A theoretical study of the polarization-independent (PI) optical gain using group III and group V interdiffusion for under- and over-strained GaAsP-AlGaAs quantum wells (QW's), respectively, is presented here. The group III interdiffusion generates a large enough Al concentration into the well of the under-strained QW for providing PI optical gain while this can be achieved in the over-strained QW through the reduction of the P concentration in the well by group V interdiffusion. When the well width increases, the required extent of interdiffusion to obtain PI optical gain increases for the case of group V diffused QW's, but, for the case of group III diffused QW's, the required extent of interdiffusion is not sensitive to the well width. In addition, the introduction of Al in the well layer of QW's can shorten and lengthen the group III and group V interdiffusion, respectively, for providing the PI optical gain. Similar results can also be obtained by increasing the P concentration in the well layer. Consequently, group III and group V interdiffusion can be used to achieve PI optical gain in the under- and over-strained QW's, respectively, for use in PI optical amplifiers. A range of the extent of interdiffusion can be used to obtain the PI gain although the value of the PI gain may reduce. Besides, the operating wavelength can be blue-shifted in group III interdiffusion and red-shifted in group V interdiffusion. Moreover, TE and TM optical gain peaks can be adjusted using interdiffusion for laser applications.