Plastic optical fiber and plastic optical fiber cable

Abstract

To provide a plastic optical fiber that simultaneously satisfies reduction of bending loss, connection loss, and transmission loss.SOLUTION: A plastic optical fiber includes: a first sheath 2 forming an outermost periphery of a core / sheath structure of the plastic optical fiber; a first core 1 forming a first sea part inside the first sheath 2. and a first island part 8 inside the first core 1. The island part has a lower refractive index than the first sea part, at least on outer periphery. The first core 1 contains a polymethyl methacrylate resin.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a plastic optical fiber and a plastic optical fiber cable. [Background technology]

[0002] A step-index optical fiber has a structure in which a core made of a transparent body is surrounded by a sheath made of a solid with a lower refractive index than the transparent body, and is a medium that transmits light within the core by reflecting the light at the boundary between the core and the sheath (see, for example, Patent Document 1). The step-index optical fibers are classified according to their material into silica glass optical fibers, multi-component glass optical fibers, polymer-clad silica fibers, plastic optical fibers, etc., and also according to their structure into single-core fibers with one core and multi-core fibers with multiple cores. The step-index optical fiber has been widely used in the past for lighting, optical data communication, photoelectric sensors, image transmission, etc., but in recent years, there has been an increasing demand for plastic optical fibers, particularly for short-distance optical transmission.

[0003] When transmitting light using these optical fibers, the larger the fiber diameter, the greater the amount of light received and the longer the transmission distance becomes possible, but on the other hand, there is a problem that bending loss increases, and even bending becomes difficult, especially when the material is quartz or glass. As a method for solving this problem, a technology has been disclosed that reduces bending loss by using a multi-core structure (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-145288 [Patent Document 2] Japanese Patent Application Publication No. 9-33737 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the optical fiber having a multi-core structure disclosed in Patent Document 2, the proportion of the sheath in the cross-sectional area increases, which leads to a decrease in the amount of light received, and furthermore, the mechanical strength of the interface between the core and the sheath is weak, which causes easy peeling. Furthermore, Patent Document 2 does not consider at all how to reduce the splice loss.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a plastic optical fiber that simultaneously satisfies the requirements for reduced bending loss, reduced splice loss, and reduced transmission loss. [Means for solving the problem]

[0007] [1] A plastic optical fiber having a core / sheath structure, a first sheath that forms the outermost periphery of the core / sheath structure of the plastic optical fiber; a first core forming a first sea portion inside the first sheath; and a first island portion formed inside the first core, at least an outer periphery of which has a refractive index lower than that of the first sea portion; the first core contains a polymethyl methacrylate resin; the first sheath is made of a fluororesin, The first sheath has a protective layer made of a thermoplastic fluorine-based resin layer on the outside. death, The refractive index of the protective layer is lower than the refractive index of the first sheath. Plastic optical fiber. [2] The plastic optical fiber described in [1], wherein the first island portion has a second sheath and a second core that forms a second sea portion inside the second sheath. [3] The plastic optical fiber described in [2] above further has a second island portion inside the second core, the second island portion consisting of a third sheath and a third core that forms a third sea portion inside the third sheath. [4] The plastic optical fiber described in [3] above, wherein the second island portion has two or more sheaths and two or more cores concentrically arranged in this order, alternating inward of the cross section of the plastic optical fiber, and has a total of four or more layers of a core / sheath structure. [5] The plastic optical fiber according to any one of [2] to [4], wherein each core constituting the plastic optical fiber is formed from the same material. [6] A plastic optical fiber according to any one of [1] to [5], wherein the diameter of the island portion of the innermost layer in the cross section of the plastic optical fiber is 20% or more and 40% or less of the diameter of the cross section of the plastic optical fiber. [7] A plastic optical fiber according to any one of [1] to [6], wherein in the cross section of the plastic optical fiber, the sheath and the core are arranged in a circular shape in a direction perpendicular to the drawing direction of the plastic optical fiber. [8] The plastic optical fiber according to any one of [2] to [7], wherein each sheath constituting the plastic optical fiber is formed from the same material. [9] The plastic optical fiber according to any one of [1] to [8], a coating layer made of a thermoplastic resin layer that coats the plastic optical fiber; Plastic fiber optic cable.

[0008] [1] a first sheath that forms the outermost periphery of the core / sheath structure of the plastic optical fiber; a first core forming a first sea portion inside the first sheath; A plastic optical fiber having a first island portion formed inside the first core, at least an outer periphery of which has a refractive index lower than that of the first sea portion; The first core contains a polymethyl methacrylate resin. Plastic optical fiber. [2] The plastic optical fiber described in [1], wherein the first island portion has a second sheath and a second core that forms a second sea portion inside the second sheath. [3] The plastic optical fiber described in [2] above further has a second island portion inside the second core, the second island portion consisting of a third sheath and a third core that forms a third sea portion inside the third sheath. [4] The plastic optical fiber described in [3] above, wherein the second island portion has two or more sheaths and two or more cores concentrically arranged in this order, alternating inward of the cross section of the plastic optical fiber, and has a total of four or more layers of a core / sheath structure. [5] The plastic optical fiber according to any one of [2] to [4], wherein each core constituting the plastic optical fiber is formed from the same material. [6] A plastic optical fiber according to any one of [1] to [5], wherein the diameter of the island portion of the innermost layer in the cross section of the plastic optical fiber is 20% or more and 40% or less of the diameter of the cross section of the plastic optical fiber. [7] A plastic optical fiber according to any one of [1] to [6], wherein in the cross section of the plastic optical fiber, the sheath and the core are arranged in a circular shape in a direction perpendicular to the drawing direction of the plastic optical fiber. [8] The plastic optical fiber according to any one of [2] to [7], wherein each sheath constituting the plastic optical fiber is formed from the same material. [9] The plastic optical fiber according to any one of [1] to [8], wherein at least one of the sheaths constituting the plastic optical fiber is made of a fluororesin.

[10] The plastic optical fiber according to any one of [1] to [9], which has a protective layer made of a thermoplastic fluorine-based resin layer on the outside of the first sheath.

