Method for manufacturing optical fiber parent material, and method for manufacturing optical fiber

1. A method for manufacturing an optical fiber preform including a core part and a cladding part, comprising:

adding an alkali metal to an inner surface of a silica-based glass pipe;

etching the inner surface of the silica-based glass pipe to which the alkali metal is added;

making a glass rod by collapsing the silica-based glass pipe after the etching; and

making an optical fiber preform using the glass rod,

wherein the silica-based glass pipe is heated in the adding such that a surface temperature of the silica-based glass pipe falls within a temperature range of 1500° C. or higher to lower than 2000° C. while a space in which the silica-based glass pipe is held is kept at a positive pressure above atmospheric pressure of greater than 0 Pa and 20 Pa or less,

wherein the silica-based glass pipe is heated by reciprocatively traversing a heat source a plurality of times, the heat source being an oxyhydrogen burner,

wherein in the adding, a heating time per traverse of the reciprocatively traversing with which a surface temperature of a predetermined area of the silica-based glass pipe falls within the temperature range of 1500° C. or higher to lower than 2000° C. is 0.5 minute or longer and shorter than 40 minutes,

wherein the heating time is a time during which a certain point in the silica-based glass pipe is heated to 1500° C. or higher, and

wherein a traverse speed of the oxyhydrogen burner is 10 mm/min or greater and 165 mm/min or less.

2. The method for manufacturing an optical fiber preform according to claim 1 , wherein in the adding, the heating time per traverse with which the surface temperature of the predetermined area of the silica-based glass pipe falls within the temperature range of 1500° C. or higher to lower than 2000° C. is 1 minute or longer and shorter than 40 minutes.

3. The method for manufacturing an optical fiber preform according to claim 1 , wherein in the adding, the heating time per traverse with which the surface temperature of the predetermined area of the silica-based glass pipe falls within the temperature range of 1500° C. or higher to lower than 2000° C. is 0.5 minute or longer and shorter than 20 minutes.

4. The method for manufacturing an optical fiber preform according to claim 1 ,

wherein in the adding, the silica-based glass pipe is heated by the oxyhydrogen burner such that the surface temperature of the silica-based glass pipe falls within the temperature range of 1500° C. or higher to lower than 2000° C., and

a width that achieves a temperature zone of 1500° C. or higher in a heating temperature profile of the oxyhydrogen burner is kept to be not more than six times a diameter of the silica-based glass pipe.

5. The method for manufacturing an optical fiber preform according to claim 4 , wherein in the adding, the silica-based glass pipe is heated such that the surface temperature of the silica-based glass pipe falls within a temperature range of 1500° C. or higher to lower than 1800° C.

6. The method for manufacturing an optical fiber preform according to claim 4 ,

wherein the alkali metal added in the adding is potassium, and

wherein the silica-based glass pipe is repeatedly heated in the adding such that the ratio d2/d1 in the optical fiber preform is 1.5 or more and less than 3.0 where d1 is the diameter of an area in which the potassium concentration in the optical fiber preform is 50 atomic ppm or more, and d2 is the diameter of an area in which the potassium concentration in the optical fiber preform is 50 atomic ppm or less and the chlorine concentration is 1000 atomic ppm or less.

7. The method for manufacturing an optical fiber preform according to claim 1 , wherein in the adding, the silica-based glass pipe is heated such that the surface temperature of the silica-based glass pipe falls within a temperature range of 1500° C. or higher to lower than 1800° C.

8. The method for manufacturing an optical fiber preform according to claim 1 ,

wherein the alkali metal added in the adding is potassium, and

wherein the silica-based glass pipe is repeatedly heated in the adding such that the ratio d2/d1 in the optical fiber preform is 1.5 or more and less than 3.0 where d1 is the diameter of an area in which the potassium concentration in the optical fiber preform is 50 atomic ppm or more, and d2 is the diameter of an area in which the potassium concentration in the optical fiber preform is 50 atomic ppm or less and the chlorine concentration is 1000 atomic ppm or less.

9. A method for manufacturing an optical fiber using an optical fiber preform including a core part and a cladding part, comprising:

adding an alkali metal to an inner surface of a silica-based glass pipe;

etching the inner surface of the silica-based glass pipe to which the alkali metal is added;

making a glass rod by collapsing the silica-based glass pipe after the etching;

making an optical fiber preform using the glass rod; and

drawing the optical fiber preform to manufacture an optical fiber,

wherein the silica-based glass pipe is heated by reciprocatively traversing a heat source a plurality of times, the heat source being an oxyhydrogen burner,

wherein in the adding, a heating time per traverse of the reciprocatively traversing with which a surface temperature of a predetermined area of the silica-based glass pipe falls within the temperature range of 1500° C. or higher to lower than 2000° C. is 0.5 minute or longer and shorter than 40 minutes,

wherein the heating time is a time during which a certain point in the silica-based glass pipe is heated to 1500° C. or higher,

wherein a traverse speed of the oxyhydrogen burner is 10 mm/min or greater and 165 mm/min or less, and

wherein in the adding, a space in which the silica-based glass pipe is held is kept at a positive pressure above atmospheric pressure of greater than 0 Pa and 20 Pa or less.