Process for thermal treatment of glass fiber preform

Abstract: The present invention provides a process for the dehydrating and purifying treatment by heating a porous glass preform for an optical fiber comprising passing the porous glass preform through a muffle tube having a SiC layer at least on its inner surface at a high temperature under an atmosphere comprising an inert gas and a silicon halogenide gas; a process for the fluorine-doping treatment by heating a porous glass preform for an optical fiber comprising passing a porous glass preform through a muffle tube having a SiC layer at least on its inner surface at a high temperature under an atmosphere comprising a fluorine compound gas and an inert gas; and a process for the vitrifying treatment by heating a porous glass preform for an optical fiber comprising passing the preform, which has been previously dehydrated and purified, through a muffle tube having a SiC layer at least on its inner surface at a high temperature under an atmosphere gas.

Description: Field of the Invention The present invention relates to a process for the thermal treatment of a porous glass preform for the production of an optical fiber preform. Description of the Related Art A preform for an optical fiber is generally produced by forming a cylindrical or an annular porous preform by the Vapor Phase Axial Deposition method or the Outside Chemical Vapor Deposition method and heating, dehydrating and sintering the porous preform in a heating furnace in an atmosphere of an inert gas such as argon or helium, a chlorine based gas or a fluorine based gas to produce a transparent and highly pure preform for the optical fiber. The operating temperature of the furnace depends on the kind of dopant to be doped and a content of the dopant in the preform, and it is generally in the range of 1200 to 1600 °C. Since impurities tend to contaminate the preform at such high temperature, a muffle tube made of a high purity quartz is typically used in the furnace to prevent the contamination. However, the quartz made muffle tube tends to rapidly get brittle due to devitrification, that is, transition from the glass phase to the crystal phase at a high temperature, and thus it is poor in durability. Recently, a carbon made muffle tube the inner surface of which is coated with silicon carbide (SiC) is often used in a heating furnace. The carbon made muffle tube can be operated at a higher temperature than the quartz made one and SiC improves a gas impermeability and oxidation resistance of the muffle tube (EP-A-0 302 121). The thermal treatment of a porous glass preform is performed in an inert gas atmosphere containing at least one fluoride selected from the group consisting of silicon fluorides and carbon fluorides. Further, in order to prevent a reaction between SiC and a reactive gas, the SiC coating on the carbon made muffle tube is treated with oxygen to form a SiO₂ layer on the SiC coating (see Japanese patent Kokai Publication No. 201634/1986). The prior arts as described above have following problems: (1) When the quartz made muffle tube is used, it is softened and deformed at a temperature above 1400 °C. In addition, the temperature of the muffle tube cannot be lowered to a temperature below a crystal transition point (lower than 300 °C) because of crystal (cristobalite) formation at a temperature above 1200 °C. Thus, once the muffle tube is heated, it should be used continuously without lowering the temperature thereof. (2) When a SiC made or SiC coated muffle tube is used, SiC reacts with a halogen based gas as a reactant to form a porous carbon, whereby the muffle tube becomes poor in gas tightness so that the halogen based gas leaks outside from the muffle tube. (3) In the case where the muffle tube having the SiO₂ layer on the SiC coating is used, thermal expansion coefficients of SiC and SiO₂ are greatly different from each other and thereby the SiO₂ layer in a thickness in the order of some microns tends to crack. Then, a gas permeates through the crack and it is impossible to stably produce the preform for a long period. SUMMARY OF THE INVENTION It is an object of the present invention to provide a process of thermal treatment for the stable production of an optical fiber preform for a long period from which preform an optical fiber having a low transmission loss can be drawn. Accordingly, the present invention provides a process for the dehydrating and purifying treatment by heating of a porous glass preform for an optical fiber, comprising passing the preform through a muffle tube having a SiC layer at least on its inner surface at a high temperature in an atmosphere consisting of an inert gas, a silicon halogenide gas and optionally a halogen based gas, wherein the volume ratio between the silicon halogenide gas and the inert gas is 0.3 to 10 parts by volume to 100 parts by volume, respectively BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 4 and 7 each schematically shows a sectional view of an heating furnace in which the present