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.
We analyze the influence of the excited states (ES) on the dynamics of optically injected quantum dot lasers. In
our model carriers from the wetting layer are first being captured into the excited state and then relax to the
ground state. Our results indicate that the dynamics of optically injected QD lasers are driven by the relaxation
time in the sense that it scales the regions where the laser exhibits distinct behaviors. It also influences the size
of the locking region. The capture time has minor effect and influences mainly static characteristics. Bifurcation
maps are studied with the main focus on self-pulsations. In particular our results show that the dynamics of
self-pulsations is consistent with experimental observations of excitable dynamics. To utilize the self-pulsations
we propose and investigate properties and limitation of the system used for all-optical signal processing. In our
approach the slave laser is switched by the information signal acting as a master laser between the locking region
and the region of self-pulsations. The maximum bit rate of such a system has been estimated to be 0.5 GHz.
This value can be improved to 1 GHz by applying correction to the detection algorithm. The correction reflects
the nature of self-pulsations and is calculated from the distribution of the time the system needs to fire a pulse.
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