Method for manufacturing glass body for optical fiber

Abstract

To provide a method of producing an optical fiber glass body that allows the production of an optical fiber glass body having a shallow relative refractive index difference with respect to pure silica glass in at least a part of a layer of a clad glass body serving as a clad, and having a small variation in the relative refractive index difference in the layer, the method inhibiting the decline in production efficiency.SOLUTION: The method of producing an optical fiber glass body includes a deposition step P11 of depositing soot that will become a predetermined glass body on an outer surface of a starting substrate, and a diffusion step P12 of heating a porous glass body 20 in a fluoride atmosphere to diffuse fluorine. A bulk density of the soot decreases from the inner to the outer circumference, and is between 0.63 g / cm3 and 0.87 g / cm3 at the inner circumference and between 0.54 g / cm3 and 0.79 g / cm3 at the outer circumference, the difference between the maximum and minimum bulk density is between 0.05 g / cm3 and 0.09 g / cm3; and in the diffusion step P12, the fluorine concentration in the atmosphere is between 1.8% and 7.4%.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a method for producing a glass body for an optical fiber. [Background technology]

[0002] An optical fiber in which at least a portion of the cladding is doped with fluorine is known. Patent Document 1 below describes such an optical fiber and a method for manufacturing such an optical fiber preform. In this document, a depressed layer is formed in the innermost region of the cladding. When preparing a glass body that will become the depressed layer in an optical fiber preform for manufacturing this optical fiber, soot that will become the depressed layer is deposited on the outer peripheral surface of a core glass body that will become the core, and the deposited soot is heated in a fluorine atmosphere to diffuse fluorine into the soot. Then, the soot with the fluorine diffused therein is sintered to turn the soot into transparent glass, thereby forming a glass body that will become the depressed layer. In this document, when depositing the soot that will become the depressed layer, the bulk density of the soot is set to 0.1 g / cm. 3 to 0.4 g / cm 3 It is preferable that: [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5342614 Summary of the Invention [Problem to be solved by the invention]

[0004] When fabricating a cladding glass body that will become the cladding of an optical fiber, the degree of fluorine diffusion into the soot that will become the cladding glass body varies depending on the bulk density of the soot and the fluorine concentration in the atmosphere. With the soot bulk density described in Patent Document 1, excessive fluorine diffusion can cause the relative refractive index difference of the depressed layer relative to pure silica to become too negative, i.e., the relative refractive index difference can become too deep. In response to this, it may be desirable to reduce the relative refractive index difference of at least a portion of the cladding layer relative to pure silica glass. Therefore, when fabricating the layer, it is possible to reduce the fluorine concentration in the atmosphere during fluorine diffusion. However, in this case, fluorine is less likely to diffuse to the innermost portion of the layer, and the refractive index at this portion may not be sufficiently reduced, resulting in excessive variation in the refractive index between the inner and outer sides of the region. Therefore, in order to sufficiently diffuse fluorine to the innermost portion of the layer while reducing the fluorine concentration in the atmosphere, it is possible to heat the soot that will become the layer deposited in the fluorine atmosphere for a long period of time. However, in this case, there is a concern that the manufacturing efficiency of the glass body for optical fiber may decrease.

[0005] Therefore, an object of the present invention is to provide a method for producing an optical fiber glass body, which can produce an optical fiber glass body in which at least a portion of a layer of a cladding glass body that becomes the cladding of an optical fiber has a shallow relative refractive index difference from pure silica glass and in which the variation in the relative refractive index difference in that layer is small, while suppressing a decrease in production efficiency. [Means for solving the problem]

[0006] In order to achieve the above object, the present invention provides a method for producing a glass body for an optical fiber, the method comprising: a specific glass body that has a relative refractive index difference with respect to pure silica glass of between −0.20% and −0.05%, and a difference between the maximum and minimum values ​​of the relative refractive index difference of between 0.03% and 0.04%, and that will become a specific layer that will be placed outside the core of an optical fiber; the method comprises a deposition step of depositing soot that will become the specific glass body on the outer peripheral surface of a starting substrate having a circular cross section; a diffusion step of heating the porous glass body, on which the soot has been deposited on the starting substrate, in a fluorine atmosphere to diffuse fluorine into the soot; and a sintering step of sintering the fluorine-diffused soot to form a transparent glass body, the bulk density of the soot decreasing from the inner peripheral side to the outer peripheral side, and being 0.63 g / cm on the inner peripheral side. 3 More than 0.87g / cm 3 or less, and 0.54 g / cm on the outer periphery 3 More than 0.79g / cm 3 The difference between the maximum and minimum bulk density values ​​is 0.05 g / cm or less. 3 More than 0.09g / cm 3 The method is characterized in that the fluorine concentration in the atmosphere in the diffusion step is 1.8% or more and 7.4% or less.

[0007] According to this method for manufacturing an optical fiber glass body, the bulk density of the soot that will become the predetermined glass body decreases toward the outer periphery, thereby suppressing cracking of the predetermined glass body during manufacturing. Furthermore, as a result of the inventor's studies, it has been found that, even when the relative refractive index difference of the predetermined glass body is shallow as described above, when the bulk density of the soot that will become the predetermined glass body and the fluorine concentration in the atmosphere during the diffusion step satisfy the above-mentioned conditions, it is possible to appropriately add fluorine to the soot that will become the predetermined glass body while preventing the diffusion step from taking too long. This is thought to be because, by satisfying the above-mentioned conditions, fluorine that penetrates from the outer periphery of the deposited soot can appropriately penetrate to the inner periphery. Therefore, according to the method for manufacturing an optical fiber glass body of the present invention, the relative refractive index difference of the predetermined glass body that will become the predetermined layer disposed outside the core with respect to pure silica glass is shallow, and a decrease in the manufacturing efficiency of an optical fiber glass body with a small variation in the relative refractive index difference can be suppressed. The optical fiber glass body is a glass body used in the manufacture of an optical fiber, and is an optical fiber preform or an intermediate optical fiber preform before being made into an optical fiber preform.

[0008] In the sintering step, the fluorine concentration in the atmosphere in which the porous glass body is placed may be 0.41% or less.

[0009] In this case, an unnecessary decrease in the refractive index on the outer periphery of the specified glass body can be suppressed. Furthermore, if the fluorine concentration is 0.18% or more, an unnecessary increase in the refractive index on the outer periphery of the specified glass body can be suppressed during the sintering process.

