JPH04119946A - Method for monitoring carbon film of optical fiber - Google Patents

Abstract

PURPOSE:To continuously measure the quality of the carbon film formed on a bare optical fiber without contact by measuring the film quality of the carbon film from a change in the dielectric loss of the resonance frequency region of the optical fiber 1 on which the carbon film is formed. CONSTITUTION:The bare fiber of the optical fiber on which the carbon film is formed is inserted into a coil in which a high-frequency current flows and the quality of the above-mentioned carbon film is measured from the change in the dielectric loss of the resonance frequency region. The reasons thereof are as follows: Since the electric conductivity of the carbon film occurs in the orbital of the pi electrons existing in the Z-axial direction of a graphite type crystal structure, the electrical conductivity in the horizontal direction of the crystal is better as the degree of crystallization of the graphite is larger and the orientability of the two-dimensional graphite crystal is larger. Since the electric conductivity of the amorphous carbon film occurs in the orbital of the pi electrons, the change in electric resistance appears as a change in the dielectric constant as well and the orientability of the two-dimensional graphite crystal appears in the form of the dielectric loss.

Description

Detailed Description of the Invention "Industrial Application Field" This invention is a carbon coating method for continuously measuring the quality of the carbon coating in a non-contact manner, such as when manufacturing optical fibers with a carbon coating formed on the surface. Regarding monitoring methods.

"Conventional Technology J When a silica-based optical fiber comes into contact with hydrogen, absorption loss due to molecular vibration of hydrogen molecules diffused within the fiber increases, and furthermore, P to s contained as a dopant increases.
, G eo IB tOs, etc., react with hydrogen and are incorporated into the fiber glass as OH groups, so there is a problem that transmission loss due to absorption of OH groups also increases.

In order to deal with such adverse effects, a method of filling an optical cable with a liquid composition having hydrogen absorption ability (Japanese Patent Application No. 1983) was proposed.
-2510808), but the effect is insufficient and the structure is complicated, resulting in economical problems.

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

By the way, it is known that the carbon wave and hydrogen resistance properties of a film formed on the surface of a bare optical fiber vary greatly depending on the quality and thickness of the film. Conventionally, in order to measure the quality and thickness of a carbon film formed on the surface of a bare optical fiber, a method has been used in which an optical fiber is cut and its cross section is observed using a microscope.

However, with this measurement method, the optical fiber has to be cut, which not only makes it impossible to perform non-destructive measurements, but also makes it impossible to perform continuous measurements.

As a method to solve such problems, the present inventors measured the electrical resistance value of the carbon film along the length of the optical fiber, and controlled the formation conditions of the carbon film based on this measured value. is proposed.

``Problem to be Solved by the Invention'' However, this method of measuring electrical resistance was proposed from the viewpoint of non-destructive and continuous quality control of optical fibers. Although the problem has been solved in some respects, direct contact between the optical fiber and the measurement electrode cannot be avoided, which may cause a decrease in the strength of the optical fiber, and measurement errors may occur due to dirt on the measurement electrode. There was room for improvement in these areas.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a carbon coating monitoring method for continuously measuring the quality of a carbon coating formed on a bare optical fiber in a non-contact manner.

"Means for Solving the Problems" In order to solve the problems described above, the present invention provides a method for inserting a bare optical fiber having a carbon coating on its surface into a coil through which a high-frequency current flows, thereby reducing the dielectric loss in the resonant frequency region. This is a method for monitoring a carbon coating of an optical fiber, characterized in that the quality of the carbon coating is measured from changes in the carbon coating.

"Operation" A bare optical fiber with a carbon coating formed on its surface is inserted into a coil through which a high-frequency current is flowing, and the quality of the carbon coating is measured from the change in dielectric loss in the resonance frequency region.

Furthermore, by controlling the conditions for forming the carbon film based on the measured values, it is possible to manufacture an optical fiber having a uniform carbon film formed thereon.

