JPH05297255A - Heat resistant optical fiber - Google Patents

Abstract

PURPOSE:To provide a polyimide covered optical fiber, which is capable of long distance transmission and excels in heat resistance, fatigue characteristic, and resistance against hydrogen. CONSTITUTION:A covering layer 3 of soft silicone resin having a low tensile modulus of elasticity is put on an optical fiber 1 as surrounding it. The tensile modulus is below 50kgf/mm<2>, while the film thickness of the covering layer lies in the extent 4-50mum. Another covering layer 2 of polyimide resin as heat resistant resin is put on the silicone layer 3 in such a way as surrounding it. In side of the silicone layer 3, a charbon layer 4 is formed. This carbon layer serves for enhancing the fatigue characteristic and resistance against hydrogen, while the soft silicon resin relieves hardening contraction of polyimide resin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant optical fiber whose coating structure is improved to improve its characteristics. In particular, a heat-resistant polarization-maintaining optical fiber coated with a polyimide resin has improved extinction ratio characteristics. Regarding things.

[0002]

2. Description of the Related Art The surface of an optical fiber made of quartz glass is liable to be scratched like ordinary glass. An ultraviolet curable resin (UV resin) is generally used as a coating material for protecting the optical fiber, and the guaranteed temperature of this resin is 100 ° C. at most.

In an environment where a normal optical fiber is used, such a temperature may be used, but for example, an optical composite overhead ground wire (OPGW)
In addition, a heat-resistant optical fiber that can withstand use at a higher temperature and higher temperature is required for those that are installed around a high temperature furnace.

Further, since the polarization-maintaining optical fiber can stably transmit information on the phase and the polarization plane in addition to the intensity information of light, it is not only used in the communication field but also in the measurement field such as a sensor using interference or polarization. It is also used in Among various sensors that apply polarization-maintaining optical fiber, the most noticeable one is an optical fiber gyro (O
FG). At present, OFG is being researched for practical use, and is partially commercialized for navigation of automobiles, automated guided vehicles, and the like. Recently, there is an increasing demand for using OFG in a direction control system of a moving body (for example, a tunnel excavator) in a high temperature environment. In order to apply OFG to such a system, a heat-resistant polarization-maintaining optical fiber capable of withstanding a high temperature of about 200 ° C. is required.

Based on the above-mentioned requirements, metal, ceramics, polyimide resin or the like has been studied as a coating material for providing high heat resistance instead of UV resin. However, metal coating has problems such as complexity of coating process and microbending due to shrinkage of coating film, and ceramic coating is extremely weak against bending due to poor ductility of the coating film, and initial strength. There is a problem such as low.

On the other hand, polyimide resin is often used as a heat-resistant coating material for ordinary optical fibers (for example, Japanese Patent Application Laid-Open No. 1-173006). A cross-sectional structure of such a conventional heat resistant optical fiber is shown in FIG. Conventionally, a single heat-resistant resin layer (polyimide,
Polytitanocarbosilane, ladder type silicone, etc.) 2 was coated to a thickness of 10 to 20 μm. That is, in this heat-resistant optical fiber, the heat-resistant resin 2 was directly coated on the surface of the bare optical fiber 1. Here, the bare optical fiber means a glass optical fiber consisting of a core and a clad in a normal single mode optical fiber, and in a polarization maintaining optical fiber, a core or a clad, a stress applying material, and It means an optical fiber composed of a support layer.

However, when the surface of the bare optical fiber 1 is directly coated with the heat resistant resin 2 such as a polyimide resin, the polyimide resin is harder than the UV resin, has a large tensile elastic modulus, and also has a shrinkage ratio upon curing. Since it is large, there is a problem that lateral pressure is applied to the bare optical fiber in the manufacturing process, and microbend is applied to increase loss or deteriorate the extinction ratio. For example, in the case of polyimide, the tensile elastic modulus (Young's modulus) is as large as 250 kgf / mm ² or more, the optical fiber receives a microbend due to the curing shrinkage in the baking oven, and the SM optical fiber of φ125 μm (single mode optical fiber) Loss is 0.03 to 0.08 dB / km
(Λ = 1.30 μm).

In the SP optical fiber (polarization-maintaining optical fiber), the extinction ratio is 10 to 15 dB / km (λ =) while the loss is increased by 2 to 3 dB / km (λ = 0.80 μm).
0.80 μm) deteriorated. In particular, the polyimide resin layer has almost no function as a buffer layer, and lateral pressure from the outside is directly applied to the bare optical fiber. Therefore, when the polyimide-coated SP optical fiber is wound in a coil, the extinction ratio is increased. There was a problem that it would deteriorate significantly.

