Polarization and transverse-mode selection in quantum-well vertical-cavity surface-emitting lasers: index- and gain-guided devices

We study polarization switching and transverse-mode competition in vertical-cavity surface-emitting lasers (VCSELs) in the absence of temperature effects. We use a model that incorporates the vector nature of the laser field, saturable dispersion, different carrier populations associated with different magnetic sublevels of the conduction and heavy hole valence bands in quantum-well media, spin-flip relaxation processes and cavity birefringence and dichroism. We consider both index-and gain-guided VCSELs and we find that spin-flip dynamics and the linewidth enhancement factor are crucial for the selection of the polarization state corresponding to a given injection current. For index-guided VCSELs the effect of spatial hole burning on the polarization behaviour within the fundamental mode regime is discussed. For gain-guided VCSELs, transverse-mode and polarization selection are studied within a Maxwell-Bloch approximation which includes field diffraction and carrier diffusion. Polarization switching is found in the fundamental mode regime. The first-order transverse mode starts lasing orthogonally polarized to the fundamental mode. At larger currents polarization coexistence with several active transverse modes occurs.