JP2018203568A - Inspection method of furnace core tube, and manufacturing method of optical fiber preform - Google Patents

Abstract

To provide an inspection method of a furnace core tube capable of inspecting buckling deformation of the furnace core tube, in a production method of an optical fiber preform for heating and sintering the optical fiber preform in the furnace core tube.SOLUTION: There is provided an inspection method of a furnace core tube 4 made of quartz glass used for producing an optical fiber preform, for detecting a radiation point of a laser beam L in the furnace core tube 4, by radiating the laser beam L into the furnace core tube from an upper end opening of the furnace core tube 4. In the inspection method of the furnace core tube 4, a distance R in the radial direction between a central axis O of the furnace core tube 4 and a buckling deformation part B of the furnace core tube 4 is measured.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a furnace core tube inspection method and an optical fiber preform manufacturing method.

  Conventionally, a sintering apparatus used for manufacturing an optical fiber preform as shown in Patent Document 1 below is known. This sintering apparatus includes a furnace core tube and a heater provided on the outer periphery of the furnace core tube. The soot contained in the optical fiber preform is sintered by heating the optical fiber preform in the furnace core tube. To vitrify. Furthermore, this patent document 1 discloses a method for inspecting the occurrence of cracks in the core tube by changing the internal pressure of the core tube.

Japanese Patent No. 5542932

By the way, as a material of this type of core tube, quartz glass may be used in order to prevent impurities from entering the optical fiber preform. In particular, when a corrosive gas such as a chlorine-based gas or a fluorine-based gas is used, a furnace 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. Further, a compressive stress acts on the core tube due to its own weight or the like. For this reason, the furnace tube is gradually buckled and deformed by repeating the sintering process of the optical fiber preform. Such buckling deformation is particularly likely to occur near the heater in the core tube. When the buckling deformation of the core tube proceeds and the buckled deformed portion comes into contact with the optical fiber preform rotating in the furnace core tube, the optical fiber preform is damaged or damaged. In order to prevent such a phenomenon from occurring, it is necessary to inspect the presence or absence of the buckling deformation of the core tube and the extent thereof.

However, in the method of Patent Document 1, it is possible to detect a crack in the core tube, but when the core tube is buckled and not cracked, the internal pressure of the core tube does not change. Therefore, such buckling deformation cannot be detected.
In recent years, the size of the core tube has been increased, 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 long products). For this reason, it is often difficult to confirm the presence or degree of buckling deformation even if it is simply visually observed from the upper end opening of the core tube.

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

  In order to solve the above-described problem, a method for inspecting a core tube according to a first aspect of the present invention is a method for inspecting a quartz core tube made of quartz glass used for manufacturing an optical fiber preform. A laser beam is irradiated into the reactor core tube from the upper end opening of the laser beam, and the irradiation point of the laser beam in the reactor core tube is detected.

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

  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 deformed portion of the core tube may be measured.

  In this case, when the optical fiber preform is rotated in the core tube by measuring the radial distance between the central axis of the core tube and the buckled deformation portion of the core tube, the optical fiber preform and the buckling deformation It can be determined whether or not the part comes into contact. As a result, it is possible to reduce the manufacturing cost and lead time of the optical fiber preform while minimizing the frequency of replacement of the core tube while suppressing the occurrence of damage and damage to the optical fiber preform.

  Moreover, the manufacturing method of the optical fiber preform which concerns on the 2nd aspect of this invention test | inspects the said core tube using the above-mentioned inspection method, and heats and sinters the said optical fiber preform in the said core tube.

  According to the method for manufacturing an optical fiber preform of the above aspect, when the optical fiber preform is sintered, the core tube and the optical fiber preform come into contact with each other, so that the optical fiber preform is damaged or damaged. And the quality of the optical fiber preform can be stabilized.

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

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

(First embodiment)
The furnace core tube inspection method and the optical fiber preform manufacturing method according to the present embodiment will be described below with reference to FIGS. In each drawing used in the following description, the scale is appropriately changed to make each member a recognizable size.

(Manufacturing method of optical fiber preform)
First, a method for manufacturing an optical fiber preform will be described. In the present embodiment, a case where a soot method such as a VAD method or an OVD method is used will be described, but another manufacturing method may be adopted.
When manufacturing an optical fiber preform by the soot method, first, an oxygen gas, a hydrogen gas, an inert gas, or the like is flowed from a burner installed in a reaction vessel, and SiCl ₄ is introduced into a flame in which these gases are reacted. Glass raw material gas such as Thereby, glass fine particles are generated. By attaching the glass fine particles to a target rotating in the reaction vessel, soot is deposited on the outer periphery of the target. Thereby, an optical fiber preform before sintering (hereinafter referred to as an unsintered preform) is obtained.

  Next, an unsintered base material is sintered by the sintering apparatus 10 as shown in FIG. 1 (sintering process). The sintering apparatus 10 includes a quartz glass core tube 4 and a sintering furnace 5 that heats the core tube 4. The sintering furnace 5 is provided on the outer periphery of the furnace core tube 4. In the sintering furnace 5, a heater 6 that is shielded from the outside air is provided.

