METHOD OF MAKING OPTICAL GLASS ARTICLE

Abstract: A refractive index profile in a glass article is easily controlled by heating an axially symmetrical glass soot preform in an atmosphere comprising a fluorine-contain­ ing compound under such conditions that a partial pressure of the fluorine-containing compound is changed as the heat­ ing proceeds so as to control a fluorine concentration in a radial direction of the optical glass article.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of making an optical glass article, especially useful as an optical transmission line. More particularly, it relates to a method of making an axially symmetrical optical glass article having a refractive index profile in a radial direc­ tion which is determined by controlled addition of fluorine. Description of the Prior Arts Hitherto, several methods have been proposed to form a refractive index profile in an optical glass article by the controlled addition of fluorine. According to the first method, the refractive index profile is formed during vapor phase axial deposition of glass soot in the VAD method, which is schematically shown in Fig. 1. In this method, a burner 61 is used for emitting a glass-forming raw material (e.g., silicon tetra­ chloride and the like) for synthesizing glass soot which forms a core part 64 of the glass soot preform. A burner 62 is used for emitting a glass-forming raw material and a fluorine-containing compound for synthesizing glass soot which forms a peripheral part 65 of the glass soot preform. Numeral 63 stands for a glass soot preform. According to this method, fluorine, however, diffuses and is inevitably homogeneously added to the glass soot preform, so that a refractive index profile is always uniform. The second method is schematically shown in Fig. 2. In this method, a glass soot preform 73 is made in such manner that a core part 74 contains a compound for lowering a vitrification temperature of glass (e.g., GeO₂, P₂O₅ and the like) in a larger concentration than a peripheral part 75. When the glass soot preform 73 is inserted in a furnace 77 kept at high temperature, since the core part 74 shrinks faster than the peripheral part 75, it is possible to add more fluorine to the peripheral part 75 so as to form a refractive index profile by selecting a concentration of the fluorine-containing compound in a heating atmosphere and timing of the addition of the fluorine-containing compound. The second method, however, has a disadvantage that the glass article contains materials other than silica and fluo­ rine although it can realize the refractive index profile. The third method is described in Preprint 2-183 for 1984 National Meeting of Denshi-Tsushin Gakkai, Hikari Denpa Bumon (Electronic Communication Society, Light and Electric Wave Section) and so on. In this method, a glass soot preform formed so that a core part and a peripheral part have different bulk densities. When fluorine is added to such glass soot preform a furnace kept at high tempera­ ture, more fluorine is added to the peripheral part than to the core part since it diffuses more easily into the peripheral part than into the core part due to difference of the bulk density. Thereby, refractive index difference is made between the core and peripheral parts. To achieve 0.3 % of refractive index difference between the core and peri­ pheral parts according to the third method, the peripheral part should have a bulk density of about 0.2 g/cm³ and the core part should have a bulk density larger than 1.5 g/cm³. However, it is extremely difficult to remove residual water from the glass soot preform having a bulk density so large as 1.5 g/cm³. The fourth method is proposed in Japanese Patent Kokai Publication (unexamined) No. 61-132532. In this method, the refractive index profile is realized by provi­ ding glass soot particles with a particle size variation in a radial direction of the glass soot preform so as to vary the vitrification temperature of the glass soot preform in the radial direction, whereby an addition amount of fluorine is controlled. Although the fourth method has several advantages such that the transparent glass preform which contains less residual water and no other material than SiO₂ and fluorine can be obtained, and the additional fluorine content can be changed in the radial direction, it requires special technique for achieving the particle size variation such as utilization of plasma flame, carbon monoxide flame and the sol-gel method. The above described four methods are developed to realize the refractive index profile in the optical glass article. On the contrary, it is sometimes desired to make the refractive index profile uniform. However, it is diffi­ cult to realize a completely uniform profile of the refrac­ tive index in the optical glass article according to the above conventional methods, since the variation of the bulk density of the glass soot perform in the radial direction influences the amount of fluorine to be added. SUMMARY OF THE INVENTION One object of the present invention is to provide a method of making a glass article having a designed refrac­ tive index profile. Another object of the present invention is to provide a method of making a glass article suitable for use in optical transmission. Further object of the present invention is to provide a method of making a glass article which is suffi­ ciently dehydrated. These and other objects are accomplished by a method of making a glass article according to the present invention, which comprises heating an axial symmetrical glass soot preform in an atmosphere comprising a fluorine-­ containing compound under such conditions that a partial pressure of the fluorine-containing compound is changed as the heating proceeds so as to control a fluorine concentration in a radial direction of the glass article. In the present invention, the axial symmetric glass soot preform may be of solid or hollow cylindrical form. