JP4183157B2 - Plastic optical fiber and optical fiber cable using the same - Google Patents

Description

表中のノズル吐出平均速度とは、１秒間にノズルから吐出される樹脂の量（単位ｍｍ³ ）をノズル出口の面積（単位ｍｍ² ）で除した値である。
【００４０】
また、表中の屈折率差とは、芯の中心領域の屈折率と外周部の屈折率との差を表している。ここで、中心領域の屈折率とは芯の中心から100[μｍ] までの範囲に観られる最小屈折率のことであり、外周部の屈折率とは芯の外周面から50[ μｍ] までの深さ範囲に観られる最大屈折率のことである。
【００４１】
［比較例１〜２］
表２に示すような通常のＰＯＦの紡糸条件で紡糸を行った以外は実施例１と同様にして、紡糸ノズルより連続的に樹脂を吐出させ、所定の速度で引き取りながら冷却、固化することによって同芯円2 層構造の未延伸ＰＯＦを得た。引き続き、この未延伸ＰＯＦを140 ℃の熱風が風速10m/sec で循環している長さ4 ｍの熱風加熱延伸炉に導入し、導入速度の2 倍の速度で引き取ることにより2 倍の加熱延伸処理を連続的に施した。表２の比較例No.1〜2 に示すような芯材中の屈折率差、ＰＯＦ破断までの繰り返し屈曲回数、伝送損失を有した、直径980[μｍ] のPMMAの芯の周りに、厚さ10[ μｍ] のフッ化ビニリデン系コポリマーの鞘が被覆された構造の直径1000[ μｍ] のＰＯＦ、及び、芯材と鞘材との吐出量比と紡糸引き取り速度との関係を変更し２倍の加熱延伸を施した、直径730[μｍ] のPMMAの芯の周りに、厚さ10[ μｍ] のフッ化ビニリデン系コポリマーの鞘が被覆された構造の直径750[μｍ] のＰＯＦを得た。
【００４２】
そして実施例３〜５と同様にケーブル化を行い直径2.2mm のＰＯＦケーブルを得た。
【００４３】
得られたＰＯＦをケーブル化したＰＯＦケーブルのＰＯＦ破断までの繰り返し屈曲回数は、78000 回、145000回と、実施例の同じ断面寸法を有したＰＯＦに比べ低いものであった。
【００４４】
【表２】

【００４５】
【発明の効果】
以上のように、本発明のプラスチック光ファイバにおいては、全体にわたって同一組成のポリマーからなる芯の外周部に芯中心領域より０．００１以上高い屈折率の部分を有するので、光学特性を損なうことなく繰り返し屈曲耐久性を向上させることができる。
【図面の簡単な説明】
【図１】本発明によるプラスチック光ファイバの一実施形態を示す模式的断面図である。
【図２】本発明による光ファイバケーブルの一実施形態を示す模式的断面図である。
【図３】繰り返し屈曲耐久性の評価方法の概略説明図である。
【図４】光ファイバ芯の屈折率分布図である。
【図５】光ファイバ芯の屈折率分布図である。
【図６】光ファイバ芯の屈折率分布図である。
【符号の説明】
(1) プラスチック光ファイバケーブル
(2) ロール
(3) 重り
(4) 光源（LED ）
(5) 光検出器（光パワーメータ）
(6) 保持金具
１２ 芯
１２ａ 芯中心領域
１２１ 芯中心部
１２２ 芯外周部
１４ 鞘
１６ 保護層
２０ プラスチック光ファイバ
２２ 被覆層[0001]
BACKGROUND OF THE INVENTION
The present invention belongs to the technical field of optical transmission, and particularly relates to a plastic optical fiber and an optical fiber cable excellent in repeated bending durability among mechanical characteristics.
[0002]
[Prior art and problems to be solved by the invention]
A plastic optical fiber and an optical fiber cable formed using the same are used for short-distance optical communication such as signal transmission between a sensor in a robot and a signal processing circuit and a data link in a moving body. . In these applications, the optical fiber is frequently subjected to a bending action based on the mechanical operation of the robot or based on inertial force or vibration external force generated when the moving body moves. Therefore, the plastic optical fiber used in such an environment is required not only to have excellent optical characteristics but also to have excellent mechanical characteristics, particularly durability against repeated bending action.
