EP1618422A1

EP1618422A1 - Optical member with protective layer, method and apparatus of producing optical member with protective layer

Info

Publication number: EP1618422A1
Application number: EP04727142A
Authority: EP
Inventors: Toru Ogura; Takahito Miyoshi
Original assignee: Fuji Photo Film Co Ltd
Current assignee: Fujifilm Corp
Priority date: 2003-04-25
Filing date: 2004-04-13
Publication date: 2006-01-25
Also published as: KR20060002981A; EP1618422A4; WO2004097487A1; JP2004341489A

Abstract

A plastic optical fiber (POF) (11) having PMMA as the main component is formed. In a resin pot (12) is contained elastomer urethane (13) that is hardened at 80 oC. When the POF (11) is fed through the resin pot (12), elastomer urethane (13) is coated on the POF (11) to form a coated wire (15). The coated wire (15) is transferred to a water tank (17) the temperature of which is adjusted to be 80 oC. The elastomer through the water tank (17) is hardened to become a protective layer having the thickness of 450μm. Since the POF (11) is heated at 80 oC during the process to form the protective layer, the POF (11) is not transformed. Thus, it is possible to prevent influence in optical properties. In addition, the protective layer has sufficient mechanical strength.

Description

DESCRIPTION

OPTICAL MEMBER WITH PROTECTIVE LAYER, METHOD AND APPARATUS FOR PRODUCING OPTICAL MEMBER WITH PROTECTIVE LAYER

Technical Field

The present invention relates to an optical member with a protective layer, and a method and an apparatus for producing such optical member. More particularly, the present invention relates to a plastic optical fiber as an optical member, and a method and an apparatus for producing such optical member.

Background Art Originally, a plastic has merits of facility in designing the composition, high machinability and light weight . Moreover, the plastic has been improved in transparency, chemical stability, mechanical strength, and so forth. Since the plastic can exhibit variety and design facility suitable for optical members, the plastic has been recently utilized as an optical member instead of a glass.

Despite larger transmission loss than quartz optical fiber, a plastic optical fiber has various merits, such as facility in fiber connection due to large diameter, facility in fiber terminal process, non-necessity for core alignment with high precision, low cost of the connecters, low danger to prick into human body, easy construction, high resistance to vibration and low price. Accordingly, it is planned to utilize the plastic optical fiber not only as household and automobile purposes but as a short-distance, high-capacity cable such as inner wirings for high-speed data processing device and a digital video interface (DVI) link.

The plastic optical fiber is composed of a core whose main component is organic compounds in which polymer forms a matrix, and a cladding composed of organic materials having different refractivity from the core . The plastic optical fiber is produced by forming a fiber including the core and the cladding at the same time by drawing a pre-polymer. It is also possible to produce the plastic optical fiber by forming an optical fiber base body (hereinafter referred to "preform"), and melt-drawing the preform.

In producing the plastic optical fiber (hereinafter referred to as "POF") by use of the preform, it is possible to adjust the outer diameter of the plastic optical fiber by melt-drawing the preform at a temperature to soften the resin. During the melt-drawing process, the lower end of the preform is drawn to extend the preform while heating the preform in a cylindrical heating chamber by use of an electric heater. For instance, after holding the preform, the preform is slowly moved down into the heating chamber to melt the preform in the heating chamber. When the preform is softened enough that the molten preform is partially moved down due to its gravity, the leading end of the molten preform is drawn and hooked to a drawing roller, so that the preform is continuously extended to form the POF. In order to apply the POF, produced in this way, to various purposes , the outer surface of the POF is coated for protection (forming a protective layer, for instance) to form a plastic optical fiber cable (optical fiber cable) , or the POF is inserted into a tube with an inner diameter enough for containing the POF, although the bare POF is used for some limited purposes, such as an optical wiring inside a device. By protecting the POF, it is possible to prevent flaw, damage, structural irregularity such as micro-bending, can decrease in optical properties , and so forth, in handling the optical fiber or in using the optical fiber in a bad environment. Examples of the materials to protect the POF are thermoplastic resins, such as polyvinyl chloride, nylon (trademark), polypropylene, polyester, polyethylene, ethylene vinyl acetate copolymer, ethylene ethylacrylate copolymer (EEA) . It is also possible to apply other thermoplastic resins than those listed above. Conventionally, as described in Japanese Laid-Open Patent Publication No.11-337781, the protective layer is formed on the POF by the following method. First, the POF passes through a chamber containing molten thermoplastic resin polymers. Then, the polymer on the POF is cooled to become solid, so that the optical fiber wire is produced. In this method, since the heat of the molten polymer causes structural irregularity of the POF, it is required to take steps or select the materials not to cause thermal damage by the molten polymer.

In order to form the protective layer without causing thermal damage, photo-polymerizing resin such as UV polymerizing resin is preferably applied (for instance, Japanese Laid-Open Patent Publication No. 2002-220261). The photo-polymerizing resin, however, is not sufficient for protection because the thickness of the protective layer is small as less than 50μm. An object of the present invention is to provide an optical member with a protective layer, and a method and an apparatus for producing such optical member that can prevent deterioration in properties and mechanical damage to the optical member by the heat of the molten thermoplastic resins during the process to cover the protective layer. Another object of the present invention is to restrain increase in transmission loss of the optical fiber as the optical member.

Disclosure of Invention

In order to achieve the above objects, the inventors has found out that the protective layer for an optical member for practical use is obtained by passing the optical member through a liquid resin bath, and then by passing the optical member coated with the resin through a region having the temperature between 50 ° C and the glass transition temperature Tg (°C) of the base material of the optical member.

Preferably, the protective layer is formed in the portion that is not the light conductive portion of the optical member, or around the optical member (on the lateral surface of the optical member that does not relate to light conduction). Then, the protective layer material is hardened at a temperature from 50 ° C to the glass transition temperature Tg (°C) of the polymer of the optical member. If a light-guide portion of the optical member has profile in the glass transition temperature with respect to the plane perpendicular to the optical transmission direction, the protective layer material is hardened at a temperature from 50 ° C to the minimum glass transition temperature Tg (°C) in the light guide portion. The heating period of the protective layer material is preferably from one second to 10 minutes. The protective layer material is preferably hardened in response to chemical reaction. The protective material layer is preferably hardened in warm water. The hardened protective layer material is preferably elastomer or polyurethane . The protective layer material preferably includes a first compound having isocyanate group and a second compound including group that includes active hydrogen, and these compounds are three-dimensionally combined to form polyurethane protective layer. The second compound is preferably polyols having at least two hydroxyl groups .

It is preferable to coat at least one additional protective layer around the above protective layer as a primary protective layer. The optical member is preferably an optical fiber. In that case, the periphery of the optical members stands for the covering portion of the optical fiber.

