We analyzed the mechanism of alkali-ions migration taking part in obtaining the frozen ionic potential responsible for the creation of second-order nonlinearity in optical fibers made of silica and other oxide-based glasses. We confirmed that the presence of alkali ions in the oxide glass, together with a high χ⁽³⁾ value, allows to obtain ionic potential, even though these ions are structural elements of glass rather than impurities. Creation of second-order susceptibility in glass is possible and, depending on alkali-ion concentration, χ⁽²⁾ can be higher than for a silica-based fiber of the same geometry. Calculated χ⁽²⁾ reached 0.58 pm/V, and the maximum value obtained for silica-based fiber for the same thermal poling conditions was only 0.33 pm/V.