Method for drawing glass preform for optical fiber

Abstract: A glass preform for an optical fiber is effectively drawn by connecting dummy rods to both ends of the glass preform and connecting the dummy rods to fitting members.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for drawing a glass preform, in particular, a glass preform having a large size to produce an optical fiber. Description of the Related Art A method for producing an optical fiber is explained by making reference to a vapor-phase axial deposition (VAD) method as an example. In the VAD method, an oxyhydrogen flame is generated using a concentric multi-port burner. Into this flame, a glass-forming raw material such as SiCl 4 , SiHCl 3 , etc. and optionally a doping raw material such as GeCl 4 , POCl 3 , BCl 3 , etc. are charged to form fine particles of glass (glass soot) through a flame hydrolysis reaction or an oxidation reaction, and the glass soot is deposited on a tip end or peripheral surface of a starting rod to form a glass soot-deposited body. Then, the glass soot-deposited body is vitrified in a heating furnace to obtain a transparent glass preform from which an optical fiber is produced. Now, a method for producing an optical fiber utilizing the above VAD method will be explained. First, by the above VAD method, a core-forming glass preform consisting of a core portion or a core portion and a part of a cladding portion is synthesized on a tip end of a starting rod, dehydrated with a halogen-containing gas and vitrified. Thereafter, the transparent glass rod (core-forming preform) is used as a starting member , and around a peripheral surface of the member , a glass soot is deposited using a burner for synthesizing fine particles of glass, which generates a flame . A composite of the glass rod and the glass soot is again vitrified in the sintering furnace to obtain a glass preform for use in the production of the optical fiber. Thereafter, to the both ends of the glass preform, dummy rods are connected by welding, and a surface of the preform is flame abrased. The glass preform is then heated by an electric furnace or an oxyhydrogen flame and drawn to obtain a preform rod having a desired diameter. Form the preform rod, an optical fiber is drawn. Hitherto, when the glass soot for the cladding portion is deposited on the glass rod consisting of the core portion or the core portion and a part of the cladding portion, to both ends of the glass rod , dummy (extension) rods and are welded, and the upper dummy rod is held by a main seed rod directly or through a fitting part . The dummy rods and , in particular, the lower dummy rod are designed so that a tapered part of a growing surface which is unavoidable in the VAD method is not formed on the glass rod for the core portion, and the glass soot is deposited up to or near the end of the dummy rods and , whereby a composite is produced. To vitrify the composite in a sintering furnace and draw it, further dummy rods 19 are welded to the ends of the preform as shown in Fig. 1. The upper dummy rod may be once cut away and rewelded. The preform 10 having the dummy rods 19 at both ends is heated and drawn in the electric furnace. In the above described conventional method, the dummy rods are welded using a horizontal glass lathe after vitrification and before drawing. As the preform is enlarged and its weight increases, it is difficult to support it by a single dummy rod. Therefore, it is necessary to hold an effective part, that is a part having a uniform diameter to be used for the production of the optical fiber. When the effective part of the glass preform is held or contacted to a foreign material, the glass surface may be flawed so that the optical fiber having large strength is not produced, or contamination with impurities increases transmission loss of the produced optical fiber. The welding manner of the dummy rods to the glass preform is one of the problems in the production of the large-size glass preform. For drawing the preform or preform rod, Japanese Patent Publication No. 167236/1987 discloses the use of a drawing apparatus which comprises a heating furnace for heating a glass preform, having a vertically extending opening, an upper chuck which holds one of the dummy rods and is provided above the heating furnace, and a lower chuck which holds the other of the dummy rods and is provided below the heating furnace, and a method for drawing the preform using such apparatus, which method comprises moving one of the chucks away from the heating furnace while moving the other of the chucks towards the heating furnace while heating the preform road in the heating furnace. When the long dummy rods are welded to and integrated with the glass preform, an overall length is very much increased so that its handling is difficult. When a fitting member is attached to the dummy rod and the fitting