Reservoir, method for producing optical fiber, and device for producing optical fiber

Abstract

This reservoir is for use in producing an optical-fiber base material, and is for placing therein a feed material including an alkali metal element or alkaline-earth metal element when the alkali metal element or alkaline-earth metal element is added to a glass pipe made of a silica-based glass by means of thermal diffusion. The reservoir is made of a silica-based glass having a total concentration of fluorine and chlorine of 1,500-20,000 ppm.

Description

Reservoir, optical fiber manufacturing method, and optical fiber manufacturing apparatus

[0001] This application claims priority to Japanese Patent Application No. 2023-092938, filed on June 6, 2023, and incorporates by reference all of the contents of that application.

[0002] When a core made of silica-based glass contains an alkali metal element or an alkaline earth metal element, the viscosity is reduced, which makes it easier for the glass network structure to be aligned even when the cooling rate is fast during drawing. In other words, the number of rings in the ring structure in the glass network structure is more likely to be uniform. This reduces Rayleigh scattering, which accounts for the majority of the transmission loss in optical fibers. As a result, the transmission loss of the optical fiber can be reduced.

[0003] Patent Documents 1 and 2 describe methods for adding alkali metal elements and alkaline earth metal elements to a glass pipe by thermal diffusion. In the thermal diffusion method, raw materials are heated and evaporated in a raw material installation section, and the raw material vapor is introduced into the glass pipe by a carrier gas while the glass pipe is heated with a burner. In this way, the alkali metal elements and alkaline earth metal elements are diffused and added to the glass pipe. This raw material installation section is called a reservoir. To prevent the raw materials from being scattered into the glass pipe by the carrier gas, the reservoir is usually formed in a saucer shape.

[0004] Furthermore, Patent Document 2 describes that in order to prevent crystallization of alkali metals, it is preferable that the glass pipe and the glass attached to the inside of the glass pipe essentially contain no chlorine.

[0005] U.S. Patent Application Publication No. 2006 / 0130530 U.S. Patent Application Publication No. 2022 / 0283363

[0006] A reservoir according to one aspect of the present disclosure is used in the manufacture of an optical fiber preform, and is a reservoir in which a raw material containing an alkali metal element or an alkaline earth metal element is placed when an alkali metal element or an alkaline earth metal element is added by thermal diffusion to a glass pipe made of silica-based glass, and is made of silica-based glass having a sum of fluorine concentration and chlorine concentration of 1,500 ppm or more and 20,000 ppm or less.

[0007] Fig. 1 is a flowchart illustrating a method for manufacturing an optical fiber according to an embodiment. Fig. 2 is a diagram illustrating an addition process using an optical fiber manufacturing apparatus according to an embodiment. Fig. 3 is a diagram illustrating an addition process using an optical fiber manufacturing apparatus according to a modified example. Fig. 4 is a graph showing a change in fluorine concentration distribution accompanying heating processing during reservoir formation. Fig. 5 is a graph showing the relationship between halogen concentration and the defective rate of reservoirs. Fig. 6 is a graph showing the relationship between chlorine concentration and the defective rate of reservoirs.

[0008] [Problem to be Solved by the Present Disclosure] When a reservoir is made of glass with a high chlorine concentration, chlorine reacts with alkali metal elements and alkaline earth metal elements in the reservoir and crystallizes. If these crystals scatter into the glass pipe as fine powder, the fine powder can act as nuclei to cause silica glass crystallization in the glass pipe. This can result in a defective core material for the optical fiber preform, which can reduce the optical fiber yield. On the other hand, if the chlorine concentration is too low, it can be difficult to control the shape of the reservoir, and the reservoir may crack during reservoir manufacturing or during diffusion doping using the reservoir.

[0009] An object of the present disclosure is to provide a reservoir, an optical fiber manufacturing method, and an optical fiber manufacturing apparatus that can improve the yield of optical fiber and the processability of the reservoir, and can reduce cracks in the reservoir.

[0010] Effect of the Present Disclosure According to the present disclosure, a reservoir, an optical fiber manufacturing method, and an optical fiber manufacturing apparatus are provided that can improve optical fiber yield and reservoir processability, and reduce reservoir cracking.

[0011] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be listed and described. (1) A reservoir according to one aspect of the present disclosure is used in the manufacture of an optical fiber preform, and is a reservoir into which a raw material containing an alkali metal element or an alkaline earth metal element is placed when adding an alkali metal element or an alkaline earth metal element to a glass pipe made of silica-based glass by thermal diffusion. The reservoir is made of silica-based glass having a sum of fluorine concentration and chlorine concentration of 1500 ppm to 20000 ppm. This reservoir can improve the yield of optical fiber and the processability of the reservoir. It can also reduce cracking of the reservoir.

