METHOD OF MAKING A PLASTIC OPTICAL FIBRE, AND A PLASTIC OPTICAL FIBRE

Abstract: The invention relates to a method of making a plastic optical fibre, in which method plastic materials with suitable optical properties are fed into an extrusion apparatus so as to form a multilayer structure where the refractive index decreases gradually in a controlled manner from the inside outwards. The invention also relates to a plastic optical fibre. In order to provide flexible and continuous manufacture and a wideband fibre, the extrusion apparatus consists of at least one multilayer conical extruder unit (1) which produces a product where the layers are formed without weld lines, are symmetrical and adhere well to each other. The number of the layers may also be high.

Description: METHOD OF MAKING A PLASTIC OPTICAL FIBRE, AND A PLASTIC OPTICAL FIBRE The invention relates to a method of making a plastic optical fibre, in which method plastic materials with suitable optical properties are fed into an extrusion apparatus, which forms a multilayer structure where the refractive index decreases gradually from the inside outwards. The invention further relates to a plastic optical fibre. At present, optical fibres are rather well known in several fields of technology. Optical fibres are normally manufactured by forming a preform from a silica tube for example by means of a Modified Chemical Vapour Deposition (MCVD) method. Gases are blown into the preform, which is simultaneously heated, so that the gases react at a high temperature (ca 2000°C) to form silica glass (Si0 2 ) and mixed oxides (Ge0 2 , P 2 0 5 , B 2 0 3 ) at the inner surface of the tube. The process is repeated in a predetermined manner. The composition of the gas is changed by means of computer control, for example, so that a desired number of layers with different refractive indices are obtained superimposed at the tube's inner surface. In the next step, the preform is collapsed so that the centre hole of the tube disappears. An optical fibre is thereafter formed from the preform in a separate stage of operation by drawing at a high temperature, and the fibre is coated in a suitable manner, for example with acrylate that hardens in UV light. The problem with the basic principle described above is that it is complicated and the fibre manufacture is discontinuous. Discontinuity of fibre production means that an optical fibre of only a limited length can be formed from a preform. The process is such that the preform is drawn to the end, whereafter the drawing is started from another preform. Before this step a new preform has naturally been produced as described above, in this manner, it is possible to achieve only limited fibre lengths of, for example, 50 to 100 km. The process is characterized by the use of very high temperatures in each step. A drawback is the rather high costs of the fibre manufacture, which result from the sensitivity and poor controllability of the manufacturing process. Due to the above drawbacks, the production of graded index (Gl) optical fibres from plastic material has been developed greatly recently. Instead of conventional step index (SI) plastic fibres graded index plastic fibres come closer to the optical properties of glass fibres. Plastic optical fibres generally provide the following advantages over glass fibres: better mountability and splicing (larger diameter, splices), better processibility, better bending properties, recyclability, easier manufacture (considerably lower temperatures, continuous process) among other things. Due to higher damping than in silica glass, plastic fibres are mainly used in short-distance applications, which include office and factory automation, consumer electronics, computers, multimedia, electronic systems in aeroplanes and vehicles, and lighting. The manufacture of graded index plastic fibres has utilized so far mainly the same technique as the production of glass fibres, wherein a rod-like preform is made in the first step, and it is later drawn to a desired thickness in a separate step. In the formation of a preform, a mixture of methyl methacrylate and a compound (dopant) with a suitable refractive index is polymerized in a tube made of polymethyl methacrylate (PMMA) to provide a desired refractive index distribution (High bandwidth, low loss polymer fibers by Y. Koike in Proc. ECOC'92, Vol 2, 1992, pp 679-686). In order to minimize damping, it is preferable to use monomers where most of the hydrogen atoms are substituted with fluoride or deuterium. The refractive index distribution is based on the diffusion of the dopant during the polymerization. However, this process has several disadvantages; for example poor controllability of the diffusion and the refractive index profile, great amount of residue monomers (impurities), and the discontinuity of the process. Therefore the refractive index profile is not stable in the longitudinal direction of the fibre. Further, due to the poor controllability of the process, the use of the process is limited to a laboratory scale and it is thus not suitable for large-scale industrial production. In addition to the aforementioned prevailing technique, graded index plastic optical fibres can also be manufactured with an extrusion method. An example of prior art arrangements is a manufacturing method disclosed in Japanese Patent Application 7-291080 (publication number 9- 133818). This reference discloses an arrangement which enables the manufacture of an optical fibre structure comprising only a few, preferably at most six, layers. The arrangement is based on a conventional coextrusion technique where melt flows intended for different layers are combined in a complicated multilayer crosshead die. However, this arrangement cannot provide, in an advantageous manner, multilayer structures where the layers are very thin and/or there is a plurality of layers. Another drawback of the above-described arrangement is the high costs of the required apparatus if the manufacture is to take place on an industrial scale; a separate extruder is required for each layer. However, the greatest disadvantage technically is the formation of a structurally weak weld line in each layer due to the distribution of the melt flow. This, in turn, leads to instability of the layer dimensions, which further deteriorates the optical properties of the product. The purpose of the invention is to provide a method of making particularly a graded index plastic optical fibre, and a graded index plastic fibre, the method comprising none of the prior art drawbacks. The method is also suitable for the production of conventional step index plastic