Method and apparatus for manufacturing optical fiber glass preform

Abstract

To provide a method for manufacturing an optical fiber porous preform.SOLUTION: A method for manufacturing an optical fiber glass preform comprises: a first step of dehydrating an optical fiber porous preform 02 while circulating gas including at least halogen or argon in a quartz furnace core tube 04 storing the optical fiber porous preform 02; a second step of, after the first step, circulating gas including helium as a main component in the quartz furnace core tube 04 to at least partially ventilate the inside of the quartz furnace core tube 04; and a third step of, after the second step, vitrifying the optical fiber porous preform 02 while circulating the gas including helium as the main component in the quartz furnace core tube 04.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a glass preform for an optical fiber, and an apparatus for producing a glass preform for an optical fiber. [Background technology]

[0002] Patent Document 1 states that "the method for manufacturing an optical fiber preform includes a dehydration process in which a dehydrating agent containing an inert gas is supplied into a furnace tube 11, 11A to dehydrate the porous glass preform 1, 1A (porous soot body), and a sintering process in which the dehydrated porous glass preform 1, 1A is sintered" (paragraph 0022), and that "argon gas is used as the inert gas mixed with the dehydrating agent, and a gas having a higher thermal conductivity than argon gas (hereinafter referred to as "high thermal conductivity gas") is left in the porous glass preform 1, 1A before the temperature of the porous glass preform 1, 1A is raised in the dehydration process" (paragraph 0024). [Prior art document] [Patent documents] [Patent Document 1] Patent No. 5298186 Summary of the Invention [Means for solving the problem]

[0003] In a first aspect of the present invention, there is provided a method for producing a glass preform for an optical fiber. The method may include a first step of dehydrating the porous preform for an optical fiber while flowing a gas containing at least a halogen or argon through a quartz core tube containing the porous preform for an optical fiber. The method may include a second step of at least partially ventilating the quartz core tube by flowing a gas containing helium as a main component through the quartz core tube after the first step. The method may include a third step of transparently vitrifying the porous preform for an optical fiber while flowing a gas containing helium as a main component through the quartz core tube after the second step.

[0004] In the second step, an integrated flow rate V2 of the gas containing helium as a main component that is caused to flow through the quartz furnace tube may satisfy the following formula [Mathematical formula 1] with respect to a volume V1 of the quartz furnace tube.

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[0005] The gas in the second step may be pure helium gas.

[0006] The gas containing at least a halogen or argon in the first step may be a gas whose main component is a halogen, or a mixed gas of a rare gas and a halogen gas.

[0007] The gas whose main component is a halogen may be a gas containing at least one of chlorine and fluorine as a main component, and the mixed gas of a rare gas and a halogen gas may be a mixed gas containing at least one of chlorine gas and fluorine gas, and at least one of helium gas and argon gas.

[0008] In the first step, of the total flow rates of gases circulated within the quartz furnace tube, the flow rate U1 of the rare gas and the flow rate U2 of the halogen may satisfy the following formula [Mathematical Expression 2].

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[0009] The first step may dehydrate the porous preform for an optical fiber while moving the porous preform for an optical fiber along the extending direction of the porous preform for an optical fiber in a quartz furnace tube.

[0010] The third step may be to heat the porous preform for the optical fiber while moving it downward along the extension direction of the porous preform for the optical fiber in a quartz furnace tube, thereby successively converting the porous preform for the optical fiber into a transparent glass from the lower end.

[0011] In the third step, after the temperature inside the quartz furnace tube reaches 1400 to 1650° C., the downward movement of the porous preform for optical fiber along the extending direction may be started.

[0012] The first step may dehydrate the optical fiber porous preform while moving the optical fiber porous preform downward in the extension direction in the quartz core tube. The second step may at least partially ventilate the quartz core tube while raising the optical fiber porous preform moved downward in the first step so that the optical fiber porous preform can be vitrified successively from the lower end by starting the downward movement of the optical fiber porous preform along the extension direction in the subsequent third step.

[0013] The first step may dehydrate the porous preform for an optical fiber by heating the entire porous preform for an optical fiber.

