Method for producing fluorine-containing silica glass

Abstract

A method for producing fluorine-containing silica glass, the method having: a degassing step in which, after a porous silica glass body is inserted into a furnace core pipe installed in an airtight container, the pressure is reduced and degassing is carried out while the inside of the furnace core pipe is heated; a fluorine addition step in which a fluorine compound gas is supplied to the inside of the furnace core pipe under reduced pressure and the porous silica glass body is heat-treated; and a transparent vitrification step in which the porous silica glass body is heat-treated under reduced pressure at a temperature higher than the temperatures of the degassing step and the fluorine addition step.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing fluorine-containing silica glass. This application claims priority to Japanese Patent Application No. 2020-162357, filed on September 28, 2020, and incorporates by reference all of the contents of said Japanese Patent Application. [Background technology]

[0002] Patent Documents 1 to 3 disclose a method of doping a glass base material with fluorine by exposing the glass base material to an atmosphere containing a fluorine compound gas and an inert gas such as He, and then sintering the glass base material to make it transparent. As disclosed in Patent Document 3, in such a conventional method of producing fluorine-containing silica glass, each step is carried out with the pressure in the production apparatus set to 1 atmosphere or more.

[0003] Patent Document 4 discloses that the dehydration reaction of a glass particle aggregate is promoted by exposing the glass particle aggregate to a halogen-containing inert gas atmosphere in a heating furnace that can be decompressed to 0.1 torr or less. Patent Document 5 discloses a method for making the glass base material transparent by heating the glass base material while blowing a halogen gas such as chlorine under reduced pressure. Patent Document 6 describes a method for producing transparent glass, which includes a step of removing OH contained in a glass particle deposit by supplying a CO2-containing gas in a heating furnace that can be evacuated, and then removing CO2 from the surface of the glass particle deposit under reduced pressure. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 60-90842 [Patent Document 2] Japanese Patent Publication No. 62-153130 [Patent Document 3] Japanese Patent Publication No. 61-247633 [Patent Document 4] Japanese Patent Publication No. 56-63833 [Patent Document 5] Japanese Patent Publication No. 1-275441 [Patent Document 6] Japanese Patent Publication No. 2008-50202 Summary of the Invention

[0005] A method for producing fluorine-containing silica glass according to one embodiment of the present disclosure includes: a degassing step of inserting a porous silica glass body into a furnace tube installed in an airtight container, and then degassing the inside of the furnace tube while heating the inside of the furnace tube; a fluorine addition step of supplying a fluorine compound gas into the furnace tube under reduced pressure and heat-treating the porous silica glass body; a transparent vitrification step of heat-treating the porous silica glass body under reduced pressure at a temperature higher than the temperatures in the degassing step and the fluorine addition step; With death, In the fluorine addition step, the exhaust of the furnace tube is stopped and the heat treatment is performed. . [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a schematic diagram showing an example of a manufacturing apparatus for fluorine-containing silica glass. [Figure 2] FIG. 2 is a graph showing the relationship between time and temperature in a production example. [Figure 3] FIG. 3 is a graph showing the relative refractive index difference distribution in the radial direction of the fluorine-containing silica glass produced in Production Example 1. [Figure 4] FIG. 4 is a graph showing the relative refractive index difference distribution in the radial direction of the fluorine-containing silica glass produced in Production Examples 2 to 4. [Figure 5] FIG. 5 is a graph showing the relative refractive index difference distribution in the radial direction of the fluorine-containing silica glass produced in Production Examples 5 to 7. [Figure 6] FIG. 6 is a graph showing the relationship between temperature and pressure during temperature increase in the degassing step. [Figure 7]FIG. 7 is a graph showing the change in pressure over time in the fluorine addition step in Production Example 8. [Figure 8] FIG. 8 is a graph showing pressure changes when the temperature was kept constant in the degassing step in Production Examples 9 to 11. [Figure 9] FIG. 9 is a graph showing the change in pressure over time in the fluorine addition step in Production Example 12. DETAILED DESCRIPTION OF THE INVENTION

[0007] [Problem to be solved by this disclosure] In the methods disclosed in Patent Documents 1 to 3, as the size of the porous silica glass body (glass base material) increases, the time required for fluorine addition increases and the amounts of fluorine compound gas and inert gas used also increase significantly. In particular, when He is used as the inert gas, the increase in the amount of inert gas used causes a significant problem in terms of increased production costs.

