JPS62119135A - 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 high frequency dielectric 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 high frequency dielectric 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.

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. An increase has been observed. 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 in gain and loss 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 deteriorates the transmission characteristics and mechanical strength of the optical fiber. This also causes the problem of worsening the situation.

[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 using high-frequency dielectric 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 a high frequency dielectric heating furnace installed in series.

Furthermore, the 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 a high frequency dielectric heating furnace installed in series and a heating furnace installed in series below the high frequency dielectric heating furnace.

[Function] Since high-frequency dielectric heating uses microwaves to induce atomic vibrations in a substance to be heated, 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 eccentricity with almost no increase in loss due to radiation or hydrogen. It is possible to obtain an optical fiber.

[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 the optical fiber, which was made from a quartz-based glass fiber base material and coated with an ultraviolet-curable acrylic resin, and as an accelerated test, it was exposed to a cumulative water atmosphere of 4 atm at room temperature for an ISO time. This is the loss characteristic of the optical fiber after heat treatment 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 overall increase in loss, regardless of the wavelength, is due to microbend loss caused by slight bending in the coated fiber due to heat treatment. Wavelength 1.4
The increase in loss (increase in hydroxyl absorption) due to the reaction between the drawing defect and hydrogen, represented by the increase in absorption of μ peaks, is an irreversible process, as is clear from the fact that it is observed after heat treatment. This poses a serious problem for the long-term stability of

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).
(increase in loss in m). 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 increase in absorption by high-speed wire drawing, the difference in temperature between the surface and the inside of the base material has been an obstacle in conventional wire drawing equipment using a resistance heating furnace.

Embodiment 1 FIG. 1 shows a first embodiment of the optical fiber drawing apparatus of the present invention. In the figure, 1 is an optical fiber base material, 2 is a drawn optical fiber, and 3 is a preheating furnace. As an example, a carbon resistance furnace using a carbon resistance 3A as a heating element is shown.

4 is a high frequency dielectric heating furnace, and an example using a rectangular waveguide type applicator is shown. 5 is a waveguide that guides the high frequency output of a high frequency oscillator (not shown) to the dielectric heating furnace 4. FIG. 2 shows an example of a device for supplying high frequency waves to the dielectric heating furnace 4. In FIG. There is a magnetron 7 within the microwave oscillator 6, and the generated microwaves are guided to the high frequency dielectric heating furnace 4 via the waveguide 5. The isolator 8 prevents reflected waves from returning to the oscillator 6, and the matching box 9 is provided to effectively convert the incident waves into heat.

The functions of the apparatus shown in FIG. 1 will be explained.

High-frequency dielectric heating utilizes the fact that a dielectric absorbs microwaves and generates heat, and the amount of heat generated is proportional to the dielectric loss of the object to be heated. The dielectric loss of quartz glass is small (
janδ=1~5XlO"' at room temperature and one frequency IMllz
) Therefore, high-frequency electric heating was conventionally considered to be inappropriate as a heating means for quartz glass. However, in Figure 6 (E
, B, 5hand: Gn assEngineerin
gHandbook, 16. McGraw-Old II (
(1958), the dielectric loss of silica glass increases rapidly as the temperature rises. Therefore, it is not possible to directly heat quartz glass from room temperature using high-frequency dielectric heating, but by pre-heating it to a temperature where the dielectric loss increases using other means, from then on, high-frequency dielectric heating alone can be used. Therefore, the winding tram can be heated to even higher temperatures.

The optical fiber can be drawn using a capstan or other suitable means.

In the example shown in Fig. 1, a carbon resistance furnace was used as the preheating furnace, but the base material was heated to a temperature that allowed high-frequency dielectric heating (
For quartz glass, any furnace can be used as long as it can be heated and maintained at a temperature of about 1000° C. or higher.

Note that once the base material is in a state where it can be constantly drawn by high-frequency dielectric heating, subsequent predictive heating is not necessarily necessary.

In addition to the example shown in FIG. 2, microwave oscillators include talistrons and traveling wave tubes, but magnetrons are generally used for industrial purposes. In this example, a magnetron oscillator with an oscillation frequency of 2.45 GHz and a maximum output of 5 kW was used. The selection of a high frequency dielectric heating furnace is important in terms of heating efficiency. A rectangular waveguide applicator, a circular waveguide applicator, an oven-type applicator, etc. can be used, but in this example, a rectangular waveguide applicator has a relatively good correspondence between the electric field distribution and the base material position. using an applicator.

The relationship between the electric field distribution in the applicator and the base material position is shown in Figure 7 (
Shown in A) and (B). In FIG. 7(B), the density of arrows indicates the strength of the electric field distribution. The efficiency of the dielectric heating furnace can be further increased by modifying the shape of the applicator.

