WO2007052814A1

WO2007052814A1 - Plastic optical fiber and production method thereof

Info

Publication number: WO2007052814A1
Application number: PCT/JP2006/322214
Authority: WO
Inventors: Kosuke Yamaki; Yukio Shirokura
Original assignee: Fujifilm Corporation
Priority date: 2005-11-02
Filing date: 2006-10-31
Publication date: 2007-05-10

Abstract

An inner clad section forming material constituted of a compound which generates amorphous polymer is poured into a hollow portion of a cylindrical outer clad section (11) formed of crystalline fluoropolymer. The inner clad forming material is polymerized while the outer clad section (11) is rotated and heated. The inner clad section forming material is diffused into an inner wall of the outer clad section (11) swelled by the heat to form a cylindrical mixed section (22) which has a lower degree of crystallinity than the outer clad section (11), and a cylindrical inner clad section (12) on an inner side of the mixed section (22). Thereafter, a core section forming material is poured in a hollow portion of the inner clad section (12) and polymerized to form a preform (14). The preform (14) is heat-drawn to obtain a POF (15) having a mixed layer (45) formed between an outer clad (43) and an inner clad (44).

Description

DESCRIPTION

PLASTIC OPTICAL FIBER AND PRODUCTION METHOD THEREOF

Technical Field

The present invention relates to a plastic optical fiber and a production method thereof.

Background Art Plastic optical fibers, hereinafter referred to as POF are widely used as plastic optical materials. Since the plastic optical fibers are formed exclusively of plastics, the plastic optical fibers have advantages such as lightweight, excellent workability, and low production cost compared to quartz based plastic optical fibers . The plastic optical fibers also have other advantages such as high bending tolerance due to high flexibility and easily increasable diameters. However, on the other hand, these advantages increase the optical transmission losses in the plastic optical fibers, making the plastic optical fibers not suitable for optical transmittance for long distance. However, the plastic optical fiber attracts attention as light transmission medium for short distance such as domestic use, vehicle-mounted use, or the like in which the transmission losses are not concerned. POF is constituted of a core and a clad. The core is formed of polymer as a matrix. The clad is formed on the outer periphery of the core and has lower refractive index than the core. The light propagates through the core, and the clad prevents the light leakage thereof. The plastic optical fiber functions as the light transmission medium by totally reflecting the light at the interface between the core and the clad having different refractive indexes. Recently, a Graded Index (GI) type POF attracts attention as the POF having high transmission capacity. In the GI POF, the refractive index is reduced from a center toward the outside in its diameter direction. By virtue of this structure, light scattering and irregular reflection are prevented and therefore the GI POF achieves high-speed transmission with low transmission loss.

The POFs including the GI POFs are produced by heat-drawing preforms each of which is constituted of a core section and a clad section. To form the preform, a polymerizable composition which forms the core is poured into a hollow portion of the cylindrical clad section, which will be rotated and polymerized to form the core section. By heat-drawing the preform, the core section and the clad section of the preform will be the core and the clad respectively. Since the POF is constituted of plastic, it exhibits excellent bending tolerance. However, on the other hand, bending causes transmission loss (hereinafter referred to as a bending loss) . The bending loss significantly lowers the function of the POF as the optical transmission medium, which is not preferable. To reduce the bending loss, a GI POF formed of fluorine-containing polymer is suggested in, for instance, Japanese Patent Laid-Open Publication No. 08-304636.

In addition, following POFs are suggested: a plastic fiber cable which has cavities between an outer periphery of a POF and a coating layer (for instance, see Japanese Patent Laid-Open Publication No.09-218327), a plastic fiber cable having a coating layer formed of resin foam around an outer periphery of a POF (for instance, see Japanese Patent Laid-Open Publication No.09-236735) , a POF provided with two coating layers having different elastic modulus around its outer periphery (for instance, see Japanese Patent Laid-Open Publication No. 11-337781, and a POF having an inner layer formed of non-crystalline fluorine-containing polymer without C-H bonds and an outer layer formed of fluorine-containing polymer (for instance, U.S. Patent Application Publication No. 2002/009276, corresponding to Japanese Patent Laid-Open Publication No. 2002-071972). Further, a method for forming a POF is suggested. In this method, a non-crystalline thermoplastic resin composition is coated around an outer periphery of a preform by extrusion-coating. Thereafter, the preform is drawn (or heat-drawn) to form the POF (see Japanese Patent Laid-Open Publication No. 2000-098144.)

However, the method disclosed in Japanese Patent Laid-Open Publication No. 08-304636 and 2002-071972 cannot reduce the bending loss sufficiently. In the case the crystalline fluorine-containing polymer is used for the outer layer, projections and rejections are formed by spherulite on the inner wall of the outer layer. As a result, the bending loss cannot be reduced by light scattering occurred at the interface between the inner layer and the outer layer. On the other hand, in the case non-crystalline fluorine-containing polymer is used for forming the outer layer, it is inferior in solvent resistance to a monomer. As a result, it becomes difficult to form a core which will be the optical transmission path out of the monomer. In Japanese Patent Laid-Open Publication No. 09-218327 and 09-236735, diameter fluctuations of POF and exacerbation of non-circularity may occur since control of the size and condition of the cavity formed between the POF and the protective layer, and of the foam in resin foam are difficult.

In Japanese Patent Laid-Open Publication No. 11-337781, since it is difficult to inhibit the light leakage and/or the optical loss caused at the interface between the clad and the core, the bending loss cannot be reduced sufficiently. In Japanese Patent Laid-Open Publication No. 2000-098144, since the diameter of the core which is the optical transmission path becomes extremely small, it becomes difficult to connect the POFs. As a result, the workability is reduced. Further, this method requires a process to coat the non-crystalline thermoplastic resin composition around the preform so that the productivity is reduced.

An object of the present invention is to provide a POF in which the bending loss caused by bending stress and the light scattering is reduced. Another object of the present invention is to provide a production method of the above POF without reduction in productivity.

Disclosure of Invention

In order to achieve the above objects and other objects, a plastic optical fiber of the present invention includes a cylindrical outer clad formed of crystalline fluorine-containing polymer, a cylindrical inner clad formed of amorphous polymer and formed in a hollow portion of the outer clad, a mixed layer formed between the outer clad and the inner clad, having a lower degree of crystallinity than the outer clad, and a core filled in a hollow portion of the inner clad, having a higher refractive index than the inner clad.

The mixed layer is formed by diffusion of a compound, which forms the inner clad, into the outer clad during thermal polymerization of the compound. A thickness t(μm) of the mixed layer satisfies 2.0 ≤ t ≤ 5.5. A refractive index of the core gradually decreases from a center toward outside in a diameter direction. A refractive index of the mixed layer gradually decreases from an inner clad side toward an outer clad side . A producing method for a plastic optical fiber includes the following steps: preparing a cylindrical outer clad section formed of crystalline fluorine-containing polymer; putting first compound which generates amorphous polymer into a hollow portion of the outer clad section; polymerizing the first compound to diffuse the first compound into the outer clad section to form a mixed section and a cylindrical inner clad section which is formed on an inner side of the mixed section, and the mixed section has a lower degree of crystallinity than the outer clad section; putting a second compound which forms a core section into a hollow portion of the inner clad section; polymerizing the second compound to form a preform; and heat-drawing the preform to form a plastic optical fiber.

The thickness t(μm) of the mixed section after the heat-drawing satisfies 2.0 ≤ t ≤ 5.5. A polymerization temperature of the first compound is in a range of 50" C to 100⁰C. The preform is drawn while being heated in a range of 220" C to 300⁰C.

According to the present invention, the POF has the cylindrical outer clad formed of crystalline fluorine-containing polymer, the cylindrical inner clad formed of amorphous polymer, the mixed layer, and the core. The mixed layer is formed by diffusion of a compound which generates the inner clad into the outer clad. The refractive indexes of the core and the mixed layer are adjusted to be gradually reduced from the center toward the outside in the diameter direction. Since this mixed layer confines the light in the core, the bending losses caused by the bending stress and the light leakage are reduced. Since the thickness of the mixed layer t (μm) satisfies the range of 2.0 ≤ t ≤ 5.5, the mixed layer sufficiently reduces the bending losses.

Since the mixed section of the preform is formed by polymerizing the first compound to diffuse the first compound into the outer clad section, new production facilities are not necessary. Moreover, the thickness and the polymerization degree of the mixed section are easily controlled by adjusting the polymerization temperature. Since the POF is formed by heat-drawing the preform, the interdiffusion of the mixed section is further promoted by the heat to form the mixed layer with appropriate thickness. Accordingly, the present invention enables to produce the POF whose bending loss is sufficiently reduced without the reduction of the productivity.

