Apparatus and method for manufacturing a hermetically coated optical fiber

Abstract: The invention relates to an apparatus and a method for manufacturing a hermetically coated optical fiber (3), the apparatus having a single reaction chamber (25) into which a bare fiber (1) and raw gas are to be introduced for applying a hermetic coating to the bare fiber (1) while the bare fiber (1) passes through the reaction chamber (25). The reaction chamber (25) has a plurality of inlet tubes (25a, 25b, 25c, 25d) to introduce the raw gas, and the plurality of inlet tubes are open to the reaction chamber (25) at different positions from each other in a direction of movement of the bare fiber (1) passing through the reaction chamber (25).

Description: The present invention relates to apparatus and method for manufacturing a hermetically coated optical fiber by coating a thin film of carbon or the like on an optical fiber spun from an optical fiber preform. (Related Background Art) As a technique to hermetically coat carbon to a spun bare fiber, a method for coating a carbon thin film on a surface of a bare fiber by exposing the optical fiber to a high temperature carbon gas atmosphere in a process of thinning the optical fiber has been known. (For example, see JP-B-38-10363) Ep 0.308.143 (published on March 22, 1989) discloses a method for manufacturing a hermetically coated optical fiber by guiding a thinned high temperature bare fiber into a carbon gas atmosphere in a process of spinning the optical fiber from an optical fiber preform to coat a carbon thin film on a surface of the bare fiber. In one embodiment of the disclosed method, a reaction tube having two reaction chambers is used, raw gas is separately introduced into each of the reaction chambers and the carbon thin films are coated in two steps in the respective reaction chambers. In the past, when a hermetic coating is applied by carbon on the surface of the optical fiber, a residual stress appears in an interface of the carbon film and the optical fiber or a tensile strength of the fiber is reduced by the unevenness created on the surface of the carbon film. In the related art, since a reaction condition such as concentration and composition of raw gas in one reaction chamber is kept constant the carbon film formed on the bare fiber has a uniform property. Where two or more reaction chambers are provided and carbon is coated with different gas atmospheres for the respective reaction chambers as proposed by the related art, heterogeneous films with interfaces are formed because the fiber temperature varies from chamber to chamber. As a result, a distortion due to a residual stress is created at a heterogeneous phase interface and the improvement of strength is not attained. An apparatus for manufacturing a hermetically coated optical fiber with two or more reaction chambers is known from JP-A-2180735 and EP-A-0 374 926. An apparatus for manufacturing a hermetically coated optical fiber according to the preamble of claim 1 is known from GB-A-2 156 858 and includes inner and outer coaxially arranged perforated tubes. Feed tubes for introduction of reactive constituents are arranged between the inner and outer tubes. It is an object of the present invention to continuously change a property of a hermetic coating formed on a bare fiber in a radial direction of the bare fiber. This object is solved according to the present invention by an apparatus and a method respectively including the features of claim 1 and claim 5. Advantageous embodiments are defined in the dependent claims. The present invention is based on the following finding of the inventors. When hydrocarbon and carbon halide are used as raw gas and a concentration of carbon halide is raised, a surface of a carbon film formed becomes smooth and a tensile strength of a fiber increases. However, as the concentration of carbon halide is raised, a hermetic effect of the carbon film formed reduces. After various studies on this, it has been found that the property of the carbon film formed in the reaction chamber depends on a temperature of a substrate (a temperature of a bare fiber) and flow rate, composition, concentration and temperature of the raw gas, through the control of those parameters, the property of the carbon film formed is successfully controlled. The method for manufacturing the hermetically coated optical fiber of the present invention is characterized by continuously changing