JPS60180927A - Production of base material for optical fiber - Google Patents

Abstract

PURPOSE:To vitrify thoroughly a large-sized base material as well to transparent glass by heating the porous optical fiber base material synthesized at a high speed by a burner having a multiple flame construction under specific conditions. CONSTITUTION:A raw material 3 is flame-decomposed (a symbol 2 is an inside flame layer, 4 an outside flame layer) by using a multiple-flame burner 1 provided with an outside nozzle for synthesis on the outside of concentrical multiple nozzles (both nozzles mentioned above are constituted by combining respectively nozzles for supplying the raw material for glass, supplying a combustible gas, supplying a combustion supporting gas and supplying an inert gas) and the pulverous particles of the resultant product are deposited to manufacture a porous glass base material 5 for an optical fiber. The resultant porous glass base material is put into an electric furnace and is heated at a heating rate of <=5 deg.C/min, by which the material is vitrified to transparent glass.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for transparently vitrifying a porous preform for optical fiber synthesized at high speed using multiple flame burners.

[Prior art]

Conventionally, the method for manufacturing optical fiber base material is MC.
VD method (Modified Chemical Vap
orDeposition), VAD method (Vapor-
Phase Axial Deposition) and the like are known, and by these methods, it is possible to manufacture optical fibers with excellent optical loss and band characteristics. The current concern in industry is to produce optical fibers and optical fiber preforms with excellent properties in large quantities and in a short period of time, which can be obtained by the above-mentioned methods, and this will reduce the cost of optical fibers. It is expected that it will be useful.

The present inventors have already developed a high-speed synthesis method for base materials using a burner with a new multiple flame structure based on the VAD method (Japanese Patent Application No. 58-219380), and clarified its effectiveness.

The porous base material synthesized at high speed using multiple flame burners is
In the next step, O is turned into a transparent gas in a high-temperature electric furnace and used for drawing optical fibers. Illustrated in the diagram. In FIG. 1, 1 is a double flame burner, 2 is an inner flame layer, 3 is a raw material layer, 4 is an outer flame, 5- is a porous base material for an optical fiber that is being synthesized, and a is a porous base material for an optical fiber that is being synthesized. Flame length due to inner flame and b means flame length due to double flame. Basic studies by the present inventors revealed that when glass raw materials are fed into an oxyhydrogen flame burner to generate fine particles, the longer the fine particles stay in the flame, the larger the particle size increases, and the deposition rate per unit time on the base material increases. It has been shown that the amount increases. The double flame burner shown in FIG. 1 allows the inner flame to retract relative to the outer flame, thereby increasing the length of the flame and increasing the time during which particulates remain in the flame.

Specifically, using the double flame burner, S i C1
4. Using GeCl4 as a raw material, a large porous base material with an outer diameter of 150 mm, a length of about 900 fi, and a weight of 2500 F was produced, and high-speed synthesis was achieved (average synthesis speed 5.5 f1 min).

However, when we put this large porous base material into an electric furnace and made it into transparent glass under the same conditions as porous base materials produced by the normal VAD method, we found that the normal VAD porous base material was completely transparent. Compared to the vitrification, the large-scale high-speed synthesis base material exhibited a translucent or almost white color, and it became clear that only incomplete transparent vitrification could be achieved.

The difference between normal VAD porous base material and large high speed synthetic base material is
(1) Fine particle size (VAD + 0.1 μm or less, high-speed synthesis)
(2) Bulk density (vAD: approx. 0.23
t/♂, high speed synthesis: about 0.5? f/3”) (3)
Dimensions (VADj approx. 60 m diameter, high speed synthesis: approx. 150
+w diameter), etc., so it is presumed that the porous base material synthesized at high speed requires transparent vitrification conditions suitable for itself.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned circumstances, and its purpose is to provide a method for transparently vitrifying a porous base material synthesized at high speed.