member is held by the chuck, a length of the dummy rod can be shortened, while the glass preform tends to be bent during drawing. JP-A-3-126633 discloses the production of optical fiber base material in which the dummy rods are connected to both ends of the target rod and the glass fine particles generation unit is traversed in the axial direction by e.g. a traverse unit while the target rod is revolved to have glass fine particles deposited around the target rod to make a deposited layer of glass fine particles, and the generation amount of glass fine particles is adjusted in the vicinity of both ends of traverse distance according to the diameter of the dummy rod. SUMMARY OF THE INVENTION The object of the present invention is to provide a method for drawing a glass preform for an optical fiber in which no long dummy rod has to be integrated with the glass preform and the glass preform is not bent during drawing. According to one aspect of the present invention, there is provided a method according to claim 1. According to another aspect of the present invention, there is provided a method according to claim 5. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a conventional glass preform having dummy rods, Fig. 2 illustrates the method for drawing a glass preform according to the present invention, Fig. 3 shows an example of a method for fitting the dummy rod to the fitting member, and Figs. 4 to 7 illustrate examples of the method for drawing a glass preform according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The method of the present invention will be explained by making reference to the accompanying drawings. According to this embodiment of the present invention, as shown in Fig. 2, a length of the dummy rod 2 or 3 can be shortened by replacing a part of the dummy rod by an upper or lower fitting member 12 or 13, whereby the handling property of the preform to which the dummy rods are connected is improved. When the fitting members are connected to the both ends of the preform, the extended body tends to bend. The present invention can prevent this bending. In this embodiment of the present invention, as shown in Fig. 2, glass rods having end structures such that the dummy rods can be fit to the glass rods are provided as upper and lower fitting members 12 and 13. To the upper and lower ends of the core rod 1, the upper and lower dummy rods 2 and 3 are welded, and the dummy rods have end structures such that they can be fit to the ends of the upper and lower fitting members 12 and 13, respectively. The dummy rods 2 and 3 which are welded to the upper and lower ends of the core rod 1 are inserted in respective fitting parts 14 and 15, and each pin made of a heat resistant material is inserted to a pair of through holes 4 and 4' and a pair of through holes 24 and 24' as shown in Fig. 3(a), whereby the upper and lower dummy rods are fixed to the upper and lower fitting members 12 and 13, respectively. Then, the fitting members 12 and 13 are held by upper and lower chucks 17 and 18 to fix the core rod 1 in a drawing apparatus and heated and drawn. The fixing method of the dummy rod to the fitting member will be explained further in detail. As shown in Fig. 3, the dummy rod 2 or 3 is inserted in the fitting part 14 or 15 of the upper or lower fitting member 12 or 13 and fixed by inserting the heat resistant pin (not shown) in the through holes 4 and 4', or 24 and 24'. In this case, a difference (clearance) between the inner diameter of the fitting member and the outer diameter of the dummy rod is from 0.2 to 4 mm, and a length of the part of the dummy rod to be inserted in the fitting member is from 50 to 150 mm. To prevent the thermal deformation of the fitting members during drawing, the length of the dummy rods is selected so that the temperature of the fitting members does not exceed 1200°C. As the difference between the inner diameter of the fitting member and the outer diameter of the dummy rod is smaller, the center axis of the fitting member is better aligned with the center axis of the dummy rod. But, when this difference is less than 0.2 mm, it is difficult to insert the dummy rod in the fitting member unless they are precisely processed. When this difference is larger than 4 mm, the center axis of the dummy rod is not well aligned with that of the fitting member so that the drawn preform tends to be bent. The center axes of the fitting member and the dummy rod are more precisely aligned with each other by the following manner: The dummy rod is inserted in the fitting member and the heat resistant cylindrical pin is inserted in the through holes of the dummy rod and the fitting member. Thereafter, as shown in Fig. 3(b), bolts 22 are tightened to fix the dummy rod more rigidly to the fitting member. The bolts 22 are preinstalled in the lower fitting member 13 and directed to the center axis of the lower