Apparatus for manufacturing glass rods for optical fiber preforms, method for manufacturing glass rods for optical fiber preforms, and method for manufacturing optical fibers.

Abstract

A production device (30) for a glass rod for an optical fiber base material comprises: a first lathe holding part (31) that is capable of holding a silica glass tube (20) at a position partway therealong; a first heating part (33) that, on one side of the first lathe holding part (31), is capable of heating the silica glass tube (20) such that an alkali compound (40) reaches a melting point or higher; a carrier gas introduction part (35) that is capable of introducing a carrier gas (CG) into a through hole (20H) of the silica glass tube (20) such that vaporized alkali compound flows therethrough and flows into the through hole (20H); and a second heating part (34) that heats the alkali compound while moving on the other side of the silica glass tube (20) along the lengthwise direction of the silica glass tube (20) and that is capable of heating the silica glass tube such that the silica glass tube (20) is doped with an alkali metal or alkali earth metal.

Description

Technical Field

[0001] The present invention relates to an apparatus for manufacturing a glass rod for an optical fiber preform, a method for manufacturing a glass rod for an optical fiber preform, and a method for manufacturing an optical fiber.

Background Art

[0002] In an optical fiber communication system, in order to increase the optical transmission distance and the optical transmission speed, the optical signal / noise ratio must be increased. Therefore, reduction of the transmission loss of an optical fiber is required. Currently, with the highly sophisticated manufacturing method of optical fibers, it is considered that the transmission loss due to impurities contained in the optical fiber has been reduced almost to the limit. The main cause of the remaining transmission loss is the scattering loss associated with fluctuations in the structure and composition of the glass constituting the optical fiber. This is inevitable because the optical fiber is made of glass.

[0003] As an optical fiber in which the scattering loss associated with fluctuations in the structure and composition of the glass as described above can be reduced, a core is composed of silica glass doped with a small amount of alkali metal oxide or alkaline earth metal oxide, and a clad is composed of silica glass doped with fluorine. Hereinafter, alkali metal oxide and alkaline earth metal oxide may be referred to as alkali oxide.

[0004] When silica glass is doped with an alkali oxide, the softening point of the silica glass is greatly lowered. That is, when compared with the case of silica glass not doped with an alkali oxide at the same temperature, the silica glass doped with an alkali oxide has a low viscosity and thus structural relaxation is easily promoted. Therefore, when an optical fiber preform is produced such that the silica glass doped with an alkali oxide becomes the core and this optical fiber preform is spun to manufacture an optical fiber, the structural fluctuations of the silica glass constituting the core are rapidly reduced. As a result, an optical fiber with reduced transmission loss can be manufactured.

[0005] Patent Document 1 discloses a method for manufacturing an optical fiber preform by doping silica glass with alkali oxide using an improved chemical vapor deposition (MCVD) method. In the method described in Patent Document 1, first, dummy glass tubes are welded to both ends of a glass tube made of pure synthetic silica glass to form a composite glass tube. Both ends of this composite glass tube are attached to a glass forming lathe used in the MCVD method, and an oxygen-containing gas is circulated as a carrier gas in the hollow part of the composite glass tube. In addition, a compound of alkali metal or alkaline earth metal, which is the raw material for alkali oxide, is placed in a solid state inside the tube on the upstream side in the direction in which the carrier gas flows within the composite glass tube. Hereinafter, alkali metal and alkaline earth metal compounds may be referred to as alkali compounds. Next, the alkali compound is heated to a temperature above its melting point using a first oxyhydrogen burner and vaporized according to its vapor pressure, and the vaporized alkali compound is cooled with the carrier gas to atomize it into fine particles and circulated toward the other side of the composite glass tube. Next, a second oxyhydrogen burner, moving relative to the glass tube from the upstream to the downstream side of the carrier gas, is used to heat the alkali compound to a temperature at which it becomes an alkali oxide through a thermal oxidation reaction (for example, the outer surface temperature of the glass tube is approximately 1300°C to 2250°C). This causes the alkali oxide to deposit on the inner surface of the glass tube and then diffuse into the silica glass that constitutes the glass tube. The alkali oxide-doped glass tube thus produced is further heated to shrink and solidify, thereby obtaining an alkali oxide-doped silica glass rod. This alkali oxide-doped glass rod can be used as the core of an optical fiber. By forming a cladding layer around such a glass rod, it can be used as an optical fiber preform.

[0006] [Patent Document 1] Patent No. 5656469 [Overview of the project]

[0007] In recent years, there has been a demand for longer optical fibers drawn from a single optical fiber preform. To achieve this, the glass rod for the optical fiber preform, manufactured according to Patent Document 1, should be lengthened. However, in the method applying the MCVD method described in Patent Document 1, both ends of the composite glass tube are attached to a lathe. Therefore, lengthening the glass tube portion that serves as the glass rod for the optical fiber preform results in a longer composite glass tube, making it more prone to bending. When the composite glass tube bends, runout occurs when the composite glass tube is rotated around its axis, leading to distortion and uneven thickness of the glass tube. For this reason, the method described in Patent Document 1 tends to make it difficult to lengthen the glass rod for the optical fiber preform. The same applies even when the composite glass tube is made from a single silica glass tube without any welded joints.