  The core tube 4 is formed in a bottomed cylindrical shape having a peripheral wall 4a and a bottom wall 4b. The core tube 4 is provided with a gas inlet and a gas outlet (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 viewed from the up and down direction, a direction orthogonal to the central axis O is referred to as a radial direction, and a direction around the central axis O is referred to as a circumferential direction.

  In the sintering step, the unsintered base material is held in the furnace core tube 4 via the support rod 1 and the dummy part 2. And the upper-end opening part of the core tube 4 is obstruct | occluded with the cover body 7, and it lowers, rotating the support rod 1. FIG. Thereby, an unsintered base material is heated and sintered from the lower part toward the upper part, and the optical fiber preform 3 is obtained. Note that the optical fiber preform 3 may be dehydrated in the furnace core tube 4 during the sintering process.

  Moreover, an optical fiber can be obtained by melting and drawing the optical fiber preform 3 thus obtained.

  Incidentally, as described above, the core tube 4 is made of quartz glass. This is to prevent impurities from entering the optical fiber preform 3 during the sintering process, but the quartz glass is softened by heating. On the other hand, the vertical compressive stress due to the dead weight of the core tube 4 acts on the peripheral wall 4 a of the core tube 4. From the above, when the sintering process of the optical fiber preform 3 is repeated, the portion near the sintering furnace 5 in the furnace core tube 4 may be buckled and deformed as shown in FIG. When the core tube 4 is buckled and the peripheral wall 4 a protrudes toward the inside of the core tube 4, the protruding portion (buckled deformed portion B) comes into contact with the optical fiber preform 3 that rotates in the core tube 4. There is a risk of it. When the buckled deformed portion B of the furnace core tube 4 and the optical fiber preform 3 come into contact with each other, the optical fiber preform 3 is damaged.

  Therefore, in this embodiment, in order to prevent contact between the buckling deformed portion B and the optical fiber preform 3 as described above, the following core tube inspection method is employed.

(Inspection method for core tube)
As shown in FIG. 2, an inspection apparatus 20 is used in the core tube inspection method of the present embodiment. The inspection apparatus 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 preform 3 and the lid 7 are removed from the core tube 4 in advance.

  The laser light source 22 emits laser light 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 light L travels downward along the vertical direction. When the laser beam L is irradiated on the bottom wall 4b of the core tube 4 and when it is irradiated on the buckling deformed portion B, when the laser beam L is observed from the upper end opening of the core tube 4, how the irradiation point is seen. Different. For this reason, the inspection operator can inspect the presence / 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.

  Note that, when the quartz core tube 4 made of quartz glass is cooled to room temperature, there is a high possibility that it will be damaged by devitrification. Therefore, a temperature of about 900 ° C. is normally maintained even at rest. Quartz glass is red-hot at about 900 ° C., but the irradiation point of the laser beam L in the core tube 4 shines brighter than the red-hot part, so the irradiation point can be easily confirmed by visual observation. In addition, when confirming the irradiation point of the laser beam L visually, the inspection operator wears laser protective glasses for safety.

  By the way, even if the core tube 4 is buckled and deformed, if the clearance (gap) between the buckled deformed portion B and the optical fiber preform 3 is secured, the core tube 4 is continuously used. be able to. For this reason, it is also effective to measure how much the buckling deformed portion B protrudes inside the core tube 4.

  Therefore, as the inspection apparatus 20, a laser marking machine including a stage 21 having a level and a laser light source 22 integrated with the stage 21 can be suitably used. In this case, the moving distance can be accurately measured while moving the laser light source 22 horizontally in the radial direction. The inspection operator moves the laser light source 22 with the central axis O as a starting point, and stops the movement of the laser light source 22 when the laser light L is applied to the buckling deformed portion B, thereby buckling in the radial direction. A distance R between the deformed portion B and the central axis O can be measured. A difference between the distance R and the radius of the optical fiber preform 3 is a clearance between the buckling deformed portion B and the optical fiber preform 3.

  Note that the central axis of the optical fiber preform 3 and the central axis of the core tube 4 are not completely coaxial, and an error occurs. Further, considering the fluctuation of the outer peripheral surface of the optical fiber preform 3 and the difference between the distance R and the radius of the optical fiber preform 3 becomes smaller than a predetermined value, the core tube 4 can be replaced. desirable. The predetermined values are the accuracy of the coincidence between the central axis of the optical fiber preform 3 and the central axis of the core tube 4, the deflection of the outer peripheral surface of the optical fiber preform 3, and the optical fiber preform 3 is heated by the heater 6. It is good to decide in consideration of thermal expansion at the time.

  As described above, according to the method for inspecting a core tube of the present embodiment, the laser light L is detected (viewed) by visual observation of the irradiation point of the laser beam L in the core tube 4 so that the laser beam L can be seen from the bottom wall of the core tube 4. Whether it is irradiated to 4b or the buckled deformation part B can be determined. Thereby, the presence or absence of buckling deformation of the core tube 4 can be easily inspected.

