JPH03131550A - Production of optical fiber coated with conductive thin film and device therefor - Google Patents

Abstract

PURPOSE:To enable uniform thin film coating over total length of an optical fiber by passing an optical fiber coated with an electrically conductive thin film through a high-frequency magnetic field and continuously measuring overcurrent generated in the conductive thin film without contact. CONSTITUTION:In the production of an electrically conductive thin film-coated optical fiber 17 by coating an optical fiber 16 drawn from preform 15 with a conductive thin film 171, the optical fiber 17 applied by the conductive thin film (e.g. in heat CVD furnace 3) is passed through a high-frequency magnetic field (generated in a hollow detection coil 7) and overcurrent generated in the conductive thin film 171 is measured in non-contact state to determine thickness of conductive thin film 171. A film thickness measuring device is provided with the detection coil 7 connected to high-frequency electric power 8 and constituted so as to determine film thickness of the conductive thin film 171 based on the change of alternative current impedance produced when the optical fiber is passed through hollow part of the detection coil 7.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a method for manufacturing an optical fiber coated with a conductive thin film;
This invention relates to a manufacturing device using this.

[Conventional technology]

Bare wires of silica-based optical fibers tend to adsorb moisture and H2, which cause deterioration in the strength of the optical fiber and increase in optical transmission loss. Conventionally, optical fibers have been precoated with UV-curable acrylate resin or the like immediately after being drawn. However, these organic polymer films alone lack airtightness, so today the precoat is divided into two or more layers, first coated with an inorganic thin film such as aluminum, and then coated with a resin. Various types of inorganic thin films have been studied, but recently amorphous carbon has attracted attention due to its excellent airtightness (for example, "0pt1cal FibreCoavunl").
cations Conference 1989”
Paper by C. M. G. Jochem et al. at the 1989 Annual Meeting of the International Society of Optical Fiber Communications).

An inorganic thin film coating on the surface of an optical fiber is often performed, for example, by chemical vapor deposition (so-called CVD) immediately after the fiber is drawn from a preform (base material). The film produced by this method is extremely thin, several hundred angstroms (A), and the film thickness and film quality are determined by the linear velocity,
It fluctuates under the influence of many factors, such as processing temperature, reaction gas concentration, pressure, and flow rate. For this reason, it is not easy to guarantee a thin film coating with uniform thickness and quality without film breakage over the entire length of the fiber, which is several tens of km, at a high spinning speed of 5 to 10 m/sec. do not have.

To control the performance of such inorganic thin films,
It would be extremely desirable to be able to continuously measure the film thickness during the manufacturing process of optical fibers and control each of the above factors based on the results.

[Problem to be solved by the invention]

However, after an optical fiber that has been drawn and coated with an inorganic thin film and a resin is wound onto a drum, a sample is taken, the resin coat is removed, and the film thickness is measured. Therefore, there was a problem to be solved in that the performance of the inorganic thin film could not be guaranteed over the entire length.

In view of the above problems, the present invention enables non-contact and continuous measurement of the thickness of a thin film in an optical fiber.
Accordingly, it is an object of the present invention to provide an optical fiber manufacturing method and apparatus that can uniformly apply a thin film coating over the entire length of the optical fiber.

[Means to solve the problem]

The present invention focuses on the fact that some of the inorganic thin films used to coat optical fibers have conductivity, and provides an optical fiber manufacturing method that makes it possible to continuously measure the electrical resistance without contact. The purpose is to provide equipment.

That is, the present invention relates to an optical fiber manufacturing method in which an optical fiber drawn from a base material is coated with a conductive thin film. It is characterized by measuring the thickness of a conductive thin film by measuring overcurrent without contact.

Further, the apparatus for manufacturing a conductive thin film coated optical fiber according to the present invention includes a drawing furnace that heats and melts a base material of a target fiber to draw an optical fiber, and a drawing furnace that coats the drawn optical fiber with a conductive thin film. In addition to the coating device, there is also a film thickness a for measuring the film thickness of the conductive thin film of the optical fiber.
In particular, this film thickness measuring device has a hollow detection coil connected to a high-frequency power source, and detects light based on the change in AC impedance that occurs when an optical fiber passes through the hollow part of this coil. It is characterized in that it measures the thickness of a conductive thin film of a fiber.

