JP4728605B2 - Optical fiber preform and manufacturing method thereof - Google Patents

Description

  The present invention relates to an optical fiber preform, a manufacturing method and an apparatus thereof, and more particularly, a tip formed by fusing a quartz tube serving as a clad and an optical fiber core rod at the tip prior to the drawing step. The present invention relates to an optical fiber preform having a fused portion, a manufacturing method thereof, and an apparatus thereof.

  In recent years, as optical fibers have been used in large quantities, their mass production and cost reduction have progressed. For mass production and cost reduction, the simplest method is to produce a large optical fiber preform and draw it. As one method for producing a large optical fiber preform, a so-called rod-in-tube method for producing an optical fiber preform has been proposed. In the rod-in-tube method, a quartz tube for the cladding part that occupies most of the volume of the optical fiber preform is fabricated, and VAD (Vapor-phase Axial) is placed inside the quartz tube (hereinafter referred to as the inside of the quartz tube). Insert an optical fiber core rod (hereinafter referred to as a core rod) including a core manufactured by Deposition method or OVD (Outside Vapor Deposition) method, and heat from the outside to fuse and integrate them to make a glass mother for optical fiber. It is a method to make a material.

  However, in recent years, the size of the quartz tube for cladding has further increased, and when the rod-in-tube method is used, cracks in the base material that occur during fusion integration and the costs associated with the increase in the size of the fusion integration equipment The rise is becoming a problem. For this reason, as proposed in Patent Document 1, a method of fusing and integrating a quartz tube and a core rod during drawing is drawing attention. In this method, since the gap between the quartz tube and the core rod is reduced and fused at the time of drawing, a large facility for fusion integration is unnecessary, and the temperature at the time of fusion integration is high. There is an advantage that the viscosity of the quartz tube is sufficiently low and cracks are hardly generated.

By the way, in the method of performing fusion integration at the time of drawing, the gap between the quartz tube and the core rod is reduced during drawing to perform fusion integration. It is necessary to block the part in advance. Further, the quartz tube end portion needs to be formed into a tapered shape in order to shorten the time from the start of drawing to the transition to the steady state. In the aforementioned Patent Document 1, it is proposed that the tip shape of the quartz tube and the tip portion of the core rod are fused (hereinafter referred to as tip fusion) through a dummy member to form the tip shape. Yes.
JP-A-60-155542

  Generally, in drawing, first, the molten base material tip falls as a small lump from the opening at the bottom of the heating furnace (hereinafter referred to as melting drop). Then, following this melting and dropping, the optical fiber is drawn and emerges from the opening at the bottom of the furnace, so that it is wound up at a relatively slow speed in the first stage, and then gradually drawn down. Raise and wind up. Since the characteristics of the optical fiber are not stable until the line speed is stabilized, the optical fiber from the time when the line speed is stabilized is shipped as a good product. For this reason, in order to increase the yield of the material, it is important to shorten the time from the melting and dropping until the linear velocity is stabilized.

  In the method of fusion integration at the time of drawing, when the cross-sectional outer diameter of the quartz tube is as thick as 100 mm or more, for example, the heat capacity of the quartz tube increases, so the time from the start of heating to melting and dropping is 60 to 90 minutes. Not as efficient. In the tip fusion, a method of fusing the tip of the core rod and the tip of the quartz tube via a dummy member has been conventionally employed. However, as the size of the quartz tube is increased, the fusion device and the dummy are joined. It is necessary to increase the size of the member, and an increase in cost is a problem.

  Further, in the tip fusion, (1) to prevent impurities that may cause fiber breakage on the surface of the core rod and the inner and outer surfaces of the quartz tube, and (2) moisture on the inner surface of the quartz tube and the surface of the core rod. Preventing adhesion and (3) preventing cracks generated during cooling after processing (tip fusion and tip molding) are also important issues for obtaining a stable optical fiber product. Regarding (2), it is important not to allow outside air to enter the quartz tube not only during the processing of the tip fused portion but also after the processing. If there is moisture, OH diffuses from the interface between the core rod and the quartz tube during heating, increasing the transmission loss of the fiber. The crack (3) is caused by the stress generated by the temperature difference generated between the quartz tube and the core rod.

  The present invention improves an optical fiber preform having a tip fused portion formed by fusing a tip portion of a quartz tube and a tip portion of an optical fiber core rod inserted into the quartz tube. An object of the present invention is to provide an optical fiber preform improved so that a stable optical fiber product with a high material yield can be obtained even without using the optical fiber.

