Method for manufacturing glass matrix for optical fibers, and apparatus for manufacturing glass matrix for optical fibers.

Abstract

The present invention provides a method for manufacturing a glass preform for optical fibers, comprising: a first step of dehydrating the porous preform for optical fibers while passing a gas containing at least halogen or argon through a quartz furnace core tube containing the porous preform for optical fibers; a second step of at least partially replacing the gas in the quartz furnace core tube by passing a gas whose main component is helium through the quartz furnace core tube after the first step; and a third step of vitrifying the porous preform for optical fibers to transparent while passing a gas whose main component is helium through the quartz furnace core tube after the second step.

Description

Technical Field

[0001] This invention relates to a method for manufacturing a glass matrix for optical fibers and an apparatus for manufacturing a glass matrix for optical fibers. Background Technology

[0002] Patent document 1 describes a method for manufacturing optical fiber mother material that includes: a dehydration process, ... supplying a dehydrating agent containing an inert gas to the core tubes 11 and 11A to dehydrate the porous glass mother material 1 and 1A (porous ash particles); and a sintering process, sintering the dehydrated porous glass mother material 1 and 1A (paragraph 0022). "Argon is used as the inert gas mixed with the dehydrating agent. Before heating the porous glass mother material 1 and 1A in the dehydration process, a gas with a higher thermal conductivity than argon (hereinafter referred to as "high thermal conductivity gas") is left inside the porous glass mother material 1 and 1A (paragraph 0024).

[0003] [Preliminary Technology Documents]

[0004] [Patent Literature]

[0005] [Patent Document 1] Japanese Patent No. 5298186 Attached Figure Description

[0006] Figure 1 This is a diagram showing a schematic view of the sintering apparatus used in the first embodiment of the present invention.

[0007] Figure 2 This is a diagram showing a schematic view of the sintering apparatus used in the second embodiment of the present invention. Detailed Implementation

[0008] The present invention will now be described through embodiments thereof, which do not limit the invention as described in the claims. Furthermore, the combinations of features described in the embodiments are not necessarily all necessary for the solution of the invention.

[0009] Figure 1 This diagram illustrates a schematic view of the sintering apparatus used in the first embodiment of the present invention. (A) shows an example of the first process, in which dehydration is performed while the apparatus moves downwards. (B) shows an example of the second process following (A), in which internal gas is exchanged. Furthermore, simultaneously with the start of the second process, the porous substrate for optical fibers (hereinafter, sometimes simply referred to as porous substrate) 02 is raised to the position required for the start of the third process. (C) shows an example of the third process following (B), in which transparent vitrification is performed while the porous substrate 02 moves downwards.

[0010] The sintering apparatus of the first embodiment is an apparatus for manufacturing optical fiber preform, and includes a quartz core tube 04 and a heating device 10. The quartz core tube 04 has a volume V1 capable of accommodating the porous preform 02. The heating device 10 is disposed around the quartz core tube 04.

[0011] The sintering apparatus may also include a moving mechanism that moves the porous preform 02, which is inserted from an opening at one end of the quartz core tube 04, along the length 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 length of the quartz core tube 04 via a shaft 03 connected to the moving mechanism.

[0012] The sintering apparatus, by performing the first step, dehydrates the porous parent material 02 by circulating a gas containing at least halogen or argon within the quartz core tube 04 that houses the porous parent material 02, using a heating device 10. Alternatively, the first step can dehydrate the porous parent material 02 while moving it along its extension direction within the quartz core tube 04. The first step can also dehydrate the porous parent material 02 by raising the internal temperature of the quartz core tube 04 to 1000°C to 1300°C.

[0013] As a specific example, the first step begins 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. Simultaneously with the start of the first step, the sintering apparatus introduces pure halogen gas, or a mixture of rare and halogen gases, into the quartz core tube 04 through the gas inlet port 01. To maintain a constant pressure within the quartz core tube 04, the sintering apparatus also discharges a fixed amount of gas through the gas outlet port 05. From the first step to the third step, the sintering apparatus continuously discharges a fixed amount of gas through the gas outlet port 05 to maintain a constant pressure within the quartz core tube 04.

[0014] Simultaneously with the start of the first process, the sintering apparatus also begins heating the quartz core tube 04 using the heating device 10. At this time, setting the internal temperature of the quartz core tube 04 to 1100℃~1300℃ is preferred for dehydrating the porous substrate 02. In the first process, the moving mechanism is used to move the porous substrate 02 from the upper part of the quartz core tube 04 downwards.