Embodiment FIG. 1 shows an example of a manufacturing apparatus suitably used for manufacturing optical fibers.

In FIG. 1, reference numeral I indicates a bare optical fiber. This bare optical fiber 1 is obtained by melt-spinning an optical fiber base material 2 in a spinning device 3, and a carbon film is formed on its surface in a CVD reactor 4 provided at the bottom of the spinning device 3. It is now possible to do so.

This CVD reactor 4 consists of a reaction tube 5 that forms a carbon film on the surface of the bare optical fiber 1 inside, and a heating element 6 that heats the reaction tube 5.

Also, in the lower stage of this CVD reactor 4, there is a bare optical fiber 1.
An evaluation device 7 is provided that measures the dielectric loss of the carbon film formed on the surface and evaluates the film quality of the carbon film based on the measured value. The results obtained by the evaluation device 7 are fed back to the CVD reactor 4.

The evaluation device 7 according to the present invention, for example, as shown in FIG. 2, has a coil 9 surrounded by an electromagnetic nozzle 10 through which a bare optical fiber 11 coated with a carbon film can be inserted.

An LCR meter 16 is connected to the coil 9 for flowing a high frequency current and measuring dielectric loss.

Furthermore, a resin liquid coating device 14 and a curing device fli 15 are successively provided at the lower stage of the evaluation device 7, so that a protective coating layer can be formed as necessary on the optical fiber on which the carbon coating has been formed. It has become.

In order to manufacture an optical fiber having a carbon film formed on its surface using such a manufacturing apparatus, the following steps are performed.

First, an optical fiber preform 2 is prepared, placed in a spinning device 3, and melt-spun into a bare optical fiber 1. Next, this bare optical fiber 1 is inserted into a CVD reactor 4, and a carbon film is formed on the surface of the bare optical fiber 1 by a CVD reaction. Next, the hydrogen resistance properties of this carbon film are evaluated using an evaluation device 7.

In general, there are two types of carbon crystal structures: diamond type and graphite type, but the carbon film formed on the surface of bare optical fiber 1 by the CVD method is usually amorphous carbon, and the diamond type crystal structure and graphite type This is a state in which crystal structures are mixed. The hydrogen resistance properties also vary greatly depending on the mixing ratio of the above two types of crystal structures.

By the way, the conductivity of the carbon film is due to the orbital of π electrons existing in the biaxial direction of the graphite crystal structure, so the higher the crystallinity of graphite and the higher the orientation of the two-dimensional graphite crystal, the higher the The conductivity in the horizontal direction of the crystal is improved. Since the conductivity of the amorphous carbon film is caused by the orbital π electrons, changes in electrical resistance also appear as changes in dielectric constant, and orientation of two-dimensional graphite crystals appears in the form of dielectric loss.

Therefore, by inserting a bare optical fiber having a carbon coating on its surface into a coil through which a high-frequency current is flowing, the quality of the carbon coating can be measured from the change in dielectric loss in the resonant frequency region.

When actually manufacturing an optical fiber, an optical fiber that exhibits good hydrogen resistance characteristics is inserted into the evaluation device 7 in advance, and the relationship between its dielectric loss and hydrogen resistance characteristics is investigated. The value of the dielectric loss measured in is sent to the CVD reactor 4 as a control signal, and based on this control signal, for example, the heating temperature of the heating element 6 that heats the reaction tube 5 is changed, The concentration of the raw material gas is controlled so as to form a carbon film exhibiting certain hydrogen resistance properties. In this way, it is possible to easily obtain an optical fiber having a carbon coating that exhibits certain hydrogen resistance characteristics.

Then, the optical fiber on which the carbon film has been formed is inserted into the resin liquid coating bag rR14, and after applying an ultraviolet curable resin liquid, etc., the resin liquid is cured in the curing device 15 to form a carbon film and a protective coating layer. An optical fiber having excellent hydrogen resistance and mechanical strength can be obtained.

