JP2004018341A - Method of drawing glass preform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly reliable preform for an optical fiber by realizing highly accurate control of the outer diameter of the preform. <P>SOLUTION: The method of drawing a glass preform 1 comprises: inserting the glass preform 1 in a furnace having a heater 6; and drawing the glass preform 1 while moving it relatively to the heater 6 to form a glass preform having a desired outer diameter. In the method, after the tapered part at the side of drawing termination end of the glass preform reaches the heater 6, the temperature of the heater 6 is lowered by a prescribed value from a temperature at which a constant part is heated. <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 the outer diameter of the reduced glass base material is measured, and the upper and lower ends of the glass base material are applied so as to apply a tensile stress to the glass base material so that the obtained measured value matches the target value. A method has been adopted in which the moving speeds of the upper and lower chucks for gripping are controlled.
[0003]
In such a stretching step, in the final stage of stretching, for example, as shown in FIG. 6A, the center of the heater 6 is located at the tapered portion 1c of the glass base material 1, and the glass softening point is It is located at the tapered portion 1c. Therefore, the temperature of the tapered portion 1c is raised to an appropriate temperature, and the stretching is performed favorably.
Then, as the drawing proceeds further, for example, as shown in FIG. 6B, when the center of the heater 6 comes near the boundary with the dummy rod 2, there is a problem that the fluctuation of the drawing outer diameter becomes large. This is because the dummy rod 2 has a smaller diameter than the tapered portion 1c of the glass base material 1 and has a smaller heat capacity, and additionally, the heat released from the tapered portion 1c of the glass base material 1 reaches this small diameter portion. This is considered to be because the dummy rod reaches the softening point as early as possible, and as a result, not the glass preform 1 but the end of the dummy rod starts to elongate, and a sufficient stretching tension cannot be applied to the stretching portion.
In addition, since the volume of the glass base material, which is the starting base material before drawing, is small near the end of drawing, the base material is more easily heated. When the temperature of the base material becomes higher than that at the start, the base material itself may sag. This also causes the end to become thicker.
As described above, in the normal stretching method, despite controlling the outer diameter, the outer diameter suddenly increases on the end side of the stretching, and this portion becomes an abnormal portion due to the variation of the outer diameter, thereby lowering the stretching yield. There was a problem.
[0004]
In order to solve this problem, a heat insulating layer made of fine glass particles is provided at the upper end of the glass base material in order to suppress the elongation of the support portion at the final stage of drawing and to reduce the fluctuation of the drawing outer diameter. A method of stretching a large diameter optical fiber preform so as to prevent heat transfer to the section has also been proposed (JP-A-11-157864).
[0005]
Further, there is also provided a method of manufacturing a glass preform for an optical fiber, in which the heating from the heating element of the drawing furnace to the small diameter portion of the glass preform and the dummy rod portion for holding the glass preform is shielded by a heat-resistant shielding jig. Proposed (Patent No. 292536)
[0006]
[Problems to be solved by the invention]
In the former method, a secondary operation of attaching glass fine particles to the glass base material is required, and it takes time and effort to prepare the base material, and handling is difficult when using a base material with glass fine particles attached. There was a problem.
[0007]
The latter method has a problem that the equipment configuration becomes complicated.
[0008]
As described above, the preparation of the base material or the configuration of the equipment is complicated in any of the methods, and there is a problem that it is an obstacle to mass production.
[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, a glass base material is put into a furnace equipped with a heater, and the glass base material is pulled while moving relative to the heater to form a glass base material having a desired outer diameter. In the method for stretching a glass base material, the heater temperature is lowered by a predetermined value from that at the time of the steady-state stretching after the tapered portion on the drawing end end side of the glass base material reaches the heater. It is characterized by having done.
[0011]
According to such a configuration, by reducing the heating temperature in the vicinity of the end of the stretching, the amount of heat applied to the base material near the end of the stretching is reduced, and the sagging of the base material and the elongation of the dummy rod can be suppressed. it can. Therefore, since the measures are taken only under the manufacturing conditions, the manufacture is easy without performing special processing on the glass base material or modifying the equipment.
