Manufacturing method of optical fiber preform and manufacturing method of optical fiber

Abstract

To provide a method for manufacturing an optical fiber preform, capable of adjusting the bulk density of soot, and a method for manufacturing an optical fiber.SOLUTION: A method for manufacturing an optical fiber preform includes a step of supplying premixed gas including organic silicon compound containing raw material gas and oxygen to a burner to deposit the formed glass particles on a start member. In the step, the mixture ratio of the raw material gas to the oxygen is controlled so as to be the maximum when the ratio of an elapsed time to the entire step is more than 0 and 0.2 or less, and when setting the average of the mixture ratio in 0.667 or more and 1 or less of the ratio of the elapsed time to R1 and the maximum to R2, R2 / R1 is more than 1 and 1.27 or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing an optical fiber preform and a method for manufacturing an optical fiber. [Background technology]

[0002] In the manufacture of an optical fiber preform, for example, a raw material gas is supplied to a burner, and the generated glass particles are deposited on a starting member to form soot, thereby obtaining a porous glass particle body. The porous glass particle body is sintered to become an optical fiber preform. The raw material gas is silicon tetrachloride (SiCl 4 ) are generally used, but in recent years, organic silicon compounds have come to be used more frequently (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-36172 A Summary of the Invention [Problem to be solved by the invention]

[0004] Since the reaction of producing glass from an organosilicon compound is an exothermic reaction, the temperature of the soot tends to be high when an organosilicon compound is used. Therefore, the bulk density of the soot tends to be high. If the bulk density of the soot is too high, the efficiency of the dehydration process may decrease. If the bulk density of the soot is too low, problems such as soot cracking may occur, or the outer diameter of the soot may become large, making it difficult for the porous glass particles to fit into a sintering furnace may occur.

[0005] An object of one aspect of the present invention is to provide a method for manufacturing an optical fiber preform and a method for manufacturing an optical fiber, which are capable of optimizing the bulk density of soot. [Means for solving the problem]

[0006] One aspect of the present invention provides a method for manufacturing an optical fiber preform, comprising the steps of: supplying a premixed gas containing a raw material gas including an organosilicon compound and oxygen to a burner; and depositing the generated glass particles on a starting member; in the step, a mixing ratio of the raw material gas and the oxygen is adjusted so that it becomes a maximum when a ratio of an elapsed time to the entire step is greater than 0 and not greater than 0.2; and when an average value of the mixing ratio in the range of the elapsed time ratio between 0.667 and 1 is R1, and the maximum value is R2, R2 / R1 is greater than 1 and not greater than 1.27.

[0007] In the manufacturing method, the mixing ratio of the premixed gas is adjusted so that R2 / R1 is greater than 1 and is equal to or less than 1.27. When R2 / R1 is greater than 1 and is equal to or less than 1.27, the ratio of the supply amount of oxygen to the supply amount of the raw material gas can be optimized in the early stage of the deposition process. Therefore, the surface temperature of the soot L can be suppressed. Therefore, the bulk density of the soot can be suppressed in the early stage of the deposition process. Therefore, chlorine is easily diffused into the soot in the dehydration process, and the efficiency of the dehydration process is improved.

[0008] The source gas may be octamethylcyclotetrasiloxane, and the mixture ratio may be 0.17 or more and 0.25 or less.

[0009] In the manufacturing method, in the process, a traverse is performed multiple times to move the burner relative to the starting member in the longitudinal direction, and by performing the traverse multiple times, the maximum mixing ratio can be made to be substantially constant over a predetermined elapsed time.

[0010] The mixture ratio that becomes the maximum value can be a local maximum value.

[0011] The mixing ratio when the elapsed time ratio is 0 and the mixing ratio when the elapsed time ratio is 1 may be substantially equal.

[0012] Another aspect of the present invention provides a method for producing an optical fiber, comprising drawing an optical fiber preform obtained by the method for producing an optical fiber preform as described above to obtain an optical fiber. Effect of the Invention

[0013] One aspect of the present invention provides a method for manufacturing an optical fiber preform and a method for manufacturing an optical fiber, which are capable of optimizing the bulk density of soot. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a schematic diagram showing a manufacturing apparatus according to the present embodiment. [Diagram 2] FIG. [Diagram 3] 4 is a diagram showing an example of the relationship between the mixture ratio of a raw material gas and oxygen in a premixed gas and the surface temperature of soot. FIG. [Figure 4] 4 is a diagram showing a change in surface temperature of soot in a manufacturing method according to an embodiment. FIG. [Diagram 5] FIG. 11 is a diagram showing a change in surface temperature of soot in a manufacturing method of a first comparative embodiment. [Figure 6] FIG. 11 is a diagram showing a change in surface temperature of soot in a manufacturing method of a second comparative embodiment. [Figure 7] FIG. 2 is a schematic diagram showing a schematic configuration of a wire drawing device. [Figure 8] 1A is a diagram showing the surface temperature of soot in an example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. [Figure 9] 1A is a diagram showing the surface temperature of soot in an example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. [Figure 10] 1A is a diagram showing the surface temperature of soot in an example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. [Figure 11] 1A is a diagram showing the surface temperature of soot in an example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. [Figure 12] 1A is a diagram showing the surface temperature of soot in an example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. [Figure 13] 1A is a diagram showing the surface temperature of soot in a comparative example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. [Figure 14] 1A is a diagram showing the surface temperature of soot in a comparative example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. [Figure 15] 1A is a diagram showing the surface temperature of soot in a comparative example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. [Figure 16] 1A is a diagram showing the surface temperature of soot in a comparative example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. [Figure 17] 1A is a diagram showing the surface temperature of soot in a comparative example, and FIG. 1B is a diagram showing the mixture ratio of raw material gas and oxygen gas in a premixed gas. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for manufacturing an optical fiber preform and a method for manufacturing an optical fiber according to an embodiment of the present invention will be described with reference to the drawings.

[0016] [Optical fiber preform manufacturing equipment] Fig. 1 is a schematic diagram showing an apparatus for producing an optical fiber preform according to an embodiment (hereinafter, referred to as a producing apparatus 1) Fig. 2 is a schematic diagram showing a supplying device.

[0017] As shown in FIG. 1, the manufacturing apparatus 1 includes a pair of rotary chucks 3, a plurality of burners 4, a support table 5, and a supply device 10. The pair of rotating chucks 3 respectively support one end and the other end of a starting member M. The rotating chuck 3 can rotate the starting member M in a direction around its axis.

