JPH07106925B2 - Method for manufacturing base material for optical fiber - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a base material of an optical fiber using a porous base material, and in particular, it uses fluorine (F) as an additive in a large amount and at high speed. The present invention relates to a method for manufacturing a base material for optical fibers added in step 1.

[Conventional technology]

Hitherto, optical fibers have been manufactured by various manufacturing methods such as the VAD method and the OVPO method, but attention has been paid to them in terms of productivity and quality. According to these methods, first, glass particles are produced by a flame hydrolysis reaction, and they are successively deposited on a rotating starting material to produce a rod-shaped porous preform. Next, the preform is heat-treated in various gas atmospheres, dehydrated and molten into glass to obtain an optical fiber base material. Furthermore, the method is a method in which this base material is spun to obtain an optical fiber.

An optical fiber is mainly composed of a core part through which light is propagated and a cladding part around it. If the refractive index of the core part is n ₁ and the refractive index of the cladding part is n ₂ , NA (numerical aperture) Is Is defined by (n ₁ and n ₂ are average values). In an optical fiber based on silica (SiO ₂ ), (i) a method of adding an additive for increasing the refractive index to the core, (ii) a method of adding an additive for decreasing the refractive index to the cladding, (iii) (i ) And (ii)
Any one of the above method, which is a combination method. Needless to say, the cladding portion in (i) and the core portion in (ii) are silica.

Commonly used additives include GeO ₂ , P ₂ O ₅ , Al ₂ O ₃ ,
Examples thereof include TiO ₂ (for increasing the refractive index), B ₂ O ₃ and F (for decreasing the refractive index) and the like. Figure 2 shows the refractive index of silica glass at a wavelength of 0.59 µm. The abscissa represents the weight% of oxide in silica, and the ordinate represents the refractive index (nα) and the refractive index Δn%. [Source: Kumamaru, Kurosaki: “Optical Transmission Material” Industrial Material 27 (1979), P39] Among these additives, fluorine is one of the most recently added additives, and the VAD method and Methods of addition have been studied and developed in other production methods.

When it is desired to obtain the same refractive index difference between the core and the cladding, the refractive index is lowered by the general cladding, and the above (ii) and (ii
The method i) requires no additive amount to be added to the core portion, or requires less amount than the method i).
It has the advantage of This is advantageous for a high NA optical fiber in that the absorption loss due to the additive in the core part is reduced. Moreover, a pure silica core optical fiber having excellent transmission loss under irradiation of radiation can be produced only by the method (ii).

As described above, the method of lowering the refractive index of the cladding portion has advantageous characteristics.

In particular, in the sintering step of the VAD method, the advantage of adding fluorine is that it can be added uniformly and a flat refractive index distribution can be provided.

The processing speed is fast. That is, a porous preform of several 100 to 1 kg can be treated and vitrified within a few hours.

2 is particularly superior to other methods.

However, in the prior art, even if the porous preform was heat-treated in an atmosphere of 100% fluorine-based gas under normal pressure, only a maximum difference of about -0.75% was added due to the difference in refractive index. Further, in another manufacturing method, for example, a method called a plasma external method, a glass raw material is blown onto a starting rod by using a flame by thermal plasma, and the glass is directly deposited and vitrified,
At this time, when a fluorine-based gas is added at the same time to add fluorine, but when a fluorine-based gas that gives a refractive index difference of -1% is contained, the deposition rate is 0.1 g / min at most. Moreover, it is known that the deposition rate decreases as the amount of addition increases.

In addition, in both the VAD method and the plasma method, when it is attempted to add fluorine by −0.5% or more due to a difference in refractive index, bubbles are left in the obtained glass base material and the amount of fluorine added is increased. However, there is a problem that this tendency becomes larger as it is done.

On the other hand, by the time of the transparent vitrification step of the glass fine particles containing quartz as the main component, the fine particles are at least temporarily
A heat treatment is performed in a gas atmosphere consisting essentially of SiF ₄ gas, and the gas is 1 atm or more;
In the above, the method for producing a glass preform for optical fibers containing fluorine, which is heat-treated while blowing SiF ₄ gas, eliminates the above-mentioned drawbacks, that is, improves the amount of fluorine added without leaving bubbles. In addition, we have realized that fluorine can be added at high speed.

[Problems to be solved by the invention]

However, the above-described method uses a highly reactive fluorine-based gas at a pressure higher than atmospheric pressure and also performs heat treatment at a relatively high temperature, and thus it is difficult to use a metal or the like as a heat treatment container. Atsuta For this reason, a material other than metal is used, but measures against gas leaks, especially leaks due to deterioration of the heat treatment container, are desired.

An object of the present invention is to solve the problem of leakage of fluorine-based gas, and to provide a method for producing an optical fiber preform capable of efficiently adding fluorine without mixing impurities. Another object of the present invention is to provide a low-cost manufacturing method capable of using a device which is cheaper than before.

