JP2003063830A - Method for manufacturing porous glass preform - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing porous glass preform capable of introducing fluorine compounds into a burner for depositing glass fine particles and depositing glass fine particles and adding fluorine simultaneously. SOLUTION: In the method for depositing glass fine particles by laminating and growing glass fine particle deposit layers radially on a starting rod, it is characterized by putting a fluorine compound gas with a flammable gas, a supporting gas, and a glass raw material gas into the burner.

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 by an OVD method, and more particularly to a method for producing a porous glass preform by depositing glass fine particles while doping a doping material.

[0002]

2. Description of the Related Art The VAD method or the OVD method is generally used as a method for synthesizing a high-purity porous glass base material (also referred to as a glass particle deposit). Among them, the OVD method, for example, as disclosed in Japanese Patent Laid-Open No. 48-73222, deposits glass fine particles produced by a hydrolysis reaction or an oxidation reaction of a glass raw material gas on the outer peripheral portion of a rotating glass rod, This is a multi-layering method in which the outer diameter of the base material is gradually increased and the glass particles are deposited in a predetermined amount, and then the deposition is stopped.

Multi-layer attachment method by OVD method (external attachment method)
For various methods, various methods have been studied and developed.As a typical method, a plurality of glass particulate deposition burners are arranged facing a rotating starting rod, and the starting rod and the burner are arranged in parallel. Divided synthesis multi-layering method in which glass particles are deposited by setting the relative movement distance of each burner in one direction to be shorter than the entire length of the porous glass preform (relative movement for each relative movement). Glass fine particle deposition layer is formed by dividing the moving range of each burner), and one or more glass fine particle deposition burners are arranged so as to face the rotating starting rod. And relatively reciprocally move in parallel, and set so that the relative moving distance of each burner in one direction is longer than the entire length of the porous glass preform. Particles is continuous synthesis multilayer with methods of depositing (a soot layer by the respective burners for each mobile each relative is formed over the entire length of the porous glass preform).

In the above-mentioned division-synthesis multi-layering method, the method of shifting the turn-back position of the reciprocating movement in one direction and repeating the operation of moving the turn-back position in the opposite direction when the turn-back position is moved by a predetermined distance is repeated. A method for efficiently producing a large-sized porous glass preform with little variation in the outer diameter of the porous glass preform in the lengthwise direction (JP-A-3-22).
8845).

On the other hand, Japanese Patent Application Laid-Open No. 2000-169172 discloses a technique of introducing a fluorine compound together with a glass raw material gas into a burner for depositing fine glass particles to produce a porous glass base material and add fluorine. In the publication, SF ₆ , SiF ₄ , C ₂ F ₆ , CF _{4 and the} like are used as the fluorine-containing gas used for the addition of fluorine, and the fluorine-containing gas is used when fluorine is added when the glass particle deposit is formed. It is described that, together with other raw material gas, it is hydrolyzed in a flame to form glass fine particles, and the glass fine particles are deposited on a quartz target to form a fluorine-containing glass fine particle deposit body.

As described above, the advantage of adding fluorine in the glass fine particle deposition step is that it is not necessary to newly provide a sintering step of adding fluorine, and fluorine can be added in a very small amount. However, the problem is that
In the technique of producing a porous glass base material having a low bulk density such as the VAD method, it is difficult to stabilize the addition amount in the radial direction of the porous glass base material (it is not impossible but very difficult to control), and the addition amount is large. Is difficult to increase.

The above publication also describes that the addition of fluorine in the step of depositing fine glass particles results in poor utilization efficiency of the reaction gas, for example, the fluorine-containing gas is exhausted unreacted. Therefore, conventionally, the addition of fluorine to the glass base material has generally been performed during the sintering of the porous glass base material. The amount of fluorine added in the step of depositing glass particles (at the time of sooting) is 0.01 to 0.2%. If a large amount of fluorine is added at the time of sooting, the cause is unknown, but the soot (glassy particle deposition layer) is broken. . On the other hand, the amount of fluorine added can be 0.15 to 0.7% during sintering.

[0008]

DISCLOSURE OF THE INVENTION The main object of the present invention is to introduce a fluorine compound into a burner for depositing fine glass particles to add a trace amount of fluorine to a porous glass base material. The contents of this fluorine addition are as follows: A low concentration of fluorine is added (the amount of added fluorine is from the level of pure silica to 0.01% or more and 0.2% or less) in the radial direction. To provide a method of performing controllability well, to provide a technique for making the amount of fluorine added to the porous glass base material uniform in the radial direction, and to change the amount of fluorine added to the porous glass base material to the radial direction. To provide a technique for making a step in the refractive index by changing the direction. In the above description, the meaning of making the amount of fluorine uniform in the radial direction means “substantially uniform” or “substantially constant”.