[0015] The pure halogen gas is an example of a gas whose main component is halogen. Additionally, both pure halogen gas and mixed gases formed by mixing rare gases and halogen gases are examples of gases that contain at least halogen or argon.

[0016] Alternatively, the gas whose main component is halogen can also be a gas containing at least one of chlorine and fluorine as its main component. A gas containing at least one of chlorine and fluorine as its main component is preferred for dehydration treatment. Furthermore, the mixture of rare gas and halogen gas can also be a mixture containing at least one of chlorine and fluorine and at least one of helium and argon.

[0017] In the first process, among all the gas flows in the quartz furnace core tube 04, the flow rate of rare gas U1 and the flow rate of halogen U2 satisfy the following [Equation 2].

[0018] [Number 2]

[0019] 0.2≦U2 / (U1+U2)≦1.0

[0020] In other words, in the first process, the sintering device ensures that 20% to 100% of all the gas flowing into the quartz furnace core tube 04 is halogen.

[0021] As described above, in the first step of the sintering apparatus, a gas containing at least halogen or argon is circulated into the quartz furnace core tube 04. In other words, in the first step, the sintering apparatus does not circulate only pure helium into the quartz furnace core tube 04. Therefore, compared to using only pure helium in the first step, the sintering apparatus can reduce the amount of helium used, which is more expensive than gases containing other elements, thereby reducing the manufacturing cost of the optical fiber preform.

[0022] In the first step, when a gas containing at least halogen or argon, such as argon, halogen gas, helium, or a mixture of these gases, is circulated into the quartz furnace core tube 04, these gases remain in the porous matrix at the end of the first step. These gases have lower thermal conductivity compared to helium.

[0023] Here, as a transparent vitrification process performed after the dehydration process, it is known that a gas mainly composed of helium, which has a higher thermal conductivity than other elements, is circulated into the quartz furnace core tube to promote transparent vitrification, thereby vitrifying the porous substrate. However, when transparent vitrification begins with a gas containing elements with relatively low thermal conductivity remaining in the porous substrate, the areas in the porous substrate that were initially vitrified in the transparent vitrification process tend to become opaque due to the gas residue. This is because, from the start of the transparent vitrification process until the vitrification of that area, sufficient helium gas is not circulated near that area.

[0024] In this embodiment, the sintering apparatus performs a second step after the first step, allowing helium-based gas to flow into the quartz core tube 04, thereby at least partially purifying the quartz core tube 04. This helium-based gas can also be pure helium. Alternatively, the second step can also purify the quartz core tube 04 at least partially while simultaneously raising the porous substrate 02 that moved downwards in the first step.

[0025] In addition, in the second process, the cumulative flow rate V2 of the gas with helium as the main component flowing into the quartz furnace core tube 04 can also satisfy the following formula [Equation 1] relative to the volume V1 of the quartz furnace core tube 04.

[0026] [Number 1]

[0027] V2≧0.5×V1

[0028] In other words, in the second process, the sintering device can replace more than half of the fluid that was filled into the quartz furnace core tube 04 at the end of the first process with a gas whose main component is helium.

[0029] As a specific example, the second process begins after the first process is completed. Simultaneously with the start of the second process, the sintering apparatus supplies pure helium gas from the gas inlet port 01 into the quartz core tube 04 to exchange the gas inside the quartz core tube 04. Also simultaneously with the start of the second process, the sintering apparatus uses a moving mechanism to raise the porous base material 02 to the position required for the subsequent third process. The sintering apparatus can either stop the moving mechanism for a fixed time after the porous base material 02 has been raised to this position, or it can begin the third process immediately after raising it and use the moving mechanism to start moving the porous base material 02.

[0030] During the period from the start of the second process to the start of the third process, the cumulative flow rate V2 of pure helium gas flowing into the quartz core tube 04, i.e., the total volume V2, is set to at least 0.5 times the internal volume V1 of the quartz core tube 04, as shown in the above formula [Number 1]. This is more preferable in terms of making the vitrified portions transparent in the early stage of the third process in the porous substrate 02.

