JPS5854097B2 - Manufacturing method of optical fiber base material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing an anhydrous optical fiber preform by subjecting a porous glass preform produced by the VAD method to a dehydroxyl treatment.

Conventionally, optical fibers used for optical communications have been manufactured by various methods, but the VAD method has recently been attracting attention.

In the VAD method (vapor phase attachment method), soot-like glass particles are attached and deposited on the lower end of a rod-shaped base material that moves upward while rotating.
In this method, a rod-shaped porous glass preform is formed by growing soot-like glass particles in the axial direction while pulling up a rod-shaped base material, and then a treatment such as dehydroxylation treatment is performed to form an optical fiber preform.

Then, this optical fiber base material is spun to form an optical fiber.

By the way, conventional dehydroxylation treatment is carried out by heat-treating a porous glass preform at 800 to 1000° C. in a halogen gas such as C12 or a halide gas such as 5OC12.

However, since the bulk density of the center and the particle size of the sooty glass particles differ depending on the porous glass preform to be dehydroxylated, the amount of hydroxyl groups in the optical fiber base material obtained by dehydroxylation also differs. It has been difficult to produce anhydrous optical fiber matrix with good reproducibility.

For example, in the case of an optical fiber base material with a square distribution type, the optical power tends to concentrate in the center, and the amount of hydroxyl groups in the center has a very large effect. There is a need for a method that can produce the same with good reproducibility.

The present invention was made in response to the above-mentioned needs, and its purpose is to provide a method for manufacturing an optical fiber preform that has advantages such as being able to manufacture an anhydrous optical fiber preform in a short time and with good reproducibility. It is about providing.

A feature of the present invention is that a porous glass preform produced by the VAD method is subjected to dehydroxyl treatment under pressure.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

First, a porous glass preform 2 is formed at the lower end of a rod-shaped substrate 1 in a reaction vessel (not shown) by the VAD method.

Then, this porous glass preform 2 is housed in a dehydroxylation furnace 7 that includes a heating tube 3, a heating element 4, a reflection plate 5, and an airtight rotary seal 6.

Then, a dehydroxylation gas mainly composed of a halogen gas such as C12 or a halide gas such as 5OC12 is introduced from the dehydration gas introduction pipe 8,
While maintaining the internal pressure at a pressure higher than atmospheric pressure by, for example, about 5 to 20 ymH20, a dehydroxyl group treatment is performed to form an anhydrous optical fiber preform.

To maintain the pressurized state as described above, for example, as shown in the drawing, the pressure inside the dehydroxylation treatment furnace γ is detected by the pressure gauge 9, and the gas flow rate is adjusted by the needle valve 11 provided in the exhaust pipe 10. This can be done by

Note that if the dehydroxylation treatment is carried out under a relatively high pressure, the diffusion rate of the dehydroxylation treatment gas becomes extremely fast, and the dehydroxylation treatment can be carried out in a short time.

Furthermore, the dehydroxylation treatment temperature varies depending on the pressure inside the dehydroxylation treatment furnace γ, the type of dehydroxylation treatment gas, the type of porous glass preform 2 to be treated, etc., but is generally 10
00°C to 1200°C is used.

By the way, the mechanism of dehydroxylation is thought to be as follows.

The dehydroxylation treatment gas, which is mainly composed of a halogen gas such as C12 or a halide gas such as 5OC12, is partially atomized under heat and pressure to generate radicals of chlorine atoms.

Under pressure, these chlorine atom radicals quickly diffuse into the center of the porous glass preform, ensuring that the 5iOH in the center becomes 5iOCA! becomes.

This 5iOCl is thermally decomposed at high temperature in the subsequent transparent vitrification step to become SiO2.

Note that the dehydroxylation treatment may be performed while the porous glass preform 2 is being lowered as shown in the drawing, but it may also be performed while being raised.

In addition, the dehydroxylation furnace 7 can be maintained in a pressurized state using a needle valve as shown in the drawing.
It is also possible to use other pressure adjusting members such as capillaries.

As explained above, in the present invention, 1. , a porous glass preform manufactured by the VAD method is subjected to dehydroxylation treatment using a dehydroxylation treatment gas under hot pressure.

Therefore, according to the present invention, the gas for dehydroxylation treatment can be rapidly diffused to the center of the porous glass preform, so that the bulk density of the porous glass preform and the particle size of the sooty glass particles can be Dehydroxylation treatment can be performed reliably not only on the outside but also on the center.

Therefore, according to the present invention, it is possible to produce a substantially water-free optical fiber base material in a short time and with good reproducibility.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples.

〔Example〕

A porous glass preform produced by the VAD method was subjected to dehydroxyl treatment using the dehydroxyl group treatment apparatus shown in the drawings.

However, the temperature inside the dehydroxylation treatment furnace 7 was set to 1500°C, and the rotational speed and descending speed of the rod-shaped base material 1 were each set to 20r2.
Set as 00TtrIL/hour・p,m.

1. Introductory pipe 8 to 5OC for gas for dehydroxylation treatment
1250CC/min, He1OA/min, 02500cc/
was introduced into the dehydroxylation furnace γ.

In addition, the needle valve 11 lowers the pressure by 5 % below atmospheric pressure.
After dehydroxyl treatment at a pressure higher than atmospheric pressure by 15 mmH20, dehydroxyl treatment was performed at a pressure higher than atmospheric pressure by 20 mmH20.

That is, the dehydroxyl group treatment was carried out in three stages.

The amount of residual hydroxyl groups in the dehydroxylated optical fiber base material was measured under each pressurized state, and the results are shown in the table below.

From the above table, it can be seen that most of the hydroxyl groups are removed when the dehydroxyl group is treated under pressure.

Furthermore, it can be seen that the amount of residual hydroxyl groups becomes extremely small when the dehydroxyl group is treated under a pressure of about 15 mmH20.

[Brief explanation of drawings]

The drawing is a schematic longitudinal sectional view of a dehydroxyl group treatment apparatus used to carry out the method of the present invention. 1... Rod-shaped base material, 2... Porous glass preform, 7... Dehydroxylation treatment furnace, 8...
...Introduction pipe for gas for dehydroxylation treatment, 9...
Pressure gauge, 10... Exhaust pipe, 11... Needle valve.

Claims (1)

[Claims] I A method for producing an optical fiber base material by dehydrating a porous glass preform produced by the VAD method with a dehydroxyl treatment gas under heating, characterized in that the dehydroxyl treatment is carried out under pressure. A method for manufacturing an optical fiber base material.