JPH082935A - Production of porous preform for optical fiber - Google Patents

Abstract

PURPOSE:To produce a porous preform for an optical fiber giving an optical fiber having excellent mechanical strength and high quality. CONSTITUTION:This production process comprises a step to obtain a molding material by adding fine particles having particle diameter satisfying the following formula to a silica-based powder and a step to produce a porous preform by the powder-forming of the molding material. d<={(2/sq. rt. 3)-1}XD (d is particle diameter of the fine particle; D is particle diameter of the silica powder).

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 Recently, it has been attempted to use a powder molding method for manufacturing an optical fiber preform for producing an optical fiber. As a method for manufacturing an optical fiber preform using the powder molding method, Extrusion molding method disclosed in Japanese Patent Application Laid-Open No. 4-1204042 and the like, Japanese Patent Application Laid-Open No. 4-124
043, etc., the pressure molding method disclosed in JP-A-64-56331, etc .; the MSP method disclosed in JP-A-60-210539; JP-A-63-195136
Centrifugal separation method disclosed in Japanese Unexamined Patent Application Publication No. Sho 5
The double process method disclosed in JP-A 8-26048 and the like can be mentioned.

Further, when the optical fiber preform is manufactured by using the above-mentioned powder molding method, it is preferable that the particle diameter of the silica-based powder used as a raw material is 0.6 to 20 μm. It is disclosed in Japanese Patent No. 208839.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In Japanese Patent Laid-Open Publication No. 9-93, when the particle diameter of the silica-based powder becomes too small, the pore diameter of the obtained porous base material becomes small,
In the subsequent purification step, it becomes difficult to diffuse purified gas, for example, chlorine gas or chloride gas obtained by reacting chlorine gas with impurities, and the purification effect of the porous base material is reduced. Also, if the particle size of the silica-based powder becomes too large,
In the degreasing step, the mechanical strength of the porous base material after removing the binder from the porous base material is lowered, the shape retention of the porous base material is lowered, and the porous base material is easily damaged.

That is, it is desirable to use a silica-based powder having a particle size as large as possible in order to give importance to the quality of the final product optical fiber. In that case, however, it is necessary to use a silica-based powder for the optical fiber. The problem is how to improve the mechanical strength.

The present invention has been made in view of the above points, and provides a method for producing a porous base material for an optical fiber, which has an excellent mechanical strength and is capable of obtaining a high quality optical fiber. The purpose is to do.

[0007]

Means for Solving the Problems The present inventors have conducted research from the viewpoint of activating the interaction between silica-based powders having a relatively large particle size, and adding silica-based fine particles to the silica-based powder. As a result, it has been found that the contact points between the silica-based powders per unit volume are increased and the mechanical strength of the porous base material is improved, and the present invention has been completed.

That is, according to the present invention, a step of adding a fine particle having a particle size satisfying the following formula (I) to a silica-based powder to obtain a molding material, and molding the molding material by a powder molding method to form a porous matrix. A method for producing a porous preform for an optical fiber, comprising the step of obtaining a material.

D ≦ {(2 / √3) −1} · D (I) (d: particle size of fine particles, D: particle size of silica-based powder) Here, as the silica-based powder in the present invention, A powder obtained by a flame hydrolysis method, a sol-gel method, a gas phase synthesis method or the like can be used. Further, as the fine particles in the present invention, it is preferable to use high-purity silica powder among the silica-based powders obtained by the method described above. As such particles, it is possible to use pure silica fine particles, silica fine particles containing a refractive index controlling element such as Ge, P and Ti, or a functional imparting element such as Er, or oxide fine particles of the above elements.

In the present invention, the spray-dried granulated powder is filled in a molding die and pressure-molded, for example, JP-A-4-12404.
It is preferable to adopt the pressure molding method disclosed in Japanese Patent Laid-Open No. 3 or the like or the MSP method disclosed in Japanese Patent Laid-Open No. 60-210539 as the powder molding method.
The present invention also relates to the extrusion molding method disclosed in JP-A-4-124042 and the like and JP-A-64-563.
It can also be adopted in the cast molding method disclosed in Japanese Patent No. 31.

