JPH08122542A - Plastic optical fiber - Google Patents

Abstract

PURPOSE: To provide a multimode SI type plastic optical fiber having a practicable transmission band, transmission loss and stability of performance in the long-term use. CONSTITUTION: This plastic optical fiber has a core-clad structure consisting of a methacrylate resin as a core and a mixture composed of 1 to 50wt.% vinylidene fluoride resin and 99 to 50wt.% methacrylate resin as a clad and has a numerical aperture NA of 0.04 to 0.35. An intermediate layer in which the content of the vinylidene flouride resin in a mixture composed of the vinylidene fluoride resin and methacrylate resin increases continuously and gently toward the outer peripheral and which has a thichness of 5 to 50μm is formed at the boundary between the core and clad of the optical fiber.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plastic optical fiber usable as an optical information communication medium.

[0002]

2. Description of the Related Art At present, practically used optical transmitters are classified into quartz type and plastic type according to the classification of materials. Also,
The transmission modes are classified into single mode type and multimode type. Further, there are a step index (SI) type in which the refractive index change along the radial direction of the cross section is discontinuous and a continuous graded index (GI) type. Also,
Optical transmission is widely used for communication trunk lines because it has a large capacity and is not affected by electromagnetic noise at all. Large-scale commercially available silica-based single-mode fiber features overwhelmingly low loss and wide bandwidth. Taking advantage of this feature, it is used for long-distance, large-capacity communication trunk lines.

Further, as the plastic type, an SI type multimode type is commercially available, and it has a larger diameter and a higher numerical aperture than silica type single mode fiber as well as silica type GI fiber and plastic clad quartz core fiber. The alignment of the light source is easy, and since it is flexible and easy to handle, the connecting operation such as connecting and disconnecting is easy.
Moreover, since the expensive device is unnecessary for such work, the connection cost is low. Taking advantage of this feature, application to short-distance communication such as data link and sensors is spreading. In the future, LA between equipment inside and outside the floor such as FA and OA
Expected to be a short-distance information transmission line with many connection points, where silica-based fibers such as N-type facility networks and terminal wiring in FTTH (Fiber To The Home) cannot be introduced due to high cost. . In addition, since it has excellent flexibility, it is less likely to be damaged or deteriorated even in a vibrating environment, and in this respect it is far superior to quartz-based, and can be applied to signal transmission lines such as networks in moving bodies such as automobiles, trains, airplanes Has been planned.

In these communication applications, high capacity can be realized by transmitting digital signals at high speed. However, the bandwidth of commercially available SI type plastic optical fibers is usually at most about 5 MHz · km, and the bandwidth is insufficient for applications such as medium and high speed LAN. In order to cope with this situation, there is an active movement to develop a GI type plastic optical fiber in which a continuous secondary refractive index distribution is formed in a cylindrical symmetry in the radial direction of the cross section.

As a method of manufacturing such a GI type plastic optical fiber, first, a cylindrical preform rod having a refractive index distribution is manufactured, and this is thermally stretched to form a fiber having a refractive index distribution. It has been known. As a method for producing such a preform rod, 1) a mixture of two or more kinds of monomers having different refractive indexes is polymerized by an interfacial gel polymerization method, and a copolymer composition ratio is changed to thereby provide a refractive index distribution in a radial direction. (Japanese Patent Application Laid-Open No. 4-973)
No. 02, No. 4-97303, No. 5-
173025 and JP-A-5-173026), or 2) non-polymerizable modified refractive index substance (dopant)
It has been proposed that a monomer is polymerized in the presence of ## STR3 ## to give a refractive index distribution in the radial direction by changing the amount of dopant present (International Publication WO93 / 08488 etc.).

[0006]

However, the GI type fiber obtained by the method of 1) above has a large scattering loss due to micro phase separation in the fiber, and thus it is difficult to obtain a low transmission loss. It's a problem. The GI type fiber obtained by the above method 2) has a possibility that the dopant in the optical fiber may gradually move during use, and there is a problem in stability of performance during long-term use, particularly durability at high temperature. Also,
The problem is that the conventional SI fiber has a too narrow transmission band.

In view of the above problems, it is an object of the present invention to provide a multimode SI type plastic optical fiber having a practical transmission band, transmission loss, and stable performance during long-term use.

