Method for drawing glass proform 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 and apparatus 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, 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, as disclosed in JP-A-3-228844 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. 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 an 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 the 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. JP-A-03126633 relates to the production of optical fiber base material. In the production of optical fiber base material according to this document, in which the dummy rods are connected to both ends of the target rod and the glass fine particle 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, 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. JP-A-03037128 describes a production device of optical fiber and its production. The teaching of this document aims at accurately carrying out the drawing of the fiber in order to produce an element strand of optical fiber having excellent characteristics. The top part of the optical fiber preform is provided with a dummy bar of small diameter which is connected to a delivery rod. The delivery rod is vertically movable in a shielded pipe on a central axis thereof by a connecting pin. Then the shielded pipe is dropped by a hoist, arranged on a heating furnace and the optical fiber preform is fixed in a heating space of the heating furnace. An inert gas is sealed in the heating furnace, the preform is drawn while heating the bottom part of the preform by a heater to form a fiber material, the delivery rod is dropped according to the drawing and the tip of the preform is maintained approximately at the central position of the heater. SUMMARY OF THE INVENTION One object of the present invention is to provide a method for producing an optical fiber from a large glass preform. According to the present invention, there is provided a method for producing an optical fiber comprising the steps: charging a glass-forming raw material in a flame generated by a burner to form glass soot through a flame hydrolysis reaction or an oxidation reaction, depositing said glass soot on the peripheral surface of a quartz glass rod forming the core portion or the core portion and a part of cladding portion of a transparent glass preform (10) for an optical fiber while relatively moving said burner and said glass rod to form a composite of said glass rod and said glass soot, sintering and vitrifying said deposited glass soot in a heating furnace to obtain said transparent glass preform for an optical fiber, drawing said preform to reduce its diameter, and further drawing said preform in a drawing furnace to obtain an optical fiber, wherein a dummy rod is welded to each end of said quartz glass rod before depositing said glass soot, each said dummy rod having, at its free end, a through hole which crosses the center axis of said dummy rod, whereby said free end of each said dummy rod is inserted in a fitting member having a depressed area at an end in which said dummy rod is inserted and a through hole which crosses the center axis of said fitting member at a position corresponding to said through hole of said dummy rod, a pin is inserted in said through holes of each said dummy rod and each said fitting member, and each said fitting member is attached to a rotating shaft so as to hold and rotate the glass preform when the glass preform is heated and drawn in an electric furnace to reduce the diameter of the preform, said glass soot is not deposited at a tip end part of each dummy rod while said glass soot is deposited in a tapered form from a middle portion of each dummy rod towards said glass rod and around the peripheral surface of said glass rod at a uniform outer diameter to form said glass rod/glass soot composite, and after reducing the diameter of the preform, the preform is heated and molten in an electric furnace while holding the end part of each said dummy rod and drawn to obtain the optical fiber. 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, Fig. 4 illustrates a first method for producing a glass preform according to the present invention, Fig. 5 shows an example of a method for fitting the dummy rod to the rotating shaft, Fig. 6 illustrates an embodiment for drawing a glass preform according to the present invention, Fig. 7 shows an example of a method for fitting the dummy rod to the fitting member, and Figs. 8 to 11 illustrate examples of a further embodiment 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. Fig. 4 illustrates one embodiment of the method of the present invention. At the upper and lower ends of the quartz glass rod 1 comprising a core portion (hereinafter referred to as "core rod"), upper and lower dummy rods 2 and 3 are connected by welding, respectively. At an end of each of the upper and lower dummy rods 2 and 3, a fitting part is provided so that the dummy rod can be fit to a main rod (rotating shaft) 6 or a fitting part of a chuck of a lathe for drawing. The fitting part has a through hole 4 for fitting through which a fixing pin is inserted. As the dummy rod, preferably a quartz glass rod which is produced by a conventional melting method or synthetic method is used. While a small amount of a dopant or impurities may be contained in the dummy rod, a glass having a very low melt viscosity is unpreferable since the dummy rod is deformed during sintering or drawing. The length of each dummy rod is