JP3050231B1 - Method and apparatus for manufacturing optical fiber preform - Google Patents

Abstract

Abstract: PROBLEM TO BE SOLVED: To prevent a fluctuation in pressure occurring in a supply path of a fluorine compound gas of an apparatus for heat-treating a porous glass base material in a fluorine compound gas atmosphere and to realize a stable production of an optical fiber base material. To provide a manufacturing method and an apparatus therefor. SOLUTION: In a reaction vessel equipped with a pipe for supplying a reduced pressure fluorine compound gas from a fluorine compound gas storage vessel through a pressure reducing valve to a fluorine compound gas from a fluorine compound gas storage vessel storing the fluorine compound gas at a high pressure, In the method for producing a preform for an optical fiber that is heat-treated in an atmosphere containing, the temperature of a pipe near the pressure reducing valve is controlled to a temperature at which the fluorine compound depressurized through the pressure reducing valve does not liquefy or solidify. A method for producing a preform for an optical fiber, and an apparatus therefor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a preform for an optical fiber in which a porous glass preform is heat-treated in a fluorine compound gas atmosphere, and an apparatus therefor.

[0002]

2. Description of the Related Art An optical fiber preform comprises a central core portion and a surrounding cladding portion, and the core portion has a higher refractive index than the cladding portion in order to facilitate light transmission. In order to make the refractive index difference between the core part and the clad part in the glass base material for a silica-based optical fiber, a dopant for increasing the refractive index is added to the core part, or a dopant for lowering the refractive index is added to the clad part. A method of adding them or using them in combination is employed. Among them, fluorine is generally used as a dopant for lowering the refractive index. There are various methods for producing an optical fiber preform to which fluorine is added. After a porous glass preform prepared by a VAD method or an OVD method is subjected to a dehydration treatment using a chlorine compound or the like as necessary. , SiF ₄ , Si ₂ F ₆ , S
F ₆ , NH ₄ F, NF ₃ , PF ₅ , CF ₄ , CCl ₂ F
There is a method of performing fluorine addition and transparency by heat treatment in a fluorine compound gas atmosphere such as ₂ .

As an example of these methods, for example, Japanese Patent Publication No. 5-26731 discloses that a laminate of glass fine particles is held or passed through a furnace kept at a high temperature to add fluorine, dehydrate and clear. And producing a preform for an optical fiber by subjecting the glass fine particle laminate to a porous state in which the laminate is pre-impregnated with fluorine at a temperature of 1100 ° C. or more and 1400 ° C. or less. A method for producing a preform for an optical fiber, characterized in that the preform is kept or passed through a high-temperature furnace in an atmosphere containing, and is made transparent. According to this method, since the impregnation with fluorine is performed at a temperature at which the glass fine particle laminate is maintained in a porous state, that is, at 1100 ° C. or more and 1400 ° C. or less, fluorine can be diffused to the center of the glass fine particle laminate. . Further, since the glass fine particle laminate is also made transparent in a mixed atmosphere of a fluorine compound gas and an inert gas, the glass fine particle laminate can be made transparent without volatilizing the fluorine at the outer peripheral portion. Further, at the time of clearing, since the fluorine is already impregnated, the speed of the clearing is not limited by the time to complete the addition of fluorine, and the speed can be increased. Further, if dehydration and removal of impurities are performed with a chlorine compound before or simultaneously with the impregnation with fluorine, a preform for an optical fiber having extremely few impurities such as Fe can be obtained.