[0014] In the first step, the porous preform for optical fiber may be heated all over using a plurality of heating devices arranged around the quartz furnace tube in a line along the extension direction of the porous preform for optical fiber.

[0015] In the third step, the temperature inside the quartz furnace tube, which has been raised in the first step, may be further raised using some of the multiple heating devices, thereby converting the porous preform for optical fiber into a transparent glass.

[0016] In the first step, the temperature inside the quartz furnace tube may be increased to 1000° C. to 1300° C. In the third step, the temperature inside the quartz furnace tube increased in the first step may be further increased to 1400° C. to 1650° C.

[0017] In the first step, the inside of the quartz furnace tube may be heated to 1000° C. to 1300° C. to dehydrate the porous preform for optical fiber.

[0018] In the third step, the inside of the quartz furnace tube may be heated to 1400° C. to 1650° C. to convert the optical fiber porous preform into a transparent glass.

[0019] In a second aspect of the present invention, there is provided an apparatus for manufacturing an optical fiber preform. The apparatus may include a quartz core tube having a volume V1 capable of accommodating a porous optical fiber preform. The apparatus may include a heating device disposed around the quartz core tube. The apparatus may perform a first step of dehydrating the porous optical fiber preform with the heating device while circulating a gas containing at least halogen or argon into the quartz core tube accommodating the porous optical fiber preform. After the first step, the apparatus may perform a second step of at least partially ventilating the quartz core tube by circulating a gas containing helium as a main component into the quartz core tube. After the second step, the apparatus may perform a third step of transparently vitrifying the porous optical fiber preform with the heating device while circulating a gas containing helium as a main component into the quartz core tube.

[0020] The apparatus may further include a moving mechanism for moving the porous optical fiber preform inserted from the opening at one end of the quartz core tube along the longitudinal direction of the quartz core tube.

[0021] In the second step, an integrated flow rate V2 of the gas containing helium as a main component that is caused to flow through the quartz furnace tube may satisfy the following formula [3] with respect to a volume V1 of the quartz furnace tube.

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[0022] The second step may include flowing pure helium gas through the quartz furnace tube to at least partially ventilate the inside of the quartz furnace tube.

[0023] The above summary of the invention does not list all of the necessary features of the present invention. Also, subcombinations of these features may also be inventions. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram showing a sintering device used in a first embodiment of the present invention. [Diagram 2] FIG. 4 is a schematic diagram showing a sintering device used in a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention according to the claims. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0026] FIG. 1 is a schematic diagram of a sintering device used in a first embodiment of the present invention, in which (A) shows an example of the first step, in which dehydration is performed while moving downward from the top. (B) shows an example of the second step following the step (A), in which internal gas is ventilated. At the same time that the second step is started, the porous preform for optical fiber (hereinafter sometimes simply referred to as the porous preform) 02 is raised to a position required for starting the third step. (C) shows an example of the third step following the step (B), in which the porous preform 02 is moved downward to be vitrified.

[0027] The sintering apparatus according to the first embodiment is an apparatus for manufacturing an optical fiber preform, and includes a quartz furnace tube 04 and a heating device 10. The quartz furnace tube 04 has a volume V1 capable of accommodating a porous preform 02. The heating device 10 is disposed around the quartz furnace tube 04.

[0028] The sintering apparatus may further include a moving mechanism for moving the porous preform 02, which is inserted from an opening at one end of the quartz core tube 04, along the longitudinal direction of the quartz core tube 04. The porous preform 02 is inserted from an opening at the top of the quartz core tube 04 along the longitudinal direction of the quartz core tube 04 by a shaft 03 connected to the moving mechanism.

[0029] The sintering apparatus performs the first step to dehydrate the porous preform 02 in the heating device 10 while circulating a gas containing at least halogen or argon in the quartz furnace tube 04 that accommodates the porous preform 02. The first step may dehydrate the porous preform 02 while moving the porous preform 02 in the quartz furnace tube 04 along the extension direction of the porous preform 02. The first step may also dehydrate the porous preform 02 by increasing the temperature inside the quartz furnace tube 04 to 1000°C to 1300°C.