not critical insofar as its end part can be held. Since the tapered part of the deposited glass soot is formed on a part of the dummy rod, its length is determined by the length of the part to be held and a length of the tapered part of the deposited glass soot. Since the length of the tapered part of the deposited glass soot is usually 0.5 to 1.0 time the outer diameter D of the glass soot deposit, the length of the dummy rod is at least 1 to 1.5 times the outer diameter of the glass soot deposit to be formed. When the dummy rod is too long, the equipment for the production of the glass preform and the sintering furnace should be made large. Then, the length of the dummy rod is preferably at most 3.0 times the outer diameter of the glass soot deposit. The outer diameter of each dummy rod is most preferably equal to the outer diameter of the core rod. When the outer diameter of the dummy rod is much larger than the outer diameter of the core rod, the glass soot deposit is deformed at the connected part between the core rod and the dummy rod and, in an extreme case, the glass soot deposit is cracked. Then, the outer diameter of the dummy rod is preferably from 0.9 to 1.3 times, more preferably from 1.0 to 1.1 times the outer diameter d of the core rod. To a lower fitting part 5 of the main rod 6 suspended in a reactor chamber for synthesizing glass soot, the end of upper dummy rod 2 is inserted, and a pin is inserted in the through hole 4 and a fitting through hole formed at a position of the fitting part 5 corresponding to the through hole 4 in a direction perpendicular to the center axis of the fitting part 5, whereby the upper dummy rod 2 and the main rod 6 are connected. Any other means for fixing the upper dummy rod and the main rod at their fixing parts may be used. For example, as shown in Fig. 5, the tip end of the upper dummy rod 2 has a larger diameter than the rest part, while a part of a side wall of the depressed area 5a of the fitting part 5 is cut out. The dummy rod is inserted in the depressed area through the cut-out part 5b, and the weight of the dummy rod is supported by the large diameter part of the dummy rod. After the upper dummy rod 2 is fit to the main rod 6, the glass-forming raw material, a combustion gas and a combustion-supporting gas are injected from a burner 7 to form glass soot in a flame 8, and the glass soot is deposited from a middle part of the upper dummy rod 2 (Fig. 1(a)). The obtained composite consisting of the core rod 1, the upper and lower dummy rods 2, 3 and the glass soot deposit 9 (Fig. 1(b)) is sintered with maintaining the upper and lower dummy rods 2, 3 to obtain a transparent glass preform 10 (Fig. 1(c)). The tip ends of the upper and lower dummy rods 2, 3 are fit to respective fitting parts 14, 15 of upper and lower fitting members 12, 13 for drawing provided in an electric furnace. Then, the upper and lower fitting members 12, 13 are fixed to respective upper and lower chucks 17, 18 of a drawing lathe (not shown), and the transparent glass preform 10 is heated and drawn to obtain a drawn preform 16 (Fig. 1(d)). Preferably, the preform 10 is fixed in the vertical direction and drawn in the vertical direction as shown in Fig. 1(d). In the above embodiment, since the glass soot is not deposited on the end parts of the dummy rods, the end parts having no deposited glass soot are used as the fitting parts, so that it is not necessary to connect any dummy rod after the sintering and vitrification of the composite of the core rod and the glass soot deposit. Thereby, it is not necessary to hold the glass surface of the preform, and the transparent glass preform can be immediately drawn without touching its surface. Then, the flaw on the surface and contamination by the impurities can be prevented, whereby the high quality optical fiber having good strength can be produced. According to the experiments on the welding of a dummy rod to the glass preform using the horizontal glass lathe, it was found that a weight of the glass preform at which it was processed by holding one dummy rod without breakage was usually from 7 to 8 kg, and at most 10 kg with very careful work. Since a quartz glass is usually used for the production of the dummy rod, it is easily cracked from a flaw by the application of a load even if the dummy rod has a large diameter. Then, the production of a large size glass preform has been limited in the above method using the horizontal glass lathe. As seen from the Examples below, it is possible to produce the glass preform having a weight of more than 7 to 8 kg, further more than 10 kg without difficulty, whereby the productivity of the glass preform and in turn the optical fiber is greatly increased. Preferred examples of the method of the present invention will be illustrated. Example 1 A core rod having a length of 800 mm and an outer diameter of 18 mm and consisting of a core portion and a part of a cladding portion was provided. At each end of the core rod, a dummy rod having an outer diameter of 18 mm and a length of 500 mm was welded. A free end of each dummy rod had a through hole having a diameter of 5 mm. In the step for synthesizing the glass soot deposit, the upper dummy rod was fit to a fitting member having, at its tip end, a cylindrical depressed area in which the dummy rod was inserted and, in a wall of the cylindrical depressed area, a through hole with a diameter of 5 mm which vertically crossed the center axis of the fitting member, at a position