Manufacturing method for glass substrates
By forming multiple glass layers with varying particle sizes and compositions, the method addresses the slow dopant diffusion issue, enhancing efficiency and reducing time for achieving a uniform refractive index in glass substrates.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- FUJIKURA LTD
- Filing Date
- 2022-08-24
- Publication Date
- 2026-06-26
AI Technical Summary
The diffusion rate of dopants in the inner part of a porous glass matrix is slower than in the outer part, leading to increased time requirements for achieving a uniform refractive index distribution, especially for larger diameters.
A method involving the deposition of glass particles with different sizes and chemical compositions to form multiple layers, followed by simultaneous dopant addition and sintering, ensuring a constant refractive index across these layers.
This approach enhances dopant adsorption efficiency, reducing the time required for dopant diffusion and achieving a uniform refractive index distribution in the glass substrate.
Smart Images
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Abstract
Description
[Technical Field]
[0001] This invention relates to a method for manufacturing a glass substrate. [Background technology]
[0002] Patent Document 1 discloses a method for manufacturing an optical fiber preform that forms a porous glass preform having a uniform bulk density distribution in the radial direction. [Prior art documents] [Patent Documents]
[0003] [Patent Document 1] Japanese Patent Publication No. 2003-300747 [Overview of the Initiative] [Problems that the invention aims to solve]
[0004] When a dopant is added to a porous glass matrix having a uniform bulk density distribution in the radial direction, the diffusion rate of the dopant in the inner part of the porous glass matrix is slower than the diffusion rate of the dopant in the outer part of the porous glass matrix. Therefore, in order to obtain a uniform refractive index distribution in the radial direction in the porous glass matrix after dopant addition, it is necessary to wait for the dopant to diffuse sufficiently in the inner part, which increases the time required for dopant addition. This problem becomes more pronounced as the outer diameter of the porous glass matrix increases.
[0005] This invention has been made in consideration of these circumstances and aims to provide a method for manufacturing a glass substrate that can reduce the time required for dopant addition. [Means for solving the problem]
[0006] To solve the above problems, Embodiment 1 of the present invention is a method for manufacturing a glass base material, comprising: depositing glass particles to form a first porous glass layer; depositing glass particles having a larger particle size than the glass particles contained in the first porous glass layer and having the same chemical composition as the glass particles contained in the first porous glass layer on the outer surface of the first porous glass layer to form a second porous glass layer; simultaneously adding a dopant to the first porous glass layer and the second porous glass layer; and sintering the porous glass base material to which the dopant has been added, thereby making the refractive index of at least the portion spanning the first porous glass layer and the second porous glass layer substantially constant in the radial direction.
[0007] Furthermore, aspect 2 of the present invention is a method for manufacturing a glass base material according to aspect 1, wherein the structure of the first burner that forms the first porous glass layer is different from the structure of the second burner that forms the second porous glass layer.
[0008] Furthermore, aspect 3 of the present invention is a method for manufacturing a glass base material according to aspect 1 or aspect 2, wherein the outer diameter of the first burner that forms the first porous glass layer is smaller than the outer diameter of the second burner that forms the second porous glass layer.
[0009] Furthermore, aspect 4 of the present invention is a method for manufacturing a glass base material in any one of aspects 1 to 3, wherein the first porous glass layer is located at the center of the porous glass base material in the radial direction.
[0010] Furthermore, aspect 5 of the present invention is a method for manufacturing a glass base material in any one of aspects 1 to 4, wherein the outer diameter of the porous glass base material is 100 mm or more.
[0011] Furthermore, aspect 6 of the present invention is a method for manufacturing a glass base material in any one of aspects 1 to 5, wherein the refractive index of the first porous glass layer is made substantially constant in the radial direction. [Effects of the Invention]
[0012] According to the above aspect of the present invention, it is possible to provide a method for manufacturing a glass base material capable of reducing the time required for adding a dopant.
