I/We Claim:
1. A method for manufacturing an optical fiber, the
method comprising:
a drawing process of drawing an optical fiber from an optical fiber preform in a drawing furnace; and
a slow cooling process of slowly cooling the optical fiber drawn in the drawing process in an annealing furnace;
wherein a temperature difference between a temperature of the optical fiber being drawn in the annealing furnace and a fictive temperature of glass included in a core of the optical fiber is 300°C or less, and
the optical fiber is slowly cooled for 0.01 seconds or more in the annealing furnace to cause the optical fiber to have a temperature of 1300°C or more and 1800°C or less.
2. The method for manufacturing an optical fiber
according to claim 1,
wherein the temperature difference between the temperature of the optical fiber being drawn in the annealing furnace and the fictive temperature of the glass included in the core of the optical fiber is 180°C or less.
3. The method for manufacturing an optical fiber
according to claim 1,
wherein, when the temperature difference between the temperature of the optical fiber being drawn in the

annealing furnace and the fictive temperature of the glass included in the core of the optical fiber is 0°C or more and 60°C or less, a preset temperature of the annealing furnace is set to a temperature lower than the temperature of the optical fiber, and
when the temperature difference between the temperature of the optical fiber being drawn in the annealing furnace and the fictive temperature of the glass included in the core of the optical fiber is higher than 60°C and equal to 300°C or less, the temperature of the optical fiber is temporarily increased and then decreased in the annealing furnace.
4. The method for manufacturing an optical fiber
according to claim 2,
wherein, when the temperature difference between the temperature of the optical fiber being drawn in the annealing furnace and the fictive temperature of the glass included in the core of the optical fiber is higher than 60°C and equal to 180°C or less, the temperature of the optical fiber is increased to a preset temperature of the annealing furnace within a time range from 0.001 to 0.1 seconds after the optical fiber is drawn in the annealing furnace and then decreased.
5. The method for manufacturing an optical fiber
according to any one of claims 1 to 4,

wherein the optical fiber is slowly cooled in the annealing furnace for 0.1 seconds or more.
6. The method for manufacturing an optical fiber
according to any one of claims 1 to 4,
wherein the optical fiber is slowly cooled in the annealing furnace for 0.02 seconds or more to cause the optical fiber to have a temperature of 1450°C or more and 1630°C or less.
7. The method for manufacturing an optical fiber
according to any one of claims 1 to 6,
wherein the optical fiber is slowly cooled in the annealing furnace for 0.1 seconds or more to cause the optical fiber to have a temperature of 1350°C or more and 1500°C or less.