optical fibres. This is achieved with a method and plastic fibre according to the invention. The method according to the invention is characterized by continuously feeding several different plastic materials with suitable optical properties into an extrusion apparatus so as to form a multilayer structure where the refractive index decreases gradually from the inside outwards, the extrusion apparatus consisting of at least one multilayer conical extruder unit with which a fibre is extruded close to the correct diameter, whereafter the fibre is drawn to the final diameter immediately after the multilayer conical extruder unit. The optical fibre according to the invention is, in turn, characterized in that it comprises superimposed layers formed without weld lines from plastic materials with different refractive indices by means of a conical extrusion apparatus, and that the fibre is characterized by a controlled layer thickness and a stable refractive index distribution. The number of the layers may also be high (e.g. 10 to 50) and it can be modified easily. A primary advantage of the invention is that it enables the manufacture of very different plastic optical fibres in an advantageous manner. The invention provides multilayer structures with several round, symmetrical layers with no weld lines, which leads to a controlled refractive index distribution, a stable fibre structure and good centricity. An important advantage of the invention is that the mass flows of the materials intended for different layers do not have to be turned 90 degrees, and no weld line resulting from the tool is formed in the layers as in case of conventional machines. The fact that the product according to the invention is formed without weld lines means that each individual layer is strong and whole and has stable dimensions even when it is thin, which is the basis for the operation of a high- quality graded index plastic fibre. In this connection, it should be noted that when multilayer products are produced with conventional extruders, the different layers must be combined in a crosshead. This means that the mass flows must be turned and guided by means of mass distributors onto other layers, which always leads to errors in centricity and to differences in the homogeneity of the mass in different parts of the layer. It is not possible to obtain a product with equal centricity and quality as in a product produced by a multilayer conical extruder where the layers are concentric due to the structure and they comprise no seams resulting from the mass distribution. In crossheads, the mass is always distributed either around the material to be coated or around a layer that has been formed previously, which means that there will always be a seam in the structure. Only the core layer can be produced with good centricity if one extruder is positioned in the direction of the line. In order to provide a wide bandwidth, the refractive index distribution must be of an accurate form. Variation of the refractive index as a function of the radius of the plastic optical fibre can be described with the following formula: n(r) = n 1 (1-2Δ (r/a) B ) 1 2 (1 ) Δ = ( nι 2 - n k 2 )/2 nι 2 (2) where n, = the refractive index of the core, n k = the refractive index of the cladding, r = the fibre radius, a = the radius of the fibre section transmitting light, g = the exponent defining the form of the refractive index distribution The value g = 2 is often used, whereupon the distribution is parabolic. The optimum bandwidth may be obtained from the used wavelength of light also with other values of g. As regards the properties of a graded index plastic optical fibre, it is essential that the number of layers is high and the refractive indices of adjacent layers differ only a little so that the distribution resembles the continuous, stepless form of formula 1. This is achieved most preferably with the method according to the invention, which results in an optimum bandwidth with each fibre structure. Another essential feature of the invention is that the entire fibre structure is manufactured in one step, which ensures the purity of the materials and the adhesion between the layers. The compatibility of the different layers is ensured not only with the selection of the materials but also with control of the process parameters, such as temperatures and viscosities. The method according to the invention is characterized by a short residence time of the melt plastic at a high temperature and by low shear forces, which are particularly advantageous in the processing of thermosensitive materials, such as PMMA. Due to the aforementioned advantages, the graded index plastic optical fibres according to the invention provide a stable and even refractive index distribution in the longitudinal direction of the fibre, which cannot be achieved with conventional techniques. Also, it is essential that the method according to the invention is continuous, wherefore the length of the fibres produced with the method is not restricted in theory. It is characteristic of the method according to the invention that it enables flexible production of very different plastic optical fibres. The method does not restrict in any way the materials to be used, which is a significant advantage compared to the prior art method disclosed above and based on the polymerization of acrylate and the diffusion of dopant. It is possible to use in the method according to the invention any extrudable plastic materials with suitable optical properties, which can be combined almost without limits to adjust suitably the optical, thermal, mechanical and other properties of the plastic fibre. The invention will be described below in more detail by means of a preferred embodiment shown in the accompanying drawing, in which Figure 1 is a general side view of a conical extruder unit used in the method according to the invention, Figure 2 is a general side view of an arrangement utilizing the method according to the invention, and Figure 3 is a general sectional view of a plastic optical fibre made with the method according to the invention. Figure 1 shows the basic features of a conical extruder unit 1 used in the method according to the invention. The conical extruder unit 1 comprises a central channel, which is surrounded by stator and rotor parts. The rotor parts are rotated by a suitable power source 2. Plastic material is supplied to the rotor surfaces by means of feed screws or pressurized air, for example. The power source may be, for example, an electric motor, a hydraulic motor or some other suitable device. The conical extruder unit shown in Figure 1 forms