Production method for optical fiber preform

Abstract

According to the present invention, a production method for an optical fiber preform is characterized in that, at a transparent vitrification step in which a transparent vitrification treatment is performed on a fine glass particle deposition body produced by blowing fine glass particles at a starting material that is rotating around the center axis thereof by suspending the fine glass particle deposition body inside a furnace tube provided with an external local heating heater and heating the fine glass particle deposition body under a helium-containing atmosphere while moving the fine glass particle deposition body relative to the heater, at least one of a pull-down speed alteration point and a helium gas flow rate alteration point is set within the movement range of the fine glass particle deposition body, when the pull-down speed alteration point has been set, the pull-down speed of the fine glass particle deposition body before the pull-down speed alteration point being greater than the pull-down speed of the fine glass particle deposition body after the pull-down speed alteration point, and when the helium gas flow rate alteration point has been set, the helium gas flow rate before the helium gas flow rate alteration point being greater than the helium gas flow rate after the helium gas flow rate alteration point.

Description

Method for manufacturing an optical fiber preform

[0001] The present invention relates to a method for manufacturing an optical fiber preform that is drawn into an optical fiber.

[0002] As a method for manufacturing an optical fiber preform, there is a method in which a glass microparticle deposit (soot) manufactured by a vapor phase synthesis method such as a vapor axial deposition method (VAD method) or an outside vapor deposition method (OVD method) is heated to be vitrified into transparent glass. An optical fiber is manufactured by heating and drawing the thus manufactured preform for an optical fiber to reduce its diameter. General disclosure

[0003] As a means for reducing the usage amount of helium gas, a method of increasing the soot lowering speed in the vitrification process higher than before can be considered. By increasing the soot lowering speed, the process time can be shortened and the usage amount of helium gas can be reduced. However, in the study by the present inventors, it was found that although the transmission loss in the optical fiber as the final product does not deteriorate, the tendency of the remaining unmelted defect of the preform to increase is observed.

[0004] As another means for reducing the usage amount of helium gas, a method of reducing the helium gas flow rate supplied in the vitrification process lower than before can be considered. However, in the study by the present inventors, it was found that although an increase in the remaining unmelted defect is not observed even when the helium gas flow rate is reduced, the transmission loss in the optical fiber as the final product tends to deteriorate.

[0005] Therefore, in order to solve the above problems, the present inventors examined the lowering speed pattern and the helium gas flow rate pattern in the vitrification process. The increase in the remaining unmelted defect caused by the increase in the lowering speed started from the upper part of the optical fiber preform after sintering, that is, the end stage of the process. Therefore, it was found that if the lowering speed during the treatment of the upper part of the optical fiber preform is returned to the same speed as before, even if the lowering speed in other ranges is increased more than before, the deterioration of the remaining unmelted defect does not occur.

[0006] Furthermore, when the helium gas flow rate was reduced during the glassmaking process, it was found that the chlorine concentration inside the reactor tube during the glassmaking process was higher than what would be expected considering the reduction in helium gas flow rate. This was thought to be because the removal of chlorine gas supplied during the dewatering process also occurred during the glassmaking process, and a certain level of helium gas flow rate was necessary to completely remove residual impurities in the soot. However, it was also found that the chlorine concentration inside the reactor tube decreased towards the end of the glassmaking process. In other words, it was determined that once the removal of residual impurities in the soot was complete, reducing the supplied helium gas flow rate did not worsen the transmission loss of the final optical fiber. By organizing and carefully considering these findings, the above problems were solved, and the present invention was achieved.

[0007] In the first embodiment, a glass microparticle deposit manufactured by blowing glass microparticles onto a starting material that rotates with its own central axis as the axis of rotation is suspended inside a furnace tube equipped with an external local heating heater, and heated in a helium-containing atmosphere while changing its relative position to the heater to perform a transparent vitrification process, at least one of a withdrawal speed change point and a helium gas flow rate change point may be provided within the movement range of the glass microparticle deposit. If a withdrawal speed change point is provided, the withdrawal speed of the glass microparticle deposit before the withdrawal speed change point may be greater than the withdrawal speed of the glass microparticle deposit after the withdrawal speed change point. If a helium gas flow rate change point is provided, the helium gas flow rate before the helium gas flow rate change point may be greater than the helium gas flow rate after the helium gas flow rate change point.