fitting member 13. Though not shown, the upper dummy rod can be fixed to the upper fitting member in the same manner. The bolts are preferably made of carbon or alumina. The number of the bolts 22 is preferably from 2 to 8. Since the dummy rod is firmly fixed to the fitting member with the bolts, the center axis of the dummy rod is precisely aligned to that of the fitting member during the drawing of preform. The use of the bolts is important for fixing the lower dummy rod to the lower fitting member. That is, since the lower dummy rod and the lower fitting member pull each other in the early stage of drawing, their center axes are easily aligned each other. As the drawing of the glass preform progresses, the upper part of the glass preform is softened, all the weight of the part below the softened part is applied to the lower fitting member. As the result, the lower dummy rod and the lower fitting member push each other. In this case, if the lower dummy rod is not firmly fixed to the lower fitting member, their center axes deviate from one another so that the drawn preform bends. As explained above, when the glass preform is drawn using the fitting members, it is important to firmly fix the dummy rod to the fitting member by the use of the difference between the inner diameter of the fitting member and the outer diameter of the dummy rod or the bolts. Further, the fitting member should not be thermally deformed. The fitting member is preferably made of quartz glass in view of the prevention of contamination of impurities. To prevent the deformation of the fitting member, the length of the dummy rod should be selected so that the temperature of the fitting member does not exceed 1200°C as explained already. According to this embodiment of the present invention, a part of the dummy rod is replaced by the fitting member so that the length of the dummy rod can be shortened as shown in Fig. 2(b). This embodiment of the present invention will be illustrated by the following Examples further in detail. Example 1 This Example will be explained by making reference to Figs. 2, and 3. (1) To both end of a core rod consisting of a core portion and a part of a cladding portion, dummy rods each having a length of 300 mm were connected. Glass soot was deposited on the peripheral surface of the core rod and vitrified to obtain a glass preform 1 having an outer diameter of 58 mm and a length of 300 mm. The dummy rods 2 and 3 had, at their ends, through holes each having a diameter of 4 mm in a direction perpendicular to the center axes of the dummy rods. (2) An apparatus for drawing the glass preform comprised a heating furnace 23 (electric resistance furnace) and chucks 17 and 18 (Fig. 2(a)). The chucks 17 and 18 vertically moved independently. To the upper chuck 17, the fitting member 12 having a length of 500 mm was fixed. The glass preform 1 was fixed to the fitting member 12 by inserting the dummy rod 2 in the fitting member 12 and inserting the alumina pin in the through holes 4 and 4 (Fig. 2(b) and Fig. 3(a)). A length of the part of the dummy rod 2 inserted in the fitting member 12 was 60 mm. The inner diameter of the depressed area of the fitting member 6 was 30.5 mm, while the outer diameter of the upper dummy rod 2 was 30.0 mm. Thereafter, the glass preform was passed through the heating furnace 23 by lowering the upper chuck 17, and the lower part of the lower dummy rod 3 protruded beyond the lower end of the furnace 23 (Fig. 2(b)). The end of the lower dummy rod 3 was inserted in the lower fitting member 13, and the alumina pin was inserted in their through holes 24 and 24' to fix the lower dummy rod 3 to the lower fitting member 13. Then, the four carbon bolts 22 were tightened up (of. Fig. 3(b)). The inner diameter of the depressed area 15 of the lower fitting member 13 was 30.5 mm, while the outer diameter of the lower dummy rod was 30.0 mm. A length of the part of the lower dummy rod inserted in the depressed area 15 of the fitting member was 100 mm. (3) After fixing the lower fitting member 13, the upper chuck 17 was lifted up so that the lower end of the preform 1 was adjusted at the level of the heater 11. Then, the lower fitting member 13 was held by the lower chuck 18 (Fig. 2(c)). After the preform 1 was thoroughly heated, the upper chuck 17 was lowered at a rate of 10 mm/min. while the lower chuck 18 was lowered at a rate of 54 mm/min. (Fig. 2(d)). When the upper chuck was lowered by a distance of 300 mm, the drawing was finished (Fig. 2(e)). In the above lowering step, the heater temperature was kept at 1950°C, and the maximum temperature of the fitting member reached 1000°C. In Fig. 2, numerals 26 and 28 stand for a shielding plate and a lower lid, respectively. By the above method, a glass preform for an optical fiber having an outer diameter of 25 mm and a length of 1.6 m which was not bent was obtained. Comparative