| abstract |
PROBLEM TO BE SOLVED: To provide an optical fiber having a wide irradiation range. SOLUTION: This is an optical fiber having a core 1 and at least one layer of clad 2, wherein the emission end face has a concave curved surface, the radius of curvature a (mm) of the concave curved surface, the optical fiber core diameter b (mm), and the core. An optical fiber having a refractive index n of (a / b) ÷ n 2 ≤ 0.5. [Selection diagram] Fig. 1 |