[0008] In the above configuration, at the point where the withdrawal speed is changed in the transparent glassmaking process, the speed change when reducing the withdrawal speed may be discontinuous. At the point where the withdrawal speed is changed in the transparent glassmaking process, the speed change when reducing the withdrawal speed may be continuous.

[0009] In the above configuration, the change in helium gas flow rate when reducing the helium gas flow rate at the point where the helium gas flow rate is changed in the transparent vitrification process may be discontinuous. Alternatively, the change in helium gas flow rate when reducing the helium gas flow rate at the point where the helium gas flow rate is changed in the transparent vitrification process may be continuous.

[0010] In the above configuration, in the transparent glassification process, a point where the helium gas flow rate is changed may be provided such that the descent distance after the change is longer than the descent distance before the change in helium gas flow rate.

[0011] In the above configuration, both a point for changing the descent speed and a point for changing the helium gas flow rate may be provided.

[0012] In the above configuration, at the point where the withdrawal speed is changed in the transparent vitrification process, the speed change when reducing the withdrawal speed of the glass microparticle deposit may be performed discontinuously, and at the point where the helium gas flow rate is changed in the transparent vitrification process, the flow rate change when reducing the helium gas flow rate may also be performed discontinuously.

[0013] In the above configuration, at the point where the withdrawal speed is changed in the transparent vitrification process, the speed change when reducing the withdrawal speed of the glass microparticle deposit may be performed discontinuously, and at the point where the helium gas flow rate is changed in the transparent vitrification process, the flow rate change when reducing the helium gas flow rate may be performed continuously.

[0014] In the above configuration, at the point where the withdrawal speed of the glass microparticle deposit is changed in the transparent vitrification process, the speed change when the withdrawal speed of the glass microparticle deposit is reduced is continuous, and at the point where the helium gas flow rate is changed in the transparent vitrification process, the flow rate change when the helium gas flow rate is reduced may be non-uniform and discontinuous.

[0015] In the above configuration, at the point where the withdrawal speed of the glass microparticle deposit is changed in the transparent vitrification process, the speed change when the withdrawal speed of the glass microparticle deposit is reduced is performed continuously, and at the point where the helium gas flow rate is changed in the transparent vitrification process, the flow rate change when the helium gas flow rate is reduced is also performed continuously.

[0016] In the above configuration, in the transparent glassmaking process, points for changing the withdrawal speed and points for changing the helium gas flow rate may be provided such that the reduction in helium gas flow rate starts before or simultaneously with the change in speed. In the transparent glassmaking process, points for changing the helium gas flow rate may be provided such that the withdrawal distance after the change is longer than the withdrawal distance before the change in helium gas flow rate.

[0017] This is a schematic longitudinal cross-sectional view showing an example of a sintering apparatus used to carry out the manufacturing method.

[0018] The following embodiments are not intended to limit the claims of the invention. Not all combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0019] In the transparent glassification process of the optical fiber preform of this embodiment, a withdrawal speed change point is provided in the movement range of the glass microparticle deposit, and the withdrawal speed of the glass microparticle deposit before the withdrawal speed change point is greater than the withdrawal speed of the glass microparticle deposit after the withdrawal speed change point. The second embodiment is characterized in that a helium gas flow rate change point is provided in the movement range of the glass microparticle deposit, and the helium gas flow rate before the helium gas flow rate change point is greater than the helium gas flow rate after the helium gas flow rate change point. The third embodiment is characterized in that a withdrawal speed change point and a helium gas flow rate change point are provided in the movement range of the glass microparticle deposit, and the withdrawal speed of the glass microparticle deposit before the withdrawal speed change point is greater than the withdrawal speed of the glass microparticle deposit after the withdrawal speed change point, and the helium gas flow rate before the helium gas flow rate change point is greater than the helium gas flow rate after the helium gas flow rate change point.

