JP2003206154A - Burner device for manufacturing porous optical fiber preform and method for manufacturing porous optical fiber preform using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a burner device for manufacturing a porous optical fiber preform, which enhances the efficiency of synthesis reaction of fine glass particles and the deposition efficiency of fine glass particles in manufacture of a porous optical fiber preform, and to provide a method for manufacturing the porous optical fiber preform using the burner device. <P>SOLUTION: In the burner device for manufacturing the porous optical fiber preform, when the diameter of the innermost one of concentric circles on which small-diameter nozzles 3, 3,... are arranged in a burner 10 for glass synthesis is represented by Dp, the number of the nozzles 3, 3,... arranged on the concentric circles by N and the outside diameter by Dn, the formula 1.0≤N.Dn/Dp≤2.2 is satisfied. When the outside diameter of a nozzle disposed at the outermost position of a first group 1 of nozzles is represented by D2, the formula 2.0≤äDp<SP>2</SP>-(N.Dn<SP>2</SP>/2)}/D2<SP>2</SP>≤4.0 is satisfied. When the outside diameter of a nozzle disposed at the innermost position of the first group 1 of nozzles is represented by D1, the formula 8≤äDp<SP>2</SP>-(N.Dn<SP>2</SP>/2)}/D1<SP>2</SP>≤35 is satisfied. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a burner device for producing an optical fiber porous preform and a method for producing an optical fiber porous preform using the burner device, and more particularly to reacting a glass raw material gas in an oxyhydrogen flame. Of the optical fiber porous preform used in the external attachment method for synthesizing glass fine particles and depositing them in the radial direction of the outer circumference of the rotating starting member and the optical fiber porous preform using the burner device. It relates to a manufacturing method.

[0002]

2. Description of the Related Art Optical fibers are manufactured by melting and drawing an optical fiber preform. Further, as a method for manufacturing the optical fiber preform, VAD method, OVD method, MCVD method, PCVD method are used.
There are methods such as law. Among them, OVD (Outside Vapo
r Phase Deposition method is based on silicon tetrachloride (SiC
glass materials such as l ₄ ), germanium tetrachloride (GeCl ₄ ) and the like, are subjected to a hydrolysis reaction or an oxidation reaction in a flame with oxygen and hydrogen to synthesize glass fine particles, and a glass material serving as a core rotating around its axis. In the radial direction of the outer peripheral portion of the cylindrical starting member provided with, glass fine particles (soot) are deposited to form a porous layer consisting of a plurality of layers to form a porous material for an optical fiber, which is used in an electric furnace. This is a method of producing an optical fiber preform by forming transparent glass while dehydrating and sintering. An optical fiber manufactured by melting and drawing such an optical fiber preform has excellent purity and other qualities.

In the OVD method, the end face of the glass synthesizing burner 10 used in the step of forming the optical fiber porous preform has a structure as shown in FIG. 1, for example. A first nozzle group 1 composed of nozzles 1a and 1b is provided at the center of the end face of the glass synthesizing burner 10, and the first nozzle group 1 and the central axis are provided around the first nozzle group 1. Similarly, a second nozzle group 2 including nozzles 2a and 2b is provided. Further, between the first nozzle group 1 and the second nozzle group 2, the first nozzle group 1
On the concentric circles, a small diameter nozzle group 4 is provided, which is composed of rows 4a and 4b of annular small diameter nozzles in which a plurality of small diameter nozzles 3, 3, ... . Further, the nozzle 1a constitutes the first ejection port 11, the portion between the nozzle 1a and the nozzle 1b constitutes the second ejection port 12, and the portion between the nozzle 1b and the nozzle 2a constitutes the third ejection port 13. None, the portion between the nozzles 2a and 2b constitutes the fourth ejection port 14, the row 4a of small diameter nozzles constitutes the fifth ejection port 15, and the row 4b of small diameter nozzles constitutes the sixth ejection port 16 Is doing.

In the OVD method, in order to synthesize fine glass particles, generally, for example, S from the first jet port 11 is used.
A glass raw material gas such as iCl ₄ and an additive gas such as oxygen or hydrogen are supplied, a combustible gas such as hydrogen is supplied from the third ejection port 13, a fifth ejection port 15 and a sixth ejection port A combustion-supporting gas such as oxygen is supplied from 16.

