KR910000731B1 - Process for production of optical fiber - Google Patents

Abstract

No content.

Description

Manufacturing method of base material for optical fiber containing fluorine in clad part

1 is a diagram showing the refractive index distribution of a representative single mode fiber.

2 is a view for explaining an example of the porous glass layer synthesizing apparatus.

3 and 4 show the refractive index distribution of the transparent glass base material obtained by the conventional method.

5 to 7 show radial distribution of refractive index of the transparent glass base materials obtained in Examples 1 and 2 of the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a base material for optical fibers, and more particularly, to a method for producing a high quality quartz base optical fiber base material containing fluorine in a clad portion.

FIG. 1 shows the refractive index distribution structure of a typical single-mode optical fiber. Conventionally, in order to form such a refractive index distribution, the method of adding the additive (dough pant) of a refractive index height to a core part has been taken a lot. As an additive for increasing the refractive index, oxides such as G ₀ O ₂ , P ₂ O ₅ , and Al ₂ O ₂ are often used. However, when these oxides are used, the transmission loss of light increases due to the increase in the ray ray scattering. ② It is easy to generate bubbles or crystal phases due to dioxide in the glass base material. ③ The thermal expansion coefficient of the glass increases, which causes problems such as the base metal being easily broken. Therefore, it is preferable that the amount of dough pants added to the glass base material is small.

For this reason, a method of increasing the refractive index difference between the core and the clad may be employed by adding a dough pants for lowering the refractive index to the clad part, for example, B ₂ O ₂ , fluorine, or the like. However, B ₂ O ₂ increases the coefficient of thermal expansion of silica glass and has an inherent absorption loss in the long wavelength region. Therefore, it is preferable to use fluorine as a refractive index decreasing component.

On the other hand, as the manufacturing method of the optical fiber, the VAD method (gas phase adhesion method) or the OVPO method (external adhesion CVD method), which forms a porous glass body by flame hydrolysis reaction, has excellent productivity and is economical. Known as the method. However, it is extremely difficult to add a sufficient amount of fluorine to quartz glass by a method using flame hydrolysis such as VAD method or OVPO method. For example, Japanese Unexamined Patent Publication No. 55-15682 describes a method of adding fluorine to a glass base material, but according to this method, the decrease in refractive index due to the addition of fluorine is only 0.2 to 0.3% at most. There is a limit to the amount of fluorine.

On the other hand, Japanese Unexamined Patent Publication No. 55-67533 proposes a method of efficiently adding fluorine by heating a laminate of glass fine particles formed by the flame hydrolysis method in an atmosphere of fluorine compound gas. However, with the method described in the above publication, it is not easy to form fluorine distribution, and it is difficult to form a refractive index distribution having a sufficient function as an optical waveguide as shown in FIG. 1 using only fluorine.

Therefore, as a method of manufacturing a fluorine-containing optical fiber base material using a method of synthesizing a porous glass body by a flame singular reaction having excellent productivity even when a refractive index distribution having a sufficient function as an optical waveguide is formed, it is shown in FIG. The method using the apparatus shown by the figure is devised.

While rotating the glass rod 2 corresponding to the core part gradually rotated upward while being mounted on the rotating and pulling apparatus 1, the burner 3 for synthesizing the glass particles on the side surface of the glass rod 2 was rotated. The glass fine particles produced by ()) are deposited to form a porous glass layer 4 corresponding to the cladding portion. The glass fine particles are formed by flame hydrolysis by simultaneously supplying H ₂ , O ₂ , SiCl _4, etc. to the burner 3 for synthesizing the glass particles. (5) is a reaction vessel and (6) is an exhaust port. By heating the composite of the glass rod and the porous glass layer thus formed in a gas atmosphere containing fluorine, fluorine is added to the porous glass layer, and the porous glass layer is transparent vitrified and is shown in FIG. It can be set as the base material for an optical fiber having the refractive index distribution as described above.

When the thickness of the cladding layer is insufficient, the transparent glass base material is extended again if necessary to form a porous base material on the outer circumference of the transparent glass base material, and then heated and transparent vitrified in an atmosphere containing fluorine. By repeating, a desired refractive index distribution can be obtained.

However, in this method using the above-mentioned FIG. 2 apparatus, when the glass rod corresponding to a core part is extended to a predetermined diameter beforehand, it is often heated in the atmosphere containing water vapor, and the glass rod The surface is likely to be contaminated by OH groups. Especially, when this glass rod is extended using the flame formed by the combustion gas containing a hydrogen atom, the contamination by the OH group on the surface of a glass rod is remarkable. In addition, even when the porous glass layer corresponding to the cladding portion is formed, the surface of the glass rod may be contaminated with OH groups by water vapor generated in the flame of the burner for synthesizing the glass particles.

