JPH10212130A - Production of optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an optical fiber applicable even when a material having low chemical and heat resistances, e.g. a transparent resin is used as a core material by subjecting a gelatinous compd. of silica to supercritical drying and sticking the resultant silica aerogel made of a porous skeleton of silica as a cladding material to the surface of a core to form a cladding layer. SOLUTION: A gelatinous compd. of silica obtd. by using alkoxysilane, water glass, etc., as starting materials is subjected to supercritical drying and treatment for imparting hydrophobic property to produce hydrophobic silica aerogel made of a porous skeleton of silica and having a refractive index of 1.008-1.18. The silica aerogel is stuck as a cladding material to the surface of a core 2 in several μm thickness to form a cladding layer 1 and the objective optical fiber is obtd. The aerogel may be stuck by an electrostatic process using static electricity. In this case, the aerogel is preferably used in the form of powder having about several μm to several mm diameter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing an optical fiber, and more particularly to a method for producing an optical fiber using silica airgel having a porous skeleton of silica as a cladding material.

[0002]

2. Description of the Related Art In general, an optical fiber comprises a core serving as a light transport path and a cladding layer covering the outer surface of the core, and the cladding material forming the cladding layer forms a core. A material having a lower refractive index than the core material is used. Conventionally, as a core material of an optical fiber, a glass such as quartz glass or a multi-component glass, an acrylic or styrene-based plastic such as methyl methacrylate, or a transparent liquid such as tetrachloroethylene has been used. I have. In addition, as the cladding material, a glass material such as a soda lime-based or borosilicate glass-based material having a lower refractive index than the core material, vinyl chloride, an acrylic-based material in which the refractive index is reduced by adding allyl diglycol carbonate or fluorine. Plastics and the like are used.

As described above, in an optical fiber, the refractive index of the cladding material is configured to be smaller than the refractive index of the core material. The magnitude of the total reflection angle of light at the interface between the material and the clad material differs.

In general, a relative refractive index difference is used as an index indicating a difference in refractive index between a core material and a clad material, and is expressed by the following equation.

Specific refractive index difference = (n ₁ −n ₂ ) / n ₁ where n ₁ is the refractive index of the core material and n ₂ is the refractive index of the cladding material.

The numerical aperture and the light receiving angle θ of the optical fiber are expressed by the following equations. Numerical aperture = n · sin θ = (n ₁₂ −n ₂₂ ) ^1/2 where n is the refractive index of the outside of the optical fiber, and usually n = 1.0 in air. Therefore, the light receiving angle of the optical fiber increases as the difference between the refractive indices of the core material and the cladding material increases, that is, as the relative refractive index difference increases. That is, in order to transmit a large amount of light, it is necessary to increase the relative refractive index difference, and this can be achieved by increasing the refractive index of the core material and decreasing the refractive index of the cladding material.

Conventionally, as a core material of a glass optical fiber, pure quartz glass having small light loss and excellent heat resistance and chemical resistance has been frequently used. However, in this case, the refractive index of pure silica glass is as low as 1.46, and there is a problem in selecting a clad material having a lower refractive index.
When glass is used for the cladding material, there is a method of adding a refractive index lowering component such as B ₂ O ₃ or fluorine in order to lower the refractive index more than pure silica glass. There is also a method of adding a dopant for raising the refractive index to quartz glass to raise the refractive index while maintaining a low optical loss. Such dopants include TiO ₂ ,
Ta ₂ O ₅ , SnO ₂ , Nb ₂ O ₅ , ZrO ₂ , Yb ₂ O ₃ , L
a ₂ O ₃ , Al ₂ O _{3 and the} like. In this case, pure quartz glass or doped quartz glass having a lower refractive index is used as the cladding material. When a plastic is used as the cladding material, a transparent resin such as a silicon resin such as polysiloxane or silicon rubber, or a fluorine-containing resin such as ethylene propylene fluoride or vinylidene fluoride may be used. These have low refractive indices of 1.2.
It is about 9 to 1.33.