[0004]

FIG. 2 shows an example of an apparatus configuration in which a porous glass base material is heat-treated in a fluorine compound gas atmosphere by such a method. In this apparatus, a porous glass base material is subjected to heat treatment of a fluorine compound gas stored at a high pressure in a fluorine compound gas storage container 1 via a pipe 4 having a pressure reducing valve 2, a pressure switch 3, and the like. The liquid is supplied to the reaction vessel 5.
In FIG. 2, 6 is a filter, 7 is a check valve, 8 and 9 are pressure gauges, 10 is a safety valve, and 11 is a purge gas line for introducing a purge gas. Usually, the pressure of the fluorine compound gas is 0.5 to 10 MPa on the primary side (the upstream side of the pressure reducing valve 2).
a, 0.1 to 0.5MP on the secondary side (the downstream side of the pressure reducing valve 2)
In order to obtain a product of stable quality, it is necessary to stabilize the pressure on the secondary side as much as possible. Many fluorine compound gases have toxicity and corrosiveness. Usually, when the pressure after decompression exceeds a predetermined pressure, the pressure switch 3 is actuated to prevent an accident and the valve 14 near the storage container 1 (cylinder) is opened. It is set to close. However, in the conventional method, the pressure on the secondary side fluctuates during the operation of the apparatus, and in the apparatus having the pressure switch, the pressure switch is operated to close the gas flow path or the flow path is closed. Even if there is no gas supply, there is a problem that the gas supply becomes unstable. In addition, the optical fiber preform manufacturing equipment stops the equipment every time one optical fiber preform is manufactured, takes out the product, and creates a new porous material. When the apparatus is restarted by charging the high-quality glass base material, in an apparatus having a pressure switch, after the apparatus is stopped (for example, after several tens of minutes), the pressure on the secondary side increases and the pressure is increased. There is also a problem that the switch operates to close the gas flow path, and it takes time to restart the operation. The present invention solves such a problem that occurs in a fluorine compound gas supply path of an apparatus for heat-treating a porous glass base material in such a fluorine compound gas atmosphere, and enables stable production of an optical fiber base material. An object of the present invention is to provide a method of manufacturing a base material for use and an apparatus therefor.

[0005]

Means for Solving the Problems The present inventors have conducted intensive studies on the causes of the above-mentioned problems occurring in the supply path of the fluorine compound gas, and as a result, the fluorine compound gas supplied at a relatively high pressure has been reduced in pressure. When the pressure is reduced by the valve, the gas is adiabatically expanded and cooled on the secondary side of the pressure reducing valve. Depending on the physical properties (critical temperature, boiling point, etc.) of the fluorine compound used and the surrounding temperature conditions, the gas is liquefied by a drop in this temperature. In some cases, the liquefied or solidified fluorine compound gasifies again due to a change in the temperature of the piping and raises the pressure in the piping, particularly when the liquefied or solidified fluorine compound remains. When stopped, the flow path during the stop of the apparatus is in a closed state, and the volume in the pipe between the pressure reducing valve and the reaction vessel is constant, so that the temperature in the pipe increases and the fluorine compound is removed. In some cases increased pressure switch pressure increase in the pipe to gasification blocks the flow path in operation, to obtain a finding that there is a phenomenon such. As a result of further study, it is necessary to prevent the liquefaction or solidification of the fluorine compound in the pipe in order to solve the above-mentioned problem. It has been found that it is effective to keep the above. That is, the present invention has the following configurations (1) to (4).