The state shown in FIG. 1A is an example in which the amount of fluorine is uniform in the radial direction. On the other hand, the state shown in FIG. 1B is an example in which the amount of fluorine is non-uniform. Figure 1
(C) is a schematic diagram showing a refractive index profile having steps, and in the figure, the portions a and b have a uniform amount of fluorine in each portion.

[0010]

The above-mentioned objects can be achieved by the following inventions according to the present invention. (1) In a glass particle depositing method in which a glass particle deposit is grown in a radial direction so that a layer is stacked on a starting rod, a combustible gas, an auxiliary gas, and a fluorine compound gas are introduced into a burner together with a glass raw material gas. A method for producing a porous glass preform characterized by the above. (2) The bulk density of the glass fine particle deposited body deposited on the starting rod exceeds 0.60 g / cm ³ and 1.0 g / cm ³
The method for producing a porous glass preform according to the above (1), characterized in that the diameter is made substantially constant in the following range.

(3) The method for producing a porous glass preform according to the above (2), characterized in that the mixing ratio of the glass raw material gas and the fluorine compound gas is kept constant. (4) The method for producing a porous glass preform according to (2) above, wherein the mixing ratio of the glass raw material gas and the fluorine compound gas is changed.

(5) Glass raw material gas and fluorine compound gas
Are mixed on the starting rod and are deposited on the starting rod.
The bulk density of the glass particulate deposit is 0.60 g / cm.³Over
1.0g / cm³Radially change within the following range
Production of the porous glass preform according to the above (1), characterized in that
Method. (6) of the glass particulate deposits deposited on the starting rod
Bulk density of 0.1 g / cm ³0.60 g / cm or more³Less than
The porous glass according to (1) above, characterized in that
Manufacturing method of base metal.

(7) The method for producing a porous glass preform according to the above (6), characterized in that the mixing ratio of the glass source gas and the fluorine compound gas or the bulk density distribution in the radial direction is changed as the glass particles grow. (8) Until the desired particle size of the glass particle deposit is reached, the mixing ratio of the glass raw material gas and the fluorine compound gas is changed as the glass particle grows, and the bulk density of the glass particle deposit layer deposited on the starting rod is changed. The volume density is set to 0.1 g / cm ³ or more and 0.60 g / cm ³ or less, and after reaching a desired diameter, the bulk density exceeds 0.60 g / cm ³ and 1.0 g / cm ³
The method for producing a porous glass preform according to the above (1), which has a constant bulk density selected from the range of ³ cm ³ or less and a constant mixing ratio of the glass raw material gas and the fluorine compound gas.

(9) Until the desired particle size of the glass particle deposit is reached, the mixing ratio of the glass raw material gas and the fluorine compound gas is changed with the growth of the glass particle, and the glass particle deposit layer deposited on the starting rod is changed. Bulk density of 0.1 g /
cm ³ and more 0.60 g / cm ³ or less in the range, desired after reaching the diameter, constant bulk selected from 1.0 g / cm ³ or less in the range in the bulk density exceeds the 0.60 g / cm ³ The method for producing a porous glass preform according to (1) above, wherein the density is changed and the mixing ratio of the glass raw material gas and the fluorine compound gas is changed. (10) The method for producing a porous glass preform according to any one of the above (1) to (9), wherein a plurality of burners for depositing glass particles are used.

(11) In the glass particulate deposition method, a plurality of glass particulate deposition burners are arranged so as to face a rotating starting rod, and the starting rod and the burner are relatively reciprocally moved in parallel to each other. (1) to (4), which is a divided synthetic multi-layering method in which glass fine particles are deposited by setting the relative movement distance of the burner in one direction to be shorter than the entire length of the glass fine particle deposit body.
(9) The method for producing a porous glass preform according to any one of (9).

(12) In the method for depositing glass particles, one or a plurality of burners for depositing glass particles are arranged facing a rotating starting rod, and the starting rod and the burner are relatively reciprocally moved in parallel. The continuous synthetic multi-layering method of depositing glass particles by setting the relative movement distance of each burner in one direction to be longer than the entire length of the glass particle deposit body. A method for manufacturing a porous glass base material. (13) The bulk density of the glass fine particle deposition layer formed on the starting rod exceeds 0.80 g / cm ³ and is 1.0 g / cm ^3.
The method for producing a porous glass preform according to the above (12), characterized in that the diameter is kept substantially constant in the range of ³ or less. (14) The method for producing a porous glass preform according to the above (13), wherein the mixing ratio of the glass raw material gas and the fluorine compound gas is constant.

(15) The above (13), wherein the mixing ratio of the glass raw material gas and the fluorine compound gas is changed.
Of the porous glass base material of the above. (16) The mixing ratio of the glass raw material gas and the fluorine compound gas is kept constant, and the bulk density of the glass fine particle deposition layer formed on the starting rod exceeds 0.80 g / cm ³ and is 1.0.
The method for producing a porous glass preform according to the above (12), characterized in that the radial direction is changed within a range of g / cm ³ or less. (17) The porous glass matrix according to the above (13), characterized in that the bulk density of the glass fine particle deposition layer formed on the starting rod is in the range of 0.1 g / cm ³ or more and 0.80 g / cm ³ or less. Method of manufacturing wood. (18) The method for producing a porous glass preform according to the above (17), characterized in that the mixing ratio of the glass raw material gas and the fluorine compound gas or the bulk density distribution in the radial direction is changed as the glass fine particles grow.

