Method for fabricating a preform for a plastic optical fibre.

Abstract: The present invention relates to a method for fabricating a preform for a plastic optical fiber and a preform fabricated thereby. Particularly, the present invention provides a new method for fabricating a preform for a plastic optical fiber, having continuous refractive index profile in a radial direction, comprising the following steps: (a) introducing a mixture consisting of at least two substances, having a different density and refractive index each other to a cylindrical reactor and polymerizing the mixture in a centrifugal field generated by revolving the reactor; (b) compensating a vacancy formed on account of a volume shrinkage caused by polymerization in the said step (a) with a mixture consisting of at least two substances, having a different density and refractive index each other, and polymerizing the mixture in a centrifugal field generated by revolving the reactor; and (c) repeating the step(b) until a vacancy is not formed after polymerization.

Description: The present invention relates to a method for fabricating a preform for a plastic optical fiber and the preform fabricated thereby. Particularly, the present invention relates to a method for fabricating a preform for a plastic optical fiber having a refractive index gradient in a radial direction and no vacancy therein, wherein additional monomer or prepolymer is charged into the vacancy formed after polymerization so as to compensate a volume shrinkage generated upon polymerizing the monomer in a centrifugal field, and then is reacted. The optical fibers for communication are generally classified into a single-mode fiber and a multi-mode fiber in terms of the transmission mode of optical signal. The currently used optical fibers for long distance high speed communication are mostly the step-index single-mode optical fibers based on quarts glass, and the fibers have a minute diameter as small as 5 µm to 10 µm. Accordingly, these glass optical fibers have a serious difficulty in alignment and connection, and thus cause the expensive costs for them. Alternatively, the multi-mode glass optical fibers having larger diameter than diameter of single-mode optical fiber may be used for a short distance communication such as local area network (LAN). However, since these multi-mode glass optical fibers require the expensive cost for alignment and connection and are fragile, they cannot be predominantly used. Therefore, they have been mainly used for a short distance communication up to 200 meters such as LAN using a metal cable, for example, the twisted pair or coaxial cable. However, since the information transmission speed or bandwidth of the metal cable is as low as about 150 Mbps and thus can not come up with the transmission speed of 625 Mbps which is a 2000 years' standard in accordance with Asynchronous Transfer Mode (ATM), it can not satisfy the future standard of transmission speed. To cope with these problems, Japan, the United States of America, and the like have made great efforts and investments for the past 10 years for development of the polymer optical fibers, which can be used in a short distance communication such as LAN. Since the diameter of polymer optical fiber can be as large as 0.5 to 1.0 mm which is 100 or more times than that of glass optical fiber, due to its flexibility, its alignment and connection will be more easier, the polymer-based connector produced by compression molding can be used for alignment and connection and thus the cost can be reduced. On the other hand, the polymer optical fiber may have a step-index (SI) structure in which a refractive index changes stepwise in a radial direction, or a graded-index (GI) structure in which a refractive index changes gradually in a radial direction. However, since SI polymer optical fiber has high modal dispersion, the transmission speed (or bandwidth) of signal can not be higher than that of cable. Meanwhile, since GI polymer optical fiber has low modal dispersion, it can have a high bandwidth. Accordingly, since GI polymer optical fiber can be fabricated in a reduced cost due to its big diameter, the bandwidth can be maintained as high as possible on account of low modal dispersion. Therefore, it is known that GI polymer optical fiber is adequate for a medium of the short distance high-speed communication because of a reduced cost derived from its large diameter and a high speed of information transmission derived from low modal dispersion. The conventional method for fabricating GI polymer optical fiber was first reported by a Japanese professor, Koike Y. et al. of Keio University in 1988[refer to " Koike, Y. et al., Applied Optics , Vol . 