JPS62119136A - Method and apparatus for drawing optical fiber - Google Patents

Abstract

PURPOSE:To obtain an optical fiber having superior reliability for a long period by preheating a base material for an optical fiber with a preheating furnace, heating it by induction heating and drawing the heated base material into an optical fiber. CONSTITUTION:A base material for an optical fiber is preheated with a preheating furnace and heated to the softening point or above by induction heating. The base material softened uniformly by the heating is drawn into an optical fiber. Thus, an optical fiber nearly free from defects caused by drawing, preventing an increase in the transmission loss due to hydrogen or radiation and having superior reliability for a long period is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical fiber drawing method and apparatus for obtaining an optical fiber with excellent reliability over a long period of time.

[Prior Art] In a conventional optical fiber drawing method, a resistance heating furnace or a high frequency induction heating furnace is used as a drawing furnace to heat and soften the optical fiber preform, and the preform is heated from the outside. However, silica glass, which is a common material for optical fibers, has very low thermal conductivity, so in order to reach the softening temperature at the center of the base material, it is necessary to heat the surface of the base material at an even higher temperature for a long time. there were.

FIG. 2 shows a conventional induction heating furnace. In the figure, 1 is an optical fiber base material, 2 is a drawn optical fiber, and 3 is an induction heating furnace, which is composed of a container 31, a solenoid-shaped coil 32, and a cylindrical zirconium oxide heating element 33. When a high frequency current is passed through the coil 32, an eddy current is generated in the zirconium oxide heating element 33, generating Joule heat. The surface of the base material 1 is heated by thermal radiation and convection from the zirconium oxide heating element, and the center of the base material is further heated by heat conduction, so there is a temperature difference between the surface and center of the base material.
Also, it took a long time to heat the surface and center to maintain the same temperature.

Just as when crystals are heated to high temperatures, defects such as vacancies and interstitial atoms occur, similar defects such as E' centers and peroxide radicals occur in silica glass when heated to temperatures above its softening point. has been observed to increase. The amount produced increases as the base material temperature during wire drawing increases and as the heating time increases. Therefore, overheating of the base material during the optical fiber drawing process will induce additional defects in the optical fiber. Recently, these drawing-induced defects are considered to be the cause of increased loss in optical fibers due to radiation and hydrogen. This increase is an irreversible change,
This impairs the long-term reliability of optical cables.

However, if the drawing furnace temperature is lowered in order to avoid overheating the base material and prevent an increase in defects, the drawing tension increases, so there is a limit to the reduction in the furnace temperature.

In addition, conventional drawing furnaces using carbon heating elements have the disadvantage that the surface of the optical fiber base material must be heated more than necessary because heating is performed from the outer periphery of the optical fiber base material.

Furthermore, overheating of the base material surface causes changes in the refractive index distribution due to diffusion of base material constituent elements, contamination of the inside of the drawing furnace and the optical fiber due to evaporation of the base material, and the transmission characteristics and mechanical properties of the optical fiber. This also causes a problem of deterioration of strength.

[Problems to be Solved by the Invention] The present invention suppresses the generation of drawing-induced defects that cause an increase in the transmission loss of the optical fiber described above, and prevents overheating of the optical fiber preform during drawing of the optical fiber. The purpose of this study is to obtain an optical fiber that can be drawn at high speed and has excellent long-term stability.

[Means for Solving the Problems] In order to achieve the above object, in the optical fiber drawing method according to the present invention, an optical fiber preform is preheated using a preheating furnace, and then the preheated preheated preform is heated. The material is heated to above its softening temperature by induction heating and drawn into optical fiber.

Further, an optical fiber drawing apparatus according to the present invention is an optical fiber drawing apparatus that heats and softens an optical fiber preform in a drawing furnace and draws it into an optical fiber. It consists of 8 conductive heating furnaces installed in series.

[Operation] Since induction heating is a method of heating by iM lightning current induced inside the base material, uniform heating can be obtained inside the optical fiber base material. Therefore, optical fibers can be drawn at substantially lower base material temperatures, and high-speed drawing is possible, resulting in very few drawing-induced defects and excellent long-term reliability with almost no increase in loss due to radiation or hydrogen. Optical fiber can be obtained.

[Example] First, FIG. 3 shows how the loss increases due to the reaction between the drawing-induced defects and hydrogen. This was done immediately after drawing an optical fiber made from a quartz-based glass fiber base material and coated with UV-curable acrylic resin, and as an accelerated test.
After being exposed to a hydrogen atmosphere of 4 atm at room temperature for 150 hours,
This is a loss characteristic of an optical fiber after being heat-treated in air at 200° C. for 15 hours. The increase in the absorption peak near the wavelength of 1.4 μm and the increase in loss at longer wavelengths are due to hydroxyl groups newly generated in the optical fiber by the hydrogen treatment. The reason why the loss increases overall regardless of the wavelength is because
Microbend loss is caused by slight bending of the coated fiber due to heat treatment. Wavelength 1.4μ
The increase in loss (increase in hydroxyl absorption) due to the reaction of hydrogen with drawing defects, represented by the increase in absorption of This poses a serious problem for long-term stability.

