JP2024141315A - Method for manufacturing silica glass and method for manufacturing optical fiber - Google Patents

Abstract

To provide a method for manufacturing silica glass that can manufacture silica glass by suppressing formation of flaws in a glass body with a simple method.SOLUTION: A method for manufacturing silica glass includes the steps of: providing a porous silica glass body M; heating the porous silica glass body in a fluorine-containing gas atmosphere; and transparently vitrifying the porous silica glass body M at a temperature of 1300°C or higher after the heating step.SELECTED DRAWING: Figure 2

Description

This disclosure relates to a method for manufacturing silica glass and a method for manufacturing optical fiber.

Non-Patent Document 1 discloses a method for producing an optical fiber preform by depositing glass particles formed by a flame hydrolysis reaction to form a porous body, and then heat-treating the porous body to convert it into transparent glass. Non-Patent Document 2 discloses a method for forming a porous body by pressure molding a silica glass powder that has been synthesized in advance. Patent Document 1 discloses a method for forming a non-porous body by depositing silica glass particles.

Japanese Patent Application Publication No. 5-229833

Hiroo Kanamori, "Optical Fiber: Half a Century of Evolution and Latest Trends," Applied Physics, Vol. 91, No. 4, p. 205-215, 2022 Kazuaki Yoshida et al., "Preparation of optical fiber base material by pressure molding method", Journal of the Ceramic Society of Japan Vol. 104, No. 6, p524-528, 1996

In the method described in Non-Patent Document 1, etc., impurity components in the raw materials or atmosphere may be mixed into the porous body, and defects such as bubbles or microcrystals may be formed in the glass body when the porous body is heat-treated to form a dense and transparent glass body. The impurity components in the flame hydrolysis method are carbon particles caused by dust in the atmosphere, or metal particles such as alkali and iron. For example, when an organic silicon material (e.g., siloxanes) is used as the raw material gas in the flame hydrolysis method, carbon particles caused by incomplete combustion may be mixed into the porous body. The impurity components in the powder molding method are dust components mixed in during powder molding, or binder components intentionally mixed in to improve the strength of the molded body. The binder components are, for example, liquid cellulose (carboxymethyl cellulose (CMC) solution) or polyvinyl acetate (PVA) solution.

The carbon impurity component can be removed by, for example, placing the porous body in an oxygen-containing atmosphere (a mixed gas of oxygen and nitrogen or air) and heating it to 500°C to 1000°C to oxidize the carbon component. However, when the porous body is large, the diffusion of oxygen molecules becomes insufficient, and the carbon component may remain. On the other hand, for impurity components that are metal components, the vapor pressure of the metal oxide must be higher than the transparent temperature of the silica porous body, so there is a possibility that metal particles will not be removed even if the heat treatment is performed as described above. Furthermore, when exposed materials (large components that are shape-stable at high temperatures, such as carbon) that cannot be used in an oxygen atmosphere are used in the device, the conditions for using oxygen must be significantly relaxed. In this case, heating must be performed up to 450°C, which is the temperature at which graphite begins to wear out due to oxidation, and there is a possibility that sufficient oxidation cannot be performed.

One method used to remove impurities, which are metal components, is to perform heat treatment in an atmosphere of halogen gas such as chlorine gas to halogenate the metal components, thereby reducing the vapor pressure and removing them by gasification. This halogenation removal method is effective for removing metal components, but the heat treatment must be performed in an airtight tube that is airtight and halogen-resistant in order to treat the atmospheric gas, which increases the equipment costs.

The present disclosure aims to provide a method for manufacturing silica glass that can easily suppress the formation of defects in the glass body.

A method for producing silica glass according to one embodiment of the present disclosure includes the steps of preparing a porous silica glass body, heating the porous silica glass body in an atmosphere of a gas containing fluorine, and converting the porous silica glass body into transparent glass at a temperature of 1300°C or higher after the heating step.

According to the present disclosure, it is possible to produce silica glass by suppressing the formation of defects in the glass body in a simple manner.

FIG. 1 is a schematic diagram showing an example of an apparatus for heating a porous silica glass body to convert it into transparent glass. FIG. 2 is a diagram for explaining the method for producing silica glass. FIG. 3 is a graph showing the relationship between the production time and the heating temperature in Experimental Example 1. FIG. 4 is a graph showing the distribution of the relative refractive index difference in the radial direction of the silica glass produced in Experimental Example 1. FIG. 5 is a graph showing the radial distribution of the relative refractive index difference of the silica glass produced in Experimental Examples 3, 4, and 5. FIG. 6 is a graph showing the radial distribution of the relative refractive index difference of the silica glasses manufactured in Experimental Examples 6, 7, and 8. FIG. 7 is a schematic diagram showing another example of an apparatus for heating a porous silica glass body to convert it into transparent glass.

[Description of the embodiments of the present disclosure]
First, the contents of the embodiments of the present disclosure will be listed and described.
[1] A method for producing silica glass according to one embodiment of the present disclosure includes the steps of preparing a porous silica glass body, heating the porous silica glass body in an atmosphere of a gas containing fluorine, and converting the porous silica glass body into transparent glass at a temperature of 1300° C. or higher after the heating step.

In this method for producing silica glass, before the porous silica glass body is vitrified to a transparent state, the porous silica glass body is heated under an atmosphere of a gas containing fluorine. In this case, before the transparent vitrification, silica (SiO ₂ ) constituting the porous silica glass body reacts with a gas containing fluorine (e.g., CF ₄ or CHF ₃ ) to form an oxygen compound (e.g., CO ₂ ). Then, the carbon impurities (e.g., C) in the porous glass body are reacted with this oxygen compound, so that the carbon impurities in the porous silica glass body are removed. If moisture remains in the porous silica glass body, the moisture (H ₂ O) reacts with a gas containing fluorine to form an oxygen compound, and similarly to the above, the carbon impurities in the porous silica glass body are reacted with the oxygen compound to remove the carbon impurities. In addition, in the above method for producing silica glass, before the transparent vitrification, metal impurities (e.g., metal oxides such as alkalis Na ₂ O or Al ₂ O ₃ ) in the porous silica glass body react with a gas containing fluorine to form fluorides (e.g., NaF or AlF ₃ ) that are easily melted, so that the metal oxides are removed. As described above, according to this method for producing silica glass, various impurities can be removed by a simple method of heating the porous silica glass body under a fluorine-containing gas atmosphere before it is vitrified into a transparent glass. As a result, according to this method, the formation of defects in the glass body can be suppressed in a simple manner.

In addition, in the above-mentioned method for producing silica glass, the porous silica glass body is heated in an atmosphere of a gas containing fluorine before being converted into transparent glass. In this case, the sintering driving force (surface free energy) during transparent glass formation can be increased due to the effect of replacing oxygen or hydroxyl groups on the surface of the porous silica glass body with fluorine (F) and the surface activation by surface etching. As a result, according to this method for producing silica glass, the heating temperature during transparent glass formation can be lowered, reducing energy consumption.

