Testing Critical Slowing Down as a Bifurcation Indicator in a Low-Dissipation Dynamical System.

We study a two-dimensional low-dissipation nonautonomous dynamical system, with a control parameter that is swept linearly in time across a transcritical bifurcation. We investigate the relaxation time of a perturbation applied to a variable of the system and we show that critical slowing down may occur at a parameter value well above the bifurcation point. We test experimentally the occurrence of critical slowing down by applying a perturbation to the accessible control parameter and we find that this perturbation leaves the system behavior unaltered, thus providing no useful information on the occurrence of critical slowing down. The theoretical analysis reveals the reasons why these tests fail in predicting an incoming bifurcation.

Extreme events in chaotic lasers with modulated parameter.

We study a theoretical model describing a laser with a modulated parameter, concentrating on the appearance of extreme events, also called optical rogue pulses. It is shown that two conditions are required for the appearance of such events in this type of nonlinear system: the existence of generalized multi-stability and the collisions of chaotic attractors with unstable orbits in external crisis, expanding the attractor to visit new regions in phase space.

Bistable and addressable localized vortices in semiconductor lasers.

We demonstrate experimentally that localized emission states in coupled broad-area semiconductor lasers can carry a finite orbital angular momentum. The resulting structures therefore possess the chirality of optical vortices together with the properties of localized structures in dissipative systems, namely, the coexistence with a low intensity homogeneous emission and the mutual independence. These results open the way to the realization of arrays of optically addressable and bistable chiral laser pixels.

Cavity soliton laser based on mutually coupled semiconductor microresonators.

We report on experimental observation of localized structures in two mutually coupled broad-area semiconductor resonators, one of which acts as a saturable absorber. These structures coexist with a dark homogeneous background and they have the same properties as cavity solitons without requiring the presence of a driving beam into the system. They can be switched individually on and off by means of a local addressing beam.

Optimization of the switch-on and switch-off transition in a commercial laser.

The response of a Class B laser to a rapid change in one of its parameters is known to be accompanied by delay and ringing. It has been theoretically and numerically shown that the transition can be modified by using adequate functional shapes for the control parameter (e.g., the laser pump) in order to steer the laser from one point of operation to another. Here we experimentally show the implementation of these ideas in a commercial device: a semiconductor laser. We establish a procedure for optimizing a controlled switch-on and switch-off and obtain a clean, fast, and reliable square pulse, either in a single shot or in a repetitive sequence. The generality of this procedure promises a wide field of application for a variety of laser systems.

Spatial structure of broad-area vertical-cavity regenerative amplifiers.

We investigate the spatial structure of broad-area vertical-cavity regenerative amplifiers injected with a homogeneous beam. The emerging patterns have a predominantly sixfold rotational symmetry, verifying the recent prediction of formation of hexagons. The length scale is controllable by means of detuning and follows the prediction for tilted waves.

Mechanisms of polarization switching in single-transverse-mode vertical-cavity surface-emitting lasers: thermal shift and nonlinear semiconductor dynamics.

We analyze polarization switching in vertical-cavity surface-emitting lasers, taking into account a proper semiconductor frequency-dependent complex susceptibility and spin-flip processes. Thermal effects are included as a varying detuning, and gain differences arise from birefringence splitting. We find that, for large birefringence, gain differences between the two linearly polarized modes are preponderant, and switching occurs owing to thermal shift. For small birefringence polarization switching from the high- to the low-gain mode occurs owing to the combined effect of birefringence and semiconductor phase-amplitude-coupled dynamics for a finite value of the carrier spin-flip rate.