JP2003300747A - Method for manufacturing porous preform for optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a porous preform for an optical fiber by which a porous glass body for its core portion having uniform bulk density distribution in the radial direction can be manufactured and an optical fiber having stable distribution of the refractive index can be produced. <P>SOLUTION: This method for manufacturing the porous preform 10 for the optical fiber is carried out by supplying combustion gas and a glass source material from a burner 1 for the core to produce glass fine particles and depositing the particles to form the porous glass body 3 for the core portion to be the core of the optical fiber. In the above method, the diameter of the gas outlet of a burner 1 for the core is set in a range of 0.5 to 1.0 time as the outer diameter of the porous glass body 3 for the core part. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a porous preform for optical fibers, and more particularly, in a oxyhydrogen flame produced by burning combustion gas and glass raw material from a burner for core and burning the combustion gas. The present invention relates to a method for producing a porous preform for optical fibers, in which a glass raw material is hydrolyzed and glass particles produced as a result are deposited on a starting material to form a core porous glass body to be the core of the optical fiber.

[0002]

2. Description of the Related Art In general, an optical fiber composed of a core and a clad is manufactured by heating a porous glass preform for the optical fiber to make it transparent and then drawing it. As a method for manufacturing a porous preform for optical fibers, for example, the VAD method (Va
por phase Axial Deposition
n: method with vapor phase axis). This VAD method blows out a combustion gas composed of a combustible gas and a combustion-supporting gas and a glass raw material from a burner having a plurality of ports, and hydrolyzes the glass raw material in an oxyhydrogen flame generated by combustion of the combustion gas. The glass particles to be the core are deposited on the axial starting material, and the glass particles to be the clad are deposited around the glass particles.

As shown in FIG. 5, for example, when a porous preform for a single mode optical fiber is manufactured, the core burner 5 is used.
1, an oxyhydrogen flame 52 is formed, a glass raw material gas is blown into the flame 52, the raw material gas is made into glass fine particles by flame hydrolysis, and the glass fine particles are deposited on a rotating starting material 55 to form a core portion porous. A quality glass body (core soot) 53 is formed. Similarly, the clad burner 56
To form an oxyhydrogen flame 57, blow a glass source gas from the center of the flame 57, and form a clad porous glass body 58 so as to surround the core soot 53. From the core soot 53 and the clad porous glass body 58, A porous base material 60 for an optical fiber is obtained.

As a technique relating to the production of this type of porous preform for optical fibers, Japanese Patent Laid-Open No. 63-123828,
JP-A-63-74931, JP-B-62-5041
The one described in Japanese Patent No. 8 is known.

(1) The technique described in Japanese Patent Laid-Open No. 63-123828 synthesizes a porous preform for an optical fiber in order to achieve both high bulk density and high synthesis rate of the preform. Set the diameter of the multi-tube burner to 0.2 to the outer diameter of the porous base material.
It is set within the range of 0.5 times. (2) The technique described in JP-A-63-74931 is
In order to make the refractive index distribution of the core porous glass body stepwise, the outer peripheral portion of the core porous glass body is heated to a temperature higher than other portions to increase the bulk density. is there. (3) The technique described in Japanese Patent Publication No. 62-50418 is
In order to improve the glass synthesis rate, the inner flame of the multi-concentric multi-tube burner is set back more than the outer flame.

[0006]

By the way, in order to improve the dispersion characteristic, which is one of the transmission characteristics of the optical fiber, it is desirable that the shape of the refractive index distribution of the core portion is stepwise. FIG. 4 shows the types of refractive index distribution. Figure 4
4A shows a case where the refractive index is inclined from the core part to the clad part, and FIG. 4B shows a case where a part where the refractive index is locally increased in the outer peripheral part of the core part.
(C) shows the case where the refractive index distribution from the core portion to the cladding portion is stepwise.

Germanium (Ge), which is a dopant, is added to the core porous glass body (core soot) in order to increase the refractive index, and the refractive index distribution of the optical fiber depends on the distribution of the dopant. Decided. However, if the bulk density of the core soot is low at the time of synthesizing the porous base material, the dopant of the core part will diffuse into the clad part during dehydration and transparency. In that case, as shown in FIG. 4A, the refractive index distribution of the core portion is not stepwise, and the refractive index of the cladding portion has an inclination.

