Method for manufacturing optical fiber preform

Abstract: A method for manufacturing a preform having steps of obtaining a plastic material by kneading silica powder, a binder, water, and a surfactant, and obtaining a porous preform (99) by extrusion-molding the plastic material. According to this method, a porous preform (99), which can obtain an optical fiber exerting high mechanical strength, can be efficiently manufactured without generating bubble.

Description: The present invention relates to a method for manufacturing an optical fiber preform to be used in the field of communication and optics. Conventionally, there has been performed a method using an extrusion molding as a method for manufacturing an optical fiber preform. In EP-A-0 476 537, published 25.03.92 and thus falling within the terms of Article 54(3) EPC, the inventors of the present invention proposed a method for forming a core portion and a clad portion at the same time by use of a vacuum extrusion machine. According to this method, as a base material, there is used a material in which binder and water is added to silica powder for applying plasticity. In the above method, since the vacuum extrusion machine is used, most of bubble in the plastic material is removed when the core and clad portions are formed. However, its deaeration effect is insufficient for the manufacture of the optical fiber preform. Due to this, in order to accelerate deaeration, molding is performed at a relative low speed, and the plastic material is retained in the extrusion machine as long as possible. However, deaeration is not sufficiently performed, and bubble remains on the obtained preform. There is an disadvantage in the optical fiber, which is obtained by providing a degreasing process and a sintering process to the preform on which bubble is left, and drawing the obtained optical fiber preform, has low mechanical strenght. Therefore, as a whole, yield is worsened in the manufacture of the optical fiber. A method of producing a catalyst carrier is known from JP-A-52 004 490 in which water is mixed with a silica powder, a binder and a surfactant to form a mixture which is subsequently extruded. DE-A-36 33 307 discloses a method of producing optical fiber preforms by extruding a mixture of water, binder and powdery oxidic material. A method of producing a glass preform from a plastic mass consisting of water, a binder and silica powder is known from EP-A-0 197 586 and EP-A-0 265 023. An object of the present invention is to provide a method for efficiently manufacturing an optical fiber preform, from which an optical fiber exerting high mechanical strength can be obtained, without generating bubble. The above object is achieved according to the present invention by a method including the features of claim 1. Detailed embodiments are defined in the dependent claims. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1A is a vertical cross sectional view showing an example of an auxiliary cylinder as an extrusion machine to be used in the method of the present invention; Fig. 1B is a horizontal cross sectional view taken along the line of A - A of Fig. 1A; Fig. 2A is a vertical cross sectional view showing the other example of an auxiliary cylinder of an extrusion machine to be used in the method of the present invention; Fig. 2B is a horizontal cross sectional view taken along the line of B - B of Fig. 2A; Fig. 3A is a vertical cross sectional view showing the other example of an auxiliary cylinder of an extrusion machine to be used in the method of the present invention; Fig. 3B is a horizontal cross sectional view taken along the line of C - C of Fig. 3A; Fig. 4A is a vertical cross sectional view showing the other example of an auxiliary cylinder of an extrusion machine to be used in the method of the present invention; Fig. 4B is a horizontal cross sectional view taken along the line of D - D of Fig. 4A; Fig. 5 is a cross sectional view showing an example of an extrusion head of the extrusion machine to be used in the method of the present invention; Fig. 6 is a cross sectional view taken along the line of E - E of Fig. 5; Figs. 7A to 7E are cross sectional views showing a manufacturing process of the method of the present invention; Fig. 8 is a cross sectional view showing an example of an extrusion head of the extrusion machine to be used in the present invention; and Fig. 9 is a view explaining an example of the extrusion machine to be used in the method of the present invention. According to the method for manufacturing an optical fiber preform of the present invention, a plastic material containing a surfactant