[0005]

By the way, in recent years, as the demand for high-speed communication has increased, the production amount of optical fibers has been increasing year by year. Therefore, in order to reduce the manufacturing cost of the optical fiber, the optical fiber preform used for manufacturing the optical fiber tends to increase in size. In the production of the optical fiber porous preform by the OVD method, the deposition rate of the glass fine particles on the starting member is increased by increasing the supply amount of the glass raw material gas, and the production time of the optical fiber porous preform is shortened. be able to.

However, the deposition rate of the glass fine particles can be increased by simply increasing the supply amount of the glass raw material gas, but the amount of the glass raw material gas exhausted regardless of the synthesis reaction of the glass fine particles also increases. As a result, the deposition efficiency of the glass particles is greatly reduced, and as a result, the manufacturing cost cannot be reduced. Therefore, in order to produce a large-sized optical fiber preform, it is very important to increase the efficiency of the synthesis reaction of the glass fine particles in the production of the optical fiber porous preform and the deposition efficiency of the glass fine particles on the starting member. Has become a problem.

The present invention has been made in view of the above circumstances, and in the production of the optical fiber porous preform, the production of the optical fiber porous preform which improves the efficiency of the synthesis reaction of the glass fine particles and the deposition efficiency of the glass fine particles. An object of the present invention is to provide a burner device for use and a method for manufacturing an optical fiber porous preform using the burner device.

[0008]

Means for Solving the Problems The above-mentioned problems are as follows: a first nozzle group consisting of one row or a plurality of rows of nozzles arranged with the same central axis, and the first nozzle group around the first nozzle group. 1
Between the first nozzle group and the second nozzle group, and a second nozzle group composed of one row or a plurality of rows of nozzles arranged with the same central axis as that of the first nozzle group. A glass synthesizing burner having a single row or a plurality of rows of small diameter nozzle groups in which a plurality of small diameter nozzles having the same inner diameter and outer diameter are arranged on a concentric circle of the nozzle group, and the glass synthesizing burner. A burner device for producing an optical fiber porous preform, comprising a glass source gas, a gas supply source for supplying oxygen, hydrogen and an inert gas, and a gas control unit for controlling the flow rate or flow velocity of these gases, In the burner for glass synthesis, among the concentric circles in which the small diameter nozzles are arranged, the diameter of the innermost concentric circle is Dp, the number of small diameter nozzles arranged on the innermost concentric circle is N, and the number of the small diameter nozzles is Outer diameter is Dn And that, these relationships 1.0 ≦ N ·
This can be solved by a burner device for producing an optical fiber porous preform with Dn / Dp ≦ 2.2. In the burner for synthesizing glass, the outer diameter of the outermost nozzle of the first nozzle group is D2, the diameter of the innermost concentric circle among the concentric circles in which the small diameter nozzles are arranged is Dp, If the outer diameter of the caliber nozzle is Dn, these relationships are 2.
^{0 ≦ {Dp 2 - (N} · Dn 2/2)} / D2 is preferably ² ≦ 4.0. In the glass synthesis burner,
The outer diameter of the nozzles arranged in the innermost side of the first nozzle group is D1, the diameter of the innermost concentric circle among the concentric circles in which the small diameter nozzles are arranged is Dp, and the outer diameter of the small diameter nozzles is Dn. Then, if these relationships are 8 ≦ {Dp ² −
^{(N · Dn 2/2)} } / D1 is preferably ² ≦ 35.

