JP2734586B2 - Optical fiber strand - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing an optical fiber. More specifically, the present invention relates to a method for producing an optical fiber which has a high strength, has a small decrease in strength over a long period of time, or has no increase in absorption by H ₂ molecules.

2. Description of the Related Art In optical communication, an optical fiber having a length of 1 km or more may be required. When such a long optical fiber is used, there is a problem that the mechanical strength of the optical fiber is not sufficient. That is, when an optical fiber is used for a special purpose such as an optical waveguide in a rapid communication system, the optical fiber is required to have a mechanical strength of 200,000 psi or more. However, the tensile strength of commercially available long optical fibers is in the range of 50,000-80,000 psi.

On the other hand, tensile strengths on the order of one million psi are typically observed in silicon oxide optical fiber materials drawn under ideal conditions. However, long optical fibers do not actually have such great mechanical strength. This is because, during and after the wire drawing process, mechanical friction or chemical erosion by contaminants in the atmosphere, such as water vapor, causes submicron-sized flaws on the surface of the optical fiber. Can be That is, an optical fiber having no scratch on its surface has a tensile strength on the order of one million psi.

As a countermeasure against the above-mentioned problem, an organic material coating is conventionally applied to the optical fiber after the optical fiber is drawn. However, these organic material coatings cannot prevent water vapor or hydroxyl ions from diffusing into the fiber. Therefore, even in the case of an optical fiber coated with an organic material, water vapor or hydroxyl group ions cause minute scratches on the surface during use or storage, and the strength of the fiber is reduced. Therefore, the optical fiber needs a hermetic coating to protect the surface from micro scratches.

As a method of forming the hermetic coating with an inorganic material such as silicon or various metals, a chemical vapor deposition (CVD) method is most frequently used today. In the CVD method, a coating material is supplied as a source gas, and a coating is formed by a surface reaction on the optical fiber surface. That is, 1
A coating is formed by reacting a seed or a plurality of source gases at a predetermined temperature.

In the CVD method, various coatings can be formed on an optical fiber. Examples of the coating that can be formed by the CVD method include silicon nitride, silicon, and phosphosilicate glass (phosphosilicate glass).
glasses), silicon, tin oxide, silicon oxynitride, boron and boron nitride. Further, a conventional polycrystalline coating such as Al or Sn can be applied on the fiber in the same manner. According to the CVD method, the coating is formed uniformly around the fiber, so that the fiber can be protected by a very thin coating. Therefore, loss due to microbending can be avoided.

FIG. 3 shows an apparatus for coating an optical fiber by a CVD method disclosed in Japanese Patent Publication No. 60-25381. The apparatus shown in FIG. 3 mainly comprises a first isolation chamber 22, a reaction chamber 23 and a second isolation chamber 24 connected by small diameter openings 26 and 27, respectively. Openings 25 and 28 each having a small diameter are formed in the first isolation chamber 22 and the second isolation chamber 24, and the optical fiber 20 enters through the opening 25, and the first isolation chamber 22 and the reaction chamber are formed.
It is pulled out from the opening 28 through the 23 and the second isolation chamber 24. While passing through the reaction chamber 23, a coating is formed on the surface of the optical fiber 20 by a chemical reaction. The first and second isolation chambers 22 and 24 are provided for isolating the reaction chamber 23 from the surrounding atmosphere, and each of them has an inert gas inlet 20.
Inert gas is introduced from 9, 210, and the pressure inside each of the isolation chambers is set to a pressure higher than the atmospheric pressure so that ambient air does not flow into the furnace from the openings 25, 28.
Raw material gas is introduced into the reaction chamber 23 from the inlet 211, and the gas after the reaction is discharged from the outlet 212. The raw material gas in the reaction chamber 23 is maintained at a predetermined temperature by the heating coil 213.

As described above, in the reaction chamber 23, the source gas chemically reacts, and a predetermined coating is formed on the surface of the optical fiber 20.
After the reaction proceeds uniformly on the surface of the optical fiber 20 and / or in the gas phase, a reaction product is deposited on the optical fiber 20. Also, activation of the reaction gas may be promoted by supplying energy into the reaction chamber 23 by microwave or high-frequency plasma or by photochemical excitation.

