JP2000044269A - Dehydrating and transparent vitrifying apparatus for porous optical fiber preform - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a dehydrating and transparently vitrifying apparatus for an porous optical fiber preform which is capable of improving the sealing performance between an upper cap and a lifting shaft and between this upper cap and a furnace core tube or furnace easing. SOLUTION: The upper aperture 2a of the furnace core tube 2 housing the porous optical fiber preform 1 is closed with the upper cap 3. The bottom end of the lifting shaft 4 freely liftably penetrating the upper cap 3 is provided with a preform clamping section 5. The porous optical fiber preform 1 in the furnace core tube 2 is heated by a heater 7 disposed at the outer periphery of the furnace core tube 2. The upper cap 3 is formed of a metal. The preform clamping section 5 is formed of quartz glass or ceramics. At least the inside surface of the upper cap 3 is provided with a corrosion resistant layer. The lifting shaft penetrating section 3a of the upper cap 3 through which the lifting shaft 4 passes is provided with a sealing material 20 made of rubber or resin in such a manner that the lifting shaft 4 may be lifted by maintaining the sealing state. The upper cap 3 is provided with a cooling means 21 for cooling with a cooling medium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for dehydrating and transparently vitrifying an optical fiber porous preform for dehydrating an optical fiber porous preform and performing vitrification.

[0002]

2. Description of the Related Art FIG. 11 is a longitudinal sectional view showing a schematic structure of a conventional apparatus for dehydrating and vitrifying an optical fiber porous preform of this kind.

[0003] A conventional optical fiber porous preform dehydration / transparent vitrification apparatus includes a furnace tube 2 made of quartz glass, carbon or ceramics such as alumina containing an optical fiber porous preform 1 to be treated. Made of quartz glass, carbon or ceramics such as alumina, which is detachably attached to the upper flange portion 2b of the furnace tube 2 so as to cover the upper opening 2a of the furnace tube 2 for taking in and out the optical fiber porous preform 1. An upper cover 3 ', an elevating shaft 4' made of quartz glass or a ceramic such as alumina penetrating the upper cover 3 'so as to be vertically movable, and an optical fiber porous preform 1 provided at a lower end of the elevating shaft 4'. And a furnace body 6 provided on the outer periphery of the furnace tube 2, and a heater 7 in the furnace body 6, and the optical fiber porous base material 1 in the furnace tube 2 is provided by a heater 7 in the furnace body 6. Heat A heating furnace 8, the gas exhaust port 1 for discharging a gas supply port 9 for supplying an internal gas from the lower portion of the core tube 2, a gas furnace heart pipe 2 at the upper side of the core tube 2
0, a gas supply port 11 for supplying an inert gas into the furnace body 6, and an upper opening 2 a of the furnace core tube 2 provided between the upper flange portion 2 b and the upper lid 3 ′ of the furnace tube 2. An annular core tube upper seal gas supply body 12 to be sealed was mainly constituted.

The elevating shaft 4 'is moved up and down by an elevating mechanism (not shown) disposed above the furnace tube 2, and is rotated around its axis by a rotating mechanism such as a motor (not shown). Exhaust gas in the furnace tube 2 discharged from the gas discharge port 10 of the furnace tube 2 is supplied to an exhaust gas treatment device (not shown) via an exhaust pipe 13 and a pressure control valve 14 provided in the middle thereof. It has become. Exhaust gas from the furnace body 6 is supplied to an exhaust gas treatment device (not shown) through an exhaust pipe 15 and a pressure control valve 16 provided in the middle of the exhaust pipe 15. The gas pressure in the furnace body 6 and the gas pressure in the furnace core tube 2 are compared by a differential pressure gauge 17, and the pressure control valves 14, 16 are controlled so that the differential pressure becomes constant.

[0005] Dehydration of such a porous optical fiber preform
The reason why the furnace tube 2 is used in the transparent vitrification apparatus is that a halogen-based gas is used at the time of heat treatment of the optical fiber porous preform 1, and this gas is diffused into the surrounding atmosphere,
This is in order not to enter the furnace body 6.

In this apparatus for dehydrating / clearing the optical fiber porous preform, the upper lid 3 'is removed from the furnace tube 2 and the optical fiber porous preform 1 is moved into the furnace tube 2 by a lowering operation of a lifting mechanism (not shown). Then, the upper cover 3 ′ is put on the upper opening 2 a of the furnace tube 2 so that the atmosphere does not enter the furnace tube 2, and the optical fiber porous preform 1 is lowered by a lowering operation of a lifting mechanism (not shown). While rotating the optical fiber porous preform 1 around its axis while rotating the optical fiber porous preform 1 around its axis while rotating the optical fiber preform 1 into the furnace tube 2 in the required gas atmosphere. The fiber porous preform 1 was subjected to a dehydration treatment or a transparent vitrification treatment.

At this time, since the porous optical fiber preform 1 is heat-treated while being rotated about its axis as described above, the raising and lowering shaft 4 'made of quartz glass or ceramics such as alumina and the upper lid are used. In the case of 3 ′, a certain gap 18 has to be provided between the lifting shaft 4 ′ and the upper lid 3 ′ due to the processing accuracy of the material.
The gas was sealed with an inert gas such as a nitrogen gas or an argon gas ejected from the core tube upper seal gas supply body 12 so as to prevent the atmosphere from entering the core tube 2.

In order to further improve the airtightness between the elevating shaft 4 'and the upper lid 3', a seal member made of an O-ring is provided at the elevating shaft penetrating portion of the upper lid 3 'through which the elevating shaft 4' passes. (See Japanese Patent Application Laid-Open No. 62-27342),
Is provided by providing a sealing material made of carbon fiber in a penetrating portion of the elevating shaft of the upper lid 3 ′ through which the sealing is performed.
No. 6).

[0009]

However, FIG.
In the sealing structure of the elevating shaft penetrating portion of the upper lid 3 'through which the elevating shaft 4' penetrates, the sealing performance when the inside of the furnace tube 2 is vacuumed or pressurized to perform heat treatment cannot be sufficiently obtained. There was a problem.

In the sealing structure described in Japanese Patent Application Laid-Open No. 62-27342, an O-ring is simply provided as a sealing material, so that the O-ring is thermally damaged by heat during heat treatment, and Has low durability and is difficult to put into practical use.

In the sealing structure described in Japanese Utility Model Laid-Open No. 4-18626, carbon fibers used as a sealing material generate dust due to abrasion of the carbon fibers when the elevating shaft 4 'moves up and down. There is a problem that foreign matter adheres to the optical fiber porous preform 1 after entering the tube 2.