[0030] As a specific example, the first step is started after the porous base material 02 is inserted into the quartz core tube 04 and the opening at the top of the quartz core tube 04 is closed. At the same time as starting the first step, the sintering apparatus introduces pure halogen gas or a mixed gas of rare gas and halogen gas into the quartz core tube 04 from the gas inlet port 01. The sintering apparatus further exhausts a constant amount of gas from the gas exhaust port 05 to keep the pressure in the quartz core tube 04 constant. The sintering apparatus continues to exhaust a constant amount of gas from the gas exhaust port 05 to keep the pressure in the quartz core tube 04 constant from the first step to the third step.

[0031] At the same time as starting the first step, the sintering apparatus starts increasing the temperature by the heating device 10 to heat the quartz core tube 04. At this time, it is preferable for the temperature inside the quartz core tube 04 to be 1100°C to 1300°C in order to dehydrate the porous base material 02. In the first step, the porous base material 02 is moved downward from the upper part of the quartz core tube 04 by the above-mentioned moving mechanism.

[0032] The above-mentioned pure halogen gas is an example of a gas whose main component is a halogen. In addition, both the pure halogen gas and a mixed gas of a rare gas and a halogen gas are examples of a gas that contains at least a halogen or argon.

[0033] The gas containing halogen as the main component may be a gas containing at least one of chlorine and fluorine as the main component. A gas containing at least one of chlorine and fluorine as the main component is preferable for dehydration treatment. The mixed gas of rare gas and halogen gas may be a mixed gas containing at least one of chlorine gas and fluorine gas, and at least one of helium gas and argon gas.

[0034] In the first step, of the total flow rates of gases circulated within the quartz furnace tube 04, the flow rate U1 of the rare gas and the flow rate U2 of the halogen satisfy the following formula [2].

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[0035] As described above, in the first step, the sintering apparatus circulates a gas containing at least a halogen or argon into the quartz furnace tube 04. In other words, in the first step, the sintering apparatus does not circulate only pure helium gas into the quartz furnace tube 04. This allows the sintering apparatus to reduce the amount of helium gas used, which is more expensive than gases containing other elements, compared to the case where only pure helium gas is used in the first step, and reduces the manufacturing cost of the optical fiber preform.

[0036] In the first step, when a gas containing at least a halogen or argon, such as argon gas, halogen gas, or a mixture of helium gas and these gases, is circulated in the quartz furnace tube 04, these gases remain in the porous base material at the end of the first step. These elements have lower thermal conductivity than helium.

[0037] Here, as a transparent vitrification step performed after the dehydration step, it is known that the porous preform is transparently vitrified while a gas mainly composed of helium, which has a higher thermal conductivity than other elements, is circulated in a quartz furnace tube in order to promote the transparent vitrification. However, if the transparent vitrification is started in a state in which a gas containing an element with a relatively low thermal conductivity remains in the porous preform, the part of the porous preform that is transparently vitrified in the early stage of the transparent vitrification step is likely to become opaque due to the remaining gas. This is because sufficient helium gas does not circulate around the part during the period from the start of the transparent vitrification step until the part is vitrified.

[0038] In contrast, the sintering apparatus according to the present embodiment executes the second step after the above-mentioned first step, thereby circulating a gas whose main component is helium in the quartz core tube 04 to at least partially ventilate the quartz core tube 04. The gas whose main component is helium may be pure helium gas. In addition, in the second step, the quartz core tube 04 may be at least partially ventilated while raising the porous base material 02 moved downward in the first step.

[0039] In the second step, the integrated flow rate V2 of the gas containing helium as a main component that is caused to flow through the quartz core tube 04 may satisfy the following formula (1) with respect to the volume V1 of the quartz core tube 04.

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[0040] As a specific example, the second step is started after the first step is completed. At the same time as starting the second step, the sintering apparatus supplies pure helium gas from the gas inlet port 01 into the quartz furnace tube 04 to ventilate the inside of the quartz furnace tube 04. At the same time as starting the second step, the sintering apparatus also uses the moving mechanism to lift the porous preform 02 to a position required at the time of starting the subsequent third step. After lifting the porous preform 02 to that position, the sintering apparatus may stop the moving mechanism for a certain period of time, or may start the third step immediately after the lift and start moving the porous preform 02 using the moving mechanism.