[Structure of the invention]

To summarize the present invention, the present invention relates to a method for manufacturing a base material for optical fiber, and includes a nozzle for supplying glass raw material in the center, a nozzle for supplying combustible gas, a combustion supporting gas and a non-flammable gas in the outer part. Consisting of a concentric multiple nozzle having each nozzle for supplying active gas, and one or more sets of nozzles for supplying raw materials, supplying inert gas, supplying combustible waste, and supplying combustion-supporting gas concentrically outside The raw material for the optical fiber base material is introduced into a multiple flame burner equipped with an outer synthesis nozzle, and the fine particles generated by the multiple flame burner are deposited to form a porous base material, which is heated at a heating rate of 5°C/min. It is characterized by being heated to transparent vitrification at a rate corresponding to the following:

The present inventors conducted experiments at various heating rates for heating transparent vitrification of the base material, and found that a heating rate of 5° C./min or less was effective.

According to the present invention, methods for obtaining a speed corresponding to 5°C/min or less include a method in which the porous base material is placed at a certain position in an electric furnace and heated, or a method in which the porous base material is heated at a certain position in an electric furnace, or a method in which the porous base material is heated at a correspondingly slow speed There is a method of inserting it into the high temperature part of the machine. In this way, by setting the temperature increase rate to 5° C./min or less, the porous base material can be made completely transparent.

〔Example〕

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

Comparative Example 1 A large porous base material with an outer diameter of 1.10 mm was produced using a double flame burner. S i C14, GeC for the inner flame
14 and further 5iC1 to the outer flame, 5 to 4
Synthesis was carried out at a rate of f/min. This base material was cut into rounds approximately 80 m thick, placed in the soaking section of an electric furnace, heated from room temperature to a predetermined temperature (maximum 1600°C) in a He gas atmosphere, and subjected to transparent vitrification treatment. went. The furnace core tube is made of quartz glass and has an inner diameter of approximately 1405 mφ.

The high-speed synthesis base material contains about 8 to 10% by weight of GeO2, and a porous base material produced by a normal VAD method can be made into transparent glass at a temperature of about 1450 to 1500°C.

These high-speed synthesis base materials were heated at 1450℃, 1500℃, 15
High temperature treatment was performed at each temperature of 50°C and 1600°C. The temperature increase rate was 10°C/min.

None of the base materials was completely transformed into transparent glass, and the base materials treated at lower temperatures had a greater degree of opacity.

Figure 2 shows 150. ,． 0℃ (Sample 1) and 1600℃ (
A transparent vitrified sample (sample thickness 21 m, 5 W, 101
1111) and a sample (Sample 3, see Example 1) that was subjected to high temperature treatment up to 1550°C at a temperature increase rate of 5°C/min or less and the relationship between wavelength and absorption coefficient. This graph shows that at a heating rate of 10° C./min, the higher the treatment temperature, the higher the transparency, but it is shown that even at 1600° C., the glass is not yet sufficiently transparent and vitrified.

It has been found that when a base material that is insufficiently transparent and vitrified is used for wire drawing, problems such as foaming occur when drawing with an oxyhydrogen burner for insertion into a jacket tube, for example.

Example 1 and Comparative Example 2 Using a high-speed synthesis porous base material produced under the same conditions as in Comparative Example 1, transparent vitrification was attempted by changing only the heating rate under the same conditions as in Comparative Example 1.

The heating rate was 7°C/min, 5°C/min, 3°C/min, 1°C/min, and the vitrification temperature was 1550°C. At a heating rate of 7°C/min, opacity remained, but at a slow heating rate of 5°C/min or less, a completely transparent glass matrix was obtained. (Sample 3 in Figure 2) Similar results were obtained at 3° C./min and 1° C./min, and in the measurement of the absorption coefficient, the sample with a thickness of 10 wI was so transparent that it could not be measured.

Similar results were obtained at 1600°C.

From the above, it has become clear that in order to obtain a transparent base material, a temperature increase rate of 5° C./min or less is required.