[0003]
In general, in order to improve the mechanical properties of a plastic optical fiber, a drawing process is performed at an appropriate temperature and at an appropriate draw ratio. However, the stretching process may cause a decrease in optical properties and an increase in heat shrinkability of the plastic optical fiber. Therefore, it is necessary to consider the suppression of these occurrences. It's not good. That is, generally speaking, increasing the draw ratio increases the mechanical properties, but increases the heat shrinkability, and when used at high temperatures, the position of the connector attached to the cable end. There arises a problem that the coupling loss tends to increase due to the occurrence of deviation and the coupling with the light source or the coupling with the light receiver.
[0004]
A normal step index type plastic optical fiber is composed of a core material and a sheath material, or a material for the protective layer when a further protective layer is used on the outer periphery of the sheath to improve repeated bending durability. An unstretched yarn is produced by melt-combined spinning using a spinning nozzle, and only the core material is melt-spun and a sheath material or protective layer material is coated around it to form a sheath or protective layer. Is manufactured through a stretching process. In spinning at this time, it has been conventionally thought that the optical characteristics are deteriorated due to distortion inside and outside the spinning nozzle. Therefore, at a low speed so as not to cause the deterioration of the optical characteristics. Spinned.
[0005]
Although the conventional plastic optical fiber manufactured as described above and the optical fiber cable using the same can be used in the above-mentioned applications, it is required to further improve the mechanical characteristics, particularly the repeated bending durability. ing.
[0006]
[Means for Solving the Problems]
According to the present invention, in order to solve the above technical problems, in the core / sheath structure plastic optical fiber in which the core is covered with the sheath, the core is made of a polymer having the same composition throughout. A plastic optical fiber is provided in which the core has a refractive index portion higher than the central region of the core by 0.001 or more at an outer peripheral portion thereof.
[0007]
In one aspect of the present invention, the outer peripheral portion has a refractive index portion higher than the central region by 0.002 or more. In one aspect of the present invention, the thickness of the outer peripheral portion is 50 μm, and the radius of the central region is the smaller value of the inner diameter of the outer peripheral portion and 100 μm. In one embodiment of the present invention, a protective layer is provided around the sheath. In one aspect of the invention, the core polymer is polymethylmethacrylate.
[0008]
According to the present invention, there is provided a plastic optical fiber cable characterized in that a coating layer is formed around the plastic optical fiber as described above as a solution to the above technical problems. The In one aspect of the present invention, a connector is attached to at least one end so that the end face of the plastic optical fiber is exposed.
[0009]
In general, the starting point of the breakdown of a material is a defect point inside the material where stress concentration is likely to occur due to external stimulation. In the case of an optical fiber, a manufacturing method is adopted in which defect points such as foreign matter and voids are not created in order to reduce light scattering to the limit. There are many cases. Therefore, in order to reduce fatigue failure due to bending or vibration of the plastic optical fiber, it is particularly important to increase the mechanical strength near the surface. Therefore, as a result of intensive studies aimed at increasing the mechanical strength of the plastic optical fiber while suppressing heat shrinkage, the present inventor has reached the present invention as described above.
[0010]
In the plastic optical fiber of the present invention, the core outer peripheral portion having a high refractive index has a larger molecular orientation than the core central region, and it is assumed that the mechanical strength near the outer surface is improved. Is done. That is, as the molecular orientation proceeds, the density increases. From the Clausius Mosotti relational expression, it is known that in a substance having the same molecular polarizability, that is, a substance having the same molecular structure, the refractive index of the substance increases as the density of the substance increases. Thereby, it is thought that the refractive index of a core outer peripheral part becomes high.
[0011]
As a result of various studies by the present inventors, a plastic optical fiber that has been spun under a certain spinning condition has a structure in which a refractive index increases outward in the radial direction in the core made of the same composition material. I have found that. In other words, it seems that the molecular orientation is increased outward in the radial direction, so that the density of the portion is also increased. The present inventors have a plastic having such a structure and excellent mechanical characteristics. We have found a method for manufacturing optical fibers.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0013]
FIG. 1 is a schematic cross-sectional view showing an embodiment of a plastic optical fiber (hereinafter sometimes abbreviated as “POF”) according to the present invention, and FIG. 2 shows an optical fiber cable according to the present invention using the same. It is a typical sectional view showing an embodiment.
