JPH0660030B2 - Method for manufacturing glass base material for optical fiber - Google Patents

Description

The present invention relates to a method for producing a base material of an optical fiber using the VAD method, and in particular, an element for adjusting the refractive index such as fluorine (F) is added. Add a large amount as a chemical agent at high speed and stably (dope)
To produce a high quality optical fiber preform.

[Conventional technology]

Conventionally, optical fibers have been manufactured by various manufacturing methods such as the MCVD method and the OVPO method, but the VAD method is drawing attention in terms of productivity and quality. In this method, first, glass microparticles are generated by a flame hydrolysis reaction and are successively deposited on a rotating starting material to form a rod-shaped porous preform (glass soot base material). Next, the porous preform is heat-treated in various gas atmospheres, dehydrated and melted into glass to obtain an optical fiber base material. Furthermore, the method is a method in which this base material is spun to obtain an optical fiber.

An optical fiber is mainly composed of a core part through which light is propagated and a cladding part around it. If the refractive index of the core part is n ₁ and the refractive index of the cladding part is n ₂ , NA (numerical aperture) Is It is defined by (n ₁ > n ₂ ) (n ₁ and n ₂ are average values). In optical fibers based on silica (SiO ₂ ), (i) the core is doped with an additive that raises the refractive index, (ii) the cladding is doped with an additive that lowers the refractive index, and (iii) (i ) And (i
Either of the methods of i), which is a combination method, is used.
Needless to say, the cladding part in (i) and the core part in (ii) are silica.

Commonly used additives include GeO ₂ , P ₂ O ₅ , and Al ₂ O.
₃ , TiO ₂ (for increasing the refractive index), B ₁ O ₃ , F (for decreasing the refractive index) and the like. Wavelength 0.59μ in FIG.
The refractive index of the silica glass at m is shown. The horizontal axis represents the oxide weight% in silica, and the vertical axis represents the refractive index (nα) and the refractive index Δn%. [Source: Kumamaru. Kurosaki: “Materials for optical transmission” Industrial material 27 (1979), P39] Among these additives, fluorine has recently attracted attention, and it is not only the VAD method but also other manufacturing methods. A method for doping is also being studied and developed.

When it is desired to obtain the same refractive index difference between the core and the cladding, the refractive index is generally lowered by the cladding, which is described in (ii) and (ii) above.
The method i) has the advantage that the amount of the additive to be doped in the core portion does not exist at all, or it can be smaller than that of the method i). This is advantageous for a high NA optical fiber in that the absorption loss due to the additive in the core portion is reduced. Moreover, a pure silica core optical fiber having excellent transmission loss under irradiation with radiation can be produced only by the method (ii).

As described above, the method of lowering the refractive index of the cladding portion has advantageous characteristics.

In particular, in the sintering process of the VAD method, the advantage of doping with fluorine is that doping can be performed uniformly and a flat refractive index distribution can be provided.

The processing speed is fast. That is, a porous preform of several hundreds to 1 kg can be treated and vitrified within several hours.

In particular, it is superior to other methods in two points, and the technology relating to this method is disclosed in JP-A-55-67533 and JP-A-60-8604.
7, 60-86049, and the like.

[Problems to be solved by the invention]

However, when fluorine, boron, etc. are added to the glass soot base material in the sintering process, the following problems have emerged.

(B) When using the inclined sintering method in which the glass soot base material is gradually moved from one end to the other end and inserted into the heating region, the obtained glass base material is transparent with no bubbles remaining, but The difference Δn in non-refractive index due to the addition of fluorine was considerably different at both ends of the base material.

(B) On the other hand, when the uniform heating and sintering method in which the entire glass soot base material is uniformly heated and heated to be transparent is used, Δn is stable and uniform in the longitudinal direction of the base material, but Tend to leave bubbles in the glass soot, and this tendency becomes remarkable as the amount of fluorine added increases, and in the glass soot base material having a high-purity quartz glass mandrel, many bubbles are left in the obtained glass base material. Was especially remarkable at the mandrel interface.

Therefore, the fluorine-containing glass base material obtained by either the gradient sintering or the uniform heating sintering may have a defect that Δn varies in the longitudinal direction or bubbles remain. There is a limit to the yield that can act as a glass base material for glass.

