We report on the observation of coherence resonance for a semiconductor laser with short optical feedback close to Hopf bifurcations. Noise-induced self-pulsations are documented by distinct Lorentzian-like features in the power spectrum. The character of coherence is critically related to the type of the bifurcation. In the supercritical case, spectral width and height of the peak are monotonic functions of the noise level. In contrast, for the subcritical bifurcation, the width exhibits a minimum, translating into resonance behavior of the correlation time in the pulsation transients. A theoretical analysis based on the generic model of a self-sustained oscillator demonstrates that these observations are of general nature and are related to the fact that the damping depends qualitatively different on the noise intensity for the subcritical and supercritical case.
Two delay-coupled semiconductor lasers are studied in the regime where the coupling delay is comparable to the time scales of the internal laser oscillations. Detuning the optical frequency between the two lasers, novel delay-induced scenarios leading from optical frequency locking to successive states of periodic intensity pulsations are observed. We demonstrate and analyze these dynamical phenomena experimentally using two distinct laser configurations. A theoretical treatment reveals the universal character of our findings for delay-coupled systems.
In this work, self-organization in semiconductor lasers with ultra-short optical feedback is investigated. Exploiting dc currents to tune the relevant feedback parameters, we have experimentally prepared and studied a number of novel nonlinear dynamical scenarios.Two different types of self-sustaining intensity-pulsations are detected depending on strength and phase of the feedback. One type of pulsations is emerging in a Hopf-bifurcation from relaxation oscillations. These oscillations become undamped due to dispersive self-Q switching. The second type of pulsations is a beating of distinct compound-cavity modes. It is also born in a Hopf bifurcation. These findings represent experimental evidence for theoretical predictions. A supplementary mode and stability analysis agrees well with measurements.Coexistence of mode beating and relaxation oscillations gives rise to the break-up of regular pulsations into chaotic emission via a quasi-periodic route to chaos. The sudden destruction of chaos is indicative of a boundary crisis scenario, in which we see a discontinuous disappearance of an attractor. The existence of chaotic saddles underlying transient chaotic dynamics which appears behind boundary crisis is experimentally verified. It is experimentally demonstrated that an excitation of chaotic transients is closely related to a conventional excitability. The experiment is supplemented by numerical simulations.The influence of external Gaussian noise close to the onset of sub-and super-critical Hopf bifurcations is studied. Noise-induced oscillations appear as a noisy precursor with Lorentzian shape peak in the power spectrum. The coherence factor defined by the product of height and quality factor exhibits non-monotonic behavior with a distinct maximum at a certain noise intensity for both types of Hopf bifurcations, demonstrating coherence resonance. Besides these similarities, the measurements reveal also qualitative differences between the two cases. Whereas the width of the noise induced peak increases monotonically with noise intensity for the supercritical bifurcation, it traverses a pronounced minimum in the subcritical case. The experimental findings are examined in terms of general model for the noise driven motion close to bifurcations.
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