27, 486(1988) "]. Since then, the related-techniques were reported in USP 5,253,323 (Nippon Petrochemicals Co.); USP 5,382,448 (Nippon Petrochemicals Co.); USP 5,593,621 to Yasuhiro Koike and Ryo Nihei; WO92/03750 (Nippon Petrochemicals Co.); WO92/03751; Japanese Patent Laid-Open No. 3-78706 (Mitsubishi Rayon Co., Ltd.); and Japanese Patent Laid-Open No. 4-86603 (Toray Ind.). The methods disclosed in these prior patents are mainly classified into two methods as follows. The first method is a batch process wherein a preliminary molding product, namely a preform in which a refractive index changes in a radial direction is fabricated, and then the resultant preform is heated and drawn to fabricate GI polymer optical fiber. The second method is a process wherein a polymer fiber is produced by extrusion process, and then the low molecular material contained in the fiber is extracted, or contrarily introduced in a radial direction to obtain GI polymer optical fiber. It is known that the first batch-type process made by a professor Koike could successfully fabricate a GI polymer optical fiber having a transmission speed of 2.5 Gbps, and that the second method could also successfully fabricate a polymer optical fiber having a relatively high bandwidth. Van Duunhoven reported another method for fabricating GI polymer optical fiber wherein when the monomers having different density and refractive index are polymerized in a centrifugal field, a concentration gradient is generated on account of density gradient, and thus the refractive index gradient is generated as disclosed in WO 97/29903. In other words, when two kinds of monomers having different density and refractive index are polymerized in a centrifugal field, a concentration gradient is generated by a density difference, and a refractive index gradient is generated by the concentration gradient, provided that the refractive index of the monomer with high density is higher than that of the monomer with low density. However, the said reference did not mention about the problem inevitably caused by volume shrinkage. In other words, since volume shrinkage occurs when monomers are polymerized (to produce a polymer), the preform for a plastic optical fiber fabricated in a centrifugal force is vacant in its center to form a shape of tube. Accordingly, when the optical fiber is fabricated by using the volume-shrunk preform, a discontinuity of refractive index profile appears as much as the vacancy occupy, which leads to a remarkable reduction of transmission amount not so as to be actually used. Alternatively, when a preform for a plastic optical fiber having a large volume is fabricated by a conventional method, since it has a tubular shape due to a volume shrinkage and a high speed revolution of the preform, the resultant plastic optical fiber deteriorates in quality. Addition to that, a preform for a plastic optical fiber cannot be fabricated at a low cost. According to one aspect of the present invention, there is provided a method for fabricating a preform for a plastic optical fiber comprising the following steps: (a) introducing a mixture consisting of at least two substances, having a different density and refractive index each other to a cylindrical reactor and polymerizing the mixture in a centrifugal field generated by revolving the reactor; (b) compensating a vacancy formed on account of a volume shrinkage caused by polymerization in the said step(a) with a mixture consisting of at least two substances, having a different density and refractive index each other, and polymerizing the mixture in a centrifugal field generated by revolving the reactor; and (c) repeating the step (b) until a vacancy is not formed after polymerization. The invention provides a method for fabricating a preform for a plastic optical fiber wherein an additional monomer and/or prepolymer is charged into the vacancy formed on account of the volume shrinkage generated upon producing the preform in a centrifugal field, and then polymerized to prevent from discontinuous refractive index profile in a radial direction. Examples of the invention will now be described in detail with reference to the accompanying drawings, in which: Fig. 1 is a diagram illustrating a series of processes for fabricating a preform for a plastic optical fiber according to the first embodiment of the present invention; Fig. 2 is a diagram illustrating a series of processes for fabricating a preform for a plastic optical fiber according to the second embodiment of the present invention; Fig. 3 is a diagram illustrating a series of processes for fabricating a preform for a plastic optical fiber according to the third embodiment of the present invention; Fig. 4 is a schematic drawing illustrating the apparatus for fabricating a preform for a plastic optical fiber according to the present invention; Fig. 5 is a schematic drawing illustrating the apparatus for continuously charging the monomers into the reactor used in the fabrication of a preform for a plastic optical fiber according to the present invention; Fig. 6 is a graph illustrating a refractive index profile of a preform for a plastic optical fiber fabricated in Examples 1 and 4 according to