Brief Description of the Drawings
[0013] [Figure 1] It is a figure which shows an example of the manufacturing apparatus used for the manufacturing method of the porous glass base material which concerns on embodiment of this invention. [Figure 2] It is a figure which shows the optical fiber base material which concerns on embodiment of this invention, and its refractive index distribution. [Figure 3A] It is a figure which shows the 1st example of the multiple ports which a burner has. [Figure 3B] It is a figure which shows the 2nd example of the multiple ports which a burner has. [Figure 3C] It is a figure which shows the 3rd example of the multiple ports which a burner has.
Embodiments for Carrying Out the Invention
[0014] Hereinafter, a method for manufacturing a glass base material according to an embodiment of the present invention will be described based on the drawings. In the present embodiment, a case where a core of an optical fiber base material is formed by using the vapor phase axial deposition method (VAD method: Vapor phase Axial Deposition Method), and a clad is formed around the core by using the outside vapor deposition method (OVD method: Outside Vapor Deposition Method) to obtain an optical fiber base material will be described. Note that the "optical fiber base material" is an example of the "glass base material".
[0015] As shown in FIG. 1, a manufacturing apparatus for a porous glass base material 5 (hereinafter simply referred to as the manufacturing apparatus 10) includes a reaction chamber 7, a first burner 3, a second burner 4, and a support portion 2. At least a part of the starting member 1 is accommodated in the reaction chamber 7. The starting member 1 is a member that serves as a starting point for the deposition of the porous glass base material 5 manufactured by the manufacturing apparatus 10. The starting member 1 is formed of quartz glass or the like. The starting member 1 is, for example, in the shape of a round bar. At least one end portion of the starting member 1 is rotatably gripped about the rotation axis O by the support portion 2. The reaction chamber 7 is provided with two exhaust ports 6 for discharging unnecessary gases. The number and position of the exhaust ports 6 can be changed as appropriate.
[0016] The burners 3 and 4 are supplied with a glass raw material gas, a combustion-supporting gas containing oxygen, a combustion gas containing hydrogen, an inert gas, and the like. Each of the burners 3 and 4 has, for example, a multi-tube structure (details will be described later). In the vicinity of the ejection ports of the burners 3 and 4, a plurality of ports through which each gas is ejected alone or in a mixed state are provided. As the glass raw material gas, for example, SiCl4, GeCl4, or an organic silicon compound can be adopted. Specific examples of the organic silicon compound include alkylcyclosiloxane. By burning the combustion gas containing hydrogen, a flame (oxyhydrogen flame) is generated in the vicinity of the ejection ports of the burners 3 and 4. By reacting the glass raw material gas in this flame, glass particles are generated. By depositing these glass particles from the tip of the rotating starting member 1 in a direction parallel to the rotation axis O, a deposition layer (soot) of the glass particles is formed. Thereby, the porous glass base material 5 is obtained.
[0017] By subjecting the porous glass base material 5 to a doping treatment and a sintering treatment, a member that becomes the core of the optical fiber base material 20 as shown in FIG. 2 is obtained. In addition to the doping treatment and the sintering treatment, a dehydration treatment may be performed on the porous glass base material 5 as necessary.
[0018] When manufacturing an optical fiber, glass particles are deposited around the obtained core 5 to form a soot. By subjecting the formed soot to a sintering treatment or the like, a cladding 8 is formed around the core 5. This yields an optical fiber preform 20 having a core 5 and cladding 8, as shown in Figure 2. An optical fiber can be obtained by drawing the optical fiber preform 20. In the example in Figure 2, the cladding 8 has a two-layer structure including a first cladding 8a covering the core 5 and a second cladding 8b covering the first cladding 8a. However, the cladding 8 may have a single-layer structure or a structure with three or more layers.
