Method for drawing glass preform

Abstract

To provide a method for processing a glass preform, capable of obtaining a glass rod stable even after drawing, small in bending amount and small in outer diameter variation at a low cost.SOLUTION: A method for drawing a glass preform comprises: welding a hanging dummy on a glass preform including characteristic defective parts at both ends and a characteristic effective part therebetween before drawing when drawing the glass preform; connecting the hanging dummy to a sending mechanism; inserting the hanging dummy into the heating furnace of a drawing device from the lower part of the glass preform; and drawing at least the characteristic effective part. When setting the longitudinal direction length of the device maintaining a temperature of 1900°C or more in the heating furnace when setting the heating furnace of the drawing device to a normal temperature when drawing the glass preform to X[mm], a relationship between a volume B [mm3] obtained by integrating a range of the glass preform welded by the hanging dummy before the drawing from the characteristic effective part end to X / 2 [mm] on the hanging dummy side and the product A [mm3] of the square of the average value of an outer diameter in the range of the characteristic effective part of the glass preform and X satisfies B≥0.2A+700,000.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a method for stretching a glass preform suitably used for spinning an optical fiber.

Background Art

[0002] In the production of a quartz glass rod typified by an optical fiber perform, after previously producing a large-sized glass preform, a method is used in which it is stretched using a stretching device equipped with a heating furnace to obtain a glass rod with a smaller diameter. Since the glass rod stretched by the stretching device has a relatively large outer diameter variation and bending, it is precisely processed again using a stretching device with a burner, which is called a glass lathe, as a heat source, so that the outer diameter variation and bending are adjusted to meet the accuracy required for the product.

[0003] In recent years, when manufacturing an optical fiber from an optical fiber perform, it is more advantageous in terms of equipment operation rate to manufacture from a larger optical fiber perform. Therefore, there is a demand for an optical fiber perform that is larger than the conventionally common one with an outer diameter of 80 mm, for example, with an outer diameter of 150 mm or more. The conventionally common optical fiber perform with an outer diameter of 80 mm was corrected for bending with a glass lathe and finished as a straight glass preform, and was used in the drawing process. However, when the preform has a large diameter such as 150 mm, it is difficult to correct the bending with a glass lathe. This is because the heating by the burner is performed in the open air, so cooling by radiation occurs simultaneously with heating. As the outer diameter increases, the effect of cooling by radiation becomes greater, making it impossible to raise the temperature sufficiently, and it is difficult to remove the strain remaining at the bending correction position. Therefore, even for a preform with a large diameter such as 150 mm, it is required to suppress the amount of bending of the glass rod obtained by the stretching device to within 2 mm / m or an amount close thereto.

[0004] An example of a stretching device used for stretching will be described with reference to FIG. 1. The stretching device generally consists of three parts: a heating furnace, a feeding section, and a take-up section. The heating furnace includes a heater 1, a water-cooled chamber 3 containing a heat insulating material 2, a top chamber 4 connected to the upper part thereof, and a lower gas seal 8 attached to the lower part of the water-cooled chamber 3. The feeding section consists of a vertically movable feeding mechanism 7 provided above the heating furnace, a suspension shaft 5 connected to the feeding mechanism 7, and a connecting jig 6. The suspension shaft 5 is inserted into the top chamber 4.

[0005] The take-up section consists of a grippable and releasable guide roller 9, a take-up roller (upper) 10, and a take-up roller (lower) 11 provided at the lower part of the furnace body. The guide roller 9 is formed of a heat-resistant roller such as carbon and serves to guide a take-up dummy 14 or a glass rod along the axis of the device. The take-up rollers 10 and 11 are driven by a motor and have the function of pulling down the take-up dummy 14 or the stretched glass rod held by the take-up rollers to appropriately stretch the glass base material 12.

[0006] The upper end of the suspension dummy 13 provided above the glass base material 12 is mechanically connected to the connecting jig 6, and the glass base material 12 is connected to the feeding mechanism 7 via the suspension shaft 5. A take-up dummy 14 is connected to the lower end of the glass base material 12. Also, in the case of the glass base material 12 manufactured via a porous glass base material, an opaque portion 15 exists in one of the tapered portions. During stretching, while pulling down the glass base material 12 via the feeding mechanism 7, the take-up dummy 14 is pulled down by the take-up rollers 10 and 11 at a speed faster than that, so that a glass rod with a smaller diameter can be obtained from the glass base material 12. The above-mentioned opaque portion 15 is left in the sintering process of vitrifying the porous glass base material into transparent glass. Since this process is performed with the porous glass base material suspended vertically, at the end of sintering, when attempting to completely vitrify the upper tapered portion with the total weight of the already vitrified ingot applied to the heated portion, it is left to prevent the tapered portion from stretching too much due to the weight.