In the present invention, the particle size of the fine particles added to the silica-based powder is expressed by the formula d≤ {(2 / √3) -1} D (d:
The particle size of the fine particles and D: particle size of the silica-based powder) are set to be satisfied. This is because when the particle size of the fine particles is too large with respect to the particle size of the silica-based powder, that is, d> {(2 / √
3) -1} · D, there is no difference in size between the fine particles and the silica-based powder, so the fine particles cannot enter the gap formed by the silica-based powders, increasing the number of contacts between the silica-based powders. Because it will not contribute to. For example,
When the particle size of the silica-based powder is 10 μm, it is necessary to set the particle size of the fine particles to 1.5 μm or less from the above formula. In order that the fine particles adhere to the vicinity of the contact point of the silica-based powder, that is, the so-called neck portion, it is necessary to make the particle size of the fine particles sufficiently smaller than the particle size obtained from the above equation.
Further, in the present invention, the particle diameters of the silica-based powder and the fine particles are the average particle diameters of the silica-based powder and the fine particles, and are generally used volume 50% particle diameters.

In the present invention, the addition amount of fine particles also affects the mechanical strength of the obtained porous base material. The lower limit of the amount of fine particles added is a value that can maintain the mechanical strength of the porous base material, and the upper limit is the amount of excess fine particles that do not contribute to the increase in the number of contacts between silica-based powders, causing aggregation of the porous base material. The value is not uniform. This is because if the porous base material is inhomogeneous, cracks or warpage will occur in the porous base material during the heat treatment in the degreasing step, the refining step, the transparent vitrification step, etc., which are the post-steps.

Thus, there is an optimum value for the amount of fine particles added, but it varies depending on the particle size of the silica-based powder used. That is, when the particle size of the silica-based powder used is large, the amount of fine particles added to maintain the mechanical strength of the porous matrix increases. Particle size is 4 ~ 20μ
For some of the silica-based powders having a particle size of m, the optimum range of the addition amount was set for the particles having a particle size of 0.04 to 0.6 μm, which was particularly effective among the particles having a particle size satisfying the formula (I). Upon examination, the results shown in FIG. 1 were obtained. The shaded area sandwiched by two solid lines in FIG. 1 is the area showing the optimum addition amount of the fine particles. According to FIG. 1, for example, a particle size of 10
When using a silica-based powder of μm, it is preferable to add fine particles in an amount of 0.5 to 1% by weight.
When the silica-based powder of 1 is used, it is preferable to add fine particles in an amount of 1 to 2% by weight. With respect to silica-based powder having a particle size of 5 μm or less, it was not necessary to add fine particles because the particle size was sufficiently small.

In the present invention, the method for adding the silica-based fine particles to the silica-based powder is to add the silica-based powder and the silica-based fine particles to pure water so that the silica-based fine particles are uniformly dispersed in the silica-based powder. The most preferable method is to disperse it in a dispersion medium such as the above to obtain a slurry, and to stir the slurry. Further, according to this method, since spray drying can be used for drying the slurry after stirring, it is possible to dry the dispersion medium in a short time, and the silica-based fine particles in the silica-based powder can be dried. In that case, it is possible to prevent segregation.

The method of the present invention is applied to PCS (Polymer Clad S
ilica) Rod-shaped porous materials such as porous base materials for fibers, porous base materials for image guides, rod lenses
Hybrid type porous base material formed by molding silica-based powder around a core rod as shown in JP-A-4-124043, etc., and porous base material in which core and clad are all manufactured by powder molding method Etc. can be applied.

[0016]

The method for producing a porous base material for an optical fiber according to the present invention is based on the formula d≤ {(2 / √3) -1} D of silica-based powder.
A feature is that fine particles having a particle diameter satisfying (d: particle diameter of fine particles, D: particle diameter of silica-based powder) are added and a molding material is molded by a powder molding method to obtain a porous base material.

Usually, when silica-based powder having a relatively large particle size is used, the number of contact points between the silica-based powders per unit volume is smaller than that when smaller silica-based powders are used, and the silica-based powders are contacted with each other. The interaction is small, and the mechanical strength of the obtained porous base material is reduced. However, by adding fine particles satisfying the above formula in a predetermined addition amount of silica-based powder, the fine particles enter the gap formed by the silica-based powders, and the number of contacts between the silica-based powders increases. In particular, the fine particles having a particle size sufficiently smaller than the particle size of the silica-based powder adhere to the vicinity of the neck portion formed when the silica-based powders contact each other. As a result, the interaction between the silica-based powders, that is, the entanglement of the silica-based powders, the cohesive force based on the interaction, and the surface tension or the capillary force due to the water remaining on the contact surface of the powder are increased. As a result, the mechanical strength of the obtained porous base material is improved.