[0019] As shown in Figure 2, the optical fiber matrix 20 manufactured in this embodiment has a step-type refractive index distribution. That is, the refractive index of the core 5, the refractive index n1 of the first cladding 8a, and the refractive index n2 of the second cladding 8b are all approximately constant in the radial direction. In this specification, "core 5," "average refractive index n0 of core 5," and "approximately constant refractive index" are specifically defined as follows. First, the optical fiber matrix 20 is divided into multiple regions (sampling regions) in the radial direction (for example, 2500 regions), and the refractive index of each sampling region is measured. Next, the change in refractive index with respect to the radial direction is calculated for each sampling region, and the sampling region located in the innermost part of the portion where the change in refractive index with respect to the radial direction is 0.000015 or more for 20 consecutive points is defined as the outermost periphery of core 5. Then, all sampling regions radially inward from the outermost periphery of core 5 are defined as "core 5." Furthermore, "average refractive index n0 of core 5" is defined as the average value of the refractive index measurements within the range of core 5 as defined in this way. Furthermore, "refractive index is approximately constant" means that within the range of core 5, the relative refractive index Δn is relative to the average refractive index n0. s This means that the variation is 0.01% or less. Note that for each sampling region, "the relative refractive index Δn relative to n0" is defined. s " is the measured refractive index in the sampling region n s It is defined as follows: Δn s [%] = 100 × (n s 2 -n02 ) / 2n s 2 Also, the relative refractive index Δn1 of n1 with respect to n0 is, for example, within the range of -0.36 to -0.32%. The relative refractive index Δn2 of n2 with respect to n0 is, for example, within the range of -0.26 to -0.22%. Note that the relative refractive indices Δn1 and Δn2 are defined as follows. Δn1 [%] = 100×(n1 2 - n0 2 ) / 2n1 2 Δn2 [%] = 100×(n2 2 - n0 2 ) / 2n2 2
[0020] (Direction definition) In the present embodiment, the direction parallel to the rotation axis O of the support portion 2, that is, the direction parallel to the central axis O of the porous glass base material 5, may be referred to as the "axial direction" or the "longitudinal direction". In the drawings, the vertical direction is represented by the Z-axis, the upper side in the vertical direction is the +Z side, and the lower side is the -Z side. In the present embodiment, the longitudinal direction coincides with the vertical direction. However, the longitudinal direction does not necessarily coincide with the vertical direction. When viewed from the longitudinal direction, the direction orthogonal to the central axis O of the porous glass base material 5 is referred to as the radial direction. Along the radial direction, the direction approaching the central axis O is referred to as the inner radial direction, and the direction departing from the central axis O is referred to as the outer radial direction. When viewed from the longitudinal direction, the direction circulating around the central axis O is referred to as the circumferential direction.
[0021] As shown in FIG. 1, in the present embodiment, the upper end portion of the starting member 1 is held by the support portion 2, and the lower end portion of the starting member 1 is located inside the reaction chamber 7. The support portion 2 has, for example, a chuck for holding the starting member 1 and a moving mechanism for moving the chuck in the vertical direction (longitudinal direction). The support portion 2 can move upward while rotating the starting member 1 around the rotation axis O.
[0022] The first burner 3 is positioned to deposit glass particles on the lower end of the starting member 1 to form a soot. In this specification, the soot formed by the first burner 3 is referred to as the first porous glass layer 5a. The first burner 3 is inclined with respect to both the vertical (longitudinal) and horizontal (radial) directions. However, the position and orientation of the first burner 3 may be changed as appropriate.
[0023] The second burner 4 is positioned to deposit glass particles onto the outer surface of the first porous glass layer 5a to form a soot. In this specification, the soot formed by the second burner 4 is referred to as the second porous glass layer 5b. The second burner 4 is positioned above the first burner 3. In other words, the burners 3 and 4 are spaced apart along the longitudinal direction of the porous glass base material 5. The second burner 4 extends along a direction substantially perpendicular to the longitudinal direction of the porous glass base material 5 (i.e., substantially radial direction). However, the position and orientation of the second burner 4 may be changed as appropriate. The manufacturing apparatus 10 may also include a plurality of second burners 4 spaced apart along the longitudinal direction of the porous glass base material 5.
