Method for manufacturing glass base material for optical fiber

Abstract

To provide a method for manufacturing an optical fiber glass preform, capable of preventing an opaque glass part or elongation from occurring by simply controlling sintering parameters.SOLUTION: A method for manufacturing an optical fiber glass preform comprises: an optical fiber porous preform preparation step of preparing an optical fiber porous preform including a porous glass layer formed in the outer periphery of a core rod; a sintering preparation step of hanging the optical fiber porous preform in the furnace core tube of a sintering apparatus; and a sintering step of heating the optical fiber porous preform while passing the mixed gas of halogen based gas and inert gas through the furnace core tube and relatively moving a heater from one end side of the core rod to the other end side to transparently vitrify the optical fiber porous preform and obtain an optical fiber glass preform. In the sintering step, the concentration of the halogen based gas in the mixed gas is reduced in the process of relatively moving the heater to the optical fiber porous preform.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a glass preform for an optical fiber by dehydrating and sintering a porous preform for an optical fiber to produce a glass preform for an optical fiber. [Background technology]

[0002] Optical fiber glass preforms are produced by depositing glass particles on a seed rod or the like to produce a porous optical fiber preform, which is then dehydrated and sintered to form a transparent glass. Typical methods for producing porous optical fiber preforms include the VAD method, MCVD method, and OVD method. These methods can be combined and used, taking into account their respective advantages. For example, a porous optical fiber preform may be produced by producing a seed rod containing a core and part of the cladding by the VAD method, and then forming the remaining part of the cladding around the outer periphery of the produced seed rod by the OVD method.

[0003] Patent document 1 describes a method for producing a porous preform for an optical fiber by depositing glass particles that will become the cladding portion on the outer periphery of a seed rod containing a core using the OVD method, and then hanging the produced porous preform for an optical fiber into a sintering device and heating it to dehydrate and sinter it, thereby producing a glass preform for an optical fiber.

[0004] The optical fiber glass preform thus produced may contain internal portions that are not sufficiently vitrified. To address this issue, it is conceivable to increase the heating temperature in the sintering apparatus in order to promote vitrification. However, if the heating temperature is too high, the softening caused by the heating causes the portion where vitrification is progressing to be stretched by the load of the portion below that portion. As a result, the optical fiber glass preform produced may be longer than the porous optical fiber preform before sintering, or the diameter of the preform may differ between the upper and lower ends. This may result in problems such as the preform being unable to be removed from the sintering apparatus, damage to the sintering apparatus, or failure to obtain desired properties.

[0005] Patent Document 1 discloses a method for solving such problems by combining several techniques, such as (1) lowering the vitrification temperature as the heating position moves from the lower end to the upper end of the base material, (2) slowing down the moving speed of the heating position, and (3) reducing the flow rate of helium gas. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-81657 Summary of the Invention [Problem to be solved by the invention]

[0007] The problem-solving method described in Patent Document 1 is complex and involves combining and optimizing three parameters: the vitrification temperature, the moving speed of the heating position, and the gas flow rate. It is difficult to optimize and manage these parameters for each of the many sintering machines installed in a factory, in accordance with the characteristics of the machine.

[0008] An object of the present invention is to provide a method for producing a glass preform for an optical fiber, which can suppress the occurrence of opaque glass portions and elongation by simply adjusting sintering parameters. [Means for solving the problem]

[0009] The method for manufacturing a glass preform for optical fiber of the present invention includes a porous preform preparation step for preparing a porous preform for optical fiber having a porous glass layer formed on the outer periphery of a core rod; a sintering preparation step for suspending the porous preform for optical fiber into a furnace tube; and a sintering step for constantly flowing a mixed gas of a halogen-based gas and an inert gas into the furnace tube and heating the porous preform for optical fiber while moving a heater relatively from one end of the core rod to the other end to convert it into transparent glass to obtain a glass preform for optical fiber, and the concentration of the halogen-based gas in the mixed gas is reduced during the process of moving the heater relatively to the porous preform for optical fiber in the sintering step.

[0010] When the concentration of the halogen-based gas at the start of the sintering step is C0 and the concentration of the halogen-based gas at the point when the other end of the core rod passes the vertical midpoint of the heater is C1, the relationship may be 0.25×C0≦C1≦0.5×C0.