at least one of flow rate, composition, concentration and temperature of the raw gas in the single reaction chamber along the direction of movement of the bare fiber when a hermetically coating is applied to the bare fiber passing through the single reaction chamber into which the raw gas is introduced. In accordance with the present invention at least one of flow rate, composition, concentration and temperature of the raw gas in the reaction chamber for applying the hermetic coating to the optical fiber while it passes through the reaction chamber is continuously changed in the direction of movement of the bare fiber guided into the reaction chamber so that the hermetic coating film having a continuously varying film property along the radial direction of the bare fiber can be applied to the bare fiber. This is based on the following principle. When hydrocarbon and carbon halide are introduced as raw gas into the single reaction chamber, the higher the concentration of hydrocarbon is, the higher is a film forming temperature and the richer is a hermetic effect of the resulting film. On the other hand, when the concentration of carbon halide which has a dehydrogenating effect is raised, the reaction temperature is lowered and a carbon film which has a smooth surface and enhance an initial strength of the fiber is formed. The film forming temperature tends to be high as the fiber temperature rises and the flow rate of the raw gas increases. In accordance with the above principle, when raw gas having a high hydrocarbon concentration is supplied to an inlet side of the high temperature bare fiber (an upstream end in the direction of movement of the bare fiber) and raw gas having a higher carbon halide concentration is supplied as the temperature of the bare fiber decreases (at it moves toward a downstream end in the direction of movement of the bare fiber) and the amount of supply of the raw gas is increased, a sufficiently thick film is formed and the carbon film having a fine structure and a high hermetic effect is formed at an interface of the bare fiber and the carbon film, and a carbon film having a smooth surface can be formed thereon without an evident boundary or step. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Fig. 1. shows a schematic view of one embodiment of an apparatus for manufacturing a hermetically coated optical fiber of the present invention, which is applicable to a method for manufacturing the hermetically coated optical fiber of the present invention, and Fig. 2. shows a schematic view of another embodiment of the apparatus for manufacturing the hermetically coated optical fiber of the present invention. Description of the Preferred Embodiment An embodiment of the present invention is now explained with reference to Fig. 1. As shown, a bare fiber 1 is formed by drawing an optical fiber preform 2 by a drawing device 10. The bare fiber 1 drawn from the preform 2 is conveyed to a hermetic coating device 20 where it is hermetically coated with carbon for instance to form a hermetically coated optical fiber 3. The hermetically coated optical fiber 3 is then conveyed to a resin application device 50 through a fiber diameter monitoring device 35 and a cooling device 40 to form a resin coated optical fiber 5. The resin coated optical fiber 5 is taken up by a drum (not shown) through a capstan, or it is taken up by the drum after it is coated with nylon. The preform 2 is hung in a tube 11 of the drawing device 10 while it is held by an arm (not shown). The tube 11 is set in a drawing oven 12 which has a heater 13 for heating and fusing a lower end of the preform 2. The drawing oven 12 may be a known electric oven or RF oven. Preferably it does not produce hydrogen molecules or dust. The bare fiber 1 having a substantial similar distribution of refractive index to that of the preform 2 is spun from the preform 2 by the drawing device 10, and it is fed to the hermetic coating device 20 as it is. The hermetic coating device 20 has a reaction tube 21 and a cooling jacket 22. The interior of the reaction tube 21 is partitioned by partitioning walls 23a and 23b into a single reaction chamber 25 and two seal chambers 26 formed on both upstream end and downstream end of the reaction chamber 25. The partitioning walls 23a and 23b have holes 24a and 24b, respectively, at centers thereof through which the fiber passes. Seal gas such as nitrogen N₂ or Ar is supplied to the seal