Method of flame abrasion of glass preform
AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPUCANT(S): Sumitomo Electric Industries, Ltd. ADDRESS FOR SERVICE: DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street, Melbourne, 3000. • • t t • . INVENTION TITLE: Method of flame abrasion of glass preform The following statement is a full description of this invention, including the best method •"'*%• of performing it known to me/us:- - la- BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of flame abrasion of a glass preform to remove unevenness on its surface and smooth its surface so as to provide a glass preform which is suitable for the fabrication of an optical fiber. Description of the Related Art As a method for producing a high purity quartz preform for an optiqal fiber, a VAD (Vapor Phase Axial Depo¬ sition) method is known and widely used. Fig. 1 schemati¬ cally illustrates the VAD method. In the VAD method, Si02 glass soot is synthesized using an oxyhydrogen flame 8 and deposited on a peripheral surface or a lower end of a star¬ ting member €. for example, a quartz rod to form a porous glass preform 7 in an axial direction of the starting member 6. Then, the porous glass preform is sintered and vitrified to prepare a transparent glass preform. Since the transparent glass preform has unevenness of several micrometers on its surface, the peripheral sur¬ face of the preform is smoothed by a flame abrasion with an oxyhydrogen flame. Fig. 2 illustrates a conventional flame abrasion method, in which a glass preform 2 is connec¬ ted to supporting rods 1 which are attached to respective » 4 t TjO<5 chucks 4, and the surface of the rotating glass preform 2 is heated with oxyhydrogen flames 3. In the conventional flame abrasion method, the glass preform is supported horizontally as shown in Fig. 2. Recendy, in view of the reduction of production cost of optical fibers, the diameter and length of preforms for optical fibers have been increased. When the glass preform is horizontally supported in the flame abrasion step, too much weight is loaded on the supporting rods due to the increased weight of the preform, so that the rods may break- As the diameter of the glass preform increases, a center part of the horizontally held preform cannot be sufficiently heated only by heat of the oxyhydrogen flame during the flame abrasion and a large temperature difference is created between the center part and the peripheral surface part of the preform, whereby a thermal strain is generated in the glass preform, so that the preform often cracks during cooling after the flame abrasion. SUMMARY QF THE INVENTION One object of the present invention is to provide £ method of flame abrasion of a glass preform with an oxyhydrogen flame, which alleviates breakage of supporting rods for the preform due to the weight of the glass preform even when the glass preform has large dimensions and a larger weight. According to the present invention there is provided a method of flame abrading a vitrified glass preform for an optical fiber, which comprises flame abrading the glass preform with an oxyhydrogen flame while the glass preform is vertically suspended and rotated about its longitudinal axis and while the glass preform and/or the oxyhydrogen flame are moved relative to each other in the vertical direction whereby the glass preform is flame abraded from its lower end to its upper end, and wherein the glass preform is not subjected to a separate pre¬ heating between a vitrification step and said flame abrading with the oxyhydrogen flame, W302,q:\c>perVp!A99-9lspe£ BRIEF DESCRIPTION OF THE INVENTION The present invention will now be described by way of example only, with reference to preferred embodiments as illustrated in the accompanying drawings in which: Fig. i schematically shows a VAD method for producing a porous glass preform for an optical fiber, Fig. 2 schematically shows a conventional method of flame abrasion of a glass preform for an optical fiber. Figs. 3 and 4 schematically show two embodiments of the method of flame abrasion of a glass preform for an optical fiber according to the present invention. Figs, 5 and 6 schematically show two other embodiments of the method of flame abrasion of a glass preform for an optical fiber according to the present invention. DETAILED DESCRIPTION OF THE DRAWINGS In the present invention, the glass preform to be flame abraded is vertically suspended and rotated. Because of the vertical suspension of the glass preform, breakage of supporting rods due to the weight of the glass preform