METHOD FOR PRODUCING GLASS PREFORM FOR OPTICAL FIBER CONTAINING FLUORINE IN CLADDING

Abstract: A method for producing a glass preform for an optical fiber comprising a core and a cladding containing fluorine, which method comprises forming a porous glass layer of substantially pure quartz on an outer surface of a fused glass rod consisting of a central portion of substantially pure quartz and a peripheral portion of quartz glass containing fluorine, and heating a compo­ site of the fused glass rod and the porous glass layer in an atmosphere containing fluorine to add fluorine to the porous glass layer and to make it transparent, by which contamination of glass with hydroyl groups can be prevented and light transmission characteristics of an optical fiber fabricated from the glass preform are improved.

Description: FIELD OF THE INVENTION The present invention relates to a method for producing a glass preform for an optical fiber containing fluorine in a cladding. BACKGROUND OF THE INVENTION A typical single mode optical fiber has a refractive index distribution as shown in Fig. 1. Hitherto, such refractive index distribution is mainly achieved by the addition of an additive which increases the refractive index of the glass in the core of the optical fiber. As the additive, an oxide such as GeO₂, P₂O₅ and Al₂O₃ is usually used. However, such additive may cause some problems such as (1) attenuation of light transmission of the optical fiber is increased by increase of Rayleigh scattering, (2) bubbles or crystal clusters are induced by the additive in the glass preform and (3) the glass preform tends to crack due to increase of the coefficient of thermal expansion of the glass. Therefore, the lower the content of the additive in the glass preform, the better. For this reason, it is proposed to increase the refractive index difference between the core and the cladding by the addition of an additive which lowers the refractive index of the glass in the cladding. Examples of such additive are B₂O₃ and fluorine and their combination. B₂O₃, however, has disadvantages that it increases the coefficient of thermal expansion of the silica glass and that it has an absorption loss in a longer wavelength region. Thus, fluorine is preferably used as the refractive index-lowering additive. The VAD method or the OVPO method in which a porous soot preform is produced by flame hydrolysis of glass raw materials is known as an economical and highly productive method for producing the optical fiber. It is however, very difficult to add fluorine in a sufficient amount to lower the refractive index of the cladding by such method utilizing flame hydrolysis. For example, Japanese Patent Kokai Publication (unexamined) No. 15682/1980 discloses a method for adding fluorine to a glass preform, by which the refractive index is lowered by only 0.2 to 0.3 %. This means that the amount of fluorine to be added has its limit in this method. Japanese Patent Kokai Publication (unexamined) No. 67533/1980 discloses a method for effectively adding fluorine to the glass preform by heating a fine glass particle deposit in an atmosphere containing a fluorine-containing compound. It is, however, difficult by this method to adequately distribute fluorine in the glass preform and thus to achieve, by the sole use of fluorine, the refractive index distribution shown in Fig. 1, which is essential to produce a practically operative optical fiber. EP-A2-0 151 438 describes a method for producing a glass preform for an optical fiber wherein fluorine is selectively added to a cladding portion of a glass preform under such conditions that an additive concentration or a bulk density of the outermost part of the core portion is to be larger than that of the inner part thereof and wherein the soot preform is heated in a atmosphere containing flourine or a fluorine-containing compound. A method schematically illustrated in Fig. 2 is proposed as a productive method for producing an optical fiber containing fluorine and having a practically operative refractive index distribution by utilizing formation of the fine glass particle deposit by flame hydrolysis. In Fig. 2, while rotating and gradually lifting up a glass rod 1 which constitutes the core and attached to a lift-up device 2, fine glass particles which are produced by means of a burner 3 are deposited on the surface of the glass rod 2 to form a porous glass layer 4 corresponding to the cladding. The fine glass particles are produced by supplying the burner 3 with hydrogen, oxygen and