JP2004210627A - Method for manufacturing preform for optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a preform for an optical fiber which reduces variation in outside diameter in the longitudinal direction of the preform called unevenness. <P>SOLUTION: In the method for manufacturing the preform for the optical fiber which comprises depositing glass fine particles 200 from a flame burner 30 on a target 100, generation of the unevenness on the surface of the target is restrained by adjusting the flow rate of a flammable gas or a gas susceptible to burn to the flame burner 30 so as to control temperature distribution of a heat zone in the target 100 generated by the flame of the flame burner 30. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
[Industrial applications]
The present invention relates to a method for manufacturing an optical fiber preform, and more particularly to a method for reducing an outer diameter variation in a longitudinal direction of a preform, which is called unevenness of an optical fiber preform.
[0002]
[Prior art]
Generally, as a method for manufacturing a preform (porous preform) for an optical fiber, a VAD method, an external method, and the like are known. In the case of this external method, both ends of a target (starting base material) are held and rotated by a chuck of a manufacturing apparatus (a glass lathe or the like), and the raw material gas is heated using one or more flame burners. To produce glass fine particles (soot), which are deposited on the outer periphery of the rotating target. After obtaining this optical fiber preform, it is sintered in a furnace tube in a high-temperature electric furnace to obtain a transparent vitreous glass base material.
[0003]
When this is illustrated, as shown in FIG. 4, both ends of a target (a transparent glass rod which will later become an optical fiber core or a core + cladding) 100 are gripped by a manufacturing apparatus, for example, chucks 20 and 20 of a glass lathe 10. A plurality of, for example, two flame burners 30 are reciprocated (traverse) with respect to the target 100, and the soot 200 generated in the flame 30a from the flame burner 30 is deposited. The desired optical fiber preform has been obtained.
[0004]
At this time, as the flame burner 30, a multi-nozzle type burner as shown in FIG. 5 or a concentric multiple pipe burner as shown in FIG. 6 is used.
Taking the example of the multi-nozzle burner shown in FIG. 5, in this burner 30A, a source gas such as SiCl ₄ gas and an additive gas such as oxygen gas and hydrogen gas are supplied to a center nozzle portion 31 at the center, and an intermediate nozzle outside the nozzle. An inert gas such as a nitrogen gas or an Ar gas is supplied to the portion 32, a combustible gas such as an oxygen gas is supplied to a small-diameter nozzle portion 33 scattered around its periphery, and a Hydrogen gas is supplied to each combustible gas.
[0005]
On the other hand, even in the concentric multi-tube burner 30B (for example, a quintuple tube structure), the center center nozzle portion 35 is supplied with a source gas such as SiCl ₄ gas and an additive gas such as oxygen gas and hydrogen gas. An inert gas such as nitrogen gas or Ar gas is supplied to the second layer nozzle portion 36, a flammable gas such as hydrogen gas is supplied to the third layer nozzle portion 37, and a nitrogen gas is supplied to the fourth layer nozzle portion 38. , And an inert gas such as Ar gas, and a supporting gas such as oxygen gas to the outermost outermost nozzle portion 39.
[0006]
[Problems to be solved by the invention]
However, there has been a problem in that the surface of the glass base material obtained by the above-described external attachment method has a variation in the outer diameter in the longitudinal direction of the base material, which is called unevenness. Usually, this outer diameter variation often appears in the form of a swell that repeats at a period of 40 to 100 mm in the longitudinal direction of the base material. Such undulations not only deteriorate the appearance, but also lead to a problem that the diameter of the optical fiber becomes unstable when the base material is spun later.
[0007]
Particularly recently, in order to reduce the cost of optical fiber manufacturing, there has been a tendency to increase the size of the preform, that is, increase the diameter of the preform, and increase the spinning speed. Irregularities are more likely to occur than before, and have become a more serious problem.
[0008]
For example, in order to increase the size of the base material, it is necessary to increase the soot deposition rate. For this reason, the number of flame burners is increased or the amount of source gas is increased. In this case, unevenness on the surface of the base material is likely to occur due to the increase in the number of burners and the amount of the source gas. Further, when a glass preform obtained from such a large preform is spun at a high speed, the high speed makes it difficult to perform fine control corresponding to the irregularities on the surface of the preform. The irregularities on the surface of the material tend to appear as variations in the optical fiber diameter.
