JP2723301B2 - Optical fiber manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention manufactures an optical fiber having excellent hydrogen resistance by spinning while monitoring the dielectric constant of a carbon coating formed on the surface of an optical fiber. About the method.

[Prior art and its problems] When a silica-based optical fiber comes into contact with hydrogen, absorption loss due to molecular vibration of hydrogen molecules diffused into the fiber increases, and P ₂ O ₅ and Ge contained as dopants further increase.
Since O ₂ , B ₂ O _3, etc. react with hydrogen and are taken into the fiber glass as OH groups, there has been a problem that transmission loss due to absorption of OH groups increases.

To cope with such adverse effects, a method of filling a liquid composition having a hydrogen absorbing ability into an optical cable (Japanese Patent Application No.
No.-2510808) is considered, but the effect is insufficient, and the structure becomes complicated, which is economically problematic.

In order to solve such problems, it has recently been announced that a carbon film can be formed on the surface of an optical fiber by a chemical vapor deposition method (hereinafter abbreviated as a CVD method), thereby improving the hydrogen resistance of the optical fiber. ing. This manufacturing method is a method in which a carbon coating is formed on the bare optical fiber surface by a CVD method, and then a protective coating layer is formed with an ultraviolet curable resin or a thermosetting resin to form an optical fiber.

By the way, it is known that the hydrogen resistance of the carbon coating formed on the surface of the bare optical fiber greatly changes depending on the film quality and thickness. Conventionally, in order to measure the film quality and thickness of the carbon coating formed on the bare optical fiber surface, a method of cutting the optical fiber and observing the cross section with a microscope has been used.

However, in this measuring method, the optical fiber has to be cut, so that not only non-destructive measurement can be performed, but also continuous measurement cannot be performed.

As a method for solving such a problem, the inventors of the present invention measure the electric resistance of the carbon coating along the length direction of the optical fiber, and control the formation conditions of the carbon coating based on the measured value. Proposed.

However, this method of measuring the electrical resistance value has been proposed from the viewpoint of nondestructively and continuously controlling the quality of an optical fiber. There is room for improvement in that the direct contact between the fiber and the measurement electrode cannot be avoided, and that the strength of the optical fiber may be reduced, and that measurement errors may occur due to contamination of the measurement electrode.

The present invention has been made in order to solve the above-mentioned problems, and it is intended to obtain an optical fiber having a certain hydrogen resistance, and to further measure the quality of a carbon coating formed on the optical fiber in a non-contact and continuous manner. It is an object of the present invention to provide a method for manufacturing an optical fiber.

[Means for Solving the Problems] In the present invention, a carbon coating is formed on the surface of a bare optical fiber, and an electromagnetic wave is irradiated while passing this through a waveguide. The means for solving the problem was to measure the dielectric constant of the carbon film and control the conditions for forming the carbon film based on the measured value.

[Operation] Both the reflection characteristic and the transmission characteristic of the electromagnetic wave in the waveguide are
Since it varies depending on the substance present in the waveguide, the base material with the carbon coating formed on the surface is passed vertically through the waveguide, and the change in the reflection or transmission characteristics of electromagnetic waves is investigated. The quality of the film can be measured.

Further, since the state of the carbon film can be known by measuring the dielectric constant of the carbon film, the conditions for forming the carbon film are controlled based on the measured value of the dielectric constant.

 Hereinafter, the present invention will be described in detail.

FIG. 1 shows an example of a manufacturing apparatus suitably used in the method for manufacturing an optical fiber of the present invention.

In FIG. 1, reference numeral 1 denotes a bare optical fiber. The bare optical fiber 1 is obtained by melt-spinning an optical fiber preform 2 in a spinning device 3, and a carbon coating is formed on the surface thereof in a CVD reactor 4 provided below the spinning device 3. It is supposed to be. The CVD reactor 4 includes a reaction tube 5 that forms a carbon coating on the surface of the bare optical fiber 1,
And a heating element 6 for heating the reaction tube 5. Further, at the lower stage of the CVD reactor 4, there is provided an evaluation device 7 for measuring the dielectric constant of the carbon coating formed on the surface of the bare optical fiber 1 and evaluating the quality of the carbon coating from the measured value. The upper CVD reactor 4 and the evaluation device 7 are connected via a controller 8, and the results obtained by the evaluation device are fed back to the CVD reactor 4.