[0008] When dehydration reactions and transparent vitrification are carried out in a vacuum vessel as in the methods disclosed in Patent Documents 4 to 6, the amount of inert gas used in these processes can be reduced. However, Patent Documents 4 to 6 do not disclose a technique for adding fluorine to a porous silica glass body in a vacuum vessel, and do not disclose any conditions for suppressing deterioration of the vacuum vessel due to fluorine compounds and for stably using the vacuum vessel for a long period of time.

[0009] An object of the present disclosure is to produce fluorine-containing silica glass with high productivity while suppressing deterioration of the container used in the production and the amount of inert gas used.

[0010] [Effects of this disclosure] According to the configuration disclosed above, fluorine-containing silica glass can be produced with high productivity while suppressing deterioration of the container used in the production and the amount of inert gas used.

[0011] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described. A method for producing fluorine-containing silica glass according to one embodiment of the present disclosure includes: a degassing step of inserting a porous silica glass body into a furnace tube installed in an airtight container, and then degassing the inside of the furnace tube while heating the inside of the furnace tube; a fluorine addition step of supplying a fluorine compound gas into the furnace tube under reduced pressure and heat-treating the porous silica glass body; a transparent vitrification step of heat-treating the porous silica glass body under reduced pressure at a temperature higher than the temperatures in the degassing step and the fluorine addition step; With death, In the fluorine addition step, the exhaust of the furnace tube is stopped and the heat treatment is performed. . According to this configuration, fluorine-containing silica glass can be produced with high productivity while suppressing deterioration of the container used in production and the amount of inert gas used. Specifically, moisture and OH groups in the porous silica glass body, which react with fluorine compound gas to produce HF gas, are desorbed in the degassing step prior to the fluorine addition step, thereby suppressing deterioration of the container. Furthermore, under reduced pressure, the fluorine compound gas easily penetrates into the interior of the porous silica glass body, improving the rate of fluorine compound addition and increasing productivity. Furthermore, since each of the above steps is performed under reduced pressure, there is no need to use an inert gas, and even if an inert gas is used, the amount of inert gas used can be reduced. The term "reduced pressure" refers to a state in which the pressure inside the furnace tube is lower than atmospheric pressure. The temperatures in the degassing step and the fluorine addition step refer to the temperatures on the surface of the furnace tube in each step. Furthermore, this configuration allows the fluorine compound gas to efficiently permeate into the porous silica glass body, and also improves the yield of the fluorine compound gas.

[0012] In the manufacturing method, The degassing step is preferably carried out at a temperature of 600°C or higher and 1200°C or lower. According to this configuration, since the degassing step is performed at a temperature of 600°C or higher, it is possible to further promote the elimination of OH groups present as silanol groups. Also, since the degassing step is performed at a temperature of 1200°C or lower, it is difficult for the porous silica glass body to be sintered and have an increased density. As a result, it is possible to prevent the fluorine compound gas from being difficult to penetrate into the porous silica glass body during the fluorine addition step.

[0013] In the manufacturing method, The maximum temperature in the degassing step is preferably 900°C or higher and 1200°C or lower. According to this configuration, the desorption reaction of moisture and OH groups proceeds more quickly at higher temperatures, so that moisture and OH groups in the porous silica glass body can be efficiently desorbed.

[0014] In the manufacturing method, The degassing step preferably has a heating time of at least 30 minutes or more at the maximum temperature. According to this configuration, the moisture and OH groups in the porous silica glass body can be more efficiently eliminated.

[0015] In the manufacturing method, The final pressure reached at the end of the degassing step is preferably less than 500 Pascals. According to this configuration, the degassing step is carried out under a sufficiently low pressure, so that the moisture and OH groups in the porous silica glass body can be more efficiently desorbed. The pressure shown here refers to the pressure inside the furnace tube.

[0016] The manufacturing method includes: In the degassing step, an inert gas may be supplied into the furnace tube. According to this configuration, impurities such as moisture desorbed from the porous silica glass body can be efficiently exhausted to the outside of the furnace tube.

[0017] The manufacturing method includes: The fluorine compound gas used in the fluorine addition step is a compound gas of a Group 14 element and fluorine, and preferably does not contain chlorine atoms or hydrogen atoms. This configuration can further reduce corrosion of the metal components of the vessel when the fluorine compound gas leaks out of the furnace tube.

[0018] The manufacturing method includes: In the fluorine addition step, the fluorine compound gas may be supplied after being diluted with an inert gas to a concentration of 1% or more and less than 100%, or the fluorine compound gas may be supplied at a concentration of 100%. According to this configuration, the pressure inside the furnace tube can be easily controlled by adjusting the concentration of the fluorine compound gas.