For example, in addition to the detection coil, this film thickness measuring device is equipped with a reference coil that has similar characteristics (for example, the same structure, the same shape, and the same size) as the detection coil, and this reference coil is left as an air core. The alternating current impedance change may be detected by connecting both coils to one high-frequency power source and detecting the difference in impedance between the two coils using a detection means such as an alternating current bridge.

[Effect]

According to the configuration of the present invention, first, the base material of the optical fiber inserted from the upper part of the drawing furnace is heated and melted in the furnace and drawn, thereby producing a bare optical fiber. This bare optical fiber immediately enters a coating device, and a conductive thin film is formed on its surface. The outer diameter of the optical fiber coated with a conductive thin film is measured using an outer diameter measuring device such as a laser wire diameter measuring device as necessary, and then the thickness of the conductive thin film on the surface is measured using a film thickness measuring device. This provides the information necessary to maintain the outer diameter and film thickness at predetermined values. Note that prior to this film thickness measurement, a resin may be coated and cured, and the thickness of the conductive thin film can also be measured in the same manner.

In the film thickness measuring device described above, when the optical fiber passes through the hollow part of the detection coil, an eddy current is generated in the conductive thin film on the surface of the fiber. This is because the surface of the fiber, which is a conductor, is placed in the magnetic field created by the high-frequency current in the detection coil. At the same time, this eddy current also creates a magnetic field, which in turn acts on the detection coil and changes its apparent AC impedance. Since the strength of this eddy current and the amount of change in AC impedance caused by it include information regarding the thickness of the conductive thin film, it is necessary to perform calibration in advance to determine the relationship between the amount of change in AC impedance of the detection coil and the film thickness. By determining the relationship and actually measuring the amount of change in this AC impedance, the film thickness can be measured.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to FIGS. 1 to 5 of the accompanying drawings. In addition, in the description of the drawings, the same elements are given the same reference numerals, and redundant description will be omitted.

FIG. 1 is a diagram showing the configuration of the main parts of an apparatus for manufacturing an optical fiber coated with a conductive thin film according to a first embodiment of the present invention. As shown in the figure, the core tube 2 of the drawing furnace 1 has an optical fiber base material 1
5 is inserted, and the tip of the base material 15 is heated and melted by a heating device while being drawn, thereby producing a bare tip 6 of the fiber. In this example, the outer diameter of this bare wire is 125 μm.

The bare wire 16 is immediately transferred to a thermal CV, which is an example of a coating device.
The reactant gas enters the reactor core tube 4 of the D furnace 3, is supplied from the reactant gas inlet 5 at the upper part of the reactor core tube 4, is heated by a heating device, and is thermally decomposed, so that a conductive thin film is formed on the surface of the bare wire 16. It is formed. The gas after the reaction is exhausted from the exhaust port 6.

In this example, the reactive gas is methane, and the generated conductive substance is carbon, but the object of the present invention is not limited to this. In other words, the conductive thin film may be made of any material as long as it can form a solid conductive airtight layer on the surface of the tip fiber and does not adversely affect the tip fiber, and the coating device can also be made of any material. You can select the appropriate format according to your needs. For example, in the case of carbon, a simple coating device such as a thermal CVD (Chemical Vapor Deposition with Pyrolysis) furnace is used to create a strong electrical conductivity using easily handled materials such as acetylene in addition to methane as a reaction gas. This is advantageous because an airtight layer can be easily formed.

Note that the carbon mentioned here is amorphous (non-crystalline).
Contains carbon and microcrystalline carbon.

