Multi-core plastic optical fiber, optical communication cable, and optical communication system

Abstract

The present invention addresses the problem of providing a multi-core plastic optical fiber, an optical communication cable, and an optical communication system, all which have a wide transmission band.　The present invention is a multi-core plastic optical fiber having a cross-sectional form comprising two or more island portions and a sea portion surrounding the island portions. The island portions each comprise a single core and cladding that surrounds the core in a one-to-one correspondence with the core. Each cladding is independent of other cladding. The refractive index (N2) of the cladding is lower than the refractive index (N1) of the core. The refractive index (N3) of the sea portion is higher than the refractive index (N2) of the cladding.

Description

Multi-core plastic optical fiber, optical communication cable, and optical communication system

[0001] The present invention relates to a multi-core plastic optical fiber, an optical communication cable, and an optical communication system.

[0002] Taking advantage of their light weight and flexibility, plastic optical fibers are used in a wide range of fields, including decorative lighting, medical applications, in-vehicle lighting, various sensors, and communications.

[0003] Single-core plastic optical fibers, which have one core in the fiber, have the disadvantage that bending loss occurs and the light retention rate decreases when the fiber is bent excessively.However, multi-core plastic optical fibers, which have multiple cores, can reduce light transmission loss when the fiber is bent.

[0004] For example, Patent Document 1 discloses an invention of an optical fiber sensor characterized by comprising one or more multi-core plastic optical fibers having a length of 50 cm to 5 m, which are manufactured by a composite spinning method so that 7 to 10,000 core fibers made of a transparent core resin with a high refractive index, a first sheath layer surrounding each of the core fibers and made of a transparent first sheath resin having a refractive index lower than that of the core resin, and a second sheath layer surrounding the outside of each of the first sheath layers and made of a second sheath resin composition in which a coloring substance is dispersed in a second sheath resin having a refractive index lower than that of the first sheath resin, are bundled together in the form of a single fiber, a light-emitting element, and a light-receiving element.

[0005] Furthermore, Patent Document 2 discloses an invention for a plastic optical fiber having a first sheath, a first core forming a first sea portion inside the first sheath, and a first island portion formed inside the first core, at least the outer periphery of which has a refractive index lower than that of the first sea portion, and the first core contains a polymethyl methacrylate resin.

[0006] JP 2009-109300 A International Publication No. 2019 / 045046

[0007] However, when optical data communication is performed using a multi-core optical fiber, the optical fiber disclosed in Patent Document 1 has a problem in that signal light leaks between cores (crosstalk), deteriorating the transmission bandwidth.

[0008] Furthermore, the optical fiber disclosed in Patent Document 2 basically transmits signal light only through the first core, and therefore has the problem that the amount of information that can be sent through one fiber is small.

[0009] Therefore, an object of the present invention is to overcome the problems of the prior art and to provide a multi-core plastic optical fiber, an optical communication cable, and an optical communication system with a wide transmission band.

[0010] The present invention provides the following to solve the above problems. (1) A multi-core plastic optical fiber having a cross-sectional configuration consisting of two or more island portions and a sea portion surrounding the island portions, wherein each of the island portions consists of a core and a cladding surrounding the core in a one-to-one correspondence with the core, each cladding is independent of the other claddings, the refractive index (N2) of the cladding is lower than the refractive index (N1) of the core, and the refractive index (N3) of the sea portion is higher than the refractive index (N2) of the cladding. (2) The multi-core plastic optical fiber according to (1) above, in which N3 is equal to or greater than N1. (3) The multi-core plastic optical fiber according to (1) or (2) above, in which the core contains a polymethyl methacrylate resin. (4) The multi-core plastic optical fiber according to any of (1) to (3) above, in which the sea is the outermost layer. (5) The multi-core plastic optical fiber according to any one of (1) to (4) above, wherein the average transmittance of the sea portion at a wavelength of 400 to 700 nm is lower than the average transmittance of the cores at a wavelength of 400 to 700 nm. (6) The multi-core plastic optical fiber according to any one of (1) to (5) above, wherein the sea portion contains a coloring material. (7) The multi-core plastic optical fiber according to (6) above, wherein the coloring material is carbon black. (8) The multi-core plastic optical fiber according to any one of (1) to (7) above, wherein the cores are arranged in a square lattice pattern in the cross-sectional form. (9) An optical communication cable comprising the multi-core plastic optical fiber according to any one of (1) to (8) above. (10) An optical communication system using the multi-core plastic optical fiber according to any one of (1) to (8) above or the optical communication cable according to (9) above, and performing spatial multiplexing communication using a plurality of signal light beams.

