JPS6086046A - Manufacture of glass preform for optical fiber - Google Patents

Abstract

PURPOSE:To manufacture the titled preform containing increased amount of fluorine and free from the contamination with impurities, by heat-treating fine glass particles composed mainly of SiO2 in a gaseous atmosphere containing both fluorine-containing gas and chlorine-containing gas. CONSTITUTION:Fine glass particles composed mainly of SiO2 and produced by the flame-hydrolysis reaction or the solution hydrolysis reaction such as the sol- gel process, are heated in an inert gas atmosphere admixed with a fluorine- containing gas (e.g. SF6) and a chlorine-containing gas (e.g. Cl2) at 1,100- 1,400 deg.C, and then converted to transparent glass by heating in an inert gas atmosphere free from the additives at >=1,400 deg.C. The amount of fluorine added to the fine glass particles can be increased, the contamination of the glass with impurities during the fluorine-addition process can be prevented, and a fluorine-containing glass preform can be manufactured without damaging the surface of the glass preform or the furnace core tube by etching. An optical fiber having stable transmission characteristics can be manufactured from the preform.

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 glass base material for optical fibers made of silica glass to which fluorine is added as a refractive index difference adjusting agent. .

(Prior art) The gas base material for the optical eyeper consists of a core part and a cladding part.The core part is located in the center and has a higher refractive index than the cladding part, for example, in order to facilitate the transmission of light. In the refractive index difference distribution structure of the optical fiber shown in FIG. 81, As is defined as the core part and part B is defined as the cladding part.

To increase the refractive index of the core (based on silica), GeO, A/2 is usually used as a refractive index increasing dopant.
03. Add TiO2 or the like to quartz glass. but,
Addition of the above-mentioned oxides causes the following drawbacks.

Light scattering (Rayleigh scattering) originating from the dopant is also increased in proportion to the dopant additive force 11jtK, which is unfavorable in terms of optical transmission.

Furthermore, when dopants are added in long and short periods, bubbles and crystal phases are generated in the glass base material. If you keep it, it will be Ge. If it is O2, it will be G.
Based on oo gas generation (if the bubble is AI!203, A
I! Clusters of 203 crystals are likely to occur. This is unfavorable for the optical transmission characteristics and the strength of the resulting optical fiber.

For this reason, it can be understood that it is preferable for the glass composition of the core-corresponding portion to have as little dopant as possible (or to use pure silica glass).

As one method of overcoming the above problems and obtaining a difference in refractive index, a glass base material for optical fibers has been considered in which fluorine, which has the effect of lowering the refractive index, is added to the quartz glass of the cladding.

An advantage of using fluorine as a dopant is that the refractive index of the cladding can be lower than that of pure silica, so pure quartz or quartz glass doped with a small amount of dopant can be used for the core. FIG. 2 shows the refractive index difference distribution of the F-doped silica glass fiber, and assumes that the refractive index difference of the silica glass is 0. For this reason, the scattering of light (Rayleigh scattering) originating from the dopant in the core, which is the part through which light passes, is reduced, making it preferable as an optical transmission path.
It is economically advantageous in that it is more abundant in resources than dopants such as O2, and the raw material can be easily purified. In addition, one of the characteristics of fluorine gas is that it is excellent not only as a dopant raw material but also as a dehydrating agent for removing water contained in soot.

Several methods have already been disclosed for adding fluorine into quartz glass.

For example, Japanese Patent Publication No. 55-15682 describes a method in which fluorine is added to glass by applying a fluoride gas in the step of forming the glass in a vapor phase. Although this method does add fluorine to the glass,
The disadvantage is that the glass deposition efficiency and the fluorine addition efficiency (doping yield) are low.

This means that in the flame hydrolysis method using H2-0□ flame, the water (H2O) present in the flame and the fluorine gas (for example, 5F6) react as shown in the equation (υ), and SF6+ 5H
This is thought to be due to the generation of 20-+ So5+6HF (17HP gas. This HF gas is stable, and at high temperatures and as long as there is moisture, most of the fluorine-based gases are converted to HF gas, leaving only a small amount. Only fluorine-based gases can be used as dopant raw materials.

