We study the polarization behavior of birefringent multi- transverse mode index-guided Vertical-Cavity Surface-Emitting Lasers during CW and modulation operation. The model we use incorporates the $alpha@-factor, different carriers populations associated with different magnetic sublevels of the conduction and heavy hole valence bands in quantum well media, spin-flip relaxation processes and cavity anisotropies. The effect of carrier diffusion and spatial hole burning on the polarization behavior within the fundamental mode regime is also considered. We find that spin-flip dynamics and the $alpha@-factor are crucial for the selection of the polarization state corresponding to a given injection current. We also analyze the direct-current polarization modulation response for both the small and large signal modulation regimes. In both cases the current is modulated around the polarization switching (PS) current, but in the large signal modulation regime the current goes from below threshold to above the PS current. A frequency doubled polarized output is obtained under large signal modulation for modulation frequencies smaller than 10 MHz. At greater frequencies (around 100 MHz) transient mode polarization partition noise becomes important. At the greatest considered frequency (1 GHz) no PS is observed. Finally the effect of multiple transverse mode operation is addressed by considering the two lowest order modes: LP$- 01$/ and LP$-11$/.
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