[0010] The starting substrate may be a core glass body that will become the core of the optical fiber.

[0011] In this case, it is possible to produce a glass body for optical fiber that can be used to produce an optical fiber having a depressed layer with a shallow refractive index relative to pure silica.

[0012] The starting substrate may also include a core glass body that will become the core of the optical fiber, and a trench layer glass body that will become a trench layer with a lower refractive index than the cladding of the optical fiber and surround the core glass body, and the soot may be deposited on the outer peripheral surface of the trench layer glass body.

[0013] In this case, it is possible to produce a glass body for an optical fiber that can be used to produce a trench optical fiber in which the relative refractive index difference between the layer surrounding the trench layer and the pure silica glass is shallow and the fluctuation in the relative refractive index difference is small.

[0014] The starting substrate may also include a core glass body that will become the core of the optical fiber, and an inner clad glass body that surrounds the core and has a refractive index higher than that of the specified glass body, and the soot may be deposited on the outer peripheral surface of the inner clad glass body.

[0015] In this case, since the specified glass body has a lower refractive index than the inner cladding glass body, it is possible to produce a glass body for an optical fiber that can be used to produce a trench-type optical fiber in which the relative refractive index difference between the trench layer and pure silica glass is shallow and the fluctuation in the relative refractive index difference is small. [Effects of the Invention]

[0016] As described above, the present invention provides a method for producing an optical fiber glass body, which can produce an optical fiber glass body in which at least a portion of a layer of a cladding glass body that forms the cladding of an optical fiber has a shallow relative refractive index difference from pure silica glass and in which the variation in the relative refractive index difference in that layer is small, while suppressing a decrease in production efficiency. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber manufactured by a manufacturing method for an optical fiber according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the refractive index profile of the optical fiber of FIG. [Figure 3]2 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber preform for manufacturing the optical fiber shown in FIG. 1. FIG. [Figure 4] 4 is a flowchart showing steps of a method for manufacturing an optical fiber, including the method for manufacturing the optical fiber preform of FIG. 3. [Figure 5] FIG. 10 is a diagram showing a deposition process. [Figure 6] 1A and 1B are diagrams illustrating the diffusion process and the sintering process. [Figure 7] 10 is a diagram similar to FIG. 2 showing the refractive index profile of the optical fiber of the second embodiment. [Figure 8] 10 is a diagram similar to FIG. 2 showing the refractive index profile of an optical fiber according to a third embodiment. [Figure 9] FIG. 2 is a diagram showing the refractive index profile of an inner cladding glass body in an example. [Figure 10] FIG. 10 is a diagram showing the refractive index profile of an inner cladding glass body in a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0018] Preferred embodiments of the method for producing a glass body for optical fiber according to the present invention will be described in detail below with reference to the drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved without departing from the spirit thereof. In the drawings referred to below, the dimensions of each component may be changed to facilitate understanding.

[0019] In the following embodiment, an example will be described in which an optical fiber glass body used in the manufacture of an optical fiber is an optical fiber preform.

[0020] (First embodiment) Fig. 1 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber manufactured from an optical fiber preform according to this embodiment. As shown in Fig. 1, in this embodiment, the optical fiber 1 mainly comprises a core 10, a cladding 11 surrounding the outer surface of the core 10, and a coating layer 12 covering the outer surface of the cladding 11. The cladding 11 has an inner cladding 11a that is a layer that contacts and surrounds the core 10, and an outer cladding 11b that is a layer that contacts and surrounds the inner cladding 11a. The outer cladding 11b includes a first region b1 that is a layer that contacts and surrounds the inner cladding 11a, and a second region b2 that surrounds the first region b1 and extends to the outermost periphery of the cladding 11.

[0021] Fig. 2 is a diagram showing the refractive index profile of the optical fiber 1 of Fig. 1. In the following diagrams showing refractive indices, the same symbols are used as for the parts whose refractive indices correspond. The refractive index of the core 10 is higher than that of the cladding 11, and in this embodiment, the core 10 is made of silica glass doped with chlorine or the like, which increases the refractive index. Therefore, the refractive index of the core 10 is higher than the refractive index n0 of pure silica glass containing no impurities.

[0022] The inner cladding 11a is made of fluorine-doped silica glass and has a refractive index lower than the refractive index n0 of pure silica glass. The refractive index of the inner cladding 11a generally decreases from the inner periphery (i.e., the core 10 side) toward the outer periphery, and the relative refractive index difference with respect to pure silica glass is generally −0.20% or more and −0.08% or less. Thus, the inner cladding 11a of this embodiment has a relatively shallow relative refractive index difference with respect to pure silica. In this embodiment, the thickness of the inner cladding 11a is slightly larger than the radius of the core 10. However, the thickness of the inner cladding 11a may be equal to or smaller than the radius of the core 10. In the following description, the term “relative refractive index difference” simply refers to the relative refractive index difference with respect to pure silica glass. The thickness of the inner cladding 11a is, for example, 20 mm or more.

[0023] The first region b1 of the outer cladding 11b is made of silica glass doped with a fluorine concentration lower than that of the inner cladding 11a, and has a refractive index lower than the refractive index n0 of pure silica glass. The refractive index of the first region b1 is highest at the inner periphery and gradually decreases from the inner periphery to the outer periphery. The lowest refractive index of the first region b1 is higher than the lowest refractive index of the inner cladding 11a. Therefore, the first region b1 has a shallower relative refractive index difference than the inner cladding 11a. Furthermore, the thickness of the first region b1 is greater than that of the inner cladding 11a. Therefore, the gradient of the decrease in the refractive index of the first region b1 from the inner periphery to the outer periphery is gentler than the gradient of the decrease in the refractive index of the inner cladding 11a from the inner periphery to the outer periphery.

[0024] The second region b2 is made of silica glass doped with fluorine at a concentration lower than that of the first region b1. The refractive index of the second region b2 is higher than the lowest refractive index of the first region b1 and lower than the refractive index n0 of pure silica glass. Therefore, the second region b2 of this embodiment has a shallower relative refractive index difference than the first region b1.

[0025] The coating layer 12 is made of a resin. Examples of the resin that constitutes the coating layer 12 include a thermosetting resin and an ultraviolet-curable resin. The coating layer 12 may have a single-layer structure made of one resin layer that surrounds the cladding 11, or may have a multilayer structure made of multiple resin layers.

