JPH06135732A - Production of optical fiber preform - Google Patents

Abstract

PURPOSE:To produce an optical fiber preform with reduced eccentricity of a cylindrical glass body in a high yield. CONSTITUTION:Molding tools 2, 3 are filled with powder 4 contg. silica powder as principal starting material and a cylindrical glass body 1 with at least a quartz glass rod la at the center part is inserted into the powder 4 and disposed at the center of the tool 3. The powder 4 in the tools 2, 3 is then compacted and vitrified by sintering.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Conventionally, as a method for producing an optical fiber preform by using a pressure molding method, there is disclosed in JP-A-61-256937.
The method shown in the issue is known. In this method, a glass rod having a predetermined refractive index is pre-installed in the center of the molding die, then silica powder is filled around the glass rod in the molding die, and the silica powder is further applied by external pressure. This is a method of manufacturing an optical fiber preform by pressure molding and then sintering.

[0003]

By the way, the molding die actually used for molding is made of a highly elastic material such as plastic or rubber. For this reason, when the optical fiber preform is manufactured by using the above-mentioned conventional method, the glass rod-shaped body previously arranged in the center of the molding die is easily moved while the silica powder is filled in the molding die. . When the optical fiber preform is actually manufactured by such a conventional method as described above and further made into an optical fiber, the deviation of the optical fiber preform from the center of the glass rod causes the eccentricity of the core of the optical fiber.

Further, when a mold for holding the glass rod-shaped body in a stationary state is used, the glass rod-shaped body is often broken during pressure molding. This is because during pressing, an unreasonable force that does not interlock with the movement of the silica powder is applied to the glass rod-like body to move the glass rod-like body, and further undesired stress is applied to the glass rod-like body due to the vertical compression of the forming die. The reason is to join.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method capable of manufacturing an optical fiber preform with a small eccentricity of a glass rod at a high yield.

[0006]

According to the present invention, a silica-based glass rod for core is provided at the center of at least the center of the molding die after filling the molding die with a powder containing silica powder as a main raw material. Insert and install the glass rod, then
In the method for producing an optical fiber preform, the powder in the molding die is pressure-molded and then sintered to be vitrified.

Hereinafter, the method of the present invention will be described in more detail.

First, a molding die is filled with a powder containing silica powder as a main raw material. At this time, preferably, the molding die is vibrated to fill the powder so that the powder becomes sufficiently uniform.

The powder containing silica powder as a main raw material may be silica powder alone or silica powder containing a dopant, or may contain a molding aid.

Then, a glass rod-shaped body having a silica glass rod for core in at least the central portion is inserted and installed at the center of the molding die. At this time, from the viewpoint of more surely inserting the glass rod into the center of the forming die, the position of the center of the forming die is grasped using a TV camera or the like, and the TV is guided so that the glass rod is guided to that position. It is desirable to insert the glass rod while monitoring with a camera or the like. By inserting the glass rod while monitoring with a TV camera, etc., the eccentricity of the glass rod can be minimized even if the molding die is slightly deformed due to the powder filling (mainly the cross section becomes elliptical). Can be controlled.
Further, it is possible to insert a glass rod-shaped body without disturbing the powder filling state, and the occurrence of cracks and the like in the glass rod-shaped body and the powder molded body after pressure molding due to the non-uniformity of the powder packing density. From the viewpoint of prevention, it is desirable to insert the glass rod while rotating and vibrating it.

The quartz glass rod for the core includes
Examples thereof include those that become the core of the optical fiber and those that become a part of the clad around the core of the optical fiber.

The glass rod-shaped body is composed of a silica glass rod for core only, or a silica glass rod for core to which glass rods having hardness equal to or higher than that of silica glass are connected to both ends or one end. Can be mentioned. The glass rod-shaped body preferably has a spire shape at the tip in the insertion direction from the viewpoint of enabling insertion without disturbing the uniformity of the powder in the molding die.

As the glass rod connected to both ends or one end of the silica glass rod for core, even if a powder having poor adhesion is used, the molded body after molding and pressing of the powder is a glass rod-shaped body. From the viewpoint of not falling, it is desirable to use a thicker rod than the silica glass rod for core.

Then, the powder in the molding die is pressure-molded, and then sintered and vitrified to manufacture an optical fiber preform.