the present invention; Fig. 7 is a graph illustrating a refractive index profile of a preform for a plastic optical fiber fabricated in Example 2 according to the present invention; Fig. 8 is a graph illustrating a refractive index profile of a preform for a plastic optical fiber fabricated in Example 3 according to the present invention; Fig. 9 is a graph illustrating a refractive index profile of a preform for a plastic optical fiber fabricated in Example 5 according to the present invention; and Fig. 10 is a graph illustrating a refractive index profile of a preform for a plastic optical fiber fabricated in Example 6 according to the present invention. The present invention provides a method for fabricating a preform for a plastic optical fiber employing a centrifugal force, characterized by that the additional monomers are compensated to the vacancy formed by volume shrinkage so as to have a continuous refractive index profile in a radial direction in its interfaces. The present invention has the following three embodiments in terms of the method for introducing the monomers. Fig. 1 is a diagram illustrating a series of processes for fabricating a preform for a plastic optical fiber according to the first embodiment of the present invention. A mixture consisting of at least two kinds of monomers having different density and refractive index each other is prepared, charged into the reactor, and then polymerized in the centrifugal field. The mixture of the said monomers is additionally introduced to the vacancy formed by volume shrinkage generated during the polymerization, and then polymerized again. By repeating these procedures of introduction of a mixture of the monomers and polymerization, the preform for a plastic optical fiber having no vacancy and continuous refractive index profile in a radial direction is obtained. Fig. 2 is a diagram illustrating a series of processes for fabricating a preform for a plastic optical fiber according to the second embodiment of the present invention. Mixtures consisting of at least two kinds of monomers having a different density and refractive index are prepared in separate proportions. One of the mixtures having a certain composition is introduced to a reactor, and then polymerized in a centrifugal field. The other mixture of which proportion is different from the first introduced mixture is introduced to the vacancy formed during the polymerization, and then polymerized. The steps of introduction of a mixture having a different composition from the mixtures previously introduced and the polymerization are repeated. As a result, a preform for a plastic optical fiber having no vacancy and a continuous refractive index profile in a radial direction is obtained. This method is similar to the first embodiment of the present invention, but there is a difference in that a continuous refractive profile can be easily attained in a radial direction with a smaller centrifugal force by using different proportions of the mixtures of the monomers charged into the vacancies. Fig. 3 is a diagram illustrating a series of processes for fabricating a preform for a plastic optical fiber according to the third embodiment of the present invention. A polymer having lower refractive index and higher density than the mixture of the monomers is crushed into particles as small as 5 mm or less, Then the particles is mixed with the mixture of the monomers, and then swelled in the monomer for 24 hours. The reason for crushing the polymer is to allow all the monomer particles to uniformly swell in the monomer and to shorten the swelling time. The resultant mixture of the monomer and the polymer is introduced to the reactor and then polymerized in a centrifugal field. When the vacancy is formed on account of the volume shrinkage, a mixture of the monomers is additionally introduced to the vacancy and polymerized again. By repeating such an introduction and polymerization of a mixture, the preform for a plastic optical fiber having a continuous refractive index profile in a radial direction and no vacancy can be obtained. Referring to the embodiments of the present invention, it is preferable that the mixture of monomers is introduced to the vacancy formed on account of volume shrinkage upon polymerization in a state of gel before the product is completely solidified. At this time, the introduction may be commenced in a continuous type, semi-continuous type or a batch type, with or without stopping rotation of the reactor. In the first and second embodiments, there has to use a pair of monomers which consisting of the mixture, one having high density and low refractive index, and the other having low density and high refractive index. On the other hand, in the third embodiment as long as the mixture of monomer have lower density and higher refractive index than the polymer, the refractive index gradient can be attained. Namely, the conditions for the reactants in the third embodiment