1. Field of the Invention
The present invention relates to a method and an apparatus for producing a coated optical fiber, and more particularly it relates to a method for producing an optical fiber which has improved strength and suffers less decrease of strength for a long time and no increase of light absorption due to hydrogen molecules, and an apparatus for producing an optical fiber coated with pyrolytic carbon by chemical vapor deposition (CVD).
2. Description of the Related Art
In some optical communication systems, an optical fiber having a length of 1 km or longer is required. In such case, one of problems is insufficient strength of the optical fiber. When the optical fiber is used, for example, as a wave guide in an undersea cable, the optical fiber is required to have a tensile strength of at least 200,000 psi. However, commercially available optical fibers have tensile strength in a range between 50,000 and 80,000 psi.
If a quartz optical fiber were made by drawing a preform under ideal conditions, it could have a tensile strength in the order of a hundred million psi. However, actually, the long optical fibers do not have such high mechanical strength. One of the reasons for the smaller mechanical strength of the commercial optical fibers is that submicron order flaws are formed on the surface of optical fiber by mechanical friction or chemical attack with impurities such as moisture in an atmosphere during and after drawing. That is, the optical fiber having no flaws may have the tensile strength in the order of a hundred million.
To overcome the above problem, the conventional optical fiber is coated with an organic coating material. The organic coating material cannot prevent diffusion of water vapor or hydroxyl ions in the optical fiber. Therefore, minute flaws can be formed on the surface of the optical fiber glass due to water vapor or the hydroxyl ions during use or storage, so that the strength of the optical fiber is decreased. In addition, the OH ions increase the transmission loss of the optical fiber. Accordingly, the optical fiber should be coated with a hermetic coating to prevent minute flaws on the surface.
Hitherto, the hermetic coating is formed from silicones or an inorganic material such as metals, and the chemical vapor deposition (CVD) is one of the most attractive methods. In the CVD method, a coating material is supplied as a raw material gas and forms a coating layer on the surface of the optical fiber. That is, one or more raw material gases are reacted at a certain temperature and deposited on the optical fiber surface.
By the CVD method, various kinds of coatings can be formed on the optical fiber surface, for example, silicon nitride, silicon, phosphosilicate glass, tin oxide, silicon oxynitride, boron, boron nitride and the like. It is also possible to form a coating of polycrystal of aluminum or tin on the optical fiber surface by the CVD method. Since the coating is formed homogeneously around the optical fiber by the CVD method, the optical fiber can be protected with the very thin coating. Thereby, light transmission loss due to microbends can be prevented.
FIG. 1 shows an apparatus for coating the optical fiber by the CVD method which is disclosed in Japanese Patent Publication No. 25381/1985. This apparatus 11 comprises a first isolation chamber 12, a reaction chamber 13 and a second isolation chamber 14. The first isolation chamber 12 and the reaction chamber 13 are connected with a small-diameter opening 16, and the reaction chamber 13 and the second chamber 14 are connected with a small-diameter opening 17. The first isolation chamber 12 and the second isolation chamber 14 have small-diameter openings 15 and 18, respectively. An optical fiber 10 is introduced in the apparatus 11 from the opening 15, passed through the first isolation chamber 12, the reaction chamber 13 and the second isolation chamber 14, and drawn out from the opening 18. During passing the reaction chamber 13, a coating is formed on the surface of the optical fiber 10 through chemical reactions. The first and second isolation chambers 12 and 14 are connected to the reaction chamber 13 to isolate the reaction chamber 13 from the ambient atmosphere. In the isolation chambers 12 and 14, inert gas is introduced through gas inlets 19 and 20, respectively, and internal pressure in the isolation chambers 12 and 14 are kept higher than the atmospheric pressure to prevent approach of the atmospheric air through the openings 15 and 28. The raw material gas is introduced in the reaction chamber 13 through an inlet 21 and the reacted gas is exhausted through an outlet 22. The raw material gas in the reaction chamber 13 is kept at a desired temperature with a heating coil 23.