[2] In the method for producing silica glass according to [1] above, the fluorine-containing gas may not contain any halogen other than fluorine. In this case, since no halogen other than fluorine is contained, the silica, carbon impurities, and metal impurities constituting the porous silica glass body react more reliably with the fluorine-containing gas, and the carbon impurities and metal impurities are more reliably removed. When halogen elements other than fluorine (chlorine (Cl), bromine (Br), iodine (I)) are contained, the bond energy between these halogen elements and carbon is weaker than the bond energy between fluorine and carbon, so that halogen gas (e.g., Cl ₂ ) is easily generated, and the carbon component is more likely to remain. In this way, various impurities are more reliably removed, so that the formation of defects in the glass body can be more reliably suppressed. In addition, when halogens other than fluorine are contained, the constituent materials of the furnace also need to have halogen corrosion resistance, which may cause an increase in equipment costs.

[3] In the method for producing silica glass according to [1] or [2] above, the fluorine-containing gas may contain at least one fluorine compound gas selected from the group consisting of perfluorocarbon and hydrofluorocarbon.

[4] In any of the above methods for producing silica glass [1] to [3], in the heating step, a fluorine-containing gas may react with silicon dioxide constituting the porous silica glass to form an oxygen-containing gas. The oxygen-containing gas may contain at least one of carbon dioxide and water vapor. In this case, the carbon dioxide or water vapor reacts with the carbon impurities in the porous silica glass body, and the carbon impurities in the porous silica glass body are more reliably removed. This makes it possible to reliably suppress the formation of defects in the glass body after it has been made into a transparent vitrified body.

[5] In any of the above methods for producing silica glass [1] to [4], in the heating step, a fluorine-containing gas may react with the silicon dioxide that constitutes the porous silica glass. The heating temperature in the heating step may be equal to or lower than the temperature at which the fluorine in the fluorine-containing gas is added to the silicon dioxide. In this case, the addition of fluorine to the porous silica glass body before sintering is suppressed, and the refractive index, etc. of the glass body in which the porous silica glass is transparently vitrified can be more reliably set to a desired value.

[6] In any of the above methods for producing silica glass [1] to [5], the heating temperature in the heating step may be 800°C or higher and 1100°C or lower. In this case, the balance between the generation of oxygen compound gas caused by the dissociation of fluorine-containing gas and reaction with silica glass particles in the porous silica glass body before sintering and the addition of the dissociated fluorine to the silica glass is excellent, making it easier to remove carbon components using the oxygen compound gas and to suppress the addition of fluorine. This makes it possible to more reliably achieve the desired refractive index and other values in the glass body in which the porous silica glass body has been transparently vitrified.

[7] In the method for producing silica glass described in [6] above, the heating temperature in the heating step may be 950°C or higher and 1050°C or lower. In this case, it becomes easier to remove carbon components using an oxygen compound gas and to suppress the addition of fluorine. This makes it possible to make the refractive index and other properties of the glass body in which the porous silica glass has been made into a transparent vitrified material even more desirable.

[8] In any of the above methods for producing silica glass [1] to [7], in the heating step, the total pressure in the furnace tube in which the porous silica glass is placed may be reduced to 1 kPa or less, and then a fluorine-containing gas may be introduced into the furnace tube until the pressure change ΔP becomes greater than 1 kPa to heat the porous silica glass body. In this case, the silica and metal impurities constituting the porous silica glass body react more reliably with the fluorine-containing gas, and carbon impurities and metal impurities are more reliably removed. By more reliably removing various impurities in this way, the formation of defects in the glass body can be more reliably suppressed.

[9] The method for producing silica glass according to any one of [1] to [8] above may further include a step of introducing an inert gas into the furnace tube while heating the furnace tube in which the porous silica glass body is placed, prior to the heating step. The heating temperature in the heating step may be lower than the heating temperature in the introducing step. In this case, the introduction of the inert gas while heating the porous silica glass body can more reliably remove moisture from the porous silica glass body.

[10] An optical fiber manufacturing method according to an embodiment of the present disclosure includes a step of producing an optical fiber preform using any one of the silica glass manufacturing methods [1] to [9] above, and a step of drawing the optical fiber preform to produce an optical fiber. In this case, various impurities are removed from the silica glass that is to be the optical fiber preform, suppressing defects, so that it is possible to reduce the outer diameter fluctuation and breakage of the glass caused by impurities during the process of processing into an optical fiber, and it becomes possible to manufacture an optical fiber with stable quality.

[Details of the embodiment of the present disclosure]
A specific example of a method for producing silica glass according to an embodiment of the present disclosure will be described below with reference to the drawings. In the following description, the same elements or elements having the same functions will be designated by the same reference numerals, and duplicated descriptions will be omitted. Note that the present invention is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

With reference to Figure 1, a manufacturing apparatus used for manufacturing silica glass will be described. Figure 1 is a schematic diagram showing an example of an apparatus for heating a porous silica glass body to turn it into transparent glass. The manufacturing apparatus 1 shown in Figure 1 includes a container 10, a furnace tube 20, a gas supply unit 30, and an exhaust system 40.

The container 10 is an airtight container. The container 10 has 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 core tube 20 is arranged inside the furnace body 14. An insertion hole 11a through which the rod 12 passes is provided at the upper end of the front chamber 11. The rod 12 is inserted into the front chamber 11 from the insertion hole 11a. An upper lid 22 is engaged with the lower end of the rod 12, and a porous silica glass body M is held further below. The rod 12 is connected to a lifting device (not shown) and can be raised and lowered, for example, on the central axis of the furnace core tube 20.

The front chamber 11 and the furnace body 14 are made of metal, such as SUS (Steel Use Stainless). When the manufacturing device 1 is not in use, the opening between the front chamber 11 and the furnace body 14 is closed by the gate valve 13. When the manufacturing device 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.

The heater 15 is a heating member for heating the porous silica glass body M arranged in the furnace core tube 20. The heater 15 is composed of, for example, a plurality of heating bodies 15a, and is arranged around the furnace core tube 20 so as to heat an area longer than the entire length of the porous silica glass body M. The heater 15 is, for example, a resistance heating type heater. By heating the furnace core tube 20 with the heater 15, various gases (for example, N ₂ , CF ₄ ) introduced into the furnace core tube 20 are also heated. A heat insulating material 16 may be arranged between the heater 15 and the furnace body 14. This can improve the heating efficiency of the furnace core tube 20 (including the porous silica glass body M and the introduced gas) by the heater 15, and makes it difficult for the heat from the heater 15 to be transmitted to the outside of the furnace body 14.