[0009]

As described above, in the conventional heat-resistant optical fiber in which the heat-resistant resin is directly coated on the optical fiber, the loss increases during the coating process and it is difficult to use it for long-distance transmission. Further, the SP optical fiber coated with a polyimide resin has a drawback that when it is wound in a coil, the extinction ratio is greatly deteriorated.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a heat-resistant optical fiber having excellent transmission characteristics. Another object of the present invention is to provide a heat-resistant optical fiber having excellent fatigue characteristics and hydrogen resistance. Another object of the present invention is to provide a heat-resistant optical fiber which can be used even in a high temperature environment and whose extinction ratio does not deteriorate even when wound in a coil shape.

[0011]

According to the present invention, a coating layer of a soft silicone resin having a low tensile elastic modulus is provided as a buffer layer on the outside of a bare optical fiber, and heat resistance is provided on the outside of the soft silicone resin coating layer. A resin coating layer is provided. As the heat-resistant resin, polyimide resin, polytitanocarbosilane, or ladder type silicone is preferable.

Further, in order to further improve the transmission characteristics, the tensile elastic modulus of the soft silicone resin coating layer is 50 kg.
less than f / mm ² , preferably 0.01 to 30 kgf / m
m ² and the film thickness of this coating layer is in the range of 4 to 50 μm,
It is preferably 6 to 50 μm. Here, the Young's modulus of the silicone is less than 50 kgf / mm ² because it is 50 k
This is because the buffering effect disappears when gf / mm ² or more.
Further, the thickness of the silicone film is set in the range of 4 μm to 50 μm because if it is less than 4 μm, the buffering effect is low and the transmission characteristics are deteriorated, and if it is 50 μm or more, peeling occurs between the silicone and the heat resistant resin layer such as polyimide and the like This is because the strength is reduced.

The present invention can also be applied to SP optical fibers. For example, a polyimide resin coating layer is used as the heat resistant resin coating layer, and a silicone resin coating layer is provided as a lateral pressure buffer layer between the bare SP optical fiber and the polyimide resin coating layer. In particular, in order to improve the extinction ratio, the thickness of the polyimide resin coating layer is preferably 15 μm or more, and in this case, the tensile elastic modulus and the film thickness of the silicone resin coating layer are preferably within the above ranges. , Does not necessarily have to be within that range.

Further, an optional layer may be provided between the bare optical fiber and the soft silicone resin coating layer, or between the soft silicone resin coating layer and the heat resistant resin coating layer. In order to improve the above, it is preferable to have a three-layer structure in which a carbon layer is provided between the optical fiber and the soft silicone resin coating layer.

[0015]

When a soft silicone resin coating layer having a predetermined thickness is provided as a buffer layer on the outside of the bare optical fiber, the shrinkage force of the heat-resistant resin, which is a protective layer having a large tensile elastic modulus, upon curing shrinkage is coated with the silicone resin. The layers relax and effectively prevent microbending from occurring in the optical fiber. This greatly improves the transmission characteristics.

Similarly, by interposing a silicone resin coating layer between the bare SP optical fiber and the polyimide resin coating layer, the lateral pressure applied to the bare optical fiber when wound in a coil is reduced. , The extinction ratio is greatly improved.

In particular, when the heat resistant SP optical fiber of the present invention is applied to a sensing coil wound around a bobbin having a small diameter of several tens of millimeters for several hundreds of meters, the thickness of the polyimide resin coating layer is as described above. It is desirable that it is 15 μm or more. The reason for this will be described with reference to FIG. 6 showing the relationship between the thickness of the polyimide resin coating layer and the extinction ratio of the sensing coil. The diameter of the bobbin is 30 mm, and the length of the wound optical fiber is 500 m. It can be seen from FIG. 6 that the extinction ratio sharply deteriorates when the thickness of the polyimide resin coating layer is 15 μm or less. This is because the rigidity of the coating layer becomes small and the lateral pressure generated by the overlapping of the optical fiber core wires is applied to the bare optical fiber.

In any heat-resistant optical fiber, by further providing a carbon layer inside the soft silicone resin coating layer, the hydrogen resistance and fatigue characteristics of the optical fiber are improved and the reliability is further improved.