[0018] The burners 4 are arranged at intervals in the longitudinal direction of the starting member M. The support table 5 carries a plurality of burners 4. The support table 5 is movable along the longitudinal direction of the starting member M. Therefore, the burners 4 are movable along the longitudinal direction of the starting member M. A part or the whole of the supply device 10 may also move together with the burners 4 and the support table 5.

[0019] The burner 4 and the support base 5 move back and forth along the longitudinal direction of the starting member M. The reciprocating motion range of the burner 4 and the support base 5 may be shorter than the overall length of the effective portion E of the starting member M. In this case, the multiple burners 4 each reciprocate within a short motion range having different positions in the longitudinal direction of the starting member M. The motion range is a range obtained by dividing the effective portion E of the starting member M into multiple parts in the longitudinal direction.

[0020] In the manufacturing apparatus 1 shown here, the burner 4 is movable in the longitudinal direction of the starting member M, but it is sufficient that the burner 4 is movable in the longitudinal direction relative to the starting member M. For example, in the manufacturing apparatus 1, the starting member M may be movable in the longitudinal direction instead of the burner 4. Both the burner 4 and the starting member M may be movable. In other words, the manufacturing apparatus 1 can be configured so that at least one of the burner 4 and the starting member M is movable in the longitudinal direction of the starting member M.

[0021] As shown in FIG. 2, a flame is generated at the outlet 4a of the burner 4 by burning a premixed gas Gm, which will be described later, or by burning a combustible gas Gf supplied to the burner 4. The flame generated by the combustion of the combustible gas Gf is called the "combustible gas combustion flame F1," and the flame generated by the combustion of the premixed gas Gm is called the "raw material combustion flame F2." The raw material gas contained in the premixed gas Gm reacts in the raw material combustion flame F2 to generate glass particles. The glass particles are deposited on the surface of the starting member M to form a deposition layer (soot L) of the glass particles. This results in a porous glass particle body. In FIG. 1, the soot L has an effective portion E. The effective portion E is a portion where the thickness of the soot L is stable and where the optical fiber can be used as a product when the optical fiber preform is drawn.

[0022] The source gas contains an organic silicon compound. An example of the organic silicon compound is alkylcyclosiloxane. Octamethylcyclotetrasiloxane (OMCTS) is particularly suitable. OMCTS is also called "D4", with the "D" standing for (CH 3 ) 2 D4(C 8 H 24 O 4 S 4 ) is widely used industrially and is easily available, but D3(C 6 H 18 O 3 S 3 ), D5(C 10 H 30 O 5 S 5 ) may be used. D3, D4, and D5 may be used alone or in combination. The organosilicon compounds do not generate hydrochloric acid even when they are oxidized, which contributes to reducing the environmental load and reducing production costs by eliminating the need for hydrochloric acid treatment equipment.

[0023] The supply device 10 includes a raw material supply line 11 , a burner line 12 , a vent line 13 , a purge gas supply line 14 , and a control unit 15 . A first valve 11a, a flow rate regulator 11b, and a vaporizer unit 11d are provided on the raw material supply line 11. In addition, a carrier gas supply line 11c and an oxygen supply line 11e are connected to the raw material supply line 11.

[0024] The upstream side of the raw material supply line 11 is connected to a supply source (not shown) of the liquid raw material. The supply source of the liquid raw material supplies the liquid raw material to the raw material supply line 11. The supply source may include, for example, a container and a pressurized gas pipe. The container contains the liquid raw material and is connected to the raw material supply line 11. The pressurized gas pipe supplies a pressurized gas having a low solubility in the liquid raw material to the container. The supply source supplies the pressurized gas from the pressurized gas pipe to the container, thereby increasing the pressure inside the container and supplying the liquid raw material from the container to the raw material supply line 11. A degassing device that degasses dissolved gas from the liquid raw material may be provided between the container and the flow rate adjustment unit 11b.

[0025] At the downstream end, raw material supply line 11 branches into burner line 12 and vent line 13. First valve 11a is located upstream of flow rate regulator 11b and vaporization unit 11d, and switches whether or not liquid raw material flows through raw material supply line 11. Flow rate regulator 11b is located between first valve 11a and vaporization unit 11d, and regulates the flow rate of the liquid raw material flowing toward vaporization unit 11d. Flow rate regulator 11b is an MFC (Mass Flow Controller) or the like.

[0026] The carrier gas supply line 11c supplies a carrier gas to the raw material supply line 11. The carrier gas may be argon (Ar), helium (He), nitrogen (N 2 ), or oxygen (O 2 The carrier gas may contain a plurality of types of gases. The oxygen supply line 11e is connected to the downstream side of the vaporization unit 11d and supplies oxygen (O2 ) is supplied to the raw material supply line 11. This produces a premixed gas Gm. 2 ) is required to make the raw gas combustible.

[0027] The burner line 12 is connected to the downstream end of the raw material supply line 11. The downstream end of the burner line 12 is connected to the burner 4. The burner line 12 supplies the premixed gas Gm to the burner 4. The burner line 12 is provided with a second valve 12a. The second valve 12a opens and closes the burner line 12. When the second valve 12a is open, the premixed gas Gm can flow toward the burner 4. A purge gas supply line 14 is connected to the burner line 12 at a portion between the second valve 12a and the burner 4.

[0028] A third valve 13a is provided in the vent line 13. The third valve 13a opens and closes the vent line 13. A treatment device (not shown) is provided downstream of the vent line 13. When the third valve 13a is open and the second valve 12a is closed, the premixed gas Gm flowing from the raw material supply line 11 flows through the vent line 13 toward the treatment device.

[0029] The purge gas supply line 14 supplies a purge gas to the burner 4. An inert gas such as nitrogen can be used as the purge gas. A fourth valve 14a is provided in the purge gas supply line 14. The fourth valve 14a opens and closes the purge gas supply line 14. When the fourth valve 14a is open, the purge gas can flow toward the burner 4.

[0030] The control unit 15 is electrically connected to the first valve 11a, the second valve 12a, the third valve 13a, the fourth valve 14a, and the flow rate adjustment unit 11b. The control unit 15 outputs signals for switching between opening and closing of the valves 11a to 14a, and signals for instructing the set value of the flow rate by the flow rate adjustment unit 11b.