[0011] When the length of the core rod is L and the distance from the other end of the core rod to the position where the decrease in concentration of the halogen-based gas starts is H, 0.05×L≦H≦0.30L may be satisfied.

[0012] When the porous preform for optical fiber hangs down, one end of the porous preform for optical fiber may be the lower end and the other end may be the upper end.

[0013] The heating temperature by the heater may be set to 1400 to 1650°C.

[0014] The halogen-based gas may be chlorine or silicon tetrachloride.

[0015] The inert gas may be helium, nitrogen or argon. [Effects of the Invention]

[0016] According to the method for producing a glass preform for an optical fiber of the present invention, the occurrence of opaque glass portions and elongation can be suppressed by simply adjusting the sintering parameters. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a diagram showing a flow of a method for manufacturing an optical fiber glass preform according to the present invention. [Figure 2] 1A and 1B are diagrams showing an example of a method for preparing a porous optical fiber preform 100 using an OVD method. [Figure 3] 1A to 1C are diagrams illustrating an example of a method for sintering a porous preform 100 for an optical fiber. [Figure 4] 1 is a diagram showing a state in which an opaque glass portion 201 remains in an optical fiber glass preform 200. FIG. [Figure 5] 1 is a diagram showing the distribution of chlorine content in an optical fiber glass preform 200. FIG. [Figure 6] 1A to 1C are diagrams illustrating a method for sintering a porous optical fiber preform 100 applied in the present invention. [Figure 7] 10A and 10B are diagrams illustrating a method for relatively reducing the volume flow rate of a halogen-based gas. [Figure 8] FIG. 1 is a diagram showing the results of an example. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the following description and drawings, the same components are denoted by the same reference numerals, and the description of components that have already been described will be omitted or will be limited to the extent necessary.

[0019] FIG. 1 is a diagram showing the flow of the method for producing an optical fiber glass preform according to the present invention.

[0020] First, a porous preform for an optical fiber is prepared, in which a porous glass layer is formed on the outer periphery of a core rod (porous preform preparation step S1).

[0021] An example of a method for preparing the porous optical fiber preform 100 using the OVD method will be described with reference to FIG. 2(a).

[0022] First, a core rod 110 is prepared, which will be the core of the optical fiber. The core rod 110 may include a core member and a part of a cladding member.

[0023] Glass handles 120 are welded to both ends of the core rod 110, and both ends are held by the rotary chuck 11 as a target rod 130. If there are no manufacturing problems, the handle 120 may be welded to only one end of the core rod 110 to form a target rod 130, and only that end may be held by the rotary chuck 11.

[0024] The burner 12 is fixed to a burner table 13 that moves in the extension direction of the target rod 130 and in the forward and backward directions toward the target rod 130. Glass raw materials such as silicon tetrachloride, trichloro(methyl)silane, and octamethylcyclotetrasiloxane, as well as oxygen and hydrogen, are supplied to the burner 12, and a flame hydrolysis reaction occurs in the flame 14, producing glass particles that serve as the cladding material.

[0025] The glass particles thus generated and released from the burner 12 are deposited on the outer periphery of a target rod 130, which rotates about its axis as the rotating chuck 11 rotates, by moving the burner stand 13 back and forth along the target rod 130, forming a porous glass layer. As the porous glass layer thickens due to the deposition of glass particles, the burner stand 13 is retracted, and when the desired thickness is obtained, the supply of glass raw materials and oxygen and hydrogen to the burner 12 is stopped. This allows the production of a porous optical fiber preform 100, as shown in FIG. 2(b), in which a porous glass layer of the desired thickness is formed on the outer periphery of the core rod 110. It should be noted that by depositing the glass particles not only on the outer periphery of the core rod 110 but also on the outer periphery of the portion of the handle 120 near the core rod 110, a larger amount of optical fiber can be obtained.

[0026] Subsequently, the prepared porous optical fiber preform 100 is suspended into the furnace tube 24 of the sintering device 20 (sintering preparation step S2).

