JPS6086045A - Manufacture of glass preform for optical fiber - Google Patents

Abstract

PURPOSE:To carry out the addition of fluorine to a glass material in high efficiency, and to manufacture a preform giving an optical fiber having stable transmission characteristics, by heat-treating fine glass particles composed mainly of quartz in a gaseous atmosphere containing both fluorine-containing gas and a chlorine-containing gas. CONSTITUTION:In the manufacture of a glass preform for optical fiber, glass fine particles composed mainly of quartz are subjected to the heat-treatment in an inert gas atmosphere containing both fluorine-containing gas an chlorine-containing gas. The heat-treatment temperature is preferably 1,100-1,400 deg.C. The deterioration of the furnace material is suppressed compared with the conventional process, and the optical fiber manufacture from the preform is free from the absorption caused by the impurities such as Cu and Fe, contains fluorine added efficiently to the glass, and has stable transmission characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a glass base material for optical fibers, and more particularly to a method for manufacturing a quartz glass base material containing fluorine. .

(Prior Art) A glass base material for an optical fiber consists of a core part and a cladding part, and the core part has a higher refractive index than the cladding part in order to facilitate the transmission of light near the center.

In order to increase the refractive index of the core portion, additives (dopants) such as TiO2, oQo, A4o3, etc. are usually added to quartz glass. Pure silica glass is often used in the cladding part of the general 7 eyeglass, and in this case, n = 1.4584 of pure silica glass is used as Δn =
Let it be O.

FIG. 1 is a graph showing the refractive index distribution of an optical fiber, where A is the core portion and B is the cladding portion. Here, the difference in refractive index between the core part and the cladding part is generally expressed as a percentage value of the relative refractive index difference. and n2, the relative refractive index difference Δnll is Δn1! Qi X 100 n2 is used.

Figure 1(a) shows a common structure for single mode fibers, in which case Δn! 2 digits is usually 0.3 to α5chi. Figure 1(b) shows a general structure for multimode fiber, and △nI for public communication fiber.
! is normally 1%, and for wide aperture angle fibers used for computer links, △n11 is approximately 2~2%.
4chi is normal.

Incidentally, an oxide dopant such as GeO2 used to increase the refractive index causes light scattering (Rayleigh scattering) due to the inherent characteristics of the dopant.

In addition, in the manufacturing process of the glass base material, bubble generation and crystal phase crystallization due to the oxide are likely to occur in the glass base material, and furthermore, the glass base material having a large coefficient of thermal expansion tends to be easily broken. Therefore, from the viewpoint of optical transmission characteristics and glass strength characteristics, it is preferable that the amount of dopant added to the glass base material is small.

For this reason, a method is generally used to increase the refractive index difference by lowering the refractive index of the cladding.For example, B2O3 is added to the cladding.
, fluorine, or a combination thereof.

B2O3 has the drawback that its coefficient of thermal expansion changes greatly with respect to the molar concentration added, and its refractive index changes depending on its thermal history.Furthermore, in terms of optical transmission characteristics, B! 03 has a unique absorption loss. Therefore, it is preferable to use fluorine as the refractive index lowering component.

It is already known that by adding fluorine to silica glass, it is possible to obtain optical 7-eyes with various refractive index distribution structures, and that depending on the structural design, 7-eyes with low dispersion can be obtained over a wide wavelength range. There is. Figure 2 is a graph showing a typical refractive index distribution structure.
C) is a step index type, (t+), (cl)
is of graded index type, (a)
- (d) all have fluorine added to the cladding portion. Regarding the J core part, (a) is quartz glass with a small amount of oxides such as G e O@, P205, etc. added to increase the refractive index, and (C) is high purity silica glass containing no additives1. , and (b), the amount of fluorine added is continuously decreased from the core periphery to the center of the core,
The center part is made only of pure silica glass that does not contain fluorine. Given the refractive index of pure silica glass n = 1.4585, let △n = "Q. (cl) i, J: :
! The amount of fluorine added is continuously decreased toward the center, and a dopant for increasing the refractive index of silica glass is added from a certain part of the core so that the amount of addition increases continuously toward the center. It is something. .

For example, GeO2, P! is added to the rad part and core part to adjust the refractive index or to facilitate glass processing.
051 ``201 At!Os It is natural that the initial additive and fluorine can coexist.

In order to obtain the same refractive index difference as shown in Figure 1 in an optical fiber made of fluorine-doped silica glass, it is necessary to reduce the amount of oxide added to the core, or to remove the oxide. Since it is not necessary to add it at all, Rayleigh scattering caused by the dopant is reduced, making it preferable as an optical transmission line.

