JP2985493B2 - Manufacturing method of optical fiber preform - Google Patents

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 an optical fiber preform, and more particularly to a method for producing an optical fiber preform as it is or an intermediate product for producing an optical fiber preform.

[0002]

2. Description of the Related Art As a method of manufacturing an optical fiber preform, a glass fine particle synthesized by a gas phase synthesis method, for example, a gas phase shaping method or an external method, is heated at a high temperature in an electric furnace to obtain a transparent vitrified material. There is a way to do that. Conventionally, transparent vitrification has been performed by heat treatment at normal pressure in an atmosphere filled with an inert gas containing a trace amount of helium gas or halogen gas. In these methods, there is a problem that, when the glass is made transparent, gas trapped between the particles of the glass fine particle deposit remains and bubbles are generated in the transparent glass body. In contrast, in recent years, Japanese Patent Application Laid-Open No. 63-210102
A method for making the film transparent under a vacuum atmosphere or a reduced-pressure atmosphere as described in Japanese Patent Application Laid-Open No. 5 (1993) -1994 has been proposed. In this method, since the atmosphere is reduced in pressure, the gas in the glass fine particle deposit is degassed, and it is expected that no bubbles remain in the glass body.

[0003]

Conventionally, an apparatus for performing a heat treatment in a vacuum or reduced-pressure atmosphere has a structure as shown in FIG. That is, the furnace core tube 2 surrounding the periphery of the glass particle deposit body 1 and the soaking furnace in which the heating heater 8 is installed outside the furnace core tube 2 are configured to enter the vacuum vessel 10 with the heat shield 9 interposed therebetween. I have. The vacuum vessel 10 has a structure in which the pressure is reduced and a vacuum atmosphere is generated by a vacuum pump 7 connected to a pipe 6 for deaeration. By increasing the heater temperature at the upper end, the vacuum vessel 10 is inserted into the furnace core tube 2. The glass particle deposit body 1 is made transparent.

When the temperature is raised to a clearing temperature (normally 1550 ° C. to 1650 ° C.) under a temperature condition shown in FIG. 6 as an example of a heat treatment by a conventional method under reduced pressure and a vacuum atmosphere by using a heating furnace, In some cases, contrary to expectations, bubbles remained. Further, the outer diameter of the transparentized glass article was not uniform in the longitudinal direction, and the shape became thick at both ends and thin at the middle as shown in FIG. In order to produce a high-quality glass article using the heating rod obtained by this method, it is necessary to stably reduce or eliminate the residual bubbles, and the finished transparent glass article has a uniform outer diameter. It is necessary to. An object of the present invention is to improve a method of manufacturing a glass fiber preform that can satisfy such requirements.

[0005]