The apparatus for producing the optical member with the protective layer comprises a first coating to apply the protective layer material on the optical member, and a heating device that is capable of controlling the temperature between 50 ° C and the smallest glass transition temperature Tg (°C) of the polymer of the optical member. The apparatus preferably comprises a line diameter control device in the downstream side of the first coating device to adjust the thickness of the protective layer. The apparatus preferably comprises a second coating device for covering at least one additional protective layer around the above primary protective layer. The apparatus preferably comprises a water tank containing warm water such that the optical member pass through the warm water inside the water tank, and the heating device preferably adjusts the temperature of the warm water.

The optical member with the protective layer comprises an optical waveguide portion formed from a polymer, and a protective layer around the optical waveguide portion that is formed from polyurethane elastomer. The protective layer (primary protective layer) is preferably covered with at least one additional protective layer. The at least one additional protective layer is preferably formed from thermoplastic resin. The optical member is preferably an optical fiber.

The optical member includes an optical transmission medium such as the optical fiber. The optical transmission medium has a fiber as optical waveguide and the protective layer around the fiber. The fiber is formed from a polymer having the glass transition temperature of Tg (° C) . In the event of producing the optical transmission medium, the protective layer material is hardened to form the protective layer at a temperature from (Tg-50) ° C to Tg (° C) . If the fiber as the optical waveguide has a profile in the glass transition temperature with respect to the plane perpendicular to the optical transmission direction, the protective layer material is hardened at a temperature from 50 ° C to the minimum glass transition temperature Tg (° C) of the fiber. The heating period of the protective layer material is preferably from one second to 10 minutes.

The main component of the polymer to form the fiber is preferably (meth)acrylic acid and/or polymer of acrylic acid ester monomer. The protective layer material is preferably elastomer, and more preferably elastomer formed by thermal hardening process . The protective layer material preferably has the main component of three-dimensionally crosslinked polyurethane. The optical transmission medium is preferably an optical fiber, especially an optical fiber in which the core has refractive index profile. The optical fiber manufactured by the above method and the apparatus used for the above method are within the scope of the present invention.

The apparatus for producing the optical transmission medium with the protective layer comprises a coating device to apply the protective layer material to the fiber as the optical waveguide, and a heater that is capable of controlling the temperature between 50 ° C and the smallest glass transition temperature Tg CO of the polymer of the fiber. The apparatus preferably comprises a wire diameter control device in the downstream side of the coating device to adjust the thickness of the protective layer.

According to the present invention, it is possible to obtain the optical member with the protective layer that has excellent mechanical strength without causing increase in transmission loss by the process to cover the protective layer around the optical member. The present invention can effectively prevent deterioration in the properties of the optical member with respect to the covering process that tends to damage the optical member, especially in the process to cover the protective layer directly to the optical member. In the event of producing a plastic optical fiber as the optical member, the plastic optical fiber exhibits excellent mechanical strength and optical properties . Moreover, by forming the protective layer from three-dimensional polyurethane that has excellent heat-resistance, the optical member may be used in a hot environment. Furthermore, in forming a additional protective layer around the three-dimensional polyurethane protective layer, the resin as the additional protective layer may have high melting viscosity. Thus, it is possible to increase the range of selection of the materials for the additional protective layer.

Brief Description of Drawings

Fig. 1 is a schematic illustration of an embodiment of an apparatus to produce an optical member with a protective layer according to the present invention;

Fig. 2 is a cross section of the optical member with the protective layer; and

Fig. 3 is a schematic illustration of another embodiment of the apparatus to produce the optical member with the protective layer.

Best Mode for Carrying Out the Invention

A plastic optical fiber (POF) is given as a preferred example of an optical member to be produced according to the present invention. First of all, polymers as the raw materials, polymerization initiators , chain transfer agents and dopants will be described. Next, as a preferred embodiment, the following description includes the method to produce the POF by melting extrusion of a base material (preform) of the graded index (GI) type POF. The GI type POF has the core in which the refractive index is gradually changed with respect to the radial direction. The following description contains a plastic optical fiber wire in which a protective layer is provided with the POF, and other embodiments of the optical member. The embodiments in the following description are to explain the present invention in detail, and thus, do not limit the scope of the invention.

Any materials are adopted for preparing the optical member, especially POF, as long as the optical member can exhibit desired optical properties. Preferably, the materials for the optical member are organic materials with high optical transmittance, such as (meth)aσrylic acid esters [(a) (meth)acrylic ester without fluoride, (b) (meta) acrylic ester containing fluoride] , (c) styrene type compounds, (d) vinyl esters, polycarbonates, or the like. The optical member may be formed from homopolymer composed of one kind of these monomers, from copolymer composed of at least two kinds of these monomers, or from a mixture of homopolymer(s ) and/or copolymer( s ) . Among them, (meth)acrylic acid ester can be used as a polymerizable monomer. Concretely, examples of the (a) (meth)acrylic ester without fluoride as the polymerizable monomer are methyl methacrylate; ethyl methacrylate; isopropyl methacrylate; tert-butyl methacrylate; benzyl methacrylate; phenyl methacrylate; cyclohexyl methacrylate, diphenylmethyl methacrylate; tricyclo [5*2' l'O²'⁶] decanyl methacrylate; adamanthyl methacrylate; isobonyl methacrylate; methyl acrylate; ethyl acrylate; tert-butyl acrylate; phenyl acrylate, and the like. Examples of

(b) (meth)acrylic ester with fluoride are 2,2,2-trifluoroethyl methacrylate; 2,2,3,3-tetrafluoro propyl methacrylate; 2 ,2,3,3,3-pentafluoro propyl methacrylate;

1-trifluoromethyl-2,2,2-trifluoromethyl methacrylate;

2,2,3,3,4,4,5, 5-octafluoropenthyl methacrylate;

2,2,3,3,4,4, -hexafluorobutyl methacrylate, and the like.

Further, in (c) styrene type compounds, there are styrene; α-methylstyrene; chlorostyrene; bromostyrene and the like. In

(d) vinylesters, there are vinylacetate; vinylbenzoate; vinylphenylacetate; vinylchloroacetate; and the like. The polymerzable monomers are not limited to the monomers listed above.

Preferably, the kinds and composition of the monomers are selected such that the refractive index of the homopolymer or the copolymer in the core is similar or higher than the refractive index in the cladding.

When the optical member is used for nearly infrared ray, the C-H bonds in the optical member cause absorption loss. By use of the polymer in which the hydrogen atom on the C-H bond is substituted by the heavy hydrogen, the wavelength range to cause transmission loss shifts to larger wavelength region. Japanese Patent Publication No. 3332922 teaches the examples of such polymers, such as deuteriated polymethylmethacrylate (PMMA-d8) , polytrifluoroethylmethacrylate (P3FMA), polyhexafluoro isopropyl-2-fluoroacrylate (HFIP2-FA), and the like. Thereby, it is possible to reduce the loss of transmission light. Note that the impurities and foreign materials in the monomers that would cause dispersion should be sufficiently removed before polymerization so as to keep the transparency at a certain level after polymerization.

