JPH0323885B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a low-loss plastic optical fiber having a core made of a polymer mainly composed of methyl methacrylate and a synthetic polymer sheath having a lower refractive index than the core, and a method for producing the same. It is related to.

Conventionally, optical fibers have been created using a concentric core-sheath structure, with a core made of a highly transparent synthetic polymer such as polystyrene or methyl methacrylate, and a sheath made of a synthetic polymer with a lower refractive index than the core. It is well known to form a fiber and transmit light incident on one end of the fiber by total internal reflection along the length of the fiber. A consideration in creating optical fibers is to minimize the factors that increase the attenuation of light by absorption or scattering as it travels through the fiber. So-called plastic optical fibers made from synthetic polymers have the advantage of being lighter and more flexible than conventional optical fibers made from inorganic glass. The disadvantage is that the light transmitted inside is attenuated to a greater extent than fiber. The present invention is directed to reducing the degree of light attenuation of plastic optical fibers using synthetic polymers.

According to the findings of the present inventors, it is clear that the cause of optical transmission loss in plastic optical fibers is due to light scattering due to the effects of impurities, dust, and minute voids contained in synthetic polymers. Summer. Figure 1 shows the optical transmission characteristics of a low-attenuation all-plastic optical fiber manufactured by a conventionally known method with a polymethyl methacrylate core and a fluororesin copolymer sheath. The minimum value is 350dB/at a wavelength of 650nm.
Km, values of 330 dB/Km at 570 nm and 400 dB/Km at 530 nm are only obtained, and the increase in loss due to scattering is particularly noticeable on the short wavelength side. According to the findings of the present inventors, absorption loss due to harmonics of infrared vibration absorption of carbon-hydrogen bonds in plastic optical fibers with polymethyl methacrylate as the core occurs at wavelengths below 580 nm.
It became clear that it was less than 10dB/Km. In other words, the main cause of the large loss on the short wavelength side in Figure 1 is scattering loss, and it is possible to reduce or eliminate impurities, dust, and minute voids in synthetic polymers that cause scattering loss by some method. Therefore, it becomes possible to produce a plastic optical fiber with low loss. As a method to improve the optical transmission properties of optical fibers, we have developed plastic optical fibers (Japanese Patent Application Laid-Open No. 2003-130012) that reduce the biacetyl content in methyl methacrylate, reduce the amount of transition metal ions, and remove particulate matter by filtration. No. 54-65555), or a method for producing plastic optical fibers (Japanese Patent Application Laid-open No. 50-83046), which involves bulk polymerizing the core component and then continuously separating volatiles, mainly residual unreacted monomers, in the polymer. No.) has been proposed.

In these methods, it is stated that the attenuation of light due to the presence of particulate matter can be improved by containing preferably 100 particles/ ^mm3 or less of vinyl monomer (Japanese Patent Laid-Open No. 55-65555). ), but according to the findings of the present inventors, in order to obtain a low-loss optical fiber, even if the number of particulates is about 10 particles/mm ³ , the light attenuation is large, and 2 particles/mm ^{3 are required} . However, it is not enough. Furthermore, it is stated that impurities in the polymer, especially impurity ions such as transition metal ions, must be at most 500 ppb, preferably at most 100 ppb (Japanese Patent Application Laid-open No. 1983-65555), but for example, if cobalt ions are present at 10 ppb, Therefore, there is an increase in loss of 50 dB/Km at a wavelength of 630 nm, and the presence of 100 ppb of nickel ions increases the loss at a wavelength of 850 nm.
There is an increase in loss of 33 dB/Km. Also,
Even in the continuous bulk polymerization method (Japanese Patent Application Laid-Open No. 50-83046), the method of adding polymerization initiators and chain transfer agents (molecular weight regulators) to monomers is not appropriate, and it is difficult to improve light attenuation. difficult. Although these methods can reduce scattering loss to a certain extent, the minimum value of attenuation is
The value remains at around 300dB/Km (wavelength 656nm).

In view of the current situation, an object of the present invention is to provide a low-loss plastic optical fiber having a core-sheath structure with extremely excellent light transmission characteristics in the visible light range.