is used. The surfactant decreases surface energy between silica powder and water. As a result, deaeration from the plastic material is accelerated in extrusion. Therefore, a preform on which bubble is little left can be obtained. It is preferable that silica powder having average grain diameter of 100 µm or less, particularly high purity silica powder having average volume diameter of 0.1 to 20 µm be used as silica powder to be used in the plastic material. Moreover, regarding the shape of silica powder, equiaxed grain, e.g., spherical grain is preferable for extrusion. Moreover, silica powder to which dopant is added may be used so as to control a refractive index. The surfactant to be used in the present invention is not limited if the surfactant decreases the surface energy between the grain of silica powder and water. In consideration of the use as a porous preform, a nonionic surfactant is preferably used. For example, an ethyleneoxide type surfactant such as polyoxyethylenealkylether having HLB (HydrophileLipophile Balance) value in the range of 6 to 10 can be used. There is no special limitation of the amount of addition of the surfactant in the plastic material. For example, in a case that the ethyleneoxide type surfactant such as polyoxyethylenealkylether having HLB value in the range of 6 to 10 is used as a surfactant, 0.01 to 0.5 parts by weight of surfactant is preferably used relative to 100 parts by weight of silica powder. A screw type vacuum extrusion machine or a piston type extrusion machine can be used as an extrusion machine. In the case that the piston type extrusion machine is used, deaeration must be performed in advance when the plastic material is kneaded. Moreover, there was used an extrusion machine having an auxiliary cylinder for stopping the rotation of the material between an extrusion screw and a die as an extrusion machine (Japanese Patent Application No. 3-218291). As a result, optical and physical characteristics of the preform was further improved by synergism with lamination prevention effect. In this case, as an auxiliary cylinder, it is possible to use a cylinder in which a resistance portion is formed by roughening the inner surface of the cylinder, or cylinders 10, 20, 30, and 40 having the shape as shown in Figs. 1 to 4. The method of the present invention may be applied to a case in which a core is formed in advance and a clad is formed in the outside of the core, or a case in which the core and clad are formed at the same time. In the case that the clad is formed in the outside of the core, the core may be formed of a core portion and a part of a clad portion. Fig. 5 is a cross sectional view showing an example of an extrusion head of the extrusion machine to be used in the method of the present invention, and Fig. 6 is a cross sectional view taken along the line of E - E of Fig. 5. The extrusion head shown in Figs. 5 and 6 is attached to the top end of an auger cylinder 61, and arranged on an extended straight line to an axis of an auger screw 62. A rod holding member 54 is supported by a suspending member 53 at the center of a body portion 52 of the extrusion head. A passage 51 for a core glass rod is provided at the center of the rod holding member 54. The passage 51 for the core glass rod and a passage for a clad plastic material are concentrically arranged. The body portion 52 and the suspending member 53 are fixed to each other by a bolt 56. A die 57 comprises a linear portion 58 and a taper portion 59, which are detachable. The manufacturing method of the porous preform using the extrusion head of Fig. 5 will be explained with reference to Figs. 7A to 7E. As shown in Fig 7A, a clad plastic material 75 is extruded to the end of the passage 51 of a core glass rod 71 through the passage 55, and the core glass rod 71 is inserted to the passage 51 through the linear portion 58 of the die 57. The core glass rod 71 comprises a silica glass rod 72, which is made of the core or the core and a part of the clad, a dummy glass rod 73, which is welded to one end of the glass rod 72 and which has the same diameter as that of the silica glass rod 72, and a support rod 74, which is welded to the other end of the silica glass rod 72 and which has the same diameter as that of the silica glass rod 72 or larger than that of the silica glass rod 72. As shown in Fig. 7B, the clad plastic material 75 is extruded to the linear portion 58 of the die 57. At this time, the clad plastic material 75 and the core glass rod 71 are integrated at the top end of the passage 51 of the core glass rod 71. Moreover, if the clad plastic material 75 is extruded, a