JP2003112938A - Producing device for optical fiber preform ingot and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method for optical fiber preform ingot and a producing device therefor which can make soot preform to sintered glass by holding air-tightness in a furnace, prevent eccentricity and curving due to contraction on the density change and prevent the deformation in the longitudinal direction due to own weight which is easily caused in accordance with a progress of transparent glass-forming. SOLUTION: This device which produces preform ingot by allowing porous preform 1 having a clad part around the core part to be dewatered and come to transparent glass is provided with an upper rotation mechanism 2 which rotates the porous preform 1, a lower shaft rotating mechanism 8 which supports a lower end part of the porous preform 1 with a lower shaft 9 and rotates the lower shaft 9 synchronously with the upper rotation mechanism 2, and a vertical moving mechanism 15 of the lower shaft which moves the lower shaft 9 in the vertical direction and, therein, the surrounding of the lower shaft 9 is sealed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber mother material having good optical characteristics, which is obtained by sintering a porous mother material (hereinafter referred to as soot mother material) formed by depositing glass fine particles into a transparent glass. The present invention relates to a manufacturing apparatus and a manufacturing method for a material ingot.

[0002]

2. Description of the Related Art Optical fiber base material ingots (hereinafter simply referred to as base material ingots) are manufactured by, for example, an external attachment method (OVD).
Method) is used to deposit soot generated by the flame hydrolysis reaction of SiCl _{4 on} the surface of the starting core base material to produce a soot base material, which is sintered in an electric furnace to obtain transparent vitrification. The sintering and transparent vitrification of the soot base material is depressurized to remove the gas remaining inside the soot base material, and further suppresses halogen gas for dehydration and foaming during transparent vitrification. Therefore, it is performed while supplying an inert gas into the furnace.

At this time, the soot base material is grasped by the upper dummy part and suspended in the furnace, and is sintered while being rotated in order to maintain the uniformity of the sintered vitrification in the circumferential direction. However, as the transparent vitrification progresses, it is stretched in the longitudinal direction due to its own weight, and deformation such as eccentricity and bending occurs due to the density change during the transparent vitrification. If the base material ingot is deformed, it adversely affects the optical characteristics of the optical fiber obtained by spinning the ingot.

As a countermeasure against this, there has been considered a method of supporting the soot base material by a lower dummy part thereof and performing vitrification into a sintered body. However, due to shrinkage accompanying the progress of transparent vitrification, the position of the lower end portion of the soot base material Since it is displaced, it is extremely difficult to maintain the airtightness of the bearing due to the high temperature of the surroundings, so it is extremely difficult to maintain the airtightness of the bearing by preventing the leakage of the gas supplied to the furnace and the inflow of outside air due to depressurization. If changed, there was a problem that bubbles remained in the base material ingot after sintering.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and the soot base material can be sintered and vitrified while maintaining the airtightness in the furnace, and when the density changes. An object of the present invention is to provide a manufacturing method and a manufacturing apparatus of an optical fiber preform ingot capable of preventing eccentricity and bending due to contraction and preventing deformation in the longitudinal direction due to its own weight which tends to occur with progress of transparent vitrification. I am trying.

[0006]

The base material ingot manufacturing apparatus of the present invention is an apparatus for manufacturing a base material ingot by dehydrating and vitrifying a porous base material having a clad portion around a core portion. An upper rotation mechanism that rotates the porous base material, a lower shaft rotation mechanism that supports the lower end of the porous base material with a lower shaft, and rotates the lower shaft in synchronization with the upper rotation mechanism, and the lower shaft. It has a lower shaft vertical movement mechanism that moves vertically.
The feature is that the periphery of the lower shaft is sealed.

In the present invention, in order to maintain the airtightness in the sintering and vitrification furnace, the magnetic seal bearing is attached to the bearing sliding portion of the lower shaft rotating mechanism for rotating the lower shaft supporting the lower end of the porous base material. And a lower shaft bellows is attached between the magnetic seal bearing and the furnace bottom so as to cover the lower shaft, thereby absorbing the displacement in the longitudinal direction of the magnetic seal bearing due to the expansion and contraction of the porous base material, and The airtightness inside is maintained. An adjustment bellows is provided around the lower shaft rotating mechanism, and the atmosphere inside the adjustment bellows and the atmosphere inside the furnace are communicated with each other, so that the adjustment bellows generated when the pressure inside the furnace is reduced and the lower shaft bellows may contract in opposite directions. It is possible to cancel the stress. The cross-sectional area of the lower shaft bellows and the total cross-sectional area of the adjusting bellows are the same.