[0026] Next, an optical fiber preform used to manufacture the optical fiber 1 according to this embodiment will be described.

[0027] Fig. 3 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber preform for producing the optical fiber 1 shown in Fig. 1. As shown in Fig. 3, the optical fiber preform 1P is composed of a rod-shaped core glass body 10P that becomes the core 10, and a clad glass body 11P that surrounds the outer peripheral surface of the core glass body 10P and becomes the clad 11. The clad glass body 11P includes an inner clad glass body 11Pa that becomes the inner clad 11a, and an outer clad glass body 11Pb that becomes the outer clad 11b, and the outer clad glass body 11Pb includes a first region glass body Pb1 that becomes the first region b1, and a second region glass body Pb2 that becomes the second region b2.

[0028] The refractive index profile of the optical fiber preform 1P is generally similar to that of the optical fiber 1. The inner cladding glass body 11Pa is in contact with and surrounds the core glass body 10P. The relative refractive index difference of the inner cladding glass body 11Pa is -0.20% or more and -0.05% or less. The position where the relative refractive index difference of the inner cladding glass body 11Pa is maximum is located approximately on the inner peripheral side, and this maximum value is -0.16% or more and -0.05% or less, and the position where the relative refractive index difference is minimum is located approximately on the outer peripheral side, and this minimum value is -0.20% or more and -0.08% or less. Furthermore, the difference between the maximum and minimum values ​​of the relative refractive index difference is 0.03% or more and 0.04% or less.

[0029] In this embodiment, the relative refractive index difference of the outer cladding glass body 11Pb is −0.20% or more and −0.05% or less, and the difference between the maximum and minimum values ​​of the relative refractive index difference of the outer cladding glass body 11Pb is 0.03% or more and 0.04% or less.

[0030] The first region glass body Pb1 of the outer cladding glass body 11Pb is in contact with the inner cladding glass body 11Pa and surrounds the inner cladding glass body 11Pa. The relative refractive index difference of the first region glass body Pb1 is, for example, −0.20% or more and −0.05% or less. In this embodiment, the relative refractive index difference on the inner peripheral side of the first region glass body Pb1 is, for example, −0.16% or more and −0.05% or less, and the relative refractive index difference on the outer peripheral side is, for example, −0.20% or more and −0.08% or less. The difference between the maximum and minimum values ​​of this relative refractive index difference is, for example, 0.03% or more and 0.04% or less. In this embodiment, this difference is smaller than the difference between the maximum and minimum values ​​of the relative refractive index difference in the inner cladding glass body 11Pa. Therefore, the average refractive index of the first region glass body Pb1 is larger than the average refractive index of the inner cladding glass body 11Pa.

[0031] The second region glass body Pb2 is in contact with the first region glass body Pb1 and surrounds the first region glass body Pb1. The refractive index of the second region glass body Pb2 gradually increases from the inner periphery side. The second region glass body Pb2 is in contact with the first region glass body Pb1 and surrounds the first region glass body Pb1. The relative refractive index difference of the second region glass body Pb2 is, for example, -0.20% or more and -0.05% or less. The difference between the maximum and minimum values ​​of this relative refractive index difference is, for example, 0.03% or more and 0.04% or less.

[0032] Here, if the inner cladding 11a of the optical fiber 1 is a predetermined layer, the inner cladding glass body 11Pa can be understood as a predetermined glass body that becomes the predetermined layer located outside the core 10. Furthermore, if the first region b1 and the second region b2 of the optical fiber 1 are each predetermined layers different from the above-mentioned predetermined layers, the first region glass body Pb1 and the second region glass body Pb2 can be understood as predetermined glass bodies that become the predetermined layer located outside the core 10. Furthermore, if the outer cladding 11b, which is a combination of the first region b1 and the second region b2, is a predetermined layer different from the above-mentioned predetermined layer, the outer cladding glass body 11Pb, which is a combination of the first region glass body Pb1 and the second region glass body Pb2, can be understood as a predetermined glass body that becomes the predetermined layer located outside the core 10.

[0033] Next, a method for manufacturing the optical fiber preform 1P and a method for manufacturing the optical fiber 1 will be described.

[0034] Fig. 4 is a flowchart showing steps in a method for manufacturing an optical fiber 1, including a method for manufacturing an optical fiber preform 1P according to this embodiment. As shown in Fig. 4, the method for manufacturing an optical fiber preform 1P according to this embodiment includes an inner cladding glass body forming step P1 and an outer cladding glass body forming step P2, and the method for manufacturing an optical fiber 1 includes a drawing step P3 of drawing the manufactured optical fiber preform 1P. In this embodiment, a first region glass body Pb1 and a second region glass body Pb2 are formed in the outer cladding glass body forming step P2.

[0035] (Inner cladding glass body forming process P1) This step is a step of forming inner cladding glass body 11Pa on the outer peripheral surface of core glass body 10P, and includes a deposition step P11, a diffusion step P12, and a sintering step P13.

[0036] <Deposition process P11> This process forms a porous glass body by depositing soot, which is glass particles, on the outer peripheral surface of a core glass body 10P having a circular cross-sectional shape using an outside deposition method. Therefore, in this process, the core glass body 10P is the starting substrate. Note that "circular" refers to a shape that appears circular, e.g., a shape with a noncircularity of 2.0% or less. Figure 5 illustrates this process. In this process, a core glass body 10P is prepared, and soot 11Sa, which will become the inner cladding glass body 11Pa, is deposited on the surface of the core glass body 10P. The soot 11Sa is sprayed from a burner 50 along with a flame. The material of the soot 11Sa is preferably one of SiCl4, OMCTS (octamethylcyclotetrasiloxane), and HMDSO (hexamethylsiloxane). The deposition temperature is lowered each time the burner 50 traverses. The deposition temperature when depositing the innermost soot 11Sa is preferably 1060°C or higher and 1230°C or lower. By setting the temperature range in this way, the bulk density of the innermost part of the soot 11Sa that becomes the inner cladding glass body 11Pa is set to 0.63 g / cm 3 More than 0.87g / cm 3 The deposition temperature when depositing the outermost soot 11Sa is preferably 1000°C or higher and 1175°C or lower. By setting the temperature in this range, the bulk density of the outermost soot 11Sa can be set to 0.54 g / cm 3 More than 0.79g / cm 3 The deposition temperature decreases from the inner circumferential side toward the outer circumferential side, so the bulk density of the deposited soot decreases from the inner circumferential side toward the outer circumferential side. In addition, the difference between the deposition temperature on the inner circumferential side and the deposition temperature on the outer circumferential side is preferably 40°C or more and 55°C or less. Within this temperature range, the difference between the maximum and minimum bulk densities of the soot that becomes the inner cladding glass body 11 Pa can be made 0.05 g / cm 3 More than 0.09g / cm 3 The deposition temperature can be as follows: Note that the deposition temperature is the surface temperature of the deposited soot 11Sa.