The above-mentioned problem is that the first nozzle group is composed of one or more rows of nozzles arranged with the same central axis, and the first nozzle group is centered around the first nozzle group. A second nozzle group consisting of one or more rows of nozzles arranged with the same axis, and between the first nozzle group and the second nozzle group on a concentric circle of the first nozzle group. In addition, the glass raw material gas is hydrolyzed in a flame by using a glass synthesizing burner equipped with one or more rows of small diameter nozzle groups in which a plurality of small diameter nozzles having the same inner diameter and outer diameter are arranged. In the method for producing an optical fiber porous glass preform, a glass fine particle is synthesized by a reaction or an oxidation reaction, and the glass fine particle is deposited in the radial direction of the outer periphery of a rotating starting member to obtain an optical fiber porous preform. The small diameter nozzle is arranged When the diameter of the innermost concentric circle is Dp, the number of small diameter nozzles arranged on the innermost concentric circle is N, and the outer diameter of the small diameter nozzle is Dn, these relationships are 1. This can be solved by the method for producing a porous glass preform with 0 ≦ N · Dn / Dp ≦ 2.2.
The outer diameter of the nozzles arranged on the outermost side of the first nozzle group is D2, the diameter of the innermost concentric circle among the concentric circles in which the small diameter nozzles are arranged is Dp, and the outer diameter of the small diameter nozzles is Dn. Then, if these relations are 2.0 ≦ {Dp ²
- It is preferable that the ^{(N · Dn 2/2)} } / D2 2 ≦ 4.0. The outer diameter of the nozzles arranged in the innermost side of the first nozzle group is D1, the diameter of the innermost concentric circle among the concentric circles in which the small diameter nozzles are arranged is Dp, and the outer diameter of the small diameter nozzles is Dn. Then, if these relations are 8 ≦ {D
p ² - is preferably a ^{(N · Dn 2/2)} } / D1 2 ≦ 35.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
FIG. 1 is a schematic configuration diagram showing an example of a glass synthesizing burner used for manufacturing an optical fiber porous preform. The burner device for producing an optical fiber porous preform of the present invention is a burner 10 for glass synthesis, and a gas supply source (not shown) for supplying glass raw material gas, oxygen, hydrogen, and an inert gas to the burner 10 for glass synthesis. And a gas control unit (not shown) that controls the flow rate or flow velocity of these gases.

The gas supply source is a gas cylinder (not shown) filled with glass material gas, oxygen, hydrogen, inert gas, etc.
And the like, and is connected to the rear end of the glass synthesizing burner 10 via a gas supply conduit (not shown). Also,
The gas control unit is composed of an electromagnetic valve, a flow rate control device, and the like, and is provided in the middle of the above-described gas supply line, and the flow rate control device controls the flow rate or flow velocity of the gas.

The glass synthesizing burner 10 has an outer diameter of 40 to 6
It has a cylindrical shape of about 0 mm, and is generally made of quartz glass. The inner diameter of the nozzle 1a of the first nozzle group 1 constituting the glass synthesizing burner 10 is about 2.5 to 6 mm,
The inner diameter of the nozzle 1b is about 4 to 10 mm. Further, the inner diameter of the nozzle 2a of the second nozzle group 2 is 25 to 45.
mm, and the inner diameter of the nozzle 2b is about 35 to 55 mm. The inner diameter of the small diameter nozzle 3 is about 1 to 2 mm, and the diameter of the row 4a of small diameter nozzles is 10 to 10.
Approximately 30 mm, the diameter of the row 4b of small diameter nozzles is 15-4
It is about 0 mm.

In the glass synthesizing burner 10, the diameter of the innermost concentric circle among the concentric circles of the above-mentioned row 4a of small diameter nozzles, that is, the first nozzle group 1 in which the small diameter nozzles are arranged, is Dp. Assuming that the number of small-diameter nozzles 3 arranged on the concentric circle is N and the outer diameter of the small-diameter nozzle 3 is Dn, it is preferable that these relations be 1.0 ≦ N · Dn / Dp ≦ 2.2. , And more preferably 1.2 ≦ N · Dn
/Dp≦2.0. When N · Dn / Dp is less than 1.0, the reaction efficiency of the glass raw material gas is lowered, the hydrolysis reaction or the oxidation reaction of the glass raw material gas is not sufficiently performed, and as a result, the deposition efficiency of the glass fine particles is also lowered.
On the other hand, when N · Dn / Dp exceeds 2.2, the resistance of the flow path to combustible gas such as hydrogen passing around the small diameter nozzle group 4 increases, and the flammability around the small diameter nozzle group 4 increases. The gas flow rate decreases. As a result, in the vicinity of the small diameter nozzle group 4, the reaction of the combustible gas with the combustion supporting gas such as oxygen ejected from the small diameter nozzle group 4 easily occurs, and as a result, the tip portion of the small diameter nozzle group 4 and the like. The glass synthesizing burner 10 may deteriorate due to abrasion.

