JPH0397632A - Production of porous glass preform for optical fiber - Google Patents

Abstract

PURPOSE:To synthesize the preform at a high rate with a multiplepipe burner without causing cracking, etc., at the time of synthesizing the preform on the periphery of a glass rod with the soot formation method by specifying the angle between the center axis of the burner and the rotating shaft of a starting material. CONSTITUTION:Glass fine particles are synthesized on the periphery of the starting material 1 from its one end with the multiplepipe flame burner 2, the burner is relatively moved in parallel with the rotating shaft of the burner 2, and a porous glass preform 5 is synthesized. In this method, since the bulk density of the preform is increased as the angle theta between the center axis of the burner 2 and the rotating shaft of the starting material 1 is increased, the angle theta is controlled to 60-90 deg.. The bulk density is not effectively increased at <60 deg., and the taper of the growth surface 4 of the preform tends to unfavorably increase at >90 deg.. The bulk density higher than a specified value is required to make the preform hard to crack and to obtain an excellent glass in vitrification and dehydration, and the synthesis is carried out at a rate to obtain the bulk density in the specified range.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a porous glass base material for optical fibers, and more specifically, a method for manufacturing a porous glass base material for optical fibers, and more specifically, a method for manufacturing a porous glass base material for optical fibers.
The present invention relates to an improvement in a method for manufacturing an optical fiber preform by synthesizing a porous glass preform on the outer periphery of a glass rod using a soot generation method such as the OvPO method (external attachment method).

[Conventional technology]

Conventionally, as a method for manufacturing a quartz-based glass tube or an optical fiber base material, there is a so-called "external attachment method" as disclosed in Japanese Patent Application Laid-Open No. 73522/1983.

This method uses SiC14 on the outer periphery of a rotating refractory starting material such as carbon, quartz glass, or alumina.
After a predetermined amount of glass particles have been deposited, the deposition is stopped and the starting material is pulled out to form a pipe-shaped glass aggregate, and this pipe-shaped glass aggregate is turned into transparent vitrification in an electric furnace. , we have obtained pipe-shaped glass.

In a similar manner, a solid optical fiber glass base material is used as a starting material to form a composite of the starting material and a porous glass base material with its outer periphery formed, and then the composite is directly heated in an electric furnace. Another possible method is to further form a transparent glass layer on the outer periphery of the starting material by heat treating the porous glass base material to make the porous glass base material transparent.

As a method for synthesizing the porous outer peripheral base material on the outer periphery of the above-mentioned starting material, the porous outer peripheral base material is deposited from near one end of the starting material, and the burner is set parallel to the starting material rotation drawing relative to the starting material. There is a way to move gradually. When using a single flame burner, such as a concentric quadruple tube burner, as shown in JP-A-61-186240, the central axis of the burner and the rotating shaft of the starting base material are The porous outer peripheral base material was synthesized with the angle set in the range of 20 to 70°.

[Problem to be solved by the invention]

Conventionally, in the welding of porous glass preforms using a single flame burner, the outer diameter of the porous glass preforms to be assembled is approximately 100 mmφ at most, and the welding speed of the porous outer peripheral preforms is also 1. ~2 g/min is the limit.

Therefore, in order to improve the economic efficiency of optical fiber production, various methods of increasing the synthesis speed were investigated, and a so-called multiple flame burner was developed as shown in Japanese Patent Publication No. 62-50418. In this method of synthesizing a porous glass base material around the outer periphery of the starting material using this multiple flame burner, it is easy to synthesize a large diameter base material, and the synthesis speed can be easily increased to 2 g/min or more. Became.

However, because the adhesion efficiency of the glass particles produced by the burner is good, if a large amount of frit gas is introduced, the outer diameter of the base material becomes too large, and the base material becomes transparent in the electric furnace during synthesis or after amalgamation. A problem arose in that it was easily broken during vitrification.