Among the problems in the above-mentioned conventional method, the present invention is particularly unstable in the longitudinal direction due to the addition of fluorine (change in relative refractive index difference Δn).
It is an object of the present invention to provide a method for producing a glass preform sufficient for use as a preform for optical fibers, which is high in quality and economical, by solving the problem of residual air bubbles.

[Means for solving problems]

As a result of earnest research on means for solving the above problems, the present inventors have found that the glass soot base material containing quartz as a main component of the present invention is a gas atmosphere containing at least one or more refractive index adjusting elements. In the method of adding the above elements to the glass soot base material by heat treatment in the furnace, a core tube having one gas supply inlet at one end is preliminarily provided with an element-containing gas for adjusting the refractive index from the gas supply inlet. While introducing a mixed atmosphere gas, the glass soot base material is gradually moved into the heated gas atmosphere to gradually shrink and become transparent from one end to the other end of the glass soot base material, and The present invention has reached a method for producing a glass base material for an optical fiber, which is characterized in that the moving direction of the glass soot base material coincides with the flow direction of the gas atmosphere. For the first time, according to the present invention described above, it is possible to achieve both stable and uniform addition of fluorine in the longitudinal direction and elimination of bubbles from the base material obtained.

In a particularly preferred embodiment of the present invention, the glass soot base material is one that has been sufficiently dehydrated with a halogen-based gas such as Cl ₂ gas before heat treatment, and the gas atmosphere is, for example, CF ₄ , C _2. Includes one type of fluorine-based or boron-based gas such as F ₆ , SF ₆ , SiF ₄ , BF ₃ etc., and the movement of the base material in these gas atmospheres and heat treatment are performed in the quartz furnace core tube to adjust the refractive index. The above method using SiF ₄ as the gas containing the element for use can be mentioned.

The glass soot base material mainly composed of quartz used in the present invention is, for example, by flame hydrolysis reaction, glass raw material gas and additive gas are introduced into a flame by using an inert gas as a carrier, and the generated glass fine particles are deposited. Blame,
Alternatively, it can be obtained by a so-called sol-gel method, that is, a method of obtaining by hydrolysis of alcoholate, and these techniques are known.

In the present invention, at least 1 of the glass soot base material is used.
Although the soot is heat-treated in a gas atmosphere containing at least one kind of refractive index adjusting element to add the refractive index adjusting element to the glass soot base material, as such a refractive index adjusting element, For example, B, F, N, P, Ge, Al, Ti and the like can be mentioned.

The present invention, in the above heat treatment, in the furnace core tube having one gas supply inlet at one end, while introducing an atmosphere gas premixed with the refractive index adjusting element-containing gas from the gas supply inlet, the glass soot base material Is gradually moved in a heated atmosphere in a direction corresponding to the flow direction of the atmospheric gas containing the refractive index adjusting element. This will be specifically described with reference to FIG. FIG. 1 is a schematic view of an embodiment of the present invention, in which 1 is a glass soot base material, 2 is a heating furnace, 3 is a core tube, 4 is a fluorine-based gas inlet, 5 is another gas inlet,
6 is a gas outlet, 7 is a heater, and 9 is a gas supply inlet when it is rotatable and vertically movable. As shown in the figure, the glass soot base material 1 placed in the core tube 3 having only one gas supply inlet 9 at the lower end is attached to the shaft 7 so that the fluorine-based gas from the inlet 4 and other gas from the inlet 5 The shaft 7 is gradually pulled upward so that the moving direction matches the flow direction, and the soot base material 1 passes through the portion of the heater 8 so that it gradually shrinks from one end to the other end and becomes transparent. To do.

Although not shown, a method may also be used in which the atmosphere gas containing a fluorine-based gas is caused to flow downward from above the pine while moving the glass soot base material from above to below.

[Action]

The present invention is based on the fact that the direction of flowing the mixed gas to which the fluorine-based gas is added and the method of moving the soot base material in the mixed gas are made to coincide with each other. Moreover, it is considered that the fluorine-based gas was impregnated into the porous material, and as a result, it was possible to stably add the fluorine in the longitudinal direction. It will be appreciated that this consideration is also true from the facts below.

That is, the soot base material is installed above the heating part in the pine cone that holds the gas atmosphere, and the mixed gas containing the fluorine-based gas is flowed upward from below the pine cone, and the soot base material faces the gas flow. While gradually lowering, the glass vitrification was carried out at the same time as the heating with fluorine, and a cylindrical glass base material without bubbles was obtained. The distribution of Δn in the longitudinal direction was measured over the entire length of the base material (30 cm). When I did
It gradually decreased from -0.2% at the front end and remained stable at -0.3% from the center to the rear end. That is, △ n is −
The variation is in the range of 0.2% to -0.3%, and this cannot be used at all for cladding materials for single mode fibers, etc., which require strict adjustment of Δn.

