Optical fiber manufacturing method

Abstract

To provide a method for manufacturing an optical fiber that suppresses deterioration in yield in the drawing speed increase process.SOLUTION: A method for manufacturing an optical fiber includes: a drawing speed increase step of drawing an optical fiber preform while increasing a drawing speed to a predetermined drawing speed; and a product manufacturing step of manufacturing an optical fiber to be used as a product while drawing an optical fiber preform at the predetermined drawing speed after the drawing speed increase step. The drawing speed increase step includes: a first drawing speed increase step of increasing the drawing speed while controlling temperature of a heating furnace on the basis of a preset temperature of the heating furnace in the product manufacturing step; and a second drawing speed increase step of increasing the drawing speed while controlling temperature of the heating furnace so that a drawing tension for an optical fiber becomes a target drawing tension for an optical fiber that is manufactured in the product manufacturing step after the first drawing speed increase step. A preset temperature for the heating furnace in the product manufacturing step is set on the basis of a temperature of the heating furnace in the product manufacturing step at the time of manufacturing an optical fiber in the past.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing an optical fiber.

Background Art

[0002] Patent Document 1 discloses a method for manufacturing an optical fiber, in which an optical fiber preform is drawn to form a glass fiber, and the glass fiber is coated with a resin on its outer periphery to manufacture an optical fiber. In this method for manufacturing an optical fiber, the cut-off wavelength of the glass fiber is set to a predetermined value by controlling the drawing tension generated in the glass fiber during the drawing of the optical fiber preform.

[0003] Patent Document 2 discloses a method for manufacturing an optical fiber having a linear velocity increase step of drawing an optical fiber preform while increasing the drawing velocity from an initial drawing velocity to a target drawing velocity, and a steady drawing step of drawing the optical fiber preform under the conditions of the target drawing velocity and the target drawing tension. In this method for manufacturing an optical fiber, in the linear velocity increase step, an optical fiber having desired characteristics is manufactured during the increase in the drawing velocity by maintaining the drawing tension at the target drawing tension while increasing the drawing velocity from the initial drawing velocity to the target drawing velocity.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present disclosure is to provide a method for manufacturing an optical fiber that suppresses a decrease in yield during the process of increasing the drawing velocity. [[ID=asc]]

Means for Solving the Problems

[0006] The method for manufacturing optical fibers in this disclosure is: A method for manufacturing optical fibers, which involves drawing a fiber optic matrix heated in a heating furnace, A line speed increase step in which the line drawing speed of the optical fiber preform is increased to a predetermined line drawing speed while drawing the optical fiber preform, The process includes a product manufacturing step, after the linear velocity increase step, in which the optical fiber to be used as a product is manufactured while drawing the optical fiber preform at the predetermined drawing speed, The aforementioned linear velocity increase step is, A first line speed increase step involves increasing the line drawing speed while controlling the temperature of the heating furnace based on the set temperature of the heating furnace in the product manufacturing step, The system includes a second linear velocity increase step, in which, after the first linear velocity increase step, the linear velocity increase step is performed while controlling the temperature of the heating furnace so that the linear tensile force of the optical fiber becomes the target linear tensile force of the optical fiber manufactured in the product manufacturing step. The set temperature of the heating furnace in the product manufacturing step is set based on the temperature of the heating furnace in the product manufacturing step during the manufacturing of optical fibers in the past. [Effects of the Invention]

[0007] According to this disclosure, it is possible to provide a method for manufacturing optical fibers that suppresses a decrease in yield during the process of increasing the drawing speed. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a conceptual diagram showing the configuration of an optical fiber manufacturing apparatus according to one embodiment. [Figure 2] Figure 2 is a diagram illustrating a method for manufacturing an optical fiber according to one embodiment. [Figure 3]Figure 3 shows the relationship between the difference in the linear tensile force of the optical fiber in the current and past optical fiber product manufacturing steps, and the difference in the temperature of the heating furnace that fluctuates due to that difference in linear tensile force. [Modes for carrying out the invention]