[0008] Therefore, the present invention aims to provide a manufacturing apparatus for glass rods for optical fiber preforms, a method for manufacturing glass rods for optical fiber preforms, and a method for manufacturing optical fibers, which can lengthen the glass rods for optical fiber preforms produced using a method that applies the MCVD method.

[0009] To solve the above problems, Embodiment 1 of the present invention includes: a first lathe gripping portion capable of gripping a silica glass tube at a predetermined distance from one end of the silica glass tube; a second lathe gripping portion capable of gripping the silica glass tube on the other side of the silica glass tube from the first lathe gripping portion; a first heating portion provided on the one side of the first lathe gripping portion and capable of heating the silica glass tube to a temperature above the melting point of an alkali metal compound or alkaline earth metal compound placed in a through-hole of the silica glass tube; and a vaporized alkali metal compound or alkaline earth metal compound flowing through the flow process The apparatus for manufacturing glass rods for optical fiber preforms is characterized by comprising: a carrier gas introduction unit capable of introducing a carrier gas from one side into the through-hole of the silica glass tube between the first lathe gripping unit and the second lathe gripping unit, so that the alkali metal compound or the alkaline earth metal compound, which is atomized by the carrier gas introduction unit, flows through the through-hole of the silica glass tube between the first lathe gripping unit and the second lathe gripping unit; and a second heating unit capable of heating the silica glass tube while moving along the longitudinal direction of the silica glass tube so that the atomized alkali metal compound or the alkaline earth metal compound reaches a temperature above the oxidation reaction temperature.

[0010] In this apparatus for manufacturing glass rods for optical fiber preforms, even when a reservoir for arranging an alkali compound is provided in a silica glass tube, the reservoir can be positioned to one side of the first lathe gripping section. Therefore, compared to the apparatus for manufacturing glass preforms described in Patent Document 1, in which both ends of the composite glass tube are attached to a lathe and the reservoir is located between the two ends, the bending of the silica glass tube can be suppressed even if the length of the silica glass tube that becomes the glass rod for optical fiber preforms is increased. Accordingly, the apparatus for manufacturing glass rods for optical fiber preforms of the present invention makes it possible to manufacture glass rods for optical fiber preforms that can be made longer.

[0011] Aspect 2 of the present invention is a manufacturing apparatus for optical fiber preform glass rods according to aspect 1, characterized in that the distance from the position where the first heating unit heats the silica glass tube to the first lathe gripping unit is greater than the distance from the position closest to the first lathe gripping unit where the second heating unit heats the glass tube to the first lathe gripping unit.

[0012] As described above, in the apparatus for manufacturing glass rods for optical fiber preforms according to Embodiment 2 of the present invention, the area between the position where the first heating section heats the silica glass tube and the first lathe gripping section, the portion where the first lathe gripping section grips the silica glass tube, and the area between the position of the second heating section closest to the first lathe gripping section that heats the glass tube and the first lathe gripping section can become a cooling section where the alkali compound circulating with the carrier gas is cooled. With the configuration of Embodiment 2, when the distance from the position where the first heating section heats the silica glass tube to the first lathe gripping section is constant, the silica glass tube to which the alkali compound is doped can be made longer compared to the case where the distance from the position where the first heating section heats the silica glass tube to the first lathe gripping section is less than or equal to the distance from the position of the second heating section closest to the first lathe gripping section that heats the glass tube to the first lathe gripping section.

[0013] A third aspect of the present invention is a manufacturing apparatus for glass rods for optical fiber preforms according to aspect 1 or aspect 2, characterized in that the first lathe gripping portion is made of metal.

[0014] In this case, the first lathe gripping section can absorb heat more efficiently from the silica glass tube compared to the case where the first lathe gripping section is made of a Teflon® coating or the like. Therefore, the alkali compound circulating in the carrier gas can be cooled efficiently.

[0015] Aspect 4 of the present invention includes a gripping step of gripping the silica glass tube with a first lathe gripping part at a predetermined distance from one end of the silica glass tube, and gripping the silica glass tube with a second lathe gripping part on the other side of the silica glass tube from the first lathe gripping part; a first heating step of heating the silica glass tube on the one side from the first lathe gripping part so that an alkali metal compound or alkaline earth metal compound placed in the through hole of the silica glass tube is heated to a temperature above its melting point; and introducing a carrier gas into the through hole of the silica glass tube from the one side, which has been vaporized in the first heating step. A method for manufacturing a glass rod for optical fiber preforms, comprising: a flow step of flowing the alkali metal compound or the alkaline earth metal compound, which is atomized during the flow process, into a through-hole of the silica glass tube between the first lathe gripping part and the second lathe gripping part; and a second heating step of heating the silica glass tube while moving along the longitudinal direction of the silica glass tube so that the atomized alkali metal compound or the alkaline earth metal compound reaches a temperature above the oxidation reaction temperature.

[0016] In the present invention, in the method for manufacturing a glass rod for optical fiber preforms, in the first heating step, the silica glass tube is heated on the side opposite to the second lathe gripping portion, with reference to the first lathe gripping portion, thereby heating the alkali compound in the through-hole of the silica glass tube. Therefore, compared to the method for manufacturing a glass member described in Patent Document 1, in which an alkali compound is placed between both ends of a composite glass tube attached to a lathe and heated, the bending of the silica glass tube can be suppressed even if the length of the silica glass tube that becomes the glass rod for optical fiber preforms is increased. Accordingly, the method for manufacturing a glass rod for optical fiber preforms of the present invention makes it possible to manufacture a glass rod for optical fiber preforms that allows for an increased length of optical fiber preform.

