Patent Description:
Currently, a plastic optical fiber, as a high-speed optical signal transmission medium, is used in, for example, local area network (LAN), factory automation (FA), office automation (OA), and the like. The low loss window of the plastic optical fiber containing a methyl methacrylate homopolymer (PMMA) as a core material exists in a visible light region, and the transmission loss thereof is low around <NUM>, <NUM>, and <NUM>. For such a plastic optical fiber, red light (<NUM>) is used from the viewpoint of balancing lifetime of an element, band, wavelength characteristics of a light-receiving element, price, versatility, and the like.

However, since red light (<NUM>) has a large transmission loss, it is not suitable for long distance communication. Therefore, for applications requiring long distance communication, for example, sensor applications for entry prevention fences of airports and warehouses, anti-theft sensor applications for solar panels and store displays, security camera applications, etc., green light (about <NUM> or <NUM>) having a smaller transmission loss than red light (<NUM>) is used.

Meanwhile, various methods have been proposed as methods for reducing the transmission loss of a plastic optical fiber. For example, Patent Literature <NUM> discloses a method of using a core material having a small sulfur atomic weight as the core material of a plastic optical fiber.

Prior art can be found e.g. in patent literature <CIT> disclosing a Plastic optical fiber and manufacturing method thereof, and plastic optical fiber cable, in document <CIT> disclosing a Plastic optical fiber strand and cable, in document <CIT> disclosing a Method for manufacturing plastic optical fiber cable, in document <CIT> disclosing a Plastic optical fiber cable and method for transmitting signal as well as in document <CIT> disclosing Plastic optical fiber, optical fiber cable, optical fiber cable with plug, method for producing methyl methacrylate based polymer and method for producing plastic optical fiber.

However, in the method disclosed in Patent Literature <NUM>, since the treatment of a sheath material is performed in a general environment, the amount of foreign matter in the sheath material is large, which causes the deterioration of transmission loss. In general, when the sheath material treated in a general environment is used for an optical fiber, a large amount of foreign matter is mixed in the sheath material, which causes the deterioration of the transmission loss of the optical fiber. For this reason, it is necessary to perform a treatment such that no foreign matter is mixed in the sheath material. Further, the optical fiber disclosed in Patent Literature <NUM> has a thick sheath, which causes the transmission loss of light in a higher-order mode or a cladding mode to increase. In particular, when the transmission loss of green light (about <NUM> or <NUM>) used for long distance communication increases, the communication distance is restricted, and thus the application of the optical fiber is limited.

In this regard, an object of the invention is to provide an optical fiber having a low transmission loss of green light (particularly, wavelength <NUM>), thereby enabling longer distance communication.

The invention provides a plastic optical fiber according to claim <NUM>.

The invention provides a plastic optical fiber cable including the plastic optical fiber having a coating layer on an outer periphery thereof.

The invention provides a sensor including the plastic optical fiber.

The optical fiber of the invention has a low transmission loss of green light (particularly, wavelength <NUM>), thereby enabling longer distance communication.

<FIG> is a schematic view illustrating cross-sectional structures of a step/index type optical fiber which is an example of an optical fiber of the invention.

Hereinafter, embodiments of the invention will be described with reference to the drawings, but the invention is not limited to the structures illustrated in these drawings.

An optical fiber of the invention has a core and a sheath consisting of at least one layer surrounding the outer periphery of the core. Examples of the kinds of the optical fiber include a step/index type optical fiber, a multi-step/index type optical fiber, a graded index type optical fiber, a multi-core optical fiber, and the like. Among these optical fibers, a step/index type optical fiber is preferable because it is thermally stable, is easy to manufacture and inexpensive, and enables longer distance communication.

In the step/index type optical fiber, light is totally reflected at the interface between the core and the sheath, and thus the light is propagated in the core.