As methods for producing quartz-based glass fibers, there are already known internal CVD methods (described in Japanese Patent Publications No. 51-23185 and Japanese Patent Publications No. 55-22423) and external CVD methods (described in Japanese Patent Application Laid-Open No. 49-10055). , VAD method (Japanese Patent Laid-Open No. 51-71316) and plasma CVD method (Japanese Patent Laid-Open No. 51-54446). Among these methods, external C using flame hydrolysis reaction
The VD method and VAD method are highly productive and economical methods.

However, although it is possible to add fluorine to quartz glass by a method using a flame hydrolysis reaction, it is extremely difficult to uniformly add a sufficient amount of fluorine into quartz glass.

For example, Japanese Unexamined Patent Application Publication No. 1997-82 describes a method of adding 7 fluorine into a glass base material. According to this method, the change in refractive index due to the addition of fluorine is The drawback is that Δn12 is only a factor of −12 at most with respect to the refractive index of glass, and there is a limit to the amount of fluorine that can be added.

The reaction in which fluorine is added to quartz particles is as shown in the following equation (1), but SiF4 (s) +38102 (e) →4SiO
1,5F (13) (1) Since moisture (H2O) generated by combustion exists in the oxyhydrogen flame used for soot production, SiF (b) also reacts as shown in equation (2). Resulting in.

5iF4(7+ 2H1O(B) →Si(%(s)+
4HF (g) (2) or +1? It is clear that 1 P4 is not only consumed as an additive amount to the quartz glass, but also consumed in reaction with the moisture present in the flame, reducing the efficiency of addition.

Furthermore, HF produced by the reaction of formula (2) has the effect of corroding glass, especially quartz (1910,), and easily reacts with fine quartz particles produced in the flame.

5i01(s)+2HF(g) →810F2Gd +
H20(7(3)Si02(Sun)+4HF(gu →
siy4(g) 10H,o(g) (4) As a result, the grain growth of the glass fine particles is suppressed, and the 1′#,
Decrease stacking. This is also clear from the fact that as the amount of fluorine compound added is increased in this method, the deposition rate of glass fine particles decreases, and eventually no glass particles are deposited at all.

Japanese Unexamined Patent Publication No. 55-67535 has the above-mentioned
As a method for overcoming the drawbacks of the method described in Publication No. 5-15682, (1) A laminate of glass particles formed by flame hydrolysis is heated at 1000°C or less in an atmosphere of fluorine compound gas, and then the laminate is 14 in an inert gas atmosphere
A method for producing a glass material for optical transmission, characterized by sintering it by heating it to 00°C or above;
A method for manufacturing a glass material for optical transmission is disclosed, which is characterized by forming a glass body containing fluorine by heating to 0° C. or higher.

Although this method can certainly add fluorine more efficiently than the method described in Japanese Patent Publication No. 55-15682, it was discovered during the research and development process of the present invention that it still has the following drawbacks. did.

In other words, in the method (2) above, the rate of addition of fluorine to the glass is slow, and impurities such as Ou+Fe are sometimes present in the resulting fiber, and the increase in transmission loss due to these impurities occurs in the 1.30 μm wavelength band. 3~
It was also 5 dB/km. (Normally, the loss value in this wavelength band was -0.20%.

On the other hand, the method (■) above is more efficient than the method (■) in that it adds fluorine at a faster rate and in a larger amount, and is more efficient with △n1 in 6 hours of treatment time! The c force was -0.25. However, the surface of the obtained glass matrix is severely corroded and becomes uneven. In addition, the furnace core tube used to maintain the gas atmosphere, such as a quartz furnace core tube, was severely eroded, and there were even cases where holes were formed in the furnace wall. The loss characteristic of the optical fiber (-) obtained by this method was as high as 10 aB/km in the 1.30 μm wavelength band.

The OH group in the fiber is less than αQ 5 ppm and IJO
Since an increase in absorption loss at 1 and 30 μm originating from the H group is not considered, the Ou′ present in the obtained phino(−
Absorption loss increase due to impurities such as p Fe is 9.5 dB
There were many cases where it was thought that the distance could be as high as /km.

One of the reasons why Ou + Fθ etc. exist in the obtained fiber is that Fe2O3 and OuO existing in the core tube wall appear on the surface due to core tube erosion and mix into the soot. It reacts as shown in (5) and (6).