[0020] In this embodiment, when the speed is changed at a point where the speed is reduced, a discontinuous change in speed means that the speed changes rapidly from the speed before the change point to a preset speed, while a continuous change in speed when reducing the speed means that the speed changes continuously from the speed before the change point to a predetermined set speed. For example, discontinuity includes cases where the change is continuous for a short period of time, such as a delay in the response to the control, ranging from a few seconds to about one minute.

[0021] Furthermore, regarding the point at which the helium gas flow rate is changed, discontinuous flow rate reduction means that the flow rate is rapidly reduced from the flow rate before the change to a preset flow rate, while continuous flow rate reduction means that the flow rate is continuously reduced from the flow rate before the change to a preset flow rate.

[0022] The embodiments will be described in detail below with reference to the drawings, through examples and comparative examples, but the present invention is not limited to these.

[0023] First, soot consisting of glass microparticles deposited using a glass microparticle deposition burner is manufactured by the VAD method or similar. The synthesized soot is heated in a zone heating furnace, dehydrated, and converted into transparent glass to serve as the optical fiber base material. Figure 1 is an example of a schematic longitudinal cross-sectional view of a sintering apparatus for dehydrating and converting soot into transparent glass. In Figure 1, soot 14 connected to a shaft 12 is suspended inside the furnace tube 16 of the sintering apparatus 10. The shaft 12 penetrates the top cover 18, and a motor (not shown) is connected to the shaft 12 to allow for vertical movement and rotation of the shaft 12 and the soot 14.

[0024] A heating furnace body 20 for heating the soot is positioned around the outer circumference of a portion of the core tube 16, and a cylindrical heating heater 22 and an insulating material 24 covering it are installed inside the heating furnace body. The temperature inside the furnace is monitored by a temperature sensor, and a temperature control device 26 controls the furnace temperature by adjusting the output of the heating heater. A sintering gas introduction pipe 28 is connected to the bottom of the core tube 16, and sintering gas necessary for dehydration and transparent vitrification is supplied. The supplied sintering gas flows upward and is exhausted through an exhaust pipe 30 provided on the top cover, regulating the pressure inside the core tube.

[0025] Using the manufacturing apparatus shown in Figure 1, Examples 1 to 4 and Comparative Examples 1 and 2 were carried out under various conditions shown in Table 1, and the length of the unmelted glass base material obtained by vitrifying a 1650 mm length of soot was investigated.

[0026] First, the soot 14 manufactured by the VAD method was suspended inside the furnace tube 16. A mixed gas of chlorine gas for dehydration and inert argon gas (chlorine gas concentration 3 mol%) was supplied as the sintering gas to the furnace tube 16 from the sintering gas introduction pipe 28 at a flow rate of 26 L / min. The heater temperature was set to approximately 1250°C to 1350°C, and the soot 14 was heated while being lowered to perform the dehydration process.

[0027] Next, the withdrawal speed before and after changing the withdrawal speed in the transparent vitrification process, and the withdrawal distance of the soot 14 before and after the change were set as shown in Table 1, and the transparent vitrification process was carried out by heating while withdrawing the soot 14. The helium gas in the transparent vitrification process was used as the sintering gas and supplied from the sintering gas introduction pipe 28 at a flow rate of 21.0 L / min, and the total withdrawal distance of the soot was set to 1800 mm in all cases.

[0028] In Example 1, the soot 14 was lowered 1000 mm at a heater temperature of 1630°C and a lowering speed of 8.0 mm / min, and then the soot 14 was lowered 800 mm at a heater temperature of 1600°C and a lowering speed of 6.0 mm / min to obtain a glass base material. The lowering speed was changed discontinuously. The remaining length of this glass base material was 65 mm, and there was no unmelted material in the effective part of the product. The total amount of helium gas used was 5425 L.

