The nonlinear gain effects due to the spectral hole burning in quantum well (QW), quantum well wire (QWW), and quantum well box (QWB) lasers are discussed. It is found that the gain nonlinearity is enhanced by the quantum confinement of carriers, and is further enhanced in the QWW and QWB lasers. These nonlinear gain effects significantly affect the modulation dynamics of the semiconductor lasers under high photon density conditions. It is emphasized that both the damping rate and the resonant relaxation frequency are degraded through reduction of the differential gain. The effects of nonlinear gain on the spectral dynamics are discussed. The result indicates that the gain nonlinearity causes enhancement of the linewidth enhancement factor with the increase of the photon density, which leads to the spectral rebroadening under high photon density conditions. >