cover those conditions for the reactants in the first and second embodiment. Further, the procedure for introducing the monomer can be reduced since the volume witch the polymer occupies is not shrunk. Alternatively, in the first, second and third embodiments of the present invention, the mixture of monomers can be made to a prepolymer, and then introduced to fill the vacancy formed on account of a volume shrinkage in a final step of repeating introduction of the mixture of monomers. Accordingly, when the last vacancy is filled with prepolymer to be polymerized, a ununiform refractive index profile in a longitudinal direction as the preform is grown, can be avoid, and thus a large preform having uniform refractive index profile in a longitudinal direction can be fabricated. At this time, the viscosity of the prepolymer adequate for prohibiting the deterioration of the yield due to a volume shrinkage of the preform as a final product is 50∼100,000 mPas (cps) (25°C), preferably 500~20,000 mPas (cps) (25°C). Since the present invention employs a principle that a refractive index gradient is formed by a centrifugal force in case that the centrifugal force is higher than the gravity, the preform for a plastic optical fiber has to satisfy the below formula: [Formula 1] ½ r 2 ω 2 > gh wherein r is radius (m) of a preform, ω is angular velocity (rad/s), g is gravitational acceleration (9.8 m/sec 2 ), and h is height (m) of a preform. The preform fabricated under the condition of low revolution speed (rpm) satisfying no such formula has the tubular shape, the diameter of the upper opening being large as it goes upward and the diameter of the lower opening being small as it goes downward. Accordingly, when the monomer is introduced again, the refractive index gradient in a longitudinal direction of the preform for a plastic optical fiber can cause a multiform of refractive index gradient. When rpm is set up in a certain value, r as a radius (m) of a preform has to be limited in accordance with the above formula. For example, in the case that a centrifugal force is 3,000 rpm and h is 100cm, it is calculated as follows: 3000 rpm = 3000 × revolution min × min 60 sec × 2π rad revolution = 314.16 rad/sec, and r 〉 2 gh ω = 2×980×100 314.16 = 1.41 cm. When r is less than 1.41, the preform has an tubular shape, the upper part being thin and the lower part being thick on account of gravity. Accordingly, the radius (r) of the preform should be higher than 1.41 cm. In general, the adequate radius of the preform is about 5~10 cm to make heat transfer for polymerization easy. The adequate length of the preform is up to 100 cm to achieve a proper thermal drawing. Two kinds of monomers having different density and refractive index used in the present invention are selected from the group consisting of methylmethacrylate, benzylmethacrylate, phenylmethacrylate, 1-methylcyclohexylmethacrylate, cyclohexylmethacrylate, chlorobenzylmethacrylate, 1-phenylethylmethacrylate, 1,2-diphenylethylmethacrylate, diphenylmethylmethacrylate, perfurylmethacrylate, 1-phenylcyclohexylmethacrylate, pentachlorophenylmethacrylate, pentabromophenylmethacrylate, styrene, TFEMA (2,2,2-trifluoroethylmethacrylate), PFPMA(2,2,3,3,3-pentafluoropropylmethacrylate), HFIPMA(1,1,1,3,3,3-hexafluoroisopropylmethacrylate) and HFBMA(2,2,3,3,4,4,4-heptafluorobuthylmethacrylate). Homopolymers or copolymers can be used as the polymers in accordance with the third embodiment of the present invention. The homopolymer is polymerized from monomer such as methylmethacrylate, benzylmethacrylate, phenylmethacrylate, 1-methylcyclohexylmethacrylate, cyclohexylmethacrylate, chlorobenzylmethacrylate, 1-phenylethylmethacrylate, 1,2-diphenylethylmethacrylate, diphenylmethylmethacrylate, perfurylmethacrylate, 1-phenylcyclohexylmethacrylate, pentachlorophenylmethacrylate, pentabromophenylmethacrylate, styrene, TFEMA (2,2,2-trifluoroethylmethacrylate), PFPMA (2,2,3,3,3-pentafluoropropylmethacrylate), HFIPMA(1,1,1,3,3,3-hexafluoroisopropylmethacrylate) and HFBMA(2,2,3,3,4,4,4-heptafluorobuthylmethacrylate). The copolymer includes, for example methylmethacrylate(MMA)-benzylmethacrylate (BMA) copolymer, styrene-acrylonitril copolymer(SAN), MMA-TFEMA(2,2,2-trifluoroethylmethacrylate) copolymer, MMA-PFPMA(2,2,3,3,3 -pentafluoropropylmethacrylate) copolymer, MMA-HFIPMA(1,1,1,3,3,3-hexafluoroisopropylmethacrylate) copolymer, MMA-HFBMA(2,2,3,3,4,4,4-heptafluorobuthylmethacrylate) copolymer, TFEMA-PFPMA copolymer, TFEMA-HFIPMA copolymer, styrene-methylmethacrylate copolymer and TFEMA-HFBMA copolymer. According to the embodiments of the present invention, the prepolymer introduced in a final step of fabricating the preporm for plastic optical fiber includes the copolymers consisted of at least two monomers selected from the group consisting of methylmethacrylate, benzylmethacrylate, phenylmethacrylate, 1-methylcyclohexylmethacrylate, cyclohexylmethacrylate, chlorobenzylmethacrylate, 