[11] The plastic optical fiber according to any one of [1] to

[10] above, a coating layer made of a thermoplastic resin layer that coats the plastic optical fiber; Plastic fiber optic cable. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a plastic optical fiber that simultaneously satisfies the requirements of reduced bending loss, reduced splice loss, and reduced transmission loss. [Brief explanation of the drawings]

[0010] [Figure 1] 1 shows a schematic cross-sectional view of an example of a plastic optical fiber according to an embodiment of the present invention. [Figure 2] 1 is a schematic cross-sectional view of another example of the plastic optical fiber of the present embodiment. [Figure 3] 1A and 1B are schematic cross-sectional views of an example of a plastic optical fiber cable according to the present embodiment and a plastic optical fiber cable according to Example 1. FIG. [Figure 4] 1 shows schematic cross-sectional views of plastic optical fiber cables according to Examples 2 and 3 of the present embodiment. [Figure 5] 1 shows a schematic cross-sectional view of a plastic optical fiber of Comparative Example 1. [Figure 6] 1 shows a schematic cross-sectional view of a plastic optical fiber of Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail. It should be noted that the following embodiments are merely examples for explaining the present invention, and are not intended to limit the present invention to the following content. The present invention can be practiced with appropriate modifications within the scope of its gist.

[0012] [Plastic Optical Fiber] The plastic optical fiber of this embodiment (for example, a step-index optical fiber can be mentioned. Hereinafter, "plastic optical fiber" may be simply referred to as "optical fiber") has a first sheath that forms the outermost periphery of the core / sheath structure of the plastic optical fiber, and a first core that forms a first sea portion inside the first sheath, and has a first island portion inside the first core, at least the outer periphery of which is formed with a refractive index lower than that of the first sea portion, and the first core contains a polymethyl methacrylate resin (PMMA resin).

[0013] Conventionally, multi-core optical fibers have tended to have large splice loss values. Without intending to be bound by any theory, this is thought to be because, when splicing multi-core optical fibers, it is difficult to accurately align the splice positions of the multiple cores in one fiber with the multiple cores in the other fiber, which results in optical loss.

[0014] On the other hand, the plastic optical fiber of this embodiment has a structure in which island portions are formed inside the first core, and the island portions have a lower refractive index than the resin of the first core, at least the outer periphery of which forms the sea portion. This allows light leaking from the island portions when the plastic optical fiber is bent or when plastic optical fibers are spliced ​​together to propagate through the surrounding sea portion, thereby improving bending loss and splice loss. Furthermore, the core as the sea portion contains PMMA (polymethyl methacrylate) resin, which imparts flexibility to the optical fiber and improves durability, thereby achieving reduced bending loss and transmission loss.

[0015] That is, in the plastic optical fiber of this embodiment, as described above, an island portion is formed inside the first core, and the island portion has a refractive index lower than that of the resin of the first core, at least the outer periphery of which becomes the sea portion.Furthermore, by using a PMMA-based resin as the core resin that forms the sea portion, it is possible to simultaneously achieve reduced bending loss, reduced connection loss, and improved communication bandwidth. Generally, the core of an optical fiber, for example, a step-index optical fiber, is the part that propagates light, and therefore it is considered best to have no foreign matter inside, except for the holes that photonic crystal fibers have as described in Patent Document 1, for example. Therefore, it is a surprising result that the above-mentioned effect can be obtained in the plastic optical fiber of this embodiment.

[0016] The plastic optical fiber of this embodiment can be used for, for example, lighting, optical data communication, and sensors such as photoelectric sensors.

[0017] (Preferred form of plastic optical fiber) In the plastic optical fiber of this embodiment, it is preferable that the sheath and the core are arranged in a circular shape in a direction perpendicular to the drawing direction of the plastic optical fiber in the cross section of the plastic optical fiber. The sheath and the core are not required to be arranged in a completely concentric shape, but it is sufficient that the sheath, which has a lower refractive index than the core, is formed in a circular shape inside the core located on the outer periphery.

[0018] FIG. 1 shows a schematic cross-sectional view of an example of a plastic optical fiber according to this embodiment. In FIG. 1, a plastic optical fiber 1A (hereinafter sometimes referred to simply as "optical fiber 1A") has a first sheath 2 that forms the outermost periphery of the core / sheath structure of the plastic optical fiber, and a first core 1 made of a PMMA-based resin that forms a first sea portion inside the first sheath 2. Inside the first core 1, there is a first island portion 8 formed to have a refractive index lower than that of the first core 1, at least the outer periphery of which is the first sea portion. A preferred form of the plastic optical fiber of this embodiment is a form having a four-layer core / sheath structure, in which a second island portion 11 is located inside the first island portion 8, and a third island portion 14 is located inside the second island portion 11. The first island portion 8 has a second sheath 9 on the outer periphery thereof, and a second core 10 that forms a second sea portion made of PMMA resin inside the second sheath 9. The second island portion 11 has a third sheath 12 on the outer periphery thereof, and a third core 13 that forms a third sea portion made of PMMA resin inside the third sheath 12. The third island portion 14 has a third sheath 15 on the outer periphery side, and a third core 16 that forms a third sea portion made of PMMA resin inside the third sheath 15.

[0019] As described above, the plastic optical fiber of this embodiment preferably has a configuration in which the islands each having a core / sheath structure are formed concentrically inward in the cross section. Although the first island portion 8 of the optical fiber 1A shown in FIG. 1 has a configuration including three sheaths and three cores, the plastic optical fiber of this embodiment is not limited to this configuration, and the first island portion 8 may be composed of two sheaths and two cores, and the entire plastic optical fiber may be composed of three sheaths and three cores. Furthermore, the first island portion 8 may have a configuration including three or more sheaths and three or more cores, and the entire plastic optical fiber may be configured with four or more sheaths and four or more cores. That is, in such a configuration, the second island portion 11 has two or more sheaths and two or more cores alternately formed concentrically in the plane direction of the cross section of the plastic optical fiber, resulting in a total of four or more layers of a core / sheath structure.

[0020] (core) The core constituting the plastic optical fiber of this embodiment is, for example, a part that transmits light, and is an optically transparent part that is surrounded by a sheath that has a lower refractive index than the core. Although there are no particular restrictions on the shape of the core, a shape having a circular cross section is preferred because it can efficiently reflect light. When the core has a structure having an inner periphery and an outer periphery, it is sufficient that only the outer periphery is surrounded by the sheath.

[0021] The material that can be used as the core must be both transparent and flexible, and is preferably a material containing a PMMA-based resin. Materials containing PMMA include polymers and copolymers of methyl methacrylate, as described below, and mixtures or compositions of multiple resins containing these (co)polymers, and a material containing 50% by mass or more of PMMA-based resin is preferred. PMMA-based resins can be used to make low-transmission-loss plastic optical fibers suitable for optical communication applications.

