JPH0583495B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing a base material for a fluoride glass optical fiber capable of transmitting infrared rays in the 2-6 μm band. 2. Description of the Related Art In recent years, optical communication systems using optical fibers have made remarkable progress. Optical communication systems have advantages over other communication systems, such as long relay distances, no need for electromagnetic induction noise countermeasures, and the ability to propagate high-density information over long distances. The system is changing. In the development of optical fibers used in such optical communications, in order to reduce optical transmission loss,
The development of materials for the core and cladding of optical fibers, as well as the development of optical devices such as light emitting and light receiving devices, is actively progressing. The optical fibers currently used in optical communication systems are made of quartz glass, and the wavelength of light is
A short wavelength band of 0.85 μm band and a long wavelength band of 1.3 μm band are used. Incidentally, light loss in optical fibers is due to light scattering and light absorption by glass substances and impurities. It is known that light loss due to light scattering has little effect on the type of material and is inversely proportional to the fourth power of the wavelength of the light used. For this reason, it is said that it is difficult to use optical fibers in short wavelength ranges such as ultraviolet rays. On the other hand, as the wavelength used becomes longer, the loss due to scattering decreases rapidly, but on the contrary, the absorption loss increases.
This kind of absorption loss is due to the lattice vibration of molecules when light hits a material, so in order to reduce this loss, it is necessary to make the material composed of heavy molecules so that it is difficult to vibrate and to use a weak force. It is preferred to use materials that are bonded. Therefore, to reduce the optical loss of optical fiber,
Using light with a longer wavelength than conventional light as a light source,
In addition, it is desired to use a material with low absorption loss. Fluoride glass and halide crystals are known as such materials with low absorption loss.
Its theoretical transmission loss is extremely small at 1/100 of that of currently used quartz fibers in a specific wavelength range. In particular, this fluoride glass is the most promising as an infrared optical fiber for communications and is being actively researched. In manufacturing such an infrared optical fiber made of fluoride glass, it is first necessary to manufacture a glass rod which will be the base material of the optical fiber. To manufacture a glass rod for an infrared optical fiber made of fluoride glass, the raw materials for the fluoride glass are prepared, placed in a crucible made of gold or vitreous carbon, heated and melted in an electric furnace, and then A method was used in which the melt was poured into a metal mold and then rapidly cooled. Problems to be Solved by the Invention In the conventional method as described above, simply pouring glass melt into a mold causes air bubbles to be drawn in due to the turbulent flow of the melt, which remains as defects after the molten glass solidifies. This has had an adverse effect on optical transmission through optical fibers using such glass rods as a base material. To deal with the problem of residual defects in glass rods as described above, for example, as described in Japanese Patent Publication No. 60-251140, a mold or a glass tube acting as a mold is rotated with its longitudinal axis as the rotation axis. A method of injecting melt while rotating is known. However, this method reduces the rotational speed to approx.
It had to be rotated at a fairly high speed of 3000 rpm, and if there was a slight deviation in the rotation axis, there was a problem that a homogeneous glass rod that was symmetrical about the cylindrical axis could not be obtained. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method for manufacturing a base material for a fluoride glass fiber, which suppresses the occurrence of defects in a glass rod without rotating a mold. Means for Solving the Problems That is, according to the present invention, in a method for producing an optical fiber preform, which includes a step of pouring a molten fluoride glass raw material into a cylindrical mold preheated to a temperature near the glass transition temperature, When starting to pour the glass raw material, the cylindrical axis of the mold should be aligned with the horizontal plane.
controlling the attitude of the mold so that the angle is less than 45°, the angle gradually increases as the frit is poured, and the cylindrical axis is substantially vertical at the end of pouring; Operation at dew point -70℃
There is provided a method for producing a fluoride glass fiber matrix, characterized in that it is carried out under a low-humidity He atmosphere as follows. Here, as the He atmosphere, it is preferable to use He gas of as high purity as possible. In addition, the above-mentioned operation in the He atmosphere is performed at least by pouring the glass raw material into the vicinity of the mold into which it is injected, or more preferably, by filling the container with high-purity He gas and performing the above-mentioned operation in the airtight container. This can be achieved by implementing the following. Note that it is effective that the He gas used as the atmosphere has a moisture content of 2.5 ppm or less (moisture dew point -70° C. or less). Such low humidity can be achieved by cooling He gas to -80°C or below. The shape of the mold is usually cylindrical. FIG. 1 is a schematic diagram of an example of an apparatus used in the method of the present invention. As shown in FIG. 1, the apparatus used in the method of the present invention consists of a cylindrical mold 1 into which the glass raw material is cast.
and a heating container 1a into which the cylindrical mold 1 is inserted and accommodated.