[0014]
FIG. 1 shows a cross section perpendicular to the longitudinal direction of the POF, which has a core / sheath structure, i.e. the core 12 is covered by a sheath 14 arranged around it. . The sheath 14 is covered with a protective layer 16 disposed around the sheath 14.
[0015]
C indicates the center (center axis) of the POF. The core 12 has a radius (distance from the POF center C) of R1, and includes the POF center C and a surrounding radius R0. The refractive index does not substantially change (for example, the maximum value of the refractive index). A central portion 121 (with a difference from the minimum value of 0.001 or less) and an outer peripheral portion 122 located around the central portion 121 and in contact with the sheath 14. The distance from the POF center C to the outer surface of the sheath 14 is R2, and the distance from the POF center C to the outer surface of the protective layer 16 is R3.
[0016]
R0 is, for example, 100 to 1400 μm, R1 is, for example, 120 to 1500 μm, R2 is, for example, 125 to 2000 μm, and R3 is, for example, 130 to 3000 μm. In order to sufficiently improve the mechanical characteristics of the POF of the present invention, the thickness (R1-R0) of the outer peripheral portion 122 is preferably 20 μm or more, and can be set to, for example, 50 μm.
[0017]
The core 12 is made of one kind of polymer (including a copolymer or the like) having the same composition composed of monomer units having the same molecular structure throughout the central part 121 and the outer peripheral part 122. As the polymer of the core 12, polymethyl methacrylate and a copolymer of methyl methacrylate and other monomers, polystyrene and styrene copolymers, polycarbonate, amorphous polyolefin, etc., which are usually used in optical fibers, are used. Of these, polymethyl methacrylate and copolymers of methyl methacrylate and other monomers are preferable from the viewpoint of optical transmission characteristics. Furthermore, polymethyl methacrylate is more preferable.
[0018]
As the material of the sheath 14, a known material can be used as long as it has a lower refractive index than the material of the core 12. For example, when polymethyl methacrylate is used as the polymer for the core 12, the material of the sheath 14 is a copolymer of fluorinated alkyl methacrylate and methyl methacrylate, short-chain fluorinated alkyl methacrylate, long-chain fluorinated alkyl methacrylate and methyl. A copolymer with methacrylate, a vinylidene fluoride polymer, a blend of a vinylidene fluoride polymer and a polymethyl methacrylate polymer, and the like can be used.
[0019]
The protective layer 16 is formed using a resin having excellent flexibility in order to further improve the bending resistance. As a material for such a protective layer 16, for example, a vinylidene fluoride polymer and a vinylidene fluoride copolymer are particularly preferable. However, this protective layer 16 may be omitted.
[0020]
FIG. 2 shows a cross section perpendicular to the longitudinal direction of a plastic optical fiber cable (POF cable) formed by arranging the coating layer 22 around the POF 20 as described above. That is, the POF 20 of the present invention is used in the form of a cable in which a coating layer 22 is formed on the outer periphery in order to improve durability and environmental resistance.
[0021]
As the material of the covering layer 22, various thermoplastic resins, thermosetting resins, photocurable resins, shape memory resins, resins containing metal fine powders, and the like can be used. Preferred resins for thermoplastic resins include vinyl chloride resin, low density polyethylene, linear low density polyethylene, chlorinated polyethylene, ethylene-vinyl acetate copolymer, blend of polyvinyl chloride and ethylene-vinyl acetate copolymer. Products, polyurethane resins and the like. Among them, a resin having a low elastic modulus such as an ethylene-vinyl acetate copolymer, a blend product of polyvinyl chloride and an ethylene-vinyl acetate copolymer is more preferably used. Also, a plasticizer can be added to various resins. In the case of vinyl chloride resin, for example, dioctyl phthalate, trioctyl trimellitate, tricresyl phosphate, etc. are used as the plasticizer. However, since the plasticizer may move to POF 20 and reduce its optical performance and mechanical properties, it is preferable to select a plasticizer that does not cause such adverse effects. In addition, as the thermosetting resin, phenol resin, urea resin, melamine resin, unsaturated polyester resin, epoxy resin, polyurethane resin, or the like is used. As the photocurable resin, for example, a combination of polymethyl methacrylate and a photopolymerization initiator is used. As the shape memory resin, acrylic resin, transisoprene, polyurethane, polynorbornene, styrene / butadiene copolymer or the like is used.