Optical fiber 17 whose surface is coated with a conductive thin film
After exiting the thermal CVD furnace 3, it passes through the detection coil 7. The detection coil 7 is a hollow coil, with an inner diameter of 500 μm in this example, and the center of the coil and the center of the optical fiber are aligned with high precision so that the optical fiber passing through the coil does not come into contact with each other. FIG. 2 shows the dimensional relationship between the detection coil and the tip fiber. As shown, optical fiber 1
7 is formed by coating the bare wire 16 with a conductive thin film 71, which is passed through the hollow part of the detection coil 7. Here, dr in the figure is the outer diameter of the optical fiber 7, d
- is the inner diameter of the detection coil 7.

The reference coil 71 is an air-core coil having the same shape and size as the detection coil 7, and has the same characteristics at least in terms of AC impedance. The detection coil 7 and the reference coil 71 are supplied with a high frequency current from a high frequency power supply 8 with a frequency of 200 MHz in this example, and are connected to an AC bridge 9, and the output of the AC bridge 9 is supplied to the signal processing device 1.
Connected to 0. The elements from the detection coil 7 to the signal processing device 10 described above correspond as a whole to the film thickness aPI determining device in the configuration of the present invention.

After passing through the detection coil 7, the optical fiber 17 has its outer diameter measured by an outer diameter measuring device 11, is coated with resin etc. in a die 12, and is exposed to UV light in a curing furnace 13.
Subjected to curing treatment by irradiation or heating. The optical fiber 18 thus completed is wound onto a drum (not shown) via the capstan 14.

Next, the operation of the apparatus of the first embodiment and the manufacturing method according to the embodiment will be explained.

In FIG. 1, a bare optical fiber 16 drawn in a drawing furnace 1 is heated by a heating device in a core tube 4 of a thermal CVD furnace 3, and a reactive gas comes into contact with its surface and is thermally decomposed, resulting in a bare optical fiber. A conductive substance such as carbon is deposited on the surface of the wire 16 to form a thin film.

When the optical fiber 17 whose surface is coated with the conductive thin film 171 passes through the hollow part of the detection coil 7, an eddy current is generated in the conductive thin film 171. This is because the surface of the fiber, which is a conductor, is placed in the magnetic field created by the high-frequency current in the detection coil 7. At the same time, this eddy current also creates a magnetic field, which in turn acts on the detection coil 7 and changes its apparent AC impedance. On the other hand, the AC impedance of the reference coil 71 does not change because it is an air core, so while observing the difference in AC impedance between the coils 7 and 71 with the AC bridge 9, the AC bridge 9 is adjusted to make the difference zero. By operating it, the amount of change in AC impedance that occurs in the detection coil 7 can be detected as the amount of operation of the AC bridge 9.

The strength of the eddy current described above and the amount of change in AC impedance caused by this include information regarding the film thickness of the conductive thin film, and if the relationship between the amount of change in AC impedance and the film thickness is calibrated in advance. As described above, the film thickness can be measured by actually measuring the amount of change in AC impedance occurring in the detection coil 7 and processing this information in the signal processing device 10. According to this device, both coils 7 and 7]
High 4p1 constant accuracy can be obtained because the external disturbance factors commonly added to the 4p1 are canceled out. Note that the electrical resistance of the conductive thin film used to determine the film thickness 4Ilj provides important clues in evaluating the performance of the film, since it includes information regarding the film quality and the like in addition to the film thickness.

FIG. 3 shows the overall configuration of an apparatus according to a second embodiment of the present invention.

In this embodiment, the vertical positions of the outer diameter measuring device 11 and the detection coil 7 are reversed, and the coil 80 that generates a magnetic field around the fiber and the detection coil 7 are separated.
This is different from the embodiment shown in FIG.

The insertion position of the detection coil 7 is preferably a place where the deflection of the fiber is small so that the coil and the fiber do not come into contact with each other. In that sense, the positions before and after the resin coating die 12 are desirable. In this example, from this point of view, the outer diameter A11
The positions of the i-determiner 11 and the detection coil 7 are reversed to bring the detection coil 7 closer to the die 12. Taking this further, the detection coil 7 and the die 12 may be integrated; in this case, the presence of a conductive substance near the coil will have a significant effect, so the die 12 and its accessories are all made of insulating material. Good.

