JPS63100038A - Method for removing impurity from natural quartz glass - Google Patents

Abstract

PURPOSE:To effectively remove impurities such as alkali elements and to obtain a quartz glass product of a pipe or the like which is suitable for optical fiber, etc., by subjecting crushed natural quartz to high temp. treatment in the atmosphere or the chlorine-contg. atmosphere. CONSTITUTION:Powder obtained by crushing natural quartz such as rock crystal, washing and selecting it, or if necessary, a quartz glass product made of natural quartz such as a pipe which is formed from the powder, is heat-treated i.e. at >=1,000 deg.C in the atmosphere or the atmosphere contg. at least chlorine (in case of the pipe, it may be heated from the outside with an oxyhydrogen burner while allowing chlorine-contg. gas to flow through the inside thereof) to remove impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a method for removing impurities from natural quartz, primarily used in the manufacture of optical fibers.

[Prior art]

Currently, optical fiber communication systems are rapidly becoming popular not only in public communications but also in various fields. The most important point in this system is its long-term reliability. What is especially important about this problem is that optical fibers experience an increase in loss due to hydrogen over time, and as the cause of this problem has been clarified, several improvements have already been made.

One of these improvements is that the jacket pipe that covers the optical fiber base material, which includes the core or core and cladding produced by the VAD method, is replaced with a conventional natural quartz pipe.
There are methods to make synthetic quartz pipes made chemically, such as the OVD method and the OVD method. This is based on the fact that when natural quartz pipes are used, they contain a large amount of impurities such as Na, which diffuse into the optical fiber base material produced by the VAD method. It is believed that the glass structure is disturbed to produce a precursor of structural defects, and as a result, the precursor reacts with hydrogen, causing an increase in loss due to hydrogen.

However, as mentioned above, when a synthetic quartz pipe is used in place of a natural quartz pipe, there is a problem in that the cost of the optical fiber increases.

[Purpose of the invention]

In view of the above-mentioned problems, an object of the present invention is to provide a method for effectively removing impurities from natural quartz glass, which is superior to conventional synthetic quartz pipes in terms of characteristics and cost when considering optical fiber applications, for example. The object of the present invention is to obtain a natural quartz pipe for coating a comparable optical fiber base material.

[Structure of the invention]

To achieve the above object, the method of the present invention includes, in the step of obtaining a quartz glass product made of natural quartz from crushed natural quartz, a step of treating the natural quartz at high temperature in the air or in an atmosphere containing at least chlorine. It is characterized by the inclusion of

[Embodiments of the invention]

The present invention will be explained in detail below with reference to the drawings.

As a result of various studies, the inventors have come to the following findings. This process will be explained with reference to FIGS. 3 and 4. Figure 3 (1) shows quartz that exists in the natural world, such as quartz, crushed, processed through various processes such as cleaning and sorting, and finally melted into a natural quartz pipe using the MCVD method. Figure 3 (2) is a cross-sectional view of an optical fiber obtained by synthesizing the cladding and core using the VAD method. It shows. In the drawings, numeral 1 indicates a core, numeral 2 a cladding, and numeral 3 a jacket portion made of a natural quartz pipe. Both of these optical fibers were cut, and the core I portion was subjected to secondary ion mass spectrometry analysis. The results are shown in Figure 4 (1) and (2), Figure 4 (1)
corresponds to Figure 3 (1), and as this figure shows, M
Although no alkali metals such as Na'PK, that is, problematic impurities, were found in the optical fiber produced by the CVD method, the optical fiber produced by the VAD method corresponding to Figure 4 (2), that is, Figure 3 (2) The fiber contains N, although it is less than ppm.
The presence of impurities such as a and K is found. So M
The amount of impurities contained in the natural quartz pipe before being coated on the optical fiber base material produced by the CVD method or the VAD method was also investigated by secondary ion mass spectrometry. The results are shown in the table below, and for comparison, the analysis results of synthetic quartz pipes produced by the VAD method or the OVD method are also shown in the lower row.

Natural quartz pipe Synthetic quartz pipe (ppm) Al
25 <0.04Ti
5 <1F e O,2
00,15 Co < 0.03 < 0.02C
r<0.15<0.1ON
a 3.5 0.04C1310
00 As this table shows, the concentration of Na is much higher in natural quartz pipes than in synthetic quartz pipes, and this Na concentration is likely to have been diffused during optical fiber manufacturing, especially during heating processes such as spinning. Estimated. Although other metal impurity elements are also present in higher concentrations in natural quartz pipes, the diffusion coefficient of alkali elements such as Na is much larger than those of other metal impurity elements, making it difficult to It is known that the diffusion coefficient of Na, which is easily movable, in glass, for example, is about 8 orders of magnitude larger than that of aluminum at 1000 t.

Now, as mentioned above, the optical fiber made by the MCVD method contains N.
In order to clarify the reason why no alkali elements such as aluminum were found, secondary ion 1tfi analysis was also conducted on the jacket portion 3 of this optical fiber made of a natural quartz pipe. However, no alkali elements were found in the jacket portion 3 either.

