Polarization behavior of vertical-cavity surface-emitting lasers: Experiments, models and applications

Abstract

Due to the emission of light perpendicular to the surface of the quantum well and the usually symmetric vertical resonator there is a priori no intrinsic polarization anisotropy mechanism in VCSELs. Small residual strain explains the emission of linearly polarized light with a common orientation along [110] or [1–10] crystallographic directions. These two modes of linear polarization are strongly anti-correlated. Experimentally, polarization switching between them can be observed with increasing the injection current. It could happen from shorter to longer wavelength mode (type 1) or in the opposite way—from longer to shorter wavelength mode (type II). The polarization switching happens through a region of mode hopping or hysteresis. In the first case, the dwell time (the average time the laser spends in one mode) scales in several orders of magnitude. Thermal (carrier) effects influence the polarization behavior of VCSELs through the red (blue) shift of the gain maximum and through the dependence of the losses on the photon energy and temperature. Also, internal or external stress strongly affects the polarization behavior of VCSELs, giving rise to refractive index and gain anisotropy. In principle, one has to consider different gain curves for different orientation of the light polarization. Such two–gain–curve gain equalization model could explain type I polarization switching followed by type II polarization switching. In order an abrupt switch to exist, gain nonlinearities have to be taken into account. It is possible that the spin-flip dynamics is playing an essential role during the polarization switch. Finally, we show one straightforward implementation of polarization switching in VCSELs, e.g. reconfigurable interconnects.