[0022] The PMMA resin that can be used as the core is a homopolymer of methyl methacrylate or a copolymer containing 50% by mass or more of units derived from methyl methacrylate. The PMMA-based resin may be a copolymer containing a unit derived from methyl methacrylate and a unit derived from a component copolymerizable with methyl methacrylate. The component copolymerizable with methyl methacrylate is not particularly limited, and examples thereof include acrylic acid esters such as methyl acrylate, ethyl acrylate, and butyl acrylate; methacrylic acid esters such as ethyl methacrylate, propyl methacrylate, and cyclohexyl methacrylate; acrylamides such as acrylamide, methylacrylamide, dimethylacrylamide, N-isopropylacrylamide, and N-methylolacrylamide; methacrylamides such as methacrylamide, methylmethacrylamide, and dimethylmethacrylamide; maleimides such as isopropylmaleimide; acrylic acid, methacrylic acid, and styrene; and these may be used alone or in combination of two or more. From the viewpoint of melt flow (ease of molding), the weight average molecular weight of the PMMA resin that is the core material is preferably 80,000 or more and 200,000 or less, more preferably 100,000 or more and 120,000 or less, in terms of polystyrene.

[0023] When the plastic optical fiber of this embodiment has multiple cores, it is preferable that each core be formed from the same material, because if each core is made of the same material, the speed of light propagating through each core will be the same, thereby improving the bandwidth of the optical fiber.

[0024] The total cross-sectional area of ​​the cores constituting the plastic optical fiber of this embodiment is preferably 50% or more of the total cross-sectional area of ​​the plastic optical fiber. When the total cross-sectional area of ​​the core is 50% or more, the light-receiving area becomes sufficiently large, enabling transmission over longer distances. From the same perspective, the total cross-sectional area of ​​the core is more preferably 60% or more, 65% or more, 70% or more, 75% or more, or 80% or more, and even more preferably 85% or more, 87% or more, 89% or more, 90% or more, 91% or more, 92% or more, 93% or more, 94% or more, or 95% or more. The ratio of the cross-sectional area of ​​the core to the total cross-sectional area of ​​the plastic optical fiber can also be expressed by the flow rate ratio of the polymer, which will be described later. The total cross-sectional area of ​​the cores constituting the plastic optical fiber relative to the total cross-sectional area of ​​the plastic optical fiber can be controlled within the above numerical range by adjusting the flow rate of the core resin in the manufacturing process of the plastic optical fiber.

[0025] (sheath) The sheath that constitutes the plastic optical fiber of this embodiment is, for example, a part that is provided to reflect light propagating within the core and has a lower refractive index than the core. The sheath may not only be disposed so as to surround the core, but may also exist as an island portion inside the core as a sea portion.

[0026] The material that can be used for the sheath is not particularly limited as long as it has a refractive index lower than that of the core formed inside the sheath, and examples thereof include glass, resin, a mixture of resins, a resin composition, etc. Among these, from the viewpoint of high transmittance to the light used, a fluororesin, a mixture of multiple fluororesins, and in particular a mixture of a fluororesin and a hydrocarbon resin are preferably used. By using a fluororesin as the sheath material, transmission loss can be further reduced. From this perspective, it is preferable that at least one of the sheaths constituting the plastic optical fiber of this embodiment is made of a fluororesin.

[0027] Examples of fluororesins used as the sheath material include fluoromethacrylate polymers, polyvinylidene fluoride resins, ethylene-tetrafluoroethylene copolymers, etc. These may be copolymers of multiple types of fluorine-containing monomers. The fluorinated methacrylate polymer is not particularly limited, but from the viewpoint of high transmittance, heat resistance, and moldability, polymers of fluorine-containing acrylate or methacrylate monomers, such as fluoroalkyl methacrylate, fluoroalkyl acrylate, and α-fluoro-fluoroalkyl acrylate, are preferred. Furthermore, the polymers may be copolymers containing units derived from fluorine-containing (meth)acrylate monomers and units derived from other components copolymerizable therewith, and copolymers of units derived from copolymerizable hydrocarbon monomers, such as methyl methacrylate, are preferred. The use of a copolymer of units derived from fluorine-containing (meth)acrylate monomers and units derived from hydrocarbon monomers copolymerizable therewith as the sheath material is preferred because it allows for easy control of the refractive index.

[0028] On the other hand, polyvinylidene fluoride resins are not particularly limited, but from the viewpoint of excellent heat resistance and moldability, preferred are homopolymers of vinylidene fluoride; copolymers of vinylidene fluoride with at least one monomer selected from the group consisting of tetrafluoroethylene, hexafluoropropene, trifluoroethylene, hexafluoroacetone, perfluoroalkyl vinyl ether, chlorotrifluoroethylene, ethylene, and propylene; and alloys of polymers containing units derived from these vinylidene fluoride components with PMMA resins. In particular, the use of perfluoroalkyl vinyl ether as a copolymerization monomer with vinylidene fluoride is preferred because it allows the sheath to have a low refractive index.

[0029] As a fluorine-based copolymer containing perfluoroalkyl vinyl ether, when a (co)polymer mainly composed of methyl methacrylate is used as the core material, from the viewpoints of good adhesion to such a core and low refractive index, it is preferably a quaternary copolymer of perfluoroalkyl vinyl ether, vinylidene fluoride, tetrafluoroethylene, and hexafluoropropene. More preferably, a copolymer composed of 20 to 40 mol% of vinylidene fluoride component, 40% or more and 60 mol% or less of tetrafluoroethylene component, 10% or more and 25 mol% or less of hexafluoropropene component, and 1% or more and 15 mol% or less of perfluoroalkyl vinyl ether component is preferred. Further, a polymer satisfying that the refractive index at 20 °C measured with a sodium D line is 1.30 to 1.37, the value of Shore D hardness (ASTM D2240) at 23 °C is 30 to 55, and the melt flow rate (MFR) (ASTM D1238, load 5 kg, diameter of orifice 2 mm, length of nozzle 8 mm, number of grams of resin flowing out in 10 minutes) at 265 °C is 1 to 75 g / 10 min is preferred from the viewpoint of facilitating the manufacturing process of the plastic optical fiber. In the present specification, the "refractive index" means a value calculated based on JIS K7142 2014.