[0011] According to the present invention, a multi-core plastic optical fiber with a wide transmission band can be provided.

[0012] Hereinafter, embodiments of the multi-core plastic optical fiber, the optical communication cable, and the optical communication system according to the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and can be modified in various ways depending on the purpose and application.

[0013] In the present invention, "equal to or greater than" means equal to or greater than the indicated numerical value, and "equal to or less than" means equal to or smaller than the indicated numerical value.

[0014] The multi-core plastic optical fiber of the present invention has a cross-sectional configuration consisting of two or more island portions and a sea portion surrounding the island portions.

[0015] Each of the islands consists of a core and a cladding surrounding it.

[0016] [Core] The core is a transmission portion that directly propagates the signal light and plays a role in efficiently transmitting the signal light.

[0017] The resin in the resin composition forming the core is preferably a material that is light-transmitting and has low transmission loss, and examples thereof include acrylic resin, modified polycarbonate resin, cycloolefin resin, styrene resin, olefin resin such as polymethylpentene, etc. Among these resins, acrylic resin is preferred because of its low transmission loss.

[0018] Examples of the acrylic resin include polymers of methacrylic acid esters and acrylic acid esters. Examples of methacrylic acid esters include alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, and butyl methacrylate; aryl methacrylates such as phenyl methacrylate; and cycloalkyl methacrylates such as cyclohexyl methacrylate and norbornenyl methacrylate. Examples of acrylic acid esters include alkyl acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate; aryl acrylates such as phenyl acrylate; and cycloalkyl acrylates such as cyclohexyl acrylate and norbornenyl acrylate. Other examples of acrylic resins include sodium polyacrylate resins, polyacrylonitrile resins, and polyacrylamide resins. Among these, polymethyl methacrylate resins containing methyl methacrylate as a main component are particularly preferred from the viewpoints of transparency and processability.

[0019] The modified polycarbonate resin may be, for example, a polycarbonate resin substituted with a lower alkyl group or a trifluoromethyl group and having an average molecular weight of 10,000 to 200,000.

[0020] Examples of the cycloolefin resin include cycloolefin polymers such as addition copolymers of ring-opening metathesis polymerization polymers and ethylene, and hydrogenated ring-opening metathesis polymerization polymers, as well as cycloolefin copolymers such as ethylene-2-norbornene.

[0021] In order to adjust the refractive index, the resin composition constituting the core may be appropriately added with a dopant that increases the refractive index, such as germanium, phosphorus, tin, boron, or the like, or a dopant fluorine-based material that decreases the refractive index, such as a fluorine-based material such as magnesium fluoride.

[0022] The refractive index (N1) of the core is preferably 1.45 or more and 1.60 or less. By making N1 1.45 or more, more preferably 1.48 or more, the refractive index difference with the cladding can be increased, thereby reducing the rate at which signal light leaks into the sea portion. Furthermore, by making N1 1.60 or less, more preferably 1.52 or less, the refractive index difference with the cladding can be suppressed, thereby reducing the number of modes of propagating light, thereby expanding the transmission bandwidth.

[0023] The refractive index can be measured in accordance with JIS K 7142:2014 using an Abbe refractometer on a 20 mm x 8 mm x 1.4 mm test piece at room temperature (25°C). If it is difficult to directly measure the refractive index of a core, the core can be heated at 210°C for 5 minutes using a press molding machine, then cooled to room temperature to produce a 20 mm x 8 mm x 1.4 mm test piece, and the refractive index can be measured. If the core composition is known, a test piece can be similarly prepared from the known composition, and the refractive index can be measured.

[0024] The cross-sectional shape of the core is preferably a perfect circle rather than a polygon because it is more uniform and has fewer irregularities, and in order to propagate signal light over long distances, the cross-sectional shape of the core should be uniformly straight and have as little cylindricity as possible. Here, cylindricity is an index showing the degree of deviation from a geometrically correct true cylinder in accordance with JIS B0621:1984.

