Optical fiber base material heat treatment method and optical fiber manufacture method

Abstract: The optical fiber base material heat treatment method can be provided that the first flame polishing heats the surface of the optical fiber base material (106) with fixing a length of the optical fiber base material (106) and the second flame polishing heats the surface of the optical fiber base material (106) by lower temperature than the temperature of the heating of the first flame polishing.

Description: BACKGROUND OF INVENTION 1. Field of the Invention The present invention is related to an optical fiber polishing method that can treat the surface of an optical fiber base material evenly by a flame. 2. Description of Related Art An optical fiber is manufactured from an optical fiber base material as the raw material. Principal ingredient of an optical fiber base material is quartz glass. An optical fiber base material is heated and elongated to the predetermined diameter to be an optical fiber preform. An optical fiber preform is drawn to be an optical fiber. If there are some damages and unevenness in the surface of an optical fiber base material, it will be cut off during drawing this material to an optical fiber. Also, the ratio between core diameter and cladding diameter of the optical fiber become uneven. If impure foreign substances are attached to the surface of the optical fiber base material, quality of the optical fiber will decrease. Furthermore, if residual strain inside an optical fiber base material is large, cracks can be caused during drawing or transporting of this material. Therefore, the surface of an optical fiber base material is preferably even and without impure foreign substances. Also, residual strain inside an optical fiber base material is preferably small. A flame polishing is usually performed to remove damages of the surface of and residual strain inside an optical fiber base material. The flame polishing heats the surface of the optical fiber base material to about 2000°C with a burner while rotating the material. However, at this temperature, part of quartz glass (SiO 2 ) at the surface of the optical fiber base material is dissociated and becomes SiO. Therefore, this part of quartz glass is dispersed. SiO reacts with the oxygen in the atmosphere, and becomes glass again. Thus, glass particles are attached on the surface of the base material heated by flame and form clouds on this surface. These clouds can be removed by heating the base material with a weak flame. But if heating is not sufficient, strain remains inside the base material. Conversely, if heating is more than necessary, clouds will occur once again. EP 0 525 681 A1 describes a method of flame abrasion of a glass preform for an optical fiber, which comprises flame abrasing the glass preform which is vertically suspended and rotated with a first oxyhydrogen flame while relatively moving at least one of the glass preform and the oxyhydrogen flame. After treating the preform with the first oxyhydrogen flame the preform is further heated by a second oxyhydrogen flame. However, the energy of the second oxyhydrogen flame is only about one sixth to one seventh of that of the first oxyhydrogen flame. SUMMARY OF THE INVENTION It is the object of the present invention to provide a method of polishing an optical fiber base material which solves the problems above, i.e. which can remove damages, unevenness, and unevenness caused by impure foreign substances of the surface of an optical fiber base material, residual strain inside an optical fiber base material, and cloud caused by the attachment of glass particles. This object according to the invention is solved by a method comprising the features of claim 1. Preferred embodiments of this method are defined in the subclaims. According to the invention a method of polishing an optical fiber base material with heat comprising a first flame polishing step in which the surface of said optical fiber base material is heated by flame while keeping the length of said optical fiber base material constant, and a second flame polishing step by which said surface of said optical fiber base material is heated by flame to a lower temperature than in said first flame polishing step is characterized in that in said second flame polishing step said surface of said optical fiber base material is heated to a temperature between 1000°C and 1800°C to remove residual stress inside said optical fiber base material. A preferred embodiment of the invention comprises that in said first flame polishing step uneven parts on said surface of said optical fiber base material are at least locally heated, and in said second flame polishing step the entire surface of said optical fiber base material is heated. Another preferred embodiment of the invention comprises that in said first flame polishing step said optical fiber base material is heated so that the temperature of said surface of said optical fiber base material reaches 1800°C to 2200°C. A further preferred embodiment of the invention comprises that said flame is generated by burning combustible gas, which contains hydrogen, and supporting gas, which contains oxygen for supporting the combustion. A still further preferred embodiment of the invention comprises that said flame is generated by burning combustible gas, which contains chain hydrocarbon, and supporting gas, which contains oxygen for supporting the combustion. A still further preferred embodiment of the invention comprises drawing said optical fiber base material to a filament-like form by heating and drawing said optical fiber base material to generate said optical fiber. BRIEF DESCRIPTION OF DRAWING Fig. 1 shows a system of an optical fiber manufacturing apparatus of the present invention. Fig. 2 shows an optical fiber manufacturing method of the present invention. Fig. 3 shows a cross section view of the glass rod second elongating apparatus shown in Fig. 1. DETAILED DESCRIPTION OF THE INVENTION The present invention will be explained in detail, using embodiments of the present invention. Fig. 1 shows a system of an optical fiber manufacturing apparatus of the present invention. The system of the optical fiber