[0030] Furthermore, from the viewpoint of heat resistance, as the sheath material, a copolymer of polyvinylidene fluoride, hexafluoropropene, and tetrafluoroethylene is preferred. More preferably, it is a copolymer composed of 40 mol% or more and 62 mol% or less of vinylidene fluoride component, 28 mol% or more and 40 mol% or less of tetrafluoroethylene component, and 8 mol% or more and 22 mol% or less of hexafluoropropene component. Further, in the copolymer, it is preferred that the refractive index measured at 20 °C with a sodium D line is 1.35 or more and 1.37 or less, the value of Shore D hardness (ASTM D2240) at 23 °C is 38 or more and 45 or less, and the MFR (ASTM D1238, load 10 kg) at 240 °C satisfies the relationship of 15 < MFR < (5 / 9)×240 - 100, from the viewpoint of facilitating the manufacturing of the plastic optical fiber of the present embodiment. The copolymer of polyvinylidene fluoride, hexafluoropropene, and tetrafluoroethylene may be a copolymer containing trifluoroethylene, hexafluoroacetone, perfluoroalkyl vinyl ether, chlorotrifluoroethylene, ethylene, propylene, or the like as a polymerization monomer, as long as the component ratio is within the above range.

[0031] The ethylene-tetrafluoroethylene copolymer is not particularly limited, but is preferably a modified ethylene-tetrafluoroethylene copolymer having a melting point in the range of 150 to 200°C, a refractive index measured at 20°C with sodium D line in the range of 1.37 to 1.41, an MFR (230°C, load 3.8 kg) in the range of 5 to 100 g / 10 min, and a reactive functional group terminal.

[0032] The various fluororesins constituting the sheath described above may be modified fluororesins. The modified fluororesin is a polymer of an ethylenic monomer (which may contain halogen atoms other than fluorine, such as chlorine; hereinafter, also referred to as a "fluoromonomer") in which all or some of the hydrogen atoms have been substituted with fluorine atoms, or a copolymer of the fluoromonomer and a monomer copolymerizable therewith, which has been modified by introducing a reactive functional group (for example, a carbonate group (carbonyldioxy group), an ester group, a haloformyl group, a carboxyl group, etc.) into the main chain or side chain, and has a reactive functional group terminal. Here, "having a reactive functional group at the end" means that a reactive functional group is present at the end of the main chain and / or side chain.

[0033] As described above, by using a modified fluororesin, which is a fluororesin having reactive functional groups at the terminals, as the sheath material, a plastic optical fiber having excellent chemical resistance, heat resistance, and the like can be obtained. From the viewpoint of chemical resistance and heat resistance, among reactive functional groups, those having a carbonate group are particularly preferred. A modified fluororesin having a reactive functional group having a carbonate group introduced therein can be easily obtained by using peroxycarbonate as a polymerization initiator during polymerization.

[0034] These reactive functional groups can be introduced by known methods, but are preferably introduced into the copolymer as a polymerization initiator. When introducing, it is preferable to use 0.05 parts by mass or more and 20 parts by mass or less of the polymerization initiator per 100 parts by mass of the resulting copolymer.

[0035] The modified fluororesin has an ethylene-tetrafluoroethylene copolymer as its main skeleton. The molar ratio of ethylene / tetrafluoroethylene in the ethylene-tetrafluoroethylene copolymer is not particularly limited, but is preferably in the range of 70 / 30 to 30 / 70 from the viewpoint of the balance between moldability and chemical resistance.

[0036] Furthermore, the modified fluororesin may be a copolymer of tetrafluoroethylene and ethylene with other monomers copolymerizable therewith, for example, olefins such as hexafluoropropylene, hexafluoroisobutene, propylene, 1-butene, 2-butene, vinyl chloride, vinylidene chloride, vinylidene fluoride, chlorotrifluoroethylene, vinyl fluoride, hexafluoroisobutene, and perfluoro(alkyl vinyl ether).

[0037] In this case, the molar ratio of ethylene / tetrafluoroethylene / other copolymerizable monomer is not particularly limited, but from the viewpoint of the balance between moldability and chemical resistance, it is preferably in the range of (10 to 80) / (20 to 80) / (0 to 40).

[0038] As the modified fluororesin, preferred are carbonyldioxy group-containing copolymers having polymer chains obtained from a monomer component consisting of 62 mol % to 80 mol % of tetrafluoroethylene, 20 mol % to 38 mol % of ethylene, and 0 mol % to 10 mol % of monomers copolymerizable therewith; and carbonyldioxy group-containing copolymers having polymer chains obtained from a monomer component consisting of 20 mol % to 80 mol % of tetrafluoroethylene, 10 mol % to 80 mol % of ethylene, 0 mol % to 30 mol % of hexafluoropropylene, and 0 mol % to 10 mol % of monomers copolymerizable therewith, because they have excellent chemical resistance and heat resistance.

[0039] The melting point of the modified fluororesin is preferably in the range of 150°C to 200°C. This is preferable because a melting point in this temperature range allows molding at a temperature of 300°C or less. If the temperature is 300°C or less, thermal decomposition of the polymethyl methacrylate resin is acceptable. The melting point can be measured by differential scanning calorimetry, for example, by using a differential scanning calorimeter (EXSTAR DSC6200) manufactured by Seiko Instruments Inc. and heating the sample at a rate of 20° C. / min.

[0040] In this embodiment, the modified fluororesin is preferably an ethylene-tetrafluoroethylene copolymer having a terminal reactive functional group. In particular, the modified ethylene-tetrafluoroethylene copolymer used as the modified fluororesin may be one obtained by copolymerizing tetrafluoroethylene and ethylene with a monomer such as propylene. Among these, those having a melting point in the range of 150°C to 200°C and an MFR (230°C, load 3.8 kg, orifice diameter 2 mm, length 8 mm) in the range of 5 to 100 g / 10 min are preferred, since they can be molded at temperatures of 300°C or less, which is the allowable temperature for thermal decomposition of the polymethyl methacrylate resin that constitutes the core resin. The above-mentioned modified fluororesin usually has a Shore D hardness (ASTM D2240) at 23°C in the range of 60 to 80. Even if the Shore D hardness is high, by introducing a reactive functional group into the sheath resin, adhesion to the core is achieved, and even a hard sheath resin is difficult to peel from the core, preventing the problem of the core popping out of the sheath.

[0041] Commercially available modified fluororesins that can be used include Neoflon EFEP RP5000 and RP4020 manufactured by Daikin Industries, Ltd. and Fluon LM-ETFE AH2000 manufactured by Asahi Glass Co., Ltd. Of these, Neoflon EFEP RP5000 and RP4020 are carbonate-modified ethylene-tetrafluoroethylene copolymers containing carbonyldioxy groups as reactive functional groups, and are suitable as sheath resins.

