JPH07187697A - Production of porous preform for optical fiber - Google Patents

Abstract

PURPOSE:To provide a method for producing a porous preform capable of producing an optical fiber without being incorporate with metallic compd. and having sufficient strength. CONSTITUTION:A silica powder is agitated in an alkaline liq. phase to remove impurity particles in this silica powder, and the preform is produced by a powder molding method by using this silica powder as a main starting material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an optical fiber porous preform for producing an optical fiber used in the fields of communication and optics.

[0002]

2. Description of the Related Art Conventionally, a method of producing a porous preform for optical fibers by using a powder molding method using silica powder as a main raw material has been performed. Specifically, JP-A-4-12404
No. 2, etc., the extrusion molding method, the pressure molding method, disclosed in JP-A-4-124043, etc., the cast molding method, disclosed in JP-A-64-56331, etc. MSP (Mechanically Sha) disclosed in Japanese Patent Laid-Open No. 60-210539
ped Preforms) method, the centrifugal separation method disclosed in JP-A-63-195136, and JP-A-58-26.
The method disclosed in Japanese Patent Publication No. 048, so-called double process method and the like can be mentioned.

[0003]

In the various powder molding methods described above, metal compounds such as ZrO ₂ and Cr ₂ O ₃ are easily mixed in the silica powder in the silica powder raw material manufacturing step and the silica powder conveying step. . If this metal compound is not removed from the silica powder, this metal compound will remain in the final product, the optical fiber, via the intermediate product, the porous preform for optical fibers. Since the metal compound serves as a fracture starting point in the optical fiber, the strength of such an optical fiber is extremely low.

The present invention has been made in view of the above circumstances, and provides a method for producing a porous preform for an optical fiber, which is capable of obtaining an optical fiber in which a metal compound is not mixed and which has sufficient strength. The purpose is to

[0005]

DISCLOSURE OF THE INVENTION The present inventors have paid attention to the charged state of metal compounds such as ZrO ₂ and Cr ₂ O ₃ which are impurities, and have the same charge of the metal compound in a specific liquid phase. The inventors have found that the metal compound is removed by utilizing the above facts and have completed the present invention.

That is, according to the present invention, silica powder is stirred in an alkaline liquid phase to remove the impurity particles in the silica powder, and the silica powder is used as a main raw material by a powder molding method to form a porous preform for optical fibers. A method for producing a porous preform for optical fibers is provided.

Here, as the silica powder, SiO ₂ ,
Alternatively, a refractive index controlling element such as Ge, B, Ti or E
Si containing an element for imparting functionality such as r or Nd as an additive
O ₂ or the like can be used. In addition, the term “impurity particles” as used herein mainly means ZrO ₂ particles and Cr ₂ O ₃ particles.

In the present invention, the alkaline liquid phase means
A liquid phase having a pH above pH 7. Particularly preferably, the pH is 9 or more. This liquid phase is formed by adjusting the pH by using an alkaline aqueous solution such as ammonia water.

Examples of the powder molding method include an extrusion molding method, a pressure molding method, a casting molding method, an MSP method, a centrifugal separation method, and a double process method.

[0010]

The present invention is characterized in that the silica powder is stirred in an alkaline liquid phase to remove the impurity particles in the silica powder.

Usually, ZrO ₂ particles and Cr ₂ O ₃ particles which are impurity particles adhere to silica powder which is the main raw material, that is, SiO ₂ particles. As can be seen from FIG. 2, in the alkaline region where the pH exceeds 7, the Si
O ₂ , ZrO ₂ and Cr ₂ O ₃ all have a negative zeta potential, that is, a negative charge. Therefore, in this alkaline liquid phase ZrO ₂ particles and Cr
_When the SiO ₂ particles having the ₂ O ₃ particles attached thereto are stirred, the charges of all the particles are negative, so that the particles repel each other and the ZrO ₂ particles and the Cr ₂ O ₃ particles are separated from the SiO ₂ particles. This makes it possible to remove impurity particles such as ZrO ₂ particles and Cr ₂ O ₃ particles from the SiO ₂ particles.

In FIG. 2, all the particles become positive in the pH range of 2 or less, but the liquid phase of pH 2 is strongly acidic, which is not preferable because handling becomes dangerous.

[0013]

EXAMPLES Examples of the present invention will be specifically described below.

(Example) Next, the step of removing impurity particles in the present invention will be described.

First, as shown in FIG. 1 (A), an alkaline aqueous solution 1 adjusted to pH 9 to 10 with aqueous ammonia 1
2 and silica powder 11 having an average particle size of 8 μm produced by the vapor phase synthesis method were mixed in equal amounts, and the mixture was put into a container 13.

