JP3286017B2 - Carbon coated optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon coated optical fiber suitably used for an optical amplifier and the like.

[0002]

2. Description of the Related Art When a quartz optical fiber comes into contact with hydrogen, absorption loss due to molecular vibration of hydrogen molecules diffused into the optical fiber increases, and P ₂ O ₅ , GeO ₂ , B ₂ O ₃ reacts with hydrogen and reacts with OH
Since it is taken into fiber glass as a base, there is a problem that transmission loss due to absorption of OH groups increases.

In order to solve such a problem, recently, a chemical vapor deposition method (hereinafter abbreviated as a CVD method) has
It has been disclosed that a carbon film is formed on the outer periphery of an optical fiber bare wire in a VD reactor to produce a carbon-coated optical fiber, thereby improving the hydrogen resistance of the optical fiber. The bare optical fiber has a structure in which the clad has a desired refractive index difference with respect to the core.In order to obtain such a bare optical fiber, erbium or the like is contained in quartz glass. Addition of a dopant for raising the refractive index or a dopant for lowering the refractive index such as fluorine has been performed. Specific examples of such an optical fiber bare wire include an erbium-doped fiber having a core made of erbium-doped quartz and a clad made of fluorine-doped quartz, and a high-density fiber having a core made of pure quartz and a clad made of fluorine-doped quartz. Silica core fibers and the like are known.

[0004]

However, when a carbon-coated optical fiber is manufactured using a bare optical fiber having a fluorine-doped quartz cladding (hereinafter abbreviated as a fluorine-doped optical fiber), an optical fiber base is required. The material is placed in a spinning furnace, and the fluorine-doped optical fiber obtained by melt-spinning the optical fiber preform is inserted into a CVD reactor,
A carbon film is formed on the outer circumference of the fluorine-doped optical fiber. At this time, since the surface temperature of the fiber in the CVD reactor is about 1000 to 1100 ° C., the fluorine in the cladding diffuses into the fluorine gas and diffuses into the cladding surface. ,
Migrate. For this reason, even if a carbon film is formed on the cladding, the fluorine on the cladding surface hinders the adhesion and formation of the carbon film, so that the efficiency of forming the carbon film deteriorates and the resulting carbon film also has poor density. Therefore, the characteristics inherent in the carbon film, that is, the characteristics of improving the hydrogen resistance of the optical fiber cannot be sufficiently exhibited.

An object of the present invention is to provide a carbon-coated optical fiber having a carbon film which is easy to adhere and form on the outer periphery of the optical fiber and has excellent denseness.

[0006]

According to the present invention, there is provided a carbon-coated optical fiber comprising: a core made of quartz doped with a dopant for increasing a refractive index;
It is characterized by comprising a clad made of the following quartz, a pure quartz layer having a thickness of 5 to 30 μm formed on the outer periphery of the clad, and a carbon film formed on the outer periphery of the pure quartz layer.

[0007]

In the carbon-coated optical fiber according to the present invention, since the core is doped with a dopant for increasing the refractive index, the amount of fluorine added to the cladding is set at 2.
An optical fiber having the desired core-cladding relative refractive index difference can be obtained even if it is as small as 5 wt% or less. Therefore,
Since the amount of fluorine to be added in the clad is small,
Fluorine gas that diffuses and migrates to the surface on which the carbon film is formed can be extremely reduced. In addition, since a pure quartz layer having a thickness of 5 to 30 μm is formed between the upper clad and the carbon film, the pure quartz layer is present even if fluorine in the clad is diffused and transferred as fluorine gas. Therefore, diffusion and transfer of the fluorine gas to the surface on which the carbon film is formed can be prevented. For this reason, the formation of the carbon film becomes easy, and the obtained carbon film becomes dense, resulting in an optical fiber having excellent hydrogen resistance.

Hereinafter, the present invention will be described in detail. FIG.
FIG. 2 is a cross-sectional view showing an example of the carbon coated optical fiber of the present invention. Reference numeral 1 in the drawing denotes an optical fiber. The optical fiber 1 includes a core 2, a clad 3, and a pure quartz layer 4. An outermost carbon film 5 is formed on the outer periphery of the pure quartz layer 4 of the optical fiber 1.

The core 2 is made of quartz to which a dopant for increasing the refractive index has been added. Examples of the dopant for increasing the refractive index include erbium, titania, alumina, germania, phosphorus pentoxide, and the like.