[0041] In the sintering apparatus, the integrated flow rate V2 of the pure helium gas flowing into the quartz core tube 04 from the start of the second step to the start of the third step, i.e., the total volume V2, is set to 0.5 times or more the inner volume V1 of the quartz core tube 04, as shown in the above formula [1]. This is preferable in order to make transparent the portion of the porous base material 02 that is to be vitrified in the early stage of the start of the third step.

[0042] The sintering apparatus further executes a third step after the second step, whereby the porous base material 02 is vitrified into a transparent state by the heating device 10 while a gas containing helium as a main component is circulated in the quartz core tube 04. In the third step, the temperature inside the quartz core tube 04, which has been heated in the first step, may be further increased to 1400°C to 1650°C, thereby vitrifying the porous base material 02 into a transparent state.

[0043] The third step may also be performed by heating the porous preform 02 while moving it downward along the extension direction of the porous preform 02 in the quartz furnace tube 04, thereby vitrifying the porous preform 02 successively from the lower end. As described above, in the second step, the porous preform 02 moved downward in the first step is raised to a position required at the time of starting the third sintering step, while at least partially ventilating the quartz furnace tube 04. In this manner, by raising the porous preform 02 moved downward in the first step to the position required in the second step, in the third step following the second step, the porous preform 02 can be vitrified successively from the lower end by starting to move the porous preform 02 downward along the extension direction of the porous preform 02.

[0044] As a specific example, the sintering device starts increasing the temperature by the heating device 10 at the same time as starting the third step. In the third step, the temperature inside the quartz furnace tube 04 is preferably set to 1400 to 1650° C. in order to transparently vitrify the porous base material 04. In the third step, it is preferable to use pure helium gas in order to transparently vitrify the entire longitudinal direction of the porous base material 04.

[0045] In the third step, the sintering apparatus uses a moving mechanism to move the porous preform 02 downward from the position at the start of the third step toward the quartz core tube 04, thereby gradually progressing the transparent vitrification from the vertically downward end of the porous preform 02. Note that, in the third step, the sintering apparatus preferably starts moving the porous preform 02 downward along the extension direction of the porous preform 02 after the temperature inside the quartz core tube 04 reaches 1400 to 1650°C.

[0046] In this way, according to the sintering apparatus of the present embodiment, before starting the transparent vitrification process of the porous preform 02, for example, before raising the temperature inside the quartz core tube 04 to about 1400°C to 1650°C, a gas mainly composed of pure helium is circulated inside the quartz core tube 04 to at least partially ventilate the inside of the quartz core tube 04. The sintering apparatus can promote the removal of gas with low thermal conductivity remaining in the porous preform 02 before starting the transparent vitrification process by at least partially ventilating the inside of the quartz core tube 04 with the gas. As a result, in the transparent vitrification process, the sintering apparatus can prevent the part of the porous preform 02 to be transparently vitrified in the early stage of the transparent vitrification process from becoming opaque. Therefore, the sintering apparatus can obtain a glass preform for optical fiber that is sufficiently transparently vitrified.

[0047] 2 is a schematic diagram of a sintering apparatus used in the second embodiment of the present invention, in which (A) shows an example of the first step, in which the porous base material 02 is moved vertically while dehydrating. (B) shows an example of the second step following step (A), in which internal gas is ventilated. (C) shows an example of the third step following step (B), in which the porous base material 02 is moved downward while transparent vitrification is performed.

[0048] The sintering apparatus according to the second embodiment has a configuration different from that of the sintering apparatus according to the first embodiment in that a heating device 10 and a heating device 11 are arranged around a quartz furnace tube 04. Other configurations of the sintering apparatus according to the second embodiment are similar to those of the sintering apparatus according to the first embodiment, and corresponding configurations are given the same reference numerals and overlapping explanations are omitted.

[0049] The first step performed in the sintering apparatus according to the second embodiment heats the porous base material 02 over its entirety, thereby dehydrating the porous base material 02. The first step may heat the porous base material 02 over its entirety using a plurality of heating devices, for example, heating device 10 and heating device 11, arranged around the quartz core tube 04 in the extension direction of the porous base material 02. In this case, the third step may further heat the inside of the quartz core tube 04, which has been heated in the first step, using only a portion of the plurality of heating devices, for example, heating device 10, thereby vitrifying the porous base material 02 into a transparent glass.