This phenomenon was inferred as follows. In other words, the alkali elements in the natural quartz pipe are diffused into the inner surface layer by heating with an external acid-hydrogen burner, which is a feature of the MCVD method. Therefore, it is presumed that it changed into chloride by reaction with the chlorine gas, and was removed by evaporation as the chloride.

Therefore, in order to confirm this inference, while flowing 1 g/win of chlorine gas into the natural quartz pipe, the pipe was heated from the outside with an acid-hydrogen burner. The acid-hydrogen burner contains 201% oxygen.
/sin, hydrogen was flowed at 601 /sin, and the burner was reciprocated several times at a feed rate of 20 m/lin.

The relationship between the heating time and the Na degree is shown in Figure 2.
As shown in this figure, after heating for about 8 hours, the Na content in the natural quartz pipe became below the detection limit using the secondary ion mass spectrometry described above. As a result, it was confirmed that the above inference was correct.

In order to reconfirm the above results, a natural quartz pipe that had been heated while flowing chlorine gas to remove Na as described above was jacketed on an optical fiber base material obtained by the VAD method, and this was spun. When an optical fiber was obtained using this method, as shown in Fig. 1D, the hydrogen loss was almost the same as that of a synthetic quartz pipe jacketed or a natural quartz pipe in which the crund and core were synthesized by the MCVD method. It was possible to obtain an optical fiber without increase.

In Fig. 1, the symbol A indicates MCVD inside the natural quartz pipe.
Symbol B indicates an optical fiber obtained by synthesizing the core and cladding using the VAD method, and symbol B indicates an optical fiber made by jacketing a natural quartz pipe without any treatment on the optical fiber base material produced by the VAD method. This figure shows an optical fiber made by jacketing a synthetic quartz pipe onto an optical fiber base material. And each optical fiber A, B,
C, and D were subjected to humidification treatment in advance to intentionally increase hydrogen loss.

In the method described above, a natural quartz pipe was heated from the outside with an acid-hydrogen burner while an appropriate amount of chlorine gas was flowing inside it, but the natural quartz pipe was heated by slowly pulling it down while flowing chlorine gas into an electric furnace at 1000°C or higher. Similar results were obtained even after treatment.

Furthermore, when we simply tried heat treatment in the atmosphere without using chlorine gas, and after the heat treatment, treated the natural quartz pipe with dilute hydrochloric acid, dilute sulfuric acid, or dilute nitric acid, similar effects were obtained. The reason for this is that when heated in the atmosphere, mainly alkali elements and alkaline earth elements diffuse to the surface of the natural quartz pipe, and react with carbon dioxide gas in the atmosphere on the surface to produce carbonates, which can be mixed with dilute hydrochloric acid, etc. This is because the carbonate is removed when soaked.

As described above, when a natural quartz glass product such as a natural quartz pipe is heat-treated in the air or in an atmosphere containing chlorine, the alkali elements in the natural quartz glass product can be efficiently removed.

Note that the above-mentioned embodiments only describe the treatment of natural quartz pipes as final products, but natural quartz, such as crystal, is crushed, then subjected to processes such as cleaning and sorting, and finally melted. It goes without saying that similar effects can be obtained if the above-mentioned treatments are performed during the series of steps to obtain a glass product made of natural quartz. That is, which process should be preceded or followed by the above-mentioned treatment process can be arbitrarily selected in consideration of the final product's shape, required characteristics, etc.

〔Effect of the invention〕

As described above, according to the present invention, impurities such as alkali elements can be effectively removed from natural quartz glass, and even when limited to applications for optical fibers, for example, it is possible to improve properties such as increased resistance to hydrogen loss, and price. It is possible to obtain a natural quartz pipe for coating an optical fiber base material, which is comparable in terms of conventional synthetic quartz pipes.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the increase in hydrogen loss resistance of an optical fiber using a natural quartz pipe according to the present invention, FIG. 2 is a diagram showing a decreasing tendency of alkali elements in the natural quartz pipe according to the present invention, FIG. 3 is a cross-sectional view of optical fibers produced by the MCVD method and the VAD method, and FIG. 4 is a diagram showing the results of secondary ion mass spectrometry of each optical fiber corresponding to FIG. 3. 1 ~ Core 2 ~ Cladding 3 ~ Jacket Patent applicant Furukawa Electric Co., Ltd. Wavelength (μm) Figure 1 Figure 3 Procedural amendment (voluntary) April 1, 1988

Claims (2)

[Claims] (1) Natural quartz characterized in that the step of obtaining a quartz glass product made of natural quartz from crushed natural quartz includes a step of treating the natural quartz at high temperature in the air or in an atmosphere containing at least chlorine. How to remove impurities from glass. (2) The method for removing impurities from natural quartz glass according to claim 1, wherein the quartz glass product is a pipe.