JPH11292557A - Method for making optical fiber porous preform transparent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain a longer service life of the vacuum furnace by reducing glass film deposition on a furnace core tube and an evacuation pipe of a vacuum furnace for use in a process of making an optical fiber porous preform transparent. SOLUTION: In the case that an optical fiber porous perform 5 is inserted into the core tube 1 of a vacuum furnace provided with a furnace core tube 1 and heated to make the preform 5 transparent, the optical fiber porous preform 5 is placed in an atmosphere such that an inert gas whose pressure is adjusted to 50 to 150 Pa is allowed to flow, in a >=1200 deg.C temp. region inside the furnace core tube 1, to make the preform 5 transparent. Also, placing the optical fiber porous preform 5 in an atomosphere such that an inert gas whose pressure is adjusted to 50 to 150 Pa is allowed to flow, over a >=800 deg.C temp. region inside the furnace core tube 1, or placing the preform 5 in an atmosphere such that an inert gas whose pressure is adjusted to 50 to 150 Pa is allowed to flow, throughout the whole temp. region inside the furnace core tube 1 over a period from the time at which the preform 5 is inserted into the core tube 1, to the time at which preform 5 is made transparent, is more effective for attaining a longer service life of the vacuum furnace.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method of heating a porous optical fiber preform to make it transparent by enabling a vacuum furnace equipped with a furnace tube to be used stably for a long period of time, and The present invention relates to a method for transparentizing an optical fiber porous preform capable of maintaining a good quality of a completed transparent preform over a long period of time.

[0002]

2. Description of the Related Art A gas phase method (VAD method) or an external method (O
The optical fiber porous preform synthesized by the VD method)
By virtue of high-temperature heat treatment in an electric furnace, the glass is made transparent and becomes a transparent base material. Conventionally, transparency has been performed at normal pressure in an atmosphere of an inert gas such as helium.However, the gas confined in the pores of the optical fiber porous preform or the gas dissolved during the transparency becomes transparent. There is a problem that it remains in the forming base material and becomes bubbles. Therefore, in order to reduce such residual gas or bubbles, the optical fiber porous preform is made transparent in a vacuum atmosphere or a reduced pressure atmosphere. JP-A-5-163038
The method described in Japanese Patent Application Laid-Open Publication No. Hei 9 (1995) -19511 is one such method.

[0003] This method is performed by a vacuum furnace having a furnace tube shown in FIG. In FIG. 1, 1 is a furnace core tube made of high-purity graphite or the like, 2 is a heater, 3 is a heat insulating material, 4 is a furnace body, 5 is an optical fiber porous preform, 6 is a seed rod, and 7 is a base material suspension. Lower rod, 8 a front chamber, 9 a door, 10 a top lid, 11 a gate valve, 12 a furnace body gas inlet, 13 a furnace tube gas inlet, 14 a furnace body gas outlet, 15 a furnace tube gas An exhaust port, 16 is a vacuum exhaust pump, 17 is a normal pressure exhaust port, and 18 is a front chamber gas exhaust port.

In the method described in Japanese Patent Application Laid-Open No. 5-163038, when a porous optical fiber preform is made transparent, a temperature rising pattern programmed under reduced pressure or vacuum is divided into two time intervals. In the first section, a gas containing an inert gas is introduced into a vacuum furnace under a reduced pressure atmosphere.
In the section, a gas containing an inert gas with a smaller flow rate than the first section is supplied to a vacuum furnace under a reduced pressure atmosphere of about 0.5 to 2 Pa or under a vacuum atmosphere in which no gas flows, The object of the present invention is to prolong the life of the furnace body by suppressing oxidation and consumption of carbon parts in the furnace body due to oxygen, moisture and the like adsorbed on the optical fiber porous preform.

In this method, the porous optical fiber preform 5 is inserted into the front chamber 8 while supporting the seed rod 6 with the preform hanging rod 7, and then the gate valve 1 is depressurized there.
1 is opened, inserted into a furnace tube 1 heated to 800 ° C., and heated and heated by a heater 2. And 1300 ° C
Then, the gas flow rate is reduced to increase the reduced pressure, or the pressure is reduced, and heating is further performed to 1600 ° C. During this time,
Gases in the furnace tube 1 and the furnace body 4 are supplied from respective gas inlets 13 and 12, and the inside of the furnace tube 1 and the inside of the furnace body 4 are depressurized by an exhaust pump 16 connected to gas outlets 15 and 14. Or, under vacuum.

