Method for manufacturing glass base material for optical fiber

Abstract

To provide a method for manufacturing an optical fiber glass preform having a refractive index distribution stable in the longitudinal direction of the glass preform.SOLUTION: A method for manufacturing a glass preform comprises the steps of: depositing a porous glass preform by a gas phase method; and inserting the porous glass preform into the vessel of a sintering furnace when sintering the porous glass preform, heating the inside of the vessel by a heater installed in the periphery of the vessel to form a heating area and sintering the porous glass preform in the heating area. The sintering is started after setting the surface temperature difference in the longitudinal direction of the porous glass preform to 50°C or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a transparent vitrification process for dehydrating and sintering a porous glass preform, and more particularly to a method for producing a glass preform for an optical fiber having uniform properties in the longitudinal direction. [Background technology]

[0002] To manufacture a glass preform for optical fiber, a porous glass preform is first prepared by various methods, including vapor axial deposition (VAD) and outside vapor deposition (OVD). The porous glass preforms prepared by these methods are either an aggregate of glass particles alone or glass particles deposited on the periphery of a transparent glass rod. The porous glass preform is then heated in a chlorine gas atmosphere at 1000-1300°C for dehydration, and then further heated in a helium gas atmosphere at 1400-1600°C to produce a transparent glass preform for optical fiber.

[0003] In the VAD method, a burner is placed below a rotating vertical starting glass rod, raw material gas is injected into an oxyhydrogen flame formed by the burner, glass particles are generated by a flame hydrolysis reaction, and the generated glass particles are deposited in the axial direction of the starting rod to produce a porous glass preform.In the OVD method, for example, a burner is placed around the outer periphery of a starting glass rod rotating in a reaction vessel, raw material gas is injected into an oxyhydrogen flame formed by the burner, glass particles are generated by a flame hydrolysis reaction, and the generated glass particles are deposited around the outer periphery of the starting glass rod to produce a porous glass preform.

[0004] A typical single-mode optical fiber preform has a high-refractive-index region called the core in the center, which is often doped with Ge, which increases the refractive index of silica glass. A cladding is also formed around the core. Glass preforms are typically manufactured by first manufacturing a portion with a cladding around the core, and then adding the remaining cladding to the outside in two steps, or by multi-step manufacturing, where the cladding is added in multiple steps. In the present invention, the glass base material may be a glass base material having a core and a part of a clad, or a glass base material having a core and all of a clad.

[0005] The porous glass preform 10 is dehydrated and vitrified into a transparent glass in a sintering furnace 1 having a furnace tube 3 made of a heat-resistant material such as carbon or quartz and a heater 13 disposed around the outer periphery of the furnace tube 3. Generally, the porous glass preform 10 is inserted into the furnace tube 3, and the lid 6 of the furnace tube 3 is closed, and gas is allowed to flow through the furnace tube 3. When using a sintering furnace (zone heating furnace) 1 shown in FIG. 1A, the sintering is performed by raising and lowering, or repeating raising and lowering, the porous glass preform 10 relative to a heating region formed by a heater 13. The sintering furnace (zone heating furnace) 1 includes a furnace body 2, a furnace core tube 3, a lower gas inlet 4, an upper gas outlet 5, a lid 6, a thermocouple 11, a temperature control device 12, and a heater 13. The porous glass preform 10 is supported by a suspension rod 8 via a dummy rod 9, and is moved up and down by an elevating device 7 connected to the suspension rod 8. The lower end of the dummy rod 9 that is embedded in the porous glass preform 10 is referred to as a dummy lower end 14. The dehydration process is carried out in a mixed gas atmosphere of chlorine-based gas and inert gas flowing inside the furnace tube 3, with the temperature of the heating region set to about 1200°C. The transparent vitrification process is carried out with the temperature of the heating region set to about 1500°C.

