KR910008934B1 - Method for producing glass preform for optical fiber - Google Patents

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Description

Manufacturing method of glass base material for optical fiber

1 is a view for explaining a heat treatment apparatus used in the present invention.

2 is a graph showing the relationship between oxides (wt%), refractive index (nα) and refractive index difference (Δn) in silica in quartz glass.

3 is a graph showing the relationship between SiF ₄ gas partial pressure and refractive index difference | Δn |.

4 is a graph showing the processing temperature dependence of the refractive index difference [Delta] n.

* Explanation of symbols for main parts of the drawings

1: support rod 2: porous preform

3: pressure vessel 4: heating

5: heating device 6: seal

7: heating device 8: gas piping

9: pressure gauge 10: gas piping (outlet)

11 valve 12 pressure reducing system

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a glass base material for an optical fiber using an aggregate of glass particles, and more particularly, to a method for producing a glass base material for an optical fiber, in which fluorine as a refractive index adjusting agent is added to the glass particle assembly when the glass particle assembly is sintered. will be.

Conventionally, optical fibers have been manufactured by various manufacturing methods such as the VAD method and the OVPO method. However, the optical fiber has attracted attention in terms of productivity and quality. These methods first generate glass fine particles by a flame gas decomposition reaction, and are sequentially deposited on a rotating starting material to form a rod-shaped porous preform. Next, the preform is heated in various gas atmospheres to dehydrate and melt vitrify to obtain an optical fiber base material. In addition, the base material is spun to obtain an optical fiber.

로 정의된다(n₁, n₂는 평균치). 실리카(SiO₂)를 베이스로 하는 광파이버에서는, (i) 코어에 굴절률을 높이는 첨가제를 첨가하는 방식, (ii) 클래드에 굴절률을 낮추는 첨가제를 첨가하는 방식, (iii) (i)과 (ii)의 방식의 합체방식의 어느 하나의 방식이 사용된다. 말할 필요도 없이, (i)에서는 클래드부가 (ii)에서는 코어부가 실리카이다.Optical fiber, and mainly the core part where the light is conveyed, and is composed of a cladding around it, assuming that the refractive index n ₁ of the core portion, the clad negative refractive index n _2, and N A (numerical aperture) is

Where n ₁ and n ₂ are mean values. In an optical fiber based on silica (SiO ₂ ), (i) a method of adding an additive that increases the refractive index to the core, (ii) a method of adding an additive that lowers the refractive index to the clad, (iii) (i) and (ii) Any one of the methods of coalescing of is used. Needless to say, in (i) the cladding portion is (ii) the core portion is silica.

(1979), P39]As the additive, typically in a well-used, GeO _2, P ₂ O _5, it can be exemplified Al ₂ O _3, TiO ₂ (refractive index increases for more than), and B ₂ O _3, F (for more than a refractive index falling), and the like. 2 shows the refractive index of the quartz glass at wavelength 0.59 mu m. The horizontal axis represents the weight percent oxide in silica, and the vertical axis represents the refractive index (nα) and the refractive index difference (Δn%). [Source: Kumamaru, Kurosaki, Japan] "Materials for Optical Transmission"

(1979), P39]

Among these additives, fluorine is an additive that has recently attracted attention, and a method of adding in the VAD method and other manufacturing methods has been studied and developed.

In the case where the difference in refractive index between the core and the clad is to be obtained, the above-described methods of (ii) and (iii) in which the refractive index is lowered in the clad generally have no additive amount added to the core portion, or (i) It has the advantage of being less than by the method of. This is advantageous in the sense that the absorption loss due to the additive of the core portion is reduced in the high NA optical fiber. In addition, the pure silica core optical fiber excellent in the transmission loss under irradiation with radiation can be produced only by the method of (ii).

As such, the method of lowering the refractive index of the clad part has advantageous properties.

In particular, in the sintering step of the VAD method, the advantage of adding fluorine is,

① Can be added uniformly and can give flat refractive index distribution.

② The processing speed is fast. That is, the porous preform of about 100-1 kg can be processed and vitrified within several hours.

It is especially good at 2 points at.

However, in the prior art, fluorine was added by heating the porous preform under atmospheric pressure in a 100% atmosphere of fluorine-based gas or in an atmosphere diluted with an inert gas. However, in this method, the defect that air bubbles remain in the obtained glass base material there was. In particular, in the VAD method, when fluorine is added to the refractive index difference by -0.5% or more, bubbles remain particularly in the glass base material obtained.

In the case of using an inert gas as a diluting gas, in the like, for example, other than N ₂ He, Ar, O _2, was left in the glass base material obtained reliably bubbles. The same thing happened in the presence of He gas. In addition, He is a preferable gas or expensive as a diluent gas, and in order to provide a glass base material at low cost, there is a problem in terms of cost, and its use is not preferable.

The present invention solves the above-mentioned drawbacks of the prior art, that is, improves the amount of fluorine added to the glass fine particle aggregate without leaving bubbles, and makes it possible to add fluorine at high speed, and also inexpensive glass. The purpose is to provide the base material.