The core tube 20 is a member for heating the porous silica glass body M to vitrify it transparently, and has an opening 21 on the upper side for inserting the porous silica glass body M. When the porous silica glass body M is inserted into the core tube 20, the opening 21 is closed by the top cover 22 that engages with the rod 12. The core tube 20 becomes airtight when the opening 21 of the core tube 20 is closed by the top cover 22. The core tube 20 may be made of carbon, for example, from the viewpoint of suppressing deformation during high-temperature heating. In order to improve the airtightness of the core tube 20, an airtight coating (for example, pyrolytic carbon, glassy carbon, silicon carbide, silicon nitride) may be applied to the material surface of the core tube 20.

The gas supply unit 30 has a first gas supply unit 31 and a second gas supply unit 32. The first gas supply unit 31 supplies a fluorine compound gas (e.g., _CF4 or _CHF3 ) or an inert gas (e.g., _N2 or He) into the furnace tube 20. The second gas supply unit 32 supplies an inert gas into the furnace body 14. By controlling the amount of gas supplied from the first gas supply unit 31 and the second gas supply unit 32, the pressure inside the furnace tube 20 and the exhaust of unnecessary components and the like to the first exhaust pipe 41 and the second exhaust pipe 43 can be controlled.

The exhaust system 40 includes a first exhaust pipe 41, a first exhaust valve 42, a second exhaust pipe 43, a second exhaust valve 44, a third exhaust pipe 45, a third exhaust valve 46, a vacuum pump 47, and a fourth exhaust valve 48.

The first exhaust pipe 41 is a pipe for exhausting the inside of the core tube 20, and is connected airtight to the core tube 20. The second exhaust pipe 43 is a pipe for exhausting the inside of the furnace body 14, and is connected airtight to the furnace body 14. The first exhaust pipe 41 is provided with a first exhaust valve 42. The second exhaust pipe 43 is provided with a second exhaust valve 44. The first exhaust valve 42 and the second exhaust valve 44 also control the pressure and exhaust in the furnace body 14 and the core tube 20. The third exhaust pipe 45 is a pipe for exhausting the inside of the front chamber 11, and is connected airtight to the front chamber 11. The third exhaust pipe 45 is provided with a third exhaust valve 46. The third exhaust valve 46 controls the pressure and exhaust in the front chamber 11.

The first exhaust pipe 41, the second exhaust pipe 43, and the third exhaust pipe 45 join downstream. A vacuum pump 47 and a fourth exhaust valve 48 are connected to the piping downstream of the joining point of the first exhaust pipe 41, the second exhaust pipe 43, and the third exhaust pipe 45. The vacuum pump 47 is a pump for evacuating the inside of the furnace body 14, the inside of the core tube 20, and the inside of the anterior chamber 11 to reduce the pressure. The fourth exhaust valve 48 is opened when exhausting. Exhaust processing is performed downstream of the fourth exhaust valve 48.

When the exhaust gas contains a fluorine compound gas, for example, the exhaust gas is sent to a scrubbing tower. When the exhaust gas does not contain a fluorine compound gas, for example, the exhaust gas is discharged into the atmosphere. The exhaust gas from the anterior chamber 11 is discharged 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. In the manufacturing apparatus 1, the furnace tube 20 is airtight, but since it is difficult to make the furnace tube 20 completely airtight, some of the gas supplied into the furnace tube 20 may flow into the furnace body 14.

Next, a description will be given of a method for producing silica glass using the above-mentioned production apparatus 1. The method for producing silica glass according to this embodiment includes the following steps (a) to (d), as shown in FIG.
(a) a step of preparing a porous silica glass body M; (b) a step of removing moisture from the porous silica glass body M; (c) a step of removing impurities from the porous silica glass body M (a step of heating the porous silica glass body M in an atmosphere of a gas containing fluorine).
(d) A step of converting the porous silica glass body into transparent glass at a temperature of 1300° C. or higher.

[Step (a)]
In the step (a) of preparing the porous silica glass body M, the porous silica glass body M is prepared by producing the porous silica glass body M by a known method such as a Vapor-phase Axial Deposition (VAD) method or an Outside Vapor Deposition (OVD) method. The porous silica glass body M has a structure in which glass particles having a particle size of 0.1 μm to 1 μm, which are generated by a flame hydrolysis reaction of a silicon compound gas (SiCl ₄ , siloxane, etc.), are deposited on a predetermined target, and is generally called a soot body. The porous silica glass body M contains moisture as adsorbed water or silanol groups. The porous silica glass body M may be prepared by a powder molding method described in Non-Patent Document 2 and the like. The porous silica glass body M prepared in this way may contain carbon-based impurity components or metal impurity components. In addition, the porous silica glass body M may have moisture adsorbed thereto.

[Step (b)]
In the step (b) of removing moisture from the porous silica glass body M, first, the porous silica glass body M prepared in the step (a) is attached to the rod 12 of the manufacturing apparatus 1 shown in FIG. 1 and placed in the furnace tube 20. Next, the furnace tube 20 and the porous silica glass body M placed in the furnace tube 20 are heated by the heater 15, and the temperature is raised to, for example, 1100° C. or higher and maintained at that temperature (see, for example, step S1 in FIG. 3. FIG. 3 will be described later). The heating temperature is, for example, 1100° C. Then, in a state in which the furnace tube 20 and the like are heated to maintain 1100° C., an inert gas is introduced from the first gas supply unit 31 into the furnace tube 20 and exhausted through the first exhaust pipe 41, thereby removing moisture adsorbed in the porous silica glass body M (soot body). At this time, nitrogen or helium may be introduced from the second gas supply unit 32 into the furnace body 14. The inert gas introduced into the furnace tube 20 is, for example, nitrogen (N ₂ ). The flow rate of the inert gas to be introduced is, for example, 2 slm. The term "slm" used here refers to the flow rate per minute under standard conditions (0° C., 1 atm). The introduction of the inert gas such as nitrogen may be performed after the furnace tube 20 is heated to 1100° C., or may be performed while the furnace tube 20 is heated toward 1100° C. After the moisture removal in step (b) is completed, the introduction of the inert gas is stopped. This step (b) corresponds to the step of introducing the inert gas. The heating temperature in step (b) is the temperature maintained for a certain period of time in step S1, and some variation is permitted.

[Step (c)]
In the step (c) of removing impurity components from the porous silica glass body M, the heating by the heater 15 is adjusted so that the temperature is lower than the heating temperature (e.g., 1100°C) in the step (b) while the porous silica glass body M is still placed in the furnace tube 20 (see, for example, step S2 in FIG. 3). In the step (c), the temperature at which the furnace tube 20 and the porous silica glass body M placed in the furnace tube 20 are heated is, for example, 800°C or higher and 1100°C or lower, and may be 950°C or higher and 1050°C or lower. The heating temperature in the step (c) is, for example, 1000°C. The heating temperature in the step (c) may be lower than the temperature at which fluorine in the fluoride compound gas is added to silica (silicon dioxide). Then, in a state in which the furnace tube 20 and the like are heated to maintain 1000°C, a fluorine compound gas (a gas containing fluorine) is introduced from the first gas supply unit 31 into the furnace tube 20. At this time, the first exhaust valve 42 is closed. That is, the fluorine compound gas introduced from the first gas supply unit 31 is sealed inside the furnace core tube 20. In the step (c), for example, after reducing the pressure inside the furnace core tube 20 in which the porous silica glass is placed until the total pressure becomes 1 kPa or less, the fluorine compound gas may be introduced so that the total pressure becomes 1 kPa or more, and as one example, the fluorine compound gas is introduced so that the total pressure becomes 2.5 kPa. Note that the heating temperature in the step (c) is a temperature maintained for a certain period of time in step S2, and some variation is allowed.

