Description and analysis of low-frequency fluctuations in vertical-cavity surface-emitting lasers with isotropic optical feedback by a distant reflector

We analyze theoretically and experimentally the polarization dynamics of vertical-cavity surface-emitting lasers exposed to isotropic optical feedback. The theoretical investigations are based on a model that takes into account different inversion populations for charge carriers with opposite spin. In the deterministic case, we observe toruslike synchronized low-frequency fluctuations of one or both polarization modes in the vicinity of the solitary laser threshold, depending on the parameters and initial conditions. Both in experiment and in simulations including spontaneous emission noise, asymmetric low-frequency fluctuations are observed above and below the solitary laser threshold as well as a transition to coherence collapse further above threshold. The amount of excitation of polarization degrees of freedom and thus the occurrence of simultaneous low-frequency fluctuations of both polarization modes is shown to depend on the magnitude of the dichroism. Apart from the correlation properties on the external-cavity time scale, we find a good qualitative agreement between experiments and simulations.