[0031] The sintering apparatus also performs a third process after the second process, in which helium-based gas is circulated into the quartz core tube 04 while the porous substrate 02 is vitrified using the heating device 10. Alternatively, the third process can further raise the temperature inside the quartz core tube 04, which has already been heated in the first process, to 1400°C–1650°C to vitrify the porous substrate 02.

[0032] The third step can also involve heating the porous substrate 02 while moving it downwards along its extension direction within the quartz core tube 04, thereby causing the porous substrate 02 to become transparent and vitrified sequentially from its lower end. As described above, as an example, in the second step, the porous substrate 02, which moved downwards in the first step, is raised to the position required for the start of the third sintering step while at least partially ventilating the quartz core tube 04. Thus, by raising the porous substrate 02, which moved downwards in the first step, to that position in the second step, and then, in the subsequent third step, by moving the porous substrate 02 downwards along its extension direction, the porous substrate 02 can be made transparent and vitrified sequentially from its lower end.

[0033] As a specific example, the sintering apparatus begins heating simultaneously with the start of the third step using the heating device 10. In the third step, setting the temperature inside the quartz furnace core tube 04 to 1400–1650°C is preferable for vitrifying the porous substrate 04. In the third step, using pure helium is preferable for vitrifying the entire length of the porous substrate 04.

[0034] In the third step of the sintering apparatus, the porous substrate 02 is moved downwards from the position at the start of the third step towards the bottom of the quartz furnace core tube 04 using a moving mechanism, so that the porous substrate 02 undergoes transparent vitrification sequentially from its vertically downward end. Furthermore, preferably, in the third step, after the temperature inside the quartz furnace core tube 04 reaches 1400–1650°C, the sintering apparatus begins to move the porous substrate 02 downwards along its extension direction.

[0035] Thus, according to the sintering apparatus of this embodiment, before starting the transparent vitrification process of the porous substrate 02, for example, before raising the temperature inside the quartz core tube 04 to approximately 1400°C to 1650°C, a gas mainly composed of pure helium is circulated into the quartz core tube 04 to at least partially ventilate the interior of the quartz core tube 04. By utilizing this gas to at least partially ventilate the interior of the quartz core tube 04, the sintering apparatus can promote the removal of gases with low thermal conductivity remaining in the porous substrate 02 before starting the transparent vitrification process. As a result, during the transparent vitrification process, the sintering apparatus can prevent the portions of the porous substrate 02 that were initially vitrified in the transparent vitrification process from becoming opaque. Therefore, according to the sintering apparatus, a sufficiently transparent vitrified glass substrate for optical fibers can be obtained.

[0036] Figure 2This diagram illustrates a schematic view of the sintering apparatus used in the second embodiment of the present invention. (A) shows an example of the first step, in which the porous substrate 02 is moved vertically while undergoing dehydration. (B) shows an example of the second step following step (A), in which internal gas is exchanged. (C) shows an example of the third step following step (B), in which the porous substrate 02 is moved downwards to achieve transparent vitrification.

[0037] In the sintering apparatus of the second embodiment, as a configuration different from that of the sintering apparatus of the first embodiment, heating devices 10 and 11 are arranged around the quartz furnace core tube 04. Other configurations of the sintering apparatus of the second embodiment are the same as those of the sintering apparatus of the first embodiment; the corresponding configurations are marked with the same reference numerals, and repeated descriptions are omitted.

[0038] In the first step performed in the sintering apparatus of the second embodiment, the porous substrate 02 is dehydrated by heating the entire porous substrate 02. The first step may also use multiple heating devices, such as heating devices 10 and 11, arranged around the quartz core tube 04 in a manner extending along the extension direction of the porous substrate 02, to heat the entire porous substrate 02. In this case, the third step may further raise the temperature inside the quartz core tube 04, which has already been heated in the first step, by using only a portion of the multiple heating devices, such as heating device 10, thereby causing the porous substrate 02 to become transparent and vitrified.

[0039] As a specific example, in the sintering apparatus of the second embodiment, the quartz core tube 04 is heated simultaneously with the start of the first step using heating devices 10 and 11. At this time, setting the internal temperature of the quartz core tube 04 to 1100°C to 1300°C is preferable for dehydrating the porous substrate 02. By using multiple heating devices 10 and 11, the time required to reach this temperature can be shortened. In the first step, the porous substrate 02 can also be moved from the upper part to the lower part of the quartz core tube 04, or from the lower part to the upper part, using a moving mechanism. The other processing steps in the first step are the same as in the first step of the first embodiment, so repeated descriptions are omitted. The same applies to the subsequent second and third steps.