<FIG> illustrates exemplary cross-sectional structures of a step/index type optical fiber <NUM>. <FIG> illustrates a case where a sheath consists of one layer and where a sheath <NUM> surrounds the outer periphery of a core <NUM>. <FIG> illustrates a case where a sheath consists of two layers and where a first sheath layer 12a (innermost layer) surrounds the outer periphery of the core <NUM> and a second sheath layer 12b (outermost layer) surrounds the outer periphery of the first sheath layer 12a. In the step/index type optical fiber of the invention, the core may be singular or plural, but it is preferable that the core is singular.

The material constituting the core (core material) is not particularly limited as long as it is a resin having high transparency, and can be selected appropriately according to the purpose of use and the like. Examples of the resin having high transparency include an acrylic resin, a styrene resin, a carbonate resin, and the like. These resins may be used alone, and may also be used in combination with two or more kinds thereof. Among these resins, an acrylic resin is preferable from the viewpoint of reducing the transmission loss of an optical fiber.

Examples of the acrylic resin include a methyl methacrylate homopolymer (PMMA), a copolymer containing <NUM>% by mass or more of a methyl methacrylate unit (a methyl methacrylate copolymer), and the like. These acrylic resins may be used alone, and may also be used in combination with two or more kinds thereof. Among these acrylic resins, a methyl methacrylate homopolymer and a copolymer containing <NUM>% by mass or more of a methyl methacrylate unit are preferable because they are excellent in optical properties, mechanical properties, heat resistance, and transparency. As the methyl methacrylate copolymer, a copolymer containing <NUM>% by mass or more of a methyl methacrylate unit is preferable, and a copolymer containing <NUM>% by mass or more of a methyl methacrylate unit is more preferable. It is particularly preferable that the core material is a methyl methacrylate homopolymer.

In the present specification, (meth)acrylate refers to acrylate, methacrylate, or both thereof.

The core material can be manufactured by using a known polymerization method. Examples of the polymerization method for manufacturing the core material include a bulk polymerization method, a suspension polymerization method, an emulsion polymerization method, a solution polymerization method, and the like. Among these polymerization methods, a bulk polymerization method or a solution polymerization method is preferable from the viewpoint of suppressing the incorporation of foreign matter.

The sheath is formed of at least one layer on the outer periphery of the core. The sheath may be formed of one layer as shown in <FIG>, or may be formed of two or more layers as shown in <FIG>. It is preferable that the sheath is formed of one layer to three layers from the viewpoint of reducing the transmission loss of an optical fiber. It is more preferable that the sheath is formed of two layers from the viewpoint of imparting heat resistance, impact resistance, and chemical resistance.

The material constituting the sheath (sheath material) is not particularly limited as long as it is a material having a lower refractive index than the core material, and can be selected appropriately according to the composition of the core material, the purpose of use, and the like.

When an acrylic resin is used as the core material, from the viewpoint of reducing the transmission loss of an optical fiber, it is preferable to use a fluorine-based resin as the sheath material. In particular, when a methyl methacrylate homopolymer or a copolymer containing <NUM>% by mass or more of a methyl methacrylate unit is used as the core material, from the viewpoint of reducing the transmission loss of an optical fiber, it is preferable to use a fluorine-based resin as the sheath material.

Examples of the fluorine-based resin include a vinylidene fluoride (VDF) homopolymer, a VDF/tetrafluoroethylene (TFE) copolymer, a VDF/TFE/hexafluoropropylene (HFP) copolymer, a VDF/HFP copolymer, a VDF/TFE/HFP/(perfluoro) alkyl vinyl ether copolymer, a VDF/hexafluoroacetone copolymer, a VDF/TFE/hexafluoroacetone copolymer, an ethylene/VDF/TFE/HFP copolymer, an ethylene/TFE/HFP copolymer, a VDFltrifluoroethylene copolymer, a fluoroalkyl (meth)acrylate polymer, a fluoroalkyl (meth)acrylate/alkyl (meth)acrylate copolymer, and the like. These fluorine-based resins may be used alone, and may also be used in combination with two or more kinds thereof. Among these fluorine-based resins, a VDF/TFE copolymer, a VDF/TFE/HFP copolymer, a VDF/HFP copolymer, an ethylene/VDF/TFE/HFP copolymer, an ethylene/TFE/HFP copolymer, a fluoroalkyl (meth)acrylate polymer, and a fluoroalkyl (meth)acrylate/alkyl (meth)acrylate copolymer are preferable because they are excellent in flexibility, impact resistance, transparency, and chemical resistance, and are low-priced.

When the sheath consists of one layer, it is preferable that the sheath material is a VDF/TFE copolymer, a VDF/TFE/HFP copolymer, a VDF/HFP copolymer, an ethylene/VDF/TFE/HFP copolymer, an ethylene/TFE/HFP copolymer, a fluoroalkyl (meth)acrylate polymer, or a fluoroalkyl (meth)acrylate/alkyl (meth)acrylate copolymer. It is more preferable that the sheath material is a VDF/TFE copolymer, a VDF/TFE/HFP copolymer, a VDF/HFP copolymer, an ethylene/VDF/TFE/HFP copolymer, or an ethylene/TFE/HFP copolymer because it is excellent in solvent resistance.

When the sheath consists of two layers, it is preferable that the material of the first sheath layer (innermost layer, 12a in <FIG>) is a fluoroalkyl (meth)acrylate polymer or a fluoroalkyl (meth)acrylate/alkyl (meth)acrylate copolymer. Further, when the sheath consists of two or more layers, the material of the second and subsequent sheath layers (outer layer, for example, 12b in <FIG>) is preferably a VDF/TFE copolymer, a VDF/TFE/HFP copolymer, a VDF/HFP copolymer, or an ethylene/TFE/HFP copolymer, and more preferably a VDF/TFE copolymer, a VDF/TFE/HFP copolymer, or a VDF/HFP copolymer.

Examples of fluoroalkyl (meth)acrylate include long-chain fluoroalkyl (meth)acrylate represented by Formula (<NUM>) below, such as <NUM>-(perfluorohexyl) ethyl methacrylate (13FM) or <NUM>-(perfluorooctyl) ethyl methacrylate (17FM); and short-chain fluoroalkyl (meth)acrylate represented by Formula (<NUM>) below, such as <NUM>,<NUM>,<NUM>-trifluoroethyl methacrylate (3FM).

In Formula (<NUM>) above, R is a hydrogen atom or a methyl group, X is a hydrogen atom or a fluorine atom, m is <NUM> or <NUM>, and n is an integer of <NUM> to <NUM>.

In Formula (<NUM>) above, R is a hydrogen atom or a methyl group, X is a hydrogen atom or a fluorine atom, m is <NUM> or <NUM>, and n is an integer of <NUM> to <NUM>.

From the viewpoint of reducing the transmission loss of an optical fiber, the fluoroalkyl (meth)acrylate polymer or the fluoroalkyl (meth)acrylate/alkyl (meth)acrylate copolymer is preferably a copolymer including <NUM> to <NUM>% by mass of unit of the long-chain fluoroalkyl (meth)acrylate represented by Formula (<NUM>) above, <NUM> to <NUM>% by mass of unit of the short-chain fluoroalkyl (meth)acrylate represented by Formula (<NUM>) above, and <NUM> to <NUM>% by mass of other copolymerizable monomer unit, and is more preferably a copolymer including <NUM> to <NUM>% by mass of unit of the long-chain fluoroalkyl (meth)acrylate represented by Formula (<NUM>) above, <NUM> to <NUM>% by mass of unit of the short-chain fluoroalkyl (meth)acrylate represented by Formula (<NUM>) above, and <NUM> to <NUM>% by mass of other copolymerizable monomer unit. Specifically, a 17FM/3FM/methyl methacrylate (MMA)/methacrylic acid (MAA) copolymer or a 13FM/3FM/methyl methacrylate (MMA)/methacrylic acid (MAA) copolymer, having the above composition ratio, is preferable.