In the conventional apparatus for dehydrating / clearing a porous optical fiber preform, the transmission characteristics of the optical fiber are reduced when the gas (mainly He gas) supplied into the furnace tube 2 is reduced. Tends to worsen. Because of this, expensive H
There was a problem that the supply amount of e gas could not be reduced. Also,
The problem of the He gas tends to become more prominent as the outer diameter of the porous optical fiber preform increases and the diameter of the core tube 2 increases.

When the furnace tube 2 is formed of a quartz material,
When the heating temperature by the heater 7 becomes 1300-1400 ° C or more,
The core tube 2 is softened and deformed. In order to prevent this deformation, the pressure in the furnace core tube 2 is reduced by several mmA from the pressure in the furnace body 6.
There is a restriction that qA to 10 mmAq must be higher. When a carbon heater is used as the heater 7,
Since the pressure in the furnace body 6 must be several mmAq higher than the atmospheric pressure, the pressure in the furnace core tube 2 is set to be more than 10 mm
Usually, Aq is increased. If the gas in the furnace tube 2 higher than 10 mmAq is supplied with an inert gas such as a nitrogen gas or an argon gas so as to prevent the air from entering through a gap between the upper lid and the elevating shaft, the gas sealing is considerably performed. There is a problem that the amount of the sealing gas is required.

An object of the present invention is to provide an apparatus for dehydrating / clearing a porous optical fiber preform capable of improving the sealing performance between the upper lid and the elevating shaft and between the upper lid and the furnace tube or the furnace body. To provide.

Another object of the present invention is to provide a porous optical fiber preform which is hardly thermally damaged even if a rubber or resin sealing material is interposed between the upper lid and the elevating shaft, wherein the upper lid is made of metal. An object of the present invention is to provide an apparatus for dehydration and transparent vitrification.

Another object of the present invention is to provide an apparatus for dehydrating and vitrifying a porous optical fiber preform, which can easily perform heat treatment in a vacuum state or a pressurized state in a furnace tube. It is in.

Another object of the present invention is to provide an apparatus for dehydrating and transparently vitrifying a porous optical fiber base material which is hardly corroded by a processing gas even with a metal upper lid and a vertical shaft.

Another object of the present invention is to provide an apparatus for dehydrating and transparently vitrifying an optical fiber porous preform which can prevent radiant heat in a furnace tube from being transmitted to an upper lid.

Another object of the present invention is to provide an apparatus for dehydrating and vitrifying a porous optical fiber preform which can suppress the flow of a sealing gas into a processing chamber in which a porous optical fiber preform exists. .

Another object of the present invention is to provide a method for heat-treating a porous optical fiber preform without moving the porous optical fiber preform in a furnace tube, and dehydrating and vitrifying the optical fiber porous preform. It is to provide a device.

[0021]

SUMMARY OF THE INVENTION The present invention is directed to a furnace tube made of quartz glass or ceramics for accommodating a porous optical fiber preform to be treated, and an upper opening of the furnace tube for inserting and removing the porous optical fiber preform. An upper lid detachably attached to an upper part of the furnace tube so as to close the portion, an elevating shaft penetrating the upper lid so as to be vertically movable, and an upper part of an optical fiber porous preform provided at a lower end of the elevating shaft. A base material gripping portion, a heating furnace provided on the outer periphery of the furnace tube and heating the optical fiber porous base material in the furnace tube with a heater, a gas supply port for supplying gas from the lower portion of the furnace tube to the inside, It is an object of the present invention to improve a dewatering / clear vitrification apparatus for an optical fiber porous preform having a gas discharge port for discharging gas in the furnace tube at an upper side of the furnace tube.

In the apparatus for dehydrating and vitrifying an optical fiber porous preform according to the present invention, the upper lid is formed of metal, and the preform holding portion is formed of quartz glass or ceramics. At least the inner surface of the upper lid is provided with a corrosion-resistant layer. A rubber or resin sealing material is provided in a vertical shaft penetrating portion of the upper lid through which the vertical shaft penetrates so that the vertical shaft can be raised and lowered while maintaining a sealed state. The space between the top lid and the furnace tube or furnace body is sealed with a rubber or resin sealing material. The upper lid is provided with cooling means for cooling the upper lid with a cooling medium.

By forming the upper lid from metal in this way,
Processing of the sealing material forming the seal portion between the upper lid and the elevating shaft is facilitated, the clearance between the upper lid and the elevating shaft can be made as small as possible, and the seal between both is made of rubber or resin. The sealing material can be easily used. Also,
When sealing is performed between the upper lid and the elevating shaft and between the upper lid and the furnace tube or the furnace body with a sealing material made of rubber or resin, the sealing can be reliably performed without emitting dust at the upper part of the furnace tube. In addition, if the seal between the upper lid and the elevating shaft and between the upper lid and the furnace tube or the furnace body can be reliably sealed, heat treatment in a vacuum state or a pressurized state in the furnace tube can be easily performed. . Further, even with a sealing material made of rubber or resin, since the upper lid is cooled by the cooling means using the cooling medium, the sealing material can be more effectively prevented from being thermally damaged.

When both the upper lid and the elevating shaft are formed of metal, the accuracy of the sealing portion can be further improved, and the sealing can be performed reliably. In this case,
It is desirable to provide a corrosion-resistant layer on the surface of the elevating shaft.

Further, even when the elevating shaft is made of metal, since the base material holding portion is formed of quartz glass or ceramics, foreign substances are removed from the base material holding portion close to the optical fiber porous base material. Intrusion into the porous base material can be avoided as much as possible. Further, even if both the upper lid and the elevating shaft are made of metal, since at least the inner surface of the upper lid and the surface of the elevating shaft are provided with a corrosion-resistant layer, they can be prevented from being corroded by the processing gas. .

In this case, it is preferable that the upper lid is provided with a heat insulating material covering the inner surface thereof. By doing so, it is possible to reduce the transmission of radiant heat from the heater to the upper lid, and to prevent the sealant from being thermally damaged.

Preferably, the upper lid is provided with an inert gas passage for covering the inner surface thereof and for flowing an inert gas over the surface of the elevating shaft protruding into the upper lid. With this configuration, the inert gas flow flowing through the inert gas passage can suppress the corrosive gas from reaching the metal upper lid and the elevating shaft.

Further, it is preferable that a heat insulating means for preventing radiant heat in the furnace tube from being transmitted to the upper lid is supported on the upper portion of the base material holding portion. By doing so, it is possible to suppress a rise in the temperature of the upper lid due to radiant heat in the furnace tube, and to more effectively prevent the rubber or resin sealing material from being thermally damaged.