Fe20B + 2F2 →2FeF2 + 57'2
02 (5) OuO-4-172Fl -40uF +
112 (h (6) FeF2 and Ou? are 1100
Although it is solid up to 1100°C, it sublimes, so it gets mixed into the soot and contaminates the soot base material.

Furthermore, if FezO3 or OuO is present in the soot base material, even if the reactions (5) and (6) occur, F at below 1100℃
Since eFl and OuF are solids, they are not removed from the soot and remain as impurities. Therefore, impurities are present in the fiber in both methods (1) and (2).

(Object of the Invention) An object of the present invention is to overcome the drawbacks of the conventional methods as described above and to provide a method for efficiently adding fluorine to a glass base material for optical fiber.

(Structure of the Invention) The gist of the present invention is that glass fine particles (soot base material) mainly composed of quartz are formed by a flame hydrolysis reaction or a sol-gel method in the manufacturing process of a glass base material for optical fibers. In order to make transparent glass, at least fluorine-based gas and chlorine-based gas are allowed to coexist, and heat treatment is performed in an atmosphere containing an inert gas.

A more preferable configuration of the present invention is that the temperature range for the heat treatment is 1100 to 1400°C.

According to the method of the present invention, ΔnI! without deteriorating the furnace material unlike the conventional method! If (3) is -1, a fiber with low loss can be obtained.

As a result of extensive research, the present inventors have discovered that heat treatment of the soot base material in an inert gas atmosphere in which at least fluorine-based gas and chlorine-based gas coexist has the following advantages, and has completed the present invention. did. For example, c4 gas and sP6
When the soot base material was heat-treated in an inert gas atmosphere in which gas coexisted, 1) corrosion of the furnace core tube was significantly suppressed;
It lasted more than a month of use.

2) Absorption originating from impurities such as O and Fe could not be confirmed in the obtained fiber even when its loss characteristics were investigated.

3) The refractive index difference Δntx (F) due to fluorine addition is the highest -
It also reached I2O3.

As a reference, when heat treatment was performed at 8F without using Okk and in an inert gas atmosphere with only gas, 1) the core tube was severely eroded, and the core tube was damaged within one day under the conditions.

2) The obtained fiber had significant absorption due to impurities derived from Ou 'p Fe.

5) Refractive index Δn1 due to addition of fluorine! (No is at most α2
It was 0chi.

The effects of adding base gas as described above are considered to be based on the following reasons.

i) Moisture (H, O) present in the gas atmosphere from the reactor core tube itself or the soot matrix mixed with outside air reacts as shown in the following equation (7) and converts into hydrochloric acid. , moisture is OF,
This suppresses the reaction as shown in equation (8) and the production of heptatonic acid HF. Therefore, corrosion of the reactor core tube by the 7-acid can also be suppressed.

HtO (p: J + 04 (b) → 2HO1 (b) ten 0
Snow @ (7) ago (g) + WEIFstm → 2HF (i
+'A B Os (g) (8 N Here, hydrochloric acid is known to have no etching effect on the furnace tube (quartz). By suppressing the reaction (8), the following equation (9
), the reaction in which fluorine is added to quartz glass becomes efficient.

4sto2(s)+87s(m → 4 S i Ot+s F(s) + s 0x(
id +Fa(i (9)11) As mentioned above, hydrofluoric acid (
If the generation of HF) is suppressed, the increase in impurities Fe and O2 in the furnace due to corrosion of the furnace tube can be suppressed. Furthermore, even if Fe+Ou is present, the reaction will occur as shown in QO, (11) below, and OuO6(b) and FeOL3 are highly volatile and will be removed from the system.

FezO5 (s) + 3014 (g) →2FeO6i
Gd+i0-@α0cuo(s) +, WO4(7-
b OuO6(g) + 'A OA) (11) Note that FeF2. Since OuF sublimates at a temperature of 1100° C. or higher, it is preferable to heat the soot base material at a temperature of 1100° C. or higher even in a gas atmosphere containing at.

Note that the fluorine-based gas is not limited to SF6 (OF4.72.E11F4.0oF2゜a)
Any material that decomposes at high temperatures and has the effect of adding fluorine to glass, such as C4F=, may be used.

Furthermore, as a chlorine gas, 5oar, coa14゜a
Anything that has a dehydrating effect, such as ct4, may be used. Further, it is preferable to add oxygen gas to the compound gas containing atoms such as carbon that cause scattering loss in the glass matrix phase.

This will be specifically explained below using examples.

