Method for producing optical fiber

Abstract

A method for producing an optical fiber, the method comprising: a drawing step in which an optical-fiber base material is heated and softened and is simultaneously drawn to thereby produce an optical fiber from a portion to be a product; and a drawing-end determination step in which an end of the optical-fiber drawing of the portion of the optical-fiber base material, which is to be a product, is determined on the basis of whether a given requirement is satisfied. In the drawing step, the linear velocity in the optical-fiber drawing is controlled so as to be a constant target value. In the drawing-end determination step, the end of the optical-fiber drawing is deemed to be at the time when the linear velocity has decreased from the target value just by a given amount.

Description

Optical fiber manufacturing method

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to methods for manufacturing optical fibers.

[0002] This application claims priority based on Japanese Application No. 2022-058765, filed on March 31, 2022, and incorporates by reference all of the contents of the aforementioned Japanese application.

[0003] Conventionally, optical fibers have been manufactured by drawing an optical fiber preform while heating and softening it. The optical fiber preform comprises an effective portion that can be used as a product and an ineffective portion at the end of the effective portion. Patent Document 1 below discloses a method for determining whether the optical fiber preform has reached the ineffective portion when manufacturing an optical fiber by drawing the optical fiber preform while heating and softening it, based on a sudden change in the drawing speed at which the optical fiber is drawn.

[0004] In Patent Document 1, when an optical fiber is being drawn while maintaining a constant drawing speed, the drawing speed decreases, and immediately thereafter the drawing speed suddenly increases in reaction. By detecting this sudden increase in drawing speed, it is determined that the optical fiber preform has reached the non-effective portion, and the drawing of the optical fiber is terminated.

[0005] Patent Publication No. 2010-13328

[0006] The method for manufacturing an optical fiber disclosed herein comprises a drawing process in which an optical fiber preform is heated and softened while being drawn to produce a portion of the optical fiber that will become a product, and a drawing completion determination process in which the completion of drawing of the optical fiber that will become the product in the optical fiber preform is determined based on whether predetermined conditions are met.In the drawing process, the drawing speed of the optical fiber is controlled with a target value of the drawing speed set to a constant, and in the drawing completion determination process, the completion of drawing of the optical fiber is determined when the drawing speed has decreased by a predetermined amount from the target value.

[0007] 1 is a schematic diagram of an optical fiber manufacturing apparatus capable of performing an optical fiber manufacturing method according to an embodiment of the present disclosure. FIG. 2 is a block diagram of a control device in the optical fiber manufacturing apparatus of FIG. 1. FIG. 3 is a schematic diagram of an optical fiber preform. FIG. 4 is a graph of the drawing speed of the optical fiber.

[0008] [Problem to be Solved by the Present Disclosure] Due to a difference in glass viscosity at the joint between the effective core glass and the non-effective core glass of the optical fiber preform, the melting state of the optical fiber preform changes, glass melting progresses rapidly, and the drawing speed of the optical fiber may increase suddenly. In the case where glass melting progresses rapidly in this way, the method described in Patent Document 1 determines that the optical fiber preform has reached the non-effective portion by the sudden increase in drawing speed, which may cause a problem when the drawing of the optical fiber is completed.

[0009] The object of the present disclosure is to provide a method for manufacturing an optical fiber that can quickly determine when an effective portion of an optical fiber preform has been switched to an ineffective portion, and can prevent the drawing speed from continuing to increase rapidly even when the drawing of the optical fiber is completed.

[0010] [Effects of the Present Disclosure] According to the present disclosure, it is possible to quickly determine when the effective portion of the optical fiber preform has been switched to the ineffective portion, and to prevent the drawing speed from continuing to increase rapidly even when the drawing of the optical fiber is completed.

[0011] [Description of Embodiments of the Present Disclosure] First, the details of the embodiments of the present disclosure will be listed and described. The present disclosure achieves the following effects through the configurations (1) to (5). (1) A method for manufacturing an optical fiber, comprising: a drawing process for manufacturing a portion of an optical fiber to be a product by drawing an optical fiber preform while heating and softening it; and a draw end determination process for determining whether the end of drawing of the optical fiber to be the product in the optical fiber preform is met based on whether a predetermined condition is satisfied. In the drawing process, the optical fiber drawing speed is controlled to a constant target value, and in the draw end determination process, the end of drawing of the optical fiber is determined when the drawing speed has decreased by a predetermined amount from the target value. This optical fiber manufacturing method configured in this manner can quickly determine the transition from the effective portion of the optical fiber preform to the ineffective portion, and can prevent the drawing speed from continuing to increase rapidly even at the end of drawing the optical fiber.

[0012] (2) In the above-described method for manufacturing an optical fiber, the optical fiber preform includes an effective portion having a core in the longitudinal center and an ineffective portion having no core in the longitudinal end, and the completion of drawing is determined by taking into account the remaining length of the effective portion of the optical fiber preform. This makes it possible to more reliably determine when the effective portion of the optical fiber preform has been switched to the ineffective portion, enabling optical fiber drawing at a stable drawing speed even at the end of drawing. Furthermore, the timing of the switch from the effective portion to the ineffective portion of the optical fiber preform can be more accurately determined without being affected by noise, etc., allowing the core glass in the effective portion of the optical fiber preform to be used up without waste.

[0013] (3) In the above-described optical fiber manufacturing method, the step of determining the end of drawing determines the conditions for determining the end of optical fiber drawing based on the amount of decrease in the drawing speed of a previously drawn optical fiber preform and the inspection results of an optical fiber obtained from the optical fiber preform. This allows the conditions for determining the end of drawing to be set based on the inspection results of the optical fiber drawn in the past, thereby enabling accurate determination of the end of drawing to be made in accordance with the specifications of the optical fiber manufacturing apparatus used, the specifications of the optical fiber, various manufacturing conditions, etc.