The reason for this is that as the outer diameter of the porous glass base material increases, the bulk density (g/a/, which represents the density of fine particle aggregates containing pores) of the porous glass base material decreases, making it unable to support its own weight. Or, it was thought that this was because it became vulnerable to thermal shock. In the past, attempts have been made to prevent cracking by adjusting the flow rate of combustion gas, but none of them have been able to provide a fundamental solution. In order to take advantage of the advantageous characteristics of the multi-tube burner in terms of composite speed, it was necessary to solve the above problems.

The present invention aims to solve the above-mentioned problems, and provides a method for synthesizing a base material at a high mixing speed using a multi-tube burner, and also capable of producing it stably without cracking. be.

[! Means to solve problem i]

In order to solve the above problems, the configuration of the present invention is a concentric multi-tube burner, which has a port for synthesizing glass particles having at least a raw material injection port, a fuel injection port, and a combustion-supporting gas injection port in the center. On this outer periphery, there is a hole protruding from the glass particle synthesis port in the gas ejection direction.
A gaseous glass raw material is ejected from a burner for synthesizing a porous glass base material using a multiple flame method, which has one or more sets of flame forming ports each having at least a fuel ejection port and a combustion-supporting gas ejection port. The glass particles produced by the hydrolysis reaction or oxidation reaction begin to be deposited near one end of the substantially cylindrical or cylindrical starting material, which is rotating around its own axis, and the burner is turned on. In the method of forming a deposit of glass particles in the axial direction on the outer periphery of the starting material by relatively moving the starting material in parallel with the axis of the starting material, the center axis of the burner and the rotation axis of the starting material are This is a method for producing a porous glass preform for an optical fiber, characterized in that the angle formed by the preform is larger than 60'' and smaller than 90°.

The angle between the central axis of the burner and the rotating axis of the starting material is particularly preferably 70 to 856, since more stable base material synthesis can be ensured.

[Effect]

By starting to synthesize glass particles on the outer periphery of the starting material near one end of the starting material using a porous glass base material synthesis burner, and moving the burner relatively parallel to the starting material rotation axis,
In the method of preparing a porous glass base material, a configuration as shown in FIG. 1 is adopted.

Generally, the burner 2 is set so that the burner center axis forms an angle θ with respect to the starting material. In the burner 2, glass raw materials such as Si C /, Si H C l's, S
iHtCb-. Si Hi, etc. are introduced, and glass fine particles SIOs are generated by a hydrolysis reaction or an oxidation reaction in a flame. At this time, in order to change the refractive index, viscosity, and thermal expansion coefficient, GeCl+, Si F+, CF
4, SF, POCb, BBr, etc. may be added.

The SiOm particles generated in the flame form a porous glass matrix 5.
The porous glass base material 5 is pulled up at a pulling speed v cml in accordance with the adhesion of the glass fine particles. At this time, the weight of glass fine particles adhering to the oblong surface 4 per minute, that is, the synthesis rate M (g/min) is the outer diameter D (cm) of the porous glass base material 5 and the bulk density ρ.
(g/crl), the relationship is as shown in equation (1) below.

M/v"D"D=11) That is, the bulk density ρ is considered to be proportional to the composite speed M and inversely proportional to the pulling speed V and the square of the outside diameter D.

The present inventors paid attention to this relationship and conducted various studies on methods for increasing the bulk density in a double flame burner. However, unlike the case of a single flame, when the burner angle θ is small in a double flame burner, the growth surface becomes flat as shown in Figure 2, and the outer diameter D of the glass base material increases, resulting in a decrease in bulk density. It was found that ρ became small (Figure 3).

Therefore, in order to increase ρ by the larger outer diameter D, +
1) From the formula, p tx M / [D '' vl, so we tried to reduce the pulling rate V. Generally, when the flame temperature is raised, the base material to be synthesized becomes harder (the porous base material has fewer pores, ), the growth rate of the base material slows down and the pulling speed decreases.Therefore, the pulling speed V
It was found that when the flame temperature was increased, that is, the flow rate of the combustion gas 1 was increased in order to reduce the diameter D, the outer diameter D became larger as shown in FIG.