In the CVD method, in order to improve the reaction rate and the reaction efficiency, preheating the gas before introducing the raw material gas into the reaction chamber is also performed. At this time, by forming the optical fiber at a higher temperature than the reaction gas, it is possible to prevent the coating from being formed on the reaction chamber wall. in this case,
Immediately after the neck-down point where the optical fiber is drawn from the base material, the optical fiber may enter the reaction chamber while the optical fiber is still sufficiently hot. Further, only the optical fiber may be heated by a method such as irradiating the optical fiber inside the reaction chamber with an infrared ray or a laser beam. FIG. 4 shows such an optical fiber heating means.
1 shows a cross section perpendicular to the axis of the reactor described in 61-32270.

The device shown in FIG. 4 is housed in a container 33, is arranged substantially parallel to the traveling direction of the optical fiber 20, and emits radiation for heating the optical fiber 20, especially infrared light.
Elongate heat sources 31 and two heat sources 31
The reflection mirror 32 has an elliptical cross section. Two elongated heat sources 31 are disposed on one focal point of the ellipse and are configured such that the optical fiber 20 passes through the other focal point of the ellipse. The radiation emitted by the heat source 31 is directly or reflected by a reflecting mirror 32 and enters a transparent window 34,
The fiber 10 drawn through 34 is illuminated. Container 33
Has a plurality of cooling water passages 35, and a cooling medium circulates in the cooling water passages 35 to cool the container 33 around the reflecting mirror 32.

FIG. 5 shows a method for forming a hermetic coating on fused silica disclosed in Japanese Patent Publication No. 38-10363. In the method shown in FIG. 5, the silica fiber preform 1
The gas is burned and melted by a gas burner oxygen flame 41 emitted from a heating ring 40 surrounding the silica fiber preform 1, and the diameter is reduced by applying tension. Immediately below the heating ring 40, a cylinder 45 configured to introduce carbonaceous gas from the lower end and flow in the direction of the arrow is arranged, and forms a thin coating of carbon on the outer surface of the expanded silica fiber. The reason why carbon was selected as the material for the hermetic coating is that the film formation rate is higher than other materials, and the effect of preventing H ₂ transmission and preventing deterioration in strength is large.

Problems to be Solved by the Invention In the above-described conventional methods, no special attention was paid to the processing temperature and the like when forming the hermetic coating. Therefore, the processing temperature at the time of forming the coating is not suitable for the optical fiber, and the initial strength of the optical fiber is sometimes reduced. In addition, the performance of the coating formed by the conventional method is insufficient, H ₂ transmission cannot be completely prevented, and the performance of the optical fiber cannot be maintained for a long period of time.

Accordingly, an object of the present invention is to provide a method for manufacturing a strand of an optical fiber, which solves the above-mentioned problems of the prior art and can apply a high-performance coating without deteriorating the strength of the optical fiber. It is in.

Means for Solving the Problems According to the present invention, in an optical fiber wire composed of a material containing SiO _{2 and} having a surface other than an end face covered with a pyrolytic carbon film, the specific resistance of the pyrolytic carbon film is , 8.0 × 10 ⁻³ Ω · cm or less is provided. In the present invention, the bare fiber is expressed by the following formula: 0.035 m / min <R × v / L ≦ 0.075 m / min ‥‥ (1) (where L is the neck-down portion of the optical fiber beam material) More preferably, the distance to the reactor, R is the diameter of the bare fiber, and v is the speed at which the bare fiber travels).

In the method of the present invention, it is preferable that the source gas for the pyrolytic carbon coating is flowed downward from the top of the reactor, that is, in the direction in which the fiber travels. Furthermore, in the method of the present invention, after forcibly cooling the fiber coated with pyrolytic carbon through the reactor,
It is preferable to coat the resin.

