Optical fiber and manufacturing method thereof

1. A method of manufacturing an optical fiber, the method comprising steps of:

inserting a core rod into a cladding pipe;

removing moisture present on the surface of the core rod and the internal surface of the cladding pipe;

closing at least one end of the cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere and/or is decompressed, heating the core rod and the cladding pipe so as to unify the core rod and the cladding pipe and to draw from the one end of the cladding pipe from the optical fiber,

wherein the concentration of hydrogen molecules or a compound containing a hydrogen atom in the dry gas is 10 volume ppm or less in total.

2. A method of manufacturing an optical fiber, the method comprising step of:

inserting a core rod into a cladding pipe;

removing moisture present on the surface of the core rod and the internal surface of the cladding pipe;

closing at least one end of the cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere and/or is decompressed, heating the core rod and the cladding pipe so as to unify the core rod and the cladding pipe and to draw from the one end of the cladding pipe to form the optical fiber, wherein

a ratio D/d of a diameter D of the core rod to a diameter d of a core portion is in the range of 1 to less than 2,

the core rod is only composed of the core portion, and

the relative refractive index difference of the core rod to the cladding pipe is 0.2% or more.

3. A method of manufacturing an optical fiber, the method comprising steps of:

inserting core rod into a cladding pipe;

removing moisture present on the surface of the core rod and the internal surface of the cladding pipe;

closing at least one end of cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere and/or is decompressed, heating the core rod and the cladding pipe so as to unify the core rod and the cladding pipe and to draw from the one end of the cladding pipe to form the optical fiber, wherein

a ratio D/d of a diameter D of the core rod to a diameter d of a core portion is in the range of 1 to less than 2,

to core rod is composed of the core portion and a first cladding portion having a refractive index smaller than that of the core portion,

the relative refractive index difference of the core portion to the first cladding portion 0.2% or more, and

the refractive index of the first cladding portion is preferably substantially equivalent to that of a region from the inner surface of the cladding pipe to a depth of one-tenth of the thickness of the cladding pipe.

4. A method of manufacturing an optical fiber, the method comprising steps of:

inserting a core rod into a cladding pipe;

removing moisture present on the surface of the core rod and the internal surface of the cladding pipe;

closing at least one end of the cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere and/or is decompressed, heating the core rod and the cladding pipe so as to unify the core rod and the cladding pipe and to draw from the one end of the cladding pipe to form the optical fiber,

wherein a ratio D 2 /d 2 of an external diameter the cladding pipe to an internal diameter d 2 thereof is in the range of 5 to 30, and the length of the cladding pipe is 500 mm or more.

5. The method of manufacturing an optical fiber, according to claim 4 , wherein the ratio D 2 /d 2 is in the range of more than 7 to 30.

6. A method of manufacturing an optical fiber, the method comprising steps of:

inserting a core rod into a cladding pipe;

removing moisture present on the surface of the core rod and the internal surface of the cladding pipe;

closing at least one end of the cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere and/or is decompressed, heating the core rod and the cladding pipe so as to unify the core rod and the cladding pipe and to draw from the one end of the cladding pipe to form the optical fiber,

wherein the eccentricity of the internal circumference of the cladding pipe to the external circumference thereof is 0.3% or less over the entire effective length.

7. An optical fiber comprising a core and a cladding, the cladding being provided around the external circumference of the core and having a refractive index smaller that that of the core,

wherein the optical fiber has only one boundary formed on a cross-section perpendicular to the axis thereof, the said boundary being formed as a result of a rod and a pipe being unified with each other by heating, and

the optical fiber has a light transmission loss of 0.5 dB/km or less at a wavelength of 1.38 μm.

8. The optical fiber according to claim 7 ,

wherein a ratio p 1 /r 1 of a distance p 1 from center of the core to the boundary to a radius r 1 of the core is in the range of 1 to less than 2.