[0031] The control unit 15 may be a part of the flow rate adjusting unit 11b. The control unit 15 may be configured to control not only the supply device 10 but also the entire manufacturing apparatus 1. As the control unit 15, for example, an integrated circuit such as a microcontroller, an IC (Integrated Circuit), an LSI (Large-scale Integrated Circuit), or an ASIC (Application Specific Integrated Circuit) can be used.

[0032] The liquid raw material is introduced into the vaporization unit 11d together with a carrier gas and vaporized therein, with the flow rate being controlled by the control unit 15 and the flow rate adjustment unit 11b. The obtained raw material gas is mixed with oxygen (O 2 ) to form premixed gas Gm. The premixed gas Gm is supplied to the burner 4 through the burner line 12. When D3, D4, or D5 is used as the raw material, the number of D units contained in the premixed gas Gm is n1, and the amount of oxygen is n2. The mixing ratio R (molar ratio) of the number of D units to oxygen in the premixed gas Gm is expressed as R=n1 / n2.

[0033] The mixture ratio R is preferably in the range of 0.68≦R≦1.00. When oxygen is contained in the carrier gas, the amount of substance n2 of oxygen in the mixture ratio R is the sum of the amounts of substance of oxygen supplied from the carrier gas supply line 11c and the oxygen supply line 11e.

[0034] For example, if the raw material is D4, one mole of the raw material contains four D units, so if the amount of substance of the raw material is m, then n1 = m x 4. If the mixing ratio R' of the premixed gas Gm is expressed by the amount of substance m of D4, then R' = m / n2. Therefore, in order to obtain 0.68 ≦ R ≦ 1.00, 0.17 ≦ R' ≦ 0.25 is used. Similarly, if the raw material is D3, then n1 = m x 3. If the raw material is D5, then n1 = m x 5. When raw materials with different numbers of D units (D3, D4, D5, etc.) are mixed and used, the value of n1 is calculated taking into account the number of D units relative to the amount of substance of each raw material.

[0035] By setting the mixture ratio R' to 0.17 or more, the soot can be heated efficiently. The reason for this can be assumed as follows. If the mixing ratio R' is less than 0.17, the amount of oxygen supplied will be large relative to the amount of raw material supplied, causing the burning speed to increase, and the flame may become unstable. Even if this flame hits the soot, it may be difficult to heat the soot sufficiently and efficiently, and the surface temperature may decrease. However, by setting the mixing ratio R' to 0.17 or more, the amount of oxygen supplied excessively can be suppressed. Therefore, the burning speed can be suppressed from becoming excessively fast, and the flame can be suppressed from becoming unstable. Therefore, it may be possible to heat the soot sufficiently and efficiently. Therefore, the decrease in the surface temperature can be suppressed.

[0036] By setting the mixture ratio R' to 0.25 or less, the carbon components in the raw material can be sufficiently combusted and the generation of soot can be suppressed. The reason for this can be assumed as follows. A large mixing ratio R' means that the amount of oxygen supplied is small relative to the amount of raw material supplied. Since a small amount of oxygen is likely to result in incomplete combustion, increasing the mixing ratio R' may result in a decrease in the surface temperature. If R' exceeds 0.25, the carbon components in the raw materials may not be completely burned and may accumulate as black soot. This may cause foreign matter to be mixed in during the subsequent sintering process. However, by keeping the mixing ratio R' at 0.25 or less, the carbon components in the raw materials can be sufficiently burned and the formation of black soot can be suppressed, thereby suppressing the introduction of foreign matter during the sintering process.

[0037] In addition to the burner line 12, a flammable gas pipe and an oxygen pipe are connected to the burner 4. The flammable gas pipe is hydrogen (H 2 ), methane (CH 3 The oxygen piping supplies a combustible gas Gf such as oxygen (O 2 ) is supplied to burner 4.

[0038] The burner 4 has a premixed gas port from which the premixed gas Gm is ejected, and a gas port for ejecting the combustible gas Gf and oxygen (O2 ) are mixed and ejected. At the outlet 4a of the burner 4, the combustible gas Gf and oxygen are mixed and burned to generate a combustible gas combustion flame F1. The premixed gas Gm is ignited by the combustible gas combustion flame F1, and the raw materials react in the raw material combustion flame F2 to generate glass particles. The combustible gas combustion flame F1 may surround the raw material combustion flame F2. In this case, it is possible to suppress the generated glass particles from diffusing outward and to stabilize the temperature of the raw material combustion flame F2. In order to configure the combustible gas combustion flame F1 to surround the raw material combustion flame F2, for example, a premixed gas port may be arranged in the center of the burner 4, and ports from which the combustible gas Gf and oxygen are ejected may be arranged around the premixed gas port.

[0039] [Manufacturing method of optical fiber preform] Next, a method for manufacturing an optical fiber preform according to an embodiment will be described. The manufacturing method of the embodiment includes (1) a deposition step and (2) a sintering step.

[0040] (1) Deposition process As shown in Fig. 1, a starting member M is attached to a rotary chuck 3. While rotating the starting member M around its axis by the rotary chuck 3, the burner 4 and the starting member M are moved back and forth relative to each other in the longitudinal direction. For example, the burner 4 is moved back and forth in the longitudinal direction of the starting member M. The control unit 15 opens the first valve 11a and the second valve 12a, and closes the third valve 13a and the fourth valve 14a.

[0041] As shown in FIG. 2, the liquid raw material is introduced into the vaporization unit 11d together with a carrier gas and vaporized. The obtained raw material gas is mixed with oxygen (O 2 ) to obtain a premixed gas Gm. The premixed gas Gm is supplied to the burner 4 through the burner line 12. Glass particles are generated by burning the premixed gas Gm at the outlet 4a of the burner 4. The glass particles are deposited on the starting material M to form soot L (see FIG. 1).

[0042] The control unit 15 and the flow rate adjustment unit 11b adjust the mixture ratio of the premixed gas Gm supplied to the burner 4. R’ The flow rate of the liquid source introduced into the vaporization unit 11d can be adjusted so that the flow rate is within an appropriate range. The mixture ratio R’ In order to adjust the amount of oxygen (O 2 ) may be adjusted to adjust the flow rate.

[0043] Hereinafter, with reference to FIG. 3 and FIG. 4, the mixing ratio of the premixed gas Gm supplied to the burner 4 will be described. R’ This article explains: Figure 3 shows the relationship between the raw gas and oxygen (O 2 ) and the surface temperature of the soot. 2 ) and the mixture ratio (OMCTS / O 2 ) (molar ratio).