[0009]
As a method of solving such irregularities on the surface of the base material, by adjusting the speed at which the flame burner moves along the target and the rotation speed of the target, the relative movement distance between the two during one rotation of the target is calculated as follows. There has also been proposed a method in which when the diameter of the raw material gas nozzle of the flame burner is set to 1.5 times, unevenness on the surface of the base material is reduced (for example, see Patent Document 1).
[0010]
[Patent Document 1]
JP 2001-287924 A (page 2-5)
[0011]
However, this method was found to be inadequate according to tests and studies by the present inventors. This is because, in addition to the raw material gas nozzle diameter, the unevenness on the surface of the base material is not limited to the diameter of the raw material gas. This is because it has been found that the above has a great effect on the occurrence of unevenness.
[0012]
Therefore, the present inventors have conducted further studies and found that the temperature distribution of the target zone heat zone generated by the flame of the flame burner is controlled by adjusting the flow rate of the flammable gas or the supporting gas to the flame burner. It has been found that the formation of irregularities on the target surface can be suppressed. Further, it has also been found that as a specific temperature distribution, the temperature difference between the highest temperature portion in the temperature distribution of the heat zone and the center temperature portion of the heat zone may be within 5% of the highest temperature portion. . Further, assuming that the flow rate of the source gas in the flame burner is V _S and the flow rate of the combustion supporting gas is V _O , 0.7 ≦ a ≦ 1.6 is satisfied in the relationship of V _O = V _S · a. By adjusting the flow rate of the raw material gas or the supporting gas as described above, it has been found that the temperature difference can be controlled within 5% of the maximum temperature portion.
The present invention has been made from such a viewpoint.
[0013]
[Means for Solving the Problems]
The present invention according to claim 1 is a method for manufacturing an optical fiber preform in which glass fine particles from a flame burner are deposited on a target, by adjusting a flow rate of a combustible gas or a supporting gas to the flame burner, A method of manufacturing a preform for an optical fiber, characterized in that by controlling a temperature distribution of a heat zone in the target which is generated by a flame of a flame burner, generation of irregularities on the surface of the target is suppressed.
[0014]
According to a second aspect of the present invention, the temperature difference between the highest temperature portion and the center temperature portion of the heat zone in the temperature distribution of the heat zone is within 5% of the highest temperature portion. The manufacturing method of the optical fiber preform described above.
[0015]
According to a third aspect of the present invention, when the flow rate of the raw material gas in the flame burner is V _S and the flow rate of the oxidizing gas is V _O , the relation of V _O = V _S · a satisfies 0.7 ≦ 0.7. The flow rate of the raw material gas or the supporting gas is adjusted so that a ≦ 1.6 is satisfied, so that the temperature difference falls within 5% of the maximum temperature portion. The manufacturing method of the optical fiber preform described above.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
First, in the present invention, the phenomenon that the generation of unevenness on the target surface can be suppressed by controlling the temperature distribution of the heat zone in the target can be described as follows.
[0017]
As described above, in the production of an optical fiber preform, a raw material gas is hydrolyzed in a flame to generate soot, and this is deposited on a target. At this time, the flame is placed on the target. Generates a heated zone. Here, the heat zone refers to a region within a temperature difference within 20% of the highest temperature portion in the flame. Therefore, if the highest temperature portion is, for example, 1000 ° C., the range up to 800 ° C. is included. Note that an additive gas such as an oxygen gas or a hydrogen gas is mixed into the raw material gas as necessary.
[0018]
In such a heat zone, if the temperature of the flame impinging on the target is high, the temperature of the heat zone naturally becomes high. Further, if the temperature of the flame is high, the soot deposited on the target is heated and deposited in a hardened state, and it is inferred that the outer diameter of the preform becomes small. Conversely, if the temperature of the flame is low, the temperature of the heat zone will also be low, and the soot deposited on the target will be heated at a low temperature, so it will be deposited in a soft state, and the outer diameter of the preform will increase. Inferred. If the temperature in the heat zone is different as described above, it is considered that the state of the soot deposited on the target differs.