[0014] (4) In the above-described optical fiber manufacturing method, in the drawing step, the drawing speed is controlled to maintain a target value for the optical fiber drawing speed at a constant value by controlling at least one of the feed rate of the optical fiber preform, the output of a heating element in a heating furnace that heats the optical fiber preform, the speed of a capstan, and the speed of a take-up bobbin. This stabilizes the drawing speed of the optical fiber in the effective portion of the optical fiber preform to follow the constant target value, making it possible to reliably detect a decrease in the drawing speed at the end of drawing and more accurately determine the end of drawing.

[0015] (5) In the above-described optical fiber manufacturing method, the predetermined decrease in the drawing speed for determining the end of optical fiber drawing in the drawing end determination step is set between 0.5% and 3% from the target value. This makes it possible to specify a range for setting the predetermined decrease in the drawing speed for determining the end of optical fiber drawing relative to a certain target value of the drawing speed in the effective portion of the optical fiber preform, and to appropriately set the conditions for determining the end of drawing. Furthermore, by setting upper and lower limits for the predetermined decrease in the drawing speed for determining the end of optical fiber drawing relative to a certain target value of the drawing speed in the effective portion of the optical fiber preform, it is possible to prevent erroneous setting of the conditions for determining the end of drawing.

[0016] [Details of the embodiments of the present disclosure] Specific examples of the method for manufacturing an optical fiber according to the present disclosure will be described below. Note that in the following description, configurations with the same reference numerals in different drawings are considered to be similar, and their description may be omitted. Furthermore, the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0017] [Embodiment 1] An optical fiber manufacturing method according to embodiment 1 of the present disclosure will be described with reference to Figures 1-4. First, an optical fiber manufacturing apparatus 10 capable of performing the optical fiber manufacturing method of this embodiment will be described with reference to Figure 1. Figure 1 is a schematic diagram of the optical fiber manufacturing apparatus 10 capable of performing the optical fiber manufacturing method.

[0018] As shown in FIG. 1 , the optical fiber manufacturing apparatus 10 includes a vertical heating furnace 13 for heating the optical fiber preform G, a feeding device 11 for feeding the optical fiber preform G into the heating furnace 13, a preform feed detector 12 for measuring the feed amount of the optical fiber preform G, a cooling device 19 for cooling the glass fiber G1 after it has been drawn, an outer diameter measuring device 20 for measuring the outer diameter of the glass fiber G1, a die 21 for coating the glass fiber G1 with an ultraviolet-curable resin, an ultraviolet irradiation device 24 for curing the ultraviolet-curable resin applied to the optical fiber G2 coated by the die 21, a tension meter 31 for measuring the tension of the coated optical fiber G2, a capstan 40 for taking up the optical fiber G2 via guide rollers 30, a first take-up bobbin 36 and a second take-up bobbin 37 for taking up the optical fiber G2 taken up by the capstan 40 via dancer rollers 34, and a control device 50.

[0019] 2, the control device 50 controls the respective control devices 52 and 53 of the optical fiber manufacturing apparatus 10 based on detection signals from the respective detectors 51, thereby adjusting each manufacturing process of the optical fiber manufacturing apparatus 10 that manufactures the optical fiber G2 having target specifications and controlling the drawing speed of the optical fiber G2 to a target value. As will be described later, the control device 50 also includes a drawing end determination unit 71 that determines a predetermined drawing end condition for the optical fiber G2. The target specifications of the manufactured optical fiber G2 are set by a target specification setting unit (not shown) of the control device 50.

[0020] The heating furnace 13 includes a cylindrical furnace tube 16 into which the optical fiber preform G is supplied, and a heating element 15 for heating the furnace tube 16. The heating element 15 raises the temperature of the furnace tube 16, forming a heating space inside the furnace tube 16. The heating space is a space where the glass of the optical fiber preform G is softened to a temperature at which it can be drawn, and is adjusted to a predetermined value, for example, 1800°C or higher, by controlling the output of the heating element 15 in response to a control command from the control device 50, although this is not particularly limited. The heating furnace 13 also includes a gas supply unit 14 that controls the amount of purge gas, such as helium or nitrogen, supplied to the heating space in response to a control command from the control device 50.

[0021] The feeding device 11 grips the upper portion of the optical fiber preform G, i.e., the end portion on the Lc side in FIG. 3 , and controls the feed rate of the optical fiber preform G into the heating furnace 13 in response to a control command from the control device 50 so that the lower end portion of the optical fiber preform G, i.e., the end portion on the La side in FIG. 3 , is positioned in the heating space inside the furnace core tube 16. From the lower end of the heating furnace 13, heated and molten glass is drawn out as a thin glass fiber G1. The feeding device 11 is provided with a preform feed detector 12 for measuring the feed rate of the optical fiber preform G and the remaining amount of the optical fiber preform G. The heating furnace 13 is also provided with a temperature detector 17 for measuring the temperature of the heating region in the furnace core tube 16. The measured values ​​of the feed rate of the optical fiber preform G by the preform feed detector 12 and the measured value of the temperature of the heating region by the temperature detector 17 are sent to the control device 50. The remaining amount of the optical fiber preform G is calculated in the control device 50 based on the effective portion length Lb of the optical fiber preform G and the measured feed rate.