[0029] Compared to the case where the entire 1800 mm of the optical fiber was glassed at a pulling speed of 6.0 mm / min (see Comparative Example 2 below), the amount of helium gas used was reduced by 14%. Furthermore, there were no problems with the transmission loss of the final optical fiber product. If the change in pulling speed is a discontinuous change that quickly changes to a preset speed, there is a higher possibility of striations occurring at that point, but it has the advantage of being simpler in terms of device control. If the change in pulling speed is a continuous change that is carried out over about 5 minutes, the control program of the device becomes more complex, but it has the advantage of suppressing the occurrence of defects at that point.

[0030] In Example 2, the soot 14 was lowered 800 mm at a heater temperature of 1630°C and a lowering speed of 8.0 mm / min, and then lowered 1000 mm at a heater temperature of 1600°C and a lowering speed of 6.0 mm / min to obtain the glass base material. The lowering speed was continuously changed over a period of 5 minutes, starting 20 mm before the lowering position of 800 mm. The length of the unmelted glass base material was 65 mm, and there was no unmelted material in the effective part of the product. The total amount of helium gas used was 5600 L.

[0031] Compared to a case where the entire 1800 mm of fiber was drawn using transparent glass at a speed of 6.0 mm / min, this method reduces helium gas usage by 11%. Furthermore, there were no problems with the transmission loss of the final optical fiber product.

[0032] In Example 3, the soot 14 was lowered 1200 mm at a heater temperature of 1615°C and a lowering speed of 7.0 mm / min, and then the soot 14 was lowered 600 m at a heater temperature of 1600°C and a lowering speed of 6.0 mm / min to obtain a glass base material. The lowering speed was changed discontinuously. The length of the unmelted glass base material was 60 mm, and there was no unmelted material in the effective part of the product. The total amount of helium gas used was 5700 L.

[0033] Compared to a scenario where the entire 1800 mm of fiber optic cable was pulled through transparent glass at a speed of 6.0 m / min, this method resulted in a 10% reduction in helium gas usage. Furthermore, there were no issues with transmission loss in the final optical fiber product.

[0034] In Example 4, the soot 14 was lowered 1000 mm at a heater temperature of 1615°C and a lowering speed of 7.0 mm / min, and then lowered 800 mm at a heater temperature of 1600°C and a lowering speed of 6.0 mm / min to obtain the glass base material. The lowering speed was continuously changed over 5 minutes starting 17.5 mm before the lowering position of 1000 mm. The length of the unmelted glass base material was 60 mm, and there was no unmelted material in the effective part of the product. The total amount of helium gas used was 5800 L.

[0035] Compared to a case where the entire 1800 mm of fiber was drawn using transparent glass at a speed of 6.0 mm / min, this method reduces helium gas usage by 8%. Furthermore, there were no problems with the transmission loss of the final optical fiber product.

[0036] In Comparative Example 1, a total descent distance of 1800 mm was achieved by lowering the material at a heater temperature of 1630°C and a descent speed of 8.0 mm / min to obtain a glass base material. The total amount of helium gas used was 4725 L.

[0037] Compared to a case where the entire 1800 mm of fiber was glassed at a drawing speed of 6.0 mm / min, this method resulted in a 25% reduction in helium gas usage. However, the remaining length of the glass base material was 150 mm, indicating that some unmelted material was present in the effective portion of the product. There were no issues with transmission loss in the optical fiber drawn outside the unmelted portion.

[0038] In Comparative Example 2, a total drawing distance of 1800 mm was lowered at a heater temperature of 1600°C and a drawing speed of 6.0 mm / min to obtain a glass base material. The length of unmelted material in this glass base material was 60 mm, and there was no unmelted material in the effective part of the product. There were also no problems with the transmission loss of the final optical fiber. However, the total amount of helium gas used was 6500 L, which was 16% more than in Example 1, resulting in increased costs.

[0039] Next, using the manufacturing apparatus shown in Figure 1, Examples 5 to 8 and Comparative Examples 2 and 3 were carried out under various conditions shown in Table 2, and the transmission loss of optical fibers obtained from optical fiber preforms obtained by glassing a 1650 mm long suit 14 was investigated.

