Method for producing porous glass preform for optical fiber

Abstract: A method for producing a porous preform for use in the fabrication of an optical fiber with at least two bur­ ners for synthesizing glass soot particles at least one of which generates a double-layer flame and at least one of which is used for forming a core part of the preform, which method comprises steps of supplying SiCl₄ and optionally GeCl₄ to an inner flame of the double-layer flame, and SiCl₄ to an outer flame of the double-layer flame to flame hydro­ lyze the supplied compounds to synthesize glass soot parti­ cles, supplying SiCl₄ and GeCl₄ as glass-forming raw mate­ rials to the burner for forming the core part to flame hydrolyze the supplied compound to synthesized glass soot particles and depositing the generated glass soot particles on the lower end of a rotating starting member and growing the porous preform in a direction of an axis of the starting member to produce the porous preform comprising a core part containing at least partly GeO₂, a porous preform produced by this method gives a transparent glass preform a surface of which is not cracked or peeled off.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for producing a porous glass preform for use in the fabrication of an optical fiber (herein after referred to as "porous preform"). Description of the Related Art Known methods for producing a preform for use in the fabrication of a quartz base optical fiber include the inside chemical vapor deposition (CVD) method, the OVD method and the VAD method. For example, the VAD method is suitable for economically producing a preform for use in the fabrication of an optical fiber having low transmission loss, an arbitrary refractive index profile in the diameter direction and a homogeneous composition in the peripheral and lengthwise directions. Fig. 1 shows a conventional embodiment of the VAD method for producing a quartz base preform. The porous preform 11 is grown from the lower end of a rotating starting member 10 with a soot stream 12 generated by an oxyhydrogen burner 13. An exhausting tube 14 removes undeposited soots. For example, a multi-port burner having a cross section shown in Fig. 2 is used as the oxyhydrogen burner 13, and a glass-forming raw material is supplied from the center port 1, hydrogen gas (H₂) is supplied from second and sixth ports 2 and 6, oxygen gas (O₂) is supplied from fourth and eighth ports, and argon gas (Ar) is supplied from third, fifth and seventh ports (namely, in the order of hydrogen, argon, oxygen, argon, hydrogen, argon and oxygen) to generate a double-layer flame. In the oxyhydrogen flame, the glass-forming raw material is flame hydrolyzed to generate glass soot particles. Then, the glass soot particles are deposited on the lower end of the rotating starting member 10 such as a glass rod, and a cylinder-form mass of glass soot particles, namely the porous preform 11 is grown in the direction of the axis of the starting member 10. Thereafter, the porous preform is heated and consolidated to obtain a transparent glass preform. By the above VAD method, it is easy to produce a large size preform with good productivity. Therefore, the VAD method is widely used in the optical fiber industry. In the above VAD method which utilizes the double-layer flame and produces a porous preform having a desired refractive index profile at a high production rate, if the glass-forming raw material contains a compound such as GeCl₄ which is added to the porous preform in the form of GeO₂, a layer containing GeO₂ at a high concentration is formed on the surface of the porous preform, and the surface of the sintered preform which is obtained by heating and vitrifying the porous preform tends to be cracked due to difference of coefficients of thermal expansion between the inside and the surface of the preform. The Table below shows the relationship between the GeO₂ concentration at the surface of the porous preform and cracks of the sintered preform. As seen from the Table, the GeO₂ concentration at the surface of the porous preform should be not larger than 5.0 % by weight. US-A-4801322 discloses a multi-flame burner which includes an inner central orifice and an outer orifice for supplying glass raw materials. Interposed between the orifices are a plurality of flame forming orifices which form a plurality of flames. Such an arrangement allows different raw materials to be supplied to different parts of the burner flame. In one particular embodiment, the multi-flame burner is used to produce a preform in which SiCl₄ and GeCl₄ are supplied to an inner burner as raw materials for synthesizing the core whereas SiCl₄ is supplied to an outer burner for synthesizing the cladding. SUMMARY OF THE INVENTION An object of the present invention is to provide an improved VAD method for producing a porous preform, which does not suffer from the