JPS6117432A - Manufacture of optical fiber preform - Google Patents

Abstract

PURPOSE:To produce a preform for single mode optical fiber having low transmission loss, by carrying out the deposition of pure glass soot using a hydrocarbon or CO as the combustion gas in the first stage, and using an oxyhydrogen flame in the second stage and thereafter. CONSTITUTION:SiCl4 is introduced into a flame generated by using a hydrocarbon or CO as the combustion gas to deposit pure SiO2 glass soot to the outer surface of a high-purity quartz glass rod. The obtained composite is heated in an atmosphere containing at least a fluorine compound gas to obtain a composite composed of the high-purity quartz rod and a fluorine-containing quartz glass layer surrounding the core quartz rod. The composite rod is drawn to a prescribed diameter. A pure SiO2 glass soot layer is deposited to the outer surface of the rod using an oxyhydrogen flame, and the product is heat-treated in an atmosphere containing at least fluorine. If necessary, the above procedure is repeated twice or more.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present FJA relates to a novel method for manufacturing a glass base material for a single mode optical fiber with low loss. %, the present invention has a structure shown in the solid line in FIG. 1, and the core is made of pure silica glass,
This invention relates to a method for producing a glass base material for a single mode optical fiber whose cladding is made of fluorine-doped quartz glass. Note that the glass preform for single mode optical fiber according to the present invention also includes one having a quartz jacket # (indicated by a broken line in FIG. 1) on the outside of the cladding layer shown in FIG.

(Prior Art) Regarding a method for manufacturing a single mode optical fiber having a structural metal as shown in FIG. 1, as already described in various publications, There is a method of adhering and coating a granular glass material having a refractive index lower than the refractive index of the material, and then heating and stretching the material.

In order to explode an optical fiber having the structure shown in Figure 1 according to the method described in the above publication, first, all pure silica glass rods were prepared and put into a flame containing 5lcz4 and a fluorine compound metal, and synthesized by a flame hydrolysis reaction. After depositing fluorine-containing silica fine particles on the outside of the quartz glass rod, the rod is heated and sintered to form transparent glass. However, the above method has a major drawback as a practical means of production. That is, when fluorine is introduced into the silica fine particle formation process, the nucleus growth of the silica fine particles is suppressed, and the substantial growth rate of the glass fine particle deposit is reduced.

FIG. 5 is a graph of experimental data showing this fact,
The horizontal axis of Figure 4 shows the fluorine atom concentration (%) in the glass raw material gas.
The growth rate (f/) of the glass particle deposit is plotted on the vertical axis.
minute] is taken.

As is clear from this graph, it can be seen that as the fluorine atom concentration in the frit gas increases, the growth rate of the glass fine particle deposit decreases.

Therefore, in the above method, it can be said that there is a practical limit to the amount of fluorine that can be contained in the quartz glass.

On the other hand, the transmittance of silica glass is highest in the wavelength band where single-mode fibers are practically used.
This is a long wavelength region of 0 μm to 1.7 μm. In order to obtain a low-loss fiber in this wavelength range, it is necessary to reduce absorption derived from OH groups as much as possible. As described above, before depositing fluorine-containing glass particles as a cladding layer on the outside of the transparent glass base material to serve as the core, the transparent glass base material for the core is stretched to a predetermined diameter. However, in this process, an oxyhydrogen flame is often used as a heat source for stretching. However, high-temperature water vapor from the oxyhydrogen flame penetrates the glass surface and remains in the glass as OI (OI), causing absorption loss in the long wavelength band.

Furthermore, in order to obtain good transmission loss characteristics of a single mode fiber, it is necessary to increase the ratio of the diameter of the synthesized cladding to the diameter of the core. This is because the single-mode core ino (in negative odor, the transmitted fundamental mode) has a large expansion, and is affected by the outer quartz pipe, resulting in deterioration of transmission loss. It is experimentally known that the diameter needs to be at least 6 to 7 times the core diameter.In particular, in the fiber structure targeted by the present invention as shown in Figure 1, the fundamental mode is strictly Since it can only propagate as a leaky mode, it is essential to have a large composite rad diameter.