Further, gas shielding means for preventing a sealing gas for sealing between the upper lid and the elevating shaft passing therethrough from flowing into the processing chamber in which the optical fiber porous preform exists is provided. It is preferably provided between the heat insulating means. With this configuration, it is possible to prevent the seal gas from flowing into the processing chamber where the optical fiber porous preform exists. Therefore, the processing gas supplied to the processing chamber (mainly He gas)
By reducing the amount of gas, deterioration of the transmission characteristics of the optical fiber can be suppressed. Therefore, the amount of expensive He gas used can be reduced. Further, even when the diameter of the core tube becomes large, it is possible to suppress the deterioration of the transmission characteristics of the optical fiber.

Preferably, the heat insulating means is always arranged below the gas discharge port during the processing of the porous optical fiber preform. In this way, the seal gas that has entered the furnace tube is discharged to the outside from the gas discharge port of the furnace tube, and the seal gas flows more effectively into the processing chamber in which the optical fiber porous preform exists. Can be reduced.

It is preferable that the gas shielding means is always disposed above the gas discharge port while the porous optical fiber preform is subjected to the heat treatment. This way,
The portion in the furnace tube between the top lid and the gas shielding means serves as a buffer chamber, and the gas pressure in the buffer chamber can be slightly higher than the portion connected to the gas discharge port,
The amount of sealing gas can be reduced.

Further, at the upper part of the preform holding portion, the radiant heat in the furnace tube is prevented from being transmitted to the upper lid, and is always disposed below the gas discharge port while the porous optical fiber preform is subjected to the heat treatment. Preferably, a gas shielding and heat insulating means is supported. By doing so, it is possible to reduce the transmission of radiant heat from the heater to the upper lid, and to prevent the sealant from being thermally damaged. In addition, it is possible to prevent the sealing gas from flowing into the processing chamber where the optical fiber porous preform exists, and therefore, the processing gas (mainly H
By reducing e-gas), it is possible to suppress the deterioration of the transmission characteristics of the optical fiber, and it is possible to reduce the amount of expensive He gas used. Further, even when the diameter of the core tube becomes large, it is possible to suppress the deterioration of the transmission characteristics of the optical fiber.

Furthermore, it is preferable that the heaters are provided in multiple stages in the direction along the longitudinal direction of the porous optical fiber preform. In this manner, by controlling the energization of the multi-stage heater, a desired position in the longitudinal direction of the optical fiber porous preform can be heated without moving the optical fiber porous preform in the furnace tube. Therefore, it is possible to reduce the friction in the shaft seal portion caused by elevating the elevating shaft, thereby prolonging the service life, and has the advantage that the processing accuracy of the elevating shaft is not strictly required.

[0034]

FIG. 1 is a longitudinal sectional view showing a schematic configuration of a first example of an embodiment of a dehydration / transparent vitrification apparatus for an optical fiber porous preform according to the present invention. Note that parts corresponding to those in FIG. 11 described above are denoted by the same reference numerals.

In the apparatus for dehydrating and vitrifying an optical fiber porous preform, the upper lid 3 is formed of a metal such as stainless steel, and the preform holding portion 5 is formed of quartz glass or ceramics. At least the inner surface of the upper lid 3 (preferably, the entire surface of the upper lid 3) is provided with a corrosion-resistant layer such as a polytetrafluoroethylene coating or a titanium coating.

The elevating shaft 4 is desirably formed of quartz glass. In this case, the surface of the elevating shaft 4 is sufficiently finished, or the surface of the elevating shaft 4 is covered with a resin such as polytetrafluoroethylene to reduce wear of a seal member 20 described later. Becomes Further, the elevating shaft 4 may be formed of the same metal as the upper lid 3.
In this case, a corrosion-resistant layer similar to that of the upper lid 3 is provided on the surface of the elevating shaft 4.

The inner periphery of a shaft through hole 19 provided in the elevating shaft penetrating portion 3a of the upper lid 3 through which the elevating shaft 4 passes is made of rubber or polytetrafluorocarbon so that the elevating shaft 4 can be moved up and down while maintaining a sealed state. A ring-shaped sealing material 20 made of a resin such as ethylene is supported. The upper lid 3 is provided with cooling means 21 for cooling the upper lid 3 with a cooling medium such as cooling water. The cooling means 21 of this example is constituted by a cooling pipe 22 wound around the outer periphery of the upper lid 3 and attached so as to be able to transfer heat by welding or the like.

The upper cover 3 is provided with a heat insulating means 23 covering the inner surface thereof. The heat insulating means 23 has a structure in which a heat insulating material 23a such as a carbon felt molded body or quartz wool is covered with a quartz cover 23b made of quartz. At the center of the heat insulating means 23, a through hole 23c through which the elevating shaft 4 passes is provided, and the inner periphery of the through hole 23c is also covered with a quartz cover 23b. When a carbon felt molded body is used as the heat insulating material 23a of the heat insulating means 23, the quartz cover 23b is not required from the viewpoint of giving the heat insulating means 23 a required shape for attaching to the inner surface of the upper lid 3. There is a quartz cover 2
3b is required.

Further, in the apparatus of this embodiment, a seal member 24 made of a resin O-ring such as rubber or polytetrafluoroethylene is also interposed between the annular core tube upper seal gas supply body 12 and the upper lid 3. Thus, a seal is provided between the furnace tube upper seal gas supply body 12 and the upper lid 3.

Under the lifting shaft 4, the upper lid 3 is provided with a heat insulating means 2.
3 is provided with a lid supporter 25 that supports the cover 3. The shaft portion 5a of the base material gripping portion 5 is connected to the lid support portion 25.

The other structure is the same as that of the conventional optical fiber porous preform dewatering / transparent vitrification apparatus shown in FIG.

Dehydration of such a porous optical fiber preform
When the upper opening 2a of the furnace tube 2 is opened by the transparent vitrification apparatus, the elevating shaft 4 is raised by operating an elevating mechanism (not shown), and a lid provided below the elevating shaft 4 in the process. The support portion 25 hits the upper lid 3 via the heat insulating means 23, the upper lid 3 moves up together and separates from the upper end of the furnace tube 2, and the upper opening 2 a of the furnace tube 2 is opened.

When the preform grasping portion 5 is stopped at a position away from the upper end of the furnace tube 2, the preform grasping portion 5 grasps the optical fiber porous preform 1 at the position of the starting rod 1a made of quartz glass. Thereafter, the elevating shaft 4 is lowered by operating a lifting mechanism (not shown), and the optical fiber porous preform 1 and the upper lid 3 are lowered together, so that the optical fiber porous preform 1 is
And the upper lid 3 is placed on the upper end of the furnace core tube 2 via the furnace tube upper seal gas supply body 12, and the upper opening 2a of the furnace tube 2 is closed.