Below the lower shaft, a weight sensor for detecting the load applied to the lower shaft, for example, a load cell, is provided, and the amount of displacement of the lower shaft in the longitudinal direction is detected by the change in the load amount of the weight sensor. Further, a lower shaft vertical movement mechanism for moving the lower shaft in the vertical direction is provided, and by this, the display load of the weight sensor for detecting the load received by the lower shaft is controlled to be constant.

The method for producing a base material ingot according to the present invention is a method for producing a base material ingot by dehydrating and vitrifying a porous base material having a clad portion around a core portion to produce a base material ingot. Is supported by a lower shaft, the lower shaft is rotated in synchronization with the porous base material, and the expansion and contraction of the porous base material is followed, and the periphery of the lower shaft is sealed to maintain airtightness in the furnace. It has a feature.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, when sintered and vitrified, deformation occurs in the longitudinal direction due to its own weight, and eccentricity and curvature occur due to density change, which greatly affects the optical characteristics of the glass preform for optical fibers. Was there. Therefore, when a preform for an optical fiber was formed by stretching the base material ingot, there was no optical characteristic defect due to deformation, and as a result of diligent examination of a method for producing a glass base material for an optical fiber with few remaining bubbles, a soot base material was obtained. By supporting the lower end of the material with a bearing, displacing the lower shaft while suppressing deformation in the longitudinal direction, and rotating the lower shaft in synchronization with the upper rotation mechanism, it is possible to prevent deformation of the soot base material due to its own weight. Finding that, the load on the lower shaft rotation mechanism is detected by a load sensor such as a load cell,
By correcting the position of the lower shaft so as to maintain the preset load, deformation such as eccentricity or bending due to contraction of the soot base material was prevented.

Further, due to the displacement of the lower shaft, the bearing sliding portion slides in the longitudinal direction. To maintain the airtightness of the bearing sliding portion at this time, a magnetic seal bearing is attached to the lower shaft. By supporting the seal bearing by the lower shaft bellows provided so as to cover the periphery of the lower shaft, the airtightness in the sintering and vitrification furnace can be maintained, and a base material ingot with few remaining bubbles can be obtained. completed.

Since the lower shaft bellows is fixed to the bottom of the furnace and the atmosphere in the lower shaft bellows communicates with the inside of the furnace, when the pressure in the furnace is reduced, the atmosphere in the lower shaft bellows is also reduced at the same time. A stress is generated that tries to shrink the. To solve this problem, an adjusting bellows is provided around the lower shaft rotating mechanism, and the atmosphere in the adjusting bellows and the atmosphere in the lower shaft bellows are communicated with each other through the atmosphere in the furnace. A stress that tries to shrink in the direction opposite to the shrinking direction of is generated, and both stresses can be offset. At this time, it is important to make the cross-sectional area of the lower shaft bellows and the total cross-sectional area of the adjustment bellows the same, which causes stresses of the same size but opposite directions in the lower shaft bellows and the adjustment bellows. Both stresses can be offset.

[0013]

(Embodiment 1) FIGS. 1 and 2 show an example of an apparatus for manufacturing a base material ingot according to the present invention. Outer diameter of 25 mm with refractive index adjusted for single mode optical fiber
A quartz glass dummy rod for core having a φ and a length of 1200 mm was welded to both ends of a quartz glass rod, which was placed in a closed reactor to deposit glass particles generated by flame hydrolysis of SiCl ₄ , A soot base material having an outer diameter of 230 mmφ was manufactured. As shown in FIG. 1, the soot base material 1 is held in an upper rotating mechanism 2 of a hermetically-sealed sinter vitrification device, and an upper dummy part 3 is grasped and suspended in a sinter vitrification furnace 4 to sinter and
Transparent vitrification was performed. Reference numeral 5 is a lower dummy part,
Reference numeral 6 is a heater for heating, and reference numeral 7 is a nozzle for introducing and exhausting gas.