The problems with the conventional method described above are: (1) a decrease in the growth rate of glass particles due to the introduction of fluorine compound gas; (2) an increase in absorption loss due to OH groups resulting from an oxyhydrogen flame as a stretching heat source; Furthermore, we have proposed a new optical fiber manufacturing method in which the diameter of the zero composite rad is equal to the core diameter. The method described in the above specification basically involves depositing pure silica glass fine particles on the outside of a surface-treated rod-shaped high-purity quartz glass, and then heat-treating it in an atmosphere containing at least a fluorine compound gas. This is an effective method for solving the above-mentioned problems (■ to ■) in the conventional method, but the resulting fiber has some residual O.
There is a problem in that transmission loss may increase due to the H group. The inventor of the present invention investigated the cause of this residual OH group and obtained the following findings.

That is, in the process of depositing pure silica glass fine particles on the outside of a rod-shaped starting quartz glass using a flame hydrolysis reaction, when horse gas gold is used as the flame fuel gas, unreacted H1 molecules contained in the flame are removed from the starting quartz glass. It was found that 5i-OH bonds were diffused into the interior of the glass rod from the surface thereof, and all 5i-OH bonds were formed in the quartz glass, which could not be completely eliminated in the subsequent heat treatment step and remained.

Here, the reason for using H gas in the pure silica glass deposition process is that the substances produced by the combustion reaction of H3 gas are only H and O. This is because it is easy to make the concentration of impurities other than H and O extremely low. As for other fuel gases, hydrocarbon gases such as OH4 gas, which are abundant in resources and easy to obtain because they are inexpensive, are sometimes used. Easily deposited in fine particle deposits. In particular, when trying to increase the glass particle deposition rate IKt by supplying a large amount of glass raw material into the flame with the aim of improving productivity, it is necessary to supply multiple combustion gases, making it more difficult to prevent carbon precipitation. become.

It is also conceivable to use CO glass, which does not contain atoms, as the combustion gas. However, when using CO glass, the vitrification reaction of 51cz4, which is a glass raw material, is 5i074 + O-tomi → 8101 +
2. This is an oxidation reaction known as snow, and the reaction efficiency is lower than the flame hydrolysis reaction when H: or hydrocarbon gas is used, and it is disadvantageous for increasing the deposition rate of glass particles aimed at improving productivity. be.

On the other hand, the advantage of using hydrocarbon gas kumquat as the combustion gas is that the probability of a 1 cHz molecule existing in the flame can be expected to be much lower than when using H, gas as the combustion gas, and H! O due to diffusion
There is a possibility that H group contamination can be reduced.

Further, an advantage of using CO glass as the combustion gas is that since no H, molecules or H,0 molecules are present in the flame, it can be expected that there will be no OH group contamination of the starting quartz glass rod.

[Problem that the invention seeks to solve]

The object of the present invention is to solve all of the above problems, reduce OH contamination of the starting quartz glass rod without reducing productivity, and provide a low-loss single-mode optical fiber preform.

[Means to solve all problems]

That is, in the present invention, (A) 5lc42 is introduced into a flame whose combustion gas is hydrocarbon gas or carbon monoxide gas, and pure S
After generating iO2 glass particles and depositing the SiO2 glass particles on the outside of a high-purity quartz glass rod,
By heat-treating the composite of the high-purity quartz glass rod and the SiO2 glass fine particle deposit in an atmosphere containing at least a fluorine compound gas, the high-purity quartz glass rod and the fluorine-containing quartz glass layer surrounding it are combined. After forming a composite, (B) the glass composite is stretched to a predetermined diameter, and the outside of the glass composite is coated with pure 810 with an acid/hydrogen flame.
The step (B) consists of forming a rich glass fine particle deposit, and heat-treating a further composite of the glass composite and the pure SiO2 glass fine particle deposit in an atmosphere containing at least fluorine. Provided is a method for manufacturing an optical fiber preform, which is characterized in that it is repeated two or more times.

The basic structure of the present invention is K SiC! in a flame consisting of hydrocarbon gas or CO glass oxygen! Introducing 4 to make pure 8
10. After generating glass fine particles and depositing the pure S10 glass fine particles on the outside of a rod-shaped high-purity quartz glass, the high-purity glass rod and pure quartz glass were combined.

The composite bar of the glass fine particle deposit layer is heat-treated in an atmosphere containing at least a fluorine compound gas to make the pure 5102 glass fine particle deposit layer contain fluorine and turn it into transparent glass, forming a high-purity quartz glass part. After forming a composite glass body consisting of a fluorine-containing quartz glass portion that surrounds the entire body, the composite glass body is stretched to a predetermined diameter, and then pure 5102 is coated on the outside of the composite glass body with an oxyhydrogen flame. The process consists of depositing glass particles and heat-treating the composite glass body and pure 5102 glass particle layer again in an atmosphere containing at least a fluorine compound gas.

