JPH0436100B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an apparatus for manufacturing an optical fiber base material that prevents contamination of impurity elements into a glass base material and has excellent durability. The VAD method is known as a general method for mass producing base materials for optical fibers. In this VAD method, glass particles generated in an oxyhydrogen flame are deposited on a rotating starting member, such as a glass plate or a glass rod, to create a cylindrical porous base material, and this porous base material is sintered. This is a method for manufacturing a transparent optical fiber base material. In this method, in order to sinter the porous base material and make it transparent, it is necessary to heat the base material to 1600°C or higher in a He or Ar gas atmosphere. A carbon furnace is usually used as this heating furnace. Points that must be kept in mind during sintering in such a heating furnace are prevention of contamination of transition elements such as Cu and Fe as well as contamination of moisture. This is because if 1 ppb or more of transition elements are mixed in, the loss wavelength characteristics of the optical fiber will be significantly impaired over all wavelengths, and if 0.1 ppm or more of water is mixed in, the characteristics in the long wavelength range will be impaired. Therefore, the porous base material is usually dehydrated, and a known method is to heat the porous base material at a high temperature in an inert gas atmosphere to which fluorine gas is added. This method not only dehydrates the porous matrix but also has the effect of adding fluorine. Adding fluorine to the porous base material has the advantage that the refractive index distribution, which is an essential element of optical fibers, can be adjusted. This point is explained in detail in Japanese Patent Publication No. 55-15682 and Japanese Patent Application Laid-open No. 55-67533. The above-mentioned treatment using fluorine gas is usually carried out in a carbon furnace simultaneously with sintering or as a preliminary step. Carbon furnaces are equipped with a furnace tube that isolates the carbon heating element from the sintering atmosphere in order to prevent the carbon heating element from being consumed by moisture and oxygen generated during heat treatment of the base material. was used. However, when an alumina core tube is used, there is a problem in that alkaline components contained in the alumina scatter around at high temperatures, and this adheres to the surface of the porous base material, forming a cristobalite layer. Therefore, quartz glass core tubes are being put into practical use. However, if the quartz glass reactor core tube contains Cu or Fe, the chlorine-based gas in the dehydration treatment atmosphere will easily chemically react with the Cu or Fe, producing volatile chlorides as shown in the equation below. A new problem arises in that it penetrates the porous matrix and significantly impairs the loss characteristics of the fiber. CuOCl ₂ ---→ Cu ₂ Cl ₂ Fe ₂ O ₃ Cl ₂ ---→ FeCl _3Furthermore , if the furnace core tube is a single piece of quartz glass,
Since Cu has the property of easily diffusing into quartz glass under high temperatures, there is also the problem that Cu volatilized from the furnace body and heating element passes through the furnace core tube and mixes into the glass base material. Furthermore, fluorine gas decomposes or reacts at high temperatures,
Produces _F2 and HF gas. These gases react with the quartz material as shown in the following equation to generate SiF ₄ gas and etch the quartz material. SiO ₂ +2F ₂ →SiF ₄ +O ₂ SiO ₂ +4HF → SiF ₄ +2H ₂ O This causes CuFe present in the quartz material to appear on the surface of the quartz material and mix into the porous base material.
Furthermore, pinholes are formed in the quartz furnace tube, which causes outside air to get mixed in and atmospheric gas to leak out of the furnace, resulting in an adverse effect on the manufacturing process. The present invention maintains the advantage that quartz tubes are denser than alumina tubes and have excellent durability due to their low coefficient of thermal expansion, while also ensuring that the above-mentioned impurities do not mix in at high temperatures and corrode by fluorine gas. The purpose of the present invention is to provide an apparatus for manufacturing an optical fiber base material that prevents the occurrence of oxidation, and its configuration is such that a porous gaseous base material is inserted into the core tube of a heating furnace, and the process is carried out in an atmosphere containing fluorine compounds. In the apparatus for dehydrating and sintering the above-mentioned glass field material, comprising a heating furnace,
The heating furnace is equipped with a quartz furnace tube, and the inner wall of the quartz furnace tube is coated with ceramic having a melting point higher than that of quartz and having corrosion resistance against fluorine gas. The present invention will be described in detail below based on embodiments shown in the drawings. The schematic configuration of the present invention is shown in the figure. A heating element 5 is provided inside the heating furnace body 6, and a furnace core tube 3 is provided at the center of the furnace body. The furnace core tube 3 is formed of a quartz glass tube, and its inner peripheral wall is coated with a ceramic layer 4 of alumina (Al ₂ O ₃ ). In addition to the above alumina, the ceramic layer 4 may be made of a ceramic having a melting point higher than that of quartz and having corrosion resistance against fluorine gas. The following table shows suitable materials for the ceramic layer.