[0050] As a specific example, the sintering apparatus according to the second embodiment starts the first step and simultaneously starts heating the quartz core tube 04 by the heating device 10 and the heating device 11. At this time, it is preferable to set the temperature inside the quartz core tube 04 to 1100°C to 1300°C in order to dehydrate the porous base material 02. By using a plurality of heating devices 10 and 11 in this way, the time required to heat up to the temperature can be shortened. In the first step, the porous base material 02 may be moved from the top to the bottom of the quartz core tube 04 or from the bottom to the top by the moving mechanism. Other processing contents in the first step are the same as those in the first step according to the first embodiment, and overlapping explanations will be omitted. The same applies to the subsequent second and third steps.

[0051] The sintering apparatus may raise the porous preform 02 to a position required at the time of starting the third step at the same time as starting the second step, and at this time, the heating of the quartz core tube 04 by the heating device 11 may be stopped. In this case, the sintering apparatus may maintain the heating of the quartz core tube 04 by the heating device 10 between the first step and the second step to maintain the temperature inside the quartz core tube 04.

[0052] The sintering apparatus starts the third step and at the same time starts increasing the temperature by the heating apparatus 10. At this time, the sintering apparatus may stop heating the quartz furnace tube 04 by the heating apparatus 11 throughout the third step.

[0053] 2, in the second embodiment, an example in which the heating device 11 is disposed directly above the heating device 10 has been described, but the effect of the invention is not impaired even if the heating device 11 is disposed directly below the heating device 10. In addition, a plurality of heating devices may be disposed directly above the heating device 10, or a plurality of heating devices may be disposed directly below the heating device 10.

[0054] The method for producing the optical fiber preform according to the present embodiment will be described in more detail below with reference to examples and comparative examples. However, the present invention is not limited to these and various modifications are possible.

[0055] [Comparative Example 1] In the first process, the gas composition ratio circulated inside the quartz core tube was set to 50% argon + 50% chlorine, the quartz core tube was heated to an internal temperature of 1000 to 1300°C, and the porous preform for optical fiber was dehydrated while being moved from the top to the bottom inside the quartz core tube.

[0056] In the second step, the temperature inside the quartz core tube was maintained at 1000 to 1300°C, and gas with the same composition ratio as the gas circulated in the first step was circulated inside the quartz core tube to ventilate the internal gas. The total volume of the gas circulated inside the quartz core tube was set to 60% of the internal volume of the quartz core tube. At the same time that the second step was started, the porous base material was raised to a position required at the time of starting the third step.

[0057] In the third process, pure helium gas consisting of 100% helium was circulated inside the quartz furnace tube while the temperature inside the quartz furnace tube was raised to 1400 to 1650°C, and the porous preform was gradually moved downward, and transparent vitrification was performed from the vertically lower end of the porous preform.

[0058] As a result, it was confirmed that there was an opaque portion at the portion of the porous base material that was to be vitrified in the early stage of the vitrification process, that is, at the vertically downward end of the porous base material.

[0059] [Comparative Example 2] In the first process, the gas composition ratio circulated inside the quartz core tube was set to 50% argon + 50% chlorine, the quartz core tube was heated to an internal temperature of 1000-1300°C, and the porous base material was dehydrated while moving from the top to the bottom inside the quartz core tube.

[0060] In the second step, the temperature inside the quartz core tube was maintained at 1000-1300°C, and pure helium gas consisting of 100% helium was circulated inside the quartz core tube to ventilate the internal gas. The total volume of the pure helium gas circulated inside the quartz core tube was set to 40% of the internal volume of the quartz core tube. At the same time that the second step was started, the porous base material was raised to a position required at the time of starting the third step.

[0061] In the third process, pure helium gas consisting of 100% helium was circulated inside the quartz furnace tube while the temperature inside the quartz furnace tube was raised to 1400 to 1650°C, and the porous preform was gradually moved downward, and transparent vitrification was performed from the vertically lower end of the porous preform.