[0015]

According to the manufacturing method of the present invention, the glass rod is inserted into the center of the molding die after the powder is filled in the molding die and the glass rod is placed in the molding die. It is possible to avoid the movement of the glass rod pre-arranged in the center of the mold during the filling of the powder. Therefore, it becomes easy to accurately install the glass rod-shaped body in the center of the mold filled with the powder, and as a result, the optical fiber preform having a small eccentricity of the glass rod-shaped body can be manufactured with a high yield. .

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings.

Example 1 First, a glass rod 1 as shown in FIG. 1 (a) was produced. This glass rod 1 has a diameter of 7.5 mm and a length of about 1
An 80 mm silica glass rod for core 1a, a silica rod 1b connected to the lower end of the silica glass rod for core 1a, having a diameter of 14 mm and a length of 20 mm and having a steeple below. Quartz rod 1c connected to the upper end of the system glass rod 2 and having a diameter of 15 mm and a length of 60 mm
Consists of. The silica glass rod 1a for core is manufactured by the VAD method which is one of the vapor phase methods, and the clad / core is about 3 times and the relative refractive index difference is about 0.35%.

Then, as shown in FIG.
After fitting the lower lid 3 into the lower opening of a cylindrical molding rubber mold 2 having a diameter of mmφ and a length of about 270 mm, the molding rubber mold 2 was filled with silica granulated powder 4 having an average particle diameter of about 150 μm. At this time, the molding rubber mold 2 and the lower lid 3 were vibrated so that the silica granulation powder 4 was uniformly filled in the molding rubber mold 2. The material of the molding rubber mold 2 and the lower lid 3 is nitrile rubber. The silica granulated powder 4 was obtained by adding polyvinyl alcohol (P
VA) and pure water are added to prepare a slurry having a slurry concentration of about 60% by weight, and then the slurry is granulated using a spray dryer. The content of polyvinyl alcohol in the silica granulated powder 4 is 2% by weight based on the silica powder. The silica powder filled in the molding rubber mold 2 was about 430 g.

Then, as shown in FIG. 1 (c), the upper part of the quartz rod 1c of the glass rod 1 is fixed to a holding jig 5a of the glass rod inserting device 5 and the quartz rod 1b is directed downward. Further, while the glass rod-shaped body 1 is being rotated and vibrated, the glass rod-shaped body 1 is slowly lowered while being aligned with the center position in the molding rubber die 2 to form the silica granulated powder 4 in the molding rubber die 2. Inserted inside. At this time, the center position of the molding die was set in advance using a television camera (not shown), and the glass rod 1 was inserted while monitoring the set position. Further, during the insertion of the glass rod 1, the molded rubber mold 2 and the lower lid 3 were also vibrated.

Next, as shown in FIG. 2A, a cylindrical supporting cylinder 6 is arranged outside the molding rubber mold 2, and the upper part of the molding rubber mold 2 is further sandwiched with the supporting cylinder 6. The upper lid 7 was fitted in the opening. The support cylinder 6 is for allowing the pressure to be applied only in the radial direction when the pressure is applied via the pressure medium in the high-voltage applying device, has a hole 6a for passing the pressure medium, and is made of metal or the like. Made of non-elastic material. The material of the upper lid 7 is nitrile rubber. These molded rubber molds 2,
The lower lid 3, the support cylinder 6 and the upper lid 7 form a molding die.

Then, as shown in FIG. 2 (b), the molding dies 2, 3, 6, 7 and the glass rod 1 and the silica granulated powder 4 are used.
Was placed in the pressure medium 8b in the pressure vessel 8a of the high-voltage applying device 8. The pressure medium 8b is made of lubricating oil. Subsequently, the pressure inside the pressure vessel 8a is increased to 1500 kg / c.
After m ² was applied for 1 minute for pressure molding, the pressure was slowly reduced for about 20 minutes. After decompression, high voltage application device 8
The molds 2, 3, 6 and 7 were taken out from the above, the upper lid 7 of the mold was removed, and the porous glass preform composed of the glass rod-shaped body 1 and the molded body of the silica granulated powder 4 was taken out. The porous glass preform thus obtained had a diameter of about 40 mm and was free from cracks in the glass rod 1 and molded articles.