Using the apparatus shown in FIG. 1, an attempt was made to draw a fiber with a diameter of 125 μm from a quartz glass rod with a diameter of 25 mm. When the magnetron output is set to the maximum 5kW, 5m/
The line could be drawn at a speed of min. The drawing tension at that time was 4 g. At a speed of 5 m/min or higher, the wire drawing tension suddenly increased, making it impossible to draw the wire stably. This includes microwave output, drawing speed,
Since there is a relationship between the temperature increase ranges as shown in the following equation, this is because a magnetron of this output cannot achieve a sufficient temperature increase with respect to an increase in the drawing speed.

η・P=c・ρ・A・■・ΔT (1) Here, P is microwave output, η is efficiency, c, P.

A and V are the specific heat, density, cross-sectional area, and drawing speed of the optical fiber, and ΔT is the temperature increase range due to dielectric heating.

Embodiment 2 Therefore, as shown in FIG. 8, the temperature gradient is increased by a second embodiment in which a heating furnace 31 similar to the preheating furnace 3 is connected in series directly below the high frequency dielectric heating furnace 4. The aim was to increase the maximum temperature. Using the apparatus shown in FIG. 8, a drawing speed of up to 20 m/min was obtained when the temperatures of the upper and lower preheating furnaces 3 and heating furnaces 31 were set at 1500° C., respectively.

The effects of the present invention were examined by comparing them with conventional examples. Compare the amount of increase in hydroxyl group absorption due to the hydrogen treatment described above for an optical fiber drawn at a speed of 5 m/min using the apparatus of the first example and an optical fiber drawn using a conventional resistance heating furnace under the same conditions. As a result, the fiber produced by the drawing apparatus of the present invention had significantly less increase in hydroxyl group absorption, that is, E' centers and peroxide radicals, than the conventional fiber. 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 20Q°C lower in the apparatus of the present invention than in the conventional apparatus.

FIG. 9 shows a second embodiment in which a preheating furnace 3 and a heating furnace 31 are provided on the upper and lower sides, the temperatures of both heating parts 3 and 31 are made equal, and the drawing speed and microwave output are adjusted using the predicted heating temperature as a parameter. Show relationships. Note that the calculation results are obtained by extrapolating the experimental results according to (1) illusion. It can be seen that the drawable speed increases as the predicted thermal temperature and microwave output increase. In fact, some 915M1lz band magnetrons have an output of 100kW, and the Talaistron type has achieved IMW (2GHz band). Therefore, by using these microwave oscillators, a drawing speed of 1000 m/min or more is possible, which, in combination with the uniform heating effect, makes it possible to manufacture high-quality optical fibers with excellent long-term stability.

Note that the effects of the present invention are not limited to silica-based optical fibers, but are also effective for multi-component glass fibers, fluoride glass fibers, etc. in the corresponding temperature ranges.

[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 as in the prior art.
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 tiltability.

[Brief explanation of drawings]

Fig. 1 is a schematic cross-sectional view of the first embodiment of the present invention, Fig. 2 is a schematic cross-sectional view illustrating the high-frequency power supply system to the high-frequency dielectric heating furnace, and Fig. 3 is a schematic cross-sectional view of the optical fiber before and after hydrogen treatment. Loss spectrum diagram. Figure 4 shows the relationship between the increase in hydroxyl absorption and drawing furnace temperature. Figure 5 shows the relationship between the increase in hydroxyl absorption and drawing speed. Figure 6 shows the temperature of dielectric loss in quartz glass. 7(^) and 7(B) are diagrams showing the relationship between electric field distribution and base material position in a rectangular waveguide applicator. FIG. 9 is a schematic cross-sectional view of an embodiment of the present invention, and FIG. 9 is a diagram showing the drawing speed possible by high-frequency dielectric heating. DESCRIPTION OF SYMBOLS 1... Optical fiber base material, 2... Optical fiber, 3... Preheating furnace, 4... High frequency dielectric heating furnace, 5... Waveguide, 6... Microwave oscillator, 31 ···heating furnace. 0.7 0.8 1. f f, 3
f, s f, 7 proper length (P tun) Figure 3: Line 1' bottom Tf ('C) Figure 4: Through the line, degree VtC noki in) Figure 5: Figure 6: 1 line 1 in 6 Figure 8

Claims (3)

[Claims] (1) Optical fiber drawing characterized by preheating an optical fiber base material using a preheating furnace, and then heating the preheated base material to a temperature higher than its softening temperature by high-frequency dielectric heating to draw an optical fiber. Method. (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 a high frequency dielectric heating furnace provided in series below the preheating furnace. (3) 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, the preheating furnace, and a heating furnace provided in series below the high frequency dielectric heating furnace.