Therefore, the refractive index distribution of the core is shown in FIG.
It is important to increase the bulk density of the core soot in order to obtain such a step shape. However, the above (1)
Although it is possible to obtain a porous body having a bulk density that is unlikely to be broken by the technique described above, when synthesizing a porous body to which a dopant is added to increase the refractive index, the vicinity of the outer peripheral portion of the porous body is Insufficient heating resulted in low bulk density, as shown in FIG.
As shown in (a), the refractive index distribution may not be stepwise.

In the technique (2), the bulk density can be increased by heating the outer peripheral portion of the core soot to obtain a stepwise refractive index distribution, but the outer peripheral portion of the core soot is better than other portions. Since the temperature becomes high, the concentration of the dopant becomes too large, and it is difficult to obtain a stable refractive index distribution. That is, the dopant has the property of being added to the glass body in a higher concentration as the temperature of the deposition surface is higher, and when the outer peripheral portion of the core soot becomes high in temperature, the dopant is likely to be locally added to the glass body in a high concentration, As a result, the refractive index of the outer peripheral portion of the core soot may locally increase as shown in FIG. When a high-concentration dopant is locally added, the composition of the glass is drastically changed, which is not preferable because structural defects easily occur during drawing.

Further, in the technique (3), the glass synthesis rate can be increased by bringing only the outer flame close to the glass deposition surface with the multi-tube burner, but like the technique (2), the core soot It is difficult to obtain a stable refractive index distribution because only the outer peripheral portion is likely to have high temperature.

The present invention has been made to solve the above-mentioned problems, and it is possible to manufacture a core porous glass body having a uniform bulk density distribution in the radial direction, thereby providing a stable refractive index. An object of the present invention is to provide a method for producing a porous preform for optical fibers, which is capable of producing an optical fiber having a distribution.

[0012]

In order to achieve the above object, a method of manufacturing a porous preform for an optical fiber according to claim 1 of the present invention comprises supplying a combustion gas and a glass raw material from a core burner to obtain a glass. In the method for producing a porous base material for an optical fiber in which fine particles are generated and deposited to form a core porous glass body to be the core of the optical fiber, the diameter of the gas outlet of the burner for the core is the core portion. It is characterized in that it is set to a range of 0.5 to 1.0 times the outer diameter of the porous glass body.

According to the method of manufacturing a porous base material for an optical fiber of claim 1, the diameter of the gas outlet of the core burner is 0.5 to 1. It is set to 0 times the range. By using the core burner set in this way, it is possible to stably manufacture a core porous glass body having a uniform bulk density distribution in the radial direction. Here, the diameter of the gas outlet refers to the maximum diameter of the ports from which any of the combustible gas and the combustion supporting gas forming the combustion gas is ejected. Therefore, when an inert seal gas other than the combustible gas and the combustion-supporting gas flows from the outermost layer port of the multi-tube burner, the diameter of the outermost layer port is not included. The combustion gas is composed of a combustible gas containing hydrogen and a combustion-supporting gas containing oxygen, and the glass raw material is, for example, silicon tetrachloride containing silicon.

Further, as described in claim 2, in the method for producing a porous base material for an optical fiber according to claim 1, the core porous glass body and the clad porous part to be the clad of the optical fiber. It is desirable to simultaneously form the quality glass body. Thereby, the manufacturing method of the present invention,
Not only when only the core porous glass body is deposited,
In addition, it can be applied to the case where the clad porous glass body is deposited so as to surround the core porous glass body. For example, a core burner forms a core porous glass body, and at the same time, a combustion gas and a glass raw material are supplied from a clad burner provided separately from the core burner to generate glass fine particles, and a core portion is produced. There is a method of forming a clad portion porous glass body by depositing the porous glass body so as to surround the porous glass body.

Further, as described in claim 3, it is desirable that the gas outlet has a plurality of concentric gas outlet ports. Specifically, it is possible to use a multi-tube burner in which all the gas outlet ports are concentric, or a burner having a plurality of non-concentric gas outlet ports between a plurality of concentric gas outlet ports.