[0040] The sintering apparatus can also raise the porous base material 02 to the position required at the start of the third process simultaneously with the start of the second process. At this time, the heating device 11 can also stop heating the quartz core tube 04. In this case, the sintering apparatus can also maintain the heating device 10 on the quartz core tube 04 during the period from the first process to the second process, maintaining the internal temperature of the quartz core tube 04.

[0041] Simultaneously with the start of the third process, the sintering apparatus begins heating using the heating device 10. At this time, the sintering apparatus can also stop heating the quartz furnace core tube 04 using the heating device 11 during the third process.

[0042] In addition, such as Figure 2 As shown in the second embodiment, an example of arranging the heating device 11 directly above the heating device 10 has been described. However, arranging the heating device 11 directly below the heating device 10 will not impair the effectiveness of the invention. Furthermore, multiple heating devices can be arranged either directly above or directly below the heating device 10.

[0043] The following examples and comparative examples illustrate the method for manufacturing the optical fiber preform of this embodiment in more detail, but the present invention is not limited to these examples and can be implemented in various forms.

[0044] [Comparative Example 1]

[0045] In the first step, the composition ratio of the gas flowing into the quartz furnace core tube is 50% argon + 50% chlorine. The quartz furnace core tube is heated to a temperature of 1000-1300°C. Meanwhile, the porous matrix material for optical fibers is moved from top to bottom inside the quartz furnace core tube for dehydration.

[0046] In the second step, the temperature inside the quartz furnace core tube is maintained at 1000–1300°C. Gas with the same composition ratio as the gas flowing through in the first step is circulated into the quartz furnace core tube to exchange the internal gas. The total volume of this gas flowing into the quartz furnace core tube is equivalent to 60% of the tube's internal volume. Furthermore, simultaneously with the start of the second step, the porous base material is raised to the position required at the start of the third step.

[0047] In the third process, while 100% pure helium gas is circulated into the quartz furnace core tube, the temperature inside the quartz furnace core tube is raised to 1400-1650°C, causing the porous matrix material to move downwards in sequence, and transparent vitrification is performed from the vertically lower end of the porous matrix material.

[0048] As a result, it was confirmed that the part of the porous substrate that was vitrified in the early stage of the transparent vitrification process, that is, the end of the porous substrate at the vertical bottom, contained an opaque part.

[0049] [Comparative Example 2]

[0050] In the first step, the composition ratio of the gas flowing into the quartz furnace core tube is 50% argon + 50% chlorine. The quartz furnace core tube is heated to a temperature of 1000-1300°C. The porous parent material is dehydrated while moving from top to bottom inside the quartz furnace core tube.

[0051] In the second step, the temperature inside the quartz furnace core tube is maintained at 1000–1300°C, and 100% pure helium gas is circulated into the quartz furnace core tube to exchange the internal gas. The total volume of pure helium gas circulating into the quartz furnace core tube is equivalent to 40% of the internal volume of the quartz furnace core tube. Furthermore, simultaneously with the start of the second step, the porous base material is raised to the position required at the start of the third step.

[0052] In the third process, while 100% pure helium gas is circulated into the quartz furnace core tube, the temperature inside the quartz furnace core tube is raised to 1400-1650°C, causing the porous matrix material to move downwards in sequence, and transparent vitrification is performed from the vertically lower end of the porous matrix material.

[0053] As a result, it was confirmed that the part of the porous substrate that was vitrified in the early stage of the transparent vitrification process, that is, the end of the porous substrate at the vertical bottom, contained an opaque part.

[0054] [Comparative Example 3]

[0055] In the first step, the composition ratio of the gas flowing into the quartz furnace core tube is 50% argon + 50% chlorine. The quartz furnace core tube is heated to a temperature of 1000-1300°C. The porous parent material is dehydrated while moving from top to bottom inside the quartz furnace core tube.

[0056] In the second step, the temperature inside the quartz furnace core tube is maintained at 1000–1300°C, and 100% pure helium gas is circulated into the quartz furnace core tube to exchange the internal gas. The total volume of pure helium gas circulating into the quartz furnace core tube is equivalent to 45% of the internal volume of the quartz furnace core tube. Furthermore, simultaneously with the start of the second step, the porous base material is raised to the position required at the start of the third step.