[0007] In recent years, various methods have been proposed for obtaining a glass rod with a small amount of bending using a stretching device. Patent Document 1 describes a method of welding a suspension dummy to a cut surface formed by cutting a part of a glass taper portion including a transparent glass portion and then stretching to align the axis of the glass base material with the center of the heating furnace.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] In the furnace stretching process of the glass base material, a suspension dummy is connected to a cut surface where a part of the glass base material is cut off, and the suspension dummy is connected to a feeding mechanism above the furnace to perform the furnace stretching process. However, in the method described in Patent Document 1, when the effective part end of the glass base material is too close to the cut surface, when trying to stretch to the effective part end, the suspension dummy and the vicinity of the welding surface between the suspension dummy and the glass taper part will be deformed by heat before the effective part end of the glass base material, resulting in problems such as the dropping of the glass base material, an increase in the diameter variation and bending after stretching. Therefore, the present invention has been made in view of the above problems, and its object is to provide a method for processing a glass base material that can obtain a glass rod with a small amount of bending and a small outer diameter variation stably and at low cost even after stretching.

Means for Solving the Problems

[0010] The method for drawing a glass base material of the present invention is a method for drawing a glass base material having characteristic defective parts at both ends and a characteristic effective part between the two characteristic defective parts into a glass rod with a smaller diameter. Prior to the drawing, a suspension dummy is welded to the glass base material, the suspension dummy is connected to a feeding mechanism, and the glass base material is inserted into the heating furnace of the drawing device from below the glass base material to draw at least the characteristic effective part. When the temperature in the heating furnace is set to the normal temperature during the drawing of the glass base material, the longitudinal length of the device maintaining the temperature in the heating furnace at 1900 °C or higher is X [mm], and the volume B [mm 3 obtained by integrating the range from the end of the characteristic effective part of the glass base material with the suspension dummy welded thereto to X / 2 [mm] on the side of the suspension dummy, and the product A [mm 3 of the square of the average value of the outer diameter of the range of the characteristic effective part of the glass base material and the X satisfy the following formula 1. [Formula 1] B ≧ 0.2A + 700,000

[0011] In the present invention, it is preferable that the distance from the end of the characteristic effective part of the glass base material on the side where the suspension dummy is welded to the welding position of the suspension dummy and the glass base material is longer than X / 2 [mm]. Also, the distance from the end of the characteristic effective part of the glass base material on the side where the suspension dummy is welded to the welding position of the suspension dummy and the glass base material may be shorter than X / 2 [mm]. In that case, it is preferable that the outer diameter of at least the range from the end of the characteristic effective part of the glass base material to X / 2 [mm] of the suspension dummy is thicker than the outer diameter of the range on the side of the suspension shaft above the range.

Advantages of the Invention

[0012] According to the method for drawing a glass base material of the present invention, it is possible to easily suppress the occurrence of diameter variation and bending of the drawn glass rod. As a result, excellent effects such as an increase in the yield rate of the drawn glass rod and a contribution to the reduction of production costs are achieved.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0014] Hereinafter, an example of an embodiment of a method for extending a glass base material according to the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the embodiments described below, and various aspects are possible. In addition, although there are places where the outer diameter of a circle is described, it is not limited to a perfect circle, and it may be a non-circle to the extent that the function as an optical fiber preform is not impaired.

[0015] As a result of intensive research, the present invention provides that at the end of the glass base material extension process, at the timing when the glass base material has been completely fed into the extension device, that is, at least at the timing when the glass base material has been fed into the extension device up to the end of the characteristic effective part of the glass base material, when the longitudinal length of the device in which the temperature in the heating furnace is maintained at 1900°C or higher is X [mm], the volume B [mm 3 obtained by integrating the range from the end of the characteristic effective part to X / 2 [mm] on the suspension dummy side of the glass base material to which the suspension dummy before extension is welded, and the product A [mm of the square of the average value of the outer diameter of the characteristic effective part of the glass base material and the X 3It has been found that when the relationship between volume B and the characteristic effective portion satisfies B≧0.2A+700,000, it is possible to suppress the occurrence of extreme diameter fluctuations and bending of the glass rod due to elongation. It has been found that when the volume B is smaller than 0.2A+700,000, near the end of the characteristic effective portion, heat is transmitted to the portion on the volume B side before the characteristic effective portion, causing it to soften and deform, making it impossible to control the shape as desired near the end of the characteristic effective portion, resulting in large diameter fluctuations and bending.