[0062] As a result, it was confirmed that there was an opaque portion at the portion of the porous base material that was to be vitrified in the early stage of the vitrification process, that is, at the vertically downward end of the porous base material.

[0063] [Comparative Example 3] In the first process, the gas composition ratio circulated inside the quartz core tube was set to 50% argon + 50% chlorine, the quartz core tube was heated to an internal temperature of 1000-1300°C, and the porous base material was dehydrated while moving from the top to the bottom inside the quartz core tube.

[0064] In the second step, the temperature inside the quartz core tube was maintained at 1000-1300°C, and pure helium gas consisting of 100% helium was circulated inside the quartz core tube to ventilate the internal gas. The total volume of the pure helium gas circulated inside the quartz core tube was set to 45% of the internal volume of the quartz core tube. At the same time that the second step was started, the porous base material was raised to a position required at the time of starting the third step.

[0065] In the third process, pure helium gas consisting of 100% helium was circulated inside the quartz furnace tube while the temperature inside the quartz furnace tube was raised to 1400 to 1650°C, and the porous preform was gradually moved downward, and transparent vitrification was performed from the vertically lower end of the porous preform.

[0066] As a result, it was confirmed that there was an opaque portion at the portion of the porous base material that was to be vitrified in the early stage of the vitrification process, that is, at the vertically downward end of the porous base material.

[0067] [Example 1] In the first process, the gas composition ratio circulated inside the quartz core tube was set to 50% argon + 50% chlorine, the quartz core tube was heated to an internal temperature of 1000 to 1300°C, and the porous preform for optical fiber was dehydrated while being moved from the top to the bottom inside the quartz core tube.

[0068] In the second step, the temperature inside the quartz core tube was maintained at 1000-1300°C, and pure helium gas consisting of 100% helium was circulated inside the quartz core tube to ventilate the internal gas. The total volume of the pure helium gas circulated inside the quartz core tube was set to 60% of the internal volume of the quartz core tube. At the same time that the second step was started, the porous base material was raised to a position required at the time of starting the third step.

[0069] In the third process, pure helium gas consisting of 100% helium was circulated inside the quartz furnace tube while the temperature inside the quartz furnace tube was raised to 1400 to 1650°C, and the porous preform was gradually moved downward, and transparent vitrification was performed from the vertically lower end of the porous preform.

[0070] As a result, it was confirmed that the portion of the porous base material that was vitrified in the early stage of the transparent vitrification process, that is, the vertically downward end of the porous base material, was also transparent.

[0071] [Example 2] In the first process, the gas circulated inside the quartz core tube was 100% chlorine, the quartz core tube was heated to an internal temperature of 1000 to 1300°C, and the porous preform for optical fiber was dehydrated while being moved from the top to the bottom inside the quartz core tube.

[0072] In the second step, the temperature inside the quartz core tube was maintained at 1000-1300°C, and pure helium gas consisting of 100% helium was circulated inside the quartz core tube to ventilate the internal gas. The total volume of the pure helium gas circulated inside the quartz core tube was set to 40% of the internal volume of the quartz core tube. At the same time that the second step was started, the porous base material was raised to a position required at the time of starting the third step.

[0073] In the third process, pure helium gas consisting of 100% helium was circulated inside the quartz furnace tube while the temperature inside the quartz furnace tube was raised to 1400 to 1650°C, and the porous preform was gradually moved downward, and transparent vitrification was performed from the vertically lower end of the porous preform.

[0074] As a result, it was confirmed that the portion of the porous base material that was vitrified in the early stage of the transparent vitrification process, that is, the vertically downward end of the porous base material, was also transparent.

[0075] [Example 3] In the first process, the gas composition ratio circulated inside the quartz core tube was set to 75% helium + 25% chlorine, the quartz core tube was heated to an internal temperature of 1000-1300°C, and the porous base material was dehydrated while moving from the top to the bottom inside the quartz core tube.

[0076] In the second step, the temperature inside the quartz core tube was maintained at 1000-1300°C, and pure helium gas consisting of 100% helium was circulated inside the quartz core tube to ventilate the internal gas. The total volume of the pure helium gas circulated inside the quartz core tube was set to 60% of the internal volume of the quartz core tube. At the same time that the second step was started, the porous base material was raised to a position required at the time of starting the third step.