  Further, when the optical fiber preform 3 is rotated in the core tube 4 by measuring the radial distance R between the central axis O of the core tube 4 and the buckling deformation portion B of the core tube 4. It can be determined whether or not the optical fiber preform 3 and the buckling deformed portion B are in contact with each other. Thereby, while suppressing the occurrence of damage or damage to the optical fiber preform 3, the frequency of replacement of the core tube 4 is minimized, and the manufacturing cost and lead time of the optical fiber preform 3 are reduced. Can do.

  Further, by adopting a method of manufacturing an optical fiber preform in which the core tube 4 is inspected using the above-described inspection method, and the optical fiber preform 3 is heated and sintered in the furnace core tube 4, an optical fiber preform is employed. 3 can be prevented from being damaged or broken, and the quality of the optical fiber preform 3 can be stabilized.

(Second Embodiment)
Next, a second embodiment according to the present invention will be described. The basic configuration is the same as that of the first embodiment. For this reason, the same code | symbol is attached | subjected to the same structure, the description is abbreviate | omitted, and only a different point is demonstrated.
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 portion, a device that can measure the distance in the vertical direction between the laser light source 22 and the reflection point by receiving the laser light L irradiated and reflected on the core tube 4 can be suitably used. As such a laser light source 22 and a light-receiving part, what is marketed as a reflection type distance sensor, for example can be used.

The distance in the vertical direction 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 furnace core tube 4 or the buckling deformed portion B. Therefore, it is possible to determine whether or not the buckling deformed portion B exists at the position of the laser light source 22 in the horizontal direction based on the detection result of the reflected light by the light receiving portion.
Moreover, according to this embodiment, the presence or absence and the extent of buckling deformation of the core tube 4 can be detected safely without visual inspection.

The above embodiment will be described below using 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 preform 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 preform 3 is 50 mm. The optical fiber preform 3 was moved into the furnace core tube 4 while being rotated and lowered to the position of the sintering furnace 5 for dehydration and sintering. Such a dehydration / sintering process was repeated 100 times, and the method for inspecting the core tube 4 described in the first embodiment was performed.

  In this embodiment, a laser pointer (class 2, 1 mW or less, red light) is used as the laser light L. When the irradiation point of the laser beam L in the furnace core tube 4 was visually confirmed, the presence of the buckling deformed portion B was confirmed in the vicinity of the heater 6. Further, when the laser light source 22 was moved in the radial direction from the central axis O toward the buckling deformed portion B, it was found that the distance R (see FIG. 2) was 130 mm. From this measurement result and the radius of the optical fiber preform 3 being 100 mm, it was found that a clearance of 30 mm was secured between the optical fiber preform 3 and the buckled deformed portion B. For this reason, the core tube 4 was buckled and deformed, but it was determined that there was no practical problem, and the dehydration and sintering treatment was performed without replacing the core tube 4. As a result, the optical fiber preform 3 was successfully manufactured without the furnace core tube 4 and the optical fiber preform 3 being in contact with each other.

  When the dehydration / sintering process 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 preform 3 and the buckling deformed portion B is 15 mm, and considering the deflection of the outer peripheral surface of the optical fiber preform 3, the optical fiber preform 3 and the buckling deformation It turned out that there exists a possibility that the part B may contact. For this reason, the core tube 4 was replaced.

(Comparative example)
When the core tube 4 was simply observed from above when the dehydration / sintering treatment was repeated 150 times under the same conditions as in the above example, it was determined that the amount of buckling deformation was small. For this reason, when the optical fiber preform 3 was inserted into the furnace core tube 4 and the dehydration / sintering process was started, abnormal noise was generated when the optical fiber preform 3 was lowered to the vicinity of the heater 6. For this reason, when the dehydration / sintering process was stopped and the state of the optical fiber preform 3 was confirmed, there were scratches on the surface, and some cracks occurred. After that, 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. In other words, the clearance between the optical fiber preform 3 and the buckled deformed portion B is only 10 mm, and the optical fiber preform 3 comes into contact with the buckled deformed portion B due to the deflection of the outer peripheral surface of the optical fiber preform 3. It is thought.

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

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

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

  In addition, the constituent elements in the above-described embodiment can be appropriately replaced with known constituent elements without departing from the gist of the present invention, and the above-described embodiments and modifications may be appropriately combined.

  DESCRIPTION OF SYMBOLS 1 ... Support rod 2 ... Dummy part 3 ... Optical fiber preform 4 ... Furnace core tube 5 ... Sintering furnace 6 ... Heater 10 ... Sintering apparatus 20 ... Inspection apparatus 21 ... Stage 22 ... Laser light source B ... Buckling deformation part L ... Laser light

Claims (3)

A method for inspecting a quartz glass core tube used for manufacturing an optical fiber preform,
A method for inspecting a reactor core tube, comprising: irradiating a laser beam into the reactor core tube from an upper end opening of the reactor core tube and detecting an irradiation point of the laser beam in the reactor core tube.   The method for inspecting a core tube according to claim 1, wherein a radial distance between a central axis of the core tube and a buckling deformed portion of the core tube is measured. Inspecting the core tube using the core tube inspection method according to claim 1 or 2,
Heating and sintering the optical fiber preform in the furnace core tube,
Manufacturing method of optical fiber preform.

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