[0022] A cooling device 19 is provided downstream of the heating furnace 13, and the glass fiber G1 exiting the heating furnace 13 is cooled by this cooling device 19. The cooling device 19 has a main body composed of two members divided in the circumferential direction of the glass fiber G1. These two members can be separated radially to open the main body. Normally, the two members are engaged together and used as a single unit. The main body of the cooling device 19 has an insertion hole through which the glass fiber G1 is inserted in the axial direction. Cooling gas is introduced into this insertion hole to cool the glass fiber G1. A cooling flow path is formed inside the main body of the cooling device 19 along the axial direction, and a cooling fluid is circulated inside this cooling flow path. The cooling gas in the insertion hole is cooled by this cooling fluid, and the glass fiber G1 is inserted through this cooling gas atmosphere, whereby the drawn glass fiber G1 is rapidly cooled from a high temperature to near room temperature, thereby stabilizing the shape of the glass fiber G1. In this embodiment, helium gas, which has high thermal conductivity, is used as the cooling gas. Because helium has high thermal conductivity, it is suitable for use as a purge gas in the heating furnace 13 and as a cooling solvent in the cooling device 19. However, since helium is more expensive than nitrogen, nitrogen may be used instead of helium when cost is a priority. For example, helium gas can be used during normal operation, and after fiber drawing is complete, nitrogen gas can be supplied to the heating furnace 13 instead of helium gas.

[0023] An upper shutter capable of opening and closing the entrance of the insertion hole is provided at the upper end of the cooling device 19, and a lower shutter capable of opening and closing the exit of the insertion hole is provided at the lower end of the cooling device 19. The upper and lower shutters are closed during drawing to increase the cooling efficiency of the cooling device 6. When the upper and lower shutters are closed, a small diameter hole is formed in their center. This small diameter hole has a diameter slightly larger than the drawn glass fiber G1. The glass fiber G1 passes through the small diameter hole while maintaining a small clearance between the upper and lower shutters. A plurality of cooling devices 19 may be provided in series on the pass line.

[0024] Downstream of the cooling device 19, there is provided, for example and without limitation, a laser-type outer diameter measuring device 20. The outer diameter dimension of the glass fiber G1 that has exited the cooling device 19 is measured by the outer diameter measuring device 20. Although there is no particular limitation, the accuracy of measuring the outer diameter of the glass fiber G1 can be improved by measuring in each of the orthogonal axial directions on a plane perpendicular to the axis of the glass fiber G1. Furthermore, the outer diameter measuring device 20 may be provided at multiple locations on the pass line. The measured values ​​of the outer diameter of the glass fiber G1 measured by the outer diameter measuring device 20 are sent to the control device 50.

[0025] Downstream of the outer diameter measuring instrument 20, there are provided a die 21 for applying an ultraviolet-curable resin to the glass fiber G1, and an ultraviolet irradiator 24 for curing the applied ultraviolet-curable resin. The ultraviolet irradiator 24 is not particularly limited, but for example, it irradiates the resin-applied optical fiber G2 with ultraviolet light using multiple UV lamps to cure the ultraviolet-curable resin. The ultraviolet-curable resin is applied by the die 21 to the periphery of the glass fiber G1 whose shape has been stabilized after being cooled by the cooling device 19, and the applied resin is cured by an ultraviolet curing reaction by the ultraviolet irradiator 24 provided downstream of the die 21, thereby obtaining an optical fiber having a uniform ultraviolet-curable resin layer provided around the periphery.

[0026] A tray 23, which is normally located away from the pass line, is provided downstream of the die 21. When the glass fiber G1 or the optical fiber G2 breaks, the tray 23 is moved to a position directly below the die 21 so that it can receive the ultraviolet-curable resin if it overflows from the die 21. A cutter 22 is provided between the die 21 and the tray 23. The cutter 22 can cut the optical fiber G2 that has been drawn and coated with resin. The die 21, cutter 22, and tray 23 are controlled in accordance with control commands from a control device 50.

[0027] The optical fiber G2, on which the ultraviolet-curable resin coating layer has been formed, is drawn into the capstan 40 via the guide roller 30. A predetermined tension is applied to the optical fiber G2 by the capstan 40. The capstan 40 includes a capstan belt 42 wound around a plurality of rollers 41, and a capstan roller 32 that is in close contact with the capstan belt 42. The capstan 40 applies tension to the optical fiber G2 by sandwiching the optical fiber G2 between the capstan belt 42 and the capstan roller 32, and the optical fiber G2 is drawn downstream.

[0028] A tension meter 31 for measuring the tension of the optical fiber G2 is provided between the guide roller 30 and the capstan 40. The measured value of the tension of the optical fiber G2 measured by the tension meter 31 is sent to the control device 50. The rotation speed of the capstan 40 is controlled in accordance with a control command from the control device 50 so that the measurement value of the outer diameter measuring instrument 20 becomes a predetermined value.

[0029] The linear speed detector 25 is incorporated into the capstan 40, and the linear speed is measured from the rotation speed of the capstan 40. The linear speed detector 25 can also be incorporated into the guide roller 30, the screening device 33, and the dancer roller 34. The measured value of the linear speed of the optical fiber G2 measured by the linear speed detector 25 is sent to the control device 50.

[0030] A screening device 33 is provided downstream of the capstan 40 to perform a strength test on the optical fiber G2. The screening device 33 applies a predetermined tension to the optical fiber G2 to perform strength tests such as pulling and bending, and tests whether the optical fiber G2 satisfies the desired strength conditions for the target specifications. If the optical fiber G2 does not break in this test, it is a good product.

[0031] The non-defective optical fiber G2 is fed to the first take-up bobbin 36 and the second take-up bobbin 37 via the dancer roller 34. For example, the non-defective optical fiber G2 is wound around one of the first take-up bobbin 36 and the second take-up bobbin 37, and for example, the non-defective optical fiber G2 drawn at the end of the drawing operation is wound around the other. Although not particularly limited, the following description will be given assuming that the non-defective optical fiber G2 is wound around the first take-up bobbin 36 and the defective optical fiber G2 is wound around the second take-up bobbin 37. The dancer roller 34 is provided with a dancer roller measuring unit 35 that detects the displacement of the dancer roller 34. The measured value of the displacement of the dancer roller 34 detected by the dancer roller measuring unit 35 is sent to the control device 50. Furthermore, the first take-up bobbin 36 and the second take-up bobbin 37 control the winding of the optical fiber G2 in accordance with a control command from the control device 50.