[0009] [Description of Embodiments in this Disclosure] First, the contents of the embodiments of this disclosure will be listed and explained. The method for manufacturing optical fibers in this disclosure is: (1) A method for manufacturing optical fibers by drawing a fiber optic matrix heated in a heating furnace, A line speed increase step in which the line drawing speed of the optical fiber preform is increased to a predetermined line drawing speed while drawing the optical fiber preform, The process includes a product manufacturing step, after the linear velocity increase step, in which the optical fiber to be used as a product is manufactured while drawing the optical fiber preform at the predetermined drawing speed, The aforementioned linear velocity increase step is, A first line speed increase step involves increasing the line drawing speed while controlling the temperature of the heating furnace based on the set temperature of the heating furnace in the product manufacturing step, The system includes a second linear velocity increase step, in which, after the first linear velocity increase step, the linear velocity increase step is performed while controlling the temperature of the heating furnace so that the linear tensile force of the optical fiber becomes the target linear tensile force of the optical fiber manufactured in the product manufacturing step. The set temperature of the heating furnace in the product manufacturing step is set based on the temperature of the heating furnace in the product manufacturing step during the manufacturing of optical fibers in the past.

[0010] In the product manufacturing step, the temperature of the heating furnace is set to a temperature at which optical fibers with desired characteristics (e.g., cutoff wavelength) can be manufactured. However, if the same set temperature is used each time the optical fiber matrix is ​​drawn, the temperature distribution inside the heating furnace may change due to the aging of the heating furnace, which may alter the characteristics of the manufactured optical fiber. In this case, for example, it is conceivable to control the tensile force of the optical fiber by changing the set temperature of the heating furnace so that the manufactured optical fiber has the desired characteristics. However, if extreme temperature fluctuations occur in the heating furnace due to tensile force control, it can lead to extreme fluctuations in the amount of glass melted, resulting in overshoot or downshoot in the drawing speed. With the above configuration, the set temperature of the heating furnace in the product manufacturing step is determined based on the temperature of the heating furnace in the product manufacturing step during past optical fiber manufacturing. This ensures that the temperature distribution is pre-determined to mitigate the effects of heating furnace degradation, and extreme temperature fluctuations of the heating furnace can be suppressed even when tensile force control is performed. Furthermore, in the linear velocity increase step, tensile force control is performed after the heating furnace temperature has been increased, thus suppressing extreme temperature fluctuations of the heating furnace due to tensile force control. Therefore, the line drawing speed can be increased stably during the line drawing speed increase process, and the decrease in yield can be suppressed.

[0011] (2) The temperature of the heating furnace in the product manufacturing step during the manufacture of the optical fiber in the past may be the temperature of the heating furnace at the time when the manufacture of the optical fiber to be used as a product was started in the product manufacturing step during the manufacture of the optical fiber in the past.

[0012] With the above configuration, since the temperature of the heating furnace at the start of manufacturing the optical fiber to be used as a product is used, the temperature at which the linear velocity stabilizes immediately after increasing can be used as the set temperature of the heating furnace.

[0013] (3) The set temperature Tb of the heating furnace in the product manufacturing step, the temperature T0 of the heating furnace in the product manufacturing step during the production of the previous optical fiber, the difference ΔS between the target wire drawing tension of the optical fiber to be manufactured in the product manufacturing step and the wire drawing tension of the optical fiber manufactured in the product manufacturing step during the production of the previous optical fiber, and the correction coefficient a may satisfy the relationship of Tb = T0 + ΔT with ΔT = aΔS.

[0014] Since the characteristics of the optical fiber preform vary for each optical fiber preform, the wire drawing tensions for manufacturing optical fibers having desired characteristics also differ. Also, the temperature of the heating furnace in the product manufacturing step reflects the wire drawing tension corresponding to the characteristics of the optical fiber to be manufactured. According to the above configuration, since the wire drawing tension that changes due to variations in the characteristics of the optical fiber preform is taken into account in advance when calculating the set temperature Tb of the heating furnace, extreme temperature fluctuations of the heating furnace can be suppressed even when wire drawing tension control is performed in the second wire speed increase step.

[0015] (4) The first wire speed increase step may be executed in a section where the wire drawing speed V during the wire speed increase step and the predetermined wire drawing speed V1 in the product manufacturing step satisfy the relationship of V < V1×0.6.

[0016] According to the above configuration, by raising the temperature of the heating furnace before performing wire drawing tension control, extreme temperature fluctuations of the heating furnace due to wire drawing tension control can be suppressed.