FIG. 2 is an enlarged schematic view of the main part of FIG. 1, and the lower end of the lower dummy part 5 is supported by a support member 10 attached to the upper end of the lower shaft 9. A lower shaft rotating mechanism 8 for rotating the lower shaft 9 is provided at a lower end portion of the lower shaft 9. Further, a magnetic seal bearing 11 is attached to the lower shaft 9, and the magnetic seal bearing 11 and a furnace bottom 12 of the sintered vitrification furnace 4 are provided. A lower shaft bellows 13 was attached between the upper and lower shafts so as to cover the lower shaft 9, and the airtightness inside the furnace was maintained.

A load cell 14 is provided below the lower shaft rotating mechanism 8.
Is installed. The lower shaft 9 is raised by the lower shaft vertical movement mechanism 15 until the lower end of the lower dummy part 5 abuts on the support member 10 attached to the upper end of the lower shaft 9 and the load display of the load cell 14 shows 1 kg. The load cell 1
The load display of No. 4 is performed in advance by the lower shaft rotating mechanism 8, the lower shaft 9, the support member 10, the magnetic seal bearing 11, and the lower shaft bellows 13.
The weight of No. 1 is canceled and only the load of the soot base material 1 is detected.

When the soot base material 1 is sintered, the internal atmosphere of the sintering and vitrification furnace 4 is replaced with He as an inert gas, and the soot base material 1 is rotated by the upper rotating mechanism 2 at a speed of 3 rpm. . At this time, the lower shaft 9 is also rotated by the lower shaft rotating mechanism 8 for 3r while synchronizing with the rotation speed of the upper rotating mechanism 2.
It was rotated at a speed of pm. Temperature controller (not shown)
The heater 6 for heating the temperature in the sintering and vitrification furnace to 1
After raising the temperature to 000 ° C and maintaining this state for about 30 minutes until the temperature of the soot base material 1 stabilizes, gas is introduced and exhausted from the exhaust nozzle 7 with a vacuum device (not shown), and the gauge pressure is increased. It takes about 1 hour from 0 Mpa to −0.
The pressure was reduced to 09 Mpa.

After maintaining this state for 1 hour, He was introduced from the gas introduction / exhaust nozzle 7 for about 1 hour, and the gauge pressure was returned to 0 Mpa. Again, the gas is introduced and exhausted from the exhaust nozzle 7 with a vacuum device, and the gauge pressure is 0 Mp.
The pressure was reduced from -a to -0.098 Mpa over about 1 hour, and the temperature in the furnace was raised to 1520 ° C by the heater 6 for heating while maintaining this state.

As the soot base material 1 increases in temperature,
The bulk density increases and contracts in the process of transparent vitrification, and the load applied to the load cell 14 decreases as it is, but the lower shaft vertical movement mechanism 15 is used to maintain the display load of the load cell 14 at 1 kg. The position of was controlled and the lower shaft was raised by 32 mm. As the vitrification further progresses, a phenomenon in which it stretches in the longitudinal direction due to its own weight appears, and if it is left as it is, the load applied to the load cell 14 increases.
The position of the lower shaft 9 is controlled by the lower shaft vertical movement mechanism 15 so that the display load of the load cell 14 is maintained at 1 kg.
Gradually descended. 8 until completion of sintering and transparent vitrification
It took time, but at this time, the position of the lower shaft vertical movement mechanism 15 was raised by 18 mm from the start of sintering.

The lower shaft 9 is displaced by the vertical movement of the lower shaft vertical movement mechanism 15. This displacement is caused by the lower shaft bellows (SUS 316L).
13), and airtightness was maintained by the magnetic seal bearing 11 even when sliding due to rotation of the lower shaft 9.
Visual inspection of the base material ingot, which had been sintered and made into vitrified glass, showed no bending, eccentricity, bubbles, etc. in the vitrified portion, and the drawbacks caused by the conventional manufacturing method were greatly improved.

(Example 2) The soot base material 1 obtained in Example 1 was sintered and vitrified by using the closed type sintered vitrification apparatus shown in FIGS. Soot base material 1 and upper rotation mechanism 2
The upper dummy part 3 is gripped and suspended in the sintering and vitrification furnace 4, and then the lower shaft rotating mechanism 8 mounted on the load cell 14 is lifted by the lower shaft vertical moving mechanism 15 to move the soot mother. The lower dummy portion 5 of the material 1 was supported by the support member 10 provided on the upper end of the lower shaft 9. Regarding the display load of the load cell 14, the tare was removed in advance so that only the load of the soot base material 1 was detected, as in Example 1. Then, the supporting load on the supporting member 10 is 1 kg at the display load of the load cell 14.
The lower shaft up-and-down moving mechanism 15 was adjusted so that