As a measure to suppress the generation of these drawing-induced defects and prevent an increase in loss due to these defects, it is effective to lower the drawing furnace temperature. Figure 4 shows the increase in hydroxyl absorption of optical fibers produced by varying the drawing furnace temperature (wavelength 1.4μ before and after hydrogen treatment, excluding microbend loss).
17). As is clear from FIG. 4, by lowering the drawing furnace temperature while keeping the drawing speed constant, the increase in hydroxyl group absorption can be reduced. However, as mentioned above, with the conventional optical fiber drawing method and apparatus, it has not been possible to draw the optical fiber while effectively lowering the drawing furnace temperature.

Another promising measure to prevent the increase in hydroxyl absorption due to drawing-induced defects is to increase the drawing speed.

FIG. 5 shows the change in the amount of increase in hydroxyl absorption with respect to the drawing speed while keeping the temperature of the drawing furnace constant. It can be seen from FIG. 5 that when the drawing temperature is kept constant, the amount of increase in absorption decreases as the drawing speed increases. This is thought to be due to the fact that the time during which the optical fiber preform is exposed to temperatures at which defects occur is shortened. In order to obtain the effect of reducing the amount of increase in absorption by high-speed drawing, the difference in temperature between the surface and the inside of the base material has been an obstacle in conventional drawing equipment using an induction heating furnace.

FIG. 1 shows an embodiment of the optical fiber drawing apparatus of the present invention. In the figure, ] indicates an optical fiber base material, 2 indicates a drawn optical fiber, 4 indicates a preheating furnace, which is a graphite resistance furnace in this embodiment, and a container 41. It consists of a cylindrical graphite resistance heating element 42. 5 is an induction heating furnace, which includes a container 51 and a coil 5.
An example consisting of 2 is shown below. Although quartz glass is an insulator at room temperature, as shown in FIG. 6, its electrical resistance decreases as the temperature rises, and it becomes a good conductor that can be heated by induction at temperatures above 1000°C. The resistance heating furnace shown in the embodiment of the present invention shown in FIG.
If the base material 1 is preliminarily heated to a temperature of 000°C or more, the high frequency current flowing through the coil 52 will directly induce an overcurrent in the base material 1, and the base material itself will generate heat, so the base material 1 can be heated uniformly. . Therefore, the optical fiber can be drawn using any appropriate means such as a winding drum or capstan. As a preheating furnace, a resistance heating furnace using a silicon carbide heating element, an oxyhydrogen flame, or the like can also be used as a predictive heat source.

When the temperature of the base material was set to 1.000° C. using the preheating furnace 4 and the base material 1 was heated using a high-frequency heating furnace with a frequency of 300 kHz and an electric power of tskw as the induction heating furnace 5, the base material 1
The temperature reached 2,000°C, and the optical fiber could be drawn at a speed of 100 m/min.

A comparison of the increase in hydroxyl group absorption by the above-mentioned hydrogen treatment with an optical fiber drawn using a conventional induction heating furnace under the same conditions revealed that the fiber produced using the drawing apparatus of the present invention had a higher hydroxyl group absorption than the conventional one. The amount of absorption gain, ie E' center and peroxide radicals, was significantly reduced. From the difference in drawing-induced defect concentration, it was estimated that the heating temperature on the surface of the optical fiber preform was approximately 200° C. lower in the apparatus of the present invention than in the conventional apparatus.

A heating furnace similar to the preheating furnace 4 may be provided below the induction heating furnace 5 shown in FIG. 1 to soften the temperature gradient of the base material 1. Note that the scope of application of the present invention is not limited to silica-based optical fibers, but also multi-component glasses and chalcogenide glasses.

It can also be used for drawing optical fibers such as fluoride glass. Further, the present invention is particularly effective in cases where heating by thermal convection is difficult, such as when drawing wire in a vacuum.

[Effects of the Invention] As explained above, the present invention can uniformly heat and soften the base material in the optical fiber drawing process, so that the surface of the base material is at a higher temperature than the center unlike the conventional technology.
It has the advantage that it is not subjected to long-term heating and therefore has fewer drawing-induced defects, that is, less increase in transmission loss due to hydrogen or radiation, and that it is possible to obtain an optical fiber with excellent long-term reliability.

[Brief explanation of drawings]

Fig. 1 is a schematic cross-sectional view of an embodiment of the present invention, Fig. 2 is a schematic cross-sectional view of a conventional induction heating device, Fig. 3 is a loss spectrum diagram of an optical fiber before and after hydrogen treatment, and Fig. 4 is an increase in hydroxyl group absorption. FIG. 5 is a diagram showing the relationship between the amount of hydroxyl group absorbed and the drawing furnace temperature. FIG. 6 is a diagram showing the change in electrical resistance of quartz glass depending on temperature. 1... Optical fiber base material, 2... Optical fiber, 3.5... 8 induction heating furnace, 4... Preheating furnace. Figure 1 0, '7 0.8 1.1 1.3
f, s f, 7 production table (Pm) Fig. 3 Line drawing temperature Tfζ・C) Fig. 4 IAIRVt (In/rrLin) Fig. 5

Claims (2)

[Claims] (1) An optical fiber drawing method characterized by preheating an optical fiber preform using a preheating furnace, and subsequently heating the preheated preheated preform to a softening temperature or higher by induction heating to draw an optical fiber. . (2) In an optical fiber drawing device that heats and softens an optical fiber preform in a drawing furnace and draws it into an optical fiber,
An optical fiber drawing apparatus characterized in that the drawing furnace comprises a preheating furnace and an induction heating furnace provided in series below the preheating furnace.