Optical fiber and method and apparatus for manufacturing optical fiber

An optical fiber manufacturing method comprising:

• melting an optical fiber preform by heating;
• drawing a bare optical fiber with a predetermined diameter from the melted optical fiber preform;
• solidifying the drawn bare optical fiber;
• applying elastic torsion to the bare optical fiber after solidification by transmitting elastic torsion to the bare optical fiber toward an upstream side in a drawing direction of the bare optical fiber;
• forming a coating layer before curing by coating an outer periphery of the solidified bare optical fiber with curable resin in a liquid state;
• forming an optical fiber, to which torsion is applied so that the elastic torsion of the bare optical fiber is held, by curing the coating layer formed on the outer periphery of the bare optical fiber to which the elastic torsion is applied;
• and
• drawing the optical fiber to which the torsion is applied, wherein
• when applying torsion to the optical fiber, a direction of the torsion applied to the bare optical fiber is periodically reversed,
• when the bare optical fiber is coated with curable resin, a viscosity of the liquid-state curable resin at the time of coating is 0.1 to 3 Pa·sec,
• an inversion period T of the torsion applied to the optical fiber in a longitudinal direction of the optical fiber is 5 to 30 m. and a maximum amplitude of an accumulated torsion angle in an inverted torsion profile is 500×T (°) to 4000×T (°),
• an inversion period T is the range of 5 to 30 m in the longitudinal direction of the optical fiber and a maximum amplitude of an accumulated torsion angle of the bare optical fiber portion in an inverted torsion profile is 100×T (°)to 1200×T (°)under the condition where a remaining elastic torsion is applied to the bare optical fiber portion in a state where the elastic torsion applied to the bare optical fiber portion is held by the elastic repulsion caused by the coating layer, and
• the elastic torsion of the bare optical fiber is held by curing the coating layer so that 20% to 30% of applied elastic torsion remains.