JP2000063141A - Production of porous glass preform for optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a porous glass preform for optical fiber, for example, in a vapor phase axial deposition (VAD method), etc., capable of reducing an excessive soot floating in a chamber without being deposited on a starting preform without greatly changing a conventional apparatus, consequently preventing the occurrence of a bad condition of qualities of the porous glass preform for an optical fiber, such as generation of foam, etc., and improving a yield. SOLUTION: In this method for producing a porous glass preform for an optical fiber by spraying an oxyhydrogen flame together with a raw material gas from plural burners for depositing a raw material so as to deposit a soot on a starting preform in the axial direction, the longitudinal sectional shape at the tip of the soot deposition layer deposited by two adjoining burners for depositing the raw material in the plural burners for depositing the raw material is adjusted so that the angle between tangents at inflection points which hold the boundary of the two soot deposition layers between the points and are closest to the boundary is made >=30 deg. but <=100 deg..

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 a porous glass preform for an optical fiber, for example, by a vapor phase axial attachment method (VAD method).

[0002]

2. Description of the Related Art Conventionally, in an optical fiber manufacturing process,
If you try to make an ultra-fine optical fiber directly, it is difficult to control it so that it has an optimal refractive index distribution. For this reason, first make a large-diameter optical fiber preform with the same refractive index distribution and then use this preform. A method of manufacturing an ultra-fine optical fiber is adopted by heating (heating) and controlling the outer diameter to be constant and stretching (drawing) a long thin fiber.

As a method for producing such an optical fiber preform, for example, a method of producing a porous glass preform for an optical fiber by a vapor phase axis attaching method, etc., and dehydrating and sintering it can be mentioned. it can. This vapor axis method (VA
The method of producing a porous glass preform for optical fibers by the D method) is necessary for controlling the refractive index of silicon tetrachloride (SiCl ₄ ) as an optical fiber raw material from below while rotating a starting preform such as a quartz rod. Germanium tetrachloride (GeCl ₄ )
And other dopants are blown from the core deposition burner together with H ₂ and O ₂ gas to form a soot deposition layer for the core in the axial direction of the quartz rod, and a light is emitted from the cladding deposition burner toward this periphery. This is a method in which silicon tetrachloride (SiCl ₄ ) or the like as a fiber raw material is sprayed together with H ₂ and O ₂ gas to form a soot deposition layer for the clad portion.

[0004]

However, not all of the soot deposited on the starting base material is deposited on the starting base material to be the porous glass base material for optical fiber, and a part of the soot is deposited. Instead, it floats in the chamber and then is discharged from the discharge port, or the soot not discharged is attached to the wall surface of the chamber. The soot that adheres to this wall will eventually drop, damage the porous glass base material for optical fiber that is being deposited, or scatter soot particles when falling, and the scattered soot particles can be deposited on the optical fiber. There is a problem that it adheres to the porous glass base material for use and causes bubbles. Further, the scattering of soot particles in this manner may interfere with the tip recognition system of the porous glass preform for optical fiber being deposited and may make it uncontrollable.

Further, the soot not deposited on the starting base material floats in the chamber before being discharged from the discharge port or before adhering to the wall surface. This means that the soot of the porous glass base material for optical fibers is itself. This is a factor in generating bubbles. Thus, the presence of excess soot that did not adhere to the starting preform has been a major problem in the manufacture of porous glass preforms for optical fibers.

Further, in recent years, it has been required to increase the size of the porous glass preform for optical fibers. With the increase in size, for example, if the production rate is doubled, the porous glass preform for optical fibers will be increased. The surplus soot that does not accumulate as a material is also doubled, and the above-mentioned problem is more likely to occur. For example, as shown in FIG. 1, the number of bubbles tends to increase as the diameter of the porous glass preform for optical fibers increases, and the excess soot attached to the chamber wall surface when the diameter exceeds 145 mm. Is observed, which shows that the quality is adversely affected.

Furthermore, as the size of the porous glass preform for optical fibers is required to be larger, the yield is also required to be improved. From this point as well, there is an urgent need to reduce this excess soot.