[0020] The manufacturing method includes: In the transparent vitrification step, it is preferable to further perform a vacuuming process in which an inert gas is supplied into the furnace tube while reducing the pressure. According to this configuration, the fluorine compound gas remaining in the furnace tube or the container can be efficiently exhausted, and adsorption of impurities into the furnace tube or the container can be suppressed.

[0021] The manufacturing method includes: At the end of the transparent vitrification step, the final pressure is preferably less than 500 Pascals. According to this configuration, for example, it is possible to reduce the number of residual bubbles in the fluorine-containing silica glass obtained by clarification.

[0022] [Details of the embodiments of the present disclosure] Hereinafter, an example of an embodiment of a method for producing fluorine-containing silica glass according to the present disclosure will be described with reference to the drawings. In this specification, directions such as upper and lower may be mentioned, but these directions are relative directions set for the convenience of explanation.

[0023] (Fluorine-containing silica glass manufacturing equipment) Fig. 1 is a schematic diagram showing an example of an apparatus for producing fluorine-containing silica glass. The apparatus 1 shown in Fig. 1 includes a vessel 10, a furnace tube 20, a furnace tube gas supply unit 31, a furnace body gas supply unit 32, a furnace tube exhaust pipe 41, a furnace tube exhaust valve 42, a furnace body exhaust pipe 43, a furnace body exhaust valve 44, a front chamber exhaust pipe 45, a front chamber exhaust valve 46, a vacuum pump 47, and an exhaust valve 48.

[0024] The vessel 10 is an airtight vessel and includes a front chamber 11, a rod 12, a gate valve 13, a furnace body 14, a heater 15, and a heat insulating material 16. A furnace tube 20 is disposed inside the furnace body 14.

[0025] An insertion hole for inserting a rod 12 is provided at the upper end of the anterior chamber 11. The rod 12 is inserted from the insertion hole into the anterior chamber 11. The upper cover of the furnace tube 20 is engaged with the lower end of the rod 12, and the porous silica glass body M is held further below that. The rod 12 is connected to an elevator (not shown) and can be raised and lowered, for example, on the central axis of the furnace tube 20.

[0026] The front chamber 11 and the furnace body 14 are made of metal such as SUS (Steel Use Stainless Steel). When the manufacturing apparatus 1 is not in use, the opening between the front chamber 11 and the furnace body 14 is closed by a gate valve 13. When the manufacturing apparatus 1 is in use, the gate valve 13 opens, creating an opening between the front chamber 11 and the furnace body 14. Then, the porous silica glass body M held by the rod 12 in the front chamber 11 descends and is inserted into the furnace core tube 20 in the furnace body 14.

[0027] The heater 15 is disposed around the furnace tube 20. The heater 15 is, for example, a resistance heating type heater. A heat insulating material 16 is disposed between the heater 15 and the furnace body 14.

[0028] The furnace muffle tube 20 has an opening on the upper side. When the porous silica glass body M is inserted into the furnace muffle tube 20, this opening is closed by an upper lid engaged with the rod 12. The furnace muffle tube 20 is made airtight when the opening of the furnace muffle tube 20 is closed by the upper lid. The furnace muffle tube 20 is preferably made of carbon, for example, from the viewpoint of suppressing deformation during high-temperature heating. Furthermore, in order to improve the airtightness of the furnace muffle tube 20, it is also preferable to apply an airtight coating (e.g., pyrolytic carbon, glassy carbon, silicon carbide, silicon nitride, etc.) to the material surface of the furnace muffle tube 20.

[0029] The muffle tube gas supply unit 31 supplies fluorine compound gas (e.g., CF4, SiF4, etc.) and inert gas (e.g., N2, He, etc.) into the muffle tube 20. The furnace body gas supply unit 32 supplies inert gas into the furnace body 14. By controlling the gas supply amounts from the muffle tube gas supply unit 31 and the furnace body gas supply unit 32, it is possible to control the pressure inside the muffle tube 20 and the exhaust of unnecessary components and the like to the muffle tube exhaust pipe 41 and the furnace body exhaust pipe 43.