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 schematically show conventional methods to form a refractive index profile in a glass arti­ cle, Fig. 3A schematically shows an apparatus for car­ rying out the method of the present invention which comp­ rises a uniform heating furnace, Fig. 3B schematically shows an apparatus for car­ rying out the method of the present invention which comp­ rises a zone heating furnace, Figs. 4A, 4B and 4C are graphs showing relation­ ships between heating time and partial pressure of a fluo­ rine-containing compound or temperature in Examples 1, 2 and 3, respectively, Figs. 5A, 5B and 5C are graphs showing relative refractive index difference to quartz of the fluorine added glass article made in Examples 1, 2 and 3, respectively, Fig. 6 is a graph showing attenuation spectrum of an optical fiber fabricated from the glass article made in Example 2, Fig. 7 is a graph showing density of a composite of a glass rod and a glass soot preform used in Example 4, Fig. 8 is a graph showing relative refractive index difference of the glass article made in Example 4, and Fig. 9 is a graph showing relative refractive index difference of the glass article made in Comparative Example. DETAILED DESCRIPTION OF THE DRAWINGS According to the present invention, the refractive index profile in the optical glass article can be designed by controlling the addition amount of fluorine in an radial direction of the glass soot preform. In one preferred embo­ diment, the fluorine concentration is such that the concent­ ration is minimum at or near a core of the optical glass article and increases toward a periphery of the glass arti­ cle. To achieve such profile of the fluorine concentration, the partial pressure of the fluorine-containing compound is adjusted to be 100 Pa or lower at the start of heating and then increased to 100 KPa (0.99 atm.) or higher at a time of completion of vitrification of the glass soot preform. In another preferred embodiment, the fluorine concentration distribution is uniform in the radial direction. Preferable fluorine-containing compounds are sili­ con fluorides such as SiF₄, Si₂F₆ and the like, although other fluorine-containing compounds such as SF₆, NH₄F, NF₃, PF₅, CF₄ and CCl₂F₂ are suitably used. The fluorine-containing compound is introduced in the heating atmosphere together with an inert gas such as helium, argon, nitrogen and so on. Two preferred apparatus for carrying out the method of the present invention are schematically shown in Figs. 3A and 3B, respectively. The apparatus of Fig. 3A comprises a uniform heating furnace and that of Fig. 3B comprises a zone heating furnace. In both apparatus, 21 stands for a rotational axis, 22 stands for a muffle, 23 stands for a furnace, 24 stands for a heater, 25 stands for a glass soot preform, 26 stands for a pressure gauge, and 27 stands for a pressure regulator. In a preferred embodiment, a muffle is made of highly pure carbon. When the uniform heating furnace is used, as the furnace temperature increases and the glass soot preform shrinks, the partial pressure of the fluorine-containing compound is changed following a changing pattern designed according to the fluorine concentration profile which is determined according to the final refractive index profile in the glass article. When the zone furnace is used, the partial pres­ sure of the fluorine-containing compound and the furnace temperature are determined each time the glass soot preform passes through a heating zone in the furnace. To decrease a content of water in the glass soot preform, it is preferably heated at a temperature at which it does not shrink prior to heating it in the atmosphere comprising the fluorine-containing compound. In the pre­ dehydration, metallic impurities are also removed. Such predehydration of the glass soot preform is useful since the glass soot preform does not shrink and the predehydration does not affect the fluorine addition. The present invention will be hereinafter exp­ lained further in detail by following examples. Example 1 In the uniform heating furnace shown in Fig. 3A, a glass soot preform having an average bulk density of 0.25 g/cm³ was held, and furnace temperature was raised at a rate of 2°C/min. In the uniform heating furnace, a pure helium was introduced till the furnace temperature reached 1,300°C. Then, SF₆ was added at a partial pressure of 3 KPa. Total pressure was atmospheric pressure. Relationships between the heating