[0027] 3(a), the sintering apparatus 20 includes a shaft 21 suspended at one end from a carriage mechanism 22, the carriage mechanism 22 that drives the shaft 21 vertically up and down while rotating it about its axis, a connecting member 23 that connects one end of the porous optical fiber preform 100 to the other end of the shaft 21, a quartz furnace tube 24 that acts as a reaction vessel, and a heating furnace 30 fixed to the outer periphery of the furnace tube 24. The heating furnace 30 also includes a heater 31 that generates heat to heat the porous optical fiber preform 100, a heat insulating material 32 that prevents the heat generated by the heater 31 from leaking to the outside, and a chamber 33 that serves as a housing.

[0028] The optical fiber porous preform 100 is inserted from above into the furnace tube 24 by driving the carriage mechanism 22, and is suspended to a position before the start of heating. After the optical fiber porous preform 100 is inserted, the upper opening of the furnace tube 24 is sealed with a lid or the like.

[0029] When the handle 120 is welded to only one end of the core rod 110, the one end of the porous preform for optical fiber 100 is connected to the connecting member .

[0030] Next, a mixed gas of a halogen-based gas and an inert gas is constantly flowed into the furnace tube 24 from the gas introduction port 25 located below the furnace tube 24, and the porous preform 100 for optical fiber is heated and vitrified while the heater 31 is moved relatively from one end side to the other end side of the core rod 110, thereby producing a glass preform 200 for optical fiber in which a cladding region is formed around the outer periphery of the core rod 110 (sintering step S3).

[0031] The halogen-based gas is preferably chlorine or silicon tetrachloride, and the inert gas is preferably, for example, nitrogen, argon, or helium. The inside of the furnace tube 24 is maintained at a constant pressure, and if the pressure becomes excessive, excess gas is discharged from the gas discharge port 26.

[0032] The porous optical fiber preform 100 suspended within the furnace tube 24 is lowered while being rotated by the carriage mechanism 22, and heated by the heater 31 from the lower end, which is one end of the core rod 110, to the upper end, which is the other end, of the porous optical fiber preform 100, successively to 1400 to 1650°C, thereby simultaneously dehydrating and sintering the porous optical fiber preform 100 to form a transparent glass (FIG. 3(b)). This allows the optical fiber glass preform 200 to be obtained (FIG. 3(c)).

[0033] Dehydration and sintering may be performed separately. For example, first, the porous optical fiber preform 100 is lowered while being rotated by the carriage mechanism 22, and heated to 1000 to 1200°C sequentially from the bottom to the top by the heater 31 to perform only the dehydration process. Next, the porous optical fiber preform 100 is raised by the carriage mechanism 22, and then lowered while being rotated again, and heated to 1400 to 1650°C sequentially from the bottom to the top by the heater 31 to perform sintering, thereby forming a transparent glass.

[0034] In addition, the porous preform 100 for optical fiber may be set below the heating furnace 30, and then rotated and raised by the carriage mechanism 22, while being heated sequentially by the heater 31 from the upper end side, which is one end of the core rod 110, to the lower end side, which is the other end, i.e., from the upper end to the lower end of the porous preform 100 for optical fiber, thereby dehydrating and sintering it to form a transparent glass.

[0035] Alternatively, the heater 31 may be moved relative to the fixed porous preform 100 for optical fiber, thereby realizing the relative movement of the heater 31 .

[0036] Simply put, inside the optical fiber glass preform 200 manufactured by the above method, an opaque glass portion 201 may remain near the tip of the core rod 110 on the side where vitrification is completed last. When vitrification is performed from the lower end to the upper end of the optical fiber porous preform 100, an opaque glass portion 201 will remain near the upper end of the core rod 110 in the optical fiber glass preform 200, as shown in Fig. 4. The opaque glass portion 201 is particularly likely to remain at the boundary between the core rod 110 and the cladding region.

[0037] The opaque glass portion 201 may induce unexpected breakage or fluctuation in outer diameter when the optical fiber glass preform 200 is spun to form an optical fiber.

[0038] A cylindrical portion near the top end of the core rod 110, where the opaque glass portion 201 remains, and a portion near the vertical midpoint of the core rod 110 were cut out from the same optical fiber glass preform 200, and the chlorine content in the glass was measured from the center of the circle (normalized radius position = 0) to the outer edge (normalized radius position = 1). The solid line shows the measurement result for the portion near the top end, and the dashed line shows the measurement result for the portion near the vertical midpoint. These measurement results show that there is no significant difference in chlorine content in the central region, which is the core rod 110, whereas in the cladding region formed around the core rod 110 by the OVD method, the chlorine content is higher in the portion near the top end than in the portion near the vertical midpoint.