The clad 3 is made of fluorine-added quartz and is formed on the outer periphery of the core 2. The amount of fluorine added to the cladding 3 is set to 2.5% by weight or less so that the core-clad relative refractive index difference becomes about 0.7% in order to shift the zero dispersion wavelength to the 1.5 μm band. The amount of addition of fluorine is adjusted within the above range according to the amount of dopant added to the core 2. 2.5 fluorine added
If the content is more than 10% by weight, when the carbon film 5 is formed on the outer periphery of the pure quartz layer 4, the fluorine in the clad 3 becomes a fluorine gas and easily diffuses and migrates to the surface of the pure quartz layer 4. A carbon film is less likely to adhere to and form on the surface of the quartz layer 4, and a carbon film having excellent denseness cannot be obtained. On the other hand, if the addition amount of fluorine is 2.5% by weight or less, diffusion and migration of the fluorine gas to the pure quartz layer 4 are reduced, so that diffusion of the fluorine gas to the surface on which the carbon film is formed is prevented. This is because the thickness of the pure quartz layer 4 can be reduced.

The thickness of the pure quartz layer 4 is 5 μm
It is not less than 30 μm. When the thickness is less than 5 μm, the possibility that the fluorine gas from the clad 3 is too thin and diffuses and migrates to the surface of the pure quartz layer 4 becomes high, while if the thickness exceeds 30 μm, This is because it becomes difficult to cut the carbon coated optical fiber.

The core 2, the clad 3, and the pure quartz layer 4 are provided in a spinning furnace with an optical fiber preform manufactured by a known technique such as a VAD method, an MCVD method, or an OVD method, and the optical fiber preform is melt spun. Can be formed.

The carbon film 5 is obtained by thermally decomposing a raw material compound such as hydrocarbon or halogenated hydrocarbon. As hydrocarbons, gaseous gas at normal temperature,
For example, ethane, propane, ethylene and the like, a mixed gas thereof, pentane, hexane, octane and the like which are liquid at ordinary temperature, a mixed solution thereof, and naphthalene which is solid at ordinary temperature can be used. Examples of the halogenated hydrocarbon include various substances such as tetrafluoromethane, dichloromethane, and dichloroethane. From the viewpoint of handling such as toxicity, those using chlorine as the halogen are preferable.

The thickness of the carbon film 5 needs to be sufficient from the viewpoint of improving the hydrogen resistance of the optical fiber, and is preferably 300 ° or more.

As a method for forming the carbon film 5 on the outer periphery of the pure quartz layer 4 of the optical fiber 1, for example, an optical fiber preform is set in a spinning furnace, and the optical fiber 1 obtained by melt-spinning the optical fiber is used. This is performed by inserting the carbon film 5 on the outer periphery of the pure quartz 4 by inserting the carbon film 5 into a CVD reactor.

Such a carbon coated optical fiber is
Since the pure quartz layer 4 is formed between the clad 3 and the carbon film 5, even if the fluorine in the clad 3 diffuses and transfers as a fluorine gas, the pure quartz layer 4 has Fluorine gas is prevented from diffusing and migrating to the surface on which the film is formed. Therefore, since fluorine gas does not diffuse and migrate on the surface of the pure quartz layer 4, a dense gas is formed on the surface of the pure quartz layer 4. Since the carbon film 5 is easily attached and formed, the obtained carbon film is, for example, 30
Even if it is as thin as about 0 ° or more, it is possible to obtain a carbon-coated optical fiber having excellent hydrogen resistance without deterioration in characteristics to hydrogen.

This carbon coated optical fiber is
Since the core is doped with a dopant for increasing the refractive index, even if the amount of fluorine to be added to the cladding 3 is reduced to 2.5% by weight or less, the desired core can be obtained.
An optical fiber having a clad relative refractive index difference is obtained. Accordingly, since the fineness of the carbon film is not impaired due to fluorine, a carbon-coated optical fiber having excellent hydrogen resistance can be obtained as described above.

[0018]

【Example】

(Examples 1 to 5) First, an optical fiber preform was set in a spinning furnace. As the optical fiber preform, as shown in FIG. 1, a core portion made of quartz doped with a dopant for increasing a refractive index and serving as a core 2 of the optical fiber 1;
Fluorine which becomes the cladding 3 of the optical fiber 1 is 0.5 to 2.
A clad portion composed of quartz added in a range of 5% by weight and a pure quartz layer portion composed of pure quartz to be the pure quartz layer 4 of the optical fiber 1 were used. Also, various thicknesses of the pure quartz layer portion were used. The dopant added to the core 2 is erbium 0.1.
07% by weight (or 23% by weight of germanium). Next, the optical fiber preform is heated to a linear velocity of 30.
The fiber was spun at 0 m / min to obtain an optical fiber 1 having the same configuration as that of FIG. Next, the optical fiber 1 is inserted into a CVD reactor in which benzene vapor diluted with argon gas to about 1 vol% as a raw material compound for forming the carbon film 5 is supplied at a flow rate of about 0.5 liter / min. Thus, a carbon film 5 was formed on the outer periphery of the pure quartz layer 4 of the optical fiber 1 to obtain a carbon-coated optical fiber.

