METHOD FOR PRODUCING GLASS PREFORM FOR OPTICAL FIBER

Abstract: Fluorine is selectively added to a cladding portion of a glass preform by producing a soot preform which comprises a core portion (9) and a cladding portion (10) under such conditions that an additive concentration or a bulk density of the outermost part (11B) of the core portion (9) is to be larger than that of the inner part thereof and heating the soot preform in an atmosphere containing fluorine or a fluorine-­ containing compound at least in some period to obtain a transparent glass preform.

Description: The present invention relates to a method for producing a glass preform for use in the fabrication of an optical fiber. An optical fiber comprises, as shown in the upper part of Figure 1 of the accompanying drawings, a core 1 through which light propagates and a cladding 2 which surrounds the core. A refractive index n₁ of the core 1 is larger than a refractive index n₂ of the cladding as shown in the lower part of Fig. 1 so as to transmit light through the core. As a relative refractive index difference δn defined by the formula: δn = (n₂ - n₁)/n₁ is increased, an incident angle on the cladding surface at which total reflection occurs increases. Thereby, for example, power loss due to bending of the optical fiber is minimized. Increase of the relative refractive index difference δn can be accomplished, for example, by increasing the refractive index n₁ of the core 1 by the addition of a metal oxide such as GeO₂, Al₂O₃ and TiO₂ to a core portion of the glass preform corresponding to the core 2, and by decreasing the refractive index n₂ of the cladding 2 by the addition of fluorine to a cladding portion of the preform corresponding to the cladding 2. In the first method by which the refractive index n₁ of the core 1 is increased, due to the increase of the amount Of the additive, following drawbacks may be caused: 1. Addition of the additive induces light scattering (i.e. Rayleigh scattering) in proportion to the amount of the additive, and the light scattering undesirably increases attenuation. 2. Addition of a large amount of the additive tends to cause the formation of bubbles or single crystals in the glass preform. For example, when GeO₂ is use as an additive, bubbles due to gaseous GeO₂ may be formed. When Al₂O₃ is used as an additive, it may form a cluster of its single crystals. The bubbles or the single crystals are cause of the light scattering resulting in attenuation. In addition, they may be cause of the fiber break. The second method by which the refractive index n₂ of the cladding is decreased can effectively overcome the above drawbacks. Typically, this method comprises adding the additive to the core portion of the soot preform to increase its refractive index to achieve predetermined refractive index difference between the core portion and the cladding portion and adding the fluorine to the cladding portion to decrease its refractive index by heating the soot preform at a high temperature in an atmosphere of the fluorine-containing compound to produce the glass preform having a larger specific refractive index difference. However, this method has some drawbacks as follows: By simply heating the soot preform at a high temperature in the atmosphere of the fluorine-containing compound, the core portion is also fluorinated so that the relative refractive index difference cannot be increased. Since fluorine is an inherently highly active material, it is very difficult to control the temperature in a furnace or to adjust the concentration of the fluorine-containing compound and the treating time so as to add fluorine only to the cladding portion of the preform. In order to add fluorine only to the cladding portion of the preform having conventional distributions of the additive and of a bulk density, it is necessary to control the temperature of the furnace within about ±30°C. Thus, it is extremely difficult to produce an optical fiber having the desired relative refractive index difference. These methods are discussed in the following publications: 1) R. Olshanski, "Propagation in glass optical waveguides", Reviews of Modern Physics, Vol. 51, No. 2, April 1979; 2) T. Izawa et al, "Materials and Processes for Fiber Preform Fabrication - Vapor Phase Axial Deposition", Proceedings of the IEEE, Vol. 68, No. 10, October 1980; and 3) Production of Glass Base Material for Light Transmission, Japanese Patent Publication JP-A-5 567 533. The latter document is acknowledged as the closest prior art reference. One object of the invention is to provide a glass preform for the fabrication of optical fibers, to a cladding portion of which, fluorine is selectively added. Another object of the invention is to provide a method for producing a glass preform by which fluorine is selectively added to the cladding portion of the preform while it is not added to the core portion. According to the present invention, there is provided a method for producing a glass preform, comprising producing a soot preform which comprises a core portion and a cladding portion 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 heating the soot preform in an atmosphere containing fluorine or a fluorine-containing compound at least in some period to obtain a transparent glass preform. According to the method of the present invention, the additive concentration or the bulk density of the outermost part of the core portion of the fine glass particle mass, i.e. the soot preform is made larger than the inner part thereof during the production of the soot preform. Thereby, when