Then, in step (c), the fluorine compound gas is sealed in the furnace core tube 20 at a predetermined pressure and kept sealed for a predetermined time. At this time, in the furnace core tube 20, silica (SiO ₂ ) constituting the porous silica glass body M reacts with the fluorine compound gas to form an oxygen compound (e.g., carbon dioxide or water vapor). Then, carbon impurities (e.g., C) in the porous silica glass body M are removed using this oxygen compound. If moisture remains in the porous silica glass body M, the moisture reacts with the fluorine compound gas to form an oxygen compound, and the carbon impurities are removed using the oxygen compound in the same manner as above. Furthermore, in step (c), metal impurities (e.g., alkalis Na ₂ O or Al ₂ O ₃ ) in the porous silica glass body M react with the fluorine compound gas to generate fluorides (e.g., NaF or AlF ₃ ) that are easily melted, and the metal impurities are removed. The time for keeping the gas sealed in step (c) is a time sufficient for the above-mentioned various reactions to occur, and can be, for example, one hour or more.

The fluorine compound gas used to remove impurities in step (c) may be, for example, perfluorocarbon (PFC) _CF4 , _C2F6 , or _C3F8 , _{hydrofluorocarbon} (HFC) _CHF3 , _CH2F2 , or _CHF2CF3 , chlorofluorocarbon (CFC _{) CF3Cl} , _{CF2Cl2 ,} or _CClF2CF3 , or hydrochlorofluorocarbon (HCFC) _CHClF2 . Another fluorine compound gas used to remove impurity components in step ( _c ) may be, for example, _SF6 , _NF3 , _SiF4 , or _HF . In terms of simplifying the gas _supply device, a fluorine compound that is gaseous at room temperature may be used. The fluorine compound gas used to remove impurities in step (c) may be PFC or HFC among the above, and may also be _CF4 or _CHF3 . Below, a reaction example in which _CF4 is used as the fluorine compound gas is shown, but since the reaction formula in the case of using other fluorine compound gases is clear to those skilled in the art, the description is omitted. Note that the fluorine compound gas used in step (c) may be one that does not contain halogens other than fluorine.

[Reaction of Silica (SiO ₂ ) with Fluorine Compound Gas (CF ₄ )]
SiO ₂ +CF ₄ → CO ₂ +SiF ₄ ...(1)

[Reaction of _H2O in silica ( _SiO2 ) with fluorine compound gas ( _CF4 ) _2H2O + _CF4 → _CO2 + 4HF ... (2)

[Removal of carbon impurities]
CO ₂ +C → 2CO...(3)
By the above reaction (1) or (2), carbon dioxide (CO ₂ ), which is an oxygen compound, is obtained from the silica or moisture in the porous silica glass body M. Then, by the reaction (3) using this oxygen compound, carbon (C), which is a carbon impurity in the porous silica glass body M, is converted to carbon monoxide (CO) and removed (exhausted). Note that, since this reaction (3) occurs inside the porous silica glass body M, the effect on the carbon members constituting the manufacturing apparatus 1 is reduced.

[Avoiding crystallization of metal impurities (alkalis)]
CF ₄ +Na ₂ O → 2NaF + COF ₂ ...(4)
3CF ₄ +2Al ₂ O ₃ → 4AlF ₃ +3CO ₂ ...(5)
By the above reaction (4) or (5), _Na2O or _{Al2O3 ,} which easily becomes a crystal nucleus, can be converted into NaF or _AlF3, which easily melts. Similar reactions are possible with other alkali metals or alkaline earth metals. In addition, such reactions are also possible when _SiF4 , which is a product of the reaction between _SiO2 and _CF4 (above reaction formula (1)), reacts with metal impurities.

When heat treatment is performed in an atmosphere of a fluorine compound gas, the driving force for sintering (increase in surface free energy) increases due to the effect of replacing oxygen or hydroxyl groups on the surface of the glass particles of the porous silica glass body M with fluorine (F) and the surface activation caused by etching the surface. This makes it possible to reduce the heating temperature in the process of forming a transparent glass, which is step (d) described below. The reaction that occurs at this time is shown in the following formula (6).

[Silica particle surface activation]
2SiOH+CF ₄ → 2SiF+2HF+CO ₂ ...(6)

[Step (d)]
In the step (d) of turning the porous silica glass body into a transparent vitrified body, the heating of the porous silica glass body M, which is still placed in the furnace tube 20 after the impurity components have been removed in the step (c), is adjusted by the heater 15 so that the temperature is 1300°C or higher, which is higher than the heating temperature (e.g., 1000°C) in the step (c) (see, for example, step S3 in FIG. 3). In the step (d), the porous silica glass body M in the furnace tube 20 is heated to 1300°C while supplying an inert gas (e.g., nitrogen (N ₂ )) from the first gas supply unit 31 into the furnace tube 20. Thereafter, the supply of the inert gas from the first gas supply unit 31 is stopped, and the porous silica glass body M is heated by the heater 15 until the temperature reaches 1450°C or higher under the condition that the pressure in the furnace tube 20 is 10 Pa or less. This heating causes the porous silica glass body M to be sintered and turned into a transparent vitrified body. As a result of the above, silica glass is produced. The heating temperature in the step (d) refers to the maximum temperature in step S3.

The silica glass produced in this way is, in one example, a base material for optical fiber. When an optical fiber base material is produced by the above-mentioned silica glass manufacturing method, this optical fiber base material may be drawn to further produce an optical fiber. A detailed description of the method for drawing an optical fiber is omitted since a known method can be used. Furthermore, the above-mentioned silica glass manufacturing method can be applied not only to the manufacturing method of an optical fiber base material, but also to the manufacturing of other optical components, for example, various optical components in semiconductor manufacturing equipment (for example, ultraviolet lenses or glass for photomasks), and is not particularly limited in its use.