In this state, the inside of the furnace core tube 2 is set to the required gas atmosphere supplied from the gas supply port 9 and heated by the heater 7 to perform dehydration processing and transparent vitrification processing of the optical fiber porous preform 1. .

In the dehydration treatment, the inside of the furnace tube 2 is switched from the gas (Ar gas) during standby to He gas, the temperature of the heating furnace 8 is heated by the heater 7 to the temperature of the dehydration treatment, and the temperature of the heater 7 is stabilized. Then, while further supplying oxygen and chlorine into the furnace tube 2, the optical fiber porous preform 1 is gradually lowered at a predetermined speed to change the heated portion of the optical fiber porous preform 1. While doing.

After the dehydration treatment is completed, the optical fiber porous preform 1 is pulled up to the dehydration start position, and then the vitrification treatment is performed. That is, in the transparent vitrification process, the gas supplied into the processing chamber 2c of the furnace core tube 2 is changed to gas conditions for the transparent vitrification process, and the heater 7 is heated to the transparent vitrification temperature. , Porous optical fiber preform 1
Is performed at a predetermined speed to change the heated portion of the porous optical fiber preform 1 to obtain a transparent optical fiber preform.

When the transparent vitrification process is completed, the elevating shaft 4 is raised by operating a raising / lowering mechanism (not shown), and the upper lid 3 is raised together. The upper lid 3 is separated from the upper end of the furnace tube 2 and the transparent optical fiber When the material comes out of the core tube 2, the transparent optical fiber preform is removed from the preform holding portion 5.

During such processing, the gas discharge port 10
The exhaust gas discharged from is processed by an exhaust gas treatment device (not shown).

As described above, in this apparatus for dehydrating / clearing the optical fiber porous preform, since the upper lid 3 is formed of metal, the sealing material for forming the sealing portion between the upper lid 3 and the elevating shaft 4 is formed. 20 can be easily processed, the clearance between the upper cover 3 and the elevating shaft 4 can be made as small as possible, and the seal between them can be easily made by the ring-shaped sealing material 20 made of rubber or resin. Can be done. In addition, top cover 3
When the space between the furnace tube 4 and the furnace tube 2 and the space between the furnace tube 2 and the upper lid 3 are sealed with sealing materials 20 and 24 made of rubber or resin, it is possible to reliably seal the upper portion of the furnace tube 2 without emitting dust. Can be. Further, if the seal between the upper lid 3 and the elevating shaft 4 and between the upper lid 3 and the furnace tube 2 can be reliably sealed, heat treatment in a vacuum state or a pressurized state in the furnace tube 2 is easily performed. be able to. Further, even with the sealing materials 20 and 24 made of rubber or resin, since the upper lid 3 is cooled by the cooling means 21 using a cooling medium, the sealing materials 20 and 24 are formed.
Can be prevented from being thermally damaged.

Even when the lifting shaft 4 is made of metal,
Since the base material holding portion 5 is made of quartz glass or ceramics, it is possible to prevent foreign matter from entering the optical fiber porous base material 1 from the base material holding portion 5 close to the porous optical fiber base material 1. Can be avoided. In addition, metal upper lid 3
Even if the elevating shaft 4 is made of metal, since at least the inner surface of the upper lid 3 and the surface of the elevating shaft 4 are provided with a corrosion-resistant layer, they can be prevented from being corroded by the processing gas.

In this case, since the heat insulating means 23 is provided on the upper lid 3 so as to cover the inner surface thereof, the transmission of the radiant heat from the heater 7 to the upper lid 3 can be reduced, and the rubber or resin sealing member 20 is formed. Can be prevented from being thermally damaged. In this case, since the surface of the heat insulating means 23 is covered with the quartz cover 23b, the fibers of the heat insulating material 23a such as the carbon felt molded body or the quartz wool fall to the optical fiber porous preform 1 side during the elevating operation. Can be prevented.

FIGS. 2 and 3 show a schematic configuration of a second embodiment of the dehydrating / transparent vitrification apparatus for an optical fiber porous preform according to the present invention. FIG. FIG. 3 is an enlarged vertical sectional view of the heat insulating means mounted on the base material holding portion in FIG. 2. Note that parts corresponding to those in FIG. 1 described above are denoted by the same reference numerals.

In this optical fiber porous preform dehydration / transparent vitrification apparatus, a furnace body 6 is provided so as to cover the entire furnace tube 2, and a furnace tube is provided between the furnace tube 2 and the furnace body 6. 2
A heat insulator 26 made of carbon felt or the like is disposed so as to cover the heater 7 and the heater 7. A flange 6 a that supports the upper lid 3 is provided on the outer periphery of the upper end of the furnace body 6.

Both the upper lid 3 and the elevating shaft 4 are made of a metal such as stainless steel as in the first embodiment.
Is formed of quartz glass or ceramics.

In this embodiment, the metal upper lid 3 is directly mounted on the flange portion 6a of the furnace body 6 via a sealing member 24 made of a resin O-ring such as rubber or polytetrafluoroethylene. Corrosion-resistant layers such as polytetrafluoroethylene coating, titanium coating, and nickel coating are provided on at least the inner surface of the upper lid 3 (preferably, the entire surface of the upper lid 3) and the surface of the elevating shaft 4 as in the first example. Have been. Further, a heat insulating means 23 is attached to the upper lid 3 so as to cover the inner surface thereof as in the first example. The heat insulating means 23 also has a structure in which a heat insulating material 23a such as a carbon felt molded body or quartz wool is covered with a quartz cover 23b made of quartz. In the center of the heat insulating means 23, a through hole 23c through which the elevating shaft 4 passes is provided.
The inner periphery of the through hole 23c is also covered with the quartz cover 23b.

As in the first example, the elevating shaft 4 can be moved up and down in the inner periphery of the shaft through hole 19 provided in the elevating shaft penetrating portion 3a of the upper lid 3 through which the elevating shaft 4 passes. Rubber or resin O such as polytetrafluoroethylene
A seal member 20 made of a ring is supported. Top lid 3
Is provided with a cooling means 21 for cooling the upper lid 3 with a cooling medium such as cooling water as in the first example. The cooling means 21 of this example is also constituted by a cooling pipe 22 wound around the outer periphery of the upper lid 3 and attached so as to be able to transfer heat by welding or the like.

A heat insulating means 28 for preventing the radiant heat in the furnace tube 2 from being transmitted to the upper lid 3 is attached to the upper part of the base material gripping portion 5. The heat insulating means 28 has a structure in which a heat insulating material 27a such as a carbon felt molded body or quartz wool is covered with a quartz cover 27b made of quartz. Insulation means 28
A through hole 29 is formed at the center of the hole to penetrate the elevating shaft 4, and the inner periphery of the through hole 29 is also covered with a quartz cover 27b.