[0030] The muffle tube exhaust pipe 41 is a pipe for evacuating the inside of the muffle tube 20. The furnace body exhaust pipe 43 is a pipe for evacuating the inside of the furnace body 14. The muffle tube exhaust pipe 41 is provided with a muffle tube exhaust valve 42. The furnace body exhaust pipe 43 is provided with a furnace body exhaust valve 44. These valves also control the exhaust inside the furnace body 14 and the muffle tube 20. The front chamber exhaust pipe 45 is a pipe for evacuating the inside of the front chamber 11. The front chamber exhaust pipe 45 is provided with a front chamber exhaust valve 46. The front chamber exhaust valve 46 controls the exhaust inside the front chamber 11.

[0031] The core tube exhaust pipe 41, furnace body exhaust pipe 43, and front chamber exhaust pipe 45 join downstream. A vacuum pump 47 and an exhaust valve 48 are provided in the piping downstream of the joining point of these exhaust pipes. The vacuum pump 47 is a pump for evacuating the interior of the furnace body 14, the core tube 20, and the front chamber 11 to reduce the pressure. The exhaust valve 48 is opened when exhausting. An exhaust process is carried out downstream of the exhaust valve 48.

[0032] If the exhaust gas contains a fluorine compound gas, for example, the exhaust gas is sent to a scrubbing tower. If the exhaust gas does not contain a fluorine compound gas, for example, the exhaust gas is released into the atmosphere. The exhaust gas from the front chamber 11 is released into the atmosphere, for example. The destination of the exhaust gas is controlled, for example, by an on-off valve (not shown) provided downstream of the vacuum pump 47.

[0033] In the manufacturing apparatus 1, the furnace core tube 20 is airtight, but since it is difficult to make the furnace core tube 20 completely airtight, some of the gas supplied into the furnace core tube 20 may flow into the furnace body 14.

[0034] (Method of manufacturing fluorine-containing silica glass) The method for producing fluorine-containing silica glass according to this embodiment will be described below. In the production method according to this embodiment, the above-described production apparatus 1 is used to produce fluorine-containing silica glass.

[0035] The method for producing fluorine-containing silica glass according to this embodiment includes the steps of: (1) a degassing step in which the porous silica glass body M is inserted into the furnace tube 20 installed in the airtight container 10, and then the inside of the furnace tube 20 is heated and degassed under reduced pressure; (2) a fluorine addition step of supplying a fluorine compound gas into the furnace tube 20 under reduced pressure and heat-treating the porous silica glass body M; (3) a transparent vitrification step of heat-treating the porous silica glass body M under reduced pressure at a temperature higher than the temperatures in the degassing step and the fluorine doping step; It has. In the transparent vitrification step, prior to the heat treatment for vitrifying the porous silica glass body M into transparent vitrification, a vacuuming process may be performed to reduce the pressure in the furnace tube 20 while supplying an inert gas into the furnace tube 20 .

[0036] The porous silica glass body M can be manufactured by known methods such as the vapor-phase axial deposition (VAD) method or the outside vapor deposition (OVD) method. The porous silica glass body M has a structure in which glass particles with a particle size of 0.1 to 1 μm, which are generated by the flame hydrolysis reaction of silicon compound gas (e.g., SiCl4, siloxane), are deposited on a predetermined target, and is generally called a soot body. The porous silica glass body M contains moisture as adsorbed water and silanol groups. When this moisture reacts with a fluorine compound gas, HF gas is generated. If HF gas leaks outside the furnace tube 20, it can corrode metal components in the furnace body 14. The vapor pressure of metal fluorides generated by corrosion is sufficiently low and their boiling points are above 1000°C, so they are rarely mixed into products. However, it is desirable to suppress the generation of HF gas from the viewpoint of preventing deterioration of the furnace body 14, etc.

[0037] Therefore, in the manufacturing method according to this embodiment, before the fluorine addition step, a degassing step is performed in which the inside of the furnace muffle tube 20 is heated and degassed under reduced pressure. In the degassing step, the moisture in the porous silica glass body M is desorbed by heat.

[0038] The desorption reaction of adsorbed water is promoted at 200°C or higher, and the desorption of OH groups from silanol groups is promoted at 600°C or higher. These desorption reactions proceed more rapidly at higher temperatures. Therefore, the degassing step is preferably performed at 600°C or higher, and the maximum temperature is preferably set to 900°C or higher. If the temperature in the degassing step is lower than 600°C, moisture desorption may not proceed sufficiently, resulting in increased transmission loss when the resulting optical fiber is fabricated. To further suppress transmission loss by sufficiently promoting moisture desorption, a heating time of 30 minutes or longer at the maximum temperature is preferred. On the other hand, if the temperature exceeds 1200°C, sintering of the porous silica glass body M proceeds, resulting in an increase in density. As a result, the fluorine compound gas is less likely to penetrate into the porous silica glass body M during the fluorine doping step. Therefore, the degassing step is preferably performed at 1200°C or lower.