[0077] In the third process, pure helium gas consisting of 100% helium was circulated inside the quartz furnace tube while the temperature inside the quartz furnace tube was raised to 1400 to 1650°C, and the porous preform was gradually moved downward, and transparent vitrification was performed from the vertically lower end of the porous preform.

[0078] As a result, it was confirmed that the portion of the porous base material that was vitrified in the early stage of the transparent vitrification process, that is, the vertically downward end of the porous base material, was also transparent.

[0079] [Example 4] In the first process, the gas composition ratio circulated inside the quartz core tube was set to 50% argon + 50% chlorine, the quartz core tube was heated to an internal temperature of 1000-1300°C, and the porous base material was dehydrated while moving from the top to the bottom inside the quartz core tube.

[0080] In the second step, the temperature inside the quartz core tube was maintained at 1000-1300°C, and pure helium gas consisting of 100% helium was circulated inside the quartz core tube to ventilate the internal gas. The total volume of the pure helium gas circulated inside the quartz core tube was set to 50% of the internal volume of the quartz core tube. At the same time that the second step was started, the porous base material was raised to a position required at the time of starting the third step.

[0081] In the third process, pure helium gas consisting of 100% helium was circulated inside the quartz furnace tube while the temperature inside the quartz furnace tube was raised to 1400 to 1650°C, and the porous preform was gradually moved downward, and transparent vitrification was performed from the vertically lower end of the porous preform.

[0082] As a result, it was confirmed that the portion of the porous base material that was vitrified in the early stage of the transparent vitrification process, that is, the vertically downward end of the porous base material, was also transparent.

[0083] The gas conditions and the results of transparent vitrification in the above Comparative Examples and Examples are summarized in Table 1. [Table 1]

[0084] The above table reveals the following. In Comparative Example 1, a mixed gas of 50% argon and 50% chlorine was circulated into the quartz core tube in the second step to ventilate the inside, resulting in the presence of opaque parts in the porous base material that were vitrified in the early stage of the vitrification step. Thus, a comparison of Comparative Example 1 with Examples 1 to 4 reveals that it is preferable to use only pure helium as the gas component circulated into the quartz core tube in the second step.

[0085] In Comparative Examples 2 to 3, the total volume V2 of the pure helium gas circulated in the quartz core tube was set to 40 to 45%, which is less than 50% of the internal volume V1 of the quartz core tube, and as a result, opaque parts were present in the part of the porous base material that was vitrified in the early stage of the vitrification process. Thus, from a comparison between Comparative Examples 2 to 3 and Examples 1 to 4, it is found that it is preferable to set the total volume V2 of the pure helium gas circulated in the quartz core tube in the second process to 0.5 times or more the internal volume V1 of the quartz core tube 04.

[0086] Thus, it is recognized that the ventilation conditions in the second step are extremely important in preventing the area heated in the early stage of the transparent vitrification step from becoming opaque.

[0087] Although the present invention has been described above using the embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It is clear to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the description of the claims that such modifications and improvements can also be included in the technical scope of the present invention.

[0088] It should be noted that the order of execution of each process, such as operations, procedures, steps, and stages, in the devices, systems, programs, and methods shown in the claims, specifications, and drawings is not specifically stated as "before," "prior to," etc., and may be realized in any order unless the output of a previous process is used in a later process. Even if the operational flow in the claims, specifications, and drawings is explained using "first," "next," etc. for convenience, it does not mean that it is essential to perform the process in this order. [Explanation of symbols]

[0089] 01 Gas inlet port 02 Porous base material 03 Shaft connected to moving mechanism 04 Quartz furnace tube 05 Gas exhaust port 10 Heating device 11 Heating device

