GB2158054A

GB2158054A - Method for dehydrating optical materials by glow discharge

Info

Publication number: GB2158054A
Application number: GB08507656A
Authority: GB
Inventors: Osamu Shinbori; Yoshinori Mimura; Hideharu Tokiwa
Original assignee: Kokusai Denshin Denwa KK
Current assignee: KDDI Corp
Priority date: 1984-03-27
Filing date: 1985-03-25
Publication date: 1985-11-06
Also published as: FR2562219B1; FR2562219A1; GB2158054B; GB8507656D0; JPS60200844A

Abstract

An optical material dehydrating method by a glow discharge, in which, in an atmosphere of halogen atoms dissociated by the glow discharge, an optical material containing OH radicals is heated by radio-frequency current up to a desired temperature for a chemical reaction of the material, by which the OH radicals are substituted by the halogen atoms, dehydrating the optical material. <IMAGE>

Description

SPECIFICATION Method for dehydrating optical materials by glow discharge The present invention relates to an optical material dehydrating method, and more particularly to a dehydrating method employing a glow discharge.

Optical communications permit wide band transmission and can be made low-loss, and hence have been spotlighted particularly in recent years. As a transmission medium for optical communication, a quartz glass fiber of low loss is mainly used, but since its transmission loss has substantially reached a theoretical limit value, new fiber materials are now being developed. Of such new materials, a fluoride glass fiber is regarded as the most promising fiber of the next generation since its transmission loss is estimated to be about one-hundredth of that of the quartz glass fiber in a 3 to 4 rtm wavelength band.Further, lenses or prisms using crystals of KBr, KCt, CaF2 and so forth which have high light transparency in the infrared ray region in the vicinity of the abovesaid wavelength, are employed as optical parts as of an optical measuring instrument.

It is well-known, however, that if a very small amount of H20 molecules or OH radicals gets mixed as an impurity in such an optical material, light in the near infrared ray region is absorbed by a molecular oscillation of the H20 molecules or OH radicals, markedly degrading the light transparency of the material.

Accordingly, it is of much importance, for constructing an optical communication system or optical part of low loss, how the H20 molecules or OH radicals are removed from the optical material used. The method for chemically removing the H20 molecules or OH radicals from the optical material is commonly referred to as a chemical dehydrating method.

As described below it has been impossible with the prior art to treat low-melting-point optical materials for dehydration without leaving orforming therein any impurities.

The present invention has been made in view of the abovedescribed defects of the prior art, and an object of the invention is to provide an optical material dehydrating method employing a glow discharge which is applicable to optical materials of both low and high melting points and permits their dehydration without leaving therein any impurities.

According to the present invention, an optical mterial dehydrating method by a glow discharge is proposed in which, in an atmosphere of halogen atoms dissociated by the glow discharge, an optical material containing OH radicals is heated by radio-frequency current up to a desired temperature for a chemical reaction of the material, by which the OH radicals are substituted with the halogen atoms, dehydrating the optical material.

Embodiments of the present invention will be described below, by way of example, in comparison with prior art with reference to the accompanying drawings, in which: Figure 1 is a section of a conventional chemical dehydrating apparatus; Figure 2 is a schematic section explanatory of the dehydrating method of the present invention using a flow discharge; and Figure 3 is a section view illustrating an embodiment of the dehydrating apparatus of the present invention using a flow discharge.

To make differences between prior art and the present invention clear, an example of the conventional art will be described first.

Figure lisa diagram schematically showing the arrangement of an apparatus utilizing a conventional chemical dehydrating method. The chemical dehydrating method shown utilizes a replacement reaction of the H20 molecules or OH radicals in a liquid phase with halogens. Reference numeral 1 indicates an inlet port for an inert gas such as He, Ar or the like; 2 designates an inlet port for fluorine, chlorine, bromine or like halogenating agent; 3 identifies a reaction tube; 4 denotes a heater; 5 represents an article to be dehydrated; and 6 shows an exhaust port. The halogenating agent supplied from the inlet port 2 and the inert gas from the inlet port 1 are mixed together and introduced into the reaction tube 3, wherein the mixture is heated by the heater 4 up to a desired temperature.Of the inert gas and the halogenating agent heated up to the desired temperature, such a halogenating agent as fluorine gas (F2) is dissociated by thermal energy from the state of molecules (F2j into the state of atoms (2F) of high reactivity (which state will hereinafter be referred to as the "nascent state" ". The halogen atoms (F) thus dissociated are replaced by OH radicals contained in the article 5 being treated. The thus substituted OH radicals further react to the free halogen atoms (F) into fluoric acid and H2O molecules, which are discharged through the exhaust port 6, along with the inert gas.