In the glass synthesizing burner 10, if the outer diameter of the nozzle 1b, that is, the outermost nozzle of the first nozzle group 1, is D2, then D2,
The relationship between Dp and Dn is 2.0 ≦ {Dp ² − (N · Dn ²
/ 2)} / D2 is preferably ² ≦ 4.0, more preferably ^{2.2 ≦ {Dp 2 - (N} · Dn 2/2)} / D2 2
≦ 3.8. ^{{Dp 2 - (N · Dn} 2/2)} / D2 2
Is less than 2.0, the flammable gas is insufficient around the small diameter nozzle group 4, and the hydrolysis reaction or the oxidation reaction of the glass raw material gas is difficult to proceed, and as a result, the deposition efficiency of the glass fine particles is reduced. On the other ^{hand, {Dp 2 - (N ·} Dn 2/2)} /
When D2 ² exceeds 4.0, the position where the flammable gas reacts with the combustion-supporting gas is too far from the glass raw material gas, so that the hydrolysis reaction or the oxidation reaction of the glass raw material gas is difficult to proceed, and as a result, the glass The particulate deposition rate is reduced.

Further, in the glass synthesizing burner 10, assuming that the outer diameter of the above-mentioned nozzle 1a, that is, the nozzle arranged at the innermost side of the first nozzle group 1, is D1, D1,
The relationship between Dp and Dn is 8.0 ≦ {Dp ² − (N · Dn ²
/ 2)} / D1 is preferably ² ≦ 35, more preferably ^{8.0 ≦ {Dp 2 - (N} · Dn 2/2)} / D1 2 ≦
24. The ^{- {Dp 2 (N · Dn} 2/2)} / D1 2 is less than 8.0, the amount of combustible gas near the glass raw material gas is reduced, the glass raw material gas hydrolysis reaction or oxidation reaction is sufficiently Will not progress to. On the other hand, {Dp ² −
When ^{(N · Dn 2/2)} } / D1 2 exceeds 35, on which hardly occur reaction of combustible gas and combustion-supporting gas in the surrounding glass material gas, the glass particles away from the burner 10 for synthesizing glass Since the particles are synthesized at different positions, the deposition rate of glass particles decreases.

Here, the deposition efficiency of the glass fine particles means the glass fine particles deposited on the surface of the starting member with respect to the total amount of the glass fine particles when it is assumed that all the glass raw material gases used have been changed into the glass fine particles by the chemical reaction. It is defined by the ratio of the total amount of. What is the deposition rate?
It is represented by the weight of the glass particles deposited on the surface of the starting member per unit time.

Although FIG. 1 shows an example of a glass synthesizing burner provided in the optical fiber porous preform producing apparatus of the present invention, it is provided in the optical fiber porous preform producing apparatus of the present invention. The burner for synthesizing glass is not limited to this. The glass synthesizing burner provided in the optical fiber porous preform manufacturing apparatus of the present invention may have a structure similar to that of the glass synthesizing burner 10 shown in FIG. This is because if the parameters D1, D2, Dp, Dn, and N are determined, other elements have almost no effect on the present invention.

The method for producing the optical fiber porous preform of the present invention will be described below. In the method for producing an optical fiber porous preform of the present invention, for example, the first ejection port of the glass synthesizing burner 10 is formed on the surface of a cylindrical starting member provided with a glass material serving as a core that rotates around its axis. 11 to SiCl
_For the glass synthesis, a mixed gas of glass raw material gas such as ₄ and oxygen is supplied, hydrogen is supplied from the third ejection port 13 and oxygen is supplied from the fifth ejection port 15 and the sixth ejection port 16. The glass fine particles are synthesized by the hydrolysis reaction of the burner 10 in the oxyhydrogen flame, and the glass fine particles are deposited in the radial direction on the surface of the starting member to obtain the optical fiber porous preform.

According to the method for producing an optical fiber porous preform of the present invention, by using the optical fiber porous preform producing apparatus provided with the glass synthesizing burner 10 as described above, the deposition efficiency of fine glass particles is improved. Is improved. Further, even if the supply amount of the glass raw material gas is increased, the deposition efficiency does not significantly decrease, so that the deposition rate is significantly improved. Therefore, the manufacturing cost of the optical fiber porous preform can be reduced. Therefore, the optical fiber preform can be increased in size at low cost.

