JP6766012B2 - Buckling deformation inspection method of core tube and manufacturing method of optical fiber base material - Google Patents

Description

The present invention relates to a method for inspecting buckling deformation of a core tube and a method for manufacturing an optical fiber base material.

Conventionally, a sintering apparatus used for manufacturing an optical fiber base material as shown in Patent Document 1 below has been known. This sintering device includes a core tube and a heater provided on the outer periphery of the core tube. By heating the optical fiber base material in the core tube, the suit contained in the optical fiber base material is sintered. And vitrify. Further, Patent Document 1 discloses a method of inspecting the occurrence of cracks in the core tube by changing the internal pressure of the core tube.

Japanese Patent No. 5542923

By the way, as a material of this kind of core tube, quartz glass may be used in order to prevent impurities from being mixed in the optical fiber base material. Further, in particular, when a corrosive gas such as a chlorine-based gas or a fluorine-based gas is used, a core tube made of quartz glass having high chemical resistance is often used.
Here, the core tube made of quartz glass is softened by heating. In addition, compressive stress acts on this core tube due to its own weight and the like. Therefore, by repeating the sintering process of the optical fiber base material, the core tube is gradually buckled and deformed. Such buckling deformation is particularly likely to occur in the vicinity of the heater in the core tube. When the buckling deformation of the core tube progresses and the buckling deformation portion comes into contact with the optical fiber base material rotating in the core tube, the optical fiber base material is damaged or damaged. In order to prevent the occurrence of such a phenomenon, it is necessary to inspect the presence or absence of buckling deformation of the core tube and its degree.

However, in the method of Patent Document 1, when a crack occurs in the core tube, it can be detected, but when the core tube is buckled and deformed without cracking, the internal pressure of the core tube does not change. Therefore, such buckling deformation cannot be detected.
Further, in recent years, the size of the core tube has been increasing, and the vertical distance from the upper end opening of the core tube to the heater may be 2 m or more (4 m or more for a long product). For this reason, it is often difficult to confirm the presence or absence and degree of buckling deformation by simply visually observing from the upper end opening of the core tube.

The present invention has been made in consideration of such circumstances, and an object of the present invention is to provide a method for inspecting a core tube capable of inspecting buckling deformation of the core tube.

In order to solve the above problems, the core tube inspection method according to the first aspect of the present invention is a quartz glass core tube inspection method used for producing an optical fiber base material, and is the core tube inspection method. The laser beam is irradiated into the core tube from the upper end opening of the core tube, and the irradiation point of the laser beam in the core tube is detected.

According to the method for inspecting the core tube according to the above aspect, by detecting the irradiation point of the laser light in the core tube, whether the laser light is irradiated to the bottom wall of the core tube or the buckling deformation portion. Can be determined. Thereby, the presence or absence of buckling deformation of the core tube can be inspected.

Further, in the core tube inspection method according to the above aspect, the radial distance between the central axis of the core tube and the buckling deformation portion of the core tube may be measured.

In this case, by measuring the radial distance between the central axis of the core tube and the buckling deformation portion of the core tube, when the optical fiber base material is rotated in the core tube, the buckling deformation with the optical fiber base material is performed. It is possible to determine whether or not the unit comes into contact with the unit. As a result, it is possible to reduce the manufacturing cost and lead time of the optical fiber base material by minimizing the replacement frequency of the core tube while suppressing the occurrence of trauma or damage to the optical fiber base material.

Further, in the method for producing an optical fiber base material according to the second aspect of the present invention, the core tube is inspected using the above-mentioned inspection method, and the optical fiber base material is heated and sintered in the core tube.

According to the method for producing an optical fiber base material according to the above aspect, when the optical fiber base material is sintered, the core tube and the optical fiber base material come into contact with each other, causing damage or damage to the optical fiber base material. It is possible to stabilize the quality of the optical fiber base material.

According to the above aspect of the present invention, it is possible to provide a method for inspecting a core tube capable of inspecting buckling deformation of the core tube.

It is the schematic of the core tube for manufacturing the optical fiber base material. It is a schematic explanatory drawing of the inspection method of a core tube.

(First Embodiment)
Hereinafter, the method for inspecting the core tube and the method for manufacturing the optical fiber base material according to the present embodiment will be described with reference to FIGS. 1 and 2. In each drawing used in the following description, the scale is appropriately changed in order to make each member a recognizable size.