The present invention is a glass base material for an optical fiber, characterized in that the glass fine particle aggregate is heat-treated at least for one period at a time under reduced pressure and substantially until the fluorine is sufficiently added into the glass in an atmosphere composed of SiF ₄ gas. It is a manufacturing method of.

In the present invention, in the above, as to provide a method for producing a glass preform for optical fiber containing fluorine to heat treatment flew emits a SiF ₄ gas, as the partial pressure of SiF ₄ is preferably 0.01 to 0.5 atm.

In the process of heat-treating a glass fine particle aggregate (porous preform), it can be easily inferred that reaction efficiency can be improved by processing under reduced pressure. However, it is difficult to obtain a good glass body even if heat treatment is carried out in a reduced pressure state in which a preform and a fluorine-based gas are introduced inside by simply using a sealed pressure vessel. The reason is heavy metal contamination from one pressure vessel (core tube), and the other is a decrease in reaction efficiency due to thermal decomposition of the atmosphere gas itself.

And further to a component other than the fluorine-based gas in the fluorine, such as CF ₄ in the C (carbon -), in the S CF ₆ it remains in the glass, the heat that causes the air bubbles. For the benefit, in the process of obtaining the present invention, when sikyeoteul added fluorine in the glass by using CF _4, it was confirmed that the air bubbles generated in the components were made of a CO _2, CO. Moreover, the presence of inert gas (He, Ar, N ₂ , O _2, etc.) was also confirmed.

Based on the above findings newly obtained by the present inventors, the present invention uses SiF ₄ for fluorination, so that the reaction with quartz glass is represented by the following reaction formula (1).

As in the case of using conventional CF ₄ or C ₂ F ₆ , CO ₂ , CO and the like does not generate extra gas.

In addition, the pressure of the SiF ₄ gas can be adjusted by blowing SiF ₄ under reduced pressure, and contaminants from the heat generating furnace are removed without reaching the free particle aggregate, thereby maintaining the cleanness of the free particle aggregate. It is possible to efficiently remove volatile surpluses in the free particle aggregates.

It has also been found that the best reaction efficiency can be maintained by always supplying fresh gas. This is the scheme of the following (2)

It is considered to be because there is an effect of suppressing the dissociation reaction represented by The partial pressure (P) of the SiF ₄ gas in the pressurized heat treatment atmosphere (treatment temperature 1200 ° C., time 3 hours) of the porous preform obtained by the VAD method, and the refractive index difference (Δn) with respect to the silica of the obtained glass base material The relationship is shown in FIG. As a result, it can be seen that fluorine is more effectively added under a reduced pressure atmosphere to lower the refractive index.

4 shows the relationship between the processing temperature T (° C.) and the refractive index difference Δn of the same processor. The higher the fluorine partial pressure and the higher the treatment temperature, the higher the refractive index difference becomes. However, as a practical problem, when the pressure exceeds 0.5 atm or the treatment temperature exceeds 1400 ° C., bubbles may remain in the glass body after the clearing, but this is not the effect of partial pressure. Therefore, the partial pressure of SiF ₄ is preferably 0.01 to 0.5 atmospheres. If the temperature is too low, the reaction does not occur 100% and is inefficient. Therefore, the treatment temperature is preferably 800 ° C or higher. In addition, it is preferable to perform dehydration in advance before performing fluorination. This is because the dehydrated fluorinated base material can be efficiently obtained by dehydration in advance. Moreover, the glass body with much less bubble compared with the conventional method can be obtained also in the range other than said suitable conditions. The example of the heat processing apparatus used for the method of this invention is shown in FIG. In Fig. 1, reference numeral 1 denotes a supporting rod, 2 denotes a porous preform, 3 denotes a pressure vessel, 4 denotes a heating part, 5 and 7 a heater, and 6 a seal. (8) denotes a gas pipe, and the apparatus is also provided with a pressure gauge (9), a gas pipe (outlet) of (10), a valve of (11), and a pressure gauge of (12). However, these are only illustrations to the last and are not limited to this structure.

Example 1

The heat treatment using the apparatus as shown in Figure 1, and the quartz glass in order to aggregate the fine particles, the pressure is heat-treated for 2 hours at temperature 1200 ℃ in a state of 0.2 atm in the SiF ₄ gas 100% atmosphere. Subsequently, the addition of SiF ₄ was stopped and the temperature was heated (1600 ° C.) until the glass particulate aggregate was melt vitrified in a vacuum atmosphere to obtain a transparent glass base material.

The negative refractive index of the obtained glass was -0.5%. The base metal was processed into a quartz tube, a preform having a core / clad was inserted into the quartz tube, collapsed to be solidified, and subsequently fiberized to obtain a low loss fiber. The fiber had a core diameter of 5 mu m, a fiber diameter of 125 mu m, a refractive index difference between the core and the cladding of 0.5%, and the loss characteristic was excellent at 0.2 dB / km at a wavelength of 1.55 mu m.