[0016] Furthermore, the minimum amount required for the volume B also varies depending on the average outer diameter of the characteristic effective part of the glass base material. When the average outer diameter of the characteristic effective part before drawing is small, the amount of heat required to soften and deform the characteristic effective part is relatively small, so that the characteristic effective part can be properly drawn in its entirety even if the volume B is small. On the other hand, when the average outer diameter of the characteristic effective part before drawing is large, the amount of heat required to soften and deform the characteristic effective part is relatively large accordingly, so that the heat transferred to the part on the volume B side becomes large and it becomes easy to soften and deform, and therefore, unless the volume B is large, the characteristic effective part cannot be properly drawn to the end. The volume B may include not only the glass base material portion to be stretched, but also the hanging dummy connected to its tip. In other words, if the distance from the characteristic effective portion end of the glass base material to the hanging dummy connection position is short and it is difficult to obtain a predetermined volume B amount, the volume in the range of the volume B can be increased by using a thick hanging dummy, and abnormal deformation of the characteristic effective portion end can be prevented. In this case, after the volume B reaches 0.2A + 700,000 or more, the outer diameter of the hanging dummy may be made thinner than in the above case. This allows the amount of material used for the hanging dummy to be reduced, contributing to cost reduction.

[0017] FIG. 1 is a cross-sectional view showing an outline of a stretching device used for stretching a glass base material in the present embodiment. The glass base material is set so that a transparent glass tapered portion is at the upper end and a glass tapered portion including an opaque glass portion is at the lower end. A suspension dummy is welded in advance to the transparent glass tapered portion at the upper end, and the suspension dummy is connected to a feeding mechanism. Further, the glass tapered portion including the opaque glass portion at the lower end is cut in advance at an appropriate location. After a take-out dummy inserted into the heating furnace from the lower part of the stretching device is welded and connected to the cut surface by a furnace heater, a glass rod stretched from below the furnace is pulled out, whereby a glass rod stretched to a predetermined diameter can be obtained.

[0018] FIG. 2 is an example of a schematic view showing the relationship between the position and temperature of a glass tapered portion including a transparent glass portion at the upper end of the base material when the glass base material reaches the stretching end position. For example, when this glass base material is an optical fiber preform obtained by vitrifying a soot produced by the OVD method into transparent glass, the distance from the characteristic effective part end 16 to the suspension dummy 13 connection part is relatively long, and the volume of the tip part of the glass base material itself often becomes large. Therefore, as the suspension dummy 13, a quartz glass rod having an outer diameter smaller than the tip diameter of the cut transparent glass tapered portion can be used. Here, the stretching end position is set as the characteristic effective part end 16 of the glass base material, and the state where the characteristic effective part end 16 reaches the center position of the heater is defined as the stretching end timing. In a heating furnace set to a heating temperature for performing stretching, in a range X [mm] in the upper half corresponding to X / 2 [mm], the volume B [mm 3 and the product A [mm of the square of the average value of the outer diameters of the range of the characteristic effective part of the glass base material before stretching and X 3 . When the relationship B ≧ 0.2A + 700,000 is satisfied, softening and deformation due to unintentional heating of this volume B portion can be prevented, so that a glass rod with a good shape can be obtained. In the case of a large-diameter glass base material where the effective portion of the characteristics exceeds 150 mm, as shown in Fig. 2, the distance from the end 16 of the effective portion of the characteristics to the hanging dummy welding position is often longer than X / 2 [mm], and a large volume B [mm 3 can be obtained.