As described above, the light receiving angle of the optical fiber changes due to the relative refractive index difference between the core material and the clad material. For example, in a glass optical fiber, when a flint type F2 glass (refractive index 1.62) is used as a core material and a soda lime type glass (refractive index 1.52) is used as a cladding material, the numerical aperture is 0.56. , The light receiving angle θ is 34 degrees. Also in the case of a plastic optical fiber, when a methacrylic resin (refractive index: 1.49) is used as the core material and a fluororesin (refractive index: 1.39) is used as the cladding material, the numerical aperture is 0.54, and the light receiving angle θ Is 32.5 degrees. As described above, in the conventional combination of the core material and the clad material, the light receiving angle of the optical fiber is about 30 to 50 degrees, and the optical fiber can carry only light incident at an angle smaller than the light receiving angle. It was difficult to transmit light. Further, since the irradiation angle of the light irradiated from the irradiation side end face of the optical fiber is equal to the light reception angle, the light cannot be irradiated at a wide angle.

[0009]

On the other hand, the applicant of the present invention has proposed an optical fiber having a larger light receiving angle and transmitting a larger amount of light than the above-mentioned conventional optical fiber as disclosed in Japanese Patent Application No. Hei.
No. -245222. In this optical fiber, the cladding layer is formed of silica airgel having a porous skeleton of silica, and the refractive index of the silica airgel can be as low as 1.2 or less. An optical fiber manufactured using silica airgel having such a low refractive index as a cladding material has a large refractive index difference, and can increase a light receiving angle up to 90 °, and transmits a large amount of light. You can do it.

In the optical fiber manufacturing method in this case, a silane gel compound obtained by hydrolytic polymerization of alkoxysilane is adhered to the surface of the core, and in this state, the gel compound is supercritically dried. A cladding layer made of silica airgel is formed on the core surface.

However, in the above-mentioned manufacturing method, when a transparent resin is used as the core material, it is not preferable in terms of chemical resistance and heat resistance of the core material in the manufacturing process. That is, in the step of attaching the silane gel compound to the surface of the core, the organic solvent contained in the gel compound erodes the core surface layer, deteriorating the state of the interface between the core and the cladding layer, and lowering the light transmission efficiency. And the core may be deformed by heat even in the supercritical drying step.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing an optical fiber having a large light receiving angle and capable of transmitting a large amount of light. It is an object of the present invention to provide a method for producing an optical fiber which can be applied even when a material having low chemical resistance and heat resistance such as a transparent resin is used.

[0013]

According to a first aspect of the present invention, there is provided a method for producing an optical fiber, comprising: a silica aerogel comprising a porous skeleton of silica by supercritically drying a gel compound of silica. And forming a clad layer by adhering the silica airgel as a clad material to the surface of a core.

According to a second aspect of the present invention, there is provided the optical fiber manufacturing method according to the first aspect, wherein the silica airgel is attached to the surface of the core by electrostatic force.

According to a third aspect of the present invention, in the method of manufacturing an optical fiber according to the first or second aspect, the core material constituting the core is a transparent resin.

According to a fourth aspect of the present invention, in the method for producing an optical fiber according to any one of the first to third aspects, the silica airgel has a refractive index of 1.
It is characterized by using the one in the range of 008 to 1.18.

According to a fifth aspect of the present invention, there is provided a method of manufacturing an optical fiber according to any one of the first to fourth aspects, wherein the silica airgel is subjected to a hydrophobizing treatment in the process of manufacturing the silica airgel. Things.

According to a sixth aspect of the present invention, in the method of manufacturing an optical fiber according to any one of the first to fifth aspects, when the silica airgel is prepared, the silica airgel is hydrolyzed and polymerized using alkoxysilane as a starting material. It is characterized in that the above-mentioned gel compound is obtained.