[0042] There is no particular limit to the thickness of the sheath, but if it is too thin, there is a risk that light will not be reflected sufficiently, and if it is too thick, there is a risk that the light-receiving area will decrease. In the plastic optical fiber of this embodiment, the thickness of the first sheath 2 is preferably 1 μm or more and 100 μm or less, more preferably 5 μm or more and 50 μm or less, and even more preferably 7 μm or more and 20 μm or less, in order to maintain the mechanical strength of the plastic optical fiber. When the plastic optical fiber of this embodiment has a plurality of sheaths, the thickness of the second and subsequent sheaths can be selected in accordance with the thickness of the first sheath.

[0043] The plastic optical fiber of this embodiment has a first sheath and a first core that form the outermost periphery of a core / sheath structure, and has a first island portion inside the first core. When the plastic optical fiber of this embodiment has one or more sheaths or one or more island portions each consisting of a core / sheath structure even in the first island portion, and has multiple sheaths overall, it is preferable that each sheath is formed from the same material, from the viewpoint of making the NA calculated by the following formula (1) of each island portion the same. Here, NA is a value calculated by the following formula (1), and determines the light reflection characteristics. By making the light reflection characteristics of each island uniform, even if a plastic optical fiber has a plurality of islands, the characteristics will be uniform as a whole, which is preferable. NA=(Ncore 2 -Nclad 2 ) 0.5 ···(1) Ncore Refractive index of the core Refractive index of Nclad sheath

[0044] (protective layer) As shown in FIG. 2, the plastic optical fiber of this embodiment preferably has a protective layer 17 on the outer periphery of the first sheath 2 that forms the outermost periphery of the core / sheath structure. By providing a protective layer 17 on the outer periphery of the first sheath 2, it is possible to improve adhesion with the coating layer 21 shown in FIG. 4 described later and to impart heat resistance to the plastic optical fiber, thereby further improving the performance of the plastic optical fiber.

[0045] The material used for the protective layer 17 is not particularly limited as long as it is a thermoplastic resin that can be spun together with the plastic optical fiber of this embodiment, but from the viewpoint of adhesion to the first sheath 2 and moldability, it is preferably a thermoplastic fluorine-based resin, and more preferably the above-mentioned ethylene-tetrafluoroethylene-based copolymer. It is particularly preferable to form the protective layer 17 as a layer having a lower refractive index than the first sheath 2. By using a material having a lower refractive index than the first sheath 2, the protective layer 17, which is a low refractive index layer, can reflect light leaking from the first sheath 2, and the above-mentioned effect can be expected. The material of the protective layer 17, which is a low refractive index layer, is not particularly limited as long as it has a refractive index lower than that of the material of the first sheath 2. The thickness of the protective layer 17 is preferably 1 μm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less, in order to impart appropriate mechanical strength within a range that does not reduce the light intensity of the plastic optical fiber.

[0046] (Island) The plastic optical fiber of this embodiment has a first island portion 8 inside the first core 1 that forms the first sea portion. The island portion has a function of reflecting light propagating through the core forming the sea portion, for example. The first island portion 8 may be formed so that the refractive index is lower than that of the first sea portion, which is the first core 1, at least at the outer periphery, and may be formed so that the refractive index is lower throughout.

[0047] A preferred embodiment of the first island portion 8 is one having a second sheath 9 and a second core 10 that forms a second sea portion inside the second sheath, as shown in FIG. The first island portion 8 has the second sheath 9 and the second core 10, so that the first island portion 8 can also propagate light. In the plastic optical fiber of this embodiment, the first island portion 8 only needs to have the second sheath 9, and does not necessarily need to have the second core 10 formed thereon.

[0048] The plastic optical fiber of this embodiment preferably has a second island portion 11 further inside the second core 10, as shown in FIG. By providing the second island portion 11 inside the second core 10, the bending loss of light propagating through the second island portion 11 can be reduced.

[0049] The second island portion 11 of this embodiment preferably has a structure in which one or more sheaths and cores are concentrically formed alternately in this order toward the inside of the cross section of the plastic optical fiber, as shown in Figure 1, for example. By having the second island portion 11 have such a structure, bending loss can be reduced more effectively. In the second island portion 11, by having a structure in which two or more cores and sheaths are alternately formed concentrically, it is possible to have a total of four or more layers of core / sheath structure, which can further effectively reduce bending loss.

[0050] As described above, the plastic optical fiber of this embodiment preferably has a concentric structure in which the core and sheath are alternately repeated, which makes it possible to reduce bending loss when bending and connection loss when connecting plastic optical fibers together. In particular, since the island portion of the innermost layer has the greatest amount of light when light is transmitted through it, the diameter of the island portion of the innermost layer is preferably 20% or more and 40% or less of the cross-sectional diameter of the plastic optical fiber of this embodiment, more preferably 25% or more and 35% or less, and even more preferably 27% or more and 33% or less. By making the diameter of the island part of the innermost layer 20% or more of the diameter of the plastic optical fiber, not only can the light intensity of the linear light source be sufficiently propagated, but also the optical axis misalignment when the optical fibers are spliced ​​together is less likely to occur, thereby reducing the splice loss.Furthermore, by making the diameter of the island part of the innermost layer 40% or less, the bending loss when the plastic optical fiber is bent can be sufficiently reduced.

[0051] [Plastic optical fiber cable] The plastic optical fiber cable of this embodiment includes the plastic optical fiber of this embodiment and a thermoplastic resin layer coating the plastic optical fiber. That is, the plastic optical fiber cable of this embodiment is coated with a coating layer made of a thermoplastic resin.

[0052] (covering layer) The coating layer constituting the plastic optical fiber cable of this embodiment is formed to coat the outer periphery of the above-mentioned plastic optical fiber. FIG. 3 shows a schematic cross-sectional view of an example of the plastic optical fiber cable of this embodiment. In FIG. 3, a plastic optical fiber cable 20 has a configuration in which a coating layer 21 is formed on the outside of the first sheath 2 of the plastic optical fiber 1A shown in FIG. The resin used as the coating layer 21 is not particularly limited, and any conventionally known thermoplastic resin can be used, such as polyethylene resin, cross-linked polyethylene resin, polypropylene resin, polyamide resin such as polyamide 6, polyamide 6T, polyamide 66, polyamide 11, polyamide 12, and polyamide 1010, vinyl chloride resin, fluororesin such as vinylidene fluoride and PFA, polyimide resin, and liquid crystal polymer obtained by condensation polymerization of 2,6-hydroxynaphthoic acid and parahydroxybenzoic acid.