In particular, the method of the present invention is carried out by the method disclosed in Japanese Patent Application No. 5-284714 which disperses a raw material powder in an alkaline solution and removes an impurity flow contained in the raw material powder due to a difference in sedimentation speed. It is effective for the obtained raw material powder. According to the method disclosed in Japanese Patent Application No. 5-284714, when the impurity particles are removed, the raw material powder having a small particle size is removed together with the impurity particles, and the number of contact points between the powder particles is reduced. However, by applying the method of the present invention, the fine particles added to the powder can increase the number of contacts between the powders.

Further, the powder synthesized by the sol-gel method, which has recently been attracting attention because of the fact that high-purity powder can be obtained, has a very narrow particle size distribution due to its synthetic characteristics. Since such a powder does not contain particles having a small particle size which plays a role of the fine particles in the present invention, it is effective to apply the method of the present invention to increase the number of contacts between the powders by the fine particles.

[0020]

Embodiments of the present invention will be specifically described below. (Example 1) 98 parts by weight of silica powder having a particle size of 15 μm produced by a gas phase synthesis method, 2 parts by weight of silica fine particles having a particle size of 0.2 μm produced by a gas phase synthesis method as fine particles to be added,
67 parts by weight of pure water as a solvent, 1.6 parts by weight of polyvinyl alcohol as a binder, and 1.2 parts by weight of glycerin are mixed and stirred to prepare a slurry, and the slurry is granulated by a spray drying method to obtain an average particle size. About 100μ
A granulated powder of m was obtained. The particle size of the added fine particles is
It satisfies the formula (I).

Then, this granulated powder was filled in a rubber mold having an inner diameter of about 70 mm, and the rubber mold was pressed under a hydrostatic pressure of 98 MPa to produce a porous base material having an outer diameter of about 58 mm and a length of about 300 mm. .

Then, the porous base material is heated in the air at 500 ° C. for 5 hours to be degreased, and thereafter, in a He gas and Cl ₂ gas atmosphere according to a conventional method.
It was dehydrated at 00 ° C., and finally, it was transparentized into glass at 1600 ° C. in a He gas atmosphere according to a conventional method to obtain a PCS optical fiber preform. Thus, 20 PCS optical fiber preforms were produced.

In the porous base material of this embodiment, since the silica fine particles enter the gap formed between the silica powders to increase the number of contact points between the silica powders and thereby have excellent mechanical strength, all 20 base materials are used. The base material for PCS optical fiber could be produced without problems such as breakage. Also, the obtained PCS optical fiber preform is drawn to obtain a core diameter of 200 μm.
When an optical fiber having a diameter of m and a cladding diameter of 230 μm was manufactured, the characteristics of this optical fiber were equivalent to those of the conventional one. (Comparative Example 1) Instead of adding silica fine particles having a particle size of 0.2 μm, a particle size of 3 μm which is a particle size not satisfying the formula (I).
Granulated powder was obtained in the same manner as in Example 1 except that the fine particles of 1 were added, and 20 porous base materials were produced using this.

Using this porous preform, a preform for PCS optical fibers was produced in the same manner as in Example 1. The porous preform had a low mechanical strength, and therefore the degreasing process was changed to the transparent vitrification process. All were damaged during the handling of the transfer. Comparative Example 2 Instead of adding 2 parts by weight of silica fine particles having a particle size of 0.2 μm, 1 part of silica fine particles having a particle size of 0.2 μm was added.
Granulated powder was obtained in the same manner as in Example 1 except that the amount was added in less than parts by weight, and 20 porous base materials were produced using this.

Using this porous preform, a preform for PCS optical fibers was produced in the same manner as in Example 1. The porous preform had a low mechanical strength, and therefore the degreasing process was changed to the transparent vitrification process. All were damaged during the handling of the transfer. (Comparative Example 3) Instead of adding 2 parts by weight of silica fine particles having a particle diameter of 0.2 μm, 2 silica fine particles having a particle diameter of 0.2 μm were added.
Granulated powder was obtained in the same manner as in Example 1 except that the addition amount was more than parts by weight, and 20 porous base materials were produced using this.

When a PCS optical fiber preform was produced using this porous preform in the same manner as in Example 1, many bubbles were generated after the transparent vitrification step, and P
It could not be used as a base material for CS optical fibers. (Example 2) 99 parts by weight of silica powder having a particle size of 10 μm produced by a gas phase synthesis method, 1 part by weight of silica fine particles having a particle size of 0.04 μm produced by a gas phase synthesis method as fine particles to be added, and pure as a solvent. 67 parts by weight of water, 1.6 parts by weight of polyvinyl alcohol as a binder, and glycerin 1.
A slurry is prepared by mixing and stirring 2 parts by weight, and the slurry is granulated by a spray drying method to obtain an average particle size of about 10
A granulated powder of 0 μm was obtained. The particle size of the added fine particles satisfies the formula (I).