[0012]
Also preferably, the heater temperature is lowered by a predetermined value from that at the time of the steady-state stretching after the tapered portion on the end-of-stretching end side of the glass base material reaches the upper end of the heater. Since the drawing is performed at a low temperature, it is possible to prevent the small-diameter part of the support part such as the dummy rod from softening, and to apply sufficient drawing tension to the glass base material even near the end. It becomes.
[0013]
Preferably, when the boundary between the tapered portion and the stationary portion reaches the upper end portion of the heater, the heater temperature starts decreasing. By defining the section where the stretching temperature is lowered in this manner, a good stretching outer diameter can be maintained. If the period for lowering the stretching temperature is shorter than this, there is no effect. If the period is too long, there is a problem that the influence is exerted even on the portion where the outer diameter was originally good and the outer diameter becomes narrower.
[0014]
Further, the heater temperature is set to be lower by about 20 to 100 ° C. than the temperature at the time when the end portion of the stretching is reached. If the reduction width is not more than 20 ° C., there is no effect, and if the reduction width exceeds 100 ° C., there is a problem that the diameter becomes smaller than the outer diameter stabilizing portion.
[0015]
In this way, an optical fiber preform having good outer diameter accuracy can be obtained.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of a method for manufacturing an optical fiber preform according to the present invention will be described in detail with reference to the drawings.
In the present embodiment, as shown in FIG. 1, the heater temperature of the stretching apparatus is set to 2,000 ° C. at the start of stretching, and the set temperature is set at a point B where the tapered portion on the drawing end end side of the glass base material reaches the upper end of the heater. The temperature is lowered by 50 ° C. so as to be 1950 ° C. at the point A, so that the small-diameter portion of the dummy rod 2 shown in FIG. 2 as the support portion is not softened, and a sufficient stretching tension is applied to the glass base material 1 even near the end. I am trying to be.
[0017]
As shown in FIG. 2, the stretching apparatus used here includes a glass preform 1 having dummy rods 2 and 3 connected to both ends thereof, and an upper driving unit 4 which grips the glass preform via the dummy rods 2 and 3. 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 1E of the glass base material 1. Control for performing feedback control of the stretching condition based on the outer diameter measuring device 7 and 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 unit 8. Although not shown here, this stretching apparatus is equipped with a furnace body having a carbon heater 6 and a carbon core tube mounted in the furnace body, and heats the glass base material 10 by the heater 6. It is stretched to form a glass base material having a desired diameter.
[0018]
The glass preform 1 is installed inside the furnace tube, and the lowering speed of the lower drive unit 5 is lowered by the glass rod 1 via the dummy rods 2 and 3 connected to both ends of the glass preform 1. The lower drive unit 5 and the upper drive unit 4 are driven and lowered so as to be stretched so as to be higher than the speed.
Further, the furnace body of the drawing furnace is formed with an inert gas introduction part so that an inert gas can be introduced into the furnace body, and by oxidizing carbon parts in the furnace body by setting the furnace body to an inert gas atmosphere. It is configured so that it can be prevented.
[0019]
This stretching step is performed as follows. First, a glass base material having dummy rods on the upper and lower parts is mounted on a drawing furnace, and the heater is heated.
[0020]
Thereafter, the lower drive unit 5 is lowered at a higher speed while the upper drive unit 4 of the glass base material is lowered at a constant speed, and the glass base material is stretched. At this time, the outer diameter of the drawn glass base material is controlled by adjusting the lowering speed of the lower drive unit by the control unit 8 based on the measurement output by the outer diameter measuring device 7.
[0021]
The amount of heat applied to the tapered glass base material in the vicinity of the end of drawing by reducing the heating temperature near the end of drawing to reduce the heat applied to the glass base near the end of drawing and near the end of drawing. Can be reduced, and the droop of the glass base material and the elongation of the dummy rod can be suppressed. Therefore, it is possible to control the outer diameter with high accuracy without performing special processing on the glass base material or modifying equipment.
[0022]
【Example】
First, a glass base material having an outer diameter of 80 mm is prepared.
After installing the base material in the stretching apparatus described in the above embodiment, the furnace temperature is raised from room temperature to 2000 ° C.
