METHOD AND APPARATUS FOR PRODUCING OPTICAL FIBER

Abstract: In a method for producing an optical fiber (7) having a resin coating, an optical fiber (7) which is drawn from a preform (1) for an optical fiber (7) heated and melt by a heating source (2) can be effectively cooled before it is coated with a resin by cooling the drawn fiber (4) by cooling gas (6) in such a manner that, in a first plane which is perpendicular to an axis of the drawn fiber (4) or a first conical surface which has a cone point on the axis of the drawn fiber (4), the cooling gas (6) is blown against the fiber (4) from one direction on said first plane or said first cone surface, and in a second plane which is perpendicular to the axis of the drawn fiber (4) and apart from said first plane or a second conical surface which has a cone point on the axis of the drawn fiber (4) and is apart from said first conical surface, the cooling gas (6) is blown against the fiber (4) from other direction on said second plane or said second conical surface so that a sum of components of all vectors of the cooling gas (6) in x and y dimensions in the three-dimensional space with a z dimension in the drawn fiber (4) direction is substantially zero (0) where a length and a direction of the vector are represented by a flow rate and a direction of the cooling gas (6), respectively.

Description: The present invention relates to a method and an apparatus for producing an optical fiber. More particularly, the present invention relates to improvement of a method and an apparatus for producing an optical fiber which comprises heating and melting a glass preform for an optical fiber (hereinafter referred to as "preform"), drawing the melt preform from its one end at a constant tension to form a fiber and coating a peripheral surface of the drawn fiber with a resin to finish an optical fiber. Description of the Related Art Usually, an optical fiber is produced as follows: As shown in Fig. 1, a preform 1 produced in a separated step is inserted in a muffle tube 3 of a heating furnace 2 and the melt glass is drawn from its lower end at a constant tension. A drawn fiber 4 is cooled by spontaneous heat radiation during drawing and then passed through a coating die 5 containing a liquid resin to coat the resin around a peripheral surface of the drawn fiber 4. Thereafter, the resin is hardened in a furnace 9 and a finished optical fiber 7 is wound by a drum 8. The reasons why the peripheral surface of the drawn fiber 4 is coated with the resin during the production of the optical fiber 7 are as follows: Since the drawn fiber 4 is a very thin and fragile glass fiber and has a diameter of 100 to 150 µm, it is so weak as to be broken under a tensile load of about 100 g. Since the drawn fiber is made of glass, different from metals, it tends to suffer from brittle facture and a minute flaw made on the fiber surface easily grows and finally breaks the optical fiber. Therefore, the surface of the drawn fiber is coated with the resin to increase tensile strength and flexural strength. To increase productivity of the optical fiber, a drawing rate of the fiber should be increased. When the peripheral surface of the drawn fiber is coated with a synthetic resin such as an ultraviolet curable resin, since the drawn fiber is drawn from the melt preform, sometimes it is not sufficiently cooled although it is cooled by radiation during drawing. If the insufficiently cooled drawn fiber is passed through the coating die 5, a coated layer of the resin is thermally affected by the drawn fiber and a viscosity of the resin around the drawn fiber is decreased too much so that a thickness of the coated layer is not uniform. To overcome such drawbacks, conventionally, a gas is blown against the drawn fiber just after it is drawn from the melt preform (cf. Japanese Patent Publication No. 39496/1979). This method is shown in Fig. 2, in which the reference 6 represents a cooling gas, and the same references represent the same parts as in Fig. 1. As the drawing rate is increased to improve the productivity, the rate of heat extraction from the drawn fiber should be increased. When the cooling gas is blown in a direction parallel to the fiber as in the method shown in Fig. 2, a gas near the fiber is not readily replaced by a fresh cool gas and so heat must be transferred from the gas near the fiber to the gas in an outside layer by heat conduction. Therefore, the cooling efficiency in this method is not satisfactory. To increase the heat conduction, helium or hydrogen can be used. However the former is expensive and the latter is dangerous due to explosion. A further prior art is that disclosed in EP-A-0079186 which involves the drawing of glass fiber whilst it is passing downwardly through a tube. In one embodiment cooling gas is supplied to an annular manifold at the base of the tube and from there it flows upwardly through the tube to cool the descending fiber. In the annular manifold is an upstanding conical member through which the descending fiber passes and this conical member, according to the drawings, causes the cooling gas to be initially directed conical-wise towards the fiber. In a further embodiment of this prior art the tube is porous and the cooling gas is supplied to a cylindrical chamber surrounding said tube. Cooling gas then flows from the chamber through the porous tube to cool the descending fiber therein. Yet a further prior art is that disclosed in