[0017] Aspect 5 of the present invention is a method for manufacturing a glass rod for optical fiber preforms according to aspect 4, characterized in that the distance from the position where the silica glass tube is heated in the first heating step to the lathe gripping portion is greater than the distance from the position closest to the first lathe gripping portion where the silica glass tube is heated in the second heating step to the lathe gripping portion.

[0018] In this case, similar to the description in Embodiment 2, the space between the position where the silica glass tube is heated in the first heating step and the first lathe gripping part, the portion where the first lathe gripping part grips the silica glass tube, and the space between the position closest to the first lathe gripping part where the second heating part heats the glass tube and the first lathe gripping part can become a cooling section where the alkali compound circulating with the carrier gas is cooled. Therefore, compared to the case where the distance from the position where the silica glass tube is heated in the first heating step to the lathe gripping part is less than or equal to the distance from the position closest to the first lathe gripping part where the silica glass tube is heated in the second heating step to the lathe gripping part, if the length of the cooling section is the same, the silica glass tube to which the alkali compound is doped can be made longer.

[0019] Aspect 6 of the present invention is a method for manufacturing an optical fiber, characterized by comprising a drawing step of drawing a fiber optic matrix having a glass rod for optical fiber matrix manufactured by the optical fiber matrix glass rod manufacturing method of Aspect 4 or Aspect 5.

[0020] This method for manufacturing optical fibers makes it possible to produce long optical fibers.

[0021] As described above, the present invention provides a manufacturing apparatus for glass rods for optical fiber preforms, a method for manufacturing glass rods for optical fiber preforms, and a method for manufacturing optical fibers, which can lengthen the glass rods for optical fiber preforms produced using a method that applies the MCVD method. [Brief explanation of the drawing]

[0022] [Figure 1] This is a cross-sectional view showing an optical fiber according to an embodiment of the present invention. [Figure 2]It is a diagram showing a silica glass tube for manufacturing a glass rod for an optical fiber preform according to an embodiment of the present invention. [Figure 3] It is a diagram showing a state where the silica glass tube of FIG. 2 is attached to a manufacturing apparatus for a glass rod for an optical fiber preform. [Figure 4] It is a flowchart showing the steps of a method for manufacturing an optical fiber according to an embodiment of the present invention. [Figure 5] It is a diagram showing the states of the first heating step and the circulation step. [Figure 6] It is a diagram showing the state of the second heating step. [Figure 7] It is a diagram showing the state of the consolidation step. [Figure 8] It is a diagram showing a glass rod for an optical fiber preform obtained through the consolidation step. [Figure 9] It is a cross-sectional view showing an optical fiber preform. [Figure 10] It is a diagram showing the state of the scribing step.

Embodiments for Carrying Out the Invention

[0023] Hereinafter, preferred embodiments of an apparatus for manufacturing a glass rod for an optical fiber preform, a method for manufacturing a glass rod for an optical fiber preform, and a method for manufacturing an optical fiber according to the present invention will be described in detail with reference to the drawings.

[0024] FIG. 1 is a cross-sectional view showing an optical fiber according to an embodiment of the present invention. As shown in FIG. 1, the optical fiber 1 of the present embodiment includes a core 11, a cladding 12 that surrounds the outer peripheral surface of the core 11 without a gap, an inner protective layer 13 that covers the outer peripheral surface of the cladding 12, and an outer protective layer 14 that covers the outer peripheral surface of the inner protective layer 13. The refractive index of the core 11 is higher than that of the cladding 12. The core 11 is made of silica glass doped with at least an alkali oxide such as an alkali metal oxide or an alkaline earth metal oxide. Further, the cladding 12 of the present embodiment is made of, for example, silica glass doped with fluorine.

[0025] Next, we will explain the manufacturing process of the glass rods used as the optical fiber preform, which will eventually become the optical fiber 1. These glass rods will ultimately become the core of the optical fiber.

[0026] Figure 2 shows a silica glass tube for manufacturing a glass rod for optical fiber preforms according to this embodiment. As shown in Figure 2, the silica glass tube 20 of this embodiment consists of an effective portion 20E, a first dummy portion 20D1, and a second dummy portion 20D2. In this embodiment, the first dummy portion 20D1 is welded to one end of the effective portion 20E, and the second dummy portion 20D2 is welded to the other end of the effective portion 20E. In the figure, the boundaries between the effective portion 20E and the first dummy portion 20D1 and the second dummy portion 20D2 are indicated by dashed lines.

[0027] For example, a commercially available synthetic silica glass tube for optical fibers can be used as the effective section 20E. The effective section 20E is the part to which alkali oxides are doped as dopants, as described later. For this reason, the effective section 20E may be a silica glass tube made of pure silica glass without dopants added, or it may be a silica glass tube to which dopants other than alkali oxides are added. Examples of such dopants include chlorine, fluorine, and germanium. Multiple types of these dopants may be doped, and the doping may be done in such a way that a concentration distribution occurs in the thickness direction. However, as will be described later, since the effective section 20E becomes the core 11 of the optical fiber 1, it is preferable that the concentration of dopant added to the effective section 20E be low from the viewpoint of reducing optical transmission loss. The size of the effective section 20E is not particularly limited, but for example, it may have an outer diameter of 32 mm and a wall thickness of 2.5 mm.

