JP2004067471A - Method of drawing glass preform and drawing apparatus used for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly reliable optical fiber preform by realizing a highly accurate control method for the outer diameter of the optical fiber preform. <P>SOLUTION: In a method of drawing a glass preform, comprising forming the glass preform having a desired outer diameter by drawing the glass preform by heating/softening it while measuring the outer diameter of the glass preform between a section where the glass preform is made soft and a position where the outer diameter of the glass preform becomes a finish diameter with an outer diameter measuring unit and while controlling the drawing speed so that the measured outer diameter D1 coincides with the target outer diameter, the target outer diameter after the finish diameter is attained is set to be larger by a prescribed value than the finish diameter at the start of drawing, and thereafter, the target outer diameter is controlled so as to gradually come close to the finish diameter D0. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for drawing a glass base material and a drawing apparatus used for the method, and more particularly to controlling the outer diameter of the glass base material.
[0002]
[Prior art]
Conventionally, as a method of stretching a glass preform for an optical fiber to a desired outer diameter, generally, a heating means such as a resistance heating furnace is used to sequentially heat and soften the glass preform from one end of the glass preform to obtain a tensile stress or the like. And moving the upper and lower chucks gripping the upper and lower ends of the glass base material while measuring the outer diameter of the reduced glass base material so that the measured value matches the target value. A method of controlling the speed has been adopted. In this method, the moving speed of the lower chuck is controlled to be higher than the moving speed of the upper chuck in order to apply a predetermined tensile stress to the glass base material.
[0003]
In such a stretching method, in order to set the outer diameter of the obtained glass base material after stretching to a desired value, the outer diameter of a predetermined position of the softened portion is measured with an outer diameter measuring instrument, and the measured outer diameter is a target. A method of adjusting the stretching speed so as to obtain a value is adopted.
[0004]
However, in such a method, there is a problem that the finished diameter after stretching varies even though the measured value is controlled to match the target diameter. This is because the difference between the target diameter and the finished diameter is not constant during stretching. The difference between the target diameter and the finished diameter depends on the length of the taper of the neck down portion where the base material is stretched. The length of this taper is greatly affected by the tension applied to the base material during stretching. That is, it is considered that the variation in the finished diameter is caused by the fact that the tension applied during stretching is not constant, so that the taper length gradually changes during stretching.
[0005]
As a countermeasure against this, for example, a method has been proposed in which the tension at the time of stretching is detected so as to control the tension, and the power supplied to the stretching furnace is controlled in accordance with the detected tension (Japanese Patent Laid-Open No. 4-325448).
[0006]
[Problems to be solved by the invention]
However, in this method, there is a problem that the production cost is increased, for example, a new device for controlling the tension is required, and the number of parameters to be controlled is increased, which complicates the production.
[0007]
Therefore, in a method of stretching a glass base material by applying a tensile stress to the glass base material once by heating and softening a predetermined position of the glass base material, the outer diameter of the softened portion is measured and the outer diameter is set in advance. There has also been proposed a method in which the tensile stress is adjusted so as to coincide with the target value, and the set outer diameter is changed in a predetermined time after the start of stretching (Japanese Patent Laid-Open No. Hei 9-221334).
[0008]
Although this method is a very simple method, desired control cannot be easily performed unless conditions are set, and it is necessary to find an optimum range in order to adjust the finished diameter.
[0009]
The present invention has been made in view of the above circumstances, and has as its object to provide a highly reliable optical fiber preform that realizes highly accurate outer diameter control.
[0010]
[Means for Solving the Problems]
Therefore, in the stretching method of the present invention, the outer diameter of the glass base material is measured by using an outer diameter measuring device between the softened portion of the glass base material and the finished diameter position of the drawing, and the measured outer diameter matches the target outer diameter. While controlling the stretching speed, the glass base material is softened and drawn, and in the glass base material drawing method for forming a glass base material having a desired outer diameter, the target outer diameter after the finished diameter comes out. Is set to be larger than the finished diameter by a predetermined value at the start of the stretching, and thereafter, the target outer diameter is controlled so as to gradually approach the finished diameter.