[0057] In the third process, while 100% pure helium gas is circulated into the quartz furnace core tube, the temperature inside the quartz furnace core tube is raised to 1400-1650°C, causing the porous matrix material to move downwards in sequence, and transparent vitrification is performed from the vertically lower end of the porous matrix material.

[0058] As a result, it was confirmed that the part of the porous substrate that was vitrified in the early stage of the transparent vitrification process, that is, the end of the porous substrate at the vertical bottom, contained an opaque part.

[0059] [Example 1]

[0060] In the first step, the composition ratio of the gas flowing into the quartz furnace core tube is 50% argon + 50% chlorine. The quartz furnace core tube is heated to a temperature of 1000-1300°C. Meanwhile, the porous matrix material for optical fibers is moved from top to bottom inside the quartz furnace core tube for dehydration.

[0061] In the second step, the temperature inside the quartz furnace core tube is maintained at 1000–1300°C, and 100% pure helium gas is circulated into the quartz furnace core tube to exchange the internal gas. The total volume of pure helium gas circulating into the quartz furnace core tube is equivalent to 60% of the internal volume of the quartz furnace core tube. Furthermore, simultaneously with the start of the second step, the porous base material is raised to the position required at the start of the third step.

[0062] In the third process, while 100% pure helium gas is circulated into the quartz furnace core tube, the temperature inside the quartz furnace core tube is raised to 1400-1650°C, causing the porous matrix material to move downwards in sequence, and transparent vitrification is performed from the vertically lower end of the porous matrix material.

[0063] As a result, it was confirmed that the part of the porous substrate that was vitrified in the early stage of the vitrification process was transparent, that is, the vertically lower end of the porous substrate was also transparent.

[0064] [Example 2]

[0065] In the first step, the composition ratio of the gas flowing into the quartz furnace core tube is 100% chlorine. The quartz furnace core tube is heated to a temperature of 1000-1300°C. Meanwhile, the porous matrix material for optical fibers is moved from top to bottom inside the quartz furnace core tube for dehydration.

[0066] In the second step, the temperature inside the quartz furnace core tube is maintained at 1000–1300°C, and 100% pure helium gas is circulated into the quartz furnace core tube to exchange the internal gas. The total volume of pure helium gas circulating into the quartz furnace core tube is equivalent to 40% of the internal volume of the quartz furnace core tube. Furthermore, simultaneously with the start of the second step, the porous base material is raised to the position required at the start of the third step.

[0067] In the third process, while 100% pure helium gas is circulated into the quartz furnace core tube, the temperature inside the quartz furnace core tube is raised to 1400-1650°C, causing the porous matrix material to move downwards in sequence, and transparent vitrification is performed from the vertically lower end of the porous matrix material.

[0068] As a result, it was confirmed that the part of the porous substrate that was vitrified in the early stage of the vitrification process was transparent, that is, the vertically lower end of the porous substrate was also transparent.

[0069] [Example 3]

[0070] In the first step, the composition ratio of the gas flowing into the quartz furnace core tube is 75% helium + 25% chlorine. The quartz furnace core tube is heated to a temperature of 1000-1300°C. The porous parent material is dehydrated while moving from top to bottom inside the quartz furnace core tube.

[0071] In the second step, the temperature inside the quartz furnace core tube is maintained at 1000–1300°C, and 100% pure helium gas is circulated into the quartz furnace core tube to exchange the internal gas. The total volume of pure helium gas circulating into the quartz furnace core tube is equivalent to 60% of the internal volume of the quartz furnace core tube. Furthermore, simultaneously with the start of the second step, the porous base material is raised to the position required at the start of the third step.

[0072] In the third process, while 100% pure helium gas is circulated into the quartz furnace core tube, the temperature inside the quartz furnace core tube is raised to 1400-1650°C, causing the porous matrix material to move downwards in sequence, and transparent vitrification is performed from the vertically lower end of the porous matrix material.

[0073] As a result, it was confirmed that the part of the porous substrate that was vitrified in the early stage of the vitrification process was transparent, that is, the vertically lower end of the porous substrate was also transparent.