[0039] As a result of intensive research based on these measurement results, it was confirmed that the lower the chlorine content in the glass, the less likely it is that opaque glass regions 201 are formed. This is thought to be because, when the chlorine content in the glass increases, chlorine gas molecules are unable to dissolve completely in the glass as chlorine atoms, and instead remain in the glass as gas molecules, forming opaque glass regions 201.

[0040] On the other hand, adding a dopant other than silicon dioxide, such as chlorine, to the glass tends to decrease the viscosity. When the heater 31 is moved relatively from the lower end to the upper end of the porous optical fiber preform 100 to proceed with the vitrification, the more the heating position moves, the more the vitrified portion below the heated portion becomes. As a result, the load on the heated portion increases as the heating position moves toward the upper end, and the degree to which the heated portion is stretched also increases. As a result, the outer diameter of the optical fiber glass preform 200 becomes smaller at the upper end than at the lower end, as shown in FIG. 3.

[0041] Therefore, to address this issue, by reducing the chlorine content in the glass, the decrease in viscosity of the glass can be suppressed, making it less likely to stretch when being vitrified into transparent glass, and the difference in outer diameter between the upper and lower ends of the optical fiber glass preform 200 can be reduced.

[0042] Therefore, in the present invention, in the sintering step S3, the chlorine content in the optical fiber glass preform 200 is reduced by the following method to suppress the occurrence of opaque glass portions 201 and the occurrence of differences in outer diameter between the upper and lower ends.

[0043] 6 shows the state in which the porous preform for optical fiber 100 is lowered while the heater 31 is moved successively from the lower end side to the upper end side of the porous preform for optical fiber 100 to proceed with transparent vitrification. In FIG. 6, the portion below the heater 31 has already been vitrified, while the portion above is in the state of the porous preform for optical fiber before transparent vitrification.

[0044] When the porous preform 100 for optical fiber is suspended in the furnace tube 24 of the sintering apparatus 20, the position of the upper end of the core rod 110 is defined as point O, the position a distance H vertically downward from point O is defined as point P, and the vertical midpoint of the heater 31 is defined as point Q. That is, point Q does not move because the heater 31 is fixed, whereas points P and O move vertically downward as the porous preform 100 for optical fiber descends, with point P passing through point Q first, followed by point O.

[0045] In the present invention, in order to reduce the chlorine content in the optical fiber glass preform 200, the concentration of halogen-based gas in the mixed gas is reduced in the process of moving the heater 31 relative to the optical fiber porous preform 100.

[0046] For example, when the optical fiber porous preform 100 descends and point P passes point Q, the concentration of the halogen-based gas in the mixed sludge can be reduced by decreasing the volumetric flow rate of the halogen-based gas in the furnace core tube 24 relative to the volumetric flow rate of the inert gas. Two methods for relatively decreasing the volumetric flow rate of the halogen-based gas are explained using FIG. 7. In FIG. 7, the solid line indicates the time change in the volumetric flow rate of the inert gas from the start of sintering, and the dotted line indicates the time change in the volumetric flow rate of the halogen-based gas from the start of sintering. FIG. 7(a) illustrates the first method, in which the volumetric flow rate of the halogen-based gas is set to 0 immediately after time T when point P passes point Q. The volumetric flow rate of the halogen-based gas does not necessarily have to be 0; it may be decreased at a rate with time. FIG. 7(b) illustrates the second method, in which the volumetric flow rate of the inert gas is increased immediately after time T when point P passes point Q. The volumetric flow rate of the inert gas does not need to be increased stepwise as shown in Figure 7(b), but may be increased at a gradient over time. Either method will produce the same effect, but since increasing the volumetric flow rate of the inert gas increases costs, it is preferable to reduce the volumetric flow rate of the halogen-based gas relative to the volumetric flow rate of the inert gas.

[0047] Specifically, for example, when the concentration of the halogen-based gas before point P passes point Q is C0 and the concentration of the halogen-based gas at the time point O passes point Q is C1, the concentration of the halogen-based gas should be reduced so that the relationship between C0 and C1 satisfies 0.25×C0≦C1≦0.5×C0.