Another object of the present invention is to propose a method for manufacturing a low-loss plastic optical fiber having a core-sheath structure with extremely excellent light transmission properties in the visible light range.

The method for manufacturing the low-loss plastic optical fiber of the present invention uses a polymer core mainly composed of methyl methacrylate, and forms a synthetic polymer sheath with a lower refractive index than the core around the core. In producing Icarfiber, a polymerization initiator and a chain transfer agent are added to methyl methacrylate monomer in the absence of oxygen in a completely closed system, and the methyl methacrylate monomer is added with the polymerization initiator and chain transfer agent. For example, dust in the acrylate monomer has a wavelength of 632.8 nm.
When observed by irradiating with Ne laser light, there are 20 or less particles per 10 cm optical path length, approximately 1 or less per 1 mm ³ , at any location in the monomer, and polymerization starts in a closed system. A polymer is obtained by bulk polymerizing the methyl methacrylate monomer to which an agent and a chain transfer agent are added at a temperature higher than the glass transition temperature of the methyl methacrylate polymer, and the transition metal ion content in this polymer is 50ppb or less for iron and manganese, 10ppb or less for copper and nickel, 5ppb or less for chromium,
For cobalt, the content should be 2 ppb or less, and the polymer is melt-spun to form a core at a temperature that does not generate air bubbles in the polymer, and a core with a refractive index lower than that of the core is formed around the core. It is characterized by having a sheath of a substantially amorphous synthetic polymer deposited at a high rate.

In the present invention, the core component of the optical fiber is a radical polymerization initiator that exhibits good activity at a temperature higher than the glass transition temperature of the methyl methacrylate polymer. The polymer may be bulk polymerized to form a polymer consisting of at least 50 mole percent methyl methacrylate units. In order to prevent dust from being mixed into the core polymer, it is not sufficient to simply distill the monomer. In other words, if a filter with a pore size of about 0.1 μm is used when adding a polymerization initiator or chain transfer agent (i.e., molecular weight regulator) to a monomer purified by distillation, a large amount of fine dust will still enter. , causing light scattering. Therefore, in the present invention,
The polymerization initiator and chain transfer agent are distilled in a closed polymerization apparatus in the absence of oxygen, and added in such a way that only the fractions are mixed with the monomers.
As a result, it is possible to significantly suppress the incorporation of minute dust particles, and further reduce loss due to light scattering. As a result, dust in the monomer to which a polymerization initiator and chain transfer agent have been added is removed by, for example, He-Ne laser light with a wavelength of 632.8 nm (the diameter of the light beam is approximately
0.5 mm), 0.02 to 20 or less per 10 cm optical path length (0.02 to 20 or less observed light spots) at any location, that is, ¹ to 1000 per 1 cm (in most cases) In other words, the concentration of dust can be approximately 1 or less per 1 mm ³ . From the above points, it is desirable that the polymerization initiator and chain transfer agent used here be easily distillable under normal pressure or reduced pressure. Examples of such polymerization initiators include organic peroxides such as di-tert-butyl peroxide, dicumyl peroxide, and cumene hydroperoxide, and azo compounds such as azo-tert-butane and azobisisopropyl. .

Mercaptans are suitable as chain transfer agents, including primary mercaptans such as n-butyl and n-propyl, secondary mercaptans such as sec-butyl and isopropyl, or
Examples include tertiary mercaptans such as tert-butyl and tert-hexyl, and aromatic mercaptans such as phenyl mercaptan.

In the present invention, the core component may be a copolymer containing at least 50 mol% or more of methyl methacrylate units, in addition to the above-mentioned polymethyl methacrylate polymer alone. Examples of the copolymer component include acrylic esters such as methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate, methacrylic esters such as ethyl methacrylate, propyl methacrylate, and butyl methacrylate, and polystyrene. In this case, since the copolymer component has a boiling point in the range of 80°C to 160°C at normal pressure, it can be easily distilled like methyl methacrylate, significantly reducing the content of impurities and dust. Therefore, a copolymer with low light scattering can be obtained. The composition of the copolymer is 2
In order to minimize light scattering due to concentration fluctuations associated with the presence of the component polymers, an azeotropic composition is preferred, but at least 90 mol% methyl methacrylate, preferably at least 95 mol %. If the copolymer contains %,
The fluidity during melt spinning is improved and excellent light transmission properties are obtained.