A concrete embodiment will be shown below with reference to FIG.
Then, the effect of the present invention is clarified. (Example) Odor of the glass synthesis burner 10 shown in FIG.
The D1, D2, Dp and Dn dimensions and the N value.
The one shown in 1 was prepared. In addition, the nozzle 1a
The outer diameter is D1 + 1.5 mm, and the inner diameter of the nozzle 1b is D2
-1.5 mm, the inner diameter of the nozzle 2a is 32 mm, the outer diameter
Is 35 mm, the inner diameter of the nozzle 2b is 38 mm, and the outer diameter is
42 mm, and the diameter of the row 4b of small diameter nozzles is Dp +
It was 7.0 mm. Next, outer diameter 30 mm, length 110
Prepare a cylindrical starting member made of 0 mm quartz glass
It was Then, grip both ends of this starting member with a gripping tool,
The starting member was placed horizontally. Then, this starting member
While rotating around its central axis, the above glass
Using the synthesis burner 10, glass fine particles are synthesized and
Place the lath synthesizing burner 10 parallel to the longitudinal direction of the starting member.
If you move the glass particles,
Radially deposited to form a cylindrical optical fiber porous preform
Obtained. At this time, the rotation speed of the starting member is set to 30 rpm.
It was In addition, from the first ejection port 11, the glass raw material gas
Silicon chloride (SiCl_Four), And spouting oxygen at 3 l / min,
Hydrogen is ejected from the third ejection port 13 at 50 l / min,
From the jet port 15 and the sixth jet port 16 of
l / min jetting, 2 l / g of argon from other jets
Minutes gushed out. SiCl ejected from the first ejection port 11_Fourof
By changing the flow rate to 4 l / min, 5 l / min, 6 l / min,
Deposit glass particles on the surface of the starting material at the flow rate
The deposition efficiency of the glass particles was examined during the measurement. Table of results
2, shown in FIGS. 2 and 3. In Table 2, X = ND
n / Dp, Y = {Dp²-(N ・ Dn²/ 2)} / D2 ²,
Z = {Dp²-(N ・ Dn²/ 2)} / D1²And

[0021]

[Table 1]

[0022]

[Table 2]

From the results shown in Table 2 and FIGS. 2 and 3, 1.0
It was confirmed that if ≦ X ≦ 2.2, 2.0 ≦ Y ≦ 4.0, and 8 ≦ Z ≦ 35, the deposition efficiency of the glass particles is 60% or more. It was also confirmed that the deposition efficiency of the glass particles was not significantly reduced even if the flow rate of the glass raw material gas was increased.

[0024]

As described above, according to the burner apparatus for producing an optical fiber porous preform of the present invention, the reaction efficiency of the glass raw material gas is improved, and as a result, the deposition efficiency of the glass fine particles is improved. Also, the tip of the nozzle is scraped off,
The burner for glass synthesis does not deteriorate. Further, even if the flow rate of the glass raw material gas is increased, the deposition efficiency of the glass fine particles does not significantly decrease, so that the deposition rate of the glass fine particles can be significantly improved. Therefore,
The manufacturing cost of the optical fiber porous preform can be reduced. Therefore, the optical fiber preform can be increased in size at low cost.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of a glass synthesizing burner used for manufacturing an optical fiber porous preform.

FIG. 2 is a graph showing the relationship between the size of a nozzle constituting a glass synthesizing burner and the deposition efficiency of glass particles.

FIG. 3 is a graph showing the relationship between the size of a nozzle that constitutes a burner for synthesizing glass and the deposition efficiency of glass particles.

[Explanation of symbols]

1 ... 1st nozzle group, 1a, 1b, 2a, 2b ... nozzle, 2 ... 2nd nozzle group, 3 ... small diameter nozzle, 4 ...
Small diameter nozzle group, 4a, 4b ... Row of small diameter nozzles, 1
0 ... Burner for glass synthesis, 11 ... First jet port, 12
... Second jet, 13 ... Third jet, 14 ... Fourth
No. 5 ejection port, 15 ... No. 5 ejection port, 16 ... No. 6 ejection port