[0039] The degassing step is also performed under reduced pressure to improve the desorption efficiency. From the viewpoint of further improving the desorption efficiency, the final pressure reached at the end of the degassing step is preferably less than 500 Pa, more preferably less than 200 Pa, and even more preferably less than 100 Pa.

[0040] Furthermore, from the viewpoint of efficiently exhausting impurities such as moisture desorbed in the degassing step to the outside of the furnace muffle tube 20 and suppressing the moisture and other impurities from re-adhering to the porous silica glass body M, the degassing step may be performed while supplying an inert gas into the furnace muffle tube 20. Here, the inert gas is not particularly limited, and N gas, Ar gas, He gas, etc. can be used. Among these, N gas is preferably used from the viewpoint of reducing manufacturing costs. The same applies to the inert gases used in other steps.

[0041] The fluorine compound gas used in the fluorine addition step is not particularly limited, but is preferably a compound gas of a Group 14 element and fluorine, specifically CF4 gas and SiF4 gas. These gases tend to be stable and difficult to decompose even at high temperatures. For example, the calculated dissociation rate of CF4 gas and SiF4 gas alone is 1 ppm or less even at 1500°C. Therefore, even if the gas leaks out of the furnace tube 20, there is little risk of corroding metal components in the furnace body 14.

[0042] CF4 is superior to SiF4 in terms of stability, since it is less likely to react with even trace amounts of moisture. On the other hand, CF4 generates CO2 as a by-product of the fluorine addition reaction to SiO2. The generated CO2 easily reacts with the carbon material that constitutes the furnace tube 20 at high temperatures of 1100°C or higher, so SiF4 is preferable from the perspective of preventing deterioration of the furnace tube 20. SF6 gas may also be used, but SF6 gas reacts with SiO2 to generate SO2. x When the gas becomes H2SO4, it is more likely to cause corrosion of metal parts, making it inferior to CF4 gas and SiF4 gas in terms of long-term stable use.

[0043] Furthermore, it is preferable that the fluorine compound gas does not contain chlorine atoms or hydrogen atoms. If gas containing chlorine or hydrogen atoms leaks out of the furnace tube 20 and reacts with metal components in the furnace body 14, and the metal impurities released from the metal components enter the furnace tube 20, the characteristics of the optical fiber obtained from the fluorine-containing silica glass product may be adversely affected. In particular, even a transition metal impurity content of 1 ppb can significantly degrade the loss characteristics of the optical fiber. Chlorine is a typical example of an element that can react with metal components to form metal impurities with high vapor pressure. Iron chloride III (FeCl3), in particular, has a boiling point in the 300°C range and a high vapor pressure, making it one of the components that easily contaminates products. Furthermore, fluorine compound gases containing chlorine atoms (e.g., CCl2F2) decompose relatively easily to form Cl2 gas, which can easily cause corrosion in the furnace body 14.

[0044] As shown in Patent Document 3, the amount of fluorine added to the porous silica glass body M is proportional to the ¼ power of the partial pressure of the fluorine compound gas during heat treatment. When a fluorine compound gas is supplied under reduced pressure, the partial pressure of the fluorine compound gas can be controlled by the pressure inside the furnace tube 20, making it possible to control the amount of fluorine added even with a 100% concentration of fluorine compound gas. However, the pressure range that can be easily controlled may vary depending on the structure of the furnace tube 20. To adjust this, a mixed gas obtained by mixing a fluorine compound gas with an inert gas may be used. In this case, it is preferable to supply the fluorine compound gas diluted with the inert gas to a concentration (volume concentration) of 1% or more but less than 100%. By controlling the concentration ratio of the fluorine compound gas flow rate to the inert gas flow rate, the fluorine concentration added to the porous silica glass body M can be adjusted to a desired concentration. Note that under reduced pressure, the fluorine compound gas easily penetrates deep into the porous silica glass body M, thereby increasing the rate of addition of the fluorine compound without having to consider interdiffusion, as occurs when supplying a mixed gas under normal pressure.

[0045] Furthermore, in the manufacturing apparatus 1, a desired partial pressure of the fluorine compound gas can be achieved by supplying a predetermined amount of gas into the furnace tube 20 while closing a valve provided in the exhaust pipe 41. In this case, the fluorine compound gas is confined within the furnace tube 20, and therefore the amount of fluorine compound gas used can be reduced.