[0012] (2) In the above (1), the reservoir may be made of silica-based glass having a total fluorine concentration and chlorine concentration of 3,500 ppm or more and 18,000 ppm or less. In this case, the yield of the optical fiber and the processability of the reservoir can be further improved. Also, the reservoir can be further reduced in cracking.

[0013] (3) In the above (1), the reservoir may be made of silica-based glass having a total fluorine concentration and chlorine concentration of 3,500 ppm or more and 13,000 ppm or less. In this case, the yield of the optical fiber and the processability of the reservoir can be further improved. Also, the reservoir can be further reduced in cracking.

[0014] (4) In any of the above (1) to (3), the reservoir may be made of silica-based glass having a chlorine concentration of 20 ppm or more. In this case, the reservoir can be further reduced from cracking.

[0015] (5) In any of the above (1) to (3), the reservoir may be made of silica-based glass having a chlorine concentration of 50 ppm or more. In this case, the reservoir can be further reduced from cracking.

[0016] (6) In either of (1) or (5) above, the reservoir may be a separate member from the glass pipe and have a connection end that can be fused to the glass pipe. In this case, the reservoir can be removed from the glass pipe and fused to another glass pipe for use.

[0017] (7) A method for manufacturing an optical fiber according to an aspect of the present disclosure may include a step of adding an alkali metal element or an alkaline earth metal element to the inner surface of a glass pipe made of silica-based glass using the reservoir described in any one of (1) to (6). In this case, the use of the reservoir described above can improve the yield of the optical fiber and the processability of the reservoir. Also, it can reduce cracks in the reservoir.

[0018] (8) An optical fiber manufacturing apparatus according to one aspect of the present disclosure may include a reservoir according to any one of (1) to (6) above, the reservoir being connected to a glass pipe, and a heat source for heating the reservoir, and may add an alkali metal element or an alkaline earth metal element to the inner surface of the glass pipe. In this case, since the optical fiber manufacturing apparatus includes the reservoir, it is possible to improve the yield of the optical fiber and the processability of the reservoir. It is also possible to reduce cracks in the reservoir.

[0019] [Details of the embodiment of the present disclosure] Specific examples of the reservoir and the method for manufacturing the optical fiber according to the present embodiment will be described with reference to the drawings as necessary. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, the same elements in the description of the drawings will be given the same reference numerals, and duplicate explanations will be omitted.

[0020] 1 is a flowchart showing a method for manufacturing an optical fiber according to an embodiment. The method for manufacturing an optical fiber includes a preparation step S1, an addition step S2, a diameter reduction step S3, an etching step S4, a collapse step S5, a draw grinding step S6, a rod-in collapse step S7, an OVD (Outside Vapor Deposition) step S8, and a drawing step S9. The optical fiber is manufactured through these steps S1 to S9 in this order. The method for manufacturing the optical fiber is carried out using an optical fiber manufacturing apparatus 10 (see FIG. 2).

[0021] The preparation step S1 is a step of preparing a glass pipe into which an alkali metal group is to be diffused as a dopant. Here, alkali metal group is a general term for alkali metal elements and alkaline earth metal elements. The glass pipe is made of silica (quartz)-based glass. Silica-based glass is primarily composed of silica and contains 90% or more silica. Silica-based glass may also contain 95% or more silica. The silica-based glass rod from which the glass pipe is made is manufactured, for example, by the vapor phase axial deposition (VAD) method. The pipe is manufactured by drilling holes in the cylindrical body and then elongating it. The silica-based glass rod from which the glass pipe is made contains a certain concentration of chlorine and fluorine. The mass fraction of other dopants and impurities is 10 ppm or less. Herein, mass fraction is the ratio of the mass of the element of interest to the total mass and is expressed as (mass of the element of interest) / (total mass). Hereinafter, mass fraction is referred to as "concentration."

[0022] The doping step S2 is a step of doping an alkali metal group by a diffusion method onto the inner surface of the glass pipe 2 (see FIG. 2) made of silica-based glass. When potassium (K) is doped as a dopant of the alkali metal group, potassium bromide (KBr) is used as the source material 5 (see FIG. 2) containing the alkali metal group. Depending on the type of alkali metal group to be doped, one or more of KBr, potassium iodide (KI), rubidium bromide (RbBr), rubidium iodide (RbI), etc. may be used as the source material 5.