Brief Description of Drawings

FIG.l is a schematicview illustrating a POF production process of the present invention;

FIG .2 is a schematic view of an example of a furnace used for heat-drawing a preform;

FIG.3A is a cross-section of an example of a preform of the present invention, and FIG.3B is a schematic drawing illustrating an example of a refractive index of the preform in a diameter direction;

FIG.4A is a cross-section of an example of a POF of the present invention, and FIG.4B is a schematic drawing illustrating an example of a refractive index of the POF in a diameter direction; FIG.5 is a cross-section of an example of a polymerization container;

FIG.6 is a schematic view of an example of a rotation polymerization device; and

FIG.7 is an explanatory view of .a rotation method of the rotation polymerization device.

Best Mode for Carrying Out the Invention

The preferable embodiments of the present invention are hereinafter described in detail, but these embodiments do not limit the scope of the present invention. In the present invention, a region formed between an outer clad section and an inner clad section is referred to as a mixed layer. The mixed layer includes a region in which an outer clad section forming material and an inner clad section forming material are mixed. Details will be described in the following.

In FIG.l, a POF 15 of the present invention is produced by heat-drawing a preform 14 having a core section 13 and a clad section. The clad section includes an outer clad section 11 and an inner clad section 12. The core section 13 is formed inside the inner clad section 12 (See FIG.3). As shown in FIG. 1, the POF production process 10 includes an outer clad section forming process 16, an inner clad section forming process 17, a core section forming process 18, and a heat-drawing process 19.

First, the outer clad section 11 is formed in the outer clad section forming process 16. The outer clad section 11 is a polymer pipe which is an outer shell of the preform 14. For instance, the outer clad section 11 can be formed as the outer clad section pipe by melt-extrusion of a polymer. This method is used in the following embodiments. However, the method for forming the outer clad section 11 is not particularly limited. An outer clad section forming material and an inner clad section forming material will be described in detail later.

Next , in a supplying process 20 of inner clad section forming material, a polymerizable composition for forming the inner clad section 12 (hereinafter referred to as an inner clad section forming material) is poured into the outer clad section 11. In a diffusion-polymerization process 21, the inner clad section forming material is polymerized.

In the diffusion-polymerization process 21, a thermal polymerization in which the inner clad section forming material is polymerized while being heated up to a desired temperature is performed. At this time, a polymerization temperature is adjusted in a range of 50" C to 100° C, more preferably, in a range of 60⁰C to 100⁰C, and especially preferably, in a range of 70° C to 100⁰C. Thereby, the polymerization of the inner clad section forming material is efficiently promoted. However, when the polymerization temperature exceeds 100° C in the diffusion- polymerization process 21, the inner clad section forming material, e.g. Methyl methacrylate (MMA), comes to a boil, generating foam during the polymerization. The foam in the polymer drastically lowers the optical properties, for instance, refractive index values may be reduced at points where the foam is generated, transmission loss is_increased, and the like, which is not preferable. On the other hand, when the polymerization temperature drops to less than 50"C, the inner clad section forming material cannot be efficiently polymerized. Moreover, this polymerization temperature is too low for swelling an inner wall of the outer clad section 11. As a result , it becomes difficult to form a mixed section by diffusing and mixing the inner clad section forming material. The polymerization time is preferably in a range of 4 to 30 hours to achieve sufficient polymerization without reducing productivity. It is more preferable that the polymerization time is in a range of 6 to 25 hours . It is especially preferable that the polymerization time is in a range of 8 to 20 hours. Polymerization temperature and polymerization time can be properly selected within the above ranges , in accordance with the sorts and compositions of the outer clad section forming material and the inner clad section forming material. In the present invention, in the diffusion-polymerization process 21, a mixed section 22 is formed concurrently with the inner clad section 12 by the polymerization of the inner clad section forming material. The mixed section 22 is a region where the outer clad section forming material and inner clad section forming material are mixed between the outer clad section 11 and the inner clad section 12. During the thermal polymerization of the inner clad section forming material in the diffusion-polymerization process 21, the inner clad section forming material permeates into the inner wall of the outer clad section 11 by the heat . This causes the polymerization of the inner clad section forming material in a state that the inner wall of the outer clad section 11 is swelled. Owing to this, the inner clad forming material is mixed into a part of the outer clad section

11 (especially the proximity of the inner wall) so that the mixed section 22 is formed. By further promoting the diffusion-polymerization process 21, the inner clad section forming material is sufficiently polymerized and thus the cylindrical inner clad section 12 is formed inside the. mixed section 22.

In the core section forming process 18, a material for forming the core section 13 (hereinafter referred to as a core section forming material) is poured into the inner clad section

12. Then, the core section forming material is polymerized to form the core section 13. Thus, the preform 14 is formed. The preform 14 has the mixed section 22 and the inner clad section 12 inside the outer clad section 11, and the core section 13 formed inside the inner clad section 12.

In the heat-drawing process 19, as shown in FIG.2, the preform 14 is set inside a furnace 25. The preform 14 is heated by the furnace 25 so that the preform 14 is partially softened. A tip 14a of the softened portion is drawn (stretched), and thus the POF 15 is obtained. The POF 15 is wound by a winding roll 27 of a winding device ( not shown ) in a roll form after passing through a diameter monitor 26. An outer diameter of the POF 15 is constantly monitored by the diameter monitor 26. In response to the monitored results, the position of the preform 14 in the furnace 25, the temperature in the furnace 25, winding speed of the winding device, and the like are properly adjusted to maintain the uniform diameter. The temperature inside the furnace 25 during the heat-drawing process 19 is in a range of 220° C to 300⁰C, and more preferably adjusted in a range of 230° C to 300° C . It is especially preferable that the temperature inside the furnace 25 is in a range of 240⁰C to 300° C. At this high temperature, the diffusion of the outer clad section forming material and the inner clad section forming material is advanced so that the polymerization of the mixed section 22 is further promoted. As a result, as shown in FIG. 4A, a mixed layer 45 with a sufficient thickness is obtained. However, if the heat-drawing temperature exceeds 300° C in the case PVDF ( polyvinylidene difluoride) is used as the outer clad section forming material, PVDF is decomposed into hydrogen fluoride gas by heat. As a result, foam is generated in the outer clad section 11. The foam inside the preform 14 lowers the optical properties of the POF 15, which is not preferable.

It is also possible to increase the thickness of the mixed layer 45 by extending the heat-drawing time. To increase the thickness of the mixed layer 45, it is preferable to extend the heat-drawing time as much as possible while lowering the furnace temperature within the above range to reduce the heating value applied to the preform 14 per unit time. However, the extended heat-drawing time may cause problems, e.g. reduction of the productivity resulting from longer production time and deterioration of the sections in the POF 15 due to longer exposure to heat . To obtain the mixed layer 45 with the sufficient thickness and to keep the excellent productivity at the same time, the heat-drawing time is preferably in a range of 3 minutes to 80 minutes. It is more preferable that the heat-drawing time is in a range of 5 minutes to 60 minutes. It is especially preferable that the heat-drawing time is in a range of 7 minutes to 40 minutes . Further, it is preferable to appropriately adjust the heat-drawing rate in a range of 3 m/min to 15 m/min in consideration to the above heat-drawing time and the desired diameter of the POF 15 to be produced.

The heat-drawing temperature and the heat-drawing time are properly determined within the above range in accordance with the material of the preform 14. A ratio of the diameter of the POF 15 to that of the preform 14 is properly determined in consideration of the diameter of the preform 14, the desired diameter of the POF 15, the material of the preform 14, and the like. Note that in the gradedindex plastic optical fiber- (GI POF) , the refractive index gradually changes from the center toward the outer periphery thereof in the diameter direction. In order to achieve such desired changes in the refractive index, the heating and heat-drawing must be performed uniformly to the preform 14. For this reason, the furnace 25 of a cylindrical form is preferable to uniformly heat the preform 14 concentrically in the diameter direction.

Inside the furnace 25, it is preferable to install an inert gas supplying device (not shown) . The inert gas is supplied inside the furnace 25 to create an inert atmosphere which prevents the deterioration of the preform 14 caused by heating. The inert gas is not particularly limited. Nitrogen gas, helium gas, neon gas, argon gas, or the like can be used. In terms of cost, nitrogen gas is preferable. In terms of thermal conductivity efficiency, helium gas is preferable. It is also possible to use a mixture gas in which plural sorts of gases are mixed such as the mixture gas of helium gas and argon gas .

Drawing tension of the preform 14 is preferably not less than 0.098N to allow the softened polymer to be oriented as disclosed in Japanese Patent Laid-Open Publication No. 7-234322. Further, the drawing tension is preferably not more than 0.98N to eliminate distortion of the molecular orientation after the heat-drawing as disclosed in Japanese Patent Laid-Open Publication No. 7-234324. Since the optimum drawing tension varies depending on the diameter or the sorts of the materials of the POF 15 to be produced, it is necessary to adjust the drawing tension accordingly. It is also possible to pre-heat the preform 14 prior to the heat-drawing as described in Japanese Patent Laid-Open Publication No. 8-106015.