[0074] [Example 4]

[0075] In the first step, the composition ratio of the gas flowing into the quartz furnace core tube is 50% argon + 50% chlorine. The quartz furnace core tube is heated to a temperature of 1000-1300°C. The porous parent material is dehydrated while moving from top to bottom inside the quartz furnace core tube.

[0076] In the second step, the temperature inside the quartz furnace core tube is maintained at 1000–1300°C, and 100% pure helium gas is circulated into the quartz furnace core tube to exchange the internal gas. The total volume of pure helium gas circulating into the quartz furnace core tube is equivalent to 50% of the internal volume of the quartz furnace core tube. Furthermore, simultaneously with the start of the second step, the porous base material is raised to the position required at the start of the third step.

[0077] In the third process, while 100% pure helium gas is circulated into the quartz furnace core tube, the temperature inside the quartz furnace core tube is raised to 1400-1650°C, causing the porous matrix material to move downwards in sequence, and transparent vitrification is performed from the vertically lower end of the porous matrix material.

[0078] As a result, it was confirmed that the part of the porous substrate that was vitrified in the early stage of the vitrification process was transparent, that is, the vertically lower end of the porous substrate was also transparent.

[0079] The gas conditions and results of the transparent vitrification of the comparative examples and embodiments are summarized in Table 1.

[0080] [Table 1]

[0081]

[0082] Based on the table above, the following situation was confirmed. In Comparative Example 1, during the second step, a mixed gas of 50% argon and 50% chlorine was circulated into the quartz furnace core tube for gas exchange. As a result, opaque portions remained in the porous substrate that were initially vitrified in the transparent vitrification process. Therefore, based on the comparison between Comparative Example 1 and Examples 1-4, it was confirmed that the composition of the gas circulated into the quartz furnace core tube during the second step is preferably pure helium only.

[0083] Furthermore, in Comparative Examples 2 and 3, the total volume V2 of pure helium flowing into the quartz furnace core tube was less than 50% of the internal volume V1 of the quartz furnace core tube, but rather 40% to 45% of it. As a result, opaque portions remained in the porous matrix material at the initial stage of the transparent vitrification process. Therefore, based on a comparison between Comparative Examples 2 and 3 and Examples 1 to 4, it was confirmed that the total volume V2 of pure helium flowing into the quartz furnace core tube in the second process is preferably at least 0.5 times the internal volume V1 of the quartz furnace core tube 04.

[0084] Thus, it is confirmed that the ventilation conditions in the second process are extremely important in preventing the heated parts from becoming opaque in the early stages of the transparent vitrification process.

[0085] The present invention has been described above using embodiments, but the technical scope of the present invention is not limited to the scope described in the embodiments. It will be understood by those skilled in the art that the embodiments can be modified or improved in various ways. As can be seen from the claims, such modified or improved methods are also included within the technical scope of the present invention.

[0086] It should be noted that the execution order of actions, sequences, steps, and stages in the apparatus, systems, programs, and methods shown in the claims, description, and drawings can be implemented in any order unless specifically stated as "before" or "prior to," and as long as the output of the previous process is not used in the subsequent process. Regarding the flow of actions in the claims, description, and drawings, even if "firstly," "secondly," etc., are used for convenience, this does not mean that they must be performed in that order.

[0087] [Explanation of Symbols]

[0088] 01 Gas Inlet Port

[0089] 02 Porous base material

[0090] 03 Shaft connected to the moving mechanism

[0091] 04 Quartz Furnace Core Tube

[0092] 05 Gas exhaust port

[0093] 10 Heating device

[0094] 11. Heating device.

Claims

1. A method for manufacturing a glass matrix for optical fibers, comprising: In the first step, the porous preform for optical fiber is dehydrated while a gas containing at least halogen or argon is passed through the quartz furnace core tube containing the porous preform for optical fiber. The second step, after the first step, involves circulating a helium-based gas through the quartz furnace core tube to at least partially ventilate it; and The third step involves, after the second step, circulating a gas with helium as the main component inside the quartz furnace core tube while simultaneously vitrifying the porous matrix material of the optical fiber. In the second step, the cumulative flow rate V2 of the helium-based gas flowing into the quartz furnace core tube satisfies the following equation [Equation 1] relative to the volume V1 of the quartz furnace core tube. [Number 1] V2≧0.5×V1.