[0046] The vacuuming process is carried out to efficiently exhaust the fluorine compound gas remaining in the furnace tube 20, the furnace body 14, etc., and to suppress the adsorption of impurities to the furnace tube 20 and the furnace body 14. Although it is preferable to carry out the vacuuming process, it is not necessary to carry out the vacuuming process because the pressure inside the furnace tube 20 is reduced at the time of the fluorine addition step.

[0047] By carrying out the transparent vitrification step under reduced pressure, residual bubbles in the product can be suppressed even if the porous silica glass body M is large. From the viewpoint of suppressing residual bubbles, the final pressure reached at the end of the transparent vitrification step is preferably less than 500 Pascals, more preferably less than 200 Pascals, and even more preferably less than 100 Pascals.

[0048] The temperature in the transparent vitrification step is not particularly limited as long as it is higher than the temperatures in the degassing step and the fluorine doping step. However, from the viewpoint of sufficiently sintering the porous silica glass body M in a short time, a temperature of 1250°C or higher is preferable, 1300°C or higher is more preferable, and 1320°C or higher is even more preferable. However, since the viscosity of the glass is reduced by the addition of fluorine to the glass base material, if the temperature in the transparent vitrification step is too high, the glass base material may stretch under its own weight. Therefore, an appropriate temperature of 1400°C or lower must be selected. By performing the transparent vitrification step under reduced pressure, residual bubbles can be reduced, and when forming a fiber in the subsequent fiber drawing step, a decrease in yield due to diameter fluctuations caused by residual bubbles can be suppressed. Furthermore, an annealing step or the like to remove residual bubbles is not required before the fiber drawing step.

[0049] In the transparent vitrification step, the supply of the fluorine compound gas may be continued or stopped. While the flow of the fluorine compound gas has the advantage of increasing the amount of fluorine added to the outer periphery of the porous body that will become the cladding of the optical fiber, it also increases the risk of damaging the furnace tube 20 and other components.

[0050] The fluorine compound gas remaining in the clarified fluorine-containing silica glass and the furnace muffle tube 20 can be exhausted to the outside of the reaction system by subjecting the furnace muffle tube 20 to a vacuum treatment (exhaust by the vacuum pump 47), and if the final pressure can be kept low at this time, the amount of residual bubbles in the clarified fluorine-containing silica glass can be reduced. Furthermore, by exhausting the fluorine compound gas while supplying an inert gas, adsorbed components in the reaction system can be efficiently desorbed, thereby suppressing damage to the furnace muffle tube 20.

[0051] If the fluorine compound gas is not present around the porous silica glass body M during transparent vitrification, the fluorine added to the outside of the porous silica glass body M will be released, increasing the refractive index of that area. However, in optical fiber applications, even if the refractive index of the outer periphery of the cladding increases slightly, this will have little effect on the optical transmission characteristics.

[0052] [Example] The present disclosure will be described in more detail below by showing Production Examples 1 to 12 as examples of the present disclosure. However, the present disclosure is not limited to the following examples.

[0053] Using the manufacturing apparatus 1, fluorine-containing silica glass in each manufacturing example was manufactured under the conditions shown in Table 1 below. The "temperature" in Table 1 refers to the surface temperature of the furnace tube 20. In even-numbered steps, the temperature was increased, decreased, or maintained, while in other steps, the temperature shown in Table 1 was maintained. The "time" in Table 1 refers to the time required for each step. "CF4" and "N2" in Table 1 indicate the amounts of CF4 gas and N2 gas supplied, respectively, in each step, with "none" indicating no supply. The unit of supply amount, "slm," is an abbreviation for "standard litre per minute," which is the flow rate per minute under standard conditions (0°C, 1 atm). In Table 1, "exhaust" indicates that the furnace tube 20 was evacuated, while "none" indicates that no evacuation was performed. In steps 5 to 7, evacuation was not performed, and CF4 gas was sealed inside the furnace tube 20. Step 8 is a vacuum process.

[0054] [Table 1]

[0055] FIG. 2 is a graph showing an example of the relationship between time and temperature shown in Table 1. Specifically, FIG. 2 is a graph corresponding to Production Example 1 below. The degassing step S1 in FIG. 2 corresponds to steps 1 to 4 in Table 1. The fluorine addition step S2 in FIG. 2 corresponds to steps 5 to 7 in Table 1. Note that, as described above, evacuation is not performed in the fluorine addition step S2. The transparent vitrification step S3 in FIG. 2 corresponds to steps 8 to 11 in Table 1.