1-phenylethylmethacrylate, 1,2-diphenylethylmethacrylate, diphenylmethylmethacrylate, perfurylmethacrylate, 1-phenylcyclohexylmethacrylate, pentachlorophenylmethacrylate, pentabromophenylmethacrylate, styrene, TFEMA(2,2,2-trifluoroethylmethacrylate), PFPMA(2,2,3,3,3-pentafluoropropylmethacrylate), HFIPMA(1,1,1,3,3,3-hexafluoroisopropylmethacrylate) and HFBMA(2,2,3,3,4,4,4-heptafluorobuthylmethacrylate). Fig. 4 is a schematic drawing illustrating an apparatus for fabricating a preform for a plastic optical fiber having a radial refractive index gradient in a radial direction with centrifugal force according to the present invention. A reactor 5 is rotatably connected to a motor. Since the direct connection of the axle of a reactor 5 to the axle of a motor 1 makes the operation unstable, the axle of a reactor 5 is connected to the axle of a motor 1 via a belt 2 as an indirect power transmission means to promote an operational stability. Although a reactor 5 can be designed so as to rotate at a maximum revolution speed of 25,000 rpm, it is excessively vibrated at a revolution speed of 10,000 rpm or more to make the reactor design difficult. And, when a reactor 5 rotates at less than 2,500 rpm, the thickness of the tubular preform becomes thicker downwardly. Accordingly, the revolution speed of a reactor 5 is preferably 3,000 to 8,000 rpm to attain an uniform refractive index gradient of the preform, and is controlled by a revolution speed-controller 10 connected to a motor 1. Air is heated by a heating coil 6 and blown to a reactor 5 through an air supplying pipe 7. The heated air stays in interior of the reactor 5 and then flows toward the exit. The temperature of the reactor 5 is measured at the exit and is controlled by a temperature controller 9. The outside of the cylinder is insulated by an insulating means 4 made of asbestos so that a reaction may be performed at a constant temperature. The rotating reactor 5 made of stainless steel consists of a top lid, a lower lid and a body which can be detached each other. A glass tube 11 is set in a reactor 5 and the reactant is introduced thereto through the glass tube 11. Fig. 5 is a schematic drawing illustrating an apparatus for continuously introducing the monomers during the course fabricating a preform for a plastic optical fiber of the present invention. As shown in Fig. 5, the glass tube 11 rotates to continuously supply the reactant. A supplier 3 is connected to the glass tube 11 in the reactor 5 and the stainless tube 14. As a stainless steel tube 14 is supported by a bearing 15 set in the supplier 3 so as to rotate in a state of interfacial contact with the belt 2, it can be retained in a fixed state even during the revolution. As an 0-ring 12 and a teflon lid 13 are provided between a glass tube 11 and the supplier 3, the supplier 3 can be retained in a fixed state even during the revolution. The reactant is continuously introduced into the reactor 5 through the supplier 3, and the motor 1 connected through the belt 2 rotates the reactor. The air heated during flowing through the pipe 7 thermally polymerizes the reactant. The monomers and the prepolymer can be continuously introduced without stopping revolution of the reactor by using the above supplier 3. As the heating medium for heating the reactant filled in the reactor 5, any one of inert gases as well as air, or the fluid, for example oil, may be used. The reactor 5 may be made of stainless steel, hastelloy, brass, aluminum, teflon, glass or other ceramics. But it will be understood that other materials may be used. A preform for a plastic optical fiber fabricated by the above process may be optionally subjected to a thermal drawing to transform it to a graded index plastic optical fiber (GI-POF) having a desired diameter, or may be processed to a relatively thick strand to make a graded index lens and an image guide for picture transmission. Examples As a pair of monomers having different density and refractive index, a styrene monomer (SM) and methyl methacrylate (MMA) were used. As a prepolymer, SM and MMA, MMA and benzylmethacrylate (BMA) were used. The refractive index of the preform for a plastic optical fiber according to the present invention was measured by using an interference microscope. Example 1: A solution consisting of 550 g of methylmethacrylate (MMA), 0.1% by weight of benzoylperoxide (BPO) as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was charged into a glass tubular reactor having 40 mm of diameter and 100 mm of length, was closed with a lid, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C for 12 hours to form a clad layer. The temperature of the reactor was lowered to a room temperature. A solution consisting of 80g of the monomers mixed with SM and MMA at a weight ratio of 40:60, 0.4% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was charged into the reactor, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C. After 7 hours, the temperature of