In order to achieve the above object, an optical fiber preform according to a first aspect of the present invention includes a tip of a quartz tube and a tip of the quartz tube of a core rod for an optical fiber inserted into the quartz tube. In the optical fiber base material having the tip fusion part formed by fusing the tip part on the same side as the part,
The tip fusion part has a taper shape, the average outer diameter of the quartz tube before fusion processing is D, and the outer diameter of the tip fusion part is 0.10 D from the longitudinal position where it is 0.95D. When the position up to the longitudinal direction is defined as the tip tapered portion after the fusion processing, the length of the tip tapered portion is in the range of 1.0D to 3.0D.

Further, the optical fiber preform according to the second aspect of the present invention is a tip fusion formed by fusing a tip portion of a quartz tube and a tip portion of an optical fiber core rod inserted into the quartz tube. In the optical fiber preform having a portion,
An inert gas is sealed between the quartz tube and the optical fiber core rod.

Furthermore, the optical fiber preform manufacturing method of the present invention includes a tip end portion of a quartz tube and a tip end portion of a core rod for an optical fiber inserted into the quartz tube, which are heated and fused in a heating furnace. Forming a landing portion;
A step of processing the tip fusion part, where D is an average outer diameter of the quartz tube before the fusion process, and 0.10D from a longitudinal position where the outer diameter of the tip fusion part is 0.95D. And having a step of processing so that the length of the tip tapered portion is in the range of 1.0D to 3.0D when the tip tapered portion after fusion processing is defined up to a certain longitudinal position. Features.

Furthermore, the optical fiber preform manufacturing apparatus according to the present invention heats and melts the tip of the quartz tube and the tip of the core rod for the optical fiber inserted into the quartz tube in a heating furnace. A fusing device for forming a landing portion;
A processing apparatus for processing the tip fusion part, wherein an average outer diameter of the quartz tube before the fusion process is D, and a diameter of the tip fusion part is 0.95D from a longitudinal position of 0.10D. Provided with a processing device for processing so that the length of the tip tapered portion is in the range of 1.0D to 3.0D when the tip tapered portion after the fusion processing is defined up to a certain longitudinal position. Features.

  According to the optical fiber preform according to the first aspect of the present invention, the cladding non-circularity of the optical fiber obtained from the optical fiber preform is reduced by setting the length of the tip tapered portion in the above range. It can be suppressed, melting and dropping time can be shortened and working time can be shortened, the length of the tip of the optical fiber that can generate bubbles can be shortened, and the linear speed stabilization time required until the linear speed can be stabilized can be shortened. The effect of improving the yield is obtained.

  In addition, according to the optical fiber preform according to the second aspect of the present invention, it is possible to prevent quality degradation during storage of the optical fiber preform, and in particular, to reduce the loss at the tip of the optical fiber that is the product. There is.

  Furthermore, according to the optical fiber preform manufacturing method and manufacturing apparatus of the present invention, it is possible to manufacture an optical fiber preform that achieves the above effects by processing the shape of the tip tapered portion into the shape.

  In the method for manufacturing an optical fiber preform of the present invention, it is also preferable to have a gradual cooling step in which the average temperature change when the temperature of the tip fused portion is lowered from 1800 ° C. to room temperature is 20 ° C./min. It is an aspect. In this case, cracking of the optical fiber preform can be prevented.

Further, in the optical fiber preform manufacturing method of the present invention, the inert gas flowing inside the heating furnace is passed from one opening of the heating furnace into which the end of the quartz tube is inserted to the other of the heating furnace. Flowing toward the opening is also a preferred aspect of the present invention. The optical fiber preform according to the second aspect of the present invention can be manufactured.

  In a preferred embodiment of the present invention, the inside of the quartz tube is kept at the same pressure as the outside of the quartz tube in the step of forming the tip fused portion and the step of processing the tip fused portion. In this case, unnecessary deformation of the quartz tube during heating can be suppressed, and fluctuations in the outer diameter of the resulting optical fiber can be reduced.

  Preferably, the optical fiber preform manufacturing apparatus of the present invention further includes a slow cooling device that sets an average temperature change at 20 ° C./min or less when the tip fusion portion is cooled from 1800 ° C. to room temperature. In this case, occurrence of cracks can be suppressed in the optical fiber preform obtained.