Then, as shown in FIG. 1B, the container 1
The lid 14 is attached to the roll 3 and the roll mill 1 is laid sideways.
The contents in the container 13 were sufficiently stirred by rotating at 5. Then, as shown in FIG. 1C, the container 1
3 was stood and left still for 8 hours. As a result, the content in the container 13 was separated into the precipitated silica powder 16 and the supernatant liquid 17. At this time, the impurity particles were dispersed in the supernatant 17 due to the difference in sedimentation speed.

Then, as shown in FIG. 1D, only the supernatant 17 in which the impurity particles are dispersed was removed. The operations shown in FIGS. 1A to 1D were repeated. Further, using pure water instead of the alkaline aqueous solution, as shown in FIG.
The operation up to (D) was repeated twice. Thus, alkali washing was performed twice and pure water washing was performed twice to obtain silica powder from which impurity particles were removed.

6 parts by weight per 100 parts by weight of this silica powder
7 parts by weight of pure water, 1.6 parts by weight of polyvinyl alcohol, and 1.2 parts by weight of glycerin were mixed and stirred to prepare a slurry. The slurry was granulated using a spray dryer to obtain granulated powder having a particle size of about 100 μm.

Next, the clad /
The core ratio is about 3, the relative refractive index difference is about 0.3%,
A dummy rod made of quartz glass having an outer diameter of about 23 mmφ and a length of 30 mm is attached to one end of a silica glass rod having an outer diameter of about 8.5 mmφ and a length of about 300 mm. A supporting rod made of quartz glass having an outer diameter of about 25 mmφ and a length of 120 mm was attached to the other end by glass welding to produce a core rod. This core rod is placed in a rubber mold having an inner diameter of about 70 mmφ, and the space is filled with the above-mentioned granulated powder.
Hydrostatic press molding was performed at a pressure of 000 kgf / cm ² to produce a porous preform for optical fibers having an outer diameter of about 56 mmφ and a length of about 300 mm.

Next, the porous preform for optical fibers is subjected to a degreasing treatment in air at 500 ° C. for 5 hours, followed by dehydration treatment at 1200 ° C. in a He, Cl ₂ atmosphere and 1600 in a He atmosphere by a conventional method. A transparent vitrification treatment was performed at a temperature of ℃ to produce a preform for optical fibers. This optical fiber base material was drawn by a well-known heat drawing method so that the outer diameter was 125 μm.
m single mode optical fiber was obtained.

When a screening test was performed on the entire length of this single mode optical fiber under the condition of elongation of 0.7%, there was no breakage of the optical fiber. It is considered that this is because the impurity particles were not attached to the silica powder as the main raw material.

In the present embodiment, the case where the pressure molding method is used as the powder molding method has been described. However, if the silica powder is stirred in an alkaline liquid phase to remove the impurity particles, another The same effect can be obtained by using the powder molding method.

(Comparative Example) A porous preform for optical fibers was prepared in the same manner as in Example except that the silica powder was not stirred to remove the impurity particles in the alkaline liquid phase, and a conventional method was used. Then, degreasing, dehydration, and transparent vitrification treatment were performed to produce a preform for optical fiber, which was drawn to obtain a single mode optical fiber.

A screening test was performed on the entire length of this single mode optical fiber in the same manner as in the example, and 0.4 breaks per 1 km occurred. this is,
It is considered that this is because the impurity particles adhered to the silica powder as the main raw material.

[0025]

As described above, in the method for producing a porous preform for optical fibers of the present invention, the silica powder is stirred in an alkaline liquid phase to remove the impurity particles in the silica powder, and the silica powder is used. Since a porous preform for optical fibers is obtained by a powder molding method using as a main raw material, an optical fiber having no metal compound and having sufficient strength can be obtained. This improves the yield of the optical fiber.

[Brief description of drawings]

1A to 1D are views showing a process of removing impurities in silica powder.

FIG. 2 is a schematic diagram showing a charged state of a metal oxide in water.

[Explanation of symbols]

11 ... Silica powder, 12 ... Alkaline aqueous solution, 13 ... Container, 14 ... Lid, 15 ... Roll mill, 16 ... Precipitated silica powder, 17 ... Supernatant liquid.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Kazuaki Yoshida 2-6-1, Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd.

Claims (1)

[Claims] 1. A silica powder is stirred in an alkaline liquid phase to remove impurity particles from the silica powder, and a porous preform for optical fibers is obtained by a powder molding method using the silica powder as a main raw material. A method for producing a porous base material for an optical fiber, which is characterized.