The refractive indexes of the core material and the sheath material is not particularly limited as long as the refractive index of the sheath material is lower than the refractive index of the core material. From the viewpoint of increasing the numerical aperture with respect to the maximum angle at which light can propagate, it is preferable that the refractive index of the core material is <NUM> to <NUM> and the refractive index of the sheath material is <NUM> to <NUM>, it is more preferable that the refractive index of the core material is <NUM> to <NUM> and the refractive index of the sheath material is <NUM> to <NUM>, and it is further preferable that the refractive index of the core material is <NUM> to <NUM> and the refractive index of the sheath material is <NUM> to <NUM>.

In the present specification, the refractive index refers to a value measured at <NUM> using a sodium D ray.

An optical fiber can be formed using a known molding method, and, for example, can be formed using a melt spinning method. The formation of an optical fiber by a melt spinning method can be carried out, for example, by melting a core material and a sheath material and performing composite spinning, respectively.

In order to suppress the amount of foreign matter in the sheath, it is preferable that a polymerization process for obtaining the sheath material, a melt-kneading process after polymerization, a pellet cutting process after melt-kneading, a drying process of the sheath material, and a supplying process of the sheath material for melt spinning are performed under an environment based on a clean room. In particular, from the viewpoint of reducing the transmission loss of an optical fiber, it is preferable to perform each step of the innermost sheath layer surrounding the outer periphery of the core under an environment based on a clean room.

The degree of cleanliness in the polymerization process for obtaining the sheath material is preferably class <NUM> or less, more preferably class <NUM> or less, and further preferably class <NUM> or less.

In the present specification, the degree of cleanliness is based on the United State Federal standards "Fed.

The degree of cleanliness in the melt-kneading process after polymerization is preferably class <NUM> or less, more preferably class <NUM> or less, and further preferably class <NUM> or less.

The degree of cleanliness in the pellet cutting process after melt-kneading is preferably class <NUM> or less, more preferably class <NUM> or less, and further preferably class <NUM> or less.

The degree of cleanliness in the drying process of the sheath material is preferably class <NUM> or less, more preferably class <NUM> or less, and further preferably class <NUM> or less.

In particular, the degree of cleanliness in the environment of drying the material constituting the innermost sheath layer is preferably class <NUM> or less, more preferably class <NUM> or less, and further preferably class <NUM> or less.

The degree of cleanliness in the supplying process of the sheath material is preferably class <NUM> or less, more preferably class <NUM> or less, and further preferably class <NUM> or less.

The amount of foreign matter having a size of <NUM> or more contained in the innermost sheath layer is preferably <NUM>/cm<NUM> or less, more preferably <NUM>/cm<NUM> or less, and further preferably <NUM>/cm<NUM> or less from the viewpoint of reducing the transmission loss of an optical fiber.

Further, the amount of foreign matter having a size of greater than <NUM> contained in the innermost sheath layer, is preferably <NUM>/cm<NUM> from the viewpoint of reducing the transmission loss of an optical fiber.

In the present specification, the size and amount of foreign matter refer to values measured using a particle counter.

From the viewpoint of reducing the transmission loss of an optical fiber, the size X (µm) of foreign matter contained in the innermost sheath layer and the amount Y of the foreign matter (number/cm<NUM>) satisfy Math Formula (<NUM>) below, more preferably satisfy Math Formula (<NUM>) below, and further preferably satisfy Math Formula (<NUM>) below. <MAT> <MAT> <MAT>.

In Math Formulae (<NUM>) to (<NUM>) above, the size X (µm) of foreign matter having a size (A size) of <NUM> to <NUM> is converted into <NUM>, the size X (µm) of foreign matter having a size (B size) of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>, the size X (µm) of foreign matter having a size (C size) of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>, the size X (µm) of foreign matter having a size (D size) of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>, the size X (µm) of foreign matter having a size (E size) of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>, and the size X (µm) of foreign matter having a size (F size) of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>. e indicates a base of natural logarithm (Napier number).

The diameter of the optical fiber is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, further preferably <NUM> to <NUM>, and particularly preferably <NUM> to <NUM>, because the transmission loss of the optical fiber can be reduced and the treatment of the optical fiber is easy.