In addition, in this example, the reason why the coil 80 that generates a magnetic field around the optical fiber and the detection coil 7 are separated is that the excitation energy is supplied to the coil 80, which is different from the detection coil 7. The current flowing can be reduced,
This is because it becomes easier to prevent temperature rise, vibration, etc. of the detection coil 7, and the measurement accuracy improves.

In order to generate a sufficiently strong eddy current under a constant high-frequency current at all film thicknesses in the embodiments shown in FIG. 1 or 3 above, the coil filling factor ν of the detection coil should be made as large as possible (1 close to). The coil filling factor ν is the ratio of the fiber cross-sectional area to the hollow cross-sectional area of the coil, and is defined by the following equation (1).

2...(1) Sea (dr/dc) Here, dr is the outer diameter of the optical fiber, and dc is the inner diameter of the detection coil. If the value of ν is small, a highly sensitive AC bridge or the like is required, and the gain of the amplifier must be increased, which increases the difficulty especially when handling high frequencies as described below. By the way, if the outer diameter of the fiber is 125 μm, the inner diameter of the detection coil is 500 μm.
(0.5mm) or less, therefore, the outer diameter is 250μ
For manufacturing equipment that makes optical fibers up to about 1.5 m long, the practical range of the inner diameter of the detection coil is 1.0 m. It can be said that it is less than mm.

On the other hand, the frequency f of the high frequency power source 8 used for film thickness measurement is determined by the conductivity σ and magnetic permeability μ of the conductive thin film so that the penetration depth δ of the eddy current in the thin film becomes an appropriate value. Here, the penetration depth δ of the eddy current is given by the following equation (2).

δ-1/yr au...(
2) It is desirable that the above δ is sufficiently larger than the thickness of the conductive thin film of the optical fiber and smaller than the outer diameter of the fiber. When calculating by inserting the constant of the carbon film into equation (2) and assuming the fiber outer diameter as 100 μm, f becomes 250 MHz or more, which is compared to 440 KHz for a copper wire of the same thickness.
The difference is clear when compared with By the way, in the case of a carbon film, a fiber outer diameter of 80 μm has a power of 390 MH.
z or more and an outer diameter of 200 μm is 60MH7 or more.

From this, it can be said that in the case of carbon films, the practical range of this frequency is 10 MHz or more.

Note that the outer diameter of the optical fiber has a large influence on the results of the film thickness 1111J described above. Therefore, one prerequisite for accurately measuring the film thickness is that the fiber outer diameter is controlled within a predetermined dimensional difference.

4 and 5 show a manufacturing apparatus according to a third embodiment.

As shown in FIG. 4, the film thickness measuring device including the detection coil 7 and the like is provided downstream of the capstan 14, that is, downstream of the resin coating device including the resin coating die 12 and the curing furnace 13. This prevents inconveniences such as the optical fiber coming into contact with a measurement coil or the like and resulting in a decrease in tensile strength. Further, in this embodiment, as shown in FIG. 5, the optical fiber 18 having a resin coating 181 on the conductive film thickness 171 is passed through a cylindrical guide vibe 20 made of Teflon or the like. A detection coil 7 is wound around the outside of the vibrator 20. The inner surface of the guide vibrator 20 is finished smoothly, and the entrance portion of the optical fiber 18 is shaped so that the inner diameter of the guide vibrator 20 is wide. Therefore, even if the optical fiber 18 comes into contact with the inner surface of the guide vibe 20 due to wire wobbling, the bare optical fiber 16 and the conductive thin film 171 are prevented from being seriously damaged.

The information regarding the film thickness obtained as described above is fed back to adjust manufacturing conditions as described below.

First, the flow rate of the reaction gas flowing into the furnace core tube 4 is adjusted to 1'J3. If the thickness of the conductive thin film is too small, the thickness can be increased by increasing the flow rate of the reaction gas, and conversely, if the film thickness is too large, the thickness can be made smaller by decreasing the flow rate of the reaction gas. can. The same applies to the concentration of the reaction gas; a high concentration allows the conductive thin film to be made thick, and a low 4° makes the conductive thin film thin.