In polymerizing monomer to form the polymer as the optical member, polymerization initiators can be added to initiate polymerization of the monomers . Examples of the polymerization initiators that generate radicals are peroxide compounds, such as benzoil peroxide (BPO); tert-butylperoxy-2-ethylhexanate (PBO); di-tert-butylperoxide (PBD); tert-butylperoxyisopropylcarbonate (PBI ) ; n-butyl-4,4-bis(tert-butylperoxy)valarate (PHV), and the like. Other examples of the polymerization initiators are azo compounds, such as 2,2 ' -azobisisobutylonitril;

2,2 ' -azobis(2-methylbutylonitril) ; 1,1' -azobis(cyclohexane-l-carbonitryl) ; 2,2 ' -azobis(2-methylpropane) ; 2,2 ' -azobis(2-methylbutane) 2,2 ' -azobis(2-methylpentane) ;

2,2 ' -azobis(2,3-dimethylbutane) ; 2,2 ' -azobis(2-methylhexane) ; 2,2' -azobis(2,4~dimethylpentane) ; 2,2' -azobis

(2,3,3-trimethylbutane) ; 2,2 ' -azobis(2,4,4-trimethylpentane) ; 3,3 ' -azobis (3-methylpentane) ; 3,3' -azobis ( 3-methylhexane) ; 3,3' -azobis(3,4-dimethypentane) ; 3,3 ' -azobis ( 3-ethylpentane) ; dimethyl-2,2'-azobis ( 2-methylpropionate) ; diethyl-2,2' -azobis( 2-methylpropionate) ; di-tert-butyl-2,2 ' -azobis (2-methylpropionate) , and the like. Note that the polymerization initiators are not limited to the above substances . More than one kind of the polymerization initiators may be combined.

For the purpose of making the property values, such as mechanical and thermal properties, uniform over the polymer, it is preferable to control degree of polymerization by adding chain transfer agents . The kind and the amount of the chain transfer agent are selected in accordance with the kinds of the polymerizable monomers. The chain transfer coefficient of the chain transfer agent to the respective monomer is described, for example, in "Polymer Handbook, 3^rd edition", (edited by J. BRANDRUP & E .H. IMMERGUT, issued from JOHN WILEY&SON) . In addition, the chain transfer coefficient may be calculated through the experiments in the method described in "Experiment Method of polymers" (edited by Takayuki Ohtsu and Masayoshi Kinoshita, issued from Kagakudojin, 1972). Preferable examples of the chain transfer agent are alkylmercaptans [for instance, n-butylmercaptan; n-pentylmercaptan; n-octylmercaptan; n-laurylmercaptan; tert-dodecylmercaptan, and the like], and thiophenols [for example, thiophenol; m-bromothiophenol; p-bromothiophenol; m-toluenethiol; p-toluenethiol, and the like]. It is especially preferable to use n-octylmercaptan, n-laurylmercaptan, and tert-dodecylmercaptan in the alkylmercaptans. Further, the hydrogen atom on C-H bond may be substituted by the fluoride atom in the chain transfer agent . Note that the chain transfer agents are not limited to the above substances . More than one kind of the chain transfer agents may be combined.

The POF as the optical member may be the graded index (GI) type POF in which the core has the refractive index profile along its radial direction. The GI type POF enables wide band optical transmission due to its high transmission capacity, the GI type POF is preferably utilized for high performance communication purpose. In order to generate refractive index profile in the POF, an additive to provide refractive index profile (hereinafter referred to as "dopant") may be contained in the polymer matrix. Otherwise, plural polymers with different refractive indices or a copolymer of such polymers may be used as the polymer to form the core.

The dopant is a compound that has different refractive index from the polymerizable monomer to be combined. The difference in the refractive indices between the dopant and the polymerizable monomer is preferably 0.005 or higher. The dopant has the feature to increase the refractive index of the polymer, compared to one that does not include the dopant . In comparison of the polymers produced from the monomers as described in Japanese Patent Publication No.3332922 and Japanese Patent Laid-Open Publication No. 5-173026, the dopant has the feature that the difference in solution parameter is 7 (cal/cm³)¹² or smaller, and the difference in the refractive index is 0.001 or higher. Any materials having such features may be used as the dopant if such material can stably exist with the polymers, and the material is stable under the polymerizing condition (such as temperature and pressure conditions) of the polymerizable monomers as described above.

This embodiment shows the method to form refractive index profile in the core by controlling the direction of polymerization by interface gel polymerizing method, and by providing gradation in density of the refractive index control agent as the dopant during the process to form the core from the polymerizable compound mixed with the dopant. Other methods, such as diffusing the refractive index control agent after preform formation, are also possible to provide refractive index profile in the core. Hereinafter, the core having the refractive index profile will be referred to as "graded index core". Such graded index core is used for the graded index type plastic optical member having wide range of transmission band. The dopant may be polymerizable compound, and in that case, it is preferable that the copolymer having the dopant as copolymerized component increases the refractive index in comparison of the polymer without the dopant . An example of such copolymer is MMA-BzMA copolymer.

Examples of the dopants are benzyl benzoate (BEN) ; diphenyl sulfide (DPS); triphenyl phosphate (TPP); benzyl n-butyl phthalate (BBP); diphenyl phthalate (DPP); diphenyl (DB); diphenylmethane (DPM); tricresyl phosphate (TCP); diphenylsoufoxide (DPSO) . Among them, BEN, DPS, TPP and DPSO are preferable. In the event that the dopant is polymerizable compounds such as tribromo phenylmethacrylate, there may be advantageous in heat resistance although it would be difficult to control various properties (especially optical property) because of copolymerization of polymerizable monomer and polymerizable dopant. It is possible to control the refractive index of the POF by controlling the density and distribution of the refractive index control agent to be mixed with the core. The amount of the refractive index control agent may be appropriately chosen in accordance with the purpose of the POF, the core material, and the like. (Other Additives) Other additives may be contained in the core and the cladding so far as the transmittance properties does not decrease. For example, the additives may be used for increasing resistance of climate and durability. Further, induced emissive functional compounds may be added for amplifying the optical signal. When such compounds are added to the monomers, weak signal light is amplified by excitation light so that the transmission distance increases. Therefore, the optical member with such additive may be used as an optical fiber amplifier. These additives may be contained in the core and/or the cladding by polymerizing the additives with the monomers .

