Polarization Dynamics of VCSELs

In this chapter we wrap up the experimental and theoretical results on polarization dynamics of solitary vertical-cavity surface-emitting lasers. Experiments have shown that VCSELs emit a linearly polarized fundamental transverse mode either along the [110] or \([1\overline 10]\) crystallographic direction. Polarization switching between these modes can occur when the injection current is increased, showing either a frequency shift from the higher to the lower frequency mode (type I) or the reverse (type II). The two modes of linear polarization are strongly anti-correlated. The switching can happen through a region of mode hopping, with a dwell time scaling over eight orders of magnitude with the switching current, or through a region of hysteresis. Thermal (carrier) effects influence the polarization behavior of VCSELs through a red (blue) shift of the gain maximum. Also, in-plane anisotropic strain can strongly modify the polarization behavior of VCSELs. All these experimental results call for explanations, as there is no a priori intrinsic polarization selection mechanism in VCSELs. We present different gain equalization models to explain type I, type II or double polarization switching. Alternatively, the spin-flip model can explain both types polarization switching by involving a microscopic spin-flip relaxation mechanism. Its predictive power has been experimentally established as, e.g., polarization switching through elliptically polarized states and dynamical instabilities. Finally, we highlight some perspective applications using polarization dynamics of VCSELs.