Example 1 In the same manner as in Example 1 except that a lower dummy rod having an outer diameter of 25 mm was used, that is, a difference between the inner diameter of the fitting member and the outer diameter of the dummy rod was 5.5 mm, and no bolt was used, the glass preform was drawn. The glass preform could be drawn under the same conditions as in Example 1 in the early stage of drawing, but after 20 minutes from the start of drawing, the center axis of the lower dummy rod deviated from the center axis of the lower fitting member so that the drawn body greatly bent. Example 2 To each end of a glass preform having a length of 400 mm and an outer diameter of 60 mm, a dummy rod having an outer diameter of 30.0 mm and a length of 300 mm was welded. At the free end of each dummy rod, a through hole having an inner diameter of 4 mm had been bored in a direction perpendicular to the center axis of the dummy rod. Thereafter, the glass preform was drawn in the same manner as in Example 1. A length of the part of the dummy rod 2 inserted in the fitting member 12 was 60 mm, the inner diameter of the depressed area of the fitting member 12 was 32.5 mm and the outer diameter of the upper dummy rod 2 was 31.0 mm. The drawn body did not bend. In addition to the ceramic or alumina pin, a carbon or SiC-coated carbon pin may be used. Now, the method according to the final aspect of the present invention will be explained. This embodiment is a modification of the above described method according to the present invention. In this embodiment, an end of each of the dummy rods which are welded to the glass preform is connected to the respective fitting members. Then, each fitting member is held by a respective chuck of the drawing apparatus, and the glass preform is heated and drawn in a heating furnace in a vertical direction. At the start of drawing, each of the upper and lower fitting members is held by each of upper and lower chucks, respectively, and thereafter, a position to be held by the upper or lower chuck is changed from the upper or lower fitting member to other part, for example, the respective dummy rod, thereby a distance in which the chuck moves is shortened. In this embodiment, the glass preform is drawn in substantially the same manner as shown in Figs. 2(a) to 2(d). Then, the position to be held by the chuck is changed to the dummy rod as shown in Fig. 4(e). The change of holding position of the lower chuck from the lower fitting member to the lower dummy rod is carried out by moving the auxiliary chuck at the same speed as the lower chuck after the lower dummy rod comes out from the lower end of the furnace and holding around the central part of the lower dummy rod with the auxiliary chuck Then, the lower chuck is loosened, and the lower fitting member is disconnected from the dummy rod. Thereafter, the lower chuck is lifted up, or the auxiliary chuck is lowered while stopping the lower chuck, whereby the lower chuck is moved to the level of the lower dummy rod. While the lower chuck is moved at the same speed as the auxiliary chuck, the lower dummy rod was held by the lower chuck, and the auxiliary chuck is loosened and its movement is stopped. As the result, the drawing is continued without stopping, and the holding position of the lower chuck is changed from the lower fitting member to the lower dummy rod. To achieve such action, the auxiliary chuck should be moved at the same speed as the lower chuck. In addition, using the auxiliary chuck, it is possible to change the holding position after the completion of drawing. When the holding position of the lower chuck is changed during drawing of the glass preform, the drawn preform may be bent if the center axis of the fitting member is only slightly deviates from the center axis of the dummy rod. The use of the auxiliary chuck can prevent such bending of the drawn preform. After the completion of drawing, the drawn glass preform is to be removed from the heating (drawing) furnace. To this end, as shown in Fig. 4(f), the upper end of the drawn preform is heated and further drawn to reduce the diameter, and the thin part 16' is broken to remove the drawn preform from the furnace (Fig. 4(g)). At the end of drawing, the weight of the drawn preform is supported only by the lower chuck. If the lower chuck is loosened, the preform is not supported and gradually drops since the part remained in the furnace is softened. In such case, the auxiliary chuck supports the drawn preform advantageously. That is, according to this embodiment of the present invention, before reducing the diameter of the upper part of the drawn preform, the drawn preform is held by the auxiliary chuck as shown in Figs. 5(i), 6(g) and 7(f) to support the drawn preform by the auxiliary chuck and the lower chuck, and the both chucks are lowered at the same speed to reduce the diameter of the upper