[0017] (5) The second wire speed increase step may be executed in a section where the wire drawing speed V during the wire speed increase step and the predetermined wire drawing speed V1 in the product manufacturing step satisfy the relationship of V1×0.6 ≤ V ≤ V1.

[0018] When the wire drawing speed is low (V < V1 × 0.6) and the control of the wire drawing tension is started, the wire drawing tension drops too much, and the optical fiber may come off the guide roller of the optical fiber manufacturing apparatus, or the optical fiber may vibrate and the amplitude may increase and contact the apparatus, resulting in the optical fiber having a low strength or breaking. According to the above configuration, since the wire drawing tension control is performed in the section where V ≥ V1 × 0.6, the occurrence of disconnection of the optical fiber can be suppressed. On the other hand, since the wire drawing tension control is performed in the section where V ≤ V1, the production of the optical fiber to be used as a product can be started immediately after reaching the predetermined wire drawing speed.

[0019] [Details of Embodiments of the Present Disclosure] A specific example of the method for manufacturing an optical fiber according to an embodiment of the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, and is shown by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In addition, in each of the drawings used in the following description, the scale is appropriately changed in order to make each member recognizable in size.

[0020] (Optical Fiber Manufacturing Apparatus) FIG. 1 is a schematic diagram showing the configuration of an optical fiber manufacturing apparatus 1 according to the present embodiment. The optical fiber manufacturing apparatus 1 is configured to draw an optical fiber preform G to form a glass fiber G1, and coat the outer periphery of the glass fiber G1 with a resin to manufacture an optical fiber G2. The optical fiber preform G is, for example, an optical fiber preform mainly composed of silica glass.

[0021] The optical fiber manufacturing apparatus 1 includes a heating furnace 2, a resin coating apparatus 3, a resin curing apparatus 4, a take-up apparatus 5, a winding apparatus 6, a tension measuring apparatus 7, and a control apparatus 8.

[0022] The heating furnace 2 is configured to heat and soften the lower end of the optical fiber preform G. The optical fiber preform G is inserted into the heating furnace 2 by the feeder 9 and fed downward. The lower end of the optical fiber preform G, softened by heating, is stretched downward to form a thin glass fiber G1.

[0023] The resin coating device 3 and the resin curing device 4 are positioned downstream of the heating furnace 2 in the direction of travel of the glass fiber G1 (downward in Figure 1). The resin coating device 3 coats the glass fiber G1 with resin, and the resin curing device 4 cures the resin. This forms an optical fiber G2 with resin formed around the glass fiber G1.

[0024] The take-up device 5 is configured to take up the optical fiber G2 via the guide roller 10 and send it to the winding device 6. For example, the take-up device 5 has a belt 5a and a roller 5b. The rotation of the belt 5a and the rotation of the roller 5b take up the optical fiber G2. The winding device 6 is configured to wind up the optical fiber G2 sent from the take-up device 5. For example, the winding device 6 winds up the optical fiber G2 while rotating.

[0025] The tension measuring device 7 is positioned near the guide roller 10. The tension measuring device 7 is configured to measure the linear tensile force acting on the optical fiber G2. For example, the tension measuring device 7 measures the linear tensile force of the optical fiber G2 by measuring the load that the optical fiber G2 exerts on the guide roller 10.

[0026] The control device 8 is electrically connected to the heating furnace 2 and the take-up device 5, and is configured to control the temperature of the heating furnace 2 and the take-up speed of the optical fiber G2 by the take-up device 5 (the wire drawing speed of the optical fiber preform G).

[0027] (Manufacturing method for optical fibers) Next, with reference to Figure 2, a method for manufacturing optical fibers using the optical fiber manufacturing apparatus 1 will be described.

[0028] As illustrated in Figure 2, once the manufacturing of the previous optical fiber is complete, a new optical fiber base material G is set in the optical fiber manufacturing apparatus 1, and the process of threading glass fibers G1 onto the optical fiber manufacturing apparatus 1 is performed. Specifically, the optical fiber base material G is inserted into the heating furnace 2 by a feeder 9, and the lower end of the optical fiber base material G is heated by the heating furnace 2. The molten glass mass falls from the heating furnace 2 due to its own weight (seed drop). The glass mass that falls from the heating furnace 2 is pulled and thinned to become a glass fiber G1 of a predetermined glass diameter. The thinned glass fiber G1 is passed through a resin coating apparatus 3 and a resin curing apparatus 4, hung on a take-up apparatus 5, and wound onto a winding apparatus 6.