Example 2 At a high temperature of 1550° C. at a temperature increase rate of 6° C./min, a chlorine-based dehydrating agent was poured and a high-speed synthesis base material was made into transparent glass. An optical fiber was drawn from this base material and its optical loss characteristics were measured.

The results are shown in FIG. In other words, Figure 3 shows the wavelength (μm
) (horizontal axis) and optical loss (dB/km ) (vertical axis).

It can be seen that an optical loss value of approximately [1L7 dB/km was obtained with a wavelength of 1.6 μm groove, which is comparable to the loss value of a normal optical fiber.

In the above example, the porous base material was installed at a certain position ('wl! center soaking part of the air furnace) and the temperature was raised to achieve transparent vitrification. After heating, it is also possible to slowly insert the porous base material into the high temperature part and vitrify it. In this case, it goes without saying that a slow insertion speed corresponding to the above-mentioned temperature increase rate of 5° C./min or less may be used.

Actual division, for example, diameter 1! IQIII+ high speed synthesis base material,
A completely transparent glass base material was obtained by inserting it into an electric furnace with a maximum temperature of 1550° C. (an example of the temperature distribution is shown in FIG. 4) at a rate of 90 cm/hour.

FIG. 4 is a graph showing an example of the temperature distribution of an electric furnace used for transparent vitrification according to the present invention. In the graph, the horizontal axis is temperature (U), and the vertical axis is distance 11m from the center of the furnace.
(,,w,) is shown.

The insertion rate can be converted into a heating rate as follows.

Based on the temperature distribution in Figure 4, shrinkage of the porous base material occurs 1
Assuming a temperature gradient of 1.5°C/m from 100 to 1400°C, an insertion speed of 90 m/hr corresponds to approximately °C/min. Needless to say, this satisfies the aforementioned temperature increase rate of 5° C./min or less.

〔Effect of the invention〕

As explained above, according to the present invention, the porous base material is heated at a slow heating rate of 5°C/min or less, or the porous base material is inserted into the hot part at a correspondingly slow insertion speed. By doing so, the porous base material synthesized at high speed using multiple flame burners can be made into completely transparent glass, which has the advantage of being able to mass-produce optical fibers with excellent properties at low cost.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of the upper part of an optical fiber base material manufacturing apparatus using an example of a double flame burner, and FIG.
Wavelengths of glass samples (sample 1, sample 2) treated at high temperature at ℃ and 1600℃ (temperature increase rate 10℃/min) and sample (sample 5) treated at high temperature up to 1550℃ at a temperature increase rate of 5℃/min or less FIG. 3 is a graph showing the relationship between the wavelength and the optical loss of an optical fiber obtained from a base material made transparent by the method of the present invention, and FIG. 4 is a graph showing the relationship between It is a graph which shows an example of the temperature distribution of the electric furnace used when transparent vitrifying according to the invention. 1: Double flame burner, 2: Inner flame layer, 3I raw material layer, 4
! Outer flame, 5: Porous base material for optical fiber Patent applicant: Nippon Telegraph and Telephone Public Corporation agent Hirotoshi Nakamoto Akira Bengami

Claims (1)

[Claims] t. Another set of concentric multiple nozzles arranged concentrically on the outside of the concentric multiple nozzle, which has a glass raw material supply nozzle in the center and nozzles for combustible gas supply, combustion-supporting scum supply, and inert gas supply on the outside part. For the above raw material supply, inert gas supply,
The raw material for the optical fiber base material is introduced into a multi-fire burner equipped with an outer synthesis nozzle consisting of a nozzle for supplying combustible gas and a nozzle for supplying combustion-supporting gas, and fine particles generated by the multi-flame burner are deposited to form a porous structure. A method for manufacturing an optical fiber base material, which comprises forming a base material and heating the base material at a rate corresponding to a temperature increase rate of 5° C./min or less to make it transparent vitrified.