part of the drawn preform. In this step, by supporting the drawn preform by the auxiliary chuck and the lower chuck, the upper part of the drawn preform is more safely drawn than when the drawn preform is supported by the lower chuck only. In particular, when the thin part 16' is broken and the drawn preform is removed from the furnace, the drawn preform may be unstabilized if it is supported by the lower chuck only. When the drawn preform is supported by the lower chuck and also the auxiliary chuck, it is stabilized and safety of the handling is improved. The above embodiment of the present invention will be illustrated by the following Examples. Example 3 In this Example, the glass preform was drawn by the method comprising the steps (a) to (d) of Fig. 2 and the steps (e), (f) and (g) of Fig. 4. After the lower opening of the furnace 23 was closed with the lower lid 28 and an inert gas was supplied into the furnace from an inlet for supplying an inert gas (not shown), the internal temperature of the furnace was raised to a determined temperature by the heater 11. The glass preform 1 for the optical fiber to which the upper and lower dummy rods 2 and 3 each having a length of 200 mm were welded was attached to the lower end of the upper fitting member 12, and the cylindrical pin made of alumina was inserted in the through holes of the upper dummy rod 2 and the upper fitting member 12, whereby the glass preform was fixed (Fig. 2(a)). The glass preform 1 for the optical fiber had an outer diameter of 55 mm and a length of 400 mm. The glass preform 1 was inserted in the heating furnace 23, and the lower lid 28 was opened. The tip end of the lower dummy rod 3 was protruded beyond the lower opening of the furnace 23 (Fig. 2(b)). Then, the end of the lower dummy rod 3 was inserted in the lower fitting member 13 and fixed with the cylindrical pin made of alumina in the same manner as above. After the opening of the lower lid 28, the upper opening was closed by the vertically movable shielding plate 26 attached to the upper fitting member 12 so that the interior of the furnace was filled with the inter gas (Fig. 2(c)). The lower end of the glass preform 1 was adjusted at the level of the heater 11, and the lower fitting member 13 was held by the lower chuck 18. Then, the upper chuck 17 was lowered at the rate of 10 mm/min. while the lower chuck 18 was lowered at the rate of 53 mm/min. to draw the glass preform 1. When the lower dummy rod 13 came out from the furnace 23 as shown in Fig. 2(d), the upper and lower chucks 17 and 18 were stopped to suspend the drawing (Fig. 2(d)). Then, the lower chuck 18 was loosened and the lower fitting member 13 was removed. Thereafter, the lower chuck 18 was lifted up to the position of the lower dummy rod 3 and grasped the end of the lower dummy rod 3, and the drawing of the glass preform was restarted (Fig. 4(e)). When the drawing of the glass preform was further carried out and the upper end of the glass preform 1 reached the level of the heater 27, the drawing was terminated (Fig. 4(f)). Only the lower chuck 12 was further lowered to reduced the diameter of the upper end of the drawn preform (Fig. 4(g)), and the thin part 16' was broken to remove the drawn preform 16 from the furnace. By the above method, the drawn preform having an outer diameter of 24 mm and a length of 2100 mm was produced. Example 4 In this Example, the glass preform was drawn by the method comprising the steps (a) to (i) of Fig. 5. After the lower opening of the furnace 23 was closed with the lower lid 28 and an inert gas was supplied into the furnace from an inlet for supplying an inert gas (not shown), the internal temperature of the furnace was raised to a determined temperature by the heater 27. As shown in Fig. 5(a), glass preform 1 for the optical fiber to which the upper and lower dummy rods 2 and 3 were connected was attached to the lower end of the upper fitting member 12 which was fixed to the upper chuck 17. The glass preform 1 was produced by integrally welding the dummy rods each having a length of 300 mm to the both end of the glass rod consisting of the core portion and a part of the cladding portion, depositing the glass soot around the glass rod and the dummy rods, heating the composite of the glass rod and the glass soot deposit in the heating furnace to vitrify the glass soot deposit. Between the heating furnace 23 and the lower chuck 18, an auxiliary chuck 27 was provided. In the same way as in Example 3, the lower dummy rod 3 was attached to the lower fitting member 13, and the lower fitting member 13 was held by the lower chuck 18. Then, the drawing of the glass preform 1 was started (Figs. 5(b) and 5(c)). When the lower dummy rod 13 came out from the furnace 23 and the central part of the dummy rod 3 reached the level of the auxiliary chuck 27, the auxiliary chuck 27 and the lower chuck 18 were lowered