(Manufacturing method of optical fiber base material)
First, a method for manufacturing an optical fiber base material will be described. In the present embodiment, a case where a suit method such as a VAD method or an OVD method is used will be described, but other manufacturing methods may be adopted.
When producing an optical fiber base material by the soot method, first, oxygen gas, hydrogen gas, an inert gas, etc. are flowed from a burner installed in a reaction vessel, and SiCl ₄ is placed in a flame in which these gases are reacted. Inject glass raw material gas such as. As a result, glass fine particles are generated. By adhering the glass fine particles to the target rotating in the reaction vessel, soot is deposited on the outer circumference of the target. As a result, an optical fiber base material before sintering (hereinafter referred to as an unsintered base material) can be obtained.

Next, the unsintered base material is sintered by the sintering apparatus 10 as shown in FIG. 1 (sintering step). The sintering apparatus 10 includes a core tube 4 made of quartz glass and a sintering furnace 5 for heating the core tube 4. The sintering furnace 5 is provided on the outer periphery of the core tube 4. A heater 6 shut off from the outside air is provided in the sintering furnace 5.

The core tube 4 is formed in a bottomed tubular shape having a peripheral wall 4a and a bottom wall 4b. Further, the core tube 4 is provided with a gas introduction port and a gas discharge port (not shown). Hereinafter, the central axis O of the core tube 4 is simply referred to as the central axis O, and the direction along the central axis O is referred to as the vertical direction. Further, the bottom wall 4b side along the vertical direction is referred to as the lower side, and the opposite side is referred to as the upper side. Further, in a plan view seen from the vertical direction, the direction orthogonal to the central axis O is called the radial direction, and the direction orbiting around the central axis O is called the circumferential direction.

In the sintering step, the unsintered base material is held in the core tube 4 via the support rod 1 and the dummy portion 2. Then, the upper end opening of the core tube 4 is closed by the lid 7, and the support rod 1 is lowered while rotating. As a result, the unsintered base material is heated from the lower part to the upper part and sintered, and the optical fiber base material 3 is obtained. At the time of the sintering treatment, the optical fiber base material 3 may be dehydrated in the core tube 4.

Further, an optical fiber can be obtained by melting and drawing the optical fiber base material 3 thus obtained.

By the way, as described above, the core tube 4 is made of quartz glass. This is to prevent impurities from being mixed into the optical fiber base material 3 during the sintering process, but the quartz glass is softened by heating. On the other hand, a compressive stress in the vertical direction due to the weight of the core tube 4 acts on the peripheral wall 4a of the core tube 4. From the above, when the sintering step of the optical fiber base material 3 is repeated, as shown in FIG. 2, the portion of the core tube 4 close to the sintering furnace 5 may buckle and deform. When the core tube 4 is buckled and deformed and the peripheral wall 4a protrudes toward the inside of the core tube 4, this protruding portion (buckling deformed portion B) comes into contact with the optical fiber base material 3 rotating in the core tube 4. There is a risk of doing so. When the buckling deformation portion B of the core tube 4 and the optical fiber base material 3 come into contact with each other, the optical fiber base material 3 is damaged.

Therefore, in the present embodiment, the following inspection method for the core tube is adopted in order to prevent the buckling deformation portion B and the optical fiber base material 3 from coming into contact with each other.

(Inspection method for core tube)
As shown in FIG. 2, the inspection device 20 is used in the core tube inspection method of the present embodiment. The inspection device 20 includes a stage 21 attached to the upper end opening of the core tube 4, and a laser light source 22 that moves in a horizontal plane along the stage 21. When inspecting the core tube 4, the optical fiber base material 3 and the lid 7 are removed from the core tube 4 in advance.

The laser light source 22 emits the laser beam L from its lower end. As the laser light L, red light (0.6 μm band) or green light (0.5 μm band) can be used. The laser beam L travels downward along the vertical direction. When the laser beam L is irradiated to the bottom wall 4b of the core tube 4 and the buckling deformation portion B is irradiated, the appearance of the irradiation point is different when observed from the upper end opening of the core tube 4. different. Therefore, the inspection worker can inspect the presence or absence of buckling deformation of the portion by visually recognizing the irradiation point of the laser beam L from the upper end opening of the core tube 4.

When the core tube 4 made of quartz glass is cooled to room temperature, there is a high possibility that it will be damaged due to devitrification. Therefore, normally, the temperature of about 900 ° C. is maintained even at rest. Quartz glass glows red at about 900 ° C., but the irradiation point of the laser beam L in the core tube 4 shines brighter than the red-hot portion, so that the irradiation point can be easily confirmed visually. When visually confirming the irradiation point of the laser beam L, the inspection worker wears laser protective goggles for safety.

