Polarization properties and instabilities of QD VCSELs

Quantum well (QW) VCSELs have a tendency to switch their polarization from one linearly polarized (LP) mode to the orthogonal one when changing the operation conditions. As polarization properties of VCSELs are governed by anisotropies, namely stress-induced birefringence and dichroism, the inherent anisotropy of quantum dots (QDs) is expected to influence the polarization properties of QD VCSELs. In this paper we summarize our experimental results on polarization properties of QD VCSELs with the main focus on polarization switching phenomena. Close to threshold the laser emits linearly polarized light which changes to elliptically polarized (EP) at some current. The main axes of these states are not aligned and the angle between them increases with current. As the current is still increased polarization switching accompanied by polarization mode hopping occurs. Distinctive feature of the observed switching is that the two EP states between which switching occurs are nonorthogonal. The angle between their major exes is 40 deg. Polarization mode hopping has been characterized in terms of the dwell time and the current-dependence of this factor examined. Apparently, the dwell time decreases when the pump current is increased which differs from what has been published for QW VCSELs. The average dwell time is 20 ns. Similarly to QW VCSELs the distribution of the dwell time is exponential. The statistics is the same for the two EP states and such symmetric switching is maintained in the whole range of currents at which the light is elliptically polarized. Large-signal modulation experiments show that the frequency at which polarization switching disappears is about 100 MHz. This indicates that the switching is of thermal origin.