Action The method of the present invention has a major feature in that a bare fiber at a temperature of 700 to 1400 ° C. is coated with pyrolytic carbon by a CVD method. Further, in the method of the present invention, the bare fiber may be allowed to reach the reaction furnace under a condition satisfying the expression (1). That is, the bare fiber reaching the reactor under the condition satisfying the expression (1) has a temperature of 700 to 1400 ° C. without exception. Since it is difficult to manufacture the optical fiber while measuring the temperature of the bare fiber immediately after drawing, the temperature at which the bare fiber reaches the reaction furnace is controlled by controlling the parameter of the above equation (1). Is controlled.

The present inventors conducted various experiments in order to solve the above-mentioned conventional problems. 1. Observation of the fracture surface of the fiber whose initial strength was reduced revealed that crystals of SiC were found.

2. Observation of the fracture surface of the fiber where the initial strength did not decrease showed no SiC crystal.

3. Pyrolytic carbon was grown on a quartz plate on a slide glass at 1000 ° C. When this was observed by X-ray, there was no spot for SiC. When the sample was heat-treated at a high temperature (treatment time: 10 minutes), a spot corresponding to SiC was formed in the sample heat-treated at 1400 ° C. or higher.

4. As a result of conducting an H ₂ test (immersing in H ₂ 100% room temperature for one week; hereinafter the same conditions), the specific resistance of the pyrolytic carbon coated fiber having no absorption peak of H ₂ molecule at 1.24 μm was 8 × 10 ⁻³ Ω · cm or less. When these fibers were heat-treated, the specific resistance was reduced by heat treatment at 800 ° C. or higher. This indicates that the pyrolytic carbon coating was performed at 800 ° C. or higher.

5.H ₂ test was carried out the results, the resistivity of the pyrolytic tic carbon coated fibers peak in the absorption of H ₂ molecules appeared to 1.24μm is at 8 × 10 ^-3 Ω · cm or higher, 700 ° C.
The specific heat was also reduced by the following heat treatment.

Got a fact. From these facts, the method of the present invention has been derived.

In the method of the present invention, the temperature of the bare fiber when coating the pyrolytic carbon must be 700 to 1400 ° C. When the bare fiber is coated with pyrolytic carbon at a temperature exceeding 1400 ° C., the initial strength of the fiber is significantly impaired by SiC fine particles formed by the reaction between SiO ₂ and C. In addition, a coating formed when the temperature of the bare fiber is 700 ° C. or lower becomes a coating (having a large specific resistance) close to an organic substance in which a large amount of H atoms remain, so that H ₂ is transmitted. In particular, in order to make the transmittance of H ₂ molecules virtually zero, the specific resistance formed at 700 ° C. or more must be 8 × 10 ⁻³ Ω · c
It is effective to coat a film of m or less.

Also, the reactor temperature at this time depends on the source gas,
For example CH ₄ if 900~1000 ℃, C ₂ H ₂ if 500-600
℃, CH ₄ + CCl ₄ may be 500 to 600 ℃.

The raw material gas is supplied from the top to the bottom of the reactor, that is,
It is more preferable to flow in the same direction as the fiber travels. By flowing the raw material gas in the same direction as the direction in which the fiber travels, it is possible to more effectively prevent the initial strength of the optical fiber from lowering. This is because, when the raw material gas flows in the same direction as the fiber travels, the raw material gas that has not yet been sufficiently heated immediately after being introduced into the reaction furnace and does not contain harmful products such as soot, has a high temperature. Reacts directly on bare fiber. Therefore, it is possible to coat the fiber without causing microscopic scratches on the surface due to harmful products.

In the method of the present invention, it is preferable that the fiber coated with the above-mentioned parolytic carbon is coated with a resin. By coating the resin, the handling of the optical fiber becomes easier and the effect of the coating is improved.

