Since the optical gain strongly affects the threshold current density of a laser and the refractive index change in the active region is directly related to the guiding behavior of optical modes, we studied theoretically: (i) the effects of uniformly biaxial strains (compression and tension) instead of uniaxial strain produced by an externally applied stress on the optical gain, and (ii) the refractive index change of InGaAs/ InGaAsP quantum well structures. We found that the optimum performance of the TE polarization can be improved by a biaxially compressive strain, because the TE mode gain is approximately enhanced by a factor 5 and the refractive index change in the active region becomes positive. On the other hand, the biaxially tensile strain increases substantially the TM mode gain but allows the refractive index change in the active region to become more negative. This implies that the optical confinement of the TM polarization is worse than the situation without strain. Therefore, a critical adjustment of the mole fraction of Ga must be strongly considered in order to have an appropriate extent of the biaxially tensile strain needed for the high performance of the TM polarization.
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