[Method of Producing Plastic Optical Fiber] The plastic optical fiber (POF) as an example the optical members may be made from the above described materials. It is to be noted, however, that any known method is applicable to produce the POF, and thus the present invention is not limited to the method described in the following paragraphs . For instance, the POF is directly produced by melt spinning described in Japanese Patent Laid-Open Publication No. 2002-40267. As the method to form the POF from the preform, there is batch molding process by melt extrusion. It is possible to form the core and the cladding by layering the cladding after forming the core, or by forming the core inside the cladding after producing the hollow tube as the cladding. As described in Japanese Patent No. 3332922, the GI type plastic optical fiber base material (GI type preform) is produced by forming hollow resin tube as the cladding, injecting resin composition as the core in the cladding, and by polymerizing the polymer by interface gel polymerization method. It is also known that the core of the GI type preform is produced by successively adding polymerizable compositions with different refractive indices after polymerization. The method to produce the GI type preform according to the present invention is not limited to the interface gel polymerization method, but it is possible to apply other method such as the method to rotate the hollow tube for polymerizing the polymer in the direction toward the center of the core from the inner surface of the hollow tube. As for the resin compositions, the resin composition with single refractive index may contain refractive index control agent. The resin composition may be the mixture of resins with difference refractive indices, or copolymer. The plastic optical fiber may have various refractive index profiles, such as GI type, step index type and pseudo-GI multi step index type. The method to producing the optical member, described below, may be applicable to various kinds of optical members , including these plastic optical fibers .

The POF may be produced by heating and drawing the preform. In that case, the heating temperature to heat the preform is appropriately selected in accordance with the property of the preform such as the quality. Generally, the heating temperature is preferably selected such that the polymer for the optical member is fluidly transformed easily with slight external force. For instance, if the preform is polymethaσryl methyl acid, the preferable heating temperature is 180 ° C to 250 °C. The drawing conditions (drawing temperature, for instance) are appropriately selected in consideration of the diameter of the obtainedpreform, desirable diameter of POF, the material to be used, and so forth. For instance, as described in Japanese Laid-Open Patent Publication H07-234322, tension in drawing may be 0.1 (N) or higher for the purpose of orientating the molten plastic. In addition, as described in Japanese Laid-Open Patent Publication HO7-234324, the tension in drawing may be 1.0 (N) or smaller for the purpose of not having strain after melt-drawing process. It is also possible to perform preliminary heating in drawing, as described in Japanese Laid-Open Patent Publication H08-106015. The flexural and lateral pressure properties of the POF produced by the above methods are improved by regulating the elongation at break and hardness thereof as described in Japanese Laid-Open Patent Publication H07-244220. [Production of Protective Layer]

Generally, the drawn POF is not used as it is. At least one protective layer is covered with the POF to form an optical fiber wire, for the purpose of improving flexural and weather resistance, preventing decrease in property by moisture absorption, improving tensile strength, providing resistance to stamping, providing resistance to flame, protecting damage by chemical agents , noise prevention from external light , increasing the value by coloring, and the like. The material of the protective layer and the method of forming the protective layer around the POF will be described. It is noted that the device used for protective layer formation may be connected to the drawing device to form the protective layer after the drawing process. The covering process may be carried out successively to cover the protective layer as the primary covering. [Protective Layer Materials]

The materials for the protective layer is selected such that the formation of the protective layer does not cause thermal damage (deformation, denaturation, thermal decompression, or the like) to the POF. Thus, the protective layer material should be hardened in reaction at a temperature between 50 ° C to the glass transition temperature Tg (°C) of the polymer for the POF. The material to be hardened at a low temperature (especially around room temperature) has short pot life in general. Moreover, such material starts to be hardened with the remaining heat of the POF when the protective layer is formed just after melt-drawing of the POF . Thus , since such material with low hardened temperature makes it difficult to manage the material and determine the covering condition, the temperature to harden the protective layer material should be 50 °C or higher. In consideration of the hardening property, in the event of the glass transition temperature Tg (°C) of the optical member to be covered with the protective layer is 100 ° C or higher, the lower limit of the hardened temperature may increase up to (Tg-50) ° C in order to increase and control the hardening speed of the protective layer material.

The forming period (the period to harden the protective layer material) is preferably between 1 second to 10 minutes, more preferably between 1 second to 3 minutes . Long forming period is not preferable because the POF is subject to heat longer. Since the protective layer material has long-term fluidity, short hardening period of the protective layer material is preferable in terms of controlling the thickness of the protective layer. Too short forming period is not preferable because of unevenness in hardening in the protective layer when the protective layer to be formed is thick. When the POF is composed of plural chemical components (for instance, when there is distribution in the amount of the additives to provide plasticity and in copolymerizing ratio of the copolymers), Tg is the smallest grass transition temperature among these chemical components. When the polymers for POF do not have glass transition temperature, Tg is the smallest phase transition temperature (melting point, for instance) . When single polymer (homopolymer) with additives to provide plasticity has distribution in the glass transition temperature, Tg is the glass transition temperature of the homopolymer. By setting the hardening temperature of the protective layer material between 50ΩC and Tg (° C) , it is possible to avoid deformation of the POF caused by thermal damage thereto during the process to harden the protective layer. Also, it is possible to prevent deterioration in properties of the POF. In the event that the POF has the GI type core, the refractive index profile is not badly changed. Since the property of the POF does not decrease by heating the POF during the covering process, it is possible to provide optical member with high quality. For some kinds of the optical member and the protective layer material, the lowest hardening temperature of the protective layer material may be (Tg-50) ° C.

The material obtained by thermal hardening the mixed liquid of a polymer precursors and reaction agent is preferably used as the protective layer material. For example, it is possible to utilize polyurethane obtained by mixing the compound with isocyanate group and the compound with reactive hydrogen. It is not necessary to supply much thermal or optical energy to such material externally because its reactivity can progress the reaction. The protective layer is thick enough not to generate the heat of reaction to cause much damage to the optical member. Some materials progress the reaction by humidity, other materials does not require heat to progress the reaction. In consideration of the descriptions above, an example of the material is one-pack type thermosetting urethane composition produced from NCO block prepolymer and powder-coated amine, as described in Japanese Patent Laid-Open Publication H10-158353.

Three-dimensionally linked thermosetting polyurethane (three dimensional polyurethane) is formed from said thermosetting urethane composition. The three dimensional polyurethane may be obtained by reacting the compound having plural isocyanate groups with polyole. In that case, the thermosetting urethane composition is kept at 80 ° C for five minutes, so the prepolymers in the thermosetting urethane composition are three dimensionally polymerized to form the three dimensional polyurethane. This method to form the three dimensional polyurethane is preferable in terms of producing cost because of water as heating medium is available.

