JP6277605B2 - Optical fiber manufacturing method and manufacturing apparatus - Google Patents

Description

  The present invention relates to a method of manufacturing an optical fiber in which an optical fiber glass preform is inserted into a drawing furnace filled with a gas containing argon or nitrogen, and the optical fiber is drawn by heating and melting in the drawing furnace. It relates to a manufacturing apparatus.

  The optical fiber tip is made by lowering an optical fiber glass base material (hereinafter referred to as a glass base material) mainly composed of quartz from above the optical fiber drawing furnace (hereinafter referred to as a drawing furnace) into the furnace core tube. Is heated and melted, and the tip of the glass base material is reduced in diameter and drawn from below the drawing furnace. Since the temperature in the drawing furnace at this time is as high as about 2000 ° C., carbon having excellent heat resistance is used for the parts in the drawing furnace.

This carbon has the property of being oxidized and consumed in a high-temperature oxygen-containing atmosphere. For this reason, it is necessary to keep the inside of a drawing furnace in the atmosphere of noble gases, such as argon gas and helium gas, and nitrogen gas (henceforth inert gas etc.).
In this case, the inside of the drawing furnace is set to a positive pressure to prevent outside air (oxygen) from entering the drawing furnace. However, if the pressure fluctuation in the drawing furnace becomes large, the outside air is entrained or the outside of the optical fiber It affects the diameter variation. Therefore, for example, Patent Document 1 discloses a structure for controlling the supply amount of an inert gas or the like so that the pressure in the drawing furnace is constant. Patent Document 2 discloses a structure in which a seal gas flow rate is controlled using a mass flow controller (MFC) so that the pressure in the drawing furnace is constant. Patent Document 3 discloses that the supply gas flow rate is adjusted so that the pressure in the drawing furnace is constant.

JP 2011-46563 A JP 2000-264670 A JP 2000-063142 A

  By the way, for example, when an inert gas such as argon gas or nitrogen gas is used, it is known that fluctuations in the pressure in the drawing furnace and the outer diameter of the optical fiber increase. A frequency analysis of the pressure fluctuation and outer diameter fluctuation measurement results in the drawing furnace revealed that there are many frequency components of about several Hz (˜2 Hz). That is, in order to adjust the pressure to be constant in response to the pressure fluctuation in the drawing furnace having such a frequency component, a response time (response time) of about several seconds as in the mass flow controller described in Patent Document 2 above. A structure having a response speed of less than 0.5 seconds is required instead of a structure having a speed). However, the above Patent Documents 1 to 3 do not disclose the specific structure and control method of the gas supply means, and there is a problem in that fluctuations in the outer diameter of the optical fiber cannot be reduced.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical fiber manufacturing method and manufacturing apparatus with less fluctuation in the outer diameter of the optical fiber.

An optical fiber manufacturing method according to the present invention includes an optical fiber glass preform inserted into a drawing furnace filled with a gas containing argon or nitrogen, and heated and melted in the drawing furnace to draw the optical fiber. An optical fiber manufacturing method, wherein the gas is drawn by the gas supply unit so that the pressure in the drawing furnace is lower than a preset target value of the pressure in the drawing furnace. A certain amount is fed into the furnace, the pressure in the drawing furnace is measured by the measuring unit, and the target value is compared with the measured value of the pressure in the drawing furnace measured by the measuring unit by the control unit. And by controlling the automatic opening and closing valve that raises the pressure in the drawing furnace by supplying gas into the drawing furnace by adjusting the opening , among the frequency components of the pressure fluctuation in the drawing furnace, 5Hz said automatic opening so as to follow the pressure variations of the following frequency components By adjusting the opening of the valve and supplies a gas such that the measurement value becomes the target value in the line drawing furnace.

An optical fiber manufacturing apparatus according to the present invention inserts an optical fiber glass preform into a drawing furnace filled with a gas containing argon or nitrogen, and heats and melts the optical fiber in the drawing furnace. An optical fiber manufacturing apparatus for supplying a certain amount of gas into the drawing furnace, a measuring unit for measuring the pressure in the drawing furnace, and adjusting the opening to draw the drawing. An automatic opening / closing valve for further supplying gas into the furnace to increase the pressure in the drawing furnace, a preset target value of the pressure in the drawing furnace, and the inside of the drawing furnace measured by the measurement unit And a control unit that controls the gas flow rate from the gas supply unit and the opening degree of the automatic opening and closing valve, and the gas supply unit is configured such that the pressure in the drawing furnace is Gas is drawn into the draw furnace so that the value is lower than the target value. Interrupt, the automatic opening and closing valve, among the frequency components of the pressure fluctuations in the line drawing furnace, and the target value the measured value by adjusting the degree of opening so as to follow the pressure variations in the frequency components lower than 5Hz The gas is supplied into the drawing furnace so that