Furthermore, this HF has the effect of corroding glass, especially quartz, and can easily be combined with silica particles generated in the flame (see (2) below).
The reaction occurs as shown in equation (3), and the produced glass particles are consumed, thereby suppressing soot deposition.

5in2(sJ+2HF(g)−+SiOF2(g)
+H20 (gJ (2) (BJ group (g) gas s iO2 (S) + 4HF (g) → SiF4 (g)
+2H20(gJ (3J) Therefore, if the amount of fluorine-based gas added IJJ is increased, the soot deposition rate will actually decrease.

In addition, Japanese Patent Application Laid-Open No. 55-67555 discloses that glass fine particles (referred to as soot base material) are produced by flame hydrolysis method, and the obtained soot base material is heat-treated in an atmosphere containing fluorine gas. A method of doping fluorine into the soot and thereby obtaining a fluorine-doped glass base material is shown.

However, this method also has some disadvantages. As one of the methods described in the above publication, soot is treated in a fluorine-based gas atmosphere at a temperature of 1000 C' or less,
The rate of fluorine addition was slow (furthermore, KCu and F6 were sometimes present in the obtained fibers. Gu and Fθ are known to cause absorption loss, which causes an increase in loss.

Furthermore, it is also described that the soot base material is treated in a gas atmosphere containing fluorine gas at a temperature of 14,000 degrees or higher, but the surface of the obtained glass base material is etched and the furnace tube used to maintain the atmosphere is made of quartz, for example. The core tube was also severely damaged by etching. It is believed that such etching was one of the factors that promoted the contamination of impurities in the core tube into the soot base material.

Furthermore, the fiber obtained by the above method has the problem that the absorption loss due to the 0H group changes over time, and when the temperature is high (the increase in absorption loss becomes significant).

As described above, the conventional methods of adding fluorine to the silica glass cladding have various problems.

(Objective of the Invention) The present invention overcomes the drawbacks of the glass base material doped with fluorine by the above-mentioned conventional method, increases the fluorine addition rate to the glass fine particles, and prevents impurities such as Fe and Gu from being added to the glass during the fluorine addition process. The purpose of the present invention is to provide an optical transmission glass fiber that prevents contamination and has more stable transmission characteristics.

(Structure of the Invention) The gist of the present invention is the flame hydrolysis reaction or F2O3-G
SiO formed by solution hydrolysis reaction such as e/method
□ Glass base material for optical fiber containing fluorine is heat-treated in a gas atmosphere containing a small amount of fluorine-based gas and chlorine-based gas. It provides a manufacturing method.

The present invention will be specifically discussed below.

Preparation of soot base material In order to generate quartz glass fine particles by a flame hydrolysis reaction, a quartz concentric multi-tube burner 1 is used as shown in Fig.
ICj'4 + or S I C14*Gθct4. h
Using a mixed gas such as zct3 and SF6.

Ar gas may be used as a carrier gas and fed into the center 5 of the oxyhydrogen flame to cause a reaction. 4 in the figure flows Ar gas as a shield so that the raw material gas reacts in a space several meters away from the tip of the burner 1. When obtaining a rod of glass fine particles, the glass fine particles are laminated in the axial direction from the tip of the rotating starting member 6. Further, when obtaining a pipe-shaped glass particle body, the glass particle body is laminated on the outer circumference of the rotating quartz rod or carbon rod 7 while traversing the burner 8, and then the central member is removed. Note that 7 may be a glass base material for the core (in this case, it is not necessary to pull out the central part. Also, a plurality of burners 8 may be used).

A soot matrix similar to the method shown in FIG. 3 can also be obtained by the Alcolade hydrolysis method, and this method is called the Soj'-Gem method.

The soot base material obtained by the above method had a refractive index distribution as shown in FIG. 4, for example.

The soot base material obtained by the above method was inserted into a furnace tube made of pure quartz, and heated at a temperature increase rate of 1400 in the range of 2 to 10 C/min in an inert gas atmosphere containing SF6 and C12 gas 'f.
After raising the temperature to C, the soot base material was transformed into transparent glass in an atmosphere containing only an inert gas such as He at a temperature that increased the soot surface to 1400 t:' or higher. The obtained glass base material has fluorine added thereto, and an example of its refractive index distribution structure is shown in FIG.

The above will be specifically explained with reference to examples.

