JP2514689B2 - Manufacturing method for optical fiber preform - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is a method for stably producing a preform for an optical fiber by making a porous glass preform for an optical fiber (hereinafter abbreviated as a porous preform) transparent glass. Regarding

[Prior Art] As a method of manufacturing an optical fiber preform,
Conventionally, vapor-phase axial deposition method (Vapour-phase Axial Depositi
on method, abbreviated as VAD method), Outside V
apour Deposition or Outside Vapour−phase Oxi
dation method, abbreviated as OVD or OVPO method), etc. to synthesize a porous matrix such as a glass particle aggregate or a composite of a glass rod and a glass rod aggregate formed on the outer periphery thereof. There is a method of obtaining a preform for an optical fiber by heating and sintering this and making it transparent glass. In addition, there is also a method of synthesizing a porous base material of a glass fine particle aggregate by a powder compacting method in which separately synthesized glass fine particles are solidified under high pressure, and heating and transparentizing the same.

As a method for making such a multilayer base material transparent by heating, a high-purity quartz container in a heating furnace (referred to as a furnace core tube)
A method in which a porous base material is inserted in the container and the inside of this container is heated and made transparent by heating it in a He atmosphere (reference;
29, No. 10, 1980, pp. 1719 to 1729), or a method of heating and transparentizing the inside of a container in a vacuum state (JP-A-54-14222).
2, gazette 56-63833). The reason for using He or a vacuum atmosphere is to prevent gas from remaining in the glass during transparent vitrification, thereby eliminating bubble generation in the preform and obtaining a high-quality transparent glass body.

[Problems to be Solved by the Invention] Among the above conventional methods, sintering in a He atmosphere is more common, but the problem of bubbles remaining in the transparent glass is completely solved even in a He atmosphere. Had not reached.

Not substituted on porous gas contained from the beginning in the vitreous, for example _{N 2, H 2, O 2} , Ar ambient gas is sufficiently He gas at the time of glass fine particle aggregates synthesis such as for gas bubble It was considered that the bubbles remained, and various conditions at the time of making transparent were improved to try to eliminate bubbles, but they could not be eliminated. Then, when the gas in this bubble was analyzed, it was found that most of the gas was He. That is, it was clarified that bubbles were generated by the He gas that was adsorbed without being completely removed from the base material during the transparentization.

Therefore, the present inventors have studied a method of making transparent in a vacuum state (under reduced pressure) as an atmosphere for making transparent.

When heating in a vacuum to produce transparent glass, it is not possible to use high-purity quartz for the entire container due to strength and structural problems, so it is common to use a carbon product. is there. However, when a container made of carbon is used, impurities such as carbon powder tend to float in the furnace and are easily contained in the glass.

Therefore, a small amount of gas is blown into the furnace core tube for the purpose of removing these impurities. Or OH contained in the base material
In order to reduce the OH group absorption peak after removing the group and forming a fiber and to obtain a fiber with a small transmission loss, a method of flowing a small amount of halogen gas, generally Cl ₂ gas is used.

However, when a transparent glass is formed by heating under reduced pressure (about 10 Pa) while flowing a gas, a problem occurs in which glass particle aggregates are cracked (so-called cracking). For the severe ones, cracks were generated in the entire base material, but especially near the gas supply port of the furnace core tube.

The present invention has been made for the purpose of solving the above-mentioned problems that occur when heating and making transparent under reduced pressure while flowing a gas.

[Means for Solving the Problems] As a means for solving the above problems, the present invention provides an inert gas or a halogen-based gas by installing a porous glass preform for optical fibers in a pressure vessel having a gas supply port. In the method of heating and transparentizing under reduced pressure in the atmosphere, heating and transparentizing so that the inert gas or halogen-based gas introduced into the pressure vessel does not directly hit the porous glass preform for optical fiber from the supply port. The present invention provides a method for manufacturing an optical fiber preform characterized by the following.

In the above, the method of installing the baffle plate between the gas supply port and the porous glass preform for optical fiber is simple and effective, and is mentioned as a particularly preferable embodiment.

As the porous base material in the present invention, as described above, VAD method, glass fine particle aggregate produced by a vapor phase synthesis method such as OVD method or OVPO method, a glass fine particle aggregate formed on the quartz rod and its periphery and It is possible to use a composite made of, or one produced by various production methods such as an aggregate of glass fine particles by a powder compacting method. The composition of the porous base material is not particularly limited.

As the inert gas or halogen-based gas used in the present invention, in addition to He gas and Cl ₂ gas, gases such as N ₂ , CF ₄ , SiF ₄ , SF ₆ and CCl ₄ can be used.

The temperature of the heat transparentization in the present invention, 1600 ℃ ~
It is generally performed at 1650 ° C, and depressurization is several Pa to 10
About 0 Pa or vacuum is preferable. Since the pressure vessel is made under reduced pressure or vacuum as described above, the pressure vessel is preferably made of metal, and since a corrosive gas containing chlorine is often contained in the porous preform for optical fiber, a metal having corrosion resistance such as SUS is used.
Manufacturing and the like are particularly preferable.

Atmospheric gas introduced into the gas supply port of this pressure vessel,
The feature of the present invention lies in the point that the material is heated and made transparent so as not to directly contact the porous base material, and the reason for doing so will be explained in the next section.

[Operation] FIG. 4 shows the structure of a conventional pressure vessel having a gas supply port. The glass fine particle aggregate 3 is fixed in a pressure vessel 1 having a gas supply port 2 and heated by a heater 5 to form a transparent glass.