The suitable temperature for coating the fiber with the above resin is
In general, when the temperature is 100 ° C. or lower, and particularly when a thermosetting resin is used, when the temperature of the fiber is high, problems such as the resin being cured in the die occur. In the method of the present invention, R × v / L>
To satisfy 0.035m / min, the drawing speed of the fiber is quite fast. Therefore, when the fiber is coated with a resin in a process immediately after the coating with the pyrolytic carbon, the height of the apparatus must be considerably increased in order to keep the fiber at an appropriate temperature by natural cooling. Therefore, it is preferable to forcibly cool the fiber coated with pyrolytic carbon to suppress the overall height of the device.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following disclosure is merely an example of the present invention, and does not limit the technical scope of the present invention.

Embodiment FIG. 1 shows an example of an apparatus for realizing the method of the present invention. 1 is a known electric furnace, a high-frequency heating furnace, etc.
The optical fiber preform 1 is melted and spun into a bare fiber 2 in a drawing furnace 15 having a heat source that does not generate H ₂ and dust. A pyrolytic carbon coating is formed on the surface of the bare fiber 2 in a reaction furnace 16 by a CVD method. The outer diameter of the fiber coated with the pyrolytic carbon is measured by the laser device 17, and the drawing speed and the like are controlled so as to have a predetermined size. The fiber that has passed through the laser device 17 is cooled by a cooling device 18, and a resin is applied by a die 19.

The reactor 16 is preferably a furnace capable of intensively heating the bare fiber 2, such as an infrared central heating furnace, in order to suppress as much as possible an extra reaction occurring on the surface of the bare fiber 2. In this embodiment, the reaction furnace 16 is constituted by a reaction tube 5 surrounded by a cooling jacket 4 made of quartz glass inside an infrared lamp 3 for intensively heating the bare fiber 2. H is provided between the cooling jacket 4 and the reaction tube 5.
e, flowing cooling gas such as N _2, it is prevented an increase in the temperature of the reaction tube 5. The reaction tube 5, the branch pipe 6 and 7 at the bottom, the branch pipe 8 and 9 are provided in the upper, N ₂ is the branch pipe 6 and 9
Flow seal gas such as A raw material gas of a hydrocarbon or a derivative thereof is supplied to the inside of the reaction tube 5 from the branch pipe 7, and the gas after the reaction is discharged from the branch pipe 8. In addition, the outside air and the reaction tube 5
Slits 10, 11 and 12 are formed at three places in the reaction tube 5 to effectively separate the atmosphere from the inside. Each slit is shaped like a funnel, all down to facilitate the passage of the bare fiber.

In the reactor 16 configured as described above, only the bare fiber 2 is intensively heated, and the wall surface of the reaction tube 5 is cooled from the outside. There is no reaction, you can continue to use without forever. The temperature of the reactor is such that the raw material gas gradually decomposes, but does not generate soot.For example, 500 ° C to 600 ° C for alloselen, 900 ° C to 100 ° C for methane.
Set to 0 ° C.

In the above apparatus, L is the distance between the neck down portion 13 of the optical fiber preform and the slit 11. In the above-mentioned apparatus, each part is adjusted so that the distance L, the outer diameter R of the bare fiber 2 and the drawing speed v satisfy the above formula (1).

Using the above apparatus, the optical fiber base material was melted and spun, and a bare fiber was coated with pyrolytic carbon, and further a resin was coated thereon to produce an optical fiber. R, L, and v are adjusted to the values shown in the following table, and the optical fiber is manufactured under the conditions satisfying the expression (1) and other conditions, and the obtained optical fibers are compared. did. The temperature of the reaction furnace was 550 ° C., N ₂ gas was supplied as a sealing gas from the branch pipes 6 and 9 at a rate of 2 l / min, and a raw material gas C ₂ H ₂ was supplied from the branch pipe 7 at a rate of 500 cc / min. The gas after the reaction was exhausted from the branch pipe 8 at a fixed rate of 2 l / min. The diameter of the bare fiber was 125 μm or 150 μm, and the thickness of the pyrolytic coating was 50 to 100 nm.

FIG. 2 shows another example of an apparatus for realizing the method of the present invention. The apparatus shown in FIG. 2 is completely the same as the apparatus shown in FIG. 1 except that the branch pipe 7 and the slit 11 are omitted, and therefore description thereof is omitted.