chambers 26 through inlet tubes 26a opening thereto. A plurality of inlet tubes 25a, 25b, 25c and 25d and an exhaust tube 25e are open to the reaction chamber 25 at different positions from each other in the direction of movement of the bare fiber 1 passing through the reaction chamber 25. Raw gas is supplied into the reaction chamber 25 through the inlet tubes 25a, 25b, 25c and 25d, and evacuated through the exhaust tube 25e. The flow rate, composition, concentration and temperature of the raw gas to be supplied to the reaction chamber 25 are independently controlled by control means 27 provided for each of the inlet tubes. Accordingly, at least one of the flow rate, composition, concentration and temperature of the raw gas in the reaction chamber 25 can be continuously changed along the direction of movement of the bare fiber 1 passing through the reaction chamber 25. The reaction chamber 25 may be of multi-tube structure having a plurality of tubes therein in order to relax the change of the raw gas blown to the bare fiber 1. Fig. 2 shows a longitudinal sectional view of another embodiment of the present invention in which a reaction chamber of double-tube structure is sectional along the direction of movement of the bare fiber. An inner tube 25f having a number of small holes formed in a surface thereof is arranged in the reaction chamber 25. As the double-tube structure, a reaction tube having an inner diameter of 40mm and quartz tubes each having a thickness of 1.5mm, an inner diameter of 25mm and a small hole diameter of 1-2mm with a pitch of 5mm may be used. The bare fiber 1 is moved along a center axis of the inner tube 25f. Thus, the raw gas passes through the small holes and is blown to the surface of the bare fiber 1. As a result, #1 the velocity difference of gas articles at the upstream end and the downstream end of the raw gas is unified, and #2 the raw gas is uniformly supplied to the bare fiber in the depthwise direction of the formed film. Accordingly, even if the openings of the inlet tubes 25a, 25b, 25c, 25d and 25e are not arranged symmetrically along the circumference, the change of the raw gas atmosphere is relaxed and the film is uniformly formed along the circumference of the bare fiber 1. The control means 27 comprises mass flow valves 28a, 28b and 28c communicated with the supply source (not shown) of raw gases (for example, hydrocarbon such as ethylene or acetylene, carbon halide such as carbon tetrachloride, hydrocarbon halide, chloroform or trichloroethylene) and inert gas such as N₂, He or Ar which is used as diluent gas, mass flowmeters 29a, 29b and 29c for metering flow rates of gases passing through the mass flow valves, and a mixing chamber 30 for mixing gases passing through the mass flowmeters. A thermometer for measuring a temperature of the internal gas and a temperature control unit such as a heater for heating the internal gas are arranged in the mixing chamber 30. As a result, the flow rate, composition, concentration and temperature of the raw gases supplied into the reaction chamber 25 from the mass flow valves through the inlet tubes 25a, 25b, 25c, and 25d can be controlled independently for each inlet tube. The raw gases having the composition thereof controlled are fully mixed in the mixing chamber 30 and then they are supplied to the reaction chamber 25. Only inert gas may be supplied into the reaction chamber 25 through at least one of the inlet tubes 25a-25d by the control of the control means 27 so that the concentration of the raw gas in the reaction chamber 25 may be continuously changed along the direction of movement of the bare fiber 1. Hydrogen or chlorine may be added to the raw gas to be introduced into the reaction chamber 25 to control the content of hydrogen atoms or chlorine atoms in the raw gas. Gas such as helium, nitrogen or air, or liquid such as water or alcohol is supplied as coolant from the inlet tube 22a into the cooling jacket 22 mounted around the reaction tube 21 to cover the reaction chamber 25, and it circulates through the exhaust tube 22b. In this manner, a side wall of the reaction tube 21 facing the reaction chamber 25 is maintained at a constant temperature. The fiber diameter monitoring device 35 arranged below the hermetically coating device 20 measures an outer diameter of the hermetically coated optical fiber 3 by a laser beam. The fiber diameter of the bare fiber 1 is controlled based on the measurement. The