above drawbacks. According to the present invention, there is provided a method for producing a porous preform for use in the fabrication of an optical fiber with at least two burners for synthesizing glass soot particles at least one of which generates a double-layer flame and at least one of which is used for forming a core part of the preform; the method comprising the steps of: supplying SiCl₄ and GeCl₄ as glass-forming raw materials to the burner which is arranged to form the core part to flame hydrolyze the supplied compounds to synthesize glass soot particles; supplying SiCl₄ as the only glass-forming raw material to the burner which is arranged to form the non-core part of the preform to flame hydrolyze the supplied compound to synthesize glass soot particles; in any event supplying SiCl₄ to the outer flame of the double-layer flame as the only glass raw material; and depositing the generated glass soot particles on the lower end of a rotating starting member and growing the porous preform in a direction of an axis of the starting member to produce the porous preform comprising a core part containing at least partly GeO₂. As result of extensive study for improving the conventional VAD method, it has been found that a surface layer having a high GeO₂ concentration is formed because of the phenomenon that undeposited SiO₂ particles and GeO₂ particles contained in a stream along the surface of the already formed porous preform react each other in the inner flame of the double-layer flame generated by the multi-port burner, and GeO₂ is contained in SiO₂ in the solid solution form. Then, a measure for preventing diffusion of the stream of undeposited particles in the inner flame, namely a measure for forming a turbulent flow has been sought, and it has been found that it is effective to supply the glass-forming material to the outer flame in addition to the inner flame. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows a conventional VAD method, Fig. 2 is a cross sectional view of a multi-port Fig. 3 schematically shows a preferred embodiment of the method of the present invention, Fig. 4 shows the refractive index profile of an optical fiber fabricated from a preform which is produced by Example 1. DETAILED DESCRIPTION OF THE DRAWINGS The present invention will be explained in detail by making reference to the accompanying drawings. Fig. 3 schematically shows one embodiment of the method of the present invention. The same numerals stand for the same elements as in Fig. 1. Numeral 9 stands for an outer flame forming a soot stream, 12 stands for an inner flame forming a soot stream, and 15 stands for a multi-port burner for supplying glass-forming raw materials. Although the outer flame forming a soot stream reaches the surface of an already deposited area of the porous preform, 70 % of said stream does not contribute to the deposition of the glass soot particles but acts to generate the turbulent flow. As the glass-forming raw material, SiCl₄ and GeCl₄ are exemplified, although other compounds which are raw materials for known additives such as TiCl₄, AlCl₃, PbCl₃ and POCl₃ may be added to the glass-forming raw material. In Fig.3, the first burner 18 for synthesizing the glass soot particles is used to form the center (core) part of the porous preform 11, while the second burner 15 is used for forming the peripheral part of the porous preform 11. One or both of the burners can be multi-port burners. For example, when the first burner 18 is a usual burner for generating a single flame and the second burner 15 is a multi-port burner for generating a double-layer flame, preferably, to the first burner 18, SiCl₄ and GeCl₄ are supplied as the glass-forming raw materials, and to the inner flame of the burner 15, SiCl₄ and GeCl₄ are supplied and to the outer flame of the burner 15, only SiCl₄ is supplied, or to the burner 18, SiCl₄ and GeCl₄ are supplied, and both to the outer and inner flames of the burner 15, only SiCl₄ is supplied. In place of the cylindrical multi-port burner as shown in Fig. 2, a multi-port burner having other cross section such as a square or an ellipse may be used. The supply rates of the glass forming materials can be easily determined by the person skilled in the art depending on the content of GeO₂ in the porous preform. Typical supply rates are explained in the following Examples. The porous preform can be dehydrated and vitrified by conventional methods to give a transparent glass preform from which an optical fiber is drawn. Preferred Embodiments of the Invention The following Examples illustrate the method of the present invention. Example 1 By using the apparatus as shown in Fig.3, and a four-port burner as the burner 18 for forming the core portion and an eight-port burner as the burner 15 for forming a cladding portion, a porous preform was produced. To the first port 1 of the burner 18, SiCl₄, GeCl₄ and argon gas as a carrier gas were supplied at rates of 120 cc/min., 15 cc/min. and 180 cc/min., respectively. To the second port 2 of the burner 18, hydrogen gas was supplied at a rate of 3.0 liter/min. To the third port 3 of the burner 18, argon gas as a sealing gas was supplied at a rate of 2.0 liter/min. To the fourth port 4 of the burner 18, oxygen gas was supplied at a rate of 5.0 liter/min. To the first port 1 of the burner 15, SiCl₄, GeCl₄ and argon gas as a carrier gas were supplied at rates of 800 cc/min., 20 cc/min. and 800 cc/min., respectively. To the second and sixth ports 2 and 6 of the burner 15, hydrogen gas was supplied at rates of 3.5 liter/min. and 40 liter/min., respectively. To the fourth and eighth ports 4 and 8 of the burner 15, oxygen gas was supplied at rates of 17 liter/min. and 27 liter/min., respectively. To the third, fifth and seventh ports 3, 5 and 7 of the burner 15, argon gas as a sealing gas was supplied at rates of 3 liter/min., 4 liter/min. and 4 liter/min., respectively. Further, as the raw material for the outer flame, SiCl₄ and argon carrier gas were supplied to the sixth port 6 of the burner 15 at rates of 120 cc/min. and 100 cc/min., respectively. Under the above conditions, the porous preform having a length of 500 mm was grown. The porous preform was heated and dehydrated in a carbon resistance furnace at 1000°C in an atmosphere of a Cl₂/helim mixture in a molar ratio of 0.01/1 and then vitrified at 1600°C in a helium atmosphere to give a transparent glass preform. Observation of the surface of transparent glass preform revealed that no crack was formed on its surface. Thereafter, the glass preform was drawn to a diameter of 5 mm and inserted in a pure SiO₂ tube made by the VAD method and having an outer diameter of 51 mm. From the outside of the tube, the composite was heated with a carbon resistance furnace to integrated them to give a preform for a 1.55 µm band dispersion shifted optical fiber. Then, the preform was drawn in a drawing furnace to fabricate an optical fiber. Its transmission loss was as low as 0.21 dB/km at a wavelength of 1.55 µm. A refractive index profile of the optical fiber is shown in Fig. 4. Comparative Example 1 In the same manner as in Example 1 but supplying no SiCl₄ to the sixth port of the burner 15, the porous preform having a length of 500 mm was grown and vitrified. The surface layer of the transparent glass form was peeled off and no good preform was produced.

Claims: A method for producing a porous preform (11) for use in the fabrication of an optical fiber with at least two burners (15,18) for synthesizing glass soot particles at least one (15) of which generates a double-layer flame and at least one (18) of which is used for forming a core part of the preform; the method comprising the steps of: supplying SiCl₄ and GeCl₄ as glass-forming raw materials to the burner (18) which is arranged to form the core part to flame hydrolyze the supplied compounds to synthesize glass soot particles; supplying SiCl₄ as the only glass-forming raw material to the burner (15) which is arranged to form the peripheral part of the preform to flame hydrolyze the supplied compound to synthesize glass soot particles; in any event supplying SiCl₄ to the outer flame (9) of the double-layer flame as the only glass raw material; and depositing the generated glass soot particles on the lower end of a rotating starting member (10) and growing the porous preform (11) in a direction of an axis of the starting member to produce the porous preform comprising a core part containing at least partly GeO₂. A method according to claim 1, wherein said burner (18) for forming said core part of said preform (11) is a burner which generates a single flame and said burner for generating the double-layer flame (15) is a multi-port burner producing inner (12) and outer (9) flames. A method according to claim 2, wherein SiCl₄ and GeCl₄ are supplied as glass-forming raw materials to both said single flame burner (18) and the inner flame (12) of said multi-port burner (15), and SiCl₄ is supplied as the only glass-forming raw material to the outer flame (9) of said multi-port burner (15). A method according to claim 2, wherein SiCl₄ and GeCl₄ are supplied as glass-forming raw materials to said single flame burner (18), and SiCl₄ is supplied as the only glass-forming raw material to both the inner (12) and outer (9) flame of said multi-port burner (15).

Inventors: DANZUKA TOSHIO C O YOKOHAMA WO, KANAMORI HIROO C O YOKOHAMA WO, OHGA YUICHI C O YOKOHAMA WORKS, YOKOTA HIROSHI C O YOKOHAMA WO, DANZUKA, TOSHIO, KANAMORI, HIROO, OHGA, YUICHI, YOKOTA, HIROSHI, DANZUKA, TOSHIO, C/O YOKOHAMA WORKS OF SUMITOMO, KANAMORI, HIROO, C/O YOKOHAMA WORKS OF SUMITOMO, OHGA, YUICHI, C/O YOKOHAMA WORKS OF SUMITOMO, YOKOTA, HIROSHI, C/O YOKOHAMA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C03B37
IPC codes: C03B37