Although the position of the lower shaft 9 is displaced as the sintering and vitrification progresses, the airtightness inside the furnace is maintained by the lower shaft bellows 13 and the magnetic seal bearing 11 provided so as to cover the lower shaft 9. . The cross-sectional area of the lower shaft bellows 13 used is 78.6 cm ² . As the inside of the furnace is depressurized, the lower shaft bellows 13 contracts, and the stress for pulling up the lower shaft 9 works, and the display of the load cell 14 becomes smaller by the weight corresponding to this stress. The stress for pulling up the lower shaft 9 was offset by providing the adjusting bellows 16 on the lower shaft rotating mechanism 8.

Specifically, ^two adjusting bellows 16 having a cross-sectional area of 39.3 cm 2 are provided and connected to the lower shaft rotating mechanism 8,
Further, a communication pipe 17 was connected so that the atmosphere inside the adjusting bellows and the atmosphere inside the sintering vitrification furnace were communicated with each other, and the atmospheric pressures inside the lower shaft bellows and inside the adjusting bellows were made the same. With such a structure, when the pressure inside the furnace is reduced, the adjusting bellows 16 contracts downward, but at the same time, the lower shaft bellows 13 also has a stress of the same magnitude with the opposite contracting direction. Occurs and both stresses cancel each other out.

The atmosphere in the sintering and vitrification furnace was replaced with He, which is an inert gas, and the soot base material 1 was heated by the upper rotation mechanism 2 to 3
It was rotated at a speed of rpm. At this time, the lower shaft 9 was also rotated by the lower shaft rotating mechanism 8 at a speed of 3 rpm in synchronization with the rotation speed of the upper rotating mechanism 2. The temperature in the furnace was raised to 1000 ° C. by the heater 6 for heating and controlled by a temperature control mechanism (not shown). After maintaining this state for about 30 minutes until the temperature of the soot base material 1 stabilizes, the gas is introduced and exhausted from the exhaust nozzle 7 with a vacuum device (not shown), and about 1 hour from a gauge pressure of 0 Mpa. The pressure was reduced to -0.09 Mpa. While maintaining this state, it was left for 1 hour, He was introduced from the nozzle 7 for about 1 hour, and the pressure in the sintering and vitrification furnace was adjusted to a gauge pressure of 0 Mp.
It returned to a.

The nozzle 7 is again evacuated by a vacuum device, and the gauge pressure is 0 Mpa, and the pressure is reduced to -0.09 in about 1 hour.
The pressure was reduced to 8 Mpa. While maintaining this state, the temperature inside the furnace was raised to 1520 ° C. by the heater 6 for heating. Although vitrification progresses and the bulk density rises as the temperature of the soot base material 1 rises, the soot base material 1 contracts at this time, the load applied to the load cell 14 decreases, and the lower shaft vertical movement mechanism 15 Rose 32 mm. Further, as vitrification progressed, a phenomenon of extending in the longitudinal direction due to its own weight appeared this time, and the lower shaft vertical movement mechanism 15 gradually descended, but the load of the load cell 14 was controlled to be maintained at 5 kg.

It took 8 hours to complete the sintering and vitrification. At this time, the lower shaft up-and-down moving mechanism 15 moves the soot base material 1
The height was 19 mm higher than the initial position. However, the lower shaft bellows (made of SUS316L) 13
The stress that contracts the lower shaft bellows 13 caused by the change in the degree of pressure reduction is canceled by the two adjusting bellows 16 that act to lower the lower shaft rotating mechanism 8, and the load display of the load cell 14 is , The base material shrinks
The stretching behavior was accurately reflected and controlled. During this time, the airtightness of the sliding portion due to the rotation of the lower shaft 9 was maintained by the magnetic seal bearing 11.

When the base material ingot produced by sintering and vitrification was confirmed, no curvature, eccentricity, bubbles, etc. were observed in the vitrified portion, and the above problems were greatly improved. The bending, optical characteristics, etc. of the base material ingot obtained by sintering and vitrification were measured, and the results are shown in Table 1.