As described above, the prior art has employed a method in which the halogenating agent is heated by a heat source such as the heater 4 up to a desired temperature for dissociation into halogen atoms of high reactivity in the nascent state and the halogen atoms are substituted by OH radicals contained in the article 5 under treatment. Accordingly, this chemical dehydrating method requires only the heating of the substance to be treated in an atmosphere containing halogens, and is now widely employed.

With the conventional chemical dehydrating method which dissociates halogen gas such as chlorine gas, by heat treatment, into nascent-state atoms, it is necessary to heat the gas to as high a temperature as about 1 200 C or more since the halogen gas in the steady state is strong in the inter-molecular binding force.

Therefore, at the heating temperature for dissociating the halogen gas, optical materials of low melting point are evaporated, and hence cannot be treated for dehydration.

Moreover, in conventional treatment of the low-melting-point optical materials for dehydration, halogenating methane gas such as carbon tetrachloride has been used for halogenation, instead of the halogen gas such as fluorine gas. The halogenating methane gas such as carbon tetrachloride (CCt4) can be dissociated into chlorine (cut) of the nascent state even when the heating temperature is about 300"C, and it also heightens the dehydration effect.

With this method, however, when molecules are dissociated by the thermal reaction, there occurs, in addition to a reaction (CCf 4 < CCf + Cf) which dissociates one chlorine atom (cut), another reaction (CCt4 < C+2Ct2) which dissociates two or more chlorine atoms simultaneously to release carbon atoms, resulting in the carbon atoms (C) getting mixed, as impurities, into the substance under treatment. Moreover, the halogenating methane such as carbon tetrachloride evolves carbon dioxide gas in the process of dehydrating reaction, and the carbon dioxide gas is mostly exhausted together with the inert gas but partly reacts to the material 5 of high reactivity, such as a fluoride, liberating carbon to form a new impurity source.

With reference to the accompanying drawings, the present invention will hereinafter be described in detail.

Figure 2 is a schematic diagram explanatory of the principles of the dehydrating method of the present invention, showing the manner in which a plasma is produced by a glow discharge in a reaction tube. In Figure 2, reference numeral 7 indicates a gas inlet port for supplying therethrough a halogenating agent and an inert gas; 3 designates a reaction tube; 8 identifies a radio-frequency coil for causing a glow discharge; and 9 represents a part where the glow discharge is being produced. For example, fluorine gas (F), supplied through the gas inlet port7 is dissociated by the glow discharge regardless of ambienttemperature, by which the plasma 9 containing F atoms in the nascent state is formed substantially uniformly inside the radio-frequency coil 8.On the other hand, the material 5 to be treated is heated up by eddy current of radio-frequency current which flows in the radio-frequency coil 8 and gives thermal energy necessary for a chemical reaction. In this way, the F atoms dissociated by the glow discharge into the nascent state are substituted by the OH radicals of the heated material under treatment, and the OH radicals further react to the F atoms into fluoric acid and H2O molecules, which are discharged through the exhaust port 6, together with the inert gas. Further, by heating material 5, the H2O molecules, which are impurities, are also evaporated and exhausted through the exhaust port 6.

As described above, when supplied into an atmosphere in which a glow discharge is being produced, the halogenating agent is dissociated into nascent-state atoms of high concentration irrespective of temperature, and by heating the material to be treated to a temperature corresponding thereto, that is, a temperature at which the solid material under treatment is allowed to chemically react but is not evaporated by heat, the OH radicals of the material being treated are substituted by the dissociated atoms and removed.

Accordingly, this method is also applicable to an optical material of a low melting point such as a fluoride glass or oxide germanium fiber for which it is impossible to employ the prior art method which dissociates a halogen gas by thermal decomposition for reaction to the OH radicals. Incidentally, Table 1 shows temperatures necessary for main optical materials to chemically react.