[0025] The average inner diameter of the core is preferably 5 to 100 μm. By setting the average inner diameter to 5 μm or more, more preferably 20 μm or more, and even more preferably 30 μm or more, stable spinning processing is possible, the cross-sectional shape of the core is stabilized, and the light transmittance of the core is improved, thereby improving propagation efficiency and enabling more effective long-distance transmission. Furthermore, by setting the average radius to 100 μm or less, more preferably 60 μm or less, and even more preferably 40 μm or less, the transmission band can be expanded.

[0026] [Cladding] The cladding protects the core from external environmental factors and reduces the rate at which signal light propagating through the core is reflected at the cladding interface and leaks into the sea portion.

[0027] Examples of the resin constituting the clad include those common to the resin constituting the core, such as acrylic resin, as well as fluororesins such as vinylidene fluoride / tetrafluoroethylene / hexafluoropropylene copolymer, vinylidene fluoride / tetrafluoroethylene / hexafluoropropylene / perfluoroalkyl vinyl ether copolymer, vinylidene fluoride / trifluoroethylene copolymer, acrylic acid fluorinated ester polymer, polyperfluorobutyl methacrylate, polyperfluoroisopropyl methacrylate, and polyhexafluoro2-propyl methacrylate.

[0028] The cladding may be doped with a dopant, such as a fluorine-based material like magnesium fluoride, to adjust the refractive index. The cladding may also contain a coloring material. By including a coloring material in the cladding, the amount of signal light leaking into the sea can be reduced.

[0029] However, since the cladding is relatively thin, if the cladding contains a coloring substance, the content (mass %) of the coloring substance in the cladding is preferably smaller than the content (mass %) of the coloring substance in the sea. If the content (mass %) of the coloring substance in the cladding is greater than the content (mass %) of the coloring substance in the sea, the melt viscosity of the resin composition when forming the cladding increases, making it difficult to form the cladding to the desired dimensions without increasing the extrusion stress, or the extrusion stress may deform the core or sea. In other words, if the cladding of the optical fiber of the present invention contains a coloring substance, the content (mass %) of the coloring substance in the cladding is preferably smaller than the content (mass %) of the coloring substance in the sea. Specifically, the content is preferably less than 3 mass %.

[0030] The refractive index (N2) of the cladding is preferably 1.35 or more, more preferably 1.38 or more, so that the difference in refractive index between the core and the cladding is small, and the number of modes of the propagating light is reduced, thereby expanding the transmission bandwidth. Furthermore, if N2 is preferably 1.50 or less, more preferably 1.42 or less, the proportion of signal light propagating through the core that is reflected at the cladding interface and leaks into the sea can be reduced.

[0031] Here, N2 can be measured in the same manner as N1. If it is difficult to collect a clad and directly measure its refractive index, the collected clad can be heated at 210°C for 5 minutes using a press molding machine, then cooled to room temperature, and molded into a test piece measuring 20 mm x 8 mm x 1.4 mm, and the refractive index can be measured. Furthermore, if the composition of the clad is known, a test piece can be similarly prepared from the known composition, and the refractive index can be measured.

[0032] In order to reflect light at the interface between the core and the cladding, the refractive index of the cladding (N2) must be lower than the refractive index of the core (N1). By making N2 lower than N1, and preferably making the difference between N1 and N2 0.02 or more, more preferably 0.05 or more, the proportion of signal light propagating through the core reflected at the cladding interface and leaking into the sea can be reduced. On the other hand, by making the difference between N1 and N2 preferably 0.20 or less, more preferably 0.09 or less, the number of modes of propagating light can be reduced, thereby expanding the transmission bandwidth.

[0033] The claddings surround the cores in a one-to-one correspondence, and each cladding is independent of the other cores. That is, one core and the cladding surrounding it constitute each of the independent island portions. This structure prevents light leaking from one core from crosstalking to another core through the cladding.

[0034] The thickness of the cladding is preferably 1 / 20 to 1 / 3 times, and more preferably 1 / 10 to 1 / 3 times, the inner diameter of the cross section of the core. By keeping the cladding thickness within this range, total reflection of light at the interface between the core and the cladding can be ensured, and a decrease in transmission efficiency can be suppressed.

[0035] [Island Portions] The multi-core plastic optical fiber of the present invention has two or more island portions per fiber. By setting the number of island portions to 2 or more, preferably 60 or more, more preferably 100 or more, and even more preferably 300 or more, the cores of each island portion can propagate different light, thereby realizing a large communication capacity per fiber. On the other hand, by setting the number of island portions to preferably 10,000 or less, more preferably 5,000 or less, even more preferably 3,000 or less, and even more preferably 1,000 or less, the inner diameter of the core is not made too small, allowing for stable spinning, stabilizing the cross-sectional shape of the core, and more effectively improving long-distance transmission.