As described above, according to the method for producing silica glass according to the present embodiment, the porous silica glass body M is heated under an atmosphere of a fluorine compound gas before the porous silica glass body M is vitrified to be transparent. As a result, before the porous silica glass body M is vitrified to be transparent, silica (SiO ₂ ) constituting the porous silica glass body M reacts with a fluorine compound gas (e.g., CF ₄ or CHF ₃ ) to form an oxygen compound (e.g., CO ₂ ). Then, the carbon impurities (e.g., C) in the porous glass body are reacted with this oxygen compound, so that the carbon impurities in the porous silica glass body M are removed. If moisture remains in the porous silica glass body M, the moisture (H ₂ O) reacts with the fluorine compound gas to form an oxygen compound, and similarly to the above, the carbon impurities in the porous glass body are reacted with the oxygen compound to remove the carbon impurities. Furthermore, in the method for producing silica glass according to this embodiment, before transparent vitrification, metal impurities (e.g., metal oxides such as alkali _Na2O or _{Al2O3 )} in the porous silica glass body M react with a fluorine compound gas to form easily meltable fluorides (e.g., NaF or _AlF3 ), thereby removing the metal oxides. As described above, according to the method for producing silica glass according to this embodiment, various impurities can be removed by a simple method of heating the porous silica glass body M under an atmosphere of a fluorine compound gas before transparent vitrification, and the formation of defects in the glass body can be suppressed by a simple method.

In the method for producing silica glass according to this embodiment, before the porous silica glass body M is vitrified, the porous silica glass body M is heated in an atmosphere of a gas containing fluorine. In addition to the above, the sintering driving force (surface free energy) during vitrification can be increased by the effect of replacing oxygen or hydroxyl groups on the surface of the porous silica glass body M with fluorine (F) and by surface activation by surface etching. As a result, according to this method for producing silica glass, the heating temperature during vitrification can be lowered, reducing energy consumption.

In the method for producing silica glass according to the present embodiment, the fluorine compound gas does not contain any halogen other than fluorine. Since it does not contain any halogen other than fluorine, the silica, carbon impurities, and metal impurities constituting the porous silica glass body M react more reliably with the fluorine compound gas, and the carbon impurities and metal impurities are more reliably removed. When halogen elements other than fluorine (chlorine (Cl), bromine (Br), iodine (I)) are contained, the bond energy between these halogen elements and carbon is weaker than the bond energy between fluorine and carbon, so that halogen gas (e.g., Cl ₂ ) is easily generated, and the carbon component is more likely to remain. As a result, according to this production method, various impurities are more reliably removed, and the formation of defects in the glass body can be more reliably suppressed. In addition, when halogens other than fluorine are contained, the constituent materials of the furnace also need to have halogen corrosion resistance, which may cause an increase in equipment costs.

In the method for producing silica glass according to this embodiment, the fluorine compound gas may include at least one of perfluorocarbon and hydrofluorocarbon fluorine compound gases.

In the method for producing silica glass according to this embodiment, in the step (c) of removing impurities, the fluorine compound gas reacts with the silicon dioxide constituting the porous silica glass body M to form a gas containing oxygen. This oxygen-containing gas may contain at least one of carbon dioxide and water vapor. This causes the carbon dioxide or water vapor to react with the carbon impurities in the porous silica glass body, making it possible to more reliably remove the carbon impurities in the porous silica glass body M. As a result, it becomes possible to reliably suppress the formation of defects in the glass body after it has been made into a transparent vitrified body.

In the method for producing silica glass according to this embodiment, in the step (c) of removing impurities, the fluorine compound gas may react with the silicon dioxide constituting the porous silica glass body M. The heating temperature in the step (c) of removing impurities is equal to or lower than the temperature at which the fluorine in the fluorine compound gas is added to the silicon dioxide. This prevents fluorine from being added to the porous silica glass body M, and the refractive index and other properties of the glass body in which the porous silica glass body M has been transparently vitrified can be more reliably set to the desired value.

In the method for producing silica glass according to this embodiment, the heating temperature in the step (c) of removing impurities may be 800°C or higher and 1100°C or lower. This provides an excellent balance between the generation of oxygen compound gas caused by the dissociation of fluorine-containing gas and reaction with silica glass particles in the porous silica glass body M, and the addition of the dissociated fluorine to the silica glass, making it easier to remove carbon components using the oxygen compound gas and to suppress the addition of fluorine. This suppresses the addition of fluorine, and more reliably makes it possible to set the refractive index, etc. of the glass body in which the porous silica glass body M is transparently vitrified to the desired value. The heating temperature in the step (c) of removing impurities may be 950°C or higher and 1050°C or lower.

In the method for producing silica glass according to this embodiment, in the step (c) of removing impurities, the total pressure in the furnace tube 20 in which the porous silica glass body M is placed may be reduced to 1 kPa or less, and then a fluorine compound gas may be introduced into the furnace tube 20 to heat the porous silica glass body M until the pressure change amount ΔP becomes greater than 1 kPa. This ensures that the silica and metal impurities constituting the porous silica glass body react with the fluorine-containing gas, and the carbon impurities and metal impurities are removed more reliably. By ensuring that various impurities are removed in this way, the formation of defects in the glass body can be more reliably suppressed.

The method for producing silica glass according to this embodiment may further include a step of introducing an inert gas (e.g., _N2 ) into the furnace tube 20 while heating the furnace tube 20 in which the porous silica glass body M is placed, prior to the step (c) of removing impurities. The heating temperature in the heating step may be lower than the heating temperature in the introducing step. In this case, the introduction of the inert gas while heating the porous silica glass body can more reliably remove moisture from the porous silica glass body.

The optical fiber manufacturing method according to this embodiment includes a step of manufacturing an optical fiber preform using the above-mentioned silica glass manufacturing method, and a step of drawing the optical fiber preform to manufacture an optical fiber. In this case, various impurities are removed from the silica glass that will become the optical fiber preform, suppressing defects, so that it is possible to reduce the fluctuation in the outer diameter of the glass and breakage caused by impurities during the process of processing it into an optical fiber, making it possible to manufacture optical fibers with stable quality.

Although the method for producing silica glass according to the present disclosure has been described in detail above, the present invention is not limited to the above embodiment and can be applied to various embodiments and modified examples. For example, in the above embodiment, the method for producing silica glass using the reduced pressure vacuum atmosphere furnace shown in FIG. 1 was described, but the present invention is not limited to this. That is, the method for producing silica glass disclosed in the above embodiment may be performed using the atmospheric pressure sintering furnace shown in FIG. 7, or the method for producing silica glass disclosed in the embodiment may be performed using a zone sintering furnace.

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

[Experimental Example 1]
As Experimental Example 1, first, a glass rod was prepared having a core in which germanium oxide (GeO ₂ ) was added to silica (SiO ₂ ), and a clad made of silica covering the outer periphery of the core. This glass rod had a diameter of 20 mm and a length (length in the longitudinal direction) of 400 mm. In addition, octamethyl-cyclotetra-siloxane (C ₈ H ₂₄ O ₄ Si ₄ ) was prepared as a glass raw material. Then, this glass raw material was supplied by nozzle spraying into a flame of H ₂ and O ₂ to form silica fine particles, and the silica fine particles were deposited on the outer periphery surface of the target glass rod. The silica fine particles correspond to the jacket covering the outer periphery of the clad. In the above manner, a porous silica glass body M was produced (prepared). The produced porous silica glass body M had a diameter of 150 mm and a weight of 2500 g.