cooling device 40 there below is used to cool the hermetically coated optical fiber to, for example, 70 °C. The cooling device 40 may comprise a cylinder having a length of 30cm and an inner diameter of 1.5cm, and cooling helium gas is supplied therein to at a rate of 10 liters per minute. The resin application device 50 arranged below the cooling device 40 comprises a die 51 which is filled with resin 52. The hermetically coated optical fiber 3 is coated with the resin 52 while it passes through the die 51 to form the resin coated optical fiber 5. An experimental result of the hermetic coating to the bare fiber 1 by carbon by the above apparatus is explained below. In the experiment, ethylene C₂H₄ was used as raw gas for hydrocarbon, and carbon tetrachloride CCl₄ was used as raw gas for carbon halide. Since carbon tetrachloride CCl₄ is in liquid phase under room temperature and room pressure, it was evaporated by an evaporator at 30°C (constant) and helium He was used as carrier gas. Nitrogen N₂ gas was used as diluent gas. The compositions and flow rates of the raw gases supplied into the reaction chamber 25 from the inlet tubes 25a, 25b, 25c and 25d were set as follows. Inlet tube C₂H₄ Carrier Gas (of CCl₄) 25a 80 cm³/min 120 cm³/min 25b 60 cm³/min 140 cm³/min 25c 40 cm³/min 160 cm³/min 25d 20 cm³/min 180 cm³/min Thus, the raw gas having a higher hydrocarbon concentration is supplied into the reaction chamber at the inlet end of the high temperature bare fiber (upstream end), and the raw gas having a higher carbon halide concentration is supplied as it goes toward the downstream end along the direction of movement of the bare fiber. The drawing speed of the bare fiber was 150 m/min, the temperature of the reaction tube wall was kept at 100°C and the flow rate of the seal gas (N₂) to the two seal chambers 26 was set to 3 liter/min. Under such a condition, carbon was hermetically coated and a carbon film having a fine structure and a high hermetic effect was formed at an interface of the bare fiber and the carbon film, and a carbon film having a smooth surface was formed thereon without clear boundary or step. As a result, a mean value of an initial strength of the resulting hermetically coated optical fiber was no smaller an 58,8 N, and a tensile strength was equal to or higher than that of a conventional optical fiber without carbon hermetic coating. This is approximately 20% increase over the related art carbon coated optical fiber. A fatigue parameter n was as high as 100 or higher and there was no increase of transmission loss (1.24 µm) by hydrogen after the optical fiber had been left in a hydrogen atmosphere (80 °C, 10⁵ Pa) for 20 hours. It exhibited a very good waterproof property and hydrogen-proof property. For comparison purpose, characteristics of a carbon coated optical fiber manufactured by supplying the raw gas into the reaction chamber 25 only from the inlet tube 25a while the inlet tubes 25b, 25c and 25d of the apparatus shown in Fig. 1 were closed are shown below. (1) Ethylene C₂H₄ as the raw gas was supplied at 200 cm³/min into the reaction chamber and CCl₄ carrier gas (He) was supplied at 300 cm³/min. Tensile strength: 44,1 - 49 N Fatigue index(n): 100 or higher Increase of transmission loss by hydrogen: 0 dB/km (2) Ethylene C₂H₂ as the raw gas was supplied at 100 cm³/min into the reaction chamber, and CCl₄ carrier gas (He) was supplied at 400 cm³/min. Tensile strength: 58,8 - 63,7 N Fatigue index (n): 100 or higher Increase of transmission loss by hydrogen: 0.1 - 0.4 dB/K

Claims: An apparatus for manufacturing a hermetically coated optical fiber having a single reaction chamber into which a bare fiber and raw material gas are to be introduced for applying a hermetic coating to the bare fiber while the bare fiber passes through the reaction chamber, the reaction chamber having a substantially unvaried cross-section in the direction of movement of the bare fiber passing therethrough and a plurality of inlet tubes to introduce the raw material gas, the plurality of inlet tubes being open to the reaction chamber at different positions from each other in a direction of movement of the bare fiber passing through the reaction chamber, and control means for controlling at least one of flow rate, composition, concentration and temperature of the raw material gas to be introduced into the reaction chamber through the plurality of inlet tubes independently for each of the inlet