[0008]

Means for Solving the Problems The gist of the present invention is to use a methacrylic acid ester resin as a core, a vinylidene fluoride resin of 1 to 50 parts by weight and a methacrylic acid ester resin 9
A core-clad structure having a mixture of 9 to 50% by weight as a clad, and a plastic optical fiber having a numerical aperture NA of 0.04 to 0.35, and a core and a clad of the optical fiber. An intermediate layer having a thickness of 5 to 50 μm, which is a mixture of vinylidene fluoride resin and methacrylic acid ester resin and has a vinylidene fluoride resin content continuously and gently increasing toward the outer peripheral direction, is formed at the interface. It is in a plastic optical fiber.

The present invention will be described in detail below. As described above, the conventional plastic optical fiber sacrifices transmission loss or long-term durability in order to achieve a wide band. The present invention provides a plastic optical fiber that achieves a wide band without sacrificing these performances.

There is a method of continuously bulk polymerizing a methacrylic acid ester resin as a core material component and directly supplying this to a spinning machine to perform composite spinning with a clad material component. Among the commercially available SI type plastic optical fibers, It is known that those manufactured by this method exhibit the lowest transmission loss. Therefore, the present inventors considered using this method to obtain a plastic optical fiber having a low loss and a wide band.

On the other hand, it is also known that the mode dispersion, that is, the band of the multimode optical fiber depends on the numerical aperture NA. The smaller the NA, the lower the mode dispersion and the wider the band.

However, the reduction of NA for the purpose of widening the band has not been studied so far. The inventors of the present invention intend to complete an optical fiber having a small transmission loss, stable long-term performance, and a wide band by controlling the refractive index of the cladding layer and ensuring the optical homogeneity of the cladding layer. It was successful.

As a factor of the transmission loss of the optical fiber, the core
There is mode conversion due to interface irregularity between the clads and scattering of light oozing into the clads. Therefore, the inventors of the present invention considered to homogenize the clad material as a means for preventing the transmission loss from increasing.

Further, when the refractive index of the core is n ₁ and the refractive index of the clad is n ₂ , the numerical aperture NA is NA = (n ₁ ² −
It is expressed as n ₂ ² ) ^1/2 and depends on the values of the refractive index of the core and the clad. Therefore, the present inventors considered controlling the refractive index of the cladding layer as a means for achieving a low NA.

In order to control the refractive index of the clad to an appropriate value, the selection of the clad material is important. For example, a material that originally has that refractive index can be used as the clad material to give a desired refractive index. Possible methods include using a copolymer obtained by copolymerizing two or more kinds of monomers with an appropriate composition, and using a blend of two or more kinds of polymers capable of giving a desired refractive index.

However, since there are rare combinations in which a polymer-polymer blend can be an optically homogeneous mixture, the clad material obtained by the general method is optically inhomogeneous and has no light scattering. As a result, this blend system is often considered unsuitable as a clad material. That is, when a polymer-polymer blend is used as a clad material, the combination and composition cannot be selected only from the viewpoint of controlling the refractive index of the blend, and the optical properties of the blend cannot be selected. It is necessary to pay attention to the homogeneity as well.

Therefore, the present inventors have considered the factors of the light scattering of the polymer blend while comparing them with the case of the copolymer. In the case of copolymerization, the number and proportion of consecutive monomers of the same type are statistically determined by the copolymerization reactivity ratio. That is, as long as there is a difference in the reactivity ratio between different monomers, the probability that the same type of monomers will be continuous has a certain distribution and is finite. There is a continuous part. This is
No matter how random a random copolymer is, if the compatibility between homopolymers is poor, it means that there is a risk of microscopic phase separation structure, resulting in increased light scattering loss. To do. Therefore, in order to reduce the scattering loss, it is important to increase the compatibility between the homopolymers rather than the randomness of the copolymerization. In other words, optical homogeneity is the same whether it is a copolymer or a mixture of homopolymers as long as the combination has compatibility at the molecular level. It is believed that the mixture of the two is superior to the random copolymer of the poorly compatible monomers.