In this example, since the furnace body 6 is provided so as to cover the entire furnace tube 2, a gas supply connection pipe 30 is supported at a lower portion of the furnace body 6, and a joint 30 at the upper end thereof is provided.
The gas supply port 9 is inserted and connected to a.

The dehydrating / transparent vitrifying apparatus for an optical fiber porous preform having such a structure can obtain the following effects in addition to the effects of the first example described above.

That is, in this embodiment, the furnace body 6 is provided so as to cover the entire furnace tube 2, and between the furnace tube 2 and the furnace body 6, the carbon felt forming is performed by covering the furnace tube 2 and the heater 7. Since the heat insulating material 26 made of a body or the like is disposed, the heat from the heater 7 for heating the optical fiber porous preform 1 is not released to the outside, and the optical fiber porous preform 1 is efficiently heated. Can be. In addition, a heat insulating means 2 for preventing radiant heat in the furnace tube 2 from being transmitted to the upper lid 3 is provided above the base material gripping portion 5.
8 is provided, the temperature of the upper lid 3 can be prevented from rising due to radiant heat in the furnace tube 2, and the rubber or resin sealing material 20 can be more effectively prevented from being thermally damaged. Can be. In this case, since the surface of the heat insulating means 28 is covered with the quartz cover 27b, the fibers of the heat insulating material 27a such as a carbon felt molded body or quartz wool fall to the optical fiber porous preform 1 side during the elevating operation. Can be prevented.

FIG. 4 shows a third embodiment of an apparatus for dehydrating and vitrifying an optical fiber porous preform according to the present invention.
It is a longitudinal section showing the schematic structure of the example. Parts corresponding to those in FIG. 2 described above are denoted by the same reference numerals.

In this apparatus for dehydrating and vitrifying an optical fiber porous preform, a plurality of heaters 7 a to 7 e are provided in multiple stages in the direction along the longitudinal direction of the optical fiber porous preform 1 on the outer periphery of the furnace tube 2. It has a multi-heater type structure.

Further, a heat insulating means 23 for covering the inner surface of the metal upper lid 3 is attached. This heat insulating means 23
A heat insulating material 23a such as a carbon felt molded body or quartz wool covering the inner surface of the upper lid 3 and a quartz cover 23b covering a surface of the heat insulating material 23a opposed to the inside of the furnace tube 2 are provided. The quartz cover 23b is provided so as to cover the inner surface where the heat insulating material 23a does not exist on the rising portion 3b side provided in the center of the upper lid 3. In addition, quartz cover 23b
Are provided so as not to contact the lifting shaft 4 as shown in the figure.

In the dehydrating / transparent vitrification apparatus having such a structure, the following effects can be obtained in addition to the effects of the first example.

That is, in this example, the plurality of heaters 7a to 7e
By controlling the energization of the optical fiber porous preform 1, it is possible to heat a desired position in the longitudinal direction of the optical fiber porous preform 1 without moving the optical fiber preform 1 in the furnace tube 2. Reduces friction due to shaft seal, extends service life,
Further, there is an advantage that the accuracy of the elevating shaft 4 is not strictly required. Further, a heat insulating material 23a covering the inner surface of the metal upper lid 3
Since the quartz cover 23b is provided on the surface facing the inside of the furnace tube 2, the fibers of the heat insulating material 23a can be prevented from dropping to the optical fiber porous preform 1 side.

As in this example, in the case of a multi-heater type dehydration / transparent vitrification apparatus in which the optical fiber porous preform 1 does not move up and down in the processing chamber 2c of the furnace tube 2 during processing, the lifting shaft 4
Can be made of metal. In this case, a corrosion-resistant layer similar to that of the upper lid 3 is provided on the surface of the metal lifting shaft 4.

FIG. 5 shows a fourth embodiment of the apparatus for dehydrating and vitrifying an optical fiber porous preform according to the present invention.
It is a longitudinal section showing the schematic structure of the example. Parts corresponding to those in FIG. 4 described above are denoted by the same reference numerals.

In the apparatus for dehydrating and vitrifying a porous optical fiber preform, the structure of the multi-heater type shown in FIG. 4 is further improved.

That is, the radiant heat in the furnace core tube 2 is placed on the upper part of the base material holding portion 5 in the same manner as in the second example shown in FIG.
A heat insulating means 28 for preventing transmission to the outside is attached. The heat insulating means 28 is made of a heat insulating material 27a such as a carbon felt molded body or quartz wool and a quartz cover 27b made of quartz.
It has a structure covered with. The heat insulating means 28 also serves as a gas shielding means for preventing gas in the furnace from flowing to the upper lid 3 in addition to the heat shielding means. At the center of the heat insulating means 28, there is provided a through hole 29 through which the elevating shaft 4 penetrates, and the inner periphery of the through hole 29 is also covered with a quartz cover 27b.
Also in this example, similarly to the third example shown in FIG. 4, a heat insulating means 23 for covering the inner surface of the metal upper lid 3 is attached. The heat insulating means 23 includes a heat insulating material 23 a such as a carbon felt molded body or quartz wool covering the inner surface of the upper lid 3,
A quartz cover 23b covers a surface of the heat insulating material 23a facing the inside of the furnace tube 2. Quartz cover 23b
Are provided so as not to contact the lifting shaft 4 as shown in the figure. Rising portion 3b provided at the center of upper lid 3
Another quartz cover 32 is provided on the inner surface where the heat insulating material 23 does not exist on the side.

Between the inner surface of the upper cover 3 facing the heat insulating means 23 and the heat insulating means 23, an inert gas passage 33 for covering the inner surface of the upper cover 3 facing the heat insulating means 23 and flowing an inert gas is provided. Is provided. In the inert gas passage 33,
An inert gas such as N2, He or Ar gas is supplied from an inert gas supply port 34 provided on the surface of the upper lid 3. The inert gas flowing through the inert gas passage 33 is blown out from between the upper end of the quartz cover 27 b and the upper lid 3 toward the shaft 5 a of the base material gripping portion 5.

Further, between the inner surface of the upper cover 3 facing the quartz cover 32 and the cover 32, there is no inert gas flowing over the inner surface of the rising portion 3b of the upper cover 3 facing the cover 32. An active gas passage 35 is provided. An inert gas such as He gas is supplied to the inert gas passage 35 from an inert gas supply port 36 provided on the surface of the upper lid 3. The inert gas flowing through the inert gas passage 35 is blown out from between the upper end of the cover 32 and the rising portion 3 b of the upper lid 3 so as to descend along the surface of the elevating shaft 4.