On the other hand, the gas was made to flow downward from above in conformity with the moving direction of the glass soot base material and the flowing direction of the fluorine-containing gas-containing atmosphere gas, and other conditions were similarly obtained by simultaneously performing fluorine addition and transparent vitrification. The Δn of the glass base material was stable at -0.3% over the entire length.

This is because the formation of Δn by adding fluorine to the glass soot base material is carried out by an equilibrium reaction as shown in the following formula (1). 3SiO ₂ (s) + SiF ₄ (g) 4SiO _1.5 F (s)・ ・ ・ (1) ｜ △ n ｜ ∝ [SiO _1.5 F] ＝ Ko [SiF ₄ ] ^1/4・ ・ ・ (2) where │ △ n ｜: absolute value of relative refractive index difference, [SiO _1.5 F ]: Concentration of SiO _1.5 F [SiF ₄ ]: Represents the concentration of SiF ₄ , if the glass soot base material is not sufficiently impregnated with the specified fluorine, the concentration of SiF _{4 is} as shown in equation (2). Therefore, it is considered that the amount of fluorine added to the glass is small, that is, | Δn | is small in the tip region where the fluorine content is insufficient.

The impregnation speed of the fluorine-based gas into the glass soot base material greatly changes depending on the bulk density of the base material, and Δn tends to increase as the bulk density increases. When the moving direction of the glass soot base material is opposed to the flow direction of the fluorine-containing gas-containing atmosphere gas, the base material with a bulk density of 0.3 g / cm ³ |
While the range of n | fluctuated from 0.2 to 0.3%,
With a glass soot base material of 0.6 g / cm ³ , 0.15 to 0.
The variation was 3%, which was unstable at the center and was 0.25%, and Δn was different at any position of the glass soot base material. This fact supports that the impregnation of the glass soot base material with the fluorine-based gas largely depends on the bulk density, and the higher the bulk density, the slower the impregnation.

On the other hand, when the insertion direction of the glass soot base material and the flow direction of the fluorine-based gas-containing atmosphere gas were matched (the gas also flowed downward from above), the bulk density of the obtained base material was 0.6.
Even in the case of g / cm ³ , Δn was stable in the longitudinal direction.
In this experiment, a glass soot base material having an outer diameter of 100 mm, a length of 300 ml, and a mandrel having a bulk density of 0.3 g / cm ³ and 0.6 g / cm ³ was used at a temperature of 1650 ° C.
Flow rate Si using a mixed gas of SiF ₄ and He at a descending speed of 4 mm / min.
F ₄ /He=0.03, and He was performed under the condition of 10 / min.

Furthermore, the glass soot base material was installed at the bottom of the pine, and the shaft was gradually pulled up and moved up to gradually move to the center of the heating furnace, and the direction in which the atmosphere gas containing fluorine-based gas was flowed was changed. When the gas flow directions are matched, the distribution of Δn in the glass soot base material obtained is stable,
The distribution of Δn was unstable when the gas flow directions were opposite to each other. When adding fluorine, pull up and pull down speeds are both 1
It is preferably 0 mm / min or less, and when it exceeds 10 mm / min, the effect of the present invention is rather reduced, which is not preferable for stabilizing Δn.

By the way, as a method of stably distributing Δn in the longitudinal direction of the glass base material, there is a method of heating and sintering the entire glass soot base material at a uniform temperature, but it is easy to leave bubbles in the base material. It was confirmed that, in the case of the glass soot base material having the mandrel, bubbles were extremely likely to remain, and the bubbles remained remarkably at the mandrel interface. Thus, although the uniform heating in the heating furnace has the effect of making Δn uniform in the longitudinal direction, it has a drawback that bubbles tend to remain.

The cause of this bubble retention phenomenon is as follows: a) Since the heating is performed uniformly, the exposure time to high temperature in a fluorine-based gas atmosphere is longer than that in the gradient sintering method. The system gas etches the mandrel gas, which roughens the interface and forms bubbles. (B) In soaking sintering, the entire soot shrinks, so excess gas exiting the system causes gradient sintering. It is considered that it is more difficult than that and bubbles are likely to remain.

