A single mode optical waveguide fiber having a refractive index profile comprising not less than four segments (16, 18, 20, 22) provides waveguide properties well suited to undersea or other long haul telecommunications systems. The novel refractive index profile is characterized by a core segment having a negative relative index, in which, the reference index is that of silica. Another feature of the invention is a cladding layer which contains refractive index increasing dopant at least in the cladding portion adjacent the outermost core segment.