JP4494325B2 - Manufacturing method of glass preform for optical fiber - Google Patents

Description

  The present invention relates to a method for producing a glass preform for an optical fiber, and more particularly to a method for producing a glass preform for an optical fiber homogeneously in the longitudinal direction.

A soot method such as a VAD method or an external method is generally used for manufacturing a glass preform for an optical fiber. In this method, a glass raw material gas such as SiCl ₄ or GeCl ₄ is introduced into an oxyhydrogen burner flame, and the generated glass fine particles are deposited on a rotating target to form a porous silica glass body. Thereafter, this porous silica glass is made into sintered glass in a sintering furnace to obtain a glass preform for optical fiber. Observing the glass preform for optical fiber thus obtained may cause defects in the glass preform. A defect here is a foreign material or a bubble resulting from a metal, an inorganic substance, etc.

  The main causes of defects are floating substances in the ambient atmosphere of the reaction vessel, metal dust generated from moving parts such as motors of manufacturing equipment, etc. entering the reaction vessel, resulting in the porous silica glass body. It seems to have adhered. When glass particles are further deposited from above in this state, some of them are roasted by a flame and volatilized or scattered. However, a part remains as a foreign substance inside the porous silica glass body. In some cases, this foreign material foams during sintering and may leave traces as bubbles. These foreign matters and bubbles can be confirmed as defects by observation after sintering if the size is 0.1 mm or more. Defects in the silica glass preform for optical fibers are not preferable because they cause breaks in the spinning process and decrease the strength of the fiber. Therefore, it is important to reduce foreign matters mixed in the quartz glass porous body.

For example, Patent Documents 1 and 2 have been proposed as conventional techniques for manufacturing such a glass preform for optical fiber.
Patent Document 1 describes a method of introducing clean air at a constant flow rate into both a reaction vessel for producing quartz glass and a sealed mantle installed so as to surround the reaction vessel. Further, it is described that defects in quartz glass can be reduced by depositing glass particles in that state.
In Patent Document 2, in the production of a porous base material, an inert gas or clean air is blown onto the surface of the starting core member in the front chamber provided on the front side of the growth side of the glass particulate deposit, A method is disclosed in which a starting core member is guided to the reaction container side from a through hole of a partition member provided between the reaction container and glass fine particles are deposited on the starting core member. Patent Document 2 also states that dust adhered to the starting core member can be effectively removed by ionizing inert gas or clean air using a commercially available static eliminator and then spraying the starting core member. Are listed.
Japanese Patent No. 3196629 JP 2000-109329 A

However, the above-described prior art has the following problems.
In Patent Document 1, clean air is supplied to both the booth and the reaction vessel. However, even in that case, the air filter cannot collect 100% of the foreign particles. The HEPA filter normally used in a clean room has a particle collection rate of about 99.97% with a particle size of 0.3 μm. Therefore, fine particles having a particle size of 0.3 μm or less may pass through the filter and be mixed into the reaction vessel. There are also high performance filters such as ULPA filters that have a higher particulate collection rate than HEPA filters. However, even in this case, there are few fine particles passing through the filter, and the pressure loss before and after the filter is large. Therefore, in order to ensure a constant air volume, a large fan may be required, which may lead to an increase in cost. In addition, the filter is clogged over time, and the collection ability decreases. Therefore, the degree of cleanliness is locally reduced, and as a result, foreign matter may be mixed in the reaction vessel.
As described above, the attempt to prevent foreign matter from mixing into the reaction vessel is very effective in suppressing defects in the optical fiber quartz glass preform, but it is difficult to prevent foreign matter from being mixed 100%.

In Patent Document 2, since ionized gas is flowed from the front end side of the base material, a local flow occurs, which may cause a decrease in deposition efficiency and an increase in soot cracking.
In addition, if the act of blowing gas to the base material is performed during deposition, a cooling effect will be manifested, which may cause distortion of the base material or breakage of the base material.
In addition, since the manufacturing apparatus illustrated in Patent Document 2 has a structure in which ionized gas flows through a narrow gap, there is a possibility that static electricity is generated again due to friction when the gas passes through the gap. Therefore, it seems that the effect of preventing foreign matter adhesion is insufficient.
Furthermore, Patent Document 2 does not have a detailed description on ionization and does not disclose how much the surface potential of the base material is, and therefore when the base material is irradiated with either a cation or an anion, The surface may be charged positively or negatively, and there is a possibility that the effect of preventing foreign matter adhesion cannot be expected.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing a glass preform for an optical fiber that can produce a high-quality optical fiber with a low yield and a high yield.