porous molding member 76 is extruded from the top end of the die 57, and the core glass rod 71 retreats as shown in Fig. 7C. At the time when the porous molding member 76 having the same length as the glass rod 72 is extruded, the porous molding member 76 is cut at the top end of the die as shown in Fig. 7D, and the dummy glass rod is extracted from the die. Finally, the linear portion 58 of the die 57 is divided and detached, and extra plastic material 77 existing in the linear portion 58 is cut along the top end of the taper portion 59 of the die 57, that is, the top end of the passage 51 of the core glass rod 71, and removed as shown in Fig. 7E. Thereafter, the linear portion 58 of the die 57 is attached, and the process is returned to the process shown in Fig. 7A, thereby making it possible to continuously manufacture the porous preform. In the extrusion head of Fig. 5, the passage 51 of the core glass rod 71 is closed at the end portion of the rod holding member 54. However, as shown in Fig. 8, the passage 51 may be formed to pass through the central axis of the the rod holding member 54, and extend in the rotation axis of an auger screw 62. In this case, the passage 51 in the rod holding member 54 and a passage 81 in the rotation axis of the auger screw 62 are connected to each other at the top end of the auger screw 62. Therefore, the portion between the rod holding member 54 and the auger screw 62 is sealed by a packing or an O-shape ring 82 so as to prevent plastic material from being introduced into the passages 51 and 81. In this case, the auger screw 62 rotates, but the rod holding member 54 does not rotate since the rod holding member 54 is fixed. Then, degreasing, dehydrating, and sintering processes are provided in the above-obtained porous preform, so that an optical fiber preform can be obtained. In degreasing process, a binder and a surfactant is removed from a porous preform. Examples of the present invention will be explained as follows. Example 1 36 parts by weight of 10% liquid solution of polyvinyl alcohol (PVA) were added to 100 parts by weight of silica fine particles having an average grain diameter of 0.7 µm synthesized by a frame hydrolyzing method. 0.3 parts by weight of polyoxyethylenealkylether as surfactant were further added thereto, and these materials were homogeneously kneaded, so that a core plastic material was obtained. 3.5 weight % of GeO 2 serving as dopant was contained in the silica fine particles in order to improve a refractive index. 3 parts by weight of methyl cellulose serving as binder and 22 parts by weight of pure water serving as solvent were added to 100 parts by weight of silica particles having an average grain diameter of 8 µm. 0.3 parts by weight of polyoxyethylenealkylether as surfactant were further added thereto, and these materials were homogeneously kneaded, so that a clad plastic material was obtained. The obtained core plastic material and clad plastic material were thrown to first and second hoppers 90 and 91 of an extrusion machine 9 shown in Fig. 9, respectively and extruded and molded. In the extrusion machine 9, a core plastic material 92 was supplied to a cross head 95 via a first kneading chamber 93 and a first vacuum chamber 94. A clad plastic material 96 was supplied to the cross head 95 via a second kneading chamber 97 and a second vacuum chamber 98. In the cross head 95, the core and clad were formed, so that a rod-shape porous preform 99 was obtained. In this case, the extrusion speed of the core plastic material was set to 0.2 kg/h, and the extrusion speed of the clad plastic material was set to 40 kg/h. Next, the obtained porous preform was dried at 110°C for 12 hours. Thereafter, degreasing process was provided to the dried preform in air at 500°C for 5 hours. Moreover, the preform was dehydrated with Cℓ 2 of 1200°C under He gas atmosphere, and sintered at 1600°C under He atmosphere, so that an optical fiber preform was manufactured. A normal drawing process was provided to the obtained optical fiber preform, so that a single mode optical fiber having an outer diameter of 125 µm was manufactured. Then, the mechanical strength was measured. A value of the mechanical strength was made by measuring 50% values of tension strength and breaking strength of the single mode optical fiber. As a result, the value was 6.7 kgf, which was the same value as the mechanical strength of the optical fiber obtained by drawing the optical fiber preform obtained by the conventional vapor method. Comparison A porous preform was manufactured in the same manner as Example 1 excepting that the material containing