[0006] Furthermore, by using a sintering furnace (soaking furnace) 1' in which a plurality of heaters 13 are arranged above and below as shown in FIG. 1B, the porous glass base material 10 can be processed without being raised or lowered. Patent Document 1 describes a technique for doping a porous glass base material with fluorine by heat treating the porous glass base material in a fluorine compound gas atmosphere in a zone heating furnace. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-47013 Summary of the Invention [Problem to be solved by the invention]

[0008] When the porous glass preform prepared by the above-mentioned method is heat-treated to produce a transparent glass preform, the refractive index distribution of the glass preform varies in the longitudinal direction, and the optical fiber produced from the glass preform has the problem that the optical properties such as the cutoff wavelength vary in the longitudinal direction. An object of the present invention is to provide a method for producing a glass preform for an optical fiber, which has a stable refractive index profile in the longitudinal direction of the glass preform. [Means for solving the problem]

[0009] The method for manufacturing a glass preform for optical fiber of the present invention includes a step of depositing a porous glass preform by a vapor phase method, and when sintering the porous glass preform, the method includes a step of inserting the porous glass preform into a container of a sintering furnace, heating the inside of the container with a heater installed on the outer periphery of the container to form a heating region, and sintering the porous glass preform in the heating region, and is characterized in that the sintering is started after the surface temperature difference in the longitudinal direction of the porous glass preform is reduced to 50°C or less.

[0010] It is preferable that the porous glass preform is sintered while the heating region is moved along the axial direction of the porous glass preform. It is also advisable to carry out the sintering after the minimum temperature of the surface of the porous glass base material is set to 200° C. or less. The sintering step further includes a dehydration step of chlorination and a vitrification step of transparent vitrification. A fluorine compound gas may be added to the atmospheric gas in the dehydration step, and the fluorine compound gas is preferably any one of SiF4, CF4, SF6, and C2F6. The porous glass base material is preferably doped with Ge.

[0011] Furthermore, the method for manufacturing a glass preform for optical fiber of the present invention includes a step of depositing a porous glass preform by a vapor phase method, and when sintering the porous glass preform, the method includes a step of inserting the porous glass preform into a container of a sintering furnace, heating the inside of the container with a heater installed on the outer periphery of the container to form a heating region, and sintering the porous glass preform in the heating region, and is characterized in that the time from the end of the step of depositing the porous glass preform to the start of the step of sintering the porous glass preform in the heating region is 2.5 hours or more, more preferably 5 hours or more.

[0012] Furthermore, the method for manufacturing a glass preform for optical fiber of the present invention includes a step of depositing a porous glass preform by a vapor phase method, and when sintering the porous glass preform, the method includes a step of inserting the porous glass preform into a container of a sintering furnace, heating the inside of the container with a heater installed on the outer periphery of the container to form a heating region, and sintering the porous glass preform in the heating region, and is characterized in that the time from inserting the porous glass preform into the container of the sintering furnace to starting the step of sintering the porous glass preform in the heating region is 1 hour or less, more preferably 0.5 hours or less. [Effects of the Invention]

[0013] According to the manufacturing method of the glass base material for optical fiber of the present invention, a glass base material having small variation in the refractive index distribution in the longitudinal direction of the base material (hereinafter referred to as longitudinal variation) can be obtained, and an optical fiber having small longitudinal variation in the cutoff wavelength, etc. can be obtained from the base material. [Brief explanation of the drawings]

[0014] [Figure 1A] FIG. 1 is a schematic diagram showing a sintering furnace having a zone heating furnace. [Figure 1B] FIG. 1 is a schematic diagram showing a sintering furnace having a soaking furnace with multiple heaters arranged above and below. [Figure 2]This figure compares a method in which the porous glass base material after deposition is directly transferred to the sintering furnace (right side of the figure), with a method in which the porous glass base material is temporarily stored in a storage facility to homogenize its temperature and then transferred to the sintering furnace (left side of the figure). [Figure 3A] FIG. 2 is a schematic diagram showing the positional relationship between a heater and a porous glass base material in a sintering furnace. [Figure 3B] FIG. 2 is a schematic diagram showing the positional relationship between a heater and a porous glass base material in a sintering furnace. [Figure 3C] FIG. 2 is a schematic diagram showing the positional relationship between a heater and a porous glass base material in a sintering furnace. [Figure 3D] FIG. 2 is a schematic diagram showing the positional relationship between a heater and a porous glass base material in a sintering furnace. [Figure 3E] FIG. 2 is a schematic diagram showing the positional relationship between a heater and a porous glass base material in a sintering furnace. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described in detail below, but the present invention is not limited to these and various embodiments are possible. Generally, the refractive index of the core in the center of the optical fiber glass preform is high, and the refractive index of the cladding surrounding the core is low. To form this refractive index profile, germanium (Ge) is sometimes used as a dopant to increase the refractive index of the core, and fluorine (F) is sometimes used as a dopant to decrease the refractive index of the cladding. Ge doping is performed by supplying a Ge-containing compound such as GeCl4 to a burner to generate Ge-containing glass particles, which are then deposited to form a porous glass preform with a Ge-doped core.F doping is performed, for example, by mixing a fluorine-containing gas into the atmospheric gas heated in the dehydration process.