The evaluation device 7 is a device suitably used in the method of manufacturing an optical fiber according to the present invention.
This apparatus measures the dielectric constant of the carbon coating formed on the surface of the bare optical fiber 1 in the CVD reactor 4, and evaluates the film quality of the carbon coating and, consequently, the hydrogen resistance property from the value of the dielectric constant. As shown in FIGS. 2 and 3, for example, the evaluation device 7 is used as an electromagnetic wave transmission device, and includes an oscillation device 9 and a predetermined wavelength oscillated from the oscillation device 9. A waveguide 10 that guides an electromagnetic wave and irradiates it with a carbon coating formed on the surface of the bare optical fiber 1, detects an electromagnetic wave guided through the waveguide 10, and obtains light from the detected value. Consisting of a detector 11 for measuring the dielectric constant of a carbon coating formed on the surface of the bare fiber 1, and coaxial cables 12, 12 connecting both ends of a waveguide 10 to the oscillator 9 and the detector 11. It is. An optical fiber insertion hole 13 is formed in the waveguide 10. The optical fiber insertion hole 13 is a through hole formed in the waveguide 10 so that the running direction of the bare optical fiber 1 inserted in the center thereof is perpendicular to the direction of electromagnetic wave propagation. An optical fiber bare wire 1 on which a carbon coating is formed is inserted through the center.

Further, a resin liquid application device 14 and a curing device 15 are continuously provided at the lower stage of the evaluation device 7 so that a protective coating layer can be formed on the optical fiber on which the carbon coating is formed, if necessary. Has become.

The following steps are used to manufacture an optical fiber having a carbon coating formed on the surface using such a manufacturing apparatus.

First, an optical fiber preform 2 is prepared, installed in a spinning device 3, and melt-spun to obtain an optical fiber bare wire 1. Next, the bare optical fiber 1 is inserted into the CVD reactor 4 to form a carbon coating on the surface of the bare optical fiber 1 by a CVD reaction. Next, the hydrogen resistance of the carbon film is evaluated by the evaluation device 7.

In order to evaluate the hydrogen resistance characteristics of the carbon coating with the evaluation device 7, first, an electromagnetic wave having a predetermined wavelength is oscillated from the oscillation device 9,
The light is guided into the waveguide 10 by the coaxial cable 12. The bare optical fiber 1 is inserted into the optical fiber insertion hole 13 of the waveguide 10 so as to be perpendicular to the direction in which the electromagnetic wave oscillated from the oscillation device 9 is guided. The carbon film formed on one surface can be irradiated with electromagnetic waves. The electromagnetic wave applied to the bare optical fiber 1 is
Further, the light is guided through the waveguide 10 and detected by the detector 11. At this time, by comparing the oscillation intensity of the electromagnetic wave oscillated by the oscillating device 9 with the intensity of the electromagnetic wave detected by the detector 11, the bare optical fiber 1 inserted through the waveguide 10 is obtained.
The dielectric constant of the carbon film formed on the surface can be measured. At this time, the electromagnetic waves used in the evaluation device 7 include:
For example, a microwave having a frequency of 5 to 20 GHz is suitable.

In general, there are two types of carbon crystal forms, a diamond type and a graphite type. The carbon coating formed on the surface of the bare optical fiber 1 by the CVD method is usually amorphous carbon. This is a state in which a crystal structure is mixed. The hydrogen-resistant property also changes greatly depending on the mixing ratio of the two crystal structures.

By the way, since the conductivity of the carbon coating is caused by the orbital of π electrons existing in the z-axis direction of the graphite crystal structure, by measuring the electric resistance value, the graphite crystal structure is mixed in the carbon coating. You can know the ratio. In addition, by irradiating electromagnetic waves together with this, the dielectric constant of the carbon coating can be measured, and from the measured value of the dielectric constant, the mixture ratio of the graphite type crystal structure in the carbon coating can be predicted.

Therefore, an optical fiber exhibiting good hydrogen resistance is inserted into the evaluation device 7 in advance to examine the relationship between the dielectric constant and the hydrogen resistance, and the value of the dielectric constant measured in the evaluation device 7 is determined. As a control signal into the CVD reactor 4, and based on the control signal, for example, a heating element 6 for heating the reaction tube 5.
The heating temperature is changed, or the concentration of the raw material gas supplied into the reaction tube 5 is changed to control the carbon film having a certain hydrogen resistance. By doing so, it is possible to easily obtain an optical fiber on which a carbon coating having a certain hydrogen resistance is formed.

Then, the optical fiber on which the carbon film is formed is inserted into the resin liquid application device 14, and after applying an ultraviolet curable resin liquid or the like,
By curing the resin liquid in the curing device 15, a carbon coating and a protective coating layer are formed, so that an optical fiber having excellent hydrogen resistance and mechanical strength can be obtained.

The measurement of the dielectric constant of the carbon coating formed on the surface of the bare optical fiber 1 is based on, for example, the transmission characteristics of electromagnetic waves of the carbon coating.
An alternative measurement can be made by measuring S ₁₁ (ω) and the reflection characteristic S ₂₁ (ω) of the electromagnetic wave.

