JP2024051832A - Plastic optical fiber and plastic optical fiber cable - Google Patents

Abstract

[Problem] To provide a plastic optical fiber and a plastic optical fiber cable that are excellent in bending resistance and long-term heat resistance and can maintain their original optical properties for a long period of time. [Solution] A plastic optical fiber having a core made of a transparent resin and cladding layers formed concentrically on the outer peripheral surface of the core in the order of a first cladding and a second cladding, wherein the material constituting the first cladding has a refractive index of 1.300 to 1.375, and the material constituting the second cladding has a hardness of 50 or more as measured by a durometer hardness test based on JIS K7215 and a melting point of 130°C or more. A plastic optical fiber cable having at least one coating layer provided on the outer periphery of this plastic optical fiber. [Selected Figure] Figure 1

Description

The present invention relates to a plastic optical fiber and a plastic optical fiber cable.

Plastic optical fibers having a core made of a highly transparent resin such as methyl methacrylate are used for optical information communication inside trains, airplanes, automobiles, and other vehicles, as well as in the field of factory automation (FA). In the above-mentioned optical information communication field, plastic optical fibers are usually used in the form of plastic optical fiber cables (hereinafter referred to as "optical fiber cables"), the outer circumference of which is coated with resin.

When optical fiber cables are used for wiring inside vehicles such as automobiles or communication wiring in the factory automation field, they are used in environments close to high-temperature bodies such as engines, or in high-temperature environments in the summer. Therefore, plastic optical fibers and optical fiber cables with excellent long-term heat resistance are desired so that transmission loss does not increase even when exposed to heat for long periods of time.

Furthermore, in the above applications, optical fiber cables are laid in a twisted state in narrow spaces, and are used as movable wiring where they are repeatedly twisted, so plastic optical fibers and optical fiber cables with excellent twisting resistance are desired.

Furthermore, in the above applications, even if the optical fiber cable is used in a vehicle such as an automobile under mechanical stress such as vibration, or in the field of factory automation (FA), where it is repeatedly bent and straightened, it is desired that the cladding of the plastic optical fiber does not crack, or that delamination between the cladding layers in a cladding having two or more layers does not occur, resulting in an increase in transmission loss. In other words, there is a demand for plastic optical fiber and optical fiber cables with excellent mechanical durability.

In Patent Document 1, the objective of the paper is to provide a plastic optical fiber with excellent long-term heat resistance, twisting resistance, and mechanical durability. It proposes a plastic optical fiber in which the cladding layer has a two-layer laminate structure of a first cladding and a second cladding formed concentrically on the outer peripheral surface of the core, the first cladding being made of a fluorinated methacrylate resin with a refractive index of 1.400 to 1.480, and the second cladding being made of a modified fluororesin with a refractive index of 1.340 to 1.395.

Patent Document 2 proposes a plastic optical fiber that uses a specific core material and cladding material, aiming to provide a plastic optical fiber with excellent optical properties, resistance to bending light loss, and resistance to moist heat. In Patent Document 2, the cladding layer is also composed of a first cladding and a second cladding, and in the examples, the refractive index of the first cladding is set to 1.412 to 1.419.

JP 2021-156984 A JP 2004-151702 A

In the plastic optical fibers of Patent Documents 1 and 2, the refractive index of the first cladding is relatively high, 1.400 to 1.480 or 1.412 to 1.419, so that the bending resistance of the obtained plastic optical fiber is insufficient.
Furthermore, although Patent Document 1 states that the melting point of the resin of the second cladding is in the range of 120 to 200°C, no consideration is given to its hardness, and in Patent Document 2, no consideration is given to the melting point or hardness of the second cladding. As a result, the heat resistance and surface hardness of the cladding layer are insufficient, and as a result, the cladding layer may deteriorate or be damaged during the manufacturing process, making it impossible to maintain the original optical properties.
That is, if the melting point of the second cladding is low, the fibers will fuse together during annealing, making it difficult to maintain the original optical properties. Furthermore, the long-term heat resistance will be insufficient. Also, if the hardness of the second cladding is low, scratches will occur on the surface during spinning (manufacturing) and processing, making it difficult to maintain the original optical properties.