The thickness of the innermost sheath layer surrounding the outer periphery of the core may be <NUM> to <NUM>, preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>, from the viewpoint of reducing the transmission loss of the optical fiber, particularly, the transmission loss of light having a wavelength of <NUM>, and from the viewpoint of the coupling efficiency with an optical element and the tolerance for the misalignment of an optical axis.

The thickness of the sheath can be appropriately set depending on spinning conditions.

When the amount of foreign matter having a size of <NUM> or greater contained in the innermost sheath layer is <NUM>/cm<NUM> or less, the thickness of the innermost sheath layer surrounding the outer periphery of the core is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, further preferably <NUM> to <NUM>, and particularly preferably <NUM> to <NUM>.

The percentage of the diameter of the core to the diameter of the optical fiber is preferably <NUM>% to <NUM>%, more preferably <NUM>% to <NUM>%, further preferably <NUM>% to <NUM>%, and particularly preferably <NUM>% to <NUM>%, from the viewpoint of reducing the transmission loss of the optical fiber and from the viewpoint of the coupling efficiency with an optical element and the tolerance for the misalignment of an optical axis.

The percentage of the thickness of the innermost sheath layer surrounding the outer periphery of the core to the diameter of the optical fiber is <NUM>% to <NUM>%, more preferably <NUM>% to <NUM>%, further preferably <NUM>% to <NUM>%, and particularly preferably <NUM>% to <NUM>%, from the viewpoint of reducing the transmission loss of the optical fiber, particularly, the transmission loss of light having a wavelength of <NUM>, and from the viewpoint of the coupling efficiency with an optical element and the tolerance for the misalignment of an optical axis.

When the sheath consists of two layers, the thickness of the first sheath layer (innermost layer, 12a in <FIG>) and the thickness of the second sheath layer (outermost layer, 12b in <FIG>) can be appropriately set.

When the sheath consists of two layers, the ratio of the thickness of the second sheath layer to the thickness of the first sheath layer is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, and further preferably <NUM> to <NUM>, from the viewpoint of reducing the transmission loss of the optical fiber.

It is preferable that the optical fiber is heated and stretched from the viewpoint of improving mechanical properties. Conditions of the heating and stretching treatment may be appropriately set depending on the material of the optical fiber, and may be continuous or batched.

When the optical fiber is used in an environment with a large temperature difference, it is preferable to anneal the optical fiber in order to suppress pistoning. Conditions of the annealing treatment may be appropriately set depending on the material of the optical fiber, and may be continuous or batched.

In order to reduce the transmission loss of the optical fiber, the optical fiber may be subjected to wet heat treatment or hot water treatment. Conditions of the wet heat treatment or hot water treatment may be appropriately set depending on the material of the optical fiber, and may be continuous or batched.

After the optical fiber is subjected to wet heat treatment or hot water treatment, the optical fiber may be subjected to drying treatment. Conditions of the drying treatment may be appropriately set depending on the material of the optical fiber, and may be continuous or batched.

In the optical fiber of the invention, the transmission loss thereof, measured by a <NUM>-<NUM> cutback method under conditions of a wavelength of <NUM> and an excitation of NA=<NUM>, is <NUM> dB/km or less, and preferably <NUM> dB/km or less.

In the present specification, the measurement of the transmission loss by the <NUM>-<NUM> cutback method is performed based on IEC <NUM>-<NUM>-<NUM>:<NUM>. Specifically, after an optical fiber of <NUM> is set in a measuring apparatus and output power P<NUM> is measured, the optical fiber is cut to a cutback length (<NUM> from incident end), output power P<NUM> is measured, and the transmission loss of light is calculated using Math Formula (<NUM>) below. <NUM>] <MAT>.

In order to make the transmission loss measured by the <NUM>-<NUM> cutback method under the conditions of a wavelength of <NUM> and an excitation of NA=<NUM> to be <NUM> dB/km or less, it is necessary to satisfy the Condition <NUM> below. Since the transmission loss of light having a wavelength of <NUM> can be further reduced, preferably one condition of Condition <NUM> and Condition <NUM> is additionally satisfied, and further preferably all of Condition <NUM> to Condition <NUM> are satisfied.

Condition <NUM>: The thickness of the innermost sheath layer is set to <NUM> to <NUM>.