[0029] Next, the optical fiber preform G is drawn. As illustrated in the upper part of Figure 2, the optical fiber preform G drawing process consists of a linear velocity increase step (times t1 to t3) and a product manufacturing step (times t3 onwards). In the linear velocity increase step, the optical fiber preform G is drawn while its drawing speed is increased to a predetermined drawing speed V1. Specifically, at time t1, the pickup device 5 starts picking up the optical fiber G2, and the optical fiber preform G is drawn while its drawing speed is increased from zero to a predetermined drawing speed V1.

[0030] The linear velocity increase step includes a first linear velocity increase step (time t1 to time t2) and a second linear velocity increase step (time t2 to time t3). In the first linear velocity increase step, the linear drawing speed is increased while controlling the temperature of the heating furnace 2 based on the set temperature Tb of the heating furnace 2 in the product manufacturing step.

[0031] Specifically, as illustrated in the lower part of Figure 2, the temperature of the heating furnace 2 is controlled to rise from the temperature Ta at the start of the linear velocity increase step to the set temperature Tb.

[0032] The set temperature Tb of the heating furnace 2 is set based on the temperature T0 of the heating furnace in the product manufacturing step of the previous optical fiber. For example, the control device 8 obtains a measurement of the temperature of the heating furnace 2 in the product manufacturing step of the previous optical fiber from a temperature measuring instrument (not shown) installed in the heating furnace 2, and calculates the set temperature Tb based on this measurement.

[0033] In the second linear velocity increase step, after the first linear velocity increase step, the drawing speed is increased while controlling the temperature of the heating furnace 2 so that the linear tensile force of the optical fiber becomes the target linear tensile force of the optical fiber manufactured in the product manufacturing step. For example, the control device 8 controls the temperature of the heating furnace based on the measurement result of the linear tensile force generated in the optical fiber G2 from the tension measuring device 7 so that the measurement result becomes the target linear tensile force. In Figure 2, T1 shows the temperature of the heating furnace 2 when the linear tensile force is controlled. The target linear tensile force of the optical fiber is set appropriately according to the characteristics of the optical fiber to be manufactured.

[0034] In the product manufacturing step, after the linear velocity increase step, optical fiber G2 having the desired characteristics for use as a product is manufactured by drawing the optical fiber preform G at a predetermined drawing speed V1. Specifically, as illustrated in the upper part of Figure 2, at time t3, after the drawing speed V reaches a predetermined drawing speed V1, the optical fiber preform G is drawn while maintaining the predetermined drawing speed V1 (steady linear velocity). Then, the optical fiber G2 having the desired characteristics is used as a product (product extraction).

[0035] Thus, the set temperature Tb of the heating furnace 2 in the product manufacturing step is determined based on the temperature T0 of the heating furnace during past optical fiber manufacturing, thereby suppressing extreme temperature fluctuations of the heating furnace 2 due to the effects of deterioration of the heating furnace 2. In other words, the set temperature of the heating furnace 2 in the product manufacturing step is set to a temperature at which optical fibers with desired characteristics (e.g., cutoff wavelength) can be manufactured. However, if the same set temperature is used each time the optical fiber matrix is ​​drawn, the temperature distribution inside the heating furnace 2 may change due to the aging deterioration of the heating furnace 2, which may change the characteristics of the manufactured optical fiber. In this case, for example, it is conceivable to control the tension force of the optical fiber so that the manufactured optical fiber has the desired characteristics. However, if extreme temperature fluctuations of the heating furnace 2 occur due to tension force control, overshoot or downshoot of the drawing speed may occur. In contrast to this, as described above, in this embodiment, the set temperature Tb of the heating furnace 2 in the product manufacturing step is determined based on the temperature T0 of the heating furnace 2 during past optical fiber manufacturing, thereby suppressing extreme temperature fluctuations of the heating furnace due to the effects of deterioration of the heating furnace. Furthermore, in the line speed increase step, the line tensile force is controlled after the temperature of the heating furnace 2 has been increased, so extreme temperature fluctuations of the heating furnace 2 due to line tensile force control can be suppressed. Therefore, the line drawing speed can be increased stably during the line drawing speed increase process, and a decrease in yield can be suppressed.