As described above, according to the coated optical fiber of the present invention, whether it is a normal optical fiber or an SP optical fiber, it has excellent high temperature characteristics and can be used for long-distance transmission. Next, the range of applications can be expanded. Further, carbon coating is also possible, which is effective as a heat-resistant optical fiber in consideration of bending loss and hydrogen resistance.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

[Example 1] A quartz optical fiber preform was melted and spun at a drawing speed of 30 m / min to give an optical fiber outer diameter φ1.
After setting the thickness to 25 μm, silicone resin (Young's modulus =
0.1 kgf / mm ² ) was applied, and the film thickness after baking and drying at 400 ° C. was set to 8 μm. Polyimide (Toray Co., Ltd., Trenice 2000) is applied to the outside thereof, and the temperature is 200 ° C to 4 ° C.
The coating was cured in a baking oven at 00 ° C. to have a final coating outer diameter of φ180 μm. FIG. 2 shows a cross-sectional structure of the heat-resistant optical fiber obtained at this time. A silicone resin coating layer 3 is formed on the outer peripheral portion of the bare optical fiber 1 so as to surround it. The polyimide resin coating layer 2 is formed so as to surround it.

The heat-resistant optical fiber thus obtained had an initial strength of 6 kgf / mm ² , and when its transmission characteristics were evaluated at room temperature, it was found to be superior to the conventional SM optical fiber coated with only a single layer. Example silicone /
The SM optical fiber coated with two layers of polyimide has a wavelength of 1.
Transmission loss in the 3 μm band is 0.38 dB / km to 0.3
It fell to 3 dB / km. Also, in the air atmosphere, 200 ℃
As a result of high-temperature treatment for 300 hours, no change was observed in the transmission loss and extinction ratio of the SM optical fiber, indicating excellent high-temperature characteristics.

[Examples 2 to 5 and Comparative Examples 1 to 4] The quartz optical fiber preform was melted and spun in the same manner as in Example 1 to obtain an optical fiber outer diameter of 125 μm, and then a silicone resin (Young's modulus 0.04 kgf / Mm ² , coefficient of thermal expansion 2.6 × 10 ^-4
/ C, OF-182 manufactured by Shin-Etsu Chemical Co., Ltd.) is applied,
Film thickness after baking and drying at 400 ℃ from less than 3μm to 60μm
I changed the above. Polyimide was applied to the outside of the silicone in the same manner as in Example 1 and cured in a baking oven at 200 ° C. to 400 ° C. to give a final coating outer diameter of φ180.
μm. The heat-resistant optical fibers having different silicone film thicknesses thus obtained were measured for peeling and transmission characteristics. The results are shown in Table 1. With a silicone film thickness of 4 μm, there is no problem although the transmission characteristics are slightly deteriorated.
If the thickness is less than μm, the fiber comes into contact with the die, which makes manufacturing difficult.

[0024]

[Table 1]

[Examples 6 to 10 and Comparative Examples 5 to 6] Similar to Example 1, the optical fiber preform was melted and spun to give an optical fiber outer diameter of 125 μm, and Young's modulus of less than 0.01 to 55 kgf / Apply different silicone of mm ² and
The film thickness after baking and drying at 00 ° C. was set to 8 μm. Then, polyimide is applied in the same manner as in Example 1, and 200 ° C to 400 ° C.
It was cured in a baking oven at ℃, and the final coating outer diameter was φ180 μm. The transmission characteristics and the coating state of the heat-resistant optical fibers having different silicone Young's moduli thus obtained were measured. The results are shown in Table 2.

[0026]

[Table 2]

[Embodiment 11] As shown in the sectional structure of FIG. 1, a carbon layer 4 is provided between the bare optical fiber 1 and the soft silicone resin coating layer 3 similar to the embodiment 1, and a carbon layer is formed from the center. It has a three-layer structure of silicone-polyimide.
That is, the outer circumference of the bare optical fiber 1 spun to an outer diameter of 125 μm is first coated with amorphous carbon by a thermal CVD method (thickness: 500 Å), and then the outer periphery is covered with a silicone resin coating layer 3 to have an outer diameter of 140 μm. After that, the polyimide resin coating layer 2 was further formed on the outer periphery thereof so that the outer diameter was 180 μm. The transmission characteristics of the heat-resistant optical fiber thus obtained are shown in Example 1.
The heat resistance was improved, and fatigue resistance and hydrogen resistance were also improved.