Condition <NUM>: The amount of foreign matter having a size of <NUM> or more contained in the innermost sheath layer is set to <NUM>/cm<NUM> or less.

Condition <NUM>: The size X (µm) of foreign matter contained in the innermost sheath layer and the amount Y of the foreign matter (number/cm<NUM>) satisfy Math Formula (<NUM>) below.

The optical fiber of the invention, as necessary, may be used as an optical fiber cable by providing a coating layer on the outer periphery thereof.

Examples of the material constituting the coating layer include olefin resins such as polyethylene resin and polypropylene resin; chlorine resins such as vinyl chloride resin and chlorinated polyethylene resin; fluorine resins; urethane resins; styrene resins; and polyamide resins. These materials constituting the coating layer may be used alone, and may also be used in combination with two or more kinds thereof.

The coating layer may be one layer or two or more layers.

In the optical fiber of the invention, the transmission loss of green light (particularly, wavelength <NUM>) which is often used for long distance communication is low, thereby enabling longer distance communication. Therefore, the optical fiber and optical fiber cable of the invention are suitable for applications requiring communication over long distance (<NUM> or more), such as sensor applications for entry prevention fences of airports and warehouses, anti-theft sensor applications for solar panels and store displays, and security camera applications.

The peak wavelength of the light source used for these applications is preferably <NUM> to <NUM>.

Hereinafter, the invention will be described in detail with reference to Examples, but the invention is not limited to these Examples.

The transmission loss of each of optical fibers obtained in Examples and Comparative Examples was measured using light having a wavelength of <NUM> by a <NUM>-<NUM> cutback method under excitation NA=<NUM>.

The measurement of the transmission loss by the <NUM>-<NUM> cutback method was performed based on IEC <NUM>-<NUM>-<NUM>:<NUM>. Specifically, after an optical fiber of <NUM> was set in a measuring apparatus and output power P<NUM> was measured, the optical fiber was cut to a cutback length (<NUM> from incident end), output power P<NUM> was measured, and the transmission loss of light was calculated using Math Formula (<NUM>) above.

The section of each of the optical fibers obtained in Examples and Comparative Examples was measured using a microscope (model name "VHX-<NUM>", manufactured by Keyence Corporation), the thicknesses of the sheath were measured at three points with respect to each <NUM>° from the center of the optical fiber, and the average value of three points was set to the thickness of the sheath.

<NUM> of resin pellets used as the sheath material was dissolved in <NUM> of acetone, and the amount of foreign matter of each size specified below was measured using a particle counter (particle counter "KL-11A", light blocking type particle sensor "KS-<NUM>", manufactured by RION Co.

In Examples and Comparative Examples, the following resins were used as the core material or the sheath material.

In a clean room of class <NUM>, a mixed solution, obtained by mixing <NUM> parts by mass of <NUM>-(perfluorooctyl) ethyl methacrylate (17FM), <NUM> parts by mass of <NUM>,<NUM>,<NUM>-trifluoroethyl methacrylate (3FM), <NUM> parts by mass of methyl methacrylate (MMA), <NUM> part by mass of methacrylic acid (MAA), <NUM> part by mass of <NUM>,<NUM>'-azobis (isobutyronitrile), <NUM> part by mass of <NUM>,<NUM>'-azobis (<NUM>,<NUM>-dimethylvaleronitrile), and <NUM> parts by mass of n-octyl mercaptan, was bubbled with nitrogen gas for <NUM> hour. Subsequently, the obtained mixed solution was supplied to a sealed polymerization container, and this polymerization container was kept in a bath at <NUM> for <NUM> hours. Thereafter, the polymerization container was placed in a steam dryer at <NUM> for <NUM> hours, so as to obtain a polymer.

In a clean booth of class <NUM>, the obtained polymer was melted and kneaded using a one-vent type single screw extruder while removing residual monomers, and, in a clean booth of class <NUM>, the molten and kneaded polymer was cut into pellets. The obtained pellets (resin B) were collected in a stainless steel container.