On the other hand, according to the VAD method, the core / cladding ratio is 1/3, the refractive index ratio is Δ = 0.3%, and the outer diameter is 8.
A quartz glass rod having a length of 5 mm and a length of about 300 mm was produced. Then, at one end of this rod,
A core rod was prepared by welding a quartz glass dummy rod having a length of 30 mm and a length of 30 mm and a quartz glass supporting rod having an outer diameter of 25 mm and a length of 120 mm at the other end.

Next, this core rod was aligned mechanically with the center of the cavity of a rubber mold having an outer diameter of 70 mm, and the granulated powder was filled in the cavity. Next, this rubber mold was pressed with a hydrostatic pressure of 98 MPa to produce a porous base material having an outer diameter of about 56 mm and a length of about 300 mm.

Next, the porous base material is heated in the air at 500 ° C. for 5 hours to be degreased, and thereafter, in a He gas and Cl ₂ gas atmosphere according to a conventional method.
It was dehydrated at 00 ° C., and finally, it was transparentized into glass at 1600 ° C. in a He gas atmosphere according to a conventional method to obtain a base material for a single mode optical fiber. In this way, 20 single mode optical fiber preforms were produced.

In the porous base material of this example, since the silica fine particles enter the gap formed between the silica powders to increase the number of contact points between the silica powders and thereby have excellent mechanical strength, all 20 base materials are used. A base material for a single mode optical fiber could be produced without problems such as breakage. Further, when the obtained base material for a single mode optical fiber was drawn to manufacture an optical fiber having an outer diameter of 125 μm, the characteristics of this optical fiber were equivalent to those of the conventional one. (Comparative Example 4) Instead of adding silica fine particles having a particle diameter of 0.04 μm, a particle diameter of 3 μ which does not satisfy the formula (I).
Granulated powder was obtained in the same manner as in Example 2 except that the fine particles of m were added, and 20 porous base materials were produced using this.

Using this porous preform, a preform for a single mode optical fiber was produced in the same manner as in Example 2. The porous preform had a low mechanical strength, so that the degreasing process to the transparent vitrification process was performed. 10 pieces were damaged during the handling to be transferred to. (Comparative Example 5) In the same manner as in Example 2 except that silica fine particles having a particle size of 0.04 μm were added in an amount of less than 0.5 part by weight instead of adding 1 part by weight of silica fine particles having a particle size of 0.04 μm. Granulated powder is obtained, and it is used as a porous matrix.
0 pieces were produced.

Using this porous preform, a preform for a single mode optical fiber was prepared in the same manner as in Example 2. The porous preform had a low mechanical strength, so that the degreasing process to the transparent vitrification process was performed. Five pieces were damaged during the handling to be transferred to. Also, three cracks were generated after the transparent vitrification step. (Comparative Example 6) The same as Example 2 except that silica fine particles having a particle size of 0.04 μm were added in an amount of more than 1 part by weight instead of adding 1 part by weight of silica fine particles having a particle size of 0.04 μm. Granulated powder was obtained by using this, and 20 porous base materials were produced using this.

Using this porous preform, a preform for a single mode optical fiber was produced in the same manner as in Example 2. As a result, a large number of bubbles were generated after the transparent vitrification step, and a single mode optical fiber was produced. It could not be used as a base material.

[0034]

As described above, the method for producing a porous preform for an optical fiber according to the present invention uses silica-based powder of the formula d≤ {(2
/ √3) -1} · D (d: particle size of fine particles, D: particle size of silica-based powder) is added, and a molding material is molded by a powder molding method to form a porous matrix. This is a method of obtaining a porous preform for optical fibers, which has excellent mechanical strength and high quality because a material is obtained. As a result, the production yield of the optical fiber can be improved.

[Brief description of drawings]

FIG. 1 is a graph for explaining the optimum range of the amount of silica-based fine particles added to silica-based powder.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tsutoo Sato 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd. (72) Inventor Kazuaki Yoshida 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd.

Claims (1)

[Claims] 1. A step of obtaining a molding material by adding fine particles having a particle diameter satisfying the following formula (I) to a silica-based powder, and a step of molding the molding material by a powder molding method to obtain a porous base material. And a method for manufacturing a porous preform for optical fibers. d ≦ {(2 / √3) −1} · D (I) (d: particle size of fine particles, D: particle size of silica-based powder)