JP7842580B2 - optical fiber base material - Google Patents

Description

This invention relates to a preform for optical fibers.

As a method for manufacturing optical fiber preforms for optical fiber production, for example, the drilling method is known, and this method is disclosed in Patent Document 1 below. In the drilling method, through holes are made in the cladding rod, which will become the cladding, using a drill or the like, and a core rod, which will become the core, is inserted into these through holes to produce the optical fiber preform. This optical fiber preform is then subjected to a welding process where both ends are cut and welded to the core rod to form sealing portions that close the ends of the through holes, and is then used for drawing optical fibers.

Japanese Patent Publication No. 2019-031427

The cutting process for optical fiber preforms is time-consuming, and there is a demand to shorten this time. Furthermore, when drawing optical fiber preforms from one end after cutting, the clad rod and core rod melt at one end, forming a tapered molten section. This molten section seals one end of the hole in the clad rod. As drawing progresses, the optical fiber preform shortens from one end, reducing the space between the clad rod and core rod within the hole. This reduction in space increases the gas pressure within that space, making it easier for air bubbles to be incorporated into the manufactured optical fiber. Therefore, there is a demand to suppress the inclusion of air bubbles in the optical fiber and improve its productivity. Consequently, there is a demand for efficient optical fiber manufacturing.

Therefore, the present invention aims to provide a preform for optical fibers that can efficiently manufacture optical fibers.

To achieve the above objective, the optical fiber base material of the present invention comprises a clad rod having a hole extending along its longitudinal direction and containing a clad glass body that forms at least a part of the cladding in the optical fiber, and a glass rod containing a predetermined glass body that forms a predetermined part different from the cladding in the optical fiber and held in the hole, wherein the clad rod includes a first section along the longitudinal direction including one end and a second section connected to the first section, and the cross-sectional area of the clad rod in the cross-section of the first section is smaller than the cross-sectional area of the clad rod in the cross-section of the second section.

In this optical fiber preform, the heat capacity per unit length of the clad rod in the first section is smaller than the heat capacity per unit length of the clad rod in the second section. Therefore, compared to the case where the cross-sectional area of the glass rod is the same in both sections and the cross-sectional area of the clad rod in the first section is greater than or equal to the cross-sectional area of the clad rod in the second section, the heat capacity per unit length of the optical fiber preform in the first section is smaller than the heat capacity per unit length of the optical fiber preform in the second section. Consequently, this optical fiber preform allows for a reduction in the heat capacity per unit length at one end of the optical fiber preform compared to the above case, and thus reduces the time required for the cutting process at one end of the optical fiber preform.

The glass rod may be surrounded by one end of the clad rod. This configuration allows for the formation of a sealing portion that closes one end of the hole by welding the glass rod to one end of the clad rod.

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 used to form the hole. Therefore, this configuration allows for a reduction in the time required for the cutting process of one end of the optical fiber base material while suppressing the difficulty in manufacturing the clad rod.

The optical fiber base material described above may comprise multiple glass rods, and the cladding rod may have multiple holes for individually holding the multiple glass rods. By adopting this configuration when the predetermined glass body is the core glass body, it becomes possible to manufacture a multicore fiber.

In this case, the clad rod may be provided with a communication hole extending along the longitudinal direction over at least a portion of the first section, where multiple holes are interconnected. This configuration reduces the heat capacity per unit length in the first section compared to a configuration without a communication hole, thereby shortening the time required for cutting one end of the optical fiber base material. Furthermore, in this case, when viewed along the longitudinal direction, all of the multiple holes in the second section may be located within the communication hole.

Furthermore, to achieve the above objective, the optical fiber base material of the present invention comprises a clad rod having a hole extending along its longitudinal direction and containing a clad glass body that forms at least a part of the cladding in the optical fiber, and a glass rod containing a predetermined glass body that forms a predetermined part different from the cladding in the optical fiber and is held in the hole. The clad rod has a first sealing portion at one end that closes one end of the hole, a second sealing portion at the other end that is welded to the glass rod and closes the other end of the hole, and a main body portion between the first and second sealing portions. The main body 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. The area of the space in the cross-section of the first section that is surrounded by the clad rod and communicates with the holding space for holding the glass rod in the second section is larger than the area of the holding space in the cross-section of the second section.