As described above, when the surface of the glass rod corresponding to the core radiates a base material contaminated with OH groups to form an optical fiber, the light propagating through the optical fiber loses absorption loss due to the presence of the OH group. In response, the transmission loss characteristic is deteriorated. In particular, when the optical fiber is used as a single mode fiber, since the power distribution of light propagating through the single mode optical fiber is widely extended to the cladding portion, the influence of the OH group contaminant layer near the boundary between the core and the clad is particularly noticeable. The degradation of the transmission loss characteristic is remarkable.

An example is given below and the degradation of the transmission characteristic by OH group contamination is demonstrated. After the pure quartz rod with a sufficiently low OH group content (~ about 10 ppb) was stretched to a diameter of 15 mm ø using an oxyhydrogen flame, it was made of pure silica on the outer circumference of the glass rod using the apparatus shown in FIG. A porous glass layer was formed. At this time, the outer diameter of the porous glass layer was 100 mm ø. When the composite of the porous glass layer and the transparent glass rod was heated and vitrified in an atmosphere containing fluorine, a transparent glass base material having an outer diameter of 45 mm ø with a refractive index distribution shown in FIG. 3 was obtained.

In addition, the base material was stretched using an oxyhydrogen flame to a diameter of 15 mm, and then a porous glass layer was formed again on the outer circumference of the transparent glass base material by the apparatus shown in FIG. The outer diameter of the porous glass layer at this time was 110 mm ø. The composite of the transparent glass base material and the porous glass layer was made of transparent glass in an atmosphere containing F. As a result, a transparent glass base material having a refractive index distribution shown in FIG. 4 was obtained.

Next, the transparent glass base material was stretched to a predetermined diameter, inserted into a commercially available quartz pipe, and integrated into a single mode fiber at 1.3 mu m. As a result, the transmission loss at 1.3 µm is 4.0 dB / km, the absorption loss due to OH groups at 1.39 µm is 150 dB / km or more, and the amount of OH groups that are thought to be incorporated at the time of stretching by the acid hydrogen salt is large. For this reason, the transmission loss characteristic was inferior.

The present invention provides a novel manufacturing method for obtaining an optical fiber in which the core part is made of pure quartz and the clad part is made of quartz glass by applying the synthesis of a porous glass layer using a flame hydrolysis reaction having excellent productivity. In order to solve the problem of deterioration in transmission loss characteristics due to OH group contamination, which is not yet solved in the conventional method, an object of the present invention is to provide a method for manufacturing an optical fiber base material containing high quality fluorine in a clad part. I am doing it.

In order to solve the above problems, the present invention uses a molten glass rod made of quartz glass, which is substantially pure in the center, and quartz glass containing fluorine in the outer part, and is made of substantially pure quartz glass on the outer periphery of the starting material. A porous glass layer is formed, and the composite of the porous glass layer and the molten glass rod is heated in an atmosphere containing fluorine, whereby the porous glass layer contains fluorine, and the porous glass layer is transparent vitrified. It is to provide a method for producing an optical fiber base material containing fluorine in the clad portion.

As mentioned in the description of the prior art, when the glass fine particle laminate is heated in a gas atmosphere containing fluorine, fluorine is easily added to the glass fine particle laminate almost uniformly, and the refractive index distribution as shown in FIG. It was difficult to get.

However, when the glass fine particle laminate is heated to a certain extent in an atmosphere containing no fluorine, and shrunk, and then heat-transparent again in an atmosphere containing fluorine, shrinkage of the porous glass body proceeds, so that fluorine is porous. A transparent glass body containing fluorine can be obtained only in the outer peripheral part without penetrating to the center of the glass body.

However, at this stage, since a sufficient cladding layer thickness cannot be obtained, it is necessary to synthesize a fluorine-added quartz glass layer on the outside of the transparent glass base material again. Then, after stretching the transparent glass base material in the shape of a glass rod, a porous glass layer is formed on the transparent glass rod by a method shown schematically in FIG. 2, and the composite of the glass rod and the porous glass layer is formed of fluorine. The porous glass layer is transparent by virtue of being heated in the atmosphere contained therein to thereby contain fluorine in the porous glass layer or after containing fluorine.

As an atmosphere containing fluorine used in the method of the present invention, for example, a mixed gas of a proton-based gas such as SF ₆ or G ₂ F ₆ , CCl ₂ F ₂ , CF ₄ and an inert gas such as He can be considered. have.

The manufacturing method of the base material for optical fibers of this invention can reduce the influence of the OH group mixed in the process of extending a glass rod. The reason for this is considered as follows.

1) The portion contaminated by the OH group is not the boundary portion between the core and the clad, but becomes a portion separated from the boundary portion to the outer circumference, thereby reducing the amount of light absorbed by the OH group.

2) When there is no fluorine in the atmosphere, it is considered that the reaction of H ₂ O incorporating into the silica glass to generate an OH group bonded to the glass structure can be represented by the following formula (1).

On the other hand, in the case of containing fluorine, even if H ₂ O is mixed in the silica glass, there is a possibility that the generation of OH groups bonded to the silica glass structure is suppressed by the reaction of the following formula (2).