9. The optical fiber according to claim 7 ,

wherein the relative refractive index difference of the core to a first cladding is 0.2% or more, the first cladding being a portion frown said boundary to the core, and

wherein the refractive index of the first cladding is substantially equivalent to the refractive index of a portion between said boundary and a circle having a radius of r+2t, the circle being concentric to the center of the core, and the thickness of the first cladding being represented by t.

10. The optical fiber according to claim 7 ,

wherein the average viscosity of the core at 1,200° C. is equivalent to or more than the average viscosity of the cladding.

11. The optical fiber according to claim 7 ,

wherein the core is made of a pure silica glass or a silica glass containing an additive, and the cladding is made of a glass primarily composed of a fluorine-containing silica glass.

12. A method of manufacturing an optical fiber, the method comprising steps of:

inserting a core rod into a cladding pipe;

closing at least one end of the cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere, heating the cow rod and the cladding pipe so as to remove moisture adhering to a surface of the core rod and an internal surface of the cladding pipe, to unify the core rod and the cladding pipe and to draw from the one end of the cladding pipe to form the optical fiber,

wherein the concentration of hydrogen molecules or a compound containing a hydrogen atom in the dry gas is 10 volume ppm or less in total.

13. A method of manufacturing an optical fiber, the method comprising the steps of:

inserting a core rod into a cladding pipe;

closing at least one end of the cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere, heating the core rod and the cladding pipe so as to remove moisture adhering to a surface of the core rod and an internal surface of the cladding pipe, to unify the core rod and the cladding pipe arid to draw from the one end of the cladding pipe to form the optical fiber, wherein

a ratio D/d of a diameter D of tile core rod to a diameter d of a core portion is in the range of 1 to less than 2,

the core rod is only composed of the core portion, and

the relative refractive index difference of the core rod to the cladding pipe is 0.2% or more.

14. A method of manufacturing an optical fiber, the method comprising the steps of:

inserting a core rod into a cladding pipe;

closing at least one end of the cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere, heating the cure rod and the cladding pipe so as to remove moisture adhering to a surface of the core rod and an internal surface of the cladding pipe, to unify the core rod and the cladding pipe and to draw from the one end of the cladding pipe to from the optical fiber, wherein

a ratio D/d of a diameter D of the core rod to a diameter d of a core portion is in the range of 1 to less than 2.

the core rod is composed of the core portion and a first cladding portion having a refractive index smaller than that of the core portion,

the relative refractive index difference of the core portion to the first cladding portion is 0.2% or more, and

the refractive index of the first cladding portion is preferably substantially equivalent to that of a region born the inner surface of the cladding pipe to as depth of one-tenth of the thickness of the cladding pipe.

15. A method of manufacturing an optical fiber, the method comprising the steps of:

inserting a core rod into a cladding pipe;

closing at least one end of the cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere, heating the core rod and the cladding pipe so as to remove moisture adhering to a surface of the core rod and an internal surface of the cladding pipe to unify the core rod and the cladding pipe and to draw from the one end of the cladding pipe to form the optical fiber,

wherein a ratio D 2 /d 2 of an external diameter D 2 of the cladding pipe to an internal diameter d 2 thereof is in the range of 5 to 30, and the length of the cladding pipe is 500 mm or more.

16. The method of manufacturing an optical fiber, according to claim 15 , wherein the ratio D 2 /d 2 is in the range of more than 7 to 30.

17. A method of manufacturing an optical fiber, the method comprising the steps of:

inserting a core rod into a cladding pipe;

closing at least one end of the cladding pipe; and

while a space formed between the core rod and the cladding pipe is connected to a dry-gas atmosphere, heating the core rod and the cladding pipe so as to remove moisture adhering to a surface of the core rod and an internal surface of the cladding pipe, to unify the core rod and the cladding pipe and to draw from the one end of the cladding pipe to form the optical fiber,

wherein the eccentricity of the internal circumference of the cladding pipe to the external circumference thereof is 0.3% or less over the entire effective length.