JP2025131521A - Plastic Optical Fiber - Google Patents

Abstract

[Problem] To provide a substantially fluorine-free plastic optical fiber at low cost, with excellent transmission loss and mechanical strength. [Solution] A plastic optical fiber formed in this order of a core, a first cladding, and a second cladding, characterized in that the first cladding is made of a polymer of methyl methacrylate and/or a copolymer mainly composed of methyl methacrylate, and the second cladding is made of a polyolefin-based resin. [Selected Figures] None

Description

The present invention relates to a plastic optical fiber that is cost-effective, highly manufacturable, and substantially fluorine-free.

Plastic optical fibers are more flexible and durable against repeated bending than glass-based optical fibers, and are used in applications such as light-guiding sensors for robots installed in drive units and bent sections, photoelectric sensors for industrial equipment, and lighting for medical endoscopes. In recent years, efforts have been made to take advantage of the flexibility and durability of plastic optical fibers against repeated bending by forming them into woven or knitted fabrics and using them as surface light emitters.

Plastic optical fiber is typically composed of two layers: a core and a cladding. The core material is generally made of a highly transparent and weather-resistant polymer, such as polymethyl methacrylate (PMMA). The sheath material, on the other hand, must have a lower refractive index than the core material in order to confine light within the core. Furthermore, in order to provide plastic optical fiber with flexibility and durability against repeated bending, it is important that this sheath material be flexible. Fluorine-containing polymers are widely used as materials with such low refractive index and flexibility.

However, in recent years, environmental pollution caused by fluorocarbon compounds (PFAS) has been pointed out, and there is a demand for the commercialization of products that do not contain fluorine compounds.

As a substantially fluorine-free optical fiber, one that uses polystyrene or polycarbonate as the core material and polymethyl methacrylate as the sheath material has been proposed (Patent Document 1). However, this configuration has the drawback of being weak against bending. Therefore, when using such plastic optical fiber for industrial purposes, it needs to be covered with a protective layer before being made into a cable, and it has been proposed to use a thermoplastic polymer as this protective material.

In addition, optical fibers obtained by composite spinning of the above-described core-sheath structure are drawn using the usual method and then coated with a protective resin to form an optical fiber cord. However, it has also been proposed to use a polypropylene-containing resin with a high melting point and high heat aging resistance as the coating resin, primarily for the purpose of improving the heat resistance of the optical fiber (Patent Document 2).

Japanese Patent Application Publication No. 58-93003 Japanese Patent Application Publication No. 6-43326

However, when forming a protective layer around the outer periphery of an optical fiber with a core-sheath structure, the heat from the coating resin increases the optical fiber's transmission loss, so careful attention must be paid to temperature increases during the coating process. Furthermore, adding a separate coating process significantly increases costs, as the additional process also impacts quality checks and yields.

As a result, it has been proposed to use multiple melt extruders to simultaneously spin the core-sheath structure of the optical fiber and the protective layer around it. However, optical fibers with protective layers obtained by simultaneous spinning cannot be stretched because the transmission loss increases significantly when stretched, and there are problems with insufficient mechanical strength.

The primary object of the present invention is to provide an inexpensive plastic optical fiber that has excellent transmission loss and mechanical strength and is substantially fluorine-free.

In order to solve the above problems, the plastic optical fiber of the present invention has the following configuration.
That is, it is a plastic optical fiber having a core, a first cladding, and a second cladding formed in that order, characterized in that the first cladding is made of a polymer of methyl methacrylate and/or a copolymer having methyl methacrylate as its main component, and the second cladding is made of a polyolefin-based resin.

In the plastic optical fiber of the present invention, it is preferable that the polyolefin resin constituting the second cladding is polypropylene, a copolymer of ethylene and propylene.

Furthermore, in the plastic optical fiber of the present invention, it is preferable that the core be made of a polymer whose main component is styrene, cycloolefin, or carbonate.

Furthermore, in the plastic optical fiber of the present invention, it is preferable that the core, first cladding, and second cladding are made of a fluorine-free polymer.

The plastic optical fiber of the present invention as described above can be used to produce woven or knitted fabrics. By forming the plastic optical fiber in this way, it can be used, for example, as a surface light emitter.

The present invention makes it possible to provide optical fibers that are inexpensive and have excellent transmission loss and mechanical strength, and that do not, at least intentionally, contain fluorine.