corresponding to the through hole of the dummy rod. A ceramic pin having a diameter of 4.5 mm was inserted through the both holes to hold the dummy rod on the fitting member. Then, the fitting member was fixed to a rotating shaft. On a peripheral surface of the core rod, glass soot deposit was synthesized at a uniform outer diameter of 220 mm. Lengths of tapered parts of the glass soot deposit formed on the upper and lower dummy rods were 160 mm and 180 mm, respectively. A total weight of the core rod and the glass soot deposit was 13.4 kg. After vitrifying the glass soot deposit in a sintering furnace, the composite of the core rod and the glass soot deposit was inserted in an electric furnace, and the end parts of the dummy rods were fit to and held by chucks with ceramic pins. While keeping the temperature of the electric furnace at 2000°C, the upper and lower chucks were traversed vertically at a rate of 10 mm/min. to draw the composite while controlling a traversing rate of the lower chuck so that the outer diameter of the preform was adjusted at 35 mm. The produced drawn preform was further drawn in a drawing furnace to produce an optical fiber. In the production of the optical fiber having a length of 100 km, the fiber was not broken and had a transmission loss of only 0.35 dB/km at a wavelength of 1.3 µm. Example 2 This Example will clarify the influence of a relationship between an outer diameter of the dummy rod and an outer diameter of the core rod on the formation of cracks on a preform. In the same constructions as in Example 5, five dummy rods having outer diameters of 16 mm, 18 mm, 19 mm, 22 mm and 24 mm were provided, and each five glass soot deposits were synthesized with each dummy rod diameter. The core rod had an outer diameter of 18 mm, and then the outer diameters of the dummy rods were 0.88, 1.0, 1.05, 1.22 and 1.33 times the outer diameter of the core rod. With the dummy rods of 18 mm and 19 mm in outer diameter, all five glass preforms were produced in good conditions. With the dummy rod of 22 mm in outer diameter, one of the preforms was cracked at its tip end when the glass soot deposit was formed on the lower molten part, while other four glass preforms were produced in good conditions. With the dummy rod of 16 mm in diameter, only two glass preforms were produced in good conditions, and with the dummy rod of 24 mm in diameter, only one glass preform was produced in good conditions. Comparative Example 1 In the same manner as in Example 1 except that a core rod consisting of a core portion and a part of a cladding portion and having an outer diameter of 18 mm was used, and to the upper and lower end of the core rod, a dummy rod having an outer diameter of 18 mm and a length of 400 mm and a dummy rod having an outer diameter of 18 mm and a length of 200 mm were connected, respectively, a glass soot deposit was formed on the composite. Since the lower dummy rod was short, a part of 20 mm in length on which no glass soot was deposited was left at the tip end of the dummy rod. A total weight of the composite was 13.3. kg. After vitrification, the upper dummy rod was held by the horizontal glass lathe, and a further dummy rod was welded to the lower dummy rod but the dummy rod was broken. Then, an effective part of the preform was chucked, and further dummy rods were welded to both ends. The preform having the welded dummy rods was drawn to an outer diameter of 35 mm and further drawn in a drawing furnace to produce an optical fiber. But, the fiber was broken eleven times per 100 km. The optical fiber which was produced from the part of the preform which had been chucked was most frequently broken. Some parts of the optical fiber had the transmission loss of 0.45 dB/km at the wavelength of 1.3 µm A preferred embodiment (A) 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. 6, 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. 6, 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. 7(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. 7, 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. 7(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. 6(b). This embodiment of the present invention will be illustrated by the following Examples further in detail. Example 3 This Example will be explained by making reference to Figs. 6 and 7. (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. 6(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. 6(b) and Fig. 7(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. 6(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 (cf. Fig. 7(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. 6(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. 6(d)). When the upper chuck was lowered by a distance of 300 mm, the drawing was finished (Fig. 6(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. 6, 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 2 In the same manner as in Example 3 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 3 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 4 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 3. 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 and apparatus according to the final aspect of the present invention will be explained. This embodiment is a modification of the above described embodiment (A) of 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. 6(a) to 6(d). Then, the position to be held by the chuck is changed to the dummy rod as shown in Fig. 8(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. 