The three dimensional polyurethane has the rubber-like feature to be deformed with small external force at room temperature and return to its original shape when such external force is removed. Since the three dimensional polyurethane is so soft and elastic that the original shape is kept after removing external force. Thus, the three dimensional polyurethane can relax the stress in a process to receive external pressure, such as the process to attach the connectors to the POF. Since the POF is not badly affected by external pressure, it is possible to prevent deterioration in optical properties such as transmission loss.

Polyurethane is formed by thermosetting pre-polymer. Such polyurethane normally has linear structure (linear PU, hereinafter referred to as "polyurethane". The upper limit of the usable temperature of polyurethane for long term use is about 60 ° C. For short term use, polyurethane may be usable up to about 80 °C. The three-dimensional polyurethane preferably used for the present invention is usable up to 120 °C for long term use, and 130 °C to 140 °C for short term use. Note that a material is usable at a predetermined temperature when the stress-strain curve (S-S curve) of the dumbbell sample kept at the predetermined temperature for a predetermined period is substantially the same as the S-S curve of the dumbbell sample kept at a room temperature for the predetermined period. The predetermined period is 200 hours or longer for long term, and 100 hours or less for short term. Thus, three-dimensional polyurethane is available at a higher temperature, compared with the linear polyurethane (usable up to 80 °C), and thermoplastic polyurethane elastomer (usable up to 100 ° C) . Moreover, compared with low density polyethylene (LDPE) , which is usable up to 60-75 ° C for long term use and 80-90 ° C for short term use, three-dimensional polyurethane has much more heat resistance. The optical fiber wire having the protective layer (primary protective layer), mainly made from three-dimensional polyurethane around the POF, may be covered with at least one additional protective layer. Such optical fiber wire with plural protective layers may be bunched to form a plastic optical fiber cable. Examples of the resin for the additional protective layer are low density polyethylene (LDPE), polypropylene (PP), soft polyvinyl chloride. In forming the additional protective layer, the resin is melted by heating. If the heat of the molten resin is conducted to the POF during covering the additional protective layer, transmission loss of the POF increases. Especially, when the POF is the GI type, the refractive index profile in the core will change so that the GI type POF loses the merit of wide range of the transmission band. The primary protective layer made from three-dimensional polyurethane, however, has low thermal conductivity, so the primary protective layer can prevent the POF from the heat of molten resin for the additional protective layer that would change the refractive index profile in the POF if three-dimensional polyurethane is not provided. Thus, there is no increase in transmission loss of the POF by the process to cover the additional protective layer.

As mentioned so far, the primary protective layer (undercoat layer) made from three-dimensional polyurethane between the POF and the additional protective layer can prevent the thermal damage to the POF by the molten resin that forms the additional protective layer. Thus, three-dimensional polyurethane is preferably adopted as the undercoat layer before forming the additional protective layer. Moreover, polyurethane has excellent slidability enough to prevent the stress to the POF when the undercoat layer is rubbed against the additional protective layer resin, such as vinyl chloride resin, thermoplastic urethane resin, and thermoplastic olefine resin. That is, the primary protective layer (undercoat layer) can relax not only the stress when the POF is bended, but the lateral stress from the additional protective layer provided for increasing mechanical strength. Therefore, it is possible to prevent deterioration in optical properties caused by unpredictable stress to the POF.

As the material for the primary protective layer, there is one-pack type thermosetting urethane composition that is composed of urethane pre-polymer with NCO group, described in WO/26374, and solid amine having the size of 20μm or smaller. For the purpose of improving the properties of the primary protective layer, additives and fillers may be added to the primary protective layer. Examples of the additives are incombustibility, antioxidant, radical trapping agent. lubricant . The fillers may be made from organic and/or inorganic compound.

If the conditions in use are appropriate, the primary protective layer may be made from liquid type rubber that exhibits liquidity at a room temperature and loses its liquidity to be hardened by heating. Concrete examples of the liquid type rubbers are polydienes (having the main structure of polyisoprene, polybutadiene, butadiene-acrylonitrile copolymer polychloroprene, or the like), polyolefines (having the main structure of polyolefine, polyisobuthylene, or the like), polyethers (having the main structure of poly(oxypropylene) , for example) , polysulfide (having the main structure of poly(oxyalkylene disulfide), for example), and polysiloxanes (having the main structure of poly(dimethyl siloxane), for example) .

[Method and Apparatus of Producing the Protective Layer]

Referring to Fig. 1, a POF 11 is supplied into a resin pot

12 that contains above descried protective layer material (resin)

13. It is preferable to provide a die in the downstream side of the resin pot 12 to make the thickness of the protective layer substantially uniform. The resin 13 is applied to the lateral surface of the POF 11 through the resin pot 12. The POF 11 coated with the resin 13 is hereinafter referred to as a coated wire 15. The POF 11 may be wound into a roll, and then supplied to the resin pot 12. Alternatively, the POF 11 to be supplied to the resin pot 12 may be obtained by drawing a preform.

The coated wire 15 is fed to a resin hardening water tank (water tank) 16 that contains warm water 17. For the purpose of keeping the temperature of the warm water 17, the warm water 17 is preferably circulated through a thermostat 18 that is connected to the water tank 16. The temperature of the warm water 17 is controlled between 50 °C to (Tg-50) °C. Preferably, the temperature of the warm water 17 is between (Tg-50) ° C to Tg (° C) , and more preferably (Tg-30) °C to Tg (°C). The coated wire 15 is preferably fed to downstream side by use of pulleys 19, 20 as depicted in the drawing. Other feeding means such as feeding roller pairs is also possible . The coated wire 15 is gradually heated by the warm water 17, so the resin 13 is hardened without causing the resin 13 to flow. By hardening the resin 13 around the POF 11, an optical fiber wire 21 having the protective layer is obtained. The feeding speed of the coated wire 15 is controlled such that the resin 13 is hardened in 1 second to 10 minutes after coating the resin 13, and that the coated wire 15 is in the water tank 16 for 1 second to 10 minutes. Thereby, it is possible to produce the protective layer without reducing the productivity of the optical fiber wire 21. The feeding speed is not limited to the above defined range.

The warm water on the optical fiber wire 21 is removed through a blowing device 22, and further removed through a water absorption device 23. In the downstream side of the water absorption device 23 with respect to the feeding path of the optical fiber wire 21 , a pair of feeding rollers 24 , 25 is provided to feed the optical fiber wire 21. A motor 26 is connected with one feeding roller 25 to rotate the pair of feeding rollers 24, 25. In order to feed the optical fiber wire 21 stably, a pressing member (a spring is illustrated in Fig. 1) 27 is preferably provided with the other feeding roller 24.