manufacturing apparatus of the present invention comprises a glass base material generating apparatus 600 which generates a glass base material 102 being a base material of an optical f iber; a glass base material dehydrating and sintering apparatus 700 which dehydrates and sinters the glass base material 102; a glass base material first elongating apparatus 900 which elongates the glass base material 102 to generate a glass rod a glass rod transportation device 380 which transports the glass rod 106; a glass rod second elongating apparatus 111 which elongates the glass rod 106 a second time to generate a preform 107; and a preform drawing apparatus 500 which draws the preform 107 to generate an optical fiber. Fig. 2 shows an optical fiber manufacturing method of the present invention. The glass base material 102 is generated by the glass base material generating apparatus 600 using the VAD method, vapor-phase axial deposition method, or the like (S200). The glass base material 102 is then dehydrated within a chlorine gas atmosphere and sintered within an inert-gas atmosphere by the glass base material dehydrating and sintering apparatus 700 (S202). The diameter of the glass base material 102 is normally 110 mm to 200 mm, compared to a diameter of 30 mm to 80 mm which is most practical for drawing to an optical fiber. Therefore, the dehydrated and sintered glass base material 102 is elongated firstly by the glass base material first elongating apparatus 900 to produce a glass rod 106 (S204), which is a base material of an optical fiber. The glass rod 106 has a diameter 3 mm to 5 mm larger than the diameter convenient for drawing to an optical fiber, which is form 30 mm to 80 mm. The glass rod 106 is transported by the glass rod transportation device 380 (S205). The glass rod 106 is then heated and elongated, the glass rod 106 is further flame polished to be removed the unevenness of the surface to be a preform 107 (S206). The preform 107 is heated and drawn to be a filament-like form by the preform drawing apparatus 500 to produce an optical fiber (S210). Fig. 3 shows an cross section view of the glass rod second elongating apparatus 111 shown in Fig. 1. The optical fiber base material heat treatment method of the present invention uses the apparatus 111 to flame polish the glass rod 106. The optical fiber base material heat treatment method of the present invention performs continuously the first flame polishing and the second flame polishing. The first flame polishing heats the surface of the optical fiber base material with flame while keeping a length of the optical fiber base material constant and the second flame polishing heats the surface of the optical fiber base material with flame by lower temperature than the temperature of the heating of the first flame polishing. The apparatus 111 has a burner 12, a chuck 18 and 20, a motor 22 and 24, and a screw bar 26. Both ends of 14, 16 of the glass rod 106 are held by the chuck 18, 20. The glass rod 106 is rotated around the axis which connects the center of the chuck 18 and the center of the chuck 20. The glass rod 106 is rotated by the motor 22. The burner 12 is connected to the screw bar 26. The screw bar 26 is rotated by the motor 24. The burner 12 heats the glass rod 106 by moving relatively in the direction of the axis of the glass rod 106 along the screw bar 26. The glass rod 106 is heated by the burner 12 with an acid hydrogen flame, which is combustion flame with hydrogen and oxygen. The first flame polishing removes unevenness of the surface of the glass rod 106. And the second flame polishing removes residual strain inside the glass rod 106 and cloud of the surface of the glass rod 106 by the heating glass rod 106 with lower temperature than the temperature of the heating of the first flame polishing. It is desirable that flame is combustion flame with combustible gas, which contains hydrogen and/or chain hydrocarbon, and supporting gas, which contains oxygen for supporting combustion. It is more desirable that flame is an acid hydrogen flame. The first flame polishing starts from heating one end A of the glass rod 106. The burner 12 heats the surface of the glass rod 106 to 2000 °C with moving along the screw bar 26. The burner 12 is moved to another end B of the glass rod 106. Therefor the burner 12 can polish the whole surface of the glass rod 106 with flame. In this process, damages, unevenness and impure foreign substances of the surface of the glass rod 106 are removed, and the surface of the glass rod 106 becomes even. At this time, glass particles are attached to the surface of the glass rod 106 so that cloud occurs on the surface of the glass rod 106. Also, the strain is remained inside the glass rod 106. The first flame polishing can be a local flame polishing which flame polish the part of the glass rod 106 which has damages. In the first flame polishing, a temperature of the surface of the glass rod 106 is made higher than 1600°C, which is softening temperature of synthesis quartz glass. Thereby, unevenness of the surface, that is damages, unevenness and impure foreign substances on the surface, are removed. However, the part of the surface of the glass rod 106 sometimes reaches about 2200°C, and cloud will be caused by glass particles attaching on the surface of the glass rod 106. In the second flame polishing, the surface of the glass rod 106 is heated to approximately softening temperature. Thereby, cloud and residual strain inside the glass rod 106 can be removed by the second flame polishing. In the case of an optical fiber, the principal ingredient is quartz glass, it is desirable that the surface of the glass rod 106 is heated to approximately between 1800 °C and 2200 °C in the first flame polishing. It is more desirable that it approximately between 1900 °C and 2100 °C. If the temperature of the surface is higher than 2200 °C dissociation of the quartz glass of the surface of the glass rod 106 becomes intense. On the other hand, if the temperature of the surface is lower than 1800°C, damages, unevenness, and impure foreign substances cannot be removed from the surface of the glass rod 106. After finishing the first flame polishing, the rotation direction of the