JPH0230633A - Production of matrix for optical fiber - Google Patents

Abstract

PURPOSE:To obtain the title matrix freed from bubbles or crystal grains by treating, in an atmosphere of inert gas of specified partial pressure, a sintered form produced by sintering a porous glass matrix in a He atmosphere. CONSTITUTION:A burner 2 for synthesizing glass fine particles is fed with a glass material gas, combustion gas and auxiliary combustion gas to subject said glass material gas to flame hydrolysis with a flame 3 from the burner 2, and the resultant glass fine particles produced are accumulated on the tip of a rotating starting material 4 to obtain a porous glass matrix 5. Thence, this matrix 5 is heated in a He atmosphere at 1,500-1,650 deg.C into a transparently vitrified sintered form, which is then heat-treated in an atmosphere of an inert gas consisting of N2 or Ar >= 0.8 in the partial pressure at 800-1,000 deg.C for >=180min.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing an optical fiber preform.

[Prior Art] Generally, in the production of a porous glass base material for optical fibers (hereinafter abbreviated as porous base material) using a flame hydrolysis reaction, as shown in Fig. burner 2
A mixture of combustion gas, frit glass, etc. is ejected from the oxyhydrogen flame 3, and glass fine particles generated by a hydrolysis reaction of the glass raw material are deposited on the tip of the rotating starting material, and are deposited in the direction of the rotating shaft 4. A method of growing a porous base material 5 is used. Then, the porous base material 5 is
- Place it in a heating furnace with a He atmosphere as shown in Figure 2,
A sintered body (transparent glass base material) that is sintered and made into transparent glass by gradually lowering it inside the heater 6 while rotating it.
is manufactured.

The porosity of the porous base material (volume % of pores in 1 cm3 of the porous base material) is approximately 80 to 90%, and in order to completely eliminate the pores during sintering, sintering in a He atmosphere is necessary. It is considered mandatory. This is because, in the progressing process of sintering, whether pores existing at the boundaries between particles of glass fine particles, that is, grain boundaries, grow or disappear depends on the permeation rate of gas within the pores. In other words, pores containing a gas with a high permeation rate are easily eliminated, and He gas has a higher permeation rate in quartz gas than other gases.

[Problems to be Solved by the Invention] The sintered body (transparent glass base material) manufactured by the above conventional method has an outer diameter of 100 to 200 μ-
is stretched into a thin optical fiber.

In this wire drawing step, the sintered body must first be processed into a shape that can be drawn with an appropriate outer diameter, so it is heated to a high temperature of 1900° C. or higher and then stretched. At this time, if the sintered body is manufactured with minute defects such as bubbles, crystal grains, impurities, etc. in the manufacturing process using the above conventional method, these defects will be used as the core of the base material after stretch forming. There was a problem in that bubbles with a diameter of about 1 to 51 mm were generated in the (line drawing base material), reducing the product yield. If a wire drawing base material containing such large bubbles is drawn, the fiber will break, making stable production impossible.

An object of the present invention is to solve the above-mentioned problems and provide a method for producing an optical fiber preform that is free from bubbles and crystal grains even when stretched and formed as a wire drawing preform, making it possible to produce a stable fiber. The object of the present invention is to improve, in particular, a method for obtaining a transparent glass preform from a porous preform.

"Means for Solving the Problems" As a result of intensive research, the present inventors have obtained a sintered body obtained by sintering a porous base material in an I4C atmosphere.
The present invention was achieved based on the discovery that the generation of bubbles in the subsequent stretch-molding process can be reduced by heat treatment in an atmosphere (excluding Ha).

That is, the present invention synthesizes a porous glass base material by depositing glass fine particles on the tip of a rotating starting material, and then sintering the porous glass base material in a He atmosphere to turn it into transparent glass. This method of manufacturing an optical fiber preform is characterized in that a sintered body is heat-treated in an atmosphere of an inert gas (excluding e) having a partial pressure of 0.8 atmospheres or more.

In a particularly preferred embodiment of the present invention, the inert gas is N or 8r, and the heat treatment is carried out at a temperature of 800 to + 000°C under an atmosphere in which the partial pressure of the inert gas is 0.8 atm or less. The above-mentioned method of holding within the range for 180 minutes or more is exemplified.

