The average refractive index of a core of this optical fiber is higher than the average refractive index of an inner cladding and the minimum refractive index of an outer cladding; and the average refractive index of a trench is lower than the average refractive index of the inner cladding and the minimum refractive index of the outer cladding. The core contains germanium dioxide; the relative refractive index difference of the core with respect to the refractive index of pure silica glass is 0.12% to 0.30%; the value obtained by subtracting the relative refractive index difference of the outer cladding with respect to the refractive index of pure silica glass from the relative refractive index difference of the core is 0.25% or more; and the diameter of the core is 4.0 m to 9.2 m.

A single mode optical fiber is provided that includes a core region having an outer radius r.sub.1 and a maximum relative refractive index .sub.1max. The single mode optical fiber further includes a cladding region surrounding the core region, the cladding region includes a depressed-index cladding region, a relative refractive index .sub.3 of the depressed-index cladding region increasing with increased radial position. The single mode optical fiber has a bend loss at 1550 nm for a 15 mm diameter mandrel of less than about 0.75 dB/turn, a bend loss at 1550 nm for a 20 mm diameter mandrel of less than about 0.2 dB/turn, and a bend loss at 1550 nm for a 30 mm diameter mandrel of less than 0.005 dB/turn. Additionally, the single mode optical fiber has a mode field diameter of 9.0 microns or greater at 1310 nm wavelength.