A plastic optical fiber cord (hereinafter referred to as a cord) is obtained by providing a protective coating layer of resin around the outer periphery of the POF 15. A method of providing the protective coating layer is not particularly limited. For instance, it is possible to use an extrusion method using thermoplastic resin. In this case, the same resin as or different sorts of resin from that used for the POF 15 can be applied. Thereby, the cord having desirable properties such as flame-resistance and weather-resistance is obtained. The protective coating layer forming process can be performed after the heat-drawing in the POF production process 10 shown in FIG.1, or in a separate process from the POF production process 10. It is also possible to form a plastic optical fiber cable (hereinafter referred to as a cable) by binding the several cords and coating the outer periphery thereof by resin. In the present invention, the optical fiber provided with the protective coating layer around the outer periphery of the POF 15 is referred to as a plastic optical fiber core wire, or the cord. The cord provided with additional coatings as necessary is referred to as a single fiber cable. When plural cords are bound with tensile strength wire and the coating is applied to the outer periphery thereof, it is referred to as a multi-fiber cable. The single fiber cable and the multi-fiber cable are collectively called as the cables. In FIG. 3A, the preform 14 has a clad section 30 which is an outer shell to confine light, and the core section 13 which is an optical transmission path. The clad section 30 is constituted of the outer clad section 11 and the inner clad section 12. The outer clad section 11 is an outer region of the clad section 30. The inner clad section 12 is a region inside the outer clad section 11. The outer clad section 11 and the inner clad section 12 are cylindrical members . Outer and inner diameters thereof and the thicknesses thereof are constant in the lengthwise direction. The outer clad section 11 is formed of crystalline fluorine-containing polymer. In the present invention, the cylindrical outer clad section 11 is formed by the melt-extrusion of PVDF. The outer clad section 11 formed by this method has low elastic modulus. Owing to this, bending deformation of the core section 13 is prevented even when the core section 13 is formed of materials with high elastic modulus. As a result, the bending loss is reduced.

The inner clad section 12 is constituted of an amorphous polymer. In this embodiment, MMA which is the inner clad section forming material is poured into a hollow portion of the cylindrical outer clad section 11 and thermally polymerized to form the inner clad section 11 whose main component is polymethyl methacrylate (hereinafter referred to as PMMA) . The outer clad section 11 and the inner clad section 12 are not limited to polymers constituted of monomers of a single type. The outer and inner clad sections 11 and 12 can be constituted of polymers formed by polymerization of oligomers such as dimers and trimers , or of plural types of polymerizable compositions.

In the inner clad section forming process 17, it is preferable to rotate the outer clad section 11 after the inner clad section forming material is poured therein prior to the start of the polymerization. Thereby, the inner wall of the outer clad section 11 is swelled and permeated into the inner clad section forming material, which improves the smoothness of the outer clad section 11. It is preferable that the rotational axis is a center of a circular cross section of the outer clad section 11 in the horizontal direction. The direction of this rotation is not particularly limited. The mixed section 22 is formed between the outer clad section 11 and the inner clad section 12. The mixed section 22 is formed by diffusing the inner clad section forming material in the inner wall of the outer clad section 11 during the formation of the inner clad section 12 by polymerization as described in POF production process 10 (see FIG. 1). Since the mixed section 22 is a region in which the crystalline outer clad section 11 and the non-crystalline inner clad section 12 are mixed, the mixed section 22 has a lower degree of crystallinity than the outer clad section 11. Disappearance of crystallization property by blending the non-crystalline polymer and the crystalline polymer disclosed in pages 115 to 120 of "Relationship between UV transmission and solubility of PVDF/PMMA blend" (Nippon Kagaku Kaishi or Journal of the Chemical Society of Japan No. 2, 2000) helps to understand the lower degree of crystallinity in the mixed section 22. To obtain the desired thickness t2 of the mixed layer 45 of the POF 15 (see FIG. 4) by heat-drawing the preform 14, the thickness tl of the mixed section 22 (see FIG. 3) is preferably in a range of 0.01 mm to 0.8 mm. The thickness tl of the mixed section 22 is controlled by the polymerization temperature and the polymerization time in the diffusion- polymerization process 21 as described above.

The core section 13 inside the inner clad section 12 has a hollow portion 31 throughout its center portion. In the present invention, the core section forming material is a mixture of MMA and DPS (diphenylsulfide) which is a refractive index modifier (dopant) . After the core section forming material is poured into the hollow portion of the inner clad section 12, the core section forming material is subjected to interface gel polymerization. Thereby, the core section whose main component is PMMA and having refractive index profile shown in FIG.3B is formed, and thus the preform 14 is obtained. The preform 14 has the clad section 30 constituted of the outer clad section 11, the mixed section 22, and the inner clad section 12. Inside the inner clad section 12, the core section 13 is formed. In FIG. 3A, boundaries between the sections are illustrated for the sake of convenience. However, the boundaries in the preform 14_may not be clearly provided or may be disappeared as the interface gel polymerization reaction proceeds . Particularly in this embodiment , since the mixed section 22 is formed between the outer clad section 11 and the inner clad section 12, there are actually no visibly identifiable boundaries between the outer clad section 11 and the inner clad section 12. Further, in FIG. 3A, the preform 14 has the hollow portion throughout its center portion. However, a ratio between the diameter of the cross-section of the hollow portion 31 and the outer diameter of the preform 14 is not particularly limited to this embodiment . The ratio may be varied depending on production conditions. The hollow portion 31 may even disappear during the production process 10.

The outer diameter of the outer clad section 11 is not particularly limited. The diameter thereof is preferably in a range of 20 mm and 32 mm. The length thereof is preferably in a range of 600 mm and 1500 mm. The thickness of the inner clad section

12 is preferably in a range of 3 mm to 10 mm. The thickness of the core section 13 is preferably in a range of 2 mm and 10 mm.

In FIG. 3B, the horizontal axis shows a diameter direction of the preform 14, and the vertical axis shows the refractive index. The value of the refractive index increases toward the upper direction of the vertical axis. A range (A) in the diameter direction of FIG.3B indicates the refractive index of the outer clad section 11 in FIG. 3A. A range (B) in FIG. 3B indicates the refractive index of the mixed section 22 in FIG. 3A. A range (C) in FIG.3B indicates the refractive index of the inner clad section 12 in FIG. 3A. A range (D) in FIG. 3B indicates the refractive index of the core section 13 in FIG. 3A. A range (E) in FIG. 3B is the hollow portion 31 in FIG. 3A so that there is no refractive index value .

In the present invention, each section is formed of properly selected material to obtain different refractive index. The refractive index of the inner clad section forming material is higher than that of the outer clad section forming material. The refractive index of the core section forming material is higher than that of the inner clad section forming material. To adjust the refractive index to obtain the desired refractive index profile, a refractive index modifier (dopant) can be added to the material of the each section. The dopant will be described in detail later.

In this embodiment, as shown in FIG.3B, the refractive index of the core section 13 is gradually lowered from the proximity of the hollow portion 31 toward the boundary between the core section 13 and the inner clad section 12. In the clad section 30, the outer clad section 11 has the lowest the refractive index. The refractive index of the inner clad section 12 is lower than that of the core section 13, but higher than that of the outer clad section 11. The refractive index of the mixed section 22 is gradually lowered from the inner clad section 12 side toward the outer clad section 11 side. Since the mixed section 22 is formed by diffusing the inner clad section forming material in the outer clad section 11, the refractive index thereof tends to be gradually lowered from the inner clad section 12 side toward the outer clad section 11 side. However, the mixed section 22 may show different refractive index profile as the diffusion- polymerization proceeds. Thus, in this embodiment, the refractive index profile of the preform 14 is gradually lowered from the center toward the outer periphery in the diameter direction. The GI POF 15 obtained from such preform 14 has extremely low transmission loss and a wide range of transmission band.

Next, the POF 15 produced by heat-drawing the preform 14 is explained. In FIG. 4A, the POF 15 has a clad 40 and a core 41. The clad 40 and the core 41 are obtained by heat-drawing the clad section 30, and the core section 13 respectively. The clad 40 is constituted of an outer clad 43, an inner clad 44, and a mixed layer 45. The hollow portion 31 in the preform 14 has been filled and disappeared while the diameter of the preform 14 is reduced by heat-drawing.

Since the POF 15 is produced by drawing the preform 14 in the lengthwise direction, each section constituting the POF 15 is thinner than the corresponding section in the preform 14. In the present invention, at the heat-drawing, the draw ratio is adjusted such that the thickness t2 (μm) of the mixed layer 45 satisfies 2.0 ≤ t2 ≤ 5.5. Thereby, the light leakage to the outside and the light scattering loss inside the outer clad 43 are prevented, and at the same time, a resistance to bending deformation is improved. As a result, the bending loss is reduced. Even if projections and depressions due to spherulite are formed on the inner wall of the outer clad 43, it becomes possible to reduce the bending loss X of the POF 15 within a range satisfying the following mathematical expression (I). D (mm) represents the diameter of the inner clad 44 (in the diameter direction).