Hereinafter, the present invention will be described in detail with reference to the drawings. In the present invention, the steps from manufacturing a porous base material to sintering it to form a sintered body are performed using conventional techniques. That is, with the configuration shown in FIG.
By introducing a combustion gas such as H, etc. and an auxiliary combustion gas such as 02, the glass raw material is subjected to a flame hydrolysis reaction in the flame 3 of this burner 2 to produce glass fine particles, which are rotated as a starting material 4. The porous base material 5 is synthesized by depositing it on the tip of the porous base material 5. Additives such as GeCl may also be added to the glass raw material.

Next, in a heating furnace as shown in FIG. 12, the porous base material 5 is heated to a temperature of 1,500 to 1,650 DEG C. in a He atmosphere to turn it into transparent glass and form a sintered body.

Although the figure shows a so-called zone heating furnace, a soaking furnace may also be used.

A feature of the present invention is that the transparent sintered body is further heated in an atmosphere where the partial pressure of an inert gas (excluding He) is 0.8 atm or more, preferably, as shown in Figures 1-3. N
, or in an atmosphere where the partial pressure of Ar is 0.8 atm or more.
It involves heat treatment within a temperature range of 0 to 1000°C. The treatment time is preferably 180 minutes or more. The higher the heat treatment temperature and the longer the treatment time, the greater the bubble reduction effect, but the conditions that are sufficient vary depending on the shape of the sintered body, sintering conditions, etc., and in experiments conducted by the present inventors, it is usually
If the temperature was maintained at ~1000°C for 180 minutes or more, the generation of bubbles could be reliably prevented.

[Function] Gas analysis of bubbles generated in a glass rod obtained by sintering a porous base material in a He atmosphere to form a sintered body and then drawing it using a conventional method revealed that the Y component was He. It turned out to be. From this fact, it is considered that the bubble generation is caused by He dissolved in the glass during sintering and foaming during stretching.

In general, the solubility of gases in glass is a function of temperature;
It tends to become smaller as the temperature increases.

Therefore, when the temperature is raised until the solubility becomes smaller than the amount of gas contained in the glass, the gas in the gas becomes supersaturated and bubbles up into the glass. It is believed that when the glass temperature at this time is equal to or higher than the softening point of the glass, the gas bubbles into the glass as bubbles centered around minute defects. In the process of drawing optical fiber base materials (transparent vitrified sintered bodies, etc.), the glass is heated to a high temperature of 1700°C or higher, which is higher than the sintering temperature, to sufficiently soften the glass. It is quite conceivable that He becomes bubbles and evaporates into the glass.

Therefore, in order to prevent the generation of bubbles in the glass during the drawing process, the present inventors have determined that
We have come up with the idea that it is effective to lower the He content and flL below the solubility of He in the glass at the drawing temperature. For example, one glass material sintered at 1500'C is
Suppose that the stretching process was carried out at 000°C. Assuming that the solubility of He in glass is 1.0 at 1600°C, it is 20
The temperature at 00°C is approximately 0°96. Therefore, in order to prevent the generation of bubbles in the drawing process, it is necessary to
It is sufficient to remove about 4% of the

In order to remove the gas from the glass in this way, an effective method is to hold the glass in an atmosphere that does not contain the target gas to be removed, and then evaporate the target gas from the glass surface into the atmosphere (outside world). This external atmosphere is l-(
Inert gas other than e, that is, N or Ar, has a partial pressure of 0,
An inert gas with a pressure of 8 atmospheres or more is preferable, and is also inexpensive (because of its low solubility in glass, N7 partial pressure of 0,
Air with a pressure of less than 8 atmospheres can also be used as the atmosphere.

Further, in order to increase the rate at which the gas existing inside the glass diffuses and moves to the glass surface, a certain degree of temperature is required, and a suitable temperature range is 800 to 1000°C. At temperatures below 800°C, a sufficient He removal effect cannot be obtained;
If the temperature exceeds 00°C, problems such as devitrification of the surface of the glass base material, diffusion of impurities, and thermal deformation occur, which is not preferable. Furthermore, it has been found that He within the glass base material diffuses toward the surface of the base material, and that a heating time of 180 minutes or more is required to reduce the Hefi degree to a desired level or less throughout the glass base material.

