JPS5929531B2 - Optical fiber diameter control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for controlling the diameter of an optical fiber during drawing, and its purpose is to more stably control the diameter of an optical fiber and obtain an optical fiber of good quality.

Research on optical fibers has progressed rapidly over the past few years, and it has become possible to realize ultra-low loss values of several dB/km.

At the same time, we are gradually entering a stage where various characteristics facing practical use, such as ease of connection between optical fibers, stress resistance characteristics, and broadband characteristics, are being discussed. One of the important keys is the uniformity of the diameter of the optical fiber.The uniformity of the diameter of the optical fiber depends on the optical fiber drawing device (
・Ru. Conventionally, the ultra-low loss optical fiber described above is drawn in the first step.
As shown in the figure, an optical fiber pararibe ohm 1 (outer diameter D) is inserted into a furnace core tube 3 heated by a heating source 2 at a constant speed Vp, and the lower end of the preform is heated and melted. A method is used in which the optical fiber 9 is made into an optical fiber 9 by pulling it out, attaching it to the drum 6, and rotating the drum 6 at a constant speed Vf using the motor controller 8.The wire diameter d of the optical fiber 9 is The outer diameter (wire diameter) of the optical fiber is detected by the tactile detector 4 from the uniaxial direction (*) of the optical fiber cross section, and is displayed on the wire diameter measuring device 5 (*). If there is a change in the wire diameter, the analog output of the wire diameter measuring device 5 is sent to the control circuit 7, and the output signal of the control circuit is fed back to the motor controller 8 to wind the optical fiber while changing the number of rotations of the drum 6. The wire diameter is controlled by changing the speed Vf.In contrast to the conventional wire drawing method as described above, the present inventors first investigated the problem of wire drawing due to imperfections in the preform structure and external disturbances. Method for suppressing wire diameter fluctuations due to fluctuations in preform melting temperature (Japanese Patent Laid-Open No. 52-65458
) was proposed.

Furthermore, a method for controlling the variation in the wire diameter of an optical fiber caused by the variation in the outer diameter of the preform while suppressing the variation in the wire diameter due to fluctuations in the preform melting temperature (Japanese Patent Laid-Open No. 52
-76044, Japanese Unexamined Patent Publication No. 52113747).
An example of the wire diameter control method described above is shown in Figure 2 a and b, in which gas is forcibly fed into the furnace core tube to maintain the flow distribution in the furnace core tube in a laminar flow state, and the melting temperature of the preform is Suppress fluctuations and prevent wire diameter fluctuations as much as possible (
- This is a method of controlling wire diameter fluctuations caused by fluctuations in the outer diameter of the preform by changing the gas flow rate, thereby reducing the range of wire diameter fluctuations. It has been experimentally confirmed that by using this method, it is possible to control the wire diameter variation of the optical fiber to around ±1% even if the outer diameter fluctuation of the optical fiber hub rib ohm is around ±2%. This method also has the following advantages over the method shown in FIG. In other words, since there is no sudden fluctuation in the melting temperature, there is almost no wire diameter fluctuation that changes with a time constant on the order of seconds (゛. However, when the cross-sectional shape of the preform is elliptical, Celsius has a complex circular structure other than a circle. It has been found that the following problems arise when this preform is delineated.
(1) If a preform with an elliptical structure as shown in Fig. 3a is drawn with the device shown in Fig. 2a, the diameter of the optical fiber will fluctuate as shown in Fig. 3b. It was recorded by a recorder (not shown) connected after the detector 4 (uniaxial measurement).

This wire diameter fluctuation is a large amplitude fluctuation in an extremely short period, and does not correspond at all to the wire diameter fluctuation of the optical fiber estimated from the preform outer diameter fluctuation.The wire diameter fluctuation is caused by another factor. It seemed to be. Therefore, the diameter of the optical fiber was determined by cross-sectional observation using a microscope.
The cross section of the optical fiber is elliptical, but in the length direction of the optical fiber there is no large amplitude fluctuation with a very short period (as shown in Figure 3b).In other words, the results shown in Figure 3b did not correspond to the variation in the diameter of the optical fiber measured from the axis in a certain direction of the cross section of the optical fiber.
It was found that the envelope obtained by connecting the maximum and minimum values of the wire diameter variation partially corresponds to the measurement results of the long and short axes of the optical fiber cross section. Since the cross section of the optical fiber was not a perfect circle but an ellipse, the way the optical fiber 9 was wound around the drum 6 was unstable and the cross section of the optical fiber 9 came into contact with the drum 6. The axis varies irregularly, so at the detector 4 that detects the diameter of the optical fiber, the detector 4 measures the long axis or short axis of the cross section of the optical fiber. It was thought that this was due to a variation in the wire diameter due to irregular measurements, such as measuring an axis somewhere in between.(2)
Similarly to (1), a preform with an elliptical structure as shown in Fig. 4a is prepared using the optical fiber diameter control device shown in Fig. 2a.
・I drew the line.