Resin C was obtained in the same manner as in Preparation Example <NUM>, except that the mixed amount of monomers was changed into <NUM> parts by mass of <NUM>-(perfluorohexyl) ethyl methacrylate (13FM), <NUM> parts by mass of <NUM>,<NUM>,<NUM>-trifluoroethyl methacrylate (3FM), <NUM> parts by mass of methyl methacrylate (MMA), and <NUM> parts by mass of methacrylic acid (MAA).

In a clean booth of class <NUM>, the resin B in the stainless steel container was dried at <NUM> for <NUM> hours, and the stainless steel container was connected with an extruder having a spinning head at the front end thereof. Then, the molten resin A, resin B, and resin D were supplied to the spinning head at <NUM>, respectively. Resin A as the core material, resin B as the first sheath layer (innermost layer) material, and resin D as the second sheath layer (outer layer) material were spun using a concentric composite spinning nozzle having a three layer structure, and were stretched twice in a fiber axis direction in a hot air heating furnace at <NUM>, so as to obtain an optical fiber having a diameter of <NUM>, where the diameter of a core is <NUM>, the thickness of a first sheath layer is <NUM>, and the thickness of a second sheath layer is <NUM>.

Evaluation results of the obtained optical fiber are shown in Table <NUM>.

Optical fibers were obtained in the same manner as in Example <NUM>, except that manufacturing conditions were changed as shown in Table <NUM>. In Example <NUM>, an optical fiber having a sheath of one layer was manufactured.

An optical fiber having a diameter of <NUM>, where the diameter of a core is <NUM>, the thickness of a first sheath layer is <NUM>, and the thickness of a second sheath layer is <NUM>, was obtained in the same manner as in Example <NUM>, except that the thickness of the first sheath layer was set to <NUM> using resin C as the first sheath layer (innermost layer) material.

Optical fibers were obtained in the same manner as in Example <NUM>, except that manufacturing conditions were changed as shown in Table <NUM>.

An optical fiber was obtained in the same manner as in Example <NUM>, except that manufacturing conditions were changed as shown in Table <NUM>, and spinning was carried out using a core-sea composite spinning nozzle in which <NUM> nozzle holes were arranged in a hexagonal array. The obtained optical fiber is a multi-core optical fiber including <NUM> cores and one sheath (sea portion, first sheath layer) surrounding the cores.

An optical fiber was obtained in the same manner as in Comparative Example <NUM>, except that manufacturing conditions were changed as shown in Table <NUM>, and spinning was carried out using a core-sea composite spinning nozzle in which <NUM> nozzle holes were arranged in a hexagonal array.

As shown in Table <NUM>, the optical fibers obtained in Examples <NUM> to <NUM>, <NUM> to <NUM>, <NUM>, and <NUM> had lower transmission loss of light having a wavelength of <NUM> than the optical fibers obtained in Comparative Examples <NUM> to <NUM>, <NUM>, and <NUM>.

Claim 1:
A plastic optical fiber comprising:
a core (<NUM>); and
a sheath (<NUM>) composed of a material having a lower refractive index than a material constituting the core, wherein the sheath consists of at least one layer,
wherein the plastic optical fiber has a transmission loss of <NUM> dB/km or less as measured by a <NUM>-<NUM> cutback method under conditions of a wavelength of <NUM> and an excitation of NA=<NUM>,
wherein
a percentage of the thickness of the innermost sheath layer surrounding an outer periphery of the core to the diameter of the plastic optical fiber is <NUM> % to <NUM> %;
a size X in µm of foreign matter contained in an innermost sheath layer and an amount Y of the foreign matter in number/cm<NUM> satisfy: <MAT> AND <MAT> wherein
for the Math Formula (<NUM>), the size X of foreign matter having a size of <NUM> to <NUM> is converted into <NUM>, the size X of foreign matter having a size of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>, the size X of foreign matter having a size of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>, the size X of foreign matter having a size of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>, the size X of foreign matter having a size of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>, and the size X of foreign matter having a size of greater than <NUM> and equal to or smaller than <NUM> is converted into <NUM>.