[0032] [Configuration of the control device 50] Next, the configuration of the control device 50 will be described with reference to Fig. 2. Fig. 2 is a block diagram of the control device 50 in the optical fiber manufacturing apparatus 10 of Fig. 1. The control device 50 is composed of a detection unit 60, a wire speed control unit 70, and a coating layer formation / cutting control unit 80. The detection unit 60 includes detection means such as a preform feed detection unit 61, a core temperature detection unit 62, an outer diameter measurement unit 63, a wire speed detection unit 64, a tension detection unit 65, and a dancer roller displacement detection unit 67.

[0033] The detection unit 60 is sent with measurement values ​​measured by the detectors 51 of each part of the optical fiber manufacturing apparatus 10, and in each detecting means of the detection unit 60, the measurement values ​​measured by the detectors 51 of each part are tallied and calculated, and stored in a memory (not shown) so that they can be used for control calculations in the wire speed control unit 70 and the coating layer formation / cutting control unit 80. For example, the measurement value from the preform feed detector 12 is sent to the preform feed detection unit 61, the measurement value from the temperature detector 17 is sent to the core temperature detection unit 62, the measurement value from the outer diameter measuring instrument 20 is sent to the outer diameter measurement unit 63, the measurement value from the wire speed detector 25 is sent to the wire speed detection unit 64, the measurement value from the tension meter 31 is sent to the tension detection unit 65, and the measurement value from the dancer roller measurement unit 35 is sent to the dancer roller displacement detection unit 67.

[0034] The drawing speed control unit 70 includes control units such as a drawing end determination unit 71, a preform feed control unit 72, a gas supply amount control unit 73, a heating element control unit 74, a capstan control unit 76, and a take-up control unit 77. The drawing speed control unit 70 determines a target value for the drawing speed of the optical fiber G2, which is set according to the specifications and manufacturing conditions of the optical fiber G2. This target value is set as a constant value, as will be described later. The drawing speed control unit 70 then calculates control commands for controlling the respective control devices 52 of the optical fiber manufacturing apparatus 10 using the collected and calculated measured values ​​stored in the memory of the detection unit 60 so that the drawing speed of the optical fiber G2 follows the target value, and sends the control commands to the respective control devices 52.

[0035] The target specifications of the optical fiber G2 to be manufactured are set by a target specification setting unit (not shown) of the control device 50. The respective control devices 52 are controlled in accordance with control commands sent from the drawing speed control unit 70, thereby manufacturing the optical fiber G2 to meet the set target specifications through each manufacturing process of the optical fiber manufacturing apparatus 10. For example, the preform feed control unit 72 sends a control command to the feed device 11, the gas supply amount control unit 73 sends a control command to the gas supply unit 14, the heating element control unit 74 sends a control command to the heating element 15, the capstan control unit 76 sends a control command to the capstan 40, and the winding control unit 77 sends a control command to the first winding bobbin 36 and the second winding bobbin 37. Each control unit of the drawing speed control unit 70 sends a control command to the respective control devices 52 of the optical fiber manufacturing apparatus 10 based on the measurement values ​​of the respective detectors 51 detected by the detection unit 60, and controls the respective control devices to control each process of the optical fiber manufacturing apparatus 10 to manufacture the optical fiber G2.

[0036] More specifically, the measured value of the outer diameter of the optical fiber G1 measured by the diameter measuring instrument 20 is acquired by the outer diameter measuring unit 63, and based on this measured value of the outer diameter of the optical fiber G1, a command value for the rotation speed of the capstan 40 is calculated in the capstan control unit 76. This command value for the rotation speed of the capstan 40 is sent from the capstan control unit 76 to the capstan 40, and the rotation speed of the capstan 40 is controlled in accordance with the command value.

[0037] The drawing completion determination unit 71 determines whether or not to terminate the drawing of the optical fiber G2 according to a drawing completion condition described below, and when it determines that drawing is terminated, it sends a termination control signal for terminating the drawing of the optical fiber G2 to each of the control units, namely, the preform feed control unit 72, the gas supply amount control unit 73, the heating element control unit 74, the capstan control unit 76, the take-up control unit 77, and the coating layer formation / cutting control unit 80. Upon receiving the termination control signal from the drawing completion determination unit 71, each control unit calculates a control command for controlling each of the control devices 52, 53 to terminate the drawing of the optical fiber G2, and this control command is sent to each of the control devices 52, 53, thereby controlling each of the control devices 52, 53 to terminate the drawing of the optical fiber G2.

[0038] The coating layer formation / cutting control unit 80 includes a die control unit 81, a cutter control unit 82, a tray control unit 83, etc., and calculates command signals to the die, cutter, and tray 53 using a control signal from the drawing speed control unit 70 and the collected and calculated measurement values ​​stored in the memory of the detection unit 60. The die control unit 81 sends, for example, a command to the die 21 to change the amount of ultraviolet-curable resin applied to the outer periphery of the glass fiber G1, specifically a command to change the temperature of the die 21 or the supply pressure of the resin supplied to the die 21. The cutter control unit 82 sends, for example, a control command to the cutter 22 to cut the optical fiber G2 when drawing of the optical fiber G2 is completed. The tray control unit 83 sends, for example, a control command to the tray 23 when the cutter 22 cuts the optical fiber G2, and controls the tray 23 to move to a position below the die 21 when the optical fiber G2 is cut.