[0022]
In the present embodiment, the core outer peripheral portion 122 has a portion having a refractive index higher by 0.001 or more than the refractive index of the core center region 12a. Here, the center region 12a indicates a region having a radius Ra including the POF center C. The center area radius Ra can be set as follows, for example. That is, Ra coincides with R0 when the radius R0 of the central portion 122 is 100 μm or less, and is 100 μm when the radius R0 of the central portion 122 exceeds 100 μm (the inner diameter R0 of the outer peripheral portion 122 is 100 μm). The smaller of them). The refractive index of the core material central region 12a refers to the minimum refractive index in the core material central region 12a (usually the refractive index at the POF center C).
[0023]
Here, an example of a method for measuring the refractive index distribution in the cross section of the POF core and a method for evaluating the repeated bending durability of the POF and POF cables will be described below.
[0024]
<Measurement method of refractive index distribution>
(1) The POF sheath 14 (and the protective layer 16 when the protective layer 16 is provided) is peeled off to obtain a measurement sample of only the core 12;
(2) Since the refractive index depends on the amount of water in the polymer, the sample in (1) is sufficiently dried to remove the water;
(3) Document Y. Kokubun and K. Iga: "Precise measurement of the refractive index profile of optical fibers by a nondestructive interference method", Trans.IECE Japan Section E, E60, 12, p.702 (1977) The refractive index profile in the radial direction is measured using the introduced lateral differential interference method.
[0025]
<Mechanical properties: Evaluation method for repeated bending durability>
FIG. 3 is a schematic explanatory diagram of a method for evaluating repeated bending durability.
[0026]
Between two rolls (2) with a radius of 15 mm separated by a gap of 2.3 mm and parallel to each other, a POF cable (1) with a diameter of 2.2 mm is dropped through the POF cable (1) below the roll (2). ) Install a 0.5 kg weight (3) on A holding bracket (6) is attached to the POF cable (1) above the roll (2), and the holding bracket (6) has an axis O parallel to the roll (2) shown in FIG. It is attached to a rotating arm (not shown) that rotates 180 degrees as the center. Here, the axis O is a line of intersection between the plane in contact with the top of the two rolls (2) and the plane of symmetry of the arrangement of the two rolls (2). That is, the rotating arm rotates to the first horizontal posture position (the state of the POF cable at this time is shown by a solid line in FIG. 3) that is tilted 90 degrees from the vertical posture position to one side about the axis O. The second horizontal posture position that has moved and then turned back to the vertical posture position, and then tilted 90 degrees to the opposite side (the state of the POF cable at this time is shown by the one-dot chain line in FIG. To the vertical posture position is repeated. This reciprocating operation is counted as one bending operation, and the bending operation is repeated at a speed of 30 times per minute.
[0027]
An LED with a wavelength of 660 nm is used as the light source (4), an optical power meter is used as the photodetector (5), and the light emitted from the light source (4) through the POF cable (1) and reaches the photodetector (5) The number of times of repeated bending until the detected value is lowered by 1 dB from the initial value (value before the start of repeated bending operation) is defined as the number of times of bending until POF fracture, thereby evaluating the bending durability. The evaluation of bending durability is an average value for five POFs.
[0028]
Specific examples of the refractive index distribution in the radial direction of the core of the POF measured as described above are shown in FIGS. The “distance from the center” on the horizontal axis in these figures is normalized by setting the distance from the POF center C to 1.0 as the core radius R1. Further, the “amount of change in refractive index” on the vertical axis in these drawings is expressed as a difference in refractive index with respect to the refractive index of the POF center C as a reference (0.000).
[0029]
FIG. 4 is a refractive index distribution diagram of a POF core having a substantially constant refractive index over the entire core, and is shown for reference. The number of times that a POF cable constructed using this POF bends in a repeated bending endurance test varies depending on the fiber diameter, draw ratio, sheath material composition, thickness, etc., but is generally from about 10,000 to 100,000. About times.
[0030]
FIG. 5 shows a refractive index distribution of a POF core having an outer peripheral portion having a portion whose refractive index is about 0.001 larger than that of the central region obtained by the present invention. The number of breaks in the repeated bending endurance test of POF having such a refractive index distribution is improved by about 30% compared to the almost uniform refractive index distribution shown in FIG.