By the way, even when the core tube 4 is buckled and deformed, if the clearance (gap) between the buckling deformed portion B and the optical fiber base material 3 is secured, the core tube 4 is still used. be able to. Therefore, it is also effective to measure how much the buckling deformation portion B protrudes inside the core tube 4.

Therefore, as the inspection device 20, a laser marking machine including a stage 21 having a spirit level and a laser light source 22 integrated with the stage 21 can be preferably used. In this case, the laser light source 22 can be moved horizontally in the radial direction, and the moving distance can be accurately measured. The inspection worker moves the laser light source 22 starting from the central axis O, and stops the movement of the laser light source 22 when the laser beam L irradiates the buckling deformation portion B, thereby causing buckling in the radial direction. The distance R between the deformed portion B and the central axis O can be measured. The difference between this distance R and the radius of the optical fiber base material 3 is the clearance between the buckling deformation portion B and the optical fiber base material 3.

The central axis of the optical fiber base material 3 and the central axis of the core tube 4 are not completely coaxial, and an error occurs. Further, considering the runout of the outer peripheral surface of the optical fiber base material 3, when the difference between the distance R and the radius of the optical fiber base material 3 becomes smaller than a predetermined value, the core tube 4 may be replaced. desirable. The above predetermined values are the accuracy of matching the central axis of the optical fiber base material 3 with the central axis of the core tube 4, the runout of the outer peripheral surface of the optical fiber base material 3, and the heating of the optical fiber base material 3 by the heater 6. It is advisable to decide in consideration of thermal expansion at the time.

As described above, according to the core tube inspection method of the present embodiment, the laser beam L is detected (visually) visually at the irradiation point of the laser beam L in the core tube 4, so that the laser beam L is the bottom wall of the core tube 4. It is possible to determine whether the 4b is irradiated or the buckling deformation portion B is irradiated. As a result, the presence or absence of buckling deformation of the core tube 4 can be easily inspected.

Further, by measuring the distance R in the radial direction between the central axis O of the core tube 4 and the buckling deformation portion B of the core tube 4, when the optical fiber base material 3 is rotated in the core tube 4. , It is possible to determine whether or not the optical fiber base material 3 and the buckling deformation portion B are in contact with each other. As a result, the manufacturing cost and lead time of the optical fiber base material 3 can be reduced by minimizing the replacement frequency of the core tube 4 while suppressing the occurrence of trauma or damage to the optical fiber base material 3. Can be done.

Further, by inspecting the core tube 4 using the above-mentioned inspection method and adopting a method for producing an optical fiber base material in which the optical fiber base material 3 is heated and sintered in the core tube 4, the optical fiber base material is manufactured. It is possible to suppress the occurrence of damage or damage to the optical fiber base material 3 and stabilize the quality of the optical fiber base material 3.

(Second Embodiment)
Next, the second embodiment according to the present invention will be described, but the basic configuration is the same as that of the first embodiment. Therefore, the same reference numerals are given to the same configurations, the description thereof will be omitted, and only the different points will be described.
In the first embodiment, the reflection point of the laser beam L is visually confirmed, but in the present embodiment, the reflected laser beam L is received by the light receiving unit.

As the light receiving unit, a unit capable of measuring the distance between the laser light source 22 and the reflection point in the vertical direction by receiving the laser light L irradiated and reflected on the core tube 4 can be preferably used. As such a laser light source 22 and a light receiving unit, for example, those commercially available as a reflection type distance sensor can be used.

The vertical distance between the laser light source 22 and the reflection point varies greatly depending on whether the reflection point is the bottom wall 4b of the core tube 4 or the buckling deformation portion B. Therefore, based on the detection result of the reflected light by the light receiving unit, it can be determined whether or not the buckling deformation portion B exists at the position of the laser light source 22 in the horizontal direction.
Further, according to the present embodiment, the presence or absence of buckling deformation of the core tube 4 and the degree thereof can be safely detected without visual inspection.

Hereinafter, the above embodiment will be described with reference to specific examples. The following examples do not limit the present invention.
(Example)
A quartz glass core tube 4 having an inner diameter of φ300 mm was used. An optical fiber base material 3 having an outer diameter of φ200 mm was set in the core tube 4. That is, the radial gap between the peripheral wall 4a of the core tube 4 and the optical fiber base material 3 is 50 mm. The optical fiber base material 3 was allowed to enter the core tube 4 while rotating, and was lowered to the position of the sintering furnace 5, where dehydration treatment and sintering treatment were performed. Such dehydration / sintering treatment was repeated 100 times, and the inspection method of the core tube 4 described in the first embodiment was carried out.