Example 2

By using the apparatus of FIG. 1, pure quartz soot prepared by the VAD method was sufficiently dehydrated in this apparatus maintained at a temperature of 900 ° C. Next, as SiF ₄ 100%, the atmosphere was 0.05 atm, the temperature was raised from 800 ° C to 1600 ° C at a rate of 3.5 ° C / min to transparent glass.

The specific refractive index difference of the obtained glass base material was -0.35%, and residual moisture was 0.1 ppm or less.

Example 3

Only the SiF ₄ partial pressure in Example 1 was changed as shown in Table 1, and the other produced the glass base material on the same conditions. The specific refractive index difference of the obtained glass base material is put together, and it shows in Table 1.

TABLE 1

Example 4

In the same manner as in Example 2, the pressure of SiF ₄ was set in the conditions shown in Table 2, and ten glass substrates were produced. The specific refractive index difference of the obtained glass base material and the presence or absence of bubbles are also polymerized and displayed in Table 2.

TABLE 2

As a result, it turns out that foam | bubble residual can be reduced by carrying out pressure reduction.

Comparative Example 1

The partial pressure was made into SF ₆ -He atmosphere conditions shown in Table 3, and the other glass substrates were produced on each of the same conditions as in Example 2.

The specific refractive index difference of the obtained glass base material and the presence or absence of bubbles are also collectively shown in Table 3.

TABLE 3

It is understood that bubbles are likely to remain in the method in which SF ₆ is diluted with He. Moreover, the glass body was etched. This is considered to be due to the following reaction.

Comparative Example 2

The partial pressure of SiF ₄ was set to SiF ₄ -He atmosphere conditions shown in Table 4, and the other conditions were the same as those in Example 2, and ten glass base materials were produced.

Table 4 also shows the specific refractive index difference of the obtained glass base material and the presence or absence of bubbles.

TABLE 4

As can be seen from Table 2, it can be seen that bubbles are likely to remain even when SiF ₄ is diluted with He. There was no etching of the vitreous body seen in the comparative example 2.

Example 5

2.6%의 고 N.A 모재를 얻었다.Using the apparatus shown in FIG. 1, the preform which adhered the silica soot around the high NA glass to which (triangle | delta) n = 2% Ge was added was processed. Condition the temperature 1250 ℃, maintaining a pressure of 0.5 atm and maintained for 1 hour by the SiF ₄ at a rate of 150cc / min, followed by vitrification under He condition of the gas atmosphere of 0.2 atm in the temperature 1700 ℃, △ in the clad part n = - △ n with 0.63%

A high NA base material of 2.6% was obtained.

Example 6

As a starting member, a 10 mmφ glass rod made of pure quartz with a center of pure quartz and a fluorine-containing quartz layer containing Δn = -0.5% was used. The soot base material was inserted from one end to the other at a rate of 4 mm / min in a coarse furnace heated at a temperature of 1,200 ° C. in a He gas atmosphere to which 2 mol% of Cl ₂ was first added. Subsequently, SiF ₄ was flown at 200 cc / min, the pressure was 0.2 atm, and the soot base material was vitrified at a temperature of 1650 ° C., and then fiberized.

The obtained fiber had no absorption derived from impurities and was sufficiently low in loss. For example, it was 0.4 dB.km in wavelength 1.30 micrometers.

The present invention has the following effects.

1) By heat treatment under reduced pressure in a fluorine-based gas atmosphere, it is possible to efficiently obtain a glass having a negative refractive index compared with silica.

2) Fluorine can be added at high speed.

3) By heat-treating SiF ₄ gas under reduced pressure, it is possible to add without leaving bubbles in the glass base material obtained without lowering the reaction efficiency of fluorine.

4) It becomes easy to prepare the base material for high NA optical fiber and the base material for pure silica core optical fiber of the type which lowered Δn of the clad.

5) A glass base material is obtained without using expensive He, without bubbles, and without OH groups.

In addition, it goes without saying that, as in the prior art, it has the advantage of adding fluorine in the sintering step of the VAD method, that is, the advantage in flat refractive index distribution and processing speed.

Claims (5)

A method for producing a glass base material for an optical fiber, in which the glass fine particle aggregate is made transparent by blowing it until the fluorine is sufficiently added to the glass in an atmosphere of at least one period of reduced pressure and substantially in an atmosphere made of SiF ₄ gas. The method for producing a glass base material for an optical fiber according to claim 1, wherein the glass is transparent by simultaneously adding fluorine. The method for producing a glass base material for an optical fiber according to claim 1, wherein the SiF ₄ gas partial pressure is 0.01 to 0.5 atm. The method of manufacturing a glass base material for an optical fiber according to claim 1, wherein the glass fine particle aggregates are dehydrated under normal pressure in advance. The method for producing a glass base material for an optical fiber according to claim 1 or 2, wherein the glass fine particle aggregates deposit glass fine particles on a high purity glass block.

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