JPH0393641A - Production of glass preform for optical fiber - Google Patents

Abstract

PURPOSE:To produce a glass preform homogeneously containing fluorine in both the radial and longitudinal directions thereof and used for producing an optical fiber by controlling the condition of a heat treatment on the heat treat ment for adding the fluorine to the SiO2 glass particle material synthesized by a gaseous phase reaction. CONSTITUTION:In a method for producing a glass preform for an optical fiber by subjecting a SiO2 glass particle material synthesized by a gaseous phase reaction to the first thermal treatment for dehydrating the material and to the second thermal treatment for adding fluorine to the material and subsequent ly converting the treated material into transparent glass, the second thermal treatment is performed by heating the SiO2 glass particle material at 1150 deg.C to 1300 deg.C for at least 20min in an atmosphere containing the fluorine gas.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a glass base material for optical fiber, and more specifically, the core is made of pure quartz (SiO*) and the cladding is made of fluorine-doped quartz (F Sing). The present invention relates to a manufacturing method suitable for manufacturing a glass preform for optical fibers.

[Conventional technology]

A so-called pure silica core optical fiber, in which the core is made of quartz glass and the cladding is made of fluorine-doped silica glass, has low transmission loss and is attracting attention as a line for long-distance communication.

The fluorine-doped silica glass used for this pure silica core optical fiber is usually JP-A-62-275035 and JP-A-62-275035.
As shown in the 90842 publications, fluorine addition is carried out in a sintering furnace. That is, SiOs glass particles synthesized by a gas phase reaction such as the VAD method or the external deposition method are dehydrated in a heating furnace, and then heat treated in a fluorine gas atmosphere, preferably at a temperature of 1000° C. to 1300° C. In this way, fluorine is added, and then the temperature of the sintering furnace is further increased and transparent vitrification is performed in a fluorine atmosphere or an inert gas atmosphere to obtain fluorine-doped quartz glass.

[Problem to be solved by the invention]

Conventionally, the fluorine addition method using high-temperature heat treatment in a fluorine gas atmosphere has had a problem in that the amount of fluorine added varies in the radial and longitudinal directions of the st Os glass particles.

In particular, when fluorine is added in the radial direction, the amount of fluorine added to the center of the Sins glass particles is small, and as a result, the refractive index is high at the center and gradually decreases toward the outer periphery, as shown in FIG. There was a problem in that the refractive index distribution shown in Fig. 1 was easily distorted.

In addition, variations in the refractive index may also occur in the longitudinal direction of the base material, and the cause could not be ascertained.

A glass base material to which fluorine is added unevenly does not have stable transmission characteristics when used as an optical fiber, which poses a major problem in manufacturing.

Conventionally, improvements to this type of problem have been made, such as increasing the fluorine concentration in the atmosphere or adjusting the furnace temperature, based on the belief that the impregnation of fluorine-based gas into the glass particles is insufficient. However, none of them have been sufficiently effective. In order to stabilize the characteristics of pure silica core optical fiber, it is essential to establish a heat treatment technique for uniformly adding fluorine.
An object of the present invention is to provide a method for manufacturing a glass preform for an optical fiber, in which fluorine can be uniformly added to glass particles both in the radial direction and in the longitudinal direction.

[Problem to be solved by the invention]

In order to solve the above problems, the present invention has a structure in which SiOs glass particles combined by a gas phase reaction are heated in a heating furnace.
In a method for producing a glass base material for an optical fiber by performing a first heat treatment for dehydration and a second heat treatment for adding fluorine, and then converting it into transparent vitrification, SiOs glass particles are treated in the second heat treatment. It is characterized by heating at a temperature of 1150° C. to 1300° C. for at least 20 minutes in an atmosphere containing a fluorine-based gas.

The heating method is to heat the entire st O glass particle body at the same time.
In the case of a so-called soaking furnace (the configuration of which is shown in Figure 1) that is exposed to a temperature range of 50°C to 1300°C, SiO*
It is preferable to expose the glass particles to the above temperature range in a fluorine atmosphere for 20 minutes or more.

Further, in the case of a so-called zone furnace (the configuration of which is shown in FIG. 2) having a narrow heating area, it is particularly preferable to carry out the heat treatment so that the heat zone, which is maintained in the above-mentioned temperature range, is moved over a period of 20 minutes or more.

[Effect]

As a result of a detailed study by the present inventors on the fluorine addition conditions and the properties of SiO* glass particles, we found that the amount of fluorine added depends not only on the fluorine concentration in the atmosphere and the heating temperature, but also on adverse effects in a certain temperature range. It turns out that it depends on the processing time.

In other words, the reason why it is difficult to add fluorine to the center of the SiO* glass particles is that the temperature at the center of the SiO* glass particles is difficult to rise. They discovered that this is because the body's thermal conductivity is low, making it difficult for the body to rise in temperature.