[Table] As a method for coating ceramics, a film forming method using a gas phase reaction, such as plasma CVD coating or CVD coating, is preferred because it can form a dense film. The thickness of the ceramic layer 4 is preferably about 5 μm to 2 mm. When the layer thickness is 2 mm or more, it is easy to peel off, and when the layer thickness is 5 μm or less, the above effect is not sufficient. Next, the ceramic layer 4 is even better if a base layer is formed on the inner wall of the quartz furnace tube in advance and the ceramic layer 4 is laminated on the surface of this base layer. As the base layer, boron nitride (BN) or the like may be used. In addition, alumina and silicon carbide are used as other base layers.
These base layers are suitable as base layers because they easily adhere to the ceramics and quartz. The thickness of the base layer may be about 2 to 10 μm, since if it is too thick, it will cause distortion and peeling between the base layer and the quartz tube. On the other hand, a supply port 7 is provided at the side end of the furnace body 6 to introduce a shielding gas such as Ar or N ₂ . Further, a supply port 8 for introducing processing gases such as He, Ar, _Cl2, etc. is provided at the lower end of the furnace core tube 3, and a porous glass base material 1 is provided above the furnace core tube 3 via a support rod 2. is suspended. In the above configuration, the quartz tube lined with the ceramic layer 4 is denser than an alumina tube or a carbon tube, has a small coefficient of thermal expansion, and has excellent durability without fear of breakage due to thermal history. In this case, in order to prevent impurities contained in the quartz tube itself from diffusing and mixing into the base material, it is desirable that the quartz tube be highly pure and transparent. As for the degree, CuO
A transparent quartz tube in which the copper content is particularly removed so that the content is 0.5 ppm or less and Fe ₂ O ₃ is 1% or less is suitable. Generally, when a quartz tube is heated to 1,500℃ or higher, a phenomenon of stretching is observed, but if the wall thickness is 5 mm or higher, no stretching occurs even at 1,600℃, and a 6-mm-thick tube made using the electric melting method does not cause stretching. Quartz tubes do not elongate even at 1650℃. Furthermore, ceramic layer 4
If a material shown in Table 1 is used as the material, the viscosity at high temperatures is lower than that of quartz, and the elongation of the tube can be suppressed. Therefore, if such a quartz tube is used, there will be no problem even at sintering temperatures of 1500° C. or higher. Further, impurities such as Cu cannot pass through the ceramic layer 4 made of alumina or the like. Therefore, in the quartz furnace tube of the present invention, the external furnace body 6 and the heating element 5 are
Impurities such as Cu diffused from the furnace are shielded by the ceramic layer 4 and do not enter the inside of the furnace tube 3. Therefore, it is possible to reliably prevent impurities from entering the porous glass base material. Furthermore, since the inner circumferential wall of the quartz tube is lined with a ceramic layer 4 made of the material shown in Table 1, even when the glass base material is sintered in a gas atmosphere containing fluorine compounds, it will not be corroded by _F2 gas or HF gas. can be prevented. The table below shows the etching effect of HF solution on quartz and alumina.

[Table] Measured by the weight _change when Therefore, there is no risk that Cu and Fe present in the stone will be exposed on the surface and cause impurity contamination, and a glass base material of higher purity can be obtained. Although the present invention has been described above based on an embodiment illustrating an example of the apparatus of the present invention, the present invention is not limited to the above embodiment. good. Example 1 A quartz furnace tube with a 10 μm thick alumina inside was heated to 1600°C using a heating element, and 50 c.c./cm of SF ₆ was placed inside the tube.
Helium was flowed at a rate of 5/min, and the porous base material 1 was inserted therein at a descending speed of 2 mm/min. When the obtained transparent glass base material was subsequently spun into a fiber, the residual moisture in the fiber was 0.01 ppm, and Cu
Absorption originating from or Fe was not observed at all. Example 2 A quartz furnace tube was used, in which the inner circumferential wall of the quartz tube was coated with BN with a thickness of 2 μm as a base layer in advance, and alumina was coated with a thickness of 10 μm on top of that, and the other conditions were the same as in Example 1. When the furnace temperature was repeatedly raised and lowered when the material was manufactured, Example 1
It has become possible to maintain a lifespan 10 times longer than before. Comparative Example A fiber was manufactured under the same conditions as in Example 1 except that a quartz tube containing 1 ppm of Cu and having no ceramic layer was used as the quartz furnace tube. The residual moisture content of the obtained fiber was 0.01 ppm. Also
Absorption originating from Cu existed up to around 1.30 μm, but this value was sufficiently low compared to the previous absorption.
It was 3dB hot. However, the inner circumferential wall of the core tube was severely etched, and it was found that there was a problem in terms of corrosion resistance. As described above in detail based on the examples, the present invention can produce an optical fiber base material that does not contain impurities, particularly Cu and water, and can obtain an optical fiber with small transmission loss. Furthermore, it can prevent erosion and wear and tear of the quartz tube caused by fluorine.
It also has the advantage of being economical due to its excellent durability.

[Brief explanation of drawings]

The figure is a schematic structural diagram showing an apparatus for manufacturing a glass preform for optical fiber according to the present invention. In the drawing, 1 is a porous base material, 2 is a support rod, 3 is a furnace tube, 4 is a ceramic layer, 5 is a heating element, 6 is a furnace body, and 7 and 8 are supply ports.

Claims (1)

[Claims] 1. An apparatus for inserting a porous glass preform into a core tube of a heating furnace and dehydrating and sintering the glass preform in an atmosphere containing a fluorine compound, comprising a heating furnace;
A base material for an optical fiber, characterized in that the heating furnace is equipped with a quartz core tube, and the inner wall of the quartz core tube is coated with a ceramic having a melting point higher than that of quartz and having corrosion resistance against fluorine gas. manufacturing equipment. 2. The apparatus for manufacturing an optical fiber base material according to claim 1, wherein the quartz tube is a high-purity quartz tube with a copper content of 0.5 ppm or less. 3. The light according to claim 1, wherein the inner wall of the quartz furnace tube is coated with boron nitride as a base layer in advance, and the ceramic is formed on the upper surface of the base layer. Fiber base material manufacturing equipment.