invention is carried out, Fig. 5 schematically shows an apparatus for measuring an oxygen concentration in a muffle tube, Fig. 6 shows results of oxygen measurements in one muffle tube with the measuring apparatus of Fig. 5, and Fig. 8 shows the transmission loss of an optical fiber produced in Example 1. DETAILED DESCRIPTION OF THE DRAWINGS The present invention will be described in detail with reference to the accompanying drawings. Fig. 1 schematically shows a sectional view of one embodiment of a heating apparatus in which the process of the present invention is carried out. The apparatus comprises a muffle tube 12, in which a porous glass preform 11 is inserted, and a furnace body 15 around the muffle tube 12 comprising a heater 13 to heat the glass preform 11, and an insulation 14 to prevent heat loss. A dehydrating and purifying gas for dehydrating and purifying the preform is supplied through a line 16 connected with the muffle tube. In addition, a silicon halogenide gas generated in a bubbler 17 is mixed with the other dehydrating and purifying gas G in a gas mixer 18. The muffle tube is made of carbon and coated with a SiC layer. Further, the SiC layer is coated with a SiO₂ layer. Each layer can be formed in a conventional manner. The term "dehydrating and purifying gas" is intended to mean a gas mixture of an inert gas such as helium (He), Argon (Ar) and nitrogen (N₂) with a silicon halogenide gas and optionally a halogen based gas. The present invention has been conceived on the basis of the following experiments: Experiment 1 A carbon made plate coated with SiC in a thickness of 100 µm was heated at 1500 °C under an atmosphere of the dehydrating and purifying gas containing 2.5 parts by volume of SiCl₄ relative to 100 parts by volume of the inert gas (He) for 10 hours. After heating, a weight loss of the SiC coating was determined to be as small as 1 %. A specific surface area of the SiC coating was 0.1 m²/g, which was substantially equal to that of the SiC coating before heating. Experiment 2 Experiment 1 was repeated except that the dehydrating and purifying gas contained 3 parts by volume of SiCl₄ and 1 part by volume of Cl₂ each relative to 100 parts by volume of the inert gas. After heating, the weight loss of the SiC coating was determined to be as small as 1.4 %. The specific surface area of the SiC coating was 0.15 m²/g, which was substantially equal to that of the SiC coating before heating. Experiment 3 Experiment 1 was repeated except that the dehydrating and purifying gas contained 5 parts by volume of Cl₂ relative to 100 parts by volume of the inert gas. After heating, the weight loss of the SiC coating was determined to be as large as 71 %. This means that SiC was completely decomposed. The specific surface area of SiC was 800 m²/g and large pores having a diameter of 30 Å were found. From the above Experiments, the following results can be induced: (1) At a high temperature, SiC reacts with Cl₂ and does hardly with SiCl₄. (2) The reaction between SiC and Cl₂ can be suppressed when SiCl₄ is added to Cl₂ in the dehydrating and purifying gas. Thus, in order to prevent the degradation of the SiC coating due to Cl₂, the dehydrating and purifying treatment is preferably carried out under an atmosphere of SiCl₄ or an atmosphere of Cl₂ mixed with SiCl₄. Those considerations can be expressed according to the following equilibrium equation: SiC + 2Cl₂ ↔ SiCl₄ + C (1) The above equation indicates the degradation of SiC by Cl₂. Therefore, in order to suppress the degradation, a concentration of the product SiCl₄ is increased on the basis of an equilibrium relation expressed by an equilibrium constant When a gas mixture of the inert gas and the silicon halogenide gas is used as the dehydrating and purifying gas, 0.3 to 10 parts, preferably 2 to 5 parts by volume of the silicon halogenide gas is used relative to 100 parts by volume of the inert gas. When a ratio of the silicon halogenide gas is less than 0.3 parts by volume, a dehydrating ability of the gas mixture is insufficient. When it is more than 10 parts by volume, increase of the effect due to addition of such amount of SiCl₄ is not so remarkable. When a gas mixture of the inert gas, the silicon halogenide and the halogen based gas is used as the dehydrating and purifying gas, 0.3 to 10 parts by volume of the silicon halogenide gas and 0.1 to 10 parts by volume of the halogen based gas are preferably used relative to 100 parts by volume of the inert gas. When the halogen based gas is contained more than 10 parts by volume, the SiC coating is degraded. When it is less than 0.1 part by volume, no effect of adding the halogen based gas is achieved. In any event, the addition of the halogen based gas further increases the dehydrating ability of the gas mixture. A heating temperature at which the dehydrating and purifying treatment can be effectively carried out is in the range of 900 to 1200 °C. When the heating temperature is lower than 900 °C, the dehydration and purification are insufficient. When it is higher