[0044] As shown in Figure 3, the surface temperature of the soot may increase or decrease depending on the mixing ratio in the premixed gas. This is for the following reason. The reaction in which glass particles are produced from organosilicon compounds (OMCTS) is, for example, as follows. This reaction is an exothermic reaction. [SiO(CH 3 ) 2 ] 4 + 16O 2 → 4SiO 2 + 8CO 2 + 12H 2 O The mixing ratio of the premixed gas affects the efficiency of the reaction, and therefore the amount of heat generated during the reaction is affected by the mixing ratio of the premixed gas, and therefore the surface temperature of the soot may be affected by the mixing ratio of the premixed gas.

[0045] In the manufacturing method of the embodiment, the mixing ratio of the premixed gas Gm R’The molar ratio is adjusted so that it reaches a maximum value (maximum value for the entire deposition process) when the ratio of the elapsed time to the entire deposition process is greater than 0 and less than 0.2. The ratio of the elapsed time to the entire deposition process is called the "relative elapsed time." "Exceeding 0" means exceeding 0.

[0046] Mixing ratio R’ When the relative elapsed time when is the maximum value is over 0 and is less than 0.2, the mixture ratio R’ reaches a maximum value at the beginning of the deposition process. Therefore, the surface temperature of the soot L also reaches a maximum value at the beginning of the deposition process. Therefore, the bulk density of the soot L tends to have a distribution that gradually decreases radially outward. Therefore, chlorine is easily diffused into the soot in the dehydration process, and the efficiency of the dehydration process is improved.

[0047] Maximum mixing ratio R’ may be a substantially constant value over a predetermined elapsed time. For example, in FIG. 11(B), the mixture ratio R’ The relative elapsed time is in the range of 0.02 to 0.17 and the mixture ratio is constant at 0.24. R’ If the maximum value of is set to a substantially constant value, the change in the gas supply conditions within the relative elapsed time becomes small, so that the gas supply amount can be determined with high accuracy. R’ can be determined with high accuracy. Therefore, the supply stability of the premixed gas Gm can be improved. In addition, since the gas supply amount can be determined with high accuracy, the flame can be stabilized. Therefore, stable soot formation is possible.

[0048] "Substantially constant value" means that the value is within ±2% of the reference value. For example, if the reference value is 0.24, the mixing ratio is R’ If is greater than or equal to 0.2352 and less than or equal to 0.2448 for the specified elapsed time, the mixture ratio R’ is essentially a constant value.

[0049] Maximum mixing ratio R’ If the mixture ratio is set to a substantially constant value, R’The length of the period (elapsed time) during which the value of the saturation voltage Vs is substantially constant may be, for example, 0.01 to 0.15 in terms of the relative elapsed time with respect to the entire deposition process. Maximum mixing ratio R’ The method to make the value of R’ When the constant value is a relatively low value, the effect of improving the supply stability of the premixed gas Gm is easily exhibited (see FIG. 11(B)). Note that the constant value is also called a steady value.

[0050] The maximum mixing ratio in the deposition process R’ In order to make the temperature of the starting material M substantially constant over a predetermined elapsed time, a method of traversing the burner 4 multiple times relative to the starting material M can be used (see FIG. 1). The supply conditions of the raw material gas and oxygen may be the same for each of the multiple traverses.

[0051] The reciprocating movement of the burner 4 includes a unidirectional path in a first direction and a unidirectional path in a second direction opposite to the first direction. A traverse is a linear movement along the unidirectional path. A traverse is, for example, a movement in which the burner moves longitudinally relative to the starting member.

[0052] Maximum mixing ratio R’ For example, in FIG. 10B, the mixture ratio R’ reaches a maximum value of 0.244 when the relative elapsed time is 0.039. R’ When takes a maximum value, the mixture ratio R’ Since the time when the gas supply amount is likely to fluctuate is short, the mixing ratio R’ This reduces the risk that the maximum value will deviate from the set value. Mixing ratio R’ The method of making the mixture ratio R’ is relatively high (see FIG. 10(B)).

[0053] In the manufacturing method of the embodiment, the mixing ratio of the premixed gas Gm R’By adjusting the above, R2 / R1 is made to exceed 1 and be 1.27 or less (preferably 1.1 or less). "R1" is the mixture ratio when the relative elapsed time is in the range of 0.667 or more and 1 or less. R’ "R2" is the average value of the mixing ratio of the premixed gas Gm in the deposition process. R’ is the maximum value of.

[0054] When the relative elapsed time is in the range of 0.667 or more and 1 or less, the mixture ratio is higher than when it is in the range of 0 or more and less than 0.667. R’ The variation of is small. Therefore, the mixing ratio in the range of relative elapsed time from 0.667 to 1 is R’ If the average value of R’ Therefore, in the manufacturing method of the embodiment, the range of the relative elapsed time of 0.667 or more and 1 or less is adopted as the calculation standard for R2 / R1.

[0055] When R2 / R1 is greater than 1 and is equal to or less than 1.27, the ratio of the supply amount of oxygen to the supply amount of raw material gas (organic silicon compound) can be optimized in the early stage of the deposition process. Therefore, the surface temperature of the soot L can be suppressed. Therefore, the bulk density of the soot L can be suppressed in the early stage of the deposition process. Therefore, chlorine is easily diffused into the soot L in the dehydration process, and the efficiency of the dehydration process is improved.

[0056] When R2 / R1 is 1.27 or less, the mixing ratio R’ is unlikely to decrease significantly. Since the ratio of the supply amount of the raw material gas to the supply amount of oxygen does not become too large, the decrease in the surface temperature of the soot L can be suppressed. Therefore, the bulk density of the soot L does not become too low even in the later stage of the deposition process. Therefore, soot cracking is unlikely to occur. In addition, since the bulk density of the soot does not become too low in the later stage of the deposition process, problems such as the soot diameter becoming large and the porous glass particles becoming difficult to enter the sintering furnace are unlikely to occur. Therefore, the equipment cost of the sintering furnace can be suppressed. In the manufacturing method of the embodiment, the amount of flammable gas used can be reduced while the decrease in the surface temperature of the soot L can be suppressed, so that costs can be reduced.

[0057] If R2 / R1 is less than 1.27, the mixture ratio R’ Since the maximum value of is suppressed, the combustion reaction of the raw gas is stabilized. As a result, a stable flame is formed and the soot deposition efficiency can be improved.