Comparative Examples 1 to 3 First, an optical fiber preform was placed in a spinning furnace. As the optical fiber preform, a core portion made of quartz doped with a dopant for increasing the refractive index serving as a core of the optical fiber and fluorine serving as a cladding of the optical fiber are added in an amount of 2.5 to 3.5% by weight. % Or a pure quartz layer composed of pure quartz and a pure quartz layer having no pure quartz layer. The dopant added to the core was erbium 0.07
% By weight (or 23% by weight of germanium). Next, the optical fiber preform was heated to a linear velocity of 300 m /
Then, an optical fiber composed of a core, a clad, and a pure silica layer, or an optical fiber composed of a core and a clad was obtained. Next, the optical fiber is inserted into a CVD reactor in which benzene vapor diluted with argon gas to about 1 vol% as a raw material compound for forming a carbon film is supplied at a flow rate of about 0.5 liter / min. A carbon film was formed on the outer periphery of the optical fiber to obtain a carbon-coated optical fiber.

The carbon coated optical fibers obtained in Examples 1 to 5 and Comparative Examples 1 to 3 were examined for carbon film thickness and hydrogen resistance. The hydrogen resistance was evaluated by a hydrogen loss test method. In this hydrogen loss test method, a bundle of 1,000 m of carbon-coated optical fiber was taken, put in a pressure vessel for measuring hydrogen loss, and the fibers at both ends were exposed.
The pressure vessel was heated to 80 ° C. and held for 4 hours. Next, the initial loss of the carbon coated optical fiber was measured using OTDR of a light source having a wavelength of 1.24 μm. After the initial loss measurement, hydrogen gas was introduced into the pressure vessel, and the hydrogen partial pressure was set to 1
atm. And 24 hours later, OTDR again
Was used to measure the loss when light having a wavelength of 1.24 μm was transmitted to the carbon-coated optical fiber to confirm the loss increment. The results are shown in Table 1 below.

[0021]

[Table 1]

As is evident from the results shown in Table 1, in Examples 1 to 3, it is found that if the amount of fluorine added to the cladding is small, a dense carbon film can be easily obtained. Further, it can be seen from Examples 4 and 5 that when the amount of fluorine added to the cladding is small, the thickness of the pure quartz layer for preventing diffusion of fluorine gas to the surface can be reduced. On the other hand, when the pure quartz layer is not provided as in Comparative Example 1 or when the pure quartz layer has a thickness of 5 μm or less as in Comparative Example 2, it is difficult to form the carbon film and the fine It turns out that the sex is poor. In addition, even when the pure quartz layer is formed to have an appropriate thickness, a dense carbon film cannot be obtained when the added amount of fluorine is 2.5% by weight or more as in Comparative Example 3.

[0023]

As described above, in the carbon-coated optical fiber of the present invention, by adding a dopant for increasing the refractive index to the core, fluorine is contained in the clad to obtain a required difference in the refractive index. A pure quartz layer having a thickness of 5 to 30 μm is formed on the outer periphery of the clad while reducing the amount of addition, thereby facilitating the attachment and formation of a dense air-tight carbon film on the surface on which the carbon film is formed. Therefore, since a carbon film having excellent hydrogen degradation resistance is obtained, for example, even when the thickness is about 300 °, it is possible to effectively prevent hydrogen from penetrating,
It has excellent hydrogen resistance. Further, because of such excellent hydrogen resistance, the carbon coated optical fiber of the present invention is suitably used for an optical amplifier or the like that requires long-term reliability.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an example of a carbon coated optical fiber of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical fiber, 2 ... Core, 3 ... Clad, 4 ... Pure quartz layer, 5 ... Carbon film

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Shinji Araki 1440 Mutsuzaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant (56) References JP-A-4-60503 (JP, A) JP-A-63-217311 (JP, A) JP-A-4-198043 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C03C 25/42 G02B 6/44 331

Claims (1)

(57) [Claims] 1. A core made of quartz to which a dopant for increasing a refractive index is added, and a fluorine addition amount is 2.5% by weight.
A carbon-coated optical fiber comprising: a clad made of the following quartz; a pure quartz layer having a thickness of 5 to 30 μm formed on the outer periphery of the clad; and a carbon film formed on the outer periphery of the pure quartz layer.