With respect to the obtained porous glass base material, a molding aid (polyvinyl alcohol) in the molded body, and further, an alkali metal, an alkaline earth metal and an alkali metal attached to the surface of the silica powder in the molded body, The following heat treatment (degreasing treatment) was performed to remove the transition metal. A heating furnace in which a chlorine-based gas (thionyl chloride is used in this experiment) flows at a ratio of about 1/10 of the flow rate of oxygen gas in a mixed atmosphere in which oxygen gas and nitrogen gas flow at a ratio of oxygen: nitrogen = 1: 4 In this heating furnace, the porous glass base material was heated from room temperature to 500 ° C. at a heating rate of 2 ° C./min, and was held at this temperature for about 5 hours to perform heat treatment.

The porous glass base material thus heat-treated is subjected to a refining treatment at a temperature of 1200 ° C. in an atmosphere of helium gas and chlorine gas using a furnace different from the above heating furnace, and then helium. In a gas atmosphere, the temperature is 16
The optical fiber preform was manufactured by raising the temperature to 00 ° C. and sintering the molded body of the porous glass preform to form a transparent glass.

The obtained optical fiber preform had no defects such as bubbles and cracks. After that, the optical fiber preform was drawn by an ordinary method to manufacture an optical fiber having a diameter of 125 μm. The obtained optical fiber had a transmission loss of 0.37 dB / km at a wavelength of 1.30 μm, which was measured by the vapor phase method. Had the same characteristics as the single mode optical fiber manufactured by. Furthermore, when the core eccentricity (distance of deviation of the core center axis) of the obtained optical fiber was examined, it was 0.2 μm, and it was confirmed that the core eccentricity was small. This is because the powder was correctly placed at the center of the powder at the time of pressure molding.

Comparative Example 1 A glass rod was placed in the center of a molding die, and then silica granulated powder was filled. Except for this, when a porous glass preform was manufactured in the same manner as in the example, the obtained porous glass preform had the glass rod-shaped body ruptured, and the subsequent process step for forming an optical fiber could be performed. could not. The reason why the glass rod-shaped body of the porous glass base material broke was that the glass rod-shaped body was not able to withstand the non-uniform pressure applied during the pressure molding because the glass rod-shaped body was displaced from the center during powder filling. .

Comparative Example 2 After a glass rod-shaped body was placed in the center of the molding die, silica granulated powder was filled. Also, the pressure molding condition is 50
The voltage was set to 0 kg / cm ² and applied for 1 minute. When a porous glass preform was manufactured in the same manner as in the Example except for this, a porous glass preform could be obtained without breaking the glass rod. Then, in the same manner as in Example 1, heat treatment (degreasing treatment), refining treatment, and transparent vitrification by sintering were performed to manufacture an optical fiber preform. afterwards,
An optical fiber having a diameter of 125 μm was manufactured by drawing the optical fiber preform by a usual method. The obtained optical fiber had a transmission loss of 0.3 at a wavelength of 1.30 μm.
It was 7 dB / km, and had characteristics equivalent to those of the optical fiber of the example. Further, when the core eccentricity of the obtained optical fiber was examined, it was 0.8 μm, and it was confirmed that the core eccentricity was larger than that of the optical fiber of the example. The cause of the eccentricity of the core is that the glass rod-shaped body is deviated from the center during powder filling.

[0027]

As described above in detail, according to the manufacturing method of the present invention, it is possible to manufacture an optical fiber preform having a small eccentricity with a high yield, and thus to manufacture a high-quality optical fiber with a small eccentricity with a high yield. It becomes possible to manufacture.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a manufacturing process of an optical fiber preform in an example.

FIG. 2 is an explanatory view showing a manufacturing process of an optical fiber preform in an example.

[Explanation of symbols]

1 ... Glass rod-like body, 1a ... Quartz glass rod for core,
1b ... Quartz rod, 1c ... Quartz rod, 2 ... Mold (molding rubber mold), 3 ... Mold (lower lid), 4 ... Silica granulated powder, 5 ...
Inserting device for glass rod, 6 ... Mold (supporting cylinder), 7 ...
Mold (upper lid), 8 ... High-voltage applying device.

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

Claims (1)

[Claims] 1. A mold is filled with a powder containing silica powder as a main raw material, and then a glass rod-shaped body having a silica glass rod for a core at least in the center is inserted and installed in the center of the mold. Then, the powder in the molding die is pressure-molded, and then sintered and vitrified, which is a method for producing an optical fiber preform.