JPS62202836A - Heating furnace for optical fiber drawing - Google Patents

Abstract

PURPOSE:The core tube in the heating furnace is coated with a layer of high melting point and high resistance to resistance to keep the atmosphere clean in the furnace whereby the quality of the optical fiber is improved and the durability of the furnace itself is increased. CONSTITUTION:The core tube 7 which runs through the drawing furnace 3 and is made of carbon or the like is coated with a layer 8 overall the inner and outer surfaces. The coating layer 8 has a melting point over 2,000 deg.C at which the tube is used and resistance to oxidation and is, e.g., TiC. The difference in heat expansion coefficient between the coating layer 8 and the core tube 7 is preferably set to 3X10<-6>deg or less. In the heating furnace, no oxidation occurs or no dust generates in the core tube 7 even at high temperature of fiber drawing and the atmosphere is kept clean in the core tube 7. Thus, optical fibers of high quality are produced stably for a long period of time.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Application> The present invention particularly relates to an optical fiber drawing apparatus for drawing a silica-based optical fiber.

<Prior Art> A quartz-based optical fiber is generally manufactured by heating and melting a rod-shaped quartz-based glass base material, and then drawing it. When manufacturing a quartz-based optical fiber, a rod-shaped quartz-based glass base material is heated to approximately 1900'C in a clean atmosphere.
It has to be heated more than that. As a heating furnace for this purpose, a carbon furnace is usually mainly used. Additionally, zirconia furnaces are also known as an alternative to carbon furnaces, but zirconia (ZrO□) is sensitive to sudden temperature changes and requires very slow temperature raising and lowering operations.
Poor productivity. Additionally, there is the problem that additives such as sintering aids and stabilizers in zirconia ceramics contaminate the atmosphere inside the furnace and have a negative effect on the optical fiber being drawn, so the mainstream heating furnace is now a carbon furnace. There is.

When drawing in such a heating furnace, particular care must be taken to clean the atmosphere within the furnace, that is, to prevent contamination of the molten quartz glass base material and the surface of the drawn optical fiber. This is done by crystallizing carbon particles and quartz glass in the furnace, or by reacting with quartz glass.In the case of a normal carbon furnace, a high-purity, dense carbon core tube is used, and clean nitrogen and A method is used in which a quartz-based glass base material is heated and wire-drawn in an inert gas such as argon.

<Problems to be Solved by the Invention> Conventional optical fiber line heating furnaces cannot have an airtight structure inside, so 1fThf in inert gas is
It is unavoidable that oxygen in the air of the fi is mixed in, and the oxidation consumption is severe especially in the high temperature part of the inner surface of the carbon furnace tube, and fine carbon particles are generated, polluting the atmosphere inside the furnace, and shortening the life of the entire carbon furnace. It was kept very short.

The present invention was made in view of the problems of the prior art, and improves the quality of optical fiber by keeping the atmosphere inside the heating furnace clean, and at the same time improves the durability of the heating furnace itself. The purpose of the present invention is to provide an optical fiber line heating furnace.

Means for Solving Problems> The optical fiber drawing heating furnace according to the present invention is a heating furnace used when drawing an optical fiber preform, and includes a carbon furnace core tube surrounding the optical fiber preform. It is characterized in that at least the inner high-temperature part of the wafer is coated with a coating layer having a melting point of 2200° C. or higher and acid resistance.

Function: When drawing optical fiber base material at high temperatures, the carbon core tube of the heating furnace is protected by a coating layer, which prevents the carbon from oxidizing and being consumed, and causing carbon powder to form and contaminate the optical fiber. There's nothing to do.

Embodiment As shown in FIG. 1, which schematically shows the structure of an embodiment of an optical fiber-coupled heating furnace according to the present invention, an annular heating element 6 is provided inside the heating furnace 3. A furnace core tube 7 surrounded by 6 is provided passing through the heating furnace 3.

The optical fiber preform 1 is supplied downward along the furnace tube 7 from the upper end of the furnace tube 7, and is melted in the high temperature region of the furnace tube 7 heated by the heating element 6 of the heating furnace 3, and is drawn by drawing means (not shown). The optical fiber 2 is formed by the following steps.

The furnace core tube 7 is made of carbon, and a coating N8 of titanium carbide (Tic) is provided all over its inner and outer surfaces. The coating layer 8 may be made of any coating material other than the above-mentioned titanium carbide, as long as it has a melting point of 2200° C. or higher, which is the serviceable temperature of the reactor core tube 7, and is acid-resistant. The chemical formula is shown in Table 1.

Table 1 Note that if the difference in thermal expansion coefficient between carbon, which is the base material of the core tube 7, and the coating N8 is large, strain will occur at the interface between them during the temperature rise and fall process, and cracks and peeling will occur in the coating layer 8. It gets cheaper. For this reason, the difference in thermal expansion coefficient between the carbon forming the furnace core tube 7 and the material of the coating layer 8 covering it becomes a problem.
It has been experimentally confirmed that the guideline is a difference in thermal expansion coefficient of 3 X IO-"m wealth/deg or less. It is desirable that this difference is as small as possible from the viewpoint of the lifespan of the coating layer 8. As a means to avoid the influence of this difference in thermal expansion coefficients, it can be solved by providing one or more coating layers with stepwise differences in thermal expansion coefficients.In this case, the difference in thermal expansion coefficients between a plurality of adjacent coating layers is 3 10-”w/deg
The following is desirable. In this case, it is possible to use a material for the intermediate coating layer even if it has poor acid resistance. In addition, the carbon of the furnace core tube 7 has a thermal expansion coefficient of approximately 3X10-'g/deg depending on the type of raw material such as coke used and processing conditions.
Since it can be adjusted between 9 x 10-''t*/deg, it is desirable to adjust it in accordance with the thermal expansion coefficient of coating N8.