[0024] A porous glass base material 5 is obtained by forming a second porous glass layer 5b so as to cover the outer surface of the first porous glass layer 5a. In this embodiment, the first porous glass layer 5a is located at the center of the porous glass base material 5 in the radial direction. The porous glass base material 5 is the part that becomes the core 5 (see Figure 2) of the optical fiber base material 20. In other words, the burners 3 and 4 in this embodiment play a role in forming the core part of the optical fiber. The outer diameter of the second porous glass layer 5b (i.e., the outer diameter of the porous glass base material 5) is, for example, 100 mm or more.
[0025] Here, the particle size of the glass particles forming the second porous glass layer 5b is larger than the particle size of the glass particles forming the first porous glass layer 5a. In other words, the particle size of the glass particles deposited by the second burner 4 is larger than the particle size of the glass particles deposited by the first burner 3. Furthermore, the chemical composition of the glass particles forming the second porous glass layer 5b is the same as the chemical composition of the glass particles forming the first porous glass layer 5a. In other words, the chemical composition of the glass particles deposited by the second burner 4 is the same as the chemical composition of the glass particles deposited by the first burner 3. Specific methods for changing the particle size of the glass particles deposited by the first burner 3 and the second burner 4 include, for example, differentiating the structure (port configuration) of the first burner 3 and the second burner 4, or differentiating the outer diameter of the first burner 3 and the second burner 4. Alternatively, the gas flow rate and temperature may be different for the first burner 3 and the second burner 4.
[0026] Next, a method for manufacturing a porous glass base material 5 using the manufacturing apparatus 10, and a method for manufacturing an optical fiber base material 20 using the porous glass base material 5 will be described.
[0027] First, a forming process is carried out to form a porous glass base material 5. In the forming process, a rotating starting member 1 is pulled upward by a support 2, while glass raw material gas is ejected from ignited burners 3 and 4. As a result, the first burner 3 deposits glass particles downward from the lower end of the starting member 1, forming a rod-shaped first porous glass layer 5a that extends in the longitudinal direction. The second burner 4 then deposits glass particles on the outer surface of the first porous glass layer 5a, forming a second porous glass layer 5b on the outer surface of the first porous glass layer 5a. This gives rise to the porous glass base material 5. As mentioned above, the chemical composition of the glass particles contained in the first porous glass layer 5a and the chemical composition of the glass particles contained in the second porous glass layer 5b are made identical. In other words, the type of glass raw material gas ejected from the first burner 3 and the type of glass raw material gas ejected from the second burner 4 are made identical. Furthermore, the size of the glass particles contained in the second porous glass layer 5b is made larger than the size of the glass particles contained in the first porous glass layer 5a.
[0028] Next, a doping process is performed. The doping process is a process of adding a dopant to the porous glass base material 5. In this embodiment, the dopant is added simultaneously to the first porous glass layer 5a and the second porous glass layer 5b in the same doping process. Note that "adding the dopant simultaneously" does not mean that the timing of dopant adsorption to the first porous glass layer 5a and the timing of dopant adsorption to the second porous glass layer 5b are not strictly simultaneous, and includes cases where the timing is slightly different within the same process (dopping process). In addition, the porous glass base material 5 may be dehydrated prior to the doping process. In the doping process, for example, the porous glass base material 5 removed from the manufacturing apparatus 10 is placed in a furnace tube (not shown), and while the porous glass base material 5 is heated with a heater or the like, a gas containing a dopant such as chlorine is introduced into the furnace tube. When a chlorine-based gas is introduced into the reactor core tube, it reacts with the glass particles of the porous glass matrix 5, thereby adding chlorine to the porous glass matrix 5. By adding chlorine to the porous glass matrix 5, the refractive index of the porous glass matrix (core) 5 can be increased. In addition to the introduction of a chlorine-based gas into the reactor core tube, an inert gas (carrier gas) such as argon may also be introduced into the reactor core tube.