[0053] (Outer coating layer) The plastic optical fiber cable of this embodiment can use the coating layer as the outermost layer, but it can also be used as a more reinforced plastic optical fiber cable by applying an outer coating layer (also called an "outer jacket") made of a thermoplastic resin such as polyamide 12, soft polyamide, polyethylene, polyvinyl chloride, polypropylene, fluororesin, or liquid crystal resin to the outer periphery.

[0054] The resin for these coating layers may contain a light-blocking agent such as carbon black to prevent the incidence of external light, or a flame retardant such as magnesium hydroxide or melamine cyanurate. Furthermore, the resin for the coating layer may contain a colorant to enhance the distinguishability and design of the plastic optical fiber cable. Known dye-based or inorganic colorants are used as colorants, but inorganic pigments are preferred from the viewpoint of heat resistance.

[0055] The resin for the coating layer and the additives may be mixed, for example, by melt-kneading using a known device such as a twin-screw extruder.

[0056] The plastic optical fiber cable of this embodiment may be a cable in which one plastic optical fiber is coated, or a cable in which two or more plastic optical fibers are bundled and coated with an outer coating layer, etc. Furthermore, a plurality of cables each coated with one plastic optical fiber may be bundled and further coated. Furthermore, the coating layer may be three or more layers, if necessary.

[0057] The plastic optical fiber of this embodiment has excellent flexibility, so it is preferably used in a curved wiring. By using a curved wiring, it becomes possible to lay it in gaps within the equipment to be used, increasing the degree of freedom in wiring design. Furthermore, when laying it in a narrow gap, it is preferable to use a multi-connected wiring, because this makes laying it easier.

[0058] [Optical fiber cable with connector] The plastic optical fiber cable of this embodiment can be used as a connectorized optical fiber cable by attaching connectors to both ends. The plastic optical fiber cable of this embodiment may be used alone, but particularly for optical transmission applications, attaching appropriate connectors to both ends facilitates connection between devices. There are no particular restrictions on the connectors that can be used, and any known connectors can be used. In particular, the plastic optical fiber cable of this embodiment can have a connector attached to the coating layer by laser welding. In this case, the plastic optical fiber cable of this embodiment is attached to the plastic optical fiber cable with the connector welded. This configuration is particularly preferable because the connector can be firmly attached to the plastic optical fiber cable by laser welding.

[0059] The plastic optical fiber of this embodiment or the plastic optical fiber cable of this embodiment is an excellent step-index plastic optical fiber and can be suitably used in optical communication systems and plastic optical fiber sensors. Therefore, the present invention includes an optical communication system having the plastic optical fiber cable of this embodiment and a plastic optical fiber sensor having the plastic optical fiber of this embodiment. From the viewpoint of reliably transmitting communication data in an optical communication system, it is preferable that the light passing through each core of the plastic optical fiber cable be light emitted from the same light source. It is possible to transmit different light, i.e., different data, through each core, but when wiring multiple cores together, precise alignment of each core is required, which results in extremely difficult multi-core connections.

[0060] [Method for manufacturing the plastic optical fiber of this embodiment] The plastic optical fiber and plastic optical fiber cable of this embodiment can be manufactured by, for example, a known composite spinning method. More specifically, materials (e.g., a predetermined resin) for each component (sheath, core, and island portions) constituting the plastic optical fiber of this embodiment are simultaneously introduced into a composite spinning die for forming a predetermined structure to obtain a plastic optical fiber (a plastic optical fiber wire).The outer periphery of this plastic optical fiber wire is coated with a heat-melted resin for forming a coating layer using a crosshead die to obtain a plastic optical fiber cable. The cross-sectional area ratio of each sheath, each core, and each island portion formed by these can be controlled by adopting a known method, for example, by changing the speed of each liquid delivery pump that sends the core and sheath resin, and adjusting the ratio (flow rate ratio) of the flow rate of the core resin to the total flow rate of the core and sheath resins. [Example]

[0061] Hereinafter, the present embodiment will be described with reference to specific examples and comparative examples, but the present embodiment is not limited to the examples described below. First, the evaluation items will be described.

[0062] <Bending loss> For each example and comparative example, the rate of reduction in light intensity was measured when the sample was bent 90 degrees along a cylinder with a radius of 5 mm. A measured value of 0.5 dB or less was deemed to be acceptable.

[0063] <Connection loss> For each example and comparative example, F07 connectors were attached to both ends of a 2 m optical fiber, and the light intensity was measured using an optical power meter (Photom205A manufactured by Gray Technos). The optical fiber was then split into two at the center of its axial direction, and an F07 connector was attached to the split section. The end faces were polished successively with lapping films of 30 μm, 9 μm, and 1 μm grain size to a flat finish. The sections were then connected using a relay adapter, and the light intensity was measured again. A test was deemed to have passed if the difference in light intensity before and after splitting was 1.5 dB or less.

[0064] <Transmission loss> For each example and comparative example, two 22 m samples were prepared, and the initial value of the transmission loss for each was measured using a 22 m-2 m cutback method with an incident NA of 0.15 and a wavelength of 650 nm as the light source. A transmission loss of 200 dB / km or less was considered to be acceptable.

[0065] <Reliability test> Of the two remaining 20m samples for which the initial transmission loss values ​​were measured, one was placed in a dry heat environment. As a reliability test of 75 The remaining one was placed in a thermostatic chamber at 105°C or 105°C as a reliability test in a humid and hot environment. 75 The samples were placed in a constant temperature and humidity chamber at 85°C and 95% RH or 85°C and 85% RH and subjected to a reliability test for 3000 hours. After 3000 hours, the samples were removed from the chamber and the transmission loss was measured using the 20m-2m cutback method in the same manner as the transmission loss measurement method described above. The reliability test was passed if the transmission loss in a dry and hot environment was 200 dB / km or less, and if the transmission loss in a humid and hot environment was 250 dB / km or less. The test conditions for the reliability test were as follows: 1. The plastic optical fiber with a protective layer was In environments of 05℃ and 85℃ 85% RH, the product without a protective layer 75 °C and 75 Reliability tests were conducted in an environment of 95% RH and 100% RH.

