ra and a part of the large-diameter rod sections 11R and 11Ra, and the second sections 22 and 22a may be shorter than the first sections 21 and 21a.

[0068] Furthermore, the manufacturing method for the optical fiber base material 1Pa of this embodiment comprises a preparation step P1 and a first cutting step P3. In the preparation step P1, a rod assembly 1P is prepared. In the first cutting step P3, one end of the rod assembly 1P is heated and cut. In the manufacturing method of this embodiment, since one end of the clad rod 20R surrounds at least a part of the small-diameter rod portion 12R, the heat capacity per unit length of one end of the rod assembly 1P is smaller than when the thickness of the small-diameter rod portion is the same as the thickness of the large-diameter rod portion. For this reason, the time required for the first cutting step P3 can be shortened compared to this case. Also, in the first cutting step P3, one end of the rod assembly 1P is heated and cut while the hole 25 is evacuated. For this reason, the time required for cutting the rod assembly 1P can be shortened even further.

[0069] (Second Embodiment) Next, a second embodiment of the present invention will be described in detail. Note that components identical or equivalent to those in the first embodiment are denoted by the same reference numerals unless otherwise specified, and redundant descriptions will be omitted.

[0070] Figure 15 is a diagram showing the optical fiber base material in this embodiment in the same manner as in Figure 4, and schematically shows the cross-section including the small-diameter rod portion 12R of the core rod 10R and the first section 21 of the clad rod 20R. The optical fiber base material 1P shown in Figure 15 is the rod assembly 1P before cutting. Figure 16 is a diagram showing the clad rod of the rod assembly 1P, which is the optical fiber base material shown in Figure 15. As shown in Figures 15 and 16, the clad rod 20R of this embodiment differs from the clad rod 20R of the first embodiment in that the diameter of the holes 25 in the first section 21 is the same as the diameter of the holes 25 in the second section 22, and a communication hole 28 is provided in the first section 21. The communication hole 28 extends along the longitudinal direction throughout the entire first section 21 and is composed of multiple interconnected holes 25. In this embodiment, when viewed along the longitudinal direction, some of the multiple holes 25 in the second section 22 are located within the communication hole 28. Furthermore, the central axis of the communication hole 28 and the central axis of the clad rod 20R are generally aligned, but may be misaligned. Also, the diameter of the communication hole 28 is generally constant in the longitudinal direction, but may vary in the longitudinal direction. In addition, the communication hole 28 only needs to extend along the longitudinal direction over at least a portion of the first section 21.

[0071] According to the rod assembly 1P, which is the optical fiber base material of this embodiment, the heat capacity per unit length in the first section 21 is reduced compared to the case where the communication hole 28 is not provided, and the time required for the melting process of one end of the rod assembly 1P can be shortened.

[0072] Figure 17 is a diagram showing the state in which the rod assembly 1P, which is the optical fiber base material shown in Figure 15, has been subjected to a cutting process, similar to Figure 5. The optical fiber base material 1Pa of this embodiment can be manufactured using the rod assembly 1P of this embodiment by the method for manufacturing the optical fiber base material of the first embodiment. As shown in Figure 17, the optical fiber base material 1Pa of this embodiment differs from the optical fiber base material 1Pa of the first embodiment in that the diameter of the hole 25a in the first section 21a of the clad rod 20Ra is the same as the diameter of the hole 25a in the second section 22a, and that a communication hole 28a is provided in the first section 21a of the clad rod 20Ra.

[0073] The first section 21a of the clad rod 20Ra is part of the first section 21 of the clad rod 20R of the rod assembly 1P. The communication hole 28a is part of the communication hole 28 of the rod assembly 1P, extends along the longitudinal direction throughout the entire first section 21a, and consists of multiple interconnected holes 25a. In the cross-section of the first section 21a, the space surrounded by the clad rod 20Ra is the space within the holes 25a and communication hole 28a in the first section 21a. This space is in communication with the space within the holes 25a in the second section 22a, which is the holding space for the core rod 10Ra in the second section 22a. Therefore, in the cross-section of the first section 21a, the area of ​​the space surrounded by the clad rod 20Ra and communicating with the holding space for the core rod 10Ra in the second section 22a is understood to be larger than the area of ​​this holding space in the cross-section of the second section 22a. Therefore, according to the optical fiber base material 1Pa of this embodiment, it is possible to suppress the inclusion of air bubbles in the optical fiber 1, similar to the first embodiment. Furthermore, according to the optical fiber base material 1Pa of this embodiment, it is possible to suppress the pressure in the space between the clad rod 20Ra and a specific core rod 10Ra in the advanced state of drawing from becoming higher than the pressure in the space between the clad rod 20Ra and other core rods 10Ra, compared to the case without the communication hole 28a. Note that the communication hole 28a only needs to extend along the longitudinal direction over at least a portion of the first section 21a.