[0040] First, the suit 14 manufactured by the VAD method was suspended in the core tube 16. While supplying a mixed gas of chlorine gas for dehydration and argon gas which is inert (chlorine gas concentration: 3 mol%) into the core tube 16 from a gas introduction tube at a flow rate of 26 L / min as a sintering gas, the heater temperature was set to about 1250°C to 1350°C and the suit 14 was heated while being lowered to perform a dehydration treatment.

[0041] Next, the helium gas flow rate before and after the change and the lowering distance of the suit 14 before and after the change as the sintering gas in the transparent vitrification process were set as shown in Table 2, and the transparent vitrification treatment was performed by heating while lowering the suit 14. The heater temperature in the transparent vitrification process was 1600°C, the lowering speed was 6.0 mm / min, and the total lowering distance of the suit 14 was all 1800 mm.

[0042] In Example 5, after lowering the suit 14 by 400 mm at a helium gas flow rate of 21.0 L / min, the helium gas flow rate was reduced to 14.7 L / min and the suit 14 was lowered by 1400 mm to obtain a glass base material. The helium gas flow rate was changed discontinuously. The total amount of helium gas used was 4830 L, and the transmission loss at 1383 nm of the optical fiber obtained by wire drawing was 0.288 dB / km.

[0043] Compared with the case where the entire total lowering distance of 1800 mm was transparently vitrified at a helium gas flow rate of 21.0 L / min, the amount of helium gas used was reduced by 23%. Also, there was no problem with the transmission loss of the optical fiber which is the final product. When the change in the helium gas flow rate is a discontinuous change that quickly changes to a preset flow rate, pressure fluctuations occur in the core tube at the corresponding location, but there is an advantage that the control of the device becomes simpler. When the change in the helium gas flow rate is a continuous change carried out over about 5 minutes, although the control program of the device becomes complicated, there is an advantage that fluctuations in the core tube pressure can be suppressed.

[0044] In Example 6, after lowering the soot 14 by 800 mm at a helium gas flow rate of 21.0 L / min, the helium gas flow rate was reduced to 10.5 L / min and the soot 14 was lowered by 1000 mm to obtain a glass base material. The helium gas flow rate was continuously changed over 5 minutes starting 15 mm before the draw distance of 800 mm. The total amount of helium gas used was 4550 L, and the transmission loss at 1383 nm of the drawn optical fiber was 0.279 dB / km.

[0045] Compared with the case where the total draw distance of 1800 mm was all vitrified into transparent glass at a helium gas flow rate of 21.0 L / min, the helium gas usage was reduced by 28%. Also, there was no problem with the transmission loss of the final product, the optical fiber.

[0046] In Example 7, after lowering the soot 14 by 600 mm at a helium gas flow rate of 21.0 L / min, the helium gas flow rate was reduced to 10.5 L / min and the soot 14 was lowered by 1200 mm to obtain a glass base material. The helium gas flow rate was changed discontinuously. The total amount of helium gas used was 4200 L, and the transmission loss at 1383 nm of the drawn optical fiber was 0.284 dB / km.

[0047] Compared with the case where the total draw distance of 1800 mm was all vitrified into transparent glass at a helium gas flow rate of 21.0 L / min, the helium gas usage was reduced by 33%. Also, there was no problem with the transmission loss of the final product, the optical fiber.

[0048] In Example 8, after lowering the soot 14 by 800 mm at a helium gas flow rate of 21.0 L / min, the helium gas flow rate was reduced to 6.3 L / min and the soot 14 was lowered by 1000 mm to obtain a glass base material. The helium gas flow rate was continuously changed over 5 minutes starting 15 mm before the draw distance of 800 mm. The total amount of helium gas used was 3850 L, and the transmission loss at 1383 nm of the drawn optical fiber was 0.281 dB / km.

[0049] Compared to a case where the entire 1800 mm of the optical fiber was glassed with helium gas at a flow rate of 21.0 L / min, this method reduces helium gas usage by 39%. Furthermore, there were no problems with the transmission loss of the final optical fiber product.

[0050] In Comparative Example 2, a glass base material was obtained by lowering a total length of 1800 mm with a helium gas flow rate of 21.0 L / min. The total amount of helium gas used was 6300 L, and the transmission loss at 1383 nm of the optical fiber obtained by drawing was 0.282 dB / km.

