JPS6112855B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Field of Application of the Invention The present invention relates to a method for manufacturing an optical fiber used for optical transmission.

(2) Prior art Until now, most low-loss optical fibers have been produced by forming a core glass layer to the required thickness on the inner wall of a quartz tube by vapor-phase chemical reaction, and then heating the glass layer to a temperature above the softening temperature of quartz. A method was used in which a tube was heated to form a rod whose cross section had a two-layer or multilayer structure with different refractive indexes, and the rod was drawn to form an optical fiber. FIG. 1 shows a conventional optical fiber manufacturing apparatus. A gas 1 containing SiCl ₄ as a raw material and an additive for controlling the refractive index is sent into a quartz glass tube 2. The quartz tube 2 is attached to a glass lathe, and as it rotates, the oxyhydrogen burner 3
As a result, the raw material gas is oxidized and becomes SiO ₂ fine powder, which is deposited on the inner surface of the tube and vitrified at the same time. A typical example of the conventional method is to repeat this process until the deposited glass film has a predetermined thickness, and then heat-weld the quartz tube to form a glass rod, which is then drawn to obtain an optical fiber. As can be understood from this process, the conventional method has the following problems. The first is that it requires three steps: glass film deposition, welding, and wire drawing.The second is that the conventional method is essentially a batch process, which makes it extremely difficult to create a continuous process from the raw material to the fiber. That's true.

(3) Purpose of the Invention The present invention is to directly produce a glass rod with a multilayer cross-sectional structure without going through welding, which was an essential process in conventional methods, and to draw the glass rod to form an optical fiber. The purpose is to simplify the process and make the process continuous.

(4) General description of the invention In order to achieve the above object, the present invention introduces raw material gas from a pipe inserted inside a glass tube, and uses a microwave cavity installed outside the tube to feed gas into the tube. Excite the plasma and vitrify the raw material gas through the resulting decomposition reaction,
This is deposited sequentially and densely in the longitudinal direction of the tube. A schematic diagram of the apparatus is shown in FIG. A source gas 2, which is a raw material, is passed through an introduction pipe 5 to a glass tube 1.
The core glass 6 is directly vitrified by the microwave cavity 4 without going through a soot state. At this time, the inside of the pipe is kept in a reduced pressure state by the pump 10. The tube is moved downward by rollers 9 at a speed corresponding to the deposition rate of the core glass, and the core glass is successively formed densely in the longitudinal direction of the tube. It is also possible to install a drawing furnace 7 below and draw lines continuously.

The present invention brings about the following effects.

1. Since the core glass is densely formed directly in the tube in the longitudinal direction, a welding process is unnecessary, and the production of the optical fiber is greatly simplified.

2 High purity raw materials can be easily obtained.
That is, since the method uses a gas such as SiCl ₄ and performs the reaction in a closed system free of impurities to obtain core glass, it is possible to easily achieve low loss.

3. Since this is a method in which the core glass is formed sequentially in the longitudinal direction of the tube, it is also possible to perform the process continuously up to the subsequent wire drawing process.

4. Since the core glass is directly vitrified by high-frequency plasma heating without going through a soot state, no air bubbles are introduced, making it easy to reduce loss.

5. By making the pipes inserted into the quartz tube that becomes the cladding into double or multiple pipes,
Optical fibers with not only step-type cores but also convergent cores can be produced.

(5) Examples Hereinafter, the present invention will be explained in detail with reference to examples.

Example 1 In the apparatus shown in FIG. 2, a quartz tube serving as a cladding was made to have a shape of 15 mmφ and a length of 3 mm, and a gas having the following composition was sent into the tube from an inlet tube (quartz tube).

SiCl ₄ 3.0sccm GeCl ₄ 0.3sccm O ₂ 30sccm When the microwave cavity is excited at 200W and 24.5GHz and the pressure inside the quartz tube is reduced to maintain approximately 10Torr, the gas introduced into the tube becomes a plasma state and the following reaction occurs. arise.

SiCl ₄ +O ₂ →SiO ₂ +2Cl ₂ GeCl ₄ +O ₂ →SiO ₂ +2Cl _2The oxides produced as a result of this reaction are directly vitrified and deposited at the bottom of the tube. The core glass deposition rate at this time was 0.5 mm/mm. After 10 hours of reaction, outer diameter 15mmφ, core diameter 9mmφ, length 260mm
I got the preform. When this was drawn and the loss was measured, a value of 28 dB/Km was obtained at a wavelength of 0.83 mμ.

Example 2 A reaction was carried out under exactly the same conditions as in the previous example to form a core glass, and then the preform was heated to about 2000°C in a drawing furnace installed at the bottom, and drawing was carried out continuously with the reaction. Ivy. The loss of the fiber obtained was almost the same as that produced under the conditions described above.

Example 3 As shown in Fig. 3, two pipes are
In the same manner as in Example 1, the amount of GeCl ₄ was reduced to about 1/2 compared to the gas introduced into the inner tube, and the same amount of SiCl ₄ and O ₂ was flowed into the outer tube. A preform was produced by carrying out a reaction. The radial refractive index distribution of the cross section of the preform thus produced was as shown in FIG. The fiber band obtained by drawing this preform is
It had a 6dB bandwidth of 560MHz/Km.

(6) Summary As explained above, according to the present invention, the welding step, which was essential in the conventional manufacturing method of high-bandwidth, low-loss fiber, is no longer necessary. Furthermore, it is also possible to draw the fiber immediately after the reaction via a preform, or to manufacture it continuously from the reaction to the fiber. Furthermore, by making the structure of the entry tube multiple, it is also possible to produce a fiber having a convergent core.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a conventional optical fiber manufacturing apparatus. FIG. 2 is a schematic diagram of an apparatus used in the manufacturing method according to the present invention. FIG. 3 is a diagram for explaining the third embodiment. FIG. 4 shows the refractive index distribution of the cross section of the fiber produced according to the present invention.

Claims (1)

[Claims] 1. Reduce the pressure inside the glass tube, feed SiCl ₄ , GeCl ₄ and O ₂ into the glass tube from a pipe inserted inside the glass tube, and at the same time heat with high-frequency plasma to prevent soot from forming inside the tube. A method for producing an optical fiber, which comprises directly filling it with glass and drawing it by successively heating it.