[0019] Fig. 3 is a schematic diagram showing another example of the relationship between the position and temperature of the glass taper portion including the transparent glass portion at the upper end of the base material when the glass base material reaches the end position of stretching. For example, when the glass base material is a core rod obtained by vitrifying a soot produced by the VAD method into transparent glass, the distance from the end 16 of the effective portion of the characteristics to the connection portion of the hanging dummy 13 is short, and the distance from the end 16 of the effective portion of the characteristics to the hanging dummy welding position may be shorter than X / 2 [mm]. Since the volume of the tip portion of the glass base material itself is often small, the hanging dummy 13 connects a thick-diameter quartz glass rod to the tip of a thin-diameter quartz glass rod, and welds the side of the connected thick-diameter quartz glass rod to the tip of the glass base material and feeds it into the stretching device. Thereby, in the heating furnace set to the heating temperature for performing the stretching process, the volume B [mm3] of the range 17 corresponding to the upper half of X / 2 [mm] within the range X [mm] maintaining 1900 °C or higher, which is the processing temperature of quartz glass, can be increased. The relationship between the volume B and the product A [mm 3 of the square of the average value of the outer diameter of the effective portion of the glass base material and the X satisfies B≧ 0.2A + 700,000, and a glass rod with a good shape can be obtained. In the case of a small-diameter glass base material where the effective portion of the characteristics is less than 150 mm, the distance from the end 16 of the effective portion of the characteristics to the welding position may be shorter than X / 2 [mm]. In that case, as described above, it is advisable to increase the outer diameter near the lower end of the hanging dummy and weld it to the tip of the glass base material to increase the volume B.

[0020] In the above case, the entire hanging dummy 13 may be made of a thick-diameter quartz glass rod. However, by combining a thin-diameter quartz glass rod and a thick-diameter quartz glass rod, it is possible to achieve cost reduction by reducing the amount of material used for the hanging dummy and simplifying handling by reducing weight. The joining of the thin-diameter quartz glass rod and the thick-diameter quartz glass rod can be easily achieved by welding with a glass lathe. Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples at all.

Example

[0021] For stretching, a large-diameter glass base material with a straight body length of 2200 mm and tapered part lengths of 500 mm at both ends was prepared, where the straight body was the characteristic effective part and the tapered parts at both ends were the characteristic defective parts, and stretching was performed with a target diameter of 150 mm. This glass base material was manufactured by depositing and sintering soot on a target in which dummy rods were connected to both ends of a core member by the OVD method. The tapered part including the opaque glass part was cut at a position 170 mm from the joint of the core rod and the dummy rod of the glass base material toward the dummy rod side. The transparent glass tapered part without the opaque glass part was cut at the tip, and a quartz glass rod was welded to the cut surface as a hanging dummy with a glass lathe. The average outer diameter of the characteristic effective part of the glass base material varies slightly depending on the deterioration state of the furnace material and the heating conditions of the sintering furnace of the porous glass base material. The average outer diameter of the characteristic effective part was calculated by averaging the outer diameters obtained by scanning at 1 mm intervals in the longitudinal direction using a laser outer diameter measuring instrument LDM-306H manufactured by Takikawa Engineering Co., Ltd.

[0022] The stretching device shown in FIG. 1 was used, and the top chamber 4 was made of quartz glass. The glass base material was set in the stretching device with the transparent glass side facing up and the side including the opaque glass part facing down, and the hanging dummy was connected to the feeding mechanism. The glass base material was set so that the lower end was at the same height as the center of the heater, heated from room temperature to 2100 °C at 40 °C / min, and then stretching was performed after welding the take-up dummy.

[0023] In the above stretching device, the in-furnace temperature distribution was measured by increasing the temperature when there was no preform, using an OMEGA TM ultra-high temperature thermocouple probe. As a result, it was measured that the longitudinal length X of the device maintaining 1900 °C or higher in the furnace was 340 mm. For the glass base material, those with various changes in the B volume from the end of the characteristic effective part on the hanging dummy connection side to the position of X / 2 = 170 mm were prepared and used as examples and comparative examples. The change in the B volume was carried out by adjusting the cutting position of the tip of the transparent glass taper part and the outer diameter of the welding end of the hanging dummy to the glass base material.

[0024] [Comparative Example 1] A glass base material with an average outer diameter of the characteristic effective part of 186 mm was stretched under the condition that the longitudinal length X / 2 of the device maintaining 1900 °C or higher in the furnace was 170 mm, with the outer diameter of the welding end side of the hanging dummy of the glass base material being 40 mm and the outer diameter of the connection side of the feeding mechanism of the hanging dummy being 40 mm. In this case, the volume B between the end of the characteristic effective part and the position of X / 2 [mm] was 2,336,947 mm 3 , and the product A of the square of the average outer diameter of the characteristic effective part and the X was 11,762,640 mm 3 was. [Comparative Example 2] A glass base material with an average outer diameter of the characteristic effective part of 179 mm was stretched under the condition that the longitudinal length X / 2 of the device maintaining 1900 °C or higher in the furnace was 170 mm, with the outer diameter of the welding end side of the hanging dummy of the glass base material being 40 mm and the outer diameter of the connection side of the feeding mechanism of the hanging dummy being 40 mm. In this case, the volume B between the end of the characteristic effective part and the position of X / 2 [mm] was 995,488 mm 3 , and the product A of the square of the average outer diameter of the characteristic effective part and the X was 10,893,940 mm 3 was. [Comparative Example 3] A glass base material with an average outer diameter of the effective portion of 160 mm was stretched under the condition that the length X / 2 in the longitudinal direction of the device maintaining 1900 °C or higher in the furnace was 170 mm, with the outer diameter of the welded end side of the suspension dummy of the glass base material being 40 mm and the outer diameter of the feed mechanism connection side of the suspension dummy being 40 mm. In this case, the volume B between the positions of X / 2 [mm] from the end of the effective portion was 1,784,554 mm 3 , and the product A of the square of the average outer diameter of the effective portion and the X was 8,704,000 mm 3 .