[0011]
The glass base material being stretched is gradually heated as the stretching progresses, and the softened state changes, so that the stretching tension applied to the base material gradually decreases. That is, actually, the difference between the target diameter and the finished diameter gradually decreases as the stretching is performed. Therefore, if the target diameter is set to a constant value, the finished diameter will gradually become relatively large even if the measured value is controlled to match the target value. On the other hand, by gradually lowering the setting of the target diameter in accordance with the change in the difference between the target diameter and the finished diameter, the finished diameter of the glass base material after stretching can be kept constant. Therefore, since the measures are taken only under the control conditions, the manufacture is easy without performing special processing on the glass base material or modifying the equipment.
[0012]
Desirably, the value of making the softened portion of the glass base material larger than the finished diameter of the softened portion at the start of the stretching is controlled to be within about 3 mm and gradually approach the finished diameter. According to various experimental results, when the diameter exceeds 3 mm, the variation in the difference between the control outer diameter and the finished diameter increases accordingly, so the outer diameter fluctuation at the start of stretching is large, and it is difficult to adjust by changing the setting of the target diameter. I know it will be.
[0013]
Further, since the change in the difference between the control outer diameter and the finished diameter changes depending on the position of the starting base material with respect to the drawing furnace, it is most effective to adjust the target diameter according to the feed amount of the base material. Therefore, it is desirable that the target outer diameter of the softened portion of the glass base material during the stretching is stretched while changing so as to gradually decrease in accordance with the feed length of the glass base material to be stretched.
[0014]
Desirably, the target outer diameter of the softened portion of the glass base material during the drawing is gradually reduced at a rate of 1.0 mm or less per 100 mm feed length of the glass base material to be drawn. The most accurate response can be made. If it exceeds 1.0 mm per 100 mm of feed length, the base material becomes too thin.
[0015]
Desirably, the target outer diameter of the softened portion of the glass base material during the stretching is more preferably gradually reduced at a rate of 0.3 mm or less per 100 mm feed length of the glass base material to be stretched.
[0016]
In this way, the method of changing the set value of the target value in accordance with the feed speed of the glass base material can most accurately cope with the change in the taper length. Becomes possible.
[0017]
In the present invention, the glass base material further includes an outer diameter measuring device for measuring the outer diameter of the softened portion of the glass base material, and control means for controlling the stretching speed so that the measured outer diameter matches the target outer diameter. Is stretched while heating and softening the glass base material to form a glass base material having a desired outer diameter, wherein the control means sets the target outer diameter at the start of the drawing to a predetermined outer diameter than the finished diameter. The target outer diameter is controlled so as to gradually approach the finished diameter.
[0018]
In this way, an optical fiber preform having good outer diameter accuracy can be obtained.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a method for stretching an optical fiber preform according to an embodiment of the present invention will be described in detail with reference to the drawings.
In the present embodiment, an apparatus having a control unit 8 as shown in FIG. 1 is used, and as shown in FIG. 2, a glass base material 1 before stretching and a glass base material 10 after stretching are soaked. And stretching the glass base material 1 after dehydration and sintering to a desired outer diameter while controlling the stretching outer diameter so that the measured value of the outer diameter measuring device 7 matches a preset target value. It is. In this method, at the start of stretching, the target outer diameter at the start of stretching is controlled to be larger than the finished diameter by a predetermined value, and the target outer diameter is controlled so as to gradually approach the finished diameter.
[0020]
As shown in FIG. 1, the stretching apparatus used here includes a glass base material 1 having dummy rods 2 and 3 at both ends, and an upper drive unit which grips the glass base material via the dummy rods 2 and 3, respectively. 4 and a lower drive unit 5, a heater 6 for heating the glass base material before stretching held by the upper drive unit 4 and the lower drive unit 5, and an outer diameter of the stretched portion 1 E of the glass base material 1 are measured. The stretching condition is feedback-controlled based on the outer diameter measuring device 7 to be performed, the measured value of the outer diameter of the stretched portion 1E of the glass base material 1 obtained by the outer diameter measuring device 7, and the target value of the outer diameter. And a control unit 8. Although not shown here, a furnace body having a carbon heater 6 and a carbon furnace tube mounted in the furnace body are often provided, and the glass base material 10 is heated and stretched by the heater 6. , To form a glass base material having a desired diameter.