  In order to achieve the above object, the present invention provides a reaction vessel installed in a booth to which clean air is supplied, and a glass source gas introduced into an oxyhydrogen burner flame on a target rotating in the reaction vessel. In the manufacturing method of depositing the generated glass fine particles to form a quartz glass porous body and then converting to sintered glass to obtain a glass preform for optical fiber, a static electricity removing device is installed in the booth, and cations and anions Glass particles are deposited while maintaining the surface potential of foreign particles suspended in the reaction vessel in the range of -0.1 to 0.1 kV by alternately or simultaneously irradiating A method for producing a base material is provided.

  In the method for manufacturing a glass preform for an optical fiber according to the present invention, the static eliminator is preferably installed at either a clean air outlet or an inlet of a reaction vessel.

  According to the present invention, a silica glass porous body is produced by depositing glass particulates while maintaining the surface potential of the foreign particulates floating in the reaction vessel in the range of −0.1 to 0.1 kV. Bonding to obtain a glass preform for optical fiber, which can reduce the adhesion of foreign matters due to static electricity during the production of the preform, and can produce high-quality optical fibers with high yield with few defects. A glass base material can be manufactured.

The present inventors paid attention to the attractive force acting between the quartz glass porous base material and the foreign material. It was found that the influence of electrostatic force is large when considering the adhesion of fine particles to the wall surface or the adhesion to the wall surface with respect to the fine particles as foreign matter.
Although glass is generally an insulator, it tends to withstand positive (+) electricity, and attached fine particles tend to withstand negative (-) electricity. The amount of charge at that time varies greatly depending on the type of foreign matter, the presence or absence of friction, humidity, room temperature, and the like, but the electrical attractive force is reduced by reducing the amount of charge. In order to reduce the electric attractive force with respect to an insulator such as a quartz glass porous body, an effect can also be obtained by reducing the charge amount of either the quartz glass porous body or the adhering foreign matter. Practically, by removing static electricity from foreign particles floating in the reaction vessel, such as fine particles that could not be collected by the clean air filter and metal dust generated from the manufacturing equipment, it adheres to the quartz glass porous body. It was effective to prevent.

  As a result of various investigations, it has been found that suppressing the amount of charge of foreign particles floating in the reaction vessel within a certain range is effective in reducing defects. The amount of electrostatic charge can be determined by measuring the surface potential using an electrostatic sensor or the like. If quartz glass fine particles are deposited in such a state, even if foreign particles are mixed in the reaction vessel, they can be discharged outside the reaction vessel without adhering to the porous silica glass. As a result, a quartz glass preform for optical fibers with few defects in the preform can be obtained after sintering, and as a result of spinning such a preform, disconnection during spinning is reduced and yield is improved. The disconnection frequency can be determined to be good if it is 1 time / 300 km or less. However, disconnection frequency = number of disconnections (times) / 1 spinning length of base material (km).

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing an example of a production apparatus suitable for carrying out the method for producing a glass preform for optical fiber according to the present invention. This manufacturing apparatus includes a reaction vessel 2 containing a quartz glass porous body 1 held with its longitudinal direction oriented vertically, a booth 3 surrounding the reaction vessel 2, and a quartz glass porous body 1 in the reaction vessel 2. A plurality of burners 4 arranged so as to impinge on the flame, an exhaust port 5 for discharging the gas in the reaction vessel 2 to the outside, a clean air supply device 6 for supplying clean air into the booth 3, and a booth 3 is provided with a clean air filter 7 provided in a clean air supply unit 3 and a static electricity removing device 8 provided outside the reaction vessel 2 in the booth 3.

