The entire fabrication process of an optical waveguide with a gradient index profile in planar soda‐lime glass substrate was analyzed by using the Nernst–Planck equation for the fluxes of ion species in the glass by considering the effect of an electric current. The derived equations were solved numerically and the effect of the diffusivity ratio of ions and the electric current on the concentration profile of the impurity were discussed. A method for the fabrication of the waveguide was presented and some conditions for getting a parabolic concentration profile of the impurity were presented for Ag–soda‐lime glass substrate system.