2. The manufacturing method according to claim 1, wherein The gas used in the second step is pure helium.

3. The manufacturing method according to claim 1 or 2, wherein The gas containing at least halogen or argon in the first step is a gas whose main component is halogen or a mixture of rare gas and halogen gas.

4. The manufacturing method according to claim 3, wherein The gas whose main component is halogen is a gas containing at least one of chlorine and fluorine as its main component, and the mixture of rare gas and halogen gas is a mixture containing at least one of chlorine and fluorine and at least one of helium and argon.

5. The manufacturing method according to claim 3, wherein In the first process, the flow rates of rare gases U1 and halogens U2 in the total flow of gas into the quartz furnace core tube satisfy the following equation [Equation 2]: [Number 2] 0.2≦U2 / (U1+U2)≦1.

0.

6. The manufacturing method according to claim 1 or 2, wherein In the first process, the porous matrix material for optical fibers is moved along the porous matrix material for optical fibers within the quartz furnace core tube. The porous matrix material moves along its extension direction while dehydrating the optical fiber.

7. The manufacturing method according to claim 1 or 2, wherein The third step involves heating the porous preform for optical fiber while moving it downwards along its extension direction within the quartz furnace core tube, thereby causing the porous preform for optical fiber to become transparent and vitrified sequentially from its lower end.

8. The manufacturing method according to claim 7, wherein The third step involves moving the porous matrix material for the optical fiber downwards along the extension direction after the temperature inside the quartz furnace core tube reaches 1400–1650°C.

9. The manufacturing method according to claim 7, wherein In the first step, while moving the porous matrix material for optical fiber downwards along the extension direction within the quartz furnace core tube, the porous matrix material for optical fiber is dehydrated. The second step involves raising the porous preform for optical fiber that was moved downward in the first step while simultaneously ventilating at least partially inside the quartz furnace core tube. This allows the porous preform for optical fiber to be vitrified sequentially from its lower end by moving it downward along the extension direction in the subsequent third step.

10. The manufacturing method according to claim 1 or 2, wherein The first step involves heating the porous substrate used for optical fibers as a whole to dehydrate the porous substrate.

11. The manufacturing method according to claim 10, wherein The first step uses multiple heating devices arranged around the quartz furnace core tube in a manner that extends along the extension direction of the porous preform for optical fiber to heat the porous preform for optical fiber as a whole.

12. The manufacturing method according to claim 11, wherein The third step involves using a portion of the plurality of heating devices to further raise the temperature inside the quartz core tube, which has already been heated in the first step, thereby making the porous matrix of the optical fiber transparent and vitrified.

13. The manufacturing method according to claim 12, wherein The first step involves heating the interior of the quartz furnace core tube to 1000℃~1300℃. The third step further heats the temperature inside the quartz furnace core tube, which has already been heated in the first step, to 1400℃~1650℃.

14. The manufacturing method according to claim 1 or 2, wherein The first step involves heating the inside of the quartz furnace core tube to 1000℃~1300℃ to dehydrate the porous matrix material for optical fibers.

15. The manufacturing method according to claim 1 or 2, wherein The third step involves heating the interior of the quartz furnace core tube to 1400℃~1650℃ to make the porous matrix material for the optical fiber transparent and vitrified.

16. An apparatus for manufacturing optical fiber preform, comprising: A quartz core tube with a volume of V1 is capable of housing porous matrix material for optical fibers; and A heating device is disposed around the quartz furnace core tube; and the device performs: In the first step, a gas containing at least halogen or argon is circulated through the quartz furnace core tube containing the porous matrix material for optical fiber, while the porous matrix material for optical fiber is dehydrated using the heating device. The second step, after the first step, involves circulating a helium-based gas through the quartz furnace core tube to at least partially ventilate it; and The third step, after the second step, involves circulating a gas with helium as the main component inside the quartz furnace core tube while using the heating device to vitrify the porous matrix material of the optical fiber to make it transparent. In the second step, the cumulative flow rate V2 of the helium-based gas flowing into the quartz furnace core tube satisfies the following equation [3] relative to the volume V1 of the quartz furnace core tube. [Number 3] V2≧0.5×V1.

17. The apparatus according to claim 16, wherein It also includes a moving mechanism that moves the porous matrix material for the optical fiber, which is inserted from the opening at one end of the quartz core tube, along the length of the quartz core tube.

18. The apparatus according to claim 16 or 17, wherein The gas used in the second step is pure helium.

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