EP-A-0235746. Fig. 1 of this prior art shows that for any given transverse plane in a cooler in which cooling gases are blown against a descending glass fiber the gases are equally opposed. Furthermore, the description makes it clear that this is important to avoid pushing the fiber to one side as it passes through the cooler and that therefore the cooling gas must pass through the wall of the cooler in any one transverse plane in a symmetrical fashion. SUMMARY OF THE INVENTION The invention is as defined in the accompanying method Claim 1 and apparatus Claim 2. Each of these claims has been divided into a two-part form based on the assumption that EP-A-0235746 is the nearest prior art. Thus the features that appear in the pre-characterizing part of these claims can also be found in EP-A-0235746. One object of the present invention is to improve a cooling manner of the drawn fiber which is drawn from the melt preform so as to increase the cooling efficiency of the drawn fiber. Another object of the present invention is to provide a method for producing an optical fiber in which the drawn fiber is efficiently cooled and coated with a resin coating and having improved quality and stability. A further object of the present invention is to provide an apparatus for carrying out the method of the present invention for producing the optical fiber. According to the first aspect of the present invention, there is provided a method for producing an optical fiber comprising the steps of: drawing a fiber at a constant tension from a preform which is heated and melted by a heating source, cooling the drawn fiber by cooling gas in such a manner that, in a first plane which is perpendicular to an axis of the drawn fiber or a first conical surface which has a cone point on the axis of the drawn fiber, the cooling gas is blown against the fiber from only one direction on said first plane or said first cone surface, and in a second plane which is perpendicular to the axis of the drawn fiber and apart from said first plane or a second conical surface which has a cone point on the axis of the drawn fiber and is apart from said first conical surface, the cooling gas is blown against the fiber from only one direction on said second plane or said second conical surface so that a sum of components of all vectors of the cooling gas in x and y dimensions in the three-dimensional space with a z dimension in the drawn fiber direction is substantially zero (0) where a length and a direction of the vector are represented by a flow rate and a direction of the cooling gas, respectively, and then coating the cooled fiber with a resin melt to obtain a finished optical fiber. According to the second aspect of the present invention, there is provided an apparatus for producing an optical fiber comprising means for drawing a fiber at a constant tension from a preform which is heated and melted by a heating source, a pipe for cooling the drawn fiber which has blowout holes arranged in such a layout that, in a first plane which is perpendicular to an axis of the drawn fiber or a first conical surface which has a cone point on the axis of the drawn fiber, the cooling gas is blown against the fiber from only one blowout hole, and in a second plane which is perpendicular to the axis of the drawn fiber and apart from said first plane or a second conical surface which has a cone point on the axis of the drawn fiber and is apart from said first conical surface, the cooling gas is blown against the fiber from only one blowout hole so that a sum of components of all vectors of the cooling gas in x and y dimensions in the three-dimensional space with a z dimension in the drawn fiber direction is substantially zero (0) where a length and a direction of the vector are represented by a flow rate and a direction of the cooling gas, respectively, and which is provided around the drawn fiber, and means for coating the cooled fiber with a resin melt to obtain a finished optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows a conventional method for producing an optical fiber, Fig. 2 schematically shows a method for producing an optical fiber disclosed in Japanese Patent Publication No. 39496/1979, Figs 3A and 3B are a schematic plan view and a conceptual vertical cross sectional view, respectively where a cooling gas is blown in directions perpendicular to a drawn fiber according to the present invention, Figs 4A and 4B are schematic plan view and a conceptual vertical cross sectional view, respectively where a cooling gas is blown in directions on conical surfaces which have their cone point on the axis of the drawn fiber according to the present invention, Fig. 5 schematically shows one embodiment of the method and apparatus for producing an optical fiber according to the present invention, Figs. 6 and 7 are a perspective view and a horizontal cross sectional view of a spiral pipe for blowing a cooling gas against the drawn fiber, respectively, Figs. 8 and 9 are a horizontal cross sectional view and a vertical cross sectional view of another embodiment of pipes for supplying a cooling gas. DETAILED DESCRIPTION OF THE INVENTION When the cooling gas is blown against the drawn fiber from a side direction, namely from a direction which is not parallel to the axis of the drawn fiber (i.e. a drawing direction) to increase the amount of heat to be deprived from the drawn fiber, it is expected that the cooling efficiency by the gas is maximized