JP4221277B2 - Optical fiber preform drawing method - Google Patents

Description

  The present invention relates to an optical fiber preform stretching method, and more particularly to a stretching method of stretching an optical fiber preform into a straight rod shape having a desired outer diameter.

  As a method of drawing the optical fiber preform, a heater as a heating means is provided inside the heating furnace, and the feeding chuck is located on the upper side of the heating furnace, and the pulling chuck is located on the lower side (stretching of the optical fiber preform). A method is known that uses a vertical stretching device that is movable in the direction (for example, Patent Document 1).

  In this drawing method, both ends of the optical fiber preform are gripped by the feeding chuck and the pulling chuck, and the optical fiber preform is heated by the heater of the heating furnace to a temperature higher than the glass softening point of the optical fiber preform. Both chucks are moved in the same direction (downward movement) with a speed difference that is faster than the moving speed of the side chuck.

  As a result, the tension in the central axis direction is applied to the optical fiber preform due to the difference in the moving speed between the pull side chuck and the feed side chuck, and in other words, the entire optical fiber preform is lowered, in other words, it is heated through the heating furnace The optical fiber preform is drawn into a straight rod shape having a desired outer diameter.

  As one method for reducing the cost of optical fibers, it has been studied to shorten the drawing time of the optical fiber preform. In the above-described drawing method, in order to shorten the drawing time, it is necessary to increase the tension applied to the optical fiber preform at the time of drawing.

  This is to shorten the stretching time without increasing the temperature in the heating furnace. In order to raise the temperature in the heating furnace, a large modification of the heating furnace is required, resulting in an increase in cost. For example, when a carbon heater is used, increasing the electric power supplied to the carbon heater increases the temperature in the heating furnace, but the life of the carbon heater is extremely shortened. In addition to this, the life of the heat insulating material around the furnace is shortened, and in the worst case, the furnace body may be melted, and as a countermeasure, it is necessary to take countermeasures against the entire heat of the furnace parts. Moreover, since the electric power to input goes up, the countermeasures, such as thickening the electric power cable to a heating furnace, are also needed.

  In addition, when the optical fiber preform is enlarged, the temperature inside the heating furnace is the same, and if it is stretched at the stretching speed (pull-side speed) before it is enlarged, the tension applied to the optical fiber preform is increased. Get higher. This is because the amount of change in the outer diameter of the optical fiber preform for achieving the target stretched diameter is increased.

  In order to make the tension the same, it is sufficient to raise the temperature in the heating furnace or lower the stretching speed. However, as described above, the temperature in the heating furnace causes an increase in cost and lowers the stretching speed. In this case, it goes against the speeding up of the stretching work.

  As described above, when the stretching speed is increased or the size of the optical fiber preform to be stretched is increased, the stretching tension increases, and costs are required to prevent this. Under such circumstances, the stretch tension tends to be higher than before.

  However, when the drawing tension becomes high, the drawing of the optical fiber preform by the drawing apparatus as described above causes bending of the drawn optical fiber preform, and the straightness of the drawn optical fiber preform cannot be obtained. Occurs.

  The bending of the optical fiber preform is evaluated by measuring the deviation from the straight line passing through the center of the cross section at both ends of the preform effective part in the longitudinal direction of the preform and using the maximum deviation or the deviation at the center of the preform. The

  One of the causes of bending of the optical fiber preform is that a feeding chuck or pulling chuck is provided at the tip of a cantilever arm, one end of which is connected to a drive means such as a feed screw mechanism, chain, or wire. The arm that holds the feeding side chuck and the pulling side chuck in a cantilevered manner (chuck support arm) is stretched due to the increased tensile tension and the increased weight of the optical fiber base material. It is conceivable that bending occurs inside.

  As a result of repeated investigations, it was found that the bending of the optical fiber preform is concentrated on the upper part (stretching end side) of the stretched optical fiber preform when stretching by descending movement. The cause is that tension is applied at the time of stretching and the chuck support arm is bent, but at the end of stretching, the movement of the chuck is stopped and the tension is not applied. At that time, it is considered that the upper part of the optical fiber preform is bent by eliminating the deflection of the chuck support arm because the upper side of the optical fiber preform in the heater portion is still hot and soft just before the end of stretching.