[0031]
FIG. 6 shows a portion obtained by the present invention having an outer peripheral portion having a portion where the refractive index is about 0.001 or more larger than the central region, and in particular, a portion where the refractive index is about 0.002 larger than the central region. It represents the refractive index distribution of a POF core having The number of breaks in the repeated bending endurance test of POF having such a refractive index distribution is about twice that of the almost uniform refractive index distribution shown in FIG.
[0032]
As can be seen from FIGS. 5 and 6, the refractive index distribution of the core monotonously increases as it progresses radially outward from the POF center. The larger the difference in refractive index between the central region of the core and the outer peripheral portion, the better, but it is usually 0.005 or less.
[0033]
In order to manufacture the plastic optical fiber of the present invention, for example, a composite spinning method is used. In this case, the shear stress difference acting on the outer peripheral polymer close to the nozzle wall surface of the resin flowing in the spinning nozzle and the inner layer polymer during spinning. Spinning is performed under an increased condition by increasing the nozzle discharge average speed or by lowering the spinning temperature and increasing the resin viscosity. From such a viewpoint, the average nozzle discharge speed is preferably 20 mm / sec or more, and more preferably 40 mm / sec or more. The spinning temperature is preferably 230 ° C. or less, and more preferably 200 ° C. or less. Further, the polymer used as the core material preferably has a melt index (MI) of 10 or less. MI is measured in accordance with JIS K7210, with an orifice diameter of 4 mmφ, an orifice length of 8 mm, a test temperature of 230 ° C., a test load of 5 kg, and a sampling time of 10 minutes. In order to facilitate the production, the average nozzle discharge speed is preferably 100 mm / sec or less, and the core polymer MI is preferably 0.1 or less.
[0034]
Also, in order not to increase the core-sheath interface irregularity during spinning, the inner surface of the nozzle is optically polished so that a slight turbulent flow is not generated in the nozzle during spinning, and the viscosity of the outer peripheral polymer, particularly the sheath material, is appropriately reduced. By devising the above, it is possible to produce an optical fiber that does not deteriorate transmission loss.
[0035]
【Example】
The present invention will be described below with reference to examples and comparative examples.
[0036]
[Examples 1-7]
Polymethylmethacrylate (PMMA) with an MI of 5 is used as the core material, a copolymer of vinylidene fluoride and tetrafluoroethylene having a lower refractive index than the core material is used as the sheath material, and these resins are extruded using an extruder. Continuous melt devolatilization, melted and devolatilized resin is quantitatively supplied to the two-layer composite spinning nozzle with a gear-type metering pump, and spinning is performed under the spinning conditions shown in Table 1, and is continuously discharged from the spinning nozzle. The unstretched POF having a concentric two-layer structure was obtained by cooling and solidifying while pulling at a spinning take-up speed. Subsequently, this unstretched POF was introduced into a 4 m long hot air heating and stretching furnace in which hot air at 140 ° C was circulated at a wind speed of 10 m / sec. The treatment was applied continuously. Thickness around the core of 980 [μm] diameter PMMA having the refractive index difference in the core material as shown in Examples Nos. 1 to 7 in Table 1, the number of repeated bending until POF breakage, and transmission loss 1000 [μm] POF with a sheath coated with a 10 [μm] sheath of vinylidene fluoride copolymer, and the relationship between the discharge ratio between the core and sheath, the total discharge and the take-up speed 750 [μm] in diameter of 750 [μm] with a sheath of a vinylidene fluoride copolymer with a thickness of 10 [μm] around a PMMA core with a diameter of 730 [μm] that was subjected to two-fold heat stretching Obtained POF.
[0037]
The obtained POF was continuously coated with molten polyethylene using a cable forming device to form a coating layer, thereby obtaining a POF cable having a diameter of 2.2 mm.
[0038]
The number of repeated bending until POF breakage of the POF cable obtained by converting the obtained POF into cable is 100,000 to 450,000 times, and it is excellent in repeated bending durability characteristics, and transmission loss is also good at 126 to 132 [dB / km]. .