[0019]
On the other hand, when the flame generated by the above-described multi-nozzle burner is observed in more detail, it is as shown in FIG. FIG. 1 schematically shows a flame. When a raw material gas is ejected from a central gas flow path (a center nozzle portion), a region (a low temperature region) 30 a ₁ having a slightly lower temperature is formed at the center. possible while a high area portion (high-temperature region portion) of the temperature on the outside 30a ₂ can be. Of course, are also spread heating area outside the high temperature region portion 30a _2, as described above, the range of the temperature difference of within 20% from the maximum temperature portion of the flame is regarded as the region 30a ₃ of the heat zones Can be That is, in the burner flame, regions having different normal temperature distributions are often generated in one heat zone.
[0020]
If there is such a temperature difference, and if the value is large, as described above, the state of the soot deposited on the target differs, so even if the same amount of soot is deposited, depending on the location, There will be a part where the soot is baked and a soft part. As a result, irregularities are generated on the surface of the base material.
[0021]
From this, by controlling the temperature distribution of the heat zone, that is, the temperature difference between the highest temperature portion in the heat zone and the central temperature portion of the heat zone, it is possible to suppress the generation of unevenness on the surface of the base material. Conceivable.
[0022]
This will be apparent from examples (experimental examples) performed by the present inventors described below. Further, from this example, it is sufficient that the temperature difference is within 5% of the maximum temperature portion, and in order to keep this temperature difference within the range of 5% or less, the flow rate of the raw material gas in the flame burner must be adjusted. Assuming that V _S and the flow rate of the supporting gas are V _O , the relationship between the raw material gas and the supporting gas is such that 0.7 ≦ a ≦ 1.6 in the relation of V _O = V _S · a. It was also revealed that the flow rate should be adjusted.
[0023]
That is, in the above equation, if a = V _O / V _S is less than 0.7, it means that the flow rate of the raw material gas is high, and the raw material gas is not easily mixed with the flame and spreads easily. This is because the tendency to diffuse to the surface becomes stronger. Further, when a = V O _/ V _S exceeds 1.6, it means that the flow velocity of the combustion assisting gas is fast, thereby, collection of the flame is deteriorated, the flame impinging on Tagetto becomes unstable, This is because the temperature difference tends to increase.
[0024]
<Example>
First, a sample optical fiber preform was manufactured using a multi-nozzle burner having the same structure as in FIG. 5 in the same manufacturing apparatus system as in FIG. At this time, the flow rate of the flammable gas, hydrogen gas, was adjusted to 1.3 to 2.0 m / sec in accordance with the increase (growth) of the outer diameter of the optical fiber preform. The flow rate of the raw material gas was adjusted so that the flow rate was 10.2 m / sec, and the flow rate of the oxygen gas, which was a combustion supporting gas, was adjusted so that the flow rate was 9.7 m / sec. At this time, a = 0.95 due to the relationship of the above expression V _O = V _S · a. In addition, 12 kg of soot was deposited around a target having an outer diameter of 30 φ with an added oxygen gas flow rate of 3 L / min.
[0025]
As a result, during soot deposition, the maximum temperature of the soot surface changed from 1100 to 800 ° C. with an increase in the outer diameter of the optical fiber preform. During this time, the temperature difference between the highest temperature portion in the heat zone and the central temperature portion of the heat zone was always within 5% as shown in the graph of FIG. In the graph of FIG. 2, a curve (solid line) I is a distribution of the center temperature portion of the heat zone that changes in response to the increase in the outer diameter of the preform, and a curve (dashed line) II is the highest in the heat zone. It is distribution of a temperature part.
[0026]
Also, optical fiber preform obtained in this way, the set preform outer diameter of the design with respect to D, and the measured average preform outer diameter in the longitudinal direction D _A, unevenness of the base metal surface _{the, {(D a -D) /} D a} if you define × 100, the unevenness degree was 0.3 or less. In addition, no irregularities were observed on the surface of the base material even in the appearance.
[0027]
<Comparative example>
Next, a sample optical fiber preform was manufactured using the same multi-nozzle burner as in FIG. 5 in the same manufacturing apparatus system as in FIG. At this time, the flow rate of the flammable gas, hydrogen gas, was adjusted to 1.3 to 2.0 m / sec in accordance with the increase (growth) of the outer diameter of the optical fiber preform. The flow rate of the source gas was adjusted so that the flow rate was 10.2 m / sec, and the flow rate of the oxygen gas, which was a supporting gas, was adjusted so that the flow rate was 20.4 m / sec. At this time, the relation of the formula _V O = _{V S} · a, was a = 2.0. In addition, 12 kg of soot was deposited around a target having an outer diameter of 30 φ with an added oxygen gas flow rate of 3 L / min.
[0028]
As a result, during soot deposition, the maximum temperature of the soot surface changed from 1100 to 800 ° C. with an increase in the outer diameter of the optical fiber preform. During this time, the temperature difference between the highest temperature part in the heat zone and the center temperature part of the heat zone changed within the range of 6 to 10% as shown in the graph of FIG. In the graph of FIG. 3, a curve (solid line) III is a distribution of the center temperature portion of the heat zone that changes in response to the increase in the outer diameter of the preform, and a curve (dashed line) VI is the highest in the heat zone. It is distribution of a temperature part.
[0029]
Also, optical fiber preform obtained in this way, the set preform outer diameter of the design with respect to D, and the measured average preform outer diameter in the longitudinal direction D _A, unevenness of the base metal surface _{the, {(D a -D) /} D a} if you define × 100, the degree of unevenness was about 0.7 to 1.8. In addition, it was confirmed that the base material surface had irregularities also in appearance.
[0030]
In the above-described embodiment and comparative example, the multi-nozzle burner having the same structure as that of FIG. 5 was used in the same manufacturing apparatus system as that of FIG. 4, but the present invention is not limited to this. A manufacturing apparatus system having one or three or more flame burners may be used, and burners having other structures may be applied.
[0031]
【The invention's effect】
As is clear from the above description, according to the method for manufacturing an optical fiber preform according to the present invention, the flow rate of the flammable gas or the supporting gas to the flame burner is adjusted, and the heat zone by the flame of the flame burner is adjusted. By simply controlling the temperature distribution of the target, it is possible to suppress the generation of irregularities on the target surface.
Therefore, in applying the present invention, it is not necessary to particularly change the manufacturing apparatus system and the flame burner to be used, and the conventional one can be used as it is, so that there is no increase in cost in implementation.
[0032]
Further, the temperature distribution may be controlled by using the temperature difference between the highest temperature portion and the central temperature portion of the heat zone in the temperature distribution of the heat zone within 5% of the highest temperature portion as a guide.
[0033]
Further, in order to keep the temperature difference within 5% of the maximum temperature portion, when the flow rate of the source gas in the flame burner is V _S and the flow rate of the combustion supporting gas is V _O , the equation V _O = V _S · In the relation of a, it can be easily performed by adjusting the flow rate of the source gas or the supporting gas so that 0.7 ≦ a ≦ 1.6 is satisfied.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing a heat zone by a flame burner.
FIG. 2 is a graph showing a relationship between a preform outer diameter and a surface temperature.
FIG. 3 is a graph showing a relationship between a preform outer diameter and a surface temperature.
FIG. 4 is a schematic explanatory view showing an example of an apparatus for manufacturing an optical fiber preform.
FIG. 5 is an end view showing an example of a flame burner.
FIG. 6 is an end view showing another example of the flame burner.
[Explanation of symbols]
Reference Signs List 10 Manufacturing apparatus 20 Chuck 30 Flame burner 30a Flame 100 Target 200 Glass fine particles (soot)

Claims (3)

In the method for producing an optical fiber preform for depositing glass fine particles from a flame burner on a target, the flow rate of a combustible gas or a supporting gas to the flame burner is adjusted to generate the flame generated by the flame of the flame burner. A method of manufacturing a preform for an optical fiber, comprising: controlling a temperature distribution of a heat zone in a target to suppress generation of irregularities on the surface of the target. 2. The optical fiber preform according to claim 1, wherein a temperature difference between a highest temperature portion and a center temperature portion of the heat zone in the temperature distribution of the heat zone is within 5% of the highest temperature portion. Method. Assuming that the flow rate of the raw material gas in the flame burner is V _S and the flow rate of the supporting gas is V _O , 0.7 ≦ a ≦ 1.6 is satisfied in the relationship of V _O = V _S · a. 3. The optical fiber preform according to claim 2, wherein the flow rate of the raw material gas or the supporting gas is adjusted so that the temperature difference falls within 5% of a maximum temperature portion. Method.

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