This can be dealt with by increasing the rigidity of the chuck support arm itself so that the chuck support arm does not bend, or by supporting the chuck on both sides, but with major equipment modifications, It will lead to cost increase. If the chuck is supported at both ends, there is a risk that the optical fiber preform mounting operation at the start of stretching and the removal of the optical fiber preform at the end of stretching may be hindered.
Japanese Patent Laid-Open No. 11-11970

  The problem to be solved by the present invention is that the optical fiber preform is not bent due to bending of the chuck support arm during stretching, and the optical fiber preform is stretched with high accuracy without any device modification. That is.

  An optical fiber preform stretching method according to the present invention includes: a heating means that grips both ends of an optical fiber preform by a feeding side gripping portion and a pulling side gripping portion that are provided so as to be movable in the stretching direction of the optical fiber preform; In the state in which the optical fiber preform is heated by the above-mentioned, the pulling side gripping part is moved in the same direction with a moving speed of the pulling side gripping part being faster than the moving speed of the sending side gripping part. In the drawing method of the optical fiber preform that stretches the optical fiber preform by applying tension to the optical fiber preform due to the difference in the moving speed of the transmission side gripping part, the optical fiber preform by the heating means at the end of stretching After the heating is stopped, a state in which tension is applied to the optical fiber preform due to a difference in moving speed between the pull-side gripping part and the sending-side gripping part is maintained.

  In the method of drawing an optical fiber preform according to the present invention, preferably, the temperature of the optical fiber preform or a temperature representative of the optical fiber preform temperature is monitored, and after the heating is stopped, the temperature is decreased to a predetermined value. Maintain the tension applied to the optical fiber preform.

  In the method for stretching an optical fiber preform according to the present invention, preferably, the heating temperature of the optical fiber preform by the heating means at the end of stretching is lower than the previous heating temperature.

  In the method for stretching an optical fiber preform according to the present invention, preferably, the moving speed of the pulling side gripping part and the moving speed of the sending side gripping part after stopping heating are gradually decreased, And the movement of the sending side gripping part are stopped simultaneously.

  In the method of drawing an optical fiber preform according to the present invention, preferably, the tension acting on the optical fiber preform is monitored, and the moving speed of the pulling gripping part and the moving speed of the sending gripping part are controlled. The tension after stopping the heating is maintained at substantially the same value as the tension before stopping the heating. Here, substantially the same value is about ± 30% of the tension during stretching.

  In the method of drawing an optical fiber preform according to the present invention, even after the heating of the optical fiber preform by the heating means is stopped at the end of the drawing, the tension is applied to the optical fiber preform due to the difference in moving speed between the pull side gripping part and the sending side gripping part. Since the optical fiber preform is still hot and the optical fiber preform is soft immediately after the heating is stopped, the pulling side gripping part and the arm that supports the sending side gripping part are eliminated. Is avoided, and bending of the optical fiber preform due to elastic restoration of the arm or the like does not occur.

  One embodiment of a vertical drawing apparatus used for carrying out a drawing method of an optical fiber preform according to the present invention will be described with reference to FIG.

  The vertical stretching apparatus has a heating furnace 10 provided with a heater 11 as a heating means inside the furnace. The heating furnace 10 has a fixed arrangement and a structure in which the optical fiber preform W can pass in the vertical direction. The heating furnace 10 is provided with a radiation thermometer 12 that measures the temperature of the heater 11.

  A feeding side chuck (feeding side gripping part) 20 for gripping the upper end part (upper dummy part Wad) of the optical fiber preform W is disposed on the upper side of the heating furnace 10, and a lower end part (lower side) of the optical fiber preform W is disposed on the lower side. Pull-side chucks (pull-side grip portions) 30 that grip the dummy portion (Wld) are arranged concentrically with each other.