[0039] [Structure of Optical Fiber Preform] The structure of the optical fiber preform G will be described with reference to Figure 3. Figure 3 is a schematic diagram of the optical fiber preform G. When drawing the optical fiber preform G, at the start of drawing, an ineffective portion La of the optical fiber preform G is drawn from the drawing start point a = 0 to a1. Then, an effective portion Lb, which is the effective portion having a core C and becoming a product, is drawn over a length Lb from a1 to a2. After drawing of the effective portion Lb having the core C is completed, an ineffective portion Lc of the length Lc from a2 to a3, which will not become a product, is drawn. The viscosity of the portion Lb containing the core C may be high while the viscosity of the portion Lc without the core C may be low, or conversely, the viscosity of the portion Lb containing the core C may be low while the viscosity of the portion Lc without the core C may be high. This may result in a large difference in glass viscosity at the joint between the core glass of the effective portion Lb and the core glass of the ineffective portion Lc. When the difference in glass viscosity becomes large in this way, there is a risk that the drawing speed will increase sharply.

[0040] [Regarding Drawing Speed ​​Control] Drawing speed control will be described with reference to FIG. 4. FIG. 4 is a graph of the drawing speed of the optical fiber. The vertical axis of FIG. 4 represents the drawing speed V [m / min] of the optical fiber G2, and the horizontal axis represents the drawing distance L [km]. The drawing distance L corresponds to the length of the optical fiber G2 taken up by the capstan 40, and is a value that can be calculated, for example, from the number of rotations of the capstan 40 or by integrating the measured value of the drawing speed detector 25 over time. The solid line represents the graph when drawing stop control is performed by the drawing end determination unit 71 of this embodiment, and the two-dot chain line represents the graph for a comparative example in which drawing stop control is not performed by the drawing end determination unit 71 of this embodiment.

[0041] When the optical fiber preform G is drawn using the optical fiber manufacturing apparatus 10 of this embodiment, the non-effective portion La at the drawing start end of the optical fiber preform G is drawn from L=0 at the start of drawing to L1, and the drawing speed is gradually increased during this period. While Fig. 4 shows the drawing speed V increasing linearly, the speed increase characteristic is not limited to a linear increase. For example, the acceleration may initially gradually increase, then remain constant, and then gradually decrease near L1.

[0042] From L1 to L2, the drawing speed V is controlled so that the target value Vc is constant. By the time a1 of the optical fiber preform G is ready to be drawn, the drawing speed V stabilizes and remains constant at approximately the target value Vc. From L1 to L2, the effective portion Lb of the optical fiber preform G, which has the core C, is ready to be drawn. From L1 to L2, at least one of the feed rate of the optical fiber preform G, the output of the heating element 15 of the heating furnace 13 that heats the optical fiber preform G, the speed of the capstan 40, and the speed of the first take-up bobbin 36 or the second take-up bobbin 37 is controlled by each of the control units 72-77 of the drawing speed control unit 70 so that the drawing speed V coincides with the constant target value Vc. For example, the rotation speed of the capstan 40 is controlled in response to a control command from the control device 50 so that the measurement value of the outer diameter measuring instrument 20 becomes a predetermined value, and other control devices are stably controlled. The feed device 11 is also controlled to increase the feed rate when the drawing speed V is low and decrease the feed rate when the drawing speed V is high. The heating element 15 of the heating furnace 13 is also controlled to increase its output when the drawing speed V is low and decrease its output when the drawing speed V is high. However, since it is desirable to control the temperature of the heating furnace 13 as constant as possible, control is performed to prioritize temperature stability in order to maintain the drawing speed V constant. By stably controlling the control devices 52 in this manner, the drawing speed V of the optical fiber G2 is controlled to be constant at the target value Vc. Control by the drawing speed control unit 70 limits fluctuations in the drawing speed V of the optical fiber G2 to within a range of less than 0.5% of the target value Vc between L1 and L2.

[0043] 4 shows an example in which the target value Vc of the drawing velocity V is always constant between L1 and L2, but it is also possible to change the target value Vc of the drawing velocity V in accordance with, for example, a change in the manufacturing conditions of the manufacturing process of the optical fiber G2 during the process. If the target value Vc of the drawing velocity V is changed to, for example, Vc', after this change, the drawing velocity V is controlled to follow the target value Vc', and fluctuations in the drawing velocity V are suppressed to a range of less than 0.5% of the target value Vc'.

[0044] Between L2 and L3, as the drawing position of the optical fiber preform G approaches the terminal end of the effective portion Lb, the position at length a2, the target value Vc of the drawing speed V remains constant, but the drawing speed V is no longer able to follow the constant target value Vc, and the drawing speed V gradually decreases from L2 onwards. This is because as the remaining amount of the optical fiber preform G decreases, the amount of glass available for drawing decreases relatively. From L2 onwards, the amount of glass falls short of the amount of glass required to make the drawing speed V follow the target value Vc, and the drawing speed V decreases. While Figure 4 illustrates an example in which the drawing speed V decreases linearly from L2 to L3, this embodiment is not limited to this. The characteristics of the decrease in drawing speed V vary depending on the manufacturing conditions and the specifications of the optical fiber G2 being manufactured. For example, rather than changing in a curved manner at L2, the drawing speed V may gradually decrease in a smoother curved manner.

[0045] At L3, the drawing speed reaches the reference drawing speed Vs, which is the condition for determining the end of drawing as defined in this embodiment. In the graph of the comparative example shown by the two-dot chain line in Figure 4, the drawing speed V continues to decrease from L2 to L5, falling below the reference drawing speed Vs. At L5, the drawing speed V suddenly rises and falls. This is because a difference in glass viscosity occurs at the joint between the core glass of the effective portion Lb and the core glass of the non-effective portion Lc at the position a2 where the drawing position of the optical fiber preform G switches from the effective portion Lb of the core C to the non-effective portion Lc. Then, from L6 onwards, the glass viscosity decreases, causing a sudden rise in the drawing speed V.