In this embodiment, a laser pointer (class 2, 1 mW or less, red light) was used as the laser light L. When the irradiation point of the laser beam L in the core tube 4 was visually recognized, the presence of the buckling deformation portion B was confirmed in the vicinity of the heater 6. Further, when the laser light source 22 was moved in the radial direction toward the buckling deformation portion B starting from the central axis O, it was found that the distance R (see FIG. 2) was 130 mm. From this measurement result and the radius of the optical fiber base material 3 being 100 mm, it was found that a clearance of 30 mm was secured between the optical fiber base material 3 and the buckling deformation portion B. Therefore, although the core tube 4 is buckled and deformed, it is judged that there is no problem in practical use, and the dehydration / sintering treatment is performed without replacing the core tube 4. As a result, the optical fiber base material 3 could be normally manufactured without contacting the core tube 4 and the optical fiber base material 3.

When the above dehydration / sintering treatment was repeated 200 times, the core tube 4 was inspected again, and it was found that the distance R was 115 mm. From this measurement result, the clearance between the optical fiber base material 3 and the buckling deformation portion B is 15 mm, and considering the runout of the outer peripheral surface of the optical fiber base material 3, the optical fiber base material 3 and the buckling deformation portion 3 are buckled. It was found that there is a risk of contact with part B. Therefore, the core tube 4 was replaced.

(Comparison example)
When the dehydration / sintering treatment was repeated 150 times under the same conditions as in the above example, the core tube 4 was simply observed from above, and it was determined that the amount of buckling deformation was small. Therefore, when the optical fiber base material 3 was inserted into the core tube 4 and the dehydration / sintering treatment was started, an abnormal noise was generated when the optical fiber base material 3 was lowered to the vicinity of the heater 6. Therefore, when the dehydration / sintering treatment was stopped and the state of the optical fiber base material 3 was confirmed, the surface was scratched and a part of the optical fiber base material was cracked. After the fact, when the core tube 4 was inspected by the same inspection method as in the example, it was found that the distance R was 110 mm. That is, the clearance between the optical fiber base material 3 and the buckling deformation portion B is only 10 mm, and the optical fiber base material 3 comes into contact with the buckling deformation portion B due to the runout of the outer peripheral surface of the optical fiber base material 3. It is thought that it was.

From the comparison between the above-described embodiment and the comparative example, according to the core tube inspection method of the present embodiment, the frequency of replacement of the core tube 4 is minimized while suppressing the occurrence of trauma or damage to the optical fiber base material 3. It was confirmed that it could be limited.

The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

For example, in the above embodiment, the heater 6 is arranged at one place, but the heater 6 may be arranged at a plurality of places. Further, the shape of the core tube 4 is not limited to the illustrated example, and may be changed as appropriate.

In addition, it is possible to replace the constituent elements in the above-described embodiment with well-known constituent elements as appropriate without departing from the spirit of the present invention, and the above-described embodiments and modifications may be appropriately combined.

1 ... Support rod 2 ... Dummy part 3 ... Optical fiber base material 4 ... Core tube 5 ... Sintering furnace 6 ... Heater 10 ... Sintering device 20 ... Inspection device 21 ... Stage 22 ... Laser light source B ... Buckling deformation part L ... Laser light

Claims (5)

A buckling deformation inspection method for a quartz glass core tube used to manufacture an optical fiber base material.
A seat of the core tube that irradiates the inside of the core tube with a laser beam from the upper end opening of the core tube, and detects the reflected light that is reflected at the irradiation point of the laser beam in the core tube and directly reaches the upper end opening. Bending deformation inspection method. The buckling deformation inspection method for a core tube according to claim 1, wherein the distance between the central axis of the core tube and the buckling deformation portion of the core tube in the radial direction is measured. The buckling deformation inspection of the core tube according to claim 1 or 2, wherein the laser beam is irradiated by using a laser marking machine provided with a stage having a spirit level and a laser light source integrated with the stage. Method. The buckling deformation inspection method for a core tube according to any one of claims 1 to 3, wherein the core tube is transparent. The core tube is inspected by using the buckling deformation inspection method for the core tube according to any one of claims 1 to 4 .
The optical fiber base material is heated and sintered in the core tube.
Manufacturing method of optical fiber base material.

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