The following describes in detail preferred embodiments of the plastic optical fiber according to the present invention, but the present invention is not limited to the following embodiments and can be modified in various ways depending on the purpose and application.

The plastic optical fiber of the present invention has a core, a first cladding, and a second cladding, in that order.

(core)
In the plastic optical fiber of the present invention, the resin used for the core material is not particularly limited as long as it is highly transparent and has a refractive index higher than that of the material used for the first cladding, but it is preferable that it be a polymer whose main component is, for example, styrene, cycloolefin, or carbonate.

When a polymer primarily composed of styrene is used for the core material, a plastic optical fiber with excellent optical transparency can be obtained. Furthermore, when a polymer primarily composed of polycarbonate is used for the core, a plastic optical fiber with excellent mechanical and thermal properties can be obtained. These can be selected appropriately depending on the performance desired for the plastic optical fiber.

Examples of the polymerization components (monomers) for such polymers include, in the case of styrene, styrene; substituted styrenes such as methylstyrene and α-methylstyrene; (meth)acrylic acid esters; (meth)acrylic acid; and N-substituted maleimides. (Meth)acrylic acid esters are a general term for acrylic acid and methacrylic acid, and examples include methyl acrylate, ethyl methacrylate, butyl methacrylate, t-butyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, phenyl methacrylate, bornyl methacrylate, and adamantyl methacrylate. Examples of N-substituted maleimides include N-isopropylmaleimide, N-cyclohexylmaleimide, N-methylmaleimide, N-ethylmaleimide, and N-o-methylphenylmaleimide. Two or more of these may be used.

In the present invention, "mainly as a component" means that it accounts for 50 mol% or more of the repeating units that make up the polymer, preferably 70 mol% or more, and more preferably 90 mol% or more of the repeating units that make up the polymer.

The core may also contain stabilizers such as antioxidants and heat stabilizers to the extent that they do not affect translucency.

(First cladding)
The plastic optical fiber of the present invention has at least two cladding layers around the core. In the plastic optical fiber of the present invention, the first cladding is the inner cladding that contacts the core and serves to totally reflect light at the core/cladding interface to prevent light propagating within the core from leaking to the outside, and therefore must have a lower refractive index than the core. In the present invention, a polymer made of methyl methacrylate and/or a copolymer mainly composed of methyl methacrylate is used as such a cladding material.

In the present invention, a polymer primarily composed of methyl methacrylate refers to a polymer in which 50 mol % or more of the repeating units constituting the polymer are derived from methyl methacrylate.

The first cladding material preferably has a lower refractive index than the core and excellent interfacial adhesion with the core. By selecting a polymer made of methyl methacrylate and/or a copolymer containing methyl methacrylate as the main component as the first cladding material, the refractive index is lower than that of the core and good interfacial adhesion can be achieved, thereby improving the flexibility of the optical fiber.

(Second cladding)
In the plastic optical fiber of the present invention, the second cladding is preferably made of a polyolefin resin, particularly a polyolefin thermoplastic resin. The thermoplastic resin may be any suitable polyolefin, such as polypropylene, polyethylene, polybutylene, or a combination or mixture thereof. However, in a preferred embodiment of the present invention, the thermoplastic resin is preferably a polyolefin selected from the group consisting of polypropylene homopolymer, polypropylene copolymer (e.g., polypropylene random copolymer), and a mixture thereof. Among these, an ethylene-propylene random copolymer, in which propylene and ethylene are copolymerized, is particularly preferred as a protective layer for optical fibers because of its excellent stretchability and flexibility. The ethylene unit content is preferably 20 mol% or less, more preferably 10 mol% or less, and even more preferably 7 mol% or less, based on the total amount of propylene units and ethylene units detected in the second cladding. By keeping the ethylene content at 7 mol% or less, a significant decrease in melting point is suppressed, thereby facilitating the temperature setting of the spinneret when spinning composite fibers with different materials. The lower limit of the ethylene unit content is preferably more than zero, more preferably 0.1 mol% or more, even more preferably 1 mol% or more, and still more preferably 4 mol% or more. By setting the ethylene unit content within the above range, the effects obtained by adding the ethylene copolymer can be fully exhibited.

The MFR (melt flow rate) of ethylene-propylene copolymer at 230°C and a load of 2.16 kg cannot be generalized and depends on the properties of the resins being co-extruded, but from the perspective of spinnability using a composite spinneret, it is preferably 0.1 g/10 min or more and 50 g/10 min or less, and more preferably 0.3 g/10 min or more and 20 g/10 min or less.