by maintaining a temperature of the cooling gas near the drawn fiber as low as possible. If the cooling gas is blown from only one direction or if an amount of the cooling gas blown from one direction is larger than that from other directions, by a force generated by the cooling gas in said one direction and accumulated in a longitudinal direction, the drawn fiber is deflected in said one direction so that a form of neck down is adversely affected. According to the present invention, since the cooling gas is blown from the blowout holes so that the sum of components of the vectors of the cooling gas in a plane perpendicular to the drawn fiber (when the length and the direction of the vector are represented by the flow rate and the direction of the cooling gas, respectively) is made substantially zero, the drawn fiber is not deflected during the cooling step from a line along which the drawn fiber can pass if no cooling gas is blown against the fiber. The present invention will be illustrated by making reference to the accompanying drawings. Figs. 3A and 3B are a conceptual plan view and a conceptual vertical cross sectional view, respectively where a cooling gas is blown in directions perpendicular to a drawn fiber according to the present invention, and Figs. 4A and 4B are a conceptual plan view and a conceptual vertical cross sectional view, respectively where a cooling gas is blown in directions on conical surfaces which have their cone point on the axis of the drawn fiber according to the present invention. In either case of Figs. 3A and 3B and Figs. 4A and 4B, each arrow represents a direction in which the cooling gas is blown, and in the plane view, the cooling gas is blown in the direction perpendicular to the drawn fiber. In addition, as seen from the vertical cross sectional views of Figs. 3B and 4B, each arrow is directed to a different point on the drawn fiber. Needless to say, the processes of Figs. 3A and 3B and of Figs. 4A and 4B can be employed simultaneously. Fig. 5 schematically shows one embodiment of the method and apparatus for producing the optical fiber according to the present invention. The references 1 through 9 have the same meanings as in Figs. 1 and 2. In Fig. 5, a cooling gas 6 flows in a spiral pipe 10 from its lower end to its upper end. The spiral pipe 10 has plural small blowout holes 11 on its inner side as shown in Fig. 6 and is provided between the heating furnace 2 and the coating die 5 with aligning a spiral axis on the drawn fiber as shown in Figs. 5 and 7. The cooling gas 6 is blown against the drawn fiber as shown in Figs. 6 and 7. The holes 11 are so arranged that, on a plane perpendicular to the drawn fiber, the gas is blown from one hole on said plane but no pair of the holes are present at the same height on a vertical plane including the fiber axis. Thereby, the sum of the components of the cooling gas vectors in x and y dimensions in a three-dimensional space with a z dimension in the drawn fiber direction can be made substantially zero. Alternatively, as shown in a horizontal cross section of Fig. 8 and a vertical cross section of Fig. 9, plural pipes 12 are vertically arranged surrounding and in parallel with the drawn fiber, and blowout holes 13 are provided on the inside walls of the pipes in such manner that no pair of the blowout holes are arranged at point-symmetric positions around the drawn fiber and all the holes are positioned at different heights. In the present invention, as the cooling gas, air, oxygen, nitrogen, helium, argon and mixtures thereof may be used. A temperature of the cooling gas may be any temperature lower than the temperature of the drawn fiber and preferably room temperature or lower. The lower temperature, the better. According to the present invention, as shown in Figs. 7 and 8, the cooling gas 6 kept at a temperature lower than the temperature of the drawn fiber is blown against the drawn fiber 4 in a direction which is not parallel to the drawn fiber. Therefore, the atmosphere near the drawn fiber is always kept at the temperature of the cooling gas 6 so that heat is effectively exchanged between the drawn fiber 4 and the cooling gas 6 near the fiber and the drawn fiber 4 is rapidly cooled. Example By using the apparatus of Fig. 5, a quartz glass fiber having a diameter of 125 µm was drawn. The apparatus had a spiral pipe which was provided from 1 meter to 3.5 meters below the heating furnace 2. From the pipe, air kept at about 20°C was blown at a flow rate of 180 l/min, and the drawn fiber was passed through the spiral pipe at a rate of 500 m/min. At the lower end of the pipe, the drawn fiber was cooled to 140°C. During cooling, no deflection of the fiber was observed. On the contrary, without the blow of the air, the temperature of the drawn fiber was 330°C at the lower end of the pipe. In the same manner as above but blowing the cooling gas in a direction upward by 45° from the horizontal plane, the drawn fiber was cooled to 100°C. As understood from the above description, by the method and apparatus according to the present invention, the cooling efficiency of the drawn fiber is greatly improved in comparison with the conventional method and the vibration of the drawn fiber during cooling is prevented. In addition, when the cooling gas is kept clean, the atmosphere near the drawn fiber which is being cooled can be kept clean, for example, in Class 10 in case where the apparatus of Fig. 5 is used.