[Means for solving problems]

The present invention is a heat treatment vessel in a double structure, and the internal pressure P ₂ of the P ₁ of the second container holding a first pressure P ₁ and the first vessel of a container holding the glass particulate matter> P _By adjusting so that ₂ > P ₁ -0.3 kg / cm ² and P ₂ > 1 atm, and by making the atmosphere in the second container a rare gas and / or nitrogen gas atmosphere, in the first container The present invention provides a method for producing a high-quality glass base material for optical fibers, which contains fluorine, while directly preventing the surrounding environment from being contaminated due to the leakage of the fluorine-based gas.

That is, according to the present invention, the fine particles are kept in the first container in a gas atmosphere substantially consisting of SiF ₄ gas for at least a period of time until the transparent vitrification step of the glass fine particles containing quartz as a main component, The first container is further provided with a _second container in which an atmosphere containing a rare gas and / or a N ₂ gas is contained.
While keeping the internal pressure of the _first container P ₁ ,
When the internal pressure of the second container is P ₂ , P ₁ > P ₂ > P ₁ −0.
P ₁ and P to be 3 kg / cm ² and P ₂ > 1 atm
_A method for producing a base material for an optical fiber, which comprises the step of adjusting ₂ and performing heat treatment.

In the present invention, it is particularly preferable to perform the heat treatment while blowing the SiF ₄ gas in the first container and the rare gas and / or the N ₂ gas in the second container. Further, in the present invention, it is preferable that the first container is made of high-purity carbon or high-purity quartz glass, and the second container is made of high-purity quartz glass because good results can be obtained.

The present invention will be specifically described below with reference to the drawings. FIG. 1 shows an example of a heat treatment apparatus used for carrying out the present invention, in which 1 is a fine glass particle body, 2 is a first pressure vessel, and 3 is a pressure vessel.
Is a second pressure vessel, 4 and 5 are hatches, 6 is a heating device,
7 and 8 are pressure gauges, 9 and 10 are valves, and 11 to 14 are gas pipes, and the arrows in the figure show the direction of gas flow.

The glass microparticles 1 are added with fluorine in a double-structured heat treatment container having a first pressure vessel 2 and a second pressure vessel 3. An atmosphere gas consisting essentially of SiF ₄ gas and having a pressure P ₁ is supplied to the first pressure vessel 2 through the valve 7 and the line 11 and exhausted through the line 13 and the valve 9 as the fluorine addition gas. . Further, an atmosphere gas consisting of a rare gas and / or N ₂ gas and having a pressure P ₂ is supplied into the second pressure vessel 3 through the valve 8 and the line 12, and is exhausted through the line 14 and the valve 10. In addition, heating is performed by the heating device 6 during this time. At this time, prepare P ₁ and P ₂ with valves 7, 8, 9 and 10 so that P ₁ > P ₂ > P ₁ −0.3 kg / cm ² and P ₂ > atmospheric pressure range. To do. It can be easily inferred that the double heat treatment container can prevent the surrounding environment from being contaminated due to the leakage of the fluorine-based gas.

However, if a double container is simply used, for example, the atmospheric gas in the outer container is not mixed with the fluorine-based gas in the inner container to efficiently add fluorine to the fine glass particles, or the inner container is It is difficult to obtain a good glass body because phenomena such as damage to the inner container occur due to the pressure inside and the pressure difference inside the outer container.

The present inventors have further studied this point. As a result, when the pressure of the atmosphere gas substantially consisting of SiF ₄ in the first container (inner container) 2 is P ₂ the pressure of the atmosphere gas consisting of the rare gas of P ₁ and / or N ₂ gas, Leakage of SiF ₄ gas to the surrounding gas can be prevented only by setting P ₂ to more than 1 atm (atmospheric pressure), but when P ₂ ≧ P ₁ , the atmosphere in the second container Gas is mixed in the first container,
As a result, the SiF ₄ gas partial pressure in the first container is lowered, and it has been found that a situation often occurs in which a glass base material in which the amount of fluorine added has not reached a predetermined amount is obtained.

On the other hand, by making P ₂ smaller than P ₁ , it is possible to prevent the atmospheric gas in the second container from mixing into the first container. However, if the pressure difference between P ₂ and P ₁ is large, P ₁ Cannot be maintained at a predetermined pressure, or in the worst case, the first container may be damaged.
As a result of the inventors' detailed experiments and studies on this point, P ₁ and P ₂ were adjusted so as to maintain the relationship of P ₁ > P ₂ > P ₁ −0.3 kg / cm ² It has been found that it is relatively easy to maintain the pressure in the inner container at a predetermined value while preventing the atmospheric gas in the container from mixing into the inner container.

In the present invention, a gas composed of a rare gas and / or N ₂ gas is used as the atmosphere gas in the second container. Examples of the rare gas include He and Ar, but Ar is more preferable. In principle, such an atmospheric gas does not require exhaust gas treatment and is relatively inexpensive, so it can be used at a large flow rate. As a result, the second container can have a large capacity within the range of pressure resistance thereof, so that it is easy to design such that the sealing portion of the lid is placed outside the heating region. That is, it is possible to prevent leakage from the outer container to the surroundings and easily control the internal pressure.