(optical fiber)
The method for producing the plastic optical fiber of the present invention is not particularly limited, but a common method is to extrude the core material and cladding material in a heated and molten state from a spinneret for concentric composite spinning to form an optical fiber wire having a two-layer core-sheath structure of core/cladding, then stretch the wire by about 1.2 to 3 times to improve mechanical properties such as breaking strength, and finally cover the outer periphery of the cladding with a protective layer. Furthermore, it is advantageous in terms of cost because it requires fewer processes to select a resin that functions as a protective layer as the second cladding material and to simultaneously form a three-layer core-sheath structure of core/first cladding/second cladding by composite spinning. In this case, the three-layer core-sheath structure is stretched by about 1.2 to 3 times to produce the plastic optical fiber.

The fiber diameter of the plastic optical fiber of the present invention is not particularly limited, but is preferably 100 μm or more and 1000 μm or less. By making the fiber diameter 100 μm or more, the amount of light required for sensors and lighting, which are suitable applications of plastic optical fiber, can be ensured. On the other hand, by making the fiber diameter 1000 μm or less, the bending resistance required of plastic optical fiber can be further improved.

Preferable cladding thicknesses are 0.5 to 30 μm for the first cladding, and more preferably 1 to 20 μm. If the thickness is less than 0.5 μm, the core/first cladding interface will become unstable, resulting in increased transmission loss. If the thickness is 30 μm or less, the thickness of the second cladding, which has excellent mechanical properties, can be made relatively large within the limited fiber diameter, improving the mechanical properties of the fiber itself.

The second cladding should have a thickness of 3 μm or more, preferably 5 μm or more, and more preferably 10 μm or more. If the thickness of the second cladding is less than 3 μm, the coating will be too thin and will not function as a protective layer.

In the plastic optical fiber of the present invention, while satisfying the above cladding thickness conditions, it is preferable that the sum of the first cladding thickness and the second cladding thickness be 20% or less of the core diameter. Increasing the thickness of the second cladding strengthens the protection of the fiber and improves its mechanical properties, but it also reduces the core diameter of the fiber and reduces the light output of the fiber. To obtain a bright fiber with excellent bendability, the sum of the first cladding thickness and the second cladding thickness should be 20% or less of the core diameter, more preferably 15% or less.

The cladding thickness is measured on the cross section of the optical fiber. It is advisable to cut the optical fiber perpendicular to the drawing direction and polish the cross section so that the core/cladding interface can be easily observed. In the cross section, the first cladding thickness is the thinnest point of the first cladding that surrounds the core. Similarly, the thickness of the second cladding is the thinnest point. If the cross section of the optical fiber is not circular, the shortest diameter of the optical fiber is taken as the fiber diameter. In the case of a multicore fiber, where a single fiber has multiple cores, the second cladding thickness is taken as the point where the distance between the first cladding and the outer circumference of the fiber is shortest.

Furthermore, depending on the quality of the spinning process, the fiber diameter of plastic optical fiber may vary in the drawing direction (hereinafter referred to as diameter variation). If the diameter variation is large, the core diameter and cladding thickness may vary, and it may not be possible to achieve the desired characteristics. The allowable diameter variation depends on the original fiber diameter and application. For example, if a plastic optical fiber is used alone for lighting or decoration, a large diameter variation will not cause a significant change in appearance and will not cause any problems. However, in applications such as inserting it into a tube, high precision is required as the gap with the inner wall of the tube will affect the diameter variation. The preferred diameter variation is 30% or less of the optical fiber diameter, preferably 20% or less, and even more preferably 10% or less.

Furthermore, it is preferable that the polymer that constitutes the plastic optical fiber of the present invention does not contain fluorine. In other words, it is preferable that the core, first cladding, and second cladding are composed of polymers that do not contain fluorine. Here, "fluorine-free" means that the polymer that constitutes the optical fiber and the composition of the additives added during the manufacturing process do not contain fluorine atoms. This does not apply to unintentional contamination during manufacturing.

The present invention will be explained in more detail below using examples. Evaluations for each example and comparative example were carried out using the following methods.

(1) Measurement of core diameter and cladding thickness (measurement of optical fiber cross section dimensions):
The optical fiber was cut perpendicular to the drawing direction with a free cutter, and the cross section was polished with lapping film of No. 600 to 15000. The cross section where the core/clad interface became clear was observed with a microscope (Keyence Corporation, VHX-7100).