As described above, the raw material gas is subjected to a chemical reaction and deposited on the surface of the optical fiber 10 to form the coating. The reaction proceeds on the optical fiber surface or in the gas phase, and the reaction product is deposited on the optical fiber surface. The raw material gas may be activated by applying energy through photochemical excitation with microwaves or radiofrequency plasma.
To increase a reaction rate or reaction efficiency in the CVD method, the raw material gas can be preheated before being introduced in the reaction chamber. In such case, the temperature of the optical fiber is kept higher than that of the raw material gas, whereby the deposition of the reaction product on the chamber wall is prevented. To this end, the optical fiber is introduced in the reaction chamber just after it is drawn and still kept at a considerably high temperature. Alternatively, the optical fiber in the reaction chamber is selectively heated by the application of IR light or a laser beam. FIG. 2 shows such heating apparatus for selectively heating the optical fiber, which is disclosed in Japanese Patent Publication No. 32270/1986.
The apparatus 2 of FIG. 2 comprises a pair of elongate heating sources 31 which are installed in a housing 33 in parallel in the progressing direction of the optical fiber and irradiate radiations such as IR light for heating the optical fiber 10, and a pair of reflective mirrors 32 each of which is combined with one of the heating source 31 and has an ellipsoidal cross section. Each of the elongate heating sources 31 is placed at one of focuses of the ellipsoidal cross section, and the optical fiber passes the other focus of the ellipsoidal cross section. The radiation irradiated from the heating sources 31 is reflected on the reflective mirrors 31 and passes a transparent window 34 and heats the optical fiber 10. The housing 33 contains many cooling water conduits 35, through which a cooling medium is circulated to cool the atmosphere near the mirror 32 in the housing 33.
However, in the conventional CVD methods, the reaction products react with the surface of the uncoated optical fiber and cause minute flaws on the surface, whereby minute cracks are formed in the uncoated fiber so that the initial strength of the optical fiber is decreased.
To overcome such defects of the CVD methods, Japanese Patent Publication No. 32270/1986 proposes a heterogeneous nucleation thermochemical deposition (HNTD), in which, on the surface of the heated bare optical fiber, fine particles of materials such as metals, glass and ceramics are directly formed. It is said that, since there were no or little reaction between the solid particles and the bare optical fiber surface in the HNTD method, the optical fiber surface would be less damaged than the CVD method.
A method for forming a hermetic coating of carbon on the optical fiber surface is disclosed by Japanese Patent Publication No. 10363/1963. This method is schematically shown in FIG. 3. In this method, a silica fiber preform 10' is heated and melt with a gas burner oxyhydrogen flame 41 made by a heating ring 40 which surrounds the preform 10'. Then, the preform is drawn under tension so as to decrease its diameter to form the optical fiber. Just below the heating ring 40, a cylinder 42 is provided, with which a carbon-containing gas is supplied in the direction indicated by arrows. Thereby, a thin carbon film is formed on the surface of the optical fiber.
EP-A-0 308 143 discloses a method for coating carbon on the optical fiber which comprises drawing a glass rod to decrease its diameter to form a bare optical fiber, introducing the optical fiber kept at high temperature in a carbonaceous atmosphere to coat the optical fiber with carbon.
The reason why carbon is selected as the material of the hermetic coating is that the carbon film is formed at a higher rate than other materials, and prevents transmission of hydrogen (H.sub.2), and deterioration of optical fiber strength.
Although the hermetic carbon coating improves the long term strength of the optical fiber, it decreases the initial strength of the optical fiber.
The above European Patent Application proposes the addition of chlorine gas to the raw material gas. However the addition of chlorine gas deteriorates the ability of hermetic carbon coating for preventing the transmission of hydrogen (hydrogen resistance) and, in an extreme case, the coated optical fiber has the same hydrogen resistance as a bare optical fiber.