8(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. 8(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. 9(i), 10(g) and 11(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 5 In this Example, the glass preform was drawn by the method comprising the steps (a) to (d) of Fig. 6 and the steps (e), (f) and (g) of Fig. 8. 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. 6(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. 6(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. 6(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. 6(d), the upper and lower chucks 17 and 18 were stopped to suspend the drawing (Fig. 6(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. 8(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. 8(f)). Only the lower chuck 12 was further lowered to reduced the diameter of the upper end of the drawn preform (Fig. 8(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 6 In this Example, the glass preform was drawn by the method comprising the steps (a) to (i) of Fig. 9. 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. 9(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 5, 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. 9(b) and 9(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. 9(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. 9(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. 9(f)). Then, the auxiliary chuck 27 was loosened and its movement was finished (Fig. 9(g)). As shown in Fig. 9(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. 9(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 7 In the same manner as the steps (a) to (d) of Fig. 9 in Example 6, 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. 10(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. 10(f)). Then, in the same manner as in Example 6, 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. 10(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 5, 6 and 7, 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 8 In this Example, the glass preform was drawn by the method comprising the steps (a) to (d) of Fig. 9 and the steps (e) and (f) of Fig. 11. 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. 10(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. 10(d)). When the upper chuck 17 was lowered by 300 mm, the drawing was terminated (Fig. 11(e)). At this stage, the drawn preform was held by the auxiliary chuck 27 as shown in Fig. 11(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. 11(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 producing an optical fiber comprising the steps: charging a glass-forming raw material in a flame (8) generated by a burner (7) to form glass soot (9) through a flame hydrolysis reaction or an oxidation reaction, depositing said glass soot (9) on the peripheral surface of a quartz glass rod (1) forming the core portion or the core portion and a part of the cladding portion of a transparent glass preform (10) for an optical fiber while relatively moving said burner (7) and said glass rod (1) to form a composite of said glass rod and said glass soot (9), sintering and vitrifying said deposited glass soot (9) in a heating furnace to obtain said transparent glass preform (10) for an optical fiber, drawing said preform (10) to reduce its diameter, and further drawing said preform in a drawing furnace to obtain an optical fiber, wherein a dummy rod (2, 3) is welded to each end of said quartz glass rod (1) before depositing said glass soot (9), each said dummy rod (2, 3) having, at its free end, a through hole (4) which crosses the center axis of said dummy rod (2, 3), whereby said free end of each said dummy rod is inserted in a fitting member (12, 13) having a depressed area at an end (14, 15) in which said dummy rod (2, 3) is inserted and a through hole which crosses the center axis of said fitting member at a position corresponding to said through hole (4) of said dummy rod (2, 3), a pin is inserted in said through holes (4) of each said dummy rod (2, 3) and each said fitting member (12, 13) and each said fitting member (12, 13) is attached to a rotating shaft so as to hold and rotate the glass preform (10) when the glass preform is heated and drawn in an electric furnace to reduce the diameter of the preform, said glass soot (9) is not deposited at a tip end part of each dummy rod (2, 3) while said glass soot (9) is deposited in a tapered form from a middle portion of each dummy rod (2, 3) towards said glass rod (1) and around the peripheral surface of said glass rod (1) at a uniform diameter to form said glass rod/glass soot composite, and after reducing the diameter of the preform, the preform is heated and molten in an electric furnace while holding the end part of each said dummy rod and drawn to obtain the optical fiber. The method according to claim 1, wherein the length of said dummy rod (2 and 3) is 1 to 3 times the outer diameter of said glass soot deposit (9). The method according to claim 1, wherein the diameter of said dummy rod (2 and 3) is 0.9 to 1.3 times the outer diameter of said quartz glass rod (1). The method according to claim 1, wherein said glass preform (10) is vertically held and drawn in a vertical direction.

Inventors: DANZUKA TOSHIO, HOSHINO SUMIO, ITO MASUMI, MUKAI KATSUZO, OHGA YUICHI, DANZUKA, TOSHIO, HOSHINO, SUMIO, ITO, MASUMI, MUKAI, KATSUZO, OHGA, YUICHI
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1993
Main classification: C03B37
IPC codes: C03B37