glass raw materials such as SiCl₄ simultaneously and flame hydrolyzing them. In Fig. 2, numerals 5 and 6 represent a reactor and an outlet, respectively. The thus formed composite of the glass rod and the porous glass layer is heated in an atmosphere containing fluorine to add fluorine to the porous glass layer and simultaneously to make it transparent to form a transparent glass preform having a refractive index distribution shown in Fig. 1. If the thickness of the cladding is not sufficient at this stage, the transparent glass preform is drawn and the fine glass particles are again deposited on the surface of the drawn glass preform and heated in an atmosphere containing fluorine. This procedure may be repeated to obtain the cladding having desired thickness. In the above-described method utilizing the apparatus of Fig. 2, the glass rod which constitutes the core is often produced by heating and drawing the rod to a desired diameter in an atmosphere containing water vapor. This may result in contamination of the glass rod surface with hydroxyl groups. Particularly, when the glass rod is drawn in a flame formed by burning combustion gas containing hydrogen, the glass rod surface is severely contaminated with the hydrogen groups. In addition, during the formation of the porous glass layer corresponding to the cladding, the glass rod surface tends to be contaminated with the hydroxyl groups derived from water vapor generated in the flame for synthesizing the fine glass particles. When the transparent glass preform in which the surface of the core is contaminated with the hydroxyl groups is drawn to form an optical fiber, light propagated through the optical fiber suffers from absorption loss due to the hydroxyl groups and light transmission characteristics of the optical fiber are deteriorated. Particularly, when the optical fiber is used as a single mode optical fiber, the light transmission is considerably affected by the presence of an interface layer contaminated with the hydroxyl group between the core and cladding, and the transmission characteristics are remarkably deteriorated since power distribution in the single mode optical fiber reaches the cladding. For example, a pure quartz rod with very low hydroxyl group content (up to about 10 ppb) is drawn to a diameter of 12 mm in an oxyhydrogen flame. Thereafter, by means of the apparatus of Fig. 2, on the surface of the drawn quartz rod, a porous glass layer of pure silica glass is formed. The outer diameter of the glass layer is 110 mm. The thus produced composite of the quartz rod and the porous glass layer is heated in an atmosphere containing fluorine to obtain a transparent glass preform of 45 mm in outer diameter having a refractive index distribution as shown in Fig. 3. Then, the glass preform is drawn in the oxyhydrogen flame to a diameter of 12 mm, on which a porous glass layer is again formed by means of the apparatus of Fig. 2. The outer diameter of the glass layer is 110 mm. The thus produced composite is heated in an atmosphere containing fluorine to obtain a transparent glass preform having a refractive index distribution as shown in Fig. 4. The glass preform is drawn to a predetermined diameter, inserted in and integrated with a commercially available quartz tube, and then drawn to form an optical fiber resulting single mode operation at a wavelength of 1.3 micrometer. Its attenuation of light transmission at a wavelength of 1.3 micrometer is 4.0 dB/km and that at wavelengths of 1.39 micrometer due to the presence of the hydroxyl groups is 150 dB/km. These results mean that the hydroxyl groups are formed in the drawing step in the oxyhydrogen flame. One object of the present invention is to provide a new method for producing a glass preform for an optical fiber comprising a core made of a pure quartz and a cladding made of quartz glass containing fluorine. Another object of the present invention is to provide a glass preform for an optical fiber, particularly for a single mode optical fiber which does not suffer from contamination with the hydroxyl groups and has improved light transmission characteristics. Accordingly, the present invention provides a method for producing a glass preform for an optical fiber comprising a core and a cladding containing fluorine, which method comprises forming a porous glass layer of substantially pure quartz on an outer surface of a starting member consisting of a fused glass rod consisting of a central portion of substantially pure quartz and a peripheral portion of quartz glass