the soot preform is sintered to convert it to a transparent glass preform, the outermost part of the soot preform is sintered at an earlier stage than the cladding portion surrounding it. The sintered part of the core prevents the penetration Of fluorine in the inner part of the core when the fluorine-containing compound is added to the atmosphere in which the soot preform is heated at a high temperature to make it transparent. In addition, the temperature range in which fluorine is selectively added to the cladding is made as wide as ±150°C, and thus it is easier to control the temperature and is possible to lower the treating temperature. It is also found that if the relative density of a part of the soot preform, namely the difference between the bulk density of said part of the soot preform and the density of said part of the transparent glass preform is more than 0.45, fluorine cannot be added to said part of the soot preform. Accordingly, when the relative density of the outermost part of the core portion is 0.45, the relative density of the cladding portion is less than 0.45. If, at such timing, the fluorine-containing compound is added to the heating atmosphere, fluorine is added to the cladding portion but not to the inner part of the core portion, thereby the refractive index distribution having a large relative refractive index difference is achieved. The present invention will be illustrated by making reference to the accompanying drawings by way of example, in which: Fig. 1 schematically shows a distribution of a refractive index of an optical fiber; Fig. 2 schematically shows a flame hydrolysis process for practicing the method of the present invention; Fig. 3 shows a distribution of a metallic additive in the glass preform; Fig. 4 shows a distribution of a bulk density of the glass preform; Fig. 5 shows distributions of a metallic additive in the soot preform; Fig. 6 shows the time-course of relative densities of the soot preforms of Fig. 5 in the radial direction; Figs. 7a, 7b and 7c show the distributions of the GeO₂ concentration and of the relative density in the soot preform and the relative refractive index difference of the glass preform produced in Example 1, respectively; Figs. 8a and 8b show the distribution of the GeO₂ concentration in the soot preform and the relative refractive index difference of the glass preform produced in Comparative Example, respectively; Figs. 9a and 9b show the distributions of the relative density in the soot proform and the relative refractive index difference of the glass preform produced in Example 2, respectively; and Fig. 10 shows an example of a processing pattern of the soot preform according to the present invention. Fig. 2 schematically shows a flame hydrolysis process for producing the soot preform to be used in the present invention. By using of multi-tube burners 3A and 3B, as a fuel gas, oxygen and hydrogen are supplied through inlets 4 and 5, respectively and gushed out through the outermost ports and the next outermost ones of the burners, respectively. Simultaneously, SiCl₄ as a raw material and GeCl₄ as an additive are supplied together with a carrier gas (eg. an inert gas such as Ar) through inlets 7 and gushed out through the center ports of the burners and reacted. In order for the raw material to react in a space several millimeters apart from the top of the burner, Ar gas is introduced as a barrier gas through an inlet 6 and gushed out through a circular port surrounding the center one. Under such conditions, glass fine particles are accumulated on a tip of a rotating starting member 8 in its axial direction to form a rod of the fine glass particle mass, namely, a soot preform. In the above procedures, the conditions including the supplying rate of each gas or material are substantially the same as in the conventional method. Preferred conditions are demonstrated by Examples. Then, to increase the additive concentration or the bulk density of outermost part of the preform core as shown in Figs. 3 and 4, oxygen is supplied through the inlet 4 of the burner 3A at a rate of 8 liters/min., Ar through the inlet 6 at a rate of 2 liters/min., hydrogen through the inlet 5 at a rate of 3 to 8 liters/ min., and SiCl₄ together with GeCl₄ co-supplied at a rate of 10 to 50 ml/min. through the inlet 7. The upper burner 3A is used not only for partially increase the additive concentration or the bulk density but also for forming the cladding portion 10. The additive concentration may be increased by raising the flow rate of the additive, and the bulk density may be increased by raising the flow rate of hydrogen to lower the surface temperature of the core portion 9. The lower burner 3B is used solely for forming the core portion 9. Through the nozzle 7, GeCl₄ and SiCl₄ are simultaneously supplied at rates of 20 ml/min. and 200 ml/min., respectively and burnt in the oxyhydrogen flame. The above described condition is one example but the method of the invention is not limited to the above described procedure. The outermost part of the core portion 9 of the thus produced soot preform has larger additive concentration or the bulk density than the inner part of the core portion 9. Then, the soot preform is dehydrated and sintered by heating it at a high temperature in a heat resistant muffle tube made of, for example, pure quartz or alumina. In the heat treatment of the soot preform, it is preferred to carry out the treatment in an atmosphere of a chlorine-containing compound primarily for dehydrating it. To this end, the soot preform is heated