[0037] This step produces a porous glass body 20 in which soot 11Sa, which will become the inner cladding glass body 11Pa, is deposited on the outer peripheral surface of the core glass body 10P. The thickness of the soot 11Sa deposited in this step is, for example, such that the thickness becomes 20 mm or more when vitrified as described below.

[0038] <Diffusion process P12> This step is a step of doping fluorine into the soot 11Sa of the porous glass body formed in the deposition step P11. FIG. 6 illustrates the process. As shown in FIG. 6, in this embodiment, fluorine is doped into the soot 11Sa of the porous glass body 20 using a heating furnace 70 including a furnace tube 71 having a storage space 75 and a heating element 72 for heating the furnace tube 71. First, the porous glass body 20 suspended from a support rod 76 is placed in the storage space 75 of the furnace tube 71. Next, a fluorine-containing diffusion gas is supplied to the storage space 75 through an air inlet (not shown), while the storage space 75 is heated by the heating element 72. The diffusion gas is a mixture of an inert gas and fluorine gas. Examples of the inert gas include He, N, and Ar. Examples of the fluorine gas include CF, C, F, SF, and SiF. The fluorine concentration in the diffusion gas atmosphere is 1.8% or more and 7.4% or less. Moreover, the temperature inside the accommodation space 75 is preferably 800° C. or higher and 1250° C. or lower, and more preferably 900° C. or higher and 1100° C. or lower. By setting such conditions, a more appropriate amount of fluorine can be added to the soot 11Sa deposited to the above bulk density in the deposition step P11.

[0039] Alternatively, a dehydration step may be performed simultaneously with this step by mixing a chlorine-based gas with the diffusion gas. Examples of chlorine-based gases include Cl2, SiCl4, SOCl2, and CCl4. Alternatively, the dehydration step may be performed before this step.

[0040] In this way, the soot 11Sa of the porous glass body 20 is doped with fluorine.

[0041] <Sintering process P13> This step is a step of sintering the soot 11Sa to which fluorine has been added in the diffusion step P12 to form a transparent vitrified body. In this embodiment, the porous glass body 20 is heated using the heating furnace 70 to form a transparent vitrified body. In this step, the porous glass body 20 is heated while a sintering gas is supplied into the accommodation space 75 through an air inlet (not shown). Examples of the sintering gas include the inert gases mentioned above. Furthermore, unnecessary fluorine in the atmosphere can be removed by flowing the sintering gas into the furnace tube 71 for a certain period of time, reducing the pressure inside the furnace tube 71 for a certain period of time, or heating the soot 11Sa deposited at 1000°C to 1200°C. The temperature at which the porous glass body 20 is heated is not particularly limited as long as it is a temperature at which the porous glass body 20 is sintered to form a transparent vitrified body. For example, a temperature of 1300°C to 1500°C is preferable. Fluorine gas may be mixed with the sintering gas. In this case, the fluorine concentration of the sintering gas is preferably 0.41% or less, and more preferably 0.18% or more from the viewpoint of adding an appropriate amount of fluorine to the inner cladding glass body 11 Pa. Note that this step is only required to sinter the porous glass body 20, and there are no particular limitations on the configuration of the heating furnace 70 or the sintering method.

[0042] The concentration of fluorine in the gas may be expressed as the partial pressure of fluorine in the atmosphere, with the same result.

[0043] In this way, a glass body is obtained in which the inner clad glass body 11Pa is formed on the outer peripheral surface of the core glass body 10P.

[0044] (Outer cladding glass body forming process P2) This process is a process of forming an outer cladding glass body 11Pb consisting of a first region glass body Pb1 and a second region glass body Pb2 on the outer surface of an inner cladding glass body 11Pa formed on the outer surface of a core glass body 10P, and includes a deposition process P21, a diffusion process P22, and a sintering process P23.

[0045] <Deposition process P21> This step is a step of forming a porous glass body by depositing soot that will become the outer clad glass body 11Pb on the outer peripheral surface of the inner clad glass body 11Pa in a manner generally similar to that of the deposition step P11. Therefore, in this step, a glass body having a circular cross section, in which the inner clad glass body 11Pa is provided on the outer peripheral surface of the core glass body 10, serves as the starting substrate. In this step as well, the deposition temperature is lowered each time the burner 50 is traversed. The deposition temperature when depositing the innermost soot is preferably 1060°C or higher and 1230°C or lower. At this time, the bulk density of the innermost part of the deposited soot is set to 0.63 g / cm. 3 More than 0.87 / cm 3 The deposition temperature when depositing the outermost soot is preferably 1000°C or higher and 1175°C or lower. By setting the temperature in this range, the bulk density of the outermost part of the deposited soot can be reduced to 0.54 g / cm. 3 More than 0.79 / cm 3 In this process, the bulk density of the deposited soot also decreases from the inner circumferential side toward the outer circumferential side. The difference between the deposition temperature on the inner circumferential side and the deposition temperature on the outer circumferential side is preferably 40°C or more and 55°C or less. By keeping the temperature within this range, the difference between the maximum and minimum bulk densities of the deposited soot can be reduced to 0.05 g / cm. 3 More than / 0.09cm 3 It can be the following:

[0046] This step forms a porous glass body in which soot that will become the outer cladding glass body 11Pb is deposited on the outer peripheral surface of the inner cladding glass body 11Pa. Note that the bulk density of the soot that will become the first region glass body Pb1 deposited in this step is preferably higher than the bulk density of the soot 11Sa deposited in the depositing step P11 within the above range, from the viewpoint of making the relative refractive index difference of the outer cladding glass body 11Pb shallower than that of the inner cladding glass body 11Pa.