That is, the transmission characteristic S ₁₁ (ω) and the reflection characteristic S
_This is because the relationship of the following equations (1) to (4) is established between ₂₁ (ω) and the relative permittivity ε.

In the above equations (1) to (4), Γ: the reflection coefficient when the length of the material placed in the waveguide 10 is infinite, and T: the material placed in the waveguide 10 Finite case transmission coefficient, μr: relative magnetic permeability, εr: relative permittivity, λ ₀ : wavelength of electromagnetic wave in free space, λc: cut-off wavelength of waveguide 10, Λ: reflection coefficient of material in waveguide, Respectively.

According to the manufacturing method as described above, the hydrogen resistance of the carbon coating can be continuously measured nondestructively, so that quality control of the manufactured optical fiber is very simple.

Further, in the manufacturing method of the present invention, the measurement of the dielectric constant, which is a control signal for the CVD reactor 4, can be performed without contact with the bare optical fiber 1, so that it is not necessary to contaminate the surface of the manufactured optical fiber. In addition, since there is no possibility that the surface of the optical fiber is damaged, an optical fiber having higher mechanical strength can be obtained.

In addition, since the quality of the carbon film can be evaluated by a non-contact method, there is no need to worry about a decrease in measurement accuracy due to contamination of the measurement electrode, which has become a problem when measuring the electric resistance value. Can be enhanced.

Example An optical fiber preform was heated by a spinning device to spin an optical fiber bare wire. Next, the bare optical fiber is supplied into a CVD reactor in which the raw material supply rate and the heating temperature are variously varied, and a carbon coating is formed on the surface thereof by a CVD method to obtain a fiber 1.
And fiber 2 were obtained.

The two kinds of fibers having different carbon coating formation conditions are run in the same evaluation apparatus as shown in FIGS. 2 and 3, and irradiated with microwaves at a frequency of 10 GHz to obtain a transmission characteristic S of the carbon coating. ₁₁ (ω) and S ₂₁ (ω) were measured. Then, these two types of optical fibers were heated at 80 ° C. and 1
The amount of increase in transmission loss at a wavelength of 1.24 μm after being left in a hydrogen atmosphere at atmospheric pressure was measured, and the hydrogen resistance characteristics of the carbon coating were examined. Further, the surface resistance values of these two types of optical fibers were measured by a four-terminal method, and the relationship between them was examined. The results are shown in Table 1.

Note that the fiber 1 has extremely high hydrogen resistance, and no increase in transmission loss was observed during the 48-hour hydrogen exposure test.
After leaving for a week, there was no increase in transmission loss.

Table 1 shows that the carbon film formed on the surface of the optical fiber was irradiated with electromagnetic waves, and the transmission characteristics and reflection characteristics of the electromagnetic waves were measured. From the measured values, it was found that the hydrogen resistance characteristics of the carbon films could be evaluated.

Therefore, the dielectric constant of the carbon coating, that is, the measured value of the transmission or reflection characteristics is sent to the CVD reactor as a control signal,
Thus, by controlling the conditions for forming the carbon film in the CVD reactor, a carbon film exhibiting good hydrogen resistance can be formed.

[Effects of the Invention] As described above, according to the method for manufacturing an optical fiber of the present invention, a carbon coating is formed on the surface of a bare optical fiber, and an electromagnetic wave is irradiated while passing the carbon coating through a waveguide. Since the dielectric constant of the carbon coating is measured from the change in the characteristics or the transmission characteristics, and the formation conditions of the carbon coating are controlled based on the measured values, a uniform carbon coating showing constant hydrogen resistance can be formed at all times. As a result, the quality control of the optical fiber can be easily performed.

Also, since the dielectric constant of the carbon coating is measured from changes in the reflection or transmission characteristics of electromagnetic waves, it can be measured continuously without contact, without damaging the carbon coating and reducing the strength of the optical fiber. There is no danger of cause.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an embodiment of a manufacturing apparatus suitably used in the method for manufacturing an optical fiber according to the present invention, and FIG. 2 is a diagram showing an evaluation apparatus preferably used in the method for manufacturing an optical fiber according to the present invention. FIG. 3 is a schematic configuration diagram showing one embodiment, and FIG.
It is a schematic sectional drawing of the waveguide suitably used for the evaluation apparatus shown in the figure. 1 ... bare optical fiber, 4 ... CVD reactor, 7 ... evaluation device.

Claims (1)

(57) [Claims] (1) forming a carbon coating on a bare optical fiber surface;
By irradiating an electromagnetic wave while passing this through a waveguide, measuring the dielectric constant of the carbon coating from a change in the reflection characteristic or transmission characteristic of the electromagnetic wave, and controlling the formation condition of the carbon coating based on the measured value. A method for producing an optical fiber, comprising: