Multiple cations intermixing in an In0.53Ga0.47As-In0.52Al0.48As quantum-well (QW) structure with 60-Å well width is being studied based on the expanded form of Fick's second law. Interdiffusion of the indium sublattice can result in a maximum compressive strain of 0.64% when annealing time reaches 3 h at 812 °C. For a small interdiffuison, i.e., 1-1.5 h, the subband separation between the lowest heavy and light hole states is at its greatest. This has a major contribution to the modified band structure and averaged density of states which can result in an enhanced optical gain up to 40%. This initial stage of intermixing provides the best lasing performance. For large interdiffusion, i.e., up to 6 h, a large blue shift of the peak gain from 0.842 eV (λ = 1.47 μm) to 1.016 eV (λ = 1.22 μm) is obtained, thus giving a high tunability of the lasing wavelength.