[0051] There were no problems with the transmission loss of the final optical fiber product. However, the total amount of helium gas used was 23% more compared to Example 1, resulting in increased costs.

[0052] In Comparative Example 3, a glass base material was obtained by lowering a total length of 1800 mm with a helium gas flow rate of 6.3 L / min. The total amount of helium gas used was 1890 L, and the transmission loss at 1383 nm of the optical fiber obtained by drawing was 0.295 dB / km.

[0053] Compared to a case where the entire 1800 mm of the optical fiber was glassed with helium gas at a flow rate of 21.0 L / min, this method reduces helium gas usage by 70%. However, the transmission loss of the final optical fiber increased.

[0054] Using the manufacturing apparatus shown in Figure 1, Examples 9 to 12 were carried out under various conditions shown in Table 3, and the length of the unmelted portion of the glass base material obtained by glassing a 1650 mm long soot 14 into transparent glass was investigated.

[0055] First, soot 14, manufactured by the VAD method, was suspended inside the furnace tube. A mixed gas of chlorine gas for dehydration and inert argon gas (chlorine gas concentration 3 mol%) was supplied as a sintering gas into the furnace tube 16 from the sintering gas introduction pipe 28 at a flow rate of 26 L / min. The heater temperature was set to approximately 1250°C to 1350°C, and the soot 14 was heated while being lowered to perform the dehydration process. The lowering speed before and after the change in the lowering speed for the transparent vitrification process, and the lowering distance of soot 14 before and after the change were set as shown in Table 3, and the transparent vitrification process was performed by heating while lowering the soot 14. The total lowering distance of soot 14 was 1800 mm in all cases.

[0056] In Example 9, the soot was lowered by 600 mm with a heater temperature of 1630°C, a lowering speed of 8.0 mm / min, and a helium gas flow rate of 21.0 L / min. Then, the soot 14 was lowered by another 200 mm with a heater temperature of 1630°C, a lowering speed of 8.0 mm / min, and a helium gas flow rate of 10.5 L / min. After that, the soot 14 was lowered by 1000 mm with a heater temperature of 1600°C, a lowering speed of 6.0 mm / min, and a helium gas flow rate of 10.5 L / min to obtain the glass base material. The changes in helium gas flow rate and lowering speed were each carried out continuously over a period of 5 minutes. Specifically, the helium gas flow rate was continuously varied from 21.0 L / min starting 20 mm before the 600 mm descent distance until it reached 10.5 L / min after 5 minutes, and the descent speed was continuously varied from 8.0 mm / min starting 20 mm before the 800 mm descent distance until it reached 6.0 mm / min after 5 minutes. The total amount of helium gas used was 3587 L, and the transmission loss at 1383 nm of the resulting optical fiber was 0.284 dB / km.

[0057] Compared to a case where the entire 1800 mm of fiber was glassed using a 6.0 mm / min retrieval speed and a helium gas flow rate of 21.0 L / min, this method resulted in a 43% reduction in helium gas usage. Furthermore, there were no problems with the transmission loss of the final optical fiber product.

[0058] In Example 10, the soot 14 was lowered by 800 mm with a heater temperature of 1615°C, a lowering speed of 7.0 mm / min, and a helium gas flow rate of 21.0 L / min. Then, the soot 14 was lowered by another 200 mm with a heater temperature of 1615°C, a lowering speed of 7.0 mm / min, and a helium gas flow rate of 10.5 L / min. After that, the soot 14 was lowered by another 800 mm with a heater temperature of 1600°C, a lowering speed of 6.0 mm / min, and a helium gas flow rate of 10.5 L / min to obtain the glass base material. The changes in helium gas flow rate and lowering speed were performed discontinuously. The total amount of helium gas used was 4100 L, and the transmission loss at 1383 nm of the optical fiber obtained by drawing was 0.277 dB / km.

[0059] Compared to a case where the entire 1800 mm of fiber was glassed using a 6.0 mm / min retrieval speed and a helium gas flow rate of 21.0 L / min, this method resulted in a 35% reduction in helium gas usage. Furthermore, there were no problems with the transmission loss of the final optical fiber product.