Next, the prepared porous silica glass body M was attached to the lower end of the rod 12 of the manufacturing apparatus 1 shown in FIG. 1 and placed in the furnace tube 20. Thereafter, the porous silica glass body M was subjected to a heat treatment under the temperature treatment conditions shown in FIG. 3, and each of the processes of removing moisture (S1), removing impurity components (S2), and making the body transparent (S3) was performed. Specifically, in step S1 shown in FIG. 3, nitrogen (N ₂ ) was flowed from the first gas supply unit 31 into the furnace tube 20 at a flow rate of 2 slm, while being exhausted by the first exhaust pipe 41. At this time, the furnace tube 20, the gas in the furnace tube 20, and the porous silica glass body M were heated by the heater 15 so that the temperature of the furnace tube 20, the gas in the furnace tube 20, and the porous silica glass body M was raised to 1100° C. As a result, the moisture attached to the porous silica glass body M was removed. In step S1, after the removal of moisture was completed, the supply of nitrogen from the first gas supply unit 31 was stopped.

3, _CF4 was introduced from the first gas supply unit 31 into the furnace tube 20 while the heating temperature by the heater 15 was lowered to 1000°C. _CF4 was introduced until the total pressure in the furnace tube 20 reached 2.5 kPa, and the heating temperature was maintained at 1000°C for 1 hour or more. At this time, the pressure was maintained without exhausting. As a result, a reaction for removing impurities from the porous silica glass body M was carried out, and carbonaceous impurities and metal oxides were removed or halogenated. After various impurities were removed, the supply of _CF4 from the first gas supply unit 31 was stopped.

Next, in step S3 shown in FIG. 3, the heating temperature by the heater 15 was increased again to 1300°C, and nitrogen was supplied from the first gas supply unit 31 into the furnace tube 20 at a flow rate of 2 slm while being exhausted by the first exhaust pipe 41. After that, the supply of nitrogen into the furnace tube 20 was stopped, and the heating temperature by the heater 15 was further increased to 1450°C or higher under the condition that the pressure inside the furnace tube 20 was 10 Pa or less, and the porous silica glass body M was transparently vitrified. As a result, transparently vitrified silica glass was obtained. There were zero bright spots in the obtained silica glass. The refractive index distribution of the glass base material for optical fiber made of the obtained silica glass is as shown in FIG. 4, and the refractive index of the jacket deposited in Experimental Example 1 was at the same level as that of the silica glass (see the relative refractive index difference Δn3 of the jacket). In FIG. 4, Δn1 is the relative refractive index difference of the core, Δn2 is the relative refractive index difference of the cladding, and Δn1, Δn2, and Δn3 mean the refractive index difference of each part with respect to the refractive index of silica glass. In addition, the optical fiber preform according to Experimental Example 1 was drawn to produce an optical fiber with a glass outer diameter of 125 μm. The local outer diameter variation of this optical fiber was 1 point/100 km, and the optical loss in the optical fiber was 0.183 dB/km at a wavelength of 1.55 μm.

[Experimental Example 2]
In Experimental Example 2, _CF4 was not supplied in the impurity removing step S2, and the other conditions were the same as those in Experimental Example 1, to obtain a transparent silica glass body. The number of bright spots in the obtained silica glass was 5 to 10/cm. In other words, a large number of bright spots remained in the silica glass of Experimental Example 2. The refractive index distribution of the silica glass obtained in Experimental Example 2 was as shown in FIG. 4, similar to Experimental Example 1, and the refractive index of the jacket deposited in Experimental Example 2 was at the same level as that of the silica glass. However, when an attempt was made to draw the optical fiber preform according to Experimental Example 2, the outer diameter fluctuated significantly, and an optical fiber could not be produced.

[Comparison between Experimental Example and Experimental Example 2]
Comparing Experimental Example 1 and Experimental Example 2, it was found that impurity components that cause bright spots in silica glass were removed by performing treatment using _CF4 . In Experimental Examples 1 and 2, it was found that the cause of bright spots in glass was thought to be organic components in the raw materials remaining as carbon or metal oxides, and that the carbon-based impurity components and metal oxides were removed by reaction through treatment using _CF4 .

[Experimental Example 3, Experimental Example 4, Experimental Example 5, Experimental Example 6, Experimental Example 7, Experimental Example 8]
For Experimental Examples 3 to 8, first, a porous silica glass body M corresponding to each Experimental Example was produced by the same method as in Experimental Example 1. Then, each of the produced porous silica glass bodies M was attached to the rod 12 of the manufacturing apparatus 1 shown in FIG. 1 and placed in the furnace tube 20, as in Experimental Example 1. Thereafter, the porous silica glass body M was subjected to a heat treatment under the temperature treatment conditions generally shown in FIG. 3, and each of the processes of removing moisture (S1), removing impurity components (S2), and making the porous silica glass body transparent (S3) was performed. In Experimental Examples 3 to 8, the heating conditions of steps S1 and S3 shown in FIG. 3 were the same as those of Experimental Example 1. Meanwhile, in Experimental Examples 3 to 8, the heating temperature in step S2 shown in FIG. 3 was set to a heating condition corresponding to six steps from 900° C. to 1150° C. in 50° C. increments. Specifically, in Example 3, the heating temperature in step S2 was set to 900° C. In Example 4, the heating temperature in step S2 was set to 950° C. In Example 5, the heating temperature in step S2 was set to 1000° C. In Example 6, the heating temperature in step S2 was set to 1050° C. In Example 7, the heating temperature in step S2 was set to 1100° C. In Example 8, the heating temperature in step S2 was set to 1150° C.

In Experimental Examples 3 to 6, the heating temperature in step S2 was in the range of 900°C to 1050°C, and no decrease in the refractive index due to the addition of fluorine to the jacket was observed (see Figures 5 and 6). On the other hand, in Experimental Examples 7 and 8, the heating temperature in step S2 was in the range of 1100°C to 1150°C, and a decrease in the refractive index due to the addition of fluorine to the jacket was observed (see Figure 6). Therefore, it was confirmed that if it is desired to avoid a decrease in the refractive index due to the addition of fluorine, the heating temperature in step S2 may be 1050°C or less.