(Comparative Example 1) The soot base material 1 produced in the same manner as in Example 1 was held in the closed type sintered vitrification apparatus shown in FIG. Hung. Next, the inside of the sintering and vitrification furnace 4 was replaced with He as an inert gas, and the soot base material 1 was heated by the upper rotation mechanism 2 to 3
It was rotated at a speed of rpm. The temperature in the furnace is set to 1000 ° C. by the heater 6 for heating by a temperature control device (not shown).
To about 30 minutes until the temperature of the soot base material 1 becomes stable. After that, the gas is introduced and exhausted from the exhaust nozzle 7 with a vacuum device (not shown), and it takes about 1 hour from a gauge pressure of 0 Mpa to −0.
The pressure was reduced to 09 Mpa.

After maintaining this state for 1 hour, He was introduced from the gas introduction / exhaust nozzle 7 for about 1 hour,
The gauge pressure was returned to 0 Mpa. The gas is introduced again by the vacuum device and exhausted from the exhaust nozzle 7, and the gauge pressure is 0M.
The pressure was reduced to -0.098 Mpa over about 1 hour from pa, and the temperature inside the furnace was raised to 1520 ° C by the heater 6 for heating while maintaining this state. It took 8 hours until the soot base material 1 was completely sintered and vitrified.

In the base material ingot thus sintered and vitrified, the lower dummy portion 5 of the soot base material 1 is not fixed by the lower shaft during the sintering as in Examples 1 and 2. As a result, axial misalignment occurred and bending that could be visually confirmed occurred. As a result of repeating the same experiment, it was found that this tendency becomes more remarkable as the outer diameter of the soot base material increases.

Table 1 shows the results of measuring the optical characteristics of the base material ingots obtained in Examples 1 and 2 and Comparative Example 1 after sintering and transparent vitrification. The numerical values in the table are average values obtained by measuring 10 base metal ingots,
The values in the curved column are the both ends of the base material ingot (the straight body part 100
(0 mm) is the swing width when it is rotated while being supported. The number of bubbles having a diameter of 0.5 mm or less in the bubble generation column is the number visually confirmed. As is clear from Table 1, the base material ingots obtained in Examples 1 and 2 are superior to Comparative Example 1 in any of the degree of bending, the generation state of bubbles, and the optical characteristics (especially eccentricity). Was there.

[0031]

[Table 1]

[0032]

According to the present invention, by controlling the load received by the lower shaft supporting the soot base material when the soot base material is sintered and made into transparent vitrification, the soot base material can be made at the time of transparent vitrification. Eccentricity, curvature, and longitudinal deformation due to its own weight caused by the contraction of the magnet, and a magnetic seal bearing is provided on the lower shaft.The magnetic seal bearing is installed between the magnetic seal bearing and the bottom of the sintered vitrification furnace. By providing the bellows so as to cover the periphery of the shaft, the airtightness in the sintering and vitrification furnace during depressurization is maintained, and further, the lower shaft bellows generated during depressurization in the furnace tries to pull up the lower shaft. By providing an adjustable bellows that contracts stress in opposite directions and properly controlling the load applied to the lower shaft, eccentricity and bending that occur during vitrification are prevented, and light with excellent optical characteristics with less bubbles etc. Obtained fiber base material ingot It is possible.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of an apparatus for manufacturing a base material ingot used in Example 1.

FIG. 2 is an enlarged schematic cross-sectional view showing a lower shaft rotating mechanism of a lower portion of the manufacturing apparatus shown in FIG.

FIG. 3 is a schematic cross-sectional view showing an example of an apparatus for manufacturing a base material ingot used in Example 2.

4 is a schematic cross-sectional view showing an enlarged lower shaft rotating mechanism of a lower portion of the manufacturing apparatus shown in FIG.

5 is a schematic diagram showing a manufacturing apparatus used in Comparative Example 1. FIG.