at the same rate (Fig. 5(d)). While the auxiliary chuck 27 and the lower chuck 18 were lowered at the same rate, the central part of the dummy rod 3 was held by the auxiliary chuck 27, that is, the holding position was changed (Fig. 5(e)). Then, the lower chuck 18 was loosened and the lower fitting member 13 was removed. Thereafter, the lower chuck 18 was lifted up to the position of the lower end of the lower dummy rod 3 and grasped the end of the lower dummy rod 3 while lowering the auxiliary chuck 27 and the lower chuck 18 at the same rate, and the lower dummy rod 3 was held by the lower chuck 18 (Fig. 5(f)). Then, the auxiliary chuck 27 was loosened and its movement was finished (Fig. 5(g)). As shown in Fig. 5(h), when the upper part of the glass preform reached the level of the heater 11, the drawing was terminated by stopping the lowering of the upper and lower chucks 17 and 18. As shown in Fig. 5(i), the drawn preform 1 was held by the auxiliary chuck 27, and the auxiliary chuck 27 and the lower chuck 18 were lowered at the same rate to reduced the diameter of the upper end of the drawn preform 16, and the thin part 16' was broken to remove the drawn preform 16 from the furnace. By the above method, the drawn preform having an outer diameter of 24 mm and a length of 2100 mm was produced. Example 5 In the same manner as the steps (a) to (d) of Fig. 5 in Example 4, the glass preform 1 was fixed to the chucks and its drawing was started. During the drawing, the holding position by the lower chuck 13 was not changed and the drawing was carried out till the upper part of the glass preform 1 reached the level of the heater 11. Then, the lowering of the upper and lower chucks 17 and 18 was stopped, and the drawn preform was held by the auxiliary chuck 27 (Fig. 6(e)). The lower chuck 18 was loosened and the lower fitting member 13 was removed. Thereafter, the lower chuck 18 was lifted up to the position of the lower end of the lower dummy rod 3 and held the end of the lower dummy rod 3 (Fig. 6(f)). Then, in the same manner as in Example 4, the upper part of the drawn preform was further drawn to reduce its diameter, and the thin part 16' was broken to remove the drawn preform 16 from the furnace (Fig. 6(g)). By the above method, the drawn preform having an outer diameter of 24 mm and a length of 2100 mm was produced. In Examples 3, 4 and 5, when a glass preform having an outer diameter of 55 mm and a length of 400 mm is drawn to the drawn preform having an outer diameter of 24 mm and a length of 2100 mm, and each dummy rod has a length of about 200 mm to 300 mm, a height of the apparatus is about 4300 mm. When no fitting member is used, each dummy rod should have a length of about 1000 mm, so that the height of the apparatus should be about 4700 mm. Example 6 In this Example, the glass preform was drawn by the method comprising the steps (a) to (d) of Fig. 5 and the steps (e) and (f) of Fig. 7. First, to the both ends of the glass preform having an outer diameter of 60 mm and a length of 400 mm, dummy rods each having an outer diameter of 30.0 mm and a length of 300 mm were welded. A free end of each dummy rod had a through hold having an inner diameter of 4 mm. The upper and lower dummy rods 2 and 3 were inserted in the upper and lower fitting members 17 and 18, and alumina pins were inserted in the through holes of the dummy rods and the fitting members, whereby the glass preform was fixed. Further, the lower dummy rod was further fixed to the lower fitting member by tightening the four carbon bolts. The depressed area of each fitting member had an inner diameter of 30.5 mm, and an outer diameter of each dummy rod was 30.0 mm. A length of the part of the upper dummy rod 2 to be inserted in the depressed area of the upper fitting member was 60 mm, and that of the lower dummy rod 3 was 100 mm. After fixing the lower fitting member, the upper chuck 17 was lifted up so that the lower end of the glass preform 1 was adjusted at the level of the heater 27, and the lower fitting member 13 was held by the lower chuck 18 (Fig. 6(c)). The heater temperature was adjusted at 1950°C. After the glass preform 1 was well heated, the upper chuck 17 was lowered at the rate of 10 mm/min. while the lower chuck 18 was lowered at the rate of 54 mm/min. to draw the glass preform 1 (Fig. 6(d)). When the upper chuck 17 was lowered by 300 mm, the drawing was terminated (Fig. 7(e)). At this stage, the drawn preform was held by the auxiliary chuck 27 as shown in Fig. 7(e). Then, the auxiliary chuck 68 and the lower chuck 18 were lowered at the same rate to reduced the diameter of the upper end of the drawn preform as shown in Fig. 7(f), and the thin part 16' was broken to remove the drawn preform 16 from the furnace. The maximum temperature of the fitting members was 1000°C. By the above method, the drawn preform which did not bend was produced.