At this time, the pressure container 1 is depressurized (or evacuated) by the rotary pump 4, and the inert gas or the halogen-based gas is supplied from the gas supply port 2. The gas pressure is as shown in FIG. Then, the pressure inside the pressure vessel 1 is rapidly reduced. Therefore, the adiabatic expansion gas sharply lowers the gas temperature (the gas that is abruptly decompressed and expanded in volume loses heat by an amount commensurate with its work amount), and the gas whose temperature has decreased is As shown by the arrow 6 in FIG. 4, the glass fine particle aggregate is directly hit. For this reason, the temperature of the portion where the gas collides decreases. It is considered that this causes thermal strain in the glass fine particle aggregate, which causes the soot to crack.

Therefore, the present inventors say that it is effective for preventing soot cracking that the expanded gas does not directly impinge on the glass fine particle aggregate and does not significantly affect the temperature distribution in the pressure vessel. It came to the present invention with the idea.

An example of the structure of the present invention is shown in FIG. The basic structure is the fourth
Similar to the figure, but the baffle plate 7 is installed between the gas supply port 2 and the glass fine particle aggregate 3, and the gas introduced into the pressure vessel 1 expands immediately after entering the vessel, The gas temperature drops sharply, but this low temperature (the furnace temperature itself
It is kept at 800 ℃ ~ 1600 ℃. ) The gas collides with the baffle plate 7 and flows in the direction indicated by the arrow 8. Therefore, the low-temperature gas does not directly hit the base material 3, and the gas whose direction is changed by the baffle plate 7 is heated by the heater 5 while bypassing the baffle plate 7 and reaching the main part of the pressure vessel 1. For,
A large temperature distribution is not formed in the furnace core tube (pressure vessel) 1, and a good optical fiber preform without soot cracking can be manufactured.

The configuration of FIG. 1 is via a jammer plate, but if the gas does not directly hit the glass fine particle aggregate,
Even with the structure shown in FIG. 2, a sufficiently large effect can be expected.

In FIG. 2, the pressure vessel has a double structure having an inner wall 9, and gas is supplied to this gap 10. Since the gap 10 is also decompressed, the temperature of the used gas drops inside the double structure, but the gas flows through the gap 10 and is heated up until it enters the main part of the pressure vessel 1. The same effect as the configuration can be obtained.

[Example] Comparative Example 1 The glass fine particle aggregate was heated with the configuration shown in FIG. The glass fine particle aggregate was a base material produced by the VAD method and had an outer diameter of 150 mmφ, a length of 500 mml, and a bulk density of 0.25 g / cm ³ . The gas used for purging in the furnace is He gas of 20
The flow rate was 0 cc / minute, and the pressure inside the container was adjusted to 10 Pa.
With the heater 4, the pressure inside the container is 8 ℃ / from room temperature to 1600 ℃.
The temperature was raised in minutes and then lowered. Under these conditions, an attempt was made to make the above three base materials transparent, but one had cracks in the entire length, and the remaining 2
The book also had a crack at the tip of the glass particle aggregate.

Example 1 As shown in FIG. 1, a glass fine particle aggregate was made transparent by vitrification with a configuration in which a baffle plate was set at the inlet of the gas supply section. All the glass fine particle aggregates were base materials manufactured by the VAD method, and the base material dimensions were almost the same as those in Comparative Example 1. For the gas, Cl ₂ gas was flowed at 100 cc / minute for dehydration, and the pressure inside the container was adjusted to 10 Pa. Cl ₂ gas was made to flow up to 1000 ° C., and when it exceeded 1000 ° C., it was switched to He gas. The temperature pattern is 6 from room temperature to 1000 ° C.
C./min. The temperature was raised from 1000.degree. C. to 1600.degree. C. at 8.degree. C./min to make it transparent. As a result of making the above two base materials transparent under these conditions, cracks did not occur, and a good optical fiber preform without bubbles could be obtained.

Example 3 A glass rod having a core portion for transmitting an optical signal having a different refractive index (showing the refractive index distribution in FIG. 3) was used on the outer periphery of a composite body in which glass particle aggregates were synthesized by the VAD method.
I tried to make it transparent. As the gas, He gas was flown, and the apparatus configuration used was that shown in FIG. The conditions are as follows. He: 200 cc / min, temperature: temperature rise from room temperature to 1600 ° C at 8 ° C / min. As a result of making a total of 6 transparent films, a high-quality optical fiber preform having good transparency could be obtained without cracks. No bubbles could be found.

In the above examples, the glass fine particle aggregates produced by the VAD method were used, but if the method of making the glass fine particle aggregates into transparent vitrification in a high temperature furnace is also effective in the production methods such as the powder compacting method and the OVPO method. is there.

[Effects of the Invention] As described above, by using the present invention, it is possible to prevent the occurrence of bubbles and cracks, and easily obtain a high-quality optical fiber preform.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention, FIG. 2 is a sectional view showing another embodiment of the present invention, and FIG. 3 is a view showing a refractive index distribution of a glass rod used in Example 2, FIG. 4 is a cross-sectional view showing an apparatus configuration of a conventional method, and FIG. 5 is a view showing a change in pressure of gas introduced into a container by the conventional method. 1: Pressure vessel, 2: Gas supply port, 3: Glass particle aggregate, 4:
Rotary pump, 5: heater, 6: gas streamline, 7: gas jam plate, 8: gas streamline, 9: inner wall, 10: gap.

Claims (2)

(57) [Claims] 1. A method in which a porous glass preform for an optical fiber is placed in a pressure vessel having a gas supply port to heat and make transparent under a reduced pressure in an inert gas or halogen-based gas atmosphere. A method for producing an optical fiber preform, characterized in that the introduced inert gas or halogen-based gas is heated and made transparent so as not to directly contact the porous glass preform for optical fiber from a supply port. 2. A method of manufacturing an optical fiber preform according to claim 1, wherein a jammer plate is installed between the gas supply port and the porous glass preform for optical fiber.