Method for producing glass preform for optical fiber

Abstract: A method for producing a glass preform for use in the fabrication of an optical fiber, which comprises adding fluorine to a soot preform in an atmosphere comprising a fluorine-containing compound at a temperature at which the soot preform is in the porous state and then keeping or insert­ ing it in an atmosphere containing a fluorine-containing com­ pound at a higher temperature to vitrify it to form a glass preform, from which glass preform, an optical fiber homogeneously containing fluorine is fabricated.

Description: FIELD OF THE INVENTION The present invention relates to a method for producing a glass preform for use in the fabrication of an optical fiber. More particularly, it relates to a method for producing a glass preform for use in the fabrication of an optical fiber which homogeneously contains fluorine and an extremely small amount of impurities. BACKGROUND OF THE INVENTION For the production of a glass preform added with fluorine, there are proposed methods comprising depositing glass soot particles to form a soot preform and then dehydrating and vitrifying the soot preform by keeping or inserting it in a furnace containing an atmosphere comprising fluorine. The first kind of such methods is described and claimed in Japanese Patent Kokai Publication (unexamined) No. 67533/1980 and comprises heating a soot preform in an atmosphere containing a fluorine-containing compound at a temperature not higher than 1,000°C and then heating and vitrifying the soot preform in an atmosphere of an inert gas at a temperature not lower than 1,400°C by keeping or inserting the soot preform 2 in an furnace 31 as shown in Fig. 1. Example 3 of Japanese Patent Kokai Publication (unexamined) No. 60938/1985 describes a similar method comprising heating a soot preform in an atmosphere containing SF₆ at 1,000°C and vitrifying it in an atmosphere not containing SF₆ at 1,600°C. In addition, Japanese Patent Kokai Publication (unexamined) No. 86045/1985 describes a method comprising heating a soot preform in an atmosphere comprising a fluorine-containing compound at a temperature not lower than 1000°C and lower than 1,400°C and then vitrifying the soot preform in an atmosphere of an inert gas at a temperature not lower than 1,400°C. The second kind of such methods comprises heating a soot preform in an atmosphere comprising a fluorine-containing compound and an inert gas at a temperature not lower than 1,400°C to produce a glass preform containing fluorine, in which the soot preform 2 is inserted in a zone furnace 41 equipped with a heater 40 as shown in Fig. 3. A method similar to the above method is described in Japanese Patent Kokai Publication (unexamined) No. 5035/1986 and comprises heating a soot preform in an atmosphere comprising a fluorine-containing compound at 1,650°C. Said publication states that "since the dopant is absorbed in the soot preform during vitrification of the glass, the treatment becomes easier at a higher temperature and vitrification proceeds at a higher rate than at a lower temperature so that the production cost is reduced". Examples 1,2 and 4 of Japanese Patent Kokai Publication (unexamined) No. 60938/1985 comprise heating a soot preform in an atmosphere comprising a fluorine-containing compound at 1,600°C. Further, claims 1 and 7 of Japanese Patent Kokai Publication No. 86049/1985 describe a method for producing dense glass by diffusing fluorine at a temperature in a sintering temperature range. As the result of the study of the above two kinds of the methods, following has been found: When, according to the first kind of the method, the soot preform is heated in an atmosphere comprising the fluorine-containing compound in an apparatus as shown in Fig. 1 at a temperature not higher than 1,000°C, the porosity of the soot preform is reserved. Thereafter, the soot preform is vitrified in an atmosphere of the inert gas. The analysis of the distribution of the refractive index of the produced glass preform revealed that the refractive index at a peripheral portion of the preform is larger than that of the central portion as shown in Fig. 2 in which R stands for an outer diameter of the glass preform. This means that the added amount of fluorine in the peripheral portion is less than that in the central portion. The reason for this may be that, since at the end of the addition of fluorine at 1,000°C, the preform is still porous, added fluorine is dissipated during subsequent heating in the inert gas atmosphere at a higher temperature. When, according to the second kind of the method, the soot preform is heated in an atmosphere comprising the fluorine-containing compound and the inert gas at a temperature not lower than 1,400°C and then vitrified, the produced glass preform has a distribution of the refractive index shown in Fig. 4, from which it is understood that the added amount of fluorine is smaller in the central portion. The reason for this may be that, since the addition of fluorine and vitrification simultaneously proceed, a period of time to add fluorine is not long so that a sufficient amount of the fluorine-containing compound does not reached the central portion. When the soot preform was inserted in the furnace at a rate less than a half of the practically employed rate, fluorine was homogeneously added to the central portion of the preform. However, the total treating time was extremely extended. EP-A-0139532 discloses a method for producing a glass preform in which a preform precursor formed of fine glass particles is initially heated to dehydrate them and to remove impurities. The preform is then further heated in an atmosphere including a fluorine-containing compound to add fluorine to the preform. Finally, the preform is vitrified in an inert atmosphere in order to render it transparent. One object of the present invention is to provide a method for producing a glass preform for use in the fabrication of an optical fiber, in which fluorine is homogeneously added. Another object of the present invention is to provide a method for producing a glass preform for use in the fabrication of an optical fiber which contains a smaller amount of impurities. Accordingly, the present invention provides a method for producing a glass preform for use in the fabrication of an optical fiber comprising the steps of: providing a transparent glass rod consisting of a pure quartz-made core and a cladding made of a quartz added with fluorine, depositing soot particles around said transparent glass rod, adding fluorine to the soot particles in an atmosphere comprising a fluorine-containing compound at a temperature at which the soot is in the porous state and for a time sufficient to allow fluorine to become homogeneously distributed in the radial direction of the soot portion of the preform, and vitrifying the fluorine containing soot preform at a higher temperature to form a glass preform by keeping or inserting the fluorine containing soot preform in an atmosphere containing a fluorine-containing compound. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 3 schematically show furnaces in which the soot preform is heated, Figs. 2 and 4 are graphs showing distributions of refractive index of conventional glass preforms, Figs. 5 and 6 are graphs showing distributions of refractive index of the glass preforms produced according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The soot preform to be treated by the method of the present invention may be produced by any of conventional methods such as the VAD method, the OVD method, the OVPD method and the like. According to the present invention, fluorine is added to the soot preform at a temperature at which the soot is still in the porous state. Thereafter, the soot preform added with fluorine is kept or inserted in a furnace having an atmosphere comprising the fluorine-containing compound at a higher temperature at which the soot preform is vitrified to give a transparent glass preform. Preferably, the fluorine-containing compounds used in the fluorine-adding step and the vitrifying step are the same. Specific examples of the fluorine-containing compound are SiF₄, Si₂F₆, SF₆, NH₄F, NF₃, PF₅ and CF₄, and chlorofluorocarbons such as CCl₂F₂. Among them, silicon fluorides such as SiF₄ and Si₂F₆ are preferred, since they do not form bubbles in the glass preform even when the soot preform is vitrified at a higher temperature and a higher rate. The addition of fluorine is preferably carried out at a temperature not lower than 1,100°C and lower than 1,400°C. When this temperature is too low, any reaction between the fluorine-containing compound and the soot preform does not proceed. On the contrary, it is too high, the soot preform too quickly shrinks. Since the fluorine is added at a temperature at which the soot preform is in the porous state, fluorine is sufficiently diffused to the central portion of the preform. At a higher temperature, the fluorine-containing compound is decomposed so as to dehydrate the soot preform. Therefore, any dehydrating agent such as a chlorine-containing compound is not necessarily used in the method of the present invention. For the purpose of dehydration, a fluorine-containing compound having no hydrogen atom such as SiF₄, Si₂F₆, SF₆ and CF₄ is preferred. When any dehydrating agent is not used, impurities such as iron tends to remain in the vitrified preform, and an optical fiber fabricated from such preform shows increase attenuation of light transmission at a wavelength of 0.8 to 1.3 µm. Therefore, it is preferred to use a chlorine-containing compound in the method of the present invention for dehydrating water and/or removing impurities. The dehydration with the chlorine-containing compound may be carried out prior to or simultaneous with the addition of fluorine. The vitrification of the preform is preferably carried out in the absence of the chlorine-containing compound in view of improvement of hydrogen resistance or radiation resistance. As the chlorine-containing compound, chlorine as such and carbon tetrachloride are preferred. The addition of fluorine and the vitrification of the soot preform may be carried out in the same furnace or in separate furnaces. Since the vitrification of the soot preform is carried out in the presence of the fluorine-containing compound, fluorine added to the peripheral portion of the preform in the fluorine adding step is not dissipated in the vitrifying step. In addition, since the soot preform to which fluorine has been added is vitrified, the vitrification rate is not influenced by a period of time for adding fluorine so that the preform can be vitrified at a higher rate. When the chlorine-containing compound is used, dehydration and removal of impurities are effectively performed so that it is possible to produce the glass preform containing a very small amount of impurities such as iron. A preferred embodiment of the present invention will be shown in the following example. Example 1 Around a transparent glass rod consisting of a pure quartz made core having an outer diameter of 2 mm and a cladding made of a quartz added with fluorine in a amount corresponding to a specific refractive index difference of -0.3 % having an outer diameter of 10 mm, glass soot particles were deposited to an outer diameter of 120 mm. The produced composite was treated in the zone furnace under following conditions: First step [Dehydration removal of impurities and addition of fluorine] Atmosphere: 97 % of helium, 3 % of SiF₄ Heater surface temperature: 1,350°C Traversing rate: 6 mm/min. Second step [Vitrification] Atmosphere: 97 % of helium, 3 % of SiF₄ Heater surface temperature: 1,650°C Traversing rate: 6 mm/min. The vitrified preform contained no bubble. The transparent glass preform was drawn to fabricate an optical fiber, which had attenuation of light transmission of 0.34 dB/km at a wavelength of 1.3 µm and transmission loss due to the hydroxyl group of 0.6 dB/km at a wavelength of 1.38 µm. The presence of impurities which were not removed by the fluorine-containing compound did not materially influence the quality of the optical fiber.

Claims: A method for producing a glass preform for use in the fabrication of an optical fiber comprising the steps of: providing a transparent glass rod consisting of a pure quartz-made core and a cladding made of a quartz added with fluorine, depositing soot particles around said transparent glass rod, adding fluorine to the soot particles in an atmosphere comprising a fluorine-containing compound at a temperature at which the soot is in the porous state and for a time sufficient to allow fluorine to become homogeneously distributed in the radial direction of the soot portion of the preform, and vitrifying the fluorine containing soot preform at a higher temperature to form a glass preform by keeping or inserting the fluorine containing soot preform in an atmosphere containing a fluorine-containing compound.

Inventors: ISHIGURO YOICHI, KANAMORI HIROO, KYOTO MICHIHISA, ISHIGURO, YOICHI YOKOHAMA WORKS OF SUMITOMO, KANAMORI, HIROO YOKOHAMA WORKS OF SUMITOMO, KYOTO, MICHIHISA YOKOHAMA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1986
Main classification: C03B37
IPC codes: G02B6, C03B20, C03B8, C03B37