According to the present invention, the gas supply unit is provided with an automatic opening / closing valve capable of opening and closing at a fast response speed while feeding gas into the drawing furnace so that the pressure in the drawing furnace is lower than the target value by the gas supply unit. Since it is provided in the air supply line separately, it is difficult to follow fluctuations of several tens of Hz or more, but the pressure in the drawing furnace can be adjusted following the pressure fluctuations of about several Hz described above. The fluctuation of the outer diameter of the optical fiber can be suppressed.
Furthermore, since the gas can be supplied into the drawing furnace and the pressure fluctuation in the drawing furnace can be maintained at a predetermined value without forcibly discharging the gas from the drawing furnace, the amount of gas used is reduced and the optical fiber is reduced. The manufacturing cost can be further reduced.

It is a figure explaining the outline of the manufacturing apparatus of the optical fiber by one form of this invention. It is a figure which shows the relationship between the ratio of argon gas, and the outer diameter fluctuation | variation of an optical fiber. It is a figure which shows the relationship between the pressure fluctuation in a drawing furnace, and the outer diameter fluctuation | variation of an optical fiber. It is a figure which shows an example of the time change of the pressure in a drawing furnace, and the outer diameter of an optical fiber. It is a figure which shows the result of the frequency analysis of FIG. It is a figure which shows the gain characteristic of the outer diameter fluctuation | variation of the optical fiber with respect to the pressure fluctuation | variation in a drawing furnace. It is a figure which shows an example of the pressure fluctuation of an upper chamber, and the pressure fluctuation of a lower chamber. It is a figure which shows the control content.

An outline of an optical fiber manufacturing apparatus to which the present invention is applied will be described with reference to FIG. In the following, a resistance furnace that heats the core tube with a heater will be described as an example. However, the present invention can also be applied to an induction furnace in which a high-frequency power source is applied to the coil to induction-heat the core tube.
In the figure, 10 is a drawing furnace, 11 is an optical fiber glass base material, 12 is an optical fiber, 13 is a dummy rod, 14 is a connecting portion, 15 is a core tube, 16 is a heater, 17 is a heat insulating material, and 18 is a furnace. Housing, 19 is a lower chamber, 20 is an upper chamber, 21 is a lid, 21a is an upper end opening, 22 is a gas introduction path, 23 is a gas supply section, 24 is a gas supply path, 25 is an automatic opening / closing valve, and 26 is A measurement unit 30 indicates a control unit.

The drawing furnace 10 includes a furnace casing 18, a lower chamber 19, and an upper chamber 20. The core tube 15 is formed in a cylindrical shape at the center of the furnace casing 18 and communicates with the lower chamber 19 and the upper chamber 20. The core tube 15 is made of carbon, and an optical fiber glass base material 11 (hereinafter referred to as a glass base material) is inserted into the core tube 15 through the upper chamber 20.
The upper chamber 20 has an inner diameter comparable to that of the core tube 15, and is sealed (sealed) with a lid 21 disposed on the upper end thereof. An upper end opening 21 a is formed in the lid 21, and a dummy rod 13 made of the same kind of glass rod as the glass base material 11 is inserted therethrough.

A heater 16 is arranged in the furnace casing 18 so as to surround the furnace core tube 15, and a heat insulating material 17 is accommodated so as to cover the outside of the heater 16. The heater 16 heats and melts the glass base material 11 inserted into the core tube 15, and causes the optical fiber 12 that has been melted and reduced in diameter to hang down from the lower chamber 19.
The glass base material 11 is welded at a connecting portion 14 connected to the dummy bar 13 or connected and integrated through a connecting member. The glass base material 11 can be moved in a drawing direction (downward) by a moving mechanism (not shown).

  The drawing furnace 10 is provided with an in-furnace gas supply mechanism such as an inert gas. Specifically, the upper chamber 20 is provided with a gas introduction path 22. For example, an inert gas or the like obtained by mixing argon gas and helium gas is fed into the furnace core tube 15 in a certain amount. This prevents oxidation and deterioration in the furnace core tube 15 and around the heater 16. The supply amount of the inert gas or the like is controlled and supplied by a gas supply unit (also referred to as a mass flow controller (MFC)) 23. In addition to this MFC, another MFC capable of controlling a small capacity may be arranged in parallel to achieve high-precision flow rate control.