[0066] ( Reference example 1) The first to fourth cores are made of polymethyl methacrylate (refractive index 1.491). The first to fourth sheaths that are in direct contact with each core are made of vinylidene fluoride. A copolymer (refractive index 1.40) of 72% by mass of ethylene and 28% by mass of tetrafluoroethylene was used. The composite die used is a two-type eight-layer composite die, with the core and sheath alternately arranged in four layers concentrically. Using the constructed die, a plastic optical fiber with a diameter of 1 mm was spun, as shown in Figure 1. A plastic optical fiber having such a cross section was manufactured. The cross section was observed using a digital microscope (Keyence VHX-8000, magnification 50x). ) and observed that sheaths were formed on all the islands in each layer, and that the conduction was observed in all the first to fourth layers. It was confirmed that the structure was as shown in the schematic cross-sectional view of Figure 1. The diameter of the innermost island of the optical fiber was measured using a digital microscope (Keyence VHX- 8000, magnification 100x) was 350 μm, and the plastic The ratio of the cross section of the optical fiber to the diameter was 35%. Tangent lines were drawn on the periphery, and the distance between the tangent lines was taken as the diameter of the island. Then, the manufactured plastic optical fiber wire was coated with polyethylene (coating diameter 2.2 mm) and used as a plastic optical fiber cable. Two plastic optical fiber cables were cut to 22 m. The initial transmission loss was measured after cutting back 2m. The results were 146dB / km and 146dB / km, which was a pass. The bending loss was 0.3 dB, which was acceptable. The connection loss was 0.9 dB, which was acceptable. The configuration and evaluation results of the bull are shown in Table 1.

[0067] Example 2 Polymethyl methacrylate (refractive index 1.491) was used as the material for the first to fourth cores, and a modified fluorocopolymer (refractive index 1.38) was used as the material for the first to fourth sheaths that are in direct contact with each core, which was a fluorocopolymer with an ethylene-tetrafluoroethylene copolymer as the main skeleton to which carbonate groups had been introduced.Furthermore, a two-type, nine-layer composite die was used that was capable of forming a layer of a copolymer of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropene (refractive index 1.37) on the outside of the first sheath, the outermost layer of the core / sheath structure.A 1mm diameter plastic optical fiber was produced, as shown in Figure 2, in which the core and sheath were alternately formed in four layers concentrically, with a protective layer on the outside. The produced plastic optical fiber wire was coated with polyamide 12 (coating diameter: 1.5 mm) to form a plastic optical fiber cable. Two plastic optical fiber cables were prepared and the initial transmission loss was measured using the cutback method. The transmission losses were 166 dB / km and 162 dB / km, respectively, which were acceptable. The bending loss was 0.3 dB, which was acceptable. The connection loss was 0.7dB, which was acceptable. The structure and evaluation results of the obtained plastic optical fiber cable are shown in Table 1.

[0068] Example 3 A plastic optical fiber cable was obtained in the same manner as in Example 2, except that the sheath resin used for the first to fourth sheaths was a fluorinated methacrylate resin (refractive index 1.42) obtained by cast polymerization of 20% by mass of tetrafluoropropyl methacrylate (4FM), 60% by mass of pentafluoropropyl methacrylate (5FM), and 20% by mass of methyl methacrylate, and the resin for the protective layer provided on the outside of the first sheath was a copolymer (refractive index 1.40) of 72% by mass of vinylidene fluoride and 28% by mass of tetrafluoroethylene. Two of the manufactured plastic optical fiber cables were prepared, and the initial value of the transmission loss of each was measured using the cutback method. The transmission losses were 141 dB / km and 142 dB / km, respectively, which were acceptable. The bending loss was 0.5 dB, which was acceptable. The connection loss was 0.8dB, which was acceptable. The structure and evaluation results of the obtained plastic optical fiber cable are shown in Table 1.

[0069] ( Reference example 4) A plastic optical fiber cable was obtained in the same manner as in Example 3, except that a quaternary copolymer (refractive index 1.36) of perfluoroalkyl vinyl ether, vinylidene fluoride, tetrafluoroethylene, and hexafluoropropene was used as the sheath resin for the first to fourth sheaths. Two manufactured plastic optical fiber cables were prepared, and transmission was performed using the cutback method. The initial loss was measured. The transmission losses were 155 dB / km and 153 dB / km, respectively, which were acceptable. The bending loss was 0.4 dB, which was acceptable. The connection loss was 0.7dB, which was acceptable. The structure and evaluation results of the obtained plastic optical fiber cable are shown in Table 1.

[0070] ( Reference example 5) The sheath resins used for the first to third sheaths are vinylidene fluoride and tetrafluoroethylene. and hexafluoropropene copolymer (refractive index 1.37) to form a concentric plastic The composite nozzle for spinning optical fiber is a two-type six-layer composite die, and the core and sheath are alternately arranged in three layers. Other than using a die with a circular shape, Reference example In the same way as in step 1, a 1 mm diameter plastic The diameter of the innermost island part and the plastic optical fiber were measured in the same manner as in Example 1. The diameter of the plastic optical fiber and the ratio of the diameter of the plastic optical fiber to the cross-sectional diameter of the plastic optical fiber were calculated. The diameter of the innermost island of the obtained plastic optical fiber was 200 μm. The fiber to diameter ratio was 20%. Polyamide 12 was added to the obtained concentric plastic optical fiber in the same manner as in Example 2. A coated plastic optical fiber cable was obtained. Two manufactured plastic optical fiber cables were prepared and each was transmitted using the cutback method. The initial transmission loss was measured. The transmission losses were 170 dB / km and 172 dB / km, respectively, which were acceptable. The bending loss was 0.2 dB, which was acceptable. The connection loss was 1.2 dB, which was acceptable. The structure and evaluation results of the obtained plastic optical fiber cable are shown in Table 1.

[0071] ( Reference example 6) The core and sheath are placed in a two-type six-layer composite die, and the flow rate ratio between the core and sheath is changed so that the core and sheath are alternately placed in three layers concentrically. A plastic die having a diameter of 1 mm was prepared in the same manner as in Example 5, except that a circular die was used. A high-quality optical fiber was obtained. The diameter of the innermost island of the obtained plastic optical fiber was 400 μm. The fiber to diameter ratio was 40%. This concentric plastic optical fiber was coated with polyamide 12 in the same manner as in Example 2. A plastic optical fiber cable was obtained. Two manufactured plastic optical fiber cables were prepared and each was transmitted using the cutback method. The initial transmission loss was measured. The transmission losses were 145 dB / km and 146 dB / km, respectively, which were acceptable. The bending loss was 0.5 dB, which was acceptable. The connection loss was 1.0 dB, which was acceptable. The structure and evaluation results of the obtained plastic optical fiber cable are shown in Table 1.

