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Polarization behavior and mode structure of vertical-cavity surface-emitting lasers with elliptical surface relief

We report on in-depth experimental and theoretical studies of the polarization behavior and mode structure of Vertical-Cavity Surface-Emitting Lasers with an elliptical surface relief. The aim of the relief is twofold, first to enhance the single mode operation by introducing spatially distributed losses, and second, to enhance polarization stability by making these losses anisotropic, i.e. polarization dependent. We first identify the transverse mode structure for different dimensions of the relief ellipse as a function of the injection current. Then we proceed with a systematic measurement of the change in orientation of the two linearly polarized (LP) fundamental modes emitted by VCSELs and their frequency splitting as a function of an externally applied strain, anisotropic in the plane of the quantum wells. We investigate two orientations of the surface relief ellipse: longer sides along [110] and [-100] directions, respectively. These studies show that whereas the polarization direction is governed by the orientation of the index ellipsoid, the selection of the lasing LP mode is mainly determined by the anisotropy in the mirror losses introduced by the elliptical surface relief. We confirm these experimental studies by theoretically estimating the effect of the relief and the anisotropic strain on both the refractive index and the losses.