[0029] Here, assuming that the particle sizes of the glass particles in the first porous glass layer 5a and the second porous glass layer 5b are the same, the bulk density of the porous glass matrix 5 will be uniform in the radial direction. However, when a dopant such as chlorine is added to a glass matrix having a uniform bulk density distribution in the radial direction, the diffusion rate of the dopant in the inner part of the porous glass matrix will be slower than the diffusion rate of the dopant in the outer part of the porous glass matrix. Therefore, in order to obtain a core with a refractive index that is approximately constant in the radial direction, it is necessary to wait for the dopant to diffuse sufficiently in the inner part, which increases the time required for the doping process. This problem becomes more pronounced as the outer diameter of the porous glass matrix 5 increases.
[0030] Therefore, in the manufacturing method of the optical fiber base material 20 according to this embodiment, the particle size of the glass particles in the first porous glass layer 5a is made smaller than the particle size of the glass particles in the second porous glass layer 5b. This makes it possible to increase the dopant adsorption efficiency in the first porous glass layer 5a located radially inward. By promoting the adsorption of dopant in the first porous glass layer 5a in this way, the time required for the dopant to diffuse throughout the porous glass base material 5 can be reduced. In particular, even when the porous glass base material 5 has a large diameter (for example, 100 mm or more), the time required for the doping process can be reduced.
[0031] After the doping process, a sintering process is carried out. In the sintering process, the porous glass base material 5 to which the dopant has been added is heated to the glass transition temperature by a heater or the like. As a result, the porous glass base material 5 becomes transparent glass. Furthermore, since the dopant is diffused throughout the porous glass base material 5 in the doping process, the refractive index of the first porous glass layer 5a and the second porous glass layer 5b becomes approximately constant in the radial direction as a result of the sintering process. In other words, a core 5 with a refractive index that is approximately constant in the radial direction is obtained.
[0032] After the sintering process, glass particles are deposited on the outer periphery of the core 5, forming a soot. By sintering this soot, a first cladding 8a is obtained. Further, by forming a soot on the outer periphery of the first cladding 8a and sintering it, a second cladding 8b is obtained. Prior to the above sintering, a dopant (for example, fluorine) that has the effect of lowering the refractive index may be doped into the soot so that the refractive indices n1 and n2 of the cladding 8a and 8b are smaller than the average refractive index n0 of the core 5. Dehydration treatment may also be performed as needed. Through the above process, an optical fiber preform 20 is manufactured. Furthermore, by drawing the optical fiber preform 20, an optical fiber is obtained.
[0033] Furthermore, the overall refractive index of the first porous glass layer 5a and the second porous glass layer 5b does not necessarily have to be approximately constant in the radial direction. For example, the refractive index of the first porous glass layer 5a may be approximately constant in the radial direction, and the refractive index of the outer periphery of the second porous glass layer 5b may decrease as it moves radially outward. Even with such a refractive index distribution in the optical fiber matrix 20, it may be possible to ensure the functionality of the optical fiber. In other words, the outer periphery of the second porous glass layer 5b does not necessarily have to be used as the core 5. By employing a manufacturing method such that the refractive index of at least the portion spanning the first porous glass layer 5a and the second porous glass layer 5b is approximately constant in the radial direction, the objective of reducing the time required for dopant addition to the first porous glass layer 5a located radially inward compared to conventional methods can be achieved.
[0034] As described above, the method for manufacturing the optical fiber base material 20 according to this embodiment involves depositing glass particles to form a first porous glass layer 5a, depositing glass particles having a larger particle size than the glass particles contained in the first porous glass layer 5a and having the same chemical composition as the glass particles contained in the first porous glass layer 5a on the outer surface of the first porous glass layer 5a to form a second porous glass layer 5b, thereby obtaining a porous glass base material 5, simultaneously adding a dopant to the first porous glass layer 5a and the second porous glass layer 5b, and sintering the porous glass base material 5 with the dopant added to make the refractive index of at least the portion spanning the first porous glass layer 5a and the second porous glass layer 5b substantially constant in the radial direction.