The inventors of the present invention have made extensive studies based on these findings, and as a result, among polymers-polymer compatible systems, polymers composed of a combination of molecularly compatible polyvinylidene fluoride resin and polymethacrylic acid resin- It was found that a system using a polymer composite system for the cladding layer is optimal for this purpose. Also, the combination of the two is optimal from the viewpoint of controlling the refractive index of the clad, and the combination of the two is optimal from the relationship with the adhesion to the core material.

That is, since the clad material of the optical fiber of the present invention is essentially compatible with the core material and contains the same kind of polymer as the core material, 1) the adhesion of the interface between the core and the clad. Very good, 2) the dynamics of the core material and the clad material during spinning are close, and the interface irregularity between the core and the clad can be reduced, and 3) the composition of the interface due to the polymer-polymer melt diffusion. Can be changed continuously, and N
It also has a very excellent feature that it can be used for a wider band without changing A.

In the present invention, the polyvinylidene fluoride resin means, for example, a homopolymer of vinylidene fluoride or a copolymer of vinylidene fluoride and a copolymerizable monomer. Examples of the copolymerizable monomer include, but are not limited to, vinyl fluoride, tetrafluoroethylene, trifluoroethylene, hexafluoropropylene, and trifluorochloroethylene.

The methacrylic acid ester resin is
A homopolymer of methyl methacrylate or a copolymer of methyl methacrylate and a monomer copolymerizable with methyl methacrylate. Copolymerizable monomers include other methacrylic acid esters such as butyl methacrylate and ethyl methacrylate, and maleimides and 5-membered ring lactones that contribute to improving heat resistance, but are not limited to these. is not.

The mixing ratio of vinylidene fluoride resin / acrylic acid ester resin used for the clad material in the present invention is 1/99 to 50/50 (% by weight). This is because when the vinylidene fluoride resin exceeds 50% by weight, the crystal growth rate of the vinylidene fluoride resin in the mixture is sufficiently high, and a quenching operation or the like is performed to prevent the growth of spherulites during hot melt shaping. Even after spinning, spherulites gradually grow depending on temperature and humidity conditions, causing interface irregularity and light scattering,
This is because it causes an increase in transmission loss. The refractive index of the clad material can be selected to obtain a desired band within this range, but in order to obtain a wider band fiber, the numerical aperture must be low, that is, the refractive index of the clad material should be the refractive index of the core material. It is desirable to be closer to. However, if the numerical aperture is too small, LD (semiconductor laser) or L used as a light source
In order to avoid a decrease in the coupling efficiency between the ED (light emitting diode) and the fiber, sufficient dimensional accuracy and optical components for condensing are required, and the ease of connection and alignment is impaired. From this viewpoint, the mixing ratio of both resins is 20/80 to 5
It is more preferably 0/50 (% by weight).

The optical fiber of the present invention may be coated with a protective layer in order to improve reliability such as handleability, heat resistance, solvent resistance, and moisture resistance. It is also possible to optimize the refractive index distribution shape so as to have the widest band by appropriately selecting and coating a transparent material having a higher refractive index than the cladding layer as the protective layer. Further, the shape of the fiber can be circular, elliptical or the like, but is not limited thereto. When used as an optical fiber, a circular cross section is preferable from the viewpoints of handleability, ease of connection with a light source, a light receiving element, and the like.

Next, a method of manufacturing the optical fiber of the present invention will be described. The core-clad structure plastic optical fiber of the present invention is a continuous bulk polymerization-direct spinning method in which a resin for a core material is continuously bulk polymerized and directly supplied to a spinning machine without pelletizing, a rod-shaped precursor (preform It can be produced by either a preform spinning method in which the core material is spun by hot drawing or the like, or a pellet spinning method in which pellets of the core material are extruded from an extruder and spun, but continuous bulk polymerization in order to keep transmission loss low. -Direct spinning method is preferred. At this time, in order to further increase the band without changing the NA, the shape of the nozzle and the spinning conditions are appropriately selected to control the residence time and temperature in the spinning nozzle, thereby fluoridating the core-clad interface by thermal fusion diffusion. It is possible to form an intermediate layer which is a mixture of a vinylidene-based resin and a methacrylic acid ester-based resin and in which the content of the vinylidene fluoride-based resin continuously and gently increases in the outer peripheral direction.