[0023]
After reaching the target temperature, stretching of the glass base material was started. At this time, the take-up speed of the glass base material is set to 40 m / min, the outer diameter in the course of stretching is measured, and the difference between the set outer diameter set in advance and the feed speed of the base material (upper chuck) is input. The outer diameter was controlled by feeding back to (speed) Va.
[0024]
Then, the elongation progresses gradually, and when the feeding length to the heater becomes 350 mm and the remaining length of the preform glass base material becomes 50 mm, the heater temperature is changed to 1950 ° C., and all the glass base material is drawn as it is. Stretching was carried out until finished.
[0025]
Here, as shown in the enlarged view of the main part in FIG. 3, when the lower end B of the tapered portion of the glass base material 1 reaches the upper end 6u of the heater 6, the temperature starts dropping, and the tapered portion of the glass base material 1 starts. When the upper end A reaches the upper end 6u of the heater 6, the heater temperature is adjusted so as to be 1950 ° C.
As a result, the outer diameter was as good as 30 ± 0.2 mm as shown in FIG. Here, the vertical axis indicates the outer diameter, and the horizontal axis indicates the stretching length.
[0026]
(Comparative example)
In this example, as in the previous example, the stretching was performed using the stretching apparatus shown in FIG. 2, and the stretching was performed to the end at a constant furnace temperature. As a result, as shown in FIG. 5, the outer diameter after stretching was as good as 30 ± 0.2 mm from the stretching start side to 80% from the stretching start side. At the end, the outer diameter was 32 mm.
[0027]
From the comparison between the example and the comparative example, by lowering the heating temperature near the end of the stretching, and reducing the heat applied to the glass base material near the end of the stretching, it is possible to easily perform the stretching without changing the outer diameter. It becomes possible.
[0028]
In the above embodiment, when the lower end B of the tapered portion of the glass base material 1 reaches the upper end 6 u of the heater 6, the temperature drop starts, but the lower end B of the tapered portion of the glass base material 1 When the lower end 6d is reached, the temperature drop may be started.
[0029]
In the above embodiment, PID may be used as a control method of the control unit.
[0030]
Although an electric heater is usually used as a heater for heating the base material, a heating medium for induction heating 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.
Further, the temperature range to be lowered can be appropriately changed.
[0031]
【The invention's effect】
As described above, by reducing the heat applied to the glass base material in the vicinity of the end of drawing, the dripping of the glass base material and the elongation of the supporting portion such as the dummy rod can be extremely easily suppressed, and the high-precision outer diameter can be obtained. Controllable stretching can be performed.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a stretching control method according to an embodiment of the present invention.
FIG. 2 is a diagram illustrating a stretching device according to an embodiment of the present invention.
FIG. 3 is an explanatory view of a main part of a method according to an embodiment of the present invention.
FIG. 4 is a diagram showing a change in outer diameter of a glass base material stretched by a method of an example of the present invention; FIG. 5 is a diagram showing a change in outer diameter of a glass base material drawn by a method of a comparative example;
FIG. 6 is an explanatory view showing stretching in a conventional example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Glass base material 2 Dummy rod 3 Dummy rod 4 Upper drive part 5 Lower drive part 6 Heater

Claims (4)

In a furnace equipped with a heater, a glass base material having a substantially constant outer diameter and a tapered portion formed so that the outer diameter is reduced at both ends is inserted, and the glass base material is passed through the heater. In a stretching method of the glass base material, which is pulled while relatively moving with respect to the glass base material so as to form a glass base material having a desired outer diameter,
After the tapered portion on the drawing end end side of the glass base material reaches the heater, the heater temperature is decreased by a predetermined value from that at the time of the steady portion drawing. Method. The heater temperature is
2. The method according to claim 1, wherein the temperature is lower than that of the stationary part when the end of the tapered part reaches the upper end of the heater. 2. The method according to claim 1, wherein when the boundary surface between the tapered portion and the stationary portion reaches the upper end of the heater, the heater temperature starts decreasing. 3. . The stretching of the glass base material according to claim 3, wherein the temperature of the heater at the end of the stretching is set to be lower by about 20 to 100C than the temperature at the time when the boundary reaches the upper end of the heater. Method.

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