[0035] This configuration enhances the dopant adsorption efficiency in the first porous glass layer 5a located radially inward. Consequently, the time required for the dopant to be added to the entire porous glass matrix 5 can be reduced.
[0036] Furthermore, the structure of the first burner 3 that forms the first porous glass layer 5a may differ from the structure of the second burner 4 that forms the second porous glass layer 5b. With this configuration, the particle size of the glass particles contained in the first porous glass layer 5a and the particle size of the glass particles contained in the second porous glass layer 5b can be easily made different.
[0037] Furthermore, the outer diameter of the first burner 3 that forms the first porous glass layer 5a may be smaller than the outer diameter of the second burner 4 that forms the second porous glass layer 5b. With this configuration, the particle size of the glass particles contained in the first porous glass layer 5a can be easily made smaller than the particle size of the glass particles contained in the second porous glass layer 5b.
[0038] Furthermore, the first porous glass layer 5a is located at the center of the porous glass base material 5 in the radial direction (i.e., at the center of the optical fiber base material 20). In other words, the porous glass base material 5 is the core component of the optical fiber base material 20. This configuration makes it possible to reduce the time required to manufacture the optical fiber base material 20, which has a large diameter core.
[0039] Furthermore, the outer diameter of the porous glass base material 5 may be 100 mm or more. Even with a porous glass base material 5 having such a large diameter, the time required for the doping process can be reduced by forming a first porous glass layer 5a and a second porous glass layer 5b with glass particle diameters that differ from each other, as described above.
[0040] The above embodiments will be described below using specific test examples. However, the present invention is not limited to the following test examples.
[0041] (Test Example 1) A manufacturing apparatus 10 was prepared as shown in Figure 1. The second burner 4 had two units, and the manufacturing apparatus 10 had a total of three burners 3 and 4. Each burner 3 and 4 had a concentric multi-tube structure including ports P1 to P4, as shown in Figure 3A. Port P1 is a port for ejecting glass raw material gas containing SiCl4. Port P2 is a port for ejecting combustion gas containing hydrogen. Port P3 is a port for ejecting inert gas such as argon or nitrogen. Port P4 is a port for ejecting combustion-supporting gas containing oxygen. The outer diameter of the first burner 3 was 15 mm, and the outer diameter of the second burner 4 was 21 mm.
[0042] Using this manufacturing apparatus 10, a porous glass matrix 5 with an outer diameter of 200 mm and a length of 1300 mm was manufactured. The glass particle size of the manufactured porous glass matrix 5 was measured using an electron microscope. The minimum particle size was 0.18 μm, and the maximum particle size was 0.25 μm. Furthermore, the manufactured porous glass matrix 5 was subjected to dehydration treatment, doping treatment, and sintering treatment to obtain a core 5, and the specific refractive index relative to SiO2 was investigated using a preform analyzer or the like. The specific refractive index Δn within the core 5 region in the radial direction was compared with the average refractive index n0 of the core 5. s The fluctuation was within 0.005%.
[0043] (Test Example 2) A manufacturing apparatus 10 was prepared as shown in Figure 1. The second burner 4 had two units, and the manufacturing apparatus 10 had a total of three burners 3 and 4. The first burner 3 had a concentric multi-tube structure including ports P1 to P9, as shown in Figure 3B. Port P1 is a port for ejecting glass raw material gas containing SiCl4. Ports P2, P4, P6, and P8 are ports for ejecting inert gases such as argon or nitrogen. Ports P3 and P7 are ports for ejecting combustion gas containing hydrogen. Ports P5 and P9 are ports for ejecting combustion-supporting gases containing oxygen. Meanwhile, each second burner 4 had a concentric multi-tube structure including ports P1 to P4, as shown in Figure 3A. The roles of each port P1 to P4 were the same as in Test Example 1. The outer diameter of the first burner 3 was 44 mm, and the outer diameter of the second burner 4 was 21 mm.