FIG. 4b shows the results of wire diameter variation showing the results of optical fiber wire diameter control in that case. Compared to FIG. 3b, the variation in wire diameter is clearly suppressed, and the effect of wire diameter control is enhanced. However, as in (1), when we observe the cross section of the optical fiber with a microscope and find the variation in the wire diameter in the length direction of the optical fiber, we find that the fourth
It was found that this partially corresponds to the wire diameter variation characteristic shown in Figure b. That is, as stated in (1), the detector 4
are the long and short axes of an optical fiber with an elliptical cross-section, and (・ is the diameter measured from an axis somewhere in between.) It has been found that the valve opening/closing device for gas flow rate control malfunctions by issuing a control signal such that the gas flow rate becomes thicker when it should be narrower, or vice versa.
The object of the present invention is to solve the problems described in 1) and (2) and provide a method for controlling the diameter of an optical fiber at degrees Celsius, which causes less variation in the diameter of the optical fiber.

In other words, even if the cross-sectional shape of the BRIFORM is an ellipse or a complex shape other than a circle, if the wire is drawn using the wire diameter control method of the present invention, there will be little variation in wire diameter and it will be a perfect circle. It is possible to obtain an optical fiber at a temperature close to 100.degree. It is.

The present invention is characterized in that the output signal of the wire diameter measuring device 5 is input to the control circuit 7 through the low-pass filter 16, and the output signal of the control circuit 7 is fed back to the valve opening/closing device 12 for controlling the gas flow rate. , less wire diameter variation,
The objective is to obtain an optical fiber having a nearly perfect circular shape. The operation shown in FIG. 5 will be explained below. The cross-sectional shape of preform 1 is an ellipse (・ indicates a complex circular structure other than a circle (・
When the optical fiber 9 is wound around the winding drum 6, the wire diameter detector (uniaxial measurement) 4 detects the long axis and short axis of the cross section of the optical fiber 9 because it is in an unstable state (
・Measure irregularly along an axis somewhere in between,
The wire diameter variation characteristics as shown in FIG. 3b are recorded on the recorder 15. Therefore, in the present invention, the wire diameter fluctuation characteristics as shown in Fig. 3b are input to a low-pass filter, and the high frequency components of the wire diameter fluctuation detected due to the fluctuation of the measurement axis shown in Fig. 3b are extracted. The attenuated and slowly fluctuating low frequency wire diameter variation is input to the control circuit 7. Then, a voltage (
or current), and if an error signal is generated, it is amplified and used as a control output signal. This output signal drives a gas flow rate control valve opening/closing device to control the flow rate of gas sent into the furnace core tube 3, thereby making the error signal of the control circuit 7 close to zero. Here, the cutoff frequency of the low-pass filter is determined by the wire diameter detector 4.
For example, it is necessary to select a frequency lower than the frequency determined by the time of transition from the long axis measurement to the short axis measurement of the cross section of the optical fiber 9 (that is, the transition time of the measurement axis of the cross section of the optical fiber). The output signal of the low-pass filter is designed to obtain a signal corresponding to the variation in the diameter of the optical fiber caused by variation in the outer diameter of the preform 1 (such as scratches, bubbles on the surface of the optical fiber, displacement of the measurement axis, etc.). (This does not include malfunctions of the optical fiber diameter detector due to Below, the fifth
We will discuss the experimental results when using the apparatus shown in the figure.

Figure 6a shows the ℃・light fur hub rib ohm 1 used in this experiment.
This is the outer diameter variation characteristic of Figure 7 shows the frequency characteristics of the low-pass filter 16 used in this experiment. Figure 6b shows the measurement results of the optical fiber diameter variation characteristics. The characteristics are clearly better than the results shown in Figures b and 4b, indicating that the wire diameter control method of the present invention is effective at °C. Although it is elliptical, the cross-sectional shape of the optical fiber drawn by the method of the present invention is almost a perfect circle (
・When it comes to cross-sectional shape, the most significant effect was obtained at ℃・U.
Although a clear explanation for this experimental fact cannot be provided at present, the inventors believe that it can be explained by the following interpretation.

That is, in the conventional method, the molten part of the preform is pulled in the axial direction and deformed into an elliptical cone shape to become an optical fiber with an elliptical cross section, but in the method of the present invention, gas is As the preform melts, it is equivalently deformed in the axial direction while external pressure is applied to the melted part of the preform to form an optical fiber.Furthermore, the external pressure fluctuates irregularly over time, resulting in an elliptical cross section. It is assumed that the shape of the preform melted part gradually deforms into a stable circular cross-sectional preform melted part and is pulled to become an optical fiber with a nearly perfect circular shape. It is another embodiment of the optical fiber wire diameter control method of the invention.