The present invention has been made in view of the above problems. For example, in the vapor phase axial attachment method (VAD method) or the like, the chamber is not deposited on the starting base metal without largely changing the conventional apparatus. Reduce the excess soot floating inside,
This provides a method for producing a porous glass preform for optical fibers, which can prevent the occurrence of defects in the quality of the porous glass preform for optical fibers such as the generation of bubbles and improve the yield. The purpose is to do.

[0009]

In order to achieve the above object, the present invention according to claim 1 is characterized in that an oxyhydrogen flame is blown together with a raw material gas from a plurality of raw material deposition burners to deposit soot in an axial direction of a starting base material. In the method for producing a porous glass preform for an optical fiber, the longitudinal cross-sectional shape of the tip of the soot deposition layer deposited by two adjacent raw material deposition burners of the plurality of raw material deposition burners is set to two. The angle between the tangents at the two inflection points that sandwich the boundary of the soot sedimentary layer and are closest to this boundary is 30 ° or more and 100 or more.
The porous glass preform for optical fibers was manufactured at a temperature of not more than °.

As described above, the soot is deposited so that the angle formed by the tangents at the two inflection points closest to the boundary between the two soot deposition layers is 30 ° or more and 100 ° or less, so that the raw material is deposited. The soot generated by the burner is efficiently deposited as a soot deposition layer, the surplus soot floating in the chamber can be reduced, and the yield can be improved.

In this case, as described in claim 2, it is preferable that both of the above-mentioned two raw material deposition burners are cladding portion deposition burners. The burner for clad part deposition usually has a larger number of soots and a larger amount of soot as compared to the burner for core part deposition. Therefore, by controlling these burners, the amount of soot floating in the chamber can be increased efficiently. This is because it can be reduced.

Further, in this case, as described in claim 3, one of the two raw material deposition burners is a first cladding portion deposition burner for depositing the first cladding portion adjacent to the core portion, It is preferable that the other one is a burner for depositing a second cladding portion, which deposits a second cladding portion adjacent to the outside of the first cladding portion. As described below,
The burner for depositing the first clad portion needs to adjust the refractive index distribution by heating the side surface of the core portion while depositing the first clad portion. The yield is low. Therefore, this first
The overall yield can be improved by improving the yield of the burner for depositing the clad portion, and the excess soot floating in the chamber can be reduced.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The embodiments of the present invention will be described below, but the present invention is not limited thereto. FIG. 2 is a diagram for explaining a method for manufacturing a porous glass preform for optical fibers, which is a burner for depositing a raw material, a burner 1 for depositing a core portion for depositing a core portion (a), a first burner for depositing a core portion. It is explanatory drawing which shows the example which used the 1st clad part deposition burner 2 for depositing a clad part (b), and the 2nd clad part deposition burner 3 for depositing a 2nd clad part (c). .

Of the two clad part deposition burners, the first clad part deposition burner 2 for depositing the first clad part (b) adjacent to the core part (a) is the first clad part deposition burner 2. In addition to depositing (b), it also has a role of heating the side surface of the core portion (a) to adjust the refractive index distribution of the core portion (a).

As described above, since it has an important role of adjusting the refractive index distribution of the core portion (a), various conditions such as the burner position and the gas flow rate of the burner 2 for depositing the first clad portion have been conventionally known. , The condition was not focused on the point of depositing the first cladding part (b), but rather on the adjustment of the refractive index distribution of the core part (a). . Therefore, considering the results of manufacturing under such conventional manufacturing conditions, when comparing the burner 2 for depositing the first cladding portion and the burner 3 for depositing the second cladding portion, the yield of the burner 2 for depositing the first cladding portion is compared. It was found that the rate was low.

Further, when the results of manufacturing under the conventional manufacturing conditions are examined, it is found that there are more obstacles such as excessive soot falling under the condition of attempting to increase the size, and further, that the size is increased or not. When the yield was examined, it was found that the yield itself was not significantly different. From this, the yield itself does not change significantly even when the size is increased, but the total soot is increasing, so the surplus soot itself is increased. Therefore, when attempting to increase the size, as shown in FIG. It can be seen that problems such as an increase in bubbles and a drop of excess soot adhering to the chamber wall occur. Therefore, in order to reduce defects such as bubbles, it is necessary to improve the yield itself, and for that purpose, it is necessary to improve the yield of the first cladding portion deposition burner 2 having a low yield. I understood it. The present inventors, as a result of studying these points, found that there is a certain relationship between the shape and the yield of the longitudinal section of the tip of the soot deposition layer of the optical glass porous glass preform,
The present invention has been completed.