[0058] R2 / R1 should be greater than 1, and is preferably 1.03 or greater. When R2 / R1 exceeds 1, the surface temperature of the soot L tends to be high in the early stage of the deposition process and low in the later stage. Therefore, the bulk density of the soot L tends to be distributed so that it gradually decreases radially outward. Therefore, chlorine is more likely to diffuse into the soot in the dehydration process, and the efficiency of the dehydration process is improved.

[0059] Fig. 4 is a diagram showing the change in the surface temperature of the soot in the manufacturing method of the embodiment. The "target temperature" in Fig. 4 is the surface temperature at which an ideal soot can be formed in which the bulk density gradually decreases radially outward, and the degree of decrease is not too large. The target temperature varies depending on the manufacturing conditions (e.g., the type of optical fiber, the size of the manufacturing device, etc.).

[0060] As shown in Fig. 4, in the manufacturing method of the embodiment, the surface temperature of the soot is suppressed in the early stage of the deposition process, and the decrease in the surface temperature of the soot is suppressed in the later stage. Therefore, the surface temperature of the soot can be brought closer to the target temperature. In Fig. 4, the difference between the surface temperature of the soot and the target temperature is small. Therefore, the soot bulk density can be suppressed in the early stage of the deposition process, and the decrease in the soot bulk density in the later stage of the deposition process can be reduced. Therefore, sufficient dehydration efficiency can be obtained in the dehydration process, and problems such as soot cracking can be suppressed.

[0061] In order to clarify the effect of the manufacturing method of the embodiment, a manufacturing method of a comparative embodiment will be described with reference to FIGS. 5 shows the change in soot surface temperature in the manufacturing method of the first comparative example, in which the supply conditions of all gases were kept constant throughout the deposition process.

[0062] As shown in Fig. 5, in the early stage of the deposition process, the soot diameter is small and the heat capacity of the soot is small, so the surface temperature of the soot is high. Therefore, the bulk density of the soot is high. If the bulk density becomes too high, it becomes difficult for chlorine to diffuse into the soot in the dehydration process, and the efficiency of the dehydration process may be reduced.

[0063] In the later stage of the deposition process, the heat capacity of the soot increases due to the increase in soot diameter, so the surface temperature of the soot tends to decrease. As a result, the bulk density of the soot decreases. If the bulk density becomes too low, problems such as soot cracking and the porous glass particles becoming difficult to enter the sintering furnace tend to occur.

[0064] FIG. 6 shows the change in the surface temperature of the soot in the manufacturing method of the second comparative embodiment. As shown in FIG. 6, in the second comparative embodiment, an attempt was made to suppress the surface temperature of the soot by reducing the burner flame by adjusting the combustible gas flow rate at the beginning of the deposition process. However, considering ensuring flame stability, there is a limit to how much the flame can be adjusted, and so it was difficult to sufficiently lower the surface temperature of the soot at the beginning of the deposition process. Therefore, in the manufacturing method of the second comparative embodiment, as in the first comparative embodiment (see FIG. 5), the bulk density of the soot increases, which may reduce the efficiency of the dehydration process in the dehydration process.

[0065] In the later stages of the deposition process, the surface temperature of the soot tends to be low, and the bulk density of the soot may be low, as in the first comparative embodiment (see FIG. 5). The surface temperature of the soot can be increased by increasing the flow rate of the combustible gas, but this increases the amount of combustible gas used, which may result in cost problems.

[0066] Thus, in the comparative manufacturing method shown in Figures 5 and 6, the bulk density of the soot tends to be high in the early stage of the deposition process, and tends to be low in the later stage of the deposition process. In contrast, in the manufacturing method of the embodiment shown in Figure 4, the soot bulk density can be suppressed in the early stage of the deposition process, and the decrease in the soot bulk density in the later stage of the deposition process can be reduced. Therefore, sufficient dehydration efficiency can be obtained in the dehydration process, and problems such as soot cracking can be suppressed.

[0067] Mixture ratio when relative elapsed time is 0 R’ and the mixture ratio when the relative elapsed time is 1 R’ It is preferable that the mixing ratio when the relative elapsed time is 0 is substantially equal to the mixing ratio when the relative elapsed time is 0. R’ is called "mixing ratio R3". The mixing ratio when the relative elapsed time is 1 R’ is called "mixing ratio R4". "Substantially equal" means that when mixing ratio R3 and mixing ratio R4 are compared, the difference between mixing ratio R3 and mixing ratio R4 is within a range of ±2% of mixing ratio R3 or mixing ratio R4.

[0068] When the mixing ratio R3 and the mixing ratio R4 are substantially equal, the surface temperature of the starting material M (see FIG. 1) can be increased in the initial stage of the deposition process. Since the surface temperature of the starting material M can be optimized in the initial stage of the deposition process, the bulk density of the soot L is good. In addition, since the mixing ratio R3 and the mixing ratio R4 are substantially equal, the amount of flammable gas used can be reduced.

[0069] As shown in FIG. 1, by completing the formation of soot L, porous glass particles are obtained. 2, after the generation of the glass particles is stopped, the second valve 12a may be closed and the fourth valve 14a may be opened to supply the purge gas to the burner 4. This allows the premixed gas Gm remaining in the burner 4 and the burner line 12 to be discharged outside the burner 4.

[0070] (2) Sintering process The porous glass particles are sintered to obtain an optical fiber preform. In addition to the sintering, the optical fiber preform may be subjected to a dehydration treatment, a doping treatment, or the like, if necessary.

[0071] [Optical fiber manufacturing method] In the method for producing an optical fiber according to the embodiment, the optical fiber preform described above is drawn to obtain an optical fiber wire (optical fiber). An example of a drawing step in the manufacturing method of the optical fiber according to the embodiment will be described with reference to FIG.

[0072] FIG. 7 is a schematic diagram showing a schematic configuration of the drawing apparatus 200. As shown in FIG. 7, in the spinning section 110, the optical fiber preform 102 is heated by a heating furnace 112. This causes the optical fiber preform 102 to be melt-spun to form a bare optical fiber 103. The outer diameter of the bare optical fiber 103 may be measured in the outer diameter measuring section 120. The bare optical fiber 103 may be cooled in a cooling section 130.