  In addition, the optical fiber of the present invention further includes a gas introduction device for flowing an inert gas from one opening of the heating furnace into which the end of the quartz tube is inserted toward the other opening of the heating furnace. This is a preferred embodiment of a base material manufacturing apparatus. It is possible to prevent dust from adhering to the outer surface of the quartz tube.

  Furthermore, it is preferable to further have a pressure holding means for keeping the inside of the quartz tube at the same pressure as the outside at the time of fusing by the fusing device and at the time of processing by the processing device. The quartz tube can be prevented from being deformed.

  Hereinafter, the principle of the present invention will be described. FIG. 3 shows the length of the taper portion of the optical fiber preform having the tip fused portion, the characteristics of the optical fiber obtained by performing fusion integration simultaneously with drawing from the optical fiber preform, and the like. The relationship is shown. Specifically, the figure shows the relationship between the length (L) of the tip tapered portion of the optical fiber preform and the cladding non-circularity of the optical fiber (Fig. (A)), the length of the tip tapered portion and the drawing time. The relationship between the melting and dropping time (Fig. (B)), the relationship between the length of the tip tapered portion and the length of the portion having bubbles in the obtained optical fiber (Fig. (C)), and the tip tapered portion The relationship between the length of the wire and the linear velocity stabilization time at the time of drawing ((d) in the figure) is shown by a graph. Here, as shown in FIG. 4, when the average outer diameter of the quartz tube 31 before processing is D (FIG. 4 (a)), the tip end tapered portion of the optical fiber preform (FIG. 4 (b)) is processed. 33 is defined as a base material portion from a longitudinal position where the outer diameter of the quartz tube 31 is 0.95D to a longitudinal position where the outer diameter of the quartz tube 31 is 0.10D, and the tip tapered portion 33 thereof. The length of is assumed to be L.

  From the graph of FIG. 3, it can be seen that in order to shorten the time from the start of heating to melting and dropping, it is only necessary to lengthen the tip tapered portion 33 (FIG. 3B). However, if the taper portion is too long, the time until the linear velocity is stabilized is extended (FIG. 3D), and bubbles are generated in the fiber for a while after the linear velocity is stabilized (FIG. 3C). ))There was found. The reason why bubbles are generated in the fiber is that when the taper portion becomes long, a portion where the core rod and the quartz tube are fused incompletely occurs, and gas tends to remain in this portion. In addition, when the taper portion is short, it has been found that, in addition to extending the melting and dropping time as described above, the cladding non-circularity immediately after the linear velocity stabilization is deteriorated (FIG. 3A). In addition, the standard upper limit value of the cladding non-circularity in FIG. 3A is generally 1%, and the definition thereof conforms to ITU-T G650.1. From the above results, it can be understood that there is an appropriate length for the length L of the tapered portion.

  When the tip tapered portion 33 is defined as described above, the appropriate length L of the tip tapered portion is in the range of L = 1.0D to 3.0D from the graphs of FIGS. 3 (a) to 3 (d). Thus, a good optical fiber can be obtained with a high yield. Regarding the tip fusion processing of the quartz tube 31 and the core rod 32 and the shape processing of the tip tapered portion 33, the quartz tube 31 and the core rod 32 are directly fused without a dummy member to reduce the cost, and the tip shape is obtained. Is preferably formed into the tapered shape described above.

  The following method was employed to obtain the shape of the tip tapered portion. First, the core rod is fixed and suspended while being inserted into the quartz tube. Both ends are heated in a vertical heating furnace, the quartz tube is extended by gravity, and the diameter of the quartz tube is reduced, whereby the end fusion between the core rod and the quartz tube is performed. During heating, a gap between the quartz tube and the core rod is obtained by flowing an inert gas (for example, Ar) from the top to the bottom of the furnace and wrapping the tip of the quartz tube with clean inert gas. And dust can be prevented from adhering to the outside of the quartz tube.

  At the time of tip fusion, an inert gas (for example, Ar) having a moisture content of 100 ppm or less is allowed to flow through the gap between the quartz tube and the core rod, thereby diffusing moisture to the core rod due to heating. Can be prevented. If the heating is further continued after the tip fusion, the heated portion will be extended by gravity. The extended unnecessary part is intermittently cut several times by a cutting machine provided under the heating furnace. The length of the tip tapered portion can be controlled by the number of times of cutting, and the tip tapered portion can be shortened by increasing the number of times of cutting. However, if the number of times of cutting is increased to some extent, the length of the tip tapered portion does not change. In addition to the number of times of cutting, the length of the tip tapered portion can be shortened by pulling the base material upward or continuously or intermittently during cutting. Also, the length of the tip tapered portion can be changed by changing the temperature distribution in the furnace.