In the present invention, when polymerizing the core component, it is not preferable to use methods such as suspension polymerization, emulsion polymerization, and solution polymerization. The reason for this is that although suspension polymerization and emulsion polymerization can produce highly pure polymers as industrial methods, they use a large amount of water, so there is a risk that foreign substances in the water may mix into the polymer. Also, there is a risk of contamination of foreign matter during the dehydration process, and since a solvent is used in solution polymerization, there is a risk of contamination of impurities or foreign matter in the solvent. Contamination with foreign matter is a concern.

Therefore, in the present invention, a polymer is formed by bulk polymerizing the core component. During bulk polymerization, by setting the polymerization temperature above the glass transition temperature of the polymer to be produced, the generation of internal strain when heating the core polymer to a high temperature during subsequent melt spinning; Alternatively, the generation of microvoids during spinning can be significantly suppressed, and the loss caused by light scattering inside the optical fiber can be significantly reduced. Polymerization temperature 100℃ to 180℃
is optimal, but the conversion rate to polymer is at least
It is necessary to gradually heat to a high temperature so that the temperature reaches 98%, preferably 99% or higher. If polymerization is carried out immediately at a high temperature without gradually heating to a high temperature, the polymerization reaction will run out of control due to the so-called gel effect, causing void generation. In addition, the polymerization temperature may be extended for a long time.
If the temperature exceeds 190°C, there is a risk that the polymer will depolymerize. In the present invention, following the polymerization process of the core component, the polymer is melt-spun without lowering the temperature below the glass transition temperature of the resulting polymer, thereby reducing the size of microvoids without causing significant volume changes. Less optical fiber can be obtained. When impurity metal ions in the polymer thus obtained were quantified by radioactivity analysis, iron and manganese were 50 ppb or less, copper and nickel were 10 ppb or less, chromium was 5 ppb or less, and cobalt was 2 ppb or less. It is also easy to keep iron and manganese at 20ppb or less, copper and nickel at 5ppb or less, and chromium at 2ppb or less.

The sheath component used in the present invention has a refractive index of at least 0.5%, preferably 2%, that of the core.
%, most preferably at least 5% lower. In particular, excellent light transmission properties can be obtained by using a substantially amorphous polymer. Such sheath components include polymers known as sheath components, such as polymers or copolymers of acrylic or fluorinated esters of methyl methacrylate, tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride, trifluoromonochloroethylene, etc. Examples include resin copolymers and elastomers such as silicone resins. In addition, when the core component is an azeotropic copolymer of methyl methacrylate and styrene, the refractive index is 1.54.
Therefore, polymers of methacrylic esters such as polymethyl methacrylate, copolymers of acrylic esters and methacrylic esters, and elastomers such as ethylene-vinyl acetate copolymers can also be used as sheath components. However, in any case, the polymer is not limited to these, and any synthetic polymer can be used as long as it is optically transparent and satisfies the condition of the difference in refractive index with the core component.

In manufacturing the plastic optical fiber of the present invention, a composite melt spinning method using a core-sheath spinneret is used, or a core component polymer formed into a fiber is immersed in a concentrated solution of a sheath component polymer. , or a method of coating through an orifice can be used. Immersion in concentrated solutions,
Alternatively, when coating, excellent light transmission characteristics can be obtained by applying coating immediately after extruding the core so that contamination of the core components by dust and the like is minimized.

EXAMPLES The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited only to these Examples. Note that a tungsten-halogen lamp was used as a light source for measuring the optical transmission characteristics of the optical fiber shown in the examples.