Means for Solving the Problems The present invention for solving the above-mentioned problems comprises: (1) synthesizing a glass particle deposit by a vapor phase synthesis method, and subjecting the glass particle deposit to a heat treatment in a vacuum or reduced-pressure atmosphere. In the method for producing an optical fiber preform by vitrification, a first heating step in which the heat treatment removes gas from the preform, and a second vitrification step in which the first heating step is performed at a higher temperature than the first heating step. heating step only contains and <br/> optical fiber base, characterized by continuing the heating step to a vacuum degree in the temperature range of the first heating step is 900 to 1200 ° C. reaches 10Pa following method of manufacturing a wood, (2) synthesizing a soot glass deposit body by vapor-phase synthesis method, the
Heats glass particle deposits in vacuum or reduced pressure atmosphere
Transformation into vitreous glass by processing
In the manufacturing method, the heat treatment removes gas from the base material.
A first heating step to remove, higher than said first heating step
A second heating step of vitrifying at a temperature, and the second heating step is performed at a degree of vacuum of 10 Pa or less,
A method for producing an optical fiber preform, wherein a temperature of 1500 to 1600 ° C. is maintained for 1 minute to 60 minutes ; (3) a glass fine particle deposit is synthesized by a gas phase synthesis method;
Heats glass particle deposits in vacuum or reduced pressure atmosphere
Transformation into vitreous glass by processing
In the manufacturing method, the heat treatment removes gas from the base material.
A first heating step to remove, higher than said first heating step
An optical fiber mother comprising a second heating step of forming a vitreous glass at a temperature and , prior to the heating treatment, preliminarily depressurizing the glass particle deposit at room temperature to a degree of vacuum of 20 Pa or less. method of manufacturing a wood, (4) to synthesize glass particles deposit by vapor phase synthesis method, the
Heats glass particle deposits in vacuum or reduced pressure atmosphere
Transformation into vitreous glass by processing
In the manufacturing method, the heat treatment removes gas from the base material.
A first heating step to remove, higher than said first heating step
Including a second heating step of vitrifying at a temperature, and
The first heat treatment step from 800 ° C. in the heat treatment
In temperature until, to heating at a 2.0 ° C. / min to 10 ° C. / minute rate
Method of manufacturing an optical fiber preform, characterized in Rukoto, (5) to synthesize glass particles deposit by vapor phase synthesis method, the
Heats glass particle deposits in vacuum or reduced pressure atmosphere
Transformation into vitreous glass by processing
In the manufacturing method, the heat treatment removes gas from the base material.
A first heating step to remove, higher than said first heating step
Including a second heating step of vitrifying at a temperature, and
From the temperature of the first heat treatment step to the second heat treatment step
To a temperature of 1.0 ° C./min to 4 ° C./min,
And when the heating rate is changed more than once during the heating process
Indicates that the heating rate after the change is lower than the heating rate before the change
Of producing optical fiber preform characterized by heat treatment
Law, (6) to synthesize glass particles deposit by vapor phase synthesis method, the
Heats glass particle deposits in vacuum or reduced pressure atmosphere
Transformation into vitreous glass by processing
In the manufacturing method, the heat treatment removes gas from the base material.
A first heating step to remove, higher than said first heating step
Including a second heating step of vitrifying at a temperature,
The heating element that heats the glass particle deposit
Is divided into multiple stages to control the temperature independently, and
The temperature of the lower heating element will be lower than the temperature of the upper heating element
Of glass fiber preform characterized by being set as follows
The manufacturing method, and (7) the glass fine particle deposit, the outer peripheral portion than the central portion
Glass with at least double waveguide structure with low refractive index
Glass particles are deposited around the rod by vapor phase synthesis
(1) to (6), wherein the composite is
The method for producing a glass fiber preform according to any one of
You.

[0006]

As a means for solving the above-mentioned problems, the present invention synthesizes a glass fine particle deposit by a gas phase synthesis method, and heat-treats the glass fine particle deposit in a vacuum or reduced-pressure atmosphere to form a transparent vitreous optical fiber. In the method of forming a base material, a first step (first heating step) of degassing the base material in a temperature range of 900 to 1200 ° C. and reducing the pressure to 10 Pa, and a transparent glass at a temperature of 1500 to 1600 ° C. It is characterized in that a two-stage heat treatment of a second step (second heating step) is performed. It is preferable that the glass fine particle deposit is subjected to a reduced pressure treatment at room temperature to 20 Pa or less before the heat treatment step. In the heat treatment step, the rate of temperature rise is also important. The temperature rising rate from 800 ° C. to the temperature of the first step is 2 ° C./min to 10 ° C./min or less, and 1 ° C. in the temperature range of the first step to the second step.
/ Min to 4 ° C / min. In the heating process from the first step to the second step, the heating rate can be changed once or more on the way, but the speed after the change needs to be lower than before the change. Further, it is preferable that the heating element for heating the glass particle deposit is divided into multiple stages in the vertical direction, and the lower part temperature is controlled to be equal to or lower than the upper part temperature.