and a fixing base 2 that supports the heating container 1a with a bearing 3 at the central axis of its cross section. Heating container 1
A has a heater embedded therein, and is connected to an external power source via a heater cord 5. In order to carry out the method of the present invention using such an apparatus, first, the heating container 1a is arranged so that the cylinder axis makes an angle of less than 45° with the horizontal direction (θ in the figure), and a cylindrical shape is placed inside the heating container 1a. Insert mold 1. Then,
The glass raw material melted by the crucible 4 is poured into the cylindrical mold 1. As the injection is carried out, the heating container 1a is rotated about the bearing 3 in an upright direction. That is, the angle between the cylindrical axis of the cylindrical mold 1 and the horizontal direction is gradually increased. It is preferable to adjust the angle of the heating container 1a with respect to the horizontal direction so that θ becomes 90° just before pouring of the glass raw material is finished. FIG. 2 shows a specific example of the mold 1 used in the present invention, which is inserted into the heating container 1a in FIG. 1 for use. FIG. 2a is a side view, partially in section, of the mold, and FIG. 2b is a plan view, partially in section, of the mold. As shown in Figures 2a and b, this mold consists of 2
Two semi-cylindrical mold parts are joined with screws to form a cylindrical shape with one end open. That is,
As shown in FIG. 2b, the mold 1 is divided into two parts parallel to the cylindrical axis, and the mold parts are connected to each other by bolts. FIG. 3 is a schematic diagram of a funnel preferably used in the method of the invention. The funnel 6 is provided with a straight pipe portion 7 long enough to reach the bottom of the mold 1. That is, when pouring the molten fluoride glass raw material into the mold 1, the funnel 6 is placed in the mold 1 while inserting the straight pipe part 7 to the bottom of the mold 1, and the molten fluoride glass raw material is poured into the funnel 6. Pour. Therefore, the glass raw material is quietly poured into the bottom of the mold 1 through the straight pipe section 7. Next, as the glass raw material is poured, the funnel 6 is gradually pulled upward, and by the time the glass melt is poured, the straight pipe part 7 of the funnel 6 is pulled up to the top of the mold hollow part, and finally the straight pipe part 7 Carry out the injection operation with care so that the glass can be pulled out of the molten glass raw material. Function The present invention is characterized in that the step of pouring the molten glass raw material into the mold is performed in a low-humidity He atmosphere. Conventionally, this process has been carried out in the atmosphere, which tends to introduce air bubbles when pouring the molten glass raw material into the mold, resulting in defects remaining in the resulting glass base material. However, since the He gas used in the present invention has high permeability to the molten glass raw material, it floats toward the liquid surface of the melt before the glass raw material solidifies, and almost no air bubbles remain in the solidified glass. It disappears. Therefore, an optical fiber base material suitable for manufacturing optical fibers with few defects can be manufactured. Conventionally, optical fibers made of fluoride glass have had a problem with absorption loss due to OH groups, which are impurities in the glass, and it is also known that OH groups react with glass components to generate hydroxides. Therefore, it has been desired to reduce such OH groups. According to the present invention, since He gas, particularly low-humidity He, is used, it is possible to suppress the contamination of moisture, which is a source of OH groups, and to obtain a fluoride glass optical fiber with less absorption loss due to OH groups. . Furthermore, in the method of the present invention, when pouring the molten frit into the mold, the mold is first arranged so that its cylindrical axis makes an angle of less than 45° with respect to the horizontal direction, and then the fluoride frit is poured into the mold. As it is poured into the mold, the angle of the cylindrical axis of the mold with respect to the horizontal direction gradually increases while standing upright.
Fill the mold with raw materials. In this way, the mold is tilted at a predetermined angle, and as raw materials are poured, the inclination of the mold with respect to the horizontal direction is increased. In this way, the injection can be carried out while keeping the injection depth of the fluoride glass raw material small, and He in the atmosphere can be easily separated from the molten fluoride glass. Therefore, bubbles are less likely to be generated in the raw material, and the effect of performing the injection in the He atmosphere becomes more effective. Further, according to one aspect of the present invention, the mold used is made of graphite or metal and has a structure that can be divided into two or more parts parallel to the cylindrical axis as shown in FIG. 2. With such a structure, it becomes possible to easily take out the fiber base material by dividing the mold without damaging the fiber base material. Furthermore, according to a preferred embodiment of the present invention, a third
The molten fluoride glass raw material is poured into the mold using a funnel having a straight pipe section that can be inserted to the bottom of the mold as shown in the figure. The use of such a funnel reduces He entrainment in the atmosphere,
Bubbles are less likely to be generated in the raw material, and the effect of injection in a He atmosphere becomes more effective. Examples Hereinafter, the present invention will be explained in detail with reference to examples, but it goes without saying that the present invention is not limited to these examples in any way. Example 1 49ZrF ₄ −25BaF ₂ −3.5LaF ₃ −2YF ₃ −2.5AlF ₃ −
For the purpose of manufacturing a base material for a fluoride glass fiber having a composition (mol%) of 18LiF, each component was prepared to have the above composition, and 20 g of the mixed raw materials were put into a metal crucible and the crucible was covered. After drying this gold crucible in a quartz hearth tube at 110℃ for 2 hours,
The glass melt was heated and held at 850°C for 2 hours, and then the temperature was lowered to 800°C. On the other hand, in a high-purity He atmosphere, a brass mold 1 with the structure shown in Fig. 2, which had been preheated to 260°C, was placed in the apparatus shown in Fig. 1, and the horizontal inclination angle of the heating container 1a was set to 20°. in dry He gas.