According to a seventh aspect of the present invention, in the method for producing an optical fiber according to any one of the first to fifth aspects, when producing the silica airgel, the pH of the aqueous solution is reduced by using water glass as a starting material. To obtain the above gel-like compound.

According to an eighth aspect of the present invention, there is provided the optical fiber manufacturing method according to any one of the first to seventh aspects, wherein a coating material is provided outside a clad layer formed by attaching the silica airgel to the surface of the core. Is provided.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
Here, FIG. 1 shows an example of an optical fiber obtained by the optical fiber manufacturing method of the present invention, wherein (a) is a side view and (b) is an end view.

In the method for producing an optical fiber according to the present invention, a silica aerogel comprising a porous skeleton of silica is prepared by supercritically drying a gel-like compound of silica, and the silica aerogel is adhered to the surface of the core 2 as a cladding material. The optical fiber is obtained by forming the cladding layer 1 by using the optical fiber.

First, a silica airgel used as a clad material will be described. The silica airgel used in the present invention is composed of a porous skeleton of silica, and the refractive index thereof can be considerably smaller than the refractive index of a material usually used as a core material. The refractive index of the silica airgel can be freely changed according to the raw material mixing ratio of the silica airgel, and is adjusted so that the refractive index of the silica airgel is smaller than that of the core material constituting the core 2 for use as a clad material. In order to set the relative refractive index difference from the core material to be large and to ensure the performance such as transparency at a good level, it is necessary to adjust the relative refractive index in the range of 1.008 to 1.18. It is preferred. As described above, silica airgel can have a very small refractive index. Therefore, in the optical fiber obtained by forming the cladding layer 1 using the silica airgel, the relative refractive index difference of the cladding layer 1 with respect to the core 2 is reduced. The light receiving angle can be increased to a maximum of 90 degrees. For this reason, the optical fiber can transmit a large amount of light by condensing the light at a wide angle from the light input end, and can also increase the light output angle of the light from the light output end.

The method for producing the silica airgel is as follows: starting materials are alkoxysilane, water glass and the like.
A method of supercritically drying the silica gel compound obtained from these is employed. Here, the supercritical drying is a drying method in which the dispersion medium is gradually removed in a critical point of the dispersion medium (solvent) contained in the gel or in an atmosphere at a higher temperature and a higher pressure than the critical point. When the gel compound is supercritically dried, the solvent is extracted and removed at a critical point of the solvent contained in the gel compound or in an atmosphere at a higher temperature and a higher pressure than the critical point. In such an atmosphere, phase transition of the solvent, that is, so-called vaporization and condensation does not occur, so that the destruction and aggregation of the structure of the gel compound can be suppressed when the solvent is removed. For this reason, the airgel obtained by the supercritical drying becomes porous.

When using alkoxysilane as a starting material,
For example, US Pat. No. 4,402,927, US Pat.
As disclosed in U.S. Patent No. 2956 and U.S. Patent No. 4,610,863, alkoxysilanes (also referred to as silicon alkoxides and alkyl silicates) are hydrolyzed and polymerized to produce a wet silica gel-like compound. A method of drying the compound in the presence of an organic solvent such as an alcohol or a dispersion medium such as carbon dioxide under supercritical conditions above the critical point of the dispersion medium (supercritical drying). In this case, as described in, for example, JP-A-5-279011 and JP-A-7-138375, it is preferable to perform a hydrophobizing treatment on the gel compound in a liquid layer or in a supercritical drying step. . That is, the silica airgel imparted with hydrophobicity in this way prevents the refractive index and the light transmittance from being deteriorated due to the adsorption of moisture, and as a result, the performance deterioration as an optical fiber is prevented. Because it is done. As the hydrophobizing agent used in the hydrophobizing treatment, those having a functional group that reacts with a silanol group and a hydrophobic group are used. Specific examples thereof include hexamethyldisilazane and hexamethyldisilazane. Organosilane compounds such as siloxane, trimethylchlorosilane, trimethylmethoxysilane, trimethylethoxysilane, triethylethoxysilane, triethylmethoxysilane, dimethyldichlorosilane, dimethyldiethoxysilane, methyltrichlorosilane, and ethyltrichlorosilane. And organic compounds such as carboxylic acids such as acetic acid, formic acid and succinic acid, and alkyl halides such as methyl chloride. One type of the hydrophobizing agent may be used alone, or two or more types may be used in combination.