Claims

1. A method for producing an optical fiber glass preform, comprising the steps of: A first step of dehydrating the porous optical fiber preform while flowing a gas containing at least a halogen or argon through a quartz furnace tube containing the porous optical fiber preform; a second step of circulating a gas containing helium as a main component through the quartz furnace tube to at least partially ventilate the quartz furnace tube after the first step; a third step of vitrifying the optical fiber porous preform into a transparent glass while circulating a gas containing helium as a main component in the quartz furnace tube after the second step; Equipped with In the second step, an integrated flow rate V2 of a gas containing helium as a main component and flowing through the quartz furnace tube satisfies the following formula (1) with respect to a volume V1 of the quartz furnace tube: The first step includes dehydrating the optical fiber porous preform while moving the optical fiber porous preform downward in the quartz furnace tube along the extending direction of the optical fiber porous preform; In the second step, the inside of the quartz furnace tube is at least partially ventilated while the porous optical fiber preform moved downward in the first step is raised so that the porous optical fiber preform can be vitrified successively from the lower end by starting to move the porous optical fiber preform downward along the extending direction and to heat the porous optical fiber preform in the subsequent third step. Manufacturing method. [0010]

2. The gas in the second step is pure helium gas. The method of claim 1 .

3. The first step comprises dehydrating the porous optical fiber preform while moving the porous optical fiber preform along an extension direction of the porous optical fiber preform in the quartz furnace tube. The method according to claim 1 or 2.

4. the third step is to heat the porous optical fiber preform while moving it downward along the extending direction of the porous optical fiber preform in the quartz furnace tube, thereby converting the porous optical fiber preform into a transparent glass successively from the lower end. The method according to any one of claims 1 to 3.

5. the third step includes starting to move the optical fiber porous preform downward along the extending direction after the temperature inside the quartz furnace tube reaches 1400 to 1650° C. The method according to claim 4.

6. The first step includes heating the entire porous optical fiber preform to dehydrate the porous optical fiber preform. The method according to any one of claims 1 to 5.

7. The first step includes heating the entire porous preform for optical fiber using a plurality of heating devices arranged around the quartz furnace tube along the extending direction of the porous preform for optical fiber. The method according to claim 6.

8. The third step is to further increase the temperature inside the quartz furnace tube, which has been increased in temperature by the first step, by using some of the heating devices, thereby converting the porous preform for optical fiber into a transparent glass. The method of claim 7.

9. The first step is to raise the temperature inside the quartz furnace tube to 1000° C. to 1300° C. The third step further increases the temperature inside the quartz furnace tube, which has been increased in the first step, to 1400° C. to 1650° C. The method of claim 8.

10. The first step is to dehydrate the porous optical fiber preform by increasing the temperature inside the quartz furnace tube to 1000° C. to 1300° C. The method according to any one of claims 1 to 9.

11. 11. The manufacturing method according to claim 1, wherein the third step is to convert the porous preform for optical fiber into a transparent glass by increasing the temperature inside the quartz furnace tube to 1400°C to 1650°C.

12. An apparatus for manufacturing an optical fiber preform, comprising: a quartz furnace tube having a volume V1 capable of accommodating a porous optical fiber preform; a heating device disposed around the quartz furnace tube; Equipped with a first step of dehydrating the porous optical fiber preform by the heating device while circulating a gas containing at least a halogen or argon in the quartz furnace tube containing the porous optical fiber preform; a second step of circulating a gas containing helium as a main component through the quartz furnace tube to at least partially ventilate the quartz furnace tube after the first step; a third step of vitrifying the optical fiber porous preform into a transparent glass by the heating device while circulating a gas containing helium as a main component in the quartz furnace tube after the second step; Run In the second step, an integrated flow rate V2 of a gas containing helium as a main component and flowing through the quartz furnace tube satisfies the following formula (1) with respect to a volume V1 of the quartz furnace tube: The first step includes dehydrating the optical fiber porous preform while moving the optical fiber porous preform downward in the quartz furnace tube along the extending direction of the optical fiber porous preform; In the second step, the inside of the quartz furnace tube is at least partially ventilated while the porous optical fiber preform moved downward in the first step is raised so that the porous optical fiber preform can be vitrified successively from the lower end by starting to move the porous optical fiber preform downward along the extending direction and to heat the porous optical fiber preform in the subsequent third step. Device. [0010]

13. a moving mechanism for moving the optical fiber porous preform inserted from an opening at one end of the quartz core tube along a longitudinal direction of the quartz core tube; 13. The apparatus of claim 12.

14. The gas in the second step is pure helium gas.

14. Apparatus according to claim 12 or 13.

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