Using the device shown in Fig. 2, the optical fiber preform is melted and spun, the bare fiber is coated with pyrolytic carbon, and the resin is further coated thereon to manufacture the optical fiber. did. The temperature of the reactor is the same as in the apparatus of FIG.
Although the temperature was set to 550 ° C., the branch pipe 9 supplied N ₂ gas as a seal gas at a rate of 2 l / min, the branch pipe 8 supplied the raw material gas C ₂ H ₂ at a rate of 500 cc / min, and the branch pipe 6 supplied a 2l /
The air was evacuated every minute. That is, the raw material gas and the seal gas were flowed from the upper part to the lower part of the reaction tube 5 in the same direction as the traveling direction of the fiber. The cooling gas for the reaction tube 5 also flowed from the upper portion to the lower portion of the reaction tube 5. Other conditions were set equal to those in the fifth embodiment performed using the apparatus shown in FIG.

In this example, the fiber coated with pyrolytic carbon was forcibly cooled by the cooling device 18 and then coated with the resin by the die 19. The cooling device 18 has a length of 30
cm, 1.5 cm inner diameter, 10 l of He flow per minute, and the bare fiber is cooled to 70 ° C or less. When the forced cooling was performed, even when the distance between the reaction furnace 16 and the die 19 was 1 m, the resin could be applied to the fiber without being cured in the die 19.

The characteristics of carbon-coated fiber (resistivity, hydrogen transmittance = Δα _1.24 , initial strength) are R × v / L
When R × v / L ≦ 0.030 m / min, no pyrolytic carbon film is formed on the fiber surface.

The pyrolytic carbon film formed at 0.030 m / min <R × v / L ≦ 0.035 m / min has a high specific resistance and permeates hydrogen considerably.

The pyrolytic carbon film formed at 0.035 m / min <R × v / L ≦ 0.075 m / min hardly permeates hydrogen and hardly decreases the initial strength of the fiber.

When a pyrolytic carbon film is formed at 0.080 m / min <R × v / L, the initial strength of the fiber is greatly reduced.

The result was obtained.

Furthermore, it was found that when the raw material gas was flowed in the same direction as the traveling direction of the fiber, the specific resistance increased and the initial strength also improved.

As a result, it was proved that the method of the present invention was effective for producing an optical fiber having no increase in loss due to hydrogen, high initial strength, and little fatigue deterioration.

Effects of the Invention As described in detail above, according to the method of the present invention, a bare fiber can be coated with a pyrolytic carbon film having an extremely low hydrogen transmittance without reducing the strength of the fiber.

Therefore, the optical fiber produced by the method of the present invention has no increase in loss due to hydrogen for a long time, has a high initial strength, and has little fatigue deterioration. Therefore, it is suitable for use in an atmosphere where water or hydrogen is highly concentrated or under stress, for example, for use in a submarine cable.

[Brief description of the drawings]

1 and 2 are longitudinal sectional views each showing an example of an apparatus for carrying out the method of the present invention, and FIGS. 3 to 5 are schematic views each showing an apparatus for carrying out a conventional method. [Main Reference Symbols] 1 ... optical fiber preform, 2 ... bare fiber, 3 ... infrared lamp, 4 ... cooling jacket, 5 ... reaction tube,
6, 7, 8, 9 ... branch pipe, 10, 11, 12 ... slit, 15
…… Draw furnace, 16… Reactor, 17… Laser device, 18 ……
Cooling device, 19: Dice, v: Fiber speed (linear speed)

Claims (2)

(57) [Claims] 1. An optical fiber comprising a material containing SiO _{2 and} having a surface other than an end face covered with a pyrolytic carbon film, wherein the specific resistance of the pyrolytic carbon film is 8.0 × 10 ⁻³ Ω. -An optical fiber which is not more than cm. 2. The optical fiber according to claim 1, wherein the specific resistance of the pyrolytic carbon film is 1.5 to 2.0 × 10 ⁻³ Ω · cm.