  After the tip tapered portion is processed, the temperature of the processed portion is gradually decreased, so that the temperature can be decreased without creating a temperature difference between the quartz tube and the core rod. Also, the heating furnace used as a heat source has a gentle temperature change in the longitudinal direction, and temperature control is easy. For this reason, unlike the slow cooling using a burner, a slow cooling is possible comparatively easily.

  When Ar gas is sealed between the quartz tube and the core rod, it is possible to prevent deterioration of fiber characteristics due to contamination of the interface during the storage period of the optical fiber preform before drawing.

  The graph of FIG. 5 shows the length from the drawing start of the optical fiber to the drawing start tip of the optical fiber preform in the optical fiber obtained by drawing the optical fiber preform according to the present embodiment. The numerical value converted from the portion to the position corresponding to the longitudinal direction is taken on the horizontal axis, and the relationship with the loss (dB / km) of the optical fiber at 1.38 μm is shown. In the optical fiber obtained from the optical fiber preform of the embodiment of the present invention, stable loss characteristics are obtained also from the portion of the optical fiber corresponding to the tip of the preform by sealing Ar gas. However, in an optical fiber obtained from a conventional optical fiber preform that does not contain Ar gas, the loss of the optical fiber corresponding to the tip of the preform is increased. The tip portion of the optical fiber having such an increase in loss needs to be made into a product excluding this in advance, which causes a reduction in product yield.

  1 (a) and 1 (b) are cross-sectional views of an optical fiber preform manufacturing apparatus 10 shown before and during processing when manufacturing an optical fiber preform according to an embodiment of the present invention. FIG.

The structure of the manufacturing apparatus is as follows. The vertical heating furnace 11 is a carbon resistance furnace, and a cutting machine 12 for removing unnecessary portions of the stretched optical fiber preform is disposed below the heating furnace 11. Although not shown in FIG. 1, there is a positioning device that suspends the quartz tube 31 at the upper part of the heating furnace 10 and determines the length for inserting the quartz tube 31 into the heating furnace 11. The lower side of the heating furnace 11 is cut off from the outside air for the purpose of preventing the entrainment of outside air into the heating furnace 11 and controlling the flow of gas in the furnace. Under the cutting machine 12, there is a camera 13 for detecting that the leading end of the base material being processed is sufficiently stretched. When the leading end is detected by the camera 13, the cutting machine 12 removes the leading end. Cut. The cut unnecessary part falls into the pedestal 15 where the upper part and the edge can be cut by the vacuum gate valve 14. After processing, the vacuum gate valve 14 is closed, the edge between the inside of the furnace and the pedestal 15 is cut, the pedestal 15 is opened to the outside air, and the dropped unnecessary part is carried out to the outside. After unloading, the inside of the pedestal 15 is replaced with an inert gas, and then the vacuum gate valve 14 is opened to perform the next processing.
Example 1

  When manufacturing the optical fiber preform of the first embodiment, first, in the manufacturing apparatus 10 shown in FIG. 1A, the optical fiber core rod 32 is inserted into the quartz tube 31, and these tips are placed in the heating furnace 11. Inserted into. The diameter of the quartz tube is φ100 to 150 mm, the diameter of the core rod is φ32 to 48 mm, and the outer diameter of the core rod and the size of the quartz tube are set so that the distance between the inner surface of the quartz tube and the outer surface of the inserted core rod is 0.3 mm or less. Was selected. A quartz dummy rod 34 is welded to the tip of the core rod 32 in the example of FIG. If an inexpensive quartz dummy rod 34 is welded to the tip of the core rod 32, the portion discarded at the time of tip processing can be used as the quartz dummy rod 34, and there is an advantage that the amount of expensive core rod discarded is reduced.