  The optical fiber preform W penetrates the heating furnace 10 up and down as shown by gripping the upper dummy portion Wad by the feeding side chuck 20 and the lower dummy portion Wld by the pulling side chuck 30. Thus, it is held vertically (stretching direction).

  The optical fiber preform W is a semi-finished preform having part or all of a core and a clad, and the core is made of synthetic quartz containing a dopant such as Ge or F. The upper dummy portion Wad and the lower dummy portion Wld are made of natural quartz.

  The feed chuck 20 is attached to the tip (one end) of the upper chuck support arm 21 via a load cell 25 for measuring tension. The upper chuck support arm 21 is a horizontal arm, and has a base end (the other end) coupled to the feed nut 22 and supported by the feed nut 22. The feed nut 22 is screw-engaged with a feed-side feed screw rod 24 provided so as to be movable in the vertical direction (stretching direction of the optical fiber preform W), and moves up and down by the rotation of the feed-side feed screw rod 24. . A feed-side drive motor 23 is connected to the feed-side feed screw rod 24, and the feed-side drive motor 23 rotationally drives the feed-side feed screw rod 24.

  The pull-side chuck 30 is attached to the tip (one end) of the lower chuck support arm 31. The lower chuck support arm 31 is a horizontal arm, the base end (the other end) is connected to the feed nut 32, and is supported by the feed nut 32. The feed nut 32 is screw-engaged with a pull-side feed screw rod 34 movably provided in the vertical direction (stretching direction of the optical fiber preform W), and moves up and down by the rotation of the pull-side feed screw rod 34. . A pull-side drive motor 33 is connected to the pull-side feed screw rod 34, and the pull-side drive motor 33 rotationally drives the pull-side feed screw rod 34.

  The vertical stretching apparatus has an electronically controlled controller 40 such as a microcomputer. The controller 40 controls the speed of the sending drive motor 23 and the pulling drive motor 33, in other words, the motor controller 41 that controls the moving speed of the sending chuck 20 and the pulling chuck 30, and the power supplied to the heater 11. And a heater power control unit 42 for controlling.

  The controller 40 receives a signal from the radiation thermometer 12 and monitors the temperature of the heater 11. Further, the controller 40 inputs a signal from the load cell 25 and monitors (monitors) the tension acting on the optical fiber preform W.

  As shown in FIG. 2 (d), the optical fiber preform W is stretched by supplying power to the heater 11 under the furnace power control by the heater power control unit 42, and the heating furnace 10 using the heater 11. The optical fiber preform W is heated inside.

  In this heated state, the speed of the feed side drive motor 23 and the pull side drive motor 33 is controlled by the motor control unit 41, and as shown in FIG. ) The chucks 20 and 30 are moved down with a speed difference Vb higher than the moving speed (feeding speed) Va of the feeding chuck 20.

  Thereby, the optical fiber preform W is given a tension (refer to FIG. 2B) in the central axis direction (axial long line direction) due to the moving speed difference (Vb−Va) between the pulling side chuck 30 and the feeding side chuck 20. The whole is stretched while moving down.

  As shown in FIG. 2C, when the extension length reaches the preset end length Le, this time is set as the extension end time Te, and the power supply to the heater 11 is cut off (power interruption). Note that the extension length has reached the preset end length Le by detecting the lowered position of the feeding side chuck 20 or the pulling side chuck 30 by a position detection switch or position detection sensor (not shown). Can be found.

  After this stretching end time Te, the speed of the feeding side drive motor 23, that is, the moving speed Va of the feeding side chuck 20, is gradually decreased, and the tension acting on the optical fiber preform W measured by the load cell 25 is shown in FIG. As shown in (b), the speed of the feed-side drive motor 23 is feedback-controlled so that it is the same level as that during stretching (within ± 30%), and the moving speed Vb of the pull-side chuck 30 is also gradually increased. Reduce. After waiting for the pulling side and the feeding side chucks 30 and 20 to have the same speed, the movement of the feeding side and the pulling side chucks 20 and 30 was stopped simultaneously.