In order to produce a cystomate base material by generating quartz glass fine particles by a flame hydrolysis reaction, the procedure shown in Fig. 3 (a) is as follows.
), using a concentric multi-tube burner 1 made of quartz, oxygen 2, hydrogen 3, etc. 4 are used as raw material gases, and Ar gas and He gas are used as carrier gases to feed into the center 5 of an oxyhydrogen flame. All you have to do is react. At 4 in the figure, Ar gas is flowed as a shield so that the raw material waste reacts in a space a few feet away from the tip of the burner. Rondo of glass imitation particles? When obtaining the glass particles, glass particles are laminated in the axial direction from the tip of the rotating starting member 6. In addition, when obtaining a pipe-shaped glass particle body, as shown in VC#i Fig. 3 (+,), while traversing the outer circumference of a rotating quartz rod or carbon rod 7 with a burner 8, glass particles are laminated at the center. Remove parts.

In addition, Ge (!4. ALO63, BBr
A raw material gas mixed with B, BF2, etc. may also be used. .

Also, a plurality of burners may be used.

A soot matrix of the same type as that produced by the method shown in FIG. 3 was also obtained by the hydrolysis method of metal alcoholade. This method is called the sol-gel method. .

The soot base material obtained by these methods, for example, Fig. 1 (
a) It has a refractive index distribution as shown in (b),
G e 02 +Tie has an increased refractive index,
, by addition of dopants such as.

Example 1 A quartz-based soot base material for a single mode fiber produced by the method shown in FIG. 3 was inserted into a quartz furnace tube and heat-treated under the following conditions (heating in a uniform heating furnace) to produce transparent vitrification. Fluorine-based gas and chlorine-based gas were flowed into the furnace together with He in combinations shown in Table 1. Furnace temperature is 1100
℃ to 1400℃ at a rate of 3 to 1400℃, and then the temperature at the soot surface was 14℃.
Heating was carried out in an atmosphere containing only He under conditions such that the temperature reached 00°C or higher. The conditions and the results obtained are summarized in Table 1.

Table 1 also shows Comparative Examples 1 and 2 in which no chlorine gas was used.

Note that Δn (Me) and Δn (F) are the refractive index differences of the obtained fibers defined as shown in Fig. 4, and in Fig. 4, Δn (Me) = -x 1o2 6 △n (F) = yloz n.

Here, nQ=1.4585 is a quartz glass refractor.

Example 2 Silica glass based soot base material for graded fiber produced by the method shown in Fig. 3? Heat treatment was performed using a quartz furnace tube in an atmosphere of each compound shown in Table 2. The temperature conditions for the heat treatment were the same as in Example 1. The results obtained were as shown in Table 2. .

Note that △n (Me) and △n (F) are the refractive index differences defined as shown in FIG.
) -n.

△n (Me) = -X 10g Q n (F) -n6 △n (F) = -X 10'' n (1 For n6, same as in Table 1 Example 3 Single mode fabricated by the method shown in Figure 3 A quartz-based soot base material for fiber was placed in a furnace preheated to 1400°C at an insertion rate of 2 to 4 minutes/minute and heat-treated (heating by zone heating). Furnace temperature distribution at this time is 800-1
Between 400℃ and 20℃/(WI), the temperature in the soaking section at 1400℃ was 10α.
C7 trous gas and chlorine gas were flowed in combinations shown in Table 943. When the soot base material reached the soaking section at 1400°C, it contracted slightly, but the
It became transparent vitrified at temperatures above ℃. In this case, the same results were obtained whether the heat-treated furnace or a completely different furnace was used.

Example 4 A quartz soot base material for a graded fiber produced by the method shown in FIG. 3 was heat-treated in a quartz furnace tube. The atmosphere in the furnace was He gas as well as SF6 at 5 occ/min and (BO
C4+02)k (50CC/molecule 5oocc/min), and the furnace temperature started at 800℃ and increased from 3 to 30℃.
b and each treatment temperature 900.1 as shown in Figure 6.
000. +100.1200°+300,140(]℃
at the above flow rate 8F'6 and 5oaz,/o,
After flowing 1t for 5 hours, the temperature was raised to the transparent vitrification temperature as Hθ glass to obtain a transparent glass body. △n (F) of the fiber obtained at each treatment temperature is 6th
Example 5 The quartz-based soot matrix for single mode fiber produced by the method shown in Fig. 6 was heat-treated in an alumina furnace tube.
Furnace atmosphere with He gas 5F61i 500cc
/min, C4 was flowed at a flow rate of 5 oar, 7 minutes. Furnace temperature is 1
The temperature was raised from 100° C. to 1400° C. at a temperature increasing rate as shown in FIG. 7, and then the steel base material was made into transparent vitrification in an atmosphere containing only an inert gas such as He. Difference in refractive index of the fiber (capital) Δn (F
) is shown in Figure 7.