[0036] In this embodiment, the temperature of the heating furnace in the product manufacturing step during the production of the optical fiber in the past is set to the temperature T0 of the heating furnace used in the product manufacturing step when drawing the previous optical fiber preform. This makes it possible to reflect the effects of the most recent aging degradation of the heating furnace 2 in the set temperature Tb of the heating furnace 2.

[0037] As illustrated in Figure 2, the temperature T0 of the heating furnace in the product manufacturing step of the previous optical fiber manufacturing process may be the temperature of the heating furnace 2 at the time (time t0) when the manufacturing of the optical fiber to be used as a product started in the product manufacturing step of the previous optical fiber manufacturing process. This allows the temperature that stabilizes immediately after the linear velocity increase to be used as the set temperature Tb of the heating furnace 2.

[0038] Furthermore, as illustrated in Figure 2, the set temperature Tb may be set such that it satisfies the relationship Tb = T0 + ΔT, where ΔT = aΔS. ΔS is the difference (ΔS = I1 - I0) between the target linear tensile force I1 of the optical fiber manufactured in the current product manufacturing step and the linear tensile force I0 of the optical fiber manufactured in the previous optical fiber manufacturing step. a (<0) is a correction coefficient, which can be set appropriately based on the heater of the heating furnace 2, the deterioration of the furnace tube and insulation material, etc. Figure 3 illustrates the relationship between the difference in linear tensile force ΔS(g) of the optical fiber in the current and past optical fiber product manufacturing steps and the difference in the temperature of the heating furnace 2 that fluctuates due to this difference in linear tensile force ΔT(°C). For example, as shown in Figure 3, if the difference in linear tensile force between the current and previous optical fiber product manufacturing steps is S1 (ΔS = S1), then the difference in the temperature of the heating furnace 2 that fluctuates due to the difference in linear tensile force S1 is ΔT = a × S1. Therefore, the set temperature Tb is set such that Tb = T0 + ΔT.

[0039] Since the properties of optical fiber preforms vary from one preform to another, the tensile force required to manufacture optical fibers with desired properties also differs. Furthermore, the temperature of the heating furnace in the product manufacturing step reflects the tensile force corresponding to the properties of the optical fiber being manufactured. As described above, by taking into account the tensile force that changes due to variations in the properties of the optical fiber preform when calculating the set temperature Tb of the heating furnace 2, extreme temperature fluctuations of the heating furnace can be suppressed.

[0040] Also, as illustrated in FIG. 2, the first wire speed increase step may be executed in a section where the wire drawing speed V during the wire speed increase step and a predetermined wire drawing speed V1 in the product manufacturing step satisfy the relationship V < V1 × 0.6. According to such a configuration, by raising the temperature of the heating furnace before performing wire tension control, extreme temperature fluctuations of the heating furnace due to wire tension control can be suppressed.

[0041] Also, as illustrated in FIG. 2, the second wire speed increase step may be executed in a section where the wire drawing speed V during the wire speed increase step and a predetermined wire drawing speed V1 in the product manufacturing step satisfy the relationship V1 × 0.6 ≤ V ≤ V1.

[0042] If the control of the wire tension is started when the wire drawing speed is low (V < V1 × 0.6), the wire tension may drop too much, causing the optical fiber to come off the guide roller 10 of the optical fiber manufacturing apparatus 1, or the optical fiber to vibrate with a large amplitude and contact the apparatus, which may result in the optical fiber having a low strength or breaking. In the present embodiment as described above, since the wire tension control is performed in a section where V ≥ V1 × 0.6, the occurrence of disconnection of the optical fiber can be suppressed. On the other hand, since the wire tension control is performed in a section where V ≤ V1, the production of the optical fiber used as a product can be started immediately after reaching the predetermined wire drawing speed.

[0043] As described above, the present invention has been described in detail and with reference to specific embodiments, but it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Also, the number, position, shape, etc. of the constituent members described above are not limited to the above embodiments, and can be changed to suitable numbers, positions, shapes, etc. for implementing the present invention.