[0006]

In the method of making transparent according to the prior art, the glass component adheres little by little to the exhaust pipe of the gas exhaust port connected to the furnace tube and the furnace. Although this is not a problem in the early stage, the following problems occur during the long-term use of the vacuum furnace. The glass component deposited inside the exhaust pipe gradually increases, and if the operation is continued as it is, the exhaust pipe will eventually be closed. Therefore, exhaust is not sufficient, and rust is generated in the stainless steel material constituting the vacuum furnace or carbon material is consumed by moisture, oxygen, and the like released from the optical fiber porous preform.

Further, a glass film is formed on the inner wall of the furnace tube, and the glass film peels off, causing a problem that the surface of the porous optical fiber preform is damaged. It is conceivable to frequently clean the exhaust pipe and remove and clean the glass film of the furnace core tube. However, in that case, the operation rate of the equipment decreases. The present invention provides a method for making an optical fiber porous preform transparent, in which the problems of the prior art are eliminated.

[0008]

According to the method for making a porous optical fiber preform transparent according to the present invention, a porous optical fiber preform is inserted into a furnace tube of a vacuum furnace having a furnace tube to heat and make it transparent. At this time, in a temperature range of 1200 ° C. or more in the furnace tube, the optical fiber porous preform is placed in an atmosphere of an inert gas at a pressure of 50 to 150 Pa to make it transparent.

Further, the optical fiber porous preform is placed in an atmosphere in which an inert gas having a pressure of 50 to 150 Pa is flowed over a temperature range of 800 ° C. or more, or the optical fiber porous preform is placed in a furnace tube. A pressure of 50 to 50 is applied over the entire temperature range in the core tube from insertion to clarification.
Placing the optical fiber porous preform in an atmosphere of an inert gas at 150 Pa has a further effect on extending the life of the vacuum furnace.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the method for making a porous optical fiber preform transparent according to the present invention, the vacuum furnace shown in FIG. 1 used in the prior art can be used as a vacuum furnace. However, it is not limited to the vacuum furnace shown in FIG.
Even if the form and material of the vacuum furnace are different, they can be used as long as they can satisfy heating conditions, pressure conditions, and the like. Further, in the present invention, the pressure of the atmosphere gas when the porous optical fiber preform is inserted into the furnace core tube of the vacuum furnace is specified. The process of obtaining such knowledge will be described below.

In a vacuum furnace, the gas is exhausted by an evacuation pump from an exhaust pipe connected to a gas exhaust port connected to a furnace tube. In this case, the gas exhaust efficiency depends on the strength of the gas flow from the furnace tube to the exhaust tube. In general, under vacuum or reduced pressure, the number of gas molecules is small, so the number of collisions between gas molecules is small, and therefore the mean free path of gas molecules is long. The lower the pressure, the longer the mean free path of the gas molecules. FIG. 2 shows the relationship between pressure and molecular mean free path in helium at 1200 ° C. and 1 ° C.
It is obtained by calculation for the case of 550 ° C.

When the mean free path of the gas molecules is smaller than the size of the gas flow point in the apparatus, the gas molecules are expected to flow easily without colliding with the inner wall of the apparatus. If the mean free path is 1/10 or less of the size of the narrowest part of the gas flow area in the apparatus, it is considered that most of the gas molecules flow without colliding with the inner wall. FIG.
In the case of the vacuum furnace described above, the narrowest part of the gas flow point is the exhaust pipe connected to the gas exhaust port of the vacuum furnace, but its inner diameter is about 25 mm. Is desirably 2.5 mm or less. From FIG. 2, it can be estimated that the pressure should be 50 Pa or more to make the mean free path 2.5 mm or less.

To reduce the mean free path, the higher the pressure of the atmospheric gas, the better. However, if the pressure is too high, another problem arises. That is, when the pressure of the atmosphere gas is high, the gas is easily confined in the transparent base material, the number of bubbles increases, and the quality of the transparent base material is reduced. When the pressure limit at which generation of bubbles due to gas confinement in the transparent base material was small was examined by experiment, it was 150 Pa. From these facts, in the present invention, the pressure range of the atmospheric gas is set to 50
１５０150 Pa.