The features of the present invention are: I) In order to obtain a sufficiently thick cladding layer containing all fluorine, pure s1 is deposited on a transparent glass rod.
o. The heating treatment for forming the glass fine particle deposit layer, nuclear purity, and adding fluorine to the glass fine particle deposit layer should be repeated two or more times. H) The first glass treatment has a large effect on OH absorption loss. To form a fine particle deposit layer, use a hydrocarbon gas with little OH contamination to the starting quartz glass rod, or a CO glass total combustion gas without OH contamination to the starting quartz glass rod, +++)on Effect on absorption loss. The purpose is to use hydrogen gas as the combustion gas, which is advantageous in improving the overall productivity of forming a small glass particle deposit layer from the second time onward.

The present invention will be specifically explained below.

A high-purity quartz glass rod, which corresponds to the core of an optical fiber, is
It can be produced using a normal VAD method using only the glass raw material f5iOt4, and the amount of residual OH groups contained in the high purity quartz glass rod can be reduced to the ppb level. This high-purity quartz glass rod is stretched to a predetermined outer diameter using a method that does not allow OH groups to penetrate from the rod surface. This processing method includes a method in which a high purity quartz glass rod is heated and stretched using a heat source that does not generate water vapor, such as a plasma flame or an electric resistance furnace. When the obtained high-purity glass rod and pure 5102 glass fine particle composite are heat-treated in a total fluorine compound gas atmosphere, S? , .

By placing the composite in a mixed gas atmosphere of a fluorocarbon gas such as 01F, , C, F, etc. and an inert gas such as helium, a sufficient amount of fluorine can be deposited in glass fine particle deposition 1-V.
C can be added substantially uniformly.

At this time, since the fluorine compound gas also serves as a dehydrating agent, it is thought that by supplying a sufficient amount of the fluorine compound gas into the atmospheric gas, water or OH groups in the glass fine particle deposit layer can be sufficiently removed. However, it is also possible to make the dehydration more complete by flowing another dehydrating agent such as a Ct winnow into the atmospheric gas until the shrinkage of the glass fine particle deposit layer does not proceed.

〔Example〕

Example 1 A fully dehydrated pure 810. A glass base material was produced.

The core base material was stretched using a high frequency plasma flame of f 4 MHz and 50 kW to obtain a quartz glass rod with an outer diameter of 12 mm. Next, using the apparatus shown in FIG. 2, a pure 810 g glass fine particle deposition layer 2 was formed on the outer circumference of the pure silica glass rod 1. 5 is a burner for synthesizing glass particles, and the burner 5 is charged with 8104 aH425t/min as a combustion gas and 0s10t/min as an auxiliary combustion gas as a glass raw material.
500 cc/min was fed, and 810-rich particles were formed in the flame and deposited on the glass rod 1. At this time, the glass rod 1 is rotated by the rotating/pulling device 4 via the connecting rod 5t- and gradually pulled upward, thereby depositing glass fine particles *2t- on the pure quartz rod 1 in the axial direction.
It was formed. The glass particle deposition rate at this time is (L
It was 5t 7 minutes.

The outer diameter of the resulting composite of the glass fine particle deposit layer 2 and the quartz glass rod 1 was 75 wmφ. the complex f8F,
Transparent vitrification was performed while heat-treating in an atmosphere with a volume ratio of 2:50 and He. The base material diameter after transparent vitrification is 30
■It was φ.

Furthermore, the obtained base material was heated again with plasma flame for 11 seconds.
φ, and a layer of pure S4O glass particles was formed on the stretched base material using the apparatus shown in FIG. 2 using H gas as a combustion gas.

The flow conditions at this time were H, 2at 1 minute, 0.16t/
min, B10t4 450 cc/min. The outer diameter of the pure SiO2 glass fine particle layer was 120 mm. In addition, the pure SiO2 glass fine particle deposition rate was 1.5
It was t/min.

The outer diameter of the obtained base material was 44 mm. After fully stretching the base material and covering it with a commercially available natural quartz pipe, the outer diameter was 125-
optical fiber. FIG. 3 shows the refractive index distribution of the optical fiber. In FIG. 5, 1' is the pure silica glass part of the starting rod, 2' is the part where the glass fine particle deposited layer synthesized using OH4'ii was heat-treated in an atmosphere containing fluorine compound gas, and 21 is the part where Ha is used. The part where the synthesized glass fine particle deposit layer was heat-treated in an atmosphere containing fluorine compound gas gold, and 6 correspond to the part of the natural quartz tube, respectively. The relative refractive index difference between the cladding and the core is 132%, and the cladding diameter/
The core diameter ratio was 110. This fiber has a single length of 5km
, the cutoff wavelength is 1.15 μm, and the transmission loss at wavelengths of 1 and 3 μm is good, less than a5 dB/km, and the wavelength is 1.3 μm.
The increase in OH absorption loss at 9 μm was also 5 dB/kI11, which was not a problem for practical use.

