Method of flame abrasion of glass preform

Abstract: 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.

Description: 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 smoothen 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 optical fiber, a VAD (Vapor Phase Axial Deposition) method is known and widely used. Fig. 1 schematically illustrates the VAD method. In the VAD method, SiO 2 glass soot is synthesized using an oxyhydrogen flame 8 and deposited on a peripheral surface or a lower end of a starting member 6, 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 surface of the preform is smoothened by a flame abrasion with an oxyhydrogen flame. Fig. 2 illustrates a conventional flame abrasion method, in which a glass preform 2 is connected to supporting rods 1 which are attached to respective 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. Recently, in view of the reduction of production cost of optical fibers, a diameter and a length of the preform for the optical fibers have been increased. When the glass preform is horizontally supported in the flame abrasion step, as a weight of the preform increases, too much weight is loaded on the supporting rods so that the rods may be broken. As the diameter of the glass rod increases, a center part of the 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 pre-form, whereby a thermal strain is generated in the glass preform, so that the preform often cracks during cooling after the flame abrasion. A prior art that is of interest is that disclosed in EP-A1-0 440 130 falling under Article 54(3) EPC wherein during the drawing of a glass rod that is horizontally arranged a carriage is driven along the length of the glass rod so as to move a detector along said length to scan the glass rod for faults. The glass rod is rotated so that every point along and around the glass rod is scanned. When a fault is detected a heater is supplied with oxygen and hydrogen so that the fault can be repaired by heating. The detector is coupled to means determining the angular location at which a fault has been found and to means determining the longigtudinal setting of the fault. SUMMARY OF THE INVENTION One object of the present invention is to provide a method of flame abrasion of a glass preform with an oxyhydrogen flame, which prevents breakage of the preform due to the weight of the glass preform even when the glass preform has large dimensions and a large weight. Another object of the present invention is to provide a method of flame abrasion of a glass preform with an oxyhydrogen flame, which can prevent deposition of white powder of SiO 2 particles generated by the flame abrasion. According to the present invention a method of flame abrasing a glass preform for an optical fiber comprises flame abrasing with an oxyhydrogen flame producing burner a glass preform which is vertically suspended and rotated while relatively moving at least one of the glass preform and the oxyhydrogen flame, there being an auxiliary burner provided below said oxyhydrogen flame for removing white powder created by said oxyhydrogen flame and wherein said glass preform is flame abrased from its lower end to its upper end. BRIEF DESCRIPTION OF THE INVENTION Fig. 1 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; and 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. DETAILED DESCRIPTION OF THE DRAWINGS In the present invention, the glass preform to be flame abrased 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 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 abrased peripheral surface of the glass soot has desired smoothness. Such relative speed is easily determined by a simple experiment, and usually from 5 to 50 mm/min., preferably from 10 to 20 mm/min. In Fig. 3, a 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 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 or all around of the glass preform as shown in Fig. 3 As the glass preform is flame abrased from its lower end to the upper end, a part which is just above a part being flame abrased is heated with an upward flow of the heated gas, whereby the crack of the glass preform is prevented. An auxiliary burner (5) is provided below the burner for generating the oxyhydrogen flame to remove the SiO 2 white powder. A distance between the burner for flame abrasion and the auxiliary burner is usually from 20 to 100 mm, preferably from 30 to 80 mm. 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 auxiliary burner but 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, SiO 2 fine particles, which were produced by hydrolysis of SiCl 4 by the VAD method, were deposited to a thickness of 250 mm and a length of 1000 mm to form a porous preform, and the porous preform was heated and vitrified in a vacuum furnace kept at 1600°C 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 µm. As shown in Fig. 3, to both ends of the glass preform 2, respective supporting rods 1 each having a diameter of 40 mm were attached and vertically supported by chucks 4 which could be vertically moved. Twelve burners 3 for generating the oxyhydrogen flame were fixed at a certain position on one side of the glass preform as shown in Fig. 3. 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). From the obtained glass preform, an optical fiber having a diameter of 125 µm was drawn. In addition to the twelve burners 3, six auxiliary burners 5 for supplying nitrogen gas were provided around the glass preform 30 mm below the burners 3 as shown in Fig.3. To each of the burners 3, hydrogen gas and oxygen gas were supplied at rates of 400 l/min. and 80 l/min., respectively, and to each of the burners 5, nitrogen gas was supplied at a rate of 50 l/min. With the nitrogen gas jetted from the burners 5, the deposition of the white powder of SiO 2 was prevented. The surface smoothness of the flame abrased glass preform was 0.5 µm. An optical fiber fabricated from this glass preform had a tensile strength at break of 7 kg. Example 2 In addition to the twelve burners 3, six auxiliary burners 9 for generating oxyhydrogen flames were provided around the glass preform 30 mm below the burners 3 as shown in Fig. 4. To each of the burners 3, hydrogen gas and oxygen gas were supplied at rates of 400 l/min. and 100 l/min., respectively to increase the abrasion speed and to each of the burners 9, hydrogen gas and oxygen gas were supplied at rates of 60 l/min. and 20 l/min., respectively. With the oxyhydrogen flames from the auxiliary burners 9, the deposition of the white powder of SiO 2 was prevented. The surface smoothness of the flame abrased glass was 0.5 µm. An optical fiber fabricated from this glass preform had a tensile strength at break of 7 kg. When a glass preform is flame abrased by the conventional method, its surface smoothness is 1.5 to 3.0 µm. 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.

Claims: A method of flame abrasion of a glass preform for an optical fiber, which comprises flame abrasing with an oxyhydrogen flame producing burner the glass preform which is vertically suspended and rotated while relatively moving at least one of the glass preform and the oxyhydrogen flame, there being an auxiliary burner provided below said oxyhydrogen flame for removing white powder created by said oxyhydrogen flame and wherein said glass preform is flame abrased from its lower end to its upper end. The method according to Claim 1 wherein a relative speed between said glass preform and said burner is from 5 to 50 mm/min. The method according to Claim 1 wherein a distance between said burner for flame abrasion and said auxiliary burner is from 20 to 100 mm. The method according to Claim 1 wherein said auxiliary burner generates an oxyhydrogen flame. The method according to any one of Claims 1 to 4 wherein said oxyhydrogen flame producing burner is provided to one side of the glass preform. the method according to any one of Claims 1 to 4 wherein a plurality of said oxyhydrogen flame producing burners and a plurality of said auxiliary burners are provided around the glass preform.

Inventors: AMEMIYA KOUJI, DANZUKA TOSHIO, OHGA YUICHI, SAITO MASAHIDE, YOKOTA HIROSHI, AMEMIYA, KOUJI, DANZUKA, TOSHIO, OHGA, YUICHI, SAITO, MASAHIDE, YOKOTA, HIROSHI, Amemiya, Kouji, c/o Yokohama Works of Sumitomo, DANZUKA, TOSHIO, C/O YOKOHAMA WORKS OF SUMITOMO, OHGA, YUICHI, C/O YOKOHAMA WORKS OF SUMITOMO, SAITO, MASAHIDE, C/O YOKOHAMA WORKS OF SUMITOMO, YOKOTA, HIROSHI, C/O YOKOHAMA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1992
Main classification: C03B37
IPC codes: C03B29, C03B37