The inert gas or the like passes through the gap between the glass base material 11 and the core tube 15 and is released to the outside along with the drawn optical fiber 12 from the shutter portion below the lower chamber 19. An exhaust path may be provided to exhaust a certain amount.
On the other hand, a measurement unit 26 is provided in the upper chamber 20 to measure the pressure in the drawing furnace 10. The measurement result is output to the control unit 30. Since there is almost no phase difference of pressure fluctuation in the drawing furnace, the pressure may be measured at any position in the furnace.

The drawing furnace 10 is provided with an in-furnace gas supply mechanism using an inert gas or the like. Specifically, a gas supply passage 24 having an automatic opening / closing valve 25 is provided near the lower end of the upper chamber 20.
The flow rate supplied into the drawing furnace 10 through the gas supply passage 24 is adjusted by the opening degree of an automatic opening / closing valve 25 such as an automatic needle valve. The automatic opening / closing valve 25 is composed of, for example, a proportional control valve, and has a response speed of less than 1 second (for example, about 0.05 seconds to 0.3 seconds). Based on a signal from the control unit 30, It is opened and closed using the magnetic force of a solenoid (not shown).

  The control unit 30 controls the amount of air supplied from the gas introduction path 22 and the gas supply path 24 into the drawing furnace 10 and holds the pressure fluctuation in the drawing furnace 10 at a predetermined value.

FIG. 2 is a diagram showing the relationship between the ratio of argon gas and fluctuations in the outer diameter of the optical fiber.
By mixing argon gas with low thermal conductivity with helium gas with high thermal conductivity, pressure fluctuations due to temperature unevenness are more likely to occur than when helium gas is used at 100%, as shown in FIG. The fluctuation of the outer diameter of the optical fiber increases.

  This variation in the outer diameter of the optical fiber can be represented by, for example, a value (3σ) obtained by triple the variation in the outer diameter of the optical fiber (standard deviation σ). When argon gas is not mixed (when helium gas is used at 100%), the outer diameter fluctuation (3σ) of the optical fiber can be suppressed to ± 0.09 μm, but when argon gas is used at 12.5% (helium) When gas is used at 87.5%), the outer diameter variation (3σ) of the optical fiber becomes ± 0.68 μm.

  When 25% argon gas is used (75% helium gas is used), the optical fiber outer diameter variation (3σ) is ± 0.61 μm, and 50% argon gas is used (50% helium gas is used) When used), the outer diameter variation (3σ) of the optical fiber is ± 1.11 μm, when 100% argon gas is used (when helium gas is not used), the outer diameter variation (3σ) of the optical fiber is ± 1.1. Increases to 40 μm. Even if nitrogen gas is used instead of argon gas, a similar result is obtained.

  FIG. 3 is a diagram showing the relationship between the pressure fluctuation in the drawing furnace (upper chamber) and the outer diameter fluctuation of the optical fiber. There is a correlation between the pressure fluctuation in the drawing furnace and the outer diameter fluctuation of the optical fiber. For example, if the pressure fluctuation in the drawing furnace is lowered to ± 2.2 Pa, the outer diameter fluctuation of the optical fiber is reduced to ± 1 μm. If the pressure fluctuation in the drawing furnace can be reduced to ± 1.0 Pa, the outer diameter fluctuation of the optical fiber can be suppressed to ± 0.40 μm. The optical fiber in this case can improve the yield of attaching the optical fiber to the multi-fiber optical connector and reduce the connection loss. Further, if the pressure fluctuation in the drawing furnace can be reduced to ± 0.5 Pa, the outer diameter fluctuation of the optical fiber can be suppressed to about ± 0.15 μm.

  The fact that there is a correlation between the pressure fluctuation in the drawing furnace shown in FIG. 3 and the outer diameter fluctuation of the optical fiber will be described in detail. When the gas in the drawing furnace is Ar 50% and He 50%, and the pressure in the drawing furnace and the outer diameter of the optical fiber are measured, the pressure in the drawing furnace (with respect to the atmospheric pressure) is shown in FIG. For example, when the differential pressure changes within a range of 9 Pa to 13 Pa, the outer diameter of the optical fiber is within a range of −1.0 μm to +1.5 μm, for example, as shown by a thin line in FIG. It turns out that it changes to an up-down direction so that the pressure in a furnace may be followed.