In this method, in order to secure a practically feasible diffusion rate, it is necessary that the vinylidene fluoride resin has a sufficiently low molecular weight and a sufficiently fast diffusion rate in the substrate. Therefore, in this case, the vinylidene fluoride-based resin used for the clad material preferably has a weight average molecular weight of 400,000 or less, and more preferably 200,000 or less. The weight average molecular weight of the methacrylic acid ester resin used for the clad material is preferably 150,000 or less,
It is more preferably 100,000 or less.

The clad material can be obtained by a method in which a methacrylic acid ester resin and a vinylidene fluoride resin are mixed and kneaded and extruded by a uniaxial or biaxial screw type extruder. In this case, the twin-screw extruder has a better mixing efficiency than the single-screw extruder, so that more uniform mixing can be achieved in a shorter residence time and deterioration of the resin due to heat history can be minimized, which is preferable. The obtained blended material for clad material may be once shaped into pellets or granules, and may be fed to the raw material supply port of the extruder for supplying the clad material in the spinning head for composite spinning. If the method of directly supplying the clad material blended by the extruder for material mixing to the spinning head portion is adopted, the process can be simplified and deterioration of the resin due to heat history can be prevented. Furthermore, when adopting the continuous bulk polymerization-direct spinning process, a methacrylic acid ester-based resin for the clad material is obtained by branching from the continuous bulk polymerization line for the core material and mixed with a vinylidene fluoride-based resin. -It is preferable to directly feed to an extruder for feeding, mix, extrude, and perform composite spinning.
That is, according to this method, the process is further simplified and the thermal deterioration of the methacrylic acid ester resin is reduced.

Further, N, N dimethylformamide, N, N dimethylacetamide and the like can be used in the dissolution and mixing with a co-solvent, but the residual solvent accelerates the deterioration of the resin during melt shaping, so that it is necessary to remove the solvent. The melt mixing method is preferable because various steps are required and the process becomes complicated.

Usually, a plastic optical fiber using a methacrylic acid ester-based resin is kept at a temperature above the glass transition point to give strength by a stretching operation or the like. However, a vinylidene fluoride-based resin and a methacrylic acid ester-based resin are used. The homogeneous mixture has excellent mechanical properties, and has excellent properties against bending and pulling even when it is not stretched. That is, since the clad material of the optical fiber of the present invention has high toughness and very good mechanical properties, it is possible to omit the drawing step performed to secure the mechanical properties of the fiber. The fact that drawing is unnecessary means that shrinkage due to heat is small even in the vicinity of the glass transition point, which means that the fiber morphology can be maintained as it is at a higher temperature than that of a drawn optical fiber. Therefore, it is possible to reduce the deformation of the fiber due to the stretching relaxation at the heat distortion temperature, which has been a problem in the past when used in a high temperature atmosphere such as in an automobile, and contribute to the improvement of the heat resistance of the optical fiber. The optical fiber after spinning can be appropriately stretched in order to impart higher mechanical strength.

The above is a method for producing an optical fiber by the spinning method. First, a columnar article having a core-clad structure having a diameter of about 10 mm or more is first produced by an extruder or the like, and this article is heated and drawn sequentially from one end. The optical fiber of the present invention can be obtained also by the method of converting the optical fiber.

The multimode plastic optical fiber obtained by these manufacturing methods is heat-treated under an appropriate temperature condition for the purpose of improving the band, and an intermediate layer having a composition distribution is formed at the core-clad interface by thermal fusion diffusion. It is also possible. In this method, as in the case of the intermediate layer forming method utilizing thermal fusion diffusion in the nozzle, each resin should have the same molecular weight as the above in order to ensure a practically feasible diffusion rate of the resin component. Is preferred.

The glass transition point of the clad material of the optical fiber depends on the blend composition, but the temperature condition for forming the intermediate layer by heat treatment must be equal to or higher than the glass transition point of the clad material. In order to shorten the heat treatment time, the heat treatment temperature is preferably 150 ° C or higher. Further, when the treatment is performed at a higher temperature, it is preferable that the optical fiber is previously coated with a heat-resistant jacket material such as silicone rubber having little or little thermal deformation, and then the heat treatment is performed.