That is, the present invention provides a method for producing a porous glass preform for optical fibers, in which an oxyhydrogen flame is blown together with the raw material gas from a plurality of raw material deposition burners to deposit soot in the axial direction of the starting preform. , The vertical cross-sectional shape of the tip of the soot deposition layer deposited by two adjacent source deposition burners of the plurality of source deposition burners is defined by the two variations that sandwich the boundary between the two soot deposition layers and are closest to this boundary. The angle between each tangent at the bending point is 30 ° or more 10
By setting the angle to 0 ° or less, the yield is improved, the excess soot floating in the chamber is reduced, and the quality of the porous glass preform for optical fibers is improved.

According to the present invention, as described above, a porous glass preform for an optical fiber is manufactured by spraying an oxyhydrogen flame together with a raw material gas from a plurality of raw material deposition burners to deposit soot in the axial direction of the starting preform. Applied to the method. As such a manufacturing method, for example, a vapor phase axial attachment method (VAD method) and the like can be mentioned.

According to the present invention, in such a manufacturing method,
The shape of the tip of the soot deposition layer deposited by two adjacent raw material deposition burners of the plurality of raw material deposition burners is controlled. The two adjacent raw material deposition burners are not particularly limited, but both are preferably cladding portion deposition burners. The clad part has a larger volume than the core part, and improving the yield of this part reduces the amount of soot floating in the chamber, and also improves the yield when viewed as a whole. Is.

Further, one of the two raw material deposition burners is a first cladding portion deposition burner for depositing the first cladding portion adjacent to the core portion, and the other one is the first cladding portion burner.
It is particularly preferable that the burner for depositing the second clad portion deposits the second clad portion adjacent to the outside of the clad portion. As described above, under the conventional conditions, the yield of the burner for depositing the first cladding portion is lower than that of other burners. Therefore, by improving the yield of the burner for depositing the first clad portion, the overall yield can be greatly improved, and the excess soot floating in the chamber can be greatly reduced. Because you can.

The term "two adjacent raw material deposition burners" as used herein means a raw material deposition burner for depositing soot deposition layers that are radially adjacent to each other in the porous glass preform for optical fibers. Indicates the relationship between the core portion deposition burner 1 and the first cladding portion deposition burner 2, or the relationship between the first cladding portion deposition burner 2 and the second cladding portion deposition burner 3. In addition, the vertical cross-sectional shape means a shape when the porous glass base material for an optical fiber is cut along a surface including a rotation axis.

According to the present invention, the vertical cross-sectional shape of the tip of the soot accumulation layer is such that the angle between the tangents at the two inflection points that sandwich the boundary between the two soot accumulation layers and are closest to the boundary is 30 ° or more and 100 or more. It has a feature in that it is set to be less than or equal to °. Hereinafter, this point will be described with reference to FIG.

The solid line in FIG. 3 shows the vertical cross-sectional shape of the tip of the soot deposition layer of the porous glass preform for optical fibers, which is the first cladding portion and the second soot deposition layer. It shows a vertical cross-sectional shape of the tip of the clad portion. Also, the broken line indicates the second derivative value in the curve shown by the solid line. The present invention is an inflection point, that is, a point where the second-order differential value becomes 0, and is closest to the boundary line between the first cladding portion and the second cladding portion in each of the first cladding portion and the second cladding portion. The feature is that the angle formed by two tangent lines at two inflection points is set to be 30 ° or more and 100 ° or less. Here, as is clear from FIG. 3, the angle formed by the tangent lines refers to the complementary angle of the angle formed by the convex portion (shoulder portion) formed by the curve that actually indicates the shape, and the curved line forms the concave portion. Also in the case, the complementary angle of the angle formed by the concave portion is similarly referred to.