When drawing this optical fiber base material from the end on the second sealing side, a molten portion is formed at that end, and this molten portion approaches the first sealing side as drawing progresses. The first section is located closer to the first sealing side than the second section. Therefore, with this optical fiber base material, 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, the space between the clad rod and the glass rod can be made larger when drawing has progressed by the same amount. Therefore, with this optical fiber base material, the increase in gas pressure 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.

The first sealing portion may be welded to the glass rod. If the first sealing portion is not welded to the glass rod, and a line is drawn from the end on the second sealing portion side, the distance between the end of the glass rod on the first sealing portion side and the first sealing portion may fluctuate during the drawing process. In this case, the amount of predetermined glass material that constitutes a predetermined portion of the optical fiber drawn per unit time changes, and the diameter of the predetermined portion of the optical fiber fluctuates in the longitudinal direction. However, by using the above configuration, the fluctuation of the diameter of the predetermined portion in the longitudinal direction can be suppressed compared to the above case.

The optical fiber base material described above, in which the clad rod has a first sealing portion, may comprise a plurality of the glass rods, and the clad rod may be provided with a plurality of holes for individually holding the plurality of glass rods. By adopting such a configuration when the predetermined glass body is the core glass body, it is possible to manufacture a multicore fiber.

In this case, the clad rod may be provided with a communication hole extending along the longitudinal direction over at least a portion of the first section, formed by multiple interconnected holes. This optical fiber base material allows for a larger space between the clad rod and the glass rod when the drawing has progressed by the same amount, compared to the case without the communication hole. Therefore, this optical fiber base material can reduce the increase in gas pressure within this space compared to the above case, thereby suppressing the inclusion of air bubbles in the optical fiber. Furthermore, this optical fiber base material can prevent the pressure in the space between the clad rod and a specific glass rod from becoming higher than the pressure in the space between the clad rod and other glass rods when the drawing has progressed, compared to the case without the communication hole. In this case, when viewed along the longitudinal direction, the entirety of the multiple holes in the second section may be located within the communication hole.

Regardless of the presence or absence of the first sealing portion, the second section is preferably longer than the first section. This configuration allows for the manufacture of longer optical fibers compared to the case where the second section is shorter than the first section.

As described above, the present invention provides a pre-fabricated material for optical fibers that can be efficiently manufactured.

This 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. Figure 1 is a cross-sectional view along the longitudinal direction of the optical fiber base material used to manufacture the optical fiber shown. Figure 2 shows a cross-sectional view perpendicular to the longitudinal direction of the optical fiber base material. Figure 2 shows another cross-sectional view perpendicular to the longitudinal direction of the optical fiber base material. Figure 2 shows the optical fiber base material after it has been subjected to a cutting process. This flowchart shows the steps for a method of manufacturing an optical fiber, including a method for manufacturing a preform for optical fibers according to the first embodiment. This diagram shows the process of welding dummy glass tubes. This diagram shows the etching process. This figure shows the state after the insertion process. This is a diagram showing the blockage process. This diagram shows the first cutting process. This diagram shows the situation after the first cutting process. This diagram shows the second cutting process. This is a diagram showing the line drawing process. This figure shows the optical fiber base material in the second embodiment in the same manner as in Figure 4. This figure shows the cladding rod, a pre-material for optical fibers, as shown in Figure 15. Figure 15 shows the optical fiber base material after it has been subjected to a cutting process, similar to Figure 5. This figure shows a modified example of a preform for optical fibers, similar to Figure 15. Figure 18 shows a cladding rod for optical fiber preforms, similar to Figure 16. Figure 18 shows the optical fiber base material after it has been subjected to a cutting process, similar to Figure 17.

