Method of manufacturing single mode optical fiber

A method of manufacturing a single mode optical fiber formed of a silica-based glass and including a glass part having a central core and a cladding region, wherein at least one element selected from the group consisting of Ge and F is added to said glass part, and said glass part has an outer diameter of about 125 μm, comprising:

• heating an optical fiber preform in a heating furnace including a first heater so as to melt said optical fiber preform;
• fiber drawing an optical fiber from said molten optical fiber preform;
• and
• continuously cooling said drawn optical fiber starting with a meniscus portion in which diameter is decreased from 90% of the preform diameter to 5% of the preform diameter to a portion where the drawn optical fiber has a temperature of 1,200° C. at a cooling rate of 1,000 to 3,000° C/sec, wherein
• a lower cylinder is arranged in contact with a bottom end side of said heating furnace, and the optical fiber is continuously cooled by a gas which is supplied into the lower cylinder and is discharged through an upper end of the heating furnace, and
• a second heater is arranged in the lower cylinder so as to heat said optical fiber drawn from the heating furnace, wherein the gas is supplied through a side wall of the lower cylinder, flows downward between the side wall of the lower cylinder and the second heater, and then flows upward in the lower cylinder and the heating furnace, and is discharged through the upper end of the heating furnace.