[0036] In the multi-core plastic optical fiber of the present invention, the cores of the island portions are preferably arranged in a square lattice pattern, which allows for a one-to-one correspondence with the arrangement of the light sources, and allows the inner diameter of the cores to be increased, thereby improving the light transmittance of the optical fiber.

[0037] In the present invention, the cores being arranged in a square lattice pattern means a structure in which the cores are regularly arranged in a lattice pattern at equal intervals in both the vertical and horizontal directions, i.e., there are multiple lines formed by the cores arranged vertically, there are multiple lines formed by the cores arranged horizontally, and the vertical and horizontal lines intersect.

[0038] [Sea portion] The sea portion protects the multiple island portions from external environmental factors, and also prevents signal light propagating through the island portions from reaching other adjacent cores even if the leaked signal light leaks into the sea portion.

[0039] Examples of the resin composition constituting the sea portion include acrylic resin, modified polycarbonate resin, cycloolefin resin, styrene resin, olefin resin such as polymethylpentene, and other thermoplastic polymers, which are used for the core, and fluororesins such as vinylidene fluoride / tetrafluoroethylene / hexafluoropropylene copolymer, vinylidene fluoride / tetrafluoroethylene / hexafluoropropylene / perfluoroalkyl vinyl ether copolymer, vinylidene fluoride / trifluoroethylene copolymer, fluorinated acrylic ester polymer, polyperfluorobutyl methacrylate, polyperfluoroisopropyl methacrylate, and polyhexafluoro2-propyl methacrylate, which are used for the cladding.

[0040] The sea portion may contain a coloring material. When the multi-core plastic optical fiber of the present invention propagates separate light beams to the cores, the light beams leaking from each core are less likely to reach the adjacent cores, thereby reducing noise such as crosstalk.

[0041] The coloring substance contained in the sea component has the function of blocking signal light from reaching other adjacent cores over the entire wavelength range of the signal light. Therefore, when the wavelength range of the signal light is outside the visible light range, it is sufficient that the coloring substance has the blocking function only over that wavelength range, and does not necessarily mean coloring for visible light. For example, if the signal light is infrared laser light, the infrared absorber may be transparent in the visible light range as long as it has the blocking function over the entire infrared light range, and such an infrared absorber is also included in the category of the coloring substance.

[0042] However, since the signal light used is generally white visible light, coloring materials in black, white, or gray that can perform the blocking function across the entire visible light range are generally preferred, and specific coloring materials include carbon black, lead oxide, titanium oxide, and organic pigments. Organic dyes can also be used, but if they are migratory, they will migrate from the sea to the core over long periods of storage, significantly reducing the transmission efficiency of the core, so it is necessary to select a material with low migration properties. Carbon black is preferred from the perspective of material cost and processability.

[0043] The average primary particle size of the carbon black is preferably 0.1 to 10 μm. By setting the particle size to 0.1 μm or more, it is possible to prevent aggregation during long-term spinning processing, which would result in an increase in the melt viscosity of the sea resin composition over time. Furthermore, by setting the particle size to 10 μm or less, it is possible to prevent clogging at the narrowest point of the sea portion.

[0044] The content of the coloring material relative to the entire resin composition constituting the sea portion varies depending on the properties of the coloring material. However, when the coloring material is carbon black having an average primary particle diameter of 0.1 to 10 μm, the carbon black content in the sea portion is preferably 0.3 to 5 mass%. By setting the content to 0.3 mass% or more, noise such as crosstalk can be effectively reduced. Furthermore, by setting the content to 5 mass% or less, a sudden increase in the melt viscosity of the resin composition in the sea portion can be prevented.

[0045] The refractive index (N3) of the sea portion is preferably 1.40 or more, more preferably 1.48 or more, so that the difference in refractive index between the sea portion and the cladding becomes large, and the leaked signal light is prevented from reaching the adjacent cores, thereby expanding the transmission bandwidth. Furthermore, when N3 is preferably 1.70 or less, more preferably 1.60 or less, the transparency of the sea portion is improved, and the light transmittance of the multi-core plastic optical fiber as a whole can be improved.

