JPS59137334A - Manufacturing apparatus of base material for optical fiber - Google Patents

Abstract

PURPOSE:To improve the durability of a carbon tube by providing a furnace core tube consisting of a carbon tube with a quartz tube fitted to the inside thereof in the center of a heating furnace to prevent the scaling of the carbon tube by oxidation. CONSTITUTION:A carbon tube 3b is inserted into the center of the main body of a heating furnace 6 equipped with a heating element 5 at the inside, and a transparent quartz tube 3a having <=0.5ppm CuO content and <=1% Fe2O3 content is fitted close to the element 5 on the inner circumference of the carbon tube 3b. A supply port 7 for introducing a shield gas such as Ar, N2 etc. is provided on the lower side of the main body of the furnace 6, and an introducing port 8 for the treating gas such as He, Ar, Cl2 etc. is provided on the bottom side. A porous glass base material 1 is hung down in the upper part of the furnace core tube 3 through a supporting rod 2. In this way, the base mterial for optical fiber free from impurities such as Cu or water can be manufactured, and the optical fiber having a small transmission loss is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for producing an optical fiber base material that prevents contamination of impurity elements into the glass matrix and has excellent durability.

V as a general method for mass producing base materials for optical fibers.
AI) method is known. This VAD method involves depositing glass particles generated in an acetic acid flame on a rotating starting member, such as a glass plate or a glass rod, to create a cylindrical porous base material, and then sintering this porous base material. This is a method for manufacturing a transparent optical fiber base material. In order to sinter and make the porous base material transparent in this method, it is necessary to heat the base material to about 1500 DEG -1600 DEG C. in a He or Arjf gas atmosphere. A carbon furnace is usually used as this heating furnace. During sintering, special attention must be paid to the prevention of contamination of transition elements such as cu+FC and prevention of moisture contamination. 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 the moisture content will be 0. This is because if more than 1 ppm is mixed in, the characteristics in the long wavelength range will be impaired.

On the other hand, since the VAD method uses a flame hydrolysis reaction to generate glass particles, some unreacted water is already mixed into the porous base material to some extent. Therefore, as a dehydration treatment to remove water present in the porous base material, the porous base material is treated with C1 or 5OC121.
High-temperature heating in a 02 atmosphere is practiced. This dehydration treatment is usually carried out in a carbon furnace as a pre-sintering step. In order to prevent the carbon heating element from being consumed by moisture and oxygen generated during the heat treatment of the base material, the carbon furnace is equipped with
A furnace tube is installed to isolate the carbon heating element from the sintering atmosphere, and conventionally it was made of alumina.

However, there is a problem in that the alkaline components contained in hepta-alumina, which uses an alumina core tube, scatter around at high temperatures and adhere 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 furnace core tube contains Cu or Fe, the chlorine-based gas in the dehydration treatment atmosphere will easily chemically react with the Cu or Fe, resulting in the formation of volatile chlorides as shown in the equation below. A new problem has also arisen in that the fibers penetrate into the porous matrix and significantly impair the loss characteristics of the fiber.

Cu　O4Cut　CA! 2 CX,.

Fel 0B-11111〉Fe C11s Furthermore,
When the furnace core tube is made of quartz glass, Cu has the property of easily diffusing into the silica glass at high temperatures, so Cu volatilized from the furnace body and heating element passes through the furnace core tube.
There is also the problem that it gets mixed into the glass base material.

The present invention provides a base material for optical fibers that reliably prevents the contamination of the above-mentioned impurities at high temperatures while maintaining the advantages that quartz tubes are denser than alumina tubes and have a smaller coefficient of thermal expansion, making them excellent in durability. The purpose of this invention is to provide a manufacturing device for manufacturing a porous material by inserting a porous base material into a core tube of a heating furnace and dehydrating and sintering it. It is characterized by forming a furnace core tube by incorporating a quartz tube inside a carbon tube.

The present invention will be described in detail below based on embodiments shown in the drawings.

The schematic configuration of this device 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 from an inner circumferential layer of a quartz glass tube 3a and an outer circumferential layer of a carbon tube 3b. That is, carbon tube 3
b is fitted in the center of the furnace, while a quartz tube 3a is installed inside the carbon tube 3b at a portion near the heating element 4 on its inner periphery. On the other hand, a supply lower for introducing shielding gas such as Ar or N2 is provided at the side end of the furnace body 6. Further, a supply port 8 for introducing processing gas such as He, Ar, C12, 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. It is suspended.

In the above structure, the quartz tube 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 being volatilized and mixed into the base material, it is desirable that the quartz tube be highly pure and transparent.

As for the extent, CuO is 0.5 ppm or less, Fe20
A transparent quartz tube in which the copper content is particularly removed so that 3% is 1% or less is suitable. Generally, when a quartz tube is heated to 1,500°C or higher, a phenomenon of elongation is observed in the quartz tube, but if the wall thickness is 5 mm or higher, no elongation occurs even at 1,600°C. 165 for mu quartz tube
No stretching occurs even at 0°C. Therefore, if these quartz tubes are used, the sintering temperature will be between 1δOO and 1600°C.
There is no problem with respect to the degree.

On the other hand, impurities such as Cu cannot pass through the carbon tube. As mentioned above, in the device of the present invention, a carbon tube is arranged around the outer periphery of the quartz tube, so impurities such as Cu volatilized from the external furnace body 6 and heating element 5 are shielded by the carbon tube 3b, and the core There is no possibility of intrusion into the inside of pipe 3.

Therefore, it is possible to reliably prevent impurities from entering the porous glass base material.

Example 1 A furnace core tube is heated to 1600°C with a heating element 5, and C1 is placed inside the tube.
2 at a rate of 5 Q cc/min and He at a rate of 5 n/min.
The porous base material 1 was inserted into it at a descending speed of 2 myv'. When the obtained transparent glass base material was subsequently spun into a fiber, the residual water content of 7 IPA was 0.01 ppm, and no absorption derived from Cu or Fe was observed.

Example 2 A fiber was manufactured under the same conditions as in Example 1 except that a quartz tube containing 1 ppm of Cu was used as the quartz tube 3a. The residual moisture in the obtained fiber was 0.01 ppm. Further, absorption originating from Cu existed up to approximately 1.30 μm, but this value was 2 to 3 dB, which was sufficiently low compared to the conventional absorption.

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 thus can provide an optical fiber with low transmission loss. . In addition, since the present invention has a quartz tube inside the carbon tube as the furnace core tube, it is possible to prevent the carbon tube from being consumed by oxidation, and the quartz tube is short and easy to replace, so it has the advantage of being highly economical. be.

[Brief explanation of the drawing]

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 drawings, 1 is a porous base material, 2 is a bar, 3 is a furnace tube, 3a is a quartz tube, 3b is a carbon tube, 5 is a heating element, 6 is a furnace body, and 7.85 is a porthole. . Patent Applicant Sumitomo Electric Industries, Ltd. Nippon Telegraph and Telephone Public Corporation Representative Patent Attorney Shibe Mitsuishi (and 1 other person)

Claims (1)

[Claims] ■ In an apparatus for inserting a porous glass base material into a core tube of a heating furnace and dehydrating and sintering it, a carbon tube is provided in the center of the heating furnace, and a quartz tube is installed inside the carbon tube to form the core. An apparatus for manufacturing an optical fiber base material, characterized in that it is formed into a tube. (2) An 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.