[Experimental Example 9, Experimental Example 10, Experimental Example 11, Experimental Example 12, Experimental Example 13, Experimental Example 14]
For Experimental Examples 9 to 14, first, a porous silica glass body M corresponding to each Experimental Example was produced by the same method as in Experimental Example 1. Then, each of the produced porous silica glass bodies M was attached to the rod 12 of the manufacturing apparatus 1 shown in FIG. 1 and placed in the furnace tube 20, as in Experimental Example 1. Thereafter, the porous silica glass body M was subjected to a heat treatment under the temperature treatment conditions generally shown in FIG. 3, and each treatment of removing moisture (S1), removing impurity components (S2), and making the porous silica glass body transparent (S3) was performed. In Experimental Examples 9 to 14, the conditions of steps S1 and S2 shown in FIG. 3 were the same as those of Experimental Example 1. Meanwhile, in Experimental Examples 9 to 14, the heating temperature in step S3 shown in FIG. 3 was set to a heating condition corresponding to six steps from 1360° C. to 1460° C. at 20° C. intervals. The heating temperature is the maximum temperature in step S3. Specifically, in Example 9, the heating temperature in step S3 was set to 1360° C. In Example 10, the heating temperature in step S3 was set to 1380° C. In Example 11, the heating temperature in step S3 was set to 1400° C. In Example 12, the heating temperature in step S3 was set to 1420° C. In Example 13, the heating temperature in step S3 was set to 1440° C. In Example 14, the heating temperature in step S3 was set to 1460° C.

In Experimental Examples 10 to 14, the heating temperature in step S3 was 1380°C or higher, and all of the porous silica glass body M was converted into transparent glass. On the other hand, in Experimental Example 9, the heating temperature in step S3 was 1360°C, and some unsintered parts were confirmed in the sintered silica glass. If there are unsintered parts, they can be sintered by heating again.

[Experimental Examples 15, 16, 17, and 18]
For Experimental Examples 15 to 18, first, a porous silica glass body M corresponding to each Experimental Example was produced by the same method as in Experimental Example 2. Then, each of the produced porous silica glass bodies M was attached to the rod 12 of the manufacturing apparatus 1 shown in FIG. 1 and placed in the furnace tube 20, as in Experimental Example 2. Thereafter, a heat treatment was performed under the same temperature conditions as in Example 2, except for the temperature conditions during transparent vitrification, to transparently vitrify each of the porous silica glass bodies M. That is, in Experimental Examples 15 to 18, unlike Experimental Examples 9 to 14, the removal of impurity components was not performed while introducing _CF4 .

In Experimental Examples 15 to 18, the heating temperature in step S3 shown in FIG. 3 was set to four heating conditions corresponding to 20°C intervals from 1400°C to 1460°C. Specifically, in Example 15, the heating temperature in step S3 was set to 1400°C. In Example 16, the heating temperature in step S3 was set to 1420°C. In Example 17, the heating temperature in step S3 was set to 1440°C. In Example 18, the heating temperature in step S3 was set to 1460°C.

In Experimental Examples 17 and 18, the heating temperature in step S3 was 1440°C or higher, and both porous silica glass bodies M were vitrified into transparent glass. On the other hand, in Experimental Example 16, the heating temperature in step S3 was 1420°C, and unsintered parts were found in the sintered silica glass. In Experimental Example 17, the heating temperature in step S3 was 1400°C, and the entire glass remained cloudy and was not sintered.

[Comparison of Experimental Examples 9 to 14 with Experimental Examples 15 to 18]
By comparing Experimental Examples 9 to 14 with Experimental Examples 15 to 18, it was found that the porous silica glass body M was easily sintered due to the surface modification of the glass particles by the treatment using _CF4 . In other words, it was found that the sintering temperature can be lowered in the method using _CF4 (Experimental Examples 9 to 14) than in the method not using _CF4 (Experimental Examples 15 to 18). Therefore, it was found that the method for producing silica glass according to the present disclosure can reduce energy consumption by lowering the vitrification temperature.

[Experimental Example 19]
As Experimental Example 19, first, a porous silica glass body M corresponding to each Experimental Example was produced by the same method as Experimental Example 1. Then, the produced porous silica glass body M was attached to the rod 12 of the production apparatus 1A and placed in the furnace core tube 20. In Experimental Example 19, the production apparatus used for transparent vitrification was the production apparatus 1A shown in FIG. 7. The production apparatus 1A was a normal pressure sintering furnace, and only the first exhaust pipe 41A and the first exhaust valve 42A were provided as an exhaust system. Then, the produced porous silica glass body M was heated by the heater 15 from 800°C to 1000°C in the normal pressure sintering furnace (production apparatus 1A) shown in FIG. 7, while supplying nitrogen from the first gas supply unit 31 to the furnace core tube 20 at 10 slm (removal of moisture) and supplying _CF4 at 100 cc/min, and holding at 1000°C for 1 hour or more to perform a heat treatment (removal of impurity components). After that, the supply of nitrogen and _CF4 into the furnace tube 20 was stopped, and the supply of helium (He) was switched to. While continuing the supply of helium, the heating temperature by the heater 15 was raised to 1420°C and maintained for 30 minutes to perform transparent vitrification. No bright spots were observed in the silica glass thus obtained. In other words, it was confirmed that various impurity components can be removed from the porous silica glass body M by supplying a fluorine compound gas even without a reduced pressure state.

[Experimental Example 20]
As Experimental Example 20, unlike Experimental Example 1, a porous silica glass body M was produced by a powder molding method. Specifically, silica glass fine particles with an average particle size of 100 nm were kneaded with a PVA aqueous solution, and particles were granulated to an average particle size of 15 μm using a spray dryer. The obtained particles were filled into a cylindrical rubber mold, and molded into a cylindrical shape with a diameter of 100 mm and a length of 300 mm using an isostatic pressing molder. This molded body was dried in the air at 100° C. to produce a porous silica glass body M. The dry weight was 1500 g.

Then, the porous silica glass body M was placed in a reduced pressure vacuum atmosphere furnace, which is the manufacturing apparatus 1 shown in FIG. 1, and heat-treated under the temperature treatment conditions shown in FIG. 3 in the same manner as in Experimental Example 1, and the processes of removing moisture (S1), removing impurity components (S2), and forming a transparent glass (S3) were performed. No bright spots were found in the silica glass obtained in this manner. In other words, it was confirmed that the method of manufacturing silica glass disclosed herein can be applied to any of the porous silica glass bodies M produced by various manufacturing methods.

[Experimental Example 21]
As Experimental Example 21, transparent vitrified silica glass was obtained under the same conditions as Experimental Example 20, except that _CF4 was not supplied in the impurity removing step S2 in Experimental Example 20. The obtained silica glass was colored black, and white turbidity due to partial crystallization was observed.