[0046] In the optical fiber manufacturing method of this embodiment, the sudden rise and fall of the drawing speed V at L5 as in the comparative example and the sudden increase in the drawing speed V from L6 onwards are avoided, and stable drawing speed control is achieved even at the end of drawing.

[0047] In the optical fiber manufacturing method of this embodiment, as shown by the solid line graph in Figure 4, when the reference drawing speed Vs, which is the condition for determining the end of drawing specified in this embodiment, is reached at L3, the drawing end determination unit 71 determines that this is the end position of drawing and sends a control command for drawing stop processing to each control unit 72-77. The reference drawing speed Vs is set to a drawing speed that is 0.5% to 3% lower than the normal target value Vc, for example, 0.8% to 1.5%, or preferably 1% lower than Vc. This is because, due to control by the drawing speed control unit 70, fluctuations in the drawing speed V of the optical fiber G2 are usually limited to less than 0.5% of the target value Vc between L1 and L2. Depending on the specifications of the optical fiber to be manufactured, the manufacturing conditions, and the manufacturing equipment, it is expected that the fluctuation range of the drawing speed V may be slightly larger than 0.5%, for example, approximately 0.7%. On the other hand, when the drawing position is near the length a2 of the optical fiber preform G, the linear velocity V, which had been decreasing, may suddenly increase, so in order to achieve stable linear velocity control, the reference linear velocity Vs is set lower than the target value Vc by not more than 3% of Vc, for example, not more than 1.5%.

[0048] At L3, when the drawing speed V reaches the reference drawing speed Vs, the drawing end determination unit 71 determines that this is the drawing end position, and the drawing end determination unit 71 transmits a control command for drawing end processing to each of the control units 72-77 in a predetermined order. When each of the control units 72-77 receives a control command for drawing end processing from the drawing end determination unit 71, it sends the control command for drawing end processing to each of the control devices 52. The time from when the drawing end determination unit 71 determines that the drawing speed V has reached the reference drawing speed Vs, which is the determination condition for the drawing end position, to when it issues the control command for drawing end processing is determined appropriately depending on the specifications of the optical fiber to be manufactured, the manufacturing conditions, the manufacturing equipment, etc., and may be simultaneous or may be a certain time, for example, 30 to 60 seconds, after determining that the determination condition for the drawing end position has been satisfied.

[0049] In the drawing termination process, first, a control command from the winding control unit 77 switches winding from the first winding bobbin 36 for winding non-defective products to the second winding bobbin 37 for winding defective products. Next, a control command from the capstan control unit 76 gradually reduces the rotation speed of the capstan 40, and a control command from the die control unit 81 gradually reduces the thickness of the ultraviolet-curable resin coating on the optical fiber G1. Next, a control command from the gas supply amount control unit 73 causes the gas supply unit 14 to switch the purge gas from helium gas to nitrogen gas, and a control command from the heating element control unit 74 gradually reduces the output of the heating element 15, gradually decreasing the temperature inside the furnace core tube 16 of the heating furnace 13. In response to this, a control command from the preform feed control unit 72 adjusts the feed rate of the optical fiber preform G so that the drawing speed V gradually decreases.

[0050] When the die control unit 81 issues a control command to sufficiently reduce the supply amount of UV-curable resin so that uncured UV-curable resin does not spill from the die 21 and the drawing speed V of the optical fiber G2 falls below a predetermined drawing speed, the cutter control unit 82 issues a control command to cause the cutter 22 to cut the optical fiber G2. Next, the tray control unit 83 moves the tray 23 to a position where uncured UV-curable resin can be caught if it spills from the die 21 after cutting the optical fiber G2. When the cut optical fiber G2 is wound onto the second take-up bobbin 37 for taking up defective products, the take-up control unit 77 issues a control command to stop the second take-up bobbin 37, thereby completing the drawing completion process. As a result, the drawing speed V of the optical fiber G2 during the drawing completion process gradually decreases from Vs to 0 in a stably controlled manner between L3 and L4. In FIG. 4, the gradient of the linear velocity V from L3 to L4 is shown to be constant, but this is an example, and in reality, the linear velocity V does not need to decrease in a straight line. The linear velocity V may be changed to draw a smooth curve at the point of L3, or the deceleration of the linear velocity V may be gradually decreased in the graph.

[0051] The optical fiber manufacturing method of this embodiment makes it possible to quickly determine the transition from the effective portion Lb of the optical fiber preform G to the ineffective portion Lc, and to prevent the drawing speed V from continuing to increase rapidly even at the end of drawing the optical fiber. Furthermore, in the drawing process, the drawing speed V is controlled by controlling at least one of the feed rate of the optical fiber preform G, the output of the heating element 15 of the heating furnace 13 that heats the optical fiber preform G, the speed of the capstan 40, and the speed of the take-up bobbins 36, 37, so that the drawing speed V is kept constant at the target value Vc of the drawing speed V of the optical fiber on the take-up bobbin. As a result, the drawing speed V of the optical fiber in the effective portion Lb of the optical fiber preform G is stabilized to follow the constant target value Vc, and therefore the decrease in the drawing speed V at the end of drawing can be reliably detected, and the end of drawing can be more accurately determined. Furthermore, since the predetermined decrease in the drawing speed V for determining the end of optical fiber drawing in the draw end determination step is set between 0.5% and 3% from the target value Vc, it is possible to specify a range for setting the predetermined decrease in the drawing speed V for determining the end of optical fiber drawing relative to a constant target value Vc of the drawing speed V for the optical fiber drawing in the effective portion Lb of the optical fiber preform G, and to appropriately set the conditions for determining the end of drawing. Also, by setting upper and lower limits for the predetermined decrease in the drawing speed V for determining the end of optical fiber drawing relative to a constant target value Vc of the drawing speed V for the optical fiber drawing in the effective portion Lb of the optical fiber preform G, it is possible to prevent erroneous setting when setting the conditions for determining the end of drawing.