[0016] As a result of intensive research, it was discovered that when sintering porous glass preforms manufactured using VAD or OVD, variations in the work and processing from the manufacture of the porous glass preform to the start of heat treatment in the sintering furnace affect the longitudinal variation in the refractive index distribution of the preform in the longitudinal direction. The following describes in detail the processing and operational variations of the porous glass base material.

[0017] First, it is considered that the temperature distribution on the surface of the porous glass preform at the start of the heat treatment is related to this. That is, even if the temperature of the heater that heats the core tube of the sintering furnace is kept constant and the moving speed of the porous glass preform moving through the heating region is controlled to be constant, if the temperature distribution in the longitudinal direction of the porous glass preform varies greatly, the degree of heat treatment will differ depending on the longitudinal position of the porous glass preform. It is known that during the dehydration process, some of the Ge doped in the core of the porous glass preform reacts with chlorine in the ambient gas, transforming into volatile GeCl4 and other compounds, which then volatilize. Different degrees of heat treatment of the porous glass preform result in different amounts of Ge volatilization, which affects the variation in the refractive index profile. Furthermore, when F doping is performed during the dehydration process, it is known that the higher the temperature, the faster the F doping rate tends to be. Since the amount of F doped varies depending on the degree of heat treatment of the porous glass base material, this also affects the variation in the refractive index profile.

[0018] When a porous glass preform is produced using the VAD method, glass particles generated by a burner are deposited from the top to the bottom of the starting rod in the axial direction. When the production of the porous glass preform is completed—that is, when the deposition of the glass particles ejected from the burner is completed and the body of the porous glass preform is separated from the flame of the deposition burner—the upper deposition portion, formed initially on the body of the porous glass preform, has been exposed to the burner flame for a long time, while the lower deposition portion has been exposed to the burner flame for a short time, resulting in a large temperature difference between the top and bottom of the body of the preform. For example, the surface temperature of the upper part of the porous glass preform immediately after deposition is about 30°C, but the lower part can exceed 200°C.

[0019] Therefore, when a porous glass preform immediately after being produced by the VAD method is placed in a sintering furnace and a sintering gas is flowed into the furnace core tube to start the dehydration process, longitudinal variation in the refractive index distribution of the preform is likely to occur. Therefore, when the porous glass preform is placed in a sintering furnace and sintering gas is started to flow through the furnace core tube to begin processing, it is desirable that the difference in surface temperature along the length of the straight body of the porous glass preform be 50°C or less. If the temperature exceeds 50°C, even if the sintering is performed under the same temperature and gas conditions, the refractive index profile will change due to differences in the fluorine doping amount and Ge doping amount between the high-temperature and low-temperature parts, and as a result, the optical properties such as MFD of the optical fiber produced from such a glass preform will vary along the length.

[0020] By placing the porous glass preform manufactured by the VAD method in an atmosphere with a stable temperature for a certain period of time, the temperature of the porous glass preform can be lowered and the temperature difference in the longitudinal direction of the preform can be reduced. The atmospheric temperature at this time can be set to a temperature below the surface temperature of the upper part of the porous glass preform immediately after deposition is completed, for example, room temperature of 25°C, to cool the porous glass preform. Natural cooling or rapid cooling by applying cold air can also be considered. Alternatively, the porous glass preform may be heated to a temperature equal to or higher than the surface temperature of the upper part thereof, or the temperature difference in the longitudinal direction of the porous glass preform may be reduced by blowing hot air onto the porous glass preform or by heating with a heater. Heating to too high a temperature increases the amount of Ge volatilized during sintering, reducing the doping efficiency. Therefore, it is preferable to start flowing sintering gas after the minimum surface temperature of the body of the porous glass preform is reduced to 200°C or below. This makes it possible to suppress longitudinal changes in the amount of fluorine doping and the amount of Ge volatilized during sintering. Therefore, it is preferable to homogenize the temperature of the porous glass preform in the longitudinal direction by temporarily storing it in an atmosphere-controlled storage cabinet shown on the left side of Fig. 2 and maintaining it in a clean, low-humidity atmosphere, rather than directly transferring the porous glass preform after deposition to a sintering furnace as shown on the right side of Fig. 2. This is also preferable because it can prevent foreign matter from adhering to the surface of the porous glass preform and moisture absorption.