The following embodiments for implementing the optical fiber preform according to the present invention are illustrated with accompanying drawings. The embodiments illustrated below are provided to facilitate understanding of the present invention and are not intended to limit its interpretation. The present invention can be modified and improved from the following embodiments without departing from its spirit. Furthermore, in this specification, the dimensions of each component may be exaggerated for the sake of clarity.

(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 the circumference of the optical fiber 1 centered on the central axis. In this cross-section, the outer shapes of the cores 10 and the cladding 20 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.

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. The dopant that changes the refractive index is not particularly limited.

The coating layer 30 is made of a resin, such as a thermosetting resin or an ultraviolet-curing resin.

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 multiple glass rods, and in this embodiment, it mainly comprises core rods 10R and cladding rods 20R including cladding glass bodies 20P that form 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.

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 section 11R and the second section of the clad rod 20R (described later) are located. As shown in Figure 3, each core rod 10R has a similar configuration to the others and includes a rod-shaped core glass body 10P that forms the core 10 as a predetermined portion distinct from the clad 20 in the optical fiber 1. In this embodiment, the core rod 10R is a glass rod in which the outer circumferential surface of the core glass body 10P is covered with a coating layer 10RL made of the same glass material as the clad glass body 20P, and the diameter of the core rod 10R is approximately constant in the longitudinal direction.

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 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 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. Furthermore, these holes 25 are through holes that open to both end faces of the clad rod 20R.

Each hole 25 holds a core rod 10R. 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. The position of the holes 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.

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. Figure 4 is another cross-sectional view perpendicular to the longitudinal direction of the optical fiber base material 1P shown in Figure 2. The diameter of each hole 25 in the first section 21 is larger 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. Also, since the length of the clad rod 20R is approximately the same as the length of the core rod 10R, both ends of the clad rod 20R surround both ends of the clad rod 20R.

Such a preform material 1P for optical fibers is used after undergoing a heat-cutting process, where 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 preform material for optical fibers in the state after this heat-cutting process will be described.

Figure 5 shows the optical fiber base material 1P shown in Figure 2 after thermal cutting. It is a cross-sectional view along the longitudinal direction of the optical fiber base material after thermal cutting. In the following explanation, 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.

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.

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

The clad rod 20Ra differs primarily from the clad rod 20R of the rod assembly 1P 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. A core rod 10Ra is held in each of the holes 25.

The clad rod 20Ra has a first sealing portion 23 at one end and a second sealing portion 24 at the other end, 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 towards the one end. It surrounds one end of each core rod 10Ra and is welded to that end, sealing one end of each hole 25a, thus connecting the core rod 10Ra 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 its outer diameter decreasing from the other end towards the one 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 generally coincide.

The second sealing portion 24 is formed in a tapered shape, with its outer diameter decreasing from one end to the other, and surrounds the other end of each core rod 10Ra, welding it to that end and sealing the other end of each hole 25a, so that 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 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, about ^10⁻⁵ Pa to ^10⁻⁸ Pa.

The outer diameter of the main body portion 20RaB is approximately constant in the longitudinal direction. The main body portion 20RaB consists of a first section 21a extending longitudinally 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. The first section 21a is a part of the first section 21 of the clad rod 20R in the rod assembly 1P. The second section 22a is a 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.

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. Furthermore, the holding space for 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 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.

Next, the manufacturing method of the optical fiber preform 1Pa and the manufacturing method of the optical fiber 1 will be described.

Figure 6 is a flowchart showing the steps of the manufacturing method for the optical fiber 1, including the manufacturing method for the optical fiber preform 1Pa according to this embodiment. As shown in Figure 6, the manufacturing method for the 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 manufacturing method for the optical fiber 1 also includes a drawing step P5 for drawing the manufactured optical fiber preform 1Pa.

<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 step P11, a dummy glass tube welding step P12, an etching step P13, and an insertion step P14.

<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.

<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.

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, and the first glass tube 41 is welded to the end face of the clad rod 20R so that the clad rod 20R and the first glass tube 41 are approximately coaxial. Furthermore, the second glass tube is welded to the other end face of the clad rod 20R in the same manner as with the first glass tube 41. In the state where the first glass tube 41 and the second glass tube are 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.