That is, when the angle θ of the multiple flame burner is small, the above equation (11) does not necessarily hold, and there is a correlation between the outer diameter D and the pulling speed V, and it is not possible to easily increase the bulk density ρ.

The reason for this was thought to be as follows. In the case of a multiple flame burner, as shown in Fig. 5, the outer flame surrounds the entire base material, which promotes the adhesion of glass fine particles. It is thought that large-diameter base materials are more likely to coalesce because of their large diameter (the phenomenon of movement toward the growth surface under the influence of force). For this reason, it is thought that if the flame temperature is raised, particles will become more likely to adhere, and particles will adhere to the outer periphery.

In the case of a single flame burner, since the above-mentioned outer flame does not exist, the adhesion area of the glass particles is limited, and therefore, even if the flame temperature is increased, the increase in the outer diameter of the base material is small. Therefore, even when the burner angle θ was small, the bulk density could be easily adjusted. On the other hand, if the burner angle θ is too large, the vicinity of the starting material cannot be heated because there is no outer flame, as shown in Figure 6, and a soft part with low bulk density is formed near the starting material. Therefore, it was not possible to form a stable matrix.

In this way, the synthesis situation of porous glass base material changes greatly due to the difference in flame structure. It was thought that

On the other hand, if the burner angle θ is greatly shifted in a multi-tube burner, the growth becomes parallel to the tip of the burner and becomes relatively sharp in the growth direction, as shown in FIG. in this case,!
It was found that when the flow rate was increased to raise the flame temperature, the growth rate did not change much and the bulk density increased as shown in FIG. 7.

That is, the growth surface of the porous glass base material changes depending on the burner angle θ, and therefore, the larger the angle θ, the easier it is to increase the bulk density ρ. Therefore, in the present invention, the burner angle θ is preferably 60 to 90 degrees, particularly preferably 70 degrees.
~85°.

If it is less than 60'', it is difficult to obtain the above-mentioned effect of increasing ρ, and if it exceeds 90'', the growth surface of the base material tends to gradually increase the taper (the taper portion becomes longer), which is disadvantageous. This is because this tapered portion is an unsteady portion and cannot be used as a product.

Generally, the bulk density ρ should be 0.0 to make it difficult to break.
2 2 Klcd or more is required, and subsequent transparency,
In order to obtain good transparent glass in the dehydration process, 0.6
g/cl or less is preferable.

Therefore, in the present invention, the angle θ is set to 60 to 90', and the composite speed M and the pulling speed V
Adjust and synthesize. The fact that the desired effect can be obtained by doing so will be specifically explained in the following examples.

〔Example〕

Example 1 Using an 8-tube burner with a concentric double flame structure,
Cladding/core ratio with refractive index difference 0.3%5. A porous glass rod was synthesized around the outer periphery of a quartz glass rod of No. 0 as a starting material. The starting quartz glass rod used had its surface smoothed and cleaned by flame polishing in advance. 8
In the heavy pipe burner, StCl+, StCl+,
As a combustion gas! 41, Ar as an inert gas, Ot as a combustion-supporting gas, and ArSH*sOx were flowed around the outer circumference in this order. Each flow rate is SiC lt:5j! / minute,
lit: 11 1/min, 8r: 31/min, Ot
:501/min. When this eight-pipe burner was set at 80 degrees with respect to the starting rod rotation axis, the pulling rate was 1. The growth surface became steady at 8 mm/min, and the outer diameter of the porous glass base material was 145 mm, allowing stable production. When the bulk density at this time was evaluated, it was found to be 0.
It was good with 2 9 grcd. After synthesizing the porous glass base material, it was made transparent in an electric furnace at a set temperature of 1600°, and a transparent optical fiber preform without bubbles could be obtained.