[0025] [Example 1] A glass base material with an average outer diameter of the effective portion of 185 mm was stretched under the condition that the length X / 2 in the longitudinal direction of the device maintaining 1900 °C or higher in the furnace was 170 mm, with the outer diameter of the welded end side of the suspension dummy of the glass base material being 40 mm and the outer diameter of the feed mechanism connection side of the suspension dummy being 40 mm. In this case, the volume B between the positions of X / 2 [mm] from the end of the effective portion was 3,593,435 mm 3 , and the product A of the square of the average outer diameter of the effective portion and the X was 11,636,500 mm 3 . [Example 2] A glass base material with an average outer diameter of the effective portion of 186 mm was stretched under the condition that the length X / 2 in the longitudinal direction of the device maintaining 1900 °C or higher in the furnace was 170 mm, with the outer diameter of the welded end side of the suspension dummy of the glass base material being 40 mm and the outer diameter of the feed mechanism connection side of the suspension dummy being 40 mm. In this case, the volume B between the positions of X / 2 [mm] from the end of the effective portion was 3,930,132 mm 3 , and the product A of the square of the average outer diameter of the effective portion and the X was 11,762,640 mm 3 . [Example 3] A glass base material with an average outer diameter of the effective portion of 179 mm was stretched under the condition that the length X / 2 in the longitudinal direction of the device maintaining 1900 °C or higher in the furnace was 170 mm, with the outer diameter of the welded end side of the suspension dummy of the glass base material being 40 mm and the outer diameter of the feed mechanism connection side of the suspension dummy being 40 mm. In this case, the volume B between the positions of X / 2 [mm] from the end of the effective portion was 3,549,456 mm 3The product A of the square of the average outer diameter of the effective characteristic part and the X is 10,893,940 mm 3 It was.

[0026] [Example 4] A glass base material with an average outer diameter of the effective characteristic part of 178 mm was stretched under the condition that the longitudinal length X / 2 of the device maintaining 1900 °C or higher in the furnace was 170 mm, with the outer diameter of the welded end side of the suspension dummy of the glass base material being 40 mm and the outer diameter of the feed mechanism connection side of the suspension dummy being 40 mm. In this case, the volume B between the positions of X / 2 [mm] from the end of the effective characteristic part was 3,548,749 mm 3 The product A of the square of the average outer diameter of the effective characteristic part and the X is 10,772,560 mm 3 It was. [Example 5] A glass base material with an average outer diameter of the effective characteristic part of 163 mm was stretched under the condition that the longitudinal length X / 2 of the device maintaining 1900 °C or higher in the furnace was 170 mm, with the outer diameter of the welded end side of the suspension dummy of the glass base material being 40 mm and the outer diameter of the feed mechanism connection side of the suspension dummy being 40 mm. In this case, the volume B between the positions of X / 2 [mm] from the end of the effective characteristic part was 2,629,018 mm 3 The product A of the square of the average outer diameter of the effective characteristic part and the X is 9,033,460 mm 3 It was. [Example 6] A glass base material with an average outer diameter of the effective characteristic part of 164 mm was stretched under the condition that the longitudinal length X / 2 of the device maintaining 1900 °C or higher in the furnace was 170 mm, with the outer diameter of the welded end side of the suspension dummy of the glass base material being 40 mm and the outer diameter of the feed mechanism connection side of the suspension dummy being 40 mm. In this case, the volume B between the positions of X / 2 [mm] from the end of the effective characteristic part was 2,999,019 mm 3 The product A of the square of the average outer diameter of the effective characteristic part and the X is 9,144,640 mm 3 It was.