[0047] <Diffusion process P22> This step is a step of adding fluorine to the soot of the porous glass body formed in the deposition step P21 in a manner generally similar to the diffusion step P12. In this embodiment, a heating furnace 70 is used. In this step, the porous glass body obtained in the deposition step P21 is placed in the storage space 75 of the furnace tube 71, and fluorine is diffused into the soot in the same manner as the diffusion step P12. The fluorine concentration in the diffusion gas atmosphere in this step is 1.8% or more and 7.4% or less, similar to the diffusion step P12. The temperature in the storage space 75 and the duration of this step are preferably similar to those of the diffusion step P12. A dehydration step may be performed simultaneously with or before this step. In this way, fluorine is added to the soot of the porous glass body formed in the deposition step P21. In addition, it is preferable that the fluorine concentration in the diffusion gas atmosphere in this step is lower than the fluorine concentration in the diffusion gas atmosphere in the diffusion step P12 within the above range, from the viewpoint of making the relative refractive index difference of the outer clad glass body 11Pb shallower than the relative refractive index difference of the inner clad glass body 11Pa.

[0048] <Sintering process P23> This step is a step of sintering the soot doped with fluorine in the diffusion step P22 to form a transparent glass. In this embodiment, this step is performed in the same manner as the sintering step P13. When fluorine gas is mixed into the sintering gas in this step, the fluorine concentration is preferably 0.41% or less, and preferably 0.18% or more, from the viewpoint of doping an appropriate amount of fluorine into the outer cladding glass body 11Pb. The soot is thus vitrified to form the outer cladding glass body 11Pb, thereby obtaining the optical fiber preform 1P shown in FIG. 3, in which the core glass body 10P, the inner cladding glass body 11Pa, and the outer cladding glass body 11Pb are integrated. Even when fluorine gas is mixed into the sintering gas in this step, the fluorine concentration is preferably lower than the fluorine concentration in the sintering gas atmosphere used in the sintering step P13, from the viewpoint of making the relative refractive index difference of the outer cladding glass body 11Pb shallower than that of the inner cladding glass body 11Pa. Furthermore, fluorine gas need not be mixed in this step.

[0049] (Drawing process P3) This process is a process of drawing an optical fiber preform 1P to obtain an optical fiber 1. This process will be explained without any particular illustration. In this process, the optical fiber preform 1P is heated in a spinning furnace, and glass is drawn from the lower end of the optical fiber preform. This drawn glass immediately solidifies, and the core glass body 10P becomes the core 10, and the cladding glass body 11P becomes the cladding 11, resulting in a bare optical fiber wire composed of the core 10 and the cladding 11. A coating layer 12 is provided on the outer peripheral surface of this bare optical fiber wire, and the optical fiber 1 shown in FIG. 1 is obtained.

[0050] As described above, this embodiment is a method for manufacturing an optical fiber preform 1P as an optical fiber glass body including a predetermined glass body that will be a predetermined layer disposed outside the core 10 of the optical fiber 1, and that has a relative refractive index difference relative to pure silica glass of between -0.20% and -0.05%, and a difference between the maximum and minimum relative refractive index differences of between 0.03% and 0.04%. As described above, the inner cladding 11a can be considered as the predetermined layer, the starting substrate can be considered as the core glass body 10P, and the inner cladding glass body 11Pa can be considered as the predetermined glass body. Alternatively, as described above, the outer cladding 11b can be considered as the predetermined layer, the starting substrate can be considered as a glass body in which the inner cladding 11a is provided on the core glass body 10P, and the outer cladding glass body 11Pb can be considered as the predetermined glass body. When viewed from these perspectives, this manufacturing method includes deposition steps P11 and P21 in which soot that will become a predetermined glass body is deposited on the outer peripheral surface of a starting substrate, diffusion steps P12 and P22 in which the porous glass body on which soot has been deposited on the starting substrate is heated in a fluorine-containing atmosphere to diffuse fluorine into the soot, and sintering steps P13 and P23 in which the fluorine-diffused soot is sintered to form transparent glass. The bulk density of the deposited soot decreases from the inner peripheral side to the outer peripheral side, and is 0.63 g / cm3 on the inner peripheral side. 3 More than 0.87g / cm 3 or less, and 0.54 g / cm on the outer periphery 3 More than 0.79g / cm 3 The difference between the maximum and minimum bulk density values ​​is 0.05 g / cm or less. 3 More than 0.09g / cm 3In the diffusion step P12, the fluorine concentration in the atmosphere is 1.8% or more and 7.4% or less.

[0051] According to this manufacturing method for the optical fiber preform 1P, the bulk density of the soot that becomes the predetermined glass body decreases toward the outer periphery, thereby suppressing cracking of the predetermined glass body during manufacturing. Furthermore, by ensuring that the bulk density of the soot and the fluorine concentration in the atmosphere in the diffusion steps P12 and P22 satisfy the above-mentioned conditions, even when the relative refractive index difference of the predetermined glass body is within the above-mentioned range, it is possible to appropriately add fluorine to the soot while suppressing an increase in the time required for the diffusion steps P12 and P22. Therefore, according to the manufacturing method for the optical fiber preform 1P of this embodiment, the relative refractive index difference of the predetermined glass body that becomes the predetermined layer disposed outside the core 10 of the optical fiber 1 with respect to pure silica is shallow, and the fluctuation of the relative refractive index difference is small, thereby suppressing a decrease in the manufacturing efficiency of the optical fiber preform 1P.

[0052] (Second embodiment) Next, a second embodiment of the present invention will be described in detail. Note that components that are the same as or equivalent to those in the first embodiment will be given the same reference numerals and will not be described again unless otherwise specified.

[0053] 7 is a diagram similar to FIG. 2 showing the refractive index profile of the optical fiber of this embodiment. The optical fiber of this embodiment differs from the optical fiber 1 shown in FIG. 1 in that it includes a trench layer 13 that surrounds the inner cladding 11a between the inner cladding 11a and the outer cladding 11b of the optical fiber 1 shown in FIG. 1. In this embodiment, the inner cladding 11a surrounded by the trench layer 13 is called the inner cladding 11a to distinguish it from the inner cladding 11a of the first embodiment. Such an optical fiber is called a trench optical fiber, and the core 10, inner cladding 11a, and trench layer 13 are sometimes called the core element. In a trench optical fiber, light propagates mainly through the core element.