Example 2 As in Example 1, the outer diameter was 40φ by the VAD method.

Length 200. A fully dehydrated pure s1o glass base material was prepared, and the base material was stretched using a high frequency plasma flame of f4 MHz and 301 cW to form a quartz glass rod with an outer diameter of 12-. A pure 810° glass fine particle deposit layer 2 was created on the outer periphery of the quartz glass rod 1 using the apparatus sound shown in FIG. Here, unlike in Example 1, CO glass was supplied as combustion gas to the burner 5 for glass particle synthesis. At this time, the supply amount of each gas raw material to burner 3 is C
O gas 10t/min, 0! Gas 10t/min, 810A
4250cc/min, and the glass particle deposition rate is α1
f/min. The outer diameter of the resulting composite of the glass fine particle deposit layer 2 and the quartz glass rod 1 was 60-. The composite was made into transparent glass in the same manner as in Example 1. The diameter of the base material after transparent vitrification was 24-.

The base material was stretched to 15tIIIφ, and using the apparatus shown in FIG. 2, a second glass fine particle deposit layer was formed and transparent vitrification was performed under the same conditions as in Example 1 using salt gas as the total combustion gas. Ta. The outer diameter of the obtained base material is 45 fins
Met.

Furthermore, the base material f 25 Bφ was stretched again, a fifth glass fine particle layer was formed, and transparent glass was obtained to obtain a transparent glass base material with an outer diameter of 50 mmφ. After covering the base material with English pipe, the outer diameter is 125
The optical fiber has the state φ.

The relative refractive index difference between the cladding and core of the obtained fiber is 0.
．． 32%, and the cladding/core diameter ratio was 12 times. This fiber has a cutoff wavelength of 1.1/ltm and a wavelength of 1.5
The transmission loss id at 11 m is good (below 145 dB/km), and the increase in OH absorption loss at a wavelength of 1.59 μm is 1 aB/km, which is even more residual oy+ than in Example 1.
We have been able to reduce the amount.

〔Effect of the invention〕

In the method of the present invention, the increase in absorption loss caused by OH groups originating from an oxyhydrogen flame is reduced by using hydrocarbon gas or CO glass combustion gas for the first pure glass particle deposition, and for the second and subsequent depositions. An oxyhydrogen flame is used to deposit pure glass particles, which improves productivity. Furthermore, the diameter of the fluorine-containing cladding is more than 6 times the diameter of the core, making it possible to stably produce single-mode fibers with excellent transmission characteristics. Valid for
And it is an economical method.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the refractive index distribution of a single mode optical fiber, which is the object of the present invention. FIG. 2 is an explanatory diagram of the process of producing a glass fine particle deposit body in an embodiment of the present invention. FIG. 5 shows the refractive index distribution of the original fiber obtained in an example of the present invention. FIG. 4 is a transmission loss spectrum of the optical fiber of FIG. 3. Figure 5 shows the fluorine atom concentration (%) in the frit gas and the glass particle deposit growth rate (
Graph showing the relationship between f/min. Figure 1 Figure 2

Claims (1)

[Claims] (1) (A) SiCl_4 is introduced into a flame with hydrocarbon gas or carbon monoxide gas as combustion gas, and pure SiO_2
After generating glass particles and depositing the SiO_2 glass particles on the outside of a high-purity quartz glass rod, the composite of the high-purity quartz glass rod and the SiO_2 glass particle deposit is placed in an atmosphere containing at least a fluorine compound gas. After forming a composite of the high-purity quartz glass rod and the fluorine-containing quartz glass layer surrounding it by heat treatment, (B) stretching the glass composite to a predetermined diameter; A pure SiO_2 glass fine particle deposit is formed on the outside of the body using an acid/hydrogen flame, and the glass composite and the pure Si
further comprising heating the composite body with the O_2 glass fine particle deposit in an atmosphere containing at least fluorine,
A method for manufacturing an optical fiber preform, characterized in that step (B) is repeated two or more times as necessary.