(19) A desired glass particle deposition body diameter is obtained.
Until the glass raw material gas grows
The starting rod.
The bulk density of the glass fine particle deposition layer formed on the top is 0.1 g.
/ Cm³0.80 g / cm or more ³The following range, desired
After reaching the diameter of 0.80g / cm³Beyond
1.0 g / cm³A certain volume selected from the following range
And the mixture of glass source gas and fluorine compound gas
The porosity of the above (12), characterized in that the mixing ratio is also constant.
Of manufacturing high quality glass base material. (20) Until the desired glass particle deposit size is reached,
With the growth of lath particles, glass raw material gas and fluorination
The mixing ratio of the product gas is changed, and the gas is deposited on the starting rod.
The glass particles have a bulk density of 0.1 g / cm³0.8 or more
0 g / cm³Set the following range, and after reaching the desired diameter,
Bulk density is 0.80 g / cm³Over 1.0g / cm³
It has a certain bulk density selected from the following range and
Changing the mixing ratio of the raw material gas and the fluorine compound gas
The method for producing the porous glass base material according to (12), which is characterized
Law.

(21) As a burner for depositing fine glass particles, a focus having a structure in which a combustible gas, an auxiliary combustion gas, a raw material gas, and a fluorine compound gas gather at one point until the burner flame reaches the porous glass deposition surface. A method for producing a porous glass preform according to any one of the above (1) to (20), characterized in that a mold burner is used.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Recently, there has been a demand for adding a very small amount of fluorine, and for that purpose, it is advisable to add fluorine when sooting. There is a close relationship between the amount of fluorine added and the bulk density, and since the bulk density can be adjusted by an external method, the amount of fluorine added in the radial direction can be controlled. This is the most important feature of the present invention. Here, the radial direction means a radial direction in a circle of a cross section perpendicular to the optical axis of the glass particle deposit body. In the production of the porous glass base material by the OVD method, the inventors have made the bulk density 0.6 g when the mixing ratio of the glass raw material gas and the fluorine compound gas is constant.
/ Cm ³ (0.8 g / in the case of the continuous synthetic multi-layering method)
It has been found that whether or not the fluorine concentration in the radial direction of the base material becomes uniform is determined at the boundary of cm ³ .

In this case, the bulk density is 0.6 g / cm.³(Communicating
0.8 g / cm for continuous multi-layering ³)
Elemental concentration becomes uniform, but 0.6g / cm³(Multiple continuous synthesis
0.8g / cm when layered³) The following should not be uniform
Yes. Therefore, if you want to add fluorine uniformly,
0.6 g / cm³(0.8g / in continuous synthetic multi-layering)
cm³) Over 1.0g / cm³The following may be done.
1.0 g / cm³If it is made larger, it becomes more transparent in the base material
Since there is a problem, the upper limit was set to 1.0. And fluorine
When it is desired to add non-uniformly, the bulk density should be 0.1 or more.
6 g / cm³(0.8g / c with continuous synthetic multilayering
m³) It should be as follows. 0.1 g / cm³Smaller
It is difficult to do so (the soot will crack), so set the lower limit.
It was set to 0.1.

The reason why the bulk density at which the fluorine addition state changes in the case of the continuous synthetic multi-layering method is higher than that in the divided synthetic multi-layering method is considered as follows. For example, in the case of the split composite multi-layering method, the reciprocating distance between the starting rod and the burner is generally set to about the interval between the burners, and the porous glass base material is always wrapped in the burner flame. Has been. Therefore, since the base material grows while being kept warm, strain is unlikely to be formed between the glass fine particle layers,
Although it is advantageous for soot cracking, the fluorine bound to the glass particles is dissociated, and the reaction of leaving is also promoted. The bulk density at which the amount of fluorine supplied from the outside of the base material and the amount of fluorine that dissociates begins to maintain a balance is 0.6 g / cm ³ in the split-synthesis multi-layering method. The reaction of dissociating fluorine bonded to the glass particles is presumed to be the following equation, and the generated HF (gas) escapes from the base material, and this reaction is accelerated at higher temperatures. SiF + H ₂ O → SiO + HF