[0073] In the coating unit 140, a coating material is applied (coated) to the outer periphery of the bare optical fiber 103 to form a coating layer, thereby obtaining a coated optical fiber 104. In the curing unit 150, the coating layer of the coated optical fiber 104 is cured by heating, UV irradiation, or the like to obtain an optical fiber strand 105 (optical fiber). The optical fiber strand 105 is taken up by a take-up unit 160 and wound up by a winding unit 170.

[0074] [Effects of the manufacturing method of the optical fiber preform and the manufacturing method of the optical fiber according to the embodiment] In the manufacturing method of the embodiment, the mixing ratio of the premixed gas Gm R’ The molar ratio is adjusted so that it reaches a maximum value (maximum value for the entire deposition process) when the relative elapsed time is between 0 and 0.2. R’ When the relative elapsed time when is the maximum value is over 0 and is less than 0.2, the mixture ratio R’reaches a maximum value at the beginning of the deposition process. Therefore, the surface temperature of the soot L also reaches a maximum value at the beginning of the deposition process. Therefore, the bulk density of the soot L tends to have a distribution that gradually decreases radially outward. Therefore, chlorine is easily diffused into the soot in the dehydration process, and the efficiency of the dehydration process is improved.

[0075] In the manufacturing method of the embodiment, the mixing ratio of the premixed gas Gm R’ By adjusting the ratio, R2 / R1 is made to be more than 1 and not more than 1.27 (preferably not more than 1.1). When R2 / R1 is more than 1 and not more than 1.27, the ratio of the supply amount of oxygen to the supply amount of raw material gas (organic silicon compound) can be optimized in the initial stage of the deposition process. Therefore, the surface temperature of the soot L can be suppressed. Therefore, the bulk density of the soot L can be suppressed in the initial stage of the deposition process. Therefore, chlorine is easily diffused into the soot L in the dehydration process, and the efficiency of the dehydration process is improved.

[0076] When R2 / R1 is 1.27 or less, the mixing ratio R’ is unlikely to decrease significantly. Since the ratio of the supply amount of the raw material gas to the supply amount of oxygen does not become too large, the decrease in the surface temperature of the soot L can be suppressed. Therefore, the bulk density of the soot L does not become too low even in the later stage of the deposition process. Therefore, soot cracking is unlikely to occur. In addition, since the bulk density of the soot does not become too low in the later stage of the deposition process, problems such as the soot diameter becoming large and the porous glass particles becoming difficult to enter the sintering furnace are unlikely to occur. Therefore, the equipment cost of the sintering furnace can be suppressed. In the manufacturing method of the embodiment, the amount of flammable gas used can be reduced while the decrease in the surface temperature of the soot L can be suppressed, so that costs can be reduced.

[0077] If R2 / R1 is less than 1.27, the mixture ratio R’ Since the maximum value of is suppressed, the combustion reaction of the raw gas is stabilized. As a result, a stable flame is formed and the soot deposition efficiency can be improved.

[0078] R2 / R1 should be greater than 1, and is preferably 1.03 or greater. When R2 / R1 exceeds 1, the surface temperature of the soot L tends to be high in the early stage of the deposition process and low in the later stage. Therefore, the bulk density of the soot L tends to be distributed so that it gradually decreases radially outward. Therefore, chlorine is more likely to diffuse into the soot in the dehydration process, and the efficiency of the dehydration process is improved. EXAMPLES

[0079] The above embodiment will be described below using specific examples, but the present invention is not limited to the following examples.

[0080] Example 1 An optical fiber preform was produced using the manufacturing apparatus 1 shown in FIG. 1. The raw material, the flow rate of which was adjusted by the flow rate adjustment unit 11b, was introduced together with a carrier gas into the vaporization unit 11d and vaporized (see FIG. 2). The obtained raw material gas was mixed with oxygen (O 2 ) to obtain a premixed gas Gm. D4 was used as the raw material. Ar was used as the carrier gas. An MFC was used as the flow rate adjustment unit 11b. The premixed gas Gm was supplied to a burner, and glass particles were deposited on a starting member (deposition process). The porous glass particles obtained in the deposition process were subjected to a dehydration process (dehydration process) and a sintering process (sintering process) to obtain an optical fiber preform.

[0081] As shown in FIG. 8B, in the deposition process, the flow rate of D4 is adjusted by the flow rate adjusting unit 11b (see FIG. 2) to adjust the mixture ratio of the premixed gas Gm. R’ The mixture ratio was adjusted. R’ is 0.191 at the beginning of the deposition process. R’ reaches a maximum value of 0.201 when the relative elapsed time is approximately 0.03. This maximum value is a local maximum value (peak value). When the relative elapsed time is 1, the mixture ratio R’ The mixing ratio for relative elapsed time in the range of 0.667 to 1 is 0.188. R’The average value of R1 is 0.188, so R2 / R1 is 1.07. The relative elapsed time is between 0.667 and 1, but not between 0 and 0.667. R’ is roughly constant.

[0082] As shown in Fig. 8(A), the maximum difference between the soot surface temperature and the target temperature was about 13°C. Therefore, the difference between the soot surface temperature and the target temperature was kept within a range of 25°C or less (more specifically, 20°C or less). No problems occurred during the sintering process, and the optical fiber preform was well dehydrated.

[0083] Example 2 As shown in FIG. 9(B), the mixture ratio R’ is 0.191 at the beginning of the deposition process. R’ The relative elapsed time is constant in the range of 0.03 to 0.08. R’ is the maximum value of this process, 0.201. When the relative elapsed time is 1, the mixing ratio R’ The mixing ratio in the range of the relative elapsed time from 0.667 to 1 was 0.188. The optical fiber preform was fabricated in the same manner as in Example 1. R’ The average value of R1 is 0.188, so R2 / R1 is 1.07. The relative elapsed time is between 0.667 and 1, but not between 0 and 0.667. R’ is roughly constant.

[0084] As shown in Fig. 9(A), the maximum difference between the soot surface temperature and the target temperature was about 13°C. Therefore, the difference between the soot surface temperature and the target temperature was kept within a range of 25°C or less (more specifically, 20°C or less). No problems occurred during the sintering process, and the optical fiber preform was well dehydrated.