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masahiro Horikoshi             Fuji Co., Ltd. 1440 Rokuzaki, Sakura City, Chiba Prefecture             Kura Sakura Office F-term (reference) 3K017 CB02 CD03 CH02                 4G014 AH16                 4G021 EA03

Claims (6)

[Claims] 1. A first nozzle group composed of one row or a plurality of rows of nozzles arranged with the same central axis, and the first nozzle group and the central axis around the first nozzle group. A second one consisting of one or more rows of nozzles arranged in the same way
Nozzle group, and between the first nozzle group and the second nozzle group, a plurality of small diameter nozzles having the same inner diameter and outer diameter are arranged on a concentric circle of the first nozzle group. A glass synthesizing burner having one or more rows of small diameter nozzle groups, a gas supply source for supplying a glass raw material gas, oxygen, hydrogen and an inert gas to the glass synthesizing burner, and flow rates of these gases. Or a burner device for manufacturing an optical fiber porous preform with a gas control unit for controlling the flow velocity, in the glass synthesis burner, among the concentric circles in which the small diameter nozzles are arranged, the innermost concentric circle Diameter is D
p, the number of small diameter nozzles arranged on the innermost concentric circle is N, and the outer diameter of the small diameter nozzle is Dn, these relationships are 1.0 ≦ N · Dn / Dp ≦ 2.2. A burner device for producing an optical fiber porous preform, which is characterized by being present. 2. In the burner for synthesizing glass, the outer diameter of nozzles arranged on the outermost side of the first nozzle group is D2, and the diameter of the innermost concentric circle among the concentric circles on which the small diameter nozzles are arranged. Is Dp and the outer diameter of the small diameter nozzle is Dn, the relationship between them is 2.0 ≦ {Dp ² − (N
^{· Dn 2/2)} /} D2 2 ≦ 4.0 optical fiber porous glass base material manufacturing burner apparatus of claim 1, wherein the a. 3. In the burner for synthesizing glass, the outer diameter of the nozzle arranged in the innermost side of the first nozzle group is D1, and the diameter of the innermost concentric circle among the concentric circles in which the small diameter nozzles are arranged. the Dp, the outer diameter of the small diameter nozzle and Dn, these relationships ^{8 ≦ {Dp 2 - (N} · D
^{n 2/2)} / D1} 2 optical fiber porous glass base material manufacturing burner apparatus of claim 1, wherein a ≦ a 35. 4. A first nozzle group consisting of nozzles of one row or a plurality of rows arranged with the same central axis, and the first nozzle group and the central axis around the first nozzle group. A second one consisting of one or more rows of nozzles arranged in the same way
Nozzle group, and between the first nozzle group and the second nozzle group, a plurality of small diameter nozzles having the same inner diameter and outer diameter are arranged on a concentric circle of the first nozzle group. Using a glass synthesizing burner having one or more rows of small-diameter nozzle groups, glass raw material gas is hydrolyzed or oxidized in a flame to synthesize glass fine particles, and the glass fine particles are rotated. In the method for producing an optical fiber porous preform by accumulating in the radial direction of the outer peripheral portion of the member to obtain an optical fiber porous preform, in the concentric circles in which the small diameter nozzles are arranged, the diameter of the innermost concentric circle is Dp. , N is the number of small diameter nozzles arranged on the innermost concentric circles, and D is the outer diameter of the small diameter nozzles.
Let n be 1.0 ≦ N · Dn / Dp ≦
2.2 The method for producing an optical fiber porous preform, characterized in that 5. The outer diameter of the nozzles arranged on the outermost side of the first nozzle group is D2, and the diameter of the innermost concentric circle among the concentric circles in which the small diameter nozzles are arranged is Dp, the small diameter nozzle the outer diameter When Dn of these relationships ^{2.0 ≦ {Dp 2 - (N} · Dn 2/2)} / D2 2 ≦ 4.0
The method for producing an optical fiber porous preform according to claim 4, wherein: 6. The outer diameter of the nozzles arranged in the innermost side of the first nozzle group is D1, and the diameter of the innermost concentric circle among the concentric circles in which the small diameter nozzles are arranged is Dp. Let Dn be the outer diameter of
^{≦ {Dp 2 - (N ·} Dn 2/2)} / D1 2 ≦ 35 and claim 4 or 5 method of manufacturing an optical fiber porous glass base material according to, characterized in that.