Dynamics and polarization effects in small-area vertical-cavity surface-emitting lasers in free-running mode and with time-delayed feedback

The polarization dynamics of vertical-cavity surface-emitting lasers operating close to threshold is investigated experimentally. For free-running devices a characterization of the dynamics is given for a scenario when two modes are excited at threshold due to a small net gain anisotropy. The polarization dynamics in this regime are found to be governed by the relaxation oscillations and to exhibit an anticorrelation of the two modes. The level of anticorrelation is strongly depending on the injection current. If isotropic feedback by a distant reflector is added, the dynamics are governed by the external cavity round-trip time scale and low frequency fluctuations are observed for both modes. These are shown to occur also for large net gain anisotropy, but without excitation of polarization degrees of freedom.