JPH02184540A - Production of optical fiber and apparatus therefor - Google Patents

Abstract

PURPOSE:To obtain an optical fiber having triangular refractive index distribution in a short time by heating a fluorine-doped quartz glass tube and drawing an optical fiber from the tube while evaporating and dissipating the doped fluorine from the surface. CONSTITUTION:A supporting quartz glass tube 2 connected to an end of a fluorine-doped quartz glass tube 1 is fixed to a feeding apparatus 3. A gas containing molecules containing halogen element other than fluorine is supplied to the glass tube 1 through a gas-feeding part 4 and the tube 1 is heated with an electric furnace 5. The glass tube 1 is drawn into an optical fiber while evaporating and dissipating the doped fluorine from the surface of the glass tube and wound with a winder 6.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method and apparatus for manufacturing an optical fiber, that is, a method and apparatus for manufacturing a high-quality optical fiber.

More specifically, the present invention relates to a method and apparatus for manufacturing a single mode optical fiber that is a fluorine-doped silica glass crat optical fiber and has a triangular refractive index distribution.

Prior Art Fluorine is the only practical additive that lowers the refractive index of silica glass without increasing optical fiber loss. Fluorine-doped silica glass clad pure silica glass core optical fibers are currently available with low film loss. The present inventors proposed a fluorine external diffusion method as a method for manufacturing a fluorine-doped silica glass clad optical fiber base material with a triangular refractive index distribution, which is promising as a low-loss 1.5 μm band zero-dispersion shifted optical fiber. (Patent Application No. 291777 of 1988). FIG. 2 shows the base material manufacturing process using this method. This method is
A fluoridated quartz glass tube is heated with an oxyhydrogen flame or the like to evaporate and diffuse fluorine from the inner surface of the glass tube, creating a diffusion layer with a low fluorine concentration on the inner surface of the tube.
This is a method of manufacturing an optical fiber preform having a high refractive index part (core) with the same volume as the diffusion layer in the center by solidifying the tube. The refractive index distribution of the optical fiber preform produced by this method is given by the following equation.

n(r)"no-Δn (1-etfc(y)+exp
(2yz+z2) ・erfc(y+z) )■ However, x-i (1+(r/R0)2)1'2-1) R.

y-ga(2(Dt)1'21 2-α(t/D)1/2 where, no = refractive index of pure silica glass Δn refractive index difference of difluorine-doped silica glass tube Ro: fluorine-doped silica glass Inner radius of tube (mm) D = Diffusion constant of fluorine (cm2/5ec) t: Diffusion time (sec) α: Constant of proportionality.

From equation (2), it can be seen that the refractive index distribution of the base material depends on the diffusion constant of fluorine in the quartz glass, the diffusion time t, and the inner radius of the fluorine-doped quartz glass tube.

Figure 3 shows the refractive index distribution of the base material, which was determined by numerical calculation from the formula (2). From the figure, it is clear that the refractive index distribution of the optical fiber preform is triangular.

The produced optical fiber preform is heated and drawn to obtain an optical fiber having a triangular refractive index distribution.

(Problems to be Solved by the Invention) In this method, the fluorine-doped quartz glass tube tends to deform due to heating, and in order to achieve a uniform refractive index distribution in the longitudinal direction, the fluorine diffusion treatment is performed at the glass softening point (1600°C). )
It was carried out at a low temperature nearby. For this reason, ■ a diffusion process of 2 hours or more is required to form a predetermined refractive index distribution;
In addition, ■ Fluorine diffusion treatment, medium heat conversion, and wire drawing are separate processes, which has the disadvantage of being uneconomical in terms of energy costs and labor costs.

The present invention has been made in view of the above problems, and provides a method and apparatus for economically manufacturing an optical fiber having a triangular refractive index distribution.

(Means for Solving the Problems) In order to solve the above problems, the optical fiber deployment method according to the present invention heats a fluorine-doped quartz glass tube, evaporates and diffuses the fluorine contained in the glass tube from the inner surface, and simultaneously Characterized by drawing a line.