[0085] Example 3 An optical fiber preform was produced using the manufacturing apparatus 1 shown in FIG. 1. The raw material, the flow rate of which was adjusted by the flow rate adjustment unit 11b, was introduced together with a carrier gas into the vaporization unit 11d and vaporized (see FIG. 2). The obtained raw material gas was mixed with oxygen (O 2 ) to obtain a premixed gas Gm. D4 was used as the raw material. Ar was used as the carrier gas. An MFC was used as the flow rate adjustment unit 11b. The premixed gas Gm was supplied to a burner, and glass particles were deposited on a starting member (deposition process). The porous glass particles obtained in the deposition process were subjected to a dehydration process (dehydration process) and a sintering process (sintering process) to obtain an optical fiber preform.

[0086] As shown in FIG. 10(B), the mixture ratio R’ is 0.222 at the beginning of the deposition process. R’ When the relative elapsed time is 0.039, the maximum value is 0.244. This maximum value is a local maximum value (peak value). When the relative elapsed time is 1, the mixture ratio R’ The mixing ratio for relative elapsed time in the range of 0.667 to 1 is 0.222. R’ The average value of R1 is 0.222, so R2 / R1 is 1.10. The relative elapsed time is between 0.667 and 1, but not between 0 and 0.667. R’ is roughly constant.

[0087] As shown in Fig. 10(A), the maximum difference between the soot surface temperature and the target temperature was about 17°C. Therefore, the difference between the soot surface temperature and the target temperature was kept within a range of 25°C or less (more specifically, 20°C or less). No problems occurred during the sintering process, and the optical fiber preform was well dehydrated.

[0088] Example 4 As shown in FIG. 11(B), the mixture ratio R’ is 0.222 at the beginning of the deposition process. R’ The relative elapsed time is constant in the range of 0.02 to 0.17. R’is the maximum value of this process, 0.24. When the relative elapsed time is 1, the mixture ratio R’ The mixing ratio in the range of the relative elapsed time from 0.667 to 1 was 0.222. The optical fiber preform was fabricated in the same manner as in Example 3 except for the above. R’ The average value of R1 is 0.222, so R2 / R1 is 1.10. The relative elapsed time is between 0.667 and 1, but not between 0 and 0.667. R’ is roughly constant.

[0089] As shown in Fig. 11(A), the maximum difference between the soot surface temperature and the target temperature was about 17°C. Therefore, the difference between the soot surface temperature and the target temperature was kept within a range of 25°C or less (more specifically, 20°C or less). No problems occurred during the sintering process, and the optical fiber preform was well dehydrated.

[0090] Example 5 As shown in FIG. 12(B), the mixture ratio R’ is 0.191 at the beginning of the deposition process. R’ reaches a maximum value of 0.201 when the relative elapsed time is approximately 0.03. This maximum value is a local maximum value (peak value). When the relative elapsed time is 1, the mixture ratio R’ The mixing ratio in the range of the relative elapsed time from 0.667 to 1 is about 0.16. The optical fiber preform was fabricated in the same manner as in Example 1 except for the above. R’ Since the average value R1 was 0.159, R2 / R1 was 1.27.

[0091] 12(A), the maximum difference between the soot surface temperature and the target temperature was about 23° C. Therefore, the difference between the soot surface temperature and the target temperature was kept within a range of 25° C. or less. When the relative elapsed time was in the range of 0.667 or more and 1 or less, the stability of the burner flame was somewhat decreased. No problems occurred during the sintering process, and the optical fiber preform was well dehydrated.

[0092] Comparative Example 1 As shown in FIG. 13(B), the mixture ratio R’ was kept constant (0.191) from the start to the end of the deposition process. Except for that, the optical fiber preform was produced in the same manner as in Example 1. R’ Since the average value R1 was 0.191, R2 / R1 was 1.00.

[0093] 13(A), the soot surface temperature was higher than the target temperature by a maximum of about 65° C. Therefore, it was not possible to keep the difference between the soot surface temperature and the target temperature within a range of 25° C. or less. The optical fiber preform was not sufficiently dehydrated in the dehydration process.

[0094] Comparative Example 2 As shown in FIG. 14(B), the mixture ratio R’ is 0.191 at the beginning of the deposition process. R’ The relative elapsed time is constant in the range of 0.22 to 0.33. R’ is the maximum value of this process, 0.201. When the relative elapsed time is 1, the mixing ratio R’ The mixing ratio in the range of the relative elapsed time from 0.667 to 1 was 0.193. The optical fiber preform was fabricated in the same manner as in Example 1 except for the above. R’ Since the average value R1 was 0.193, R2 / R1 was 1.04.

[0095] 14(A), the soot surface temperature was higher than the target temperature by a maximum of about 61° C. Therefore, it was not possible to keep the difference between the soot surface temperature and the target temperature within a range of 25° C. or less. The optical fiber preform was not sufficiently dehydrated in the dehydration process.

[0096] Comparative Example 3 As shown in FIG. 15(B), the mixture ratio R’ is 0.191 at the beginning of the deposition process. R’reaches a maximum value of 0.26 when the relative elapsed time is 0.039. This maximum value is a local maximum value (peak value). When the relative elapsed time is 1, the mixture ratio R’ The mixing ratio in the range of the relative elapsed time from 0.667 to 1 was 0.188. The optical fiber preform was fabricated in the same manner as in Example 1. R’ Since the average value R1 was 0.188, R2 / R1 was 1.38.

[0097] 15(A), the soot surface temperature was lower than the target temperature by a maximum of about 40° C. Therefore, it was not possible to keep the difference between the soot surface temperature and the target temperature within a range of 25° C. or less. During the deposition process, the soot was contaminated with black soot, which affected the appearance of the optical fiber preform.

[0098] Comparative Example 4 As shown in FIG. 16(B), the mixture ratio R’ At the beginning of the deposition process, 0.191 Mixing ratio R’ indicates that the relative elapsed time is 0.02~0.22 This constant value of the mixture ratio is R’ is the maximum value of this process, 0.203. When the relative elapsed time is 1, the mixing ratio R’ The mixing ratio in the range of the relative elapsed time from 0.667 to 1 was 0.189. The optical fiber preform was fabricated in the same manner as in Example 1. R’ Since the average value R1 was 0.189, R2 / R1 was 1.07.

[0099] 16(A), the soot surface temperature was lower than the target temperature by a maximum of about 26° C. Therefore, it was not possible to keep the difference between the soot surface temperature and the target temperature within a range of 25° C. or less. The bulk density of the soot became lower and the soot diameter became larger, which made it difficult for the porous glass particles to enter the sintering furnace.