In this multi-heater type dehydrating / transparent vitrifying apparatus, it is desirable that the elevating shaft 4 is made of metal in terms of cost and the like. In this case, a corrosion-resistant layer is provided on the surface of the metal lifting shaft 4.

The other structure is the same as that of the third embodiment of the present invention shown in FIG.

The dehydrating / transparent vitrification apparatus for an optical fiber porous preform having such a structure can obtain the following effects in addition to the effects of the third example described above.

As in the second example, the heat insulating means 28 for preventing the radiant heat in the furnace tube 2 from being transmitted to the upper lid 3 is disposed above the base material gripping portion 5. The temperature rise of the upper lid 3 due to the radiant heat can be suppressed, and the rubber or resin sealing material 20 can be more effectively prevented from being thermally damaged. In addition, since the surface of the heat insulating material 27a such as a carbon felt molded product or quartz wool is covered with the quartz cover 27b, the heat insulating means 28 causes the fibers of the heat insulating material 27a to move toward the optical fiber porous preform 1 during the elevating operation. Falling can be prevented. Further, the upper lid 3 is provided with inert gas passages 33 and 35 covering the inner surface thereof, so that the inert gas flows over the inner surface of the upper lid 3 so that the corrosive gas is made of metal. Reaching the upper lid 3 and the elevating shaft 4 can be suppressed.

FIG. 6 shows a fifth embodiment of the apparatus for dehydrating and vitrifying a porous optical fiber preform according to the present invention.
It is a longitudinal section showing the schematic structure of the example. Parts corresponding to those in FIG. 5 described above are denoted by the same reference numerals.

This optical fiber porous preform dewatering / transparent vitrification apparatus is a modification of the fourth example shown in FIG. In this apparatus, Ar, N2 are used as seal gases.
When an inert gas such as a gas is used, a heat insulating means 28 also serving as a gas shielding means for covering the base material gripping portion 5 is arranged below the gas discharge port 10, and in this state, the optical fiber porous preform 1 Heat treatment is performed.

When the heat insulating means 28 also serving as a gas shielding means is disposed below the gas discharge port 10 and heat treatment is performed, a sealing gas such as Ar or N 2 gas may enter the furnace tube 2. The gas is discharged from the gas discharge port 10 and hardly enters the processing chamber 2 c containing the optical fiber porous preform 1.

The inventors of the present invention have conducted intensive studies on the phenomenon that the transmission characteristics of the optical fiber deteriorate when the gas (mainly He gas) supplied into the furnace tube 2 is reduced. Was found to occur when the gap between the core tube 2 and the optical fiber porous preform 1 was considerably large.

This is because the He gas in the processing gas is lighter than the N 2 gas which is the sealing gas, and the density of the gas further decreases by heating, so that the density difference from the sealing gas becomes very large. It was found that a density difference flow occurred in the furnace tube 2 and N2 gas flowed into the vicinity of a heating portion of the furnace tube 2 by a heater. This tendency is considered to increase when the processing gas is small.
It is considered that this is likely to occur even when the distance between the optical fiber preform 1 and the optical fiber preform 1 is large, and this is one of the causes of the increase in the transmission loss of the optical fiber.

As a countermeasure against this, the sealing gas is
A gas shielding means for making it difficult to flow to the lower side of the inside, or a heat insulating means 28 also serving as a gas shielding means is provided. The gas shielding means, or the heat insulating means 28 also serving as the gas shielding means,
Preferably, it is located below the gas discharge port 10.
By doing so, the sealing gas hardly flows below the gas shielding means or the heat insulating means 28 also serving as the gas shielding means. Further, in addition to the gas shielding means or the heat insulating means 28 which also serves as a gas shielding means, by further providing a gas shielding means on the gas discharge port 10 in the furnace tube 2, the sealing gas can be reduced below the furnace tube 2. The flow to the side can be further reduced. In this case, the space between the upper lid and the gas shielding means is a buffer chamber for storing the seal gas, and the pressure is slightly higher than the pressure of the buffer chamber for exhausting, so that the amount of the seal gas can be reduced.

FIG. 7 is a longitudinal sectional view showing a schematic configuration of a sixth example of the embodiment of the dehydration / transparent vitrification apparatus for an optical fiber porous preform according to the present invention provided with the above gas shielding means. It is. Note that parts corresponding to those in FIG. 1 described above are denoted by the same reference numerals.

In this apparatus for dehydrating and vitrifying a porous optical fiber preform, a heat insulating means 37 which also functions as a gas shielding means is provided so as to surround the preform holding part 5. The heat insulating means 37 also serving as the gas shielding means is constituted by an annular heat insulating material 38 and a gas shielding material 39 covering the surface of the heat insulating material 38. The outer diameter of the heat insulating means 37 also serving as the gas shielding means is formed, for example, about 5 to 20 mm smaller than the inner diameter of the furnace tube 2. The gap 40 between the heat insulating means 37 also serving as the gas shielding means and the furnace tube 2 is
The size of the seal gas is set to be smaller than the size at which the flow of the processing gas in the processing chamber 2c can prevent the sealing gas from falling into the processing chamber 2c due to the density difference flow. The gap 40 is changed depending on the processing gas amount, the sealing gas amount, the gap between the furnace core tube 2 and the optical fiber porous preform 1, and the like. Further, the heat insulating means 37 also serving as the gas shielding means is mounted on the base material gripping portion 5 so as to move up and down together when the elevating shaft 4 moves up and down. Other configurations are the same as those in FIG.

The dehydration of such a porous optical fiber preform
In the transparent vitrification apparatus, the optical fiber porous preform 1 is inserted to a predetermined position in the furnace tube 2, and in this state, the heat insulating means 37 serving also as a gas shielding means is positioned below the gas discharge port 10. Then, the gas supplied into the furnace tube 2 is switched to the processing gas, and the temperature of the heater 7 is raised to the processing temperature. Thereafter, processes such as dehydration and transparent vitrification are performed in the same manner as in the above-described example and the conventional example.

