JPH01179735A - Production of optical fiber base material - Google Patents

Abstract

PURPOSE:To obtain the title optical fiber porous base material with reduced residual OH group and transmission loss and coated with a thin deposit by depositing the fine glass particle current distant from a flame on a supporting surface isolated from the flame in the production of an optical fiber base material by VAD. CONSTITUTION:Core materials 4 and 4' for graded index-type fibers, for example, are connected through an intermediate dummy rod 9. One end of a center glass body with a working dummy rod fixed on both ends is fixed to a lifting machine 8 of the reaction vessel 6 of the VAD device, the uppermost part of the core material 4 is placed in front of a burner 1, then the lifting machine 8 is raised while injecting a raw material and combustion gas from the burner 1, and fine glass particles 3 enveloped in the flame 2 are deposited on the core material 4 as the center glass body directly covered with the flame 2 to form a porous glass body 5. Meanwhile, the surplus fine glass particle current 3 not depositing on the core material 4 and distant from the flame 2 is deposited on the core material 4' isolated from the flame 2 to form a porous glass body 5' coated with a thin deposit layer.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing an optical fiber preform, and in detail, O.
The present invention relates to a method for producing a porous base material that has low absorption loss due to H groups and has a small thickness of glass fine particles deposited on a central glass body.

[Conventional technology]

The VAD method (Va
por phase Axial Deposition
n Method (vapor phase axial deposition method, vapor phase axial deposition method) is known, and as shown in Fig. 5, combustion gas and frit gas are ejected from burner 1 to form Glass raw materials are flame-hydrolyzed in the flame 2 to synthesize glass fine particles 5, which are adhered and deposited on the outer periphery of the central glass body 4 to form a deposit 5 of glass fine particles. and deposit 5
This is a method to obtain a transparent glass body by heating a porous glass body consisting of υ. In addition, 6 in FIG. 4 and each drawing shows the deposition surface, and the white arrow 7 shows the growth direction of the porous body.

In the synthesis of porous glass bodies by the VAD method, the adhesion efficiency of glass particles to the porous glass body is greatly influenced by the positional relationship between the flow of glass particles and the flame. That is, the adhesion efficiency is increased because the glass particle flow is narrowed down compared to the spread of the flame when the glass particle flow is recessed into the flame. Furthermore, the bulk density of the porous glass body to be synthesized is determined by the atmosphere during synthesis and the temperature of the porous glass body itself. That is, the higher the temperature, the higher the bulk density. On the other hand, in actual production, it is advantageous in terms of manufacturing costs to synthesize a porous glass body at the highest possible deposition rate and with less cracking, deformation, etc. For this purpose, it is preferable that the adhesion efficiency and bulk density are high. Therefore, in the conventional synthesis of porous glass bodies, there is a narrow flow of glass particles in the flame, and in order to deposit them in a high-temperature atmosphere, as shown in Fig. 3, the glass particles 3 wrapped in the flame 2 are This was done in such a way that it adhered and deposited on the central glass body 4 that was in direct contact with it.

[Problem that the invention seeks to solve]

However, the conventional porous glass body synthesis method described above has the following problems.

As shown in FIG. 3, since the flame 2 directly hits the deposition surface 6, water will seep into the central glass body 4, which has been previously dehydrated. No matter how much the synthesized porous glass body 5 (including the central base material) is dehydrated, it is difficult to completely dehydrate the water that has seeped into the central glass body 4, and as a result, OH There is a risk of increased absorption loss due to residual groups, impairing the quality of the fiber.

By the way, the refractive index distribution of the core of a graded index (GI) type fiber base material has two shapes: one without a hem as shown in Figure 4-(a), and the other with a hem as shown in Figure 4-(a). There is a shape that can be drawn. When attempting to manufacture a GI type fiber base material using the former core material without hem draw, air bubbles are likely to occur at the interface between the core base material and the vibrator during the cobb process where a quartz pipe for cladding is covered and melted into one piece. . As a means to prevent this, there is a method of synthesizing a pure quartz layer (synthetic rad layer) on the outside of the core base material. In this case as well, it is preferable to prevent OH group contamination into the core base material, and in addition to this, the pure quartz (SiOl) layer described above is as thin as possible in order to prevent a significant change in manufacturing conditions and a drop in productivity. However, there is a need for a means that can satisfy both requirements.

The present invention was made in view of the above-mentioned current situation, and an object of the present invention is to solve the problem of water infiltration into the central glass body and provide a method for manufacturing a porous base material with a reduced amount of residual OH groups. do. In addition, the present invention eliminates water intrusion into the central glass body.
The present invention also provides a method for producing a thin porous base material for a glass particle deposit.

[Problem that the invention seeks to solve]

The present invention synthesizes glass fine particles by subjecting frit gas and combustion gas to a flame hydrolysis reaction in a flame formed by blowing out frit gas from a burner, and deposits the glass particles on a central glass body. After forming a porous glass body, in a method for manufacturing a transparent glass base material by making the porous glass body transparent, a flow of glass fine particles at a place away from the flame is attached to a deposition surface that is not hit by the flame. -Relates to a method for manufacturing an optical fiber preform, which is characterized by depositing the preform.