On the other hand, in the case of the continuous synthetic multi-layering method, every burner reciprocates so as to pass through the entire effective portion of the starting rod every time it moves in one direction, so that the base material exists outside the flame. Time will increase. Since the base material is cooled during this period, the temperature of the base material itself is kept lower than in the case of the split-synthesis multi-layering method, and it is considered that the dissociation reaction is small.
Comparing the continuous synthetic multi-layering method and the split-synthesis multi-layering method, the continuous synthetic multi-layering method has less dissociation reaction at the same bulk density, and the fluorine supply amount is larger than the dissociation amount until the equilibrium state is reached. Will take longer. That is, the bulk density, which immediately settles in an equilibrium state, is higher than that of the split-synthesis multi-layering method, and is 0.8 g / cm ³ . However,
Since new glass fine particles are deposited on the cooled base material, strain is likely to accumulate between the glass fine particle layers, and soot cracking is likely to occur. In addition, the synthesis speed is faster with the split-synthesis multi-layering method because all burners are used for depositing glass particles, but the continuous synthesis multi-layering method in which each layer is formed by the same burner is longer in the base metal length direction. The change in the amount of deposit is small.

When the bulk density is reduced (for example, 0.
(Less than 3 g / cm ³ ), a split-synthesis multi-layering method using a plurality of burners is effective. In the continuous synthetic multi-layering method, the soot may cool and crack when the burner moves away. It is advantageous to use multiple burners because the soot will not get cold and will not crack.

In the present invention, when a fluorine compound is added to a burner for depositing glass particles to add fluorine,
Since the mechanism of fluorine addition changes depending on the bulk density of the glass particulate deposit, it will be described below. The addition of fluorine is roughly divided into the following three patterns and added to the porous glass base material. 1) The glass raw material undergoes a hydrolysis reaction to form glass particles, and the fluorine of the fluorine compound is bonded to the glass particles until it reaches the deposition surface. 2) After the glass particles are deposited, they exist in the gaps between the glass particles. Fluorine is bonded to the glass particles. Since the size of the gap varies depending on the bulk density of the glass fine particles, the amount added here has a large dependency on the bulk density. 3) After the glass particles are deposited, fluorine enters through the gaps between the glass particles and bonds with the glass particles. In this case as well, the degree of reaction depends on the bulk density of the glass particle deposit, and further, from the relationship between the atmosphere in which the glass particle deposit exists (base material temperature, H ₂ O concentration, etc.) and the dissociation reaction,
6 g / cm ³ (0.8 g / c in the continuous synthetic multi-layering method)
In a region where the bulk density is higher than that of m ³ ), the equilibrium state of the addition reaction and the dissociation reaction is immediately settled, so that the change in the amount of fluorine added due to this reaction is not observed. Although it will settle into an equilibrium state of the dissociation reaction and the addition reaction even in the range of the bulk density or less, the state in which the amount of fluorine supplied through the gaps of the glass particles is larger than the amount of dissociated fluorine is relatively long Exists across. Here 1) ~
The respective inventions (1) to (21) will be described based on 3).

(1), (12) The technical idea of supplying a fluorine compound gas to the burner, which is the object of the present invention, to produce porous glass fine particles and to add fluorine to the porous glass part is defined. . The invention is a basic feature of the present invention, in which (1) is a general multi-layering method, and (11) and (12) are inventions specified in a division composite multi-layering method and a continuous composite multi-layering method, respectively. OV
The method D is a method in which the bulk density control in the radial direction is easier to perform and the controllability of the amount of fluorine added is higher than that in the VAD method. Comparing the divided composite multi-layering method and the continuous composite multi-layering method, the former has a higher composition speed than the latter (cost can be reduced),
Soot cracking is less likely to occur, and in the latter case, the stability of the glass particulate deposition amount in the longitudinal direction is better than in the former case.

(2), (13) First, when the bulk density exceeds 0.60 g / cm ³ (0.8 g / cm ³ for continuous synthetic multilayering) and 1.0 g / cm ³ or less, The reaction of 3) can be ignored. When the bulk density is kept constant in the radial direction, the reaction of the above 2) is constant in the radial direction, and as a result, fluorine can be uniformly added in the radial direction. Since the reaction amount of the above 2) varies depending on the bulk density in the range of more than 0.60 g / cm ³ (0.8 g / cm ³ in the case of continuous synthetic multilayering) and 1.0 g / cm ³ or less, an appropriate bulk density is obtained. With this, it is possible to adjust to a desired amount of fluorine added. (3), (14) When the mixing ratio of the glass raw material gas and the fluorine compound gas is constant, the fluorine concentration around the glass fine particles is constant, so that the reaction of 1) is equal in the radial direction.

(4), (15) Glass fine particles are deposited until the desired porous glass diameter is reached, and when the desired porous glass diameter is reached, the glass raw material gas is increased or decreased, or the fluorine compound gas is increased or decreased. Change the fluorine concentration. By doing this 1)
Fluorine addition due to the change in the amount of fluorine added by the reaction of 2 and the change in the amount of fluorine added by the reaction of 2 (the size of the gap does not change because the bulk density is the same, but the amount of fluorine trapped in the gap changes) The amount changes, and it becomes possible to create a step in the refractive index [see FIG. 1 (C)]. In addition, since the reaction suddenly changes from the layer where the flow rate is changed, the step of the refractive index changes sharply. The refractive index can be continuously changed by gradually changing the mixing ratio of the glass raw material gas and the fluorine compound gas little by little.