the reactor was lowered to a room temperature. A solution consisting of 20g of the monomers mixed with SM and MMA at a weight ratio of 50:50, 0.4% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was introduced to the vacancy formed due to a volume shrinkage generated by a centrifugal force, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C. The said procedures were repeated several times to obtain a preform for a plastic optical fiber having no vacancy. The refractive index profile in a radial direction for the preform was determined, and the results were shown in Fig. 6. Example 2: A solution consisting of 550 g of MMA, 0.1% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was charged into a glass tubular reactor having 40 mm of diameter and 100 mm of length, was closed with a lid, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C for 12 hours to form a clad layer. The temperature of the reactor was lowered to a room temperature. A solution consisting of 20g of the monomers mixed with SM and MMA at a weight ratio of 40:60, 0.4% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was charged into the reactor, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C. After 5 hours, the temperature of the reactor was lowered to a room temperature. A solution consisting of 20g of the monomers mixed with SM and MMA at a weight ratio of 45:55, 0.4% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was introduced to the vacancy formed due to a volume shrinkage generated by a centrifugal force, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C. The said procedures were repeated by using the monomers mixed with SM and MMA at a weight ratios of 50:50, 55:45, and 60:40, respectively to obtain the preforms for plastic optical fiber having no vacancy. The refractive index profile in a radial direction for the preform was determined, and the results were shown in Fig. 7. Example 3: A solution consisting of 40g of the monomers mixed with SM and MMA at a weight ratio of 40:60, 0.4% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was charged into a glass tubular reactor, and was reacted at a temperature of 65°C. After the reaction was completed, the obtained copolymer was crushed in a small size, and then dipped for 24 hours in 40g of the monomers mixed with SM and MMA at a weight ratio of 60:40. The obtained mixture was charged into a glass tubular reactor having 40 mm of diameter and 100 mm of length, was closed with a lid, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C for 7 hours. After the temperature of the reactor was lowered to a room temperature, 13g of the monomers mixed with SM and MMA at a weight ratio of 60:40, 0.4% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent were mixed. The mixture was introduced to the vacancy formed due to volume shrinkage generated by a centrifugal force, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C. The said procedures were repeated several times to obtain preforms for plastic optical fiber having no vacancy. The refractive index profile in a radial direction for the preform was determined, and the results were shown in Fig. 8. Example 4: The same procedure as Example 1 was conducted except that after forming a clad layer, the monomers mixed with SM and MMA at a weight ratio of 40:60 were polymerized at a temperature of 65°C and a revolution speed of 5,000 rpm for 7 hours to obtain a preform for a plastic optical fiber having no vacancy. The refractive index profile in a radial direction for the preform was determined to know a refractive index depending on a revolution speed (rpm), and the results were shown in Fig. 6. Example 5: A solution consisting of 550 g of MMA, 0.1% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was charged into a glass tubular reactor having 60 mm of diameter and 300 mm of length, was closed with a lid, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C for 12 hours to form a clad layer. The temperature of the reactor was lowered to a room temperature. A solution consisting of 150g of the monomers mixed with SM and MMA at a weight ratio of 5:95, 0.1% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was charged into the reactor, was closed with a lid, and then was reacted at a revolution speed of 4,000 rpm and a temperature of 65°C. A solution consisting of 70g of the monomers mixed with SM and MMA at a weight ratio of 10:90, 0.1% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was introduced to the vacancy formed due to a volume shrinkage generated by a centrifugal force, was closed with a lid, and then was reacted at a revolution speed of 5,000 rpm and a temperature of 65°C. Finally, A solution consisting of 100g of the monomers mixed with SM and MMA at a weight ratio of 15:85, 0.1% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was introduced into another reactor, and then was reacted at a temperature of 65°C for 3 hours to fabricate a prepolymer having a viscosity of 1,000 cps. After