This is done by allowing a constant flow of gas to flow through the furnace core tube 3, and feeding back the signal detected by the wire diameter detector 4 to the motor controller 8 through the low-pass filter 16 and control circuit 7. This is a control method that reduces variations in the diameter of the optical fiber while adjusting the winding speed. It is clear that the wire diameter control method of the present invention can also be applied to preforms with circular, square, and polygonal structures.

Furthermore, the wire diameter control method of the present invention is not limited to the above-mentioned embodiments.
JP 52113747, JP 52-119949,
JP-A-52-129532, JP-A-52-129533
) can also be applied by inserting a low-pass filter between the wire diameter measuring device 5 and the control circuit 7. As mentioned above, when the cross-sectional shape of the preform is an ellipse or a complex circular structure other than a circle, the cross-sectional shape of the drawn optical fiber is also close to the cross-sectional shape of the preform. Depending on the surface condition of the drum when the optical fiber is wound onto the drum, the optical fiber is wound while rotating in the radial direction.

The rotation in the radial direction is random, and the diameter detection section of the optical fiber may measure the long axis of the optical fiber cross section, the short axis, or some axis in between. The optical fiber diameter detector has a fast response, so it can respond to slight scratches, bubbles, etc. on the optical fiber surface in the longitudinal direction of the optical fiber and output a signal. Against this background, the response speed of the optical fiber wire diameter detector and the control response speed of the wire diameter control device shown in Figure 2 a and b are extremely fast. For wire diameter detection, insert a low-pass filter to measure the long and short axes of the cross-section of the optical fiber, and irregularly measure the axis somewhere in between. easily respond to the output signal of the device.

It also responds to extremely rapid (impulse-like) fluctuations such as scratches and bubbles on the surface of the optical fiber, and it issues a control signal to make the wire diameter thinner when it should be made thicker, and conversely, when it should be made thinner. It was found that incorrect control was carried out by issuing a control signal to increase the thickness of the optical fiber.As a result, the fluctuation in the diameter of the optical fiber was increased, and the data on the measured wire diameter fluctuation characteristics became unreliable. To solve this problem, the present invention inserts a low-pass filter between the wire diameter detector and the control circuit to give the control system low-pass filter characteristics, thereby making it possible to measure the cross section of the optical fiber. It is designed to respond only to true optical fiber diameter fluctuations by attenuating the high frequency components of fluctuations due to shaft fluctuations, scratches on the optical fiber surface, and bubbles, and slowly fluctuating.

As a result, we have achieved a degree Celsius optical fiber with little variation in wire diameter and a nearly perfect circle. Here, the cutoff frequency of the low-pass filter is determined from the time when the wire diameter detector moves from the long axis of the cross section of the optical fiber to the short axis measurement (i.e., the time of transition of the measurement axis of the cross section of the optical fiber). Since the response frequency is selected to be lower than that and closer to the response frequency caused by extremely rapid C. impulse fluctuations such as scratches and bubbles on the surface of the optical fiber, the effects of the present invention as described above can be obtained. In the example, since the drawing speed was approximately several meters per minute, the cutoff frequency was selected to be 43 Hz as shown in FIG. The above effect can be obtained without inserting a low-pass filter (Fig. 3b)
, FIG. 4b) and the optical fiber diameter variation characteristic when a low-pass filter is inserted (FIG. 6b).

As described above, the wire diameter control device of the present invention has the effect of making it possible to realize an optical fiber with small wire diameter fluctuations and a cross-sectional shape close to a perfect circle, and to connect optical fibers together. It has great effects in terms of ease of processing, improved stress resistance, and better reproducibility of band characteristics.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a schematic diagram of an optical fiber drawing device using the conventional preform method, FIGS. 2 a and b are schematic diagrams of an optical fiber diameter control device previously proposed by the present inventor, and FIG. Figure 3a, 4th
Figure a, Figure 6a are diagrams showing the outer diameter variation characteristics of the optical fiber hub rib ohm used by the present inventor for experiments, Figures 3b and 4.
Figure b is a diagram showing the optical fiber diameter variation characteristics obtained through experiments with the drawing device previously proposed by the present inventor, and Figures 5 and 8 are implementations of the optical fiber diameter control method of the present invention. A schematic diagram of an example, FIG. 6b is a diagram showing an example of the diameter variation characteristics of an optical fiber obtained using the apparatus of FIG. 5, and FIG. FIG. 3 is a diagram showing frequency characteristics of a low-pass filter.

Claims (1)

[Claims] 1. An optical fiber material is fed into a heating source at a predetermined speed, and while gas is forcibly fed between the optical fiber material and the heating source, the heated and melted material is drawn out and wound around a drum, and the drum is When drawing an optical fiber at a predetermined speed by rotating it at a predetermined speed, the wire diameter of the optical fiber is detected and the above-mentioned A method for controlling the diameter of an optical fiber, the method comprising controlling at least one of the amount of gas fed and the drawing speed. 2. The method for controlling the diameter of an optical fiber according to claim 1, wherein the cutoff frequency of the control system including the circuit having the low-pass filter characteristic is selected to be lower than the commercial frequency.