[0048] Furthermore, for example, when the length of the core rod 110 is L, the distance H from point P to point O may be determined so that the relationship between the length L and the distance H satisfies 0.05×L≦H≦0.30L.

[0049] It is desirable to determine the concentration C1 of the halogen-based gas and the distance H from point P to point O so that the transmission loss at a wavelength of 1383 nm in the optical fiber obtained by spinning the optical fiber glass preform manufactured by the optical fiber glass preform manufacturing method of the present invention near point O is 0.31 dB / km or less.

[0050] According to the manufacturing method of the glass preform for optical fiber of the present invention described above, the chlorine content in the glass preform for optical fiber can be reduced by simply adjusting the sintering parameters, for example, by reducing the volumetric flow rate of chlorine gas to 0 immediately after point P passes point Q, so that the occurrence of opaque glass portions and the occurrence of a difference in outer diameter between the upper end and the lower end due to elongation of the upper end can be easily suppressed. [Example]

[0051] Below are examples of the present invention and comparative examples. For each example, chlorine gas was used as the halogen-based gas and helium gas was used as the inert gas, and the length of the core rod 110 was set to L, and the chlorine concentration in the furnace core tube 24 before point P reached point Q was set to C0. Figure 8 summarizes the results.

[0052] Example 1 Point P was determined by setting the distance H from point O to 0.08 L, and immediately after point P passed point Q, the volumetric flow rate of chlorine gas was reduced to 0 to obtain an optical fiber glass preform 200. The chlorine concentration in the furnace tube 24 immediately after point O passed point Q was 0.48C0. No opaque glass portion 201 was formed near point O, and the optical fiber obtained by spinning this portion had a transmission loss of 0.28 dB / km at a wavelength of 1383 nm. The outer diameter of this portion was 153 mm.

[0053] <Example 2> Point P was determined as a distance H from point O of 0.17 L, and immediately after point P passed point Q, the volumetric flow rate of chlorine gas was reduced to 0 to obtain an optical fiber glass preform 200. The chlorine concentration in the furnace tube 24 immediately after point O passed point Q was 0.43C0. No opaque glass portion 201 was formed near point O, and the optical fiber obtained by spinning this portion had a transmission loss of 0.28 dB / km at a wavelength of 1383 nm. The outer diameter of this portion was 156 mm.

[0054] Example 3 Point P was determined by setting the distance H from point O to 0.20 L, and immediately after point P passed point Q, the volumetric flow rate of chlorine gas was reduced to 0 to obtain an optical fiber glass preform 200. The chlorine concentration in the furnace tube 24 immediately after point O passed point Q was 0.37C0. No opaque glass portion 201 was formed near point O, and the optical fiber obtained by spinning this portion had a transmission loss of 0.29 dB / km at a wavelength of 1383 nm. The outer diameter of this portion was 161 mm.

[0055] Example 4 Point P was determined as a distance H from point O of 0.26 L, and immediately after point P passed point Q, the volumetric flow rate of chlorine gas was reduced to 0 to obtain an optical fiber glass preform 200. The chlorine concentration in the furnace tube 24 immediately after point O passed point Q was 0.26C0. No opaque glass portion 201 was formed near point O, and the optical fiber obtained by spinning this portion had a transmission loss of 0.30 dB / km at a wavelength of 1383 nm. The outer diameter of this portion was 163 mm.

[0056] <Comparative Example 1> Sintering was completed without reducing the volumetric flow rate of the chlorine gas, yielding an optical fiber glass preform 200. An opaque glass portion 201 was formed near point O. The optical fiber obtained by spinning this portion had a transmission loss of 0.28 dB / km at a wavelength of 1383 nm. The outer diameter of this portion was 145 mm.

[0057] <Comparative Example 2> Point P was determined by setting the distance H from point O to 0.03 L, and immediately after point P passed point Q, the volumetric flow rate of chlorine gas was reduced to 0 to obtain an optical fiber glass preform 200. The chlorine concentration in the furnace tube 24 immediately after point O passed point Q was 0.87C0. An opaque glass portion 201 was formed near point O. The optical fiber obtained by spinning this portion had a transmission loss of 0.28 dB / km at a wavelength of 1383 nm. The outer diameter of this portion was 147 mm.