(2) Transmission loss (transmission loss per unit length (dB/km) calculated using the cutback method):
An LED light source was connected to one end (incident side) of an optical fiber cut to a sample length Ls = 10 m, and a spectrometer and optical power meter were installed on the other end (exit side) to measure the optical power value _P1 (dBm) at a wavelength of 650 nm. Next, with the incident side fixed, the optical fiber was cut to a reference length Lr = 2 m, and the optical power value _P2 (dBm) on the exit side was measured again, and the transmission loss per unit length (dB/km) was calculated using the following formula (1).
Transmission loss (dB/km)=(P ₁ −P ₂ )/(Ls−Lr)×1000 (Equation 1)

(3) Breaking strength (breaking strength of plastic optical fiber):
The plastic optical fibers obtained in each of the examples and comparative examples were subjected to a tensile test at a tensile speed of 100 mm/min in accordance with JIS C 6837 (2015), and the load at which the plastic optical fiber broke (breaking strength) was measured. Three tests were conducted for each sample, and the average value was used.

(4) Diameter fluctuation (measurement of the outer diameter variation of optical fiber):
The outer diameter of the optical fiber during spinning was measured using a Keyence LS-9006M dimension measuring instrument. Measurements were made 1 million times at 0.2 mm intervals, and the standard deviation (σ) of the outer diameter fluctuation was calculated. σ<20 μm was rated as good, 20 to 30 μm was rated as acceptable, and σ>30 μm was rated as unacceptable.

(5) Composition ratio (measurement of composition ratio of clad materials used in each example and comparative example):
The composition ratio was determined using solid 19F-NMR (AVANCE NEO 400 manufactured by Bruker) and FT-IR (FT-IR manufactured by Bio-Rad Digilab).

(6) Bending loss (loss of light intensity due to bending of optical fiber) (flexibility):
For the plastic optical fiber obtained in each example and comparative example, a 650 nm wavelength LED was connected as a light source to one end of the fiber cut to 1 m, and the amount of light emitted from the other end of the fiber was measured with a power meter. The bending state was measured by wrapping the optical fiber 360 degrees around a metal rod with a radius of 5 mm. The value obtained by subtracting the initial optical power value from the optical power value in the bent state was used as an index of bending resistance. The smaller the absolute value of the difference in optical power values, the better the bending resistance. A variation in the absolute value of less than 0.25 was rated as good, 0.25 to 0.5 as fair, and more than 0.5 as unsatisfactory.

(7) Heat resistance (heat resistance evaluation of optical fiber):
A plastic optical fiber cut to a length of 10 m was placed in a thermo-hygrostat (PL-1J manufactured by Espec Corporation), and 50 cm of each end of the optical fiber was pulled out from the cable hole on the side of the device. An LED with a wavelength of 650 nm was connected as a light source to one end of the pulled-out optical fiber, and a power meter was connected to the other end of the fiber to measure the optical loss. Next, the thermo-hygrostat was set to 85°C, and the optical loss was measured every 24 hours until 200 hours had passed, and changes in transmission loss were confirmed. A change of 1 dB or more was considered to be a significant change.

[Example 1]
According to the composition table in Table 1, 100 wt% polystyrene with a styrene residue content of 100 mol% was used as the core material, 3 wt% methyl methacrylate/methyl acrylate copolymer was used as the first cladding material, and 100 wt% polypropylene homopolymer with 100 mol% propylene was used as the second cladding material. These materials were fed into a composite spinning machine and subjected to core-sheath composite spinning at 240°C to obtain a plastic optical fiber with a fiber diameter of 300 μm (core diameter 240 μm, first cladding thickness 10 μm, second cladding thickness 20 μm). The resulting optical fiber had a good transmission loss of 770 dB/km. Next, the fiber was stretched 1.5 times in a heating furnace at 130°C. The breaking strength of the optical fiber improved, but the transmission loss decreased to 2400 dB/km. The diameter fluctuation at this time was σ = 16.8 μm, which was good. The bending loss was -0.39 dBm, which was judged to be acceptable. The polypropylene coating was in good condition, and there were no particular problems with handling the fiber. Based on the above results, there were no problems with transmission loss, processability (fiber diameter fluctuation), or bending loss. However, since transmission loss decreased during stretching, it was determined that the fiber was unsuitable for stretching. In a heat resistance test for 200 hours in an 85°C environment, there was a slight tendency for transmission loss to increase, but the change was less than 1 dB, so there was no problem.