(5), (16) Since the bulk density range is the same as before, the reaction of 3) above can be ignored. Furthermore, since the mixing ratio of the glass raw material gas and the fluorine compound is not changed, the reaction of 1) may be considered to be constant. Therefore, glass particles are deposited under the same conditions up to the desired porous glass diameter, the temperature is controlled from the diameter at which a step in the refractive index is desired to be created, and the reaction of 2) above is changed by changing the bulk density. A step is created. Since the reaction of 1) above is constant, the change in the step difference in the refractive index is small, which is suitable for delicate adjustment. The refractive index can be continuously changed by gradually changing the bulk density little by little.

(6), (17) The bulk density of the glass fine particle deposit is 0.10 g / cm ³ or more and 0.60 g / cm ³ (0.8 g / cm ^{3 in} continuous synthetic multilayering).
A method for increasing the amount of fluorine added is provided as cm ³ ) or less. (7), (18) In the present invention, the amount of fluorine added is increased and the amount added is made uniform in the radial direction. Since the porous glass base material according to the present invention has a low bulk density, the reaction of 3) above also occurs. In this method, since fluorine enters through the gaps between the glass particles and is added to the glass, the more the glass particles attached earlier, the more fluorine is added.
Therefore, the fluorine compound gas is increased or the glass raw material gas is decreased toward the latter half of the deposition of the glass particles to increase the fluorine concentration, and the reaction of 1) and 2) is changed to add in the radial direction. The fluorine concentration is made uniform. In the situation where the reactions 1) to 3) can be performed, it is considered that 3) has the greatest effect on the amount of fluorine added, and the bulk density is lowered as the glass particles are further deposited [the effect of 2) is used]. This is because the addition amount can be made uniform.

(8), (19) In the present invention, a large and uniform amount of fluorine is added by the above methods (7) and (18) until a desired diameter is obtained. The bulk density is 0.60 g / cm ³ (0.8 g / cm ³ for continuous synthetic multi-layering) and 1.0
By increasing the amount to g / cm ³ or less, the reaction of the above 3) is suppressed, and a small amount of fluorine is added in the radial direction without change in the addition amount. It is possible to realize a step having a large refractive index that cannot be realized by the above inventions (5) and (16). (9), (20) According to the present invention, it is possible to provide a plurality of steps in the refractive index due to the difference in the amount of fluorine added, not in one location but in the radial direction.

(10) According to the present invention, the deposition rate of glass particles can be improved, a porous glass base material having a low bulk density can be produced, and the efficiency of fluorine addition can be increased. It is known that it is difficult to produce a glass particulate deposit of low bulk density by the continuous synthetic multi-layering method.
This is because the burner for depositing glass fine particles lowers the temperature of the portion where the glass fine particles are not deposited, causing the porous glass base material to crack. The cause of the cracks is that the glass particles before shrinkage are deposited on the glass particles that have shrunk at low temperature, and distortion is created here, which causes the bonding of the glass particles to strain as many layers of glass particles are deposited. It is thought that it will be unbearable. In order to prevent this, it is necessary to produce a glass particle deposit having a high bulk density and a firm bond between glass particles.

A bulk density of 0.1 g / cm ³ is difficult to achieve. The cracking frequency of the glass particulate deposit depends on the size of the base material, but at most about 0.3 g / cm ³ may be considered as the limit that can be produced by one burner. On the other hand, in the multi-layer coating method using a plurality of burners, since the fine glass particles are always deposited in a state where the porous glass base material is wrapped in a flame, the temperature change of the fine glass particle deposits is small, and thus strain is less likely to be accumulated, Since it is hard to break, a low bulk density porous glass base material can be manufactured.

A further effect is that the efficiency of fluorine addition is increased. Since fluorine is generated from a plurality of burners, the porous glass base material is always exposed to a fluorine atmosphere, and thus in the state where the reaction of 3) above occurs,
Fluorine addition efficiency improves. Further, as compared with the case of a single burner, the fluorine compound gas is less likely to diffuse before reaching the glass fine particle deposit body, and the amount of fluorine contained in the gap between the glass fine particles is increased. ) Reaction efficiency is also improved.

(21) In the multi-tube burner, the glass raw material gas, the fluorine compound gas, the combustible gas, and the supporting gas are mixed in a natural flow. Especially, in the focus type burner, they are forcibly mixed. It has a good shape and good mixing efficiency. Therefore, the reaction amount in 1) above increases, and the reaction efficiency between the glass particles and fluorine improves. An example of the focus type burner is described in JP-A-10-101343.