can be prevented. As shown in Fig. 3, a vitrified glass preform 2 is supported by a pair of supporting rods 1 which are attached to respective chucks 4 of a lathe (not shown) and rotated. The rotating glass preform 2 is heated with oxyhydrogen flames generated by burners 3 from its lower end (a) to its upper end (b). A relative speed between the glass preform and the burners is adjusted so that the abraded peripheral surface of the glass soot has desired smoothness. Such relative speed is easily determined by a simple experiment, and is usually from 5 to 50 mm/min., preferably from 10 to 20 mm/min. In Fig. 3, the position of the burner 3 is fixed, while the glass preform is moved downwardly, though it is possible to move the burner upwardly while fixing 940302,q:\oper\phh,20499-9Zspe,3 the glass preform, or to move the burner upwardly and the glass preform downwardly. The burner may be the same as used in the conventional flame abrasion of the glass preform. The burner may be provided on one side of the glass preform as shown in Fig. 3 or all around of the glass preform as shown in Fig. 4. When the glass preform is flame abraded from its lower end to the upper end, a part which is just above a part being flame abraded is heated with an upward flow of the heated gas, whereby cracking of the glass preform is alleviated without the need for a separate pre-heating between a vitrification step and the flame abrasion. 940302,$ \&per\phtiWW-9ZspM In one preferred embodiment, an auxiliary burner is provided below the burner for generating the oxyhydrogen flame to remove the Si02 white powderi A distance between the burner for flame abrasion and the auxiliary burner is usually from 20 to 100 mm, preferably from 30 to 80 nun. To the auxiliary burner, any gas can be supplied insofar as deposition of the white powder generated by the oxyhydrogen flame is prevented. For example, nitrogen gas is blown from the auxi¬ liary burner buc the temperature of the glass preform may be decreased. Preferably/ the auxiliary burner is one for generating an oxyhydrogen flame to heat the glass preform. PREFERRED EMBODIMENTS OF THE INVENTION The flame abrasion method of the present invention will be explained further in detail by following Examples. Example 1 Around a quartz rod having a diameter of 30 mm, Si02 fine particles, which were produced by hydrolysis of SiCl4 by the VAD method, were deposited to a thickness of 250 mm and a length of 1000 nun to form a porous preform, and the porous preform was heated and vitrified in a vacuum furnace kept at 1600aC to obtain, a transparent glass preform having a diameter of 120 mm, a length of 1000 mm and a weight of 36 kg. A surface roughness (center line average height) was measured by a surface roughness meter to find that it was 3 to 5 pm. As shown in Fig. 3, to both ends of thevglass preform 2* respective supporting rods 1 each having a dia¬ meter of 40 mm were attached and vertically supported by chucks 4 which could be vertically moved. Thirteen burners 3 for generating the oxyhydrogen flame were fixed at a cer¬ tain position on one side of the glass preform as shown in Fig. 3. To each burner, hydrogen gas and oxygen gas were supplied at rates of 300 1/min. and 60 1/min., respectively. The glass preform was downwardly moved at a rate of 10 mm/min, and flame abrased from its lower end (a) to its' upper end (b). After the flame abrasion, the surface ;;"/ roughness of the glass preform was 1.0 ym. From the obtained glass preform, an optical fiber having a diameter of 125 ym was. drawn. Its tensile strength at break was 6.5 kg. Example 2 In the same manner as in Example 1, a glass pre¬ form having a length of 1000 mm and a diameter of 120 mm was produced. Then, in the same manner as in Example 1 but using twelve burners 3 which were- provided around the glass pre¬ form as shown in Fig. 4 and supplying hydrogen gas and oxy¬ gen gas at rates of 300 1/min. and 60 1/min., respectively to each burner, theiglass preform was flame abrased from its lower end to its upper end, whereby the moving speed of the glass preform could be increased to 15 mm/min. The surface roughness of the flame abrased glass preform was 0.8 \im. v« An optical fiber fabricated from this glass pre¬ form had a tensile strength at break of 7.0 kg. Example 3 In addition to the twelve burners 3, six auxiliary burners 5 for supplying nitrogen gas were provided around thej,glass preform 30 nun below the burners 3 as shown in Fig. 5. To each of the burners 3, hydrogen gas and oxygen gas were supplied at rates of 400 1/min. and 80 1/min,, respec¬ tively, and to each of the burners 5, nitrogen gas was supp¬ lied at a rate of 50 1/min. With the nitrogen gas jetted from the burners 5, the deposition of the white powder of Si02 was prevented. But, a temperature of flame abrasion was decreased. The surface smoothness of the flame abrased glass preform was 0.5 pm. An optical fiber fabricated from this glass pre¬ form had a tensile strength at break of 7 kg. Example 4 In addition to the twelve burners 3, six auxiliary burners 9 for generating oxyhydrogeh flames were provided around the.glass preform 30 mm below the burners 3 a& shown in Fig. 6. To each of the burners 3, hydrogen gas and oxy¬ gen gas were supplied at rates of 400 1/min. and 100 1/min., respectively to increase the abrasion speed and to each of the bufnets 9, hydrogen gas and oxygen gas were supplied at rates of 60 l/min and 20 1/min., respectively. With the oxyhydrogen flames from the auxiliary burners 9, the deposi¬ tion of the white powder of SiC was prevented. The surface smoothness of the flame abrased glass was 0.5 \im. An optical fiber fabricated from this glass pre¬ form had a tensile strength at break of 7 kg. When a glass preform is flame abrased by the con¬ ventional method, its surface smoothness is 1.5 to 3.0 pm. When an optical fiber is fabricated from the glass preform which is flame abrased by the conventional method, .. its tensile strength at break is only 5.5 to 6.0 kg. THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A method of flame abrading a vitrified glass preform for an optical fiber, which comprises flame abrading the glass preform with an oxyhydrogen flame while the glass preform is vertically suspended and rotated about its longitudinal axis and while the glass preform and lot the oxyhydrogen flame are moved relative to each other in the vertical direction whereby the glass preform is flame abraded from its lower end to its upper end, and wherein the glass preform is not subjected to a separate pre-heating between a vitrification step and said flame abrading with the oxyhydrogen flame. 2. The method according to claim 1, wherein a relative speed between said glass preform and said burner is from 5 to 50 mm/min. 3. The method according to claim 1 or claim 2, wherein an auxiliary burner is provided below said burner for flame abrasion. 4. The method according to claim 3, wherein a distance between said burner for flame abrasion and said auxiliary burner is from 20 to 100 mm. 5. The method according to claim 3 or claim 4, wherein said auxiliary burner generates an oxyhydrogen flame. • tm-m 6. A method of flame abrasion of a glass preform substantially as hereinbefore described with reference to Figures 3 to 6 of the drawings and/or the Examples. DATED this 2nd day of March, 1994. SUMITOMO ELECTRIC INDUSTRIES, LTD. By its Patent Attorneys DAVES COLLISON CAVE 940302,q:\oper\i)hh»2049imspe,9 A glass preform (2) for an optical fiber is flame abrased with an oxyhydrogen flame while vertically suspending and rotating the glass preform (2) and relatively moving one or both of the glass preform (2) and the oxyhydrogen flame, whereby a surface of the glass preform (2) is smoothened. <IMAGE> 1. A method of flame abrading a vitrified glass preform for an optical fiber, which comprises flame abrading the glass preform with an oxyhydrogen flame while the glass preform is vertically suspended and rotated about its longitudinal axis and while the glass preform and lot the oxyhydrogen flame are moved relative to each other in the vertical direction whereby the glass preform is flame abraded from its lower end to its upper end, and wherein the glass preform is not subjected to a separate pre-heating between a vitrification step and said flame abrading with the oxyhydrogen flame. 2. The method according to claim 1, wherein a relative speed between said glass preform and said burner is from 5 to 50 mm/min. 3. The method according to claim 1 or claim 2, wherein an auxiliary burner is provided below said burner for flame abrasion. 4. The method according to claim 3, wherein a distance between said burner for flame abrasion and said auxiliary burner is from 20 to 100 mm. 5. The method according to claim 3 or claim 4, wherein said auxiliary burner generates an oxyhydrogen flame.
• tm-m 6. A method of flame abrasion of a glass preform substantially as hereinbefore described with reference to Figures 3 to 6 of the drawings and/or the Examples.
DATED this 2nd day of March, 1994.
SUMITOMO ELECTRIC INDUSTRIES, LTD.
By its Patent Attorneys DAVES COLLISON CAVE 940302,q:\oper\i)hh»2049imspe,9