[Experimental Example 22, Experimental Example 23,]
In Experimental Example 22 and Experimental Example 23, the porous silica glass body M corresponding to each Experimental Example was produced by the same method as Experimental Example 1. Then, each of the produced porous silica glass bodies M was attached to the rod 12 of the manufacturing apparatus 1 shown in FIG. 1 and placed in the furnace core tube 20, as in Experimental Example 1. Thereafter, the porous silica glass body M was subjected to a heat treatment under the temperature treatment conditions shown in FIG. 3, and each of the processes of removing moisture (S1), removing impurity components (S2), and making the porous silica glass body transparent (S3) was performed. Specifically, in step S1 shown in FIG. 3, nitrogen (N ₂ ) was flowed from the first gas supply unit 31 into the furnace core tube 20 at a flow rate of 2 slm, and exhausted by the first exhaust pipe 41. At this time, the furnace core tube 20, the gas in the furnace core tube 20, and the porous silica glass body M were heated by the heater 15 so that the temperature of the furnace core tube 20 and the gas in the furnace core tube 20 was raised to 1100° C. As a result, the moisture attached to the porous silica glass body M was removed. In step S1, after the removal of moisture was completed, the supply of nitrogen from the first gas supply unit 31 was stopped.

3, CHF3 was introduced into the furnace core tube 20 from the first gas supply unit 31 while the heating temperature by the heater ₁₅ was lowered to 900°C (in the case of Experimental Example 22) or 1000°C (in the case of Experimental Example 23). _CHF3 was introduced into the furnace core tube 20 until the total pressure in the furnace core tube 20 reached 3.2 kPa, and the same heating temperature was maintained for one hour or more. At this time, the pressure was maintained without exhausting. As a result, a reaction for removing impurities in the porous silica glass body M was carried out, and carbonaceous impurities and metal oxides were removed or halogenated.

After that, in step S3 shown in FIG. 3, the gas supply was stopped, and the exhaust valves 42 and 44 were opened, and transparent vitrification was performed under heating conditions of a maximum furnace temperature of 1400°C under a pressure of 100 Pa or less. In both experimental examples 22 and 23, the porous silica glass body M was transparently vitrified. There were zero bright spots in the obtained silica glass.

[Experimental Examples 24 and 25]
In Experimental Example 24 and Experimental Example 25, the porous silica glass body M corresponding to each Experimental Example was produced by the same method as Experimental Example 1. Then, each of the produced porous silica glass bodies M was attached to the rod 12 of the manufacturing apparatus 1 shown in FIG. 1 and placed in the furnace core tube 20, as in Experimental Example 1. Thereafter, the porous silica glass body M was subjected to a heat treatment under the temperature treatment conditions shown in FIG. 3, and each of the processes of removing moisture (S1), removing impurity components (S2), and making the porous silica glass body transparent (S3) was performed. Specifically, in step S1 shown in FIG. 3, nitrogen (N ₂ ) was flowed from the first gas supply unit 31 into the furnace core tube 20 at a flow rate of 2 slm, and exhausted by the first exhaust pipe 41. At this time, the furnace core tube 20, the gas in the furnace core tube 20, and the porous silica glass body M were heated by the heater 15 so that the temperature of the furnace core tube 20 and the gas in the furnace core tube 20 was raised to 1100° C. As a result, the moisture attached to the porous silica glass body M was removed. In step S1, after the removal of moisture was completed, the supply of nitrogen from the first gas supply unit 31 was stopped.

3, C2F6 was introduced into the furnace core tube 20 from the first gas supply unit 31 while the heating temperature by the heater ₁₅ was lowered to 950°C (in the case of Experimental Example 24) or 1050°C (in the case of Experimental Example ₂₄ ). The introduction of _C2F6 was continued until the total pressure in the furnace core tube 20 reached 1.6 _kPa , and the same heating temperature was maintained for one hour or more. At this time, the pressure was maintained without exhausting. As a result, a reaction for removing impurities in the porous silica glass body M was carried out, and carbonaceous impurities and metal oxides were removed or halogenated.

After that, in step S3 shown in FIG. 3, the gas supply was stopped, and the exhaust valves 42 and 44 were opened, and transparent vitrification was performed under heating conditions of a maximum furnace temperature of 1400°C under a pressure of 100 Pa or less. In both experimental examples 24 and 25, the porous silica glass body M was transparently vitrified. There were zero bright spots in the obtained silica glass.

As explained above, it was confirmed that defects in silica glass can be reduced by a simple method of performing heat treatment under an atmosphere of a fluorine compound, _CF4, under appropriate processing conditions. Furthermore, it was confirmed that this manufacturing method can reduce the temperature at which glass becomes transparent.

Reference Signs List 1, 1A... Manufacturing apparatus 10... Container 11... Front chamber 11a... Insertion hole 12... Rod 13... Gate valve 14... Furnace body 15... Heater 15a... Heating body 20... Furnace tube 21... Opening 22... Top lid 30... Gas supply section 31... First gas supply section 32... Second gas supply section 40... Exhaust system 41, 41A... First exhaust pipe 42, 42A... First exhaust valve 43... Second exhaust pipe 44... Second exhaust valve 45... Third exhaust pipe 46... Third exhaust valve 47... Vacuum pump 48... Fourth exhaust valve M... Porous silica glass body Δn1... Relative refractive index difference of core Δn2... Relative refractive index difference of cladding Δn3... Relative refractive index difference of jacket

Claims (10)

Providing a porous silica glass body;
heating the porous silica glass body in an atmosphere of a gas containing fluorine;
a step of converting the porous silica glass body into a transparent glass at a temperature of 1300° C. or more after the heating step;
A method for producing silica glass comprising the steps of: The fluorine-containing gas does not contain any halogen other than fluorine.
The method for producing silica glass according to claim 1 . The fluorine-containing gas includes at least one fluorine compound gas of perfluorocarbon and hydrofluorocarbon.
The method for producing silica glass according to claim 1 . In the heating step, the fluorine-containing gas reacts with silicon dioxide constituting the porous silica glass to form an oxygen-containing gas,
The oxygen-containing gas contains at least one of carbon dioxide and water vapor.
The method for producing silica glass according to claim 1 . The heating temperature in the heating step is equal to or lower than the temperature at which the fluorine in the fluorine-containing gas is added to the silicon dioxide.
The method for producing silica glass according to claim 4 . The heating temperature in the heating step is 800° C. or more and 1100° C. or less.
The method for producing silica glass according to claim 1 . The heating temperature in the heating step is 950° C. or higher and 1050° C. or lower.
The method for producing silica glass according to claim 6 . In the heating step, the total pressure in a furnace tube in which the porous silica glass is placed is reduced to 1 kPa or less, and then the fluorine-containing gas is introduced into the furnace tube until a pressure change ΔP becomes greater than 1 kPa, thereby heating the porous silica glass body.
The method for producing silica glass according to claim 1 . The method further includes, before the heating step, a step of introducing an inert gas into a furnace tube while heating the furnace tube in which the porous silica glass body is disposed;
The heating temperature in the heating step is lower than the heating temperature in the introducing step.
The method for producing silica glass according to claim 1 . A step of producing an optical fiber preform by using the method for producing silica glass according to any one of claims 1 to 9;
a step of drawing the optical fiber preform to produce an optical fiber;
A method for manufacturing an optical fiber comprising the steps of:

Images