3) The OH group may be bonded in the form of filling the bonding portion in the silica glass. However, since the O bonding portion is already filled by F due to the presence of F, the probability that the OH group is bonded to the glass is lowered, so that the OH group is bonded to the glass structure. The production of OH groups is suppressed. In the method of the present invention, in addition to being able to obtain a transparent glass body in which only fluorine is not added to the central portion of the porous glass body and contained only in the outer peripheral portion, the influence of the mixed OH group can also be reduced. It is possible to obtain a high quality optical wave base material having a distribution, excellent transmission loss characteristics and low absorption loss due to OH group with good productivity.

Hereinafter, an Example demonstrates this invention concretely.

Example 1

The porous base material made of pure silica produced by the VAD method was once inserted into a furnace at 1200 ° C. At this time, He was flowed into the furnace at 5 L / min and Cl ₂ at a capacity of 100 cc / min, and the OH group in the porous base material was sufficiently reduced.

Next, this porous base material was inserted into a furnace at 1400 ° C. to shrink. At this time, only He was allowed to flow at a flow rate of 5 L / min.

In addition, this contracted porous base material was inserted into a furnace at 1640 ° C to obtain a transparent glass base material. At this time, the furnace was allowed to flow at a flow rate of He 5 l / min and SF ₆ 100 cc / min. As a result, the refractive index distribution of the obtained transparent glass base material is shown in FIG.

Next, after the transparent glass base material was stretched to a diameter of 15 mm using an oxyhydrogen flame, a porous glass layer made of pure silica was formed on the outer circumference of the transparent glass base material by the apparatus shown in FIG. The outer diameter of the porous glass layer at this time was 110 mm ø. The composite of the porous base material and the transparent glass base material was dehydrated in an atmosphere containing fluorine to be transparent glass again. Dehydration was performed in an atmosphere of 1200 ° C., He 5 L / min, Cl ₂ 500 cc / min and SFC 100 cc / min. As a result, a transparent glass base material having a refractive index distribution shown in FIG. 6 was obtained. The outer diameter of this base material was 45 mm / ø.

After the base material was stretched using an oxyhydrogen flame to a diameter of 15 mm, a porous glass layer made of pure silica was again formed on the outer circumference of the transparent glass base material by the apparatus shown in FIG. The outer diameter of the porous glass layer at this time was 110 mm ø. The composite of the transparent glass base material and the porous glass layer was dehydrated under the same conditions as the above conditions in an atmosphere containing fluorine, and then transparent glass was formed. As a result, a transparent glass base material having a refractive index distribution shown in FIG. 7 was obtained. The outer diameter of this base material was 45 mm ø.

Next, the transparent glass base material was stretched to a predetermined diameter, inserted into a commercially available quartz pipe, and then integrated into a single mode fiber at 1.3 mu m. As a result, the transmission loss at 1.3µm is 1.0dB / km, the absorption loss at 1.39µm dlm in OH group is 10dB / km, and the effect of the mixing amount of OH group and the transmission loss of OH group is not used in the present invention. It is considerably smaller than that.

Example 2

In the method of Example 1, the glass base material was stretched using a plasma flame instead of an oxyhydrogen flame. All other method conditions were the same as those of Example 1, and the refractive index distribution and the dimensions of the obtained base material were the same as those shown in FIG.

As a result of fiberizing the base material, an optical fiber having excellent transmission loss characteristics with a transmission loss of 1.3 µm at 0.6 dB / km and an absorption loss of 1.39 µm with an OH group at 6 dB / km was obtained.

This improvement in the transmission loss characteristic is an effect of not using an oxyhydrogen flame which causes OH group mixing at the time of stretching.

In addition to the plasma flame, it is also effective to use an electric resistance furnace or an induction furnace.

Claims (4)

The starting material is a molten glass rod made of substantially pure quartz glass in the center part and a quartz glass containing fluorine in the outer part, and forms a porous glass layer made of substantially pure stone glass on the outer peripheral part of the starting material. The composite of the layer and the molten glass rod is heated in an atmosphere containing fluorine, whereby the porous glass layer contains fluorine, and the porous glass layer is vitrified. The manufacturing method of the base material for optical fibers containing. Substantially pure porous porous glass produced by VAD method is heated and shrunk not to be transparent vitrified, and then the porous glass is heated in an atmosphere containing fluorine, whereby a portion of the porous glass is fluorine. And a fluorine which is a starting material of a molten glass rod obtained by transparent vitrification of this porous glass body to a predetermined diameter. The method for producing a base material for an optical fiber according to claim 1, wherein the cladding portion contains fluorine which is used to extend the glass rod or the transparent glass base material using a plasma flame. The method of manufacturing a base material for an optical fiber according to claim 2, wherein the cladding portion contains fluorine which is used to extend the glass rod or the transparent glass base material using a plasma flame.

Images