Thus, the angle formed by the two tangents is 30
By depositing soot so as to be not less than 100 ° and not more than 100 °, the yield can be improved and the excess soot can be reduced. Here, 30 ° or more 10
The reason why it is set to 0 ° or less is that the required yield cannot be obtained when the angle is less than 30 °, and when the angle exceeds 100 °, the soot deposition layer is cracked or deformed, so that the production cannot be performed.

For example, by using the first clad part deposition burner and the second clad part deposition burner, the shapes of the first clad part and the second clad part are changed by changing the deposition conditions, and the above-mentioned tangent lines are used. An experiment was conducted to change the angle formed by. FIG. 4 shows the result of measuring the angle and the yield of the first cladding portion at this time. As is clear from FIG. 4, the yield is less than 50% below 30 °, which is not preferable,
On the other hand, when the angle exceeds 100 °, cracking and deformation occur and the porous glass preform for optical fibers cannot be manufactured. As described above, in the present invention, the angle formed by the two tangents is 30 °.
It is preferable that the shape is 100 ° or less.

As described above, the burner for depositing the first cladding portion has the role of depositing the first cladding portion and also the role of burning (heating) the side surface of the core portion to adjust the refractive index distribution. However, it is also possible to adjust the refractive index distribution while changing the shape of the first cladding portion, that is, the angle formed by the two tangent lines, as described below. That is,
The shape of the clad portion is determined by the gas conditions mainly including the flow rate of the fuel gas. On the other hand, the adjustment of the refractive index distribution, that is, the method of heating the side surface of the core part, depends on the flow rate of the combustion gas, the amount of protrusion of the flame throttle cover of the first cladding deposition burner from the burner, and the first cladding deposition burner. The positional relationship between the flame and the core portion, that is, the position of the burner for depositing the first cladding portion, is determined by three factors.

Therefore, by adjusting the shape of the cladding portion according to the gas conditions and then adjusting the refractive index distribution by setting the burner, the shape satisfying the requirements of the present invention can be obtained. It is possible to obtain a porous glass preform for optical fibers having a uniform refractive index distribution. By doing so, it is possible to produce a porous glass preform for optical fibers which has a high yield and has few defects such as bubbles and which has an ordered refractive index distribution. it can.

[0028]

EXAMPLES Next, examples of the present invention and comparative examples will be described. (Example) Oxygen 6.5 L / mi was added to the burner for core deposition.
n, hydrogen 3.2 L / min, argon 1.5 L / mi
n, germanium tetrachloride 11 cc / min, silicon tetrachloride 0.12 L / min, and oxygen 13 L / min, hydrogen 13 L / min, argon 2.0 L / min, silicon tetrachloride 0 in the burner for depositing the first cladding. Flow rate of 4 L / min, and oxygen of 23 is added to the burner for depositing the second cladding portion.
L / min, hydrogen 40 L / min, argon 6.0 L /
min., 1.1 L / min of silicon tetrachloride was flown to manufacture a porous glass preform for a single mode optical fiber.

As a result, the yield of the first cladding is 70.
%, And the yield of the second clad portion was 75%. When 10 pieces were manufactured, no excess soot adhering to the wall surface was observed, and the average number of bubbles was 0.7. At this time, the angle formed by the tangents at the inflection point closest to the boundary between the first cladding portion and the second cladding portion is 85 °, and the outer diameter of the obtained porous glass preform for optical fiber is 152 mmφ.
Met.

(Comparative Example) Oxygen was added to the burner for core deposition.
5 L / min, hydrogen 3.2 L / min, argon 1.5
L / min, germanium tetrachloride 11 cc / min, and silicon tetrachloride 0.12 L / min were flown, and oxygen 14 L / min and hydrogen 14.5 L / min were supplied to the first cladding deposition burner.
min, argon 2.0 L / min, silicon tetrachloride 0.4
L / min, oxygen 23 L / min, hydrogen 40 L / min, argon 6.0 L / min, and silicon tetrachloride 1.1 L / min were further passed through the second cladding deposition burner for single mode optical fiber. The porous glass base material of was manufactured.

As a result, the yield of the first cladding portion is 55.
%, And the yield of the second clad portion was 75%. When 10 pieces were manufactured, the excess soot adhering to the wall surface was dropped four times, and the average number of bubbles in the case where such drop did not occur was 5.3. At this time, the angle formed by the tangents at the inflection point closest to the boundary between the first cladding portion and the second cladding portion was 20 °, and the outer diameter of the obtained porous glass preform for optical fiber was 146 mmφ.