[0044] Using this manufacturing apparatus 10, a porous glass matrix 5 with an outer diameter of 200 mm and a length of 1300 mm was manufactured. The glass particle size of the manufactured porous glass matrix 5 was measured using an electron microscope. The minimum particle size was 0.25 μm, and the maximum particle size was 0.33 μm. Furthermore, the manufactured porous glass matrix 5 was subjected to dehydration treatment, doping treatment, and sintering treatment to obtain a core 5, and the specific refractive index relative to SiO2 was investigated using a preform analyzer or the like. The specific refractive index Δn within the core 5 region in the radial direction was compared with the average refractive index n0 of the core 5. s The fluctuation was approximately 0.01%.
[0045] (Test Example 3) A manufacturing apparatus 10 was prepared as shown in Figure 1. The second burner 4 had two units, and the manufacturing apparatus 10 had a total of three burners 3 and 4. Each burner 3 and 4 had the structure shown in Figure 3C. Specifically, each burner 3 and 4 had a structure combining a concentric multi-tube including ports P1 to P5 and multiple nozzles Q arranged inside port P3. The multiple nozzles Q were spaced apart around the burners 3 and 4. Port P1 is a port for ejecting glass raw material gas containing SiCl4. Port P2 is a port for ejecting inert gas such as argon or nitrogen. Port P3 is a port for ejecting combustion gas containing hydrogen. Port P4 is a port for ejecting inert gas such as argon or nitrogen. Port P5 is a port for ejecting a combustion-supporting gas containing oxygen. Nozzle Q ejects a combustion-supporting gas containing oxygen. Furthermore, the outer diameter of the first burner 3 was 21 mm, and the outer diameter of the second burner 4 was 25 mm.
[0046] Using this manufacturing apparatus 10, a porous glass matrix 5 with an outer diameter of 200 mm and a length of 1300 mm was manufactured. The glass particle size of the manufactured porous glass matrix 5 was measured using an electron microscope. The minimum particle size was 0.20 μm, and the maximum particle size was 0.27 μm. Furthermore, the manufactured porous glass matrix 5 was subjected to dehydration treatment, doping treatment, and sintering treatment to obtain a core 5, and the specific refractive index relative to SiO2 was investigated using a preform analyzer or the like. The specific refractive index Δn within the core 5 region in the radial direction was compared with the average refractive index n0 of the core 5. s The fluctuation was within 0.005%.
[0047] (Test example 4) A manufacturing apparatus 10 was prepared as shown in Figure 1. The second burner 4 had two units, and the manufacturing apparatus 10 had a total of three burners 3 and 4. Each burner 3 and 4 had the structure shown in Figure 3C. Specifically, each burner 3 and 4 had a structure combining a concentric multi-tube including ports P1 to P5 and multiple nozzles Q arranged inside port P3. The multiple nozzles Q were spaced apart in the circumferential direction of the burners 3 and 4. The roles of ports P1 to P5 and nozzles Q were the same as in Test Example 3. The outer diameter of the first burner 3 was 40 mm, and the outer diameter of the second burner 4 was 25 mm.
[0048] Using this manufacturing apparatus 10, a porous glass matrix 5 with an outer diameter of 200 mm and a length of 1300 mm was manufactured. The glass particle size of the manufactured porous glass matrix 5 was measured using an electron microscope. The minimum particle size was 0.25 μm, and the maximum particle size was 0.40 μm. Furthermore, the manufactured porous glass matrix 5 was subjected to dehydration treatment, doping treatment, and sintering treatment to obtain a core 5, and the specific refractive index relative to SiO2 was investigated using a preform analyzer or the like. The specific refractive index Δn within the core 5 region in the radial direction was compared with the average refractive index n0 of the core 5. s The fluctuation was approximately 0.01%.