[0023] The diameter-reducing step S3 is a step of reducing the diameter of the glass pipe to which the alkali metal group has been added. The etching step S4 is a step of etching the inner surface of the glass pipe. The etching step S4 allows the inner surface of the glass pipe, which contains a high concentration of impurities added together with the alkali metal group, to be scraped and the impurities to be removed. The collapse step S5 is a step of collapsing the glass pipe to form a glass rod.

[0024] The draw grinding process S6 is a process of drawing the glass rod and grinding the outer periphery of the glass rod to form a core rod that will become a core portion. The rod in-collapse process S7 is a process of providing a first cladding portion outside the core portion. The OVD process S8 is a process of drawing a rod formed by integrating a core portion and a first cladding portion to a predetermined diameter, and then synthesizing a second cladding portion containing fluorine on the outside of the rod by the OVD method. This produces an optical fiber preform. The drawing process S9 is a process of drawing the optical fiber preform. This produces an optical fiber.

[0025] FIG. 2 is a diagram illustrating a doping process using an optical fiber manufacturing apparatus according to an embodiment. The optical fiber manufacturing apparatus 10 according to an embodiment includes a reservoir 1, a heat source 3, and a heat source 4. The optical fiber manufacturing apparatus 10 is used in the manufacturing of an optical fiber, specifically in the doping process S2. That is, the reservoir 1 according to an embodiment is used in the manufacturing of an optical fiber, specifically in the doping process S2. The reservoir 1 is made of silica-based glass and contains fluorine and chlorine. The silica-based glass is primarily composed of silica and contains 90% or more silica. The silica-based glass may contain 95% or more silica. A silica-based glass rod, which is the base of the glass pipe of the reservoir 1, is manufactured, for example, by the vapor phase axial deposition (VAD) method. The reservoir 1 is manufactured by drilling holes in the cylindrical body and then elongating it. The sum of the fluorine concentration and chlorine concentration in the reservoir 1 is 1500 ppm or more and 20000 ppm or less. This improves the yield of the optical fiber and the processability of the reservoir 1. Furthermore, cracking of the reservoir 1 can be reduced. The sum of the fluorine concentration and the chlorine concentration in the reservoir 1 may be 3500 ppm or more and 18000 ppm or less, or 3500 ppm or more and 13000 ppm or less. The chlorine concentration in the reservoir 1 is 20 ppm or more. This further reduces cracking of the reservoir 1. The chlorine concentration in the reservoir 1 may be 50 ppm or more.

[0026] As described above, when the reservoir 1 is made of glass with a high chlorine concentration, chlorine reacts with alkali metal elements and alkaline earth metal elements in the reservoir 1 to crystallize. If these crystals are dispersed as fine powder into the glass pipe 2, the fine powder acts as nuclei to cause silica glass crystallization in the glass pipe 2. This can result in a defective core material for the optical fiber preform, which can reduce the yield of optical fibers. The sum of the fluorine and chlorine concentrations in the reservoir 1 is 20,000 ppm or less, and may be 18,000 ppm or less, or even 13,000 ppm or less, thereby improving the yield of optical fibers.

[0027] Generally, glass materials with low chlorine concentrations are manufactured by insufficient removal of impurities using chlorine gas or by replacing chlorine with fluorine-based gas. Optical fibers manufactured using reservoirs made of the former glass material suffer from increased transmission loss due to absorption loss caused by impurities. The latter glass material contains a large amount of fluorine, and heating during reservoir shaping causes fluorine to desorb from the surface. If too much fluorine is desorbed, viscosity increases only on the surface, generating tensile stress. This causes cracks to form on the surface, making the reservoir more susceptible to breakage during reservoir manufacturing or diffusion doping. If the reservoir cracks during diffusion doping, the yield of core material for optical fiber preforms may decrease, which in turn may result in a decrease in the yield of optical fiber.

[0028] The sum of the fluorine concentration and chlorine concentration in the reservoir 1 is 1500 ppm or more, and may be 3500 ppm or more, thereby reducing the deterioration of the transmission loss of the optical fiber due to insufficient removal of impurities by chlorine gas. Furthermore, the occurrence of cracks on the surface due to the desorption of halogen elements is reduced. Therefore, breakage of the reservoir 1 is reduced. As a result, the deterioration of the yield of the optical fiber is reduced.