[0072] (Comparative Example 1) The core resin is polymethyl methacrylate (refractive index 1.491), and the sheath resin is Copolymer of vinylidene fluoride, tetrafluoroethylene and hexafluoropropene (refractive index) The resins were then introduced into the core and sheath resin distribution chambers of the 19-island composite spinning die using a ratio of 1.37. A 19-island plastic optical fiber strand with a diameter of 1 mm was produced by composite spinning. The manufactured optical fiber Reference example As with 1, a plastic optical fiber cable was used. When the cross section was observed, it was confirmed that all 19 islands were guiding light, as shown in the schematic cross section of Figure 5. The structure shown is as follows. The plastic optical fiber 31 shown in FIG. 5 is made up of a sheath 29 and a 19-type fiber formed inside the sheath 29. It is composed of a core 28. Two manufactured plastic optical fiber cables were prepared and each was transmitted using the cutback method. The initial transmission loss was measured. The transmission losses were 161 dB / km and 164 dB / km, respectively, which were acceptable. The bending loss was 0.3 dB, which was acceptable. The connection loss was 1.8 dB, which was a failure. The structure and evaluation results of the obtained plastic optical fiber cable are shown in Table 1.

[0073] (Comparative Example 2) The core resin is polymethyl methacrylate (refractive index 1.491), and the sheath resin is Copolymer of vinylidene fluoride, tetrafluoroethylene and hexafluoropropene (refractive index) The resins were placed in the core resin distribution chamber and sheath resin distribution chamber of the single-core composite spinning die, respectively, using a ratio of 1.37. A single-core plastic optical fiber wire with a diameter of 1 mm was produced by composite spinning. The manufactured plastic optical fiber wire Reference example A plastic optical fiber cable was fabricated in the same manner as in 1. When the cross section was observed, it was confirmed that the core, which is the sea portion, was guiding light, and the structure was as shown in the schematic cross section of Figure 6. The plastic optical fiber 34 shown in Figure 6 is composed of a sheath 33 and a core 32 formed inside the sheath 33. Two manufactured plastic optical fiber cables were prepared and each was transmitted using the cutback method. The initial transmission loss was measured. The transmission losses were 139 dB / km and 141 dB / km, respectively, which were acceptable. The bending loss was 1.2 dB, which was unacceptable. The connection loss was 0.9dB, which was acceptable. The structure and evaluation results of the obtained plastic optical fiber cable are shown in Table 1.

[0074] ( Reference example 7) The flow rate ratio of the core and sheath was changed to four layers, i.e., two types and eight layers. Reference example A plastic optical fiber was obtained in the same manner as in 1. The diameter of the innermost island of the obtained plastic optical fiber was 500 μm. The ratio of the optical fiber to the diameter was 50%. Two manufactured plastic optical fiber cables were prepared and each was transmitted using the cutback method. The initial transmission loss was measured. The transmission losses were 141 dB / km and 139 dB / km, respectively, which were acceptable. The bending loss was 0.3 dB, which was acceptable. The connection loss was 1.0 dB, which was acceptable. The structure and evaluation results of the obtained plastic optical fiber cable are shown in Table 1.

[0075] ( Reference example 8) The flow rate ratio of the core and sheath was changed to four layers, i.e., two types and eight layers. Reference example A plastic optical fiber was obtained in the same manner as in 1. The diameter of the innermost island of the obtained plastic optical fiber was 150 μm. The fiber to diameter ratio was 15%. Two manufactured plastic optical fiber cables were prepared and each was transmitted using the cutback method. The initial transmission loss was measured. The transmission losses were 156 dB / km and 158 dB / km, respectively, which were acceptable. The bending loss was 0.2 dB, which was acceptable. The connection loss was 1.6 dB, which was a failure. The structure and evaluation results of the obtained plastic optical fiber cable are shown in Table 1.

[0076] In Table 1 below, the resin materials used for the first sheath and the protective layer are shown below. A: Copolymer of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropene (refractive index 1.37) B: Fluorinated methacrylate resin (refractive index 1.42) C: Perfluoroalkyl vinyl ether, vinylidene fluoride, tetrafluoroethylene, and hexafluoropropene quaternary copolymer (refractive index 1.36) D: Copolymer of vinylidene fluoride and tetrafluoroethylene (refractive index 1.40) E: Modified fluoropolymer (refractive index 1.38)

[0077] [Table 1] [Explanation of symbols]

[0078] 1...First core 2...First sheath 8...First island 9...Second sheath 10...Second core 11...Second island 12...Third sheath 13...Third core 14...Third Island 15...Fourth sheath 16...Fourth core 17...Protective layer 20...Plastic optical fiber cable 21…Covering layer 28...Core 29...scabbard 31...Plastic optical fiber 32...Core 33...scabbard 34...Plastic optical fiber 1A, 2A...Plastic optical fiber

Claims

1. A plastic optical fiber having a core / sheath structure, a first sheath that forms the outermost periphery of the core / sheath structure of the plastic optical fiber; a first core forming a first sea portion inside the first sheath; and a first island portion formed inside the first core, at least an outer periphery of which has a refractive index lower than that of the first sea portion; the first core contains a polymethyl methacrylate resin; the first sheath is made of a fluororesin, a protective layer made of a thermoplastic fluorine-based resin layer on the outside of the first sheath; The refractive index of the protective layer is lower than the refractive index of the first sheath. Plastic optical fiber.

2. the first island portion has a second sheath and a second core forming a second sea portion inside the second sheath; 2. The plastic optical fiber according to claim 1.

3. a second island portion formed inside the second core and including a third sheath and a third core forming a third sea portion inside the third sheath; 3. The plastic optical fiber according to claim 2.

4. The second island portion has two or more sheaths and two or more cores concentrically arranged in this order, alternately arranged inward of the cross section of the plastic optical fiber, and has a total of four or more layers of a core / sheath structure. The plastic optical fiber according to claim 3.

5. Each core constituting the plastic optical fiber is formed of the same material.

5. The plastic optical fiber according to claim 2.

6. the diameter of the island portion of the innermost layer in the cross section of the plastic optical fiber is 20% or more and 40% or less of the diameter of the cross section of the plastic optical fiber; The plastic optical fiber according to any one of claims 1 to 5.

7. In the cross section of the plastic optical fiber, the sheath and the core are arranged in a circular shape in a direction perpendicular to the drawing direction of the plastic optical fiber.

7. The plastic optical fiber according to claim 1.

8. Each sheath constituting the plastic optical fiber is formed of the same material. The plastic optical fiber according to any one of claims 2 to 7.

9. A plastic optical fiber according to any one of claims 1 to 8; a coating layer made of a thermoplastic resin layer that coats the plastic optical fiber; Plastic fiber optic cable.

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