(1) A porous glass base material is placed in a reaction vessel equipped with a pipe for supplying a reduced pressure fluorine compound gas through a pressure reducing valve from a fluorine compound gas storage vessel storing the fluorine compound gas at a high pressure. In the method for producing a preform for an optical fiber in which heat treatment is performed under an atmosphere containing a compound gas, the temperature of a pipe near the pressure reducing valve is controlled to a temperature at which a fluorine compound depressurized through the pressure reducing valve does not liquefy or solidify. A method for producing a preform for an optical fiber, comprising: (2) The method for producing a preform for an optical fiber according to claim 1, wherein the temperature of the pipe near the pressure reducing valve is controlled to be higher than the critical temperature of the fluorine compound by 20 ° C or more. (3) The fluorine compound is SF ₆ , SiF ₄ , CCIF
_{3. The} method for producing a preform for an optical fiber according to claim 1, wherein the preform is ₃ (CFC 13) or CHClF ₂ (CFC 22). (4) a fluorine compound gas storage container for storing a fluorine compound gas at a high pressure, a reaction container for heating a porous glass base material in an atmosphere containing a fluorine compound gas, and a reaction container from the fluorine compound gas storage container to the reaction container Temperature control for controlling the temperature of a pipe near the pressure reducing valve in a manufacturing apparatus for an optical fiber preform having a pipe for supplying a fluorine compound gas to the pipe, and a pressure reducing valve and a pressure switch installed in the middle of the pipe. An apparatus for manufacturing a preform for an optical fiber, characterized by comprising means.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the method and the apparatus of the present invention will be described with reference to FIG. 1 which shows an example of the apparatus configuration when a porous glass base material is heat-treated in a fluorine compound gas atmosphere according to the present invention. This will be described in detail. 1, the same parts as those in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted. Although the supply pipe system of the fluorine compound gas in the present invention is substantially the same as the case of FIG. 2 described above, the temperature of the pipe near the pressure reducing valve 2 is changed to liquefaction or solidification of the fluorine compound depressurized through the pressure reducing valve 2. It is characterized in that it is controlled to a temperature that does not occur. The heating means is not particularly limited, but a heater 12 such as a tape heater (ribbon heater) or a lead heater is provided in the primary side (high pressure side) and secondary side (low pressure side) pipes 4 near the pressure reducing valve 2.
And the temperature is controlled by the temperature control means 13 to a predetermined temperature. The appropriate heating range is about 25 to 50 cm before and after the pressure reducing valve 2, but there is no problem if the heating range is wider.

The heating temperature depends on the physical properties of the fluorine compound used.
(Critical temperature, boiling point, etc.) and ambient temperature conditions
But the fluorine compound depressurized through the pressure reducing valve 2
It is necessary to control and manage to a temperature that does not solidify or solidify
You. Critical temperature Tc of fluorine compound gas and ambient temperature Ta
(= Tc <Ta) when gas is close (= gas temperature Tg)
Liquefaction or sublimation when adiabatic expansion occurs (Tc> Tg)
Heating is performed at the temperature of the pipe near the pressure reducing valve.
At least 20 ° C. higher than the critical temperature of the fluorine compound.
Preferably, the temperature is reached. Preform for optical fiber
The fluorine compound gas used in the production of
F₆, SiF_Four, CCIF_Three(CFC13) and CHC
IF_Two(CFC22) is a typical one.
The physical properties of the gas are as shown in Table 1.
The temperature range is SF₆Is 75-95 ° C, SiF_FourIs 2
0-40 ° C, CCIF _Three(CFC 13) 50-70 ° C
And CHClF_Two(CFC22) is about 115-135 ° C
Degrees. The data in Table 1 is 1 atmosphere (0.1MP).
The value in a), higher pressure in the pipe
(Usually 0.1 to 0.5 MPa) so the boiling point (sublimation point)
, The difference from the pressure on the primary side is large (for example,
(1MPa or more) and the pressure on the primary side is less than 5MPa
Liquefaction or solidification of gas on the secondary side depending on the ambient temperature
Will occur.

[0009]

[Table 1]

By maintaining the pipe temperature near the pressure reducing valve in the above range throughout the operation of the apparatus, the fluorine compound gas can be supplied into the reaction vessel at a stable pressure. Does not block the flow path, and the reaction conditions are stable, so that a homogeneous product can be obtained. In addition, it is possible to prevent an accident in which the pressure switch is activated due to an increase in the pressure in the piping while the apparatus is stopped. Note that heating is not required during stoppage of the apparatus where no fluorine compound gas flows.

[0011]

EXAMPLES Next, the effects of the present invention will be demonstrated by Examples.
However, the present invention is not limited to this embodiment. (Embodiments 1 to 3) Using the apparatus having the configuration shown in FIGS.
SF as fluorine compound gas₆, SiF _FourOr Freon 1
In the process of manufacturing a preform for optical fiber using 3
Pressure fluctuation in the pipe 4 on the secondary side (the rear side of the pressure reducing valve 2).
I checked the condition. In this example, the inner diameter of the pipe 4 is 4.35 m.
m, the length from the pressure reducing valve 2 to the reaction vessel 5 is 30 m.
Operating pressure of the pressure switch 3 is 0.3 ± 0.01MP
a was set.