time and the partial pressure of SF₆ or the temperature are shown in Fig. 4A. The preform removed from the furnace at 1,500°C was transparently vitrified. The transparent glass article had a refractive index profile shown in Fig. 5A. Example 2 Through the zone heating furnace of Fig. 3B, a glass soot preform having an average bulk density of 0.25 g/cm³ was passed three times under conditions shown in the following table: Note *1) Relative length = A ratio of length of a soot preform after each pass to length of the soot preform the first pass. Relationships between the heating time and the partial pressure of SiF₄ or the temperature are shown in Fig. 4B. The produced transparent glass article contained no bubble and had a refractive index profile shown in Fig. 5B. A single mode optical fiber fabricated from this transparent glass article had 0.4 dB/km of absorption due to water at a wavelength of 1.38 µm and 0.19 dB/km of attenu­ ation of light transmission at a wavelength of 1.55 µm. An attenuation spectrum of the optical fiber is shown in Fig. 6. Example 3 In the uniform heating furnace shown in Fig. 3A having a muffle made of highly pure carbon, a glass soot preform having an average bulk density of 0.25 g/cm³ was held, and furnace temperature was raised at a rate of 5°C/min. In the uniform heating furnace, a pure helium was introduced till the furnace temperature reached 1,300°C. Then the fluorine compound was added with gradually increa­ sing its partial pressure up to 300 KPa in a temperature range of 1,300°C to 1,500°C. Relationships between the heating time and the partial pressure of the fluorine com­ pound or the temperature are shown in Fig. 4C. The trans­ parently vitrified glass article had a refractive index profile shown in Fig. 5C. A graded index type optical fiber fabricated from this transparent glass article had a wave­ band of 500 MHz. Example 4 Around a glass rod consisting of a core part of pure silica and a peripheral part of SiO₂ containing fluo­ rine, glass soot was deposited by the VAD method to form a composite. Distribution of bulk density of the composite was measured undestructively by means of X-ray CT. It is as shown in Fig. 7. It is understood that the bulk density near the rod surface was relatively large. In Fig. 7, "a" stands for a thickness of the deposited soot and "b" stands for a diameter of the glass rod. The composite was heated by passing it through the zone heating furnace of Fig. 3B three times under conditions shown in the following table: The transparent glass article has a refractive index profile as shown in Fig. 8. Comparative Example In the same manner as in Example 4 but heating the composite under conditions shown in the following table, a transparently vitrified glass article was produced: The transparent glass article had a refractive index profile as shown in Fig. 9, from which it is under­ stood that, near the glass rod region, the addition amount of fluorine was smaller than in the peripheral region. According to the present invention, it is possible to make a glass article having a refractive index profile of a designed pattern using fluorine as a sole additive. Therefore, when the glass article made by the method of the present invention is used as a glass preform for fabricating an optical fiber, the optical fiber having improved hydrogen resistance and radiation resistance can be fabricated. When the glass soot preform is dehydrated prior to the addition of fluorine at lower temperature, the water content in glass article can be extremely reduced without affecting the amount of fluorine to be added. Further, the method of making a glass article according to the present invention does not require any special technique such as carbon monoxide flame, plasma flame or the sol-gel method.

Claims: 1. A method of making a glass article which comp­ rising heating an axially symmetrical glass soot preform in an atmosphere comprising a fluorine-containing compound under such conditions that a partial pressure of the fluo­ rine-containing compound is changed as the heating proceeds so as to control a fluorine concentration in a radial direc­ tion of the optical glass article. 2. The method according to claim 1, wherein the fluorine-containing compound is a silicone fluoride. 3. The method according to claim 1, wherein the heating is carried out in a muffle made of highly pure car­ bon. 4. The method according to claim 1, wherein the partial pressure of the fluorine-containing compound is 100 Pa or lower at the start of heating and 100 KPa or higher at the completion of vitrification of the glass soot preform. 5. The method according to claim 1, wherein the fluorine concentration in the glass soot preform is minimum at the core of the preform and gradually increases towards the periphery of the glass soot preform. 6. The method according to claim 1, wherein the fluorine concentration in the transparently vitrified glass article is homogeneous in the radial direction.

Inventors: ISHIGURO, YOICHI C/O YOKOHAMA WORKS, KANAMORI, HIROO C/O YOKOHAMA WORKS, KYOTO, MICHIHISA C/O YOKOHAMA WORKS, URANO, AKIRA C/O YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1986
Main classification: C03B37
IPC codes: C03B37