[0056] (Production Example 1) In Production Example 1, a soot body (porous silica glass body) was used for processing, in which a porous glass layer was deposited by the VAD method on a starting rod with a core and cladding structure. Among the conditions listed in Table 1, the temperature in Step 3 was 1100°C, the time was 120 minutes, and the temperature in Step 5 was 1100°C, the time was 20 minutes, and the CF4 supply amount was 5 slm. The total CF4 supply amount was 100 liters. Steps 6 and 7 were at the same temperature as Step 5, and the processing time for Step 7 was 240 minutes. The CF4 supply amount in Steps 6 and 7 was 0 slm. The temperature rise time in Step 8 was 100 minutes, the holding time for Step 9 was 1300°C and 70 minutes, the temperature rise time for Step 10 was 90 minutes, and the temperature in Step 11 was 1350°C. The soot body was made transparent to produce transparent silica glass.

[0057] The relative refractive index difference of the transparent silica glass produced in Production Example 1 is shown in Figure 3. In Figure 3, the area up to a radius of ±25 mm represents the starting rod, and the area from ±25 to 65 mm represents the fluorine-doped silica glass produced in this Production Example. The fluorine doping amount, converted into a refractive index difference, was 0.26% in the inner part and 0.18% in the outer periphery, and these were doping amounts and refractive index distributions that would not pose any problems in terms of characteristics for an optical fiber preform.

[0058] (Manufacturing Examples 2 to 4) As in Production Example 1, a soot body was used for processing, in which a porous glass layer was deposited by the VAD method on a starting rod with a core and cladding structure. In Production Examples 2 to 4, the CF4 supply amount in Step 5 was 5 slm, and the required time was 15, 30, or 45 minutes, respectively. The transparent silica glass of Production Examples 2 to 4 was produced in the same manner as Production Example 1, except that the time for Step 7 was 480 minutes. Figure 4 shows the relative refractive index differences of the transparent silica glass of Production Examples 2 to 4. As shown in Figure 4, the fluorine doping amount can be adjusted by changing the total amount of gas supplied. Furthermore, by extending the processing time of Step 5, the decrease in the fluorine doping amount in the outer periphery can be suppressed compared to Production Example 1, as shown in Figure 3.

[0059] (Production Examples 5 to 7) As in Production Example 1, a soot body was used for processing, in which a porous glass layer was deposited by the VAD method on a starting rod with a core and cladding structure. In step 5, the CF4 supply rate was 5 slm and the required time was 45 minutes. In Production Examples 5 to 7, the set temperature in steps 5 to 7 was changed to three levels: 1050, 1100, and 1150°C, with the other conditions being the same as in Production Examples 2 to 4. The relative refractive index differences of the transparent silica glass in Production Examples 5 to 7 are shown in Figure 5. As shown in Figure 5, the amount of fluorine added can be adjusted by changing the temperature in the fluorine addition process.

[0060] (Production Example 8) In Production Example 8, the conditions in the degassing step were adjusted. In Production Example 8, the conditions were the same as in Production Example 1, except that the temperature in Step 3 in Table 1 was set to 1200°C. Figure 6 shows the change in furnace pressure as the temperature was increased. Figure 6 shows that the furnace pressure increases as the temperature increases, but becomes constant near 1100°C and tends to decrease above 1150°C. This indicates that the furnace pressure increases due to H2O desorbed from the soot body, but maintaining the temperature at a high temperature reduces the moisture in the soot body, and therefore no longer contributes to the increase in pressure.

[0061] Next, Figure 7 shows the pressure change during the fluorine addition process. The pressure was approximately 10 kPa when a total of 100 liters of CF4 was introduced into the furnace, and increased as the treatment time progressed, reaching 21 kPa after 240 minutes. This is due to the combined effect of the reaction between CF4 and SiO2 to form SiF4 and CO2, and the reaction between CO2 and carbon in the furnace tube to form CO gas.

[0062] With the changes in conditions during the degassing process, the refractive index distribution was roughly the same as that shown in Figure 3, and increasing the temperature during the degassing process had almost no effect on the amount of fluorine added.