[Example 2]
A plastic optical fiber was produced in the same manner as in Example 1, except that a random copolymer polypropylene obtained by copolymerizing 98.5% by weight of propylene and 1.5% by weight of ethylene was used as the second cladding material according to the composition table in Table 1. The transmission loss of the obtained optical fiber was a good 760 dB/km, and the result after a 1.5-fold heat drawing treatment was 1300 dB/km. The diameter fluctuation was good at σ=18.6 μm. The bending loss was -0.24 dBm, which was judged to be good. The state of the polypropylene coating was good, and there were no particular problems with handling the optical fiber. In the heat resistance test, as in Example 1, no significant change in transmission loss was observed.

[Examples 3 to 6]
Plastic optical fibers were produced in the same manner as in Example 1, except that a random copolymer polypropylene in which the copolymerization ratio of propylene and ethylene was adjusted was used as the second cladding material according to the composition table in Table 1. The transmission loss of the obtained optical fibers was good at 750 dB/km in all cases, and the results after 1.5x heat drawing treatment were also good at 830 to 860 dB/km. The diameter fluctuation was good at σ<30 μm in all cases. The bending loss was -0.20 to -0.21 dBm and was judged to be good. The condition of the polypropylene coating was good, and there were no particular problems with handling the optical fiber. No significant change in transmission loss was observed in the heat resistance test.

[Comparative Example 1]
A plastic optical fiber with an outer diameter of 260 μm was produced in the same manner as in Example 1, except that a second cladding was not formed according to the composition table in Table 1. The resulting optical fiber had a good transmission loss of 690 dB/km, and even after 1.5x heat drawing, the result was also good at 820 dB/km. The diameter fluctuation was good at σ = 19.0 μm, but the bending loss was unacceptable at -0.56 dBm. Because the optical fiber was not covered with a second cladding, the optical fiber was prone to deterioration in transmission loss due to dirt or scratches caused by handling, and the optical fiber was prone to breakage when bent, making it difficult to handle. In a heat resistance test, the transmission loss gradually increased after heating to 85°C, and after 72 hours, it had deteriorated by more than 1 dB.

[Example 7]
A plastic optical fiber was fabricated in the same manner as in Example 1, except that a random copolymer polypropylene, copolymerized with 79% by weight of propylene and 21% by weight of ethylene, was used as the second cladding material according to the composition table in Table 1. The resulting optical fiber had a good transmission loss of 750 dB/km, but after a 1.5x heat stretching treatment, the loss tended to decrease to 1080 dB/km. Furthermore, possibly due to the low melting point of the second cladding, the diameter fluctuation was large at σ = 43.8 μm, and no improvement in transmission loss was observed after heat stretching treatment. Therefore, it was determined that further increase in the amount of ethylene units would be outside the preferable range. The bending loss was -0.20 dBm, which was determined to be good. The condition of the polypropylene coating was good, and there were no particular problems with handling the optical fiber. No significant change in transmission loss was observed in the heat resistance test.

[Example 8]
According to the composition table in Table 1, a polycarbonate resin containing 99% or more by weight of carbonate resin as the core material, 3% by weight of a 97% by weight methyl methacrylate/methyl acrylate copolymer as the first cladding material, and 100% by weight of a polypropylene homopolymer containing 100 mol% propylene as the second cladding material were fed into a conjugate spinning machine and subjected to core-sheath conjugation spinning at 245°C to obtain a plastic optical fiber having a fiber diameter of 260 μm (core diameter 220 μm, first cladding thickness 5 μm, second cladding thickness 15 μm). The resulting optical fiber had a transmission loss of 1420 dB/km and a breaking strength of 5.0 N. Next, when the optical fiber was stretched 2.0 times in a heating furnace at 135°C, the breaking strength of the optical fiber was significantly improved to 8.3 N, but the transmission loss decreased to 8420 dB/km. The fiber diameter fluctuation at this time was σ = 24.5 μm. The bending loss was -0.48 dBm, which was judged to be acceptable. The condition of the polypropylene coating was good, and there were no particular problems with handling the fiber. In a heat resistance test for 200 hours in an 85°C environment, there was almost no change in the transmission loss, which was good.