[0054] The refractive index of the trench layer 13 is lower than those of the inner cladding 11a and the outer cladding 11b, and the relative refractive index difference of the trench layer is, for example, not less than −0.20% and not more than −0.05%.

[0055] A method for manufacturing an optical fiber preform for producing such a trench-type optical fiber includes a trench layer glass body forming step, which becomes the trench layer 13, between the inner cladding glass body forming step P1 and the outer cladding glass body forming step P2 in the method for manufacturing an optical fiber preform of the first embodiment. However, since the inner cladding 11a is replaced with the inner cladding 11a as described above, in this embodiment, the inner cladding glass body 11Pa is replaced with the inner cladding glass body 11Pa, and the inner cladding glass body forming step P1 is replaced with the inner cladding glass body forming step P1. The inner cladding glass body 11Pa becomes the inner cladding 11a in the optical fiber. In the trench layer glass body forming step, the inner cladding glass body forming step P1 is completed, and the glass body with the inner cladding glass body 11Pa formed on the outer peripheral surface of the core glass body 10P is inserted into a through-hole of a glass tube having a refractive index similar to that of the trench layer 13 and collapsed. In this way, a trench layer glass body is formed on the outer peripheral surface of the inner cladding glass body 11Pa. Then, an outer cladding glass body forming step P2 is performed. In the outer cladding glass body forming step P2 of this embodiment, the starting substrate includes a core glass body 10P that will become the core 10 of the optical fiber, and a trench layer glass body that will become the trench layer 13 with a lower refractive index than the cladding 11 of the optical fiber and surrounds the core glass body 10P, and soot that will become the outer cladding glass body 11Pb is deposited on the outer peripheral surface of the trench layer glass body. In this way, an optical fiber preform used in manufacturing the optical fiber of this embodiment is obtained. By performing a drawing step P3 using this optical fiber preform, an optical fiber having the refractive index profile shown in FIG. 7 is obtained.

[0056] In the trench layer glass body forming step, the trench layer glass body may be formed by performing steps generally similar to the deposition step, diffusion step, and sintering step described above on a glass body having an inner cladding glass body 11Pa formed on the outer peripheral surface of a core glass body 10P. In this case, for example, in the deposition step, the bulk density is made lower than in the deposition step P11 of the first embodiment, and in the diffusion step, the fluorine concentration in the atmosphere is made higher than in the diffusion step P12 of the first embodiment, thereby increasing the concentration of fluorine added to the soot.

[0057] In the method for manufacturing an optical fiber preform according to this embodiment, the inner cladding 11a surrounding the core 10 inside the trench layer 13 can be considered as the predetermined layer, the starting substrate as the core glass body 10P, and the inner cladding glass body 11Pa as the predetermined glass body. In this way, when a predetermined layer is located between the core 10 and the trench layer 13, the relative refractive index difference of the predetermined glass body that becomes the predetermined layer is shallow, and the fluctuation in the relative refractive index difference can be suppressed from decreasing in manufacturing efficiency of an optical fiber preform. Alternatively, as described above, the starting substrate can be considered as a glass body including the core glass body 10P and the trench layer glass body, the outer cladding 11b as the predetermined layer, and the outer cladding glass body 11Pb as the predetermined glass body. In this case, when a predetermined layer is located outside the trench layer 13, the relative refractive index difference of the predetermined glass body that becomes the predetermined layer is shallow, and the fluctuation in the relative refractive index difference can be suppressed from decreasing in manufacturing efficiency of an optical fiber preform.

[0058] In the method for manufacturing an optical fiber glass body for producing an optical fiber having a trench layer as in this embodiment, the glass body that becomes trench layer 13 may be a predetermined glass body. In this case, the starting substrate includes a core glass body 10P and an inner cladding glass body 11Pa that surrounds core glass body 10P, and soot is deposited on the outer peripheral surface of inner cladding glass body 11Pa. In this case, inner cladding glass body 11Pa has a refractive index that is lower than that of core glass body 10P and higher than that of the predetermined glass body. In this case, the refractive index of inner cladding glass body 11Pa that becomes inner cladding 11a may be constant.

[0059] (Third embodiment) Next, a third embodiment of the present invention will be described in detail. Note that components that are the same as or equivalent to those in the first embodiment will be given the same reference numerals and will not be described again unless otherwise specified.

[0060] Fig. 8 is a diagram showing the refractive index profile of the optical fiber of this embodiment, similar to Fig. 2. The optical fiber of this embodiment differs from the optical fiber 1 shown in Fig. 1 in that it includes an intermediate cladding 14 and a trench layer 13 between the core 10 and inner cladding 11a of the optical fiber 1 shown in Fig. 1. Therefore, the optical fiber of this embodiment is a trench-type optical fiber, just like the second embodiment. However, the optical fiber of this embodiment differs from the optical fiber of the second embodiment in that the inner cladding 11a is located on the outer peripheral surface of the trench layer 13.

[0061] In a method for manufacturing an optical fiber preform for producing such a trench-type optical fiber, a rod-shaped core element glass body that will become the core element is prepared. The core element glass body is a glass body with a circular cross section, in which an intermediate clad glass body that will become the intermediate clad 14 is formed on the outer peripheral surface of a core glass body 10P that will become the core 10, and a trench layer glass body that will become the trench layer 13 is formed on the outer peripheral surface of the intermediate clad glass body. In this embodiment, in the inner clad glass body formation step P1 in the manufacturing method of an optical fiber preform of the first embodiment, soot 11Sa is deposited on the outer peripheral surface of the trench layer glass body in this core element glass body, instead of the core glass body 10P. Therefore, in this embodiment, the core element glass body is the starting substrate on which the soot 11Sa that will become the inner clad glass body 11Pa is deposited. Thereafter, other steps are performed in the same manner as in the manufacturing method of an optical fiber preform of the first embodiment. In this way, an optical fiber preform used to manufacture an optical fiber having the refractive index profile of FIG. 8 is obtained. By performing the drawing step P3 using this optical fiber preform, an optical fiber having the refractive index profile of FIG. 8 is obtained.

[0062] According to the manufacturing method of the optical fiber preform of this embodiment, when the inner cladding 11a, which is the layer around the core element in the trench-type optical fiber, is a predetermined layer, the relative refractive index difference of the predetermined glass body that becomes the predetermined layer is shallow, and the fluctuation of the relative refractive index difference is small, so that the decrease in manufacturing efficiency of the optical fiber preform can be suppressed.