[0039]
[Table 1]

The average nozzle discharge speed in the table is a value obtained by dividing the amount (unit mm ³ ) of resin discharged from the nozzle per second by the area (unit mm ² ) of the nozzle outlet.
[0040]
Further, the refractive index difference in the table represents a difference between the refractive index of the central region of the core and the refractive index of the outer peripheral portion. Here, the refractive index of the central region is the minimum refractive index found in the range of 100 [μm] from the center of the core, and the refractive index of the outer peripheral portion is from the outer peripheral surface of the core to 50 [μm]. It is the maximum refractive index observed in the depth range.
[0041]
[Comparative Examples 1-2]
Except for spinning under normal POF spinning conditions as shown in Table 2, in the same manner as in Example 1, the resin was continuously discharged from the spinning nozzle, and cooled and solidified while being taken up at a predetermined speed. An unstretched POF having a concentric circle two-layer structure was obtained. Subsequently, this unstretched POF was introduced into a 4 m long hot air heating and stretching furnace in which hot air at 140 ° C was circulated at a wind speed of 10 m / sec. The treatment was applied continuously. Thickness around the core of PMMA with a diameter of 980 [μm] having a difference in refractive index in the core material as shown in Comparative Examples No. 1 and No. 2 in Table 2, the number of repeated bending until the POF break, and transmission loss. Changed the relationship between the discharge rate of the core material and the sheath material and the take-up speed of the yarn with a diameter of 1000 [μm] with a sheath coated with a 10 [μm] vinylidene fluoride copolymer sheath. A POF with a diameter of 750 [μm], in which a sheath of a vinylidene fluoride copolymer with a thickness of 10 [μm] is coated around a PMMA core with a diameter of 730 [μm] that has been subjected to double heat stretching It was.
[0042]
Then, cable formation was performed in the same manner as in Examples 3 to 5 to obtain a POF cable having a diameter of 2.2 mm.
[0043]
The number of repeated bending until POF breakage of the POF cable obtained by converting the obtained POF into a cable was 78000 times and 145000 times, which were lower than those of the POF having the same cross-sectional dimensions of the examples.
[0044]
[Table 2]

[0045]
【The invention's effect】
As described above, the plastic optical fiber of the present invention has a refractive index portion higher than the core central region by 0.001 or more on the outer peripheral portion of the core made of the polymer having the same composition throughout, so that the optical characteristics are not impaired. Repeated bending durability can be improved.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing an embodiment of a plastic optical fiber according to the present invention.
FIG. 2 is a schematic cross-sectional view showing an embodiment of an optical fiber cable according to the present invention.
FIG. 3 is a schematic explanatory diagram of a method for evaluating repeated bending durability.
FIG. 4 is a refractive index distribution diagram of an optical fiber core.
FIG. 5 is a refractive index distribution diagram of an optical fiber core.
FIG. 6 is a refractive index distribution diagram of an optical fiber core.
[Explanation of symbols]
(1) Plastic optical fiber cable
(2) Roll
(3) Weight
(4) Light source (LED)
(5) Optical detector (optical power meter)
(6) Holding metal 12 Core 12a Core center region 121 Core center portion 122 Core outer peripheral portion 14 Sheath 16 Protective layer 20 Plastic optical fiber 22 Coating layer

Claims (7)

In a plastic optical fiber having a core / sheath structure in which a core is covered with a sheath, the core is made of a polymer having the same composition throughout, and the core is higher than the central region of the core by 0.001 or more at the outer periphery. A plastic optical fiber having a refractive index portion. 2. The plastic optical fiber according to claim 1, wherein the outer peripheral portion has a refractive index portion higher than the central region by 0.002 or more. The thickness of the said outer peripheral part is 50 micrometers, and the radius of the said center area | region is a smaller value of the internal diameter of the said outer peripheral part, and 100 micrometers, The said any one of Claims 1-2 characterized by the above-mentioned. Plastic optical fiber. The plastic optical fiber according to claim 1, wherein a protective layer is provided around the sheath. The plastic optical fiber according to claim 1, wherein the core polymer is polymethyl methacrylate. A plastic optical fiber cable, wherein a coating layer is formed around the plastic optical fiber according to claim 1. 7. The plastic optical fiber cable according to claim 6, wherein a connector is attached to at least one end so as to expose an end face of the plastic optical fiber.

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