On the other hand, the thickness of the conductive thin film can also be adjusted by the temperature of the bare wire 16 in the furnace tube 4, and generally, the higher the temperature, the thicker the conductive thin film. Such temperature adjustment of the bare wire 16 can be easily achieved by moving the furnace tube 4 closer to or farther from the base material 15.

〔Effect of the invention〕

According to the configuration of the present invention, changes in the film thickness of an optical fiber coated with a thin film of a conductive substance such as carbon can be continuously measured as changes in electrical resistance, and this film thickness measurement can be performed completely. Because it is done in a non-contact manner,
Will not damage the surface of the optical fiber. In this way, film thickness can be continuously monitored during the production of optical fibers, making it possible to quickly determine the production conditions for the conductive thin film and, if necessary, controlling the thin film coating equipment in real time. Thus, good film performance can be guaranteed over the entire length of the manufactured optical fiber. Moreover, as a result, the step of inspecting membrane performance can be omitted.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing a manufacturing apparatus for a conductive thin film coated optical fiber according to the first embodiment of the present invention, FIG. 2 is a partially enlarged view of FIG. 1, and FIG. 3 is a conductive thin film-coated optical fiber manufacturing apparatus according to the second embodiment FIG. 4 is an explanatory diagram showing an apparatus for manufacturing a conductive thin film coated optical fiber according to the third embodiment, and FIG. 5 is a diagram showing the main parts of FIG. 4. be. DESCRIPTION OF SYMBOLS 1... Wire drawing furnace, 3... Thermal CVD furnace, 7... Detection coil, 8... High frequency power supply, 9... AC bridge, 1
0... Signal processing device, 1 Co... Fiber outer diameter measuring device, 12... Resin coating die, 16...
Bare optical fiber, 17... Optical fiber coated with a conductive thin film, 171... Conductive thin film, dr.
...outer diameter of optical fiber, do...inner diameter of detection coil,
18...Resin-coated tip fiber.

Claims (1)

[Claims] 1. A method for manufacturing an optical fiber coated with a conductive thin film, in which an optical fiber drawn from a base material is coated with a conductive thin film, the optical fiber coated with the conductive thin film being exposed to a high frequency magnetic field. 1. A method of manufacturing a conductive thin film-coated optical fiber, the method comprising measuring the thickness of the conductive thin film by passing the conductive thin film through the conductive film and measuring the eddy current generated in the conductive thin film in a non-contact manner. 2. The conductivity according to claim 1, wherein after coating the conductive thin film, the conductive thin film is further coated with a resin and cured, and then passed through the high frequency magnetic field to measure the thickness of the conductive thin film. A method for manufacturing thin-film coated optical fiber. 3. A drawing furnace that heats and melts the base material to draw an optical fiber, a coating device that coats the drawn optical fiber with a conductive thin film, and a film thickness that measures the thickness of the coated conductive thin film. a measuring device, the film thickness measuring device having a hollow detection coil connected to a high frequency power source, and a change in AC impedance of the detection coil that occurs when the optical fiber passes through the hollow part of the detection coil. An apparatus for manufacturing an optical fiber coated with a conductive thin film, characterized in that the thickness of the conductive thin film is measured based on the following. 4. The film thickness measuring device detects a reference coil that has substantially the same characteristics as the detection coil and is connected to the high frequency power source with an air core, and a difference in AC impedance between the detection coil and the reference coil. 4. The apparatus for manufacturing a conductive thin film-coated optical fiber according to claim 3, further comprising a detection means for detecting a change in the AC impedance in the detection coil by detecting a change in the AC impedance in the detection coil. 5. The apparatus for manufacturing a conductive thin film coated optical fiber according to claim 3, wherein the film thickness measuring device is disposed downstream of a resin coating device that coats a resin on the conductive thin film. .