As a result of optimizing the setting of the burner, the refractive index distribution of the core portion was the same in both the examples and the comparative examples. FIG. 5 shows a schematic shape of a vertical cross section of the tip of the soot deposition layer in the above-mentioned Examples and Comparative Examples. In the figure, (A) shows the shape of the example, and (B) shows the shape of the comparative example. Further, (a) shows a core portion, (b) shows a first cladding portion, and (c) shows a second cladding portion.

The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, has substantially the same configuration as the technical idea described in the scope of the claims of the present invention, and has any similar effect to the present invention. It is included in the technical scope of the invention.

For example, in the above-described embodiment, the tip shape of the first clad portion and the second clad portion is used for explanation, but the present invention is not limited to this, and for example, the core portion and the first clad are used. The present invention also applies to the relationship between the two cladding parts and the relationship between two adjacent cladding parts outside the first cladding part.

[0035]

As described above, according to the method for producing a porous glass preform for optical fibers according to the present invention, an oxyhydrogen flame is blown together with the raw material gas from a plurality of raw material deposition burners soot in the axial direction of the starting preform. In the method for producing a porous glass preform for optical fiber to be deposited, the vertical cross-sectional shape of the soot deposition layer tip deposited by two adjacent raw material deposition burners of the plurality of raw material deposition burners, The angle formed by the tangents at the two inflection points that sandwich the boundary between two soot sedimentary layers and are closest to this boundary is 30 ° or more 10
The angle is set to 0 ° or less.

According to the present invention, the soot is deposited such that the angles formed by the respective tangents at the two inflection points closest to the boundary between the two soot deposition layers are 30 ° or more and 100 ° or less. The soot generated by the raw material deposition burner is efficiently deposited as the porous glass preform for optical fibers, the excess soot floating in the chamber can be reduced, and the yield can be improved.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the diameter of a porous glass preform for optical fibers and the number of bubbles.

FIG. 2 is an explanatory diagram illustrating an example of a method for manufacturing a porous glass preform for optical fibers.

FIG. 3 is a graph showing a vertical cross-sectional shape of a tip of a porous glass preform for an optical fiber and a second derivative value.

FIG. 4 is a graph showing the relationship between the yield of the first cladding portion and the angle formed by two tangent lines.

FIG. 5 is a schematic cross-sectional view showing a vertical cross-sectional shape of the tip of a porous glass preform for optical fibers, (A) showing an example and (B) showing a comparative example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Core part deposition burner, 2 ... 1st clad part deposition burner, 3 ... 2nd clad part deposition burner, 4 ... Optical fiber porous glass base material, (a)
... Core part, (b) ... 1st clad part, (c)
... The second clad portion.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hideo Hirasawa             2-13-1, Isobe, Annaka-shi, Gunma Shin-Etsu             Gaku Kogyo Co., Ltd. Precision Materials Research Laboratory F-term (reference) 4G021 EA01 EB14

Claims (3)

[Claims] 1. A method for producing a porous glass preform for an optical fiber, wherein an oxyhydrogen flame is blown together with a raw material gas from a plurality of raw material deposition burners to deposit soot in the axial direction of the starting preform. Of the two adjacent raw material deposition burners, the vertical cross-sectional shape of the tip of the soot deposited layer at the two inflection points that sandwich the boundary between the two soot deposited layers and are closest to this boundary A method for producing a porous glass preform for an optical fiber, characterized in that an angle between each tangent line is 30 ° or more and 100 ° or less. 2. The method for producing a porous glass preform for an optical fiber according to claim 1, wherein each of the two raw material deposition burners is a cladding portion deposition burner. 3. One of the two raw material depositing burners comprises a first cladding portion for depositing a first cladding portion adjacent to a core portion.
The optical fiber according to claim 2, wherein the optical fiber is a burner for depositing a cladding portion, and the other one is a burner for depositing a second cladding portion that deposits a second cladding portion adjacent to the outside of the first cladding portion. For manufacturing porous glass preform for use in manufacturing.