[0046] Here, N3 can be measured in the same manner as N1 and N2. When it is difficult to sample a sea portion and directly measure its refractive index, the sampled sea portion can be heated at 210°C for 5 minutes using a press molding machine, and then cooled to room temperature to prepare a test piece measuring 20 mm x 8 mm x 1.4 mm, and the refractive index can be measured. Furthermore, if the composition of the sea portion is known, a test piece can be prepared in the same manner from the known composition, and the refractive index can be measured.

[0047] The refractive index (N3) of the sea portion must be higher than the refractive index (N2) of the cladding. By making N3 higher than N2, preferably by making the difference between N3 and N2 0.05 or more, and more preferably 0.10 or more, signal light leaking from an island is reflected at the sea-cladding interface and does not enter other islands, thereby preventing crosstalk and expanding the transmission bandwidth. On the other hand, there is no particular upper limit for the difference between N3 and N2, but from the viewpoints of material versatility and cost, the upper limit is thought to be about 0.30.

[0048] In the multi-core plastic optical fiber of the present invention, the refractive index of the sea (N3) is preferably equal to or greater than the refractive index of the core (N1). By making N3 equal to or greater than N1, and more preferably making the difference between N3 and N1 equal to or greater than 0.01, and even more preferably equal to or greater than 0.09, the possibility that signal light leaking from an island portion enters another island portion can be reduced, and the transmission bandwidth can be expanded.

[0049] Furthermore, it is preferable that the average transmittance of the sea portion in the wavelength range of 400 to 700 nm is lower than the average transmittance of the core in the wavelength range of 400 to 700 nm. This can improve transmission efficiency by preventing signals transmitted through the sea from being transmitted as noise. Examples of means for adjusting these average transmittances include adjusting the transparency of the coloring substance or material mentioned above.

[0050] In the multi-core plastic optical fiber of the present invention, the sea portion is preferably the outermost layer, so that signal light leaking into the sea portion is attenuated and lost on the fiber surface, and the leaked signal light can be prevented from reaching other adjacent cores.

[0051] [Optical fiber] The cross section of the multi-core plastic optical fiber of the present invention is preferably circular from the viewpoint of handling, and its diameter is preferably 0.2 mm to 10 mm. A diameter of 0.2 mm or more provides appropriate rigidity and ease of handling, while a diameter of 10 mm or less provides appropriate flexibility and ease of handling.

[0052] [Manufacturing Method] As a method for manufacturing the multi-core plastic optical fiber of the present invention, for example, a method of continuously molding the resin compositions constituting the core, the cladding, and the sea portion into a predetermined shape can be mentioned. As the continuous molding method, it is preferable to form the multi-core plastic optical fiber by a composite spinning method using an extrusion die. Unlike other resin molding dies, the extrusion die does not cool or solidify any of the resin compositions inside the extrusion die, and the resin composition constituting the core, the resin composition constituting the cladding, and the resin composition constituting the sea portion are extruded from a core discharge part, a cladding discharge part, and a sea portion discharge part, respectively, and then cooled and solidified to be molded into a predetermined shape.

[0053] The extrusion die is not particularly limited as long as it can form the multi-core plastic optical fiber of the present invention, but it is preferable to arrange a plurality of sea component discharge portions so as to surround the outer peripheries of the core discharge portions and the cladding discharge portions. By arranging in this manner and performing composite spinning under appropriate conditions, it is possible to make the extrusion stress caused by the resin composition constituting the sea discharged from the sea discharge portion to be applied with approximately uniform strength from any direction, up, down, left, or right, to the resin compositions constituting each core discharged from each core discharge portion and the resin compositions constituting each cladding discharged from each cladding discharge portion.

[0054] When the balance of the extrusion stress is always maintained, the cross-sectional shape of each core continues to be the same as the cross-sectional shape of each core discharge part as it cools and solidifies, and the cross-sectional shape of each core discharge part continues to be reflected in the cross-sectional shape of the core. If the cross-sectional shape of each core discharge part is processed into a perfect circle with extremely high precision, it becomes possible to form a multi-core plastic optical fiber with a perfect cylindrical shape with extremely little deformation of the core.

[0055] The clad discharge portion may be shaped as a ring having an inner diameter equal to the inner diameter of the clad and an outer diameter equal to the outer diameter of the clad, or an inner diameter slightly larger than the inner diameter of the clad and an outer diameter slightly smaller than the outer diameter of the clad. That is, the former may be formed with the same width as the clad, or the latter may be formed with a ring shape slightly narrower than the clad. Alternatively, the clad discharge portion may be formed as multiple spots at equal intervals on the centerline circle of the ring-shaped clad.