a longitudinal layer structure where the layers are superimposed coaxially. The aforementioned facts are characteristic of a conical extruder, wherefore they will not be described in greater detail herein. Reference is made to WO 89/11961 , which describes the structure and operation of the aforementioned conical extruder. The basic idea of the invention is that the aforementioned conical extruder is used to manufacture preferably a graded index plastic optical fibre. In the manufacture, plastic materials with suitable optical properties are fed into an extrusion apparatus, which is used to form a multilayer structure where the refractive index decreases gradually from the inside outwards. An essential feature of the invention is that the extrusion apparatus consists of at least one conical extruder unit 1. The fibre is compressed in the multilayer conical extruder unit close to the correct diameter and the fibre is drawn to the final diameter immediately after the multilayer conical extruder unit at a suitable temperature. All the aforementioned steps are carried out in one stage of operation. Figure 2 shows generally a preferred embodiment of the invention utilizing two conical extruder units 1 placed one after the other. A product 3 to be prepared is drawn, either without cooling or after suitable cooling, from the previous conical extruder unit 1 to the next conical extruder unit 1. When the product is drawn through several extrusion units and hose tools are used, it is rather difficult to control the layer thicknesses. When hose tools are used, the product must be stretched slightly at each new layer, which also results in changes in the inner layer(s). It should also be noted that all the layers constitute a stiff mass if the product is not cooled occasionally. The layer thicknesses are also very small. Therefore the successive conical extruders 1 must also be synchronized together so that the layer thicknesses are reduced in the correct proportion from the inside outwards. It should be noted that the layers are formed in the invention such that the tool does not touch the inner layer in order not to damage it. The plastic material used for forming the layers is supplied to the conical extruder in a manner known per se. These matters constitute conventional technology to a person skilled in the art, wherefore they will not be described in greater detail herein. The process of Figure 2 is carried out in a clean space at a standardized pressure. Standard conditions can be provided, for example, by means of a suitable protective gas. The protective gas may be nitrogen, for example. The different refractive indices of the different layers are achieved by means of the properties of the plastic materials. Different refractive indices can be obtained, for example, by mixing together different plastic components. The components can be mixed either in a separate step or as a part of the fibre manufacture. However, it has been found particularly advantageous that the mixing is carried out by means of the conical extruder unit 1 , i.e. the same apparatus which forms the final product. The different refractive indices of the layers can naturally also be achieved by means of different plastic materials. In the example of Figure 2, 12 layers are formed in each conical extruder unit 1 , which means that in this example the final product 3 comprises 24 layers. The layers are formed by supplying selected plastic materials to the inner and outer surfaces of the rotors. The invention can therefore provide rather high numbers of layers. In principle, the final product may comprise an unlimited number of layers. Practical examples of numbers of layers include 10, 20 and even 50, which provides considerable bandwidths. Figure 3 shows generally a graded index plastic optical fibre according to the invention comprising 24 layers whose refractive index decreases gradually from the inside outwards. This product is manufactured by means of the arrangement shown in Figure 2. Figure 3 shows generally how the refractive index decreases from the inside outwards. The layers can be formed from any plastic material with suitable optical properties. Examples of such materials include polymethyi methacrylate (PMMA), polystyrene (PS), polycarbonate (PC), cyclic olefin copolymer (COC), polymethylpentene (PMP), silicon polymers, micro- crystalline polyamide (PA) or derivatives thereof, such as copolymers with suitable, for example fluoridated or deuterated, monomers. Different materials can naturally be modified for different layers suitably by mixing therein other polymers, softeners (plasticizing) or other suitable additives. The modification of the materials can be carried out either beforehand in a separate step, or in a mixer integrated into the conical extruder or in the extruder itself. The properties of a high-quality plastic optical fibre require great purity of both the materials and the process, which must be taken into account in the processing of the materials. The continuous nature of the method according to the invention and the opportunity of producing the entire multilayer structure in one stage of operation make the method particularly advantageous for the purity requirements of the product. The embodiment disclosed above is not intended to restrict the invention in any way, but the invention can be modified freely within the scope of the claims. Therefore it is clear that the implementation according to the invention or the details thereof do not have to be exactly as shown in the figures, but other kinds of arrangements are also possible. Figure 2 shows an arrangement with two conical extruder units. However, this is not the only alternative but the invention can naturally be applied also by means of for example one, three, four or any other number of conical extruder units. The invention can also be applied in connection with conical extruder units forming for example two, three, four or some other number of layers. It is also possible in the invention that a fibre or rod made from a homogenous plastic material with suitable optical properties in a separate step is coated by a desired number of layers extruded by a conical extruder to provide either a desired plastic optical fibre or a preform thereof. Further, according to the invention the plastic layers transmitting light can be coated by a cladding extruded in the same step from a suitable plastic material to provide mechanical, thermal and chemical protection and to prevent the contamination of the optical fibre already during the manufacture. The cladding is shown generally in Figure 3 by means of reference numeral 4.