Another object of the present invention is to provide an apparatus for carrying out the above manufacturing method, which includes: a heating electric furnace for evaporating and diffusing fluorine contained in the inner surface of a fluorine-doped quartz glass tube; An optical fiber comprising a fluorine-doped quartz glass tube feeding device for feeding the fluorine-doped quartz glass tube into the electric furnace, and an optical fiber winding device for drawing the fluorine-doped quartz glass tube heated in the electric furnace. The manufacturing apparatus is characterized by comprising a gas supply section that supplies a gas containing molecules containing a halogen element other than fluorine into a fluorine-doped quartz glass tube, and an exhaust section that exhausts the gas containing halogen-containing molecules. .

(effect) The optical fiber manufacturing method of the present invention is shown in FIG.

The method of the present invention evaporates and diffuses fluorine in a short time by heating at a high temperature of 2000° C. or higher, and simultaneously performs solidification and wire drawing.

FIG. 4 shows the relationship between the diffusion constant of fluorine and the reciprocal of absolute temperature in fluorine-doped silica glass.

By increasing the diffusion temperature from the glass softening point of 1600°C to 2000°C, the diffusion constant becomes 4 × 10−”
cm2/sec to 5 X 10-9cm2/sec
100 times or more, and the diffusion time required to form the refractive index distribution is significantly shortened. Therefore, a desired refractive index distribution is formed in the single mode optical fiber depending on the temperature and temperature soaking section passage time conditions used in a normal drawing process.

According to this method, since the external diffusion treatment and solidification are performed simultaneously with the drawing of the fluorine-doped quartz glass tube, it is possible to obtain a highly accurate and uniform refractive index distribution without causing uneven deformation of the glass tube. can.

Furthermore, since the glass tube softens at the high temperature of wire drawing and becomes solid naturally due to surface tension, it is possible to omit the external diffusion treatment step and solidification step by heating the fluorine-doped quartz glass tube.

According to the manufacturing method of the present invention, when manufacturing an optical fiber, a gas or gaseous substance containing molecules containing a halogen element other than fluorine is introduced into a fluorine-containing quartz glass tube, and OH groups are mixed in from the atmosphere during the heating drawing process. By preventing this, optical absorption loss of <1.39 μm based on OH groups can be reduced, making it possible to reduce the loss of the optical fiber. That is, in the present invention, in which fluorine is evaporated, diffused, and solidified at the same time as described above, the removal of OH groups is important. This makes it possible to produce virtually usable optical fibers.

Examples of gases containing halogen elements other than fluorine include chlorine, thionyl chloride, and bromine.6For example, the above-mentioned thionyl chloride and bromine are liquid at room temperature, and inert gases such as helium can be used. Bubble it with water and use it as a gas or gas. Therefore, herein (including the claims), the term "gas" is used to include gases and gaseous substances.

By using the manufacturing apparatus of the present invention, high-quality optical fibers can be manufactured safely and easily.

Hereinafter, it will be explained in detail using examples.

(Example) FIG. 5 shows an optical fiber manufacturing apparatus used in this example.

As is clear from Fig. 5, the fluorine-doped quartz glass tube 1
It has a fluorine-doped quartz glass tube feeding device W3 for conveying the fluorine-doped quartz glass tube 1 to the heating electric furnace 5, and this feeding device 3 includes a supporting quartz glass tube 2 for supporting the fluorine-doped quartz glass tube 1. The fluorine-doped quartz glass tube 1
It is designed to be installed. Such heating electric furnace 5
A device (winding device) 6 for drawing the heated fluorine-doped quartz glass tube 1 is provided below the electric furnace 5. Further, the upper part of the manufacturing apparatus is provided with a gas supply section 4 for supplying a gas containing molecules containing a halogen element other than fluorine into the fluorine-doped quartz glass tube 1, and an exhaust section 7 for discharging the gas. It is being

Optical fibers were manufactured using such equipment.

First, a fluorine-doped quartz glass tube 1 (
Relative refractive index difference 0.7% based on pure silica glass, outer diameter 2
5 mm, inner diameter 15 mm, and length 200 mm).