at a temperature of 800 to 1,100°C in an atmosphere to which helium gas and the chlorine-containing compound are supplied at rates of 10 liters and 100 ml per minute, respectively. The primary object of the use of the chlorine-containing compound is to dehydrated the soot preform. Specific examples thereof are Cl₂, SOCl₂, COCl₂, CCl₄, etc. When the dehydration temperature is lower than 800°C, impurities contained in the preform are not completely removed and dehydration takes longer time. When the soot is heated at a temperature higher than 1,100°C in the atmosphere of the chlorine-containing compound, it shrinks and the bulk density increases throughout the preform so that it is difficult to add fluorine thereto. Further, since the chlorine-containing compound volatilizes the additive as a chloride thereof, it may be used for adjusting the distribution of the refractive index. For this end, preferred temperature is from 1,000 to 1,100°C. After the first heating step, the second heating step is carried out primarily for adding fluorine to the preform. In this step, the temperature depends on the distributions of the additive and of the bulk density of the soot preform produced by the flame hydrolysis. Preferably, it is 1,150±150°C. Accordingly, the outermost part of the core portion is sintered at about 1,000°C, and then the whole soot preform is sintered at about 1,700°C in an atmosphere of the fluorine-containing compound to add fluorine only to the cladding portion 10. That is, in order not to add fluorine to the core portion 9 of the soot preform, it is essential to start the supply of the fluorine-containing compound when the relative density of the outermost part of the core portion 9 reaches 0.45 or higher. When the soot preform is heated at a constant temperature rising rate in the muffle tube having a homogeneous temperature distribution, the fluorine-containing compound is supplied at a temperature between a temperature T₁ at which the relative density of the outermost part of the core portion reaches 0.45 and a temperature T₂ at which that of the cladding portion reaches 0.45. Thus, when the difference ΔT between the temperatures T₁ and T₂ is larger, a temperature range in which the fluorine-containing compound may be supplied is wider. In other words, as the temperature difference ΔT is smaller, it is required to more precisely control the temperature at which the fluorine-containing compound is supplied. For larger temperature difference ΔT, larger difference Δ₁ of the additive concentration in Fig. 3 or larger difference Δ₂ of the bulk density in Fig. 4 is preferred. However, the differences are too large, the bubbles undesirably tends to form during sintering. Preferred difference is 0.1 to 0.3 % by weight for the additive concentration and 0.1 to 0.3 for the bulk density. Now, two cases in which the distributions of the additive concentration are as shown in Fig. 5 are compared. In the Figures (A) and (B), r represents a distance in the radial direction from the center axis of the soot preform, r₁ is a radius of the core portion 9, namely the position of the outermost part 11B of the core portion in Fig. 3, Δr is a microdistance, and r₁ + Δr is a radius of the innermost part 11A of the cladding portion 10 in Fig. 3. Fig. 6 shows change of the relative density against the temperature of the soot preforms when two preforms A and B having different additive concentrations are heated at a temperature rising rate of 16°C/min. The curves Ar₁ and Br₁ represent the changes of the relative densities of the outermost parts 11B of the core portions 9 of the soot preforms A and B, respectively. The curves Ar₁ + Δr and Br₁ + Δr represent the changes of the relative densities of the innermost parts 11A of the cladding portions 10 of the soot preforms A and B, respectively. As is seen from Fig. 6, the temperature difference ΔT between the temperatures T₁ and T₂ is larger in the soot preform A than the preform B. Thus, it is more easy to control the temperature of the preform A than that of the preform B. The same may apply to the bulk density. Although both of the additive concentration and the distribution of the bulk density of the outermost part 11A of the core portion are preferably made larger, either one of them can be made larger to obtain the same result. In the flame hydrolysis, so-called tailing of the additive distribution may be observed in the innermost part of the cladding portion under certain conditions. To prevent the tailing, the distribution of the refractive index may be adjusted by supplying chlorine in advance. PREFERRED EMBODIMENTS OF THE INVENTION EXAMPLE 1 To a furnace kept at 800°C, there was charged a soot preform for a single mode optical fiber having the distributions of the additive concentration and of the bulk density shown in Figs. 7a and 7b, respectively. Then, it was heated at a rate of 3°C/min. to 1,100°C with supplying chlorine gas and helium gas at rates of 100 ml/min. and 10 liters/min., respectively according to the processing pattern of Fig. 10. Thereafter, the supply of the chlorine gas was terminated. The soot preform was heated to 1,300°C with supplying SF₆ gas a rate of 150 ml/min, and further heated to 1,500°C with supplying pure helium gas to produce a transparent glass preform. The distribution of the refractive index of the thus produced glass preform is shown in Fig. 7c. The core portion had the distribution of the refractive index which corresponded to that of the additive concentration, and the refractive index of the cladding portion decreased by the