[0021] Once placed in a sintering furnace, the porous glass preform is exposed to radiant heat from the heater or the inner wall of the furnace tube heated by the heater, and the temperature on the side closer to the heated area may gradually increase. Therefore, if the porous glass preform is left in the sintering furnace for a long time and then the dehydration process is started by flowing sintering gas, the refractive index distribution of the preform is likely to vary in the longitudinal direction. Therefore, if the porous glass preform, once placed in the sintering furnace, starts to be raised / lowered to the heating zone as soon as possible, the temperature on the side that was closest to the heating zone when placed in the furnace can be prevented from becoming too high, and the refractive index distribution shape of the preform in the longitudinal direction can be stabilized.

[0022] In addition, by extending the time from the end of the production of the porous glass base material (when the deposition of glass particles ejected from the burner is completed and the straight body of the porous glass base material is separated from the flame irradiation of the deposition burner) to the start of sintering, the longitudinal variation of the refractive index distribution can be suppressed, and the longitudinal variation of the optical properties can also be suppressed. That is, the time from when the deposition of the porous glass preform is completed and the porous glass preform is removed from the flame irradiation of the deposition burner until the porous glass preform is introduced into the furnace tube and the sintering gas starts to flow is preferably 2.5 hours or more, and more preferably 5 hours or more. Furthermore, longitudinal fluctuation can also be suppressed by shortening the time from placing the porous glass in the sintering vessel to starting sintering. The time from placing the porous glass in the sintering vessel to starting sintering should preferably be within 1 hour, and more preferably within 0.5 hours. [Example]

[0023] (Examples 1 and 2 and Comparative Examples 1 and 2) Example 1 A porous glass preform having a total length of 2000 mm was produced by the VAD method. The porous glass preform 10 was then naturally cooled for 24 hours and sintered in a sintering furnace 1 with a heater length of 300 mm. Before the start of sintering after natural cooling, the difference in surface temperature of the porous glass preform 10 in the longitudinal direction was 20°C. Figures 3A to 3E show the positional relationship between the heater 13 and the porous glass preform 10. First, with the porous glass preform 10 waiting at the top of the sintering furnace 1 (FIG. 3A), a mixed gas consisting of Cl2: 0.7 L / min, Ar: 30 L / min, and SiF4 gas: 0.1 L / min was flowed into the sintering furnace 1, and the temperature was simultaneously raised and controlled to 1300°C. Then, the porous glass preform 10 was moved downward at a speed of 10 mm / min (FIG. 3B), and a fluorine doping and dehydration process was carried out. The porous glass preform 10 was lowered to a predetermined position, and He gas was introduced into the sintering furnace 1 at 20 L / min while the porous glass preform 10 was raised to the upper end of the sintering furnace 1 (FIG. 3C). The temperature was then raised and controlled to 1500°C, and the porous glass preform 10 was moved downward from the upper end of the sintering furnace 1 at a speed of 5 mm / min (FIG. 3D). After the porous glass preform 10 was lowered to the desired position and the vitrification process was completed, the transparent porous glass preform 10 was raised (FIG. 3E). After sintering, the difference in the cutoff wavelength (maximum value - minimum value) in the longitudinal direction of the porous glass preform 10 was 1 nm, and the average MFD was 9.15 μm.

[0024] Example 2 A porous glass preform 10 with a total length of 2000 mm was produced using the VAD method. The porous glass preform 10 was then naturally cooled for 12 hours and then sintered in a soaking furnace 1' with a heater length of 1300 mm. Before the start of sintering, the surface temperature difference along the length of the porous glass preform 10 was 40°C. A mixed gas consisting of Cl2: 0.7 L / min, Ar: 30 L / min, and SiF4 gas: 0.1 L / min was flowed into the soaking furnace 1'. The temperature was simultaneously raised and controlled to 1300°C, and the preform was heated for 4 hours to perform a fluorine doping and dehydration process. Thereafter, He gas was introduced into the soaking furnace 1' at a rate of 20 L / min, and the temperature was raised and controlled to 1500°C, thereby turning the porous glass preform 10 into a transparent glass. After sintering, the cutoff wavelength had a numerical difference (maximum value-minimum value) of 2 nm in the longitudinal direction of the porous glass preform 10, and the average MFD was 9.16 μm.