In addition to forming a resin protective film layer after measuring the film quality of the carbon film, it is also possible to measure the film quality of the carbon film layer by inserting an optical fiber into the coil after the resin protective film layer has been formed. be.

Incidentally, as will be described later, FIG. 3 shows the relationship between the frequency and the transmitted power ratio in the resonant frequency region for calculating the load Q, which is a guideline for hydrogen resistance characteristics.

Here, Δf=f(H)-rcL'1 fo=(f(L)+rcH))/2Q(L
)=f,/Δf In addition, f(L) and f(H) are frequencies that are 1/2 of the transmitted power at the minimum insertion loss value, and Q(L) is called load Q.

(Measurement Example) Under the following conditions, the load Q was derived from the measured dielectric loss of an optical fiber actually coated with carbon, and the relationship between the load Q and the hydrogen resistance characteristics is shown in FIG.

Coil ・Conductor diameter φ0.41 Number of turns 70 turns Measurement frequency: 10M to 40MHz Resonance frequency + 25MHz Measurement core wire: Outer diameter 250μm, UV resin double-layer coating treatment: Hydrogen partial pressure: latm, 80℃, 24 hours or more The carbon film monitoring method according to the invention monitors changes in dielectric loss, which directly reflects changes in carbon film quality (crystallinity, crystal orientation), and is therefore extremely sensitive to changes in carbon film quality. .

Furthermore, since the measurement is performed without contacting the carbon coating, accurate measurements are possible without being affected by dirt on the electrodes, etc. Furthermore, there is no need to worry about contaminating the surface of the manufactured optical fiber, and there is no need to worry about scratches on the surface of the optical fiber. Since there is no longer any possibility of this occurring, it becomes possible to obtain an optical fiber with higher mechanical strength.

Furthermore, since the hydrogen resistance properties of the carbon coating can be continuously measured in a non-destructive manner, the quality control of manufactured optical fibers is greatly simplified.

Furthermore, the measured values obtained by the monitoring method of the present invention are sent to the CVD reactor as a control signal, thereby
By controlling the conditions for forming the carbon film in reactor D, it is possible to uniformly form a carbon film that exhibits good hydrogen resistance.

In the example shown in FIG. 1, a bare optical fiber is used as the base material on which the carbon film is formed, but the method of monitoring the carbon film of the present invention is not limited to this.

"Effects of the Invention" As explained above, the present invention measures the film quality of the carbon film by inserting a bare optical fiber with a carbon film formed on the surface into a coil through which a high-frequency current flows. The quality of the carbon coating can be measured in a contact and continuous manner, without damaging the carbon coating, causing no reduction in the strength of the optical fiber, and without causing measurement errors due to dirt on the measurement electrode.

In addition, by controlling the conditions for forming the carbon film based on these measured values, it is possible to form a uniform carbon film that always exhibits constant hydrogen resistance, and it is also possible to easily control the quality of optical fibers. can.

[Brief explanation of drawings]

Fig. 1 is a schematic configuration diagram of an embodiment of an optical fiber manufacturing device, Fig. 2 is a diagram showing an evaluation device, Fig. 3 is a diagram showing the relationship between frequency and transmitted power ratio, and Fig. 4 is a diagram showing the relationship between load Q and FIG. 3 is a diagram showing the relationship between hydrogen resistance characteristics. l... Optical fiber bare wire, 2... Optical fiber base material, 3
... Spinning device, 4... CVD reactor, 5... Reaction tube, 6... Heating element, 7... Evaluation device, 8... Controller, 9... Coil, 10...・Electromagnetic Knoll, 1
1... Optical fiber with carbon coating formed, 14...
Resin coating device, 15... Curing device, ]6... LCR
meter.

Claims (1)

[Claims] A carbon fiber optical fiber characterized in that a bare optical fiber having a carbon film formed on its surface is inserted into a coil through which a high-frequency current flows, and the film quality of the carbon film is measured from changes in dielectric loss in a resonant frequency region. How to monitor coatings.