[0021]
Further, the upper drive unit 4 holds the dummy rod 2 with the upper chuck 4c, and drives the motor 4m by the control unit 8 so as to apply a predetermined pulling speed to the glass base material. On the other hand, the lower drive unit 5 holds the dummy rod 3 with the lower chuck 5c, and drives the motor 5m by the control unit 8 so as to apply a predetermined pulling speed to the glass base material.
[0022]
As described above, the glass preform 1 is installed inside the furnace tube, and is held by the upper drive unit 4 and the lower drive unit 5 via the dummy rods 2 and 3 exposed at both ends of the glass preform 1. I have. The control unit 8 is configured to lower the lower driving unit 5 so that the lowering speed of the upper driving unit 4 is higher than the lowering speed of the upper driving unit 4 and perform stretching. Further, an inert gas introduction section for introducing an inert gas as required is formed in the furnace tube, so that oxidation is prevented by supplying the inert gas.
[0023]
This stretching step is performed as follows. First, the glass base material is mounted on the upper drive unit 4 and the lower drive unit 5 of the stretching device via dummy rods provided at both ends, and the temperature of the heater is raised to heat the glass base material.
[0024]
Thereafter, while lowering the upper drive unit 4 of the glass base material, the lower drive unit 5 is lowered at a higher speed to stretch the glass base material. At this time, when performing stretching while adjusting the stretching speed by the control unit 8 based on the output measured by the outer diameter measuring device 7, the target outer diameter is increased by 3 mm at the stretching start point, and gradually the target outer diameter is increased by about 0.1 mm per 100 mm. Make the outer diameter closer to the finished diameter.
[0025]
The neck-down shape of the base material during stretching is as shown in FIG. During the stretching, the outer diameter D1 of the base material, which is larger than the finished diameter D0 after the stretching and is in the middle of the stretching, is measured by the outer diameter measuring device 7 so that the measured value matches the preset target diameter D0 + α. The control unit 8 controls the stretching speed, that is, the lowering speed difference between the upper drive unit and the lower drive unit. At this time, as the elongation time elapses, the base material is heated and softened, so that the length of the taper of the neck-down portion changes, and the difference between the measured outer diameter D1 and the finished diameter D0 gradually decreases. Therefore, when the target value is set to a constant value, even if the measured outer diameter D1 is controlled so as to coincide with the target diameter, the finished diameter D0 increases.
Therefore, conversely, as shown in FIG. 4, the target diameter D0 + α is set such that α becomes smaller as the stretching proceeds, that is, the target diameter D0 + α gradually approaches the value of the finished diameter D0, so that the finished diameter is gradually increased. The diameter is prevented from becoming large.
[0026]
In this way, the target diameter, as the stretching progresses, by gradually reducing the value of the difference α between the target diameter and the finished diameter, it is possible to cancel the change in the taper length of the neck-down portion, and easily, Stretching without variation in outer diameter can be performed. Therefore, it is possible to control the outer diameter with high accuracy without performing special processing on the glass base material or modifying equipment.
[0027]
(Example)
First, a glass base material having an outer diameter of 80 mm is prepared.
Then, this glass base material is mounted on the stretching apparatus of FIG. 1 described in the above embodiment. In this state, the heater 6 is turned on, and the furnace temperature is raised from room temperature to 2000 ° C.
[0028]
When the target temperature is reached in this way, the glass base material take-up speed is initially set to 40 m / min, the target outer diameter at the start of drawing is set to 31 mm with respect to the finished diameter of 30 mm, and the base material is fed after the start of drawing. The target outer diameter set at a rate of 0.2 mm per 100 mm length is gradually lowered as shown in the example of FIG. 4, set to 30.2 mm at the end of stretching, and the outer diameter is measured by the outer diameter measuring device 7. Then, the measured value was fed back to the feed speed (upper chuck speed) Va of the base material for feeding the difference between the preset target outer diameter and the target outer diameter, and the outer diameter was controlled to perform stretching. As a result, the outer diameter of the drawn glass base material was as good as 30 ± 0.2 mm as shown in FIG.