[0028] The first dummy section 20D1 and the second dummy section 20D2 are made of silica glass tubes having approximately the same outer diameter and wall thickness as the effective section 20E. The first dummy section 20D1 and the second dummy section 20D2 are made of silica glass tubes, and dopants may or may not be added. Note that the first dummy section 20D1 and the second dummy section 20D2 are not welded to the effective section 20E, and the first dummy section 20D1, the effective section 20E, and the second dummy section 20D2 may all consist of a single silica glass tube. In this embodiment, the first dummy section 20D1 is provided with a pair of reduced-diameter sections 21a and 21b. The space between the reduced-diameter sections 21a and 21b in the first dummy section 20D1 becomes a reservoir section 22 on which an alkali compound, such as an alkali metal compound or an alkaline earth metal compound, can be placed on the inner circumferential surface. The side of the reduced-diameter section 21a opposite the reservoir section 22 is a connecting section 23. The connecting portion 23 includes the end of the silica glass tube 20. The space between the reduced diameter portion 21b and the effective portion 20E of the first dummy portion 20D1 is a cooling portion 24, and as will be described later, the alkali compound flowing through the through-hole 20H of the silica glass tube 20 is cooled in the cooling portion 24.

[0029] Next, we will describe the manufacturing equipment for optical fiber 1.

[0030] Figure 3 shows the silica glass tube 20 shown in Figure 2 attached to the manufacturing apparatus for optical fiber preform glass rods. As shown in Figure 3, the manufacturing apparatus 30 for optical fiber preform glass rods in this embodiment mainly comprises a first lathe gripping section 31, a second lathe gripping section 32, a first heating section 33, a second heating section 34, and a carrier gas introduction section 35.

[0031] The first lathe gripping section 31 and the second lathe gripping section 32 are provided at a predetermined distance from each other and are configured to grip the silica glass tube 20. The first lathe gripping section 31 and the second lathe gripping section 32 are made of metal and have three claws provided at equal intervals in the circumferential direction on the silica glass tube 20, which grip the outer surface of the silica glass tube 20. The first lathe gripping section 31 is capable of gripping the silica glass tube 20 at a predetermined distance from one end of the silica glass tube 20, and the second lathe gripping section 32 is capable of gripping the silica glass tube 20 on the other side of the first lathe gripping section 31. Details of the position in which the first lathe gripping section 31 grips the silica glass tube 20 will be described later. Furthermore, the first lathe gripping section 31 and the second lathe gripping section 32 are mechanically connected to a drive section (not shown), and are configured to rotate together with the silica glass tube 20 around its central axis while gripping the silica glass tube 20. In this embodiment, the second lathe gripping section 32 can move along the longitudinal direction of the silica glass tube 20 so that the silica glass tube 20 can be attached to the optical fiber preform glass rod manufacturing apparatus 30.

[0032] The first heating section 33 is located on the side opposite to the second lathe gripping section 32, which is one side of the first lathe gripping section 31. In the example shown in Figure 3, the first heating section 33 is an electric furnace and is capable of heating the silica glass tube 20. The first heating section 33 is configured to heat at least the reservoir section 22 of the silica glass tube 20 so that the alkali compound inside the silica glass tube 20 reaches a temperature above its melting point, as will be described later. For example, the silica glass tube 20 can be heated to approximately 780°C. The first heating section 33 may be an oxyhydrogen burner or a plasma heater, in addition to an electric furnace.

[0033] The second heating section 34 is located between the first lathe gripping section 31 and the second lathe gripping section 32, on the other side of the first lathe gripping section 31. In the example shown in Figure 3, the second heating section 34 consists of an oxyhydrogen burner and is configured to heat the silica glass tube 20 while moving along the longitudinal direction of the silica glass tube 20. The second heating section 34 is mechanically connected to a drive unit (not shown) and can move so as to heat at least the effective portion 20E. Furthermore, the second heating section 34 is configured to heat the silica glass tube 20 to the extent that alkali compounds inside the silica glass tube 20 react with oxygen in the carrier gas (thermal oxidation reaction) to form alkali oxides, thereby doping the silica glass tube 20 with alkali metals or alkaline earth metals. For example, the outer surface of the silica glass tube 20 can be heated to approximately 1300°C to 2250°C. Note that the second heating section 34 may be an electric furnace or a plasma heater in addition to an oxyhydrogen burner.

[0034] The carrier gas introduction section 35 is connected to a carrier gas supply source (not shown) via piping and is also connected to the connection section 23 of the silica glass tube 20. The carrier gas introduction section 35 is configured to allow the introduction of carrier gas from one side into the through-hole 20H of the silica glass tube 20 so that the alkali compound vaporized in the reservoir section 22 of the silica glass tube 20 flows through it, and the alkali compound, which is atomized during the flow process, flows into the inner hole of the effective section 20E. The vaporized alkali compound is cooled in the cooling section 24 and atomized. Even when the silica glass tube 20 rotates around its central axis, the carrier gas introduction section 35 can introduce carrier gas into the through-hole 20H without rotating, while suppressing the inflow of external gas into the through-hole 20H. Details of the carrier gas will be described later.