[0025] (Comparative Example 1) A porous glass preform 10 having a total length of 2000 mm was manufactured by the VAD method. After manufacturing, it was immediately placed in a sintering furnace 1 having a heater length of 300 mm and left in the state shown in FIG. 3A for 3 hours before starting sintering. The difference in surface temperature of the porous glass preform 10 in the longitudinal direction before starting sintering was 300°C. Thereafter, it was sintered in the same manner as in Example 1. After sintering, the cutoff wavelength had a numerical difference (maximum value-minimum value) of 30 nm in the longitudinal direction of the porous glass preform 10, and the average MFD was 9.25 μm.

[0026] (Comparative Example 2) A porous glass preform 10 having a total length of 2000 mm was produced by the VAD method. The porous glass preform was then naturally cooled for 12 hours. The porous glass preform 10 was then placed in a soaking furnace 1' with a heater length of 1300 mm and left there for 3 hours before the start of sintering. The surface temperature difference in the longitudinal direction of the porous glass preform 10 before the start of sintering was 40°C, but there were places where the minimum temperature of the porous glass preform 10 reached 260°C. The porous glass preform 10 was then sintered in the same manner as in Example 1. After sintering, the cutoff wavelength had a numerical difference (maximum value-minimum value) of 5 nm in the longitudinal direction of the porous glass preform 10, and the average MFD was 9.34 μm. The conditions and properties of Examples 1 and 2 and Comparative Examples 1 and 2 are summarized in Table 1.

[0027] [Table 1]

[0028] When the temperature difference in the longitudinal direction of the porous glass preform 10 before the start of sintering is small as in Examples 1 and 2, the amount of fluorine doped can be made uniform in the longitudinal direction, and the amount of Ge volatilized can also be made uniform, resulting in suppression of longitudinal fluctuations in optical properties. This was the same whether the sintering furnace 1 (Example 1) had a zone heating furnace or the sintering furnace 1' (Example 2) had a soaking furnace. On the other hand, when the difference in surface temperature in the longitudinal direction of the porous glass preform 10 before the start of sintering is large as in Comparative Example 1, the amount of fluorine doped and the amount of Ge volatilized in the longitudinal direction of the porous glass preform 10 become non-uniform, and the longitudinal variation in optical properties is large. Therefore, the present invention is characterized in that the sintering is started after the difference in surface temperature in the longitudinal direction of the porous glass preform 10 is reduced to 50°C or less.

[0029] Furthermore, in Comparative Example 2, although the difference in surface temperature in the longitudinal direction before the start of sintering was small, there were some places where the minimum temperature on the surface of the porous glass preform 10 before the start of sintering exceeded 200°C, which facilitated the volatilization of Ge during sintering, resulting in a large MFD. Therefore, it is preferable to perform sintering after lowering the minimum temperature on the surface of the porous glass preform 10 to 200°C or below. Even for the porous glass base material 10 that is not doped with fluorine, the volatilization of Ge is relevant, so by applying the conditions of the present invention, it is possible to suppress the fluctuation of the optical properties in the longitudinal direction. Furthermore, in either the case where fluorine is doped uniformly in the radial direction or where there is a difference in fluorine concentration in the radial direction, by applying the conditions of the present invention, it is possible to suppress fluctuations in the optical characteristics in the longitudinal direction. Furthermore, by controlling the surface temperature, the work becomes much easier than controlling the inside of the porous glass, and the conditions of the present invention can be applied to all products.