[0029]
(Comparative example)
In this example, similarly to the above-described example, stretching was performed using the stretching apparatus shown in FIG. 1, and the target outer diameter was 34 mm with respect to the finished diameter of 30 mm. As a result, as shown in FIG. 5B, the outer diameter after stretching has a large variation in the outer diameter immediately after the start of stretching in the base material longitudinal direction, and the outer diameter gradually increases in the length direction. Occurred.
[0030]
From the comparison between the embodiment and the comparative example, the target diameter is increased by a predetermined value in the vicinity of the stretching start time, and by gradually decreasing the increase width, the stretching speed is adjusted in the vicinity of the stretching start end, so that the stretching can be easily performed. In addition, it is possible to perform stretching without variation in outer diameter.
[0031]
Although the target diameter is gradually reduced, the target diameter may be gradually reduced.
[0032]
In the stretching apparatus, an electric heater is generally used as a heater, but a heating medium that is induction-heated may be used. Further, although the glass base material is moved with respect to the heater, it goes without saying that the heater may be moved.
[0033]
【The invention's effect】
As described above, according to the method for stretching a glass base material of the present invention, the target diameter is set to gradually decrease during stretching in order to prevent the glass base material from becoming thicker as the stretching proceeds, and Since the control is performed, it is possible to perform the stretching capable of controlling the outer diameter with high accuracy.
Further, according to the stretching apparatus of the present invention, it is possible to easily obtain a glass base material having a small outer diameter variation.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing a stretching device according to an embodiment of the present invention.
FIG. 2 is a view showing a glass base material before drawing and a glass base material after drawing drawn by the drawing method according to the embodiment of the present invention.
FIG. 3 is an explanatory view of a main part of a stretching method according to an embodiment of the present invention.
FIG. 4 is a diagram showing a relationship between a base material feeding speed and a target outer diameter in a stretching control method according to an embodiment of the present invention.
FIG. 5 is a view showing a change in outer diameter of a glass base material stretched by a stretching control method according to an example of the present invention and a comparative example.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 glass base material before stretching 2 dummy rod 3 dummy rod 4 upper drive unit 5 lower drive unit 6 heater 7 outer diameter measuring device 8 control unit 10 glass base material after drawing 100 glass base material after drawing

Claims (6)

Measure the outer diameter of the glass base material between the softened portion of the glass base material and the position that is the finished diameter of the drawing with an outer diameter measuring device, and control the drawing speed so that the measured outer diameter matches the target outer diameter. While stretching the glass preform while softening the glass preform, forming a glass preform having a desired outer diameter,
The target outer diameter after the finished diameter is set is set to be larger than the finished diameter by a predetermined value at the start of the stretching, and then the target outer diameter is controlled so as to gradually approach the finished diameter. A method for stretching a glass base material, comprising: 2. The method according to claim 1, wherein a target outer diameter of the softened portion of the glass base material at the time of starting the stretching is set so that a difference from the finished diameter is within 3 mm, and is controlled so as to gradually approach the finished diameter. The method for stretching a glass base material according to the above. The target outer diameter of the softened portion of the glass base material during the stretching is stretched while changing so as to gradually decrease according to the feed length of the glass base material to be stretched. 3. The method for stretching a glass base material according to item 2. 3. The method according to claim 1, wherein the target outer diameter of the softened portion of the glass base material during the drawing is gradually reduced at a rate of 1.0 mm or less per 100 mm feed length of the glass base material to be drawn. The method for stretching a glass base material according to the above. 3. The method according to claim 1, wherein the target outer diameter of the softened portion of the glass base material during the drawing is gradually reduced at a rate of 0.3 mm or less per 100 mm feed length of the glass base material to be drawn. The method for stretching a glass base material according to the above. An outer diameter measuring device for measuring the outer diameter of the softened portion of the glass base material, and a control means for controlling the stretching speed so that the measured outer diameter matches the target outer diameter, are heated and softened by heating the glass base material. In a glass preform stretching device that forms a glass preform having a desired outer diameter while stretching while
The control means is characterized in that the target outer diameter at the start of stretching is set to be larger than the finished diameter by a predetermined value, and the target outer diameter is controlled so as to gradually approach the finished diameter. Glass preform stretching equipment.

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