Figure 2-1 shows the results of investigating the relationship between the processing time and the number of bubbles generated when a glass sintered body was processed in an atmosphere of Nff1 O and 8 atm while the heating temperature was kept constant at 800°C. . From Figure 2-1, at a temperature of 800°C, for more than 180 minutes,
It can be seen that bubbles can be reduced by H treatment.

Figure 2-2 shows the results of examining the number of bubbles generated when the treatment temperature was varied while the treatment time was kept constant at 180 minutes in a nitrogen atmosphere of 0.8 atm. From this figure, the temperature is 800℃
It can be seen that a sufficient bubble reduction effect can be obtained with the following steps. In addition, when treated in the same manner at a temperature of +200°C,
Although no bubbles were observed, a problem occurred in which the surface of the base material became devitrified.

[Example] SiC1, as a gaseous glass raw material, is introduced into the oxyhydrogen flame of a combustion burner to generate glass fine particles, which are deposited on the tip of the rotating starting material as shown in Fig. 11 and then rotated around the rotating shaft. By growing in the direction of 150 ffi diameter
A porous base material for an optical fiber having a length of +s and a length of 700 mm was synthesized. This was inserted into a heating furnace with a 100% He atmosphere as shown in FIG. 12, and heated by gradually lowering the inside of the heater without rotating, to obtain a sintered body with a diameter of 7 ff1e and a length of 400 mm. When this sintered body was visually observed, no residual air bubbles were found. However, when this sintered body was subjected to a stretching process at 2000°C for 30 minutes, four bubbles were found in the glass rod obtained by the stretching process (comparative example).

Therefore, as an example of the present invention, a sintered body prepared in the same manner as the comparative example described above was heated to 800°C by a heater 7 in an atmosphere of N, 0° and 8 atm as shown in Figure 1-3. A treatment was performed in which the sample was held in a furnace for 5 hours.

Next, this heat-treated sintered body was heated to 2000°C as in the comparative example.
When the glass was stretched, not a single bubble was found in the formed glass (Example).

From the above results, it is clearly seen that the heat treatment of the present invention is very effective in reducing the generation and growth of bubbles in the subsequent stretch forming process of the sintered body (transparent glass base material).

[Effects of the Invention] As explained above, a porous base material synthesized by depositing glass particles by a flame hydrolysis reaction on the tip of a starting material is sintered in an He atmosphere, and the obtained sintered body is heated at a high temperature. By heat-treating in an inert gas atmosphere other than Hc before stretching, the present invention can prevent bubbles from forming in the glass base material during the stretching process, making it possible to improve the production of optical fiber base materials. The yield can be significantly improved and a high-quality wire base material can be obtained. Therefore, by using the wire-coating base material according to the present invention, it is possible to stably manufacture optical fibers without problems such as disconnection.

[Brief explanation of the drawing]

1-1 to 1-3 are diagrams explaining embodiments of the present invention, in which FIGS. 1 to 1 are steps for creating a porous base material, and FIG. sintered body] - Cheng,
1-3 are schematic explanatory diagrams of the process of heat treatment in an inert gas other than He. FIG. 2-1 is a chart showing the relationship between the processing time and the number of bubbles generated, and FIG. 2-2 is a chart showing the relationship between the processing temperature and the number of bubbles generated.

Claims (1)

[Claims] 1) A porous glass base material is synthesized by depositing glass particles on the tip of a rotating starting material, and the porous glass base material is heated with He
For optical fibers, which is characterized in that after sintering in an atmosphere to make it transparent and vitrifying, the obtained sintered body is heat-treated in an atmosphere in which the partial pressure of an inert gas (excluding He) is 0.8 or more. Method of manufacturing base material. 2) The light according to claim 1, wherein the heat treatment in an atmosphere with an inert gas partial pressure of 0.8 or more is maintained within a temperature range of 800 to 1000°C for 180 minutes or more. Method for manufacturing fiber base material. 3) The method for manufacturing an optical fiber preform according to claim 1, wherein the inert gas is N_2 or Ar.