containing fluorine, said starting member being produced by heating and shrinking a porous glass body of pure quartz produced by the VAD method to such extent that the body is not made transparent, heating the partially shrunk glass body in an atmosphere containing fluorine so as to add fluorine to the peripheral portion of the porous glass body and to make it transparent and optionally drawing it to a certain diameter, and heating the composite of the fused glass rod and the porous glass layer in an atmosphere containing fluorine to add fluorine to the porous glass layer and to make it transparent. Fig. 1 shows a refractive index distribution of a typical single mode optical fiber, Fig. 2 schematically shows one example of an apparatus for synthesizing a porous glass layer on a glass rod, Figs. 3 and 4 show refractive index distributions of conventional transparent glass preforms, and Fig. 5 to 7 show refractive index distributions in a radial direction of transparent glass preform produced in Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION As described in connection with the conventional methods, when the fine glass particle deposit is heated in the atmosphere containing fluorine, fluorine tends to be homogeneously added throughout the layer so that it is difficult to achieve the refractive index distribution of Fig. 1. On the contrary, when the fine glass particle deposit is heated and shrunk to some extent in an atmosphere not containing fluorine and then heating it in the atmosphere containing fluorine to make it transparent, fluorine is added only to the peripheral portion of the porous glass layer but not to the central portion of it since the porous glass layer is partially shrunk. The heating step in the atmosphere not containing fluorine may be carried out at a temperature of 1,200 to 1,450°C, and the heating step in the atmosphere containing fluorine may be carried out at a temperature of 1,400 to 1,700°C. The heating atmosphere comprises an inert gas (e.g. helium) and optionally a chlorine compound (e.g. Cl₂). Specific examples of the fluorine-containing compound to be contained in the heating atmosphere are SF₆, C₂F₆, CCl₂F₂, CF₄ and the like. When the thickness of the cladding layer is not sufficient at this stage, it is necessary to deposit further layer of quartz glass containing fluorine. To this end, the transparent glass preform produced in the previous step is heated and drawn, and on its surface, fine glass particles are deposited by means of the apparatus of Fig. 2. The formed composite of the transparent glass rod and the porous glass layer is then heated in the atmosphere containing fluorine to add fluorine to the porous glass layer and to make it transparent. In this heating step, the conditions are substantially the same as in the previous heating step in the atmosphere containing fluorine. The method according to the present invention can reduce the influence of the hydroxyl groups which may be formed in the drawing step of the glass rod. The reason for this may be explained as follows: (1) The interfacial portion between the core and the cladding is not contaminated with the hydroxyl groups but a part outer than the interfacial potion is contaminated so that quantity of light absorbed by the hydroxyl groups is decreased. (2) If fluorine is not contained in the heating atmosphere, water penetrates into silica glass and forms the hydroxyl group together with the glass structure according to the following reaction equation: On the contrary, when fluorine is contained in the heating atmosphere, the formation of the hydroxyl groups may be suppressed by the following equation, even if water penetrates into silica glass: (3) The hydroxyl group is thought to be formed so as to occupy the oxygen deficient part in glass. Since, however, the oxygen deficient part is occupied by fluorine present in the atmosphere, the hydroxyl group is less combined in the glass structure. The present invention will be hereinafter explained further in detail by following Examples. EXAMPLE 1 A porous soot preform of pure silica with an outer diameter of 100 mm and a length of 600 mm was produced by the VAD method by supplying oxygen at 35 l/min., hydrogen at 30 l/min., SiCl₄ at 1 l/min. and argon 12 l/min. from a multi-tube burner and pulling up the formed preform at a rate of 70 mm/hr. The thus formed porous preform was inserted in a furnace kept at 1,200°C for 90 minutes. In the furnace, as atmospheric gasses, helium and chlorine were introduced at rates of 5 l/min. and 100 