<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.

<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 core rod 10R is inserted into the corresponding hole 25. In this embodiment, the core rods 10R are inserted into the holes 25 such that one end of the 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. Also, since the length of the clad rod 20R and the length of the core rod 10R are approximately the same, the other end of the core rod 10R is surrounded by the other end of the clad rod 20R.

<Closing 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.

<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 at 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 at 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 that they move apart in the longitudinal direction, and one end of the rod assembly 1P is cut off starting from the constricted portion 26 to form the first sealing portion 23 as shown in Figure 12. In this embodiment, one end of the clad rod 20R surrounds one end of the core rod 10R, so the core rod 10R is located at the end of the rod assembly 1P that is cut off, and the formed first sealing portion 23 is welded to the core rod 10R. In this embodiment, one end of the rod assembly 1P is cut 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.

<Second fusing process P4>
This step 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 of this step. As shown in Figure 13, similar to the first cutting step 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. In this embodiment, since the other end of the clad rod 20R surrounds the other end of the core rod 10R, the formed second sealing portion 24 is welded to the core rod 10R.

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.

<Drawing process P5>
This process involves drawing a fiber optic base 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 fiber optic base material 1Pa opposite to the first sealing portion 23 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 the molten portion ND. This molten portion ND seals the end of the hole 25a in the clad rod 20Ra 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 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 1N using a coating device 70 to obtain the optical fiber 1 shown in Figure 1.

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, and the core rod 10R is held in the hole 25. The clad rod 20R includes a first section 21 along its longitudinal direction that includes this one end, and a second section 22 connected to the first section 21. 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. Therefore, in the rod assembly 1P of this embodiment, the heat capacity per unit length of the clad rod 20R in the first section 21 is smaller than the heat capacity per unit length of the clad rod 20R in the second section 22. Therefore, compared to the case where the cross-sectional area of the core rod 10R is the same in the first section 21 and the second section 22, and the cross-sectional area of the clad rod 20R in the first section 21 is greater than or equal to the cross-sectional area of the clad rod 20R in the second section, the heat capacity per unit length of the optical fiber base material 1P in the first section 21 is smaller than the heat capacity per unit length of the optical fiber base material 1P in the second section 22. Therefore, according to the rod assembly 1P of this embodiment, compared to the above case, the heat capacity per unit length at one end of the rod assembly 1P can be reduced, the time required for the cutting process at one end of the rod assembly 1P can be shortened, and the optical fiber base material 1Pa can be manufactured efficiently.

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, according to the rod assembly 1P of this embodiment, the time required for the cutting process at one end of the rod assembly 1P can be shortened 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.

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, and the core rod 10Ra is held in the hole 25a. The clad rod 20Ra has a first sealing portion 23 that closes one end of the hole 25a at one end, a second sealing portion 24 that is welded to the core rod 10Ra at the other end and closes the other end of the hole 25a, and a main body portion 20RaB between the first sealing portion 23 and the second sealing portion 24. The main body portion 20RaB consists of a first section 21a that runs along its longitudinal direction from the end on the first sealing portion 23 side to the second sealing portion 24 side, and a second section 22a that is connected to the first section 21a. When drawing the optical fiber base material 1Pa of this embodiment from the end on the second sealing portion 24 side, 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 first sealing portion 23 side as the drawing progresses. In the optical fiber base material 1Pa of this embodiment, the first section 21a is located closer to the first sealing portion 23 than the second section 22a. 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 that holds the core rod 10Ra in the second section 22a is 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, by drawing the line from the end on the second sealing portion 24 side, 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, according to the optical fiber base material 1Pa of this embodiment, the 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 1 can be suppressed.