Example 1-1 A pure 10 mmφ quartz glass rod doped with 17 wt% of GeO2 was used as the starting member A, and a pure 5 in2 quartz glass rod was deposited on its outer periphery using a flame hydrolysis reaction. A soot deposit 1c having the structure shown in FIG. 4 (aJK) was obtained.
% and 20 mol of SF6! The temperature was raised from 800C to 1400C in an atmosphere containing He 2 to form a transparent glass. The refractive index distribution of the obtained glass base material was as shown in FIG. 5 (aJ).

Example 1-2 Using carbon rod A with a diameter of about 6 mm as a starting member,
A layer of carbon powder is made in advance on the rond using an acetylene flame, and a layer of soot B made of pure 5102 is placed on top of this.
was deposited to obtain a soot deposit with the structure shown in FIG.
The temperature was raised from 800 C to 1400 p in a He 2 atmosphere to which 10 mob % of SF6 and at% were added to form transparent glass. The refractive index distribution of the obtained glass base material is shown in Figure 5 (b
It was as shown in J.

Example 1-5 A silica glass rod A having a refractive index distribution shown in M4 diagram (0) doped with cteO2 in a range of 0 to 17 wt% was used as a starting member, and a flame hydrolysis layer reaction was applied to the outer periphery of the rod. was used to deposit pure 5in2 sulfur). The mantissa soot deposit is O/2 at 1 mo1%.
The temperature was raised to 800 to 1,400 C in an Hθ gas atmosphere to which 20 mat% of SF6 was added, and transparent glass was formed.

The refractive index distribution of the obtained glass base material was as shown in FIG. 5(c).

Characteristics of the Obtained Fibers The characteristics of the fibers obtained from the glass base materials according to the methods of Examples 1-1 to 1-3 above are such that there is no increase in absorption due to impurities, and the loss is sufficiently low (for example, 1. 50μm
(about 0.5 dB/hs), and the absorption peak due to OH group did not change over time.

Here, the method of the present invention is not limited to the above description (OF4.F2°SiF4. and Cj o F2, etc. can be used as the fluorine-based gas, and 5oal!2.aoal!2. ccz4 etc. are used.

Furthermore, similar fluoridation caps and fiber properties can be obtained even when fluoridation treatment and transparent vitrification are performed using different heat-generating furnaces.

Experimental Example 1 Figure 6 shows 1 moI of chlorine gas in inert gas! %, S
10 mol of F6! % of the refractive index difference (Δn-%) obtained as a result of an experiment held at a predetermined temperature shown on the horizontal axis of the figure for 5 hours in an atmosphere containing . From this result, the temperature is 110
It has been found that it is efficient to fluorine the soot in the range of 0 to 1400C.

Experimental Example 2 (A) In a gas atmosphere where Hθ gas flows at a flow rate of 151/min, C12 gas is 1 rno1% SF6 is 5 m
If it is o/%, (5 mol of BJ OJ gas!
5mol of %SF6! %, and (0) CI
! 2 gas 1 rnol! The refractive index difference -Δn- was measured by changing the heating rate in the range of 2 to IOC/min for the case where 20 mo1% of %8F6 was contained. The results of the experiment are shown in FIG. It can be seen from this graph that the slower the temperature increase rate, the greater the decrease in the refractive index, that is, the greater the amount of fluorine added to the glass.

Comparative Example 1 A soot base material was heat-treated in an atmosphere of fluorine-based gas (for example, only 8F6) without adding chlorine gas. in this case,
Etching of the core tube significantly shortened the life of the core tube compared to when chlorine gas was added. When the glass base material treated in such an atmosphere was made into a fiber, the diameter was 1.1μ.
An absorption peak believed to be derived from Fe or Cu was observed near the m wavelength. Furthermore, when the fiber obtained by this method was heated to 200C and held for 2 hours, the absorption loss due to OH groups increased more than 10 times.

Further, the glass base material was etched.

This is thought to be because the moisture in the soot and the moisture in the mixed air react with the fluorine gas to produce fluorine (HF), which corrodes the glass base material and the quartz glass tube.

On the other hand, the life of the quartz tube is extended when C12 gas is added because O12 gas removes moisture in the atmosphere from hydrochloric acid (8
This is thought to be to suppress the production of hydrofluoric acid (Cl) and hydrofluoric acid. It is known that H(7?) has almost no etching effect on quartz.