[Embodiment 12] An elliptic jacket type SP optical fiber preform was spun to have an outer diameter of 125 μm by a drawing furnace, silicone resin was applied to the outer periphery, and this was baked and cured by passing through an electric furnace at 350 ° C. Then, a coating layer having an outer diameter of 140 μm was formed. Further, a polyimide (Treney 2000 manufactured by Toray Industries, Inc.) was applied to the outer periphery thereof and passed through an electric furnace set at 500 ° C. to be cured to form a coating layer having an outer diameter of 180 μm (polyimide film thickness was 20 μm). The cross-sectional structure of the heat resistant SP optical fiber manufactured in this manner is shown in FIG. In the figure, 1 is an elliptical jacket type SP optical fiber bare wire, which comprises a core 5, a clad 6, an elliptical jacket layer 7, and a support layer 8. Reference numeral 13 is a silicone resin coating layer, and 12 is a polyimide resin coating layer.

The characteristics of the heat-resistant SP optical fiber thus obtained were measured in a bundled state, and the wavelength was 0.8 μm.
The transmission loss at m is 2.8 dB / km to 2.3 dB / km
The extinction ratio is improved from -16dB / km to -28d.
It was improved to B / 1km. This characteristic is almost the same as the characteristic of an ordinary SP optical fiber coated with UV resin. Next, a sensing coil in which 1 km of this heat-resistant SP optical fiber was wound around a bobbin (material: Invar alloy) with a diameter of 30 mm was prototyped and its characteristics were evaluated, and there was almost no increase in loss due to overlapping winding and deterioration of extinction ratio. It was Further, the extinction ratio of this sensing coil was as good as -26 dB or less in the temperature range of -40 ° C to 200 ° C. Also, 200
Even after heating at 400 ° C. for 400 hours, no change was observed in the transmission loss and extinction ratio of the SP optical fiber, indicating excellent high temperature characteristics.

[0030]

[Table 3]

In this embodiment, the elliptical jacket type SP is used.
Although an optical fiber is used, another SP optical fiber, such as an elliptic core fiber or a PANDA fiber, may be used. The outer diameter of the bare SP optical fiber is 125 μm.
The diameter may be larger or smaller.

[Embodiment 13] The outer circumference of an SP optical fiber bare wire spun to an outer diameter of 125 μm is first coated with amorphous carbon by a thermal CVD method (thickness: 500 Å), and then a silicone resin coating layer is provided on the outer circumference thereof. After the outer diameter was formed to be 140 μm, a polyimide resin coating layer was further formed on the outer periphery so that the outer diameter was 180 μm. The heat-resistant SP optical fiber thus obtained had the same loss and extinction ratio as those of Example 12.
Furthermore, when the dynamic fatigue characteristics of the heat-resistant SP optical fiber of this example were evaluated, the fatigue coefficient n value was 200 or more (normal UV
The n value of the resin-coated fiber was 20 to 30).

Comparative Example 7 S spun to an outer diameter of 125 μm
A polyimide resin coating layer was formed directly on the outer circumference of the bare P optical fiber so that the outer diameter was 160 μm. The sectional structure is shown in FIG. The extinction ratio of this heat-resistant SP optical fiber was at most about -18 dB (fiber 1 km).

Comparative Example 8 S spun into an outer diameter of 125 μm
A silicone resin coating layer was formed on the outer circumference of the bare P optical fiber so that the outer diameter was 140 μm, and then a polyimide resin coating layer was further formed on the outer circumference so that the outer diameter was 160 μm. Is 10 μm). The heat-resistant SP optical fiber thus obtained had the same loss and extinction ratio as those of Example 12 measured in the bundled state. However, when this heat-resistant SP optical fiber was wound around a bobbin with a diameter of 30 mm, the extinction ratio deteriorated significantly.

Example 14 (Polytitanocarbosilane,
Examples of Ladder Type Silicone)] In Examples 1 to 13 and Comparative Examples 1 to 8, when polytitanocarbosilane or ladder type silicone was coated instead of the polyimide resin coating, polytitanocarbosilane was coated. The same result as that of the case of coating with the polyimide resin was obtained for both the case and the case of coating with the ladder type silicone.

The polytitanocarbosilane coating was formed by adding 100 parts by weight of polytitanocarbosilane to 20 parts of fumed silica.
Parts by weight, 1.0 parts by weight of tetrastearoxytitanium was added to the composition, which was diluted with a mixed solvent of xylene and methyl cellosolve, coated on a spun optical fiber using a coating die, and passed through a heating furnace at 400 ° C. Allowed to cure.

