SINGLE-MODE OPTICAL FIBER

PROBLEM TO BE SOLVED: To provide a single-mode optical fiber having excellent bending loss characteristics and low transmission loss characteristics.SOLUTION: In a single-mode optical fiber, having a trench-type refractive index profile, when an outer diameter of a first cladding layer is represented as r1 and an outer diameter of a second cladding layer is represented as r2, a differential value d (r)/dr of a relative refractive index difference (r) in the radial direction relative to a distance r from the center of a core layer satisfies -0.10≤d (r)/drd (r)/dr (r1