[0027] With the configurations of Comparative Examples 1 to 3 and Examples 1 to 6 above, the glass base material was stretched and the shape of the effective characteristic part after stretching was evaluated. The results are summarized in Table 1. ○ in the evaluation means good, and × means there is a problem. Furthermore, the results of these evaluations were plotted and shown in FIG. 4. Here, the horizontal axis is the product A of the square of the average outer diameter of the characteristic effective part and X, and the vertical axis is the volume B between the positions of X / 2 [mm] from the end of the characteristic effective part.

[0028]

Table 1

[0029] In the case of Comparative Examples 1 to 3, all of the volume B was smaller than the value obtained by calculating 0.2A + 700,000. At the end of stretching, the hanging dummy welding end was heated and caused an unintended large deformation. The outer diameter variation of the characteristic effective part was as large as -2.1 mm to +2.6 mm compared to the target stretching diameter, and the amount of bending was also as large as 2.3 mm / m to 3.4 mm / m. In the case of Examples 1 to 6, all of the volume B was larger than the value obtained by calculating 0.2A + 700,000. At the end of stretching, the hanging dummy welding end did not undergo excessive deformation. The outer diameter variation of the product part was as small as -0.2 mm to +0.7 mm compared to the target stretching diameter, and the amount of bending was also suppressed to as small as 0.4 mm / m to 1.3 mm / m. As shown in FIG. 2, Examples 1 to 6 are cases where the distance from the end of the characteristic effective part on the side where the hanging dummy is welded to the welding position of the hanging dummy and the glass base material of the glass base material is longer than X / 2 [mm]. As a result, when the volume B between the end of the characteristic effective part and the position of X / 2 [mm] satisfies the relationship of B ≧ 0.2A + 700,000 with respect to the product A of the square of the average outer diameter of the characteristic effective part and the X, it can be seen that good evaluations are obtained. In FIG. 4, B = 0.2A + 700,000 is shown by a broken line. Examples 1 to 6, for which the evaluations were good, are plotted above the Y-axis from this broken line, and Comparative Examples 1 to 3, for which there were problems with the evaluations, are plotted below the Y-axis from this broken line.

[0030] Next, stretching was performed on relatively small glass base materials with a diameter of less than 150 mm under changed conditions. The glass base material used had a straight body part with a length of 800 mm and tapered parts at both ends with a length of 100 mm. The straight body part was the characteristic effective part, and the tapered parts at both ends were the characteristic defective parts. Drawing was performed with a target drawn diameter of 50 mm. This glass base material was manufactured by sintering a porous glass base material deposited by the VAD method. The tapered part including the opaque glass part was cut at a position 50 mm from the joint of the core rod and the dummy rod of the glass base material toward the dummy rod side. The transparent glass tapered part side without the opaque glass part had its tip cut, and a suspension dummy was welded to the cut surface using a glass lathe. The average outer diameter of the characteristic effective part of the glass base material varies slightly depending on the deterioration state of the furnace lining of the sintering furnace for the porous glass base material and the heating conditions. The average outer diameter of the characteristic effective part was calculated by averaging the outer diameters obtained by scanning at 1 mm intervals in the longitudinal direction using a laser outer diameter measuring instrument LDM-306H manufactured by Takikawa Engineering Co., Ltd.

[0031] The drawing device shown in FIG. 1 was used, and the top chamber 4 was made of quartz glass. The glass base material was set in the drawing device with the transparent glass side up and the side including the opaque glass part down, connecting the suspension dummy to the feeding mechanism. It was set so that the lower end of the glass base material was at the same height as the center of the heater, heated from room temperature to 2100 °C at 40 °C / min, and then drawing was performed after welding the take-out dummy.

[0032] In the above drawing device, the in-furnace temperature distribution was measured by heating and measuring using an OMEGA TM ultra-high temperature thermocouple probe when there was no preform. As a result, it was measured that the longitudinal length X of the device maintaining 1900 °C or higher in the furnace was 300 mm. Drawing was performed while changing the volume B from the end of the characteristic effective part on the suspension dummy connection side of the glass base material to a position of X / 2 = 150 mm. The change in the volume B was performed by adjusting the tip cutting position of the transparent glass tapered part and the outer diameter of the welding end of the suspension dummy to the glass base material. The detailed conditions are shown below.