The present invention was made in consideration of the above-mentioned conventional technology, and aims to provide a plastic optical fiber and a plastic optical fiber cable that have excellent bending resistance and long-term heat resistance and can maintain their original optical characteristics for a long period of time.

The above problems are solved by the present invention described below.
The first gist of the present invention is
A plastic optical fiber having a core made of a transparent resin and a cladding layer formed concentrically on an outer peripheral surface of the core, the cladding layer being a first cladding layer and a second cladding layer in this order,
The refractive index of the material constituting the first cladding is 1.300 to 1.375;
a plastic optical fiber, the second cladding material having a hardness of 50 or more as measured by a durometer hardness test based on JIS K7215 and a melting point of 130° C. or more;
It is in.
A second gist of the present invention is a plastic optical fiber cable comprising the plastic optical fiber and at least one coating layer provided around the outer periphery of the plastic optical fiber.

According to the present invention, it is possible to provide a plastic optical fiber and a plastic optical fiber cable that are excellent in bending resistance and long-term heat resistance and can maintain their original optical characteristics for a long period of time.
The plastic optical fiber of the present invention is suitable for various optical information and communication applications, medical applications, and lighting applications due to its excellent bending resistance, long-term heat resistance, and ability to maintain optical properties.

1 is a schematic cross-sectional view showing an example of a plastic optical fiber of the present invention.

The present invention will be described in detail below. However, the following description is an example of an embodiment of the present invention, and the present invention is not limited to the contents of the following description as long as it does not exceed the gist of the present invention.
In the present invention, unless otherwise specified, a numerical range expressed using "to" in this specification means a range including the numerical values before and after "to" as the lower and upper limits, and "A to B" means A or more and B or less.

[Plastic Optical Fiber]
The plastic optical fiber of the present invention (hereinafter, appropriately abbreviated as "optical fiber") has a core made of a transparent resin and a cladding layer formed concentrically on the outer peripheral surface of the core in the order of a first cladding and a second cladding. Specifically, as shown in Fig. 1, an optical fiber 10 having a cladding layer 12 formed concentrically on the outer peripheral surface of a core 11 in the order of a first cladding 12a and a second cladding 12b can be given.

<Optical fiber diameter and thickness of each layer>
The diameter of the plastic optical fiber of the present invention is preferably 0.1 mm to 5 mm, more preferably 0.2 mm to 3 mm, even more preferably 0.3 to 2 mm, and particularly preferably 0.9 to 1.1 mm, from the viewpoints of excellent handleability of the optical fiber, coupling efficiency with optical elements, and tolerance to optical axis misalignment.

From the viewpoint of the coupling efficiency with the optical element and the tolerance for optical axis misalignment, the core diameter of the optical fiber of the present invention is preferably 85% or more of the diameter of the optical fiber, and more preferably 90% or more. On the other hand, from the viewpoint of the tolerance for uneven thickness of the cladding layer, the core diameter is preferably 99.9% or less of the diameter of the optical fiber.

The thickness of the first cladding is preferably 10% or less, more preferably 7% or less, and even more preferably 5% or less of the diameter of the optical fiber, from the viewpoints of total reflection of the light passing through the core, sufficient occupancy of the cross-sectional area of the core of the optical fiber, coupling efficiency with the optical element, and tolerance for optical axis misalignment. On the other hand, the thickness of the first cladding is preferably equal to or greater than the wavelength of the propagating light, from the viewpoint of the propagation of light within the optical fiber. For example, when propagating visible light, the thickness of the first cladding is preferably equal to or greater than the wavelength of the propagating light, from the viewpoint of the propagation of light within the optical fiber.
nm, the thickness of the first cladding is preferably 0.4 to 50 μm, more preferably 1.0 to 25 μm, and even more preferably 2.0 to 15 μm.

The thickness of the second cladding is preferably such that the ratio of the thickness of the first cladding to the second cladding is 1:0.1 to 1:5, more preferably 1:0.1 to 1:4, even more preferably 1:0.1 to 1:3, and particularly preferably 1:0.2 to 1:3, in order to fully obtain the effect of forming a second cladding with a high melting point and high hardness and to fully ensure the cross-sectional area occupancy rate of the first cladding and core of the optical fiber. For example, when propagating visible light, the thickness of the second cladding is preferably 2 to 30 μm, more preferably 3 to 20 μm, and even more preferably 4 to 15 μm.

<Core>
The transparent resin forming the core is not particularly limited as long as it is a resin with high transparency, and examples thereof include acrylic resin, styrene resin, carbonate resin, etc. These transparent resins may be used alone or in combination of two or more. Among the above-mentioned materials, acrylic resin and polycarbonate resin are preferred because they have excellent transparency at a wavelength of about 650 nm, and acrylic resin is more preferred because they have excellent long-term heat resistance and are suitable for longer distance communication.

Examples of the acrylic resin include homopolymers of methyl methacrylate (PMMA) and copolymers of methyl methacrylate and one or more vinyl monomers. Specific examples of the copolymers include copolymers containing 50% or more by mass of methyl methacrylate units. These acrylic resins may be used alone or in combination of two or more. Among these acrylic resins, methyl methacrylate homopolymers and copolymers containing 50% or more by mass of methyl methacrylate units (methyl methacrylate copolymers) are preferred because of their excellent optical properties, mechanical properties, heat resistance, and transparency. As the methyl methacrylate copolymers, copolymers containing 60% or more by mass of methyl methacrylate units are preferred, and copolymers containing 70% or more by mass of methyl methacrylate units are more preferred. The homopolymers of methyl methacrylate are particularly preferred as the core material.
In this specification, the term "(meth)acrylate" refers to an acrylate, a methacrylate, or both.

The refractive index of the core material such as an acrylic resin is preferably from 1.485 to 1.50, and more preferably from 1.490 to 1.495.
In this specification, the refractive index is a value measured according to the method described below.

The core material can be manufactured by a known polymerization method. Examples of polymerization methods for manufacturing the core material include bulk polymerization, suspension polymerization, emulsion polymerization, and solution polymerization. Among these polymerization methods, bulk polymerization and solution polymerization are preferred because they can prevent the inclusion of foreign matter.

<Cladding layer>
The cladding layer of the plastic optical fiber of the present invention is a two-layer cladding layer in which a first cladding and a second cladding are concentrically formed in this order on the outer circumferential surface of the core.
By making the cladding layer a two-layer structure, it is possible to suitably select the material constituting the first cladding and adjust the optical properties of the plastic optical fiber, such as heat resistance, light receiving amount, and transmission bandwidth, while by suitably selecting the material constituting the second cladding, it is possible to achieve excellent bending resistance, long-term heat resistance, and ability to maintain optical properties.

(First clad)
In the present invention, the refractive index of the material constituting the first cladding is characterized by being 1.300 to 1.375.
If the refractive index of the material constituting the first clad is 1.375 or less, the refractive index of the material is low, so that the loss when bending or the twisting loss is small, and the bending resistance is excellent. On the other hand, if the refractive index is 1.300 or more, the mode dispersion is reduced, and the communication performance is excellent, especially during wideband transmission. In addition, if the refractive index is within the above range, the light propagates within the core by repeating total reflection between the core with a high refractive index and the first clad, and the optical characteristics are guaranteed even if the second clad (outermost layer) is scratched. From this viewpoint, the refractive index of the material constituting the first clad is 1.300 to 1.375, preferably 1.310 to 1.373, and more preferably 1.320 to 1.370.

As the material constituting the first clad, from the viewpoint of excellent flexibility, impact resistance, transparency, and chemical resistance, a fluorine-based polymer such as a vinylidene fluoride-based polymer, a perfluoroalkyl methacrylate-based polymer, a methacrylic acid ester-based polymer, a copolymer of a perfluoroalkyl methacrylate-based compound and a (meth)acrylate-based compound, and the like are used.
Examples of vinylidene fluoride polymers include polyvinylidene fluoride, copolymers containing vinylidene fluoride units, such as vinylidene fluoride-tetrafluoroethylene copolymers, vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-hexafluoroacetone copolymers, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymers, vinylidene fluoride-trifluoroethylene copolymers, and other ternary or higher copolymers containing vinylidene fluoride units. These may be used alone or in combination of two or more.
The refractive index of these fluorine-based polymers and copolymers can be adjusted to the above range by controlling the type and composition of the monomer units that constitute them.

(Second Clad)
In the present invention, the material constituting the second clad is characterized by having a hardness of 50 or more according to a durometer hardness test based on JIS K7215 and a melting point of 130° C. or more.

If the material constituting the second clad has a hardness of 50 or more as measured by a durometer hardness test based on JIS K7215, it is possible to suppress the occurrence of scratches on the surface during spinning (manufacturing) and processing, and to maintain the original optical properties. From this viewpoint, the material constituting the second clad has a hardness of preferably 60 or more as measured by a durometer hardness test based on JIS K7215, more preferably 62 or more, and even more preferably 64 or more. Furthermore, if the material constituting the second clad has a hardness of 90 or less as measured by a durometer hardness test based on JIS K7215, it has high processability, and therefore the hardness is usually 90 or less, preferably 85 or less, more preferably 80 or less, and particularly preferably 75 or less.

In addition, if the melting point of the material constituting the second cladding is 130° C. or higher, it is possible to suppress fusion between the fibers during annealing, and it is possible to maintain the original optical properties and have excellent long-term heat resistance. From this viewpoint, the melting point of the material constituting the second cladding is preferably 131° C. or higher, and more preferably 133° C. or higher.
On the other hand, from the viewpoint of high workability, the melting point of the material constituting the second clad is usually 180° C. or lower.
The melting point of the material constituting the second cladding is measured by the method described in the Examples section below.

The refractive index of the material that constitutes the second cladding is preferably greater than that of the material that constitutes the first cladding, so that defects such as scratches in the second cladding do not affect the optical properties of the optical fiber. Therefore, it is preferable that the refractive index of the second cladding is greater than 1.375.

From the viewpoint of optical characteristics, materials constituting the second cladding include fluorine-based polymers such as vinylidene fluoride-based polymers, perfluoroalkyl methacrylate-based polymers, methacrylic acid ester-based polymers, and copolymers of perfluoroalkyl methacrylate-based compounds and (meth)acrylate-based compounds.
Examples of vinylidene fluoride polymers include polyvinylidene fluoride, copolymers containing vinylidene fluoride units, such as vinylidene fluoride-tetrafluoroethylene copolymers, vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-hexafluoroacetone copolymers, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymers, vinylidene fluoride-trifluoroethylene copolymers, and other ternary or higher copolymers containing vinylidene fluoride units. These may be used alone or in combination of two or more.
These fluoropolymers and copolymers can be adjusted in hardness as measured by a durometer hardness test based on JIS K7215, melting point, and refractive index by controlling the type and composition of the monomer units that constitute them.

<Manufacturing method of plastic optical fiber>
The plastic optical fiber can be manufactured by a known manufacturing method, for example, a melt spinning method.
The plastic optical fiber can be manufactured by melt spinning, for example, by melting a core material and a cladding material separately and composite spinning the melted materials.
When an optical fiber cable is used in an environment with a large temperature difference, it is preferable to anneal the plastic optical fiber in order to suppress pistoning. The processing conditions for the annealing process may be set appropriately depending on the material of the plastic optical fiber. The annealing process may be performed continuously or in batches.

[Plastic optical fiber cable]
The plastic optical fiber of the present invention can be formed into a plastic optical fiber cable having at least one coating layer around the outer periphery of the optical fiber, if necessary, to mechanically protect the optical fiber when the optical fiber is used for wiring inside a building or for connecting various devices inside a vehicle such as an automobile, and to protect the optical fiber from exposure to gasoline, battery acid, windshield washer fluid, etc.

<Coating Layer>
As the material for the coating layer, various thermoplastic resins that are commonly used as coating layers for optical fibers can be used, but depending on the environment and requirements in which the optical fiber cable is used, one or a mixture of two or more resins selected from the group consisting of polyamide-based resins, polyethylene-based resins, polypropylene-based resins, polyvinylidene chloride-based resins, chlorinated polyethylene-based resins, polyurethane-based resins, and vinylidene fluoride-based resins can be used.