[0035] Next, a method for manufacturing an optical fiber according to this embodiment will be described.

[0036] Figure 4 is a flowchart showing the steps of a method for manufacturing an optical fiber according to an embodiment of the present invention. As shown in Figure 4, the method for manufacturing an optical fiber according to this embodiment comprises a gripping step P1, a first heating step P2, a flow step P3, a second heating step P4, a solidification step P5, a base material formation step P6, and a wire drawing step P7.

[0037] <Gripping process P1> This step involves gripping the silica glass tube 20 with the first lathe gripping section 31 and the second lathe gripping section 32. In this step, the first lathe gripping section 31 grips the silica glass tube 20 at a predetermined distance from one end of the silica glass tube 20. Specifically, the silica glass tube 20 is passed through the first lathe gripping section 31 so that the first lathe gripping section 31 is positioned at the cooling section 24 of the first dummy section 20D1 of the silica glass tube 20, and the reservoir section 22 is positioned at the first heating section 33, and the silica glass tube 20 is gripped by the claws of the first lathe gripping section 31. At this time, the second lathe gripping section 32 is kept sufficiently far from the first lathe gripping section 31 so that the silica glass tube 20 does not come into contact with the second lathe gripping section 32. Furthermore, if the first heating unit 33 is configured to be movable in the longitudinal direction of the silica glass tube 20, it is preferable that the first heating unit 33 be moved so that it is positioned in the reservoir unit 22 while the first lathe gripping unit 31 is gripping the silica glass tube 20. Next, the second dummy unit 20D2 of the silica glass tube 20 is gripped by the second lathe gripping unit 32. In this way, the silica glass tube 20 is attached to the optical fiber preform glass rod manufacturing apparatus 30.

[0038] Furthermore, it is preferable that the distance L1 from the position where the first heating section 33 heats the silica glass tube 20 to the first lathe gripping section 31 is greater than the distance L2 from the position closest to the first lathe gripping section 31 where the second heating section 34 heats the silica glass tube 20 to the first lathe gripping section 31. In this case, if the lengths of the effective section 20E and the cooling section 24 are constant, the distance between the first lathe gripping section 31 and the second lathe gripping section 32 can be reduced compared to the case where the first lathe gripping section 31 grips the silica glass tube 20 such that the distance L1 is less than or equal to the distance L2, thereby further suppressing the deflection of the silica glass tube 20.

[0039] <First heating step P2> Figure 5 shows the process and the distribution process P3. This process involves heating the silica glass tube 20 on one side of the first lathe gripping section 31 so that the alkali metal compound or alkaline earth metal compound placed in the through hole 20H of the silica glass tube 20 is heated to a temperature above its melting point.

[0040] Prior to this step, an alkali compound 40, which is solid at room temperature, is placed in the through-hole 20H of the reservoir section 22. In this embodiment, potassium chloride (KCl) is used as the alkali compound 40. When potassium chloride is used as the alkali compound 40, the diffusion rate into the silica glass is easily set to an appropriate rate, mainly due to the atomic weight of potassium. As alkali metals, lithium, sodium, potassium, rubidium, cesium, etc. can be used, and as alkaline earth metals, beryllium, magnesium, calcium, strontium, barium, etc. can be used. In addition, halides (chlorides, bromides, fluorides, iodides), sulfides, carbonates, bicarbonates, etc. can be used as these compounds. It is preferable that the alkali compound 40 is in particulate form from the viewpoint of easily melting the alkali compound 40 by heating. Furthermore, it is sufficient that the alkali compound 40 is placed in the reservoir section 22 by this step, and the alkali compound 40 may be placed in the reservoir section 22 before the gripping step P1.

[0041] In this process, first, the carrier gas CG is circulated through the through-hole 20H of the silica glass tube 20 from one side to the other via the carrier gas introduction section 35. As the carrier gas CG, for example, a gas containing dry oxygen heated to room temperature or a temperature of about 80°C to 120°C is used. Furthermore, with the alkali compound 40 placed in the reservoir section 22, the reservoir section 22 is heated by the first heating section 33 to dry the alkali compound 40. At this time, for example, the alkali compound 40 is heated at 150°C for 15 minutes or more.

[0042] Next, while carrier gas CG is circulated from the carrier gas introduction section 35, the silica glass tube 20 is rotated around its central axis by the rotation of the first lathe gripping section 31 and the second lathe gripping section 32, and the reservoir section 22 is further heated by the first heating section 33, heating the alkali compound 40 above its melting point. At this time, the silica glass tube 20 is heated to, for example, about 780°C. Due to this heating, the alkali compound 40 melts, and according to the vapor pressure, the alkali compound 40 vaporizes, generating alkali compound vapor 41.