On the other hand, when water glass is used as a starting material, for example, US Pat. No. 5,137,297 and US Pat.
As disclosed in No. 4, a gelling agent such as sulfuric acid is added to an aqueous solution of an alkali metal silicate (an aqueous solution of an alkali metal silicate), or a water glass aqueous solution is brought into contact with an acid ion exchange resin. A method in which a gel-like compound of silica is prepared by lowering the ph value, and this is subjected to supercritical drying in the same manner as in the above-mentioned case is exemplified. Also in this case, it is preferable to perform a hydrophobic treatment in the step of producing the silica airgel for the same reason as described above.

In the present invention, the material used for the core material constituting the core 2 is not particularly limited, and those generally used for the core material of an optical fiber can be used. Glasses such as glass, and acrylic or styrene-based transparent resins such as polymethyl methacrylate (PMMA) can be used, but are particularly effective when a transparent resin is used as described later.

The method of manufacturing an optical fiber according to the present invention comprises forming a clad layer 1 around a core 2 by using the silica airgel after producing the silica airgel as described above. A good optical fiber can be obtained regardless of the material used.

That is, in the present invention, even when a transparent resin having poor chemical resistance and heat resistance is used as a core material in manufacturing an optical fiber, the surface of the core 2 is formed using silica aerogel which has already been manufactured as a material. By forming the clad layer 1 on the core 2, the surface of the core 2 is not affected by the organic solvent contained in the silica gel compound which is an intermediate product in the process of producing the silica airgel,
Further, it is not necessary to be deformed by exposure to heat in the supercritical drying step. Of course, it goes without saying that the optical fiber manufacturing method of the present invention can also be applied to a case where a glass material having no problem in chemical resistance and heat resistance is used as the core material.

An example of a method for attaching the silica airgel as a clad material to the surface of the core 2 is an electrostatic process using an electrostatic force. That is, in this electrostatic process, one or both of the core 2 and the silica airgel are charged so as to attract each other, and the silica airgel is attached to the surface of the core 2 by the action of the electrostatic force generated between the core 2 and the silica airgel. It is to let. In this case, the shape of the silica allogel is preferably in the form of powder in order to be easily adsorbed by electrostatic force, and the particle diameter is preferably about several μm to several mm.

Examples of a method for charging the core 2 and the silica airgel include corona discharge charging and friction charging. As the simplest specific example, there is a method in which the surface of the core 2 is charged by rubbing with a suitable insulating material, and the silica airgel powder is adhered to the surface. As shown in FIG. 2, an apparatus including a grounded core 2, a spray gun 3 for discharging the silica airgel powder 10 by charge, and a high-voltage power supply 4 connected to the spray gun 3 is prepared. The silica airgel powder 10 is charged by corona discharge generated by the high voltage of 4 and friction between the powder particles, and the silica airgel powder 10 is charged from the spray gun 3 to the grounded core 2 and electrostatically adhered to the surface of the core 2. And a method in which the adhesion layer is used as the cladding layer 1. The upper limit of the thickness of the cladding layer 1 formed of silica airgel is not particularly limited as long as the cladding has a thickness capable of achieving the optical transmission function of the optical fiber. I just need.