(I) -0.015 + 0.1 x D¹-⁵ < X < 0.3 + 0.1 x D^1-5 During the production of the POF 15, the draw ratio is adjusted to satisfy 300 < D < 710.

A vertical axis and a horizontal axis in FIG. 4B are equivalent to those in FIG.3B so that the explanations thereof are omitted. InFIG.4B, a range (F) indicates the refractive index of the outer clad 43 in FIG. 4A. A range (G) indicates the refractive index of the mixed layer 45. A range (H) indicates the refractive index of the inner clad 44. A range (I) indicates the refractive index of the core 41.

As shown in FIG. 4B, the refractive index in each section of the POF 15 in the diameter direction is approximately equivalent to that in the preform 14 shown in FIG. 3B. The outer clad 43 has the lowest refractive index. The refractive index increases in the mixed layer 45, the inner clad 44, and the core 41 in this order. The refractive index of the core 41 gradually increases toward the center of the POF 15. In the mixed layer 45, the refractive index gradually decreases from the inner clad 44 side toward the outer clad 43 side. The refractive index profile coefficient of the preform 14 is approximately equivalent to that of the POF 15. The refractive index profile coefficients of the preform 14 and the POF 15 are represented by "g" in the following equation (II) . "R" is an outer diameter of the preform 14 or the POF 15, "r" is a distance between the center of the preform 14 or the POF 15 in the diameter direction and a measurement position, "nl" is a maximum refractive index value in the preform 14 or the POF 15 in the diameter direction, "n2" is a minimum refractive index value in the preform 14 or the POF 15 in the diameter direction, and Δ is a value obtained by (nl - n2)/nl.

(II) n(r) = nl {1 - 2(r/R)⁹ x Δ}^1/2 = nl (1-2Δ)^1/2 The refractive index profile coefficients of the preform 14 and the POF 15 exemplified in this embodiment are preferably in a range of 0.5 and 4.0, more preferably in a range of 1.5 and 3.0, and ideally 2.0.

Next, the production method of the preform 14 is explained in detail. Note that this embodiment is an example of the present invention and the present invention is not limited to this example .

In FIG.5, a polymerization container 50 has a main body 50a and a pair of lids 50b. In this embodiment, the polymerization container 50, the main body 50a, and the lids 50b are made of stainless steel. As shown in FIG. 5, it is preferable that the inner diameter of the polymerization container 50 is slightly larger than the outer diameter of the outer clad section 11. The polymerization container 50 is configured such that the outer clad section 11 rotates in accordance with the rotation of the polymerization container 50, which will be described later. It is preferable to provide support members for supporting the outer clad section 11 inside the polymerization container 50 so as to support the outer clad section 11 to follow the rotation of the polymerization container.

The cylindrical outer clad section 11 is previously accommodated in the polymerization container 50. The cylindrical outer clad section 11 is formed by melt-extrusion of PVDF. This melt-extrusion is performed by using a commercially available melt-extrusion apparatus. The melt-extrusion method is not particularly limited. Any known melt-extrusion method can be used. First, a side end of the outer clad section is sealed by a plug 51. After the inner clad section forming material 52 is poured into the outer clad section 11, the other side end into which the inner clad section forming material 52 is poured is also sealed. Thereafter, the polymerization container 50 is set in a rotation polymerization device 60 shown in FIG. 6 to perform thermal polymerization. Thereby, the inner clad section 12 is formed. The plug is formed of a material which is not soluble to the core section forming material, and does not contain compounds which dissolve the plasticizers . For instance, Polytetrafluoroethylene (PTFE) can be used. Note that it is also possible to perform decompression processing to the outer clad section 11 and/or to the poured inner clad section forming material 52 as necessary before or after the inner clad section forming material 52 is poured. As shown in FIG.6, the rotation polylmerization device 60 has plural rotation support members 62 disposed in its housing 61, a driving section 63 which rotates the rotation support members 62, and a thermostat 64 for measuring the temperature in the housing 61 and for controlling the temperature based on the measured result. The rotation support member 62 has a cylindrical shape. The rotation support members 62 are approximately horizontally installed and approximately parallel to each other in the lengthwise direction such that at least one polymerization container 50 is supported by circumferential surfaces of adjacent rotation support members 62. One end of each of the rotation support members 62 is rotatably attached to a side wall of the housing 61. Each of the rotation support members 62 is independently rotated by the driving section 63. The driving section 63 is provided with a motor, a decompression device, a controller, and the like (not shown). The driving section 63 is activated and controlled by the controller.

As shown in FIG. 7, during the polymerization, each polymerization container 50 is held by two adjacent rotation support members 62. The polymerization container 50 is rotated in accordance with the rotation of the rotation support member 62 controlled by the driving section 63. In this embodiment, the polymerization container 50 is rotated by a surface-drive method. However, the rotation method is not particularly limited. Note that the rotation speed of the rotation support members 62 is preferably in a range of 500 rpm and 4000 rpm, more preferably, in a range of 1500 rpm and 3500 rpm. The rotation speed is preferably adjusted within these ranges on the basis of the polymerization conditions.

As shown in FIG. 7, a magnet 50c is attached to each of the lids 50b sealing the side ends of the polymerization container 50. A magnet 62a is also attached inside each of the rotation support members 62. These magnets 62a prevent a lift of the polymerization container 50 from the rotation support members 62 during the rotation. Other than the above method, it is also possible to place a rotation member similar to the rotation support member 62 touching the upper surface of the polymerization container 50 set in the appropriate position in the rotation polymerization device 60, and rotate this rotation support member in the same direction as the rotation support members 62. It is also possible to prevent the lift by providing a holding device above the polymerization container 50, pressing against the polymerization container 50. Since the present invention does not depend on the lift preventing method, the lift preventing method does not limit the scope of the present invention.

After pouring the inner clad section forming material 52 into the outer clad section 11, the polymerization container 50 is set in the rotation polymerization device 60 with its lengthwise direction approximately horizontal, and rotated about its axis by a desired number of times while being heated. Thereby, the polymerization of the inner clad section forming material 52 is promoted. Before this polymerization or concurrently with this polymerization, the inner clad section forming material 52 is diffused and permeated into the inner wall of the outer clad section 11. A layer formed by this diffusion is the mixed section 22.

The inner clad section 12 is formed inside the mixed section 22 along with the formation of the mixed section 22. Owing to this , the refractive index profile of the mixed section 22 is gradually reduced from the inner clad section 12 side toward the outer clad section side 11 as shown in FlG. 3B. To form the core section 13, the core section forming material is poured into the inner clad section 12. Thereafter, the polymerization container 50 is set at the appropriate position in the rotation polymerization device 60 with its lengthwise direction approximately horizontal, and rotated about its axis by a desired number of times while being heated in the same manner as above. Thereby, the inner wall of the inner clad section 12 is swelled or dissolved by the contact of the core section forming material so that a swelled layer in gel form is formed in an initial stage of the polymerization. The swelled layer in gel form accelerates the polymerization speed (that is, a gel effect) to promote the reaction at the interface of the inner clad section 12 and the core section forming material, which is referred as interfacial gel polymerization. As a result, the interfacial gel polymerization initiates from the inner wall of the inner clad section 12 and proceeds toward the center of the core section 13 in the diameter direction. At this time, smaller the molecular volume of the compound, the faster it is embedded inside the swelled liquid. As a result, the dopant with the larger molecular weight compared to other compounds is pushed from the swelled liquid toward the center of the core section 13 as the polymerization proceeds to form the core section having the refractive index profile in which the refractive index increases toward the center thereof., Before the thermal polymerization for forming the inner clad section 12, a pre-polymerization can be performed with the outer clad section 11 set in an upright position. To perform the pre-polymerization, it is also possible to rotate the outer clad section 11 around its cylindrical axis by using a rotation mechanism. Thereby, the mixed section 22 can be easily formed over the inner side of the outer clad section 11. In this embodiment, the mixed layer 22 and the inner clad section 11, and the core section 13 are uniformly formed over the inner wall of the outer clad section 11 by keeping the outer clad section 11 approximately horizontal in the lengthwise direction when rotated. A degree of parallelism with the horizontal direction is not particularly limited. However, it is especially preferable when an angle of the rotation axis is approximately 5° or less with respect to the horizontal direction.

After the inner clad section 52 is formed, the core section forming material is poured into the inner clad section 52, and polymerized to form the core section 13. At this time, interfacial gel polymerization is performed. Thus, the preform 14 having the structure shown in FIG. 3A is produced.