[0056] The timing of discharging the resin compositions constituting the core, clad, and sea portions is preferably as simultaneous as possible, and even if there is a difference, it is preferably less than one second, which is substantially the same as when they are discharged simultaneously.The amount of each resin composition discharged is preferably such that the extrusion stress applied to the resin composition constituting each core, which is caused by the resin composition constituting the sea discharged from the sea discharge portion surrounded by each core, and the conversely, the extrusion stress applied to the resin composition constituting each sea, which is caused by the resin composition constituting each core discharged from each core discharge portion, are the same at every location, and these stresses continue to cancel each other out, so that the cross-sectional shape of the core can be maintained as a perfect circle.

[0057] [Optical communication cable] The multi-core plastic optical fiber of the present invention can be suitably used for an optical communication cable, that is, an optical communication cable in which a coating of polyethylene or the like is applied to the outer periphery of the optical fiber of the present invention.

[0058] [Optical communication system] The multi-core plastic optical fiber or the optical communication cable of the present invention can be suitably used in an optical communication system. The optical communication system of the present invention uses the multi-core plastic optical fiber or the optical communication cable of the present invention and performs spatial multiplexing communication using multiple signal beams, thereby enabling large-capacity communication.

[0059] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0060] [Measurement Method] (1) Refractive Index of Core, Cladding, and Sea Test pieces measuring 20 mm x 8 mm x 1.4 mm were prepared from the core, cladding, and sea used in each Example and Comparative Example, and the refractive indexes were measured using an Abbe refractometer in an atmosphere at room temperature of 25°C.

[0061] (2) Transmission Bandwidth (GHz) A network analyzer (MS46122B, manufactured by Anritsu) was connected to a 670 nm wavelength VCSEL laser and an O / E converter (SPA-2_650 nm, manufactured by Graviton). Next, two multi-core plastic optical fibers, 30 m and 1 m in length, were cut from each of the examples and comparative examples, and both end faces were mirror-polished using a polishing sheet. The light emitted from the VCSEL laser was collimated to a width of 2 mm and focused onto one core of the multi-core plastic optical fiber using an objective lens with a numerical aperture (NA) of 0.3 and a pupil diameter of 10 mm. The other end of the optical fiber was connected to an O / E converter. The frequency was swept from 0 GHz to 2 GHz, and the transmission characteristics (S21) were measured at 0.1 GHz intervals. A differential profile was created by subtracting the measurement value at 1 m from the measurement value at 30 m. In this profile, the frequency at which the transmission characteristics at 0 GHz were reduced by 3 dB was measured as the transmission band.

[0062] [Materials] A: Polymethyl methacrylate, refractive index 1.49 B: Copolymer of 75% by mass of vinylidene fluoride / 25% by mass of tetrafluoroethylene, refractive index 1.41 C: Fluorinated acrylate polymer, refractive index 1.42 D: Polystyrene, refractive index 1.59 E: Copolymer of 18% by mass of vinylidene fluoride / 62% by mass of tetrafluoroethylene / 16% by mass of hexafluoropropylene / 4% by mass of perfluoropropyl vinyl ether, refractive index 1.35.

[0063] Example 1: A resin composition A was prepared as the core, B as the cladding, and C as the sea portion. Each was poured into a spinning pack incorporating an extrusion die, melted at 250°C, and then a polymer flow was discharged from each discharge port to obtain a multi-core plastic optical fiber. The extrusion die used had distribution holes for a core discharge port, a cladding discharge port surrounding the core, and a sea discharge port surrounding the island portions. The fiber cross section contained 64 closely packed cores, with one core per independent cladding region. The obtained optical fiber had a core diameter of 40 μm, a core center-to-center spacing of 50 μm, and a cladding thickness of 3.0 μm. The results of evaluation using the above-described method are shown in Table 1.

[0064] [Example 2] An optical fiber was produced in the same manner as in Example 1, except that the resin composition constituting the sea portion was changed to B. The obtained optical fiber had a core diameter of 40 μm, a core center-to-center spacing of 50 μm, and a cladding thickness of 3.0 μm. The results of evaluation using the above-mentioned method are shown in Table 1. Since the refractive index of the sea portion was the same as that of the core, the transmission band was good.