In the present invention, as shown in FIGS. 3A and 3B, in order to prevent the cladding layer 1 formed of the silica airgel on the surface of the core 2 from peeling off.
It is preferable to provide a coating material on the outside. As the coating material, for example, a resin such as polyethylene, cross-linked polyethylene, polypropylene, polyvinyl chloride, or polyolefin elastomer, or a material made of metal such as stainless steel or aluminum can be used.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to embodiments.

Example 1 First, a powdery silica airgel was prepared as follows. That is, an oligomer of tetramethoxysilane (manufactured by Colcoat, product name: methylsilicate 51, average molecular weight: about 470), ethanol, water, and aqueous ammonia in a molar ratio of 1:43:20:
A sol was prepared by mixing at a ratio of 0.12, and a gel compound dispersed in a liquid phase was prepared. Next, this gel compound was heated and stirred at a temperature of 40 ° C. for about 2 hours in an ethanol solution of 0.2 mol / L of hexamethyldisilazane (a reagent manufactured by Toray Dow Corning Silicone Co., Ltd.).
A hydrophobic treatment was performed. Next, the gelled compound after the hydrophobizing treatment is placed in carbon dioxide at 18 ° C. and 55 atm, and an operation of replacing ethanol contained in the gelled compound with carbon dioxide is performed for about 2 hours. Supercritical drying is carried out at 40 ° C and 80 atm, which are supercritical conditions, and about 2
This was performed for 4 hours to obtain a powdery silica airgel comprising a porous skeleton of silica. The resulting powdered silica airgel had a particle size of 100 to 500 μm and a refractive index of 1.03. Next, an optical fiber was produced as follows.
That is, polymethyl methacrylate (refractive index 1.4)
9) A core material having a diameter of 3 mm and a length of 30 cm is used as a core material and charged by rubbing the surface thereof, and the silica airgel is adhered to the surface of the core to a thickness of 1 mm by electrostatic force to form a clad layer. Then, an optical fiber was manufactured by inserting it into a stainless steel flexible tube having a diameter of 5 mm prepared as a coating material.

(Example 2) An optical fiber was produced as follows using a powdered silica airgel produced in the same manner as in Example 1 and using the same core as in Example 1. That is, the silica airgel is charged by a corona discharge of 30,000 volts and friction between powder particles, and is discharged by a spray gun, and is electrostatically attached to the surface of the grounded core to form a clad layer. An optical fiber was prepared by inserting the fiber into a stainless steel flexible tube having a diameter of 5 mm prepared as a coating material.

(Comparative Example 1) Oligomer of tetramethoxysilane (manufactured by Colcoat, product name: methyl silicate 5)
1, average molecular weight: about 470), ethanol, water and aqueous ammonia in a molar ratio of 1: 43: 20: 0.12 to prepare a sol, and a gel-like compound dispersed in a liquid phase is prepared. Prepared. Furthermore, this gel compound was added at 0.2 mol /
L in an ethanol solution of hexamethyldisilazane (a reagent manufactured by Toray Dow Corning Silicone Co., Ltd.) at a temperature of 70 ° C.
Hydrophobic treatment was performed by heating and stirring for about an hour. The same core as in Example 1 was placed in a sol solution tank in which the gel-like compound thus hydrophobized was dispersed in the liquid phase.
After immersion for 2 seconds, a gel-like compound layer having a thickness of 20 μm was formed on the core surface by dip coating. The core on which the layer of the gel compound is formed is placed in carbon dioxide at 18 ° C. and 55 atm, and the operation of replacing ethanol contained in the gel compound with carbon dioxide is performed for about 5 minutes. Supercritical drying was performed for about 15 minutes at a critical condition of 40 ° C. and 80 atm to form a cladding layer made of silica airgel on the core surface. Further, this was inserted into a stainless steel flexible tube having a diameter of 5 mm prepared as a coating material to prepare an optical fiber.

Comparative Example 2 Using the same core as in Example 1, a 20 μm thick cladding layer was formed on the surface of this core using a fluororesin (refractive index: 1.40) as a cladding material to obtain an optical fiber. Was.