[0030] (Examples 3 to 6 and Comparative Example 3) A porous glass preform 10 having a total length of 2000 mm was manufactured by the VAD method. It was then sintered in a sintering furnace 1 having a heater length of 300 mm. Figures 3A to 3E show the positional relationship between the heater 13 and the porous glass preform 10. First, the porous glass preform 10 shown in Fig. 3A was placed on standby at the top of a sintering furnace, and a mixed gas of Cl2: 0.7 L / min, Ar: 30 L / min, and SiF4 gas: 0.1 L / min was flowed into the furnace, and the temperature was simultaneously raised and controlled to 1300°C. Then, the porous glass preform 10 was moved downward at a speed of 10 mm / min (Fig. 3B), and a fluorine doping and dehydration process was carried out. When the porous glass preform 10 was lowered to the desired position, He gas was introduced into the sintering furnace 1 at a rate of 20 L / min, and the preform was simultaneously raised to the upper end (FIG. 3C). The temperature was then raised and controlled to 1500°C, and the porous glass preform 10 was moved downward from the upper end at a speed of 5 mm / min (FIG. 3D). After the porous glass preform 10 was lowered to the desired position and the vitrification process was completed, the transparent porous glass preform 10 was raised (FIG. 3E). In this case, the porous glass preform 10 was produced with a time period from completion of production of the porous glass preform 10 by the VAD method to the start of sintering of 28.3 hours in Example 3, 5 hours in Example 4, 3 hours in Example 5, 2.5 hours in Example 6, and 1.9 hours in Comparative Example 3. In Examples 3 to 6 and Comparative Example 3, the time period from placing the porous glass preform 10 in a sintering vessel to the start of sintering was uniformly set to 1 hour. Table 2 shows the results of Examples 3 to 6 and Comparative Example 3.

[0031] [Table 2]

[0032] In Examples 3 and 4, where the time from the end of production by the VAD method to the start of sintering was 5 hours or more, the variation in the cutoff wavelength in the longitudinal direction of the base material was small. On the other hand, in Examples 5 and 6, where the time was shorter than 5 hours, the variation in the cutoff wavelength in the longitudinal direction was larger than in Examples 3 and 4, and it can be seen from Table 2 that the variation was significantly larger in Comparative Example 3, where the time was shorter than 2.5 hours. This is thought to be because in Comparative Example 3, the temperature at the VAD end side in the longitudinal direction of the porous glass preform 10 is high, resulting in a large temperature difference from the start side, and if the time until sintering starts is short, the preform is not cooled sufficiently and the preform cannot be sufficiently heated uniformly. If the numerical difference in cutoff wavelength in the longitudinal direction of the preform (cutoff longitudinal difference) is large, there is a possibility that a region will emerge that does not meet the product specification value. Therefore, in the present invention, the time from the end of the process of depositing the porous glass preform to the start of the process of sintering the porous glass preform in the heating region is set to 2.5 hours or more, more preferably 5 hours or more.

[0033] (Examples 7 to 10 and Comparative Example 4) A porous glass preform with a total length of 2000 mm was manufactured using the VAD method, and then sintered in sintering furnace 1 with a heater length of 300 mm. First, with the porous glass preform 10 shown in Fig. 3A waiting at the top, a mixed gas consisting of Cl2: 0.7 L / min, Ar: 30 L / min, and SiF4 gas: 0.1 L / min was flowed into the sintering furnace 1, and the temperature was simultaneously raised and controlled to 1300°C. Then, the porous glass preform 10 was moved downward at a speed of 10 mm / min (Fig. 3B), and a fluorine doping and dehydration process was carried out. The porous glass preform 10 was lowered to the desired position, and He gas was introduced into the sintering furnace 1 at 20 L / min while the porous glass preform 10 was raised to the upper end of the sintering furnace 1 (FIG. 3C). The temperature was then raised and controlled to 1500°C, and the porous glass preform 10 was moved downward from the upper end of the sintering furnace 1 at a speed of 5 mm / min (FIG. 3D). After the porous glass preform 10 was lowered to the desired position and the vitrification process was completed, the transparent porous glass preform 10 was raised (FIG. 3E). In this case, the glass preforms were produced with a time period from placing the porous glass preform in the sintering furnace to the start of sintering of 0.4 hours in Example 7, 0.5 hours in Example 8, 0.8 hours in Example 9, 1.0 hour in Example 10, and 1.9 hours in Comparative Example 4. In Examples 7 to 10 and Comparative Example 4, the time period from the completion of production of the porous glass preform 10 by the VAD method to the start of sintering was uniformly 5 hours. Table 3 shows the results of Examples 7 to 10 and Comparative Example 4.