The inventors of the present invention heat-treated the glass fine particle deposit under a reduced pressure atmosphere, and examined changes in the degree of vacuum and the bulk density of the glass fine particle deposit. As shown in FIG.
The degree of vacuum is degraded by degassing from the glass fine particle deposit in the range of ° C to 1200 ° C. The bulk density of the glass fine particle deposit increases from 1200 ° C. and becomes vitrified at 1550 ° C. Next, an experiment was performed using a composite deposit in which glass fine particles were deposited on the outer periphery of a glass rod as an object of the present invention by a vapor phase synthesis method. The composite deposit is
In a process of performing decompression treatment at a temperature of 200 ° C., and then further raising the temperature to 1550 ° C. to form a transparent glass, a correlation between the ultimate vacuum degree at 1200 ° C. and the number of bubbles in the transparent glass body was examined. The results are shown in FIG.
When the pressure is reduced to a or less and the glass is transparently vitrified, almost no bubbles are generated. However, when the temperature is raised before reaching 10 Pa and the glass is transparently vitrified, the number of bubbles rapidly increases. These bubbles mainly occur around the glass rod on which the glass particles are deposited. When the number is small, the bubbles can be confirmed as minute bubbles, but when the number is large, the bubbles appear white and opaque. Therefore,
The most temperature range of outgassing from the glass particulate deposits,
That is, from 900 ° C. to 1200
It is important to reduce the pressure to 10 Pa or less in the range of ° C., raise the temperature, and turn the glass into a transparent glass to obtain a transparent glass body without bubbles.

In the manufacture of the optical fiber preform, as important as the absence of bubbles is the uniform outer diameter. As shown in FIG. 5, the present inventors have found that the rate of temperature rise from the first step to the second step is the outer diameter difference (lower end diameter−center diameter).
And a strong correlation. In this temperature range, as shown in the experimental results of FIG. 5, the glass particle deposit rapidly shrinks. For this reason, the lower end portion has a large heating surface area, and is more likely to be heated and contracted than the central portion. Therefore, the lower end has a larger diameter than the center.
With respect to the outer diameter difference generated in this way, it is necessary to reduce the temperature difference over the entirety and gradually contract the temperature difference. In order to use it as an optical fiber preform, it is necessary to suppress the difference in outer diameter to about 6 mm.
FIG. 5 shows that the temperature is between 1 ° C./min and 4 ° C./min. Furthermore, the transparentization temperature range of 1500 to 1600 ° C.
In, when the glass fine particle deposit is heated for an unnecessarily long time, the glass is easily softened and hangs down due to its own weight.
Therefore, minimal heating is sufficient. The minimum heating time depends on the outer diameter of the sediment and the bulk density.
Within minutes is desirable. It is also effective to lower the temperature of the lower end in order to suppress the increase in the diameter of the lower end, and as shown in FIG. It is also possible to keep the temperature below. In the case of FIG. 1, the heater is divided into three stages of heaters 3, 4, and 5.

On the other hand, paying attention to the insertion process from the room temperature to the first step, the glass fine particle deposit is previously removed at room temperature by 20P.
It is important to reduce the pressure to a degree of vacuum of a or less for the purpose of maintaining the heating furnace. This is because the glass fine particle deposit contains a large amount of water, hydrochloric acid, and the like. Also, 80
At a temperature of 0 ° C. or lower, the glass fine particle deposit does not change at all, but care must be taken in the process of raising the temperature from 800 ° C. to the first step. If heated rapidly in this temperature range, the gas inside the deposit is rapidly released, causing the deposit to crack. However, lowering the heating rate more than necessary lowers the productivity, which is not desirable, and a range of 2 ° C./min to 8 ° C./min is appropriate according to various studies.