SiO*glass particle body is 0. It is formed by aggregation of fine glass particles of 1 to 0.5 microns, and contains a considerable proportion of pores. For example, the bulk density (expressing the density relative to the volume including holes; g/cd) is 0.
．． In the case of 3 teal, the volume ratio occupied by glass particles and pores is approximately 1:6. It is 4.

Therefore, the thermal conductivity of the glass particle body depends on that of the atmospheric gas, and is about one order of magnitude smaller than that of the transparent glass body.

For this reason, it is thought that the inside of the glass particle is difficult to heat, and fluorine addition in the center is difficult to progress, resulting in a refractive index distribution.

On the other hand, it was found that fluctuations in the refractive index distribution in the longitudinal direction of the village were caused by the temperature distribution in the longitudinal direction during the heat treatment for fluorine addition, and that there were parts where the temperature did not rise sufficiently. That is, fluorine is converted into SiO! In order to uniformly fill the glass particles, it is necessary to make the heating conditions of the entire base material uniform, and this condition requires that the temperature at which the SiO* glass particles themselves are substantially heated is 1150°C or higher and 1300°C.
It has been found that it is satisfactory if the heating time is 20 minutes or more.

At temperatures lower than 1,150°C, the fluorine addition reaction does not proceed sufficiently, and at temperatures higher than 1,300°C, the viscosity of the glass decreases and transparent vitrification begins, which on the contrary inhibits fluorine addition.

On the other hand, the treatment time is the time necessary for the reaction, and even if it was longer, the problem of excessive addition of fluorine did not occur.

Next, the configuration of the present invention will be explained based on examples.

FIG. 1 shows a so-called soaking furnace with a long heating zone. In Fig. 1, l is a heater, 2 is a heat treatment furnace core tube,
3 represents a Sift glass particle body. Figure 2 shows the temperature distribution when the set temperature was maintained at 1250°C in this soaking furnace.

In this case, the set temperature of 1250°C is only in the central part of the 800m total length of the furnace, and the heat zone where the temperature is 1150°C or higher is about 700°C.
It is n. From this, simply set the furnace temperature to 1150.
Even if you try setting it to ℃ or 1200℃, the si
It can be seen that the entire length of the Os glass particles is not heated at the set temperature. Therefore, Si
O*The total length of the glass body is 1150℃~1300℃ at the same time
By heating the base material so that it can be maintained within the temperature range for 20 minutes or more, it becomes possible to uniformly add fluorine over the entire base material.

On the other hand, FIG. 3 shows a so-called zone furnace with a short heating zone. In the figure, the meanings of 1 to 3 are the same as in the case of FIG. The temperature distribution in this case is shown in FIG. In the case of this zone furnace, the area where the temperature is 1150℃ or higher is 2
00 m, so in order to keep the base material within this region for 20 minutes or more, it is sufficient to perform the heat treatment while moving at a traverse speed of 10 m/min or less.

By the heat treatment of the present invention explained above, a base material to which fluorine is uniformly added can be obtained.

〔Example〕

Comparative Example 1 Synthesized by vAD method, outer diameter 140-φ, length 700
The SiOs glass particle body of m was heat-treated in the soaking furnace shown in FIG. The temperature distribution and the position of the Sins glass particles at this time are shown in FIG. 1 for set temperature 1200℃
The length of the Sins glass particles heated above 150° C. was about 500 μl. The heat treatment conditions are shown in Table 1.

After the glass was made transparent, the refractive index distribution in the glass was measured in the longitudinal direction, and as shown in FIG. 7, it was found that there were areas with a small amount of fluorine added at both ends where the processing temperature was low.

Example 1 Same length as the comparative example, 700 wj! In the same soaking furnace, heat treatment was performed using the same soaking furnace, raising the furnace temperature setting for the second heat treatment to 1250 °C, and setting the length of the area in the furnace that can be heated to 1150 °C or higher to be approximately 700 m+. I did it. Other conditions were the same as in Table 1.

As a result, there was no change in refractive index in the longitudinal direction, and a homogeneous fluorine-doped glass was obtained. Furthermore, the refractive index profile in the radial direction was relatively uniform, as shown in FIG.

Comparative Examples 2 to 4 In Example 1, the second heat treatment time was shortened to 15 minutes and 10 minutes (Comparative Examples 2 and 3). The refractive index distribution of the obtained base material is shown in FIG. 9 and FIG. As can be seen from both figures, when the heating time is shorter than 20 minutes, the difference in refractive index between the center and the outer periphery increases, making it impossible to obtain homogeneous fluorine-doped glass.

In addition, in Example 1, the second heat treatment temperature was 1350°C.
The temperature in the center of the furnace is 1350℃, and the temperature at both ends is 1350℃.
When the temperature was adjusted to 300° C., a nonuniform refractive index distribution as shown in FIG. 11 was obtained (Comparative Example 4).