[Examples 9 to 13]
A plastic optical fiber was fabricated in the same manner as in Example 8, except that a random copolymer polypropylene, in which the copolymerization ratio of propylene and ethylene was adjusted, was used as the second cladding material according to the composition table in Table 1. The resulting optical fiber had a transmission loss of 1350 to 1380 dB/km and a breaking strength of 5.0 to 5.1 N. When the fiber was then stretched 2.0 times in a heating furnace at 135°C, the breaking strength of the optical fiber improved to 8.1 to 8.3 N, but the transmission loss decreased to 3520 to 4810 dB/km. The diameter fluctuation at this time was σ = 17.3 to 24.8 μm. The bending loss was -0.43 to -0.40 dBm. The polypropylene coating was in good condition, and the fiber was easy to handle. A heat resistance test at 85°C for 200 hours showed almost no change in transmission loss, indicating good performance.

[Example 14]
A plastic optical fiber was fabricated in the same manner as in Example 8, except that a random copolymer polypropylene copolymerized with 79% by weight of propylene and 21% by weight of ethylene was used as the second cladding material according to the composition table in Table 1. The resulting optical fiber had a transmission loss of 1350 dB/km and a breaking strength of 5.1 N. Next, when the fiber was stretched 2.0 times in a heating furnace at 135°C, the breaking strength of the optical fiber was 8.3 N and the transmission loss was 3990 dB/km. The diameter fluctuation at this time was σ = 32.6 μm, and the bending loss was -0.40 dBm. While the diameter fluctuation increased, no improvement in transmission loss after the thermal stretching treatment was observed, so it was determined that further increases in the amount of ethylene units would deviate from the preferable range. The polypropylene coating was in good condition, and the fiber was easy to handle. A heat resistance test for 200 hours at 85°C showed almost no change in transmission loss, indicating favorable results.

[Comparative Example 2]
A plastic optical fiber with an outer diameter of 230 μm was fabricated in the same manner as in Example 8, except that the second cladding was not formed according to the composition table in Table 1. The resulting optical fiber had a transmission loss of 1160 dB/km and a breaking strength of 3.3 N. After a 2.0-fold heat stretching treatment, the transmission loss changed to 1210 dB/km and the breaking strength to 7.7 N. The diameter fluctuation was good at σ = 11.2 μm, but the bending loss was unacceptable at -0.68 dBm. Because the optical fiber was not covered with a second cladding, contamination or scratches during handling of the optical fiber were likely to reduce the transmission loss, making the optical fiber difficult to handle. In a heat resistance test, the transmission loss gradually increased after heating at 85°C, and after 96 hours, it had deteriorated by more than 1 dB.

[Reference example]
A plastic optical fiber was produced in the same manner as in Example 1, except that a fluororesin consisting of 75% by weight of vinylidene fluoride and 25% by weight of tetrafluoroethylene was used as the second cladding material in accordance with the composition shown in Table 1.

The properties of the optical fibers obtained in the above Examples, Comparative Examples, and Reference Examples are shown in Table 2. The abbreviations in Table 1 have the following meanings:
St: polystyrene, PC: polycarbonate, MMA: methyl methacrylate, MA: methyl acrylate, PP: propylene, Et: ethylene, 2F: vinylidene fluoride, 4F: tetrafluoroethylene

The plastic optical fiber of the present invention can be suitably used for wiring inside moving vehicles such as automobiles, aircraft, ships, and trains; short-distance communication wiring for AV (audio-visual) equipment, home appliances, office equipment, etc.; medical endoscopic lighting; ophthalmic surgical lighting; laparoscopic surgical lighting; catheter lighting; microscope lighting; light-guiding sensors for robots; photoelectric sensors for industrial equipment; automobile collision sensors; decorative wall lighting; and interior lighting.

Claims (6)

A plastic optical fiber having a core, a first cladding, and a second cladding formed in that order, wherein the first cladding is made of a polymer of methyl methacrylate and/or a copolymer containing methyl methacrylate as the main component, and the second cladding is made of a polyolefin resin. The plastic optical fiber according to claim 1, wherein the polyolefin resin constituting the second cladding is polypropylene, a copolymer of ethylene and propylene. The plastic optical fiber according to claim 1 or 2, wherein the core is made of a polymer whose main component is styrene, cycloolefin, or carbonate. A plastic optical fiber according to claim 1 or 2, wherein the core, first cladding, and second cladding are made of a fluorine-free polymer. A fabric made using the plastic optical fiber described in claim 1 or 2. A knitted fabric using the plastic optical fiber described in claim 1 or 2.