Further, the furnace tube of the vacuum furnace is heated to about 800 ° C., the porous optical fiber preform inserted into the front chamber is inserted into the furnace tube by opening the gate valve, and the temperature is gradually increased by about 154.
The temperature was raised to 0 ° C. to make the optical fiber preform transparent and used as a transparent base material. When the relationship between the release of glass components and the temperature in the furnace tube was examined, the release of most of the glass components was 1200
It was found that it occurred in the temperature range of not less than ℃. Therefore, by controlling the pressure of the atmospheric gas only in the temperature range of 1200 ° C. or more, a considerable effect on the extension of the life of the vacuum furnace can be achieved. It is needless to say that the life of the vacuum furnace can be further extended by controlling the pressure in the entire temperature region until the optical fiber preform is inserted into the furnace tube and made transparent.

As the atmosphere gas, an inert gas is used so as not to react with the optical fiber porous preform, the furnace tube and the like. From the viewpoint of gas exhaust efficiency, the pressure of the atmosphere gas is preferably higher. However, if the pressure is increased, bubbles are easily generated. Therefore, the atmosphere gas is preferably a gas that hardly generates bubbles. Helium gas is preferable as the atmosphere gas because helium gas has a high transmittance among the inert gases and is unlikely to become bubbles.

[0016]

EXAMPLE As Example 1, the vacuum furnace of FIG. 1 was used, and the optical fiber porous preform was made transparent under the following conditions. The inside of the furnace was evacuated in advance to maintain the temperature at 800 ° C., and the optical fiber porous preform was inserted into the front chamber. After evacuating the front chamber, the gate valve is opened and the optical fiber preform is inserted into the furnace tube, and the temperature inside the furnace tube is raised to 80 over about 8 hours.
The temperature was raised from 0 ° C. to 1540 ° C. to make the base material of the optical fiber porous chamber transparent. During this time, 5 SLM of helium gas was introduced into the furnace tube and the furnace (the flow rate per minute was set to a standard state of 0 ° C,
The pressure was maintained at about 100 Pa. As a result, the resulting transparent base material had no bubbles and was good. After the same treatment of 100 optical fiber preforms was continuously performed, the inside of the furnace tube and the inside of the exhaust tube were observed. However, adhesion of the glass film was hardly found. In addition, the average number of air bubbles in the 100 transparent base materials was 0.2 / bubble, and no abnormality such as a scratch was found on the surface of the transparent base material.

As Example 2, the vacuum furnace of FIG. 1 was used, and the optical fiber porous preform was made transparent under the following conditions different from Example 1. 800 ° C inside furnace tube
During the heating from 800 ° C to 1200 ° C, the pressure was maintained at about 100 Pa by flowing 5 SLM of helium gas while the temperature was rising from 800 ° C to 1200 ° C.
Helium gas was flown at 8 SLM while the temperature was raised to 0 ° C., and the pressure was maintained at about 120 Pa. After the same treatment of 100 porous optical fiber preforms continuously, the inside of the furnace tube and the inside of the exhaust tube were observed, but almost no glass film adhesion was found. In addition, the average number of air bubbles in 100 pieces of the transparent base material was 0.3 / book, and no abnormality such as a scratch was found on the surface of the transparent base material.

In Example 3, the vacuum furnace of FIG. 1 was used, and the optical fiber porous preform was made transparent under the following conditions different from Example 1. 800 ° C inside furnace tube
During the heating from 800 ° C. to 1540 ° C., the pressure was maintained at a vacuum of about 5 Pa without flowing helium gas during the heating from 800 ° C. to 1200 ° C., and the helium gas was heated at 4 SLM during the heating from 1200 ° C. to 1540 ° C. The pressure was maintained at about 80 Pa by flowing. Continuously use one porous optical fiber preform
After the same treatment as above, the inside of the furnace tube and the inside of the exhaust tube were observed, but adhesion of the glass film was hardly found.
The average number of bubbles of 100 transparent base materials is 0.2 /
In the book, no abnormality such as a scratch was found on the surface of the transparent base material.

As Comparative Example 1, the vacuum furnace of FIG. 1 was used, and the optical fiber porous preform was made transparent under the following conditions different from Example 1. 800 ° C inside furnace tube
While the temperature was raised from 1 to 1540 ° C., the pressure was maintained at a vacuum of about 1 Pa without flowing helium gas. Although there were no air bubbles in the obtained transparent preform, the same process was continuously performed on 20 optical fiber porous preforms, and the transparent preform was examined. Many wounds were found. Also, when observing the inside of the furnace core tube and the inside of the exhaust tube,
A glass film was formed on the inner wall of the furnace core tube, and several places where the glass film was peeled were found. In addition, foreign substances considered to be glass components also accumulated in the exhaust pipe, and the exhaust pipe was closed.