  Moreover, FIG. 2 is a block diagram which shows the other example of the manufacturing apparatus suitable in order to implement the manufacturing method of the glass preform for optical fibers by this invention. This manufacturing apparatus accommodates a porous silica glass body 1 that is held with its longitudinal direction oriented horizontally, a reaction vessel 2 having an open lower side, a booth 3 surrounding the reaction vessel 2, and a porous silica glass porous body. A burner 4 disposed so as to be exposed to a flame from below toward the body 1, an exhaust port 5 provided at the upper part of the booth 3 for discharging the gas in the reaction vessel 2 to the outside, and the booth 3 from the lower part of the booth 3 A clean air supply device 6 for supplying clean air to the booth 3, a clean air filter 7 provided on the lower side of the booth 3, and a static electricity removing device 8 provided on the clean air filter 7 in the booth 3. It is configured.

  In the production method of the present invention, the glass produced from the glass raw material gas introduced into the oxyhydrogen burner flame on the target rotating in the reaction vessel 2 using the production apparatus shown in FIG. 1 or the production apparatus shown in FIG. When the quartz glass porous body 1 is produced by depositing fine particles, a static eliminating device 8 is installed in the booth 3, and foreign substances floating in the reaction vessel 2 are irradiated by cation and anion alternately or simultaneously. The glass fine particles are deposited while maintaining the surface potential of the fine particles in the range of -0.1 to 0.1 kV.

  As the static eliminator 8, it is desirable to use a static eliminator such as an ionizer. Due to the ions generated from the ionizer, the surface potential of foreign particles floating in the reaction vessel, such as particles that could not be collected by the clean air filter 7 or metal generated from the manufacturing apparatus, can be reduced. What is necessary is just to select suitably the installation location of the static eliminating device 8 for every manufacturing apparatus. Generally, it is desirable to install near the blowout port of clean air or the intake port of the reaction vessel because the effect is high. In addition, as for the ion generated here, it is desirable to irradiate a cation and an anion alternately or simultaneously. If only ions of either polarity are used, static electricity of the opposite polarity cannot be removed. In the case of alternately irradiating positive ions and negative ions, it is necessary to adjust the generation frequency, but there is usually no problem if the frequency is about 10 Hz.

  As a result of various changes in the conditions, if the surface potential of the foreign particle floating in the reaction vessel 2 is in the range of −0.1 to 0.1 kV, the foreign material adheres less to the porous silica glass body 1. As a result, a quartz glass preform for optical fiber with few defects after sintering could be obtained. In the case of spinning the base material, the disconnection frequency can be set to 1 time / 300 km or less. On the other hand, if the surface potential of the foreign particles floating in the reaction vessel 2 is outside the above range, defects after sintering increase, the frequency of disconnection increases, the manufacturing yield deteriorates, and the manufacturing cost increases.

Next, the quartz glass porous body 1 produced as described above is sintered and made into a transparent glass to obtain a glass preform for an optical fiber for an optical fiber.
This sintering step can be performed using a conventionally known method and apparatus in the optical fiber manufacturing field.

  According to the present invention, a silica glass porous body is produced by depositing glass particulates while maintaining the surface potential of the foreign particulates floating in the reaction vessel in the range of −0.1 to 0.1 kV. Bonding to obtain a glass preform for optical fiber, which can reduce the adhesion of foreign matters due to static electricity during the production of the preform, and can produce high-quality optical fibers with high yield with few defects. A glass base material can be manufactured.

[Examples 1-3, Comparative Examples 1-2]
A quartz glass porous body was manufactured using the apparatus shown in FIG. Here, it manufactured in the state which installed the reaction container in the booth supplied with clean air. A static eliminator (Clean Air Barrier Eliminator SJ-G manufactured by Keyence Corporation) was installed in the vicinity of the inlet of the reaction vessel as shown. The ion irradiation time was appropriately changed, and the surface potential of the fine particles floating in the reaction vessel at that time was measured. The surface potential was measured using an electrostatic sensor (High-precision electrostatic sensor SK manufactured by Keyence Corporation).
Thereafter, using a plurality of burners, glass fine particles were deposited on a target rotating at 30 rpm in the reaction vessel to produce a porous silica glass body having a diameter of 230 mm and a length of 1900 mm. The gas flow rates at this time were SiCl ₄ flow rate: 5.5-7.5 SLM, hydrogen gas flow rate: 40-100 SLM, oxygen gas flow rate 15-40 SLM, and argon gas flow rate 1 SLM.
After producing a porous silica glass body, it was put into a sintering furnace and subjected to sintering vitrification to obtain an optical fiber glass preform. With respect to the glass preform for optical fiber, the number of defects after sintering was confirmed.
Thereafter, the glass preform for optical fiber was spun and the frequency of disconnection was investigated. The results are shown in Table 1.