The coating 11J8 can be formed densely on the surface of the furnace tube 7 by a CVD method, a plasma CVD method, a sputtering method, or the like. Further, the thickness of the coating layer 8 is preferably about 5 micrometers to 1 millimeter. This is because if the thickness is 1 mm or more, it will easily peel off, and if the thickness is 5 micrometers or less, the effect of the present invention will not be sufficiently obtained.

In this embodiment, the coating N8 was formed on the entire inner and outer surfaces of the furnace tube 7, but the coating N8 was formed only on the inner surface or as shown in FIG. 2, which shows the structure of another embodiment of the present invention shown in FIG. It is also effective to form the layer 8 only on the particularly high-temperature portion of the inner peripheral surface of the furnace tube 7. Note that the same reference numerals in FIG. 2 as in FIG. 1 indicate the same parts as in FIG.

In these embodiments shown in FIGS. 1 and 2, the heating furnace 3
Inert gas is supplied to the inlet 4 to protect the heating element 6 inside.
The inert gas is supplied into the furnace and into the furnace core tube 7 from the inside of the furnace, and the inert gas supplied into the furnace is exhausted from the exhaust port 5. On the other hand, the inert gas supplied into the furnace core tube 7 prevents air from entering the furnace core tube 7.

Next, an experimental example of the present invention and a comparative example conducted using a heating furnace as shown in FIG. 1 will be shown below.

Experimental Example 1 A quartz-based light beam with an outer diameter of 20 mm was heated using a heating furnace capable of heating to 2100°C while supplying nitrogen gas to the inside using a carbon core tube with an inner diameter of 30 mm and whose entire surface was coated with titanium carbide 20 micrometers thick. An optical fiber having an outer diameter of 125 micrometers was drawn for 10 kilometers from the fiber base material. The resulting optical fiber was extremely strong with a minimum tensile strength of 3.5 kg over its entire length. Even after continuous use for 10 days, no dust was observed in this furnace core tube, and a clean atmosphere could be maintained.

Experimental Example 2 A carbon core tube with a thermal expansion coefficient of 3.2 x 10-'w/deg, whose inner surface was coated with tantalum carbide with a thickness of 50 micrometers, was heated from room temperature to 2000°C for three times. When the test was repeated, peeling occurred at the boundary between the core tube and the coating layer. Therefore, when an intermediate layer of tungsten carbide with a thickness of 20 micrometers having a coefficient of thermal expansion between that of carbon and tantalum carbide was provided, no peeling occurred at all. Using this furnace core tube, wire was drawn under the same conditions as in Experimental Example 1, and a strength test was conducted over the entire length of the obtained single mode optical fiber with an outer diameter of 125 micrometers, and the minimum tensile strength was 5. A very high value of .2 kg was obtained.

Comparative Example A core tube made only of high-purity, dense carbon was used, and wire was drawn under the same conditions as in Experimental Example 1, with an outer diameter of 1
A 25 micrometer optical fiber with a length of 10 kilometers was obtained. As a result of a strength test over the entire length, the minimum tensile strength was less than 0.6 kg. Moreover, the central part of the inner surface of this reactor core tube was significantly worn out after five consecutive days of use. Although nitrogen gas was fed into the core tube to prevent air from entering, some air was found to be mixed in, and it was also observed that the carbon was oxidized. Through this comparative experiment, it was confirmed that the effect of the coating layer of the core tube according to the present invention is extremely significant.

<Effects of the Invention> According to the optical fiber drawing heating furnace of the present invention, a coating layer having a melting point of 2200°C or more and acid resistance is provided on the core tube of the heating furnace, so that high drawing temperatures can be achieved. Even in this case, the furnace core tube does not oxidize or generate dust, and the atmosphere inside the furnace core tube can be kept extremely clean. Since optical fibers can be drawn in a non-contaminated environment, it has become possible to manufacture optical fibers of excellent quality with extremely high tensile strength. Furthermore, there is no oxidation consumption due to 1ffi air mixed into the furnace core tube, and it is possible to produce optical fiber of excellent quality stably for a long period of time, and the durability of the heating furnace is dramatically increased, which is also economically effective. .

[Brief explanation of drawings]

FIGS. 1 and 2 are cross-sectional views showing the schematic structure of one embodiment of the optical fiber line heating furnace of the present invention. In the drawings, 1 is an optical fiber base material, 2 is an optical fiber, 3 is a heating furnace, 4 is an inert gas supply port, 5 is an inert gas discharge port, 6 is a heating element, 7 is a furnace core tube, and 8 is a furnace tube. It is a covering layer.

Claims (1)

[Claims] 1. A heating furnace used for drawing an optical fiber preform, which heats at least the high-temperature inner surface of a carbon furnace core tube surrounding the optical fiber preform to a melting point of 2200°C or higher. An optical fiber heating furnace characterized by being coated with a durable and acid-resistant coating layer. 2. The difference in thermal expansion coefficient between the coating layer and the core tube is 3×10^-
The optical fiber wire drawing heating furnace according to claim 1, characterized in that the heating temperature is 6 mm/deg or less. 3. The optical fiber heating furnace according to claim 1 or 2, wherein the coating layer is made of a plurality of layers made of different materials. 4. The optical fiber line heating furnace according to claim 3, characterized in that the difference in coefficient of thermal expansion between the plurality of adjacent coating layers is 3 x 10^-^6 mm/deg or less. .