[Explanation of symbols]

1. …… Soot base metal, 2. ...... Upper rotation mechanism, 3. ...... The upper dummy part, 4. ...... Sintered vitrification furnace, 5. ...... Lower dummy part, 6. ... Heating heater, 7. ... Nozzles for gas introduction and exhaust, 8. ...... Lower shaft rotation mechanism, 9. ...... Lower axis, Ten. ...... Supporting member, 11. ... Magnetic seal bearings, 12. ...... The hearth, 13. ...... Lower shaft bellows, 14. ...... Load cell, 15. ...... Lower shaft vertical movement mechanism, 16. ...... Adjustable bellows, 17. ...... Communication pipe.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Seiichiro Otsuka             2-13-1, Isobe, Annaka-shi, Gunma Shin-Etsu             Gaku Kogyo Co., Ltd. Precision Materials Research Laboratory (72) Inventor Hirofumi Kase             2-13-1, Isobe, Annaka-shi, Gunma Shin-Etsu             Gaku Kogyo Co., Ltd. Precision Materials Research Laboratory F-term (reference) 4G021 CA12

Claims (14)

[Claims] 1. An apparatus for producing a base material ingot by dehydrating and vitrifying a porous base material having a clad portion around a core portion, wherein an upper rotating mechanism for rotating the porous base material, porous The lower end of the base material is supported by the lower shaft, and has a lower shaft rotating mechanism that rotates the lower shaft in synchronization with the upper rotating mechanism, and a lower shaft vertical moving mechanism that moves the lower shaft in the vertical direction,
An optical fiber preform ingot manufacturing apparatus, characterized in that the periphery of the lower shaft is sealed. 2. A magnetic seal bearing is provided at a bearing sliding portion of a lower shaft rotating mechanism for rotating a lower shaft supporting a lower end of the porous base material, and the lower shaft is provided between the magnetic seal bearing and the furnace bottom. 2. The optical fiber preform ingot manufacturing apparatus according to claim 1, wherein a lower shaft bellows is attached so as to cover it. 3. The optical fiber preform ingot manufacturing apparatus according to claim 1, wherein an adjusting bellows is provided around the lower shaft rotating mechanism, and an atmosphere in the adjusting bellows communicates with an atmosphere in the furnace. 4. The cross-sectional area of the lower shaft bellows and the total cross-sectional area of the adjusting bellows are the same.
An apparatus for manufacturing an optical fiber preform ingot according to any one of 1. 5. The apparatus for manufacturing an optical fiber preform ingot according to claim 1, further comprising a weight sensor below the lower shaft for detecting a load applied to the lower shaft. 6. The apparatus for manufacturing an optical fiber preform ingot according to claim 5, wherein the weight sensor is a load cell. 7. The optical fiber preform ingot manufacturing apparatus according to claim 1, further comprising a lower shaft vertical movement mechanism for moving the lower shaft in the vertical direction. 8. A method of producing a base material ingot by dehydrating and vitrifying a porous base material having a clad portion around a core portion, wherein a lower end portion of the porous base material is supported by a lower shaft, While rotating the shaft in synchronization with the porous preform and following the expansion and contraction of the porous preform, the circumference of the lower shaft is sealed to maintain the airtightness in the furnace. Production method. 9. A magnetic seal bearing is provided on a bearing sliding portion of a lower shaft rotating mechanism for rotating a lower shaft supporting a lower end of the porous base material, and the lower shaft is provided between the magnetic seal bearing and the furnace bottom. The optical fiber mother board according to claim 8, wherein a lower shaft bellows is attached so as to cover, and the lower shaft bellows absorbs displacement in the longitudinal direction of the magnetic seal bearing due to expansion and contraction of the porous base material, thereby maintaining hermeticity in the furnace. Method of manufacturing lumber ingot. 10. An adjusting bellows is provided around the lower shaft rotating mechanism, and the atmosphere in the adjusting bellows and the atmosphere in the furnace are communicated with each other, whereby the adjusting bellows and the lower shaft bellows generated in depressurizing the furnace are in opposite directions. The method for producing an optical fiber preform ingot according to claim 8 or 9, wherein the stress that tends to shrink is canceled out. 11. The method for manufacturing an optical fiber preform ingot according to claim 8, wherein the lower shaft bellows and the adjustment bellows have the same cross-sectional area. 12. A weight sensor for detecting a load applied to the lower shaft is provided below the lower shaft, and a displacement amount of the lower shaft in a longitudinal direction is detected by a change in a load amount of the weight sensor. A method for manufacturing an optical fiber preform ingot according to any one of 1. 13. The method of manufacturing an optical fiber preform ingot according to claim 12, wherein the weight sensor is a load cell. 14. A lower shaft vertical movement mechanism for moving the lower shaft in the vertical direction is provided, and the lower shaft is vertically moved so that the display load of a weight sensor installed to detect the load received by the lower shaft becomes constant. The method for manufacturing an optical fiber preform ingot according to claim 8, wherein the mechanism is controlled.