[0033] [Comparative Example 4] A glass base material with an average outer diameter of the effective portion of 107 mm was stretched under the condition that the longitudinal length X / 2 of the apparatus maintaining 1900 °C or higher in the furnace was 150 mm, with the distance from the end of the effective portion to the welded end of the hanging dummy being 50 mm, the outer diameter of the welded end side of the hanging dummy being 40 mm, and the outer diameter of the feed mechanism connection side of the hanging dummy being 40 mm. In this case, the volume B between the end of the effective portion and the position of X / 2 [mm] was 213,628 mm 3 , and the product A of the square of the average outer diameter of the effective portion and the X was 3,434,700 mm 3 was obtained. [Comparative Example 5] A glass base material with an average outer diameter of the effective portion of 103 mm was stretched under the condition that the longitudinal length X / 2 of the apparatus maintaining 1900 °C or higher in the furnace was 150 mm, with the distance from the end of the effective portion to the welded end of the hanging dummy being 50 mm, the outer diameter of the welded end side of the hanging dummy being 80 mm, and the outer diameter of the feed mechanism connection side of the hanging dummy being 40 mm. In this case, the volume B between the end of the effective portion and the position of X / 2 [mm] was 901,718 mm 3 , and the product A of the square of the average outer diameter of the effective portion and the X was 3,182,700 mm 3 was obtained. [Comparative Example 6] A glass base material with an average outer diameter of the effective portion of 102 mm was stretched under the condition that the longitudinal length X / 2 of the apparatus maintaining 1900 °C or higher in the furnace was 150 mm, with the distance from the end of the effective portion to the welded end of the hanging dummy being 50 mm, the outer diameter of the welded end side of the hanging dummy being 90 mm, and the outer diameter of the feed mechanism connection side of the hanging dummy being 40 mm. In this case, the volume B between the end of the effective portion and the position of X / 2 [mm] was 1,127,867 mm 3 , and the product A of the square of the average outer diameter of the effective portion and the X was 3,121,200 mm 3 was obtained.

[0034] [Example 7] A glass base material with an average outer diameter of the effective part of 104 mm was suspended with the outer diameter of the welded end of the suspension dummy thickened to 120 mm and the distance from the end of the effective part to the welded end of the suspension dummy set to 50 mm under the condition that the longitudinal length X / 2 of the device maintaining 1900 °C or higher in the furnace was 150 mm, and the outer diameter of the connection side of the feed mechanism of the suspension dummy was set to 120 mm for stretching. In this case, the volume B between the end of the effective part and the position of X / 2 [mm] was 1,841,651 mm 3 , and the product A of the square of the average outer diameter of the effective part and the X was 3,244,800 mm 3 was obtained. [Example 8] A glass base material with an average outer diameter of the effective part of 102 mm was suspended with the outer diameter of the welded end of the suspension dummy thickened to 120 mm and the distance from the end of the effective part to the welded end of the suspension dummy set to 50 mm under the condition that the longitudinal length X / 2 of the device maintaining 1900 °C or higher in the furnace was 150 mm, and the outer diameter of the connection side of the feed mechanism of the suspension dummy was set to 40 mm for stretching. In this case, the volume B between the end of the effective part and the position of X / 2 [mm] was 1,719,374 mm 3 , and the product A of the square of the average outer diameter of the effective part and the X was 3,121,200 mm 3 was obtained. [Example 9] A glass base material with an average outer diameter of the effective part of 106 mm was suspended with the outer diameter of the welded end of the suspension dummy thickened to 140 mm and the distance from the end of the effective part to the welded end of the suspension dummy set to 50 mm under the condition that the longitudinal length X / 2 of the device maintaining 1900 °C or higher in the furnace was 150 mm, and the outer diameter of the connection side of the feed mechanism of the suspension dummy was set to 40 mm for stretching. In this case, the volume B between the end of the effective part and the position of X / 2 [mm] was 2,533,334 mm 3 , and the product A of the square of the average outer diameter of the effective part and the X was 3,370,800 mm 3 was obtained.

[0035] With the configurations of Comparative Examples 4 to 6 and Examples 7 to 9 above, the glass base material was stretched and the shape of the effective part after stretching was evaluated. The results are summarized in Table 2. ○ in the evaluation means good, and × means there is a problem. Furthermore, the results of these evaluations are plotted and shown in FIG. 5. Here, the horizontal axis is the product A of the square of the average outer diameter of the characteristic effective part and X, and the vertical axis is the volume B between the positions X / 2 [mm] from the end of the characteristic effective part.