By the way, adding fluorine accelerates the contraction of soot,
This is also a cause of making it difficult to discharge the surplus gas from the system. For example, the glass soot base material containing fluorine corresponding to Δn = -0.3% shrinks faster and the vitrification temperature is about 15%.
A decrease of 0 ° C. was confirmed during the experiment of the present invention. In the sintering of the glass to which fluorine is added as described above, the soot contracts quickly and an excess gas is likely to remain, so that it is easier to leave bubbles in the base material as compared with the sintering of pure quartz soot. How to shrink the glass soot base material is very important.

From this point of view, it is clear from the results and consideration of the above-mentioned experiments that the selection of the gradient sintering method is of great significance in producing the glass base material to which fluorine is added.

Next, it is important to select the raw material gas when adding fluorine,
The use of SiF ₄ gas is most effective and preferable for preventing bubbles from remaining. The use of other fluorine-based gases such as SF ₆ and CF ₄ makes it possible to add (3) and
Excess gases such as CO ₂ and SO ₂ F ₂ are generated by the reaction of the equation (4), and these gases cause bubbles.

SF ₆ + SiO ₂ → SiF ₄ + SO ₂ F ₂ (3) CF ₄ + SiO ₂ → SiF ₄ + CO ₂ (4) On the other hand, in the case of SiF ₄ , excess gas is generated as is clear from the above reaction formula (1). There is no. In addition, in the soot with a mandrel, the use of SF ₆ and CF ₄ is compared with the etching by the reactions of the above formulas (3) and (4), whereas SiF ₄ does not have such an etching reaction. Generation of bubbles is also suppressed.

Furthermore, it is important to select the material for the sintering pineapple. For example, when using alumina, AlF ₃ as an impurity reacts with _{SF 6, CF 4, CuF 2} , CuF, generates FeF _3, FeF _2, etc., which enters the soot obtained glass preform If it becomes a fiber, it will cause the absorption of impurities. From this point, it is preferable to use a high-purity quartz tube as the pine tube. In addition, even when using a quartz tube, SF ₆ and CF ₄ gases should be as described in (3) above.
Since the core tube is etched by the reaction of (4) and (4), it cannot be used for a long time. When the method of the present invention was investigated, a temperature of 1400 ° C. and a fluorine content of Δn was −0.3% to obtain a glass body. In order to obtain a glass body, SF ₆ gas was lengthened, and the core tube was 2 mm thick in 8 hours. It was etched. Therefore, after all
The use of SiF ₄ gas that does not etch SiO ₂ is preferred.

It is preferable to sufficiently dehydrate the soot with a gas such as Cl ₂ before adding fluorine to the soot, because the effect of the present invention is further enhanced. This is to prevent the moisture in the soot from reacting with the fluorine-based gas as shown in the following formula (5) to generate HF gas having an etching action.

SF ₆ + H ₂ O → SOF ₄ + 2HF (5) The present invention will be specifically described below with reference to Examples.

〔Example〕

Example 1 Quartz having an outer diameter of 10 mm is a quartz step-like core containing 6% by weight of GeO _{2 in the} central portion, and fluorine is added to the peripheral portion to make Δn = -0.3%. A soot composed of pure SiO ₂ was deposited on the outer periphery of the system rod by using a flame hydrolysis reaction, and the outer diameter 10 having the refractive index distribution structure shown in FIG.
A 0 mm soot base material was used. The soot base metal was constructed according to the configuration shown in FIG. 1 and gradually in a He atmosphere containing 3 mol% of SiF ₄ in a furnace core tube at a temperature of 1600 ° C. at a speed of 4 mm / min in accordance with the flow direction of the atmosphere gas. It was pulled up to a transparent glass. The refractive index distribution of the obtained glass base material was as shown in FIG. The vertical axis in Fig. 2 is the quartz standard relative refractive index difference (%)
Where A corresponds to the starting quartz rod portion and B corresponds to the SiO ₂ soot deposited portion.

Example 2 A soot base material having an outer diameter of 100 mm and having a refractive index distribution structure shown in FIG. 4 was prepared by depositing a soot of pure SiO _{2 on} the outer periphery of a carbon rod having an outer diameter of about 6 mm by utilizing a flame hydrolysis reaction. . The base material was heated to 1600 ° C. in a He atmosphere containing 10 mol% of SiF ₄ according to the constitution shown in FIG. The refractive index of the obtained glass base material was as shown in FIG.