As a result of continuing operation by the apparatus having the configuration shown in FIG. 2, the temperature in the pipe 4 fluctuates during operation of the apparatus in winter, when the temperature around the pipe becomes low (−5 to + 5 ° C.). When activated, the flow path of the fluorine compound gas was frequently interrupted (see Table 2). Therefore, as shown in FIG. 1, a heater 12 is attached to the pipe 4 before and after the pressure reducing valve 2 over a length of 300 mm on the primary side and 400 mm on the secondary side, and the temperature of the pipe 4 is maintained at a predetermined temperature. The pressure in the piping was stable, and there was no interruption of the operation due to the operation of the pressure switch 3 through ten repetitions of stop and operation. Further, in the apparatus without the heater, no trouble occurred after stopping the apparatus, which occurred 2-3 times out of 10 times of stopping and operating. Table 2 shows the properties of the fluorine compound used, the operating conditions, the situation of occurrence of trouble, and the like.

[0013]

[Table 2]

[0014]

According to the method of the present invention, the fluorine compound gas is supplied into the reaction vessel at a stable pressure, so that a homogeneous product can be obtained. It is also possible to prevent an accident in which the pressure switch rises and the pressure switch is activated. It is particularly effective to heat the pipe in the vicinity of the pressure reducing valve so as to be at least 20 ° C. higher than the critical temperature of the fluorine compound used.
In the method of the present invention, the fluorine compound gas is SF ₆ , SiF ₄ ,
CCIF ₃ (CFC 13) or CHClF ₂ (CFC 2)
It is particularly suitable in the case of 2). Further, according to the apparatus of the present invention, the method can be easily performed.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an apparatus configuration in a case where a porous glass base material is subjected to a heat treatment in a fluorine compound gas atmosphere according to the present invention.

FIG. 2 is a diagram showing an example of an apparatus configuration in a case where a porous glass base material is subjected to heat treatment in a fluorine compound gas atmosphere according to a conventional technique.

[Explanation of symbols]

1 storage container 2 pressure reducing valve 3 pressure switch
4 piping 5 reaction vessel 6 filter 7 check valve
8, 9 Pressure gauge 10 Safety valve 11 Purge gas line 12 Heater 13 Temperature control means 14 Valve

Claims (4)

(57) [Claims] 1. A reaction vessel provided with a pipe for supplying a reduced pressure fluorine compound gas from a fluorine compound gas storage vessel through a pressure reducing valve to a fluorine compound gas from a fluorine compound gas storage vessel storing the fluorine compound gas at a high pressure. In the method for producing a preform for an optical fiber that is heat-treated in an atmosphere containing a gas, the temperature of a pipe near the pressure reducing valve is controlled to a temperature at which a fluorine compound depressurized through the pressure reducing valve does not liquefy or solidify. A method for producing a preform for an optical fiber, comprising: 2. The method of manufacturing an optical fiber preform according to claim 1, wherein the temperature of the pipe near the pressure reducing valve is controlled to a temperature higher than the critical temperature of the fluorine compound by 20 ° C. or more. . 3. The method according to claim 1, wherein the fluorine compound is SF ₆ , SiF ₄ ,
CCIF ₃ (CFC 13) or CHClF ₂ (CFC 2)
The method for producing a preform for an optical fiber according to claim 1 or 2, wherein 2) is used. 4. A fluorine compound gas storage container for storing a fluorine compound gas at high pressure, a reaction container for heating a porous glass base material in an atmosphere containing a fluorine compound gas, and In an apparatus for manufacturing a preform for optical fiber, comprising: a pipe for supplying a fluorine compound gas to a reaction vessel, and a pressure reducing valve and a pressure switch installed in the middle of the pipe, the temperature of the pipe near the pressure reducing valve is controlled. An apparatus for manufacturing a preform for an optical fiber, comprising a temperature control means.