  A chamber 16 surrounding this upper end was provided at the end (upper end) on the non-heated side of the quartz tube 31, and Ar gas having a moisture content of 100 ppm or less was supplied into the chamber 16 from the Ar supply port 17. Further, the chamber 16 is provided with a jet port 18 for blowing Ar gas so that the quartz tube 31 is not deformed by the increase in the internal pressure of the quartz tube 31 after the end fusion. Ar gas flows until after the slow cooling is completed, and then the jet port 18 and the supply port 17 are closed, and Ar is sealed inside the quartz tube 31. This prevents outside air containing moisture from entering the inside of the quartz tube 31 from the end of processing to the drawing. An inert gas, Ar gas in this example, was supplied to the heating furnace 11 from an Ar supply port 19 near the upper opening of the heating furnace 11. Ar gas is divided into two parts after hitting the quartz tube 31, and the flow of one Ar is directed upward to prevent the outside air from entering the heating furnace 11, and the dust in the heating furnace 11 becomes quartz. Prevents seizing near the top of the machined part of the tube. A pump 20 was installed on the lower side of the heating furnace 11 so that the other Ar flow would flow downward, and the gas in the heating furnace 11 was discharged out of the heating furnace 11.

  Since the lower side of the heating furnace 11 is sealed, the pressure is reduced by discharging the gas using the pump 20, and the gas in the heating furnace 11 becomes a downward flow. In this embodiment, only one Ar gas supply port is provided. However, a plurality of supply ports such as a supply port for flowing Ar gas upward and a supply port for flowing downward Ar gas are provided. May be. By supplying Ar gas, it is possible to prevent dust from adhering to the outside of the quartz tube 31 by flowing so that the tip of the quartz tube 31 is wrapped with clean Ar gas.

  FIG.1 (b) has shown the time of shaping | molding of a front-end | tip shape. When the quartz tube 31 melts, its tip is pulled by gravity and contracts in the radial direction, and the core rod 32 and the tip of the quartz tube 31 are fused. At this time, the temperature of the heating furnace 11 was 1900-2100 degreeC. When the tip portion extended, the camera 13 recognized the tip portion, and the tip extended by the cutting device 12 was cut. By repeating this melting and cutting, the length of the tip tapered portion can be controlled. In this example, the number of times of cutting was 4 and the length of the tip tapered portion was 2.2D. After processing, in order to prevent deformation of the quartz tube 31 and the core rod 32 during slow cooling, the temperature of the heating furnace 11 was lowered to 1800 ° C. as quickly as possible.

  In the temperature range of 1800 ° C., the quartz glass is still soft, and even if a temperature difference occurs between the quartz tube 31 and the core rod 32 and a stress is generated, it can be deformed by itself so that no cracks are generated. In order to eliminate the temperature difference between the quartz tube 31 and the core rod 32, the temperature was maintained at 1800 ° C. for 5 minutes, and then the temperature was lowered to room temperature at a rate of 20 ° C./min. Cracks did not occur. However, when the temperature change was cooled at 30 ° C./min, the quartz tube 31 sometimes cracked. Moreover, when processing is performed continuously, if the temperature of the heating furnace 11 is lowered too much, it takes time to raise the temperature next, which is not economical. In order to avoid this, even if the quartz tube 31 is gradually pulled up from the heating furnace 11 when the temperature of the heating furnace 11 reaches a certain temperature (for example, 1800 ° C.), there is an effect of gradual cooling. The pulling speed depends on the temperature of the heating furnace 11 during pulling and the temperature distribution in the furnace, but cracks do not occur if the temperature change in the processed portion is 30 ° C./min or less.

  The optical fiber preform manufactured by the method shown in this example was drawn. FIG. 3 shows the characteristics of an optical fiber obtained from the optical fiber preform. For example, for a preform base material having a length L of the tip tapered portion 33 of 2.2D, the time from the start of heating to the melting and dropping at the time of drawing is, for example, 23 minutes ((b) in the figure), the linear velocity The time to stabilization was 57 minutes (FIG. (D)), and no bubbles were generated in the fiber (FIG. (C)). For comparison, a base material formed with a tapered portion length of 4.0D was drawn. The time from the start of heating to melting and dropping was as short as 20 minutes ((b) in the figure), but the time to linear velocity stabilization was 98 minutes ((d) in the figure). Bubbles were generated in the fiber ((c) in the figure). Further, the clad non-circularity was within the standard and there was no problem ((a) in the figure). Also, the transmission loss at a wavelength of 1.38 μm caused by OH was within the standard (FIG. 5).

Example 2
If the side opposite to the side to be subjected to the tip welding process is sealed, the internal pressure of the quartz tube 31 increases during the tip fusion, and in the worst case, the quartz tube 31 may be deformed. For this reason, it is necessary to keep the pressure in the quartz tube 31 constant. In Example 2, the pressure difference was controlled by monitoring the differential pressure between the atmospheric pressure and the quartz tube 31 during heating and slow cooling, and discharging the gas from the quartz tube 31. FIG. 2 shows the upper structure of the heating furnace 11 when processing the optical fiber preform of the second embodiment. In this embodiment, the target pressure is atmospheric pressure. After cooling sufficiently, an inert gas (eg, Ar gas) with less dust is sealed.