[0058] <Comparative Example 3> Point P was determined as a distance H from point O of 0.39 L, and immediately after point P passed point Q, the volumetric flow rate of chlorine gas was reduced to 0 to obtain an optical fiber glass preform 200. The chlorine concentration in the furnace tube 24 immediately after point O passed point Q was 0.15C0. An opaque glass portion 201 was formed near point O. The optical fiber obtained by spinning this portion had a transmission loss of 0.34 dB / km at a wavelength of 1383 nm. The outer diameter of this portion was 167 mm.

[0059] From the results of each of the above implementations, the following was confirmed:

[0060] If the distance H from point O is shorter than 0.05 L, the amount of chlorine contained in the glass near point O does not decrease, and an opaque glass region is formed. In addition, this region is prone to stretching, and the outer diameter also becomes thinner.

[0061] Furthermore, if the distance H from point O is longer than 0.30L, the amount of chlorine contained in the glass near point O decreases, forming transparent glass and making it difficult for the outer diameter to expand. However, the degree of decrease in chlorine concentration is so great that the area near point O cannot be sufficiently dehydrated, and the transmission loss at a wavelength of 1383 nm becomes higher than 0.31 dB / km.

[0062] On the other hand, if the distance H from point O is determined to satisfy 0.05×L≦H≦0.30L and the optical fiber glass preform 200 is manufactured under the condition that C1 satisfies 0.25×C0≦C1≦0.5×C0, the optical fiber obtained by spinning the portion near point O will have a transmission loss of 0.31 dB / km or less at a wavelength of 1383 nm, and will be able to obtain the characteristics required for the product.

[0063] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention. In other words, appropriate modifications are possible within the scope of the technical idea expressed in the present invention, and forms incorporating such modifications and improvements are also included within the technical scope of the present invention. [Explanation of symbols]

[0064] 11 Rotating chuck 12 burners 13 Burner stand 14 Flame 20 Sintering equipment 21 Shaft 22 Carriage mechanism 23 Connecting member 24 furnace tube 25 Gas inlet port 26 Gas exhaust port 30 Furnace 31 Heater 32 Insulation 33 Chamber 100 Porous preform for optical fiber 110 Core Rod 120 Handle 130 Target Stick 200 Glass base material for optical fiber 201 Opaque glass area H distance L length

Claims

1. a porous optical fiber preform preparation step of preparing an optical fiber porous preform having a porous glass layer formed on the outer periphery of a core rod; a sintering preparation step of suspending the porous optical fiber preform in a furnace tube of a sintering apparatus; a sintering step in which a mixed gas of a halogen-based gas and an inert gas is constantly flowed into the furnace tube, and the porous optical fiber preform is heated while a heater is relatively moved from one end side to the other end side of the core rod, thereby vitrifying the porous optical fiber preform to obtain a glass preform for an optical fiber; Run A method for manufacturing a glass preform for an optical fiber, wherein in the sintering step, the concentration of the halogen-based gas in the mixed gas is reduced during the process of moving the heater relative to the porous preform for an optical fiber.

2. The concentration of the halogen-based gas at the start of the sintering step is C ０ The concentration of the halogen-based gas at the time when the other end of the core rod passes through the vertical midpoint of the heater is defined as C １ When this is done, 0.25 × C ０ ≦C １ ≦0.5×C ０ 2. The method for producing an optical fiber glass preform according to claim 1, wherein

3. 3. The method for manufacturing a glass preform for optical fiber according to claim 2, wherein, when the length of the core rod is L and the distance from the other end of the core rod at which the decrease in concentration of the halogen-based gas starts is H, 0.05 × L≦H≦0.30L.

4. 4. The method for manufacturing a glass preform for an optical fiber according to claim 1, wherein, when the porous preform for an optical fiber is suspended, the one end of the core rod is the lower end of the core rod and the other end is the upper end of the core rod.

5. 2. The method for manufacturing an optical fiber glass preform according to claim 1, wherein the heating temperature by the heater is 1400 to 1650°C.

6. 2. The method for producing an optical fiber glass preform according to claim 1, wherein the halogen-based gas is chlorine or silicon tetrachloride.

7. 2. The method for producing an optical fiber glass preform according to claim 1, wherein the inert gas is helium, nitrogen, or argon.

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