[0036]

EXAMPLES The present invention will be described in more detail with reference to examples below, but the invention is not intended to be limited thereto. (Example 1) Four burners for depositing glass particles were used, the intervals between the burners were set to 200 mm, and the glass particles were deposited by the split synthesis multilayer method. Burner and starting rod (outer diameter: 36 mm, effective length: 440 mm)
The relative movement with and is performed by moving the starting rod up and down, and the moving distance in one direction at a time is approximately the burner interval, and the folding position is moved by 20 mm and the folding position is shifted by 200 mm. The operation of starting to move in the opposite direction and returning the folded position to the initial position was repeated 40 times to produce a porous glass preform. Add SiCl ₄ to each burner at 6.0 SLM and CF
₄ was added at 1.0 SLM, and the bulk density was 0.8 g / cm ³ ,
0.7 g / cm ³ and 0.6 g / cm ³ of porous glass base material (outer diameter of 230 mm, 240 mm and 25, respectively)
0mm, effective length is 440mm for all),
After the sinter was made transparent, the refractive index of the obtained glass base material was measured.

As a result, 0.8 g / cm ³ , 0.7 g /
The indices of refraction of the base materials of cm ³ and 0.6 g / cm ³ were 0.013%, 0.042% and 0.07%, respectively, below the level of pure silica. The refractive index profile of each base material showed the pattern shown in FIG. 1 (A), and there was almost no variation in the refractive index difference in the radial direction. The bulk density was adjusted by changing the amount of hydrogen supplied to the burner to change the temperature of the glass particle deposition surface (the same applies to the following examples). Specifically, the bulk density is 0.7 g / cm
Temperature of the glass particle deposition plane compared to that of ^3, if the bulk density is 0.8 g / cm ³ set higher about 15 ℃, if the bulk density is 0.6 g / cm ³ to about 15 ℃ I tried to lower it.

Example 2 Bulk density of Example 1 0.7 g /
The CF ₄ flow rate under the conditions for producing the porous glass base material of cm ³ is set to 0.
A porous glass preform was produced in the same manner as in Example 1 except that 5 SLM was used, and the refractive index was measured after the sinter was made transparent. As a result, the refractive index was decreased by 0.005% from the pure silica level. After all, there was almost no change in the refractive index in the radial direction.

Example 3 A porous glass base material having a bulk density of 0.4 g / cm ³ was prepared under the same conditions as in Example 1, and after sintering and making transparent, the refractive index was measured. The refractive index was decreased from the silica level by 0.14%, the refractive index was increased toward the surface of the base material, and the surface was in a state where the refractive index was decreased from the pure silica level by 0.06%. It corresponds to FIG. 1 (B)].

Example 4 CF _{4 under the} same conditions as in Example 3
The porous glass base material was prepared by gradually increasing only the flow rate of 1 → 2.5 SLM, and the refractive index was measured after sinter transparency, and the starting rod interface was 0.15% from the pure silica level. The refractive index is decreasing, and the refractive index is gradually increasing toward the surface of the base material, and the surface is in a state where the refractive index is decreasing from the pure silica level by 0.12%. The fluctuation was greatly reduced and the refractive index in the radial direction became almost uniform.

(Embodiment 5) Glass fine particles were deposited under the same conditions as in Embodiment 4, and after the flow rate of CF ₄ reached 2.5 SLM, the bulk density of Embodiment 1 was changed to 0.7 g / cm ^3. Then, a porous glass preform was prepared, and the refractive index was measured after the sinter was made transparent. The refractive index of the starting rod interface is 0.14% lower than the pure silica level, and is 0.13% even before changing the glass particle deposition conditions.
At the position after the change, the refractive index was lower than the 0.041% pure silica level, and there was almost no change, and the refractive index was up to the surface of the base material [corresponding to FIG. 1 (C)].

Example 6 A porous glass base material having a bulk density of 0.2 g / cm ³ was prepared under the same conditions as in Example 4, and the refractive index was measured. As a result, the starting rod interface is 0.2%
The refractive index was lowered to the level of 0.17% pure silica on the surface of the base material, and the refractive index in the radial direction was almost uniform. The burners used in Examples 1 to 6 are all focus burners.

(Example 7) The burner used was changed to a multi-tube burner, and a bulk density of 0.3 g / cm ^{3 was} obtained by a divided synthetic multilayering method using four glass particle deposition burners.
The porous glass base material was manufactured. The dimensions of the starting rod used and the conditions of reciprocal movement were the same as in Example 1, and
C ₄ is still 6.0 SLM, the flow rate of CF ₄ is 1,
Glass particulate deposition was performed with a few SLMs. as a result,
The refractive index of the starting rod interface when CF ₄ is 1 SLM is
The refractive index is 0.04% lower than the pure silica level,
When CF ₄ is 2SLM, it is 0.07%, and 3SLM
Was 0.082%. Compared with Examples 3 and 6, the amount of fluorine added was small. This indicates that the focus burner can add more fluorine.