Referring to Fig.2, the thickness of the protective layer 13a of the optical fiber wire 21 is preferably from 20μm to 3mm, more preferably from 50μm to 2mm, and most preferably 80um to 1mm. If the thickness of the protective layer 13a is less than 20μm, the protective layer 13a does not work sufficiently as a protective covering. On the other hand, the protective layer 13a with the thickness more than 3mm will remain non-reacted portion in the protective layer 13a because of slow hardening or incompletion in reaction. Although the average diameter L2 of the POF 11 is preferably 0.2mm to 2.0mm, the diameter L2 is not limited to this range. The diameter of the core 11a is preferably 0.1mm to 1.0mm. The thickness of the cladding lib is preferably 0.01mm to 1.9mm.

In Fig.3 , another embodiment of producing the optical fiber wire as the optical member is depicted. The POF 31 is supplied and fed through a tension measuring device 50 that measures the tension to the POF 31. The POF 31 is fed toward the resin pot 32 while the tension to the POF 31 is controlled by use of the tension measuring device 50. In order to increase optical properties, it is preferable to provide a dust removing device 52 between the tension measuring device 50 and the resin pot 32 to remove dust on the surface of the POF 31. A resin supply device 52 supplies resin 33 as the protective layer material continuously to the resin pot 32. The resin pot 32 is surrounded by a dry chamber 53. A dry air supply device 54 blows dry air (nitride air, for instance) to protect the resin pot 32 from dust in coating the resin 33 around the POF 31. It is preferable to provide a resin pot level meter 63 to control the amount of the resin 33 in the resin pot 32.

The POF 31 through the die 35 becomes a coated fiber in which the protective layer with preferable thickness is formed around the POF 31. The thickness of the protective layer is adjusted by controlling the size of lip aperture formed in the outlet of the resin pot 12, 32. Also, the thickness of the protective layer is adjustable by controlling the viscosity of the resin and the feeding speed of the POF 31. Accordingly, the diameter of the coated fiber 34 is adjusted by controlling various parameters, such as the temperature of the resin pot 32 and the feeding seed of the POF 31, based on the measured diameter of the coated fiber 34 by use of a diameter measuring device 55. The coated fiber 34 is fed to the heating chamber 56 in which the resin around the POF 31 is hardened by heat to form the protective layer. A hot air supply device 57 connected to the heating chamber 56 blows hot air into the heating chamber 56 such that the hot air flows in the direction opposite to the feeding direction of the coated fiber 34. The hot air to the heating chamber 56 may flow in the feeding direction. In order to uniform the thickness of the protective layer by preventing projection of the protective layer material, it is preferable to monitor and control the temperature in the heating chamber 56 by providing a pair of thermometers 58, 59 near the entrance and exit of the heating chamber 56. When the coated resin does not exhibit fluidity, the resin becomes solid as the protective layer of the optical fiber wire 60. As described in the previous embodiment, the heating period of the protective layer material is preferably between 1 second and 10 minutes .

The temperature in the heating chamber 56 is controlled based on existence of projections in the protective layer of the optical fiber wire 60 that is monitored by a projection detecting device 61. In the downstream side of the projection detecting device 61, a pair of feeding rollers 42, 43 is provided to feed the optical fiber wire 60. A motor 44 is connected with one feeding roller 43 to rotate the pair of feeding rollers 42, 43. As mentioned in the previous embodiment, a spring 45 is provided with the other feeding roller 42. A fiber length measurement device 62 is preferable connected with the other feeding roller 42 to measure the length of the optical fiber wire 60. The above described protective layer may be the primary protective layer, and at least one additional protective layer may be formed around the primary protective layer. When the primary protective layer is thick enough to decrease thermal damage to the POF in forming the additional layer, the condition of temperature to harden the protective layer material is not so strict as the case in which the protective layer is formed directly on the POF. Besides the materials listed in the preceding paragraphs , the material for the additional protective layer may be thermoplastic resin, such as polyolefines (polyethylene and polypropylene, for example), polyvinyl chloride, nirons, polyester, ethylene vinyl acetate copolymer, EEA (ethylene- ethyl acrylate copolymer) .

Concretely, the following materials are applicable as the additional protective layer. Due to excellent elasticity, these materials have merits in terms of providing mechanical property such as bending resistance. Examples of the materials form the additional protective layer are isoprene type rubber (natural rubber, isoprene rubber, for example), butadiene type rubber (styrene-butadiene compolymerized rubber, butadiene rubber, for example), diene type special rubber (nitryl rubber, chloroprene rubber, for example), olefine type rubber (ethylene-propylene rubber, acrylic rubber, butyl rubber, halogen type butyl rubber, for example) , ether type rubber, polysulfide rubber and urethane type rubber) . It is also possible to thermoplastic elastomer (TPE) as the material for the additional protective layer. The thermoplastic elastomer exhibits rubber-like elasticity at room temperature, and is plasticized and becomes easy in extrusion. For example, the material is styrene type TPE, olefine type TPE, vinyl chloride type TPE, urethane type TPE, ester type TPE, amide type TPE. The polymers for the additional covering layer are not limited to those listed above as long as the polymer is extruded at a temperature Tg (°C) or less. It is possible to use copolymer or mixed polymer of the above materials .

The additional protective layer may contain additives such as, for example, flame-retardant, UV-stabilizer, antioxidant, radical capturing agent and lubricant. These additives may be also contained in the first layer, so far as the first layer does not lose the moisture permeability. Some frame retardants contain resins with halogen like bromine, additives and phosphorus . Recently, metal hydroxide is mainly used as the frame retardant for the purpose of reducing toxic gas emission. The metal hydroxide contains water of crystallization, which is not removed during the manufacture of the POF. Thus , it is preferable to provide a moisture proof cover around the primary protective layer and to form the metal hydroxide as the frame retardant around the moisture proof cover. The POF may be coveredwith plural cover layers with multiple functions. Examples of such cover layers are a frame retardant layer described above, a barrier layer to prevent moisture absorption, moisture absorbent (moisture absorption tape or gel, for instance) between the protective layers or in the protective layer, a flexible material layer and a styrene forming layer as shock absorbers to relax stress in bending the POF, a reinforced layer to increase rigidity. The thermoplastic resin as the cover layer may contain structural materials to increase the strength of the optical fiber cable. The structural materials are a tensile strength fiber with high elasticity and/or a metal wire with high rigidity. Examples of the tensile strength fibers are an aramid fiber, a polyester fiber, a polyamid fiber. Examples of the metal wires are stainless wire, a zinc alloy wire, a copper wire. The structural materials are not limited to those listed above. It is also possible to provide other materials such as a metal pipe for protection, a support wire to hold the optical fiber cable. A mechanism to increase working efficiency in wiring the optical fiber cable is also applicable .

In accordance with the way of use, the POF is selectively used as a cable assembly in which the POFs are circularly arranged, a tape core wire in which the POFs are linearly aligned, a cable assembly in which the tape core wires are bundled by using a band or LAP sheath, or the like.