Further, as described in claim 4, it is desirable to set the flow rate of the oxygen gas blown out from the core burner in the range of 4 to 16 m / sec. By setting the flow velocity of the oxygen gas which influences the spread of the flame in such a range, the spread of the flame can be suppressed and the deposition efficiency of the glass particles can be kept good.

Further, as described in claim 5, in the core burner according to claim 3, the mutual deviation amount in the gas flow direction of the tip positions of the plurality of gas blowout ports is 30 mm.
It is desirable that it is contained within. By using such a core burner, a part of the core porous glass body does not become hotter than other parts,
The bulk density distribution in the radial direction of the core porous glass body can be made uniform.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the method for producing a porous preform for optical fibers according to the present invention will be described below with reference to FIGS. FIG. 1 is a schematic diagram showing an embodiment of a method for producing a porous preform for optical fibers of the present invention, FIG. 2 is a schematic diagram of the core burner shown in FIG. 1, and FIG. FIG. 3B is a plan view, and FIG. Figure 3 is the second
It is a top view which shows the burner for cores used for the Example.
Further, FIG. 4 is an example of the refractive index distribution, where (a) shows a case where the refractive index is inclined toward the clad portion, and (b) is a portion where the refractive index is locally increased in the outer peripheral portion of the core portion. And (c) is a diagram showing a case where the refractive index distribution from the core portion to the clad portion is stepwise.

As shown in FIG. 1, in the method for manufacturing a porous preform for optical fibers according to the present embodiment, the core porous glass body (core soot) 3 constituting the porous preform for optical fiber 10 is formed.
And the clad portion porous glass body 8 are simultaneously formed by using different core burners 1 and clad burners 6, respectively. An oxyhydrogen flame 2 is formed by the core burner 1, a glass raw material gas is blown into the flame 2, the raw material gas is made into glass fine particles by flame hydrolysis, and the glass fine particles are deposited on a rotating shaft-shaped starting material 5. Then, the core soot 3 is formed. Similarly, an oxyhydrogen flame 7 is formed by the clad burner 6, a glass material gas is blown from the center of the flame 7, and a clad porous glass body 8 is formed so as to surround the core porous glass body 3. A porous preform 10 for an optical fiber, which is composed of the core soot 3 and the porous glass body 8 of the clad portion, is obtained.

By the way, the present inventor pays attention to the relationship between the size of the core burner 1 and the size of the core soot 3 shown in FIG. 1 in order to manufacture the core soot 3 having a uniform bulk density distribution in the radial direction. investigated. As a result, they have found that the size of the core burner 1 has an appropriate range as a method of making the entire core soot 3 have a uniform bulk density.

That is, when the diameter of the core burner 1 becomes larger than the outer diameter of the core soot 3, the flame of the core burner 1 heats up to the side surface of the core soot 3, and the bulk density of the outer peripheral portion of the core soot increases. become. Further, if the diameter of the core burner 1 becomes 0.5 times or less of the outer diameter of the core soot 3, heating near the outer peripheral portion of the core soot 3 becomes weak, and the bulk density of the outer peripheral portion of the core soot 3 decreases. I understood.

Therefore, the diameter of the core burner 1 is preferably in the range of 0.5 to 1.0 times the outer diameter of the core soot 3. In that case, in order to suppress the spread of the flame, it is desirable to set the flow velocity of the oxygen gas within an appropriate range. That is, even if the diameter of the core burner 1 is proper, it is difficult to obtain a uniform bulk density when the flame spreads. Therefore, it is necessary to suppress the spread of the flame. Therefore, the flow velocity of oxygen gas is increased to some extent. There is a need.

Specifically, it has a structure as shown in FIG.
Use concentric multi-tube burner as core burner 1
In the case of doing, silicon tetrachloride (Si
Cl _Four) And germanium tetrachloride (GeCl_Four) And hydrogen (H
₂) To the second and sixth ports P2 and P6.₂)
Supply oxygen to the fourth and eighth ports P4 and P8 (O₂)
Supply to the third, fifth and seventh ports P3, P5, P7
Supply the inert gas argon (Ar) gas
It And at that time, the flow of the oxygen gas in the fourth port P4
Speed up to 4m / sec or more. By doing so, the flame spread
Can be suppressed, and the core soot 3 is dense in the radial direction.
The degree distribution can be made uniform. Where the oxygen gas
If the flow velocity is less than 4 m / sec, the flame will spread wider than necessary.
And the bulk density of the outer peripheral portion of the core soot 3 is low.
turn into. In addition, the flow velocity of oxygen gas exceeds 16 m / sec.
As a result, the heating efficiency of the core soot 3 decreases,
It is preferable to set the flow rate of oxygen gas to 4 to 16 m / sec.
Yes.