  Thereby, even after the heating of the optical fiber preform W by the heater 11 is stopped at the stretching end time Te, the tension is applied to the optical fiber preform W due to the moving speed difference between the pull side chuck 30 and the feed side chuck 20. The upper chuck support arm 21 and the lower chuck support arm that support the feeding side chuck 20 and the pulling side chuck 30 while the optical fiber preform W is still hot and the optical fiber preform W is soft immediately after the heating is stopped. It is avoided that 31 is lost.

  As a result, the optical fiber preform W is not bent due to the elastic recovery of the upper chuck support arm 21 and the lower chuck support arm 31 due to the release of tension, and the optical fiber preform is not accompanied by device modification. The material W can be drawn with high accuracy.

  Further, after the stretching end time Te, that is, after the heating is stopped, the period C in which the tension is applied to the optical fiber preform W is set to a temperature representative of the temperature of the optical fiber preform W or the optical fiber preform temperature. After monitoring and heating, the monitoring temperature can be limited to a predetermined value, for example, until the temperature drops below the softening temperature (glass transition temperature) of the optical fiber preform W. This monitored temperature may be the temperature of the heater 11 measured by the radiation thermometer 12.

  As another embodiment, as shown in FIGS. 3 (a) to 3 (d), the furnace power is gradually reduced at a time Ta slightly before the drawing end time Te, and the optical fiber preform by the heater 11 is used. The heating temperature of W can be made lower than the previous heating temperature, and the time required for the optical fiber preform W to cool below the glass transition temperature can be shortened.

  Also in this case, the tension acting on the optical fiber preform W is controlled so as to maintain substantially the same value as before the time Ta after the time Ta. Therefore, after the time Ta, the moving speed Va of the feeding side chuck 20 and the pulling side chuck are controlled. Both the moving speeds Vbt of 30 gradually decrease.

  If the heating temperature of the optical fiber preform W by the heater 11 is too low, the movement speeds Va and Vbt of the feeding side chuck 20 and the pulling side chuck 30 are reduced accordingly, and the drawing time becomes longer. There is no point in making it longer than shortening the time for the base material W to cool. This temperature drop is preferably within about 20% of the steady state and within 300 mm from the stretching end time Te.

  In the above-described embodiment, the vertical movement apparatus of the descending movement method is used. However, the method of stretching the optical fiber preform according to the present invention can be similarly implemented by the vertical stretching apparatus of the ascending movement system, The same can be applied to a horizontal stretching apparatus that stretches in the direction.

  A vertical stretching apparatus as shown in FIG. 1 was used, and at the end of stretching, tension was continuously applied until the optical fiber preform was cooled and solidified. The stretching speed at this time was 62.5 mm / min on the pulling side and 40 mm / min on the feeding side, and the temperature inside the heating furnace could not be measured directly. Therefore, when the heater temperature was measured with a radiation thermometer, it was 2500 ° C. . The stretching tension at this time was 1000 N, and an optical fiber preform with an outer diameter of 100 mm before stretching was stretched to a target outer diameter of 80 mm.

  At the end of stretching (when the feed side chuck or pull side chuck moves to the set position), the heater power of the heating furnace is turned off. Then, gradually lower the moving speed of the feeding side chuck and change the speed of the pulling side chuck so that the tension becomes the same as that during stretching, so that the pulling side and the feeding side chuck become the same speed. wait. At the same time, the movements of the feeding side and pulling side chucks were stopped simultaneously. As a result, the optical fiber preform could be stretched without bending.

  Note that the outer diameter of the upper part of the base material (still soft part at the end) changes due to the tension applied after the end of stretching. The lower the tension, the thinner the optical fiber preform, but the greater the bending. In general, if the tension is up to ± 30% of the tension applied to the optical fiber preform at the time of stretching, a very large preform bending does not occur and the optical fiber preform does not become thin.