Furthermore, in the present invention, the first or inner container is completely shielded from the oxygen atmosphere, and
Since the differential pressure between the second container and the second container is extremely small, it is also advantageous that a highly heat-resistant and inexpensive material such as high-purity carbon can be used.

Further, in the present invention, the second container has heat resistance,
Since pressure resistance, oxidation resistance, and airtightness are required, there is a problem in oxidation resistance and airtightness with carbon (when carbon has a large pressure difference between the inside and outside, gas permeates). It is preferable to use quartz glass as a material that satisfies all requirements. Further, from the viewpoint of heat resistance, high-purity quartz glass is particularly preferable.

The method of the present invention can prevent leakage of SiF ₄ gas to the periphery and efficiently add F when heat treatment is performed in a SiF ₄ atmosphere in which glass fine particles are pressurized.

Although the description has been made with reference to FIG. 1, the method of the present invention is not limited to this.

〔Example〕

Example 1 Pure silica glass fine particles of 100 mmφ × 400 mml prepared by the VAD method using the apparatus shown in FIG. 1 were added with fluorine and made transparent according to the present invention. The inside of the first container was set to 100% SiF ₄ atmosphere and the inside of the second container was set to 100% N ₂ atmosphere. The internal pressure of each container, the first container is 4.0 kg / cm ^2, the second internal pressure of the container Atsuta at 3.95kg / cm ^2. After heat treatment was carried out at a temperature of 1200 ° C. for 2 hours, it was melted and vitrified. The negative refractive index of the obtained glass was 1.06%. There was no gas leak to the surrounding environment, and fluorine could be added at the specified value.

Comparative Example 1 With respect to the same glass fine particle body as that used in Example 1,
Except that the P ₁ was 4.0kg / cm _^2, P ² and 4.1 kg / cm ² was obtained by following Example as well as means pursuant molten glass body. The negative refractive index of the obtained glass was 0.98%, which was 0.08% smaller than that in Example 1 (large in absolute value). Also, some bubbles were found in the obtained glass. This is N ₂ in the second container
It seems that the gas was mixed in the first container.

The same glass soot body and used in Comparative Example 2 Example 1 and Comparative Example 1, a P ₁ to 4.0 kg / cm ^2, P ₂ similarly except that the 3.5 kg / cm ² from Example 1 KoTsuta. When the pressure inside the first container was monitored during the heat treatment for 2 hours, it was extremely unstable, and the average SiF ₄ flow rate as in Example 1 was 3.7 atm.
The negative refractive index of the obtained glass was 1.04%. Example 1
It can be seen that the addition efficiency of fluorine is lower than that of -1.06% in the case of.

From the above examples and comparative examples, it is clear that the present invention is effective. Needless to say, the present invention is not limited to the above-mentioned embodiments.

〔The invention's effect〕

As is clear from the above description, the present invention has the following effects.

(1) The container is made into a double structure, the inside of the first container is made into a SiF ₄ gas atmosphere, and the atmosphere gas consisting of a rare gas and / or N ₂ gas is blown into the inside of the second container, so that the SiF ₄ gas It is possible to heat-treat the gas fine particles in a pressurized atmosphere while preventing the leakage of the surrounding area.

(2) The first container pressure is P ₁ , and the second container pressure is P _2.
In this case, by setting P ₁ > P ₂ > P ₁ −0.3 kg / cm ² , it is possible to prevent gas other than SiF _{4 from} entering the first container and
The internal pressure of the container can be kept stable.

(3) The first container is in a state where oxygen is blocked, and the second container
Since the pressure difference from the internal pressure of the container is less than 0.3 kg / cm ^2, it is possible to use an inexpensive high-purity carbon container having high heat resistance.

(4) Since the second container may have a considerably large capacity, the seal portion of the upper lid can be designed outside the heating area.
Leak prevention can be performed more effectively.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of an apparatus used to carry out the present invention.

Claims (3)

[Claims] 1. A method for producing a glass fine particle containing quartz as a main component at least for a period of time before the step of vitrifying the fine glass.
The first container is kept in a first container having a gas atmosphere consisting essentially of SiF ₄ gas, and the first container is further provided with a second container whose inside contains a rare gas and / or N ₂ gas. While holding in the container, when the internal pressure of the first container is P ₁ and the internal pressure of the _second container is P ₂ , P ₁ > P ₂ > P ₁ −0.3 kg /
A method for producing a base material for an optical fiber, which has a step of performing heat treatment by adjusting P ₁ and P ₂ so that the pressure is cm ² and P ₂ > 1 atm. 2. The light according to claim 1, wherein heat treatment is carried out while blowing SiF ₄ gas in the first container and rare gas and / or N ₂ gas in the second container. Manufacturing method of base material for fiber. 3. The method according to claim 1, wherein the first container is made of high-purity carbon or high-purity quartz glass, and the second container is made of high-purity quartz glass. The method for producing a base material for optical fibers described in 1.