Claims: A method for drawing a glass preform (1) for an optical fiber comprising the steps of: integrally welding two dummy rods (2, 3) with their first ends to the ends of the glass preform (1) and connecting the second ends of the dummy rods (2, 3) to a respective fitting member (12, 13), holding each of said fitting members with a respective chuck (17, 18), and heating and drawing said glass preform in a heating furnace (23), wherein the second end part of each dummy rod (2, 3) has a through hole (4, 24) which is formed across the center axis of said dummy rod and in which a pin is to be inserted, each dummy rod (2, 3) is connected with said respective fitting member (12, 13) having a depressed area (14, 15) in which said dummy rod is inserted and a through hole (4', 24) which is provided at a position corresponding to said through hole (4, 24) of said dummy rod and crosses the center axis of said fitting member at right angles, a heat-resistant pin is inserted in both through holes of said dummy rod and said fitting member to hold both ends of said glass preform by said fitting members, the difference between the inner diameter of said depressed area and the outer diameter of said dummy rod is in a range between 0.2 mm and 4 mm, and the length of that part of said dummy rod to be inserted in said depressed area is in a range between 50 mm and 150 mm. The method according to claim 1, characterized in that said fitting member (12, 13) having the depressed area (14, 15) and the through hole (4', 24') has 2 to 8 bolts (23) which are tightened from the periphery of said fitting member to the center axis of said fitting member, and said dummy rod (2, 3) is fixed to said fitting member by said pin and said bolts. The method according to claim 1, characterized in that the temperature of said fitting member (12, 13) is kept at a temperature not higher than 1200 °C. The method according to claim 1, characterized in that the glass preform (1) is vertically held and drawn in a vertical direction. A method for drawing a glass preform for an optical fiber comprising the steps of: integrally welding two dummy rods (2, 3) with their first ends to the ends of the glass preform (1) and connecting the second ends of the dummy rods to a respective fitting member (12, 13), holding each of said fitting members by a respective chuck (17, 18), and heating and drawing said glass preform in a vertical direction in a heating furnace (23), wherein after drawing has started, said lower chuck releases said lower fitting member and then holds another part of the combination of the preform and the dummy rods, for example, the dummy rod. The method according to claim 5, characterized in that at the final stage of drawing said lower chuck releases said lower fitting member and is then moved to said lower dummy rod to hold it, said lower fitting member is removed, said lower chuck holding said lower dummy rod is further moved to draw an upper end of said drawn preform to reduce its diameter, and a thinned part (16') of said drawn preform is broken to remove said drawn preform from said upper dummy rod. The method according to claim 5, characterized in that the step wherein said upper or lower chuck releases said upper or lower fitting member and then holds another part comprises drawing said glass preform using an auxiliary chuck (27) which is provided between said heating furnace and said lower chuck (18) and moves independently from said lower chuck and finishing drawing. The method according to claim 7, characterized in that at the final stage of drawing said lower chuck (18) releases said lower fitting member (13) and is then moved to said lower dummy rod to hold it, said lower fitting member is removed, said lower chuck holding said lower dummy rod is further moved to draw an upper end of said drawn preform to reduce its diameter, and a thinned part (16') of said drawn preform is broken to remove said drawn preform from said upper dummy rod. The method according to claim 7, characterized in that while said auxiliary chuck (27) is lowered at the same rate as said lower chuck (18), a middle part of said lower dummy rod (3) is held by said auxiliary chuck during drawing; then, said lower chuck is loosened and said lower fitting member is removed; said lower chuck is brought relatively close to said auxiliary chuck; while said lower chuck and said auxiliary chuck are lowered at the same rate, the lower end of the lower dummy rod is held by said lower chuck; said auxiliary chuck is loosened; whereby said lower chuck releases said lower fitting member to said lower dummy rod without interrupting drawing. The method according to claim 7, characterized in that at the final stage of drawing the upper part of said drawn preform is held by said auxiliary chuck (27), said auxiliary chuck and said lower chuck are lowered at the same rate to further draw an upper end of said drawn preform to reduce its diameter, and a thinned part (16') of said drawn preform is broken to remove said drawn preform from said upper dummy rod.

Inventors: DANZUKA TOSHIO, HOSHINO SUMIO, ITO MASUMI, MUKAI KATSUZO, OHGA YUICHI, DANZUKA, TOSHIO, HOSHINO, SUMIO, ITO, MASUMI, MUKAI, KATSUZO, OHGA, YUICHI, DANZUKA, TOSHIO, C/O YOKOHAMA WORKS, HOSHINO, SUMIO, C/O YOKOHAMA WORKS, ITO, MASUMI, C/O YOKOHAMA WORKS, Mukai, Katsuzo, c/o Yokohama Works, OHGA, YUICHI, C/O YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1993
Main classification: C03B37
IPC codes: C03B37