  When the feeding was stopped immediately after the end of stretching, the tension increased momentarily (due to the time difference from the deceleration on the take-off side), and the outer diameter of that portion became thinner. It is also possible to apply tension by stopping the sending side and pulling side at the same time, and then moving the sending side in the opposite direction to that during stretching. However, this method causes bending of the optical fiber preform. It was. This is because the tension applied to the optical fiber preform before the feed is moved in the opposite direction is lowered, and the optical fiber preform is bent as the chuck returns to its original position.

  Also, as a comparative example, the upper dummy part of the optical fiber preform was thickened and extended to the dummy part, so that the effective part of the optical fiber preform was sufficiently swept to the position where it would not bend at the end of stretching. . As a result, the bending of the optical fiber preform was reduced, but after the stretching of the optical fiber preform on the drawing end side, the dummy part (natural quartz) and the optical fiber preform (synthetic quartz) had different viscosities at high temperatures. The outer diameter of the has fluctuated greatly.

  When a base material having an outer diameter of 100 mm before stretching was stretched to a target of 80 mm, the position of about 100 mm fluctuated from the boundary with the dummy part, and the vicinity of the boundary was the narrowest and the outer diameter was about 78 mm. The stretching speed at this time was 62.5 mm / min on the pulling side and 40 mm / min on the feeding side, and the temperature inside the heating furnace could not be measured directly. Therefore, when the heater temperature was measured with a radiation thermometer, it was 2500 ° C. . The stretching tension at this time was 1000N.

  This method requires a large number of dummy portions, leading to an increase in cost. Even if the viscosity is extended using a dummy portion similar to that of the optical fiber preform, the cost is increased.

It is a block diagram which shows one Embodiment of the vertical extending | stretching apparatus used for implementation of the extending | stretching method of the optical fiber preform | base_material by this invention. (A)-(d) is a time chart which shows one Embodiment of the extending | stretching method of the optical fiber preform | base_material by this invention. (A)-(d) is a time chart which shows other embodiment of the extending | stretching method of the optical fiber preform | base_material by this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Heating furnace 11 Heater 12 Radiation thermometer 20 Feed side chuck 21 Upper chuck support arm 22 Feed nut 23 Feed side drive motor 24 Feed side feed screw rod 25 Load cell 30 Pull side chuck 31 Lower chuck support arm 32 Feed nut 33 Pull side Drive motor 34 Feed-side feed screw rod 40 Controller 41 Motor controller 42 Heater power controller

Claims (5)

In the state where both ends of the optical fiber preform are gripped by the feeding side gripping portion and the pulling side gripping portion provided movably in the extending direction of each optical fiber preform, and the optical fiber preform is heated by the heating means, The two gripping parts are moved in the same direction with a speed difference that is faster than the moving speed of the sending side gripping part, and the pulling side gripping part and the sending side gripping part move according to the moving speed difference between the pulling side gripping part and the sending side gripping part. In an optical fiber preform stretching method that applies tension to the optical fiber preform and stretches the optical fiber preform.
At the end of stretching, heating of the optical fiber preform by the heating means was stopped, and even after the heating was stopped, tension was applied to the optical fiber preform due to a difference in moving speed between the pull-side gripping portion and the sending-side gripping portion. An optical fiber preform stretching method that maintains the state.   The temperature of the optical fiber preform or a temperature representative of the optical fiber preform temperature is monitored, and after the heating is stopped, a state where tension is applied to the optical fiber preform is maintained until the temperature drops to a predetermined value. 2. A method for stretching an optical fiber preform according to 1.   The method of drawing an optical fiber preform according to claim 1 or 2, wherein a heating temperature of the optical fiber preform by the heating means at the end of the drawing is lower than a previous heating temperature.   4. The movement speed of the pulling side gripping part and the movement speed of the sending side gripping part after heating stop are gradually decreased, respectively, and the movement of the pulling side gripping part and the sending side gripping part is stopped simultaneously. The method for stretching an optical fiber preform according to any one of the above.   The tension acting on the optical fiber preform is monitored, and the tension after stopping heating is maintained at almost the same value as the tension before stopping heating by controlling the moving speed of the pulling gripping part and the moving speed of the sending gripping part. The method for stretching an optical fiber preform according to any one of claims 1 to 4.

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