[0063] In this embodiment, the intermediate cladding may be omitted, and the core 10 may be in contact with the trench layer 13. In this case, the glass body prepared in the inner cladding glass body forming step P1 is a glass body in which a trench layer glass body that will become the trench layer 13 is formed on the outer peripheral surface of a core glass body 10P that will become the core 10.

[0064] Although the present invention has been described above using the above-mentioned embodiments as examples, the present invention is not limited to these.

[0065] For example, in the above embodiment, an example was described in which the relative refractive index difference of the outer cladding glass body 11Pb was −0.20% or more and −0.05% or less, and the difference between the maximum and minimum values ​​of the relative refractive index difference of the outer cladding glass body 11Pb was 0.03% or more and 0.04% or less. However, unlike the above embodiment, the entire outer cladding glass body 11Pb may deviate from this relative refractive index difference. In this case, it can be considered that only the inner cladding 11a is the specified layer, the inner cladding glass body 11Pa is the specified glass body, and the outer cladding 11b is not the specified layer, and the outer cladding glass body 11Pb is not the specified glass body. Furthermore, some layers of the outer cladding may have this relative refractive index difference, while other layers do not. For example, the outer peripheral side of the second region glass body Pb2 may not have this relative refractive index difference. In this case, the region satisfying the above relative refractive index difference can be considered as the specified glass body that becomes the specified layer. Also, for example, only the first region glass body Pb1 may be a predetermined glass body.

[0066] In the above-described embodiments, an example has been described in which the optical fiber glass body used in the manufacture of an optical fiber is an optical fiber preform. However, the present invention is not limited to this. For example, in the first embodiment, an intermediate preform in which an inner cladding glass body 11Pa is provided on the outer peripheral surface of a core glass body 10P is an optical fiber glass body used in the manufacture of an optical fiber of the present invention. Alternatively, in the third embodiment, an intermediate preform in which an inner cladding glass body 11Pa is provided on the outer peripheral surface of a trench layer glass body is an optical fiber glass body used in the manufacture of an optical fiber of the present invention.

[0067] In view of the above, the present invention provides a method for producing a glass body for an optical fiber, which includes a predetermined glass body that will become a predetermined layer disposed outside the core 10 of the optical fiber 1, and which has a relative refractive index difference with respect to pure silica glass of between −0.20% and −0.05%, and a difference between the maximum and minimum values ​​of the relative refractive index difference of between 0.03% and 0.04%, the method comprising: a depositing step of depositing soot that will become the predetermined glass body on the outer peripheral surface of a starting substrate; a diffusion step of heating the porous glass body, on which the soot has been deposited on the starting substrate, in a fluorine-containing atmosphere to diffuse fluorine into the soot; and a sintering step of sintering the fluorine-diffused soot to form a transparent glass body, wherein the bulk density of the deposited soot decreases from the inner peripheral side to the outer peripheral side, and is 0.63 g / cm on the inner peripheral side. 3 More than 0.87g / cm 3 or less, and 0.54 g / cm on the outer periphery 3 More than 0.79g / cm 3 The difference between the maximum and minimum bulk density values ​​is 0.05 g / cm or less. 3 More than 0.09g / cm 3 In the diffusion step P12, the fluorine concentration in the atmosphere is 1.8% or more and 7.4% or less.

[0068] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0069] Example 1 A φ20 core rod glass body was prepared, made of silica glass with a chlorine concentration of 1.5 wt% and produced by the VAD method. A deposition step P11 was carried out on the outer peripheral surface of this core rod glass body, depositing soot to obtain a porous glass body. In this step, the initial deposition temperature was 1100°C, and the temperature was gradually lowered to a final deposition temperature of 1040°C. As a result, the bulk density of the soot on the inner peripheral side was 0.68 g / cm. 3 The bulk density of the soot on the outer periphery is 0.61 g / cm 3 The bulk density was calculated by measuring the thickness of each soot layer using a laser and calculating the increase in soot weight for each soot layer. Next, this porous glass body was placed in a heating furnace and heated at 1050°C for 5 hours in a 150 sccm Cl2 and 5 SLM He gas atmosphere for dehydration. Next, the heating furnace was heated at 1050°C for 8 hours in a 200 sccm SiF4 and 5 SLM He gas atmosphere for diffusion step P12, in which fluorine was added to the soot. The fluorine concentration in this atmosphere was 2.8%. After 10 hours, the SiF4 was increased to 12.5 sccm, and the atmosphere in the heating furnace was fully replaced. Then, the soot was heated at 1450°C for sintering step P13, and the soot was vitrified into a transparent glass. The fluorine concentration in the atmosphere in sintering step P13 was 0.25%. It took 20 hours from the start of the dehydration treatment to the end of sintering step P13. In this way, a glass body was obtained in which the inner clad glass body 11Pa was formed on the outer peripheral surface of the core glass body 10P.

[0070] Next, this glass body was elongated to φ25, and then subjected to a deposition step P21 to deposit soot, thereby obtaining a porous glass body. In this step, the initial deposition temperature was 1165°C, and the temperature was gradually lowered to a final deposition temperature of 1120°C. Next, a dehydration treatment, a diffusion step P22, and a sintering step P23 were performed under the same conditions as the dehydration treatment, the diffusion step P12, and the sintering step P13. In this way, an optical fiber preform was obtained. The relative refractive index difference from the inner peripheral side to the outer peripheral side of the inner cladding glass body 11Pa of this optical fiber preform is shown in Table 1 and FIG. 9. The normalized radius is the radius normalized by the thickness of the inner cladding glass body 11Pa of each optical fiber preform.

[0071] (Examples 2-7) The deposition step P11 was carried out in the same manner as in Example 1, with the initial deposition temperature and the final deposition temperature set to the temperatures shown in Table 1, to obtain a porous glass body having soot bulk densities on the inner circumferential side and on the outer circumferential side set to the bulk densities shown in Table 1. Next, the fluorine concentration in the atmosphere was set to the concentration shown in Table 1, and the diffusion step and sintering step were carried out in the same manner as in Example 1, and the subsequent steps were carried out in the same manner as in Example 1 to obtain an optical fiber preform. The relative refractive index difference from the inner circumferential side to the outer circumferential side of the inner cladding glass body 11Pa of this optical fiber preform is shown in Table 1 and FIG.