Furthermore, the reason why impurities do not enter in a chlorine atmosphere is that impurities such as Cu 'p Fe that were originally contained in the soot base material are
By the reaction shown in the following formula, cu cl! 2 or Fe
This is probably because it becomes a volatile gas such as 015 and is easily transported to the outside of the processing system and removed efficiently.

cuo(sJ +Cl2-au012 + !A02F
es+03(s)+5012→2Fe(j15+
7'202-10,000F2 For gas, use 0uF2 or Fe
Even if Fs is generated, it is solid and not volatile.

The reason why the absorption peak due to OH groups in fibers treated using only fluorine gas in the sintering process becomes unstable is that H atoms are not completely released from the glass due to the above-mentioned embedding, and 5i- It exists in a metastable form other than an OH bond, and this H atom forms an S knee OH bond under heating, so it is thought that the absorption peak decreases with heating. On the other hand, when a heptadium-based gas or a chlorine gas is allowed to coexist, the H atoms are almost completely removed from the glass, so it is considered that no fluctuation occurs in the OH group absorption of the obtained fiber.

According to experiments by the present inventors, it is preferable that the chlorine gas added to the inert gas is up to 2 mo/%, and the fluorine gas added to the inert gas is preferably up to 50 mo/% so that bubbles are less likely to remain.

Comparative Example 2 Using OF4 as the fluorine-based gas, the flow rate was set at 5 mo1% with respect to He as the inert gas, and the flow rate was 80%.
Heat treatment was performed by raising the temperature from 0 to 1400C at a rate of 5CI. When the obtained glass base material is made into a fiber, it has extremely large loss characteristics and its origin is due to structural irregularities.
One possibility was thought to be the presence of carbon particles in the fiber. This fiber has a step-type refractive index profile and a loss characteristic of 56B / 1.30 μm.
It was b.

Example 2 In the method of Comparative Example 2 above, 02 gas was further added to 7
The heat treatment was performed under the same conditions except that 1% of mo was added.

The loss characteristic of the obtained fiber was 0.5 dB/ at L50 μm, which was much lower than that of the comparative example.

As is clear from the results of Example 2 and Comparative Example 2, when OF4 containing carbon atoms is used, an additional 0□ is added to the gas atmosphere.
By adding , it is possible to suppress an increase in transmission loss due to structural irregularities caused by carbon particles and achieve low loss.

(Effects of the Invention As detailed above, the method of the present invention increases the fluorine addition rate to glass fine particles, prevents the contamination of impurities during fluoride addition treatment, and etches the surface of the glass base material and the furnace tube. It is an excellent method to obtain a fluorine-doped glass matrix without any damage, and the fibers made from the resulting matrix have stable fc transmission properties.

[Brief explanation of the drawing]

Figure 1 (aJ is a single mode fiber, (b is a diagram showing the general refractive index distribution of a 7-hammer multimode fiber, A diagram showing the distribution, Figure 3 (b) is an explanatory diagram of the method of producing a soot base material by the flame hydrolysis method, and Figure 4 (a) to (0) are examples 1-1 to
FIG. 5 is a diagram showing the structure when soot is deposited on the starting member in Example 1-5, and FIG.
1-3 shows the refractive index distribution of the glass base material obtained by heat-treating the glass base material in Fig. 4 (aJ ~ (0) The graph showing the relationship between fiber refractive index difference Δn (FJ and FIG.
This is a graph showing the relationship between tD W Mr "4 difference Δn (F). Agents 1) Akira Agent Ryo Hagiwara - Figure 1 Figure 2 (Q) Figure 4 Figure 5

Claims (1)

[Scope of Claims] (υ Tuf?, glass particles mainly composed of 5jO2 formed by a hydrolysis reaction or a solution hydrolysis reaction such as the E3o14el method, in which at least a fluorine-based gas and a chlorine-based gas coexist A method for producing a glass base material for an optical fiber containing fluorine, which is characterized by heat treatment in a gas atmosphere of
A method for producing a glass preform for optical fiber containing fluorine according to claim 1, characterized in that C. (3) A method for producing a glass base material for optical fiber containing fluorine as set forth in claim 1, characterized in that CF4 is used as the fluorine-based gas.