Supporting quartz glass tubes 2 (outer diameter 25 mm) are attached to both ends of this glass tube.
mm, inner diameter 15 mm, length 100100 O and a dummy quartz glass rod were connected and used for wire drawing. The support quartz glass tube 2 is fixed to the fluorine-doped quartz glass tube feeding device 3, and chlorine gas diluted with helium gas is introduced into the tube from the upper end of the support glass tube through the gas supply section 4 to completely fill the inside of the glass tube. After the gas has been replaced, the fluorine-doped quartz glass tube 1 is heated to 2000°C in the carbon resistance furnace 5 to soften it, the dummy quartz glass rod is dropped under its own weight, and wire drawing is started using the winder 6. The drawing is carried out by feeding the fluorine-doped quartz glass tube 1 into the furnace from the upper part of the carbon resistance furnace (heating electric furnace) 5 at a constant speed, and winding the optical fiber from the lower part of the furnace using the winder 6. An optical fiber with an outer diameter of 125 μm was obtained.

The refractive index distribution of the produced optical fiber is shown in FIG. 6, which shows that the optical fiber has a triangular refractive index distribution. The relative refractive index difference Δ between the core and claddings was 0.55%, and the core diameter 2a was 8.0 μm.The loss of the fabricated optical fiber was 0.3 dB at a wavelength of 1.60 μm.
It was Am.

Figure 7 shows the absorption loss of 1.39 μm due to OH groups and the chlorine flow rate ratio C12/(C1
2+He), and as the chlorine flow rate ratio increases, 1
．． It has become clear that the absorption loss at 39 μm decreases rapidly.

Moreover, when chlorine is not used, the absorption loss at 1.5 μm is 1100 dBA or more, which is a major drawback in practical use.

(Effects of the Invention) As explained above, high-quality optical fibers can be manufactured in a short time with a small number of steps by simultaneously performing fluorine evaporation and diffusion treatment and solidification by high-temperature heating in the drawing process. Therefore, significant savings in energy costs and labor costs associated with external diffusion treatment and solidification processes can be expected.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the steps of the optical fiber manufacturing method of the present invention, Figure 2 is a diagram showing the optical fiber manufacturing process using the conventional fluorine external diffusion method, and Figure 3 is an optical fiber matrix obtained by numerical calculation. Figure 4 is a diagram showing the refractive index distribution of the material, Figure 4 is a diagram showing the diffusion constant of fluorine in fluorine-doped silica glass versus the reciprocal of absolute temperature, and Figure 5 is the optical fiber of the present invention used in Examples. Figure 6 is a diagram showing the manufacturing equipment, Figure 6 is a diagram showing the refractive index distribution of the optical fiber manufactured in the example, and Figure 7 is the relationship between the absorption loss of 1.39 μm due to OH groups and the chlorine flow rate ratio in the atmosphere. FIG. 1... Fluorine-doped quartz glass tube, 2... Supporting quartz glass tube, 3... Fluorine-doped quartz glass tube feeding device,
4... Gas supply section, 5... Carbon resistance furnace, 6...
- Winding machine. Figure 1

Claims (2)

[Claims] (1) A method for producing an optical fiber, which comprises heating a fluorine-doped quartz glass tube to evaporate and diffuse fluorine from the inner surface of the glass tube, while simultaneously drawing the tube. (2) a heating electric furnace for evaporating and diffusing fluorine contained from the inner surface of the fluorine-doped quartz glass tube; a fluorine-doped quartz glass tube feeding device for feeding the fluorine-doped quartz glass tube into the electric furnace; An optical fiber manufacturing device comprising an optical fiber winding device for drawing a fluorine-doped quartz glass tube heated in an electric furnace, wherein the fluorine-doped quartz glass tube contains molecules containing a halogen element other than fluorine. An optical fiber manufacturing apparatus comprising: a gas supply section that supplies gas; and an exhaust section that exhausts gas containing halogen-containing molecules.