Disclosed is a single mode optical waveguide having a core refractive index profile defined by upper and lower boundary profiles on a chart of relative index percent versus radius. The relative refractive index as a function of radial dimensions of the core are selected to provide an optical waveguide fiber having properties suitable for a high performance telecommunication system operating in the wavelength window of about 1530 nm to 1625 nm. Refractive index profile designs encompassing the wavelength window 1250 nm to 1350 nm are also disclosed. The embodiments of the core waveguide have exceptionally low total dispersion slope and attenuation over these wavelength windows.