[0034] [Table 3]

[0035] In Examples 7 and 8, where the time from placing in the sintering furnace to the start of sintering was 0.5 hours or less, the variation in cutoff wavelength in the longitudinal direction of the base material was small. On the other hand, in Examples 9 and 10, where the time from placing in the sintering furnace to the start of sintering was longer than 0.5 hours, the variation in cutoff wavelength was larger than in Examples 6 and 7, and it can be seen from Table 3 that the variation was significantly larger in Comparative Example 4, where the time was longer than 1 hour. The results of Comparative Example 4 are thought to have occurred because the VAD end side of the porous glass preform 10 in the longitudinal direction was heated, creating a large temperature difference with the start side. If the numerical difference in the cutoff wavelength in the longitudinal direction of the preform is large, there is a possibility that a region will emerge that does not meet the product specification values. Therefore, in the present invention, the time from inserting the porous glass preform into the container of the sintering furnace to the start of the process of sintering the porous glass preform in the heating region is set to 1 hour or less, more preferably 0.5 hours or less.

[0036] The process of lowering the porous glass preform 10 into the sintering vessel is automated, after which manual tasks such as closing the lid 6 and connecting the piping are performed before sintering begins. Because the automated process described above takes time, workers typically perform other tasks during that time, which tends to lengthen the time from placing the preform in the sintering vessel to starting sintering. To achieve this time within one hour or even 0.5 hours, the process must be performed efficiently and without gaps. [Explanation of symbols]

[0037] Sintering furnace (zone heating furnace): 1, sintering furnace (soaking furnace) 1', furnace body: 2, furnace core tube: 3, lower gas inlet: 4, upper gas exhaust port: 5, lid: 6, lifting device: 7, hanging rod: 8, dummy rod: 9, porous glass base material: 10, thermocouple: 11, temperature control device: 12, heater: 13, dummy lower end: 14.

Claims

1. A method for manufacturing a glass base material, comprising a step of depositing a porous glass base material by a vapor phase method, and a step of, when sintering the porous glass base material, inserting the porous glass base material into a vessel of a sintering furnace, heating the inside of the vessel with a heater installed on the outer periphery of the vessel to form a heating region, and sintering the porous glass base material in the heating region, the sintering step is started after the surface temperature difference in the longitudinal direction of the porous glass base material is set to 50°C or less and the minimum surface temperature is set to 200°C or less; the time from when the porous glass base material is inserted into the vessel of the sintering furnace to when the sintering step is started is set to 1 hour or less; The method for manufacturing a glass preform for an optical fiber includes: raising the temperature of the sintering furnace while the porous glass preform is waiting at the upper end of the sintering furnace.

2. 2. The method for manufacturing a glass preform for an optical fiber according to claim 1, wherein the porous glass preform is sintered while being moved in the axial direction of the porous glass preform relative to the heating region.

3. 2. The method for manufacturing an optical fiber glass preform according to claim 1, wherein the sintering step further comprises a dehydration step of chlorinating the glass preform and a vitrification step of forming a transparent glass preform, each of which is a separate step.

4. 4. The method for producing an optical fiber glass preform according to claim 3, wherein a fluorine compound gas is added to the atmospheric gas in the dehydration step.

5. The fluorine compound gas is SiF ４ , C.F. ４ , SF ６ , C ２ F ６ 5. The method for producing an optical fiber glass preform according to claim 4, wherein either one of the following is performed:

6. 6. The method for manufacturing a glass preform for an optical fiber according to claim 1, wherein the porous glass preform is doped with Ge.

7. A method for manufacturing a glass base material, comprising a step of depositing a porous glass base material by a vapor phase method, and a step of, when sintering the porous glass base material, inserting the porous glass base material into a vessel of a sintering furnace, heating the inside of the vessel with a heater installed on the outer periphery of the vessel to form a heating region, and sintering the porous glass base material in the heating region, wherein the time from the end of the step of depositing the porous glass base material to the start of the step of sintering the porous glass base material in the heating region is 2.5 hours or more, the time from when the porous glass base material is inserted into the vessel of the sintering furnace to when the sintering step is started is set to 1 hour or less; The method for manufacturing a glass preform for an optical fiber includes: raising the temperature of the sintering furnace while the porous glass preform is waiting at the upper end of the sintering furnace.

8. 8. The method for manufacturing a glass preform for an optical fiber according to claim 7, wherein the time from the end of the step of depositing the porous glass preform to the start of the step of sintering the porous glass preform in the heating region is 5 hours or more.

9. 8. The method for manufacturing a glass preform for optical fiber according to claim 1, wherein the time from inserting the porous glass preform into a vessel of a sintering furnace to starting the process of sintering the porous glass preform in the heating region is 0.5 hours or less.

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