tubes, characterized in that the plurality of inlet tubes include a first inlet tube in an upstream portion of the reaction chamber for a first raw material gas and a second inlet tube in a downstream portion of the reaction chamber for a second raw material gas, and that the control means is arranged to continuously vary at least one of the flow rate, the composition, the concentration and the temperature of the raw material gas in the reaction chamber from the upstream portion to the downstream portion. An apparatus for manufacturing a hermetically coated optical fiber according to claim 1, characterized in that the control means includes a plurality of flow rate valves for controlling flow rates of constituent gases of the raw material gas, a mixing chamber for mixing the constituent gases passing through the flow rate valves and temperature control means for controlling a temperature of the raw material gas mixed in the mixing chamber. An apparatus for manufacturing a hermetically coated optical fiber according to claim 2, characterized in that the temperature control means includes a thermometer for measuring the temperature of the raw material gas in the mixing chamber and a heater for heating the raw material gas in the mixing chamber. An apparatus for manufacturing a hermetically coated optical fiber according to claim 1, characterized in that the reaction chamber has an inner tube having a number of small holes in a surface thereof, and the bare fiber passes through the inner tube. A method of manufacturing a hermetically coated optical fiber comprising the steps of: passing a bare optical fiber through a single reaction chamber having a substantially unvaried cross-section in the direction of movement of the bare fiber, controlling at least one of flow rate, composition, concentration and temperature of a raw material gas before the raw material gas is introduced into the reaction chamber; and introducing the raw material gas into the reaction chamber through a plurality of inlet tubes, wherein a first raw material gas is introduced into an upstream portion of the reaction chamber through a first inlet tube and a second raw material gas is introduced into a downstream portion of the reaction chamber through a second inlet tube, the first raw material gas is different from the second raw material gas in at least one of flow rate, composition, concentration and temperature before the raw material gasses are introduced into the reaction chamber, and whereby at least one of the flow rate, the composition, the concentration and the temperature of the raw material gas in the reaction chamber varies continuously from the upstream portion to the downstream portion. A method for manufacturing a hermetically coated optical fiber according to claim 5, characterized in that the raw material gas to be introduced into said reaction chamber is ethylene, acetylene, carbon halide or hydrocarbon halide, or a mixture of two or more thereof. A method for manufacturing a hermetically coated optical fiber according to claim 5 or 6, characterized in that chlorine or hydrogen is mixed with the raw material gas to be introduced into the reaction chamber. A method for manufacturing a hermetically coated optical fiber according to claim 5, 6 or 7, characterized in that a carbon film is hermetically coated on the bare fiber. A method for manufacturing a hermetically coated optical fiber according to claim 5, characterized in that the raw material gas having a high hydrocarbon concentration is supplied to the inlet tube opening to the reaction chamber at the upstream portion and raw material gas having a high carbon halide concentration is supplied to the inlet tube opening to the reaction chamber at the downstream portion with an increased amount of supply of the raw gas, in the direction of movement of the bare fiber.

Inventors: AIKAWA HARUHIKO, ISHIGURO YOICHI, KATSUYAMA YUTAKA, NAGAYAMA KATSUYA, YOSHIMURA ICHIRO, YOSHIZAWA NOBUYUKI, AIKAWA, HARUHIKO, ISHIGURO, YOICHI, KATSUYAMA, YUTAKA, NAGAYAMA, KATSUYA, YOSHIMURA, ICHIRO, YOSHIZAWA, NOBUYUKI, AIKAWA, HARUHIKO, C/O YOKOHAMA WORKS, ISHIGURO, YOICHI, C/O YOKOHAMA WORKS, Nagayama, Katsuya, c/o Yokohama Works, YOSHIMURA, ICHIRO, C/O YOKOHAMA WORKS
Applicants: NIPPON TELEGRAPH & TELEPHONE, SUMITOMO ELECTRIC INDUSTRIES, NIPPON TELEGRAPH AND TELEPHONE CORPORATION, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1992
Main classification: C03C25
IPC codes: C03C25