When synthesizing a porous glass body using the VAD method, in contrast to the conventional method in which a stream of glass particles wrapped in flame adheres and accumulates on the central glass body that is directly hit by the flame, the present invention A stream of glass particles located far away adheres and accumulates on the central glass body, which is not exposed to the flame.

FIG. 1 shows a conceptual diagram of the present invention. However, the relative positional relationship between the burner 1 and the central glass body 4 is not particularly limited as long as the above conditions are satisfied.

In FIG. 1, the burner 1 and the central glass body 4 are arranged so that the flame 2 emitted from the burner 1 does not directly hit the central glass body 4.

[Effect]

In the method of the present invention, when fine glass particles are deposited on a quartz-based central glass body by ejecting frit gas and combustion gas from a burner, fine glass particles 3 located away from a flame 2 are exposed to the flame. 2 adheres and deposits on the deposition surface 6 that is not directly in contact with it, so firstly, the adhesion and deposition efficiency becomes low. Furthermore, since the ambient temperature during synthesis and the temperature of the porous glass body itself are lower than when synthesis is performed within the flame 2, the bulk density of the porous glass body 5 to be synthesized is lowered. Therefore, it is possible to manufacture a porous base material 5 with a thin deposited thickness.

Note that the difference between being directly hit by the flame and being away from the flame means a difference that can be determined visually.

Furthermore, since the flame 2 does not directly hit the central glass body 4 on which the glass particles 3 are deposited, there is no possibility of water seeping into the central base material 4. Further, water remaining in the synthesized porous glass body 5 can be completely removed by a subsequent dehydration treatment. Therefore, it is possible to manufacture an optical fiber preform in which an increase in absorption loss due to residual OH groups is reduced.

At that time, there are no limitations on the composition, shape, dimensions, etc. of the central glass body. Furthermore, the frit gas and combustion gas include all gases that can synthesize water at the same time as glass particles through reaction.

Hereinafter, the present invention will be specifically explained using examples.

〔Example〕

Embodiment 2 - As shown in Fig. 2(a), the core materials 4 and 4' for gradient index type fibers are connected via an intermediate dummy rod 9, which is used as a central glass body, and working dummy rods are attached to both ends of the central glass body. and one side was attached to the pulling machine 8 of the VAD device.

The starting position is such that the top of the core material 4 is in front of the burner 1, and the raw material and combustion gas are transferred to the burner 1.
The pulling machine 8 was raised while ejecting water from the air. At this time, the length of the intermediate dummy rod 9 was set in advance so that the flame 2 from the burner would not hit the core material 4'. In this way, as shown in Figure 2-0), the adhesion and accumulation of the glass particles 3 to the core material 4 can be achieved using the conventional method, that is, flame 2.
The glass particles 3 encapsulated in the flame 2 are attached and deposited on the central glass body directly hit by the flame 2 to form the porous glass body 5. On the other hand, the excess glass particles that did not adhere to the core material 4 adhere to and accumulate on the core material 4', but in this case, the glass particles located away from the flame 2 are deposited on the deposition surface that is not directly hit by the flame 2. It becomes a mold that is attached and deposited to form the porous glass body 5I. Note that 6 is a reaction vessel, 1 is a burner, and 7 and 7' are exhaust gases.

Specific manufacturing conditions are shown below.

■ Bar: Round 8-pipe burner ■ Pulling machine lifting speed near 0 m+/hour Next, the porous glass body obtained under the above conditions and the transparent glass body obtained by dehydrating and sintering it. The data are summarized in Table 2.

Table 2 From the results in Table 2, it can be seen that the transparent glass body from the porous glass body 5', in which the glass particle flow far from the flame is deposited on the deposition surface not exposed to the flame, has a higher hydroxyl group content. It can be seen that there is little (below the detection limit) and that a thin layer can be formed.

〔Effect of the invention〕

As described above, in the synthesis of a porous glass body onto a central glass body by the VAD method, the present invention allows glass particles to be attached and deposited on the central glass body away from the flame and not directly exposed to the flame. Since it is possible to prevent H groups from remaining, it is possible to manufacture an optical fiber preform in which the increase in absorption loss due to OH groups is reduced. At the same time, it is possible to manufacture a porous base material with a thin deposited thickness. Therefore, it is effective when used in the production of optical fiber preforms with low loss or strict structural specifications.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram illustrating a method of forming a porous glass body according to the present invention. FIG. 2-(a) and FIG. 2-(a) are explanatory diagrams of embodiments of the present invention, and FIG.
Figure a) shows the state at the start of deposition, and Figure 2-A shows the state just before the completion of formation of the porous body. Fig. 6 is a conceptual diagram explaining the synthesis method of porous glass bodies by the conventional vAD method, Fig. 4-(a) and Fig. 4-(b).
) are all diagrams showing the refractive index distribution of the core base material, and FIG. 4-(b) shows a shape without a hemline, and FIG. 4-(b) shows a shape with a hemline.

Claims (1)

[Claims] (1) Fine glass particles are synthesized by subjecting the frit gas to a flame hydrolysis reaction in a flame formed by blowing out frit gas and combustion gas from a burner, and depositing the glass particles on the central glass body. After forming a porous glass body, in a method for manufacturing a transparent glass base material by making the porous glass body transparent, a flow of glass fine particles at a place away from the flame is directed onto a deposition surface that is not hit by the flame. A method for producing an optical fiber preform, which comprises adhering and depositing the preform.