[0060] In Example 11, the soot 14 was lowered 800 mm with a heater temperature of 1615°C, a lowering speed of 7.0 mm / min, and a helium gas flow rate of 21.0 L / min. Then, the soot was lowered 1000 mm with a heater temperature of 1600°C, a lowering speed of 6.0 mm / min, and a helium gas flow rate of 10.5 L / min to obtain the glass base material. The changes in helium gas flow rate and lowering speed were each carried out continuously over a period of 5 minutes. Specifically, the helium gas flow rate was continuously changed from 21.0 L / min and the lowering speed from 7.0 mm / min starting 17.5 mm before the 800 mm lowering distance, until they reached 10.5 L / min and 6.0 mm / min, respectively, after 5 minutes. The total amount of helium gas used was 4150 L, and the transmission loss at 1383 nm of the obtained optical fiber was 0.280 dB / km.

[0061] Compared to a case where the entire 1800 mm of fiber was glassed using a 6.0 mm / min retrieval speed and a helium gas flow rate of 21.0 L / min, this method resulted in a 34% reduction in helium gas usage. Furthermore, there were no problems with the transmission loss of the final optical fiber product.

[0062] In Example 12, the soot 14 was lowered by 800 mm at a heater temperature of 1615°C, a lowering speed of 7.0 mm / min, and a helium gas flow rate of 21.0 L / min. Then, the soot was lowered by 1000 mm at a heater temperature of 1600°C, a lowering speed of 6.0 mm / min, and a helium gas flow rate of 6.3 L / min to obtain the glass base material. The changes in helium gas flow rate and lowering speed were performed discontinuously. The total amount of helium gas used was 3450 L, and the transmission loss at 1383 nm of the optical fiber obtained by drawing was 0.279 dB / km.

[0063] Compared to a case where the entire 1800 mm of fiber was glassed using a 6.0 mm / min retrieval speed and a helium gas flow rate of 21.0 L / min, this method resulted in a 45% reduction in helium gas usage. Furthermore, there were no problems with the transmission loss of the final optical fiber product.

[0064] In Examples 9 and 10, the point of change in the descent speed and the point of change in the helium flow rate are different. More specifically, the point of change in the helium flow rate is located below the point of change in the descent speed, and therefore, in the transparent vitrification process, the point of change in the helium flow rate starts earlier than the point of change in the descent speed. In contrast, in Examples 11 and 11, the point of change in the descent speed and the point of change in the helium flow rate are the same. Therefore, in the transparent vitrification process, the change in the descent speed and the change in the helium flow rate start simultaneously. Alternatively, the point of change in the helium flow rate may be located above the point of change in the descent speed, so that in the transparent vitrification process, the change in the helium flow rate starts later than the change in the descent speed.

[0065] Examples 1, 3, 9, 11, and 12 are provided with a point for changing the lowering speed such that the lowering distance after the change is longer than the lowering distance before the change. Examples 2 and 10 are provided with a point for changing the lowering speed such that the lowering distance after the change is shorter than the lowering distance before the change. Alternatively, the point for changing the lowering speed may be provided so that the lowering distance before the change and the lowering distance after the change are the same.

[0066] In Examples 5 to 12, the helium gas flow rate change point is provided such that the descent distance after the change is longer than the descent distance before the change. Alternatively, the helium gas flow rate change point may be provided such that the descent distance after the change is shorter than or the same as the descent distance before the change.

[0067] Although the invention has been described using embodiments, the technical scope of the present invention is not limited to the scope described in the embodiments above. It will be apparent to those skilled in the art that various modifications or improvements can be made to the embodiments described above. It will be clear from the claims that such modified or improved forms may also be included in the technical scope of the present invention.

[0068] It should be noted that the execution order of operations, procedures, steps, and stages in the devices, systems, programs, and methods shown in the claims, specifications, and drawings is not explicitly stated as "before" or "prior to," and can be implemented in any order unless the output of a previous process is used in a later process. Even if the operation flow in the claims, specifications, and drawings is described using phrases such as "first," "next," etc., for convenience, this does not mean that it is mandatory to perform the operations in that order.