[0074] Although the present invention has been described above with reference to the above embodiments, the present invention is not limited thereto.

[0075] For example, in the second embodiment described above, clad rods 20R, 20Ra were described as having connecting holes 28, 28a in the first sections 21, 21a, each containing a portion of a plurality of holes 25, 25a. However, the connecting holes 28, 28a only need to be formed by the communication of a plurality of holes 25, 25a in the first sections 21, 21a. For example, the connecting holes 28 may be as shown in Figures 18, 19, and 20. Note that Figure 18 is a diagram showing a modified optical fiber base material in the same manner as Figure 15, and the optical fiber base material 1P shown in Figure 18 is a rod assembly 1P before cutting. Figure 19 is a diagram showing the clad rod of the rod assembly 1P, which is the optical fiber base material shown in Figure 18, in the same manner as Figure 16. Figure 20 is a diagram showing the state after cutting has been performed on the rod assembly 1P, which is the optical fiber base material shown in Figure 18, in the same manner as Figure 17. In this modified example, similar to the second embodiment, the central axes of the communication holes 28, 28a and the central axes of the clad rods 20R, 20Ra are roughly coincided, and the only holes in the first section 21, 21a are the communication holes 28, 28a, and the shape of the first section 21, 21a is cylindrical. Furthermore, when viewed along the longitudinal direction, the entirety of the multiple holes 25, 25a in the second section 22, 22a is located within the communication holes 28, 28a. According to the rod assembly 1P of this modified example, the strength of the first section 21 can be reduced, and the time required for cutting one end can be shortened. In addition, from the viewpoint of shortening the time required for cutting, by providing another hole in the first section 21 that is not connected to the hole 25, the cross-sectional area of ​​the clad rod 20R in the first section 21 may be made smaller than the cross-sectional area of ​​the clad rod 20R in the second section 22. Furthermore, according to the modified optical fiber base material 1Pa, the inclusion of air bubbles in the optical fiber 1 can be suppressed, similar to the second embodiment.

[0076] Furthermore, in the first cutting step P3 of the above embodiment, one end of the rod assembly 1P was cut so that a part of the small-diameter rod portion 12R and a part of the first section 21 of the clad rod 20R remained. However, one end of the rod assembly 1P may be cut so that the small-diameter rod portion 12R does not remain in the hole 25. In this case, the optical fiber base material 1Pa manufactured will have a configuration in which the large-diameter rod portion 11Ra is connected to the first sealing portion 23. Alternatively, one end of the rod assembly 1P may be cut so that the first section 21 does not remain. In this case, the optical fiber base material 1Pa manufactured will have a configuration in which the main body portion 20RaB of the clad rod 20Ra consists only of the second section 22a.

[0077] Furthermore, in the above embodiment, clad rods 20R and 20Ra made of clad glass body 20P were described as an example. However, the clad rods 20R and 20Ra only need to include clad glass body 20P, and may further include a predetermined glass body that is different from the clad 20 in the optical fiber 1. Examples of such predetermined parts include the core 10, markers, and stress-applying parts that apply stress to the core 10. In addition, the clad rod 20R may be provided with voids extending along the longitudinal direction where no glass rods are housed.

[0078] Furthermore, in the above embodiment, a rod assembly 1P comprising a core rod 10R as a glass rod and a base material 1Pa for optical fibers were described as examples. However, the glass rods provided in the rod assembly 1P and the base material 1Pa for optical fibers may be glass rods comprising a large-diameter rod portion containing a predetermined glass body that is a predetermined part different from the cladding 20 in the optical fiber 1, and a small-diameter rod portion connected to one end of the large-diameter rod portion and thinner than the large-diameter rod portion. For example, the glass rod may be a glass rod in which the large-diameter rod portion contains a glass body that serves as a marker for the predetermined part, or a glass rod in which the large-diameter rod portion contains a glass body that serves as a stress-applying part for the predetermined part. Also, the glass rod may be a glass rod in which the core glass body that becomes the core 10 is covered with a glass body that serves as a low refractive index layer surrounding the core 10. Furthermore, if there are multiple glass rods, the predetermined glass body contained in the large-diameter rod portion of at least one of the multiple glass rods may be different from the predetermined glass body contained in the large-diameter rod portion of at least one other glass rod.

[0079] Furthermore, 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 viewpoint of reducing processing waste, it is preferable to perform the cutting of the rod assembly 1P in the first cutting step P3 and the second cutting step P4 using an oxyhydrogen burner 50. Generally, the heat spot of an oxyhydrogen burner is narrower than the heat spot of an electric furnace. Therefore, by using an oxyhydrogen burner, the constricted portions 26 and 27 formed in the first cutting step P3 and the second cutting step P4 can be made smaller compared to when an electric furnace is used, the length of the first sealing portion 23 and the second sealing portion 24 can be shortened, and processing waste can be reduced. The heating source may also be light with a high absorption rate for glass, such as a CO2 laser. When a laser is used as the heat source, it is possible to make the heat spot have an arbitrary distribution, and it is possible to achieve optimal heating for the size and material of the base material.