[0052] [Embodiment 2] A method for manufacturing an optical fiber according to embodiment 2 of the present disclosure will be described. Figures 1 to 4 will be referenced in common with embodiment 1. In embodiment 1, the condition for determining the end position of drawing by the drawing end determination unit 71 was explained as being that the drawing speed V of the optical fiber G2 has decreased to the reference drawing speed Vs. However, in this embodiment, an embodiment will be described in which an additional condition for determining the end position of drawing by the drawing end determination unit 71 is added.

[0053] In an actual manufacturing apparatus, various noises may be generated depending on the specifications of the optical fiber G2 to be manufactured, the manufacturing conditions, and the manufacturing apparatus. In order to prevent the end-of-drawing determination unit 71 from erroneously determining that the optical fiber G2 is at the end of drawing even when the drawing speed V of the optical fiber G2 temporarily drops below the reference drawing speed Vs due to such noise, in this embodiment, multiple determination criteria are provided, and the end-of-drawing determination unit 71 correctly determines that the optical fiber G2 is at the end of drawing when all of the multiple determination criteria are satisfied.

[0054] The first judgment condition is, as in the first embodiment, that the drawing velocity V of the optical fiber G2 has decreased to the reference drawing velocity Vs or less.

[0055] The second judgment condition is that the preform feed detector 12 detects that the remaining amount of the optical fiber preform G has fallen to Ls or less, where Ls = Lc + ΔL (ΔL ≥ 0) (Equation 1). In Equation 1, ΔL is a positive real number corresponding to the remaining margin of the effective portion Lb from a2, which is the position between the effective portion Lb where the core C is located and the non-effective portion Lc.

[0056] In this embodiment, when both the first and second determination conditions are satisfied, the drawing end determination unit 71 determines that the drawing end position has been reached. As a result, in this embodiment, even if the first determination condition is satisfied, that is, the drawing speed V of the optical fiber G2 has temporarily dropped to or below the reference drawing speed Vs due to noise or erroneous detection by the linear speed detector 25, the drawing end determination unit 71 will not determine that the drawing end position has been reached unless the second condition is satisfied, that is, the remaining amount of the optical fiber preform G has dropped to or below Ls. This makes it possible to prevent erroneous determination of the drawing end position due to noise, etc.

[0057] In this embodiment, a third determination condition can be added. The third determination condition is whether the winding length of the optical fiber G2 around the capstan 40, i.e., the drawing distance L, is greater than or equal to the reference length L Ｇ It is that we have reached the Ｇis calculated from the volume of glass in the optical fiber preform G from the tip a=0 of the optical fiber preform G to the end position a2 of the effective portion Lb having the core C, the outer diameter of the optical fiber G2, and a predetermined amount of excess preform glass.

[0058] When the first, second, and third judgment conditions are used, the drawing end judgment unit 71 judges that the drawing end position is reached when all of the first, second, and third judgment conditions are satisfied. The detection signal from the preform feed detector 12 may have a margin set to detect early that the remaining amount of the optical fiber preform G has become less than a predetermined amount. Even in such a case, the third judgment condition is added in this embodiment, so that even if the first judgment condition is met, in which the drawing speed V of the optical fiber G2 temporarily drops below the reference drawing speed Vs due to noise or erroneous detection by the linear speed detector 25, or even if the second judgment condition is met early due to the margin set in the detection signal from the preform feed detector 12, the drawing end judgment unit 71 judges that the drawing end position is reached when the drawing distance L of the optical fiber G2 has reached the reference length L. Ｇ If the third condition, that is, the position Lb of the optical fiber preform G has reached the effective portion Lc, is not satisfied, the draw end determination unit 71 does not determine that the draw end position has been reached, thereby preventing erroneous determination of the draw end position due to noise, a detection margin, or the like. In this embodiment, as in the first embodiment, the drawing speed V of the optical fiber G2 during the draw end process gradually decreases in a stably controlled manner from Vs to 0 between L3 and L4, as shown in Fig. 4. As a result, according to the optical fiber manufacturing method of this embodiment, it is possible to more reliably determine that the optical fiber preform G has switched from the effective portion Lb to the non-effective portion Lc. This makes it possible to draw the optical fiber at a stable drawing speed V even at the end of draw. In addition, it is possible to more accurately determine the time when the optical fiber preform G switches from the effective portion Lb to the non-effective portion Lc, regardless of the influence of noise, etc., and therefore it is possible to use up the effective portion core glass of the optical fiber preform G without waste.

[0059] [Embodiment 3] A method of manufacturing an optical fiber according to embodiment 3 of the present disclosure will be described. Figures 1 to 4 are commonly referred to in conjunction with embodiments 1 and 2. In embodiment 1, when the drawing speed V reaches the reference drawing speed Vs, the drawing end determination unit 71 determines that this is the drawing end position and transmits control commands for drawing end processing to the control units 72-77 in a predetermined order. However, in this embodiment, the reference drawing speed Vs is determined from historical data of past drawing of optical fiber preforms G.

[0060] When drawing an optical fiber G2 from an optical fiber preform G, the specifications, manufacturing conditions, and manufacturing equipment of the optical fiber G2 are used to determine the distance L3 at the boundary between the effective portion Lb having the core C of the optical fiber preform G and the ineffective portion Lc having no core C, which is the distance L3 that should be the judgment condition for the drawing distance L, and the value of the linear speed V of the optical fiber G2 that corresponds to the reference linear speed Vs that is the judgment standard when the drawing distance L is L3, and the history of this is stored in the memory (not shown) of the linear speed control unit 70.