[0044] In the above embodiment, the temperature T0 of the heating furnace 2 in the product manufacturing step of the previous optical fiber is used as the temperature of the heating furnace 2 in the product manufacturing step of the optical fiber in the past. However, the temperature of the heating furnace in the product manufacturing step of the optical fiber two, three, four, or five previous optical fiber manufacturing processes may be used, or the average value of these temperatures may be used. Alternatively, the temperature of the heating furnace in the product manufacturing step of the optical fiber manufacturing process when the optical fiber was manufactured using the same type of optical fiber preform may be used from among the temperatures of the heating furnace in the past optical fiber manufacturing processes. By using the same type of optical fiber preform, the variation in line tensile force due to variations in the characteristics of the optical fiber preform is reduced, thereby suppressing extreme temperature fluctuations of the heating furnace.

[0045] In the above embodiment, the set temperature Tb is set such that it satisfies the relationship Tb = T0 + ΔT, where ΔT = aΔS. However, for example, the set temperature Tb may be set to the temperature T0 of the heating furnace in the product manufacturing step during the manufacturing of the previous optical fiber. Even in this case, extreme temperature fluctuations of the heating furnace due to the effects of furnace degradation can be suppressed.

[0046] In the above embodiment, the tension measuring device 7 is located near the guide roller 10. However, the tension measuring device 7 may be located between the heating furnace 2 and the resin coating device 3. For example, the tension measuring device 7 may be an optical device configured to measure the tensile force generated in the glass fiber G1 during wire drawing. [Explanation of symbols]

[0047] 1: Optical fiber manufacturing equipment 2:Heating furnace 3: Resin coating device 4:Resin curing equipment 5: Collection device 5a: Belt 5b: Laura 6: Winding device 7:Tension measuring device 8: Control device 9: Feeder 10: Guide roller a: Correction factor G: Optical fiber base material G1: Glass fiber G2: Optical fiber ΔS: Difference t0: Time t1: Time t2: time t3: Time T0: Temperature T1: Temperature Ta:Temperature Tb: Set temperature ΔT: Temperature difference V: Drawing speed V1: Drawing speed

Claims

1. A method for manufacturing optical fibers, which involves drawing a fiber optic matrix heated in a heating furnace, A line speed increase step in which the line drawing speed of the optical fiber preform is increased to a predetermined line drawing speed while drawing the optical fiber preform, The process includes a product manufacturing step, after the linear velocity increase step, in which the optical fiber to be used as a product is manufactured while drawing the optical fiber preform at the predetermined drawing speed, The aforementioned linear velocity increase step is, A first line speed increase step involves increasing the line drawing speed while controlling the temperature of the heating furnace based on the set temperature of the heating furnace in the product manufacturing step, The system includes a second linear velocity increase step, in which, after the first linear velocity increase step, the linear velocity increase step is performed while controlling the temperature of the heating furnace so that the linear tensile force of the optical fiber becomes the target linear tensile force of the optical fiber manufactured in the product manufacturing step. The set temperature of the heating furnace in the product manufacturing step is set based on the temperature of the heating furnace in the product manufacturing step during the manufacturing of optical fibers in the past. The set temperature Tb of the heating furnace in the product manufacturing step, the temperature T0 of the heating furnace in the product manufacturing step during the manufacturing of the optical fiber in the past, the difference ΔS between the target linear tensile force of the optical fiber manufactured in the product manufacturing step during the manufacturing of the optical fiber in the past, and the correction coefficient a satisfy the relationship Tb = T0 + ΔT, where ΔT = aΔS. The first line speed increase step is performed in a section where the line drawing speed V during the line speed increase step and the predetermined line drawing speed V1 in the product manufacturing step satisfy the relationship V < V1 × 0.

6. The method for manufacturing an optical fiber, wherein the second linear velocity increase step is performed in an interval where the linear velocity increase step's drawing speed V and the predetermined linear velocity V1 in the product manufacturing step satisfy the relationship V1 × 0.6 ≤ V ≤ V1.

2. The method for manufacturing an optical fiber according to claim 1, wherein the temperature of the heating furnace in the product manufacturing step during the manufacture of the optical fiber in the past is the temperature of the heating furnace when the manufacture of the optical fiber to be used as the product was started in the product manufacturing step during the manufacture of the optical fiber in the past.

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