than 1200 °C, the porous glass preform contracts whereby diffusion of the dehydrating and purifying gas into the preform inside and volatilization of impurities to be removed from the preform inside are suppressed. In an alternative manner to prevent the degradation of SiC in the presence of the halogen based gas contained in the dehydrating and purifying gas, the porous glass preform is dehydrated and purified in a heating furnace having a quartz made muffle tube firstly, and then heated in the muffle tube coated with SiC. The quartz made muffle tube is not degraded by the halogen based gas such as Cl₂, but the temperature thereof cannot be lowered since crystallization happens at a temperature above 1200 °C as described above. In order to avoid the crystallization, the dehydrating and purifying treatment in the quartz made muffle tube is preferably performed at a temperature lower than 1100 °C. The muffle tube preferably used in the present process has an inner layer made of a highly pure silicon carbide or has an inner and an outer layer each made of the highly pure silicon carbide. The highly pure silicon carbide used for the muffle tube has preferably a purity of not less than 99.999 % and preferably contains iron of not more than some ppm and copper of not more than 1 ppm. The muffle tube in the heating furnace used in the present invention has the highly pure silicon carbide layer as its inner layer or as its inner and outer layers. The silicon carbide layer is preferably formed by a coating method with vapor phase reaction (CVD method) such as plasma CVD (PCVD) coating method or chemical CVD coating method since a highly pure and dense coating can be formed by such method. A thickness of the highly pure silicon carbide coating can be selected depending on the operation temperature of the muffle tube and the atmosphere in the muffle tube. Generally, the coating has a thickness of at least 1 µm, preferably at least 5 µm, more preferably at least 25 µm, for example 50 µm. Carbon, alumina or SiC sintered material can be exemplified as a material for the muffle tube substrate. As the carbon material, especially a highly pure carbon material is preferred. In the case where the highly pure carbon is used for the muffle tube substrate, the impurity of the carbon expressed as a total ash content is not more than 50 ppm, preferably not more than 20 ppm. For example, when the carbon has the ash content of more than 1000 ppm, it cannot be used for making the muffle tube substrate in view of the impurities such as iron and copper. The impurities and their amounts contained in the carbon having the total ash content of not more than 20 ppm are shown in the following Table: B < 0.1 ppm Ca < 0.1 ppm Mg < 0.1 ppm Ti < 0.1 ppm Al < 0.1 ppm V < 0.1 ppm Si < 0.8 ppm Cr < 0.1 ppm P < 0.2 ppm Fe < 0.1 ppm S < 0.1 ppm Cu < 0.1 ppm Ni < 0.1 ppm Fig. 2 Schematically shows a sectional view of a heating furnace with which the present process is performed. In Fig. 2, the reference number 21 indicates a porous glass preform, 22 a supporting rod for the preform, 23 a muffle tube, 24 a heater, 25 a furnace body, 26 an inlet for introducing an inert gas inside the furnace body and 27 an inlet for introducing an atmosphere gas (for example, SF₆, helium and so on) into the muffle tube. The reference number 231 indicates a muffle tube substrate made of carbon, 232 a coating layer of the highly pure silicon carbide. In the embodiment as shown in Fig. 2, the coatings of the highly pure silicon carbide are present as an inside layer and an outside layer of the muffle tube 23. Fig. 3 schematically shows a sectional view of a heating furnace for the optical fiber preform which is also used in accordance with the present invention. In the embodiment shown in Fig. 3, a member 31 is shown which is made of a heat resistant material having a small gas permeability such as a ceramic or a metal material and in which a muffle tube 23 is inserted in order to prevent penetration of a contaminant into the muffle tube through a wall thereof. At least an inner surface of the muffle tube 23 is coated with the highly pure carbon (not shown). A preferred material used for the member 31 is the ceramic or the metal material having a nitrogen permeability in the order of 10⁻⁶ cm²/sec. or less. Examples of the ceramic material are, in addition to silicon carbide, quartz glass, Al₂O₃, BN and so on. In another embodiment of an heating furnace which is used in the present invention, a muffle tube comprises an upper portion, a middle portion and a lower portion which are detachably connected each other. At least the middle portion is made of the highly pure carbon, and the upper and the lower portion are made of a heat and corrosion resistant material. One example of the above embodiment is Schematically shown in Fig. 4. A heater 24 is located inside a heating furnace body 25. A muffle tube 23 comprises an upper portion 434, a middle portion 435 and a lower portion 436 and they are detachably connected each other by, for example, screwing. At