Example 1 10% of methyl methacrylate monomer was distilled as an initiator in a completely closed polymerization apparatus.
mmol/azo-tert-butane and 40 mmol/n-butyl mercaptan as a chain transfer agent were added by distillation in the absence of oxygen.
When the dust in this monomer was detected using a He-Ne laser, only 1 to 0.02 light spots or less, and in most cases less than 0.02 light spots, were observed per 10 cm of optical path length. After thoroughly mixing these monomers and carrying out bulk polymerization at 130°C for 24 hours, the temperature was gradually raised to increase the polymerization rate, and finally the polymerization was completed at 180°C to form the core component polymer. I got it. When a portion of this polymer was taken out and the transition metal ion content was measured, it was found that iron was 20 ppb, and cobalt and nickel were less than 2 ppb. This polymer is melt-spun at 190℃ to obtain diameter
A 0.90 mm fiber was obtained, silicone resin was used as the sheath component, the core fiber was guided through an orifice into the silicone resin, the sheath component was continuously coated on the core fiber, and the sheath component was cured. A composite fiber with a core-sheath structure with a film thickness of 0.10 mm was obtained. FIG. 2 shows the optical transmission characteristics of this optical fiber.
As is clear from Figure 2, the wavelengths are 520nm and 570nm.
There is a low loss window at m and 650nm, especially for wavelength
At 520nm and 570nm, the loss was 85dB/Km.
This loss is less than 100 dB/Km for a plastic optical fiber, which is comparable to the loss of an inorganic glass core optical fiber whose sheath is made of plastic.
Furthermore, as is clear from the comparison with the optical transmission characteristics of the low-attenuation all-plastic optical fiber manufactured by the conventional method shown in Figure 1,
It can be seen that an increase in loss due to scattering on the shorter wavelength side is hardly observed in the plastic optical fiber according to the present invention, and the effect of significantly reducing dust, minute voids, etc. that cause scattering loss is apparent. .

Example 2 In Example 1, di-tert was used as a polymerization initiator.
- Core component polymer obtained in the same manner as in Example 1 except that 5 mmol/butyl peroxide was used (the number of dust particles in the monomer was 2 to 0.02 or less per 10 cm of optical path length,
Transition metal ion content is 30ppb for iron and 30ppb for nickel.
Using a copolymer of vinylidene fluoride and tetrafluoroethylene as the sheath component, each polymer was extruded at 210°C using a core-sheath spinneret, and the core diameter was 0.65 mm. A composite fiber with a sheath component thickness of 0.10 mm was obtained. The window with the lowest loss of this optical fiber was at a wavelength of 570 nm, and its attenuation was 190 dB/Km. A low loss window of 210 dB/Km was also observed at a wavelength of 650 nm.

Example 3 In Example 1, 95 mol% of methyl methacrylate and 5 mol% of ethyl acrylate were used as core components.
Using mol% copolymer, tert as chain transfer agent
- The core component polymer was prepared in the same manner as in Example 1 except that 40 mmol/butyl mercaptan was used (dust in the monomer was 1 to 0.02 or less per 10 cm of optical path length, transition metal ion content was 20 ppb iron, nickel and less than 2 ppb of cobalt). Using trifluoroethyl methacrylate as the sheath component, it was extruded together with the core polymer at 200℃ using a sheath-core spinneret, with a core diameter of 0.80 mm and a sheath component film thickness of 0.08 mm.
A composite fiber was obtained. The window with the lowest loss of this optical fiber is at a wavelength of 570 nm,
The amount of attenuation was 110dB/Km. Also, the wavelength
Attenuation of 120dB/Km for 520nm and 650nm, respectively.
A low loss window of 180 dB/Km was observed.