In this embodiment, since the heat insulating means 37 also serving as a gas shielding means is disposed below the gas discharge port 10 as shown in the figure, even if a part of the sealing gas comes down due to the density difference flow. In addition, it can only reach the upper surface of the heat insulating means 37 which also serves as a gas shielding means. Therefore, the optical fiber porous preform 1
Almost never becomes a seal gas atmosphere. Further, according to the heat insulating means 37 also serving as the gas shielding means, the heat insulating means 3
7 can be prevented from being heated to a high temperature by heating the atmosphere above the metal lid 7 and the resin sealing material 20 such as rubber or polytetrafluoroethylene. And the accuracy of the gap between them can be improved. As a result, the amount of the sealing gas could be reduced. In addition, it was found that the heat insulation effect was improved, the radiant heat to the upper lid 3 side was reduced, the heater power was also reduced, and there was also an energy saving effect. Further, it is possible to prevent the base material gripping portion 5 made of quartz from being deformed by heat.
Furthermore, the transmission loss of the optical fiber is reduced from 3/4 to 1 /
It could be reduced to about 2.

In this example, as the optical fiber porous preform 1 is raised and lowered, the heat insulating means 37 also serving as a gas shielding means is moved up and down.
In the case where the gap 40 between the gasket 7 and the elevating shaft 4 is bent due to thermal deformation, the heat insulating means 37 also serving as a gas shielding means is provided.
May touch the inner surface of the core tube 2. Glass fine particles may adhere to the inner surface of the furnace tube 2, and the adhering material on the inner surface of the furnace tube 2 is peeled off by raising and lowering the heat insulating means 37 also serving as a gas shielding means, and the optical fiber porous preform 1 is removed. There is a possibility that it may adhere and cause surface defects in the optical fiber porous preform 1. Therefore, the heat insulating means 3 also serving as a gas shielding means is provided below the gas discharge port 10 on the inner surface of the furnace tube 2.
7, a stopper for receiving the heat insulating means 37 serving also as a gas shielding means, and the heat insulating means 37 serving also as a gas shielding means is not lowered below the stopper so as to prevent the optical fiber porous preform 1 from being lowered. Almost no surface defects.

FIG. 8 is a longitudinal sectional view showing a schematic configuration of a seventh example of an embodiment of a dehydrating / transparent vitrifying apparatus for an optical fiber porous preform according to the present invention provided with the above gas shielding means. It is. Parts corresponding to those in FIG. 7 described above are denoted by the same reference numerals.

In this apparatus for dehydrating / clearing the optical fiber porous preform, the heat insulating means 37 which is supported by the preform holding portion 5 in the furnace tube 2 below the gas discharge port 10 and also serves as a gas shielding means is provided. The gas shielding means 41 is disposed above the gas discharge port 10 and the stopper 4 on the inner surface of the core tube 2 is disposed.
The gas shielding means 41 is positioned so that it does not descend downward. A hole 43 through which the elevating shaft 4 penetrates is provided at the center of the gas shielding means 41. Is provided, and a buffer chamber 45 is provided between the heat insulating means 37 serving also as a gas shielding means and the gas shielding means 41, and the gas shielding means 41 and the upper lid 3 are provided.
And a buffer chamber 46 is provided between them. Other configurations are the same as those in FIG.

The dehydration of such a porous optical fiber preform
In the transparent vitrification apparatus, the sealing gas is rectified by the gas passage hole 44 of the gas shielding means 41, flows into the buffer chamber 45, and flows out from the gas discharge port 10. Also, since the flow of the seal gas is restricted by the gas passage hole 44,
The gas pressure in the buffer chamber 46 between the gas shielding means 41 and the upper lid 3 increases, whereby the sealing performance increases and the amount of the sealing gas can be reduced. Further, it is possible to prevent the processing gas from entering the buffer chamber 46.

FIG. 9 is a longitudinal sectional view showing a schematic configuration of an eighth embodiment of an apparatus for dehydrating / transparent vitrifying an optical fiber porous preform according to the present invention provided with the gas shielding means as described above. It is. Note that parts corresponding to those in FIG. 8 described above are denoted by the same reference numerals.

In the apparatus for dehydrating and vitrifying a porous optical fiber preform, a gas shielding means 41 for supporting a stopper 42 is provided on the elevating shaft 4. Other configurations are the same as those of the seventh example shown in FIG.

With such a configuration, the same effect as in the seventh example shown in FIG. 8 can be obtained.

FIG. 10 is a longitudinal sectional view showing a schematic configuration of a ninth embodiment of an apparatus for dehydrating / transparent vitrifying an optical fiber porous preform according to the present invention provided with the above gas shielding means. It is. Note that parts corresponding to those in FIG. 8 described above are denoted by the same reference numerals.

In the apparatus for dehydrating and vitrifying an optical fiber porous preform, a multi-heater type heat insulating means 37 which also serves as a gas shielding means and a gas shielding means 4 shown in FIG.
1 is provided. In this example, the core tube 2
The upper flange 2b also serves as a stopper 42 and supports the gas shielding means 41.

Even with such a multi-heater type, the same effect as that of the seventh example shown in FIG. 8 can be obtained.

The elevating shaft 4 shown in each of the above examples has a structure in which the cooling medium flows from the outward path to the inward path by, for example, a double pipe, and the elevating shaft 4 is cooled.
Such a structure is easy to process when the elevating shaft 4 is made of metal and easy to maintain the mechanical strength, and can be implemented without any trouble.

[0097]

In the apparatus for dehydrating / transparent vitrifying an optical fiber porous preform according to the present invention, since the upper lid is made of metal, the processing of the sealing material forming the sealing portion between the upper lid and the elevating shaft is performed. And the clearance between the upper lid and the elevating shaft can be made as small as possible, and the sealing between them can be easily performed with a sealing material made of rubber or resin. In addition, by sealing between the upper lid and the elevating shaft and between the upper lid and the furnace tube or the furnace body with a sealing material made of rubber or resin, the upper part of the furnace tube is reliably sealed without emitting dust. be able to. In addition, since the seal between the upper lid and the elevating shaft and between the upper lid and the furnace tube or the furnace body can be reliably performed, heat treatment in a vacuum state or a pressurized state in the furnace tube can be easily performed. Can be. Furthermore, even if the sealing material is made of rubber or resin, since the upper lid is structured to be cooled by the cooling means using the cooling medium, the sealing material can be prevented from being thermally damaged.

When both the upper lid and the elevating shaft are formed of metal, the accuracy of the sealing portion can be further improved, and the sealing can be performed reliably. Further, even when the elevating shaft is made of metal, since the base material holding portion is formed of quartz glass or ceramics, foreign substances are removed from the base material holding portion close to the optical fiber porous base material. Intrusion into the material can be avoided as much as possible. Further, even if both the upper lid and the elevating shaft are made of metal, since at least the inner surface of the upper lid and the surface of the elevating shaft are provided with a corrosion-resistant layer, they can be prevented from being corroded by the processing gas. .

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a schematic configuration of a first example of an embodiment of a dehydrating / transparent vitrification apparatus for an optical fiber porous preform according to the present invention.