In the optical fiber base material 1Pa of this embodiment, the first sealing portion 23 is welded to the core rod 10Ra. If the core rod 10Ra is not welded to the first sealing portion 23, drawing a line from the end on the second sealing portion 24 side may cause the distance between the end of the core rod 10Ra on the first sealing portion 23 side and the first sealing portion 23 to fluctuate during drawing. In this case, the amount of core glass body 10P drawn per unit time changes, and the diameter of the core of the optical fiber 1 fluctuates in the longitudinal direction. However, with the optical fiber base material 1P of this embodiment, the fluctuation of the diameter of a predetermined portion in the longitudinal direction can be suppressed compared to the above case. Note that the first sealing portion 23 does not necessarily have to be welded to the core rod 10Ra.

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, compared to the case where the pressure inside the hole 25a is above atmospheric pressure, the pressure in the space between the cladding rod 20Ra and the core rod 10Ra in the advanced state can be lowered, thereby suppressing the inclusion of air bubbles in the optical fiber 1. Note that the pressure inside the hole 25a may be above atmospheric pressure.

The rod assembly 1P and optical fiber base material 1Pa of this embodiment are provided with multiple core rods 10R and 10Ra, and the cladding rods 20R and 20Ra are provided with multiple holes 25 and 25a that individually hold the multiple core rods 10R and 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.

In this embodiment, the rod assembly 1P and optical fiber base material 1Pa have first sections 21 and 21a that are shorter than the second sections 22 and 22a. Therefore, compared to the case where the second sections 22 and 22a are shorter than the first sections 21 and 21a, it is possible to manufacture longer optical fibers 1. Note that the second sections 22 and 22a may also be shorter than the first sections 21 and 21a.

Furthermore, the manufacturing method of the optical fiber preform 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,
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 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. Therefore, compared to the case where the cross-sectional area of the core rod 10R is the same in the first section 21 and the second section 22, and the cross-sectional area of the clad rod 20R in the first section 21 is greater than or equal to the cross-sectional area of the clad rod 20R in the second section, the heat capacity per unit length at one end of the rod assembly 1P can be reduced. Consequently, the time required for the cutting process of one end of the rod assembly 1P can be shortened compared to this case. Furthermore, in the first cutting process P3, one end of the rod assembly 1P is heated and cut while the hole 25 is evacuated. Therefore, the time required for cutting the rod assembly 1P can be further shortened.

(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.

Figure 15 is a diagram showing the optical fiber base material in this embodiment, similar to Figure 4, and schematically shows the cross-section of the clad rod 20R including the first section 21. The optical fiber base material 1P shown in Figure 15 is a 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, a portion of the multiple holes 25 in the second section 22 are located within the communication hole 28. Furthermore, while the central axis of the communication hole 28 and the central axis of the clad rod 20R are generally aligned, they may be misaligned. Also, while the diameter of the communication hole 28 is generally constant in the longitudinal direction, it may vary in the longitudinal direction. Additionally, the communication hole 28 only needs to extend along the longitudinal direction over at least a portion of the first section 21.

According to the rod assembly 1P of this embodiment, similar to the first embodiment, the heat capacity per unit length in the first section 21 is reduced, and the time required for the cutting process at one end of the rod assembly 1P can be shortened.

Figure 17 shows the rod assembly 1P, which is the optical fiber base material shown in Figure 15, after it 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 and the manufacturing method for 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.

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 housing 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, the inclusion of air bubbles in the optical fiber 1 can be suppressed, similar to the first embodiment. Furthermore, according to the optical fiber base material 1Pa of this embodiment, compared to the case without the communication hole 28a, it is possible to suppress the pressure in the space between the clad rod 20Ra and a specific core rod 10Ra becoming higher than the pressure in the space between the clad rod 20Ra and other core rods 10Ra when the drawing has progressed. Note that the communication hole 28a only needs to extend along the longitudinal direction over at least a portion of the first section 21a.

The present invention has been described above using the above embodiments as examples, but the present invention is not limited thereto.

For example, in the second embodiment described above, clad rods 20R, 20Ra were described as having communication holes 28, 28a in the first sections 21, 21a, each containing a portion of a plurality of holes 25, 25a. However, the communication 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 communication 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 1P in the same manner as in 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 in 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 in 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 coincide, 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, the cross-sectional area of the core rod 10R in the first section 21 may be smaller than the cross-sectional area of the core rod 10R in the second section 22. Also, according to this modified optical fiber base material 1Pa, similar to the second embodiment, the inclusion of air bubbles in the optical fiber 1 can be suppressed.