screw bar 26 is reversed, so that the burner 12 is moved to the end A, which is a start point of heating. Then, the second flame polishing is performed. The burner 12 heats the surface of the glass rod 106 to 1500 °C with moving along the screw bar 26. The burner 12 moves to other end B of the glass rod 106, with flame polishing the whole surface of the glass rod 106. The whole surface of the glass rod 106 is heated with lower temperature than temperature of the heating of the first flame polishing. Thereby, glass particles of the surface of the glass rod 106 can be removed. The surface of the glass rod 106 can be transparent, and residual strain inside the glass rod 106 can be removed. It is desirable that the surface of the glass rod 106 is heated to approximately between 1000 °C and 1800 °C in the second flame polishing. It is more desirable that the surface of the glass rod 106 is heated to approximately between 1200 °C and 1600 °C. If the temperature of the surface is higher than 1800 °C, new cloud occurs on the surface of the glass rod 106. On the other hand, if the temperature of the surface of the glass rod 106 is lower than 1000°C, residual strain inside the glass rod 106 cannot be removed. (EXAMPLE) The glass rod 106 having an average diameter of 60 mm was installed on the glass rod second elongating apparatus 111. Black spots of impure foreign substances, the diameter of which were about 1 mm, were attached to the surface of the glass rod 106. The Black spots could not be removed by wiping with cloth. The glass rod 106 was rotated at the speed of 25 rpm. In the first flame polishing, the part of the glass rod 106, where the impure foreign substances was attached with, to 2000 °C by acid hydrogen flame with hydrogen 300 L/m and oxygen 170 L/m. By the first flame polishing, the impure foreign substances were removed, but glass particles were attached to the surface of the glass rod 106 so that white cloud was occurred on the surface of the glass rod 106. Next, in the second flame polishing, the surface of the glass rod 106 was heated to 1500°C by acid hydrogen flame with hydrogen 230 L/m and oxygen 130 L/m. The surface of the glass rod 106 was observed by irradiating the light of a halogen lamp or fluorescent light, then the surface of the glass rod 106 was transparent and even without damages, unevenness, impure foreign substances, and glass particles. Also, the strain inside the glass rod 106 was tested by qualitative test using a polarized light board, it was confirmed that the strain inside the glass rod 106 was removed. The glass rod 106, the surface of which had a hole of about diameter 20 µm, then the surface of the glass rod 106 was transparent and even, and the strain inside the glass rod 106 was removed. (COMPARATIVE EXAMPLE) Impure foreign substances attached to the surface of the glass rod 106, and had unevenness. Only the second flame polishing was performed heating the glass rod 106 to 1500°C. Then, the impure foreign substances and unevenness on the surface of the glass rod 106 couldn't be removed. According to the optical fiber base material heat treatment method of the present invention, the glass rod, which is an optical fiber base material, of the high quality can be obtained. The surface of the optical fiber base material becomes transparent without cloud caused by glass particles. The surface of the optical fiber base material becomes even without damages, unevenness, and impure foreign substances. Furthermore, the optical fiber base material can be obtained without residual strain inside can be obtained. Because the optical fiber manufacture method of the present invention uses the optical fiber base material treated by the heat treatment method of the present invention, the high quality optical fiber can be obtained.

Claims: A method of polishing an optical fiber base material (106) with heat comprising: a first flame polishing step in which the surface of said optical fiber base material (106) is heated by flame while keeping the length of said optical fiber base material (106) constant, and a second flame polishing step by which said surface of said optical fiber base material (106) is heated by flame to a lower temperature than in said first flame polishing step, characterized in that in said second flame polishing step said surface of said optical fiber base material (106) is heated to a temperature between 1000°C and 1800°C to remove residual stress inside said optical fiber base material (106). The method of polishing an optical fiber base material (106) as claimed in claim 1, characterized in that in said first flame polishing step uneven parts on said surface of said optical fiber base material (106) are at least locally heated, and in said second flame polishing step the entire surface of said optical fiber base material (106) is heated. The method of polishing an optical fiber base material (106) as claimed in claim 1, characterized in that in said first flame polishing step said optical fiber base material (106) is heated so that the temperature of said surface of said optical fiber base material (106) reaches 1800°C to 2200°C. The method of polishing an optical fiber base material (106) as claimed in claim 1, characterized in that said flame is generated by burning combustible gas, which contains hydrogen, and supporting gas, which contains oxygen for supporting the combustion. The method of polishing an optical fiber base material (106) as claimed in claim 1, characterized in that said flame is generated by burning combustible gas, which contains chain hydrocarbon, and supporting gas, which contains oxygen for supporting the combustion. The method of polishing an optical fiber base material (106) as claimed in one of claims 1 to 5, further comprising drawing said optical fiber base material (106) to a filament-like form by heating and drawing said optical fiber base material (106) to generate said optical fiber.

Inventors: HIDEKI FUJII, HIRASAWA HIDEO, HOSHINO SHOJI, SHIMADA TADAKATSU, TAKAAKI NAGAO, HIDEKI, FUJII, HIRASAWA, HIDEO, HOSHINO, SHOJI, SHIMADA, TADAKATSU, TAKAAKI, NAGAO
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 2000
Main classification: C03B37
IPC codes: C03B37, C03B29