FIG. 2 is a longitudinal sectional view showing a schematic configuration of a second example of the embodiment of the apparatus for dehydrating and vitrifying an optical fiber porous preform according to the present invention.

FIG. 3 is an enlarged vertical sectional view of a heat insulating material cover mounted on a base material gripping portion in FIG. 2;

FIG. 4 is a longitudinal sectional view showing a schematic configuration of a third example of the embodiment of the dehydrating / transparent vitrification apparatus for an optical fiber porous preform according to the present invention.

FIG. 5 is a longitudinal sectional view showing a schematic configuration of a fourth example of the embodiment of the optical fiber porous preform dehydration / transparent vitrification apparatus according to the present invention.

FIG. 6 is a longitudinal sectional view showing a schematic configuration of a fifth example of the embodiment of the dehydration / transparent vitrification apparatus for an optical fiber porous preform according to the present invention.

FIG. 7 is a longitudinal sectional view showing a schematic configuration of a sixth example of the embodiment of the dehydration / transparent vitrification apparatus for an optical fiber porous preform according to the present invention.

FIG. 8 is a longitudinal sectional view showing a schematic configuration of a seventh example of the embodiment of the dehydration / transparent vitrification apparatus for an optical fiber porous preform according to the present invention.

FIG. 9 is a longitudinal sectional view showing a schematic configuration of a main part of an eighth example of the embodiment of the dehydrating / transparent vitrification apparatus for an optical fiber porous preform according to the present invention.

FIG. 10 shows dehydration of a porous optical fiber preform according to the present invention.
It is a longitudinal section showing the schematic structure of the 9th example of an embodiment in a transparent vitrification device.

FIG. 11 is a longitudinal sectional view showing a schematic configuration of a conventional apparatus for dehydrating / clearing an optical fiber porous preform of this type.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 Porous optical fiber preform 1a Starting rod 2 Furnace tube 2a Upper opening 2b Upper flange 2c Processing chamber 3,3 'Upper lid 3a Elevating shaft penetrating part 3b Rising part 4,4' Elevating shaft 5 Base material gripping part 5a Shaft Part 6 Furnace body 6a Flange part 7, 7a to 7e Heater 8 Heating furnace 9 Gas supply port 10 Gas discharge port 11 Gas supply port 12 Furnace core upper seal gas supply body 13 Exhaust line 14, 16 Pressure control valve 15 Exhaust line Reference Signs List 17 Differential pressure gauge 18 Gap 19 Shaft through hole 20, 24 Sealing material 21 Cooling means 22 Cooling pipe 23 Heat insulating means 23a Heat insulating material 23b Quartz cover 23c Through hole 24 Sealing material 25 Lid support 26 Heat insulating material 27a Heat insulating material 27b Quartz cover 28 Heat insulating Means 29 Through-hole 30 Gas supply connection pipe 30a Joint part 32 Quartz cover 33, 35 Inert gas Passages 34, 36 Inert gas supply port 37 Insulating means also serving as gas shielding means 38 Insulating material 39 Gas shielding material 40 Gap 41 Gas shielding means 42 Stopper 43 Hole 44 Gas through hole 45, 46 Buffer chamber

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Junichi Takeda 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd. (72) Inventor Satoshi Sugiyama 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd.

Claims (9)

[Claims] 1. A furnace tube made of quartz glass or ceramics for accommodating an optical fiber porous preform to be treated, and an upper opening of the furnace tube for inserting and removing the optical fiber porous preform. An upper lid detachably attached to an upper part of the furnace tube, an elevating shaft penetrating the upper lid so as to be vertically movable, and a base material gripping part provided at a lower end of the elevating shaft to grip an upper part of the optical fiber porous preform. A heating furnace provided on the outer periphery of the furnace tube and heating the optical fiber porous preform in the furnace tube with a heater; a gas supply port for supplying gas from a lower portion of the furnace tube to the inside; An optical fiber porous preform dehydration / transparent vitrification apparatus comprising a gas discharge port for discharging gas in the furnace core tube at an upper side of the tube, wherein the upper lid is formed of metal, and the base material gripping portion is made of quartz. Glass Or at least an inner surface of the upper lid is provided with a corrosion-resistant layer, and a rubber is provided on a vertical shaft penetrating portion of the upper lid through which the vertical shaft penetrates so that the vertical shaft can be raised and lowered while maintaining a sealed state. Alternatively, a resin sealing material is provided, the space between the upper lid and the furnace tube or the furnace body is sealed with a rubber or resin sealing material, and the upper lid is provided with cooling means for cooling the upper lid with a cooling medium. A dewatering / transparent vitrification apparatus for an optical fiber porous preform, characterized in that it is used. 2. The apparatus for dehydrating and vitrifying a porous optical fiber preform according to claim 1, wherein the upper cover is provided with a heat insulating material covering an inner surface thereof. 3. An upper cover having an inert gas passage for covering an inner surface thereof and for flowing an inert gas covering a surface of the elevating shaft protruding into the upper cover. The optical fiber porous preform dehydration / transparent vitrification apparatus according to claim 1 or 2. 4. A heat insulating means for preventing radiant heat in the furnace tube from being transmitted to the upper lid is supported on an upper portion of the base material gripping portion. An apparatus for dehydrating and vitrifying a porous optical fiber preform as described in the above. 5. A gas shielding means for suppressing a seal gas sealing between the upper lid and the elevating shaft passing therethrough from flowing into a processing chamber in which the optical fiber porous preform exists. 5. The apparatus for dehydrating and vitrifying a porous optical fiber preform according to claim 4, wherein a device is provided between the upper lid and the heat insulating means. 6. The heat insulating means according to claim 4, wherein said heat insulating means is always disposed below said gas discharge port while said optical fiber porous preform is being processed.
4. The dehydration / transparent vitrification apparatus for an optical fiber porous preform according to item 1. 7. The gas shielding means according to claim 4, wherein the gas shielding means is always disposed above the gas discharge port while the porous optical fiber preform is subjected to the heat treatment. Equipment for dehydration and transparent vitrification of porous optical fiber preform. 8. An upper portion of the preform holding portion, which prevents radiant heat in the furnace tube from being transmitted to the upper lid, and is always higher than the gas discharge port during the heat treatment of the optical fiber porous preform. 3. An apparatus for dehydrating and vitrifying a porous optical fiber preform according to claim 1, wherein a gas shielding and heat insulating means disposed below is supported. 9. The optical fiber according to claim 1, wherein the heater is provided in multiple stages in a direction along a longitudinal direction of the porous optical fiber preform. 4. The dehydration / transparent vitrification apparatus for an optical fiber porous preform according to item 1.