In this embodiment, the outer clad section 11 and the inner clad section forming material 52 are diffused to form the mixed section 22 at the time of the polymerization of the inner clad section forming material 52. This reaction is bulk polymerization. In the bulk polymerization, the polymerization is promoted while the generation of foam in each section is prevented so that each section can be formed without reducing the optical properties. To form the core section 13 inside the inner clad section 12, a reaction similar to the reaction for forming the inner clad section 12, that is, an interfacial gel polymerization is occurred at the interface between the core section 13 and the inner clad section 12. As a result, it becomes possible to obtain the core section 13 in which the foam is prevented. In the core section forming process 18, the polymerization temperature is preferably set in a range of 6O⁰C and 120° C, and the rotation speed of the polymerization container 50 during the rotation gel polymerization is preferably in a range of 500 rpm and 3000 rpm. Note that the polymerization temperature and the rotation speed can be selected to satisfy the above range in accordance with the type of the core section forming material and the like. Hereinafter, various materials used for forming the POF of the present invention are explained.

The material of the clad section including the outer clad section and the inner clad section, and the material of the core section of the preform are not limited as long as having the optical transmission function. The preferable material is organic material with high optical transparency. To prevent the light scattering, the core material is preferably amorphous polymer. It is preferable that the clad material and the core material have excellent fitness to each other. Moreover, it is preferable that the clad material and the core material are excellent in mechanical properties such as toughness. It is also preferable that the clad material and the core material are excellent in heat resistance and moisture resistance. In the present invention, crystalline fluoropolymer is used for the outer clad section forming material. The crystalline fluoropolymer is a fluorine-containing resin in which a crystalline structure area exists in a non-crystalline structure area. The crystalline structure area is an area in which long molecular chains are orderly aligned, and the non-crystalline structure area is an area in which the long molecular chains are randomly aligned. With the use of the crystalline fluoropolymer in forming the outer clad section, it becomes possible to form the POF with excellent solvent-resistance. An example of the crystalline fluoropolymer is polyvinylidene fluoride (PVDF) which is used in this embodiment. Other than PVDF, it is also possible to use polytetrafluoroethylene (PTFE) . Moreover, it is also possible to use copolymer of the fluorine resin, for instance, PVDF-based copolymer, tetrafluoroethylene-perfluoro alkylvinyl ether (PFA) random copolymer, chlorotrifluoroethylene (CTFE) copolymer, and so forth. Further, copolymer of methylmethacrylate (MMA) and fluoro(meth)acrylate such as trifluoroethyl methacrylate (FMA), hexafluoro isopropyl methacrylate, or the like.

The polymer which makes the refractive index of the outer clad section lower than that of the inner clad section is used for the outer clad section forming material. It is preferable to select the clad section forming material, especially, the outer clad section forming material with low water absorption rate to protect the core from moisture as much as possible. The outer clad may be formed from the polymer having the saturated water absorption (water absorption) of less than 1.8%. More preferably, the water absorption of the polymer is less than 1.5%, and most preferably less than 1.0%. The water absorption (%) is obtained by measuring the water absorption after soaking the sample polymer in the water of 23 ⁰C for one week, pursuant to the ASTM D 570 experiment .

A compound which generates amorphous polymer is used for the inner clad section forming material. This amorphous polymer is a resin which does not include crystallized region and in which molecular orientation is random. For instance, there are PVC (polyvinyl chloride), PS (polystyrene), ABS (acrylonitrile butadiene styrene copolymer) , PMMA (polymethyl methacrylate) , PC (polycarbonate) , and the like. These materials have high elastic modulus so that the bending deformation of the preform can be reduced. The above described crystalline polymer is excellent in solvent resistance, but it has higher mold shrinkage and is sensitive to acids and alkalis. On the other hand, the amorphous polymer has lower mold shrinkage and excellent resistance to acids and alkalis, but is inferior in solvent resistance. According to the present invention, the mixed section 22 in which the crystalline polymer and the amorphous polymer are mixed is formed between the outer clad section 11 and the inner clad section 12, and the POF 15 having the mixed layer 45 is formed by heat-drawing the preform 14 so that the mold shrinkage is reduced as much as possible while the resistance of the POF 15 to the acids and the alkalis is improved.

Examples of the inner clad section forming material and the core forming material are (meth)acrylic acid esters [(a) (meth) acrylic ester without fluorine, (b) (meta) acrylic ester containing fluorine], (c) styrene type compounds, (d) vinyl esters, bisphenol-A as the raw material of polycarbonate, and the like. In addition, homopolymer composed of one of these monomers, copolymer composed of at least two kinds of these monomers, or a mixture of the homopolymer(s) and/or the copolymer(s) can be used. When the mixture of polymers is used, a boiling point Tb thereof is defined as the lowest boiling point of the plural raw material compounds which make the mixture , or a reduced boiling point if the boiling point is reduced by making an azeotropic mixture. In addition, when the copolymer or the blend polymer is obtained from the mixture, the glass transition temperature of the copolymer or the blend polymer is defined as Tg. Among them, (meth)acrylic acid ester or fluorine-containing polymer are preferably used in forming the light transmission medium.

Examples of the (a) (meth) acrylic acid ester without fluorine are methyl methacrylate; ethyl methacrylate ; isopropyl methacrylate ; tert-butyl methacrylate; benzyl methacrylate; phenyl methacrylate; cyclohexyl methacrylate, diphenylmethyl methacrylate; tricyclo [5'2¹I¹O²'⁶] decanyl methacrylate; adamanthyl methacrylate; isobornyl methacrylate; norbornyl methacrylate; methyl acrylate; ethyl acrylate; tert-butyl acrylate; phenyl acrylate, and the like. Examples of (b) (meth) acrylic ester with fluorine are 2,2,2-trifluoroethyl methacrylate; 2, 2 , 3 , 3-tetrafluoro propyl methacrylate; 2,2,3, 3, 3-pentafluoro propyl methacrylate; 1-trifluoromethyl-2 , 2 , 2-trifluoroethyl methacrylate; 2,2,3,3,4,4,5, 5-octafluoropenthyl methacrylate;

2 , 2 , 3 , 3 , 4 , 4 , -hexafluorobutyl methacrylate, and the like.

Further, examples of (c) styrene type compounds are styrene, a-methylstyrene, chlorostyrene, bromostyrene, and the like. Examples of (d) vinylesters are vinylacetate, vinylbenzoate, vinylphenylacetate, vinylchloroacetate, and the like. Although the present invention is not limited to the above kinds of the polymerizable compounds, it is preferable that the kinds and relative proportions of the polymerizable compounds are selected such that the homopolymer or the copolymer formed of the polymerizable compounds has a desired refractive index profile in the light transmission medium formed therefrom.

When the polymer constituting the preform includes hydrogen atom (H) , it is preferable to substitute deuterium atom (D) for the hydrogen atom. By virtue of this, transmission loss may be reduced. Especially, the transmission loss in wavelengths of a near-infrared region may be reduced.

In order to use the POF 15 for the near-infrared rays, polymers such as described in Japanese Patent No. 3332922 and Japanese Patent Laid-Open Publication No. 2003-192708 are utilized. In this polymer, deuterium atom, fluorine and so forth are substituted for the hydrogen atom of a C-H bond to prevent absorption loss caused by the C-H bond. By using this kind of the polymer, it becomes possible to reduce the loss of the transmission signal light by shifting the wavelength region causing the transmission loss to the longer-wavelength. Examples of such polymers are, for instance, deuteriated polymethylmethacrylate (PMMA-d8), polytrlfluoroethylmethacrylate (P3FMA), and polyhexafluoro isopropyl-2-fluoroacrylate (HFIP 2-FA) . It is desirable that the impurities and foreign materials in the raw compound causing diffusion are sufficiently removed before polymerization so as to keep the transparency of the POF after polymerization.

Weight-average molecular weight of the polymer for forming the preform 14 is preferable to be from ten thousands to one million , in consideration of suitable drawing of the preform 14. Much preferably, the weight-average molecular weight is from thirty thousands to a half of one million. Drawing properties concern molecular weight distribution (MWD: weight-average molecular weight / number average molecular weight) as well. In a case that the MWD is too large, the drawing properties deteriorate when a constituent having extremely large molecular weight is mixed. As a result, it may become impossible to perform drawing. MWD is preferably four or less, and the more preferably three or less.