[0010]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Example 1 A glass fine particle deposit made of pure SiO ₂ synthesized by the VAD method was made transparent according to the present invention. The size of the glass particle deposit was φ160 mm × 700 mm. After reducing the pressure of this base material to 20 Pa at room temperature, it was inserted into the furnace shown in FIG. 1 and heated to 1000 ° C. at a rate of 4 ° C./min. At this time, the furnace internal pressure was 18 Pa, but when it was held for one hour, it reached 5 Pa.
The temperature was raised to ° C. However, the heating element of the furnace was divided into three parts, and the temperature was raised to 1500 ° C. only in the lowermost part. During this heating, the furnace pressure was constant at approximately 5 Pa. After reaching the maximum temperature, the temperature was maintained for 5 minutes, and the temperature was lowered. As a result, the obtained glass body was a good transparent body without air bubbles over the entire length. A very uniform outer diameter of φ70 mm ± 0.3 mm was obtained over the entire length.

Comparative Example 1 A glass particle deposit synthesized by the VAD method in the same manner as in Example 1 was reduced in the furnace to 20 Pa and the temperature was increased at 8 ° C./min with the configuration shown in FIG. It was kept at 1600 ° C. for 30 minutes and then taken out of the furnace after cooling. As a result, microbubbles having a diameter of 0.1 mm or less were observed over the entire length, and the outer diameter was as thin as φ65 mm at the center and as thick as φ73 mm at the lower part.

Example 2 In the same manner as in Example 1, a glass fine particle deposit synthesized by the VAD method was used. After reducing the pressure of the base material to 20 Pa at room temperature,
It was inserted into the furnace shown in FIG. The inside of the furnace was previously heated to 800 ° C. The temperature of the first step is 1100 ° C. and 80
The temperature was raised from 0 ° C. at a rate of 3 ° C./min. At this time, the furnace pressure was 20 Pa. After holding for 1 hour, 5Pa
, The temperature was raised to the second step. Second
The temperature of the step was 1550 ° C. In the temperature raising process, first, the temperature is raised from 1100 ° C. to 1400 ° C. at 3 ° C./min,
The temperature was raised from 400 ° C. to 1550 ° C. at 2 ° C./min. The heating element of the furnace was divided into three parts, and only the lowermost part was stopped at 1520 ° C. After reaching the maximum temperature, the temperature was maintained for 3 minutes, and the temperature was lowered. During this heating process, the furnace pressure was constant at 5 Pa. As a result, the obtained glass body has no bubbles over the entire length,
It was a transparent good body. Outer diameter is φ68m over the entire length
m ± 0.3 mm.

[0013]

As described above, according to the present invention,
Since it is manufactured under conditions that sufficiently remove the gas present in the glass particle deposit, there is no air bubble remaining in the glass after clarification, and the temperature rise rate in the temperature range where the glass particle deposit contracts. By limiting the above, a glass body having a uniform glass diameter can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating a manufacturing method using a heating furnace having a divided heating element according to an embodiment of the present invention.

FIG. 2 is a diagram schematically illustrating a heating furnace used in the present invention or a conventional method.

FIG. 3 shows the degree of vacuum (Pa) and the bulk density (g / c) of a glass particle deposit in a heat treatment under a reduced pressure atmosphere.
m ³⁾ is a graph showing changes in the.

FIG. 4 is a graph showing the correlation between the ultimate vacuum at 1200 ° C. and the number of bubbles in a transparent glass body.

FIG. 5 is a graph showing a correlation between a heating rate from a first step to a second step and an outer diameter difference.

FIG. 6 is a graph showing temperature conditions of a conventional heat treatment.

FIG. 7 is an explanatory view showing a shape of a glass article having an outer diameter difference manufactured by a conventional method.