Claims: 1. A method for producing an optical fiber comprising steps of: drawing a fiber (4) at a constant tension from a preform (1) which is heated and melted by a heating source (2); cooling the drawn fiber (4) by cooling gas (6); and coating the cooled fiber (4) with a resin melt to obtain a finished optical fiber (7); characterized in that cooling of the drawn fiber is carried out in such a manner that, in a first plane which is perpendicular to an axis of the drawn fiber (4) or a first conical surface which has a cone point on the axis of the drawn fiber (4), the cooling gas (6) is blown against the fiber (4) from only one direction on said first plane or said first cone surface, and in a second plane which is perpendicular to the axis of the drawn fiber (4) and apart from said first plane or a second conical surface which has a cone point on the axis of the drawn fiber (4) and is apart from said first conical surface, the cooling gas (6) is blown agianst the fiber (4) from only one direction on said second plane or said second conical surface so that a sum of components of all vectors of the cooling gas in x and y dimensions in the three-dimensional space with a z dimension in the drawn fiber (4) direction is substantially zero (0) where a length and a direction of the vector are represented by a flow rate and a direction of the cooling gas (6), respectively. 2. An apparatus for producing an optical fiber comprising: means for drawing a fiber (4) at a constant tension from a preform (1) which is heated and melted by a heating source (2), a pipe means (10; 12) for cooling the drawn fiber (4), said pipe means having blowout holes (11, 13) therein; and means (5) for coating the cooled fiber (4) with a resin melt to obtain a finished optical fiber (7); characterized in that said pipe means (10; 12) has blowout holes (11; 13) arranged in such a layout that, in a first plane which is perpendicular to an axis of the drawn fiber (4) or a first conical surface which has a cone point on the axis of the drawn fiber (4), the cooling gas (6) is blown against the fiber (4) from only one blowout hole (11; 13), and in a second plane which is perpendicular to the axis of the drawn fiber (4) and apart from said first plane or a second conical surface which has a cone point on the axis of the drawn fiber (4) and is apart from said first conical surface, the cooling gas (6) is blown against the fiber (4) from only one blowout hole (11, 13) so that a sum of components of all vectors of the cooling gas (6) in x and y dimensions in the three-dimensional space with a z dimension in the drawn fiber (4) direction is substantially zero (0) where a length and a direction of the vector are represented by a flow rate and a direction of the cooling gas (6), respectively, and which is provided around the drawn fiber (4). 3. The apparatus according to claim 2, wherein the pipe means (10; 12) for cooling the drawn fiber (4) consists of a spiral pipe (10) whose spiral axis is aligned on the drawn fiber (4). 4. The apparatus according to claim 2, wherein the pipe means (10; 12) for cooling the drawn fiber consists of plural pipes (12) which are vertically arranged surrounding and in parallel with the drawn fiber (4), and the blowout holes (13) are provided on inside walls of the pipes (12) in such manner that no pair of the blowout holes (13) are arranged at point-symmetric positions around the drawn fiber (4) and all the holes (13) are positioned at different height.

Inventors: SAKAMOTO, KATSUJI C/O YOKOHAMA WORKS OF, TAKIMOTO, HIROAKI C/O YOKOHAMA WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: C03B37
IPC codes: C03B37, C03C25