least an inner surface of the middle portion 435 is coated with a layer 432 made of the highly pure silicon carbide. Since the upper portion 434 and the lower portion 436 are not subjected to so high temperature as in the middle portion 435, they are not necessarily made of such highly pure material as in the middle portion 435, but they only need to have the gas impermeabilities. Thus, although the upper and the lower portions are shown to have the layers 432 made of the highly pure silicon carbide as shown in Fig. 4, the layers are preferably made of a usual silicon carbide material from a view point of economy which does not have such high purity as the silicon carbide material as described above. For example, a silicon carbide material having a purity of 99.9 % is satisfactory. In addition, since the upper and lower portions are heated at a temperature not higher than 1000 °C, they may be made of a quartz material which is not resistant to a fluorine based gas. Even in such case, the contents of copper and iron, especially copper content should be taken into account and they are preferably not more than 0.1 ppm. The muffle tube having the middle portion at least the inner surface of which is coated with the highly pure silicon carbide is suitably used in the present invention since it does not react with the halogen based gas such as SiF₄, Si₂F₆, CF₄ and C₂F₆, and it is highly heat resistant. In the case where the heating furnace as shown in Fig. 2, 3 or 4 is used, a large amount of air around the muffle tube (or environmental operation atmosphere) penetrates into the muffle tube, when the glass preform is inserted in or removed from the muffle tube. Fig. 5 schematically shows an apparatus which measures an amount of air inflow into a muffle tube comprising the muffle tube 51, an inlet 52 for introducing a purging gas, a sampling tube 53 for a gas in the muffle tube, a device 54 which measures an oxygen concentration in the gas and a pump 55. An inner diameter of the muffle tube 51 is 150 mm and a tip portion of the sampling tube is located at a position by 1 m from an opening of the muffle tube. Results of the oxygen concentration measurement carried out with respect to one embodiment with the above measuring device are shown in a graph in Fig. 6. As understood from the graph, the air around the muffle tube penetrates into the muffle tube and it is impossible to prevent the inflow of the air even when an amount of the purging nitrogen is increased. The inflow of the air causes problems as follows: Firstly, inside of the muffle tube is contaminated with dusts in the air. The dusts comprises SiO₂, Al₂O₃, Na₂O, Fe₂O₃ and so on. Among them, Al₂O₃ and Na₂O cause devitrification of the preform, and Fe₂O₃ causes increase of a transmission loss. Such inflow of the air is prevented in an additional embodiment of the heating furnace in which the present invention is performed. The heating furnace comprises a heater, a muffle tube and a front chamber which previously or subsequently accommodates the glass preform before or after a thermal treatment of the preform, and from or in which the preform is inserted in or removed from the muffle tube. Fig. 7 schematically shows a heating furnace which is the same as that shown in Fig. 4 except that it comprises the front chamber 71. When the furnace is used, the glass preform 21 to be thermally treated is inserted in the front chamber 71 and a top cover (not shown) of the front chamber is closed. Then, nitrogen is supplied in the front chamber which is separated from the muffle tube 23 with a partition means 76 for nitrogen replacement. The partition means 76 is opened, the porous glass preform is inserted in the muffle tube 23 of which inside atmosphere has been previously replaced with a desired atmosphere gas, the partition means is closed and then the thermal treatment is initiated. After the thermal treatment, the partition means is opened, the partition means is closed after the glass preform is lifted from the muffle tube into the front chamber, and then the glass preform is removed from the front chamber after opening the top cover. The front chamber is preferably constructed so that it can be evacuated to a pressure of 10⁻² Torr and be heated to a temperature of 800 °C. The front chamber is preferably made of a material which is heat resistant and which does not liberate any contaminant. For example, quartz glass, SiC, Si₃N₄ or BN is preferred for making the front chamber. The material of the chamber may the same as or different from that of the muffle tube. When the front chamber is evacuated, for example a rotary pump may be used. In order to prevent a back flow of a pump oil, a trap cooled with liquid nitrogen may be located between the front chamber and the pump. A rotary installing mechanism having a magnetic seal is disposed at the top of the front chamber. EXAMPLES The present invention will be further described with the following examples. Example 1 A heating furnace comprising a muffle tube 23 having a SiC coating 