(Example 8) The burner used was a multi-tube burner, and one burner for depositing glass particles was used to carry out multi-layering by the continuous synthesis multi-layering method. The starting rod used had the same dimensions as in Example 1, and the relative movement between the burner and the starting rod was carried out by moving the starting rod in the vertical direction, the reciprocating movement was 800 times, and the outer diameter was 250.
mm, and an effective portion length of 440 mm to prepare a porous glass base material. The bulk density was 0.3 g / cm ³ , SiCl ₄ was 6.0 SLM, and CF ₄ was 3 SLM. As a result, the index of refraction at the starting rod interface is 0.
It was down 075%.

(Example 9) Three glass burner for depositing glass particles (focus type burner) were installed at an interval of 150 mm, and glass particles were deposited by a continuous synthetic multilayering method.
The starting rod used had the same size as in Example 1, and the relative movement between the burner and the starting rod was performed by moving the starting rod in the vertical direction. The number of reciprocating movements was 267, and 6.0 SLM and CF ₄ of SiCl ₄ were added to each burner.
1.0 SLM, and a porous glass base material having a bulk density of 0.8 g / cm ³ and 0.7 g / cm ³ (outer diameters of 230 mm and 440 mm, effective portion lengths of 24 respectively).
0 mm and 440 mm) were prepared and sintered and made transparent. As a result of measuring the refractive index of the obtained glass base material, it was found that the base material having a bulk density of 0.8 g / cm ³ had a profile of FIG. 1 (A) and the refractive index was decreased by 0.02% from the level of pure silica. . The base material having a bulk density of 0.7 g / cm ³ is shown in FIG.
The refractive index is 0.055% lower than the pure silica level at the starting rod interface, and the refractive index increases toward the surface of the base material, and the refractive index is 0.035% from the pure silica level on the surface. It was falling.

That is, when the bulk density of the porous glass base material was 0.8 g / cm ³ , no difference in the amount of fluorine added in the radial direction was observed, but the bulk density was 0.7 g / cm ³ . There was a difference in the amount added. This difference in the amount added increases as the bulk density further decreases.

Table 1 below summarizes the relationship between the examples and the claims together with the data of bulk density and refractive index for easy understanding.

[0048]

[Table 1]

[0049]

[Table 2]

[0050]

According to the method for producing a porous glass preform of the present invention, (1) the low concentration of fluorine can be added in the radial direction, and the amount of fluorine added can be accurately controlled. Can be made uniform in the radial direction, (3) At least one step in refractive index can be easily formed, (4)
By using multiple burners, it is possible to produce glass particle deposits with a low bulk density, which has never existed in the past, in multi-layering, and the excellent effect of improving the controllability of the amount of fluorine added is achieved. Sent.

[Brief description of drawings]

FIG. 1A shows that the amount of fluorine added is constant in the radial direction.
Schematic diagram of the refractive index showing an example, (B) is a conceptual diagram of the refractive index showing one example in which the amount of fluorine added is not uniform in the radial direction, (C) is a uniform amount of fluorine added in each part of a and b It is a conceptual diagram of the refractive index indicating that.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G02B 6/00 356 G02B 6/00 356A (72) Inventor Motonobu Nakamura 1 No. 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Factory F-term (reference) 4G014 AH12 AH15 4G021 EA03 EB05 EB14 EB26

Claims (21)