Further, an appropriate oxygen gas flow rate (4 to 16 m /
The setting of (sec) is not limited to the case of ejecting from the fourth port P4, but may be applied to the case of ejecting from the eighth port P8 in the above example, and the case of ejecting oxygen from another port. Can also be applied to.

Further, preferably, the distance from the gas outlet of the core burner 1 to the deposition surface of the glass particles is 5 to 5.
It is good to set it to 30 cm. Thereby, the spread of the flame can be suppressed within an appropriate range, and the bulk density distribution of the core soot 3 in the radial direction can be made uniform.

As the flammable gas used in the present invention, hydrocarbons such as CH ₄ , C ₂ H ₆ and C ₃ H ₈ can be used in addition to hydrogen. It is also possible to mix oxygen and supply it to the raw material port instead of hydrogen. Furthermore, as a preferred mode, it is preferable that the tip positions of the plurality of gas ejection ports of the core burner 1 are substantially aligned with the gas flow direction. That is, in order to improve the glass synthesis rate, there is a method in which in the case of a double flame burner, the tip of the ejection port of the outer flame is made to protrude more than the ejection port of the inner flame, but in such a case only the outer flame is used. This is because the core soot is brought closer to the deposition surface and the outer peripheral portion of the core soot is heated more than other portions. Therefore, the inventor
As a result of the examination, in order to make the bulk density distribution in the radial direction of the core soot uniform, as shown in FIG. Was confirmed. This length L is 30
If it exceeds mm, the outer peripheral portion of the core soot 3 has a higher temperature than the other portions, and the bulk density of the heated portion becomes higher than that of the other portions.

(First Embodiment) Next, a first embodiment according to the present invention will be described. In this example, as in the above-described embodiment, the porous preform for optical fibers was manufactured in the mode shown in FIG. The concentric multi-tube burner shown in FIG. 2 is used as the core burner 1, and the first port P at the center is used.
Supplied with 1 to silicon tetrachloride (SiCl ₄₎ and germanium tetrachloride (GeCl ₄₎ and hydrogen (H _2), supplying a hydrogen (H ₂₎ in the second, sixth port P2, P6, fourth, While supplying oxygen (O ₂ ) to the eight ports P4 and P8 and supplying argon (Ar) gas, which is an inert gas for sealing, to the third, fifth and seventh ports P3, P5 and P7, the core soot 3
Was manufactured. At the same time, the clad burner 6 formed the clad porous glass body 8 so as to surround the core soot 3 to obtain an optical fiber porous preform 10 composed of the core soot 3 and the clad porous glass 8.

As the structure of the core burner 1, the eighth port P8 has a diameter of 30 mm, and the outer flame ejection ports (P5 to P8) are connected to the inner flame ejection ports (P1 to P4).
It is projected forward by 5 mm. Further, the flow rate of oxygen in the fourth port P4 was set to 14 m / sec. Further, the distance from the inner flame ejection ports (P1 to P4) to the deposition surface of the core soot 3 was set to 20 cm.

The core soot 3 produced under the above conditions had a diameter of 35 mm. As a result, the bulk density of the core soot 3 in the radial direction is within the range of 0.15 to 0.18 g / cm ³ , and the refractive index distribution of the optical fiber is stepwise as shown in FIG. 4 (c). became.