  In this example, since the pressure control is performed, the structure of the manufacturing apparatus is complicated compared to Example 1, but since Ar gas is not used during heating and slow cooling, the amount of Ar gas used is small, Economical.

  As mentioned above, although this invention was demonstrated based on the suitable embodiment example, the optical fiber preform | base_material of this invention, its manufacturing apparatus, and a method are not limited only to the structure of the said embodiment and Example. Also, various modifications and changes made from the configurations of the above-described embodiments and examples are also included in the scope of the present invention.

(A) And (b) is the partial cross section side view of a manufacturing apparatus shown in the state before the front-end | tip part processing of the optical fiber preform | base_material of Example 1, respectively, during a front-end | tip part processing, respectively. Sectional drawing of the upper part of a heating furnace shown in the state before the process of the optical fiber preform | base_material of Example 2. FIG. (A)-(d) is a graph which respectively shows the relationship between the length of a front-end | tip taper part, and the characteristic of the optical fiber obtained by drawing from this preform | base_material for optical fibers. (A) And (b) is sectional drawing which shows the front-end | tip part of the preform | base_material for optical fibers in the state before and after a process, respectively, in order to show the definition of the front-end | tip taper part of the preform | base_material for optical fibers. The graph which shows the difference of the loss characteristic of the preform | base_material for optical fibers of one Embodiment of this invention, and the preform | base_material for optical fibers of a prior art.

Explanation of symbols

10: Manufacturing apparatus 11: Heating furnace 12: Cutting device 13: Camera 14: Vacuum gate valve 15: Pedestal 16: Chamber 17: Ar supply port 18: Ar outlet 19: Ar supply port 20: Pump 31: Quartz tube 32: Rod 33: Tapered end portion 34: Dummy rod

Claims (7)

In the optical fiber preform having a tip fusion portion formed by fusing a tip portion of a quartz tube and a tip portion of an optical fiber core rod inserted into the quartz tube,
The quartz tube has an outer diameter of 100 mm or more,
Having a gap between the inner surface of the quartz tube and the outer surface of the core rod for optical fiber,
The tip fusion part has a taper shape, the average outer diameter of the quartz tube before fusion processing is D, and the outer diameter of the tip fusion part is 0.95D from a longitudinal position of 0.10D. An optical fiber preform characterized in that when a position in a longitudinal direction is defined as a tip tapered portion after fusion processing, the length of the tip tapered portion is in the range of 1.0D to 3.0D. .   2. The optical fiber preform according to claim 1, wherein an inert gas is sealed in the gap between the quartz tube and the optical fiber core rod. Preparing a quartz tube having a cross-sectional outer diameter of 100 mm or more;
Inserting a core rod for an optical fiber so as to have an inner surface with a gap inside the quartz tube of the quartz tube,
A step of forming a tip fused portion by heating and fusing a tip portion of the quartz tube and a tip portion of an optical fiber core rod inserted into the quartz tube in a heating furnace;
A step of processing the tip fusion part, where D is an average outer diameter of the quartz tube before the fusion process, and 0.10D from a longitudinal position where the outer diameter of the tip fusion part is 0.95D. And having a step of processing so that the length of the tip tapered portion is in the range of 1.0D to 3.0D when the tip tapered portion after fusion processing is defined up to a certain longitudinal position. A manufacturing method of a base material for optical fiber, which is characterized.   The method for manufacturing an optical fiber preform according to claim 3, wherein the temperature of the heating furnace in the step of forming the tip fusion part is 1900 to 2100 ° C.   5. The method according to claim 4, further comprising a gradual cooling step in which an average temperature change when the temperature of the tip fused portion is lowered from 1800 ° C. to room temperature is 20 ° C./min or less after the processing step. The manufacturing method of the preform | base_material for optical fibers as described in any one of.   The inert gas that flows inside the heating furnace flows from one opening of the heating furnace into which the end of the quartz tube is inserted toward the other opening of the heating furnace. The manufacturing method of the preform | base_material for optical fibers as described in any one of -5.   The optical fiber according to any one of claims 3 to 6, wherein in the step of forming the tip fused portion and the step of processing, the inside of the quartz tube is maintained at the same pressure as the outside of the quartz tube. A manufacturing method of a base material.

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