[0080] Furthermore, in the above embodiment, a rod assembly 1P comprising a clad rod 20R with holes 25 open at both ends was described as an example. However, both ends of the holes 25 may be closed with, for example, a resin film or the like.

[0081] Furthermore, in the above embodiment of the method for manufacturing the optical fiber base material, a preparation step P1 including a glass member preparation step P11, a dummy glass tube welding step P12, an etching step P13, and an insertion step P14 was 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 first cutting step P3 may be performed after the second cutting step P4. Furthermore, a closure step P2 in which the entire opening on one end side of the hole 25 of the clad rod 20R is closed with a dummy rod 43 as a closing member was described as an example. However, in the closure step P2, it is sufficient to close at least a part of the opening on one end side of the hole 25, and the closing member is not particularly limited.

[0082] According to the present invention, a preform for optical fibers that can efficiently manufacture optical fibers is provided, and it can be used in various fields related to optical fibers. [Explanation of Symbols]

[0083] 1. Optical fiber 1P... Fiber optic base material (rod assembly) 1 Pa... Pre-fabricated material for optical fibers 10 cores 10P...Core glass body 10R, 10Ra... Core Rod 11R, 11Ra...Large diameter rod section 12R, 12Ra... Small diameter rod section 20.. Clad 20P... Clad glass body 20R, 20Ra... Clad Rod 20RaB...Main unit 21,21a...Section 1 22,22a...Second section 23...First sealing section 24. Second sealing section 25,25a...hole 28,28a...Communication hole

Claims

1. A clad rod comprising a clad glass body having holes extending along the longitudinal direction, which form at least part of the cladding in an optical fiber, The optical fiber includes a large-diameter rod portion containing a predetermined glass body that is a predetermined part different from the cladding, and a glass rod that is held in the hole and includes a small-diameter rod portion connected to one end of the large-diameter rod portion and thinner than the large-diameter rod portion, Equipped with, One end of the clad rod surrounds at least a portion of the small-diameter rod section, The clad rod includes a first section along the longitudinal direction including the one end, and a second section connected to the first section. The diameter of the hole in the first section is greater than the diameter of the hole in the second section. A preform for optical fibers characterized by the following features.

2. The sum of the cross-sectional areas of the clad rod and the glass rod in the cross-section of the first section is smaller than the sum of the cross-sectional areas of the clad rod and the glass rod in the cross-section of the second section. The optical fiber base material according to feature 1.

3. A clad rod comprising a clad glass body having holes extending along the longitudinal direction, which form at least part of the cladding in an optical fiber, The optical fiber includes a large-diameter rod portion containing a predetermined glass body that is a predetermined part different from the cladding, and a glass rod that is held in the hole and includes a small-diameter rod portion connected to one end of the large-diameter rod portion and thinner than the large-diameter rod portion, Equipped with, One end of the clad rod surrounds at least a portion of the small-diameter rod section, The clad rod has a first sealing portion welded to the side of the small-diameter rod portion opposite to the large-diameter rod portion at one end to close one end of the hole, and a second sealing portion welded to the side of the large-diameter rod portion opposite to the small-diameter rod portion at the other end to close the other end of the hole. The main body portion between the first sealing portion and the second sealing portion of the clad rod consists of a first section along the longitudinal direction from the end on the first sealing portion side toward the second sealing portion side, and a second section connected to the first section. In the cross-section of the first section, the area of ​​the space surrounded by the clad rod and communicating with the holding space for holding the glass rod in the second section is greater than the area of ​​the holding space in the cross-section of the second section. A preform for optical fibers characterized by the following features.

4. The diameter of the hole in the first section is greater than the diameter of the hole in the second section. The optical fiber preform according to feature 3.

5. The glass rods are provided in multiple locations, The clad rod is provided with a plurality of holes, each holding a plurality of glass rods individually. A preform for optical fibers according to any one of claims 1 to 4.

6. The clad rod is provided with a communication hole that extends along the longitudinal direction over at least a portion of the first section and is formed by multiple holes communicating with each other. The optical fiber preform according to feature 5.

7. The large-diameter rod portion and the small-diameter rod portion are made of the same material. The maximum refractive index is constant from the large-diameter rod portion to the small-diameter rod portion. A preform for optical fibers according to any one of claims 1 to 6.

8. The large-diameter rod portion is longer than the small-diameter rod portion. The optical fiber preform according to any one of claims 1 to 7.

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