[0065] [Example 3] An optical fiber was produced in the same manner as in Example 1, except that the resin composition constituting the sea region was changed to D. The obtained optical fiber had a core diameter of 40 μm, a core center-to-center spacing of 50 μm, and a cladding thickness of 3.0 μm. The results of evaluation using the above-mentioned method are shown in Table 1. Since the refractive index of the sea region was higher than that of the core, the transmission band was better.

[0066] [Example 4] An optical fiber was produced in the same manner as in Example 1, except that the resin composition constituting the core was changed to D and the resin composition constituting the sea region was changed to D. The obtained optical fiber had a core diameter of 40 μm, a core center-to-center spacing of 50 μm, and a cladding thickness of 3.0 μm. The results of evaluation using the above-mentioned method are shown in Table 1. Since the refractive index of the sea region was the same as that of the core, the transmission band was good.

[0067] Comparative Example 1 An optical fiber was produced in the same manner as in Example 1, except that the resin composition constituting the core was changed to E. The obtained optical fiber had a core diameter of 40 μm, a core center-to-center spacing of 50 μm, and a cladding thickness of 3.0 μm. The results of evaluation using the above-mentioned method are shown in Table 1. The refractive index of the sea portion was smaller than the refractive index of the cladding, so the transmission band was poor.

[0068] Comparative Example 2: A resin composition A was prepared as the core composition, and B was prepared as the cladding composition. Each was poured into a spinning pack incorporating an extrusion die, melted at 250°C, and then a polymer flow was discharged from each discharge port to obtain a multi-core plastic optical fiber. The extrusion die used had distribution holes for the core discharge port and the cladding discharge port surrounding the cores. The fiber cross section had 64 cores arranged in a staggered pattern. The obtained optical fiber had a core diameter of 40 μm and a core center-to-center spacing of 50 μm. The results of evaluation using the above-mentioned method are shown in Table 1. The obtained optical fiber did not include a sea region, and therefore had a poor transmission bandwidth.

[0069] Comparative Example 3: A resin composition for the core, B resin composition for the cladding, and A resin composition for the sea portion were prepared. Each was poured into a spinning pack incorporating an extrusion die, melted at 250°C, and then a polymer flow was discharged from each discharge port to obtain a multi-core plastic optical fiber. The extrusion die used had distribution holes for the core discharge port, the cladding discharge port surrounding the cores, and the sea discharge port surrounding the cladding. The cross section of the fiber contained 64 cores in a staggered arrangement, with 64 cores per independent cladding region. The resulting optical fiber had a core diameter of 40 μm and a core center-to-center spacing of 50 μm. The results of evaluation using the above-described method are shown in Table 1. The resulting optical fiber had poor transmission bandwidth because the cladding did not individually surround each core.

[0070]

Claims

1. A multi-core plastic optical fiber having a cross-sectional shape consisting of two or more island portions and a sea portion surrounding the island portions, wherein each of the island portions consists of a core and a cladding surrounding the core in one-to-one correspondence with the core, each of the claddings is independent of the other claddings, the refractive index (N2) of the cladding is lower than the refractive index (N1) of the core, and the refractive index (N3) of the sea portion is higher than the refractive index (N2) of the cladding.

2. The multi-core plastic optical fiber according to claim 1, wherein N3 is equal to or greater than N1.

3. The multi-core plastic optical fiber according to claim 1 or 2, wherein the core contains a polymethyl methacrylate resin.

4. The multi-core plastic optical fiber according to claim 1 or 2, wherein the sea portion is the outermost layer.

5. A multi-core plastic optical fiber according to claim 1 or 2, wherein the average transmittance of the sea portion in the wavelength range of 400 to 700 nm is lower than the average transmittance of the cores in the wavelength range of 400 to 700 nm.

6. The multi-core plastic optical fiber according to claim 1 or 2, wherein the sea portion contains a coloring material.

7. The multi-core plastic optical fiber according to claim 6, wherein the coloring material is carbon black.

8. The multi-core plastic optical fiber according to claim 1 or 2, wherein in said cross-sectional configuration, said island portions are arranged in a square lattice pattern.

9. An optical communication cable comprising the multi-core plastic optical fiber according to claim 1 or 2.

10. An optical communication system using the multi-core plastic optical fiber according to claim 1 or 2, or the optical communication cable according to claim 9, for performing spatial multiplexing communication using a plurality of signal lights.

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