no surfactant is used as a clad plastic material. Similar to Example 1, degreasing and sintering processes were provided to the obtained porous preform, thereby obtaining an optical fiber preform. Then, the optical fiber preform was drawn, so that a single mode optical fiber was manufactured, and the mechanical strength was measured in the same manner as Example 1. As a result, the mechanical strength was 1.1 kgf. Example 2 Regarding a clad plastic material, 3 parts by weight of methyl cellulose serving as binder and 22 parts by weight of pure water serving as solvent were added to 100 parts by weight of silica particles having an average grain diameter of 8 µm. 0.3 parts by weight of polyoxyethylenealkylether as surfactant were further added thereto, and these materials were homogeneously kneaded, so that the clad plastic material was obtained. A core glass rod was manufactured by VAD method. The core glass rod to be used had a ratio of clad to core of about 3 : 1, a specific refractive index difference of about 0.3%, an outer diameter of about 8.5 mm, and a length of about 300 mm. A dummy rod made of silica glass having a diameter of about 8.5 mm and a length of 50 mm was connected to one end of the core glass rod, and a support glass rod having a diameter of about 25 mm and a length of 150 mm was connected to the other end. The above-mentioned clad plastic material and the core glass rod were used and extruded and molded by the extrusion head of Fig. 5 in accordance with the processes of Figs. 7A to 7E, thereby a porous preform was obtained. Then, the obtained porous preform was dried at 110°C for 12 hours. Thereafter, degreasing process was provided to the dried preform in air at 500°C for 5 hours. Moreover, the preform was dehydrated with Cℓ 2 of 1200°C under He atmosphere by the normal method, and sintered at 1600°C under He atmosphere, so that an optical fiber preform was manufactured. In these processes, neither crack nor warp was generated in the preform. A normal drawing process was provided to the obtained optical fiber preform, so that a single mode optical fiber having an outer diameter of 125 µm was manufactured. Then, the mechanical strength was measured similar to Example 1. As a result, the value was the same value as the mechanical strength of the optical fiber obtained by drawing the optical fiber preform obtained by the conventional vapor method. According to the above embodiment, since the passage of the plastic material in the extrusion machine is linear, and the direction is not changed, the flow of the plastic material is extremely homogeneous. Therefore, the optical fiber porous preform having neither distortion nor variation of density can be obtained. Thereby, after forming the porous preform and the following heating process, it is possible to control the generation of the crack and the warp. According to the above-explained method for manufacturing an optical fiber preform of present invention, since a plastic material containing a surfactant is used, a preform, which can obtain an optical fiber exerting high mechanical strength, can be efficiently manufactured without generating bubble.

Claims: A method of manufacturing an optical fiber preform including the steps of: obtaining a plastic material by kneading silica power, a binder, water, and a surfactant; and obtaining an optical fiber preform (76) including a core portion (71) and a clad portion (75) by extrusion-molding the plastic material onto an outside portion of the core portion (71) to thereby form the clad portion (75). The method according to claim 1, characterized in that the core portion (71) is a plastic material, and is extruded at the same time as extruding the clad portion (75). The method according to claim 1, characterized in that the core portion (71) is a glass rod formed of a core portion and a part of a clad portion. The method according to claim 1, characterized in that a nonionic surfactant is used as a surfactant. The method according to claim 1, characterized in that the amount of addition of the surfactant is that 0.01 to 0.5 parts by weight of surfactant is used relative to 100 parts by weight of silica powder.

Inventors: KOINUMA YOSHIAKI, SATOH TSUGUO, YAGI TAKESHI, YOSHIDA KAZUAKI, KOINUMA, YOSHIAKI, SATOH, TSUGUO, YAGI, TAKESHI, YOSHIDA, KAZUAKI, Koinuma, Yoshiaki, c/o THE FURUKAWA ELEC. CO., LTD, Satoh, Tsuguo, c/o The Furukawa Electric Co., Ltd., Yagi, Takeshi, c/o The Furukawa Electric Co., Ltd., Yoshida, Kazuaki, c/o THE FURUKAWA ELEC. CO., LTD
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1993
Main classification: C03B37
IPC codes: B29D11, C03B37