the obtained prepolymer was filled to the vacancy, the reactor was closed with a lid, and the reaction was commenced at a temperature of 65°C for 12 hours without a revolution. Additionally, the reaction was commenced at a temperature of 80°C for 24 hours to fabricate a large preform having no vacancy. The refractive index profile in a radial direction for the preform was determined, and the results were shown in Fig. 9. Example 6: A solution consisting of 550 g of MMA, 0.1% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was charged into a glass tubular reactor having 60 mm of diameter and 300 mm of length. The reactor was closed with a lid, and then was reacted at a revolution speed of 3,000 rpm and a temperature of 65°C for 12 hours to form a clad layer. The temperature of the reactor was lowered to a room temperature. A solution consisting of 150g of the monomers mixed with SM and MMA at a weight ratio of 5:95, 0.1% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was charged into the reactor, was closed with a lid, and then was reacted at a revolution speed of 4,000 rpm and a temperature of 65°C. A solution consisting of 70g of the monomers mixed with SM and MMA at a weight ratio of 10:90, 0.1% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was introduced to the vacancy formed due to a volume shrinkage generated by a centrifugal force, was closed with a lid, and then was reacted at a revolution speed of 5,000 rpm and a temperature of 65°C. Finally, a solution consisting of 100g of the monomers mixed with SM and MMA at a weight ratio of 15:85, 0.1% by weight of BPO as a reactive initiator and 0.2% by weight of 1-butanethiol as a chain transfer agent was introduced into another reactor, and then was reacted at a temperature of 65°C for 3 hours to fabricate a prepolymer having a viscosity of 2,000 mPas (cps). After the obtained prepolymer was filled into the vacancy, the reactor was closed with a lid, and the reaction was commenced at a temperature of 65°C for 12 hours without a revolution. Additionally, the reaction was commenced at a temperature of 80°C for 24 hours to fabricate a large preform having no vacancy. The refractive index profile in a radial direction for the preform was determined, and the results were shown in Fig. 10. Comparative Example 1: SM and MMA were mixed at a weight ratio of 40:60, and polymerized at a temperature of 65°C and a revolution speed of 3,000 rpm for 7 hours to obtain a preform for a plastic optical fiber. The obtained preform for a plastic optical fiber had vacancy in its center, and the diameter of the empty vacancy in the preform was about a half of the total diameter of preform. Example 7: Fabrication of Optical Fiber The preforms fabricated in Examples 2 and 5 were subjected to a thermal drawing at a temperature of 220°C to produce plastic optical fibers having thickness of 0.5mm, respectively. The loss of transmission for the optical fibers at 650 nm was 180 dB/km and 200 dB/km, respectively. Effects of the Invention The present invention provides the new method for fabricating a preform for a plastic optical fiber having a continuous refractive index gradient and thus no discontinuity of refractive index profile in a radial direction.

Claims: A method for fabricating a preform for a plastic optical fiber comprising the following steps: (a) introducing a mixture consisting of at least two substances, having a different density and refractive index each other to a cylindrical reactor and polymerizing the mixture in a centrifugal field generated by revolving the reactor; characterised by , (b) compensating a vacancy formed on account of a volume shrinkage caused by polymerization in the said step(a) with a mixture consisting of at least two substances, having a different density and refractive index each other, and polymerizing the mixture in a centrifugal field generated by revolving the reactor; and (c) repeating the step (b) until a vacancy is not formed after polymerization. A method for fabricating a preform for a plastic optical fiber according to Claim 1, wherein at least two substances having a different density and refractive index each other used in steps (a), (b) and (c) are two kinds of monomers. A method for fabricating a preform for a plastic optical fiber according to Claim 2, wherein the proportion of the monomers of the mixture used in steps (a), (b) and (c) is same each other. A method for fabricating a preform for a plastic optical fiber according to Claim 2, wherein the proportion of the monomers of the mixtures used in steps (a), (b) and (c) are different each other. A method for fabricating a preform for a plastic optical fiber according to Claim 1, wherein at least two kinds of substances consisting of the mixture having a different density and refractive index each other in step (a) are two kinds of monomers and polymer, and at least two kinds of substances consisting of the mixture having a different density and