ml/min., respectively to sufficiently reduce the amount of the hydroxyl groups. The thus heated porous soot preform was inserted in a furnace kept at 1,400°C for 30 minutes to shrink it. In the furnace, only helium was introduced at a rate of 5 l/min. Thereafter, the shrunk preform was inserted in a furnace kept at 1,640°C to make it transparent. In the furnace, helium and SF₆ were introduced at rates of 5 l/min. and 100 ml/min., respectively. A refractive index distribution of the thus produced transparent glass preform is shown in Fig. 5. After drawing the transparent glass preform to a diameter of 12 mm in an oxyhydrogen flame generated by a multi-tube burner, a porous glass layer of pure silica was formed on the outer surface of the drawn preform under following conditions by means of the apparatus of Fig. 2. The outer diameter of the porous glass layer was 110 mm. Conditions: Oxygen: 35 l/min. Hydrogen: 35 l/min. SiCl₄: 1.1 l/min. Argon: 12 l/min. Pulling up rate: 65 mm/hr. The composite of the transparent glass preform and the porous glass layer was dehydrated in an atmosphere containing fluorine at 1,200°C and then made transparent at 1,640°C. In the dehydration step, helium, chlorine and SF₆ were introduced at rates of 5 l/min., 50 ml/min. and 100 ml/min., respectively, and in the transparent step, helium and SF₆ were introduced at rates of 5 l/min. and 100 ml/min., respectively. The thus produced transparent glass preform had an outer diameter of 45 mm and refractive index distribution shown in Fig. 6. The glass preform was again drawn to a diameter of 12 mm in the oxyhydrogen flame and then on its outer surface, a porous glass layer of pure silica was formed under the same conditions as above by means of the apparatus of Fig. 2. The outer diameter of the porous glass layer was 110 mm. The composite of the transparent glass preform and the porous glass layer was dehydrated and made transparent under the same conditions as above to obtain a transparent glass preform having an outer diameter of 45 mm and refractive index distribution as shown in Fig. 7. The finally produced transparent glass preform was drawn to a desired diameter, inserted in and integrated with a commercially available quartz tube and then spun to form an optical fiber of 125 micrometer in diameter. Its attenuation of light transmission at a wavelength of 1.3 micrometer was 1.0 dB/km and that at a wavelength of 1.39 micrometer due to the presence of the hydroxyl groups was 10 dB/km. EXAMPLE 2 In the same manner as in EXAMPLE 1 but using a plasma flame in place of the oxyhydrogen flame for drawing the glass preform, a transparent glass preform was produced. The finally produced glass preform had a refractive index distribution as shown in Fig. 7. The glass preform was spun in the same manner as in EXAMPLE 1 to form an optical fiber. Its attenuation of light transmission at a wavelength of 1.3 micrometer was 0.6 dB/km and that at a wavelength of 1.39 micrometer due to the presence of the hydroxyl groups was 6 dB/km. The glass preform may be drawn in an electric resistance heater or an induction heater.

Claims: A method for producing a glass preform for an optical fiber comprising a core and a cladding containing fluorine, which method comprises forming a porous glass layer of substantially pure quartz on an outer surface of a starting member consisting of a fused glass rod consisting of a central portion of substantially pure quartz and a peripheral portion of quartz glass containing fluorine, said starting member being produced by heating and shrinking a porous glass body of pure quartz produced by the VAD method to such extent that the body is not made transparent, heating the partially shrunk glass body in an atmosphere containing fluorine so as to add fluorine to the peripheral portion of the porous glass body and to make it transparent and optionally drawing it to a certain diameter, and heating the composite of the fused glass rod and the porous glass layer in an atmosphere containing fluorine to add fluorine to the porous glass layer and to make it transparent. A method according to claim 1, wherein the drawing of the glass rod is carried out in a plasma flame.

Inventors: KANAMORI, HIROO C/O YOKOHAMA WORKS, KYOTO, MICHIHISA C/O YOKOHAMA WORKS, MIZUTANI, FUTOSHI C/O YOKOHAMA WORKS, TANAKA, GOTARO C/O YOKOHAMA WORKS, YOSHIOKA, NAOKI C/O YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1985
Main classification: C03B37
IPC codes: C03C13, B66C11, C03B37, B66C1, B65G47, B66C13, B25J5, B25J19, B23Q7, G02B6