[0072] (Comparative Examples 1-6) The deposition step P11 was carried out in the same manner as in Example 1, with the initial deposition temperature and the final deposition temperature set to the temperatures shown in Table 1, to obtain a porous glass body having soot bulk densities on the inner periphery and the outer periphery as shown in Table 1. Next, the fluorine concentration in the atmosphere was set to the concentration shown in Table 1, and the diffusion step and sintering step were carried out in the same manner as in Example 1, and the subsequent steps were carried out in the same manner as in Example 1 to obtain an optical fiber preform. Note that the diffusion step in Comparative Example 6 was carried out for twice the time of Example 1. The relative refractive index difference from the inner periphery to the outer periphery of the inner cladding glass body 11Pa of this optical fiber preform is shown in Table 1 and FIG. 10. TIFF0007777461000001.tif240170

[0073] In Examples 1-7, by preparing an optical fiber preform under the conditions in Table 1, the bulk density of the soot that becomes the inner cladding glass body 11 Pa decreases from the inner circumferential side toward the outer circumferential side, and is 0.63 g / cm 3 on the inner circumferential side. 3 More than 0.87g / cm 3 or less, and 0.54 g / cm on the outer periphery 3 More than 0.79g / cm 3 The difference between the maximum and minimum bulk density values ​​is 0.05 g / cm or less. 3 More than 0.09g / cm 3It was also found that the relative refractive index difference of the inner cladding glass body 11Pa was -0.20% or more and -0.05% or less, and the difference between the maximum and minimum values ​​of the relative refractive index difference was 0.03% or more and 0.04% or less. In other words, it was found that the inner cladding glass body 11Pa could have a shallow relative refractive index, and fluctuations in the refractive index of the inner cladding glass body 11Pa could be suppressed.

[0074] On the other hand, Comparative Examples 1 and 2 resulted in a large change in the relative refractive index difference of the inner cladding glass body 11Pa. This is thought to be due to the high fluorine concentration in the sintering step P13 in Comparative Example 1, and the low fluorine concentration in the diffusion step P12 in Comparative Example 2, although the bulk density was roughly the same as in the Examples. Comparative Examples 3 and 4 resulted in an excessively large relative refractive index difference of the inner cladding glass body 11Pa. This is thought to be due to the bulk density of the deposited soot being too low. Comparative Example 5 resulted in a large change in the relative refractive index difference of the inner cladding glass body 11Pa. This is thought to be due to the fact that, although the bulk density was low, the fluorine concentration in the diffusion step P12 was too low, preventing fluorine from properly penetrating the inner periphery of the soot. Furthermore, as described above, Comparative Example 6, which achieved suitable properties, required a long time for the diffusion step P12.

[0075] From the above, it has been found that, according to the examples within the scope of the present invention, a method for manufacturing an optical fiber preform is provided that can suppress a decrease in the manufacturing efficiency of the optical fiber preform, which is an optical fiber glass body used to manufacture an optical fiber having a shallow relative refractive index difference in the region on the inner side of the cladding. [Industrial Applicability]

[0076] According to the present invention, there is provided a method for manufacturing an optical fiber glass body, which can produce an optical fiber glass body in which at least a part of a layer of a cladding glass body that becomes the cladding of an optical fiber has a shallow relative refractive index difference from pure silica glass and in which the variation in the relative refractive index difference in that layer is small, while suppressing a decrease in manufacturing efficiency, and it is expected that this method will be used in fields such as the manufacture of optical fibers. [Explanation of symbols]

[0077] 1. Optical fiber 1P···Optical fiber base material 10 cores 10P...Core glass body 11. Clad 11a... Inner cladding (inner cladding) 11b...Outer cladding 11P... Clad glass body 11Pa···Inner cladding glass body (inner cladding glass body) 11Pb...Outer cladding glass body 20. Porous glass body P1: Inner cladding glass body forming process (inner cladding glass body forming process) P2: Outer cladding glass body forming process P3...Drawing process P11,P21...Deposition process P12, P22... Diffusion process P13, P23...Sintering process

Claims

1. A method for producing a glass body for an optical fiber, the method comprising: a specific glass body that will be a specific layer disposed outside a core of an optical fiber, the specific glass body having a relative refractive index difference with respect to pure silica glass of -0.20% or more and -0.05% or less, and a difference between the maximum and minimum relative refractive index differences of 0.03% or more and 0.04% or less, the method comprising: a depositing step of depositing soot that will become the predetermined glass body on an outer peripheral surface of a starting substrate having a circular cross section; a diffusion step of heating the porous glass body, in which the soot has been deposited on the starting substrate, in a fluorine atmosphere to diffuse fluorine into the soot; a sintering step of sintering the fluorine-diffused soot to form a transparent glass; Equipped with The bulk density of the soot decreases from the inner circumferential side toward the outer circumferential side, and is 0.63 g / cm3 at the inner circumferential side. ３ 0.87g / cm or more ３ or less, and 0.54 g / cm ３ 0.79g / cm or more ３ or less, and the difference between the maximum and minimum bulk density values ​​is 0.05 g / cm ３ 0.09g / cm or more ３ is as follows: In the diffusion step, the fluorine concentration in the atmosphere is 1.8% or more and 7.4% or less.

1. A method for producing a glass body for an optical fiber, comprising:

2. In the sintering step, the fluorine concentration in the atmosphere in which the porous glass body is placed is 0.41% or less. The method for producing a glass body for an optical fiber according to claim 1 .

3. The starting substrate is a core glass body that will become the core of the optical fiber.

3. The method for producing a glass body for an optical fiber according to claim 1 or 2.

4. the starting substrate includes a core glass body that will become the core of the optical fiber, and a trench layer glass body that will become a trench layer having a refractive index lower than that of the cladding of the optical fiber and that surrounds the core glass body; The soot is deposited on the outer peripheral surface of the trench layer glass body.

3. The method for producing a glass body for an optical fiber according to claim 1 or 2.

5. the starting substrate includes a core glass body that will become the core of the optical fiber, and an inner clad glass body that surrounds the core and has a refractive index higher than that of the predetermined glass body; The soot is deposited on the outer peripheral surface of the inner cladding glass body.

3. The method for producing a glass body for an optical fiber according to claim 1 or 2.

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