[0043] <Distribution process P3> This process involves introducing a carrier gas CG into the through-hole 20H of the silica glass tube 20 from one side, circulating the alkali metal compound or alkaline earth metal compound vaporized in the first heating step P2, and circulating the alkali metal compound or alkaline earth metal compound, which becomes finely atomized during the circulation process, into the through-hole 20H of the silica glass tube 20 on the other side of the first lathe gripping section 31. As described above, in the first heating step P2, the silica glass tube 20 is rotated while the carrier gas CG circulates through the through-hole 20H of the silica glass tube 20. Therefore, the vapor 41 of the alkali compound 40 flows from one side to the other through the through-hole 20H of the silica glass tube 20, carried by the carrier gas CG. When the vapor 41 flows from the reservoir section 22 to the cooling section 24, the vapor 41 is cooled, and the vapor 41 becomes fine particles 42 of the alkali compound 40. The fine particles 42 are carried by the carrier gas CG to the other side of the first lathe gripping section 31. In this case, if the first lathe gripping portion 31 is made of metal, it is preferable from the viewpoint of easily cooling the alkali compound 40 flowing through the through hole 20H, as the first lathe gripping portion 31 can easily absorb heat from the cooling portion 24 of the silica glass tube 20.

[0044] <Second heating step P4> This process involves heating the silica glass tube 20 while moving the other side of the first lathe gripping portion 31 along the longitudinal direction of the silica glass tube 20 so that the alkali metal compound or alkaline earth metal compound fine particles 42 reach an oxidation reaction temperature or higher. Figure 6 shows the process. As described above, alkali compound fine particles 42 flow through the through hole 20H of the effective portion 20E. In this process, as shown in Figure 6, the silica glass tube 20 is rotated by the first lathe gripping portion 31 and the second lathe gripping portion 32, and the second heating portion 34 is moved along the longitudinal direction of the silica glass tube 20 from the upstream side to the downstream side in the flow direction of the carrier gas CG, thereby heating the silica glass tube 20 with the second heating portion 34. At this time, the second heating portion 34 heats the silica glass tube 20 so that the alkali compound reacts with oxygen in the carrier gas CG (thermal oxidation reaction) to become an alkali oxide. For example, the second heating section 34 heats the silica glass tube 20 so that its outer surface reaches a temperature of approximately 1300°C to 2250°C. The fine particles 42 are heated by the moving second heating section 34 in this manner. As described above, the carrier gas CG contains oxygen. Therefore, alkali compounds are thermally oxidized by the oxygen contained in the carrier gas CG, alkali metal compounds are converted into alkali metal oxides, and alkaline earth metal compounds are converted into alkaline earth metal oxides, which are deposited on the inner wall of the silica glass constituting the silica glass tube 20 downstream of the heating section. The deposited alkali metal oxides or alkaline earth metal oxides are heated again by the moving second heating section 34 and diffuse into the interior of the silica glass constituting the silica glass tube 20. This process can be repeated by traversing the second heating section 34 until the concentration of alkali metals doped into the silica glass tube 20 reaches a desired value. In this way, alkali metal oxides or alkaline earth metal oxides are doped into the silica glass tube 20.

[0045] <Solidification Process P5> This process involves further heating and reducing the diameter of the silica glass tube 20, which has been doped with an alkali metal or alkaline earth metal after the second heating process P4, and finally solidifying it to form a glass rod 20R for optical fiber preforms. Figure 7 shows the process, and Figure 8 shows the glass rod 20R for optical fiber preforms obtained by this process.

[0046] As shown in Figure 7, in this process, the silica glass tube 20 is heated to approximately 2000°C or higher from the outer surface side by the second heating unit 34 while rotating the silica glass tube 20 around its axis, thereby reducing the diameter of the effective portion 20E of the silica glass tube 20, and finally solidifying it. By heating the silica glass tube 20 while relatively moving the second heating unit 34 along the longitudinal direction of the effective portion 20E of the silica glass tube 20, the entire effective portion 20E of the silica glass tube 20 is gradually reduced in diameter and solidified. At this time, heating may be performed while reducing the pressure inside the through hole 20H of the silica glass tube 20. In that case, the process temperature can be reduced, and thus the energy required for the process can be reduced. In this way, a cylindrical glass rod 20R for optical fiber preforms made of silica glass doped with alkali metal oxide or alkaline earth metal oxide is obtained.

[0047] In this process, it is preferable to etch the inner surface of the effective portion 20E of the silica glass tube 20 before solidifying the effective portion 20E of the silica glass tube 20. In this case, for example, this is done by traversing the second heating section 34 while flowing a fluorine-based gas such as sulfur hexafluoride (SF6) gas or ethane hexafluoride (C2F6) gas through the through hole 20H. The above alkali compound may contain impurities such as transition metals, but while alkali oxides diffuse deep into the interior of the silica glass tube 20, these impurities tend to remain on the inner surface of the silica glass tube 20 because they do not diffuse as easily within the silica glass tube 20 as alkali oxides. Therefore, these impurities can be removed by etching the inner surface of the silica glass tube 20.

[0048] <Base material formation process P6> This process involves surrounding the outer surface of the glass rod 20R for the optical fiber preform with a silica glass layer that will serve as cladding for the optical fiber 1. Figure 9 is a cross-sectional view showing the optical fiber preform obtained in this process. Although not specifically illustrated, for example, a silica glass soot can be attached to the outer surface of the glass rod 20R for the optical fiber preform by the soot method, and then sintered in a fluorine compound-containing atmosphere to form a silica glass layer 12P with added fluorine on the outer surface of the glass rod 20R for the optical fiber preform. Alternatively, the glass rod 20R for the optical fiber preform may be manufactured by inserting it into a through-hole of a silica glass tube containing fluorine and then integrating them. Furthermore, the soot method and the integration method may be appropriately combined to produce the product. In this way, an optical fiber preform 1P is obtained in which the outer surface of the glass rod 20R for the optical fiber preform is surrounded by a silica glass layer 12P containing fluorine.