(Second Embodiment) Next, a second embodiment according to the present invention will be described. In this example, as in the above-described embodiment, the porous preform for optical fibers was manufactured in the mode shown in FIG. However, the difference between this embodiment and the first embodiment is the form of the core burner used. Figure 3
As shown in (1), the core burner 11 used is one in which a plurality of nozzles Q, which are small-diameter ports, are arranged between concentric ports. Silicon tetrachloride (SiCl ₄ ), germanium tetrachloride (GeCl ₄ ), and hydrogen (H ₂ ) are supplied to the first port P1 at the center of the core burner 11, and the third port P1 is supplied.
3 is supplied with hydrogen (H ₂ ), small-diameter nozzle Q and fifth port P 5 are supplied with oxygen (O ₂ ), and second and fourth ports P are supplied.
2, P4 is an inert gas for sealing argon (A
r) The core soot 3 was manufactured while supplying gas.

As the structure of the core burner 11, the outer diameter of the fifth port P5 was 30 mm, and all the ports and the tip surfaces of the nozzles were aligned. Also, the fifth port P5
The oxygen flow rate was set to 9 m / sec. Furthermore, the distance from the ejection port to the deposition surface of the core soot 3 was set to be 10 cm.

The core soot 3 produced under the above conditions had a diameter of 30 mm. As a result, the bulk density of the core soot 3 in the radial direction is within the range of 0.20 to 0.22 g / cm ³ , and the refractive index distribution of the optical fiber is stepwise as shown in FIG. 4 (c). became.

[0033]

As described above, according to the method for producing a porous base material for an optical fiber of the present invention, as a burner for a core, the diameter of the gas outlet is 0 of the outer diameter of the porous glass body of the core portion. Use the one set in the range of 0.5 to 1.0 times.
By using the core burner set in this way, it is possible to stably manufacture a core porous glass body having a uniform bulk density distribution in the radial direction. Further, by setting the flow rate of the oxygen gas blown out from the core burner within the range of 4 to 16 m / sec, it is possible to suppress the spread of the flame and keep the deposition efficiency of the glass particles good. Further, as the core burner, the mutual deviation amount of the tip positions of the plurality of gas blowout ports in the gas flow direction is set to 30.
By using the glass having a diameter of less than mm, a part of the core porous glass body does not become hotter than the other part, and the bulk density distribution in the radial direction of the core porous glass body is made uniform. be able to.

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment of a method for producing a porous preform for optical fibers of the present invention.

FIG. 2 is a schematic diagram of the core burner shown in FIG.
(A) is a plan view seen from the gas outlet, and (b) is a cross-sectional view of a main part.

FIG. 3 is a plan view showing a core burner used in a second embodiment.

FIG. 4 is an example of a refractive index distribution, in which (a) shows a case where the refractive index is inclined toward the clad portion, and (b) shows a portion where the refractive index is locally increased in the outer peripheral portion of the core portion. FIG. 6C is a diagram showing a case where there is a stepwise refractive index distribution from the core portion to the cladding portion.

FIG. 5 is a schematic view showing an embodiment of a conventional method for producing a porous preform for optical fibers.

[Explanation of symbols]

Burner for 1 core Dioxygen flame 3 core soot (core porous glass body) 5 starting materials 6 Clad burner 7 oxyhydrogen flame 8 Porous glass body in the clad part 10 Porous preform for optical fiber 11 Core burner (second embodiment) P1 to P8 ports (gas outlet ports) Q nozzle

Claims (5)

[Claims] 1. A porous mother for an optical fiber, in which a combustion gas and a glass raw material are supplied from a core burner to generate and deposit fine glass particles to form a core porous glass body to be a core of the optical fiber. In the method for producing a material, the diameter of the gas outlet of the core burner is set to a range of 0.5 to 1.0 times the outer diameter of the core porous glass body. For manufacturing a porous base material for use. 2. The porous mother material for an optical fiber according to claim 1, wherein the core porous glass body and the clad porous glass body to be the cladding of the optical fiber are simultaneously formed. Method of manufacturing wood. 3. The method for producing a porous preform for an optical fiber according to claim 1, wherein the gas outlet has a plurality of concentric gas outlet ports. 4. The method for producing a porous preform for an optical fiber according to claim 1, wherein the flow rate of oxygen gas constituting the combustion gas is set in a range of 4 to 16 m / sec. 5. The optical fiber according to claim 3, wherein the core burner has a mutual deviation amount of the tip positions of the plurality of gas outlet ports in the gas flow direction within 30 mm. For manufacturing a porous base material for use.