Method and device for screening optical fiber core, and method for manufacturing optical fiber core

A method for screening an optical fiber core including a resin coating layer, the method comprising:

• a pre-strain applying step of adding a tensile force while feeding a portion of the optical fiber core retained at both ends of the portion, and applying a pre-tensile strain larger than zero and smaller than a guaranteed tensile strain set as a guaranteed value, the optical fiber core including the resin coating layer and being wound by a bobbin and fed out from another bobbin;
• a guaranteed strain applying step of adding a tensile force while feeding the portion of the optical fiber core retained at both ends of the portion and applied with the pre-tensile strain, and applying the guaranteed tensile strain only for a predetermined time;
• and
• a guaranteed strain releasing step of releasing the optical fiber core from the guaranteed tensile strain, wherein:
• at the pre-strain applying step, the pre-tensile strain is applied by applying tensile strain stepwise,
• at the pre-strain applying step, when applying the tensile strain in N stages (N is an integer of 2 or more), and a tensile force added at the n-th stage (n is an integer of 1 or more and less than N) is set to the tensile force n, the tensile force is added so as to achieve the relationship of (tensile force 1)>(tensile force (n+1)−tensile force n).