When the polymerizable compound is polymerized to produce a polymer, polymerization initiators can be used. For instance, there are various polymerization initiators which radicals, e.g. benzoil peroxide (BPO), and peroxide compound [such as tert-butylperoxy-2-ethylhexanate (PBO), di-tert-butylperoxide (PBD), tert-butylperoxyisopropylcarbonate (PBI), n-butyl-4,4-bis(tert-butylperoxy)valarate (PHV), and the like]. Other examples of the polymerization initiators are azo compounds, such as 2,2 ' -azobisisobutylonitril, 2 , 2 ' -azobis (2- methylbutylonitril) , 1, 1 ' -azobis (cyclohexane-l-carbonitryl) , 2,2' -azobis(2-methylpropane) , 2,2' -azobis( 2-methylbutane) 2,2 ' -azobis(2-methylpentane) , 2 , 2 ' -azobis( 2 , 3- dimethylbutane) , 2,2 ' -azobis( 2-methylhexane) ,

2,2' -azobis(2,4-dimethylpentane) , 2,2 ' -azobis

(2,3, 3-trimethylbutane) , 2,2' -azobis ( 2 , 4 , 4-trimethylpentane) , 3,3' -azobis ( 3-methylpentane) , 3,3' -azobis ( 3-methylhexane) , 3,3' -azobis(3,4-dimethypentane) , 3,3 ' -azobis (3-ethylpentane) , dimethyl-2, 2 ' -azobis ( 2-methylpropionate) , diethyl-2, 2 ' -azobis ( 2-methylpropionate) , di-tert-butyl-2,2' -azobis(2-methylpropionate) , and the like. Note that the polymerization initiators are not limited to the above substances. It is also possible to combine more than one kind of the polymerization initiators . In order to keep the physical properties , such as mechanical properties , thermal properties and so forth of the polymer uniform over the whole plastic optical fiber to be manufactured, it is preferable to control the polymerization degree by use of the chain transfer agent . The kind and the amount of the chain transfer agent are selected in accordance with the kinds of the polymerizable monomer. The chain transfer coefficient of the chain transfer agent to the respective monomer is described, for example, in "Polymer Handbook, 3^rd edition", (edited by J. BRANDRUP & E . H . IMMERGUT , published by JOHN WILEY&SON) . In addition, the chain transfer coefficient may be calculated through the experiments in the method described in "Experiment Method of Polymers" (edited by Takayuki Ohtsu and Masayoshi Kinoshita, published by Kagaku-Dojin Publishing Company, Inc. , 1972) . Preferable examples of the chain transfer agent are alkylmercaptans [for instance, n-butylmercaptan; n-pentylmercaptan; n-octylmercaptan; n-laurylmercaptan; tert-dodecylmercaptan, and the like], and thiophenols [for example, thiophenol; m-bromothiophenol; p-bromothiophenol; m-toluenethiol; p-toluenethiol, and the like] . It is especially preferable to use n-octylmercaptan, n-laurylmercaptan, and tert-dodecylmercaptan in the alkylmercaptans. Further, the hydrogen atom on C-H bond may be substituted by the fluorine atom (F) or a deuterium atom (D) in the chain transfer agent. Note that the chain transfer agents are not limited to the above substances. It is also possible to combine more than one kind of chain transfer agents. The dopant is a compound that has different refractive index from the polymerizable compounds to be combined. The difference in the refractive indices between the dopant and the polymerizable monomer is preferably 0.005 or larger. The dopant has the feature to increase the refractive index of the polymer, compared to one that does not include the dopant . In comparison to the polymers produced from the monomers as described in Japanese Patent Publication No.3332922 and Japanese Patent Laid-Open Publication No. 5-173026, the dopant has the feature that _the difference in solution parameter is 7 (cal/cm³)^1/2 or smaller, and the difference in the refractive index is 0.001 or larger. Any materials having such features may be used as the dopant if such material can stably exist with the polymers, and the material is stable under the polymerizing condition (such as heat and pressure conditions) of the polymerizable monomers as described above.

The dopant may be polymerizable compound, and in that case, it is preferable that the copolymer having the dopant as the copolymerized component increases the refractive index in comparison to the polymer without the dopant . Any materials having such features may be used as the dopant if such material can change the refractive index and stably exists with the polymers, and the material is stable under the polymerizing condition (such as temperature and pressure conditions) of the polymerizable compound which is the core monomer or the raw material of the inner clad. This embodiment shows the method to form refractive index profile in the core by mixing the dopant with the polymerizable compound for the core, by controlling the direction of polymerization according to the interface gel polymerizing method, and by providing gradation in concentration of the refractive index control agent as the dopant during the process to form the core. In addition, there are other methods, one example is a method in which the refractive index control agent is diffused in the previously-formed preform. Hereinafter, the core having the refractive index profile will be referred to as "graded index core" . Such graded index core realizes the graded index type plastic optical member having wide range of transmission band. Examples of the dopants are benzyl benzoate (BEN) ; diphenyl sulfide (DPS); triphenyl phosphate (TPP); benzyl n-butyl phthalate (BBP); diphenyl phthalate (DPP); diphenyl (DB); diphenylmethane (DPM); tricresyl phosphate (TCP); diphenylsulfoxide (DPSO) . Among them, BEN, DPS, TPP and DPSO are preferable. In addition, the dopant may be the polymerizable compound such as tribromo phenylmethacrylate. In this case, although it would be difficult to control various properties (especially optical property) because of copolymerization of the polymerizable monomer and the polymerizable dopant for forming a matrix, the produced dopant may be advantageous in heat resistance. It is possible to alter the refractive index of the POF 15 to a desired value by controlling the concentration and the distribution of the dopant in the core section 13.

With respect to adding quantity of the polymerization initiator, the chain transfer agent, and the dopant, it is possible to properly determine a preferable range in accordance with the kind and so forth of the core monomer. In this embodiment, the polymerization initiator is added so as to be 0.005 to 0.050 mass% relative to the core monomer. It is much preferable to set this additive rate within a range of 0.010 mass% to 0.020 mass%. Meanwhile, the chain transfer agent is added so as to be 0.10 to 0.40 mass% relative to the core monomer. It is much preferable to set this additive rate within a range of 0.15 mass% to 0.30 mass*. In addition, when the dopant is added, its additive rate is preferably set in a range of 1 mass% to 50 mass% relative to the core monomer.

Other additives may be contained in the core and the clad so far as the transmittance properties do not decrease. For example, the additives may be used for increasing weather resistance and durability. Further, induced emissive functional compounds may be added for amplifying the optical signal. When such compounds are added to the monomer, attenuated signal light is amplified by excitation light so that the transmission distance increases. Therefore, the optical member with such additive may be used as an optical fiber amplifier in an optical transmission link. These additives may be contained in-the core, the clad and a part thereof by polymerizing the additives with the various polymerizable compounds being used as the raw material .

One of production methods for the preform of the GI POF is described in Japanese Patent No. 3332922. In this method, a cylindrical resin pipe to be a clad is formed, a resin compound to be a core is poured into the hollow portion of the cylindrical pipe, and then the interface gel polymerization, which is one of the bulk, polymerization, is performed to form the core. Polymerization conditions, in this case, the polymerization temperature and the polymerization time can be properly selected according to monomers and polymerization initiators used. In addition, there are polymerization conditions described in WO 03/19252, to form a core with no concentration fluctuation. The core may be formed by another known method, in which plural kinds of polymerizable compounds are sequentially applied to form layers having different refractive indices . Note that the production method for the preform of the GI POF is not limited to the interface gel polymerization described above. As the resin compound, there are a resin compound having single refractive index in which a refractive index control agent is applied, a mixture of plural resins having different refractive indices, a copolymer and so forth. The present invention can be applied to various types of the POFs having various refractive index profiles such as a single mode type and a step index type, in addition to the GI POFs.

The POF is normally coated with at least one protective layer, for the purpose of improving flexural and weather resistance, preventing decrease in property by moisture absorption, improving tensile strength, providing resistance to stamping, proving resistance to flame, protecting from damage by chemical agents, noise prevention from external light, increasing the value by coloring, and the like _to enhance the marketability of the POF 15.

Examples of the material for the protective layer are as follows. These are effective in providing mechanical property (such as bending property) due to high elasticity. There are rubbers as the polymer, such as isoprene rubbers (for example, natural rubber and isoprene rubber), butadiene rubbers (for example, styrene-butadiene copolymer rubber and butadiene rubber) , diene special rubbers (for example, nitrile rubber and chloroprene rubber), olefin rubbers (for example, ethylene-propylene rubber, acrylic rubber, butyl rubber and halide butyl rubber), ether rubbers, polysulfide rubbers and urethane rubbers .