[0063] (Examples 9-11) In Production Examples 9 to 11, the conditions were the same as in Production Example 1, except that the temperatures in Step 3 in Table 1 were set to 1000, 1050, and 1100°C, respectively. Figure 8 shows the change in furnace pressure when the temperature was held after heating. Figure 8 shows that the furnace pressure is held at a constant pressure at the beginning of the holding time and then begins to decrease. It can also be seen that the higher the temperature, the shorter the time held at a constant pressure, and the lower the final pressure reached after a holding time of 2 hours. This indicates that the increase in furnace pressure due to H2O desorbed from the soot body is greater at higher temperatures, and that the time required to reduce the moisture content of the soot body can be shortened.

[0064] Even with the changes in conditions during the degassing process, the refractive index distribution was roughly the same as that shown in Figure 3, and changing the temperature during the degassing process had almost no effect on the amount of fluorine added. However, in the treatment at 1000°C, SiO2 powder was found to adhere to part of the anterior chamber after the base material was produced, which is presumed to have been due to the influence of residual H2O.

[0065] (Production Example 12) Transparent silica glass for Production Example 12 was produced in the same manner as Production Example 1, except that the conditions listed in Table 1 were changed to those listed in Table 2 below. In Production Example 12, the fluorine compound gas was SiF4. Figure 9 shows the pressure change during the fluorine addition process. The pressure was approximately 10 KPa when a total of 100 L of SiF4 was introduced into the furnace, and it decreased over the treatment time, eventually reaching a pressure of approximately 4 KPa. This corresponds to the reaction between SiF4 and SiO2, which adds fluorine to the soot, consuming SiF4 and reducing the number of gas molecules in the furnace, resulting in a decrease in pressure. The refractive index profile obtained was roughly the same as that shown in Figure 3, and the change in fluorine compound had little effect on the amount of fluorine added.

[0066] [Table 2]

[0067] As shown in FIGS. 3 to 9, the transparent silica glasses obtained in Production Examples 1 to 12 all contained sufficient fluorine in the cladding portion.

[0068] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Furthermore, the number, position, shape, etc. of the components described above are not limited to the above embodiments, and can be changed to any number, position, shape, etc. that is suitable for implementing the present invention. [Explanation of symbols]

[0069] 1: Manufacturing equipment 10: Container 11: Front room 12: Rod 13: Gate valve 14:Furnace body 15: Heater 16:Insulation material 20: Furnace tube 31: Furnace tube gas supply section 32: Furnace gas supply section 41: Furnace core tube exhaust pipe 42: Furnace tube exhaust valve 43: Furnace exhaust pipe 44: Furnace exhaust valve 45: Front chamber exhaust pipe 46: Front chamber exhaust valve 47: Vacuum pump 48: Exhaust valve S1: Degassing process S2: Fluorine addition process S3: Transparent vitrification process M: Porous silica glass body

Claims

1. a degassing step of inserting a porous silica glass body into a furnace tube installed in an airtight container, and then degassing the inside of the furnace tube while heating the inside of the furnace tube; a fluorine addition step of supplying a fluorine compound gas into the furnace tube under reduced pressure and heat-treating the porous silica glass body; a transparent vitrification step of heat-treating the porous silica glass body under reduced pressure at a temperature higher than the temperatures in the degassing step and the fluorine addition step; and A method for producing fluorine-containing silica glass, wherein the heat treatment is performed while the evacuation of the furnace tube is stopped in the fluorine addition step.

2. The method according to claim 1 , wherein the degassing step is carried out at a temperature of 600° C. or higher and 1200° C. or lower.

3. 3. The method according to claim 1, wherein the maximum temperature in the degassing step is 900°C or higher and 1200°C or lower.

4. The method according to claim 3 , wherein the degassing step has a heating time of at least 30 minutes or more at the maximum temperature.

5. The method according to claim 1 , wherein the final pressure reached at the end of the degassing step is less than 500 Pascals.

6. The manufacturing method according to claim 1 , wherein an inert gas is supplied into the furnace tube in the degassing step.

7. 7. The method according to claim 1, wherein the fluorine compound gas used in the fluorine addition step is a compound gas of a Group 14 element and fluorine, and does not contain chlorine atoms or hydrogen atoms.

8. 8. The manufacturing method according to claim 1, wherein in the fluorine addition step, the fluorine compound gas is supplied after being diluted with an inert gas to a concentration of 1% or more but less than 100%, or the fluorine compound gas is supplied at a concentration of 100%.

9. A manufacturing method described in any one of claims 1 to 8, wherein the transparent vitrification process further comprises a vacuum process in which an inert gas is supplied into the furnace tube while reducing the pressure.

10. The method according to claim 1 , wherein the final pressure reached at the end of the transparent vitrification step is less than 500 Pascals.

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