(Evaluation of Performance of Optical Fiber)
2 and one end of the optical fiber obtained in Comparative Examples 1 and 2.
Irradiate e-Ne laser light (wavelength 543.5 nm),
The light receiving angle θ was measured. In addition, a 10 W tungsten lamp was used as a light source, and light was emitted from one end of the optical fiber to enter the optical fiber, and the illuminance of light emitted from the other end was measured with an illuminometer to evaluate the amount of transmitted light.

[0039]

[Table 1]

As can be seen from the results in Table 1, Examples 1 and 2
In Example 2, since the illuminance at the emission end was larger than that in Comparative Examples 1 and 2, it can be said that the transmitted light amount was large. Considering this, first, in Comparative Example 2, the light receiving angle was small because the fluororesin having a higher refractive index was used as the cladding material than the silica airgel used in the example, and the transmitted light amount was Is getting smaller. In Comparative Example 1, since the silica airgel was used as the cladding material as in the example, the light-receiving angle had a maximum value of 90 °, but a step in which the core surface formed a gel-like compound layer in the manufacturing process At the same time, it is attacked by the ethanol contained in the gelled compound, and the heat in the subsequent supercritical drying step softens the core surface, and as a result, the interface state between the core and the clad layer deteriorates, It is considered that loss was caused in the optical transmission process in the optical fiber due to this, and the amount of transmitted light was reduced.

[0041]

As described above, according to the optical fiber manufacturing method of the present invention, an optical fiber having a large light receiving angle and capable of transmitting a large amount of light can be obtained. Even if a material with low chemical resistance or heat resistance is used, the core surface can be formed using a finished silica airgel to form a cladding layer on the core surface. Since the optical fiber can be manufactured without including a step of exposing to heat such as drying, an optical fiber which does not reduce the light transmission amount can be obtained even in such a case.

[Brief description of the drawings]

FIG. 1 shows an example of an optical fiber obtained by a method for producing an optical fiber of the present invention, wherein (a) is a side view and (b) is an end view.

FIG. 2 is a schematic explanatory view showing one example of an electrostatic process for attaching silica airgel to a core surface in the present invention.

FIGS. 3A and 3B show another example of the optical fiber obtained by the optical fiber manufacturing method of the present invention, wherein FIG. 3A is a side view and FIG. 3B is an end view.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cladding layer 2 Core 5 Coating material 10 Silica airgel powder

Continued on the front page (72) Inventor Kenji Sonoda 1048 Odakadoma, Kadoma City, Osaka Prefecture Matsushita Electric Works, Ltd.

Claims (8)

[Claims] 1. A silica airgel comprising a porous skeleton of silica is prepared by supercritically drying a gel-like compound of silica, and the silica airgel is adhered to a surface of a core as a cladding material to form a cladding layer. Optical fiber manufacturing method. 2. The method according to claim 1, wherein the silica airgel is attached to a surface of the core by electrostatic force.
The manufacturing method of the optical fiber described in the above. 3. The method for producing an optical fiber according to claim 1, wherein the core material constituting the core is a transparent resin. 4. The method for producing an optical fiber according to claim 1, wherein the silica airgel has a refractive index in the range of 1.008 to 1.18. 5. The method for producing an optical fiber according to claim 1, wherein the silica airgel is subjected to a hydrophobizing treatment in the process of producing the silica airgel. 6. The optical fiber according to claim 1, wherein said silica airgel is prepared by hydrolyzing and polymerizing an alkoxysilane as a starting material to obtain said gel-like compound. Manufacturing method. 7. The optical fiber according to claim 1, wherein in preparing the silica airgel, water gel is used as a starting material to lower the pH of the aqueous solution to obtain the gel-like compound. Recipe. 8. The method for producing an optical fiber according to claim 1, wherein a coating material is provided outside a clad layer formed by attaching the silica airgel to the surface of the core.