Example 2 Same structure as Example 1, burner angle set at 75°,
A porous glass rod was synthesized around the outer periphery of a quartz glass rod with a cladding/core ratio of 4.7. The burner structure, gas flow rate, etc. were kept under the same conditions. As a result, the pulling speed was 1.
At a speed of 65 (1) m/min, the long plane became steady, and the outer diameter of the porous glass preform was 148B, allowing stable production. After manufacturing, the bulk density was evaluated and found to be 0. 3 1
It was tr/cnf. This base material is also heated to 1600℃.
When the material was made transparent in a high-temperature furnace, it was possible to obtain a good preform for optical fiber with no bubble irregularities.

Comparative example! A dual flame 8M tube burner similar to Examples 1 and 2 was used, with a burner angle of 40' and a clad/core ratio of 5. A porous glass base material was synthesized on No. 0 quartz glass. Example 1.2
In order to make a comparison, the gas flow rate was also set to the same conditions. As a result, the outer diameter of the base material was as large as 195 mm, and it was predicted that the bulk density would be small. After applying soot for about 2 hours, cracks appeared on the outer surface of the base material. When the bulk density of the synthesized porous glass base material was evaluated, it was found to be 0. It is as low as 19 grad, especially near the surface.
．． It was found that the soot was soft, about 1 grad.

〔Effect of the invention〕

As explained above, in the method of synthesizing a porous glass base material on the outer periphery of a starting material using a burner with a double flame structure, the angle θ between the central axis of the burner and the rotation axis of the starting material is set to 60 to 9
0°, preferably thicker than 70° and smaller than 90';
More preferably, by setting it, there is an effect that the porous glass base material can be easily synthesized.

In the embodiment of the present invention, the clad/core ratio is 4.7 to 5.
Although we have described the case where a porous glass rod is synthesized on the outer periphery of a lath rod made of 0 quartz, even if the starting rod is a core rod for a Gl-type optical fiber with a parabolic refractive index distribution, or a fire-resistant material such as carbon Even if it is a rod,
The effects of the present invention remain unchanged.

Naturally, this also applies to cases where steps such as dehydration or addition of fluorine are performed in the heat treatment after synthesis of the porous glass base material.

The effects of the present invention are not limited to the case of a concentric eight-pipe burner, but also to a burner having a double flame structure or a multiple flame structure.

The present invention is particularly effective when synthesizing large base materials at a synthesis rate of 4 g/min or more.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing the configuration of the present invention, Fig. 2 is a diagram illustrating the growth surface of the base material when the burner angle θ is reduced as in the case of a single flame burner, and Fig. 3 is an explanatory diagram showing the outer diameter in general. Figure 4 shows the relationship between 111 flow rate and outer diameter D when the burner angle θ is small for a double flame burner, and Figure 5 shows the relationship between D and bulk density ρ. Figure 6 shows the state of glass particle adhesion in the case of a double flame burner when θ is increased. It is a figure showing the relationship between flow volume and outer diameter D. In the figure, l is the starting material, 2 is the burner, 3 is the flame formed by the burner, 4 is the growth surface of the porous glass base material, 5 is the porous glass base material, and 6 is the flame formed by the double flame burner. 7 is the outer flame, 7 is the inner flame, 8 is the direction of the thermophoresis force acting on the glass particles, and 9 is the glass particles formed in the flame.

Claims (1)

[Claims] (1) A concentric multi-tube burner, which has a port for synthesizing glass particles having at least a raw material injection port, a fuel injection port, and a combustion-supporting gas injection port in the center, and the port for synthesizing glass particles on its outer periphery. A multi-flame type porous glass base material synthesis burner that has one or more sets of flame forming ports having at least a fuel injection port and a combustion-supporting gas injection port protrudes in the gas injection direction. One end of a substantially cylindrical or cylindrical starting material that is rotating around its own axis by ejecting glass raw materials and subjecting them to a hydrolysis or oxidation reaction in a flame, thereby producing fine glass particles. In a method in which a deposit of glass fine particles is formed in the axial direction on the outer periphery of the starting material by starting the deposition from nearby and moving the burner relatively in parallel with the axis of the starting material, the burner A method for producing a porous glass preform for an optical fiber, characterized in that the angle between the central axis of the starting material and the rotational axis of the starting material is greater than 60° and smaller than 90°.