TABLE 1 Optical material Temperature PC7necessary for chemical reaction Fluoride glass 400 - 500 Oxide germanium 700 - 800 Chalcogenide glass fiber 700 - 800 Multicomponent-system glass fiber 700 - 800 Potassium bromide 700 - 800 Potassium chloride 700 - 800 Quartz glass fiber 12001400 Figure 3 illustrates a specific example of an apparatus for carrying the dehydration method of the present invention into practice. In Figure 3, reference numeral 10 indicates a susceptor support rod made of quartz glass alumina or like insulator; 11 designates a susceptor made of metal or graphite; 12 identifies a crucible; 13 denotes fluoride glass to be treated; and 14 represents a coupling portion having a flange coupled through a seal. As an initialization, the height of the radio-frequency coil 8 is adjusted so that the crucible may be disposed inside the radio-frequency coil 8. Further, the reaction tube 3 is evacuated to a vacuum of 10- torr (1 torr 1/760 atm). After the above initialization, an inert gas such as argon (Ar) or helium (He) is introduced from the gas inlet port 7, holding the pressure in the reaction tube 3 between 0.03 to 1 torr.Next, when applying a radio-frequency current to the radio-frequency coil 8, a glow discharge is produced in the reaction tube 3 and, at the same time, the susceptor 11 is heated by an eddy current resulting from the radio-frequency current By adjusting the amount of radio-frequency current, a temperature is obtained which is higher than the melting point of the fluoride glass (about 460 C) but lower than the temperature at which the fluoride glass does not evaporate (about 900"C). In this state, a gas mixture containing a halogenating agent in the inert gas is supplied from the gas inlet port 7.The halogenating agent thus introduced into the reaction tube is dissociated by the glow discharge, and nascent-state ionized halogen atoms of high concentration react to OH radicals in the fluoride glass 13, forming fluoric acid and H2O molecules in the plasma. The fluoric acid and H2O molecules thus formed are exhausted from the exhaust port 6, along with the inert gas, thus performing the dehydration of the fluoride glass.

While the above embodiment has been described in connection with the case of treating the fluoride glass for dehydration, it is needless to say that the present invention is applicable to the dehydration of quartz fiber and like high-melting-point optical materials as well as such low-melting-point optical materials as oxide germanium fiber, chalcogenide glass fiber, multicomponent-system glass fiber, potassium bromide and potassium chloride fibers.

As described above, according to the present invention, a halogenating agent is dissociated, through utilization of a glow discharge, into nascent-state halogen atoms, and a material to be treated is heated by radio-frequency current which is applied for generating the glow discharge. Accordingly, the dissociation of the halogenating agent into the nascent-state halogen atoms is independent of the temperature for treatment, and it is necessary only to heat the solid optical material to a temperature necessary for its chemical reaction. Thus, the present invention is also applicable to the low-melting point optical materials that the prior art cannot treat for dehydration, and further, the invention permits dehydration without leaving any impurities in the treated materials, and hence is of great utility for obtaining low-loss optical materials for use in the near infrared ray region.

Claims

1. A method for dehydrating an optical material by a glow discharge, characterised in that, in an atmosphere of halogen atoms dissociated by the glow discharge, an optical material containing OH radicals is heated by radio-frequency current up to a desired temperature for a chemical reaction of the material, by which the OH radicals are substituted by the halogen atoms, dehydrating the optical material.

2. A method according to claim 1, in which the desired temperature is established at a temperature at which the optical material is allowed to chemically react but is not evaporated by heat.

3. A method according to claim 1 or 2, in which the optical material is fluoride glass while the desired temperature is from 400 to 500 C.

4. A method according to claim 1 or 2, in which the optical material is any of: oxide germanium; chalocogenide glass fiber; multicomponent-system glass fiber; potassium bromide, and potassium chloride, while the desired temperature is from 700 to 800 "C.

5. A method according to claim 1 or 2, in which the optical material is quartz glass fiber while the desired temperature is from 1200 to 1400 "C.

6. A method for dehydrating an optical material by a glow discharge substantially as herein described with reference to Figure 2 with or without reference to Figure 3.