[0049] As described above, it was confirmed that the particle size of the glass particles can be changed by making the composition and outer diameter of the first burner 3 and the second burner 4 different. And as previously stated, by changing the particle size of the glass particles, the time required for the dopant to be added to the entire porous glass matrix 5 can be reduced.
[0050] The technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention.
[0051] For example, in the above embodiment, the porous glass base material 5 was the core 5 of the optical fiber base material 20, but the porous glass base material 5 may also be the first cladding 8a or the second cladding 8b of the optical fiber base material 20. In other words, in the manufacturing of the first cladding 8a or the second cladding 8b, two porous glass layers with different glass particle sizes may be formed, as in the above embodiment.
[0052] Furthermore, the dopant added to the porous glass matrix 5 in the doping process is not limited to chlorine. The type of dopant can be appropriately changed according to the design of the refractive index distribution of the optical fiber matrix 20. For example, when applying the above embodiment to the manufacture of the first cladding 8a or the second cladding 8b as described above, fluorine or the like can be used as the dopant.
[0053] Furthermore, the porous glass base material 5 may have layers other than the first porous glass layer 5a and the second porous glass layer 5b. In other words, in the above embodiment, there were two porous glass layers with a substantially constant refractive index (forming the core 5), but the refractive index may be made substantially constant for three or more porous glass layers, and the core 5 may be manufactured using three or more porous glass layers. In this case, a porous glass base material 5 with a larger outer diameter can be manufactured. Also, even if the diameter of the porous glass base material 5 is not increased, the glass particle size can be controlled with higher precision in the radial direction of the porous glass base material 5 by controlling the glass particle size for each of the three or more porous glass layers. This makes it easier to adjust the fluctuation in the refractive index in the radial direction.
[0054] Furthermore, the refractive index distribution shown in Figure 2 is merely an example. For instance, the refractive index may change in a rectangular shape at the boundary between the core 5 and the first cladding 8a.
[0055] Furthermore, although the above embodiment describes a method for manufacturing the optical fiber preform 20, the above embodiment may also be applied to the manufacture of glass preforms other than the optical fiber preform 20. The manufacturing method according to the above embodiment can be suitably used when manufacturing a glass preform that includes a layer with a substantially constant refractive index in the radial direction.
[0056] Furthermore, without departing from the spirit of the present invention, the components in the above-described embodiments may be replaced with well-known components as appropriate, and the above-described embodiments and modifications may be combined as appropriate. [Explanation of Symbols]
[0057] 3…First burner 4…Second burner 5…Porous glass base material 5a…First porous glass layer 5b…Second porous glass layer 20…Optical fiber base material (glass base material)
Claims
1. A first porous glass layer is formed by depositing glass particles. A porous glass base material is obtained by depositing glass particles having a larger particle size than the glass particles contained in the first porous glass layer and having the same chemical composition as the glass particles contained in the first porous glass layer onto the outer surface of the first porous glass layer to form a second porous glass layer. A method for manufacturing a glass substrate, comprising simultaneously adding a dopant to the first porous glass layer and the second porous glass layer, and sintering the porous glass substrate to which the dopant has been added, thereby making the refractive indices of at least both the first porous glass layer and the second porous glass layer continuously substantially constant in the radial direction.
2. The method for manufacturing a glass base material according to claim 1, wherein the structure of the first burner that forms the first porous glass layer is different from the structure of the second burner that forms the second porous glass layer.
3. The method for manufacturing a glass base material according to claim 1 or 2, wherein the outer diameter of the first burner that forms the first porous glass layer is smaller than the outer diameter of the second burner that forms the second porous glass layer.
4. The method for manufacturing a glass base material according to claim 1 or 2, wherein the first porous glass layer is located at the center of the porous glass base material in the radial direction.
5. The method for manufacturing a glass substrate according to claim 1 or 2, wherein the outer diameter of the porous glass substrate is 100 mm or more.