[0036]

Table 2

[0037] Since the object of the above study is a relatively small glass base material manufactured by the VAD method, the distance from the end of the characteristic effective part to the welded end of the suspension dummy is shorter than the vertical length X / 2 = 150 mm of the device that maintains 1900 °C or higher in the furnace. Therefore, if the outer diameter of the welded end side of the suspension dummy is thin, the volume B between the positions X / 2 [mm] from the end of the characteristic effective part is small, and at the end of stretching, the welded end of the suspension dummy is heated and an unintended large deformation occurs. Therefore, in Comparative Examples 4 to 6, the amount of outer diameter variation of the characteristic effective part was as large as -5.1 mm to +16.2 mm compared to the target stretching diameter, and the amount of bending was also as large as 3.5 mm / m to 7.1 mm / m. Therefore, in the case of Examples 7 to 9, as shown in FIG. 3, by increasing the outer diameter of the welded end side of the suspension dummy and increasing the volume B between the end of the characteristic effective part and the position X / 2 [mm], it is possible to prevent the deformation of the welded end of the suspension dummy at the end of stretching, and the amount of outer diameter variation of the product part is as small as -0.4 mm to +0.3 mm compared to the target stretching diameter, and the amount of bending is also suppressed as small as 0.2 mm / m to 0.6 mm / m. In Examples 8 and 9, the outer diameter of the side connected to the feeding mechanism of the suspension dummy is thinner than the outer diameter of the welded end side. Since the outer diameter of the welded end side of the suspension dummy affects the volume B between the end of the characteristic effective part and the position X / 2 [mm], by reducing the outer diameter of the side connected to the feeding mechanism, the cost of the entire suspension dummy can be reduced.

[0038] In FIG. 5, B = 0.2A + 700,000 is drawn as a broken line. Examples 7 to 9 in which the above evaluation was good are plotted above the Y-axis from this broken line, and Comparative Examples 4 to 6 in which there were problems with the above evaluation are plotted below the Y-axis from this broken line. From these, it can be seen that when the volume B between the characteristic effective part end and the position of X / 2 [mm] has a relationship of B ≧ 0.2A + 700,000 with the product A of the square of the average outer diameter of the characteristic effective part and the X, a good evaluation can be obtained.

[0039] Note that the present invention is not limited to the above-described embodiments, and can be freely deformed, improved, etc. as appropriate.

Explanation of Reference Numerals

[0040] 1: Heater 2: Heat Insulating Material 3: Water Cooling Chamber 4: Top Chamber 5: Suspension Shaft 6: Connection Fixture 7: Feeding Mechanism 8: Lower Gas Seal 9: Guide Roller 10: Take-up Roller (upper) 11: Take-up Roller (lower) 12: Glass Base Material 13: Suspension Dummy 14: Take-up Dummy 15: Tapered Opaque Part 16: Characteristic Effective Part End 17: Range corresponding to X / 2 [mm] in the upper half of the range X [mm] maintaining 1900 °C or higher

Claims

1. When stretching a glass base material having characteristic defective portions at both ends and a characteristic effective portion between the two characteristic defective portions into a glass rod with a smaller diameter, prior to the stretching, a suspension dummy is welded to the glass base material, the suspension dummy is connected to a feeding mechanism, and the glass base material is inserted into a heating furnace of a stretching device from below the glass base material to stretch at least the characteristic effective portion. A stretching method, wherein when the heating furnace of the stretching device is set to a normal temperature during glass base material stretching, the vertical length of the device in which the temperature in the heating furnace is maintained at 1900 °C or higher is X [mm], the volume B [mm ３ obtained by integrating the range from the end of the characteristic effective portion of the glass base material with the suspension dummy welded before stretching to X / 2 [mm] on the suspension dummy side, and the product A [mm ３ of the square of the average value of the outer diameters in the range of the characteristic effective portion of the glass base material and X satisfy the following formula (1). A method for stretching a glass base material, characterized in that: [Formula 1] B ≧ 0.2A + 700,000

2. 2. The method for elongating a glass base material according to claim 1, wherein the distance from the end of the characteristic effective portion of the glass base material on the side to which the hanging dummy is welded to the welding position of the hanging dummy and the glass base material is longer than X / 2 [mm].

3. 2. The method for elongating a glass base material according to claim 1, wherein the distance from the end of the characteristic effective portion of the glass base material on the side to which the hanging dummy is welded to the welding position of the hanging dummy and the glass base material is shorter than X / 2 [mm].

4. 4. The method for elongating a glass base material according to claim 3, wherein the outer diameter of the hanging dummy is greater than the outer diameter of the hanging shaft side range above the end of the characteristic effective portion of the glass base material up to X / 2 mm.

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