[Claims] 1. A method for depositing glass fine particles in which a glass fine particle deposit is grown in a radial direction so that a layer is stacked on a starting rod, wherein a burnable gas, a combustion supporting gas,
A method for producing a porous glass preform, which comprises introducing a fluorine compound gas together with a glass raw material gas. 2. The bulk density of the glass fine particle deposition layer formed on the starting rod exceeds 0.60 g / cm ³ and 1.0 g.
The method for producing a porous glass preform according to claim 1, wherein the ratio is substantially constant in the radial direction in the range of / cm ³ or less. 3. The method for producing a porous glass preform according to claim 2, wherein the mixing ratio of the glass raw material gas and the fluorine compound gas is kept constant. 4. The method for producing a porous glass preform according to claim 2, wherein the mixing ratio of the glass raw material gas and the fluorine compound gas is changed. 5. A mixture of glass source gas and fluorine compound gas
Glass with a constant mixing ratio and formed on the starting rod
The bulk density of the fine particle deposition layer is 0.60 g / cm ³Beyond
1.0 g / cm³Radially change within the following range
The manufacture of the porous glass preform according to claim 1,
Method. 6. The bulk density of the glass fine particle deposition layer formed on the starting rod is 0.1 g / cm ³ or more and 0.60 g / c or more.
The method for producing a porous glass preform according to claim 1, wherein the range is m ³ or less. 7. The method for producing a porous glass preform according to claim 6, wherein the mixing ratio of the glass raw material gas and the fluorine compound gas or the bulk density distribution in the radial direction is changed with the growth of the glass fine particles. 8. The volume of the glass particle deposition layer formed on the starting rod is changed by changing the mixing ratio of the glass raw material gas and the fluorine compound gas along with the growth of the glass particles until the desired glass particle deposition body diameter is obtained. Density 0.1g / cm
^The volume density is set to ³ or more and 0.60 g / cm ³ or less, and after reaching the desired diameter, the bulk density exceeds 0.60 g / cm ³ and 1.
2. The production of a porous glass preform according to claim 1, wherein a constant bulk density selected from the range of 0 g / cm ³ or less is used, and a mixing ratio of the glass raw material gas and the fluorine compound gas is also fixed. Method. 9. Until the desired particle size of the glass particle deposit is reached, the mixing ratio of the glass raw material gas and the fluorine compound gas is changed with the growth of the glass particles, and the volume of the glass particle deposit layer formed on the starting rod is increased. Density 0.1g / cm
^The volume density is set to ³ or more and 0.60 g / cm ³ or less, and after reaching the desired diameter, the bulk density exceeds 0.60 g / cm ³ and 1.
The method for producing a porous glass preform according to claim 1, wherein a constant bulk density selected from the range of 0 g / cm ³ or less is used and the mixing ratio of the glass raw material gas and the fluorine compound gas is changed. 10. A burner for depositing a plurality of glass fine particles is used, and the burner for depositing a plurality of glass fine particles is used.
The method for producing a porous glass preform according to item. 11. A plurality of burners for depositing glass particles are arranged facing a rotating starting rod in the glass particulate depositing method, and the starting rod and the burner are relatively reciprocally moved in parallel, and each burner is moved. 10. The divided synthetic multi-layering method of depositing glass fine particles by setting the relative movement distance of the glass fine particles in one direction to be shorter than the entire length of the glass fine particle deposit body. The method for producing a porous glass preform according to item. 12. One or a plurality of burners for depositing glass particles are arranged facing a rotating starting rod in the method for depositing glass particles, and the starting rod and the burner are relatively reciprocally moved in parallel. 2. The continuous synthetic multi-layering method of depositing glass fine particles by setting the relative movement distance of each burner in one direction to be longer than the entire length of the glass fine particle deposit body. A method for manufacturing a porous glass base material. 13. The bulk density of the glass fine particle deposition layer formed on the starting rod is more than 0.80 g / cm ³ and 1.0 or more.
The method for producing a porous glass preform according to claim 12, wherein the radial direction is substantially constant within a range of g / cm ³ or less. 14. The method for producing a porous glass preform according to claim 13, wherein the mixing ratio of the glass raw material gas and the fluorine compound gas is kept constant. 15. The method for producing a porous glass preform according to claim 13, wherein the mixing ratio of the glass raw material gas and the fluorine compound gas is changed. 16. The glass raw material gas and the fluorine compound gas are mixed at a constant mixing ratio, and the bulk density of the glass fine particle deposition layer formed on the starting rod exceeds 0.80 g / cm ³ and 1.0 g / cm ^{3. The} method for producing a porous glass preform according to claim 12, wherein the diameter is changed in the range of ³ or less in the radial direction. 17. The bulk density of the glass fine particle deposition layer formed on the starting rod is 0.1 g / cm ³ or more and 0.80 g / cm ^3.
The method for producing a porous glass preform according to claim 12, wherein the range is not more than cm ³ . 18. The method for producing a porous glass preform according to claim 17, wherein the mixing ratio of the glass raw material gas and the fluorine compound gas or the bulk density distribution in the radial direction is changed with the growth of the glass fine particles. 19. The volume of the glass particle deposit layer formed on the starting rod is changed by changing the mixing ratio of the glass raw material gas and the fluorine compound gas as the glass particles grow until the desired glass particle deposit diameter is reached. Density 0.1g / c
m ³ or more and 0.80 g / cm ³ or less, and after reaching a desired diameter, the bulk density exceeds 0.80 g / cm ³ and a certain volume selected from the range of 1.0 g / cm ³ or less. The method for producing a porous glass preform according to claim 12, wherein the density is set and the mixing ratio of the glass raw material gas and the fluorine compound gas is also set to be constant. 20. The mixing ratio of the glass raw material gas and the fluorine compound gas is changed with the growth of the glass fine particles until the desired glass fine particle deposition body diameter is obtained, and the bulk density of the glass fine particles deposited on the starting rod is changed. The volume density is set to 0.1 g / cm ³ or more and 0.80 g / cm ³ or less, and after reaching the desired diameter, the bulk density exceeds 0.80 g / cm ³ and 1.0 g
The method for producing a porous glass preform according to claim 12, wherein a constant bulk density selected from the range of / cm ³ or less is used, and the mixing ratio of the glass raw material gas and the fluorine compound gas is changed. 21. A burner for depositing fine glass particles,
A focus type burner having a structure in which a combustible gas, a combustible gas, a raw material gas, and a fluorine compound gas are gathered at one point until the burner flame reaches the porous glass deposition surface. 21. The method for producing a porous glass preform according to any one of 20.