Example 2 Approved by VAD method, outer diameter 140mφ, length 700
A fluorine-containing glass was produced by heat-treating the Sift glass particles of 50 m in size in the zone furnace shown in FIG. The furnace temperature was set at 1250° C., and the temperature distribution inside the furnace at this time was as shown in FIG. II 200 areas with temperatures over 50℃
A heat treatment for fluorine addition was performed by traversing at a speed of 1/min. Table 2 shows the conditions at this time. Traverse is si
This was done so that the entire Os glass particles uniformly passed through the heater. As a result, a good refractive index distribution similar to that shown in FIG. 8 was obtained. It was also uniform in the longitudinal direction.

Comparative Example 5 For comparison, the traverse speed of the second heat treatment in Example 2 was 15 cm/min, the time to be heated to 1150'C or higher was less than 20 minutes, and the other conditions were the same as in Example 2. When fluorine was added and the refractive index distribution was evaluated, the refractive index distribution was non-uniform in the radial direction as shown in FIG.

From the results of the above examples and comparative examples, it is clear that the total length of the SiO* glass particles is substantially 1150 to 1 in a fluorine atmosphere.
It can be understood that uniform fluorine addition is possible by setting the furnace structure and heating conditions in the furnace so that heating can be performed at a temperature of 300° C. for 20 minutes or more.

In the present invention, the outer periphery of the glass starting rod is coated with 81
A similar effect can be expected for a composite in which 0 g of glass particles are adhered and deposited.

In the present invention, the fluorine-based gas is SiF. Although the explanation has been given with reference to an example using CF4, SF4, etc., the effects of the present invention are the same.

In addition, SiO* glass particles that are not doped with additives are often used, but GeO*
, hos, B*Os, etc., there is no particular change in the effect.

〔Effect of the invention〕

As explained above, SiOx glass particles are 115
By setting the heat treatment conditions such that the temperature is maintained at a temperature of 0° C. or more and 1300° C. or less for 20 minutes or more, the fluorine addition reaction proceeds uniformly, and a fluorine-doped glass with a uniform refractive index can be obtained.

A glass body with a uniform amount of fluorine added is suitably used as an intermediate product for pure silica core optical fiber, and it is possible to obtain an optical fiber with stable characteristics.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the configuration of a soaking furnace according to the present invention, and FIG.
The figure is a temperature distribution diagram of the soaking furnace, Figure 3 is a schematic diagram showing the structure of the zone furnace according to the present invention, Figure 4 is a temperature distribution diagram inside the zone furnace, and Figure 5 is a diagram showing the temperature distribution in the radial direction. A refractive index distribution diagram when fluorine is added. Figure 6 is a diagram showing the temperature distribution in the furnace and the position of the st Os glass particles used in a comparative example. Figure 7 is a longitudinal temperature distribution diagram in the comparative example. , FIG. 8 is a radial refractive index distribution diagram based on the working example, FIG. 9 is a refractive index distribution diagram when the holding time is 15 minutes as a comparison with Example 1 (comparative example 2), and FIG. As a comparison with Example 1, the refractive index distribution diagram when the holding time is 10 minutes (Comparative Example 3), 1st
Figure 1 is a refractive index distribution diagram when the set temperature is 1350°C (Comparative Example 4) as a comparison with Example 1, and Figure 12 is a refractive index distribution diagram when the traverse speed is 15 km/min (Comparative Example 4) as a comparison with Example 2. It is a refractive index distribution map of Example 5). In the figure, 1 is the heater, 2 is the furnace core tube for heat treatment, and 3 is SI0m
A glass particle body is shown.

Claims (3)

[Claims] (1) SiO_2 glass particles synthesized by gas phase reaction are subjected to a first heat treatment for dehydration and a second heat treatment for fluorine addition in a heating furnace, and then made into transparent glass to form a glass matrix for optical fibers. In a method of manufacturing a material,
In the second heat treatment, the SiO_2 glass particles are heated at 1150°C or higher and 1300°C in an atmosphere containing fluorine gas.
1. A method for producing a glass preform for optical fibers, which comprises heating at a temperature of .degree. C. or lower for at least 20 minutes. (2) In the second heat treatment, 1150°C or higher 13
2. The method for producing a glass preform for optical fiber according to claim 1, characterized in that a heating furnace having a structure capable of simultaneously storing the entire SiO_2 glass particle body in a temperature range of 00° C. or lower is used. (3) In the second heat treatment, the heat treatment is performed while moving so that each part of the SiO_2 glass particles is exposed to a temperature range of 1150°C to 1300°C in the heating furnace for 20 minutes or more. Claims (
1) The method for producing a glass preform for optical fiber according to item 1).