232 in a thickness of 50 µm as shown in Fig. 2 was used. A porous glass preform 21 was inserted in the muffle tube at a temperature of 1100 °C. 5 l/min. of He and 300 cc/min. of SiCl₄ were supplied in the muffle tube as the dehydrating and purifying gas. The preform was lowered and passed along a side portion of a heater 24 through the muffle tube at a lowering rate of 5 mm/min. and thereby the preform was dehydrated. Subsequently, the temperature of the furnace was raised to 1650 °C and the preform was again passed along the side portion of the heater at a rate of 2 mm/min. while supplying He at 5 l/min., whereby the preform was vitrified. An optical fiber was drawn from the glass preform obtained by the above treatment. A residual water content in the optical fiber was less than 10 ppb to show that substantially no water was present in the optical fiber. A transmission loss was measured and the results thereof showed no absorption increase of the optical fiber due to a transition metal such as Fe, Cu, Cr or Ni (see Fig. 8, curve I). The above treatment was carried out to vitrify thirty preforms. After the thirtieth treatment, a surface of the muffle tube was observed. No degradation of the SiC coating was observed. Example 2 Example 1 was repeated except that a gas mixture consisting of 5 l/min. of He, 200 cc/min. of SiCl₄ and 50 cc/min. of Cl₂ was used as the dehydrating and purifying gas. The surface of the muffle tube was observed after treating thirty preforms. No degradation of the SiC coating was observed as in Example 1. Comparative Example 1 Example 1 was repeated except that 5 l/min. of He and 200 cc/min. of Cl₂ were supplied as the dehydrating and purifying gas. The treatment was carried out on ten preforms and an optical fiber was drawn from each preform. The water content of each optical fiber was determined and was as large as 0.2 to 1 ppm in the optical fibers drawn from the third to tenth preforms. The wavelength transmission loss characteristic of each optical fiber was measured and showed not only an absorption peak due to OH at 1.4 µm but also an absorption peak of 3 to 10 dB/km due to Cu²⁺ at 0.85 µm (see Fig. 8, Curve II). The surface of the muffle tube was observed after the treatment of the tenth preform. The SiC coating in a heating portion of the muffle tube was completely changed and green crystal of CuCl₂ was deposited on electrodes (made of copper) of the heater in the furnace body due to corrosion with Cl₂. As explained with reference to the Examples and the Comparative Example, when the muffle tube having the coating of SiC is used in accordance with the present process, a preform for optical fibers having substantially no water and substantially no impurity can be stably formed so that the optical fiber having a less transmission loss can be drawn from such preform. In addition, since the muffle tube does not destroy due to the temperature lowering operation in comparison with the conventional quartz made muffle tube, the muffle tube can be economically and stably used for a long period.

Claims: A process for the dehydrating and purifying treatment by heating of a porous glass preform (11) for an optical fiber, comprising passing the porous glass preform (11) through a muffle tube (12) having a SiC layer at least on its inner surface at a high temperature in an atmosphere consisting of an inert gas, a silicon halogenide gas and optionally of a halogen based gas, wherein the volume ratio between the silicon halogenide gas and the inert gas is 0.3 to 10 parts by volume to 100 parts by volume, respectively. The process according to claim 1, wherein the silicon halogenide gas is SiCl₄. The process according to claim 1 or 2, wherein the halogen based gas is Cl₂. The process according to any one of the claims 1 to 3, wherein a volume ratio between the halogen based gas and the inert gas is 0.1 to 10 parts to 100 parts, respectively. The process according to any one of the claims 1 to 4, wherein a heating temperature of the preform is in the range of 800 to 1200°C. The process according to any one of the claims 1 to 5, wherein the SiC layer has a purity expressed in terms of a total ash content of not more than 50 ppm. The process according to any one of the claims 1 to 6, wherein the muffle tube further has a SiC layer on its outer surface. The process according to any one of the claims 1 to 7, wherein the SiC layer is formed with a method selected from the group consisting of PCVD coating method and CVD coating method. The process according to any one of the claims 1 to 8, wherein the SiC layer has a thickness of not less than 1 µm.

Inventors: ISHIKAWA SHINJI, KANAMORI HIROO, KYOTO MICHIHISA, OHGA YUICHI, TSUCHIYA ICHIRO, YOKOTA HIROSHI, ISHIKAWA, SHINJI, C/O YOKOHAMA WORK, KANAMORI, HIROO, C/O YOKOHAMA WORK, KYOTO, MICHIHISA, C/O YOKOHAMA WORK, OHGA, YUICHI, C/O YOKOHAMA WORK, TSUCHIYA, ICHIRO, YOKOTA, HIROSHI, C/O YOKOHAMA WORK, TSUCHIYA, ICHIRO, C/O YOKOHAMA WORK
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C03B37
IPC codes: G02B6, C03B37