Further, it is preferable to ensure to fix the end of the POF as the optical member according to the present invention by using an optical connector. The optical connectors widely available on the market are PN type, SMA type, SMI type, F05 type, MU type, FC type, SC type and the like.

A system to transmit optical signals through the POF, the optical fiber wire and the optical fiber cable as the optical member comprises optical signal processing devices including optical components, such as a light emitting element, a light receiving element, an optical switch, an optical isolator, an optical integrated circuit , an optical transmitter and receiver module, and the like . Such system may be combined with other POFs . Any know techniques can be applied to the present invention. The techniques are described in, for example, "'Basic and Practice of Plastic Optical Fiber' (issued from NTS Inc.)", "Optical members can be Loaded on Printed Wiring Assembly, at Last' in Nikkei Electronics, vol. Dec. 3, 2001", pp. 110-127", and so on. By combining the optical member according to with the techniques in these publications, the optical member is applicable to short-distance optical transmission system that is suitable for high-speed and large capacity data communication and for control under no influence of electromagnetic wave. Concretely, the optical member is applicable to wiring in apparatuses (such as computers and several digital apparatuses) , wiring in trains and vessels , optical linking between an optical terminal and a digital device and between digital devices , indoor optical LAN in houses , collective housings, factories, offices, hospitals, schools, and outdoor optical LAN.

Further, other techniques to be combined with the optical transmission system are disclosed, for example, in "'High-Uniformity Star Coupler Using Diffused Light Transmission' in IEICE TRANS. ELECTRON., VOL. E84-.C, No.3, MARCH 2001, pp. 339-344", "'Interconnection in Technique of Optical Sheet Bath' in Journal of Japan Institute of Electronics Packaging., Vol.3, No.6, 2000, pp.476-480". Moreover, there are am optical bus (disclosed in Japanese Patent Laid-Open Publications No.10-123350, No.2002-90571, No.2001-290055 and the like); an optical branching/coupling device (disclosed in Japanese Patent Laid-Open Publications No.2001-74971, No.2000-329962, No.2001-74966 , No.2001-74968 , No.2001-318263, No.2001-311840 and the like); an optical star coupler (disclosed in Japanese Patent Laid-Open Publications No.2000-241655) ; an optical signal transmission device and an optical data bus system (disclosed in Japanese Patent Laid-Open Publications No.2002-62457, No.2002-101044 , No.2001-305395 and the like); a processing device of optical signal (disclosed in Japanese Patent Laid-Open Publications No.2000-23011 and the like); a cross connect system for optical signals (disclosed in Japanese Patent Laid-Open Publications No.2001-86537 and the like); a light transmitting system (disclosed in Japanese Patent Laid-Open Publications No.2002-26815 and the like); multi-function system (disclosed in Japanese Patent Laid-Open Publications No.2001-339554, No .2001-339555 and the like); and various kinds of optical waveguides, optical branching, optical couplers, optical multiplexers, optical demultiplexers and the like. When the optical system having the optical member according to the present invention is combined with these techniques , it is possible to construct an advanced optical transmission system to send/receive multiplexed optical signals . The optical member according to the present invention is also applicable to other purposes, such as for lighting, energy transmission, illumination, and sensors . The above embodiment recites the method and apparatus to produce the optical fiber wire from the POF (the optical transmission medium) as the optical member. The method according to the present invention is applicable to other method for covering the protective layer, such as dipping method. Thus, besides forming the protective layer on the POF, the method according to the present invention is appropriately utilized for covering a short-length optical member like an optical connector, and for covering the edges of lenses and optical films . [Experiments] The present invention will be described in detail with reference to Experiments ( 1 ) - ( 3) as the embodiments of the present invention and Experiments (4) -(7) as the comparisons. The materials, contents, operations and the like will be changed so far as the changes are within the spirit of the present invention. Thus, the scope of the present invention is not limited to the Experiments described below. The description below explains Experiment (1) in detail. Regarding Experiments (2) -(7), the portions different from Experiment (1) will be explained. The protective layer material, the heating method, the thickness of the protective layer, increase in transmission loss after covering and the hardenability in each experiment are listed in Table 1 below. In Table 1, the leftmost column shows the number of the Experiment .

In Experiment (1), predetermined amount of solution of monomer (methacryl acid-methyl (in which water is decreased to lOOOppm or less)) is poured into a cylindrical and rigid polymerizing pot having the inner diameter of 22 mm and the length of 600 mm. The inner diameter of the polymerizing pot corresponds to the outer diameter of the preform to be produced. As the polymerization initiator, dimethyl-2, 2 ' -azobis (2-methylpropyonate) of 0.5 wt.% of the monomer solution is contained. In addition, as the chain transfer agent, n-laurylmercaptan of 0.62 wt.% of the monomer solution is contained. While the polymerizing pot is concussed in 60 ° C water bath, the monomer solution is subject to preliminary polymerization for 2 hours. Thereafter, the polymerizing pot is kept horizontally (the axial direction of the cylindrical pot is kept horizontally) at 65 ° C, and then heat polymerization process is carried out for three hours while rotating the cylindrical pot at a speed of 3000 rpm. Thereafter, the heat process at 90 ° C is performed for 24 hours, so that a cylindrical tube formed from high polymer (PMMA) is obtained.

Next, the solution of the monomer (methacryl acid-methyl (in which water is decreased to lOOOppm or less)) as the core material is mixed with dibutyl phthalate as the refractive index control component . The amount of the dibutyl phthalate is 10 wt . % of the monomer solution. After the monomer mixture solution is filtered through membrane filter made from polytetrafluoroethylene with the accuracy of 0.2μm, the filtered solution is directly poured into the hollow portion of the cylindrical tube. As the polymerization initiator, di-t-butylperoxide of 0.016 wt.% of the monomer mixture solution is added. As the chain transfer agent, n-laurylmercaptan of 0.27 wt.% of the monomer mixture solution is added. The cylindrical tube containing this monomer mixture solution is inserted in a glass tube having the diameter larger by 9% than that of the cylindrical tube, and then the glass tube is kept vertically and stationary in a pressure polymerization chamber. Then, in nitrogen atmosphere, the pressure polymerization chamber is pressurized into 0.1 MPa, and the monomer mixture solution is subject to thermal polymerization at 90 °C for 48 hours. Thereafter, the pressure in the pressure polymerization chamber increases to 0.4 MPa, and then the monomer mixture solution is subject to thermal polymerization at 120 °C for 24 hours. After thermal polymerization, heat treatment is performed to obtain the preform. The weight average molecular weight of the preform is 106,000, and the molecular weight distribution ((weight average molecular weight)/(number average molecular weight)) is 2.1. The glass transition temperature of the preform takes the lowest value of 100 "C (=Tg) in the center of the core. The glass transition temperature in the core gradually increases accordance with the refractive index profile. The glass transition temperature in the outermost region of the core is 105 °C.