  FIG. 5 is a diagram showing the results of frequency analysis of FIG. 4. The pressure fluctuation period in the drawing furnace indicated by the thick line in FIG. 5 and the outer diameter fluctuation period of the optical fiber indicated by the thin line in FIG. In the region where the frequency is relatively low, which is about 2 Hz or less, they are almost the same. Further, as shown in FIG. 5, the pressure in the drawing furnace and the outer diameter of the optical fiber are less than 2 Hz and the strength is increased. For this reason, it can be seen that if the pressure fluctuation at less than 2 Hz in the drawing furnace is reduced, the outer diameter fluctuation of the optical fiber can be further reduced. On the other hand, the higher the frequency, the smaller the correlation between the pressure fluctuation and the optical fiber outer diameter fluctuation. Therefore, it can be said that it is almost sufficient to reduce the pressure fluctuation to about 5 Hz. This will be described next.

  FIG. 6 is a diagram illustrating gain characteristics of the outer diameter variation of the optical fiber with respect to the pressure variation. The gain characteristic indicates the frequency characteristic of amplitude when the transfer function G (s) is G (s) = glass outer diameter / furnace pressure. From this figure, it can be seen that the amplitude (gain) becomes smaller as the frequency becomes higher, and therefore, even if the pressure fluctuation of the high frequency component is controlled, the outer diameter fluctuation of the optical fiber is hardly influenced (ineffective). Since the gain when the frequency is 5 Hz is reduced to about −11 dB (amplitude is about ¼), it can be said that it is sufficient to control and reduce the pressure fluctuation up to about 5 Hz in combination with the result of FIG. FIG. 6 shows the gain characteristics when Ar is 50% and He is 50%. The gain characteristics slightly change depending on the gas components, but not greatly.

  Further, when the pressure in the upper chamber and the pressure in the lower chamber are measured at a sampling period of 0.1 second, as shown in FIG. 7A, the pressure in the upper chamber (differential pressure from the atmospheric pressure) is 40 Pa. The pressure in the lower chamber is measured in the range of 0 Pa (corresponding to atmospheric pressure) to 14 Pa in the range of ~ 54 Pa, and the pressure in the lower chamber is smaller than the pressure in the upper chamber. The fluctuation cycle of pressure is almost the same. FIG. 7 (B) shows an enlarged view of the section of 5 to 8 seconds in FIG. 7 (A). The fluctuation cycle of the pressure in the upper chamber and the fluctuation cycle of the pressure in the lower chamber are the same. I understand that well. That is, there is almost no phase difference of pressure fluctuation in the drawing furnace, and the pressure may be measured at any position in the furnace.

  Thus, the pressure fluctuation in the upper chamber or the lower chamber can be considered in the same way as the pressure fluctuation in the drawing furnace, and the pressure fluctuation in the drawing furnace is correlated with the outer diameter fluctuation of the optical fiber. There is. On the other hand, as described with reference to FIG. 2, the outer diameter variation of the optical fiber increases as the ratio of argon gas increases.

In order to keep the outer diameter fluctuation small even when the ratio of the argon gas is increased, it is necessary to reduce the pressure fluctuation in the drawing furnace 10. In order to control this pressure fluctuation, the control unit 30 first sets a target value Pset of the pressure in the drawing furnace 10. This Pset is set to a value that is higher than the minimum required pressure, such as keeping the drawing furnace at a positive pressure.
And this control part 30 controls the opening degree of the automatic opening-and-closing valve 25 so that the pressure (measurement value P) in the drawing furnace 10 measured by the measurement part 26 may correspond with the set target value Pset. For the control to set the opening degree of the automatic opening / closing valve 25 from the deviation between the target value Pset and the measured value P, P control (Proportional Control), I control (Integral Control), D control (Derivative Control) : Differential control), or various controls combining these appropriately.

  Specifically, in order to suppress (correct) pressure fluctuation due to natural convection in the drawing furnace 10, the control unit 30 expects this pressure fluctuation (for example, a value about 3 Pa lower than Pset). To the gas supply unit 23. The gas supply unit 23 supplies an inert gas or the like that satisfies the command pressure of the control unit 30 (a value lower than the two-dot chain line (Pset) in FIG. 8 and indicated by a broken line in FIG. 8) into the drawing furnace 10. To send.