(Effects of the Invention) The effects of the present invention that are clear from the above Examples and Comparative Examples are as follows.

0) According to the heat treatment of the present invention in a gas atmosphere coexisting with a fluorine-based gas and a chlorine-based gas, transmission loss can be significantly reduced compared to the case where a chlorine-based gas does not coexist (Comparative Examples 1 and 2). This is considered to be because the addition of the 4 gases eliminated absorption originating from impurities (Example 1 and Comparative Examples 1 and 2).

(b) Regarding fluorine gases, even 81F4 is SF@
However, there is no difference in the amount of fluorine added to the obtained glasses (see Δn(F) in Examples 2-2 and 2-3), and there is no difference in transmission loss either, and the glass has the same effect.

(U) The method of the present invention is applicable when heating uses a uniform heating furnace (
Both in Example 1) and in the case of using a zone heating furnace (Example 3), the same effect is shown with respect to the addition of fluorine.

(b) It can be seen that the heat treatment temperature range in which fluorine is effectively added by the method of the present invention is 1100 to 1400°C (Example 4).

(e) The slower the temperature increase rate in the heat treatment, the more effective it is for fluorine addition. In Example 5, when the heating rate is 2°/min, the amount of fluorine added is approximately four times that when the heating rate is 100/min.

In other words, according to the method of the present invention, the deterioration of the furnace material is suppressed compared to the conventional method, the obtained fiber does not absorb impurities derived from Ou and Fθ, fluorine is efficiently added, and the obtained fiber is Fibers made from this material have stable transmission properties.

[Brief explanation of drawings]

Figure 1 (a) shows the general refractive index distribution of single-mode fiber, (b) shows the general refractive index distribution of multimode fiber Z-, and Figures 2 (a) to (d) show the fluorine-doped fiber in the cladding. A diagram showing the refractive index distribution of a low-dispersion fiber, Figures 3 (a) and (kl) are explanatory diagrams of the method for producing a cysuto base material by flame hydrolysis, and Figure 4 shows Example 1 and Comparative Example. Figure 5 shows the refractive index distribution of the fibers obtained in Examples 2 and 3. Figure 6 shows the heat treatment temperature and temperature of Example 4. FIG. 7 is a graph showing the relationship between the refractive index difference Δn (F) of the obtained fiber, and FIG. 7 is a graph showing the relationship between the heating rate of Example 5 and the refractive index difference Δn (F') of the obtained fiber. be. Agents 1) Akira's agent Ryo Hagiwara - Figure 1 Figure 3 (a) Figure 4 Figure 5 Procedural amendments Manabu Shiga, Commissioner of the Patent Office 1, Indication of the case September 1, 1988, Application No. No. 194102 No. 2, Name of the invention, Method for manufacturing a glass base material for optical fiber 3, Person making the correction Relationship with f'l! t, 'j patent applicant (1 ('li
5-15 Higashikyochohama, Osaka-shi Yorokane (213) Sumitomo Electric Industries, Ltd. (N) I4, J Representative's Kamitetsubu (1 (Gaichishima) 4, Agent 11 +'li Minato-ku, Tokyo) Toranomon-Cho 1” 16 No. 2 l Subject of amendment (1) “Detailed description of the invention” column of the specification (2) Contents of amendment to drawing a (1) Table 1 on page 18 of the specification, Table 2 on page 20 and Table 3 on page 25 are corrected as shown in the attached sheet. (2) Page 6 and Figure 7 of the drawings are corrected as shown in the attached sheet.

Claims (3)

[Claims] (1) In the transparent vitrification process of glass fine particles whose main component is quartz, the fine particles are heat-treated in a gas atmosphere in which at least a fluorine-based gas and a chlorine-based gas coexist. A method for manufacturing a glass base material for optical fiber. (2) The method for producing a glass preform for optical fiber according to claim 1, wherein the heat treatment temperature is 1100 to 1400°C. (3) The method for producing a glass base material for optical fiber according to claim 1, characterized in that the heat treatment is performed by raising the temperature to 1100 to 1400° C. at a temperature increase rate shown in FIG. ,.