Among these, polyamide resins have excellent heat resistance, bending resistance, and solvent resistance, making them suitable as optical fiber coating materials for applications that require heat resistance and environmental resistance. In addition, they are easy to process and have a moderate melting point, making them easy to coat optical fibers without reducing the transmission performance of the optical fiber.

Examples of polyamide resins include homopolymers consisting of monomer units such as nylon 10, nylon 11, nylon 12, nylon 6, and nylon 66, copolymers consisting of combinations of these monomer units, and nylon elastomers containing nylon monomer units with soft segments. These may be used alone or in combination of two or more types. Also, if necessary, polymers or compounds other than polyamide resins may be mixed. When other polymers or compounds are mixed, it is preferable to mix them so that the content of polyamide resin in the coating material is 20% by mass or more and 70% by mass or less.

Among polyamide resins, homopolymers or copolymers of nylon 11 and nylon 12 are particularly preferred. These have excellent adhesion, good moldability during the coating process, are less likely to cause thermal or mechanical damage to the optical fiber, and have excellent dimensional stability. The synergistic effect of this adhesion and dimensional stability can effectively prevent pistoning.

In addition, in the optical fiber cable of the present invention, the coating layer can contain a colorant such as carbon black to prevent external light from entering the optical fiber.

In the optical fiber cable of the present invention, a flame retardant may be contained in the material forming the coating layer in order to impart or improve flame retardancy. Known flame retardants such as metal hydroxides, phosphorus compounds, and triazine compounds can be used as the flame retardant. In particular, when a polyamide resin is used as the main component of the coating layer, it is preferable to use a triazine compound, and among these, melamine cyanurate is more preferable.

In addition, in the optical fiber cable of the present invention, a coloring agent or the like may be added to at least the coating layer constituting the outermost layer. This makes it easy to improve the distinguishability and design of the optical fiber cable. Although known coloring agents can be used, it is preferable to use inorganic pigments, since dye-based coloring agents may migrate into the optical fiber under high temperatures and increase transmission loss.

The coating layer may have a single-layer structure or a multi-layer structure having two or more layers.
The thickness of the coating layer varies depending on the diameter of the optical fiber, the constituent material of the coating layer, and the required characteristics. From the viewpoint of the long-term durability and twist resistance of the coated cable, the thickness is preferably 50 to 700 μm per layer, and more preferably 100 to 350 μm.

<Method of manufacturing optical fiber cable>
As a method for coating the outer circumference of an optical fiber with a coating layer, for example, a method using an extrusion coating device equipped with a crosshead die can be mentioned. In particular, when coating a plastic optical fiber with a coating layer, a method using an extrusion coating device equipped with a crosshead die is preferable because it can obtain an optical fiber cable with a uniform diameter.

[Application]
The optical fiber and optical fiber cable of the present invention have a wide transmission band and are excellent in bending resistance, long-term heat resistance, and optical property maintenance performance, and therefore can be suitably used for optical information communication, for example, in trains, airplanes, automobiles, and other vehicles, and in buildings, i.e., for indoor wiring applications, and for optical information communication in the field of factory automation (FA). In particular, they are suitable for use in a bent state in a narrow space, such as the inside of an automobile or other vehicle, where they are exposed to mechanical stress and a high-temperature environment.

The optical fiber and optical fiber cable of the present invention are also useful for illuminating medical equipment, such as endoscopes, ophthalmic surgery, and catheters.