refractive index each other in the steps (b) and (c) are the said two kinds of monomers. A method for fabricating a preform for a plastic optical fiber according to Claim 5, wherein the proportion of the monomers of the mixtures in the steps (b) and (c) are same each other. A method for fabricating a preform for a plastic optical fiber according to Claim 5, wherein the proportion of the monomers of the mixtures used in the steps (b) and (c) are different each other. A method for fabricating a preform for a plastic optical fiber according to Claim 2, wherein the said two kinds of monomers are a monomer and another monomer having lower refractive index and higher density than that. A method for fabricating a preform for a plastic optical fiber according to Claim 5, wherein the mixture of the said two kinds of monomers has higher refractive index and lower density than the polymer. A method for fabricating a preform for a plastic optical fiber according to Claim 5, wherein the mixture of the step (a) is obtained by crushing the polymer, mixing the crushed polymer with two kinds of monomers, and then swelling it for 24 hours. A method for fabricating a preform for a plastic optical fiber according to Claim 2 or 5, wherein two kinds of monomers are selected from the group consisting of methylmethacrylate, benzylmethacrylate, phenylmethacrylate, 1-methylcyclohexylmethacrylate, cyclohexylmethacrylate, chlorobenzylmethacrylate, 1-phenylethylmethacrylate, 1,2-diphenylethylmethacrylate, diphenylmethylmethacrylate, perfurylmethacrylate, 1-phenylcyclohexylmethacrylate, pentachlorophenylmethacrylate, pentabromophenylmethacrylate, styrene, TFEMA(2,2,2-trifluoroethylmethacrylate), PFPMA(2,2,3,3,3-pentafluoropropylmethacrylate), HFIPMA(1,1,1,3,3,3-hexafluoroisopropylmethacrylate) and HFBMA(2,2,3,3,4,4,4-heptafluorobuthylmethacrylate). A method for fabricating a preform for a plastic optical fiber according to Claim 5, wherein the said polymer is a homopolymer of a monomer selected from the group consisting of methylmethacrylate, benzylmethacrylate, phenylmethacrylate, 1-methylcyclohexylmethacrylate, cyclohexylmethacrylate, chlorobenzylmethacrylate, 1-phenylethylmethacrylate, 1,2-diphenylethylmethacrylate, diphenylmethylmethacrylate, perfurylmethacrylate, 1-phenylcyclohexylmethacrylate, pentachlorophenylmethacrylate, pentabromophenylmethacrylate, styrene, TFEMA(2,2,2-trifluoroethylmethacrylate), PFPMA(2,2,3,3,3-pentafluoropropylmethacrylate), HFIPMA(1,1,1,3,3,3-hexafluoroisopropylmethacrylate) and HFBMA(2,2,3,3,4,4,4-heptafluorobuthylmethacrylate). A method for fabricating a preform for a plastic optical fiber according to Claim 5, wherein the said polymer is a copolymer selected from the group consisting of methylmethacrylate(MMA)-benzylmethacrylate(BMA) copolymer, styrene-acrylonitril copolymer(SAN), MMA-TFEMA (2,2,2-trifluoroethylmethacrylate) copolymer, MMA-PFPMA(2,2,3,3,3-pentafluoropropylmethacrylate) copolymer, MMA-HFIPMA(1,1,1,3,3,3-hexafluoroisopropylmethacrylate) copolymer, MMA-HFBMA(2,2,3,3,4,4,4-heptafluorobuthylmethacrylate) copolymer, TFEMA-PFPMA copolymer, TFEMA-HFIPMA copolymer, styrene-methylmethacrylate copolymer and TFEMA-HFBMA copolymer. A method for fabricating a preform for a plastic optical fiber according to any one of Claims 2 to 7, wherein a prepolymer is filled in vacancy finally formed and then polymerized in the step (c). A method for fabricating a preform for a plastic optical fiber according to Claim 14, wherein the said prepolymer is copolymer made of at least two monomers selected from the group consisting of methylmethacrylate, benzylmethacrylate, phenylmethacrylate, 1-methylcyclohexylmethacrylate, cyclohexylmethacrylate, chlorobenzylmethacrylate, 1-phenylethylmethacrylate, 1,2-diphenylethylmethacrylate, diphenylmethylmethacrylate, perfurylmethacrylate, 1-phenylcyclohexylmethacrylate, pentachlorophenylmethacrylate, pentabromophenylmethacrylate, styrene, TFEMA(2,2,2-trifluoroethylmethacrylate), PFPMA(2,2,3,3,3-pentafluoropropylmethacrylate), KFIPMA(1,1,1,3,3,3-hexafluoroisopropylmethacrylate) and HFBMA(2,2,3,3,4,4,4-heptafluorobuthylmethacrylate). A method for fabricating a preform for a plastic optical fiber according to Claim 14, wherein the said prepolymer has a viscosity of 50∼100,000 cps (25°C). A method for fabricating a preform for a plastic optical fiber according to Claim 1, wherein the centrifugal field generated by the revolution of the reactor satisfy the following formula: [Formula 1] ½ r 2 ω 2 > gh wherein r is a radius (m) of the preform, ω is an angular velocity (rad/s), g is a gravitational acceleration (9.8 m/sec 2 ) , and h is a height (m) of the preform.

Inventors: CHO HAN SOL, HWANG JIN TEAK, IM SANG HYUK, PARK O OK, SUH DUCK JONG, CHO, HAN SOL, HWANG, JIN TEAK, IM, SANG HYUK, PARK, O OK, SUH, DUCK JONG
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 2001
Main classification: B29D11
IPC codes: C08F212, B29C39, C08F2, C08F291, B29D11, C08F220, B29C37, G02B6