The preform does not have any bubbles that would be generated due to volume shrinkage at the time when polymerization is completed. The preform is heated at 230 ° C and drawn to obtain the POF having the diameter of 300 μm. The measured transmission loss of the POF is 160 dB/km at the wavelength of 650nm, and 1250 dB/km at 850nm. In the resin pot 12 in Fig. 1, one-pack type thermosetting urethane (manufactured by Sunstar Engineering Inc. , Penguin Cement RD-8014GA (hereinafter referred to as "elastomer urethane")) is poured. The POF is feed through the resin pot 12 at the speed of 3m/min to apply the elastomer urethane on the POF. The temperature of the warm water 17 in Fig. 1 is set at 80 °C. Then, the elastomer urethane is hardened by feeding the POF in the warm water for 10 seconds, so that the protective layer around the POF is formed (water tank method) . The thickness of the protective layer is 3 mm. The increase in the transmission loss after coating the protective layer is 0 dB/km at both 650nm and 850nm. The hardening reaction in the protective layer is completed. The cross section of the protective layer shows that the protective layer is solid (o) over the whole region.

In Experiment ( 2 ) , the heating method to harden the protective layer is the same as Experiment (1) (water tank method) . The POF in Experiment (2) is covered such that the thickness of the protective layer becomes 450 μm. In Experiment (3) , heat wind is applied to the POF to harden the protective layer. The POF in Experiment (3) is covered such that the thickness of the protective layer becomes 20 μm. In Experiment (4) as the comparison, the same protective material as Experiment (1) is used. The temperature of the warm water 17 in Fig. 1 is kept at 40 ° C (= Tg-60 °C). When the POF is kept in such warm water for 5 minutes, the protective layer is not hardened (x) . Thus, the protective layer in Experiment (4) does not work properly. In Experiment (5), which uses the same protective material as Experiment (1), the temperature of the warm water is kept at 105 °C (= Tg+5 °C). The POF is kept in such warm water for 1 minute . The protective layer is hardened (o) , and the thickness of the protective layer is 3 mm. However, the increase in transmission loss after covering is 20 dB/km at the wavelength of 650nm, and 100 dB/km at 850nm. In Experiment (6) , the protective layer material is polyethylene containing 40% of magnesium hydroxide. The protective layer is coated on the POF under the temperature of 140 °C. The thickness of the protective layer is 450μm. Although the protective layer is hardened (o) over whole region, the increase in transmission loss after covering is 80 dB/km at the wavelength of 650nm, and 200 dB/km at 850nm. In Experiment (7) as the comparison, polyetherurethane is used as the protective layer material that is polymerizable composition to be hardened by application of ultraviolet rays . After polyetherurethane of 200μm in thickness is coated, ultraviolet rays are applied for 10 seconds in total by use of plural high pressure hydrogen lamps having the power of 40 W/cm². The polymerizable composition becomes sticky, and not hardened to serve as the protective layer. Thus, in Experiment (7), it is not possible to measure the transmission loss. [Table 1]

1(*) : Water tank method

2 (*) : Hot air blowing method

3(*): UV ray application method

The above Table 1 shows that the optical fiber wire according to Experiments 1-3, in which the three-dimensional polyurethane (elastomer urethane) as the protective layer has the thickness between 20μm to 3mm, has excellent optical properties and mechanical strength.

Industrial Applicability

The present invention is applicable to an optical member such as a plastic optical fiber, an optical connector, lenses, optical films, and so forth.

Claims

1. Amethod for producing an optical member with a protective layer, said protective layer being made from a protective layer material around an optical waveguide portion of said optical member, said method comprising the steps of: coating said protective layer material on said optical member; and hardening said protective layer material at a hardening temperature between 50 ° C and Tg (°C); wherein Tg is the glass transition temperature of said polymer.

2. The method according to claim 1 , said protective layer is made from a protective layer material on a portion different from a light guide portion of said optical member.

3. The method according to claim 1, wherein said optical waveguide portion has profile in the glass transition temperature with respect to the plane perpendicular to the optical transmission direction.

4. The method according to claim 1 , wherein said hardening temperature is determined in a range from (Tg-50) ° C to Tg (°C).

5. The method according to claim 1, wherein the heating period of said protective layer material is from 1 second to 10 minutes .

6. The method according to claim 1, wherein the step of hardening said protective layer material is carried out in warm water.

7. The method according to claim 1, wherein hardened said protective layer material is elastomer.

8. The method according to claim 1 , wherein said protective layer material is hardened by chemical reaction.

9. The method according to claim 1 , wherein hardened said protective layer material is polyurethane.

10. The method according to claim 1 , wherein said protective layer material comprises a first compound having isocyanate group and a second compound including group that includes active hydrogen, and wherein said first and second compounds are three-dimensionally combined to form polyurethane layer as said protective layer.

11. The method according to claim 8, wherein said second compounds is polyols having at least two hydroxyl groups.

12. The method according to claim 1 , wherein said protective layer is a primary protective layer, and said method further comprising the step of: forming at least one additional protective layer to cover said primary protective layer.

13. The method according to claim 1, said optical waveguide portion has refractive index profile with respect to the plane perpendicular to the optical transmission direction.

14. The method according to claim 1, wherein said optical member is an optical fiber.

15. An optical member with a protective layer in which said optical member has an optical waveguide portion made from a polymer, and polyurethane elastomer as said protective layer is formed around said optical member.

16. The optical member according to clam 15, wherein the protective layer is a primary protective layer, and said optical member further comprising: at least one additional protective layer formed around said primary protective layer.

17. The optical member according to claim 16, wherein the additional protective layer is made from thermoplastic resin.

18. The optical member according to claim 15, wherein said optical member is an optical fiber.

19. An apparatus for producing an optical member with a protective layer, said optical member being made from a polymer, and said protective layer being formed on the surface of said optical member while said optical member is fed along a feeing path, said apparatus comprising: a first coating device to coat said protective layer material on the optical member; and a heating device to heat said protective layer material at a hardening temperature between 50 ° C and Tg (°C) to harden said protective layer; wherein Tg is the smallest glass transition temperature of said polymer of said optical member.

20. The apparatus according to claim 19, further comprising a line diameter control device, located in the downstream side of said first coating device with respect to said feeding path, to control the thickness of said protective layer.

21. The apparatus according to claim 19, wherein the protective layer is a primary protective layer, and said apparatus further comprising: at least one additional coating device to coat at least one additional protective layer around said primary protective layer.

22. The apparatus according to claim 19, further comprising: a water tank containing warm water such that said optical member passes through said warm water inside said water tank, said heating device controlling the temperature of the warm water at said hardening temperature.