As illumination used in medical devices, there is a combination of a light source and a light guide that transmits light from the light source to the tip and irradiates the object to be irradiated. The light guide is made of a single optical fiber or a bundle of multiple optical fibers to increase the amount of light transmitted. In recent years, the miniaturization of image pickup elements such as CCDs has made it possible to reduce the diameter of the observation probe of an endoscope, and it is desired that the light guide placed in the probe also be reduced in diameter, and an optical fiber that transmits a large amount of light even with a small diameter is desired. In addition, by making it possible to reduce the diameter of the observation probe of an endoscope, it becomes possible to directly observe narrow ducts such as the bile duct and pancreatic duct, which could not be entered by the previous probe. Since the bile duct and pancreatic duct are very narrow lumens of 5 to 10 mm, the outer diameter of the endoscope inserted into these lumens is limited to 3 to 4 mm. In addition, the observation probe of the endoscope to be inserted into the bile duct or pancreatic duct is usually inserted into the duodenal papilla through an opening on the side of the observation probe of a side-viewing duodenoscope, and then inserted into the bile duct or pancreatic duct. Since the probe is guided in a direction facing 90 degrees to the side at the opening of the duodenoscope, it is desirable to use an optical fiber that does not reduce the amount of light even when bent.
The optical fiber and optical fiber cable of the present invention can be suitably used for such medical equipment illumination applications.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as they do not exceed the gist of the invention.

[Abbreviations of compounds, etc.]
The abbreviations for compounds and the like used in the following Examples and Comparative Examples are as follows.
2F: vinylidene fluoride 4F: tetrafluoroethylene 6F: hexafluoropropylene 8F: octafluorobutene 3FM: 2,2,2-trifluoroethyl methacrylate MAFO: 3,3,4,4,5,5,6,6,7,7,8,8,8,-tridecafluorooctyl methacrylate MAA: methacrylic acid

[Materials used]
The following materials were used for the core material constituting the core of the optical fiber, the cladding material constituting the first cladding and the second cladding, and the coating layer.
First clad material (B-1): Fluorine resin (2F/4F/6F/8F) copolymer, refractive index 1.35)
First clad material (B-2): Fluorine resin (2F/4F/6F copolymer, refractive index 1.363)
First clad material (B-3): Fluorine resin (2F/4F/6F copolymer, refractive index 1.377)
First clad material (B-4): Fluorine resin (3FM/MAFO/MMA/MAA copolymer, refractive index 1.418)
Second clad material (C-1): Fluorine resin (2F/4F copolymer, refractive index 1.405, melting point 135°C, hardness 65.8 according to durometer hardness test)
Second clad material (C-2): Fluorine resin (2F/4F/6F polymer, refractive index 1.377, melting point 129°C, hardness 49.3 according to durometer hardness test)
Polyamide resin (N-1): Polyamide 12 resin (product name "Grilamid XE3926", manufactured by EMS-GRIVORY)
Polyamide resin (N-2): Polyamide 12 resin (product name "Grilamid XE3823", manufactured by EMS-GRIVORY)

[Measurement and evaluation method]
The methods for measuring and evaluating various physical properties and characteristics are as follows.

<Measurement of refractive index>
A film-like test piece having a thickness of 200 μm was prepared by melt pressing, and the refractive index of sodium D line at 23° C. was measured in accordance with ISO 13468 using an Abbe refractometer (model name “NAR-3T”, manufactured by Atago Co., Ltd.).

<Measurement of hardness by durometer hardness test>
Measurement was performed in accordance with JIS K7215.

<Melt point measurement>
The melting point was measured by differential scanning calorimetry, using a differential scanning calorimeter (model: EXSTAR DSC6200, manufactured by Seiko Instruments Inc.) by raising the temperature of the sample at a heating rate of 20° C./min.

<Evaluation of long-term heat resistance>
The optical fibers obtained in the examples and comparative examples were exposed for 3,000 hours in an environment with a temperature of 105°C or an environment with a temperature of 85°C and a humidity of 95%, and were measured by a 25m-1m cutback method under conditions of a wavelength of 650 nm and an excitation NA of 0.1.
Measurements using the 25m-1m cutback method were performed in accordance with IEC 60793-1-40:2001. Specifically, a 25m optical fiber was set in a measuring device, and the output power _P2 was measured. The optical fiber was then cut to the cutback length (1m from the input end), and the output power _P1 was measured. The optical transmission loss (unit: dB/km) was calculated using the following formula (1). The above measurements were performed in a light-shielded environment.

<Evaluation of bending resistance (bending loss)>
An LED light with a wavelength of 660 nm was input from one end of a 10 m long optical fiber cable. In this state, the center of the optical fiber cable was wrapped 360° around the outer circumference of a rod-shaped object with a radius of 10 mm (R10 mm), and the amount of light output from the other end was measured. The bending loss was calculated from the difference between the amount of output light from the optical fiber cable bent in this way and the amount of output light measured in the same manner for the same optical fiber cable in a non-bent state.