[0029] The reservoir 1 is a glass pipe connected to a glass pipe 2 to which an alkali metal group is added. The reservoir 1 has a large-diameter portion 11, a small-diameter portion 12, a first connecting portion 13, and a second connecting portion 14. The reservoir 1 functions as a storage portion for a raw material 5 containing an alkali metal group when the alkali metal group is added to the glass pipe 2 by thermal diffusion. In the reservoir 1, the raw material 5 is mainly stored in the large-diameter portion 11. The outer diameter of the large-diameter portion 11 is equal to the outer diameter of the glass pipe 2. The small-diameter portion 12 is located between the first connecting portion 13 and the second connecting portion 14. The outer diameter of the small-diameter portion 12 is smaller than the outer diameter of the glass pipe 2 and also smaller than the outer diameter of the large-diameter portion 11.

[0030] The first connecting portion 13 connects the large diameter portion 11 and the first end of the small diameter portion 12. As described above, the outer diameter of the small diameter portion 12 is smaller than the outer diameter of the large diameter portion 11, so the first connecting portion 13 forms a step. The height of the step caused by the first connecting portion 13 is, for example, about 1 mm. The first connecting portion 13 has a tapered shape. The outer diameter of the first connecting portion 13 gradually decreases from the large diameter portion 11 toward the small diameter portion 12.

[0031] The second connecting portion 14 connects the second end of the thin-diameter portion 12 to the glass pipe 2. As described above, the outer diameter of the thin-diameter portion 12 is smaller than the outer diameter of the glass pipe 2, so the second connecting portion 14 forms a step. The height of the step formed by the second connecting portion 14 is equal to the height of the step formed by the first connecting portion 13. The second connecting portion 14 has a tapered shape. The outer diameter of the second connecting portion 14 gradually decreases from the glass pipe 2 toward the thin-diameter portion 12.

[0032] In this embodiment, the reservoir 1 and the glass pipe 2 are made of a single member and have the same composition before the addition step S2. The reservoir 1 and the glass pipe 2 are formed, for example, from a single glass pipe. The portion of the glass pipe that will become the narrow-diameter portion 12 is heated to narrow the diameter. This results in the reservoir 1 and the glass pipe 2 being formed in a connected state.

[0033] A heat source 3 is disposed outside the reservoir 1. The heat source 3 is an external heat source for heating the reservoir 1. More specifically, the heat source 3 is an external heat source for heating the raw material 5 placed in the reservoir 1. A heat source 4 is disposed outside the glass pipe 2. The heat source 4 is an external heat source for heating the glass pipe 2. The heat sources 3 and 4 are, for example, oxyhydrogen burners. The heat sources 3 and 4 may be induction furnaces, resistance furnaces, or the like.

[0034] In the addition step S2, the raw material 5 is heated by the heat source 3 to generate raw material vapor. The heating temperature is, for example, 600°C or higher and 1000°C or lower. The generated raw material vapor is introduced into the glass pipe 2 together with a carrier gas, while the glass pipe 2 is heated from the outside by the heat source 4. The carrier gas contains, for example, oxygen. The flow rate of the carrier gas is 1 SLM or higher (volume of gas flowing per minute under standard conditions (25°C, 100 kPa)) and 3 SLM or lower. The reservoir 1 includes a first connecting portion 13 with a step on the larger-diameter portion 11 closer to the glass pipe 2, which reduces the scattering of the raw material 5 into the glass pipe 2 by the carrier gas.

[0035] In the doping step S2, the glass pipe 2 is heated by moving the heat source 4 along the longitudinal direction of the glass pipe 2. The glass pipe 2 is heated by traversing the heat source 4 at a speed of 30 mm / min to 60 mm / min for a total of 8 turns to 15 turns so that the temperature of the outer surface of the glass pipe 2 reaches 1400° C. to 2000° C. In this way, the alkali metal group is diffused and doped into the inner surface of the glass pipe 2.

[0036] FIG. 3 is a diagram illustrating an addition process using an optical fiber manufacturing apparatus according to a modified example. The optical fiber manufacturing apparatus 10A according to the modified example differs from the optical fiber manufacturing apparatus 10 in that it includes a reservoir 1A instead of the reservoir 1 (see FIG. 2). The reservoir 1A according to the modified example has a different shape from the reservoir 1. The reservoir 1A has the same composition as the reservoir 1. That is, the reservoir 1A is made of silica-based glass and contains fluorine and chlorine. The sum of the fluorine concentration and the chlorine concentration in the reservoir 1A is 1500 ppm or more and 20000 ppm or less. The sum of the fluorine concentration and the chlorine concentration in the reservoir 1A may be 3500 ppm or more and 18000 ppm or less, or may be 3500 ppm or more and 13000 ppm or less. The chlorine concentration in the reservoir 1A is 20 ppm or more. The chlorine concentration in the reservoir 1A may be 50 ppm or more.