[0049] <Drawing process P7> This process involves drawing a fiber optic fiber from the fiber optic fiber preform 1P, which has a glass rod 20R for the fiber optic fiber preform. Figure 10 shows this process. In this process, the fiber optic fiber preform 1P is placed in a spinning furnace 110, and the heating section 111 of the spinning furnace 110 heats the fiber optic fiber preform 1P. This heating causes the lower end of the fiber optic fiber preform 1P to melt, and glass is drawn from the fiber optic fiber preform 1P. As soon as the drawn molten glass leaves the spinning furnace 110, it solidifies, and the glass rod 20R for the fiber optic fiber preform becomes the core 11, and the silica glass layer 12P surrounding the glass rod 20R becomes the cladding 12. In this way, a bare fiber optic fiber is obtained. After that, this bare fiber optic fiber passes through a cooling device 120 and is cooled to an appropriate temperature. The cooled bare fiber optic fiber passes through a coating device 130, where an inner protective layer 13 and an outer protective layer 14 are formed, resulting in the fiber optic fiber 1 shown in Figure 1. Then, the optical fiber 1 is reoriented by the turn pulley 141 and wound up by the reel 142. In this way, the optical fiber 1 is manufactured.

[0050] As described above, the manufacturing apparatus 30 for optical fiber preform glass rods of this embodiment includes a first lathe gripping section 31 capable of gripping the silica glass tube 20 at a predetermined distance from one end of the silica glass tube 20, a second lathe gripping section 32 capable of gripping the silica glass tube 20 on the other side of the first lathe gripping section 31, and a first heating section 33 provided on the other side of the first lathe gripping section 31, capable of heating the silica glass tube 20 so that the alkali compound 40 placed in the through hole 20H of the silica glass tube 20 is above its melting point. The device includes a carrier gas introduction section 35 that can introduce a carrier gas CG into the through-hole 20H of the silica glass tube 20 from one side, so that vaporized alkali compounds are circulated and the alkali compounds, which are atomized during the circulation process, circulate within the through-hole 20H of the silica glass tube 20 between the first lathe gripping section 31 and the second lathe gripping section 32; and a second heating section 34 that can heat the silica glass tube 20 so that the alkali compound particles move along the longitudinal direction of the silica glass tube 20 and the atomized alkali compound particles reach an oxidation reaction temperature or higher.

[0051] Furthermore, the manufacturing method for the optical fiber preform glass rod of this embodiment includes a gripping step P1 in which the silica glass tube 20 is gripped by a first lathe gripping part 31 at a predetermined distance from one end of the silica glass tube 20, and the silica glass tube 20 is gripped by a second lathe gripping part 32 on the other side of the first lathe gripping part 31, and a first heating step P2 in which the silica glass tube 20 is heated on the one side of the first lathe gripping part 31 so that the alkali compound 40 placed in the through hole 20H of the silica glass tube 20 is above its melting point, and the silica glass tube 2 The device comprises a flow step P3 in which a carrier gas CG is introduced into the through-hole 20H of the silica glass tube 20 from one side, the alkali compound vaporized in the first heating step P2 is flowed through it, and the alkali compound, which is atomized during the flow process, is flowed through the through-hole 20H of the silica glass tube 20 between the first lathe gripping part 31 and the second lathe gripping part 32, and a second heating step P4 in which the silica glass tube 20 is heated so that the alkali compound particles flowing through the through-hole 20H while moving along the longitudinal direction of the silica glass tube 20 are heated to a temperature above the oxidation reaction temperature.

[0052] The above-described manufacturing apparatus 30 for optical fiber preform glass rods allows for the implementation of the above-described manufacturing method for optical fiber preform glass rods. Furthermore, the manufacturing apparatus 30 and manufacturing method for optical fiber preform glass rods allow for suppression of bending of the silica glass tube 20, even when the length of the silica glass tube 20 that becomes the optical fiber preform glass rod 20R is increased, compared to the manufacturing apparatus and manufacturing method for optical fiber preform glass rods of glass members where the alkali compound 40 is placed between the first lathe gripping part 31 and the second lathe gripping part 32 and heated. In addition, in the manufacturing apparatus for optical fiber preform glass rod 20R of this embodiment, the alkali compound circulating in the carrier gas CG can be cooled between the position where the first heating part 33 heats the silica glass tube 20 and the first lathe gripping part 31, at the part where the first lathe gripping part 31 grips the silica glass tube 20, and between the position closest to the first lathe gripping part 31 where the second heating part 34 heats the silica glass tube 20 and the first lathe gripping part 31. Therefore, compared to the manufacturing apparatus and method for glass rods for optical fiber preforms where the alkali compound 40 is placed between the first lathe gripping part 31 and the second lathe gripping part 32 and heated, the alkali compound can be cooled more appropriately. Accordingly, the manufacturing apparatus 30 and method for glass rods for optical fiber preforms of this embodiment can manufacture glass rods 20R for optical fiber preforms that can lengthen the optical fiber preform 1P.