<Evaluation of bending resistance (twisting loss)>
An LED light with a wavelength of 660 nm was input from one end of a 2 m long optical fiber cable. In this state, one point of the optical fiber cable was fixed and rotated at a position 25 cm away from the fixed point to twist the optical fiber cable. After 25 turns, the amount of light emitted from the other end was measured. The twisting loss was calculated from the difference between the amount of light emitted from the twisted optical fiber cable and the amount of light emitted from the same optical fiber cable in a non-twisted state, which was measured in the same manner.

[Example 1]
<Production and evaluation of plastic optical fiber>
The core material was polymethyl methacrylate (refractive index 1.492), the first clad material was the first clad material (B-1), and the second clad material was the second clad material (C-1). The fibers were spun using a three-layered concentric composite spinning nozzle and stretched twice in the fiber axis direction in a hot air heating furnace at 140°C to obtain a stretched optical fiber with a first clad thickness of 5 μm, a second clad thickness of 10 μm, and a diameter of 1.0 mm. The stretched optical fiber was annealed in a 1 m long heating furnace with heated air at a temperature of 85°C while maintaining a constant length to obtain a plastic optical fiber. The obtained optical fiber was used to evaluate long-term heat resistance. The results are shown in Table 2.

<Production and evaluation of plastic optical fiber cables>
The material constituting the primary coating layer was polyamide resin (N-1), and the material constituting the secondary coating layer was polyamide resin (N-2). These materials were fed to a resin coating crosshead type 40 mm cable coating device (manufactured by Hijiri Seisakusho Co., Ltd.), and the outer circumference of the above-mentioned plastic optical fiber was coated with a primary coating layer (thickness 255 μm) and a secondary coating layer (thickness 345 μm), to obtain an optical fiber cable with a diameter of 2.20 mm. The obtained optical fiber cable was used to evaluate bending resistance. The results are shown in Table 2.

[Example 2, Comparative Examples 1 and 2]
Except for using the cladding material shown in Table 1, optical fibers and optical fiber cables were manufactured and evaluated in the same manner as in Example 1. The results are shown in Table 2.

From the above results, it can be seen that the plastic optical fiber of the present invention, which uses a first clad material having a refractive index specified in the present invention and a second clad material having a hardness and melting point specified in the present invention, has excellent bending resistance, long-term heat resistance, and ability to maintain optical properties.
In contrast, Comparative Example 1, in which the first clad material has a high refractive index, is inferior in long-term heat resistance and durability, while Comparative Example 2, in which the second clad material has a low melting point and hardness, is inferior in flex resistance.

10 Plastic optical fiber 11 Core 12 Cladding layer 12a First cladding 12b Second cladding

Claims (6)

A plastic optical fiber having a core made of a transparent resin and a cladding layer formed concentrically on an outer peripheral surface of the core, the cladding layer being a first cladding layer and a second cladding layer in this order,
The refractive index of the material constituting the first cladding is 1.300 to 1.375;
A plastic optical fiber, wherein the material constituting the second cladding has a hardness of 50 or more as measured by a durometer hardness test based on JIS K7215, and a melting point of 130° C. or more. The plastic optical fiber according to claim 1, wherein the melting point of the material constituting the second cladding is 133°C or higher. The plastic optical fiber according to claim 1, which is used for indoor wiring or for communication wiring in automobiles. The plastic optical fiber according to claim 1, which is a plastic optical fiber for illuminating medical equipment for endoscopes, ophthalmic surgery, or catheters. A plastic optical fiber cable having at least one coating layer provided around the outer circumference of the plastic optical fiber according to any one of claims 1 to 4. 6. The plastic optical fiber cable according to claim 5, wherein the coating layer is formed of a resin mainly composed of at least one resin selected from the group consisting of polyamide-based resins, polyethylene-based resins, polypropylene-based resins, polyvinylidene chloride-based resins, chlorinated polyethylene-based resins, polyurethane-based resins, and vinylidene fluoride-based resins.

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