As Comparative Example 2, the vacuum furnace of FIG. 1 was used, and the optical fiber porous preform was made transparent under the following conditions different from Example 1. 800 ° C inside furnace tube
Helium gas for 20S while heating from
The pressure was maintained at about 200 Pa by LM flow. Five bubbles were generated in the obtained first transparent base material. The same treatment was continuously performed on 20 optical fiber porous preforms, and a transparent preform was examined. As a result, no scratch was found on the surface of the transparent preform, but bubbles were averaged at 3.5. One / piece was accepted.
When the inside of the furnace tube and the inside of the exhaust tube were observed, no formation or adhesion of a glass film was observed. However, in the case of Comparative Example 2, since the number of bubbles was large and the portion had to be discarded as defective, the productivity was inferior to Examples 1 to 3.

[0021]

According to the method for transparentizing an optical fiber porous preform of the present invention, when the optical fiber porous preform is inserted into the furnace tube of a vacuum furnace having a furnace tube and heated to make it transparent, 1
In a temperature range of 200 ° C. or higher, the optical fiber porous preform is placed in an atmosphere in which an inert gas is supplied at a pressure of 50 to 150 Pa to make it transparent. Adhesion of the glass film to pipes, exhaust pipes, etc. is small, and the life of the vacuum furnace can be extended. In addition, it is possible to reduce problems related to product quality such as damage to the porous optical fiber preform due to peeling of the glass film adhered to the furnace tube and generation of many bubbles in the transparent preform.

Further, during the step of clearing the porous optical fiber preform, the pressure is set to 50 to over a temperature range of 800 ° C. or more.
Putting the optical fiber porous preform under an atmosphere in which an inert gas was flowed at 150 Pa, or inserting the optical fiber porous preform into the furnace core tube and clarifying it after the entire temperature range in the furnace core tube until it became transparent. The life of the vacuum furnace can be further extended by placing the optical fiber porous preform under an atmosphere in which an inert gas is supplied at a pressure of 50 to 150 Pa.

[Brief description of the drawings]

FIG. 1 is a sectional view of a vacuum furnace.

FIG. 2 is a graph showing the relationship between the molecular mean free path of helium gas and pressure.

[Explanation of symbols]

 1: Furnace tube 2: Heater 3: Heat insulation material 4: Furnace body 5: Optical fiber porous base material 6: Seed bar 7: Base material hanging rod 8: Front room 9: Door 10: Top cover 11: Gate valve 12 : Furnace body gas inlet 13: Furnace tube gas inlet 14: Furnace body gas outlet 15: Furnace tube gas outlet 16: Vacuum exhaust pump 17: Normal pressure exhaust port 18: Front chamber gas exhaust port

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Fumio Yoshimura 1st Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Inside Sumitomo Electric Industries Co., Ltd. Electric Manufacturing Co., Ltd.

Claims (3)

[Claims] 1. A method of inserting a porous optical fiber preform into a furnace tube of a vacuum furnace having a furnace tube to heat and make it transparent, wherein a pressure of 50-200 ° C. is set in a temperature region of 1200 ° C. or more in the furnace tube. A method for making a porous optical fiber preform transparent, comprising placing an optical fiber porous preform in an atmosphere of an inert gas at 150 Pa and flowing the same. 2. A method of inserting a porous optical fiber preform into a furnace tube of a vacuum furnace having a furnace tube to heat and make it transparent, wherein a pressure of 50 to 800 ° C. or more in the furnace tube is used. A method for making a porous optical fiber preform transparent, comprising placing an optical fiber porous preform in an atmosphere of an inert gas at 150 Pa and flowing the same. 3. A method for inserting a porous optical fiber preform into a furnace tube of a vacuum furnace having a furnace tube and heating and making the optical fiber transparent, and then inserting the optical fiber porous preform into the furnace tube to make the fiber tube transparent. Pressure in all temperature ranges up to
A method for making a porous optical fiber preform transparent, comprising placing the optical fiber porous preform in an atmosphere in which an inert gas at a pressure of up to 150 Pa is flown.