Claims: CLAIMS 1. A method of making a plastic optical fibre in one stage of operation, characterized by continuously feeding several different plastic materials with suitable optical properties into an extrusion apparatus so as to form a multilayer structure where the refractive index decreases gradually from the inside outwards, the extrusion apparatus consisting of at least one multilayer conical extruder unit (1) with which a fibre is extruded close to the correct diameter, whereafter the fibre is drawn to the final diameter immediately after the multilayer conical extruder unit. 2. A method according to claim 1, characterized in that the extrusion apparatus consists of two or more multilayer conical extruder units (1) placed one after another, a product being made in one step by drawing the fibre, either without cooling or after suitable cooling, from a previous conical extruder unit (1) to the next conical extruder unit (1). 3. A method according to claim 2, characterized in that the space between the conical extruder units (1) is closed for example in order to cool the product or to use a protective gas. 4. A method according to claim 2 or 3, characterized in that the successive conical extruder units (1) are synchronized such that the layer thicknesses decrease in a desired proportion when moving from the inside outwards. 5. A method according to any one of the preceding claims 1 to 4, characterized in that the different refractive indices of the different layers are obtained by mixing the plastic components of the layers in the conical extruder unit (1). 6. A method according to any one of the preceding claims 1 to 4, characterized in that a plastic material with a suitable refractive index is manufactured in a separate device connected to the conical extruder, from which it is supplied to the extruder either in a molten form or as a liquid, powder or granulate. 7. A method according to claim ^ characterized in that the extrusion apparatus consists of one conical extruder, and the desired number of layers are provided, if necessary, in several extrusion steps. 8. A method according to any one of the preceding claims 1 to 7, characterized in that a fibre or rod made from a homogenous plastic material with suitable optical properties in a separate step is coated by a desired number of layers extruded by a conical extruder to obtain either a desired plastic optical fibre or a preform thereof. 9. A plastic optical fibre where the refractive index decreases gradually from the inside outwards, characterized in that it comprises superimposed layers formed without weld lines from plastic materials with different refractive indices by means of a conical extrusion apparatus, and that the fibre is characterized by controlled layer thickness and stable refractive index distribution. 10. A plastic optical fibre according to claim 9, characterized in that the number of the layers exceeds 10. 11. A plastic optical fibre according to claim 9 or 10, characterized in that the number of the layers is between 20 and 50. 12. A plastic optical fibre according to any one of the preceding claims 9 to 11, characterized in that the plastic material is polymethyi methacrylate, polystyrene, polycarbonate, polymethylpentene, cyclic olefin copolymer, microcrystalline polyamide, polysiloxane or a copolymer thereof or some other polymer with suitable optical properties. 13. A plastic optical fibre according to any one of the preceding claims 9 to 12, characterized in that in order to adjust the refractive indices, the plastic materials are modified suitably by mixing therein other polymers with suitable optical properties, softeners or other additives adjusting the refractive index. 14. A plastic optical fibre according to claim 9, character- i z e d in that the plastic layers transmitting light are coated by a cladding (4) extruded from a suitable plastic material in the same step to provide mechanical, thermal and chemical protection.

Inventors: HEINONEN JOUNI, KIRJAVAINEN KARI, SUVANTO MARKKU, HEINONEN, JOUNI, KIRJAVAINEN, KARI, SUVANTO, MARKKU
Applicants: HEINONEN JOUNI, KIRJAVAINEN KARI, NEXTROM HOLDING SA, NK CABLES OY, SUVANTO MARKKU, HEINONEN, JOUNI, KIRJAVAINEN, KARI, NEXTROM HOLDING S.A., SUVANTO, MARKKU
Application year: 1999
Main classification: G02B
IPC codes: B29C47, B29D11, G02B6