Ladder type silicone is a polymer represented by the following structural formula (1),

[0038]

[Chemical 1]

(R ¹ , R ² , R ³ and R ⁴ each represent an alkyl group or an aryl group and may be the same or different. The alkyl group represented by R ¹ , R ² , R ³ and R ⁴ is, for example, A methyl group and an ethyl group, R ¹ , R ² , R ³ ,
The aryl group represented by R 4 is, for example, a phenyl group. r is a positive integer representing the degree of polymerization. ) In each of the examples and comparative examples, R ¹ , R ² , R ³ and R in the structural formula (1) are used.
A precursor xylene solution in which all ⁴ are methyl groups was applied to a spun optical fiber, passed through an electric furnace at a temperature of 400 ° C., and dried and cured.

[0040]

The present invention has the following effects.

(1) According to the heat-resistant optical fiber described in claims 1 and 2, a silicone layer is provided as a buffer layer on the surface of the optical fiber, and the shrinkage force of the heat-resistant resin as the protective layer during curing shrinkage is relaxed. Therefore, the transmission characteristics and the tensile strength can be improved.

(2) According to the heat-resistant optical fiber described in claims 3 to 4, by coating the surface with a silicone resin and a polyimide resin, heat-resistant polarization plane preservation with a good extinction ratio even at a high temperature of 200 ° C. An optical fiber is obtained. Furthermore, since this polarization-maintaining optical fiber has a good extinction ratio even if it is wound in a coil shape, it can be applied to a sensing coil. Therefore, it becomes possible to measure the angular velocity, the magnetic field, the electric field, etc. in a high temperature environment, and the industrial contribution is extremely large.

(3) According to the heat resistant fiber of the fifth aspect, since the carbon layer is further provided, fatigue characteristics and hydrogen resistance are also improved.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a silicone / polyimide-coated optical fiber having a carbon layer added according to an embodiment of the present invention.

FIG. 2 is a sectional view showing the structure of a silicone / polyimide coated optical fiber according to an embodiment of the present invention.

FIG. 3 is a sectional view showing the structure of a heat-resistant optical fiber of a conventional example.

FIG. 4 is a cross-sectional view showing the structure of a heat-resistant polarization-maintaining single-mode fiber of the present embodiment.

FIG. 5 is a cross-sectional view showing the structure of a conventional heat-resistant polarization-maintaining single-mode fiber.

FIG. 6 is a characteristic diagram of extinction ratio showing the thickness dependency of a polyimide resin coating layer in a sensing coil using a polarization-maintaining optical fiber.

[Explanation of symbols]

 1 Optical Fiber Bare Wire 2 Polyimide Resin Coating Layer 3 Soft Silicone Resin Coating Layer 4 Carbon Layer 5 Core 6 Clad 7 Elliptical Jacket Layer 8 Support Layer 11 Polarization Preserving Optical Fiber Bare Wire 12 Polyimide Resin Coating Layer 13 Silicone Resin Coating Layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Go Okubo 5-1-1 Hidaka-cho, Hitachi-shi, Ibaraki Hitachi Cable Co., Ltd., Optro System Laboratories (72) Inventor Masashi Nakamura Hidaka-cho, Hitachi-shi, Ibaraki 5-1-1 Hitachi Electric Cable Co., Ltd. Hidaka Plant (72) Inventor Yuetsu Azuma 5-1-1 Hidaka-cho, Hitachi City, Ibaraki Hitachi Cable Electric Co., Ltd.

Claims (5)

[Claims] 1. A coating layer made of a soft silicone resin having a low tensile elastic modulus is provided outside the bare optical fiber, and a coating layer made of a heat resistant resin is provided outside the soft silicone resin coating layer. Heat resistant optical fiber. 2. The elastic modulus of the soft silicone resin coating layer is in the range of 0.01 kgf / mm ^{2 to} 50 kgf / mm ² , and the thickness of the coating layer is in the range of 4 to 50 μm. The heat-resistant optical fiber according to claim 1, which is characterized in that. 3. The heat resistant optical fiber according to claim 1, wherein the bare optical fiber is a polarization maintaining optical fiber bare wire. 4. The heat-resistant optical fiber according to claim 3, wherein the heat-resistant resin coating layer is a polyimide resin coating layer and has a thickness of 15 μm or more. 5. The heat resistant optical fiber according to claim 1, further comprising a carbon layer provided inside the soft silicone resin coating layer.