In FIGS. 4 and 5, B represents a portion where SiO ₂ soot is deposited.

The outer periphery of the silica-based Garasurotsudo outer diameter 100mm with a refractive index profile of FIG. 6, which is added to Example 3 GeO ₂ in the range of 0-17 wt%, consisting of pure SiO ₂ by using a flame hydrolysis Soot was deposited to obtain a soot base material having the refractive index distribution structure shown in FIG. The soot base material was dehydrated in advance by heating it to a temperature of 1200 ° C. in a He gas atmosphere containing 1 mol% of Cl ₂ in the constitution shown in FIG. Next, the atmosphere was changed to a He gas atmosphere in which 20 mol% of SiF ₄ was added, and the temperature was gradually raised to 1600 ° C. to obtain transparent vitrification.
It was as shown in the figure.

In any of Examples 1 to 3 above, the refractive index in the longitudinal direction of the obtained base material was stable over the entire length, and no bubbles were observed in the base material.

In addition, the properties of the fibers obtained from the glass base materials of Examples 1 to 3 are sufficiently low loss without any increase in absorption due to impurities (for example, 1.30 μm).
, About 0.5 dB / km), and the absorption peak due to the OH group did not change with time.

〔The invention's effect〕

As apparent from the above, the present invention is a glass soot base material is heated in an atmosphere gas containing a refractive index adjusting element to add the element in the soot base material, a glass base material containing the element In the method of manufacturing the base material, the addition amount becomes uniform in the longitudinal direction and the radial direction of the base material, so that there is no fluctuation in the refractive index and no bubbles remain in the base material.SiF ₄ is used especially when fluorine is added. For example, the effect of preventing residual bubbles is great, and the use of a quartz tube as the core tube makes it possible to reduce the residual water content of the base material very easily. Therefore, the present invention is an advantageous method that is applied to the production of glass preforms to improve the productivity and the quality of the glass preforms.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention, and FIGS. 2 to 7 are diagrams showing a glass soot base material or a refractive index distribution structure of the glass base material of the present invention, in which the vertical axis represents Represents the relative refractive index difference Δn (%) based on quartz. Fig. 2: Glass soot base material of Example 1 Fig. 3: Glass soot base material of Example 1 Fig. 4: Glass soot base material of Example 2 Fig. 5: Glass base material of Example 2 Fig. 6: Glass soot base material of Example 3 FIG. 7: Glass base material of Example 3

Claims (7)

[Claims] 1. A glass soot base material containing quartz as a main component is heat-treated in a gas atmosphere containing at least one or more refractive index adjusting elements in a furnace core tube, whereby the glass soot base material is obtained. In the method of adding the above element, the glass soot base material is introduced into the furnace core tube having one gas supply inlet at one end, while introducing an atmosphere gas premixed with a gas containing the refractive index adjusting element from the gas supply inlet. Is gradually moved into the heated gas atmosphere to gradually shrink and become transparent from one end of the glass soot base material to the other end, and the moving direction of the glass soot base material is the flow direction of the gas atmosphere. A method for producing a glass preform for optical fibers, which is performed so as to agree with each other. 2. The glass base material for optical fibers according to claim 1, wherein the glass soot base material has been sufficiently dehydrated in advance using a halogen-based gas before the heat treatment. Method of manufacturing wood. 3. The method for producing a glass base material for an optical fiber according to claim 1, wherein the gas atmosphere contains one of a fluorine-based gas and a boron-based gas. 4. A fluorine-based gas or a boron-based gas is CF ₄ , C ₂ F ₄ , S
The method for producing a glass preform for an optical fiber according to claim (3), which is any one of F ₆ , SiF ₄ , and BF ₃ . 5. The glass soot base material has a cylindrical high-purity quartz glass mandrel at the center thereof, and the mandrel itself can be a part of the optical fiber base material. The method for producing a glass preform for optical fibers according to item (1). 6. The movement and heat treatment of the glass soot base material in the gas atmosphere are performed in a quartz furnace core tube, and SiF ₄ is used as a gas containing a refractive index adjusting element.
The method for producing a glass preform for optical fibers according to item (1). 7. The glass soot base material for optical fibers according to claim (6), wherein the glass soot base material is dehydrated in advance by using Cl ₂ gas before heat treatment. Production method.