  Subsequently, the control unit 30 instructs the opening degree of the automatic opening / closing valve 25 so as to correct the deviation of the measured value P so that the measured value P of the in-furnace pressure becomes the target value Pset. When the measured value P is large, the automatic opening / closing valve 25 reduces the opening to supply a small amount of gas, and when the measured value P is small, the automatic opening / closing valve 25 increases the opening to supply a large amount of gas. In addition, the air supply amount is made to follow the magnitude of the measured value P. As shown in FIG. 5, the pressure fluctuation has a higher intensity as the frequency is lower, and as shown in FIG. 6, the lower frequency component affects the outer diameter fluctuation of the optical fiber. It is set to 0.3 s or less, more preferably 0.2 s or less, and control is performed so as to follow the pressure fluctuation of the low frequency component.

  The drawing furnace gas was Ar 50% and He 50%, and the above air supply control was performed. As a result, the pressure fluctuation of 5 Hz or less could be reduced to about 1/10 before the control. As a result, the outer diameter fluctuation (3σ) of the optical fiber, which was conventionally ± 1.11 μm, can be reduced to ± 0.15 μm or less.

  As described above, an automatic opening / closing valve is provided in the gas supply path separately from the gas supply unit, and the opening degree of the automatic opening / closing valve is set from the deviation between the target value Pset of the pressure in the drawing furnace and the measured value P. The gas supply unit supplies gas by opening and closing the automatic open / close valve at a fast response speed while sending the gas into the drawing furnace so that the pressure in the drawing furnace is lower than the target value. Thus, for example, even when argon or nitrogen is mixed, it is possible to reduce the pressure fluctuation in the drawing furnace, and to suppress the outer diameter fluctuation of the optical fiber.

  Furthermore, since the gas can be supplied into the drawing furnace and the pressure fluctuation in the drawing furnace can be maintained at a predetermined value without forcibly discharging the gas from the drawing furnace, the amount of gas used is reduced and the optical fiber is reduced. The manufacturing cost can be further reduced.

DESCRIPTION OF SYMBOLS 10 ... Drawing furnace, 11 ... Glass base material for optical fibers, 12 ... Optical fiber, 13 ... Dummy rod, 14 ... Connection part, 15 ... Core tube, 16 ... Heater, 17 ... Heat insulating material, 18 ... Furnace housing, DESCRIPTION OF SYMBOLS 19 ... Lower chamber, 20 ... Upper chamber, 21 ... Cover body, 21a ... Upper end opening, 22 ... Gas introduction path, 23 ... Gas supply part, 24 ... Gas supply path, 25 ... Automatic opening / closing valve, 26 ... Measurement part, 30: Control unit.

Claims (3)

An optical fiber manufacturing method for drawing an optical fiber by inserting a glass base material for an optical fiber into a drawing furnace filled with a gas containing argon or nitrogen, and heating and melting in the drawing furnace.
A certain amount of gas is fed into the drawing furnace by a gas supply unit so that the pressure in the drawing furnace is lower than a preset target value of the pressure in the drawing furnace. The pressure in the drawing furnace is measured, and the control unit compares the target value with the measured value of the pressure in the drawing furnace measured by the measurement unit, and adjusts the opening degree. Controls an automatic open / close valve that supplies gas into the drawing furnace to increase the pressure in the drawing furnace, and follows pressure fluctuations of frequency components of 5 Hz or less among frequency components of pressure fluctuations in the drawing furnace An optical fiber manufacturing method for supplying gas into the drawing furnace so that the measured value becomes the target value by adjusting the opening of the automatic opening and closing valve. The method for producing an optical fiber according to claim 1, wherein the gas filling the drawing furnace contains 50% or more of argon or nitrogen .   An optical fiber manufacturing apparatus for drawing an optical fiber by inserting a glass base material for an optical fiber into a drawing furnace filled with a gas containing argon or nitrogen, and heating and melting in the drawing furnace.
  A gas supply unit for feeding a certain amount of gas into the drawing furnace, a measurement unit for measuring the pressure in the drawing furnace, and adjusting the opening to further supply gas into the drawing furnace An automatic open / close valve for raising the pressure in the drawing furnace, and comparing a preset target value of the pressure in the drawing furnace with a measured value of the pressure in the drawing furnace measured by the measurement unit; A control unit that controls a gas flow rate from the gas supply unit and an opening degree of the automatic opening and closing valve;
  The gas supply unit feeds gas into the drawing furnace so that the pressure in the drawing furnace is lower than the target value, and the automatic open / close valve is configured to reduce pressure fluctuations in the drawing furnace. An apparatus for manufacturing an optical fiber that supplies gas into the drawing furnace so that the measured value becomes the target value by adjusting the opening degree so as to follow the pressure fluctuation of the frequency component of 5 Hz or less among the frequency components. .

Images