[Description of sign]

 DESCRIPTION OF SYMBOLS 1 Glass fine particle deposit body 2 Furnace core tube 3 Heater 4 Heater 5 Heater 6 Deaeration piping 7 Vacuum pump 8 Heater 9 Heat shield 10 Vacuum container

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C03B 37/014

Claims (7)

(57) [Claims] 1. A method for producing an optical fiber preform by synthesizing a glass fine particle deposit by a vapor phase synthesis method and heat-treating the glass fine particle deposit in a vacuum or reduced-pressure atmosphere to form a transparent glass. a first heating step of heating to remove gas from the preform, the second heating step <br/> viewed free to vitrified at a higher temperature than the first heating step, and the first The heating process is 900-1200 ° C
Until the degree of vacuum reaches 10 Pa or less in the temperature range,
A method for producing an optical fiber preform, comprising continuing a heating step . 2. A method for producing a glass fine particle deposit by a gas phase synthesis method.
After synthesizing, deposit the glass particle deposit in a vacuum or reduced pressure atmosphere.
Transformation into glass by heat treatment in air
In the method for producing a base material, whether the heat treatment is a base material or not.
A first heating step of removing these gases, and the first heating
The second heating step to make the glass viscous at a higher temperature than the process
Wherein, and the second heating step is in the degree of vacuum below 1 0 Pa, 1,500 to 1,600 method for manufacturing an optical fiber preform temperature you characterized in that retaining 1 to 60 minutes of ° C.. 3. A method for producing a glass fine particle deposit by a gas phase synthesis method.
After synthesizing, deposit the glass particle deposit in a vacuum or reduced pressure atmosphere.
Transformation into glass by heat treatment in air
In the method for producing a base material, whether the heat treatment is a base material or not.
A first heating step of removing these gases, and the first heating
The second heating step to make the glass viscous at a higher temperature than the process
Wherein, and prior to the heat treatment method in advance the glass particles deposit optical fiber preform it characterized that you vacuum treatment to a vacuum degree 20Pa at room temperature. 4. A method for producing a glass fine particle deposit by a gas phase synthesis method.
After synthesizing, deposit the glass particle deposit in a vacuum or reduced pressure atmosphere.
Transformation into glass by heat treatment in air
In the method for producing a base material, whether the heat treatment is a base material or not.
A first heating step of removing these gases, and the first heating
The second heating step to make the glass viscous at a higher temperature than the process
Wherein, and the heat treatment odor from Te 8 00 ° C. to a temperature of the first heat treatment step, 2.0 ° C. / min to 10 ° C. / min in the optical fiber preform you characterized by raising the temperature Production method. 5. A method for producing a glass fine particle deposit by a gas phase synthesis method.
After synthesizing, deposit the glass particle deposit in a vacuum or reduced pressure atmosphere.
Transformation into glass by heat treatment in air
In the method for producing a base material, whether the heat treatment is a base material or not.
A first heating step of removing these gases, and the first heating
The second heating step to make the glass viscous at a higher temperature than the process
And the temperature is raised from the temperature of the first heat treatment step to the temperature of the second heat treatment step at a rate of 1.0 ° C./min to 4 ° C./min, and at least once during the temperature rise process method for manufacturing an optical fiber preform you characterized in that heating rate after the change is heat treated such that the following heating rate before the change in case of changing the rise rate of temperature. 6. A method for producing a glass fine particle deposit by a gas phase synthesis method.
After synthesizing, deposit the glass particle deposit in a vacuum or reduced pressure atmosphere.
Transformation into glass by heat treatment in air
In the method for producing a base material, whether the heat treatment is a base material or not.
A first heating step of removing these gases, and the first heating
The second heating step to make the glass viscous at a higher temperature than the process
The heating element for heat-treating the glass fine particle stack is divided into multiple stages in the vertical direction to independently control the temperature, and so that the temperature of the lower heating element is equal to or lower than the temperature of the upper heating element. features and to Ruga Las fiber manufacturing method of the preform to be set. 7. A composite in which the glass fine particle deposit is formed by vapor-phase synthesizing glass fine particles on the outer periphery of a glass rod having at least a double waveguide structure having a lower refractive index at an outer peripheral portion than at a central portion. The method for producing a glass fiber preform according to any one of claims 1 to 6 , wherein the glass fiber preform is a body.