Apparatus for drawing a glass preform for optical fibers

Abstract: An apparatus for drawing a glass preform for an optical fiber is disclosed.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an apparatus for vertically 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, as shown in Fig. 1, the transparent glass rod (core-forming preform) is used as a starting member 1, and around a peripheral surface of the member. 1, a glass soot 9 is deposited using a burner 7 for synthesizing fine particles of glass, which generates a flame 8. A composite of the glass rod and the glass soot 9 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 1, dummy (extension) rods 2 and 20 are welded as shown in Fig. 1, and the upper dummy rod 2 is held by a main seed rod 6 directly or through a fitting part 5. The dummy rods 2 and 20, in particular, the lower dummy rod 20 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 1 for the core portion, and the glass soot 9 is deposited up to or near the end of the dummy rods 2 and 20, whereby a composite shown in Fig. 2 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. 3. 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. An apparatus for drawing a preform or preform rod is disclosed in Japanese Patent Publication No. 62-167 236, wherein the use of a drawing apparatus is described 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 rod 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, the length of the dummy rod can be shortened, while the glass preform tends to be bent during drawing. SUMMARY OF THE INVENTION The object of the present invention is to provide an and apparatus for vertically drawing a glass preform in which no long dummy rod is integrated with the glass preform and the glass preform is not bent during drawing. According to the present invention, there is provided an apparatus for vertically drawing a glass preform for an optical fiber comprising a heating furnace, upper and lower chucks which independently move in the direction of the center axis of said heating furnace, and an auxiliary chuck which is provided between said heating furnace and said lower chuck and independently moves in the direction of the center axis of said heating furnace. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows the VAD method for producing a glass preform for an optical fiber, Fig. 2 shows a glass soot preform produced by the VAD method, Fig. 3 shows a conventional glass preform having dummy rods, Figs. 4 to 6 illustrate examples of the apparatus for drawing a glass preform according to the present invention. DETAILED DESCRIPTION OF THE INVENTION In the apparatus an end of each of the dummy rods (2 and 3) which are welded to the glass preform (1) is connected to the respective fitting members (12 and 13). Then, each fitting member is held by a respective chuck (17 and 18) of the drawing apparatus, and the glass preform (1) is heated and drawn in a heating furnace (23) in a vertical direction. At the start of drawing, each of the upper and lower fitting members (12 and 13) is held by each of upper and lower chucks (17 and 18), respectively, and thereafter, a position to be held by the upper (17) or lower chuck (18) is changed from the upper (12) or lower (13) fitting member to other part, for example, the respective dummy rod (2 and 3), whereby a distance in which the chuck moves is shortened. The change of holding position of the lower chuck (18) from the lower fitting member (13) to the lower dummy rod (3) is carried out by moving an auxiliary chuck (27) at the same speed as the lower chuck (18) after the lower dummy rod (3) comes out from the lower end of the furnace and holding around the central part of the lower dummy rod (3) with the auxiliary chuck (27). Then, the lower chuck (18) is loosened, and the lower fitting member (13) is disconnected from the dummy rod (3). Thereafter, the lower chuck (18) is lifted up, or the auxiliary chuck (27) is lowered while stopping the lower chuck (18), whereby the lower chuck (18) is moved to the level of the lower dummy rod (3). While the lower chuck (18) is moved at the same speed as the auxiliary chuck (27), the lower dummy rod (3) was held by the lower chuck (18), and the auxiliary chuck (27) is loosened and its movement is stopped. As the result, the drawing is continued without stopping, and the holding position of the lower chuck (18) is changed from the lower fitting member (13) to the lower dummy rod (3). To achieve such action, the auxiliary chuck (27) should be moved at the same speed as the lower chuck (18). In addition, using the auxiliary chuck (27), it is possible to change the holding position after the completion of drawing. When the holding position of the lower chuck (18) is changed during drawing of the glass preform (1), 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 (16) is to be removed from the heating (drawing) furnace (23). To this end, as shown in Fig. 4h, the upper end of the drawn preform (16) 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. 4i). At the end of drawing, the weight of the drawn preform (16) is supported only by the lower chuck (18). If the lower chuck (18) is loosened, the preform (16) is not supported and gradually drops since the part remained in the furnace is softened. In such case, the auxiliary chuck (27) supports the drawn preform (16) advantageously. That is, in using the apparatus according to the present invention, before reducing the diameter of the upper part of the drawn preform (16), the drawn preform (16) is held by the auxiliary chuck (27) as shown in Figs. 4(i), 5(g) and 6(f) to support the drawn preform (16) by the auxiliary chuck (27) and the lower chuck (18), 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 (16) by the auxiliary chuck (27) and the lower chuck (18), the upper part of the drawn preform (16) is more safely drawn than when the drawn preform (16) is supported by the lower chuck (18) only. In particular, when the thin part 16' is broken and the drawn preform (16) is removed from the furnace, the drawn preform (16) may be unstabilized if it is supported by the lower chuck (18) only. When the drawn preform (16) is supported by the lower chuck (18) and also the auxiliary chuck (27), it is stabilized and safety of the handling is improved. The above apparatus of the present invention will be illustrated by the following Examples. Example 1 In this Example, the glass preform was drawn by the apparatus comprising the steps (a) to (i) 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 27. As shown in Fig. 4(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. 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. 4(b) and 4(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. 4(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. 4(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. 4(f)). Then, the auxiliary chuck 27 was loosened and its movement was finished (Fig. 4(g)). As shown in Fig. 4(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. 4(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 2 In the same manner as the steps (a) to (d) of Fig. 4 in Example 1, 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. 5(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. 5(f)). Then, in the same manner as in Example 1, 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. 5(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 1 and 2, 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 3 In this Example, the glass preform was drawn by the apparatus comprising the steps (a) to (d) of Fig. 4 and the steps (e) and (f) of Fig. 6. 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. 5(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. 5(d)). When the upper chuck 17 was lowered by 300 mm, the drawing was terminated (Fig. 6(e)). At this stage, the drawn preform was held by the auxiliary chuck 27 as shown in Fig. 6(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. 6(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. With the above apparatus, the drawn preform which did not bend was produced.

Claims: An apparatus for vertically drawing a glass perform (1) for an optical fiber comprising i) a heating furnace (23), ii) upper and lower chucks (17, 18) which independently move in the direction of the center axis of said heating furnace (23), characterized by further comprising an auxiliary chuck (27) which is provided between said heating furnace (23) and said lower chuck (18) and independently moves in the direction of the center axis of said heating furnace (23). The apparatus according to claim 1 which comprises means for moving said auxiliary chuck (27) and said lower chuck (18) at the same rate.

Inventors: DANZUKA TOSHIO, HOSHINO SUMIO, ITO MASUMI, MUKAI KATSUZO, OHGA YUICHI, DANZUKA, TOSHIO, HOSHINO, SUMIO, ITO, MASUMI, MUKAI, KATSUZO, OHGA, YUICHI, Danzuka, Toshio Yokohama Works of, Hoshino, Sumio Yokohama Works of, Ito, Masumi Yokohama Works of, Mukai, Katsuzo Yokohama Wors of, Ohga, Yuichi Yokohama Works of
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1993
Main classification: C03B37
IPC codes: C03B37