[0061] The optical fiber G2 wound on the take-up bobbin is subjected to a subsequent inspection process in which its transmission characteristics, such as transmission loss, chromatic dispersion, cutoff wavelength, mode field diameter, and polarization mode dispersion, are measured and the optical fiber G2 is classified as either a good product that satisfies the required specifications or a defective product that does not. This determination data for good product or defective product is stored in the memory (not shown) of the detector 60 together with data on the drawing distance L.

[0062] The reference drawing speed Vs corresponding to the L3, which is the draw end determination condition, when drawing the optical fiber G2 from the optical fiber preform G in the current drawing of the optical fiber G2 having the target specifications from the optical fiber preform G is calculated by statistical processing based on the drawing distance L3 and the corresponding reference drawing speed Vs, which are the determination criteria, stored in the memory of the detection unit 60. In this statistical processing, it is desirable to consider various conditions, such as the specifications of the optical fiber preform G and the optical fiber G2, the manufacturing conditions, and the manufacturing equipment. While not particularly limited, for example, the reference drawing speed Vs corresponding to the L3, which is the draw end determination condition, is calculated by averaging only past historical information under the same or similar specifications and manufacturing conditions of the optical fiber preform G and the optical fiber G2. For example, the reference drawing speed Vs is calculated from the amount of reduction in drawing speed corresponding to the drawing distance at the boundary between the effective portion Lb and the non-effective portion Lc of the optical fiber preform G. Alternatively, the reference drawing speed Vs is calculated from the amount of reduction in drawing speed corresponding to the drawing distance at the boundary between the good and bad optical fiber G2. Therefore, according to the optical fiber manufacturing method of this embodiment, by using the reference drawing speed Vs, which is calculated from the past history and serves as a condition for the drawing end determination unit 71 when drawing an optical fiber G2 with target specifications from the current optical fiber preform G, it is possible to accurately determine the end of drawing in accordance with the specifications of the optical fiber manufacturing apparatus 10 being used, the specifications of the optical fiber, various manufacturing conditions, etc. This makes it possible to more reliably, more accurately, and more stably determine the end of drawing.

[0063] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above. Furthermore, the elements of the above-described embodiments can be combined as far as technically possible, and such combinations are also included within the scope of the present disclosure as long as they include the features of the present disclosure.

[0064] REFERENCE SIGNS LIST 10: Optical fiber manufacturing apparatus 11: Feeding device 12: Preform feed detector 13: Heating furnace 14: Gas supply unit 15: Heating element 16: Furnace tube 17: Temperature detector 19: Cooling device 20: Outer diameter measuring device 21: Die 22: Cutter 23: Receptacle 24: Ultraviolet irradiation device 25: Line speed detector 30: Guide roller 31: Tension meter 32: Capstan roller 33: Screening device 34: Dancer roller 35: Dancer roller measuring unit 36: First winding bobbin 37: Second winding bobbin 40: Capstan 41: Roller 42: Capstan belt 50: Control device 51: Detectors for each part 52: Control equipment for each part 53: Die, cutter, receptacle 60: Detection unit 61: Preform feed detection unit 62: Core temperature detection unit 63: Outer diameter measurement unit 64... Wire speed detection unit 65... Tension detection unit 66... ​​Optical fiber strength detection unit 67... Dancer roller displacement detection unit 70... Detection unit 71... Drawing end determination unit 72... Base material feed control unit 73... Gas supply amount control unit 74... Heating element control unit 76... Capstan control unit 77... Winding control unit 80... Coating layer formation / cutting control unit 81... Die control unit 82... Cutter control unit 83... Receptacle control unit C... Core G... Optical fiber base material G1... Glass fiber G2... Optical fiber V... Optical fiber drawing speed Vc... Target value of drawing speed Vs... Reference value of drawing speed La... Non-effective portion Lb... Effective portion Lc... Non-effective portion L... Drawing distance

Claims

1. a drawing process in which the optical fiber preform is heated and softened while being drawn to produce an optical fiber that will become a product; a drawing completion determination step of determining whether or not a predetermined condition is satisfied to end drawing of the optical fiber to be the product of the optical fiber preform; Equipped with In the drawing process, the drawing speed is controlled to a constant target value for the optical fiber drawing speed; In the step of determining completion of drawing, the end of drawing of the optical fiber is determined when the drawing speed is reduced by a predetermined amount from the target value.

2. the optical fiber preform includes an effective portion that is a central portion in a longitudinal direction and has a core, and a non-effective portion that is an end portion in a longitudinal direction and has no core, 2. The method for producing an optical fiber according to claim 1, wherein the step of determining completion of drawing further determines completion of drawing of the optical fiber by taking into consideration a remaining length of the effective portion of the optical fiber preform.

3. 3. The optical fiber manufacturing method according to claim 1, wherein in the drawing completion determination step, a condition for determining the completion of drawing of the optical fiber is determined based on an amount of decrease in drawing speed of an optical fiber preform previously drawn and an inspection result of an optical fiber obtained from the optical fiber preform.

4. 3. The method for manufacturing an optical fiber according to claim 1, wherein in the drawing process, the drawing speed is controlled to keep a target value of the drawing speed of the optical fiber on the take-up bobbin constant by controlling at least one of the feed rate of the optical fiber preform, the output of a heating element of a heating furnace that heats the optical fiber preform, the speed of a capstan, or the speed of a take-up bobbin.

5. 3. The method for producing an optical fiber according to claim 1, wherein a predetermined reduction amount of the drawing speed for determining completion of drawing of the optical fiber in the drawing completion determination step is set to a value between 0.5% and 3% from the target value.