[0037] The reservoir 1A has a large-diameter portion 21, a first small-diameter portion 22, a second small-diameter portion 23, a first connecting portion 24, and a second connecting portion 25. In the reservoir 1A, the raw material 5 is placed mainly in the large-diameter portion 21. The outer diameter of the large-diameter portion 21 is larger than that of the glass pipe 2. The outer diameters of the first small-diameter portion 22 and the second small-diameter portion 23 are equal to that of the glass pipe 2 and smaller than that of the large-diameter portion 21. The first small-diameter portion 22 is connected to the glass pipe 2.

[0038] The first connecting portion 24 connects the large diameter portion 21 and the first small diameter portion 22. As described above, the outer diameter of the first small diameter portion 22 is smaller than the outer diameter of the large diameter portion 21, so the first connecting portion 24 forms a step. The height of the step caused by the first connecting portion 24 is, for example, about 1 mm. The first connecting portion 24 has a tapered shape. The outer diameter of the first connecting portion 24 gradually decreases from the large diameter portion 21 toward the first small diameter portion 22.

[0039] The second connecting portion 25 connects the large diameter portion 21 and the second small diameter portion 23. As described above, the outer diameter of the second small diameter portion 23 is smaller than the outer diameter of the large diameter portion 21, so the second connecting portion 25 forms a step. The height of the step formed by the second connecting portion 25 is, for example, about 1 mm. The second connecting portion 25 has a tapered shape. The outer diameter of the second connecting portion 25 gradually decreases from the large diameter portion 21 toward the second small diameter portion 23.

[0040] The reservoir 1A is formed, for example, from a single glass pipe. The portion of the glass pipe that will become the large-diameter portion 21 is heated while increasing the gas pressure inside the glass pipe, thereby expanding and enlarging the diameter. This process results in the reservoir 1A. The reservoir 1A and the glass pipe 2 are separate components. The reservoir 1A is integrated with the glass pipe 2 by fusion-connecting the first small-diameter portion 22 to the glass pipe 2. The first small-diameter portion 22 has a connection end 22a that is fusion-connected to the glass pipe 2.

[0041] The reservoir 1A and the glass pipe 2 do not necessarily have the same composition, but may have different compositions, even before the adding step S2. The reservoir 1A has a stepped first connecting portion 24 on the larger-diameter portion 21 closer to the glass pipe 2, which reduces the scattering of the raw material 5 into the glass pipe 2 by the carrier gas. After the adding step S2, the reservoir 1A can be detached from the glass pipe 2 and re-fused to another glass pipe 2 for use.

[0042] The following describes the evaluation results of the change in the concentration distribution of the added elements due to the heating process during reservoir formation. Figure 4 is a graph showing the change in the fluorine concentration distribution due to the heating process during reservoir formation. Here, reservoir 1A was used from the perspective of ease of production. The heating temperature during processing was set to 1000°C or higher and 2000°C or lower. By setting the temperature within this range, the viscosity of the glass decreases, improving the processability of the glass.

[0043] The vertical axis in Fig. 4 represents the fluorine concentration. The horizontal axis represents the difference between the radial position from the central axis of the reservoir and the inner radius of the reservoir, divided by the thickness (wall thickness) of the reservoir. The position of 0 on the horizontal axis corresponds to the inner surface of the reservoir, and the position of 1 on the horizontal axis corresponds to the outer surface of the reservoir. As shown in Fig. 4, the fluorine concentration before heating was approximately 20,000 ppm at the inner surface of the reservoir, gradually decreasing toward the outer surface of the reservoir, and reaching approximately 17,500 ppm at the outer surface of the reservoir.

[0044] In contrast, the fluorine concentration after heating is zero on the inner and outer surfaces of the reservoir. A sudden change in concentration occurs near the inner and outer surfaces of the reservoir. As such, it can be confirmed that the fluorine concentration on the inner and outer surfaces decreases when the reservoir is heated to a high temperature by heating. Although not shown in the figure, the change in the chlorine concentration distribution before and after heating was similar to the change in the fluorine concentration distribution.