Furthermore, in the first cutting step P3 of the above embodiment, one end of the rod assembly 1P was cut so that a portion 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 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.

Furthermore, in the above embodiment, clad rods 20R and 20Ra made of clad glass body 20P were described as an example. However, 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. Also, the clad rod 20R may be provided with voids extending along the longitudinal direction where no glass rods are housed.

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 containing predetermined glass bodies that are different from the cladding 20 in the optical fiber 1. For example, the predetermined glass bodies contained in the glass rod may be, for example, glass bodies that serve as markers as predetermined parts, or glass bodies that serve as stress-applying parts as predetermined parts. Also, the glass rod may be a glass rod in which a core glass body that forms the core 10 is covered with a glass body that forms a low refractive index layer surrounding the core 10. Furthermore, if there are multiple glass rods, the predetermined glass body contained in at least one of the multiple glass rods may be different from the predetermined glass body contained in at least one of the other glass rods.

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 the oxyhydrogen burner 50. Generally, the heat spot of an oxyhydrogen burner is narrower than that 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 using an electric furnace, the lengths 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 create an arbitrary distribution of heat spots, making it possible to achieve optimal heating for the size and material of the base material.

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

Furthermore, in the above embodiment, a rod assembly 1P in which both ends of the clad rod 20R surround the core rod 10R was described as an example. However, at least one end of the clad rod 20R does not need to surround the core rod 10R. For example, in the insertion step P14 described above, by inserting the core rod 10R into the hole 25 in such a way that the core rod 10R is not surrounded by one end of the clad rod 20R, a rod assembly in which one end of the clad rod 20R does not surround the core rod 10R can be obtained, and from this rod assembly, a base material for optical fibers in which the first sealing portion 23 is not welded to the core rod 10Ra can be obtained.

Furthermore, the manufacturing method for the optical fiber base material of the above embodiment was described using a preparation step P1 as an example, which includes a glass member preparation step P11, a dummy glass tube welding step P12, an etching step P13, and an insertion step P14. 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 with an etched inner surface defining the hole 25, 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 was described as an example, in which the entire opening at one end of the hole 25 of the clad rod 20R is closed with a dummy rod 43 as a closing member. However, in the closure step P2, it is sufficient to close at least a part of the opening at one end of the hole 25, and the closing member is not particularly limited.

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.

1... Optical fiber 1P... Optical fiber base material (rod assembly)
1Pa... Optical fiber base material 10... Core 10P... Core glass body 10R, 10Ra... Core rod 20... Cladding 20P... Cladding glass body 20R, 20Ra... Cladding rod 20RaB... Main body 21, 21a... First section 22, 22a... Second section 23... First sealing section 24... Second sealing section 25, 25a... Hole 28, 28a... Communication hole

Claims (5)

A clad rod comprising a clad glass body that forms at least part of the cladding in an optical fiber, having multiple holes extending along the longitudinal direction,
The optical fiber includes a predetermined glass body that forms a predetermined portion different from the cladding, and a plurality of glass rods that are individually held in a plurality of holes,
Equipped with,
The clad rod includes a first section along the longitudinal direction including one end, and a second section connected to the first section.
The cross-sectional area of the clad rod in the cross-section of the first section is smaller than the cross-sectional area of the clad rod in the cross-section of the second section.
The diameter of each of the holes in the first section is greater than the diameter of the same hole in the second section.
A preform for optical fibers characterized by the following features. The optical fiber base material according to claim 1, characterized in that the glass rod is surrounded by one end of the clad rod. The optical fiber base material according to claim 1 or 2 , characterized in that 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 a plurality of interconnected holes. The optical fiber base material according to claim 3 , characterized in that, when viewed along the longitudinal direction, the entirety of the plurality of holes in the second section is located within the communication holes. The second section is longer than the first section.
A preform for optical fibers according to any one of claims 1 to 4 .