[0032]

EXAMPLES The present invention will be specifically described below based on examples. Example 1 Using a twin-screw kneading extruder, a uniform mixture of polyvinylidene fluoride / polymethylmethacrylate = 48/62 (% by weight) was obtained and used as a clad material. In addition, polymethylmethacrylate produced by continuous bulk polymerization was used as the core material. These clad materials and core materials are supplied to a spinning machine, and the fiber diameter is 1000 μm and the core diameter is 980 μm.
An optical fiber with a clad structure was obtained. The core material was continuously bulk polymerized and directly supplied to the spinning machine.

When a -3 dB band was measured with a 100 m long fiber, it was 150 MHz (measuring device; optical sampling oscilloscope manufactured by Hamamatsu Photonics KK, light source;
Toshiba semiconductor laser TOLD9410 emission wavelength 650
nm, excitation NA; 0.85 all modes excitation). In addition, FFP (Far Field) at 0.5 m of this fiber
Full Width 5% M from Pattern)
NA calculated from the maximum value was 0.310.
Furthermore, the transmission loss of this plastic optical fiber is 135
dB / km (measurement method: 25 m-5 m cutback method, wavelength: 650 nm, incident NA = 0.1). The coupling efficiency with the semiconductor laser (TOLD9201) is -2.7d.
B was -1.2 dB with respect to Comparative Example 1 described later.

This fiber was placed in an atmosphere at 85 ° C. for 10 minutes.
The increase in transmission loss after being left for 00 hours is 30 dB / km.
And showed good stability over time.

The plastic optical fiber of this embodiment has a band of 150 MHz or more required for medium and high speed LANs. Further, the transmission loss is relatively small, and the performance is sufficient to withstand use at 100 m or longer. Furthermore, low NA
However, the increase in the coupling loss of the optical fiber is extremely small. Example 2 An optical fiber was produced in the same manner as in Example 1 except that a uniform mixture of polyvinylidene fluoride / polymethyl methacrylate = 41.7 / 58.3 (wt%) was used as the cladding material. 125m for -3dB band with 100m fiber
It was Hz. The NA was 0.324 and the transmission loss was 135 dB / km. The coupling efficiency with the semiconductor laser was -0.7 dB as compared with Comparative Example 1.

Example 3 An optical fiber was manufactured in the same manner as in Example 1 except that a uniform mixture of polyvinylidene fluoride / polymethyl methacrylate = 50/50 (wt%) was used as the cladding material.
The -3 dB band was 160 MHz for 100 m long fiber. Also, the NA is 0.341, and the transmission loss is 1.
It was 35 dB / km. The coupling efficiency with the semiconductor laser was -0.8 dB as compared with Comparative Example 1.

Example 4 An optical fiber manufactured in the same manner as in Example 3 was heat-treated at 190 ° C. for 30 minutes to form a melt-mixed intermediate layer having a thickness of 45 μ at the interface between the core and the clad. The thickness of the intermediate layer was measured by cutting out a thin sample at a portion of 96 μm from the outer peripheral portion of the fiber toward the center and measuring the refractive index distribution by a differential interference method using an inter-faco interference microscope.

A -3 dB band is 1 for a 100 m long fiber.
The transmission band was 80 MHz, and the transmission band was wider than that of the third embodiment.

Comparative Example 1 According to the method of Example 1, a commercially available plastic optical fiber (Mitsubishi Rayon Esca Premier, product number: GH400) is used.
1, fiber diameter: 1000 μm, core diameter: 980 μ
It was 50 MHz when the −3 dB band of m) was measured. The NA is 0.511 and the transmission loss is 135d.
B / km. Semiconductor laser (TOLD9201)
The coupling loss with was -1.5 dB.

[0040]

The SI type optical fiber of the present invention has a wide transmission band, low transmission loss, and excellent performance stability during long-term use.

Claims (2)

[Claims] 1. A core-clad structure comprising a methacrylic acid ester-based resin as a core and a mixture of 1 to 50 parts by weight of vinylidene fluoride-based resin and 99 to 50% by weight of a methacrylic acid ester-based resin as a clad. , Numerical aperture NA
Is a plastic optical fiber having a value of 0.04 to 0.35. 2. A mixture of a vinylidene fluoride resin and a methacrylic acid ester resin at the interface between the core and the clad, wherein the content of the vinylidene fluoride resin increases continuously and gently toward the outer peripheral direction. Thickness 5-50 μm
The plastic optical fiber according to claim 1, wherein an intermediate layer is formed.