[0045] Table 1 shows the concentrations of the additive elements in the glass materials used in the above evaluation and the defective rate of the reservoirs. The additive elements are chlorine (Cl), fluorine (F), and OH (hydroxyl group), a molecule that is easily doped with impurities. In Table 1, the sum of the chlorine and fluorine concentrations is shown as the halogen concentration. The defective rate of the reservoirs (production defective rate) represents the probability that a reservoir that cannot be used for doping with alkali metals is formed when reservoirs are manufactured 20 times by heating processing for each concentration condition.

[0046]

[0047] Figure 5 is a graph showing the relationship between halogen concentration and the defective rate of reservoirs. The vertical axis of Figure 5 represents the defective rate of reservoirs, and the horizontal axis represents halogen concentration, i.e., the sum of fluorine concentration and chlorine concentration. In regions with high halogen concentration, the number of defective reservoirs increased. This is presumably due to the occurrence of cracks caused by a sudden concentration difference near the reservoir surface, as shown in Figure 4. In regions with low halogen concentration, reservoir defects occurred not due to cracks but due to poor shape control. This is presumably due to the significant decrease in the added concentration, which increased the viscosity of the glass as a whole and worsened processability.

[0048] From the results of FIG. 5, it can be seen that the halogen concentration in the reservoir may be 1500 ppm or more and 20000 ppm or less, 3500 ppm or more and 18000 ppm or less, or 3500 ppm or more and 13000 ppm or less.

[0049] FIG. 6 is a graph showing the relationship between chlorine concentration and the reservoir defect rate. The vertical axis of FIG. 6 represents the reservoir defect rate, and the horizontal axis represents the chlorine concentration. FIG. 6 shows the results for conditions 2 to 9, excluding conditions 1 and 10, which had high defect rates. The results of FIG. 6 confirm that the defect rate increases even when the chlorine concentration is low, so the chlorine concentration may be 20 ppm or higher, or even 50 ppm or higher. Under conditions where the halogen concentration is within a certain range, a decrease in chlorine results in a corresponding increase in the proportion of fluorine. Since fluorine has a greater viscosity fluctuation effect and thermal expansion coefficient change than chlorine, it is presumed that an increase in the proportion of fluorine further promotes the viscosity fluctuation effect and thermal expansion coefficient change due to the detachment of halogen elements from the surface, increasing tensile stress and resulting in an increase in defects due to cracks.

[0050] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the spirit of the present disclosure.

[0051] The reservoir 1A and the glass pipe 2 may be formed as separate members and fused together, or may be formed as a single member.

[0052] REFERENCE SIGNS LIST 1, 1A... Reservoir 2... Glass pipe 3, 4... Heat source 5... Raw material 10, 10A... Manufacturing device 11... Large diameter portion 12... Small diameter portion 13... First connecting portion 14... Second connecting portion 21... Large diameter portion 22... First small diameter portion 22a... Connecting end 23... Second small diameter portion 24... First connecting portion 25... Second connecting portion

Claims

1. A reservoir used in the manufacture of an optical fiber preform, in which a raw material containing an alkali metal element or an alkaline earth metal element is placed when an alkali metal element or an alkaline earth metal element is added to a glass pipe made of silica-based glass by thermal diffusion, comprising: The glass is made of silica-based glass having a total fluorine concentration and chlorine concentration of 1500 ppm or more and 20000 ppm or less. Reservoir.

2. The glass is made of silica-based glass having a total fluorine concentration and chlorine concentration of 3500 ppm or more and 18000 ppm or less. The reservoir of claim 1 .

3. The glass is made of silica-based glass having a total fluorine concentration and chlorine concentration of 3500 ppm or more and 13000 ppm or less. The reservoir of claim 1 .

4. It is made of silica-based glass having a chlorine concentration of 20 ppm or more. A reservoir according to any one of claims 1 to 3.

5. It is made of silica-based glass having a chlorine concentration of 50 ppm or more. A reservoir according to any one of claims 1 to 3.

6. a connecting end that is a separate member from the glass pipe and is fusion-connected to the glass pipe; A reservoir according to any one of claims 1 to 3.

7. 4. The method of claim 1, further comprising the step of adding an alkali metal element or an alkaline earth metal element to the inner surface of a glass pipe made of silica-based glass by using the reservoir of claim 1. Optical fiber manufacturing method.

8. The reservoir according to any one of claims 1 to 3, wherein the reservoir is connected to the glass pipe; a heat source for heating the reservoir; adding an alkali metal element or an alkaline earth metal element to the inner surface of the glass pipe; Optical fiber manufacturing equipment.