The glass melt in the crucible 4 was poured at a constant flow rate. When the amount of glass melt remaining in the crucible 4 becomes small, the angle between the cylindrical axis of the mold 1 and the horizontal direction is gradually increased while pouring the melt, and just before the melt is poured out, The angle was adjusted to 90°. Thereafter, the mold temperature was set to 250°C and annealing treatment was performed for 2 hours. After the annealing treatment, the temperature of the furnace was lowered to room temperature and the glass rod was taken out from the mold 1. A homogeneous glass rod with a diameter of 8.5 x 100 mm and no bubbles was obtained. Example 2 For the purpose of manufacturing a base material for a fluoride glass fiber having the same composition as in Example 1, each component was prepared and a glass melt was heated and held in the same manner as in Example 1. Subsequently, the temperature was lowered to 800°C, and in a high-purity He atmosphere, the brass mold 1 with the structure shown in Fig. 2, which had been preheated to 260°C, was placed in the apparatus shown in Fig. 1, and the heating vessel 1a was heated. The glass melt in the crucible 4 was poured into the crucible at a constant flow rate using a gold funnel 6 as shown in FIG. 3 in dry He gas with the horizontal tilt angle of 90°. At this time, first use funnel 6.
Insert the straight pipe part 7 to the bottom of the mold, and then gradually pull it upward as the glass melt is poured, so that the straight pipe part 7 is located at the top of the hollow part of the mold by the time the glass melt is poured. The melt was poured while being careful to allow the straight pipe part 7 to be finally pulled out from the melt. Thereafter, it was annealed in the same manner as in Example 1 to obtain a uniform glass rod of φ8.5×100 mm that did not contain air bubbles. The side surfaces of the glass rod thus obtained were precisely polished and cleaned, and then jacketed with Teflon FEP to produce a glass base material for optical fiber. Comparative Test Using Atmospheric Gas and Casting Angle In order to confirm the effects of He atmosphere and casting angle according to the present invention, a glass melt was The atmosphere during injection into the mold was N ₂ atmosphere or He atmosphere, and the casting angle (the angle between the cylindrical axis of the mold and the water direction at the initial stage of injection) and the heating temperature of the mold were as shown in Table 1. Then, the same treatment as in Example 1 was carried out to produce a glass rod. The resulting glass rods were observed for residual air bubbles, and the results are shown in Table 1. The evaluation is shown in Table 1 as follows: glass with many remaining bubbles is marked as x, glass with few remaining bubbles is marked as △, and glass with almost no remaining bubbles is marked as ○.

Table 1 shows that by using the He atmosphere and casting angle of the present invention, a glass rod with fewer bubbles can be obtained. Effects of the Invention According to the production method of the present invention, impurities, especially OH
It is possible to obtain a homogeneous base material for a fluoride glass fiber with less contamination of groups and with suppressed generation of defects such as bubbles. Therefore, by introducing the manufacturing method of the present invention into the optical fiber manufacturing process, it becomes possible to manufacture high-quality optical fibers with less light absorption loss.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a specific example of an apparatus used in the manufacturing method of the present invention. Figure 2 is a specific example of a cylindrical mold used in the manufacturing method of the present invention, and Figure 2a
is a side view thereof, and FIG. 2b is a sectional view thereof.
FIG. 3 is a perspective view of a funnel with a straight tube section used in accordance with a preferred embodiment of the invention. Main reference numbers, 1...mold, 1a...heating container, 2...fixing base, 3...bearing, 4...crucible, 5...heater cord, 6...funnel, 7...funnel straight pipe Department.

Claims (1)

[Scope of Claims] 1. In a method for manufacturing an optical fiber base material, which includes a step of pouring a molten fluoride glass raw material into a cylindrical mold preheated to a temperature near the glass transition temperature, when starting to pour the glass raw material, The cylinder axis of the mold makes an angle of less than 45° with respect to the horizontal plane,
The angle is gradually increased as the glass raw material is poured, and the posture of the mold is controlled so that the cylindrical axis becomes substantially vertical at the end of pouring. A method for producing a fluoride glass fiber matrix, characterized in that the method is carried out in a low-humidity He atmosphere.