To provide a single mode optical fiber with having reduced attenuation.

A single mode optical fiber comprises from center to periphery: a core having a radius (a) and an index difference (Dn₁) with outer cladding (Dn_cl) comprised between-0.5x10^-3 and 0.5x10^-3; an inner depressed cladding having a radius (r_ring1) and an index difference (Dn_inner) with outer cladding; a ring having an inner radius (r_ring1) comprised between 21 μm and 35 μm, an outer radius (r_ring2) and an index difference (Dn_ring) with the outer cladding comprised between -0.5x10^-3 and 0.5x10^-3; an outer depressed cladding having a radius (r_out) and an index difference (Dn_out) with outer cladding. The fiber has a ratio of the volume of the core over the width of the ring (W_ring) comprised between 0.12 μm and 0.2 μm and the outer depressed cladding has a volume comprised between 15 μm² and 30 μm². A low cost preform with increased capacity can be obtained.