The material for the protective layer may be a liquid rubber that exhibits fluidity in a room temperature and becomes solidified by application of heat . Examples of the liquid rubber are polydiene rubbers (basic structure is polyisoprene, polybutadiene, butadiene-acrylonitril copolymer, polychloroprene , and so forth), polyorefin rubbers (basic structure is polyorefin, polyisobutylene, and so forth), polyether rubbers (basic structure is poly(oxypropylene) , and so forth) , polysulfide rubbers (basic structure is poly(oxyalkylene disufide), and so forth) and polysiloxane rubbers (basic structure is poly(dimethyl siloxane) , and so forth). In addition, thermoplastic elastomer (TPE) can be used for the material of the protective layer. The thermoplastic elastomer exhibits rubber elasticity at a room temperature, and becomes plasticized at a high temperature, which facilitates the molding. Examples of the thermoplastic elastomer are styrene thermoplastic elastomers, olefin thermoplastic elastomers, vinyl chloride thermoplastic elastomers, urethane thermoplastic elastomers, ester thermoplastic elastomers, amide thermoplastic elastomers, and so forth. Other materials than those described above can be used as long as the coating layer is formed at a temperature of equal to or less than the glass transition temperature Tg (⁰C) of the POF polymer. For example, it is possible to use copolymer and mixed polymer of the above described materials or other materials . A substance obtained by thermal hardening of the mixed liquid of a polymer precursors and reaction agent is also preferably used for the material of the protective layer. An example of such material is one-component thermosetting urethane composition produced from NCO block prepolymer and powder-coated amine, as described in JP-A No. 10-158353. Another example is one-component thermosetting urethane composition that is composed of urethane pre-polymer with NCO group, described in WO 95/26374, and solid amine having the size of 20μm or smaller. For the purpose of improving the properties of the primary protective layer, additives and fillers may be added. Examples of the additives are incombustibility, antioxidant, radical trapping agent, lubricant. The fillers may be made from organic and/or inorganic compound.

The POF may have a second (or more) protective layer around the above described protective layer as the first protective layer. If the first protective layer has a sufficient thickness to decrease the thermal damage to the POF, the requirement of the hardening temperature of the second protective layer becomes less strict compared with the first protective layer. The second protective layer may be provided with the additives such as fire retardants, UV absorbent, antioxidant, radical trapping agent, and lubricant. The flame retardants are a resin and an additive with halogen like bromine or with phosphorus . Metal hydroxide is preferably used as the flame retardant for the purpose of reducing toxic gas emission. The metal hydroxide contains water of crystallization, which is not removed during the manufacture of the POF. Thus, it is preferable to provide a moisture proof coat around the first protective layer and to form the metal hydroxide as the flame retardant around the moisture proof coat .

The POF may be coated with plural coat layers with multiple functions . Examples of such coat layers are the flame retardant layer described above, a barrier layer to prevent moisture absorption, moisture absorbent (moisture absorption tape or gel, for instance) between the protective layers or in the protective layer, a flexible material layer and a foaming layer as shock absorbers to relax stress in bending the POF, a reinforced layer to increase rigidity, and the like. The thermoplastic resin as the coat layer may contain structural materials to increase the strength of the optical fiber cable. The structural materials are a tensile strength fiber with high elasticity and/or a metal wire with high rigidity. Examples of the tensile strength fibers are an aramid fiber, a polyester fiber, a polyamide fiber. Examples of the metal wires are stainless wire, a zinc alloy wire, a copper wire. The structural materials are not limited to those listed above. It is also possible to provide other materials such as a metal pipe for protection, a support wire to hold the optical fiber cable. A mechanism to increase working efficiency in wiring the optical fiber cable is also applicable.

The POF manufactured according to the present invention is suitable for a plastic optical cable. In accordance with the way of use, the POF is selectively used as a cable assembly in which the POFs are circularly arranged, a tape core wire in which the POFs are linearly aligned, a cable assembly in which the tape core wires are bundled by using a band or LAP sheath, or the like. When an optical device containing the POF according to the present invention is used, it is preferable to ensure to fix the terminal of the optical device by using an optical connector. The optical connectors widely available on themarket are PN type, SMA type, SMI type, FO5 type, MU type, FC type, SC type and the like.

The POF of the present invention can be used with various kinds of light emitting elements. Preferable one is Vertical Cavity Surface-Emitting Laser (VCSEL) described in Japanese Patent Laid-Open Publications No.7-307525, No.10-242558 , No.2003-152284, which has a narrow divergence angle and a high accumulation ratio, is actuated by relatively low-current, and can change a laser emission wavelength according to an elemental ratio. In addition, the present invention can be applied to a system to transmit optical signals, which uses optical signal processing devices including optical components, such as a light receiving element, an optical switch, an optical isolator, an optical integrated circuit, an optical transmitter and receiver module, and the like. Any known techniques can be applied to such system. The techniques are described in, for example, " 'Basic and Practice of Plastic Optical Fiber' (published by NTS Inc.)", "'Optical members can be Loaded on Printed Wiring Assembly, at Last' in Nikkei Electronics, vol. Dec. 3, 2001", pp. 110-127", and so on. By combining the optical member having the POF of the present invention with the techniques in the above publications , the optical member is applicable to short-distance optical transmission system that is suitable for high-speed and large capacity data communication and for control under no influence of electromagnetic wave. In particular, the optical member is applicable to wiring in apparatuses (such as computers and several digital apparatuses), wiring in trains and containers, optical linking between an optical terminal and a digital device and between digital devices , indoor optical LAN in houses , collective housings, factories, offices, hospitals, schools, and outdoor optical LAN.

Further, other techniques to be combined with the optical transmission system are disclosed, for example, in "'High-Uniformity Star Coupler Using Diffused Light Transmission' in IEICE TRANS. ELECTRON., VOL. E84-C, No.3, MARCH 2001, pp. 339-344", "'Interconnection in Technique of Optical Sheet Bath' in Journal of Japan Institute of Electronics Packaging., Vol.3, No.6, 2000, pp.476-480". Moreover, there are an arrangement of light emitting elements on a waveguide surface (disclosed in Japanese Patent Laid-Open Publication No.2003-152284) , an optical bus (disclosed in Japanese Patent Laid-Open Publications No.10-123350, No .2002-90571 , No.2001-290055 and the like); an optical branching/coupling device (disclosed in Japanese Patent Laid-Open Publications No.2001-74971, No .2000-329962 , No .2001-74966 , No.2001-74968 , No.2001-318263, No .2001-311840 and the like); an optical star coupler (disclosed in Japanese Patent Laid-Open Publications No .2000-241655 ); an optical signal transmission device and an optical data bus system (disclosed in Japanese Patent Laid-Open Publications No.2002-62457, No.2002-101044 , No.2001-305395 and the like); a processing device of optical signal (disclosed in Japanese Patent Laid-Open Publications No.2000-23011 and the like); a cross connect system for optical signals (disclosed in Japanese Patent Laid-Open Publications No.2001-86537 and the like) ; a light transmitting system (disclosed in Japanese Patent Laid-Open Publications No.2002-26815 and the like); multi-function system (disclosed in Japanese Patent Laid-Open Publications No.2001-339554 , No.2001-339555 and the like); and various kinds of optical waveguides, optical branching, optical couplers, optical multiplexers, optical demultiplexers and the like. When the optical system having the optical member according to the present invention is combined with these techniques, it is possible to construct an advanced optical transmission system to send/receive multiplexed optical signals . The optical member according to the present invention is also applicable to other purposes, such as for lighting (light transmission), energy transmission, illumination, and sensors.

The present invention will be described in detail with reference to Examples (l)-(3) as the embodiments of the present invention. Note that these examples do not limit the scope of the present invention. Common production conditions such as those of the preform 14 are only described in Example (1) in detail. Regarding Examples (2) -(3), the portions different from Example (1) will be explained. [Example ( 1) ] In Example (1) (in Experiments 1-1 to 1-3), three types of POFs 15 having different outer diameters are produced from the preform 14 by changing conditions of heat-drawing. The preform 14 was formed by the method described below. All of the produced POFs 15 are GI POFs. The materials and -the production methods of the preform 14 are the same. [Experiment 1-1]

By the melt-extrusion molding, the cylindrical outer clad section 11 was formed of PVDF with the inner diameter of 20 mm, the length of 905 mm, and the inner diameter of 20.5 mm. The inner clad section forming material was poured into the tube. The inner clad section forming material was 185g of MMA in which the polymerization initiator and the chain transfer agents were mixed. The polymerization initiator was dimethyl-2, 2'-azobis (2-methylpropionate) (V-601 produced by Wako Pure Chemical Industries, Ltd.) of 0.022 mol% (a half-life at 70⁰C: 5 hours). The chain transfer agent was n-laurylmercaptan of 0.1 mol%. The outer clad section 11 in which the inner clad section forming material 52 were poured was accommodated in the polymerization container 50. The polymerization container 50 was set in the rotation polymerization device 60 such that the lengthwise direction of the polymerization container 50 became horizontal. The thermal polymerization was performed for 8 hours in an atmosphere of 70° C while the polymerization container 50 was rotated at 2000 rpm. Then, the thermal polymerization was continued for 4 hours at 90° C with the same rotation speed to form the mixed section 22 and the inner clad section 12 inside the mixed section, which were inside the outer clad section 11. The thickness of the mixed section 22 was 0.02 mm.