[0100] Comparative Example 5 As shown in FIG. 17(B), the mixture ratioR’ is 0.222 at the beginning of the deposition process. R’ The relative elapsed time is constant in the range of 0.02 to 0.27. R’ is the maximum value of this process, 0.24. When the relative elapsed time is 1, the mixture ratio R’ The mixing ratio in the range of the relative elapsed time from 0.667 to 1 was 0.222. The optical fiber preform was fabricated in the same manner as in Example 3 except for the above. R’ Since the average value R1 was 0.222, R2 / R1 was 1.08.

[0101] 17(A), the soot surface temperature was lower than the target temperature by a maximum of about 36° C. Therefore, it was not possible to keep the difference between the soot surface temperature and the target temperature within a range of 25° C. or less. The bulk density of the soot became lower and the soot diameter became larger, which made it difficult for the porous glass particles to enter the sintering furnace.

[0102] The test results will be considered. As shown in Fig. 13(A) and Fig. 13(B), in the comparative example 1, dehydration was insufficient in the dehydration process. In the comparative example 1, the mixing ratio of the premixed gas Gm was R’ Since the adjustment of the temperature was not performed, the surface temperature of the soot became high in the initial stage of the deposition process, which may have resulted in an increase in the bulk density of the soot, which may have resulted in insufficient dehydration in the dehydration process.

[0103] As shown in Figures 14(A) and 14(B), in the comparative example 2, dehydration was insufficient in the dehydration process. R’ Although the mixture ratio was adjusted, R’ reached a maximum value when the relative elapsed time exceeded 0.2. Therefore, the surface temperature of the soot was high in the early stage of the deposition process, as in Comparative Example 1. This increased the bulk density of the soot, which may have resulted in insufficient dehydration in the dehydration process.

[0104] As shown in FIGS. 15(A) and 15(B), in Comparative Example 3, the mixture ratio R’ It is possible that the large temperature difference between the source gas and the soot caused the combustion of the raw gas to become unstable, lowering the surface temperature of the soot. It is also believed that the soot was generated due to the unstable combustion of the raw gas.

[0105] As shown in FIG. 16(A) and FIG. 16(B), in Comparative Example 4, the mixture ratio R’ Since the maximum value of extends beyond a relative elapsed time of 0.2, it is possible that the bulk density of the soot was low overall. As shown in FIG. 17(A) and FIG. 17(B), in Comparative Example 5, the mixture ratio R’ Since the maximum value of extends beyond a relative elapsed time of 0.2, it is possible that the bulk density of the soot was low overall.

[0106] In Examples 1 to 5, unlike Comparative Examples 1 and 2, the mixing ratio is set to reach a maximum value when the relative elapsed time is greater than 0 and less than 0.2. R’ As a result, the bulk density of the soot was suppressed, and the processing efficiency in the dewatering process was improved.

[0107] In Examples 1 to 5, unlike Comparative Example 3, R2 / R1 is greater than 1 and is 1.27 or less, so the combustion of the raw material gas is stabilized and the surface temperature of the soot is optimized. Therefore, the bulk density of the soot is good. In Examples 1 to 5, the combustion of the raw material gas is stabilized, so that almost no soot is generated.

[0108] In Examples 1 to 5, unlike Comparative Examples 4 and 5, the mixing ratio is set to reach a maximum value when the relative elapsed time is greater than 0 and less than 0.2. R’ As a result, the bulk density of the soot was suppressed, and the processing efficiency in the dewatering process was improved.

[0109] The present invention has been described above based on the best mode, but the present invention is not limited to the above mode, and various modifications are possible without departing from the gist of the present invention. 1 employs a system in which the burner 4 reciprocates in a short operating range (short-distance reciprocating system), but the manufacturing apparatus may also employ a system in which the burner forms soot while reciprocating over the entire length of the effective part of the starting member (full-length reciprocating system). Even in the case of the full-length reciprocating system, the starting member, rather than the burner, may be movable in the longitudinal direction.

[0110] The manufacturing apparatus may employ a method (orbital method) in which a burner moves orbitally along a circular orbit having a partial orbit along the starting member. In the orbital method, the burner forms soot by a deposition process while moving along the partial orbit. Even when the full-length reciprocating method or the circulating method is adopted, the mixing ratio of the premixed gas can be adjusted in the deposition process.

[0111] In the orbital system, the movement of the burner going around once along the orbit is called a traverse. The reciprocating motion of the burner in the full length reciprocating mode includes a unidirectional path in a first direction and a unidirectional path in a second direction opposite the first direction. Traverse refers to linear movement along the unidirectional path. In addition, although the manufacturing apparatus 1 shown in FIG. 1 has a plurality of burners 4, the number of burners 4 may be one. R’ teeth, Relative elapsed time 0 Koshi, It may be adjusted to have a maximum value when it is 0.2 or less. [Explanation of symbols]

[0112] 1... manufacturing apparatus, 4... burner, 102... optical fiber preform, 105... optical fiber wire (optical fiber), M... starting member.

Claims

1. The method includes a step of supplying a raw material gas containing an organosilicon compound and a premixed gas containing oxygen to a burner, and depositing the generated glass particles on a starting member, In the process, a mixing ratio of the amount of substance of the raw material gas to the amount of substance of the oxygen is adjusted so as to be maximum when a ratio of an elapsed time to an entire process is more than 0 and not more than 0.2; a mean value of the mixing ratio in the range of the elapsed time ratio of 0.667 or more and 1 or less is R1, and the maximum value is R2, where R2 / R1 is greater than 1 and is 1.27 or less.

2. the source gas is octamethylcyclotetrasiloxane, 2. The method for producing an optical fiber preform according to claim 1, wherein the mixture ratio is 0.17 or more and 0.25 or less.

3. In the step, a traverse is performed a plurality of times to relatively move the burner in a longitudinal direction with respect to the starting member, 3. The method for producing an optical fiber preform according to claim 1, wherein the mixture ratio having the maximum value is a substantially constant value over a predetermined elapsed time by performing the traverse a plurality of times.

4. 3. The method for producing an optical fiber preform according to claim 1, wherein the mixing ratio that becomes the maximum value is a local maximum value.

5. 5. The method for manufacturing an optical fiber preform according to claim 1, wherein the mixing ratio when the elapsed time ratio is 0 and the mixing ratio when the elapsed time ratio is 1 are substantially equal.

6. A method for producing an optical fiber, comprising drawing an optical fiber preform obtained by the method for producing an optical fiber preform according to any one of claims 1 to 5 to obtain an optical fiber.

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