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Diffused quantum-well (QW) distributed feedback (DFB) lasers and optical amplifiers will be theoretically analyzed in this paper. For DFB lasers, a design rule will be proposed and the validity of the design rule will be discussed with respect to changes in the injected carrier density. The range of grating period, which can be used in the design, is discussed. As a consequence, the maximum tuning range of the emission wavelength can be estimated without involving the time-consuming self-consistent simulation. The features of polarization independence of optical amplifiers achieved by using diffused QWs are also discussed. Our theoretical results successfully explain why polarization independence can achieve in the long-wavelength tail of the modal gain and absorption coefficient but not at photon energies above the transition edge. This explanation applies to other tensile-strained QWs for polarization-independent applications. The understanding is crucial for optimizing polarization-independent devices. To conclude, our analysis of the diffused QW optical devices demonstrates that QW intermixing technology is a practical candidate for not only realizing monolithic photonic integrated circuit, but also enhancing optical device performance.