As in Examples 1 to 3 shown in Table 1, if the surface potential of the fine particles floating in the reaction vessel is in the range of -0.1 to 0.1 kV, the number of defects after sintering is 2 or less. As a result, the frequency of disconnection when spinning the produced optical fiber glass preform could be reduced to 1 time / 300 km or less.
On the other hand, in Comparative Examples 1 and 2 in which the surface potential of the fine particles floating in the reaction vessel deviated from the above range, the number of defects after sintering was increased to 7 or more, and the produced optical fiber glass preform was spun. The frequency of disconnection when doing so increased to 2.5 times / 300 km or more.

[Comparative Example 3]
An optical fiber glass preform was prepared in the same manner as in each of the examples except that the static particles floating in the reaction vessel were not removed, and the number of defects after sintering was confirmed. Thereafter, the glass preform for optical fiber was spun and the frequency of disconnection was investigated.
When static electricity was not removed, the surface potential of the fine particles floating in the reaction vessel was 1.5 kV. The number of defects after sintering was 8, and the fracture frequency was 5.1 times / 300 km or more, which was worse than Examples 1-3.

[Comparative Example 4]
An optical fiber glass preform was prepared in the same manner as in each of the above examples except that static electricity removal of fine particles floating in the reaction vessel was performed only with cations, and the number of defects after sintering was confirmed. Thereafter, the glass preform for optical fiber was spun and the frequency of disconnection was investigated.
When the static electricity was removed only with cations, the surface potential of the fine particles floating in the reaction vessel was 0.3 kV. The number of defects after sintering was 5, and the fracture frequency was 2.6 times / 300 km or more, which was worse than Examples 1-3.

[Comparative Example 5]
An optical fiber glass preform was prepared in the same manner as in each of the above examples except that static electricity removal of fine particles floating in the reaction vessel was performed only with anions, and the number of defects after sintering was confirmed. Thereafter, the glass preform for optical fiber was spun and the frequency of disconnection was investigated.
When static electricity was removed only with anions, the surface potential of the fine particles floating in the reaction vessel was -0.35 kV. The number of defects after sintering was 4, and the fracture frequency was 2.1 times / 300 km or more, which was worse than Examples 1-3.

It is a block diagram which shows an example of a suitable manufacturing apparatus in order to implement the manufacturing method of the glass preform for optical fibers by this invention. It is a block diagram which shows the other example of the manufacturing apparatus suitable in order to implement the manufacturing method of the glass preform for optical fibers by this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Silica glass porous body, 2 ... Reaction container, 3 ... Booth, 4 ... Burner, 5 ... Exhaust port, 6 ... Clean air supply apparatus, 7 ... Clean air filter, 8 ... Static electricity removal apparatus.

Claims (2)

A reaction vessel is installed in a booth to which clean air is supplied, and glass fine particles generated from the glass raw material gas introduced into the oxyhydrogen burner flame are deposited on a target that rotates in the reaction vessel to obtain a porous quartz glass. In a manufacturing method for obtaining a glass preform for an optical fiber after forming into a sintered body,
A static eliminator is installed in the booth, and the surface potential of the foreign particles suspended in the reaction vessel is kept in the range of −0.1 to 0.1 kV by alternately or simultaneously irradiating positive ions and negative ions. A method for producing a glass preform for optical fibers, comprising depositing glass particles. The method for producing a glass preform for an optical fiber according to claim 1, wherein the static eliminating device is installed at either a clean air outlet or an inlet of a reaction vessel.

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