Transition from oscillatory to excitable regime in a system forced at three times its natural frequency.

The effect of a temporal modulation at three times the critical frequency on a Hopf bifurcation is studied in the framework of amplitude equations. We consider a complex Ginzburg-Landau equation with an extra quadratic term, resulting from the strong coupling between the external field and the unstable modes. We show that, by increasing the intensity of the forcing, one passes from an oscillatory regime to an excitable one with three equivalent frequency-locked states. In the oscillatory regime, topological defects are one-armed phase spirals, while in the excitable regime they correspond to three-armed excitable amplitude spirals. Analytical results show that the transition between these two regimes occurs at a critical value of the forcing intensity. The transition between phase and amplitude spirals is confirmed by numerical analysis and it might be observed in periodically forced reaction-diffusion systems.

Growth dynamics of noise-sustained structures in nonlinear optical resonators.

The existence of macroscopic noise-sustained structures in nonlinear optics is theoretically predicted and numerically observed, in the regime of convective instability. The advection-like term, necessary to turn the instability to convective for the parameter region where advection overwhelms the growth, can stem from pump beam tilting or birefringence induced walk-off. The growth dynamics of both noise-sustained and deterministic patterns is exemplified by means of movies. This allows to observe the process of formation of these structures and to confirm the analytical predictions. The amplification of quantum noise by several orders of magnitude is predicted. The qualitative analysis of the near- and far-field is given. It suffices to distinguish noise-sustained from deterministic structures; quantitative informations can be obtained in terms of the statistical properties of the spectra.

Polarization switching in quantum-well vertical-cavity surface-emitting lasers.

Switching between linearly polarized states of slightly different optical frequencies with the same transversemode pattern is found, as the injection current is increased. Switchings found here for semiconductor rateequation models incorporating a vector electric field, birefringence, and the alpha factor are similar to previously reported experimental results.

Pulse statistics in single-mode semiconductor lasers modulated at gigahertz rates.

The statistics of switch-on time, maximum light intensity, and pulse width of single-mode lasers modulated at gigahertz rates are analyzed. Numerical results obtained from noise-driven rate equations are reported. Pulse statistics, and in particular timing jitter, are shown to be rather insensitive to the bias current at this high-speed modulation. In addition, pulse statistics become rather independent of the modulation period when biasing slightly below threshold.