Claims

1. A method for manufacturing an optical fiber preform, comprising a transparent vitrification process in which a glass microparticle deposit, manufactured by blowing glass microparticles onto a starting material that rotates with its own central axis as the axis of rotation, is suspended in a furnace tube equipped with an external local heating heater, and heated in a helium-containing atmosphere while changing its relative position to the heater, wherein at least one of a withdrawal speed change point and a helium gas flow rate change point is provided within the movement range of the glass microparticle deposit, and when the withdrawal speed change point is provided, the withdrawal speed of the glass microparticle deposit before the withdrawal speed change point is greater than the withdrawal speed of the glass microparticle deposit after the withdrawal speed change point, and when the helium gas flow rate change point is provided, the helium gas flow rate before the helium gas flow rate change point is greater than the helium gas flow rate after the helium gas flow rate change point.

2. The method for manufacturing an optical fiber preform according to claim 1, wherein, at the point where the withdrawal speed is changed in the transparent glassing step, the speed change when the withdrawal speed is reduced is performed discontinuously.

3. The method for manufacturing an optical fiber preform according to claim 1, wherein, at the point where the withdrawal speed is changed in the transparent glassification process, the speed change when the withdrawal speed is reduced is performed continuously.

4. The method for manufacturing an optical fiber preform according to claim 1, wherein, at the point where the helium gas flow rate is changed in the transparent glassification process, the flow rate change when reducing the helium gas flow rate is performed discontinuously.

5. The method for manufacturing an optical fiber preform according to claim 1, wherein, at the point where the helium gas flow rate is changed in the transparent glassification process, the flow rate change when reducing the helium gas flow rate is performed continuously.

6. The method for manufacturing an optical fiber preform according to claim 1, wherein in the transparent glassification step, the helium gas flow rate change point is provided such that the drop distance after the change is longer than the drop distance before the change in helium gas flow rate.

7. The method for manufacturing an optical fiber preform according to claim 1, wherein both the lowering speed changing point and the helium gas flow rate changing point are provided.

8. The method for manufacturing an optical fiber preform according to claim 7, wherein, at the point of changing the withdrawal speed in the transparent glassification process, the speed change when reducing the withdrawal speed of the glass microparticle deposit is performed discontinuously, and at the point of changing the helium gas flow rate in the transparent glassification process, the flow rate change when reducing the helium gas flow rate is performed discontinuously.

9. The method for manufacturing an optical fiber preform according to claim 7, wherein, at the point of changing the withdrawal speed in the transparent glassification step, the speed change when reducing the withdrawal speed of the glass microparticle deposit is performed discontinuously, and at the point of changing the helium gas flow rate in the transparent glassification step, the flow rate change when reducing the helium gas flow rate is performed continuously.

10. The method for manufacturing an optical fiber preform according to claim 7, wherein, at the point of changing the withdrawal speed of the glass microparticle deposit in the transparent glass formation process, the speed change when reducing the withdrawal speed of the glass microparticle deposit is performed continuously, and at the point of changing the helium gas flow rate in the transparent glass formation process, the flow rate change when reducing the helium gas flow rate is performed discontinuously and not uniformly.

11. The method for manufacturing an optical fiber preform according to claim 7, wherein at the point of changing the withdrawal speed of the glass microparticle deposit, the speed change is performed continuously when the withdrawal speed of the glass microparticle deposit is reduced, and at the point of changing the helium gas flow rate of the transparent glass process, the flow rate change is performed continuously when the helium gas flow rate is reduced.

12. The method for manufacturing an optical fiber preform according to claim 7, wherein in the transparent glassification process, the reduction in helium gas flow rate is initiated before or simultaneously with the change in velocity, by providing the drop velocity change point and the helium gas flow rate change point.

13. The method for manufacturing an optical fiber preform according to claim 7, wherein in the transparent glassification step, the helium gas flow rate change point is provided such that the drop distance after the change is longer than the drop distance before the change in helium gas flow rate.

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