degree corresponding to the amount of added fluorine. Elemental analysis with an X-ray microanalyzer (XMA) revealed that the core portion did not contain any fluorine. An optical fiber fabricated from the thus produced glass preform had δn values of +0.30 % and -0.15 % in the core and the cladding, respectively. COMPARATIVE EXAMPLE 1 To confirm the effects achieved by Example 1, a soot preform was produced by supplying hydrogen through the nozzle 5 at a rate of 2 liters/min. but no GeCl₄ through the nozzle 7. The thus produced soot preform had distributions of the bulk density and the additive concentration shown in Figs. 7b and 8a, respectively, which did not have any peak. The soot preform was treated in the same manner as in Example 1 to produce the glass preform. XMA elemental analysis revealed that fluorine was uniformly distributed throughout the core and cladding portions. The distribution of the refractive index is shown in Fig. 8c, from which it is understood that the distribution of the refractive index corresponds to that of the additive concentration and δn is not increased. EXAMPLE 2 To a furnace kept at 800°C, there was charged a soot preform for a single mode optical fiber having the distributions of the additive concentration and of the bulk density shown in Figs. 7a and 9b, respectively. Then, it was heated at a rate or 3°C/min. to 1,050°C with supplying chlorine gas and helium gas at rates of 100 ml/min. and 10 liters/min. Thereafter, the supply of the chlorine gas was terminated. The soot preform was heated to 1,300°C with supplying SF₆ gas a rate of 150 ml/min, and further heated to 1,500°C in a pure helium gas to produce a transparent glass preform. The distribution of the refractive index of the thus produced glass preform is shown in Fig. 9c. The core portion had the distribution of the refractive index which corresponded to that of the additive concentration, and the refractive index of the cladding portion decreased by the degree corresponding to the amount of added fluorine. XMA elemental analysis revealed that the core portion did not contain any fluorine. An optical fiber fabricated from the thus produced glass preform had δn values of +0.3 % and -0.15 % in the core and the cladding, respectively. COMPARATIVE EXAMPLE 2 To confirm the effects achieved by Example 2, a soot preform for a single mode optical fiber having the same distributions of the additive concentration and the bulk density as that of Comparative Example 1 was treated in the same manner as in Example 2. XMA elemental analysis revealed that fluorine was uniformly distributed throughout the core and cladding portions of the produced glass preform. The only difference between Example 2 and Comparative Example 2 was that the preform used in the latter had a smaller refractive index at the outermost part of the core portion. Thus, it is understood that the refractive index of this part greatly influences the distribution of the refractive index. EXAMPLE 3 A soot preform made of pure quartz having relative bulk densities of 0.45 and 0.25 at the outermost part of the core portion and the cladding portion, respectively was heated to 1,050°C and dehydrated in the same conditions as in Example 2. Thereafter, the supply of the chlorine gas was terminated. The soot preform was heated to 1,650°C with supplying helium gas and SF₆ gas a rate of 10 liters and 300 ml per minute to produce a transparent glass preform. δn of the core and cladding portions of the thus produced glass preform were 0 % and -0.3 %, respectively. Although above Examples were carried out by using the soot preform for a single mode optical fiber, a soot preform for a multi mode optical fiber may be produced by the present invention. Further, a soot preform produced by the "outer deposition method" in which the fine glass particles are accumulated on a cylindrical mandrel may be treated by the method of the invention. Thus, there has been disclosed a method for producing glass preform for use in the fabrication of an optical fiber comprising a core and a cladding in which a cladding portion of a soot preform, i.e. a fine glass particle mass, is selectively added with fluorine during sintering the soot preform to convert it into a transparent glass preform at a high temperature.

Claims: A method for producing a glass preform, characterised by producing a soot preform which comprises a core portion (9) and a cladding portion (10) under such conditions that an additive concentration or a bulk density of the outermost part (11B) of the core portion (9) is to be larger than that of the inner part thereof and heating the soot preform (9, 10) in an atmosphere containing fluorine or a fluorine-containing compound at least in some period to obtain a transparent glass preform. A method according to claim 1 characterised in that before heating, the soot preform is heated in an atmosphere containing a chlorine-containing compound to dehydrate it. A method according to claim 1 or 2 characterised in that the soot preform is heated at a temperature of 1,000 to 1,300°C. A method according to any one of claims 1 to 3 characterised in that the soot preform is produced under such condition that both the additive concentration and the bulk density of the outermost part (11B) of the core portion (9) are larger than those of the inner part of the core portion (9).

Inventors: KAWAUCHI, HIROSHI C/O YOKOHAMA WORKS OF SUMITOMO, KYOTO, MICHIHISA C/O YOKOHAMA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1985
Main classification: C03B37
IPC codes: G02B6, C03B37, C03B19, C03C13