Example 4 In Example 1, the azeotropic composition ratio of methyl methacrylate and styrene as core components (methyl methacrylate 52 mol%, styrene 48
A monomer containing 5 mmol/azo-tert-butane as a polymerization initiator and 30 mmol/n-butyl mercaptan as a chain transfer agent was used. Dust in this monomer is He−Ne
When detected by laser, 2 to 0.02 per 10 cm of optical path length
It was less than one. This monomer was bulk polymerized at 120°C for 24 hours in the same manner as in Example 1, and then the temperature was gradually raised and the polymerization was finally completed at 180°C to obtain a core component polymer. Ta. When a portion of this polymer was taken out and the transition metal ion content was measured, it was found to be 30 ppb of iron, 5 ppb or less of nickel, and 2 ppb or less of cobalt. This polymer was melt-spun at 180°C to obtain a fiber with a diameter of 0.85 mm, and an ethylene-vinyl acetate copolymer was used as the sheath component.
The obtained core fiber is introduced into the molten sheath component through an orifice, and the core fiber is continuously coated with the sheath component to a film thickness of 0.10.
A composite fiber with a core-sheath structure of mm was obtained.
FIG. 3 shows the optical transmission characteristics of this optical fiber. As is clear from Figure 3, this fiber has low-loss windows at wavelengths of 520nm, 580nm, and 655nm, with attenuations of 220dB/Km and 155dB/Km, respectively.
Km and 200dB/Km, achieving extremely low loss optical transmission characteristics.

As explained above, according to the present invention, a polymer mainly composed of methyl methacrylate with low scattering loss is used as a core component, and bulk polymerization is performed under special conditions that reduce minute voids, dust, and impurities. , melt spinning the polymer to form a core;
By using a synthetic polymer with a lower refractive index than the core as the sheath component, it has extremely superior optical transmission characteristics in the visible region compared to conventional plastic optical fibers, and has wavelengths of 520 nm and 520 nm.
At 570nm, a plastic optical fiber with extremely low transmission loss of less than 85dB/Km has been obtained, and as an optical fiber for long distances, it can take advantage of its advantages of being lighter and more flexible than inorganic glass. It has the advantage that it can be used as Further, according to the present invention, the wavelength is 570n.
It has an extremely low loss window at m and 650nm compared to conventional plastic optical fibers, so it has the advantage of being able to construct an economically efficient optical transmission system using inexpensive yellow-green and red light-emitting diodes as light sources. be.

In addition, in the method for producing plastic optical fiber according to the present invention, the core component polymer obtained as an intermediate product can be used as a material for optical waveguides etc. by processing such as casting or pressing, in addition to being used as an optical fiber. It is possible to use this method for various applications, and it is also possible to create optical circuits and optical paths with extremely low loss.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the measurement results of the optical transmission characteristics in the visible light range of a low-attenuation all-plastic optical fiber manufactured by a conventional method, and FIG. A graph showing the measurement results of the optical transmission characteristics in the visible light range of a low-loss plastic optical fiber containing methacrylate as a core component. 1 is a graph showing measurement results of optical transmission characteristics in the visible light region of a low-loss plastic optical fiber having a core component of a copolymer of the following.

Claims (1)

[Scope of Claims] 1. In producing a plastic optical fiber having a core made of a polymer mainly composed of methyl methacrylate and a synthetic polymer sheath having a lower refractive index than the core formed around the core, Next (A)
~(E) Step (A) Step of adding a monomer mainly composed of methyl methacrylate into a polymerization reaction vessel by distillation under an argon or nitrogen atmosphere, (B) Glass transition temperature of the polymer produced by polymerization A radical polymerization initiator that shows activity at a temperature higher than that and can be distilled under normal pressure or reduced pressure, and a chain transfer agent that can be distilled under normal pressure or reduced pressure.
(C) adding the monomer by distillation to the monomer in the polymerization reaction vessel containing methyl methacrylate as the main component; (D) melt-spinning the polymer under temperature conditions that do not lower the temperature below the glass transition temperature and do not generate bubbles in the polymer; and (E) continuously depositing an amorphous synthetic polymer having a refractive index lower than that of the core around the core, all of which are performed in a completely closed system. A method of manufacturing a low-loss plastic optical fiber. 2. In the method for manufacturing a low-loss plastic optical fiber according to claim 1,
Production of a low-loss plastic optical fiber, characterized in that the core is a copolymer containing 50 mol% or more of methyl methacrylate units and monomer units having a boiling point in the range of 80 to 160°C at normal pressure. Method.