[0053] The present invention has been described above with reference to the above embodiments, but the present invention is not limited thereto. For example, in the above embodiments, the alkali compound was heated to a temperature above the temperature at which it reacts with oxygen, and the alkali metal oxide or alkaline earth metal oxide was doped into the silica glass tube 20. However, the present invention is not limited to this, and an alkali compound such as an alkali metal compound or alkaline earth metal compound may be doped into the silica glass tube 20. In this case, in the second heating step P4, the alkali compound is heated to a temperature above the melting point of the alkali compound by the second heating unit 34, and the molten alkali compound is moved while in contact with the silica glass tube 20, thereby doping the alkali compound into the silica glass tube 20. In this case, the carrier gas CG does not have to contain oxygen, and the carrier gas CG in this case may be an inert gas such as argon, helium, or nitrogen. Alternatively, the carrier gas CG may be a mixed gas of silicon tetrachloride and oxygen. Therefore, in this case, the carrier gas introduction section 35 is configured to introduce the carrier gas into the through-hole 20H of the silica glass tube 20 so that the vaporized alkali compound flows through the through-hole 20H and the alkali compound, which is atomized during the flow process, accumulates on the inner wall of the silica glass tube 20 between the first lathe gripping section 31 and the second lathe gripping section 32.

[0054] Furthermore, although the above embodiment was described using an example where the cladding 12 is doped with fluorine, the cladding 12 does not necessarily have to be doped with fluorine. If the core 11 is doped with a dopant that increases the refractive index, such as germanium, the cladding 12 may be pure silica glass that is not doped with any dopant. In addition, to lower the refractive index of the cladding 12, the cladding 12 may be doped with boron or the like.

[0055] Furthermore, in the above embodiment, a reservoir portion 22 was formed by a pair of diameter-reduced portions 21a and 21b, but the reservoir portion 22 only needs to contain an alkali compound 40 and be heated by the first heating portion 33, and does not need to have the same configuration as in the above embodiment.

[0056] As described above, the present invention provides a manufacturing apparatus for glass rods for optical fiber preforms, a method for manufacturing glass rods for optical fiber preforms, and a method for manufacturing optical fibers, which can be used in optical fiber communications and other fields that use optical fibers.

Claims

1. A first lathe gripping part capable of gripping a silica glass tube at a predetermined distance from one end, A second lathe gripping portion capable of gripping the silica glass tube is located on the other side of the silica glass tube from the first lathe gripping portion, A first heating section is provided on one side of the first lathe gripping section and is capable of heating the silica glass tube to a temperature above the melting point of an alkali metal compound or alkaline earth metal compound placed in the through-hole of the silica glass tube. A carrier gas introduction unit is provided that allows a carrier gas to be introduced into the through-hole of the silica glass tube from one side, so that the vaporized alkali metal compound or alkaline earth metal compound is circulated and the alkali metal compound or alkaline earth metal compound, which is atomized during the circulation process, flows through the through-hole of the silica glass tube between the first lathe gripping unit and the second lathe gripping unit. A second heating unit is provided that can heat the silica glass tube while moving along the longitudinal direction of the silica glass tube so that the fine particles of the alkali metal compound or the alkaline earth metal compound reach an oxidation reaction temperature above the temperature of the silica glass tube. Equipped with A manufacturing apparatus for glass rods for optical fiber preforms, characterized by the following features.

2. The distance from the position where the first heating unit heats the silica glass tube to the first lathe gripping unit is greater than the distance from the position closest to the first lathe gripping unit where the second heating unit heats the silica glass tube to the first lathe gripping unit. The apparatus for manufacturing glass rods for optical fiber preforms according to feature 1.

3. The first lathe gripping portion is made of metal. A manufacturing apparatus for glass rods for optical fiber preforms according to claim 1 or 2.

4. A gripping step in which the silica glass tube is gripped by a first lathe gripping part at a predetermined distance from one end of the silica glass tube, and the silica glass tube is gripped by a second lathe gripping part on the other side of the silica glass tube from the first lathe gripping part, A first heating step involves heating the silica glass tube on one side of the first lathe gripping portion so that the alkali metal compound or alkaline earth metal compound placed in the through-hole of the silica glass tube reaches a temperature above its melting point, A flow step is to introduce a carrier gas into the through-hole of the silica glass tube from one side, to flow the alkali metal compound or alkaline earth metal compound vaporized in the first heating step, and to flow the alkali metal compound or alkaline earth metal compound, which is atomized during the flow process, into the through-hole of the silica glass tube between the first lathe gripping part and the second lathe gripping part. A second heating step involves heating the silica glass tube while moving along its longitudinal direction so that the fine particles of the alkali metal compound or alkaline earth metal compound reach an oxidation reaction temperature above the temperature of the silica glass tube. Equipped with A method for manufacturing glass rods for optical fiber preforms, characterized by the following:

5. In the first heating step, the distance from the position where the silica glass tube is heated to the first lathe gripping portion is greater than the distance from the position closest to the first lathe gripping portion where the silica glass tube is heated in the second heating step to the first lathe gripping portion. A method for manufacturing a glass rod for optical fiber preforms according to feature 4.

6. The invention comprises a drawing step of drawing a fiber optic matrix having a glass rod for optical fiber matrix manufactured by the method for manufacturing a glass rod for optical fiber matrix according to claim 4 or 5. A method for manufacturing optical fibers, characterized by the following:

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