The core section forming material was poured in the hollow portion of the inner clad section 12 at room temperature and atmospheric pressure. The core section forming material was MMA of 8Og in which the polymerization initiator, the chain transfer agent, and the dopant were mixed. The polymerization initiator was dimethyl-2, 2 ' -azobis (2-methylpropionate) (V-601). The chain transfer agent was n-laurylmercaptan. The dopant was diphenyl sulfide (DPS), which was a non-polymerizable compound. Additive ratios of Dimethyl-2, 2' -azobis (2-methylpropionate), n-laurylmercaptan, and DPS were 0.04 mol%, 0.15 mol%, and 7 mass% relative to MMA respectively. Next, the clad section 30 was set in the rotation polymerization device 60 such that the lengthwise direction of the clad section 30 became horizontal. The clad section 30 was rotated at 2000 rpm for 10 hours in an atmosphere of 70^βC. Thereafter, the interfacial gel polymerization was performed at the same rotation speed for 24 hours in an atmosphere of 12O⁰C. Thus, the preform 14 of the GI POF 15 was obtained. In the preform 14, the hollow portion is formed in the center in the diameter direction, and the core section 13 was formed inside the clad section 30. The POF 15 was produced by heat-drawing the preform 14 in the furnace 25 shown in FIG. 2 while the temperature inside the furnace 25 was controlled at 280⁰C. The furnace 25 was of a cylindrical shape with a height of 480 mm, and an inner diameter of 80 mm. The drawing speed (of the preform 14) was 15 m/minute, and a residence time of the preform 14 in the furnace 25 for heat-drawing (drawing residence time) was 7 minutes. The, outer diameter of the POF 15 was 316 μm. The outer diameter of the inner clad 44 was 300 μm. The thickness of the mixed layer 45 was 2.1 μm. The thickness of the mixed layer 45 formed between the outer clad 43 and the inner clad 44 of the POF 15 was measured by TOF-SIMS with PHI-TRIFT II (produced by ULVAC-PHI Inc) . The thickness of the mixed_/ layer 45 was defined as the thickness of a region in which each of F-element of PVDF and 0-element of PMMA had a concentration gradient in the concentration profile and in which F-element and 0-element were mixed. The bending loss of the produced POF 15 was extremely low (0.002 dB). The bending loss was measured in conformance with JIS C6861. The bending loss was a difference between a transmission loss value after bending and that before bending when the POF 15 of 5 m was bent with a bending radius R of 10 mm.

[Experiment 1-2]

The preform 14 for producing the POF 15 was formed of the same material and by the same method as those in the Experiment 1-1. At the time of heat-drawing of the preform 14 in the furnace 25, the drawing speed was 5 m/minute and the drawing residence time was 9 minutes. The produced POF 15 had the outer diameter of 470 μm. The outer diameter of the inner clad 44 was 440 μm. The thickness of the mixed layer 45 was 2.4 μm. The bending loss of the POF 15 was extremely low (0.015 dB) . [Experiment 1-3]

The preform 14 for producing the POF 15 was formed of the same material and by the same method as those in the Experiment 1-1. At the time of heat-drawing of the preform 14 in the furnace 25, the drawing speed was 3 m/minute and the drawing residence time was 8 minutes. The outer diameter of the produced POF 15 was 750 μm. The outer diameter of the inner clad 44 thereof was 712 μm. The thickness of the mixed layer 45 was 3.2 μm. The bending loss of the POF 15 was extremely low (0.045dB). [Example (2) ]

In Example (2) , the preform 14 for producing the POF 15 was formed of the same material and by the same method as those in Example ( 1) except that the temperature in the furnace 25 was 230° C. In the following experiments , the drawing speed and the drawing residence time are changed to produce three types of POFs 15 having different outer diameters .

[Experiment 2-1] The POF 15 was produced by heat-drawing the preform 14 in the furnace 25. The drawing speed was 15 m/minute and the drawing residence time was 24 minutes. The outer diameter of the produced POF 15 was 316 μm. The thickness of the mixed layer 45 was 2.0 μm. The bending loss of the POF 15 was extremely low (0.003dB). [Experiment 2-2]

The POF 15 was produced by heat-drawing the preform 14 in the furnace 25. The drawing speed was 5 m/minute and the drawing residence time was 28 minutes. The outer diameter of the produced

POF 15 was 470 μm. The thickness of the mixed layer 45 was 2.3 μm. The bending loss of the POF 15 was extremely low (O.OlβdB).

[Experiment 2-3]

The POF 15 was produced by heat-drawing the preform 14 in the furnace 25. The drawing speed was 3 m/minute and the drawing residence time was 26 minutes. The produced POF 15 had the outer diameter of 750 μm. The thickness of the mixed layer 45 was 2.9 μm. The bending loss of the POF 15 was extremely low (0.046 dB). [ Example ( 3 ) ]

In Example (3) , the preform 14 for producing the POF 15 was formed of the same material and by the same method as those in Example (1) except that the temperature in the furnace 25 was 230° C. In the following experiments, the drawing speed and the drawing residence time are changed to produce three types of POFs 15 having different outer diameters . [Experiment 3-1]

The POF 15 was produced by heat-drawing the preform 14 in the furnace 25. The drawing speed was 15 m/minute and the drawing residence time was 7 minutes. The produced POF 15 had the outer diameter of 316 μm. The thickness of the mixed layer 45 was 0.9 μm. The bending loss of the POF 15 was relatively high (0.320 dB) .

[Experiment 3-2]

The POF 15 was produced by heat-drawing the preform 14 in the furnace 25. The drawing speed was 5 m/minute and the drawing residence time was 9 minutes. The produced POF 15 had the outer diameter of 470 μm. The thickness of the mixed layer 45 was 1.4 μm. The bending loss of the POF 15 was relatively high (0.430 dB) .

[Experiment 3-3] The POF 15 was produced by heat-drawing the preform 14 in the furnace 25. The drawing speed was 3 m/minute and the drawing residence time was 7 minutes. The produced POF 15 had the outer diameter of 750 μm. The thickness of the mixed layer 45 was 1.7 μm. The bending loss of the POF 15 was relatively high (0.570 dB) . According to the results of the Examples ( 1 ) and ( 2 ) , regardless of the outer diameter of the POF 15, the bending loss was significantly reduced in the POF 15 having the mixed layer of 2.5 μm or more. On the other hand, in Example (3), the POF 15 was formed with the mixed layer having smaller thickness than those of Examples (1) and (2). As a result, the bending loss of the POF 15 is largely increased regardless of its diameter compared to those of Examples (1) and (2). As described above, the inner clad section forming material 52 is poured into the hollow portion of the outer clad section 11, and thereafter, the thermal polymerization is performed to form the preform 14 having the mixed section 22 of the specified thickness between outer clad section 11 and the inner clad section 12. Then the POF 15 is produced by heat-drawing the preform 14. By the heat -drawing, the mixed section 22 of the preform 14 becomes the mixed layer 45 of the POF 15. The bending loss of the produced POF 15 is reduced since the mixed layer 45 prevents the light leakage and the bending deformation caused by the bending stress.

Industrial Applicability

The present invention is preferably applied to a production method of a plastic optical fiber used for optical transmission, lighting, energy transmission, illumination, sensor and so forth.

Claims

1. A plastic optical fiber formed by heat-drawing a preform comprising: a cylindrical outer clad formed of crystalline fluorine-containing polymer; a cylindrical inner clad formed of amorphous polymer, said inner clad being formed in a hollow portion of said outer clad; a mixed layer formed between said outer clad and said inner clad, said mixed layer having a lower degree of crystallinity than said outer clad; and a core filled in a hollow portion of said inner clad, said core having a higher refractive index than said inner clad.

2. A plastic optical fiber described in claim 1 , wherein said mixed layer is formed by diffusion of a compound, which forms said inner clad, into said outer clad during thermal polymerization of said compound.

3. A plastic optical fiber described in claim 1 , wherein a thickness t (μm) of said mixed layer satisfies 2.0 ≤ t ≤ 5.5.

4. A plastic optical fiber described in claim 1 , wherein a refractive index of said core gradually decreases from a center toward outside in a diameter direction.

5. A plastic optical fiber described in claim 2, wherein a refractive index of said mixed layer gradually decreases from an inner clad side toward an outer clad side.

6. A producing method for a plastic optical fiber comprising: preparing a cylindrical outer clad section formed of crystalline fluorine-containing polymer; putting first compound which generates amorphous polymer into a hollow portion of said outer clad section; polymerizing said first compound to diffuse said first compound into said outer clad section to form a mixed section and a cylindrical inner clad section on an inner side of said mixed section , said mixed section having a lower degree of crystallinity than said outer clad section; putting a second compound which forms a core section into a hollow portion of said inner clad section; polymerizing said second compound to form a preform; and heat-drawing said preform to form a plastic optical fiber.

7. A producing method described in claim 6 , wherein a thickness t (μm) of said mixed section after said heat-drawing satisfies 2.0 ≤ t ≤ 5.5.

8. A producing method described in claim 6 , wherein a polymerization temperature of said first compound is in a range of 50° C to 100° C.

9. A producing method described in claim 6 , wherein said preform is drawn while being heated in a range of 220° C to 300° C.